Metallic Poisons: Arsenic, Mercury, Lead and Thallium

NTA tests the four heavy metals as a flat list of source plus mechanism plus signature sign plus classical test. Memorise them in that order for each metal and you will land most one-mark and two-mark items in this bullet.

Arsenic. Trivalent As III (arsenite) is the dangerous species; pentavalent As V (arsenate) is reduced to As III in vivo. Arsenic binds vicinal thiol groups on pyruvate dehydrogenase and other lipoic-acid enzymes, shutting down mitochondrial ATP synthesis. Acute oral poisoning gives violent gastroenteritis with rice-water stools, abdominal cramps, and a garlic odour on the breath. Chronic exposure (the dominant Indian picture) gives Mees transverse white lines on the nails, palmar and plantar hyperkeratosis, "raindrop" hyperpigmentation on the trunk, and (years later) Bowen's disease and cutaneous and bladder cancers. Indian sources: contaminated groundwater in the West Bengal, Bihar, Assam and Jharkhand arsenicosis belt (more than 100,000 affected in West Bengal alone since the 1980s), Paris green (copper acetoarsenite) pesticide, opium adulteration, and traditional Ayurvedic and Unani preparations.

Mercury. Three chemical forms with three different toxic profiles. Elemental mercury vapour from broken thermometers, sphygmomanometers and dental amalgam crosses the lung-blood and blood-brain barriers and damages the CNS (tremor, erethism, the historical Mad Hatter syndrome from felt-hat makers using mercuric nitrate). Inorganic salts (mercuric chloride or corrosive sublimate, HgCl2) cause caustic gastroenteritis and acute tubular necrosis (the suicidal and homicidal poison). Organic mercury (methylmercury from microbial methylation in aquatic sediments and biomagnification up the food chain) is the Minamata Bay (Japan, 1956) poison: it crosses the placenta and causes congenital cerebral palsy, ataxia and visual-field constriction. Ethylmercury (thimerosal preservative) is the vaccine-related controversy and has very different kinetics from methylmercury.

Lead. Chronic, cumulative, multisystem poison. Industrial sources: battery recycling, smelter work, leaded petrol (phased out in India by 2000), leaded paint (still ubiquitous in older Indian housing), lead-glazed pottery, and contaminated cosmetics like low-grade sindoor, surma and kohl. Ayurvedic and Unani preparations with intentional lead content (US-FDA recalls have repeatedly flagged Indian-origin products). Mechanism: lead inhibits delta-aminolevulinic acid dehydratase and ferrochelatase in haem synthesis, causing microcytic hypochromic anaemia with basophilic stippling of red cells. Pathognomonic signs: Burton's line (a blue-black sulphide deposit at the gum margin), lead colic (cramping abdominal pain), wrist-drop and foot-drop from peripheral neuropathy, and (in children) encephalopathy from blood lead levels above about 70 micrograms per decilitre.

The wet tests are out of fashion at the bench, where ICP-MS does everything in one run, but they are very much in fashion at NTA, which tests Reinsch and Marsh in almost every Paper 2 cycle. Learn the reagents, the products, the colour and shape, and the one diagnostic trick that separates similar metals.

Reinsch test. A polished strip of pure copper foil is dipped in the viscera filtrate after digestion with concentrated HCl, and boiled gently for about 30 minutes. Any arsenic, mercury, antimony or bismuth in solution displaces onto the copper as a grey to black deposit. The foil is rinsed, dried, transferred to a Reinsch tube and heated. The sublimate that forms on the cooler upper part of the tube is examined under the microscope: octahedral colourless As2O3 crystals confirm arsenic, while mercury gives small metallic globules. The test is a screen, not a confirmation; a negative Reinsch rules out the four metals, a positive Reinsch demands confirmation by Marsh (for As) or Gutzeit (for As) or instrumental analysis.

