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Evaluation of a pulsed flash-tube for inductively-coupled plasma atomic-fluorescence spectrometry
T~hra, Vol. 36, No. l/2, PP. 311-314, 1989 Printed in Great Britain. All tights reserved
0039-9140/89$3.00+ 0.00 Copyright 0 1989Pergamon Press plc
EVALUATION OF A PULSED FLASH-TUBE FOR INDUCTIVELY-COUPLED PLASMA ATOMIC-FLUORESCENCE SPECTROMETRY KRIJPA* and J. D. WINEFORDNER~ Department of Chemistry, University of Florida, Gainesville, FL 32611, U.S.A.
M. A. MIGNARDI, B. W. SMITH,B. T. JONES,R. J.
(Received 6 May 1988. Revised 22 August 1988. Accepted 8 September 1988) Summary-A pulsed continuum xenon flash-tube is used as an excitation source in ICP-AFS. The simultaneous excitation of many elements is attractive and avoids the one source per element as in conventional ICP-AFS. In this study, the pulsed continuum flash-tube is evaluated as an excitation source for multielement atomic fluorescence snectrometrv in an ICP. Analytical figures of merit are given for the elements studied.
The most commonly used multielement atomic spectroscopy technique is inductively-coupled plasma atomic-emission spectrometry (ICP-AES), which achieves @ml detection limits, linear calibration over a wide range, excellent precision, and measurements free from matrix effects for most elements.‘-3 More recent techniques include the multielement hollow-cathode l~~inductively~oupied plasmaato~c-~uores~nce spectrometer (HCL-ICP-AFS), inductively-coupled spectrometer plasma-mass (ICP-MS), and continuum source-furnace-atomicabsorption spectrometer (CSF-AAS) combinations-’ The fluorescence approach results in lower detection limits, greater spectral selectivity, and a reduced emission background;4*5 the furnace atomicabsarption approach is applicable to smaller sample amounts than the ICP-AES approach. In our present study, we decided to capitalize on the simplicity of the atomic-fluore~nce spectra of virtually all elements, as well as the possibility of exciting all atoms (and/or ions) simultaneously by means of a spectralcontinuum light-source.@ To increase the source spectral irradiance,9 especially in the ultraviolet, we used a repetitively pulsed xenon dash-Tut, and a gated detector to increase the measured signal-tonoise ratio. Here, we describe the experimental system and give some initial analytical figures of merit.
fnstrumentation A schematic diagram of the system used is shown in Fig. 1. The experimental components and manufacturers are listed in Table 1. Source radiation from the pulsed flash-tube was focused into the ICP by using two lenses, Li and L2 (both with diameter and focal lengths of JO mm): L, collimated the light from the flash-tube and Lr focused the collimated light onto the center of the ICP, above the load coils. The diameter of the focused beam was approximately *Present address: Baird Corp., Redford, MA 01730. tAuthor to whom correspondence should be sent. TAL. 34/1-2-u
8 mm. Several precautions were taken to reduce the amount of scattered light reaching the monochromator. A light-trap was placed opposite the ICP, and blackened bafIIes were set up around the ICP. The collection lens, 5 (focal length 178 mm), was also enclosed within a blackened tube to further reduce scattered light. The limiting noise of the system was due to scattered light from the excitation source. The 300-W xenon flash-tube was enclosed within a fancooled housing having a front-surface spherical mirror (focal length 31.5 mm) and collimating lens (L,). The lamp was operated from a pulsed 5-kV power supply. A 0.2~PF discharge capacitor provided an input energy of 5 J per flash, a flash half-width of cu. 680 nsec, and a peak power of 4.2 kW. The lamp was pulsed at 20 Hz. To help reduce radiofrequency (rf) noise, the entire lamp housing was surrounded and grounded with copper wire cloth acting like a Faraday cage. The fluorescence radiation was collected at 90” to the excitation beam and a 1: 1 image of the ICP was focused onto the entrance slit of the monochromator (focal length 350 mm, reciprocal dispersion 20 &mm, and aperture f 6.8). So as to not overfill the monochromator collimator, an iris diaphragm was placed between Ls and the mon~hromator. A small fraction of the excitation light from the flash-tube was reflected to a photodiode which triggered the boxcar averager. The photocurrent pulse produced by the photomultiplier tube (PMT) was passed through a 1000-R load resistance directly into the boxcar input. The resulting signal pulse had an FWHM of cu. 1.5 psec. The boxcar gate delay time (the time between the trigger pulse and the start of the measurement) was 700 nsec. The gate width (the time during which fluorescence was measured) was 1.8 ~sec for all cases. Thirty signals (i.e., 30 lamp flashes) were averaged for each output signal. The “busy out” of the boxcar averager triggered the Stanford analog-to~i~tal (A/D) system to measure the output signal. Horizontal and vertical movement of the ICP torch was accomplished with two single-axis mounts allowing horizontal translation up to ca. 100 mm and vertical translation up to cu. 80.mm. The ICP concentric pneumatic nebulizer was fed by peristaltic pump to permit a lower sample uptake rate and thus reduce salt encrustation in the torch. The plasma operating conditions are listed in Table 2. Reagents and procedure All components of the experimental system were operated according to the directions given in the manufacturers’ manuals. The stock solutions were made with reagentgrade compounds recommended by Parsons et uL,‘~ and 311
