The Absorption of Radiation What happens to an excited atom/molecule after absorbing a UV/vis photon ? The excited atom or molecule will eventually return to the ground state. Several possibilities exist for energy loss: 1) The extra energy is lost by collision (heat) 2) A photon is emitted as the electron falls back to the ground state. NB. electron will have jumped to a high vibrational level in the higher orbital. It will rapidly go down to the lowest vibrational state in the higher orbital. Therefore, the photon emitted will be of lower energy (higher λ) than the absorbed photon. Emissions will be in all directions (fluorescence). The electron may go down to a metastable state in a slightly lower orbital, from which the dropping back to ground state orbital is “forbidden” and therefore slow. This is phosphorescence.

A Related Process to Fluorescence/Phosphorescence In which the photon is not absorbed. This process is called scattering of light. No Change of Wavelength

Tyndell scattering - particles somewhat larger than molecules (colloids)

Rayleigh scattering - by molecules.

Turbidimetry - amount lost in a straight line. Nephelometry - amount scattered to 90°. Determination of molar mass (polymers) and particle size (very important in industry). Air pollution monitoring (scattering by smog). With a Change of Wavelength Photon transfers some energy to a molecule or vice-versa, i.e. scattered photon is of slightly higher or lower energy than the incident photon. Wavenumber JR = Ji ± ΔJ Raman scattered incident Raman shift* *This is the energy difference between two vibrational levels (or possibly rotational levels). Raman is complementary to IR (largely). Because Raman needs a change in polarisabilty (shape) but not necessarily dipole moment. IR- production of dipole moment is required for vibrational axis.

Main analytical atomic and molecular UV/vis spectroscopic techniques are:

Emission, Absorption and Fluorescence. Useful Schematics

Atomic emission spectroscopy - arc spectroscopy (steel works) plasma spectroscopy flame photometry (visible) Molecular emission spectroscopy CHEMILUMINESCENCE (+bioluminescence) A + B → C* → C + hν Molecular Fluorescence (molecular luminescence) Fluorimetry Phosphorimetry Atomic fluorescence Can be/is used * Atomic Absorption Spectroscopy (AAS) • Flames • Plasmas • Carbon Furnaces

* UV/ Visible Molecular Absorption Spectroscopy UV/Visible Molecular Absorption Spectroscopy Determinand absorbs light in visible or UV region. We react determinand with a reagent (derivatise it) to form a chemical species/product which does absorb visible or UV light. NB. The reagent must not absorb at the wavelength used for measurement. How Do We Measure the “Colour” of the Solution ? Visible light - can use a colour comparator, or could use a coloured comparator disc (glass or plastic). OK for chlorine in swimming pools for example, but not very precise and what do we do in the UV region ? For visible region, can use a filter photometer instrument.

More advanced instruments are totally enclosed. Clearly, even if a high concentration of determinand is present, only a fraction of the light is absorbed. Therefore, use a filter which passes light of wavelength similar to the determinand. Red dye absorbs green light, therefore needs green filter to get maximum sensitivity (green filter more or less closely the colour absorbed). How Do We Make the Measurement ? SHUTTER CLOSED 0% light WATER IN CELL 100% light SAMPLE IN CELL <100% light Light Detection - Photocell

Photomultiplier tube , e.g. 200-650nm

Suppose we want to restrict the wavelengths passed through the sample cell more carefully, we use a monochromator. PRISM

Fused (vitreous) quartz : 200nm - 4µm in IR. Optical glass gives better dispersion in visible. DIFFRACTION GRATING

e.g. 2000 lines cm-1 ⇒ d = 5 x 10-4 cm At θ = 6.00° , ! = 522.5

n nm

1st order 2nd order 3rd order 4th order 522.5nm 261.2nm 174.2nm 130.6nm

REFLECTION GRATING

Thin metal plate or glass plate with metallic film.

α + θ = β - θ

Light Sources Continuous spectrum - “white light source”. (line spectra ,e.g. mercury lamp - when single narrow line is required, e.g. for calibration of instruments). Tungsten filament lamp : 320nm - 3500nm (tungsten-iodine, quartz-iodine) Halogen light. Hydrogen or deuterium discharge lamp , 160-360nm : more intense, longer life, more expensive. Neon discharge lamp - fluorescence. Helium cathode lamps - line spectra - atomic absorption. Single / Double Beam Instruments Single Beam Instruments

Single wavelength measurement. e.g. at 520nm. Set 0% transmittance with shutter closed. Put water in cell to get 100% transmittance. Put sample or standard in cell to read %transmittance (or rather absorbance). NB. Blank must be done at each wavelength as the intensity of the lamp is not uniform (emission spectrum of lamp). Used to plot manually. More recently , scan all λ with blank, then scan all λ with standard, and then subtract. Better to use double beamed instruments.

