The gas chromatographic detector best suited to trace organochlorine pesticides and PCBs is the

flame photometric detector for sodium

Atomic absorption spectrophotometry measures

the absorption, by ground-state atoms, of light from a hollow-cathode lamp of the element being determined

the diffraction of X-rays by crystal planes

the light emitted by atoms excited in a flame

the mass-to-charge ratio of ions formed in a plasma

the absorption, by ground-state atoms, of light from a hollow-cathode lamp of the element being determined

Which of the following techniques determine the elemental composition of an environmental sample? Select all that apply.

Atomic absorption spectrophotometry

Assertion (A): HPLC rather than GC is preferred for non-volatile or heat-sensitive compounds. Reason (R): In GC the sample must be vaporised and carried through a heated column by a gas. Choose the correct option.

Both A and R are true, and R is the correct explanation of A

Both A and R are true, but R is not the correct explanation of A

The most suitable technique for determining trace mercury in drinking water is

cold-vapour atomic absorption spectrophotometry

Which statements about electron microscopy and infrared spectroscopy are correct? Select all that apply.

SEM images the surface morphology of particles using scanned electrons

TEM needs an ultra-thin specimen because the electrons must pass through it

SEM cannot give any information about elemental composition

FTIR measures electronic transitions of atoms in a flame

SEM images the surface morphology of particles using scanned electrons

TEM needs an ultra-thin specimen because the electrons must pass through it

Environmental Chemistry IV: The Principles of Analytical Methods — Titrimetry, Gravimetry, Bomb Calorimetry, Chromatography (Paper, TLC, GC, HPLC), Flame Photometry, Spectrophotometry (UV-VIS, AAS, ICP-AES, ICP-MS), Electrophoresis, XRF, XRD, NMR, FTIR, GC-MS, SEM and TEM

Unit II ends with a list of instruments, and the examiner asks the same three things of each: what physical principle it uses, what it measures, and which environmental analyte it is the method of choice for. This chapter answers those questions for every technique the syllabus names, grouped by principle: the classical wet methods of titrimetry, gravimetry and bomb calorimetry; the separation methods of chromatography; the atomic and molecular spectroscopies from flame photometry to ICP-MS, all resting on the Beer-Lambert law or on emission; and the structural and imaging methods of electrophoresis, X-ray fluorescence and diffraction, NMR, FTIR, GC-MS and electron microscopy. Hardness by EDTA, the Rf value, the Beer-Lambert law, a calorimeter calculation and Bragg's law are worked.

1. Titrimetry, gravimetry and bomb calorimetry

Titrimetry determines an analyte by measuring the volume of a standard reagent that reacts with it completely, the end point being shown by an indicator or an electrode. Four reaction types cover most environmental titrations. Acid-base titrations give alkalinity and acidity. Complexometric titration with EDTA at pH 10, with Eriochrome Black T as indicator (wine-red to blue), gives total hardness; with 0.01 M EDTA, 1 mL is equivalent to 1 mg of CaCO₃, so if a 50 mL sample needs 10 mL, hardness = 10 × 1000 ÷ 50 = 200 mg/L as CaCO₃. Redox titrations include the Winkler method for DO (iodine liberated by the fixed oxygen titrated with thiosulphate) and the back-titration of excess dichromate with ferrous ammonium sulphate in COD. Precipitation titration includes Mohr's method for chloride, titrating with silver nitrate using potassium chromate, which gives a red-brown silver chromate at the end point.

Gravimetry determines an analyte by weighing: either the residue left after drying or ignition, or a pure precipitate of known composition. Total suspended solids are the residue retained on a glass-fibre filter dried at 103 to 105 °C; total dissolved solids the residue of the filtrate evaporated and dried (at 180 °C in standard methods); volatile solids the loss on ignition at 550 °C, a measure of organic content. Sulphate can be precipitated and weighed as barium sulphate, and airborne particulate matter is measured by weighing a filter before and after sampling a known volume of air. Gravimetry is slow but needs no calibration, which makes it a primary method.

A bomb calorimeter measures the heat of combustion of a fuel, biomass or solid waste. A weighed sample is burnt in pure oxygen at about 25 to 30 atm inside a sealed steel "bomb" immersed in a known mass of water, and the temperature rise is recorded. Because the volume is constant, the heat measured is the change in internal energy. The calorimeter is first calibrated with benzoic acid, whose heat of combustion is about 26.45 kJ/g, to find its heat capacity; then heat released = heat capacity × temperature rise. If the calorimeter's heat capacity is 10 kJ/°C and burning 1.0 g of refuse-derived fuel raises the temperature by 2.5 °C, the fuel releases 25 kJ/g. The bomb gives the gross calorific value, because the water formed condenses; the net value subtracts the latent heat of that water.

