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

Atomic absorption measures absorption by a suitable population of free atoms at characteristic wavelengths. Sample introduction and atomisation determine which elements and matrices can be addressed. Calibration relates the instrument response to concentration under the method's conditions.

Flame AAS introduces a suitable sample into a flame. Graphite-furnace AAS uses controlled electrothermal stages and can provide useful sensitivity from a small aliquot. Furnace temperature programmes, matrix modifiers and background correction must suit the actual assay rather than a universal biological-sample recipe.

Cold-vapour methods exploit mercury's volatility and suitable conversion chemistry to transport mercury vapour to a detection cell. The preparation must account for the forms intended to be measured. Mercury is unusually volatile for a metal at room temperature; it is not scientifically correct to say that every other metal has literally no measurable vapour pressure.

Hydride generation converts suitable elements into volatile hydrides under controlled chemical conditions before detection. Conversion efficiency can depend on oxidation state and matrix. Hazardous hydrides require appropriate containment and an approved procedure, not an improvised test on an unknown digest.

Detection capability varies by element, matrix and setup. Flame, furnace and vapour-generation approaches cannot be assigned fixed universal ppm or ppb limits. Select the complete method using demonstrated performance, especially where low concentrations influence an exposure assessment.

A high expected concentration does not automatically make a particular flame method adequate. Consider specimen dilution, contamination, working range and uncertainty. Equally, an instrument with a lower brochure detection limit can fail if the preparation loses the target.

ICP-OES introduces material into an inductively coupled plasma and measures characteristic light emitted by excited atoms or ions. The plasma and optical configuration support measurement of multiple elements, but the actual panel and sensitivity depend on the method.

Suitable line selection, background correction and assessment of spectral overlap are essential. A bright signal from another element or the matrix can affect the selected measurement. Several wavelengths may help investigate an unexpected result, but they do not replace method validation.

Dissolved solids, acid composition and residual organic material can influence sample transport and plasma behaviour. A digest may need a validated dilution or another preparation. The plasma being very hot does not mean it automatically measures every element without interference.

ICP-OES can be useful for defined environmental, occupational or biological questions. Its suitability depends on the required range and specimen. Do not infer that a multi-element instrument automatically distinguishes poisoning from an underlying metabolic disorder or identifies the source of an exposure.

ICP-MS measures ions produced in the plasma after suitable introduction and separation. It can offer sensitive multi-element measurements and isotope information. Instrument design, tuning, contamination and the specimen determine the practical detection and quantitative limits.

Spectral interference includes atomic isobars, polyatomic ions and, in some conditions, doubly charged species. For example, argon–oxygen ions can overlap iron-56 at nominal m/z 56. A nominal-mass signal is not necessarily entirely due to the target element.

Collision and reaction cells manage interference by mechanisms that depend on the gas and configuration. Helium collision methods can exploit energy discrimination, while reactive gases can alter the chemistry of an interfering or target ion. They do not universally break every polyatomic ion while leaving all analytes unchanged.

Other approaches include appropriate isotope selection, mathematical corrections and higher-resolution or tandem-MS configurations. Each has limitations and requires verification. A correction that works in clean water may be inadequate in a chloride-rich biological digest.

Elemental isotope ratios can support comparisons when precision, fractionation and reference materials are addressed. Shared ratios do not necessarily establish one unique source. Chemical speciation instead distinguishes forms such as inorganic and methylmercury through a species-preserving separation with appropriate detection.

HPLC–ICP-MS can separate chemical species before elemental measurement. Identification requires suitable species references and evidence that extraction or storage has not converted them. Total-element digestion followed by an isotope ratio cannot reconstruct all original mercury species.

Ion chromatography separates suitable ions through interactions with a stationary phase. Conductivity is a common detector, often with suppression to reduce eluent background, but amperometric, optical and mass-based detection are also used for particular applications.

Fluoride, nitrate, nitrite and oxalate are examples of possible targets in suitable methods. Cyanide analysis requires particular attention to chemical form, stability, preparation and detection; not every generic conductivity method measures it adequately. Cation analysis uses a suitable separation and measurement scheme of its own.

A chromatographic peak must be evaluated for identity, separation and interference against the validated method. Matrix composition, high concentrations of other ions and sample preparation can alter performance. A retention-time match alone does not exclude every alternative.

Oxalate can support investigation of ethylene glycol exposure, but also has endogenous and other causes. Urinary oxalate, renal crystals and metabolic acidosis do not uniquely “lock” the diagnosis. Parent-compound analysis, timing, clinical findings and alternative explanations remain relevant.

Do not compare a spot-urine concentration in mg/L directly with a 24-hour excretion reference in mg/day. Those quantities require different information. Similarly, drinking-water fluoride or arsenic limits cannot be applied directly to a blood or tissue concentration.

Interpretation begins with the exact specimen, units, collection time, chemical form and analytical capability. Background exposure can produce detectable concentrations without acute poisoning. “Any thallium detected means poisoning” is therefore not a defensible universal rule.

In the United States, CDC currently uses a children's blood lead reference value of 3.5 µg/dL to identify concentrations higher than those in most children in the reference population. It is not a safe threshold, a universal treatment trigger or a legal limit. No safe blood lead level in children has been identified.

UK, Indian, Canadian and Australian clinical, occupational and environmental guidance can use different action frameworks. Consult the current authority for the particular population and setting. Do not combine unrelated historical numbers into a table labelled worldwide medico-legal cutoffs.

Arsenic interpretation illustrates why chemical form matters. Seafood can raise total urinary arsenic through less toxic organic forms. Speciation and exposure history may be needed. Hair arsenic also raises contamination and reference-range problems; a single number does not prove chronic criminal poisoning.

Mercury matrix choice depends on the form and timing of exposure. Urine commonly supports elemental or inorganic mercury assessment; it should not be labelled a universal marker of organic mercury exposure. Total measurements and species-specific measurements need separate interpretation.

An elevated result can justify further investigation without establishing who caused it, the original dose or cause of death. Keep exposure assessment, medical management and forensic attribution as distinct conclusions, supported by the appropriate evidence.

Contamination control is critical at low concentrations. Evaluate collection devices, reagents, vessels and the preparation environment. A method blank follows the relevant process; a suitable reference or control material helps assess measurement performance in a more representative matrix.

Matrix spikes, duplicates and certified reference materials answer complementary questions. A good spike does not necessarily demonstrate complete recovery of native analyte, and agreement between duplicates does not rule out shared bias. The batch design and acceptance criteria belong to the validated quality procedure.

Read the reference-material certificate for analytes, assigned-value status, uncertainty, expiry or validity conditions and wet/dry basis. An older product name does not guarantee current availability or that every listed metal has a certified value in the desired matrix.

External calibration can be suitable when matrix behaviour is adequately controlled. Standard addition can address some proportional matrix effects by adding known target amounts to aliquots. It does not remove contamination, spectral overlap or analyte loss that occurred before the additions.

For a valid standard-addition extrapolation, preserve comparable matrix conditions and account for final volumes. A negative concentration-axis intercept estimates the original contribution under the model's assumptions. Poor linearity or changing matrix composition undermines that interpretation.

Report the measurand, matrix, units, relevant method and limitations. Scientific quality guidance such as ANSI/ASB validation standards and UKIAFT guidance supports practice within its scope; national legal requirements vary. Neither a trace-metal finding nor an accreditation label decides a case outcome.

Related reading: Dry Ashing and Wet Digestion for Metallic Poisons.

Related reading: Toxic Metals and Anions: Arsenic, Lead, Mercury, Thallium and Beyond.

Related reading: Biological, Non-Biological and Viscera Matrices in Toxicology.