Anal. Chem. 1994,66, 685-691
Monodisperse Dried Microparticulate Injector for Analytical Instrumentation J. Barry French,. Bernard Etkin, and Raymond Jong University of Toronto Institute for Aerospace Studies, 4925 Dufferin Street, North York, Ontario, Canada, M3H 5T6
This paper reports on a theoretical and experimental research and development program aimed at improving the way liquid samples are introduced into plasma torches for analytical and research purposes. The result is the MDMI, a system that generates a train of small identical droplets of the sample, dries them, and injects the resulting train of dried particles into the plasma. A high degree of uniformity and regularity is achieved in the final particle train. Advantages include less interference from oxide ions In ICPMS applications, 100% sample utilization and lower signal noise leading to better precision. The paper presents both theoretical and experimental results.
This paper reports results on an improved version of the old concept of monodisperse droplet introduction of liquid samples into ICPMS systems, namely, monodisperse dried microparticulate injection (MDMI). As such, it builds on the work of a number of researchers who have applied the concept of monodisperseaerosol generation tostudies of flames and plasmas for both basic research and potential analytical applications. In this approach, the sample stream is maintained in a monodisperse,ordered droplet array by introducing it into a highly laminar flow of heated carrier gas in which the microdroplets evaporate to yield a uniform stream of microparticles for injection into the plasma. Much prior work has shown that the presence of water vapor in the injected sample plays a primary role in the formation of interfering oxide ions. Samples introduced without solvent, as for example by laser ablation, have been shown by Hagar,' Gray? and Arrowsmith3 to produce negligible oxides. A similar result for a nebulizer is obtained by Houk and co-workers4 employing repeated heating and wall condensation to thoroughly remove water vapor. This reference also lists the work of many others who have employed spray chamber cooling, or heating followed by condensation, to achieve partial oxide reduction, often at the expense of some sample loss and/or memory effects. Part of the objective of the present work was to determine the reduction in oxide ions which should result using MDMI from the combination of two effects, the elimination of the saturated water vapor, which is a very significant part of the water burden in a sample from a nebulizer and spray chamber, and the additional effect of the extra water vapor in the microregion surrounding a droplet evaporating in the plasma. (1) Hagar, J. Anal. Chem. 1989, 61, 1243-1248. ( 2 ) Gray, A. L. Analyst 1985, 551-556. ( 3 ) Arrowsmith, P. Anal. Chem. 1987, 5, 1437-1443. (4) Alves, L. C.; Wiederin, D. R.; Houk, R. S . Anal. Chem. 1992,64,1164-1169.
0003-2700/94/0366-0685$04.50/0 0 1994 American Chemical Socletv
Figure 1. Cross sectlon of assembled MDMI apparatus.
Previously, monodispersedroplet generators have been used to study droplet-flame interactioms However, the larger droplet size range they inherently produce (40-80-pm diameter) was deemed in ref 6 to be too large, so shear flow was introduced to shatter the drops into a polydisperse range of smaller sizes. It was expected in this work that, because predrying the sample droplets results in microparticulates of diameters from submicrometers to a few micrometers, depending on solute concentration, phenomena related to paticulate evaporation should not present major analytical difficulties. The overall motivation for this work was to begin to delineate the potential of the MDMI approach in comparison to standard nebulization techniques, for improving two analytically important performance parameters-reduction in the oxide ion mass spectral interference problem without causing sample loss or memory effect and signal noise reduction via elimination of the random noise inherent with nebulizers, while inherently providing 100% sample utilization.
EXPERIMENTAL APPARATUS 1. Overall Design Considerations. An overall crosssectional schematic view of the assembled MDMI apparatus is shown in Figure 1. The design considerations for the droplet generator and laminar flow oven are described in more detail below. Basically, the droplet generator injects a stream of uniformly spaced drops into a coflowing argon stream at about 3 m/s approximately isokinetically. The argon is preheated to the desired operating temperature (450-800 "C) and maintained as laminar flow throughout. The oven temperature is normally selected to make the drop evaporate to dryness just before injection into the plasma. ( 5 ) Hieftje, 0.M.; Malmstadt, H. V. Anal. Chem. 1968, 40, 1860-1867. ( 6 ) Winlder, P. C.; Perkins, D. D.; Williams, W. K.; Browner, R. F. Anal. Chem. 1988.60. 489-493.
