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GC Flashcards

1
Q

GC Instrument

A
  1. Injector port/oven
  2. column and column oven
  3. detector
  4. Computer
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2
Q

GC Columns

A
  1. Packed
  2. WCOT - wall coated open tubular
  3. SCOT - solid-support coated tubular
  4. PLOT - polymer layer open tubular
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3
Q

Open tubular benefits

A
  • much lower flow resistance
  • can be much longer
  • can be much narrower
  • very high plate counts
  • high efficiencies
  • better separtion
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4
Q

Increasing Resolution

A

Smaller internal diameter

longer column

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5
Q

Selectivity

A

How strongly the analyte interacts with the stationary phase

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6
Q

Choosing Column

A
  1. choose stationary phase that compliments polarity of analyte
  2. retention depends on structure of analyte and stationary phase
  3. low retention leads to poor separation
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7
Q

GC phases

A

mobile phase is a carrier gas

stationary phase is a non-volatile liquid bound to column or solid particles/polymer

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8
Q

Partition Coefficient factors

A

activity constant (intermolecular force strength)

vapour pressure

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9
Q

selectivity calculation

A

ratio between two analytes distribution coefficients (or activity coefficients)

determined by interaction between analytes and stationary phase

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10
Q

Non polar SP

A

low selectivities

separates by boiling point/distillation

e.g. dimethyl polysiloxane

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11
Q

polar SP

A

separates by dipole differences additionally

e.g cyanopropyl phenyl

or carbowax

or phenyl groups

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12
Q

Efficiency calculation

A

Column length divided by height equivalent of a theoretical plate

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13
Q

Golay Equation

A

Height of separation

no eddy diffusion

only longitudal diffusion and resistance to mass transit

C term for both liquid and gas phases

(for thin films liquid term is 0)

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14
Q

factors of liquid C term

A
  • stationary film thickness
  • diffusion coefficient in stationary phase
  • retention
  • flow rate
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15
Q

factors of B term

A
  • diffusion of analyte in mobile phase
  • obstruction factor
  • flow rate
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16
Q

factors of gas C term

A
  • diffusion coefficient in gas
  • column diameter
  • retention
  • flow rate
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17
Q

Overloading

A

Compound mass exceeds capacity of WCOT

peak fronting

lower efficiency and resolution

poor reproducibility

peak distortion

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18
Q

Film Thickness Advantages

A

Increased Interaction

Increased Retention

better resolution of early peaks

reduced tailing (silanol shielding)

better for volatilve

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19
Q

Film Thickness cons

A

Increased column bleed (deteriation)/ noise

poorer resolution (longer retention)

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20
Q

Volatile vs non volatile compounds column

A

volatiles should be analysed on thick film columns to increase retention

high molecular weight compounds should be analysed thinner film columns to decrease run time and minimize bleed from higher temps

