Deck 25: Gas Chromatography

Question 1

Cold trapping and solvent trapping is used with____________________ injection to give____________________ peaks.

A)split;sharper
B)on-column;faster eluting
C)splitless;sharper
D)split;faster eluting
E)splitless;faster eluting

Answer 1

splitless;sharper

Question 2

Which statement is NOT true for flame ionization detectors?

A)The detector responds to hydrocarbon analytes,which are burned in a mixture of H₂ and air after eluting from the column.
B)During combustion the sample carbon atoms produce CH radicals,which are thought to produce CHO⁺ ions and electrons in flames.
C)In the absence of analyte,a current of ~10⁻¹⁴ A flows between the flame tip and the collector.Analytes produce a current of ~10⁻¹² A.
D)Flame ionization detectors respond to non-hydrocarbon compounds,but to a lesser degree than hydrocarbons.
E)The detector responds proportionally to analyte mass over seven orders of magnitude.

Answer 2

Flame ionization detectors respond to non-hydrocarbon compounds,but to a lesser degree than hydrocarbons.

Question 3

Which is NOT a sample preparation technique for use with gas chromatography?

A)purge and trap
B)solid-phase microextraction
C)thermal desorption
D)stir-bar sorptive extraction
E)matrix matching

Answer 3

matrix matching

Question 4

Which statement is NOT true for gas chromatography detectors?

A)Thermal conductivity detectors respond to the thermal conductivity difference between analyte and carrier gas.
B)Flame ionization detectors are used only with inorganic compounds.
C)Thermal conductivity detectors respond to all analytes.
D)Flame ionization detectors generate signal by burning the analyte,generating electrons.
E)For thermal conductivity and flame ionization detectors,detector response is proportional to analyte concentration.

Question 5

To maximize solid-phase microextraction analyte recovery,keep all of the following in mind,except:

A)"like dissolves like" in selecting fiber.
B)agitate or stir the sample.
C)select thick or porous film for volatiles.
D)add 25% NaCl and adjust pH,reducing pH for acids and increasing pH for bases.
E)purchase solid-phase microextractors from reputable suppliers.

Question 6

As the temperature of a GC column increases,the____________________ of the analytes increases,the retention time________________________________________and the peak shapes____________________ .

A)vapor pressure;;increases;;sharpen
B)solubility;;decreases;;sharpen
C)vapor pressure;;decreases;;sharpen
D)equilibrium;;increases;;broaden
E)temperature;;increases;;sharpen

Question 7

For quantitative analysis with gas chromatography:

A)the retention time reported by the instrument for a compound is proportional to the quantity of that compound.
B)the area of a peak reported by the instrument for a compound is proportional to the quantity of that compound.
C)the calculated response factor value for a compound is proportional to the quantity of that compound.
D)the peak height reported by the instrument for a compound is inversely proportional to the quantity of that compound.
E)the peak shape reported by the instrument for a compound is proportional to the quantity of that compound.

Question 8

Which statement is NOT true for GC carrier gases?

A)N₂,He,and H₂ are the most common carrier gases.
B)Optimal linear velocity increases in the order N₂ < He < H₂.
C)The fastest separations can be achieved with N₂ as carrier gas.
D)N₂ gives a lower detection limit than He with a flame ionization detector.
E)Helium is the most common carrier gas and is compatible with most detectors.

Question 9

The temperature program for a separation starts at a temperature of 50 ^oC and ramps the temperature up to 270 ^oC at a rate of 10 ^oC/minute.Which statement is NOT true for this separation?

A)At 10 ^oC/minute,a total of 22 minutes is needed to reach 270 ^oC.
B)Strongly retained solutes will remain at the head of the column while the temperature is low.
C)Weakly retained solutes will separate and elute early in the separation.
D)The vapor pressure of strongly retained solutes will increase as temperature increases.
E)Strongly retained analytes will give broad peaks.

Question 10

For method development,multiple decisions must be made.Order the development decisions below in the proper order.
I detector
II goal of analysis
III injection
IV sample preparation
V column

A)II,III,I,IV,V
B)II,I,V,III,IV
C)II,V,III,I,IV
D)II,IV,I,V,III
E)II,IV,III,I,V

Question 11

The thermal conductivity detector functions by measuring the________________________________________for a hot filament that is the result of a(n)____________________ to compensate for the____________________ of the filament by the mixture of analyte and carrier gas that elutes from the column.

