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Analytical characterisation of a capacitively coupled plasma torch with a central tube electrode
Talanta 48 (1999) 827 – 837
Analytical characterisation of a capacitively coupled plasma torch with a central tube electrode Emil A. Cordos a,*, Tiberiu Frentiu a, Ana-Maria Rusu a, Sorin D. Angel b, Alpar Fodor a, Michaela Ponta a a
Uni6ersity of Cluj, Department of Chemistry, 3400 Cluj-Napoca, Romania Uni6ersity of Cluj, Department of Physics, 3400 Cluj-Napoca, Romania
b
Received 18 July 1997; received in revised form 22 October 1997; accepted 24 October 1997
Abstract A new type of radiofrequency capacitively coupled plasma torch is presented. The torch electrode geometry is coaxial with a tubular central electrode and one or two outer ring electrodes. The argon plasma is generated at 275 W radiofrequency power and 27.12 MHz and it has a very good stability and a low gas consumption of 0.4 l min − 1. The nebulized sample is introduced through the tubular electrode into the core of the annular shaped plasma thus achieving a better atomisation and a lower background. The limits of detection for 20 elements are in the range of ng ml − 1 and the dynamic range between 2.5 and 3.5. The best results are obtained with the torch with two outer ring electrodes. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Atomic spectroscopy; Plasma sources; Capacitively coupled plasma.
1. Introduction The analytical performance in plasma atomic spectrometry is strongly dependent on the form and on the procedure by which the sample is introduced into the plasma discharge. The process involves two steps. The first is the dispersion of the sample by an appropriate method: ablation or sputtering for the solids, nebulization for the liquids, etc. The second step consists in the conditioning of the dispersed particles or droplets which are then carried further into the plasma * Corresponding author. Fax: + 40 64 420667; e-mail: [email protected]
discharge by a flow of gas. Most of the work in the sample introduction was devoted to the first step of the process. However, the second step has its critical points, one of which is the region of the plasma where the dispersed sample is introduced. It implies that the plasma torch has to be designed accordingly. A good example is the radio frequency inductively couple plasma (ICP). The exceptional analytical qualities of this plasma are the results of a o combination of two factors: plasma higher temperature and the plasma annular shape which allows the sample to be introduced right into the core of the discharge. Consequently the atomic and ionic excitation is very efficient; the spectral background is relatively
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low, the detection limits are very good, the dynamic range is large and the matrix effect is not important [1 – 8]. The torches developed later for microwave capacitively coupled plasma (CMP) and microwave induced plasma (MIP) were designed such that the plasmas have an annular shape so that a better interaction between sample and plasma could be achieved. Winefordner et al. [9–11] were the first to build CMP torches of medium power (up to 500 W) with tantalum tube electrodes and of high power (1 kW) with graphite electrode, that allows for the sample aerosol to be introduced in the centre of the plasma. The detection limits obtained with these torches were 1 – 2 order of magnitude better that those obtained with torches with platinum wolfram tip electrode and side introduction of the sample aerosol. Okamoto et al. [12], by introducing the sample in the core of an annular shape, high power (1.3 kW), argon MIP have achieved in atomic emission, detection limits similar with those in Ar-ICP-AES. The same authors [13,14] using detection by mass spectrometry (MS) have obtained detection limits for nitrogen microwave plasma (N2-MIP-MS) similar with those from ArICP-MS. Jin et al. [15,16] have built a new microwave argon plasma torch, similar with an ICP torch but operated at less than 100 W. This plasma has also an annular shape which in combination with an ultrasonic nebulization system yields good detection limits for refractory elements in spite of its low power. In the last years a number of radio frequency coupled plasma torches (r.f.-CCP) of low and medium power, were developed [17]. Cordos et al. [18–23] have built an atomic emission spectrometer based on a r.f.-CCP torch with coaxial electrodes in tip-ring geometry. The torch was low power (85– 275 W at 27.12 MHz), adapted for pneumatic nebulized liquid samples. The sample aerosol was introduced at a distance of 5 mm from the plasma base using a teflon piece with 12 holes. For this type of torch a part of the sample remains in the plasma mantle and does not reach in the hot plasma core where the dissipated power is at maximum [20]. Obviously the plasma capabilities are not fully used. Therefore the authors of the present paper have designed a new type of
Ar-r.f. CCP torch with a tubular central electrode and one or two ring counter electrodes. This new torch could be an alternative to low power CMP and MIP torches. A single argon gas flow is used both for sample nebulization and as support gas. By introducing the sample directly in the plasma core a more efficient atomisation and excitation is expected. In the present paper some figures of merit of this type of torch are compared with those of torch with electrodes in tip to ring geometry. The optimisation of torch parameters and the detection limits are presented.
2. Experimental
2.1. Instrumentation The schematic diagram of the plasma torch is given in Fig. 1. The torch main body is made of brass and it is water-cooled. In the central channel of the torch is placed a PTFE tube with an inner
Fig. 1. Schematic diagram of the r.f. CCP torch with central tubular electrode and two outer ring electrodes.
