surface concentration of chlorpyrifos evaluated by electromagnetic discharge imaging technique

Surface concentration of chlorpyrifos evaluated byelectromagnetic discharge imaging technique

Y.E. Wu a,*, C.R. Yu a, G.C. Sih b

a Department of Mechanical Engineering, National Taiwan University of Science and Technology, Taiwan, ROCb Institute of Fracture and Solid Mechanics, Lehigh University, Bethlehem, PA, USA

Abstract

Image analysis is used in conjunction with the Electromagnetic Discharge Imaging (EDI) technique to evaluate the

surface concentration and the characteristic discharge intensity of chlorpyrifos. Nearly identical surface condition of

gypsum specimens were subjected to chlorpyrifos solutions of various concentrations. The EDI discharges were then

applied to detect the changes. Experimental results show that the EDI discharge intensity increases with increasing

chlorpyrifos concentration up to 0.4% where a saturation limit is reached. Since the changes of the ionization property

and the dielectric property caused by the concentration variation have opposing e�ects on the EDI discharge intensity,

saturation by the foregoing two e�ects occur at high concentration. Further analysis shows that three characteristic gray

levels were found in the gray-level distribution curve for each chlorpyrifos concentration of 0.1%, 0.4%, and 1.6%.

These characteristic gray levels can be used to rank concentration of chlorpyrifos. Ó 1998 Elsevier Science Ltd. All

rights reserved.

1. Introduction

The EDI technique involves applying a highfrequency and high voltage AC signal to the platesof capacitor-like arrangement between the speci-men and the electrode. This results in the forma-tion of an electrical discharge from the surface ofthe specimen that depends on its geometry andproperties, e.g., dielectric strength, conductivity,and chemical composition, etc. The discharge isrelatively uniform and stable. The so-called``emission image'' or ``discharge image'' is thenexposed onto the commercial ®lm placed betweenthe specimen and the electrode. The embodying

information about the specimen can be manifestedin many ways, e.g., intensity, pattern, spectrumdistribution, etc. The EDI process is similar toKirlian photography, which is named after Seyonand Valentina Kirlian from Soviet Union. A his-torical account of EDI can be found in [1±3].However, the EDI method should be carefullydistinguished from the corona discharge techniquewhen used in Nondestructive testing (NDT) [1].The applications of EDI to NDT are numerous[5,6,8±15]. The method can be used for detectingthe variations of physical property and chemicalproperty as well as other imperfections in thematerial, including organic matters.

Application of EDI to evaluate the pesticideresidue on a material relies on ionization in theelectrical breakdown. It involves the collisions ofelectrons, ions, and photons with gas molecule,

Theoretical and Applied Fracture Mechanics 30 (1998) 39±49

* Corresponding author. Tel.: +8862 7376451; fax: +8862

7376460; e-mail: [email protected].

0167-8442/98/$ ± see front matter Ó 1998 Elsevier Science Ltd. All rights reserved.

PII: S 0 1 6 7 - 8 4 4 2 ( 9 8 ) 0 0 0 4 2 - 1

and electrode processes that occurred at or nearelectrode surface. The rate of ionization is directlyproportional to the ionization current. Dependingon the energy levels, electrons or ions can be re-moved from specimens to take part in electricaldischarges. The amount would depend on thegeometrical variations of the specimen besides itsmaterial properties. By relating the changes tothose in the specimen, a procedure can be devel-oped for assessing the chemical and electricalproperty changes, both qualitatively and/orquantitatively.

Changes of chemical compositions will alterelectrical behavior of specimens. During the elec-trical discharge process, material emits electronshaving their own energies due to the variations of``workfunction'' and ``dielectric property'' viastreamer. Therefore, di�erent discharge patternand intensity can be observed. In addition to thechanges of composition, discharge intensity canalso be a�ected by the variation of concentration.For example, in air, the EDI image appears to beblue or a reddish-purple (depending on electric®eld strength) where the ionization and excitationfavor the arc spectrum of N2 and nitric oxide.While yellow can be identi®ed with the presence ofC2 [6].

