MS detector troubleshooting

[Irina]

Hi,

My GC-MS background is very high. When I run a blank it is about 100000 uV.

I try to clean it like a week ( baking it and flushing it) but still looks the same.

Any suggestions ? (except changing septa - I did that)

Thanks

[fsistere]

What m/z do you see in thespectrum of the background? It's important to know if it's a contamination or column problem.

[Irina]

the main m/z are 253 and 73, which are specific for siloxanes, but my problem is not just one peak . the background is going up with the temperature. I don't know how to attach a picture for showing you what I'm talking about.

[Don_Hilton]

What you describe is common in many columns. This results from depolymerization of the stationary phase in the GC column. This can be caused by oxygen in the carrier gas or residues left on the column that degrade the stationary phase.

Question #1: What grade of carrier gas are you using? It should be something like 99.995% or better. For GC/MS I prefer 99.999% or better. Lower grades of carrier gas can include small quantities of oxygen. The level may be low - but over time it will distroy a column.

#2: do you have an indicating trap at the back of the instrument? and is the indicator for oxygen used up? (No indicating trap - I strongly reccomend get one that shows both water and oxygen in the carrier gas.

#3: Have you throughtly cleaned the inlet? Septum crumbs lying on the bottom of the inlet will generate silanes. It may be time to change the seal on the bottom of the inlet - and a wipedown of the bore of the inlet with solvent may not be a bad idea. Replace the liner with a new clean liner as well as the septum.

#4 What is the maximum operatign temperature for the column (isothermal and programed?) and how hot have you been running it? Have you been running the column above the isothermal limit for extended periods of time? I know some folks who run the column above the upper temperature limit - and the expect to change the column frequently. Longer time at hiigher temperatures contributes to bleed - and this will not be reversed by baking.

In my experience, once a column starts to bleed badly, the best solution is to replace it with a new column (and place the old one in the trash - not save it for "just in case"). If the column will not stop bleeding after being baked out, you can try to remove a short portion from the inlet end of the column. This may add life to the remainder of your column. But it may be sufficiently short that it is hardly worth the trouble of adjusting retention times in your mehods.

Keep in mind that all columns bleed. I can not judge how bad your background is from your description.

[Irina]

Thank you Don for your post > it was useful for learning more about columns

1) I'm not sure of the gas purity, but it was the highest purity I could find on our market special for GC

2) I don't have a trap. Until a month ago I didn't know that exists something like that (I'm still trying to find one suitable for my GC, but also to be for moisture and oxygen just one)

3) I replaced the liner and the septum, but no improvement

4) usually I'm not using the highest temperature because I'm doing VOC on that column

The problem seems to be that the EI source needed a clean (I didn't know that the source should be cleaned from time to time). After that maintenance the background drop at less than 10 000. Now I'm waiting for the vacuum to be good and then to make another tunning and baking one more time because I get my hand in it .

I hope I will get a trap for helium as soon as posible. Do you have a suggestion for me? (I'm using a Shimadzu GC-MS QP2010 Plus)

Thanks

[Ron]

Columns optimized for volatiles analysis have a thicker stationary phase and tend to bleed more than thin phase columns. You may have a somewhat higher background than an intrument running a thin phase column, but that is normal.

As Don said above put traps on the carrier in line, this is critical if the purity of the gas is questionable. It is probably best to put a high capacity trap first (closest to the gas cylinder), and a high purity trap between the high capacity trap and the instrument.

Once the column starts to depolymerize after exposure to oxygen the process is autocatalytic, and as Don said the best thing is to replace with a new column when this happens. With the proper traps you will maximize the column lifetime as well as produce better data.

[Don_Hilton]

I do not know what vendors you have available to you, so I will list a few URL's of vendors in the US:

http://www.restek.com/restek/class/5004225.asp
http://www.sigmaaldrich.com/analytical- ... ement.html
http://www.chem.agilent.com/en-us/produ ... fault.aspx (look under Gas Purification)

There are others - but these come to mind at the moment. Look at what is on these pages for ideas and then look at what you can find through suppliers available to you locally.

Be absolutely sure of your gas quality. If you put one of these purifiers on poor grade gas, you will discover that they will remove a percentage of the contaminants, leaving you with gas that is not as poor -- until the trap is exhausted.

[Irina]

Thank you all for advices. I hope everything will be fine with my gc