Engineer Chemical Analysis cheat sheet

SI base units for analytical results: amount of substance is the mole (mol); mass is the kilogram (kg); the mole contains exactly 6.02214076×10^23 elementary entities (Avogadro constant, exact by definition).

Traceability chain: result → calibration standard → certified reference material → SI unit, each link with stated uncertainty; a result without an unbroken chain to a stated reference is not metrologically traceable.

Accuracy = closeness to the true value (includes bias); precision = closeness of replicate results to each other (random error only). High precision does not imply accuracy.

Control chart rules (Shewhart/Westgard): 1 point beyond ±3s = rejection; 2 consecutive points beyond ±2s on the same side = warning/rejection; 4 consecutive points beyond ±1s on one side = shift.

Limit of detection: LOD = 3s0 (s0 = SD of blank or low-level replicates); limit of quantification LOQ = 10s0. Report the basis used (blank SD, calibration curve residual SD, or IUPAC k·s).

Internal quality control: at least one blank, one certified reference material, one duplicate, and one spiked sample per analytical batch; recovery acceptance typically 80–120% for trace analysis, tighter near 100% for major analytes.

Proficiency testing (PT) score z = (x − x_assigned)/σ_PT; |z| ≤ 2 satisfactory, 2 < |z| < 3 questionable, |z| ≥ 3 unsatisfactory.

Significant figures and rounding: carry full precision through calculations, round only the final result; round to the least precise input for multiplication/division.

Representative sampling requires the sample to reflect the parent population in all relevant properties; the sampling plan must define population, sampling unit, sample size, and frequency before collection.

Gy's sampling theory: fundamental sampling error variance decreases with increasing sample mass and with decreasing particle size; sample mass needed scales with d^3 (d = nominal top particle size) for a given heterogeneity.

Composite sampling: combine n increments to reduce random sampling error by a factor of √n relative to a single increment; do not composite samples that may differ systematically (e.g., different strata or depth intervals).

Sample preservation: cool (2–8 °C) and dark for most organics and microbiological samples; acidify with HNO3 to pH < 2 for dissolved metals; minimize headspace for volatile organics; analyze within holding time stated in the method.

Contamination control in trace analysis: use pre-cleaned PTFE/PFA or quartz vessels, high-purity acids, laminar-flow clean benches, and procedural blanks run through the whole preparation.

Chain of custody: every transfer records sample ID, date/time, custodian, condition, and purpose; any break invalidates defensibility of the result.

Drying and homogenization: report results on a defined basis (dry mass, wet mass); dry at a specified temperature (e.g., 105 °C) and record moisture content for conversion.

Sub-sampling error can exceed analytical error; verify homogeneity by analyzing replicate sub-samples and comparing variance with method repeatability.

Volumetric: moles analyte = C × V (L) × stoichiometric ratio from the balanced equation; equivalence point is the stoichiometric point, endpoint is what the indicator shows.

Primary standards must be high purity, stable, non-hygroscopic, and high molar mass (e.g., potassium hydrogen phthalate, sodium carbonate, potassium dichromate); standardize secondary solutions against them.

Gravimetric: mass of analyte = mass of weighed form × gravimetric factor (molar mass analyte / molar mass weighed form × stoichiometric ratio); weighings must reach constant mass.

Gravimetric requirements: precipitate must be sparingly soluble, filterable, pure, and convertible to a stoichiometric weighing form by ignition or drying at a defined temperature.

Acid–base indicator selection: choose indicator whose transition range brackets the equivalence pH; phenolphthalein (8.2–10.0) for weak base titrations, methyl orange (3.1–4.4) for strong acid–weak base.

Redox titrations: permanganate in acidic medium (MnO4^- + 8H+ + 5e^- → Mn2+ + 4H2O, 5-electron change); dichromate 6-electron change; iodometry uses liberated I2 titrated with thiosulfate.

Complexometric titration with EDTA: 1:1 metal–EDTA stoichiometry; buffer pH to selectivity (e.g., pH 10 for Ca/Mg, pH 5–6 for Zn); use masking agents or back-titration for interfering metals.

Karl Fischer titration measures water: stoichiometric 1:1 water–iodine reaction; volumetric for high water content, coulometric for trace water (µg range).

Beer–Lambert law: A = ε·b·c; absorbance is linear in concentration only at low c, at fixed wavelength, with monochromatic light, and when the analyte does not associate or react.

Transmittance and absorbance: A = −log10(T) = log10(I0/I); A = 2 corresponds to 1% transmittance; stray light limits upper measurable absorbance (typically A ≤ 2–3).

