USEPA 8260 - 'New' Instrument Issues

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

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A few months ago I set up another purge and trap/GC-MS instrument to run samples for USEPA 8260. It had been mothballed for probably at least a year. After a few teething issues the instrument is running, but I have a couple of problems that are being a bit tenacious:

1. Significant carryover of high-boiling compounds (e.g. about 50% for naphthalene). I bumped up the desorption time somewhat but to no avail.

2. Surrogate response gradually increases over a sequence of runs, but internal standard response remains fairly stable. I found this odd because the internal standard and the surrogate are dosed similarly.

The instrument is a Tekmar Stratum with an Aquatek 100 autosampler, injecting into an Agilent 6890/5975 combo.

Any ideas what could cause these issues? I don't remember whether or not I replaced the trap in the Tekmar when I installed the instrument, but just wondering if there's anything that immediately stands out as possibly wrong.

I had another unusual issue as well in that the internal pressure was well in excess of 30 psi and adjusting the internal regulator in the Aquatek did nothing to throttle it back. I ended up having to put an additional regulator in the line to restrict it to the 6 psi that the Tekmar wants. Not sure what could have caused this aside from a faulty internal regulator in the Aquatek autosampler.
The last four analytes there around Naphthalene are always troublesome, not easy to bake them out and any type of contamination like char or clay residue in the system can hold on to them.

Does the Aquatek have a hot water rinse or heated bake of the sparge tube? I use both on the Encon Evolution to help reduce carryover for these analytes.

Any residue in the flow path from a former foaming sample can cause the carryover so may want to take it apart and clean the sparge tube mount and lines between it and the 6 port valve and any Tee fittings in the flow path. Might also want to replace the 1/16" tubing with some fresh silcosteel tubing too if there is the possibility of contamination.
The past is there to guide us into the future, not to dwell in.
What trap are you running?

What does your runs and bake look like?

At what concentration are you seeing carryover? I would consider 50% at 1000ppb normal but 50% at 10 ppb indicative of a major problem.

Did you clean the source? I've had that reliably solve 2. for me on specific instruments.
DReggio wrote:
What trap are you running?

What does your runs and bake look like?

At what concentration are you seeing carryover? I would consider 50% at 1000ppb normal but 50% at 10 ppb indicative of a major problem.

Did you clean the source? I've had that reliably solve 2. for me on specific instruments.


We use the Vocarb3000 traps.

Normally about 10 minute purge at 40ml/min, desorb at 240C for 0.5 to 1.0 minutes, bake 8 minutes at 240C (anything over 250C triggers the need to look for trap breakdown products per the method)

At the most 5% carryover should be expected, so 50ppb after a 1000ppb with a second blank being non-detect. If you can heat the sparge tube during bake and are using hot water then the carryover is even less.

The drifting Surrogate recovery, especially if it is 1,2-Dichloroethane-d4 is a common problem. We eliminated that surrogate just because it performed terribly. It's recovery is dependent on the total mass of target analytes in a sample/standard so the curve is skewed and when looking at blanks and samples that are clean you get higher values. The CCV would look correct but blanks and samples would recover high. Source cleaning can help, but I think it is also something inherent in P&T devices themselves.
The past is there to guide us into the future, not to dwell in.
James_Ball wrote:
DReggio wrote:
What trap are you running?

What does your runs and bake look like?

At what concentration are you seeing carryover? I would consider 50% at 1000ppb normal but 50% at 10 ppb indicative of a major problem.

Did you clean the source? I've had that reliably solve 2. for me on specific instruments.


We use the Vocarb3000 traps.

Normally about 10 minute purge at 40ml/min, desorb at 240C for 0.5 to 1.0 minutes, bake 8 minutes at 240C (anything over 250C triggers the need to look for trap breakdown products per the method)

At the most 5% carryover should be expected, so 50ppb after a 1000ppb with a second blank being non-detect. If you can heat the sparge tube during bake and are using hot water then the carryover is even less.

