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A heated-electrode discharge lamp for trace analysis by atomic emission spectroscopy
Chimica Acta, 110 (1979) 291-300 Scientific Publishing Company, Amsterdam
Analytica OEkevier
A HEATED-ELECTRODE BY ATOMIC EMISSION
DISCHARGE SPECTROSCOPY
KUNIYUKI
KITAGAWA*
Department Chikusa-ku,
of Synthetic Chemistry, Nagoya (Japan)
TSUGIO
and RYOICHI
Printed in The Netherlands
LAMP
FOR TRACE
ANALYSlS
NARITA
Facutty
of Engineering,
Nagoya
University,
Furo-cho.
Iiibarigaoka,
Tenpaku-
TAKEUCHI
School of Materials Science. Toyohashi cho, Toyohashi. Aichi (Japan) (Received
-
University
of Technology.
16th May 1979)
S~MARY Graphite electrodes heated by d.c. power alone, or a combination of d.c. and r.f. power, are investigated for generating atomic emission from cadmium with specially designed discharge lamps. Best results are achieved if the pkma produced by both power sources can be concentrated within a cylindrical electrode.
Techniques which may be applied to the multi-element au;alysis of samples of small volume include d.c. glow discharge [ 11, atomic emission spectrometry with electrothermal atomizers [ 21, atomic fluorescence spectrometry with electrothermal atomizers and continuum light sources [ 31, and the inductively coupled plasma fitted with an electrothermal atomizer [4 1. Although the d-c. glow discharge lamp is a versatile emission source, it has been applied chiefly to measurements at high concentrations, partly because of the low atomization rate by sputtering. In addition, the low excitation temperature (up to 4,000 IS) leads to inadequate sensitivity for small sample sizes.
In this study, in order to overcome these problems, a discharge lamp has been constructed which incorporates a heated graphite electrode. The electrode serves as an atomizer, is heated by dissipation of the supplied power and is concurrently located in a discharge plasma. Aspects of the power supply are discussed. The technique is applied to the determination of trace amounts of cadmium. EXPERZMENTAL
Instrumen to tion Figure 1 is a schematic diagram of the experimental of the components is described below.
arrangement.
Each
292
Fig. 1. Schematic diagram of the experimetdal arrangement: (a) r-f. power supply; (b) supply; (c) r.f. power meter; (d) coupling circuit; (e) discharge lamp; (f) Pirani gauge; (g) rotary pump; (h) lens; (i) monochromator; (j) iris diaphragm; (k) photomultiplier tube; (1) pre-amplifier; (m) strip chart recorder.
d-c power
Discharge lamps. Two types of discharge lamp were designed for this work (Fig. 2). Lamp A was used almost exclusively. The anode is located at right angles to the cathode. Lamp B was designed to stabilize the r-f. discharge. The anode and cathode are placed face to face co-axially. Figure 3 shows in detail the two cathode shapes used. The cathode functions simultaneously as an atomizer and as an electrode for the discharge. One of the cathodes is a hollow graphite tube; the other is a serni~y~nd~~~ graphite tube. Both were made of graphite tube, (2.4 mm o.d., 1.5 mm i.d.) carved from graphite rod of high purity used for spark/arc spectroscopy (Hitachi Kasei). The hollow graphite tube has an opening of 1 mm diameter for sample introduction on the lateral face, In lamp A, the graphite is fitted to a supporting electrode of graphite rod (5mm diameter). The electrode is covered with a quartz tube to shield it from the discharge, so that the discharge is concentrated at the atomizing graphite cathode. In lamp B, the atomizing graphite cathode is fitted to a tungsten electrode through a graphite connector which thermally insulates the glass plug from the cathode. The graphite connector is covered with a quartz tube. The anode is made of tungsten rod (2 mm diameter)_ Both lamps have inlet and outlet capillary tubes of 05mm diameter for stabilizing the inner pressure of the lamp. DC and r-f. power supplies. The d.c, power supply used is a current regulator (Hitachi) for the old type of hollow-cathode Iamp, which could provide currents up to 100 mA. Figure 4 shows the circuit diagram of the r-f, power supply and matching circuit. This supplies the discharge lamp with an effective power of 75 W when the anode voltage of the valve is 1,300 V and the frequency is 21 MHz. The output power is transmitted to the discharge lamp through a co-axial cable of 50 ohms impedance. The matching circuit includes a function to
293
rI 0
_J Fig. 2. Two types of discharge lamp: (1) heated graphite cathode; (2) quartz tube; (3) graphite rod; (4) tungsten anode; (5) quartz window; (6) capillary tube.
