purge and trap loss of analyte

[chloesabrina]

I am baffled, I run EPA 8260b by purge and trap - centurion autosampler, encon concentrator and agilent 6890/5973 GC/MS. I have not changed any conditions and all of a sudden 1,1-dichlorethene is all over the place. Running a 50 ppb standard I can have results of 48 and then next run will be 33. My ical for 1.1-dce looks great and all other analytes that are from the same mix are looking consistent and very good recoveries.

Any ideas/suggestions????

[James_Ball]

How is your recover for 1,1,1-Trichloroethane? Is it higher when 1,1-DCE is lower?

If so do you see similar problems with Pentachloroethane and Tetrachloroethene or 1,1,2,2-Tetrachloroethane and Trichloroethene? This is a common problem if you have certain types of contamination in the system, the haloalkane losing a chlorine and forming the corresponding haloalkene with one less chlorine.

[chloesabrina]

All other compounds look GREAT! Thats why this is so strange. Do you think it could be a bad sorbent trap? I had replaced the trap the same day I performed the new ical and this problem started happening a couple of days later. There was no evidence of inconsistency in the ical though.

[James_Ball]

Trap problems are always possible, if you have the one you took out you can put it back in and see what happens, or if you have a new one try that too.

Is your room temperature steady? Sometimes breakdown can happen if the vials sit too long when too warm.

Are you analyzing waste waters or drinking waters? Have you analyzed any salt water samples?

You could have some type of contamination in the sparge tube itself causing problems. I have had samples that have suspended carbon in them that have caused problems later on because of the residue left behind, usually stuck in the frit of the sparger.

Also if someone preserved a sample with HCl that was also preserved with Sodium Thiosulfate you can get elemental sulfur precipitating out which can cause problems and one sign that has happened will be that the stainless steel needle in the sparger will turn black(which can be cleaned off with abrasive).

[chloesabrina]

On this particular instrument it is mostly groundwaters, but some of those can be pretty loaded. I did think about the frit in the sparge tube - I think I'll change the trap tomorrow and then if that doesn't help replace the sparge tube.

Thanks for advice

[James_Ball]

I just remembered another problem I had once when analyzing waste waters. I have had samples that were water filtered through a carbon bed and there were carbon particles in the sample. Those can contaminate the sparger and adsorb target analytes in later samples and it can drive you crazy trying to find that problem

[chloesabrina]

I have had better recoveries and more consitency since replacing the MORT, but still not as good as it was. When you had the problems with the sparge tube did you replace the tube or did you clean it? Did you use anything special to clean with?

[James_Ball]

We have had trouble recently with the MORT also, but it is affecting the last few compounds to elute(Naphthalene, Trichlorobenzenes), not sure yet what contaminates it.

If I have it handy I use the no-chromix solution in H2SO4, if not I will soak in conc. HNO3. Then wash with Citranox and finally with DI. It seems to do well cleaning them, but eventually you just can't get the crud out of those frits. I prefer the fritless spargers now and seem to get the same sensitivity at the same flow rate even though the bubbles are larger.

Purge and Trap systems have to be the biggest pain ever to troubleshoot. Autosampler, P&T, Transfer lines, inlet, column, detector, far too many places for a problem to occur.

[01310040231]

I posted a similar problem about a year ago. Somebody suggested that I look for H2O at the retention time for 1,1 DCE. It turned out that there was a different amount of H2O where 1,1 DCE was eluting. I replaced the water manger and rinsed the sample path with Methanol. This solved the problem. I regularly rinse the sample path at least once a month to avoid this problem. Also, I am not sure what kind of split ratio you are using. I use at least 1:100 and it helps to keep the system in good shape.