What is the accurate procedure to calibrate gas chromatograph?
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).