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Table 1 Instrumentation and operating conditions for plasma torch and spectrometer Rf. Generator
EOP model, 27.12 MHz free running oscillator, 275 W (Research Institute for Analytical Instrumentation, Cluj-Napoca, Romania). Plasma torch Laboratory constructed. Watercooled torch body. Capacitively coupled, coaxial geometry with molybdenum tubular electrode, 2–4 mm i.d., 5 mm o.d. One or two outer ring electrodes, 25 mm diameter, made of 11 gauges copper wire. Support gas: argon spectral grade (Azo Mures, Tg. Mures, Romania), 0.4 l min−1 gas flow at atmospheric pressure. Sample introPneumatic nebulizer and 4 rollers peristaltic pump, 0.8 ml min−1 solution intake, 120 ml glass nebulization duction sys- chamber, 10% nebulization efficiency (Research Institute for Analytical Instrumentation, Cluj-Napoca, Romatem nia). Aerosol introduction into the plasma core via central tube electrode. Optics Monochromator Heath EU 700, 0.35 m focal length, 1200 grooves per mm grating. Photomultiplier R14 14 (Hamamatsu, Japan) operated at 700 V. Photomultiplier power supply Heath EU 701 (Heath Co. Benton Harbour, MI, USA) Recorder K 201 (Zeiss-Jena, Jena Germany)
diameter of 5 mm. Further, into this tube is introduced a molybdenum tubular electrode with the inner diameter of 2 – 4 mm. The electrode is centred and secured in position by 6 screws and connected to the r.f. generator. Over the whole torch assembly is placed a quartz tube, 15 mm inner diameter and 200 mm length. One or two rings, 25 mm diameter, made out of copper wire, are placed outside of the quartz tube and connected to the common of the r.f. generator. The rings position is adjustable. The plasma was generated using an r.f. generator of free running type, operated at 275 W and 27.12 MHz. The support gas was spectral grade argon at atmospheric pressure. This single gas flow is used both for sample nebulization and as support gas for the discharge. The plasma develops between the tubular electrode and the outer ring electrodes. The samples were nebulized using a pneumatic nebulizer and a 4 rollers peristaltic pump and introduced into plasma core via the tubular electrode. The optical signal was measured by a Heath EU 700 monochromator, equipped with a Hamamatsu R1414 photomultiplier. The photocurrent was recorded on a Zeiss Jena K201 recorder. Details of the equipment were given in Table 1.
3. Reagents Stock solutions of 1000 mg ml − 1 of Al, Ba, Ca, Cd, Cr, Cu, Fe, Eu, Bi, Mg, Mn, Ni, Co, Pb, Zn,
Hg, Yb, Li, Na, Ag were prepared by dissolution of high-purity metals, salts or oxides in HCl or HNO3. Single element working standards were obtaining by diluting the stock solutions with 2% v/v HNO3. For the blank measurements a 2% v/v HNO3 solution was used.
4. Results and discussion
4.1. Optimisation of plasma torch working parameters The discharge obtained with tubular electrode torch looks like a long and slim flame with an inner cone at the base. The cone is shorter than the plasma core obtained with a tip-ring torch [18]. Like in a regular chemical flame the emission is at maximum above the inner cone. The stability of the discharge and its analytical performance are dependent of the material and the dimensions of the tubular electrode, the placement of ring electrodes, plasma power and support gas flow. Since the intensity of the electric field is at maximum in the centre of the plasma the quartz tube is protected against excessive heating, in contrast with the torches having outer parallel plane electrodes, where the electric field is at maximum close to the tube walls. The role of the ring electrodes are different. The first electrode (lower ring electrode) influences the uniformity and density of electrical field lines in the lower part of the
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Fig. 2. The geometry of the r.f. CCP torches compared in the present paper. T-R. Torch with electrodes in tip-ring geometry SR. Torch with central tubular electrode and single ring outer electrode DR. Torch with central tubular electrode and double ring outer electrode.
plasma. A proper diameter and position of this electrode insures the discharge stability and its adherence to the electrode. The second (upper) ring electrode concentrates the electric field lines in the centre of the plasma, with the improvement of power transfer from the r.f. field to the plasma. This increases the atomic and ionic excitation. When using the second ring electrode the inner cone of the plasma discharge gets shorter and brighter. A schematic of the three torches compared in the present paper is given in Fig. 2. In the first stage, the optimisation of plasma torch parameters was accomplished for the tubular central electrode torch with a single outer ring electrode. The optimisation implies the position of the ring electrode, maximum plasma power, argon gas flow and diameter of the tubular electrode. The influence of the second electrode upon the plasma properties was determined only as function of its relative position to the first one and it will be discussed later. The optimum position of the ring electrode, in single ring configuration, is one close to the tubular electrode and it was established empirically. It
was found that a distance of 5 mm between the top of the tubular electrode and the first ring electrode yields a stable discharge. At this distance between electrodes the divergence of the electric field lines provides an uniform heating of the top circumference of the tubular electrode and a good plasma adherence to the electrode. It was established by trials that the maximum power of the discharge that do not damage the electrodes is 275 W. The optimum values for argon gas flow and for the diameter of the tubular electrode were established by monitoring the intensity of Na I 588.9 nm line. The influence of argon gas flow on Na I 588.9 nm line intensity, for two tube diameters, is presented in Fig. 3. The emission increases abruptly when gas flow changes from 0.3 to 0.4 l min − 1, but further, when the gas flow increases from 0.4 to 0.8 l min − 1, the signal growth do not exceed 25% in spite of the fact that nebulization efficiency increases two times. The relative signal plateau in this gas flow range, is due both to the decrease of analyte residence time in the zone of maximum excitation and to the cooling of the
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Fig. 3. Emission signal for Na I 588.9 nm line (solution concentration: 50 mg ml − 1 Na) as function of argon gas flow. Inner diameter of the central molybdenum tubular electrode 3.0 mm (A) and 3.5 mm (B)
plasma. A more detailed picture is given in Table 2 where some figures of merit are presented for the above mentioned sodium line, as function of tubular electrode diameter and argon gas flow. The relative standard deviation of the background (RSDB) is an indicator of plasma mechanical stability. A very stable plasma is obtained at 0.4 l min − 1 argon gas flow and 3.5 mm tube diameter where RSDB has a minimum values of 1.2. The gas flow and the tube diameter affect the signal to background ratio (SBR) by influencing both the signal and the background but in different ways and proportions. For example, as shown before, an increase of gas flow from 0.4 to 0.8 l min − 1 produces a moderate increase of the signal (10 – 25%) but, in exchange, the background increases two times for tubes having 3 or 3.5 mm diameter. If the tube diameter increases over 3.5 mm the background increases dramatically. The overall effect is a decrease of the SBR. For a given gas flow the decrease is a moderate one as long the tube diameters changes from 3 to 3.5 mm (546 – 450 for 0.4 l min − 1 and 341– 206 for 0.8 l min − 1). For a given diameter the doubling of the gas flow produces a more important decrease of the SBR (546 – 341 for 3 mm
diameter and 450–206 for 3.5 mm diameter). Increasing the tube diameter to 4 mm decreases the SBR with almost an order of magnitude. The limits of detection (LOD) follow the same pattern as the SBR. The best LOD’s are obtained for argon gas flow of 0.4 l min − 1. Therefore the increase of gas flow much over 0.4 l min − 1 and the use of tube diameters larger than 3.5 mm are not recommended. The optimum constructive and operating condition for the plasma torch are: 275 W r.f. power, 0.4 l min − 1 argon gas flow, 3.5 mm inner diameter for the tubular electrode and a 5 mm distance between this electrode and the lower ring electrode. These conditions were also valid for other elements. The figures of merit for the torch with a single ring electrode, in the above mentioned conditions are listed in Table 3. As expected, the LOD’s improved for all elements as compared with the plasma torch in tip-ring electrode geometry, where the sample is not introduced in the centre of the discharge. Since the improvement is more notable for the elements with a lower dissociation energy of the oxides, Ag, Cd, Mn, Cd, Zn, Co, one may assume that the improvement is in the atomisation process.