The technique for analyzing pesticide residueshas advanced rapidly in recent years with ex-tremely high sensitivity. Stringent requirements aretherefore imposed on the reliability and repro-ducibility. The characteristics of the corona en-hancement caused by the inhomogeneities resultedfrom the chemical compositions and/or concen-tration variations can thus be used to detect thepesticide residue on foods. The advantage of EDIover conventional methods of chemical analysis isthat information can be gathered for the entire®eld. The discharge intensity a�ected by pesticideresidues can be recorded photographically and/ordigitally to an atomic scale level. Various colorsand intensities can be correlated to the type andamount of foreign chemicals or energy states.Chlorpyrifos is a drug with long lasting e�ect andextensive usage. It is suitable for removing envi-ronmental pest, especially for black tiny mosqui-toes, ¯ea, and stripped-mosquitoes both adult andlarva. Since the residual e�ect of this pesticide can

last up to 1�2 months, it is necessary to know theamount of dosage on the food prior to harvest toavoid any harmful e�ect. The purpose of this studyis to evaluate the residual surface concentration ofchlorpyrifos on a gypsum specimen.

2. EDI process and basic parameters

2.1. EDI process

All materials emitted electrons under excitation.Their characteristics depend on the materialproperties such as chemical compositions, homo-geneity, defects, crystal structure, etc. They willa�ect the discharge pattern, frequency, color, andlight intensity. Di�erent EDI images are thus ob-tained and o�er information on the changes ofmechanical, chemical and physical properties ofthe material.

Discharge processes involve various kinds ofparticle collision and electron carriers' propaga-tion mechanisms. During the discharge process,the quantities of such carriers and their intensitiesof emitted light are varied. Discharge process canbe divided into the ``gas process'' and ``electrodeprocess'' [1]. The ionization process associatedwith the discharge involves only collision of elec-tron, ions, photons and gas molecules and is re-ferred to as the ``gas process'', while the ``electrodeprocess'' involves discharge phenomenon nearbyor directly on the electrode surface. The distinctionlies mainly in the origin of the ionization and/ordeionization electron carriers as the initiator. Inmost situations, these two processes occur simul-taneously, unless in an extreme condition wherethe ®eld frequency or discharge gap is very high. Inthe discharge process, the electrons can escapefrom the electrode. That is when the kinetic energyobtained from applied ®eld is su�ciently high sothat they could exceed the potential barrier. Thispotential barrier is called the ``work function''. Itcorresponds to the minimum energy that is re-quired for electrons to escape from the metal sur-face [1,7]. Work function can be a�ected bytemperature, light, impurities, strain, and electric®eld. The carriers generated from discharge processwill reenter the discharge process. The avalanche of

40 Y.E. Wu et al. / Theoretical and Applied Fracture Mechanics 30 (1998) 39±49

electrons and ions between electrodes can thus beaccelerated.

The specimen geometry can also a�ect the ion-ization procedure. Di�erent specimen shape andthickness require di�erent value of external voltageto start the discharge process, and will a�ect the®nal electric ®eld distribution, eddy current loss,dielectric loss and gas breakdown voltage. All ofthese would in¯uence the results.

2.2. Basic parameters

There are many parameters, which will a�ectthe EDI processes. They include the waveformpattern of generator, amplitude, frequency andduration of applied electric ®eld, specimen geom-etry, physical property and chemical property, etc.Displayed in Fig. 1 is a block diagram showing theparameters that would in¯uence the EDI results.

As the molecules from the chlorpyrifos solutionand air are mixed together, the equivalent dielec-tric constant ee of the mixed gas can be expressedby [1]

ee � b�e0k0 ÿ e1k1�2

�e0 � 2e1��k0 � 2k1�2; �1�

where b is a constant. Note that e0 and e1 are thedielectric constants of molecules from chlorpyrifossolution and air respectively, while k0 and k1 arethe corresponding thermal conductivities.

