Thanks for your reply.
Okay, I will let you know our test procedure in detail.
We have cryogenic trap(Tenax TA 20mg) + GC/FID system to check VOCs concentrations in the our customer field site.
Our GC/FID online monitoring system has pressurized standard cylinder inside of instrument to make calibration curve automatically. Our calibration range was 0, 5ppbv, 20ppbv, 50ppbv. For example, if I want to make 10ppbv level sample, N2 flow was 1000mL, and our cylinder standard gas flow was 1mL.
Target concentration was made in the mixing chamber.
In order to compare the results between cryogenic GC/FID and thermal desorption GC/MS for 10ppbv mixing chamber sample, sampling was conducted as follows;
One sample was passed through cryogenic Tenax TA trap for 15min at the rate of 0.1L/min(Total volume was 1.5L) and other sample was collected using air sampler at the rate of 0.1L/min for 120min(to analyze sample using GC/MS) individually. GC/FID online system run time was about 40min. GC/FID system data was expressed as average for 3 time analyses (3 cyles for 120min). GC/MS offline data was expressed as one data for 2hrs sampling period. In this case, thermal desorption GC/MS sampling adsorbent is Tenax GR type(200mg packed in the stainless steel tube to desorb using high temp helium carrier gas).
Finally, when we compare Toluene concentration between GC/FID and GC/MS, concentration showed very similar result for each. The ratio(MS/GC-FID) was about 1.1. However, m,p-xylene ratio(MS/GC-FID) was about 1.7. m,p-xylene's concentration was also calculated using Toluene equivalents.
Our customer also use Toluene equivalents to quantitate Toluene and m,p-xylene to make comparison.
If possible, would you let me know the reason of difference and some advises?
If I use individual standard for m,p-xylene instead of Toluene equivalents, concentration would be same?
Our crogenic GC/FID online system's column was HP-5(30m).
GC/MS column may be 60m length, and......
I don't know detail information due to limited information from our customer.