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Application of pattern recognition and piezoelectric sensor array for the detection of organic compounds
Falanta, Vol. 42, No. 3, pp. 475482, t995
Pergamon
Copyright '~5 1995 ElsevierScience Ltd Printed in Great Britain. All rights reserved 0039-9140/95 $9.50 + 0.00
0039-9140(95)01435-7
APPLICATION PIEZOELECTRIC
OF
PATTERN
SENSOR OF
ARRAY
ORGANIC
RECOGNITION FOR
THE
AND
DETECTION
COMPOUNDS
GYORGY BARK6, BAL~,ZS PAPP and JOZSEF HLAVAY* University of Veszpr6m, Department of Analytical Chemistry 8201, Veszpr6m, P.O. Box 158, Hungary
(Received 20 January 1994. Revised 26 August 1994. Accepted 16 September 1994) Summary--Organic vapours were measured by an array of piezoelectric crystal detectors. Quartz crystals were coated by different GC stationary phases. Four coated crystals were placed in an array and pattern recognition was used for identification of the compounds including acetone, benzene, chloroform and pentane. A computer program was developed for the measurement of the frequency changes and data processing. Pattern recognition method using feature extraction was applied for identification of analytes.
The determination of organic compounds can mostly be accomplished by gas chromatography (GC), however, some other chemical sensors have also been used. 1 These sensors could be applied for the monitoring of the organic vapours at trace concentration. Two kinds of chemical sensors have mostly been applied: the metal-oxide semiconductor gas sensors] and the piezoelectric quartz crystal sensors. The quartz crystal might be used as surface acoustic wave (SAW) 3 or as piezoelectric quartz crystal microbalance (PQM). 4 The frequency of the SAW sensor ranges from 30 to 200 MHz, while that of the PQM lies from 8 to 15 MHz. Owing to the Sauerbrey equation the change of frequency of a quartz crystal is proportional to the change of the mass deposited on the surface of crystals. 5 AF=--2.3,106,F2,
AM A '
(1)
where AF is the change in frequency (Hz), F is the basis frequency of the quartz crystal (MHz), AM is the change in mass (g), A is the area coated (cm2). In the development of the piezoelectric chemical sensors, the first work of King 6 included the investigation of some liquidcoated crystals that can be operated by partition of the analyte between the gas and liquid phases. From equation (1), King estimated a detection limit of 10 ~ g.7 Guilbault and Tomita 8 developed a piezoelectric device sensitive for the *Author to whom correspondence should be addressed.
organophosphorus compounds and pesticides. They found a mixture of 3-PAD, Triton X-100 and sodium hydroxide that had excellent selectivity and fast response. Karmarkar and Guilbault 9 used iridium(I) complex as a coating material for aromatic hydrocarbons in air. Hahn and co-workers ~° developed a crystal detector for the determination of the acetoin in air. The quartz crystal was coated with semicarbazide. Ho ~ developed a portable device with the piezoelectric crystal coated by Carbowax550. This sensor was used for the monitoring of the toluene in ambient air. Edmonds and West ~2 built a detector for the determination of the hydrocarbons using Pluronic 64 GC stationary phase as coating material. Two crystals were applied: a coated crystal for measurements and an uncoated reference one. It was found that the coating substance has given poor selectivity for hydrocarbons. Coating such as Pluronic L64, Carbowax 20M and squalane, although sensitive, were not particularly selective, and adsorbed many different organic compounds. An array of detectors was suggested providing that the sensitivity of each detector to each analyte was known, and the signal from each detector could be analysed mathematically and the concentration of each component could be estimated, even in multicomponent mixtures. Fraser and co-workers ~3 developed a multisensor piezoelectric crystal detector system for the determination of airborne contaminants such as ammonia. Klinkhachorn and co-workers ~4 designed a PQM system that was able to detect 475
476
GYORGY BARK0 et al.
