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Forensic Terminology

The following terms are frequently used to discuss forensic analysis. The definitions below explain the term as it is used in the context of forensic analysis. In 2018, the Organization of Scientific Area Committees for Forensic Science (OSAC) created a Lexicon of Forensic Science Terminology to help facilitate communication across many disciplines. Additional terms can be accessed in the OSAC Lexicon.

If you would like to view a short video on scientific terminology recorded by Dr. Jay Gehlhausen and Logan Johnson, please see below.

Absorbance - the measurement of how much light is absorbed by a substance. The inverse of transmittance.

Accessioning - the process of receiving, sorting, and labeling samples. The specimen accessioner is also responsible for the accurate distribution of the samples to the correct departments for testing.

Accreditation - a process by which a laboratory must prove to an accrediting agency that their processes, equipment, and employees are competent, credible, and accurate. The accrediting agency will inspect the laboratory and observe its operation. If the laboratory meets the accrediting agency's expectations, they will receive an accreditation from that accrediting agency. Lab accreditation shows that the lab meets certain minimum standards, but does not guarantee that no errors will occur.

Accurate - the proximity of measurement to the actual, correct value. For example, if your machine weighs a sample at 4.55 grams when the actual weight of the sample is 4.55 grams, then your machine is accurate. Compare to precise.

Aliquot - a smaller portion of the whole sample. Instead of using the entire sample for testing, the scientist will take a small portion of the blood, urine, or serum to run tests. If the aliquot is destroyed by human or mechanical error or consumed by the testing process, the original sample is still intact and the scientist may take a new aliquot for additional testing.

Analyte - the molecule of interest in the assay.

Assay - refers to the entire process of analyzing a sample. Rather than calling the process an experiment, this term is used when the scientist is trying to figure out something. An assay is a scientific method that is proven, and thus, should produce accurate and precise results if performed correctly.

Bands - can be used somewhat synonymously with "peaks" in infrared spectroscopy. While band represents a very complex analytical chemistry reaction, it will be sufficient to know that a band represents that a specific functional group is present in a molecule.

Base Peak - the most intense, tallest, peak on a mass spectrum. This peak is assigned the abundance of 100.

Blank - a blank is part of the quality control portion of the experiment. A blank is a solution containing none of the analyte of interest and is used to test the instrument. A blank should produce a negative result and will help demonstrate that the machine is functioning properly and that cross-contamination is not occurring in the machine.

Calibration curve - a tool used in scientific analysis to calculate the concentration of a substance in an unknown sample. Several samples with a "known" concentration, typically starting with a smaller amount then increasing to a larger amount (for example, 5 mg/mL, 10 mg/mL, 25 mg/mL, etc.) are run prior to the unknown sample, and the results are plotted on a graph. (X-axis is concentration, while the Y-axis is the machine's response). The plotting of this data should create a straight line (or close to it). The slope of this line can be used to calculate the concentration of the unknown sample.

Certification - A scientist must be able to prove that they are knowledgeable in the field in which they are practicing and they must be able to prove that they are able to perform their assigned assays competently to receive a certification. The certifying body sets the standards and requirements for certification. A scientist may be certified in a specific field, or more specifically on a particular assay.

Compound - a mixture; two or more of different things: molecules, solvents, reagents, etc.

Confirmatory test - a test that is performed after a screening/presumptive test. This test will be able to identify a substance to the exclusion of all others. For example, gas chromatography/mass spectrometry is a confirmatory test in the field of drug analysis. These tests are expensive and typically time consuming. Typically these tests are only performed when a (quicker and less expensive) presumptive/screening test yields positive results.

Frequency - the number of times a point on a wave passes a fixed reference point in one second. The unit for frequency is Hertz (Hz).

Functional group - groups of atoms that share the same characteristics. These molecules as a group will produce specific, identifiable peaks on an IR spectrum.

Homogeneous - uniform throughout. In drug chemistry and toxicology, liquids will typically be stirred or mixed thoroughly to ensure that all portions of the liquid contain the same analytes and same concentration of analytes. Complete homogeneity may not be possible in all situations.

Immunoassay - a biochemical test used to determine the presence of a specific molecule or compound in a sample. An immunoassay uses and measures antibodies present in the sample to indirectly determine the presence of a compound in a sample.

Internal standard - a substance that is added to the blank, calibration standards, and unknown sample to improve the precision of the assay. The internal standard contains a known concentration of a reference standard. The internal standard is part of the quality control of the experiment, and is included to test the adequacy of the individual assay run. If the response from the machine for the internal standard are unexpected, it is likely that the other results from that test are compromised.

