define gas chromatography
- a separation technique
- the mobile phase is a gas
- GC is carried out in a column
define analytes
gaseous and vaporizable (volatile or semi-volatile) molecules that are difficult to separate
what is the mobile phase
an inert gas
what is the stationary phase
liquid or solid phase immobilized in the column
define gas-liquid chromatography
liquid (non-volatile) stationary phase is adsorbed onto a solid inert packing or immobilized on the capillary tubing walls
what is the mechanism behind gas-liquid chromatography?
compounds separated based on their volatility and interactions with the stationary phase
what are important components of the GC instrument?
- carrier ggas flow control equipment
- injection port with heater
- separation column with stationary phase
- oven
- detector
- data recording system
what are examples of common mobile phases?
eg. helium, argon, nitrogen
what is the column flow rate in a packed column?
in an open tubular column?
packed column: 25-150mL/min
open tubular column: 1-25ml/min
what is the pressure of the inlet?
10-50psig
what are 2 most common injection modes?
- liquid samples are injected using a microsyringe manually or with autoinjection
- thermo-desorption unit: sample is heated for desorption of volatiles. Ca be solid or liquid
what are the 2 main parts of the injection port?
- rubber septum: syringe needle is inserted to inject sample
- glass insert: vaporization chamber
what temperature is the injection port usually maintained at?
high temp
what are 3 diff types of injections?
describe each.
- split injection:
- only a small fraction will enter the column; remainder is sent to waste
- used for routine analysis and high concentration samples - splitless injection
- all samples injected moves into the column
- used for quantitative trace analysis - on-column injection:
- column is first at low temp to condense sample in narrow band
- for examples that decompose above boiling point
- no heated injection port used
what are packed columns?
- glass or stainless steel coils
- filled with the stationary phase or packed coated with the stationary phase
examples of phase
- carbon based
- zeolite
- synthetic polymers
- silica and alumina based
what are capillary columns?
- thin fused silica (purified silicate glass) capillary
- has the stationary phase coated on inner surface, which provides fast equilibrium b/w mobile and stationary phase
what are 2 types of GC capillary columns?
describe each
WCOT: wall coated open tubular: column coated w/ liquid
SCOT: support coated open tubular: porous support coated w/ liquid
what are main differences between packed and capillary columns?
compared to packed columns, capillary columns have
- larger: column size, theoretical plates, solute capacity
- smaller: flow rates, head pressure, film thickness
with a nonpolar PDMS coating on the stationary phase, how can the polarity of the polymer be increased?
by increasing percentage of phenyl groups on the polymer
what are chiral GC columns?
- function: enantiomer separation
- physiological activity may be diff for diff isomers
common chiral stationary phases use derivatives of what?
- cellulose
- chiral silane
- cyclodextrin (especially beta-cyclodetrin)
in GC, what mechanism is very dependant on temperature?
the partitioning/separation mechanisms
what compounds can’t be analyzed simply with GC? why?
what can be done about this?
compounds with functional groups w/ active hydrogens
they tend to form intermolecular hydrogen bonds which results in
- lower inherent volatility
- tendency to interact with column materials
for these compounds, derivatization (eg. alkylation, acylation and silylation) are used to modify properties of the compound to make them more amenable in GC
what is an example of derivatization for GC?
conversion of FAs to fatty acid methyl esters (FAME)
