not appearing peaks

[gandreou]

hello!

does anybody have any idea why when I am injecting 2 microliters of a 50ppm standard solution (containing 33 alkanes in cyclohexane) to my hp58920-trio 1000 ms, I get to see only the 5 first alkanes?

The column is DB-5MS, 30m, 0.25mm, and the oven program 50 C (1min), 50-150 C(10 C/min), 150-290 (5C/min), 290 (20min), and injector 250C.

Thanks for your help in advance

Georgia

[Kajjo]

Very strange, indeed.

Excuse the simple questions...

1) Are you sure the sample is authentic? (other devices like GC give all peaks?)
2) Are you sure the injector actually heats properly? (do the peaks you see decrease a lot in intensity? do other low-volatile samples elute properly?)
3) Are you sure the oven actually executes your temperature program? (oven is hot when opening during run -- might be crude method but safe)

4) How does the chromatogram look like after the first five peaks?
+ high-temperature column bleeding is visible? (yes: even more strange then > check sample, no: check temperature control)
+ Flat baseline: MS maybe stops acquisition?

Should I come up with better ideas, I will post again...

Kajjo

[gandreou]

hello Kajjo

I am sure that the sample is authentic and it is brand new. The peaks decrease in intensity (they look broadening and getting flat) and after the 6th peak I get to see nothing, just flat baseline.

I don't know how to check if there is a problem with the ms during acquisition.

Filament, trap and source current are ok at the beginning.

thank you for your help

[Kajjo]

Well, that looks like the lower volatile compounds do not actually get onto the column. Most probably the injector does not heat properly, please check! If the injector temperature is far below boiling point of the analytes, they will not be evaporated and brought onto the column (more or less, at least). The decreasing intensity and peak broadening points clearly towards injector problems!

>> check liner is clean (free of dirt and septum particles) and injector heats properly

I figure from your reply that the high-temperature column bleeding is visible? If yes, both the MS and the oven temperature control are fine, so please have a short look on the chromatogram!

Also, the mass spectra of the high-temperature range should exhibit the typical polysiloxane peaks. That would ensure that the MS works. However, as you describe the problem, it is most likely not due to the MS.

By the way, but not relevant for your problem:
I would use smaller amounts than 2 ul for injection, if possible, since too large injection volumes increase peak width and decrease resolution. Of course this depends on your injector liner dimensions. About 0.5-1.0 ul should be fine, normally, given a split rato of approx. 1:25 to 1:75.

Kajjo

[gandreou]

well, I have changed the liner and raised the injector temperature to 295C. I don't know how to check if the injector is working properly. the boiling points of the alkanes range from 216-467C. I can see up to C18 (bp 329).

The mass mostly seen after the peaks is 207 (polysilosaxane).

I have also changed the oven temperature program, but still the same result.

[Kajjo]

Well, with your temperature program you should easily see peaks up to C28 and beyond.

After which time do the peaks you see elute? (e.g. C15 at 10 min?)

Don't you have a digital multimeter with a metallic temperature probe? That's affordable and really easy to use! (< $100)

Is the injector port (septum holder/cap) extremely hot to the swift touch? (do not touch really!) It should be...!

Kajjo

[gandreou]

Dear Kajjo,

if it is ok with I could send you an email with a chromatogram attached, so as to have a better look on that.

to answer your previous question C15 appears at 22 min and after that my peaks are very broad and with no intensity until they disappear, and the injector port is too hot (i touched it, sinse our digital thermometer has broken down).

thank you for your time and help

Georgia

[Kajjo]

Sure, an email is OK.

Kajjo

[gandreou]

Well,because I don't have your email, either send reply with your email or email me ( you can access to my email address) and I ll reply back with the chromatogram.

Thanks

Georgia

[Kajjo]

Sorry, I've activated the feature now!
Please ZIP if large file, send either screenshot of chromatogram or HP-MS data file.

Kajjo

[Kajjo]

I've seen the chromatogram. The first five alkanes have typical sharpness and are more or less equidistant. Fine!

After that, they decrease in intensity, get very broad and the peak-peak distance increases strongly with each peak. However, the peaks are actually visible, so the injector is probably OK.

All this might be best explained with a decreasing oven temperature. Probably, the temperature program is not executed correctly. Have you watched the temperature rise and stay high? Check again carefully!

Maybe some sort of "overheating emergency shut down" because of a wrongly set maximum temperature? We once had that with a very old GC.

What about the FLOW?
Alternatively, maybe a decrease in carrier-gas pressure or flow-rate could give rise to the symptoms. Are there any time-programmed features in your GC that could cause carrier gas flow to break down or to stop?

Is there anything time-programmed at 10 to 15 min? (that is the time range where the problem most probably occurs)

Kajjo

[CE Instruments]

Could be a stuck flap on the GC. GC program runs but the GC does not get over about 110C as the heater cannot cope with the open flap. Certainly sounds like the GC does not temperature program.

[gandreou]

You are right about that. There is a change in the ramp. At 11 min it changes from 10c/min to 5C/min. Thats why I tried different oven programs today. Regardless of the temperature or the ramp there is always a decrease in intensity at around 13 min ( it also happened at 8 min and at 16min).

that's why I think it is the column problem. I have watched the oven and temperatures seem to increase according to the oven program.

I just changed the column with one that used to work fine in the past (4 months ago). I ll condition it tonight and [b]hopefully[/b] I ll have some good news tomorrow....

Georgia

[gandreou]

how can I understand if it is a stuck flap in the gc?

From what I can hear ( I can hear the flapping inside the gc) and see the oven is wroking according to the program.

[JI2002]

What's the carrier gas flow rate? Are you using constant flow rate or constant pressure?