792 (28) (29) (30) (31) (32) (33) ~I
(34) (35) (36)
Anal. Chem. 1980, 52, 792-796 C. W. Allen, "Astrophysical Quantities", Athlone, London, 1963. C. H. Corliss and B. Warner, J. Res. Natl. Bur. Stand., 70A, 325 (1966). C. H. Corliss and J. L. Tech., J . Res. Nat. Bur. Stand., 71A, 567 (1967). F. P. Banfield and M. C. E. Huber, Astrophys. J., Part 7 , 186, 335 ( 1973). V. K . Prokof'ev, 2. Physik, 50, 701 (1928). Y. I. Ostrovskii. N. P. Penkin. and L. N. Shabanova. Sov. Phvs.-Dokl.. 3, 538 (1958). D. J. Halls, Spectrochim. Acta, Part 6 ,32, 221 (1977). D. J. Halls, Spectrochim. Acta, Part 6 ,32, 397 (1977). E. Hinnov and H. Kohn, J . Opt. SOC. Am., 47, 156 (1957).
(37) S.R. Koirtyohann and E. E. Pickett. Proc. 13th Coll. Spectrosc. I n t . , Ottawa, 1967,Hilger, London, 1968, p 270.
for review February 8,1979. Accepted January 23, 1980. Supported in part by the National Science Foundation through grants CHE-77-22152 and CHE-79-18073 and by the National Institutes of Health through G r a n t PHS GMl7904-06. Taken in part from the Ph.D. thesis of K.A.S. RECEIVED
Characterization of Droplet Sprays Produced by Pneumatic Nebulizers John W. Novak, Jr. and Richard F. Browner* School of Chemistry, Georgia Institute of Technology, Atlanta, Georgia 30332
The results of a comprehensive study of liquid droplet sprays from common atomic absorption and Inductively coupled plasma nebulizers are presented. Droplet size distributions In the 0.00- to 10-pm diameter range are measured as a function of nebulizer operating conditions. Special attention is given to the aerosol transport systems (e.g., spray chambers) and their effects on droplet distributions. An attempt is also made to correlate particle distributions with analytical sensitivity
.
T h e fundamental importance of the sample introduction process in analytical atomic spectrometry is well recognized. Consequently, i t is rather surprising that more effort has not been directed toward elucidating the mechanisms which control the sample transport process from the liquid bulk phase, through the intermediate aerosol state, into the flame or plasma. There are many aspects of the transport process worthy of detailed investigation but one fundamental property of droplet sprays which determines sample transfer efficiency, interference freedom, and measurement precision is the droplet size distribution (I-&?), A number of workers have recognized the importance of this factor but, unfortunately, the techniques used did not give reliable information on droplets less than approximately 10 km in diameter ( I , 4 , 5 , 8). T h e major part of the sample mass in analytical atomic spectrometry, with liquid sample introduction, is typically transported in droplets less than 10 pm in diameter. Consequently data from many of these earlier studies are not directly applicable to an understanding of nebulization related phenomena as they apply to analytical flames and plasmas. The importance of gaining accurate information about particle sizes has been recognized recently by Skogerboe and Olson ( 3 ) ,who examined desolvated sprays used for sample introduction into microwave and dc plasmas. The present study is a n attempt t o obtain reliable comparative data on droplet size distributions of sprays produced by a wide range of pneumatic nebulizers, using the techniques recently developed by the authors (9). EXPERIMENTAL Aerosol Monitoring System. The monitoring system and its associated components (sampling heads, dilution apparatus, 0003-2700/80/0352-0792$01,00/0
etc.) have been described previously (9). The major components are a cascade impactor (Andersen Model 2000) and an electrical mobility aerosol analyzer (Thermo-Systems). Unless specifically stated otherwise, dioctylphthalate (DOP) was used to generate the various aerosols. The accuracy of the measurement system has been estimated as &lo%, with a relative standard deviation of 2% (9). When nebulizers were used a t low liquid flow rates, i.e., less than 0.2 mL min-l, the droplet density was sufficiently low that the aerosol did not warrant further dilution prior to entering the cascade impactor. In these instances, the total aerosol spray was either taken directly into the monitoring system through a glass tube of the same diameter as the spray