Sustainability & Environment (Agri-Food Systems) · Pollution Control & Environmental Engineering · also Environmental Analytical Techniques, Environmental Instrumentation, Analytical Methods in Environmental Science
How environmental samples are taken and analysed and how good the numbers are: sampling plans, gravimetric and volumetric methods, spectroscopy, chromatography, quality assurance and monitoring networks.
In the exams: UGC NET Environmental Sciences Unit 2; ICAR AIEEA-PG Physical Science Unit II (instrumental analysis); ICAR AICE Aquatic Environmental Management Unit 3
Every environmental decision, from issuing a factory permit to declaring a drought or certifying organic produce, depends on measurements. Environmental monitoring is the systematic sampling and analysis of air, water, soil, sediment and living things to describe environmental conditions, detect changes and check compliance. This subject covers how monitoring programmes and networks are designed, how samples are taken and preserved, the classical gravimetric and volumetric methods, the main instrumental techniques (UV–visible spectrophotometry, atomic absorption and ICP methods, chromatography and mass spectrometry, electrochemical sensors), isotopes as tracers, how good the numbers are (detection limits, accuracy, precision, calibration), quality assurance and laboratory accreditation, continuous and low-cost sensor networks, and biological indicators and biomonitoring.
It complements Environmental Chemistry, which explains the chemistry behind each parameter, and Air & Noise Pollution, Water Quality and Soil Science subjects, which interpret results against standards. Here the focus is method: what each technique measures, how it works, its strengths and limits, and how to make results defensible. Methods are described by principle; operating procedures should follow recognised compendia such as Standard Methods for the Examination of Water and Wastewater, US EPA methods, ISO standards and Indian standards and CPCB guidelines.
The subject is examined in UGC NET Environmental Sciences (Unit 2, analytical techniques), ICAR AIEEA-PG (instrumental analysis in the Physical Science group) and ICAR AICE Environmental Sciences, and it underpins Pollution Control Board laboratory recruitment and laboratory accreditation. Calculations that recur are worked through: sample size for a sampling plan, solids and titration results, absorbance and calibration, isotope δ values, detection limits, recoveries and relative standard deviations, and biotic indices.
Monitoring programmes are designed backwards from their purpose. Common objectives are compliance monitoring (checking whether discharges, emissions or ambient levels meet standards); trend monitoring (detecting long-term change, such as declining groundwater levels or improving air quality); baseline monitoring before a project or policy (essential for environmental impact assessment); surveillance and early warning (detecting pollution episodes, algal blooms or disease outbreaks quickly); effectiveness monitoring (checking whether a control measure works); and research monitoring to understand processes. The US EPA's data quality objectives (DQO) process formalises this: state the problem, identify the decision, define the inputs, boundaries and decision rules, specify tolerable errors, and then design the sampling to meet them. Each objective implies different choices of parameters, locations, frequency, methods and data handling. Compliance monitoring needs legally defensible, standard methods with documented chain of custody; trend monitoring needs consistent methods and long records more than high precision on any single day; early warning needs rapid, often automated measurement. A programme without clear objectives tends to collect data that answer no question.
A monitoring network is a set of stations chosen to represent conditions over an area or system. Air quality networks site stations to represent population exposure (residential and commercial areas), background levels, maximum impact near sources (located with dispersion modelling and wind roses) and transport between regions. India's National Air Quality Monitoring Programme (NAMP) operates manual stations across hundreds of cities, supplemented by continuous ambient air quality monitoring stations (CAAQMS) reporting in real time. Water quality networks cover river reaches upstream and downstream of cities and major discharges, reservoirs, groundwater wells and coasts; India's National Water Quality Monitoring Programme, run with state boards, monitors thousands of locations, and the UNEP GEMS/Water programme compiles global data. Sampling designs include judgemental (expert choice, efficient but not statistically representative), simple random, stratified random (dividing a heterogeneous area into more uniform strata and sampling each, which improves precision), systematic or grid sampling (regular spacing, useful for mapping contamination and for geostatistics such as kriging), and composite sampling (mixing several sub-samples to estimate an average cheaply, at the cost of losing information on variability). Sampling frequency must capture the variability of interest: diurnal cycles for air pollutants, seasonal cycles for rivers, storm events for runoff. The number of samples needed to estimate a mean within a margin of error E at a given confidence is approximately n = (zσ/E)², where σ is the standard deviation and z about 1.96 for 95 per cent confidence. For σ = 20 mg/kg and E = 5 mg/kg, n ≈ (1.96 × 20/5)² ≈ 62 samples. Halving the margin of error requires four times as many samples, which is why sampling costs dominate many programmes. Detecting trends requires long, consistent records, and statistical tests such as the Mann–Kendall test are used for non-parametric trend detection in environmental time series.
