The 125I label in a radioimmunoassay tube is counted with which detector?

Well-type sodium iodide crystal gamma counter

Liquid scintillation counter with a cocktail

In a swinging-bucket (horizontal) centrifuge head, the tubes during the run are:

Horizontal, so the sediment packs flat at the base

Tilted at about 25 degrees from the vertical

Held at a fixed angle of about 45 degrees

Vertical, parallel to the axis of rotation

A 45-year-old man with dyspepsia drinks a capsule of 14C-labelled urea, and 20 minutes later his breath sample contains a high level of 14CO2. This result indicates:

Active Helicobacter pylori infection of the stomach

Bacterial overgrowth in the small intestine

Lactose malabsorption from lactase deficiency

Exocrine pancreatic failure with fat malabsorption

A 4-year-old child living near a battery recycling unit has abdominal colic and basophilic stippling of red cells. Which technique is standard for measuring blood lead?

Atomic absorption spectrophotometry

Instrumentation and radioisotopes

58 Kerala MSc MLT Entrance practice questions on Instrumentation and radioisotopes, part of Biochemistry. The first 25 are below; the first 5 with answers and explanations (tap Show answer after you have chosen), the rest in Learn with a free account.

Question 1 · Easy
The 125I label in a radioimmunoassay tube is counted with which detector?
1. ALiquid scintillation counter with a cocktail
2. BPhotographic film badge
3. CGold-leaf electroscope
4. DWell-type sodium iodide crystal gamma counter
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Answer: D · Well-type sodium iodide crystal gamma counter
125I decays by electron capture and gives off low-energy gamma and X-ray photons. These pass through the tube wall and are detected by a thallium-activated sodium iodide crystal with a well that surrounds the tube. No sample preparation is needed. Weak beta emitters such as 3H and 14C need a liquid scintillation counter instead.
- A: Liquid scintillation counting is used for weak beta emitters such as tritium and 14C.
- B: Film badges monitor a worker's personal dose, not sample activity.
- C: An electroscope only shows ionisation qualitatively; it is not a counter.
Question 2 · Easy
In a swinging-bucket (horizontal) centrifuge head, the tubes during the run are:
1. ATilted at about 25 degrees from the vertical
2. BHorizontal, so the sediment packs flat at the base
3. CHeld at a fixed angle of about 45 degrees
4. DVertical, parallel to the axis of rotation
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Answer: B · Horizontal, so the sediment packs flat at the base
The buckets hang vertically at rest and swing out to the horizontal as the rotor turns. Sediment therefore forms a flat, even layer at the bottom of the tube, which makes it easy to remove the supernatant. Fixed-angle heads hold tubes at a constant angle and sediment along the side and bottom. Swinging-bucket heads are used for serum separation and urine sediment.
- A: Swinging buckets move fully out to the horizontal during the spin.
- C: A fixed angle describes the angle-head rotor.
- D: Vertical tubes are held in vertical rotors used for density gradients.
Question 3 · Medium
In liquid scintillation counting, a yellow, haemolysed sample counts much lower than an equal amount of isotope in a clear sample. This loss is called:
1. ADead time
2. BBackground
3. CQuenching
4. DChemiluminescence
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Answer: C · Quenching
Quenching is any process that reduces the light output reaching the photomultiplier. Coloured samples absorb the scintillation light (colour quenching), and some chemicals take energy from the solvent before it reaches the fluor (chemical quenching). Counts fall, so efficiency must be corrected with an internal or external standard. Chemiluminescence has the opposite effect and gives falsely high counts.
- A: Dead time is the recovery period of a detector at very high count rates.
- B: Background is the count with no sample and is subtracted, not caused by colour.
- D: Chemiluminescence adds spurious light and raises counts.
Question 4 · Medium
A 45-year-old man with dyspepsia drinks a capsule of 14C-labelled urea, and 20 minutes later his breath sample contains a high level of 14CO2. This result indicates:
1. ABacterial overgrowth in the small intestine
2. BLactose malabsorption from lactase deficiency
3. CActive Helicobacter pylori infection of the stomach
4. DExocrine pancreatic failure with fat malabsorption
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Answer: C · Active Helicobacter pylori infection of the stomach
