Chromatography Forum

what detector voltage would produce molecular ions for caffeine detection? and what type of ionisation? currently the method is set on 1200V EM and all I'm getting is 2 ion fragments

Hi
A bit more info first please.

Instrument you are using and ionisation mode.
Does it tune
What ions are you seeing
Method details

It could be a whole host of issues including leaks, inlet, temperature, gases..the list is endless at this point.

the instrument is a GC/MS Agilent 6890N GC with a 5793 MS detector.

instrument does tune.
basically just seeing the original ion - no change in the mw
not sure what ionisation mode its in or how to check or change it

method

oven
90oC @ 4min
ramp 25oC/min to 300oC

inlet
split mode
10:1 split ratio

column
HP-5MS 0.25mm * 30m * o.25um

thermal aux 2
MSD transfer line heater
280oC

3uL injection
em absolute voltage = 1200
low mass 40 high 400

ms quad 150
ms source 230

Hi

You are probably working in EI.

My inital thinking is the MS is ok as I have never come across a 5973MS EI ionisation not smashing a molecule to bits and also it tunes!

First off, are you setting the EM voltage to 1200 or is the tune setting this for you? What may be happening is that you are only seeing the molecular ion as it is the biggest and you have in fact got a sensitivity issue either due to the MS ( 1200MV is too low ) or the GC.

the voltage was set from a earlier method when the software was installed on a different computer. the default method has a voltage of 1529. also should it be set on relative or absolute? If i click on relative it goes to 0 in the input box but at the end says "= 1200". if i click on absolute it goes to 1200 in the input box

I think that the detector voltage has little or no influence on the fragmentation of the molecule. As far as I know, it can be either fixed (never did that) or changed accordingly to daily tuning. High voltages mean stronger signal but also more background noise - ideal should be something around 1200/1400. The relative abundance of the molecular and fragment ions depends first on the source and then on the quad tuning. Assuming you are working in Electron impact (and not in chemical ionization), the filament is almost always set at 70 eV, so that it produces a "standard" fragmentation which is comparable with the libraries. The only factor then that could modify slightly the relative abundance is quad tuning, depending on whether you used standard spectra, max abundance or high mass tune. But the difference between those is small, ans it is unlikely that you are going to completely loose a peak from your spectrum. By the way, assuming you are not using derivation, your spectrum should look like this http://www.massbank.jp/jsp/Dispatcher.j ... 77&site=10

Hmmm... really? Are you seeing a peak and assuming it is caffeine or do you see the molecular ion at 194 and one other?
The first requirement for you to get a signal out of a GCMS is to have an analyte volatile enough to get through your GC
column. I don't know where caffeine boils but it's hard to imagine it is volatile enough to get through a GC.

Which ions are you seeing? 194 and 109? Can you post a photo of the mass spectrum? You should definitely be seeing fragments below 109 m/z. Caffeine should be readily detectable via GC-MS. What is the ratio of 194 to 109?

Caffeine should be readily detectable via GC-MS. ?

Indeed; in simple extracts of coffee caffeine gives a huge peak under very generic conditions - 5% phenyl capillary column, temperature programmed from 40C to 240C at 5C/min so detection should not be a problem. Unless it is at trace levels the problems that the OP reports are probably symptoms of a hardware or method problem.

Peter