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ANALYST, APRIL 1993, VOL. 118 439 Skin Contact Electrodes for Medical Applications* Brian R. Eggins Department of Applied Physical Sciences, University of Ulster, Newtownabbey, Co. Antrim, UK BT37 OQB Skin contact electrodes require electrolyte gels between the skin and the electrode in order t o ensure good electrical contact. The effect of different types of electrolyte gel on skin impedance was studied. The main types of gels used were wet gels, karaya-gum based hydrogels and synthetic copolymer-based hydrogels [2-acrylamido-2-methylpropanesulfonic acid-N,N'-methylenebis(acry1amide) copolymers]. The effect of variation in gel composition on the impedance of the skin was investigated. Keywords: Electrodes; skin; impedance; hydrogels A number of medical procedures involve the measurement of electrical signals from the human body or the application of signals to the body.The best known of the former example is probably the electrocardiogram (ECG) for monitoring the behaviour of the heart and detecting possible abnormalities. An example of the latter is in heart failure, where the application of a substantial pulse of electricity may cause defibrillation, i.e., cause the heart beat to return to normal. Other important procedures are: electroencephalography (EEG), measurement of brain waves; electromyography (EMG), measurement of muscle behaviour; and electrooculo- graphy (EOG), measurement of eye behaviour. 1 When astronauts go into space there are abnormal stresses o n their bodies and it is important to study how the body responds under conditions of weightlessness over an extended period.To this end a special multi-electrode harness has been designed and constructed in our centre for use in the Juno Project. I t is well known that people perspire when under psycholo- gical stress. This is one of the principles behind the so-called polygraph or lie detector test. The sweating produces changes in the electrical impedance of the skin which can be monitored. 2.3 In all of these methods an electrode is involved in contact with the skin. Contact is usually made not directly, but via an electrolyte gel. It is with some of the properties of suitable electrolyte gels that this paper is concerned. Background and Theory First of all some understanding of the structure of the skin is necessary; a schematic diagram of the skin is given in Fig.1.4 Skin consists of the epidermis, the dermis and the subcu- taneous layers. The major impedance is due to the epidermis. This is, in turn, divided into three layers, the stratum corneum which consists largely of dead cells that make it non-conduc- tive, has a high impedance and is semi-permeable to ions.5 The impedance can be up to 5 MQ,"7 at low frequency with surface bioelectrodes. The other layers are the stratum granulosum and the stratum germinativum with a combined impedance of We can model the impedance of the total system as shown in Fig. 2. Fig. 2 shows the electrode lead resistance, the electrode-gel interface impedance (the double layer capacitance, CDL in parallel with the charge transfer resistance, Rcr), the gel resistance, the skin impedance (Rp in parallel with Cp) and the underlying tissue impedance.0.1-1 kQ.2.' * Prcscnted at thc Scnsors and Signals Symposium at Thc Royal Society of Chemistry Autumn Meeting, Dublin, Ireland. September 16-18, 1992. The impedance of the skin dominates the others and ha5 the form(~31() j = m; CIJ = frequency in rad s-1; a = a constant 0 < a < 1. Usually a =: 0.8 for human skin.8 This represents the parallel combination of RP and a pseudo-capacitive impe- dance , ZCpA, where and K = Rp/Tm (3) The usual way to display data for a complex impedance is to use an Argand diagram in which X,, skin reactance, the imaginary part of the impedance, is plotted against skin resistance, Rs, as shown in Fig. 3. One observes a semicircular relationship as frequency increases from right to left round the semicircle.Actually it is not a true semicircle unless a = 1.0, when the phase angle 8 = d 2 . At zero frequency the impedance is totally resistive. As frequency increases the reactive component increases and the resistive component decreases, until the reactance X s reaches a maximum at oo, where m0 = UT. Subsequently, as frequency continues to increase, reactance tends towards zero and resistance tends towards the relatively small, high-fre- quency limiting value R, due to the resistance of the leads, gel and underlying tissues. Manufacturers and distributors of pre-gelled electrodes at present use performance standards drawn up by the American Association of Medical Instrumentation (AAMI).11 These consist of tests carried out on electrode pairs connected gel to gel. These are, therefore, independent of skin contact effects. It is clear that these AAMl tests are largely irrelevant. The contribution of the patient's skin is ignored, although the skin impedance is much higher than that of the electrode-gel interface. One of the tests is the measurement of the gel-to-gel impedance at 10 Hz. The 10 Hz impedances measured on unprepared human skin do not correlate well with those measured gel to gel.l2 Some of the 'worst' electrodes in the AAMI bench tests were found to be the best in vivo. Conversely some of the 'best' tested electrodes were proved to be among the worst in vivo. A 99% correlation has, however, been found between electrode impedance measure- ments carried out on abraded skin (i.e., where the epidermal impedance layer has been effectively removed) and those on electrodes connected gel to gel.The proper measurement of skin impedance in contact with an electrode therefore requires the use of a frequency response analyser, although the standard AAMI tests use only one frequency, i.e., 10 Hz.440 ANALYST, APRIL 1993, VOL. 118 I Lead Stratum corneum Stratum g ranu losum ratum germinativum 1 Epidermis Dermis I Fig. 1 Schematic diagram of the skin. (Reproduced with permission from ref. 4) Fig. 2 Simple equivalent circuit model of the electrode-skin impedance. (Reproduced with permission from ref. 11) Experimental A new set of in vivn tests were devised with patients using a frequency response analyser (Solatron 1172) and a Skin Electrode Impedance Tester (SKEIT).13314 A three-electrode configuration was used on the forearm (test, control and reference). An applied voltage of 0.01 V (root mean square; r.m.s.) is used giving a current source from the SKEIT of 10 pA (r.m.s.). This avoids non-linear behaviour of the system. The parallel skin resistance, Rp, is especially non-linear.15 It decreases markedly with applied signal amplitudes greater than 10 pA cm-2. The RP dominates the lower frequencies, producing most of the non-linear effects at low frequencies, although fewer effects at high frequencies. All impedance measurements were carried out on the left forearm of the same 34 year old Caucasian male. The procedure used monitored the skin impedance for 30 min.The variation of RP and K with time can also be followed.16 A nrul tifrequency impedance locus was then plotted by sweeping between 1 Hz and 9.99 kHz (the limit of the Solatron 1172). Wet gels consist of an electrolyte (usually NaCl) a thick- ener, a preservative, wetting agents and abrasives.17 Table 1 shows the sample composition of a wet gel.18 Natural hydrogels consist typically of a hydrophilic colloid such as karaya gum which acts as an elastic adhesive cross-linked with a hydrating agent such as propylene glycol, together with a water-soluble hydrating agent such as glycerol. Dissolved in this are salts such as sodium Chloride and calcium chloride which promote the cross-linking. Water is also added.19--21 Table 2 gives the composition of a typical karaya hydrogel.The AMPS-MBA copolymer hydrogel22 consists of the 2-acrylamido-2-methylpropanesulfonic acid (AMPS) mono- RSIQ Fig. 3 model of the skin. (Reproduced with permission from ref. 11) Argand diagram (impedance locus) and equivalent circuit mer, a hydrophilic electrolyte; N,N’-methylenebis(acry1- amide) (MBA), the cross-linking component; polyacrylic acid (PAA), a polyelectrolyte; glycerol, a hydrating agent; sodium chloride, the major electrolyte; water, as the solvent. (See Table 3 for composition ranges of an AMPS copolymer hydrogel .) Results and Discussion The true values of R p , K and a can only be properly ascertained from the frequency locus plots and these change with time. Fig. 4 shows some actual impedance loci from a commercial wet gel (Marquette ECG) electrode.The monofrequency curves for 1 and 10 Hz decrease with time, for the most part, forming semicircular arcs whose centres lie on or near the vertical axis. This indicates that RP is the major source of the observed non-linear behaviour’3 and that RP tends to decrease with time after the first 4-5 min following electrode applica- tion. The impedance of the skin depends on a number of factors. Between individuals RP ranges from 10 kS2 cm-2 to 5 MQ cm-2 using a gelled ECG electrode 10 mins,*,l3 after application. The parallel capacitance is in the range 0.02-0.06 pF cm-25 or using the constant phase angle impedance, K is in the range 2-10 MB s-” cm2.338 Thus RP varies much more wideJy than Kp. Generally, females have higher impedances than males and dark skinned subjects higher impedances than those with fair skins.23 Other factors also need to be considered.The site of the electrode placement has a significant effect; the forehead has aANALYST, APRIL 1993, VOL. 118 44 1 " 10 Hz A'' .. _,..... t..... i- Table 1 Composition of a typical wet gel Component Amount present Unibase 295 g Hydroxymcthylcellulose 2g Propyl p-hydroxybenzoate 