Thermo desorption problem

[KCl34]

Hi all,
I’m working on a 3800 GC, 2200 GCMS Varian Saturn system. We have a seven valves trapping system coupled with two capillary columns (30 meters). The EFC1 mass flow system is installed and a vacuum system for the trapping phase is present.
Since I arrived in the January 2007 we always use a PAH liquid standard in order to calibrate the system (our field is the identification of diesel soot absorbed PAH in a underground mine environment).
In the past the calibration was performed spiking a quartz filter with the liquid standard (50 microl) and heating up a stainless steel reactor connected with the sampling probe (and the trap): Considering that I never detected till now any PAH from the standard using this set up, two months ago I started not to trust anymore the health of the trap and I start to consider the replacement (of the trap) with a quartz reactor. The idea was to insert in the quartz reactor quartz filter loaded with standard and in the future with soot, the idea of thermodesorption (Meuzelaar in primis). The method should presents a very quick desorption (till to 300 celsius) and then a 8 degrees/minutes ramp in the column with a flow of 1 ml/min.
After a lot of work in order to obtain the right pressure (leaks in the quartz reactor) in the column (5 psi at least) I’ve been able to have the right setup (I reduced the split ratio to zero…is it a problem?). Despite that I’m still not able to detect any PAH peaks from the standard but only a lot of strange peaks (intensity around 10 Mcounts for the peaks) . I performed also blank test (quartz reactor empty and warm) with a lot of noise results and bypassing the quartz reactor at all with very good baseline.
Of course my standard could be poisoned (acetone the solvent) but I would like to be sure that there are no nice tricks for a beginner user of a GC in order to understand what is wrong on this system.

Thank you for reading till here and I’m looking for your help

[Peter Apps]

The set-up that you describe sounds extremely complex for thermal desorption from soot. Unless there are other applications for the same hardware, or other constraints, you could do this analysis with an off-the-shelf thermal desorber connected directly to a single capillary column.

Peter

[KCl34]

Thank you Peter for the reply. I completely agree with you, I found the system too much complex for our purpose. The main and starting idea was to sampling directly in a lab-mine where we test a diesel engine (we live in a huge dilution tunnel): so the system should help to sample enough material (concentration of soot low, dilution high) in a 3-4 hours test. But it didn't due too the low concentration or our aftertreatment devices tested.
I was looking to preserve this setup for the future (possible long run test ans so more material to sample) but I start to think that simplify the setting would be one of the best solution.
Thank you

Emanuele Cauda

[MikeD]

Do you have to show quantitative monitoring data to a mines regulator? If not then a commercial thermal desorber would probably perform better than your current setup as Peter says. However, although I am generally supportive of thermal desorption, I don't believe TD equivalence to the PAH reference methods of liquid extraction and HPLC/fluorescence or GC/MS has been demonstrated yet for all the usual target PAHs. That's not to say people haven't tried TD. At least two manufacturers have data notes out.

Here is a reference to the UK experience with PAH monitoring in general workplaces. Unwin J., Cocker J., Scobbie E and Chambers H (2006). An Assessment of Occupational Exposure to Polycyclic Aromatic Hydrocarbons in the UK. Annals of Occupational Hygiene, 50(4), 395-403.

Since no one has a single agreed limit value for benzo[a]pyrene in air most people (in Europe anyway) normally monitor diesel particulates by measuring organic and elemental carbon. Carbon monoxide or Bosch smokemeter blackness have also been used as surrogates.