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A2 - Chemistry & Immunology - Harr > Chem 6.1 Instrumentation > Flashcards

Chem 6.1 Instrumentation Flashcards

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1
Q

Which formula correctly describes the relationship between absorbance and percent transmittance (%T)?
A. A = 2 – log %T
B. A = log T
C. A = –log %T
D. A = 2 – %T

A

A. A = 2 – log %T

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2
Q

A solution that has a transmittance of 1.0%T would have an absorbance of:
A. 1.0
B. 2.0
C. 1.0%
D. 99.0%

A

B. 2.0

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3
Q

In absorption spectrophotometry:
A. Absorbance is directly proportional to transmittance
B. Percent transmittance is directly proportional to concentration
C. Percent transmittance is directly proportional to the light path length
D. Absorbance is directly proportional to concentration

A

D. Absorbance is directly proportional to concentration

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4
Q

Which wavelength would be absorbed strongly by a red-colored solution?
A. 450 nm
B. 585 nm
C. 600 nm
D. 650 nm

A

A. 450 nm
##Footnote
A solution transmits light correponding in wavelength to its color, and usually absorbs light of wavelengths complementary to its color. A red solution transmits light of 600 to 650 nm and strongly absorbs 400 to 500 nm light.

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5
Q

A green-colored solution would show highest transmittance at:
A. 475 nm
B. 525 nm
C. 585 nm
D. 620 nm

A

B. 525 nm
##Footnote
Green light consists of wavelengths from 500 to 550 nm. A green-colored solution with a transmittance maximum of 525 nm and a 50-nm bandpass transmits light of 525 nm and absorbs light below 475 nm and above 575 nm. A solution that is green would be quanititated using a wavelength that it absorbs strongly, such as 450 m,.

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6
Q

SITUATION: A technologist is performing an enzyme assay at 340 nm using a visible range spectrophotometer with a tungsten light source. After setting the wavelength and adjusting the readout to zero %T with the light path blocked, a cuvette with deionized water is inserted. With the light path fully open and the 100%T control at maximum, the instrument readout will not rise above 90%T. What is the most appropriate first course of action?
A. Replace the source lamp
B. Insert a wider cuvette into the light path
C. Measure the voltage across the lamp terminals
D. Change the wavelength to 335 nm

A

A. Replace the source lamp

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7
Q

Which type of monochromator produces the purest monochromatic light in the ultraviolet (UV) range?
A. A diffraction grating and a fixed exit slit
B. A sharp cutoff filter and a variable exit slit
C. Interference filters and a variable exit slit
D. A prism and a variable exit slit

A

D. A prism and a variable exit slit

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8
Q

Which monochromator specification is required to measure the true absorbance of a compound having a natural absorption bandwidth of 30 nm?
A. 50-nm bandpass
B. 25-nm bandpass
C. 15-nm bandpass
D. 5-nm bandpass

A

D. 5-nm bandpass

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9
Q

Which photodetector is most sensitive to low levels of light?
A. Barrier layer cell
B. Photodiode
C. Diode array
D. Photomultiplier tube

A

D. Photomultiplier tube

The photmultiplier tube uses dynodes of increasing voltage to amplify the current produced by the photosensitive cathode. It is 10,000 times as sensitive as a barrier layer cell, which has no amplification. A photomultiplier tube requires a DC-regulated lamp because it responds to light fluctuations caused by the alternating current (AC) cycle.

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10
Q

Which condition is a common cause of stray light?
A. Unstable source lamp voltage
B. Improper wavelength calibration
C. Dispersion from second-order spectra
D. Misaligned source lamp

A

C. Dispersion from second-order spectra

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11
Q

A linearity study is performed on a visible spectrophotometer at 650 nm and the following absorbance readings are obtained:
Concentration of Standard: Absorbance
10.0 mg/dL: 0.20
20.0 mg/dL: 0.41
30.0 mg/dL: 0.62
40.0 mg/dL: 0.79
50.0 mg/dL: 0.92

The study was repeated using freshly prepared standards and reagents, but results were identical to those shown. What is the most likely cause of these results?