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21
Q

Increasing length advantages

A

Increased interation

increased retention

higher efficiency

resolution increases by square of length

though longer run times

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22
Q

Column Temperature

A

Alters distribution constants/partitioning towards mobile phase

increases vapour pressure

decreases retention

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23
Q

Gradient temperature advantages

A

same resolution

sharper late eluting peaks

faster run time

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24
Q

Carrier gases

A

H2, He, N2

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25
Optimal flow rates of carrier gases
N \< He \< H
26
Anayltes Diffusion carrier gases
H2 \> He \> N2
27
NItrogen gas
Best efficiency but smallest range of optimal flows (high C term) gives lower detection limit with ionisation
28
Hydrogen Carrier Gas
highest optimal flow rates highest diffusion explosive
29
Helium carrier gas
in between N2 and H2 good efficiency and good flow rates most common but expensive
30
Injection Types
Split Splitless On Column
31
Split injection
Only a fraction of sample goes to column rest to waste
32
Split injection advantages
simple small sample sizes less overloading high flow rate high resolution inject without dilution inject dirty samples higher temps
33
Split injection disadvantages
no trace analysis split may not represent sample
34
Splitless injection
Entire sample introduced to column
35
Splitless Advantages
high sensitivity trace analysis
36
Splitless Disadvantages
Prone to overloading Time Consuming colder temps temperature program to vaporise entire dilution required optimisation required not well suited to volatile compounds (must be 30˚C below solvent)
37
On column
Used for samples that deompose above thier boiling points no hot injector injected straight on to column solvent trapping column temp vaporises sovent after injection
38
solvent trapping
analyte and solvent condense solute molecules become trapped in solvent band leads to sharper peaks
39
Cold Trapping
low temperature high BP anlytes condense at head of column low BP analytes and solvent elute early separated then column heated to separate high BP
40
Kovat's Retention Index
for comparing retention of hydrocarbons I = 100n higher index means higher retention and eluted later
41
Detectors
Thermal Conductivity Detector Flame Ionisation Detector Electron Capture Detector
42
Thermal Conductivity Detector
- Anlayte changes thermal conductivity - change measured by electrodes relative to carrier gas - requires significant difference (e.g. H2) - higher flow rates decrease sensitivity - least sensitive - but universl
43
Flame Ionisation Detector
- analyte ionised by flame - hydrocarbons selectively oxidised creating current - current proportional to carbon atoms - selective for organics
44
Electron Capture Detector
Carrier gas ionised by beta-radiation ejecting electrons electronegative analytes absorb free electron combine with cations decreasing current selective for inorganics
45
Detector sensitivity
ECD \> FID \> TCD
46
Increasing film thickness
Reduced analyte wall interactions increased sample capacity broader peaks increased column bleed good for analytes with low BPs good for high conc samples
47
decreasing film thickness
sharper peaks (retention) decreased bleed increased signal to noise decreased sample capacity shorter retention good for high BP analytes increased wall interactions
48
Phase ratio
Ranks retention ability of columns smaller ratio means more retention (low BP volatiles) higher ration means less retention (heavier analytes)
49
Separation mechanisms
separations are effected by partitioning the partitioning depends on the analyte and the column non-polar columns are mainly van der waals and higher boiling points have longer retention More polar columns have dipole-dipole interactions which affect retention (not just BP)
50
Sample Preparations Methods
Derivatisation pyrolysis Solid phase extraction (SPE) purge and trap
51
Derivatisation
chemically react analyte functional groups to improve: - sample volatility (decrease polarity) - selectivity - detectability - stability
52
Derivatisation Reactions
- silylation (less surface adsorption as well) - acylation - alkylation - esterification (FAME) e. g. acids, hydroxyls, O, S, N, P etc
53
Pyrolysis
Thermal decomposition of sample creates more volatile analytes characteristic of original structure though may not work e.g DNA proteins, micro-organisms
54
Solid Phase MicroExtraction
55
SPME
solid phase micro extraction put fibre in liquid sample/head space to collect volatile analytes by adsorption then put fibre in injector and thermally desorb
56
Purge and trap
purge liquid sample with gas trap volatile analytes in absorbent tube layers of increasing absorbent strength aim to collect 100% of volatiles from sample thermally desorb in injector and cold trap
57
Non polar interactions/retention dimethyl polysiloxane
- dispersion forces or van der Waals - increase with size - elute in order of BP
58
Intermediate Polar Columns Interactions/Retention methylphenylpolysiloxan
Dipole dipole π-π bonding hydrogen bonding induced dipole basic interactions combination of BP and force strength
59
Polar Column Interactions/Retention carbowax
strong dipole dipole strong induced dipole hydrogen bonding basic interactions separation determined by differences in strength of analyte interactions/polarity
60
Smaller internal diameter
decreased phase ratio higher retention higher efficiency N=L/dc smaller capacity shorter columns with same efficiency
61
Increasing Column Length
increased efficiency N=L/dc smaller than proportional increase in resolution run time increases faster increased back pressure
62
Total Ion Chromatogram
Sums all mass peaks of scan over time
63
Extracted Ion Chromatogram
Single mass monitored over time
64
Selective Ion Monitoring
Monitor a few m/z more sensitive target analysis
65
Selected Reaction Monitoring
Delivers specific fragment to detector sensitive and quantitive
66
Time of Flight
Mass of ion assigned based on time to reach detector - larger m/z slower than small m/z - sensitive and full scan
67
Spectral Matching
Identify unknown compound by fragmentation

Analytical (7 decks)

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