Anal. Chem. 1990, 62, 1861-1866
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Variable Flow Rates and a Sinusoidal Flow Pump for Flow Injection Analysis Jaromir Ruzicka,* Graham D. Marshall,' and Gary D. Christian
Department of Chemistry, BG-10, University of Washington, Seattle, Washington 98195
The use of constant flow rates have, to date, dominated the theory and the praxls of flow Injection analysis. A cbser look at the theoretical prlnclples underlylng this technique suggests, however, that a reproduclble flow pattern Is the only prerequlslte for successful lmplementatlon of the this technique. Thls leads to a conslderatlon of stopped flow and flow reversal based on slnusoldal flow patterns, generated by a slmple robust cam-drlven computer-controlled plston pump. The reproduclblllty of the slnusoldal flow Injection system, designed prlmarlly for process control applications, Is experimentally demonstrated.
The trend toward automation and miniaturization of analytical instrumentation favors the use of flow-based methodologies. Many existing analytical techniques make use of computer-controlled sample manipulation in flowing streams with flow-through detectors. Examples of such established and emerging techniques are the coupling of flow injection analysis (FIA) with detection methods such as UV-vis spectrometry, atomic absorption spectrometry ( U S ) , inductively coupled plasma (ICP) with optical emission spectrometry (OES) or mass spectrometric (MS) detection, and Fourier transform infrared (FTIR) spectroscopy. These methods all rely on the injection of a well-defined sample zone into a carrier stream and the subsequent detection of a signal that has been modulated by a combination of physical and chemical interactions (1-4), and they must have a well-designed solvent delivery system. In this paper, following a short review of solvent delivery systems and the present status of flow programming in FIA, the random walk model is used to broaden the basis for the introduction of the sinusoidal flow concept for sample propulsion. The Experimental Section of the paper presents the details of the construction and the utilization of a sinusoidal flow pump and emphasizes its simplicity and its reproducibility.
FLOW PROGRAMMING IN FIA AND SINUSOIDAL FLOW The carrier stream transporting the injected zone toward the detector can be propelled continuously at a constant (monotonous) flow rate (1-41, at a linearly changing flow rate (positive or negative ramp) ( 5 , 6 ) ,or as suggested here, in a sinusoidal pattern. While abrupt steps (1) or linear ramps (5,s)were used in the past to facilitate the rapid washout of the system or the kinetics of dispersion and of chemical reactions (5, 6), the mechanical complexity of the pumping systems used then did not seem to outweigh the benefits of this approach. Thus, the use of a constant flow rate has been dominating the theory and the praxis of flow injection (1-4). It is difficult to say whether this has been due to trivial practicality, since the generation of a monotonous flow rate can be accomplished by a variety of means, or whether the Present address: Council for Mineral Technology (Mintek), Private bag x3015, Randburg, 2125, South Africa.
Table I. Characteristics of the Sinusoidal Flow Pump advantages
disadvantages
no pump tubing refilling reservoirs requires no check valves time and therefore inert components reduces sampling no pulses frequency not as flexible as perfectly reproducible flow pattern simple construction peristaltic pump low cost long maintenance-free lifetime inexpensive maintenance computer controllable adjustable flow rate accommodates higher back pressures stable flow rate quiet operation
use of a constant flow rate developed from analogy with chromatography or in air-segmented continuous-flow analyzers. Indeed, more than 95% of all flow injection methods developed so far use a continuous linear flow (Figure l, left top). Although for many applications this kind of simple operation is sufficient, it has a number of recognized disadvantages: (1) it is wasteful (the reagents are pumped continuously), (2) the requirement of increased reaction time implies the use of longer tubes, (3) the chemical kinetics are concealed within the physical process of dispersion, and (4)it is difficult (and costly) to maintain for long periods of time. Also, it is difficult to generate flow in a perfectly linear fashion (e.g., pulse free) and to maintain it for long periods of time (7). Flow programming avoids some of these drawbacks and can be summarized as follows: Stopped-Flow FIA (Figure 1,left center) was introduced (8)to decrease reagent consumption and to allow reaction rates to be directly monitored. With a selected section of the dispersed sample zone stopped in the detector, the physical dispersion and the chemical reaction rate are decoupled, allowing the measurement of reaction rates and the execution of kinetic assays (1-3, 9, 10). Reversed or oscillating Plow (11-14) has been applied to transport a sample zone repeatedly through the same detector, with the goal of monitoring reaction rate. The result is, however, a rather convoluted readout (11). Linearly oscillating flow has been found useful in improving the contact of the reactants in heterogeneous FIA systems such as enzyme reactors (12)and solvent extraction systems (13)and, most recently, for liquid/gas equilibration (14),in allowing the use of short flow channels. As already emphasized, the above alternatives have been achieved by using constant linear flow rates. However, each of the above flow programs can be accomplished on the basis of a sinusoidal flow rate (Figure 1, right). The only requirement to be met is that the injection and the flow pattern be reproducibly synchronized and maintained. The sinusoidal flow pattern comes to mind, since it is much simpler to generate by a piston pump than is a linear flow rate by using a
0003-2700/90/0362-1861$02.50/0 0 1990 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 17, SEPTEMBER 1, 1990 LINEAR FLOW
SINUSOIDAL FLOW
Figure 1. Schematic representation of various flow programs at a linear flow rate and a sinusoidal flow rate. Fwd, forward direction of flow: off, pump stopped; rev, reverse direction of flow.
