Why you don't know the concentration unless you do a standard curve for each compound

Intensity is related to concentration, but not equivalent.

In mass spectrometry, each single compound has a different response factor from other compounds, and it also depends on the conditions during acquisition. That means the equation to convert signal to concentration is unique to each compound and batch. That is why a calibration curve with pure standards of the targets and internal standards is required for every batch analyzed if absolute quantification is needed.

Note that response factors might also be widely different between compounds. So the signal intensity of one compound does not tell us anything about its relative amount compared to another compound. (if compounds are very close chemically, with the same functional groups, etc, then response factors tend to be similar).

So as an example comparing glutamine and glucose. Glucose is not ionizing easily, while glutamine is. so if in an example you would have the same intensity for glucose and glutamine, the glutamine concentration in the sample would be much lower than glucose.

You can however compare signal of one compound between samples (this is what is done in untargeted metabolomics). So if glucose signal in sample A is higher than in sample B, then you know there was more glucose in A than in B.

But note that the equations to convert signal to concentrations usually don’t have a slope of 1, nor power of 10. So that mean that a doubling in signal doesn’t usually mean a doubling in absolute concentration. It just mean an increase. It might mean the concentration is 1.2x higher, or it might mean it is 50x higher (as example).