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Topic: Why use Internal Standard for GC-MS?  (Read 13392 times)

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Offline LabRat01

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Why use Internal Standard for GC-MS?
« on: February 09, 2011, 04:54:56 PM »
If peak area is proportional to the amount of compound present, and I record a GC-MS of my starting material (a known amount) before I run a reaction, and find a peak area of lets say 50 (whatever units), then after the reaction is over I find a peak area of my starting material to be 25 (whatever units). Then obviously I've only used 50% of my starting material...

Why the hell do I need an internal standard? Is it something to do with the injected volume not being the same?

Surely all of the other conditions like temperature etc are constant between runs...!

Can I not directly compare to the two peak areas of my two GC-MS printouts?

Offline Grundalizer

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Re: Why use Internal Standard for GC-MS?
« Reply #1 on: February 09, 2011, 08:21:20 PM »
Internal standards are great because they allow you to compare those two chart printouts.  If you didn't have an internal standard, how do you know the temp hasn't changed?  How do you know some nearby power-line wasn't turned on and pumped in a bunch of noise to the machine?  A multitude of things could have gone wrong between the time you ran your initial sample and your final sample.

In a real research lab, a GC-MS will probably have many people using it if your samples are run hours apart--so who knows what they did to it?  Maybe they peed in the mobile phase (sorry that's LC).

If you run internal standards each time, and THOSE peaks come out to be the same, then you know your good to go and you can compare the peaks from your two readouts, if not...who knows if something happened that effected the 2nd reading.

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