Note 09-10_AA-AE

1

Atomic Absorption, Atomic

Fluorescence and Atomic Emission

Spectrometry

Chapters 9 & 10

Basic Principles of AA

All atoms can absorb light.

The wavelength at which light is absorbed is specific for each element.

The amount of light absorbed at this wave-length

Increases as the number of atoms of the selected element in the light path increases

Is proportional to the concentration of absorbing atoms

The relationship between the amount of light absorbed and the concentration of the analyte present in known standards can be used to determine unknown concentrations by measuring the amount of light they absorb.

2

General Analytical Procedure

Convert the sample into solution, if it is not already in solution form.

Make up a solution which contains no analyte element (the analytical blank).

Make up a series of calibration solutions containing known amounts of analyte element (the standard).

Atomize the blank and standard in turn and measure the response for each solution.

Plot a calibration graph showing the response obtained for each solution.

Atomize the sample solution and measure the response.

Determine the concentration of the sample from the calibration, based on the absorbance obtained for the unknown.

3

Atomization

Atomization is the process of forming

free atoms by applying heat to a sample

Continuous atomizers [the atomization

conditions (temperature) are constant

with time]

Flames, plasmas

Noncontinuous atomizers (the

atomization conditions vary with time)

Electrothermal atomizers (furnaces)

Spark discharges 4

Atomization Devices

Flame: combustion of an oxidant and a fuel

Electrical furnaces

Plasma: partially ionized gases maintained

either by an electrical discharge or by

coupling to a microwave or RF field

Arc: a continuous electrical discharge

between conducting electrodes

Spark: an intermittent discharge

5

6

Flame Atomizer

Processes occurring during atomization.

7

Flame Atomization

Processes for the Salt MX

Asterisk (*) indicates

excited states

MX

solution

Nebulization

Desolvation

Volatilization

Dissociation

Ionization

Recombination

Reaction

Dissociation

Emission Emission

(AES, AFS)

Excitation

(AAS) Emission Excitation Excitation

Excitation

Emission

Association

MX soln

aerosol

Discard

MX solid

aerosol

MX (g)

M (g)

MX* (g)

M + + e- MO, MH

MOH, etc

M +* M* (g) MO*, MH*

MOH*, etc

8

Properties of Flames

Fuel Oxidant Temp ( C) Maximum burning

velocity (cm s-1)

Natural gas Air 1700 1900 39 43

Natural gas Oxygen 2700 2800 370 390

Hydrogen Air 2000 2100 300 440

Hydrogen Oxygen 2550 2700 900 1400

Acetylene Air 2100 2400 158 266

Acetylene Oxygen 3050 3150 1100 2480

Acetylene Nitrous oxide 2600 2800 285

When gas flow rate < burning velocity, flashback occurs.

When gas flow rate > burning velocity, burner blows off.

When gas flow rate ~ burning velocity, flame is stable.

9

Flame Structure & Temp Profiles

Primary combustion zone: characterized by blue luminescence (due to radicals C2, CH, H3O

+, HCO+), not in thermal equilibrium, not used for flame spectroscopy Interzonal region: narrow, rich in free atoms, in thermal equilibrium, hottest, used for flame spectroscopy Secondary combustion zone: the products of the inner core converted to stable oxides and then dispersed into the surroundings.

Temp profile for a natural gas/air flame Regions in a flame

Flame Absorption Profiles

Mg

Initial increase due to an increased number of Mg atoms produced by the longer exposure to the heat of the flame

