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Analytical > GC > Flashcards

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
Q

Optimal flow rates of carrier gases

A

N < He < H

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26
Q

Anayltes Diffusion carrier gases

A

H2 > He > N2

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27
Q

NItrogen gas

A

Best efficiency

but smallest range of optimal flows (high C term)

gives lower detection limit with ionisation

28
Q

Hydrogen Carrier Gas

A

highest optimal flow rates

highest diffusion

explosive

29
Q

Helium carrier gas

A

in between N2 and H2

good efficiency and good flow rates

most common

but expensive

30
Q

Injection Types

A

Split

Splitless

On Column

31
Q

Split injection

A

Only a fraction of sample goes to column rest to waste

32
Q

Split injection advantages

A

simple

small sample sizes less overloading

high flow rate

high resolution

inject without dilution

inject dirty samples

higher temps

33
Q

Split injection disadvantages

A

no trace analysis

split may not represent sample

34
Q

Splitless injection

A

Entire sample introduced to column

35
Q

Splitless Advantages

A

high sensitivity trace analysis

36
Q

Splitless Disadvantages

A

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
Q

On column

A

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
Q

solvent trapping

A

analyte and solvent condense

solute molecules become trapped in solvent band

leads to sharper peaks

39
Q

Cold Trapping

A

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
Q

Kovat’s Retention Index

A

for comparing retention of hydrocarbons

I = 100n

higher index means higher retention and eluted later

41
Q

Detectors

A

Thermal Conductivity Detector

Flame Ionisation Detector

Electron Capture Detector

42
Q

Thermal Conductivity Detector

A
  • 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
Q

Flame Ionisation Detector

A
  • analyte ionised by flame
  • hydrocarbons selectively oxidised creating current
  • current proportional to carbon atoms
  • selective for organics
44
Q

Electron Capture Detector

A

Carrier gas ionised by beta-radiation ejecting electrons

electronegative analytes absorb free electron

combine with cations decreasing current

selective for inorganics

45
Q

Detector sensitivity

A

ECD > FID > TCD

46
Q

Increasing film thickness

A

Reduced analyte wall interactions

increased sample capacity

broader peaks

increased column bleed

good for analytes with low BPs

good for high conc samples

47
Q

decreasing film thickness

A

sharper peaks (retention)

decreased bleed

increased signal to noise

decreased sample capacity

shorter retention

good for high BP analytes

increased wall interactions

48
Q

Phase ratio

A

Ranks retention ability of columns

smaller ratio means more retention (low BP volatiles)

higher ration means less retention (heavier analytes)

49
Q

Separation mechanisms

A

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
Q

Sample Preparations Methods

A

Derivatisation

pyrolysis

Solid phase extraction (SPE)

purge and trap

51
Q

Derivatisation

A

chemically react analyte functional groups to improve:

  • sample volatility (decrease polarity)
  • selectivity
  • detectability
  • stability
52
Q

Derivatisation Reactions

A
  • silylation (less surface adsorption as well)
  • acylation
  • alkylation
  • esterification (FAME)
    e. g. acids, hydroxyls, O, S, N, P etc
53
Q

Pyrolysis

A

Thermal decomposition of sample

creates more volatile analytes

characteristic of original structure

though may not work

e.g DNA proteins, micro-organisms

54
Q

Solid Phase MicroExtraction

A
55
Q

SPME

A

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
Q

Purge and trap

A

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
Q

Non polar interactions/retention

dimethyl polysiloxane

A
  • dispersion forces or van der Waals
  • increase with size
  • elute in order of BP
58
Q

Intermediate Polar Columns Interactions/Retention

methylphenylpolysiloxan

A

Dipole dipole

π-π bonding

hydrogen bonding induced dipole

basic interactions

combination of BP and force strength

59
Q

Polar Column Interactions/Retention

carbowax

A

strong dipole dipole

strong induced dipole

hydrogen bonding

basic interactions

separation determined by differences in strength of analyte interactions/polarity

60
Q

Smaller internal diameter

A

decreased phase ratio

higher retention

higher efficiency N=L/dc

smaller capacity

shorter columns with same efficiency

61
Q

Increasing Column Length

A

increased efficiency N=L/dc

smaller than proportional increase in resolution

run time increases faster

increased back pressure

62
Q

Total Ion Chromatogram

A

Sums all mass peaks of scan over time

63
Q

Extracted Ion Chromatogram

A

Single mass monitored over time

64
Q

Selective Ion Monitoring

A

Monitor a few m/z

more sensitive

target analysis

65
Q

Selected Reaction Monitoring

A

Delivers specific fragment to detector

sensitive and quantitive

66
Q

Time of Flight

A

Mass of ion assigned based on time to reach detector

  • larger m/z slower than small m/z
  • sensitive and full scan
67
Q

Spectral Matching

A

Identify unknown compound by fragmentation

Analytical (7 decks)

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