2. Chromatography: paper, TLC, GC and HPLC

Every chromatographic method separates the components of a mixture by their different distribution between a stationary phase and a mobile phase that moves past it. A component that spends more time in the stationary phase moves more slowly. In paper chromatography the stationary phase is water held in the cellulose fibres and separation is mainly by partition; in thin-layer chromatography (TLC) it is a thin layer of silica gel or alumina on a plate and separation is mainly by adsorption. In both, a spot is identified by its retardation factor, Rf = distance travelled by the component ÷ distance travelled by the solvent front, which always lies between 0 and 1; a spot that moves 3.0 cm while the solvent front moves 7.5 cm has Rf = 0.4.

Technique	Mobile phase and principle	Environmental use
Gas chromatography (GC)	An inert carrier gas (helium, nitrogen) through a heated capillary column; separation by volatility and partition into a liquid film	Volatile and semi-volatile organics: VOCs, pesticides, PCBs, hydrocarbons
High-performance liquid chromatography (HPLC)	A liquid pumped at high pressure through a packed column, often reverse-phase C18	Non-volatile or heat-sensitive compounds: PAHs, many pesticides and herbicides, pharmaceuticals, dyes
Ion chromatography	An aqueous eluent through an ion-exchange column with a conductivity detector	Anions such as fluoride, chloride, nitrate, sulphate and phosphate in one run

In GC and HPLC a compound is identified by its retention time, the time it takes to reach the detector, and quantified by the area of its peak against standards. The detector is chosen for the analyte. In GC the flame ionisation detector responds to almost all organic compounds and is used for hydrocarbons; the electron capture detector is extremely sensitive to halogenated compounds and is the classic detector for organochlorine pesticides and PCBs; the thermal conductivity detector is universal but insensitive. HPLC commonly uses ultraviolet absorbance or fluorescence, the latter very sensitive for PAHs. Retention time alone cannot prove identity, because two compounds can co-elute; that is what mass spectrometry adds.

3. Flame photometry and spectrophotometry: UV-VIS, AAS, ICP-AES and ICP-MS

Absorption spectrophotometry rests on the Beer-Lambert law: absorbance A = log(I₀ ÷ I) = εbc, where ε is the molar absorptivity, b the path length and c the concentration. Absorbance is therefore proportional to concentration, and an unknown is read from a calibration line of standards: if a 2.0 mg/L standard gives A = 0.40 and a sample gives A = 0.30, the sample contains 2.0 × 0.30 ÷ 0.40 = 1.5 mg/L. Transmittance and absorbance are not proportional: T = 10% corresponds to A = 1 and T = 1% to A = 2. In UV-visible spectrophotometry, many analytes are first converted to coloured compounds: phosphate to molybdenum blue, ammonia by the phenate or Nessler methods, nitrite by diazotisation to an azo dye, and fluoride by the SPADNS method.

Technique	Principle	Typical environmental use
Flame photometry	Atoms excited in a flame emit light at characteristic wavelengths (sodium 589 nm, potassium 766 nm); emission intensity is proportional to concentration	Sodium, potassium, lithium and calcium in water and soil extracts
Atomic absorption spectrophotometry (AAS)	Ground-state atoms in a flame or graphite furnace absorb light from a hollow-cathode lamp of the same element	Individual heavy metals; the graphite furnace reaches lower limits than the flame; cold-vapour AAS for mercury and hydride generation for arsenic and selenium
ICP-AES (also called ICP-OES)	An argon plasma at about 6,000 to 10,000 K excites atoms, which emit at many wavelengths at once	Many elements simultaneously at mg/L to µg/L levels
ICP-MS	The plasma ionises the atoms, and a mass spectrometer separates the ions by mass-to-charge ratio	Ultra-trace metals down to ng/L, and isotope ratios for source tracing

⚠️ Emission or absorption?

Flame photometry and ICP-AES measure light emitted by excited atoms; AAS measures light absorbed by atoms in the ground state. ICP-MS measures neither: it counts ions by mass. An option that describes AAS as "measuring the light emitted by the flame" has confused it with flame photometry.

