Background Correction Method in Atomic Absorption Spectrometry

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Background Correction
Methods in Atomic Absorption
Spectrometry
Margaretha T.C. de Loos-Vollebregt
Delft University of Technology, Delft,
The Netherlands
Ghent University, Ghent, Belgium
1 Introduction 1
2 Deuterium-Lamp Background Correction 3
2.1 Brief History of Background Correction 3
2.2 Principle of Deuterium-lamp
Background Correction 3
2.3 Instrumentation 3
2.4 Analytical Performance and
Applications 5
3 Zeeman-Effect Background Correction 5
3.1 Zeeman Effect 5
3.2 Principle of Zeeman-effect Background
Correction 7
3.3 Instrumentation 9
3.4 Analytical Performance and
Applications 10
4 Pulsed-Lamp Background Correction 12
4.1 Principle 12
4.2 Instrumentation 12
4.3 Analytical Performance and
Applications 12
5 Background Correction in High-Resolution
Continuum Source Atomic Absorption
Spectrometry 13
6 Comparison of Methods 13
6.1 Sensitivity and Dynamic Range 13
6.2 Frequency of Background Correction 14
6.3 Accuracy of Background Correction 15
Abbreviations and Acronyms 15
Related Articles 15
References 16
Update based on the original article by Margaretha T.C. de Loos-
Vollebregt, Encyclopedia of Analytical Chemistry, © 2000, John Wiley
& Sons, Ltd.
Atomic absorption spectrometry (AAS) is based on the
absorption of element-specific primary source radiation
by analyte atoms. If part of the radiation is absorbed
by molecules or lost owing to scattering, a higher gross
absorbance is measured. The difference between the net
absorption of the analyte atoms and the measured gross
absorbance is called background absorbance. Background
absorption and scattering effects have much more serious
effects on the results produced in electrothermal atomic
absorption spectrometry (ETAAS) than flame atomic
absorption spectrometry (FAAS).
The amount of incident light deflected or absorbed
by nonatomic species must be measured to obtain the
correct net absorbance of the analyte atoms only. Perfect
background correction (BC) can be obtained only when
the background absorbance measurement corresponds
exactly in space, time, and wavelength with the atomic
absorbance measurement. Since exact coincidence of all
three parameters is obviously impossible, it is customary
to give priority to the equality in space and to make
the difference in time and/or wavelength as small as
possible. Although molecular absorption and radiation
scattering are both broadband phenomena, there is no
spectral range where constant background attenuation can
be guaranteed.
In all BC systems, two measurements are made. The total
or gross absorbance is measured at the wavelength of the
atomic absorption line, and the background attenuation is
then subtracted to obtain the analyte absorbance. There
are several ways in which the nonatomic absorption at
the resonance wavelength can be estimated, including
continuum-source (CS) or deuterium-lamp (D2-lamp) BC,
Zeeman-effect BC, and pulsed-lamp or Smith–Hieftje BC
correction methods. The principle of each method, the
instrumentation, and applications are discussed. D2-lamp
BC is widely used in FAAS and ETAAS. Pulsed-lamp BC
is sometimes used in FAAS but more often in ETAAS.
Zeeman-effect BC is only used in ETAAS.
1 INTRODUCTION
AAS is based on the absorption of element-specific
primary source radiation by analyte atoms. If part of
the radiation is absorbed by molecules or lost owing to
scattering, a higher gross absorbance is measured. The
difference between the net absorption of the analyte
atoms and the measured gross absorbance is called
background absorbance. In fact, we do not know which
part of the background absorbance arises from scattering
and which part is due to molecular absorption.
Scattering of primary source radiation from nonvolat-
ilized particles much smaller than the wavelength (λ)
Encyclopedia of Analytical Chemistry, Online © 2006–2013 John Wiley & Sons, Ltd.
This article is © 2013 John Wiley & Sons, Ltd.
This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd.
DOI: 10.1002/9780470027318.a5104.pub2
2 ATOMIC SPECTROMETRY
200
0.7
0.8
T
ra
ns
m
itt
an
ce
0.9
1.0
250 300
Wavelength (nm)
350 400 450
Figure 1 Transmission spectrum of the air–acetylene flame with bands of OH, NH, and NO. (Reproduced with permission from
Ref. 1. Copyright 2002, Elsevier.)
follows Rayleigh’s law. The intensity of the scattering
depends on the wavelength as λ−4, so that short
wavelengths are scattered more than long wavelengths
radiation. Scattering is directly proportional to the
number of scattering particles per unit volume and
to the square of the particle volume. Consequently,
radiation scattering occurs more strongly with increasing
particle size. In FAAS, well-designed nebulizers and
spray chambers are used and therefore, virtually, no
unvolatilized particles pass into the optical beam.
The flame is transparent for wavelengths >230 nm
but scattering of radiation and absorption is observed
below 230 nm (Figure 1). The flame contains oxygen
and hydrogen and therefore absorption by diatomic
molecular species such as OH, NH, and NO is
observed at specific wavelength positions.(1) The reduced
transparency of the flame contributes to increased noise,
and adequate BC is needed when analyte absorption
lines are located close to such molecular absorption
bands.
In ETAAS, considerable numbers of particles can be
observed. They may arise from organic sample materials
during the pyrolysis step or from volatilized inorganic
materials that condense in the cooler parts of the atom-
izer. Small solid particles can also appear owing to
boiling, sputtering, and incomplete vaporization. Broad-
band molecular absorption of radiation is caused by
molecular species formed or vaporized in the atomizer
during the atomization step, particularly alkali and alka-
line earth halides. Sharp fine structure of the vibrational
bands in electronic spectra of the molecules can be super-
imposed on broadband background absorption.
The effects of molecular absorption in FAAS have
long been recognized. The CaOH+ band has a maximum
near the barium resonance line at 553.6 nm. Molecular
absorption spectra of a number of halides, nitrates, and
sulfates have also been reported. Molecular absorption
by diatomic molecules with high dissociation energy, such
as CS or PO, is quite common when atomization takes
place in a graphite furnace. As an example, Figure 2
shows the molecular absorption spectrum resulting
200
0
0.2
0.4
0.6
A
bs
or
ba
nc
e
0.8
1.0
1.2
1.4
220 240 260
Wavelength (nm)
280 300 320 340
240.0 nm
280.0 nm
296.1 nm
300.1 nm
Figure 2 Molecular absorption spectrum observed during
vaporization of magnesium sulfate in a graphite furnace.
The numbers refer to positions of SO2 absorption bands.
(Reproduced with permission from Ref. 2. Copyright 1992,
Royal Society of Chemistry.)
from vaporization of magnesium sulfate in a graphite
furnace.(2) Massmann and Güçer(3) measured spectra
resulting from the CN system with band heads at 386.2,
387.1, and 388.3 nm using nitrogen as the inert gas in
ETAAS. Background absorption and scattering effects
have much more serious effects on the results produced
in ETAAS than in FAAS and therefore appropriate BC
should always be applied in AAS when a graphite furnace
is used for atomization.
The amount of incident light deflected or absorbed
by nonatomic species must be measured to obtain the
correct, net absorbance of the analyte atoms only. In
electrothermal atomization, the spatial and temporal
distribution of the atoms and the background-generating
species may be entirely different.(4) Perfect BC can
be obtained only when the background absorbance
measurement corresponds exactly in space, time, and
wavelength with the atomic absorbance measurement.
Encyclopedia of Analytical Chemistry, Online © 2006–2013 John Wiley & Sons, Ltd.
This article is © 2013 John Wiley & Sons, Ltd.
This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd.
