Air, Water and Land Pollution

Air, Water and Land Pollution
Chapter 9: Atomic Spectroscopy for Metal Analysis Copyright © 2010 by DBS

Contents Introduction to the Principles of Atomic Spectroscopy
Instruments for Atomic Spectroscopy Selection of the Proper Atomic Spectroscopic Technique Practical Tips to Sampling, Sample Preparation, and Metal Analysis

Atomic Spectroscopy for Metal Analysis Introduction to the Principles of Atomic Spectroscopy
Molecular (UV-VIS and IR) spectroscopy: deals with inorganic or organic molecules in solution Atomic spectroscopy: mainly deals with high energy absorption/emission of individual atoms

In molecular spectroscopy, low-energy radiation (IR, VIS, UV) causes molecules to vibrate/rotate or outer electrons to transit from low to high energy states In atomic spectroscopy, higher energy radiation is used to transit inner electrons from low to high energy states High energy radiation is provided by: (a) flame in flame atomic absorption spectroscopy (FAA) (b) electrothermal furnace in flameless graphite furnace atomic absorption spectroscopy (GFAA) (c) plasma in inductively coupled plasma-optical emission spectroscopy (ICP-OES) (d) X-rays in X-ray fluorescence spectroscopy (XRF)

3 major types of atomic spectroscopy: Absorption – light of a wavelength characteristic of the element of interest radiates through the atom vapor. The atoms absorb some of the light. The amount absorbed is measured. Emission – sample is heated to excitation/ionization of the sample atoms. Excited and ionized atoms decay to a lower energy state through emission. Intensity of the light emitted is measured. Fluorescence – a short wavelength is absorbed by the sample atoms, a longer wavelength (lower energy) radiation characteristic of the element is emitted and measured

Liquid sample is aspirated to become aerosols of fine particles (nebulization) Flame vaporizes the aerosols (atomization) Elevated temperatures in a flame or furnace changes the chemistry of atoms Temperature affects the ratio of excited and unexcited atoms Beer’s law is used to calculate concentration

Flame and Flameless Atomic Absorption Processes Occurring in Flame and Flameless Furnace Solution is introduced into a high-temperature flame or furnace, molecules containing the elemental atoms become gaseous atoms through a series of reactions Flame and flameless furnaces are two common radiation sources used in atomic spectroscopy

Flame and Flameless Atomic Absorption Processes Occurring in Flame and Flameless Furnace e.g. Calcium present as a salt (CaCl2): 1. removal of water produces gaseous CaCl2 2. gaseous CaCl2 is further dissociated into gaseous Ca0 atoms At elevated temperatures Ca can have other electronic states: 3/4. Ca0* (excited Ca atom), 5. Oxide/Hydroxide formation 6. Ca+ (ionic Ca), 7. Ca+* (ionic Ca with excited e-)

Flame and Flameless Atomic Absorption Processes Occurring in Flame and Flameless Furnace Ca analyzed by atomic absorption spectroscopy the radiation absorption of gaseous atom (Ca0) is measured (Fig. 9.1 reaction 3) Ca can also be analyzed by atomic emission spectroscopy (Fig. 9.1 reaction 4)

Flame and Flameless Atomic Absorption Processes Occurring in Flame and Flameless Furnace If M is used to denote the vapor form of any atom (metals), M + hν → M* (for FAAS) M * → M + hν (for FAES) Note: Formation of metal oxide/hydroxide (5) and ionization of gaseous atom (6) are common interferences that must be miimized

Flame and Flameless Atomic Absorption Processes Occurring in Flame and Flameless Furnace Flame used in abs/emission spectroscopy is around K e.g. air-acetylene 2250 ºC, nitrous oxide-acetylene 2955 ºC Flameless graphite furnace – electrically heated graphite boat

Flame and Flameless Atomic Absorption The Boltzmann Equation: Understanding the Temperature Effect and the Sensitivity Difference Between Absorption and Emission Spectrometry Temperature effects ratio between excited and unexcited atoms (ground state) Where N* and N are the number of atoms in an excited state and the ground state, k = Boltzmann constant (1.38 x J/K), T = Temperature (K), E* - E is the energy difference between excited and ground states, P* and P are the statistical weights of the excited and ground states respectively (determined by the no. states having equal energy at each quantum level n) e.g. in Hydrogen there are P = 2 ways that an atom can exist at the n = 1 energy level (1s2), and P = 8 ways an atom can arrange itself at n = 2 (2s2, 2p6)

