MLT GlobeLaboratory Operations: Instrumentation & methods
39 Laboratory Operations MCQs on Instrumentation & methods with answers and explanations.
After using an oil immersion objective, how should the lens be cleaned?
Lens paper with a recommended cleaner or a small amount of alcohol removes oil safely. Xylene can dissolve lens cement and is toxic; paper and gauze can scratch.
A light microscope objective forms an image because a glass lens bends light rays passing through it. This bending is called:
Refraction is the change in direction of light passing between media of different refractive index. Resolution is the ability to separate two close points.
The 100× objective on a laboratory microscope is also known as the:
The 100× lens requires immersion oil to raise numerical aperture and resolution. Scanning is 4×, low power 10× and high-dry 40×.
Unstained cells are nearly transparent under a bright-field microscope. What is usually done to increase the contrast of their structures?
Stains bind cellular components selectively and create contrast. Brighter light actually reduces contrast, and a cover slip or higher eyepiece does not add it.
According to Beer's law, absorbance is directly proportional to:
A = a × b × c: absorbance equals absorptivity × light path × concentration. Transmittance falls logarithmically as concentration rises, so %T is not linear.
In a modern spectrophotometer, which component most commonly isolates a narrow band of wavelengths from the light source?
A monochromator such as a diffraction grating (or prism) disperses light and selects a narrow band. The photomultiplier tube is the detector, not the wavelength selector.
How does nephelometry differ from turbidimetry when measuring immune complexes?
Nephelometry detects light scattered by particles, usually at an angle to the beam. Turbidimetry measures the reduction in light passing straight through, similar to absorbance.
A mass spectrometer identifies and quantifies ions according to their:
Mass spectrometers ionize molecules and separate the ions by mass-to-charge ratio (m/z). Isoelectric point is the basis of isoelectric focusing, not mass spectrometry.
In the polymerase chain reaction, during which step do the primers bind to complementary sequences on the single-stranded template?
Annealing is performed at a lower temperature so primers hybridize to their targets. Denaturation separates strands; extension is when the polymerase synthesizes new DNA.
A refractometer is used in the laboratory to estimate urine specific gravity or serum total protein by measuring:
Refractive index (the bending of light) increases with dissolved solids, so it estimates specific gravity or total protein. Freezing point depression is used by osmometers.
In gas chromatography, a drug in a sample is identified mainly by comparing its:
Each compound passes through the column in a characteristic time (retention time) under set conditions; matching it with a standard identifies the compound. Peak area gives the quantity.
A reagent blank is included in a colorimetric assay mainly to correct for:
The reagent blank contains reagents without sample, so its absorbance is subtracted to remove colour from the reagents. A sample blank corrects for the sample's own colour, such as haemolysis or icterus.
Before a centrifuge is started, tubes must be placed so that:
An unbalanced rotor vibrates, can damage the centrifuge and may break tubes, creating aerosols.
Most laboratory reagents and kits that need refrigeration are stored at:
Refrigerated storage is 2–8 °C; temperatures should be recorded daily.
When reading the volume of a clear aqueous solution in a graduated pipette or cylinder, you should read:
For clear liquids the lowest point of the curved meniscus is read with the eye level with the mark to avoid parallax error.
A pH meter is calibrated before use with:
Buffers of known pH, such as 7.0 and 4.0 or 10.0, are used to calibrate the electrode across the working range.
Which part of a light microscope focuses light onto the specimen?
The condenser below the stage gathers and focuses light on the specimen; its iris diaphragm controls the amount of light.
The 'useful' magnification of a light microscope is approximately 500–1000 times which value of the objective?
Useful magnification ≈ 500–1000 × NA; beyond this, 'empty magnification' enlarges the image without revealing more detail. NA also determines resolving power.
Which material is commonly used to check the wavelength accuracy of a spectrophotometer?
Holmium oxide (or didymium) glass has sharp absorbance peaks at known wavelengths, used to verify wavelength calibration. Neutral density filters check photometric (absorbance) accuracy.
In a fluorometer, how does the measured emitted light compare with the excitation light?
Fluorescent molecules lose some energy before emitting, so emitted light has a longer wavelength (Stokes shift). The detector is placed at 90° to reduce interference from the excitation beam.