Quiz 4 Flashcards
(89 cards)
What is the carrier gas?
is what transports gaseous analyte through the GC column, has a high purity and is less than 1 ppb O2.
What is the stationary phase in gas-liquid partition chromatography?
This is a nonvolatile liquid bonded to the inside of the column or to a fine solid support (such as silica or aluminum particles)
Where is the analyte adsorbed in gas-solid adsorption chromatography?
The analyte is adsorbed directly on the solid particles of the stationary phase
Describe how a gas chromatogram works?
volatile (liquid or gas) sample is injected through a septum (rubber disk) into a heated port to make it into a gas. The vapor is swept through the column located in the column oven by He, N2, or H2 carrier gas,
The column must be hot enough to provide sufficient vapor pressure for the analytes to be eluted.
The detector which is hotter than the column shows the response of the separated analytes
What are open tubular columns?
These are the most frequently used S phase in GC, have fused silica in them.
What are open tubular columns coated in?
They are coated with polyamide (can deal with temps upto 350 C) to protect it from atmopsheric moisture and breaking
What are the benefits of open tubular columns?
Give higher resolution (reduced A term)
Shorter analysis time (as analyte doesn’t have to go through packing material like in a packed column)
Greater sensitivity (reduced A term)
Whats a negative about open tubular columns?
They have a lower sample capacity (due to their small diameter)
What is the length of a open tubular column?
15-100m
How thick is the stationary phase in an OT column?
0.1-5 um
How big is the diameter in a OT column?
0.1-0.53 mm
Name three types of OT columns?
Wall coated open tubular column, support coated open tubular column (solid support coated with stationary liquid phase), porous layer open tubular column (stationary solid phase particles)
What does a narrower column in GC indicate?
This indicates a higher resolution/ seperation
What do narrow columns require on terms of pressure and what do they have in terms of sample capacity?
They require higher pressure and have less sample capacity.
At what diameter can u not use Mass spec as a detector and why?
If the column diameter is greater than 0.32 mm you cant use mass spec as it tends to overload the vaccum system of the MS
As column length increases, what also increases?
The number of theoretical plates increases (N), as resolution is prportional to sqr root N, than as column length increases resolution also increases a long with rentention time.
what increases as stationary phase thickness increases in an OT column?
The retention time and resolution of peaks with a retention factor (K) less than 5 also increases.
How do you decide what liquid s phase to go with?
By polarity, if analyte is non polar choose s phase that’s non polar and vice versa
As the GC column ages, what happens to the stationary phase?
the stationary phase gets lost as silanol groups are exposed which can increase tailing (distorts peaks shape due to analyte lingering on s phase) which increases bleeding (when s phase decomposes/evaporates in mobile phase) which then decreases performance.
In non polar analytes and s phase, what is retention time determined by?
Volatility, with more volatile ones (goes by BP) eluting the fastest
In polar analytes and s phase, what is retention time determined by?
Determiend by polar interactions ex: H bonding
How can we make our column selective?
Can use certain non polar or polar s phases to control which compounds elute fastest or not.
How do you reduce the tendency of the stationary phase to “bleed” from a column at high temp?
you chemically bond (covalently attach) it to a silica surface and have it covalently cross linked to itself
How do you monitor column performance?
You peridoically measure rention factor (K) or N