Gas Chromatography Flashcards
Column length and inner diameter of capillary column
Length: 30 or 60 m
Inner diameter: 0.1 to 1.0 mm
(2 columns can be in one GC oven)
High efficiency column size
0.1 mm (narrow peaks observed)
How to increase sample capacity
Increase column diameter, column length, thickness (more) stationary phase
What is the mobile phase in GC?
Carrier gas
Properties of carrier gas
- 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
What do different carrier gases have?
Different analysis time and pressure restrictions (due to different gas viscosities)
Need to consider the separation but also best choice for detector
Other considerations for carrier gas
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
How to reduce analysis time
Increase flow rate (increase column internal diameter)
How to control the carrier gas
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
Injection port components
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
Properties of liquid stationary phase
Chemically inert
Low vapour pressure
Thermally stable
Wide operating temperature range
Low viscosity
Similar properties with analyte
Non-polar stationary phase interaction with solute
London-dispersion
Elution order based on boiling point
Retention order increases with bp
What is the most important factor for determining retention time?
Oven temperature
Role of injector
Introduce sample
No discrimination
No sample decomposition
Solvent peak should not interfere with solute peaks
Which injection methods are better for quantitative analysis
Solvent flush method
Air plug method
Flash evaporation
Injection port temperature is heated at least 20-40 degrees (Celsius) higher than start column to ensure vaporization
Which injector type would be used for low analyte concentration?
Splitless mode
(all injected sample goes into column)
Which injector type would be used for high analyte concentration?
Split mode
(not all injected sample goes into column)
Split Ratio
Determined by flow rates and valves in injection port
S.R. = (split vent flow + column flow) divided by column flow
Benefits of PTV injector
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
Benefits of cold-on-column injector
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
Disadvantages of cold-on-column injector
Can have non-volatile material on column
Require more maintenance
Role of detector
Register presence of target analytes
Obtain qual/quantitative info.
Universal vs selective detectors
Universal - theoretically detects “all” compounds that elute from the column
Selective - only detects compounds with a specific property
Mass-flow sensitive vs concentration sensitive
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)
Destructive vs Non-destructive
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)
Thermal conductivity detector
Not good for trace analysis
Good for difficult to detect analytes
Flame ionization detector
‘Universal’ for carbon containing compounds
Good reliability/rugged
Flow rate of hydrogen/air can impact response
Nitrogen phosphorus detector
Silylated derivatives and silylation reagents can reduce detector response
Bead has lifetime so periodically needs to be replaced
Great for analysis for nitrogen/phosphorus
Flame photometric detector
Specific for phosphorus/sulfur
Hydrogen enriched flame excited atoms/molecules
Counts/monitors emission
Electron capture detector
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
