Analytical Chemistry

What are three processes that only occur with atoms?
1) Atomic absorption
2) Atomic Emission
3) Atomic Fluorescence: excitation is by absorbing light
Name the three factors that contribute to atomic line width
1. uncertainty principle: can’t measure energy and time infinitely well
2. Doppler broadening: when atom moves closer to the light source, it absorbs more light and therefore, has a longer line width.
3. Pressure broadening: atoms collide so their electric fields interact and modify the line width.
What are three tasks that must be completed by the Atomizer?
1) Get sample into the atomic state
2) Get atoms into the gas phase
3) Remove interferences (largest source of error)
Name seven processes that may occur with a Flame as an atomizer
1) Nebulization
2) Desolvation
3) Liquefacation
4) Vaporization
5) Atomization
6) Excitation
7) Ionization
What is the main difference between the Graphite Furnace versus Flame?
You can pyrolyze, which means you can remove that matrix without removing the analyte.
Name three characteristics of a Hallow Cathode Lamp (HCL)
1) Made of metal/material containing atoms we want to measure
2) Emit wavelengths which are unique to our element of interest
3) Lamp uses atomic emission to emit wavelenghts your’re interested in.
Name some advantages and disadvantages of HCL
Adv: Maintain selectivity
Disadv: you must change light bulb everytime you test a different element, horrible for qualitative analysis
Name three characteristics of an Electrodeless Discharge Lamp (EDL)
1) Good for volatile elements
2) Metal of interest in a low P bulb in the gas phase.
3) Radio frequency is used to excite the atoms.
Name an advantage and disadvantage for EDL
Adv: stable, very intense, better detection limit
Disadv: expensive

