Chromatography Forum

Good day everyone,
We have an environmental chamber in my lab. We want to test products for VOC's by GC-MS (we have a thermal desorption unit and a cryogenic trap). We will capture the VOC's on Tenax (stainless steel cartridges). What I would like to know is :
- can I do a toluene calibration curve by direct injection, or it will not represent the way the VOC's will be trapped and injected?

Thanks for your help!
Julia

Julia,

Direct injection of toluene will not be representative of the sample trapping system. What you have to do is inject your toluene standards onto the tenax tubes.

Julia,

July in earlier posts was proposing to do something similar. I would suggest making a gas phase standard in a static dilution bulb and then injecting that into a stream of clean air or nitrogen that is passing into your thermal desoprtion tube. Use about the same amount of air that you would take for your sample. This will most closely replicate what you are trying to analyze for.

Best regards.

Generally using known volumes of an independently verified test gas is best, but if not available then the following is just as valid for toluene, assuming your sample tubes contain 100-200 mg Tenax. It would also be an advantage if your tube dimensions are 89 mm x 6.4 mm od. because you can buy certified reference tubes for toluene at 1 ug and 0.056 ug.

Using a calibrated microlitre syringe you can spike the toluene in 1-10 uL dilute methanol solution onto the sorbent tube. There are several equivalent variations on a theme to introduce the calibration sample(s). They should all allow prior vaporization in a air/gas stream and the purge volume should be sufficient to remove >99.5 % of the methanol. This is realistic enough for calibration purposes. You can ignore humidity effects, oxidation by ozone and other anomalies. At least for toluene on Tenax they are trivial.

1) Inject onto the retaining plug/gauze etc. above the Tenax while drawing clean air through with a pump at 50-100 ml/min for 4 mins.

2) Adapt a surplus GC injector, or make one, to fit the sample tube, inject while passing air/gas through it at 50-100 ml/min for 4 mins . Heating is not necessary, but OK provided you are using a plunger-in-needle syringe. A conventional plunger-in-barrel syringe has to be cool, otherwise you might evaporate half the dead volume in the needle as well.

3) Inject into a small glass reservor connected to the tube while drawing clean air through with a pump at 50-100 ml/min for 4 mins.

Some of this is described in EPA compendium method TO-17 (clause 9.3).
http://www.epa.gov/ttn/amtic/files/ambi ... to-17r.pdf

Similar versions are in ASTM method D6196-03 clause 8.3 or ISO 16017-1 (4.7-4.9). These two standards are very closely related.