Marsh test (1836). James Marsh devised the test for the LaFarge poisoning trial after frustration that earlier arsenic tests gave evanescent precipitates that juries could not see. The apparatus generates nascent hydrogen by zinc and dilute sulphuric acid in a flask; the test solution is added; any arsenic reduces to arsine gas, AsH3, which is dried, swept through a heated section of glass tubing, and decomposed to deposit a shiny black mirror of metallic arsenic on the cool end of the tube. The Marsh mirror is the canonical evidentiary object in 19th century arsenic cases. The two MCQ tricks: antimony gives an identical-looking mirror, but the arsenic mirror is soluble in sodium hypochlorite (NaOCl) while the antimony mirror is not; and a mirror forms on the cool side of the tube, not at the heated zone where the arsine actually decomposes.

Gutzeit test. A simpler colour version of the Marsh chemistry. Zinc plus acid generates hydrogen plus arsine; the gas is passed over filter paper impregnated with mercuric chloride (HgCl2). Arsine reduces HgCl2 to yellow or yellow-brown mercury arsenide species, staining the paper. Sensitivity is lower than Marsh but the apparatus is field-portable.

Mercury wet tests.

Heavy metals sit locked inside the organic matrix of viscera, hair and nails. Before any instrument can see them, the matrix has to be destroyed and the metal liberated into dilute acid solution. Three routes are tested.

Dry ashing. The simplest and cleanest method. Weighed viscera is dried in an oven, transferred to a porcelain crucible, and incinerated in a muffle furnace at 450 to 550 degrees Celsius for several hours until a white or grey ash remains. The ash is dissolved in dilute HNO3 and made up to volume. Dry ashing works well for stable metals (Pb, Cu, Zn, Cd) but loses mercury entirely (Hg boils at 357 degrees Celsius and volatilises long before ashing temperatures) and partially loses arsenic, antimony and selenium. For Hg and As work, dry ashing is the wrong choice and the MCQ trap.

Wet digestion. The traditional viceral-analysis workhorse. Viscera is digested in concentrated HNO3 alone, or in HNO3 + H2SO4 + HClO4 (the Kjeldahl-style mixture), or in HNO3 + H2O2, on a hot plate or in a Kjeldahl block. The digestion destroys organic matter by oxidation and yields a clear, faintly yellow solution ready for instrumental analysis. Two safety flags: HClO4 forms explosive perchlorate salts with organics if it goes dry, so it is added late and never alone; and the digestion is done under a perchloric-acid fume hood with a water-wash trap, not a normal chemical hood.

Microwave-assisted digestion. The modern standard at DFSS and CFSL toxicology divisions. Viscera plus HNO3 (and sometimes H2O2) is sealed in a Teflon bomb and heated under pressure in a microwave digester. Closed-vessel digestion preserves volatile metals (Hg, As, Se), eliminates contamination from the open hot-plate environment, and finishes in 30 to 60 minutes instead of 8 to 12 hours. It is the route of choice for any sample where Hg or As is on the analyte list.

After digestion, a final clean-up step (centrifugation, filtration through 0.45 micron, or chelation extraction) removes residual particulates before the digest hits the AAS nebuliser or the ICP plasma.

Once the digest is in dilute nitric acid, four instrument families compete for the analysis. NTA tests the strengths and the typical detection limits of each.

Atomic Absorption Spectrometry (AAS). A hollow-cathode lamp made of the target metal emits the resonance line; the digest is atomised in a flame (acetylene-air for most metals) or in a graphite furnace (for sub-ppb sensitivity on Pb, Cd, As). Ground-state atoms in the atomiser absorb the resonance line in proportion to concentration. For mercury, the cold-vapour technique (CV-AAS) reduces Hg(II) with stannous chloride to elemental Hg vapour at room temperature and sweeps the vapour through a long quartz cell; sensitivity reaches sub-ppb. For arsenic, antimony and selenium, hydride generation (HG-AAS) reduces the analyte to the volatile hydride (AsH3, SbH3, SeH2) with sodium borohydride and sweeps it into a heated quartz cell. The book chapter on AAS covers the optics and the matrix correction in detail.

Inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES). An argon plasma at 6,000 to 10,000 kelvin atomises and excites every element in the sample simultaneously. Emission lines are dispersed by an echelle grating onto a CCD detector. ICP-OES is the screening workhorse: a single run gives quantitative data on 20 or more elements in two minutes. Detection limits sit at low ppb to high ppt, an order of magnitude better than flame AAS but not as low as graphite furnace AAS or ICP-MS.

