Anal. Chem. 2003, 75, 5262-5266
Selective Detection of HFC and HCFC Refrigerants Using a Surface Acoustic Wave Sensor System Florian Bender,*,† Aleksandr Skrypnik,† Achim Voigt,† Joachim Marcoll,‡ and Michael Rapp*,†
Forschungszentrum Karlsruhe, Institute for Instrumental Analysis, Postfach 3640, 76021 Karlsruhe, Germany, and Dra¨gerwerk AG, Moislinger Allee 53-55, 23542 Lu¨beck, Germany
Halogenated hydrocarbons are the generic base of most refrigerants. They are known to be greenhouse gases; some of them are even suspected to have an ozone depleting potential. Thus, an urgent need exists to detect and identify these compounds. However, refrigerants usually have very low boiling points as well as low electrochemical activities. The latter problem is a serious obstacle to the development of appropriate electrochemical sensors. It is circumvented by using mass sensitive methods of detection. To solve the former problem, the analytical system has to be optimized with regard to effective collection and detection of the refrigerants. In this work, a system for detection and identification of refrigerants is presented, based on a surface acoustic wave sensor array. The system is using a two-step analyte preprocessing unit containing a molecular sieve filter to minimize humidity and a carboxen trap for preconcentration of the analytes. Refrigerants R22, R134a, and R507 have been selected for analysis. The respective detection limits of 10, 20, and 50 ppm indicate the system is useful for leak detection in air conditioning and refrigerating systems, surveillance of exposure limits, etc. Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) have been used for commercial refrigeration and airconditioning applications for many years.1-4 They have been investigated as substitutes for chlorofluorocarbons (CFCs) since the detrimental effect of the latter on the stratospheric ozone layer was discovered.5,6 Today, HCFCs are also suspected to have a potential as ozone depleting compounds,3,5 in contrast to HFCs.1-3,5,6 However, the latter still have a strong global warming potential.2,6 * Corresponding author. E-mail:
[email protected] (F.B.) and rapp@ ifia.fzk.de (M.R.). † Institute for Instrumental Analysis. ‡ Dra¨gerwerk AG. (1) de Brito, F. E.; Gurova, A. N.; Nieto de Castro, C. A.; Mardolcar, U. V. High Temp. - High Pressures 2000, 32, 631-651. (2) Siegl, W. O.; Wallington, T. J.; Guenther, M. T.; Henney, T.; Pawlak, D.; Duffy, M. Environ. Sci. Technol. 2002, 36, 561-566. (3) Dieterle, F.; Belge, G.; Betsch, C.; Gauglitz, G. Anal. Bioanal. Chem. 2002, 374, 858-867. (4) Busche, S.; Dieterle, F.; Kieser, B.; Gauglitz, G. Sens. Actuators, B 2003, 89, 192-198. (5) Boudouris, D.; Prinos, J.; Bridakis, M.; Pantoula, M.; Panayiotou, C. Ind. Eng. Chem. Res. 2001, 40, 604-611. (6) Takita, Y.; Tanabe, T.; Ito, M.; Ogura, M.; Muraya, T.; Yasuda, S.; Nishiguchi, H.; Ishihara, T. Ind. Eng. Chem. Res. 2002, 41, 2585-2590.
