ANALYST, JANUARY 1993, VOL. 118 35 Fast-responding, Fibre-optic Based Sensing System for the Volatile Anaesthetic Halothane, Using an Ultraviolet Absorption Technique and a Fluorescent Film Judith A. Barnard Howie and Peter Hawkins* Sensor Research Group, Faculty of Applied Sciences, University of the West of England , Coldharbour Lane, Bristol, UK BS16 IQY An improved version of a fast-responding sensing system for the widely used volatile anaesthetic halothane (2-bromo-2-iodo-I ,I ,I-trifluoroethane) is described. The concentration of halothane is determined using an ultraviolet radiation (UV) absorption technique. Ultraviolet radiation at 230 nm is conveyed by a silica optical fibre to a gas flow-through cell containing the halothane, and the intensity of the UV reaching the other side of the cell is measured using a fluorescent polymer film. This paper describes the development of an efficient fluorescent polymer film for the sensor based on poly(ethy1ene glycol) containing two fluorophores { 2,5-diphenyloxazole and tris[4,4,4-trifluoro-I -(2-thieny1)butane-I ,3-diono]europium(iii)). The film fluoresces strongly with a red line spectrum when excited in the range from about 200 to 380 nm.There is evidence of direct energy transfer between the two fluorophores. A similar effect is observed in films prepared from Carbowax 20M and the europium chelate. An advantage of this approach is that the fluorescent radiation can be transmitted back to a silicon photo-detector using an inexpensive polymer optical fibre bundle. Two experimental sensor systems for halothane are described and the results show that, although the response does not obey the Beer-Lambed law, a reliable system for determining halothane can be constructed, which operates over the medically important range &3%.This paper also describes how the signal-to-noise ratio of the system can be improved by using the long fluorescence lifetime of the film. A system for relaying the fluorescent radiation efficiently to polymer optical fibres is described, which is based on a clear film of poly(methy1 methacrylate) containing the two fluorophores. The sensor head is compact and has a fast response time, thereby making it suitable for end-tidal respiratory gas measurements. The sensing system could also be used with other gases or liquids that absorb in the range 200-380 nm.Keywords: Ultraviolet fibre optic; anaesthetic gas analysis; fluorescence quenching; end-tidal respiratory measurement; europium (3- dike tone chelate Halothane (2-bromo-2-iodo-l,l,l-trifluoroethane) is a com- monly used inhalation anaesthetic. Several different types of anaesthetic gas analysers are available'-2 for measuring inhalation levels of halothane (in the range 0-3%) in oxygen and dinitrogen oxide. Some of these have the disadvantages of being bulky, expensive or unsuitable for routine clinical use. Most are also too slow in their response times to measure fast changes in concentration, particularly the end-tidal value. The end-tidal value is the final measurement of the exhalation prior to the start of inhalation and is related to the gas level in the lungs and the concentration in the arterial blood.3 The aim of this work was to construct a fast-responding measuring system for halothane within the constraints of a number of design criteria.These were that the device should be: fast and reliable enough for end-tidal measurements; safe and simple enough for use in a routine hospital environment; and have a small, remote, sensor head to prevent interference in the surgical procedures. A fibre-optic technique was chosen as the most likcly to fulfil all the above criteria. Many fibre-optic chemical sensors have already been reported.4.5 These include a number with medical uses such as blood gas,6 glucose7 and ion8 analysers and as immunosensors.c) A fibre-optic halothane sensor has also been reported,*O which utilizes the effect of fluorescence quenching by the halothane, but its response time is far too slow for end-tidal measure- ments.A rapid-response halothane sensor based on ultra- violet radiation (UV) absorption spectroscopy has been reported by Tatnall et al.11 This design incorporates a bulky sensor head containing the UV source, sample cell, reference cell, photo-detectors, heaters and associated electronics. The aim of this project could be achieved by improving on this * To whom correspondence should be addrcssed. design by using optical fibres to relay UV, from an external source to the sample cell and to transmit a signal back to an external detector, so reducing the size of the sensor head and making it intrinsically safe.The experimental design described in this paper incorpo- rates a silica optical fibre to transmit UV from a deuterium lamp to a sample cell through which the gas mixture passes. Unabsorbed UV passes out of the cell, through a silica window on the opposite face and strikes a fluorescent film. The amount of UV reaching the film varies with the concentration of halothane in the gas mixture, which in turn affects the intensity of fluorescent radiation produced by the film. There are several potential advantages in using this arrange- ment. An inexpensive non-UV transmitting polymer fibre bundle can now be used to relay the fluorescent radiation to a remote semiconductor detector, thereby reducing errors caused by UV scattered inside the cell. Semiconductor detectors work well in polymer fibre-optic systems and have peak sensitivities in the visible or near-infrared regions that are well matched to fluorescent radiation in the red part of the spectrum. A method of improving the signal-to-noise ratio of the detected signal by using the fluorescent film is also described.This paper discusses the nature of the fluorescent films used and their use in a syster , suitable for the basis of a fast-responding halothane gas analyser. Experimental Reagents The fluorophores used for the sensor films were tris[4,4,- 4-trifluoro-l-(2-thienyl)butane-1,3-diono]europium(111) (Eu- TTA, from Kodak Laboratory Research Products, Liverpool, UK) and 2,5-diphenyloxazole (PPO, Scintran grade from BDH, Poole, Dorset, UK). The polymers used as a solid36 ANALYST, JANUARY 1993, VOL. 118 matrix were Carbowax 20M (Phase Separations, Clwyd, UK), poly(ethylene glycol) with an average relative molecular mass of 10 000, poly(methy1 methacrylate) (PMMA, medium rela- tive molecular mass) and ethylene glycol diglycidyl ether (all from Aldrich, Gillingham, Dorset, UK).The solvents used, chloroform, ethanol, propan-2-01, methanol and acetone, were all of spectroscopic-grade purity (Aldrich). Halothane was used in liquid form (Aldrich). Other fluorophores used were Rhodamine 6G and Coumarin 152 (Kodak). Instrumentation The instruments used for preliminary fluorescence and absorption studies were a Perkin-Elmer LS-5 fluorimeter (Beaconsfield, Buckinghamshire, UK) fitted with a front- surface accessory, and a Perkin-Elmer Lambda 5 spectropho- tometer.Some sensor measurements were performed with a Perkin-Elmer MPF-3 fluorimeter modified to accept a gas- flow cell. In another experimental halothane sensor, UV was produced by a deuterium lamp (Ealing, Watford, UK), and, after passing through a frequency-stabilized light chopper (Model 1000 from Monolight Instruments, Weybridge, UK), was relayed to a 2 m long, 1 mm diameter UV-transmitting silica fibre (manufactured by Mitsubishi and supplied by Oriel, Leatherhead, UK). The fibre was connected to a gas-flow cell containing the sensor film and finally to a large-area (100 mm2) silicon photo-voltaic detector (RS Components, Corby, UK). In the final study, polymer optical fibres used to transmit the fluorescence to a remote detector were also obtained from RS Components.Calibration of the sensor was carried out by a gas-chromato- graphic method using a Pye Unicam 104 (Cambridge, UK) fitted with a 1.5 m Porapak Q column and a flame-ionization detector. Development of the Sensor Film Halothane absorbs UV over a band of approximately 100 nm with a maximum at 208 nm [see Fig. l(A)] and in the range 220-250 nm, and absorbs strongly enough for detection without interference from the other major components in the end-tidal gas mixture, which are oxygen, nitrogen, dinitrogen oxide and water vapour.11 The film is required to fluoresce when excited with UV in the range 220-250 nm and it is important that it emits in the visible region with a high quantum efficiency to avoid any possible problems with low sensitivity and, if possible, to enable an inexpensive photo- diode rather than a photomultiplier tube to be used as the detector. Several fluorophores, polymers and film-making techniques were investigated for preparing films with high fluorescence 200 280 360 440 600 