insmeth lecture 6 - aas

ATOMIC ABSORPTION SPECTROSCO

PY

2

Atomic Spectroscopy

• Types:1. Atomic Absorption Spectrometry (AAS)2. Atomic Fluorescence Spectrometry (AFS)3. Atomic Emission Spectrometry (AES)4. Atomic Mass Spectrometry (many types)*

Inductively coupled plasma mass spectrometry (ICP-MS) Thermal Ionization Mass Spectrometry (TIMS) Secondary Ion Mass Spectrometry (SIMS) Laser Microprobe Mass Spectrometry (LMMS)

5. Atomic X-Ray Spectrometry* Absorption, fluorescence, and emission forms

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Characteristics of Atomic Spectroscopy

• Involves EMR in the UV and visible regions of the EMR spectrum

• Based on excitation of a (valence) electron to an excited state

• Involves atoms or ions in the gaseous state

• Involves a process called “atomization”

• Capable of analyzing ~ 70 elements

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Atomization

• A process by which molecular constituents (analyte) of a sample are simultaneously decomposed and converted to atoms (and ions) in the gaseous state

• Atomic spectroscopy methods are categorized based on the type of atomization

• The atomization process is the “signal generator”

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Atomization

From Skoog et al. (2004); Table 28-1, p.840

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The Atomization Process

[M+,X-]aq [M+,X-]aq

nebulization

solution mist

[MX]solid

vaporizationdesolvation

[X0]gas[M0]gas

[MX]gas

atom

izat

ion

[M+]gas [X+]gas

ato

miz

atio

n

[M*]gas[M0]gas

emission

excitation or

absorption

(via heat or light)ground state excited state

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Atomization and Excitation

• The excitation step involves promotion of an electron to a higher energy state

• Excitation can occur several ways, including– Flame– Electrical discharge– Inductively coupled plasma– Absorption of EMR

• Involves outer bonding electrons• Several transitions are possible• Excitation involves discrete spectral lines

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Atomization and Excitation

Atomic Emission Spectroscopy• The heat from a flame or an

electrical discharge promotes an electron to a higher energy level

• As the electron falls back to ground state, it emits a wavelength characteristic of the excited atom or ion

From Skoog et al. (2004); Figure 28-1, p.840

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Atomic Line Spectra

Each spectral line is characteristic of an individual energy transition

E = h

nc

h

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Atomizers

• Atomization occurs in an electrically heated graphite tube

• The graphite tube is flushed with an inert gas (Ar) to prevent the formation of (non-absorbing) metal oxides

Electrothermal or Graphite Furnace AtomizerElectrothermal or Graphite Furnace Atomizer

graphite tube

FORMATION OF ATOMIC VAPOR

Four methods used to vaporize sample from solution: • Ovens: Sample placed in an oven; after evaporating

solvent, sample vaporized into irradiation area by rapidly increasing temperature.

• Electric arc or spark: Sample subjected to high current or high potential A.C. spark.

• Ion bombardment: Sample placed on cathode and bombarded with + ions (Ar+). Sputtering process dislodges them from cathode and directs them to irradiation region.

• Flame atomization: Sample sprayed into flame where it undergoes atomization and irradiation.

FLAME ATOMIZERS

• Total consumption burner: Separate channels bring sample, fuel, and oxidant to combustion area. All of the sample, that is carried into the burner, is burned;

• Sensitivity is greater than in a burner where the sample is not completely burned.

• extra turbulence in the flame from variations in droplet size increase noise. Undergraduate Instrumental Analysis,

Robinson, p. 267.

Premix (laminar flow) burner• Sample, fuel, and oxidant mixed prior to entering flame.• Solution drawn into pneumatic nebulizer.• Liquid breaks into a fine mist then directed against a

glass bead.• Nebulization (formation of small droplets) occurs.• Mixing baffles insure only fine mist makes it through to

burner. • Excess liquid collects at spray chamber.

