T. Kono and S. Kobayashi, BunsekiKagaku, 19,1491 (1970);Chem. Abstr.,
74 (18),94017 (1971). J. L. Robinson, R. D. Barnekow and P.F. Lott, At. Absorp. News/., 8,60
(27)J. Stary, "The Solvent Extraction of Metal Chelates", Macmillan Company, New York, 1964. (28)P. T. Gilbert, Anal. Chem., 34,210R (1962).
(1969). R. R. Brooks, B. J. Presley. and I . R. Koplan, Anal. Chim. Acta, 38,321
(1967). J. Nix and T. Goodwin, At. Absorp. News/., 9,(6),119 (1970). F. Roth and E. Gilbert, Mitt. Rebe Wein, ObstbauFruechteverwet?, 19,430 (1969);Chem. Abstr., 73 (I), 2727 (1970). J. H. Culp, R. L. Windham, and R. D. Whealy, Anal. Chem., 43, 1321 ( 1971). J. D. Winefordner and T. J. Vickers, Anal. Chem., 46, 192R (1974). G. M. Hieftje, T. R. Copeland, and D. R. de Olivares, Anal. Chem., 48, 14213
(1976). J. E. Allan, Spectrochim Acta, 17,459 (1961).
RECEIVEDfor review June 30,1976. Accepted September 16, 1976. Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for support of this research. Presented in part before the Southeastern Regional Meeting of the American Chemical Society in Birmingham, Ala., November 1972, and before the 4th International Conference on Atomic Spectroscopy in Toronto, Ontario, Canada, November 1973.
Analog Data Treatment of Spectra in Flame Absorption and Emission Spectrometry Hiroki Haraguchi' and Naoki Furuta Department of Chemistry and Physics, National lnstitute for Environmental Studies, Yatabe, lbaraki 300-2 1, Japan
Etsuro Yoshimura and Keiichiro Fuwa' Department of Argicultural Chemistry, The University of Tokyo, Yayoi, Bunkyo-ku, Tokyo 1 13, Japan
An analog data processing system using magnetic tapes has been investigated to measure absorption and emlssion spectra in flame absorption and emission spectrometry. The system has been found to be very useful and convenient for the spectral baseline and background (due to absorptions and emissions of the flame components and co-existing species) corrections. Several absorptlon and emission spectra for solutions of H3P04,indium, and MgCI2, respectively, aspirated into the air-acetylene flame have been measured by utilizing the system connected to a usual type of atomic absorption spectrophotometer. Photodissociation processes of molecules in the flame and some problems in atomic spectrometry can be discussed from the spectra observed by the present system.
Molecular absorptions in the flame have been investigated to characterize the molecular species in the flame ( I ) , to analyze some elements (2-4), and to elucidate the background absorptions and the interfering mechanisms in atomic flame spectrometry (5-13).In spite of the importance of these investigations, there have not been many studies of molecular absorption flame spectrometry. This is probably due to the inconvenience of the measurement of absorption spectra in the flame because of the difficulty of the spectral baseline correction, which originates from the variation of the intensity of the continuum light source with wavelength. As a consequence, the difficulty of baseline correction has prevented us from measuring easily and conveniently the absorption spectra in the flame by a usual spectrophotometer. A digital data processing system has the feasibility to resolve the problems mentioned above. Johnson, Plankey, and Winefordner applied a computer to atomic fluorescence spectrometry ( 1 4 ) . Recently, Spillman and Malmstadt reported a computer-controlled programmable monochromator system for atomic emission and fluorescence multielement
Present address, Department of Chemistry, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan. 2066
flame spectrometry, which has the capability of automated wavelength calibration and background correction ( 1 5 ) . Furthermore, TV-type multichannel detectors such as a vidicon tube, a silicon photodiode array, and so forth have been used in flame atomic emission, absorption, and fluorescence spectrometry (16-20). Busch, Howell, and Morrison used an image vidicon tube with an optical multichannel analyzer (OMA) for the elimination of background emission and spectral interference in flame emilion spectrometry (16), where the molecular emission bands of OH and CaOH were corrected for the measurements of Bi and Ba lines, respectively. Haraguchi, Fowler, Johnson, and Winefordner applied a silicon intehsified tube with an OMA to a study of molecular fluorescence of phosphorous monoxide in flames ( 2 0 ) . However, no application of such digital data processing systems has been