I am doing a mechanochemical synthesis and trying to find the induction time of the reaction. Thus, I take samples every 30 seconds and inject each sample into the HPLC to detect the time required for the product to form. I see the baseline of HPLC is just mostly noise for all the samples that have been reacted for 2 mins. But at 2.5 min, I start to see a very small rise in the retention time that corresponds to the compound. The height of the peak is then further increased as the reaction time increases.

The problem is, that this peak (reaction proceeded for 2.5 min) at 6.25 min is noticeable but very, very small. The baseline noise fluctuates at an amplitude of 0.075 mAu while the peak height is merely 0.1 mAu (measured from the supposed baseline to the tip of the peak). I have been using this method to do quantitative analysis of the product and it is 100% sure that it is the compound, but not some ghost peak, injection peak or random noise. So, can I say it takes about 2.5 minutes for the product to form? Is the peak too small to be counted as a "real" peak? I don't think the traditional LOD works here because I am not trying to quantify it but only to do qualitative analysis.

To determine the induction time, the product concentration would be very very low and the peak would thus be very small as well and outside the calibration curve. But is there a standard such as "if the product concentration reaches 1 mg/ml? 1 ug/ml? 1 ng/ml? the product starts to form" or "if the peak reaches 1 mAu? 2 mAu? then the product starts to form at this X number of time approximately?"

Note 1: I know that people use time-resolved PXRD to determine the change in crystal structure, but wouldn't that also depend on the "concentration of change in crystal structure"? If the concentration is too low, you won't be able to see on the PXRD diagram. Also, HPLC is the only method to analyse this sample. PXRD, GC-MS and NMR can not do it.

Note 2: The compound only absorbs UV strongly at 195 nm. An increase beyond 205 nm would essentially have no peak.

Note 3: The HPLC method is a 100% aqueous salt solution with an ion-pairing reagent on a polar C18 column

Note 4: In the picture, the peak is very small and the baseline noise is huge it is because I zoomed in a lot to see it and it is only the start of product formation. After 30 mins of reaction time, the product peak height reaches 50 mAu and you won't see a noisy baseline anymore.