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Therefore, it is important to provide an analytical method to control the use of HCFCs and HFCs in several ways. First, leakage of these compounds is a significant problem.2 Thus, a portable instrument is needed which can detect HCFCs and HFCs in small concentrations. Second, since different halogenated hydrocarbons have very different impacts on the environment, it is important to have a method to identify a given compound. Third, since a demand exists to recycle refrigerants, a method is needed to monitor the purity of a refrigerant, again necessitating its exact identification.3,4 Several attempts have been made to detect and identify refrigerants using existing sensor systems. This includes polymercoated quartz crystal resonators5 and optical sensing methods.3,4 These attempts have been successful in concentrations down to 0.44%.3 However, for some applications such as the monitoring of industrial refrigerating systems, detection limits of 100 ppm or below are desired. Among the many different types of sensor devices on the market, surface acoustic wave (SAW) sensors have proven advantageous for gas-phase analysis.7,8 Often, SAW sensors are coated with polymer layers both in order to increase the number of analyte molecules which can be ad-/absorbed from the gas phase and to provide a certain chemical selectivity to the sensor device. A number of SAW sensors, each coated with its individual polymeric material, can then be combined into a sensor array. Thus, the presence of an analyte will cause a characteristic pattern of sensor responses which, with the help of pattern recognition software, can be used to identify the analyte. For the present study, an array of eight polymer-coated SAW sensors was combined with a preconcentration stage to detect and identify refrigerants at low concentrations. It has been demonstrated that a hand-held version of this type of device can be designed.9 Three refrigerants have been chosen for investigation: two HFCs (R134a and R507) and a HCFC (R22). R22 (chlorodifluoromethane) has a boiling point of -40 °C. It has been widely used not only for commercial refrigeration and air-conditioning applications1,3,4 but also as a blowing agent for the foaming of plastics.5 However, it is being gradually replaced by less harmful compounds and/or mixtures, such as R134a (1,1,1,2-tetrafluoroeth(7) Grate, J. W.; Martin, S. J.; White, R. M. Anal. Chem. 1993, 65, 940A948A. (8) Grate, J. W.; Martin, S. J.; White, R. M. Anal. Chem. 1993, 65, 987A996A. (9) Biswas, S.; Heindselmen, K.; Wohltjen, H.; Staff, C. Food Contr. 2003, in press. 10.1021/ac034542l CCC: $25.00
© 2003 American Chemical Society Published on Web 08/16/2003
ane)1,3,4 which has a boiling point of -26 °C. The largest market for R134a is vehicle air conditioning.2 The azeotropic blend R507 is another promising candidate for substitution of R22. R507 is a 50/50 mixture of 1,1,1-trifluoroethane (R143a) and pentafluoroethane (R125) with a boiling point of -47 °C. It is already covering a wide range of applications, including supermarket display cases, transport refrigeration, and ice machines.1 EXPERIMENTAL SECTION Materials. Refrigerants R22, R134a, and R507 were purchased from Messer Griesheim, Krefeld, Germany. All refrigerants were of purity 98.5% or higher. Carboxen 564 and carboxen 1000 sorbent materials were purchased from Sigma-Aldrich Chemie, Supelco division, Taufkirchen, Germany. Molecular sieve 0.3 nm (sodium aluminum silicate, 2 mm pearls) was obtained from Merck, Darmstadt, Germany. The following coating materials have been used for the SAW sensor array (suppliers in parentheses): poly(butyl methacrylate) and polyisobutylene (Fluka, Buchs, Switzerland); ethylene glycol adipate (Riedel-de Hae¨n, Seelze, Germany); glycidoxypropylmethyl dimethylsiloxane (ABCR, Karlsruhe, Germany); L grease (Apiezon, London, UK); poly(chlorotrifluoroethylene-co-vinylidene fluoride) (Aldrich, Milwaukee, WI); polyurethane alkyd resin with trace isocyanates (RS Components, Corby, UK); and silar-10C (Alltech, Deerfield, IL). Sensor Array. The sensor devices used for this work are based on a SAW resonator design. The frequency of operation is 433 MHz. Eight SAW devices are combined into a sensor array, each device being coated with a different coating material. The frequencies of the devices are read out consecutively using a multiplexing technique.10 A ninth, uncoated device is used as a reference, so only the difference frequencies must be processed instead of the high frequencies of operation. It takes one second to read out the eight frequency values. The sensor devices are inserted into a miniaturized sensor board with incorporated gas channels containing a total gas volume of 80 µL.11 Trap. The sensor array is combined with a preconcentration unit (“trap”).11 The trap contains a sorbent material which ad-/ absorbs the analyte from the surrounding gas atmosphere. Periodically, the sorbent material is heated to a temperature sufficient to effectively desorb the analyte. The thermally desorbed sample is then aspirated by the sensor array. In this way, many analytes can be preconcentrated by several orders of magnitude.12 For preconcentration of refrigerants, carboxen was selected as sorbent material.11-13 Different amounts of carboxen (40-600 µL) were packed into a sorbent tube made of either PTFE (poly(tetrafluoroethylene)) or glass. The temperature of desorption was varied between 150 °C and 300 °C. Resistive heating and air cooling of the sorbent tube are employed. The gas sample is aspirated through the sensor array for 2-4 min using a flow rate of approximately 500 mL/min. Next, the gas flow is stopped, and the sorbent tube is heated to the desired temperature of desorption. Finally, the desorbed sample is pumped through the sensor array at reversed flow for 40-100 s using a (10) Rapp, M.; Reibel, J.; Voigt, A.; Balzer, M.; Bu ¨low, O. Sens. Actuators, B 2000, 65, 169-172. (11) Bender, F.; Barie´, N.; Romoudis, G.; Voigt, A.; Rapp, M. Sens. Actuators, B 2003, 93, 135-141. (12) Lu, C.-J.; Zellers, E. T. Anal. Chem. 2001, 73, 3449-3457. (13) Groves, W. A.; Zellers, E. T.; Frye, G. C. Anal. Chim. Acta 1998, 371, 131143.