Wavelengthinm Fig.1 A, Absorption spectrum of halothane (hatched area) in propan-2-01. Absorption (solid line) and fluorescence emission (broken line) spectra of B, EuTTA and C, PPO in ethanol quantum efficiencies. Early experiments were performed using the laser dyes, Rhodamine 6G and Coumarin 152, as they seemed promising. However, the work was abandoned as it proved difficult to make reproducible films with these dyes. This is because they are prone to solvent effects and produce shifts in the fluorescence spectrum, depending on the polarity of the polymer matrix.Rhodamine 6G and Coumarin 152 also have relatively small Stokes' shifts (22 and 109 nm, respec- tively, in ethanol) and exhibit self-absorption at high concen- trations, so the thickness of the film and the concentration of the dye in the film are critical. A europium chelate was found to be a more suitable fluorophore. The luminescence characteristics of lanthanide chelates have been extensively studied12 and europium(ii1) (3-diketone chelates, in particular, have interesting fluores- cence properties and high quantum efficiencies. l3 Fig. 1(B) shows that EuTTA has a strong absorption spectrum in the near-UV region, which is characteristic of the ligands, and a strong fluorescence spectrum consisting almost entirely of a number of narrow lines in the red region of the spectrum characteristic of the ion (the strongest being at 614 nm corresponding to the sDo + 7F2 transition in the Eu"' ion).The europium ion is excited by energy transfer from the ligands. The excited europium 4f valence electrons are well screened by the outer filled 5s and 5d shells, and the bulky ligands surrounding the ion also shield it against intermolecular collisions, which encourage non-radiative processes. The result of these factors is that the europium chelate has a narrow-band, ionic-type emission well separated from its excitation spectrum and which is largely unaffected by changes in its chemical environment. The energy transfer process is generally believed to occur through the triplet state and results in the fluorescence having a relatively long lifetime (0.4-0.6 ms for EuTTA).Films prepared from poly(ethy1ene glycol) were found to produce the highest fluorescence quantum yield for a given concentration of europium chelate, and this material was studied in detail. Poly(ethy1ene glycol), however, tends to form spherulites as the solvent evaporates, producing an uneven distribution of the chelate. Good films can be prepared by using methanol-acetone (1 + 1) as solvent to which is added 2.5% v/v ethylene glycol diglycidyl ether, followed by rapid evaporation of the solvent from the film. A commercial product based on poly(ethylene glycol), Carbowax 20M, also produces satisfactory films. These were prepared by dissolving 0.8 g of Carbowax 20M in 8 cm3 of 4 x 10-3 mol dm-3 EuTTA in chloroform.The solution was left for 1 h at 60 "C to ensure that all the polymer had dissolved and was then stirred thoroughly. Films were formed by a simple dipping technique onto either a glass slide for front-surface illumination or a silica-glass slide for rear-surface illumina- tion. The films were dried rapidly in a hot air stream and then stored overnight at 40 "C to ensure complete evaporation of the solvent. Carbowax 20M contains an additive, 2,2'- [ (methylethylidene)-bis(4,1 -phenyleneoxymethylene)]bisoxi- ran, which is itself fluorescent with an absorption band extending to shorter wavelengths than that of EuTTA and an emission spectrum overlapping the absorption band of EuTTA.The additive appears to act as a donor fluorophore to EuTTA and usefully extends the excitation band of the film to shorter wavelengths as the red emission from the EuTTA in the film on its own is relatively weak when excited in the range 220-250 nm (the region required for halothane detection in end-tidal gases). However, as Carbowax 20M is a commercial product, it was not possible to vary the donor concentration to achieve optimum results so this work could not be carried any further, and other possible donor molecules for use in a poly(ethy1ene glycol) and ethylene glycol diglycidyl ether film were investigated instead. The requirements for the donor fluorophore are that it absorbs strongly in the region 220-250 nm and emits in theANALYST, JANUARY 1993, VOL.118 37 range 300-380 nm where the