Sampling is often the largest source of error in environmental data. A grab sample represents one place and time; a composite sample combines sub-samples over time (time- or flow-proportional) or space to represent average conditions; and automatic samplers collect sequences, for example during storms. Sample containers must not contaminate or adsorb the analyte: glass with PTFE-lined caps for organic compounds and oils, acid-washed plastic for metals, sterile bottles for microbiology (with sodium thiosulphate to neutralise chlorine in treated water). Many parameters change after collection through biological activity, chemical reactions, volatilisation or adsorption, so samples are preserved and analysed within holding times. Typical practice, following Standard Methods and EPA guidance, includes: cooling to about 4 °C (≤6 °C) for most samples; acidifying with nitric acid to pH below 2 for metals (holding time up to six months, except mercury); acidifying with sulphuric acid for nutrients such as ammonia, nitrate plus nitrite, total Kjeldahl nitrogen and COD (about 28 days); analysing BOD within 48 hours of cooled storage; analysing microbiological samples within about 8 hours (6 hours' transport plus 2 hours to processing) for regulatory work; and measuring temperature, pH, dissolved oxygen and conductivity in the field or fixing DO on site (Winkler reagents). Volatile organic compounds are collected in vials without headspace. Soil sampling defines depth (for example 0–15 cm for plough-layer fertility, deeper for nitrate or contaminants), uses augers or cores, and commonly combines 10–20 sub-samples from a field or zone into a composite in a zig-zag or grid pattern. Samples are air-dried, ground and sieved through 2 mm for most chemical analyses; samples for volatile contaminants or microbial activity are kept field-moist and cool.
Ambient air sampling uses high-volume samplers for total suspended particulate, respirable dust samplers for PM10, and PM2.5 samplers with size-selective inlets and filters at defined flow rates (the US federal reference method uses 16.7 L/min). Filters are weighed before and after sampling under controlled temperature and humidity. Gaseous pollutants were traditionally collected by drawing air through absorbing solutions in impingers and analysing them colorimetrically: CPCB methods include the improved West and Gaeke method for SO2 (with pararosaniline) and the modified Jacobs and Hochheiser method for NO2. Passive samplers rely on diffusion onto an absorbent over days or weeks and are cheap for mapping NO2, SO2 and ozone. Stack sampling uses isokinetic sampling, in which the sampling velocity equals the stack gas velocity so that particles are not over- or under-collected. Volatile organic compounds are sampled with sorbent tubes or evacuated canisters. Biota sampling, for contaminants in fish, shellfish, plants or bioindicators, specifies species, size, tissue (such as muscle, liver or whole body) and number of individuals, and samples are frozen. Sediments are collected with grabs or cores. Throughout, documentation makes data defensible. Field records note location (with GPS), date, time, conditions and observations. Chain-of-custody forms record every transfer of a sample from collection to analysis, with signatures, so that results can be used in legal proceedings. Field quality control samples, such as field blanks, trip blanks and field duplicates, detect contamination and variability introduced during sampling, as chapter 10 explains.
Gravimetric analysis determines an analyte by weighing. In environmental work it is used mainly for solids and particulate matter. Total suspended solids (TSS) are measured by filtering a known volume of water through a pre-weighed glass-fibre filter and drying at 103–105 °C: TSS (mg/L) = (mass of filter plus residue − mass of filter, mg) × 1000 ÷ sample volume (mL). For example, if a filter weighs 1.2210 g before and 1.2345 g after filtering 250 mL, TSS = 13.5 mg × 1000/250 = 54 mg/L. Total dissolved solids (TDS) are measured by evaporating filtered water and drying the residue at 180 °C. Volatile solids (VSS, VS) are the mass lost on igniting the residue at 550 °C, an estimate of organic matter used for sludges and biomass; fixed solids remain after ignition. Settleable solids are measured volumetrically in an Imhoff cone. Gravimetric methods also determine sulphate (precipitated as barium sulphate), oil and grease (extracted with a solvent and weighed after evaporation), and particulate matter on filters from air samplers, where the mass concentration equals the mass gain divided by the volume of air sampled. Gravimetry is accurate and needs no calibration against standards, but it is slow and requires careful drying, desiccation and weighing on analytical balances, with filters conditioned at constant temperature and humidity for PM work.
Titrimetry measures an analyte by reacting it with a standard solution of known concentration (the titrant) until an endpoint, detected by an indicator colour change or by an instrument such as a pH meter. Four types are common in environmental analysis. Acid–base titrations measure alkalinity and acidity, as the chemistry subject explains. Complexometric titrations with EDTA measure total hardness and calcium: at pH 10 with Eriochrome Black T indicator, EDTA complexes calcium and magnesium, and the colour changes from wine-red to blue at the endpoint. Total hardness (mg/L as CaCO3) = volume of EDTA (mL) × molarity × 100,000 ÷ sample volume (mL); for 12.5 mL of 0.01 M EDTA used on a 50 mL sample, hardness = 12.5 × 0.01 × 100,000/50 = 250 mg/L as CaCO3. Precipitation titrations, such as Mohr's method, measure chloride with silver nitrate and potassium chromate indicator, the endpoint marked by red-brown silver chromate. Redox titrations include the Winkler method for dissolved oxygen (oxygen oxidises manganese(II), which in acid liberates iodine titrated with sodium thiosulphate using starch indicator), the dichromate COD method (excess dichromate back-titrated with ferrous ammonium sulphate using ferroin indicator), and iodometric determination of residual chlorine. Titrimetric methods are inexpensive, precise in skilled hands and remain the reference for several parameters, but they need relatively high concentrations, can suffer from interferences, and are labour-intensive. Standardisation of titrants against primary standards, such as anhydrous sodium carbonate for acids or potassium hydrogen phthalate for bases, is essential, as are blanks and duplicate titrations.