The drifting Surrogate recovery, especially if it is 1,2-Dichloroethane-d4 is a common problem. We eliminated that surrogate just because it performed terribly. It's recovery is dependent on the total mass of target analytes in a sample/standard so the curve is skewed and when looking at blanks and samples that are clean you get higher values. The CCV would look correct but blanks and samples would recover high. Source cleaning can help, but I think it is also something inherent in P&T devices themselves.

I use a vocarb3000 also. 11 minute purge, desorb 1 minute at 250C, and bake 8 minutes at 260C. I get a little bromomethane only if doing methanol extractions of hot samples. 50uL MeOH in the vial gives ~2-3 ppb.
DReggio wrote:
What trap are you running?

What does your runs and bake look like?

At what concentration are you seeing carryover? I would consider 50% at 1000ppb normal but 50% at 10 ppb indicative of a major problem.

Did you clean the source? I've had that reliably solve 2. for me on specific instruments.

It's a Vocarb 3000 trap. At least I assume it is, it should have been set up identically to our other ones at the site that we got it from.

Runs and bakeout look fine. Seeing about 50% carryover at all concentrations, once it gets down to 2 ppb it sort of hovers there..

I cleaned the source not long prior to running the last calibrations, probably let it bake out over the weekend.

I think next I'll try changing the trap and sparger. It'd be quite an old trap so if it's degraded in performance that might be an issue.

We don't have the same carryover issue on any of our other instruments, which have been in continuous use.

The Aquatek does do a bake/rinse cycle. I may also try changing as much of the tubing as I can if I can find some spare tubing...

It's funny - I got this instrument back online because we had a large surge in samples arriving and our other two instruments were fully booked, but as soon as I got it back to operating condition sample numbers went down and we no longer needed a third instrument... always seems to be the way.
wss wrote:
DReggio wrote:
What trap are you running?

What does your runs and bake look like?

At what concentration are you seeing carryover? I would consider 50% at 1000ppb normal but 50% at 10 ppb indicative of a major problem.

Did you clean the source? I've had that reliably solve 2. for me on specific instruments.

It's a Vocarb 3000 trap. At least I assume it is, it should have been set up identically to our other ones at the site that we got it from.

Runs and bakeout look fine. Seeing about 50% carryover at all concentrations, once it gets down to 2 ppb it sort of hovers there..

I cleaned the source not long prior to running the last calibrations, probably let it bake out over the weekend.

I think next I'll try changing the trap and sparger. It'd be quite an old trap so if it's degraded in performance that might be an issue.

We don't have the same carryover issue on any of our other instruments, which have been in continuous use.

The Aquatek does do a bake/rinse cycle. I may also try changing as much of the tubing as I can if I can find some spare tubing...

It's funny - I got this instrument back online because we had a large surge in samples arriving and our other two instruments were fully booked, but as soon as I got it back to operating condition sample numbers went down and we no longer needed a third instrument... always seems to be the way.


A 2ppb background means something is really not working.
The purge and trap is likely the source of the issue, but as a sanity check do a gc only run to rule out that side of the system.
Replacing the trap and sparge is the right place to start. You should then verify all the flows and double check temperatures. Something else may have failed when the regulator did.
If the carryover peaks are coming out as actual peaks, then it would have to be the P&T or GC, if it was contamination in the MS it would not give peaks corresponding to the oven temperature program.

I would most likely suspect some type of adsorbing contamination in the P&T. A little carbon in the valve or lines will hold on to traces of the analytes then when gas sits static in them between runs it will build up then pass through to the GC once the flow is re-established.
The past is there to guide us into the future, not to dwell in.
I changed the trap and the sparger, carryover has improved somewhat, down from 30-50% to about 10%, but still way too high. Going to look at changing the lines next, not sure where the naphthalene and other heavies might be getting stuck...
Contamination in the concentrator could be almost anywhere.
When you flush the 6 port valve (or equivalent in your system) switch it to both positions. If sample touches it, clean it.
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