Analysis of Ethyl Alcohol in Beverages, Blood and Breath

Ethanol cases dominate Indian forensic-toxicology casework: drunk driving under Section 185 of the Motor Vehicles Act 1988, hooch tragedies in prohibition states (Bihar, Gujarat), brewing and excise prosecutions, and post-mortem alcohol estimation. The syllabus bullet packages all of this into three matrices the candidate must handle separately.

Beverages and liquors. Seized country liquor, suspected methanol-spiked hooch, or excise samples are analysed for ethanol percentage (volume by volume, abbreviated v/v) and for methanol as an impurity or adulterant. The work-horse method is direct-injection or headspace gas chromatography with FID, which separates methanol from ethanol cleanly and quantifies both. Specific-gravity hydrometry and refractometry are older field tests that still appear in MCQs.

Biological fluids. Whole blood is the standard medical-legal matrix, drawn into a grey-top vacutainer with sodium fluoride (1 percent, anti-glycolysis) and potassium oxalate (anticoagulant). Urine, vitreous humor and (rarely) cerebrospinal fluid are used as alternative matrices, mainly in post-mortem cases.

Breath. Non-invasive estimation of BAC via breath alcohol concentration (BrAC), using the blood-breath partition ratio of 2100 to 1 (approximate, at 34 degrees Celsius). This is what traffic police use at highway roadblocks; evidential confirmation in court still requires a blood sample or a calibrated evidential-grade IR breath analyser.

The full chemistry, pharmacology and clinical picture of ethanol (and the related volatiles methanol, isopropanol, acetone) sits in the book deep dive on volatile and gaseous poisons, which the NET candidate should skim once for context.

Headspace GC-FID is the method an Indian SFSL or CFSL toxicology division reports in court. It is the textbook gold standard for blood alcohol and the syllabus expects you to know the schematic, the internal standard, the column class and the detector.

The principle is simple. A measured aliquot (typically 100 microlitres) of blood, urine or vitreous humor is pipetted into a 10 mL glass vial with a measured volume of internal standard (n-propanol or t-butanol). The vial is sealed with a PTFE-lined septum and equilibrated at 60 to 65 degrees Celsius for about 15 minutes. Ethanol partitions between the liquid and the vapour above it according to Henry's law; an automated headspace sampler draws a fixed-volume vapour aliquot and injects it onto a polar capillary column (DB-ALC1, DB-ALC2, or a Carbowax-class stationary phase) that resolves methanol, ethanol, acetone, isopropanol and the n-propanol internal standard. The flame ionisation detector responds linearly to carbon-hydrogen mass; the ethanol-to-internal-standard peak-area ratio gives a quantitative BAC against a four-point calibration curve.

Two MCQ-grade facts to lock down. The internal standard is n-propanol (sometimes t-butanol), used to correct for injection volume and headspace variability. The detector is FID, which responds to all hydrocarbons but is selective enough when the column resolves the volatiles cleanly. Limit of quantitation is around 10 mg per 100 mL, well below the Indian legal cut-off of 30 mg per 100 mL.

For seized beverages, the same instrument is used with direct liquid injection (after dilution) and the answer is reported as percent ethanol v/v. The same chromatogram flags methanol, the marker of toxic hooch.

The alcohol dehydrogenase (ADH) assay is the fast hospital screen and the field-lab back-up to GC-FID. Ethanol plus NAD+ is converted by yeast or bacterial ADH to acetaldehyde plus NADH; the NADH absorbs strongly at 340 nm, and the rate of absorbance increase is proportional to ethanol concentration over a calibration range of about 10 to 400 mg per 100 mL. Reaction is run at pH 9.0 in a glycine or Tris buffer with a semicarbazide trap to pull acetaldehyde out of the equilibrium.

