CH. 21 Atomic Spectroscopy

Transcription

1 CH. 21 Atomic Spectroscopy 21.1 Anthropology Puzzle? What did ancient people eat for a living? Laser Ablation-plasma ionization-mass spectrometry CH. 21 Atomic Spectroscopy 21.2 plasma In Atomic Spectroscopy Samples 2000~8000K emission vaporized or absorption of characteristic λ Concentration range: μg/g pg/g ppm ppb

2 21-1. An Overview 21.3 Consider Atomic flame methods Three types Flame Temp: 2000~3000K Sample aspirated to flame. liq. evaporated & remaining solid becomes ATOMIZED An Overview 21.4 Atomic Absorption 10cm Hollow Cathode Lamp : cathode made with the same element to be analyzed Bandwidths of gaseous atom (~0.001 nm vs. ~100nm for liq. solid) So sharp, little overlap between diff. atoms

3 21-1. An Overview 21.5 Atomic Fluorescence : laser irradiate atoms from ground state to excited state atom emits radiation: fluorescence more sensitive than AA Atomic Emission : under very hot flames like plasma, atoms are excited directly then excited atoms emit photons no lamp needed Atomization: Flames, Furnaces & Plasmas 21.6 Flame type, shifted to Inductively coupled plasma or graphite furnace 1. Flames premix burner (fuel + oxidant + sample) mixed pneumatic nebulizer -- aerosol Low efficiency: only about 5% reach flame

4 21-2. Atomization: Flames, Furnaces & Plasmas 21.7 Fuel type When droplet enters flame, solid vaporizes and decomposes into atoms Molecules emit radiation Atoms Under fuel rich condition, excess carbon reduces metal oxides, hydrides -- affect sensitivity Atomization: Flames, Furnaces & Plasmas Graphite furnace : eletrictrically heated, more sensitive than a flame 1~100 μl sample used : to prevent oxidation of graphite : Ar passed : max Temp 2550 o C less than 7s : residence time of analyte < 1s : precision 5-10% manual, ~1% automation Heating step (i.e. Fe in ferritin) ~0.1ppm Fe containing sample (10μL) injection: ~ 90 o C drying : 125 o C for 20 min solvent evap charring : 1400 o C for 60s - destroy organic atomization: 2100 o C for 10s absorbance meas. cleaning : 2500 o C for 10s purging : Ar or N 2 at each step except during atomization

5 21-2. Atomization: Flames, Furnaces & Plasmas Matrix modifiers for furnace Matrix modifiers : substance to help matrix more volitile & analyte less volitle during pyrolysis some sample lost during charring NH 4 NO 3 is added to seawater to make NaCl volatile Reduction of Interference by matrix modifier a. Mn in seawater b. absorbance : ~10μL 0.5M NaCl from optical scatter by smoke created by heating NaCl c. NH 4 NO 3 addition NH 4 Cl + NaNO 3 evaporated Atomization: Flames, Furnaces & Plasmas Inductively Coupled Plasma (ICP) Adv.: much hotter than flame eliminate much interferences due to high T, stable, inert Ar simultaneous multi-element analysis ICP-AES: Replace AAS

6 21-2. Atomization: Flames, Furnaces & Plasmas ICP torch spark from Tesla coil ionize Ar e- are accelerated 6,000~10,000K Ultrasonic Nebulizer : better atomization. Vibrating piezoelectric crystal Fine aerosol passing through condenser : no energy loss of plasma for solvent sensitivity enhanced (3-10 times) Ultrasonic nebulizer Atomization: Flames, Furnaces & Plasmas 21.12

7 21-3. How Temp. Affects Atomic Spectroscopy Temp. Influences : atomization : proportion of atoms in ground st, excited st., ionized state 1. The Boltzamann Distribution : relative population of different states at thermal equilibrium degeneracy N * N o = g * Exp( ΔE / kt) g o K=1.38x10-23 J/K) How Temp. Affects Atomic Spectroscopy Effect of Temperature on Excited-State Population At 2,600K flame fraction of Na in ex. State, degeneracy g*/g o =2 lowest excited state of Na line = 3.371x10-19 J/atom N * No = Exp( x10 1 J/( x10 J/K 2600K) = 1.67x % excited state, 99.98% ground state N * At 2610K 4 = No 4% increase (no big change) So, flame for emission important for Emission Spectroscopy ICP is normally used