Molecular absorption spectrophotometry measures how much light of a given wavelength a solution absorbs. In the ultraviolet (about 190–400 nm) and visible (about 400–800 nm) regions, absorption corresponds to electronic transitions in molecules. Many analytes do not absorb strongly themselves, so colorimetric methods add reagents that react with the analyte to form a coloured compound whose absorbance is proportional to concentration. The Beer–Lambert law states that absorbance A = εbc, where ε is the molar absorptivity (L mol⁻¹ cm⁻¹), b the path length (cm, usually 1 cm) and c the concentration. Absorbance is related to transmittance T (the fraction of light transmitted) by A = −log10 T, so 40 per cent transmittance corresponds to A ≈ 0.40, and A = 1 means only 10 per cent of the light passes. The law holds best for dilute solutions and monochromatic light; deviations occur at high concentrations, with stray light and with chemical changes, so samples are diluted to stay within the linear range, typically absorbances below about 1. A spectrophotometer consists of a light source (deuterium lamp for UV, tungsten for visible), a monochromator (prism or grating) to select wavelength, a sample cell (quartz for UV, glass or plastic for visible), a detector (photodiode or photomultiplier) and readout. Measurements are made at the wavelength of maximum absorbance for best sensitivity. A calibration curve of absorbance against standard concentrations is prepared, and sample concentrations are read from it: with a calibration slope of 0.15 absorbance units per mg/L and a sample absorbance of 0.45 (after subtracting the blank), the concentration is 3.0 mg/L.
Many standard water and soil methods are colorimetric. Orthophosphate reacts with ammonium molybdate and antimony in acid, and ascorbic acid reduces the complex to molybdenum blue, measured at about 880 nm; it is used for water, soil-test phosphorus (Olsen and Bray extracts) and total phosphorus after digestion. Ammonia is measured by the phenate (indophenol blue) method, or historically with Nessler's reagent. Nitrate is measured by reduction to nitrite in a cadmium column followed by diazotisation to an azo dye, by direct UV absorbance near 220 nm with correction for organic matter, or by other methods; nitrite by diazotisation. Fluoride is measured by the SPADNS method, in which fluoride bleaches a zirconium–dye lake, or by ion-selective electrode. Hexavalent chromium reacts with diphenylcarbazide in acid to form a red-violet complex measured at about 540 nm. Iron is measured with 1,10-phenanthroline, and silica, sulphate (turbidimetric, as barium sulphate) and anionic surfactants (methylene blue active substances) have standard methods. In air, CPCB methods measure SO2 and NO2 colorimetrically after collection in absorbing solutions. Turbidimetry and nephelometry measure scattered rather than absorbed light: turbidity in NTU uses a nephelometer measuring light scattered at 90°. Fluorescence spectroscopy, which measures light emitted by excited molecules, is more sensitive and selective for fluorescent compounds such as chlorophyll a (used to estimate algal biomass), polycyclic aromatic hydrocarbons and dissolved organic matter. Spectrophotometric methods are inexpensive, robust and suitable for field kits and portable photometers, which is why they dominate routine monitoring in many laboratories. Their limitations include interferences from colour, turbidity and other substances, the need for reagent preparation and blanks, and detection limits higher than those of instrumental techniques such as ion chromatography for anions.
Atomic spectrometric methods measure elements, particularly metals and metalloids, by converting them to free atoms or ions and measuring their interaction with light. In atomic absorption spectrometry (AAS), a sample is atomised and light from a hollow cathode lamp, made of the element being measured and emitting its characteristic wavelengths, passes through the atom cloud; ground-state atoms of that element absorb the light in proportion to their concentration. Because each element needs its own lamp, AAS usually measures one element at a time. Flame AAS sprays the sample into an air–acetylene flame (or nitrous oxide–acetylene for refractory elements such as aluminium and silicon); it is robust and inexpensive, with detection limits typically in the range of tens to hundreds of µg/L. Graphite furnace (electrothermal) AAS places a small volume in a graphite tube heated in stages to dry, ash and atomise the sample, giving detection limits about 100 to 1000 times lower, in the µg/L or sub-µg/L range, at the cost of slower analysis and more matrix interference. Hydride generation AAS converts arsenic, selenium and antimony to volatile hydrides with sodium borohydride, improving sensitivity for these elements, important for groundwater arsenic. Cold vapour AAS measures mercury by reducing it to elemental mercury vapour at room temperature. Interferences include chemical interferences (formation of stable compounds in the flame, overcome with releasing agents such as lanthanum), ionisation and spectral or background absorption, corrected with deuterium lamp or Zeeman background correction.