The strengths are speed (results in 5 minutes) and instrument simplicity (any UV-Vis spectrophotometer). The weaknesses are specificity (other primary alcohols including methanol and isopropanol are oxidised by some ADH preparations) and matrix interference (post-mortem blood gives noisy baselines). For court evidence in India, the ADH result is always confirmed by GC-FID.

The two historical chemical methods still appear as MCQ distractors. The Kozelka and Hine dichromate oxidation method oxidises ethanol to acetic acid in acidic dichromate; residual orange dichromate is titrated iodometrically or measured colorimetrically. The Cavett method uses a similar dichromate-sulfuric system on diffused ethanol vapour. Both are obsolete in casework, both are tested for historical recall.

For broader poisoning context (signs, antidotes, lethal doses), the book chapter on signs, symptoms and antidotes of common poisons is the deep dive. Ethanol's lethal blood concentration is taken as 400 mg per 100 mL (0.4 percent) in most Indian texts, with respiratory depression dominating above 300 mg per 100 mL.

The Widmark equation (Erik Widmark, 1932) lets you back-calculate the total dose of ethanol consumed, or the BAC at an earlier time, from a measured BAC and body weight. The exam form is:

A = r × W × C

where A is total alcohol absorbed (g), W is body weight (kg), C is the BAC expressed in g per kg (or sometimes g per L, watch the units), and r is the Widmark factor (the apparent volume of distribution of ethanol relative to body water). Standard values are r = 0.68 for men and r = 0.55 for women, reflecting the higher fat fraction (and lower body water) in women.

For back-calculation to the time of the offence, the elimination rate (beta) is taken as 15 to 20 mg per 100 mL per hour (0.15 to 0.20 g per L per hour) on the linear, zero-order elimination phase. If a blood sample drawn 2 hours after the offence reads 50 mg per 100 mL, the BAC at the offence time is approximately 50 + (2 × 18) = 86 mg per 100 mL. NTA has used back-calculation arithmetic in Paper 2 numericals; practise one or two by hand.

1. Measure BAC at sampling time
   Whole blood drawn in NaF/oxalate vacutainer, analysed by headspace GC-FID. Record the value in mg per 100 mL.
2. Note the time gap
   Hours between the alleged offence (e.g. road accident) and the blood draw. Document on the requisition form.

Breath analysers exploit the blood-breath partition ratio for ethanol (about 2100 to 1 in alveolar air to blood, at 34 degrees Celsius). The subject delivers a deep end-expiratory breath of at least 1.5 litres into a mouthpiece; the instrument samples the last fraction (alveolar) and reports BrAC, which is multiplied by the partition ratio to estimate BAC. Three sensing technologies dominate the syllabus.

Infrared (IR) breath analysers measure absorbance at one or two diagnostic wavelengths of the ethanol molecule, typically near 3.4 micrometres (C-H stretch) and 9.5 micrometres (C-O stretch). Acetone, a common interferent in diabetics on ketogenic diets, absorbs at different wavelengths and is rejected. The Intoxilyzer 8000 and the Drager Alcotest 9510 are evidential-grade IR stationary units used in many Western jurisdictions; in India, evidential confirmation is still typically a blood draw. IR spectroscopy theory and the underlying infrared absorption principle sit in the book instrumentation tree.

Fuel-cell (electrochemical) breath analysers use a platinum-electrode fuel cell: ethanol diffuses across a porous membrane to the platinum surface where it is oxidised to acetic acid, releasing electrons; the current is proportional to ethanol mass. Specificity for ethanol is high (methanol and acetone give negligible signal), drift is low, and the unit is small enough for handheld traffic-police use. The Drager Alcotest 6810 / 7510 and the Lion Alcolmeter 500 sit in this class and are the typical Indian highway-roadblock instruments.

Semiconductor (metal-oxide, SnO2) breath analysers use a heated tin-dioxide sensor whose surface conductivity changes when reducing gases (ethanol, but also other volatiles) adsorb. Cheap and small but non-specific: tobacco smoke, methanol, ketones and household solvents all give a signal. Used for personal-use screening, not evidential casework.