NB. there is a loss at all interfaces apart from absorbance of solution. MATCHED CELLS

The Absorption of Radiation Consider an assembly of independent absorbers of N m-3 having a beam of monochromatic photons incident upon it of power P0.

Probability of a photon colliding with an absorber is dependent on N. Provided that the energy of the photon corresponds to the energy of a spectroscopic transition within the absorber, there is a finite probability that the photon will be absorbed and the energy converted to internal energy in the absorber. Two questions : (i) How much power is lost from the beam, and what is the emergent power P ? (ii) What happens to the lost power. (1) Having reached an excited state by absorption, it can be depopulated by 3 mechanisms: -spontaneous emission of a photon → fluorescence phophorescence Photons emitted equally in all space (radiation is isotopic) ∴ only small contribution to P. - stimulated emission Photons emitted in the same direction with same phase as incident photons, ∴ do contribute to P, BUT at normal temperatures

Ni

N0

is very low, ∴ effect is weak.

- a quenching collision occurs. e.g. M* + Q → M + Q* (2) Power is also lost by scattering - Rayleigh Scattering is an elastic process : λi = λs.

Raman scattering is an inelastic process with the molecule being excited to a very short lifetime virtual state. i.e. λi ≠ λs. For both Rayleigh and Raman : ! = 1

"4

(ii) How much power is lost from the beam ?

a) the absolute power absorbed by a given increment. ΔN of absorption if proportional to the power, ΔP∝P. b) Successive equal increments in the number of absorbers absorb equal fractions of the incident power, this is the formal statement of Beers Law. These observations can be expressed mathematically by considering very small increments in the number of absorbers.

-dP ! P dN

or dPP

= - k dN

Integrating from Po → P and from 0 → N,

dPPP 0

P

! = - k dN0

N

!

ln PP 0

= - kN

The way of expressing k and N are different for atomic and molecular spectroscopy. Molecular Spectroscopy Express N in terms of the molar concentrations c and absorption path length b (cm), and k in terms of the molar absorptivity ε.

log P0

P = !bc

ε = molar absorptivity (mol-1 cm-1 dm-3) c = molar concentration (mol dm-3) b = absorption path length (cm) We normally write

A = log P 0

P

where A is the absorbance, i.e. A = εbc. ∴ A ∝ c. Note, c = 0 - no absorption. P = P0, ∴ A = 0.

If A = 4, P 0

P = 10-4 (P hard to measure)

If A = 0.001, P 0

P = 0.997 (hard to differentiate between P and P0)

Also note : Transmittance T =

P0P

, i.e. %T = PP0x100 .

Used mainly in infra-red spectroscopy. ∴ A = -log T

Limitations to Beers Law 1) Fundamental i) fails at high concentrations because electron clouds interact and ε is altered. ii) refractive index changes at higher conc., replace ε with εn/(n2+2)2. 2) Instrumental Limitation Applies strictly only to monochromatic radiation -with finite bandwidth curvature towards the concentration axis occurs at high concentration. 3) Chemical Deviations Chromophore concentration may be affected by a shift in the chemical equilibrium. e.g. Cr2O7 + H2O 2HCrO4

- 2H+ + 2CrO42-

Atomic Absorption Spectrometry Spectroscopic Processes Used in Analytical Atomic Spectrometry

E

E1 2 3

1

0 1. Atomic absorption 2. Atomic emission 3. Atomic fluorescence Wavelength range used for atomic absorption (AA) E1-E0 = 1.6 eV ⇒ 800nm E1-E0 = 6.5 eV ⇒ 180nm Population of Excited State

For atoms in thermal equilibrium, and in the absence of strong external radiation sources, the number of atoms in an excited state, N1, compared with the number of atoms in the ground state, N0, is given by the Boltzmann equation.

E1 - the energy of the excited state k - the Boltzmann constant gx - the statistical weight of the population = 2J + 1 NB. N1 << N0, even at 5000K, ∴ greatest sensitivity is obtained by using absorption lines starting from the ground state, i.e. resonance lines. Shape and Width of Atomic Absorption Lines At low concentrations, i.e. in the absence of self-absorption and in the absence of strong electrical fields, such as those created by gaseous ions in a plasma, the physical width of an atomic absorption line is determined by a combination of two processes: i) pressure or collisional broadening ii) Doppler broadening i) Pressure Broadening

collisions perturbthe energylevels

!"#$p = 1

where Δνp is the pressure or collisional 1/2 width and τ is the mean period between collisions.

