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Quantitative Mass Spectrometry at IMSERC

Quantitation of Environmental Pollutants

Liquid chromatography-mass spectrometry (LC-MS) is a combination of a High-Performance Liquid Chromatography (HPLC) system and a Mass Spectrometer (MS) that allows for the isolation and quantitative measurement of the analytes of interest in highly complex mixtures. LC separates compounds by their physicochemical properties, and MS differentiates ionized compounds by mass-to-charge ratio. Thus, the mass spectrometer acts not only as the LC detector, but it also helps to identify the species corresponding to each chromatographic peak through its unique mass spectrum.

These two techniques used in tandem provide a unique capability for highly sensitive and cost-effective quantitative determination of organic molecules in a variety of applications, including analysis of trace organic compounds in environmental, food, and biological matrices. Combining the two separation mechanisms of LC and MS allows particular analytes to be isolated from the mixture and gives confidence that the correct component is being measured. In addition, the technique allows for the use of isotopically labelled internal standards, which do not separate by LC from the analyte of interest, but these standards can be separated by their mass difference. The use of stable isotopically labelled (SIL) internal standards help control variability in a quantitative assay.

IMSERC is equipped with walkup quantitative capabilities to enable any researcher to gain easily quantitative results. IMSERC primarily utilizes a SCIEX 6500+ QQQ or a Bruker AmaZon X Ion Trap for quantitative experiments using MS and/or MS/MS. The resulting selectivity of such quantitative techniques is demonstrated below.

In the above graphic, (TOP)when using just a single stage of MS detection, it is oftentimes impossible to distinguish ion signals of interest from the total ion chromatogram (TIC); however (BOTTOM) when using MS/MS quantitative techniques to monitor specific fragmentation patterns of the analytes of interest, low levels of detection can be reached in the most complicated of matrices. In both of examples shown above, separation and quantitation of chiral isomers of the antidepressant drug, Fluoxetine, and metabolites detected in blood are demonstrated.

Specifically, the Sciex 6500+ in IMSERC is an industry leading instrument in regard to sensitivity, having the ability to quantitate to very low levels (sub ppt). Below is an example of an environmental quantitative analysis method built in IMSERC, wherein detection of Perfluorooctanoic Acid (PFOA) in water is performed (PFOA is part of a class of compounds called PFOS/PFOAs typically used as a fire retardant). Routine detection at low ppt levels can be achieved.