thenoyltrifluoroacetonate ligands in EuTTA absorb strongly. A fluorophore meeting this requirement is PPO, which is commonly used in liquid scintillation counters and has absorption and emission spectra as shown in Fig. l(C). Films containing PPO and EuTTA were prepared from 8 cm3 of 0.6 X 10-3 mol dm-3 EuTTA in methanol-acetone (1 + 1) and included 2.5% of ethylene glycol diglycidyl ether to which was added 0.02 g of PPO and 0.8 g of poly(ethy1ene glycol). A dip coating technique similar to that previously described for Carbowax 20M was used to prepare the films. When a film containing both PPO and EuTTA is excited at 230 nm, the PPO fluorescence is considerably quenched and the europium emission enhanced, as shown in Fig.2. Fig. 2(A) shows the broad absorption spectrum for a film containing only PPO extending to wavelengths below 200 nm and the strong fluorescence spectrum covering a broad band from about 330 to 500 nm with a negligible emission at 614 nm when the film is excited at 230 nm. A film containing just EuTTA has an absorption spectrum, as shown in Fig. 2(C), and when excited at 230 nm produces an emission spectrum consisting almost entirely of the characteristic Eu"' ion emission similar to that shown in Fig. l(B). The corresponding absorption and fluorescence spectra for a film containing both EuTTA and PPO are shown in Fig. 2(B). As expected, the absorption spectrum is dominated by the much stronger absorption produced by the PPO molecules with a small contribution from the EuTTA molecules.When the film is excited with UV at 230 nm, the characteristic emission spectrum in the UV from the PPO is considerably quenched and extends now from about 360 to 500 nm, whereas the characteristic emission from the Eu'" ion at 614 nm is greatly enhanced. Unfortunately, it was not possible to make accurate quantitative measurements as to the extent of enhancement produced by adding PPO to the film containing EuTTA because of difficulties in preparing films of consistent thickness and in placing them in the same place each time in the fluorimeter; however, there is no doubt that a film 1.2 al C m e a 0.8 0.4 1 1 I I 200 280 360 Wavelengthlnm ( b) 440 300 340 380 420 460 500 540 590 630 Emission wavelengthhm Fig. 2 Absorption and emission spectra of three poly(ethy1ene glycol) and ethylene glycol diglycidyl ether films containing A.PPO only; B , PPO and EuTTA; and C, EuTTA only. The emission spectra were recorded with the films excited at 230 nm containing EuTTA and PPO glows much redder when compared visually with a film containing just EuTTA if both are irradiated at 230 nm. The mechanism for the energy transfer between the molecules is being investigated further. Halothane Detection Using the Sensor Film Fluorescence is emitted uniformly in all directions so that, in a practical sensor, the sensing film could be viewed either from the front with the return fibre placed alongside the fibre carrying the exciting radiation or viewed from the rear with the return fibre behind the sensing film. Two experimental arrangements were constructed to investigatc these options.First Experimental Arrangement A gas-flow cell was specially made to fit into a Perkin-Elmer MPF-3 fluorimeter. The fluorescent film on a glass slide was attached to a small adjustable plug and placed inside the flow cell. Ultraviolet radiation at a wavelength of 230 nm (band- width 36 nm) entered the cell through a silica window, passing through the gas sample (effective pathlength about 35 mm) to strike the sensor film. The film angle was adjusted to about 45" to the direction of the UV so that the maximum fluorescence and minimum scattered light entered the emission slit (slit- width 5 nm) of the fluorimeter. The cell also contained a septum fitting to allow gas samples to be removed by syringe for analysis by gas chromatography.The halothane concentra- tion in the gas flow was controlled by using a vaporizer system similar to that of Zbinden et al.1 A stream of nitrogen gas, controlled by a mass flow controller, was passed through liquid halothane in a gas-jar maintained at a constant 0 "C in an ice-bath. The gas stream, saturated with halothane, was then joined by a faster nitrogen flow and thoroughly mixed before passing into the gas-flow cell. Halothane concentrations were measured by gas chromatography and calibration was carried out using liquid halothane