DOI: 10.1002/9780470027318.a5104.pub2BACKGROUND CORRECTION METHODS IN ATOMIC ABSORPTION SPECTROMETRY 3
Since exact coincidence of all three parameters is
obviously impossible, it is customary to give priority
to the equality in space and to make the difference in
time and/or wavelength as small as possible. Although
molecular absorption and radiation scattering are both
broadband phenomena, there is no spectral range where
constant background attenuation can be guaranteed.
In all BC systems, two measurements are made. The
total or gross absorbance (Ameasured) of the atoms and
the nonatomic species is measured at the wavelength
of the resonance line emitted by the hollow cathode
lamp (HCL). The background attenuation (Abg) is then
subtracted from the measured absorbance to obtain the
analyte absorbance (Aanalyte) (Equation 1)
Aanalyte = Ameasured − Abg (1)
There are several ways in which the nonatomic
absorption at the resonance wavelength can be estimated.
2 DEUTERIUM-LAMP BACKGROUND
CORRECTION
2.1 Brief History of Background Correction
The first BC method was proposed by Willis(5), in which
a nonresonant analyte line emitted by the source that
is not absorbed by the analyte is used to measure the
nonspecific absorption in the vicinity of the analyte
absorption wavelength. In the two-line BC method, argon
or neon lines can also be used. Alternatively, another
source emitting radiation of an element that is not
found in the sample, can be inserted for the background
measurement. The background spectral character must
be known to be the same at the second line as it is at the
primary line, and therefore, the closer the second line is to
the primary line, the better will be the correction. These
conditions cannot be met for all elements and all matrices.
In some wavelength regions, the steep slope of molecular
absorption bands makes it difficult to find a nonabsorbing
line close enough to the analyte line to ensure accurate
BC. Two-line BC went into practice earlier than the other
methods but it has not widely been used.
To improve the sensitivity of FAAS, Koirtyohann and
Pickett(6) used a 40-cm long, 10-mm i.d. tube that was
heated in a hydrogen–oxygen flame, providing a much
longer effective absorption path than was previously used
in the more conventional burners. They observed that,
at higher sensitivities, absorption by matrix salts at the
wavelength of an elemental resonance line can cause
significant errors, and proposed to use a hydrogen lamp
to correct for matrix absorption. In fact, this BC system
is still widely used to correct for background absorption
in FAAS and ETAAS.
2.2 Principle of Deuterium-lamp Background
Correction
The gross absorbance Ameasured is the sum of the
atomic absorbance of the analyte atoms Aanalyte and
the background attenuation Abg. The gross absorbance
is measured with the narrow HCL emission line of
the element at the selected specific wavelength of
the resonance line. An estimate of the background
absorbance is obtained as an average over the spectral
band-pass of the monochromator using a CS of radiation,
e.g. a hydrogen lamp or D2-lamp in the ultraviolet (UV)
wavelength region or a tungsten-halogen lamp in the
visible part of the spectrum. The width of the atomic
absorption line is about 100 times smaller than the band-
pass of the monochromator, so that the analyte absorption
of the CS radiation can be neglected.
2.3 Instrumentation
For the correction of background attenuation, Koirty-
ohann and Pickett(6) used a dual-channel system in which
the radiation from an HCL was passed alternately with
the radiation of a continuum hydrogen lamp through
the flame. Nowadays, a D2-lamp is most frequently used
as the CS. Figure 3 shows the basic schematic config-
uration of a D2-lamp BC system for FAAS. By using
lamp pulsation or by means of a rotating chopper with a
sector mirror, the radiation from the HCL with intensity
I 0
HCL and the radiation from the D2-lamp with inten-
sity I 0
D2
is passed alternately through the atomizer. After
passing the monochromator, both radiation beams reach
the same detector and the net atomic absorbance is
derived.
The operation of the D2-lamp BC system is explained
in Figure 4(a–f). The monochromator separates the
resonance line from the emission spectrum of the
element-specific HCL. The width of this line is a
few picometers. From the D2-lamp emission spectrum,
the monochromator isolates a band equivalent to
the spectral band-pass, usually 0.2–1 nm. The primary
0
2
IHCL
IHCL
ID 2
ID
0
Figure 3 Schematic configuration of a D2-lamp BC system.
Radiation from the HCL (IHCL) is passed alternately with
radiation from the D2-lamp (ID2 ).
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This article is © 2013 John Wiley & Sons, Ltd.
This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd.
DOI: 10.1002/9780470027318.a5104.pub2
4 ATOMIC SPECTROMETRY
In
te
ns
ity
In
te
ns
ity
Hollow cathode lamp
Lamp
emission
D2-lamp
(a)
(b)
Spectral
band-pass1 nm
IHCL
IHCL
ID
ID
0 0
2
In
te
ns
ity
In
te
ns
ity
(c)
In
te
ns
ity
In
te
ns
ity
A
bs
or
ba
nc
e
A
bs
or
ba
nc
e
(d)
Atomic
absorption
In
te
ns
ity
A
bs
or
ba
nc
e
A
bs
or
ba
nc
e
In
te
ns
ity
(e)
Background
absorption
In
te
ns
ity
A
bs
or
ba
nc
e
A
bs
or
ba
nc
e
In
te
ns
ity
(f)
2
Atomic +
background
absorption
Figure 4 Mode of operation of a D2-lamp BC system. (a) The
line emission spectrum of the HCL and the continuum emitted
by the D2-lamp. (b) The monochromator isolates the resonance
line from the spectrum of the HCL and passes a band of
radiation from the D2-lamp corresponding with the spectral
band-pass. (c) The intensities I 0
HCL and I 0
D2 are equalized.
(d) For atomic absorption by the analyte element, I 0
HCL is
attenuated by an amount corresponding to its concentration,
whereas I 0
D2 is not significantly attenuated. (e) Broadband
background attenuates the intensity of both sources to the
same degree. (f) Atomic absorption by the analyte in addition
to the background attenuates the HCL intensity further as in
(d), whereas the D2-lamp intensity is not further attenuated.
(Reproduced with permission from Wiley-VCH from Ref. 7.)
intensities of the two beams of radiation are equalized
to obtain I 0
HCL = I 0
D2
. When analyte atoms absorb
HCL radiation, the intensity I 0
HCL decreases with
an amount corresponding to the atom concentration.
Naturally, I 0
D2 is also attenuated at the resonance
wavelength, but since the half-width of the atomic
absorption line is about 5 pm, whereas the continuum
has a width equal to the spectral band-pass of 0.2–1 nm,
the absorption of continuum radiation by the analyte
atoms can be neglected. If nonspecific attenuation of the
radiation occurs, whether through scattering or molecular
absorption, both radiation beams will be attenuated to the
same extent because background attenuation is usually a
broadband phenomenon. In D2-lamp BC, it is assumed
that the background is constant over the observed spectral
range, i.e. the attenuation of I 0
D2 is measured as the
average over the spectral band-pass and used for BC at
the wavelength position of the analyte absorption line. If
the background attenuation is not continuous within the
observed spectral range, positive or negative BC errors
will occur.
The two lamps are observed by the detector alternately
in time. Instrument electronics separate the signals and
compare the absorbances from both sources. A net
absorbance will be displayed only when the absorbance of
the two lamps differs. Since the continuous background
attenuates both sources equally, it is ignored. Analyte
atoms absorb the HCL radiation and negligibly absorb the
broadband CS emission, and it is therefore still measured
and displayed as usual without BC. Most modern AAS
instruments permit almost simultaneous measurement
of the uncorrected signal or background signal and the
corrected signal. The speed of the BC systemshould be
high enough to follow the fast transient signals in ETAAS
(see Section 6.2).
BC with a CS has some disadvantages. The alignment
and superposition of the HCL or electrodeless discharge
lamp (EDL) and the CS are critical. The lamps are
located at different positions in the instrument, the
shape of the beams is different, and the intensity
distribution in the beams is not the same. Even if it is
possible to align the beams exactly, different absorption
volumes may be irradiated. The application of two light
beams requires a more complicated optical system. In
many instruments, double-beam operation is available
to compensate for intensity changes of the lamps. The
D2-lamp emits sufficient radiation in the short wavelength
range from 190 to 330 nm, whereas a halogen lamp is
used for BC above this range. In general, the signal-
to-noise ratio (S/N) is worsened through the use of
two radiation sources and a more complicated optical
system is used to measure their intensities. If a D2-lamp
is used outside its optimum range, the radiant intensity
of the line source must be reduced, leading to further
deterioration of the S/N. Finally, it should be noted
that the principle of operation of D2-lamp BC limits
Encyclopedia of Analytical Chemistry, Online © 2006–2013 John Wiley & Sons, Ltd.
This article is © 2013 John Wiley & Sons, Ltd.
This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd.
DOI: 10.1002/9780470027318.a5104.pub2
BACKGROUND CORRECTION METHODS IN ATOMIC ABSORPTION SPECTROMETRY 5
its successful use to continuous background absorption
only.
2.4 Analytical Performance and Applications
Many commercial AAS instruments are based on
D2-lamp BC. In FAAS and hydride generation AAS, it
is the most widely applied BC system. It is fully adequate
for all applications of FAAS, except in some very
unusual circumstances. During methods development,
BC is considered. In many FAAS applications, there
is no need to use BC at all. In general, in the
lower UV wavelength region (<230 nm), BC is required
for compensation of scattering and absorption by the
flame.
BC systems employing CSs are incapable of correcting
background attenuation caused by electronic excitation
spectra because these comprise many narrow lines. The
actual BC that is required depends on the degree
of overlap between the elemental spectral line and
individual molecular rotational lines. An example is the
absorption line of Au at 267.6 nm, which lies exactly in the
middle between two rotational lines of indium chloride
so that the actual background attenuation is much lower
than the mean background absorbance found over the
spectral range. Several molecular absorption spectra have
been studied in the vicinity of atomic absorption lines.(1,8)
Interferences have been reported for the determination
of Bi at 306.8 nm and Mg at 285.2 nm through absorption
bands of OH in an air–acetylene flame and also for
Fe at 247.3 nm, Pd at 244.8 and 247.6 nm, and Yb at
246.4 nm owing to PO molecular absorption.(8) Many
of the possible interferences are very unlikely to be
serious in FAAS. Although problems with D2-lamp BC
in FAAS seldom occur, CS BC is not free from pitfalls.
The limited energy of the D2-lamp lowers the S/N and
the correction becomes inaccurate when the background
is structured.
In the graphite furnace, the amount of matrix is
typically much larger relative to the analyte and
interferences can be more severe. Spectral interference
in ETAAS with D2-lamp BC may result from atomic
absorption of the CS radiation by massive amounts of
metals in the matrix, usually at secondary absorption lines
of these elements. Probably, the first identified example
of this type of BC error has been reported by Manning,
for the determination of Se at the 196-nm line in the
presence of iron.(9) Potential interferences of this type
can be predicted based on the wavelengths of atomic
absorption lines. However, during method development,
much time and effort are usually spent in obtaining
an acceptable program for heating of the graphite
furnace, and the application of chemical modifiers is
also considered. Owing to the improved graphite furnace
design, the optimization of the temperature program,
and the application of chemical modifiers, potential
interferences, and BC errors are not always found in
real analysis.
BC errors are hard to identify when the background
absorbance takes place simultaneously with the atomic
absorption. If there is some time delay between the two
processes, a positive or negative disruption of the baseline
may be observed just before the atomic absorption starts
or just after the atomic absorption is finished. It should
be noted that the method of standard additions does not
control BC errors.
Most ETAAS users distrust their results when the
background level exceeds 0.5 absorbance unit. The
limitations are probably related to the different geometry
and possible misalignment of the optical beams of the
HCL and the D2-lamp. The increasing interest in ETAAS
as a selective and highly sensitive analytical method has
stimulated the development of improved BC systems.
3 ZEEMAN-EFFECT BACKGROUND
CORRECTION
3.1 Zeeman Effect
In 1897, the Dutch physicist Pieter Zeeman(10) (Figure 5)
reported the observation that if a source of light is
placed between the poles of a magnet, the lines appear
to broaden and, if the magnetic field is strong enough
and the resolving power of the spectroscope is sufficiently
high, they appear to be split into a number of components.
The first splitting patterns studied by Zeeman follow from
lines of singlet series. A single line was split into three
polarized components.
Both splitting and polarization were explained by
Lorentz with the classical electromagnetic theory. A
few years later, it was demonstrated that the lines of
other series do not split into the simple triplet but
to a group of four or more components. The classical
theory was unable to account for this. Therefore, the
two types of splitting were labeled the normal and the
anomalous Zeeman effects. An example of the normal
Zeeman effect is presented in Figure 6. Figure 6(a) refers
to the normal Zeeman effect, and Figure 6(b) shows an
example of the anomalous Zeeman effect.(11) The splitting
pattern is always symmetrical around the position of the
nondisplaced line. The components polarized parallel to
the magnetic field are referred to as π-components, and
the components polarized perpendicular to the magnetic
field are referred to as σ-components (σ deriving from
the German senkrecht). In the case of a normal Zeeman
triplet, the π-component is located at the position of
the original line at frequency ν, and two σ-components
Encyclopedia of Analytical Chemistry, Online © 2006–2013 John Wiley & Sons, Ltd.
This article is © 2013 John Wiley & Sons, Ltd.
This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd.
DOI: 10.1002/9780470027318.a5104.pub2
6 ATOMIC SPECTROMETRY
Figure 5 Dr Pieter Zeeman (1865–1943). Dutch physicist,
who discovered the influence of a magnetic field on radiation in
1896.
are found at frequencies ν ± �ν. Such a Zeeman
splitting is observed from the direction perpendicular
to the magnetic field (transverse Zeeman effect). In the
direction of the magnetic field, the central component
is not observed, whereas the two displaced components
are circularly polarized and rotate in opposite directions
(longitudinal Zeeman effect). Lines that do not belong
to a singlet series split into more than three components.
Their pattern is always symmetrical with respect to the
position of the original line, with a central group of π-
components and groups of σ-components on either side
as illustrated in Figure 6(b) for a 2S1/2 –2P3/2 transition.
All Zeeman splitting patterns are observed in absorption
as well as in emission.
The sum of the intensities of the π-components is equal
to the sum of the intensities of the σ-components,and
if the various components are combined, the resulting
beam is nonpolarized and the intensity is equal to the
π
π
σ σ
σ
(a)
(b)
σ
1P1
1S0
Zero field 1 T
0
−1
0
+1
MJ
MJ
+3/2
+1/2
+1/2
−1/2
−1/2
−3/2
2P3/2
2S1/2
Zero field 1 T
Figure 6 Magnetic splitting of atomic energy levels and the
corresponding Zeeman splitting patterns. (a) 1S0 –1P1 transition,
normal Zeeman effect; (b) 2S1/2 –2P3/2 transition, anomalous
Zeeman effect. (Reprinted from Ref. 11. Copyright with
permission from Elsevier Science, 1985.)
intensity observed without the magnetic field. The energy
states generated around an original energy level E0 are
described by Equation (2)
E = E0 + μBB MJ g (2)
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DOI: 10.1002/9780470027318.a5104.pub2
BACKGROUND CORRECTION METHODS IN ATOMIC ABSORPTION SPECTROMETRY 7
where μB is the Bohr magneton (9.274 × 10−24 J T−1),
B is the magnetic flux density (T), MJ is the magnetic
quantum number, and g is the Landé factor. Both
emission and absorption transitions between energy levels
are governed by the selection rule �MJ = 0,±1 where
�MJ = 0 represents the π-components and �MJ = ±1
the σ-components. The displacement of each Zeeman
component is proportional to the strength of the magnetic
field. In a magnetic field of 1 T, the displacement of the
σ-components in the normal Zeeman effect is 0.467 cm−1,
which is equal to 4 pm at a wavelength of 300 nm. The
normal Zeeman effect is shown by Be 234.9 nm, Mg
285.2 nm, Ca 422.7 nm, Sr 460.7 nm, Ba 553.6 nm, Cd
228.8 nm, and Zn 213.9 nm. Several lines that belong to
a triplet series also split into one π-component and two
σ-components. However, the shift of their σ-components
varies from half to twice the shift in the normal Zeeman
effect, e.g. Ge 265.2 nm (1.5), Hg 253.7 nm (1.5), Pb
217.0 nm (0.5), Pb 283.3 nm (1.5), Pd 244.8 nm (1.0), Si
251.6 nm (1.5), and Sn 286.3 nm (1.5). All other transitions
used in AAS show anomalous Zeeman splitting patterns
that consist of at least two π-components and at least
two σ-components. The Zeeman splitting pattern in
Figure 6(b) is observed for Ag 328.1 nm, Cu 324.8 nm,
Au 242.8 nm, Na 589.0 nm, and K 766.5 nm. Zeeman
splitting patterns of the absorption lines used in AAS were
classified into several groups by Koizumi and Yasuda.(12)
Figure 7 shows the Zeeman splitting patterns for various
transitions that are used in AAS.
3.2 Principle of Zeeman-effect Background Correction
In AAS, the nonshifted or slightly shifted π-components
are used to measure the total absorbance of the analyte
atoms and the background, whereas the shift of the
(groups of) σ-components from the original line position is
used to measure the background absorbance. Figure 8(a)
shows the resonance line emitted by the HCL and the
absorption line of the atoms in the graphite furnace
at zero magnetic field strength. Note that the width
of the absorption line is about twice the width of the
lamp emission line owing to the higher temperature and
higher pressure in the atomizer compared to those in the
HCL.
Figure 8(b) shows the situation where the graphite
furnace is in a transverse magnetic field of 1 T. Absorption
takes place within the π-components of the absorption
line profile ka
π, whereas the σ-components of the
absorption line profile ka
σ are shifted away to wavelengths
λ1 and λ2. When a π-component-rejecting polarizer is
positioned in the optical beam and when the magnetic
field is switched on and off at high enough frequency,
e.g. a 50 Hz alternating current (AC) magnetic field, the
gross absorbance and the background absorbance can be
measured alternately. The net analyte absorbance follows
then from Equation (1). The background absorbance kb
is measured exactly at the wavelength of the analyte
resonance line λa and within the width of the HCL
emission line profile. Consequently, the background
absorbance is measured correctly for any type of
background, even if it is highly structured. The alternative
approach is to use a constant magnetic field from a
permanent magnet or a direct current (DC) magnet and
to rotate the polarizer to permit alternate measurements
with π-polarized radiation and σ-polarized radiation.
The π-polarized radiation is then absorbed by the π-
component of the analyte absorption line profile ka
π
and the background kb, providing the gross absorbance.
The σ-polarized radiation is absorbed by the background
only because the σ-components of the analyte absorption
line profile are shifted to wavelengths λ1 and λ2, and
they are too far away to absorb the lamp emission line.
Combination of the two measurements again provides
the net analyte absorption.
Another possibility is to use a longitudinal magnetic
field rather than a transverse magnetic field. Figure 8(c)
shows that in the direction of the magnetic field, only
the circularly polarized σ-components of the analyte
absorption line profile ka
σ+ and ka
σ− are observed. In
a longitudinal AC-modulated magnetic field, the gross
absorbance is measured at zero field (see Figure 8a) and
the absorbance of the background is measured at exactly
the wavelength of the lamp emission line during the
magnet on period. A longitudinal AC Zeeman-effect BC
system does not require a polarizer and therefore the full
intensity of the HCL resonance emission line can be used
in both measurements.
In all AAS instruments using Zeeman-effect BC, two
alternate intensity measurements are performed. One
refers to the unshifted zero-field analyte absorption line
(in an AC field) or the slightly shifted and broadened
π-components (in a constant field). The intensity is given
by Equation (3)
I1 = I 0
1 exp(−ka
1) exp (−kb
1) (3)
where I 0
1 is the incident intensity, kb
1 is the absorption
coefficient of the background, and ka
1 is the analyte
absorption coefficient. The other measurement refers to
the situation when the σ-components are shifted away
from the original line position (AC field) or when σ-
polarized radiation is used (constant field). The intensity
is given by Equation (4)
I2 = I 0
2 exp(−ka
2) exp(−kb
2) (4)
where I 0
2 is the incident intensity, kb
2 is the absorp-
tion coefficient of the background, and ka
2 is the
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8 ATOMIC SPECTROMETRY
1S0−1P1 4S3/2−4P1/2
4S3/2−4P3/2
3P0−3P1, 
1S0−3P1, 
3P2−3P2
2S1/2−2P3/2
2S1/2−2P1/2
3F4−3G5
6S5/2−6P7/2
Be ∗2348.6 Å
Mg
Ca
Sr
Ba
∗2852.1
∗4226.7
∗4607.3
∗5535.5
Zn ∗2138.6
Cd ∗2288.0
Li ∗6707.8
Na ∗5890.0
5895.9
K ∗7664.9
7699.0
Rb ∗7800.2
7947.6
As 1972.0 Å
∗1937.0
Sb 2311.5
∗2175.8
Bi 3067.7
∗2230.6
Si ∗2516.1 (3P2−3P2)
Ge∗2651.6 (3P0−3P1)
Sn 2863.3 (3P0−3P1)
Pb ∗2833.1 (3P0−3P1)
Zn 3075.9 (1S0−3P1)
Hg ∗2536.5 (1S0−3P1)
Cd 3261.0 (1S0−3P1)
Cs ∗8521.1
8943.5
Cu ∗3247.5
3274.0
Ag ∗3280.7
3382.9
Au ∗2428.0
2676.0
Hg 1849.0
Ni ∗2320.0 Å
Al ∗3092.7 (2P3/2−2D5/2) (4)
Co ∗2407.3 (4F9/2−4G11/2) (10)
Fe ∗2483.3 (5D4−5F5) (9)
Rh ∗3434.9 (4F9/2−4G11/2) (10)
V ∗3184.0 (4F7/2−4G9/2) (8)
Ru ∗3498.9 (5F5−5G6) (11)
Y ∗4102.4 (2D5/2−2F7/2) (6)
Zr ∗3601.2 (3F4−3G5) (9)
Cr ∗3578.7 (7S3−7P4) (7)
Mo∗3132.6 (7S3−7P4) (7)
Se ∗1960.3 (3P2−3S1) (3)
Te ∗2142.8 (3P2−3S1) (3)
Mn∗2794.8
Mn 2798.3 (6S5/2−6P5/2) (6)
Mo 3170.4 (7S3−7P3) (6)
Cr 3593.5 (7S3−7P3) (6)
3601.2
Similar patterns
Similar patterns
Zr
Number of fine lines
of the π-component
( ):
Figure 7 Classification of Zeeman patterns for spectral lines of various elements. *indicates lines most frequently used in AAS.
10 Å = 1 nm. (Reprinted from Ref. 12. Copyright with permission from Elsevier Science, 1976.).
remaining analyteabsorption coefficient of the shifted
σ-components, which should be close to zero. A simple
subtraction of the absorbances measured in both situa-
tions corresponds to taking the log ratio of the intensities
(Equation 5).
ln(I2/I1) = ln(I 0
2/I
0
1) + (ka
1 − ka
2) + (kb
1 − kb
2) (5)
This expression forms the basis for all Zeeman-effect
BC systems in AAS. When the incident intensities I 0
1 and
I 0
2 are equal and when the same background attenuation
is measured, the net analyte absorption follows from
Equation (6)
A = log(I2/I1) = 0.43(ka
1 − ka
2)∝. NA (6)
It is clear that a linear analytical response proportional
to the number of analyte atoms NA is obtained. The
assumption that I 0
1 is equal to I 0
2 is quite reasonable
because it is the same beam of radiation in case of
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BACKGROUND CORRECTION METHODS IN ATOMIC ABSORPTION SPECTROMETRY 9
kb
(a) λ1 λa λ2
ka
IHCL
In
te
ns
ity
A
bs
. c
oe
ffi
ci
en
t
kb
(b) λ1 λa λ2
ka
IHCLIn
te
ns
ity
A
bs
. c
oe
ffi
ci
en
t
σka
πka
σ
kb
ka
σ+ ka
IHCL
σ−
(c) λ1 λa λ2
A
bs
. c
oe
ffi
ci
en
t
In
te
ns
ity
Figure 8 Profiles of the HCL emission line (IHCL) and the absorption coefficient of the analyte (ka) at wavelength λa in the
presence of background absorbance (kb) for the situation of a normal Zeeman effect. (a) Conventional AAS, no magnetic field;
(b) in the presence of a transverse magnetic field; (c) in the presence of a longitudinal magnetic field.
the AC magnetic field or the π-polarized and the σ-
polarized parts of the same beam in case of the constant
magnetic field. The background absorbance is measured
within the analyte emission line profile in all Zeeman-
effect BC systems with the magnetic field applied to the
atomizer. The only requirement is that the background is
not influenced by the magnetic field when an AC magnet
is used, which is usually the case (see Section 3.4). From
Equation (6), it can be concluded that the σ-components
in the Zeeman splitting pattern should be shifted as
far away as possible from the original line position to
reduce ka
2 to a minimum, i.e. a strong magnetic field
should be applied, especially when the analyte absorption
line is broad by nature or when its Zeeman splitting
pattern is relatively narrow. In contrast, maximum
sensitivity also requires that ka
1 is large, i.e. close to
the conventional AAS absorption coefficient. Optimum
sensitivity is obtained when a strong AC magnetic field is
used. If a constant magnetic field is applied, the optimum
magnetic field strength compromises a large shift for
the σ-components simultaneous with minimum shift in
the π-components. Even at optimum field strength, the
sensitivity is significantly reduced by about a factor of
two for many of the analyte absorption lines that show
the anomalous Zeeman effect. This explains why the
AC-modulated magnetic field is preferred in many of the
commercially available AAS instruments with Zeeman-
effect BC.
In principle, Zeeman-effect BC systems can have the
magnetic field either around the atomizer or around the
primary source of radiation. When the primary source
of radiation is in the magnetic field, σ-components of
the lamp emission line are located at the wavelength
positions λ1 and λ2. These lines are then used to measure
the background absorbance at both sides of the analyte
atomic absorption line. Equations (3–6) apply also to this
situation.
3.3 Instrumentation
The application of the Zeeman effect for BC in AAS was
proposed by Prügger and Torge(13) in 1969. The authors
claimed that a shift of the primary source resonance line of
about 10 pm is required to allow successive measurements
of the gross AAS signal and the background absorbance
and to propose a system based on magnetic splitting of the
source line in a DC or AC magnetic field. However, AAS
instruments based on this type of Zeeman-effect BC have
not been developed. The main reason is that the HCLs
and EDLs traditionally used in AAS do not fit into the gap
of a magnet that has to provide a magnetic field strength
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10 ATOMIC SPECTROMETRY
close to 1 T at reasonable size, power requirements, and
price. Specially designed lamps are required for stable
operation in a magnetic field.
The more useful alternative is to apply the magnetic
field to the atomizer, as discussed in the previous
section. This approach has been followed by Koizumi
and coworkers incorporating a 1.1-T permanent magnet
around a metal atomizer or a graphite furnace atomizer
respectively.(12,14) The plane of polarization is rotated
at 100 Hz to provide for BC, whereas the intensity of
the HCL radiation is modulated at 1.5 kHz to eliminate
the signal caused by emission of radiation from the
atomizer. Several laboratories have built and tested
ETAAS instruments with Zeeman-effect BC(15 – 18) and
since that time, instrument companies have marketed
ETAAS instruments with Zeeman-effect BC. Only one
FAAS instrument has become available, with a 0.95-T
field of a permanent magnet applied to a conventional
10-cm burner.(19) The Zeeman-effect BC system for
FAAS is complicated and, in general, there is no need
for it.
The transverse Zeeman-effect BC system presented
in Figure 9(a) is marketed by various manufacturers
and most of them use an AC-modulated magnetic field
of 0.8–1.0 T in combination with a fixed polarizer
to reject π-polarized radiation and transmit only σ-
polarized radiation. The longitudinal Zeeman-efffect BC
system presented in Figure 9(b) has been described by
Liddell and Brodie,(17) De Loos-Vollebregt et al.,(20)
and Shuttler(21) and is also commercially available. A
transverse heated atomizer is placed in the longitudinal
Hollow
cathode
lamp
Hollow
cathode
lamp
Polarizer
(a)
(b)
Magnet
Graphite
furnace
Magnet
Magnet
Graphite
furnace
Monochromator,
detector
Monochromator,
detector
Figure 9 Schematic diagrams of different Zeeman-effect BC
systems for AAS. (a) Transverse magnetic field, i.e. the magnetic
field is perpendicular to the optical axis of the spectrometer. The
polarizer provides alternately π- and σ-polarized radiation if the
magnetic field is constant, or transmits only σ-polarized radiation
if an AC-modulated magnetic field is applied. (b) Longitudinal
magnetic field oriented parallel to the optical axis of the
spectrometer. No polarizer is required in this configuration.
magnetic field and there is no polarizer in the optical
system (see Section 3.2). This gives a considerable
improvement in light throughput and simplifies the optical
system so that detection limits are lower although the
length of the graphite furnace is somewhat compromised.
The tube has to fit in the gap of the magnet and the optical
beam has to pass through holes drilled in the core of the
magnet.
3.4 Analytical Performance and Applications
Zeeman-effect BC is nowadays widely used in ETAAS.
The BC is performed exactly at the wavelength of
the analyte atomic absorption line. High nonspecific
absorbance (up to about two) can be corrected and in
the AC Zeeman system, sensitivities are similar to those
obtained in conventional AAS. Nevertheless, there are
a few elements that show somewhat reduced sensitivity
because their lines are relatively broad and/or the Zeeman
splitting pattern is relatively narrow. An overview of the
early applications of Zeeman-effect BC shows that a
wide variety of elements have been measured in different
samples and certified reference materials,(11) and in recent
years, many applicationsfollowed. The relatively volatile
elements, e.g. Pb and Cd, have been measured by many
analytical chemists using ETAAS with Zeeman-effect
BC. This is not surprising because in the determination
of volatile elements, the pyrolysis temperature is not high
enough to remove most of the matrix compounds that
are responsible for the background absorption during
atomization. Pb and Cd are often determined in a
rather difficult matrix, such as blood and urine, and
the low concentrations of the analyte elements do not
permit dilution of the samples. The results obtained
with Zeeman-effect BC have been compared with those
obtained with D2-lamp BC, looking at the baseline of
the net AAS signals. Zeeman-effect BC is often superior,
although a fair comparison would require that the same
atomizer and atomization conditions are used. There
are an increasing number of solid sampling applications
where the Zeeman-effect BC system is the key to
success.
In general, the dynamic range is slightly limited when
Zeeman-effect BC is used. In a strong magnetic field,
the σ-components are shifted almost completely away
from the absorption line profile so that, according
to Equation (6), the shape of the AC Zeeman AAS
analytical curve is completely determined by the zero-
field absorbance. The analytical curve will therefore
be very similar to the corresponding conventional
AAS analytical curve measured without BC. The
relatively small contribution of the σ-components to
the measurement of the background has only a minor
influence on the slope and the curvature of the AC
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BACKGROUND CORRECTION METHODS IN ATOMIC ABSORPTION SPECTROMETRY 11
Zeeman AAS analytical curve. This is equally valid for the
normal Zeeman effect and the anomalous Zeeman effect.
The same is essentially true when a constant magnetic
field is used as long as the spectral line displays a normal
Zeeman effect. For the majority of the transitions that
show an anomalous Zeeman effect, the splitting in the
π-components broadens the absorption line profile and
therefore the linearity of the Zeeman AAS analytical
curve is somewhat improved, although the slope is
lower.
At very high analyte concentrations, all the ETAAS
signals show a dip (see Figure 10) when Zeeman-
effect BC is applied, which can be explained from the
phenomenon that is known as roll-over. From the very
early days when Zeeman-effect BC was used, rumors
were around that the analytical curve first increases,
then reaches the maximum absorption level at relatively
high analyte concentration, and beyond the maximum,
decreases again toward the concentration axis. Such
a double-valued behavior has indeed been reported
for FAAS systems with Zeeman-effect BC.(19) From
a theoretical analysis of the roll-over phenomenon,(11)
it became clear that the nonabsorbed radiation that
is emitted by the primary source and reaches the
detector is responsible for this. With increasing analyte
concentration, the zero-field absorbance or π-component
absorbance, ka
1 in Equation (6), increases much more
rapidly than the σ-component absorbance, ka
2. With
increasing analyte concentration, the ka
1 curve levels
off. The σ-component absorbance is subject to the same
curvature, but at much higher concentration. At a certain
concentration, a situation is reached where the ka
1 and ka
2
curves show the same rate of increase, or equal derivative
with respect to concentration. At that concentration,
the difference signal according to Equation (6) shows
a zero slope, i.e. the analytical curve reaches a maximum.
Thereafter, a FAAS analytical curve would bend back to
the concentration axis. The absorbance and concentration
of roll-over are indicated in Figure 10, where the dotted
line illustrates the decreasing absorbance values. The
1.0
2.0
A
bs
or
ba
nc
e
2.0 3.01.0
Roll-over 
absorbance
Integrated absorbance
Peak absorbance
Dip level
Cu, concentration (mg L−1)
σ
σ
σ
Figure 10 Analytical curve for Cu 324.8 nm measured in
ETAAS with Zeeman-effect BC at magnetic field strength 0.8
T. The peak absorbance reaches a maximum at high analyte
concentration, whereas the integrated absorbance increases
slowly beyond the roll-over absorbance.
height and the position of the maximum depend on
the strength of the magnetic field and the amount of
nonabsorbed source radiation (so-called stray light). A
list of typical roll-over absorbances for ETAAS has been
published by Slavin and Carnrick.(22)
Figure 10 shows the net ETAAS analyte absorbance
signals. At lower analyte concentrations, the signals are
similar to those obtained with D2-lamp BC. However,
beyond the concentration where the maximum peak
absorbance is reached, the signals show a dip. The
dip becomes deeper with increasing concentrations. In
peak absorbance, the analytical curve levels off when the
absorbance of roll-over is reached, whereas in integrated
absorbance, the analytical curve continues to increase but
the slope becomes much smaller. It is clear that there is no
risk of misunderstanding the measured peak absorbance
or integrated absorbance. At high analyte concentrations,
the signals are analytically useless but no double-valued
behavior is observed in ETAAS. In FAAS, a warning
system for roll-over is used that is based on the detection
of the short periods of high absorbance at the start of
nebulization and at the end, where the atom density is
increasing beyond the value that corresponds with the
roll-over concentration. Detection of these peaks alerts
the analyst that sample dilution is necessary to obtain the
correct concentration values.(19)
Zeeman-effect BC systems allow the extension of the
analytical curve by taking measurements at three different
field strengths, i.e. zero, intermediate, and maximum field
strength.(23,24) The analytical curves derived from three-
field measurements allow the sensitivity to be decreased
so that high analyte concentrations otherwise measured
by FAAS can be determined by ETAAS. The three-field
operation provides up to about 10-fold extension of the
dynamic range, which is advantageous for direct solid
sampling ETAAS.
A few problems with Zeeman-effect BC have been
reported. Molecules such as OH, NO, NO2, and SO2
indeed show a Zeeman effect. Therefore, the background
measured in two different polarization planes (with
constant magnetic field) or measured with and without
a magnetic field present (with AC-modulated magnetic
field) may differ.(22,25 – 27) If matrix absorption lines or
molecular bands very close to the analyte wavelength
exhibit a Zeeman splitting, correction errors may occur.
A matrix absorption line that does not overlap with
the lamp emission line at zero-field strength may do so
at maximum field strength or the other way around.(22)
Massmann(25) reported a BC error for Zeeman-effect
BC in FAAS from OH rotational lines at Bi 306.8 nm,
using a DC magnetic field applied to the acetylene–air
flame. However, in ETAAS, such OH interferences do
not generally occur.
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12 ATOMIC SPECTROMETRY
Wibetoe and Langmyhr(26) systematically investigated
spectral interferences and BC errors. They have surveyed
atomic line tables to find pairs of elements that
have contiguous lines, and found several cases of
possible spectral interferences at recommended analyte
wavelengths, e.g. the 459.4 nm Eu line (interference from
V and Cs), the 247.6 nm Pd line (interference from Pb),
and the 265.9 nm Pt line (interference from Eu).
Overcorrection problems for Pb caused by Zeemansplitting of the PO molecular absorption band has
been discussed by Zong et al.(27) Phosphate causes an
overcorrection for Cd at the alternate 326.1 nm line.
BC errors have also been reported for determination
of Fe at the alternate 271.9 nm line in the presence
of Pt and for determination of Hg at 253.6 nm in the
presence of Co.(22,26) The observed BC errors depend
on the strength of the magnetic field and they can be
diminished or avoided by optimization of the temperature
program and modifier selection. Moreover, it should
be noted that many listed interferences do not take
place at the recommended analyte absorption line or
only when the analyte is determined in a very unusual
matrix.
4 PULSED-LAMP BACKGROUND
CORRECTION
4.1 Principle
Ling(28) first advanced the idea of BC based on the
subtraction of the absorbances measured alternately with
a normal lamp emission line and a broadened lamp
emission profile showing a dip in the center. The author
described a simple method to correct for nonatomic
absorption in a portable mercury photometer that
included two mercury vapor lamps emitting the mercury
resonance line broadened to different extents. Smith and
Hieftje(29) proposed a method for BC in AAS based on the
broadening, which occurs in an HCL emission line when
the lamp is operated at very high currents. Under such
conditions, the absorbance measured for a narrow atomic
line is low, whereas the apparent absorbance caused by a
broadband background contributor remains the same as
when the lamp is operated at conventional current level.
BC can therefore be effected by taking the difference
in absorbances measured with the lamp operated at
high and low currents. The so-called Smith–Hieftje BC
system applies its correction very near the atomic line of
interest.
4.2 Instrumentation
The HCL power supply and drive circuitry(29) generate
a 9-ms low-current pulse of 5–10 mA, followed by a
La
m
p 
cu
rr
en
t
0 50 100
Time (ms)(a)
Wavelength
λa
In
te
ns
ity
(b)
Low current
High current
Figure 11 Pulsed-lamp BC. (a) Modulation of the lamp
current that drives the HCL. (b) Corresponding lamp emission
line profiles generated under low- and high-current operation.
0.3-ms high current pulse of 200–300 mA, as presented
in Figure 11(a). A total pulse cycle repetition time
of 50 ms allows the atomic cloud generated in an
HCL during the high-current mode to clear before
measurement is made again in the low-current mode
for a broad range of elements. Figure 11(b) presents
the conventional lamp emission line profile observed at
low lamp current and the broadened and self-reversed
lamp emission line during the high-current operation
of the lamp. It is clear that the narrow line profile
emitted under low-current operation is affected equally
by atomic absorption or a broadband spectral feature. In
contrast, the broadened and self-reversed profile obtained
at high current is affected by a broadband absorber or
scatterer but there is not so much overlap with the atomic
absorption line profile. Absorbances calculated under
low- and high-current operation are subtracted to yield a
difference value that is free of any broadband background
contribution.
4.3 Analytical Performance and Applications
The pulsed-lamp BC method is equally applicable to
FAAS, hydride generation AAS, and ETAAS. For
effective operation, it is necessary to broaden appreciably
the emission line of the element under investigation.
However, the degree of broadening is different for each
element and also depends on the peak current at which
the HCL is driven. To achieve the desired degree of
line broadening and line reversal while maintaining HCL
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BACKGROUND CORRECTION METHODS IN ATOMIC ABSORPTION SPECTROMETRY 13
reliability and acceptable lifetime, a special design of
the lamp modulation circuitry and optimum high lamp
current adjustment for each element are required. The
slope of the analytical curves is decreased to some extent
when pulsed-HCL BC is applied. The measurement at
high lamp current is intended to measure the background.
Unfortunately, the broadening of the lamp emission line
profile is limited and self-reversal is not complete. The
remaining radiation in the central part of the lamp
emission line profile is therefore absorbed by analyte
atoms and the corresponding absorbance is subtracted
from the gross absorbance together with the background.
The loss of sensitivity ranges from about 13% for Cd to
84% for Hg.(29) Similarly to the Zeeman-effect BC system,
the analytical curves show roll-over at high analyte
concentrations and ETAAS signals show a dip at very high
analyte concentrations.(30,31) The ability of the pulsed-
HCL BC system to overcome background interferences
in real samples has been demonstrated. Similarly to the
Zeeman approach, the pulsed-lamp BC method requires
only a single primary source of radiation and single beam
optics so that optical alignment is simplified.
5 BACKGROUND CORRECTION IN
HIGH-RESOLUTION CONTINUUM
SOURCE ATOMIC ABSORPTION
SPECTROMETRY
High-resolution continuum source atomic absorption
spectrometry (HR-CS AAS) has been discussed in
detail in High-resolution Continuum Source Atomic
Absorption Spectrometry – Theory and Applications.
The broadband spectral distribution of a CS conve-
niently permits observation of the background spectrum
in the vicinity of the analyte absorption line. Correc-
tion for background with a flat spectral distribution is
based on simultaneous intensity measurements outside
the analysis line. The analyte atomic absorption is typi-
cally measured in 3–5 pixels of a linear charge-coupled
device detector, at the position of the analysis line.
Intensities measured at other selected pixels are used
for BC but also for compensation of lamp fluctuations
and emission of radiation from the atomizer. After
correction for continuous background absorption, the
spectrum may still be disturbed by nearby atomic absorp-
tion of concomitant elements or molecular absorption
with rotational line structure. Such background may
arise from the atomizer, e.g. the background absorp-
tion by OH, NH, and NO in FAAS (Figure 1). The
interfering molecular spectrum can be measured with
a blank flame and subtracted from the sample spec-
trum using a least squares-based BC approach. Molecular
absorption caused by the sample matrix or modifier is
more common in ETAAS, e.g. molecular absorption by
PO as discussed in Section 3.4 or SO2 (Figure 2). Only
few diatomic molecules with relatively high dissociation
energy have been observed during the atomization step.
The CS, NO, and PO absorption spectrum can be created
from H2SO4, HNO3, and NH4H2PO4, respectively. The
software should select the appropriate reference spec-
trum or combination of spectra. An example of correction
for structured molecular background in HR-CS AAS is
shown in Figure 12 for the determination of Sb in sedi-
ment reference material at the less-sensitive resonance
line at 231.147 nm, using direct solid sampling and elec-
trothermal atomization.(32)
HR-CS AAS shares with the Zeeman-effect and
pulsed-lamp BC correction systems the advantage of
using only a single light source, so that lamp alignment is
not so much of a problem as in the D2-lamp BC method.
The structured background is measured at the wavelength
of the analysis line, although in a separate run using the
spectrum of the appropriate interferent(s).
6 COMPARISON OF METHODS
6.1 Sensitivity and Dynamic Range
From the principle of D2-lamp BC (Section 2.1), it is clear
that the conventional AAS sensitivity is not influenced
by the BC system. The additional measurement with
the D2-lamp provides the background absorbance and
231.30
231.25
231.20
231.15
Wavelength
(nm)
231.10
231.05
231.000.00
A
bs
or
ba
nc
e
0.05
0.10
1
0
2
3
4
Sb
Fe
Time (s)
5
6
7
Figure 12 Time- and wavelength-resolved absorbance spec-
trum for NIST SRM 8704 Buffalo River Sediment in the
vicinity of the Sb line at 231.147 nm after automatic correc-
tion of continuous background absorption. (Reproduced with
permission from Ref. 32. Copyright 2009, Elsevier.)
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14 ATOMIC SPECTROMETRY
the analyte atoms do not contribute significantly to the
background signal because of their narrow absorption
line profile.
In all Zeeman-effect BC systems, the measurement
that is intended to provide the background absorbance is
slightly influenced by the absorption of the analyte atoms
owing to incomplete shift of the Zeeman σ-components
away from the lamp emission line profile. The contribu-
tion of the remaining σ-components decreases the net
analyte absorbance by about 10% for most elements
and by up to 45% for a few elements (Cu, Be, Bi). A
further decrease in sensitivity is observed in Zeeman-
effect BC systems on the basis of a constant magnetic
field from a permanent magnet or a DC magnetic field for
all analyte absorption lines that show anomalous Zeeman
splitting patterns. The sensitivity reduction is more severe
when a central π-component is missing. The best Zeeman
AAS sensitivity for all elements is obtained in a strong
AC magnetic field, as nowadays used in most commer-
cially available ETAAS instruments with Zeeman-effect
BC.
Pulsed-lamp BC provides sensitivity close to conven-
tional AAS for a few elements only. It is hard to achieve
sufficient broadening and self-reversal in the lamp emis-
sion line profile during the high-current pulse. Similarly
to Zeeman-effect BC, the contribution of the remaining
part of the lamp line profile that is insufficiently shifted is
subtracted from the gross absorbance together with the
estimated background. The loss of sensitivity varies from
13% for Cd to 84% for Hg.(29)
In HR-CS AAS, there is no influence of BC on
the sensitivity. Correction for continuous background
is derived from simultaneous measurements at correc-
tion pixels on both sides of the absorption line,
whereas correction for structured background is based on
subtraction of the reference spectrum of the interfering
compound(s). Further explanation is found in High-
resolution Continuum Source Atomic Absorption Spec-
trometry – Theory and Applications.
Additional analytical curves of reduced sensitivity
can be obtained in AC Zeeman-effect BC by the
three-field approach discussed in Section 3.4, whereas
in HR-CS AAS, the sensitivity can also be selected
between the sensitivity of conventional AAS and lower
sensitivities by using selected pixels at the wings of the line
profile. The various calibration graphs become available
simultaneously.
