Capillary Electrophoresis: Power Tool in Forensic Science
Capillary Electrophoresis (CE) has revolutionised the field of DNA analysis in forensic science over the past decades. It is an instrumental evolution of traditional slab gel electrophoretic techniques. CE is highly sensitive, rapid, and an automated technique used to separate DNA fragments based on size. Lately, CE has become the gold standard technology in every modern forensic DNA laboratory, providing reliable evidence in courts of law.
Principle of Capillary Electrophoresis
Capillary Electrophoresis is based on the principle that charged particles migrate in an electric field at different rates based on their charge-to-size ratio. The components of capillary electrophoresis include an injection system, capillary, high voltage source, electrode, and a detector. This method works for charged inorganic molecules. The inorganic molecules will move based on their charge towards the oppositely charged end.
In the case of DNA fragments: Smaller DNA fragments migrate faster, and larger DNA fragments move more slowly through the matrix. This difference in migration rates allows the DNA fragments (of different lengths) to be separated and detected individually.
Mechanism of Capillary Electrophoresis in Forensic DNA Analysis
- Sample Preparation: The DNA sample is extracted and amplified using PCR, and the primers are tagged with fluorescent dyes of different colours.
- Sample Injection: A small amount of the amplified DNA is injected into the capillary, which contains a viscous polymer matrix.
- Separation: High voltage is applied across the capillary, and the negatively charged DNA molecules migrate towards the positive electrode.
- Detection: At the detection window, the fluorescent labels are excited by a laser, and a detector measures the emitted light.
- DNA Analysis: The system software converts the fluorescence signals into an electropherogram, a graph showing peaks for each STR allele. The peak heights and positions are used to determine the DNA profile.
Significance of Capillary Electrophoresis in Forensic Science
- High Sensitivity and Accuracy: This method can detect DNA even if very low concentrations are present and also from mixed samples.
- Standardized and Valid in Court of Law: CE-based STR analysis results are accepted in legal procedures globally.
- Rapid Analysis: Modern CE instruments can process a large quantity of samples in a single run, which is crucial for handling large case backlogs.
- Nationalised Database: National databases in India rely on STR profiles generated by CE for DNA matching and identification.
New Developments in Capillary Electrophoresis in Forensic Toxicology
Capillary electrophoresis, although best known for its role in forensic DNA analysis, is a booming tool used in forensic toxicology as well. CE is gaining attention due to its unique advantages: low sample volume, high separation efficiency, and compatibility with biological fluids.
One of the biggest evolutions is the integration of CE with MS detectors, which is used to detect toxic substances such as basic drugs, synthetic cannabinoids, and peptide-based toxins. It is also used in the differentiation of chiral compounds in drugs to separate and quantify drug enantiomers.
In What Type of Forensic Cases Can Capillary Electrophoresis Be Used?
- Sexual Assault Cases: In such cases, CE is majorly used to generate DNA profiles from minute biological evidence from semen or vaginal swabs to precisely identify the suspect.
- Mass Disaster Victim Identification: In some unusual events or natural disasters, CE is used to match DNA from recovered remains to identify the victim’s relevance from their relatives.
- Identify Missing Persons: Sometimes it is crucial to identify and locate missing persons. CE-based DNA analysis can be used to solve such cases by comparing DNA from unidentified remains to family reference samples.