chamber or was passed through the spray chamber itself, with baffles, etc. in place. Aerosol Generation. Both atomic absorption (AA) and inductively coupled plasma (ICP) nebulizers were used in this study. The AA nebulizers used were: (1)A Jarrell-Ash concentric type with adjustable sample capillary, (2) a Perkin-Elmer concentric type, also with adjustable sample capillary. The Jarrell-Ash nebulizer was operated without an impact bead. The ICP nebulizers tested were: (1)A concentric all glass model commercially made by J. Meinhard Associates (Tustin, Calif.), and (2) two versions of the cross-flow nebulizer, made in slightly different forms by Plasma-Therm (Model TN-1) and Jarrell-Ash. Only two burner chamber assemblies were used: (1) A Perkin-Elmer AAS premix unit, and (2) a Plasma-Therm ICP concentric tube spray chamber (Model SC-2). A Plasma-Therm ICP torch (Model T1.O) was used in several experiments to test its influence on droplet size distribution. Data Presentation. Droplet size distribution graphs are plotted to show the log of the droplet diameter (d,) as a function of the mass distribution function (mass % / A log d,). It is important t o note that the mass distribution is a time integrated distribution, taken over an entire test run. It is obtained by first converting the average droplet count of the electrical mobility aerosol analyzer to a mass-per-size interval measured, then summing these masses and the masses from the cascade impactor stages. The mass percent of each size interval is then calculated, and finally normalized against A log d, which is the difference in logs of the maximum and minimum effect cutoff diameters (50% collection efficiency) of the size range. The droplet diameters measured with the present apparatus are aerodynamic diameters. However, as the droplets will be nearly spherical in the gas stream, and as DOP has a density of 0.981 g ~ m - their ~ , aerodynamic diameters will be effectively equivalent to their geometric diameters. For desolvation and solute vaporization processes, the geometric diameter is the relevant parameter, whereas for impaction or settling processes 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 6, MAY 1980
the aerodynamic diameter is needed. The near equivalence of the two diameters in the data presented herein simplifies the use of these data in calculations involving any of the above processes. I t should be emphasized that droplet distributions obtained with DOP as test liquid will differ from those obtained with aqueous solutions, as we have discussed previously (9). In general, the lower surface tension of DOP will result in distributions with a smaller median droplet diameter than that to be expected with aqueous solutions. Nevertheless, the trends observed should be very similar for DOP and aqueous samples when the influence of variables such as gas flow, mixer paddles, etc. are considered. The only other significant differences to be anticipated between data obtained with aqueous samples and with DOP relate to the relatively low maximum sample aspiration rate possible with DOP ( 1 1 . 2 mL min-I). This again will tend to favor smaller droplet formation with DOP than with water (11). Nevertheless, in spite of these limitations, the present technique is the only one to our knowledge which readily allows the measurement of size distributions in the range 0.1-10 pm with the high particle-numberdensity sprays produced by pneumatic nebulizers. V i t h each figure, values for the mass median diameter (a,) ana the geometric standard deviation (sg) are shown. The mass median diameters were calculated from plots of cumulative mass percent collected on the plates of the cascade impactor vs. droplet cutoff diameters for the collection stages. The mass median diameter corresponds to a cumulative mass of 50%. Geometric standard deviations were calculated as the ratio of the droplet diameters corresponding to cumulative mass percentages of 84.13 and 50.0, respectively. This relationship is strictly only true for distribution curves with a log normal shape, which is not always the case here. Nevertheless, the numerical values do give an indication of the degree of polydispersity of the distribution. Large sg values correspond to highly polydisperse aerosols.