A. Wrong wavelength used
B. Insufficient chromophore concentration
C. Matrix interference
D. Stray light

A

D. Stray light

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12
Q

Which type of filter is best for measuring stray light?
A. Wratten
B. Didymium
C. Sharp cutoff
D. Neutral density

A

C. Sharp cutoff

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13
Q

Which of the following materials is best suited for verifying the wavelength calibration of a spectrophotometer?
A. Neutral density filters
B. Potassium dichromate solutions traceable to the National Bureau of Standards reference
C. Wratten filters
D. Holmium oxide glass

A

D. Holmium oxide glass
##Footnote
Wavelength accuracy is verified by determining the wavelength reading that gives the highest absorbance (or transmittance) when a substance with a narrow natural bandwidth (sharp absorbance or transmittance peak) is scanned. ..

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14
Q

Why do many optical systems in chemistry analyzers utilize a reference light path?
A. To increase the sensitivity of the measurement
B. To minimize error caused by source lamp fluctuation
C. To obviate the need for wavelength adjustment
D. To reduce stray light effects

A

B. To minimize error caused by source lamp fluctuation

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15
Q

Which component is required in a spectrophotometer to produce a spectral absorbance curve?
A. Multiple monochromators
B. A reference optical beam
C. Photodiode array
D. Laser light source

A

C. Photodiode array

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16
Q

The half bandwidth of a monochromator is defined by:
A. The range of wavelengths passed at 50% maximum transmittance
B. One half the lowest wavelength of optical purity
C. The wavelength of peak transmittance
D. One half the wavelength of peak absorbance

A

A. The range of wavelengths passed at 50% maximum transmittance

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17
Q

The reagent blank corrects for absorbance caused by:
A. The color of reagents
B. Sample turbidity
C. Bilirubin and hemolysis
D. The intrinsic absorbance of both the reagents and sample matrix

A

A. The color of reagents

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18
Q

A plasma sample is hemolyzed and turbid. What is required to perform a sample blank to correct the measurement for the intrinsic absorbance of the sample when performing a spectrophotometric assay?
A. Substitute deionized water for the sample
B. Dilute the sample 1:2 with a standard of known concentration
C. Substitute saline for the reagent
D. Use a larger volume of the sample

A

C. Substitute saline for the reagent

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19
Q

Which instrument requires a highly regulated direct current (DC) power supply?
A. A spectrophotometer with a barrier layer cell
B. A colorimeter with multilayer interference filters
C. A spectrophotometer with a photomultiplier tube
D. A densitometer with a photodiode detector

A

C. A spectrophotometer with a photomultiplier tube

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20
Q

Which statement regarding reflectometry is true?
A. The relation between reflectance density and concentration is linear
B. Single-point calibration can be used to determine concentration
C. 100% reflectance is set with an opaque film called a white reference
D. The diode array is the photodetector of choice

A

C. 100% reflectance is set with an opaque film called a white reference

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21
Q

Bichromatic measurement of absorbance can correct for interfering substances if:

A. The contribution of the interferent to absorbance is the same at both wavelengths
B. Both wavelengths pass through the sample simultaneously
C. The side band is a harmonic of the primary wavelength
D. The chromogen has the same absorbance at both wavelengths

A

A. The contribution of the interferent to absorbance is the same at both wavelengths

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22
Q

Which instrument requires a primary and secondary monochromator?
A. Spectrophotometer
B. Atomic absorption spectrophotometer
C. Fluorometer
D. Nephelometer

A

C. Fluorometer

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23
Q

Which of the following statements about fluorometry is accurate?
A. Fluorometry is less sensitive than spectrophotometry
B. Fluorometry is less specific than spectrophotometry
C. Unsaturated cyclic molecules are often fluorescent
D. Fluorescence is directly proportional to temperature

A

C. Unsaturated cyclic molecules are often fluorescent

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24
Q

Which of the following components is not needed in a chemiluminescent immunoassay analyzer?
A. Source lamp
B. Monochromator
C. Photodetector
D. Wash station

A

A. Source lamp

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25
Q

Which substance is used to generate the light signal in electrochemiluminescence?
A. Acridinium
B. Luminol
C. Dioxetane phosphate
D. Ruthenium

A

D. Ruthenium

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26
Q

Light scattering when the wavelength is greater than 10 times the particle diameter is described by:
A. Rayleigh’s law
B. The Beer–Lambert law
C. Mie’s law
D. The Rayleigh–Debye law

A

A. Rayleigh’s law

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27
Q

Which statement regarding nephelometry is true?