superimpose d.c. power on r.f. power. An r.f. power meter is connected between the r.f. power supply and the matching circuit for monitoring the reflected power or the standing wave ratio. Optics. A Littrow quartz prism spectrograph (Shimadzu QL-170) of focal length 170 cm was employed. A photomultiplier tube (HTV R306) is located on the focal plane where the photographic plate is usually fitted. The photomultiplier tube is mounted on a scanning block which moves on a screw bar rotated by a synchronous motor, in order to adjust the wavelength.
294
lmm
Fig. 3. Two types of graphite cathode.
Fig.
4. Circuit
diagram
of the r.f. power
supply
and coupling
circuit.
An exit slit (0.1 mm ) is placed 2 mm in front of the photomultiplier tube, by attaching it to the block. The position of the slit is precisely adjusted to coincide with the focal plane. These elements are assembled in a cabinet. The width of the entrance slit is 0.1 mm. In order to reduce the black body radiation from the heated cathode and the continuum emission from the discharge, an iris of l-mm aperture is located in front of the entrance slit. The image of the heated cathode is focused on the entrance slit with a magnifying ratio of unity. The analytical line used was Cd I (228.8 nm). A very simple circuit was assembled for Photocurrent preamplifier_ preamplifying the resulting photocurrent. An integrating circuit (Intersil LF 356) is incorporated into the circuit, which has a very high input impedance and low enough noise to connect to the photomultiplier tube. The preamplifier is located in the cabinet together with the dry battery power suPPlYDischarge
gas
Argon (normal and of high purity, 99.99%) is used for the discharge gas. A rotary pump removes the gas at a pressure of 4.5-16 torr. The pressure in the discharge lamp is measured by a Pirani gauge (Okano). The sensor is fitted to a T-branch of the discharge lamp. The flow rate of argon was 150 ml min-’ at STP.
Reagents and procedure A stock solution of cadmium was prepared by dissolving a suitable amount of cadmium metal in the minimum of nitric acid. Standard solutions were prepared just before use by diluting the stock solution with distilled and deionized water, adjusting the acidity to 0.1 M in nitric acid. Sample solution was transferred to the atomizing graphite cathode by a microsyringe, dried by evacuating for 3 min and atomized by discharge. The temperature of the heated graphite cathode was measured by an optical pyrometer. RESULTS
AND DISCUSSION
D.c. glow discharge Several basic characteristics were estimated for d.c. glow discharge with the hollow graphite cathode in lamp A. Figure 5 shows the dependence of the voltage on the current and pressure. The internal electrical resistance, aV/iH, increases as the pressure decreases. This suggests that the discharge lamp operates as an abnormal glow discharge under the given conditions. In the region beyond the broken line, the graphite tube cathode was heated to a visible dark red (ca. 800°C). Figure 6 shows the relation between the outer surface temperature of the hollow graphite cathode, the power and the internal pressure. Except at 16 torr, similar temperatures were attained at any internal pressure, for any given input power. This indicates that the temperature of the heated graphite cathode is defined almost entirely by the
‘ol--_.
.-.-
-1
-.--_._
-L____r
50
25 Lamp
75
currtnt(m/\)
Fig. 5. Relation between the discharge current and voltage for lamp A at the pressures (tom) indicated on the curves.
296
. 614’ 09 012 0!6
7LL___.---_~-_15
20
-3o---
35
?owcr(VJ:
Fig. 6. Relation between the surface temperaturn power at different pressures.
