Chem 241 Exam UNC Tiani

Extraction

the physical transfer of a solute from one phase to another

some goals of a separation

100% recovery: product from a synthesis

100% purity: diff. forms of same compound (enantiomers)

Separations involve phase transfers:

1. analyte out of sample matrix

2. other compounds out besides analyte

precipitation

liquid to solid

solvent-solvent extraction

solvent 1 to solvent 2

distillation

liquid to gas

sublimation

gas to solid

distribution constant (Kd)

organic over aq, characters of the analyte, organic phase, and aq phase

A solute in an aq solution is to be extracted using 150 mL of organic solvent. What method will extract the maximum amount of solute?

many extractions with smaller volumes of organic solvent

What is important to remember about extractions number?

ROUND UP

what is q?

fraction left behind in the water/aq phase

In a solvent-solvent extraction, charged acid-base species (e.g. BH+ and A-), as compared to neutral species (e.g. HA and B) are...

more soluble in the aqueous phase than the organic phase

What is the impact of the extraction of a weak acid (HA) into an organic solvent as the pH of the aq phase increases?

As pH increases, the value of D decreases and the percentage of the weak acid left in the aq phase increases.

pH effects in solvent-solvent extraction

HA - D decreases as pH increases

B - D increases as pH increase

When trying to calculate the fraction of weak base in the form B, extracted from a buffered solution into an organic solvent, what is the distribution coefficient you would use?

Kd partical fraction A-

You are trying to develop an extraction scheme to separate a neutral, non-acid-base organic compound (N) from a weak acid (HA, pka = 4.5). Both species are present in an aqueous solution. What strong species do you add first before adding organic solvent?

NaOH bc the charges of the analyte and other compound need to be diff. for their to be an extraction

What would happen to the peak profile if the flow were stopped and the column were allowed to sit?

It would broaden and diminish in height bc of diffusion

Affinity chromatography

specific groups are attached to the stationary phase to attract the solute

adsorption chromatography

solute molecules equilibrate btwn the mobile phase and the surface of the stationary phase

gas-solid adsorption chromatography

Semi Permanent retention of active or polar analytes

Not very common

Non-linear adsorption resulting in peak tailing

partition chromatography

solute dissolved in liquid phase bonded to the surface of the column

size exclusion chromatography

large molecules are not retained

ion exchange chromatography

1. mobile anions held near cations that are covalent attached to stationary phase

2. anion-exchange resin; only anions can be attracted to it

What is usually tm in GC?

CH4 and air bc they are known not to be retained, but you will still see a peak

What is the peak area under a chromatogram from a liquid chromatography exp. proportional to?

concentration

What it the expression for the retention factor/capacity factor (k)?

tr-tm/tm

What is true if K is very large? >30

1. the solute may not elute from the column bc it is strongly retained

2. the solute spends majority of time in the stationary phase

What is the separation factor/relative retention/selectivity coefficient?

t'r2/t'r1 --- always greater than 1

What will an increase in the separation factor do to a chromatogram?

increase separation btwn peaks

What will an increase in retention factor do to a chromatogram?

Shift it to the right, peaks broaden

What will an increase in N do to a chromatogram?

Increase peak height, narrow peaks

We want plate height to be --- so that we have ---- in our chromatogram and thus achieve --- separations.

small; narrower peaks; more efficient

What variables measure separation efficiency?

N, H, and R

In chromatography, you can increase column efficiency by (H, L, N)

increasing N and L (L to a point) & decreasing H

Why can we only increase L to a point to increase separation efficiency?

bc increasing L will increase the diffusion coefficient bc it increases the time on the column

What resolution do you want?

greater than 1.5 bc 1.5 is baseline

T/F: resolution is proportional to the number of plates on the column.

FALSE: R ~ square root N ~ square root L

How do you change resolution in GC?

vary column temperature

How do you change resolution in LC/HPLC?

change MP composition

How do you change resolution in GC or LC/HPLC?

change SP composition

What layer are charged species soluble in?

aqueous MP, not in organic SP

What layer are neutral species soluble in?

partition btwn MP and SP

What are the 3 primary modes of band broadening that we need to consider in chromatography?

multiple solute paths in column, longitudinal diffusion, and mass transport issues

Multiple path term (A) depends on what

quality of packing and stationary phase particle diameter

What is the van Diameter equation for GC exp. w/ an open tubular column

H = B/ux +Cux

open tubular vs packed columns

OTC: higher resolution, shorter analysis time, increased sensitivity, lower sample capacity

