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A polymer support for pyridoxine hydrochloride used as a sorbent for the piezoelectric quartz crystal detection of ammonia
Analytica Chimica Acta, 155 (1983) 225-229 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
Short Communication
A POLYMER SUPPORT FOR PYRIDOXINE HYDROCHLORIDE USED AS A SORBENT FOR THE PIEZOELECTRIC QUARTZ CRYSTAL DETECTION OF AMMONIA
G. J. MOODY, J. D. R. THOMAS* and M. A. YARMOa Applied Chemistry (Great Britain)
Department,
Redwood
Building,
UWIST, Cardiff CFl 3XA
(Received 1st June 1983)
Summary. A high molecular mass polyalkoxylate, Antarox CO-880 (nonylphenoxypolyethoxylate with 30 ethoxylate units) is a suitable matrix for supporting pyridoxine hydrochloride (an ammonia sorbent) on a quartz piexoelectric crystal detector. The matrix helps considerably to extend the useful lifetime of the detector from ca. 10 to >53 days.
The scope of the piezoelectric crystal detector in analytical chemistry is greatly widened by using absorbent coatings of appropriate selectivity and sensitivity on the quartz crystal [l] . The analyte is selectively sorbed by the coating, thereby increasing the mass on the crystal and decreasing the frequency of vibration. The frequency change is linearly related to the mass sorbed according to AF = -AmF/Adt, which by substituting appropriate parameters for commercially available crystals simplifies to AF = -2.3 X lo6 F’Am/A, where AF is the change in frequency (Hz), F is the initial frequency of the quartz plate (MHz), Am is the mass sorbed (g), A is the area of the coating (cm2), d is the density of the quartz (g cmS3) and t is the thickness of the crystal (cm). Thus, for a particular experimental set-up, the change in frequency can be expressed as AF = KC, where C is the analyte concentration (mg dmM3)and K is a constant which includes the basic frequency of the quartz crystal, the area coated and a factor to convert the mass of analyte sorbed to its gas-phase concentration. For the detection of ammonia in air, Ucon 75-H-90,000 and Ucon LB3000X were first used and found to have good sensitivity [2]. These were followed by coatings of extracts of Cupsicum annum pods and ascorbic acid, with and without silver nitrate [ 31, and later by L-glutamic acid hydrochloride and pyridoxine hydrochloride (vitamin B6 hydrochloride) [ 41, which showed exceptional sensitivity. aPresent address: Chemistry Department, National University of Malaysia, Bangi, Selangor, Malaysia. 0003-2670/83/$03.00
o 1983 Elsevier Science Publishers B.V.
226
During evaluation of a laboratory-constructed piezoelectric apparatus, [ 51, it was found that simply coating the quartz crystal with pyridoxine hydrochloride from an ethanol/water solution led to a coating with an effective lifetime of only ca. 10 days. However, application of the pyridoxine hydrochloride in a matrix of nonylphenoxypolyethoxylate (Antarox CO880) gave greatly improved stability over a much longer period. Experimental Apparatus and detector design. The measuring unit consisted of a frequency oscillator with a buffered output powered by a Weir 400 power supply set at 9 V d.c. The frequency output from the oscillator was measured by a Marconi type 2431A 200-MHz digital frequency meter. A digital-toanalog converter selected the last two digits of the frequency meter output for conversion to an analog signal to a Bryans Model 28000 chart recorder, reading to f 1 Hz. The quartz crystal (AT-cut) with gold electrode (Quartz Crystal Co., Wellington Crescent, New Malden, Surrey) had a resonant frequency of 9 MHz. The detector cell incorporating this crystal was based on the design of Karmarkar and Guilbault [6] wherein the gas sample was split into two streams impinging directly on opposite faces of the coated crystal. The glass-encased cell detector was immersed in a water bath at 25 f O.l”C. Crystal coatings. A saturated solution of 1% (w/v) pyridoxine hydrochloride in ethanol/water (1 + 1) was dip-coated on the quartz crystal for the first series of experiments. For the second series, the solution for dipcoating consisted of a (1 + 1) mixture of a 0.2% (w/v) solution of pyridoxine hydrochloride in ethanol and a 0.2% (w/v) solution of Antarox CO-880 in acetone. Each coating was readily removed by soaking the crystal in ethanol. The crystal was dried before re-coating. Ammonia test samples. The ammonia gas test samples were obtained from ammonia vapour over ammonia liquor equilibrated at 25°C. Serial dilution of the headspace gas was effected by syringe dilution [7] with ambient air. Successive dilutions were delayed