of the heated graphite cathode and the
input power. It is well known that the differential potential in the dx. glow is largest near the cathode. In this event, the power supplied seems to be converted to Joule heat around the cathode. At 16 torr, the temperature is lower than at the other pressures, partly because the increased loss of heat by thermal conduction through argon gas cannot be ignored. Figure 7A shows the dependence of the relative emission intensity of the Cd I (228.8 nm) line on the distance between the anode and cathode. The temperature of the cathode surface was controlled at l,lOO”C, at 9 torr. The
50 63 70 Curren:W@f
Pressure(torr)
Fig. 7. (A). Dependence of the relative intensity of cadmium emission at 228.8 nm on distance between the electrodes; 1 ~1 of 0.5 ppm Cd solution; 9 torr; 1,lOO”C. (B) pendence of the relative intensity on the sample volume taken. (C) Dependence of relative intensity on the d.c. current. (D) Dependence of the signal/noise ratio on pressure and flow rate of argon: (a) 1 IO ml min-’ ; (b) 50 ml min-’ _
the Dethe the
297
relative
depends inversely on the distance. The empirical equation proposed for the glow discharge [ 51; here m is the amount of cathode material sputtered by the discharge, P the pressure and d the distance between the anode and cathode. If the pressure is constant, m 0: d-l. Although, strictly, the variation of the excitation temperature and degree of ionization should be taken into account, it has been suggested that the sputtering rate is the predominant factor affecting the intensity of emission from the glow discharge. In this sense, the behaviour of the glow discharge in this experiment agrees with the equation_ This was further proved by the fact that the loss of the heated graphite cathode increased as the cathode approached the anode. Figure 7B shows the dependence of the relative intensity on the sample volume taken. The intensity for a sample volume of 2 ~1 is reduced; because of the small volume of the hollow cathode, the larger sample solution tends to spread towards the mouth of the tube, so that the residence time of the atoms within the tube becomes shorter and the emission intensity is reduced. In order to overcome this, the volume of the hollow might be increased, but the power introduced for heating the cathode would then also have to be increased, and this would intensify the loss of the graphite cathode. For this reason, a larger graphite cathode cannot be employed if only d.c. power is available for heating. Despite this limitation, the heated graphite cathode seems to be useful in that its single-ended configuration gives a very uniform Any temperature gradient at the terminal distribution of temperature. position could not be distinguished by eye. Figure 7C shows the relation between the relative emission in’tensity and the current. As the current increases, the relative intensity increases. This is attributable to the increase in the sputtering rate and the temperature of the graphite cathode. Figure 7D shows the dependence of the signal-to-noise ratio (S/N) on the pressure and the flow rate of argon. This indicates that S/N is not significantly affected by the pressure but is affected by the flow rate of argon; this happens partly because the background emission increases as the flow rate of argon decreases, and partly because the fluctuation in the background emission intensity increases. It is not clear what is responsible for the background increase when the flow rate is decreased; presumably, the yield of nitrogen oxide, which is responsible for the background emission (see below) changes with the flow rate. The detection limit (WV = 2) is lo-” g for cadmium. m?‘d
intensity
= constant
R. f. discharge
has been
The d-c. glow discharge with the heated hollow graphite cathode gave some desirable improvement in sensitivity. However, a problem is the loss of graphite from the cathode. In order to overcome this, another way of enhancing the excitation temperature was examined. For this purpose, an r-f. discharge was combined with the technique stated above. Owing to the
298
high-frequency alternating electromagnetic field, the ions in the plasma are confined within a limited region. Since the number of collisions between species is increased, the excitation temperature should be raised. In preliminary experiments, the mode of coupling between the d-c. and r.f_ power was investigated. Figure 8 shows the different coupling configurations. In configuration 1, the r-f. power is inductively coupled, in 2 semiinductively coupled, and in 3 and 4 capacitively coupled. It was found that
in the fit two configurations, the r-f. plasma was repelled by the d-c. glow plasma near the heated graphite cathode. There existed a dark region, ca 3 mm thick between the red r-f. and blue-white d-c. plasmas, and con-
sequently the r-f. power did not contribute to increase in the cadmium emission intensity. In the latter two configurations, the plasmas appeared to unite. Because a common earth can be taken, configuration 4 is recommended. Figure 9 shows typical traces of the cadmium emission peak obtained with lamp A, incorporating the hollow or semicylindrical graphite cathode. It was noticed that with the hollow graphite cathode, the atomic emission intensity remains constant while the background increases by a factor of 3.5 when the r-f. power is superimposed. As stated above, in configuration 4, the r-f. plasma appeared to be in contact with the d-c. glow. In practice, however, the result suggests that the r-f. plasma cannot progress into the hollow but contacts a sheath of the d-c. glow around the cathode. This contact is reflected in the increase in the background emission intensity because the r-f. plasma excites impurity molecules. Because of the quasiclosed configuration of the hollow graphite cathode, the r-f. power is shielded from the atomic cloud in the hollow_ In contrast, with the semicylindrical graphite cathode, the atomic emission intensity is enhanced concurrently with the background emission intensity when the r-f. power is applied. This suggests that this shape of cathode is not shielded from the r-f. plasma, because of its open configuration, and that the r-f. and d-c. plasmas amalgamate in the region where the atomic cloud is present. Nevertheless, the atomic emission intensity obtainable with the semicylindrical design is lower than that obtainable with the hollow cathode under the same conditions of operation. This happens partly because the semicylindrical cathode does not have the effect of imprisoning the atomic cloud, so that the residence time in the region measured is shortened.