M. A. MIGNARDIet nl.
312
I
HV
I
nun
L!OD
Fig. i. Schematic diagram of the experiment set-up.
Table 1. Exuerimental components Equipment
Model
Manufacturer
Micropulser power supply
457A
Xenon Corporation,
Flash-tube
Novatron-722
Xenon Corporation
SRS gated integrator and boxcar averager
SR 250
Stanford Research Systems, Inc., Palto Alto, CA 94306
SRS computer interface
SR 245
Stanford Research Systems, Inc.
Digital oscilloscope
2430A
Tektronix, Inc., Beaverton, OR 97077
Monochromator
EU-700-77
GCA/McPherson
R 928
Hamamatsu, Wattham, MA 02145
412A
John Fluke Mfg., Co., Inc., Seattle, WA
Photomultiplier
tube
PMT high-voltage power supply
Wobum, MA 01801
Co., Acton, MA 01720
Trigger photodiode
FND 100
EG & G Electra-Optics, Salem, MA 01970
Radiofrequency generator
HFP 1500D
Plasma-Therm Inc., Kresson, NJ 08053
ICP Plasma-Therm torch
Standard and tong
Precision Glassblowing of Colorado, Parke, CO 80134
-
ICP Plasma-Therm concentric nebulizer
Precision Glassblowing of Colorado
Microcomputer
PC-XT
International Business Machines Corp., Boca Raton, FL
Peristaltic pump
Rabbit
Rainin Instrument Co., Inc., Boston, MA
Table 2. Optimal experimental conditions* Element line?
I AFS3
rf power, W
Observation height, mm
Slit-width,
nm
Ba(II) Ca(il) Cd(I) Na(1) VI) Cd(V§ Mixture$
455.4 393.4 228.6 589.6 292.4 228.6 200-800
;: 500 500 700 700 600
15 27 15 IS 15 45 15
2000 1500 900 2000 1100 1000 1500
Pm
*Other experimental conditions: sample uptake rate 1.15 ml/m& nebulizer pressure 31 psig; plasma Ar flow-rate 15 l./min; auxiliary Ar flow-rate 1-3 Ljmin. t(I) indicates an atomic line, (II) an ionic line (singly ionized). $Analysed with a long torch. $A mixture of the 5 elements at concentrations of 20 ppm for each element.
