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GC Flashcards

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1
Q

In GC lowering the temperature will do what to retention times

A

Will increase

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2
Q

Two methods to control the temperature in GC

A

Isothermally, temperature programming

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3
Q

Isothermally

A

Constant column temperature

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4
Q

When would you use Isothermally programming

A

Only useful if components have a similar boiling point. Otherwise will produce overlapping peaks which means the mixture wasn’t well separated. This method would also have long run times

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5
Q

Temperature programming

A

Changing the temperature during the separation.

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6
Q

When to use temperature programming

A

If components have a similar boiling point

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7
Q

Improving separation conditions will have what affect

A

Provide better separation
reduce the runtimes
Allow complex mixtures to be separated

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8
Q

Types of columns used in GC

A

Packed or capillary

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9
Q

Examples of capillary columns in GC

A

SCOT and WCOT (both examples of fused silica glass capillary columns)

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10
Q

SCOT

A

Extra layer of support
Support costed open tubular column

there is an extra layer of support material inside the column which the stationary phase is coated to

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11
Q

WCOT

A

Wall coated open tubular column

The stationary phase is bound to the capillary coating.

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12
Q

GC:
Mobile phase
Columns
Stationary phase
Temperature control
Detectors
Mode

A

MP: inert gas eg helium
Packed or capillary columns
High boiling point liquid eg PEG
Via oven (temperature programming or Isothermally)and injector port
FID
Partition

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13
Q

Benefits of using an FID

A

Similar response to different kinds of compounds, detects a wide concentration range, linear response, good resolution and is non destructive

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14
Q

Packed column
Material
Dimensions
Contents
Efficiency
Diagram

A

Glass or steel
5mm diameter 1-10m long
Crushed fire brick support w liquid coating SP ie PEG
300-3000 plates per metre
Packed (not open)
Uneven SP coating

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15
Q

Capillary column
Material
Dimensions
Contents
Efficiency
Diagram

A

Silica glass
0.5mm diameter, 10-100m long
1um SP layer
SP has high MW, heat stable polymer
High efficiency More than 10,000 plates per metre
polyimide used as coating to strengthen silica glass

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16
Q

Why are capillary columns more efficient?

A

The even coating of SP on silica ensures even amounts of sample hitting the detector

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17
Q

SCOT

A

Has a solid support coated with SP on the capillary column

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18
Q

WCOT

A

a thin layer of stationary phase, is coated on the capillary’s inner wall.

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19
Q

Split injection

A

Controls the fraction of the sample that enters the colimn

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20
Q

Splitless injector

A

Loads the entire sample into the column which is useful when completing trace analysis

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21
Q

SP in GC

A

High MW
Heat stable
Polymer eg silica

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22
Q

Things that affect quality of separation in GC

A

Flow rate and temperature

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23
Q

How an FID works

A

The eluent mixes with H and is burned in air, this produces a flame that contains electrons and the cation. Applying a potential between the flame’s tip and the collector gives a current that is proportional to the concentration of cations in the flame.

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24
Q

Schematic GC diagram

A

MP and ideal characteristics
Injector split or split-less
Oven with temperature regulation
Columns (packed or capillary) holds SP
SP ideal characteristics
Detector FID/mass spec

