Absorbance
In plain words. How much light a sample soaks up at a given wavelength, on a log scale.
For example. An absorbance of 1 means only 10% of the light gets through; 2 means only 1%.
Why you should know this. Every UV-visible and many detector methods report it; its log scale explains why very high readings are unreliable.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage.
Accuracy
In the international vocabulary, both at once: good precision and good trueness. (from Is the calibration still holding? An internal standard plus a CRM as your QC)
In plain words. How close a result is to the true value.
For example. Five darts scattered around the bullseye are accurate on average but not precise.
Why you should know this. A precise but inaccurate method passes its control chart and still reports the wrong answer; accuracy is checked against a known value.
Sources. ICH Q2(R2), Validation of Analytical Procedures (2023). Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Adiabatic
In plain words. Happening too fast for heat to move in or out.
For example. A sound wave squeezes the liquid so quickly that no heat escapes.
Why you should know this. It explains why sound gives the adiabatic, not the isothermal, compressibility.
Sources. Kinsler, L. E.; Frey, A. R.; Coppens, A. B.; Sanders, J. V. Fundamentals of Acoustics. Wiley. Atkins, P.; de Paula, J. Atkins’ Physical Chemistry. Oxford University Press.
Aliquot
In plain words. A measured portion taken from a larger amount of sample.
For example. Pipetting exactly 10 mL out of a 1 L bottle gives you a 10 mL aliquot.
Why you should know this. Every dilution, spike and replicate starts with an aliquot, and its volume sets the uncertainty that follows.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Analyte
In plain words. The thing you are actually trying to measure in the sample.
For example. In a test for lead in drinking water, lead is the analyte; the water and everything else in it is not.
Why you should know this. Every method, result and report is about one analyte; naming it clearly is the first step in choosing how to measure it.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. IUPAC. Compendium of Chemical Terminology (the “Gold Book”), online edition.
API gravity
An inverse density scale for petroleum defined by the American Petroleum Institute; higher API means a lighter (less dense) oil, with water at 10. (from The power of the densitometer)
In plain words. The oil industry's density scale. Higher API means lighter oil; water is 10.
For example. A light crude might be 40° API; a heavy crude might be 15°.
Why you should know this. Crude oils and fuels are traded and graded by it.
Sources. ASTM D1250 / API MPMS Chapter 11.1, Petroleum Measurement Tables. ASTM D4052, Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter.
Apparent molar volume
The effective volume one mole of solute appears to add to a solution, capturing the fact that volumes do not simply add on mixing. (from Reading concentration from a single number)
In plain words. The volume a dissolved substance seems to add to a solution, which is often not the volume it took up on its own.
For example. Dissolving salt in water adds less volume than the dry salt occupied, because the water packs tighter around the ions.
Why you should know this. It explains why density-concentration curves differ between solutes and must be calibrated, not calculated.
Sources. Millero, F. J. “The molal volumes of electrolytes.” Chemical Reviews 71 (1971) 147–176. Atkins, P.; de Paula, J. Atkins’ Physical Chemistry. Oxford University Press.
Base oil
The refined oil that forms the bulk of a lubricant, before additives; graded by viscosity and characterised partly by density. (from Fingerprinting a fluid)
In plain words. The refined oil that makes up most of a lubricant before the additives go in.
For example. Motor oil is mostly base oil plus a package of additives.
Why you should know this. Lubricant blending and quality control start from base-oil properties.
Sources. ASTM D2270, Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 °C and 100 °C. ASTM D445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids.
Baseline
The new oil’s own acid or base number, measured by the same laboratory and method. (from Reading an oil's acid and base numbers)
In plain words. The known-good starting value that later results are compared against.
For example. The new oil measured 100 cSt; a later 95 cSt only means something against that baseline.
Why you should know this. Monitoring is trend-reading; without a baseline a result cannot be called good or bad.
Sources. ASTM D5185, Standard Test Method for Multielement Determination of Used and Unused Lubricating Oils and Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES).
Baumé
Historical density scales expressed in the units a trade cared about — general solutions, sugar, and petroleum respectively. (from Reading concentration from a single number)
In plain words. An old hydrometer scale for liquid strength, still seen with acids and caustics.
For example. Sulfuric acid strength in some plants is still quoted in degrees Baumé.
Why you should know this. You will still meet it on acid and caustic specifications and old plant procedures.
Sources. Green, D. W.; Southard, M. Z. (eds.) Perry’s Chemical Engineers’ Handbook. McGraw-Hill.
Beer–Lambert law
In plain words. The rule that absorbance rises in step with concentration, which holds at modest absorbance.
For example. Twice the dye in a cuvette absorbs twice as much light, until the reading gets too high.
Why you should know this. It is why absorbance methods are calibrated at modest absorbance and high samples are diluted.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage.
Binary system
A mixture of exactly two components — one solute in one solvent. (from Reading concentration from a single number)
In plain words. A mixture of exactly two things: one dissolved in another.
For example. Salt in water, or ethanol in water.
Why you should know this. Only in a true two-component mixture can one number like density name the concentration.
Sources. Atkins, P.; de Paula, J. Atkins’ Physical Chemistry. Oxford University Press.
Blank
The same titration with everything except the sample; its volume is subtracted. (from Quality control for titration)
In plain words. A sample with everything except the analyte, run to see what the instrument reads when there is nothing there.
For example. Pure reagents taken through the whole method; their small reading is the background you subtract.
Why you should know this. Every trace method subtracts a blank, and its scatter sets your detection limit.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. Currie, L. A. “Limits for qualitative detection and quantitative determination.” Analytical Chemistry 40 (1968) 586–593.
Bracketing
The main calibration strategies — an external curve; a ratio to an added reference; spiking the sample itself; and standards run tight around the result. (from The number the instrument can't give you)
In plain words. Running standards just above and just below the expected result, close in time to the sample.
For example. For a sample near 50 ppm, run 45 and 55 ppm standards right before and after it.
Why you should know this. Used where tight accuracy near a specification limit matters, such as release testing.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
°Brix
A density-based scale for dissolved sucrose; 1 °Brix ≈ 1 g sucrose per 100 g of solution. (from The power of the densitometer)
In plain words. A scale for how much sugar is dissolved: 1 °Brix is about 1 g sugar per 100 g of solution.
For example. Grape juice at 24 °Brix is about 24% sugar by weight.
Why you should know this. The standard way food, beverage and sugar labs report dissolved sugar.
Sources. ICUMSA Methods Book (International Commission for Uniform Methods of Sugar Analysis): sucrose and °Brix tables.
Bulk property
A property of the material as a whole rather than of its individual components — density, refractive index and viscosity are all bulk properties. (from The power of the densitometer)
In plain words. A property of the whole sample, not of any one ingredient in it.
For example. Density tells you how heavy the whole liquid is, not which molecules made it so.
Why you should know this. It reminds you that one bulk number cannot tell you which components are present.
Sources. Atkins, P.; de Paula, J. Atkins’ Physical Chemistry. Oxford University Press.
Calibration
The deliberate mapping of instrument response onto concentration, built from standards of known value. (from The number the instrument can't give you)
In plain words. Teaching the instrument what its signal means by showing it samples whose answer you already know.
For example. Run standards of 1, 5 and 10 ppm, note the signal for each, and you can turn any later signal into a concentration.