Flame and Flameless Atomic Absorption

Flame and Flameless Atomic Absorption Understanding “Nebulization” and “Atomization” Process: Why Higher Sensitivity is Achieved in Flameless GFAA Than Flame FAA Nebulization In FAAS a liquid sample is nebulized – aspirated into small liquid particles (aerosols), remaining larger droplets condense out (only around 10 % of fine aerosols reach the burner) Atomization = conversion of element into atomic vapor In FAAS nebulization takes place prior to atomization making the process far less efficient than GFAAS In GFAAS the entire sample is atomized inside the graphite boat leading to lower detection limits

Graphite furnace AAS Sample injection into graphite tube Drying Decomposition Atomization Absorbance is measured during atomization Advantages of FAAS Advantages of GFAAS Simple technique Increased sensitivity (μg L-1) Solvent extraction removes interferences Not needed Readily available equipment Smaller samples Shorter instrument time Unattended operation possible Lower instrument cost Reduced contamination

Flame and Flameless Atomic Absorption Quantitation and Qualification of Atomic Spectroscopy Concentration of an element present in the sample is described by Beer’s law Absorption depends on the number of ground state atoms in the optical path A = klC Where A = absorbance, C = concentration, l = path length of the flame, k = coefficient unique to each element For emission spectroscopy, the emitted light intensity I of a population of n excited atoms depends on the number of atoms dn that return to the ground state during an interval time dt (dn/dt = kn). As n is proportional to C the concentration of the element, the emitted light intensity I, is also proportional to concentration: I = klC

Inductively Coupled Plasma (ICP) Atomic Emission Same process occur as with flame atomic emission (Fig. 9.1) ICP (plasma) is an ionized gas at extremely high temperature Ar → Ar+ + e- The energy in the plasma is transferred by collision of Ar+ with the atoms of interest Enough to ionize many metals with I.E. 7-8 eV Most metals are ionizable emitting in the UV range, non- metals do not form ions in the ICP (require a vacuum)

Inductively Coupled Plasma (ICP) Atomic Emission ICP can theoretically analyze almost all elements FAAS and FES can only measure around 70 elements ICP can also measure multiple elements simultaneously, whereas flame techniques can only measure one at a time Note: ICP-AES (Atomic Emission Spectroscopy) is used interchangeably with ICP-OES (Optical Emission Spectroscopy)

Atomic X-Ray Fluorescence X-ray fluorescence is a two-step process 1. Excitation of inner electrons via X-rays 2. “jump ins” of the electrons from higher energy levels to fill vacancies Atom is stabilized – emits characteristic X-ray fluorescence unique to the element XRF instrument measures the photon energy from the fluorescence to identify the element and the intensity of the photon to measure the amount of element in the sample

Atomic Spectroscopy for Metal Analysis Instruments for Atomic Spectroscopy
Flame and Flameless Atomic Absorption Basic instrument components:

Flame and Flameless Atomic Absorption Basic instrument components: Light source: hollow cathode lamp (HCL) of the element being measured. Provides the spectral line for the element of interest. Inside the lamp, filled with argon or neon gas, is a cylindrical metal cathode containing the metal for excitation, and an anode. When a high voltage is applied across the anode and cathode, gas particles are ionized. As voltage is increased, gaseous ions acquire enough energy to eject metal atoms from the cathode. Some of these atoms are in an excited states and emit light with the frequency characteristic to the metal.

