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Encyclopedia of Physical Science and Technology EN016H-959 August 1, 2001 11:6

908 Toxicology in Forensic Science

Two or more independent tests are normally required, or the testing method used is appropriately validated; that
the use of a more powerful analytical test, such as mass is, it has been shown to accurately and precisely detect
spectrometry (MS) may often be preferred. Because of the particular substance(s), there is little or interference from
need to perform a rigorous analysis, the analytical schema other drugs or from the matrix, and a useful detection
is often broken up into two steps. The identification stage limit has been established. Moreover, it is essential that
is termed the screening or initial test, while the second the method is rugged and will allow any suitably trained
analytical test is the confirmation process. The confirma- analyst to conduct the procedure and achieve the same
tion process often also provides a quantitative measure results as another analyst. To achieve these aims, it will
of how much substance was present in the sample; other- be necessary to trial the method in the laboratory with
wise, a separate test is required to quantify the amount of specimens of varying quality before full validation can be
substance present in the specimen (see later). achieved.
In all processes, it is important that no analytical in- It is recommended to include internal quality controls
consistency appears, else a result may be invalidated. For with each batch of samples to enable an internal check of
example, in the identification of amphetamine in a blood the reliability of each assay. These controls contain known
specimen, an immunoassay positive to the amphetamine drugs at known concentrations. Suitable acceptance crite-
class is expected to be positive for one or more am- ria are required for these controls before results of un-
phetamines in the confirmation assay. The apparent de- known cases can be accepted and released to a client.
tection of a drug in one analytical assay but not in another Acceptance criteria vary depending on the analyte and
means that the drug was not confirmed, providing both application. For example, blood alcohol estimations have
assays are capable of detecting this drug or one or more acceptance criteria less than 5%, while postmortem blood
∗
members of a drug class. procedures may be 10 to 20%.
While MS is the preferred technique for confirma- An important feature of analytical assays in forensic
tion of drugs and poisons, some substances display poor toxicology is the use of internal standards. These are drugs
mass spectral definition. Compounds with base ions at with chemical and physical characteristics similar to the
mass/charge ratios of less than 100 or with common drug(s) being analyzed and, when added at the start of the
ions such as m/z 105 and with little or no ions in the extraction procedure, provide an ability to negate the ef-
higher mass range are not recommended for confirma- fects of variable or low recoveries from the matrix. Hence,
tion by MS alone. Derivitization of a functional group even when recoveries are low, the ratios of analyte and
to produce improved mass spectral properties can often drug are essentially the same as for situations of higher
be successful. Common derivatives include acyl esters, recovery. An ideal recovery marker is when the inter-
silyl ethers, etc. Alternatively, reliance on other chromato- nal standard is a deuterated analog of the analyte. When
graphic procedures can provide adequate confirmation. deuterated internal standards are used, it may not be nec-
It is important when using any chromatographic proce- essary to match the calibration standards with the same
dure (HPLC, GS, CE, etc.) that the retention time of the matrix as the unknown samples. It is important, however,
substance being identified matches that of an authentic that absolute recoveries are reasonable (i.e., at least over
standard. 30%). This ensures less variability between samples and
Some apparent analytical inconsistencies may provide optimizes the detection limit.
important forensic information. For example, if a result From time to time, it will be important to run unknown
for opiates is negative in urine, but positive in blood, it samples prepared by another laboratory or by a person
is quite likely that heroin was administered shortly be- not directly involved in laboratory work to establish profi-
∗
fore death and the metabolites had not yet been excreted. ciency. These are known as proficiency programs or qual-
This situation is often found in acute sudden death among ity assurance programs. These trials are often conducted
heroin users in whom substantial urinary excretion has not with many other laboratories conducting similar work and
yet occurred. provide an independent assessment of the proficiency of
the laboratory to detect (and quantify) specific drugs. The
performanceofthelaboratoryshouldberegularlyassessed
VII. QUALITY ASSURANCE from these results and any corrective action implemented,
AND VALIDATION if appropriate. This process provides a measure of con-
tinuous improvement, an essential characteristic of any
Essential components of any form of toxicological testing laboratory.
are validation and quality assurance. It is important that
Normally, the coefficient of variation (CV) of the mean is calculated
∗
Heroin is rapidly metabolized to morphine. as a standard deviation divided by the mean of the result.
∗