Maximum absorption at the middle of the flame

Decrease after maximum absorp-tion due to formation of oxide particles

Ag

Ag oxides not formed

Continuous increase in the number of Ag atoms

Cr

Formation of very stable oxides

Maximum absorbance immediately above the burner

Continuous decrease in the number of free Cr atoms 10

11

Processes leading to atoms, molecules and ions with continuous

sample introduction into a plasma or flame

Conversion into spray (mist of fine droplets) by nebulizer

Evaporation of solvent by the flame or plasma

Volatilization of the dry aerosol particles to produce

atomic, molecular and ionic species, which are in

thermodynamic equilibrium at least in localized regions

Spray

Solution

sample Dry aerosol

Vapor

Free atoms Nebulization Desolvation Volatilization

Molecules Ions

Sample Introduction: Continuous

12

Formation of atoms, molecules, & ions with discrete sample

introduction

Transportation of a discrete sample to the atomization

device

Evaporation of solvent by the flame or plasma

Volatilization of the solid particles to produce atomic,

molecular and ionic species, which are in

thermodynamic equilibrium at least in localized regions

Discrete

droplet

Solution

sample Solid

particle

Vapor

Free atoms Transport Desolvation Volatilization

Molecules Ions

Sample Introduction: Discrete

Laminar Flow Burner

Laminar flow is a mode of gas flow in which the lines of flow are approximately parallel and change smoothly, if at all, in time & space

Laminar flow burner produces Relatively quiet flame

Long path length

Enhanced sensitivity & reproducibility

13 A laminar-flow burner

Electrothermal Atomizer

14

(a) Cross-sectional view of a

graphite furnace with integrated

(b) Longitudinal configuration of the

graphite furnace. Note the temp

profile shown in blue along the

path of the furnace. In the

longitudinal configuration, the

temperature varies continuously

along the path, reaching a

maximum at the center.

(c) Transverse configuration of the

furnace. The temperature profile

is relatively constant along the

path.


Tube dimensions:

length, 5 cm; i.d. ~1 cm or less

Made of graphite (may be coated with a layer of

pyrolytic carbon to seal the pores of the tube)

15


Two (2) inert gas streams:

Internal stream to exclude air and to carry away vapors

External stream to prevent entrance of outside air and to protect

the tube from burning

16 Cross-sectional view of a graphite furnace

Major Steps Using Electrothermal Atomizer

Three steps (~45 90 s): Drying or desolvation step (110-125 C, 20-30 s). The solvent is evaporated, leaving a solid residue in the furnace. The T is chosen to evaporate the solvent, water, as rapidly as possible without foaming or splattering.

Ashing or charring step (350-1200 C, 45 s). Organic matter in the sample is ashed or converted to H2O and CO2, and volatile inorganic components are vaporized. The temperature should be ideally set high enough to remove all volatile components without loss of the analyte.

Atomization step (2000-3000 C, 3-10 s). The sample is vaporized and atomized to produce atomic vapor that is probed by the source. The atomic vapor is produced rapidly and diffuses out of the observation zone to produce a transient, peak-shaped response.

clean step involving a higher current and tube T employed after the atomization step to remove any remaining sample residue in the tube.

Adv.: very little sample required

17

Temp Profile with Ramp Heating

18

cool down

time

tem

pe

ratu

re

dry

30 s

atomize

3.5 s

clean-out

25 s

20 s

ash

8 s

ambient

5 C/s

50 C/s

500 C/s 105 C

500 C

2400 C

19

Typical output from a

spectrophotometer

equipped with an

electrothermal atomizer

The sample was 2 L of

canned orange juice

Drying time: 20 s

Ashing time: 50 s

How much Pb

was present in

the sample?

AA analysis of Pb

Time, s

Example

Performance Characteristics of

Electrothermal Atomizers

A discrete sample is deposited

Small sample volume (0.5-10 L)

The atomizer electrically heated to produce a transient cloud of atomic vapor

Entire sample atomized in short time at a temperature of 2000-3000 C

Sample spends up to 1 s in analysis volume

Superior absolute DL (10-10 10-13 g analyte)

Less reproducible (5-10% compared with 1% for flame and plasma)

20

Atomic Absorption Instrumentation

A light source used to generate light at the wavelength which is characteristic of the analyte element [for example, Na emission 2P1/2

2S1/2 at 589.6 nm (D1 line) used to probe Na in analyte.]

An atomizer to create a population of free analyte atoms from the sample.

An optical system to direct light from the source through the atom population and into the monochromator.

A monochromator to isolate the specific analytical wavelength of light emitted by the hollow cathode lamp from the non-analytical lines including those of the fill gas.

A light-sensitive detector (usually a photomultiplier tube) to measure the light accurately.

Suitable electronic devices which measure the response of the detector and translate this response into useful analytical measurements.

