Analytical Chemistry
Analytical chemistry is the science of obtaining, processing and interpreting information about the composition and structure of matter. It answers two basic questions: What is in this sample? (qualitative) and How much is there? (quantitative). Modern instrumental methods have transformed analytical chemistry from a wet-bench craft into a high-throughput, automated discipline central to industry, medicine and the environment.
Accuracy is the closeness of a measurement to the true value; precision is the closeness of repeated measurements to one another. A method can be precise without being accurate (systematic error), and vice-versa (random error around a true mean).
Sampling and the analytical workflow
A useful analysis follows a chain of steps, each with its own error budget:
- Define the problem — analyte, matrix, required precision.
- Collect a representative sample — often the largest source of error.
- Prepare the sample — dissolution, extraction, derivatisation.
- Apply the analytical method — gravimetric, volumetric, instrumental.
- Calibrate — external standards, internal standards, standard additions.
- Process data — statistics, error analysis, reporting.
Classical (wet) methods
Gravimetric analysis
The analyte is converted into a pure precipitate of known stoichiometry, filtered, dried and weighed. Example: Ag⁺ as AgCl(s) for determining chloride. Despite their age, gravimetric methods remain the most accurate analytical techniques for many ions.
Volumetric (titrimetric) analysis
A solution of known concentration (the titrant) is added until reaction with the analyte is complete, signalled by an indicator or instrumental detector. Major categories:
| Type | Reaction | Indicator example |
|---|---|---|
| Acid–base | H⁺ + OH⁻ → H₂O | Phenolphthalein, methyl orange |
| Redox | e.g., MnO₄⁻ / Fe²⁺ | KMnO₄ self-indicating |
| Precipitation | Ag⁺ + Cl⁻ → AgCl | K₂CrO₄ (Mohr's method) |
| Complexometric | EDTA + M²⁺ | Eriochrome Black T |
Concentration relations: C₁V₁ = C₂V₂ (one-to-one stoichiometry); generalise via the balanced equation.
Instrumental methods
Spectroscopy
- UV-visible absorption: Beer–Lambert law A = εcl, where A is absorbance, ε molar absorptivity, c concentration, l path length.
- Atomic absorption spectroscopy (AAS): atomised sample absorbs light from a hollow-cathode lamp of the same element. Sub-ppm sensitivity for many metals.
- Infrared (IR): identifies functional groups by vibrational transitions.
- Nuclear magnetic resonance (NMR): probes the magnetic environment of nuclei; structural elucidation in organic and biochemistry.
- Mass spectrometry (MS): molecular weight + fragmentation; coupled to GC or LC for complex mixtures.
Electrochemistry
- Potentiometry: ion-selective electrodes (e.g., glass pH electrode) measure activity through the Nernst equation.
- Conductometry: measures total ion concentration.
- Voltammetry / polarography: measures current as a function of applied potential; trace-metal analysis.
Chromatography
A separation technique exploiting differential partitioning between a stationary phase and a mobile phase:
| Method | Mobile phase | Stationary phase | Typical use |
|---|---|---|---|
| GC | Inert gas | Liquid on capillary | Volatile organics |
| HPLC | Liquid | Packed silica (C-18) | Drugs, pesticides |
| TLC | Liquid | Silica on plate | Quick qualitative |
| Ion-exchange | Aqueous | Charged resin | Cations/anions |
| Size-exclusion | Aqueous | Porous gel | Proteins, polymers |
Detectors include flame ionisation (FID), thermal conductivity (TCD), UV-Vis, electrochemical, and mass spectrometers.
- Limit of detection (LOD): the smallest analyte concentration distinguishable from a blank with stated confidence (often 3σ above blank).
- Linear dynamic range: concentrations over which detector response is proportional to amount.
- Selectivity: ability to discriminate analyte from interferents.
- Robustness: insensitivity to small method changes.
Statistical evaluation of data
Analytical results are reported with their uncertainty. Key statistics:
- Mean x̄ = (Σxᵢ)/n.
- Standard deviation s = √[Σ(xᵢ − x̄)²/(n − 1)].
- Relative standard deviation RSD = s / x̄ (×100 for %).
- Confidence interval μ = x̄ ± t · s/√n.
- Q-test for rejecting outliers; t-test for comparing means; F-test for comparing variances.
A simple rule: report results to the same number of significant figures as your absolute uncertainty allows. Reporting 23.4567 g when your balance is good to 0.01 g misleads readers into a false sense of precision.
Quality assurance
Reliable analysis requires:
- Calibration with certified reference materials traceable to SI units.
- Blank corrections to subtract contamination from reagents and apparatus.
- Standard addition when the sample matrix interferes.
- Internal standards to correct for sample loss during preparation.
- Inter-laboratory comparisons and accreditation (ISO 17025).
Analytical chemistry underpins drug safety (HPLC of impurities), forensic science (DNA, drug residues), environmental monitoring (pesticides, heavy metals, air quality), food safety (mycotoxins, adulteration) and clinical diagnosis (glucose, cholesterol, blood gases). Every quantitative claim in chemistry ultimately rests on the analytical chemist's apparatus and statistics.