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25
State one advantage and one disadvantage of metal columns used in GLC.
Are robust but may react with acids eg steel
26
State one advantage and one disadvantage of glass columns used in GLC
are less reactive and see-through but they are fragile
27
Isothermal chromatogram
constant temperature can lead tooverlapping peaks ie not as good separation and long run times e.g. 60 minutes
28
Temperature programming chromatogram
provides a good baseline, good separation of peaks, and shorter runtimes e.g. 20 minutes rather than 60 minutes .
29
Benefit of using temperature programming
Better separation of complex mixtures.
30
If you have a complex sample which will take too long to elute, what is wrong with simply raising the temperature? Why is temperature programming more useful?
If you just increase the temperature this will provide a shorter run time. However, it will not provide better separation as the peaks will be overlapping. Temperature programming is better in this example. If you are using temperature program between 30 and 90° does a provide better separation in the beginning and delays the later analytes.
31
Packed columns v capillary column characteristics:
Packed columns: are made of glass or steel, are large and short and provide low efficiency Capillary columns: are made of silica glass, are thin and long. With a layer of stationary phase coated on the capillary and provide high efficiency
32
Which column provides better efficiency
a capillary column is better then a packed column. Capillary columns are more efficient i.e. they provide thin narrow peaks on the chromatogram.
33
Ideal characteristics of GC stationary phase
Dissolves analytes Forms a thin film Inert (does not react) Thermally stable (doesn’t degrade) Lower volatility (does not evaporate ) Able to form a thin film
34
Mobile phase ideal characteristics GC
Gas is the mobile phase. Pure Free from oxygen Compatible with detector
35
If you believe a peak is component a. What what should happen if you add some to the sample and re-run? What is this called?
If it is a: the peak should get bigger If it is not a: a new peak at a different retention time should appear and no change to the original sample This is called “spiking “
36
What does the flame do in FID. Explain how flames and ionisation are involved in the working of FID use a diagram
the analytes are mixed with hydrogen, they come into contact with oxygen and are burned in the flame. This creates ions and they are counted. The ions are detected across the anode and cathode.
37
Advantages FID:
Good for linear response Good for a wide range of concentrations Good at detecting low concentrations
38
Disadvantages of a FID:
I had of the components have a low molecular weight Not good at detecting halogens Destroys the sample
39
The characteristics of a good detector are the same for GLC and HPLC. Learn top six
Similar response to all kinds of compounds Wide concentration range Low detection limit Linear response Good resolution Fast response to change Stability Reliability Ease of use Working temp. range ambient – 400oC And be non-destructive
40
Why are capillary columns more efficient and packed columns:
Capillaries columns have an even layer of stationary phase does allows analytes to hit the detector more evenly
41
Partition chromatography SP MP Interactions Diagram Uses
Support material eg C18 Gas/liquid eg methanol and water Solution/ polarity Diagram sample dissolved in liquid phase coated on surface of solid support) HPLC paper
42
Ion Exchange SP MP Interactions Diagram Uses
Polymer SO3- N(CH3)3+ Ionic solution Electrostatic Diagram + molecule bonds to - beads and - molecule elute quickly HPLC/ column
43
Size exclusion SP MP Interactions Diagram Uses
Porous gel or beads Liquid Exclusion based on size Diagram large molecules eluding faster as they push through the beads)
44
What does the flame do in FID
Counts the number of carbon atoms, once the hydrogen in an organic compound is burned, the FID can tell us about the analyses concentration
45
ideal characteristics of gases to be used in GLC.
Volatile Different molecular weights
46
What is the effect on retention time of a polar sample in GC if you..’ halving column length doubling solvent flow rate decreasing stationary phase particle size increasing density of mobile phase increasing the temperature Increasing the polarity of the mobile phase
Shorter rt :halving column length Shorter rt: doubling solvent flow rate Longer rt: decreasing stationary phase particle size Longer rt: increasing density of mobile phase increasing the temperature Longer rt: Increasing the polarity of the mobile phase
47
Benefit of using temperature programming
Good baseline Good peak separation Short run times eg 20 v 60 minutes
48
Packed v capillary column
Both fused silica glass columns PACKED large and short Low efficiency CAPILLARY thin and long Layer of SP coated on capillary High efficiency
49
What does good efficiency mean?
Thin narrow peaks
50
GLC Mode sp mp
Partition mode sp: liquid eg peg in a column either packed or capillary Packed: liquid coated on a solid support and pa led into column Capillary column - liquid bonded to wall of glass silica capillary
51
Restrictions on mobile phase in GLC
The mobile phase carrier has must be: Pure Compatible with the detector Oxygen free
52
Injector in GC
Heated 1ul Splitter used with capillary columns
53
Types of capillary column
(SCOT) Core is empty, stationary phase on particles attached to wall. Less common, can give problems. OR Core is empty, stationary phase is covalently bonded to wall. More common
54
GC sp desirable properties
Low volatility (high boiling point) Thermally stable Chemically inert Dissolves analyte Able to form a thin film Dissolves the analytes to be separated
55
Oven in GC How to operate it
Oven temperature very important - must be controlled within 0.1oC Isothermally or temperature programmed Oven can operate from 30 – 400oC and can heat up or cool down at 0.5 – 50oC/min Care – packed columns must not be heated fast – lag in temperature
56
Isothermally
one temperature only Suitable for components which have similar boiling points
57
Temperature Programmed
(Ramped) – temperature increases during the separation Used for mixtures with a broad range of boiling points, allows much better chromatograms
58
An ideal detector for GLC would have:
Similar response to all kinds of compounds Wide concentration range Low detection limit Linear response Good resolution Fast response to change
59
Flame ionisation detector
60
How are analytes identified in GC
The analytes are identified by comparing Retention Times with standards under the same conditions
61
Spiking
adding the standard. If the peak you think is the standard is, it will increase
62
Advantages of GC
Linear response Wide concentration range Can detect as low as 1 ng Suitable for use with capillary columns Easier to use than Mass Spectrometer (see later) Robust
63
Disadvantages of gc
Can not detect low molecular weight compounds such as CO2, Poor detection of halogens Destructive, so can not collect eluent after detection
64
Applications of GC
Pollutants in water and air samples Volatiles in wine samples Urine and blood samples for drug residues
65
Restrictions on analytes in GC
Volatile fairly non-polar thermally stable
66
Split v splitless injector
The most common inlet for capillary GC can be operated in two modes, spilt or splitless. during theinjection process, the liquid sample is vapourised into the gas phase prior to transfer onto the capillar column.
67
Fid
It's high sensitivity and linear range for carbon-containing compounds make it very popular in organic analysis. & volage of horun 200 and 300V across these components The flame ionisation detector produces a proportional response to the number of carbon ators in a molecule.

Analytical (9 decks)

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