0.02% Methyl p-hydroxybenzoate 0.01% Sodium chloride (0.05 mol dm-3) 80 cm3 Table 2 Composition of a typical karaya hydrogel Component Property Amount present Karaya gum Elastic, adhesive 40 g Glycerol Hydrating agent 57 g Propylene glycol Plasticizer, hydrating agent 3 g Sodium chloride Electrolyte 0.007-1.5 g Water Swelling agent, solvent 3-10 g Calcium chloride Electrolyte, cross-linking agent 0-1.5 g Table 3 Composition ranges of an AMPS copolymer hydrogel Component Property Amount present AMPS Hydrophilic component , electrolyte 10 g MBA Cross-linking agent 0.3-1.6 g PAA Pol yelectrol yte 6.7-8.1 g Glycerol Hydrating agent 19-23 g Sodium chloride Electrolyte 0.02-0.64 g Water Solvent &12% 60 50 t A 0 20 40 60 80 100 120 140 R&Q Fig.4 Impedance loci from a commercial wet gel electrode (Marquette 'wet' electrode; 10 pA inner left forearm). A, t = 6 min; B, f = 0.5 min; and C, t = 45 min low impedance,2723 but the outer forearm is higher.2,23 The effect of electrode area is obvious, as is electrode contact pressure. Skin condition, minute cuts, etc., can short out large epidermal impedances, and abrasion, stripping or puncturing can have the same effect.Thus abrasion of the epidermis can make a huge difference. Mild abrasion is a useful method of preparing the skin before application of electrodes, provided it does not cause pain, bleeding or irritation or open the skin to irritation by electrode gels.24 From Fig. 5 we can see there is a considerable time variation with a, RP or K.677723 RP decreases exponentially, with a time constant of about 10 min depending on the gel and skin condition. This is due to the gradual penetration of the electrolyte into the skin. The initial increase is possibly due to the contraction of the skin pores on application of the relatively cold gel. Initially Cp increases ( K decreases) due t o electrolyte spread but then remains relatively ~onstant."~~~3 The emotional state of a patient, e.g., tenseness or stress, can cause an alteration in the degree of perspiration.25 Perspiration moistens the epidermis and causes a decrease in impedance.This effect can be used to monitor psychophysio- logical behaviour. This is one of the factors monitored by the now unpopular lie detector. 1.4 1 1.4 1 1.4 1 1 0.2 j 0.; j 0.2 1 , I , I 1 0 0 5 10 15 20 25 30 35 Ti me/min Fig. 5 Variation with time of impedance loci for a commercial wet gel electrode (Marquette; 10 pA inner left forearm). A, R,; B, a; and C, K 16 14 12 10 ' 6 8 8 4 2 '. . ... *A.. , . . . . . . . . . . . . . . , ... I I -. '.. I t I 0 5 10 15 20 25 30 35 R F J ~ ~ Fig. 6 Impedance locus of a hydrogel electrode (HP 13951C; 10 pA inner left forearm).A, t = 45 min; B. t = 0.5 min; and C, t = 4.5 min 3.5 1 3.5 1 3.5 I I 0.5 0 j O.: j 0.5 0 ~ 10 15 20 25 30 35 5 Ti me/mi n Fig. 7 Variation with time of impedance loci for a commercial hydrogel electrode (HP 13951C; 10 pA inner left forearm). A, K ; B, R,; and C, a In biopotential monitoring a gel is used: (a) to ensure good electrical contact; ( b ) to minimize skin impedance; and (c) to decrease motion artefacts.13 There are two general types of biopotential electrode gel, a 'wet' type which has been used in most commercial electrodes and a solid hydrogel. Gelled skin impedance depends on ( a ) skin preparation and (b) gel composition and concentration. In our studies we did not abrade the skin but treated it with atropine which minimizes the variable effect due to perspira- tion.Previous studies14 have considered commercially available wet gels and some hydrogels. The current studies have been with two types of hydrogels, one based on the natural polysaccharide karaya gurn2o721 and the other based on a synthetic copolymer between AMPS and MBA.22 Considera- tion was given to all the components of each hydrogel. The composition of each was varied independently and the effect observed on the impedance variables Rp, a, and K , taking account of the time after application of the electrode t o the skin. Although these gels have favourable impedance properties they have a number of drawbacks. As a result of the variable source of the natural polymers, there is great inconsistency in their physical and chemical properties, including variable amounts of impurities. This leads to a significant variation in their electrical properties.23.24442 Also, karaya-based gels tend to creep and flatten out exposing the skin to the bare electrode.They are also not particularly cheap to manufacture and tend to irritate the skin especially if the concentration of electrolytes differs very much from physiological levels. 13,’s Hydrogels have a number of advantages over the wet gels. 13 They leave no residue, they are repositionable, they are relatively non-drying. They make firm electrical contact and a simply designed electrode is possible. Hydrogel based elec- trodes are generally less expensive to manufacture than wet-gel based electrodes. However, they have the disadvan- tages that they are less conductive and less ‘wet’ than the standard gels.The impedance locus of a commercial hydrogel (HP 13951C) electrode is plotted in Fig. 6. Although the impedance loci form arcs as before (Fig. 4), the monofrequency curves for 1 and 10 Hz fluctuate back and forth with time indicating that although R P is the major source of non-linearity it does not decrease monotonically as before. This effect is shown more clearly in Fig. 7. We see a much more uneven response. Hydrogel electrodes do not wet the skin much and the variations in R P are dependent to a large extent on sweat gland activity, which is controlled by the autonomic nervous system and hence dependent on the emotional state of the subject.25 The magnitude ( K ) of the pseudo-capacitive impedance, ZcpA, remains relatively constant apart from an initial increase as observed before.Treating the skin with atropine eliminates the variable sweating effect26 and makes RP and K rise gently but steadily for about 1 h. It then remains constant. Consequently a is between 0.8 and 0.9 and is fairly constant. From the studies carried out with our own hydrogel electrodes it was found that increasing the salt concentration causes a dramatic decrease in Rp. This effect is more marked with AMPS than with karaya gels. lncreasing concentrations of water lower the RP of AMPS but have little effect on karaya. Increasing concentrations of glycerol, propylene glycol or pol yacrylic acid generally cause decreases in RP with karaya gels, but have little effect on AMPS. Conclusion It is clear that in vivo measurements of electrode-gel-skin impedance are more relevant to electrode performance than the widely practised AAMI bench tests, but that the wide variations in the response of different skin, gel, environmental and psychological conditions make it difficult to draw quanti- tative conclusions from the data.Qualitatively we have shown that the synthetic polymer hydrogel is much more reliable and less prone to fluctuations due to variable composition of the gel let alone the karaya. The RP for AMPS is generally 1110 to 11100 that of karaya. A four-channel technique is being developed and our sampling technique is being refined in order to obtain data ANALYST, APRIL 1993, VOL. 118 simultaneously from up to four electrode-gel-skin interfaces.This, it is anticipated, will result in more meaningful prccision and accuracy. 1 2 3 4 5 6 7 8 9 10 I 1 12 13 14 1.5 16 17 18 19 20 21 22 23 24 25 26 References Cromwell, L., Weibell, F. J., and Pfeiffer, E . A . , Biomedical Instrumentation and Measurements, Prentice Hall, London. 1980. Grimncs, S., Med. B i d . Eng. Comput., 1983, 21, 750. Yamamoto, Y., and Yamamoto, T., Med. Biol. Eng. Comput.. 1977, 15, 219. Neuman, M. R. ~ in Medical Instrumentation: Application and Design, ed. Webster. J . (3.. Houghton Mifflin, Boston, 1978. Edelberg, R., in A Treatise of the Skin, cd. Elden, H. R., Wiley. New York, 1971, vol. J. Gatzke, R. D., in Biomedical Electrode Technology, eds. Millar, H. A., and Harrison, D. C., Academic Press. New York, 1974, p. 99. Yamamoto, Y., and Yamamoto, T., Med.Biol. Eng. C’omput.. 1978, 16, 592. Tregear, R. T., Physical Functions of the Skin. Academic Press, London, 1966. Cole, K. S . , Cold Spring Harbour Symp. Quant. Biol., 1940,8, 110. Salter, D. C., D.Phil. Thesis, University of Oxford, 1980. Standard f o r Pregelled ECG Disposable Electrodes, Association for the Advancement of Medical Instrumentation, Arlington, VA, 1983. Shoenberg, A. A., Klingler. D. R., Baker, C. D.. Worth, N. P., Booth, N. P., and Lyon, P. C., UBTL T e c h . Rep. No. TK 1605-00-5, University of Utah Research Institute, Salt Lake City, UT, 1979. McAdams, E . T., and Jossinet, J.. Automedica, 1990, 13. 187. Eggins, B. R., McAdams, E . T., and Jossinct, J., unpublished work. Yamamoto, T., and Yamamoto, Y., Med. Biol. Eng. Comput., 1981, 19, 302. Olson. W. H.. Schmincke, D. R., and Henley, B. L., Med. Inmum., 1979, 13, 269. Yamamoto, Y., Yamamoto, T., and Ozawa. T., Med. Biol. Eng. Comput., 1986, 24, 71. Geddcs, L. A., and Baker, L. E., Principles of Applied Biomedical Instrumentation, Wiley, New York, 2nd edn., 1975. Hymes, A . C., US Pat. 4 12.5 110, November 14, 1978. Cahalan, P. T.. and Coury, A. .I., US Pat. 3 640 741, July 5 , 1983. Rolf, D., US Pat. 4674512, June 23, 1987. Cahalan, P. T., and Coury. A . J . , US Put. 4391 278, July 5 , 1983. Almasi. J . J . , and Schmitt. 0. H., Ann. New York Acad. Sci.. 1970, 170, 509. Yamamoto, T., and Yamamoto. Y., Med. Hiol. Eng. Comput., 1976, 14, 151. Kennard, D. W., J. Physiol., 1963,457. Goodall, M.. J . Clin. Phrmacol., 1970, 10, 235. Paper 2105442 F Received October 10, 1992 Accepted February 24, I993