I
2
3
4
i
+ e
Fig. 8. Various coupling configurations
between the r-f. and d.c. powers.
-
299
B ,
A
1 min
w
C
HOI low
u-shaped
J
-
-
9. Typical emission peaks for the two types of heated graphite cathode incorporated into lamp A. Sample, 1 ql of 0.8 ppm Cd; pressure, 9 torr. Curves A and C, 80 mA d.c.; curves B and D, 80 mA d.c., 39 W r.f. Fig.
Concentration
of r-f. plasma in the hollow
cathode
The results described above were obtained with the use. of lamp A. The hollow cathode rejected the r-f. plasma and even the use of the semicylindrical cathode did not give an overall increase in the atomic emission intensity. In order to overcome the former problem, an attempt was made to concentrate the r.f. power in the hollow cathode. In lamp A, part of the r-f. power is lost between the anode and the supporting electrode_ An insulator (a quartz tube) is not useful for isolating the supporting electrode from the r.f. power; it simply results in the r-f. plasma covering the quartz tube. In order to avoid this, lamp B was designed by placing the cathode opposite the anode. Figure 10 shows typical traces of the emission peak for different modes of power supply with lamp B; the smallest peak from the d.c. glow discharge is the same as that shown in Fig. 9. Thus the effect of the r.f. power is surprisingly pronounced in this type of discharge lamp. The shielding effect seems to be overcome by the converging r-f. power. It is clear from Fig. 10 that changes in the direct current scarcely affect the emission intensity; this was confirmed by applying only the r-f. power to the discharge lamp. The hollow graphite cathode is heated to a temperature somewhat lower than that attained with the d-c. and r-f. power, where part of the r-f. power is converted into heat. However, the atomic emission intensity is similar to those obtained by the other power supplies. Because of the absence of the d-c. glow plasma, the r-f. plasma was observed to enter the hollow. Thus, the effect of the r.f. power on the atomic cloud is increased and the excitation temperature is raised. Probably, this compensates for the lower temperature
300
Fig. 10. Typical emission peaks for lamp B. Sample, 1 ~1 of 0.5 ppm Cd; pressure, 9 torr. Curve A. 60 mA d-c.; curves B-E, 70 W r-f. with 60, 40. 20 and 0 mA d.c., respectively.
Fig. 11. Spectrum of the backgroutld emission.
of the cathode.
Thus the hollow
cathode
can be heated by the r.f. power
alone. Backflound
emission
There remains S/.. and therefore
the problem of the background emission, which the detection limit. Figure 11 shows the spectrum
limits
of the background emission in the U.V. region. When the graphite cathode was replaced by a copper cathode of the same dimensions, the resulting spectrum was the same, except that copper lines were also present. Consequently, the
background emission is attributable
to the emission from molecuies present
in the surrounding gas. Analysis of the spectrum indicates that it consists chiefly of the y-band of NO with a trace of a CO band. These species might be produced by reactions in the plasma from nitrogen, oxygen and CO2 impurities_ When 99.999% argon was used for the plasma gas, the background emission was five times less intense. The background emission was found in the positive column around the anode, thus a configuration of the discharge lamp should be designed so as to remove the positive column from the measurement region _ REFERENCES 1 C. 2 H. 3 D. 4 A. 5 G.
G. Bruhn and W. W. Harrison, Anal. Chem., 50 (1978) 16. Kawaguchi and B. L. Vallee, Anal. Chem., 47 (1975) 1029. R. Demers, Appl. Spectrosc., 22 (1968) 797. M. Gunn. D. L. Millard and G. F. Kirkbright, Analyst, 103 (1978) Grancis, Handbuch der Physik, Vol. 22, Springer, Berlin, 1956.
