CHEM SEP final 2 Flashcards
Moment Analysis (Method of Moments)
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Every distribution function (Gaussian or otherwise) has a set
of statistical moments characterizing its position and shape.
Go through The different variations you can have with moment analysis
Continuous vs discrete (summation vs integral),
Raw vs central moments (raw just alues as is, central is value - mean (make the mean the origin so the population is deviations from the mean); in raw moment the origin is the center
Population vs sample: sample is value /numebr of samples , population is integration over the function
REVIEW GUIDE FOR WHAT THESE LOOK LIKE
What are the 4 sample moments
Mean, variance, Skewness, kurtosis
4 sample moments how to write equations for each
Review how to write equations for each
How to interpret skewness?
0 - gaussian, > 0 is fronting (right side) and < 0 is tailing (left side)
How to interpret kurtosis?
excess = kurtosis - 3; gaussian is 3 , smallest is 1 (makes it a rectangle) - higher you go the more narrow (a measure of how flat)
GC can analyze compounds that are ..?
Voltalie or can be derivitized to become volatile
Parts of a GC system
: a gas source with pressure
and flow regulators, an injector, a column in an oven, a detector, and a
computer for data acquisition.
So GC using a compressible fluid means what
As we get a pressure drop (poissons/darcys law) - we get a density change - which means the gas expands (via Boyles law) - so this causes an increase of linear local velocity and local flow
In GC what determines the change in rate of velocity from inlet vs outlet
Ratio of Pressure in/Pressure out - the higher - the steeper the change close to end of column
Know how to derive pressure in out equation equation
Difference in GC velocity van deemter vs experiemntal
in van deemter it is average ; in experimental its at the outlet
What are 3 things that need to be corrected for , for experimental GC average flow rate/veloctiy measurement
1) pressure gradient drop (j) (multiply it by V out to get V average)
2) Temperature correction (Tc/Tout) because outlet is a lower temp causing reduction in flow rate (because measure outside of GC with bubble flowmeter)
3) Bubble flowmeter causes gas to be saturated with water so need to account for addition of pressure from water vapor (Pout - Pw/ Pout)
2 ways to determine average velocity in GC
Multiply measured velocity out by j, pressure correction factor and temp correction factor OR just use retention time (L/rt))
How is volume corrected for in GC
Similarly since based off flow and we know flow is affected - need correction terms j for compressibility , NET parameters are tr- tm, Vr - vm
What is corrected and NET volume in GC
So volume usually got by tr * F HOWVER F is affected in GC (F at outlet vs average) - so we need to adjust it instead of V = t * F it’s now V = t *j *F and this is now called corrected volume NET volume is uncorrected volume - corrected volume
What is specific retention volume
Specific retention volume is commonly used in GC to permit
inter-comparisons of retention of an analyte i in columns
containing different weights of the same stationary phase at
the same column temperature . Calculated as NET volume / weight of stationary phase multiplied by 273/ column temp
What is specific retention volume related to in thermodyanmic terms
on top R * 273 on Botton inverse with MW of stationary phase, the activity coefficient of analyte I in stationary phase and Vapor pressure of pure I in standard state (analyte I(
How is GC from other chromatographic methods in terms of retention
Not based on partition or seperation but on vapor pressure and volatility because the gaseous mobile phase is a lot more than concentration fo analyte and has a much lower density so we assume it doesn’t interact with the solute (especially incomparison to the stationary phase) - so K is dependant on temperature here
How is chemical potential split for GC
So depends on uio which is the same but then the dilution of entropy terms which inis activity term * concentration so when we consider this in the mobile phase of GC - we assume ideal gas so y =1 and the concentration is just the PARTIAL PRESSURE of component I
so this simplifies to u = uio + RTln(Pi)
What is selectivity in GC sepeartion based off of and why
Vapor pressure of sample components (when we do u - uio + RTln(C *y) in an ideal gas this just turns into partial pressure of analyte ; AND
Solute solvent interactions with Stationary phase
What is yis dependant on
PiM - partial vapor pressure of analyte ,, inversely related to MOLE FRACTION of I in SP and PiO - vapor pressure of pure component I at column temp
Distribution coefficeint in GC
(moles in SP / Weight SP) / (moles MP / corrected mobile phase volume) (in other words MOLALITY / MOLARITY)
No how to derive thermodynamics of retention showing that Specific retention volume depends on yis and Pio (inverse related)