A. Nephelometry is less sensitive than absorption spectrophotometry
B. Nephelometry follows Beer’s law
C. The optical design is identical to a turbidimeter except that a helium–neon (HeNe) laser light source is used
D. The detector response is directly proportional to concentration

A

D. The detector response is directly proportional to concentration

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28
Q

The purpose of the nebulizer in an atomic absorption spectrophotometer that uses a flame is to:
A. Uniformly distribute the sample in the flame
B. Cause ejection of an outer shell electron
C. Reduce evaporation of the sample
D. Burn off organic impurities

A

A. Uniformly distribute the sample in the flame

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29
Q

A flameless atomic absorption spectrophotometer dehydrates and atomizes a sample using:
A. A graphite capillary furnace
B. An electron gun
C. A thermoelectric semiconductor
D. A thermospray platform

A

A. A graphite capillary furnace

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30
Q

When measuring lead (Pb) in whole blood using atomic absorption spectrophotometry, what reagent is required to obtain the needed sensitivity and precision?
A. Lanthanum
B. Lithium
C. Triton X-100
D. Chloride

A

C. Triton X-100

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31
Q

Interference in atomic absorption spectrophotometry caused by differences in viscosity is called:
A. Absorption interference
B. Matrix effect
C. Ionization interference
D. Quenching

A

B. Matrix effect

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32
Q

Which of the following is required when measuring magnesium by atomic absorption spectrophotometry?
A. A mercury vapor source lamp
B. A chopper to prevent optical interference from magnesium emission
C. A neutral density filter
D. A 285-nm reference beam to correct for background absorption

A

B. A chopper to prevent optical interference from magnesium emission

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33
Q

When measuring calcium by atomic absorption spectrophotometry, which is required?
A. An organic extraction reagent to deconjugate calcium from protein
B. An internal standard
C. A magnesium chelator
D. Lanthanum oxide to chelate phosphates

A

D. Lanthanum oxide to chelate phosphates

34
Q

Ion selective analyzers using undiluted samples have what advantage over analyzers that use a diluted sample?
A. Can measure over a wider range of concentration
B. Are not subject to pseudohyponatremia caused by high lipids
C. Do not require temperature equilibration
D. Require less maintenance

A

B. Are not subject to pseudohyponatremia caused by high lipids

35
Q

Select the equation describing the potential that develops at the surface of an ion selective electrode (ISE).

A. van Deemter equation
B. van Slyke equation
C. Nernst equation
D. Henderson–Hasselbalch equation

A

C. Nernst equation

36
Q

The reference potential of a silver–silver chloride electrode is determined by the:
A. Concentration of the potassium chloride filling solution
B. Surface area of the electrode
C. Activity of total anion in the paste covering the electrode
D. The concentration of silver in the paste covering the electrode

A

A. Concentration of the potassium chloride filling solution

37
Q

The term RT/nF in the Nernst equation defines the:
A. Potential at the ion-selective membrane
B. Slope of the electrode
C. Decomposition potential
D. Isopotential point of the electrode

A

B. Slope of the electrode

38
Q

The ion-selective membrane used to measure potassium is made of:
A. High-borosilicate glass membrane
B. Polyvinyl chloride dioctylphenyl phosphonate ion exchanger
C. Valinomycin gel
D. Calomel

A

C. Valinomycin gel

39
Q

The response of a sodium electrode to a 10-fold increase in sodium concentration should be:
A. A 10-fold drop in potential
B. An increase in potential of approximately 60 mV
C. An increase in potential of approximately 10 mV
D. A decrease in potential of approximately 10 mV

A

B. An increase in potential of approximately 60 mV

The Nernst equation predicts an increase of approx 60 mV per 10-fold increase in sodium activity. For sodium:

E = E degree + RT/nF x 2.3 log10[ Na+]
RT/nF x 2.3 = 60 mV at 37C

Therefore:
E =E degree + 60 mV x log10[Na+]