Inductively Coupled Plasma Mass Spectrometry (ICP-MS). The same argon plasma feeds a quadrupole, time-of-flight or sector-field mass analyser. Ions are sorted by mass-to-charge ratio (m/z) and counted on an electron multiplier. Detection limits reach low ppt (parts per trillion), three to four orders of magnitude better than flame AAS. ICP-MS also resolves isotopes, which lets the forensic toxicologist distinguish natural-abundance lead from leaded-petrol lead by Pb-206 / Pb-207 / Pb-208 ratios, and lets the geochemist source-attribute arsenic in groundwater. ICP-MS is the gold standard at CFSL Hyderabad and the central DFSS toxicology lab. The full primer is in the book chapter on ICP-OES and ICP-MS

Hair and nail grow at predictable rates (scalp hair about 1 cm per month, nails about 3 mm per month) and incorporate circulating metals into the keratin matrix in proportion to blood levels at the time of growth. Cutting a hair shaft into 1 cm segments from root to tip and analysing each segment by hair analysis protocols gives a month-by-month record of exposure, with the most recent month at the root.

The classical case is the re-examination of Napoleon Bonaparte's hair: 19th century analyses found elevated arsenic, and later neutron-activation and ICP-MS studies on multiple authenticated locks showed segmental peaks consistent with chronic environmental exposure (probably from wallpaper pigments) rather than acute poisoning. Modern Indian use cases include occupational lead exposure in battery-recycling workers (a Tamil Nadu cohort showed clear segmental gradients), chronic arsenic exposure in West Bengal arsenicosis patients, and forensic time-lining in suspected slow poisoning cases.

Sample prep for hair is critical: external decontamination by washing with detergent, water and acetone is mandatory to remove surface deposition that would otherwise inflate the apparent exposure. Acid digestion (HNO3 + H2O2 in a microwave bomb) follows, then ICP-MS quantification. The forensic toxicologist reports each segment as ppm dry weight against a reference range and notes which segments exceed the toxic threshold.

BIS IS 10500 drinking-water limits. The Bureau of Indian Standards drinking-water specification sets the acceptable and permissible limits that every state water-supply utility is required to meet. The four NTA metals: arsenic 0.01 mg/L (10 ppb), lead 0.01 mg/L, mercury 0.001 mg/L, cadmium 0.003 mg/L. These match the WHO guideline values for As, Pb and Cd, and the more stringent WHO value of 0.006 mg/L for total Hg. Memorise the four numbers; "0.01 mg/L for As and Pb" is the workhorse MCQ.

West Bengal and Bihar arsenicosis. Groundwater arsenic in the Ganga-Brahmaputra delta is geogenic (released from natural sediments by reductive dissolution), not industrial. Affected districts in West Bengal include Malda, Murshidabad, Nadia, North and South 24 Parganas; in Bihar, the belt runs through Bhojpur, Buxar, Patna, Bhagalpur and Khagaria; Assam (Brahmaputra valley) and Jharkhand have similar problems. Public-health surveillance through SWID Bengal and the Central Ground Water Board has identified more than 100,000 confirmed arsenicosis cases in West Bengal since the 1980s. The forensic angle is the chronic-exposure differential: distinguish geogenic groundwater arsenic from acute homicidal arsenic by clinical pattern (hyperkeratosis and Mees lines for chronic, rice-water stools and garlic breath for acute) and by speciation analysis (inorganic As III/V vs organic monomethylarsonic acid in chronic exposure).

Lead in sindoor, surma and Ayurvedic preparations. Multiple US-FDA recalls have flagged Indian-origin low-grade sindoor (the vermilion powder worn at the parting of the hair) with lead content in the percent range, and Ayurvedic and Unani preparations made by the rasa-shastra tradition that intentionally incorporate lead, mercury and arsenic. The Central Council for Research in Ayurvedic Sciences acknowledges the issue and has issued cleaner-manufacturing guidelines. Forensic casework arises when chronic encephalopathy or anaemia is traced back to one of these products.

Thallium poisoning cases. Rare but headline-grabbing in Indian press: a handful of intra-family homicidal cases in the last two decades, exploiting the tasteless, odourless property and the slow clinical onset. Detection by ICP-MS on urine and on hair segments is the modern gold standard.