Essay example --

In the analysis of environmental samples, advances in instrumentation have allowed laboratories to routinely report sample results into the parts per billion (ppb) range, but this is sometimes insufficient to meet project goals. This article will illustrate a technique that can be used to increase the sensitivity of the mass selective detector, and thereby lower the reporting limit of GC/MS methods such as EPA 8260 for VOCs, or EPA 8270 for SVOCs. The technique is called selected ion monitoring, or SIM, and is specific to the operation of the mass selective detector (MSD). Before describing the advantages and disadvantages of the SIM technique, a brief overview of the principles of operation of the MSD is in order. The MSD is a two-part instrument, comprised of a filter stage and a signal processing, or detection stage. Analytes that have been separated on the gas chromatograph (GC) enter the MSD and encounter a beam of electrons that ionize the analyte molecule into reproducible fragments, each having characteristic properties of mass and charge that allow them to be separated in the filter stage of the MSD. At any point in time, only fragments with the allowed mass to charge ratio (m/z) will pass through the filter and are counted in the detector stage of the MSD. In scan mode, the filter operates dynamically, sequentially scanning m/z ions between the upper and lower limits of m/z specified by the analyst. A full-range spectrum of several hundred m/z ions (specified as atomic mass units, or AMU) is typically scanned in this way. The filter cycles through the m/z range at radio frequencies (109 cycles per second), so each ion in the fragmentation pattern is counted billions of times. The sens itivity of the detector is directly ... ... quant ion. The analyst must be aware of this in order to avoid reporting false or inflated values. Another disadvantage is that the analyte list must be drastically reduced from the 50+ analytes typically reported in a GC/MS analysis. In SIM mode, the detector must be programmed to look for a specific m/z ion (the quant ion) at any particular point in time. Each unknown analyte that is separated on the GC and enters the MSD may potentially have a unique quant ion, so detector limitations come into play for any analyte list that has more than 15 to 20 analytes. Finally, the MSD detector operated in SIM mode is not capable of identifying unknowns or tentatively identified compounds (TICs). SIM analysis is a powerful tool, but one that must be used carefully. Please contact your project manager at Kiff Analytical if you would like further information on SIM analysis