Ana&-ticalChemistry, Vol. 66,No. 5, March 1, 1994 686
\FIOW
constraint
Figure 2. Droplet generator deslgn.
Many heating methods, flow configurations, and droplet generation methods were considered before this combination was selected. Some systems tried and rejected are described below. Injecting the droplets at right angles to the flow was studied both theoretically and experimentally. In that case, some of the evaporated solvent is left in a part of the flow field different from the dried sample particulates. However, the potential advantage in solvent separation was found not to be worth the complexity or the variability in particulate position arising from small variations in launch velocity, droplet evaporation rate, and argon velocity. Droplet heating by microwave and infrared radiation was studied theoretically. However, besides their unnecessary complexity, these approaches are flawed. We learned in the course of this work that it is beneficial not only to heat the particle to dryness, but also to maintain it in a hot stream so that no recondensation onto the sample microparticle occurs between the oven and the plasma. Since a hot stream is needed, the design task was to see whether the temperatures and distances needed for evaporation were feasible for droplet sizes typically produced by monodisperse generators, utilizing simply the heat transfer to the droplet under laminar flow conditions plus irradiation from the hot oven walls. Perhaps the most commonly used monodispersegenerators are based on natural or induced instability in a liquid jet, first studied by Lord R a ~ l e i g h .There ~ have been many others who have employed this technique; refs 8 and 9 list some of them. However, Rayleigh instability entails an inconvenient combination of drop spacings, drop sizes, and drop injection velocities. Drop spacings are so small and initial velocities areso high that drop wakeinteractions occur, leading toscatter or agglomeration, unless some drops are removed by charging and electrostatic deflection. We found that this can be done, and we even studied systems in which charged sample droplets were deflected electrically into a region of dry argon. However, all these systems were rejected as impractically complex compared to the use of uncharged droplets from a suitably designed drop-on-demand droplet generator. 2. Droplet Generator. The droplet generator finally adopted in this work is of the axisymmetric type shown in Figure 2. It follows the principles presented by Zoltan.lo It is basically a glass tube with an internal contour variation as indicated surrounded by an annular piezoceramic. It is critical that the exit taper terminate in a sharp-edged orifice so that (7) Rayleigh, Lord. On the Instabilityof Jets. Proc. London Math. Soc. 187%.10, 4-13. (8) Galley, P. J.; Hieftje, G. M. Appl. Specrrosc. 1992, 46, 146C-1463.
(9) Berglund, R. N.; Liu, B. Y. H. Enuiron. Sci. Technol. 1973, 7, 147-153. (IO) Zoltan, S. J. Pulsed Droplet Ejecting System U S . Patent 3,683,212, 1972.
