Gas Chromatography

Question 1

Column length and inner diameter of capillary column

Answer 1

Length: 30 or 60 m
Inner diameter: 0.1 to 1.0 mm

(2 columns can be in one GC oven)

Question 2

High efficiency column size

Answer 2

0.1 mm (narrow peaks observed)

Question 3

How to increase sample capacity

Answer 3

Increase column diameter, column length, thickness (more) stationary phase

Question 4

What is the mobile phase in GC?

Answer 4

Carrier gas

Question 5

Properties of carrier gas

Answer 5

• non-reactive towards analyte (inert gas)
• gas does not impact selectivity
• non-flammable (however H2 with air is flammable,
  would need leak sensor if H2
  used)
• cheap and available in high purity (99.999% or better)
• compatible with detector

Question 6

What do different carrier gases have?

Answer 6

Different analysis time and pressure restrictions (due to different gas viscosities)

Need to consider the separation but also best choice for detector

Question 7

Other considerations for carrier gas

Answer 7

May also have to consider the gases needed for
detection systems. For example He is frequently
used as carrier gas in mass spectrometry and it
allows faster analysis times than N2 without
significant change in plate height.

Helium often preferred over H2 due to additional
safety requirements to prevent explosion from
reaction of H2 with air if a leak occurs

Question 8

How to reduce analysis time

Answer 8

Increase flow rate (increase column internal diameter)

Question 9

How to control the carrier gas

Answer 9

Pressure regulators are used to control the gas supply
for carrier gas as well as additional gases for
detectors

Typically use a double stage regulator, 50-350 psig to reduce pressure of gas reaching GC and regulate flow rate of gas

Pressure measured with gauges

Question 10

Injection port components

Answer 10

Septum
- problems (coring, septum bleeding)

Injection port liners (glass/quartz)
- prevent decomposition of sample

Injection port
- introduces liquid/gas sample into the system using microsyringe

Question 11

Properties of liquid stationary phase

Answer 11

Chemically inert
Low vapour pressure
Thermally stable
Wide operating temperature range
Low viscosity
Similar properties with analyte

Question 12

Non-polar stationary phase interaction with solute

Answer 12

London-dispersion
Elution order based on boiling point
Retention order increases with bp

Question 13

What is the most important factor for determining retention time?

Answer 13

Oven temperature

Question 14

Role of injector

Answer 14

Introduce sample
No discrimination
No sample decomposition
Solvent peak should not interfere with solute peaks

Question 15

Which injection methods are better for quantitative analysis

Answer 15

Solvent flush method
Air plug method

Question 16

Flash evaporation

Answer 16

Injection port temperature is heated at least 20-40 degrees (Celsius) higher than start column to ensure vaporization

Question 17

Which injector type would be used for low analyte concentration?

Answer 17

Splitless mode
(all injected sample goes into column)

Question 18

Which injector type would be used for high analyte concentration?

Answer 18

Split mode
(not all injected sample goes into column)

Question 19

Split Ratio

Answer 19

Determined by flow rates and valves in injection port

S.R. = (split vent flow + column flow) divided by column flow

Question 20

Benefits of PTV injector

Answer 20

Reduces discrimination between analytes with range of bp’s
Reduces breakdown of analytes
Prevents non-volatile material from entering the column
Can perform a pre-separation of target analytes from solvent or other components of the sample

Question 21

Benefits of cold-on-column injector

Answer 21

Good for high volatility analytes to elute near solvent front
May minimize decomposition of sample in injector
Good with respect to discrimination towards analytes of higher bp

Question 22

Disadvantages of cold-on-column injector

Answer 22

Can have non-volatile material on column
Require more maintenance

Question 23

Role of detector

Answer 23

Register presence of target analytes
Obtain qual/quantitative info.

Question 24

Universal vs selective detectors

Answer 24

Universal - theoretically detects “all” compounds that elute from the column

Selective - only detects compounds with a specific property

Question 25

Mass-flow sensitive vs concentration sensitive

Answer 25

Mass-Flow Sensitive Detector - signal is proportional to the absolute mass of an analyte reaching the detector cell per unit of time (i.e. g/s)

Concentration Sensitive Detector – signal is proportional to the mass of an analyte in the detector cell per unit of volume of carrier gas (i.e. g/mL)

Question 26

Destructive vs Non-destructive

Answer 26

Destructive - sample becomes completely consumed during analysis (i.e. burnt up in a flame)

Non-Destructive – sample is not consumed, the sample can go on for further analysis (i.e. to another detector in series or collected if sample is valuable)

Question 27

Thermal conductivity detector

Answer 27

Not good for trace analysis
Good for difficult to detect analytes

Question 28

Flame ionization detector

Answer 28

‘Universal’ for carbon containing compounds
Good reliability/rugged
Flow rate of hydrogen/air can impact response

Question 29

Nitrogen phosphorus detector

Answer 29

Silylated derivatives and silylation reagents can reduce detector response
Bead has lifetime so periodically needs to be replaced
Great for analysis for nitrogen/phosphorus

Question 30

Flame photometric detector

Answer 30

Specific for phosphorus/sulfur
Hydrogen enriched flame excited atoms/molecules
Counts/monitors emission

Question 31

Electron capture detector

Answer 31

Selective and very high sensitivity for electrophilic species –halogenated and nitro compounds (detection limit 10-16mol/mL)

Radiative source emits beta particles
Detector detects secondary thermal electrons ionized by the beta particles