Instrumental Analysis Exam 3

Optical Spectrometry
elements converted to gaseous atoms or elementary ions then absorption, emission, fluro of species is measured
Atomic Mass Spectrometry
samples are atomized, gaseous atoms converted to positive ions, ions separated by mass to charge ratio
X-Ray Spectrometry
no atomization required
Absorpt, Emiss, Fluro of sample directly measured
Is triplet state more or less energetic than singlet state?
Less energetic than singlet state
Explain the splitting of orbitals
In triplet excited state two spins are unpaired: effect of orbital magnetic moment on B-field causes splitting.
What conditions apply to no splitting.
In singlet excited state-spins are paired and magnetic effects cancel.
What is effective line width of absorption/emission line?
Because of line broadening its width is one half of the maximal signal.
4 Causes of line broadening?
1. HUP – natural line width
2. Doppler Effect
3.Collisions – small changes in E-level cause range of abs/emit wavelength
4. Electric and B-field effects
5 Steps of Flame Atomization
1. sample nebulized by gaseous oxidant
2. solvent evaporates to product finely divided solid aerosol
3. aerosol volatized to form gaseous molecules
4. dissociation of molecules produce atomic gas
5. some of atoms in as ionize to form cations and electrons
3 zones of a flame
Interzonal – free atoms prevalent most used in spectrometry
Secondary combustion – inner core products oxidized
Primary combustion – no thermal equilibrium
Laminar flow burners in AAS [atomic absorption spectrometry]
quiet flame with long path length for max absorption which enhances sensitivity and reproducibility
Electrothermal Atomization process
1. few uL of sample evaporate at low temp
2. sample ashed as higher temp
3. current rapidly increased to 100s of amperes, temp increased to 3000 to atomize
4.absorption or fluoro of atomic vapor measured in region immediately above heated surface
Advantages/Disadvantages of Electro-thermal atomization
Advantages: enhanced sensitivity, entire sample is ionized in short period of time
Disadvantage: 5-10% precision : flame 1% precision, method is slow, analytical range is narrow
-Method of choice when flame/plasma have inadequate detection limits.
Spectral Interferences in Atomic Absorption Spectrometry
Abs/Emiss of interfering species lies so close that it cant be resolved with monochrometer.
-organic solvents used to dissolve analyte that aren’t combusted.
-scattering from refractory oxides Ti W Zn
Chemical Interferences in Atomic Absorption Spectrometry
More common than spectral
-chemical processes occur during atomization that alter absorption of analyte
-anions form cmpds with low VP reducing amt atomized
-reversible diss rxn
-ionization equilibria
Most important and widely used sources of AES [Atomic Emission Absorption]
ICP [inductively coupled argon plasma]
DCP [direct current argon plasma]
MIP [microwave induced argon plasma]
GD [glow discharge plasma]
-convert sample to atoms/ions AND in process excite fraction of species to an excited state
What is Plasma?
-collection of free moving electrons and ions. with no sustaining power plasma recombines into neutral gas.
-makes up to 99% of visible universe
-range cool tenuous [aurora] to hot dense [star]
Advantages of plasma/arc/spark emission spec over flame/electrothermal absorp methods
-lower susceptibility to chem interferences bc of higher temps
-good emission spectra result for most elements under 1 set of excitation conditions
-determination of nonmetals
-conc ranges of several orders of magnitude
ICP torch [inductively coupled plasma]
1. argon ions/electrons are principle conducting species in argon plasma with cations of sample in present amts.
2. ionization of Ar initiated by a spark from Tesla coil
3. ions interact with fluctuating B-field causes ion and electron in coil to flow in annular path
4. resistance of ion/electron to flow causes heating of plasma
ICP sample introduction
high velocity breaks up liquid into fine droplets which are carried into plasma. furnace only for sample introduction
micro-sampling and low detection limits
Advantages of plasma source
-atomization more complete and fewer chemical interferences occur
-chemically inert environment enhances lifetime of analyte by preventing oxidation
-self absorption less frequent b/c temp cross section of plasma uniform
-calibration curves liner over several order of magnitude for concentration
DCP characteristics [Direct current plasma source]
3 electrodes : 2 graphite 1 tungsten
plasma jet formed by bringing cathode in momentary contact with anode
-argon ionizes and current develops generating ions to sustain current
-much less argon required but higher maintenance : incomplete sample volatilization
Desirable properties of emission spectrometer
1. high resolution
2. rapid signal acquisition
3. low stray light
4. wide dynamic range
5. accurate/precise wavelength ID
6. high stability with environment
Application of plasma sources
Limited to approx. 60 elements. Elements have several lines to choose from.