8 Atomic Linewidths Linewidth of source < Linewidth of Absorbing sample 10-4 nm Based on Heisenberg Uncertainty Principle, The shorter the lifetime of excited st. More uncertain its energy h δe δt 4π δe: uncertainty in energy diff. between gr.st. & ex.st. δt: lifetime of excited state. If δt, δe. uncertainty δt ~ 10-9 s, h 6. 6x10 δe = 4πΔt 4π( J s 10 J If λ = 500 nm, ΔE = hc/λ = 4.0x10-19 J -25 δe 10 J ΔE δλ λ 19 δ = 2 10 ΔE 2 10 J 7 E 7 4 δλ = 10 ) nm Inherent linewidth broadening linewidths of AA or AE signals : two mechanisms to broaden the line width to 10-3 ~10-2 nm i) Doppler effect Atom moving toward retardation source sees radiation more frequently. thus, it sees higher ν light than that moving away Doppler linewidth: 7 T δλ = λ( 7 10 ) M:mass M : Emission near λ=300nm from Fe (56 amu) at 2500K δλ = 300nm( 7 10 ) = nm 56 pressure broadening By collision between atoms, collision shorten the lifetime of excited state δt decrease δe increase δλ increase

9 Hollow-Cathode Lamp (HCL) If monochromator used, could it generate lines narrower than 10-3 to 10-2 nm? For narrow lines, HCL provides sharp line. HCL contains same element (cathode) to be analyzed. filled with Ne or Ar (1~5 torr) : When 500V, gas ionized first : Ar + is accelerated to cathode, and it strikes cathode to sputter metal atoms. Hight E. e collide with gaseous target element atomic radiation same λ (or ν) with the λ for sample absorption : monochromatic. Each HCL for each sample to analyze is required Multielement Detection with ICP ~ 70 elements simultaneously, atomic emission dispersed at polychromator (by prism one dimension, by grating at another dimension) 2D signal monitored by charge injection device (CID) or by charge coupled device (CCD) 262,000 pixels

10 Background Correction Signals mixed with analyte signal from absorption emission, optical scattering of matrix Graphite furnace AA bronze in HNO 3 background signal must be removed. signal mixed with background A: ~0.3 Ex) Emission spectrum with CID detector Pixel 1-2: background 7-8: peak 14-15: background Ex) AA in HCL beam chopping : lamp + flame flame only it corrects flame emisssion not for scattering

11 21.21 electrical modulation 1) D 2 lamp correction one from D 2 Absorption by scattering from background the other from HCL Absorption from analyte + scattering (abs. of D 2 light by sample : negligible) 2) Zeeman effect: G.F. By magnetic field parallel to light path through furnace Absorption light splits into 3 comp. : 2 shifted to longer and shorter λ : 1 unshifted (no electromagnetic polar.) When B is on, background alone B is off, sample + background Detection Limits : concentration of an element that gives a signal ~ 2 x noise level S/N > 2 For G.F., 2 orders of magnitude lower D.L.: Furnace < ICP < flame

12 Interference Interference: Any effect that changes signal while analyte concentration remains unchanged. How to remove? - remove source of interference - prepare standard exhibiting the same interference 1. Types of Interference i) Spectral interference when signal overlaps with other element signal or flame signal, choose another line, or high resolution oxide formation by heating under H 2 prior to atomization

13 21-5. Interference ii) Chemical interference : SO 4 2-, PO 4 3- hinder atomization of Ca 2+ salts use releasing agents : EDTA, 8-hydrozyquinoline protect Ca 2+ from SO 4 2-, PO 4 3- Use masking : La 3+ reacts with PO 4 3- Use fuel rich flame for reducing oxidation iii) Ionization interference In case of Alkali metals at low temp. M(g) M + (g) + e - (g) Add more electrons, rxn moves Insert easily ionizable atoms like K: ionization suppressor For K, 1000 ppm of CsCl electrons from Cs + suppress ionization of K Interference A general way of bypassing interferences : method of standard addition - by adding known quantities of analyte to the unknown in its complex matrix Difference of slopes: units/ppm vs units/ppm In distilled water, absorbance increases 1.64 times more than in aquarium water due to less interferences

14 21-5. Interference Virtues of ICP Flame emission : self-absorption concentration non-linear calibration ICP features hotter, longer residence time more complete atomization enhanced signal less formation of oxides & hydroxides remarkably free of background linear range : ~ 8 orders of magnitude Interference 21.28

15 21-6. ICP-Mass spectrometry More power in detection -- coupled with Mass spectrometry ICP-MS of coffee beans ~ 15 ng Pb/mL ICP-Mass spectrometry Need a good resolution to distinguish isobaric matter 40 Ar 16 O Fe + by checking multiple isotopes, 40 Ar Se + can be eliminated 138 Ba Ga + More on MS with the next chapter