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Knowledge-based system for the provision of an analytical strategy for simultaneous determination of metals by differential-pulse polarography
ANALYTICA
CHIMICA
ACT!4
ELSEVIER
Analytica Chimica Acta 316 (1995) 47-56
Knowledge-based system for the provision of an analytical strategy for simultaneous determination of metals by differential-pulse polarography M.P. Garcia-Armada
a2*, J. Losada a, S. de Vicente-Perez
aDepartamento de Ingenieria Quhica Industrial, Universidad Polittknica de Madrid, C. Jo& Gutierrez Abascal2, b Departamento
de Ciencias Analiticas,
Uniuersidad National de Educacihn a Distancia, C. Senda de1 Rey s/n,
Received 10 November
b 28006 Madrid, Spain 28040 Madr% Spain
1994; revised 24 April 1995; accepted 26 June 1995
Abstract A knowledge-based system for differential-pulse polarography has been developed. This system allows to store all polarographic data (supporting electrolyte-metal-peak potential) and to process them in order to facilitate its handling, and provides the possibility to determine simultaneously in electrolytes or combinations of electrolytes a maximum number of metals with a minimum number of steps. Moreover the system is able to interpret the experimental data provided by a polarogram in order to identify the sample constituents. The system’ program is written in commercially available Q-PRO 4 and allows a very easy user communication. To use the system only a personal computer with MS DOS and a polarograph are required, which means that no extra investments are necessary. Keywords:
Polarography;
Knowledge-based
system; Metals
1. Introduction One of the most interesting differential pulse polarographic (DPP) applications is the simultaneous determination of electroactive species. Many scientists have made contributions to this field for mixtures with several metals [l-3] in various materials, and also some deconvolution programs have been developed to solve problems concerned with overlapping peaks [4-71. DPP allows to perform qualitative and quantita-
* Corresponding
author.
0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0003-2670(95)00357-6
tive analyses in the same solution and suitable sensibility and also peak resolution ties. It is therefore very interesting to find that applies general DPP to any sample appear.
it has a possibilia system that may
2. The knowledge base Usually, the work scheme to be followed by the analyst can be summed up as is presented in Fig. 1. The first difficulty with actual multicomponent samples appears when a complicated polarogram, with many peaks and interferences between the sample constituents, must be interpreted since the informa-
M.P. Garcia-Armada
48
et al. /Analytica
tion for each particular mixture is rarely tabulated or described. The next difficulty to overcome by the analytical chemist is the handling and data compilation in order to design the suitable analytic method for each specific problem. By using the suitable supporting electrolyte combination or by addition of complexing agents [8] the identification and determination of the whole electroactive species present in a sample is possible by a few steps without a previous separation stage. The choice of a suitable combination requires to handle a lot of data concerned with the electrochemical behaviour of metals in several media and also to take into account: 1. chemical interferences between supporting electrolyte and each sample constituent; 2. mutual interferences between the present ions as overlapping or peak potential displacements; and 3. minimal analysis times and efforts This all means that it is very difficult to bring this technique into general use, and usually, the analyst therefore prefers to use chemical separation methods. In recent papers it has been described that a series of expert systems have been applied to instrumental techniques like spectrophotometry [9,10], X-ray fluorescence [ll], chromatography [ 121, atomic absorption spectrometry [13] and voltammetry [4,14,15]. However, they have only been applied to groups of a few metals under specific conditions.
-1
1piiqz-q
Choice of suitable methodology for quantitative detemination
F
. Ouantitative
Fig. 1. Systematics
determination
usually followed by the analyst.
Chimica Acta 316 (1995) 47-56
Fig. 2. Aims system schedule.
Our purpose is to develop an optimization tool for general application of the differential-pulse technique to analyse every metal mixture that can occur. The aims of the present expert system are basically (Fig. 2) to store series of experimental polarographic data on supporting electrolytes, metals and peak potentials into files with easy access in order to process them either to identify the constituents or to obtain the best method for quantitative determination when the constituents are known. 2.1. Data Data from the literature can be stored in the database, but generally these have been obtained under specific conditions that may not be applicable to the analysis of complicated mixtures. The system utility depends mainly on database contents. We have filled them with our own experimental data, obtained from mixtures of a great number of cations in several supporting electrolytes [16], and therefore interferences related to the above have been accounted for. Since bibliographic data for very complicated cases are not available, this expert system has not been designed as a polarographic data source but as a tool and for this reason each user can fill the database with his own experimental data according to the experimental conditions to be employed during the performance of the analytical procedure (supporting electrolyte, pH, ionic strength, reference and auxiliary electrode types, etc.). It is important that in order to obtain accurate results, the data have to be actual and valid.
MP. Garcia-Armada
et al./Analytica
Chimica Acta 316 (1995) 47-56
2.2. Identification
49
2nd analyst step: selection of suitable supporting electrolyte. 1st expert system step: ‘cleanness’ of cations for each supporting electrolyte: la, consultation of databases; lb, ordering of cations in numerical order of peak potentials; Ic, rejection of metals which are not present in the sample; Id, rejection of non-electroactive or precipitate cations (an arbitrary peak potential has been assigned to these cations: 99.99 and 88.88 respectively, to this end) and evaluation of the difference between adjacent peak potentials in order to reject which are overlapped; 5. le, storage of determinable cations in each electrolyte into a new database; 6. lf, go to second optimization. 2nd expert system step: ‘cleanness’ of supporting electrolytes: 1. 2a, it consults in database created in 1st step and compares determinable cations in different electrolytes; 2. 2b, it selects the electrolyte or minimum combination of electrolytes to determine the whole of electroactive cations present in the sample, by
The system substitutes the visual comparison of polarograms and automatically carries out the mixture constituents identification. The main error source in the peak assignments is due to the non-coincidence between the experimental peak potential and the stored values. In order to avoid errors in the assignment of polarographic peaks, the system searches into the database’s potential values with intervals of +0.05 V. Intervals can be extended by multiplying by a factor chosen by the user, in such a way that by introducing a certain potential, the program searches, within the fixed interval, the cation which appears at the nearest potential to that introduced. 2.3. Selection of suitable methodology The method selection procedure is carried out in two steps: first and second optimizations which reproduce in an automatic way the ‘steps followed by the analyst: 1st analyst step: bibliographic search to determine the electrochemical behaviour of constituents all together in each electrolyte to be considered. COMPUTER
USER
for each supporting, all the cations in Ep numerical order and the Ep difference between the cation marked with cursor and the next. Shows,
Rejects with the keyboard: less suitable electrolytes . Shows results screen: suitable electrolvtes with - determinable cations - Ep to be observed in the polarogram.