ISSN:0003-2654    

DOI:10.1039/AN9931800439

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST, APRIL 1993. VOL. 118 COMMUNICATION 443 Material for publication as a Communication must be on an urgent matter and be of obvious scientific importance. Rapidity of publication is enhanced if diagrams are omitted, but tables and formulae can be included. Communications receive priority and are usually published within 5-8 weeks of receipt. They are intended for brief descriptions of work that has progressed to a stage at which it is likely to be valuable to workers faced with similar problems. A fuller paper may be offered subsequently, if justified by later work. Manuscripts are usually examined by one referee and inclusion of a Communication is at the Editor‘s discretion. Liquid Nitrogen Cooling in Microwave Digestion Helen J. Reid, Stanley Greenfield and Tony E.Edmonds* Department of Chemistry, Lo ug h bo ro ug h University of Techno logy, Lo ug h bo ro ug h, L eiceste rs h ire, UK LE77 3TU In an attempt to minimize the delay in opening Teflon pressure vessels following microwave acid digestion, and thus significantly reduce sample preparation time, various approaches to vessel cooling have been investigated. These include the feasibility of carrying out digestions with the pressure vessels immersed in liquid nitrogen and the use of liquid nitrogen as a pre- and post-digestion coolant. Liquid nitrogen cooling in the microwave unit was found to decrease digestion rates considerably, although the prevention of rapid and uncontrollable increases in pressure during digestion of organic material could be useful in some cases. Liquid nitrogen cooling subsequent to or between heating cycles was found to be very effective, especially where several heating, cooling and venting cycles were required.As well as enabling the vessels to be cooled rapidly and opened, it stopped any continuing increase in pressure which could otherwise cause the hot vessels to vent with potential loss of sample. Pre-digestion cooling also helped to delay the onset of rapid increases in pressure. Keywords: Microwave digestion; sample preparation; liquid nitrogen cooling The use of microwave heating for the rapid, reproducible and controllable digestion of a variety of samples, using concen- trated acids under pressure, is now well established and has found many applications.1.2 However, one limitation of this technique is the necessary delay in opening the digestion bombs, as the contents must be cooled to room temperature before opening, both for safety reasons and in order to avoid loss of volatile analytes.The digests retain their heat for a considerable period of time after the microwave power is switched off, partly due to the nature of the microwave heating effect (the equivalent of ‘standing time’ in microwave cookery) and partly because the Teflon digestion vessels are such poor conductors of heat. In addition, for larger (about 1 g) organic samples the internal pressure often continues to rise after the cessation of microwave heating, as a result of the evolution of carbon dioxide from continuing decomposition reactions. Hence the time delay in order to allow the vessel contents to cool and the internal pressure to be reduced to a safe level adds a significant period to the over-all time for sample preparation by this technique. Reported post-digestion cooling methods include water,3-8 ice-water,Y-12 a freezer,13,14 or occasionally liquid nitrogen.15 However, the need for a more effective cooling method has been recognized.8 As liquid nitrogen is transparent to microwaves, it will only be heated through contact with the hot vessel walls (which are, in turn, indirectly heated through contact with their contents) and thus the cooling effect should be retained for the duration of a typical digest.The vessels used for this work were made of * To whom correspondence should be addressed. periluoroalkoxy Teflon (Teflon PFA) ; hence they may be immersed safely in liquid nitrogen.Therefore, it was decided to investigate the feasibility of the use of liquid nitrogen for vessel cooling in situ. Experimental Apparatus This work was carried out using a Model MDS-81D micro- wave unit [CEM Corporation, Matthews, NC, USA, supplied by CEM (Microwave Technology), Buckingham, UK], incor- porating a removable Teflon turntable holding up to 12 vessels. The full power output of the unit is rated at 630 -t 70 W and was measured at 675 -t 15 W using the method described in the operation manual supplied with the unit.16 The power can be adjusted in 1% increments and can be programmed over three stages. Digestions were performed in CEM 120 ml Teflon PFA digestion vessels and sealed to a reproducible torque of 16.3 N m using a CEM capping station.The vessels are designed to operate at pressures of up to 830 kPa (120 Ib in-2); a pressure relief mechanism in the vessel cap causes venting of the vessels if the internal pressure exceeds this. In order to monitor the increase in pressure during digestions, one of the vessels was attached to a pressure transducer (Sensym SX15ODN) via a pressure line, consisting of gll 0.d. Teflon tubing and incorporating a pressure relief valve set to open at 830 kPa. The pressure transducer was calibrated by application of pressures of 0-830 kPa from a nitrogen cylinder. In order to isolate the pressure sensor apparatus from the acid digests and thus prevent corrosion, the tubing was left partially filled with distilled water after flushing.17444 ANALYST, APRIL 1993, VOL. 118 A precision miniature two-way valve (Omnifit , Cambridge, UK) was installed in the pressure line in the cavity to enable the disconnection of the monitoring vessel without venting, thus allowing the safe removal of the turntable of hot vessels from the cavity for cooling. The valve (2-way7 rated to 3450 kPa), which incorporates a Teflon body and a Tefzel valve key, joins a short length (about 10 cm) of tubing from the monitor vessel to the main pressure line tubing. The Teflon- Teflon seals at each side of the valve are made using gripper fittings (Omnifit) which are rated to 6900 kPa (1000 lb in-2) and are designed to allow frequent connection and disconnec- tion, via finger-tight screw caps, without damaging the seals.After completion of a heating cycle, the valve is closed by a 90" rotation of the valve key and disconnected from the pressure line. The vessel, with the short tubing and valve still attached, may then be removed from the cavity, cooled and vented safely in a fume hood by rotating the valve key to the open position. Polystyrene, which is a good insulator and effectively transparent to microwaves, was used to form a containment vessel for liquid nitrogen in the microwave unit. The vessel could hold four pressure bombs plus a fifth uncapped vessel to act as a venting trap. The outer surface was wrapped in polythene film to minimize evaporation of nitrogen through the porous walls; in addition, four depressions were made in the base of the container to enable it to locate in the turntable drive mechanism in the cavity floor.Reagents Digestions were carried out using AnalaR concentrated nitric acid (density 1.42 g ml-1, BDH, Poole, Dorset, UK). Samples of tomato puree, pet food, milk powder and sweet bay were provided by the Metallic Impurities in Organic Matter Sub-committee of the Analytical Methods Committee of the Royal Society of Chemistry (Analytical Division). Initial Experiments With Acid Blanks Applying microwave radiation to concentrated nitric acid in uncapped vessels immersed in liquid nitrogen showed that very cold (even frozen) nitric acid will couple with the microwave energy to produce rapid heating. Two 10 ml aliquots of acid were heated, at 675 W, from frozen to 60 "C in 2 min, and from ambient to >70 "C in 1 min.On cessation