found in molecular absorption flame spectrometry. So far, it has been recognized that an analog data processing system is generally less advantageous than a digital one, especially, in terms of versatility and precision. However, in spectrochemical instruments, which are usually inexpensive, it is very difficult to use the digital processing system, for example, for baseline correction, because of the necessity of sampling many data points, and it is expensive. Therefore, the applications of the digital data processing system to spectrochemical instruments have been restricted to some definite and versatile purposes mentioned above. Even in the present "state of the art" scientific instruments, the analog data processing system seems capable of handling simple data treatment such as subtraction, differentiation, background correction, baseline correction, and so forth, where a subtraction circuit and some simple additional circuits are required, if the system can be operated with good precision. As mentioned earlier, in molecular absorption flame spectrometry using a continuum light source, the spectral baseline correction has been a serious problem. Therefore, flame absorption spectra have been usually obtained by the manual measurements of the absorbances a t fixed wavelengths in the whole wavelength range of interest, and the wavelength range observed has not been wide because such measurements were very tedious. Recently, Haraguchi, Toda, Hirabayashi, and
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Fuwa reported a two-flame double-beam automatic scanning spectrophotometer for flame absorption spectra, in which a digital memory circuit with 3000 points was employed for baseline correction (13).In spite of the employment of a second flame (reference cell), a data processing system for the baseline correction was still required in the spectrophotometer. Therefore, the present authors have sought another simple and convenient method for flame absorption spectrometry, and examined the use of an analog data processing system utilizing magnetic tapes, which was originally developed for data treatment in molecular absorption and fluorescence spectroscopy in solution. It has been found, as reported previously as a short communication (21), that this analog system connected to a usual atomic absorption spectrophotometer is useful and convenient for the investigation of absorption spectra in the flames. This system is also useful for background corrections in flame emission spectrometry. Hence, applications of the analog data processing system to flame absorption and emission spectrometry will be shown and discussed in detail in this paper.
i.
I
0.4
300 400 500 600 Wavelength, nm
200
700
Figure 1. Spectral baselines of spectrophotometer and baseline correction in an air-acetylene flame
EXPERIMENTAL Samples. All chemicals used were of guaranteed reagent grade. Indium oxide (Inz03; 99.9%) was purchased from Mitsuwa Chemical Co., Ltd. Deionized water was used as a reference in all measurements of spectra. Instrumentation. An atomic absorption spectrophotometer (Model AA-650 from Shimadzu Scientific Instruments, Inc., Japan), which is of a double-beam type, was used without further modification. The spectrophotometer is equiped with a deuterium lamp (L613K from Hamamatsu TV Co., Japan) as a continuum light source for background correction in atomic absorption spectrometry. A 10-cm slot burner was used for an air-acetylene flame. A Spectral Band Analyzer (SBA; Shimadzu-Yasec SBA-1 from Shimadzu Scientific Instruments, Inc.), which is a data processing system with two magnetic tapes, was connected to the spectrophotometer through an adaptor (Shimadzu-Yasec DIF-4). Generally, one of the tapes (Tape A) is used for data storage directly from the spectrophotometer, and another one (Tape B) for calculation-performed (or corrected) data storage. However, the uncorrected or corrected data can be stored in both tapes, when required. Procedure. Some basic absorption spectra obtained by the spectrophotometer with and without the SBA are shown in Figure 1. The output of the spectrophotometer is proportional to absorbance, Le., a 1-V output of the spectrophotometer is equal to an absorbance of 1.0. The output in absorbance units from the spectrophotometer was fed into the SBA. Therefore, in the following procedure, all the calculations (subtraction) of spectral data in the SBA were carried out in absorbance units without any scale expansion. Spectrum