Primary reaction zone = blue cone

Complete combustion = outer cone

*Many elements form oxides/hydroxides in the outer cone

RICH FLAME – rich in fuel; more sensitive

LEAN FLAME – with excess oxidant; hotter

• Graphite Furnace more sensitive than flame

• Requires less sample• Graphite does not

oxidize due to Ar(g)• Confines atomized

sample in the optical path for several sec

FURNACE

INDUCTIVELY COUPLED PLASMA (ICP)

• Twice as hot as flame

• Lower interferences– High temperature– High stability– Inert environment

• Simultaneous multielement analysis

• More expensive than FAAS

ICP BURNER

• High purity Ar fed through plasma gas inlet

• Ar ionizes and free electrons accelerate through RF field

• Temp range: 6000-10000 K

Modification in ICP

• Ultrasonic nebulizer – lowers LOD for most elements

• Piezoelectric crystal (quartz) – a material whose dimensions change in an electric field

• Mist is created when the sample is sprayed against the vibrating crystal

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Radiation Sources for AAS

Hollow Cathode Lamps (HCL)• Lamps receive an applied

potential of ~ 300 V DC• Filled with an inert gas (Ne or

Ar) at low vacuum (1-5 torr)• Exit window composed of

pyrex or quartz; depending on wavelength produced

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How a HCL Works• An applied potential of ~ 300 V DC ionizes the inert gas

Ne (g) -----> Ne+* (g) + e-

• The ionized gas generates a current flow in the lamp• Metal cations (e.g. Fe, Mn, Ca) on the cathode acquire

(kinetic) energy from the ionized gas and dislodge into the vacuum

• A “cation cloud” forms around the cathode (a process called sputtering)

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How a HCL Works• Some of the sputtered cations are in excited states (M*)

and emit light (h) as they return to ground state (M0)M* -----> M0 + h

• sputtered cations redeposit; this occurs mostly on the cathode, but some also deposit on the inner glass surface

• Light intensity limitation –> self absorption– As the current increases, M sputtered increases, but the % M*

decreases. Unexcited gaseous atoms (M0) absorb light produced within the lamp, preventing it from exiting the lamp

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Radiation Sources for AAS

Electrodeless Discharge LampsCharacteristics• The EDL houses a sealed

quartz tube (lamp) containing argon gas and a metal (or metal salt) of interest

• The quartz tube is under high vacuum

• A radio frequency (or microwave) coil surrounds the lamp

• ~ 10 times more intense than a hollow cathode lamp

• Unstable output• Only available for about 15

elements

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How an EDL Works

• An intense RF (or microwave) field is applied to the sealed quartz tube within the lamp

• Ar gas within the tube ionizes and gains kinetic energy from the RF field

• Energy is transferred to the metal upon collision• Excited metal returns to ground state, emitting light (h)

Ar (g) ---------> Ar* (g) + M (s) -------> M* (g) + Ar (g)

M (s)

h

• Selects one line from the HCL and rejects as much emission from the flame or furnace as possible

MONOCHROMATOR

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FAAS - Instrumental Components and Layout

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FAAS - Instrumental Components and Layout

Double Beam Light Transmission and Signal Processing Techniques

• Modulating the lamp (e.g. on and off) is a means to alternate the signal getting to the detector

• A chopper is used to split the light beam into a reference and signal beam path

• The lock-in amplifier coordinates the signal between the chopper and the detector; it also identifies the signal being received at the detector

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Flame Atomic Absorption Spectrometry (FAAS)

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GFAAS - Instrumental Components and Layout

• Very similar to FAAS

• Most use auto samplers

• Can involve unique background correction methods

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Graphite Furnace Atomic Absorption Spectrometry (GFAAS)

• The atomization process is achieved and temperature is rigorously controlled using a series of heating steps– Dry (remove water)– Ash or Char (destroy organics)– Atomize

• Small sample size– Typically 10-50 µL

• At const. furnace temp, the area under absorbance peak curve is a reliable measure of the analyte