flow rate of about 5 mL/min. The sorbent tube is cooled while the cycle restarts. Gas Chamber. For refrigerant detection, a 130 L gas chamber was constructed from PMMA (poly[methyl methacrylate]). Gas inlet and outlet of the SAW sensor system were connected to the chamber which was filled with ambient air. Refrigerants were injected using a glass syringe. A fan provided for rapid distribution of the injected analyte. RESULTS AND DISCUSSION Due to their high volatility, the detection of refrigerants using a SAW sensor system is not a trivial task. Considerable efforts have been made to find coating materials which exhibit a strong interaction with these compounds. Some interesting candidates have been proposed, including paracyclophanes14 and cyclodextrines.15 However, even when using sophisticated sensor coatings, the use of some means of analyte preconcentration seems to be unavoidable. In the choice of a solvent phase for a suitable preconcentration stage, similar problems are encountered. Porous polymers, like tenax, are not suitable because their affinity for highly volatile analytes is too low. Carboxen, a carbon molecular sieve, was found to have a higher affinity for refrigerants. Unfortunately, carboxen is also known for its high water retention.13 This entails the difficulty to separate a small analyte signal from a largesand often fluctuatingshumidity background. The problem was eventually solved by using a two-step analyte preprocessing unit comprising two traps. The first contains a molecular sieve based on sodium aluminum silicate. This material was found to reduce humidity decisively, but it will not retain significant amounts of the refrigerants. The latter are successively collected in the second trap containing a carboxen bed. Both traps are heated periodically to reactivate them, but only the sample desorbed from the carboxen trap is aspirated by the sensor array. As a result, the array will give no significant response to humidity while still maintaining a high sensitivity to refrigerants. In a first series of experiments, a SAW sensor system with small exchangeable carboxen trap was used. To minimize the power consumption of the system, the PTFE sorbent tube was filled with only 40 µL of carboxen 564. A temperature of desorption of 200 °C and a total cycle time of 4 min were selected. Figure 1 shows a measurement for detection of R507 in concentrations of 1600-100 ppm. The continuous frequency responses of all eight sensor devices are plotted; each peak corresponds to the desorption phase of one cycle of measurement. To visualize the response pattern of the array, the maxima of the eight sensor responses are shown as a radial plot for selected peaks. Due to the small sorbent volume of the carboxen trap, the frequency responses of individual devices are quite small. However, evaluation of the radial plots helps to confirm the presence of the refrigerant. Although the characteristic response pattern for R507 appears slightly smeared out for small concentrations, it is still markedly different from the response pattern for ambient air (first and last radial plots in Figure 1). In fact, the criterion for (14) Dickert, F. L.; Haunschild, A.; Kuschow, V.; Reif, M.; Stathopulos, H. Anal. Chem. 1996, 68, 1058-1061. (15) Dickert, F. L.; Landgraf, S.; Sikorski, R. J. Mol. Model. 2000, 6, 491-497.
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Figure 1. Detection of refrigerant R507 using a 40 µL carboxen-564 trap. Concentrations and corresponding radial plots are indicated in the figure, together with radial plots for two selected cycles with no R507 being present.