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Table 2 Analytical characteristics of sodium emission (Na I 588.9 nm) used for torch optimisation Tubular electrode inner diameter, mm
Argon gas flow, l min−1
SBRa
RSDBb
LODc
3 3 3.5 3.5 4
0.4 0.8 0.4 0.8 0.8
546 341 450 206 42
1.5 1.8 1.2 1.4 1.5
4.0 8.0 4.0 10.0 54.0
SBR, signal to background ratio for 50 mg ml−1 Na. RSDB, relative standard deviation of the background (determined on ten successive measurements). c LOD, limit of detection (3 s criteria), ng ml−1. a
b
4.2. Influence of the second ring electrode The presence of the second ring electrode changes the shape of the plasma and the optimum observation height for the analytical signal. This changes are function of the distance between the lower and the higher electrode. As an example, in Fig. 4, the emission intensity for the Al I 396.15 nm line is represented as function of distance between electrodes and observation height. It could be seen that the optimum emission intensity could be obtained in a spot that covers about 4 mm in the observation height and 10 mm in the ring distance. The optimum distance between ring electrodes, the optimum observation height and the detection limits are listed in Table 4, for the same elements as in Table 3. The elements were divided into 3 groups as function of distance between electrodes. The first group includes elements for which the upper electrode is relatively close to the lower electrode, 60 mm, and the optimum observation height is generally higher, 12 – 20 mm. All ionic lines are in this group. The oxides dissociation energy is above 4 eV. For this group the improvement in LOD’s are generally higher than those obtained for the other two groups. The most notable increase in emission signal is for Ba and Yb. The elements from the second group exhibit the best emission when the electrodes are farther, 70 mm, but the optimum observation height is lower, 6–12 mm. The dissociation energy for the oxides is B 4.3 eV. The decrease of LOD is generally smaller than for the first group, with an increasing factor of 1.4 – 1.8.
The third group includes elements with lower excitation energy so that they exhibit a good LOD even in plasma with a single ring electrode or in plasma of the tip-ring torch. The emission signal improvement is illustrated in Fig. 5 where the actual experimental traces are given for Cd and Ba. For both these elements the reduction in the background level and background noise could be noticed, when changing from tip-ring torch to a torch with tubular central electrode. The increase in the signal level could also be seen when using two outer ring electrodes as compared with single outer electrode torch. At the element concentration of 250 and 500 ng ml − 1 Ba yields an appreciable signal only for the tubular central electrode torch with two outer ring electrode. Increasing plasma temperature or electron number density does not increase significantly the atomisation for this elements but rather increases the excitation efficiency or slightly decreases the background. As a matter of fact, the LOD’s improvement, when switching from tip-ring torch to single ring tubular torch and further to double ring tubular torch, is the results of two effects: increase of the emission and decrease of the background. The contribution of these two effects is not the same for all elements. As an example, the changes in emission and background intensities are given in Table 5, for a number of selected elements. The intensities are relative, the column with higher values being equal with 100 units. One of the first things to be noticed is the lower background intensity of the tubular torches as compared with tip-ring torch. As for the listed
Data from [21].
422.67 588.99 670.79 285.21 328.07 393.36 324.74 279.55 228.81 425.43 405.78 403.08 257.61 352.45 213.85 371.99 396.15 345.35 459.40 253.65 223.06 346.44 214.44 493.41
Ca(I) Na(I) Li(I) Mg(I) Ag(I) Ca(II) Cu(I) Mg(II) Cd(I) Cr(I) Pb(I) Mn(I) Mn(II) Ni(I) Zn(I) Fe(I) Al(I) Co(I) Eu(I) Hg(I) Bi(I) Yb(I) Cd(II) Ba(I)
a
l nm
Element
2.93 2.11 1.85 4.34 3.78 3.15 3.82 4.43 5.41 2.91 4.38 3.08 4.81 3.54 5.80 3.33 3.14 3.54 2.70 4.88 5.55 3.58 5.78 2.51
Eex., EV
5.0 — — 4.4 1.4 — 4.9 — 3.8 4.2 4.1 4.0 — 4.3 4.0 4.0 5.0 3.7 — — 4.0 — — 6.0
Eox.dis., eV
24 10 10 20 18 25 18 22 16 24 14 18 20 18 12 14 26 16 28 12 18 26 24 28
Optimum observation height mm 200 450 300 225 128 46 60 38 24 15.5 13.5 12.2 10.0 9.1 26 7.6 5.4 4.0 5.0 2.0 2.0 1.4 1.4 0.8
SBR (for a 50 mg ml−1 solution)
Torch tubular electrode single ring (SR)
Table 3 Figures of merit for a CCP torch with central tubular electrode and single ring outer electrode
9 4 5 6 13 32 25 35 65 100 110 125 155 170 60 200 290 390 300 750 800 1000 1100 1900
LOD (SR) ng ml−1
15 13 9 9 430 — 28 — 770 120 450 750 — 1700 700 240 — 2500 440 750 — — — 5700
LODa (T-R) ng ml−1
Torch tip-ring (T-R)
1,6 3,2 1,8 1,5 33 — 1,1 — 11,9 1,2 4,1 6 — 10 11,7 1,2 — 6,4 1,5 1 — — — 3
(T-R) (SR)
LOD’s ratio
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l nm
422.67 393.36 285.21 279.55 324.74 493.41 425.43 396.15 403.08 257.61 346.44 371.99 459.40 223.06 214.44 228.81 405.78 352.45 213.85 345.35 253.65 588.99 670.79 328.07
Element
Ca(I) Ca(II) Mg(I) Mg(II) Cu(I) Ba(I) Cr(I) Al(I) Mn(I) Mn(II) Yb(I) Fe(I) Eu(I) Bi(I) Cd(II) Cd(I) Pb(I) Ni(I) Zn(I) Co(I) Hg(I) Na(I) Li(I) Ag(I)
14 16 12 14 14 20 18 16 12 16 20 10 16 14 18 12 10 8 6 10 6 6 6 12
Optim. Obs. height mm 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 70 70 70 70 70 70 80 80 80
Distance between ring electrodes mm
tubular electrode double ring (DR)
Torch
530 300 450 150 90 44 28 21 21 17 14 15 25 4 3.6 40 20 13 38 7 3 540 360 161
SBR(for a 50 mg ml−1 solution)
Table 4 Figures of merit for a CCP torch with central tubular electrode and double ring outer electrode.