1066.
Analytica OEkevier
A HEATED-ELECTRODE BY ATOMIC EMISSION
DISCHARGE SPECTROSCOPY
KUNIYUKI
KITAGAWA*
Department Chikusa-ku,
of Synthetic Chemistry, Nagoya (Japan)
TSUGIO
and RYOICHI
Printed in The Netherlands
LAMP
FOR TRACE
ANALYSlS
NARITA
Facutty
of Engineering,
Nagoya
University,
Furo-cho.
Iiibarigaoka,
Tenpaku-
TAKEUCHI
School of Materials Science. Toyohashi cho, Toyohashi. Aichi (Japan) (Received
-
University
of Technology.
16th May 1979)
S~MARY Graphite electrodes heated by d.c. power alone, or a combination of d.c. and r.f. power, are investigated for generating atomic emission from cadmium with specially designed discharge lamps. Best results are achieved if the pkma produced by both power sources can be concentrated within a cylindrical electrode.
Techniques which may be applied to the multi-element au;alysis of samples of small volume include d.c. glow discharge [ 11, atomic emission spectrometry with electrothermal atomizers [ 21, atomic fluorescence spectrometry with electrothermal atomizers and continuum light sources [ 31, and the inductively coupled plasma fitted with an electrothermal atomizer [4 1. Although the d-c. glow discharge lamp is a versatile emission source, it has been applied chiefly to measurements at high concentrations, partly because of the low atomization rate by sputtering. In addition, the low excitation temperature (up to 4,000 IS) leads to inadequate sensitivity for small sample sizes.
In this study, in order to overcome these problems, a discharge lamp has been constructed which incorporates a heated graphite electrode. The electrode serves as an atomizer, is heated by dissipation of the supplied power and is concurrently located in a discharge plasma. Aspects of the power supply are discussed. The technique is applied to the determination of trace amounts of cadmium. EXPERZMENTAL
Instrumen to tion Figure 1 is a schematic diagram of the experimental of the components is described below.
arrangement.
Each
292
Fig. 1. Schematic diagram of the experimetdal arrangement: (a) r-f. power supply; (b) supply; (c) r.f. power meter; (d) coupling circuit; (e) discharge lamp; (f) Pirani gauge; (g) rotary pump; (h) lens; (i) monochromator; (j) iris diaphragm; (k) photomultiplier tube; (1) pre-amplifier; (m) strip chart recorder.
d-c power
Discharge lamps. Two types of discharge lamp were designed for this work (Fig. 2). Lamp A was used almost exclusively. The anode is located at right angles to the cathode. Lamp B was designed to stabilize the r-f. discharge. The anode and cathode are placed face to face co-axially. Figure 3 shows in detail the two cathode shapes used. The cathode functions simultaneously as an atomizer and as an electrode for the discharge. One of the cathodes is a hollow graphite tube; the other is a serni~y~nd~~~ graphite tube. Both were made of graphite tube, (2.4 mm o.d., 1.5 mm i.d.) carved from graphite rod of high purity used for spark/arc spectroscopy (Hitachi Kasei). The hollow graphite tube has an opening of 1 mm diameter for sample introduction on the lateral face, In lamp A, the graphite is fitted to a supporting electrode of graphite rod (5mm diameter). The electrode is covered with a quartz tube to shield it from the discharge, so that the discharge is concentrated at the atomizing graphite cathode. In lamp B, the atomizing graphite cathode is fitted to a tungsten electrode through a graphite connector which thermally insulates the glass plug from the cathode. The graphite connector is covered with a quartz tube. The anode is made of tungsten rod (2 mm diameter)_ Both lamps have inlet and outlet capillary tubes of 05mm diameter for stabilizing the inner pressure of the lamp. DC and r-f. power supplies. The d.c, power supply used is a current regulator (Hitachi) for the old type of hollow-cathode Iamp, which could provide currents up to 100 mA. Figure 4 shows the circuit diagram of the r-f, power supply and matching circuit. This supplies the discharge lamp with an effective power of 75 W when the anode voltage of the valve is 1,300 V and the frequency is 21 MHz. The output power is transmitted to the discharge lamp through a co-axial cable of 50 ohms impedance. The matching circuit includes a function to
293
rI 0
_J Fig. 2. Two types of discharge lamp: (1) heated graphite cathode; (2) quartz tube; (3) graphite rod; (4) tungsten anode; (5) quartz window; (6) capillary tube.