Figure 2. Detection of refrigerants R134a, R22, and R507 using a 600 µL carboxen-1000 trap. Concentrations are indicated in the figure, together with the radial plots for the responses of the array to 100 ppm of each analyte.
detection of R507 could be as simple as to check if the response vector pointing straight up in the radial plots is the largest. All three refrigerants under investigation can be detected with the above system at concentrations down to 100 ppm or below. However, due to the small frequency shifts obtained, the response patterns may still be subject to significant random fluctuations. If reliable discrimination between closely related analytes becomes a system requirement, repeatability and robustness of the systems response behavior will be crucial issues.16,17 Therefore, a second system was designed, using a larger sorbent tube filled with 600 µL of carboxen 1000. To permit temperatures of desorption up to 400 °C, a glass tube was used to hold the sorbent material. Figure 2 shows a measurement for successive detection of all three refrigerants. A temperature of desorption of 210 °C and a total cycle time of 5 min were selected. (16) Wessa, T.; Ku ¨ ppers, S.; Rapp, M.; Reibel, J. Sens. Actuators, B 2000, 70, 203-213. (17) Frank, M.; Hermle, T.; Ulmer, H.; Mitrovics, J.; Weimar, U.; Go ¨pel, W. Sens. Actuators, B 2000, 65, 88-90.
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With the larger carboxen trap capacity, the sensitivity of the system has become much larger, as it was to be expected. For R507, the resulting increase in frequency shift amounts to about a factor of 8. Also given in the figure are the radial plots for the responses to 100 ppm of each of the respective analytes. The radial plot of the HCFC compound, R22, can be easily distinguished from those of the other refrigerants, while the more similar analytes, R134a and R507, produce similar response patterns. For a given analyte, the radial plots will change very little with increasing concentration (not shown). It was found that with the help of methods of pattern recognition, the system can even discriminate between R134a and R507. However, at concentrations below 100 ppm, this task becomes more difficult. A close look at Figure 2 reveals that the frequency response of the array is not strictly proportional to concentration. In particular, for high concentrations, a saturation effect is observed. Since this behavior occurs with all sensor devices, the reason is very likely due to saturation of the sorbent material. This was to
Figure 3. Frequency response of the most sensitive device as a function of concentration for three different refrigerants. The dotted line indicates the response to background atmosphere (ambient air).
Figure 4. Frequency response of the most sensitive device to 2300 ppm R134a as a function of temperature of desorption. Experimental values (b) fall close to a second-order polynomial fit.
be expected since the sorbent material will have only a moderate capacity for very volatile analytes. Thus, it may be necessary to focus on the range of concentrations most important for a given application. For this work, the system was designed to give a nearly linear response in the concentration range of 100-1000 ppm for the three refrigerants under investigation. Figure 3 shows the frequency response of the most sensitive device of the sensor array as a function of refrigerant concentration. Below 1000 ppm, the curves are still close enough to linear behavior to facilitate evaluation by pattern recognition. At the same time, for all three analytes, the frequency responses are well above the background level (response to ambient air) at a concentration of 100 ppm. If detection in another concentration range is desired, the design parameters of the system can be adjusted accordingly. For example, if a nearly linear behavior at higher concentrations is desired, the cycle time of the measurement and/or the gas flow rate may be reduced. This will decrease the sensitivity of the system but increase the concentration at which saturation of the sorbent material occurs.
Another parameter which strongly affects the sensitivity of the system is temperature of desorption. This is shown in Figure 4 for the response of the most sensitive device to a concentration of 2300 ppm of R134a. Increasing the temperature of desorption from 150 °C to 300 °C results in an increase in frequency shift of about a factor of 5. At first glance, this may seem surprising since even the lowest temperature of desorption is already far above the boiling point of R134a (-26 °C). It is interesting to note that for water, only a weak dependence of the frequency response on temperature of desorption was found. Certainly, the sorbent material contains a variety of binding sites with different adsorption affinities. Thus, it is to be expected that increasing the temperature of desorption will free more binding sites in the sorbent material which otherwise would remain permanently blocked. However, one would expect that this will affect water adsorption as well. The decisive difference may lie in recondensation of thermally desorbed water in the short stainless steel tube connection between carboxen trap and sensor array, an effect which will be negligible for the very volatile refrigerants. Analytical Chemistry, Vol. 75, No. 19, October 1, 2003
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Table 1. Approximate Detection Limits and Other Relevant Data for Three Refrigerantsa refrigerant
boiling point [°C]
detection limit [ppm]
exposure limit [ppm]
R22 R134a R507
-40 -26 -47
10 20 50
500 1000
a A system with a 600 µL carboxen-1000 trap was used. For R507, no exposure limit was found.
