When using mass spec to detect and identify chemical compounds in food samples, MRM (multiple reaction monitoring) on triple quad mass spec systems is typically used, where 2 MRM transitions (at least) are monitored for each compound to provide high selectivity to confirm a compound's identity once it is detected.
Figure 1. MRM detection schematic. The parent ion is selected in Q1, fragmented in Q2, and the fragment ion is selected in Q3 and detected.
Regulations require that the ion ratio of the 2 MRM transitions monitored falls within 30% relative to that of the calibration standard for the compound (or the expected ion ratio). (Reference: SANCO 12571/2013)
If the ion ratio does not meet this criterion, then that must mean that the peak detected is not the compound of interest, or is it?
In the case of food samples, the analysis can become complicated by the chemical components present in the sample - otherwise known as the matrix components. These compounds can do tricky things to the MRM transitions detected by the mass spectrometer, causing signal suppression or enhancement, which can affect the detected ion ratios.
Here is an example of a compound we detected in a grape sample. We detected both MRM transitions, but the qualifying MRM transition is outside of the acceptable ion ratio range we expect for the compound Pyrimethanil.
Based on the MRM ion ratio alone, this result would NOT MEET the performance criteria for identification and would be reported as NEGATIVE for Pyrimethanil. But that report would be wrong!
Lucky for us, in this analysis, we didn't just collect 2 MRM transitions - we acquired full MS/MS fragment spectra for this detected peak. That means we can compare these results to a library spectrum of Pyrimethanil. This allows us to look at the complete molecular profile of the detected compound for more confidence beyond simply monitoring 2 MRM transitions.
When comparing those results to the library spectrum of Pyrimethanil, we obtained a library score of 99.5 - a very good match of our detected peak to the compound Pyrimethanil.
The comparison of our collected data to the compound library spectrum prevented us from reporting a false negative result!
And best of all, setting-up your acquisition methods to collect full MS/MS spectral data on detected peaks is just as easy as setting-up a method to detect 2 MRM transitions. And, processing data to compare to the compound library is even easier - just 5 simple mouse-clicks, and processing is complete.