INFICON Explorer Portable Gas Chromatograph - Manual (Page 208)

Method Development
206
TVOC Methods
The TVOC method will provide accurate response in the approximate range 10 to
150 ppm when you use nitrogen carrier and are calibrating using 100 ppm
isobutylene.
Carrier Gas and Injection Time
The easiest way to adjust TVOC sensitivity is to change the injection time. However
this will also effect the range. The useful dynamic range is limited with nitrogen by
the presence of a negative dip in the baseline caused by air in the sample. This
negative peak is approximately 60 mV in height and it co-elutes with the VOC peak.
Any peak which is to be accurately integrated must be greater than 100 mV in
height. You can determine the peak height from the graph view.
Explorer cannot
quantitate a peak greater than 2500 mV in height.
The PID response is non-linear and this is seen as a disproportionate amount of
signal at higher concentrations. When these factors are taken into account the useful
dynamic range is about 15:1. The exact useful range will depend on the compound
being analyzed. If the injection time is adjusted to give a 100 mV peak for 1 ppm
benzene the working concentration range will be approximately 1 to 15 ppm. If the
injection time is adjusted to give a 100 mV peak for 10 ppm benzene the working
concentration range will be approximately 10 to 150ppm.
Explorer’s sensitivity with nitrogen carrier gas is approximately 400 mV/ppm of
benzene so the lower detection limit could be sub-ppm benzene if, for instance
the injection time is set to 6 seconds.
The negative baseline dip also makes the TVOC less linear than the GC because the
TVOC peak must first fill the air dip before a positive peak is obtained. This
amounts to a sample loss which has the greatest effect on quantitation accuracy at
low concentrations.
The ranges listed in the table below are approximate and are based on isobutylene
samples.