Report
Flow
Injection Techniques for Water
Monitoring
916 A Analytical Chemistry, Vol. 66, No. 18, September 15, 1994
On-site automated FI monitors provide near-continuous, reliable, and low-cost data for assessing water quality
S
ince its development in the mid1970s, flow injection (FI) has be come a routine laboratory tech nique for sample analysis and on-line sample treatment (1,2). It is capable of a fast response (typically 10-120 s) and a high sample throughput (typically 30120/h) with low reagent consumption (typical flow rates are 0.5-2.0 mL/min) and low operational costs and is easily adapted to automated analysis. Such fea tures are well suited to the requirements of industrial process analysis, and the po tential of FI for on-line process monitoring has been discussed (3, 4). Recently, interest has also focused on the use of FI for in situ monitoring of envi ronmental matrices, particularly natural waters. With the trend toward increas ingly stringent legislation pertaining to the quality of surface water and ground water, the incidence of transient pollution events, and the desire to understand complex biogeochemical cycling pro cesses, there is a growing need to obtain reliable quantitative analytical data in situ. Automated FI monitors operating in the field represent a low-cost option for obtain ing near-continuous, quantitative data for a wide range of dissolved aquatic chemi cal parameters. In this Report we outline the basic features of FI that have made it so attractive for laboratory and process analysis and discuss how these charac teristics are applicable to monitoring natu ral and polluted waters.
Reagent consumption is generally low in FI systems (an important consideration for field applications) and can be re duced still further by using reagent injec tion manifolds, as shown in Figure 2. Re verse FI, as this configuration is often called, is suitable for applications in which the sample is in abundant supply (as is the case in many environmental situa tions) and is particularly useful when ex transported to a flow-through detector for pensive reagents are necessary. Reverse FI also minimizes the quantity of re measurement. The response is in the form of a peak, the height of which is usu agents) discharged to waste, which is ad vantageous if environmentally question ally directly related to analyte concentra tion. The degree of sample dispersion is able reagents are used. highly reproducible and is controlled by Simultaneous FI can be performed by factors such as flow rate, manifold geome designing split-line manifolds in which the try, and tubing length and diameter. sample is injected into more than one flow channel and undergoes a different A simple FI manifold (Figure 1) typi cally consists of a propulsion unit (such as reaction in each channel. This can be achieved either by splitting the carrier a peristaltic pump), a six-port rotary sam ple injection valve, and a flow-through de stream after injection or by connecting two injection valves in series in two separate tector (such as a spectrophotometer). reaction systems. Other components that Narrow-bore poly(tetrafluoroethylene) can easily be incorporated into FI systems (PTFE) tubing (typically 0.8 mm i.d.) is used for sample and reagent transport, and include gas dialysis units, for the diffu sion of a gaseous analyte from a carrier tightly wound coils are often included to aid mixing. A number of options are avail (donor) stream through a microporous membrane into a reagent (acceptor) able for the basic FI components (Table 1). The manifold shown in Figure 1 is a stream, and solid-phase reaction columns, in which the injected sample reacts with single-channel system in which the car rier stream (which can also contain a re (or selected components are retained by) a column packed with solid material. agent) transports the sample to the de tector. If the method requires more than one reagent, additional streams can be Process FI merged with the carrier stream at suit Recently there has been considerable in able points in the manifold. terest in process analytical chemistry. The ability to acquire analytical information Basic principles during the various stages of an industrial FI was first described by Ruzicka and process is seen as a means of increasing Hansen (5) as an unsegmented flow tech Kevin N. A n d r e w efficiency and maximizing economic per nique in which a volume of liquid sam Nicholas J . Blundell formance (6). Process analytical chemis ple (typically 10-200 μι) is inserted into a David Price try can also play a role in the environ moving liquid carrier stream, where it Paul J . Worsfold mental control of industrial processes undergoes physical dispersion as it is University of Plymouth
0003-2700/94/0366-916A/$04.50/0 © 1994 American Chemical Society
