Chromatography Forum

Dear GC/MS Users and Others,

I worked on a GC/MS as a graduate student. Now that I graduated and my first task at work is to write specifications for purchasing a GC/MS – I feel like I am lost. I wonder what would be the requirements of an ideal GC/MS for trace level organics analysis in water samples. I would appreciate any pointers from the forum. I would also appreciate for your time and kind help in this matter.

Sincerely – SA

PS: The GC/MS that I used in school was an old model and no more on the market. So I cannot really use those specifications as a template.

Your first step has to be clarity on what you need the instrument to do - trace level organincs in water could be any concentration from mg/l to ng/l. What kind of organics - pesticides, petrochemicals, natural products etc ? What sample prep are you planning to use ? What throughput in terms of samples per day are you going to need, and are you working in a regulated lab. Do you want to identify unknowns or will you always have target analytes ?

Then you need to scan the literature for similar analyses to the ones that you want to do, to get an idea of the instrumental set ups that were used. You will find that one particular make of instrument is quite popular - this does not necessarily mean that it is the best for you.

Knowing the concentrations that you need to reach, and sample sizes that you can work with, whether or not you can use SIM to pull target analytes out of complex backgrounds, etc will allow you to specify the minimum quantity that the MS must be able to detect ( in either SIM or full scan, or both depending on your analysis). For a high throughput lab oven cycle time will be important, for a research lab running two-hour separations on complex natural materials maybe less so. The cabability of the pneumatics in terms of maximum and minimum flows and pressures and split ratios all depend on the type of analyses.

All GC-MSs are run by software - and often its ease of use for various operations is what best determines which instrument you should buy.

Peter

SAGCMS,
In total agreement with Peter. What Methods are you going to be running (EPA, Standard Methods, custom), how many samples, what kind of water (waste, finished drinking, ground) should be your first considerations...i know from my personal experience that the EPA methods usually include what type of system the method was validated on and the settings they used to generate their results. Next, I would go to software, call some vendors and get some demos. For the most part a GC/MS is a GC/MS the software is what you're going to be using day to day. make sure it does what you want it to.

It appears to me that you are trying to put the cart before the horse First you need to decide what your applications are and if there are any restrictions defined by the methods you must use and the people for whom you run the analyses. Then you should consider your budget, will you need to compromise and buy lower cost ? Is there a future need for additional analysis that might necessitate a better system e.g. one that can do MS/MS ? Once you have your requirements defined contact the vendors and get them to visit/send info. Any that don't seem interested ? Those that meet your budget and application requirements should then offer you a demonstration and run some test samples. The biggest difference between systems is the software and you can't write a spec to decide which system is best without using it Once you have evaluated the systems it should then be easy to take the specification of your chosen system as the basis for the spec you need. It can easily be adjusted to be used for tenders and purchase justification.
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