u = l(n)'/2
Inject
Forward flow
4, pattern
Signal
Reverse flow
Cam driven syringe
"
..I
cycle
negative ramp), showed that, as long as the injection is synchronized with the start of the flow change cycle, reproducible results can be obtained. In this work, one step further is made by examining the dispersion process under a periodically changing nonlinear flow rate. Since dispersion is a random process, it can be described by the random walk model. While Giddings (15) used this model to describe the dynamics of chromatographic processes, it was Betteridge and co-workers (16,17) who used it to describe zone dispersion in FIA, yet their considerations were limited to conditions of constant linear forward flow. Briefly, let us consider the dispersing element of fluid as being displaced in any direction in a step of fixed length. Provided that there is an equal chance of moving in any direction, the random walk is symmetrical. Since all molecules within the said element of fluid cannot be expected to move through the same step of a Tied length (their movement is indeed erratic), the length of a fixed step must be taken as the average length of actual displacement (viz., its square root). Thus, if a large number of molecules start together on a random walk of many steps, the standard deviation, u, of the resulting (Gaussian) concentration profile is
cycle
Piston action
Figure 2. Conceptual representation of a sinusoidal flow pump.
circular cam-driven plunger (Figure 2). Such a simple and robust construction has a number of advantages over propulsion systems used thus far (Table I). At this point, the question arises why sinusoidal flow, if so simple and advantageous, has not yet been implemented to FIA. The answer is that not only the praxis but also the theory of FIA has, until now, been based on the assumption that a linear and preferrably constant forward flow is necessary for successful operation of this technique. This central assumption is examined and challenged in the next section where aspects of sinusoidal zero net flow conditions are discussed.
THE RANDOM WALK MODEL AND SINUSOIDAL FLOW The successful operation of a flow injection analyzer requires that the sample and the reagent be brought together, mixed, and allowed to react in a perfectly reproducible manner time and again. The practice of calibrating the analyzer and executing the analysis of unknown samples with intermittent recalibration necessitates this requirement. Following from this, nearly all techniques based on the injection of a sample into a flowing stream have used constant linear flow. The reason is that, as long as a constant linear flow rate is maintained throughout the entire measurement period, the sample may be conveniently injected into the system at any time. In this way, all injected samples disperse in exactly the same way and the ensuing chemical reactions proceed for exactly the same time period, before being monitored by a flow-through detector, regardless of when the sample is injected into the carrier stream. However, it was Rios et al. ( 5 ) and Toei (6) who, when using linearly changing flow rates (positive or
where 1 is a fixed step length and n is the number of steps taken. It is noteworthy that the zone spread-and consequently, the degree of mixing with surrounding carrier stream-is proportional to the step length, 1. This is easy to understand, since the final displacement of the molecule from the origin depends on the step length in one direction. However, the final displacement increases only with the square root of the number of steps taken, n, since a cancellation effect must be considered, due to the fact that the molecule is equally likely to move in the direction opposite to the original displacement. If several random steps occur simultaneously or consecutively, as they do in chromatography and in flow injection, the final displacement of any given molecule is determined by the sum of displacements in every direction. In chromatography, the key relation is the height of a theoretical plate, H = L / a 2 . This equation, in fact, gives a measure of the generation of variance per unit length of a column traveled by the dispersing zone. Clearly, the zone spreading increases with the square root of the distance traveled, L, while the separation on the components increases with the number of theoretical plates, N , unidirectionally traversed and therefore linearly with distance traveled (since NH = L for uniform plate dimension). Resolution, which is the key issue of chromatography, is