8260 - Toluene-d8 loss

[cjm]

To all,

My coworker has been struggling with an issue on several 8260 volatiles instruments (6890/5973 with Enchon concentrator). The problem is Toluene-d8 (25 ppb) abundance, which should stay consistent for every run, drops out low with increased concentration of target analytes during a calibration, range 0.4-200 ppb. Toluene-d8 drops dramatically from target concentration 50-200. This also causes problems with linearity of toluene in the curve.

This problem is being seen across several instruments. I don't personally work with purge and trap systems and thought it could be an issue with the MS, something like saturation in the analyzer supressing signal for ions in toluene-d8, or EM too high. I'll list as many conditions as I have in front of me. Perhaps some setting is not quite right or someone here has experienced this before. Any thoughts would be greatly appreciated! Any additional info needed please let me know. Thank you.

purge vol = 5mL, purge temp = 35C, valve temp = 130C, line temp = 130C
trap type K (EST), purge ready = 35C, purge 11 min @ 35C, dry purge = 1 min @ 31C, desorb column pressure = 20 psi, desorb preheat 255C, desorb = 1 min @ 260C, bake 8 min @ 260C, purge flow 40 mL/min, trap back pressure 0 psi

6890N split, 2mm splitless liner, ratio 50:1, col flow 1.0 mL/min, 20.1 psi @ 45C

Oven initial temp 45C for 3 min, 15C/min to 230 for 3.3 min.

Column DB-624 20m, 0.18 mm, 1.0 um

MS 5973 BFB autotune EM=1565, foreline 219 mtorr, scan 35-260 amu, 5.98 scans/sec, transfer line 180C

[Steve Reimer]

I have seen similar behavior after running nasty samples, but instead of toluene the effect is on 1,4-dichlorobenzene-d4 but not the 1,2-dichlorobenzene-d4. The response becomes very dependent on the amount of methanol in the sample so that adding 100uL instead of 10uL results in a ~25% increase in response. The fix has been to flush out the transfer line from the concentrator to the inlet. (Cold methanol rinse followed by hot water)
This often appears after running samples with large amounts of creosote and/or chlorinated alkenes.

[mckrause]

Does your toluene become non-linear at the higher concentrations? In other words, is your toluene response higher than expected for your higher concentration standards?

[cjm]

Both responses for toluene and toluene-d8 decrease with increased concentrations. It remains linear from 0.4-20 ppb then starts dropping off from 50-200 ppb.

[JI2002]

The fact that this happened to all the instruments may indicate the problem is standard related, not instrument related. However, the foreline pump pressure seems to be high at 219 mtorr. I thought it should be ~ 50 mtorr. What happens if the 200 standard is diluted 10X and analyzed again? Could you list the responses of toluene and toluene-d8 for all the calibration standards?

[cjm]

I will list a calibration when I gather the data together. One interesting thing happened this past Friday when running a test. Mass spec was cleaned and two new filaments installed. Continuing calibration standard was run with filament 1. Toluene/d-8 was junk. Very low response (<25% initial calibration). When switched to filament 2 it was perfect. We switched back to filament 1 and results were even worse than the first run. What's interesting is looking at the data, all compounds with quant ion 91 suffered too, not just toluene. I don't know why a faulty filament would act this way. The bigger question is, could we really be seeing multiple faulty filaments across several instruments over the course of several months, all with this same strange behavior? Research continues....

[cjm]

Update: Toluene now failing with filament 2. back to original issue. I'm stumped.

[JI2002]

If the filaments are new and from the same supplier as before you had this issue, filaments are unlikely the culprit. Check tune report and make sure the tune is good. Also scan down to m/z 10 to see if anything else elutes at the retention time of toluene and toluene-d8. More importantly, think about what changed before this problem, instrument? sample preparation procedure? standards? traps?

[mckrause]

Thanks for the updates; they help.

Two things jump to mind here.

1. Agreed, the foreline pressure is WAY too high. Check this. You have to be able to efficiently pump away the carrier gas, and you simply can't at that kind of foreline pressure.

2. I think there is a really good chance that you are saturating your detector, since you are seeing this with the aromatics (they respond the best in the detector). Try running at -100 or -200 V from your autotune values and see what happens.

[cjm]

I'm now not so sure it's saturation, but I'm testing that theory now. We have several instruments running very differently and I'm still trying to understand them. Tune reports from one to another have 69 ion abundances from 300k to 1.8M. Even before the standards are injected the tuning of the mass specs is not consistent from one instrument to another. I am going to try and get all instruments tuned appropriately so all the methods have the same starting point.

