District Of Columbia Department Of Forensic Sciences FBS02 ✓ Solved

District of Columbia Department of Forensic Sciences FBS02 – P

District of Columbia Department of Forensic Sciences FBS02 – Phenolphthalein Presumptive Chemical Test for the Presence of Blood. This procedure is used to determine the possible presence of blood on evidentiary material.

To establish the practices for documenting the examination of evidence to conform to the requirements of the Department of Forensic Sciences (DFS) Forensic Science Laboratory (FSL) Quality Assurance Manual, the accreditation standards under ISO/IEC 17025:2005, and any supplemental standards. Blood can usually be located by the visual appearance of the stain (red-brown color). This is augmented by testing with presumptive tests for blood, such as the phenolphthalein (Kastle-Meyer) test. This test relies on the peroxidase-like activity of the heme groups associated with the hemoglobin contained in red blood cells. In the presence of hydrogen peroxide, this peroxidase-like activity will catalyze the oxidation of phenolphthalin, which is colorless in solution, into phenolphthalein resulting in a pink colored solution. The presence of a pink color is a positive test result indicating the presumptive presence of blood.

Wear personal protective equipment (e.g., lab coat, gloves, mask, eye protection) when carrying out standard operating procedures. Read Material Safety Data Sheets to determine the safety hazards for chemicals and reagents used in the standard operating procedures.

The materials required for this test include: Phenolphthalin Working Solution, 3% Hydrogen Peroxide, and Positive Control-Blood. Note that solutions should not be used directly from the stock bottles; Reagent SOPs should be followed for preparation and labeling instructions.

The Positive and Negative Controls must be tested prior to daily use, with results recorded in casework documentation. The Positive Control (FBR02) must exhibit a pink color within 10 to 15 seconds upon the addition of the 3% Hydrogen Peroxide up to the 1:512 dilution or greater. If this dilution tests negative, the reagents may need to be replaced. The Negative Control must show no pink color after 15 seconds of adding 3% Hydrogen Peroxide.

Procedures for testing involve locating stain areas visually or with an alternate light source, taking a small sample of the suspected bloodstain, and adding 1-2 drops of the phenolphthalin working solution. If a pink color develops at this stage, it should be recorded as inconclusive. After that, 1-2 drops of 3% Hydrogen Peroxide are added and observed for 10-15 seconds. A pink color indicates a presumptive positive result for blood; no color change indicates a presumptive negative result. The phenolphthalein test is used for presumptive blood identification, and a confirmatory test may be required.

The phenolphthalein test relies on the catalytic peroxidase-like activity of the heme group in hemoglobin. The test can react to blood from both animals and humans. Insufficient sample quality or quantity can limit the development of a positive reaction; a color change must be observed within 15 seconds. Plant peroxidases may react similarly to blood, but can typically be visually distinguished.

Documentation of the test results should be recorded on the FBU Serology Examination Worksheet and in the FBU Report of Results.

Paper For Above Instructions

The Phenolphthalein Presumptive Chemical Test for the Presence of Blood, as outlined by the District of Columbia Department of Forensic Sciences (DFS), serves as a critical methodology in forensic science for identifying potential blood stains at crime scenes. This test exploits the chemical characteristics of hemoglobin in red blood cells to yield an observable color change indicative of blood's presence. Its implementation aligns with rigorous quality assurance standards, notably ISO/IEC 17025:2005, ensuring meticulous adherence to scientific protocols in forensic analyses.

To conduct this test, forensic scientists first undertake a visual inspection or utilize an alternate light source to identify suspected bloodstains. This process is essential as many biological stains may mimic the color and appearance of blood, necessitating cautious examination before proceeding to confirmatory tests.

Upon locating a suspected stain, a sample is taken via a non-invasive method, such as a swab or a small cutting. For the phenolphthalein test, it is crucial to follow the prescribed protocols meticulously to avoid contamination and ensure the reliability of results. The procedure stipulates the addition of 1-2 drops of phenolphthalin Working Solution to the sample, followed by careful observation. An inconclusive pink color at this stage suggests interference from other oxidizing substances and further assessment is required (Gaensslen, 1983).

Subsequently, the addition of 3% hydrogen peroxide to the sample initiates the chemical reaction that can confirm or negate the presence of blood. The reaction relies on the peroxidase-like activity inherent in hemoglobin, which facilitates the oxidation of phenolphthalin into phenolphthalein, producing a pink hue if blood is indeed present (Culliford, 1971). The critical observation period of 10-15 seconds is designed to balance accurate detection with the risk of false positives which can arise from prolonged reaction times.

Significantly, while the phenolphthalein test is effective in presumptive blood detection, it is imperative to understand its limitations. As noted in the standard operating procedures, the test may react to other substances, including plant materials that possess peroxidase activity—thereby complicating interpretations and necessitating confirmatory tests to unequivocally identify blood (Lee, 2000).

Moreover, scientists must document their findings meticulously. Each test's results are recorded on the FBU Serology Examination Worksheet, serving as a critical tool for maintaining chain-of-custody and ensuring compliance with legal standards (Camps, 1968). This level of documentation not only validates the testing process but also aids in future case reviews and appeals.

The comprehensive understanding of blood's biochemical properties ensures forensic scientists apply these tests judiciously. With rigorous preparation and precautionary measures in place—such as wearing personal protective equipment and adhering to Material Safety Data Sheets—laboratory safety and integrity are significantly enhanced (FBR16, Current Version).

The implications of the phenolphthalein test extend beyond mere identification; they are pivotal in the broader framework of criminal investigations. Accurate identification of blood can guide subsequent investigative actions, from securing crime scenes to directing further forensic analysis and interpretations (FSL Departmental Operations Manuals, Current Versions).

In conclusion, the phenolphthalein test stands as an invaluable tool within forensic serology, marrying scientific rigor with real-world application in criminal justice. Its systematic approach, rooted in established protocols and quality assurance standards, underscores its importance in forensic investigative processes.

References

• Camps, F. E. (1968). Gradwohl’s Legal Medicine. Baltimore: Williams and Wilkins.
• Culliford, B. J. (1971). The Examination and Typing of Bloodstains in the Crime Laboratory. National Institute of Law Enforcement and Criminal Justice PR71-7, Washington, DC, 41-52.
• Gaensslen, R. E. (1983). Sourcebook in Forensic Serology, Immunology, and Biochemistry. U.S. Government Printing Office, Washington, DC, 103.
• Lee, H. C. (2000). Identification and Grouping of Bloodstains. In R. Saferstein (Ed.), Forensic Science Handbook. Prentice-Hall.
• FBR02 - Positive Control – Blood (Current Version).
• FBR16 - Phenolphthalin Working Solution (Current Version).
• Forensic Science Laboratory Quality Assurance Manual (Current Version).
• FSL Departmental Operations Manuals (Current Versions).
• FSL Laboratory Operations Manuals (Current Versions).