Targeted screening describes a mass spectrometry (MS) work flow used for screening complex solutions for trace levels of known compounds and, when detected, quantifying their concentration. Accurate MS quantitation at trace levels is challenging and requires careful method development. For difficult targeted screening problems we consult with our quantitation experts to create accurate and robust methods. Ultra-high performance liquid chromatography/quadrupole time of flight mass spectrometry (UPLC/QToF-MS) and APGC/QTof-MS are both well suited for targeted screening since these modern instrument platforms provide impressive mass accuracy (3-to-5 ppm) for selectivity and sensitivity to as low as 10 ppb depending on the target molecule. Both mass accuracy and sensitivity are essential requirements for effective targeted screening.
First, authentic standards of target compounds are prepared in a similar matrix to the sample (i.e., matrix-matched standards) and are then analyzed with MSE (“MS everything” which is a data acquisition method yielding molecular ions and fragment data in a single run). MSE standard data provide the molecular and fragment ion MS data required for target analyte entry into Mass Spec Lab’s searchable compound library. MSE also provides the quantitative data for analyte standard curves and limits of quantitation generated by our quantification software. Mass Spec Lab currently uses the MetaScope Mass Spectral Library Search Engine within the Progenesis QI (Waters) software suite for targeted screening and QuanLynx (Waters) for quantification. Neutral exact mass data, a minimum of three fragment ion masses, and retention time are included in the compound database entries which are screened during analysis of the test sample.
Test samples are analyzed using the chromatographic conditions of the library standard analysis. Progenesis QI algorithms allow alignment of multiple test replicates, peak identification, deconvolution of detected analyte mass spectra, and scoring of compound matches against the compound library. Detected targeted analytes are then quantitated. Non-detection is revealed by library match scores below a predefined level determined during method development. A QC standard mix is analyzed with the test sample to verify that the instrumental response is within defined limits, typically 85-115% of optimal.