JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 813 Transport Effects With Dribble and Jet Ultrasonic Nebulizers" Matthew A. Tarr Guangxuan Zhu? and Richard F. Browner$ School of Chemistry and Biochemistry Georgia Institute of Technology Atlanta GA 30332-0400 USA Ultrasonic nebulizers improve analyte transport compared with pneumatic types but require partial or total solvent removal. Aerosol desolvation using a heated chamber followed by a condenser not only reduces the solvent loading in the plasma but also increases the analyte transport relative to transport without desolvation. The influence on emission signals of the interaction of each component in the sample introduction system in terms of analyte and solvent transport losses are reported. Significant analyte losses in the spray chamber and condenser are observed and various approaches to improving analyte transport are presented.Two configurations of sample injection into the ultrasonic nebulizer the dribble and jet type are compared with the jet type showing greater repeatability and improved peak shape for flow injection measurements. Keywords Inductively coupled plasma atomic emission spectrometry; ultrasonic nebulizer; analyte transport; solvent transport The use of ultrasonic nebulizers (USNs) has improved detection capabilities of inductively coupled plasma atomic emission spectrometry (ICP-AES) and mass spectrometry (ICP-MS) primarily through increased analyte transport to the Although USNs have recently become more widely used in atomic spectrometry there are still many fundamental characteristics of these nebulizers that are poorly understood.The aerosol properties of a USN have recently been characterized and shown to produce a broad primary aerosol distribution with significant aerosol vol- ume contained in droplets >300 pm in a diameter.2 However despite these large droplets a significant en- hancement in small droplet generation compared with pneumatic nebulizers (PNs) was also achieved. Increased primary aerosol output at small droplet sizes is not the sole cause for improved detection limits with a USN. The use of a desolvation apparatus can improve detection power irrespective of the nebulizer type used. Zhu et aL3 and Jakubowski et aL4 both reported improved signal intensities and lower background interferences in ICP-MS using pneumatic nebulization with desolvation.With a USN a major increase in analyte transport compared with a PN results from the initial production of a primary aerosol with droplets generally smaller than those from the PN. Aerosol desolvation shifts the primary droplet distribution to smaller sizes3 by evaporation then further increases the analyte transport to the plasma relative to transport without desolvation. Montaser et aL5 have mea- sured detection limits in ICP-AES for a number ofelements comparing PNs used with and without desolvation with USNs used with desolvation. With PNs they report 2-10- fold improvements in limits of detection by using desolva- tion. When comparing normal pneumatic nebulization to ultrasonic nebulization with desolvation they report en- hancement of between 6- and 87-fold.When both systems are compared using desolvation the USN improves detec- tion limits over the PN by a factor of only between 3 and 8. Although the USN still provides significant enahancements these results indicate that the desolvation process itself plays an important role in improving detection limits. This work reports data on solvent and analyte transport the interaction of the heated chamber and condenser on analyte loss processes and the use of USN for flow injection (FI) analysis. *Presented at the 1992 Winter Conference on Plasma Spectro- ?On leave from the Dalian Institute of Chemical Physics Dalian *To whom correspondence should be addressed. chemistry San Diego CA USA January 6- 1 1 1992.People's Republic of China. Experimental A USN was fabricated in-house based on the design of Fassel and Bear.6 A Model CPMT transducer from Channel Products (Chesterland OH USA) was powered by a PlasmaTherm (Vorhees NJ USA) UN:PS- 1 generator operating nominally at 1.35 MHz. The generator was tuned to the resonant frequency of the transducer by maximizing incident power while minimizing reflected power. The power generator had an output range of 0-55 W. In addition a second commercially available USN (CETAC Technologies Omaha