GC/MS response decreased 4 times

[wl_yang]

Hello,

I have a problem again. I was running some herbicides using GC/MS half year ago, and now the responses decrease about four times for each compound! What happened? Please help me. Thank you.

[jimmyhu]

The response of GCMS does change over time, it may depend on: (1) instrument tuning, (2) instrument setup parameters, (3) standards degradation, (4) etc, So an employment of internal standard can minimize the variation of the GCMS response if it is not controlled.

[wl_yang]

Thanks a lot, Jimmy.
The solutions I analyzed are standard solution I made. The response for each compound decreased after several months later using the solution from the same source. Then I made a new stock solution and new standard solutions, and they could not give me higher responses. I autotuned and the result is good. I am changing the inlet septum now.

[Consumer Products Guy]

You need to first determine whether the low response is a GC issue or an MS issue.

[wl_yang]

because the tuning of MSD looks good.

[Ron]

If there has been no instrument maintenance performed in the last 6 months the instrument may need some work. As Consumer Products Guy pointed out the first step is to determine if the problem is in the GC or MS. Do routine maintenance of the inlet and column and see if response goes back up. If not, the ion source and lenses probably need to be cleaned. The tune may still look good, but have the repeller, focusing lens, or detector values changed significantly? If so the MS probably needs cleaning even though it tunes properly.