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Development of a fully integrated microdistillation flow injection system for the determination of trace level ammonia
Laboratory automation and information management
ELSEVIER
Laboratory Automation and Information Management 33 (1998) 199-206
Development of a fully integrated microdistillation flow injection system for the determination of trace level ammonia C.W.K. Chow *, R. Lane, T.C.W. Yeow, D.E. Davey, D.E. Mulcahy Analysis and Sensors Group, School of Chemical
Technology,
University of South Australia, P.O. Box 1, Ingle Farm SA 5095, Australia
Received 26 February 1998; accepted 4 May 1998
Abstract An in-housed designed computerised flow injection system comprised a fully integrated microdistillation flow injection (MDFI) system for low level ammonia analysis was reported. In this system, the microdistillation separation step was incorporated into the flow injection manifold and the ammonia gas sensing probe sensing element was replaced by a flow-through micro-pH electrode which sensed the change in pH of a flowing collector solution caused by the dissolution of distilled ammonia gas, in a process analogous to that occurring in the internal solution of the gas sensing probe. A
computerised control and data acquisition system was constructed for this system using a commercially available data acquisition card which offered many advantages such as improved data acquisition rates and control over the system components, as well as good graphics display and data processing options. The system was optimised using a multi-variable simplex optimisation technique. 0 1998 Elsevier Science B.V. All rights reserved. Keywords:
Ammonia analysis; Flow injection analysis; Ammonia ion selective electrode; Microstill pre-concentration; Simplex optimisation
1. Introduction Previous potentiometric ammonia/ammonium flow injection analysis systems have involved either direct exposure of the sensor to the sample matrix, as for the ammonium ion selective electrode [l], or a separation step which can be performed at the sensor, i.e., the gas permeable membrane of the ammonia gas sensing probe [2-41, or prior to the sensor, in a gas diffusion unit [5-91. Both of these approaches have inherent limitations. In the first case the direct exposure of the sensor to complex sample matrices
* Corresponding author 0925.5281/98/$
can result in poor performance and deterioration of the sensor due to adsorption of high molecular weight species on the electrode surface [9]. The use of a membrane separation step protects the sensing element from the sample matrix [9,10], but the membrane itself is fragile and prone to clogging by particulate matter in the sample [lo]. In addition, the use of a membrane in a device such as the ammonia gas sensing probe places a major restraint on the analysis time of the sensor, as its response and recovery time is determined by the rate of diffusion of ammonia through the membrane and more importantly the rate of diffusion of ammonia away from the sensor in the recovery phase [11,12]. The system described in this paper attempts to address these problems by the integration of a mi-
- see front matter 0 1998 Elsevier Science B.V. All rights reserved. PII: S1381-141X(98)00005-7
200
C. W.K. Chow et al./L.aboratory Automation and Information Management 33 (1998) 199-206
crodistillation unit into a flow injection system to form a microdistillation flow injection (MDFI) system for the detection of ammonium. In the system described previously [13], the microstill was used as part of a preconcentration unit in which the sample was prepared for subsequent introduction to the flow injection analysis system. In the system described here, however, the distillation process is fully integrated into the flow injection analysis system as a separation/preconcentration device, in an analogous manner to the way in which the gas dialysis technique has been integrated into flow injection systems [12,14-161.
internal filling solution of the gas sensing probe. Changes of pH in this collector solution due to dissolution of distilled ammonia gas can then be directly sensed at a flow-through micro pH electrode. In this manner the main advantage of the gas sensing probe, the separation of the sensor from the sample matrix, is retained, while the limitation of diffusioncontrolled response and recovery is avoided. In addition, the ability to concentrate the analyte from a large sample volume into a small collector volume in the system reported earlier 113) is retained, thereby ensuring a low detection limit for the system. A brief description of the operation of this system is given below.
2. Theory of operation
2.1. Instrumentation
This ammonia monitoring system was designed with the purpose of addressing some of the shortcomings of the previously reported system [13], in particular its low throughput [ 17,181. This was addressed by fully integrating the microdistillation process into the flow injection analysis system as a separation/preconcentration device (Fig. 11, in an analogous manner to the way in which the gas dialysis technique has been integrated into flow injection systems [12,14-161. The detection method used in this system is also different from that used in conventional gas sensing probe flow injection systems. The separation of the ammonia analyte from the sample matrix achieved by the microdistillation process renders the extra separation step at the gas permeable membrane of the ammonia gas sensing probe unnecessary. Consequently, the HCl collector solution employed in the preconcentration system described in the previously reported system [13] can be replaced by a weakly acidic, flowing collector solution which acts in a manner analogous to the
The ammonium microdistillation flow injection (MDFI) system operates by using the microstill to separate the ammonia from a discrete volume of sample periodically introduced to the microstill by means of a flow injection system (Fig. 1). The microdistillation unit, pumps, flow injection switching valve, manifold tubing and connectors were all as described in [ 131. The sample is mixed with sodium hydroxide solution in the microstill to ensure conversion of ammonium ion to ammonia and then distilled off at approximately 100°C. The volatilised ammonia gas is carried by a stream of acid-scrubbed air to a condenser where it is collected in a flowing stream of slightly buffered, acidic collector solution. The dissolution of the basic ammonia gas in the flowing collector solution causes a pH shift in a similar manner to the response of the ammonia gas sensing probe described in the previous system [ 131. The collector solution is then passed through a liquid/gas phase separator where a bubble-free portion of the collector solution is resampled and swept past
4-Sample loop Pump 2
Sample Carrier NaOH
\ /)
Still waste
Fig. 1. Schematic
diagram of the flow injection section of the sample manifold.