4. Electrophoresis, XRF, XRD, NMR, FTIR, GC-MS, SEM and TEM

Technique	Principle	What it tells the environmental scientist
Electrophoresis	Charged molecules migrate through a gel or buffer in an electric field at rates set by charge and size	Separation of proteins and DNA fragments: microbial community profiling, biomarkers of exposure
X-ray fluorescence (XRF)	Primary X-rays eject inner-shell electrons, and the atoms emit secondary X-rays whose energies identify the elements	Non-destructive elemental composition of soils, sediments, filters and solid waste, including portable field units
X-ray diffraction (XRD)	X-rays are diffracted by the regular planes of a crystal according to Bragg's law, nλ = 2d sin θ	Identification of crystalline phases: clay minerals, asbestos, the minerals in fly ash and dust
Nuclear magnetic resonance (NMR)	Nuclei with spin, such as ¹H and ¹³C, in a strong magnetic field absorb radio waves at frequencies set by their chemical environment	Molecular structure of organic pollutants and of humic substances
Fourier-transform infrared spectroscopy (FTIR)	Molecules absorb infrared radiation at frequencies of their bond vibrations; an interferometer and a Fourier transform give the spectrum	Functional groups and identity of organic compounds, and the polymer type of microplastics
GC-MS	GC separates the mixture; the mass spectrometer fragments each compound and records its mass spectrum	Confirmed identification and quantification of pesticides, dioxins, PAHs and VOCs; the reference method for trace organics
Scanning electron microscopy (SEM)	A focused electron beam scans the surface and secondary electrons form the image; with energy-dispersive X-ray analysis it also gives elemental composition	Surface morphology of particles, fibres, fly ash spheres and biofilms
Transmission electron microscopy (TEM)	Electrons pass through an ultra-thin specimen, giving much higher resolution of internal structure	Nanoparticles, asbestos fibre identification, viruses and the internal structure of cells

Bragg's law is the one formula in this list that is set as a number. For first-order diffraction (n = 1) of copper Kα radiation (λ = 0.154 nm) from planes 0.154 nm apart, sin θ = λ ÷ 2d = 0.5 and θ = 30°. In practice the diffraction angles measured give the d-spacings, and the set of d-spacings is matched to reference patterns to name the mineral: kaolinite's basal spacing of about 0.7 nm distinguishes it from illite's 1.0 nm and from swelling smectite, whose spacing grows when it takes up water or glycol.

🧠 Which method for which question

"How much of which element?" AAS, ICP-AES, ICP-MS or XRF. "Which mineral or crystal?" XRD. "Which organic compound, confirmed?" GC-MS. "Which functional groups or which plastic?" FTIR. "What does the particle look like?" SEM for the surface, TEM for the inside.

Key takeaways

Titrimetry covers alkalinity (acid-base), hardness (EDTA with Eriochrome Black T), DO and COD (redox) and chloride (Mohr's method); with 0.01 M EDTA, 1 mL equals 1 mg of CaCO₃.
Gravimetry weighs residues and precipitates and needs no calibration; the bomb calorimeter, calibrated with benzoic acid, gives the gross calorific value of fuels and wastes.
Chromatography separates by partition or adsorption between phases; Rf identifies spots on paper and TLC, retention time identifies peaks in GC and HPLC, and the ECD suits halogenated pesticides.
A = εbc underlies UV-VIS; flame photometry and ICP-AES measure emission, AAS absorption by ground-state atoms, and ICP-MS counts ions by mass for ultra-trace metals.
XRF gives elements and XRD crystalline minerals (nλ = 2d sin θ); NMR and FTIR give organic structure, GC-MS confirms trace organics, and SEM and TEM image surfaces and interiors.

