final exam 2 stuff Flashcards
whats the difference between capillary and packed column GC’s
capillary columns are long and thin and have a stationary “waxy” coating on the inside
packed columns have “balls” of stationary phase
what’s the differen
how do we adjust the retention in a GC?
we can adjust the temperature, the flow rate, and the chemical nature of the stationary phase
what does adjusting the temperature do to a GC?
if we heat the colum, it will cause less retention since more of the molecules will be in the gaseous phase
how do we adjust the chemical nature of the stationary phase
we can make it more polar or nonpolar
how do we generally adjust the retention of analytes in LC and GC
it’s easier to adjust the mobile phase rather than the stationary phase
what affects the retention in gas chromatography
a combination of the boiling point and the polarity
how does the boiling point play into gas chromatography
analytes with low boiling points are more volatile and will go into the mobile phase easier
in GC, the separation is based on the difference in partitioning between:
the stationary and gas (mobile phase)
When we increase the column temperature, what happens to the gas phase concentration? To what solutes?
The gas phase concentration of ALL solutes increases with increased temperatures
why is it a problem when we increase the temperature?
all the analytes are affected and may not be completely resolved
why is it a problem when we decrease the temperature
it is inefficient we get better resolution and separation but the peaks become broader and it takes more time
what is temperature programmed chromatography?
it is where we increase the temperature of the column during a run
when we don’t use temperature programmed chromatography what happens when we get resolution with early eluters
the late eluters get too broad or do not elute
when we don’t use temperature programmed chromatography what happens when we get resolution/narrow peaks with late eluters?
the early eluters do not get resolved
what is a gradient in liquid chromatography
it is when we alter the composition of the mobile phase during the run in order to achieve better separation in less amounts of time
what is the difference in mobile phases for HPLC and GC
in GC we have a gas mobile phase
in HPLC we have a liquid mobile phase
is HPLC isothermal?
yes, it is difficult to change the temperature
is GC isothermal
no, we often change the temperature to change the mobile phase
in a GC, how do the analytes interact with the mobile and stationary phases
the analyte interacts with the stationary phase only, the mobile phase serves as a carrier
in a HPLC, how do the analytes interact with the mobile and stationary phases?
the analytes interact with both the mobile and stationary phases
if we make the mobile phase and stationary phases in a HPLC more similar what happens to the retention times?
the times decrease
if we make the mobile phase and stationary phase in a HPLC less similar, what happens to the retention times?
the times increase
what are the best analytes for LC
analytes that are non-volatile
what are the best analytes for GC
analytes that are volatile
why do we not use normal phase HPLC very often?
normal phase HPLC has a nonpolar mobile phase,
usually nonpolar analytes are more volatile and can be used more easily in a GC separation
