An instrumental technique and detector widely used in forensic science. Mass spectrometry can be applied to organic analysis (drugs and arson, for example) or to inorganic analysis (bullets, paint, and so on) depending on the type of sample introduction system it is coupled to. Mass spectrometry as used in forensic science is not a stand alone technique, but rather a mass spectrometer is coupled to different sample introduction instruments or devices, yielding what is called a hyphenated technique. Examples include gas chromatography/mass spectrometry (GC/MS), high-pressure liquid chromatography/mass spectrometry (HPLC/MS), inductively coupled plasma/mass spectrometry (ICP-MS), and pyrolysis mass spectrometry. Note that a slash (/) is often used in place of a hyphen, but the meaning is the same, and both notations are used interchangeably. Mass spectrometry is used in forensic science in drug analysis, arson, analysis of plastics and paints, toxicology, gunshot residue (GSR), and explosives to name a few applications. The greatest advantage of mass spectrometry is that in almost all cases, it can provide definitive identification of the compounds or elements that make up a given sample. This is called qualitative analysis, and in addition, mass spectrometry is a powerful quantitative tool in that it can be used to determine how much of each component is present, down to trace level concentrations.
Mass Spectrometer Designs
The first figure illustrates the basic design of a generic mass spectrometer. The sample is introduced through an inlet into a chamber that is kept at a very low pressure, in the range of 1 x
10″5 torr (about 1 one-billionth of normal atmospheric pressure). This low pressure is essential to prevent collisions between ions (charged particles) created by the instrument and atmospheric components. The sample molecule (if it is an organic compound) is then ionized to form charged fragments (Fj”1″
, and so on) of the original molecule. The M+ ion is called the molecular ion in that it is created by stripping a single electron away from the original compound. In organic mass spectrometry, the type used for the analysis of drugs, for example, this molecular ion is important because it will have the same molecular weight as the parent molecule. Smaller fragments also form that can be as small as individual atoms. The process of ionization is different for inorganic materials such as mercury and arsenic, when the inductively coupled plasma creates the ions before they are introduced into the mass spectrometer. Thus, in a sample containing gold (Au), the plasma will create gold ions (Au+) that will have the characteristic atom weight of gold (197 atomic mass units). However, these atoms do not fragment and are detected as is. Ions may be positively or negatively charged. In forensic work, most applications focus on the positive ions such as M+ and fragments.
A number of different types of ionization schemes are available for mass spectrometers used for organic molecules. The most common is electron impact (El), described below. Other types include chemical ionization (CI), particularly useful when a strong M+ signal is needed; electrospray, common with HPLC systems; fast atom bombardment (FAB) and matrix assisted laser desorption/ionization (MALDI), used for large molecules like proteins; and laser ablation, for surface analysis of organic and inorganic materials.
In organic mass spectrometry, if instrument conditions are standardized and the same type of ionization is used, the pattern of fragmentation for a given molecule is reproducible from instrument to instrument. This is what provides the qualitative analysis of mass spectrometry. For example, if a cocaine sample is introduced into the mass spectrometer, the overall pattern of fragmentation will always be the same as long as the same instrumental settings are used. This standardization is the basis of spectral libraries that forensic scientists use to identify individual compounds. With inorganic mass spectrometry, the task is easier since the detected mass is the same as the mass of an element found on the periodic table of the elements. For example, if a signal is recorded at a mass of 74.9, the element is arsenic.
As shown in the figure, once ions are produced, they are directed into a region called the mass filter, where they are separated out into individual masses and the abundances recorded. The separation is based on size of the ion as well as the charge. The results are plotted in a mass spectrum that shows the relative abundance of each mass. The actual mass spectrum shown is for cocaine, and this spectrum will always look the same under standard instrument conditions. Thus, spectra obtained in any one forensic lab are directly comparable to those obtained by any other lab using the same standard conditions. There are a number of different types of mass filters used in mass spectrometers including TOF (time-of-flight), quadrupoles, ion traps, and magnetic sector. The most common type found in forensic labs for both organic and inorganic applications is the quadrupole, illustrated in the figure.
Quadrupole Mass Spectrometers
The original molecule is introduced into the mass spectrometer by an inlet such as a gas chromatograph (GC) or high pressure liquid chromatograph (HPLC). It passes by a plate that has a positive charge, and ionization occurs. These ions are created by collision with a stream of electrons created by a tiny filament much in the way the filament in a light bulb produces light. The electrons are drawn toward the target that is positively charged since unlike charges attract. Conversely, since like charges repel, the positive ions are driven into a stack of focusing lenses by the positive charge on the repeller. The ions are focused into a tight beam that enters the quadrupole area, which consists of four metallic rods. Manipulation of the electrical fields allows the ions to be filters so that only one mass is detected at any given time. The emerging ions are directed into an electron multiplier that amplifies the signal sent to the detector. The quadrupole is nearly standard for GC; ion traps and TOF designs are found in many HPLC systems. There are even MS/MS systems in which fragment ions that emerge from the mass filter are directed into another mass spectrometer that repeats the process and produces another series of smaller fragments. This type of information is extremely useful when attempting to identify a material that was not found in the mass spectral library. MS/MS is also useful for very large molecules such as proteins, but this application is not widespread yet in forensic science.