obtained only when the gap between component zone centers is outdistanced by zone spreading. This situation is bound to happen sooner for lower values of H and higher values of L. This is exactly why no chromtographic separation will be achieved with no forward motion and why the symmetrical random process must overcome the asymmetry of forward flow, because the use of a unidirectional monotonous flow rate is the condition sine qua non for chromatographic separation. In contrast, FIA does not aim at separation of the analyte components but rather a t the effective derivatization of analyte molecules suitable for subsequent detection, a process which requires adequate interdispersion of sample and reagent molecules. Therefore, the key issue of FIA is reproducible dispersion of the injected zone within the carrier stream and the timing of the arrival of the reacted zone at the detector. The transition of the originally square input, through skewed peaks, to a final symmetrical shape is indicative of the degree of mixing of the reacting components, and this process has also been described by a random walk model in two papers published so far (16, 17). Yet these works have missed one key point, namely, that mixing of reacting components can
ANALYTICAL CHEMISTRY. VOL. 62. NO. 17, SEPTEMBER 1. 1990
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F l w a 3. shaoidal fbw pattern ganerated b y h rotation o l h cam: b. rotation through an arc horn 120 to 60'.
a, rotation through full 360':
be achieved without traveling a net distance L. Indeed, by moving the zone backward and forward in steps of equal length as described by the symmetrical random walk model, the inevitable result of complete mixing of zone and carrier stream components, as described by eq 1, will be achieved. Increase of I will increase the intermixing of the adjacent zones, while increase of n will promote their homogenization. By selecting the step length, 1, and the number of flow reversals, n, any degree of mixing and any desired length of reaction time can be conveniently achieved without the need to displace the sample a net distance, L, along the channel. T h e obvious consequence of this is that the consumption of reagents will be drastically reduced since no net flow means zero reagent (or carrier stream) consumption. In reality, this is not attainable, since the zone has to be transported from the injeetor into the detector through some minimum distance, L. Even if the detector were placed within the injector loop (1, 11-14), some net forward flow would have to be used to flush out the system prior to the next measurement cycle. Nevertheless, the importance of this consideration lies in the fact that it points the way to novel options in the construction of more effective and minaturized flow injection systems. Finally, having accepted the symmetry of the random zone movement and having abandoned the necessity for unidirectional flow, the flow pattern to which each individual step is subjected can be considered. Constant length steps that are traversed at linear flow at constant speed (I2-14,18)are one possibility, yet, as long as each step follows the same flow pattern, the use of sinusoidal flow can equally well be considered.
SINUSOIDAL FLOW INJECTION SYSTEM The integration of the sinusoidal flow pump with an electrically actuated multiport injection valve allows simultaneous
and synchronized sample zone injection and stream switching. This design eliminates the need for check valves, which are the most troublesome part of any syringe pump. The volumes of the syringes, the analyzer channels, the sample loop, and the detector system, as well as the pump speed, will have to be carefully matched with the range of desired flow rates and the chemistry to he performed. Following the initial experiments, a decision can he made on whether the full sinusoidal curve of flow rates or a selected arc will be used (Figure 3). In any case, though, the stroke volume will have to he larger than the combined volumes of the injector loop, the reactor, and the detector. The time interval selected for the piston movement will be at least as long as the measurement cycle. Note that all previous sinusoidal flow pumps designed for chromatography have a stroke volume that is a very small fraction of the volume of the system and, therefore, the full stroke time also is a very small fraction of the analysis time. A sinusoidal flow pump powered by a computer-controllable motor and coupled to an eight-port valve fulfills