nanogram level mass changes of sorbed com- The four crystals were coated with stationary pounds. Five oscillator modules were used to phases and the coated area was 0.2 cm 2. The construct an array of four sensors and pattern thin film of the coating was formed by solvent recognition was applied for the multi-com- evaporation. The mass of the coating caused a ponent analysis. Chang j5 has developed an frequency shift of about 8 kHz. The crystals odour sensing system with PQM using a number were placed in a dry nitrogen stream for about of different lipid-coated crystals. The identifi- 2 hr and then were built into the sensor array. cation of odorant has been found to be depen- Coatings were purged with dry nitrogen until dent on the species of lipids. Schmautz j6 used a the resonant frequency of the crystals reached a quartz sensor array with non-selective but steady state. Organic vapours of standard comdifferent sensitive coating material for the pounds were injected into the nitrogen carrier analysis of the anaesthetic gases. The sensor gas. The nitrogen (T 45, supplied by Messer signals were processed with pattern recognition Griesheim, Hungary) contained only 30 ppm methods. Carey ~7 applied pattern recognition water vapour and was dried with CaC12 to methods for selection of stationary phases. A remove the traces of water. The flow rate generally useful procedure was proposed, tested, was measured by rotameter and kept at 20 l./hr and the best sensitive coating materials were (Fig. 2). Thermostatted sample holders were selected. The major problem using PQM sensors applied and the concentrations of the organic is the interference of water vapour. Most of the compounds were calculated using the ideal gas sensing compounds are also sensitive to moist- law. The vapours of the compounds were inure, so it has to be removed from the air without jected by a syringe into the flow of the carrier the reduction of the analyte vapours. Among gas and the analytes reached the four crystals many different solutions, one effective method is simultaneously in the detector. The decrease of the application of the Nation tubing with a 13X the frequency proportional to the change in molecular sieve as drying agent packed on the weight was recorded (equation 1). Frequencies outside of the membrane in a closed container. 2° of the crystals were measured with a four chanOthers include the use of chromatographic nel frequency counter, and compared to the column filled with silica gel or molecular sieves, clock of a computer as reference frequency. A this has to be set into the sample introduction computer program was developed for the line. 4,8 measurement of the change in frequency and for In our work a sensor array has been devel- the application of the pattern recognition oped for detection of the organic vapours. It method. The pattern recognition uses feature consists of four quartz crystals coated by GC extraction and the K-nearest neighbour method stationary phases. Pattern recognition method for the processing of the saved frequency files, j8 using feature extraction has been applied for An IBM AT386 DX2 computer with math identification of the analyte. Model experiments coprocessor was used and an AX5216 counwere carried out to find the optimal conditions ter/timer board (AXIOM) was used for the for the analysis of organic contaminants in frequency measurement. different workshops. RESULTS AND DISCUSSION EXPERIMENTAL
Analytical grade benzene, n-pentane, acetone, chloroform, cyclohexane, toluene and methanol (Reanal, Hungary) were used. Nine MHz AT-cut quartz crystals, resonating in thickness-shear mode, were used. Silver electrodes were deposited to the both sides of the crystals. The crystals were manufactured by Gamma Co., Hungary. Stationary phases of OV1 and OV275 (Supelco), ASI50 (Applied Science Laboratories Inc.), and the polyphenilether (Carlo Erba) were used. The oscillator and data handling system was home-built. The sensor array consisted of four crystals (Fig. 1).
The responses of four quartz crystals coated by different GC stationary phases were investigated as an organic vapour sensor. A characteristic response was given by each sensor placed in the array. Seven different organic vapours were measured. The changes of raw frequency values of a quartz crystal coated by OVI for 25 #mol organic compounds as a function of time are listed in Table I. Adsorption and, consequently desorption, took place in only some hundreds of msec. As can be seen from the data, the recovery rate was the same for all volatile compounds. The reversibility, selectivity and sensitivity of the piezoelectric sensors to vapours rely on the
477
Piezoelectric crystal detectors
Ouadz crystal Top ~ew:
N 2and sample out
/
connection
N2 and sample input Side view:
)uadz crystal
Ouadz
~
N2 and -~ sample out
- - N 2 and sample out
Fig. 1. Piezoelectric sensor array.
coating materials. The recorded frequency change was equilibrium response, and it depends greatly on the flow rate of gas mixture. However, the flow rate was kept constant (20 l./hr) for the reproducible responses. The maximum frequency change of the quartz sensors is listed in Table 2. The GC stationary phases show different sensitivity to organic vapours. From the results four compounds were chosen for the characterization of the group of the organic materials. Benzene was applied as a representative compound of the aromatic hydrocarbons, acetone for ketones, n-pentane for the hydrocarbon groups, and chloroform for the chlorinated
hydrocarbons. The frequencies of the compounds have been drawn as a bar chart t9 (Fig. 3). These values were used for the preparation of the data matrices. The frequency was converted to pattern with feature extraction. The feature extraction method consists of signal processing. The PQM sensor signal can be processed by extraction of the most distinctive feature parts of the signal. The lowest and highest frequencies of four crystals were converted into the feature space (see Table 1). Not only the highest frequency change, but also the lowest one contains valuable information concerning the reaction between the coating materials and analytes.
GYORGY BARK6 et al.
478
Computer
\
Oscillators and data handling card
FIHzl
/ %1 \
.
Drying tube Rotameter
\
Detector array
Carrier gas
Fig. 2. T h e e x p e r i m e n t a l set-up.
The feature space presents data files in the computer's memory. In the present case it consists of eight frequency values (two for each sensor). For classification of vapours the signals of the sensor array were transformed into a concentration independent plane. Linear transformation was applied for normalization of the signal. The process is shown by equations 2 and 3.