of microwave heating the cooling is very rapid (from over 70 to 20 "C in under 1 min). Microwave heating of four sealed vessels containing con- centrated nitric acid, with and without immersion in liquid nitrogen, showed that the internal pressure rise is slowed significantly by the presence of liquid nitrogen during the heating cycle (pressure rises of 140 and 480 kPa, respectively, were produced by 3 min of heating at 675 W). Liquid nitrogen cooling at the end of the heating cycle lowers the temperature and pressure very rapidly and enables safe opening of the vessels within 2-3 min. The liquid nitrogen boiled off fairly rapidly in the poly- styrene container as the acid was heated; the 700 ml needed to immerse the vessels up to their caps evaporated within 5 rnin at 675 W.To avoid overheating (and possible melting of the polystyrene) it was necessary to top up the container periodically with liquid nitrogen. This was carried out while the container was in the cavity, using a glass funnel and flexible poly(viny1 chloride) tubing passed through the open door. Liquid Nitrogen Cooling During Digestions Microwave digestions were carried out on 1 g samples of tomato puree and sweet bay powder. These two samples showed very different behaviour when subjected to standard (non-cooled) microwave pressure digestion; the tomato puree produced a modest pressure increase and rapid dissolution , the sweet bay produced a rapid increase in pressure requiring multiple heating, cooling and venting cycles for complete dissolution.This difference is probably a result of the higher water content of the tomato puree which meant that only about 250 mg of organic material had to be decomposed. In each case, four sealed vessels, three containing 1 g of sample and 10 ml of concentrated nitric acid, the fourth an acid blank, were immersed in liquid nitrogen in the microwave unit. With tomato puree, full power was applied until a pressure of 690 kPa was attained (3 rnin 20 s, about twice the time taken without cooling). This produced a yellow-green solution that indicated complete dissolution but incomplete decomposition; this was confirmed on further examination of the digests, when significant amounts of organic residues were found. Performing the sweet bay digestions whilst the vessels were immersed in liquid nitrogen reduced the pressure rise dram- atically by comparison with an uncooled digestion, and enabled stabilization of the pressure at 480 kPa at an applied power of 170 W (following initial heating at 510 W) on the first heating cycle, which could not be achieved with uncooled digests, even at low applied powers.However, two 10 min heating cycles failed to produce complete dissolution; thus it appeared that the rate of digestion was being decreased. It was found that when liquid nitrogen was used for cooling during the microwave digestion, vapour from the digestion mixture froze onto the walls of the vessel instead of refluxing back into the liquid. As microwaves heat only the liquid and not the vapour, the latter cools rapidly and freezes.Wurfels et aZ.18 indicated that nitrogen dioxide, in vapour form, plays a vital role in nitric acid pressure digestions and will freeze at these temperatures (f.p. -10 "C). This will naturally slow down the rate of digestion. In an attempt to reduce the increase in pressure without slowing down the digestion unduly, vessels were cooled in liquid nitrogen before digestion of the sweet bay samples. This postponed the onset of the pressure increase, but once started it was again rapid and no improvement in digestion efficiency was obtained. Post-digestion Cooling The cooling of hot pressure vessels after digestion by immersion in liquid nitrogen for 1 min in the polystyrene container proved a very efficient means of cooling the digests down to (or below) room temperature withcut freezing them.The vessels could then be vented safely and returned to the microwave unit for further heating, or opened and transferred to calibrated flasks for dilution with minimum delay. Using this cooling technique, four 1 g tomato puree samples could be prepared in 7 min (from the start of heating to opening the first vessel). Samples such as pet food and milk powder which require a two-stage digestion, with cooling and venting to relieve excess pressure between two heating periods, could be processed in 20 min. Although simple and convenient to construct, the poly- styrene container was not ideal for holding vessels in the cavity for several reasons. First, it is limited to four vessels; second, the vessels often become hot enough to melt the polystyrene when no coolant is present; third, the polystyrene acts as insulation preventing air cooling of the vessels as recommen- ded by the manufacturers; and finally, the constrained position of the vent tubes of the vessels that is necessary for them to fit into the cavity, renders the pressure relief valves liable to open prematurely.Undoubtedly these problems could be overcome by an alternative design in a different material, for example Teflon. However, the container was useful for immersing four vessels in liquid nitrogen for post-digestion cooling following their transfer from the TeflonANALYST, APRIL 1993, VOL. 118 445 turntable and was successfully used for short digestions involving rapid increases in pressure at relatively low tem- peratures, when the rapid addition of liquid nitrogen in situ was effective in controlling the pressure increase and thus stopping the relief valves opening.Conclusions The use of liquid nitrogen for cooling during microwave digestion slows down the digestion due to freezing of the reactants, although moderation of the increase in pressure when digesting samples of high organic content may be useful. An alternative is to use the liquid nitrogen for post-digestion cooling, when it counteracts any continuing pressure increase and significantly reduces the delay in opening the vessels and processing the digests. H. J . R. thanks the Trustees of the Analytical Chemistry Trust Fund of the Royal Society of Chemistry for the award of a SAC Studentship.References Matusiewicz, H., and Sturgeon, R. E . , Prog. Anal. Spectrosc., 1989, 12, 21. Kuss, H. M., Fresenius’J. Anal. Chem., 1992, 343, 788. Bettinelli, M., Baroni, U., and Pastorelli, N., J . Anal. At. Spectrom., 1987, 2, 485. Fernando, L. A., Heavner, W. D.. and Gabrielli, C. C., Anal. Chem., 1986, 58, 511. 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Littlejohn, D., Egila, J. N., Gosland, R. M., Kunwar, U. K., Smith, C., and Shan, X. Q., Anal. Chim. Acta, 1991,250, 71. Matusiewicz, H., Sturgeon, R. E., and Berman, S. S . , J . Anal. At. Spectrom., 1989, 4, 323. Rantala, R. T. T., and Loring, D. H . , Anal. Chim. Acta, 1989, 220, 263. Suzuki, T., and Sensui, M., Anal. Chim. Acta, 1991, 245, 43. Aysola, P., Anderson, P. W., and Langford, C. H., Anal. Chem., 1987,59, 1582. Lamothe, P., Fries, T., and Consul, J., Anal. Chem., 1986, 58, 1881. Smith, F., Cousins, B., Bozic, J., and Flora, W., Anal. Chim. Acta, 1985, 177, 243. Ybanez, N., Cervera, M. L., Montoro, R., and de la Guardia, M., J . Anal. At. Spectrom., 1991. 6, 379. Cabrera, C., Lorenzo, M. L., Gallego, C., Lopez, M. C., and Lillo, E., J . Agric. Food Chem.. 1992,40, 1631. Friel, J. K., Skinner, C. S . , Jackson, S. E., and Longerich, H. P., Analyst, 1990, 115,269. Kingston, H. M., and Jassie, L. B., Introduction to Microwave Sample Preparation, American Chemical Society, Washington, DC. 1988, pp 118-119 and 157-158. Operation and Service Manual for Microwave Digestion System Model MDS-81D, CEM Corporation, Matthews, NC, pp. 15- 16. Kingston, H. M., and Jassie, L. B., Anal. Chem., 1986, 58, 2534. Wurfels, M., Jackwerth. E., and Stoeppler, M., Anal. Chim. Acta, 1989, 226, 1. Paper 3100569K Received January 29, 1993

