Spectroehimica Acra. Vol. 380. No. S/6. pp. 913-975. 1983.

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Pwymon F’resr Ltd.

RESEARCH NOTE

Correlation of the analytical signal to the characterized nebulizer spray

(Received 4 January 1983)

Abstract-The droplet-size distribution of the aerosol produced by different ICP nebulizers are determined by employing a Mie scattering technique. A definite relationship exists between the signal magnitude and the total number of particles in the small fraction. A narrow districution corresponds to a high signal-noise ratio.

1. INTRODUCTION

THE IMPORTANCE of knowing the droplet size distributions produced by pneumatic nebulizers used in atomic spectroscopy has been recognized by a number of workers [l-lo]. NOVAK and BROWNER [8] mention that the absolute emission signal produced increased with a decrease in the median droplet diameter of the spray. A more detailed investigation of the relationship between the analytical signal and the measured droplet size distributions is presented in this research note.

2. EXPERIMENTAL

The ICP nebulizers tested were a concentric glass nebulizer made by the CSIR, a Jarrell-Ash fixed cross-flow nebulizer, a Babington high solids nebulizer and a MAK high pressure fixed cross-flow nebulizer. A Scott double walled chamber [l l] and the same ICP torch (Plasma-Therm) were used throughout so that the path traversed by the aerosol was identical for all the nebulizers.

Nebulizer gas flow rates could not be varied much because of the narrow nebulizer gas orifices, but

the supply or back pressure was varied. Optimum signal-background ratios were obtained for nebulizer gas flow rates of 0.7 I/min through the concentric glass nebulizer and 0.5 I/min through the Jarrell-Ash Babington (4 ml/min solution feed rate) and cross-flow nebulizers. Back pressures to the nebulizers were 200,370 and 420 kPa, respectively. The MAK nebulizer was operated at a flow rate of 0.5 I/min and 1400 kPa supply pressure.

A Forward Scattering Spectrometer Probe (FSSP-100, Particle Measuring Systems, Boulder, Colorado) which employes Mie scattering, was used to characterize the nebulizer sprays. The FSSP detects light pulses produced by scattering from individual aerosol droplets as they pass through a sample area of 3 x 0.02 mm, which is illuminated by a focused laser beam [12]. Each light pulse within the sample area is counted and sized by a 16 channel pulse height analyser. The Mie calibration curve of the probe was checked with glass beads of known size distribution (10-l 5 pm) prior to use. The aerosol stream 5 cm from the ICP torch tip was passed through the sampling area of the beam. FSSP ranges of 2-47 pm and OS-8 pm were employed. All measurements were made in triplicate while aspirating distilled water.

A Jarrell-Ash 0.5 m monochromator (model 82-000) and a Plasma-Therm ICP-5000 operating at 1.5 kW were employed to observe the radiation 16 mm above the coil. The plasma gas flow rate was

[1] J. A. DEAN and W. J. CARNES, Analyr. Gem. 34, 192 (1962). [Z] J. B. WILLIS Speclrochim. Acta 23, 811 (1967). [3] J. STUPAR and J. B. DAWSON, Appl. Opt. 7, 1351 (1968). [4] K. SZIV& L. P&AS and E. PUNGOR, Specrrochim. Acta JIB, 289 (1976). [S] R. K. SKOGERB~E and K. W. OLSON, Appl. Spectrosc. 32, 181 (1978). [6] C. TH J. ALKEMADE and R. HERRMANN, in Fundamentals of Analytical Flame Spectroscopy. Chapter 4, John

Wiley, New York (1979). [7] J. W. NOVAK, JR. and R. F. BROWNER, Analyt. Chem. 52,287 (1980). [8] J. W. NOVAK, JR. and R. F. BROWNER, Analyr. Gem. 52, 792 (1980). [9] M. W. ROUTH, Appl. Spectrosc. 35, 170 (1981).

[lo] L. R. LAYMAN and F. E. LICHTL Analyr. Chem. 54, 638 (1982). [11] R. H. SCOIT, V. A. FASSE~ R. N. KNISELEY and D. E. NIXON, Analyf. Chem. 46,75 (1974). [ 121 R. G. KNOLLENBERG, Three New Instrumentsfor Cloud Physics Measurements. p. 554. Preprints International

Conference on Cloud Physics, 1976, Boulder, Colorado, U.S.A.

