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Comparison of nebulizers working below 0.81 min−1 in inductively coupled plasma atomic emission spectrometry
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*I mc&mica Acre,Vol. 41B,Nos l/Z. pi. 4961, 19%. printed in Great Britain.
0 1986.Pcrgamon RcssLtd.
Comparison of nebulizers working below 0.8 1 min- ’ in inductively coupled plasma atomic emission spectrometry E. MICHAUD-POUSSEL and J. M. MERMET CNRS,BP 22,69390 Vemaison, France
Service Central d’Analyse du
(Received 2 January 1985; in revisedform 28 June 1985) Abstrret-Several nebulii of concentric,cross-flow, Vee, frit and ultrasonic types have been compared in terms of signal-to-noise ratio (for both analyte and background signal), uptake rate, efficiencyand memory effects. Especially, the dependence of these figures of merit at gas flow rates below 0.8 I min - ’ is investigated so as to allow an evaluation of these.nebuliiers for use together with low consumption torches in ICP-AES. This study was done mainly with a 56MHz ICP, but additional measurements for a 27-MHz indicate that the results are applicable to a 27-MHz system.
1. INTR~OUC~I~N THERE has been a constant trend to design lower power and lower flow rate torches in inductively coupled plasma atomic emission spectrometry (ICP-AES) in order to use low power (< 1 kW) and reduced size generators at 27 MHz [l-13] or higher frequencies [14-183. This decreases both the capital investment and the running cost. With the aid of conventional but optimized torches, it is presently possible to work below 10 1min-’ and 1 kW [3,5, 7, 133, and a toroidal shaped plasma can be sustained down to 4-5 1min- ‘. Further, a ball shaped plasma associated with external (air or water) cooling, can be operated down to plasma gas flow rates as low as 11 min - ’ [16,17]. Our work centres on toroidal shaped plasmas which presently are preferred for routine analysis, since they provide good analytical results and ease of operation. However, with such torches, nebulizers with low aerosol gas flows must be used. With a torch running between 5 and 8 1min- ‘, the speed of the carrier gas and the carrier gas flow rate as well, can be reduced so as to increase the residence time compared to 12-16 1min- ’ torches. Consequently, with injectors having an inner diameter between 1 and 2 mm, the carrier gas flow rate must be below 1 1min-‘. It is known that pneumatic nebulizers are difficult to run below this value, because of the reduced diameters and accurate adjustment of the capillaries required, but also because of the stronger dependence of the performance on the physical properties of the solution (especially the viscosity) and the possibility of blocking at low aerosol carrier gas flow rates. Several pneumatic nebulizer designs have been proposed either to increase the efficiency at
[l] C. D. ALLEMANDand R. M. BARNES, Appl. Spectrosc. 31,434 (1977). [2] C. D. ALLEMAND, R. M. BARNESand C. D. WOHLERS, Anal. Chem. 51,2392 (1979). [3] R. N. SAVAGEand G. M. HIEFTJE,Anal. Chem. 51,408 (1979). [4] H. KAWAGUCHI, T. ITO, S. RUBIand A. MIZUIKE,Anal. Chem. 52,244O (1980). [5] A. D. WEISS,R. N. SAVAGE and G. M. HIEFTJE,Anal. Chim. Acta 124,245 (1981). [6] M. D. LOWE, Appl. Spectrosc. 35, 126 (1981). [7] R. REZAAIYAN, G. M. HIEFTJE, H. ANDERSON, H. KAISERand B. MEDDINGS, Appl. Spectrosc. 36,627 (1982). [S] H. KAISER and B. MEDDINGS,presented at the 21th Eastern Analytical Symposium, New York (1982). [9] H. KAWAGUCHI, T. TANAKA,S. MIUM, J. Xu and A. MIZUIKE,Spectrochim. Acta 38B, 1319 (1983). [lo] G. M. HIEFTJE, Spectrochim.Acta 38B, 1465 (1983). [11] G. M. ALLENand D. M. COLEMAN, Anal. Chim. Acta 158,267 (1984). [ 123 A. MONTASER,G. R. HUSE, R. A. WAX, S. K. CHAN, D. W. GOLIGHTLY, J. S. KANEand A. F. DORRZAPF,Anal. Cbem. !56,283 (1984). [13] G. ANGLEYS and J. M. MERMET, Appl. Spectrosc. 38,647 (1984). [14] J. M. MERMET and C. TRA~SY,Appl. Spectrosc. 31,237 (1977). [ 153 R. REZMIYAN, J. W. OLESIKand G. M. HIEFCIE, 35th Pittsburgh Conference, Atlantic City (1984), paper 905. [16] G. R. KORNBLUM, W. VANDERWAAand L. DE GALAN, Anal. Chem. 51,2378 (1979). [17] P. A. M. R~pso~ and L. DE GALAN, Spectrochim. Acta 38B, 707 (1983).
50
E. MICHAUDPOUSSELand J. M. MERMET
low flow rate or to avoid clogging, namely the Vee nebulizer [20-291 whose design is a modification of the one proposed by BABINGTON [30], and the frit nebulizer [31-331. Besides these nebulizers which are still using a gas flow to produce the aerosol and are therefore gas flow rate dependent, the ultrasonic nebulizer has been proposed for a long time C34-473. Recently, jet impact nebulizers have been described [48] and are similarly independent of the gas flow rate for the formation of aerosol. At the moment, only pneumatic (cross-flow or concentric) and Vee nebulizers are readily commercially available. With conventional ICP systems comparisons between pneumatic (concentric, cross-flow and Vee) nebulizers have been published [49, 503. Subsequent to the optimization of low power, low consumption torches, realized with a 27MHz Plasma-Therm unit [ 131, the present work describes a comparison of various nebulizers either commercially available (Meinhard concentric and Perkin-Elmer cross-flow nebulizers) or built in the laboratory (Vee, frit and ultrasonic nebulizers). Several figures of merit have been used for the comparison, namely the signal-to-noise (of the signal) ratio SNsR and the signal-to-noise (of the background) ratio SN,R. The RMS value of the first ratio is known to be the reciprocal of the relative standard deviation RSD of the net signal. The second ratio is the reciprocal of the RSD of background value and is linked to detection limit cL. Further, we have also considered efficiency and memory effects of the various nebulizers and applied these results to some detection limits. 2.
EXPERIMENTAL
For this study, a 56 MHz Durr generator but also a 64 MHz Durr generator have been used. The ICP stands of the systems used allowed easy interchange of the different nebulizers as large space below the
[18] [19] [ZO] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [SO]
R. M. BARNESand R. G. SCHLEICHER,Spectrochim. Acta 3OB, 109 (1975). P. A. M. RIPSON, L. DE GALANand J. W. DE RUITER, Spectrochim. Acta 37B, 733 (1982). R. F. SUDDEND~RFand K. W. BOYER, Anul. Chem. SO, 1769 (1978). J. F. WOLCO~ and C. BUTLER-S• BEL,Appl. Spectrosc. 32,591 (1978). J. R. GARBARINOand H. E. TAYLOR, Appl. Spectrosc. 34, 584 (1980). B. THELIN, Analyst 106,54 (1981). P. J. MCKINNON,K. C. GtEssand T. V. KNIGHT,Deu. Atom. Plasma Spectrochem. Analysis, Ed. R. M. BARNES, p. 287. Heyden, London (1981). P. A. M. RIPSONand L. DE GALAN,Spectrochim. Acta 36B, 71 (1981). L. EBXIN and M. R. CAVE, Anulyst 107, 172 (1982). D. R. HEINE, M. B. DENTONand T. D. SCHLABACH,Anal. Chem. 54,81 (1982). J. F. WOLCO~ and C. BUTLER-NOBEL,Appl. Spectrosc. 36,685 (1982). M. D. WICHMAN,R. C. FRY and N. MOHAMED,Appl. Spectrosc. 37,254 (1983). R. S. BABINGTON,Popular Science (May issue), 102 (1973). C. T. APEL, T. M. BIENIEWSKI,L. E. Cox and D. W. STEINHAUS,ICP Informution Newslett. 3, 1 (1977). L. R. LAYMANand F. E. LICHTE, Anal. Chem. 54,638 (1982). L. M. FAIRES,B. A. PALMER and J. W. BRAULT,Spectrochim. Acta 4OB, 135 (1985). R. H. WENDTand V. A. FASSEL,Anal. Chem. 37,920 (1965). H. DUNKENand G. PFORR,Z. Phys. Chem. 230,48 (1965). H. C. HOAREand R. A. MOSTYN,Anal. Chem 39, 1153 (1967). J. M. MERMETand J. ROBIN,Anal. Chem. 40, 1918 (1968). G. W. DICKINSONand V. A. FASSEL,Ad. Chem. 41, 1021 (1969). P. W. J. M. BOUMANSand F. J. DE BOER,Spectruchim.Acta 278,391 (1972). J. C. SOUILLIART and J. P. ROBIN,Anulusis 1, 427 (1972). P. W. J. M. B~UMANSand F. J. DE BOER,Spectrochim. Acta 3OB, 449 (1975). K. W. OLSON,W. J. HAASand V. A. FASSEL,Anal. Gem. 49,632 (1977). H. UCHIDA,H. MATNJI,T. UCHIDAand C. IIDA,Spectrosc. ht. 11, 1 (1978). R. DIEMIASZONEK, J. L. MOUTONand C. TRASSY,Analusis 7,96 (1979). R. E. STURGEON, S. S. BERMAN, J. A. H. DESAULNIERS, A. P. MYKYTIUK,J. W. MCLARENand D. S. RUSSELL, Anal. Ckm. 52, 1585 (1980). C. E. TAYLORand T. L. FLOYD,Appl. Spectrosc. 35,408 (1981). J. M. MERMET,C. TRASSYand P. RIPOCHE,Deuelopments in Atomic Plasma Spectrochmical Analysis, Ed. R. M. BARNES, p. 245. Heyden, London (1981). M. P. D~HERN and G. M. HIEFTJE, Appl. Spectrosc. 38,405 (1984). R. M. BARNES,H. S. MAHAN~, M. R. CAVE and L. FERNANDO, ICP Infirm&on Newslett. 8, 562 (1983). F. J. M. J. MAESSEN,P. COEVERT and J. BALKE,Anal. Gem. 56,899 (1984).