ISSN:0003-2654    

DOI:10.1039/AN9931800443

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST, APRIL, 1993, VOL. 118 COMMU NlCATlON 447 Material for publication as a Communication must be on an urgent matter and be of obvious scientific importance. Rapidity of publication is enhanced if diagrams are omitted, but tables and formulae can be included. Communications receive priority and are usually published within 5-8 weeks of receipt. They are intended for brief descriptions of work that has progressed to a stage at which it is likely to be valuable to workers faced with similar problems. A fuller paper may be offered subsequently, if justified by later work. Manuscripts are usually examined by one referee and inclusion of a Communication is at the Editor's discretion. Effective Laboratory Monitoring for the Abuse of the Beta-agonist Clenbuterol in Cattle Christopher T.Elliott, John D. G. McEvoy, William J. McCaughey, Desmond H. Shortt and Steven R. H. Crooks Residue Laboratory, Veterinary Science Division, Stone y Road, Stormont, Belfast, UK BT4 3SD The use of the beta-agonist clenbuterol (CBL) as a growth promoter has been outlawed in European meat production. The detection of its illegal use is dependent on CBL residues persisting in animal tissues for longer than the withdrawal times given by abusers. A comparison of urine, bile and liver matrices indicated that analysis of the liver offered the best possibility for CBL detection. However, an experimental study showed that CBL detection following withdrawal could be further extended (up t o 56 d) if the retina was used as the target tissue. Analysis of 703 retina and liver samples from cattle suspected of CBL medication revealed that 96 cattle had CBL residues present in their retinas, only 46 of these were liver positive. There were no instances of liver CBL residues being detected without the associated retina also being positive.Keywords: Clenbuterol; liver; retina; immunoassa y The beta-agonist clenbuterol (CBL) is a potent anabolic and lipolytic agent in many species,lJ but is not licensed as a growth promoting compound. However, as substantial finan- cial gains can be attained from its illegal use in cattle, there is a strong black market in its use. In extreme cases this illegal use of CBL has led to outbreaks of food poisoning in humans."." Under the random National Surveillance Schemes, which each member state implements to satisfy EC directives, tests on urine are used to detect CBL residues.A number of studies have shown that this matrix does not contain detectable concentrations for more than S d after CBL removal from the This withdrawal period can be easily observed by producers to avoid detection. Furthermore, a significant proportion of' the benefits of growth promotion gained as a result of medication are retained.7 In an initial survey carried out between June and September 1990, 90 animals suspected o f CBL treatment were urine tested for residues of the drug; none were positive, yet strong 'rumours' persisted that abuse was occurring. This laboratory has carried out extensive experimental and monitoring programmes designed to develop an effective screening test and to evaluate different matrices for the detection o f CBL residues.h The matrices assessed during these studies included urine, bile and liver.Positive results produced by the screening test were all confirmed by gas chromatography-mass spectrometry (GC-MS).X Experimental results indicated that urine was the least effective matrix for the detection of CBL residues with bile only slightly better. Liver was found to be substantially more effective for the detection of CBL residues. A comparison of residue concentrations detected in paired liver and bile samples taken between January and April 1991 from 286 cattle suspected of CBL treatmcnt was undertaken. This exercise showed that in 127 confirmed positive livers, corresponding bile samples were negative in 53 cases (Table 1). There were no recorded instances of a positive bile with a paired negative liver. The data generated indicated that a monitoring pro- gramme based on bile or urine would fail to detect an unacceptable proportion of true positives.Following a report5 that CBL residues could be more readily detected in the eyes of medicated animals, persistence of CBL residues in the eye and liver tissues of medicated cattle was ascertained. Experimental Eyes were obtained from a number of animals known to be negative. The fluid layers of the eyes were removed by means of a needle and syringe. (Earlier studies had shown these to be poor sites for CBL accumulation).7 The eyes were cut open and the retina and choroid layers lining the back of the eyeball were removed by scraping.Aliquots of these tissues were spiked with CBL (1-10 ng g- I of retinal tissue) to produce positive reference material. Unspiked material was used as the negative control. Retina and liver samples (n = SO) were taken, following slaughter, from cattle raised on Department of Agriculture for Northern Ireland farms. These were known to be free from CBL medication. These samples were applied to the outlined imm unoassay . Following a proteolytic digest and organic solvent extrac- tion an enzyme immunoassay was applied to both positive and negative tissue extracts (as described in ref. 6). A group of Friesian heifers ( n = 8) were medicated with448 ANALYST, APRIL 1993, VOL. 118 Table 1 Comparison of bile and liver immunoassay screening for the presence of CBL residue in cattle suspected of previous medication.Data generated between January and April 1991 Total All tests Bile +ve Bile +ve Bile -ve tested negative liver +ve liver -ve liver +ve 286 159 74 0 53 Table 2 Summary of CBL residue concentrations found in a variety of matrices taken from cattle experimentally fed on ten times the therapeutic doses of clenbuterol for 30 d Matrix Withdrawalld 0 0 16 16 39 39 56 56 Retinal ng g- 1985 1357 222 139 137 94 55 123 Liver/ 100.6 71.1 2.8 3.2 0.9 0.5 0.3 0.4 ngg-' Bile/ ng ml-1 12.3 2.8 <0.3 <0.3 <0.3 <0.3 (0.3 ~ 0 . 3 Urine/ ng ml-1 - 1.3 <0.3 <0.3 <0.3 <0.3 <0.3 <0.3 Table 3 Comparison of retina and liver screening for the presence of CBL residue in cattle suspected of previous medication.Data generated between August 1992 and January 1993 Samples Tests Retina Sve Retina +ve Retina -ve tested negative liver +ve liver -ve liver +ve 703 607 46 50 0 16 pg kg-1 of CBL (ten times the therapeutic dose of CBL) for 30 d. Following medication, a serial kill procedure was employed from 0 to 56 d withdrawal. Urine, bile, liver and eyeballs were taken from each animal. Paired eyeball and liver samples (n = 703) were taken at meat plants from cattle suspected of CBL treatment. These samples were tested by the immunoassay procedure. Results and Discussion Recoveries from spiked retinas ranged from 77 to 83%. This range is consistent with those from other spiked matrices applied to the test procedure. Analysis of known negative samples yielded limits of detection of 1.0 and 0.15 ng g-1 in retina and liver samples, respectively.No sample analysed gave higher concentrations than these defined limits indicating an absence of false negative results. The higher detection limit for retinal tissue was obtained as a result of the smaller sample size available for analysis. The concentrations of the drug found in each matrix tested at each of the withdrawal times are shown in Table 2. The test detected CBL in bile and urine only at day 0. Liver samples remained positive for 56 d; however, concentrations detected at day 56 were close to the sensitivity of the assay. Confirma- tory methods of analysis such as GC-MS would be unlikely to confirm the low concentrations present. However, in spite of the small amount of tissue available, the high concentrations of the drug residue in retinal samples that were still detected at the end of the trial make this tissue eminently suitable for GC-MS confirmation.Results from the intensive sampling programme using both retinal and liver screening tests, applied to field samples taken from suspect CBL treated animals slaughtered between August 1992 and January 1993 (Table 3), confirmed the findings of the experimental study. In 50 cases CBL residues were detected only in retinal tissue, the associated livers being negative. In contrast, no liver was found to be positive without the associated retinas also being positive. 'The use of bile or urine as a matrix for testing may be attractive as it offers simple collection and analysis procedures and lower costs.However, it offers the consumer less protection and allows the abuser a greater opportunity to continue with the abuse. The results obtained in this study indicate that analysis of retinal material is the most reliable way of detecting CBL abuse, especially when a long withdrawal period may have been observed in an attempt to avoid laboratory detection. Screening programmes based on any other matrix may be ineffective in deterring the abuse of this compound. References Baker, P. K . , J . Anim. Sci., 1984, 59, 1256. Jones, R. W., Easter, R. A., McKcith, F. K., Dalrymple, R. H., Maddock, H. M., and Betchel, D., J . Anirn. Sci.. 1985, 61, 905. Martinez-Navarro, J . F., Lancet, 1990, 336, 1311. Pulce, C., Lamaison, D.. Keck, G . , Bostvironnais, C., Nicolas, J., Descotes, J., Mora, M., and Colmant, A., Bull. Epidemiol. Hebdomadaire, 1991, 5 , 17. Meyer, H. H. D., and Rinke, L., J . Anim. Sci., 1991,69,4538. Elliott, C. T., McCaughey, W. J . , and Shortt, H. D., Food Addit. Contam., 1993, in the press. Elliott, C. T., unpublished results. Blanchflower, W. J., Hewitt, A., Elliott, C. T., and Kennedy, G., Biol. Mass Spectrom., 1993. in the press. Paper 3100678F Received February 3, 1993

