Gas chromatography: specifying an instrument, a detector and a gas chromatography machine, and what gas chromatography services should deliver against a named method

Specifying the system

1. The detector decides the instrument. A flame ionisation detector is robust and near-universal for organics; an electron capture detector is exquisitely sensitive to halogenated compounds; a mass spectrometer gives identification as well as quantitation and costs and demands far more. Fix the detector from the analytes and the required detection level first.
2. Inlet and sample introduction. Split, splitless, on column and headspace introduce very different amounts of sample and treat thermally labile analytes very differently. Discrimination at the inlet is a common and invisible source of quantitative error, so choose the mode from the analyte rather than the convenience.
3. Column phase and dimensions. Stationary phase polarity, film thickness, internal diameter and length set resolution, capacity and run time. A longer, thinner column resolves better and runs slower; a thicker film retains volatiles better and bleeds more at high temperature.
4. Carrier gas supply. Helium supply and price have been volatile, and hydrogen and nitrogen are both viable alternatives with consequences for efficiency and for safety. If hydrogen is chosen, the laboratory ventilation and detection arrangements are part of the purchase rather than an afterthought.
5. Data system and records. The chromatography data system holds the raw data and its audit trail, and it is where a regulated laboratory's integrity questions land. Confirm how raw data is retained, who can reprocess it and what the audit trail captures before the system is chosen.

Outsourcing to a gas chromatography service

Ask which method the laboratory will run by its published designation, whether it is within their accredited scope, what the reporting limit is for your matrix, and what the turnaround is. A laboratory quoting a method outside its accredited scope may still be competent, but the certificate carries less weight.

Ask for the raw chromatogram alongside the result. A number without a trace cannot be assessed, and any laboratory confident in its work will provide it.

Running cost and consumables

Carrier gas, inlet liners, septa, ferrules and columns are the recurring spend, and inlet consumables are what most affects data quality day to day. A maintenance schedule kept properly prevents most of the problems that otherwise present as method failures.

Budget for a spare column of the phase in routine use. A column failure mid-campaign with a long lead time is far more expensive than holding one.

Common questions

What can gas chromatography analyse?

Compounds that can be vaporised without decomposing, which covers most volatile and semi-volatile organics. Thermally labile, very polar or non-volatile compounds usually need derivatisation or a liquid phase technique instead.

Which detector should I choose?

Flame ionisation for general organics, electron capture for halogenated compounds at low levels, and mass spectrometry where identification as well as quantitation is needed. The detector determines the detection level and most of the cost.

Helium, hydrogen or nitrogen?

Helium is conventional and its supply has been unreliable. Hydrogen gives faster, more efficient separations and requires ventilation and leak detection arrangements. Nitrogen is cheap and slower. The decision affects the laboratory as well as the instrument.

What should I ask a testing laboratory?

Which method by designation, whether it is within their accredited scope, the reporting limit in your matrix, the turnaround, and whether they will supply the raw chromatogram with the certificate.

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