X+
=
INT
where X + is the position of the cluster at the highest value in rows ( 1 . . . 32); Y ÷ the position of the cluster at the highest value in columns ( 1 . . . 4 ) (OV1 = 1, ASI50 = 2, OV275 = 3 and PPh-ether = 4, see Fig. 4); AF~w the highest value of the measured frequency on crystal coated by OV1; AFAsJs0 the highest value of the measured frequency on crystal coated by ASI50; +
AFt, + + + . AF &l + AF ASlSO+ AF ov27s+ AF PPh-ether
) R
(2)
Y+=I,
T a b l e 1. T h e c h a n g e in f r e q u e n c y ( H z ) o f a q u a r t z c r y s t a l s e n s o r c o a t e d b y O V I for 25 # m o l o f o r g a n i c c o m p o u n d s as a f u n c t i o n o f s a m p l i n g time Time
(msec) 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150
Benzene 1 1 I 1 -20 -70 -96 -71 --61 -50 -30 --4 -2 --3 -2 -2 --2 -- 1 -I -2 -l 0 - 1 -- 1
Toluol 1 1 --20 -47 -30 -20 -20 - 10 - 1 0 - 1 0 -- 1 0 0 --l 0 0 0 0 0 0 0 1
Cyclohexane 1 1 1 2 1 -80 -160 - 140 - 100 -80 -70 --50 --5 --4 --4 --2 --3 --2 -2 -2 -2 -I - 1 --1
n-Pentane 1 1 0 -
-90 150 177 170 122 -80 -6 -5 --3 - 3 -2 -- 1 --2 I 0 1 l l l 1 1
Acetone
Methanol
1 0 1 - 30 -90 - 110 -50 -30 -2 -1 - 1 --1 0 0 I 0 1 1 1 0 0 l 1 0
0 1 0 1 - 1 -30 - 32 - 75 -25 -20 -1 --1 1 0 1 l 0 1 0 I l l 0 I
Chloroform 0 0 1 0 0 1 -40 - 120 - 150 -140 - 120 --100 --80 --8 -6 0 0 1 0 0 1 0 0 1
Piezoelectric crystal detectors Table 2. The maximum values of frequency changes (Hz) of quartz crystals coated by GC stationary phases for 25 #mol of organic compounds OV1 OV275 ASI50 PPh-ether 1 Benzene 96 25 40 125 2 Toluol 47 10 20 54 3 Cyclohexane 160 175 0 100 4 n-Pentane 177 48 30 170 5 Acetone 110 260 250 390 6 Methanol 75 30 50 50 7 Chloroform 150 120 178 450
of the feature extraction (R = 32). It can be seen that each transformed response is calculated as the ratio of the summary of the four sensor replies. The patterns of the benzene are shown in Fig. 4. This method could eliminate the effects of absolute amounts of vapours on pattern recognition. The patterns were stored as binary data set. The data set gives a value of 1 when a cluster has been found in the column of a sensor. In any other case the values are 0. The size of the saved pattern file was about 10% of the file of frequencies recorded. The number of columns of the data set was equal to the number of the sensors (Fig. 4). The number of rows of the data set was proportional to the resolution of the feature
+
AFov275 the highest value of the measured fre+
quency on crystal coated by OV275; AFpph.ether the highest value of the measured frequency on crystal coated by PPh-ether, R the resolution of the feature extraction (R = 32) and
X- = INT
479
AFovl ) AFov) + AFAs150 + AFov275 + AFpeh-ether " R
(3)
Y---l, where, X - is the position of the cluster at the lowest value in rows (1 . . . 32); Y - the position of the cluster at the lowest value in columns (1 . . . 4) (OV1 = 1, ASI50 = 2, OV275 = 3 and P P h - e t h e r = 4, see Fig. 4); AFow the lowest value of the measured frequency on crystal coated by OVI; AFAs~50 the lowest value of the measured frequency on crystal coated by ASI50; AFov275 the lowest value of the measured frequency on crystal coated by OV275; AFpph.ether the lowest value of the measured frequency on crystal coated by PPh-ether and R the resolution
extraction. With an increasing resolution the calculation time of pattern recognition should considerably be increased. However, large differences in the frequency changes are not necessary for the distinction of the patterns. ~6 The repeatability of the patterns has been found to be the most important criteria of the appropriate recognition. The pattern recognition approach can be seen in Fig. 5a and b. At first, the computer program was taught. Sample of organic vapours was injected and the changes in the frequencies of
450 ;
-
400 •
.
.
.
.
.
•
i
.
I 350 ~ !
I
300 III Acetone 250
IIBenzene ~]Chloroform
200
I Pentane
150 100 50 0 OVl
ASI50
0V275 b
PPh-ether
Fig. 3. Frequency changes of piezoelectric quartz crystals for organic vapours coated by different GC stationary phases.
