Practice with national-level exam (FACT, FACT Plus, NET, CUET, etc.) mocks, learn from structured notes, and get your doubts solved in one place.
The four chromatographic workhorses of a forensic toxicology lab, the detectors that go with each and the Indian SOPs that govern their use.
Chromatography is the central craft of a forensic toxicology laboratory. The toxicologist receives a viscera tin, a post-mortem blood syringe or a sealed packet of unknown white powder, and the first job is to push that mixture through a separating medium until each component travels at a different speed and arrives at a detector one at a time. Four techniques carry almost all of this work: thin layer chromatography (TLC) for a cheap visual screen, high performance liquid chromatography (HPLC) for thermally fragile drugs, high performance thin layer chromatography (HPTLC) for quantitative herbal and dye fingerprinting, and gas-liquid chromatography (GLC, usually shortened to GC) for volatile poisons and pesticides.
The reason all four still coexist is economic and practical. A district SFSL with a single GC-FID and a single HPLC-DAD cannot afford to discard TLC because TLC costs almost nothing per sample and tells the analyst whether to bother with the bigger instrument at all. NIPER Mohali and NIN Hyderabad run HPTLC because the technique fingerprints complex herbal mixtures in a way HPLC cannot. AIIMS, NIMHANS and a handful of metro hospitals run LC-MS/MS for confirmation, but the bulk of routine Indian casework still moves through GC-MS at CFSL Chandigarh, CFSL Hyderabad, FSL Madhuban and the better resourced state SFSLs. This page walks through the shared principle, then takes each technique in turn with its mobile phase, detector, Indian deployment pattern and the troubleshooting moves that separate a clean chromatogram from a worthless one.
Twenty rupees of silica gel and a glass tank still triages most viscera samples.
TLC is the simplest and oldest of the four. A glass or aluminium plate is coated with a thin layer of silica gel 60 F254 (the F254 means the silica contains a fluorescent indicator that lights up under 254 nm UV; analytes that absorb in that range appear as dark spots against a bright green background). Standard plate sizes are 20 by 20 centimetres for full screening and 5 by 10 centimetres for spot checks. The sample extract is applied as a small dot one centimetre from the bottom edge, the plate is placed in a glass tank containing a few millilitres of solvent, the solvent climbs the plate by capillary action and the analytes ride along at different speeds.
The choice of solvent system depends on the chemistry of the suspected analyte. Indian toxicology manuals (Modi's Medical Jurisprudence and Toxicology, the BPRD forensic handbook and the CFSL SOP set) standardise on four mobile phases that between them cover most drug classes.
| System | Solvent ratio | Best for |
|---|---|---|
| A (general drugs) | Methanol : ammonia 100 : 1.5 | Initial screen across most basic drugs |
| B (opioids) | Chloroform : methanol : ammonia 90 : 10 : 1 | Morphine, codeine, heroin, tramadol |
| C (basic drugs) | Ethyl acetate : methanol : ammonia 80 : 10 : 10 | Alkaloids, amphetamine, ephedrine |
| D (acidic and neutral drugs) |
A reverse-phase C18 column with a diode array detector handles the thermally fragile half of forensic toxicology.
HPLC moved the planar idea into a closed steel column under pressure. The stationary phase is a finely packed bed of silica beads, chemically modified so the surface is non-polar. Reverse-phase C18 columns (a layer of octadecyl chains bonded to the silica) are the default for forensic drug analysis. The typical analytical column is 250 millimetres long and 4.6 millimetres in internal diameter, packed with five micrometre particles. UHPLC variants run shorter and narrower (100 by 2.1 millimetres) with 1.8 micrometre particles, trading higher backpressure for faster runs and better resolution.
The mobile phase is a mixture of water and an organic modifier (methanol, acetonitrile) often pulsed across a gradient so the solvent strength rises during the run and pulls progressively more retained analytes off the column. A small concentration of formic acid or an acetate buffer keeps the pH stable so weakly basic drugs do not develop tailing peaks. A high pressure pump (Shimadzu Prominence LC-2030, Agilent 1100 or 1260, Waters Alliance are the three platforms most state SFSLs and medical college toxicology departments actually own) drives the mobile phase at one to two millilitres per minute.