Human tissues have no urease. H. pylori makes a powerful urease that splits the labelled urea into ammonia and 14CO2, which is absorbed and exhaled. A high breath count therefore shows active infection, and the test turns negative after successful treatment. The tiny dose of 14C is a useful example of a clinical radioisotope tracer.
- A: Small bowel overgrowth is tested with glucose or lactulose hydrogen breath tests.
- B: Lactose malabsorption is shown by a lactose hydrogen breath test.
- D: Pancreatic fat malabsorption uses faecal elastase or labelled triglyceride tests.
Question 5 · Easy
A 4-year-old child living near a battery recycling unit has abdominal colic and basophilic stippling of red cells. Which technique is standard for measuring blood lead?
1. AAtomic absorption spectrophotometry
2. BFlame emission photometry
3. CIon-selective electrode
4. DNephelometry
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Answer: A · Atomic absorption spectrophotometry
Blood lead is measured by atomic absorption spectrophotometry, usually with a graphite furnace, or by ICP-mass spectrometry. Ground-state lead atoms absorb light from a lead hollow cathode lamp, which makes the method sensitive and specific. Flame emission suits easily excited elements such as sodium, potassium and lithium. Lead is not measured by nephelometry or routine ion-selective electrodes.
- B: Lead atoms are poorly excited in a flame, so flame emission is insensitive for lead.
- C: Routine ion-selective electrodes measure Na, K, Cl and Ca, not blood lead.
- D: Nephelometry measures light scatter by particles, such as immune complexes.
Question 6 · Medium
In internal-standard flame photometry for sodium and potassium, lithium is added to every diluted sample. It improves accuracy mainly because it:
1. ASuppresses ionisation of sodium and potassium
2. BRaises the flame temperature for better excitation
3. CBinds phosphate that would trap the analytes
4. DCorrects for changes in flame and aspiration rate
Answer and explanation in LearnFree account
Question 7 · Hard
Gross count 4,500 cpm; empty-vial background 500 cpm; detection efficiency 40%. Expressed in disintegrations per minute, the true activity is:
1. A4,000 dpm
2. B10,000 dpm
3. C1,600 dpm
4. D11,250 dpm
Answer and explanation in LearnFree account
Question 8 · Hard
A 52-year-old man has milky serum: triglycerides 2,800 mg/dL. Sodium is 124 mmol/L by flame photometry and 139 mmol/L by direct ion-selective electrode; measured osmolality is 290 mOsm/kg. What explains the low flame photometer result?
1. ALipid particles scatter the emitted light in the flame
2. BDirect ISE overestimates sodium in lipaemic serum
3. CHypertriglyceridaemia draws water out of the cells
4. DLipid displaces plasma water and the total volume is analysed
Answer and explanation in LearnFree account
Question 9 · Medium
Fluorimetric methods are often 100 to 1,000 times more sensitive than absorption photometry. The main reason is that fluorimetry:
1. AUses light of a shorter wavelength for detection
2. BIs unaffected by temperature, pH and quenching
3. CMeasures emitted light against a near-zero background
4. DUses compounds that absorb far more incident light
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Question 10 · Hard
On an older colorimeter that reads only percent transmittance, a 100 mg/dL glucose standard reads 60% T and a diabetic patient's test reads 30% T, both against a reagent blank. What is the patient's glucose?
1. A50 mg/dL
2. B200 mg/dL
3. C236 mg/dL
4. D42 mg/dL
Answer and explanation in LearnFree account
Question 11 · Hard
Serum calcium by atomic absorption with an air-acetylene flame reads lower and lower as phosphate is added to spiked samples, although the calibration is correct. Which addition to the diluent corrects this?
1. ACaesium chloride, which suppresses ionisation
2. BSodium oxalate, which keeps calcium in solution
3. CLithium chloride, used as an internal standard
4. DLanthanum chloride, which binds the phosphate
Answer and explanation in LearnFree account
Question 12 · Easy
Lambert's law, taken on its own, states that absorbance is directly proportional to the:
1. AWavelength of the incident light
2. BIntensity of the incident light
3. CConcentration of the absorbing substance
4. DPath length of light through the solution
Answer and explanation in LearnFree account
Question 13 · Hard
During a run on a direct-reading flame photometer with no internal standard, sodium and potassium readings of the standard and the controls fall steadily over 30 minutes. The flame looks normal and the gas pressure is steady. What is the most likely cause?
1. AWrong interference filter for potassium
2. BIonisation of sodium in a cool flame
3. CLithium contamination of the diluent
