- Email: [email protected]
Transmission line structure of a coherent forward scattering resonance monochromator measured by high resolution laser spectroscopy
Spectrochimica Acta? Vol. 48B. No. 9, pp. 1093-1099, Printed in Great Bntain.
1993 0
0584-8547/93 $6.00 + zoo 1993 Pergamon Ress Ltd
Transmission line structure of a coherent forward scattering resonance monochromator measured by high resolution laser spectroscopy* HIDEYUKI MATWTA and KICHINOSUKE HIROKAWA Institute for Materials Research, Tohoku University, Katahira 2-1-1, 980 Sendai, Japan
and GERD HERMANN and CARSTEN SCHWABE I. Physikalisches Institut, Justus-Liebig-Universitat, Heinrich-Buff-Ring 16, D-W-6300 Giessen, F.R.G. (Received 26 January 1993; accepted 10 March 1993)
Abstract-Transmission line structures of a copper coherent forward scattering resonance monochromator (CFSRM) with d.c. glow discharge atomization have been measured by high resolution laser spebroscopy. Based on these measurements, optimti conditions have been chosen and were used for a Cu CF8RM using atomization in a pulsed glow discharge. Spectra emitted by a Grimm-type source with a Cu-Fe cathode were filtered through the Cu CFSRM. In consequence, suppression of spectral interferences from an extremely close-lying Fe emission line could be better achieved than by using a conventional grating monochromator.
1. INTRODUCTION COHERENT forward scattering resonance monochromator (CFSRM), employing sputtering atomization by an argon glow discharge (GD), has been developed [l-3]. The bandwidth of the CFSRM is narrow, comparable to the linewidth of a hollow cathode lamp (HCL) [4], and the transmission efficiency is relatively high [3, 41. Thus, the CFSRM is expected to improve the detection limits (LODs) and the accuracy of atomic emission spectrometry by suppressing continuum background as well as spectral line interferences. In this work, the spectral transmission of a CFSRM for the atomic emission line of an HCL has been investigated in order to optimize the transmission profile. Employing high resolution laser spectroscopy, the transmission line profiles of a copper CFSRM were measured and the influence of the magnetic field strength and of the GD current on the transmission line structure has been studied. Based on these results, optimal operation conditions were chosen to which the Cu CFSRM was set when the analytical performance was proven. Cu in Fe has been determined by measuring the emission on the 324.8 nm resonance line and suppressing interferences by an extremely closelying Fe line (separation less than 0.002 nm), which is observed in the gap between the two copper hyperfine structure (hfs) components. A
2. APPARATUS The CFSRM was the same as previously reported [3]. For analytical application as a filter for emission spectrometry, our Cu CFSN has been operated in a pulsed mode of the GD to allow measuring in the afterglow, and to avoid strong emission from its discharge. Pulsing was performed at 18 P%xwith a peak discharge current of 200 mA in Ar under a pressure of 670 Pa (5 torr) and with a pulse duration of 1.5 ms. The leakage light owing to the finite polarizer extinction ratio and stray light were corrected [3]. The block diagram of the set-up is shown in
* This paper is dedicated to Prof. Dr D.Sc. Dr h.c.mult. A. Scharmann on the occasion of his 65th birthday. 1093
1094
H.
MATSUTA
et al.
SUPPlY +
Y
Fig. 1. Block diagram of the CFSRM as used as an optical filter to measure atomic emission spectroscopy with high suppression of spectral interferences. (PMT: photomultiplier tube; light source: Grimm-type GD lamp for analytical applications (Section 3.2) or laser for the fundamental investigations (Section 3.1); a, b: control signals; c: PMT output signal.)
1. All results in this paper have been achieved on the 324.8 nm D2 resonance line of copper. For the high resolution measurements by laser spectroscopy, a GD was used in d.c. mode, which continuously produced atomic copper vapour. The GD was operated with Ar under the same pressure of 670 Pa (5 torr) and a current of 20-30 mA. The laser set-up for high resolution spectroscopy is shown in Fig. 2. An argon ion laser (Coherent Innova lOO-18UV) pumped an actively stabilized cw dye-ring laser (Spectra 380D). Additional details have been reported [5]. The frequency of laser radiation was doubled by a lithium iodate crystal [6] and the linewidth of the radiation at 324.8 nm was 7 x 1W7 nm (2 MHz at 923 THz). For precise spectral calibration, a marker cavity (separation of calibration marks, i.e. free spectral range: 150.4 MHz or 5.017 x 10m3 cm-‘) was used. Its optical length could be piezoelectrically locked to a He+Ne laser that is absolutely stabilized on the transmission lamb-dip of a lz9b cell. A laboratory computer was operated as a 4096-channel analyser, recording simultaneously the CFS transmission spectrum with a 1Zbit resolution, as well as the frequency marks and, in addition, the absorption of another cell filled with I2 for coarse absolute calibration of the spectra. As the analytical light source, a laboratory-constructed Grimm-type GD source with an alloy Fig.
