PDA

View Full Version : GC/MS Question



PlurredChemistry
04-09-2001, 05:51
Hey All
I've used a GC before.
I (kinda) have access to an MS.
I know how they work seperately.
When people say that you need to test your pills with a GC/MS does that mean that it is a single machine that does both GC and MS analysis, or are they doing both seperately?
Just curious...
~mateo~

johnboy
04-09-2001, 11:15
um whenever i say it i mean it as shorthand for "use GC or MS or HPLC or whatever, so long as its more accurate than these damn spot test kits we have to use"
anyways i'm sure someone will correct me but AFAIK GC/MS testing is just doing one after the other and cross referencing the results.

MotionBlur
04-09-2001, 16:08
GC/MS means the machines are joined together...So you just inject the sample once into the GC and it makes its way thru the column and into the MS. The bridge between the two machines is a pretty complex piece of engineering due to the requirement of the MS to work in a vacuum.
Usually a computer will interpret the results and give you a %hit rate based on its sample library. So what you get is the standard GC trace but you can click on a peak and it gives you the MS data for that peak!
Needless to say, these machines are very expensive.
Mb

Quirks
06-09-2001, 00:31
They are used together because MS needs nearly pure samples and the GC works by seperating the components which are then spit into the MS. Together they are a very powerful analytical tool.

Roches
07-09-2001, 07:14
gas chromatography is necessary to determine the composition of the original sample (in terms of several unknowns) and to separate the mixture into very pure fractions suitable for mass spectrometry.
mass spectrometry then provides you with a conclusive identification of the sample.
MS is probably best suited for simple identification. you can, in theory, elucidate structures from it, but it'd be an unnecessarily difficult math problem - especially because of the effects of isotopes (which is why you get those staggered groups of peaks..)
you could just as easily use a more primitive method like IR for pill testing, considering that the number of possible unknowns is quite small.
if the MS doesn't identify the substance (as sometimes seems to happen with Dancesafe) then you're in a very interesting position. you'd then, theoretically anyway, elucidate the structure by NMR.

Quirks
07-09-2001, 17:43
The problem with nmr is it's difficult to prepare a sample from a single pill, let alone a scrapping. Even ir needs a relatively pure sample and would require too much preping from a single pill, and really isn't useful with just a scraping. The only practical method of incontrevertable quick onsite analysis is going to be gc/ms.

Quirks
07-09-2001, 17:46
And out of curiosity roches, do you ever go to system or turbo on the weekends?

PoMo
10-09-2001, 06:00
Quirks, I'm sure you've seen him there... pasty-faced white guy with a stained lab coat, and those splash-proof chem glasses...