среда, 6 февраля 2019 г.
Essay example --
In the digest of environmental samples, advances in orchestration have allowed laboratories to routinely report sample results into the parts per billion (ppb) kitchen range, plainly this is sometimes insufficient to meet project goals. This article will elaborate a technique that can be utilise to increase the esthesia 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 special to the operation of the mass selective detector (MSD). Before describing the advantages and disadvantages of the SIM technique, a legal 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 isolated on the turgidity chromatograph (GC) enter the MSD and encounter a beam of electrons that ionize the analyte mole cule into uniform 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 with(predicate) the filter and are counted in the detector stage of the MSD. In gaze mode, the filter operates dynamically, sequentially scanning m/z ions between the top(prenominal) and lower limits of m/z specified by the analyst. A full-range spectrum of several(prenominal) 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 signifier is counted billions of times. The sensitivity of the detector is directly ... ... quant ion. The analyst must be informed of this in order to avoid reporting false or exalted values. Another disadvantage is that the analyte list must be drast ically cut back from the 50+ analytes typically reported in a GC/MS analysis. In SIM mode, the detector must be programmed to look for a proper(postnominal) 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 head for the hills 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 equivalent further information on SIM analysis