Control samples are collected by taking swabbings or cuttings from an unstained area of the same material near the recovered stain.
These control samples, also sometimes referred to as substrate samples, are crucial for comparison and analysis in forensic science and other fields. They serve as a baseline to determine if a substance found in a sample is inherent to the material itself or if it originated from an external source. Properly collected and labelled control samples are critical for accurate test results.
Here's a breakdown of the process:
- Location: Choose an area of the same surface or material as the stain that appears to be free from contamination or staining. The closer to the stain, the better, as it will likely represent the background composition more accurately.
- Method: The method of collection should match the method used to collect the actual evidence sample. If the stain was collected via swabbing, then the control sample should be collected via swabbing as well. If the stain was collected by cutting out a piece of the substrate, then the control should be collected in the same manner.
- Tools: Use sterile tools and equipment to prevent cross-contamination. This includes using fresh swabs, cutting instruments, or collection containers.
- Size: Collect a sufficient amount of material for analysis. The size will depend on the type of analysis being performed but should be comparable to the size of the stain sample.
- Packaging: Package the control sample separately from the stain sample to prevent cross-contamination.
- Labeling: Clearly label the control sample with all relevant information, including the date, time, location, a description of the sample (e.g., "unstained carpet"), and the case number. Be sure to label it distinctly as a "Control Sample."
Example:
Imagine a bloodstain is found on a carpet. To collect a control sample, you would:
- Identify an unstained area of the carpet close to the bloodstain.
- Use a sterile scalpel or scissors to cut out a piece of the unstained carpet. Alternatively, you could use a sterile swab moistened with distilled water to swab an unstained section.
- Place the cutting or swab in a clean, properly labelled container.
- Ensure the container is sealed and labeled as "Control Sample - Unstained Carpet near Bloodstain."
By following these steps, a reliable control sample can be obtained and compared to the stain sample, enabling accurate conclusions to be made.
