In forensic chemistry, qualitative and quantitative analysis are two fundamental approaches used to study substances, but they focus on different aspects of the evidence.
1. Qualitative Analysis
- Purpose: To identify what a substance is.
- Focus: Determines the chemical composition or presence of specific compounds.
- Examples in Forensics:
- Identifying whether a powder is cocaine, sugar, or baking soda.
- Detecting the presence of arsenic or cyanide in a poison sample.
- Checking for fingerprint residues using ninhydrin or cyanoacrylate fuming.
- Techniques Used:
- Chromatography (TLC, GC, HPLC) – separates components to identify them.
- Spectroscopy (IR, UV-Vis, MS) – identifies chemical structures.
- Spot tests – simple chemical reactions that indicate presence of a substance.
In simple terms: Qualitative analysis answers the question, “What is it?”
2. Quantitative Analysis
- Purpose: To measure how much of a substance is present.
- Focus: Determines the concentration or amount of a chemical or compound.
- Examples in Forensics:
- Measuring blood alcohol concentration (BAC) in a suspected drunk-driving case.
- Determining drug levels in blood, urine, or tissues in toxicology.
- Measuring the amount of explosive residue in fire debris.
- Techniques Used:
- Gas Chromatography with detectors – quantifies drugs or accelerants.
- High-Performance Liquid Chromatography (HPLC) – measures precise concentrations.
- Spectrophotometry – calculates concentrations based on light absorption.
In simple terms: Quantitative analysis answers the question, “How much is there?”
Key Difference
- Qualitative: Focuses on identity (what substance it is).
- Quantitative: Focuses on amount (how much of it is present).
Summary
Both types of analysis are essential in forensic chemistry. Often, investigators first perform qualitative tests to identify a substance and then conduct quantitative tests to measure its amount. Together, they provide a complete chemical profile that is crucial for criminal investigations, toxicology, and legal evidence.