480
GY6RGY BARK0 et al.
1
2
m
---ram" m
©
m m
©
.m m#
Fig. 4. The piezoelectric sensor array and the pattern for benzene.
the crystals were measured, converted to patterns and saved in the memory (Fig. 5a). After the teaching process, the unknown organic vapour was measured. The pattern of unknown compound was compared with the learned patterns. The two binary data sets were processed by the K-nearest neighbour method. The unknown sample set was grouped to its nearest neighbour learning point. During the learning process the data base of organic compounds can be formed by the given sample set. The maximum score was saved to the matched pattern and the unknown organic compound was identified (Fig. 5b).
The partition of a solute vapour between the surrounding gas phase and the coating material is represented by the adsorption of organic vapour into the sensitive layer. ~7 The solubility coefficient, K, is a parameter to compare different coating materials referred to the sensitivity of test vapours:
AF=k,Vc,Cc=
AFt
,Cg,K (4) Pc AF • Pc K (5) Cg* AFt' where AF is the change in frequency (Hz); AFt the frequency shift of the coating (Hz); Pc the Conversion of the spectra to pattern
121.,
The hequ©ncy apectrn of Ihe benzene
Sensor nrrayvrith fourmystals
N
•
m
j
am
Memorization of the pattern
Investigation on the learned patterns
b,,
Convc~ ~ e
unknown spedrum to pattern
m
m - -
--mm
m
~mmzm
11111-ram--
mm-- m'_
--m
m
Learned pagerns
m
m - - m --
Fig. 5. The process of pattern recognition for identification of benzene.
Identification of the b e n z e n e
Piezoelectric crystal detectors
481
120
1°° I i , i 80 ~ ~f
IAcetone
[
IBBenzen e 60
~Chloroforrn [ Pentane _
40
] I
20
ASIS0
OVI
0V275
PPh-ether
Fig. 6. Solubility coefficients calculated for different GC stationary phases.
density of the coating (g/cm3); Cc the vapour concentration in the coating (g/cm3); Cg the vapour concentration in the gas phase (g/cm3); k is a constant and lie the gas volume in the coating (cm 3). The values of Cg were calculated by the ideal gas law. The solubility coefficients of the coating materials were compared and the results are shown in Fig. 6. Comparing the values of the dF and K (Figs 3 and 6), it can be seen that only the PPh-ether shows a different pattern. This coating proved to be the most sensitive material for the organic vapours. The ASI50 and the OV275 gave poor sensitivity to the aromatic compounds and the hydrocarbons, but they were found to be sensitive coatings for acetone. The OV 1 is generally used in GC for the separation of compounds having different polarity. Therefore, the analytes can appropriately be differentiated by the K values on the four coatings for the pattern recognition. CONCLUSIONS
The array of piezoelectric quartz crystals coated with gas chromatographic stationary phases may be a suitable sensor for identifying selected compounds of organic vapours. A computer program was developed for the measurement of the frequency changes and data processing. The array of the chemical sensor consisted of four coated quartz crystals and the model system can be used for detection of different vapours. On the basis of the results of model experiments the identification of several pollutants may be carried out and characterization of a polluted environment may be performed. The application of the chemical sensor
developed needs the removal of moisture content of the sample. The sensing elements are usually sensitive to water vapours. The effect of this interference can be overcome by incorporating a drying unit set into the analysis line. Further improvements can be expected by using further crystals coated with different GC stationary phases and applying calibration curves for the determination of the amount of the compounds in air. Acknowledgements--The authors wish to express their gratitude to OTKA 2544 for the financial support.
REFERENCES 1. S. J. Haswell and A. D. Walmsley, Anal. Proc., 1991, 28, 115. 2. T. Aishima, Anal. Chim. Acta, 1991, 243, 293. 3. J. T. Wood and J. F. Alder, Talanta, 1992, 39, 1505. 4. M. H. Ho and G. G. Guilbault, Anal. Chem., 1982, 54, 1998. 5. G. Z. Sauerbrey, Z. Physik, 1959, 155, 206. 6. W. H. King, Jr., Anal. Chem., 1964, 36~ 1735. 7. W. H. King, Environ. Sci. Technol., 1970, 4, 1136. 8. G. G. Guilbault and Y. Tomita, Anal. Chem., 1980, 52, 1489. 9. K. H. Karmarkar and G. G. Guilbault, Environ. Lett., 1975, 10, 237. 10. E. C. Hahn, A. A. Suleiman, J. R. Cavanaugh and G. G. Guilbault, Anal. Chim. Acta, 1987, 197, 195. 11. M. H. Ho, B. Reitz and G. G. Guilbault, Anal. Chem., 1980, 52, 1489. 12. T. E. Edmonds and T. S. West, Anal. Chim. Acta, 1980, 117, 147. 13. S. M. Fraser, T. E. Edmonds and T. S. West, Analyst, 1986, 111, 1183. 14. P. Klinkhachorn, B. Huner, E. B. Overton, H. P. Dharmasena and D. A. Gustowski, IEEE Trans. Inst. Meas., 1990, 39, 264.