C. W.K. Chow et al. / Laboratory
Automation
and Informafion Management
33 (1998) 199-206
very high data acquisition rates, multi-channel acquisition, and pump speed control if required. Control of the FIA valve was achieved by using the digital I/O lines of the card to control an on/off power point unit. The solenoid-operated valve could then be switched as required. The operating speed of the Gilson Minipuls 3 peristaltic pumps was controlled via the 0 to 5 V analog output ports of the card. Data acquisition from the potentiometric sensor was carried out using an Orion 720A pH/mV meter to process the high impedance signal. The analog chart recorder output of the meter was fed to one of the analog input channels of the data acquisition card where it was converted and stored in the PC. A schematic diagram of the hardware has been illustrated in Fig. 2.
the flow-through micro pH electrode and reference electrode where the pH shift is detected as a conventional flow injection peak. The sensor was a flowthrough glass pH microelectrode with an internal dead volume of 11 ~1 (Model 16-705, Microelectrodes, Londonderry, NH, USA). A matching flowthrough Ag/AgCl reference electrode (Model 16 702) was placed in-line immediately after the pH microelectrode. A stainless steel tube was inserted in-line immediately in front of the sensor so that any streaming potential or charge build-up was fed to ground via the pH/mV meter. The height of this peak in millivolts can then be related to the ammonium concentration in the sample, within an operating range determined by the experimental conditions. 2.1.1. Hardware Data acquisition and system control were achieved using an IBM compatible 486-SX PC fitted with a commercial data acquisition card (National Instruments, Austin, TX, USA, Model AT-MIO-16F-5). The card included a 16 channel (16 single ended, 8 differential) 12 bit analog to digital converter, 200 kHz throughput, two analog output ports and an 8 bit digital input/output port. This gave the possibility of
2.1.2. Software The software for this system was developed using LabWindows (Version 2.0, National Instruments, Austin, TX, USA) which is provided with the ATMIO-16F-5 data acquisition board. LabWindows is technically a version of Basic, however, it includes graphics and data acquisition features which greatly reduce the software development time. In addition,
Solenoid
Orion 70:2A Meter IBM PC
Sample Pump FIA Controller AT-MIO-16F-5 Data Acquisition Card
201
Carrier Pump 1
Fig. 2. A schematic diagram of the hardware
for the flow injection analysis
Isystem.
202
C. W.K. Chow et al./L.aboratory
Fig. 3. Flow chart of the controlling injection analysis system.
software
Automation and Information Management
for the flow
the AT-MIO-16F-5 board provides hardware data acquisition control and background data processing functions that further simplify the software development. The sequence of the operating cycle is shown schematically in Fig. 3. To begin operation of the system, the operator must first enter the duration of the load and inject cycles (step 1). For example these might typically be 2.5 min and 1 min respectively. The cycle is then initiated (step 2) and the flow injection valve turned to the load position (step 3). The PC then begins to read the data from the meter (step 4) at a rate of 5 data points per second for the duration of the load time (in this case 2.5 mm). During this time the sensor output data is saved to the hard drive and plotted on the screen in real-time (step 5). At the conclusion of the load time, acquisition is momentarily suspended (1 s) while the FIA valve is turned to the inject position (step 6) and then data acquisition and display is resumed (steps 7 and 8) for the duration of the inject period (1 mm). After the specified inject time has elapsed, the process loops back to step 3. This cycle continues until the end of the experiment when it is ended by the user (step 9).
experimental variables [ 191. It has been used successfully a number of times in the optimisation of flow injection systems and is ideally suited to this type of application [20-231. In the optimisation experiment a low collector concentration was selected (3 X 1O-4 M NH,Cl) and the response to a very low concentration ammonia sample (25 pg/l NH:-N) was optimised according to a response function which gave greater weighting to the signal size (0.6) over baseline return (0.4). The four factors for simultaneous optimisation were constrained by boundary conditions chosen from the information gained from the single variable experiments. The main aim being to select a set of conditions which provide a low limit of detection (N 5 kg/l) at the expense of some analysis speed. For this reason the maximum allowed sample loop volume was increased to 6 ml. The four factors were therefore constrained to the following regions and normalised across this range: Distillation temperature: Allowed range = 96103°C Sample loop volume: Allowed range = 3.0-6.0 ml Carrier flow rate: Allowed range = 4.5-7.5 ml/min
0.70 1
0.60
0.10
0.00
3. Results and discussion 3.1. Multi-uariable
simplex optimisation
The multi-variable simplex optimisation technique was employed to simultaneously optimise the critical
33 (1998) 199-206
4
A ! , 0
2
I 4
/
I
6
/
I
8
’
I
I
I
IO
12
14
I
I
1
SIMFLEX VERTD( NUMBER Fig. 4. The progress of the response function values towards convergence as the simplex optimisation progresses. Shown are the peak width or baseline return component of the response function: 0, the peak height component: 0 and the total response function: A.