ISSN:0003-2654    

DOI:10.1039/AN9931800447

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST, APRIL 1993, VOL. 118 COM M U NlCATlON 449 Material for publication as a Communication must be on an urgent matter and be of obvious scientific importance. Rapidity of publication is enhanced if diagrams are omitted, but tables and formulae can be included. Communications receive priority and are usually published within 558 weeks of receipt. They are intended for brief descriptions of work that has progressed to a stage at which it is likely to be valuable to workers faced with similar problems. A fuller paper may be offered subsequently, if justified by later work. Manuscripts are usually examined by one referee and inclusion of a Communication is at the Editor's discretion. Polymer Characterization Using Laser Desorption-Ion Mobility Spect rom et ry Michael Simpson, David R.Anderson, Cameron W. McLeod and Michael Cooke* Environmental Research Centre, School of Science, Sheffield Hallam University, Pond Street, She ffield, UK S l 7WB A series of polymer composite materials has been classified using laser desorption-ion mobility spectrometry. The pulse from an Nd: YAG laser ablates the surface of the composite polymer generating molecular fragments in the vapour phase that are representative of the target material. These vapour phase components are drawn into the ion mobility spectrometer source where they are ionized. Both positive and negative ions are formed and the spectrometer can operate in either mode. The ions are resolved into a pattern or signature characteristic of the polymer. As the spectrometer can sample at a rate of approximately 1 s-1 a rapid method of identifying polymers results.Similar materials such as high and low density polyethylenes can be differentiated in this way. Keywords: Laser desorption; ion mobility spectrometry; polymer characterization Ion mobility spectrometry (IMS) has existed as an analytical technique for more than two decades but has yet to become established as a major technique for vapour analysis. It is a gas-phase electrophoretic technique, which was referred to as plasma chromatography when it was first introduced. 132 The early plasma chromatography systems used a 63Ni radioactive source ((S-emitter) to ionize organic vapours by iodmolecule interactions and the resultant charged species were separated in a drift tube and collected in the form of an ion current.The different charged species had characteristic drift times and thus the response consisted of a series of peaks plotted against times. Hence the name plasma chromatography. Early interest in the technique focused on its wide applicability to organic vapours, its speed of response (typical drift times are of the order of 1-100 ms and hence a typical cycle time is less than 1 s) and its sensitivity (low ppb for many compounds). Much later its robustness and portability were recognized as key features. To date only a few specific applications have been described but the technique has been the subject of several reviews.3-6 These papers tend to emphasize the limitation of IMS as a laboratory-based technique where it is less effective at separation than capillary gas chromatography (GC) and less structurally informative than mass spectrometry (MS).Recently, however, there has been a trend towards screening techniques and field-based measurements and it is in this area that IMS can excel. An ion mobility spectrometer consists of two tubular components joined together. These are the reaction tube and the drift tube. The reaction tube contains the ionization source (usually 43Ni) and produces the ions, which enter the drift tube * To whom correspondence should be addressed. via an electronic gate that pulses open. The ions pass along the drift tube and are acquired by the collector in the form of an ion current, the magnitude of which represents the ion concentration (and charge). The technique has been com- pared with time-of-flight mass spectrometry.However, the information given is not fragmentary but resembles the picture or signature7 given by a gas chromatograph where a range of peaks represents a mixture of components. Both positively and negatively-charged ionic species can be produced in the reaction tube and hence both positive and negative signatures can be collected. Additional ionization can be introduced by doping the reactant tube carrier gas (air in most applications) with a species such as acetone, which gives greater discrimina- tion to the ion chemistry and is thus similar to the chemical ionization principle in conventional mass spectrometry. Although the h3Ni source is most commonly used for ion production some reports of the use of lasers as ionization sources have appeared.8.9 The laser is used at relatively low power and serves both to ablate the organic material from the surface or from within the matrix and to ionize the vaporized material.This presents a complex ion chemistry to the drift tube. The use of the laser as a vapour preparation technique from solid samples with ionization by the IMS instrument presents a simpler alternative. Ion mobility spectrometry has been applied to the study of a range of organic substances where specific targets are identi- fied and the instrument configured to respond to these targets. Aliphatic and aromatic amines have been studied10 as have other airborne organic vapoursl' and some inorganic pollutant gases. 12 Explosives,g narcotics'3.14 and polycyclic aromatic hydrocarbons (PAHs)~ have all been measured using IMS.It has also been used to monitor chemical warfare agents. Two areas of application of considerable importance are the450 - J m ANALYST, APRIL 1993, VOL. 118 evaluation of organic contamination in soils and the identifica- tion of polymers and composites. In the former case the identification of the presence of contamination together with information about the chemical class of compound present (polychlorinated biphenyl, PAH, phenol, etc. ) will prevent the detailed analysis of clean soils and highlight the most appropriate sampling regime for the type of pollutants present. For polymers the laser provides a sample generation capability far superior to pyrolysis in both speed and control and IMS should provide a signature that identifies the material.In this paper we report the use of laser desorption- ion mobility spectrometry (LD-IMS) to characterize polymers and hence promote their sorting and re=cycling. Experimental The experimental system consists of a Q-switched Nd: YAG laser (Spectra Physics DCR 11) operating at 1064 nm together with a flow cell fitted with a suitable window. The flow cell is coupled to an ion mobility spectrometer (Graseby Ionics, Bushey Road, Watford, UK) capable of operating in both positive and negative ion modes. The flow cell is clamped in position so that samples of materials can be butted up to the flow cell in a reproducible way using a moveable (x-y-z) stage. This ensures that every sample is positioned at the same place in relation to the laser beam.The laser operates in either single shot or repetitive (10 Hz) modes, long pulse and Q-switched modes and in both focused and unfocused conditions. A carrier gas (air, argon, helium) is used to transfer ablateddesorbed material