A)voltage increase;decreased resistance;;heating
B)resistance change;increased voltage;;cooling
C)voltage decrease;increased resistance;;heating
D)resistance change;increased voltage;;heating
E)voltage increase;increased resistance;;cooling

Question 12

____________________ columns are used with____________________ chromatography.

A)Wall-coated open tubular;gas-solid
B)Porous-layer open tubular;liquid-solid
C)Wall-coated open tubular;liquid-liquid
D)Porous-layer open tubular;liquid-liquid
E)Wall-coated open tubular;liquid-solid

Question 13

Under what conditions are split,splitless,and on-column injection used?
I Split injection is preferred for samples where the analyte of interest constitute > 0.1% of the sample.
II On-column injection is used for samples that decompose above their boiling point.
III Splitless injection is preferred for introducing small volumes of sample onto the column.
IV Split injection is preferred for samples with high boiling point solutes in low boiling point solvents.
V Splitless injection is preferred for trace analysis of analytes that are less than 0.01% of the sample.

A)I,II,and III
B)II,III,and IV
C)II,IV,and V
D)II,III,and V
E)I,II,and V

Question 14

The response factor between an alcohol and an alkane is 2.54.An internal standard with a concentration of 25.9 nM has a peak area of 92832.The peak area for the alcohol is 23422.What is the concentration of alcohol in the sample?

Question 15

The elution order for solutes from a gas chromatography column is loosely in order of:

A)decreasing molar mass.
B)decreasing freezing point.
C)decreasing dielectric constant.
D)increasing boiling points.
E)increasing entropy change.

Question 16

The first 3 to 10 meters of a capillary column serves as a guard column and retention gaps.Guard columns:

A)improve peak shape by separating volatile solvent from less volatile solutes prior to chromatography.
B)divert a portion of the sample to waste to improve peak shape.
C)accumulate nonvolatile substances to prevent column contamination
D)begin separation by retaining less volatile solutes from volatile solutes.
E)increase the number of theoretical plates,reducing plate height.

Question 17

To quantify the amount of ethanol in an organic sample,a technician prepares a standard solution containing 0.1473 grams ethanol and 0.1847 grams decane in 1.203 grams dichlormethane.GC analysis returns peak areas of 9,930 for ethanol and 7,893 for decane.Next,0.9831 grams unknown and 0.2932 grams decane are added to 1.2119 grams dichloromethane.GC analysis returns peak areas of 6,382 for ethanol and 11,283 for decane.What is the percent by mass ethanol in the sample?

Question 18

Which is NOT true for split injection?

A)The sample is completely vaporized and mixed in the mixing chamber by a combination of high temperature and brisk carrier flow.
B)At the split point,a small fraction of vapor enters the column,but most passes through the waste vent.
C)The proportion of sample that does not reach the column is the split ratio.
D)Split ratios typically range from 50:1 to 500:1.
E)After injection,the waste vent is closed and flow rate increased.

Question 19

Which of the following is INCORRECT for quantitative or qualitative analysis with gas chromatography?

A)The confirmation ion collected by a mass spectrometer is used to quantify a peak.
B)Selected ion monitoring identifies a peak by monitoring the m/z characteristic for the compound.
C)Internal standards quantify compounds by adding a known amount of compound to the sample and calculating the concentration from peak areas for the compound and internal standard,and the response factor for the two compounds.
D)Spiking identifies a peak by a known compound to the unknown.If the peak area increases,the peak has been identified.
E)Linear response for concentration and detector response is required for quantitative analysis.

Question 20

Which of the following statements are true for GC columns?
I GC columns are capillaries composed of fused silica and coated with a polyimide.
II Column inner diameters are typically 0.10 cm to 0.53 cm and the columns are 15 cm to 100 cm in length,with 30 cm the most common.
III A wall-coated open tubular column has a liquid stationary phase coating the inside wall of the column.
IV A porous-layer column is filled with a highly porous stationary phase.
V A packed column is a column filled with a solid stationary support coated in liquid stationary phase.

A)I,III,and V
B)II and IV
C)III,IV,and V
D)I and II
E)I,II,III,IV,and V