PV = nRT runs through every gas chromatograph: how much sample a gas loop delivers, whether a liquid injection fits its liner, and how the carrier flows.
By Medrado Analytical Innovations · Foundations, No. 23 · for the working analyst
Most analysts last met the ideal gas law in a first-year chemistry exam: PV = nRT, pressure times volume equals the amount of gas times a constant times the temperature. It seems to belong to the textbook, not the lab. But a gas chromatograph runs on it. It describes how much sample a gas loop delivers, how much room a liquid injection needs in the inlet, and how the carrier gas flows. And for a gas loop filled to the room's atmosphere, it is one reason a QC chart can follow the barometer.
The law in one line
Pressure, volume, temperature and the amount of gas are tied together:
P × V = n × R × T n = amount of gas (moles), R = 8.314 J/(mol·K), T in kelvin, P absolute
for a fixed volume filled to a set pressure: n is proportional to P / T
Read the second line in plain words. A container of fixed size, filled until it reaches a given pressure, holds more gas when that pressure is higher, and less gas when it is warmer. (This is different from the classroom picture of a sealed flask, where the amount is fixed and the pressure rises when you heat it.) Two things to keep straight: P must be the absolute pressure, not the gauge reading, and T must be in kelvin.
Part of the GC course
This lesson belongs to From signal to result, a course on the gas chromatograph from detector to reported number, coming soon. The rest of the lesson, its questions and the course run will be part of it.
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Check yourself
Answer in your head first, then open the answer. Any question can go into your Quiz me.
A gas loop is filled to the room's pressure. The barometer rises. What happens to the sample delivered?
Show the answer
“A gas sample loop holds a fixed volume, so the amount of sample it holds rises and falls with the pressure and temperature of the gas inside it.”
Plain-language explanations, examples, where you would use each term, and the sources are in the full glossary.
Absolute pressure
The gauge reading plus the atmosphere; the pressure the ideal gas law needs.
Sample loop
A short tube of known volume that fills with a gas sample and is then switched into the carrier stream.
Liner
The glass sleeve in a GC inlet where a liquid injection vaporises.
Compressibility factor
The correction that describes how far a real gas departs from PV = nRT; close to 1 near atmospheric pressure.
Sources
P. Atkins, J. de Paula and J. Keeler, Atkins' Physical Chemistry, 11th ed. (Oxford University Press, 2018): the perfect gas equation; real gases and the compressibility factor.
Agilent Technologies, "Tips on Making your GC System and Analysis more Robust" (e-seminar, 2016; solvent vapour volumes at 250 °C and 20 psig, liner volumes), agilent.com/cs/library/eseminars/public/Tips_on_Making_your_GC_System_and_Analysis_more_Robust_10112016.pdf; and the Agilent GC Vapor Volume Calculator (OpenLab GC help).
H. M. McNair, J. M. Miller and N. H. Snow, Basic Gas Chromatography, 3rd ed. (Wiley, 2019): carrier gas, viscosity and flow control.