Using IR to detect / quantify a molecule at low concentrations I want to detect / quantify a specific terpenoid molecule from a microbiological growth medium. The molecule will however be present at very low levels approx. 10-100 mg/L. 

Is it viable to use the IR spectrum to detect this molecule at such low levels? If not quantitatively at least in a qualitative sense? The idea is to have some sort of method that is amenable to reasonably high throughput screening.

A GC / GCMS which is the typical assay method is too time consuming for screening a lot of samples. 

The IR spectrum is shown below. Can I just monitor a specific wavelength with max absorbance, (say) 3000 cm-1? That way I won't have to scan the whole wavelength range. 

Any other ideas?
There isn't enough information to answer the question with certainty.  We'd need an IR spectrum of the culture medium (possibly dried/lyophilized medium)

I suspect that the peak at 3000 $\mathrm{cm}^{-1}$ is an alkyl CH stretch.  You're going to see similar absorption peaks in lipids and proteins, so I don't think that particular peak is going to be too promising for you.

You might look into headspace mass spectrometry.  Many terpenes are volatile and can be analyzed directly from headspace air (i.e. the air directly above your cultures).  You don't necessarily need to do gas chromatography, so the method can be quite fast.  It won't be as diagnostic as to the identity of the particular terpenes formed, however.