Dropping IS & Surr responses

[EnviroChrom]

Hi there,

I have a 5975 coupled with the ATOMX P&T and I'm having issues with the IS & Surr. responses. Both IS and Surr. responses are dropping consistently. I started with 250k+ for Pentaflurobenze after running about 60 blanks the response was dropped to 43k. The affect on the surr. was consistent; what I mean is the Surr read 56 at 250k+ and it was 55at 43k. I have tried several different things with the ATOMX like changing the pressure regulator and IS valves, checked for leaks no leak.
Could it be the tune? Any suggestions.

P.S.: IS & Surr. are in the same mix.

Thanks a lot in advance.

[nvalentine]

Hello,

I'm sorry to hear you're having issues, but will try my best to help you get resolution. I do have a couple of questions before I can offer any suggestions.

What concentration and injection volume are you using for the IS/SS? Sometimes when priming a standard, especially at a high concentration, the first few runs may have a higher response. Depending on the concentration, this can take a long time to evacuate and stabilize at low injection volumes, but there are ways to speed it up and get it to stabilize quickly if this is the case.

Has there ever been a point at which the IS/SS has stabilized? From the sounds of it, you appear to be gradually losing response over time and there isn't really any point at which it levels off. One way you can troubleshoot this is to manually spike a vial with your IS/SS (with the appropriate concentration) and determine what your IS/SS response "should be".

This might also help to isolate from the MS. I guess it could be the tune, but that usually take a lot longer to progress in the decline of the IS/SS response. It would be relatively easy to test with a set of standards to gauge a target response versus the IS/SS response. There could also be other indicators in your tune printouts, bfb pass/fail, etc. if this were the culprit.

The first goal is really to isolate the issue no matter where the problem is originating from. And hopefully, that just takes a few runs with some modified sample prep.

Please feel free to respond here, email me at nvalentine@teledyne.com with any questions, or call our technical support line at 800.874.2004.

Best Regards,

[EnviroChrom]

Thanks a lot for your quick response. Here is some more info that i got.

IS/SS conc. is 50ppb in reservoir and ATOMX takes 5uL for the purge volume of 5mL of sample. I can assure that priming is not the case because this issue is going on for days, weeks & months.

If I run the instrument continuously for days the response drop is very gradual. As soon as it sits idle for some time or day, days, weeks, it will act up. What I mean is there are times when I have seen slight rise and then gradual decrease.

Yesterday I did 20 instrument spike and the responses were consistent around 37k’s range then i did 10 manual spike and the responses were consistent around 48k’s .

Thanks for your help.

[nvalentine]

Have you tried running spiked standards to see if this gradual decline in response is across all compounds or just the IS/SS?

A gradual decline in response over the course of days of contunuous analysis is normal to running a mass spec. This should happen regardless of the type of purge and trap/autosampler used. Allowing the mass spec to sit idle will serve as sort of a reset button on the response, which will come back up at the start of the next run, and slowly decrease over time again. The speed at which this decline occurs, or the initial response declining, could speak to the condition of the mass spec components, whether they would need cleaned, replaced, etc.

The telling part would be seeing responses for spike samples and making sure the relative responses don't change. A 5ppb sample should come back at 5ppb no matter how high or low the IS/SS, hence the idea of relative response factors and use of internal standards.

Have you run a full calibration and done any check standards during your evaluations? This is probably the best way to understand what is going on. As long as your IS/SS are behaving the same way the compounds are, then there really shouldn't be an issue. If they are, then I would go down the line of making method changes, physical instrument changes, etc.

I hope this helps explain things a little more clearly, but feel free to pass along any additional information or ask any pertinent questions.

Best Regards,

[LindseyPyron]

Since you mentioned running 60 blanks, I am going to assume that is is probably a moisture issue. There are several things to try and optimize your GC/MS conditions. If you give our tech support team a call at 800-283-3510 we can walk you through a few things to try.

[tlahren]

Lindsey is right. It's almost certainly a water issue. We had the same issue in the past. Like Lindsey said, call the tech support to help you establish a rugged method.

Also, Lindsey noted that the time it takes for the "decline" to occur could represent issues on the MS side. We were using an older 5973 with inert source. The inert source is crucial so I'm assuming you have it on your 5975. We also ended up switching the drawout plate from the standard 3mm to the 6mm hole (called Extended Linearity Kit I think). I'm not sure why, but this seemed to help resolve issues we were having with decreased response.

I have to agree with everything Lindsey said. I saw the same issues with the same results. Especially with letting the MS sit overnight to "rest." It allows excess water to be removed from the MS so you can start fresh the next day with a good vacuum and great ion transmission again.

[PeterB]

The current batch of AtomX seem to also have a standard addition bug going through them. The early ones we have are amazingly consistent with IS and Surrogate additions, however we've just sent one back, and it's replacement is showing the same issues.

As the earlier guys have said though, if it's the addition that is the issue, then runs with manually added standards are completely unaffected.