Algorithm for locating analytes of interest based on mass spectral similarity in GC × GC–TOF-MS data: analysis of metabolites in human infant urine

The developed algorithm reported herein, referred to as “DotMap,” addresses the need to rapidly identify analyte peak locations in gas chromatography × gas chromatography–time of flight mass spectrometry (GC × GC–TOF-MS) data. The third-order structure of GC × GC–TOF-MS data is such that at each point in the GC × GC chromatogram, a complete mass spectrum is measured. DotMap utilizes this third-order structure to search for the location of a given spectrum of interest in a complete data set, or in a user selected portion of the complete data set. The algorithm returns a contour plot indicating the location of signal(s) with the most similar mass spectra to the analyte of interest. A spectrum from the region indicated is then subjected to an automated mass spectral search to give immediate feedback on the accuracy of the analysis. This algorithm was investigated with a trimethylsilyl (TMS) derivatized human infant urine sample that contained organic acid metabolites. One hundred percent of 12 selected TMS derivatized organic acid metabolites in human infant urine were located with the DotMap algorithm. A typical automated DotMap analysis takes 30 s on a 1.6 GHz PC with 1024 MB of RAM. Vanillic acid (TMS) was located by DotMap, but also exhibited overlap with other organic acids. The presence of vanillic acid (TMS) was confirmed by subjecting the appropriate GC × GC region to chemometric signal deconvolution by PARAFAC to yield pure component information suitable for subsequent quantification.