What separates HPLC from GC is the detector zoo at the far end. Each detector picks up a different physical signature and the choice depends on the analyte.
| Detector | Signal it reads | Forensic strength | Forensic limitation |
|---|---|---|---|
| UV-DAD (diode array) | Absorbance across 190 to 800 nm at every time point | Library matching by UV spectrum; identifies drugs with a chromophore | Blind to compounds with no UV chromophore (some sugars, some aliphatic poisons) |
Small particles, automated application and densitometric scanning turn TLC into a quantitative tool.
HPTLC is not a different technique from TLC so much as TLC done with engineering and automation. The silica particle size drops from ten to fifteen micrometres in regular TLC to three to five micrometres in HPTLC, which sharpens spots and roughly doubles resolution. The sample is applied as a narrow band (not a dot) by an automated band applicator like the Camag Linomat 5, which sprays a precisely metered volume across a defined band width using a syringe and a nitrogen stream. The plate is developed in an automated developing chamber (Camag ADC2) that controls humidity, saturation and solvent front travel. After development, the plate goes onto a densitometric scanner (Camag TLC Scanner 4) that reads absorbance or fluorescence at chosen wavelengths across each band, producing a chromatogram that looks much like an HPLC trace but with band position as the x-axis.
The result is a planar technique that is quantitative (peak area on the densitogram correlates linearly with analyte mass over a calibration range), reproducible (because everything from application to scanning is instrumental rather than manual) and capable of running up to seventy samples in parallel on a single 20 by 10 centimetre plate. That throughput is why HPTLC is the workhorse of Indian institutions that screen herbal and food matrices.
In forensic and regulatory practice HPTLC is concentrated at NIPER Mohali for pharmaceutical fingerprinting and adulteration cases, NIN Hyderabad for nutritional and food safety profiling, CCRAS for ayurvedic and herbal fingerprinting under the Drugs and Cosmetics Rules, and FSSAI-empanelled laboratories for food adulterant screening (synthetic dyes in chilli powder, melamine in milk, mineral oil in edible oils).
The relevance to a forensic toxicology lab is twofold. Suspected herbal poisoning cases (aconite from Aconitum napellus, Cerbera odollam from yellow oleander, abrin from Abrus precatorius) often need a fingerprint comparison rather than a single-analyte assay, and HPTLC is faster and cheaper than HPLC-MS for that job. Dye and pigment forensics in ink, paint and textile fibre cases also lean heavily on HPTLC because dyes are non-volatile and separate cleanly on silica.
A 30-metre capillary column and a flame at the end is still where forensic toxicology meets the courtroom.
Gas-liquid chromatography pushes a vaporised sample through a long thin capillary column with a flowing inert gas. The forensic standard column is fused silica, 30 metres long, 0.25 millimetres in internal diameter, with a 0.25 micrometre liquid film coated on the inner wall. The stationary phase varies with target chemistry: DB-1 (100 percent polydimethylsiloxane, non-polar, broad screen), DB-5 (5 percent phenyl, the universal forensic phase, used for drugs and pesticides) and DB-WAX (polyethylene glycol, polar, ideal for alcohols and short-chain volatiles).
Helium is the traditional carrier gas in Indian labs. Hydrogen is the cheaper and faster alternative and is now common at CFSL Hyderabad and several refurbished state SFSLs, with appropriate cylinder safety controls.
The detector dictates what the GC actually does for a case.
| Detector | Selectivity | Forensic target | Indian deployment |
|---|---|---|---|
| FID (flame ionisation) | Universal for combustible organics; carbon-hydrogen response | General drug and hydrocarbon screen; blood alcohol | At least one GC-FID in every state SFSL |
| ECD (electron capture) | Halogen and nitro group selective | Organochlorine pesticides (DDT, endosulfan, lindane), nitro explosives (TNT, RDX) | All explosive units and pesticide-heavy state SFSLs (Punjab, Haryana, AP, Telangana) |
What goes on the shelf at a district SFSL, a state SFSL and a CFSL is different. The case routing follows the equipment.
