pulsed amperometric detection of furan compounds in transformer oil

Analytica Chimica Acta 450 (2001) 253–261

Pulsed amperometric detection of furancompounds in transformer oil

Timothy Boswortha, Steven Setforda,1, Richard Heywoodb, Selwayan Sainia,∗a Cranfield Centre for Analytical Science, Cranfield University at Silsoe, Silsoe, Beds MK45 4DT, UK

b National Grid Company plc., Kelvin Avenue, Leatherhead, Surrey KT22 7ST, UK

Received 11 July 2001; received in revised form 22 August 2001; accepted 29 August 2001

Abstract

The failure of high voltage transformers can result in significant cost and supply implications to both power supplier andconsumer alike and in extreme cases may result in explosion, serious injury or death. Transformer failure can be predictedby measuring furanics present in the oil, produced by the thermolytic breakdown of cellulosidic insulators. Failing units canhave furanic levels of up to 10�g ml−1. The use of pulsed amperometric detection (PAD) to measure furanics in transformeroils in real time is reported here. Oils were examined by pre-extraction or direct suspension in aqueous measurement solutionor by solubilisation and direct PAD measurement in organic solvents. Linear relationships between PAD response and furanicconcentration was found for 2-furaldehyde and furfuryl alcohol (F-OH) across the range of 0–10�g ml−1, with PAD provingmost sensitive to the latter compound. PAD was performed directly in the organic phase int-butanol with 0.1 M tetramethylammonium hydroxide, with aged oils containing >2�g ml−1 of 2-furaldehyde yielding data within close agreement (<9%)of a standard chromatographic method. The simplicity and rapidity of this method offers the power transmission industry ameans of monitoring furanic levels in transformers in real time, thereby reducing the risk of uncontrolled transformer failure.© 2001 Elsevier Science B.V. All rights reserved.

Keywords: PAD; Transformer oil; Furanics

1. Introduction

Electrical power transformers are used to step upor step down voltage and are an integral componentof any efficient power distribution network. A typi-cal transformer incorporates coils of conducting wirewrapped around a core and covered with a paper-basedinsulator. Essential to the operation of these units aretransformer oils that serve both an insulating and heat

∗ Corresponding author. Tel.:+44-0-1525-863513;fax: +0044-0-1525-863533; URL:http//www.ccas.org.uk.E-mail address: [email protected] (S. Saini).

1 Co-corresponding author.

dissipation function. Regrettably, there are instancesof transformers failing whilst in service, creating sig-nificant cost-implications for the power supplier and,in extreme cases, explosion with a consequent threatof severe injury or death. Most failures are initiated bytransformer overload, a state that can be predicted bymeasuring indicator or ‘priority’ compounds producedin the transformer oil as the overload is generated.

One such group of priority compounds are thefuranics, aliphatic heterocycles produced by the ther-molytic breakdown of cellulose, a main constituentof the paper-based insulator. The commonest furanicsfound in failing units are 2-furaldehyde (2-FA) andfurfuryl alcohol (F-OH). Essentially, temperatures

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254 T. Bosworth et al. / Analytica Chimica Acta 450 (2001) 253–261

in excess of 80◦C thermolytically degrade cellulosepolymer chains in the paper into their constituentglucose monomer units, which are then cleaved andfurther degraded to yield furanics. The postulatedmechanism of paper breakdown is well-documented[1–4]. The chromatographic analysis of thermolyti-cally degraded oil extracts has shown that that thetotal furanic content in poorly cooled transformerscan be up to 10�g ml−1 [5].

Due to the considerable cost and safety implica-tions inherent in operating failing transformers, muchwork has focussed on measuring furanics in order topredict failure. Off-line diagnostic methods centre onmeasuring dissolved gases in oils [4,6] or chromato-graphic techniques [5]. However, since transformerfailure may be rapid, the ideal test method should besimple, low-cost and fast, allowing repeated and rou-tine operation in an on-line or at-line capacity. Mindfulof these issues, Blue et al. [7,8] have described fluores-cent methods and also an opto-electronic instrumentthat are responsive to the accumulation of 2-FA in oil.