superimpose d.c. power on r.f. power. An r.f. power meter is connected between the r.f. power supply and the matching circuit for monitoring the reflected power or the standing wave ratio. Optics. A Littrow quartz prism spectrograph (Shimadzu QL-170) of focal length 170 cm was employed. A photomultiplier tube (HTV R306) is located on the focal plane where the photographic plate is usually fitted. The photomultiplier tube is mounted on a scanning block which moves on a screw bar rotated by a synchronous motor, in order to adjust the wavelength.
294
lmm
Fig. 3. Two types of graphite cathode.
Fig.
4. Circuit
diagram
of the r.f. power
supply
and coupling
circuit.
An exit slit (0.1 mm ) is placed 2 mm in front of the photomultiplier tube, by attaching it to the block. The position of the slit is precisely adjusted to coincide with the focal plane. These elements are assembled in a cabinet. The width of the entrance slit is 0.1 mm. In order to reduce the black body radiation from the heated cathode and the continuum emission from the discharge, an iris of l-mm aperture is located in front of the entrance slit. The image of the heated cathode is focused on the entrance slit with a magnifying ratio of unity. The analytical line used was Cd I (228.8 nm). A very simple circuit was assembled for Photocurrent preamplifier_ preamplifying the resulting photocurrent. An integrating circuit (Intersil LF 356) is incorporated into the circuit, which has a very high input impedance and low enough noise to connect to the photomultiplier tube. The preamplifier is located in the cabinet together with the dry battery power suPPlYDischarge
gas
Argon (normal and of high purity, 99.99%) is used for the discharge gas. A rotary pump removes the gas at a pressure of 4.5-16 torr. The pressure in the discharge lamp is measured by a Pirani gauge (Okano). The sensor is fitted to a T-branch of the discharge lamp. The flow rate of argon was 150 ml min-’ at STP.
Reagents and procedure A stock solution of cadmium was prepared by dissolving a suitable amount of cadmium metal in the minimum of nitric acid. Standard solutions were prepared just before use by diluting the stock solution with distilled and deionized water, adjusting the acidity to 0.1 M in nitric acid. Sample solution was transferred to the atomizing graphite cathode by a microsyringe, dried by evacuating for 3 min and atomized by discharge. The temperature of the heated graphite cathode was measured by an optical pyrometer. RESULTS
AND DISCUSSION
D.c. glow discharge Several basic characteristics were estimated for d.c. glow discharge with the hollow graphite cathode in lamp A. Figure 5 shows the dependence of the voltage on the current and pressure. The internal electrical resistance, aV/iH, increases as the pressure decreases. This suggests that the discharge lamp operates as an abnormal glow discharge under the given conditions. In the region beyond the broken line, the graphite tube cathode was heated to a visible dark red (ca. 800°C). Figure 6 shows the relation between the outer surface temperature of the hollow graphite cathode, the power and the internal pressure. Except at 16 torr, similar temperatures were attained at any internal pressure, for any given input power. This indicates that the temperature of the heated graphite cathode is defined almost entirely by the
‘ol--_.
.-.-
-1
-.--_._
-L____r
50
25 Lamp
75
currtnt(m/\)
Fig. 5. Relation between the discharge current and voltage for lamp A at the pressures (tom) indicated on the curves.
296
. 614’ 09 012 0!6
7LL___.---_~-_15
20
-3o---
35
?owcr(VJ:
Fig. 6. Relation between the surface temperaturn power at different pressures.