C. W.K. Chow et al. /Laboratory
Automation and information
Collector flow rate: Allowed range = 0.10-0.40 ml/min The Response Function was chosen as: R.F. = 0.6(Peak
Height) + 0.4( 1 - Peak Width)
where Peak Height was normalised across the range 0 to 50 mV and Peak Width (at 20% peak height) was normalised across the range 0 to 200 s. The low concentration collector solution provides superior low level sensitivity and detection limit at the expense of a limited range. Fig. 4 shows the response function again converging to a maximum value after 15 iterations. 3.2. Analytical
system output
From the information obtained in these experiments, a set of condition was for future work. These are shown in Table 1. Raw calibration data from this Ammonia Microdistillation Flow Injection System obtained under conditions set out in Table 1 are shown in Fig. 5a. These data have been collected at a rate of 5 data points per second and a sample throughput of 15 samples per hour. The calibration graph constructed from these data is shown as Fig. 5b. 3.3. Control mance
and data acquisition
system
pegor-
This system has great flexibility due to the multiple input and output functions. The ability to control pump speed through the O-5 V D/A output together
Table 1 Final operating conditions selected after the optimisation procedures for the ammonium microdistillation flow injection system Variable Collector concentration Sample loop volume Distillation temperature Carrier flow rate Collector flow rate Air flow rate Microstill NaOH concentration Analysis time
3 X 10.’ M NH,Cl 6.0 ml 1 101.0”c 6.0 ml/min 0.25 ml/min 500 ml/min 0.01 M 4 min
Management
33 (1998) 199-206
203
with the availability of high sampling rate and full hardware controlled data acquisition were significant improvements on the previously reported flow injection system for ammonia [13]. The flexibility and time saving offered by the use of the LabWindows software was a further significant advantage. However, the purchase of the data acquisition card and LabWindows software package considerably increased the cost associated with the development of the system. The LabWindows software package was purchase with the data acquisition card as a package. LabWindows is a software development system for BASIC programming. It provides a specialised programming environment for data acquisition and instrument control applications. Software developed from this package can be either run within the programming environment or complied and linked with the stand alone libraries to create stand alone application software which is the preferred mode for running most instrumentation software.
3.3.1. FIA timing control Control of the FIA valve was achieved by using the digital I/O lines of the data acquisition card to control an on/off power point. The solenoid operated valve could then be switched as required. This system proved satisfactory in controlling the timing of the FIA valve. The operating speed of the Gilson Minipuls 3 peristaltic pump could also be controlled via the 0 to 5 V analog output port of the card, which provided further control over the analytical system.
3.3.2, Data acquisition The use of a commercial data acquisition card for data acquisition. Unlike the previously reported system [13], the pH/mV meter was used only as a high input impedance amplifier to handle the high impedance signal from the pH electrode. Consequently it is not necessary to have a pH/mV meter with RS232 output for this system. With the use of the LabWindows package and the data acquisition card, complete instrumental control can be readily achieved. A number of data acquisition and control functions are provided by the LabWindows package for
204
C. W.K. Chow et al./L.aboratory
Automation and Information Management
(a)
33 (1998) 199-206
(b)
0.16
, 3omin
120
1
0.a
-
1
0
II 0.8
0.4 o.cQ -
Fig. 5. Ammonium calibration data in the range 5 to 100 kg/l in pg/l NH:-N beneath the relevant peaks.
100 NH&N.
log,, Concentration
The sample concentration
1.2 Ammonium
1.6
2.0
(pgh-N)
for each group of three repeats is given
the control of this particular data acquisition card. In this application, the DAQ_Start command was used extensively. This function initiates an asynchronous data acquisition operation and stores the data in an array, as well as providing sampling rate and duration controls. During the analysis, this command issues a software trigger to initiate the data acquisition operation over a specified period of time before returning the control to the computer. During this period of time, the acquired data is stored in it’s own RAM until the end of the data acquisition period. When the computer regained control, the block of data is transferred to the computer’s RAM which then saved to the hard-drive. The file loading command provided by LabWindows for data retrieval is much more sophisticated than the equivalent function provided by BASIC, removing the need for a sub-routine to create and name new files at regular intervals as described for the earlier system [13].
was created using the PLOTXY statement (plot x-y graph function), which allowed the data to be plotted at the conclusion of the experiment with an auto scale control to maximise the resolution. With the use of LabWindows, the graphics display module is very easy to create with each function requiring only one line code to complete the operation. This offers tremendous time savings in comparison to the BASIC programming used for the previous system [ 131, as well as providing far superior on-screen graphical presentation.