UV-Vis chromophores: π→π* and n→π* transitions; conjugated systems shift λmax to longer wavelength (bathochromic) and increase ε (hyperchromic).

IR selection rule: a vibration is IR-active only if the dipole moment changes; symmetric stretches of symmetric molecules (e.g., N2, O2) are IR-inactive.

IR region assignments: O–H broad 3200–3600 cm^-1; N–H 3300–3500; C–H 2850–2960; C=O 1650–1750; C=C 1600–1680; fingerprint region below 1500 cm^-1.

Fluorescence: emission at longer wavelength than excitation (Stokes shift); intensity proportional to concentration at low absorbance (A < 0.05) to avoid inner-filter effects; quenching reduces signal.

Quantitation: build a calibration curve with ≥5 standards spanning the working range, blank-correct, and verify linearity (r² alone is insufficient; inspect residuals).

Cuvette and solvent: choose solvent transparent in the measured region (water, hexane for UV; IR uses KBr, NaCl, or ATR crystal); match cuvette pathlength and material to wavelength.

AAS: Beer–Lambert-type relation A = k·c in the linear range; hollow cathode lamp source, atomization by flame (~2000–3000 K) or graphite furnace (~2500–3000 K); element-specific, one element per lamp.

AAS interferences: spectral (overlap, background), chemical (refractory compound formation, e.g., Ca–P), ionization (suppress with ionization buffer such as K or Cs); use Zeeman or deuterium background correction.

ICP-OES: argon plasma ~6000–10000 K; multi-element, radial or axial viewing; detection limits typically µg/L–mg/L; matrix and spectral interferences corrected by inter-element correction factors.

ICP-MS: plasma ionizes, quadrupole or sector separates by m/z; detection limits ng/L–µg/L; isobaric overlaps (e.g., 40Ar on 40Ca) and polyatomic ions (40Ar16O on 56Fe) require collision/reaction cells or interference equations.

Internal standards (e.g., Sc, Y, In, Rh, Re) correct for drift, matrix suppression, and transport variation; add at constant concentration to all standards and samples.

Quantitation modes: external calibration with matrix-matched standards, standard additions for matrix-heavy samples, isotope dilution for highest accuracy in ICP-MS.

Sample introduction: nebulizer + spray chamber for solutions; solid sampling via laser ablation or electrothermal vaporization; hydride generation for As, Se, Sb, Hg; cold vapor for Hg.

Quality checks: check continuous calibration verification (CCV) recovery 90–110%, method blanks, duplicate RSD, and interference check solution per batch.

Retention factor k = (tR − t0)/t0; selectivity α = k2/k1; resolution Rs = 2(tR2 − tR1)/(w1 + w2); baseline resolution requires Rs ≥ 1.5.

van Deemter equation: H = A + B/u + C·u; minimum plate height gives optimum linear velocity; smaller particles (HPLC, sub-2 µm) reduce the C term and allow higher flow.

Efficiency: N = 16(tR/w)^2 or N = 5.54(tR/w1/2)^2; plate count increases with column length and decreases with particle size in HPLC.

GC: mobile phase is a gas (carrier: He, N2, H2); separation depends on volatility and column polarity; oven temperature program controls retention; detectors: FID (universal for organics), ECD (halogenated), NPD, MS.

HPLC modes: reversed-phase (C18, polar mobile phase, nonpolar analytes), normal-phase, ion-exchange, size-exclusion; gradient elution changes mobile-phase composition with time to elute broad polarity range.

Ion chromatography: suppressed conductivity detection; anion exchange with carbonate/bicarbonate or hydroxide eluents; cation exchange with methanesulfonic acid; suppression reduces eluent background conductance.

Hyphenation: GC-MS (electron ionization, library match), LC-MS/MS (electrospray or APCI, MRM transitions for quantitation), LC-ICP-MS for elemental speciation.

System suitability: tailing factor, resolution, plate count, and retention time RSD must meet method criteria before sample injection; injection volume and column temperature are critical variables.

Nernst equation: E = E° − (RT/nF)·ln Q; at 25 °C, E = E° − (0.05916/n)·log Q for the cell reaction; potentiometry measures potential at zero current.

pH electrode: glass membrane potential responds to H+ activity; calibrate with at least two buffers bracketing the sample pH; slope should be 95–105% of theoretical 59.16 mV/pH at 25 °C.

Ion-selective electrodes: response slope ~59.16/z mV per decade; detection limit set by membrane solubility; use standard additions or known-addition for complex matrices.

Conductometry: conductivity depends on ion concentration, charge, and mobility; temperature compensation required (typically 2%/°C); used in IC detection and water purity.