Comparison of nebulizers
51
torch is available. Besides these generators, a 27-MHz Plasma-Therm unit has been used to compare the results obtained at higher frequencies and to provide detection limits on a commercial system (Perkin-Elmer ICP 5500 system). Torches have been studied that enable working at low flow rates, mainly by reducing the annular space between the outer and the intermediate tubes [l, 7,8,10,13,14]. In the case of the 27-MHz ICP, a space of 0.5 mm was selected, providing a minimum plasma gas flow rate of 4 1min- ’ and a working plasma gas flow rate of 6.5 1min- ‘. Using a higher frequency (56 or 64 MHz), it is not necessary to use reduced size torches to obtain low flow rates [Sl]. With an o.d. of the intermediate tube of 24 mm and
i.d. of the outer tube of 26.5 mm, corresponding to an annular space of 1.25 mm, it is possible to work with a flow rate of 10 1 min-‘. Torch dimensions and operating parameters are summarized in Table 1. The ICP Durr generators are set up with a Jobin-Yvon l-m monochromator with a 3600 line/mm interferometric grating 1521. With the 27-MHz Plasma-Therm ICP, the optical system and readout of the Perk&Elmer ICP 5500 were used. In each case, mass flow controllers (Brooks 5810) were utilized for the carrier and sheathing gas flow rates. The Meinhard, cross-flow and frit nebulizers were fed at a pressure of 3 bars. No attempt was made to optimize the spray chamber for each nebulizer. A Scott spray chamber [53] was used for the experiments. The observation height in the case of the 56 and 64 MHz was selected just above the top of the coil. With the 27 MHz system, the observation height was 10 mm above the coil. A test solution containing manganese at a concentration of 100 ng ml- 1was used and the Mn II 257.6 nm line was selected for the experiments in this study.
3. RESULTS 3.1. Meinhard and cross-jlow nebulizers Two concentric type A Meinhard nebulizers (#292 and 1442) and a Perkin-Elmer cross flow nebulizer were used. Meinhard nebulizers can be operated with or without a force feeding peristaltic pump. The influence of the pump has been studied at rates between 0.25 and 3 ml min - I. A maximum increase of the signal (30 “/,)was found near 0.75-1.0 ml min- ’ for carrier gas flow rates between 0.6 and 0.7 1min- ’ (Fig. 1). Below 0.6 1 min- ‘, the influence is relatively small. Therefore, no peristaltic pump was used with the Meinhard nebulizers. For the PE nebulizer, however, a pump is used. As with a rate of 0.75 ml min- ‘, the PE nebulizer (analyte) signal intensity was similar to that observed with Meinhard nebulizers without a pump (Fig. 2), a 0.75 ml min- ’ pump rate was used in further experiments. It is to be noted that the lowest carrier gas flow rate which can be used for any of these three nebulizers is 0.3-0.35 lmin- ‘. 3.2. Vee nebulizers Further a Vee nebulizer is investigated. Similar to other Vee-type nebulizers, it has been developed [25] in order to reduce. the uptake rate compared to Babington design. The gas capillary tube may have an i.d. as small as 0.1 mm. This design has been suggested [25] to be
Table 1. Operating parameters of the two ICP systems used in this work Generator Frequency Power Torch: external tube internal tube Plasma gas Carrier gas Sheathing gas
Plasma-Therm 27 MHz 600W
Durr 56 MHz l@OOW
19-16 mm 30-26.5 mm ls-12mm 24-21 mm 6.5lmin-’ 101min-1 0-0.8lmin-* O-0.81min-L
[Sl] E POUSEL and J. M. MERMET,Spectrochim. Acta 41B (1986). [SZ] J. W. MCLARENand J. M. MERMFT,Spectrochim. Actu 398, 1307 (1984). [S3] R. H. SCOIT, V. A. FASSEL,R. N. KNISELEY and D. E. NIXON, Anal. Gem. 46,75 (1974).
E. MICHAUDPOU~~EL and J. M. MERMET
52
1
2
3 QL mL/min
Fig. 1. Variation of the signal obtained with a concentric Meinhard nebulizer as a function of the peristaltic pump rate QL, (Mn II 257.61 mu, 100 ng ml-‘).
150 -
100 -
50 -
I 04
06
I F,
L/min
Fig. 2. Variation of the signal obtained with two Meinhard nebulizers (M~and MO) and a Perk&Elmer cross-flow nebulizer (PEB) as a function of the carrier gas flow rate F, (Mn 100 ng ml-‘). Only the PE nebulizer is fed with a peristaltic pump (0.75 ml min- ‘).
used with very low carrier gas flow rates. The dimensions of the Vee nebulizer constructed are shown in Fig. 3. Several materials have been used in the construction. It was possible to machine in glass ceramic (Macor from Corning) a gas inlet of 0.3 mm. For smaller diameters, quartz capillary tubes (i.d. 0.07 mm) were inserted into a PTFE base. With an i.d. of 0.3 mm, the working range of the nebulizer is similar to that of concentric or cross-flow nebulizers (minimum gas flow rate of 0.35 1min- ‘). In the case of an i.d. of 0.07 mm, the working range (with a pressure of 3 bars) is between 0.1 and 0.5 1min -I. The latter type has been used in this study. Since there is no Venturi effect, it is necessary to use a peristaltic pump. As the ratio between the width of the rectangular channel through which the sample flows (1 mm) and the i.d. of the gas inlet (0.07 mm) is rather large, the optimal uptake rate may be expected to be rather high. As may be seen from Fig. 4, it is necessary to use a pump rate of at least 2.4 ml min - 1 to be relatively independent of the rate. However, then, the increase of signal is
53
Comparison of nebulizers Solution
-
Ar 07Opm
Fig. 3. Vee nebulizer design.