Several of the key furan indicator compounds canbe detected electrochemically, using methods such asamperometry. However, a main drawback is foulingof the working electrode (WE) surface due to theadsorption of oxidised species. Such considerationsled Dilleen et al. [9] to propose pulsed amperomet-ric detection (PAD), a self-cleaning electro-analyticalmethod, for the measurement of 2-FA in aqueoustransformer oil extracts [10,11]. The method incorpo-rated 2-FA extraction into 0.1 M NaCl/0.1 M NaClO4via a macrodialyser cell and the potential for assayautomation through use of flow injection. Whilst de-tection was possible, extraction efficiencies of∼15%were recorded.

The aim of this study was to address the sam-ple preparation issue regarding the PAD detectionof furanics in transformer oil. Tests were performedon artificially spiked and genuinely aged trans-former oils and results were compared against anindustry-standard method. Two routes were investi-gated. Firstly, direct suspension of oil samples intopredominantly aqueous measurement solution withsimultaneous extraction and measurement of furanicsby PAD. Secondly, a novel method recently devel-oped in our laboratories in which PAD is performeddirectly in the organic phase by solubilisation of oilsample into a supporting organic solvent containing

organic base/electrolyte. Both methods were com-pared against PAD experiments performed on standardspiked aqueous solutions and batch extracted samples.

2. Experimental

2.1. Reagents and solutions

Sodium perchlorate, sodium hydroxide, 2-fural-dehyde, furfuryl alcohol, furan and tetramethylammonium hydroxide (TMAH) were supplied bySigma–Aldrich, Poole, UK. Transformer oil samples,all of the same oil type and originally produced bythe same major global manufacturer, were donatedby the National Grid Company (NGC, Leatherhead,UK). Supplied were an unused oil containing no fu-ran derivatives, an oil aged artificially in absence ofpaper insulant and hence contained no furanics anda number of used oils, with known amounts of 2-FAas determined by NGC using the CEI/IEC standardmethod 1198, 1993 [12]. Deionised reverse osmosis(DRO) water was prepared using an Elgastat system(Elga, High Wycombe, UK).

2.2. Preparation of supporting solutions used inPAD measurements

2.2.1. Aqueous measurement solutionAqueous measurement solution consisted of 23

parts 0.1 M NaOH/0.1 M NaClO4 + 1 part solvent.Solvents examined included hexane, tetrahydrofu-ran, t-butanol (tert form) and acetonitrile (all Sigma,Poole, UK) with the latter proving most suitable forfurans dissolution. Tests were performed at ambienttemperature (∼20◦C).

2.2.2. Organic measurement solutionOrganic measurement solution consisted of 10 mM

base/electrolyte in 99% (v/v) solvent and 1% (v/v)DRO water. Solvent-base/electrolyte combinationsexamined were acetonitrile-NaOH/NaClO4 andt-butanol–TMAH.

2.2.3. Furanics in measurement solutionFuranics were dissolved in the chosen measurement

solution at the 50�g ml−1 level and diluted in the sameas appropriate.

T. Bosworth et al. / Analytica Chimica Acta 450 (2001) 253–261 255

2.2.4. Furanics in transformer oilA hundred microlitres of 5 mg ml−1 furanic in

aqueous measurement solution was added to 9.9 mltransformer oil to give a stock solution containing50�g ml−1 of furanic (2-FA or F-OH). The spikedoils were diluted in the same oil as appropriate.

2.3. Sample preparation

2.3.1. Extraction methodFuranics were extracted from transformer oil prior

to PAD by adding 1 ml of oil (spiked/unspiked) to 1 mlaqueous measurement solution in Eppendorf tubes,vigorously shaking for 1 min and bench centrifugingat 6500 rpm for 5 min. After phase separation, the up-per (oil) layer was discarded and the residual aqueousphase reconstituted to 1 ml with aqueous measurementsolution and measured by PAD.

