SnO2-based gas sensors as chromatographic detectors

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Sensors and Actuntors

B, 18-19 (1994) 470-473

SnO,-based gas sensors as chromatographic detectors N. Birsan and R. Ionescu InstiMe of Physics and Technology of Materials, PO Box MG-7, Buchamt-Mdgurele (Romania)

Abstract Being unselective, SnO,-based gas sensors can be used by means of appropriate methods. Various techniques, e.g., pattern recognition or time modulation of the sensor temperature are used. The lack of selectivity is compensated also when one uses these sensors in chromatographic systems. The chromatographic column itself separates the gas mixture into a time sequence, so the sensor receives the different gases at different moments; the first report was made by Figaro Engineering. We used Sn02-based gas sensors as chromatographic detectors; in this paper a complete description of our chromatographic system and of the chromatograms is given. The gas samples we analysed were CO/air mixtures in the 1 to 100 ppm range. The reproducibility was very good; the CO concentration was measured with a maximum error of 10% during a working day. The detection limit was below 1 ppm which is a very good result.

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Being unselective, SnOz-based gas sensors can be used by means of appropriate methods. Various techniques, e.g., pattern recognition [l] or time modulation of sensor temperature [2] are used, The lack of selectivity is compensated also when one uses these sensors in chromatographic systems. The chromatographic column itself separates the gas mixture into a time sequence, so the sensor receives the different gases at different moments; the first report was made by Figaro Engineering, Japan [3]. In this way, one can take full advantage of SnO,-based gas sensors’ remarkable sensitivity and simplicity of the measuring circuit. We used our SnO,-based gas sensors as chromatographic detectors; in this paper a complete description of our chromatographic system and of the chromatograms is given. Some partial results were previously reported [4, 51.

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Fig. 1. The setup of the standard chromatographic system.

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Fig. 2. The chromatographic detector: (1) header; (2) electrodes; (3) lead wires; (4) sensitive layer.

Chromabgraphic system The experimental setup is presented in Fig. 1. Its components are: (i) carrier gas source (a): cylinder containing compressed oxygen (technical grade); (ii) flow regulator (b) supplied by ITIM Cluj-Napoca; (iii) six-way tap (c) for sample injection; (iv) oven and temperature regulator of the chromatographic column (d) supplied by ITIM Cluj-Napoca;

0925-4005/94/$7.00 0 1994Elsetier Sequoia. All rights reserved SSDI 092%4005(93)01042-3

(v) chromatographic column (e); (vi) oven and temperature regulator of the detector (f) supplied by ITIM Cluj-Napoca; (vii) detector (g), the sensitive layer was made as a porous thick layer of polycrystalline SnO, (0.2 wt.% Pd). The sensor was mounted onto a standard header in order to fit in the oven (Fig. 2); (viii) measuring circuit, out of balance signal type; (ix) X-r recorder (h).

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The way in which SnO,-based gas sensors work requires an oxygen-containing gas carrier; the oxygen from the ambient atmosphere is chemisorbed inducing the appearance of intergranular Schottky barriers which control the resistance of the sensing layer. The reducing gas reacts with the chemisorbed oxygen lowering the barriers and, as a consequence, the resistance of the sensor. We found that technical grade oxygen is a better gas carrier in comparison with air or purified oxygen.

Experimental The experimental procedures was: (i) Carrier gas flow is started through the pneumatic circuit. (ii) Temperature controllers were set to the desired vaIues. (iii) Detector measuring circuit is turned on; when the sensor signal is stabilized, the voltage across R, V,, is set to zero using the variable resistor P. The time needed for stabilization is 4 to 5 h in the first day of use and 1 h in each following day. Any change in the sensor resistance, R,, is recorded as a change in V,,; the decrease of R, induces the increase of K?,’ (iv) Gas sample is injected using the six-way tap; due to the separation properties of the chromatographic column, the components of the gas sample reach the detector at different moments. In this way, the wmposition of the ambient atmosphere of the detector changes from the carrier gas to mixtures between the carrier gas and the individual components of the gas sample. The time-concentration profiles of these wmponents, being usually of gaussian lype, and each change of the sensor resistance related to their presence in the carrier gas is modulated by this time dependence and is recorded as peaks in the signal(voltage)-time plot - named chromatogram. Clearly, the appearance of a reducing gas in the ambient atmosphere of the detector will determine the appearance of a positive peak in the chromatogram.

