Atomic Absorption and Atomic Fluorescence Yongsik Lee

Introduction ► AAS and AFS similarity  Sample introduction  Atomization ► AFS has not gained widespread general use for routine elemental analysis

9A. Sample atomization tech ► Flame atomization ► Electrothermal atomization ► Specialized atomization techniques  Glow discharge atomization  Hydride atomization  Cold vapor atomization

Flame Atomization ► Neubulization  the sample solution is dispersed into tiny droplets. ► Desolvation  the solvent of the solution is evaporated.  The finely divided solid aerosol is mixed with a fuel and an oxidant. ► common fuels: natural gas, hydrogen, acetylene ► common oxidants: air, oxygen, nitrous oxide ► Volatilization  The sample is burned in a flame produced by the fuel and oxidants to form gaseous molecules.  The sample is burned in a flame produced by the fuel and oxidants to form gaseous molecules.  Temperatures ranging from 1700 C to 3150 C are produced depending on the fuel/oxidant combination. ► At such a high temperature, the gaseous molecule (MX) can be  atomized (MX --> M)  and ionized (M --> M+).  and ionized (M --> M+). ► Energy of a particular wavelength will be used to excite the molecule, atom, and ions. Changes in the energy level can be measured for quantitative determination.

Scheme of flame atomization

Types of Flames ► Fuel  Natural gas  Hydrogen  Acetylene ► Oxidant  Air ℃  Oxygen ℃  Nitrous oxide ℃ ► Table 9-1 Properties of Flames  Burning velocity – important for flame stability  Flashback – flame propagate back into the burner  Blowing off the burner at higher flow rates

Flame structure ► Flame profile  Figure 9-2 & 9-3  Primary combustion zone ► Blue luminescence of C2, CH, and other radicals  Internal region ► Rich in free atoms  Secondary combustion zone ► Products of the inner core are converted to stable molecular oxides

Temperature profile ► Figure 9-3 ► Max temperature  Location about 1 cm above the primary combustion zone  Optical focus to this region

Flame absorbance profiles ► Ag (not readily oxidized)  Continuous increase from the flame ► Cr (forms stable oxides)  Continuous decrease  Oxide formation dominant ► Mg  Have a maximum ► Use proper portion of the flame for maximum absorbance

Flame atomizer laminar flow burner

Multiple-slot premixed CH4/air burner

Flame Diagnostics ► Laser-induced fluoresence measurements of OH, CH, CN and H2CO radicals in low and atmospheric pressure flames ► rag.web.psi.ch/htdz/Www_Homepage/ Comb_Diag_Exp.ht

Performance Characteristics ► Reproducible behavior  Best to all other methods for liquid samples ► Sampling efficiency (= Sensitivity)  bad ► Sample flows down the drain ► Residence time in the optical path in the flame is brief (= 1/10000 s)

Electro-thermal Atomization ► An Electro-thermal Atomizer consists a cylindrical graphite tube connected to an electrical power supply. ► An Electro-thermal Atomizer consists a cylindrical graphite tube connected to an electrical power supply. ► A small amount (0.5 to 10mL) of sample is introduced and heated electrically.  Two stage heating: ► At lower temperature: the sample is evaporated and ashed ► Rapid increase in temperature: volatilization and atomization ► Used in ICP, AAS, AFS

Electrothermal atomization ► Advantage  less reproducible results than flame atomization  Atomizie in short time  residence time in the optical path is long(<1sec)  Higher sensitivity

Electrothermal Atomizers ► ► entire sample atomized short time ( °C) ► ► sample spends up to 1 s in analysis volume ► ► superior sensitivity ( g analyte) ► ► less reproducible (5-10 %)   Flame method - 1% or better ► Used when flame or plasma atomization provides inadequate detection limit

ETA

Electrothermal Graphite Furnace ► ► Gas flow   external Ar gas prevents tube destruction   internal Ar gas circulates gaseous analyte ► ► Three step sample preparation for graphite furnace   Dry - evaporation of solvents (10->100 s)   Ash - removal of volatile hydroxides, sulfates, carbonates ( s)   Fire/Atomize - atomization of remaining analyte (1 s)

ETA output signal ► sample  Canned orange juice 2  L  Drying 20 s  Ashing 60 s  Standards lead ► High speed data acquisition possible  Rapid (<1 s) response ► Quantitative analyses  Based on peak height  Peak area is also used

