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Instrumental Test 3
Terms in this set (28)
- probes quantum state transitions associated with molecular vibrations in the physical forms of gas, liquid, solutions, and crystalline or amorphous solids
-based on scattering of radiation
- changes the frequency because of interaction between the vibrational energy level and incident radiation of the molecules
plot- intensity of the scattered radiation produces Stokes
Rayleigh - electron falls down to original ground state and there is no energy change, light from same wavelength is re-emitted
Stokes Scattering - electron is excited and then falls to vibrational level - molecule absorbed a certain amount of light
Anti-Stokes - electron excited from vibrational level reaches a virtual level and then falls to ground level
-Antistokes and Rayleigh scattering has the same wavelength as the radiation source
-IR and Raman spectroscopy yields complementary formations.
-Raman active vibration may be inactive in the IR range and vice versa.
-Raman yields more useful information than IR in certain regions
-Raman is insensitive to water and so high-quality spectra for hydrated samples can be determined
Advantages of FT-Raman over Dispersie Raman
- use infrared which eliminates fluorescence and photo decomposition
- good frequency precision for high-resolution
Disadvantages of FT-Raman over Dispersive
-detectors have low sensitivity
- difficulties with aqueous solutions
- Rayleigh scattering requires a lot of filtering
1. laser source of visible or near-infrared monochromatic radiation
2. Common sources:
Neodymium YAG source
3. sample illumination device
- similar to fluorescence but with high power illumination.
- FT Raman instrument use interferometry and Fourier transform methods
-super spectral resolution
- frequency accuracy
- high throughput advantages over dispersive based instrument.
explores absorption of radiofrequency radiation by atomic nuclei with non-zero spin in a strong magnetic field
- used mainly for identification although few quantitative applications have been reported
- NMR CLASSIFIED AS
Solution State - liquid samples which are analyzed
Solid - State - dry solid samples packed into zirconium rotors and then analyzed
- NMR FREQUENCY DEPENDS ON
identity of the nucleus and the magnetic field strength
-solvent and internal standard required
Two types of NMR
Continuous Wave NMR
FT-NMR ( most common today) -
more useful structural information,
expensive but produce a lot of information
Most common measured atomic species in NMR
^1H, ^13C, ^19F, ^31P (s=1/2)
- these nuclei possess a magnetic moment
-^1H and I = 1/2 nuclei have magnetic moment which can line up w/applied field or against
- nuclei w/ I= 1/2 nuclei is exposed to external magnetic field, it can exist in one of two energy levels
1.radio frequency source to irradiate the sample: frequency and power are stable
2.highly sensitive RF receiver
3 SUPER CONDUCTING MAGNET(discriminator):
must produce strong and steady field: is environmentally sensitive and must be refilled with helium and nitrogen to keep superconductivity
4. SAMPLE PROBE:
holds sample tube in fixed spot in bore of the magnet and houses a turbine assembly for spinning the sample and coils for excitation and detection
5. Sensitive RF Receiver:
detects RF signal produced by resonating nuclei
6. Signal processing electronics
7. Powerful Computer
Properties of NMR Standards
The standard should be:
chemically inert, low volatility, similar solubility as analyte, and reasonable reaction time
Spectral Signature of NMR
unique for each compound: proton or carbon chemical shifts are detected and used to identify functional groups
Mass Spectrometry (MS)
a technique for identifying and quantifying organic and biological molecules
- Provides useful information about:
molecular weight, elemental composition, empirical formula, functional groups
- Consists of 5 basic processes:
vaporization, ionization, acceleration, deflection, and detection
instrument that converts samples into charged gas molecules or ions then it separates them according to mass-to-charge (m/z) ratio magnetically or electrically, or by a combination of both
- ions w/ high (m/z) values are deflected to a lower extent relative to ions w/low mass-to-charge ratio
- produces a plot showing relative abundance of each ion as a function of mass-to-charge value known as mass spectrum
- base peak: most intense peak in mass spectrum
- relative abundance of other peaks expressed as % of abundance of the base peak
Useful peaks in a mass spectrum and molecular ion
molecular ion peak, peaks due to fragmentation of molecular ion, peaks due to isotopes
molecular ion gives clues about molecular weight of compound
Mass Spectrometer Instrumentation
1. Ion source - devise for sample ionization
2. Mass-Selective Analyzer - device for separating ions according to mass-to-charge ratio
3.Ion detector and readout system
4. Vacuum system
Mass Spectrometry Sample
solid, liquid, or gas but must be volatized prior to ionization
allows introduction of representative pure sample into ionization area of spectrometer w/ minimal vacuum loss
An ion source converts molecules into gaseous ions.
