Chromatography Forum

Hi all this is ashok from india. I hope all are good. Here in our GC-MS we are running standards every day at least 10 injection.The area of standard decreasing day to day let see today I am getting 30000 area for 50 ppb STD but the next day i am getting 29000 and the next day (third day) i am getting 28000 area. Let me know what may be the reason? Please help me.....

Hi all this is ashok from india. I hope all are good. Here in our GC-MS we are running standards every day at least 10 injection.The area of standard decreasing day to day let see today I am getting 30000 area for 50 ppb STD but the next day i am getting 29000 and the next day (third day) i am getting 28000 area. Let me know what may be the reason? Please help me.....

Air leaks can degrade sensitivity over time and also if you inject aqueous samples you can lose sensitivity. Column temperature too high can cause excessive column phase bleed which will also cause lower sensitivity over time.

Usually lower sensitivity comes from changes in vacuum or a dirty source if it is that fast. Longer periods of time it can just be normal wear of the electron multiplier.

Mr.James
Thanks for your comment. But we checked instrument leak also there is no leak in instrument and we are using organic samples(Hydrocarbons).there is no column bleed ions in the TIC and we are using same method in other instrument there we are getting very good repeatable result so do you think problem in oven temperature?.

Recently service engineer cleaned the source so we are suspecting it is not the problem of ion source. we need to monitor our vacuum system from today on words..

So shall i wait for some more days? then it will automatically become normal?

Thanks
Ashok

How old is the electron multiplier? If it is near the end of its life, it can give these symptoms, slowly losing sensitivity over short periods of time.

What model and manufacturer of MS do you have?

Mr.James,
It is new agilent GC 7890B & ms 5977 six month back only we purchased. previously we didn't face this much high response decrease within one day last 2 week on words we are facing this problem (after source clean). but after source clean we got very good response for PFTBA 219 ion nearly 140%. now today i did Etune again the now we are getting PFTBA 213 ion response only 80%. Slight variation in EM voltage also but very less.

our analysis is SIM mode we are monitering ions from 200 to 250 m/z only.

Regards
Ashok

Mr.James,
It is new agilent GC 7890B & ms 5977 six month back only we purchased. previously we didn't face this much high response decrease within one day last 2 week on words we are facing this problem (after source clean). but after source clean we got very good response for PFTBA 219 ion nearly 140%. now today i did Etune again the now we are getting PFTBA 213 ion response only 80%. Slight variation in EM voltage also but very less.

our analysis is SIM mode we are monitering ions from 200 to 250 m/z only.

Regards
Ashok

With the drop in 219 percentage it could be an air leak causing oxidization of the source over time, or there could be a problem with the column having high bleed and making the source dirty quickly, it has been known to happen that you get a bad column though it is rare.

If you set it to scan from 18m/z up to 510m/z you will see if there is any air and water getting into it, sometimes just scanning like that shows more than the air water check tune does.

Also try scanning with the cal gas valve closed and threshold set to about 10 and see how much background you have. You should only have traces of 18, 28, 32, 40, 44 and maybe a small amount of 207 from the column bleed, but not much else.

If you do see air and water try tightening the column interface fitting about 1/4 turn, as those ferrules can shrink after they are first installed.

Hi Ashok,

did you solve the problem and could tell us what parameters were responsible causing the problems?

I have quite the same problems...

Dennis