the essential functions of a flow injection system in a novel way (Figure 4). During the single revolution of the circular cam, two events take place sequentially (see Figure 2): the load cycle and the measurement cycle, which together comprise the analysis time. The direction of flow during these two cycles is opposite, and the flow rates are the mirror image of each other, together forming the familiar sinusoidal pattern. In the simplest case, two paired, synchronously moving syringes are used, coupled to an eight-port valve (Figure 4). These components are connected to a reactor (L) and a flow-through detector (D). During the load cycle, the backward movement of the two pistons aspirates the sample solution into the loop of the injector and the reagent from the reagent reservoir into the reagent syringe. When the backward limit of the piston is reached and the flow rate is zero, the valve is switched to the inject position and the measurement cycle commences with the forward motion of the pistons. As the flow rate increases according to a sinusoidal pattern, the sample zone is propelled through the reactor to the detector and finally is expelled from the system to waste (W). When
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ANALYTICAL CHEMISTRY, VOL. 62, NO. 17, SEPTEMBER 1, 1990
Table 11. System Parameters and Variables" syringe radius, mm reaction coil, mm tube diam, mm pump frequency, Hz a,, deg a2,deg stroke vol, mL flow rate at injection, mL/min flow rate at detector, mL/min maximum flow rate, mL/min
6.25 600 0.8 0.01 165 90
6.25 6.25 600 600 0.8 0.8 0.01 0.005 120 120 60 60
5.0 600
0.8 0.005 120 60
5.0 1000 0.5 0.005 120 60
1.78 1.80 6.60 6.94
1.84 6.01 6.86 6.94
1.18 1.92 1.89 2.22
1.18 1.92 2.19 2.22
1.84 3.00 3.43 3.47
The pump variable(s) that are changed from left to right in the table are given in bold type in the top block. The effect of this change on the stroke volume and the respective flow rates is given in the bottom block. the piston reaches the extreme discharge position, the flow rate having returned to zero, the valve is turned and the process is repeated for the next sample. In order to wash the sample and the spent reagent out of the flow injection system, the reagent syringe volume must be at least 4 times larger than the combined injection loop and sample aspiration line volumes. This will prevent any carry-over in the sample aspiration lines and the analyzer channels between measurements. The cam may be used to trigger an electrically actuated valve when the piston has reached either of its two extreme positions. While this simple approach will reliably secure perfectly reproducible synchronization of the sample zone injection with the flow pattern, computer control of the system offers several important advantages. (1)With the cam movement stopped after a suitable time delay following the sample injection, the flow is stopped, so that additional reaction time is gained for chemical derivatization to proceed. If the sample/reagent mixture is stopped within the flow cell, the reaction rate can be measured ( I , 8-1 0). (2) With the cam repeatedly oscillating after a suitable time delay following the sample injection, the sample will be pulsed back and forth. A small amplitude of repeated flow reversals will result in an oscillating zone within the reactor, ensuring mixing of the sample with the reagent and increasing the time for reaction. High-viscosity samples (as often encountered in process control applications) will be effectively mixed and derivatized. (3)Under computer control, it will not be necessary to rotate the cam unidirectionally through a full circle to accomplish the load and measurement cycle. By selecting a suitable arc of cam movement (Figure 3b), the piston stroke length, and therefore, the stroke volume, can be selected and synchronized with the valve switching. Also, the initial flow rate may be varied. Thus, the desired range of flow rates and the flow pattern can be selected without the need to change the cylinder and the piston or the pump speed. (4) Suitable software control will allow all of the above sample manipulations to be combined by designing a suitable flow program. Such a program could conceivably result in a system where a sample and reagent are aspirated, the sample injected, the flow oscillated within the reactor to provide thorough mixing of the reactants, the resultant mixture propelled into the detector where it is stopped for reaction rate measurements, and then finally, is discharged to waste.
