Data Evaluation

Mass spectrometry is used as an analytical tool in many forensic situations. In some instances, however, it can be regarded only as a complement to other chemical methods.

FIGURE 8 Detector: schematic figure of electron multiplier.

Three examples have been selected to show how MS has become an indispensable analytical tool.

A. Toxicology

The search for drug substances, pesticides, poisons, and their metabolites in body fluids from living persons or in postmortem organs presents an important and difficult task for the chemist. In addition to the fact that the forensic scientist most often does not know what intoxicant to look for, the main reason for using MS to begin with is the large number of possible toxic substances. In toxicology work at the Poison Center in Munich, as many as 8000 different substances have, in fact, been reported in 40,000 investigated objects.

Figure 9 shows an outline of the usual MS approach for searching a biological sample taken from a human for alien compounds with pharmacological effects. The example selected is a real-life incident of reckless driving by a motorist apprehended by the police on the suspicion of being under the influence of some drug or drugs. An extract of the blood sample was injected into a gas chro-matograph/mass spectrometer focused on a broad range of mass fragments. The mass chromatogram at A in the figure is made up of the total ion current (the sum of all fragments recorded) and showed no clear peaks indicative of any drug substances. To raise the signal-to-noise ratio, the total ion current was reconstructed with the sum of the m/z 91 and 92, and then a peak appeared on the new mass chromatogram at B.

In the next step of the analytical process, the substance generating the peak at B was to be identified, which was achieved by comparing the mass spectrum of the analyte at C with mass spectra in an on-line library. Out of ten candidates picked by the program, three possible ones are shown at D, E, and F. Even though the mass spectrum of the analyte best fitted that of methylbenzene, it also matched nearly as well the mass spectra of the two other candidates. In addition to the recorded fragments at m/z 91 and 92, the final identification of the analyte was based on the fact that the retention time for the analyte was the same as that for methylbenzene. To hold up to legal scrutiny,

FIGURE 9 Search of blood sample for toxics. The mass chromatogram at A shows the total ion current (the sum of all fragments recorded) and the reconstructed mass chromatogram at B shows the ions with the sum of m/z 91 and 92. The mass spectra at C-F depict the library search for identifying the peak at B. The unknown analyte's mass spectrum is, after background subtraction, displayed at C. The three hottest candidates in the library along with their names and CAS (Chemical Abstracts Service) numbers are shown at D-F. At G are shown the chemical formula of the first ranked candidate and the value for how well the mass spectrum of the candidate fits with that of the analyte and vice versa. A value of 1000 indicates identical mass spectra; zero, no fragments in common.

FIGURE 9 Search of blood sample for toxics. The mass chromatogram at A shows the total ion current (the sum of all fragments recorded) and the reconstructed mass chromatogram at B shows the ions with the sum of m/z 91 and 92. The mass spectra at C-F depict the library search for identifying the peak at B. The unknown analyte's mass spectrum is, after background subtraction, displayed at C. The three hottest candidates in the library along with their names and CAS (Chemical Abstracts Service) numbers are shown at D-F. At G are shown the chemical formula of the first ranked candidate and the value for how well the mass spectrum of the candidate fits with that of the analyte and vice versa. A value of 1000 indicates identical mass spectra; zero, no fragments in common.

proof of the analyte identity generally must indicate that at least two fragments and the retention time are the same as for the suggested substance. In the example here, the motorist suspected of being under the influence of drugs was probably a "sniffer," who had inhaled paint thinner or some other solvent containing toluene (methylbenzene) before driving his car.

B. Arson Analysis

The term arson analysis implies the search of materials taken from a fire scene for accelerant residues to establish

TABLE I Common Accelerants

Main type (approximative boiling point)

Major components

Examples of commercial products

Abundant mass fragments

Light petroleum distillates

Medium petroleum distillates

Heavy petroleum distillates

(120-410° C) Varia n-Alkanes, branched alkanes Alkylbenzenes, naphthalenes

^-Alkanes, branched alkanes, alkylbenzenes ^-Alkanes, branched alkanes, alkylbenzenes, naphthalenes ^-Alkanes, branched alkanes, naphthalenes Alcohols, ethers, a-pinene

Petroleum ethers, pocket lighter, rubber cement solvents, lacquer thinners Automotive gasoline, lantern fuels

Charcoal starters, paint thinners, mineral spirits, torch fuels, dry-cleaning solvents Fuel oil, aviation fuel, insect sprays, charcoal starters Fuel oil, diesel fuel

Solvents, turpentine

91,92, 105, 106, 115, 116, 118, 119, 120, 134, 141, 142, 148, 156 43,55,57,71,82, 83,91,105,

whether the incident was of incendiary origin or not. The U.S. National Fire Protection Association reported that over 100,000 fires in 1994 were arson related and more than 500 persons lost their lives in these fires. In terms of loss of human life, houses, properties, and goods, this type of incidence, extracts a large toll from society as well as from individuals, and makes the forensic task urgent.