to the ion mobility spec- trometer. Vapours produced by the action of the laser on materials butted up to the flow cell are swept down a tube to a glass funnel where the ion mobility spectrometer samples the vapours. A small coarse Whatman 97 filter-paper is used to prevent particles entering the spectrometer. Direct connec- tion to the ion mobility spectrometer has also been performed but requires the carrier gas flows to be carefully matched to the internal pump in the spectrometer to prevent overload.The spectra were accumulated on a PC using a Graseby Analytical ASP board and software designed specifically for this purpose. Default parameters for shutter pulse-width, frequency of sampling and drift region field were employed. In a typical experiment the spectrometer was set to acquire spectra over a 30 s period and the laser was fired approxi- mately 5 s after the start. Accumulated spectra were recorded on disk and on video tape. The IMS instrument was operated in the negative ion mode. Spectra of individual target compounds were obtained from neat liquids by a similar procedure to that described by Eiceman et al. 11 Results When running in the normal operating mode the IMS instrument is recycling air through the internal filter system. Some ionization of this air takes place in the reactant cell giving rise to a background spectrum, which consists essen- tially of a single peak known as the reactant ion peak (RIP).This peak had a characteristic drift time of 6530 ps in our system. The introduction of a volatile component into the spectrometer produces additional ions provided that ioniza- tion of the compound does occur. Hence the system does not respond (at ambient temperature) to simple alkanes but does respond to compounds that contain electron-rich moieties. An additional feature of the current software is that the total area for the peaks (ion species) present remains constant. Hence the presence of a component gives rise to one or more peaks at characteristic drift times but as the area of these peaks increases so the area under the RIP decreases.In this mode, therefore, the spectrometer is operating as a whole air screening device where the decrease in RIP area is a reflection of the total level of pollution of the air being sampled. As such, Table 1 Details of polymeric samples Elements detected by Sample Commercial details LIPS PVC Nylon 6,6 ‘Maranyl’ (ICI Advanced Materials) Ti HDPE ‘Rigidex’ 146 (BP Chemicals LDPE ‘Carlona’ Grade 18-200MA (Shell ABS ‘Darvic’ Industrial Grade (Glynwed Plastics Ltd.) Ti, Mg International Ltd.) Mg, Ca Chemicals) Mg, Ca Developments Ltd.) Mg ‘Formid’ ABS 350 Natural (Polymon 1 1 Time - Fig. 1 Ion mobility signatures of (a) ABS, (b) PVC and (c) nylon 6,6. Negative ion mode was used in each case I I Time- Fig. 2 Ion mobility signatures of (a) LDPE and (b) HDPE.Negative ion mode in both cases it shows promise as a general monitoring device for screening air for non-natural components such as exhaust fumes and is capable of monitoring transient events due to the rapid (about 1 s) sampling frequency.15 Some indication as to the nature of the pollutants present can be obtained from comparison of the drift times for the peaks present with those obtained from standards. Plastic materials are not generally volatile but are formed from individual, volatile substances. Plastics formation is essentially a process of polymerization and cross-linking to yield the required physical properties. However, most poly- meric materials are complex mixtures and can even be heterogeneous. Apart from the polymer itself the material often contains the by-products of the polymerization reaction, plasticizers? extender oils, inorganic and organic colourants, catalysts and fillers of various types.The polymeric materials studied include high and low density polyethylene, poly- propylene, nylon 6,6, ABS, (acrylonitrile-butadiene- styrene), PETP [poly(ethylene terephthalate)] and PVC [poly(vinyl chloride)]. The samples were qualitatively ana-ANALYST, APRIL 1993, VOL. 118 lysed by laser-induced plasma emission spectrometry (LIPS) (see Table 1). All polymers except the PETP produced a characteristic signature following laser ablation. The PETP sample used was highly transparent and thus failed to attenuate the laser beam sufficiently to ablate material from the surface.Signatures (spectra) for three types of polymer are shown in Fig. 1. Clear differences in pattern are discernible. Under similar conditions for the laser operation the pattern is reproducible. When the laser operating conditions are varied then the result pattern changes as different molecules are generated at the surface of the plastic. Hence variations of laser operating parameters produce differing signatures and thus allow maximization of differences between similar materials such as low density polyethylene (LDPE) and high density polyethylene (HDPE) (Fig. 2). Fig. 1 shows that both ABS and nylon 6,6 produce three major peaks in their respective signatures but both the relative intensity and position along the x-axis (drift time) are different. The PVC produces a far more complex yet characteristic signature.The logical approach to interpreta- tion of data of this type is through pattern recognition software. A further development would be a neural network so that the system can ‘learn’ to recognize patterns, identify materials and initiate an appropriate response. Conclusions This concept forms the basis of a polymer characterization technique, which is far simpler to operate and more flexible in its range of applications than the more conventional pyrolysis GC-MS approach. An additional advantage is the rapid cycle time for spectrum acquisition which, at about 1 s-1, is comparable to GC-MS. Further development focusing on improving IMS resolution, the characterization of the ablated 45 1 species by GC-MS, and the compilation of a signatures database are in progress.We thank Graseby Ionics for technical support and encour- agement. Two of us (M.S. and D.A.) thank the School of Science, Sheffield Hallam University, for financial support. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 1s References Karasek, F. W., Res. Dev., 1970,21, 330. Cohen, M. J., and Karasek, F. W., J. Chromatogr. Sci., 1970,8, 330. St. Lewis, R. H., and Hill, H. H. Jr, CRC Crit. Rev. Anal. Chem., 1990,21, 321. Spangler, G. E., Campbell, D. N., Vora, K. N., and Carrico, J. P., ISA Trans., 1984,23, 17. Hill, H. H., Siems, W. F., Louis, R. H. and McMinn, D. G., Anal. Chem., 1990,62, 1201A. Spangler, G. E., Carrico, J. P., and Campbell, D. N., J Test. Eval., 1985, 13, 234. Schumate, C., and Hill, H. H., Jr, Anal. Chem., 1989,61,601. Huang, S. D., Kolastis, L., and Lubman, D. M., Appl. Spectrosc., 1987,41, 1371. Eiceman, G. A., Anderson, G. K., Danen, W. C., Ferris, M. J., and Tiee, J. J., Anal. Lett., 1988, 21, 539. Karpas, Z., Anal. Chem., 1989, 61, 684. Eiceman, G. A., Snyder, A. P., and Blyth, D. A., Znt. 1. Environ. Anal. Chem., 1990, 38 415. Eiceman, G. A., Leasure, C. S., and Vandiver, V. J., Anal. Chem., 1986,58, 76. Lawrence, A. H., and Elias, L., Bull. Narc., 1985, 27, 3. Lawrence, A. H., Anal. Chem., 1986, 58, 1269. Cooke, M., and Baldwin, C., unpublished work. Paper 3100781 B Received February 9, 1993

