Loss of Sensitivity in GC-MS

[lpenney]

Our lab routinely uses GC-MS to analyze for pesticides in water and other matrices, and we used to achieve detection limits in the 1 ppb range. We have recently purchased a new pesticide standard mix and a new GC column designed for pesticide analysis. Upon injection of pure standard solutions, we are now having difficulty seeing compounds at a concentration of 100 ppb. We have done many routine checks (eg checking for leaks, baking the column, changing the injector syringe, etc) and can't find anything wrong.

Our system consists of an Agilent 6890 GC with 5973 MSD. We are currently using a cool-on-column injector and hydrogen as a carrier gas.

Does anyone have any suggestions as to how we might improve our sensitivity?

Thanks,
Lisa

[oscarBAL]

Dear Lisa.
I just want to know; did you checked the distant of the column in the Source?
When Was the Last time you cleaned the source?
maybe you can increase the voltage for the source but that is not a good Idea; any way I never has used a Agillent GC/MS.
What about your retention time.
Has you messured the flow at the end of the column?
I am not a specialist but I think the problem is related with the source

good Look.

[sanjay]

Hello Lisa

If everything cleaned injector, Septa & column is fitted properly possibly you need to cleaned the source. You are using Hydrogen as acarrier gas how is the performance. Is this is your choice to use h2 as acarrier gas or agilent support h2 as acarrier gas

[lpenney]

Thanks for the help thus far.

I forgot to mention in my original post that we cleaned the source about six months ago. While we have been running samples on a semi-regular basis, the instrument has not been used that often since the source was cleaned - it certainly has not been running 24/7. So we didn't think the source should need cleaning already, but maybe this is the next thing to look at.

Lisa

[AICMM]

lpenney,

Can you tell us what column you switched to and what flow parameters you are using?

Best regards.

[sassman]

A few questions:

Do you regularly perform mass and resolution calibration?

If yes, is the sensitivity for PFTBA also decreasing?

When was the last time you replaced the multiplier?

As already mentioned, cleaning the source might help.

[lpenney]

I would like to try and answer some of the questions that have been asked. I'm sorry if I'm not answering them satisfactorily, but I'm not an expert in GC-MS!

- the new column is a Zebron ZB-50 from Phenomenex, 30m x 0.25 mm ID, 0.25 um film thickness
- we are using constant pressure mode with a starting flow of 1.1 mL/min
- we do an autotune on a regular basis (I'm not sure what the difference is between an autotune and a mass or resolution calibration). There have been no signs from the autotune that the source needs a cleaning (ie from the last autotune, the EM volts were approximately 1700, the abundance of the peak at 502 m/z was 64000, etc).
- I'm not sure when the multiplier was last replaced, if ever (I just started in this lab about 8 months ago).

Does any of this help to explain a loss in sensitivity? Is there something obvious that we're missing?

Thanks again for the help,
Lisa

[AICMM]

lpenney,

Have you always used hydrogen as the carrier? Or have you switched. When you say direct on column, are you using a section of transfer line since you are using a 0.25 id column? What is the level of air in the tune relative to 69?

Based on your comments, I would be more inclined to look at the inlet rather than the MS. Have you tried putting standard directly on column by hand and looking at the response?

Best regards.

[astroguille]

Hi Lisa

If the Autotune is OK, I think that the problem is in the inyection port.
Verify the liner and the injector hardware.

[Rick]

Autotune find a em voltage......1700
So, in your method, add a relative voltage of 400 to increase signal for your product.
You can increase the em voltage find by autotune too...for example, you can change the em voltage 1700 for 1800.