Presentation on theme: "Elemental Analysis - Atomic Spectroscopy"— Presentation transcript:
1Elemental Analysis - Atomic Spectroscopy A) IntroductionBased on the breakdown of a sample into atoms, followed by the measurement of the atom’s absorption or emission of light.i. deals with absorbance fluorescence or emission (luminescence) of atoms or elemental ions rather then molecules- atomization: process of converting sample to gaseous atoms or elementary ionsii. Provides information on elemental composition of sample or compound- UV/Vis, IR, Raman gives molecular functional group information, but no elemental information.iii. Basic process the same as in UV/Vis, fluorescence etc. for moleculesAbsorbanceFluorescence
2iv. Differences for Molecular Spectroscopy - no vibration levels much sharper absorbance, fluorescence, emission bands- position of bands are well-defined and characteristic of a given element - qualitative analysis is easy in atomic spectroscopy (not easy in molecular spectroscopy)Examples:carbonoxygennitrogen
3B) Energy Level Diagrams energy level diagram for the outer electrons of an element describes atomic spectroscopy process.i. every element has a unique set of atomic orbitalsii. p, d, f split by spin-orbit couplingiii. Spin (s) and orbital (l) motion create magnetic fields that perturb each other (couple)- parallel higher energy; antiparallel lower energy• Similar pattern between atoms butdifferent spacing• Spectrum of ion different to atom• Separations measured inelectronvolts (eV)1eV = 1.602x10-19 J= kJ ×mol-1• As number of electrons increases,number of levels increasesemission spectra more complexLi 30 linesCs 645 linesCr 2277 linesNaMg+Note slight differences in energy due to magnetic fields caused by spin
4C) Desire narrow lines for accurate identification Broadened byi. uncertainty principleUncertainty principal:Dt . DE ≥ h\Dt . Dn ≥ 1Dt – minimum time for measurementDn – minimal detectable frequency differencePeak line-width is defined as width in wavelength at half the signal intensity
5C) Desire narrow lines for accurate identification Broadened byii. Doppler effectDoppler effect- emitted or absorbed wavelength changes as a result of atom movement relative to detector- wavelength decrease if motion toward receiver- wavelength increases if motion away from receiverUsage in measurement of velocity of galaxies, age of universe and big bang theory
6C) Desire narrow lines for accurate identification Broadened byiii. Pressure broadeningPressure broadening:Collisions with atoms/molecules transfers small quantities of vibrational energy (heat) - ill-defined ground state energyEffect worse at high pressures:• For high pressure Xe lamps (>10,000 torr) turns lines into continua!
7D) Effect of Temperature on Atomic Spectra - temperature changes number of atoms in ground and excited states- need good temperature controlBoltzmann equationN1 and No – are the number of atoms in excited and ground statesk – Boltzmann constant (1.28x10-23 J/K)T – temperatureDE – energy difference between ground and excited statesP1 and Po – number of states having equal energy at each quantum levelNa atoms at 2500 K, only 0.02 % atoms in first excited state!Less important in absorption measurements % atoms in ground state!
8E) Sample Atomization – expose sample to flame or high-temperature Need to break sample into atoms to observe atomic spectraii. Basic steps:a) nebulization – solution sample, get into fine droplets by spraying thru thin nozzle orpassing over vibrating crystal.b) desolvation - heat droplets to evaporate off solvent just leaving analyte and othermatrix compoundsc) volatilization – convert solid analyte/matrix particles into gas phased) dissociation – break-up molecules in gas phase into atoms.e) ionization – cause the atoms to become chargedf) excitation – with light, heat, etc. for spectra measurement.