Inductively coupled plasma (ICP) techniques use an argon plasma at about 6000–10,000 K to atomise and ionise the sample efficiently. In ICP optical emission spectrometry (ICP-OES, also called ICP-AES), excited atoms and ions emit light at characteristic wavelengths, and a spectrometer measures many elements simultaneously over a wide linear range, with detection limits typically in the low µg/L range. In ICP mass spectrometry (ICP-MS), ions from the plasma are separated by mass-to-charge ratio, usually in a quadrupole mass analyser, giving the lowest detection limits (often ng/L), simultaneous multi-element analysis and the ability to measure isotope ratios, such as lead isotopes for source tracing. ICP-MS is subject to polyatomic interferences (for example, argon chloride at mass 75 interfering with arsenic), overcome with collision/reaction cells or high-resolution instruments, and to matrix effects from high dissolved solids, which require dilution and internal standards. Most atomic methods require samples in solution. Waters are acidified and may be filtered (dissolved metals) or digested (total recoverable metals); soils, sediments, sludges and tissues are digested with acids, such as nitric acid with hydrogen peroxide, aqua regia, or hydrofluoric acid for total dissolution, often in closed-vessel microwave systems. Sequential extraction schemes estimate metal fractions (exchangeable, carbonate-bound, oxide-bound, organic, residual), and single extractants such as DTPA estimate plant-available micronutrients (zinc, iron, copper, manganese) in soil testing. X-ray fluorescence (XRF), including portable field instruments, measures elements in solid samples non-destructively and is useful for rapid screening of soil contamination, though its detection limits are higher. The choice among techniques depends on the elements, required detection limits, number of samples and budget: flame AAS for routine metals at moderate levels, graphite furnace or hydride AAS for specific trace elements, ICP-OES for many elements in many samples, and ICP-MS for ultra-trace work and isotopes.
Chromatography separates the components of a mixture as they are carried by a mobile phase past a stationary phase, to which different compounds are attracted to different extents. Each compound emerges (elutes) at a characteristic retention time, and the detector response (peak area) is proportional to the amount present. Separation quality is described by resolution, which depends on the column's efficiency (number of theoretical plates), selectivity and retention. Gas chromatography (GC) uses an inert carrier gas (helium, nitrogen or hydrogen) and a capillary column coated with a stationary phase, in an oven whose temperature is programmed to elute compounds of increasing boiling point. It suits volatile and semi-volatile, thermally stable compounds: solvents, hydrocarbons, PAHs, organochlorine and organophosphorus pesticides, PCBs and some greenhouse gases. Detectors include the flame ionisation detector (FID, sensitive to most organic compounds), the electron capture detector (ECD, extremely sensitive to halogenated compounds such as organochlorine pesticides and PCBs), the nitrogen–phosphorus detector (for organophosphates and nitrogen pesticides), the flame photometric detector (sulphur and phosphorus), the thermal conductivity detector (universal but less sensitive, used for permanent gases), and the mass spectrometer. GC with FID and ECD measures methane and nitrous oxide in greenhouse gas flux studies. High-performance liquid chromatography (HPLC) pumps a liquid mobile phase at high pressure through a column packed with fine particles; reversed-phase HPLC, with a non-polar stationary phase (such as C18) and polar mobile phase (water–methanol or acetonitrile), is most common. It suits non-volatile, polar or thermally unstable compounds: many modern pesticides and herbicides, pharmaceuticals, mycotoxins such as aflatoxins, and PAHs (with fluorescence detection). Detectors include UV and diode-array, fluorescence, and mass spectrometers. Ion chromatography separates ions on ion-exchange columns with conductivity detection and is the standard method for anions such as fluoride, chloride, nitrite, nitrate, phosphate and sulphate in a single run.
Mass spectrometry ionises molecules and separates the ions by mass-to-charge ratio, giving both identification (from the mass spectrum) and quantification. In GC-MS, electron ionisation fragments molecules into reproducible patterns that can be matched against spectral libraries. In LC-MS, softer ionisation methods such as electrospray ionisation keep molecules intact. Mass analysers include quadrupoles, ion traps, time-of-flight and Orbitrap instruments. Tandem mass spectrometry (MS/MS, for example a triple quadrupole) selects a precursor ion, fragments it and monitors a specific product ion, greatly improving selectivity and sensitivity; LC-MS/MS is now the workhorse for pesticide residues, pharmaceuticals, per- and polyfluoroalkyl substances (PFAS) and other emerging contaminants at ng/L levels. High-resolution mass spectrometry supports non-target screening for unknown contaminants. Organic analysis requires sample preparation to extract, clean up and concentrate analytes. Liquid–liquid extraction partitions analytes from water into an immiscible solvent; solid-phase extraction (SPE) passes water through a cartridge that retains analytes, which are then eluted with a small volume of solvent, achieving large concentration factors; Soxhlet and pressurised liquid extraction are used for soils and sediments; headspace and purge-and-trap methods transfer volatile compounds into the gas phase for GC; and QuEChERS (quick, easy, cheap, effective, rugged and safe), based on acetonitrile extraction with salts and dispersive SPE clean-up, is widely used for multi-residue pesticide analysis in food and soil. Derivatisation makes polar compounds volatile for GC. Quality control for chromatography includes calibration with standards over the working range, internal standards and surrogate compounds to correct for losses, isotopically labelled standards for isotope dilution in MS, blanks, matrix spikes and confirmation of identity using retention time and mass spectral ion ratios. Regulatory pesticide residue analysis compares results with maximum residue limits (MRLs), set nationally (in India by FSSAI) and internationally by Codex Alimentarius, as the pesticides subject explains.