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974 Research Note

16 I/min. A Keithley Model 417 high speed pica-ammeter was used to monitor the signals. The signal variation over 5 min recorded on a strip chart recorder (JJ Instruments CR-600) was used to obtain the signal-noise ratios.

3. RESULTS AND DISCUSSION

The frequency of occurrence of various sized droplets is shown by the solid line histograms in Fig. 1. Droplet size distributiohs when employing the range 0.5 to 8 pm are shown to the left while those obtained using the range 2 to 47 brn are shown to the right in Fig. 1. The corresponding ~01s. calculated from the number ofdroplets measured are shown by the broken line histograms. It is evident from Fig. 1 that by far the greatest number of droplets have diameters smaller than 5 pm. With the exception of the high pressure MAK, only a very small fraction of the total vol. of the aerosol material is carried by these small droplets.

A photomicrograph of a gelatine coated slide, which was over-exposed to aerosol from the concentric glass nebulizer, is shown in Fig. 2. It can be seen that the frequency of occurrence of small droplets is much larger than the occasional large ones. The analytical importance of these occasional large droplets becomes apparent when it is considered that a single 50 pm diameter droplet has a vol. equal to 125 000 droplets with a 1 pm dia.

Table 1 shows the signal-background ratio (SBR) obtained when aspirating a 1 fig/ml solution of manganese while monitoring the Mn 257.6 nm line. A definite relationship exists between the signal magnitude and the total number and vol. of the particles in the small size fraction, but not for those in the larger one. This direct correlation between the OS-8 pm size fraction and the SBR indicates that this fraction largely determines the magnitude of the analytical signal.

The signal-noise ratios (SNR) obtained with the different nebulizers when aspirating a 25 pm/ml Mn solution is shown in Fig. 1. The narrow size distribution of the MAK nebulizer corresponds to a high SNR. Conversely, the wide distribution of the JAC nebulizer corresponds to a low SNR. This

8 SNR

6

46 b CGN :

h

Fig. 1. Particle size distributions of ICP pneumatic nebulizers tested. MAK, High pressure cross-Row nebulizer. CGN, Concentric glass nebulizer. JAB, Jarrell-Ash Babington nebulizer. JAC, Jarrell-Ash fixed cross-flow nebulizer. Solid line histograms show the number of particles measured by the FSSP during the 20 seccounting time. The vols. calculated from theaveragedia. ofa size binand the number

of particles are shown by the broken line histograms. SNR, Signal-noise ratios obtained.

Research Note

Fig. 2. Photomicrograph showing a portion of a gelatinc coated slide which was over exposed to aerosol from the concentric glass ncbulizer.

Table 1. Measured signal-background ratio (SBR) related to the total vol. and number of particles for two FSSP size ranges

Nebulixer SBR Total no. of particles x lo3 Total vol. in pm3 x lo6

OS-8 pm 2-47 pm 0.5-8 pm 2-47 pm

CGN 45 22 17 4.7 226 JAB 59 26 12 1.6 170 JAC 67 33 16 12.2 960

MAK 14 56 1 8.1 1.5

corresponds to the excellent signal-noise ratios obtained by HIEFTJE and MALMSTADT [13], when viewing the radiation from the small region in which single droplets of uniform size were atomized. It is evident that the instability or noise will be increased by supplying the plasma with an aerosol with a wide particle size distribution.

4. CONCLUSION

The emission signal (SBR) has been found to be correlated to the total vol. of droplets smaller than 8 pm which are produced by a nebuhzer. Flicker noise decreased considerably when a narrow particle size distribution was supplied to the plasma. Irregular supply ofdroplets, particularly those larger than 8 pm, results in a deterioration of the SNR.

National Physical Research Laboratory, CSIR, P.O. Box 395, Pretoria, 0001, South Africa

S. D. OLSEN and A. STRASHEIM

[13] G. M. HIEFTJE and H. V. MALMSTADT, Anulyf. C’km. 41. 1735 (1969).