Fig. 2. Experimental
set-up for measuring the transmission line profiles of the CFSFUA with high resolution.
CFSRM: transmission line structure
1095
4
3 300mT
H
0.004 nm a
Wavelength Fig. 3. Cu CFSRM transmission line structure measured with high resolution at different magnetic fields. Vertical lines indicate the positions of the two resolved Cu Dz line (hfs) components.
cathode was used. Measurements were carried out with cathodes of Fe-Cu of JSS materials (Japanese Standard Samples of iron and steel): FXS350 (2.0 mg g-’ Cu in Fe), FXS351 (5.0 mg g-l Cu in Fe), and FXS352 (10.0 mg g-l Cu in Fe). Also, an Fe-Si alloy, FXS302 (0 mg g-l Ca, 2.0 mg g-’ Si in Fe), was used. The spectra of the Grimm-type GD lamp were recorded with a spectrometer (Hitachi P-5200 ICP instrument, grating 3600 lines mm-‘, focal length 75 cm, resolution 0.007 nm, 10 km slit).
3. MEASUREMENTS, RESULTSAND DISCUSSION 3.1. High resolution laser measurements
First, the dependence of the transmission profile on the applied magnetic field, B, investigated. When the applied magnetic field was increased, the intensity transmitted by the Cu CFSRM at the wavelengths of the (hfs) components of the & emission line reached a maximum at about B = 300 mT. At higher field strength, the transmitted intensity decreased, passed a local minimum at about 500 mT and then increased again [3]. High resolution transmission spectra of the Cu CFSRM measured at 100 mT, 300 mT and 500 mT are shown in Fig. 3. The spectra are normalized so that the maximum peak intensity of each profile is unity. The vertical lines indicate the positions of the (hfs) components of the copper resonance line at (h,(D,)) = 324.754 nm (centre of mass air wavelength, vacuum wavelengths determined by comparison with the IZ spectrum hv(DZ,Fg = 2) = 324.8492, hv(DZ,Fg = 1) = 324.8452, and (A,(&)) = 324.8478 -C 0.0004 nm) with a separation of 0.0040 nm owing mainly to the ground state (hfs) structure. The peak transmission at 100 mT is about 5% of the source intensity transmitted in the open pair polarizer position (i.e. with the analyser adjusted parallel to the polarizer). At 100 mT, the transmission spectrum of the CFSRM is overlapping very well with zero-field (hfs) components of the Cu line. At 300 mT, the peak transmission increases was
1096
H. MATSUTAer al.
0.004
nm
.
Wavelength Fig. 4. Cu CFSRM transmission line structure measured with high resolution at different number densities of Cu atoms: (a) appropriate; (b) excessive density.
to more than 10% at the position of the (hfs) components, but the CFSRM is also transmitting all wavelengths between the (hfs) components. At a magnetic field strength of 500 mT, transmission decreases at the wavelengths of the (hfs) components but increases (maximum about 18%) at the line peaks, which are shifted far to the red wings of the (hfs) components. This is due to many-fold “line-crossings” of perpendicularly polarized n- and o-components, and also corresponds to the wellknown reduction of sensitivity in Zeeman atomic absorption spectrometry at such field strengths. By changing the GD current, the number of resonant atoms was varied [7]. Figure 4(a) shows a transmission spectrum measured at B = 100 mT with an appropriate Cu atomic density (discharge current 20 mA). At an excessive Cu density (discharge current 30 mA), the transmission spectrum of the Cu CFSRM is drastically changed as shown in Fig. 4(b). The transmission at the zero-field (hfs) components of the Cu D2 line decreases and the transmission profile of the Zeeman (hfs) components is spread over a broader wavelength range around the (hfs) components (also between the zero-field (hfs) line components). The given results show that Cu CFSRM should work at 100 mT and with appropriate number density of Cu atoms to obtain a narrow transmission profile and high reduction of spectral interferences. A detailed theoretical discussion of the spectral peculiarities, which are different from that shown for sodium [8], taking into account magnetic dipole and electric quadrupole (hfs) interaction of both states, will be presented in a future paper [9]. 3.2. Analytical examples The radiation from a Grimm-type GD source, with cathodes of certified standard materials of Fe alloy containing trace impurities of copper, was filtered by a Cu CFSRM in order to investigate the performance to suppress spectral interferences by closely neighbouring lines (Fe emission). Emission spectra of the same material and of Fe-Si alloy recorded with a conventional spectrometer (resolution’0.007 nm) are presented in Fig. 5. The spectrum in Fig. 5(a) is obtained from Fe containing 2 mg g -l Si, which is certified to contain no Cu, and Fig. 5(b) shows a spectrum of Fe with 10 mg g-l Cu. Both spectra are normalized so that the maximum peak intensity is unity. With this conventional spectrometer, the emission line obtained from Fe-Si alloy at 324.75 nm (all wavelengths in this paper
CF?SRM:transmission line structure
I
1097
!