GC vs. LC/HPLC

GC:

-OTC- very common

-packed columns

-no guard column

LC/HPLC:

-packed columns only

-guard columns used often

-shorter columns

-large column diameters

Mass transfer (Cux)

depends on diameter of particles (packed column) or column diameter if OT, diffusion coefficient of solute in the MP or SP, and thickness of SP

Higher mass transfer (C):

particle diameter

increase diameter of particles (packed column) or column diameter if OT

Lower mass transfer (C):

particle diameter

decrease diameter of particles (packed column)or column diameter if OT

Higher mass transfer (C):

diffusion coefficient

decrease diffusion coefficient

Lower mass transfer (C):

diffusion coefficient

increase diffusion coefficient

Higher mass transfer (C):

SP thickness

increase thickness of SP

Lower mass transfer (C):

SP thickness

decrease thickness of SP

What part of the curve is governed by mass transport?

right

What part of the curve is governed by longitudinal diffusion?

left

What part of the curve is governed by multiple paths (eddy diffusion)?

middle (up or down)

What does overloading in GC cause?

fronting

Overloading

solute moves through column more slowly at higher concentrations

Fronting

gradual increase at front end of peak and normal gaussian shape at back end of peak

liquid SP film is saturated w/ solute so SP looks like solute

Fronting does what to Kd

increases it

tailing does what to Kd

decreases it

What does overloading in LC/HPLC cause?

tailing

What does "hot spots" cause?

tailing

tailing

limited retention capacity. at high solute conc's, solute saturates a significant portion of SP sites leaving fewer sites available for retention

Overall, retention of solute reduced

Tailing and hot spots

Si-O-H sites from silica surface "hot spots" - provide site of possible secondary interaction w/ solute

Silanization

solves tailing issue by capping the hot spot

GC MP

carrier gas, has no polarity, just comes simply through column

GC SP

liquid (partition) or solid (adsorption)

What samples can be analyzed in GC?

volatile liquids or gas samples

What temp must T-injector and T-detector be above?

~ 50 degrees C above least volatile compound

GC has ---- flow rates than LC/HPLC

faster bc OTC

GC has ---- column (capillary) lengths than LC/HPLC

longer

GC has ---- inner column (capillary) diameters than LC/HPLC

smaller

As the temperature of a GC increases, the vapor pressure of the analytes -----, the retention time -----, and the peak shapes ----

increases, decreases, sharpen

What compound will elute first in GC?

lowest boiling point

OTC for GC. Increasing the thickness of the stationary phase film affects which terms and how?

increase C and thus H, and increases the retention factor

Large diameter packed column

large sample capacity

greater surface area

higher pressure

smaller length

Full van equation

Narrow capillary GC OTC

lower sample capacity

higher resolution/

reduced plate height

smaller diameters

larger length

no A term

Gas-liquid partition chromatography

high BP liquid SP or solid support particles coated w/ a thin film

SPME

Sample preparation method where a stationary phase coated on a needle is exposed to sample for collection of analyte prior to analysis

0.32 mm inner diameter and 0.25 mm diameter OTC, the ---- column has better resolution

smaller

Butanol was found to elute later than octane despite having a lower BP on a GC w/ polar SP. Why?

Butanol is polar and can interact more strongly w/ the stationary phase

Requirements for GC stationary phase

high BP (non-volatile) bc "column bleeding" which is the decomposition and evaporation of SP at elevated column temperatures

Polar columns are good for ---- analytes

polar

non-polar columns are good for --- analytes

non-polar

GC non polar SP is based typically on ---

Boiling points

GC polar SP is based

polarity

When analyzing a mixture of samples by GC, all with a range of boiling points, what is used to obtain the most efficient separation?

temperature programing

Temperature programming should go from ----- bc

low to high bc otherwise all the compounds would elute at the start

The GC detector most common for organic compounds is...

Flame ionization

Flame Ionization Detectors (FID)

mass detectors w/ peak area proportional to # of susceptible carbon atoms

The GC detector particularly sensitive to halogen containing molecules is...

electron capture

What are the destructive GC detectors?

Flame ionization and mass spectrometry

What are the nondestructive GC detectors?

thermal conductivity and electron capture

What does thermal conductivity detector measure?

voltage

What does electron capture detector measure?

current

What does flame ionization detector measure?

current

What does mass spectrometry measure?

intensity (abundance) vs. m/z

What type of device is thermal conductivity detector?

concentration sensitive (peak area ~ con.)