ratio = signal analyte/signal standard
Steps of atomic mass spectrometry
1. atomization
2. converting substantial portion of atoms into stream of ions
3. separating ions on basis of M/Z ratio
M = mass # of ion in AMU
Z = # of fundamental charges of the ion
4. counting # of ions of each type/measuring ion current produced with ions formed strike suitable transducer
[3&4 performed by a mass spec]
Unique important feature of mass spectrometers
Differentiate between masses of isotopes
nominal mass
whole number precision in mass measurement
chemical atomic mass [ average atomic mass ]
sum of atomic masses of isotopes times fractional abundance
3 types of mass spectrometers used in atomic MS
Quadrapole [most common], Double focusing, Time of flight
Components of mass spectrometer
1.introduce micro amount of sample into ion source
2. positive gas ions generated and accelerated into mass analyzer
3. mass analyzer disperses ions based on M/Z ratio
4. transducer converts stream ions into electrical signal
Quadrupole mass analyzers [MS]
Ratio of alternating AC and DC currents determine what ions make it through – increased while maintaining their ratio
Time of flight MS
1. positive ions produced by bombardment of sample with pulses of electron/laser
2.ions accelerated by E-field pulse and pass into drift tube
3. All ions have same KE , lighter ions arrive first
Flight takes micro-second
Time of flight MS advantages/disadvantages
Disadvantages=less resolution and reproducibility
Advantages=simple rugged unlimited mass range rapid data acquisition
Double focusing mass spectrometer
1. ions from source accelerated through slit
2. curved electrostatic field focuses beam of ions with narrow band of KE into another slit
3. in curved B-field lighter ions deflected more and dispersed ions fall onto photographic plate
Why is MS better than optical spectrometery
MS spectra much simpler less interference spectra
4 types of spectroscopic interferences with MS
-occur with ionic species have same M/Z value as analyte ion
1. isobaric ion: two elements have same mass
2. polyatomic/abduct ions: blank can sometimes correct
3. doubly charged ions
4. refractory oxide ions: oxides and OH- formed from analyte etc peaks overlap with analyte peaks
What does mass spectrometry provide information about?
1. elemental composition of samples of matter [Atomic Mass Spectrometry]
2. structures of organic, inorganic, and biomolecules
3. qualitative/quantititive composition of complex mixtures
4. structure and composition of solid surfaces
5. isotopic ratios of atoms in samples
The molecular ion
The charged radical species with the same molecular mass as the molecule
Base Peak
Arbitrarily defined as the largest beak and intensity is set to 100
Molecular Mass Spectrometry procedure
Bombardment with electrons generates excited state, relaxation leads to fragmentation and ions with lower masses.
Sample first vaporized then ionized. 3 Types of ion sources.
3 types of ion sources [basic gas phase]
Electron impact [EI] Chemical Ionization [CI] Field Ionization [FI]
When are desorption techniques applicable
to nonvolatile thermally unstable samples. does not require volatilizaton so applicable to analytes as large as 10^5
hard ion sources
impart enough energy to leave analyte in high energetic excited state which leads to bond rupture and lots of ions smaller than molecular ion
soft ion sources
cause very little fragmentation and the spectrum usually consist of the molecular ion and a small number of other peaks
Electron Impact
sample brought to high temp that can produce a molecular vapor which is ionized with a beam of energetic electrons.
not very efficient. 1 in 1 million undergoes reaction.
daughter ions formed from subsequent relaxation and fragmentation
-hard ion source
Chemical Ionization Source
gaseous ions of sample are ionized by collision with ions produced by electron bombardment of an excess of reagent gas
-electron beam interacts exclusively with reagent molecules
-soft ion source
Field Ionization Source
Ions formed under the influence of a large E-field generated by applying high voltage to specific formed emitters that consist of fine tips have diameters of 1 uM
-Ions formed by quantum mechanical tunnel mechanism in which electrodes from analyte are extracted from microtips of anode.
-Little energy imparted and little fragmentation occurs
How do desorption sources operate?
Energy introduced to solid/liquid sample in a way to cause direct formation of gaseous ions
-spectra often only 1 molecular ion
-like FI except that the micro-needle is coated with sample solution
4 Methods of desorption sources
1. Field desorption
3. Electrospray
4. Fast Atom Bombardment
Matrix Assisted Laser Desorption Ionization –
-low conc. of analyte is uniformly dispersed in solid/liquid matrix
-laser beam focused on sample, matrix material must strongly absorb laser radiation
-matrix and sample then desorbed/ionized to create ion plume
-most common mass analyzer used with MALDI is TOF
Electrospray Ionization
gentle of cmpd.
-impr for analyzing biomolecules
-sample soln pumped through capillary needle uL/min
-charged spray of fine droplets passed through desolvating capillary – where evaporation of solvent and attachment of charge occurs
-droplets get smaller and charge increases until surface tension cant support charge left with a charged analyte molecule
Resolution of mass spectrometer
capability of a MS to differentiate between two mass/charge ratios
R = nominal mass / mass difference between two adjacent peaks that are just resolved
Tandem MS
Mass spectrum of pre-selected ion are obtained
Daughter peaks of daughter peaks
Some applications of MS
1.structure of organic/bio molecules
2. determine MM of peptide protein
3. ID of components on this layer and paper chromatography
4. determine AA sequence
5. ID drug in bodily fluids
6. Determine pesticide in food