As the chlorpyrifos concentration in solution isaltered, the electrical property (e.g. dielectric con-stant, conductivity, etc.) of the specimen will bechanged accordingly. Since the whole electrodecon®guration (system) can be regarded as a simpleseries R-L-C equivalent circuit, according to elec-

trical theory, the local electric current will bevaried with the changes of electrical properties ofthe specimen and air gap. Thus, the total imped-ance ZT can be expressed as (where the inductanceimpedance is neglected)

ZT � �R2e � X 2

c �1=2 � �R2e � �1=xCe�2�1=2

; �2�where Re, Ce are the equivalent resistance andequivalent capacitance of the electrode con®gura-tion (system); Xc (� 1/xCe) is the equivalent ca-pacitance impedance, and x is the sourcefrequency. Thus, the discharge current Ie in theEDI electrode con®guration can be obtained by

Ie � V �t�=�R2e � �1=xCe�2�1=2; �3�

where V(t) is the applied electric voltage.Some of the molecules from chlorpyrifos solu-

tion could di�use to the air gap that will alter thewhole dielectric constant of air gap and will a�ectthe total ionization current. Since chlorpyrifoscontains a large amount of molecules having highdielectric constant, the total capacitance imped-ance can thus be decreased and result in an in-crease in ionization current and light intensity. Onthe other hand, the increase in resistance of thespecimen will result in a decrease of the ionizationcurrent and light intensity as shown in Eq. (3).

2.3. Image processing

The streamers generated from electrical dis-charge will produce di�erent intensity and fre-quency of photons. These photons will generatevarious degrees of brightness on the ®lm. The imageprocessing technique digitizes the EDI dischargeimage; it can be used with the quasi-spectrum

Fig. 1. In¯uencing parameters associated with EDI.

Y.E. Wu et al. / Theoretical and Applied Fracture Mechanics 30 (1998) 39±49 41

analyzing method to enhance the di�erence betweenimages. The EDI image is usually expressed by``mean gray-level value'' because it is produced bylights having various frequencies and intensities.More speci®cally, it can be shown that

Mean Gray-level Value

�Xn�255

n�0

�Gn � Ngn�" #

=�Px � Py�; �4�

where Gn is the gray-level value of the nth pixel andNgn the global number of pixels that have gray-level values of Gn; while, Px and Py are the numberof pixels in the horizontal and vertical directions,respectively.

3. EDI instrumentation and procedure

3.1. EDI setup

A block diagram of the EDI system used in thiswork is shown in Fig. 2. It consists of a functiongenerator, pulse generator, power ampli®er, oudincoil and electrode. The direct contact procedure isadopted. The arrangement for the electrode systemconsists of a sheet of conductive material (I.T.O.)[4,5] and a grounded specimen. Film is insertedbetween the two electrodes with the emulsion sidefacing the specimen maintaining a carefully con-trolled dielectric gap distance of L. The gas dis-charge image is exposed directly to the ®lm.Processing of EDI photos includes the image in-put, data acquisition, and analysis.

3.2. Experiment procedure

3.2.1. Specimens preparationThe gypsum specimen is 10 mm ´ 30 mm. Since

the discharge intensity is very sensitive to surface

conditions, the specimen surface is very carefullyprepared and polished (using #600 of sandpaper)prior to in®ltration of the chlorpyrifos solutions.The commercial grade of chlorpyrifos (a kind oforganophosphatic pesticide) was used in thisstudy, where the compositions of chlorpyrifos is:O, O-diethyl-O, 3,5,6-trichloro-2-pyridyl phos-phorothioate (40.8%) + solvent (59.2%, including20% of high alcohol polyethylene glycol ether and39.2% of Kerosene). 5 ml of chlorpyrifos is mixedwith di�erent amount of distilled water to obtainconcentrations of 0.1%, 0.2%, 0.4%, 0.8%, and1.6%. Parts of the solutions with concentration of0.1%, 0.2%, and 0.4% were further diluted to so-lutions with concentrations of 10±50 ppm. Afterthat, suction tube was used to applied 0.5 ml of thechlorpyrifos with di�erent concentrations to thegypsum specimens. Every 30 s, a solution dropletwas applied to the same position of the gypsumspecimen to insure a good di�usion e�ect.