B in Figure 1 is a spectral baseline obtained in the absorption mode of the spectrophotometer with neither the flame or data-correction. In spite of the use of a double-beam type of spectrophotometer, the spectral baseline is not a straight line. This is because the two divided beams cannot be completely balanced, probably because the qualities of the mirrors and lenses used in the two optical paths are not exactly the same. Spectrum A in Figure 1also shows a spectral baseline without any baseline correction, when an air-acetylene flame is burning. Owing to some absorption or scattering due to the flame components, spectrum A is different from spectrum B. Spectrum C shown in Figure 1 was obtained by a subtraction of spectrum B from spectrum A, as mentioned below: In the first step, spectrum B in Figure 1 was measured, and the data were stored in Tape A (Memory I) of the SBA without any data treatment. In the next step, spectrum A in Figure 1 was measured, and a t the same time the data correction, Le., a subtraction of spectrum B (in Tape A; Memory I) from spectrum A in Figure 1,was stored in Tape B, where the subtraction was carried out in the subtraction circuit in the SBA. During this procedure, spectrum C is stored in both Tape A and Tape B (Memory 11). These memories in the tapes can be used to obtain the derivative spectrum shown in Figure 1, D. In the steps mentioned above, the starting times of the wavelength scanning and the data-sampling in both the tapes must be synchronized, and the tape speeds for the spectral recording also controlled. The SBA is equipped with a 500-Hz pulse generator, and the start, sampling, and speed of the tapes are controlled using this pulse; namely, the gate circuit, the pulse generator (standard oscillator) in
(A) Spectral baseline with the flame and without data treatment. (E) Spectral baseline without the flame and data treatment. (C)Absorption spectrum of the air-acetylene flame (A B) obtained by the use of the SEA. (D) Firstderivative spectrum of spectrum C
-
the SBA, and the wavelength scanning of the monochromator start 3 s, 4 s, and 10 s, respectively, after the tapes start. During and after this operation, the starting times and tape-speeds of both tapes are synchronized by monitoring the phase differences between the standard oscillator and the tapes. This mechanism in the SBA suppresses the time lag between the tapes to within %oa s in 1 s. The repeatability in the present experiment was such that the wavelength difference between several spectra re( orded in Tape A and Tape B was less than 0.1 nm, when the scan speed of the monochromator was 100 nm min-1. Spectrum D in Figure 1 is a first derivative spectrum of spectrum C. It was obtained by differentiating the starting times of the tapes by 0.5 s and by measuring the difference of the output between Tape A and Tape B. The calculation of the derivative can be carried out during or after the observation of the flame spectra. In all the measurements of the absorption and emission spectra, the flow rates of acetylene (2.25 1. min-l) and air (10 1. min-I) were maintained constant, and all the spectra were observed at a height of 8 mm above the burner head. The spectral bandwidth of monochromator was 0.2 nm in the absorption spectra and 0.03 nm in the emission spectra. The scanning speed was 100 nm min-' in all the spectra. The aspiration rate of the sample solutions was kept at 10 ml min-l.
RESULTS AND DISCUSSION As mentioned in the Experimental section, the SBA employed in this experiment has been found useful for the data treatment of flame spectra. Therefore, more applications of the S B A to the measurements of flame absorption and emission spectra are shown in this section. Spectrum C in Figure 1,which was obtained by subtracting spectrum B from spectrum A in Figure 1,shows an absorption spectra of the air-acetylene flame used. In the spectrum, molecular absorption of the OH radical and other broad absorption of some flame components (including the oxygen molecule) in the air-acetylene flame are clearly observed near 285 and 306 nm, and below 250 nm, respectively. These molecular absorptions cannot be identified so clearly in the uncorrected spectrum (Figure 1, A). The broad absorptions in the wavelength range from 380 to 500 nm are due to the 2dorder diffraction of the bands below 250 nm by the grating used. Such apparent absorptions could be corrected by using an uv filter, but were not carried out in the present experiment.