Fig. 3. Work made by computer
and user in manual mode.
50
M.P. Garcia-Armada
et al. /Analytica
rejecting those which are less adequate either because it allows to determine a few cations or because the same cations can be determined by using another electrolyte with easier preparation. To this end, the supporting electrolyte database has been fed with the electrolytes in increasing order of difficulty and so, if the same cations can be determined with several electrolytes, the program will reject the most difficult one. In this way, only the necessary electrolyte or combination of electrolytes for determining the cations present in the sample will be shown on screen. The whole process is carried out by the expert system in a few seconds. The only data to be introduced by the user are the cations to be determined and the minimum separation between adjacent peaks to be considered by the system. If the user, for any reason, wants to perform the selection manually instead of automatically, the system allows this. In this case, the system asks the user if he wants to use the manual or automatical mode before starting the procedure. On the basis of the information supplied by the user, and data retrieved from the databases, the results of the inference are reported to the user in a results expression screen, In the manual mode the user does not have to introduce any data and the optimizations can be carried out by two interactive screens. In this case, the work done by both user and computer is shown in Fig. 3.
3. Polarographic
procedure
The polarographic data handled in this work have been provided by a Metrohm Polarecord E-506. The dropping mercury electrode (DME), glassy carbon auxiliary electrode and Ag/AgC1/3 M KC1 reference electrode were used for current-potential measurements.
3.1. Quantitative
determinations
The multiple standard addition method is proposed to carry out all the quantitative determinations.
Chimica Acta 316 (1995) 47-56
4. The language Given the nature of the work to do, it was chosen to use the Q-PRO 4 (registered trademark of QUICK-N EASI PRODUCTS Inc.). This language allows to work with any type of database that suits the programmer and the application; and also because of its simplicity in numeric or alpha data handling in nonlimited fields. The language allows a very easy communication between the program and user. The user does not have to carry out a tedious question and answer session to use the program but simply, to accept or refuse the options which appear bar-shaped on the computer screen. The system has been developed for manual acquisition of polarographic data in order to be applied without needing expensive instrumentation or suitable software. Q-PRO 4 allows to read from any widely used database (DBase, LOTUS, etc.) by means of a very easy modification of the starting of the program in order to carry out the direct data acquisition.
5. System design The knowledge-based system consists of six programs with corresponding interactive screens and three programs for automatic selection; five databases with corresponding .FID (file item descriptor generator) and four .IDX (indexed), and three helps (.HLP) to user. A general scheme of the system is shown in Fig. 4. The whole knowledge-based system, which includes the system and its interpreter, uses 446499 bytes. 5.1. Main menu Besides date and other controls in 9 alpha fields, the 8960-bytes menu (O-MEN) offers the work options: - Exit to MS DOS - See help texts - Use data or supporting-metal combinations - Use the supporting-metal combination number by defaults
M.P. Garcia-Armada
et al. /Analytica
Chimica Acta 316 (1995) 47-56
51
- Use another existent supporting-metal combination number - Create, see or print supporting-metal combinations - See existent data or create new data - Select existent data - Build polarogram with selected data - Carry out an identification - Exit and delete the databases contents - Exit, copy and delete the databases contents - Exit without deleting the databases contents - Cancel the data work option - Cancel the exit option Each appears in bar-form by pressing any key. Routines are of the form IF and tables which recognize any key. 5.2. Data input
Fig. 4. General scheme of system architecture.
Flow diagram.
The program destined to choose supporting-metals combinations (ELEC_CAT.FCH) shows a 4%fields and 19072-bytes interactive screen and feeds to two databases: a counter with &bytes record (random type) and another three key indexed with 232-bytes record. This program allows to create up to 99 files or
INPUT OF ELBCTROLYTB-UBTAL COMBINATIONS
Elec_cat.fch
combination n*: [ ELNcTNOLYTBS
1 [MDT-Q-]
To
l]
USE
[Al+DMG ] [MDT-I)-]
[CLO4-5-l
[ABDT+AH]
[Dl+AcAa]
I I NBTALS TO USE /-(As+ I 1A13+1 IA93+1 [Aa3+1 [Bi3+1 [Bi3+] [adz+] [co2+] [cr3+] [cr3+] ] 1 [cU2+1 lc”2+1 lFe3+1 IH92+1 IfinZ+l [NiZ+] [Pb2+] (Sb3+] [znZ+] [ 1
f.2 BEGIN THIS IS THE ACTUALLY
USED ELECTAOLYTE/METAL
COMBINATION
. ..
Fig. 5. Interactive screen for consulting and input of supporting/metal combinations (ELEC_CAT.FCH program). Combination stored as number 1. Abbreviations used are: NH3/NH4, 1 M ammonia buffer; Al + DMG, ammonia buffer with addition of dimethylglyoxime; CLO4-5, perchlorate at pH 5; AEDT + AH, EDTA with acetic acid; Dl + A&s, EDTA with acetic acid and ascorbic acid; AEDT-6-, EDTA at pH 6; and AEDT-9-, EDTA at pH 9.