2.3.2. Direct injection methodOil was added directly to the aqueous measurement

solution. Stirring of the system resulted in the disper-sion of the oil phase into discrete droplets surroundedby aqueous measurement solution, hence providing alarge surface area for dynamic extraction of analytes.The acetonitrile in the measurement solution enhancedsolubilisation of furanics into the aqueous phase inwhich PAD was performed.

2.3.3. Organic-phase PADOil was added to stirred organic measurement solu-

tion and PAD performed directly in the organic phase.

2.4. Electrochemical measurements

The electrochemical cell consisted of a 10 ml glassbeaker containing 5 ml of measurement solution, aplatinum (Pt) disc WE (1.6 mm diameter), glassycarbon counter electrode (3.2 mm diameter) and sil-ver/silver chloride reference electrode (all BioAna-lytical Systems, Congleton, UK). The platinum WEwas polished using a BAS PK4 polishing kit, rinsedwith DRO water and sonicated (5 min) in DRO waterprior to each experiment. During measurement, thesystem was allowed to equilibrate (∼150 s) prior toaddition of 10�l of sample and the increase in currentresponse recorded after 50 s. Tests were performed at30◦C to prevent solidification oft-butanol.

2.5. Pulsed amperometric detection (PAD)

Measurements were performed under strong alka-line conditions to ensure efficacy of the cleaning pro-cess. The WE was first poised at+0.8 V (Eox) for 0.4 s(tox) to oxidise adsorbed species on the electrode sur-face. The potential was then changed to−1.2 V (Ered)for 0.4 s (tred) to reduce the oxidised species, result-ing in their removal and hence cleaning and regenera-tion of the WE surface. The electrode was then poisedat the optimum furanics detection potential,+0.3 V(Edet), for 0.2 s (tdet) with no delay time. The cyclewas repeated over the requisite analysis time. The WEcurrent was sampled at the end of theEdet step and thecurrent–time profile recorded. PAD experiments wereperformed in stirred solutions using a potentiostat con-structed in our laboratories [13]. Organic-phase PADincorporated anEox of +1.6 V.

2.6. Dual pulse staircase voltammetry (DPSV)

DPSV was used to identify the electrochemicalproperties of the furanics and, like PAD, incorpo-rated a pulsed cleaning step [14,15]. First, the WEwas poised at+0.7 V for 1 s to oxidatively desorbsurface adsorbed organic compounds with simultane-ous formation of surface oxide on the Pt WE. Theelectrode was then stepped to−0.9 V for 1 s to re-duce and remove this surface oxide and regeneratea clean Pt surface. The potential was then scannedfrom −1 to +1 V (scan rate of 0.5 V s−1) in 10 mVpotential steps. The WE current was sampled at eachpotential and the voltammogram recorded. DPSVexperiments were performed on quiescent solutionsusing a PSTAT10 Autolab electrochemical analyserwith GPES 3 software (Ecochemie, Utrecht, TheNetherlands).

3. Results and discussion

3.1. Selection of optimum detection potential

DPSV, performed on F-OH, 2-FA and furan in bothaqueous and organic measurement solutions, indicatedan optimum PAD potential window (Edet) of +0.2to +0.4 V (versus Ag/AgCl) for both measurement


Fig. 1. DPSV voltammograms of 10 ml aliquots of 10�g�l−1 (A)aqueous measurement solution, (B) 2-furaldehyde and (C) furfurylalcohol, in 5 ml aqueous measurement solution.

solutions. NoiE distortions were observed in the or-ganic measurement solutions tested and no shift inoxidative peakmaxima were observed for F-OH and2-FA when tested in the organic and aqueous measure-ment solutions. The solvents used in the organic mea-surement solutions (acetonitrile andt-butanol) were ofsufficient polarity to disassociate the electrolytes usedand hence, provide adequate conductivity in thesesolutions. Typical voltammograms for F-OH, 2-FAand blank aqueous measurement solution is shown inFig. 1. Furan was found to be non-electroactive underthe conditions employed. Individual PAD experimentswere then performed at fixed detection potentialsof between+0.1 and+0.5 V in which it was con-firmed that the optimumEdet for F-OH and 2-FA was+0.3 V based on the criterion of the greatest signal,and greatest signal/noise ratio (S/N). Introduction of10�l of 2 �g ml−1 F-OH and 2-FA yielded responsesof 0.200±0.010 and 0.174±0.0015�A, respectively(n = 5) at anEdet of +0.3 V. In organic conditionswith the same furanic concentrations, F-OH and 2-FAyielded responses of 0.150 ± 0.012 and 0.120 ±

