Mass analyzer the same in GC-MS versus LC-MS

[chris.singleton]

Greetings,


In our lab we are considering a purchase of a GC-MS. We usually do LC-MS/MS, but have some assays that require GC-MS. Since we would use the GC-MS sparingly, we thought about getting one of the MSs that can change the source and go from LC to GC and back (Thermo and Varian). I had talked to another vendor (whose company did not make MSs that could switch back and forth) and they asked why would I want to do that, it is unadvisable, the MSs are different etc. I was pretty skeptical, especially since that person mentioned other things about quadrupoles that I know is incorrect.

So my question is, are there any significant differences between the triple-quad mass analyzers for GC and LC? I know that the sources will be different (API for LC vs. EI/CI for GC), but is there any real difference in the way the quadrupoles/mass analyzers are designed?

As a side note, I know one manufacturer actually makes an atmospheric pressure GC, which is a simple probe change from the API in an LC (http://www.waters.com/waters/nav.htm?cid=10100362). And this was also done in a paper 5 years ago (McEwen and McKay, J. Am. Soc. Mass. Spectrom. 2005, 16, 1730-1738) where they modified an LC-source to take a GC inlet. I can't think of a difference, anyone else know of any?

[Don_Hilton]

The API interface for GC will give API type ionization - which will be soft. Expect to have the same kinds of limits on which kinds of molecules will ionize as with API and you will not be able to use spectral libraries to identify compunds.

While the mass analyzers are similar in LC and GC instruments - the source is a major difference. And the interface between the source and analyzer section is a part of that difference.

If you are interested in a MS that will allow you to switch from LC to GC and back, get references and a demo.

My experience with an instrument that was supposed to do both LC MS and GC MS, years ago was that yes it did do both - but not as well as instruments that would do one or the other.

[chris.singleton]

Thanks Don. The instrument that we are looking at will have a dedicated source for both LC and GC, so we will be switching the sources on an occasional basis and will be able to get EI fragmentation for library studies.


Has anyone else had experience with these instruments?

[CE Instruments]

Whilst I cannot comment on current instruments , systems that could run GC and LC data on the same MS platform have been around for years. There is no difference in the Mass analyser for Quads , single /triple. All the difference is in the Ion source. Systems in the past used to work OK , especially if you did not change too often. Where the results get bad is usually where the LC eluent can contaminate the Mass analyser causing problems with EI and leak problems with continued source swaps. How can anyone design a source with two completely different technologies that optimises the operation of both techniques in a single housing where the distance from the ion source is a factor in the sensitivity ? Any dual use source will always be a compromise. If you need to do GC/MS, I doubt you need a triple, buy a low cost single quad for GC and do not risk a big increase in downtime on one of your triples from problems trying to get GC to work.
Just my 2C

[Widebore]

My recommendation is that you should avoid the dual platform. Get a dedicated GCMS. You'll have less problems in the end.

[R13]

The physics laws "working" in both quadrupols are the same, so these basicaly the same.

BUT general instrument hardware design optimisation in both cases will be different because of nature of ionisation, ionisation chamber pressure differences and so on. So some instrument performance (sensitivity being first) must be sacrificed to make instrument variable.

It is like with "flying cars". You can buy one, but:
-it is expensive,
-it is prone to many problems and repairs because of complexity
-it it is a bad car
-it is a miserable plane

[MassSpecM]

i work in an academic lab, where flexibility is key (more instrument for your money principle as financing is tight)
most of the platforms in my lab is interchangeble - in theory!

we have an old Quattro 1 instrument capable of switching between EI and API. this is actually very easy and requires minimal effort and downtime. exept when i forget to pull back the GC column and snap it off.
but we do not look at low levels of anything so i wouldn't know about sensitivity changes.

we have a thermo DFS system (magnetic sector) that can pretty much do everything from ESI to CI to FAB - IN THEORY!
this switch has proved more troublesome. EI to ESI - nightmare! the rough pump we received with the ESI unit did not do the job and we had to upgrade (at our cost) to a industrial vacuum pump. The ESI manifold shocks the living daylights out of you when you touch the syringe pump and the cappilary is a b!t@h to clean. the fab gun is inconveniently positioned and makes the source very very dirty! duh!
Also, switching back to EI resulted in a day's downtime and backing out the source to get rid of all the junk that accumulated in the source.
however, when working in ESI I get beautiful results within 1ppm mass accuracy in comarison to the 5 - 8 ppm in EI

so yes in theory its nice to have, but if you are not in an acedemic invironment and there are deadlines around the corner and managers breathing in your neck..... stay away far away!
you need to be very hardware savvy to operate it efficiently.

i suggest if you want to buy the waters one - ask them for a demo - time them from removing the GC interface and connecting the API interface - times that by 3 or 4 - thats the time it will take you to do it! if you can live with the downtime - go for it!

@r13 - but its a flying car