NE USA) was also used in these studies. Solvent was introduced to the nebulizers with a high- performance liquid chromatography (HPLC) pump (Con- stametric Model IIG). Samples were introduced via a Rheodyne 7 125 injection valve using various sample loops and directed through polymeric tubing (0.01 in i.d.) and introduced to the nebulizer using one of two methods.In the first method of sample injection termed the dribble USN (DUSN) the tubing [polymeric or poly(tet- rafluoroethylene) 0.04 in i.d.1 is cut at an angle of approximately 45" and placed nearly in contact with the glass surface of the nebulizer. The liquid is then allowed to dribble across the surface of the nebulizer where nebuliza- tion can occur. This method is commonly cited in the literature and is the method used in most commercially available systems. As an alternative sample injection method ajet USN was employed. This method utilizes a length of 50 pm i.d. fused silica tubing (except as noted). The fused silica is positioned approximately 1 cm from the nebulizer surface.At the liquid flow rates used in these studies the back pressure of the HPLC pump causes a stable liquid jet to form at the tip of the fused silica tubing. The liquid jet impacts on the nebulizer surface initially with an interaction diameter only marginally greater than the original jet diameter and then forms a thin film of liquid which is subsequently nebulized through interaction with the acoustical energy of the nebulizer. It is important to note that the liquid jet energy is not adequate to cause direct nebulization through impact with the transducer surface. In the absence of r.f. energy applied to the transducer no detectable aerosol is formed. The nebulizer designed in-house referred to as the Jet USN is illustrated in Fig.l(a). It is possible to convert any conventional USN to jet operation by replacing the large bore sample introduction tubing with 50 pm or smaller i.d. fused silica tubing and using a pump capable of sustaining a stable liquid jet. The commercial unit was operated both in the dribble814 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 (a) Water To condenser f To plasma f Condenser // Air-cooled - USN Samde ' ' introdiction - Spray chamber Fig. 1 Schematic diagrams of USNs used (a) Jet USN; and (b) CETAC USN. Point 'I' indicates intermediate solvent collection point and point 'T' indicates position for measuring aerosol temperature in the CETAC spray chamber and jet modes and is referred to as the CETAC DUSN or CETAC Jet USN respectively.Fig. l(b) illustrates the CETAC USN design. Unless otherwise noted after leaving the spray chamber aerosols were passed first through a heated glass chamber and then through a condenser in order to remove excess solvent. A Perkin-Elmer ICP/6000 ICP atomic emission spectro- meter was used for data collection taking raw data through an analogue-to-digital converter and storing it on an IBM PC/XT computer. Data were collected and stored at 0.5 s intervals. Analyte and solvent transport measurements were car- ried out in order to determine the amounts of analyte and solvent reaching the plasma and the locations of analyte loss. Several different collection methods both direct and indirect were used in order to gain a thorough understand- ing of the mass transport processes and obtain a mass balance for all the sample introduced.Direct collections were performed as previously reported* using solutions of 100 ppm of Cd in 1 O/o HN03 (as) for analyte transport and 1% HN03 (as) for solvent transport. Direct analyte collec- tions were made only after the desolvation system while solvent collections were made both after the spray and after desolvation. Indirect analyte and solvent transport mea- surements were also carried out using the same solutions as for the direct collections. The solvent was collected at the first drain position (spray chamber) and was measured by weighing. The analyte was collected from both drains (spray chamber and condenser) by collecting the liquid from each drain. The glassware associated with each drain was rinsed and the rinse was added to the drain collection.For all analyte collections concentrations were determined by analysis using ICP-MS with blank subtraction. Experiments using supplementary heat in the spray chamber region of the CETAC Jet USN were also con- ducted. The aerosol temperature was measured