If sodium concentration is 10 mmol/L, then:

E =E degree + 60 mV x log10[Na+] = E degree + 60 mV

If sodium concentration increases from 10 mmol/L, then:

E =E degree + 60 mV x log10[Na+] =

E degree + 60 mV x 2 = E degree + 120 mV

40
Q

Which of the electrodes below is a current-producing (amperometric) rather than a voltage-producing (potentiometric) electrode?
A. Clark electrode
B. Severinghaus electrode
C. pH electrode
D. Ionized calcium electrode

A

A. Clark electrode

The Clark electrode is composed of two half cells that generate current, not voltage. It is used to measure partial pressure of oxygen (PO2), and is based on an amperometric method called polarography. When -0.8 V is applied to the cathode, O2 is reduced, causing current to flow. Current is proportional to the PO2 of the sample.

41
Q

Which of the following would cause a “response” error from an ISE for sodium when measuring serum but not the calibrator?
A. Interference from other electrolytes
B. Protein coating the ion-selective membrane
C. An over-range in sodium concentration
D. Protein binding to sodium ions

A

B. Protein coating the ion-selective membrane

Response is the time required for an electrode to reach steady-state potential. Ion-selective analyzers use a microprocessor to monitor electrode response, slope, drift, and noise. When an electrode gives an acceptable response time when measuring an aqueous calibrator, but not when measuring serum, the cause is often protein buildup on the membrane.

42
Q

In polarography, the voltage needed to cause depolarization of the cathode is called the:
A. Half-wave potential
B. Isopotential point
C. Decomposition potential
D. Polarization potential

A

C. Decomposition potential

In polarography, a minimum negative voltage must be applied to the cathode to cause reduction of metal ions (or O2) in solution. This is called the decomposition potential. It is concentration dependent (dilute solutions require greater negative voltage) and can be determined by using the Nernst equation.

43
Q

Persistent noise from an ISE is most often caused by:
A. Contamination of the sample
B. Blocked junction at the salt bridge
C. Over-range from high concentration
D. Improper calibration

A

B. Blocked junction at the salt bridge

44
Q

Which element is reduced at the cathode of a Clark polarographic electrode?
A. Silver
B. Oxygen
C. Chloride
D. Potassium

A

B. Oxygen

The Clark electrode is designed to measure O2. O2 diffuses through a gas-permeable membrane covering the electrode. It is reduced at the cathode, which is made of platinum or other inert metal. Electrons are supplied by the anode, which is made of silver. The net reaction is:

4 KCl + 2H2O + O2 + 4 Ag° —> 4 AgCl + 4 KOH

45
Q

Which of the following statements accurately characterizes the coulometric titration of chloride?
A. The indicator electrodes generate voltage
B. Constant current must be present across the generator electrodes
C. Silver ions are formed at the generator cathode
D. Chloride concentration is inversely proportional to titration time

A

B. Constant current must be present across the generator electrodes

46
Q

In the coulometric chloride titration:
A. Acetic acid in the titrating solution furnishes the counter ion for reduction
B. The endpoint is detected by amperometry
C. The titrating reagent contains a phosphate buffer to keep pH constant
D. Nitric acid (HNO3) is used to lower the solubility of AgCl

A

B. The endpoint is detected by amperometry

Reduction of Ag+ back to Ag(o) generates the current, which signals the endpoint. The titrating reagent contains HNO3, acetic acid, H2O, and either gelatin or polyvinyl alcohol. the HNO3 furnishes nitrate, which is reduced at the generator cathode, forming ammonium ions. The ammonium becomes oxidized back to nitrate at the indicator anode. Gelatin or polyvinyl alcohol is needed to prevent pitting of the generator anode. Acetic acid lowers the solubility of AgCl, preventing dissociation back to Ag+.

47
Q

Which of the following compounds can interfere with the coulometric chloride assay?
A. Bromide
B. Ascorbate
C. Acetoacetate
D. Nitrate

A

A. Bromide

Chloride assays based on either coulometric or chemical titration are subject to positive interference from other anions and electronegative radicals that may be titrated instead of chloride ions. These include other halogens, such as bromide, cyanide, and cysteine.