4. DPartial blockage of the aspirator by protein
Answer and explanation in LearnFree account
Question 14 · Medium
Nephelometry is preferred to turbidimetry for measuring very low protein concentrations, such as CSF IgG, because nephelometry:
1. AUses ultraviolet light, which proteins absorb strongly
2. BMeasures the fall in light passing straight through
3. CMeasures a small signal against a nearly dark background
4. DNeeds no antibody to form immune complexes
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Question 15 · Hard
In a competitive RIA for a hormone, the zero standard binds 10,000 cpm. Standards give bound/zero-bound (B/B0) values of 80% at 5 ng/mL, 50% at 20 ng/mL and 30% at 50 ng/mL. A patient's tube binds 3,000 cpm. What is the patient's result?
1. A20 ng/mL
2. B5 ng/mL
3. CBelow 5 ng/mL
4. D50 ng/mL
Answer and explanation in LearnFree account
Question 16 · Easy
Carbon-12 and carbon-14 are isotopes of the same element. Their nuclei differ in the number of:
1. ANeutrons
2. BElectrons
3. CPositrons
4. DProtons
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Question 17 · Medium
A student holds a cuvette by its clear optical faces and leaves fingerprints on them before reading. The likely effect on the result is:
1. AFalsely low absorbance from extra transmitted light
2. BNo effect, because the blank corrects for it
3. CFalsely high absorbance from light scatter
4. DA shift of the reading to a longer wavelength
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Question 18 · Medium
Atomic absorption spectrophotometry is highly specific for the element measured mainly because:
1. AThe hollow cathode lamp emits lines of that same element
2. BA wide-band filter separates the element's colour
3. CThe flame excites only the atoms of that element
4. DThe detector responds to only one wavelength
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Question 19 · Hard
Radioactive decay is random, and the standard deviation of a count of N is the square root of N. How many counts must be collected for a counting error (1 SD) of 1%?
1. A1,000,000
2. B1,000
3. C100
4. D10,000
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Question 20 · Medium
A deeply jaundiced patient's serum is tested by a colorimetric method that is read near 450 nm. Which control best prevents a falsely high result?
1. AA serum blank read without the colour-forming reagent
2. BA reagent blank read with water in place of the serum
3. CA brighter lamp and a wider-bandpass filter
4. DDoubling the volume of serum taken for the test
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Question 21 · Medium
Fluorescence readings of the same sample fall steadily as the laboratory warms through the afternoon. The best explanation is that:
1. AWarm cuvettes scatter more of the light
2. BRising temperature increases collisional quenching
3. CWarm samples absorb less of the exciting light
4. DHeat shifts emission to a shorter wavelength
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Question 22 · Hard
A urea standard of 40 mg/dL, read in a 1 cm cuvette, gives an absorbance of 0.40. A patient's test, read by mistake in a 0.5 cm cuvette under otherwise identical conditions, gives 0.30. What is the true urea concentration?
1. A120 mg/dL
2. B30 mg/dL
3. C60 mg/dL
4. D15 mg/dL
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Question 23 · Hard
A student measures DNA at 260 nm. She uses the visible-range cuvettes from the colorimetry practical. The water blank reads above 2.0 absorbance, and the instrument cannot be set to zero. What is the problem?
1. AThe wavelength filter is set too far into the red
2. BOrdinary glass cuvettes absorb ultraviolet light
3. CDNA from the last sample is left in the cuvette
4. DThe tungsten lamp burns out at 260 nm
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Question 24 · Medium
In atomic absorption of calcium with a hot nitrous oxide-acetylene flame, an excess of potassium salt is added to samples and standards. Its purpose is to:
1. AAct as an internal standard for the reading
2. BBind phosphate so that calcium is released
3. CSuppress ionisation of calcium atoms in the flame
4. DRaise the flame temperature for better atomisation
Answer and explanation in LearnFree account
Question 25 · Medium
Technetium-99m is the most widely used isotope for diagnostic imaging (bone, thyroid and renal scans). The main reason is that it:
1. AEmits high-energy beta particles that kill tumour cells
2. BEmits alpha particles that are easy to detect
3. CHas a half-life of several months for long studies
4. DEmits 140 keV gamma rays and has a 6-hour half-life
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33 more Instrumentation and radioisotopes questions