Also, can have single or multielement light sources

Name two non-spectral interferences that can occur with AAS
1) Chemical interferences
2) Ionization
3) Matrix interferences
What are some ways to fix the chemical interferences for AAS?
Problem: Formation of non-volatile compounds
Solution:
1) Add a better cation (stronger affinitiy for the anion than analyte)
2) standard addition
3) change the solvent
4) Add a better anion: binds to analyte and volatilizes
5) Hotter flame
6) Match matrix with standards
What are some ways to fix the ionization issue in AAS?
Problem: ions don’t absorb the same atomic wavelengths.
Answer: 1) Lower the flame temperature
2) Use ionization suppressor: compunds which will ionize easily, supply electrons (alkali metals: Cs)
What are some ways to deal with matrix intereferences for AAS?
Problem: organic compunds in the matrix can burn; causing an increase in flame T.
Answer: match matrix, sample pre-treatment, standard addition
Name three spectral intereferences that can occur for AAS
1) Overlapping atomic peaks (NOT A PROBLEM)
2) Combustion products (cause broad peaks)
3) Molecular products
What is one way you can eliminate the interference from the combustion products in AAS?
Run a blank
Name four ways to correct for the intereferences caused by molecular products in AAS
1) Match the blank matrix better
2) Change flames: get it hotter
3) 2-line method
4) Continuous source (HCL)
Explain the 2-line correction method
-Get 2 spectral lines
1) One is absorbed by analyte and interferencec
2) The second is close by in wavelength but is only absorbed by the interferences
So, you subtract the interferences.
Explain the continous source correction process
The broadband source gives us background absorption. If you subtract what you get from the moleculark you get the atomic absorption.
-Don’t need to worry about the two wavelengths ( like 2 line method)
Can we make a qualitative application for AAS?
No, we don’t worry about it; each elements needs a different HCL.
What must you remember when applying a quantitative analysis to AAS?
1) analytes must be atomized
2) absorb in UV and visible
3) exclusively metals
How can we prepare a solution for AAS
Flame–> solutions only
Graphite furnace –> both solids and liquids
1. SA digestion
2. Combustion in a bomb
3. Microwave decomposotion (might lose sample, remove interefereces)
What type of standards would you run for AAS
External or standard addition
What are two basic principles of AES?
1) Measure emission of atoms as they relax from exicted to ground state.
2) Excite atoms
What factors take temperature into account for AES?
1) excitation source
2) Variable in Pressure and Doppler broadening
3) Produce atoms
4) Determine number of atoms in ground state vs. excited state.
What are some characterisitcs of the flame used for atomic emission?
1) simple, but relatively low temperature: few atoms are excited, restricted to atoms w/ lower energy levels.
2) Temp fluctuations are a large source of error
What are two electrical excitation sources?
We want to impact our sample with high energy electrons (transfer KE)
1) DC arc
2) High Voltage spark
What are some characteristics of the DC arc?
1) plasma forms: free electrons, positive ions, and neutral species
2) excite sample w/ electrical thermal energy
3) poor stability
4) volatile atoms emitting before non-volatile ones. can do solids, and have 5-10% precision; 10-100 ppb
What are the advantages and disadvantages of using a spark source?
General: use avg. measurement because sparks hit different areas of sampple.
Adv: analyze most elements, including Ne, Ar, O2, N2, halogens; can do liquids, solids, and gas.
Disadv: poor sensitivity, lots of lines yielding possible interferences
Name the 5 steps involved for Plasma (Inductively coupled plasma)
1) Introduce Air
2) radiofrequency will produce magnetic field
3) Ignite the plasma
4) ion and electrons accelerate in magnetic field –>increase T
5) sample introduced
What are the advantages for using ICP?
1) plasma is stable (need to make sure signal intensity is consistent: helps w/ precision. Important to matrix match standards
2) minimal number of chemical interferences
3) high atomization efficiencies
What’s a major disadvantage to ICP>
Background emission
How can we get high resolution from our monochromator?
1) Increase inverse linear dispersion
2) use mutliple orders of refraction
Name three types of luminescence. How do they differ from one another?
1) Fluorescence
2) Phosphorescence
3) Chemiluminescence
Probability of smething happening is related to ______ scale. _____ processes are more likely to occur than ______ processes.
time / Fast / slow
Name six different relaxation processes.
1) Vibrational relaxation
2) Internal conversion
3) Intersystem crossing
4) Fluorescence
5) Phosphorescence
6) External conversion
Explain vibrational relaxtion briefly.
Lose energy to solvent during collsions, so there’s a slight temperature change within vibrational transitions.
– 10^-12 lifetime
Internal conversion?
Switches over to another excited state (could be rotational).
It happens when lowest energy vibronic state of one envelope overlaps with energy of another. BUT they need to have same multiciplity.
Intersystem corssing?
Same as internal conversion, except change in multiplicity (its a measure of electron spin)
Fluorescence versus phosphorescence?
Fluorescence: 10^-9, occurs in singlet state.
Phosphorescence: 10^-3, come from quantum mechanically forbidden
What are the 6 factors which intensity?
1) Quantum yield: Fi = rate of fluorescence / rate all of all deactivation or relaxtion processes < or equal 1
2) Temperature: as temp goes up, fluorescence goes down due to quenching (by O2).
3) Viscosity: as it increases, F increases
4) pH (change in F between protonated and deprotonated form)
5) Dissolved O2
6) Molecular structure: more efficient with extended Pi bonds, more rigid, and subsituent groups (ex: Halogens)
What is the fluorescence equation?
F = 2.3(Fi)EbcPo