The single most asked legal fact in this bullet is the threshold under Section 185 of the Motor Vehicles Act 1988: driving with BAC above 30 mg per 100 mL of blood (or the equivalent BrAC) is an offence. The penalty (since the 2019 amendment) is up to 6 months imprisonment and a fine of up to Rs 10,000 for the first offence; up to 2 years and Rs 15,000 for a second offence within 3 years.

Where alcohol-impaired driving causes death or grievous hurt, the Bharatiya Nyaya Sanhita (BNS) 2023 culpable-driving provisions are invoked. BNS Section 106 (causing death by negligence) and Section 105 (culpable homicide not amounting to murder) replace the older IPC Sections 304A and 304 Part II respectively; the prosecution must establish both BAC above the threshold and a causal link to the death or injury.

Sampling procedure is governed by the Bharatiya Nagarik Suraksha Sanhita (BNSS) 2023 medico-legal provisions (the BNSS-equivalent of the old CrPC Section 53 examination, with sampling protocols broadly retained from the Form 21 / police-surgeon practice). The blood is drawn by a registered medical practitioner, the skin site is cleaned with povidone-iodine, never an alcohol swab (alcohol swabs contaminate the sample), and the blood enters a grey-top vacutainer with sodium fluoride (anti-glycolytic, prevents in-vitro glucose fermentation to ethanol) and potassium oxalate (anticoagulant). The tube is labelled, sealed, and dispatched in a cold chain to the SFSL or CFSL toxicology division. Chain-of-custody breaks are the first line of defence attack.

Prohibition states (Bihar and Gujarat) prosecute under their state excise acts in addition to (and in some cases instead of) Section 185 MVA, with stricter thresholds and longer custodial sentences. Cases in these states route through the state SFSLs and CFSL Chandigarh handles the technically difficult casework (methanol-spiked hooch tragedies, for example). AIIMS Delhi forensic-medicine protocols are widely cited in viva.

Post-mortem ethanol is the trickiest sub-topic in this bullet because the body can generate ethanol after death through microbial fermentation, giving a false-positive BAC that defence counsel will weaponise. Anaerobic gut and tissue bacteria (Candida albicans, Clostridium species, some lactobacilli) ferment glucose, glycogen, lactate and amino acids to ethanol during putrefaction. Reported putrefactive ethanol levels can reach 70 mg per 100 mL in heavily decomposed cases.

Three forensic safeguards address this.

First, vitreous humor from the eye is the matrix of choice when putrefaction is suspected. The vitreous is anatomically isolated, low in glucose and substrates, and bacterially sterile in the early post-mortem interval; ethanol diffuses into the vitreous from blood antemortem and persists there with minimal post-mortem production. A vitreous-to-femoral-blood ratio above 1.2 supports antemortem ingestion; a ratio below this, or a blood result without vitreous corroboration, weakens the case.

Second, femoral vein blood (drawn from the leg, not the heart) is preferred over heart blood because the femoral compartment is further from the gut and less subject to post-mortem redistribution and bacterial fermentation. The sample is drawn into a NaF/oxalate tube even though the subject is dead, because in-vitro fermentation continues at room temperature.

Third, fermentation markers are run alongside ethanol. n-Propanol, n-butanol and isobutanol are markers of microbial fermentation (yeasts and clostridia produce them); their presence on the headspace GC-FID chromatogram alongside ethanol signals putrefactive origin and the ethanol value is qualified or rejected. The book chapter on post-mortem and viscera collection in poisoning covers the collection SOP in full; the matrix-selection rationale is in biological, non-biological and viscera matrices.

The cross-examination playbook on post-mortem alcohol is predictable: the defence will ask whether vitreous humor was tested, whether femoral or heart blood was used, whether NaF was present, and whether fermentation markers were quantified. Every answer must be on the file.