Evaluation of a pulsed flash-tube
313
diluted as required. Distilled demineralized water was used throughout. For each element studied, the optimal ICP rf power, observation height above the ICP load coil, monochromator slit-width, and ICP gas flows were determined. Calibration graphs and limits of detection were also determined for each element. For measurement of the synthetic mixture containing five elements, compromise values of the operating parameters were necessary. A comparison was also made of the results obtained with an extended ICP torch and the conventional torch used for most parts of this study. An extended ICP torch has an outer sleeve that is 40 mm longer than that of the conventional torch. Finally, we looked at
the possibility of double resonance excitation of Ca by use of the flash-tube. RESULTS AND DISCUSSION
Molecular species of non-refractory elements typically have low molecular dissociation energies and are easily atomized by the plasma at lower rf powers, whereas those of the refractory elements typically have high molecular dissociation energies and are atomized by the plasma at higher rf powers. Also, the atomic and ionic populations of the sample species in the plasma are often greatly affected by the choice of rf power and observation height.‘*‘OFor these initial studies, the dependences of the fluorescence on the rf power and observation height were examined independently, as shown in Figs. 2 and 3 respectively. Figure 4 shows the variation in fluorescence signalto-noise with monochromator slit-width for each element (at optimal rfpower and observation height). Table 2 lists the optimal conditions (found by univariate search) for each element, based on Figs. 2-4. Typical calibration plots obtained under the optimal conditions are shown in Fig. 5. Table 3 lists the analytical figures of merit. The detection limit is defined as the concentration in pg/ml of the element in pure aqueous solution resulting in a signal that is three times the standard deviation of the blank
k-
1808
4 8
StF
PONNR
fNl
Fig. 3. The effect of rf power on the fluorescence signal for each element.
I@
V
‘00 SLIT-WIDTH
15'00
20'00
( ,u,m)
Fig. 4. The variation in fluorescence signal-to-noise ratio for each element as a function of spectrometer slit-width.
.
I-6 if
2O
25 SO 86 OBS~RVATSON
4O 46 50 66 HlliIGIiT (mm)
6@
66
Fig. 2. The efkct of observation height above the load coil on the fluorescence signal for each element.
~ LOG [CONCENTRATION
(ppm)l
Fig. 5. Analytical calibration curves for each element.
314
M. A.
MIGNARDI et al.
Table 3. Analvtical fiaures of merit Element line*
Sensitivity,t mV.ml.fig-
Ba(I1) Ca(II) Cd(I) Na(I) V(II) Cd(I)§
LOD, Lcnlml
Literature LOD,” IIKlml
0.09 0.03 0.02 0.04 0.4 0.04
0.05 0.0004 0.0005 0.0003 0.1 -
14.1 18.4 26.3 10.7 1.88 14.9
*See footnote t to Table 2. tReferred to the boxcar input. $Measured with a long torch.
next most intense fluorescence line of vanadium at 292.4 nm, was observed. We were unable to detect any instances of double resonance fluorescence of calcium as described by Omenetto et al., I3 who observed double resonance excitation of Ca with two different dye lasers (pumped by an excimer laser), one laser giving excitation at 396.847 nm and the other at 370.603 nm, the fluorescence being observed at 373.690 nm. CONCLUSION
measurements. The electronic band-width for all measurements was cu. 1 Hz. Previously reported ICP-AFS detection limits for the same elements are also listed in Table 3. All the log-log calibration plots have slopes between 0.96 and 1.06. The effect of using a long torch was examined for cadmium as model element. The results are shown in Figs. 2-5 and the corresponding curves are labeled Cd(L). With the long torch, the fluorescence signal was practically constant at observation heights 45560 mm above the load coil, whereas with the standard torch the signal rapidly decreased with increase in observation height (Fig. 2). The variation in signal with increase in rf power was similar in pattern for both torches, but the signal was significantly larger with the standard torch (Fig. 3). A similar difference was observed for the effect of monochromator slit-width on the signal-to-noise ratio, and again the standard torch gave much the better performance (Fig. 4). Despite this, the detection limits for cadmium were essentially the same with both torches (Table 3). Figure 6 shows a multielement scan of a synthetic mixture of the five elements (20 pg/ml each), with a standard torch. The most intense fluorescence line of each element except vanadium was excited and observed (see Table 2). The most intense fluorescence signals for vanadium are at 309-310 nm, i.e., in the middle of a strong OH band.” Therefore, the
The work presented thus far shows promising results for the use of a flash-tube in ICP-AFS. The present system would appear to have considerable use in multielement analysis even though the detection limits for each element were at least an order of magnitude poorer than the best reported in the literature. Future work on this project will involve studies to improve the detection limits so as to be competitive with the present commercial atomic spectrometry systems. Improvements could be made by better rf shielding, decreased excitation source scatter, and better and more efficient collection of the fluorescence signal. Since the limiting noise in our system was due to scattered light from the flash-tube, efforts will also be made to reduce scattered light by using non-resonance fluorescence emission filters and better light-traps. An ultrasonic nebulizer will also be used for the more efficient sample uptake into the ICP. Acknowledgements-This research was supported by NIHGM-38434-01. The authors wish to thank Thomas J. Manning and Dr. Moi B. Leong for their initial help in setting up the ICP. REFERENCES
1. G. Tolg, Analysf, 1987, 112, 365. 2. J. A. C. Broekaert, Anal. Chim. Acta, 1987, 196, 1. 3. J. A. C. Broekaert and G. Tolg, Z. Anal. Chem., 1987, 326, 495.