0.002�A, respectively (n = 5) at anEdet of +0.3 V.Higher detection potentials resulted in a decrease insignal due to increased formation of oxide on theWE surface.

4. Analysis of spiked solutions by PAD

4.1. Experimental data

Calibration data relating to the measurement of2-FA and F-OH spiked directly into aqueous measure-ment solution or unused transformer oil, (see Section2) is given in Table 1. Actual PAD experimental datais shown in Fig. 2.

4.2. Spiked aqueous measurement solutions

Linear relationships for 2-FA and F-OH withcorrelation coefficient values of 1.000 and 0.9982,respectively were observed across the concentra-tion range of 0–10�g ml−1. The response reached aplateau at around 25�g ml−1 for both compounds.PAD proved∼90% more sensitive for F-OH (gra-dient value of 1.93 × 10−7 A �g−1 ml−1) than 2-FA(1.01 × 10−7 A �g−1 ml−1). No measurable re-sponse was noted for the non-substituted furan parentcompound.

4.3. Furanics spiked transformer oilaqueous extracts

Linear relationships (correlation coefficient valuesof 0.9881 and 0.9919) across the concentration rangeof 0–10�g ml−1 with limits of detection of 0.77 and0.47�g ml−1 could be described for 2-FA and F-OH,respectively. Since a total furanic level of 10�g ml−1

is indicative of critical transformer failure, this methodis capable of providing useful quantitative analyticaldata. Again, PAD proved more responsive to F-OH(9.2 × 10−7 A �g−1 ml−1) than 2-FA (6.2 × 10−7

A �g−1 ml−1), by almost 50%.The transformer oil artificially aged in the absence

of paper material should contain no furan derivatives.Extracts of this oil were prepared in an identical man-ner to the spiked oil. No PAD response was observedwhen introducing aliquots of these extracts into the


Table 1Trend data relating to the PAD of 2-furaldehyde and furfuryl alcohol across the range of 0–10�g ml−1 analytea

Analyte Trend data Relativeresponse (%)

R.S.D.range (%)

Correlation(R2)

LOD(�g ml−1)

Spiked aqueous measurement solution2-FA y = 1.01 × 10−7x + 1.25 × 10−6 100.0 2.4–7.3 1.0000 0.56F-OH y = 1.93 × 10−7x + 1.13 × 10−6 100.0 2.3–5.9 0.9982 0.30

Spiked transformer oil aqueous extracts2-FA y = 6.20 × 10−8x + 1.02 × 10−6 61.4 2.8-8.9 0.9881 0.77F-OH y = 9.22 × 10−8x + 7.35 × 10−7 47.8 2.1–8.7 0.9919 0.47

Direct addition of oil to aqueous measurement solution2-FA y = 4.66 × 10−8x + 6.98 × 10−8 46.1 2.3–6.2 0.9902 0.61F-OH y = 5.75 × 10−8x + 6.60 × 10−8 29.8 2.5–7.1 0.9996 0.24

Oil solubilisation in organic measurement solution and direct PAD analysis2-FA y = 4.85 × 10−8x + 6.37 × 10−8 48.0 1.9–5.9 0.9841 0.42F-OH y = 6.07 × 10−8x + 4.55 × 10−8 31.5 2.1–6.8 0.9907 0.23

a Relative response relates the signal versus concentration gradient obtained by a given method against the equivalent furanic responsein spiked aqueous measurement solution gradient;y = PAD response (A),x = analyte concentration (�g ml−1), n = 3, LOD = limit ofdetection.

measurement vessel indicating that no electroactivespecies capable of interfering with the furanic PADresponse were generated during the artificial agingprocess.