Why you should know this. Every number your instrument reports depends on it; you calibrate when a method starts, after maintenance, and whenever QC says the map has moved.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. ICH Q2(R2), Validation of Analytical Procedures (2023).
Calibration curve
In plain words. The line (or curve) that links signal to concentration, built from your standards.
For example. Plot peak area against known concentration for five standards; that plot is the calibration curve you read unknowns from.
Why you should know this. You read every unknown from it, so you check its shape, its range and its low end before you trust it.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry. Pearson.
Capillary viscometer
In plain words. A glass tube you time liquid flowing through; slower flow means thicker liquid.
For example. The classic method for motor-oil viscosity at 40 °C and 100 °C.
Why you should know this. The reference method for oil and fuel kinematic viscosity.
Sources. ASTM D445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids.
carryover
Response that changes with run order rather than concentration: early injections conditioning the flow path (priming), or residue from one injection biasing the next (carryover). (from The number the instrument can't give you)
In plain words. Traces of one sample left in the system that show up in the next run.
For example. A blank run right after a strong sample shows a small peak that should not be there.
Why you should know this. A blank after your highest standard tells you whether one sample is contaminating the next.
Sources. McNair, H. M.; Miller, J. M.; Snow, N. H. Basic Gas Chromatography. Wiley. ICH Q2(R2), Validation of Analytical Procedures (2023).
Censored (non-detect)
A result below the decision threshold is "less-than," not zero; summarising such data needs methods for censoring, not a substituted fill value. (from Where the signal loses to the noise)
In plain words. A result reported as 'less than' a limit, not as a number.
For example. '< 5 ppb' does not mean zero, so you cannot just average it in as 0.
Why you should know this. Averaging non-detects as zero biases results low; censored data need proper handling.
Sources. Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry. Pearson.
Certified reference material (CRM)
A material with an independently assigned value and a stated uncertainty, traceable to a reference. (from Is the calibration still holding? An internal standard plus a CRM as your QC)
In plain words. A standard sold with a certificate stating its value, its uncertainty and how it traces back to national references.
For example. A NIST or other certified material you run to prove your method gives the right answer, not just a repeatable one.
Why you should know this. Run one to prove a method is accurate, not just repeatable, and to trace your results to a recognised reference.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. IUPAC. Compendium of Chemical Terminology (the “Gold Book”), online edition.
Cetane improver
An additive that raises the measured cetane number without changing density or distillation — so the calculated index does not register it. (from Calculated, not measured)
In plain words. An additive that makes diesel ignite more readily.
For example. It raises the real cetane number but not the calculated index.
Why you should know this. It explains why a calculated index and a measured cetane number can disagree.
Sources. ASTM D613, Standard Test Method for Cetane Number of Diesel Fuel Oil. ASTM D4737, Standard Test Method for Calculated Cetane Index by Four Variable Equation.
Cetane index
An estimate of diesel ignition quality computed from density and distillation temperatures (ASTM D4737), not measured in an engine. (from The power of the densitometer)
In plain words. An estimate of the cetane number worked out from density and boiling points, not measured in an engine.
For example. Cheap and fast, but it cannot see a cetane-improver additive.
Why you should know this. Labs report it when an engine test is not available; it must be labelled as calculated.
Sources. ASTM D4737, Standard Test Method for Calculated Cetane Index by Four Variable Equation.
Cetane number
How readily a diesel ignites under compression; measured in a CFR engine (ASTM D613). Higher means shorter ignition delay. (from Calculated, not measured)
In plain words. How quickly a diesel fuel ignites when compressed. Higher means quicker and smoother.
For example. Road diesel is typically in the 40s to 50s.
Why you should know this. Diesel specifications set a minimum cetane number.
Sources. ASTM D613, Standard Test Method for Cetane Number of Diesel Fuel Oil.
CFR engine
A single-cylinder Cooperative Fuel Research engine used to measure cetane and octane numbers under standard conditions. (from Calculated, not measured)
In plain words. A standard single-cylinder test engine used to rate fuels.
For example. The reference way to measure cetane or octane numbers.
Why you should know this. It is the reference test behind cetane and octane numbers.
Sources. ASTM D613, Standard Test Method for Cetane Number of Diesel Fuel Oil.
Chromatogram
In plain words. The trace from a chromatograph: signal against time, with a peak for each separated compound.
For example. Each bump on the line is one compound coming out of the column.
Why you should know this. Looking at it, not just the report, catches missing peaks, overlaps and integration errors.
Sources. McNair, H. M.; Miller, J. M.; Snow, N. H. Basic Gas Chromatography. Wiley.
Co-elute
In plain words. Two compounds coming out of the column at the same time, so their peaks overlap.
For example. Two peaks sitting on top of each other in a chromatogram.
Why you should know this. Co-eluting peaks give wrong areas; recognising them tells you when to change the separation or use a selective detector.
Sources. McNair, H. M.; Miller, J. M.; Snow, N. H. Basic Gas Chromatography. Wiley.
Compressibility
In plain words. How much a liquid shrinks when you squeeze it.
For example. Liquids barely compress, but the small amount they do tells you about their molecules.
Why you should know this. It carries information about molecular forces that density alone does not.
Sources. Atkins, P.; de Paula, J. Atkins’ Physical Chemistry. Oxford University Press. Kinsler, L. E.; Frey, A. R.; Coppens, A. B.; Sanders, J. V. Fundamentals of Acoustics. Wiley.
Critical value (LC)
The threshold each result is judged against; above it, declare "detected." Controls false positives (α). As a signal it is ≈ 1.65·σ for a normal blank of known σ (α = 0.05); in concentration, ≈ 1.65·σ/S — the form used when reporting "< LC." (from Where the signal loses to the noise)
In plain words. The cut-off a single result must clear before you call the analyte 'detected'.
For example. If the critical value is 5 ppb, a result of 4 ppb is reported as not detected.
Why you should know this. It is the line you judge each individual result against when deciding 'detected' or 'not detected'.
Sources. Currie, L. A. “Limits for qualitative detection and quantitative determination.” Analytical Chemistry 40 (1968) 586–593.
Degree of freedom
The number of composition variables you may set independently. A binary solution has one (at fixed T and P). (from Reading concentration from a single number)
In plain words. How many things about the mixture you can change independently.
For example. Salt water at a fixed temperature has one: once you know how salty it is, everything else follows.
Why you should know this. It is the reason one density reading can name the strength of a simple solution and not a complex one.
Sources. Atkins, P.; de Paula, J. Atkins’ Physical Chemistry. Oxford University Press.
Density (ρ)
Mass per unit volume of a substance (g/mL, kg/m³). (from Reading concentration from a single number)
In plain words. How much mass is packed into a given volume.
For example. Water is about 1.00 g/mL and gasoline about 0.74 g/mL, which is why gasoline floats on water.
Why you should know this. It identifies materials, converts volume to mass for custody transfer, and reads concentration in simple solutions.
Sources. ASTM D4052, Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Drift
How far the baseline wanders over time, usually stated per hour. (from The detector is the constant)
In plain words. A slow, steady change in readings over time or over run order, not caused by the sample.
For example. The same check standard reads 100, 101, 102, 103 as the run goes on: that steady climb is drift.
Why you should know this. Spotting drift early (with check standards through the run) saves rerunning a whole batch.
