Test 4 Flashcards
Components of fluorescence, phosphorescence and scattering spectrophotometers
Source > wavelength selector > sample > detector > readout
Components of emission and chemiluminescence spectrophotometers
Source and sample > wavelength selector > detector > readout
Emittance (M)
- measured in W/m^3
- M = sigma T^4
- sigma = Stefan Boltzmann constant = 5.6698 x 10^-8 Wm^-2K^-4
- T= temperature
Laser defn
Light Amplification by Stimulated Emission of Radiation
Laser Properties
- high intensity
- monochromatic (1 color)
- collimated (travels in a straight line)
- coherent (emitted waves are in phase)
- plane polarized (electric fields line up)
Laser characteristics
- energy pump
- spontaneous emission
- stimulated emission
- absorption
- population inversion
Population Inversion
More species in excited (metastable) state than in the ground state
How laser works
- Pumping causes a population inversion
2. Lazing occurs when rate of stimulated emission exceeds the rate of absorption
Diffraction
Observation that light passing through a small aperture spreads out and does not travel in a straight line
Constructive interference
Electric field vectors are in the same direction
Destructive interference
Electric field vectors are in opposite directions
Components of absorption spectrophotometers
Source>wavelength selector> sample > detector > readout
Stray light
Radiation that reaches the detector without going through the sample or monochromator (unwanted radiation)
Detectors
Convert electromagnetic radiation into a measurable quantity (voltage, current, etc)
Photomultiplier tube (PMT)
- very sensitive due to inherent amplification
- easy to damage
- Gains of 10^6 to 10^7 electrons/proton typical
- fast response times
- limited only by dark current. But thermal portion of this is gone at -30 degrees Celsius
Thermocouple
Junction between dissimilar metals generates potential difference which is temperature dependent
Thermopile
Group of thermocouples
Photoconductive
Semiconductor conductivity changed with infrared radiation
Fourier Transform Infrared Spectrometry (FTIS)
- all frequencies measured simultaneously
- spectrum computed by FFT
- operated by generating a complex interference pattern as function of moving mirror distance
Advantages of FTIR
- Fellgetts advantage
- jacquinots advantage
- wavelength accuracy
- Stray light advantage
FTIR Advantage - Fellgetts Advantage
Signal-to-noise ratio increases with sq. root of number of scans added
FTIR Advantage Jacquinots advantage
High energy striking detector because slits not used
FTIR Advantage - wavelength accuracy
Laser fringe reference is best method of distance measurement. Wavelength accuracy +/- 0.01 cm^-1
FTIR Advantage - Stray Light Advantage
Only modulated radiation passing through interferometer detected. Stray light results in DC offset
Atomic absorbable width
0.2-0.5 nm wide
Molecular absorbance width
10-100nm
Absorption
Excite atoms with energy required for excitation transition
Emission
Source provides atomization and excitation - relaxation provides emission
Atomization
Process by which liquid samples are converted into aerosols, then molecules, and finally into atoms
Nebulizer
Device used to convert liquid samples into aerosols
Laminar flow burner - Advantages
- long (5-10 cm) flame path length
- quiet flame - stable/reproducible
Laminar flow burner - disadvantages
- low sampling efficiency (most goes down the drain)
- flashback (explosive mixtures)
Non-Flame Atomizers -
Electro thermal (graphite furnace)
Advantages
- no sample loss during nebulizing
- longer residence time (secs)
- large (>10 muL) volumes
- lower detection limits (0.1-100 pg)
Non-Flame Atomizers -
Electro thermal (graphite furnace)
Disadvantages
- matrix interference problems (matrix diffusing into graphite)
Blaze wavelength
Wavelength at which incidence angle = reflected angle.
Grooves/mm
UV-VIS : 300-2000 (1500 typ.)
IR : 10-200 (100 typ.)
AAS vs AES (flame)
- AES requires no source but AAS source lines are so narrow that monochromator needs are not too demanding
- necessary operator skill generally less for AAS
- background correction usually easier for AES
- precision/accuracy approximately equal for both
- linear range typically wider for AES
- interferences. AAS subject to less spectral. Chemical interferences are approximately the same for both.
