what is the reason?

[che313]

Yesaterday I run a sample with my GC-MS and it gives this chromatogram


today, I re-run it again but it gives me diffierent chromatogram!!


what could be the reason is?

Instrument: Agilent 5975B GC-MS
Carier gas: He
Flow: 1 ml/min
Column: HP-5ms
length: 30m
dia= 25 um
thi = 0.25 um

Method
initial T = 70 C for 2 min
then 25 C/min to 300 C hold for 4.8 min

source= 230 C
quad = 150 C
transfer = 250 C

[Consumer Products Guy]

Looks like a chromatography issue, not an MS issue. Could be do to clogged syringe needle (even bent needle or plunger, take a look), a leaky septum, a column leak, sample level in vial below needle tip, carrier gas cylinder empty, stuff like this. Are you doing manual or autosampler injections? OK, if I didn't find any of that - what would I do next? I'd rinse a syringe with something like alcohol or hexane then dispense it, then inject THAT empty syringe into GCMS, and over-ride any solvent delay, to see if and when my essentially-unretained peak takes to elute.

[che313]

it is auto injection.

how can I treat a clogged syringe needle?
I'll check for others and till you

thanks

[Peter Apps]

What are you expecting to be in the sample ?. What happens if you run a standard ? Run a search on the MS library to identify the peaks - my guess is that they are siloxanes from column bleed or the inlet.

Peter

[JI2002]

Assume the first chromatogram is what it should be, then I agree with CPG that the second chromatogram is clearly just the baseline, which means the sample didn't go into the column. TO check if it's an autosampler problem, just do a manual injection.

[che313]

Assume the first chromatogram is what it should be, then I agree with CPG that the second chromatogram is clearly just the baseline, which means the sample didn't go into the column. TO check if it's an autosampler problem, just do a manual injection.

Yes, the first chromatogram is what it should be.

And as far as the syringe clogged many times before, I assume it is clogged again.

Heating the needle is the procedure I was follow for solving this situation, I'm right?

[Consumer Products Guy]

Take the syringe out of the autosampler, see if you can manually aspirate and eject a solvent such as methanol. If no, replace the syringe, or replace the needle if it's a such a type, unclog the needle with a tiny metal "cat whisker" cleaning wire, or replace the syringe. And try a manual injection as well before re-installing into the autosampler. I've seen it all: bent plungers, ends of plungers coming off, bent needles, clogged needles, liquid too low in autosampler vials, leaky septa...

[AICMM]

che313,

Best to use a cleaning wire as per CPG.

If you are continuously clogging your needle you are either using the wrong needle type or syringe or you are tightening your septa way too tight. I will assume you are using a 7673 or some derivative and a 5 or 10 uL syringe made for said autosampler. Based on my experience (having been around numerous versions of this autosampler and rarely seeing them clog needles) then you are tightening your septa way too tight. Pop a new septum in, tighten the nut until you first start to see a pressure rise and then a 1/2 turn more and no more.

Best regards.

[Peter Apps]

Also check that your wash solvent dissolves everything in the sample - poor washing usually causes jammed plungers but can also lead to blocked needles.

Peter