793
ow O D
, ; prr
DiaFlet 0 1 5 ,d l a. m e , ttr
Figure 1. Effect of capillary position on droplet size distribution. Perkin-Elmer AAS nebulizer, natural sample uptake rate. 7 L min-l air flow at 180 kPa differential, 0.054 mL min-’ DOP flow. (e) 4.5 turns = 1.7 Wm, sg = 2.2. (0)0 turns on capillary. on capillary. = 1.5 Fm, s g = 2.3. Note: For explanation of terms, see Data Presentation section
a,
a,
RESULTS A N D D I S C U S S I O N AA N e b u l i z e r s . Two nebulizers, the Perkin-Elmer and Jarrell-Ash models, were subjected to a detailed examination t o determine t h e influence of all the normally variable parameters, such as capillary position, liquid uptake rate and nebulizing gas flow on the droplet size distributions. Effect of Sample Capillary Position. Perkin-Elmer Nebulizer. T h e effect of varying the position of the sample capillary in the throat of the nebulizer was first studied. Moving the sample capillary has the effect of varying the gas velocity in t h e region of the capillary tube end, and consequently changing the pressure drop across the capillary, thereby influencing sample uptake rate. In order to measure the effect of capillary position (i.e., nebulizing gas velocity) alone, the liquid uptake rate was regulated with a syringe pump. A Perkin-Elmer nebulizer was used for this study at a constant gas flow of 5.6 L min-’. T h e 0-turn position was designated to be with the capillary tube screwed fully into its furthermost position (extending slightly through the throat of t h e nebulizer body). T h e other extreme measurement position was with the capillary screwed 4.5 turns back from this position. This corresponds to a lateral retraction of 1.7 m m through the nebulizer throat. Droplet sizes measured at these two operating positions showed a shift in the mean from 2.4 pm a t zero turns to 1.9 pm a t 4.5 turns. Similar experiments were performed with the nebulizer aspirating a t its natural uptake rate. However, this procedure produced size distributions that were considerably different from those measured with the fixed uptake rate. As the sample capillary was moved from the 0-turn to the 4.5-turn position (higher air stream velocity), more sample was aspirated which caused a higher fraction of large droplets to be produced compared t o the previous experiment at fixed solution uptake rate. Consequently, the mass median droplet diameter remained essentially unchanged a t the various capillary positions. This is quite surprising when one considers the significant change in the shape of the distribution curves
1
0 2
05
Droalet diameter
,.m
Figure 2. Influence of liquid uptake rate on droplet size distribution. Perkin-Elmer AAS nebulizer. 5.6 L min-’ air flow at 124 kPa differential. ( 4 ) 0.054 mL min-’ DOP. 3, = 1.7 Fm, sg = 2.2. (0)0.16 mL min-’ = 1.9 k m , sg = 2.3. (m) 0.40 mL min-‘ DOP. = 2.1 DOP. Mm, s g = 2.4
a,
a,
as illustrated in Figure 1. As can be seen, the curve resulting from the 4.5-turn position has far more bimodal character than the curve from the 0-turn position. Also the curve obtained a t 4.5 turns shows a much greater mass of droplets with diameters greater than 4 pm. However, this mass is counterbalanced by the greater mass of droplets with diameters below 2 pm, which results in very little net shift of the droplet median diameter from the 0-turn value. As the critical vaporization and desolvation processes taking place in sample transport and atomization are dependent on rate-determined steps, which in turn are dependent on either the droplet diameter or the square of t h e droplet diameter ( 4 , IO), the simple numerical value of the median droplet diameter can be a very misleading means of estimating the likely magnitude of these effects. Clearly, unless the droplet distribution curves follow a symmetrical pattern, which in our experience they rarely do, it is essential to have a detailed plot to correlate observed flame and plasma phenomena and droplet size data. E f f e c t o f Liquid Uptake Rate. T h e influence of liquid uptake rate (controlled with a syringe pump) on droplet size distributions is shown in Figure 2 for a Perkin-Elmer nebulizer and in Figure 3 for a Jarrell-Ash nebulizer. As would be anticipated, a t the low liquid flow rates used in these studies