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the equilibrium position of the meniscus is well-defined. A compressive pulse from the piezoelement (typically 100 V applied radially) causes a momentary contraction of the glass tube, resulting in an expulsion of the nearly incompressible liquid in both directions. By using a suitable flow constraint in the backward direction, the desiredvolume is directed toward the orifice, where it is expelled as a droplet (the system we adoptedproduced 57-pmdrops from a 30-pmorifice). Surface tension causes the meniscus at the orifice to regain its initial position, and since a drop has been ejected, the device acts as a micropump, driven by the surface tension, without need for any external peristaltic or other pumping device. It was found experimentally that the micropump can cope with a head pressure variation of typically 10 cm of water above or below atmospheric pressure. The micropump must be primed to permit pumping action to start; any air bubbles in the sample transport line cause it to lose its prime. Pump flows can be varied at will, utilizing an appropriate variable-frequency square wave pulse generator, from a single drop (e.g., 57-pm diameter, 0.1 nL) up to 6000 drops/s (0.58 pL/s). For comparison purposes, the flow at 57 pm and 1000 hertz is of the same order as the net sample transport rate into the plasma from a standard cross-flow nebulizer (ca. 20 pL/ min) and is also typical of flows from liquid chromatography columns. Of course, if larger sample flows were to be involved, as for example from an automated flow injection system, it would only be necessary to ensure that the inlet to the micropump is flooded with continuous liquid flow, with excess flow going to drain, so arranged that the pump inlet pressure is nearly atmospheric. The drop velocity and drop size measurement employed a 180X binocular microscope equipped with an instant camera and a strobe light synchronized to the micropump’s pulse generator frequency. The strobe light gives the impression of stationary droplets frozen in space so that a calibrated microscope eyepiece micrometer can measure the diameter and the spacing between droplets. Instant photographs of the droplet train revealed identical results. In either method, the measurements were taken within 5 mm from the micropump orifice face. This is the maximum distance the droplet travels through the oven-micropump interface before it is captured by the laminar flow. The velocity is calculated from the measured droplet spacing and the pulse generator frequency. The drop size and initial speed can bevaried within a useful range by altering the width and height of the pulse applied to the piezoceramic (Figure 3). In our system we could in this way generate drops from 48- to 64-pm diameter. The corresponding range of initial speeds was 0.6-3.5 m/s. At one stage of the development, we designed and made our own micropumps, trying several different design configurations. To assist in this process, we developed an analytical model of the type illustrated in Figure 2 (to be published). This model yielded drop sizes and drop speeds similar to those obtained experimentallyand demonstrated correctly the trends associated with changes in design. Ultimately, we used micropumps obtained from On Target Technologies, Santa Clara, CA, and Microdrop GmbH, Norderstedt, Germany. All the results reported in this paper were obtained with samples simply aspirated from an open container via a smallbore capillary connecting tube.
65 h
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Pulse width (micro sec)
i:
B
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Pulse w'd:n (micro sec) Figure 3. Variation of drop diameter (A) and drop velocity (B) as a function of pulse width and pulse height applied to the piezoceramic. Test solution, distilled water.
3. Oven Design. The laminar, coflow oven design had to meet simultaneously several operational design criteria: ( 1) maintain a near-zero root-mean-square turbulence level in the stream so that the droplets could maintain their spatial and temporal uniformity; (2) permit the drops to evaporate to dryness in a practical distance (say 20 cm), using standard argon operating flowsof order 1 L/min; (3) yield a sufficiently small gravity-inducedvertical deflectionof the drying particle trajectory that horizontal operation would be feasible, Le., that particles not hit the wall. It was not at all obvious at the outset that these design goals could be achieved. We therefore decided to develop an analytical model of the trajectory and evaporation of the droplets to guide us in the design of the experimentalapparatus. The objective was to gain some insight into the physical processes at work, especially to identify those that are critical, and to be able to specify in advance approximately the correct dimensions and temperatures to be used in the oven. This would enable us to avoid some of the expensive cut-and-try that would otherwise be necessary and to predict the trends to be expected when design variables werechanged. The model described below achieved these objectives. It grew from very crude beginnings to that presented here as more detail was added at various stages in its evolution. Although it appears to be relatively complicated and sophisticated, it is still only a rough approximation to the real physics of the process. The model was used both for studies of the coflow system reported here and for other configurations in which the drops were injected at right angles to the gas flow. The physical effects included in the model are as follows: (1) the aerodynamic and gravitational forces acting on the