! (torr) 1p

"

ii) Doppler Broadening Doppler broadening is caused by the relative movement of the emitted or absorber with respect to the absorber.

ν ←⎯ red shift ν = ν0 - Δν ↑ no shift ν = ν0 ↓ ⇐observer ν ⎯→ blue shift ν = ν0 + Δν Atoms and molecules are in random motion with a Maxwellian distribution of velocities. This leads to a Gaussian distribution of shifts; i.e. the peak has a Gaussian shape with half-width given by

where ν0 = frequency of the line centre and R = universal gas constant and m = atomic / molecular weight.

NB. !" 0

12

Tm

# $%&

'()

For typical atom cells used in AA, p = 1atm, T = 2000-3000K, Δλ = 0.001 - 0.01nm For Beers Law to be obeyed over a wide concentration range, Δλsource << Δλabsorption i.e. Δλsource << 0.001nm Specificity of Atomic Absorption Factors Influencing Specificity: 1. Absorption spectra of elements at the temperatures encountered in atomic absorption

spectrometry are simple, comprising only the resonance lines. Transitions from the ground state to the lowest excited states are generally the strongest absorbers.

2. Absorption spectra are unique to each element and because the lines are very narrow, and there are few of them, overlap of absorption lines in comparatively rare.

3. The light source in AA is normally a hollow cathode lamp (HCL) and the cathode is

made from, or lined with, the analyte element. The HCL therefore only emits the atomic lines (including the resonance lines) of the analyte, plus some of the fill gas, e.g. Ne.

AA has a very high specificity because of the lock (absorption line) and key (emission line) nature of the process.

emission line

absorption line

Wavelength

Background Radiation The atom cells used in atomic absorption spectrometry unfortunately contain not only the analyte atoms, but also molecules and radicals. These species have associated with the absorption bands which may cover several nanometres of the spectrum. If they overlap with analyte resonance lines they will cause absorption to occur which is not related to the concentration of the analyte. Steps must be taken therefore to correct for this background absorption. Sensitivity and Detection Limit - The Characteristic Concentration (sensitivity) This is the concentration of analyte which gives rise to a 1% absorption of radiation, i.e. an absorbance of 0.0044. This depends primarily on the absorption line strength. Therefor, for equal hollow cathode lamp line width the spectral bandpass absorption path length (L) and number density (N) should be approximately equal for all instruments. - The Detection Limit This is the concentration of the analyte which gives a signal (absorbance) “n” times the standard deviation of the blank, where n = 2, or 3.

This is a more realistic estimate because it take account of the “noise” in the system, i.e. from the sample introduction, flame, etc. Linear Concentration Range 1. The lower limit is set by the noise in the system because the system has to discriminate between two very similar numbers. e.g. A = 0.0044 I ≈ 0.99I0 2. The upper limit is set by deviations from Beers Law and ultimately deteriorating precision because of the low intensity of transmitted radiation. e.g. A = 2 I ≈ 0.01I0 The normal linear range is 1-2 orders of magnitude of concentration. Instrumentation for Atomic Absorption Spectrometry The analytical application of atomic absorption was the subject of a patent in 1953 from the CSIRO, Australia, and the first paper by Sir Alan Walsh was published in 1955. The basic components are:

The Atomic Line Source The Hollow Cathode Lamp (HCL) - mechanism - The cathode is made from a hollow cylinder of the analyte element or from a metal such as Ta or W into which the analyte element is deposited to form a liner. - A voltage of 150-300V (current 2-20mA) is applied between the anode and the cathode.

- A discharge is established in the fill has, position ions of the fill gas are accelerated into the cathode and remove analyte atoms by a collisional process known as sputtering. - The analyte atoms are excited by collisions with the fill has ions and the electrons in the discharge. - The output spectrum comprises of the atomic spectrum (mainly resonance lines and lines originating from excited states close to the ground state) of the analyte and the fill gas. - Some multi-element lamps, e.g. Ca-Mg, Fe-Ni-Cr are available, but the intensity of the individual lines are reduced. - HCL’s have a limited lifetime, dependant upon the operating current. As the lifetime decreases the current increases and as the lifetime decreases, the volatility increases. - The factors responsible for limiting the lifetime : (a) The adsorption of the fill gas on the inner surfaces of the lamp - discharge efficiency is reduced. (b) The deposition of the analyte, e.g. for volatile elements Cd, As, Se on the inner surfaces of the lamp leading to enhanced fill gas absorption. (c) The evolution of hydrogen from the analyte leading to an intense hydrogen discharge spectrum. The Hollow Cathode Lamp The low pressure leads to small collisional broadening and the low temperature leads to small Doppler broadening. Δλ < 0.001nm