EXPERIMENTAL SECTION Reagents. Bromothymol blue dye solution and sodium tetraborate carrier solution were prepared in the usual way ( 1 ) . Apparatus. The flow injection system depicted in Figure 4 was used in all experiments and consisted of the following components: a 10-port electrically actuated Valco valve (a suitable eight-port valve is not commercially available),a prototype Alitea
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variable-speed two-piston sinusoidal flow pump (Alitea USA, P.O. Box 26, Medina, WA 98039) equipped with disposable syringes (diameter 6.5 mm-see Table 11) and rigid syringe plungers with Teflon endpieces, an AB1 analytical-Kratos division Spectroflow 783 spectrophotometer, a Labpro-I analog-bdigital converter and control interface (Lab Data Systems, Seattle, WA), an IBM PC (compatible)personal computer, and software for data acquisition and pump and valve control (19). The injection valve was fitted with a 25-pL sample loop. A reaction coil of 100 cm (for 0.5mm-i.d. tubing) or 60 cm (for 0.8-mm-i.d. tubing) was woven around a reaction manifold to promote radial mixing. The spectrophotometer was equipped with optical cables that were connected to a tubular flow-through cell with a path length of 2 mm. The absorbance of the flowing stream was measured at 620 nm. The output from the detector was recorded on a Radiometer strip chart recorder and was also digitized and stored by the computer. Sinusoidal Flow Pump. The parameters of prime interest are the volume of a stroke, the volume of the sample loop, the volume of the flow system, and the flow rates at which the carrier and sample streams are propelled. Equation 2 gives the volume, V, of a stroke of the piston with radius, r, when the cam, which has a radius of R , is rotated through an arc of a2 - a1radians. a2is the starting angle of the cam, measured from the axis of the piston (see Figure 5), and a1is the ending angle and represents the point at which the injection of the sample takes place. V = xr2R(cos ( x - a l ) + cos a 2 ) (2) In the special case where a1 = x and volume of the cylinder is given by
a2 =
0, the maximum
VO = 2Rxr2 (3) Determining the flow rate, Q, at time, t , rests on the fact that VO = JoTI2Qdt. Now Q = QD sin a , and Qo is the maximum flow rate at cam position, a = x / 2 . For any angle of rotation, a, a = 2xvt, where v is the frequency of the pump in hertz. Integration between the limits of a half-revolution and substitution of eq 4 yield the flow rate at any angle, a. The relationship between the flow rates and the various dimensions of the pump is given by Q = 2Rx2r2vsin a (4) with the maximum flow rate being achieved when (provided that ail 5 r / 2 Ia 2 ) ,viz.
Q
= x/2
Qo = 2Rx2r2v (5) Using these equations, it is possible to design a system to meet the requirements of the particular method under development. Table I1 gives the values of some of the pertinent parameters at certain selected settings and serves to illustrate the wide range of values some of the important parameters may take on. Figure 3b demonstrates the flow pattern that is achieved when the pump is operated in an arc between 'I3. and 2 / 3 x . A significant characteristic of this pump is that, when the arc of rotation is chosen to be from 5 / 3 ~to 2 / 3 x ,the flow rate varies by less than 14% from the maximum ((sin x - sin 2/3x)sinx). This fact should be born in mind for systems where it may be desirable that the changes in the flow rate be minimized. Where a large change in flow rates is required, an appropriate arc of rotation may be chosen, say from 11/6xto x . The flow rate is proportional to the pump speed, and as with a peristaltic pump, this parameter is easily manipulated to provide a range of desired flow rates. The other variable that may be
ANALYTICAL CHEMISTRY, VOL. 62, NO. 17, SEPTEMBER 1, 1990
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1 min
0
I fi
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Time
I;
Figure 8. Typical peak shape obtained at sinusoidal flow (left) and calibration plot of bromothymol blue solution, where the following concentrations of bromothymol blue samples were injected in triplicate: A, 0.01 glL; 0, 0.02 g/L; C, 0.04 g/L; D, 0.08 g/L; E, 0.16 g/L.
altered to give a more significantchange in flow rates is the syringe diameter; the flow rate is proportional to the square of the syringe radius. Of course, changing this parameter will also have an effect on the stroke volume. The stroke volume must be balanced against the flow injection system volume in order to ensure thorough washout of the system after each measurement. The rule of thumb is that the stroke volume, as defined by the combined volumes of the sample loop, the reactor, and the detector, should be at least 4 times the system volume. The prototype pump is based on an Alitea C4-V peristaltic pump, which is fully computer controllable with respect to stop/forward/reverse and the pump speed. The rollers of the peristaltic pump were replaced by a circular cam, and the pump motor was regeared to give a gear ratio of 1:625.
RESULTS AND DISCUSSION A series of experiments was carried out to demonstrate the usefulness of the sinusoidal flow pump. First, the reproducibility of the pump was tested by the repeated injection of a dye sample into a buffered carrier stream (Figure 6). This experiment necessitated the optimization of various system parameters such as the tubing diameter, the pump speed, and the length of the reaction coil, resulting in the dimensions mentioned above. Parameters such as the cylinder volume, the arc of cam rotation, and the cam diameter were not varied for practical reasons. Once optimum conditions were identified, calibration standards were injected and the linearity of the obtained calibration curve was determined. In the second experiment, the viability of stopped-flow experiments was demonstrated by stopping the flow when a portion of the sample bolus was in the detector (Figure 7 ) . Because no reaction was taking place, the absorbance remained constant until the flow was resumed, whereupon the original base-line absorbance was achieved. A third experiment demonstrated the ability of the system to accommodate flow reversal for the purpose of extending the reaction time and promoting sample mixing with the carrier stream. In these experiments, the sample bolus was propelled into the coiled reaction tube and pulsed back and forth for a selected number of times (Figure 8). The amplitude of the oscillation was about 1 s, and the delay time before reversal was 2 s. In the fourth experiment, the long-term reliability of the pump was tested by allowing it to operate continuously for 24 h.