3 5 3 9 16 40 50 70 75 90 100 105 60 400 400 40 80 120 40 210 500 3 4 50
LOD (DR) ng ml−1
9 32 6 35 25 1900 100 290 125 155 1000 200 300 800 1100 65 110 170 60 390 750 4 5 13
LOD (SR) ng ml−1
Single ring (SR)
3 6.4 2 3.9 1.6 47.5 2 4.1 1.7 1.7 10 1.9 5 2 2.7 1.6 1.4 1.4 1.5 1.8 1.5 1.3 1.3 1.3
SR/DR
LOD’s ratio
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Fig. 4. Emission intensity of Al I 396.15 nm line as function of distance between outer ring electrodes and observation height.
Fig. 5. The recorded emission signal for Cd and Ba. a, background; b, 250 ng ml − 1; c, 500 ng ml − 1; T-R., Torch with electrodes in tip-ring geometry; SR., Torch with central tubular electrode and single ring outer electrode; DR., Torch with central tubular electrode and double ring outer electrode.
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Table 5 Line and background intensities for r.f. CCP torches with different electrode geometry Element
Relative line intensity
Ba Mn Cd Ni Co Pb Ag
Relative background intensity
T-Ra
SRb
DRc
T-Ra
SRb
DRc
1 15 32 36 39 44 35
1.4 32 48 50 53 65 71
100 100 100 100 100 100 100
100 100 100 100 100 100 100
45 36 13 14 18 35 5.4
60 63 17 18 23 41 6
a
T-R, CCP torch with tip ring electrode geometry (data according to [21]). SR, CCP torch, central tubular electrode single outer ring electrode. c DR, CCP torch, central tubular electrode, double outer ring electrode. b
elements, two extremes could be mentioned. In the case of Ba, the LOD improvement is due to the increase of the emission signal, especially when the double ring tubular torch is used. At the other end is Ag for which the decrease of background intensity is responsible for LOD decrease.
In comparing the tip-ring torch with the single ring tubular torch, the improvement of the analytical performance is due rather to the background decrease and in the lesser degree to the increase of the emission. Adding a second ring electrode to the tubular torch does not further reduce the
Table 6 Calibration curves parameters Element
a
Standard deviation of ‘a’
n
Standard deviation of ‘n’
Correlation coefficient
Dynamic range
Ag Al Ba Bi Ca Cd Co Cr Cu Eu Fe Hg Li Mg Mn Na Ni Pb Yb Zn
1.88 1.31 2.14 2.48 2.63 1.88 1.87 1.43 1.70 1.80 1.71 1.10 1.97 1.87 2.48 2.49 1.85 1.83 1.82 2.33
0.034 0.016 0.026 0.017 0.013 0.014 0.033 0.013 0.014 0.025 0.024 0.011 0.026 0.030 0.017 0.021 0.021 0.028 0.020 0.011
0.98 0.82 0.96 1.03 0.87 0.89 0.95 0.97 0.88 1.03 0.86 0.96 0.91 1.01 0.88 1.03 0.98 0.88 1.04 0.85
0.035 0.014 0.020 0.020 0.022 0.016 0.023 0.010 0.013 0.020 0.013 0.005 0.018 0.034 0.016 0.025 0.015 0.016 0.016 0.011
0.993 0.997 0.995 0.996 0.996 0.998 0.995 0.999 0.996 0.997 0.998 0.999 0.997 0.994 0.997 0.995 0.998 0.997 0.997 0.999
2.5 3 3 3 3 2.5 3.5 3 2.5 3 3 3.5 3.5 2.5 2.5 3 3 3.5 2.5 2.5
E.A. Cordos et al. / Talanta 48 (1999) 827–837
background but significantly increases the emission. The dynamic range was established using the calibration curve: log y= a+ n log C where y is the emission intensity in arbitrary units, a is the log (signal /concentration unit), C represents the concentration in mg ml − 1and n is the calibration curve slope. The values of the above mentioned coefficients are listed in Table 6. The calibration curves were recorded starting from a minimum concentration five times LOD. The dynamic ranges are between 2.5 and 3.5 for all studied elements.
5. Conclusions The analytical performance of a capacitively coupled plasma spectral source could be substantially improved by using a torch with a tubular central electrode instead of a tip electrode. To the enhancement of the analytical signal contribute both the increase of the emission, due to sample introduction into the plasma core, and the lowering of the background. The capacitively coupled plasma could be an attractive source for atomic spectrometry since it characterised by low power, good stability and a low gas consumption. The best figures of merit are obtained for the torch with tubular central electrode and two outer ring electrodes, properly positioned. The detection limits are in the range of ng ml − 1 with a dynamic range from 2.5 to 3.5 orders of magnitude.
.