M.P. Garcia-Armada
52
et al. /Analytica
Chimica Acta 316 (1995) 47-56
EXPERI"ERTAL DATA INPUT DIFFERENTIAL.-PULSB POLAROGlt&PHY ____________________~~~~~~~~~~~~
l_Datoa.fch
cculbination tap: 01 1 y$yJyLYTBs
rl+::: MDT-g-
cLO4-5HCl 1n
Dl+AcAs
AEDT+AH
1
1
I
I
Al
PEAR POTENTIAL rr
7
a
1
WRITE THE POTENTIAL
(O)(frm
-2.99 to +2_99)(8=Precip~tate)(9=NR)
f.2 Delete numeric field f.3 go to optimize Confirm with / Change with
Fig. 6. Interactive screen for consulting and input of polarographic data (l_DATOS:FCH user if he wants to correct any data or simply to consult some record card.
supporting-metal combinations numbered from 1 to 99. Each file is capable to contain 20 supporting electrolytes and 20 metals, that is to say, it is possible to stock information from ca. 1980 supporting electrolytes into 39 600 records with supporting electrolyte, metal and peak potential data.
i= li
ELECTROLYTES TO USE Al+DIIG AEDT-I)-
Record card format to handle by the
To create a file or supporting-metal combination it is only needed to choose this option and write the supporting electrolyte and metal names to be considered. One of the stored supporting-metal combinations is shown in Fig. 5. Each file is chosen independently by the user, and
IDENTIFICATION ______________
Posibles.fch
program).
CombLnation N*x
[Ol] I
CI.Q4-SBCl 1H
ABDT+AR
Dl+AcAm I
-1 CATIONS TO USE Mn2+
r
POTENTIALS ----=-=q -0.05 -0.04 -0,oz II-0.03 B [ -0,Ol +0,01 1 +0,02 +0,03 II 1 +0,04 +o,os -1.05 1 -1.04 J,
m
Pb2+
Sb3+
2x12+
Peak potential: [-1.101 to search into a margin of +/- 0,OS
Multiple
(to +/- 0,05): [ 11
CONFIRM THE MULTIPLE TC MARGIN OF SEARCH +/- 0,OS OR CRANGE IT f.2 Delete numeric field END OF SEARCHING........
Fig. 7. Interactive
screen for metals identification
(POSIBLES.FCH
program).
M.P. Garcia-Armada
et al. /Analytica
2wjor.fch
rCa+v-t1
K-Z+ ILxxz+ G>cdZ+ S>Sb3+ 1 Q>NiZ+I HXo2+ 1T>EnZ+ I (E>Bi3+ IIWZr3+ M>Fe3+p N>HgZ+I IR>PbZ+I IAMg+ I lB>A13+[ C>As3+[ P>Nnz+l
I
I
I
-0.19 -0.45 -0.75 -0.80 -1.05 -1.24 -1.30 88.88 88.88 88.88 88.88 88.88 99.99 99.99 99.99 99.99
1:
II
1: -
II
NH3/N”4*Al+DHG l
Chimica Acta 316 (1995) 47-56
l
I
53
?IKST OPTINISATION To SELECT SUITABLE METHOD ____________________---
The .c+een will lhew, for each elec trolyte, all the peAk potentials ob tained for the difetent cations in numerical order. It will qhow too the potentials dif ference between the cureor poeition potential and the next below.
I 1 I 1 1 1 [ 1 1 1 i [
. .. . Go to 2 Dptimitation . . . . Co to Data program eF.10~ .. . Exit to Mean I&mu .. . .. . Eliminate the Aesay .. . . Accept the Away . . .. .. Save those accepted . . .. .. Choose another Support. Pot.: 99.99 -NR 80.88 =Precipit. ____________----------
I
II SUP/CAT N*Ol DIFFERENCE: +0.26 OPTION-> SUPPDRT.rNH3fNH4 l *CLO4-5-*AEDT+AK*Dl+AcAs*AEDT-6-*AEDT-9-+HCl 1H l . l l l l l l
Fig. 8. Interactive screen for first optimization in manual mode (2_MEJOR.FCH program). Elimination M ammonia buffer medium for the mixture considered in this work (Fe, Cr, Ni, Mn, Co and Cu).
when the expert system finishes the work with one certain file, the user must choose another if it is necessary. The input and store data program (1 DATOS.FCH) has a 92-field interactive screen which offers the input, modify, search and print options from a bar-form menu as described before. It
ELKCl'KGLYTES . NK3/NH4 4 Al+DMG 8' CLo4-s- < AEDT+A?l *r Dl+AcAs d AEDT-C- < AEDT-9- J tic1 1x
of non determinable
cations in 1.0
uses 23296-bytes and the supported database is indexed with 6-searching keys corresponding to possible potential-electrolyte-metal combinations, with ll-bytes record. This program is provided with code-decode tables for record contents. In both ELEC_CAT.FCH and l_DATOS.FCH program operations bar-form menu and screen map
I_opti.m&fch SELECTION OF SDPPDRTING ELECTKOLYTES Ag-Al-As-A8-Ei-Bi_ r-Zr-C~-Zu-F+?+l4+Ni-Pb-Sb-Zn4 # J yr 7 $ q J g ti J q q I J q J J J q I ti J v J J 11 I
i + + + + .
I
:-------------------A1
=DELETE
Fig. 9. Interactive screen for second optimization the mixture considered.
=END
in manual mode (3_0PTIMA.FCH
program).