3.3.3. Graphics display The real-time graphics display component was created using the PLOTSTRIPCHART statement (strip chart function) provided by LabWindows. This allowed the electrode potential to be displayed on the screen in real-time, on fixed scale axes, as the experiment proceeded. The non-real time graphics display
This system described is much more sophisticated and flexible than the previous system [13], although at a significantly increased cost. The use of a commercially available data acquisition card and associated software package to create a control and data acquisition system which allows control of the peristaltic pump operations as well as the FIA valve,
3.3.4. Data processing In general, the data processing module in this system is very similar to the previous system [ 131. Further details of either the hardware or the software can be provided if desired.
4. Conclusion
C. W.K. Chow et al. /Laboratory
Automation and Information Managemenf
while enabling rapid data acquisition rates, excellent real-time and non real-time data displays and a wide range of data processing options. The use of the software package associated with the data acquisition card greatly reduces the time and difficulty of the software development component due to the availability of a range functions for activities such as charting and data transfer. The cost of setting up this system, including the purchase of the data acquisition card, was of the order of $10,000. This system has been successfully used to control a microdistillation flow injection (MDFI) system for the detection of ammonia in water which has a limit of detection of 2 kg/l, and a throughput of 15 or more samples per hour. This system has much better throughput than the previously reported system [13] and requires less sample while retaining the merits of low detection limit, robustness, isolation of the detector from the sample matrix, and no hazardous or expensive reagents. The system has been shown to be stable over time, provide good selectivity, good precision, and excellent correlation with the standard calorimetric method.
Acknowledgements The financial assistance of the DEET is gratefully acknowledged. The authors wish to thank Dr. S. McLeod, Mr. J.W.K. Wong, Mr. P. Souter, Mr. K. Randell and Mr. C. Smith for their contributions in this development.
References [I] M.L. Balconi, F. Sigon, R. Ferraroli, F. Realini, Applications of flow injection analysis in a power plant determination of pH, ammonia and hydrazine in an avt-conditioned water steam cycle, Anal. Chim. Acta 214 (1988) 367-374. [2] H. Hara, A. Motoike, S. Okaxaki, Alternate washing method for flow-through determination of ammonium ion using an ammonia gas electrode, Anal. Chem. 59 (1987) 1995-1999. [3] M.D. Stewart, Continuous monitoring for ammonia and nitrate using ion-selective electrodes, Ultrapure Water 5 (10) (1988) 24-38. [4] H. Hara, S. Matsumoto, Continuous-flow system for the accurate determination of low concentrations of ammonium ions using a gas permeable Poly(tetrafluoroethylene) tube
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decontaminator and an ammonia gas-sensing membrane electrode, Analyst 119 (1994) 1839-1842. 151 M.E. Meyerhoff, Y.M. Fraticelli, Flow injection determination of ammonia-N using a polymer membrane electrodebased gas sensing system, Anal. Lett. 14 (B6) (1981) 415432. [61 H.L. Lee, M.E. Meyerhoff, Comparison of tubular polymeric pH and ammonium ion electrodes as detectors in the automated determination of ammonia, Analyst 110 (1985) 371376. [71G. Schulze, C.Y. Liu, M. Brodowski, 0. Elsholz, Different approaches to the determmation of ammonium ions at low levels by flow injection analysis, Anal. Chim. Acta 214 (1988) 121-136. [81 S. Alegret, J. Alonso, J. Bartroli, E. Martinez-Fabregas, Flow injection system for on-line potentiometric monitoring of ammonia in freshwater streams, Analyst 114 (1989) 14431447. and conductimetric t91 W. Frenzel, C.Y. Liu, Potentiometric determination of ammonium by gas diffusion flow injection analysis, Fresenius J. Anal. Chem. 342 (1992) 276-280. flow injection analysis: [lOI V. Kuban, Gas diffusion/permeation Part 1. Principles and instrumentation, Crit. Rev. Anal. Chem. 23 (5) (1992) 323-354. [ill M.A. Arnold, Improved dynamic response of potentiometric ammonia sensors using pure teflon membranes, Anal. Chim. Acta 154 (1983) 33-39. 1121G.G. Guilbault, J.P. Czamecki, M.A. Nabi Rahni, Performance improvements of gas-diffusion ion-selective and enzyme electrodes, Anal. Chem. 57 (1985) 2110-2116. 1131C.W.K. Chow, R. Lane, T.C.W. Yeow, DE. Davey, D.E. Mulcahy, Development of an automated flow-injection system for the determination of trace level ammonia, Lab. Autom. Inf. Manage. 33 (1997) 129-136. [I41M. Valcarcel, M.D. Luque de Castro, Flow-injection Analysis, Principles and Applications, Ellis Horwood, Chichester, UK, 1987. [I51Z. Fang, Z. Zhu, S. Zhang, S. Xu, L. Guo, L. Sun, On-line separation and preconcentration in flow injection analysis, Anal. Chim. Acta 214 (1988) 41-55. in the flow[I61V. Kuban, Gas permeation and preconcentration injection determination of acid available cyanide in waste water, Anal. Chim. Acta 259 (1992) 45-52. 1171R. Lane, Development of a single sensor microdistillation flow injection system for speciation of inorganic nitrogen in water, PhD Thesis, University of South Australia, 1995 I181R. Lane, C.W.K. Chow, D.E. Davey, D.E. Mulcahy, S. McLeod, An on-line microdistillation-based preconcentration technique for ammonia measurement, Analyst 122 (1997) 1549-1552. [191J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, 2nd. edn., Ellis Horwood, Chichester, UK, 1988. t201T.A.H.M. Janse, P.F.A. Van Der Wiel, G. Kateman, Experimental optimization procedures in the determination of phosphate by flow-injection analysis, Anal. Chim. Acta 155 (1983) 89-102. 1211 J. Alonso, J. Bartroli, J. Coello, M. de1 Valle, Simultaneous
