Chromatography Forum

I'm work at a Agilent 6890/5973MSD in my laboratory
and also I have sampling tube(packed with Tenax TA) for thermal desorption.

My major working is analysis of VOCs in air matrix samples.

As you know, offline GC/MS(Agilent) is time-consuming for sample analysis. However, portable GC/FID/PID is more rapid than offline GC/MS method for sample analysis.

Recently, I bought portable GC/FID/PID.
The brand name is Toxic Vapor Analyzer(TVA)-1000B.
They have two detectors.


My questions are as follows;
Total volatile organic compounds(TVOC) concentration is different between GC/MS and portable GC/FID/PID.
Of course, I know the difference between two systems because portable GC/FID/PID has different calibration gas compared to thermal desorption GC/MSD.

However, TVOC concentration diffrence is about from 10 to 40times.
If GC/MS value is 1 (for ratio), GC/FID has about 30, GC/PID has about 50.

Please tell me how can I correct concentration between different measurement system?

Do you have any idea or good suggestion?

Best regards.

Jae,

Very interesting. Here's why. PID should not see lots of things FID can see (like methane, for example) while PID can see some things FID cannot see (like H2S for example.) MS can nominally see them all depending on what mass cut off you are running. Methane for example at mass 16 is probably below your mass spec mass cutoff.

Furthermore, your trap will not collect everything (methane for example has a breakthrough volume of a few mLs or 10's of mLs) but your FID will not have that issue.

Then there is response factors. For PID these vary all over the map according to ionization potential so using one calibration compound will mean lots of response factor modification for others. PID vendors have these tables for review if you want to compare them to your expected target list. For FID, basically a carbon counter except there are correction factors for functional groups that will alter the overall response somewhat.

If you are doing GC in all three cases, you should consider a standard mixture that will elute and respond for all three instruments to obtain a cross sensitivity value and work from there. Having said that, unless you calibrate for every component, total hydrocarbons does not really hold up completely due to these factors.

Best regards.

Hi, ACIMM

Thank you for your kind reply.

I think you're right.
I'll consider your reply and then try it.

Thank you.

Best regard