Selection of suitable electrolytes
to anaiyse
M.P. Garcia-Armada
54
et al. /Analytica
Chimica Acta 316 (1995) 47-56
enter into the first optimization by pressing a function-key. The program (2 MEJOR.FCH) uses 35 200 bytes and shows a 528-fields screen (Fig. 8). It has got an indexed auxiliary database with 404-bytes records for the different selections carried out. The second program (3_0PTIMA.FCH) uses 44800 bytes and shows a 889-fields screen (Fig. 9). This program allows to carry out the final selection by means of routines of the form FOR...NEXT, BEGIN and tables. The eventual selection is incorporated into an indexed database (232 bytes in each record) where the code results will be stored. When the second optimization is finished, by pressing a function key, a 10752-bytes third program (4_POLARO.RES) shows, by means of an interactive screen with 148 fields, which polarogram or polarograms must be done (Fig. 10): supporting electrolytes must be used, determinable metals and peak potentials must be observed. When the automatic mode is chosen by means of AUTO-MAN program (6784 bytes) and the above cited parameters are put in, then both programs 2_MEJOR.AUTO and 3 OPTIMA.AUTO (30464 and 44800 bytes respec&ely) run consecutively. They are 2_MEJOR.FCH and 3_0PTIMA.FCH
tables have been used to control cursor positions. Once into a file, the data input is carried out by fixing in the screen (Fig. 6), both supporting electrolyte and metal with the cursor and enter-key, and writing the peak potential value. 5.3. Identification program A 17 664-bytes program (P~SIBLES.FCH) allows, by means of a 102~fields interactive screen, to search one of the three variables (supporting electrolyte, metal or peak potential) by introducing two of them. An example is shown in Fig. 6, one peak at a potential of - 1.10 V is observed in 1 M ammonia buffer as supporting electrolyte. The program identifies it as Ni’ + and shows the peak potential recorded for this cation (- 1.05 V> and, on its right, the difference between both recorded and introduced values ( + 0.05 V). 5.4. Selection of suitable methodology The selection is carried out by two programs (2_MEJOR.FCH and 3_0PTIMA.FCH) which make first and second optimizations possible. From the screen shown in Fig. 7 it is possible to
Q_POLRRO.RES POXAROGRAMS
(Peak Potentiala)
~~Loz?T~z,::-~TSs _s I= CATIONS AND POTENTIALS Co2+(-1.14)~Ca2+(-1.24)~Cr3+(-1.19) qx Ecu2+(-0.19)~Cu2+(+0.05)~Cu2+(-0.19) ~Cu2+(-0.45)~Mn2+(-1.46)~Fe3+(-0.10) ( INiZ+(-l.OS)[NiZ+(-l.Ol)l ( ) (
t (
)I
EXIT
1 1
PRINT OPTION: [
THESE AFZ ALL THE CHOSEN ELECTROLYTES
Fig. 10. Results considered.
expression
screen
in manual
and automatic
modes
(4_POLAROXES
program).
Analysis
method
for the mixture
h4.P. Garcia-Armada
et al./Analytica
(manual mode) modified with nested FOR...NEXT loops and using the criteria related above, in such a way that they analyse, compare and select the screen fields contents. Once the selection is finished the result is shown on the screen by means of the 4_POLARO.RES program like the manual mode. In each program of the knowledge-based system the option exists of printing either the screens or to print out database contents, with the corresponding routines of format and bearing in mind the printer type to use.
6. Conclusions In comparison with other known systems, this work contributes some new features. The language is very easy for programmers and allows a convenient communication with the user by means of interactive screens. Furthermore it is not necessary to store the interferences between peaks because the system detects them by means of subtractions. This expert system does not require expensive or complex instrumentation, only a personal computer IBM compatible with at least 256 Kbytes of RAM and MS DOS is required, and the user does not need to have knowledge of computers. Nevertheless the system may connect with an external database that acquires directly the data from an analog polarograph linked to the computer. The system applications are wider because it allows to store 39600 records with supporting electrolyte, metal and peak potential data. It is very important to tell again that the applicability of the system depends fundamentally on database contents. The stored peak potentials must have been obtained under the same instrumental conditions and so it is better to use experimental values rather than bibliographic ones.
Table 1 Results of quantitative system indications
%, w/w E, %
determination
by following
the expert
Fe3+
Cr3+
Ni2+
Mn2+
co2+
cu2+
51.84 0.096
24.35 1.35
19.72 -0.91
1.289 -0.70
0.159 - 19.50
0.060 0.00
Chimica Acta 316 (1995) 47-56
55
If the databases are filled with valid values, then the system will provide the best method to solve any problem that may appear during the polarographic analysis of metal mixtures.
7. Validation This expert system has been used for the simultaneous determination of Fe, Cr, Ni, Mn, Co and Cu in a steel sample with certified composition from: - Bundesanstalt fur materialprtifung (BAM), Berlin-Dahlem - Staatl. materialpriifungsamt, Nordheim-Westfalen (MPA) - Max Plank Institut fur eisenforschung, Diisseldorf The supporting electrolytes suggested by the system were ammonia buffer with addition of dimethylglyoxime, sodium perchlorate at pH 5 and EDTAacetic acid buffer (Fig. 10). The quantitative determinations were carried out by the multiple standard addition method. Table 1 shows the relative errors of the results. Moreover the system has been successfully applied to the determination of several metal mixtures in water and in pharmaceutical products samples.
References [ll D.B. McDonnell, Analyst, 106 (1981) 790. 121G. Somer, G. Ozyariik and M.E. Green, Analyst, 110 (1985) 151. [31 A.C. Almon, Anal. Chim. Acta, 249 (1991) 447. 141 M. Esteban, I. Ruisanchez, MS. Larrechi and F.X. Rius, Anal. Chim. Acta, 268 (1992) 95, 107. [51 A.M. Bond and B.S. Grabaric, Anal. Chem., 48 (1976) 1624. [61 P. Lanza, J. Chem. Educ., 67 (1990) 704. 171 G. Tunes, A. Cladera, E. Gomez, J.M. Estela and V. Cerda, J. Elecroanal. Chem., 338 (1992) 49. [81 A. Bobrowski and M. Walczak, Chem. Anal. (Warsaw), 29 (1984) 663. [91 K. Jansen and P. Van Espen, Anal. Chim. Acta, 191 (1986) 169. M.R. Detaevernier and D.L. Massart, DO1 J. Seneyers-Verbeke, Anal. Chim. Acta, 191 (1986) 181. Ml B. Vander Bogaert, J.B.W. Morsink and H.C. Smit, Anal. Chim. Acta, 270 (1992) 107, 115. G. Kateman, M. [l-a J.A. Van Leenwen, B.G.M. Vandeginste,
56
M.P. Garcia-Armada
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Mulholland and A. Cleland, Anal. Chim. Acta, 228 (1990) 145. [13] W.R. Browett, T.A. Cox and M.J. Stillman, in B.A. Hohne and T.H. Pierce (Eds.), Expert Systems Applications in Chemistry (ACS Symposium Series, Vol. 4081, American Chemical Society, Washington, DC, 1989, p. 210.
Chimica Acta 316 (1995) 47-56 [14] M. Esteban, C. Ariho, I. Ruisanchez, MS. Larrechi and F.X. Rius, Anal. Chim. Acta, 284 (1993) 435. [15] M. Esteban, C. Ariho, 1. Ruisanchez, MS. Larrechi and F.X. Rius, Anal. Chim. Acta, 285 (1994) 193, 377. [16] M.P. Garcia-Armada, J. Losada, S. Vicente-PCrez, J. Chem. Educ., in press.