9E) Sample Atomization – expose sample to flame or high-temperature iii. Types of Nebulizers and Atomizers
10F) Atomic Absorption Spectroscopy (AAS) – commonly used for elemental analysis– expose sample to flame or high-temperature– characteristics of flame impact use of atomic absorption spectroscopyFlame AAS:• simplest atomization of gas/solution/solid• laminar flow burner - stable "sheet" of flame• flame atomization best for reproducibility (precision) (<1%)• relatively insensitive - incomplete volatilization, short time in flame
11Different mixes and flow rates give different temperature profile in flame - gives different degrees of excitation of compounds in path of light source
12- not in thermal equilibrium and not used b) interconal region ii. Types of Flame/Flame Structure – selection of correct flame region is important for optimal performancea) primary combustion zone – blue inner cone (blue due to emission from C2, CH &other radicals)- not in thermal equilibrium and not usedb) interconal region- region of highest temperature (rich in free atoms)- often used in spectroscopy- can be narrower in some flames (hydrocarbon) tall in others (acetylene)c) outer cone- cooler region- rich in O2 (due to surrounding air)- gives metal oxide formationTemperature varies across flame –need to focus on correct part of flamePrimary region for spectroscopyNot in thermal equilibrium and not used for spectroscopyFlame profile: depends on type of fuel and oxidant and mixture ration
13Most sensitive part of flame for AAS varies with analyte Consequences:- Sensitivity varies with element- must maximize burner position- makes multi-element detection difficult
14iii. Basic instrument design (Flame atomizer) Single beamDouble beam
15a) atomizer1) Laminar Flow Burner- adjust fuel/oxidant mixture for optimum excitation of desiredcompounds- usually 1:1 fuel/oxidant mix but some metals forming oxides useincrease fuel mix- different mixes give different temperatures.Laminar – non-turbulent streamline flowsample, oxidant and fuel are mixedonly finest solution droplets reach burnermost of sample collects in wasteprovides quite flame and a long path length
16Po P 2) Electrothermal (L’vov or Graphite furnace) - place sample drop on platform inside tube- heat tube by applying current, resistance to current creates heat- heat volatilizes sample, atomizers, etc. inside tube- pass light through to measure absorbancePoPPlace sampledroplet on platform
173) Comparison of atomizers a) Electrothermal (L’vov or Graphite furnace) :advantages:- all sample used- longer time of sample in light beamlower limit of detection (LOD)can use less sample (0.5 – 10)disadvantage:- slow (can be several minutes per element or sample)- not as precise as flame (5-10% vs. 1%)- low dynamic range (< 102, range of detectable signal intensity)\ use only when there is a need for better limit of detection or have less sample than Laminar flow can useb) Laminar Flow Burneradvantages:- good b (5-10 cm)- good reproducibilitydisadvantages:- not sample efficient (90-99% sample loss before flame)- small amount of time that sample is in light path (~10-4 s)- needs lots of sample
18b) Light source- need light source with a narrow bandwidth for light output- AA lines are remarkably narrow (0.002 to nm)- separate light source and filter is used for each elementproblem with using typical UV/Vis continuous light source- have right l, but also lots of others (non-monochromatic light)- hard to see decrease in signal when atoms absorb in a small bandwidth- only small decrease in total signal area- with large amount of elements bad sensitivity
191. ionizes inert gas to high potential (300V) 2) Solution is to use light source that has line emission in range of interest- laser – but hard to match with element line of interest- hollow cathode lamp (HCL) is common choiceHollow Cathode LampCoated with element to be analyzedProcess: use element to detect element1. ionizes inert gas to high potential (300V)Ar Ar+ + e-2. Ar+ go to “-” cathode & hit surfaces3. As Ar+ ions hit cathode, some of deposited element is excited anddislodged into gas phase (sputtering)4. excited element relaxes to ground state and emits characteristic radiation- advantage: sharp lines specific for element of interest- disadvantage: can be expensive, need to use different lamp for each element tested.
20c) Source Modulation (spectral interference due to flame) - problem with working with flame in AA is that light from flame and light source both reach detector- measure small signal from large background- need to subtract out flames to get only light source signal (P/Po)i. done by chopping signal:ii. or modulating P from lamp:Flame + PFlame onlyPFlame + PFlame onlytime
21d) Corrections For Spectral Interferences Due to Matrix - molecular species may be present in flame- problem if absorbance spectra overlap since molecular spectrum is much broader with a greater net absorbance- need way of subtracting these factors out
22Methods for Correction 1) Two-line method- monitor absorbance at two l close together> one line from sample one from light source> second l from impurity in HCL cathode, Ne or Ar gas in HCL, etc- second l must not be absorbed by analyte> absorbed by molecular species, since spectrum much broader- A & e are ~ constant if two l close- comparing Al1, Al2 allows correction for absorbance for molecular speciesAl1 (atom&molecule) – Al2 (molecule) = A (atom)Problem: Difficult to get useful second l with desired characteristics
232) Continuous source method - alternatively place light from HCL or a continuous source D2 lamp thru flame- HCL absorbance of atoms + molecules- D2 absorbance of moleculesadvantage:-available in most instrumentseasy to dodisadvantage:difficult to perfectly match lamps (can give + or – errors)
24- only absorb light with same orientation 3) Zeeman Effect- placing gaseous atoms in magnetic field causes non-random orientation of atoms- not apparent for molecules- splitting of electronic energy levels occurs (~ 0.01 nm)- sum of split absorbance lines original line- only absorb light with same orientation- can use Zeeman effect to remove background> place flame polarized light throughsample in magnetic field getabsorbance (atom+molecule) orabsorbance (molecule) dependingon how light is polarizedBackgroundzz***Background+Absorbancez*z
25A e) Chemical Interference - more common than spectral interference 1) Formation of Compounds of Low Volatility- Anions + Cations SaltCa2+ +SO42- CaSO4 (s)- Decreases the amount of analyte atomized decreases the absorbance signal- Avoid by:> increase temperature of flame (increase atom production)> add “releasing agents” – other items that bind to interfering ionseg. For Ca2+ detection add Sr2+Sr2+ + SO42- SrSO4 (s)increases Ca atoms and Ca absorbance> add “protecting agents” – bind to analyte but are volatileeg. For Ca2+ detection add EDTA4-Ca2+ + EDTA4- CaEDTA2- Ca atoms2) Formation of Oxides/HydroxidesM + O » MOM + 2OH » M(OH)2- M is analyte> use less oxidantnon-volatile & intense molecular absorbanceA
263) IonizationM » M+ + e-- M is analyte- Avoid by:> lower temperature> add ionization suppressor – creates high concentration of e- suppresses M+ by shifting equilibrium.