I
02
04
FA
L/min
Fig. 4. Variation of the signal obtained with the Vee nebulizer (V) with different peristaltic pump rates, as a function of the carriergas, and comparison with a Perk&Elmer cross-flow nebulizer (PE) also fed with a peristaltic pump (0.75 ml min- I).
quite large as compared with the PE cross-flow nebulizer. The ratio is 6 at 0.5 1min- ’ and 9 at 0.4lmin-‘. 3.3. Frit nebulizers Frit disc nebulizers up to now have hardly been investigated [31-331. They may be used in two ways. Either the solution is fed onto the surface of the frit material along with the gas (Fig. 5) and goes through it or the solution is fed onto the opposite side (Fig. 6). A disc with pore sizes of 5-10 pm has been found to provide the best results. A compromise diameter of 18 mm was selected to achieve good aerosol production at an acceptably low gas flow rate. The first design, called Fl, includes a spray chamber and the aerosol outlet tube extends into the spray chamber to avoid the formation of any droplets from the spray chamber wall at the outlet. Since the signal intensity is relatively independent of the pump rate in the range 0.03-0.3 ml min - I, a value of 0.03 ml min - a has been adopted. The flow rate thus is much lower than that used with other nebulizers. The best signal stability is obtained when the exit of the capillary tube is set at 2 mm above the surface of the frit disc. Except for the way the solution is fed onto the frit in the second design, called F2, its behavior in terms of signal, efficiency and pumping rate is similar to Fl, as shown in Fig. 7. Fla and Flb refer to two devices which are in theory identical. 3.4. Ultrasonic nebulizer Several designs of ultrasonic nebulizers have been proposed for ICP-AES C34-471. Either the solution is fed directly onto one side of a transducer which is water cooled on the other
54
E. MICHALIDPOUSSEL and J. M. MERMET Solution V
a Ar
Fig. 5. Frit nebulizer (Fl) design.
Solution V
Fig. 6. Frit nebulizer (F2) design.
side or a liquid transmitting bath is used for ultrasonic waves. In the first case, a continuous feed is provided, and in the second, a given amount of solution (typically 2-5 ml) is brought into the nebulization cell. In the latter case, the solution to be nebulixed is separated from the liquid bath by a thin membrane. We preferred to use this latter system because of a better coupling between the transducer and the solution and developed a type as described earlier [37,47]. A minimum volume of 3 ml is required in the cell, although only a small part is used in aerosol production. Even at low flow rates ( < 0.5 1 min- I), the amount of aerosol taken up by the gas can be so large that the discharge extinguishes. Therefore, a gas flow rate of 0.1 1min- ’ was selected and an external gas was added to have enough gas velocity to penetrate the plasma. If this gas is added to the carrier gas at the exit of the cell and if the flow of both gases is laminar, the
Comparison of nebulizers
55
s
o-
o-
02
0.4
0.6
F~
L/min
Fig. 7. Variation of the signal (Mn 100 ng ml -I) with the two different designs of the frit nebulizer (Fl and F2) as a function of the carrier gas. Fla and Flb are two different buildings using the same design Fl.
added gas acts as sheathing gas preventing the diffusion of aerosol towards the inner wall of the injector [47]. Accordingly, memory effects and partial clogging in the injector are minimized, with a better constriction of the sample at the base of the discharge. The influence of the sheathing gas flow rate has been described [54]. In contrast to the other nebulizers, the carrier gas flow rate is kept constant (0.1 1min- ‘) and the variation of the total gas flow rate was obtained by adding sheathing gas flow. The signal decreases with the flow rate since the residence time is shorter for the same amount of solution. That is why the signal is very large at low flow rates even as compared to the Vee nebulizer (Fig. 8). The ratio is only 1.5 at 0.5 1min-’ but 6 at 0.4 1min-’ and 20 at
‘S
4
3
2
1
0.2
04
0.6
F,
L/min
Fig. 8. Variation of the signal (Mn 100 ng ml-‘) obtained with the ultrasonic nebulizer (US) as a function of the total carriergas. The aerosol is uptaken with a constant flow rate of 0.1 1min- t and the total gas flow rate is adjusted with the sheathing gas flow rate. Results are compared with the Vet nebulii (V) fed with a peristaltic pump rate of 2.4 ml mitt-‘. [54]
E. MKXAUD, Thesis, University of Lyon (1983).
56
E. MICHAUDPOUSSEL and J. M. MERMET
0.3 lmin- ‘. Compared to pneumatic nebulizers (Fig. 4), the ratio is more than 50 at 0.4 1min- ‘. The aerosol production of such nebulizer is very large below 0.5 1min- ’ whereas other nebulizers can only be used with a much lower efficiency. The variation of the signal as a function of the flow rate, for various nebulizers is given on the same scale in Fig. 9. It is to be noted that below 0.55 1mim- ‘, the signal for the ultrasonic nebulizer is out of scale. 3.5. Study of the SNsR The noise N, of the signal was determined as the standard deviation of the signal from a chart recorder. Obviously, this value depends on the response time (0.6 s) of the readout system which was kept constant during all experiments. With a concentration of 100 ng ml- i, the noise is mainly provided by the nebulization and atomization processes and is of the flicker noise type. The best values are obtained with the ultrasonic nebulizer (Fig. 10).
6x10’-
5
-
1
02
04
0.6
Fh.
L/min
Fig 9. Summary of the variations of the signal (Mn 100 ng ml-‘) obtained with different nebulizers (frit nebuhzers Fl and F2, Meinhard nebulizer M, Perkin-Elmer cross-Row nebulizer PE, Vee nebulizer V and ultrasonic nebuhzer US) as a function of the carrier gas flow rate.
SN,R
80-
0.2
0.3
04
0.5
0.6
FA
L/mln
Fig. 10. Comparison of the different values of the signal-to-noise of the signal ratio, SNsR, as a function of the carrier gas for different nebulizers as described above.
Comparison of nebulizers
57
This is due to the high value of the signal rather than the low value of the noise. The Meinhard, the cross-flow and the Vee nebulizers provide similar values. However with the Vee-nebulizer, these values can be obtained below 0.5 1min- I. Both frit nebulizers provide similar SNsR at a low gas flow and show a rather small variation with flow rate. When using a peristaltic pump with Meinhard nebulizers, the SNsR, especially near 0.5 1min- ‘, are almost independent of the pumping rate between 0.25 and 3 ml min- ‘. 3.6. Study of the SN,R The variation of the SN,R (Fig. 11) is very similar to the variation of the signal (Fig. 9). Highest values are obtained for the ultrasonic nebulizer followed by the Vee nebulizer. Between the frit nebulizers and the Meinhard and cross-flow nebulizers, there is a crossing point near 0.5 1mitt-‘. Accordingly, above this value, these both pneumatic nebulizers will provide better detection limits than frit nebuhzers, with the reverse behavior below 0.5 1min- ‘. This is confirmed at different carrier gas flow rates the detection limit values for Pt (Table 2). Above 0.5 1 min- I, the Meinhard nebulizer provides better detection limits than the frit nebulizer and near 0.5 1min- ‘, the detection limits are the same in both cases. At 0.35 1 min- ‘, the frit nebulizer still has an acceptable SN,R and therefore the detection limits are almost independent of the flow rate for this nebulizer. Table 2 conRrms also that the ultrasonic nebulizer provides the best detection limits. The Vee nebulizer is also shown to give lower detection limits than the Meinhard nebulizer even at a flow rate as low as 0.35 1min- l. 3.7. Study of the uptake rate ana’ of the eficiency
The uptake rate is easy to determine, not however the amount of aerosol produced. The validity of several of the methods proposed [25, 26, 55, 561 has been the subject of some SN,R
0.2
0.4
0.6
FA
L/mln
Fig. 11. Comparison of the different values of the signal-to-noise of the background ratio, SN,R, as a function of the carrier gas for different nebulizers as described above. Table 2. Comparison of the detection limits obtained with Pt II 214.42 (ng ml-‘) using different nebulizers as described in the text and for different carrier gas flow rates (I min-‘)
[SS]
Carrier gas
Meinhard
Frit
Vee
us
0.35 0.47s 0.60
560 90 70
100 90 110
SO 2s -
2 S 10
P. SCHUTYSERand E. JANSSENS,Spectrochim. Acta 34B, 443 (1979). [S6] D. D. SMITH and R. F. BROWNER,Ad. Chem. 54,533 (1982).
