Evaluation of Matrix Complexity in Nontargeted Analysis of Small-Molecule Toxicants by Liquid Chromatography-High-Resolution Mass Spectrometry

Complex mixtures, characterized by high density of compounds,challengetrace detection and identification. This is further exacerbated innontargeted analysis, where a compound of interest may be well hiddenunder thousands of matrix compounds. We studied the effect of matrixcomplexity on nontargeted detection (peak picking) by LC-MS/MS(Orbitrap) analysis. A series of similar to 20 drugs, V-type chemicalwarfare agents and pesticides, simulating toxic unknowns, were spikedat various concentrations in several complex matrices including urine,rosemary leaves, and soil extracts. Orbitrap TraceFindersoftware was used to explore their peak intensities in relation tothe matrix (peak location in an intensity-sorted list). Average practicaldetection limits of nontargets were determined. While detection amongthe first 10,000 peaks was achieved at 0.3-1 ng/mL levels inthe extract, for the more realistic top 1000 list,much higher concentrations were required, approaching 10-30ng/mL. A negative power law functional dependence between the peaklocation in an intensity-sorted suspect list and the nontarget concentrationis proposed. Controlled complexity was explored with a series of urinedilutions, resulting in an excellent correlation between the powerlaw coefficient and dilution factor. The intensity distribution ofmatrix peaks was found to spread (unevenly) on a broad range, fittingwell the Weibull distribution function with all matrices and extracts.The quantitative approach demonstrated here gives a measure of theactual capabilities and limitations of LC-MS in the analysisof nontargets in complex matrices. It may be used to estimate andcompare the complexity of matrices and predict the typical detectionlimits of unknowns.

Automated summary

This study investigated the impact of matrix complexity on nontargeted detection by LC-MS/MS (Orbitrap) analysis. The results showed that detection limits for nontargets were higher in complex matrices and required higher concentrations. A quantitative method was used to assess and compare the complexity of matrices and predict the typical detection limits of unknowns.