at location T in Fig. l(b). With no supplementary heat the aerosol temperature in the spray chamber was found to be 45 "C. Heat was applied to achieve aerosol temperatures of 62 and 86 "C in the spray chamber. Height profile measurements for Mn emission were carried out with and without supplementary heat using three different nebulizer gas flow rates (0.68 0.82 and 0.96 dm3 min-l of Ar). Cadmium transport measurements as described above were also carried out with a spray chamber aerosol temperature of 86 "C and a nebulizer gas flow rate of 0.82 dm3 mine'.Filtered de-ionized water Fisher TraceMetal Grade HN03 and Fisher certified atomic absorption reference standard solutions ( 1000 ppm) of Mn and Cd were used in this study. Results and Discussion Visual Observations When both the ultrasonic nebulizers were operated under conditions of greatest operating efficiency corresponding to a power of >45 W for the water-cooled transducer and 25-30 W for the air-cooled transducer nearly all of the introduced sample was nebulized and a pattern of nodes and antinodes could clearly be observed on the surface of the transducers. At the nodal regions droplets of approxi- mately 1 mm in diameter formed on the transducer and remained stationary.The aerosol emanated from the antinodal regions between these stationary droplets. The nodes and antinodes appear to correspond to a standing wave formed on the surface of the transducer with a wavelength determined by measuring the spacing of the nodal distribution of approximately 1 mm. This value agrees fairly well with the calculated value of 1.1 mm for the wavelength of 1.35 MHz waves in distilled water. The nodal pattern on the transducer surface is shown in Fig. 2. The over-all dimensions of the nodal pattern are fairly large compared with the capillary waves observed by Lang.' For the transducer used in this study the theoretical value for capillary waves is 10 ,urn far too small to be observed with the magnification used in Fig.2. Transport Measurements Solvent and analyte transport measurements for the Jet USN the CETAC DUSN as supplied and the CETAC Fig. 2 Photograph of USN during jet nebulization process. Aerosol is slightly visible as a foggy region and the liquid jet is visible as a thin white line travelling vertically from the bottom to the centre of the transducer. Major &ale divisions are in centi- metres and liquid flow rate- 1 cm3 min-'JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 815 Table 1 and condenser= 2 "C Solvent transport for USNs liquid flow rate= 1 cm3 min-'; nebulizer gas flow rate=0.82 dm3 min-I of Ar; heated zone= 140 "C; Nebulizer Collection Transport efficiency Solvent mass No. of (O/O) transport/g min-I measurements 2 Jet USN Point I [Fig.l(a)] no heat 31.2k0.2 Jet USN After desolvator - 0.0 100 2 0.0005 4 Jet USN After desolvator no aerosol - 0.005 1 6 Jet USN Spray chamber drain 27. I * 0.6 CETAC DUSN After desolvator - 0.04 8 f 0.00 1 3 CETAC Jet USN After desolvator - 0.0 180 * 0.0009 3 - (saturated argon) - Table 2 Analyte transport for USNs liquid flow rate= 1 cm3 min-'; heated zone= 140 "C; and condenser=2 "C Transport efficiency Nebulizer Collection Nebulizer gas flow/dm3 min-I of Ar (O/O) No. of measurements Jet USN After desolvator CETAC DUSN After desolvator CETAC Jet USN After desolvator CETAC Jet USN After desolvator CETAC Jet USN After desolvator 0.82 0.82 0.68 0.82 0.96 15.0k0.7 21.1 f 0 . 4 18.4 * 0.6 22.0 f 1 .o 22.7 f 0.5 converted to jet operation are reported in Tables I 2 and 3.Several important observations can be made based on these data. While values as high as 85% for analyte transport have been claimed in the literature,* the present data show a maximum analyte transport of about 22%. This value is based on a sample flow rate of 1 cm3 min-l and is somewhat higher than the 11% analyte transport for a USN at 2.8 cm3 min-' reported by Olson et al.' Based on previous studies,* the more common flow rates for a USN of 2-3 cm3 min-' would probably result in a lower percentage transport but a higher over-all mass transport. Further- more as evidenced by both the analyte and solvent collections of the spray chamber drain approximately 70-80% of the introduced sample is lost in the spray chamber. Although nearly 100% of the sample is nebulized many of the droplets are large and are lost either owing to settling or through collisions with