794 * ANALYTICAL CHEMISTRY, VOL. 52, NO. 6, MAY 1980
I
0.1
0 2
0.5
Droplet diavete‘
2
1
5
10
Figure 3. Influence of liquid uptake rate on droplet size distribution. Jarrell-Ash AAS nebulizer. 5.6 L min-’ air flow at 76 kPa differential. (A)0.012 mL min-’ DOP. a, = 2.2 pm, sg = 2.3. (M) 0.054 mL min-’ DOP. d, = 2.5 pm, sg = 2.1. ( 0 )0.16 mL min-’ DOP. 3, = 2.6 pm, sg = 2.0
0,
0 2
0 5
O r o p l ~ t dlarneter
2
5
0
pm
Figure 4. Influence of spray chamber on droplet size distribution. Perkin-Elmer AAS-nebulizer. (0)With chamber. 7 L min-’ air at 180 kPa differential. d , = 2.5 pm, sg-= 1.7. (+) Without chamber. 7
L min-’ air at 180 kPa differential. d , = 2.8 pm, sg = 1.6. ( 0 )With chamber. 4 L min-’ air at 69 kPa differential. a, = 3.3 p m , s g = 1.4. (A)Without chamber. 4 L min-‘ air at 69 kPa differential. d , = 3.3 pm, sg = 1.4
(total range 0.012-0.16 mL min-’ DOP), liquid flow has very little effect on the droplet size distributions measured. This is in accordance with the prediction of the Nukiyama and Tanasawa equation (11):
Do =
1850 (u)’/*
v,- Vl
+
4661 ( (
77 up)”2
)’“ (
1000 -
where Do= Sauter median droplet diameter (pm); Vg, VI = gas and liquid velocities, respectively (m s-l); u = liquid surface tension (N m-*); 11 = liquid viscosity (N s m-7; Qg,QI= volume flow rates of gas and liquid respectively (mL s-l); and p = liquid density (kg m-3). In this instance the gas velocity is so high (always supersonic) that the relative velocities of gas and liquid streams are not significantly altered by these changes in the liquid stream velocity. Influence of Nebulizing Gas Flow on Droplet Distributions. The effect of increasing nebulizing gas flow rate and, hence, gas velocity a t the sample introduction position is to shift the droplet distribution peak toward smaller droplets. For example, with the Perkin-Elmer nebulizer, a change in gas flow from 4 to 7 L m i d produces a shift in mass median diameter
2
0s
Dropler
vv
dlarneler
5
10
ym
Figure 5. Effect of nebulizing gas flow on droplet size distribution. Jarrell-Ash AAS nebulizer, natural sample uptake rate. (A)Gas flow, 10 L min-’. 3 , = 1.8 pm, sg = 2.3. ( 0 )Gas flow, 7.3 L min-‘. d , = 2.2 pm, sg = 1.9
from 3.3 to 2.8 pm as shown in Figure 4. This trend again agrees qualitatively with the predictions of the Nukiyama and Tanasawa equation. Effect of Perkin-Elmer Spray Chamber and Mixing Paddles. The spray chamber in an AAS spectrometer may play an important role in modifying the droplet size distribution produced by the nebulizer. In general, the bulk of the nebulized solution is lost a t this stage of the transport process and passes to waste. It is therefore of some interest to examine the efficiency of the spray chamber in removing large droplets from the aerosol stream and also to examine the influence of other components, such as mixing paddles on the droplet size distribution. In Figure 4,the droplet size distributions for a Perkin-Elmer nebulizer, operated with s n d without the spray chamber/ mixing paddle combination are shown. Curves are shown for the nebulizer operated under low and high gas flow conditions, which were selected to produce droplet sprays with inherently large and small populations of large diameter droplets, respectively. At high gas flow rates, the population of large droplets is relatively low, and the chamber/paddle assembly appears to make little difference t o the droplet size distribution. However, it should be remembered that the measurement system itself involves passage of the droplet spray through a glass concentric cylinder where the very large droplets will be lost by settling or impaction. This configuration was in fact selected deliberately in order to emulate to a degree the spray chamber in AA spectrometers. One can deduce from these curves, therefore, that the spray chamber itself performs the major function of large droplet removal from the aerosol spray, with the mixer paddles performing a relatively minor function in this respect. Furthermore, although removal of large droplets is relatively efficient under normal operating conditions, if the population density of large droplets is sufficiently great, there may be significant breakthrough t o the flame. This effect may be further emphasized in the case