Two types: Gas-phase ion sources and Desorption ion sources
Ionization - Gas Phase (3 Types)
1. Electron Impact (EI) Ionization - uses a beam of energetic electrons generated by a filament to cause fragmentation, which provides structural information, of molecules.
methane reacts with high energy electrons to produce several reactive ions such as CH4 + , CH3 + and CH2 + .
-The fragment ions provide structural information.
2.Chemical Ionization (CI) - uses a large excess of reagent gas such as methane or ammonia and energetic electrons to ionize the analyte.
-produces less fragmentation of the analyte.
-Molecular weight information is obtained
from ions in the mass spectrum.
-methane reacts with high energy electrons
to produce several reactive ions such as
CH4 + , CH3 + and CH2 + .
-The predominant ions (CH4 +● and CH3 + )
react rapidly with additional reagent gas
-species collide w/analyte and ionize analyte
by proton or hydride transfer
-Proton transfer produces (M+1)+ ions and
hydride transfer produces (M-1)+ ions. These
ions undergo less fragmentation due to their
3. Field Ionization (FI) - uses a powerful electric field concentrated in the tip of an emitter wire to vaporize and ionize a sample.
-Field Desorption (FD) - utilizes a powerful electric field concentrated in the tip of an emitter wire to ionize a sample coated on the emitter.
the majority of the solvent must be allowed to evaporate before the emitter is introduced into the mass spectrometer source.
application of a high potential between the emitter and another electrode not to far away from the emitter causes desorption of ions and ionization of the analyte
Purpose - to distinguish between minute mass differences, separate ion beams according to their mass-to-charge ratio, and limit the ions arriving at the detector to a narrow range of mass.
Identification of Formulas and Structure from Mass Spectra
Step 1: Most important - Identify the molecular ion peak, the (M+1)+ or M-1)+ peak
Step 2: Study the isotope distribution pattern and look for elements with abundant and distinctive isotopic patterns in the spectrum.
M+1 peak - C, N, H
M+2 peak - O, Br, Cl
Fragmentation pattern in mass spectrometry depends on:
1) Identity (and chemistry) of parent compound
2) Ionization method
Tandem Mass Spectrometry (MS-MS)
also known as MSn .
- involves multiple stages of mass spectrometry
- enables mass spectrum of fragmented and ions to be produced.
-Identifying components when chromatographic separation is not possible.
-where fragmentation is not abundant.
In this mode
-first quadrupole (Q1) select ions of specific mass - - - - second quadrupole (Q2) collide with gas or collision cell causing fragmentation producing ions called product ions.
-third quadrupole (Q3) ions analyzed and detected by the detector.
Resolving Power of Mass Spectrometers
- describes the capability of a mass spectrometer to differentiate between ions of adjacent masses (m/z).
- can range from 500 to over 1× 106 .
- depends on MS instrument design.
- R= m/deltam
Resolution is ΔM at a given M.
Two methods of defining resolution:
- Method 1 - Two peaks of equal intensity, each contributing 5%, are resolved if the overlap between the peaks is 10 % or less of the height of each peak
- Method 2 - Measures peak width at 50% peak height and produces values approximately twice that. It was calculated using the 10 % valley method.
Resolution = 20,000-100,000 or higher range. Accuracy = about ± 0.005%.
Useful for the determination of precise molecular masses of substances.
HE-MS instruments must be calibrated using known compounds such as:
Mass Spectrometry Detectors
1. Electron Multiplier (EM)
-One ion can produce 105 electrons or more.
- very fast response
- very sensitive.
- inexpensive detector
- a metal or carbon cup that captures ions
and store the charge
- current generated is measured and
Surface Spectroscopic Methods - XPS, SIMS
1. X-Ray Photoelectron Spectroscopy
- specific technique.
-used to identify and quantify
-DETECTS: Elemental composition, Oxidation states, and Functional group composition.