- detection limit depends on the element
AES more sensitive than AAS (in these elements)
Al, Ba, Ca, Eu, Ga, Ho, In, K, La, Li, Lu, Na, Nd, Pr, Rb, Re, Ru, Sm, Sr, Tb, TI, Tm, W, Yb
AAS vs AES (sensitivity about the same)
Cr, Cu, Dy, Er, Gd, Ge, Mn, Mo, Nb, Pd, Rh, Sc, Ta, TI, V, Y, Zr
AAS vs AES (AAS more sensitive)
Ag, As, Au, B, Be, Bi, Cd, Co, Fe, Hg, Ir, Mg, Ni, Pb, Pt, Sb, Se, Si, Sn, Te, Zn
Plasma defn
A plasma is a conducting, ionized gas
AAS advantage
Cheaper, easier to operate, lower operating costs (compared to Plasma too)
Plasma vs AAS
- Compared to flame and electro thermal, much higher temperatures
- high temperature minimizes interelement interferences
- high temperature allows access to difficult metals and non-metals
- emission allows for simultaneous multi-element analysis
- generally complimentary
- plasma emission has advantage due to higher operating temperatures
Separation methods- Extraction
Using a solvent to selectivity remove the analyte
Separation methods - Chromatography
Using a separation column to isolate mixture components
Chromatography general principles
- Mobile phase (gas, liquid, or supercritical fluid) flows over/through immiscible stationary phase (liquid, solid) held in a column or solid surface
- sample introduced at the beginning of the column
- sample components exhibit various affinities for stationary phase
- sample components with strong affinities for stationary phase move slowly through the column. Those with weak affinities move rapidly
- individual components elute in “bands”
3 major components of chromatography
- mobile phase: analyses “dissolve” in the mobile phase. Also, it serves as the transport medium
- stationary phase: immobile phase to which the individual analyses have varying degrees of attraction
- equilibrium: the nature of attraction of individual analysts can be described as equilibrium processes, such as adsorption and solvent-solvent partition
Elution defn
Process by which analyzes are passed through the column
Eluent defn
Mobile phase
Partition coefficient (partition ratio)
For a given eluting species, A, the ratio of the concentration of A in the stationary phase to that in the mobile phase.
HETP (H)
Height Equivalent to a Theoretical Plate
Resolution defn
Ability of column to separate to analytes
Qualitative analysis by chromatography
TR is characteristic of analyte under fixed chromatography conditions
Quantitative analysis by chromatography
Both peak height and peak area are proportional to analyte concentration under fixed chromatography conditions
mass spectrometry
Process for analysis of single ions. Basically, molecules are converted into gas phase ions, separated according to mass to charge ratio, and then detected by an ion sensitive detector
Base peak
- Largest peak in the mass spectrum
- arbitrarily given an intensity of 100
- represents the most favorable fragmentation pathway
Exact mass
Involves particular isotopes of element or compound containing a particular set of isotopes
Nominal mass
Referring to MS ion masses measured to the nearest whole number
MS structural information
- fragmentation pattern is representative of molecular structure
- exact mass measurements give elemental composition of fragments and molecular ions
- recognition of isotope abundance patterns
Radical cation
- radical cation know as the molecular ion
- it’s usually the highest mass speak in spectrum
- also called the “parent” ion
- accurate mass analysis of the molecular ion can determine elemental composition
M + 1 for H
0.015
M+ 1 for C
1.08
M+ 1 for N
0.37
M+ 1 for O
0.04
M+2 for O
0.20
M+2 for Cl
32.5
M+2 for Br
98.0
Effects of rings and double bonds
More rings and double bonds means a more stable molecular ion
Branching effects
More branching equals less stable molecular ion
Because branches are places of weakness and so places for molecules to work
Basic instrumentation for mass spectrometry
Inlet>source>mass analyzer>detector
Source»detector vacuum (10^-5 - 10^-8)
Sources for mass spec.
- gas phase sources: sample volatilizations; then ionization
- desorption sources: simultaneous volatilization and ionization
Sources for mass spec. (Hard and soft)
- hard sources; causes lots of fragmentation
- soft sources; causes little fragmentation
Mass analyzers - features of a quadrupole mass analyzer
- rapid scan (50msec/decade)
- high transmission (good sensitivity)
- low voltage (0-50 V)
- no accelerating voltage dependence on trajectory
- linear mass scan
- low resolution (R
Mass analyzers time of flight mass spectrometer features
- Msec per scan
- high mass analysis (> 10000 amu)
- low resolution (R