482
GYt3RGY BARK0 et al.
15. S.M. Chang, Y. lwasaki, M. Suzuki, E. Tamiya, 1. Karube and H. Muramatsu, Anal. Chim. Acta, 1991, 249, 323. 16. W. P. Carey, K. R. Beebe and B. R. Kowalski, Anal. Chem., 1986, 58, 149. 17. A. Schmautz, Sensors Actuators B, 1992, 6, 38. 18. B. R. Kowalski, Pattern recognition in chemical research, in computers and biochemical research, C. E.
Klopfstein (ed.), Vol. 2, p. 74. Academic Press, New York, 1974. 19. A. D. Walmsley, S. J. Haswell and E. Metcalfe, Anal. Chim. Acta, 1991, 242, 31. 20. Y. S. Fung, 83rd Annual Meeting & Exhibition, Pittsburgh, Pennsylvania, Air & Waste Management Association, 1990, 90 170.7.
Pergamon
Copyright '~5 1995 ElsevierScience Ltd Printed in Great Britain. All rights reserved 0039-9140/95 $9.50 + 0.00
0039-9140(95)01435-7
APPLICATION PIEZOELECTRIC
OF
PATTERN
SENSOR OF
ARRAY
ORGANIC
RECOGNITION FOR
THE
AND
DETECTION
COMPOUNDS
GYORGY BARK6, BAL~,ZS PAPP and JOZSEF HLAVAY* University of Veszpr6m, Department of Analytical Chemistry 8201, Veszpr6m, P.O. Box 158, Hungary
(Received 20 January 1994. Revised 26 August 1994. Accepted 16 September 1994) Summary--Organic vapours were measured by an array of piezoelectric crystal detectors. Quartz crystals were coated by different GC stationary phases. Four coated crystals were placed in an array and pattern recognition was used for identification of the compounds including acetone, benzene, chloroform and pentane. A computer program was developed for the measurement of the frequency changes and data processing. Pattern recognition method using feature extraction was applied for identification of analytes.
The determination of organic compounds can mostly be accomplished by gas chromatography (GC), however, some other chemical sensors have also been used. 1 These sensors could be applied for the monitoring of the organic vapours at trace concentration. Two kinds of chemical sensors have mostly been applied: the metal-oxide semiconductor gas sensors] and the piezoelectric quartz crystal sensors. The quartz crystal might be used as surface acoustic wave (SAW) 3 or as piezoelectric quartz crystal microbalance (PQM). 4 The frequency of the SAW sensor ranges from 30 to 200 MHz, while that of the PQM lies from 8 to 15 MHz. Owing to the Sauerbrey equation the change of frequency of a quartz crystal is proportional to the change of the mass deposited on the surface of crystals. 5 AF=--2.3,106,F2,
AM A '
(1)
where AF is the change in frequency (Hz), F is the basis frequency of the quartz crystal (MHz), AM is the change in mass (g), A is the area coated (cm2). In the development of the piezoelectric chemical sensors, the first work of King 6 included the investigation of some liquidcoated crystals that can be operated by partition of the analyte between the gas and liquid phases. From equation (1), King estimated a detection limit of 10 ~ g.7 Guilbault and Tomita 8 developed a piezoelectric device sensitive for the *Author to whom correspondence should be addressed.
organophosphorus compounds and pesticides. They found a mixture of 3-PAD, Triton X-100 and sodium hydroxide that had excellent selectivity and fast response. Karmarkar and Guilbault 9 used iridium(I) complex as a coating material for aromatic hydrocarbons in air. Hahn and co-workers ~° developed a crystal detector for the determination of the acetoin in air. The quartz crystal was coated with semicarbazide. Ho ~ developed a portable device with the piezoelectric crystal coated by Carbowax550. This sensor was used for the monitoring of the toluene in ambient air. Edmonds and West ~2 built a detector for the determination of the hydrocarbons using Pluronic 64 GC stationary phase as coating material. Two crystals were applied: a coated crystal for measurements and an uncoated reference one. It was found that the coating substance has given poor selectivity for hydrocarbons. Coating such as Pluronic L64, Carbowax 20M and squalane, although sensitive, were not particularly selective, and adsorbed many different organic compounds. An array of detectors was suggested providing that the sensitivity of each detector to each analyte was known, and the signal from each detector could be analysed mathematically and the concentration of each component could be estimated, even in multicomponent mixtures. Fraser and co-workers ~3 developed a multisensor piezoelectric crystal detector system for the determination of airborne contaminants such as ammonia. Klinkhachorn and co-workers ~4 designed a PQM system that was able to detect 475
476
GYORGY BARK0 et al.