droplet; (2) heat transfer to the drop from the hot gas and the hot wall; (3) evaporation of water from the drop; (4) diffusion of water vapor from the droplet into the gas. There were necessarily many approximations made, some explicit, some implicit, the effect of which could not be known in advance. As it turned out, in spite of the approximations, the model gave reasonably good agreement with our observations, and we were able to use it with advantage for its intended purposes. The model takes the form of seven first-order ordinary nonlinear differential equations with numerous supporting relations. They are solved by numerical integration using a fourth-order Runge-Kutta procedure. The motion takes place in the ( x , y ) plane, with gravity in the negative y direction. The gas is at a uniform temperature equal to that of the wall except in the boundary layer of the drop. Everywhere, except in the immediate neighborhood of a drop, the gas has a velocity parallel to theduct, but of a magnitude that varies with position both along and across the duct; the former because of the local entry transient, and the latter because of the parabolic velocity profile across the duct. At each computing step the heat transfer and diffusion are treated as though the drop were alone in a uniform infinite stream of gas having the local velocity and irradiated by hot walls of infinite extent infinitely far away. The water temperature is assumed to be uniform within the droplet. The theory used for the heat and mass transfer is that presented in ref 11, pp 409 and 647 ff. (The variables are defined in the list of symbols.) 3a. Differential Equations
(2)
y=u u =f x / m
(3)
i, = & / m
(4)
b = -K/d[2 + 0.6S~'/~Re'/~]
(5)
+ Lm)/mc
(7)
T = (Q
Equations 1-4 express the dynamics of a drop of mass m acted on by the external forces f y ) . Equations 5 and 6 represent the loss of mass from the drop by diffusion of water vapor away from it, and eq 7 describes the heating and cooling of the drop that results from heat transfer and evaporation. To completethe system, we need relations for all the quantities that occur on the right-hand sides of eqs 3-7 as functions of the seven main variables. These are as follows: 3b. External Forces
vx,
fv = -Fu/Vr
- mg - Sa
(9)
vr= [(U-u)2 + u2]1/2
(10)
( 1 1 ) Bird, R. B., Stewart, W. E., Lightfoot, E. N . Trunsporr Phenomena; John Wiley & Sons Inc.: New York. 1960.
Analytical Chemistry, Vol. 66, No. 5, March 1, 1994
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I
C, = 24(1
+ 0.15Re0.687)/Re
Sa = 1.615(u - LI)lo1'/2d2/(pp)1/2sgn w
(12) (13)
(14) In the above relations, the drag coefficient c d is given by a commonly used form of function of the Reynolds number ReI2 and Sa is the Saffman force, an aerodynamic lift that occurs when V, is nonzero and when there is a lateral gradient in the gas speed." 3c. Heat Transfer. In eq 7 Q is the heat transfer to the drop, being the sum of that from the gas, given by a heattransfer coefficient h, and that from the wall radiation, in terms of a constant derived from the Stefan-Boltzmann constant, and the fourth-power law for radiation: = -au/ay
Q = d 2 [ h ( T , - T ) + al(Tw+ 273)4] h = k/d(2
(15)
+ 0.6PrIf3Relf2)
(16) The heat transfer coefficient h, for forced convection between a sphere and a gas, is given in terms of the coefficient of thermal conductivity of the gas and a well-known correlation with the Prandtl and Reynolds numbers." We estimated a1 for our geometry to be roughly a, = 0.68 X lo-* cal s-'
K4
(17)
3d. Diffusion of Water Vapor. The diffusion of water vapor away from the drop is the mechanism by which mass transfer takes place. In a steady state, which is implicitly assumed, this rate is just equal to the rate of evaporation. The governing equation ( 5 ) is seen to be analogous to that for heat transfer (see h above) with the Schmidt number replacing the Prandtl number. The mass-transfer coefficient K is derived from Fick's law of diffusion in ref 11 as
K = kxo/(1 - x,)
(18)
h = 2pM,a)/ppU
(19)
where
For xo we used the ratio of partial pressure of water vapor to that of the gas at the temperature of the drop. The estimation of a) was uncertain. We used the formula given in ref 11 (p 505) for H20 in a nonpolar gas. The vapor pressure was not adjusted to allow for the presence of any solutes, Le., the droplet was modeled as pure water. It is appreciated that this assumption departs substantially from the true physics as the droplet shrinks toward complete dryness. However, the diameter shrinks very rapidly at the end (the rate tends to infinity as d 0), and hence the distance traveled while it is drying is not sensitive to the assumptions that are used in this terminal phase. At any rate, the model with this assumption served its purpose well enough. 3e. Physical Constants. The various physical properties needed were obtained by fitting analytical expressions to handbook data. The four physical properties (p, p , k, a)) are
-
(12) Rudinger,G.Fundamentals ofGas-Particle F1ow;ElsevierScientificPublishing Co.: New York, 1980;p 9.