9B Atomic Absorption Instrumentation ► ► AAS should be very selective   each element has different set of energy levels   lines very narrow ► ► BUT for linear calibration curve (Beers' Law) need bandwidth of absorbing species to be broader than that of light source difficult with ordinary monochromator ► ► Solved by using very narrow line radiation sources   minimize Doppler broadening   pressure broadening   lower P and T than atomizer ► ► and using resonant absorption   Na emission 3p  2s at nm used to probe Na in analyte

Radiation Sources ► Hollow cathode lamp  The most common source for AAS  W anode, cylindrical cathode of specific metal, 1-5 torr Ne or Ar ► Electrodeless discharge lamp

Hollow cathode lamps ► ► 300 V applied between anode and metal cathode (-) ► ► Ar ions bombard cathode and sputter cathode atoms   Fraction of sputtered atoms excited, then fluoresce ► ► Cathode made of metal of interest   Na, Ca, K, Fe...   different lamp for each element   restricts multi-element detection ► ► Metal mixture ► ► Hollow cathode   to maximize probability of redeposition on cathode   restricts light direction ► ► High potentials (high currents)   Lead to greater intensities   Self-absorption by unexcited atoms

Electrodeless Discharge Lamps ► Greater Radiation intensity  times than HCL ► Sealed quartz tube + a few torr of Ar + metal (or its salt) ► Light by RF (27 MHz) or microwave radiation  Ionization of Ar to excite the metal ► Less reliable than HCL

Jedis holding EDLs

Spectrophotometers ► Single beam design  Dark current is nulled with a shutter  100 %T adjustment with a blank is aspirated into the flame

Double beam design AAS

Scheme of double beam ► ► Beam usually chopped or modulated at known frequency ► ► Signal then contains constant (background) and dynamic (timevarying) signals

Interferences in AAS ► Signal at one wavelength often contains luminescence from interferents in flame  Spectral interferences  Chemical interferences

Interferences ► Spectral interferences  Spectrum is close (AA lines in 0.1 Å )  Cannot be resolved by the monochromator ► Chemical interferences  By chemical processes during atomization  Alter the absorption characteristics

Chemical interferences ► ► reverses atomization equilibria ► ► reacts with analyte to form low volatility compound   releasing agent - cations that react preferentially with interferent - Sr acts as releasing agent for Ca with phosphate   protecting agent - form stable but volatile compounds with analyte (metal-EDTA formation constants)

Ionization equilibria ► Ionization of atoms and molecules  Can be neglected in air flame  Significant in higher temperatures ► ► hotter atomization means   more ionization   emission from interferents

Degree of ionization at flame

Detection limit ► ► AA/AE comparable (ppb in flame) ► ► AAS less suitable for   weak absorbers (forbidden transitions)   metalloids and non-metals (absorb in UV)   metals with low IP (alkali metals)

Detection limit 1-20 ng/mL

시험 일정 변경 ► 3/2 강의소개 ► 3/30 1 쿼터 중간시험 ► 4/22-23 대한 화학회 학술대회 ( 여의도 ) ► 4/27 1 쿼터 기말시험 ► 5/18 개교기념일 휴강 ► 5/21 → 5/25 2 쿼터 중간시험 (7-10 chapt ► 6/15 → 6/18 2 쿼터 기말 시험

Interferences in AAS ► Signal at one wavelength often contains luminescence from interferents in flame  Spectral interferences  Chemical interferences

Spectral interference ► Overlap of atomic spectral lines  Very rare for atoms  V line at Å with Al line at Å  Use Al line at Å ► Combustion by-products other than analyte and particulate  most significant ► fuel and oxidant interferences  Background correction can correct those ► Sample matrix interferences more challenging  Oxides and hydroxides; may need to change flame chemistry  Organic material species  incomplete combustion products scattering  Most dependent on flame chemistry and temperature  Add excess to standards for “radiation buffer effect”

Correction of spectral interference ► ► Historically, spectral interference in graphite furnace most severe, very specialized corrections applied to minimize problem ► ► Two Line Correction   An additional spectral line from the source,   close in frequency to the analyte wavelength can be employed.   Impurity in HLC, NE or Ar in HLC, or sample (non-resonant emission)   special case & rare ► ► Continuum Source Correction   Signal from a continuous (deuterium lamp) is alternately passed through the analyte zone.   Limited value. ► ► Zeeman Effect Correction   In a strong magnetic field (=10 KG), the magnetic field generated by the spinning electron alters the “energy” or wavelength of transitions.   For Singlet transitions, 3 lines -s, p, +s result. p lines absorb radiation polarized parallel to magnetic field, s perpendicular to field ► ► Smith-Hieftje correction

Zeeman Effect – Magnetic

The magnet of our love…

Zeeman effect correction ► B - Rotating polarizer ► E – absorption at // polarization ► Very sensitive correction technique

Atomic Absorption Techniques US EPA method 7000A ► ► 4.8 Glassware   All glassware, polypropylene, or Teflon containers, including sample bottles, flasks and pipets, should be washed in the following sequence:   detergent, tap water, 1:1 nitric acid, tap water, 1:1 hydrochloric acid, tap water, and reagent water.