3.2.2. EDI photographingThe direct contact procedure is adopted to

minimize information loss. An air gap of 0.05 mmis used; the exposure duration is 2 s where fre-quency is set at 420 Hz. The Film is Konica superXG 100. Shown in Fig. 3 is the specimen andelectrode con®guration of EDI photographingarrangement.

3.2.3. Image processingAlthough the original EDI image obtained by

direct contact can be used to analyze the charac-teristic variation of the specimen, the photographscan only provide a qualitative analysis. More de-tailed information can be obtained by the appli-cation of image analysis and pseudo coloringmethod [16] to digitize the electromagnetic dis-charge image. The images can be converted to

Fig. 2. Block diagram of EDI system.


digitized signals, or be converted into any usablepseudo images. Fig. 4 describes the EDI imageprocess procedure; the original image is convertedto a 256-graylevelized image. Density slicingtechnique and pseudo-color image processingmethods were used in this study to enhance thefeature extraction capability.

4. Discussions and results

4.1. EDI image intensity distribution

EDI photos of gypsum specimens in®ltratedwith various concentrations of the chlorpyrifossolutions are converted into digital ®les to obtainthe gray-level values and distribution curves. Themean gray-level value of each photo is calculated.Three (3) of the EDI photos of the gypsum spec-imen and their gray-level distribution curves withchlorpyrifos concentration of 10, 30, and 50 ppm,are shown respectively in Fig. 5(a)±(c) to serve asan example. Displayed in Fig. 6 are the variationsof mean gray-level of the EDI discharge intensitywith the chlorpyrifos concentration ranging from10 to 50 ppm. It can be seen that the increase ofmean gray-level value is linearly proportional tothe increase of chlorpyrifos concentration.

The breakdown voltage of electrical dischargecan be altered by many parameters, such as, theelectrode property, distance between electrodes,and the property of the air gap between the elec-trodes etc. Changes of the chlorpyrifos concen-tration could a�ect the density of electrons, thelocal current ¯ow, air gap characteristic and thelocal electric potential-®eld distribution. Accord-ing to the discharge theory [1,8], the breakdownvoltage is inversely proportional to the densities ofelectrons and ions, while the discharge intensity isproportional to the electron density. Breakdownvoltage could decrease as the density of electrons isincreased with more chlorpyrifos molecules dif-fused into the air gap. This can be re¯ected by thedischarge intensity where more streamers wouldoccur during the discharge process. In addition,the dielectric constants of the chlorpyrifos andwater molecules are greater than that of the air. Asthese molecules di�use into the air gap, theequivalent dielectric constant could be increased.According to Eqs. (2) and (3), for a ®xed sourcefrequency, the total equivalent impedance of airgap could decrease. This will result in a linear in-crease of the ionization/discharge current. Conse-quently, the mean gray-level value is increasedlinearly with increasing chlorpyrifos concentrationin solution as shown in Fig. 6.

Changes of the dielectric property also a�ect thelocal current ¯ow in the EDI discharge process.The increase of dielectric strength could decreasethe local ®eld intensity and the available energy;this will result in a decrease of local current ¯ow.As the molecules of the chlorpyrifos solutioncontinue to di�use into the air gap, the dielectricstrength of the mixed air gap may increase, re-sulting in an increase of the ionization potential.When the amount of larger molecules in the airgap reaches a certain value, the in¯uence of di-electric constant of the mixed air gap on the dis-charge intensity increase becomes less signi®cant.

Fig. 4. EDI image processing procedure.

Fig. 3. Specimen and electrode con®guration of EDI photo-

graphing arrangement.


The discharge intensity would not increase linearlywith the increase of chlorpyrifos concentration.Instead, the increase of the dielectric constant andelectrical resistivity of the gypsum specimen by themolecules from chlorpyrifos solution would have

much stronger e�ect on the discharge process. Theincrease of electrical resistivity caused by increas-ing chlorpyrifos concentration in the gypsumspecimen tends to decrease the available potential®eld strength for ionization. Thus, the discharge

Fig. 5. Three of EDI photos and their gray-level value distribution curves with chlorpyrifos concentration of: (a) 10 ppm; (b) 30 ppm.

and (c) 50 ppm.


intensity will be decreased with the increase of thechlorpyrifos concentration. These two factors haveopposing e�ects on the discharge intensity suchthat the saturation of discharge would occur athigher concentration.