CHIMICA
ACT!4
ELSEVIER
Analytica Chimica Acta 316 (1995) 47-56
Knowledge-based system for the provision of an analytical strategy for simultaneous determination of metals by differential-pulse polarography M.P. Garcia-Armada
a2*, J. Losada a, S. de Vicente-Perez
aDepartamento de Ingenieria Quhica Industrial, Universidad Polittknica de Madrid, C. Jo& Gutierrez Abascal2, b Departamento
de Ciencias Analiticas,
Uniuersidad National de Educacihn a Distancia, C. Senda de1 Rey s/n,
Received 10 November
b 28006 Madrid, Spain 28040 Madr% Spain
1994; revised 24 April 1995; accepted 26 June 1995
Abstract A knowledge-based system for differential-pulse polarography has been developed. This system allows to store all polarographic data (supporting electrolyte-metal-peak potential) and to process them in order to facilitate its handling, and provides the possibility to determine simultaneously in electrolytes or combinations of electrolytes a maximum number of metals with a minimum number of steps. Moreover the system is able to interpret the experimental data provided by a polarogram in order to identify the sample constituents. The system’ program is written in commercially available Q-PRO 4 and allows a very easy user communication. To use the system only a personal computer with MS DOS and a polarograph are required, which means that no extra investments are necessary. Keywords:
Polarography;
Knowledge-based
system; Metals
1. Introduction One of the most interesting differential pulse polarographic (DPP) applications is the simultaneous determination of electroactive species. Many scientists have made contributions to this field for mixtures with several metals [l-3] in various materials, and also some deconvolution programs have been developed to solve problems concerned with overlapping peaks [4-71. DPP allows to perform qualitative and quantita-
* Corresponding
author.
0003-2670/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0003-2670(95)00357-6
tive analyses in the same solution and suitable sensibility and also peak resolution ties. It is therefore very interesting to find that applies general DPP to any sample appear.
it has a possibilia system that may
2. The knowledge base Usually, the work scheme to be followed by the analyst can be summed up as is presented in Fig. 1. The first difficulty with actual multicomponent samples appears when a complicated polarogram, with many peaks and interferences between the sample constituents, must be interpreted since the informa-
M.P. Garcia-Armada
48
et al. /Analytica
tion for each particular mixture is rarely tabulated or described. The next difficulty to overcome by the analytical chemist is the handling and data compilation in order to design the suitable analytic method for each specific problem. By using the suitable supporting electrolyte combination or by addition of complexing agents [8] the identification and determination of the whole electroactive species present in a sample is possible by a few steps without a previous separation stage. The choice of a suitable combination requires to handle a lot of data concerned with the electrochemical behaviour of metals in several media and also to take into account: 1. chemical interferences between supporting electrolyte and each sample constituent; 2. mutual interferences between the present ions as overlapping or peak potential displacements; and 3. minimal analysis times and efforts This all means that it is very difficult to bring this technique into general use, and usually, the analyst therefore prefers to use chemical separation methods. In recent papers it has been described that a series of expert systems have been applied to instrumental techniques like spectrophotometry [9,10], X-ray fluorescence [ll], chromatography [ 121, atomic absorption spectrometry [13] and voltammetry [4,14,15]. However, they have only been applied to groups of a few metals under specific conditions.
-1
1piiqz-q
Choice of suitable methodology for quantitative detemination
F
. Ouantitative
Fig. 1. Systematics
determination
usually followed by the analyst.
Chimica Acta 316 (1995) 47-56
Fig. 2. Aims system schedule.
Our purpose is to develop an optimization tool for general application of the differential-pulse technique to analyse every metal mixture that can occur. The aims of the present expert system are basically (Fig. 2) to store series of experimental polarographic data on supporting electrolytes, metals and peak potentials into files with easy access in order to process them either to identify the constituents or to obtain the best method for quantitative determination when the constituents are known. 2.1. Data Data from the literature can be stored in the database, but generally these have been obtained under specific conditions that may not be applicable to the analysis of complicated mixtures. The system utility depends mainly on database contents. We have filled them with our own experimental data, obtained from mixtures of a great number of cations in several supporting electrolytes [16], and therefore interferences related to the above have been accounted for. Since bibliographic data for very complicated cases are not available, this expert system has not been designed as a polarographic data source but as a tool and for this reason each user can fill the database with his own experimental data according to the experimental conditions to be employed during the performance of the analytical procedure (supporting electrolyte, pH, ionic strength, reference and auxiliary electrode types, etc.). It is important that in order to obtain accurate results, the data have to be actual and valid.
MP. Garcia-Armada
et al./Analytica
Chimica Acta 316 (1995) 47-56
2.2. Identification
49
2nd analyst step: selection of suitable supporting electrolyte. 1st expert system step: ‘cleanness’ of cations for each supporting electrolyte: la, consultation of databases; lb, ordering of cations in numerical order of peak potentials; Ic, rejection of metals which are not present in the sample; Id, rejection of non-electroactive or precipitate cations (an arbitrary peak potential has been assigned to these cations: 99.99 and 88.88 respectively, to this end) and evaluation of the difference between adjacent peak potentials in order to reject which are overlapped; 5. le, storage of determinable cations in each electrolyte into a new database; 6. lf, go to second optimization. 2nd expert system step: ‘cleanness’ of supporting electrolytes: 1. 2a, it consults in database created in 1st step and compares determinable cations in different electrolytes; 2. 2b, it selects the electrolyte or minimum combination of electrolytes to determine the whole of electroactive cations present in the sample, by
The system substitutes the visual comparison of polarograms and automatically carries out the mixture constituents identification. The main error source in the peak assignments is due to the non-coincidence between the experimental peak potential and the stored values. In order to avoid errors in the assignment of polarographic peaks, the system searches into the database’s potential values with intervals of +0.05 V. Intervals can be extended by multiplying by a factor chosen by the user, in such a way that by introducing a certain potential, the program searches, within the fixed interval, the cation which appears at the nearest potential to that introduced. 2.3. Selection of suitable methodology The method selection procedure is carried out in two steps: first and second optimizations which reproduce in an automatic way the ‘steps followed by the analyst: 1st analyst step: bibliographic search to determine the electrochemical behaviour of constituents all together in each electrolyte to be considered. COMPUTER
USER
for each supporting, all the cations in Ep numerical order and the Ep difference between the cation marked with cursor and the next. Shows,
Rejects with the keyboard: less suitable electrolytes . Shows results screen: suitable electrolvtes with - determinable cations - Ep to be observed in the polarogram.