Electrochemical methods measure electrical properties (potential, current or conductance) related to analyte concentration. Potentiometric methods measure the potential of an indicator electrode relative to a reference electrode (such as silver/silver chloride or calomel) at near-zero current. The glass pH electrode responds to hydrogen-ion activity across a thin glass membrane; its response follows the Nernst equation, about 59.16 mV per pH unit at 25 °C, so a change of 2 pH units shifts the potential by about 118 mV. pH meters are calibrated with at least two buffers bracketing the expected range (for example pH 4, 7 and 10) and corrected for temperature; the slope should be close to the theoretical value. Electrodes need careful storage, cleaning and replacement as they age. Ion-selective electrodes (ISEs) respond to specific ions: the fluoride electrode, with a lanthanum fluoride crystal membrane, is a standard method for fluoride in drinking water (with a total ionic strength adjustment buffer, TISAB, to fix ionic strength and pH and release complexed fluoride); others measure nitrate, ammonium, chloride, calcium and potassium, though selectivity and interferences limit their accuracy in complex matrices. The oxidation–reduction potential (ORP) electrode, usually platinum, indicates redox conditions, as the chemistry subject explains. Conductivity measures the ability of water to conduct electric current, which rises with dissolved ion concentration; it is reported in µS/cm or dS/m at 25 °C (instruments compensate for temperature, about 2 per cent per °C) and is used to estimate TDS and salinity of irrigation water and soil extracts.
Amperometric sensors measure the current produced when an analyte is oxidised or reduced at an electrode held at a fixed potential. The membrane (Clark-type) dissolved oxygen electrode is the classic example: oxygen diffuses through a gas-permeable membrane and is reduced at the cathode, producing a current proportional to its partial pressure. Optical (luminescent) DO sensors, which measure the quenching of a luminescent dye by oxygen, have largely replaced membrane electrodes for continuous monitoring because they need less maintenance and do not consume oxygen. Electrochemical gas sensors used in portable and low-cost air monitors work amperometrically for CO, NO2, SO2, O3 and H2S. Voltammetric methods scan the electrode potential and measure current. Anodic stripping voltammetry (ASV) first deposits trace metals such as lead, cadmium, copper and zinc onto an electrode by reduction, then strips them back into solution by scanning the potential positively; the stripping current peaks identify and quantify each metal at very low (µg/L or lower) concentrations, and the method can distinguish labile metal species relevant to bioavailability. Polarography with a dropping mercury electrode is historically important but largely replaced. Biosensors combine a biological recognition element (enzyme, antibody, nucleic acid or whole cell) with a physical transducer. Examples include enzyme-inhibition sensors for organophosphate and carbamate pesticides (based on acetylcholinesterase), immunosensors for specific contaminants, and whole-cell bioluminescent sensors for toxicity. Biosensors offer speed and portability for screening but generally need confirmation by laboratory methods. Electrochemical sensors are also used in multiparameter water quality sondes, which combine pH, conductivity, DO, ORP, temperature, turbidity, chlorophyll and nitrate sensors for in-situ and continuous measurement.
Isotopes are atoms of the same element with different numbers of neutrons. Stable isotope ratios vary slightly in nature because physical, chemical and biological processes discriminate between light and heavy isotopes (isotope fractionation), and these variations act as natural tracers. Ratios are measured by isotope ratio mass spectrometry or laser spectroscopy and expressed in delta notation relative to international standards: δ (per mil, ‰) = (Rsample/Rstandard − 1) × 1000, where R is the ratio of heavy to light isotope. Standards include Vienna Standard Mean Ocean Water (VSMOW) for hydrogen and oxygen, Vienna Pee Dee Belemnite (VPDB) for carbon, and atmospheric nitrogen for nitrogen. Positive δ values mean the sample is enriched in the heavy isotope relative to the standard. Applications are wide. In hydrology, δ18O and δ2H of water trace sources of groundwater recharge, evaporation and mixing; precipitation worldwide falls close to the global meteoric water line, δ2H = 8 δ18O + 10 (Craig, 1961), and evaporated waters plot below it with lower slopes. In nitrogen pollution studies, δ15N and δ18O of nitrate help distinguish sources: nitrate from manure and sewage is typically enriched in 15N compared with nitrate from synthetic fertiliser, and denitrification enriches both isotopes in the remaining nitrate, though source signatures overlap. Carbon isotopes (δ13C) distinguish C3 and C4 plant inputs to soil organic matter (C4 plants such as maize are less depleted in 13C), track fossil carbon in the atmosphere, and identify methane sources. In ecology, δ15N increases by about 3–4‰ per trophic level, so it indicates food-web position. Isotopes of lead (and of strontium and others) fingerprint sources of metals and the geographic origin of food and timber.
Radioactive isotopes provide clocks and tracers. Carbon-14, produced in the atmosphere and taken up by living things, decays with a half-life of about 5730 years; radiocarbon dating measures ages up to about 50,000 years, and the absence of 14C in fossil fuels lets 14C measurements separate fossil and modern carbon in atmospheric CO2 and in aerosols and biofuels. 'Bomb carbon' from atmospheric nuclear tests in the 1950s–1960s provides a time marker for recent decades. Tritium (3H, half-life about 12.3 years), also elevated by bomb tests, dates young groundwater, together with tritium–helium-3 methods; chlorofluorocarbons and sulphur hexafluoride are also used as groundwater age tracers. Sediment dating uses lead-210 (half-life about 22 years), from the decay of atmospheric radon, to date sediments deposited over the last 100–150 years, with caesium-137 peaks from the 1963 weapons-testing maximum and from accidents such as Chernobyl (1986) as independent markers; dated cores reconstruct histories of pollution such as metal or nutrient loading. Caesium-137 is also used to estimate soil erosion and deposition rates since the 1960s by comparing its inventory in a field with that at an undisturbed reference site. Radon-222 is used to locate groundwater discharge to rivers and coastal waters, because groundwater is rich in radon. Radioactive tracers can also be added deliberately in experiments, such as 15N-labelled fertiliser (a stable isotope) or 32P, to track nutrient fate, subject to radiation safety rules. Measuring radionuclides requires counting instruments: gamma spectrometry with high-purity germanium detectors for gamma emitters such as 137Cs, liquid scintillation counting for beta emitters such as tritium and 14C, alpha spectrometry, and accelerator mass spectrometry for very small 14C samples. Radioactive decay arithmetic, with activity halving each half-life, is explained in the chemistry subject.