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Interferences in AAS

Two Types1. Spectral – when the absorption or emission

spectra of an interfering species overlaps or lies close to that of the analyte

2. Chemical – species in the sample matrix interfere with the atomization of the analyte• Enhances or decreases the volatility of the

analyte

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Spectral Interferences in AASExample: Determination of Ba in

the presence of Ca– Both Ca and Ba atomize

simultaneously– Ca (g) + oxidant ----> CaOH (g)

– CaOH (g) exhibits broad band molecular absorption

– The observed absorbance is in error due to the non-atomic signal coming from BaOH (g)

Sig

nal

Wavelength

λBa

Sig

nal

Wavelength

λBa

Non-atomic signal

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Chemical Interferences in AAS

• Most chemical interferences result from a change in the atomization behavior of the analyte

• Usually the atomization signal is depressed

• The interference usually comes from analysis of an analyte in low concentration in a complex matrix (e.g. seawater)

Matrix Modifier• Substance added to the

sample to reduce the loss of the analyte during the charring by making the matrix more volatile or the analyte less volatile

• Ex: Seawater analysis:– NH4NO3 added to increase

volatility of NaCl.

– Pd(NO3)2 decreases volatility of analyte Sb

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Chemical Interferences in AAS (cont)

Example: Determination of Calcium in the presence of phosphate

• An equilibrium exists in aqueous solutions between calcium and calcium phosphate

Ca2+ + PO4-3 <-----> CaPO4

-1

• CaPO4-1 is less volatile (i.e. more difficult to atomize)

than Ca2+ • This equilibrium tells us that as [PO4

-3] increases, [CaPO4

-1] increases and [Ca2+] decreases• Net result, the absorbance of atomic calcium decreases

as phosphate content in the sample increases

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Interferences in AAS

Approaches to Background Correction• Use of a continuous light source as a

reference beam (FAAS)• Zeeman Background correction (GFAAS)• Calibration using the method of standard

additions– An alternative means of calibration that

accounts for interferences in the calibration process

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Background Correction in

FAAS• Monitor a wavelength

nearby the atomic line of interest using a continuous light source to generate the nearby wavelength

• AbsTotal – AbsBack = AbsAtomic

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GFAAS - Zeeman Background Correction

• Light from a HCL or EDL is passed through a rotating polarizer producing two perpendicular radiation beams

• A strong magnetic field is applied to the atomic vapor in the furnace

• The magnetic filed splits the electronic energy levels of the atoms into several closely spaced (~0.01 nm) absorption lines

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GFAAS - Zeeman Background Correction (cont.)

• Original absorbance = sum of induced absorbances• Absorption by the analyte only occurs during one phase

of the rotation of the polarizer• Background absorption occurs during both phases of the

rotating polarizer• The analyte absorption is determined by subtracting the

signal observed during the two phases of the rotating polarizer

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GFAAS - Zeeman Background Correction (cont.)

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AAS Atomization Suppression – The Method of Standard Additions

• An analysis method used to overcome suppression of the atomization signal

• Procedure/Characteristics– Add standards into sample

• Requires equal proportions of sample and standard

– Assumes that signal suppression is the same in all samples

– Requires more analyses• 3+ runs per sample

– Differences in slope due to matrix suppression

– Extending the linear regression line to x-axis gives the concentration of the (diluted) analyte

Comparison of calibration curves of strontium Comparison of calibration curves of strontium in pure water and in aquarium water by the in pure water and in aquarium water by the method of standard additionsmethod of standard additions

ANALYTICAL TECHNIQUES

• Beer's law, A = k×C, not always true making a calibration curve necessary.

• Standard addition method is used to minimize the effects from the matrix

• Anion- height of the absorbance peak is influenced by type and concentration of anion. It can reduce the number of atoms made. An unknown matrix is thus hard to correct for

• Cation: The presence of a second cation sometimes causes stable compounds to form with the cation being analyzed. e.g. Al + Mg produces low results for Mg due to the formation of an Al/Mg oxide.