E. MICHAUD-POUSSEL and J. M. MERMET
58
controversy [57,58]. In this work, we compared the amount of aerosol in the case of different nebulizers by measuring the absorption of water by silicagel at the exit of the nebulization system. Results are summarized in Table 3. The efficiencies are usually below 3 % for the Meinhard and cross-flow nebulizers. Also for the Vee nebulizer, the efficiency is very low. This fact can be explained by the large ratio between the size of the central channel and the i.d. of the gas inlet. On the other hand, the efficiency is very high for frit nebulixers of whatever the type. A value of 20 % can be easily achieved. This enables a very low consumption rate (at least a factor 10 lower than for the other nebulizers). If a frit nebulizer with uptake rate similar to a Meinhard nebulizer, i.e. about 0.3 ml min-’ at 0.35 1 mm-‘, and with the same 20% efficiency could be designed, the amount of aerosol produced would be greater than that obtained with the ultrasonic nebulizer. For this last system, it is not possible to speak of efficiency since the totality of the aerosol produced reaches the plasma. With a carrier gas of 0.1 1min- ‘, the amount of aerosol is 0.03 g mm-i. The amounts of aerosol produced by the various systems are shown in Fig. 12. It is appreciated [59,60] that the aerosol concentration, i.e. the concentration of liquid per gas volume, is an important parameter. The highest aerosol concentration is obtained with the ultrasonic nebulizer and then with the Vee nebulizer. Although the signal is not related directly to the amount of aerosol, since a spray chamber is used to select the appropriate droplet diameter and since various nebulizers can produce different diameter distributions, by comparison with the results in Table 4, it may be seen that signal and detection limits are closely connected to this amount of aerosol produced and the aerosol concentration. Note that the flicker noise is directly related to the droplet size distribution [61]. 3.8. Memory effects We have carried out some experiments to check the importance of memory effects for each nebulizer including the associated spray chamber. Manganese was still used as the test element at a concentration of 100 ng ml-‘. For each nebulizer, the gas flow rate giving the
Table 3. Comparison of the uptake rate Q,,the amount of aerosol produced Qs the efficiency ( “/,)and the aerosol concentration (@ l- t) using different nebnlixers (of the frit, cross-flow, concentric and Vee types as described in the text) for different carrier gas Bow rates Carrier gas (1 min-‘)
Q. g&--l 0.15
:t:‘?-’ . 00 aer.
cont. (pl I-‘)
0.35
$ 2 aerosol
0.475
: 2 aerosol
0.60
: 2 aerosol
Frit Fl
Frit F2
0.003 0.035 8 20 0.0075 0.035 21 21 0.0095 0.035 27 20 0.009 0.035 26 15
0.003 0.035 8 20 o.ao7 0.035 20 20 0.009 0.035 26 19 0.010 0.035 29 17
Cross-flow PE
0.0055 0.75 0.7 16 0.010 0.75 1.3 21 0.014 0.75 2 23
[57’j P. A. M. RIBON and L. DE GALAN,And. Ckm. 55,372 (1983). [58] R. F. BROWNER and D. D. SMITH,Anal. Chem. 55,373 (1983). [59] A. GUSTAVSON, Anal. Gem. 55,94 (1983). [60] A. GUSTAVWIN,Anal. CIteat.S6,815 (1984). [61] S. D. OMENand A. STRASHEIM, Spectrochim.Acta 38B, 973 (1983).
Meinhard 1442 292
0.0045 0.23 2 13 0.010 0.51 2 21 0.014 0.68 2. 23
0.006 0.27 2 17 0.010 0.45 2 21 0.0145 0.62 2.3 24
Vee 0.0035 2.4 0.15 23 0.010 2.4 0.4 29 0.020 2.4 0.8 42
Comparison of nebulizers
59
Qa g/min
0.03-
_
'3
I
I
us
,
I
I
0.2
0.4
FA
L/min
Fig 12. Comparison of the amount of aerosol Q. obtained using various nebulizers as described above, as a function of the carriergas flow rate. With the ultrasonic nebulizcr (US), the uptake rate is kept constant using a gas flow rate of 0.1 I min-‘, the total gas llow rate is obtained by adding a sheathing gas.
maximum value of the signal was selected (0.55 1min - 1 for Meinhard and PE nebulizers, 0.5 for the Vee nebulizer and 0.45 for the frit nebulizer). It was found that, with the Meinhard nebulizer, the signal decreased to 1% of its original value after 60 s. With the PE cross-flow nebulizer 30 s and with the Vee nebulizer 45 s were required. With both frit nebulizers which show the same behavior, the 1% level is reached only after 25 min. When, a few ml of water are used to clean the frit material, i.e. by using the nebulizer no longer as a nebulizer but as a filter, memory effects become similar to those for Meinhard nebulizers. This may easily be done by using a higher flow from the peristaltic pump. 3.9. Application to the 27-MHz ICP system set up with a reduced size torch In a previous paper [ 131, we have presented the design of a low power (600 W) low flow rate torch (6 1min- i) torch with an annular space of 0.5 mm. Results were presented using a Meinhard nebulizer and an ultrasonic nebulizer. Several experiments have confirmed that the results described above are also applicable to a 27-MHz system. With gas flow rates as reported in Table 4, detection limits (based on a 2a concept) for several elements have been determined (Table 5) without any attempt to optimize parameters such as time constant. In the same table, detection limits are compared with the ones obtained with a conventional torch (12 1 min-i) and a Meinhard nebulizer (0.6 1mm-‘). Once again the ultrasonic nebulizer provides the best results although the difference is not so large with the Meinhard nebulizer because of the limited amount of water which could be injected into the discharge. 4. CONCLUSIONS Studies of the signal, signal-to-signal noise ratio and signal-to-background noise ratio have provided information for the selection of nebulizers when working at a carrier gas flow below Table 4. Car&r and sheathing gas flow rates used with Merent nebulizers and with the 27-MHz plasma-Therm unit Nebulizer Carriergas(Imin-1) Sheathing gas
Meinhard 0.6 -
Frit Fl
Vee
US
0.2 0.25
0.4 -
0.1 0.3
60
E. MICHAUDPOUSSBL and J. M. MERME~ Table 5. Comparison of the detection limits (ng ml-‘) obtained with the 27-MHz ICP system with a conventional torch and with a low power (6OOW), low consumption rate (6.5 I mitt-i) torch using different nebulixers described in the text. With the conventional torch, a Vee nebulixerwith a diameter of 300 jun for the gas inlet was used in contrast with the Vee nebulixer (70 m) used with the low consumption torch Conventional torch Low power, low consumption torch Meinhard Vee-300 Meinhard Vee-70 Frit US As Mn P Pb Pt Sb Sn Sn V
I II I II II I I II II
197.20 257.61 213.62 220.35 214.42 206.83 283.99 189.99 292.40
70 1 50 40 30 30 70 40 5
60
350 5 340 240 140 100
250 4 400 300 100 100 60
20
100 1 100 80 70
40
40 150 8
30
Table 6. Summary of the figures of merits of various nebulixers used in this work with the main requirement to work at low carrier gas gow rates (< 0.8 1 mitt-t). One star means that the nebulixer is acceptable for the figure which is considered, two stars mean that the nebulixer is good and three that it is excellent Meinhard Cross-flow Low flow rate Detection limit Uptake rate El&iency Clogging Easeofuse. Memory effects cost
Frit
Vee
us l **
l
*
l **
l **
L
L
l*
l*
***
l*
**
***
l
l **
l
l
l
l **
l
l*
l
l **
l **
***
l **
l*
***
l
l **
***
l
l **
l*
**
l **
l **
l **
l
***
1min- 1. They have confirmed that available pneumatic nebulizers of either the concentric or cross flow type provide their best results above 0.5 1 min - ‘. Below this value, both frit and Vee nebulizers provide better results. Both of the latter systems need improvements. Vee nebulizers must have a reduced uptake rate which can be obtained by decreasing the size of the channel through which the sample flows and by having several holes instead of one so as to cover most of the channel. Frit nebulizers seem to be very attractive because of the simplicity of their construction. It could be thought that memory effects are the main drawback of this nebulizer but we have seen that they can be minimized by a cleaning of the fi-it material which is easy to carry out between samples. On the other hand, using this last nebulizer for routine analysis, we have found that the reproducibility may change considerably each time the nebulizer is wet again. The response really depends on the way the material is initially wet. Moreover, after a few days, a deterioration of performance was observed resulting from a gradual clogging of the frit material even when using prefiltered solutions. Moreover, instead of using glass material, some more inert materials would be preferable for this purpose. However, their availability with high porosity may be problematic. The ultrasonic nebulizer, in terms of figures of merit is presently the best system but it is the most difficult to operate on a routine basis at its present stage of development. The development of such a nebulizer really usable for routine analysis would require a large engineering effort, which explains why this system is not readily available. 0.8
Comparison of nebulizers
61
If we compare analytical figures of merit and physical properties of the various nebulizers (with corresponding spray chambers) as summarized in Table 6, we can conclude that, presently, the best compromise for work with low power, low flow rate torches would be the Vee nebulizer, especially if improvements can be obtained in reducing the solution consumption.