To be fair, this problem we're seeing only started back in November, and it is odd that we can run a calibration with "ok" results then run a midpoint standard and have it fail only for toluene-d8 and toluene. I overlayed two 20 ppb standard chromatograms run only a couple hours apart on the same day. the only two compounds that failed were toluene-d8 and toluene. concentrations dropped from about 20 (of true value 25) to 6 ppb.

We currently see this issue on three instruments. new filaments, old filaments, refurbished filaments don't seem to make a difference.

[cjm]

The pressure fluctuations we're seeing may be a faulty guage. I have looked at the tune very closely and I'm satisfied the vacuum is ok. So I have lowered the EM on one instrument to obtain IS counts similar to another (working) instrument. The problem persists. When we run a 0.5 ppb standard, toluene and toluene-d8 responses are ok. When we run a 50 ppb standard, their abundances drop by 80%. If the introduction of the surrogate and ISTD compounds are the same every time, how in the world are we seeing such a reduction in response? I have to believe there's some weird chemistry going on in the system somewhere.

[cjm]

Well we have narrowed down our search for the likely cause. We performed a manual injection of standard through the GC inlet with fantasic results. Toluene and -d8 were right on the money. What's interesting too is the majority of target compounds showed a response increase 5-10x normal values, with little change in internal standard values. So we know the problem lies in the purge and trap system....a system I know very little about. I'm not sure what to try next. I would like to isolate different parts of the system to narrow it down further.

[James_Ball]

Try increasing your desorb time to 2 minutes and see if that helps any.

You are running an Encon purge and trap, what is the bake and ready temperatures of the MoRT(Moisture Reduction Trap)? I have found that if you bake the MoRT at too high temperatures it causes a greater bias in internal standard recovery versus concentration, possibly from a microscopic layer of char forming from high boiling compounds during the bake. Though what you are seeing with the response biased low with increasing concentration is the opposite of what I normally see.

I run my MoRT bake at 180-150C since all you really need to cook off is the moisture. Also over time the trap becomes contaminated and should be replaced, it is simply an empty Silcosteel treated tube but it does need to be changed out at times. Try getting a new one and see if that helps at all if you haven't done it yet. Trouble shooting purge and trap problems is an order of magnitude worse than with simple injection port introduction of samples.

Somewhere on here I posted results I did of a study on calibration concentrations versus internal standard responses. With purge and trap, responses seem to vary depending on the total mass of organic analytes, not amount of methanol as was previously thought. I was trying to calibrate from 0.05ppb to 50ppb but my 1,4-Dichlorobenzene-d4 internal standard would almost double in response from low to high standard when I ran with a 5ppb internal standard concentration. I increased internal standard concentration to 50ppb and the response increase became less than 20% over the calibration range. It seems if you take the total mass of all of your analytes in each standard, the less change in total mass you have the more linear your calibrations will be. If you have 100 compounds at 0.05ppb you have a total concentration of 5ppb plus you add 8 internal standards/surrogates at 5ppb you have a total of 45ppb. Then you have 100 analytes at 50ppb you have a concentration of 5000ppb plus 8 internal standards/surrogates at 5ppb the total is 5040ppb. But increase the internal standard concentration you then have with the same standards a low of 5ppb targets plus 400ppb internal/surrogates of 405ppb and a high standard concentration of 5400ppb. Low internal spikes gives a difference of 1008X between low and high total concentration, high internal spikes gives a difference of 135X between low and high total concentrations and reduces internal standard bias on the 1,4-Dichlorobenzene-d4 from 100% to less than 20%.

I only noticed this when trying to work with super low concentrations, but I always had similar problems with normal concentration ranges where I was seeing sometimes a 50% increase in the response of 1,4-Dichlorobenzene-d4. I worked for years to figure out what was causing this but I seem to be on the right track now to solving the problem. It is something unique to purge and trap and I believe it may have something to do with the efficiency of the trapping material to adsorb and release components depending on the total loading of the organic analytes, but I need time to do more study on this. The trick seems to be to limit the total difference between high and low concentrations as total mass of analytes, which can be normalized over a larger range of calibration levels by increasing the concentration used in the internal standard/surrogate mix. It would mean a departure from the traditional wisdom of making your internal standard concentration fall in the middle of your calibration curve, but it is similar to what we do with our ICPMS for metals analysis were we spike the internal standards near the high point of the calibrations so that they will overwhelm any possible contribution from trace amounts that could be in the samples themselves since with metals we don't have the luxury of using labeled isotopes.

If I ever manage to get caught up on all my projects I plan to get back to studying this in more depth. I wish I had known about this during grad school, it would have made a great thesis research project.

[cjm]

James,

Thank you for the information. I have passed it on to our voa department. I am going to have my coworker join the forum and put up his thoughts on this as it is his instruments that are being affected (now 4 out of 5). He will be able to give more detail as I'm not experienced with purge and trap systems.