Analytical Chemistry and Instrumental Methods
Scope
This domain tests quantitative measurement: calibration and Beer's law, spectroscopic identification (UV-Vis, IR, NMR, MS), chromatographic separation, titrimetry, and the quality-control practices that make a result defensible.
Core rules
- Accuracy is closeness to truth; precision is reproducibility. A biased method can be precise but inaccurate.
- Beer–Lambert: A = εbc, valid at low concentration and at a fixed path length; absorbance is linear only over the calibrated range.
- Dilution factor multiplies the measured concentration back to the original sample.
- IR identifies functional groups (broad O–H near 3300 cm⁻¹, sharp C=O near 1700 cm⁻¹); NMR reports chemical shift, integration, and n + 1 splitting; MS gives molecular ion and isotope patterns.
- Chromatography separates by differential partitioning; resolution depends on selectivity and efficiency.
- QC includes blanks, duplicates, matrix spikes, certified reference materials, control charts, and written SOPs.
Worked example
A 2.00 mL water sample is diluted to 50.0 mL. The diluted solution gives A = 0.400 in a 1.00 cm cell, and the calibration slope εb = 0.500 (μg/mL)⁻¹cm⁻¹. Then c_diluted = A/(εb) = 0.400/0.500 = 0.800 μg/mL. The dilution factor is 50.0/2.00 = 25.0, so the original sample is 0.800 × 25.0 = 20.0 μg/mL.
Traps
- Treating absorbance as concentration without a calibration curve or blank subtraction.
- Forgetting the dilution factor, which understates results by the dilution ratio.
- Reading NMR integration as absolute concentration; it is a ratio unless an internal standard is used.
- Reporting a result without a method blank, which hides contamination and background signal.