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MASS SPECTROMETRY

المؤلف:  Max M. Houck، Jay A. Siegel

المصدر:  Fundamentals of Forensic Science

الجزء والصفحة:  MASS SPECTROMETRY

2026-07-01

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MASS SPECTROMETRY

Thus far we have discussed the interactions of electromagnetic radiation with matter. In the regions of the spectrum that are the main interest of forensic science; UV/visible and IR, energy is absorbed, electrons are promoted or bonds are caused to vibrate, but there is no lasting change to the substance. Electromagnetic radiation is not the only type of energy that can be absorbed by molecules which then undergo changes. Energy from lasers, a beam of electrons, or a beam of small molecules can be trans ferred to matter. The result of this energy absorbance is usually the loss of one or more electrons by the substance, normally creating positive ions (although negative ion mass spectrometry is also sometimes employed). Depending upon the amount of energy from the source, these ions may stay intact or may undergo fragmentation into more stable ions. The resultant array of ions is called the mass spectrum and the techniques used to create the mass spectrum are collectively called mass spectrometry. Mass spectrometry has two very important properties that make it a valuable tool in analytical chemistry and thus in forensic chemistry.

1. If the energy of the source is carefully controlled then the fragmentation pattern for a given substance will be very reproducible.

2. The fragmentation pattern for a given substance is unique. The mass spectrum of a pure substance is a reliable way of identifying it. The basic components of a mass spectrometer are shown in Figure 5.14. There are several types of instruments that vary by sampling method, type of source energy, and methods of separating the fragment ions formed. These are described briefly below.

FIGURE 5.14 Diagram of the major parts of a mass spectrometer. There are several ways that analytes can be ionized and sometimes fragmented as shown in the diagram. The ionization is accomplished in a vacuum. There are also a number of ways that the ions can be separated. Detection is done by an electron multiplier.