4. A. Montaser and V. A. Fassel, Anal. Chem., 1976, 48, 1940. 5. L. A. Davis, R. J. Krupa and J. D. Winefordner, Anal. Chim. Acta, 1985, 173, 512.
6. N. Gmenetto and J. D. Winefordner,
in Inductively Coupled Plasmas in Analytical Atomic Spectrometry,
A. Montaser and D. W. Golightly - _.(eds.), p. 323. VCH, New York, 1987. 7. D. J. Johnson, W. K. Fowler and J. D. Winefordner, Talanta, 1977, 24, 227.
I
200
300
Fl uorescence
I 400
I 500
8. D. J. Johnson, F. W. Plankey and J. D. Winefordner, Anal. Chem., 1974, 46, 1898. 9. G. Beck, Reo. Sri. Instrum., 1974, 45, 318. 10. M. L. Parsons. B. W. Smith and G. E. Bentlev, Handbook of Flame Specwoscopy, p. 16. Plenum Press, New York. 11. D. R. Demers, D. A. Busch and C. D. Allemand,
I
Am. Lab., 1982, 22, No. 3, 167.
600
Wave1 ength
(nm)
Fig. 6. A multielement analysis of a synthetic mixture of the five elements studied.
12. R. J. Krupa, G. L. Long and J. D. Winefordner, Spectrochim. Acra, 1985, 4OB, 1485. 13. N. Omenetto, B. W. Smith, L. P. Hart, P. Cavalli and G. Rossi, ibid., 1985, 4OB, 1411.
0039-9140/89$3.00+ 0.00 Copyright 0 1989Pergamon Press plc
EVALUATION OF A PULSED FLASH-TUBE FOR INDUCTIVELY-COUPLED PLASMA ATOMIC-FLUORESCENCE SPECTROMETRY KRIJPA* and J. D. WINEFORDNER~ Department of Chemistry, University of Florida, Gainesville, FL 32611, U.S.A.
M. A. MIGNARDI, B. W. SMITH,B. T. JONES,R. J.
(Received 6 May 1988. Revised 22 August 1988. Accepted 8 September 1988) Summary-A pulsed continuum xenon flash-tube is used as an excitation source in ICP-AFS. The simultaneous excitation of many elements is attractive and avoids the one source per element as in conventional ICP-AFS. In this study, the pulsed continuum flash-tube is evaluated as an excitation source for multielement atomic fluorescence snectrometrv in an ICP. Analytical figures of merit are given for the elements studied.
The most commonly used multielement atomic spectroscopy technique is inductively-coupled plasma atomic-emission spectrometry (ICP-AES), which achieves @ml detection limits, linear calibration over a wide range, excellent precision, and measurements free from matrix effects for most elements.‘-3 More recent techniques include the multielement hollow-cathode l~~inductively~oupied plasmaato~c-~uores~nce spectrometer (HCL-ICP-AFS), inductively-coupled spectrometer plasma-mass (ICP-MS), and continuum source-furnace-atomicabsorption spectrometer (CSF-AAS) combinations-’ The fluorescence approach results in lower detection limits, greater spectral selectivity, and a reduced emission background;4*5 the furnace atomicabsarption approach is applicable to smaller sample amounts than the ICP-AES approach. In our present study, we decided to capitalize on the simplicity of the atomic-fluore~nce spectra of virtually all elements, as well as the possibility of exciting all atoms (and/or ions) simultaneously by means of a spectralcontinuum light-source.@ To increase the source spectral irradiance,9 especially in the ultraviolet, we used a repetitively pulsed xenon dash-Tut, and a gated detector to increase the measured signal-tonoise ratio. Here, we describe the experimental system and give some initial analytical figures of merit.