Fig. 2. Actual PAD responses to the addition of 10�l aliquots of oil aged in the absence of insulating paper and spiked with 2-FA (5�g ml−1)to (a) 5 ml of aqueous measurement solution and (b) 5 mlt-butanol containing 10 mM TMAH and 1% (v/v) distilled-reverse osmosis water.

The sample extraction process resulted in a decreasein system sensitivity to 61 and 48% of that recorded for2-FA and F-OH, respectively when spiked in aqueousmeasurement solution. Two factors were considered


primarily responsible: incomplete furanics extractionfrom the oil sample, and interaction of co-extractedoil matrix components with the WE. The latter factorshould be minimal due to the self-cleaning nature ofPAD and not vary significantly since the same oil sam-ple was used in all experiments. When comparing thesize of the background current in the absence of spikedanalyte, a mean decrease of 41.5±3.4 nA, presumablydue to co-extracted oil matrix effects was recorded.By subtracting this current value from the values ob-tained for the spiked measurement solution standards,mean extraction efficiencies of 87.8 and 68.5% wererecorded for 2-FA and F-OH, respectively.

4.4. Direct addition of oil to aqueousmeasurement solution

Direct addition of transformer oil sample intothe aqueous measurement solution and consequentfuranics determination by PAD negates the require-ment for an extraction step and removes the issue ofvariability in extraction efficiency due to the basicphysio-chemical differences between the furanics andalso inter-batch variability factors. However, successrequires the non-miscibility of the sample with thepredominantly aqueous measurement solution to beconsidered.

Direct addition of microlitre quantities of furanic-spiked transformer oil resulted in the dispersionof the oil into droplets in the stirred continuousaqueous phase. Such an approach offers the capa-bility of in situ extraction of furanics across theoil/aqueous-phase interface due to the considerablesurface area of the sample/extractant interface. How-ever, the benefits of the in situ extraction procedurecome at a price: an increase in sample matrix effectsdue to the presence of bulk oil components. The gra-dients of the linear trend-lines were 4.66× 10−8 and5.75×10−8 A �g−1 ml−1 for 2-FA and F-OH, respec-tively across the concentration range of 0–10�g ml−1

(Table 1). This equated to a decrease in responseto 46 and 30% of that recorded in the spiked aque-ous measurement solution standards for 2-FA andF-OH, respectively. As before, the alcohol elicitedthe greater response by the system (23% larger sig-nal than for 2-FA). Again, the response of the sys-tem was found to plateau at∼25�g ml−1 for bothanalytes.

4.5. Oil solubilisation and direct PAD analysis

Recent work in our laboratories has demonstratedthat PAD can be performed in certain organic solvents,provided that specific conditions are adhered to: (1) asmall amount of water, typically 1%, is present; (2)base is present at a concentration of at least 10 mM;(3) an increased cleaning oxidation potential is re-quired, typically+1600 to+1800 mV; (4) the chosensupporting solvent does not undergo electrochemicalreactions at the working electrode.

We have identified a number of solvents that donot elicit a PAD response and hence are suitable assupporting solvents, including acetonitrile, dioxane,benzonitrile andt-butanol. Whilst many alcohols aresusceptible to oxidation by PAD, it is evident thatthe tert nature of the latter solvent acts to protect thehydroxyl group from oxidation. The more open struc-ture of other alcohols such as the linear aliphaticsand F-OH ensures that similar steric shielding effectsare not evident when these compounds are subjectedto PAD. The requirement for small quantities ofwater in the measurement solvent is unclear, but isbelieved to provide appropriate solubilisation of theelectrolyte/base.