Ion - a molecule that has a number of electrons that does not match the number of protons. If an ion has fewer electrons than protons, it will be positively charged (known as a cation). If an ion has more electrons than protons, it will be negatively charged (known as an anion).

Metabolite - a compound produced as a result of metabolism or due to a metabolic reaction.

Molecular ion - the molecular ion is the peak on the mass spectrum that represents the molecular weight of the compound being analyzed. The molecular ion may or may not be present, and could be the base peak in some scenarios. This peak will often called the parent peak, and also will be symbolized as the M+ peak (since the molecular ion is a radical cation - a positively charged molecule due to the removal of one electron). Often, the molecular ion is accompanied by what are called isotopic peaks, which are labeled M+1 and M+2.

Molecule - a molecule is a group of atoms bonded together that is the smallest possible amount of a particular substance that has all of the same characteristics unique to that substance.

Negative control - part of the quality control for the assay. In toxicology testing, the negative control is blood, urine, or serum that contains none of the targeted analyte (drug). The negative control should receive no response from the machine and is used to check that the machine is working correctly.

Positive control - part of the quality control of the assay. In toxicology testing, the positive control is blood, urine, or serum that contains a known amount the targeted analyte (drug). The positive control should receive a positive response from the machine for the drug in question. In ELISA testing, the positive control is used as the minimum threshold amount to test positive for a drug. If the concentration of a drug in a person's blood is higher than the positive control, then the lab will report a positive result. In a confirmatory toxicology assay, the positive control is used for quality control and comparison, but is not the only factor used to make a conclusion.

Precise - the proximity of two or more measurements to each other. For example, if your machine weighs a sample at 4.55 grams the first time, 4.56 grams the second time, and 4.55 grams a third time, your machine is fairly precise. Even if the true weight of the sample is 5.55 grams, the machine would be considered precise. Compare to accurate.

Presumptive test - a screening test that is performed to determine if a substance may be present. Presumptive tests may produce false positive results, so results must be confirmed using a confirmatory test. In general, presumptive tests are faster and less expensive than confirmatory tests. Presumptive tests help analysts determine which samples should receive additional confirmatory testing.

Proficiency test - a proficiency test is a sample provided by a proficiency testing company which should be tested in the exact manner as all other samples are tested. The laboratory sends their results back to the proficiency testing company for evaluation. The proficiency test is designed to test the laboratory's procedures and performance to ensure that the laboratory results are accurate. Ideally, the analyst will not know that she is completing a proficiency test. This is called a blind proficiency test. However, proficiency testing is rarely conducted blindly in forensic labs. Proficiency test samples may not be an adequate test of an analyst's ability to conduct forensic testing because forensic samples are often more complex than proficiency test samples.

Quality assurance (QA) - a set of activities working to ensure the quality of the work of the entire laboratory. QA focuses on how well assays are running as a whole, consistency of results, and the adequacy of the scientists' performance. QA is proactive - it attempts to develop and improve the scientific processes that used in the laboratory so that errors are prevented. Compare to Quality Control.

Quality control (QC) - a set of activities performed on individual lab tests to ensure that the results being obtained are accurate. QC is reactive, whereby it aims to identify problems in the run and to correct the defects in individual results. QC may include the use of blanks, internal standards, negative controls, positive controls, etc. to measure the accuracy of the test on a certain batch of sample. Compare to Quality Assurance.

Sensitive - when a machine or technique is referred to as sensitive it means that the machine or technique can detect very small amounts of a substance. Sensitivity is a quantitative characteristic. Compare to specific.

Specific - when a machine or technique is referred to as specific it means that the machine or technique can detect certain molecules with great accuracy. Specificity is a qualitative characteristic. Compare to sensitive.

Transmittance - the measurement of how much light travels through a sample. The inverse of absorbance.

Validation/method validation - when a new machine, technology, or technique is introduced into the laboratory it must be validated via a validation procedure. While the method of validation changes based on what is being validated, validation typically entails the running of many known samples, which are compiled together to create a portfolio. This portfolio is examined to determine if the machine/technique is working properly and consistently. If it is determined to be working correctly, then the machine/technique will be implemented into the lab's procedures. Note: many known samples may include tens or hundreds of samples, and the entire validation procedure may take some time.

Volatile - evaporates or changes from liquid to gas easily at normal temperatures.

Wavelength - Sound and light are made up of waves. A wave has both crests, the highest point, and troughs, the lowest point. A wavelength is a measure of distance between two successive points on a wave, as in: crest to crest or trough to trough. Wavelengths are typically measured in centimeters (cm).

Wavenumber - the number of waves that exist over a specific distance. Wavenumbers are measured in inverse centimeters (cm-1).