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ARGON TEMPERATURE (Celcius)
Figure 4. Theoretical tube length required to evaporate all the water from a 5-pmdlameter droplet as a function of argon temperature and flow rate.
all strong functions of temperature, and hence their values are very different at the surface of the drop, where the temperature is less than 100 OC,from those in the body of the gas, where the temperature might be as high as 800 OC.To accommodate this difference we used the approximation suggested in ref 11 (p 747); i.e., the four gas properties are calculated as "film" values, the simple average of those at infinity and those at the surface of the drop. 3f. Results. The model indicates that the controlling factors in evaporation are the large latent heat of the water and the thermal conductivity of the argon. The rate of heat transfer to the drop is relatively small so that the water remains relatively cool even when surrounded by a very hot gas. The results obtained did not turn out to be very sensitive to the initial temperature of the droplet. However, to allow for the inevitable heating that would occur in the water just upstream of the very hot oven (actually we had to cool it to prevent boiling), we usually started the calculation with the droplet at an elevated temperature. In a typical calculation, with a 60-rm drop in argon at 750 OC,the drop cools in the first millisecond from an initial temperature of 80 to 49 OC,and then cools further to a steady-state temperature of 44 OC until all evaporation of liquid water is completed at the dryness point D . Figure 4 showssome typical results obtained with the model, in which the calculated length of tube required to evaporate all the water from a 57-pm diameter droplet of analyte in solution is plotted against argon temperature and flow rate. The drying tube is 5 mm in diameter, and the droplets are launched axially at 3 m/s at the center of the tube. For example, for a droplet injected into a 500 OC argon stream flowing at 0.9 L/min (defined at room temperature), the distance to achieve the dryness point D is calculated to be 20 cm, a practical length for design purposes. The associated gravity drop would be 1.3 mm, which indicates that the drop does not hit the wall. One constraint that has to be met when the drying tube is disposed horizontally is that the droplet must not hit the wall. This can occur by virtue of the curvature of the trajectory induced by gravity. Increasing the initial drop diameter and decreasing the gas temperature both favor this occurrence, as does starting the drop below the axis of the tube or pointing
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Flgure6. Comparisonof MDMIand nebulizer analyte response curves. Nebulizer with an uptake rate of 1 mL/min and estimated 2 % efficiency and MDMI with an uptake and injection rate of 6 pL/min with 100% efficiency at 1000 drops/s in and 800 OC oven: 10 ng/pL solution. 0
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Figure 5. Photograph(A) of droplet stream producedby the micropump and time-resohred Sr ion emission intensity (B) 16 mm above the load coil at 1000 drops/s. The time-resolved data were acquired with a monochromator viewed through a horizontal slit. Courtesy of 0 l e ~ i k . l ~ Photograph courtesy of Microdrop GmbH.
