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A gas that is hot enough to contain ions and free electrons
How does atomic spectroscopy work?
Sample is vaporized by a laser at 2,000-8,000 K, decomposes into atoms. Plasma ionizes some of the atoms after laser, which pass into mass spectrometer that separates ions by mass and measures the quantity.and ions whose concentrations are measured by emission or absorption of characteristic wavelengths of radiation.
What is the accuracy of atomic spec?
Bandwidth of absorption and emission spectra's of:
- Solids and Liquids: 10-100 nm
- Gaseous Atoms: 0.001 nm
Laser irradiates atoms in the flame to promote the atoms to an excited electronic state from which the fluoresce to return to ground state.
Why is Atomic Fluorescence more sensitive than Atomic Absorption?
x1000 more sensitive than AA
Easier to observe a weak fluorescence signal against dark background than look for a small difference between large amounts of light.
3 types of Atomic Spectroscopy
3 ways of atomizing an analyte
Common fuel-oxidizer combination for flame
Acetylene / Air
If hotter temp needed, Acetylene / Nitrous Oxide used.
Rich vs Lean Flame
- Rich: flame is rich in fuel (excess carbon reduce metal oxides and hydroxides and increase sensitivity)
- Lean: has excess oxidant (flame is hotter)
What are some things that affect the flame atomization?
- -"rich" or "lean"
- -molecules emit broad radiation that must be subtracted from sharp atomic signals
- -height of flame at which maximum atomic absorption / emission observed
- -flow rate of fuel, sample, oxidizer
Matrix: everything in sample except analyte
Matrix Modifier: substance added to sample to reduce loss of analyte during charring by making matrix more volatile or making analyte less volatile
What is the principle difference between atomic and ordinary molecular spectroscopy?
- -light source
- -sample container (flame, furnace, or plasma)
- -the need to subtract background emission
Concentration of an element that gives a signal equal to three times the standard deviation of signal from blank.
Detection Limit for Furnaces
Two orders magnitude lower than what is observed with flame (sample confined to small volume for a long time)
Detection Limit for ICP
Intermediate between flame and furnace.
Closer to graphite furnace with ultrasonic nebulization and axial plasma viewing.
What kind of standard solutions are used for flame, furnace, and plasma analysis?
Plasma and Furnace: a purer grade standard for most sensitive analysis
Standards also have a shelf life and concentrations change over time depending on storage.
any effect that changes the signal while analyte concentration stays the same.
Overlap of analyte signal with signal due to other elements or molecules in sample or with signals due to flame or furnace.
Interference from a molecule has broader spectrum (because of vibrational and rotational transitions with electronic transitions)
Sometimes heating sample prior to atomization can reduce oxides, and eliminate interferences
Caused by any component in sample that decreases the extent of atomization of analyte.
Addition of releasing agents decrease interference's. They react with the interference of the sample freeing up the sample. (Adding La3+ to free up Ca2+ from sulphate and phosphate)
Higher flame temps also reduce this
Ionization of ground state element in flame prevents it from going to the excited state: doesn't absorb light of appropriate wavelength. (desired signal decreases)
Ionization Suppressors that decrease ionization of analyte used. Preferably something that is easier at ionization to produce high concentration electrons that reduce ionization. (desirable at low temp flame)
Inductively Coupled Plasma Interferences
Negligible - twice as hot as conventional flame and residence time of analyte twice as long.
Effect of Self Absorption
Emission from the central region is absorbed by the outer region causing a non linear calibration curve.
- Flame / Furnace: linear by two orders of magnitude
- Plasma Emission: linear by five orders of magnitude
- ICP: linear by eight orders of magnitude
Physical transfer of a solute from one phase to another.
What are some common reasons to use extraction?
- -isolate or concentrate the desired analyte
- -separate it from a species that would interfere with analysis
Solute partition is based on "like dissolves like" which means that solute is more soluble in a phase whose polarity is similar to that of the solute.
- K is the equilibrium constant from the reaction:
- S (in phase 1) <=> S (in phase 2)
Partition Coefficient Equation
K = AS2
- -AS1 = activity in phase 1
- -Phase 2 is above phase 1
Fraction remaining in phase 1 after n extraction.
) / (V1
- - ml's of solvent 1 (V1)extracted with ml's solvent 2 (V2)
- -n = number extractions (better to do many small extractions that 1 big extraction)
D = (total concentration in phase 2) / (total concentration in phase 1)
- D = (K*Ka)/(Ka + [H+]) = K*αBαB = fraction weak base in neutral form
- (αB = ([B]aq) / ([B]aq + [BH+]aq)
When is the distribution coefficient used instead of K?
You use D when dealing with a species that has more than 1 chemical form (B and BH+)
What would you do to extract a base into water? An acid into water?
Use a pH low enough to convert B into BH+ or HA in A-
Solvent moving through column (either liquid or gas)
The one that stays in place inside column. Commonly the most viscous liquid chemically bonded to the inside of the capillary tube or onto solid surface.
What gives rise to the separation in chromatography)
- -Equilibration of solutes between mobile and stationary phase
- -Unequal distribution coefficients between two phases
Process of passing liquid or gas through a chromatography column.
Solid stationary phase and liquid / gas mobile phase used. Solute adsorbs on on surface of solid particles. The stronger it adsorbs, the slower it moves.
Liquid stationary phase bonded to solid surface (typically fused silica / SiO2 in GC).Solute equilibrates between stationary liquid and mobile phase, which is flowing in GC.