SA(B) 41:1/2-E
*I mc&mica Acre,Vol. 41B,Nos l/Z. pi. 4961, 19%. printed in Great Britain.
0 1986.Pcrgamon RcssLtd.
Comparison of nebulizers working below 0.8 1 min- ’ in inductively coupled plasma atomic emission spectrometry E. MICHAUD-POUSSEL and J. M. MERMET CNRS,BP 22,69390 Vemaison, France
Service Central d’Analyse du
(Received 2 January 1985; in revisedform 28 June 1985) Abstrret-Several nebulii of concentric,cross-flow, Vee, frit and ultrasonic types have been compared in terms of signal-to-noise ratio (for both analyte and background signal), uptake rate, efficiencyand memory effects. Especially, the dependence of these figures of merit at gas flow rates below 0.8 I min - ’ is investigated so as to allow an evaluation of these.nebuliiers for use together with low consumption torches in ICP-AES. This study was done mainly with a 56MHz ICP, but additional measurements for a 27-MHz indicate that the results are applicable to a 27-MHz system.
1. INTR~OUC~I~N THERE has been a constant trend to design lower power and lower flow rate torches in inductively coupled plasma atomic emission spectrometry (ICP-AES) in order to use low power (< 1 kW) and reduced size generators at 27 MHz [l-13] or higher frequencies [14-183. This decreases both the capital investment and the running cost. With the aid of conventional but optimized torches, it is presently possible to work below 10 1min-’ and 1 kW [3,5, 7, 133, and a toroidal shaped plasma can be sustained down to 4-5 1min- ‘. Further, a ball shaped plasma associated with external (air or water) cooling, can be operated down to plasma gas flow rates as low as 11 min - ’ [16,17]. Our work centres on toroidal shaped plasmas which presently are preferred for routine analysis, since they provide good analytical results and ease of operation. However, with such torches, nebulizers with low aerosol gas flows must be used. With a torch running between 5 and 8 1min- ‘, the speed of the carrier gas and the carrier gas flow rate as well, can be reduced so as to increase the residence time compared to 12-16 1min- ’ torches. Consequently, with injectors having an inner diameter between 1 and 2 mm, the carrier gas flow rate must be below 1 1min-‘. It is known that pneumatic nebulizers are difficult to run below this value, because of the reduced diameters and accurate adjustment of the capillaries required, but also because of the stronger dependence of the performance on the physical properties of the solution (especially the viscosity) and the possibility of blocking at low aerosol carrier gas flow rates. Several pneumatic nebulizer designs have been proposed either to increase the efficiency at
[l] C. D. ALLEMANDand R. M. BARNES, Appl. Spectrosc. 31,434 (1977). [2] C. D. ALLEMAND, R. M. BARNESand C. D. WOHLERS, Anal. Chem. 51,2392 (1979). [3] R. N. SAVAGEand G. M. HIEFTJE,Anal. Chem. 51,408 (1979). [4] H. KAWAGUCHI, T. ITO, S. RUBIand A. MIZUIKE,Anal. Chem. 52,244O (1980). [5] A. D. WEISS,R. N. SAVAGE and G. M. HIEFTJE,Anal. Chim. Acta 124,245 (1981). [6] M. D. LOWE, Appl. Spectrosc. 35, 126 (1981). [7] R. REZAAIYAN, G. M. HIEFTJE, H. ANDERSON, H. KAISERand B. MEDDINGS, Appl. Spectrosc. 36,627 (1982). [S] H. KAISER and B. MEDDINGS,presented at the 21th Eastern Analytical Symposium, New York (1982). [9] H. KAWAGUCHI, T. TANAKA,S. MIUM, J. Xu and A. MIZUIKE,Spectrochim. Acta 38B, 1319 (1983). [lo] G. M. HIEFTJE, Spectrochim.Acta 38B, 1465 (1983). [11] G. M. ALLENand D. M. COLEMAN, Anal. Chim. Acta 158,267 (1984). [ 123 A. MONTASER,G. R. HUSE, R. A. WAX, S. K. CHAN, D. W. GOLIGHTLY, J. S. KANEand A. F. DORRZAPF,Anal. Cbem. !56,283 (1984). [13] G. ANGLEYS and J. M. MERMET, Appl. Spectrosc. 38,647 (1984). [14] J. M. MERMET and C. TRA~SY,Appl. Spectrosc. 31,237 (1977). [ 153 R. REZMIYAN, J. W. OLESIKand G. M. HIEFCIE, 35th Pittsburgh Conference, Atlantic City (1984), paper 905. [16] G. R. KORNBLUM, W. VANDERWAAand L. DE GALAN, Anal. Chem. 51,2378 (1979). [17] P. A. M. R~pso~ and L. DE GALAN, Spectrochim. Acta 38B, 707 (1983).
50
E. MICHAUDPOUSSELand J. M. MERMET
low flow rate or to avoid clogging, namely the Vee nebulizer [20-291 whose design is a modification of the one proposed by BABINGTON [30], and the frit nebulizer [31-331. Besides these nebulizers which are still using a gas flow to produce the aerosol and are therefore gas flow rate dependent, the ultrasonic nebulizer has been proposed for a long time C34-473. Recently, jet impact nebulizers have been described [48] and are similarly independent of the gas flow rate for the formation of aerosol. At the moment, only pneumatic (cross-flow or concentric) and Vee nebulizers are readily commercially available. With conventional ICP systems comparisons between pneumatic (concentric, cross-flow and Vee) nebulizers have been published [49, 503. Subsequent to the optimization of low power, low consumption torches, realized with a 27MHz Plasma-Therm unit [ 131, the present work describes a comparison of various nebulizers either commercially available (Meinhard concentric and Perkin-Elmer cross-flow nebulizers) or built in the laboratory (Vee, frit and ultrasonic nebulizers). Several figures of merit have been used for the comparison, namely the signal-to-noise (of the signal) ratio SNsR and the signal-to-noise (of the background) ratio SN,R. The RMS value of the first ratio is known to be the reciprocal of the relative standard deviation RSD of the net signal. The second ratio is the reciprocal of the RSD of background value and is linked to detection limit cL. Further, we have also considered efficiency and memory effects of the various nebulizers and applied these results to some detection limits. 2.
EXPERIMENTAL
For this study, a 56 MHz Durr generator but also a 64 MHz Durr generator have been used. The ICP stands of the systems used allowed easy interchange of the different nebulizers as large space below the
[18] [19] [ZO] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [SO]
R. M. BARNESand R. G. SCHLEICHER,Spectrochim. Acta 3OB, 109 (1975). P. A. M. RIPSON, L. DE GALANand J. W. DE RUITER, Spectrochim. Acta 37B, 733 (1982). R. F. SUDDEND~RFand K. W. BOYER, Anul. Chem. SO, 1769 (1978). J. F. WOLCO~ and C. BUTLER-S• BEL,Appl. Spectrosc. 32,591 (1978). J. R. GARBARINOand H. E. TAYLOR, Appl. Spectrosc. 34, 584 (1980). B. THELIN, Analyst 106,54 (1981). P. J. MCKINNON,K. C. GtEssand T. V. KNIGHT,Deu. Atom. Plasma Spectrochem. Analysis, Ed. R. M. BARNES, p. 287. Heyden, London (1981). P. A. M. RIPSONand L. DE GALAN,Spectrochim. Acta 36B, 71 (1981). L. EBXIN and M. R. CAVE, Anulyst 107, 172 (1982). D. R. HEINE, M. B. DENTONand T. D. SCHLABACH,Anal. Chem. 54,81 (1982). J. F. WOLCO~ and C. BUTLER-NOBEL,Appl. Spectrosc. 36,685 (1982). M. D. WICHMAN,R. C. FRY and N. MOHAMED,Appl. Spectrosc. 37,254 (1983). R. S. BABINGTON,Popular Science (May issue), 102 (1973). C. T. APEL, T. M. BIENIEWSKI,L. E. Cox and D. W. STEINHAUS,ICP Informution Newslett. 3, 1 (1977). L. R. LAYMANand F. E. LICHTE, Anal. Chem. 54,638 (1982). L. M. FAIRES,B. A. PALMER and J. W. BRAULT,Spectrochim. Acta 4OB, 135 (1985). R. H. WENDTand V. A. FASSEL,Anal. Chem. 37,920 (1965). H. DUNKENand G. PFORR,Z. Phys. Chem. 230,48 (1965). H. C. HOAREand R. A. MOSTYN,Anal. Chem 39, 1153 (1967). J. M. MERMETand J. ROBIN,Anal. Chem. 40, 1918 (1968). G. W. DICKINSONand V. A. FASSEL,Ad. Chem. 41, 1021 (1969). P. W. J. M. BOUMANSand F. J. DE BOER,Spectruchim.Acta 278,391 (1972). J. C. SOUILLIART and J. P. ROBIN,Anulusis 1, 427 (1972). P. W. J. M. B~UMANSand F. J. DE BOER,Spectrochim. Acta 3OB, 449 (1975). K. W. OLSON,W. J. HAASand V. A. FASSEL,Anal. Gem. 49,632 (1977). H. UCHIDA,H. MATNJI,T. UCHIDAand C. IIDA,Spectrosc. ht. 11, 1 (1978). R. DIEMIASZONEK, J. L. MOUTONand C. TRASSY,Analusis 7,96 (1979). R. E. STURGEON, S. S. BERMAN, J. A. H. DESAULNIERS, A. P. MYKYTIUK,J. W. MCLARENand D. S. RUSSELL, Anal. Ckm. 52, 1585 (1980). C. E. TAYLORand T. L. FLOYD,Appl. Spectrosc. 35,408 (1981). J. M. MERMET,C. TRASSYand P. RIPOCHE,Deuelopments in Atomic Plasma Spectrochmical Analysis, Ed. R. M. BARNES, p. 245. Heyden, London (1981). M. P. D~HERN and G. M. HIEFTJE, Appl. Spectrosc. 38,405 (1984). R. M. BARNES,H. S. MAHAN~, M. R. CAVE and L. FERNANDO, ICP Infirm&on Newslett. 8, 562 (1983). F. J. M. J. MAESSEN,P. COEVERT and J. BALKE,Anal. Gem. 56,899 (1984).