It can be observed from Figs. 7 and 8 that themean gray-level value and the discharge intensity(bright dots) increases linearly with increasingconcentration of the chlorpyrifos solution from0.1% to 0.4%. The trend is the same as that shownin Fig. 6 for low chlorpyrifos concentration.Within this range, the increase of ionization cur-rent caused by the molecules of chlorpyrifos so-lution di�used into the air gap still plays animportant role in the discharge process. Beyond0.4% of concentration, the increase of electricalresistivity in the gypsum specimen caused by theabsorption of large molecules from the chlorpyr-ifos solution also becomes an important factorthat a�ects discharge intensity. Since these twofactors a�ect discharge intensity in opposite ways,a saturation value for the mean gray-level prevails.It follows that a slight increase of discharge in-tensity with chlorpyrifos concentration is observedfor specimens having the chlorpyrifos concentra-tion from 0.4% to 1.6%.

Fig. 7. EDI photos of gypsum specimen with chlorpyrifos

concentration of 0.1%, 0.2%, 0.4%, 0.8%, and 1.6%, respec-

tively.

Fig. 6. Variations of mean gray-level value of EDI discharge

intensity with chlorpyrifos concentration from 10 to 50 ppm.


4.2. The characteristic gray level (dischargeintensity)

A signi®cant feature was observed in this study.The number of bright dot on the EDI photo wasincreased with increasing concentration of chlor-pyrifos as shown in Fig. 7. The same applies to themean gray-level value of discharge intensity. Acorrelation between the ®lm intensity and chlor-pyrifos concentration can be established based on

the distribution curve of discharge intensity underthe same experimental conditions, since the ®lmintensity is proportional to the exposure time andthe amount of energy received.

Shown in Fig. 9 is the gray-level value distri-bution curves of specimens having chlorpyrifosconcentrations of 0.1%, 0.4% and 1.6%. Fivecharacteristic values were found. They correspondto gray-level values of 74, 116, 132, 150 and 248.The optical intensity for those areas having gray-level of 74 results from the background signal, andcan be neglected [16]. The other four characteristicgray-level values, i.e., 116, 132, 150 and 248, re-quire further study.

The application of EDI to NDE relies on ion-ization in electrical breakdown. It involves thecollisions of electrons, ions and photons with gasmolecules inside the air gap, and the electrodeprocesses take place at or near the electrode sur-face. The rate of ionization is directly proportionalto the discharge current. Depending on the energylevels, the electrons and ions can be removed fromthe solid to take part in the electrical discharges.The amount would depend on the geometricalvariations of the solid in addition to its materialproperties. The ¯ow chart shown in Fig. 10 out-lines the cause and e�ect in the EDI process.

Fig. 9. The graylevel value distribution curve of gypsum specimen with chlorpyrifos concentration of 0.1%, 0.4%, and 1.6%.

Fig. 8. Variations of mean gray-level values of EDI discharge

intensity with chlorpyrifos concentration of 0.1%, 0.2%, 0.4%,

0.8%, and 1.6%, respectively.


The available energy associated with the elec-trical and chemical properties of specimen iscombined and re¯ected via the discharge stream-ers. The discharge spectrum is composed of excitedmolecules and radicals formed within the dis-charge in addition to the electrons and photons.Based on the quantum theory, the emission spectraof molecules or electrons depend on the micro-structure and the composition of the test specimenas well as the experimental parameters. Thus, theEDI photos are produced by photons from dif-ferent origins with various amounts of energies.The advantage of EDI photo is that informationcan be gathered for the entire ®eld regardless of theenergy source arisen from di�erent energy ¯uxesemitted at varying rates and magnitudes. The in-tensity and character of photon emissions as re-¯ected by inhomogeneities in the material and byforeign chemicals can be recorded photographi-cally. The intensity and pattern of the dischargeimage recorded on the ®lm can be analyzed to sortout the characteristic optical intensity I1, I2, etc.,with the corresponding energy level U1, U2, etc.,via intensity analysis [6].