21

Monochromator Flame/furnace Line source Chopper Detector

Line Width Effects

Atomic absorption lines are remarkably narrow (0.002 to 0.005 nm)

Monochromators have effective bandwidths significantly greater than the width of atomic absorption lines

The use of radiation isolated from a continuum source by a monochromator causes instrumental departures from

Poor sensitivity results because only a small fraction of the radiation from the monochromator slit is absorbed by the sample

Solution:

Usage of line sources with bandwidths even narrower than the absorption line width

Disadvantage:

A separate source lamp needed for each element (or group of elements)

22 Absorption of a resonance line by atoms

23

Radiation Sources - HCL

HCL: Use element to detect element

Construction A hollow cup cathode (-) made of the desired element (Na, Ca, Pb

A tungsten (W) anode (+)

Low-pressure (1-5 torr) Ar or Ne

Principle Apply 300 V between anode and cathode

Form argon cations (Ar+)

Blast atoms of the element into the gas phase and to excite them

Emit lines of the element coating the cathode

Characteristics One element, one HCL

Bandwidth 3 times smaller than that of the absorption in the flame or furnace

Operation at near room temperature and low pressure

A up to 0.4

Sharp lines specific for element of interest

Disadv.: can be expensive, need to use different lamp for each element tested

Ne or Ar

at 1-5 torr Glass shield

W anode Hollow cathode

Quartz or

Pyrex window

Radiation Sources - EDL

Construction A quartz tube contains metal and iodine (or more volatile metal iodide salt)

The tube is placed inside a ceramic cylinder on which an antenna from a microwave generator (2450 MHz, 200 W) is coiled

Low-pressure Ar or Ne atmosphere

Principle Apply an alternating field of sufficient power

Vaporize to excite the atom inside the tube by the coupled energy

Produce their characteristic emission spectrum

Characteristics One element, one EDL

Temperature control required (a 130 C increase results 1000-fold increase in line intensity)

A separate power supply required

Warm-up periods of 30 min required

24

Cutaway of an electrodeless

discharge lamp (EDL)

open mirror

Source Modulation

Modulation The changing of some property such as. frequency, amplitude, & wavelength) of a carrier wave by the desired signal in such a way that the carrier wave can be used to convey information about the signal

In AAS, the source radiation is amplitude modulated, but background and analyte emission are not and are observed as DC signals

Purpose To eliminate interferences caused by emission of radiation by the flame or graphite furnace

In AAS to distinguish between the component of radiation arising from the source and the component of arising from the flame background

Device Light chopper, a mechanical device, such as a circular metal disk with alternate quadrants being removed, that alternately transmits and blocks radiation from a source 25

Light Chopper

Construction Interpose a chopper between the source and the flame

Operation Rotate the chopper at a constant speed, resulting in an alternating atomic absorption & atomic emission signal

The signal from emission of radiation will be continuous and can be subtracted from the modulated AA signal

How it works? -mirror so that detector

sees alternating light intensities

In closed position, only light emitted by flame is read since the light from the source is cut off

In open position, light from both the flame emission and

The elements of the detector is such that the emission signal is subtracted from the total signal and this difference is what we measured

26

27

(a) Single-beam design HCL

Chopper (or a pulsed power

supply)

Atomizer

Grating monochromator

Photomultiplier transducer

(b) Double-beam design The beam from HCL split by

a mirrored chopper, one half

passing through the flame

and the other half around it.

The 2 beams are then

recombined by a half-

silvered mirror and pass into

a nonochromator.

Typical Flame Spectrophotometer

Interferences

Spectral interference: emission or absorption

by species other than the analyte within the

band-pass of the wavelength selection device.

Chemical interference: due to various chemi-

cal processes occurring during atomization

that alter the absorption characteristics of the

analyte.

