Chapter 02· 5 min read
Analytical Instruments
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The forensic laboratory's purpose is to convert physical evidence into legally defensible identifications. Different evidence types call for different instruments; matching the technique to the sample is the analyst's central skill. This chapter covers the instrument families that appear repeatedly in casework: microscopy, spectroscopy, mass spectrometry, X-ray methods, thermal analysis, and capillary electrophoresis.
2.1Microscopy — Choose by Specimen
| Microscope | Best for | Forensic use |
|---|---|---|
| Compound | Thin / transparent specimens | Hair, fibre, pollen identification |
| Stereo | 3-D opaque specimens, low magnification | Bullets, tool marks, evidence sorting |
| Comparison | Side-by-side comparison | Firearms ID, tool-mark comparison |
| Polarised (PLM) | Birefringent materials | Fibre ID (Δn), asbestos, minerals |
| Fluorescence | Fluorophore-labelled samples | Stained latent prints, dye-tagged DNA |
| SEM | High-magnification surface imaging | GSR particles, fingerprint detail, paint cross-sections |
The comparison microscope is the workhorse of firearms and tool-mark examination. Two complete stereo columns are joined optically through a beam-splitter bridge that combines the two image-paths. The examiner sees a single eyepiece view divided by a vertical seam: questioned specimen on one side, known on the other. Striations are aligned across the seam by rotating both stages until the patterns are continuous.
2.2The Beer-Lambert Law
The foundational quantitative law in absorption spectroscopy:
The law holds linearly between absorbance ~0.1 and ~1.0:
When an unknown gives absorbance > 1.0, dilute by a known factor (e.g., 1:10) and re-measure within the linear region. Compute the diluted concentration via Beer-Lambert, then multiply back by the dilution factor.
2.3FTIR — Functional-Group Fingerprinting
Fourier-Transform Infrared spectroscopy identifies organic compounds by their functional-group absorption pattern. Different bonds vibrate at characteristic wavenumbers (cm⁻¹).
| Wavenumber (cm⁻¹) | Functional group | Forensic example |
|---|---|---|
| 3200–3550 | O-H stretch | Alcohols, water |
| 3300 | N-H stretch | Amines, amides |
| 3050 | =C-H aromatic | Benzene rings |
| 2960–2870 | C-H aliphatic | Alkanes |
| 1715 | C=O ester | PET, esters |
| 1640 | C=O amide | Nylon, proteins |
| 1540 | N-H bend (amide-II) | Nylon, peptides |
| 1240 | C-O-C asymmetric | Esters, ethers |
| 720 | Aromatic out-of-plane | Para-substituted benzenes |
PET fingerprint trio: 1715 + 1240 + 720 cm⁻¹ — diagnostic of polyethylene terephthalate.
Sampling techniques
- ATR (Attenuated Total Reflectance): solid pressed against diamond / ZnSe / Ge crystal; non-destructive; ~0.5–2 µm penetration. Modern default for forensic chips and films.
- KBr-pellet transmission: ground sample mixed with KBr powder, pressed into transparent pellet. Gold standard for ground powders but destructive.
- Direct transmission: thin films / liquids in a cell.
2.4Mass Spectrometry — GC-MS Confirmation
Gas chromatography coupled to mass spectrometry (GC-MS) is the gold-standard confirmatory technique for organic controlled substances.
The GC separates the sample into peaks by volatility on a heated capillary column. Each peak enters the MS where electron-impact ionisation (70 eV is the EI standard) fragments the molecules into characteristic patterns. The mass spectrum is matched against library references (NIST, Wiley, SWGDRUG); a match score above 700 / 1000 is considered strong.
The trio 303 + 182 + 82 is the cocaine fingerprint. Confirmation requires: (1) library mass-spectrum match + (2) co-injected reference retention time match (within ±0.5%). This is Category-A SWGDRUG confirmation, court-admissible.
LC-MS/MS handles polar / thermolabile / non-volatile compounds that GC-MS cannot vaporise: drug glucuronides (e.g., morphine glucuronide MRM 462→286), peptides, proteins. Tandem MS adds a second mass-selection stage for high specificity at picogram detection.
2.5Atomic Spectroscopy Hierarchy
For trace metals in forensic toxicology, the choice of atomic spectroscopy depends on sensitivity and multi-element needs:
| Technique | Sensitivity | Multi-element | Isotopic | Cost |
|---|---|---|---|---|
| ICP-MS | ppt (10⁻¹² g/mL) | Yes | Yes | High |
| ICP-AES (OES) | ppb–ppm | Yes | No | Medium |
| Flame AAS | ppb | No (single lamp) | No | Low |
| Flame photometry | mg/L | No (alkali only) | No | Very low |
For forensic GSR identification (ASTM E1588), SEM-EDX gives both morphology (1–5 µm spheroidal particle) and elemental composition (Pb + Sb + Ba simultaneously). Detection limits ~0.1–1 wt% for elements heavier than sodium — sufficient for the primer-residue triad.
2.6X-Ray Methods
XRF — X-ray Fluorescence
Each element has unique inner-shell binding energies, so X-rays emitted when atoms relax after ionisation identify which element produced them. Fe K-α at 6.4 keV, Cu K-α at 8.05 keV, Au L-α at 9.71 keV, Pb L-α at 10.5 keV. Used for GSR, glass discrimination, paint composition, counterfeit metals.
XRD — X-ray Diffraction (Bragg's Law)
Crystalline materials identified by characteristic d-spacings via Bragg's law: nλ = 2d sin θ. Pattern matched against ICDD-PDF reference library (~100 000 entries). Distinguishes polymorphs (cocaine HCl vs free base, NH₄NO₃ phases I-V).
LIBS — Laser-Induced Breakdown Spectroscopy
High-energy pulsed laser creates a microplasma; atomic emission lines identify elements. Portable, near-non-destructive (μm crater), and capable of depth-profiling through paint layers. Standard for handheld field analysis.
2.7TLC and HPLC — Chromatographic Separation
Thin-layer chromatography (TLC) is the simplest separation technique. Sample is spotted at the bottom of a silica plate; a mobile phase travels up by capillary action; analytes partition between silica and mobile phase, separating by polarity.
HPLC is the high-pressure liquid version, with detection by UV-Vis or mass spectrometry. Reversed-phase HPLC uses a non-polar C18 stationary phase + polar water-acetonitrile mobile phase. More acetonitrile = shorter retention; more water = longer retention.
2.8Capillary Electrophoresis — STR Profiling
CE has displaced slab-gel electrophoresis for forensic DNA STR profiling. Single-base-pair resolution across the 75–450 bp STR allele range; multi-colour fluorescence detection allowing 20+ STR loci in a single multiplexed PCR + CE run; 96 samples processed per day per multi-capillary instrument (ABI 3500 / 3500xL).
Microscope choice: thin transmitted = compound; 3-D opaque = stereo / comparison; birefringent = PLM; surface morphology = SEM. Beer-Lambert: A = εcl. Linear 0.1–1.0; above 1.0 → dilute. FTIR sampling: ATR for solids (modern default); KBr for ground powders. GC-MS confirmation: library + co-injected retention time. Both required. LC-MS/MS: polar / thermolabile / non-volatile. GSR: Pb + Sb + Ba on a 1–5 µm spheroid by SEM-EDX. XRD: Bragg's law nλ = 2d sin θ; match against ICDD-PDF. Rf: spot distance / solvent distance, reproducible on same plate.
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