48
Q

Which of the following compounds contributes to the osmolality of plasma?
A. Cholesterol
B. Protein
C. Drug metabolites
D. Triglyceride

A

C. Drug metabolites

Osmolality is the concentration ( in moles [mol]) of dissolved solute per kilogram solvent. Proteins and lipids are not in solution, and do not contribute to osmolality. The nonionized solutes, such as glucose and urea, contribute 1 osmole per mole per kilogram (Osmol/mol/kg) water, whereas dissociated salts contribute 1 Osmol/mol of each dissociated ion or radical.

49
Q

One mole per kilogram water (H2O) of any solute will cause which of the following:
A. Lower the freezing point by 1.86°C
B. Raise vapor pressure by 0.3 mm Hg
C. Lower the boiling point by 0.52°C
D. Lower osmotic pressure by 22.4 atm

A

A. Lower the freezing point by 1.86°C

Both freezing point and vapor pressure are lowered by increasing solute concentration. Boiling point and osmotic pressure are raised. Increasing solute concentration of a solution opposes a change in its physical state and lowers the concentration of H2O molecules.

50
Q

What component of a freezing point osmometer measures the sample temperature?
A. Thermistor
B. Thermocouple
C. Capacitor
D. Electrode

A

A. Thermistor

A thermistor is a temperature-sensitive resistor. The resistance to current flow increases as temperature falls. The temperature at which a solution freezes can be determined by measuring the resistance of the thermistor. Resistance is directly proportional to the osmolality of the sample.

51
Q

What type of measuring circuit is used in a freezing point osmometer?
A. Electrometer
B. Potentiometer
C. Wheatstone bridge
D. Thermal conductivity bridge

A

C. Wheatstone bridge

The resistance of the thermistor is measured using a network of resistors called a Wheatstone bridge. When the sample is frozen, the bridge is balanced by using a calibrated variable resistor so that no current flows to the readout. The resistance required to balance the meter is equal to the resistance of the thermistor.

52
Q

Which measurement principle is employed in a vapor pressure osmometer?
A. Seebeck
B. Peltier
C. Hayden
D. Darlington

A

A. Seebeck
##Footnote
The term Seebeck effect refers to the increase in voltage across the two junctions of a thermocouple caused by a difference in the temperature at the junctions. Increasing osmolality lowers the dew point of a sample. When sample is cooled to its dew point, the voltage change across the thermocouple is directly proportional to osmolality.

53
Q

The freezing point osmometer differs from the vapor pressure osmometer in that only the freezing point osmometer:
A. Cools the sample
B. Is sensitive to ethanol
C. Requires a thermoelectric module
D. Requires calibration with aqueous standards

A

B. Is sensitive to ethanol

54
Q

The method for measuring iron or lead by plating the metal and then oxidizing it is called:
A. Polarography
B. Coulometry
C. Anodic stripping voltometry
D. Amperometry

A

C. Anodic stripping voltometry

55
Q

The term isocratic is used in high-performance liquid chromatography (HPLC) to mean that the:
A. Mobile phase is at constant temperature
B. Stationary phase is equilibrated with the mobile phase
C. Mobile phase consists of a constant solvent composition
D. Flow rate of the mobile phase is regulated

A

C. Mobile phase consists of a constant solvent composition

An isocratic separation uses a single mobile phase of constant composition, pH, and polarity, and requires a single pump. Some HPLC separations use a gradient mobile phase to increase distance between peaks. Gradients are made by mixing two or more solvents by using a controller to change the proportions of solvent components.

56
Q

The term reverse phase is used in HPLC to indicate that the mobile phase is:
A. More polar than the stationary phase
B. Liquid and the stationary phase is solid
C. Organic and the stationary phase is aqueous
D. A stronger solvent than the stationary phase

A

A. More polar than the stationary phase

57
Q

What is the primary means of solute separation in HPLC using a C18 column?
A. Anion exchange
B. Size exclusion
C. Partitioning
D. Cation exchange

A

C. Partitioning

Stationary phases (column packings) used in HPLC separate solutes by multiple means, but in reverse-phase HPLC using C18, the relative solubility between the mobile phase and stationary phase is most important and depends on solvent polarity, pH, and ionic strength. Stationary phases with particles less than 2.5 microns in diameter are designated as ultraperformance liquid chromatography (UPLC) because they increase column resolution over standard size sorbets.

