Engineer Chemical Analysis flashcards

33 cards. Read the term and the definition here. The deck at the top of this page is optional.

Beer-Lambert law

Absorbance A = ε·b·c, where ε is molar absorptivity, b is pathlength in cm, and c is molar concentration. Linear only at low concentration with monochromatic light and non-interacting analyte.

Limit of detection

Smallest analyte amount giving signal statistically distinguishable from blank, typically LOD = 3s0 where s0 is the standard deviation of blank or low-level replicate measurements.

Limit of quantification

Lowest concentration reliably measurable with acceptable precision and accuracy, commonly LOQ = 10s0. Must be validated with replicate measurements at that level.

Traceability chain

Unbroken sequence of calibrations, each with stated uncertainty, linking a measurement result to a certified reference material or SI unit. Any missing link breaks traceability.

Gy's sampling theory

Fundamental sampling error variance decreases as sample mass increases and particle size decreases. Required sample mass scales with the cube of nominal top particle size for heterogeneous material.

Composite sampling

Combining n increments into one sample reduces random sampling error by √n. Do not composite samples from systematically different strata or depth intervals.

Chain of custody

Chronological record of sample possession and handling: each transfer lists sample ID, date, time, custodian, condition, and purpose. Gaps undermine legal defensibility.

Primary standard

Highly pure, stable, non-hygroscopic reagent with high molar mass used to standardize a secondary solution, e.g., potassium hydrogen phthalate for base or potassium dichromate for oxidant.

Gravimetric factor

Ratio of analyte molar mass to weighed-form molar mass, multiplied by stoichiometric coefficients, converting mass of precipitate to mass of analyte.

Equivalence versus endpoint

Equivalence point is the stoichiometric completion of the reaction; endpoint is the observable signal change (indicator color). A good indicator makes these coincide.

Karl Fischer titration

Water-specific titration with iodine in the presence of sulfur dioxide and a base. Stoichiometric 1:1 water–iodine reaction; volumetric for high water, coulometric for trace water.

EDTA complexometric titration

EDTA binds most metal ions 1:1. Buffer pH controls selectivity (pH 10 for Ca/Mg, pH 5–6 for Zn). Masking agents or back-titration handle interfering metals.

Stokes shift

Difference between excitation and emission wavelengths in fluorescence. Emission occurs at longer wavelength because vibrational relaxation precedes photon emission.

IR selection rule

A molecular vibration absorbs infrared radiation only if the dipole moment changes during the vibration. Symmetric stretches of symmetric molecules are IR-inactive.

Inner-filter effect

At high absorbance, fluorescence intensity is no longer proportional to concentration because excitation and emitted light are absorbed. Keep A below about 0.05.

Hollow cathode lamp

Element-specific line source in atomic absorption spectroscopy. Emits narrow atomic lines of the analyte, giving high selectivity against background absorption.

Internal standard

Element or compound added at constant concentration to all standards and samples to correct for drift, matrix suppression, and transport variation in atomic and chromatographic methods.

Polyatomic interference

Ions formed from plasma gas and matrix, e.g., 40Ar16O on 56Fe in ICP-MS. Corrected with collision/reaction cells, interference equations, or matrix separation.

Isotope dilution

Adding a known amount of an enriched isotope and measuring the altered isotope ratio by ICP-MS. Provides highest accuracy because recovery cancels out.

van Deemter equation

H = A + B/u + C·u describes plate height as a function of linear velocity. Minimum H gives optimum velocity and maximum efficiency.

Resolution in chromatography

Rs = 2(tR2 − tR1)/(w1 + w2). Baseline resolution requires Rs ≥ 1.5. Increases with selectivity, efficiency, and retention factor.

Retention factor

k = (tR − t0)/t0, where t0 is the unretained marker time. Measures how strongly the analyte partitions into the stationary phase.

Ion chromatography suppression

A suppressor device reduces eluent background conductance while enhancing analyte conductance, enabling sensitive conductivity detection of anions or cations.

Nernst equation

E = E° − (RT/nF)·ln Q. At 25 °C this becomes E = E° − (0.05916/n)·log Q. Relates cell potential to reaction quotient.

pH electrode slope

Potential change per pH unit, theoretically 59.16 mV at 25 °C. Acceptable calibration slope is 95–105% of theoretical; lower slope indicates aging or contamination.

Thermogravimetric analysis

Measures mass change versus temperature under controlled atmosphere. Quantifies moisture, volatiles, decomposition steps, and ash residue.

Differential scanning calorimetry

Measures heat flow versus temperature. Determines melting point, glass transition, crystallinity, and reaction enthalpy. Calibrated with indium (mp 156.6 °C).

Method validation

Documented confirmation that a method meets requirements for its intended use: specificity, linearity, accuracy, precision, LOD/LOQ, robustness, and stability.

Expanded uncertainty

U = k·u_c, where u_c combines Type A and Type B contributions in quadrature. k = 2 gives approximately 95% coverage probability.

Horwitz criterion

Empirical relationship between acceptable relative standard deviation and analyte concentration. Predicts precision limits for interlaboratory studies across concentration ranges.

ALCOA+ principles

Data integrity criteria: Attributable, Legible, Contemporaneous, Original, Accurate, plus Complete, Consistent, Enduring, Available. Applied to all laboratory records and electronic data.

Fume hood face velocity

Air speed through the hood opening, typically 0.3–0.5 m/s (60–100 ft/min). Verified with a calibrated anemometer; low sash height maintains containment.

Instrument qualification sequence

Design Qualification, then Installation Qualification, Operational Qualification, and Performance Qualification. Each stage documented before routine use of analytical instrumentation.