Sources. ASTM D6299, Standard Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Dynamic range
The span over which the detector gives an increasing, usable response — from the detection limit up to where response tops out; wider than the straight part, which is the "linear dynamic range." Beyond linearity it must be calibrated with a curve. (from Where the line stops being straight)
In plain words. The whole range where the signal still goes up with more analyte, even if no longer in a straight line.
For example. A detector may still respond at 10 %, but only the lower part of that range is straight.
Why you should know this. It tells you how far you can push a detector, and that only the straight part is easy to calibrate.
Sources. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage.
Dynamic viscosity (μ)
The internal resistance to shear itself, independent of density (Pa·s, or centipoise). (from Fingerprinting a fluid)
In plain words. The liquid's own resistance to being sheared, regardless of how heavy it is.
For example. Measured in centipoise (cP); water at room temperature is about 1 cP.
Why you should know this. Engineering calculations of pumping and flow need it; many meters report it directly.
Sources. ASTM D7042, Standard Test Method for Dynamic Viscosity and Density of Liquids by Stabinger Viscometer. Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology. Elsevier (1989).
Electrostriction
In plain words. Water molecules squeezing in tightly around charged ions, so the solution takes up less room than expected.
For example. One reason 1 L of water plus salt makes slightly less than the volumes would suggest.
Why you should know this. It explains why mixing volumes do not simply add, which matters whenever you prepare solutions by volume.
Sources. Millero, F. J. “The molal volumes of electrolytes.” Chemical Reviews 71 (1971) 147–176.
External standard
The main calibration strategies — an external curve; a ratio to an added reference; spiking the sample itself; and standards run tight around the result. (from The number the instrument can't give you)
In plain words. The everyday calibration: run standards separately from the samples and read samples off that curve.
For example. Run 1, 5 and 10 ppm standards, then run samples and read them from the line.
Why you should know this. The default calibration; knowing it well tells you when you need something stronger.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage.
Extrapolation
Reading beyond the highest standard, or off a straight line in a curved region — outside the calibration, not allowed. (from Where the line stops being straight)
In plain words. Reading beyond the range your standards actually covered.
For example. Calibrating to 50 ppm and reporting a sample at 200 ppm from the same line.
Why you should know this. Results outside the calibrated range are not calibrated; you dilute or report them as above range.
Sources. ICH Q2(R2), Validation of Analytical Procedures (2023). Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
False positive
Calling a blank "detected" (controlled by LC) versus missing a real low sample (controlled by placing LD above LC). (from Where the signal loses to the noise)
In plain words. A false positive says something is there when it is not; a false negative misses something that is.
For example. A smoke alarm going off for toast is a false positive; one that stays silent in a real fire is a false negative.
Why you should know this. Every detection decision balances these two errors; regulators often care more about one than the other.
Sources. Currie, L. A. “Limits for qualitative detection and quantitative determination.” Analytical Chemistry 40 (1968) 586–593.
Flexural mode
A standing bending vibration of the tube — fixed at the clamps, moving most in the middle — not a rigid swing. (from The oscillating U-tube, in depth)
In plain words. A bending vibration: the tube flexes in the middle while its ends stay held.
For example. Like a ruler clamped at both ends and plucked in the middle.
Why you should know this. It explains why viscous samples need a correction in vibrating-tube meters.
Sources. ASTM D4052, Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter.
Fuel dilution
Unburned fuel entering the oil, lowering its viscosity and density. (from Reading wear from bulk properties)
In plain words. Unburned fuel leaking into the engine oil and thinning it.
For example. Short trips and worn injectors are common causes.
Why you should know this. Falling oil viscosity with a fuel smell points here, and it is a common cause of engine wear.
Sources. ASTM D445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids.
Gibbs phase rule
In plain words. A rule that counts how many things you can vary in a mixture before everything else is fixed.
For example. It is why one number, such as density, can be enough to pin down the strength of a simple two-part solution.
Why you should know this. It tells you how many measurements you need to pin down a mixture.
Sources. Atkins, P.; de Paula, J. Atkins’ Physical Chemistry. Oxford University Press.
Heteroscedasticity
Scatter that grows with concentration, breaking the equal-variance assumption of ordinary least squares — the case that calls for weighted regression. (from The number the instrument can't give you)
In plain words. Scatter that grows as concentration grows, so the high standards wobble more than the low ones.
For example. Repeat injections at 100 ppm vary by ±2 ppm while those at 1 ppm vary by ±0.02 ppm.
Why you should know this. If your scatter grows with concentration, an ordinary fit will short-change your low standards; this tells you when to weight.
Sources. Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry. Pearson.
Hydrometer
A weighted float that sinks to a depth set by a liquid's density; the oldest practical density-to-concentration instrument. (from Reading concentration from a single number)
In plain words. A weighted glass float that sinks to a depth set by the liquid's density; you read the scale where it floats.
For example. Brewers float a hydrometer in wort to see how much sugar is in it.
Why you should know this. Still used in plants and breweries; knowing its limits helps you decide when a digital meter is worth it.
Sources. Green, D. W.; Southard, M. Z. (eds.) Perry’s Chemical Engineers’ Handbook. McGraw-Hill.
IAPWS-95
The internationally agreed formulation fixing the density of pure water — the anchor for density calibration. (from The oscillating U-tube, in depth)
In plain words. The internationally agreed equations for the properties of pure water, including its exact density.
For example. Why calibrating a density meter with pure water ties it to a reference nobody argues with.
Why you should know this. It is the reference behind water calibration of density meters.
Sources. Wagner, W.; Pruß, A. “The IAPWS Formulation 1995 for the Thermodynamic Properties of Ordinary Water Substance for General and Scientific Use.” J. Phys. Chem. Ref. Data 31 (2002) 387–535.
ICP (inductively coupled plasma)
In plain words. Inductively coupled plasma: a very hot gas flame used to break samples into atoms so their metals can be measured.
For example. ICP instruments measure trace metals in water, oils and digested solids.
Why you should know this. Most trace-metal results you read come from an ICP instrument.
Sources. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage.
ICP-OES
Inductively coupled plasma optical emission — identifies and quantifies wear metals in the oil. (from Reading wear from bulk properties)
In plain words. An instrument that heats a sample in a plasma until the elements glow, and reads which metals are there from the colours of light.
For example. Finding iron from gears and copper from bearings in used oil.
Why you should know this. The routine way labs measure wear metals and additives in oil.
Sources. ASTM D5185, Standard Test Method for Multielement Determination of Used and Unused Lubricating Oils and Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage.
Ill-conditioned
A calculation where small input errors cause large output errors — here, two calibration relationships too similar to solve cleanly. (from When one number isn't enough)
In plain words. A calculation where a tiny error going in becomes a big error coming out.
For example. Two nearly parallel lines: nudge one slightly and the crossing point jumps a long way.
Why you should know this. It warns you when two measurements are too similar to solve a mixture reliably.
Sources. Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry. Pearson.
Independent equations
Two relationships that carry different information, so together they can solve for two unknowns. Two relationships that say the same thing cannot. (from When one number isn't enough)
In plain words. Two relationships that tell you different things, so together they can solve two unknowns.
For example. Density and refractive index react differently to sugar and to alcohol, so together they can split the two apart.
Why you should know this. It is the test for whether a second measurement actually adds information.
Sources. Atkins, P.; de Paula, J. Atkins’ Physical Chemistry. Oxford University Press.
Intercept
In plain words. Where the line crosses the axis when the other value is zero.
For example. A calibration intercept is the signal the line predicts at zero concentration.