The choice between TLC, HPLC, HPTLC and GC in Indian forensic practice is rarely free. It is dictated by what the lab actually owns, by the chemistry of the suspected analyte and by the evidentiary standard the prosecution will need to meet.
Tailing peaks, poor resolution and ghost peaks are the three failure modes every analyst sees in the first month.
A forensic chromatogram has to survive cross-examination. Three classic failure modes account for most of the rejected runs in Indian SFSL audits.
Tailing peaks are the commonest complaint, especially with basic drugs (amphetamines, ephedrine, codeine, alprazolam). The cause is almost always silanol activity on the column packing: free Si-OH groups on the silica surface form hydrogen bonds with basic nitrogen atoms in the analyte, delaying its release. The fixes are predictable. Add an ammonia or triethylamine modifier (about 0.1 percent) to the HPLC mobile phase to mask the silanols. Switch to a deactivated or end-capped C18 column. On GC, use a base-deactivated capillary phase. If the tail persists, the column is degraded and needs replacement; a long, drifting tail with each successive injection on the same column is the telltale.
Poor resolution between two adjacent peaks (Rs below 1.5) is the second failure mode. The fixes are method-dependent. On HPLC, run a longer gradient with a shallower slope so the two analytes spend more time at their respective elution strengths. On GC, raise the initial oven temperature more slowly and increase the column length or split ratio. On TLC, switch to a less polar solvent system so the spots travel less far and have more room to separate. Resolution problems that survive method adjustment usually mean the column is approaching end-of-life.
Ghost peaks are extra peaks with no analyte in the injection. The causes split between carryover (the previous sample is still bleeding off the injector or column) and contamination (dirty inlet liner, degraded septum, impure mobile phase). Fixes are blanks between samples, a fresh inlet liner and septum every fifty injections on GC, and HPLC-grade solvents only. Ghost peaks at the retention time of the target are dangerous because they produce false positives; an unexplained peak in a procedural blank is a reason to invalidate the whole batch.
A district SFSL receives a viscera sample in a suspected oleander seed poisoning. The lab has TLC, GC-FID and HPLC-DAD but no mass spectrometry. The first analytical step is:
| Chloroform : acetone 80 : 20 |
| Barbiturates, benzodiazepines, salicylates |
Once the plate has developed, the analyst dries it and visualises the spots. UV at 254 nm is the first look, picking up any analyte that quenches the fluorescent silica indicator. Spray visualisation follows, chosen for the suspected class. Marquis reagent (formaldehyde plus concentrated sulphuric acid) gives a purple colour with opioids and an orange colour with amphetamines. Mayer's reagent (potassium mercuric iodide) precipitates alkaloids as a cream-white spot. Dragendorff's reagent (potassium bismuth iodide) gives an orange-brown spot with most nitrogenous bases. Ninhydrin develops amino acids and amphetamines as purple-pink. Iodine vapour in a sealed tank gives a generic brown spot for any unsaturated organic compound.
Rf comparison with co-spotted reference standards is the working logic. A diazepam reference run on the same plate at the same time as the unknown gives an internal Rf yardstick that cancels out plate-to-plate variation in solvent strength, humidity and temperature. The unknown spot at Rf 0.42 next to a diazepam reference also at Rf 0.42 is a strong screening result. It is not, by itself, a confirmation; the limitation is sensitivity (approximately five micrograms of analyte per spot is the lower threshold of reliable visualisation) and specificity (several drugs share Rf values in any given system, which is why two solvent systems should always be run in parallel).
| Fluorescence (FLD) | Emission after excitation at a chosen wavelength | Picogram sensitivity for quinine, polycyclic aromatic hydrocarbons and a handful of fluorescent metabolites | Only useful when the analyte fluoresces; less than ten percent of forensic targets |
| Electrochemical (ECD) | Oxidation or reduction current at a working electrode | Catecholamines, phenolic drugs, certain antibiotics | Narrow analyte set; baseline drift if mobile phase oxygen is not removed |
| MS / MS-MS (single quadrupole or triple quad) | Mass-to-charge ratio and fragmentation pattern | Confirmatory: structure-level identification, quantitation at picogram level | Capital cost above one crore; consumable cost; needs trained operator |
The standing Indian SFSL workflow on HPLC-DAD reads roughly like this. Barbiturates in viscera or post-mortem blood are quantitated on an isocratic phosphate-buffer methanol method at 220 nm. Paracetamol overdose cases run on a 250 nm channel. Alprazolam in drug-facilitated crime samples is screened on a reverse-phase gradient at 230 nm and confirmed against a deuterated internal standard for high-value cases. Antidepressants, antihistamines and beta-blockers each have a worked-up SOP at CFSL and the better state SFSLs.