Comparison of nebulizers
51
torch is available. Besides these generators, a 27-MHz Plasma-Therm unit has been used to compare the results obtained at higher frequencies and to provide detection limits on a commercial system (Perkin-Elmer ICP 5500 system). Torches have been studied that enable working at low flow rates, mainly by reducing the annular space between the outer and the intermediate tubes [l, 7,8,10,13,14]. In the case of the 27-MHz ICP, a space of 0.5 mm was selected, providing a minimum plasma gas flow rate of 4 1min- ’ and a working plasma gas flow rate of 6.5 1min- ‘. Using a higher frequency (56 or 64 MHz), it is not necessary to use reduced size torches to obtain low flow rates [Sl]. With an o.d. of the intermediate tube of 24 mm and
i.d. of the outer tube of 26.5 mm, corresponding to an annular space of 1.25 mm, it is possible to work with a flow rate of 10 1 min-‘. Torch dimensions and operating parameters are summarized in Table 1. The ICP Durr generators are set up with a Jobin-Yvon l-m monochromator with a 3600 line/mm interferometric grating 1521. With the 27-MHz Plasma-Therm ICP, the optical system and readout of the Perk&Elmer ICP 5500 were used. In each case, mass flow controllers (Brooks 5810) were utilized for the carrier and sheathing gas flow rates. The Meinhard, cross-flow and frit nebulizers were fed at a pressure of 3 bars. No attempt was made to optimize the spray chamber for each nebulizer. A Scott spray chamber [53] was used for the experiments. The observation height in the case of the 56 and 64 MHz was selected just above the top of the coil. With the 27 MHz system, the observation height was 10 mm above the coil. A test solution containing manganese at a concentration of 100 ng ml- 1was used and the Mn II 257.6 nm line was selected for the experiments in this study.
3. RESULTS 3.1. Meinhard and cross-jlow nebulizers Two concentric type A Meinhard nebulizers (#292 and 1442) and a Perkin-Elmer cross flow nebulizer were used. Meinhard nebulizers can be operated with or without a force feeding peristaltic pump. The influence of the pump has been studied at rates between 0.25 and 3 ml min - I. A maximum increase of the signal (30 “/,)was found near 0.75-1.0 ml min- ’ for carrier gas flow rates between 0.6 and 0.7 1min- ’ (Fig. 1). Below 0.6 1 min- ‘, the influence is relatively small. Therefore, no peristaltic pump was used with the Meinhard nebulizers. For the PE nebulizer, however, a pump is used. As with a rate of 0.75 ml min- ‘, the PE nebulizer (analyte) signal intensity was similar to that observed with Meinhard nebulizers without a pump (Fig. 2), a 0.75 ml min- ’ pump rate was used in further experiments. It is to be noted that the lowest carrier gas flow rate which can be used for any of these three nebulizers is 0.3-0.35 lmin- ‘. 3.2. Vee nebulizers Further a Vee nebulizer is investigated. Similar to other Vee-type nebulizers, it has been developed [25] in order to reduce. the uptake rate compared to Babington design. The gas capillary tube may have an i.d. as small as 0.1 mm. This design has been suggested [25] to be
Table 1. Operating parameters of the two ICP systems used in this work Generator Frequency Power Torch: external tube internal tube Plasma gas Carrier gas Sheathing gas
Plasma-Therm 27 MHz 600W
Durr 56 MHz l@OOW
19-16 mm 30-26.5 mm ls-12mm 24-21 mm 6.5lmin-’ 101min-1 0-0.8lmin-* O-0.81min-L
[Sl] E POUSEL and J. M. MERMET,Spectrochim. Acta 41B (1986). [SZ] J. W. MCLARENand J. M. MERMFT,Spectrochim. Actu 398, 1307 (1984). [S3] R. H. SCOIT, V. A. FASSEL,R. N. KNISELEY and D. E. NIXON, Anal. Gem. 46,75 (1974).
E. MICHAUDPOU~~EL and J. M. MERMET
52
1
2
3 QL mL/min
Fig. 1. Variation of the signal obtained with a concentric Meinhard nebulizer as a function of the peristaltic pump rate QL, (Mn II 257.61 mu, 100 ng ml-‘).
150 -
100 -
50 -
I 04
06
I F,
L/min
Fig. 2. Variation of the signal obtained with two Meinhard nebulizers (M~and MO) and a Perk&Elmer cross-flow nebulizer (PEB) as a function of the carrier gas flow rate F, (Mn 100 ng ml-‘). Only the PE nebulizer is fed with a peristaltic pump (0.75 ml min- ‘).
used with very low carrier gas flow rates. The dimensions of the Vee nebulizer constructed are shown in Fig. 3. Several materials have been used in the construction. It was possible to machine in glass ceramic (Macor from Corning) a gas inlet of 0.3 mm. For smaller diameters, quartz capillary tubes (i.d. 0.07 mm) were inserted into a PTFE base. With an i.d. of 0.3 mm, the working range of the nebulizer is similar to that of concentric or cross-flow nebulizers (minimum gas flow rate of 0.35 1min- ‘). In the case of an i.d. of 0.07 mm, the working range (with a pressure of 3 bars) is between 0.1 and 0.5 1min -I. The latter type has been used in this study. Since there is no Venturi effect, it is necessary to use a peristaltic pump. As the ratio between the width of the rectangular channel through which the sample flows (1 mm) and the i.d. of the gas inlet (0.07 mm) is rather large, the optimal uptake rate may be expected to be rather high. As may be seen from Fig. 4, it is necessary to use a pump rate of at least 2.4 ml min - 1 to be relatively independent of the rate. However, then, the increase of signal is
53
Comparison of nebulizers Solution
-
Ar 07Opm
Fig. 3. Vee nebulizer design.