27\ I (emission) µ N1, so signal increases with increase in temperature G) Atomic Emission Spectroscopy (AES) – similar to AA with flame now being used for atomization and excitation of the sample forlight production1) Atomic ProcessesheatDegree of Excitation Depends on Boltzmann Distribution:N1 and No – are the number of atoms in excited and ground statesk – Boltzmann constant (1.28x10-23 J/K)T – temperatureDE – energy difference between ground and excited statesP1 and Po – number of states having equal energy at each quantum levelIncrease Temperature increase in N1/No (more excited atoms)\ I (emission) µ N1, so signal increases with increase in temperature
282) Comparison of AA and AES Applications Need good temperature control to get reproducible signaleg. For Na, temperature difference of 10o 2510results in a 4% change in N1/NoTemperature Dependence Comparison between AA and AES:- AA is relatively temperature independent. Need heat only to get atoms, not atoms in excited state.- AA looks at ~ 99.98% of atoms- AES uses only small fraction (0.02%) of excited atoms2) Comparison of AA and AES ApplicationsAES - emission from multiple species simultaneouslyComparison of Detection LimitFlame Emission More SensitiveSensitivity About the SameFlame Absorption More SensitiveAl, Ba, Ca, Eu, Ga, Ho, In, K, La, Li, Lu, Na, Nd, Pr,Rb, Re, Ru, Sm, Sr, Tb, Tl, Tm, W, YbCr, Cu, Dy, Er, Gd, Ge, Mn, Mo, Nb, Pd, Rh, Sc, Ta, Ti, V, Y, ZrAg, As, Au, B, Be, Bi, Cd, Co, Fe, Hg, Ir, Mg, Ni, Pb, Pt, Sb, Se, Si, Sn, Te, ZnSome better by AA others better by AES
293) Instrumentation- Similar to AA, but no need for external light source (HCL) or chopper > look at light from flame> flame acts as sample cell & light sourceAtomization Sources:SourceTemperature (oC)FlamePlasma4,000-6,000Arc/Spark4,000-5,000/40,00Electrothermal usually not used – too slow and not as precise
30a) Flame Source:- used mostly for alkali metals> easily excited even at low temperatures- Na, K- need internal standard (Cs usually) to correct for variations flameAdvantages- cheapDisadvantage- not high enough temperature to extend to many other elements
31b) Plasma (inductively coupled plasma - ICP) - plasma – electrically conducting gaseous mixture (cations & electrons)- temperature much higher than flame- possibility of doing multiple element analysis> elements in 5 minutesAdvantages- uniform response- multi-element analysis, rapid- precision & accuracy (0.3 – 3%)- few inter-element interferences- can use with gas, liquid or solids sample
32(high voltages at high frequency) Inductively Coupled Plasma (ICP) Emission Spectroscopy- involves use of high temperature plasma for sample atomization/excitation- higher fraction of atoms exist in the excited state, giving rise to an increasein emission signal and allowing more types of atoms to be detectedIons forced to flow in closedpath, Resistance to flowcauses heatingTemperature Regionsin Plasma TorchMagnetic fieldAr chargesby Tesla coil(high voltages at high frequency)
33Overall Design for ICP Emission Spectrometer Rowland circle:- curvature corresponds to focal curve ofthe concave grating.- frequencies are separated by gratingand focused onto slits/photomultipliertubes positioned around the Rowlandcircle- slits are configures to transmit lines fora specific element
34Arc & Spark Emission Spectroscopy - involves use of electrical discharge to give high temperature environment- higher fraction of atoms exist in the excited state, giving rise to an increasein emission signal and allowing more types of atoms to be detected- can be used for solids, liquids or gas phase samples- types of discharge used:DC arc: high sensitivity, poor precisionDC spark: intermediate sensitivity and precisionAC spark: low sensitivity, high precisionBecause of difficulty in reproducing the arc/spark conditions, all elements of interest are measured simultaneously by use of appropriate detection scheme.Arc created by electrodes separated by a fewmm, with an applied current of 1-30 AConcave grating disperse frequencies,photographic film records spectra
35Comparison of ICP and Arc/Spark Emission Spectroscopy - Arc/Spark first instrument used widely for analysis- all capable of multielement detection with appropriate instrument design (e.g elements in 5 min for ICP- ICP tends to have better precision and stability than spark or arc methods- ICP have lower limits of detection than spark or arc methods- ICP instruments are more expensive than spark or arc instruments
36Example 11: For Na atoms and Mg+ ions, compare the ratios of the number of particles in the 3p excited state to the number in the ground state in a natural gas-air flame (2100K)