ISTD area count drop over time GC/MS

[mah1986]

Shimadzu 17A/QP5000

I am having issues with my ISTD area count dropping over time during a run. 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?? Has anyone experienced this before??? I have performed direct injections, with similar area counts, so I have ruled out the Purge and Trap as an issue

[nvalentine]

How dramatic of a drop? A gradual drop in response over consecutive runs (over hours/days) is expected, but anything drastic would be an issue.

[mah1986]

It is gradual, within a run, but dramatic throughout a day. For example, yesterday I ran a batch and my counts for 1,4-Difluorobenzene (ISTD) started at 3 million and by the end of the run (30 samples), it was around 600,000. At the beginning of each day after the instrument has not ran, it will start back high again, dropping throughout the run.

I should also note that for all of my ISTDs/Surrogates/Spikes are responding in the same way, only to varying degrees, thus leaving me with erratic results.

[nvalentine]

Do the compounds associated with the IS respond accordingly or is the calculated response also varying? If everything (all response) is dropping over time, that is normal, but there are some things you can do to slow this drop off down. The higher the split you can run and the less stuff (especially water) you can put into the mass spec, the longer the IS/SS values tend to hold. The IS values should hold for much more than 30 runs. I don't see anywhere near that amount when doing runs of 60+ samples.

[Bigbear]

I bet it's water build up in your source. As was stated increase split ( if you purge 25 ml you should be able to split 120-150:1). Use a 0.18 column, hold your oven at end temp for as long as you can (with my P&T setup I can hold the oven for 10 min and still have it ready to go before desorb preheat).

How long has it been since the last source cleanning?

[James_Ball]

I fought this for years with my Agilent systems, but since you mentioned pre-rods I doubt it is an Agilent.

Do you have a turbo or oil diffusion pump?

Does your IS area go up with increasing calibration standard concentrations when running a curve then drop off with samples?

Do you use a dry purge to clean out some of the water?
What are the trap materials?(having silica gel and charcoal in it can lead to a lot of water in the system)

Water loading as some said would be my first guess, but it can also come from the Electron Multiplier with a mixture of water and methanol loading it up. Since it is just after a PM I would assume that the roughing pump oil was changed as well as diffusion pump oil if it has one. If not try changing the oil.

If you have a diffusion pump, how stable is your lab room temperature over the course of the run? I also had trouble once with a 5971 that had a diffusion pump and the areas would decrease during the day and increase at night. The room temperature was varying about 15F sometimes more. We had to improve the temperature control in that room to fix the problem.

I found after many years that changing from a K&M monolithic EM to the ETP folded ribbon type EM that most of my variability over a run went away. Not sure if they do not catch as much water during a run or what, but I had one instrument that ran well after years of it varying wildly over time and when I checked the guy I had running it had switched to the ETP, once I switched all my others over the problem went away on them also(mixture of 5973 and 5975 systems)