by 30-60 s in order to allow ammonia to diffuse throughout the air in the syringe. The concentration of ammonia was checked by titration. For example, 3, 5 and lo-cm3 samples removed from the headspace over the ammonia liquor were taken by syringe and slowly injected into 20 cm3 of 0.025 M sulphuric acid. The excess of sulphuric acid was titrated with 0.1 M sodium hydroxide using methyl orange as indicator. Ten replicate samples of 5 and 10 cm3 of the headspace gas contained 30.8 + 0.2 and 30.4 + 0.2 mg drn-j (mean f std. dev.), respectively, of ammonia. Diluted headspace gas samples were similarly checked; for example, 4 cm3 of headspace gas diluted to 10 cm3 was found to contain 12.1 f 0.1 mg drn-j ammonia. The responses of the coated crystal were tested on triplicate l-cm3 samples of appropriate dilutions of the headspace ammonia test samples and the mean decrease in frequency was measured. The diluted samples were injected into a carrier stream of dry (anhydrous calcium chloride) air, and passed
227
through the crystal compartment at 30 cm3 min-’ by the pump of a Pitman Instruments model 7069 air sampler. Results and discussion The sample injection method essentially results in a gas-phase flow injection mode of analysis. Typical responses are shown in Fig. 1, from which the fast response of the detector is evident. There is a fast initial return of the frequency towards the baseline for 20 s after the sample has passed through the detector, although full return to the original frequency takes much longer depending on the concentration of ammonia in the sample (at least 30 min for the higher concentrations). Both modes of detector behaved similarly with regard to response speed and desorption rate. For 6 and 12 mg dme3 ammonia, the desorption times were of the order of 10 min for both modes. These correspond to response times of <30 s and desorption times of ca. 4 min noted by Hlavay and Guilbault [4] for <1 mg dmm3ammonia. However, fresh samples may be injected before return to the baseline frequency for it is the immediate decrease in frequency caused by the injected sample that is analytically significant (Fig. 2). Of course, injections of aliquots of samples greater than the 1 cm3 used here would lead to larger frequency changes as observed in other studies of pyroxidine hydrochloride coatings [4] . Slowing the air carrier stream would be expected to have a similar effect, but at the expense of longer times required to sweep the previous sample away. This study was expressly devoted to the effect of the polymer (Antarox CO-880) matrix to support the sorbent. Figure 2(b) shows clearly that the use of Antarox CO-880 as a supporting matrix helps to maintain the effective sensitivity of the pyridoxine hydrochloride coating on the crystal for a prolonged period; there was only slight loss of sensitivity (0.14 Hz dm3 mg-‘) between the attainment of equilibrium by the crystal (day 3) and the end of
Fig. 1. Example of calibration responses to ammonia gas of quartz crystal coated only with pyroxidine hydrochloride (day 6). Arrows indicate the injection points. Air carrier at 30 cm3 min-‘. Numbers on outputs are mg drnm3ammonia.
[NH$mg
dram3
Fig. 2. Calibration with ammonia gas of quartz crystal coated with pyroxidine hydrochloride: (a) alone; (b) in a matrix of Antarox CO-880. (X) Control on crystal coated with Antarox CO-880 on day 5. The correlation coefficient (Fig. 2b) for 6,12, 18 and 24 mg dm” NH, for days 3-12 together (from runs on day 3, 5, 6, 7, 8, 10 and 12) is 0.9912 (AHZ = 6.41 [NH,] + 20.0); the correlation coefficient for the same NH, concentration for day 53 is 0.9856 (AHZ = 6.27 [NH,] + 20.4).
the study period (day 53). The pyridoxine hydrochloride-coated crystal without the polymer support steadily and quickly lost its sensitivity, until at day 10 there was a mere 30 Hz decrease in frequency for the 30 mg dmm3 sample (Fig. 2a) compared with 198 Hz for the crystal with pyroxidine hydrochloride supported by Antarox CO-880. The response of the crystal with the Antarox support tended to approach a limiting value above 24 mg dmT3 ammonia. The blank of ca. 20 Hz for the crystal with the Antarox matrix containing pyridoxine hydrochloride is attributed to background moisture, for such a reading was obtained by injecting l-cm3 control samples of uncontaminated air equilibrated over water. Dry air gave blank readings of <3 Hz. A control experiment with Antarox CO-880 coated on the crystal without any pyridoxine hydrochloride confirmed that the polymer matrix was not significantly involved in ammonia sorption but did sorb water (Fig. 2b).
