Gas chromatography: choosing the instrument, the detector and the column

Specifying gas chromatography systems

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 laboratory, or buying from gas chromatography manufacturing companies

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.

Gas chromatography equipment: 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.

The gases: the hidden half of a gas chromatography price

A gas chromatograph needs a carrier gas at high purity, and a flame detector needs hydrogen and air as well. That means cylinders or generators, regulators, traps, tubing that does not leak and a supply arrangement, and in many laboratories the gas installation costs more than the instrument's cheaper modules.

Generators remove cylinder handling and add maintenance; cylinders are simpler and need storage, changing and safety provision. Decide this at the same time as the instrument, because it determines where the instrument can be sited.

Gas chromatography FID: why the flame detector is still the default

A flame ionisation detector responds to almost all organic compounds roughly in proportion to carbon content, is linear over a very wide range, is robust and is cheap to run. That combination is why it remains the workhorse even where more selective detectors exist.

It is destructive, blind to water and to most permanent gases, and gives no structural information. Where identity has to be confirmed, a mass detector is the answer, and many laboratories run both on one instrument for exactly that reason.

In house on your own gas chromatography apparatus, or sent out

A single method run occasionally rarely justifies an instrument once gases, columns, standards, maintenance and an operator's time are counted. A laboratory with a steady stream of similar samples reaches the opposite conclusion quickly.

Where the result must be accredited, sending out also buys the accreditation, the validated method and the reporting. Bringing that in house is a larger project than buying the instrument, and pricing it honestly is what makes the decision defensible.

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.

What does a gas chromatography machine really cost to run?

Carrier and detector gases or generators, columns as consumables, standards, a service contract and an operator's time. The gas installation alone is frequently comparable with the cheaper modules of the instrument.

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