I
,
324.65
324.75
324.85
Wavelength ( nm ) Fig. 5. Emission spectra of a Grimm-type GD lamp measured with a conventional grating monochromator (0.007 nm resolution): (a) 2 mg g-* Si; (b) 10 mg g-’ Cu.
are given as air values if not otherwise remarked) was observed. The relative intensity of this line peak increased with a cathode of Fe alloy containing 10 mg g-l Cu as shown in Fig. 5(b). Although Ar also has an emission line at 324.755 nm [lo], the observed line at 324.75 nm is not a result of Ar emission, because it was not observed when Ar was used as discharge gas in combination with a Ni cathode in the same Grimm-type lamp. Since there is no emission line in Si at 324.75 nm, this line is assigned to Fe. When a conventional spectrometer is used, it is extremely difficult to suppress or even to reduce this interference by Fe emission. Cu emits two Doppler-broadened & (hfs) components (if the (hfs) structure of the excited state is neglected) at the wavelengths of 324.752 and 324.756 nm. When the Cu emission spectrum is measured with a conventional spectrometer, one unresolved peak is observed roughly at the centre of mass wavelength at 324.754 nm of the f\yo Cu (hfs) components. If the Fe interference is a result of a single emission line component and if this line has almost the same peak wavelength as the centre of mass wavelength of the Cu line, the Fe emission line will be removed or strongly suppressed by the Cu CFSRM. In the pulsed operation, during the discharge phase of the period, atomic Cu is produced to such an excess that the decay of the atomic density leads to optimum density in the afterglow phase. Thus, the correct excessive GD current must be found to realize an optimal, narrow CFSRM bandpass after pulse termination. In the afterglow period, the transmission of the Cu CFSRM for & resonance radiation first increases, when the number density of Cu atoms in the atomic vapour is still in the excekive range (see Fig. 4). Then, transmission reaches its maximum during the decay of the Cu den&y, and decreases again after the optimum is past. The time dependence of the transmission in pclsed operation measured at B = 100 mT with a boxcar averager (Statiford Research Systems SRS250; gate time: 15 IJ.S)is shown in Fig. 6. The duiation of the excessive atomic density after pulse termination is 5.5 ms. In the analytical experiments, the signal is suppressed during the pre-selected delay time in the afterglow period by a laboratory-constructed gated amplifier (gate time: 17 ms) in order to obtain an optimal transmission profile and optimal suppression of spectral interferences. Light from the Grimm-type GD source with Fe-Q alloy cathodes containing Cu with 0, 2, 5 and 10 mg g-l was filtered by the Cu CFSRM. Using delay times of 1.5 and 6.5 ms, the intensity transmitted from the Grimm-type source has been measured for each cathode material. In order to compensate for the
1098
H.
0
MATWJTA
2
et al.
4
8
Cu
IO
&h3~
Fig. 6. Transmission intensity measured as a function of the delay time after termination pulsed GD atomization. Magnetic field: 100 mT; sampling gate width: 15 t.rs.
of
0.80.6-
0.20.0-f, 2
I
4
8.
6
I.