UHPLC has crept into Indian forensic practice over the last decade. Shorter columns and smaller particles cut a 30-minute LC run to under eight minutes, which matters when a state SFSL clears two hundred backlog cases a quarter. The trade-off is higher pressure (over 800 bar), shorter column life and a more demanding sample preparation step.
| NPD (nitrogen-phosphorus) | Selective for N (drugs) and P (organophosphate pesticides) | Alkaloids, benzodiazepines, organophosphate pesticides | CFSL and tier-one SFSLs; less common at district level |
| MS (mass spectrometry) | Universal with fragment-level structural information | Confirmatory identification across drugs, pesticides, volatile poisons | CFSL Chandigarh, CFSL Hyderabad, FSL Madhuban, AIIMS, NIMHANS, NDTL Delhi |
Two GC sample-introduction modes carry most forensic volatile work. Headspace GC takes a sealed glass vial containing the biological matrix (blood, urine, gastric contents) and heats it to a fixed temperature. Volatile analytes partition into the vapour above the liquid, and a sample of that headspace gas is injected onto the column. Ethanol in blood is the canonical headspace assay, run on every drink-drive case routed through an SFSL. The same workflow handles methanol (Bihar 2016, Gujarat 2022, Tamil Nadu 2023 hooch tragedies), chloroform (the classical criminal stupefying agent), petrol and other organic solvents.
For non-volatile drugs the trick is the reverse: make the analyte volatile by chemistry. Derivatisation converts polar hydroxyl, carboxyl or amino groups into less polar silyl or methyl groups so the molecule survives the GC inlet. Silylation with BSTFA (N,O-bis(trimethylsilyl)trifluoroacetamide) is the workhorse for hydroxylated drugs (cannabinoids, opioid metabolites, benzodiazepine metabolites). Methylation with diazomethane covers carboxylic acid drugs (some NSAIDs, valproate). Derivatisation is the bottleneck of GC sample prep and is one reason LC-MS/MS has gradually eaten into GC casework at tier-one Indian labs.
GC-MS remains the courtroom gold standard for drug and poison identification in Indian forensic practice. The combination of retention time on a defined column, electron-impact mass spectrum and library match (NIST and Wiley libraries) is the evidentiary trifecta. AIIMS forensic toxicology, NIMHANS Bangalore, CFSL Chandigarh, CFSL Hyderabad and FSL Madhuban (Haryana) all run GC-MS as their primary confirmatory platform. LC-MS/MS at NDTL (the WADA-accredited National Dope Testing Laboratory), AIIMS and a handful of metro labs handles the cases GC-MS cannot reach: thermally labile drugs, polar metabolites and ultra-trace post-mortem casework.
Equipment distribution matters when planning casework. Every state SFSL has at least one GC-FID and one HPLC-DAD as a working minimum. GC-MS is concentrated at CFSL Chandigarh, CFSL Hyderabad, FSL Madhuban, AIIMS and NIMHANS, with a slow rollout to tier-one state labs. LC-MS/MS is rarer still, anchored at NDTL Delhi, AIIMS and the CFSL trio. HPTLC sits outside the formal SFSL network at NIPER Mohali, NIN Hyderabad and CCRAS-funded centres. TLC is universal because the consumables cost almost nothing.
The four chromatographic techniques between them carry every routine forensic toxicology workflow in India, from a TLC screen at a district SFSL to a GC-MS confirmation at CFSL Chandigarh and an LC-MS/MS quantitation at AIIMS. Pick the technique for the chemistry, the detector for the analyte, and write the report so a defence counsel and a sessions judge can both follow the reasoning.