the walls of the spray chamber.Collection of solvent at the intermediate point I [see Fig. l(a)] for the Jet USN with no heat applied indicates a transport of about 30% which is in agreement with the collections of analyte and solvent from the spray chamber drain. At room temperature total solvent transport is approximately equal to aerosol transport and therefore close correspondence of these values is to be expected. Drain collections from the condenser showed that approximately 7% of the sample originally introduced is Table 3 Analyte loss' in spray chamber and condenser drains liquid flow rate=1 cm3 min-'; nebulizer gas flow rate=0.82 dm3 min-' of Ar; heated zone= 140 "C; and condenser= 2 "C Original analyte lost to drain No.of Nebulizer Collection (O/O) measurements Jet USN Spray chamber 76*4 3 Jet USN Condenser drain 7.3 f 0.8 3 drain lost in the condenser. These losses are presumably due to settling and collisional processes. As the supersaturated gas enters the condenser the solvent begins to condense and the dried aerosol particles act as nucleation sites for water condensation; the particles can actually increase in size at this point. Such an increase in size might lead to loss of aerosol particles on the condenser surfaces. In addition the wet inner surface of the condenser might cause changes in the interaction between particles (wet or dry) in the aerosol and the walls of the condenser.The analyte losses in the condenser indicate that the conventional design is not the most efficient for desolvating aerosols. Not only is the analyte transport poor but the system is also limited in its capacity to remove solvent. A condenser is only capable of reducing the solvent loading to its vapour pressure value at the operating temperature of the condenser. Furthermore condensation of water onto the aerosol particles increases the solvent loading beyond the saturation point. In order to illustrate this point the solvent loading for Ar passed through a wet desolvation system is reported in Table 1. This value which should approach the saturation value for Ar at 2 "C accounts for only half of the solvent loading when 1% HNO is nebulized. The remaining solvent is present as condensed water on the aerosol particles.In order to calculate the mass balance the values for analyte transport found by collections from the spray chamber drain and condenser drain and the final aerosol transport can be added and should total 100%. For these studies using the in-house Jet USN design the results are summarized as follows over-all transport 1 5.0 If 0.7%; loss to spray chamber drain 76*4%; loss to condenser drain 7.3fO.8%. The sum of the experimental values was therefore 98 f 6% which accounts for all the analyte within experimental error. Owing to the difficulties of performing direct analyte collections on a wet aerosol no direct measure of the losses Table 4 Analyte transport for CETAC Jet USN with and without supplementary spray chamber heat liquid flow rate= 1 cm3 min-I; nebulizer gas flow rate=0.82 dm3 min-' of Ar; heated zone= 140 "C; condenser=2 "C; and 50 pm capillary Spray chamber Transport efficiency Nebulizer temperature/"C Collection (Yo) No.of measurements CETAC Jet USN 45 (no heat) After desolvator 24.4 0.1 CETAC Jet USN 86 After desolvator 3 2 + 2 2 3816 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 in the heated region could be obtained. However since the mass balance accounted for nearly all the analyte intro- duced it can be assumed that losses in this region are relatively small. This assumption is well founded based on the processes occurring in the heated zone. As the aerosol particles are heated solvent rapidly evaporates thereby reducing the size of the particles.Small particles are much less vulnerable to settling or collisional losses so transport in the heated region is expected to be high. Although the actual values differ all of the transport measurements reported here are in general agreement with the work of Weber et af.,9 using a USN system. These workers report that 93% of the analyte is lost in the spray chamber and 12.8% of the remaining analyte is lost in the desolvator yielding an over-all transport of 5.2% at a liquid flow rate of 2.24 cm3 min-l. Weber et aL9 also suggested I 1 I I I 1 9 10 11 12 13 14 15 0.6 I 0.7 I I I 1 I 1 10 11 12 13 14 15 1.1 I 1 1 .o 1 (c) A 0.9 1 1 I I I I 10 11 12 13 