Why you should know this. A large intercept can reveal contamination, a blank problem or a curve that does not pass through zero.
Sources. Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry. Pearson.
Internal standard
A reference added equally to every sample and standard; the analyte/reference ratio cancels effects that hit both — in ICP the primary, routine control for multiplicative matrix effects. (from Calibrate inside the sample)
In plain words. A known amount of a reference compound added to every vial, so you measure the analyte against it instead of trusting the raw signal.
For example. If a shaky injection delivers 5% too little, both the analyte and the internal standard drop 5%, and their ratio stays right.
Why you should know this. Reach for it when injection volume, sample preparation or instrument drift varies from run to run.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage.
Interpolate
In plain words. Reading a value between your standards, inside the calibrated range.
For example. A signal between your 10 and 20 ppm standards, read off the line as 14 ppm.
Why you should know this. Reading inside your standards is what a calibration supports.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Isothermal compressibility
Compressibility measured slowly, at constant temperature; larger than the adiabatic value and not what sound speed gives. (from The second number your density meter already measures)
In plain words. How much a liquid shrinks when squeezed slowly at constant temperature.
For example. A little larger than the fast, adiabatic value that sound speed measures.
Why you should know this. Knowing the difference keeps you from comparing two different quantities.
Sources. Atkins, P.; de Paula, J. Atkins’ Physical Chemistry. Oxford University Press.
Isotope dilution
Quantitation with a labelled analog as internal standard, from the natural/labelled ion-signal ratio. In its classical gravimetric form a reference-grade method; the routine labelled-internal-standard-plus-curve version is the same idea, not the same rigour. (from The reference that rides along)
In plain words. Measuring an analyte against a heavier copy of itself, so almost every loss and interference cancels.
For example. Used for reference-grade measurements where the answer has to stand up to any audit.
Why you should know this. Used for reference values and the most demanding trace analyses.
Sources. de Hoffmann, E.; Stroobant, V. Mass Spectrometry: Principles and Applications. Wiley. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Isotopically labelled analog
A copy of the analyte with atoms replaced by a heavier isotope; chemically almost identical (¹³C/¹⁵N closer than ²H), it co-elutes and differs mainly in mass. (from The reference that rides along)
In plain words. A copy of the analyte made with heavier atoms: it behaves almost identically but weighs a little more.
For example. Caffeine with three heavy-carbon atoms, used to measure ordinary caffeine by mass spectrometry.
Why you should know this. The best internal standard for mass spectrometry, when one is available.
Sources. de Hoffmann, E.; Stroobant, V. Mass Spectrometry: Principles and Applications. Wiley.
Kinematic viscosity (ν)
Dynamic viscosity divided by density — flow under the fluid's own weight (mm²/s, or centistokes). (from Fingerprinting a fluid)
In plain words. How fast a liquid flows under its own weight: dynamic viscosity divided by density.
For example. Timing oil draining through a glass capillary gives kinematic viscosity, in centistokes (cSt).
Why you should know this. Most oil and fuel specifications are written in it (centistokes at 40 °C and 100 °C).
Sources. ASTM D445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids.
Limit of detection (LOD)
In plain words. The smallest amount the method can reliably tell apart from nothing.
For example. With an LOD of 10 ppb, a sample truly at 10 ppb will be detected almost every time.
Why you should know this. It tells a customer the lowest level your method can see, and is required in method validation.
Sources. ICH Q2(R2), Validation of Analytical Procedures (2023). Currie, L. A. “Limits for qualitative detection and quantitative determination.” Analytical Chemistry 40 (1968) 586–593.
Limit of quantitation (LOQ)
The lowest concentration measurable with acceptable precision; conventionally ≈ 10·σ/S (≈ 10% RSD only if the scatter there equals the blank's). (from Where the signal loses to the noise)
In plain words. The smallest amount the method can put a trustworthy number on.
For example. Between the LOD and the LOQ you can say 'it is there', but not reliably 'how much'.
Why you should know this. It is the lowest value you should report as a number; below it you report a qualified result.
Sources. ICH Q2(R2), Validation of Analytical Procedures (2023).
Linear range
The concentration span over which response rises in a straight line (with or without an intercept) — where the calibration is genuinely linear. (from Where the line stops being straight)
In plain words. The range where doubling the amount doubles the signal: a straight line.
For example. A detector straight from 0.5 to 2.5 % but bending above that has a linear range up to about 2.5 %.
Why you should know this. You keep samples inside it, or dilute them, so the calibration line applies.
Sources. ICH Q2(R2), Validation of Analytical Procedures (2023). Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
m/z (mass-to-charge ratio)
In plain words. Mass-to-charge ratio: how a mass spectrometer sorts ions, roughly by their mass.
For example. An ion of mass 78 with one charge appears at m/z 78.
Why you should know this. Mass spectrometry identifies and separates compounds by it.
Sources. de Hoffmann, E.; Stroobant, V. Mass Spectrometry: Principles and Applications. Wiley.
Matrix
Everything in the sample besides the analyte, which can change the response. (from Standard addition in GC, and mass versus volume results)
In plain words. Everything in the sample that is not the analyte: the liquid it is dissolved in and whatever else came along.
For example. Measuring iron in used engine oil: the oil, soot and additives are the matrix.
Why you should know this. Most wrong results that look right come from the matrix, so you check it whenever a sample type changes.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. IUPAC. Compendium of Chemical Terminology (the “Gold Book”), online edition.
Matrix effect
Anything in the sample other than the analyte that changes the measured response — its sensitivity (multiplicative) or an added background (additive). (from Calibrate inside the sample)
In plain words. When the rest of the sample changes how strongly the instrument responds to your analyte, so the same amount reads high or low.
For example. Salt in seawater can dim a metal's signal, so 1 ppm in seawater reads lower than 1 ppm in clean water.
Why you should know this. When a spiked sample recovers 75% instead of 100%, a matrix effect is the first suspect.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage.
Matrix factor
The in-matrix slope divided by the clean-standard slope — the sensitivity remaining (below 1) or gained (above 1) in the matrix. Not a spike-recovery, which also includes preparation losses. (from Calibrate inside the sample)
In plain words. How much of the instrument's sensitivity survives in the real sample, compared with clean standards.
For example. A matrix factor of 0.77 means the sample matrix hid about 23% of the signal.
Why you should know this. It quantifies how much a sample type suppresses or enhances your signal.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Matrix-matched standards
Standards prepared in a matrix that mimics the sample's, so they respond like it — the first remedy for a matrix effect when the matrix can be reproduced. (from Calibrate inside the sample)
In plain words. Standards made up in something that looks like your sample, so they respond the way the sample does.
For example. Making zinc standards in clean base oil to measure zinc in lubricating oil, instead of in water.
Why you should know this. The first fix to try when clean standards and real samples disagree.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage.
Method detection limit (MDL)
In plain words. A detection limit measured through the whole procedure, in the real sample type.
For example. Spike real soil extracts at a low level seven times, run them all the way through, and calculate from their scatter.
Why you should know this. Environmental labs reporting under US EPA rules must determine it by the official procedure.
Sources. US EPA, 40 CFR Part 136, Appendix B: Definition and Procedure for the Determination of the Method Detection Limit (Revision 2, 2016).
Molality
Moles of solute per kilogram of solvent; it does not change with temperature. (from Concentration: the ways labs say how much)
In plain words. Concentration as moles of substance per kilogram of solvent (not of solution).
