Decreased response in high points, GC-MS

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

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Hello,

I run EPA 8260 and have been experiencing a strange problem for months now. During calibration the first few points will be linear (.5-20 ppb) but the high points (50-100ppb) show extreme suppression of the chromatogram sometimes a highpoint will how a lower area count than a point below it. It happens to HP5890, 6890 and 5975c GCs, and to Tekmar 3100, and SOLATek 72 concentrators.

The problem arises suddenly, and is difficult to rid. For the older GCs reducing the purge time for 2 minutes to 30 seconds helped. For the newer GCs a full referb of the concentrator sometimes helps.

My first though was that it was a water issue, but increasing the split dramatically or letting it bake for 72 hours causes no change.

Some compounds show a significant increase:
Benzene
Chloroform

many show a huge decrease:
Styrene
Bromoform
Ethylbenzene
m,p-Xylenes
o-Xylene
Isopropylbenzene
1,1,2,2-Tetrachloroethane
Bromobenzene


Below is a comparison of a 75ppb run where that looks normal (black) and a suppressed one (blue). Only standards were run between these two runs.
Image

Internal standard comparision for these runs, over 100% is an increase from normal:

Pentafluorobenzene 160%
1,4-Difluorobenzene 167%
Chlorobenzene-d5 120%
1,4-Dichlorobenzene-d4 20%

It is mainly happening to water instruments but has happened to one soil instrument. We purge with Nitrogen and run with Helium. Gas was tested by hooking everything directly to a new tank helium.

Please let me know if you have any ideas or would like any additional information
Cleaning the source with a nonpolar solvent (hexane) then methanol as opposed to solely methanol got my responses back.
It looks like you have your gain (EM Volts) too high.

Lower the gain until you can just get responses for a sample at about half to one third the concentration of your lowest standard in your curve, using a threshold of 20 or so. You will have lower overall signal but improve the signal to noise and improve the linearity of your calibrations.

If you look at the problem peaks in the high standards, do you notice that the quant ion in QEdit appears to be split, with it dropping to baseline in the middle of the peak? If so it is over ranging the detector and it is shutting down in the middle of the peak for your mass of interest. The TIC will still appear to have a regular looking peak since it is a total of all the ions present, but with the over ranged peaks cut out, the overall height will be smaller.

This article from Agilent has a great write up on the problem with linearity and EM voltages starting on page 11

http://www.chem.agilent.com/Library/app ... 0603EN.pdf

I started following this suggestion and it really helped my linearity. It is very counter intuitive to lower the EM volts to gain sensitivity, but it actually does work.
The past is there to guide us into the future, not to dwell in.
Thank you for the reply.

My peaks look good and I don't think my gain is too high. EM Volts are only at 1600 with a 69 area count of 500,000 (HP 5975c). But I will try anything at this point. I lowered the area count to 250,000 and am running a test now.

While cleaning the source with hexane made my chromatograph look normal again I am still having trouble obtaining a linear response factor past 20ppb, but only for certain analytes.

I have been running 10ppb then 75ppb spikes as diagnostics and consistently have considerably higher response factors (1.2X-2X) in the high point for the following compounds:

associated with *Chlorobenzene-d5*
1,1,1,2-Tetrachloroethane
Ethylbenzene
o-Xylene
Bromoform

associated with *1,4-Dichlorobenzene-d4*
Isopropylbenzene
1,1,2,2-Tetrachloroethane
Propylbenzene
Bromobenzene

and the surrogate Bromofluorobenzene (always spiked at 50 ppb)

Chlorobenzene-d5 is stable while 1,4-Dichlorobenzene-d4 increases by about 20% between 10ppb and 75 pb spikes

The rest of the compounds are linear, most with single digit RSDs.

So it seems like some sort of active site but I have replaced the transfer line, all the sample lines. sonicated the block in hexane then methanol, and replaced the mount.

Please let me know if you have any other suggestions.
What split ratio are you using?
----suffers separation anxiety----
The runs with ion 69 at 250,000 look the same but with the area count halved. The split is 25:1 with a column ID of 0.25 mm. I know that is low for a 5975c but it has been working for us for years (at least as far as passing calibrations is concerned). My first thought was that too much water was getting on the system. I have changed the method to try and eliminate that by:

raising the split to 50:1
reducing the desorb time from .75 minutes to .3 minutes
reducing the purge time from 11 minutes to 4 minutes

None of these changes had any positive effect on problem compounds. I tested this by comparing RFs of 10ppb points to 75 ppb points.

Replacing the transfer line, moisture trap, sample pathway and cleaning the mount, block and 6-port valve helped but did not alleviate the problem.
This problem pops up here now and then. James is correct that keeping the EM low helps.

The last time I could not resolve this issue I replaced the repellor assembly. The problem cleared up.
Could you go into more detail about what you replaced? Did you replace just number 11 in this picture or the EI block it and the filaments mount to as well?

Thank you,
Damien
The Repeller #11 and the insulators #10 can give problems at times. Also the insulator #5 can give problems if it becomes contaminated.

Dropping the purge time can cause lower response on some of the heavier compounds and with more water soluble ones like the ketones and acetone. The shorter desorb time can cause problems if the analytes are making it to the lower portion of the trap.

I normally can run with 10 minute purge and 1 minute desorb and as low as 20:1 split on my instruments, though I usually have enough sensitivity at 40:1 and higher. We use the Encon/Encon Evolution purge and traps and they remove the water pretty well.

One trick I was shown a long time ago was to try using something like a piece of 0.53 DB-1 column as the transfer line. It will focus the water and methanol into a narrow band which gets split out more efficiently by the injection port. I have never needed to use it myself, but was told it helps a lot when using an ion trap instrument.

When a source gets really dirty, I clean the metal parts with a mixture of the alumina grit(white powder they used to ship with all Agilent MSs) and citranox, mixing it to a tooth past consistance in a cup and using a swab to rub the parts well. The swab will turn black the first few times you clean the part, then not so much once it is clean. Then was the metal parts with hot water to remove the grit and citranox, then sonicate with tap water, followed by DI water, then twice in Methanol before trying at 50C in the GC oven before reassembly.

Never put the insulators or filaments in the water or solvents, just replace them if they are very dirty, they soak up the solvents or water too much and take forever to bleed it back out.
The past is there to guide us into the future, not to dwell in.
Thank you for the response. I never considered the repeller causing problems unless it was scratched, I will look into obtaining a replacement.

Does citranox have an ingredients list or have you tested it? Methanol is the only solvent allowed in my building due to past contamination issues. Just using hexane was a hassle but we are going to give pentane a try as it is not on the 8260 list.

Still no progress with the RSD issue. I'm going to do a complete refurbishment of the concentrator next week.
DReggio wrote:
Thank you for the response. I never considered the repeller causing problems unless it was scratched, I will look into obtaining a replacement.

Does citranox have an ingredients list or have you tested it? Methanol is the only solvent allowed in my building due to past contamination issues. Just using hexane was a hassle but we are going to give pentane a try as it is not on the 8260 list.

Still no progress with the RSD issue. I'm going to do a complete refurbishment of the concentrator next week.


I have never found any problems using Citranox, it is what we use to clean our glassware also, before rinsing with methanol then baking at 105C.

We even have the HPLC in the same room with the volatiles instrument and it uses Acetonitrile and we have little to no problems with it showing up in blanks, even when we calibrate down to 1ppb. Unless of course someone were to dump a beaker of it directly into the sink while I am preparing standards, that does cause problems :( and he wont be doing that again.
The past is there to guide us into the future, not to dwell in.
Just to close this out. Replacing the electron multiplier resolved the problem.
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