229
Conclusions A polyethoxylate polymer of high relative molecular mass, such as the Antarox CO-880 used here, is a suitable matrix for maintaining the useful life of piezoelectric crystal detectors coated with selective sorbents. Its humectant properties cause responses to sample moisture, but methods are available for overcoming this, such as the use of a gas chromatographic precolumn packed with an appropriate drying agent placed between the sample injection port and the crystal. The National University of Malaysia, Kuala Lumpur, Malaysia is thanked for financial support to M.A.Y. REFERENCES 1 2 3 4 5 6 7
G. G. Guilbault, Ion-Selective Electrode Rev., 2 (1980) 3. K. H. Karmarkar and G. G. Guilbault, Anal. Chim. Acta, 75 (1975) 111. L. M. Webber and G. G. Guilbault, Anal. Chem., 48 (1976) 2244. J. Hlavay and G. G. Guilbault, Anal. Chem., 50 (1978) 1044. A. J. Cannard, G. J. Moody, J. D. R. Thomas and M. A. Yarmo,unpublished K. H. Karmarkar and G. G. Guilbault, Anal. Chim. Acta, 71 (1974) 419. F. W. Karasek and J. W. Tienay, J. Chromatogr., 89 (1974) 31.
work, 1983.
Short Communication
A POLYMER SUPPORT FOR PYRIDOXINE HYDROCHLORIDE USED AS A SORBENT FOR THE PIEZOELECTRIC QUARTZ CRYSTAL DETECTION OF AMMONIA
G. J. MOODY, J. D. R. THOMAS* and M. A. YARMOa Applied Chemistry (Great Britain)
Department,
Redwood
Building,
UWIST, Cardiff CFl 3XA
(Received 1st June 1983)
Summary. A high molecular mass polyalkoxylate, Antarox CO-880 (nonylphenoxypolyethoxylate with 30 ethoxylate units) is a suitable matrix for supporting pyridoxine hydrochloride (an ammonia sorbent) on a quartz piexoelectric crystal detector. The matrix helps considerably to extend the useful lifetime of the detector from ca. 10 to >53 days.
The scope of the piezoelectric crystal detector in analytical chemistry is greatly widened by using absorbent coatings of appropriate selectivity and sensitivity on the quartz crystal [l] . The analyte is selectively sorbed by the coating, thereby increasing the mass on the crystal and decreasing the frequency of vibration. The frequency change is linearly related to the mass sorbed according to AF = -AmF/Adt, which by substituting appropriate parameters for commercially available crystals simplifies to AF = -2.3 X lo6 F’Am/A, where AF is the change in frequency (Hz), F is the initial frequency of the quartz plate (MHz), Am is the mass sorbed (g), A is the area of the coating (cm2), d is the density of the quartz (g cmS3) and t is the thickness of the crystal (cm). Thus, for a particular experimental set-up, the change in frequency can be expressed as AF = KC, where C is the analyte concentration (mg dmM3)and K is a constant which includes the basic frequency of the quartz crystal, the area coated and a factor to convert the mass of analyte sorbed to its gas-phase concentration. For the detection of ammonia in air, Ucon 75-H-90,000 and Ucon LB3000X were first used and found to have good sensitivity [2]. These were followed by coatings of extracts of Cupsicum annum pods and ascorbic acid, with and without silver nitrate [ 31, and later by L-glutamic acid hydrochloride and pyridoxine hydrochloride (vitamin B6 hydrochloride) [ 41, which showed exceptional sensitivity. aPresent address: Chemistry Department, National University of Malaysia, Bangi, Selangor, Malaysia. 0003-2670/83/$03.00
o 1983 Elsevier Science Publishers B.V.