For example. 1 mol of salt in 1 kg of water is 1 molal, whatever the final volume turns out to be.
Why you should know this. Used where temperature changes volume, since it does not depend on volume.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Molarity
Ways to state concentration: moles per litre of solution; reactive equivalents per litre; moles per kilogram of solvent; grams of solute per 100 g of solution. (from Reading concentration from a single number)
In plain words. Concentration as moles of substance per litre of solution.
For example. 1 M sodium chloride is 58.44 g of salt made up to 1 litre.
Why you should know this. The most common concentration unit for preparing reagents and standards.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Monotonic
Always moving in one direction — here, density that only rises (or only falls) as concentration increases, so each density maps to a single concentration. (from Reading concentration from a single number)
In plain words. Always moving the same way: only going up, or only going down.
For example. If density only ever rises as you add more salt, each density reading points to exactly one concentration.
Why you should know this. If a curve doubles back, one reading could mean two concentrations; you check monotonicity across your range.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Multi-point calibration
Calibration from several standards (usually ≥5) spanning the range, so slope, intercept, and linear limits are measured, not assumed. (from The number the instrument can't give you)
In plain words. A calibration built from several standards across the range instead of just one.
For example. Five standards from 1 to 100 ppm show whether the response is really a straight line, which one standard never can.
Why you should know this. It is how you prove a response is linear and find where it stops being linear.
Sources. ICH Q2(R2), Validation of Analytical Procedures (2023). Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry. Pearson.
Multigrade oil
An oil meeting a cold grade and a hot grade at once (e.g. 10W-40), enabled by a high VI. (from Viscosity index, and what it really tells you)
In plain words. An oil that meets a cold-weather grade and a hot-running grade at once.
For example. 10W-40 behaves like a thin oil when cold and a thicker one when hot.
Why you should know this. Most engine oils are multigrade; the grade tells you cold and hot behaviour.
Sources. SAE J300, Engine Oil Viscosity Classification.
Newtonian
A fluid whose viscosity is a single number at a given temperature, independent of shear rate. (from When one viscosity number isn’t enough)
In plain words. A liquid with one viscosity, however fast you stir it.
For example. Water, gasoline and most light oils.
Why you should know this. If a fluid is Newtonian, one viscosity number is enough; that decides which instrument you need.
Sources. Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology. Elsevier (1989).
Newton–Laplace equation
In plain words. The equation linking sound speed, density and how squeezable a liquid is.
For example. Measure sound speed and density, and it gives you the compressibility.
Why you should know this. It turns sound speed and density into compressibility.
Sources. Kinsler, L. E.; Frey, A. R.; Coppens, A. B.; Sanders, J. V. Fundamentals of Acoustics. Wiley.
Non-destructive
A measurement that does not consume or alter the sample. (from The power of the densitometer)
In plain words. A test that leaves the sample unchanged, so you can use it again.
For example. A density reading leaves the sample ready for the next test.
Why you should know this. It means the same sample can go on to other tests.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Non-Newtonian
A fluid whose viscosity changes with shear rate, so one number cannot describe it. (from When one viscosity number isn’t enough)
In plain words. A liquid whose thickness changes with how hard or fast you push it.
For example. Ketchup is thick in the bottle but flows once you shake it.
Why you should know this. Paints, greases, foods and slurries need viscosity reported at a stated shear rate, or the number is meaningless.
Sources. Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology. Elsevier (1989).
Normality
Ways to state concentration: moles per litre of solution; reactive equivalents per litre; moles per kilogram of solvent; grams of solute per 100 g of solution. (from Reading concentration from a single number)
In plain words. Concentration counted by how much it can react, per litre. Older, still in some methods.
For example. 1 M sulfuric acid is 2 N, because each molecule can give up two hydrogen ions.
Why you should know this. Older methods and titrations still use it; converting correctly avoids factor-of-two errors.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Oil condition monitoring
Tracking an in-service oil’s properties over time to catch problems before failure. (from Reading wear from bulk properties)
In plain words. Testing a machine's oil regularly to spot problems before they cause a breakdown.
For example. A falling viscosity trend in an engine oil warning of fuel leaking into it.
Why you should know this. It catches engine and machine problems early and cheaply, before a failure.
Sources. ASTM D5185, Standard Test Method for Multielement Determination of Used and Unused Lubricating Oils and Base Oils by Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES). ASTM D445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids.
Oscillating U-tube
A vibrating glass tube whose resonant period depends on the density of the fluid filling it; the basis of modern digital density meters. (from Reading concentration from a single number)
In plain words. A small glass tube that vibrates; the heavier the liquid inside, the slower it vibrates, and that gives the density.
For example. The heart of a modern digital density meter, which needs a few drops and under a minute.
Why you should know this. It is the working part of most modern density meters, and its error sources (bubbles, temperature, viscosity) are the ones you guard against.
Sources. ASTM D4052, Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter.
Paraffinic
Saturated chain hydrocarbons (lighter, higher cetane) vs ring hydrocarbons (denser, lower cetane) at the same boiling range. (from Calculated, not measured)
In plain words. Paraffinic: chain-shaped hydrocarbons, lighter and quick to ignite. Aromatic: ring-shaped, denser and slower to ignite.
For example. Two diesels with the same boiling range: the denser one is usually more aromatic.
Why you should know this. Hydrocarbon type drives ignition quality, density and how a fuel or oil behaves.
Sources. Watson, K. M.; Nelson, E. F.; Murphy, G. B. “Characterization of petroleum fractions.” Industrial & Engineering Chemistry 27 (1935) 1460–1464.
Peak area
In plain words. The area under a peak on the chromatogram; it grows with the amount of that compound.
For example. Twice the compound gives roughly twice the area, inside the linear range.
Why you should know this. It is the raw measurement behind most chromatographic results.
Sources. McNair, H. M.; Miller, J. M.; Snow, N. H. Basic Gas Chromatography. Wiley.
Peltier thermostat
In plain words. A small solid-state heater-cooler that holds the cell at an exact temperature.
For example. The same idea that cools some portable drink coolers.
Why you should know this. Precise temperature control is what makes five-decimal density possible.
Sources. ASTM D4052, Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter.
Period (T)
The time for one full vibration — the reciprocal of frequency; timed precisely by averaging many cycles. (from The oscillating U-tube, in depth)
In plain words. The time one full vibration takes.
For example. A vibration of 500 cycles per second has a period of 2 milliseconds.
Why you should know this. Density meters time the period because it can be measured very precisely.
Sources. ASTM D4052, Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter.
Polarizability
How easily a molecule's electron cloud distorts in an electric field such as light; it sets how much the molecule slows and bends light, and so drives refractive index. (from When one number isn't enough)
In plain words. How easily a molecule's electrons get pushed around by light. More easily pushed means more bending of light.
For example. It is why oils bend light more than water does.
Why you should know this. It explains why refractive index and density respond differently to composition, which makes them good partners.
Sources. Hecht, E. Optics. Pearson. Atkins, P.; de Paula, J. Atkins’ Physical Chemistry. Oxford University Press.
ppb (parts per billion)
In plain words. Parts per billion: one part in a billion, a thousand times smaller than ppm.
For example. 1 ppb is about 1 microgram in a litre of water.
