MLT GlobeLaboratory Operations: Instrumentation & methods – page 2
39 Laboratory Operations MCQs on Instrumentation & methods with answers and explanations.
During serum protein electrophoresis at pH 8.6, gamma globulins move slightly toward the cathode even though they carry a small negative charge. This is caused by:
Electroendosmosis is the bulk flow of buffer toward the cathode caused by fixed negative charges on the support medium. Weakly charged gamma globulins are carried with it.
In reversed-phase HPLC with a nonpolar C18 column and a polar mobile phase, which compounds elute first?
In reversed-phase chromatography the stationary phase is nonpolar, so polar compounds interact least and elute first. Nonpolar compounds are retained longer.
In a time-of-flight (TOF) mass analyzer, such as in MALDI-TOF identification of bacteria, which ions reach the detector first?
All ions receive similar kinetic energy, so lighter ions (lower m/z) travel faster through the flight tube and arrive first. Neutral molecules are not accelerated or detected.
In real-time PCR, sample A has a cycle threshold (Ct) of 22 and sample B has a Ct of 32 for the same target. What does this indicate?
Ct is inversely related to the starting amount of target: fewer cycles are needed to cross the threshold when more template is present. A 10-cycle difference is roughly a 1000-fold difference.
A 100 µL pipette is checked gravimetrically with water (assume density 1.000 g/mL). The mean delivered mass is 0.0980 g. The inaccuracy (percent error) is:
Delivered volume = 98.0 µL. Error = (98.0 − 100)/100 × 100 = −2.0%. The gravimetric method converts the weight of delivered water to volume.
Most clinical osmometers measure osmolality by detecting which colligative property of the sample?
Freezing point depression osmometers measure how much solutes lower the freezing point (1 osmol/kg lowers it by 1.86 °C). Refractive index is not a colligative property.
In atomic absorption spectrophotometry for lead, the light source is a hollow cathode lamp whose cathode is made of:
The cathode is made of the same element being measured so it emits that element's exact wavelengths, which ground-state atoms in the flame absorb.
In an ion-selective electrode analyser, the potassium electrode membrane usually contains:
Valinomycin is an ionophore that binds K⁺ selectively and is used in potassium ISE membranes. Sodium is measured with a special sodium-selective glass membrane.
In a Severinghaus pCO₂ electrode, carbon dioxide is measured by detecting:
CO₂ diffuses through the membrane into bicarbonate solution, forming carbonic acid and lowering pH, which a glass electrode senses (potentiometric). Current at a platinum cathode describes the Clark pO₂ electrode.
The Clark electrode used to measure pO₂ works on which principle?
The Clark electrode applies a fixed voltage; oxygen is reduced at the platinum cathode and the current is proportional to pO₂. Potentiometric glass electrodes measure pH and pCO₂.
The main advantage of a double-beam spectrophotometer over a single-beam instrument is that it:
By splitting light between sample and reference paths and comparing them, a double-beam instrument cancels variations in source intensity. It still needs a monochromator and appropriate blanks.
Isoelectric focusing separates proteins according to their:
In isoelectric focusing each protein migrates in a pH gradient until it reaches the pH where its net charge is zero (pI), then stops. SDS-PAGE, not IEF, separates mainly by size.
A glass pipette marked 'TD' is calibrated:
TD (to deliver) pipettes deliver the marked volume when drained; TC (to contain) pipettes hold the marked volume and must be rinsed out.
Which light source is commonly used in spectrophotometers for measurements in the ultraviolet range?
Deuterium lamps give a continuous UV spectrum; tungsten lamps are used for the visible range, and hollow cathode lamps for atomic absorption.
Which grade of laboratory water, with the highest purity, is used for sensitive tests such as trace analysis and molecular methods?
Type I (CLRW) water has the highest resistivity and lowest contamination; Type III is used for rinsing glassware.
At high absorbance values, a spectrophotometer's standard curve bends toward the concentration axis and loses linearity. The most likely instrumental cause is:
Stray light adds unabsorbed light at the detector, making absorbance read falsely low; the effect is largest in highly absorbing samples, causing negative deviation from Beer's law.
A patient with severe hypertriglyceridemia has sodium 126 mmol/L by indirect ISE but 140 mmol/L by direct ISE on a blood gas analyzer. The best explanation is:
Indirect ISE dilutes the sample and assumes normal plasma water; excess lipid lowers plasma water, giving falsely low sodium. Direct ISE measures activity in plasma water and is unaffected.
A chloridometer generates silver ions at a constant rate; the time to reach the endpoint is proportional to the chloride concentration. This method is:
In coulometric-amperometric titration, generated Ag⁺ combines with Cl⁻; excess Ag⁺ at the endpoint increases current and stops the timer. ISE methods measure a voltage instead.
A patient on peritoneal dialysis with icodextrin has high glucose meter readings but a normal plasma glucose on the laboratory analyser. Which meter enzyme is most likely used?
GDH-PQQ also reacts with maltose, which is formed from icodextrin, giving falsely high readings. Glucose oxidase meters are specific for glucose and not affected by maltose.