A gas chromatograph is an apparatus consisting of an injection port connected to a column that has a detector at its outlet end. The column is contained in an oven that is electrically heated, either isothermally or at a programmed rate. A stream of inert carrier gas, usually helium, is introduced into the injection port and flows through the column and detector. The injection port is a heated region that is sealed from the outside environment by a silicone rubber septum through which the sample is injected using a hypodermic syringe. Individual sample components are vaporized and travel through the column at a rate dependent on their interaction with the material used to pack the column. The detector registers and records the output from the column. In this case the detector is a mass spectrometer.
Mass spectrometers are sophisticated instruments that produce, separate, and detect positively charged gas phase ions. (Negatively charged ions can also be investigated but the abundances of negative ions are 10 to 1,000 times less than those of positive ions and negative ion mass spectrometry is less frequently used.) Pure compounds can be identified by their characteristic ions and the result is nearly always unambiguous.
The mass spectrometer consists of an inlet system, an ionization device, a mass analyzer, and an ion detector, and the system is kept at a vacuum of about 10-4 to 10-7 Torr. (There are various options available for all four basic components which illustrates the versatility of the technique.) The following describes an electron impact-quadruple mass spectrometer.
Neutral gaseous molecules entering the ionization area are bombarded with electrons to "smash" the compound of interest to yield positively charged ions. Ionization is often followed by a series of spontaneous competitive decomposition reactions (fragmentation) which produce additional ions. The instrument operates under a high vacuum to prevent absorption of the charged particles by air molecules.
The ions are then extracted from the ion source, collimated, and accelerated by an electric potential applied across a series of metal plates with exit apertures, before entering the quadrupole mass analyzer. The quadrupole mass analyzer consists of four circular parallel rods. The rods are electrically connected in diagonally opposite pairs and mass separation is achieved by applying a DC potential, positive to one pair and negative to the other and superimposing a radio frequency AC potential which differs in phase by 180° between the pairs of rods. The peak values of the AC voltage are greater than the DC voltage so that the "positive" pair is sometimes negative, and vice versa.
This produces a dynamic arrangement of electromagnetic fields resulting in certain ions taking a stable path through the analyzer and reaching the detector, whereas other ions take unstable paths and are filtered out before reaching the detector. The detector consists of an electron multiplier that magnifies the current generated by the ions striking the detector by a factor of 106 to 109. The event is recorded by some form of data system. The mass spectrum is a plot of the electron multiplier output (intensity) versus the quadrupole electrical settings and produces a characteristic pattern of fragments for the compound under investigation.
Mass spectrometry on its own is not suitable for FDR work because generally speaking "pure" compounds must be analyzed. Samples for FDR examination taken from skin and clothing surfaces are complex mixtures containing many unpredictable contaminants from occupational and
impact accelerator analyzer
Figure 16.5 Quadrupole mass spectrometer.
impact accelerator analyzer
environmental sources as well as skin salts and lipids. The problem is solved by separation of the constituents using gas chromatography prior to introduction to the mass spectrometer.
Combined gas chromatography-mass spectrometry (GC-MS) takes advantage of the separating power of the gas chromatograph and the identification power of mass spectrometry. The gas chromatograph separates the components and provides retention time data and the mass spectrometer identifies the components. The combined instrumentation has the potential to provide very useful information in FDR casework. Figure 16.5 illustrates a quadrupole mass spectrometer.
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