Why you should know this. Environmental and trace limits are often set in ppb.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
ppm (parts per million)
Parts per million: an ambiguous shorthand that can mean mg/kg, mg/L or other fractions; write the unit instead. (from Units and dimensional analysis)
In plain words. Parts per million: one part in a million.
For example. 1 ppm is about 1 mg in a litre of water.
Why you should know this. The everyday unit of trace analysis; mixing up ppm and ppb is a thousand-fold error.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Precision
The random variation between repeat measurements of the same thing — distinct from accuracy, which is closeness to the true value. (from The number the instrument can't give you)
In plain words. How close repeat measurements are to each other, whether or not they are right.
For example. Five darts in a tight cluster off to one side are precise but not accurate.
Why you should know this. Repeatability and reproducibility limits in every standard method are precision numbers; you compare your scatter against them.
Sources. ICH Q2(R2), Validation of Analytical Procedures (2023). Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
priming
Response that changes with run order rather than concentration: early injections conditioning the flow path (priming), or residue from one injection biasing the next (carryover). (from The number the instrument can't give you)
In plain words. The first injections coating active spots in the system so later ones read fully.
For example. The first injection of the day reads low because some analyte sticks to a fresh liner; by the third it settles.
Why you should know this. It explains why the first injection of a sequence often reads low, and justifies a conditioning injection.
Sources. McNair, H. M.; Miller, J. M.; Snow, N. H. Basic Gas Chromatography. Wiley.
Proportional
In plain words. A straight line that passes through zero: no analyte, no signal.
For example. Only then can one standard and a single response factor give you the answer.
Why you should know this. Single-point calibration is only valid when the response is proportional.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Pycnometer
A flask of precisely known volume; weighed empty and full, it gives the mass of a known volume of liquid — the older way to measure density. (from The oscillating U-tube, in depth)
In plain words. A small flask of exactly known volume, weighed empty and full to get a liquid's density.
For example. The classic way to measure density before digital meters.
Why you should know this. Still a reference way to check density meters and measure density by weighing.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
QC (quality control)
In plain words. Checks run alongside samples to prove the instrument and method are still working.
For example. A known check sample run every shift: if it reads off, results since the last good check are suspect.
Why you should know this. QC results decide whether a batch of results can be released.
Sources. ASTM D6299, Standard Practice for Applying Statistical Quality Assurance and Control Charting Techniques to Evaluate Analytical Measurement System Performance. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Quality factor (Q)
A measure of how lightly a vibration is damped; a viscous sample lowers it, and the meter uses that to correct the density. (from The oscillating U-tube, in depth)
In plain words. How long a vibration keeps ringing before it dies away; thick liquids damp it faster.
For example. The meter uses it to correct density readings for viscous samples.
Why you should know this. Meters use it to correct density for viscous samples automatically.
Sources. ASTM D4052, Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter.
Recovery
A measured value as a percentage of its known true value; reading standards — especially the lowest — back through the curve exposes bias R² hides. (from The number the instrument can't give you)
In plain words. How much of a known amount you actually measured, as a percentage.
For example. You add 10 ppm and measure 9.5 ppm: that is 95% recovery.
Why you should know this. Spike recoveries and read-back recoveries are how methods prove accuracy in validation and routine QC.
Sources. ICH Q2(R2), Validation of Analytical Procedures (2023). Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Refractive index
How strongly a material bends light; a bulk optical property that varies with composition independently of density. (from The power of the densitometer)
In plain words. How much a liquid bends and slows light. It changes with what is dissolved in it.
For example. A refractometer reads sugar in a drop of fruit juice this way.
Why you should know this. A fast second measurement that, with density, can split a three-part mixture.
Sources. Hecht, E. Optics. Pearson. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Relative response factor (RRF)
The analyte's sensitivity divided by the internal standard's (k_a/k_IS); the relative sensitivity read from the standards (rebuilt and checked each sequence). It equals the plot's slope only when you plot ratio against the concentration ratio. (from The reference that rides along)
In plain words. How strongly the instrument responds to the analyte compared with the internal standard.
For example. If the analyte gives twice the signal of the internal standard at the same amount, its relative response factor is 2.
Why you should know this. Internal-standard and normalization methods use it to put different compounds on one scale; it must be checked, not assumed.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. McNair, H. M.; Miller, J. M.; Snow, N. H. Basic Gas Chromatography. Wiley.
Residuals
The signed distance of each point from the fitted line. A bend (smile/frown) means the wrong model; a widening fan means non-constant scatter. (from The number the instrument can't give you)
In plain words. How far each point sits above or below the fitted line.
For example. If the residuals form a smile (low in the middle, high at the ends), a straight line is the wrong shape for your data.
Why you should know this. Plotting them is the quickest honest check that a straight line is the right model.
Sources. Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry. Pearson.
Resolved
Separated in the signal (a distinct peak and m/z) so analyte and a different-compound internal standard do not overlap. A labelled analog instead co-elutes and is split only by mass. (from The reference that rides along)
In plain words. Separated cleanly, so each compound has its own peak.
For example. Two peaks with flat baseline between them are resolved.
Why you should know this. Integration is only trustworthy for resolved peaks, or with a detector that can tell overlapping compounds apart.
Sources. McNair, H. M.; Miller, J. M.; Snow, N. H. Basic Gas Chromatography. Wiley.
Resonant frequency
The rate at which a system vibrates on its own after a push, set by its stiffness and its mass. (from The oscillating U-tube, in depth)
In plain words. The rate something naturally vibrates at after a tap.
For example. A guitar string's note; a heavier string vibrates slower.
Why you should know this. It is what a vibrating density meter actually measures.
Sources. ASTM D4052, Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter.
Response
What the instrument actually measures — a peak area, absorbance, current, or frequency — before it is mapped to a concentration. (from The number the instrument can't give you)
In plain words. The raw number the instrument gives you before it is turned into a concentration.
For example. A peak area of 48,200 counts on a chromatograph is a response; '12 ppm' is the answer you get after calibration.
Why you should know this. Knowing what the instrument actually measures tells you what can disturb it: a peak area, a current and an absorbance each fail in their own ways.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Response factor
The response per unit concentration (response ÷ concentration), fixed from one standard; an unknown is read as concentration = response ÷ response factor. Valid only where the response is linear through zero. (from The number the instrument can't give you)
In plain words. How much signal you get per unit of concentration. Divide a signal by it to get concentration.
For example. If 1 ppm gives 500 counts, the response factor is 500 counts per ppm, so 2,000 counts means 4 ppm.
Why you should know this. Quick single-point work relies on it, and it is only valid where the response is proven straight through zero.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. McNair, H. M.; Miller, J. M.; Snow, N. H. Basic Gas Chromatography. Wiley.
Response ratio
Analyte response divided by internal-standard response; the quantity you calibrate and read, because a shared run-to-run factor cancels in it. (from The reference that rides along)
In plain words. The analyte's signal divided by the internal standard's signal.
For example. Analyte area 4,820 ÷ internal standard area 9,910 = 0.486.
Why you should know this. It is the number you calibrate with when using an internal standard.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
rheometer
Shears a fluid over a range of rates to map its whole flow curve. (from When one viscosity number isn’t enough)
In plain words. An instrument that stirs or shears a liquid at many speeds to map how its thickness changes.
For example. Used for paints, foods and polymers that do not have just one viscosity.
Why you should know this. It is the instrument for any fluid where viscosity depends on how it is handled.