standards. The vapour pressure data of Bottomley and Seiflow14 were used to convert the measured molar concentrations of the gas samples, taken from the flow cell, into volume percentages of halothane.Several different fluorescent films were tested and they all produced similar results. Fig. 3 shows a typical plot of the logarithm of initial fluorescence intensity (i.e., with nitrogen flow only) divided by the fluorescence intensity with halo- thane present (log Zo/l) plotted against the concentration of halothane (% v/v) in the cell, obtained using a sensor film containing EuTTA in Carbowax 20M. The graph over this range is non-linear and, therefore, the sensor does not appear to adhere to the Beer-Lambert law of absorbance. 0.4 0.3 c ,a - cn 0.2 -I 0.1 0 2 3 Halothane concentration (% vlv) Fig. 3 Typical response curve for the first experimental halothane gas sensor using a fluorescent film of Carbowax 20M containing EuTTA, showing log(lo/l) versus halothane concentration (% v/v), where lo is the intensity of the fluorescent light with no halothane present and I is the intensity with halothane present38 ANALYST, JANUARY 1993, VOL.118 output G~~ flow i n Summing amplifier I Deuterium [3 I Coupling optics Gas Chopper UV-transmitting fibre flow 1 L- Gas flow out Fig. 4 Arrangement for the second experimental halothane sensor test rig Second Experimental Arrangement The apparatus shown in Fig. 4 was assembled with a silica fibre to transmit a chopped beam of UV to the cell so that a noise reduction technique could be used to improve the signal-to- noise ratio. The halothane gas-flow system was the same as used previously. Ultraviolet radiation, produced by a deuterium source, passed through a chopper maintained at 3 kHz and was then coupled into the fibre.The coupling optics and the large numerical aperture fibre were chosen in order to capture as much incident radiation as possible. The far end of the fibre was connected to a glass flow cell with a pathlength of 35 mm (total internal volume about 1.8 cm3) and with a silica window at the opposite end. The window was coated on the outside with a fluorescent film that was in contact with a large-area photo-diode. In this experimental arrangement, use was made of the relatively long fluorescence lifetime of the europium ion emission (about 0.4 ms) to improve the signal-to-noise ratio of the detecting system. The light chopper was running at a sufficiently high frequency to provide a measurable signal from the photo-detector during the closed periods of the chopper produced by the afterglow of the europium fluores- cence.As the concentration of halothane increases, the amount of UV reaching the film decreases and there is a corresponding decrease in the intensity of the afterglow. The signal at the output of the detector can be analysed electrically into a varying (a.c.) component and a steady (d.c.) com- ponent, which contains the afterglow from the europium ion. The a.c. component can be isolated from the signal by passing it through an a.c. inverting amplifier with unity gain. When this signal is now added electronically, using the summing amplifier, to the original signal coming from the photo- detector the remainder contains only the d.c. component and much of the noise is cancelled out, producing a considerable improvement in the signal-to-noise ratio.Fig. 5 shows a typical plot of the logarithm of the final signal without halothane (Vo) divided by the signal with halothane present (V) versus halothane concentration (% v/v) for a film prepared from poly(ethy1ene glycol) and ethylene glycol diglycidyl ether with EuTTA and PPO. The response curve is very similar to that obtained in Fig. 3, showing that the sensing system again does not obey the Beer-Lambert law. The reproducibility of the response of the sensing system to halothane was investigated by combining the results obtained in the first experimental arrangement with those obtained in the second (see Fig. 6). The results were first normalized at the 1% halothane level and then a polynomial fit was performed to obtain the equation of the best curve through all the points. 0.002 S 2 8 0.001 -I 0 0.5 1 .o 1.5 2.0 Halothane concentration (% v/v) Fig.5 Typical response curve for the sensing system shown in Fig. 4 using a fluorescent film of poly(ethyiene glycol) and ethylene glycol diglycidyl ether containing PPO and EuTTA. showing log(V&’) versus halothane concentration (% v/v), where V, is the output with no halothane present and V is the output with halothane present 0.4 0.3 - Q) C 0 a. * 0.2 . P, -I ? 