of the heated graphite cathode and the
input power. It is well known that the differential potential in the dx. glow is largest near the cathode. In this event, the power supplied seems to be converted to Joule heat around the cathode. At 16 torr, the temperature is lower than at the other pressures, partly because the increased loss of heat by thermal conduction through argon gas cannot be ignored. Figure 7A shows the dependence of the relative emission intensity of the Cd I (228.8 nm) line on the distance between the anode and cathode. The temperature of the cathode surface was controlled at l,lOO”C, at 9 torr. The
50 63 70 Curren:W@f
Pressure(torr)
Fig. 7. (A). Dependence of the relative intensity of cadmium emission at 228.8 nm on distance between the electrodes; 1 ~1 of 0.5 ppm Cd solution; 9 torr; 1,lOO”C. (B) pendence of the relative intensity on the sample volume taken. (C) Dependence of relative intensity on the d.c. current. (D) Dependence of the signal/noise ratio on pressure and flow rate of argon: (a) 1 IO ml min-’ ; (b) 50 ml min-’ _
the Dethe the
297
relative
depends inversely on the distance. The empirical equation proposed for the glow discharge [ 51; here m is the amount of cathode material sputtered by the discharge, P the pressure and d the distance between the anode and cathode. If the pressure is constant, m 0: d-l. Although, strictly, the variation of the excitation temperature and degree of ionization should be taken into account, it has been suggested that the sputtering rate is the predominant factor affecting the intensity of emission from the glow discharge. In this sense, the behaviour of the glow discharge in this experiment agrees with the equation_ This was further proved by the fact that the loss of the heated graphite cathode increased as the cathode approached the anode. Figure 7B shows the dependence of the relative intensity on the sample volume taken. The intensity for a sample volume of 2 ~1 is reduced; because of the small volume of the hollow cathode, the larger sample solution tends to spread towards the mouth of the tube, so that the residence time of the atoms within the tube becomes shorter and the emission intensity is reduced. In order to overcome this, the volume of the hollow might be increased, but the power introduced for heating the cathode would then also have to be increased, and this would intensify the loss of the graphite cathode. For this reason, a larger graphite cathode cannot be employed if only d.c. power is available for heating. Despite this limitation, the heated graphite cathode seems to be useful in that its single-ended configuration gives a very uniform Any temperature gradient at the terminal distribution of temperature. position could not be distinguished by eye. Figure 7C shows the relation between the relative emission in’tensity and the current. As the current increases, the relative intensity increases. This is attributable to the increase in the sputtering rate and the temperature of the graphite cathode. Figure 7D shows the dependence of the signal-to-noise ratio (S/N) on the pressure and the flow rate of argon. This indicates that S/N is not significantly affected by the pressure but is affected by the flow rate of argon; this happens partly because the background emission increases as the flow rate of argon decreases, and partly because the fluctuation in the background emission intensity increases. It is not clear what is responsible for the background increase when the flow rate is decreased; presumably, the yield of nitrogen oxide, which is responsible for the background emission (see below) changes with the flow rate. The detection limit (WV = 2) is lo-” g for cadmium. m?‘d
intensity
= constant
R. f. discharge
has been
The d-c. glow discharge with the heated hollow graphite cathode gave some desirable improvement in sensitivity. However, a problem is the loss of graphite from the cathode. In order to overcome this, another way of enhancing the excitation temperature was examined. For this purpose, an r-f. discharge was combined with the technique stated above. Owing to the
298
high-frequency alternating electromagnetic field, the ions in the plasma are confined within a limited region. Since the number of collisions between species is increased, the excitation temperature should be raised. In preliminary experiments, the mode of coupling between the d-c. and r.f_ power was investigated. Figure 8 shows the different coupling configurations. In configuration 1, the r-f. power is inductively coupled, in 2 semiinductively coupled, and in 3 and 4 capacitively coupled. It was found that
in the fit two configurations, the r-f. plasma was repelled by the d-c. glow plasma near the heated graphite cathode. There existed a dark region, ca 3 mm thick between the red r-f. and blue-white d-c. plasmas, and con-
sequently the r-f. power did not contribute to increase in the cadmium emission intensity. In the latter two configurations, the plasmas appeared to unite. Because a common earth can be taken, configuration 4 is recommended. Figure 9 shows typical traces of the cadmium emission peak obtained with lamp A, incorporating the hollow or semicylindrical graphite cathode. It was noticed that with the hollow graphite cathode, the atomic emission intensity remains constant while the background increases by a factor of 3.5 when the r-f. power is superimposed. As stated above, in configuration 4, the r-f. plasma appeared to be in contact with the d-c. glow. In practice, however, the result suggests that the r-f. plasma cannot progress into the hollow but contacts a sheath of the d-c. glow around the cathode. This contact is reflected in the increase in the background emission intensity because the r-f. plasma excites impurity molecules. Because of the quasiclosed configuration of the hollow graphite cathode, the r-f. power is shielded from the atomic cloud in the hollow_ In contrast, with the semicylindrical graphite cathode, the atomic emission intensity is enhanced concurrently with the background emission intensity when the r-f. power is applied. This suggests that this shape of cathode is not shielded from the r-f. plasma, because of its open configuration, and that the r-f. and d-c. plasmas amalgamate in the region where the atomic cloud is present. Nevertheless, the atomic emission intensity obtainable with the semicylindrical design is lower than that obtainable with the hollow cathode under the same conditions of operation. This happens partly because the semicylindrical cathode does not have the effect of imprisoning the atomic cloud, so that the residence time in the region measured is shortened.