US EPA method 7000A ► ► 5.7 Calibration standards   preparation of standards which produce an absorbance of 0.0 to 0.7.   Calibration standards are prepared by diluting the stock metal solutions at the time of analysis.   calibration standards should be prepared fresh each time a batch of samples is analyzed.   Prepare a blank and at least three calibration standards in graduated amounts in the appropriate range of the linear part of the curve. The calibration standards should be prepared using the same type of acid or combination of acids and at the same concentration as will result in the samples following processing.   Calibration curves are always required.

US EPA method 7000A ► ► 7.2 Direct aspiration (flame) procedure: ► ► In general, after choosing the proper lamp for the analysis, allow the lamp to warm up for a minimum of 15 minutes ► ► Align the instrument, position the monochromator at the correct wavelength, ► ► select the proper monochromator slit width, and adjust the current according to the manufacturer's recommendation. Subsequently, ► ► Light the flame and regulate the flow of fuel and oxidant. Adjust the burner and nebulizer flow rate for maximum percent absorption and stability. ► ► Run a series of standards of the element under analysis. ► ► Construct a calibration curve by plotting the concentrations of the standards against absorbances. ► ► Aspirate the samples and determine the concentrations either directly or from the calibration curve. ► ► Standards must be run each time a sample or series of samples is run.

US EPA method 7000A ► ► 8. QUALITY CONTROL ► ► 8.2 A calibration curve must be prepared   each day with a minimum of a calibration blank and three standards.   After calibration, the calibration curve must be verified by use of at least a calibration blank and a calibration check standard (made from a reference material or other independent standard material) at or near the mid- range.   The calibration reference standard must be measured within 10 % of it's true value for the curve to be valid.

US EPA method 7000A ► ► 8. QUALITY CONTROL ► ► 8.3 If more than 10 samples per day are analyzed,   the working standard curve must be verified by measuring satisfactorily a mid-range standard or reference standard after every 10 samples.   This sample value must be within 20% of the true value, or the previous ten samples reanalyzed.

► ► Dilution test   For each analytical batch select one typical sample for serial dilution to determine whether interferences are present.   The concentration of the analyte should be at least 25 times the estimated detection limit.   Determine the apparent concentration in the undiluted sample.   Dilute the sample by a minimum of five fold and reanalyze. ► ► Test results   Agreement within 10% between the concentration for the undiluted sample and five times the concentration for the diluted sample indicates the absence of interferences, and such samples may be analyzed without using the method of standard additions.

EPA Test Methods ► Q: What are test methods? A: Test methods are approved procedures to measure  the presence and concentration of physical and chemical pollutants;  evaluating properties, such as toxic properties, of chemical substances;  or measuring the effects of substances under various conditions. ► Q: Why an index? A: This Index was developed to improve access to US EPA test methods. It is not an official EPA publication nor does inclusion or exclusion of methods indicate EPA approval or disapproval of any method.

What information is included? ► Method Number  the official method number or a compiler assigned number if un-numbered.  Examples: or 3810 or TO-15 or 8080A* ( * indicates not available) or SAMPLIN. ► Chemical or Method Description  chemical, analyte, group of chemicals or name of protocol.  If you don't find the one you want, try a broader term such as metals for mercury or pesticides for DDT. Examples: asbestos or absorption or Maneb or larval survival or mercury. ► Reference Source of where to get the method by one of four categories:  EPA Report # - EPA report number. Examples: 600/ or SW-846 Ch 3.3.  40 CFR Part - Title 40 of the Code of Federal Regulations part numbers. Examples: 136 App A = 40 CFR 136 Appendix A.  Region 1 # - EPA Region 1 Library local call number. Examples: 01A a or 01A  Electronic Version - abbreviated reference to an electronic version if available. The full web address is provided in the Sources of EPA Test Methods list. Examples: www or ttn/emc or NEMI; the full web address provided in the Source List; CD indicates included in the Water Methods CD ROM (EPA 821/C ). ► Date Issued - date method was published. Examples: 09/25/1996 or /// (date unknown).

Homework ► 9-12, 9-20, 9-21