Ion Exchange Chromatography
Anions or cations covalently attach to stationary solid phase, usually resin. Solute ions of opposite charge attracted to stationary phase. Mobile phase is a liquid.
Molecular Exclusion Chromatography (gel filtration, size exclusion, gel permeation chromatography)
Separates molecules by size with larger solutes passing through more quickly. Liquid or gas pass through a porous gel. Pores are small enough to exclude large solutes, streaming past the pores. Small molecules take longer because they have to do more traveling.
Most selective chromatography employing interaction between one kind of solute molecule an second molecule covalently attached to solid phase.
Molecule wanted passes through but is attracted and attached to stationary phase. The rest of the molecules flow through. Changing the pH dislodges the molecules bound to stationary phase.
Tells how many centimeters are traveled in 1 min by the solvent.
The time that elapses between injection of the mixture onto the column and arrival of the component into the detector.
Volume mobile phase required to elute particular solute from the column.
Adjusted Retention Time Equation
- -tr: total time
- -tm: amount of time mobile phase or solute travels through the column
Relative Retention Time Equation
α = (adjusted retention time of standard) / (adjusted retention time of sample)
It is an expression of retention time of the sample relative to the standard. Greater the relative retention, greater the separation.
α should be greater than 1
- k = (tr - tm) / tmk = (time solute spends in stationary phase) / (time solute spends in mobile phase)
- k = (moles solute in stationary phase) / (moles solute in mobile phase)
- k = csVs / cmVm
Time required to elute that peak minus the time (tm
) required for mobile phase to pass through the column.
Longer a component is retained, greater the retention factor.
Relation of Retention Time to Partition Coefficient
k = K(Vs/Vm)
Relating relative retention time to partition coefficient
α = K2 / K1
Volume of mobile phase required to elute particular solute from column. It is constant over range of flow rates.
Vr = tr * uv
uv = volume flow rate
What two factors contribute to how well compounds are separated by chromatography?
- 1) differences in ellution time (farther apart, better the separation)
- 2) how broad the peaks are (narrow the pictures, better the separation)
How well two peaks can be differentiated in chromatography separation.
- Resolution: Δtr / wav
- Resolution: ΔVr / wav
- Resolution: 0.589Δtr / w1/2av
- Δtr or ΔVr: separation between peaks
- wav: average width of two peaks
Net transport of a solute from a region of high concentration to a region of low concentration caused by random movement of molecules.
One main cause of band spreading.
- H = σ2 / x
- Constant of proportionality between variance (σ2) of the band and the distance traveled (x). These are theoretical plate height equivalents.
Smaller the plate height, narrower the band width. Better separation BUT smaller efficiency.
As sum this is like the observed standard deviation. You can square and add them all up to get a total observed variance / total standard deviation.
That is when the fluid flows fastest along the sides and decreases to zero towards the walls.
That is when the peak gets wider decreasing the resolution.
How can band broadening be decreased by manipulating the connecting tubing?
The connecting tube is kept narrow and short.
Van Deemter Equation
H ~ A + B/ux
- A: multiple paths
- B: longitudinal diffusion
- C: equilibration time
The optimal linear velocity is when the H value (plate height) is as low as it possibly could be.
What aspect of the van Deemter equation is contributes most to plate height bellow, above, and at optimal velocity?
- Below: B
- Above: C
- At: B and C contribute equally to plate height
What aspect of the van Deemter equation is contributes most to plate in a packed column, tubular column, and capillary electrophoresis?
- Packed Column: all three contribute equally
- Tubular Column: A is 0, so bandwidth decreases and so does resolution
- Capillary Electrophoresis: A and C goes to 0, reducing plate height and providing extraordinary separation powers
- -diffusional broadening of a band
- -band slowly broadens as the molecules diffuse from high concentration to low concentration ahead and behind the band
Movement of solute from one phase to another.
Solute must diffuse from mobile phase to the surface of the stationary phase for equilibration to occur.
How does the speed of the mobile phase affect affect equilibration?
The faster the mobile phase, less time available for transfer to occur.
How does decreasing the column radius affect plate height?
Decreasing the radius reduces plate height because the solute has to diffuse through less to reach the stationary phase.
How does decreasing stationary phase thickness affect plate height?
Reduces the plate height because the solute diffuses faster from stationary phase to mobile phase.
How does temperature affect GC / LC mass transfer plate height?
It is decreased because increasing temperature increases the diffusion coefficient. It also allows the linear velocity to increase without affecting resolution.
How do multiple paths from the Van Deemter Equation affect band spreading?
Since some flow paths become longer as it passes through a porous column, the molecules elute through at different times broadening the band.
Which has better resolution for smaller columns:
-open tubular column
Open tubular column provides higher resolution and increased sensitivity for smaller columns. An open tubular column is longer for the same plate height value, increasing the number of theoretical plates.
Plate height is reduced also because multiple flow paths cannot occur.
You can also get the same resolution for less time by just reducing column length.
But since they have a small capacity, they can't be used for preperative seperations.
A graph of concentration of stationary phase versus concentration of mobile phase.
What are three common isotherms and their resulting peaks?
- Fronting: cs v cm has a positive slope (Gaussian curve goes past ideal)
- Ideal: cs v cm slope is 1
- Tailing: cs v cm has a negative slope (Gaussian curve goes bellow ideal)
Gaussian curve is signal vs. time
What actions resulting in fronting and tailing?
- Fronting: overloading the sample
- Tailing: retention time (k) decreases, or if the column has sites that bind to solute strongly
How can you reduce tailing?
Through the process of salinization where silanol groups are blocked by nonpolar groups.