I
02
04
FA
L/min
Fig. 4. Variation of the signal obtained with the Vee nebulizer (V) with different peristaltic pump rates, as a function of the carriergas, and comparison with a Perk&Elmer cross-flow nebulizer (PE) also fed with a peristaltic pump (0.75 ml min- I).
quite large as compared with the PE cross-flow nebulizer. The ratio is 6 at 0.5 1min- ’ and 9 at 0.4lmin-‘. 3.3. Frit nebulizers Frit disc nebulizers up to now have hardly been investigated [31-331. They may be used in two ways. Either the solution is fed onto the surface of the frit material along with the gas (Fig. 5) and goes through it or the solution is fed onto the opposite side (Fig. 6). A disc with pore sizes of 5-10 pm has been found to provide the best results. A compromise diameter of 18 mm was selected to achieve good aerosol production at an acceptably low gas flow rate. The first design, called Fl, includes a spray chamber and the aerosol outlet tube extends into the spray chamber to avoid the formation of any droplets from the spray chamber wall at the outlet. Since the signal intensity is relatively independent of the pump rate in the range 0.03-0.3 ml min - I, a value of 0.03 ml min - a has been adopted. The flow rate thus is much lower than that used with other nebulizers. The best signal stability is obtained when the exit of the capillary tube is set at 2 mm above the surface of the frit disc. Except for the way the solution is fed onto the frit in the second design, called F2, its behavior in terms of signal, efficiency and pumping rate is similar to Fl, as shown in Fig. 7. Fla and Flb refer to two devices which are in theory identical. 3.4. Ultrasonic nebulizer Several designs of ultrasonic nebulizers have been proposed for ICP-AES C34-471. Either the solution is fed directly onto one side of a transducer which is water cooled on the other
54
E. MICHALIDPOUSSEL and J. M. MERMET Solution V
a Ar
Fig. 5. Frit nebulizer (Fl) design.
Solution V
Fig. 6. Frit nebulizer (F2) design.
side or a liquid transmitting bath is used for ultrasonic waves. In the first case, a continuous feed is provided, and in the second, a given amount of solution (typically 2-5 ml) is brought into the nebulization cell. In the latter case, the solution to be nebulixed is separated from the liquid bath by a thin membrane. We preferred to use this latter system because of a better coupling between the transducer and the solution and developed a type as described earlier [37,47]. A minimum volume of 3 ml is required in the cell, although only a small part is used in aerosol production. Even at low flow rates ( < 0.5 1 min- I), the amount of aerosol taken up by the gas can be so large that the discharge extinguishes. Therefore, a gas flow rate of 0.1 1min- ’ was selected and an external gas was added to have enough gas velocity to penetrate the plasma. If this gas is added to the carrier gas at the exit of the cell and if the flow of both gases is laminar, the
Comparison of nebulizers
55
s
o-
o-
02
0.4
0.6
F~
L/min
Fig. 7. Variation of the signal (Mn 100 ng ml -I) with the two different designs of the frit nebulizer (Fl and F2) as a function of the carrier gas. Fla and Flb are two different buildings using the same design Fl.
added gas acts as sheathing gas preventing the diffusion of aerosol towards the inner wall of the injector [47]. Accordingly, memory effects and partial clogging in the injector are minimized, with a better constriction of the sample at the base of the discharge. The influence of the sheathing gas flow rate has been described [54]. In contrast to the other nebulizers, the carrier gas flow rate is kept constant (0.1 1min- ‘) and the variation of the total gas flow rate was obtained by adding sheathing gas flow. The signal decreases with the flow rate since the residence time is shorter for the same amount of solution. That is why the signal is very large at low flow rates even as compared to the Vee nebulizer (Fig. 8). The ratio is only 1.5 at 0.5 1min-’ but 6 at 0.4 1min-’ and 20 at
‘S
4
3
2
1
0.2
04
0.6
F,
L/min
Fig. 8. Variation of the signal (Mn 100 ng ml-‘) obtained with the ultrasonic nebulizer (US) as a function of the total carriergas. The aerosol is uptaken with a constant flow rate of 0.1 1min- t and the total gas flow rate is adjusted with the sheathing gas flow rate. Results are compared with the Vet nebulii (V) fed with a peristaltic pump rate of 2.4 ml mitt-‘. [54]
E. MKXAUD, Thesis, University of Lyon (1983).
56
E. MICHAUDPOUSSEL and J. M. MERMET
0.3 lmin- ‘. Compared to pneumatic nebulizers (Fig. 4), the ratio is more than 50 at 0.4 1min- ‘. The aerosol production of such nebulizer is very large below 0.5 1min- ’ whereas other nebulizers can only be used with a much lower efficiency. The variation of the signal as a function of the flow rate, for various nebulizers is given on the same scale in Fig. 9. It is to be noted that below 0.55 1mim- ‘, the signal for the ultrasonic nebulizer is out of scale. 3.5. Study of the SNsR The noise N, of the signal was determined as the standard deviation of the signal from a chart recorder. Obviously, this value depends on the response time (0.6 s) of the readout system which was kept constant during all experiments. With a concentration of 100 ng ml- i, the noise is mainly provided by the nebulization and atomization processes and is of the flicker noise type. The best values are obtained with the ultrasonic nebulizer (Fig. 10).
6x10’-
5
-
1
02
04
0.6
Fh.
L/min
Fig 9. Summary of the variations of the signal (Mn 100 ng ml-‘) obtained with different nebulizers (frit nebuhzers Fl and F2, Meinhard nebulizer M, Perkin-Elmer cross-Row nebulizer PE, Vee nebulizer V and ultrasonic nebuhzer US) as a function of the carrier gas flow rate.
SN,R
80-
0.2
0.3
04
0.5
0.6
FA
L/mln
Fig. 10. Comparison of the different values of the signal-to-noise of the signal ratio, SNsR, as a function of the carrier gas for different nebulizers as described above.
Comparison of nebulizers
57
This is due to the high value of the signal rather than the low value of the noise. The Meinhard, the cross-flow and the Vee nebulizers provide similar values. However with the Vee-nebulizer, these values can be obtained below 0.5 1min- I. Both frit nebulizers provide similar SNsR at a low gas flow and show a rather small variation with flow rate. When using a peristaltic pump with Meinhard nebulizers, the SNsR, especially near 0.5 1min- ‘, are almost independent of the pumping rate between 0.25 and 3 ml min- ‘. 3.6. Study of the SN,R The variation of the SN,R (Fig. 11) is very similar to the variation of the signal (Fig. 9). Highest values are obtained for the ultrasonic nebulizer followed by the Vee nebulizer. Between the frit nebulizers and the Meinhard and cross-flow nebulizers, there is a crossing point near 0.5 1mitt-‘. Accordingly, above this value, these both pneumatic nebulizers will provide better detection limits than frit nebuhzers, with the reverse behavior below 0.5 1min- ‘. This is confirmed at different carrier gas flow rates the detection limit values for Pt (Table 2). Above 0.5 1 min- I, the Meinhard nebulizer provides better detection limits than the frit nebulizer and near 0.5 1min- ‘, the detection limits are the same in both cases. At 0.35 1 min- ‘, the frit nebulizer still has an acceptable SN,R and therefore the detection limits are almost independent of the flow rate for this nebulizer. Table 2 conRrms also that the ultrasonic nebulizer provides the best detection limits. The Vee nebulizer is also shown to give lower detection limits than the Meinhard nebulizer even at a flow rate as low as 0.35 1min- l. 3.7. Study of the uptake rate ana’ of the eficiency
The uptake rate is easy to determine, not however the amount of aerosol produced. The validity of several of the methods proposed [25, 26, 55, 561 has been the subject of some SN,R
0.2
0.4
0.6
FA
L/mln
Fig. 11. Comparison of the different values of the signal-to-noise of the background ratio, SN,R, as a function of the carrier gas for different nebulizers as described above. Table 2. Comparison of the detection limits obtained with Pt II 214.42 (ng ml-‘) using different nebulizers as described in the text and for different carrier gas flow rates (I min-‘)
[SS]
Carrier gas
Meinhard
Frit
Vee
us
0.35 0.47s 0.60
560 90 70
100 90 110
SO 2s -
2 S 10
P. SCHUTYSERand E. JANSSENS,Spectrochim. Acta 34B, 443 (1979). [S6] D. D. SMITH and R. F. BROWNER,Ad. Chem. 54,533 (1982).
