STP Technical Skills & Application 5 — Questions and Answers
Question 1: Which term describes the ability of an analytical method to measure only the intended analyte without interference from other substances?
- Precision
- Accuracy
- Analytical specificity (selectivity) (Correct answer)
- Sensitivity
Correct answer: Analytical specificity (selectivity)
Analytical specificity (or selectivity) is the ability of an assay to measure the target analyte alone, even in the presence of other potentially interfering compounds.
Question 2: A laboratory scientist uses a haemocytometer to count cells. Under ×10 objective with a 10× eyepiece, the scientist counts 60 cells in a 1 mm² area at a depth of 0.1 mm. What is the cell concentration per mL?
- 60 cells/mL
- 600 cells/mL
- 6 × 10⁵ cells/mL (Correct answer)
- 6 × 10⁴ cells/mL
Correct answer: 6 × 10⁵ cells/mL
Volume counted = 1 mm² × 0.1 mm = 0.1 mm³ = 1×10⁻⁴ mL; concentration = 60 / 1×10⁻⁴ = 6×10⁵ cells/mL.
Question 3: In gel electrophoresis, what is the purpose of loading a DNA ladder alongside experimental samples?
- To stain the DNA bands for visualisation
- To serve as a molecular weight reference for estimating fragment sizes (Correct answer)
- To prevent the DNA from degrading during the run
- To neutralise the ionic strength of the buffer
Correct answer: To serve as a molecular weight reference for estimating fragment sizes
The ladder contains DNA fragments of known sizes that migrate predictably, allowing the sizes of experimental bands to be estimated by comparison.
Question 4: Which biosafety level (BSL) is required for work with Mycobacterium tuberculosis?
- BSL-1
- BSL-2
- BSL-3 (Correct answer)
- BSL-4
Correct answer: BSL-3
M. tuberculosis is a BSL-3 pathogen because it can cause serious or potentially lethal disease and is transmissible via aerosol, requiring respiratory protection and negative-pressure rooms.
Question 5: When using reverse transcription PCR (RT-PCR) to detect an RNA virus, what is the first enzymatic step?
- DNA polymerase extends primers on the viral genome
- Reverse transcriptase converts viral RNA into complementary DNA (cDNA) (Correct answer)
- Ligase joins DNA fragments together
- Helicase unwinds the double-stranded RNA
Correct answer: Reverse transcriptase converts viral RNA into complementary DNA (cDNA)
Reverse transcriptase uses the viral RNA as a template to synthesise cDNA, which then serves as the template for conventional PCR amplification.
Question 6: A method validation study shows a coefficient of variation (CV) of 2.1% for intra-assay precision and 8.4% for inter-assay precision. What is the most likely explanation for the difference?
- The intra-assay samples were stored incorrectly
- Between-run variation from factors such as different reagent lots, operators, and calibrations contributes to higher inter-assay CV (Correct answer)
- The inter-assay samples contained more interfering substances
- Intra-assay precision is always lower because fewer samples are measured
Correct answer: Between-run variation from factors such as different reagent lots, operators, and calibrations contributes to higher inter-assay CV
Inter-assay (between-run) precision captures additional sources of variation — including operator, reagent lot, calibration, and environmental differences — that do not affect a single run.
Question 7: Which component of a HPLC system is responsible for maintaining a constant flow rate of mobile phase through the column?
- The detector
- The injector
- The pump (Correct answer)
- The column oven
Correct answer: The pump
The high-pressure pump delivers mobile phase at a precise, constant flow rate, which is essential for reproducible retention times and quantitative results.