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optimization of variables in FIA systems by means of the simplex method, Anal. Lett. 20 (8) (1987) 1247-1263. [22] D.E. Davey, D.E. Mulcahy, R. Lane, S. McLeod, An on-line microdistillation based preconcentration technique for ammonia measurement, Proceedings, 12th. Australian Symposium
on Analytical Chemistry, Perth, WA, 26th. Sept.-1st. Oct., 1993. [23] P. Di, D.E. Davey, Trace gold determination by on-line preconcentration with flow injection atomic absorption spectrophotometry, Talanta 41 (4) (I 994) 565-57 1.
ELSEVIER
Laboratory Automation and Information Management 33 (1998) 199-206
Development of a fully integrated microdistillation flow injection system for the determination of trace level ammonia C.W.K. Chow *, R. Lane, T.C.W. Yeow, D.E. Davey, D.E. Mulcahy Analysis and Sensors Group, School of Chemical
Technology,
University of South Australia, P.O. Box 1, Ingle Farm SA 5095, Australia
Received 26 February 1998; accepted 4 May 1998
Abstract An in-housed designed computerised flow injection system comprised a fully integrated microdistillation flow injection (MDFI) system for low level ammonia analysis was reported. In this system, the microdistillation separation step was incorporated into the flow injection manifold and the ammonia gas sensing probe sensing element was replaced by a flow-through micro-pH electrode which sensed the change in pH of a flowing collector solution caused by the dissolution of distilled ammonia gas, in a process analogous to that occurring in the internal solution of the gas sensing probe. A
computerised control and data acquisition system was constructed for this system using a commercially available data acquisition card which offered many advantages such as improved data acquisition rates and control over the system components, as well as good graphics display and data processing options. The system was optimised using a multi-variable simplex optimisation technique. 0 1998 Elsevier Science B.V. All rights reserved. Keywords:
Ammonia analysis; Flow injection analysis; Ammonia ion selective electrode; Microstill pre-concentration; Simplex optimisation
1. Introduction Previous potentiometric ammonia/ammonium flow injection analysis systems have involved either direct exposure of the sensor to the sample matrix, as for the ammonium ion selective electrode [l], or a separation step which can be performed at the sensor, i.e., the gas permeable membrane of the ammonia gas sensing probe [2-41, or prior to the sensor, in a gas diffusion unit [5-91. Both of these approaches have inherent limitations. In the first case the direct exposure of the sensor to complex sample matrices
* Corresponding author 0925.5281/98/$
can result in poor performance and deterioration of the sensor due to adsorption of high molecular weight species on the electrode surface [9]. The use of a membrane separation step protects the sensing element from the sample matrix [9,10], but the membrane itself is fragile and prone to clogging by particulate matter in the sample [lo]. In addition, the use of a membrane in a device such as the ammonia gas sensing probe places a major restraint on the analysis time of the sensor, as its response and recovery time is determined by the rate of diffusion of ammonia through the membrane and more importantly the rate of diffusion of ammonia away from the sensor in the recovery phase [11,12]. The system described in this paper attempts to address these problems by the integration of a mi-
- see front matter 0 1998 Elsevier Science B.V. All rights reserved. PII: S1381-141X(98)00005-7
200
C. W.K. Chow et al./L.aboratory Automation and Information Management 33 (1998) 199-206
crodistillation unit into a flow injection system to form a microdistillation flow injection (MDFI) system for the detection of ammonium. In the system described previously [13], the microstill was used as part of a preconcentration unit in which the sample was prepared for subsequent introduction to the flow injection analysis system. In the system described here, however, the distillation process is fully integrated into the flow injection analysis system as a separation/preconcentration device, in an analogous manner to the way in which the gas dialysis technique has been integrated into flow injection systems [12,14-161.
internal filling solution of the gas sensing probe. Changes of pH in this collector solution due to dissolution of distilled ammonia gas can then be directly sensed at a flow-through micro pH electrode. In this manner the main advantage of the gas sensing probe, the separation of the sensor from the sample matrix, is retained, while the limitation of diffusioncontrolled response and recovery is avoided. In addition, the ability to concentrate the analyte from a large sample volume into a small collector volume in the system reported earlier 113) is retained, thereby ensuring a low detection limit for the system. A brief description of the operation of this system is given below.