8
I
10
I
I2
Time (ms) Fig. 7. Normalized analytical curve for Cu obtained with B = 100 mT.
variation of transmission efficiency, the data were normalized so that the slopes of both analytical curves for 1.5 and 6.5 ms delay became unity. Then, the value at zero Cu concentration is the normalized signal of Fe at 324.75 nm. The results are shown in Fig. 7. By suppressing the first part of the signal during the excessive atomic density period (O-6.5 ms), the relative transmission of the Fe emission line could be reduced to about half of that obtained by suppressing the period during O-l.5 ms. By applying a magnetic field strength B = 300 mT to the CFSRM, which also yields high transmission for the & resonance line, the Fe emission signal could not be reduced regardless of the delay time of the sampling gate. The normalized transmission for the Fe line at B = 300 mT is always roughly the same as for 1.5 ms in combination with B = 100 mT. These results are in good agreement with the laser spectroscopic measurements.
4. CONCLUSIONS When appropriate operating conditions are used, the CFSRM works as a narrow optical band-filter, which can reduce interferences from extremely close-lying lines. Its bandwidth and its structure depends on the applied magnetic field and on the atomic vapour density, but can be matched to the zero-field Cu emission line. Thus, the (hfs) components of the resonance line of the respective element can pass the CFSRM with relatively high transmission efficiency. REFERENCES [l] H. [2] H. (31 H. [S] G.
Matsuta Matsuta Matsuta S. Jolly
and and and and
K. K. K. R.
Hirokawa, Specrrochim. Actu 43B, 159 (1988). Hirokawa, Spectrochim. Acta 43B, 1255 (1988). Hirokawa, Anal Chem. 63, 1747 (1991). Stephens, Anal. Chim. Actu 139, 323 (1982).
CFSRM: transmission line structure [5] [6] [7] [8] [9]
1099
M. Grexa, G. Hermann, G. Lasnitschka and B. Fricke, Phys. Rev. 38, 1263 (1988). H. Buesener, A. Renn, M. Brieger, F. von Moers and A. Hese Appl. Phys. B 39, 77 (1986). P. W. J. M. Boumans, Anal. Chem. 44, 1219 (1972). M. Gross and G. Hermann, Spectrochim. Actu 48B, 1079 (1993). G. Hermann, C. Schwabe, T. Udem, K. Hirokawa and H. Matsuta, to be submitted to Specrrochimicu Acta Part B. [lo] M. I. T. Wavelength Tables, Vol. 2: Wavelengrhs by Elements. The MIT Press, MA, U.S.A. (1982).
1993 0
0584-8547/93 $6.00 + zoo 1993 Pergamon Ress Ltd
Transmission line structure of a coherent forward scattering resonance monochromator measured by high resolution laser spectroscopy* HIDEYUKI MATWTA and KICHINOSUKE HIROKAWA Institute for Materials Research, Tohoku University, Katahira 2-1-1, 980 Sendai, Japan
and GERD HERMANN and CARSTEN SCHWABE I. Physikalisches Institut, Justus-Liebig-Universitat, Heinrich-Buff-Ring 16, D-W-6300 Giessen, F.R.G. (Received 26 January 1993; accepted 10 March 1993)
Abstract-Transmission line structures of a copper coherent forward scattering resonance monochromator (CFSRM) with d.c. glow discharge atomization have been measured by high resolution laser spebroscopy. Based on these measurements, optimti conditions have been chosen and were used for a Cu CF8RM using atomization in a pulsed glow discharge. Spectra emitted by a Grimm-type source with a Cu-Fe cathode were filtered through the Cu CFSRM. In consequence, suppression of spectral interferences from an extremely close-lying Fe emission line could be better achieved than by using a conventional grating monochromator.