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Analytical characterisation of a capacitively coupled plasma torch with a central tube electrode Emil A. Cordos a,*, Tiberiu Frentiu a, Ana-Maria Rusu a, Sorin D. Angel b, Alpar Fodor a, Michaela Ponta a a
Uni6ersity of Cluj, Department of Chemistry, 3400 Cluj-Napoca, Romania Uni6ersity of Cluj, Department of Physics, 3400 Cluj-Napoca, Romania
b
Received 18 July 1997; received in revised form 22 October 1997; accepted 24 October 1997
Abstract A new type of radiofrequency capacitively coupled plasma torch is presented. The torch electrode geometry is coaxial with a tubular central electrode and one or two outer ring electrodes. The argon plasma is generated at 275 W radiofrequency power and 27.12 MHz and it has a very good stability and a low gas consumption of 0.4 l min − 1. The nebulized sample is introduced through the tubular electrode into the core of the annular shaped plasma thus achieving a better atomisation and a lower background. The limits of detection for 20 elements are in the range of ng ml − 1 and the dynamic range between 2.5 and 3.5. The best results are obtained with the torch with two outer ring electrodes. © 1999 Elsevier Science B.V. All rights reserved. Keywords: Atomic spectroscopy; Plasma sources; Capacitively coupled plasma.
1. Introduction The analytical performance in plasma atomic spectrometry is strongly dependent on the form and on the procedure by which the sample is introduced into the plasma discharge. The process involves two steps. The first is the dispersion of the sample by an appropriate method: ablation or sputtering for the solids, nebulization for the liquids, etc. The second step consists in the conditioning of the dispersed particles or droplets which are then carried further into the plasma * Corresponding author. Fax: + 40 64 420667; e-mail: [email protected]
discharge by a flow of gas. Most of the work in the sample introduction was devoted to the first step of the process. However, the second step has its critical points, one of which is the region of the plasma where the dispersed sample is introduced. It implies that the plasma torch has to be designed accordingly. A good example is the radio frequency inductively couple plasma (ICP). The exceptional analytical qualities of this plasma are the results of a o combination of two factors: plasma higher temperature and the plasma annular shape which allows the sample to be introduced right into the core of the discharge. Consequently the atomic and ionic excitation is very efficient; the spectral background is relatively
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low, the detection limits are very good, the dynamic range is large and the matrix effect is not important [1 – 8]. The torches developed later for microwave capacitively coupled plasma (CMP) and microwave induced plasma (MIP) were designed such that the plasmas have an annular shape so that a better interaction between sample and plasma could be achieved. Winefordner et al. [9–11] were the first to build CMP torches of medium power (up to 500 W) with tantalum tube electrodes and of high power (1 kW) with graphite electrode, that allows for the sample aerosol to be introduced in the centre of the plasma. The detection limits obtained with these torches were 1 – 2 order of magnitude better that those obtained with torches with platinum wolfram tip electrode and side introduction of the sample aerosol. Okamoto et al. [12], by introducing the sample in the core of an annular shape, high power (1.3 kW), argon MIP have achieved in atomic emission, detection limits similar with those in Ar-ICP-AES. The same authors [13,14] using detection by mass spectrometry (MS) have obtained detection limits for nitrogen microwave plasma (N2-MIP-MS) similar with those from ArICP-MS. Jin et al. [15,16] have built a new microwave argon plasma torch, similar with an ICP torch but operated at less than 100 W. This plasma has also an annular shape which in combination with an ultrasonic nebulization system yields good detection limits for refractory elements in spite of its low power. In the last years a number of radio frequency coupled plasma torches (r.f.-CCP) of low and medium power, were developed [17]. Cordos et al. [18–23] have built an atomic emission spectrometer based on a r.f.-CCP torch with coaxial electrodes in tip-ring geometry. The torch was low power (85– 275 W at 27.12 MHz), adapted for pneumatic nebulized liquid samples. The sample aerosol was introduced at a distance of 5 mm from the plasma base using a teflon piece with 12 holes. For this type of torch a part of the sample remains in the plasma mantle and does not reach in the hot plasma core where the dissipated power is at maximum [20]. Obviously the plasma capabilities are not fully used. Therefore the authors of the present paper have designed a new type of
Ar-r.f. CCP torch with a tubular central electrode and one or two ring counter electrodes. This new torch could be an alternative to low power CMP and MIP torches. A single argon gas flow is used both for sample nebulization and as support gas. By introducing the sample directly in the plasma core a more efficient atomisation and excitation is expected. In the present paper some figures of merit of this type of torch are compared with those of torch with electrodes in tip to ring geometry. The optimisation of torch parameters and the detection limits are presented.
2. Experimental
2.1. Instrumentation The schematic diagram of the plasma torch is given in Fig. 1. The torch main body is made of brass and it is water-cooled. In the central channel of the torch is placed a PTFE tube with an inner
Fig. 1. Schematic diagram of the r.f. CCP torch with central tubular electrode and two outer ring electrodes.
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Table 1 Instrumentation and operating conditions for plasma torch and spectrometer Rf. Generator
EOP model, 27.12 MHz free running oscillator, 275 W (Research Institute for Analytical Instrumentation, Cluj-Napoca, Romania). Plasma torch Laboratory constructed. Watercooled torch body. Capacitively coupled, coaxial geometry with molybdenum tubular electrode, 2–4 mm i.d., 5 mm o.d. One or two outer ring electrodes, 25 mm diameter, made of 11 gauges copper wire. Support gas: argon spectral grade (Azo Mures, Tg. Mures, Romania), 0.4 l min−1 gas flow at atmospheric pressure. Sample introPneumatic nebulizer and 4 rollers peristaltic pump, 0.8 ml min−1 solution intake, 120 ml glass nebulization duction sys- chamber, 10% nebulization efficiency (Research Institute for Analytical Instrumentation, Cluj-Napoca, Romatem nia). Aerosol introduction into the plasma core via central tube electrode. Optics Monochromator Heath EU 700, 0.35 m focal length, 1200 grooves per mm grating. Photomultiplier R14 14 (Hamamatsu, Japan) operated at 700 V. Photomultiplier power supply Heath EU 701 (Heath Co. Benton Harbour, MI, USA) Recorder K 201 (Zeiss-Jena, Jena Germany)
diameter of 5 mm. Further, into this tube is introduced a molybdenum tubular electrode with the inner diameter of 2 – 4 mm. The electrode is centred and secured in position by 6 screws and connected to the r.f. generator. Over the whole torch assembly is placed a quartz tube, 15 mm inner diameter and 200 mm length. One or two rings, 25 mm diameter, made out of copper wire, are placed outside of the quartz tube and connected to the common of the r.f. generator. The rings position is adjustable. The plasma was generated using an r.f. generator of free running type, operated at 275 W and 27.12 MHz. The support gas was spectral grade argon at atmospheric pressure. This single gas flow is used both for sample nebulization and as support gas for the discharge. The plasma develops between the tubular electrode and the outer ring electrodes. The samples were nebulized using a pneumatic nebulizer and a 4 rollers peristaltic pump and introduced into plasma core via the tubular electrode. The optical signal was measured by a Heath EU 700 monochromator, equipped with a Hamamatsu R1414 photomultiplier. The photocurrent was recorded on a Zeiss Jena K201 recorder. Details of the equipment were given in Table 1.