2. Theory of operation
2.1. Instrumentation
This ammonia monitoring system was designed with the purpose of addressing some of the shortcomings of the previously reported system [13], in particular its low throughput [ 17,181. This was addressed by fully integrating the microdistillation process into the flow injection analysis system as a separation/preconcentration device (Fig. 11, in an analogous manner to the way in which the gas dialysis technique has been integrated into flow injection systems [12,14-161. The detection method used in this system is also different from that used in conventional gas sensing probe flow injection systems. The separation of the ammonia analyte from the sample matrix achieved by the microdistillation process renders the extra separation step at the gas permeable membrane of the ammonia gas sensing probe unnecessary. Consequently, the HCl collector solution employed in the preconcentration system described in the previously reported system [13] can be replaced by a weakly acidic, flowing collector solution which acts in a manner analogous to the
The ammonium microdistillation flow injection (MDFI) system operates by using the microstill to separate the ammonia from a discrete volume of sample periodically introduced to the microstill by means of a flow injection system (Fig. 1). The microdistillation unit, pumps, flow injection switching valve, manifold tubing and connectors were all as described in [ 131. The sample is mixed with sodium hydroxide solution in the microstill to ensure conversion of ammonium ion to ammonia and then distilled off at approximately 100°C. The volatilised ammonia gas is carried by a stream of acid-scrubbed air to a condenser where it is collected in a flowing stream of slightly buffered, acidic collector solution. The dissolution of the basic ammonia gas in the flowing collector solution causes a pH shift in a similar manner to the response of the ammonia gas sensing probe described in the previous system [ 131. The collector solution is then passed through a liquid/gas phase separator where a bubble-free portion of the collector solution is resampled and swept past
4-Sample loop Pump 2
Sample Carrier NaOH
\ /)
Still waste
Fig. 1. Schematic
diagram of the flow injection section of the sample manifold.
C. W.K. Chow et al. / Laboratory
Automation
and Informafion Management
33 (1998) 199-206
very high data acquisition rates, multi-channel acquisition, and pump speed control if required. Control of the FIA valve was achieved by using the digital I/O lines of the card to control an on/off power point unit. The solenoid-operated valve could then be switched as required. The operating speed of the Gilson Minipuls 3 peristaltic pumps was controlled via the 0 to 5 V analog output ports of the card. Data acquisition from the potentiometric sensor was carried out using an Orion 720A pH/mV meter to process the high impedance signal. The analog chart recorder output of the meter was fed to one of the analog input channels of the data acquisition card where it was converted and stored in the PC. A schematic diagram of the hardware has been illustrated in Fig. 2.
the flow-through micro pH electrode and reference electrode where the pH shift is detected as a conventional flow injection peak. The sensor was a flowthrough glass pH microelectrode with an internal dead volume of 11 ~1 (Model 16-705, Microelectrodes, Londonderry, NH, USA). A matching flowthrough Ag/AgCl reference electrode (Model 16 702) was placed in-line immediately after the pH microelectrode. A stainless steel tube was inserted in-line immediately in front of the sensor so that any streaming potential or charge build-up was fed to ground via the pH/mV meter. The height of this peak in millivolts can then be related to the ammonium concentration in the sample, within an operating range determined by the experimental conditions. 2.1.1. Hardware Data acquisition and system control were achieved using an IBM compatible 486-SX PC fitted with a commercial data acquisition card (National Instruments, Austin, TX, USA, Model AT-MIO-16F-5). The card included a 16 channel (16 single ended, 8 differential) 12 bit analog to digital converter, 200 kHz throughput, two analog output ports and an 8 bit digital input/output port. This gave the possibility of
2.1.2. Software The software for this system was developed using LabWindows (Version 2.0, National Instruments, Austin, TX, USA) which is provided with the ATMIO-16F-5 data acquisition board. LabWindows is technically a version of Basic, however, it includes graphics and data acquisition features which greatly reduce the software development time. In addition,
Solenoid
Orion 70:2A Meter IBM PC
Sample Pump FIA Controller AT-MIO-16F-5 Data Acquisition Card
201
Carrier Pump 1
Fig. 2. A schematic diagram of the hardware
for the flow injection analysis
Isystem.