14 15 0.5 ' Height above load coil/mm Fig. 3 Height profile emission characteristics for Mn at 257.610 n m using CETAC Jet USN plasma gas= 14 dm3 min-' of Ar; auxiliary gas=1 dm3 min-' of Ar; power=1.25 kW; heated zone= 140 "C; and condenser=2 "C.Nebulizer gas flows are (a) 0.68 (6) 0.82 and (c) 0.96 dm3 min-' of Ar. Temperatures of aerosol in spray chamber are A 45 (no heat); B 62; and C,,86 "C that condenser losses occur in the system due to condensa- tion of solvent onto dry particles followed by settling or impaction losses. The transport studies obtained in the present work clearly indicate that the major part of the aerosol produced by a USN is lost in the desolvation and transport system. In order to reduce these losses two approaches are possible the production of a primary aerosol that has smaller droplets which are therefore more efficiently transported to the plasma; and design of a desolvation system that efficiently desolvates a large percentage of the aerosol including the larger droplets initially present.For illustrative purposes an experiment was devised to increase analyte transport through the spray chamber. Supplementary heat was applied to the glass spray chamber of the CETAC Jet USN utilizing a 50 pm capillary for sample introduction. Height profiles for Mn emission at three aerosol temperatures and three nebulizer gas flow rates are given in Fig. 3. With no supplementary heat the aerosol initially formed had a temperature of about 45 "C presumably owing to heat transfer from the transducer. By averaging signal enhancements over all the measurement heights for nebulizer gas flows of 0.68 0.82 and 0.96 dm3 min-I the signals increased by 14 29 and 35% respectively. Since the sample is desolvated regardless of the temperature of the spray chamber it is assumed that signal enhancements are mainly the result of increased analyte transport.Furthermore it is important to note that the emission signals show some variation with nebulizer gas flow rate. Although the nebulization process is basically independent of gas flow it is believed that transport is considerably more sensitive to gas flow. In addition it is important to consider the residence time in the plasma which is also a function of gas flow. In order to confirm that the signal enhancements were in fact due to increased transport measurements were made to compare the analyte transport with and without supple- mentary heat at the spray chamber.These transport results (Table 4) indicate an increase in transport of approximately 30% when the spray chamber aerosol is heated to 86 "C. This value is very close to the average emission intensity increase of 29% under comparable conditions [Fig. 3(6)]. Although applying heat to the spray chamber dramatic- ally increases the analyte transport this method has practical drawbacks. Heating the spray chamber causes increased background noise and signal noise due to inter- mittent flash vaporization of droplets that come into contact with the hot glass. Furthermore deposition of the analyte on the walls of the spray chamber can also occur resulting in memory effects. Despite the fact that the method used here is not appropriate for analytical use it does provide evidence that large droplets can be modified to bring about higher transport.With attention paid to the design of the spray chamber heating the aerosol as early as possible above the transducer face might be a viable method for increasing the transport. Owing to the limita- tions in producing small droplets it is probable that with current systems the most effective way of producing higher analyte transport is by improving the transport of larger primary aerosol droplets. Flow Injection Studies The two primary reasons for using a liquid Jet USN are to improve the uniformity of the liquid film formed on the transducer surface; and to minimize the band broadening for low flow rate FI studies and for coupling with micro- HPLC. In order to study band broadening and repeatability effects FI studies were carried out at relatively low flow rates.The use of FI-ICP using a 25 pm capillary Jet USN at a liquid flow rate of 150 mm3 min-' is illustrated inJOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY SEPTEMBER 1992 VOL. 7 817 5 I 1 0 200 400 600 Time/s Fig. 4 Flow injection ICP emission traces for ( a ) Jet USN (25pm capillary) and (6) DUSN (0.0 1 in