nanogram level mass changes of sorbed com- The four crystals were coated with stationary pounds. Five oscillator modules were used to phases and the coated area was 0.2 cm 2. The construct an array of four sensors and pattern thin film of the coating was formed by solvent recognition was applied for the multi-com- evaporation. The mass of the coating caused a ponent analysis. Chang j5 has developed an frequency shift of about 8 kHz. The crystals odour sensing system with PQM using a number were placed in a dry nitrogen stream for about of different lipid-coated crystals. The identifi- 2 hr and then were built into the sensor array. cation of odorant has been found to be depen- Coatings were purged with dry nitrogen until dent on the species of lipids. Schmautz j6 used a the resonant frequency of the crystals reached a quartz sensor array with non-selective but steady state. Organic vapours of standard comdifferent sensitive coating material for the pounds were injected into the nitrogen carrier analysis of the anaesthetic gases. The sensor gas. The nitrogen (T 45, supplied by Messer signals were processed with pattern recognition Griesheim, Hungary) contained only 30 ppm methods. Carey ~7 applied pattern recognition water vapour and was dried with CaC12 to methods for selection of stationary phases. A remove the traces of water. The flow rate generally useful procedure was proposed, tested, was measured by rotameter and kept at 20 l./hr and the best sensitive coating materials were (Fig. 2). Thermostatted sample holders were selected. The major problem using PQM sensors applied and the concentrations of the organic is the interference of water vapour. Most of the compounds were calculated using the ideal gas sensing compounds are also sensitive to moist- law. The vapours of the compounds were inure, so it has to be removed from the air without jected by a syringe into the flow of the carrier the reduction of the analyte vapours. Among gas and the analytes reached the four crystals many different solutions, one effective method is simultaneously in the detector. The decrease of the application of the Nation tubing with a 13X the frequency proportional to the change in molecular sieve as drying agent packed on the weight was recorded (equation 1). Frequencies outside of the membrane in a closed container. 2° of the crystals were measured with a four chanOthers include the use of chromatographic nel frequency counter, and compared to the column filled with silica gel or molecular sieves, clock of a computer as reference frequency. A this has to be set into the sample introduction computer program was developed for the line. 4,8 measurement of the change in frequency and for In our work a sensor array has been devel- the application of the pattern recognition oped for detection of the organic vapours. It method. The pattern recognition uses feature consists of four quartz crystals coated by GC extraction and the K-nearest neighbour method stationary phases. Pattern recognition method for the processing of the saved frequency files, j8 using feature extraction has been applied for An IBM AT386 DX2 computer with math identification of the analyte. Model experiments coprocessor was used and an AX5216 counwere carried out to find the optimal conditions ter/timer board (AXIOM) was used for the for the analysis of organic contaminants in frequency measurement. different workshops. RESULTS AND DISCUSSION EXPERIMENTAL
Analytical grade benzene, n-pentane, acetone, chloroform, cyclohexane, toluene and methanol (Reanal, Hungary) were used. Nine MHz AT-cut quartz crystals, resonating in thickness-shear mode, were used. Silver electrodes were deposited to the both sides of the crystals. The crystals were manufactured by Gamma Co., Hungary. Stationary phases of OV1 and OV275 (Supelco), ASI50 (Applied Science Laboratories Inc.), and the polyphenilether (Carlo Erba) were used. The oscillator and data handling system was home-built. The sensor array consisted of four crystals (Fig. 1).
The responses of four quartz crystals coated by different GC stationary phases were investigated as an organic vapour sensor. A characteristic response was given by each sensor placed in the array. Seven different organic vapours were measured. The changes of raw frequency values of a quartz crystal coated by OVI for 25 #mol organic compounds as a function of time are listed in Table I. Adsorption and, consequently desorption, took place in only some hundreds of msec. As can be seen from the data, the recovery rate was the same for all volatile compounds. The reversibility, selectivity and sensitivity of the piezoelectric sensors to vapours rely on the
477
Piezoelectric crystal detectors
Ouadz crystal Top ~ew:
N 2and sample out
/
connection
N2 and sample input Side view:
)uadz crystal
Ouadz
~
N2 and -~ sample out
- - N 2 and sample out
Fig. 1. Piezoelectric sensor array.
coating materials. The recorded frequency change was equilibrium response, and it depends greatly on the flow rate of gas mixture. However, the flow rate was kept constant (20 l./hr) for the reproducible responses. The maximum frequency change of the quartz sensors is listed in Table 2. The GC stationary phases show different sensitivity to organic vapours. From the results four compounds were chosen for the characterization of the group of the organic materials. Benzene was applied as a representative compound of the aromatic hydrocarbons, acetone for ketones, n-pentane for the hydrocarbon groups, and chloroform for the chlorinated
hydrocarbons. The frequencies of the compounds have been drawn as a bar chart t9 (Fig. 3). These values were used for the preparation of the data matrices. The frequency was converted to pattern with feature extraction. The feature extraction method consists of signal processing. The PQM sensor signal can be processed by extraction of the most distinctive feature parts of the signal. The lowest and highest frequencies of four crystals were converted into the feature space (see Table 1). Not only the highest frequency change, but also the lowest one contains valuable information concerning the reaction between the coating materials and analytes.