202
C. W.K. Chow et al./L.aboratory
Fig. 3. Flow chart of the controlling injection analysis system.
software
Automation and Information Management
for the flow
the AT-MIO-16F-5 board provides hardware data acquisition control and background data processing functions that further simplify the software development. The sequence of the operating cycle is shown schematically in Fig. 3. To begin operation of the system, the operator must first enter the duration of the load and inject cycles (step 1). For example these might typically be 2.5 min and 1 min respectively. The cycle is then initiated (step 2) and the flow injection valve turned to the load position (step 3). The PC then begins to read the data from the meter (step 4) at a rate of 5 data points per second for the duration of the load time (in this case 2.5 mm). During this time the sensor output data is saved to the hard drive and plotted on the screen in real-time (step 5). At the conclusion of the load time, acquisition is momentarily suspended (1 s) while the FIA valve is turned to the inject position (step 6) and then data acquisition and display is resumed (steps 7 and 8) for the duration of the inject period (1 mm). After the specified inject time has elapsed, the process loops back to step 3. This cycle continues until the end of the experiment when it is ended by the user (step 9).
experimental variables [ 191. It has been used successfully a number of times in the optimisation of flow injection systems and is ideally suited to this type of application [20-231. In the optimisation experiment a low collector concentration was selected (3 X 1O-4 M NH,Cl) and the response to a very low concentration ammonia sample (25 pg/l NH:-N) was optimised according to a response function which gave greater weighting to the signal size (0.6) over baseline return (0.4). The four factors for simultaneous optimisation were constrained by boundary conditions chosen from the information gained from the single variable experiments. The main aim being to select a set of conditions which provide a low limit of detection (N 5 kg/l) at the expense of some analysis speed. For this reason the maximum allowed sample loop volume was increased to 6 ml. The four factors were therefore constrained to the following regions and normalised across this range: Distillation temperature: Allowed range = 96103°C Sample loop volume: Allowed range = 3.0-6.0 ml Carrier flow rate: Allowed range = 4.5-7.5 ml/min
0.70 1
0.60
0.10
0.00
3. Results and discussion 3.1. Multi-uariable
simplex optimisation
The multi-variable simplex optimisation technique was employed to simultaneously optimise the critical
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4
A ! , 0
2
I 4
/
I
6
/
I
8
’
I
I
I
IO
12
14
I
I
1
SIMFLEX VERTD( NUMBER Fig. 4. The progress of the response function values towards convergence as the simplex optimisation progresses. Shown are the peak width or baseline return component of the response function: 0, the peak height component: 0 and the total response function: A.
C. W.K. Chow et al. /Laboratory
Automation and information
Collector flow rate: Allowed range = 0.10-0.40 ml/min The Response Function was chosen as: R.F. = 0.6(Peak
Height) + 0.4( 1 - Peak Width)
where Peak Height was normalised across the range 0 to 50 mV and Peak Width (at 20% peak height) was normalised across the range 0 to 200 s. The low concentration collector solution provides superior low level sensitivity and detection limit at the expense of a limited range. Fig. 4 shows the response function again converging to a maximum value after 15 iterations. 3.2. Analytical
system output
From the information obtained in these experiments, a set of condition was for future work. These are shown in Table 1. Raw calibration data from this Ammonia Microdistillation Flow Injection System obtained under conditions set out in Table 1 are shown in Fig. 5a. These data have been collected at a rate of 5 data points per second and a sample throughput of 15 samples per hour. The calibration graph constructed from these data is shown as Fig. 5b. 3.3. Control mance
and data acquisition
system
pegor-
This system has great flexibility due to the multiple input and output functions. The ability to control pump speed through the O-5 V D/A output together
Table 1 Final operating conditions selected after the optimisation procedures for the ammonium microdistillation flow injection system Variable Collector concentration Sample loop volume Distillation temperature Carrier flow rate Collector flow rate Air flow rate Microstill NaOH concentration Analysis time
3 X 10.’ M NH,Cl 6.0 ml 1 101.0”c 6.0 ml/min 0.25 ml/min 500 ml/min 0.01 M 4 min
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203
with the availability of high sampling rate and full hardware controlled data acquisition were significant improvements on the previously reported flow injection system for ammonia [13]. The flexibility and time saving offered by the use of the LabWindows software was a further significant advantage. However, the purchase of the data acquisition card and LabWindows software package considerably increased the cost associated with the development of the system. The LabWindows software package was purchase with the data acquisition card as a package. LabWindows is a software development system for BASIC programming. It provides a specialised programming environment for data acquisition and instrument control applications. Software developed from this package can be either run within the programming environment or complied and linked with the stand alone libraries to create stand alone application software which is the preferred mode for running most instrumentation software.
3.3.1. FIA timing control Control of the FIA valve was achieved by using the digital I/O lines of the data acquisition card to control an on/off power point. The solenoid operated valve could then be switched as required. This system proved satisfactory in controlling the timing of the FIA valve. The operating speed of the Gilson Minipuls 3 peristaltic pump could also be controlled via the 0 to 5 V analog output port of the card, which provided further control over the analytical system.