i.d. tubing); liquid flow rate= 150 mm3 min-I. Injections (50 mm3) of 10 ppm Mn are indicated by arrows Fig. 4(cr). Such flow rates are too low for the effective operation of the DUSN as illustrated in Fig. 4(b). In Fig. 4 the peaks in each trace are normalized with the first peak height set to unity. Normalization was carried out because of intensity differences between the two nebulizers used.In the case of the Jet USN reasonable repeatability was achieved at 150 mm3 min-l although other flow rates gave somewhat poorer precision. With the DUSN (with 0.00 1 in i.d. tubing) repeatability between peaks became steadily worse as the flow rate was lowered. The instabilities in the DUSN at these flow rates arise owing to the finite gap between the sample tubing and the nebulizer. In order to achieve stable operation there must be a constant and steady supply of solution flowing onto the nebulizer face. As flow rates become low the dribble design cannot provide a continuous flow of sample and instabilities arise. The Jet USN system was not optimized with respect to the total volume of the system for the low flow rates used in these experiments and therefore the data are probably not the best that could be achieved.However a preliminary conclusion is that effective coupling to an ICP at a low flow rate might be possible with minimal band broadening by using an ultrasonic nebulizer with jet sample introduction. Conclusion Data presented here illustrate that typical USNs deliver only about 20% or less of the analyte introduced to the plasma. The percentage transport is dependent on the sample flow rate and generally decreases with increasing flow rate. Improved analytical performance of USNs is a function of both increased aerosol output and the use of desolvation. Although the aerosol particle size distribution in a USN indicates significant amounts of large droplets this aerosol can be effectively utilized for atomic spectro- metry by altering the particle size before losses occur.Such an approach is applicable to other types of nebulizers and can lead to significant improvements in detection limits. The use of a fused silica capillary in place of standard tubing can enhance the performance of USN. Improvement comes from the ease of alignment and the ability to perform low flow experiments such as FI and micro-HPLC. How- ever this technique has certain disadvantages. The usable liquid flow rate range is restricted at high values by back pressure and at low values by the required minimum liquid velocity needed to form a jet. In addition an HPLC pump with minimal pulse fluctuations is required. Clogging may also present a problem for unfiltered solutions especially if very small diameter (< 50 pm) capillaries are used.Finally it is important to pay close attention to all components of a sample introduction system for ICP. As illustrated in this report previous workers have neglected the importance of the desolvation system in enhancing both transport and detection limits. Many assumptions have been made as to transport values without sound experi- mental evidence. Only through close attention to such details can a steady path to improvements in sample introduction for atomic spectrometry be made. This research was supported by the National Science Foundation under Grant No. CHESS-08 183. The authors gratefully acknowledge the following companies for donat- ing or lending equipment used in the study Channel Products; CETAC Technologies; and Perkin-Elmer. M.A.T. acknowledges financial support from an American Chemi- cal Society Analytical Division Fellowship sponsored by Perkin-Elmer. References Olson K. W. Haas W. J. Jr. and Fassel V. A. Anal. Chem. 1977 49 632. Tarr M. A. Zhu G. and Browner R. F. Appl. Spectrosc. 199 1,459 1424. Zhu G. Pan C. and Browner R. F. paper No. 1271 presented at the 1989 Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy Atlanta GA USA March 6-10 1989. Jakubowski N. Feldmann I. and Stuewer D. Spectrochim. Acta Part B 1992 47 107. Montaser A. Tan H. Ishii L. Nam S.-H. and Cai M. Anal. Chem. 1991,63 2660. Fassel V. A. and Bear B. R. Spectrochim. Acta Part B 1986 41 1089. Lang R. J. J. Acoust. SOC. Am. 1962 34 6. Petrucci G. A. and Van Loon J. C. Spectrochim. Acta Part B 1990 45 959. Weber A. P. Keil R. Tobler L. and Baltensperger U. Anal. Chem. 1992,64 672. Paper 2/01 789J Received April 3 1992 Accepted June 12 I992