-electron spectrometer is used to measure the kinetic energy of emitted electron and
binding energy of the atomic orbital
-large range of application because it can get to core electrons
- can identify all but 2 elements
- it can identify chemical state
- good for quantitative analysis
- differentiate between oxygen states
-has to work with high vacuum
- provides limited organic information
- cant detect Helium or Hydrogen
- takes a lot of time to produce spectra
- X-ray tube which serves as source
- sample surface in vacuum chamber
- electron energy analyzer which acts as discriminator
- detector: can be electron multiplier or Faraday cup
- scan and readout system
2. Secondary-Ion Mass Spectrometry (SIMS) -
- DETECTS: Elemental, Molecular, and Isotopic composition
HOW IT WORKS:
- sample placed in ultra-high vacuum and exposed to a flux of primary ions
- causes ejection of secondary ions from a sample surface
- flux of ions are sent to a mass analyzer for analysis
Static SIMS - delivers a low dose of primary ions which are used to maintain chemical integrity of the sample surface .
- Dynamic SIMS - delivers higher dose of a primary ions to bombard a sample
Atomic Force Microscopy (AFM)
Atomic Force Microscopy (AFM)
- Structural information.
- Molecular interaction
- Frictional composition
- Mechanical composition.
- USES a sharp silicon or silicon nitride tip at the end of a cantilever spring TO PROBE and SCAN sample
- TOPOGRAPHIC IMAGE is made by plotting the deflection of the cantilever versus its position on the sample.
- great for measuring small samples with a lot of accuracy
- does not need a vacuum
- the sample is not damaged by extra treatments
- has only 1 scan image size
- slow scan time so thermal drift can occur
- poor signal-to-noise ratio
1. Contact AFM - use of constant force or height mode.
2. Tapping mode - oscillating the tip with oscillation amplitude
3. Non-contact - measures and maps the force gradient between the tip and sample surface.magnifies details in samples.
Transmission Electron Microscope (TEM) AND Scanning Electron Microscope (SEM)
MONOENERGETIC BEAM OF ELECTRONS TO PROBE SAMPLE
-FORM TWO-DIMENSIONAL IMAGE
Atomic resolution lattice images
Particle size distribution
- ELECTRON BEAM KNOCKS OFF PARTICLES
- FORMS 3D IMAGES
- ELECTRON PROBE: a SEM When fitted with wavelengthdispersive X-ray spectrometer
- uses migration of charged and uncharged species influenced by electrical field
- Common modes
Capillary zone electrophoresis (CZE) - based on free solution mobility
. - simple and most widely used
- performed in a capillary filled with an electrolyte solution of ionic strength and chosen pH
Capillary gel electrophoresis (CGE) - based on size and charge.
Capillary isoelectric focusing electrophoresis (CIEF) - based on isoelectronic point (pH value).
Capillary isotachophoresis electrophoresis (CITP) - based on moving boundaries.
- driving force for the separation is a high voltage power supply
- Low and high voltages give longer and rapid analysis time
- applied constant voltage generates electrophoretic migration of the ionizes species based on m/z charge and electroosmotic flow
Capillary Electrophoresis continued
Order of Elution
First - Cations with highest mobility first
Second - All unseparated neutrals
Last - Anions with highest mobility
Column that uses fused silica
Injection volume is 5 to 50 nL.
-Sample introduction methods
1. ELECTROKINETIC INJECTION based on use of electric field that drives sample solution into the capillary
2. PRESSURE INJECTION based on the use of pressure difference between the two ends of the capillary which drives sample solution into the capillary.
- Detection Methods
Supercritical Fluid Chromatography (SCF)
supercritical fluid used as a mobile phase for chromatographic separation.
The fluid is neither a true liquid nor a true gas.
-is faster and has better resolution than liquid chromatography
-formed when a volatile material is heated above the critical point
-Properties of a Supercritical fluid
1.density is similar to its liquid state
2. Several times than its gas state.
3. density and power can be modified by changing temperature and pressure.
4. can dissolve large, nonvolatile molecules such like n-alkanes with carbon atoms
Supercritical Fluid Chromatography
- most used supercritical fluid.
-used to extract caffeine from coffee beans, producing decaffeinated coffee.
Supercritical CO2 Properties
-Low critical temperature
-transmits in the ultraviolet range.
-excellent solvent for nonpolar organic
Most widely used detectors
- GC detectors: flame-ionization, and electron capture.
- HPLC detectors: UV-visible absorption and fluorescence.
infrared spectrum is made up of :
The functional group region
The finger print region
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