Fig. 3. Work made by computer
and user in manual mode.
50
M.P. Garcia-Armada
et al. /Analytica
rejecting those which are less adequate either because it allows to determine a few cations or because the same cations can be determined by using another electrolyte with easier preparation. To this end, the supporting electrolyte database has been fed with the electrolytes in increasing order of difficulty and so, if the same cations can be determined with several electrolytes, the program will reject the most difficult one. In this way, only the necessary electrolyte or combination of electrolytes for determining the cations present in the sample will be shown on screen. The whole process is carried out by the expert system in a few seconds. The only data to be introduced by the user are the cations to be determined and the minimum separation between adjacent peaks to be considered by the system. If the user, for any reason, wants to perform the selection manually instead of automatically, the system allows this. In this case, the system asks the user if he wants to use the manual or automatical mode before starting the procedure. On the basis of the information supplied by the user, and data retrieved from the databases, the results of the inference are reported to the user in a results expression screen, In the manual mode the user does not have to introduce any data and the optimizations can be carried out by two interactive screens. In this case, the work done by both user and computer is shown in Fig. 3.
3. Polarographic
procedure
The polarographic data handled in this work have been provided by a Metrohm Polarecord E-506. The dropping mercury electrode (DME), glassy carbon auxiliary electrode and Ag/AgC1/3 M KC1 reference electrode were used for current-potential measurements.
3.1. Quantitative
determinations
The multiple standard addition method is proposed to carry out all the quantitative determinations.
Chimica Acta 316 (1995) 47-56
4. The language Given the nature of the work to do, it was chosen to use the Q-PRO 4 (registered trademark of QUICK-N EASI PRODUCTS Inc.). This language allows to work with any type of database that suits the programmer and the application; and also because of its simplicity in numeric or alpha data handling in nonlimited fields. The language allows a very easy communication between the program and user. The user does not have to carry out a tedious question and answer session to use the program but simply, to accept or refuse the options which appear bar-shaped on the computer screen. The system has been developed for manual acquisition of polarographic data in order to be applied without needing expensive instrumentation or suitable software. Q-PRO 4 allows to read from any widely used database (DBase, LOTUS, etc.) by means of a very easy modification of the starting of the program in order to carry out the direct data acquisition.
5. System design The knowledge-based system consists of six programs with corresponding interactive screens and three programs for automatic selection; five databases with corresponding .FID (file item descriptor generator) and four .IDX (indexed), and three helps (.HLP) to user. A general scheme of the system is shown in Fig. 4. The whole knowledge-based system, which includes the system and its interpreter, uses 446499 bytes. 5.1. Main menu Besides date and other controls in 9 alpha fields, the 8960-bytes menu (O-MEN) offers the work options: - Exit to MS DOS - See help texts - Use data or supporting-metal combinations - Use the supporting-metal combination number by defaults
M.P. Garcia-Armada
et al. /Analytica
Chimica Acta 316 (1995) 47-56
51
- Use another existent supporting-metal combination number - Create, see or print supporting-metal combinations - See existent data or create new data - Select existent data - Build polarogram with selected data - Carry out an identification - Exit and delete the databases contents - Exit, copy and delete the databases contents - Exit without deleting the databases contents - Cancel the data work option - Cancel the exit option Each appears in bar-form by pressing any key. Routines are of the form IF and tables which recognize any key. 5.2. Data input
Fig. 4. General scheme of system architecture.
Flow diagram.
The program destined to choose supporting-metals combinations (ELEC_CAT.FCH) shows a 4%fields and 19072-bytes interactive screen and feeds to two databases: a counter with &bytes record (random type) and another three key indexed with 232-bytes record. This program allows to create up to 99 files or
INPUT OF ELBCTROLYTB-UBTAL COMBINATIONS
Elec_cat.fch
combination n*: [ ELNcTNOLYTBS
1 [MDT-Q-]
To
l]
USE
[Al+DMG ] [MDT-I)-]
[CLO4-5-l
[ABDT+AH]
[Dl+AcAa]
I I NBTALS TO USE /-(As+ I 1A13+1 IA93+1 [Aa3+1 [Bi3+1 [Bi3+] [adz+] [co2+] [cr3+] [cr3+] ] 1 [cU2+1 lc”2+1 lFe3+1 IH92+1 IfinZ+l [NiZ+] [Pb2+] (Sb3+] [znZ+] [ 1
f.2 BEGIN THIS IS THE ACTUALLY
USED ELECTAOLYTE/METAL
COMBINATION
. ..
Fig. 5. Interactive screen for consulting and input of supporting/metal combinations (ELEC_CAT.FCH program). Combination stored as number 1. Abbreviations used are: NH3/NH4, 1 M ammonia buffer; Al + DMG, ammonia buffer with addition of dimethylglyoxime; CLO4-5, perchlorate at pH 5; AEDT + AH, EDTA with acetic acid; Dl + A&s, EDTA with acetic acid and ascorbic acid; AEDT-6-, EDTA at pH 6; and AEDT-9-, EDTA at pH 9.
