Scientific Practices and Laboratory Applications
Domain 7 - Scientific Practices and Laboratory Applications
This domain tests hands-on competence: safe and accurate measurement, standard laboratory techniques, and correct treatment of data.
Measurement and solution math
- Use SI units and report to the precision the instrument allows; read a graduated cylinder at the meniscus.
- Micropipettes are chosen by volume (P20, P200, P1000) and never set beyond their range.
- Dilution: C₁V₁ = C₂V₂. Percent solutions: (mass solute ÷ volume solution) × 100. Molarity = moles solute ÷ liters solution. Serial dilutions multiply dilution factors.
- Magnification: total magnification = ocular × objective. As magnification rises, field of view shrinks; resolution is set by wavelength and lens quality, not by magnification alone.
Techniques
Spectrophotometry uses absorbance against a standard curve (Beer–Lambert: absorbance is proportional to concentration). Chromatography separates by polarity/solubility; Rf = distance solute traveled ÷ distance solvent traveled. Centrifugation separates by density and size. Gel electrophoresis moves negatively charged DNA toward the anode; small fragments travel farther. PCR cycles denaturation, primer annealing, and extension; aseptic technique and proper biohazard disposal are nonnegotiable.
Data analysis
Report mean and standard deviation; overlapping error bars suggest no significant difference. Chi-square compares observed and expected counts (χ² = Σ(O−E)²/E) with degrees of freedom = categories − 1. The t-test compares two means.
Worked example
You need 100 mL of 0.5% NaCl from a 4% stock. C₁V₁ = C₂V₂, so V₁ = (0.5% × 100 mL) ÷ 4% = 12.5 mL of stock, made up to 100 mL with 87.5 mL of distilled water. For a 1:10 dilution series instead, each step takes 1 mL of sample into 9 mL of diluent, and the third tube is 10⁻³ of the original concentration.
Traps
Adding 12.5 mL stock to 100 mL water (that yields 112.5 mL); forgetting to dilute to final volume; reporting more significant figures than the instrument supports; using categories − 1 incorrectly or raw counts instead of the ratio in chi-square; expecting large DNA fragments to move farthest; assuming higher magnification automatically means better resolution; skipping controls in an assay.