3. Reagents Stock solutions of 1000 mg ml − 1 of Al, Ba, Ca, Cd, Cr, Cu, Fe, Eu, Bi, Mg, Mn, Ni, Co, Pb, Zn,
Hg, Yb, Li, Na, Ag were prepared by dissolution of high-purity metals, salts or oxides in HCl or HNO3. Single element working standards were obtaining by diluting the stock solutions with 2% v/v HNO3. For the blank measurements a 2% v/v HNO3 solution was used.
4. Results and discussion
4.1. Optimisation of plasma torch working parameters The discharge obtained with tubular electrode torch looks like a long and slim flame with an inner cone at the base. The cone is shorter than the plasma core obtained with a tip-ring torch [18]. Like in a regular chemical flame the emission is at maximum above the inner cone. The stability of the discharge and its analytical performance are dependent of the material and the dimensions of the tubular electrode, the placement of ring electrodes, plasma power and support gas flow. Since the intensity of the electric field is at maximum in the centre of the plasma the quartz tube is protected against excessive heating, in contrast with the torches having outer parallel plane electrodes, where the electric field is at maximum close to the tube walls. The role of the ring electrodes are different. The first electrode (lower ring electrode) influences the uniformity and density of electrical field lines in the lower part of the
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Fig. 2. The geometry of the r.f. CCP torches compared in the present paper. T-R. Torch with electrodes in tip-ring geometry SR. Torch with central tubular electrode and single ring outer electrode DR. Torch with central tubular electrode and double ring outer electrode.
plasma. A proper diameter and position of this electrode insures the discharge stability and its adherence to the electrode. The second (upper) ring electrode concentrates the electric field lines in the centre of the plasma, with the improvement of power transfer from the r.f. field to the plasma. This increases the atomic and ionic excitation. When using the second ring electrode the inner cone of the plasma discharge gets shorter and brighter. A schematic of the three torches compared in the present paper is given in Fig. 2. In the first stage, the optimisation of plasma torch parameters was accomplished for the tubular central electrode torch with a single outer ring electrode. The optimisation implies the position of the ring electrode, maximum plasma power, argon gas flow and diameter of the tubular electrode. The influence of the second electrode upon the plasma properties was determined only as function of its relative position to the first one and it will be discussed later. The optimum position of the ring electrode, in single ring configuration, is one close to the tubular electrode and it was established empirically. It
was found that a distance of 5 mm between the top of the tubular electrode and the first ring electrode yields a stable discharge. At this distance between electrodes the divergence of the electric field lines provides an uniform heating of the top circumference of the tubular electrode and a good plasma adherence to the electrode. It was established by trials that the maximum power of the discharge that do not damage the electrodes is 275 W. The optimum values for argon gas flow and for the diameter of the tubular electrode were established by monitoring the intensity of Na I 588.9 nm line. The influence of argon gas flow on Na I 588.9 nm line intensity, for two tube diameters, is presented in Fig. 3. The emission increases abruptly when gas flow changes from 0.3 to 0.4 l min − 1, but further, when the gas flow increases from 0.4 to 0.8 l min − 1, the signal growth do not exceed 25% in spite of the fact that nebulization efficiency increases two times. The relative signal plateau in this gas flow range, is due both to the decrease of analyte residence time in the zone of maximum excitation and to the cooling of the
E.A. Cordos et al. / Talanta 48 (1999) 827–837
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Fig. 3. Emission signal for Na I 588.9 nm line (solution concentration: 50 mg ml − 1 Na) as function of argon gas flow. Inner diameter of the central molybdenum tubular electrode 3.0 mm (A) and 3.5 mm (B)
plasma. A more detailed picture is given in Table 2 where some figures of merit are presented for the above mentioned sodium line, as function of tubular electrode diameter and argon gas flow. The relative standard deviation of the background (RSDB) is an indicator of plasma mechanical stability. A very stable plasma is obtained at 0.4 l min − 1 argon gas flow and 3.5 mm tube diameter where RSDB has a minimum values of 1.2. The gas flow and the tube diameter affect the signal to background ratio (SBR) by influencing both the signal and the background but in different ways and proportions. For example, as shown before, an increase of gas flow from 0.4 to 0.8 l min − 1 produces a moderate increase of the signal (10 – 25%) but, in exchange, the background increases two times for tubes having 3 or 3.5 mm diameter. If the tube diameter increases over 3.5 mm the background increases dramatically. The overall effect is a decrease of the SBR. For a given gas flow the decrease is a moderate one as long the tube diameters changes from 3 to 3.5 mm (546 – 450 for 0.4 l min − 1 and 341– 206 for 0.8 l min − 1). For a given diameter the doubling of the gas flow produces a more important decrease of the SBR (546 – 341 for 3 mm
diameter and 450–206 for 3.5 mm diameter). Increasing the tube diameter to 4 mm decreases the SBR with almost an order of magnitude. The limits of detection (LOD) follow the same pattern as the SBR. The best LOD’s are obtained for argon gas flow of 0.4 l min − 1. Therefore the increase of gas flow much over 0.4 l min − 1 and the use of tube diameters larger than 3.5 mm are not recommended. The optimum constructive and operating condition for the plasma torch are: 275 W r.f. power, 0.4 l min − 1 argon gas flow, 3.5 mm inner diameter for the tubular electrode and a 5 mm distance between this electrode and the lower ring electrode. These conditions were also valid for other elements. The figures of merit for the torch with a single ring electrode, in the above mentioned conditions are listed in Table 3. As expected, the LOD’s improved for all elements as compared with the plasma torch in tip-ring electrode geometry, where the sample is not introduced in the centre of the discharge. Since the improvement is more notable for the elements with a lower dissociation energy of the oxides, Ag, Cd, Mn, Cd, Zn, Co, one may assume that the improvement is in the atomisation process.