the initial velocityvector downward. Raising the launch point above the tube center considerably extends the range in temperature-diameter space that is useable. Figure 4 shows that the furnace length is not as sensitive to gas temperature as might be supposed. The gas flow rate, which together with tube diameter controls the dwell time of the droplet, is more important. The curious behavior of the graphs below 500 OC results from the interaction of trajectory with temperature. The dwell times at lower gas temperatures are longer because the drop sinks into a region of lower gas velocity. At a gas flow of 1 L/min, the dwell time in a 5-mm-diameter oven for 20-cm travel varies from 82.5 ms at 400 OC to 48.2 ms at 600 OC. The MDMI system can of course be used with its axis vertical, in which case the gravity and trajectory effects are negligible. The position of the dryness point D is controllable slightly by oven temperature but mostly by flow rate. Thus, as could be important for analytical samples containing volatile analytes, the oven can be operated to position the dryness point just ahead of the point of injection into the plasma, so that the sample remains near room temperature in spite of the elevated oven temperature, to avoid differential diffusional spreading of analytes of varying volatilities. EXPERIMENTAL RESULTS Figure 5 shows a photograph of a typical droplet stream as produced by the micropump and, below it, a record of a typical ion emission recorded from a point in the plasma, obtained byOlesik,14using the MDMI system in his laboratory, and reproduced by his courtesy here. It is immediately apparent that the highly ordered nature of the droplet stream has been preserved through the laminar flow oven and into the plasma. ~~
(13) Saffman, P. G. J. Fluid Mech. 1965, 22, 385; 1968, 31 (corrigendum). (14) Olesik, J. W., unpublished.
Initial tests on the MDMI system were carried out using a standardized test solution containing 10 ng/mL rhodium as a representative analyte and 10 ng/mL cerium as a representative element with a known strong tendency to produce oxide ions in the operations of ICPMS systems with conventional pneumatic nebulizers and double-pass spray chamber. The general picture of what happens to a dried particle is as follows. After entry into the plasma, it is rapidly heated to the point of vaporization. The vaporization may continue over several millimeters of travel, depending on the size of the parti~1e.l~ This reference indicates that 1-2 mm is required for particles formed from low concentration, and 3-4 mm for particles from higher concentration. Vaporization is followed rapidly by dissociationof molecules and ionization of molecules and atoms. The plume of vaporization products expands laterally as it travels downstream in the plasma. The concentration of oxides in the microregion around the vaporizing particle is proportional to the local concentration of oxygen15and, hence, is diminished if diffusion of the water vapor has lowered its concentration in this region. A second factor known to be of great importance for oxide ion concentration measured in an ICPMS is the local temperature of the plasma at the "sampler", Le., the location where the plasma is sampled into the vacuum. This temperature governs the equilibrium concentration of oxides in the dissociation reaction
+
MO+ e M+ o Lower temperature favors the presence of oxides. Figure 6 presents the ion count rates for Rh+ and percent CeO+ as functions of the argon sample gas flow rate Q,using 1000drops/s. The ICPMS system used was a Perkin Elmer/ Sciex Model 5000 Elan system. This figure shows the main features of the performance. The characteristic curve for a conventionalpneumatic nebulizer is seen, with a rather broad peak and a level of about 1.8% for oxides on the low-flow side of the peak. The MDMI result, for a maximum Rh count of (1 5) Douglas, D. J. Fundamental Aspects of Inductively Coupled Plasma-Mass Spectrometry In Inductively Coupled Plasmas in Analytical Atomic Spectroscopy, 2nd ed.; Montaser, A., Golightly, D. W., Eds.; VCH Publishers Inc.: New York, 1992; Chapter 13, pp 613-650.
AnalyticalChemistry, Vol. 66, No. 5, March I, 1994
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20-
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0.5
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Drop Frequency (kHz) Figure 7. MDMI variable sample loadlng of 57-pmdiameter droplets by varying drop frequency. Normalized sensitivity relative to cross-flow nebulizer at 1 mUmin uptake rate.
