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Anal. Chem. 1982, 5 4 , 638-642
Glass Frit Nebulizer for Atomic Spectrometry Lawrence R. Layman" and Frederick E. Llchte US. Department of the Interlor, U S . Oeologlcal Survey, Box 25046, MS 928, Denver Federal Center, Lakewood, Colorado 80225
The nebullzatlon of sample solutions Is a critical step In most flame or plasma atomlc spectrometrlc methods. A novel nebulization technlque, based on a porous glass frit, has been Investigated. Basic operating parameters and characteristics have been studied to determine how this new nebulizer may be applied to atomlc spectrometric methods. The results of prellmlnary comparlsons wlth pneumatlc nebuilzers lndlcate several notable dlflerences. The frit nebulizer produces a smaller droplet size distribution and has a hlgher sample transport efflclency. The mean droplet size Is approximately 0.1 pm, and up to 94% of the sample Is converted to usable aerosol. The most significant ilmltations in the performance of the frit nebulizer are the slow sample equlllbratlon time and the requirement for wash cycles between samples. Loss of solute by surface adsorption and contamination of samples by leaching from the glass were both found to be llmitatlons only in unusual cases. This nebulizer shows great promise where sample volume Is limited or where measurements require long nebullzatlon times.
Current nebulization systems can be a limiting factor in the performance of both flame and plasma atomic spectrometric methods. Popular pneumatic nebulizers, both cross-flow and concentric, suffer from short-term noise, long-term drift, and sample transport inefficiency. Erratic performance due to nebulizer clogging frequently results when aspirating samples which contain either a high dissolved salt content or suspended particulates (1). A wide range of droplet sizes is produced by these nebulizers. Because the larger droplets are lost in transport, this results in inefficient sample delivery (2). The process of droplet size discrimination within the spray chamber may also be related to several interelement interferences (3). Droplet size also appears to be related to desolvation and atomization processes in the atom cell ( 4 ) . Ultrasonic nebulizers produce a very narrow droplet size distribution and therefore exhibit much higher transport efficiency (5). However, the ultrasonic nebulizer is more complex and expensive than a pneumatic nebulizer. The recently introduced Babington nebulizer has been successful in reducing problems due to nebulizer clogging (1, 6). I t produces a droplet size distribution similar to other pneumatic nebulizers and sample transport efficiency is not improved. The limitations noted above have stimulated the search for better, more efficient nebulization systems. The use of a sintered glass frit as an analytical sample nebulizer has been investigated in this study. The use of a fritted glass disk as a nebulizer was introduced by Ape1 et al. (7, 8). The disk works on a principle similar to that of the Babington nebulizer. Nebulizing gas is passed through the multitude of small holes in the disk, and the sample solution is allowed to flow over the surface. This nebulizer has several features which make it attractive: it is simple to make and to operate, it is not prone to clogging with samples having high salt or particulate concentrations, it is highly efficient, up to 94% of sample is transported to the atom cell, and it generates an unusually fine sample mist. The unique properties of this nebulizer should be applicable to several analytical situations. The very high efficiency
coupled with a low sample consumption rate would be useful, for example, with multielement sequential atomic emission spectrometers which may demand a long aspiration time. Samples with limited sample volume available (Le., less than 500 pL) could also be analyzed. The fine mist generated also promises improvements in performance. Droplet size has been related to several interelement interferences ( 4 ) ,with smaller droplets generating fewer problems.
EXPERIMENTAL SECTION Apparatus. The sintered glass frits used are Corning fine, medium, and coarse porosity Pyrex brand glass frits, 10,20, and 30 mm in diameter. Samples are delivered to the frit by a Gilson Minipuls I1 varible speed peristaltic pump using Fisher brand standard manifold tubing. Wash solution is delivered by a Hamilton digital diluter, with a 10-mL Hamilton syringe, used as a syringe pump. The nebulizer gas flow is controlled by a Matheson Model 8240 mass flow controller. The laser scattering studies were made with a Jodon Engineering Associates Model SN 254 helium neon laser and a Jarrell Ash */2-mEbert monochromator and recorded on a strip chart recorder using a 0.3 s time constant. The studies using the inductively coupled plasma (ICP) were done on a Jarrell-Ash Model 1160 Plasma Atomcomp with a 63-channel direct reading polychromator, using a 6-8 integration time and dynamic background intensity correction. The Babington nebulizer used for comparison studies was described by Garbarino et al. (9). The atomic rlbsorption studies were done on a Perkin-Elmer Model 306 spectrometer with a Perkin-Elmer Model 0303-0299 concentric nebulizer with a 10-cm slot air-acetylene burner. Droplet size distribution measurements were made with an Anderson Cascade Impactor, operated following manufacturers instructions. Nebulizer Optimization. Samples were applied to the front (high pressure) or to the back (low pressure) sides of the frit. The arrangement of the frit, sample solution tube, wash tube, and drain is shown in Figure 1. The performance of the nebulizer was evaluated with laser scattering measurements, atomic emission measurements using the ICP, and atomic absorption measurements. Memory effects were measured using the ICP. A wash of the frit between samples was tested. The volume, composition, and placement of this wash solution were considered with various combinations used. For reduction of evaporation of solvent from the frit, a system to presaturate the carrier gas with water vapor was used. Laser Scattering Experiments. The intensity of light scattered at 90' from an incoming laser beam was measured as shown in Figure 2. The aerosol was forced out of the end of a 4 mm i.d. glass tube, and the 2 mm diameter laser beam was passed through the column of aerosol. At the laser wavelength of 632.8 nm, significant scattering at 90' is obtained only from particles which exhibit Raleigh scattering or large particle scattering. These phenomena occur only for particles in the 0.1-1 pm diameter range (10). Larger particles than 3/2X will exhibit only macroscopic reflection and refraction. These interactions cause primarily low angle deflection of the incident beam, and they do not produce a significant right angle dispersion of the incident light (11). Scattered intensity is proportional to the density of droplets (in this size range) present in the aerosol. Relative nebulization efficiencies were determined from scattered intensity and sample flow rate. Noise and drift in the nebulizers were measured. The effect of samples containing high dissolved salts or particulates on the nebulizers was determined. The effect of using water-saturated argon carrier gas was also observed. Spectrometric Experiments. The aerosol from this nebulizer was fed into an ICP system, as shown in Figure 3, and data were
This article not subJectto U S . Copyright. Published 1982 by the American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 4,APRIL 1982
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