Gas Chromatography Flashcards
IUPAC definition of gas chromatography:
separation technique; mobile phase is gas (carried out in column)
The analytes in GC must be _____. Otherwise, they may be ______ in order to be compatible.
gaseous/vaporizable (volatile or semi-volatile)
derivatized (modify into structure that is volatile)
What are common mobile phases for GC?
inert gas (He, argon, N2)
What is the stationary phase for GC?
LIQUID or solid phase immobilized in column
gas-solid chromatography has limited applications - not common
Gas-_____ chromatography is the most common technique. Describe it.
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
GC separates compounds based on ____ and _____
volatility (vapor pressure)
interaction with stationary phase (partitioning)
Basic parts of a GC: (6)
carrier gas flow control injection port + heater separation column w/ stationary phase oven detector + data recording
What is the usual flow rate in a packed GC column?
25-150mL/min
What is the usual flow rate in a open tubular GC column?
1-25mL/min
What is the usual pressure of the inlet?
10-50 psig
common injection modes for liquid samples for GC:
injection with microsyringe (manual or autoinject)
common injection modes for solid or liquid (or SPME) samples for GC:
thermo-desorption unit: heat sample for desorption of volatiles
2 main parts of the injection port for GC:
rubber septum (needle inserted thru)
glass insert (vaporization chamber)
T/F: temperature does not matter for the injection port
False; usually maintained at high temp
The different types of injections:
- split injection
- splitless injection
- on-column injection
What is a split injection? What is it used for?
only small fraction (0.1-1%) of injected will actually enter column (rest is waste)
used for routine analysis/high concentration samples
What is splitless injection? What is it used for?
all injected sample moves into column
for quantitative analysis of trace analytes
what is on-column injection? what is it used for?
NO HEATED injection port (low temperature to condense sample in narrow band)
for samples that decompose above boiling point (temp sensitive)
____ can be used to directly absorb analytes from the sample headspace, and inject to ____ into GC
headspace SPME
desorb analytes
2 main types of columns for GC:
packed columns
capillary columns
Describe the typical GC packed column (materials, dimensions)
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
examples of typical GC packed columns:
C based (carbotrap, carbopack, carboxen)
zeolite (aluminosilicate); synthetic polymers (tenax, chromosorb);
silica/alumina
Describe the typical GC capillary column (materials, dimensions)
thin fused-silica (purified silicate glass)
10-100m length, 250um inner diameter
stationary phase coated (few um) on inner surface
capillary GC columns have a (thin/thick) coating on the inner surface. This allows for a (slow/fast) equilibrium between ___ and ____
thin
fast
mobile vs stationary phaes
What is WCOT and SCOT?
coating types in capillary columns:
WCOT: wall coated open-tubular (column coated w/ liquid)
SCOT: support-coated open-tubular (porous support coated w/ liquid)
T/F: a packed column is shorter than a capillary column
True
Which has a smaller diameter, a packed or capillary column? How does this affect flow rate?
capillary
slower flow rate (10-60mL/min vs 0.5-15mL/min)
T/F: a capillary column can be up to 100m in length
True
T/F: the column head pressure can be similar in packed vs capillary column
True; but capillary column range can be lower (10-40 vs 3-40 psig)
Which has a higher theoretical plate #? packed or capillary column?
capillary
Which can tolerate a higher sample load? packed or capillary column?
packed
the stationary phase in GC is a non-____ liquid coated on a ______
volatile (and inert)
solid support
Give some common examples of GC stationary phases
polydimethyl siloxane (gen purpose) 5% phenyl-PDMS 50%phenyl-PDMS 50% trifluoropropyl-PDMS polyethylene glycol 50% cyanopropyl-PDMS
A 100% polydimethyl siloxane coating is (polar/nonpolar)
nonpolar
What effect does increasing % of phenyl groups on polymer PDMS achieve?
increases polarity
polyethylene glycol is used for (polar/nonpolar) compounds
polar
what stationary phase is used to analyze FAMEs?
5% phenyl-PDMS
what stationary phase is used to analyze drugs or steroids?
PDMS or 50% phenyl-PDMS
what stationary phase is used to analyze PUFAs?
50% cyanopropyl-PDMS
For what compounds is PEG used as a stationary phase to analyze?
free acids, alcohols, ethers, essential oils, glycols
___ GC columns have been developed for enantiomer separation, which is important in the ____ industry
chiral
pharmaceutical (different enantiomers have different physiological activity)
chiral stationary phases use derivatives of: (3)
cellulose
chiral silane
cyclodextrin (B-cyclodextrin - a 7 sugar ring molecule)
The separation column in GC is usually conducted inside a ______. Why?
thermostat-controlled oven
partitioning mechanisms are dependent on temperature
What is the usual temp range of the liquid gas (CO2) vs the oven?
liquid gas: -50C
oven: 400C
Why is precise temperature control of GC oven important?
will determine the separation precision
Compounds with what functional groups cannot be analyzed directly with GC? Why?
functional groups with active hydrogens (COOH, OH, NH, SH)
tend to form intermolecular H bonds -> lower volatility + interact w/ column material
What can be done to make compounds containing active H groups compatible with GC?
derivatization (modify property of compounds)
alkylation
acylation
silylation
T/F: fatty acids are directly analyzable with GC
False; need to convert into FAMEs
What are FAMEs?
fatty acid methyl esters (derivatized fatty acids)
FA + BF3/methanol -> methyl FA
derivatization will lower the _____ of a compound
boiling point (vapor pressure)
