Gas Chromatography Flashcards
What is the most common carrier gas?
Helium
What kind of compounds are used in gas chromatograpy?
Compounds that can be vaporized without decomposition
What are the requirments for analytes in GC?
Compounds must have sufficient volatility (bp < 500 degrees celsius) and thermal stability
What are the two ways analytes are introduced into the instrument?
- Through organic, unpolar solvent where the compounds are extracted from the aquatic phase and into the organic phase.
- Through gas phase where the volatille analyte transfer into the headspace from the liquid
What are the three capillary GC columns and what is the stationary phase?
- Wall-coated open tubular column (stationary liquid phase)
- Porous-layer open tubular column (stationary solid phase)
- Packed column (solid support coated with stationary liquid phase)
What type of chromatography is wall-coated open tubular column?
Partition chromatography
What stationary phase does wall-coated open tubular column have?
It has a liquid stationary phase bonded to the inside of the capilary wall
What type of chromatography is porous-layer open column?
Adsorption chromatography
What stationary phase does porous-layer open tubular column have?
It has a solid stationary phase particles on the inside wall of the column
What does open tubular columns offer compared with packed columns?
- Higher resolution
- Shorter analysis time
- Greater sensitivity
- Lower sample capacity
What are the standard dimensions for a WCOT column?
length = 25 m
inner diameter = 0,25 mm
thickness of stationary phase = 0,25 micrometer
What are molecular sieves?
Molecular sieves are inorganic or organic material with cavities into which small molecules enter and are partially retained
What column has molecular sieves?
Porous-layer open tubular
What does PLOT stand for?
Porous-layer open tubular column
What does WCOT stand for?
Wall-coated open tubular column
How does the column diameter affect the resolution and why?
With decreased column diameter the volume of mobile phase also decreased. This will mean shorter diffusion distances, faster equilibrium stabilization and less mass transfer.
Plate height will decrease. Plate number and resolution will increase. This will mean a more efficient column
How does a decreased column diameter affect the retention time?
With decreased column diameter the volume of mobile phase will decrease more than the stationary phase. This means more analyte will disolve in the stationary phase and increases the retention factor and retention time
How does a larger column length affect the resolution?
Longer column will increase the plate number and create narrower peaks and higher resolution.
How much does the resolution of a double column length increase by?
Increases by the factor of square root of 2
How does the film thickness affect the resolution for volatile compounds?
Increased film thickness, increased stationary phase volume, increased retention factor, increased resolution for volatile compounds
What are analytes mainly seperated on?
Boiling points, analytes with low boiling points move faster through column
What is isothermal analysis?
Isothermal analysis is when you have a constant temperature during analysis
What is thermal programming?
Thermal programming is when you increase the temperature during analysis
What is the perk of temperature programing?
It will fit more compounds and will therefor have a faster analysis time and narrower peaks.
When is split injection used?
When analytes of interest is more than 0,1% of the sample
How does split injection work?
- Sample is injected into the hot injector
- Sample is vaporized
- Small portion of sample is allowed into the column
What is the split ratio?
The portion of the sample the does not reach the column
When is the splitless injection used?
Used for trace analysis (less than 0,01% of the sample)
How does splitless injection work?
- The sample is injected into the hot injector and is vaporized
- Most of the sample is allowed to enter the column
- After a certain time (splitless time) the splitvent is opened and the remaining parts of the sample is sweept out through the split vent
What size is the starting band in split injection?
The starting band is narrow
What size is the starting band in splitless injection?
Starting band is broad
What is focusing techniques used for?
To focus the results in narrow starting bands
What kind of compounds are solvent trapping used on and how does it work?
Volatile compounds.
Initial oven tempertures are 20-40 degrees celsius below the boiling point of the solvent. The solvent condenses at the begining of the column and traps the analytes.
What kind of compound is coldtrapping used on and how does it work?
Used with compounds with high boiling point.
Initial column temperature is 150 degrees celsius below boiling point of the analyte. The analytes condense at the beginning of the column and are trapped by the stationary phase.
What are the four most common detectors for GC?
- Flame ionization
- Electron capture
- Nitrogen-phosphorous
- Mass spectrometic
How does flame ionization work and what does it detect?
The sample passes through a hydrogen flame and as the analytes are burned in the flame ions are formed creating a signal.
It detects analytes containing carbon
How does the electron capture detector work and what does it detect?
- The carrier gas is ionized by electrons of high energyy which gives rise to a current that is kept constant by a variable frequence pulse
- When analytes with high electron affinity enter the detector they capture electrons
- The detector then increases the frequency of pulses to maintain a constant current
- The detector signal is the frequency of pulses
Selective for eg. nitiles, halogens and conjugated carbonyls
How does the nitrogen-phosphorous detector work and what does it detect?
A glasbead containing rubidium sulfate convert N and P containing compounds into ions which give rise to a current (detection signal)
Selective for compounds containing N and P
What is sample preparing?
It’s the process of turing a sample into a form that is suitable for analysis
What is liquid-liquid extraction?
It is the process where organic compounds are tranferred to the organic phase
How does solid-phase microextraction work?
Analytes are absorbed to the stationary phase of the column. The analytes are then eluated with successively stronger solvents and collected in fractions
How does solid-phase microextraction work?
It extracts compouds from liquid or air without using solvents by using a syringe with a
fused silica fiber coated with stationary phase which can be extended or retracted using the attached needle.
How can you determine the identity of a peak?
- Retention time
- NPD, ECD as they provide som degree of specificity
- MS, as it can identify the compound by its mass spectrum
How can you identify the concentration of a compound?
- Calibration curve (peak area (y) vs concentration (x))
- Internal standard
What is an internal standard?
It is a compound which is added in the same amount to all samples and standards which therefor corrects for carations of the injected sample volumes
