We have found over 40 peaks in spoiled beef. Because the SPME extraction
that we are currently using is selective and in many cases, competitive,
the compounds' concentration that SPME extracted might not be proportional
to the compounds' concentration in headspace. Therefore, what are the
appropriate or reasonable criteria for choosing the "right" indicator
compounds?

I am not sure what you mean by "indicator" compound. Could you explain what you are after?

You are absolutely right about spme fibers being selective. If you have a compound in the headspace that is very very much higher than others, it can outcompete the minor compounds for adsorption on the fiber and result in an underestimation of the minor components.

HI
Thanks for your message. By "indiactor" compounds, I mean certain compounds which can be used as "indicators" of food spoilage or food contamination. For example in our experiments on meat spoilage, we come across a lot of compounds. We are analysing the headspace volatiles from meat daily for a certain period of storage. As meat spoils, some compounds are emitted more than the others. But some compounds increase over a certain period, say for 4-5 days we observe an increasing trend, and later it starts decreasing. We reason that this must be due to the reaction of this compound (say Compound A) with compound B and hence the quantity decreases after 6 days. (like acetic acid reacting with ethanol etc..). I am confused because, if I select such a compound as an indicator of meat spoilage, then after 6 days when I see the quantity decrease, then I will misinterpret it as meat is "not spoiled".

So, I am looking for some advice as to how should I proceed in selecting the 'unique" compounds which can be used as an indicator.

I'd appreciate your help in this matter. Thanks!

This is off-topic, but it might help; if not, I apologize.

The compounds that decrease after 4-5 days might actually be consumed by bacteria other than those that produced the original chemicals. Bacteria do this all the time, with fast-growing organisms munching the readily available nutrients, followed by more specialized ones utilizing the first group's waste products. This makes more sense to me than a bunch of chemical reactions going on in the heaspace, for what that's worth.

Are you letting your sample sit in a closed container for 6 days or are you allowing the meat to spoil elsewhere and then periodically placing a sample in a headspace vial for spme?