Inorganic Poison Analysis: AAS, ICP-OES, ICP-MS and Ion Chromatography

Atomic absorption rests on a single idea. Free ground-state atoms in the light path of a hollow-cathode lamp emitting the same element's characteristic line absorb a fraction of the light proportional to their number, and Beer-Lambert converts the reading into a concentration. What changes between the four modes is the way the sample is persuaded to give up its atoms.

Flame AAS uses a burner fed with fuel and oxidant. Air-acetylene at about 2300°C atomises lead at 283.3 nm, copper at 324.7 nm, zinc at 213.9 nm, cadmium at 228.8 nm. Refractory elements need nitrous oxide-acetylene at about 2800°C (aluminium, chromium, vanadium). Flame AAS sits at ppm and is the proportionate tool for acute high-dose cases.

Graphite-furnace AAS swaps the flame for a graphite tube. A microlitre aliquot is pipetted in, the tube is heated through a programmed ramp (drying 110°C, ashing 400 to 1200°C, atomisation up to 2700°C), and the atomic vapour sits in the light path long enough for the reading. Detection drops to ppb, the regime needed for As, Pb, Cd and Hg in routine blood and urine.

Cold-vapour AAS is the mercury-specific mode. Mercury is the only metal with a measurable vapour pressure at room temperature. Stannous chloride reduces mercuric ion to elemental mercury, the vapour is swept into a quartz cell, and absorbance is read at 253.7 nm.

Hydride-generation AAS exploits the hydride chemistry of As, Sb, Se, Bi and Sn. Sodium borohydride in acid reduces the metal ion to its volatile hydride (arsine AsH3, stibine SbH3), swept by argon into a heated quartz cell at about 900°C where it dissociates to free atoms.

ICP-OES replaces single-element sequential AAS with multi-element parallel analysis. A radio-frequency field couples to an argon stream in a quartz torch, heating the gas to a partially ionised plasma at 6000 to 10000 K, hot enough to atomise any element. Excited atoms emit characteristic wavelengths and a polychromator reads all wavelengths simultaneously.

The standard inorganic forensic panel (Pb, As, Cd, Hg, Cu, Zn, Cr, Ni, Co, Mn, Fe, Al) runs in one injection at ppb, comparable to GFAAS but with simultaneous readout. Mercury is the weak link because of a poor optical emission line, and is usually still done by CVAAS or ICP-MS. ICP-OES is the natural fit for open-question cases: industrial pollution, chronic occupational exposure, Wilson disease versus copper poisoning, environmental groundwater screening. NABL-accredited FSLs at Madhuban, Mahabaleshwar, Kalina and Mohali run ICP-OES or ICP-MS as the routine panel.

ICP-MS uses the same argon plasma as ICP-OES but replaces the optical detector with a mass spectrometer. Ions are extracted through a sampling cone, separated by quadrupole or sector field according to m/z, and counted by an electron multiplier. Detection drops to ppt, three to four orders below ICP-OES, and the instrument delivers what no AAS or ICP-OES can: isotope ratios.

Isotope ratios matter for two recurring questions. First, source attribution for lead: industrial and ore-source lead carries characteristic ²⁰⁸Pb to ²⁰⁶Pb and ²⁰⁷Pb to ²⁰⁶Pb signatures, so a blood lead in a child can be tied back to a specific paint, solder or battery-scrap stream. Second, methylmercury versus inorganic mercury, where the ²⁰²Hg to ²⁰⁰Hg ratio with HPLC-ICP-MS tells the toxicologist whether a fish-eating cohort case is organic mercury from methylated fish or inorganic from dental amalgam or industry.

Interference is the practical challenge. The argon plasma creates polyatomic ions that mimic analyte masses. The classical interferent is ⁴⁰Ar¹⁶O at m/z 56 sitting on ⁵⁶Fe. Modern collision-reaction cells use helium or hydrogen to break up the polyatomic without affecting the monoatomic analyte. The Agilent 7700 at CFSL Chandigarh, the PerkinElmer NexION at CFSL Hyderabad and the Thermo iCAP RQ at several state SFSLs run collision-cell technology as standard.

AAS and ICP see the metal, not the counter-ion. A case driven by an anion (ethylene glycol metabolised to oxalate, fluoride in fluorosis water, sodium fluoroacetate in a rodenticide) needs a different instrument. Ion chromatography fills that gap.