Voltammetry: current vs. applied potential; diffusion-limited peak current proportional to concentration (Randles–Ševčík); useful for redox-active species at trace levels.

Thermogravimetric analysis (TGA): mass change vs. temperature; quantifies moisture, volatiles, decomposition, and ash; heating rate affects onset and resolution.

Differential scanning calorimetry (DSC): heat flow vs. temperature; measures melting point, glass transition (Tg), crystallinity, and reaction enthalpy; calibrate with indium (mp 156.6 °C).

Thermal method variables: heating rate, atmosphere (N2 vs. air), sample mass, and pan type must be fixed and reported for reproducibility.

Validation parameters (ICH/ISO 17025 context): specificity, linearity and range, accuracy, precision (repeatability and intermediate precision), LOD/LOQ, robustness, and stability.

Linearity: assess over the working range with ≥5 concentration levels, triplicate injections; evaluate slope, intercept, residual plot, and lack-of-fit; r² ≥ 0.995 is a common criterion but not sufficient alone.

Accuracy: compare with certified reference material, spiked recovery, or independent method; report bias and its uncertainty.

Precision: repeatability (same analyst, same day) and intermediate precision (different days/analysts/instruments); express as RSD; Horwitz criterion relates acceptable RSD to concentration.

Robustness: deliberately vary method parameters (pH, temperature, flow, wavelength) within defined ranges and confirm results remain within acceptance criteria.

Measurement uncertainty: combine Type A (statistical, from replicates) and Type B (certificates, manufacturer specs, calibration) in quadrature: u_c = √(Σu_i²); expanded U = k·u_c with k = 2 for ~95% coverage.

Method development sequence: define analyte and matrix → select technique → optimize separation/detection → establish calibration → validate → transfer to routine QC.

Documentation: validation report must state acceptance criteria, raw data, deviations, and revalidation triggers (e.g., column change, instrument change, new matrix).

Chemical hazard communication: GHS labels with pictograms, signal words, hazard and precautionary statements; Safety Data Sheets (SDS) accessible for every hazardous chemical.

Fume hood face velocity typically 0.3–0.5 m/s (60–100 ft/min); verify with a calibrated anemometer and keep the sash low during work with volatile or toxic substances.

Compressed gas cylinders: secure upright with chain or bracket; use flashback arrestors for flammable gases (H2, acetylene); acetylene must never be used above ~15 psi delivery pressure or in copper lines.

Waste segregation: halogenated vs. non-halogenated solvents, heavy-metal waste, and aqueous acid/base waste kept separate; label with contents and date; dispose via licensed contractor.

Instrumentation qualification: Design Qualification (DQ) → Installation Qualification (IQ) → Operational Qualification (OQ) → Performance Qualification (PQ); maintain calibration and maintenance logs.

Data integrity (ALCOA+): Attributable, Legible, Contemporaneous, Original, Accurate, plus Complete, Consistent, Enduring, Available; audit trails must be enabled and immutable.

Electronic records: unique user accounts, password controls, role-based access, and audit trail review; 21 CFR Part 11 and EU GMP Annex 11 define compliance expectations for computerized systems.

Personal protective equipment: chemical splash goggles, lab coat, nitrile gloves (check compatibility), and face shield for corrosive handling; eyewash and safety shower within 10 seconds' travel.

Validation parameters (ICH/ISO 17025 context): specificity, linearity and range, accuracy, precision (repeatability and intermediate precision), LOD/LOQ, robustness, and stability.

Linearity: assess over the working range with ≥5 concentration levels, triplicate injections; evaluate slope, intercept, residual plot, and lack-of-fit; r² ≥ 0.995 is a common criterion but not sufficient alone.

Accuracy: compare with certified reference material, spiked recovery, or independent method; report bias and its uncertainty.

Precision: repeatability (same analyst, same day) and intermediate precision (different days/analysts/instruments); express as RSD; Horwitz criterion relates acceptable RSD to concentration.

Robustness: deliberately vary method parameters (pH, temperature, flow, wavelength) within defined ranges and confirm results remain within acceptance criteria.

Measurement uncertainty: combine Type A (statistical, from replicates) and Type B (certificates, manufacturer specs, calibration) in quadrature: u_c = √(Σu_i²); expanded U = k·u_c with k = 2 for ~95% coverage.

Method development sequence: define analyte and matrix → select technique → optimize separation/detection → establish calibration → validate → transfer to routine QC.

Documentation: validation report must state acceptance criteria, raw data, deviations, and revalidation triggers (e.g., column change, instrument change, new matrix).