Chlorpyrifos solution is composed of four typesof molecules; they are water molecule, chlorpyrifosmolecule, kerosene molecule, and high alcoholpolyethylene glycol ether molecule, respectively.Since there are four characteristics gray-level val-

ues left for identi®cation, it is natural to assumethat each characteristic gray-level is associatedwith one type of molecules. The chemical compo-sitions of chlorpyrifos reagent used in this experi-ment is 40.8% of chlorpyrifos, 39.2% of keroseneand 20% of high alcohol polyethene glycol ether,and the arithmetic volume ratio of these threecomponents is 2.04 : 1.96 : 1.

The pixel numbers related to the gray-level of116, 132 and 150 are 1750, 1450, and 860, re-spectively, for chlorpyrifos concentration of 1.6%.The arithmetic ratio of pixel number at these threegray-level value is 2.03 : 1.69 : 1. Similar resultsare also obtained for chlorpyrifos concentration of0.1% and 0.4%, the arithmetic ratios of pixelnumber at these three gray-level values for 0.1%are 2.02 : 1.62 : 1, and for 0.4% are 2.02 : 1.75 : 1.Comparing these three arithmetic ratios of pixelnumber with the arithmetic ratio of chemicalcontent for chlorpyrifos, it is evident that the gray-level of 116, 132, and 150 are the characteristicgray-level values of the chlorpyrifos. The gray-level of 116 is associated with chlorpyrifos mole-cules, while the gray-level values of 132 and 150can be related to kerosene and high alcohol poly-ethylene glycol ether molecule, respectively. Fur-thermore, the pixel numbers at these three gray-level values is increased with the increases ofchlorpyrifos concentration as shown in Fig. 11. A

Fig. 10. The cause and e�ect in the EDI process.


linear relationship was observed for gray-levelvalue of 150 that is associated with the chlorpyr-ifos molecule. It further indicates that these threecharacteristic gray levels can be used to rank theconcentration of the chlorpyrifos reagent.

The gray-level value of 248 is associated withthe water molecule. For the characteristic graylevel of water molecule, due to the water content ofchlorpyrifos-solution decreases with increasing ofthe chlorpyrifos concentration, the conductivity ofchlorpyrifos solution is thus proportional to theincreases of water content. Consequently, the dis-charge intensity re¯ected by water is higher for thechlorpyrifos solution of low concentrations. Thiscan be seen in Fig. 9, the pixel number of graylevel 248 for 0.1%, 0.4%, and 1.6% are 650, 250,and 200, respectively. The pixel number at gray-level value of 248 is decreased with the increase ofchlorpyrifos concentration; the decrement of pixelnumber is much larger for the high chlorpyrifosconcentration. Therefore, the mean gray-levelvalues for specimens having 0.4% concentrationand higher increase very slowly. It should be notedthat these characteristic gray-level values could bealtered if the EDI conditions were changed. Ifhigher discharge duration or frequency settingwere chosen, those characteristic gray-level values

would be shifted to higher region, and vice versa.Therefore, the experimental conditions should bekept constant so as to minimize the error.

5. Conclusions

It is demonstrated that the EDI technique canbe used to evaluate the concentration of pesticide.Based on the experimental results, the followingconclusions can be made:· The EDI discharge intensity increases with in-

creasing chlorpyrifos concentration up to 0.4%when a saturation limit is reached.

· Since the changes of the ionization property andthe dielectric property caused by the concentra-tion variation have opposing e�ects on the EDIdischarge intensity, the discharge intensitywould be saturated by these two interactive ef-fects at high concentration.

· Further analysis shows that three characteristicgray levels were found in the gray-level distribu-tion curve for each chlorpyrifos concentrationof 0.1%, 0.4%, and 1.6%. These characteristicgray levels can be used to rank the concentrationof the chlorpyrifos.

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surface concentration of chlorpyrifos evaluated by electromagnetic discharge imaging technique

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