illustrated because the larger particles are a consequence of reduced nebulization gas flow. The droplets will therefore travel a t a lower velocity in the gas stream and the efficiency of any impaction loss of large particles will be significantly reduced. On the other hand, the lower gas velocity will result in a larger transit time for the droplets, and so gravitational settling losses could increase a t lower gas flows. This would tend to counteract the impaction losses discussed above. Which effect actually predominates will depend on the gas velocities of the nebulizer and auxiliary air flows through the burner chamber and burner, the geometries of the burner chamber and burner, the
ANALYTICAL CHEMISTRY, VOL 52. NO 6, MAY 1980
u-t
01
0 2
1
3 5
:rOPlef
Oldme'cr
5
0
"71
Flgure 6. Effect of gas flow rate on droplet size distribution. Plasma-therm crossflow nebulizer. (A)Gas flow, 1.6 L min-' (207 kPa differential). a, = 1.6 pm, sg = 2.1. (W) Gas flow, 1.2 L min-' (138 kPa differential). a, = 1.9 p m , sg = 1.9. ( 0 )Gas flow, 0.8 L min-' (69 kPa differential). a, = 2.0 pm, sg = 1.8. Liquid uptake rate set at 0.12 mL min-' with a syringe pump
droplet size distribution, and the droplet transit time through t h e burner chamber. Comparison between Perkin-Elmer and Jarrell-Ash Nebulizers. Samples of Perkin-Elmer and Jarrell-Ash nebulizers operated at the same nebulization gas flow rate (5.6 L min-') and sample uptake rate (0.054 m L min-l) were compared. Under these operating conditions, the Perkin-Elmer nebulizer produces a more favorable droplet distribution, with a higher proportion of small droplets. However, operating a t natural sample uptake rates and similar nebulizing gas flows (see Figures 1and 5 ) the Jarrell-Ash nebulizer produces a higher proportion of smaller droplets. This does not, however, result directly in a greater mass of sample reaching the flame per second because the higher transport efficiency of the smaller droplets produced by the Jarrell-Ash nebulizer is almost exactly counterbalanced by the great natural sample uptake rate of the Perkin-Elmer nebulizer (almost twice that of the Jarrell-Ash nebulizer). The net rate of sample transport for both systems is therefore very similar, as determined by the mass of sample collected per unit time on the plates of the cascade impactor. Correlation of Transport Efficiency, M e a n Droplet Size, and Analytical Signal. T o relate the information discussed above to a more realistic analytical situation, a n aqueous solution of NaCl was nebulized into a n air/acetylene flame using a Perkin-Elmer nebulizer. The nebulizer was operated at a fixed low liquid uptake rate but varying gas velocity by changing the sample capillary position in the nebulizer throat. T h e shift in droplet distribution with gas velocity change for t h e aqueous Na solution should be similar to that observed with DOP under the same operating conditions, even though the absolute position of the distribution curve for the aqueous solution will be shifted to larger droplet sizes than that for DOP. There was indeed found to be an inverse linear relationship between the Na emission signal and the median droplet diameter of the spray, over the range 1.9-2.4 pm. While the linearity of this trend is probably fortuitous, it does indicate clearly the importance of obtaining a small average droplet size in order to maximize sample transport efficiency to the flame and obtain optimum signals. ICP Nebulizers. Influence of Nebulizer Gas Flow. The effect of nebulizer gas flow on droplet size distribution for a concentric all-glass nebulizer, operating at its natural liquid uptake rate, and for a cross-flow nebulizer operating a t a fixed liquid flow rate were examined. Data for the crossflow neb-
795