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Table 2 Analytical characteristics of sodium emission (Na I 588.9 nm) used for torch optimisation Tubular electrode inner diameter, mm
Argon gas flow, l min−1
SBRa
RSDBb
LODc
3 3 3.5 3.5 4
0.4 0.8 0.4 0.8 0.8
546 341 450 206 42
1.5 1.8 1.2 1.4 1.5
4.0 8.0 4.0 10.0 54.0
SBR, signal to background ratio for 50 mg ml−1 Na. RSDB, relative standard deviation of the background (determined on ten successive measurements). c LOD, limit of detection (3 s criteria), ng ml−1. a
b
4.2. Influence of the second ring electrode The presence of the second ring electrode changes the shape of the plasma and the optimum observation height for the analytical signal. This changes are function of the distance between the lower and the higher electrode. As an example, in Fig. 4, the emission intensity for the Al I 396.15 nm line is represented as function of distance between electrodes and observation height. It could be seen that the optimum emission intensity could be obtained in a spot that covers about 4 mm in the observation height and 10 mm in the ring distance. The optimum distance between ring electrodes, the optimum observation height and the detection limits are listed in Table 4, for the same elements as in Table 3. The elements were divided into 3 groups as function of distance between electrodes. The first group includes elements for which the upper electrode is relatively close to the lower electrode, 60 mm, and the optimum observation height is generally higher, 12 – 20 mm. All ionic lines are in this group. The oxides dissociation energy is above 4 eV. For this group the improvement in LOD’s are generally higher than those obtained for the other two groups. The most notable increase in emission signal is for Ba and Yb. The elements from the second group exhibit the best emission when the electrodes are farther, 70 mm, but the optimum observation height is lower, 6–12 mm. The dissociation energy for the oxides is B 4.3 eV. The decrease of LOD is generally smaller than for the first group, with an increasing factor of 1.4 – 1.8.
The third group includes elements with lower excitation energy so that they exhibit a good LOD even in plasma with a single ring electrode or in plasma of the tip-ring torch. The emission signal improvement is illustrated in Fig. 5 where the actual experimental traces are given for Cd and Ba. For both these elements the reduction in the background level and background noise could be noticed, when changing from tip-ring torch to a torch with tubular central electrode. The increase in the signal level could also be seen when using two outer ring electrodes as compared with single outer electrode torch. At the element concentration of 250 and 500 ng ml − 1 Ba yields an appreciable signal only for the tubular central electrode torch with two outer ring electrode. Increasing plasma temperature or electron number density does not increase significantly the atomisation for this elements but rather increases the excitation efficiency or slightly decreases the background. As a matter of fact, the LOD’s improvement, when switching from tip-ring torch to single ring tubular torch and further to double ring tubular torch, is the results of two effects: increase of the emission and decrease of the background. The contribution of these two effects is not the same for all elements. As an example, the changes in emission and background intensities are given in Table 5, for a number of selected elements. The intensities are relative, the column with higher values being equal with 100 units. One of the first things to be noticed is the lower background intensity of the tubular torches as compared with tip-ring torch. As for the listed
Data from [21].
422.67 588.99 670.79 285.21 328.07 393.36 324.74 279.55 228.81 425.43 405.78 403.08 257.61 352.45 213.85 371.99 396.15 345.35 459.40 253.65 223.06 346.44 214.44 493.41
Ca(I) Na(I) Li(I) Mg(I) Ag(I) Ca(II) Cu(I) Mg(II) Cd(I) Cr(I) Pb(I) Mn(I) Mn(II) Ni(I) Zn(I) Fe(I) Al(I) Co(I) Eu(I) Hg(I) Bi(I) Yb(I) Cd(II) Ba(I)
a
l nm
Element
2.93 2.11 1.85 4.34 3.78 3.15 3.82 4.43 5.41 2.91 4.38 3.08 4.81 3.54 5.80 3.33 3.14 3.54 2.70 4.88 5.55 3.58 5.78 2.51
Eex., EV
5.0 — — 4.4 1.4 — 4.9 — 3.8 4.2 4.1 4.0 — 4.3 4.0 4.0 5.0 3.7 — — 4.0 — — 6.0
Eox.dis., eV
24 10 10 20 18 25 18 22 16 24 14 18 20 18 12 14 26 16 28 12 18 26 24 28
Optimum observation height mm 200 450 300 225 128 46 60 38 24 15.5 13.5 12.2 10.0 9.1 26 7.6 5.4 4.0 5.0 2.0 2.0 1.4 1.4 0.8
SBR (for a 50 mg ml−1 solution)
Torch tubular electrode single ring (SR)
Table 3 Figures of merit for a CCP torch with central tubular electrode and single ring outer electrode
9 4 5 6 13 32 25 35 65 100 110 125 155 170 60 200 290 390 300 750 800 1000 1100 1900
LOD (SR) ng ml−1
15 13 9 9 430 — 28 — 770 120 450 750 — 1700 700 240 — 2500 440 750 — — — 5700
LODa (T-R) ng ml−1
Torch tip-ring (T-R)
1,6 3,2 1,8 1,5 33 — 1,1 — 11,9 1,2 4,1 6 — 10 11,7 1,2 — 6,4 1,5 1 — — — 3
(T-R) (SR)
LOD’s ratio
E.A. Cordos et al. / Talanta 48 (1999) 827–837 833
l nm
422.67 393.36 285.21 279.55 324.74 493.41 425.43 396.15 403.08 257.61 346.44 371.99 459.40 223.06 214.44 228.81 405.78 352.45 213.85 345.35 253.65 588.99 670.79 328.07
Element
Ca(I) Ca(II) Mg(I) Mg(II) Cu(I) Ba(I) Cr(I) Al(I) Mn(I) Mn(II) Yb(I) Fe(I) Eu(I) Bi(I) Cd(II) Cd(I) Pb(I) Ni(I) Zn(I) Co(I) Hg(I) Na(I) Li(I) Ag(I)
14 16 12 14 14 20 18 16 12 16 20 10 16 14 18 12 10 8 6 10 6 6 6 12
Optim. Obs. height mm 60 60 60 60 60 60 60 60 60 60 60 60 60 60 60 70 70 70 70 70 70 80 80 80
Distance between ring electrodes mm
tubular electrode double ring (DR)
Torch
530 300 450 150 90 44 28 21 21 17 14 15 25 4 3.6 40 20 13 38 7 3 540 360 161
SBR(for a 50 mg ml−1 solution)
Table 4 Figures of merit for a CCP torch with central tubular electrode and double ring outer electrode.
