Can derive the quantity of a substance from a GC/MS report if I know:
the ratio to another substance in the data,
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: $300 \:\pu{mg}$ (Know already, not part of GC/MS report)
- Mol. Mass: $500 \:\pu{g/mol}$ (found in external source)
- Peak Proportion: 3
Substance X:
- Quantity: Unknown
- Mol. Mass: $425 \:\pu{g/mol}$ (again, external source)
- Peak Proportion: 1
To be clear, the ratio of 1:3 means that substance X has a horizontal peak $\frac13$ 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($\rm M$) of substance X is 0.85 of substance Y, can I derive the actual mass of substance X using the formula:
\begin{equation}\mathrm{qty}_X=\frac{\mathrm{qty}_A\times(M_X/M_A)}{\mathrm{peak}_A/\mathrm{peak}_X}\end{equation}
with data being:
\begin{equation}\rm qty_X=\frac{300\:\pu{mg}\times(425/500)}{3/1}\end{equation}
Which simplifies to:
\begin{equation}\rm qty_X=(300\:\pu{mg}\times 0.85)/3\end{equation}
and finally the result of $85 \:\mathrm{mg}$.
So I guess in the end there are 3 questions:
- Is this even how GC/MS results work?
- If so, is my assumption to derive the mystery quantity using the molecular mass correct?
- 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 $\frac13$ "peak" of substance A doesn't let me know if I should let my dog/child/self ingest it.