M.P. Garcia-Armada
52
et al. /Analytica
Chimica Acta 316 (1995) 47-56
EXPERI"ERTAL DATA INPUT DIFFERENTIAL.-PULSB POLAROGlt&PHY ____________________~~~~~~~~~~~~
l_Datoa.fch
cculbination tap: 01 1 y$yJyLYTBs
rl+::: MDT-g-
cLO4-5HCl 1n
Dl+AcAs
AEDT+AH
1
1
I
I
Al
PEAR POTENTIAL rr
7
a
1
WRITE THE POTENTIAL
(O)(frm
-2.99 to +2_99)(8=Precip~tate)(9=NR)
f.2 Delete numeric field f.3 go to optimize Confirm with
Fig. 6. Interactive screen for consulting and input of polarographic data (l_DATOS:FCH user if he wants to correct any data or simply to consult some record card.
supporting-metal combinations numbered from 1 to 99. Each file is capable to contain 20 supporting electrolytes and 20 metals, that is to say, it is possible to stock information from ca. 1980 supporting electrolytes into 39 600 records with supporting electrolyte, metal and peak potential data.
i= li
ELECTROLYTES TO USE Al+DIIG AEDT-I)-
Record card format to handle by the
To create a file or supporting-metal combination it is only needed to choose this option and write the supporting electrolyte and metal names to be considered. One of the stored supporting-metal combinations is shown in Fig. 5. Each file is chosen independently by the user, and
IDENTIFICATION ______________
Posibles.fch
program).
CombLnation N*x
[Ol] I
CI.Q4-SBCl 1H
ABDT+AR
Dl+AcAm I
-1 CATIONS TO USE Mn2+
r
POTENTIALS ----=-=q -0.05 -0.04 -0,oz II-0.03 B [ -0,Ol +0,01 1 +0,02 +0,03 II 1 +0,04 +o,os -1.05 1 -1.04 J,
m
Pb2+
Sb3+
2x12+
Peak potential: [-1.101 to search into a margin of +/- 0,OS
Multiple
(to +/- 0,05): [ 11
CONFIRM THE MULTIPLE TC MARGIN OF SEARCH +/- 0,OS OR CRANGE IT f.2 Delete numeric field END OF SEARCHING........
Fig. 7. Interactive
screen for metals identification
(POSIBLES.FCH
program).
M.P. Garcia-Armada
et al. /Analytica
2wjor.fch
rCa+v-t1
K-Z+ ILxxz+ G>cdZ+ S>Sb3+ 1 Q>NiZ+I HXo2+ 1T>EnZ+ I (E>Bi3+ IIWZr3+ M>Fe3+p N>HgZ+I IR>PbZ+I IAMg+ I lB>A13+[ C>As3+[ P>Nnz+l
I
I
I
-0.19 -0.45 -0.75 -0.80 -1.05 -1.24 -1.30 88.88 88.88 88.88 88.88 88.88 99.99 99.99 99.99 99.99
1:
II
1: -
II
NH3/N”4*Al+DHG l
Chimica Acta 316 (1995) 47-56
l
I
53
?IKST OPTINISATION To SELECT SUITABLE METHOD ____________________---
The .c+een will lhew, for each elec trolyte, all the peAk potentials ob tained for the difetent cations in numerical order. It will qhow too the potentials dif ference between the cureor poeition potential and the next below.
I 1 I 1 1 1 [ 1 1 1 i [
I
II SUP/CAT N*Ol DIFFERENCE: +0.26 OPTION-> SUPPDRT.rNH3fNH4 l *CLO4-5-*AEDT+AK*Dl+AcAs*AEDT-6-*AEDT-9-+HCl 1H l . l l l l l l
Fig. 8. Interactive screen for first optimization in manual mode (2_MEJOR.FCH program). Elimination M ammonia buffer medium for the mixture considered in this work (Fe, Cr, Ni, Mn, Co and Cu).
when the expert system finishes the work with one certain file, the user must choose another if it is necessary. The input and store data program (1 DATOS.FCH) has a 92-field interactive screen which offers the input, modify, search and print options from a bar-form menu as described before. It
ELKCl'KGLYTES . NK3/NH4 4 Al+DMG 8' CLo4-s- < AEDT+A?l *r Dl+AcAs d AEDT-C- < AEDT-9- J tic1 1x
of non determinable
cations in 1.0
uses 23296-bytes and the supported database is indexed with 6-searching keys corresponding to possible potential-electrolyte-metal combinations, with ll-bytes record. This program is provided with code-decode tables for record contents. In both ELEC_CAT.FCH and l_DATOS.FCH program operations bar-form menu and screen map
I_opti.m&fch SELECTION OF SDPPDRTING ELECTKOLYTES Ag-Al-As-A8-Ei-Bi_ r-Zr-C~-Zu-F+?+l4+Ni-Pb-Sb-Zn4 # J yr 7 $ q J g ti J q q I J q J J J q I ti J v J J 11 I
i + + + + .
I
:-------------------A1
Fig. 9. Interactive screen for second optimization the mixture considered.
=END
in manual mode (3_0PTIMA.FCH
program).
Selection of suitable electrolytes
to anaiyse
M.P. Garcia-Armada
54
et al. /Analytica
Chimica Acta 316 (1995) 47-56
enter into the first optimization by pressing a function-key. The program (2 MEJOR.FCH) uses 35 200 bytes and shows a 528-fields screen (Fig. 8). It has got an indexed auxiliary database with 404-bytes records for the different selections carried out. The second program (3_0PTIMA.FCH) uses 44800 bytes and shows a 889-fields screen (Fig. 9). This program allows to carry out the final selection by means of routines of the form FOR...NEXT, BEGIN and tables. The eventual selection is incorporated into an indexed database (232 bytes in each record) where the code results will be stored. When the second optimization is finished, by pressing a function key, a 10752-bytes third program (4_POLARO.RES) shows, by means of an interactive screen with 148 fields, which polarogram or polarograms must be done (Fig. 10): supporting electrolytes must be used, determinable metals and peak potentials must be observed. When the automatic mode is chosen by means of AUTO-MAN program (6784 bytes) and the above cited parameters are put in, then both programs 2_MEJOR.AUTO and 3 OPTIMA.AUTO (30464 and 44800 bytes respec&ely) run consecutively. They are 2_MEJOR.FCH and 3_0PTIMA.FCH
tables have been used to control cursor positions. Once into a file, the data input is carried out by fixing in the screen (Fig. 6), both supporting electrolyte and metal with the cursor and enter-key, and writing the peak potential value. 5.3. Identification program A 17 664-bytes program (P~SIBLES.FCH) allows, by means of a 102~fields interactive screen, to search one of the three variables (supporting electrolyte, metal or peak potential) by introducing two of them. An example is shown in Fig. 6, one peak at a potential of - 1.10 V is observed in 1 M ammonia buffer as supporting electrolyte. The program identifies it as Ni’ + and shows the peak potential recorded for this cation (- 1.05 V> and, on its right, the difference between both recorded and introduced values ( + 0.05 V). 5.4. Selection of suitable methodology The selection is carried out by two programs (2_MEJOR.FCH and 3_0PTIMA.FCH) which make first and second optimizations possible. From the screen shown in Fig. 7 it is possible to
Q_POLRRO.RES POXAROGRAMS
(Peak Potentiala)
~~Loz?T~z,::-~TSs _s I= CATIONS AND POTENTIALS Co2+(-1.14)~Ca2+(-1.24)~Cr3+(-1.19) qx Ecu2+(-0.19)~Cu2+(+0.05)~Cu2+(-0.19) ~Cu2+(-0.45)~Mn2+(-1.46)~Fe3+(-0.10) ( INiZ+(-l.OS)[NiZ+(-l.Ol)l ( ) (
t (
)I
1 1
THESE AFZ ALL THE CHOSEN ELECTROLYTES
Fig. 10. Results considered.