Analytical Chemistry, Vol. 66, No. 18, September 15, 1994 917 A
Report when applied to the on-line monitoring of gaseous and liquid emissions. The requirements of an on-line process monitor include rapid analysis and high sampling frequency; robust construction to withstand harsh chemical matrices; a simple design that can be easily maintained; the ability to perform automated, unattended analyses and undertake regular self-calibration; and minimal capital and operating costs. FI can meet all of these requirements and can be applied to a wide range of chemical parame-
Peristaltic pump
Sample Reagent -
ters. In addition to single-parameter moni- variables in response to feedback from the toring, FI has the potential for multicomon-line monitor. The monitor can be deponent process monitoring (7), parsigned to incorporate sample pretreatticularly in combination with multichannel ment procedures such as filtration, diludetection systems and chemometric softtion, and preconcentration. Solenoid ware routines. valves can also be incorporated to switch Figure 3, a typical on-line process FI between the sample and standards for regmonitor, shows how each component is ular self-calibration. controlled automatically by a simple sinProcess FI applications that use molecgle-board computer, which in turn commu- ular spectroscopic detection (Table 2) nicates with a central process control have been discussed in a number of publicomputer. This arrangement allows appro- cations (8-24). The primary applicapriate adjustments to be made to process tions, which demonstrate both the versatility of FI and its ability to withstand harsh sample matrices, are in the areas of biotechnology, industrial chemistry processes, and water quality. One of the most promising new applications of FI is its use for front-end sample treatment and deDetector livery for spectroscopic detection (e.g., Mixing coil inductively coupled plasma MS). •Waste Environmental monitoring
Injection valve Results
Figure 1. Simple single-channel FI manifold.
Table 1 . Options for a FI system Component
Options
Description
Propulsion system
Peristaltic pump
Set of rollers on a revolving drum that squeezes flexible tubing to produce a constant, pulsing flow
Sinusoidal flow pump
Cam-driven piston that produces a variable pulseless flow
Gas-pressure reservoir
Pressurized inert gas vessel connected by a flow regulator to each reagent or carrier reservoir, producing pulseless flow
Rotary valve
Six-port unit with sample loop; electronic or pneumatic operation
Injection system
Hydrodynamic injection Selective stopping and starting of both sample and reagent pumps; sample enters reagent stream while the reagent pump is stopped and is then transported to the detector when it is restarted
Detection system
Syringe
Manual injection through septum
Optical
UV-vis, diode array, and IR spectrophotometry; solid-state photometry (LED source, photodiode detector); fluorometry; chemiluminescence; atomic spectrometry
Electrochemical
Potentiometry (ion-selective and pH electrodes), conductimetry, amperometry, coulometry, voltammetry
918 A Analytical Chemistry, Vol. 66, No. 18, September 15, 1994
The traditional approach to monitoring dissolved chemical species in natural waters involves periodic manual sample collection followed by laboratory analysis. However, this approach is susceptible to errors arising from contamination and/or degradation of the sample during collection, transportation, and storage (25). In addition, it is very time consuming and labor intensive, particularly when large numbers of samples are involved, and only provides intermittent data, which may exclude short-term fluctuations of environmental significance. The ability to conduct analyses in situ and provide a pseudo-continuous profile of chemical fluxes is therefore important, not only because of the limitations of the conventional approach but also because of escalating legislative restrictions and the increasing need for greater management and control of industrial, environmental, and agricultural processes. Approaches to in situ monitoring
Syringe-based, fixed-site monitors that incorporate wet chemistries and spectrophotometric detection are commercially available for the determination of a variety of chemical species. Deployment of this type of instrumentation is restricted, however, by its power requirements and lack of portability. Consequently, the majority