58
Q

The most commonly used detector for clinical gas–liquid chromatography (GLC) is based on:
A. UV light absorbance at 254 nm
B. Flame ionization
C. Refractive index
D. Thermal conductance

A

B. Flame ionization

59
Q

What type of detector is used in high-performance liquid chromatography with electrochemical detection (HPLC–ECD)?
A. Calomel electrode
B. Conductivity electrode
C. Glassy carbon electrode
D. Polarographic electrode

A

C. Glassy carbon electrode

60
Q

In GC, the elution order of volatiles is usually based on the:
A. Boiling point
B. Molecular size
C. Carbon content
D. Polarity

A

A. Boiling point

61
Q

Select the chemical that is used in most HPLC procedures to decrease solvent polarity.
A. Hexane
B. Nonane
C. Chloroform
D. Acetonitrile

A

D. Acetonitrile

62
Q

In thin-layer chromatography (TLC), the distance the solute migrates divided by the distance the solvent migrates is the:
A. tR
B. Kd
C. Rf
D. pK

A

C. Rf

63
Q

Which reagent is used to extract alkaline drugs, such as cocaine metabolites, from urine?
A. Acid and sodium chloride
B. Alkali and organic solvent
C. Chloroform and sodium acetate
D. Neutral solution of ethyl acetate

A

B. Alkali and organic solvent

64
Q

What is the purpose of an internal standard in HPLC and GC methods?
A. To compensate for variation in extraction and injection
B. To correct for background absorbance
C. To compensate for changes in flow rate
D. To correct for coelution of solutes

A

A. To compensate for variation in extraction and injection

65
Q

What is the confirmatory method for measuring drugs of abuse?
A. HPLC
B. Enzyme-multiplied immunoassay technique (EMIT)
C. Gas chromatography with mass spectroscopy (GC-MS)
D. TLC

A

C. Gas chromatography with mass spectroscopy (GC-MS)

66
Q

The fragments typically produced and analyzed in methods employing mass spectroscopy are typically:
A. Of low molecular size ranging from 10 to 100 daltons
B. Cations caused by electron loss or proton attachment
C. Anions caused by bombarding the molecule with an electron source
D. Neutral species formed after excited molecules form a stable resonance structure

A

B. Cations caused by electron loss or proton attachment

67
Q

What component is used in a GC-MS but not used in an LC-MS?
A. Electron source
B. Mass filter
C. Detector
D. Vacuum

A

A. Electron source

68
Q

What process is most often used in LC-MS to introduce the sample into the mass filter?
A. Electrospray ionization (ESI)
B. Chemical ionization
C. Electron impact ionization
D. Fast atom bombardment

A

A. Electrospray ionization (ESI)

HPLC instruments use solvent rather than gas’s to separate molecules. The sample is converted into a gaseous state by ESI before it enters the mass filter. ESI uses a small-bore tube that forms a 1- to 4- micron nozzle at the mass filter inlet and which is charged by several kilovolts. The sample enters the tube along with inert drying gas. The tube is heated to evaporate the solvent, but unlike the electron impact used in GC-MS, the ionizer is not under vacuum. When a droplet of the sample reaches the nozzle, it becomes highly charged. The size of the droplet is decreased as a result of evaporation. This causes the charge density to become excessive, and the droplets break apart. The tiny charged droplets repel each other and break apart again, forming a plume. These particles are drawn into the mass filter by “ion optics” (a system of repeller plates, counter-electrode, and magnets). ESI does not result in extensive fragmentation, producing mostly the parent or “molecular” ion, a process called soft ionization.

