UV-Vis spectroscopy is a common analytical technique used in forensic chemistry to detect and identify substances based on their absorption of ultraviolet (UV) and visible (Vis) light. It is widely applied because it is fast, non-destructive, and relatively simple.
How UV-Vis Spectroscopy Works
- A sample is exposed to UV (200–400 nm) or visible light (400–700 nm).
- Molecules with conjugated double bonds or aromatic rings absorb specific wavelengths of light.
- The instrument measures the absorbance at each wavelength, producing a spectrum.
- Each compound has a characteristic absorption pattern, which can help identify it.
Applications in Forensic Analysis
1. Drug Identification
- UV-Vis can detect the presence of drugs in powders, tablets, or liquids.
- By comparing the absorbance spectrum of a sample to known standards, chemists can confirm the identity of substances like barbiturates, analgesics, or illegal narcotics.
2. Analysis of Blood and Biological Fluids
- Certain components in blood, like hemoglobin, absorb visible light.
- UV-Vis can help detect blood traces and estimate concentration of compounds like drugs or alcohol in blood and urine.
3. Detection of Poisons and Chemicals
- Many toxic compounds absorb UV light.
- For example, cyanide, phenols, or aromatic compounds can be detected in forensic toxicology using UV-Vis.
4. Ink and Document Analysis
- UV-Vis spectroscopy can differentiate inks on questioned documents by comparing absorption spectra.
- Helps detect forgeries, alterations, or counterfeit documents.
5. Monitoring Chemical Reactions
- In arson or chemical cases, UV-Vis can track changes in chemical composition of residues over time.
- For example, it can help detect accelerants or degradation products in fire debris.
Advantages in Forensics
- Non-destructive: the sample is usually not destroyed.
- Fast and relatively inexpensive.
- Can provide both qualitative (what substance) and quantitative (how much) information.
Limitations
- Not very specific for complex mixtures—other techniques like GC-MS or HPLC may be needed for confirmation.
- Only works for compounds that absorb UV or visible light.
Summary
UV-Vis spectroscopy in forensic analysis is used to:
- Identify drugs, toxins, and chemicals.
- Detect blood or biological fluids.
- Analyze inks and documents.
- Monitor chemical reactions in evidence.
It is a rapid, reliable screening tool that helps forensic chemists quickly narrow down potential substances before using more sophisticated methods for confirmation.