GYORGY BARK6 et al.
478
Computer
\
Oscillators and data handling card
FIHzl
/ %1 \
.
Drying tube Rotameter
\
Detector array
Carrier gas
Fig. 2. T h e e x p e r i m e n t a l set-up.
The feature space presents data files in the computer's memory. In the present case it consists of eight frequency values (two for each sensor). For classification of vapours the signals of the sensor array were transformed into a concentration independent plane. Linear transformation was applied for normalization of the signal. The process is shown by equations 2 and 3.
X+
=
INT
where X + is the position of the cluster at the highest value in rows ( 1 . . . 32); Y ÷ the position of the cluster at the highest value in columns ( 1 . . . 4 ) (OV1 = 1, ASI50 = 2, OV275 = 3 and PPh-ether = 4, see Fig. 4); AF~w the highest value of the measured frequency on crystal coated by OV1; AFAsJs0 the highest value of the measured frequency on crystal coated by ASI50; +
AFt, + + + . AF &l + AF ASlSO+ AF ov27s+ AF PPh-ether
) R
(2)
Y+=I,
T a b l e 1. T h e c h a n g e in f r e q u e n c y ( H z ) o f a q u a r t z c r y s t a l s e n s o r c o a t e d b y O V I for 25 # m o l o f o r g a n i c c o m p o u n d s as a f u n c t i o n o f s a m p l i n g time Time
(msec) 0 50 100 150 200 250 300 350 400 450 500 550 600 650 700 750 800 850 900 950 1000 1050 1100 1150
Benzene 1 1 I 1 -20 -70 -96 -71 --61 -50 -30 --4 -2 --3 -2 -2 --2 -- 1 -I -2 -l 0 - 1 -- 1
Toluol 1 1 --20 -47 -30 -20 -20 - 10 - 1 0 - 1 0 -- 1 0 0 --l 0 0 0 0 0 0 0 1
Cyclohexane 1 1 1 2 1 -80 -160 - 140 - 100 -80 -70 --50 --5 --4 --4 --2 --3 --2 -2 -2 -2 -I - 1 --1
n-Pentane 1 1 0 -
-90 150 177 170 122 -80 -6 -5 --3 - 3 -2 -- 1 --2 I 0 1 l l l 1 1
Acetone
Methanol
1 0 1 - 30 -90 - 110 -50 -30 -2 -1 - 1 --1 0 0 I 0 1 1 1 0 0 l 1 0
0 1 0 1 - 1 -30 - 32 - 75 -25 -20 -1 --1 1 0 1 l 0 1 0 I l l 0 I
Chloroform 0 0 1 0 0 1 -40 - 120 - 150 -140 - 120 --100 --80 --8 -6 0 0 1 0 0 1 0 0 1
Piezoelectric crystal detectors Table 2. The maximum values of frequency changes (Hz) of quartz crystals coated by GC stationary phases for 25 #mol of organic compounds OV1 OV275 ASI50 PPh-ether 1 Benzene 96 25 40 125 2 Toluol 47 10 20 54 3 Cyclohexane 160 175 0 100 4 n-Pentane 177 48 30 170 5 Acetone 110 260 250 390 6 Methanol 75 30 50 50 7 Chloroform 150 120 178 450
of the feature extraction (R = 32). It can be seen that each transformed response is calculated as the ratio of the summary of the four sensor replies. The patterns of the benzene are shown in Fig. 4. This method could eliminate the effects of absolute amounts of vapours on pattern recognition. The patterns were stored as binary data set. The data set gives a value of 1 when a cluster has been found in the column of a sensor. In any other case the values are 0. The size of the saved pattern file was about 10% of the file of frequencies recorded. The number of columns of the data set was equal to the number of the sensors (Fig. 4). The number of rows of the data set was proportional to the resolution of the feature
+
AFov275 the highest value of the measured fre+
quency on crystal coated by OV275; AFpph.ether the highest value of the measured frequency on crystal coated by PPh-ether, R the resolution of the feature extraction (R = 32) and
X- = INT
479
AFovl ) AFov) + AFAs150 + AFov275 + AFpeh-ether " R
(3)
Y---l, where, X - is the position of the cluster at the lowest value in rows (1 . . . 32); Y - the position of the cluster at the lowest value in columns (1 . . . 4) (OV1 = 1, ASI50 = 2, OV275 = 3 and P P h - e t h e r = 4, see Fig. 4); AFow the lowest value of the measured frequency on crystal coated by OVI; AFAs~50 the lowest value of the measured frequency on crystal coated by ASI50; AFov275 the lowest value of the measured frequency on crystal coated by OV275; AFpph.ether the lowest value of the measured frequency on crystal coated by PPh-ether and R the resolution
extraction. With an increasing resolution the calculation time of pattern recognition should considerably be increased. However, large differences in the frequency changes are not necessary for the distinction of the patterns. ~6 The repeatability of the patterns has been found to be the most important criteria of the appropriate recognition. The pattern recognition approach can be seen in Fig. 5a and b. At first, the computer program was taught. Sample of organic vapours was injected and the changes in the frequencies of
450 ;
-
400 •
.