69
Q

In mass spectroscopy, the term base peak typically refers to:
A. The peak with the lowest mass
B. The peak with the most abundance
C. A natural isotope of the molecular ion
D. The first peak to reach the mass detector

A

B. The peak with the most abundance

70
Q

Which of the following is the most useful method when screening for errors of amino acid and organic acid metabolism?
A. Two-dimensional TLC
B. GC
C. Electrospray ionization tandem-mass spectroscopy (MS/MS)
D. Inductively coupled-mass spectroscopy (ICP-MS)

A

C. Electrospray ionization tandem-mass spectroscopy (MS/MS)

71
Q

In MS/MS, the first mass filter performs the same function as:
A. The ion source
B. The chromatography column
C. Extraction
D. The vacuum system

A

B. The chromatography column

72
Q

SITUATION: A GC-MS analysis using nitrogen as the carrier gas shows an extensively noisy baseline. A sample of the solvent (ethyl acetate) used for the extraction procedure was injected and showed the same noise. Results of an Autotune test showed the appearance of a base peak at 16 with two smaller peaks at 17 and 18. These results indicate that:

A. The solvent is contaminated
B. The carrier gas is contaminated
C. There is electrical noise in the detector
D. The ion source is dirty

A

B. The carrier gas is contaminated

73
Q

Why is vacuum necessary in the mass filter of a GC-MS?
A. Ionization does not occur at atmospheric pressure
B. It prevents collision between fragments
C. It removes electrons from the ion source
D. It prevents contamination

A

B. It prevents collision between fragments

74
Q

What method is used to introduce the sample into a mass spectrometer for analysis of a trace element?
A. ESI
B. Laser desorption
C. ICP ionization
D. Direct injection

A

C. ICP ionization

Mass spectrometers can be used to measure trace metals, but the atoms need to be vaporized and ionized like molecules before they enter the mass filter. This is done by introducing the sample into a very hot plasma (6k-10k K) called a torch. The torch is made by circulating argon through inner and outer quartz tubes. The tubes are wrapped with a coil of wire that receives a radio frequency. This creates current flow through the wire and a magnetic field at the torch end. Argon atoms are excited by the current and magnetic field and ionize. When the argon is ignited by a spark, it forms a plasma. The sample is mixed with argon at the other end to create an aerosol. When it reaches the torch, the solvent is evaporated and the energy from the torch and collisions with argon ions cause ejection of outer-shell electrons, forming cations of the element.

ICP-MS is used to measure any trace element that readily forms cations.

75
Q

Which component is needed for a thermal cycler to amplify DNA?
A. Programmable heating and cooling unit
B. Vacuum chamber with zero head space
C. Sealed airtight constant-temperature chamber
D. Temperature-controlled ionization chamber

A

A. Programmable heating and cooling unit

76
Q

In real-time polymerase chain reaction (PCR), what value is needed to determine the threshold?
A. Background signal
B. Melt temperature
C. Maximum fluorescence
D. Threshold cycle

A

A. Background signal

77
Q

Given the following real-time PCR amplification curve, what is the threshold cycle (Ct)? (pg 306)

A. 15
B. 20
C. 25
D. 30

A

C. 25

78
Q

In addition to velocity, what variable is also needed to calculate the relative centrifugal force (g force) of a centrifuge?
A. Head radius
B. Angular velocity coefficient
C. Diameter of the centrifuge tube
D. Ambient temperature in degrees Centigrade

A

A. Head radius

79
Q

Which of the following situations is likely to cause an error when weighing with an electronic analytical balance?
A. Failure to keep the knife edge clean
B. Failure to close the doors of the balance before reading the weight
C. Oxidation on the surface of the substitution weights
D. Using the balance without allowing it to warm up for at least 10 minutes

A

B. Failure to close the doors of the balance before reading the weight

80
Q

When calibrating a semiautomatic pipet that has a fixed delivery of 10.0 µL using a gravimetric method, what should be the average weight of deionized water transferred?
A. 10.0 µg
B. 100.0 µg
C. 1.0 mg
D. 10.0 mg

A

D. 10.0 mg

Gravimetric and spectrophotometric analyses are the two methods used to verify pipet volume accuracy and precision. Since spectrophotometric analysis involves dilution, gravimetric analysis is associated with greater certainty. At 20°C, the density of pure water is 0.99821 g/mL. Therefore, each microliter weighs almost exactly 1.0 mg.

A2 - Chemistry & Immunology - Harr (18 decks)

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