# Gas chromatography/mass spectrometry: Can I use peak ratios and molecular mass to derive quantity? [closed]

This may be more of a chem question, but until there is a SE dedicated to chemistry, I'll rely on my philosophy that most natural sciences are applied physics and try my luck here.

So, without going into too much detail, I'm wondering if I can derive the quantity of a substance from a GC/MS report if I know the ratio to another substance, the quantity of that second substance, and the molecular mass of both substances. The site was very dense and vague regarding how GC/MS test work, but fairly upfront regarding how they derive their ratios.

Here are the numbers (slightly tweaked, but more or less in proportion):

Substance A:

• Quantity: 300mg
• Mol. Mass: 500g/mol (found in external source)
• Peak Proportion: 3

Substance X:

• Quantity: Unknown
• Mol. Mass: 425g/mol (again, external source)
• Peak Proportion: 1

To be clear, the ratio of 1:3 means that substance X has a horizontal peak 1/3 of substance A. I'm not clear on what the x-axis is actually a measurement of, which is part of why I'm at a loss. The site does make it very explicit that the ratios are not directly proportional to mass ratio (so if it found a ratio of 3:1 of glucose to arsenic, this doesn't mean the substance is 75% glucose and 25% arsenic, only that the glucose "peaks" 3 times higher, which I've taken to mean "3 times the oomph", but that may be incorrect, as well).

So, if I know that the molecular mass of substance X is .85 of substance Y, can I derive the actual mass of substance X using the formula:

$$qty_X=\frac{qty_A\times(molmass_X/molmass_A)}{peak_A/peak_X}$$

with data being:

$$qty_X=\frac{300mg\times(425/500)}{3/1}$$

Which simplifies to:

$$qty_X=(300mg\times .85)/3$$

and finally the result of 85mg.

So I guess in the end there are 3 questions:

1. Is this even how GC/MS results work?
2. If so, is my assumption to derive the mystery quantity using the molecular mass correct?
3. Is the math itself in order? (I'm specifically worried that I should invert either the mass ratio or the peak ratio or both).

Of course, if the answer to the first questions is no, then my true question is: can I derive the quantity of substance X with the given data, and if so, what would be the right approach?

If anyone is curious for some context, I need to know the actual quantity of substance X as I know that, by mass, it has a threshold between harmless and toxic, so just knowing substance X is 1/3 "peak" of substance A doesn't let me know if I should let my dog/child/self ingest it.

Thanks for your patience and help.

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Migration to chemistry.SE? – Qmechanic May 5 '12 at 14:46
Hi Anthony, and welcome to Physics Stack Exchange! Your philosophy actually does not work in that way - the sciences which build on physics, in particular chemistry, are not on topic here. We do have a chemistry site which is currently in private beta, so you could try to get access and ask there, or you could wait until they go public and come back and flag this for migration to that site. – David Zaslavsky May 5 '12 at 15:23
Glad to know it's in the works. My rep at SO might be good enough to get me a sneak peek. Will try. – Anthony May 5 '12 at 16:08
Ah rats. Looks like rep alone is not enough. Had to get in on the ground floor. You think it will really be public in a few days, or is that a boilerplate meant to entice me? – Anthony May 5 '12 at 16:10
@Anthony for what it's worth, I did think the site would be going public in a few days. But then I heard that they are not getting very many questions so I really can't be sure now. – David Zaslavsky May 6 '12 at 20:59