Assignment• The nickel content in river

water was determined by AA analysis after 5.00 L was trapped by ion exchange. Rinsing the column with 25.0 mL of a salt solution released all of the nickel and the wash volume was adjusted to 75.00 mL; 10.00 mL aliquots of this solution were analyzed by AA after adding a volume of 0.0700 g Ni/mL to each. A plot of the results are shown below. Determine the concentration of the Ni in the river water.

Determination of Nickel Content by AA

y = 5.6x + 20

0

40

80

120

0 5 10 15

Volume of Nickel Added(mL)

Ab

sorb

ance

Un

its

Sample Problem 1

• Potassium standards gave the following emission intensities at 404.3 nm. Emission from the unknown was 417. Find [K+] and its uncertainty in the unknown.

Sample (g/mL)

Relative emission

0 0

5.00 124

10.00 243

20.00 486

30.00 712

Sample Problem 2

Free cyanide in aq soln can be determined indirectly by AA on the basis of its ability to dissolve silver as it passes through a porous silver membrane filter at pH 12.

4Ag + 8CN- + 2H2O + O2 4Ag(CN)2- + 4OH-

A series of silver standards gave a linear calibration curve in FAAS with a slope of 807 meter units per ppm Ag in the standard. An unknown cyanide solution passed through the silver membrane gave a meter reading of 198 units. Find the molarity of CN- in the unknown.

Internal Standard• A known amount of a compound, different

from analyte, that is added to the unknown.

• Compares the intensity of the absorbance of an internal standard compared to that of the analyte

• Important when instrument response may vary slightly from one run to the next

Internal Standard• Widely used in chromatography since the

small quantity of sample solution injected is not reproducible.

• Ex: HPLC/GC - flow rates vary by a few % causing a change in the detector response. Thus, relative response is useful.

Examples of IS• Deuterated chlorobenzene

(C6D5Cl) is an internal standard used in the analysis of volatiles on GC-MS because it is similar to Chlorobenzene but does not occur naturally.

• Nor-Leucine is an internal standard for the analysis of amino acids via GC-MS.

• Tetramethylsilane as IS in NMR

Internal Standard

• Preparation Procedure– Prepare a sample of unknown analyte concentration– Add an amount of known standard to the solution

• Analyze the sample• Compare the area under each signal - Response

Factor (F)

Sample Problem• In a preliminary experiment, a solution

containing 0.0837 M X and 0.0666 M S gave peak areas of Ax = 423 and As = 347. To analyze the unknown, 10.0 mL of 0.146 M S were added to 10.0 mL unknown and the mixture was diluted to 25.0 mL in a volumetric flask. This mixture gave the signals Ax = 553 and As = 582. Find [X].

Sample Problem• Answer: 0.143 M

Practice Problem1. A solution was prepared by mixing 5.00 mL

of unknown (element X) with 2.00 mL of solution containing 4.13 g of standard (element S) per mL, and diluting to 10.0 mL. The measured signal ratio in an AA experiment was 0.808 (signal X/signal S). In a separate experiment, for equal concentrations of X and S, the signal due to X was found to be 1.31 times more intense than the signal due to S. Find [X].

Practice Problem2. A solution containing 3.47 mM X (analyte) and 1.72

mM S (standard) gave peak areas of 3473 and 10222, respectively, in a chromatographic analysis. The 1.00 mL of 8.47 mM S was added to 5.00 mL of unknown X, and the mixture was diluted to 10.0 mL. This solution gave peak areas of 5428 and 4431 for X and S, respectively.

a. Calculate F for analyte.

b. Find [S] in mM in the 10.0 mL of mixed solution.

c. Find [X] in mM in the 10.0 mL of mixed solution.

d. Find [X] in the original sample.

Answers

1. 5-C 1.02 g/mL

2. 5-29 a. 0.168

b. 0.847 mM

c. 6.18 mM

d. 12.35 mM