Sources. Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology. Elsevier (1989).
RSD (relative standard deviation)
The scatter of repeated results, as a percentage of their mean. (from Quality control for titration)
In plain words. Scatter as a percentage of the average, so you can compare small and large numbers fairly.
For example. A standard deviation of 0.2 on an average of 10 is a 2% RSD.
Why you should know this. Method precision is usually stated as %RSD, so you can compare it across concentrations.
Sources. Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry. Pearson. ICH Q2(R2), Validation of Analytical Procedures (2023).
R² (coefficient of determination)
How much of the variation in the data the fitted line explains (0 to 1). Over a wide span it is almost automatically high, and an unweighted fit lets the largest points dominate — so it is blind to consistent low-end bias. (from The number the instrument can't give you)
In plain words. A score from 0 to 1 for how well a line fits the points. It can look excellent while the line is still wrong where it matters.
For example. An R² of 0.9995 can hide a 20% error at your lowest standard, because the big standards dominate the score.
Why you should know this. Auditors and software report it; knowing its blind spots stops a high R² from hiding a bad low end.
Sources. Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry. Pearson.
Detector saturation
A distinct ceiling — a flooded detector, an overloaded inlet or column, crowded ionization — where more analyte no longer gives proportional signal. (from Where the line stops being straight)
In plain words. When the detector is flooded and cannot give a bigger signal for more analyte.
For example. A peak with a flat, cut-off top is a classic sign.
Why you should know this. A flat-topped peak or capped signal means the result is wrong low; dilute and rerun.
Sources. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage. McNair, H. M.; Miller, J. M.; Snow, N. H. Basic Gas Chromatography. Wiley.
Shear rate
How fast adjacent layers of fluid slide past each other — the speed gradient across the fluid. (from When one viscosity number isn’t enough)
In plain words. How fast one layer of liquid slides past the next.
For example. Spreading paint with a fast brush stroke is a high shear rate; paint sitting on the wall is near zero.
Why you should know this. Viscosity of non-Newtonian fluids must be reported with it.
Sources. Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology. Elsevier (1989).
Shear stress
The force per unit area applied to slide one layer over another. (from When one viscosity number isn’t enough)
In plain words. The push, per unit area, that slides one layer of liquid over another.
For example. The force your brush applies to the paint.
Why you should know this. With shear rate it defines viscosity and the shape of a flow curve.
Sources. Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology. Elsevier (1989).
Shear-thickening (dilatant)
Stiffens under stress — cornstarch suspensions. (from When one viscosity number isn’t enough)
In plain words. Gets thicker the harder you push it.
For example. Cornstarch and water: stir slowly and it flows, punch it and it goes stiff.
Why you should know this. It matters in pumping concentrated slurries, where flow can suddenly stiffen.
Sources. Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology. Elsevier (1989).
Shear-thinning
Losing viscosity under mechanical stress — temporary or permanent. (from Viscosity index, and what it really tells you)
In plain words. Gets thinner the harder or faster you push it.
For example. Paint, ketchup and many polymer solutions.
Why you should know this. It explains why paints spread, why polymer-thickened oils thin in bearings, and why one viscosity number is not enough.
Sources. Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology. Elsevier (1989).
Signal-to-noise (S/N)
An instrument estimate — LOD at S/N ≈ 3, LOQ at S/N ≈ 10 — valid only with a genuinely characterised baseline; gameable by smoothing or a chosen noise window. (from Where the signal loses to the noise)
In plain words. How tall a peak or signal is compared with the random wiggle of the baseline.
For example. A peak three times taller than the noise is roughly where you start to trust it is real.
Why you should know this. Instrument software reports it; knowing its limits stops it being mistaken for a validated detection limit.
Sources. ICH Q2(R2), Validation of Analytical Procedures (2023).
Slope
In plain words. How steeply the line rises: extra signal per extra unit of concentration.
For example. A slope of 0.65 means each extra ppm adds 0.65 units of signal.
Why you should know this. It is your method's sensitivity; a falling slope over time can warn of a failing detector or column.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry. Pearson.
Snell's law
The rule that fixes how much a light ray bends at a boundary, set by the refractive indices of the two media. (from When one number isn't enough)
In plain words. The rule for how much light bends when it passes from one material into another.
For example. Why a straw looks bent where it enters a glass of water.
Why you should know this. It is the principle inside every refractometer.
Sources. Hecht, E. Optics. Pearson.
speciation
The specific chemical form of an element or compound; a spike must match the native analyte's form to behave identically. (from Calibrate inside the sample)
In plain words. Working out which chemical forms are present, not just the total.
For example. Telling chromium(III) from the more toxic chromium(VI) instead of reporting total chromium.
Why you should know this. Toxicity and reactivity often depend on the form, not the total.
Sources. IUPAC. Compendium of Chemical Terminology (the “Gold Book”), online edition.
Specific gravity
A sample's density divided by the density of water at a stated reference temperature; dimensionless. (from Reading concentration from a single number)
In plain words. A liquid's density compared with water's, so it has no units.
For example. A battery acid with specific gravity 1.28 is 28% denser than water.
Why you should know this. Many specifications and older methods quote specific gravity rather than density.
Sources. ASTM D4052, Standard Test Method for Density, Relative Density, and API Gravity of Liquids by Digital Density Meter. IUPAC. Compendium of Chemical Terminology (the “Gold Book”), online edition.
Speed of sound
How fast a sound wave travels through a liquid; with density it yields the fluid's compressibility, a second independent composition signal. (from The power of the densitometer)
In plain words. How fast sound travels through the liquid. It depends on how stiff and how heavy the liquid is.
For example. About 1,480 m/s in water at room temperature.
Why you should know this. Many density meters report it; it can measure concentration where density alone cannot.
Sources. Kinsler, L. E.; Frey, A. R.; Coppens, A. B.; Sanders, J. V. Fundamentals of Acoustics. Wiley.
Spike
A known, deliberate amount of analyte added to the sample. (from Calibrate inside the sample)
In plain words. A known amount of analyte deliberately added to a sample.
For example. Adding 5 ppm of lead to a water sample to see whether the method finds it.
Why you should know this. Spike recoveries are the routine proof that a method finds what is really there.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. ICH Q2(R2), Validation of Analytical Procedures (2023).
Standard
A material of accurately known concentration used to teach the instrument the response-to-concentration relationship. (from The number the instrument can't give you)
In plain words. A sample whose concentration you know accurately, used to calibrate or check the instrument.
For example. A certified solution labelled 10.00 mg/L sodium is a standard.
Why you should know this. A curve is only as true as the standards behind it; their purity, preparation and age decide your accuracy.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. IUPAC. Compendium of Chemical Terminology (the “Gold Book”), online edition.
Standard addition
Calibrating inside the sample: spike equal aliquots of it with known increments, at constant final volume, plot response vs added concentration, and read C₀ = intercept ÷ slope. (from Calibrate inside the sample)
In plain words. Calibrating inside the sample itself by adding known amounts of the analyte to portions of it.
For example. Split a sample into four, add 0, 5, 10 and 15 ppm, measure all four, and the line tells you what was there to begin with.
Why you should know this. Reach for it when the sample matrix changes the signal and you cannot make standards that behave like the sample.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage.
Standard deviation
In plain words. A measure of how much repeat results scatter around their average.
For example. Results of 9.8, 10.0 and 10.2 have a standard deviation of 0.2.