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Study Biochemistry

Instrumentation and radioisotopes

The 125I label in a radioimmunoassay tube is counted with which detector?

In a swinging-bucket (horizontal) centrifuge head, the tubes during the run are:

In liquid scintillation counting, a yellow, haemolysed sample counts much lower than an equal amount of isotope in a clear sample. This loss is called:

A 45-year-old man with dyspepsia drinks a capsule of 14C-labelled urea, and 20 minutes later his breath sample contains a high level of 14CO2. This result indicates:

A 4-year-old child living near a battery recycling unit has abdominal colic and basophilic stippling of red cells. Which technique is standard for measuring blood lead?

In internal-standard flame photometry for sodium and potassium, lithium is added to every diluted sample. It improves accuracy mainly because it:

Gross count 4,500 cpm; empty-vial background 500 cpm; detection efficiency 40%. Expressed in disintegrations per minute, the true activity is:

A 52-year-old man has milky serum: triglycerides 2,800 mg/dL. Sodium is 124 mmol/L by flame photometry and 139 mmol/L by direct ion-selective electrode; measured osmolality is 290 mOsm/kg. What explains the low flame photometer result?

Fluorimetric methods are often 100 to 1,000 times more sensitive than absorption photometry. The main reason is that fluorimetry:

On an older colorimeter that reads only percent transmittance, a 100 mg/dL glucose standard reads 60% T and a diabetic patient's test reads 30% T, both against a reagent blank. What is the patient's glucose?

Serum calcium by atomic absorption with an air-acetylene flame reads lower and lower as phosphate is added to spiked samples, although the calibration is correct. Which addition to the diluent corrects this?

Lambert's law, taken on its own, states that absorbance is directly proportional to the:

During a run on a direct-reading flame photometer with no internal standard, sodium and potassium readings of the standard and the controls fall steadily over 30 minutes. The flame looks normal and the gas pressure is steady. What is the most likely cause?

Nephelometry is preferred to turbidimetry for measuring very low protein concentrations, such as CSF IgG, because nephelometry:

In a competitive RIA for a hormone, the zero standard binds 10,000 cpm. Standards give bound/zero-bound (B/B0) values of 80% at 5 ng/mL, 50% at 20 ng/mL and 30% at 50 ng/mL. A patient's tube binds 3,000 cpm. What is the patient's result?

Carbon-12 and carbon-14 are isotopes of the same element. Their nuclei differ in the number of:

A student holds a cuvette by its clear optical faces and leaves fingerprints on them before reading. The likely effect on the result is:

Atomic absorption spectrophotometry is highly specific for the element measured mainly because:

Radioactive decay is random, and the standard deviation of a count of N is the square root of N. How many counts must be collected for a counting error (1 SD) of 1%?

A deeply jaundiced patient's serum is tested by a colorimetric method that is read near 450 nm. Which control best prevents a falsely high result?

Fluorescence readings of the same sample fall steadily as the laboratory warms through the afternoon. The best explanation is that:

A urea standard of 40 mg/dL, read in a 1 cm cuvette, gives an absorbance of 0.40. A patient's test, read by mistake in a 0.5 cm cuvette under otherwise identical conditions, gives 0.30. What is the true urea concentration?

A student measures DNA at 260 nm. She uses the visible-range cuvettes from the colorimetry practical. The water blank reads above 2.0 absorbance, and the instrument cannot be set to zero. What is the problem?

In atomic absorption of calcium with a hot nitrous oxide-acetylene flame, an excess of potassium salt is added to samples and standards. Its purpose is to:

Technetium-99m is the most widely used isotope for diagnostic imaging (bone, thyroid and renal scans). The main reason is that it: