Further Reading
6.3 Interferences
6.3.2 Spectral Interferences
Any kind of radiation that falls into the monochromator bandwidth, causing an emis- sion or absorption signal at the detector, may be termed as a spectral interference.
Naturally, stray light reaching the detector, which is the extraneous radiation that falls outside the intended monochromator bandwidth, is also a source of error; however, this phenomenon will not be discussed here since it is common to all spectrometric techniques.
A very simple kind of spectral interference is emission from atomizer, which may saturate the detector in some cases. Precautions are taken in particular with the graphite atomizers, as these will have a high blackbody emission from the furnace wall, probe or platform. For the ideal optical designs, the entrance slit of the mono- chromator is conveniently shielded from this sort of emission. This problem is actu- ally solved as the spectrometer is designed. However, the user should be reminded that excessive misalignments of furnace atomizers might create this type of compli- cation, which shows itself by excessive noise on signal profiles.
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Figure 6.11 Effect of Ni on volatilization of Se in ETAAS, shown by ashing plots. A: 1.0 ng Se(IV) in aqueous solution; B: 1.0 ng Se (IV) and 5000 ng Ni as NiNO3
The spectral interferences may be combined in two important classes; namely, lineand broadband interferences. It should be remembered that the spectral slit size, typically 0.5–2.0 nm, is much larger than the linewidth of radiation sources, about 0.005 nm, which determines the resolving power. Therefore, any species absorbing within the spectral slit range will not cause any interference unless a direct overlap with the source emission profile takes place. Such a direct overlap is called line interferenceand the best solution is to employ another analytical wavelength where there is no such overlap. One of the well-known line interferences occur during the determination of Al at 308.215 nm, where the presence of vanadium, which absorbs at 308.211 nm, causes line interference by a direct overlap. The remedy is to use the line 309.271 nm for Al. Line interferences are relatively rare and can be detected by the presence of positive systematic errors on analyte signal.
The presence of some species, such as halides of alkali and earth alkaline metals, some oxides and other volatile molecules cause a broadband absorption. These molec- ular species, unlike the atomic species, have rather broad absorption bands; the fine structure due to vibrational states may not be resolved by the monochromator; there- fore, the absorption profiles appear as a continuum in a small spectral interval where the atomic absorption is to be measured. Occasionally, this continuum may be struc- tured. In addition, the presence of particles in atomizer cause light scattering; this is different than absorption, but its net effect is again in the form of a broadband. This type of interference may be called broadband interference; it is also frequently named as background absorption. Since the problem is distributed all over the spectral slit range, the analyte beam power is also reduced, resulting in a positive error. Several types of broadband absorption by matrix components are shown in Figure 6.12. Flat, sloped and structured background absorption profiles may be encountered where the structure may bed due to other element atoms as well as the molecular vibrational bands. It should be stressed that broadband absorption problem is much more serious for ETAAS as compared to flame atomizers, especially in the region of 180–350 nm.
A group of instrumental designs have been suggested and these are known as background correction techniquesin AAS. In this text, it has been noted in several
Figure 6.12 Schematic illustration of several types of broadband absorption. EP: source emission profile; AP: absorption profile; shaded area represents analyte atomic absorption. (a) Flat background; (b) sloped background; (c) sloped and struc- tured background
occasions that a narrow source linewidth provides high wavelength selectivity in AAS that is not limited by the monochromator bandwidth. However, this spectral selectivity may be lost in some cases of background correction since these tech- niques require the consideration of the whole range covered by the spectral slit. This loss of spectral selectivity may cause other errors during the background correction.
The performance and success of different background-correction approaches have been the subject of continuous study and discussion in literature and up to date no single background correction technique seems to perfectly solve all the broadband absorption problems in AAS. Background correction techniques most used in AAS are given below.
Two-line method. This is an approach used as the first solution to broadband absorption errors. Another wavelength in the vicinity of analyte line is used to meas- ure the broadband absorption. It is assumed that the background absorption is flat, as shown in Figure 6.12(a). Analyte source radiation is absorbed both by the analyte and background species. A second measurement performed at a nearby wavelength provides the value of broadband absorption that is to be subtracted from the value found on analyte wavelength to give the net atomic absorption. The second line may be selected by using a source lamp of another element, which has a line very close to the analytical line; naturally this element should be absent in the sample.
Alternatively, a non-absorbing nearby line of the analyte source lamp may be used.
A third way is to use a nearby line from the inert gas in source lamp. In any case, two separate measurements have been necessary that rendered this approach rather impractical since the use of separate sample portions degrades precision and increases analysis time. Presently, this technique is not employed at all in the com- mercial AA spectrometers. Nevertheless, new instruments are being manufactured with the capability of simultaneous multi-wavelength measurements, using CCD or CID array detectors. Therefore, although now it seems to be obsolete, the two-line technique may find new applications with these novel instruments since only a sin- gle measurement would be sufficient for background correction. In addition, with the array detectors an accurate spectral characterization of background absorption and thus a proper selection of background wavelength can be made so that a good background correction can be realized. Recently, a portable AAS instrument using a W-coil atomizer and equipped with an array detector system has used the two-line technique for background correction. Recommended lines for this technique have been given in several sources.31,32
Continuum source technique. This approach was suggested by Koirtyohann and Pickett.8Two separate light beams are alternately sent through the measurement zone, from a line source of analyte and a continuum source such as a D2or an H2 lamp. The principle of the technique is illustrated in Figure 6.13. Line source emis- sion is absorbed by analyte atoms and background species, providing a signal which is the sum of atomic and broadband absorptions (AA ⫹BA). When the mirror sec- tor of the chopper is on the optical axis, continuum source radiation passes through the atomizer. Continuum radiation is absorbed by both the analyte atoms and the background constituents; however, the atomic absorption is a very small fraction of the total value and thus may be neglected; the signal represents broadband absorp- tion (BA). The difference is taken and the net atomic absorption (AA) signal is
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obtained. Instead of mechanical chopping, a suitable pulsing regime for both the lamps may be used in most of the recently manufactured AA spectrometers employ- ing continuum source technique for background correction.
D2 arc or D2 HCLs have been used in the UV region where the background absorption errors are most significant. Some instruments have an additional contin- uum source for VIS region, typically a W-lamp. Continuum source background cor- rection technique has been commercialized and used widely. As mentioned before, when a continuum source is used for background correction, the wavelength selec- tivity of an atomic absorption measurement is practically lost. In this case, any inter- fering atom whose absorption profile falls in the range of monochromator bandwidth will cause an excessively and erroneously high background absorption signal and the overcorrection of background absorption will take place; both the sensitivity and accuracy will be degraded. Interference of Fe during Se determination has been a typical example for this type of error.33On the other hand, the volume in which the analyte atoms are observed should be exactly the same for both the line and the con- tinuum sources; this requires a perfection in optical design; otherwise errors will Figure 6.13 Continuum source background correction technique. AA: Atomic absorption;
BA: Broadband absorption. (A) Instrumental setup. a: Line source; b: contin- uum source; c: sector mirror (chopper); d: atomizer; e: monochromator; f:
detector. (B) Modulated signals. a: Line source (AA ⫹BA); b: continuum source (BA); d: Dark current. (C) Source emission (EP) and absorption (AP) profiles.
a: Line source (AA ⫹BA); b: continuum source (BA). Shaded area represents analyte atomic absorption
occur due to any misalignment. A modulation between two sources will create no time-based errors when a steady-state signal from a flame atomizer is used.
However, a transient obtained from a graphite atomizer or HGAAS, CVAAS,etc., will need a fast monitoring especially on the ascending and descending sides of the peak-shaped signal. Earlier continuum source background correction designs used a modulation frequency of 30–70 Hz. Becoming aware of these time-based errors with peak-shaped signals, most novel AA instrument manufacturers now employ modu- lation frequencies as high as 200 Hz; therefore, the measurements of total and back- ground absorption are made almost simultaneously.
Self Reversal (Smith–Hieftje)Technique. When an HCL is operated by sufficiently high currents, the emission profile is splitted into two peaks due to self-absorption.
Ground-state atoms are relatively more abundant in the cooler section of the lamp discharge; therefore, suffering the broadening effects in lesser extent, they have a narrower bandwidth as compared to the excited-state atoms, which are relatively more abundant in the hotter sections. Consequently, when self-absorption takes place, the result is a self-reversal of the emission peak and a splitting at extreme cur- rent values. In this technique, the line source is pulse modulated by low and high cur- rents. For the low-current mode, both atomic and broadband absorption (AA ⫹BA) are measured.10The splitted emission profile is obtained with high current and is used for background absorption (BA) only, since the intensity is almost zero at the centre where the analytical line is located. The subtraction will give the net atomic absorption (AA), as illustrated in Figure 6.14.
This technique requires a single source and thus does not have the problems of source profile alignment as in the continuum source technique. However, specially designed HCLs are needed to standwith current modulation with acceptable life times. Since the splitting may not be perfect, background absorption contains some of the analyte atomic absorption signal, resulting in overcorrection. The use of this technique therefore causes non-linearity in the calibration plot, reduced sensitivity and even self-reversal at high absorbance values resulting in two concentration val- ues for an absorbance reading.
Zeeman effect background correction techniques. Zeeman discovered in 1897 that spectral lines undergo splitting when atoms are placed in a magnetic field.34This is caused by the changes in atomic energy levels; a typical Zeeman splitting is shown in Figure 6.15 for Mg atoms. πand σcomponents are polarized in planes perpendi- cular to each other. With respect to the original resonance wavelength,πcomponent has no shift, while the σcomponents are shifted about ±0.01 nm around the centre.
The total radiation power of π component is equal to the sum of σ components.
When only 3 lines are formed, this splitting is termed as normal Zeeman effect, such as for Cd (228.8 nm), Hg (253.7 nm), Mg (285.2 nm), Pb (283.3 nm), Si (251.6 nm), Sn (224.6 nm), Sr (460.7 nm) and Zn (213.9 nm).
The Zeeman splitting may result in more than three lines; in these cases there are more than one πlines, one of which may or may not be coincident with the resonance wavelength. In these cases, the term anomalous Zeeman effectis used. Many impor- tant analytical lines, such as Se (196.0 nm), Al (308.2 and 396.2 nm), As (197.2 and 193.8 nm), Sb (217.6 and 231.1 nm), Cr (357.9 nm), Fe (248.3 nm), Ni (232.0 nm), Ag (328.1 nm), Co (240.7 nm), Cu (324.8 nm) and Mn (279.5 nm) show anomalous
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Figure 6.14 Self-reversal background correction technique. (A) Splitting of an HCL emission profile by using a high current. a: Normal operation, low current; b: medium current, partial splitting; c: maximum current, maximum splitting. (B) Modulated signals. a: Low current, atomic and broadband absorption (AA ⫹ BA); d: Dark current; b: high current, broadband absorption (BA). (C) Source emission (EP) and sample absorption (AP) profiles. a: Low current (AA ⫹BA);
b: high current (BA). Shaded area represents analyte atomic absorption
Figure 6.15 The normal Zeeman effect for magnesium. (a) No magnetic field; (b) magnetic field, 10 kG, Zeeman splitting; (c) Zeeman effect causing a splitting of spectral lines
Zeeman behaviour. The sorts of Zeeman splitting and splitted line patterns can be found in Welz’ monograph.35In case of significant abundance of isotopes that have slightly shifted wavelengths, each isotope exhibits its own Zeeman-splitting pattern.
Regarding the instrument designs, there are several types of Zeeman background cor- rection. The magnetic field can be applied to the source of radiation; in this case, emission line profile undergoes Zeeman splitting; this configuration is called direct Zeeman. On the other hand, the magnet can be placed around the atomizer; absorp- tion line profile splits; this configuration is called inverse Zeeman. During the Zeeman splitting, the polarization of the πand the σcomponents is termed according to the direction of observation, as shown in Figure 6.16 and described below.
Transverse Zeeman effect. In this case, the magnetic field is on the atomizer and applied perpendicular with respect to the optical axis of spectrometer. Therefore, the direction of observation is perpendicular to the magnetic field; πcomponent is polar- ized in a direction parallel to the magnetic field; the σcomponents, on the other hand, are polarized in a plane perpendicular to the magnetic field.
Longitudinal Zeeman effect. The magnetic field situated around the atomizer is applied parallel to the optical axis. πcomponents practically absent since its plane of polarization does not match with that of the πcomponent of the emission from the source. σ components are displaced on the frequency axis and are circularly polarized.
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Figure 6.16 Transverse and longitudinal Zeeman effect. H: Magnetic field direction; O: opti- cal axis and direction of observation; G: graphite cuvette atomizer. (A) Transverse Zeeman effect:πcomponent is polarized parallel, and σcomponents are polarized perpendicular to H. Radiation power is distributed as σ⫺(25%), π(50%),σ⫹(25%). (B) Longitudinal Zeeman effect:πcomponent is absent and σ components are circularly polarized. Radiation power is distributed as σ⫺(50%),σ⫹(50%)
After the review of common Zeeman configurations and definitions, the instru- mental applications and their working principles can be discussed. Depending on the location of the magnet, application of magnetic field in a dc or ac and the modes of measurements, among the terms of direct, inverse, transverse and longitudinal, sev- eral kinds of Zeeman effect background correction designs are possible.36These are explained in the paragraphs below and in Figure 6.17.
Direct and transverse Zeeman AAS with constant magnetic field. The magnetic field is applied continuously (dc) and perpendicular to the optical axis and the mag- net is on the emission source. A rotating polarizer is placed between the source and the atomizer, allowing πand σcomponents to be transmitted sequentially through the atom cloud. Only the emission profile is splitted. As the πcomponent is used, both the broadband (BA) and atomic absorption (AA) are measured; σcomponent is used in another phase to measure BA only; the difference provides the net AA sig- nal. This approach is usually called briefly as direct Zeemanin literature.
Inverse and transverse Zeeman AAS with continuous magnetic field. The magnetic field is applied continuously (dc) and perpendicular to the optical axis; the magnet is on the atomizer. Only the absorption profile is splitted. A rotating polarizer is situated
Figure 6.17 Four important configurations for Zeeman background correction in AAS. EP:
Source emission profile; AP: absorption profile. (A) Direct and transverse Zeeman AAS; (B) inverse and transverse Zeeman AAS-continuous magnetic field; (C) inverse and transverse Zeeman AAS-alternating magnetic field; (D) inverse and longitudinal Zeeman AAS-alternating magnetic field
between the source and the atomizer, allowing alternatively the πand the σcompo- nents from the source, both on the same wavelength as the unsplitted analyte resonance line. The πcomponent will be giving both the BA and AA signals; σcomponent will not be absorbed by the atoms whose absorption profiles are in πconfiguration, which is polarized in a different plane; therefore, only the BA signal will be registered. The difference is used to give the net A signal.
Inverse and transverse Zeeman AAS with alternating magnetic field. The magnet is on the atomizer and the magnetic field is applied alternatingly (ac), perpendicular to the optical axis; only the absorption profile is splitted. A static polarizer allows the transmission of only the σcomponent of the emission source; its wavelength is not shifted. Two phases are encountered with the magnetic field,on and off. When it is off, the absorption profile is not splitted, both πand σcomponents absorb at the cen- tral wavelength; (BA ⫹AA) signals are registered. When it is on, the σcomponents from the source cannot be absorbed by the atoms that have the πconfiguration for the absorption profile; therefore, only BA is measured. The difference of these sig- nals from the two phases provides the net AA signal.
Inverse and longitudinal Zeeman AAS with alternating magnetic field. The mag- netic field is applied in an ac mode and parallel to the optical axis; the magnet is on the atomizer. In the absence of the magnetic field, (BA ⫹AA) is measured. When the magnetic field is on, splitting takes place for the absorption profile; however, the πcomponent for the absorption profile and the πcomponent for the emission profile are on the different planes; no interaction occurs and therefore the πcomponent for the analyte atoms is practically absent. The σcomponents of the absorption profile are circularly polarized and shifted in wavelength; therefore, there is no interaction between them and the σcomponents from the source emission. As a result, only BA is measured when the magnetic field is applied; this signal is subtracted from the (BA ⫹AA) signal registered in the absence of the magnetic field, the net AA signal is thus obtained.
Each one of the four designs described above has their relative advantages and dis- advantages. The direct Zeeman technique corrects the background signal using the σ components; although these are very close to the analytical wavelength, this correc- tion is not exactly made at the resonance line where the atomic absorption signal is taken; in the case of structured background absorption, significant error may result.