Practice questions (10)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

A 50 mL water sample needs 10 mL of 0.01 M EDTA to reach the Eriochrome Black T end point at pH 10. What is its total hardness in mg/L as CaCO₃?
Numerical answer — type the value.
Show answer
Answer: 200
With 0.01 M EDTA, 1 mL reacts with 0.01 mmol of Ca²⁺ plus Mg²⁺, equivalent to 1 mg of CaCO₃ (molar mass 100). So 10 mL is 10 mg of CaCO₃ in 50 mL, which is 10 × 1000 ÷ 50 = 200 mg/L, a hard water.
In a spectrophotometric determination obeying the Beer-Lambert law, a 2.0 mg/L standard gives an absorbance of 0.40 and a sample gives 0.30 in the same cell. What is the sample concentration in mg/L, to one decimal place?
Numerical answer — type the value.
Show answer
Answer: 1.5
Absorbance is proportional to concentration at fixed path length, so c = 2.0 × 0.30 ÷ 0.40 = 1.5 mg/L. The proportionality holds for absorbance, not for transmittance, which falls logarithmically with concentration.
The gas chromatographic detector best suited to trace organochlorine pesticides and PCBs is the
1. flame photometric detector for sodium
2. refractive index detector
3. thermal conductivity detector
4. electron capture detector
Show answer
Answer: D — electron capture detector
The electron capture detector is extremely sensitive to electronegative halogen atoms, which is why it became the standard for chlorinated pesticides and PCBs. The thermal conductivity detector is universal but insensitive, and a refractive index detector belongs to liquid chromatography.
Atomic absorption spectrophotometry measures
1. the diffraction of X-rays by crystal planes
2. the light emitted by atoms excited in a flame
3. the mass-to-charge ratio of ions formed in a plasma
4. the absorption, by ground-state atoms, of light from a hollow-cathode lamp of the element being determined
Show answer
Answer: D — the absorption, by ground-state atoms, of light from a hollow-cathode lamp of the element being determined
In AAS the atomised sample absorbs the element's own resonance lines from the hollow-cathode lamp, and the absorbance measures concentration. Emission describes flame photometry and ICP-AES, mass-to-charge ICP-MS, and diffraction XRD.
Which of the following techniques determine the elemental composition of an environmental sample? Select all that apply.
1. X-ray fluorescence
2. Atomic absorption spectrophotometry
3. Nuclear magnetic resonance
4. ICP-MS
Show answer
Answer: A — X-ray fluorescence; B — Atomic absorption spectrophotometry; D — ICP-MS
ICP-MS, XRF and AAS all measure elements: by ion mass, by characteristic secondary X-rays and by atomic absorption respectively. NMR probes the chemical environment of nuclei such as ¹H and ¹³C in molecules, so it gives molecular structure rather than elemental composition.
Assertion (A): HPLC rather than GC is preferred for non-volatile or heat-sensitive compounds. Reason (R): In GC the sample must be vaporised and carried through a heated column by a gas. Choose the correct option.
1. Both A and R are true, and R is the correct explanation of A
2. Both A and R are true, but R is not the correct explanation of A
3. A is true, but R is false
4. A is false, but R is true
Show answer
Answer: A — Both A and R are true, and R is the correct explanation of A
Both are true and R explains A. A compound that will not vaporise, or that decomposes when heated, cannot be carried through a GC column intact, whereas HPLC separates it in a liquid at or near room temperature, which is why PAHs, many herbicides and pharmaceuticals are run by HPLC.
On a TLC plate a pesticide spot moves 3.0 cm while the solvent front moves 7.5 cm from the origin. Its Rf value is
1. 0.60
2. 0.40
3. 4.50
4. 2.50
Show answer
Answer: B — 0.40
Rf = distance moved by the spot ÷ distance moved by the solvent front = 3.0 ÷ 7.5 = 0.40. Rf can never exceed 1, so 2.50, the inverted ratio, is impossible, and 4.50 is the difference of the two distances.
In X-ray diffraction, first-order reflection (n = 1) of radiation of wavelength 0.154 nm occurs from crystal planes 0.154 nm apart. What is the Bragg angle θ in degrees?
Numerical answer — type the value.
Show answer
Answer: 30
Bragg's law nλ = 2d sin θ gives sin θ = 1 × 0.154 ÷ (2 × 0.154) = 0.5, so θ = 30°. The measured diffraction angle on an instrument is usually 2θ, here 60°, which is why reading the instrument's axis directly as θ doubles the answer.
The most suitable technique for determining trace mercury in drinking water is
1. gravimetry after precipitation
2. cold-vapour atomic absorption spectrophotometry
3. flame photometry
4. paper chromatography
Show answer
Answer: B — cold-vapour atomic absorption spectrophotometry
Mercury is reduced to elemental mercury vapour, which is swept into an absorption cell at room temperature, giving detection at very low levels; ICP-MS is the other modern choice. Flame photometry suits alkali metals, and gravimetry and paper chromatography are far too insensitive for µg/L concentrations.
Which statements about electron microscopy and infrared spectroscopy are correct? Select all that apply.
1. SEM cannot give any information about elemental composition
2. FTIR measures electronic transitions of atoms in a flame
3. SEM images the surface morphology of particles using scanned electrons
4. TEM needs an ultra-thin specimen because the electrons must pass through it
Show answer
Answer: C — SEM images the surface morphology of particles using scanned electrons; D — TEM needs an ultra-thin specimen because the electrons must pass through it
SEM builds an image of the surface and TEM of the interior of a thin section. FTIR measures molecular bond vibrations in the infrared, not atomic electronic transitions, and an SEM fitted with energy-dispersive X-ray analysis does report elemental composition.