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Separation Techniques > Gas Chromatography > Flashcards

Gas Chromatography Flashcards

1
Q

IUPAC definition of gas chromatography:

A

separation technique; mobile phase is gas (carried out in column)

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2
Q

The analytes in GC must be _____. Otherwise, they may be ______ in order to be compatible.

A

gaseous/vaporizable (volatile or semi-volatile)

derivatized (modify into structure that is volatile)

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3
Q

What are common mobile phases for GC?

A

inert gas (He, argon, N2)

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4
Q

What is the stationary phase for GC?

A

LIQUID or solid phase immobilized in column

gas-solid chromatography has limited applications - not common

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5
Q

Gas-_____ chromatography is the most common technique. Describe it.

A

gas-liquid

liquid (NON-VOLATILE) stationary phase adsorbed onto solid inert packing or immobilized on capillary tubing

separate compounds based on volatility + interaction w/ stationary phase

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6
Q

GC separates compounds based on ____ and _____

A

volatility (vapor pressure)

interaction with stationary phase (partitioning)

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7
Q

Basic parts of a GC: (6)

A 
carrier gas flow control
injection port + heater
separation column w/ stationary phase
oven
detector + data recording
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8
Q

What is the usual flow rate in a packed GC column?

A

25-150mL/min

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9
Q

What is the usual flow rate in a open tubular GC column?

A

1-25mL/min

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10
Q

What is the usual pressure of the inlet?

A

10-50 psig

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11
Q

common injection modes for liquid samples for GC:

A

injection with microsyringe (manual or autoinject)

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12
Q

common injection modes for solid or liquid (or SPME) samples for GC:

A

thermo-desorption unit: heat sample for desorption of volatiles

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13
Q

2 main parts of the injection port for GC:

A

rubber septum (needle inserted thru)

glass insert (vaporization chamber)

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14
Q

T/F: temperature does not matter for the injection port

A

False; usually maintained at high temp

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15
Q

The different types of injections:

A
  1. split injection
  2. splitless injection
  3. on-column injection
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16
Q

What is a split injection? What is it used for?

A

only small fraction (0.1-1%) of injected will actually enter column (rest is waste)

used for routine analysis/high concentration samples

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17
Q

What is splitless injection? What is it used for?

A

all injected sample moves into column

for quantitative analysis of trace analytes

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18
Q

what is on-column injection? what is it used for?

A

NO HEATED injection port (low temperature to condense sample in narrow band)

for samples that decompose above boiling point (temp sensitive)

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19
Q

____ can be used to directly absorb analytes from the sample headspace, and inject to ____ into GC

A

headspace SPME

desorb analytes

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20
Q

2 main types of columns for GC:

A

packed columns

capillary columns

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21
Q

Describe the typical GC packed column (materials, dimensions)

A

materials: glass or stainless steel
dimension: 1-5m length, 5mm inner diameter

filled/coated w/ stationary phase

ex: C based (carbotrap, carbopack, carboxen); zeolite (aluminosilicate); synthetic polymers (tenax, chromosorb); silica/alumina

22
Q

examples of typical GC packed columns:

A

C based (carbotrap, carbopack, carboxen)
zeolite (aluminosilicate); synthetic polymers (tenax, chromosorb);
silica/alumina

23
Q

Describe the typical GC capillary column (materials, dimensions)

A

thin fused-silica (purified silicate glass)

10-100m length, 250um inner diameter

stationary phase coated (few um) on inner surface

24
Q

capillary GC columns have a (thin/thick) coating on the inner surface. This allows for a (slow/fast) equilibrium between ___ and ____

A

thin
fast
mobile vs stationary phaes

25
Q

What is WCOT and SCOT?

A

coating types in capillary columns:

WCOT: wall coated open-tubular (column coated w/ liquid)

SCOT: support-coated open-tubular (porous support coated w/ liquid)

26
Q

T/F: a packed column is shorter than a capillary column

A

True

27
Q

Which has a smaller diameter, a packed or capillary column? How does this affect flow rate?

A

capillary

slower flow rate (10-60mL/min vs 0.5-15mL/min)

28
Q

T/F: a capillary column can be up to 100m in length

A

True

29
Q

T/F: the column head pressure can be similar in packed vs capillary column

A

True; but capillary column range can be lower (10-40 vs 3-40 psig)

30
Q

Which has a higher theoretical plate #? packed or capillary column?

A

capillary

31
Q

Which can tolerate a higher sample load? packed or capillary column?

A

packed

32
Q

the stationary phase in GC is a non-____ liquid coated on a ______

A

volatile (and inert)

solid support

33
Q

Give some common examples of GC stationary phases

A 
polydimethyl siloxane (gen purpose)
5% phenyl-PDMS
50%phenyl-PDMS
50% trifluoropropyl-PDMS
polyethylene glycol
50% cyanopropyl-PDMS
34
Q

A 100% polydimethyl siloxane coating is (polar/nonpolar)

A

nonpolar

35
Q

What effect does increasing % of phenyl groups on polymer PDMS achieve?

A

increases polarity

36
Q

polyethylene glycol is used for (polar/nonpolar) compounds

A

polar

37
Q

what stationary phase is used to analyze FAMEs?

A

5% phenyl-PDMS

38
Q

what stationary phase is used to analyze drugs or steroids?

A

PDMS or 50% phenyl-PDMS

39
Q

what stationary phase is used to analyze PUFAs?

A

50% cyanopropyl-PDMS

40
Q

For what compounds is PEG used as a stationary phase to analyze?

A

free acids, alcohols, ethers, essential oils, glycols

41
Q

___ GC columns have been developed for enantiomer separation, which is important in the ____ industry

A

chiral

pharmaceutical (different enantiomers have different physiological activity)

42
Q

chiral stationary phases use derivatives of: (3)

A

cellulose
chiral silane
cyclodextrin (B-cyclodextrin - a 7 sugar ring molecule)

43
Q

The separation column in GC is usually conducted inside a ______. Why?

A

thermostat-controlled oven

partitioning mechanisms are dependent on temperature

44
Q

What is the usual temp range of the liquid gas (CO2) vs the oven?

A

liquid gas: -50C

oven: 400C

45
Q

Why is precise temperature control of GC oven important?

A

will determine the separation precision

46
Q

Compounds with what functional groups cannot be analyzed directly with GC? Why?

A

functional groups with active hydrogens (COOH, OH, NH, SH)

tend to form intermolecular H bonds -> lower volatility + interact w/ column material

47
Q

What can be done to make compounds containing active H groups compatible with GC?

A

derivatization (modify property of compounds)
alkylation
acylation
silylation

48
Q

T/F: fatty acids are directly analyzable with GC

A

False; need to convert into FAMEs

49
Q

What are FAMEs?

A

fatty acid methyl esters (derivatized fatty acids)

FA + BF3/methanol -> methyl FA

50
Q

derivatization will lower the _____ of a compound

A

boiling point (vapor pressure)

Separation Techniques (12 decks)

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