I am currently working on a project looking for pesticides in herbal tea. At this point, I am using the contents of one tea bag (~1.5g) and 10mL of freshly boiled water with a QuEChERS extraction. I am still working through the details on the impact of the hot water and what type of cleanup will be necessary for pesticide determination, so that will be a blog for another day. I first decided to do some split injections of the raw extract to see what type of natural products are in some of the herbal teas I picked up at the grocery store.
Using GCxGC-TOFMS with a 30m x 0.25mm x 0.25µm Rxi-5Sil MS in the first dimension and a 1m x 0.25mm x 0.25µm Rtx-200 in the second, I processed the data by doing a “peak find” and library search for anything with a S/N of at least 100. I then get my peak list and sort according to the NIST library similarity number, so that the highest matches are at the top of the list. I start scanning through the list and see what names sound either like a pesticide or something else interesting. For example, I came across the name niacinamide with a similarity of 912, and a spectra that seemed fairly unique. I did a quick google search on the name and (according to Wikipedia) it is a main ingredient in an acne medication and has shown to have anti-anxiety properties among other potential medicinal uses. This type of peak find is made possible by the spectral deconvolution of the time-of-flight mass spectrometer.
I also mine through the data by taking advantage of the ordered chromatograms produced by GCxGC. This means that compounds with a similar structure, or a homologous series, will elute in a band across the chromatogram. I found another compound, lupeol, which is a triterpenoid that has been noted to have several medicinal properties. By looking at peaks that are eluting near lupeol, I found sitosterol which is a phytosterol that may reduce cholesterol.
While I don’t have standards to positively confirm peak identity, a high NIST match similarity with a spectra that contains abundant high m/z ions increases my confidence in the peak assignments. I really enjoy digging through the data and seeing if anything interesting pops up. Using GCxGC-TOFMS can give you A LOT of data to mine through, but with spectral deconvolution and ordered chromatograms, it makes it much easier.