of currently available portable instrumentation for monitoring chemical water quality parameters in situ is based on coupling ion-selective electrodes with datalogging devices. The ion-selective electrode approach has recently been used by the U.K. National Rivers Authority to develop small, hand-held meters and larger monitors intended for long-term field deployment (26). However, the sensing surface is prone to biological fouling, and electrodes are available only for a limited number of chemical species. Biological early warning systems (i.e., monitors incorporating living organisms) have also been proposed for fixed-site deployment. Their operation is based on a pollutant-induced change in the status of the organism, such as enhanced movement or ventilatory frequency, and these monitors can be useful as general indicators of water quality. However, they are not sufficiently selective to provide the specific chemical information that is often required. FI, particularly in conjunction with solid-state detection systems, can potentially offer a rugged, portable, and selective alternative to the approaches mentioned. In situ FI instrumentation
Solid-state photometric detectors (27) incorporate light-emitting diodes as sources (and photodiodes as detectors) and are now available at minimal cost for most of the visible region of the electromagnetic spectrum. These low-power, low-cost detectors have enabled standard FI spectrophotometric methods to be adapted for field applications. Power consumption is an important consideration for an in situ monitor. For deployments of up to 30 days, batteries provide the most rugged and reliable power supply. A range of nonrechargeable cells, such as zinc-carbon, lithium, and alkaline manganese, and rechargeable cells, such as nickel-cadmium and lead-acid, are commercially available. Rechargeable cells based on the traditional lead-sulfuric acid cell are the most widely used infield-basedequipment because of their low cost, large range of available power capacities, and excellent recharging capability. Their main limitation is their power-to-weight ratio, which reduces
portability for longer term deployments (e.g., more than one week of intermittent operation). In some remote locations, solar panels are a cost-effective and reliable power supplement to batteries. The microcomputer control/data acquisition system can be a significant part of the total cost and power requirement of an in situ monitor. However, modern CMOS-based microcontrollers have an average power consumption in the microamp range, which is negligible in compari-
son to the power required by the system's components (typically 200-400 mA). Sample delivery to and data transmission from the monitor are important aspects of in situ measurement; biofouling and suspended solids are the two most significant problems. Sample delivery and data transmission generally involve the incorporation of commercially available inlinefiltrationdevices; biofouling can be managed by including biocides in the reagent streams. Various options are avail-
Reagent 1
Carrier Injection valve
Mixing coil
Detector Waste
Reagent 2 Mixing coil Diluent
Sample
Figure 2. Four-channel FI manifold with reagent injection.
Standard
Switching valve
Pump
Waste
Sample delivery and pretreatment
Injection waive / Reaction manifold
Reagents
Detector
Single-board computer Local output / Dynamic \ chemical process
Central process control computer
Figure 3. Process FI monitor. Solid lines represent the flow of liquid. Dotted lines represent communication links. Analytical Chemistry, Vol. 66, No. 18, September 15, 1994 9 1 9 A
Report able for remote communication: satellite, land-line, VHF/UHF radio, and cellular telephone. The particular method used depends on the specific requirements of a given application, such as cost, monitor location, and frequency of transmission. The industry standard output from on-line monitoring devices is a 4-20-mA loop, but an RS-232 interface can also be used. Freshwater monitoring
Within the freshwater environment, key application areas for in situ FI monitoring include effluent discharges (point and diffuse sources), supply intake, data acqui sition for environmental databases, nutri ent budget studies, and leachate monitor ing of soils and landfills. Each of these de ployment scenarios places different requirements on the design consider ations of any monitoring system. For ex ample, iffixed-sitemonitoring is appropri ate for a point discharge into a river or within a water treatment plant, ac power is likely to be available. In contrast, for nutrient budget studies at remote locations or for soil leachate monitoring, a portable battery-powered monitor would be essential. In the latter context, in situ monitoring is particularly important for those species that are highly mobile and pass relatively easily between soil water, groundwater, and surface wa ter, because a conventional (manual)