fnstrumentation A schematic diagram of the system used is shown in Fig. 1. The experimental components and manufacturers are listed in Table 1. Source radiation from the pulsed flash-tube was focused into the ICP by using two lenses, Li and L2 (both with diameter and focal lengths of JO mm): L, collimated the light from the flash-tube and Lr focused the collimated light onto the center of the ICP, above the load coils. The diameter of the focused beam was approximately *Present address: Baird Corp., Redford, MA 01730. tAuthor to whom correspondence should be sent. TAL. 34/1-2-u
8 mm. Several precautions were taken to reduce the amount of scattered light reaching the monochromator. A light-trap was placed opposite the ICP, and blackened bafIIes were set up around the ICP. The collection lens, 5 (focal length 178 mm), was also enclosed within a blackened tube to further reduce scattered light. The limiting noise of the system was due to scattered light from the excitation source. The 300-W xenon flash-tube was enclosed within a fancooled housing having a front-surface spherical mirror (focal length 31.5 mm) and collimating lens (L,). The lamp was operated from a pulsed 5-kV power supply. A 0.2~PF discharge capacitor provided an input energy of 5 J per flash, a flash half-width of cu. 680 nsec, and a peak power of 4.2 kW. The lamp was pulsed at 20 Hz. To help reduce radiofrequency (rf) noise, the entire lamp housing was surrounded and grounded with copper wire cloth acting like a Faraday cage. The fluorescence radiation was collected at 90” to the excitation beam and a 1: 1 image of the ICP was focused onto the entrance slit of the monochromator (focal length 350 mm, reciprocal dispersion 20 &mm, and aperture f 6.8). So as to not overfill the monochromator collimator, an iris diaphragm was placed between Ls and the mon~hromator. A small fraction of the excitation light from the flash-tube was reflected to a photodiode which triggered the boxcar averager. The photocurrent pulse produced by the photomultiplier tube (PMT) was passed through a 1000-R load resistance directly into the boxcar input. The resulting signal pulse had an FWHM of cu. 1.5 psec. The boxcar gate delay time (the time between the trigger pulse and the start of the measurement) was 700 nsec. The gate width (the time during which fluorescence was measured) was 1.8 ~sec for all cases. Thirty signals (i.e., 30 lamp flashes) were averaged for each output signal. The “busy out” of the boxcar averager triggered the Stanford analog-to~i~tal (A/D) system to measure the output signal. Horizontal and vertical movement of the ICP torch was accomplished with two single-axis mounts allowing horizontal translation up to ca. 100 mm and vertical translation up to cu. 80.mm. The ICP concentric pneumatic nebulizer was fed by peristaltic pump to permit a lower sample uptake rate and thus reduce salt encrustation in the torch. The plasma operating conditions are listed in Table 2. Reagents and procedure All components of the experimental system were operated according to the directions given in the manufacturers’ manuals. The stock solutions were made with reagentgrade compounds recommended by Parsons et uL,‘~ and 311
M. A. MIGNARDIet nl.
312
I
HV
I
nun
L!OD
Fig. i. Schematic diagram of the experiment set-up.
Table 1. Exuerimental components Equipment
Model
Manufacturer
Micropulser power supply
457A
Xenon Corporation,
Flash-tube
Novatron-722
Xenon Corporation
SRS gated integrator and boxcar averager
SR 250
Stanford Research Systems, Inc., Palto Alto, CA 94306
SRS computer interface
SR 245
Stanford Research Systems, Inc.
Digital oscilloscope
2430A
Tektronix, Inc., Beaverton, OR 97077
Monochromator
EU-700-77
GCA/McPherson
R 928
Hamamatsu, Wattham, MA 02145
412A
John Fluke Mfg., Co., Inc., Seattle, WA
Photomultiplier
tube
PMT high-voltage power supply
Wobum, MA 01801
Co., Acton, MA 01720
Trigger photodiode
FND 100
EG & G Electra-Optics, Salem, MA 01970
Radiofrequency generator
HFP 1500D
Plasma-Therm Inc., Kresson, NJ 08053
ICP Plasma-Therm torch
Standard and tong
Precision Glassblowing of Colorado, Parke, CO 80134
-
ICP Plasma-Therm concentric nebulizer
Precision Glassblowing of Colorado
Microcomputer
PC-XT
International Business Machines Corp., Boca Raton, FL
Peristaltic pump
Rabbit
Rainin Instrument Co., Inc., Boston, MA
Table 2. Optimal experimental conditions* Element line?
