GC Theory

Question 1

What is the mobile phase in gas chromatography?

Answer 1

Carrier gas

Question 2

What is the stationary phase in gas chromatography?

Answer 2

Thin liquid coated film capillary column

Question 3

What are the best samples for GC analysis?

Answer 3

-Volatile substances that vaporise/evaporate at normal temps and pressure
-analytes that are chemically stable in heat

Question 4

What are bad samples for GC analysis?

Answer 4

-substances with low volatility
-substances that are thermally labile

Question 5

How are polarity, volatility, and boiling point connected?

Answer 5

-Polarity dictates the strength of intermolecular forces
-Boiling point measures the energy required to break the forces
-Volatility measures how easily a substance evaporates

Question 6

What are the applications for gas chromatography?

Answer 6

-Analysis of solvents and oils in home care products
-Explosives and GSR analysis
-Routine drug and alcohol testing
-Analysis of petroleum mixtures

Question 7

What is the criteria for the carrier gas?

Answer 7

-Chemically inert
-Stable
-Doesn’t react with other compounds

Question 8

What are the types of carrier gases?

Answer 8

-Helium
-Nitrogen
-Hydrogen

Question 9

What is the injection part of the GC system?

Answer 9

-Introduces sample to GC
-Rapidly vaporizes
-Mixes with carrier gas
-Transfers part of sample into the column

Question 10

What is the septum?

Answer 10

Rubber seal where syringe needle enters - prevents gas leaks

Question 11

What is the role of the glass liner in the GC system?

Answer 11

-Helps vaporize the sample evenly
-Prevents sample loss
-Can be packed or unpacked depending on application

Question 12

Why is split injection used?

Answer 12

-Prevents peak broadening
-Avoids column overload
-Better for concentrated samples

Question 13

How are liquid samples analysed using GC?

Answer 13

-On-column injector
-Heated flash vaporization
-Autosamplers
-Heated split/splitless

Question 14

How are solid samples analysed using GC?

Answer 14

-Thermal desorption
-Heated static headspace
-Programmed temperature vaporisation

Question 15

How are gas samples analysed using GC?

Answer 15

-Gas sampling valve
-Method analysis

Question 16

What is split mode injection?

Answer 16

-Split valve open
-Sample is vaporised in injection port
-Only a small amount of sample goes to column

Question 17

What is splitless mode injection?

Answer 17

-Split valve closed
-All sample enters column
-Useful for trace amounts of analyte

Question 18

What is the effect of splt ratio on peak shape?

Answer 18

-As split injection ratio decreases, more analyte is introduced to the capillary
-Higher injection volumes = band broadening and tailing

Question 19

Why are GC columns wrapped circularly?

Answer 19

-coiled to fit long tubing in a compact oven
-ensures uniform heating
-protects column
-maintains optimal gas flow

Question 20

What is a Wall-coated open tubular?

Answer 20

-Stationary phase = thin liquid film coated directly on wall
-Most common
-High efficiency
-Best for most GC

Question 21

What is a Support-coated open tubular?

Answer 21

-Inner wall coated with solid support
-Support holds liquid stationary phase

Question 22

What are the advantages of Wall-coated open tubular?

Answer 22

-High resolution
-Fast separations
-Good sensitivity

Question 23

What are the advantages of support-coated open tubular?

Answer 23

-Higher sample capacity than WCOT
-less common

Question 24

What is Porous-layer open tubular?

Answer 24

-Inner wall coated with solid support
-Used for gas separations

Question 25

What is the flame ionization detector?

Answer 25

-Most common -Eluent enters an air flame -Compound burn forms ions and electrons -Voltage applied between flame and collector electrode -ions collected and current produced

Question 26

What are the advantages of the flame ionization detector?

Answer 26

-Very sensitive to organic compounds -Wide linear range -Stable and reproducible

Question 27

What are the disadvantages of the flame ionization detector?

Answer 27

-Doesn't detect water, CO2, noble gases, NH3

Question 28

What is the thermal conductivity detector (TCD)?

Answer 28

-Universal detector -Heated filament kept at constant power -Analyte exits column and changes gas thermal conductivity -less sensitive than FID but non-destructive

Question 29

What are the advantages of the thermal conductivity detector?

Answer 29

-Universal -Non-destructive -Simple and robust -Good for permanent gases -Wide linear range -No flame required

Question 30

What are the disadvantages of the thermal conductivity detector?

Answer 30

-Lower sensitivity -Carrier gas dependant -Sensitive to flow and temperature changes -Low selectivity

Question 31

What are the other types of detectors?

Answer 31

-Nitrogen phosphorous detector -Mass spectrometry -Fourier Transform Infrared

Question 32

What is the rate of separation based on?

Answer 32

-Boiling point of components -Analyte partitioning and interaction with stationary phase

Question 33

What is the rate of migration based on?

Answer 33

-Carrier gas pressure (flow rate) -Oven temp

Question 34

How does separation work in the GC column?

Answer 34

-Mobile phase carries compounds through column from injector to detector -Different compounds interact different with the stationary phase -Causes compounds to move at different speeds and leads to separation

Question 35

What does interaction strength depend on?

Answer 35

-Polarity -Molecular size -Functional groups -Stationary phase chemistry

Question 36

Which analytes tend to have low boiling points?

Answer 36

Non-polar

Question 37

Which analytes tend to have high boiling points?

Answer 37

Polar

Question 38

What analytes elute faster?

Answer 38

-lower boiling points -due to high volatility

Question 39

What is the partition coefficient (K')?

Answer 39

partition ratio of the analyte

Question 40

What is the partition coefficient (K') equation?

Answer 40

K' = Cs/Cm

Question 41

What is Cs in the partition coefficient (K') equation?

Answer 41

Concentration of analyte in stationary phase

Question 42

What is Cm in the partition coefficient (K') equation?

Answer 42

Concentration of analyte in mobile phase

Question 43

What does peak resolution (Rs) depend on?

Answer 43

-Retention -Efficiency -Selectivity

Question 44

What are the key parameters to change to improve peak resolution?

Answer 44

-Temperature -Gas flowrate -Type of gas -Stationary phase thickness -Diameter of column

Question 45

What is the equation to change the retention factor in GC?

Answer 45

K' = tR - t0 / t0

Question 46

What is tR in the retention factor equation?

Answer 46

retention time of the analyte

Question 47

What is t0 in the retention factor equation?

Answer 47

Dead time - time for unretained compound to pass through column

Question 48

How does increased oven temperature affect retention time?

Answer 48

-compounds vaporise easier -weaker sp-analyte interactions -analytes spend more time in mp -retention time decrease as peaks elute faster -K' decreases

Question 49

What is isothermal GC?

Answer 49

-GC capillary kept at same temp through the run

Question 50

What is temperature programming GC?

Answer 50

-Temp increased over the run

Question 51

How does increasing the diameter of the capillary column affect peak resolution?

Answer 51

decreases peak resolution

Question 52

How do you calculate retention phase ratio?

Answer 52

B = dc/4df

Question 53

What is dc in retention phase ratio?

Answer 53

Column inner diameter

Question 54

what is df in retention phase ratio?

Answer 54

Stationary phase film thickness

Question 55

How does phase ratio affect retention time?

Answer 55

Higher = shorter retention time

Question 56

How does phase ratio affect resolution?

Answer 56

Lower = better resolution

Question 57

How does phase ratio affect peak broadening?

Answer 57

Higher = more peak broadening

Question 58

How does phase ratio affect sensitivity?

Answer 58

Lower = higher sensitivity