3 5 3 9 16 40 50 70 75 90 100 105 60 400 400 40 80 120 40 210 500 3 4 50
LOD (DR) ng ml−1
9 32 6 35 25 1900 100 290 125 155 1000 200 300 800 1100 65 110 170 60 390 750 4 5 13
LOD (SR) ng ml−1
Single ring (SR)
3 6.4 2 3.9 1.6 47.5 2 4.1 1.7 1.7 10 1.9 5 2 2.7 1.6 1.4 1.4 1.5 1.8 1.5 1.3 1.3 1.3
SR/DR
LOD’s ratio
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Fig. 4. Emission intensity of Al I 396.15 nm line as function of distance between outer ring electrodes and observation height.
Fig. 5. The recorded emission signal for Cd and Ba. a, background; b, 250 ng ml − 1; c, 500 ng ml − 1; T-R., Torch with electrodes in tip-ring geometry; SR., Torch with central tubular electrode and single ring outer electrode; DR., Torch with central tubular electrode and double ring outer electrode.
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Table 5 Line and background intensities for r.f. CCP torches with different electrode geometry Element
Relative line intensity
Ba Mn Cd Ni Co Pb Ag
Relative background intensity
T-Ra
SRb
DRc
T-Ra
SRb
DRc
1 15 32 36 39 44 35
1.4 32 48 50 53 65 71
100 100 100 100 100 100 100
100 100 100 100 100 100 100
45 36 13 14 18 35 5.4
60 63 17 18 23 41 6
a
T-R, CCP torch with tip ring electrode geometry (data according to [21]). SR, CCP torch, central tubular electrode single outer ring electrode. c DR, CCP torch, central tubular electrode, double outer ring electrode. b
elements, two extremes could be mentioned. In the case of Ba, the LOD improvement is due to the increase of the emission signal, especially when the double ring tubular torch is used. At the other end is Ag for which the decrease of background intensity is responsible for LOD decrease.
In comparing the tip-ring torch with the single ring tubular torch, the improvement of the analytical performance is due rather to the background decrease and in the lesser degree to the increase of the emission. Adding a second ring electrode to the tubular torch does not further reduce the
Table 6 Calibration curves parameters Element
a
Standard deviation of ‘a’
n
Standard deviation of ‘n’
Correlation coefficient
Dynamic range
Ag Al Ba Bi Ca Cd Co Cr Cu Eu Fe Hg Li Mg Mn Na Ni Pb Yb Zn
1.88 1.31 2.14 2.48 2.63 1.88 1.87 1.43 1.70 1.80 1.71 1.10 1.97 1.87 2.48 2.49 1.85 1.83 1.82 2.33
0.034 0.016 0.026 0.017 0.013 0.014 0.033 0.013 0.014 0.025 0.024 0.011 0.026 0.030 0.017 0.021 0.021 0.028 0.020 0.011
0.98 0.82 0.96 1.03 0.87 0.89 0.95 0.97 0.88 1.03 0.86 0.96 0.91 1.01 0.88 1.03 0.98 0.88 1.04 0.85
0.035 0.014 0.020 0.020 0.022 0.016 0.023 0.010 0.013 0.020 0.013 0.005 0.018 0.034 0.016 0.025 0.015 0.016 0.016 0.011
0.993 0.997 0.995 0.996 0.996 0.998 0.995 0.999 0.996 0.997 0.998 0.999 0.997 0.994 0.997 0.995 0.998 0.997 0.997 0.999
2.5 3 3 3 3 2.5 3.5 3 2.5 3 3 3.5 3.5 2.5 2.5 3 3 3.5 2.5 2.5
E.A. Cordos et al. / Talanta 48 (1999) 827–837
background but significantly increases the emission. The dynamic range was established using the calibration curve: log y= a+ n log C where y is the emission intensity in arbitrary units, a is the log (signal /concentration unit), C represents the concentration in mg ml − 1and n is the calibration curve slope. The values of the above mentioned coefficients are listed in Table 6. The calibration curves were recorded starting from a minimum concentration five times LOD. The dynamic ranges are between 2.5 and 3.5 for all studied elements.
5. Conclusions The analytical performance of a capacitively coupled plasma spectral source could be substantially improved by using a torch with a tubular central electrode instead of a tip electrode. To the enhancement of the analytical signal contribute both the increase of the emission, due to sample introduction into the plasma core, and the lowering of the background. The capacitively coupled plasma could be an attractive source for atomic spectrometry since it characterised by low power, good stability and a low gas consumption. The best figures of merit are obtained for the torch with tubular central electrode and two outer ring electrodes, properly positioned. The detection limits are in the range of ng ml − 1 with a dynamic range from 2.5 to 3.5 orders of magnitude.
.
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