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The Journal of the Royal Institute of Thailand Vol. 30 No. 4 Oct.-Dec. 2005 1134 Scientific Instrumentation in Research molecular formula information. For research work the mass spectrometry is usually coupled to gas chromatography commonly known as GC/MS. Flash Animation of GC/MS The movie shows a short series of steps for the process of a single analyte (already separated from the other analytes in the chromatographic mixture) denoted as ABC exiting the chromato- graphic column and: • the analyte (A-B-C) undergoing ionization and fragmentation • the charged fragments (A + B + C + ) being separated by mass • the fragments which are focused on the mass filter’s exit slit passing into the detector • and the charged ions being detected. In this example, the lightest fragment is B + ; the heaviest A + . The last frame of the movie is a mass spectrum displaying only these three fragments. Their relative mass to charge ratios are specified by their relative position on the x axis (low mass/charge to left, high mass/charge to right). The relative amounts (commonly called peak intensity) of each of these fragments determined during the mass analyzer’s scan is reflected by the y axis. The suite of gas chromatographic detectors includes: the flame ionization detector (FID), thermal conductivity detector (TCD) or hot wire detector), electron capture detector (ECD), photoionization detector (PID), and others. Another GC detector that is also very expensive but very powerful is a scaled down version of the mass spectrometer. When coupled to a GC the detection system itself is often referred to as the mass selective detector or more simply the mass detector. This powerful analytical technique belongs to the class of hyphenated analytical instrumentation (since each part had a different beginning and can exist independently) and is called gas chromatograhy/mass spectrometry (GC/MS). Placed at the end of a chromatographic column in a manner similar to the other GC detectors, the mass detector is more complicated than, for instance, the FID because of the mass spectrometer’s complex requirements for the process of creation, separation, and detection of gas phase ions. A capillary column most often used in the chromatograph because the entire MS process must be carried out at very low press- ures (~10 -5 torr) and in order to meet this requirement a vacuum is maintained via constant pumping using a vacuum pump. It is difficult for packed GC columns to be interfaced to an MS detector because they have carrier gas flow rates that cannot be as successfully pumped away by normal vacuum pumps; however,