roughly the sameorder as that of the nebulizer, yields a similar peak, displaced to the right by about 0.05 L/m, and with an oxide level of about 0.4%. The increased ion count with increasing gas flow on thelow-flow sideof the peak is attributed to the diminished time for lateral diffusion by the ion cloud and, for the nebulizer, possibly an increase in sample transport rate. On the high-flow side of the peak, the diminishing ion count is attributed to the reduction of the time available for ionization of the analyte atoms. The peaks in Figure 6 for the nebulizer and the MDMI havecomparable widths. The breadth of that for the nebulizer is attributed to the extended region over which the random spray of droplets of many sizes vaporize, even though each very small droplet may vaporize essentially at a point. The breadth of that for the MDMI is attributed to the much larger size of the monodisperse dried particles that were present in this case. Unpublished data of Olesik has suggested that vaporization occurs over a track length of approximately 2 mm, which is enough to account for the observed width of the mountain. The reduction in oxide percentage for the MDMI compared to the nebulizer is attributed primarily to the fact that the droplets have been predried, with consequent diffusion of the H20 radially outward across the stream, leaving a lower concentration of oxygen where the solute ions are formed. The concentration is also lower because the total water burden from MDMI does not contain a contribution due to saturating the carrier gas with water vapor as occurs in the nebulizer plus spray chamber combination. The steeply rising values of oxide percentage on the highflow side of the mountain are attributed to the lower temperature of the plasma at the sampler that is associated with the larger momentum of the gas flow. The latter results in a downstream movement of the isotherms of the plasma. This explanation is consistent with visual observation of the plasma obtained when yttrium is added. A pink sheath region indicative of yttrium oxide and yttrium atoms precedes the central blue plasma dominated by atomic emission and can be seen to be pushed downstream with higher flows. Figure 7 presents typical sensitivity results, obtained on Rh+ ion intensity as a function of sample transport rate. The MDMI system permits a wide variation in transport rate, and hence sensitivity, at a fixed sample gas flow, by varying the drop frequency. 690
Analytical Chemistry, Vol. 66, No. 5, March 1, 1994
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Sample Dwell Time (sec) Figure 8. Noiseanalysis, in relativestandard deviation(RSD), between MDMI and the cross-flow nebulizer with a doubiapassspray chamber. Measurementdwell time represents the amount of time the given mass is measured. Statistical curve calculated from 1/N"*.
It is seen that a linear response exists as expected, up to about 6000 drop/s, and that the resulting sensitivity exceeds that available with conventional nebulizers by a factor of more than 2. This result has been obtained without any optimization with respect to drop size and drop-generator parameters. It is uncertain at this time what causes the falloff in response at higher loadings; this point is currently under investigation. It seems clear that MDMI provides an excellent research tool to explore the upper loading/sensitivity limits. Figure 8 presents the results of a comparative noise analysis (relative standard deviation, RSD) between MDMI and the cross-flow nebulizer. These were obtained from 10 repeat samples for Rh+ at an ion count rate of 5400 Rh+ ions/s. The resulting factor of 3 noise reduction is very encouraging in that it implicitly corroborates that a high degree of laminarity and thus regularity of drop spacing was achieved. This was confirmed by hot-wire anemometer tests, which indicated a remaining turbulence level below 0.3%root-mean-squarevalue relative to bulk velocity (extremely low). This result also confirms the generallyaccepted explanation that the random diameter and position of the droplets produced by a nebulizer is the dominant noise source in conventional ICPMS operation. We have been unable to determine what the remaining source or sources of noise in the total system were to cause the noise to plateau at a value of about 3 times the ion counting statistics limit. It is emphasized that the ICPMS system used was also used for various other experimental purposes and was not fully tuned and optimized, so that it may have contributed to the noise. Nevertheless, the combination of 100%sample utilization, good sensitivity and reduced noise suggests that isotope ratio determinations of better than 0.1%RSD precision with very small samples may be within reach by utilizing this approach, and this aspect is under investigation at present. The size of the microparticulates found upon taking the sample solution droplets to dryness depends on total solute concentration. As indicated on Figure 9, a 57-pm diameter drop originally at 1% sodium chloride concentration yields a 13-pm particle (assuming a sphere of density corresponding to solid NaCI). Since this is in the size range at which particle size effects have been observed in direct solids injection work,
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those reported. This version has not been observed to produce any contamination or sample memory artifacts. At this stage it is suggestedthat the MDMI approach has been demonstrated to provide a useful research tool for investigating s a m p l e plasma interactions. Because of the combination of attributes demonstrated, namely, 100%sample utilization, substantially lowered oxide interference levels, and lowered noise leading to improved precision, it may also afford analytic advantages in future.
