major concerns exist about the quality of water for such uses as human consumption and crop irrigation . thus , the number of samples as well as of analytes ( both normal components and pollutants ) increases continuously , challenging the capacity of analytical laboratories . in uruguay , the school of chemistry of the universidad de la repblica has been engaged in water analysis due to an agreement with the uruguayan water regulatory body ( ursea , unidad reguladora de servicios de energa y agua ) with the school acting as a contract laboratory for the analysis of water samples from the public water supply . samples are collected on a regular basis from hundreds of sampling points throughout the country and analysed for almost 60 analytes both organic and inorganic , as well as turbidity , ph , and microbiological parameters . with the exception of a few determinations made on the field , the determinations are carried out in the school 's laboratories by means of apha standard methods . with regard to the inorganic analytes , this means that a large amount of volumetric , colorimetric , and spectrometric determinations are carried out manually , thus presenting a heavy workload for the analysts and resulting in a low throughput . with the goal of solving this problem , although commercial automatic analysers for water samples are available , they are expensive . working in a university environment , it makes sense to engage in the design of the analyser as students , both graduate and nongraduate , can take part in the task . automation of the analysis of water samples has been the subject of a significant number of papers published in the last years . many of these works resort to some kind of flow - based technique [ 24 ] . however , most of the flow - based analysers proposed in the literature are dedicated systems focusing on the determination of one or two parameters in water samples [ 58 ] , mainly for oceanographic research . recently , the concept of a computer supported system of networked automated water analysers has been introduced , the european awacss system [ 10 , 11 ] being an example of a continent - wide network of analysers .   sequential injection analysis ( sia ) [ 12 , 13 ] was proposed in 1990 as a flow - technique oriented to process analysis but presenting some properties which make it increasingly interesting for the automated routine analysis of water samples . the technique is robust , highly versatile and features low - reagent consumption , resulting in a low volume of chemical residues . thus , it was decided that the analyser to be designed should be based on this technique . the analyser should have multiparametric capability , that is , it should be capable of carrying out not less than three methods without the need of physical modification or reagent changes . it should also be flexible , allowing a fast and easy change of the analytes and methods under software control , and should be capable of interaction with an autosampler . in order to accommodate the analytical methods currently in use , the proposed analyser should resort to photometric detection based on a multiwavelength light - emitting diode ( led )   light source . this light source has been extensively investigated and found to be suitable for low cost and portable analytical equipment [ 1520 ] . this work presents the design , construction , and evaluation of an automatic multiparametric analyser based on sia for the analysis of drinking and surface water as well as groundwater samples . the sia flow system ( figure 1 ) was based on a syringe pump ( cavro xp-3000 , tecan , mnnedorf , switzerland ) with a two - position valve and a 2.5  ml barrel , and a 10-position cheminert selector valve with microelectric actuator ( valco instruments , houston , tex , usa , model   c25 - 3180emh ) . each of these devices was fed by its own dedicated 24  v dc power supply . both the pump and the syringe were controlled from a notebook computer via the rs232 serial ports of the devices . since the notebook did not have any conventional serial port , commercial plug - and - play usb - serial adapters were used to connect the notebook to the pump and the valve , thus generating virtual com1 and com2 serial ports . connections and coils of the flow system were made with 0.8  mm ( internal diameter ) fep tubing . a photometric detector was built around a glass flow cell ( optical path 10  mm , internal volume 200  l ) , a photodiode detector ( vtb1013 , perkinelmer ) , and a common - anode tricolour high - brightness led ( rl5-rgb , superbrightleds , st . the signal from the photodiode was processed by an amplifier implemented with two tl-081 operational amplifiers , and then sent to a 14-bit analogue to digital ( a / d ) interface ( usb-1408fs , measurement computing , norton , mass , usa ) controlled via a usb port of the computer . each component of the rgb tricolour led ( red , green , blue , nominal wavelengths 630  nm , 525  nm , and 472  nm ) was driven by a lm-334 constant - current source , in turn fed by one section of a ln-2803 octal darlington driver ic . the 3 relevant input gates of this driver were controlled from ttl - level signals from the input / output ( i / o ) ports of the a / d interface . in this way , each section of the led array could be turned on or off at will , allowing selective use of the led with the appropriate wavelength under computer control . a regulated power supply employing lm317 and lm337 voltage regulators provided 12  v for the operational amplifiers and leds . the prototype was built in an industrial plastic box with a hinged lid , measuring 31  30  19  cm ( wdh ) ( figure 3 ) . the front of the syringe pump and the stator of the valve protrude via holes in the case , thus being accessible for fluidic connections and inspection . the detector flow cell is the only component of the flow path situated inside the case . two teflon bulkhead connectors ( omnifit , cambridge , uk ) allow input and output connections to the cell .   