CONCLUSIONS Refrigerants have some crucial chemical and physical characteristics which must be taken into account in the design of a detection system. These include their low boiling points, their low chemical reactivities, and the existence of a number of chemically similar compounds. Thus, few sorbent materials are suitable for preconcentration of refrigerants. Among the materials tested, carboxen offered the best performance for this purpose. A major drawback of this sorbent material is its high water retention which may result in a strong influence of background humidity on the systems response to the presence of refrigerants. However, this problem was effectively eliminated by the use of a two-step analyte preprocessing unit containing a molecular sieve filter to minimize humidity in the aspirated gas sample and a carboxen trap for preconcentration of the analytes. It was found that for refrigerants, the amount of sorbent material needed is considerably larger than for other analytes.11 An amount of 600 µL is considered appropriate. A pronounced dependence of sensitivity on temperature of desorption was found. Depending on the system requirements regarding power consumption, a value between 200 °C and 300 °C may be chosen. With these design parameters, the system was able to detect the three different refrigerants at concentrations well below the values considered critical for occupational exposure. A summary of the approximate detection limits is given in Table 1. Of the three refrigerants investigated, the system is most sensitive to R22. Since SAW sensors are predominantly mass sensitive devices, this may in part be due to the higher density of the HCFC compound in condensed state. Since HCFCs are considered more hazardous for the environment than HFCs, this system preference is considered advantageous. Of the two HFCs, the system is more sensitive to R134a, most likely due to its higher boiling point. The other major criterion for the performance of the system is its ability to discriminate between different refrigerants. Again,
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it was found that the system can easily distinguish between the more hazardous HCFC and the other compounds. Discrimination between more closely related analytes is more difficult. However, it was found that at concentrations of 100 ppm and above, the system can even differentiate between R507 and R134a which are very similar. In addition, the system was able to both identify and quantify a given sample by means of quantitative pattern recognition. For this task, saturation effects of the sorbent material have to be taken into account. If possible, the system parameters (gas flow rate, cycle time, amount of sorbent material, temperature of desorption) may be adjusted such that saturation effects are avoided. If this is not possible, nonlinear methods of quantification have to be used, such as pattern recognition based on nonlinear quantitative neural networks. Even if just a single refrigerant is to be detected, methods of pattern recognition may still be useful in confirming the presence of the analyte. This was found when a system with a small trap containing just 40 µL of carboxen 564 was used. In this case, a frequency shift of about 100 Hz is sufficient for evaluation and detection of the analyte. This is very helpful for the design of a portable system with low power consumption, for example, for leak detection. In conclusion, it can be said that the SAW sensor system described above is suitable for detection of small concentrations of refrigerants down to 10 ppm for R22. This is sufficient for leak detection and monitoring of refrigerating and air conditioning systems. It was found that by means of pattern recognition, the system can discriminate between similar compounds. This makes the system useful for identification of unknown analytes but may help in monitoring the purity of a refrigerant in recycling procedures as well. The sensitivity of the system to HCFC compounds tends to be higher than that for HFC compounds. This fact is regarded advantageous since HCFCs are considered more hazardous to the environment; therefore, lower exposure limits must be imposed on them. In addition, the system can most easily distinguish between HCFCs and HFCs, which is important because the former are gradually withdrawn and replaced by the latter.1,5 ACKNOWLEDGMENT The authors wish to thank M. Dirschka for valuable technical assistance and H. Matthiessen for fruitful discussions. Received for review May 22, 2003. Accepted July 15, 2003. AC034542L