Accuracy is the closeness of a result to the true value; precision is the closeness of repeated results to one another. A method can be precise but inaccurate (biased), or accurate on average but imprecise. Systematic errors (bias) arise from calibration errors, interferences, contamination or losses and shift all results in one direction; random errors scatter results around the mean. Precision is expressed as standard deviation or relative standard deviation (RSD, also called the coefficient of variation): RSD (%) = standard deviation ÷ mean × 100; replicate results with a mean of 10 mg/L and standard deviation of 0.5 mg/L have an RSD of 5 per cent. Repeatability refers to precision under the same conditions in a short time; reproducibility to precision across laboratories or over time. Accuracy is assessed with certified reference materials and spike recoveries. Recovery of a matrix spike is calculated as recovery (%) = (spiked sample result − unspiked sample result) ÷ amount added × 100: if a sample contains 8.4 µg/L, and after adding 10 µg/L it reads 18.2 µg/L, recovery = 9.8/10 × 100 = 98 per cent. Acceptable recovery ranges, often about 80–120 per cent for many trace analyses, are set by method and regulation. The limit of detection (LOD) is the lowest concentration that can be distinguished reliably from a blank, commonly estimated as three times the standard deviation of replicate blank (or low-level) measurements, converted to concentration with the calibration slope: LOD = 3σ/slope. The limit of quantification (LOQ) is the lowest concentration that can be measured with acceptable precision, often 10σ/slope. With a blank standard deviation of 0.002 absorbance units and a calibration slope of 0.05 absorbance units per mg/L, LOD = 0.006/0.05 = 0.12 mg/L and LOQ = 0.02/0.05 = 0.4 mg/L. The method detection limit (MDL) used by the US EPA is estimated from replicate spiked samples processed through the whole method. Results below the LOD are reported as 'not detected' with the limit stated, not as zero; the LOD must be well below the regulatory limit to be useful.
Most instrumental methods are calibrated by measuring standards of known concentration and fitting a calibration function, usually linear by least squares. Good practice uses at least five standards spanning the working range, a calibration blank, checks of linearity (the correlation coefficient alone, such as r² > 0.995, is not sufficient evidence; residuals should be examined), and independent check standards from a different source. Samples outside the calibrated range are diluted, not extrapolated. Calibration is verified during a run with continuing calibration verification standards. Matrix effects, where other components of the sample enhance or suppress the signal, are addressed by matrix-matched standards, internal standards (a known amount of a similar compound added to every sample and standard, correcting for instrument drift and losses), or the method of standard additions, in which known amounts of analyte are added to portions of the sample itself and the original concentration is found by extrapolating the response line to zero signal. Method validation demonstrates that a method is fit for purpose by establishing its selectivity, linearity and range, LOD and LOQ, accuracy (trueness), precision (repeatability and reproducibility), robustness and measurement uncertainty. Measurement uncertainty, estimated following the Guide to the Expression of Uncertainty in Measurement (GUM), combines all significant sources of error into a stated interval, such as 25 ± 2 mg/L at 95 per cent confidence; ISO/IEC 17025 requires accredited laboratories to estimate it. When comparing a result with a limit, the uncertainty determines whether exceedance can be stated with confidence. Data handling also matters: reporting appropriate significant figures, identifying outliers with defined statistical tests (such as Grubbs' test) rather than discarding inconvenient data, and treating censored values below detection limits appropriately in statistical summaries.
Quality assurance (QA) is the overall management system that ensures data are of known and documented quality fit for their purpose; quality control (QC) is the set of operational techniques and checks used within it. A QA programme includes a quality manual, standard operating procedures (SOPs) for every method, staff training and competence records, sample management and chain of custody, equipment calibration and maintenance, document and record control, data review, corrective actions, internal audits and management review. QC samples detect specific problems. Blanks detect contamination: field blanks (analyte-free water taken through field handling), trip blanks (for volatile compounds, travelling with samples), and method or reagent blanks (taken through the laboratory procedure). Duplicates assess precision: field duplicates reflect sampling plus analysis variability, and laboratory duplicates analytical variability, often expressed as relative percent difference (RPD) = |x1 − x2| ÷ mean × 100. Matrix spikes and spike duplicates assess recovery and matrix effects. Certified reference materials (CRMs), with traceable certified values in a matrix similar to samples (such as river sediment or plant tissue), check accuracy. Calibration verification and instrument checks confirm stability during analytical runs. Control charts track QC results over time. A Shewhart chart plots a control sample's results against its mean, with warning limits at ±2 standard deviations and action (control) limits at ±3 standard deviations. A result outside the action limits, or patterns such as several consecutive points on one side of the mean or a steady trend, signals that the method is out of control; analysis stops, the cause is found and corrected, and affected samples are reanalysed.