28

402 403 404 405 406 407

Ga

403.3

K

404.4, 404.7 Mn

403.1, 403.3, 403.5

Pb

405.8

308.21 308.22

V

308.211

Al

308.215

Spectroscopic Interferences

Prevent accurate absorbance measurements

Result from the following:

Unresolved lines: A line of interest cannot be

resolved from a line of another element due to the

resolving power of the monochromator

Molecular absorption: Molecules such as CaOH in

the flame have broad absorption bands (543-622

nm) which often overlap with the Na doublet

(589.0, 589.6 nm) and the Ba line (553.6 nm)

Light scattering from particles and droplets:

Particles originate from incomplete atomization

while droplets result from poor nebulization

29

30

Direct Spectral

Overlap Interferences

in Atomic Absorption

Elements/Interferent Wavelengths (nm)

Cu/Eu Cu 324.7540

Eu 324.7530

Si/V Si 250.6899

V 250.6905

Fe/Pt Fe 271.9025

Pt 271.9038

Al/V Al 308.2155

V 308.2111

Hg/Co Hg 253.652

Co 253.649

Mn/Ga Mn 403.3073

Ga 403.2882

Zn/Fe Zn 213.856

Fe 213.859

Fe/Cr Fe 302.064

Cr 302.067

31

Choose a reference line from the source as close as possible to

the analyte line but must not be absorbed by the analyte.

Any decrease in power of the reference line from that observed

during calibration arises from absorption or scattering by the

matrix products of the sample.

This decrease is then used to correct the absorbance of the

analyte line.

Analyte line background + analyte absorption (AT)

Reference line background absorption (AB)

Difference the corrected atomic absorption (AA = AT AB)

Problems: a suitable reference line is often not available.

reference

line

analyte

line

Correction Methods: Two-Line

total measured A (AT) = analyte A (AA) + background A (AB),

i.e. AT = AA + AB

32

Correction Methods: Continuum-Source

Reference is a

deuterium (D2) lamp.

Use a chopper so that

the D2 lamp and HCL

alternatively pass

through the graphite-

tube atomizer. Slit width

is wide.

HCL background + analyte absorption (AT)

D2 lamp background absorption (AB)


Problems: S/N decreases due to the addition of a lamp & chopper,

imperfect alignment of two lamps. Low power is not suitable for

visible region (> 350 nm)

33

Correction Methods: Zeeman-Effect

Zeeman effect: the splitting of spectral lines in a magnetic field.

Central line background + analyte absorption (AT)

Satellite lines background absorption (AB)


Adv.: single radiation source & only one optical path through the atomizer,

more accurate correction. Disadv.: instable magnetic field results in

irregular emission signal; large background correction, significant error.

Field on

Magnetic quant # ML

+1

-1

0

Field off

1S0

1P0

-

+

I

34

Schematic of an electrothermal atomic absorption instrument that

provides a background correction based upon the Zeeman effect

Correction Methods: Zeeman-Effect

Self-absorption & Self-reversal

35

Effects of both emission and absorption by the same element

(a) Unobstructed emission line

(c) Self-absorption (with further

(d) Self-reversal (single line

split into two separate

lines)

(b) Self-absorption (with increasing amounts

of atomic vapor placed between the source

and a high-resolution spectrometer)

36

Correction Methods: Self-Reversal

0.3 ms

9.7 ms Cu

rre

nt

Time

5-20 mA

10

0-5

00

mA

The current waveform

A

Concentration

Alow

Ahigh

Alow - Ahigh

The analyte abs cal curve for

low and high i measurements

The measurements are made at two currents:

low (normal) and high (pulse)

Low current background + analyte absorption (AT = Alow)

High current absorbing radiation produced from excited species)

(AB = Ahigh)


Ra

dia

nt

po

wer

Emission profile of HCL

Wavelength

Chemical Interferences

Result from various chemical processes

occurring during atomization that alter the

absorption characteristics of the analyte

Formation of compounds of low volatility

Dissociation reactions

Ionization

Affect the concentration of ground state atoms

in the flame/furnace

37

Chem. Interferences: Forming Stable Cpds

Formation of stable compounds (di- or triatomic molecules), i.e. refractory (thermally stable) oxides of B, P, W, U, Zr, Nb or refractory salts (CaPO4). [An increase in concns of sulfate & phosphate results in decrease in Ca absorbance; presence of Al causes low results in Mg due to formation of MgAlO2]

Possible cures: Apply hotter flame such as N2O/C2H2 to break apart the compounds

Use a fuel-rich flame to reduce metal oxides

Avoid certain anions with certain analyte cations using releasing agents which are cations that react preferentially with the interferent and

43-]

Chelate the metal analyte using protective agents to form stable but volatile species with the analyte [EDTA (Ca/Al-Si-PO4

3--SO44-, 8-

hydroxyquinoline (Al/Ca-Mg), APDE (the ammonium salt of 1-pyrrolidine-carbodithioic acid)].