E: molar absorbtivity, Po = more intense light
F = kC

What the three deviations from linearity that could occur for fluorescnece?
1) A is less than 0.05
2) self-quenching
3) self-absorbance
2+3 eliminated when you work at mM concentration of below.
What are the two strategies for analyzing non-fluorescent compounds?
1) Quenching: one molecule collides with another molecule and deactivates fluorescent compound w/o emitting light: need constant amount of fluorophore and varying amounts of analyte
2) Derivitization with fluorescing compound: Analyte + fluorophore –> analyte:fluorophore
What is a disadvantage of qualitative analysis for Fluoresence molecular spectroscopy?
You get broad peaks, and therfore minmial structural information.
What are some advantages for fluorescence spectroscopy?
1) not as many compounds fluorescence: fewer interferences, narrower range of functional groups
2) can measure both emission and excitation spectra
With respect to fluorescence spec, discuss the differences between emission and excitation?
Emission: 1 band of excitation wavelengths, measure fluoresence intensity versus emission wavelengths
Excitation: scan wavelengths, and where you get Abs you detect fluorescence; one band of emission wavelengths, mreasure FI vs. excitiaton wavelength.
With respect to Fluorescence instrument, discuss differences with excitation and emission monochromator.
1) For emission scan, excitation monochromator is fixed at lamba max for absorbance, and emission monochromator scans.
2) For excitation scan, emission mono is fixed and excitation mono scans.
What is the sample placed at 90 degrees in a fluorescence instrument?
Because F takes place in full degress equally, and the only configuration we DON’T want to use is 180 degrees because you get scattered light due to the excitation light going trhough.
What are two types of light sources that can be used for fluorescence spectroscopy?
Remember F is proportion is Po!
1) Continuum source: Deuterium and W (all wavelengths, but not all powerful at all wavelengths).
2) Line source: cannot scan exciation spectrum, but its powerful (Hg lamp, and Lasers)
What are two types of vibrational spectroscopy?
1) Infrared absorbance
2) Raman
Paint the classical picture of infrared absorbance!
It’s a simple molecule: a spring with force constant, k attached to 2 balls, m.
2 things that affect vibration:
1) Force constant single < double < triple
2) Mass of atoms
Paint the quantum mechanical picture of Infrared [email protected]
Vibrational energies are quantized; E = (n+1/2)hv
delta n = +/- 1 (can only go from 0 to 1)
really looks like an anharmonic potential.
delta n = 2, 3, 4 are weak overtones.
selectrion rule 2: must be dipole moment and is must change.
What are the qualitative aspects of IR spectroscopy?
a. use intensity and peak position to identify compound
b. computer search algorithms
c. need pure compound
What are some quantitative aspects of IR spectroscopy?
1) Beer’s law
2) use same quantitative techniques of Uv-Vis.
3) Sharp peaks: more susceptible to non-linearity.
What are the solid sampling methods for IR spec?
1) mull: small amt of solid mixed with hydrocarbon and put between salt plates. its not quantitive and oil has its own peaks.
2) KBr pellets
3) Solution: fixed pathlength salt cell, but solvent has peaks
4) Attenuated Total Reflectance
Explain Attenuated Total Reflectance.
The crystal is either Zn, Se, or diamond.
Basically, part of the electric field of photon gets into sample and is absorbed. So it just interacts with the surface; not transmitting light.
What are the liquid sampling methods for IR spec?
1) ATR
2) Salt windows: can’t control pathlength
3) fixed pathlength cell for quantitative. Spacer ensures the pathlength is fixed.
Two types of instruments for gas sampling methods for IR spec?
1) Dispersive: need all components to transmit and detect IR light. Having a monochromator to detect IR light is difficult.
2) Fourier Transform
What are the advantages of FT-IR?
1) Multiplex advantage = acquire entire spectrum at one time (no drift).
2) High wavelength accuracy
3) Through put advantage: no monochromator, but you use a Michaelson-Interferometer.
What is the basic purpose of the Michaelson-Interferomater?
Takes the waveform from 10^14 Hz to 10^3 Hz. It splits the light intensity in 1/2: 1/2 goes to the fixed mirror, 1/2 goes to movable mirror.
The movable mirror delays the waveform, so when it comes back to beam splitter, they recombine.
What gives you a measure of how much you slow down the translation process with a Michaelson-interferomater?
The translation factor: f = (2Vm/C)* V

and you can get resolution = 1/ retardation.
Retardation = 2 (M – F)

What are the requirements for the movable mirror?
1) speed must be constant
2) position of mirror must be known exactly at any time
3) must be planar as it moves; it moves at a rate of 0.01 – 10cm/sec
4) must be able to determine a precise sampling interval ( have to measure in exact same place)
5) must be able to determine accurate retardation = 0.
Is the laser the light source in a Michaelson-interferogram?
NO! The laser is used to ensure that the measurement is made in the exact same place.
White light source is determine an accurate retardation = 0.
At more _____________, the interferogram has one sharp peak, and gives an accurate reading.
wavelengths
What are the 2 most common light sources for Michaelson interferograms?
composed of inert solids; heat 1500 K to 2200 K (blackbody radiation)
1) Nernst glower: rare earth oxide
2) Globar: silican carbide
3) Tungsten lamp: could use it
What are three types of detectors for FTIR?
1) Thermal Detector
2) Pyroelectric detector
3) Photoconducting
Give me some info on thermal detectors and two types of them.
Need a material with a low heat capacity (sensitivity) and need to minimize thickness (get rid of extra heat quickly)
1) Thermo coupler: temp dependent voltage
2) Bolometer: Temp dependent resistance
Give me some info on pyroelectric detectors.
They are the most common and inexpensive; based on time dependent capacitance.
Fast time response
What is the best type of detector for FTIR?
Photoconducting (or a semiconductor); must be cooled to avoid thermal noise (prefer liq. N2).
Why isn’t FTIR good for quantitative analysis?
1) can get evaporation
2) selective interaction with diamond interface
3) low sensitivyt (only interact with first few nm of sample)
4) polychromatic deviations due to broad IR
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