mica s hields anodehol low cathode

connect ingpin s pl as tic

base

insu latingsupports

gl ass envelope

n eon at 3 torrgraded s eal

s ilica window

Operating Characteristics (a) Generally P ∝ cn n = 2, 3 (b) As P is increased, the signal-to-noise ratio in measurement of P and P0 increases. (c) However, as i increases, two undesirable phenomena occur : Self-absorption - causes broadening of atomic line Self-reversal - causes a loss of intensity at the atomic line centre.

The Atom Reservoir

The function of the atom reservoir is to convert all the chemical forms of an element present in a liquid sample into free atoms in the gaseous state. The absorption depends on the number density N (atoms m-3) and the absorption path length L. Therefore, for a given chemical concentration, c:- - N should be constant regardless of the chemical form of the element. - All the atoms of the analyte entering the atom cell should be converted to free atoms on the same time scale and with the same spatial distribution. Two types of atom cell are commonly used in AAS:- - the chemical flame (detection limits 0.01-2µg ml-1) - the electrothermal atomiser (detection limits 0.001-10ng ml-1) The Chemical Flame The samples in liquid form cannot be directly introduce into the flame. They are first converted to a fine aerosol (dmax < 6µm) by a nebulizer and spray-chamber. The nebulizer produces a primary aerosol, 0 < d < 100µm and the spray chamber filters out the large droplets (d > 6µm). Only about 5-7% of the sample entering the nebulizer reaches the flame. An impact bead is usually incorporated into the spray chamber, it also acts as a filter for large droplets and can produce some secondary fragmentation, thereby increasing the aerosol flux to the flame. Desirable Properties of a Flame to be used for AAS :- (a) The temperature and chemical environment must be suitable for producing efficient atomisation of the analyte element. (b) The ability to tolerate a wide variety of solvents, e.g. H2O, EtOH, MIBK. (c) They have low levels of background emission and absorption. (d) They must be stable and reproducible from day to day and have low “noise” characteristics. (e) They must be safe, convenient and inexpensive to operate. These properties are provided by a Laminar pre-mixed flame stabilised on a slot burner. A Typical 10cm Slot Burner For a laminar pre-mixed flame.

the fuel and oxidant gases are pre-mixed in the spray chamber. The flame is stable provided that: i) The flow velocity of gases must be greater than the flame burning velocity. ii) The burner temperature must be low enough to quench the flame reaction.

made of titanium Common Analytical Flames Oxidant Fuel Temp / °C Air CH4 1850-1900 Air natural gas 1700-1900 Air H2 2000-2050 Air C2H2 2125-2400 N2O C2H2 2600-2800 Function of the Sample Introduction / Atomisation System. See diagram (next page). The Laminar Pre-Mixed Flame

.

Flame Mechanisms - Pre heating zone :- The flame gas is heated to ignition temperature by conduction from the combustion zone above. - Primary reaction cone :- This is the bright luminescent zone <0.1mm thick where primary combustion occurs. The residence time is too short for thermodynamic equilibrium. There are many excited molecules and radicals produced by exothermic chemical reactions, e.g. for air oxidant : CO2, CO, H2O, H2, N2, H•, O•, OH•. This region is unsuitable for analytical use. - Interconal zone :- This is the hotter part of the flame and the zone used for analytical spectroscopy. No new reactants are added so that thermal equilibrium is rapidly stabilised (e.g. through recombination of radicals and dissociation of unstable reaction products).

