Centrifuges, pipettes, balances and laboratory water

6 DOH Abu Dhabi (HAAD) lab licensing practice questions on Centrifuges, pipettes, balances and laboratory water, part of General Laboratory, Quality and Safety. The first 6 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

1. Question 1 · Medium

   A crucible taken from a drying oven is weighed while still warm on an analytical balance. The reading is lower than expected and creeps upward as the crucible cools. What causes this?

   1. AStatic charge on the balance draft shield
   2. BVibration from a centrifuge on the bench
   3. CRising convection currents of warm air
   4. DEvaporation of water from the crucible

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   Answer: C · Rising convection currents of warm air

   A warm object heats the air around it, and the rising air pushes upward on the pan, so the object appears lighter than it is. As the object cools to room temperature, the upward force fades and the reading climbs to the true mass. Hot items should therefore be cooled to room temperature in a desiccator before weighing. Analytical balances also need a level, vibration-free bench away from drafts, and regular checks with certified weights.

   • A: Static charge gives unstable readings, not a steady rise as the object cools.
   • B: Vibration makes readings fluctuate rather than drift in one direction.
   • D: Evaporation would make the reading fall, not rise, over time.

2. Question 2 · Hard

   A procedure calls for centrifugation at 1,500 x g. The rotor radius, measured from the center to the bottom of the tube, is 15 cm. Using RCF = 1.118 x 10^-5 x r x rpm^2, the speed to set is about:

   1. A2,100 rpm
   2. B3,000 rpm
   3. C950 rpm
   4. D1,500 rpm

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   Answer: B · 3,000 rpm

   Rearranging gives rpm = square root of [RCF / (1.118 x 10^-5 x r)] = square root of [1,500 / (1.118 x 10^-5 x 15)] = square root of about 8.9 million, which is about 3,000 rpm. Using the rotor diameter (30 cm) instead of the radius gives about 2,100 rpm, and entering the radius in mm gives about 950 rpm. Procedures should state g-force, not rpm, because the same rpm gives a different force on a rotor of another size.

   • A: 2,100 rpm uses the 30 cm diameter instead of the 15 cm radius.
   • C: 950 rpm enters the radius in millimeters (150) instead of centimeters.
   • D: 1,500 rpm wrongly treats rpm and g-force as the same quantity.

3. Question 3 · Easy

   Under CLSI GP40, clinical laboratory reagent water (CLRW) must have a resistivity of at least:

   1. A18.2 MΩ·cm at 25 °C
   2. B1 MΩ·cm at 25 °C
   3. C0.1 MΩ·cm at 25 °C
   4. D10 MΩ·cm at 25 °C

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   Answer: D · 10 MΩ·cm at 25 °C

   CLSI GP40 sets CLRW limits of resistivity of at least 10 MΩ·cm at 25 °C, total organic carbon below 500 ppb, bacteria below 10 CFU/mL, and filtration through a 0.2 µm filter. It replaced the older type I, II and III grades; 1 and 0.1 MΩ·cm were the old type II and III limits. 18.2 MΩ·cm is the theoretical maximum for pure water, reached by ultrapure systems but not required for CLRW. Resistivity is monitored at the outlet, and a fall warns that the deionizing cartridges are exhausted.

   • A: 18.2 MΩ·cm is the theoretical maximum for pure water, not the CLRW minimum.
   • B: 1 MΩ·cm was the old type II limit, too impure for CLRW.
   • C: 0.1 MΩ·cm was the old type III limit, used for tasks such as rinsing glassware.

4. Question 4 · Medium

   With an air-displacement pipette, viscous or foaming liquids such as whole blood are dispensed most accurately by:

   1. AReverse pipetting
   2. BUsing a larger pipette set near its minimum
   3. CForward pipetting with fast plunger release
   4. DPre-wetting the tip with distilled water

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   Answer: A · Reverse pipetting

   In reverse pipetting the plunger is pressed to the second (blow-out) stop before aspirating, so extra liquid is drawn up. Only the volume to the first stop is dispensed, and the excess left in the tip, including the film that clings to the tip wall, is discarded. This avoids the low delivery and bubbles that viscous and foaming liquids cause with forward pipetting. For very viscous or volatile liquids, a positive-displacement pipette is an alternative.

   • B: Pipettes are least accurate near the bottom of their volume range.
   • C: Fast release causes bubbles and incomplete filling with viscous liquids.
   • D: Pre-wetting with water dilutes the sample; pre-wetting is done with the liquid itself.

5. Question 5 · Easy

   What must be done before starting a centrifuge?

   1. AMix different tube sizes in the same bucket
   2. BRemove the caps from all tubes to release pressure
   3. CBalance equal tubes opposite each other, caps on
   4. DLoad all the tubes on one side of the rotor

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   Answer: C · Balance equal tubes opposite each other, caps on

   An unbalanced load causes vibration that can damage the centrifuge and break tubes. Opposite positions must carry equal weights, using balance tubes if needed. Capped tubes and closed lids contain aerosols if a tube breaks. The lid should not be opened until the rotor has stopped.

   • A: Mixed tube sizes can break or unbalance the load.
   • B: Uncapped tubes release aerosols.
   • D: Loading one side unbalances the rotor.

6. Question 6 · Medium

   How is the volume delivered by a pipette checked by the gravimetric method?

   1. ABy measuring the liquid height in a tube with a ruler
   2. BBy weighing dispensed water and converting mass to volume
   3. CBy counting the number of drops it delivers
   4. DBy weighing the empty pipette tip before and after use

   Answer and explanation in LearnFree account