Laboratory accreditation provides independent recognition of technical competence. The international standard ISO/IEC 17025 (General requirements for the competence of testing and calibration laboratories, current edition 2017) covers management and technical requirements, including impartiality, method validation, measurement uncertainty, metrological traceability, proficiency testing and reporting. In India, the National Accreditation Board for Testing and Calibration Laboratories (NABL) accredits laboratories to ISO/IEC 17025, and the Ministry of Environment, Forest and Climate Change recognises environmental laboratories under the Environment (Protection) Act, 1986, for analyses with legal standing; other countries have their own accreditation bodies linked through the International Laboratory Accreditation Cooperation. Proficiency testing (inter-laboratory comparison) sends identical samples to many laboratories and compares their results with the assigned value, often using z-scores: z = (laboratory result − assigned value) ÷ standard deviation for proficiency assessment, with |z| ≤ 2 satisfactory, 2–3 questionable and > 3 unsatisfactory. Regular participation reveals biases a laboratory cannot see on its own. Good Laboratory Practice (GLP), an OECD system, governs non-clinical safety studies, such as pesticide registration tests, and focuses on study organisation and record-keeping. Data validation and verification review results before release: checking holding times and preservation, QC results against acceptance criteria, calculations and transcription, and flagging data that do not meet requirements. Data management systems store results with metadata so that they can be traced back to samples, methods and QC. Well-documented data can be shared and reused; poorly documented data often cannot be defended or compared, however carefully they were measured.
Continuous monitoring provides high-resolution time series for real-time public information, early warning and compliance. Continuous ambient air quality monitoring stations use automated analysers based on reference or equivalent methods: beta attenuation monitors (BAM), which measure the attenuation of beta radiation by particles collected on a filter tape, and tapered element oscillating microbalances (TEOM) for PM10 and PM2.5; chemiluminescence analysers for NO and NO2 (NO reacts with ozone to emit light; NO2 is first converted to NO); UV fluorescence for SO2; UV photometry for ozone (absorption at 254 nm); non-dispersive infrared (NDIR) for CO and CO2; and gas chromatographs for benzene, toluene and xylene. Data are transmitted to central servers and used to calculate the AQI. Continuous emission monitoring systems (CEMS) measure pollutants and flow in stacks continuously. In India, the Central Pollution Control Board directed 17 categories of highly polluting industries to install online continuous emission and effluent monitoring systems connected to regulators from 2014 onwards, enabling real-time compliance checks, though data quality depends on calibration, maintenance and audit. Continuous water quality monitoring uses multiparameter sondes and online analysers at river stations, intakes and treatment plants for pH, conductivity, DO, turbidity, temperature, ammonium, nitrate, chlorophyll and organic matter indicators such as UV absorbance at 254 nm. Automatic weather stations provide the meteorological context, as the agrometeorology subject explains.
Low-cost sensors, costing a small fraction of regulatory analysers, have expanded monitoring to many more locations, including villages, schools and homes. Optical particle sensors count and size particles by light scattering and estimate PM mass; electrochemical sensors measure gases such as NO2, O3, CO and SO2; and metal-oxide sensors respond to many gases. They have important limitations: optical PM sensors overestimate mass at high humidity because particles absorb water and swell, they do not measure particle density or composition, and their response varies with aerosol type; electrochemical gas sensors drift, respond to interfering gases (cross-sensitivity, such as ozone affecting NO2 sensors) and are sensitive to temperature and humidity. Calibration by collocation with reference instruments, correction models (including machine learning), regular recalibration and quality checks are essential, and data should be presented as indicative rather than regulatory. Evaluations by agencies such as the US EPA and South Coast AQMD publish performance targets for sensors. Satellite remote sensing complements ground networks over large areas. Instruments such as TROPOMI on Sentinel-5P measure tropospheric columns of NO2, SO2, CO, methane and formaldehyde daily at kilometre-scale resolution, revealing emission hotspots and trends; MODIS and other sensors measure aerosol optical depth, which can be related to surface PM2.5 with models and ground data; fire detection products track crop residue burning; and multispectral sensors map water turbidity, chlorophyll and algal blooms. Satellite data are column or surface-reflectance measurements, not ground-level concentrations, so they need validation. Combining dense low-cost networks, reference stations, satellites and models through data fusion provides better spatial coverage than any one source. Citizen science projects using low-cost sensors raise awareness and fill data gaps, provided data quality is communicated transparently. Internet of Things platforms, open data portals and dashboards make data accessible, but data governance, maintenance funding and long-term continuity are common weaknesses of sensor networks.