a program was written and compiled in visual basic 6.0 ( microsoft ) . a graphic language programming add - on ( softwire 3.1 , softwire technology ) was used together with visual basic to allow an easier programming from a graphic environment . the program controlled the whole operation of the system including the data acquisition , as discussed below . the data ( series of absorbance data points ) obtained were stored on hard disk as ascii files . these data files were later processed with the peak simple program ( sri , torrance , calif , usa ) which provided baseline correction , peak - height measurement , and hard copy printout . astm type i water was obtained from a millipore ( so paulo , brazil ) direct - q5 water purifier . as schematised in figures 1 and 2 , the prototype of the analyser basically consists of the following sections : the flow system comprising the syringe pump , the selector valve , and the flow cell ; the light source and its control unit composed by the tricolour led , three adjustable constant - current sources and the uln-2803 driver ; the photo detector and its amplifier ; a power supply for the analogue portion ( amplifier , photodetector , and leds ) ; the a / d interface and its associated ttl i / o ports ; the notebook computer , with windows xp operating system , and the usb - serial adapters ; the custom - written software .   for fluid handling , the cavro pump not only can be easily controlled via a rs232-compliant serial port , but also it has a number of commands available to control speed and acceleration allowing for optimum fluidic handling . it also has its own memory capable of storing complete subroutines that can be called and run when necessary . the 10-position selector valve can accommodate several reagents and auxiliary devices such as minicolumns or mixing coils , allowing for the implementation of several methods simultaneously . it is also controlled via a rs232 serial port by means of simple commands such as  go9 the photometric detector was designed around a tricolour led comprising red , green and blue components , and a silicon photodiode as detector . 10-turn potentiometer to a value such that , when water was filling the flow cell , the output signal of the analogue amplifier was about 70% of the saturation voltage . given the high gain of the amplifier , the value of this current was around 1  ma for each led . the use of low currents was chosen to avoid the overheating of the semiconductor and the consequent variations in light output . the program was a critical component as the whole operation of the instrument relied on it . also , the desired flexibility depended heavily on its correct design . visual basic 6.0 was chosen as programming language because of the previous experience of the group with this language . the use of the softwire graphic programming environment ( in some way reminiscent of the labview language ) was decided because it greatly simplifies programming providing a graphic environment based on activex controls , while simultaneously keeping the possibility of command - based programming of visual basic . the program designed has two modules presented on the screen as formularies : the operating ( main ) form ( figure 4 ) and the setup form ( figure 5 ) . the first one has several command buttons and a scrolling screen simulating a strip - chart recorder , where real - time data is plotted . it also allows calling service routines such as initialisation of the syringe pump and selection of the serial port ( com1 or com2 ) to be connected to the pump and to the valve . it also allows the selection of the a / d board and channel numbers , sampling rate , scale , as well as data paths and file names . other functions currently available ( but not essential for the operation ) include the possibility of manually turning individual leds on or off and manually setting the photometric scale . after that , each line consists of three parts : a command for the selector valve , a command for the syringe pump , and a third part containing time intervals which may be necessary to wait . once loaded from the setup screen , they are ready to run . since the analyser was to be programmed and operated by trained personnel , no effort was made to use a  natural language  which would require an interpreter increasing the complexity of the program . in fact , the set of alphanumeric commands necessary to operate the valve and the pump is small and easy to learn by the user . these commands are provided by the manufacturers in the respective manuals and are as simple as  go9  to send the valve to position 9 , d1500r  to dispense a volume equivalent of 1500 steps of the syringe pump , and so forth . the first active line of a method stores the led number ( 1 to 3 ) allowing for selection of the led with the appropriate wavelength for the intended method . once the method is loaded and started by means of a command button on the operation form , each line is parsed into its three components . first , a valve command is issued via the corresponding serial com port , sending the selector valve to a given position . then , after waiting a fixed time period of 400  ms ( configurable from the setup screen ) , the pump command is issued via the second serial com port . then , if necessary , the system waits for the specified time interval before executing the next line of the script . the current valve and pump commands , time interval , and line number this screen also supports limited edition capabilities , allowing  on the fly  modifications of the script to be tested and stored . this feature turned out to be useful during methods development . data acquisition runs as a separate process , which can be started / stopped at any time by command buttons on the operation screen . the program triggers a / d conversions from the a / d interface as required . in order to be able to acquire real - time data , this is done at the beginning of the cycle while pumping water through the flow cell . the program first measures and memorises the  dark response  with all leds off . , the program can then calculate the absorbance each time the a / d interface is triggered . absorbance values are then plotted on the scrolling screen and saved to a raw - data file in ascii format for later processing .   