P
d
Figure 7. Stopped-flow experiment obtained at sinusoidal flow by the repeated injection of 0.08 g/L bromothymol blue solution (SI-S,) with a 3-s stopped-flow period after a 3.1-s (S&) and 3.5-s(S,-S,) delay after the moment of injection. Since a longer delay period results in the arresting of a more dilute element of the dye solution in the flow cell, the horizontal portion of the flow injection profile (stop period) exhibits lower absorbance than that with the shorter delay period. The high reproducibility of sequential injections (S1-SB and S,-S,) documents the perfect repeatability of zone dispersion at synchronized injection into the sinusoidal flow.
N=4
Time
Figure 8. Influence of sinusoaal flow reversal on the peak shape and the residence time. Peaks S,-S, were recorded by injecting a 0.08 g/L bromothymol blue solution. Note how the peak becomes progressively more symmetrical with increasing number of flow reversals
(N). Finally, the ability of the pump to operate against a higher than normal back pressure was tested. In the optimization of the system components, the diameter of the tubing was found to have a significant effect on the performance of the system. When 0.5-mm-i.d. tubing was
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used, the resultant underpressure during aspiration, due to the constriction imposed by the tubing, caused a serious deterioration in the precision of the system, since the evolution of dissolved gases from the solution during the reservoir loading cycle introduced an uncontrollable factor in the observed flow rates. Replumbing of the system with 0.8-mm-i.d. tubing resulted in improved precision but required shortening of the reaction coil to allow adequate flushing of the system. This is not perceived to be a limitation as it was shown in the third experiment that longer coils could be simulated by using flow reversal. The data depicted in Figure 6 present a typical recorder tracing of successive triplicate injections of five calibration solutions. An indication is also given of the shape of the peak obtained by using this pump. A calibration using dye samples with concentrations spanning 3 orders of magnitude (data not depicted graphically) gave a correlation coefficient of 1.000 when a linear least-squares calibration was applied. The precision of the system (including the pump) was determined by calculating the percent relative standard deviation of 12 injections of a dye solution, which gave an average absorbance of 0.604 absorbance unit. It was found to be 0.54%. Standard deviations of similar magnitude were obtained for less concentrated samples. In the stopped-flow experiments, in spite of a relatively high flow rate a t the time of detection, it was possible to reproducibly stop a portion of the sample bolus in the eye of the detector (Figure 7 ) . The height of the plateau is experimentally identical for each stopped-flow experiment. This confirms that, for kinetically controlled reactions, it would be possible to undertake kinetic assays. By stopping the sample bolus a t some point in the reactor, these tests demonstrate an excellent reproducibility of this method for increasing reaction time. The flow-reversal experiments illustrate the power of this approach for prolonging the reaction time and promoting sample mixing, as can be seen in Figure 8. The shape of the peaks with increasing number of reversals gives an indication of the improved mixing achieved when the flow was pulsed back and forth in the reaction coil before being propelled through the detector. This phenomenon has been discussed in detail in a recent publication (18). In the published work, however, a constant flow rate was generated by a peristaltic pump, and the flow was periodically reversed. The use of flow reversal to facilitate sample/reagent homogenization has the advantage that mixing perpendicular to the flow axis is promoted as the sample zone is folded back on itself with each reversal of the flow direction. A decrease in sensitivity caused by a decrease of peak height can, however, be compensated by measuring the peak area. In the pressure tolerance tests, a back pressure of 80 psi presented no problem to the pump. It is possible that higher pressures could be tolerated if necessary. When the pump was allowed to operate continuously for a period of 18 h, no deterioration of the system’s performance was observed. The relative standard deviation of the peaks obtained over this period was 0.63% at an absorbance level of 0.23 absorbance unit. The piston used is claimed by Alitea to be able to withstand 1OOOOO strokes. The most vulnerable component of the pump is the plastic syringe, which is easily replaced at a very low cost.