ACKNOWLEDGMENT
0
NaCl CONCENTRATION (ng/pl)
Flgure 9. Effect of micropartlculate slze on slgnai intensity for 10 ng/& solutions of Rh. Concentration of NaCl was varied to obtain different dried particle sizes.
it is encouraging that no apparent effect due to particle size, induced by adding NaCl to a constant concentration rhodium solution, was observed. If solutes more refractory than NaCl were to be present then, depending on their concentration and volatility, there might be effects on analytical results deriving from variations in vaporization time and consequent width of the peak. This point is under further investigation As a related operational issue, it was noted that at the highest salt concentrations salt microparticles were observed to build up over many hours of operation around the drop injection orifice at the entrance to the oven but did not cause operational difficulties. They were readily removed by flushing with distilled water so that pendant drops form at the tip to dissolve the buildup.
This work is part of a joint research program of Sciex Division of MDS Ltd., Toronto, and the Institute for Aerospace Studies, University of Toronto, with support from the Province of Ontario under its URIF and Technology Fund programs. Support of the inorganic chemistry division of Perkin-Elmer Corp. under the PE-Sciex joint venture is also acknowledged. GLOSSARY C
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K L
m
DISCUSSION These results confirm that it is feasible to design a laminar flow oven which can take drops of the order of 60-pm diameter to dryness in a practical distance. Since this is the size of drop conveniently within the range of monodisperse droplet generation technology, the combination appears to offer an intriguing alternate possibility to nebulizers both for research purposes and for analytical purposes as well. The results obtained at sample loadings commensurate with normal nebulizer loadings (Le., 1000drop@ of about 60-1m diameter) indicate, as expected, that basic analytical behavior is similar. However, the effect of predrying the sample is that the floor level of the oxide curve can be reduced by more than 0.5 order of magnitude. This result is explained by the local excess of water vapor that is present around a wet droplet (as exists around the larger drops in the size distribution from a pneumatic nebulizer, as observed in the work of Olesik16). The noise reduction results confirm that nebulizer-induced diameter and position randomness is the dominant noise source when one is used, so that the MDMI approach offers significant noise reduction potential. The apparatus used in some of this work had an oven constructed of stainless steel which was, as expected, a source of contamination at elevated temperature. We have also used an all-quartz version that yielded results generally similar to (16) Hobbs, S . E.; Olesik, J . W . A n a l . Chem. 1992, 6 4 , 274-283.
M Mi Pr
Q Re S Sa
sc T
TB T W
U u, v
V, x, Y XO
P
P PI W
specific heat of droplet liquid drag coefficient specific heat of the gas diameter of droplet binary diffusion coefficient of H20 in the gas forces acting on droplet in the x and y directions, respectively aerodynamic force on droplet acceleration of gravity heat-transfer coefficient thermal conductivity of the gas mass-transfer coefficient latent heat of vaporization mass of droplet molecular weight of the gas molecular weight of the liquid C#/k, the Prandtl number heat-transfer rate Vplplp, the Reynolds number droplet projected area, u@/4 Saffman force p/pD, the Schmidt number temperature of droplet temperature of the gas temperature of the walls speed of the gas velocity componentsof droplet in the x and y directions, respectively speed of droplet relative to gas coordinates of droplet concentration of water vapor in gas at the drop surface coefficient of viscosity of gas density of the gas density of the liquid velocity gradient in gas (see eq 14)
Received for review June 25, 1993. Accepted December 2, 1993.@ Abstract published in Advance ACS A b s f r a c f s ,January IS, 1994.
AnalytlcaI Chemistty, Vol. 66,No. 5, March 1, 1994
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