STP Technical Skills & Application 2 — Questions and Answers
Question 1: When calibrating a spectrophotometer, which solution should be used to set the 100% transmittance (zero absorbance) baseline?
- Distilled water
- The sample solution
- A blank reagent solution (Correct answer)
- A standard of known concentration
Correct answer: A blank reagent solution
A blank reagent solution (containing all reagents except the analyte) is used to zero the instrument, accounting for any absorbance from the reagents themselves.
Question 2: In a laboratory performing PCR, a technician observes amplification in the no-template control (NTC). What is the most likely cause?
- Incorrect primer design
- Contamination of reagents or workspace with DNA (Correct answer)
- Insufficient annealing temperature
- Too many PCR cycles
Correct answer: Contamination of reagents or workspace with DNA
A positive NTC result indicates DNA contamination of reagents or the workspace, as no template was added to that reaction.
Question 3: Which statistical measure is most appropriate for describing the spread of a skewed dataset in a clinical laboratory context?
- Standard deviation
- Variance
- Interquartile range (IQR) (Correct answer)
- Coefficient of variation
Correct answer: Interquartile range (IQR)
The IQR is preferred for skewed distributions because it is not influenced by extreme outliers, unlike standard deviation.
Question 4: A scientist needs to prepare 500 mL of a 0.1 M NaCl solution (MW = 58.44 g/mol). How many grams of NaCl are required?
- 2.92 g (Correct answer)
- 5.84 g
- 29.22 g
- 58.44 g
Correct answer: 2.92 g
Moles needed = 0.1 mol/L × 0.5 L = 0.05 mol; mass = 0.05 × 58.44 = 2.92 g.
Question 5: In Western blotting, what is the primary purpose of the blocking step?
- To denature the protein sample
- To prevent non-specific antibody binding to the membrane (Correct answer)
- To transfer proteins from gel to membrane
- To visualise molecular weight markers
Correct answer: To prevent non-specific antibody binding to the membrane
Blocking (e.g., with milk or BSA) saturates non-specific binding sites on the membrane so that antibodies only bind the target protein.
Question 6: Which type of pipette is most appropriate for accurately dispensing 0.75 mL of a reagent?
- Pasteur pipette
- Graduated (serological) pipette
- Micropipette set to 750 µL (Correct answer)
- Volumetric pipette calibrated at 0.75 mL
Correct answer: Micropipette set to 750 µL
A micropipette set to 750 µL provides the greatest accuracy and precision for volumes in the sub-millilitre range.
Question 7: When performing a Gram stain, what is the role of the decolorizing agent (acetone-alcohol)?
- It kills the bacteria
- It fixes the crystal violet to Gram-positive cell walls
- It removes crystal violet from Gram-negative cells (Correct answer)
- It enhances counterstain uptake in all cells
Correct answer: It removes crystal violet from Gram-negative cells
The decolorizer extracts crystal violet from Gram-negative cells (which have thinner peptidoglycan and an outer membrane), leaving them colourless before counterstaining.
STP Technical Skills & Application 2
When calibrating a spectrophotometer, which solution should be used to set the 100% transmittance (zero absorbance) baseline?