38

N

OH 8-hydroxyquinoline

N

S SH

1-pyrrolidine-carbodithioic acid

Chem. Interferences: Dissociation Equilibria

Dissociation equilibria

Dissociation rxns involving metal oxides & hydroxides:

MO M + O

M(OH)2 M + 2OH

Presence of HCl decreases Na lines due to:

NaCl Na + Cl

Presence of Al or Ti enhances absorption by V:

VOx V + Ox

AlOx Al + Ox

TiOx Ti + Ox

where Ox is any oxygen-bearing species.

Possible cures: Fuel-rich combustion mixture will shift the equilibria involving O to the right, resulting in an increase in M concentration as well as A.

39

Chem. Interferences: Ionization

Ionization (M M+ + e-) Especially a problem for Group I metals which have low ionization energies.

Cures: Raise the free electron concentration in the gas phase by adding a large excess of Cs salt (ca. 100 fold, ionization suppressor) to the sample; the excess e- from the Cs atoms suppress ionization of other metal atoms. K or Na, 2000-5000

g/mL.

40

K 0 g/mL

K 1000 g/mL

K 10000 g/mL

K 25000 g/mL

Strontium, g/mL A

bs

orb

an

ce

(N2O-acetylene)

Effect of K concentration on the

calibration curve for strontium

41

M M+ + e-

PK 1[M]

]][eM[ 2-

Initial: P 0 0

Equil: (1 - )P P P

where is the fraction of M that is ionized, and P is the partial pressure of

the metal in the gaseous solvent before ionization.

[e-] = [B+] + [M+]

B B+ + e-

where B is an ionization suppressor, which provides a relatively high

concentration of electrons to the flame

Ionization Equilibria

Degree of Ionization of Metals at

Flame Temperatures

Ionization

potential

eV

Fraction ionized @ the indicated P & T

P = 10-4 atm P = 10-6 atm

Element 2000 K 3500 K 2000 K 3500 K

Cs 3.893 0.01 0.86 0.11 >0.99

Rb 4.176 0.004 0.74 0.04 >0.99

K 4.339 0.003 0.66 0.03 0.99

Na 5.138 0.0003 0.26 0.003 0.90

Li 5.390 0.0001 0.18 0.001 0.82

Ba 5.210 0.0006 0.41 0.006 0.95

Sr 5.692 0.0001 0.21 0.001 0.87

Ca 6.111 3 10-5 0.11 0.0003 0.67

Mg 7.644 4 10-7 0.01 4 10-6 0.09

42

DL (ng/mL = ppb) for Selected Elements Element AAS flame AAS

electrothermal

AES flame AES ICP AFS

flame

Al 30 0.1 5 0.2 5

As 200 0.5 2 15

Ca 1 0.25 0.1 0.0001 0.4

Cd 1 0.01 2000 0.07 0.1

Cr 4 0.03 5 0.08 0.6

Cu 2 0.05 10 0.04 0.2

Fe 6 0.25 50 0.09 0.3

Hg 500 5 5

Mg 0.2 0.002 5 0.003 0.3

Mn 2 0.01 0.01 1

Mo 5 0.5 100 0.2 8

Na 0.2 0.02 0.1 0.1 0.3

Ni 3 0.5 600 0.2 0.4

Pb 8 0.1 200 1 5

Sn 15 5 300 200

V 25 1 200 0.06 25

Zn 1 0.005 5000 0.1 0.1 43

Atomic Emission Spectrometry

Inductively coupled plasma (ICP) is the most important method of atomization and excitation among other methods: flame, electric arc, electric spark, direct current plasma and microwave-induced plasma

A plasma is an electrical conducting gaseous mixture containing a significant concentration of cations (for example, Ar+ in argon plasma) and electrons

ICP has the advantages: Low interelement interference

Simultaneous multielements determination (good emission spectra for most elements under a single set of condition)