ulizer are shown in Figure 6. The effect of increasing gas flow in both instances was to decrease the average droplet size considerably, as a consequence of a reduction in the density of large droplets and an increase in the density of small droplets. All the curves are unimodal. A cery interesting comparison with the AA nebulizers, in addition to the lack of bimodality in the curves, is the shift to smaller average droplet size with increasing nebulizing gas flow when liquid uptake is a t its natural rate. T h e Perkin-Elmer AA nebulizer, on the other hand, maintains a droplet distribution that is virtually unchanged by increasing gas velocity. The shift to smaller droplet sizes with the ICP nebulizer arises largely because the natural liquid uptake rate with the ICP nebulizers is significantly less (4 to 5 times) than with the AA nebulizers. Consequently, the liquid flow rate is never sufficient, under natural uptake conditions, to cause the formation of large droplets which would counterbalance the effect of the increased production of smaller droplets, as with the Perkin-Elmer AA nebulizer. Effect of Liquid Uptake Rate. An increase in liquid flow rate from 0.12 to 1.2 mL min-' produced a significant change in mass median diameter with the crossflow designs. For the Plasma-Therm nebulizer, the median droplet diameter increased from 1.9 to 2.5 pm over this flow rate range, with the nebulizer operated at 138 kPa pressure differential (1.2 L m i d Ar flow). This could be of some relevance to attempts to couple high performance liquid chromatographs and ICPs. Clearly, a drastic reduction in mobile phase flow can lead to significant improvements in droplet production characteristics. Effect of Droplet Settling Chamber. ICP nebulizers typically require the removal of large droplets from their polydisperse spray with a concentric chamber. The mode of operation is probably by gravitational settling of large droplets, leaving smaller droplets to pass to the plasma torch. T h e chamber was shown to be effective in its operation, by a comparison of droplet size distributions with and without the chamber. Addition of the plasma torch to the assembly resulted in a further slight shift of the curve to smaller droplet sizes, indicating that some impaction on the torch surfaces occurs, resulting in preferential removal of large droplets. The constriction at the top of the sample introduction capillary is the most likely site for this process to occur.
CONCLUSION The trends found by this study indicate the following: (1) AA concentric nebulizers produce a very polydisperse droplet spray. The distribution shifts to lower droplet diameters when these devices are operated a t high nebulizing gas flows and/or low liquid flows. (2) AA spray chambers aid in removal of large droplets at high uptake rates of sample solution; however, their effectiveness is questionable a t low solution uptake as would be the case if AA is used for liquid chromatography decection. (3) ICP nebulizers produce droplets with relatively narrow size distributions. Lower solution uptake rates resulted in smaller spray droplets with both ICP nebulizers tested. In contrast to AA nebulizers, the combination of higher nebulizing gas flow and natura! solution uptake rate produces a droplet distribution with a smaller mass median diameter. (4) The concentric spray chamber designed for use with ICP nebulizers appears to be a valuable aid in the removal of large droplets from the sample aerosol. ACKNOWLEDGMENT T h e loan of nebulizers and spray chambers by the Perkin-Elmer Corporation and by the Jarrel-Ash Ilivision, Fisher Scientific Company, are gratefully acknowledged. LITERATURE CITED (1) Willis, J. 6.Spectrochim. Acta, Part A 1967, 811. (2) SzivBs, K.; Pblas, L.; Pungor, E. Spcctrochim. Acta, Part B 1976, 289.
796
Anal. Chem. 1980, 52, 796-800
(3) Skogerboe, R . K.; Olson, K. W. Appl. Specfrosc. 1978, 3 2 , 181. (4) Hieftje, G. M.; Malmstadt, H. V. Anal. Cbem. 1969, 47, 1735. (5) Stupar, J.; Dawson. J. B. Appl. Opt. 1968, 7 , 1351. (6) Dean, J. A.; Carnes, W. J. Anal. Cbem. 1962, 3 4 , 192. (7) Alkemade. C. Th. J.: Hermann. R. "Fundamentals of Analvtical Flame Spectroscopy", Wiley: New York, 1979; Chapter 4. (8) Kolrtyohann, S.R.; Pickett, E. E. Anal. Cbem. 1966, 38, 1087. (9) Novak, J. W. Jr.: Browner. R. F. Anal. Cbem. 1980, 52, 287. (10) Bastiaans, G. J.; Hieftje, G. M. Anal. Cbem. 1974, 46, 901. I
( 1 1) Nukiyama, S.; Tanasawa, T. "Experiments on the brtomization of Liquids in an Air Stream", Hope, E., Transl.; Defense Research Board, Department of National Defense: Ottawa, Canada, 1950.
RECEIVED for review September 6, 1979. Accepted January 31, 1980. This material is based on work supported by the National Science Foundation under Grant No. CHE77-07618.
