Discipline-Specific Scientific Subject Knowledge
The discipline paper is set at honours-degree level in the subject you applied under - chemistry, biology, physics, agriculture, geology and so on - but a common set of competencies decides many marks: selecting a valid method, calibrating it, running quality controls, and reporting results with correct units, dilution factors and uncertainty.
Core rules and concepts
- A method is only valid within its validated range, matrix and detection limit; outside that range you dilute, concentrate or change method.
- Calibration uses at least a blank plus three or more standards bracketing the sample; the calibration blank must be subtracted before reading a concentration.
- Traceability means every result links back to a certified reference material or a recognised national standard; ISO/IEC 17025 accreditation and good laboratory practice codify this.
- Quality control includes reagent blanks, duplicate or replicate analyses, spiked samples for recovery, and certified reference materials analysed alongside unknowns.
- Biological work adds aseptic technique, positive and negative controls, and the distinction between technical replicates (same sample run twice) and biological replicates (independent samples).
Worked example
A spectrophotometric nitrate determination gives a calibration slope of 0.0850 absorbance units per mg/L (blank-corrected). A ten-fold diluted sample reads 0.510 absorbance and the reagent blank reads 0.010. Net absorbance = 0.510 − 0.010 = 0.500. Concentration in the cuvette = 0.500 / 0.0850 = 5.88 mg/L. Undo the dilution: 5.88 × 10 = 58.8 mg/L in the original sample.
Traps that cost marks
- Forgetting the reagent blank, which inflates every result.
- Applying the dilution factor in the wrong direction, or applying it twice.
- Reading absorbance above the linear range (roughly 1.0), where the curve flattens.
- Using a calibration built in a different matrix from the sample.
- Treating replicate runs of one sample as evidence of reproducibility across samples.