I AFS3
rf power, W
Observation height, mm
Slit-width,
nm
Ba(II) Ca(il) Cd(I) Na(1) VI) Cd(V§ Mixture$
455.4 393.4 228.6 589.6 292.4 228.6 200-800
;: 500 500 700 700 600
15 27 15 IS 15 45 15
2000 1500 900 2000 1100 1000 1500
Pm
*Other experimental conditions: sample uptake rate 1.15 ml/m& nebulizer pressure 31 psig; plasma Ar flow-rate 15 l./min; auxiliary Ar flow-rate 1-3 Ljmin. t(I) indicates an atomic line, (II) an ionic line (singly ionized). $Analysed with a long torch. $A mixture of the 5 elements at concentrations of 20 ppm for each element.
Evaluation of a pulsed flash-tube
313
diluted as required. Distilled demineralized water was used throughout. For each element studied, the optimal ICP rf power, observation height above the ICP load coil, monochromator slit-width, and ICP gas flows were determined. Calibration graphs and limits of detection were also determined for each element. For measurement of the synthetic mixture containing five elements, compromise values of the operating parameters were necessary. A comparison was also made of the results obtained with an extended ICP torch and the conventional torch used for most parts of this study. An extended ICP torch has an outer sleeve that is 40 mm longer than that of the conventional torch. Finally, we looked at
the possibility of double resonance excitation of Ca by use of the flash-tube. RESULTS AND DISCUSSION
Molecular species of non-refractory elements typically have low molecular dissociation energies and are easily atomized by the plasma at lower rf powers, whereas those of the refractory elements typically have high molecular dissociation energies and are atomized by the plasma at higher rf powers. Also, the atomic and ionic populations of the sample species in the plasma are often greatly affected by the choice of rf power and observation height.‘*‘OFor these initial studies, the dependences of the fluorescence on the rf power and observation height were examined independently, as shown in Figs. 2 and 3 respectively. Figure 4 shows the variation in fluorescence signalto-noise with monochromator slit-width for each element (at optimal rfpower and observation height). Table 2 lists the optimal conditions (found by univariate search) for each element, based on Figs. 2-4. Typical calibration plots obtained under the optimal conditions are shown in Fig. 5. Table 3 lists the analytical figures of merit. The detection limit is defined as the concentration in pg/ml of the element in pure aqueous solution resulting in a signal that is three times the standard deviation of the blank
k-
1808
4 8
StF
PONNR
fNl
Fig. 3. The effect of rf power on the fluorescence signal for each element.
I@
V
‘00 SLIT-WIDTH
15'00
20'00
( ,u,m)
Fig. 4. The variation in fluorescence signal-to-noise ratio for each element as a function of spectrometer slit-width.
.
I-6 if
2O
25 SO 86 OBS~RVATSON
4O 46 50 66 HlliIGIiT (mm)
6@
66
Fig. 2. The efkct of observation height above the load coil on the fluorescence signal for each element.
~ LOG [CONCENTRATION
(ppm)l
Fig. 5. Analytical calibration curves for each element.
314
M. A.
MIGNARDI et al.