Atomic Spectroscopy Instruments for Atomic Spectroscopy
Flame and Flameless Atomic Absorption Basic instrument components: Nebulizer and atomizer: In a flame system (a), the nebulizer sucks up the liquid sample, creates a fine aerosol, mixes the aerosol with fuel/air. Flame creates vaporized atoms. In a flameless graphite furnace system (b) both liquid and solid samples are deposited into a graphite boat using a syringe inserted through a cavity. Graphite furnace can hold an atomized sample in the optical path for several seconds, compared with a fraction of a second for a flame system – results in higher sensitivity of the GFAA compared to FAA

Flame and Flameless Atomic Absorption Basic instrument components: Monochromator: Isolates photons of various wavelengths that pass through the flame or furnace. Similar to the monochromator in UV-VIS spectroscopy in that it uses slits, lenses, mirrors and gratings/prisms. 3. Detector: The PMT detector determines the intensity of photons in the analytical line exiting the monochromator

Flame and Flameless Atomic Absorption Basic instrument components: 3. Detector: The PMT detector determines the intensity of photons in the analytical line exiting the monochromator. Before an analyte is atomized, a measured signal is generated by the PMT as light from the HCL passes through the flame/furnace. When analyte atoms are present – some part of that light is absorbed by those atoms. This causes a decrease in PMT signal that is proportional to the amount of analyte.

Cold Vapor and Hydride generation Atomic Absorption FAAS and GFAAS can measure most elements Cannot measure: mercury, selenium and arsenic All too volatile to be measured by flame or furnace techniques

Cold Vapor and Hydride generation Atomic Absorption Cold Vapor Atomic Absorption (CVAA) Spectroscopy for Hg Free mercury atoms exist at room temperature, no requirement for heating Sample may contain Hg0, Hg22+ or Hg2+ In CVAA Hg is chemically reduced to the atomic state by reaction with a strong reducing agent (e.g. SnCl2 or NaBH4) in a reaction flask Hg is then carried via gas stream to the absorption cell

Cold Vapor and Hydride generation Atomic Absorption Hydride Generation Atomic Absorption (HGAA) Spectroscopy for As and Se AsH3 and SeH3 generated by reaction samples containing As and Se with NaBH4 Uses same setup as FAAS except it switches nebulizer for the hydride generation module

Cold Vapor and Hydride generation Atomic Absorption Hydride Generation Atomic Absorption (HGAA) Spectroscopy for As and Se Sample is reacted in the external hydride generator with reducing agent (NaBH4) Hydride generated is then carried via inert gas to the sample cell in the light path of the FAAS Unlike CVAA product is not free atoms but AsH3 / SeH3 which are not measurable Sample cell must be heated to dissociate the hydride into free atoms (As0) and Se0) Higher sampling efficiency leads to lower detection limits: ppb vs ppm for regular FAAS and GFAAS

Inductively Coupled Plasma Atomic Emission (ICP-OES)

Inductively Coupled Plasma Atomic Emission (ICP-OES) Sample is nebulized and entrained in the flow of plasma support gas (Ar) Source:

Inductively Coupled Plasma Atomic Emission (ICP-OES) Plasma torch inner tube contains the sample aerosol and Ar support gas Radio frequency generator produces a magnetic field which sets up an oscillating current in the ions and electrons of the support gas (Ar) Produces high temperatures (up to 10,000 K) Source:

Inductively Coupled Plasma Atomic Emission (ICP-OES) Atomizes the sample and promotes atomic and ionic transitions which are observable at UV and visible wavelengths Excited atoms and ions emit their characteristic radiation, which are collected by a device that sorts the radiation by wavelength Intensity of the emission is detected and turned into a signal that is output as concentration

Atomic X-Ray Fluorescence Three main types: wavelength dispersive energy dispersive Consists of a polychromatic source (X-ray tube or radioactive material), sample holder, photon detector (Si-semiconductor) Source: wikipedia

Atomic X-Ray Fluorescence Wavelength dispersive XRF (WDX) – fluorescence radiation is separated according to wavelength by diffraction on an analyzer crystal before being detected, can detect multiple elements at the same time Energy dispersive XRF (EDX) – energy of a photon of a specific wavelength is detected Source: wikipedia

Atomic X-Ray Fluorescence Energy dispersive XRF – consists of a polychromatic source (X-ray tube or radioactive material), sample holder, detector Smaller and cheaper than wavelength dispersive XRF Ideal for field investigations No moving parts