.
.
.
.
•
i
.
I 350 ~ !
I
300 III Acetone 250
IIBenzene ~]Chloroform
200
I Pentane
150 100 50 0 OVl
ASI50
0V275 b
PPh-ether
Fig. 3. Frequency changes of piezoelectric quartz crystals for organic vapours coated by different GC stationary phases.
480
GY6RGY BARK0 et al.
1
2
m
---ram" m
©
m m
©
.m m#
Fig. 4. The piezoelectric sensor array and the pattern for benzene.
the crystals were measured, converted to patterns and saved in the memory (Fig. 5a). After the teaching process, the unknown organic vapour was measured. The pattern of unknown compound was compared with the learned patterns. The two binary data sets were processed by the K-nearest neighbour method. The unknown sample set was grouped to its nearest neighbour learning point. During the learning process the data base of organic compounds can be formed by the given sample set. The maximum score was saved to the matched pattern and the unknown organic compound was identified (Fig. 5b).
The partition of a solute vapour between the surrounding gas phase and the coating material is represented by the adsorption of organic vapour into the sensitive layer. ~7 The solubility coefficient, K, is a parameter to compare different coating materials referred to the sensitivity of test vapours:
AF=k,Vc,Cc=
AFt
,Cg,K (4) Pc AF • Pc K (5) Cg* AFt' where AF is the change in frequency (Hz); AFt the frequency shift of the coating (Hz); Pc the Conversion of the spectra to pattern
121.,
The hequ©ncy apectrn of Ihe benzene
Sensor nrrayvrith fourmystals
N
•
m
j
am
Memorization of the pattern
Investigation on the learned patterns
b,,
Convc~ ~ e
unknown spedrum to pattern
m
m - -
--mm
m
~mmzm
11111-ram--
mm-- m'_
--m
m
Learned pagerns
m
m - - m --
Fig. 5. The process of pattern recognition for identification of benzene.
Identification of the b e n z e n e
Piezoelectric crystal detectors
481
120
1°° I i , i 80 ~ ~f
IAcetone
[
IBBenzen e 60
~Chloroforrn [ Pentane _
40
] I
20
ASIS0
OVI
0V275
PPh-ether
Fig. 6. Solubility coefficients calculated for different GC stationary phases.
density of the coating (g/cm3); Cc the vapour concentration in the coating (g/cm3); Cg the vapour concentration in the gas phase (g/cm3); k is a constant and lie the gas volume in the coating (cm 3). The values of Cg were calculated by the ideal gas law. The solubility coefficients of the coating materials were compared and the results are shown in Fig. 6. Comparing the values of the dF and K (Figs 3 and 6), it can be seen that only the PPh-ether shows a different pattern. This coating proved to be the most sensitive material for the organic vapours. The ASI50 and the OV275 gave poor sensitivity to the aromatic compounds and the hydrocarbons, but they were found to be sensitive coatings for acetone. The OV 1 is generally used in GC for the separation of compounds having different polarity. Therefore, the analytes can appropriately be differentiated by the K values on the four coatings for the pattern recognition. CONCLUSIONS
The array of piezoelectric quartz crystals coated with gas chromatographic stationary phases may be a suitable sensor for identifying selected compounds of organic vapours. A computer program was developed for the measurement of the frequency changes and data processing. The array of the chemical sensor consisted of four coated quartz crystals and the model system can be used for detection of different vapours. On the basis of the results of model experiments the identification of several pollutants may be carried out and characterization of a polluted environment may be performed. The application of the chemical sensor
developed needs the removal of moisture content of the sample. The sensing elements are usually sensitive to water vapours. The effect of this interference can be overcome by incorporating a drying unit set into the analysis line. Further improvements can be expected by using further crystals coated with different GC stationary phases and applying calibration curves for the determination of the amount of the compounds in air. Acknowledgements--The authors wish to express their gratitude to OTKA 2544 for the financial support.
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