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What is the accurate procedure to calibrate gas chromatograph?

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2009-04-16 04:40:42

This is an extremely simplified overview of the process assuming

the GC has been programmed appropriately with proper oven

temperatures, flow rates and timings.

Typically gas chromatographs are calibrated using a mixture of

compounds of known concentrations. One example of this would be a

"Standard Gas" which is specially mixed by a supplier to the

customer's specifications to match component gasses that they would

be testing for. As an example, 21% Oxygen, 75% Nitrogen, 2% Carbon

Dioxide and 2% Carbon Monoxide might be used if the GC is to be

used for testing breathing air. After being mixed, the supplier

samples the gas and measures it on their own test equipment and

issues a tracable certificate with the Standard Gas that allows the

tracing of standards used all the way back to the metric standard

weights and measures.

A chromatography run is then performed on the GC watching for

peaks on all the various detectors. Once the run has been

completed, the peaks for each compound are isolated, by the

operator, and a cursor is placed to bracket each peak. The idea is

to tell the GC where on the chromatography graph to start and stop

calculating the area under each peak.

Once these cursors are placed, the GC calculates the area under

each peak in the graph, and the operator informs the GC what the

concentration of each component gas was in the Standard Gas, and

the GC uses simple math from then on to calculate what a given

component concentration in a sample is based on the pixel count

under the peak for each component.

When running a sample, a base-line is first run using the

Standard Gas, and the values compared to the Standard Gas's

certificate values. If they agree, then you're ready to run a

sample. The sample is run, and the results produced, but they are

not final until a second base-line run is performed. If the values

from the first and second base-line runs match, then the

calibration on the instrument hasn't changed, and the results from

the sample run are valid. Multiple samples can be batched between

base-line runs, but their results should not be considered valid

until a base-line has been run both some time before and some time

after the sample(s).

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