-
- Posts: 7
- Joined: Mon Aug 28, 2017 5:33 pm
We're currently using a Tekmar 3000 with a Vocarb 3000 trap for the extraction of the following volatile organic compounds :
Chlorure de vinyle / Vinyl chloride
Dichlorométhane / Dichloromethane
Trihalométhanes / Trihalomethanes:
Bromoforme / Bromoform
Chloroforme / Chloroform
Bromodichlorométhane / Bromodichloromethane
Dibromochlorométhane / Dibromochloromethane
Tetrachlorure de carbone / Carbone tetrachloride
Benzène / Benzene
1,2-Dichloroéthane / 1,2-Dichloroethane
Trichloroéthylène / Trichloroethylene
Toluène / Toluene
Tétrachloroéthylène / Tetrachloroethylene
Ethylbenzène / Ethylbenzene
Xylènes totaux / Total Xylenes
1,4-Dichlorobenzène / 1,4-Dichlorobenzene
1,2-Dichlorobenzène / 1,2-Dichlorobenzene
This is an old obsolete system that's always worked well, but for the past year or so we've been having problems with the loss of a large portion of the methylene chloride somewhere during the extraction step (the signal is about 95% lower than expected) from historical results. The problem has progressed to the point that we can't analyse client samples because even trace amounts of methylene chloride are quantified as reportable due to the quality of the quantification curve.
The purge and trap extraction method uses most of the parameters recommended in EPA method 524.3 (some slight modifications to the desorb temperature and time), but we've tried significantly different parameters with success for all VOCs except methylene chloride who's signal is always greatly diminished compared to the other compounds.
Injection of a standard directly into the GC column or placing the standard directly on top of the trap gives the expected signal. Extraction of a sample from the sparge unit (either in water or a standard placed directly into the unit) gives us the much smaller signal. We've replaced the both the trap and sparge unit without improvement.
Finding replacement parts for that unit is getting costly, so we decided to purchase a refurbished unit from an independant supplier to "replace everything at once" (Tekmar doesn't refurb these units anymore). Anyhow, we're been testing the new unit for a week or so and we have the exact same problem!
Tekmar tech support gave us some leads to follow, but nothing has been successful. We're pretty certain that active sites are the culprit, but we just can't locate the source. The only things the 2 units have in common is the water (which we've tried eliminating without success), the trap (which was replaced without success), the sparge unit (which was replaced without success) and the teflon O-rings used to install the sparge unit (though Tekmar tells us those do not come in contact with the sample so they shouldn't be the cause of the problem).
Tekmar recommended we clean the moisture trap (please note that this is something we removed from our old system long ago and replaced with a small length of tubing), so we removed the trap from the refurbed system, pumped some methanol through the trap and dried it in the oven for about 2 hours at 120 degrees C (we couldn't find a recommended procedure for cleaning this part).
Anyhow, this leaves us with Tekmar's final recommendation which is cleaning the mount, but we want to do this right if we're going to remove it.
As anyone experienced a situation similar to this one, or does anyone have any ideas on how to isolate/resolve the problem?
Best regards,
Mathieu