1. INTRODUCTION COHERENT forward scattering resonance monochromator (CFSRM), employing sputtering atomization by an argon glow discharge (GD), has been developed [l-3]. The bandwidth of the CFSRM is narrow, comparable to the linewidth of a hollow cathode lamp (HCL) [4], and the transmission efficiency is relatively high [3, 41. Thus, the CFSRM is expected to improve the detection limits (LODs) and the accuracy of atomic emission spectrometry by suppressing continuum background as well as spectral line interferences. In this work, the spectral transmission of a CFSRM for the atomic emission line of an HCL has been investigated in order to optimize the transmission profile. Employing high resolution laser spectroscopy, the transmission line profiles of a copper CFSRM were measured and the influence of the magnetic field strength and of the GD current on the transmission line structure has been studied. Based on these results, optimal operation conditions were chosen to which the Cu CFSRM was set when the analytical performance was proven. Cu in Fe has been determined by measuring the emission on the 324.8 nm resonance line and suppressing interferences by an extremely closelying Fe line (separation less than 0.002 nm), which is observed in the gap between the two copper hyperfine structure (hfs) components. A
2. APPARATUS The CFSRM was the same as previously reported [3]. For analytical application as a filter for emission spectrometry, our Cu CFSN has been operated in a pulsed mode of the GD to allow measuring in the afterglow, and to avoid strong emission from its discharge. Pulsing was performed at 18 P%xwith a peak discharge current of 200 mA in Ar under a pressure of 670 Pa (5 torr) and with a pulse duration of 1.5 ms. The leakage light owing to the finite polarizer extinction ratio and stray light were corrected [3]. The block diagram of the set-up is shown in
* This paper is dedicated to Prof. Dr D.Sc. Dr h.c.mult. A. Scharmann on the occasion of his 65th birthday. 1093
1094
H.
MATSUTA
et al.
SUPPlY +
Y
Fig. 1. Block diagram of the CFSRM as used as an optical filter to measure atomic emission spectroscopy with high suppression of spectral interferences. (PMT: photomultiplier tube; light source: Grimm-type GD lamp for analytical applications (Section 3.2) or laser for the fundamental investigations (Section 3.1); a, b: control signals; c: PMT output signal.)
1. All results in this paper have been achieved on the 324.8 nm D2 resonance line of copper. For the high resolution measurements by laser spectroscopy, a GD was used in d.c. mode, which continuously produced atomic copper vapour. The GD was operated with Ar under the same pressure of 670 Pa (5 torr) and a current of 20-30 mA. The laser set-up for high resolution spectroscopy is shown in Fig. 2. An argon ion laser (Coherent Innova lOO-18UV) pumped an actively stabilized cw dye-ring laser (Spectra 380D). Additional details have been reported [5]. The frequency of laser radiation was doubled by a lithium iodate crystal [6] and the linewidth of the radiation at 324.8 nm was 7 x 1W7 nm (2 MHz at 923 THz). For precise spectral calibration, a marker cavity (separation of calibration marks, i.e. free spectral range: 150.4 MHz or 5.017 x 10m3 cm-‘) was used. Its optical length could be piezoelectrically locked to a He+Ne laser that is absolutely stabilized on the transmission lamb-dip of a lz9b cell. A laboratory computer was operated as a 4096-channel analyser, recording simultaneously the CFS transmission spectrum with a 1Zbit resolution, as well as the frequency marks and, in addition, the absorption of another cell filled with I2 for coarse absolute calibration of the spectra. As the analytical light source, a laboratory-constructed Grimm-type GD source with an alloy Fig.
Fig. 2. Experimental
set-up for measuring the transmission line profiles of the CFSFUA with high resolution.
CFSRM: transmission line structure
1095
4
3 300mT
H
0.004 nm a
Wavelength Fig. 3. Cu CFSRM transmission line structure measured with high resolution at different magnetic fields. Vertical lines indicate the positions of the two resolved Cu Dz line (hfs) components.
cathode was used. Measurements were carried out with cathodes of Fe-Cu of JSS materials (Japanese Standard Samples of iron and steel): FXS350 (2.0 mg g-’ Cu in Fe), FXS351 (5.0 mg g-l Cu in Fe), and FXS352 (10.0 mg g-l Cu in Fe). Also, an Fe-Si alloy, FXS302 (0 mg g-l Ca, 2.0 mg g-’ Si in Fe), was used. The spectra of the Grimm-type GD lamp were recorded with a spectrometer (Hitachi P-5200 ICP instrument, grating 3600 lines mm-‘, focal length 75 cm, resolution 0.007 nm, 10 km slit).