monitoring scheme may not detect shortterm changes, such as those caused by storms, between sampling intervals. A battery-powered, microcontrollerbased FI monitor has been shown to pro vide a versatile and reliable system for the determination of chemical parameters in natural waters (28) and, when used in con junction with a miniature microporous suction cup, has the potential for measur ing chemicalfluxesin soil cores. Figure 4 shows the monitor and a concentration profile that depicts the passage of nitrate fertilizer through a core of saturated clay soil. The detector incorporates a green light-emitting diode (Amax = 565 nm) to detect the pink diazo compound (Xmax = 542 nm) formed from the reaction of ni trite (produced by the reduction of nitrate using a copperized cadmium column) with sulfanilamide and JV-1-naphthylethylenediamine. Marine monitoring
The in situ monitoring of marine environ ments (estuarine, coastal, and open ocean waters) can involve shipboard tech niques or remote instrumentation, includ ing submersible and buoy-mounted de vices. As mentioned earlier, the major problem with the conventional "send-it-tothe-lab" approach is contamination and/or degradation of the sample prior to analysis. This is generally more serious
Table 2. Process FI applications Area Chemical production
Analyte
Azo dyes Morphine Metal production lron(ll and III) Aluminum in steel Paper production Calcium Fish farming Ammonia and nitrite Hydroponic cultivation Nitrate Wastewater monitoring Phosphate, ammonia, and nitrate Total phosphorus Treated water monitoring Fluoride Aluminum and iron Biotechnology L- Phenylalanine Glucose, lactic acid, and protein Ammonium, glucose, and protein Acetate and phosphate β-Galactosidase Penicillin V
Detection method
Reference
Spectrophotometry Chemiluminescence Spectrophotometry Spectrophotometry Spectrophotometry Spectrophotometry LED photometry Spectrophotometry
8 9 10 11 12 13 14 15
Spectrophotometry Fluorometry LED photometry Fluorometry LED photometry
16 17 18 19 20
Diode array
21
Spectrophotometry Spectrophotometry LED photometry
22 23 24
920 A Analytical Chemistry, Vol. 66, No. 18, September 15, 1994
for marine samples than for freshwater samples because of the lower concentra tions of analytes and the high and variable dissolved solids content of the matrix (29). The refractive index of the matrix can also be particularly problematic for wet chemical methods that use optical de tection. The remoteness of open ocean locations also means that sampling is par ticularly expensive. Real-time data sup plied by in situ monitoring can maximize the spatial and temporal information ob tained. FI techniques therefore offer the same advantages for marine sample analy sis as they do for freshwater sample anal ysis. Applications for laboratory and ship board monitoring have been comprehen sively reviewed (30, 31). Since the early 1980s, a wide variety of analytes have been successfully deter mined by FI with optical detection on board research vessels (32-40), as shown in Table 3. Thefirstshipboard application was a reverse FI method (reagent injec tion) for the determination of nitrate or nitrite, in which seawater was pumped di rectly into the manifold (32). Since that time, several wet chemical procedures based on UV-vis, fluorescence, and chemiluminescence detection have been reported. The logical progression from ship board monitoring is the deployment of submersible systems. This approach has been pioneered by Johnson (41) via the use of "scanners" (submersible chemi cal analyzers) capable of housing continu ous flow (as opposed to FI) manifolds (e.g., for the determination of silicate and sulfide in the Rose Garden hydrothermal vent field of the Galapagos Rift). The manifolds were pressure compensated and oil filled to withstand depths of up to 2500 m. Submersible monitors such as these prevent contamination and/or sam ple degradation by eliminating sample transportation to the ship. The main ad vantage, however, is the much greater number of samples that can be analyzed in situ as compared with the limited num ber of samples that can be collected and returned to the ship in a single day. This has been demonstrated by another sub mersible continuousflowanalyzer (42) used for the determination of nitrate. The analyzer used positive displacement pumps rather than peristaltic pumps. The
high-definition depth profile data obtained from a cruise in the Sargasso Sea are in excellent agreement with data obtained from bottle-collected samples from the same location. Groundwater monitoring
FI instrumentation can potentially be used for in situ groundwater monitoring, provided the sample is pumped to the surface. However, there is a growing demand for monitoring devices that are physically located within boreholes. At present, FI monitors cannot meet this requirement because of their physical size (the typical diameter of an inspection borehole is 2 in.). Nonetheless, recent advances in the miniaturization of FI components (e.g., micromachined silicon structures incorporating pumps and flow manifolds) suggest the feasibility of in situ FI monitoring within boreholes in the foreseeable future (43).