ISSN:0003-2654    

DOI:10.1039/AN9931800449

出版商:RSC    

年代:1993

数据来源: RSC

摘要:

ANALYST, APRIL 1993, VOL. 118 453 CUMULATIVE AUTHOR INDEX JANUARY-APRIL 1993 Adams. Michael J., 229 Alder, J. F., 395 Allag, Houssein, 401 Anderson, David R., 449 Andrew, B. E., 153 Andrews, William J., 425 Arrigan, Damicn W. M., 355 Ashok Kumar, T., 293 Avidad. Ramiro, 303 Bae, Yea-Ling, 297, 301 Bangar Raju, G., 101 Bannon, Thomas, 361 Barclay, David A., 245 Barjat, HervC, 73 Barker, Philip G., 347 Barnard Howie, Judith A., 35 Bartlett, Philip N.. 371 Bayo, Javier, 171 Bell, Jimmy D., 241 Belton, Peter S., 73 Benmakroha, Farida, 401 Bhaskar, Nilam, 1 Biondi, Cinzia. 183 Birmingham, John. J.. 1 Blair, Neil, 371 Bos, Albert, 323 Bos, Martinus. 323 Boudjerda, Tarik, 401 Boufenar, Rabah, 401 Breen, William, 415 Brinkman, Udo A. Th., 11 Brown, Marc B., 407 Bruns, Roy E., 213 Cai.Xiaohua, 53 Canela, Ramon. 171 Capitan-Vallve y , Luis Fermin, Carlson, Robert G . , 257 Casey, Vincent, 389 Cassidy, John F.. 415 Cermak, Josef, 79 Chadima, Radko, 79 Chaisuksant, Rasamcc. 179 Chartier, A., 157 Chen, Liang, 277 Chokshi, Hitesh P., 257 Clarke, Colin G., 229 Clinton, Cathriona. 415 Cooke, Michael, 449 Corbini, Gianfranco, 183 Cordero, Bernard0 Moreno, 209 Corti, Piero, 183 Cotsaris, Evangelo, 265 Crean, G. M.. 429 Crooks. Steven R. H., 447 Cummins, Diane, 1 Cummins, Phillip G., 1 Cunningham, K.. 341 Daenens, P., 137 Davis, Willard E., 249 de Andrade, Joso Carlos, 213 de la Guardia, Migucl, 23 de Paula Eiras, Sebastiso, 213 Deasy, Brian. 355 Dcbrabandere, Lode, 137 Dempsey. Eithnc, 411 Diamond, Dermot, 347 Diaz, Susana, 171 Diewald, Wolfgang, 53 Djerboua, Ferhat, 401 Domanskq, Karel, 335 Dominguez, Lucas, 171 303 Dowle, Chris J., 17 Dreassi, Elena, 183 Economou, Anastasios, 47 Edmonds.Tony E., 407,443 Egan, Dcnise, 411 Egan, Denise A., 201 Eggins, Brian R., 439 Elliott, Christopher T., 447 Espinosa-Mansilla, Anunciacion, 89 Fearn, Tom, 235 Fernandez Laespada, Ma. Esther, 209 Feygin, Ilya, 281 Fielden, Peter R., 47 Fitzgerald, Catherine, 361 Flaherty, T., 429 Foster, Robert, 415 Fox, C. G., 157 Fraidias Bccerra, Antonio J., Friel, Sharon, 371 Frutos, G., 59 Fu, Chengguang, 269 Gaind, Virindar S., 149 Garcia Gomez de Barreda, Garcia-Lopez, Trinidad, 303 Gardner, Julian W., 371 Gcorgcs, J., 157 Ghijsen, Rudy T., 11 Gibney, Patrick M., 425 Givens, Richard S . , 257 Glennon, Jeremy D., 355 Goodfellow, Brian J., 73 Greenfield, Stanley, 443 Greenway, Gillian, 17 Gregory, Donald P., 1 Grob, Robcrt, 11 Gu, Zhi-cheng, 105 Harris, S.J., 341 Hartnett, Margaret, 347 Haswell, Stephen J., 245 Hawkesworth, K . , 395 Hawkins, Peter, 35 Hembree, Jr., Doyle M., 249 Hidalgo de Cisneros, Jose L. Hokari. Norihisa, 219 Howard, Vyvyan C., 1 Huang, Ka-lin, 205 Hunt, Terence P., 17 Idriss, Kamal A., 223 Iizuka, Ryuji, 165 Ishida, Junichi. 165 Iwachido, Tadashi, 273 Janata, Jifi, 335 Jedrzcjczak, Kazik, 149 Johnston, Brian, 355 Jones, Carol L., 1 Josowicz, Mira, 335 Ju, Doweon. 253 Kalcher, Kurt, 53 Kallury, Krishna M. R., 309 Kasumimoto, Hanac, 131 Katz, Stanley E., 281 Kessler, Margalith, 235 Keyes, Emmetine T., 385 Kinoshita. Toshio, 161 Kobayashi, Atsushi, 273 Kobayashi, Shouichi, 131 Kotrlq, Stanislav, 79 175 Daniel, 175 Hidalgo, 175 Kovanic, Pavel, 145 Krishan Puri, Bal, 85 Kubal, Gina, 241 Kumar, Manjeet, 193 Lan, Chi-Ren, 189 Lang, Mark J., 425 Li, Xiang-Ming, 289 Liang, Wei-An, 97 Lin, Yuehe, 277 Lopez Ruiz, B., 59 Lunte, Susan M., 257 Lyons, Cormac H., 361 Lyons, Michael E.G., 361 Mc Monagle, James B., 389 McArdlc, Fiona A., 419 McCallum, John J., 401 McCaughey, William J . , 447 MacCraith, Brian D., 385 McDonagh, Colette M., 385 McEvoy, John D. G., 447 McGilp, John F., 385 McKervey, M. A., 341 McLeod, Cameron W., 449 Magce, Robcrt J., 53 Martin, J . P., 59 Mathieu, Jacques, 11 Mcllidis, Antonios S . , 179 Mertens, Bart, 235 Midgley, Derek, 41 Miller, James N.. 407 Miller, Richard M., 1 Moreno, Miguel A., 171 Moriyama, Youichi, 29 Moss, Martin C., 1 Muiioz Leyva, Juan A., 175 Nabekura, Tomiko, 273 Nagahiro, Tohru, 85 Nakagawa, Genkichi, 219 Nakamura, Kayoko, 29 Narayanaswamy, Ramaier, 317 Navalon, Alberto, 303 Neuhold, Christian, 53 Nicholson, Brenton C., 265 Nimura, Noriyuki, 161 O’Bcirn, Brendan, 389 O’Donoghuc, Eilish, 415 O’Keeffe, Gcrard, 385 O’Kelly, Brendan, 385 O’Kennedy, Richard, 201,411 O’Neill, Robcrt D., 433 O’Sullivan, Ciara, 411 Palaniappan, R., 293 Papageorgiou, Vassilios P., 179 Paukert, TomaS, 145 Paynter, J., 379 Pearcc, Timothy C., 371 Perez Pavon, Jose Luis, 209 Peris Cardells, Empar, 23 Persaud.Krishna C., 419 Petelenz, Danuta, 335 Pitre, Krishna S., 65 Pramauro, Edmondo, 23 Preston, Gaynor, 245 Prevot, Alcssandra Bianco, 23 Prieta. Javier, 171 Proictti, Daniela, 183 Pyo, Dongjin, 253 Radulovic, Stojan, 241 Raurich, Joscp Garcia, 197 Reckhow, David A., 71 Reid, Helen J., 443 Riley, David P., 407 Roe, Merrion P., 425 Ruan, Fu-Chang, 289 RubeSka, Ivan, 145 Rubio Leal, Amparo, 89 Sadler, Peter J., 24 I Saleh, Magda M.S . , 223 Salinas, Francisco, 89 Salvatore, Michael J . , 281 Sanchis, Vicente, 171 Sanz Pedrcro, P., 59 Satake, Masatada, 85 Savarino, Piero, 23 Seare, Nichola J., 407 Shallow, A., 429 Sheppard, Robcrt C., 1 Shimoishi, Yasuaki, 273 Shortt, Desmond H., 447 Simpson. Michael, 449 Singleton, Scott, 1 Slater, Jonathan M., 379 Smyrl, Norman R., 249 Smyth, Malcolm R . , 411 Srimkova, Jitka, 79 Srivastava, P. K., 193 Su, Hongbo, 309 Suarez, Guillermo. 171 Svehla, G., 341 Svehla, Gyula, 355 Svendsen, C. N., 123 Tang, Gui-na, 205 Taniguchi, Hirokazu, 29 Teasdale, P.R. , 329 Thompson, Michael, 235, 309 Torrades, Francesc, 197 Toyo’oka, Toshimasa, 257 Tsai, Suh-Jen Jane, 297, 301 Tsuzuki, Wakako, 131 Tucker, Alan, 241 Van Boven, M., 137 van der Linden, Willem E., 323 Veiro, Jeffrey A., 1 Verma, Neerja, 65 Vijaya Raju, K., 101 Vilchez, Jose Luis, 303 Viscardi, Guido, 23 Vos, Johannes G., 385 Voulgaropoulos, Anastasios, Wada. Hiroko, 219 Wallace, G. G., 329 Walton, Philip W., 425 Wang, Bao-ning, 205 Wang, Joseph, 27’7, 41 1 Watt, E. J., 379 Williams, David M., 249 Williams, Kathleen E., 245 Wong, Kwok-Yin, 289 Wuchner, Klaus, 11 Xic, Yuefeng, 71 Xu, Hongda, 269 Yamaguchi, Masatoshi, 165 Yamauchi, Shuji, 161 Yang, Mengsu, 309 Yoshida, Tomohiko, 29 Yuchi, Akio, 219 Zenki, Michio, 273 Zhang, D., 429 Zheng, Minghui, 269 Zhou.Jie, 97 Zhu, Zhong-Iiang, 105 Zou, Shi-Fu. 97 179454 ANALYST, APRIL 1993, VOL. 118 EIRELEC 1993 Electrochemistry to the year 2000 September 11-15 Adare, Co. Limerick, Ireland Scientific Programme An international conference dealing with recent advances in electrochemical methodology, technology and sensors will be held at the Dun Raven Arms Hotel, Adare, Co. Limerick, Ireland. The programme will consist of plenary, invited and contributed oral papers and posters, and will be organized to allow for maximum discussion of papers. Plenary lectures will be given by: Professor A. Bard (USA) Professor J.O.M. Bockris (USA) Professor J. Wang (USA) A strong programme of invited and contributed lectures is currently being organized.Location Adare is a small, picturesque, historical village located just a short drive from Shannon International Airport (approx. 35 min). The village has been chosen because of the large range of activities it can provide, both cultural and sporting, and ample opportunity will be given to savour the local environment. The conference will be based in the "Olde Worlde" atmosphere of the Dun Raven Arms Hotel; a large range of accommodation is available within and close to Adare, ranging from the world renowned Adare Manor to pleasant bed and breakfast type lodgings. An option has been taken on several family holiday cottages within the town and these will be available to those delegates wishing to be accompanied by their families. Publication We have planned to publish a Special Issue of The Analyst based on the papers and posters that will be presented at this conference, and authors of invited and contributed papers will be encouraged to submit their papers to Professor M.R. Smyth at the address given below before, or at, the meeting. Those wishing to submit abstracts (1 page A4) or to obtain further details of the meeting should contact Professor Smyth. The second circular for this meeting will be available in April 1993 and will contain full details of registration fees, accomm odati on, etc. Contact Address : Professor M.R. Smyth, School of Chemical Sciences, Dublin City University, Dublin 9, Ireland. Tel: +353-1-7045308; Fax: +353-1-7045503

ISSN:0003-2654    

DOI:10.1039/AN9931800453

出版商:RSC    

年代:1993

数据来源: RSC