Why you should know this. Control limits, detection limits and method precision are all built from it.
Sources. Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry. Pearson.
Stray light
In plain words. Unwanted light that reaches the detector without passing through the sample properly.
For example. It caps how high an absorbance reading can honestly go.
Why you should know this. It sets the practical top of an absorbance method's range.
Sources. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage.
Surrogate
A reference that tracks recovery through preparation and is reported as a QC check; unlike an internal standard, it usually does not correct the result. (from The reference that rides along)
In plain words. A compound added before preparation to check how much survives the work-up. It flags losses but usually does not correct the result.
For example. A surrogate recovered at 60% warns you that the extraction lost material.
Why you should know this. Surrogate recoveries are reported with environmental results as a check on sample preparation.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
TAN / TBN (acid and base number)
Milligrams of potassium hydroxide needed to neutralise the acidic material in one gram of oil. (from Acid and base number: the methods)
In plain words. TAN measures acids building up in an oil; TBN measures how much acid-fighting reserve is left.
For example. Rising TAN and falling TBN together say the oil is wearing out.
Why you should know this. Together they tell you when an oil is chemically worn out.
Sources. ASTM D664, Standard Test Method for Acid Number of Petroleum Products by Potentiometric Titration. ASTM D2896, Standard Test Method for Base Number of Petroleum Products by Potentiometric Perchloric Acid Titration.
Ternary system
A mixture of three components — for example two solutes in one solvent. (from The power of the densitometer)
In plain words. A mixture of three things: usually two dissolved in a third.
For example. Sugar and acid in water, as in a soft drink.
Why you should know this. Three components need two independent measurements; knowing this stops you trusting one number for a mixture.
Sources. Atkins, P.; de Paula, J. Atkins’ Physical Chemistry. Oxford University Press.
Thixotropic
Thins over time under shear and recovers at rest — greases, gels. (from When one viscosity number isn’t enough)
In plain words. Thins the longer you stir it and slowly thickens again at rest.
For example. Some greases and gels.
Why you should know this. Greases and gels need time-dependent testing, not a single reading.
Sources. Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology. Elsevier (1989).
Transducer
A crystal that turns an electrical pulse into an ultrasonic wave and the echo back into a signal. (from The second number your density meter already measures)
In plain words. A part that turns an electrical signal into sound (or another form) and back.
For example. The crystal in an ultrasonic meter that sends a pulse and hears its echo.
Why you should know this. It is the part that sends and receives the pulse in an ultrasonic meter.
Sources. Kinsler, L. E.; Frey, A. R.; Coppens, A. B.; Sanders, J. V. Fundamentals of Acoustics. Wiley.
VI improver
A polymer that raises viscosity index but can shear-thin under stress. (from Viscosity index, and what it really tells you)
In plain words. A polymer additive that helps oil stay thick when hot; it can be broken down by use.
For example. Why a multigrade oil can lose viscosity over time in a hard-working engine.
Why you should know this. It explains permanent viscosity loss in multigrade oils.
Sources. ASTM D2270, Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 °C and 100 °C. Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology. Elsevier (1989).
Viscosity
A fluid's resistance to flow; kinematic viscosity equals dynamic viscosity divided by density. (from The power of the densitometer)
In plain words. How thick a liquid is: how much it resists flowing.
For example. Honey has high viscosity; water has low viscosity.
Why you should know this. It grades lubricants and fuels, flags contamination and degradation, and sets how a fluid pumps and sprays.
Sources. ASTM D445, Standard Test Method for Kinematic Viscosity of Transparent and Opaque Liquids. Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology. Elsevier (1989).
Viscosity index (VI)
A standard measure of how little a lubricant's viscosity changes with temperature; higher is more stable. (from Fingerprinting a fluid)
In plain words. A number for how little an oil thins as it heats up. Higher is steadier.
For example. A VI of 150 oil holds its thickness across temperature much better than a VI of 90 oil.
Why you should know this. It is printed on lubricant specifications and tells you how an oil will behave from cold start to full heat.
Sources. ASTM D2270, Standard Practice for Calculating Viscosity Index from Kinematic Viscosity at 40 °C and 100 °C.
Watson characterization factor (K)
K = (mean boiling point)^⅓ ⁄ specific gravity; ~12.5 marks a paraffinic stock, ~10 an aromatic one. (from Calculated, not measured)
In plain words. A number from boiling point and density that says whether an oil is mostly chains or mostly rings.
For example. About 12.5 for a waxy paraffinic oil, about 10 for a highly aromatic one.
Why you should know this. Refiners use it to classify crude oils and fractions quickly.
Sources. Watson, K. M.; Nelson, E. F.; Murphy, G. B. “Characterization of petroleum fractions.” Industrial & Engineering Chemistry 27 (1935) 1460–1464.
Weight %
Ways to state concentration: moles per litre of solution; reactive equivalents per litre; moles per kilogram of solvent; grams of solute per 100 g of solution. (from Reading concentration from a single number)
In plain words. Grams of the substance in every 100 grams of the whole mixture.
For example. A 10 wt% sugar syrup has 10 g sugar in every 100 g of syrup.
Why you should know this. Common on certificates and in industry specifications; it avoids volume and temperature effects.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman.
Weighted regression
Least-squares fitting that weights each standard (commonly by 1/response² when relative scatter is roughly constant) so the low end is not swamped by the high standards. (from The number the instrument can't give you)
In plain words. A curve fit that gives the low standards a fair say instead of letting the big numbers dominate.
For example. Weighting by 1/response² keeps a 1 ppm standard from being ignored next to a 1,000 ppm one.
Why you should know this. Use it when calibrations span orders of magnitude and the low end matters, as it does near a detection limit.
Sources. Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry. Pearson.
Working (calibration) range
The full span of concentration a calibration covers, floor to ceiling. Over a wide span, log-spaced levels keep each one informative; the validated range of a real method is usually narrower, chosen around where you report. (from The number the instrument can't give you)
In plain words. The span of concentrations a method has been shown to measure well, from its lowest to its highest reliable value.
For example. A method validated from 1 to 50 ppm should not be trusted for a sample reading 200 ppm without dilution.
Why you should know this. It tells you when to dilute a sample or report it as above range, instead of trusting a number outside what you proved.
Sources. ICH Q2(R2), Validation of Analytical Procedures (2023).
x-intercept
Where the fitted line crosses zero response; its distance from the origin, |b/m|, is the analyte already present. (from Calibrate inside the sample)
In plain words. Where the line crosses zero signal. In standard addition its distance from zero is the amount already in the sample.
For example. A line crossing at −8.2 ppm means the sample held 8.2 ppm before anything was added.
Why you should know this. In standard addition it is the answer.
Sources. Harris, D. C. Quantitative Chemical Analysis. W. H. Freeman. Skoog, D. A.; West, D. M.; Holler, F. J.; Crouch, S. R. Fundamentals of Analytical Chemistry. Cengage.
yield stress
Behaves as a solid until a minimum (yield) stress is exceeded, then flows — toothpaste, grease. (from When one viscosity number isn’t enough)
In plain words. Won't flow at all until you push past a minimum force.
For example. Toothpaste sits on the brush until you squeeze the tube.
Why you should know this. It decides whether a paste will sag, pump or stay put.
Sources. Barnes, H. A.; Hutton, J. F.; Walters, K. An Introduction to Rheology. Elsevier (1989).