Table 3. Analvtical fiaures of merit Element line*
Sensitivity,t mV.ml.fig-
Ba(I1) Ca(II) Cd(I) Na(I) V(II) Cd(I)§
LOD, Lcnlml
Literature LOD,” IIKlml
0.09 0.03 0.02 0.04 0.4 0.04
0.05 0.0004 0.0005 0.0003 0.1 -
14.1 18.4 26.3 10.7 1.88 14.9
*See footnote t to Table 2. tReferred to the boxcar input. $Measured with a long torch.
next most intense fluorescence line of vanadium at 292.4 nm, was observed. We were unable to detect any instances of double resonance fluorescence of calcium as described by Omenetto et al., I3 who observed double resonance excitation of Ca with two different dye lasers (pumped by an excimer laser), one laser giving excitation at 396.847 nm and the other at 370.603 nm, the fluorescence being observed at 373.690 nm. CONCLUSION
measurements. The electronic band-width for all measurements was cu. 1 Hz. Previously reported ICP-AFS detection limits for the same elements are also listed in Table 3. All the log-log calibration plots have slopes between 0.96 and 1.06. The effect of using a long torch was examined for cadmium as model element. The results are shown in Figs. 2-5 and the corresponding curves are labeled Cd(L). With the long torch, the fluorescence signal was practically constant at observation heights 45560 mm above the load coil, whereas with the standard torch the signal rapidly decreased with increase in observation height (Fig. 2). The variation in signal with increase in rf power was similar in pattern for both torches, but the signal was significantly larger with the standard torch (Fig. 3). A similar difference was observed for the effect of monochromator slit-width on the signal-to-noise ratio, and again the standard torch gave much the better performance (Fig. 4). Despite this, the detection limits for cadmium were essentially the same with both torches (Table 3). Figure 6 shows a multielement scan of a synthetic mixture of the five elements (20 pg/ml each), with a standard torch. The most intense fluorescence line of each element except vanadium was excited and observed (see Table 2). The most intense fluorescence signals for vanadium are at 309-310 nm, i.e., in the middle of a strong OH band.” Therefore, the
The work presented thus far shows promising results for the use of a flash-tube in ICP-AFS. The present system would appear to have considerable use in multielement analysis even though the detection limits for each element were at least an order of magnitude poorer than the best reported in the literature. Future work on this project will involve studies to improve the detection limits so as to be competitive with the present commercial atomic spectrometry systems. Improvements could be made by better rf shielding, decreased excitation source scatter, and better and more efficient collection of the fluorescence signal. Since the limiting noise in our system was due to scattered light from the flash-tube, efforts will also be made to reduce scattered light by using non-resonance fluorescence emission filters and better light-traps. An ultrasonic nebulizer will also be used for the more efficient sample uptake into the ICP. Acknowledgements-This research was supported by NIHGM-38434-01. The authors wish to thank Thomas J. Manning and Dr. Moi B. Leong for their initial help in setting up the ICP. REFERENCES
1. G. Tolg, Analysf, 1987, 112, 365. 2. J. A. C. Broekaert, Anal. Chim. Acta, 1987, 196, 1. 3. J. A. C. Broekaert and G. Tolg, Z. Anal. Chem., 1987, 326, 495.
4. A. Montaser and V. A. Fassel, Anal. Chem., 1976, 48, 1940. 5. L. A. Davis, R. J. Krupa and J. D. Winefordner, Anal. Chim. Acta, 1985, 173, 512.
6. N. Gmenetto and J. D. Winefordner,
in Inductively Coupled Plasmas in Analytical Atomic Spectrometry,
A. Montaser and D. W. Golightly - _.(eds.), p. 323. VCH, New York, 1987. 7. D. J. Johnson, W. K. Fowler and J. D. Winefordner, Talanta, 1977, 24, 227.
I
200
300
Fl uorescence
I 400
I 500
8. D. J. Johnson, F. W. Plankey and J. D. Winefordner, Anal. Chem., 1974, 46, 1898. 9. G. Beck, Reo. Sri. Instrum., 1974, 45, 318. 10. M. L. Parsons. B. W. Smith and G. E. Bentlev, Handbook of Flame Specwoscopy, p. 16. Plenum Press, New York. 11. D. R. Demers, D. A. Busch and C. D. Allemand,
I
Am. Lab., 1982, 22, No. 3, 167.
600
Wave1 ength
(nm)
Fig. 6. A multielement analysis of a synthetic mixture of the five elements studied.
12. R. J. Krupa, G. L. Long and J. D. Winefordner, Spectrochim. Acra, 1985, 4OB, 1485. 13. N. Omenetto, B. W. Smith, L. P. Hart, P. Cavalli and G. Rossi, ibid., 1985, 4OB, 1411.