after finishing a run , the user can choose to keep the raw - data file with a new name or to delete it . in the first case , the routine creates a new text file containing a header with information to make it compatible with the peak simple chromatography program . this information includes the number of data points and the sampling rate used by the a / d interface , allowing for the establishment of a real - time scale in the peak simple program . at this stage of the project , the use of a commercial chromatography program for data processing and evaluation was preferred in order to save development time . however , it is possible for future versions of the software to provide new routines for data processing such as the establishment of baselines , calculation of peak heights , areas and widths , handling of calibration curves , and so forth . the system price was kept at a minimum , the syringe pump and selector valve being the most expensive items ( about us $ 3000 for both ) followed by the notebook computer ( purchased locally for us $ 600 ) . the cost of the rest of the components , mainly electronics and hardware ( including the a / d interface ) , was under us $ 1000 . in order to assess the system 's capabilities , several methods , based on the well - established apha standard methods , chloride was determined by the colorimetric mercuric thiocyanate method ( based on apha method 4500-cl g ) , using the blue led for detection . nitrite was determined using the green led by reaction with n-(1-naphtyl)-ethylenediamine dihydrochloride and sulphanilamide ( based on apha method 4500-no2   b ) . similarly the method of online reduction in a cadmium minicolumn and determination as nitrite ( based on apha method 4500-no3   e ) was used for nitrate . the latter apha methods , in their batch form , were used as reference methods for validation , while for chloride , the argentometric method apha 4500-cl b was used . table 1 shows the analytical figures of merit of the methods as obtained in the validation stage . for each method , special attention was given to medium - term stability , as there is the possibility of drift from electronic components and temperature changes in the flow cell , especially as components temperature raises during operation . for this purpose , repeated injections were carried out for a period of 60 minutes , at concentrations of 30  mg l ( chloride ) , 0.60  mg l ( nitrite ) , and 10  mg  l   ( nitrate ) . baseline drift was under 0.010 ( absorbance ) , and sensitivity drift ( measured as the relative difference of the average of the height of the first 5 peaks and of the last 5 ones ) was 1.9% ( chloride ) , 0.55% ( nitrite ) , and 3.9% ( nitrate ) . only in the latter case the slight baseline drift has no effect as the program used for data processing allows for baseline correction . the number of methods that could be simultaneously run depends on the number of ports in the selector valve .   with the 10-position selector valve used and depending on the complexity of the method , it was possible to determine up to 4 analytes sequentially without changing reagents or flow configuration . in order to determine a greater number of simultaneous parameters , it would be necessary to use a selector valve with a higher number of ports . one apparent limitation of the system resides in the use of a tricolour led which limits the number of wavelengths available . however , the system can be used to implement a number of methods involving colorimetric detection , as long as the wavelength required by the method is close to one of the wavelengths emitted by the rgb led . depending on the difference of wavelengths , some loss of sensitivity may be expected if compared to the ideal situation of a perfect wavelength match ; however , this will not necessarily diminish the usefulness of the system for the intended purpose , as demonstrated by the three methods tested in this work . a lab - made autosampler has been designed and is being built at this time . however , a commercial autosampler could conceivably be added to the system with slight modifications in the software allowing for serial communications with the device . further modifications of the system , beside those previously mentioned , would be mainly based on modifications of the software and thus easily implemented . some of these modifications , currently under study , include allowing led operations at any method line , allowing a more complex scheme such as using two leds for background correction by the two - wavelength technique , or periodically resetting the photometric scale , in order to minimise the effects of drift in the zero signal or in the leds emission intensity . the total cost was low enough to make it affordable to a low - budget university laboratory . the analytical figures of merit found for the methods tested were satisfactory for the intended purpose .