CONCLUSION The suitability of variable flow for FIA as generated by the described sinusoidal flow pump, together with the possibilities of performing stopped-flow or flow-reversal experiments, has been shown. The advantages of using this method of stream propulsion are the simplicity of the pump construction, the
absence of check valves, the ruggedness resulting in low maintenance, and the capability of handling aggressive liquids such as organic solvents and strong acids and bases. The extraordinary smoothness of all flow injection profiles (Figures 6-8) and the stability of the base lines, as well as the high reproducibilty of sample zone dispersion as documented by long-term repetitive dye injections, confirm the exceptional stability of the propulsion and injection system, making it suitable for process control applications. Although the present work deals only with the investigation, the optimization, and the verification of the physical parameters of the sinusoidal flow injection system by dye injection, it is believed, based on previous experience ( I ) , that the system will successfully handle a wide variety of chemical assays, since it is capable of bringing together readants in a highly reproducible manner. The drawback of the present system, compared to conventional flow injection schemes, is a lower sampling frequency, due to the fact that the sample and reagent aspiration cycle is as long as the measurement cycle, because the cam and the piston move at the same speed during both operations. Since some time is needed to load the sample, even in the conventional flow injection schemes, it can be estimated that the sampling rate of sinusoidal flow injection is less than halved. For process control applications, this reduction in sampling frequency is not a drawback, since in process control sampling frequencies in excess of one sample per minute are not desired. For laboratory applications, however, this factor should be considered along with all other attributes of the sinusoidal flow system (Table I). Finally, the use of sinusoidal flow propulsion for sensor injection and for chromatography is presently under investigation in our laboratory.
ACKNOWLEDGMENT The Council for Mineral Technology (Mintek) is acknowledged for its support of G.D.M. The staff of the chemistry department’s precision workshop is acknowledged for their willingness to make repeated modifications to the prototype pump, as is Alitea Sweden for donation of some of the pump components. LITERATURE CITED Ruzicka, J.; Hansen, E. H. Flow Injection Analysis, 2nd ed.; Wiley 8 Sons: New York, 1988. Valcarcel, M.; Luque de Castro, M. D. Flow Injectbn AnalysisPrinciples and Applbtiom; Ellis Hotwood: Chichester, 1987. Moller, J. Flow Injection Analysls ; Springer Verlag: Berlin/Heldelberg, 1988. Burgnera, J. L. Flow I n p i o n Atomic Spectroscopy; Marcel Dekker: New York, 1989. Rios, A.; Luque de Castro, M. D.; Valcarcel, M. Talanta 1985, 3 2 , 845. Toei, J. Talanta 1988, 35, 425. Clark, G. D.; Zable, J.; Ruzicka, J.; Christian, G. D. Tabnta, in press. Wada, H.; Sawa, Y.; Morimoto, M.; Ishuzuki, T.; Nakagawa, G. Anal. Chim. Acta 1988, 2 2 0 , 293. Ruzicka, J.; Hansen, E. H. Anal. Chim. Acta 1978, 9 9 , 37. Hungerford, J.; Christian, G. D.; Ruzicka, J.; Wings, J. C. Anal. Chem. 1985, 57, 1974. Olsen, S.; Ruzicka, J.; Hansen. E. H. Anal. Chim. Acta 1982, 136, 101. Ruz, J.; Rios, A.; Luque de Castro. M. D.; Valcarcel, M. Talanta 1986. 33, 199. Ruzicka, J.; Hansen, E. Anal. Chim. Acta 1988, 2 1 4 , 1. Canete, F.; Rios, A.; Luque de Castro, M. D.; Valcarcel, M. Anal. Chem. 1988, 60, 1540. Canete, F.; Rios, A.; Luque de Castro, M. D.; Valcarcel, M. Anal. Chlm. Acta 1989, 224, 127. Giddlngs, J. C. Dynamics of Chromatography;Marcel Dekker: New York, 1965. Betteridge, D.; Marczewskl, C. 2.; Wade, A. P. Anal. Chim. Acta 1984, 165, 227. Crowe. C. D.; Levin, H. W.; Bettrldge, D.; Wade, A. P. Anal. Chlrn. Acta 1987. 194, 49. Clark, G. D.; Christian, G. D.; Ruzicka, J.; Anderson, G. F.;van Zee, J. A. Anal. Instrum. (New York) 1989, 18(1). 1.
RECEIVED for review March 5,1990. Accepted May 16,1990.