12 10
Conclusions
1
8
I
6
•5
4 2
0
2
4
6
8
10
12
14
Sample number
Figure 4. Determination of nitrate concentration gradient in soil. Top: Battery-powered microcontroller-based monitor. Bottom: Concentration profile depicting the passage of nitrate fertilizer through a core of saturated clay soil.
T a b l e 3 . Shipboard a n d submersible a p p l i c a t i o n s of FI Area of deployment
Analyte
Detection method
Santa Barbara channel Gulf Stream Mediterranean Santa Monica Basin Mid-Atlantic Ridge California coast California coast Juan de Fuca Ridge Cariaco and Manus Basins
Nitrate and nitrite Ammonia Hydrogen peroxide Cobalt Manganese and silicate Manganese Copper Iron and manganese Silicate, phosphate and sulfate
Spectrophotometry Spectrophotometry Spectrophotometry Chemiluminescence Spectrophotometry Chemiluminescence Chemiluminescence Spectrophotometry Spectrophotometry
Reference 32 33 34 35 36 37 38 39 40
FI is now accepted as a powerful, routine tool for the automation of wet chemical methods in the laboratory and is being increasingly used in process situations. The full potential of the technique for in situ environmental monitoring, however, has yet to be realized. Novel chemistries are required to provide lower detection limits and information on chemical speciation. Nonetheless, advances in computer hardware, software, power sources, communication systems, solid-state light sources, detectors, and other hardware, together with the proven versatility and reliability of FI, have made such deployments feasible for the monitoring of natural and polluted waters. We would like to thank the following organizations in the U.K. for their support: ICI Chemicals and Polymers, Runcorn; ICI Engineering, Winnington; Brixham Environmental Laboratories (Zeneca pic), Brixham; Institute of Freshwater Ecology, Wareham; and Plymouth Marine Laboratory, Plymouth. We'd also like to thank AFRC/NERC (grant number GST/02/ 587) and NERC (reference number GT4/91/ MPS/43). References
(1) Ruzicka, J. Fresenius. Z. Anal. Chem. 1986,324,745. (2) Valcàrcel, M.; Luque de Castro, M. D. Flow Injection Analysis: Principles and Application; Ellis Horwood: Chichester, U.K., 1987.
Analytical Chemistry, Vol. 66, No. 18, September 15, 1994 921 A
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(3) Benson, R L.; Worsfold, P. J.; Sweeting, (41) Johnson, K S.; Beehler, C. L; SakamotoF. W. Anal. Proc. 1989,26, 385. Arnold, C. M. Anal. Chim. Acta 1986,179, 245. (4) Luque de Castro, M. D. Talanta 1989, (42) Taylor, C. D.; Howes, B. L.; Doherty, K. W. 36,591. Mar. Technol. Soc.J. 1993,27, 32. (5) Ruzicka, J.; Hansen, Ε. H. Anal. Chim. (43) van der Schoot, B. H. et al. Anal. Method Acta 1975, 78,145. Instrumen. 1993,1,38. (6) Callis, J. B.; Illman, D. L.; Kowalski, B. R. Anal. Chem. 1987,59, 624 Λ (7) MacLaurin, P.; Worsfold, P. J. Microchem. J. 1992,45,178. (8) Gisin, M.; Thommen, C. Trends Anal. Chem. 1989,8,62. (9) Barnett, N. W. et al. Anal. Chim. Acta 1993,282,551. (10) Lynch, T. P.; Kernoghan, N. J.; Wilson, J. N. Analyst (London) 1984,109, 843. (11) Bergamin, H. et al. Anal. Chim. Acta 1986,190,177. (12) Nyman, J.; Ivaska, A Talanta 1993,40,95. (13) Ariza, A. C. et al./. Autom. Chem. 1992, Paul J. Worsfold (left) is a professor of analyt 