Determination of elements (B, P, W, U, Zr, Nb) that tend to form refractory (thermally stable) compounds

Analysis of nonmetal elements (Cl, Br, I, S)

2-3 orders of magnitude higher concentration range than the absorption methods

44

ICP Source

Construction 3 concentric quartz tubes

Top of outer tube (15-30 mm in diameter) surrounded by a water-cooled induction coil that is connected to a high-frequency generator

Generator operated at frequencies of 4 50 MHz (commonly 27 MHz) & at output powers of 1 5 kW

An inert gas (usually Ar) flowing through the tube. (This flow of gas as the support gas for the plasma and as the coolant for the quartz tube.)

Operation To form the plasma, the spark from a Tesla coil is

region of the induction coil

Once the Ar conducts, the plasma forms spontaneously if the flow patterns are proper inside the tube

The induced current, composed of ions & electrons flowing in a closed circular path, heats the support gas to temperature up to 10,000 K and sustains the ionization necessary for achieving a stable plasma

45 A typical ICP

Radial viewing

of torch

axial viewing

of torch

Two different Ar flows are used:

A relatively low flow velocity, about 1 L/min, used to transport the sample to the plasma.

A much higher flow velocity, typically 10 L/min, introduced tangentially. (This flow of Ar thermally isolates the plasma from the outer quartz con-finement tube and prevents overheating).

The plasma itself located near the exit end of the concentric tubes.

46

Temperatures in a typical

ICP source

47

Sample Introduction to ICP

A typical nebulizer (cross-flow)

for sample injection into a

plasma source

Device for electrothermal vaporization

For sample introduction only, not for

atomization that occurs in the plasma

Little sample (~5 L)

Low absolute DL (~1 ng)

Freedom from interference

Multielement capabilities

48

Block diagram of a typical ICP atomic emission spectrometer

Viewing geometries for ICP sources.

(a) Radial geometry used in ICP atomic

emission spectrometers.

(b) Axial geometry used in ICP mass

spectrometers and in several ICP

atomic emission spectrometers.

Schematic Diagram of ICP

Characteristics of ICP

High temperatures: T as high as 10,000 K, measurement around 6,000 K, in this case, all compounds break apart

Long residence times: sample residence times in the plasma are usually 2 to 3 ms. (The combination of high T and long residence times leads to nearly complete solute vaporization and a high atomization efficiency.)

High e- number densities (few ionization interferences) suppress ionization

Ar Ar+ + e-

Free atoms formed in nearly chemically inert environment. (The inert chemical environment means that free atoms should have relatively long lifetimes in the plasma)

Molecular species absent or present at very low levels

No electrodes/no explosive gas

ICP is an emission form of spectroscopy, and thus requires very good monochromator

As concentration of analyte 0, intensity of radiation 0

49

Summary about ICP

Advantages Complete atomization

Fewer chemical interferences (due to high temp)

Uniform temperature cross section leading to fewer self-absorption and self-reversal effects

Little ionization (large and constant electron concentration from Ar suppressing M from ionization: M+ + e- M)

Wide dynamic range (up to 106 units).

Low detection limit.

Simultaneous multielements determination.

Disadvantages: Samples must be in solution

AA and AE methods are complementary.

50

51

Direct Current Plasma Source (DCP)

(tungsten)

Diagram of a 3-electrode DCP source

(graphite)

Construction An inverted Y shape

3-electrode system

Operation Argon flowing from 2 anodes toward cathode

Plasma jet formed by bringing the cathode into momentary contact with the anodes

T: at arc core > 8000 K, at the viewing region ~5000 K.

Sample aspirated into the area between the arms of the Y, where it is atomized, excited, and detected.

Characteristics (vs. ICP) fewer spectral lines

much less Ar assumption

shorter residence time, incomplete volatilization

replacement of C electrode every few hours

Desirable Properties of an Emission Spectrometer

1. High resolution (0.01 nm or / > 100,000)

2. Rapid signal acquisition and recovery

3. Low stray light

4. Wide dynamic range (>106)

5. Accurate and precise wavelength identification and selection

6. Precision intensity readings (

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Note 09-10_AA-AE