GC-MS problem (methanol is converted to formaldehyd?)

[Chacer]

Here is the spektra of the mystery peak for those who have trouble seeing the picture

[Don_Hilton]

On seeing signals at 135, 136, 138, and 139. Umm... apparently trouble counting (or my cat was helping by walking on the keyboard). What I'd like to do with 135 - 138 is overlay them in the same plot, if the software will do that, and display from just before the start of the peak to just after the end of the peak. The point being to see that the ions all have the same shape profile through the peak. If they all rise and fall together, they are likely to be from the same compound. If they rise and fall differently, they are from two closely eluting compounds.

[Chacer]

It seems that throuwout the peak, there is the same shape profile for the ions.
The only thing that looks weird, is the "smear" between the peaks



Im still learning this software, so i dont know how to filter out certin ions yet.

[Chacer]

[Don_Hilton]

In your picture with masses like 56 and 89, one would not expect to see much - at least in the region of the mystery peak -- because these ions do not show much or any signal in the spectrum. My interest in masses 135-138 was to see if the signal in the mass spectrum corresponds to something going on the chromatogram that would indicate that there is a significant M+2 ion in the spectrum.

That "smear" you see looks to be several compounds that eleute close together -- additional mystery compunds. Each would have a spectrum, but the spectra are likely to be mixed because the peaks coelute.

My thinking as I commented on the M+1 and M+2 intensities was to move away from expecting the problem to be a reaction between formaldehyde and metamphetamine. If there is a sulfur (or silicon - but the M+2 does not look to be sufficiently intense - and I don't have numbers to work with) in the molecule, then we are absolutely looking at something else. If this takes you in the direction of remembering something that has been done with the instrument that would account for different compounds, it might help. I suspect that the history of the instrument would have the best clue as to what is going on. Something happened differently between the two instruments before the problem started. So, in troubleshooting the problem - do you have run logs for the instruments which might show something about the history of the instruments during the period before the problem began?

[Don_Hilton]

Actually this could be a silicon compound - I was thinking the M+1 rato for silicon while looking at the M+2 abundance of the peak. (The problem with going by memory... )

[Chacer]

The so called experts from Agilent called back today.
They told us to clean everything, and tomorrow we will start to do that and see what happenes.
Im not sure how to explain this in english, but they said that the pipe that leads the gas out, could be partially blocked/dirty, causing a "suckback" ( ). Which is causing the smear. We will clean and see if it makes a diffrent.

[Don_Hilton]

I would be interested to know which tube they think is blocked... Keep us informed.

[Consumer Products Guy]

Agilent thinks you could have some crap in the split vent line. Clean it out or replace it, and clean the inlet weldment as well (the part the split vent line screws into.

[Chacer]

We have cleaned everything and its still the same...
We have ordered new tubing and will replace it when it arrives.
Ill keep you informed.

[Chacer]

New tubing has arrived, however this did not solve the problem...
Moreover, the other GC has started doing the same now...
We are now quite sure that methanol is causing the problem...
We tried removing the MeOH under nitrogen and redissolved in EtOH and there was no problem with methamphetamine...
However the Rt of all the compounds changed. So dissolving in EtOH is not disirable as we would have to remake the entire database.
Suggestions are still welcome.

[Peter Apps]

Looking back through the thread, you have never actually told us your operating conditions apart from the inlet temperature and a highly implausible column temperature programme in the first post.

If a switch from methanol to ethanol changes retention times there must be some odd solvent effect happening - solvent effects can also give peak splitting, so maybe at last we have a clue.

So please give us full details on the GC settings.

Peter

[Chacer]

Sry didnt think about that.
Here is the info:


Temp Prog:


Max oven temp: 325

MSD Temp:
Setpoint Quad = 150 C
Setpoint Source = 230 C

Front inlet temp = 220

Gas type: He

Front inlet:
Split/Splitless Inlet:
injection mode: pulsed splitless
inlet pressure (kpa): 97.488
purge flow (ml/min): 50
purge time (min): 1.98
pulse pressure (kpa): 241.312
pulse pressure time (min): 2.00
gas saver flow (ml/min): 20
gas saver time (min): 3.00
gas saver on: Yes

Injection Volume = 1uL

Collum:
HP-5MS 5% Phenyl Methyl Siloxane (Agilent)
Lenght: 30m
Internal Diameter: 250 um
Film Thickness: 0,25 um

[Chacer]

I see that the temp in my first post was wrong...
Sorry for any confusion...

[Chacer]

ALSO it is a GC-MS and not a GC-MS/MS
Im still new