14,181. (14) Clinch, J. R et al. Anal. Proc. 1988,25,71. ical science at the University of Plymouth (Department of Environmental Sciences, (15) Pedersen, K M.; Kiimmel, M.; Seieberg, H. Anal. Chim. Acta 1990,238,191. Drake Circus, Plymouth, PU 8AA, U.K.). (16) Benson, R. L. et al. Anal. Chim. Acta He received his B.Sc. degree from Loughbor 1994,291, 233. ough University of Technology (U.K.) and (17) Chen, D.; Luque de Castro, M. D.; Valcârhis Ph.D. from the University of Toronto cel, M. Anal. Chim. Acta 1990,230,137. (18) Benson, R L.; Worsfold, P. J. Sci. Total En- (Canada). His research interests are the ap plication of FI techniques, molecular spec viron. 1993,135,17. (19) Nalbach, U. et al. Anal. Chim. Acta 1988, troscopy, and environmental analysis. 213, 55. (20) Nikolajsen, K.; Nielsen, J.; Villadsen, J. Kevin N. Andrew (right) received his B.Sc. Anal. Chim. Acta 1988,214,137. degree in environmental science in 1992from (21) Chung, S. et al. Anal. Chim. Acta 1991, the University of Plymouth and is working 249,77. (22) Forman, L. W.; Thomas, B. D.; Jacobson, on his doctorate there. His research concerns F. S. Anal. Chim. Acta 1991,249,101. the application ofFI and chemometric tech (23) Kracke-Helm, Η-A. et al./. Biotechnol. niques to the on-line monitoring of industrial 1991,20,95. liquid effluent streams. (24) Carlsen, M. et al. Anal. Chim. Acta 1993, 279,51. (25) Hunt, D.T.E.; Wilson, A L. The Chemical Analysis of Water: General Principles and Techniques; Royal Society of Chemistry: London, 1986. (26) Adams, J.RW.; Dolby, J. C; Williams, P. N. /. Inst. Water Environ. Manage. 1992,6,64. (27) Dasgupta, P. K et al. Talanta 1993,40,53. (28) Blundell, N. J. et al./. Autom. Chem. 1993,15,159. (29) Johnson, K. S.; Coale, K. H.; Jannasch, NicholasJ.Blundell (left) is a postdoctoral H. W.Anal. Chem. 1992, 64,1065. (30) Atienza, J. et al. Crit. Rev. Anal. Chem. fellow at the University of Plymouth and 1991,22,331. received his Ph.D. in inorganic chemistry (31) Atienza, J. et al. Crit. Rev. Anal. Chem. from the University of Leicester (U.K.) in 1992,23,1. (32) Johnson, K. S.; Petty, R L. Limnol. Ocean- 1991. His research interests are in the design and deployment of FI instrumenta ogr. 1983,28,1260. (33) Willason, S. W.; Johnson, K S. Mar. Biol. tion for remote environmental monitoring 1986, 91,285. and in the use of computers in analytical (34) Johnson, K. S. et al. Anal. Chim. Acta chemistry. 1987,201,83. (35) Johnson, K. S. et al. Nature 1988,332,527. David Price (right) is a postgraduate stu (36) Kolotyrkina, I. Y.; Shpigun, L. K.; Zolotov.Y. k. J. Anal. Chem. (USSR) 1988, dent working jointly at the University of Ply mouth and Plymouth Marine Laboratory. 43,223. (37) Chapin, T. P.; Johnson, K S.; Coale, Κ. Η. He received a B.Sc. degree from the Univer Anal. Chim. Acta 1991,249,469. sity of Liverpool (U.K.) in 1991. His re (38) Coale, K. H. et al. Anal. Chim. Acta 1992, search is in the area of marine analytical 266,345. (39) Coale, K H. et al. Nature 1991,352,325. chemistry and specifically the application of FI with chemiluminescence detection to the (40) Shpigun, L K; Kolotyrkina, I. Y.; Zolotov, shipboard monitoring of H202. Y A Anal. Chim. Acta 1992,261,307.
9 2 2 A Analytical Chemistry, Vol. 66, No. 18, September 15, 1994
