Chromatography Forum

Hello all,
I've been running a SIM method to analyze urine sample for propoxyphene for about 6 months now. I recalibrate every two weeks and follow all of agilents recommended maintenance guidelines. I autotune everyday samples are run, and the EM Voltage is currently reading about 1650. I've been looking over all of my old calibration data and have noticed approximately a 50% drop in ion abundance since the method was initially validated. Could this be a natural loss in sensitivity as the electron multiplier ages, or is it more likely that my analytical standards are deteriorating (stored as methanolic solutions at -20). Has anyone else seen something like this? Any help would be greatly appreciated.
Thanks

Hello all,
I've been running a SIM method to analyze urine sample for propoxyphene for about 6 months now. I recalibrate every two weeks and follow all of agilents recommended maintenance guidelines. I autotune everyday samples are run, and the EM Voltage is currently reading about 1650. I've been looking over all of my old calibration data and have noticed approximately a 50% drop in ion abundance since the method was initially validated. Could this be a natural loss in sensitivity as the electron multiplier ages, or is it more likely that my analytical standards are deteriorating (stored as methanolic solutions at -20). Has anyone else seen something like this? Any help would be greatly appreciated.
Thanks

Probably, the ion source has been contaminated. It needs cleaning.

chemboy831,

Your autotune is trying to maintain a threshold value for the 69 ion and it will raise the EM voltage to try and get to that value. So if you look at your 69 ions over time they are probably very similar in response (probably 250K or so.) Plus, an EM voltage in the 1600's is pretty healthy. However, the ion you are looking at in Sim is probably not close to 69 so look at the relative stability of the closer ion 131, 219, etc.... relative to the 69 and see if it is maintaining a reasonably consistent value. Finally, 502 is a pretty good indicator of source cleanliness on the Agilent MS's, should easily bust 1% relative abundance.

Coming from an environmental background, I am a believer in running a fairly stable compound at a fixed concentration as a means of assessing overall sensitivity/stability of the MS.

Best regards.

I would suggest making a resh standard and comparing the resut. For some materials six months is a long life for a standard after it is opened or prepared. Over 6 months, some transesterification would not surprise me. While reactions are slower at -20 degrees, they still continue.

Chemboy831,

From time to time, i have noticed the same thing with my 5973. My detector is relatively aged though with the EM at around 1800 Volts. While troubleshooting, i keep two things in mind. One is what AICMM indicates (EM is decided based on m/z 69 and the analyte m/z may be different from m/z 69). Second is that the analytes pass through the injection port and the column while the calibrant enters the mass spec directly at the ionisation source. Any problem at the inlet including septa particles in the liners, leaking septa, old inlet seal and column clogging at the injector side could lead to the kind of result you are seeing.

I always wondered if i could use a compound with m/z 69 as a fragment ion and inject it as a standard check. Perhaps we could use PFTBA!!

How long has it been since the ion source has been cleaned? Urine extracts usually still have a substantial concentration matrix components. I would imagine that at a minimum the source would need to be cleaned every three months to maintain performance.

Don't forget the injection port. Urine extracts can contaminate liners and seals, which can contribute to compound losses.

Don't forget the injection port. Urine extracts can contaminate liners and seals, which can contribute to compound losses.

Most probably is a dirty source,but before venting the system I would:
-prepare a fresh standard,as Don suggested
-if it doesn't get better, check the inlet namely the liner (after a certain time and type of sample it can get contaminated).
-if neither of the first 2 suggestions work I would then vent the system and clean the ion source.

good luck!
Regards,
At_om

Thanks everyone for their advice. Just to follow up, cleaning the ion source cleared up all of my problems. I went back through my autotunes and noticed the isotopic ratio of the m/z 502 peak has been outside of the laboratories +/-20% window for about a week. I now know that this is a good indicator that it's time for a cleaning.
chemboy831

There are many possible reasons. BTW did you know that it is possible to upgrade a 5973 to a Triple Quad? (which can still run in SIM or Scan mode with almost no loss in sensitivity). Chromsys LLC (Chromtech GmbH in Europe) apparently does this. Quite an impressive product and the specs are as good as most top-of-the-line Triple Quads. It is by far the cheapest way of getting a Triple Quad instrument. I saw them at the ASMS show in Philadelphia last year.