Abundance and horn aging

[thohry]

Hi all,

I just have this question relating the abundance of a GC/MS. Let's say I have a new GC/MS instrument, with certain compound with the same concentration, I always get a signal of one million counts or similar. After half year or so, the abundance gets lower, so I clean the source and do the autotune (of course the EMV is higher : from 1300 to 1700). I expect the compound signal still gets around one million. But it is only 700k or so. Is there anything wrong ?
Thanks for any inputs.

[Yama001]

I do not believe an autotune can achieve that level of precision. I would simply push the EMV up a bit from the autotune (or whatever type of tune process you like) to get a similar area. Sometimes this is a meaningless act, as the S/N may stay the same.

Keep in mind the tuning process is using a compound (often PFTBA) that is in rough vapor pressure equilibrium with the vacuum. Not really a precise setup.

I usually try to achieve a response that is within 50 percent of whatever I was getting when the MDL, LOD, etc was determined, biased towards the high side. If I get at least 2 years out of a horn, I do not mind replacing it.

[thohry]

I do not believe an autotune can achieve that level of precision. I would simply push the EMV up a bit from the autotune (or whatever type of tune process you like) to get a similar area. Sometimes this is a meaningless act, as the S/N may stay the same.

Keep in mind the tuning process is using a compound (often PFTBA) that is in rough vapor pressure equilibrium with the vacuum. Not really a precise setup.

I usually try to achieve a response that is within 50 percent of whatever I was getting when the MDL, LOD, etc was determined, biased towards the high side. If I get at least 2 years out of a horn, I do not mind replacing it.

Thanks

[James_Ball]

I agree with Yama, best way is to do the autotune then inject a standard and manually adjust the EMV up or down to bring the area counts back to near what you originally had. I often step it up a few volts when needed to keep the areas up between cleanings.

[MSCHemist]

I find you can't really tell to much from the abundances except that if they are low you have a big problem. I had a cracked repeller insulator causing a 90% drop in signal and the abundances looked fine on the tune.