Microcomputer-Automated On-Line Reagent Dilution System for Stopped-Flow Instrumentation Scott Stieg' and Timothy A. Nieman" School of Chemical Sciences, University of Illinois, Urbana, Illinois 6 180 1
A simple, yet elegant approach to reagent dilution has been developed to reduce the time requlrements for manual preparation of reagents. The system consists of three-way proportioning valves and homogenlzlng mixers that are on-line wlth the sample loops of a stopped-flow delivery device. Alternate slugs of reagent and diluent are delivered by the valve to the homogenizing mixer. The reagent to diluent ratio Is determined by the duty cycle of the valve control waveform. The system is controlled by an 8080-based microcomputer. Characterization has shown most aspects of instrument performance to center about the choice of the perlod of the valve contrd waveform. The lower limit for this period is determined by the valve response time. The upper limit is determined by the ability of the mixer to homogenize the delivered volumes of reagent and diluent. Dynamic range of dilution varies between 17 and 91 dependlng on flow rate. Dllution predslon Is typically 0.2-0.5 % RSD.
It is a common situation in analytical research to be involved with optimization of reagent concentrations and construction of working curves, both of which devote significant time to the preparation of the many necessary solutions. Such a situation exists in our work in analytical chemiluminescence (CL); the number of required solutions is particularly extensive in those studies involving simplex optimization of reagent concentrations, and generation of CL response profiles as a function of reagent concentration (1-3). T o alleviate this aggravation, we have developed a microcomputer-controlled CL research instrument which integrates a stopped flow delivery system with a unique reagent dilution system based on a digital approach to dilution. The philosophy of the method and the general aspects of the design are totally adaptable t o most situations where automated reagent preparation/ dilution systems are desirable. In addition t o the obvious goals of good accuracy and precision, our design goals for the dilution system included ease of computer control, on-line operation, small size, a minimum of moving parts, and low cost per channel. The last three objectives stem from the requirement t h a t our system have four channels to prepare the four reactant solutions for 'Present address: Laboratory of the Government Chemist, Cornwall House, Stamford St., London, SE19NQ, United Kingdom. 0003-2700/80/0352-0796$01 .OO/O
the CL reactions we have been studying (e.g., gallic acid, OH-, H202,analyte). Automated sample preparation methods have been reviewed (4-6)and fall into two classifications: continuous flow and discrete manipulations. T h e discrete methods essentially mimic manual operations and are by nature off-line. A four-channel system of this type would require a multitude of moving parts and analog measurements (volume of solution displaced or weight of solution delivered). In continuous flow systems, the ratio of reagent flow rate to diluent flow rate is the ratio of reagent dilution after mixing. A four-channel system using standard continuous flow technology would require eight variable flow devices (infusion pumps, peristaltic pumps, etc.): four for reagents and four for diluent, with possibly a flow rate transducer for each to provide feedback, plus nontrivial calculation to keep track of the resultant reagent concentrations in a flow cell being fed by eight lines a t different flow rates. A continuous flow dilution method can be devised in which the dilution ratio is not determined by a ratio of flow rates but rather by the duty cycle of a proportioning valve. This paper describes the design and characterization of such an instrument. The paper following this one discusses work done with that instrument to investigate the CL reaction of gallic acid for possible use in simultaneous multicomponent analysis.
THE INSTRUMENT The reagent dilution and stopped-flow delivery systems are shown schematically in Figure 1. They are subunits of our total CL research instrument. Stopped Flow Delivery System-. The reagent delivery system shown in Figure 2 is based on a system described by Pardue (7) in which the syringes serve only to contain water or other solvent which drives the actual reagents from sample loops. The reagent solutions and analyte enter from the reagent dilution system described below and are drawn through the sample loops (1.5-mm i.d. Teflon tubing) by a vacuum into a vacuum ballast waste container, also described below. When the preparation/rinsing cycle ends, the threeway valves Al-A4 and B1-B4 (Angar Scientific, Model 2503-12) open the sample loops on-line with the syringes. The syringes are filled with a driving/rinsing solvent (water in our work). The check valves (Lee Co., Model TCKA 6201050A) ensure proper flow into and out of the syringes. When the reaction is to be initiated, the syringes deliver, forcing the water into the sample loops and displacing the reagents first into the reaction mixer and then into the flow cell for CL 0 1980 American Chemical Society