The list of the most common accelerants, shown in Table I, may at first give the impression that these would rapidly gasify along with the burning solid materials and thus not become detectable, but it is not always so. Traces of the fuel often remain in some closed areas of the fire scene even after the temperature has reached perhaps 1000° C, and these accelerant residues may be detectable after proper sample collection.

By virtue of its high sensitivity and specificity, GC/MS is well suited for searching trace amounts of test materials for residues of accelerant components. As reported in the literature on material spiked with accelerants, specific GC/MS patterns of different types of fuels may be used to identify an accelerant. A problem with this approach is that burning plastics and other solid materials release hydrocarbons that are the same as those in accelerants and thus may contribute to a false positive result by tainting the test material. The use of fuel labels may overcome this hurdle. Methyl-tertbutyl-ether (MTBE), which is an additive in gasoline, has been suggested as such a marker. Owing to its low boiling point (55.2°C), however, it disappears rapidly during a fire and is therefore only detectable in postmortem materials from a fire victim who has inhaled MTBE. Figure 10 shows such an example from the real-life fire of a villa, which was completely destroyed. No accelerant residues were spotted in the ashes around the body of a fire victim, so blood samples of the de ceased were also searched for fuel components. On the chromatogram monitoring the sum of all ions (TOT), no peaks appeared, but after it had been reconstructed with the selected ions at m/z 78 + 91 + 92 a number of aromatic hydrocarbons showed up. These could, however, stem either from a fuel or from pyrolyzed plastics. The presence of MTBE at m/z 73 indicated, however, that gasoline had been used to set the fire. This piece of evidence pointing to arson could not have been brought light without MS.

C. Environmental Issue

Growing global use and transportation of chemicals with potential deleterious effects on the environment have prompted forensic scientists to develop methods for

FIGURE 10 Arson analysis of blood sample. The mass chro-matogram at A shows the total ion current (the sum of all fragments recorded). The reconstructed mass chromatogram at B shows the ions with the sum at m/z 78, 91, and 92 to monitor aromatic hydrocarbons, and the reconstructed mass chromatogram at C shows the ions at m/z 73 to monitor MTBE.

FIGURE 10 Arson analysis of blood sample. The mass chro-matogram at A shows the total ion current (the sum of all fragments recorded). The reconstructed mass chromatogram at B shows the ions with the sum at m/z 78, 91, and 92 to monitor aromatic hydrocarbons, and the reconstructed mass chromatogram at C shows the ions at m/z 73 to monitor MTBE.

detecting the origin of illicit waste discharge. A common issue is to link the constituents of, for instance, an oil spill sample with its original source. GC or GC/MS are usually used to do the analysis when the individual hydrocarbons are separated and displayed on a chromatogram, whose pattern is compared with that of the suspected original source. A major problem in such a survey, however, is that some time usually has passed between the waste discharge and the specimen collection. Owing to weathering by evaporation and biodegradation of the components during this period, the composition of the oil spill may have changed to the extent that it bears only limited similarities with that of the origin.

Monitoring the contents of stable isotopes by MS in the whole sample or in its individual constituents after GC has isolated them is an approach used to get around the weathering problems. It has been shown that the most common stable isotope parameter used (i.e., the 13C/12C ratio) may be specific for a source and not very much influenced by weathering effects. Technically, the analysis is done after the test material has been combusted to carbon dioxide and water, and the 13C/12C ratio is then determined in the carbon dioxide fraction by MS. To determine the 13C/12C ratio of the individual components of a sample, they are separated by GC, each isolated hydrocarbon goes directly into a combustion chamber, and the carbon dioxide, after having been freed from water, enters the mass spectrometer. An illustration of the use of this tool to identify oil spill sources is the stable isotopes of samples taken from the area of Prince William Sound several years after the Exxon Valdez disaster. The isotope distribution in the samples as measured on the bulk contents correlated with two distinct origins of the oil spill: one coming from the Exxon Valdez oil and the other from Californian crude oil. The latter source had probably been released in to the sea from storage tanks by an earthquake that had taken place in the area 40 years earlier.

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