Improving Detection limits using Exact mass chromatograms on a GCT Mass Spectrometer

Library Number:
Waters Technical Brief
Content Type:
Application Notes
Content Subtype:
Technical Notes
The detection of low levels of analytes in GC-MS is primarily limited by the resence of chemical interference ions close to the mass of the target ion. Chemical background comes from a variety of sources including, source contamination, column bleed and sample matrix. The contribution, from these background ions, to the reconstructed ion chromtogram for a particular target ion, can lead to poor signal to noise ratios. By decreasing the mass window specified for the reconstructed ion chromatogram a significant proportion of this backgroun interference can be removed from the chromatogram, leading to higher signal to noise ratios. For quadruple mass spectrometers mass accuracy is usually within 0.2da. To be certain of capturing a target ion, reconstructed mass chromatograms are usually generated with a mass window of +/- 0.25da (i.e. 500mda). A higher degree of specificity is afforded using a magnetic sector instrument in high-resolution SIR mode. Using resolution of 10,000 (10% valley definition) for a target ion at m/z 250 a mass window of 25mda can be monitored. Using the exact mass capability of the GCT exact mass chromatograms can be generated with a mass window of junst 20mda centered on the calculated exact mass of the target ion. At m/z 250, this is an equivalent selectivity to a magnetic sector instrument in SIR mode operating at a resolution of 12500 (10% vally definition)