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
2067
200
300
400
500
600
Wavelength, nm Flgure 2. Flame absorption spectrum for H3P04aspirated into the airacetylene flame (A) Uncorrected spectrum (upper). Uncorrected baseline including flame background absorptions (lower). (6)Corrected spectrum. Concentration of H3P04. 1 M
0
Wavelength, nm Flgure 4. Flame emission and absorption spectra for the magnesium chloride solution aspirated into the air-acetylene flame (A) Uncorrected emission spectrum (upper). Flame background emission spectrum (lower). (B) Corrected emission spectrum. (C) Corrected absorption spectrum. Concentration of MgCI?, 0.8 M
200
300
400
500
600
Wavelength, nm Figure 3. Flame absorption spectrum for the 3 N HCI solution of indium aspirated into the air-acetylene flame Concentration of indium, 0.2 M
In spectrum D, a derivative spectrum of the flame absorption spectrum (Figure 1,C) is shown as an example of the capability of the SBA. As mentioned above, the spectrum was obtained with a time lag of 0.5 s between the starting times of the tapes, which corresponds to a wavelength difference of 0.833 nm. Since the time lag employed was small, only the spectral components with steep gradients were observed as the derivative curves. Therefore, only the derivative spectra of OH (near 285 and 306 nm) can be seen clearly in Figure 1, D. Although more examples are not shown here, this technique was useful for the observation of sharp atomic lines and the analysis of molecular hyperfine structure, adjusting the time lag and the monochromator slit width appropriately. Applications to Flame Absorption Spectrometry. In Figure 2, B, an absorption spectrum for phosphoric acid solution aspirated into the air-acetylene flame is shown along with the uncorrected spectrum (Figure 2, A) for the same solution. In this spectrum, the spectral baseline and background absorption were corrected by the SBA. Namely, spectrum B in Figure 2 was obtained by subtracting the lower spectrum from the upper spectrum in Figure 2, A, which were observed without and with the aspiration of phosphoric acid into the air-acetylene flame, respectively. Therefore, molecular absorption of the OH radical and some flame constituents, which are observed in Figure 1,D, are also corrected and cannot be seen in Figure 2, B. In the spectrum in Figure 2, B, molecular 2068
absorption bands are observed near 246 and 325 nm, and, in addition, broad absorptions are observed between 200 and 350 nm. The absorption bands near 500 nm and the broad absorption bands between 400 and 600 nm are due to the 2dorder diffraction of the absorptions near 246 nm and between 200 and 300 nm, respectively. The sharp absorption line at 590 nm is due to sodium in the solution. The molecular absorptions near 246 nm, which show the hyperfine structure with several bands near 230,238,246,254, and 260 nm, originate from the y system of phosphrous monoxide (PO). The broad absorptions between 200 and 350 nm are due to phosphorous oxides other than PO, as reported previously (4,22). The absorption maximum at 246.0 nm can be applied to the determination of phosphorus ( 4 ) .The absorption bands near 325 nm can be assigned to the fl system of PO ( I ) , which is produced by the electronic transitions of 211-22and 211-211. The fl system of PO in a flame has been observed in absorption for the first time in this experiment. An absorption spectrum for the 3 N HC1 aqueous solution of 0.2 M indium aspirated into the air-acetylene flame, is shown in Figure 3. The spectrum in the wavelength range from 200 to 300 nm was reported previously, and was observed by the manual technique at fixed wavelengths (11).The spectrum in the wavelength range 200-300 nm is consistent with the previous spectrum. As assigned in the previous paper ( I I ) , the molecular absorption bands of InCl near 267 nm and many sharp atomic lines a t 451,410,326,304,293,275.5 nm and so forth are observed in the absorption spectrum in Figure 3, where the apparent absorption lines and bands due to the 2d-order diffraction by the grating are also observed above 400 nm. The broad absorption in the wavelength range from 260 to 300 nm (absorption maximum at 273 nm) have been ascribed to the molecular bands of indium monoxide (InO) ( I I ) , while the broad bands below 230 nm have not been identified in the previous work. One possible explanation for the identification of the bands below 230 nm might be given by the consideration of the photodissociation process of I n 0 in the flame. This photodissociation process may be interpreted as follows:
ANALYTICAL CHEMISTRY, VOL. 48, NO. 14, DECEMBER 1976
Table I. Spectral Parameters Obtained from the Absorption Spectrum of Indium Monoxide in the Air-Acetylene Flame Observed maximal peaks, nma
Observed, cm-1
Species
A1
Xz
A3
In0
273
207
(165)d
v2
- u1
Calculated, cm-1 b u3
- u1
. . . .d
11680
v2
- v1
15868
Dissociation energy
- v1
eV'
24373
3.3
v3
a XI, XZ, and X3 are the wavelengths corresponding to V I , u2, and u3, respectively. C. E. Moore, "Atomic Energy Levels", Nat. Bur. ) ] In (2P1/2),and between Stand. U.S.A., 1971. These values were just estimated as the energy differences between [In ( 2 P 1 / p ) + 0 ( ~ P zand [In ( * P 1 / 2 ) 0 ( 3 P p ) ] and [In ( 2 S 1 / 2 ) 0 ( 3 P 2 ) ] . Actually, these values do not show the exact values of ( u 2 - q )and (ut3 - V I ) , because the shapes of potential curves for the excited molecular states and the electronic distributions in the vibrational levels of the unexcited molecule (InO) should be taken into consideration. A. G. Gaydon, "Dissociation Energies and Spectra of Diatomic Molecules", 3d ed., Chapman and Hall Ltd., London, 1968. Not observed. The value in the parentheses is an expected wavelength for X.1.
+
+
+ hup I n 0 + hug In0
-
In(2P1/2)
+ O*(lD) +
In*(2S1/2) O(3P2)
where V I , up, and u3 are the frequencies of the absorption maxima for InO, 2PI/p and 2S1/p the ground and excited states of the indium atom, respectively, 3P2and lD the ground and excited states of the oxygen atom, respectively. The frequencies v 1 and 19correspond to the absorption maxima a t 273 and 207 nm, respectively. The spectral parameters for this process are summarized in Table I. The calculated and observed values for up - u l in Table I are not in good agreement. This is probably because some energy of the excited molecule is consumed as kinetic energy. Therefore, it would be more accurate to assign the frequencies not a t the absorption maxima, but at the lower frequency end of the continuum band. In fact, if the observed wavelength hp could be assigned a t 190 nm, the observed value of 16002 cm-l for u p - VI,which is close to the calculated value, could be obtained experimentally. Since the absorption maximum for v3 is below 200 nm, as predicted from the data in Table I, the value of v3 could not be obtained experimentally in this work. The photodissociation processes for simple organic and inorganic molecules excited in vacuum discharge tubes have been investigated extensively after the work of Franck et al. (23, 24), but the photodissociation process for I n 0 has not been reported so far. Recently, the present authors have pointed out for the first time that the photodissociation processes can take place even in flames in the case of sodium halides (21,25).According to the results in Table I, it seems reasonable that the absorption bands of I n 0 in the uv region also arise from the photodissociation process in the air-acetylene flame. The absorption bands of I n 0 could be observed for all the "03, HF, HCl, HBr, and HI solutions of indium in the previous work (11). However, further study should be required to verify the photodissociation of I n 0 more rigorously. From the analytical point of view, the findings of the absorption bands of InCl in the flame are important to explain the mechanism of the chemical interference in indium atomic absorption spectrometry (9,10). Application to Flame Emission Spectrometry. The emission and absorption spectra, which were observed with the aspiration of the aqueous solution of 0.8 M MgC12 into the air-acetylene flame, are shown in Figure 4, B and C. In Figure 4, A, the uncorrected emission spectrum for the same solution (upper), and the emission spectrum of flame background (lower) are shown. Usually, the clear continuum emission or absorption bands due to OH, CH, Cp, CH, NH, and so forth are observed in the spectra of flames ( 2 6 ) .In Figure 4, A, the OH emission bands are observed near 285 and 306 nm, and the continuum bands, which are usually explained by the reaction CO 0 Con hv,are observed between 260 and 500 nm.