E. MICHAUD-POUSSEL and J. M. MERMET
58
controversy [57,58]. In this work, we compared the amount of aerosol in the case of different nebulizers by measuring the absorption of water by silicagel at the exit of the nebulization system. Results are summarized in Table 3. The efficiencies are usually below 3 % for the Meinhard and cross-flow nebulizers. Also for the Vee nebulizer, the efficiency is very low. This fact can be explained by the large ratio between the size of the central channel and the i.d. of the gas inlet. On the other hand, the efficiency is very high for frit nebulixers of whatever the type. A value of 20 % can be easily achieved. This enables a very low consumption rate (at least a factor 10 lower than for the other nebulizers). If a frit nebulizer with uptake rate similar to a Meinhard nebulizer, i.e. about 0.3 ml min-’ at 0.35 1 mm-‘, and with the same 20% efficiency could be designed, the amount of aerosol produced would be greater than that obtained with the ultrasonic nebulizer. For this last system, it is not possible to speak of efficiency since the totality of the aerosol produced reaches the plasma. With a carrier gas of 0.1 1min- ‘, the amount of aerosol is 0.03 g mm-i. The amounts of aerosol produced by the various systems are shown in Fig. 12. It is appreciated [59,60] that the aerosol concentration, i.e. the concentration of liquid per gas volume, is an important parameter. The highest aerosol concentration is obtained with the ultrasonic nebulizer and then with the Vee nebulizer. Although the signal is not related directly to the amount of aerosol, since a spray chamber is used to select the appropriate droplet diameter and since various nebulizers can produce different diameter distributions, by comparison with the results in Table 4, it may be seen that signal and detection limits are closely connected to this amount of aerosol produced and the aerosol concentration. Note that the flicker noise is directly related to the droplet size distribution [61]. 3.8. Memory effects We have carried out some experiments to check the importance of memory effects for each nebulizer including the associated spray chamber. Manganese was still used as the test element at a concentration of 100 ng ml-‘. For each nebulizer, the gas flow rate giving the
Table 3. Comparison of the uptake rate Q,,the amount of aerosol produced Qs the efficiency ( “/,)and the aerosol concentration (@ l- t) using different nebnlixers (of the frit, cross-flow, concentric and Vee types as described in the text) for different carrier gas Bow rates Carrier gas (1 min-‘)
Q. g&--l 0.15
:t:‘?-’ . 00 aer.
cont. (pl I-‘)
0.35
$ 2 aerosol
0.475
: 2 aerosol
0.60
: 2 aerosol
Frit Fl
Frit F2
0.003 0.035 8 20 0.0075 0.035 21 21 0.0095 0.035 27 20 0.009 0.035 26 15
0.003 0.035 8 20 o.ao7 0.035 20 20 0.009 0.035 26 19 0.010 0.035 29 17
Cross-flow PE
0.0055 0.75 0.7 16 0.010 0.75 1.3 21 0.014 0.75 2 23
[57’j P. A. M. RIBON and L. DE GALAN,And. Ckm. 55,372 (1983). [58] R. F. BROWNER and D. D. SMITH,Anal. Chem. 55,373 (1983). [59] A. GUSTAVSON, Anal. Gem. 55,94 (1983). [60] A. GUSTAVWIN,Anal. CIteat.S6,815 (1984). [61] S. D. OMENand A. STRASHEIM, Spectrochim.Acta 38B, 973 (1983).
Meinhard 1442 292
0.0045 0.23 2 13 0.010 0.51 2 21 0.014 0.68 2. 23
0.006 0.27 2 17 0.010 0.45 2 21 0.0145 0.62 2.3 24
Vee 0.0035 2.4 0.15 23 0.010 2.4 0.4 29 0.020 2.4 0.8 42
Comparison of nebulizers
59
Qa g/min
0.03-
_
'3
I
I
us
,
I
I
0.2
0.4
FA
L/min
Fig 12. Comparison of the amount of aerosol Q. obtained using various nebulizers as described above, as a function of the carriergas flow rate. With the ultrasonic nebulizcr (US), the uptake rate is kept constant using a gas flow rate of 0.1 I min-‘, the total gas llow rate is obtained by adding a sheathing gas.
maximum value of the signal was selected (0.55 1min - 1 for Meinhard and PE nebulizers, 0.5 for the Vee nebulizer and 0.45 for the frit nebulizer). It was found that, with the Meinhard nebulizer, the signal decreased to 1% of its original value after 60 s. With the PE cross-flow nebulizer 30 s and with the Vee nebulizer 45 s were required. With both frit nebulizers which show the same behavior, the 1% level is reached only after 25 min. When, a few ml of water are used to clean the frit material, i.e. by using the nebulizer no longer as a nebulizer but as a filter, memory effects become similar to those for Meinhard nebulizers. This may easily be done by using a higher flow from the peristaltic pump. 3.9. Application to the 27-MHz ICP system set up with a reduced size torch In a previous paper [ 131, we have presented the design of a low power (600 W) low flow rate torch (6 1min- i) torch with an annular space of 0.5 mm. Results were presented using a Meinhard nebulizer and an ultrasonic nebulizer. Several experiments have confirmed that the results described above are also applicable to a 27-MHz system. With gas flow rates as reported in Table 4, detection limits (based on a 2a concept) for several elements have been determined (Table 5) without any attempt to optimize parameters such as time constant. In the same table, detection limits are compared with the ones obtained with a conventional torch (12 1 min-i) and a Meinhard nebulizer (0.6 1mm-‘). Once again the ultrasonic nebulizer provides the best results although the difference is not so large with the Meinhard nebulizer because of the limited amount of water which could be injected into the discharge. 4. CONCLUSIONS Studies of the signal, signal-to-signal noise ratio and signal-to-background noise ratio have provided information for the selection of nebulizers when working at a carrier gas flow below Table 4. Car&r and sheathing gas flow rates used with Merent nebulizers and with the 27-MHz plasma-Therm unit Nebulizer Carriergas(Imin-1) Sheathing gas
Meinhard 0.6 -
Frit Fl
Vee
US
0.2 0.25
0.4 -
0.1 0.3
60
E. MICHAUDPOUSSBL and J. M. MERME~ Table 5. Comparison of the detection limits (ng ml-‘) obtained with the 27-MHz ICP system with a conventional torch and with a low power (6OOW), low consumption rate (6.5 I mitt-i) torch using different nebulixers described in the text. With the conventional torch, a Vee nebulixerwith a diameter of 300 jun for the gas inlet was used in contrast with the Vee nebulixer (70 m) used with the low consumption torch Conventional torch Low power, low consumption torch Meinhard Vee-300 Meinhard Vee-70 Frit US As Mn P Pb Pt Sb Sn Sn V
I II I II II I I II II
197.20 257.61 213.62 220.35 214.42 206.83 283.99 189.99 292.40
70 1 50 40 30 30 70 40 5
60
350 5 340 240 140 100
250 4 400 300 100 100 60
20
100 1 100 80 70
40
40 150 8
30
Table 6. Summary of the figures of merits of various nebulixers used in this work with the main requirement to work at low carrier gas gow rates (< 0.8 1 mitt-t). One star means that the nebulixer is acceptable for the figure which is considered, two stars mean that the nebulixer is good and three that it is excellent Meinhard Cross-flow Low flow rate Detection limit Uptake rate El&iency Clogging Easeofuse. Memory effects cost
Frit
Vee
us l **
l
*
l **
l **
L
L
l*
l*
***
l*
**
***
l
l **
l
l
l
l **
l
l*
l
l **
l **
***
l **
l*
***
l
l **
***
l
l **
l*
**
l **
l **
l **
l
***
1min- 1. They have confirmed that available pneumatic nebulizers of either the concentric or cross flow type provide their best results above 0.5 1 min - ‘. Below this value, both frit and Vee nebulizers provide better results. Both of the latter systems need improvements. Vee nebulizers must have a reduced uptake rate which can be obtained by decreasing the size of the channel through which the sample flows and by having several holes instead of one so as to cover most of the channel. Frit nebulizers seem to be very attractive because of the simplicity of their construction. It could be thought that memory effects are the main drawback of this nebulizer but we have seen that they can be minimized by a cleaning of the fi-it material which is easy to carry out between samples. On the other hand, using this last nebulizer for routine analysis, we have found that the reproducibility may change considerably each time the nebulizer is wet again. The response really depends on the way the material is initially wet. Moreover, after a few days, a deterioration of performance was observed resulting from a gradual clogging of the frit material even when using prefiltered solutions. Moreover, instead of using glass material, some more inert materials would be preferable for this purpose. However, their availability with high porosity may be problematic. The ultrasonic nebulizer, in terms of figures of merit is presently the best system but it is the most difficult to operate on a routine basis at its present stage of development. The development of such a nebulizer really usable for routine analysis would require a large engineering effort, which explains why this system is not readily available. 0.8
Comparison of nebulizers
61
If we compare analytical figures of merit and physical properties of the various nebulizers (with corresponding spray chambers) as summarized in Table 6, we can conclude that, presently, the best compromise for work with low power, low flow rate torches would be the Vee nebulizer, especially if improvements can be obtained in reducing the solution consumption.
SA(B) 41:1/2-E