I
2
3
4
i
+ e
Fig. 8. Various coupling configurations
between the r-f. and d.c. powers.
-
299
B ,
A
1 min
w
C
HOI low
u-shaped
J
-
-
9. Typical emission peaks for the two types of heated graphite cathode incorporated into lamp A. Sample, 1 ql of 0.8 ppm Cd; pressure, 9 torr. Curves A and C, 80 mA d.c.; curves B and D, 80 mA d.c., 39 W r.f. Fig.
Concentration
of r-f. plasma in the hollow
cathode
The results described above were obtained with the use. of lamp A. The hollow cathode rejected the r-f. plasma and even the use of the semicylindrical cathode did not give an overall increase in the atomic emission intensity. In order to overcome the former problem, an attempt was made to concentrate the r.f. power in the hollow cathode. In lamp A, part of the r-f. power is lost between the anode and the supporting electrode_ An insulator (a quartz tube) is not useful for isolating the supporting electrode from the r.f. power; it simply results in the r-f. plasma covering the quartz tube. In order to avoid this, lamp B was designed by placing the cathode opposite the anode. Figure 10 shows typical traces of the emission peak for different modes of power supply with lamp B; the smallest peak from the d.c. glow discharge is the same as that shown in Fig. 9. Thus the effect of the r.f. power is surprisingly pronounced in this type of discharge lamp. The shielding effect seems to be overcome by the converging r-f. power. It is clear from Fig. 10 that changes in the direct current scarcely affect the emission intensity; this was confirmed by applying only the r-f. power to the discharge lamp. The hollow graphite cathode is heated to a temperature somewhat lower than that attained with the d-c. and r-f. power, where part of the r-f. power is converted into heat. However, the atomic emission intensity is similar to those obtained by the other power supplies. Because of the absence of the d-c. glow plasma, the r-f. plasma was observed to enter the hollow. Thus, the effect of the r.f. power on the atomic cloud is increased and the excitation temperature is raised. Probably, this compensates for the lower temperature
300
Fig. 10. Typical emission peaks for lamp B. Sample, 1 ~1 of 0.5 ppm Cd; pressure, 9 torr. Curve A. 60 mA d-c.; curves B-E, 70 W r-f. with 60, 40. 20 and 0 mA d.c., respectively.
Fig. 11. Spectrum of the backgroutld emission.
of the cathode.
Thus the hollow
cathode
can be heated by the r.f. power
alone. Backflound
emission
There remains S/.. and therefore
the problem of the background emission, which the detection limit. Figure 11 shows the spectrum
limits
of the background emission in the U.V. region. When the graphite cathode was replaced by a copper cathode of the same dimensions, the resulting spectrum was the same, except that copper lines were also present. Consequently, the
background emission is attributable
to the emission from molecuies present
in the surrounding gas. Analysis of the spectrum indicates that it consists chiefly of the y-band of NO with a trace of a CO band. These species might be produced by reactions in the plasma from nitrogen, oxygen and CO2 impurities_ When 99.999% argon was used for the plasma gas, the background emission was five times less intense. The background emission was found in the positive column around the anode, thus a configuration of the discharge lamp should be designed so as to remove the positive column from the measurement region _ REFERENCES 1 C. 2 H. 3 D. 4 A. 5 G.
G. Bruhn and W. W. Harrison, Anal. Chem., 50 (1978) 16. Kawaguchi and B. L. Vallee, Anal. Chem., 47 (1975) 1029. R. Demers, Appl. Spectrosc., 22 (1968) 797. M. Gunn. D. L. Millard and G. F. Kirkbright, Analyst, 103 (1978) Grancis, Handbuch der Physik, Vol. 22, Springer, Berlin, 1956.
1066.