expression
screen
in manual
and automatic
modes
(4_POLAROXES
program).
Analysis
method
for the mixture
h4.P. Garcia-Armada
et al./Analytica
(manual mode) modified with nested FOR...NEXT loops and using the criteria related above, in such a way that they analyse, compare and select the screen fields contents. Once the selection is finished the result is shown on the screen by means of the 4_POLARO.RES program like the manual mode. In each program of the knowledge-based system the option exists of printing either the screens or to print out database contents, with the corresponding routines of format and bearing in mind the printer type to use.
6. Conclusions In comparison with other known systems, this work contributes some new features. The language is very easy for programmers and allows a convenient communication with the user by means of interactive screens. Furthermore it is not necessary to store the interferences between peaks because the system detects them by means of subtractions. This expert system does not require expensive or complex instrumentation, only a personal computer IBM compatible with at least 256 Kbytes of RAM and MS DOS is required, and the user does not need to have knowledge of computers. Nevertheless the system may connect with an external database that acquires directly the data from an analog polarograph linked to the computer. The system applications are wider because it allows to store 39600 records with supporting electrolyte, metal and peak potential data. It is very important to tell again that the applicability of the system depends fundamentally on database contents. The stored peak potentials must have been obtained under the same instrumental conditions and so it is better to use experimental values rather than bibliographic ones.
Table 1 Results of quantitative system indications
%, w/w E, %
determination
by following
the expert
Fe3+
Cr3+
Ni2+
Mn2+
co2+
cu2+
51.84 0.096
24.35 1.35
19.72 -0.91
1.289 -0.70
0.159 - 19.50
0.060 0.00
Chimica Acta 316 (1995) 47-56
55
If the databases are filled with valid values, then the system will provide the best method to solve any problem that may appear during the polarographic analysis of metal mixtures.
7. Validation This expert system has been used for the simultaneous determination of Fe, Cr, Ni, Mn, Co and Cu in a steel sample with certified composition from: - Bundesanstalt fur materialprtifung (BAM), Berlin-Dahlem - Staatl. materialpriifungsamt, Nordheim-Westfalen (MPA) - Max Plank Institut fur eisenforschung, Diisseldorf The supporting electrolytes suggested by the system were ammonia buffer with addition of dimethylglyoxime, sodium perchlorate at pH 5 and EDTAacetic acid buffer (Fig. 10). The quantitative determinations were carried out by the multiple standard addition method. Table 1 shows the relative errors of the results. Moreover the system has been successfully applied to the determination of several metal mixtures in water and in pharmaceutical products samples.
References [ll D.B. McDonnell, Analyst, 106 (1981) 790. 121G. Somer, G. Ozyariik and M.E. Green, Analyst, 110 (1985) 151. [31 A.C. Almon, Anal. Chim. Acta, 249 (1991) 447. 141 M. Esteban, I. Ruisanchez, MS. Larrechi and F.X. Rius, Anal. Chim. Acta, 268 (1992) 95, 107. [51 A.M. Bond and B.S. Grabaric, Anal. Chem., 48 (1976) 1624. [61 P. Lanza, J. Chem. Educ., 67 (1990) 704. 171 G. Tunes, A. Cladera, E. Gomez, J.M. Estela and V. Cerda, J. Elecroanal. Chem., 338 (1992) 49. [81 A. Bobrowski and M. Walczak, Chem. Anal. (Warsaw), 29 (1984) 663. [91 K. Jansen and P. Van Espen, Anal. Chim. Acta, 191 (1986) 169. M.R. Detaevernier and D.L. Massart, DO1 J. Seneyers-Verbeke, Anal. Chim. Acta, 191 (1986) 181. Ml B. Vander Bogaert, J.B.W. Morsink and H.C. Smit, Anal. Chim. Acta, 270 (1992) 107, 115. G. Kateman, M. [l-a J.A. Van Leenwen, B.G.M. Vandeginste,
56
M.P. Garcia-Armada
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Mulholland and A. Cleland, Anal. Chim. Acta, 228 (1990) 145. [13] W.R. Browett, T.A. Cox and M.J. Stillman, in B.A. Hohne and T.H. Pierce (Eds.), Expert Systems Applications in Chemistry (ACS Symposium Series, Vol. 4081, American Chemical Society, Washington, DC, 1989, p. 210.
Chimica Acta 316 (1995) 47-56 [14] M. Esteban, C. Ariho, I. Ruisanchez, MS. Larrechi and F.X. Rius, Anal. Chim. Acta, 284 (1993) 435. [15] M. Esteban, C. Ariho, 1. Ruisanchez, MS. Larrechi and F.X. Rius, Anal. Chim. Acta, 285 (1994) 193, 377. [16] M.P. Garcia-Armada, J. Losada, S. Vicente-PCrez, J. Chem. Educ., in press.