ISTD sensitivity issues

[kst1]

Hi! I have been successfully analyzing for a particular compound using an ISTD (deuterated isotope) on my GC-MS. But lately, the ISTD peak is barely showing up on my chromatograms and its abundance is way down. It appears as if the internal standard is not there. I have cleaned out the inlet thoroughly as per Agilent's recommendations since they said that active sites might have built up in the inlet. But still dont see any difference. I did quantify another compound using a different ISTD and there was no problem. Could anybody suggest what is going on? I have made fresh ISTD solutions a couple of times and also opened a new vial to eliminate any issues at that end.

Thanks!

[vballchemist]

What system parameters, Column, compound, method?
Is the Total ion peak decreasing or just the peak for the quant ion?
Is it possible the tune is drifting so as to miss the quant ion?
I have seen this before. I do a daily mass axis calibration.

[kst1]

I am quantifying the compound under SIM mode. The compound at 212 m/z and the ISTD at 217 m/z. I have been running this method for past 4 months and this issue has just popped up. At 50 ppt. concentration for the ISTD, I would normally see a peak area around 35000 but now the peak area I get for the same conc. is ~ 200. This problem does not exist with the compound at 212.

[jaj]

I don't think that you are giving us enough information to help you.
If your information is not proprietary, we would like to know the column,
method, sample introduction device, etc.

[kst1]

OK. I am using SPME (PDMS fibre) along with Gerstel MPS2. It is a Agilent 6890 GC with a HP-5ms column and a 5973 mass spec. I have already switched fibres to no avail. My samples are white wine. 20 min. extraction with SPME fibre under agitation and then desorption for 2 min. Inlet temp is 260 and GC to MS tranfer line temp is 280. Oven parameters are 45C for 2 min and 12C/min increase to 250. He flow rate is 1.2 ml/min. I dont think there is anything wrong with the method as I have run done hundreds of runs without any problem. But now since last week, this issue has popped up. Thanks!