topic 6 gas chrom Flashcards
what is basic principle of chromatographic separation (3)
- analytical technique that can be used to separate a mixture into individual components
- involves distribution of sample components between two phases: stationary and mobile
- components that are distributed preferentially in mobile phase will elute out of chromatographic system faster than those distributed preferentially in stationary phase
stationary phase
place where sample mixture is separated into individual components
mobile phase
transport to deliver sample from injection port to column to detector
what makes a sample able to be analysed by gas chromatograph (2) + examples
- volatile, low boiling point (low molecular mass)
- thermally stable under gc operating temperatures
- eg. petrol chemicals, pesticides, solvents, hydrocarbons
components of gc (5)
- carrier gas system (mob. phase)
- sample introduction system
- column (stat. phase)
- detection system
- recorder/computer workstation
requirements for carrier gas (2) and why
what are commonly used carrier gases? (4)
requirements:
- chemically inert: cannot react with stat phase/sample
- high purity: impurities give rise to background signals, noisy baseline, or degrade stat phase
commonly used: nitrogen, helium, argon, hydrogen
why is autosample preferred over manual injection when introducing sample (4)
- autosampler can hold up to 150+ samples
- instantaneous injection
- same amount of sample injected every time
- each operator might perform manual injection differently which affects components’ retention time and reproducibility of results
packed vs capillary column (6)
- packed is physically packed with particles; capillary has packing material coated onto wall with the inside hollow
- packed has shorter column length, larger diameter than capillary
column length: packed 2-3m; capillary 5-60m
diameter: packed 3-6mm; capillary 0.2-0.6mm - packed has higher stationary phase loading
- packed has larger sample size, capillary has smaller
- packed has poorer resolving power
- packed has poorer separation efficiency
why does packed column have poorer separation efficiency than capillary column? (5)
- packed column is physically packed with particles
- sample components follow different paths and reach detector at different times
- broader peak obtained
- in capillary column, sample components follow same path and reach detector around the same time
- sharper peak obtained
factors that affect elution rate of sample and how (2)
- boiling point: higher bp = slower
2. polarity: more similar polarity to stat phase = slower
two modes of column temperature operation
isothermal and temperature programmed
function of detector
senses presence of a component different from carrier gas and converts the information into an electrical signal
pros (3) and cons (1) of thermal conductivity detector
advantages
• general purpose, non-destructive
• no fuel gases required, only carrier gas
• simple, robust, cheaper
disadvantages
• not sensitive: not suitable for trace analysis
pros (3) and cons (2) of flame ionisation detector
advantages
• highly selective for organic compounds with C-H bonds
• high sensitivity (lower limit of detection
• rugged, reliable, easy to maintain and operate
disadvantages
• destructive
• requires hydrogen, air, and carrier gas to operate
mechanism of operation of flame ionisation detector (4)
C-H + O → CHO+ + e-
• current produced is proportional to amount of sample being burnt
• current sensed by electrometer is converted to digital form and sent to an output device
• more C-H bonds = higher signal intensity