This zone may be several millimetres long because the hot gases expand after the initial combustion. The inter-conal zone is characterised by low luminescence and low noise. When N2O is used as the oxidant it has a characteristic red colour due to the emission of the CN• radical - Secondary reaction zone :- the major region of the flame where air diffuses in and the final products are formed, e.g. CO + 1/2O2 → CO2 + hν. The characteristic blue colours arise from hydrocarbons. Factors Affecting Atomisation Efficiency (i) Temperature affect on the dissociation equilibrium (M-X)n M + X (n=1 - 10) As the temperature increases, the equilibrium is shifted to the right. (ii) Chemical environment in the flame effect on the dissociation equilibrium. M-O M + O An oxidising flame (fuel lean) pushes the equilibrium to the left, while a reducing flame (fuel rich) pushes it to the right. Application of Different Flame Types Air/C2H2 This is the most widely used type of flame. It is appropriate for a wide range of non-refactory elements which do not have a high affinity to oxygen. i.e. Mg, Cu, Zn, Cd. N2O/C2H2 This is a very high temperature flame. The inner conal zone is red (the red feather) because of the CN• emission. CN• is an efficient scavenger of oxygen. It is appropriate for the involatile elements and those having an affinity for oxygen, i.e. Al, B, Ba, Mo, Si, Ti. Air/H2 This is a low temperature, low ultra-violet absorption, low quenching flame. The applications are for As, Se, hydride atomisation and AFS. Interferences in Flame Atomic Absorption

Spectral Interference This includes the overlap of an atomic absorption line of a matrix element with the analyte. There are only 29 cases known and they are only significant at high matrix levels. e.g. Cu on Eu and Si on V. Another possibility is the overlap of an emission line on the matrix with the atomic absorption line of the element. e.g. Fe on Se in ETA-AAS. The final possibility is the uncorrected background absorption. Physical Interference This is a variation in the physical properties, e.g. viscosity, surface tension, between the samples and the standards, leading to changes in nebulisation efficiency. Therefore the samples and standards need to be matrix matched, e.g. in acid content. NB. The samples and standards should be at the same temperature. Ionisation Interferences These are caused by differing levels of low ionisation potential elements, e.g. Na, K, Al between the samples and the standards. This affects the position of the ionisation equilibrium:-

M M+ + e-

It also affects low ionisation potential elements which are significantly ionised in the flame. Therefore excess ionisation suppresser (buffer) is added, i.e. an element of low ionisation potential, e.g. 1000-5000µg ml-1 of Na or K, to the samples and the standards. Chemical Interference (i) Due to the presence in the samples of chemical species which combine in the gas phase with the analyte to form thermally stable compounds which are not readily atomised. e.g. Ca + PO4

3- → calcium pyrophosphate

e.g. Ca + Al → Ca(AlO2)2 (calcium aluminate)

NB. A non-linear interference curve with a pronounced “knee” is characteristic of a chemical interference which involves a reaction stoichiometry. A linear interference curve is characteristic of a non-specific matrix interference. (ii) Due to the presence in the samples of species which combine with the analyte to give more volatile compounds, e.g. F- with refactory elements such as Si, Ti; EDTA with Cu; 8-hydroxyquinoline with Al or Cr. (iii) Due to the presence in the samples of refactory elements, e.g. Zr, U - rare earth elements which are not significantly atomised and can acclude the analyte into the matrix particles of clotlets. NB. The interference curve tends to be approximately linear without the “knee”. To remove such interference smaller particles need to be generated and the flame needs to be hotter. (iv) Due to the acclusion of the analyte into a more volatile matrix, e.g. NH4Cl sublimes explosively and therefore increases the atomisation efficiency. Interferences can be removed by matrix matching all samples and standards with NH4Cl. Methods of Overcoming Chemical Interferences These include the use of a hotter flame. Ca/PO4

3-; Co/Al interferences are eliminated in N2O2/C2H2 flames. Also the use of a releasing agent, e.g. La, Sr at 1000-2500ppm. These have a high affinity for bonding with oxygen or oxo-anions and therefore preferentially combine with the interferrants. You can use a protective agent, e.g. a chelating complex, which preferentially complexes the analyte and prevents the formation of the thermally stable compound, e.g. EDTA complexes Ca and reduces PO4

3- interferences. The atomisation species is Ca(EDTA). The nebuliser uptake rate can be reduced so that smaller particles are produced. Electrothermal Atomic Absorption Spectroscopy (ETA-AAS)

Dimensions, L=30mm, I.D. = 5mm Material : Pyrolytically coated electro-graphite or totally pyrolytic graphite (TPC). Mode of Operation Furnace Programme: (1) Drying Stage T ≈ 105-115°C for 30-40 secs (20µl sample) If T is too high, sample is “splattered” away from the deposition point.

Ramp heating is preferred for high organic matrix samples, e.g. blood, high sugar samples, e.g. fruit juice, to avoid spitting and frothing during drying which leads to poor precision.