The principle is ion exchange. A quaternary ammonium column retains anions, a sulfonate column retains cations. A continuous eluent (sodium carbonate or bicarbonate for anions, methanesulfonic acid for cations) flows through, sample ions elute at characteristic retention times and pass through a suppressor that reduces the eluent background before the conductivity detector reads each peak.

The anions of forensic interest are fluoride (chronic fluorosis from the Nalgonda and Anantapur belts, acute HF spills), chloride, sulphate, nitrate and nitrite (methaemoglobinaemia workup), phosphate, cyanide and most importantly oxalate. Oxalate is the metabolite of ethylene glycol, which turns up regularly in Indian urban poisoning either as accidental ingestion or as a homicidal vehicle. Urinary oxalate well above the physiological 10 to 40 mg per 24 hours, with calcium oxalate crystals on microscopy and a high anion-gap acidosis, locks the diagnosis without needing the parent on GC. Cations (Na, K, NH4, Ca, Mg) are used for electrolyte profiling in mass-poisoning incidents with adulterated milk, water or food.

1. 1. Wet digestion of the biological matrix
   About 1 g of liver, kidney or whole blood is digested in concentrated nitric acid with hydrogen peroxide in a microwave-assisted closed vessel at 180 to 200°C for 15 to 30 minutes, giving a clear aqueous digest.
2. 2. Calibration setup
   External calibration with NIST-traceable standards for clean digests, standard addition for matrix-heavy samples. A method blank, reagent blank and at least one CRM (SRM 955c for blood lead, SRM 1577c bovine liver for As, Pb, Hg, Cd) run with every batch.
3. 3. Instrument analysis
   Flame AAS for ppm acute cases, GFAAS for ppb biological matrix, CVAAS for mercury, HG-AAS for arsenic and antimony, ICP-OES for multi-element panels at ppb, ICP-MS for ppt and isotope ratios. Ion chromatography in parallel for anions.
4. 4. QC and reporting
   Duplicates on at least 10 percent of samples, spike recovery at 80 to 120 percent, CRM result within certificate uncertainty. The report carries matrix, method, LOD, LOQ, result with uncertainty, CRM value and the relevant medico-legal cut-off.

A toxicology report is useful only when the analyst can place the numerical result on a clinical and legal scale. Indian SFSL practice uses cut-offs drawn from WHO, BIS and ICMR guidance.

Blood lead is the most frequently cited number. WHO holds there is no safe threshold, but operational cut-offs are 5 µg/dL as a paediatric screening trigger, 10 µg/dL as intervention level (used at AIIMS Delhi and PGI Chandigarh), and 40 µg/dL as adult occupational action under Factories Act guidance for battery, smelter and lead-paint workers. 80 µg/dL is acute encephalopathy territory.

Hair arsenic above 1 ppm is chronic exposure. Urine arsenic above 50 µg per 24 hours flags recent exposure. WHO caps drinking-water arsenic at 10 ppb while BIS IS 10500 permits 50 ppb without an alternate source, the gap that drives the West Bengal, Bihar, Assam and Jharkhand arsenicosis work and the WBHIDCO groundwater programme. Urine mercury above 50 µg/L suggests organic mercury exposure. Thallium has no normal physiological level and any detection is presumptively poisoning.

NABL accreditation under ISO/IEC 17025 sets the quality baseline at most modern Indian FSLs. Every batch runs with a method blank, a reagent blank, at least one CRM (SRM 955c blood lead, SRM 1577c bovine liver, SRM 2670a urine), duplicates on at least 10 percent of samples, and a spike-recovery check.

Calibration follows the matrix. A clean aqueous digest is quantitated against external calibration with NIST-traceable standards. A matrix-heavy digest (urine, incompletely digested viscera) uses standard addition: the digest is spiked at increasing concentrations, the calibration line is built within the matrix itself, and the concentration is read from the negative x-intercept.

CFSL Chandigarh runs an Agilent 7700 ICP-MS as the trace-metal workhorse. FSL Madhuban runs a flame plus GFAAS battery alongside an ICP-MS. PHED (Public Health Engineering Department, Government of West Bengal) uses ICP-MS for West Bengal groundwater arsenic mapping. AIIMS Delhi handles occupational lead from battery and smelter workers via GFAAS with ICP-MS for source attribution. PGI Chandigarh runs the Wilson versus copper sulphate workup, where the differential matters: copper sulphate is a homicidal and suicidal vehicle in north India while Wilson is a treatable genetic disease.