Rights: The University of Waikato Published 26 November 2007 Download

A gas chromatograph is a highly specialised piece of laboratory equipment that measures levels of different gases in air samples. Teacher Fellow Phil Kendon and NIWA technician Anitra Fraser explain how the gas chromatograph works.


Phil Kendon (Teacher Fellow): A gas chromatograph is a way of separating mixtures of gases in a sample of air.

Anitra Fraser (NIWA): A gas chromatograph can basically measure a lot of different gases. In our lab we measure methane, carbon monoxide, carbon dioxide and nitrous oxide.

Phil Kendon (Teacher Fellow): So what happens is the sample in the glass flask is attached to a carousel; a special carousel that’s been designed by the scientists here at NIWA.

Anitra Fraser (NIWA): And then there’s basically a bit of computer set-up, so I’ve just got to make sure that the computer works properly. Then I start everything going.

Phil Kendon (Teacher Fellow): The sample is then injected - a small amount of the sample is injected - into a stream of what’s called a carrier gas, which is nitrogen, and that then flows into the gas chromatograph. Inside the gas chromatograph is what’s called a column, a chromatograph column, which is packed with a special powder. As the air with the sample in it, the nitrogen with the sample in it, passes into the column different gases in the air travel through the powder at different speeds. At the end, after the column stage, the air then goes through to a detector, a special detector; there are different detectors for different gases. For example, methane uses something called a flame ionisation detector. Once the methane arrives at that detector, it’s ionised in the detector and the ions generate an electric voltage which is transferred to the computer and produces a peak, and that tells the scientists how much gas - how much methane there is, or whatever the gas is they’re looking at in the sample of air.

Anitra Fraser (NIWA): Now after a sample has been run I have to process the data and check that it’s all working properly. So basically it gets loaded into another program, and then we do a series of measurements. We measure the peak height of the different chemicals that we are looking at, and compare them against a standard. These standards have been measured really accurately and we know, sort of, what level they should be at. So if there is some funny result I can go back and find out why. Typically we measure each sample five times, but then we also intersperse it with standards. So that basically means it takes about 55 minutes to an hour just to get one sample. The great beauty about the carousel is that we can put eight samples on all at the time, and set it up, and then leave it running overnight. So we can collect samples when we’re not at work, which is really good.