Organic solvents tend to “wet” the tube and run over surface, and also soak into the tube, so use half the usual amount. (2) Ashing Stage Purpose of this stage is to remove as much of the sample matrix and possible and to leave the analyte in a form which yields the higher atomisation efficiency and produces reproducible signals. e.g. removal of organic matter which gives a dense pyrolysis smoke which would “interfere” with, and produces “noisy” atomic absorption. Generally, the analyte is preferred in a thermally stable form so that ashing temperature can be as high as possible. Therefore, prefer HNO3 over HCl as the MO oxide form is stable whereas the MCl chloride form is volatile,e.g. Al, Fe, Ni, As, Ca, Pb. Can use a matrix modifier, e.g. Pd to convert a volatile analyte, e.g. As, Se, Cd, Pb, to a more stable form. Optimisation of Ashing Temperature:

Ashing programme - start at 150°C → 500-1200°C and hold for 30 seconds. - ramp rate, if used, 50°C s-1 for controlled pyrolysis of organic matter.

(2a) Cool Down Stage It has been shown that heating rate is proportional to temperature rise, ∴ pre-cooling enables dT/dt to be increased which gives sharper peaks and less tailing for involatile elements. (3) Atomisation Stage The purpose of this stage is to atomise the sample as rapidly as possible to produce sharp, reproducible absorption.

Peak height or peak area can be measured.

Atomisation programme - start at ashing temperature - 1500-2900°C for 5 seconds.

Ramp mode is not used because sharp peaks are required. (4) Cleaning Stage Purpose : to remove residual analyte, e.g. carbide forming elements, e.g. Mo, V, Ta, and residual matrix before next sample - “memory effects” cause interference. Background Correction Background correction is required to correct for the affects of molecular absorption (and scattering) on the atomic absorption lines, which otherwise leads to a positive error in the calculation of the concentration. It is applied to both flame and ETA- AAS, but is more important in the latter and technically more difficult because of the differing time varying nature of the atomic and molecular signals. The Effect of Simultaneous Atomic and Molecular Absorption ( a trivial example) Consider two set of absorbing material, each of length L, but having different absorption coefficients k (m-1). Let the ratio of the transmitted to incident power be 0.8 and 0.6 for slabs (a) and (b) respectively.

A(a) = -log 0.8 = 0.0969 A(b) = -log 0.6 = 0.228 A(a+b) = - log (0.8 x 0.6) = 0.3187 NB. 1. The order of blocks (a) and (b) does not matter. 2. Whether (a) or (b) is atomic or molecular absorption does not matter. What about mixing blocks (a) and (b) so that absorption occurs simultaneously ? Firstly, divide blocks (a) and (b) into 2 slabs, i.e. (a1), (a2), (b1) and (b2). Then order them (a1)(b1)(b2)(a1). Because A = kL, halving L halves A, but the transmission changes as

12

log I0

I =

kL2

logI0

I! "

# $

12

= kL2

logII0

! " % #

$ &

12

= - kL2

i.e. if

II0

= 0.8 for slab (a), then II0

= 0.89 (i.e. 0.8 ) for (a1) and (a2).

i.e. A(a1) = - log(0.89) = 0.0506 A(b1) = - log(0.77) = 0.1135 A(b2) = - log(0.77) = 0.1135 A(a1) = - log(0.46) = 0.0506 A(a1+b1+b2+a2) = 0.3282 This argument can be extended by considering an infinite number of slabs each of thickness dl, used in any order. Therefore it is concluded that simultaneous absorption of 1 wavelength by independent absorbers can be quantified by adding the absorbances of the individual components. e.g. as in simultaneous detection of two components in spectrophotometry. NB. If two lines of differing wavelength are being simultaneously absorbed, the absorbances are not additive. Background Correction Techniques Continuum Sources Background Correction Principle: For the D2 hollow cathode lamp the measured absorption Am hasn’t changed as the atomic absorption is over a small bandwidth so only a small percentage is absorbed. The D2 HCL measures only molecular absorption. In practice the HCL and the D2HCL are pulsed alternatively.

For transient ETA signals, the pulsing frequency must be high enough to effect correction is a time short compared with significant changes in the signal. e.g. for a 0.4s signal, the pulsing frequency should be >50Hz, usually 200Hz. Limitations of Continuous Source Background Correction (1) Limited range of background correction, <1A. (2) Inaccurate with structured background because it measures the average absorption in the spectral bandpass, not specifically what’s under the line. (3) Requires an additional light source which should be of approximately the same intensity as the hollow cathode lamp. It is different in the UV region. (4) Alignment of light sources is critical, particularly in ETA-AAS because both beams must sample the same atomic population.