MLS Laboratory Equipment & Techniques 3 — Questions and Answers
Question 1: Which class of fire extinguisher should be used for a burning electrical equipment fire in the laboratory?
- Class A
- Class B
- Class C (Correct answer)
- Class D
Correct answer: Class C
Class C extinguishers use non-conductive agents (CO2 or dry chemical) suitable for electrical fires where water-based agents would be dangerous.
Question 2: When using a refractometer to measure urine specific gravity, the instrument measures:
- Osmolality directly
- The refractive index, which correlates with solute concentration (Correct answer)
- The electrical conductivity of the sample
- pH of the urine
Correct answer: The refractive index, which correlates with solute concentration
A refractometer measures how much light bends (refractive index) as it passes through the sample, which correlates with dissolved solute concentration and specific gravity.
Question 3: Which of the following describes the correct procedure when using a swinging-bucket rotor centrifuge for density gradient separations?
- Load tubes asymmetrically to create differential force
- Balance opposing tubes to within manufacturer tolerances before spinning (Correct answer)
- Always spin at maximum RPM first
- Remove the gradient medium before centrifugation
Correct answer: Balance opposing tubes to within manufacturer tolerances before spinning
Proper balancing of opposing buckets is essential to prevent rotor damage, instrument vibration, and potential centrifuge failure.
Question 4: An autoclave that sterilizes at 121°C for 15 minutes uses which combination of conditions?
- Dry heat at 15 psi
- Moist heat (steam) at 15 psi (Correct answer)
- UV radiation at atmospheric pressure
- Ethylene oxide at 121°C
Correct answer: Moist heat (steam) at 15 psi
Standard autoclaving uses saturated steam (moist heat) at 15 psi (103 kPa) above atmospheric pressure, achieving 121°C for effective sterilization.
Question 5: A laboratory balance displays 5.003 g when weighing a 5.000 g reference weight. The balance is:
- Precise but not accurate (Correct answer)
- Accurate but not precise
- Both accurate and precise
- Neither accurate nor precise
Correct answer: Precise but not accurate
If the balance consistently reads 5.003 g (reproducible), it is precise; however, since it deviates from the true 5.000 g value, it is not accurate.
Question 6: Which microscopy technique uses oblique illumination to produce a three-dimensional appearance without staining, making it ideal for viewing unstained wet preparations?
- Darkfield microscopy
- Phase contrast microscopy (Correct answer)
- Fluorescence microscopy
- Brightfield microscopy
Correct answer: Phase contrast microscopy
Phase contrast microscopy converts phase shifts in light passing through transparent specimens into visible brightness changes, allowing visualization of unstained cells.
Question 7: What is the primary purpose of a HEPA filter in a biological safety cabinet?
- To remove chemical vapors and gases
- To trap particulates ≥0.3 µm with ≥99.97% efficiency (Correct answer)
- To sterilize air using UV light
- To regulate airflow temperature
Correct answer: To trap particulates ≥0.3 µm with ≥99.97% efficiency
HEPA (High Efficiency Particulate Air) filters remove at least 99.97% of airborne particles ≥0.3 µm in diameter, protecting against biological aerosols.
MLS Laboratory Equipment & Techniques 3
Which class of fire extinguisher should be used for a burning electrical equipment fire in the laboratory?