3.3.2, Data acquisition The use of a commercial data acquisition card for data acquisition. Unlike the previously reported system [13], the pH/mV meter was used only as a high input impedance amplifier to handle the high impedance signal from the pH electrode. Consequently it is not necessary to have a pH/mV meter with RS232 output for this system. With the use of the LabWindows package and the data acquisition card, complete instrumental control can be readily achieved. A number of data acquisition and control functions are provided by the LabWindows package for
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Automation and Information Management
(a)
33 (1998) 199-206
(b)
0.16
, 3omin
120
1
0.a
-
1
0
II 0.8
0.4 o.cQ -
Fig. 5. Ammonium calibration data in the range 5 to 100 kg/l in pg/l NH:-N beneath the relevant peaks.
100 NH&N.
log,, Concentration
The sample concentration
1.2 Ammonium
1.6
2.0
(pgh-N)
for each group of three repeats is given
the control of this particular data acquisition card. In this application, the DAQ_Start command was used extensively. This function initiates an asynchronous data acquisition operation and stores the data in an array, as well as providing sampling rate and duration controls. During the analysis, this command issues a software trigger to initiate the data acquisition operation over a specified period of time before returning the control to the computer. During this period of time, the acquired data is stored in it’s own RAM until the end of the data acquisition period. When the computer regained control, the block of data is transferred to the computer’s RAM which then saved to the hard-drive. The file loading command provided by LabWindows for data retrieval is much more sophisticated than the equivalent function provided by BASIC, removing the need for a sub-routine to create and name new files at regular intervals as described for the earlier system [13].
was created using the PLOTXY statement (plot x-y graph function), which allowed the data to be plotted at the conclusion of the experiment with an auto scale control to maximise the resolution. With the use of LabWindows, the graphics display module is very easy to create with each function requiring only one line code to complete the operation. This offers tremendous time savings in comparison to the BASIC programming used for the previous system [ 131, as well as providing far superior on-screen graphical presentation.
3.3.3. Graphics display The real-time graphics display component was created using the PLOTSTRIPCHART statement (strip chart function) provided by LabWindows. This allowed the electrode potential to be displayed on the screen in real-time, on fixed scale axes, as the experiment proceeded. The non-real time graphics display
This system described is much more sophisticated and flexible than the previous system [13], although at a significantly increased cost. The use of a commercially available data acquisition card and associated software package to create a control and data acquisition system which allows control of the peristaltic pump operations as well as the FIA valve,
3.3.4. Data processing In general, the data processing module in this system is very similar to the previous system [ 131. Further details of either the hardware or the software can be provided if desired.
4. Conclusion
C. W.K. Chow et al. /Laboratory
Automation and Information Managemenf
while enabling rapid data acquisition rates, excellent real-time and non real-time data displays and a wide range of data processing options. The use of the software package associated with the data acquisition card greatly reduces the time and difficulty of the software development component due to the availability of a range functions for activities such as charting and data transfer. The cost of setting up this system, including the purchase of the data acquisition card, was of the order of $10,000. This system has been successfully used to control a microdistillation flow injection (MDFI) system for the detection of ammonia in water which has a limit of detection of 2 kg/l, and a throughput of 15 or more samples per hour. This system has much better throughput than the previously reported system [13] and requires less sample while retaining the merits of low detection limit, robustness, isolation of the detector from the sample matrix, and no hazardous or expensive reagents. The system has been shown to be stable over time, provide good selectivity, good precision, and excellent correlation with the standard calorimetric method.
Acknowledgements The financial assistance of the DEET is gratefully acknowledged. The authors wish to thank Dr. S. McLeod, Mr. J.W.K. Wong, Mr. P. Souter, Mr. K. Randell and Mr. C. Smith for their contributions in this development.
References [I] M.L. Balconi, F. Sigon, R. Ferraroli, F. Realini, Applications of flow injection analysis in a power plant determination of pH, ammonia and hydrazine in an avt-conditioned water steam cycle, Anal. Chim. Acta 214 (1988) 367-374. [2] H. Hara, A. Motoike, S. Okaxaki, Alternate washing method for flow-through determination of ammonium ion using an ammonia gas electrode, Anal. Chem. 59 (1987) 1995-1999. [3] M.D. Stewart, Continuous monitoring for ammonia and nitrate using ion-selective electrodes, Ultrapure Water 5 (10) (1988) 24-38. [4] H. Hara, S. Matsumoto, Continuous-flow system for the accurate determination of low concentrations of ammonium ions using a gas permeable Poly(tetrafluoroethylene) tube
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optimization of variables in FIA systems by means of the simplex method, Anal. Lett. 20 (8) (1987) 1247-1263. [22] D.E. Davey, D.E. Mulcahy, R. Lane, S. McLeod, An on-line microdistillation based preconcentration technique for ammonia measurement, Proceedings, 12th. Australian Symposium
on Analytical Chemistry, Perth, WA, 26th. Sept.-1st. Oct., 1993. [23] P. Di, D.E. Davey, Trace gold determination by on-line preconcentration with flow injection atomic absorption spectrophotometry, Talanta 41 (4) (I 994) 565-57 1.