Bioindicators are organisms or communities whose presence, abundance, condition or behaviour reveals environmental conditions, integrating effects over time in ways that chemical snapshots cannot. Lichens are classic air quality indicators: many species are sensitive to sulphur dioxide and nitrogen compounds, so lichen diversity and the index of atmospheric purity (IAP), based on the number, frequency and sensitivity of lichen species on trees, map air pollution; nitrogen-tolerant species increase near ammonia sources such as livestock farms. Mosses accumulate metals from the air and are used in moss surveys and moss bags to map metal deposition. Tobacco cultivar Bel-W3 shows visible injury from ozone and has been used as a bioindicator plant. In fresh waters, benthic macroinvertebrates are the most widely used indicators because they are relatively sedentary, live for months to years and differ in pollution tolerance. Mayflies (Ephemeroptera), stoneflies (Plecoptera) and caddisflies (Trichoptera) are generally sensitive, while chironomid midge larvae, oligochaete worms (Tubifex) and leeches tolerate organic pollution and low oxygen. Indices include the EPT index (the number or proportion of EPT taxa: for example, 45 EPT individuals in a sample of 120 gives 37.5 per cent), biotic scores such as the Biological Monitoring Working Party (BMWP) score and average score per taxon, the saprobic system developed in Central Europe, and diversity indices. Algae are also used: diatom indices for nutrient and organic pollution, and the Palmer algal pollution index, which scores genera tolerant of organic pollution. Fish communities are assessed with the index of biotic integrity (IBI), developed by James Karr (1981), which combines measures of species richness, composition, trophic structure and condition. In India, CPCB has used biological water quality criteria based on saprobic and diversity scores alongside chemical classes.
Biomonitoring also measures contaminants in organisms (bioaccumulation monitoring). Filter-feeding bivalves such as mussels and oysters accumulate metals and organic contaminants and are used in programmes such as the US 'Mussel Watch' to track coastal pollution trends; fish tissues, bird eggs, human blood and urine and plant leaves serve similar purposes. Bioaccumulation reveals biologically available contamination even when water concentrations are below detection. Toxicity tests (bioassays) expose standard organisms to samples or chemicals under controlled conditions. Acute tests measure the median lethal concentration (LC50, the concentration killing 50 per cent of test organisms in a set time such as 48 or 96 hours) or the median effective concentration (EC50) for sublethal effects; chronic tests measure effects on growth or reproduction and give no-observed-effect concentrations (NOEC). Standard organisms include the water flea Daphnia, fish such as fathead minnow or zebrafish, algae, and luminescent bacteria (Aliivibrio fischeri, formerly Vibrio fischeri), whose light output falls with toxicity in rapid screening tests. Whole effluent toxicity testing evaluates complex discharges whose individual components are unknown. Toxicity is expressed as toxic units, TU = 100/LC50 (with LC50 as per cent effluent), so an effluent with LC50 of 25 per cent has 4 TU; higher TU means more toxic. Biomarkers are measurable biochemical, physiological or histological responses indicating exposure to or effects of contaminants: inhibition of acetylcholinesterase in fish or invertebrates indicates exposure to organophosphate and carbamate pesticides; induction of the EROD enzyme activity (cytochrome P450 1A) indicates exposure to PAHs, PCBs and dioxins; metallothionein proteins rise with metal exposure; vitellogenin in male fish indicates endocrine-disrupting oestrogenic compounds; and DNA damage can be measured with the comet assay. Biomarkers give early warning before population effects appear, but they must be interpreted with knowledge of natural variability. Combining chemical, toxicological and ecological evidence, an approach sometimes called the 'triad', gives the most reliable assessment of environmental quality, especially for sediments.
A student uses this subject to understand what lies behind every number in other courses and to answer exam questions on instrument principles (which detector for which compound, which AAS mode for which element), sampling and preservation rules, and QA terms. The numerical core includes sample size estimation, TSS and hardness calculations, absorbance from transmittance and concentration from calibration, δ values, LOD and LOQ, spike recovery, RSD and RPD, z-scores, EPT percentages and toxic units.
A five-year practitioner, such as an analyst in an accredited environmental laboratory, a pollution control board field officer or a monitoring consultant, uses the subject to design sampling plans, collect and preserve samples correctly, run and calibrate instruments, maintain control charts, take part in proficiency tests, validate methods, calibrate low-cost sensor networks against reference monitors, and report results with detection limits and uncertainty that will stand up in an audit or court.
A senior professional with 25 years' experience, such as a laboratory director, a head of a state monitoring network or a regulator's chief scientist, uses the subject to plan networks that answer policy questions, choose technologies that balance cost and quality, oversee accreditation and data integrity, integrate continuous, sensor and satellite data, and decide when data are good enough to support enforcement or public health action.
A researcher with a PhD and 25 years' experience develops analytical methods for emerging contaminants such as pharmaceuticals and PFAS, isotope and receptor-model approaches to source tracing, sensor calibration and data fusion methods, and integrated biomonitoring frameworks combining chemistry, toxicology and ecology. Such a researcher trains analysts and advises on national monitoring standards.
Methods are described by principle; holding times, preservation and detection limits are typical values from standard compendia and vary by method, matrix and regulation, so the cited method must be followed in practice. National monitoring network sizes and programme details change and are described qualitatively. Specific instrument models and vendors are not named. Advanced topics such as chemometrics, non-target screening workflows, remote sensing retrieval algorithms and environmental DNA (eDNA) monitoring are mentioned only briefly. Biotic index scoring tables (BMWP, saprobic values) are region-specific and are not reproduced.
Original practice questions written to the syllabus and pattern of the exams named in each question's examTags. They are not past papers and do not reproduce any board's questions.
These notes are free to read here, with no account.
What the Laisha app adds:
Free for ten days, then a subscription. We would rather write that here than have you find it later.