0 1 2 3 Halothane concentration (% v/v) Fig. 6 with the values normalized to the 1% halothane concentration Data shown in Figs. 3 (El) and 5 ( X) plotted on the same axes The equation of the best-fit curve through all the points is y = 5.34 X 10-3 + 0 .3 1 2 ~ - 0.0315~2 with a correlation coefficient of 0.9984. This suggests that the apparent deviation from the Beer-Lambert law is indepen- dent of the arrangement of the sensing film and that a simple calibration procedure would suffice to allow reproducible results to be obtained. The results published by Tatnall etal.11 show a similar non-linear response, but they do not commentANALYST, JANUARY 1993, VOI,. 118 39 Front view of sensor w Front silvered mirror Bundle of eight plastic fibres Gas flow in , -- I : '\ I output Summing amplifier r y iensorfitm Plastic optical fibres I I I e' Unity gain a.c. inverting amplifier Chopper L- . Gas flow out Fig. 7 Arrangement for the experimental halothane sensor with a return polymer fibre optical bundle to a remote photodiode on it.Diprose et a1.15 observed a similar non-linear response and derived an experimental equation: V , = 129.7 [ l - exp(-l.lx)] + 4860 [l - exp(-O.O32x)] where V, is the reading on the output meter of their instrument. A good agreement is obtained between these two equations (normalized at x = 1.5) with a correlation coeffi- cient of 0.9926. They attribute the non-linearity to the UV used, which consisted mainly of two spectral lines from a mercury lamp as the Beer-Lambert law assumes monochro- matic radiation. A similar explanation could be used to account for the results obtained in this investigation as the UV used has a wide spectral bandwidth. The results show that a reliable measurement system could be assembled with a UV lamp placed outside the sensing head and linked to it through a silica fibre, and with a fluorescent film and a simple silicon photo-detector inside the sensing head, which would be an improvement on the system described by Tatnall et aZ.11 The next stage in the development is to incorporate a return fibre so that the photo-detector can be placed externally.Design of a Sensing System With a Return Optical Fibre A drawback with using fluorescence is that as it is scattered in all directions it makes efficient collection by a return fibre difficult. A means of collecting the fluorescent radiation efficiently, based on some earlier work,l6 was investigated where it was found that, when a clear fluorescent film containing EuTTA and PPO in PMMA is coated directly onto the plastic cover of a semiconductor photo-detector, it forms a waveguide so that a large proportion of the fluorescence emerges from the side of the film.Use was made of this to improve the coupling of the fluorescence into a return fibre bundle (Fig. 7) by positioning eight polymer optical fibres (about 2 m long) radially onto a front-face silvered mirror and then coating the area between the fibres and the ends of the fibres with the fluorescent film. The film was prepared by applying a few drops of a solution containing 0.8 g of PMMA and 0.02 g of PPO in 4 cm3 of chloroform, to which was added 4 cm3 of a 2 x 10-3 mol dm-3 EuTTA solution in chloroform, onto the mirror, spreading the solution and allowing the solvent to evaporate. The other ends of the fibres were bound together tightly to form a bundle and then positioned carefully over the large-area photodiode.The photodiode and the rest of the equipment used were the same as described previously, although additional electronic filtering was added at the output to compensate for a loss in sensitivity that reduced the 0.4 0.3 a, C 0 a * 0.2 2 0 -4 0.1 0 0.5 1 .o 1.5 2.0 2.5 3.0 Halothane concentration (% v/v) Fig. 8 Response of the sensing system shown in Fig. 7 to different concentrations of halothane. The results of four repeat experiments are shown and these have been normalized to obtain the best fit with the curve shown in Fig. 6 signal-to-noise ratio. The loss in sensitivity was probably caused by imperfect coupling between the fluorescent film and the fibres and by the fact that films prepared from PMMA appeared to have a lower fluorescence output than