Emerging spectrometric techniques for the forensic analysis of body fluids

Highlights

• Body fluids are exceptionally useful evidence in forensic investigations.

• Identification of body fluids is problematic and partially unsolved.

• Spectroscopy and spectrometry have good potential to identify body fluids.

• Modern analytical techniques with statistics have a promising future.

Abstract

Body fluids are evidence of great interest in forensics because they allow identification of individuals through the study of DNA. After reviewing the tests and the methods that are currently being used by forensic practitioners for the detection of body fluids (e.g., blood, semen, saliva, vaginal fluid, urine and sweat), and after showing their main drawbacks and limitations, this work focuses on the review of emerging spectrometric techniques applied for the forensic analysis of body fluids. These techniques include the use of ultraviolet-visible, infrared (IR), Raman, X-ray fluorescence and nuclear magnetic resonance spectroscopy and mass spectrometry for investigating blood, semen, saliva, urine, vaginal fluid or sweat. Although all these spectrometric techniques seem to have a high potential to differentiate body fluids prior to DNA extraction, IR and Raman spectroscopy have shown the most promising results for discriminating stains from body fluids.

Introduction

Body fluids are exceptionally useful forensic evidence because they provide information that allows police to discover their corresponding owner by analyzing the DNA (deoxyribonucleic acid) content. Since DNA testing has acquired huge relevance for solving crimes, the detection of biological evidence at the crime scene has become one of the priorities of law-enforcement officers during ocular inspection. According to Locard's principle, “every contact leaves a trace”, so a small part of the offender usually stays in the place while a small part of the place goes with the offender (e.g., in homicide, resulting from confrontation between victim and aggressor, residues from the victim are probably found on the attacker and vice versa) [1]. After trace detection, the forensic process continues with evidence analysis to obtain information about the source (identity) or activity (why the trace is where it is) and how it might relate to a proposed crime scene.

Technically, a biological fluid comes from a living being. In the forensic field, the biological fluids of interest are body fluids, which come from a human being, especially those from the attacker. Body fluids generated by human beings include blood, semen, saliva, vaginal fluid, urine, sweat, breast milk, tears and mucus. Undoubtedly, blood, semen and saliva are found in larger amounts than the others at crimes scenes. Consequently, these body fluids have been the most studied samples [2], [3], [4].

Tests currently used to analyze body fluids are classified according to their specificity in two different categories (presumptive and confirmatory) [2]. Presumptive tests provide a large number of false positives (i.e., the test is positive although the body fluid is not present in the sample). Presumptive tests are unspecific to a single body fluid, so a positive response is due only to the suspicion that the fluid may be present in the stain. Thus, it is always necessary to apply a confirmatory test to confirm the presence of a body fluid in a stain because, by contrast, confirmatory tests are specific to identify a particular body fluid. A positive response certainly ensures the presence of the searched body fluid in the stain under examination.

Presumptive and mainly confirmatory tests used in body-fluid identification are based on each body fluid having a unique composition, which is the result of specific components of each body fluid and the difference in the relative ratio of common components found in several body fluids [2]. As Lednev and Virkler indicate [2], urea is found in urine, semen and sweat, its concentration in urine being larger than in either of the other two fluids. Table 1 shows the principal components present in blood, semen, saliva, vaginal fluid, urine and sweat [2].

Table 2 shows the most relevant tests currently used to detect or to confirm blood, semen, saliva, vaginal fluid, urine and sweat. Table 2 contains the current available tests to detect blood, semen, saliva, vaginal fluid, urine and sweat. The columns indicate, respectively, test name, type of test (chemical, spectroscopic, microscopic, crystal test, immunological, chromatographic or electrophoretic), specificity (presumptive or confirmatory), main characteristics of the test and references. All the tests pursue the detection of specific components, ratios of components or characteristics, such as fluorescence, to detect presumptively or to confirm (identify) each body fluid.

As seen in Table 2, the six body fluids can be presumptively detected by different tests, most of them chemical. However, there are available confirmatory tests solely for blood and semen, which involve immunological interactions, microscopic visualization of specific components or formation of specific crystals by chemical reaction. In addition to the lack of confirmatory tests for saliva, vaginal fluid, urine and sweat, there are other disadvantages, such as:

• most confirmatory tests (for blood and semen) are destructive; and,

• it is necessary to apply different tests to confirm each type of body fluid; this limitation requires division of a sample into several parts, and a portion of the sample having to be kept for possible future analyses.

To date, there is no test or method used by forensic practitioners able to detect and to confirm a body-fluid source without destroying the evidence [2], [5]. This situation makes it necessary to develop confirmatory and non-destructive methods applicable to different types of body fluids. Nowadays, there are two principal lines of active research in body-fluid identification. One is dedicated to the development of mRNA (messenger ribonucleic acid) markers [6], [7], [8], [9], [10], [11], [12] based on the different mRNA sequences in each body fluid. The other is based on the use of spectrometric analytical techniques.

In this work, our goal is to provide a critical review of the works using emerging spectrometric techniques to analyze body fluids.