Atomic X-Ray Fluorescence XRF is unique among all atomic spectroscopic techniques in that it is non-destructive Good for elemental composition analysis For quantitative analysis require reference standard with similar matrix to that of the sample Detection limits are in the ppm (mg/kg) range

Atomic Spectroscopy for Metal Analysis Selection of the Proper Atomic Spectroscopic Techniques
Important factors: Detection limit Working range Sample throughput Cost Interferences Ease of use Availability of proven methodology

Ions Found in Natural Waters
Conc. Range (mg L-1) Cations Anions 0-100 Ca2+, Na+ Cl-, SO42-, HCO3- 0-25 Mg2+, K+ NO3- 0-1 Fe2+, Mn2+, Zn2+ PO43- 0-0.1 Other metal ions NO2- Artiola, J.F., Pepper, I.L., and Brusseau, M. (2004) Environmental Monitoring and Characterization. Elsevier, Amsterdam. Reeve, 2002

FAAS > ICP-OES > HGAAS > GFAAS > ICP-MS
Atomic Spectroscopy for Metal Analysis Selection of the Proper Atomic Spectroscopic Techniques Comparison of Detection Limits and Working Range Low detection limit is essential for trace analysis Without low level capability – sample pre-concentration is required FAAS > ICP-OES > HGAAS > GFAAS > ICP-MS

Comparison of Detection Limits and Working Range

Comparison of Detection Limits and Working Range Analytical range is the concentration range over which quantitative results can be obtained without the need for recalibration Ideal working range minimizes analytical effort (e.g. dilutions, pre-concentration)

Comparison of Interferences and Other Considerations Interference Three types: (i) spectral, (ii) chemical, (iii) physical

Comparison of Interferences and Other Considerations Interference Spectral: in spectroscopy, interference occurs when another emission line (e.g. from other elements in the sample) is close to the emitted line of the test element and is not resolved by the monochromator Chemical: formation of undesired species during atomization Physical: variation of instrument parameters such as uptake in the burner and atomization efficiency (gas flow rate, sample viscosity etc.)

Comparison of Interferences and Other Considerations Other Considerations Sample throughput, cost, ease of use, availability of proven methodology Single-element (FAAS and GFAA) vs. multi-element (ICP-OES/MS) Single: Change of lamp Run time ~1 min Multi: 10-40 elements per minute

FAAS < GFAAS > ICP-OES << ICP-MS
Atomic Spectroscopy for Metal Analysis Selection of the Proper Atomic Spectroscopic Techniques Comparison of Interferences and Other Considerations Other Considerations Cost: FAAS < GFAAS > ICP-OES << ICP-MS

Comparison of Interferences and Other Considerations Other Considerations ICP-OES and ICP-MS are multi-element techniques favored when there is a large number of samples and cost is not a concern

Comparison of Interferences and Other Considerations Other Considerations ICP-OES has become the dominant instrument for routine analysis of metals Compared to FAAS: Lower interferences (due to higher temperatures) Spectra for most elements can be recorded simultaneously under the same conditions Higher temperature allows compounds (e.g. metal oxides) to be measured Determination of non metals (e.g. Cl, Br, I, S) Wider linear working range Cons: cost, carrier gas consumption (runs overnight), slightly more complicated to run, limited use for group one metals (Li, Na, K, etc.) (emission lines are near IR)

Atomic Spectroscopy for Metal Analysis Practical Tips to Sampling
Correction for Sample Moisture and Dilution For solid samples moisture content (w), dry weight of sample (m), and volume of digestate (V) is required in order to calculate concentration of element:

Instrumental Drift and Run Sequence QA/QC Instrument drift is a common problem Inexperienced analysts are often frustrated by broad variation of results for the same sample tested in different batches Very unlikely to get the exact same results for the same sample Follow QA/QC protocols and report margin of error (SD) Usual for operator to run the instrument blank and standards several times between samples

