The Walker test is a presumptive test used in forensic science to identify gunshot residue (GSR) on clothing.
Understanding the Walker Test
The Walker test is specifically designed to detect the presence of nitrite ions, a key component of gunpowder residue, on fabrics. This is achieved using a specialized photographic paper that has been soaked in a mixture of naphthylamine and sulfanilic acid. When this treated paper is pressed against the questioned area of clothing, a chemical reaction occurs if nitrite ions are present. The reaction causes the formation of a red colouration on the paper, indicating the presence of GSR.
How the Walker Test Works:
- Chemical Reaction: The test relies on the reaction between nitrite ions and the chemicals on the photographic paper.
- Color Indication: The presence of a red coloration indicates a positive result for nitrites. This confirms the presumptive presence of gunshot residue in the tested area.
- Presumptive Nature: It is important to remember that the Walker test is presumptive. It suggests the possible presence of GSR, but further confirmatory testing is necessary for absolute identification.
Key Features of the Walker Test:
| Feature | Description |
|---|---|
| Purpose | Presumptively detects the presence of nitrite ions associated with gunpowder residue. |
| Method | Naphthylamine-sulfanilic acid soaked photographic paper is used. |
| Reaction | Nitrite ions react with the chemicals on the paper, producing a red color. |
| Interpretation | Red colouration signifies the presumptive presence of GSR. Absence of color suggests the absence of significant nitrites. |
| Limitations | Presumptive test; requires further confirmation with more specific techniques. Not specific to GSR. |
Advantages of the Walker Test:
- Non-destructive: It’s a non-destructive testing method, which preserves the original state of the evidence for further analysis.
- Relatively quick: The test is relatively fast and easy to perform making it suitable for quick analysis of suspected GSR areas.
- Cost effective: Requires materials that are relatively inexpensive and commonly available in laboratories.
Practical Example:
Imagine a piece of clothing is suspected to have gunshot residue. The forensic analyst would:
- Soak photographic paper in a naphthylamine-sulfanilic acid solution.
- Place the treated paper on the suspect area of the clothing.
- Apply slight pressure to the paper.
- If red coloration appears on the paper, it indicates a possible GSR location and a presumptive positive result.
Conclusion
The Walker test is a simple, presumptive method used to identify areas on clothing that might contain gunshot residue based on the presence of nitrite ions. The red colouration provides visual confirmation of the possible presence of GSR. It is important to note that it is a preliminary test and requires further confirmation for final conclusions.
