During 2013 we will present a series of short articles on gas chromatography (GC) troubleshooting and maintenance. In this
first installment, we concentrate on the first three important stages in any GC analysis, namely the quality and type of gases
supplied to the instrument, key sample preparation considerations, and common problems encountered with the most popular sample
inlet for GC: the split–splitless injector.
Gases commonly used in GC include nitrogen, hydrogen, and helium for the carrier gas, hydrogen for flame ionization detection
(FID), and nitrogen, or less favorably helium, as the make-up gas for ionizing detectors. The carrier gas should be free from
moisture and oxygen, both of which cause increased stationary-phase bleed, poor peak shape, and reduced sensitivity. Alumina
in-line filters (traps) should be fitted as close as possible to the instrument. Self-indicating traps are preferred as the
indicator is a tell-tale to signal that the trap is exhausted and needs to be changed. FID and other ionizing detector gases
should be free of hydrocarbons that can reduce sensitivity; a hydrocarbon trap is recommended. Other detector types have specific
gas purity requirements, and mass spectrometry (MS) detectors, for example, need to have large-capacity oxygen traps fitted.
In general, the gases used should be 99.999% pure or better. A periodic "pressure drop" test should be carried out on gas
lines to test for leaks in the supply lines — usually achieved by switching off all GC systems, closing the cylinder head
regulator (or switching off the gas generator production), and then monitoring the line pressure for a significant pressure
drop, which indicates a non-pressure-tight line between the gas supply and the instrument.
Sample solvents should be chosen to ensure compatibility with the GC column stationary phase to avoid peak shape effects such
as broadening, splitting, and shouldering. Match the sample-solvent polarity with the stationary-phase polarity to avoid these
effects: For example, the use of water or methanol as sample solvent with methyl silicone stationary phase or hexane with
PEG or Wax phases is not recommended. Different sample solvents produce different volumes of vapor in the inlet, and one should
test that the amount of vapor created does not exceed the available volume within the liner; otherwise "backflash" may occur
in which analyte is deposited in the carrier gas and septum lines that can lead to carry-over and irreproducible quantitative
results. There is a dedicated software calculator available within CHROMacademy to assist with this test.
Leaking septa give rise to baseline shifts during injection, pressure problems, and poor quantitative reproducibility. Check
or replace the septum before each campaign of analyses. Periodically verify the septum purge flow with a flowmeter to avoid
noisy baselines with spurious peaks and reduced sensitivity.
The inlet liner should be appropriate to the injection mode and will, over time, become active toward analytes with polar
functional groups. The glass wool and quartz glass surface are typically silylated (deactivated) when new; however, the derivatizing
reagent hydrolyses over time and exposure to moisture exposes silanol groups, which causes a secondary, unwanted, retention
to a proportion of the analyte molecules and, consequently, significant peak tailing. Clean, deactivate, or replace liners
or glass wool as necessary to reduce peak tailing and overcome problems with peak area reproducibility.
Ensure that the column is positioned within the inlet according to manufacturers' instructions to avoid problems with quantitative
Clean the inlet walls and any metallic seals at the lower and upper part of the inlet periodically using both polar and nonpolar
solvents to ensure good peak shape and quantitative reproducibility.
Periodically check the flow rate of the split gas from the inlet in both the split and splitless (where the flow should be
zero) modes with a flowmeter to check for issues with blocking of the split gas lines, the viability and accuracy of the split
flow regulator and to test the operation of the solenoid valve that toggles between split and splitless modes.
To optimize peak efficiency (sharp Gaussian peaks), reduce tailing of the solvent peak, and avoid rising baselines during
the analysis, ensure that during splitless injection the initial oven temperature is at least 10 °C below the sample solvent
boiling point and that the inlet switches to split mode at an optimum time after injection to purge inlet of residual vapors.
The optimum split-on (purge) time can be estimated by monitoring peak area with purge times starting at around 20 s after
injection and increasing in 10-s steps. The purge time corresponding to the first repeatable peak area measurement should
be chosen as the split-on time — that is, if peak areas are reproducible for the 30- and 40-s purge times then 40 s is chosen
as the split-on time.
Check out the on-line interactive CHROMacademy GC Troubleshooter for a wealth of GC troubleshooting and maintenance information.