Simple experiments were performed in which equalvolumes of transformer oil and supporting solventwere shaken together, with botht-butanol and acetoni-trile proving completely miscible with the oil. Furtherexamination identified suitable electrolyte/base sys-tems soluble at the 10 mM level int-butanol–TMAH(to provide the necessary hydroxyl ions for PAD)and acetonitrile-NaCl/NaClO4. Correspondingly, so-lutions containing these solvent–salt systems with1% (w/v) DRO water were prepared. Transformer oilpreparations spiked with furanics were then addeddirectly to stirred solvent-electrolyte/base solutionsand the PAD responses recorded, the results for thet-butanol–TMAH system being shown in Table 1. Theacetonitrile-NaCl/NaClO4 system yielded less reli-able data (high S.D.) and was, therefore, eliminatedfrom the study.

As with the previously described predominantlyaqueous-phase supporting solvent systems, a lin-ear concentration response profile was recorded forboth F-OH and 2-FA int-butanol–TMAH acrossthe range 0–10�g ml−1. The relative response ofthe organic-phase PAD system to that obtained for


the furanics spiked aqueous measurement solutionyielded values of 48 and 32% for 2-FA and F-OH,respectively with the latter compound again elicitinga greater response by PAD (∼25%).

It is pertinent to compare the mono-phasic or-ganic PAD data with the spiked transformer solutionsextracted into, then analysed within, the aqueousmeasurement solution, also a mono-phasic system.Despite the fact that there were extraction efficiencyand matrix issues surrounding the latter system, sig-nificantly reduced PAD responses were observed forthe organic-phase PAD measurement system. Whilstmatrix effects may have an effect on the response, itis probable that the reduced response of the methodwas due to organic solvent specific factors suchas the increased ohmic drop that is manifested inless-conductive supporting electrolyte preparations.Attempts to minimise this factor included ensuringa minimal distance between the reference and work-ing electrodes. Interestingly, similar data trends wereapparent between the organic-phase method and thebi-phasic approach in which the oil was added directlyto aqueous measurement solution.

4.6. Selection of optimum method forspiked samples analysis

All three of the measurement methods investigatedwere able to provide quantitative data of acceptablerepeatability (Table 1 — %R.S.D. values were all

Table 2Determination of accuracy of PAD compared with the standard CEI/IEC method for the determination of furanics in used transformer oilsamples by the organic-phase PAD method (n = 3)

Concentration of 2-furaldehyde byCEC/IEC method (�g ml−1)

Measured responsevalue (×108 �A)

R.S.D. (%) Calculated concentra-tion (�g ml−1)a

Accuracy (%)b

0.64 1.13 11.2 1.02 +37.11.01 1.21 13.4 1.18 +14.61.05 1.21 13.2 1.18 +11.21.14 1.27 11.6 1.31 +12.71.49 1.30 11.2 1.36 −9.41.68 1.32 9.4 1.41 −19.21.84 1.54 8.7 1.86 +1.22.25 1.69 11.4 2.17 −3.62.48 1.74 8.1 2.28 −9.02.58 1.82 5.2 2.44 −5.72.73 1.97 4.3 2.75 +0.7

a Concentration calculated from equation:y = 4.85× 10−8x + 6.37× 10−8 (from Table 1).b Accuracy: 100%× [(measured response− calculated response)/calculated response].

<9% for each method evaluated) across the requiredconcentration range. However, in selecting the opti-mum measurement method for furanics in transformeroil, the desirable operational criteria for the measure-ment process require consideration. Given that furaniclevels can rise very rapidly in failing transformers,the system must be capable of essentially real-timeanalysis. Consequent to this, the measurement processshould be rapid, low-cost and simple to perform withthe sample preparation method being compatible withreal-time data generation. Given these constraints, thedirect mono-phasic solubilisation of oil into organicsolvent is simplest to perform and is most amenableto construction of a real-time system. There are nosample extraction issues to consider and no com-plications arising from the bi-phasic nature of theaqueous-phase oil suspension method.

5. Real samples analysis

The organic-phase PAD method was used to mea-sure furanics levels in real aged transformer oil sam-ples. The samples had known 2-FA concentrationsdetermined by the CEI/IEC 1198:1993 method [12].Results are shown in Table 2.

