Spectroscopy in Forensics, Medicine and Industry

Introduction

The techniques in this unit are not only tools for exam questions. Mass spectrometry confirms drug use in athletes and road users, the physics of NMR produces detailed images of the brain, and infrared and UV-visible spectrometers check medicines and foods every day. This page surveys how the same principles — absorption of radiation, nuclear spin and ion masses — are applied where accuracy really matters.

Core explanation

Forensic and sports drug testing. Screening tests, often based on antibodies, are fast but can give false positives. A positive screen is therefore confirmed by GC–MS or LC–MS . Chromatography separates the sample; the retention time and the full mass spectrum of each component are compared with a certified reference standard. Matching both gives very strong evidence of identity. In sports testing, isotope-ratio mass spectrometry can even tell whether testosterone was produced in the body or came from a synthetic source, because synthetic material generally has a slightly different ¹³C:¹²C ratio.

Breath alcohol testing. Evidential breath analysers measure the absorption of infrared radiation by ethanol, typically in the C–H stretching region near 2950 cm⁻¹ or the C–O region near 1050 cm⁻¹. By the Beer–Lambert law, absorbance is proportional to concentration in the sample chamber, so the reading converts to an alcohol concentration.

Magnetic resonance imaging (MRI). MRI is based on ¹H NMR. The patient lies in a strong magnetic field, typically 1.5 or 3 tesla, and radio-frequency pulses excite the hydrogen nuclei, mainly in water and fat. Magnetic field gradients make the resonance frequency depend on position, so the signal can be turned into a map. Tissues differ in water content and in how quickly the nuclei relax back to equilibrium, which produces contrast between, for example, grey and white matter. MRI uses no ionising radiation; the main hazards are the powerful magnet, which can pull ferromagnetic objects violently, and effects on some implanted devices.

Pulse oximetry. A clip on the finger shines red and infrared light through tissue. Oxygenated and deoxygenated haemoglobin absorb these wavelengths differently, so the ratio of absorbances gives the oxygen saturation of the blood — a direct medical use of visible and near-infrared absorption.

Industrial quality control. Pharmaceutical companies confirm the identity of every batch of raw material, often by comparing its IR spectrum with a reference. UV-visible spectroscopy measures the concentration of active ingredients during dissolution tests. NMR checks purity and detects isomeric impurities. Environmental monitors use IR absorption to measure CO₂ and CO in air, and UV-visible methods to measure pollutants in water.

Step-by-step reasoning

How a confirmatory drug test reaches a conclusion:

1. A screening test flags a sample as possibly positive. 2. The sample is separated by chromatography. 3. Retention time is compared with a reference standard. 4. The mass spectrum (molecular ion and fragments) is compared with the standard. 5. Only if both match within set limits is the result reported as confirmed.

Visual explanation

Picture an MRI scanner as a giant NMR spectrometer with a person in place of the sample tube. The strong magnet aligns the hydrogen nuclei; radio pulses tip them; the weak signal they emit as they relax is recorded by coils and rebuilt by a computer into cross-sectional images.

Real-world analogy

A confirmatory test is like checking both a person's photograph and fingerprints before releasing a parcel. Either check alone could be fooled occasionally; together, a false match becomes extremely unlikely. Retention time and mass spectrum play the roles of photograph and fingerprint.

Real-world example

Food authentication laboratories use NMR and isotope-ratio mass spectrometry to detect fraud, such as honey diluted with cheap syrups or wine labelled with a false region of origin. Natural products carry characteristic isotope ratios and minor components that act as a chemical signature.

Why?

Why is the word "nuclear" usually left out of the name MRI? The technique is exactly nuclear magnetic resonance, but the word "nuclear" alarmed patients who associated it with radioactivity. In fact no radioactive material or ionising radiation is involved — only magnetic fields and radio waves.

Common misconception

"Spectroscopy gives a certain identification every time." Every method has detection limits, possible interferences and uncertainty. This is why forensic laboratories use reference standards, controls and two independent measurements before reporting a result.

Worked example

Question: An IR breath analyser reads an absorbance of 0.30 for a calibration gas containing 35 µg of ethanol per 100 cm³ of breath. A driver's breath gives an absorbance of 0.24 under the same conditions. Estimate the ethanol level.

Reasoning: Absorbance is proportional to concentration (Beer–Lambert law, same path length). Concentration = 35 × (0.24 ÷ 0.30) = 35 × 0.80 = 28 µg per 100 cm³.

Answer: About 28 µg of ethanol per 100 cm³ of breath.

Quick check

1. Which nuclei produce most of the signal in a medical MRI scan, and in which substances? Answer: Hydrogen-1 nuclei, mainly in water and fat molecules in the body's tissues.

Exam focus

Be ready to link each application to its underlying principle: GC–MS to molecular ions and fragmentation, breathalysers to IR absorption by bonds, MRI to ¹H NMR, and colorimetry to the Beer–Lambert law. Explain why the technique suits the task.

Advanced insight

Functional MRI detects brain activity indirectly. Deoxygenated haemoglobin is paramagnetic while oxygenated haemoglobin is not, so changes in local blood oxygenation slightly alter the NMR relaxation of nearby water. Active brain regions receive extra oxygenated blood, producing a small signal change that can be mapped over time.

Summary

Spectroscopy underpins modern analysis. GC–MS confirms drugs by retention time and mass spectrum, and isotope ratios reveal synthetic substances. IR absorption measures breath alcohol and atmospheric gases. MRI applies ¹H NMR to water and fat in the body without ionising radiation. IR, UV-visible and NMR spectroscopy confirm identity, purity and concentration in industry.

Practice questions

1. Why is a positive screening test for a drug followed by GC–MS? Answer: Screening tests can give false positives; GC–MS is highly specific because both retention time and the full mass spectrum must match a reference standard. 2. State one safety hazard associated with MRI and explain it. Answer: The very strong magnetic field can pull ferromagnetic objects into the scanner at high speed and can affect some implanted devices, so people and equipment are screened first. 3. Which law allows a breath analyser to convert IR absorbance into ethanol concentration? Answer: The Beer–Lambert law, because absorbance is proportional to concentration for a fixed path length. 4. Suggest how IR spectroscopy can be used to check a delivery of a pharmaceutical raw material. Answer: Its IR spectrum is recorded and compared with a reference spectrum; matching bands, especially in the fingerprint region, confirm the identity of the material.