sample volume (1 cm3), and (v) X-r speed (12 mmlmin) and sensitivity (1 mV/cm). In Fig. 3, a typical chromatogram is presented; the CO concentration was 20 ppm. Its elements are: (i) The first perturbation of the baseline, a, wrresponds to the pressure changes in the pneumatic circuit due to the injection of the unpressurized gas sample in the pressurized flow of carrier gas. (ii) The first peak, b, corresponds to the gases less retarded by the column, i.e., 0, and H, if it is the case. The peak is due to the way in which works the chromatographic wlumn: when in the gas sample are present gases which are also present in the carrier gas, in the ambient atmosphere of the detector at their corresponding moments appear the differences between the concentrations in the gas sample and in the carrier gas. For oxygen it results a Gaussian-type decrease of the concentration recorded as a positive peak. (iii) The second peak, c, corresponds to NP SnO,based gas sensors do not react with N,; its massive presence determines a non-negligible decrease of O2 concentration which results in a positive peak. (iv) The third peak, d, corresponds to CH,. It is a negative one due to the presence in the carrier gas of a small amount of CH, approximately 40 ppm. In the gas sample the amount of Cl& is lower, maybe zero, so we have a Gaussian-type decrease of its concentration in the ambient atmosphere of the sensor, recorded as a negative peak. (v) The fourth peak, e, corresponds to CO. When one evaluates the performances of a chromatographic system, there are some important checkpoints to pass: (i) baseline noise and drift; (ii) lowest detectable concentration and resolution, and (iii) reproducibility. Baseline dr$f is a measure of detector stability if it operates in stable conditions, i.e., a stable flow rate of

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Results The performance of the chromatographic system performances were tested using CO/air mixtures obtained in a dynamic calibration setup. The CO concentration was measured using an IR gas analyser (Junkalor Dessau-Infralyt 4). The test conditions were: (i) detector temperature (350 “C); (ii) carrier gas flow (20 cm3/ min); (iii) 2 m length, 2 mm inner diameter stainlesssteel column filled with 5 8, molecular sieve; (iv) gas

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Fig. 3. micaI chromatogram obtained for CO in air detecting using the chromatographic system presented in Fig. 1 (CO concentration 20 ppm). The scale is the same for Figs. 4-6.

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a stable carrier gas. The baseline drift is approximately 2 mV in an eight-hour working day, which means 10% of the initial value of V,. Such drift values did not require baseline adjustment and could not be related only to the time evolution of the sensor. In the same time interval the flow-rate drift was approximately 6% of its initial value and the temperature regulator of the coh~mn oven has a 2% estimated drift. However, even in these conditions the sensor stability is quite remarkable if compared with the one obtained in usual test conditions. The baseline noise is extremely low and seems to be the electronic noise of the recorder. Lowest detectable concenkafion and resohtion. In Fig. 4 is presented the chromatogram corresponding to 1 ppm CO concentration. This is the lowest controlled concentration available in our laboratory due to the limits of the IR gas analyser. The low noise of our detector permits the use of amplifiers, so we expect that the detection limit to be under 1 ppm. If compared with the detection limit of the standard thermal conductivity detector, approximately 100 ppm, our result is quite remarkable. In Fig. 5 is presented a series of chromatograms which correspond to three different CO concentrations. One can easily distinguish between the corresponding peaks. The ~pmducibiliryis also very good; during a working day, if one takes as concentration parameter the height of the peak, at the same concentration of the gas sample the relative differences between the heights of the corresponding peaks did not exceed k 10%. On different days, Fig. 6, the deviation is less than 30%. We can say that using only one calibration procedure for a working day, one can measure with a maximum error of 10%. The problem of a parameter associated with the gas concentration remains open; usually, in gas chromatography one uses the peak area which is proportional

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Fig. 4. Chromatogratn obtained for CO in air detecting using thechromatographicsystempresentedin Fig. 1 (COmncentration: 1 ppm).

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Fig. 5. Chromatograms obtained for CO in air detection for different CO concentrations during a working day: (a) 5 ppm; (b) 10 ppm; (c) 20 ppm.

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oven can be miniaturized and the carrier gas is easy to obtain and unexpensive.

Conclusion

d Fig. 6. Chromatogram obtained for Co in air detection using thechromatographicsystempresentedin Fig. 1 (COconcentration: 10 ppm, next day).

It is possible to eliminate the disadvantages related to the lack of selectivity of SnO,-based gas sensors by using them in chromatographic systems with oxygen as carrier gas; at least for CO, their performances as chromatographic detectors made them interesting due to their high sensitivity and simplicity in use. There is a hope that in this way one can obtain reliable pollutionmonitoring systems. Much more work is needed for this purpose in order to identify the applications which can be solved in this manner, and the specific technical solutions.

References to the concentration. This is not the case for our detectors due to their non linear relationship between resistance and gas concentration. For the sake of simplicity, we used the height of the peak as concentration parameter. This is an unsatisfactoq compromise, so we are still looking for a nice calibration procedure. The results obtained make us confident with the possibility of the use of simplified chromatographic systems, using SnO,-based gas sensors, as reliable pollution monitoring systems: there is no need for sophisticated temperature regulators or large ovens as well as expensive electronics. The column and detector

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