Eleven used oils were tested. The oil having thelowest 2-FA concentration (0.64�g ml−1) was leastaccurate, over-estimating 2-FA levels by 37%. Thoseoils containing 1–2�g ml−1 of 2-FA had accuracies


varying from −19 to +15%. Oils with 2-FA levels>2�g ml−1 were most accurate yielding PAD val-ues within 9% of the standard method. It is com-mon in many analytical procedures for accuracy to bepoorer at low analyte concentrations due to S/N andgeneral experimental error factors. At concentrations>2�g ml−1, the method lends itself to the reliablemeasurement of 2-FA, concentrations that are consis-tent with the early diagnosis of transformer failure.

6. Discussion

The fact that different furanics yield different PADresponses raises issues when analysing furanic mix-tures, as may be expected when measuring real trans-former oils on-line. It should be noted that the keyaspect of the current study was to provide a meansof rapid low-cost on-line measurement. The electro-chemical route satisfies these demands but the inherentnon-specificity of the PAD method means that resolu-tion of the electrochemical signal of a furan derivativemixture into its component parts is not possible. It isalso evident from Fig. 1 that the use of alternative de-tection potentials, at which the various signals can beindividually determined, does not exist.

The proposed method should, therefore, be used asa semi-quantitative screening tool whereby the signalobtained is compared against a threshold value or se-ries of indicator values in order to assess the remaininguseful lifetime of the oil. Given the price that must bepaid should a fully loaded transformer fail, it would bewise to calibrate the system against furanics to whichthe PAD technique is most responsive. For example,the real samples tested were compared against 2-FAsince this is the commonest furanic found in failingtransformers [6].

However, if desired, PAD data could be comparedagainst other (more responsive) furanics, to ensureminimisation of false-negative diagnoses. For exam-ple, the other common furanic found in failing trans-formers is F-OH, which proved 23% more sensitiveto PAD than 2-FA. Calibrating against F-OH would,therefore, provide some useful indication of 2-FA con-centrations whilst reducing the possibility of yieldingfalse-negative data. Selection of a lower 2-FA thresh-old value (<10�g ml−1 guideline) would provide asimilar safeguard.

A concern in this study was that the ‘ageing’ of anoil within a power transformer may result in the gener-ation of species other than furanics that could undergoelectro-oxidation at the selected PAD detection poten-tial. Given that the aged oils containing >2�g ml−1

yielded 2-FA values within 9% of the standard method,it is evident that electroactive interferents were notgenerated in significant amounts during ageing of the11 oils studied. Furthermore, PAD analysis of the agedoil known to contain no furanics yielded no current re-sponse by PAD, indicating that non-furanic electroac-tive species are not formed in significant quantitiesduring oil usage.

7. Conclusions

Both the direct addition of transformer oil to aque-ous measurement solution and the direct solubilisationof oil into organic solvent with added organic elec-trolyte/base and 1% (v/v) water represent advancesin the detection of furanic compounds in transformeroils. Neither method requires a separate sample ex-traction step and hence both are amenable to on-lineanalysis. Of the two methods, the organic-phase PADprocedure offers greater operational advantages inthat it is a mono-phasic system, hence, there are noissues with regard to the suspension of discrete oildroplets in the aqueous matrix and the consequentin situ extraction. Tests performed on oils removedfrom in-service transformers by organic-phase PADyielded concentrations within 9% of those deter-mined by a standard off-line liquid chromatographicmethod.

The rapidity, coupled to the self-cleaning natureof the organic-phase PAD process, lends itself ide-ally to on-line analyses. It is envisaged that theon-line system could incorporate 1�l min−1 flowingt-butanol–TMAH stream to which transformer oil wascontinually fed at appropriate flow-rates. Samplescould be passed through a mixing coil and fed into aflow-cell containing the three-electrode assembly.

Acknowledgements

TB gratefully acknowledges support from the Engi-neering and Physical Sciences Research Council, UK


and National Grid Company plc., UK for funding inthe form of a total technology grant. This paper is ded-icated to the memory of David Dobson, a pioneer inthe science of organic-phase PAD.

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pulsed amperometric detection of furan compounds in transformer oil

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