3. MEASUREMENTS, RESULTSAND DISCUSSION 3.1. High resolution laser measurements
First, the dependence of the transmission profile on the applied magnetic field, B, investigated. When the applied magnetic field was increased, the intensity transmitted by the Cu CFSRM at the wavelengths of the (hfs) components of the & emission line reached a maximum at about B = 300 mT. At higher field strength, the transmitted intensity decreased, passed a local minimum at about 500 mT and then increased again [3]. High resolution transmission spectra of the Cu CFSRM measured at 100 mT, 300 mT and 500 mT are shown in Fig. 3. The spectra are normalized so that the maximum peak intensity of each profile is unity. The vertical lines indicate the positions of the (hfs) components of the copper resonance line at (h,(D,)) = 324.754 nm (centre of mass air wavelength, vacuum wavelengths determined by comparison with the IZ spectrum hv(DZ,Fg = 2) = 324.8492, hv(DZ,Fg = 1) = 324.8452, and (A,(&)) = 324.8478 -C 0.0004 nm) with a separation of 0.0040 nm owing mainly to the ground state (hfs) structure. The peak transmission at 100 mT is about 5% of the source intensity transmitted in the open pair polarizer position (i.e. with the analyser adjusted parallel to the polarizer). At 100 mT, the transmission spectrum of the CFSRM is overlapping very well with zero-field (hfs) components of the Cu line. At 300 mT, the peak transmission increases was
1096
H. MATSUTAer al.
0.004
nm
.
Wavelength Fig. 4. Cu CFSRM transmission line structure measured with high resolution at different number densities of Cu atoms: (a) appropriate; (b) excessive density.
to more than 10% at the position of the (hfs) components, but the CFSRM is also transmitting all wavelengths between the (hfs) components. At a magnetic field strength of 500 mT, transmission decreases at the wavelengths of the (hfs) components but increases (maximum about 18%) at the line peaks, which are shifted far to the red wings of the (hfs) components. This is due to many-fold “line-crossings” of perpendicularly polarized n- and o-components, and also corresponds to the wellknown reduction of sensitivity in Zeeman atomic absorption spectrometry at such field strengths. By changing the GD current, the number of resonant atoms was varied [7]. Figure 4(a) shows a transmission spectrum measured at B = 100 mT with an appropriate Cu atomic density (discharge current 20 mA). At an excessive Cu density (discharge current 30 mA), the transmission spectrum of the Cu CFSRM is drastically changed as shown in Fig. 4(b). The transmission at the zero-field (hfs) components of the Cu D2 line decreases and the transmission profile of the Zeeman (hfs) components is spread over a broader wavelength range around the (hfs) components (also between the zero-field (hfs) line components). The given results show that Cu CFSRM should work at 100 mT and with appropriate number density of Cu atoms to obtain a narrow transmission profile and high reduction of spectral interferences. A detailed theoretical discussion of the spectral peculiarities, which are different from that shown for sodium [8], taking into account magnetic dipole and electric quadrupole (hfs) interaction of both states, will be presented in a future paper [9]. 3.2. Analytical examples The radiation from a Grimm-type GD source, with cathodes of certified standard materials of Fe alloy containing trace impurities of copper, was filtered by a Cu CFSRM in order to investigate the performance to suppress spectral interferences by closely neighbouring lines (Fe emission). Emission spectra of the same material and of Fe-Si alloy recorded with a conventional spectrometer (resolution’0.007 nm) are presented in Fig. 5. The spectrum in Fig. 5(a) is obtained from Fe containing 2 mg g -l Si, which is certified to contain no Cu, and Fig. 5(b) shows a spectrum of Fe with 10 mg g-l Cu. Both spectra are normalized so that the maximum peak intensity is unity. With this conventional spectrometer, the emission line obtained from Fe-Si alloy at 324.75 nm (all wavelengths in this paper
CF?SRM:transmission line structure
I
1097
!
I
,
324.65
324.75
324.85
Wavelength ( nm ) Fig. 5. Emission spectra of a Grimm-type GD lamp measured with a conventional grating monochromator (0.007 nm resolution): (a) 2 mg g-* Si; (b) 10 mg g-’ Cu.
