ISTD area count drop over time GC/MS

[LWC]

Hello,

Shimadzu 17A/QP5000

I am still having issues with my ISTD area count dropping over time. The inlet flow controller has been replced, there are no leaks, and the MS has recently had it's PM including having the pre rods cleaned. Any one have any other suggestions as to what is going on with this instrument??

(example today for 7 samples area counts dropped form 2.5 million to 1.5 million. )

[Don_Hilton]

There are a number of possibilities - and they break down pretty much into into two groups: 1) the sensitivity of the MS is fallign away or 2) material is not getting to the MS.

So, do you have any diagnostics like voltage required to get a given signal strength of a tuining standard (like PFTBA)?

Have you tried running the vendor's check standard for the instrument?

Material not getting to the MS could be 1) it was not in the solution going into the instrument 2) a bad syringe or septum 3) destruction in the linlet or 4) degredation on the column.

If you will tell more about your method, what the internal standard is, and what you have done to rule out these possibilities, we might be of better help.

[LWC]

The sample comes from a purge and trap so that knocks out the septum and syringe (I have replaced the septum multiple times just in case); the standard is in the samples, it's area count just decreases over time; The column is new and was installed in Aug.; the method has not changed at all method parameters are the same now as when it used to work fine.

The ISTD is 1-Chloro 2- florobenzene, method is set up for 524.2 analytes.

One "maintenance company" suggests it could be that the quads are either very dirty or going bad. The instrument is 7 yrs old and to my knowledge have never been cleaned like the pre rods (I have olny been working with it for 1 1/2 yrs.

[LWC]

Just to add something else I also did a multiple CCV run and all of the analytes I look at drop over time.

[yangz00g]

I am not surprsied to see if it happend to a Thermo GC-MS, but to Shimadzu, no experinec.

It may need some break-in time after source cleanings. Try more injections to see if drop slows and eventually levels off.

[Don_Hilton]

If the problem is quads, you should be able to detect a change in the instrument from tune parameters. I am not familiar with this particular instrument so I will assume that it has a solenoid and PFTBA vial for tuining. If so, the detector voltage to obtain a specific signal strength on a mass in the PFTBA spectrum (like m/z 69) will show the state of the mass spec. So, step 1: Do you have tune data to show that the mass spectrometer is consistant or is losing sensitivity?

[AICMM]

LWC,

Since it seems you are set up with the injector in line here are two things I would suggest you try. First, run a set of blank samples with a much higher split flow which should put less water on column and see if that improves the stability (even though it will decrease the sensitivity.) Second, try a direct injection in the inlet of the surrogate in methanol over the course of the day to see if that stays more stable than when purging. This will start to divide the problem/system in half, suggesting either purge-and-trap or GC/MS.

Best regards.

[seb9]

I have the same problem with the same setup.
LWC, could you be so kind to post how you solved your problem`?
I would be very grateful!
Thanks in advance

[nkg]

follow on from Don

in the past when using thermo single quads the pre filters could be cleaned / exchanged but in general this only got you going for another couple of weeks / months depending on usage.

once again not that famililiar with the software but can you acquire ms data with the reference vial open, scanning 50-650, acquire some data for at least 30mins? if so you should have a pretty level chromatagram a slight drop of 10% or so is reasonable, if not i would be going for a new / refurb quad , this is what i would have done with the thermo system anyway