+
-
+
In the spectra in Figure 4, B and C, corrections were made for spectral baseline and background emission and absorptions using the SBA data processing system. Thus, for example, in the corrected emission spectrum shown in Figure 4, B, the emission bands of the flame components disappear, and only those from the MgClp solution are observed. As is well known, the sharp line a t 285.2 nm is due to atomic magnesium, and the broad bands observed between 340 and 420 nm are due to MgO (or MgOH) ( 2 6 ) .The weak bands near 500 nm are also due to MgO (24,26). In the corresponding absorption spectrum for the same solution, the atomic line a t 285.2 nm and the molecular bands near 365 nm are observed in absorption, while the bands near 500 nm could not be observed, probably because of their weak intensity. The broad absorption in the uv region, which is observed only in the absorption spectrum, may be due to the photodissociation process of MgCl. Further experiments concerning the photodissociation process of MgCl are in progress using the present data processing system, and will be published in the near future.
LITERATURE CITED (1) W. 8. Pearce and A. G. Gaydon, "The Identification of Molecular Spectra", Chapman and Hall, London, 1965. (2)J. A. Fiorino, R. N. Kniseley, and V. A. Fassei, Spectrochim. Acta, Part 6, 23, 413 (1968). (3) K. Fuwa and 8.L. Vallee, Anal. Chem., 41, 188 (1969). (4)H. Haraguchi and K. Fuwa, Anal. Chem., 48, 784 (1976). (5)S.R . Koirtyohann and E. E. Pickett, Anal. Chem., 37, 601 (1965). (6)S.R. Koirtyohann and E. E. Pickett, Anal. Chem., 38, 585 (1966). (7)H. Haraguchi and K. Fuwa, Chem. Lett., 1972, 913. (8)H. Haraguchi, M. Shiraishi, and K. Fuwa, Chem. Lett., 1973, 251. (9)K. Fujiwara, H. Haraguchi, and K. Fuwa, Anal. Chem., 47, 743 (1975). (IO)H. Haraguchi and K. Fuwa, Bull. Chem. Soc. Jpn., 48, 3056 (1975). (1 1) H. Haraguchi and K. Fuwa, Spectrochim. Acta, Part B, 30,535 (1975). (12)T. Nakahara and S. Musha, Anal. Chim. Acta, 80, 47 (1975). (13)H. Haraguchi, S.Toda, N. Hlrabayashi, and K. Fuwa, BunsekiKagaku, 24, 392 (1975). (14)D. J. Johnson, F. W. Piankey. and J. D. Winefordner. Anal. Chem., 47, 1739 (1975). (15)R. W. Spillman and H. V. Maimstadt, Anal. Chem., 48, 303 (1976). (16)K. W. Busch, N. G. Howell, and G. H. Morrison, Anal. Chem., 46, 2074 (1974). (17)D. 0.Knapp, N. Omenetto, L. P. Hart, F. W. Piankey, and J. D. Winefordner, Anal. Chim. Acta, 69, 455 (1974). (18)Y. Talmi, Anal. Chem., 47, 658A.699A (1975). (19) K. M. Aldous, D. G. Mitchell, and K. W. Jackson, Anal. Chem., 47, 1034 (1975). (20)H. Haraguchi, W. K. Fowler, D. J. Johnson, and J. D. Winefordner, Spectrochlm. Acta, Part A, 32, 1539 (1976). (21)H. Haraguchi, N. Furuta, E. Yoshimura, and K. Fuwa, BunsekiKagaku, 24, 733 (1975). (22)K. Fuwa, H. Haraguchi, K. Okamoto, and T. Nagata, Bunseki Kagaku, 21, 945 (1972). (23)T. R . Hogness and J. Franck, 2.Phys., 44, 26 (1927). (24)G.Hertzberg, "Molecular Spectra and Molecular Structure. I. Spectra of Diatomic Molecules", 2d ed., Van Nostrand Co., Inc., Toronto, 1950. (25)N. Furuta, E. Yoshimura, Y. Nemoto. H. Haraguchi, and K. Fuwa, Chem. Lett., 1976, 539. (26)R . Herrman and C. T. J. Alkemade, "Chemical Analysis by Flame Photometry", 2d ed., Translated by P. T. Gilbert, lnterscience Publishers, New York, 1963.
RECEIVEDfor review May 10,1976. Accepted September 16, 1976.
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