The dynamic range of AAS is limited owing to
the presence of a small amount of radiation from
the HCL within the spectral band-pass, which cannot
be absorbed by the analyte atoms. Starting from the
detection limit, absorbance can be measured over about
three orders of magnitude, i.e. from about 0.001 up to
1 absorbance unit. This range is not influenced by the
D2-lamp BC system. In Zeeman-effect and pulsed-lamp
BC systems, the dynamic range is further limited owing
to the roll-over phenomenon discussed in Section 3.4.
The analytical curves in peak absorbance level off at
somewhat lower absorbances than in conventional AAS,
whereas the analytical curves in integrated absorbance
continue to increase, slowly, up to very high analyte
concentrations.
6.2 Frequency of Background Correction
BC systems involving sequential measurements of the
gross absorbance and the background according to
Equation (1) should operate fast enough to follow
fluctuations in the background absorbance. In the D2-
lamp BC, this requires switching, at a certain frequency,
between the HCL and the D2-lamp. The frequency
should be high enough to obtain a good estimate of
the background at the time of the measurement of the
gross absorbance. The nearest (temporally) measured
background absorbance is subtracted from the gross
absorbance. The background can also be estimated
from two measurements, before and after the gross
absorbance measurement, which are combined using
bracketing techniques.(33) In ETAAS, the atomic and
background absorption signals may rise to a maximum
in times ranging from 0.1 s or less to several seconds,
depending on the furnace design, the heating rate, and
the composition of the sample. The beam switching
frequency should be high enough to enable the fastest
rates of change of the atomic and background absorption
signals to be followed. The upper limit to the chopping
frequency follows from practical considerations, whereas
HCLs can be pulsed up to very high frequencies. A
high enough BC frequency of 200–400 Hz can easily be
achieved.
In Zeeman-effect BC systems, the frequency is related
to the modulation frequency of the AC magnetic field
or the frequency of rotating the polarizer in the case of
a constant magnetic field. In most of the instruments,
the BC frequency is 50–100 Hz. It is complicated to
rotate a polarizer at high speed and it is expensive
to modulate an AC magnetic field at a frequency
higher than the mains frequency. In the pulsed-lamp
BC system, the HCL is pulsed repetitively with a cycle
of 50 ms, but the actual time between low-current and
high-current measurements is only 4.5 ms, indicating
the temporal proximity of BC.(29) Oppermann et al.(34)
described a high-speed self-reversal BC method using
pulsed HCLs with a frequency of 100 Hz and lamp
currents up to 600 mA. In all BC approaches, an increase
in the operating frequency decreases the time available
to measure the signals and consequently decreases the
S/N.
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This article is © 2013 John Wiley & Sons, Ltd.
This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd.
DOI: 10.1002/9780470027318.a5104.pub2
BACKGROUND CORRECTION METHODS IN ATOMIC ABSORPTION SPECTROMETRY 15
6.3 Accuracy of Background Correction
The accuracy of the correction for background
absorbance must be perfect up to high levels of back-
ground absorption. In the absence of analyte, the base
line should be a straight line even at high background
absorbance levels. Three conditions should be fulfilled
to accomplish this. The background must be measured
at the wavelength of the analyte absorption line or very
close to the line. In the previous section, we have also
seen that the transient ETAAS signal requires a good
approximation of the background at the time of measure-
ment of the gross absorbance signal. In addition, the
background must be measured at the same position in
the atomizer. This is easily achieved if only one source of
radiation is used as in the Zeeman-effect BC system and
the pulsed-lamp BC system. In D2-lamp BC, lamp align-
ment is always difficult because of the different positions
of the lamps in the instrument, the different optical paths,
the different geometries of the beams, and the different
intensity distributions. Since the atomic vapor in both the
flame and the graphite furnace is not truly homogeneous,
nor is the radiant cross-section of either source, perfect
matching is difficult.
Background absorbance in FAAS is usually low and it
can be corrected easily with the D2-lamp BC system. In
ETAAS, the D2-lamp BC system is often also successful.
BC problems have been observed when the background is
higher than about 0.5–1 absorbance unit, often owing to
misalignment of the radiation beams. Many instruments
are fitted with both a D2-lamp for the UV and a visible
lamp for the visible wavelength region. Theintensity
of the D2-lamp is quite low when a low spectral band-
pass is selected, which may be a limiting factor for the
HCL intensity also because most instruments require
more or less equal intensities of the two radiation
beams. D2-lamp BC systems are not able to correct
for structured background. The background is measured
as an average background absorbance level over the
spectral band-pass of the monochromator, i.e. within
0.2–2 nm. Numerous over- and undercorrections have
been reported. Several of them are well known and can
be found in textbooks on AAS; others are only observed
under very special experimental conditions. Zeeman-
effect BC systems, pulsed-lamp BC systems, and CS
AAS instruments measure the background at exactly the
wavelength position of the analyte absorption line or very
close to the line, i.e. within an interval of 10 pm. Although
perfect BC correction is not guaranteed, there is a better
chance that the right background absorbance is measured
at a wavelength position close to the absorption line in
comparison to the average value over the full spectral
band-pass. The Zeeman-effect BC system that applies
the magnetic field to the atomizer is the only system
that provides correction for the background exactly at
the wavelength of the analyte absorption line. Even here,
perfect BC is not guaranteed because, in a very few
cases, the background may change with the strength
of the magnetic field in AC Zeeman-effect BC systems
(Section 3.4). Most of the errors reported for the D2-lamp
BC system are not found when Zeeman-effect or pulsed-
lamp BC is applied or when CS AAS is used. Finally, it
should be noted that high background absorbance should
be avoided when possible so as to minimize light losses
and maintain the best S/N available.
ABBREVIATIONS AND ACRONYMS
AAS Atomic Absorption Spectrometry
AC Alternating Current
BC Background Correction
CS Continuum Source
CS AAS Continuum Source Atomic
Absorption Spectrometry
DC Direct Current
D2-Lamp Deuterium Lamp
EDL Electrodeless Discharge Lamp
ETAAS Electrothermal Atomic Absorption
Spectrometry
FAAS Flame Atomic Absorption
Spectrometry
HCL Hollow Cathode Lamp
HR-CS AAS High-resolution Continuum Source
Atomic Absorption Spectrometry
S/N Signal-to-Noise Ratio
UV Ultraviolet
RELATED ARTICLES
Atomic Spectroscopy (Volume 11)
Atomic Spectroscopy: Introduction • Flame and Vapor
Generation Atomic Absorption Spectrometry • Graphite
Furnace Atomic Absorption Spectrometry
Clinical Chemistry (Volume 2)
Atomic Spectrometry in Clinical Chemistry
Environment: Water and Waste (Volume 3)
Flame and Graphite Furnace Atomic Absorption Spec-
trometry in Environmental Analysis
Food (Volume 5)
Atomic Spectroscopy in Food Analysis
Steel and Related Materials (Volume 10)
Atomic Absorption and Emission Spectrometry, Solu-
tion-based in Iron and Steel Analysis
Encyclopedia of Analytical Chemistry, Online © 2006–2013 John Wiley & Sons, Ltd.
This article is © 2013 John Wiley & Sons, Ltd.
This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd.
DOI: 10.1002/9780470027318.a5104.pub2
16 ATOMIC SPECTROMETRY
Modifiers in Graphite Furnace Atomic Absorption
Spectrometry – Mechanisms and Applications
High-resolution Continuum Source Atomic Absorption
Spectrometry – Theory and Applications
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DOI: 10.1002/9780470027318.a5104.pub2
BACKGROUND CORRECTION METHODS IN ATOMIC ABSORPTION SPECTROMETRY 17
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Encyclopedia of Analytical Chemistry, Online © 2006–2013 John Wiley & Sons, Ltd.
This article is © 2013 John Wiley & Sons, Ltd.
This article was published in the Encyclopedia of Analytical Chemistry in 2013 by John Wiley & Sons, Ltd.
DOI: 10.1002/9780470027318.a5104.pub2