are given as air values if not otherwise remarked) was observed. The relative intensity of this line peak increased with a cathode of Fe alloy containing 10 mg g-l Cu as shown in Fig. 5(b). Although Ar also has an emission line at 324.755 nm [lo], the observed line at 324.75 nm is not a result of Ar emission, because it was not observed when Ar was used as discharge gas in combination with a Ni cathode in the same Grimm-type lamp. Since there is no emission line in Si at 324.75 nm, this line is assigned to Fe. When a conventional spectrometer is used, it is extremely difficult to suppress or even to reduce this interference by Fe emission. Cu emits two Doppler-broadened & (hfs) components (if the (hfs) structure of the excited state is neglected) at the wavelengths of 324.752 and 324.756 nm. When the Cu emission spectrum is measured with a conventional spectrometer, one unresolved peak is observed roughly at the centre of mass wavelength at 324.754 nm of the f\yo Cu (hfs) components. If the Fe interference is a result of a single emission line component and if this line has almost the same peak wavelength as the centre of mass wavelength of the Cu line, the Fe emission line will be removed or strongly suppressed by the Cu CFSRM. In the pulsed operation, during the discharge phase of the period, atomic Cu is produced to such an excess that the decay of the atomic density leads to optimum density in the afterglow phase. Thus, the correct excessive GD current must be found to realize an optimal, narrow CFSRM bandpass after pulse termination. In the afterglow period, the transmission of the Cu CFSRM for & resonance radiation first increases, when the number density of Cu atoms in the atomic vapour is still in the excekive range (see Fig. 4). Then, transmission reaches its maximum during the decay of the Cu den&y, and decreases again after the optimum is past. The time dependence of the transmission in pclsed operation measured at B = 100 mT with a boxcar averager (Statiford Research Systems SRS250; gate time: 15 IJ.S)is shown in Fig. 6. The duiation of the excessive atomic density after pulse termination is 5.5 ms. In the analytical experiments, the signal is suppressed during the pre-selected delay time in the afterglow period by a laboratory-constructed gated amplifier (gate time: 17 ms) in order to obtain an optimal transmission profile and optimal suppression of spectral interferences. Light from the Grimm-type GD source with Fe-Q alloy cathodes containing Cu with 0, 2, 5 and 10 mg g-l was filtered by the Cu CFSRM. Using delay times of 1.5 and 6.5 ms, the intensity transmitted from the Grimm-type source has been measured for each cathode material. In order to compensate for the
1098
H.
0
MATWJTA
2
et al.
4
8
Cu
IO
&h3~
Fig. 6. Transmission intensity measured as a function of the delay time after termination pulsed GD atomization. Magnetic field: 100 mT; sampling gate width: 15 t.rs.
of
0.80.6-
0.20.0-f, 2
I
4
8.
6
I.
8
I
10
I
I2
Time (ms) Fig. 7. Normalized analytical curve for Cu obtained with B = 100 mT.
variation of transmission efficiency, the data were normalized so that the slopes of both analytical curves for 1.5 and 6.5 ms delay became unity. Then, the value at zero Cu concentration is the normalized signal of Fe at 324.75 nm. The results are shown in Fig. 7. By suppressing the first part of the signal during the excessive atomic density period (O-6.5 ms), the relative transmission of the Fe emission line could be reduced to about half of that obtained by suppressing the period during O-l.5 ms. By applying a magnetic field strength B = 300 mT to the CFSRM, which also yields high transmission for the & resonance line, the Fe emission signal could not be reduced regardless of the delay time of the sampling gate. The normalized transmission for the Fe line at B = 300 mT is always roughly the same as for 1.5 ms in combination with B = 100 mT. These results are in good agreement with the laser spectroscopic measurements.
4. CONCLUSIONS When appropriate operating conditions are used, the CFSRM works as a narrow optical band-filter, which can reduce interferences from extremely close-lying lines. Its bandwidth and its structure depends on the applied magnetic field and on the atomic vapour density, but can be matched to the zero-field Cu emission line. Thus, the (hfs) components of the resonance line of the respective element can pass the CFSRM with relatively high transmission efficiency. REFERENCES [l] H. [2] H. (31 H. [S] G.
Matsuta Matsuta Matsuta S. Jolly
and and and and
K. K. K. R.
Hirokawa, Specrrochim. Actu 43B, 159 (1988). Hirokawa, Spectrochim. Acta 43B, 1255 (1988). Hirokawa, Anal Chem. 63, 1747 (1991). Stephens, Anal. Chim. Actu 139, 323 (1982).
CFSRM: transmission line structure [5] [6] [7] [8] [9]
1099
M. Grexa, G. Hermann, G. Lasnitschka and B. Fricke, Phys. Rev. 38, 1263 (1988). H. Buesener, A. Renn, M. Brieger, F. von Moers and A. Hese Appl. Phys. B 39, 77 (1986). P. W. J. M. Boumans, Anal. Chem. 44, 1219 (1972). M. Gross and G. Hermann, Spectrochim. Actu 48B, 1079 (1993). G. Hermann, C. Schwabe, T. Udem, K. Hirokawa and H. Matsuta, to be submitted to Specrrochimicu Acta Part B. [lo] M. I. T. Wavelength Tables, Vol. 2: Wavelengrhs by Elements. The MIT Press, MA, U.S.A. (1982).