those prepared from poly(ethy1ene glycol).The concentration of halothane in the cell was changed and the response of the measurement system was noted. The results of four repeat experiments (Fig. 8) show a greater scatter in the points on the graph than found previously because of the lower signal level. When the results are normalized there is a reasonable agreement with the earlier results as shown by the solid line which represents the equation determined previously. The signal-to-noise level would be greatly improved by using a more intense UV source and a photomultiplier as the detector. Discussion A major problem with sensors that rely simply on t h e absorption of radiation for detection is drift in the output caused by instabilities in the source and the detector and by the mechanical rigidity of the system.A method often used to correct for drift is to use a reference ce11,11,12 but this adds to the bulk of the detector head. In this instance a proposed method for correcting for drift would be to excite the film alternately at a wavelength where halothane absorbs ( e . g . , between 220 and 250 nm) and at a wavelength where40 ANALYST, JANUARY 1993, VOL. 118 none of the gases in the sample cell absorb (e.g., between 300 and 380 nm) and then subtracting one signal from the other. The response time of the experimental sensing system was not measured because it was not possible to change the halothane concentrations fast enough in the flow system to carry out an accurate measurement. Tatnall et al.*1 reported a response time of about 40 ms for a 90% change in output to a step change in halothane concentration for their system.No information is given about the response time of the optical and electronic measurement system they used, but it is reasonable to assume that this is fast so that the response time is largely controlled by the wash-out time of the sample cell and connecting tube. In our sensing system, the response time of the optical and electronic measurement system is ultimately limited by the decay time of the fluorescence of the film (about 0.4 ms), which is sufficiently fast, so that if a similar sample cell design is used with our system, then a comparable response time would be expected.The only details given in the paper of Tatnall et al. on the dimensions of the sample cells used are the volumes of the adult cell (4.52 cm3) and infant cell (0.56 cm3), and no information is given about the optical pathlength of the cells. As the volume of our experimental cell (1.8 cm3) falls between these two volumes, the wash-out time of the experimental cell should fall between the values for these two cells. Tatnall et al.11 heated the whole of the sensor head to 42 k 0.5 "C to prevent condensation of water on the silica windows of the sample cell and to stabilize the photo-detectors. A small heater would also be required in our system to heat the sample cell to prevent condensation. The temperature stability of the fluorescent films was not investigated, but if this is found to produce a significant drift in the output, then the film too could be heated separately at a constant, but lower, tempera- ture.The over-all size and weight of the sensing head should be substantially smaller in our design than theirs as it does not contain the UV source, filters, iris and photo-detectors. A magnetically shielded casing is also not necessary to reduce interference problems as all of the signals are conveyed by optical fibres. This system was developed originally for use with halo- thane, although it could be adapted to analyse for other gases or liquids that absorb in the range 2W350 nm. 1 2 7 8 9 10 11 12 13 14 15 16 References Zbinden, A., Westenskow, D., Thomson, D., Funk, B., and Macrtens, J . Int. Clin. Monit. Comput., 1986, 2, 151, Hayes. J., Westcnskow, D., and Jordan, W., Anaesthesiology, 1983, 59, 435. Eger, E., and Bahlman, S., Anuesthesiology, 1971, 35, 301. Scitz, W., CRC Crit. Rev. Anal. Chem., 1988, 19, 135. Wolfbeis, O., TrAC, Trends Anal. Chem., 1985, 4, 184. Gerhich, J.. Lubbers, D., Opitz, N., Hansmann, D., Miller, W., Tusa, J., and Yafuso, M., IEEE Trans. Biomed. Eng., Narayanaswamy, R., and Sevilla, Y . , 111, Anal. Lett., 1988,21, 1165. Roe, J. N., Szoka, F. C., and Verkman, A. S., Analyst, 1990, 115,353. Tromberg, B. J., Sepaniak, M. 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