Instrumental Drift and Run Sequence QA/QC

Erroneous Data and Methods of Compensation Erroneous results arise due to one or more of the sources of interference described above Compensation methods: background correction, higher temperatures, release agent, alternative wavelength, internal standard, matrix spike, etc. e.g. Chemical interferences Refractory salts e.g. PO43-, SO42- and silicate ion e.g. Ca2+ forms refractory insoluble Ca3(PO4)2 Add release agent (10% lanthanum solution or EDTA) Complex solutions (matrix) require method of standard additions Add small volumes higher concentration standards (change in volume is negligable) Graph of concentration vs. absorbance Concentration of sample is x-intercept Overcomes problem of matrix effects

Erroneous Data and Methods of Compensation Simple solutions (e.g. water) use standard curve technique to find unknown concentration Complex solutions (matrix) require method of standard additions Add small volumes higher concentration standards (change in volume is considered negligible) Graph of concentration vs. absorbance Concentration of sample is x-intercept Overcomes problem of matrix effects

Question A series of solutions is made up by adding 0.1, 0.2, 0.3, 0.4 and 0.5 mL of a 10 mg L-1 lead standard to 100 mL aliquots of the unknown solution. The following results were obtained: Volume std. (mL) Abs Plot a calibration graph and determine the concentration of the unknown Assuming constant volume of 100 mL, the concentration increase in the 5 solutions are 10, 20, 30, 40, and 50 μg L-1. (e.g. 0.5 mL of 10 mg/L = 5 x 10-3 mg in 100 mL = 5 μg/0.1 L = 50 μg L-1) Absorbance = ( x conc) Unknown = 21.8 g μL-1 lead

Quantification

References Csuros, M. and Csuros, C. (2002) Environmental Sampling and Analysis for Metals. CRC press, Boca Raton, Fl. Tatro, M.E. (2000) Optical Emission Inductively Coupled Plasma in Environmental Analysis. Encyclopedia of Analytical Chemistry, Edited by Meyers, R.A. John Wiley & Sons, West Sussex, UK.

Questions 1. Explain: (a) The difference in the electronic configuration among various species of calcium, that is, Ca in CaCl2, Ca0, Ca0*, and Ca2+; (b) of these species, which one(s) are desired for AAS measurement of Ca and which one(s) are unwanted species that may cause interference for AES measurement of Ca? 2. For the following atomic absorption spectrometers – FAA and ICP-OES: (a) Sketch the schematic diagram; (b) Describe the principles (functions) of major components. 15. Explain why NH4NO3 is added to seawater when Pb and Ca are analyzed by FGAA. (hint: removes interference due to high salinity – show chemistry) 23. A groundwater sample is analyzed for its K by FAA using the method of standard additions. Two 500 µL aliquots of this groundwater sample are added to 10.0 mL DI water. To one portion, 10.0 µL of 10 mM KCl is added. The net emission signals in arbitrary units are 20.2 and What is the concentration of K in this groundwater in mg/L? (hint: use example 9.2) 24. A 5-point calibration curve was made for the determination of Pb via FAAS. The regression equation was: y = 0.155x , where y is the signal output as absorbance, and x is the Pb concentration in mg/L. (a) A contaminated groundwater sample was collected, diluted from 10 to 50 mL, and analyzed without digestion. The absorbance reading was for the sample. Calculate the concentration of Pb in this groundwater sample.

Questions 23. A groundwater sample is analyzed for its K by FAA using the method of standard additions. Two 500 µL aliquots of this groundwater sample are added to 10.0 mL DI water. To one portion, 10.0 µL of 10 mM KCl is added. The net emission signals in arbitrary units are 20.2 and What is the concentration of K in this groundwater in mg/L? (hint: use example 9.2) Assume x millimols (mmols) of K in 500 µL sample Total mmols K in spiked sample = x + 10 mmol/L x 10µL x 1 L / 106 µL = x + 1 x 10-4 mmols Using ratio technique: x/20.2 = (x + 1 x 10-4 mmols) / 75.1 Solve for x, x = 3.89 x 10-5 mmols in 500 µL aliquot x = 3.89 x 10-5 mmols / 500 x 10-6 L x 1 mol/1000 mmols = 7.78 x 10-5 mols/L 7.78 x 10-5 mols/L x g/mol x 1000 mg/g = 3.04 mg/L = 3.04 ppm

Air, Water and Land Pollution

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