226
During evaluation of a laboratory-constructed piezoelectric apparatus, [ 51, it was found that simply coating the quartz crystal with pyridoxine hydrochloride from an ethanol/water solution led to a coating with an effective lifetime of only ca. 10 days. However, application of the pyridoxine hydrochloride in a matrix of nonylphenoxypolyethoxylate (Antarox CO880) gave greatly improved stability over a much longer period. Experimental Apparatus and detector design. The measuring unit consisted of a frequency oscillator with a buffered output powered by a Weir 400 power supply set at 9 V d.c. The frequency output from the oscillator was measured by a Marconi type 2431A 200-MHz digital frequency meter. A digital-toanalog converter selected the last two digits of the frequency meter output for conversion to an analog signal to a Bryans Model 28000 chart recorder, reading to f 1 Hz. The quartz crystal (AT-cut) with gold electrode (Quartz Crystal Co., Wellington Crescent, New Malden, Surrey) had a resonant frequency of 9 MHz. The detector cell incorporating this crystal was based on the design of Karmarkar and Guilbault [6] wherein the gas sample was split into two streams impinging directly on opposite faces of the coated crystal. The glass-encased cell detector was immersed in a water bath at 25 f O.l”C. Crystal coatings. A saturated solution of 1% (w/v) pyridoxine hydrochloride in ethanol/water (1 + 1) was dip-coated on the quartz crystal for the first series of experiments. For the second series, the solution for dipcoating consisted of a (1 + 1) mixture of a 0.2% (w/v) solution of pyridoxine hydrochloride in ethanol and a 0.2% (w/v) solution of Antarox CO-880 in acetone. Each coating was readily removed by soaking the crystal in ethanol. The crystal was dried before re-coating. Ammonia test samples. The ammonia gas test samples were obtained from ammonia vapour over ammonia liquor equilibrated at 25°C. Serial dilution of the headspace gas was effected by syringe dilution [7] with ambient air. Successive dilutions were delayed by 30-60 s in order to allow ammonia to diffuse throughout the air in the syringe. The concentration of ammonia was checked by titration. For example, 3, 5 and lo-cm3 samples removed from the headspace over the ammonia liquor were taken by syringe and slowly injected into 20 cm3 of 0.025 M sulphuric acid. The excess of sulphuric acid was titrated with 0.1 M sodium hydroxide using methyl orange as indicator. Ten replicate samples of 5 and 10 cm3 of the headspace gas contained 30.8 + 0.2 and 30.4 + 0.2 mg drn-j (mean f std. dev.), respectively, of ammonia. Diluted headspace gas samples were similarly checked; for example, 4 cm3 of headspace gas diluted to 10 cm3 was found to contain 12.1 f 0.1 mg drn-j ammonia. The responses of the coated crystal were tested on triplicate l-cm3 samples of appropriate dilutions of the headspace ammonia test samples and the mean decrease in frequency was measured. The diluted samples were injected into a carrier stream of dry (anhydrous calcium chloride) air, and passed
227
through the crystal compartment at 30 cm3 min-’ by the pump of a Pitman Instruments model 7069 air sampler. Results and discussion The sample injection method essentially results in a gas-phase flow injection mode of analysis. Typical responses are shown in Fig. 1, from which the fast response of the detector is evident. There is a fast initial return of the frequency towards the baseline for 20 s after the sample has passed through the detector, although full return to the original frequency takes much longer depending on the concentration of ammonia in the sample (at least 30 min for the higher concentrations). Both modes of detector behaved similarly with regard to response speed and desorption rate. For 6 and 12 mg dme3 ammonia, the desorption times were of the order of 10 min for both modes. These correspond to response times of <30 s and desorption times of ca. 4 min noted by Hlavay and Guilbault [4] for <1 mg dmm3ammonia. However, fresh samples may be injected before return to the baseline frequency for it is the immediate decrease in frequency caused by the injected sample that is analytically significant (Fig. 2). Of course, injections of aliquots of samples greater than the 1 cm3 used here would lead to larger frequency changes as observed in other studies of pyroxidine hydrochloride coatings [4] . Slowing the air carrier stream would be expected to have a similar effect, but at the expense of longer times required to sweep the previous sample away. This study was expressly devoted to the effect of the polymer (Antarox CO-880) matrix to support the sorbent. Figure 2(b) shows clearly that the use of Antarox CO-880 as a supporting matrix helps to maintain the effective sensitivity of the pyridoxine hydrochloride coating on the crystal for a prolonged period; there was only slight loss of sensitivity (0.14 Hz dm3 mg-‘) between the attainment of equilibrium by the crystal (day 3) and the end of
Fig. 1. Example of calibration responses to ammonia gas of quartz crystal coated only with pyroxidine hydrochloride (day 6). Arrows indicate the injection points. Air carrier at 30 cm3 min-‘. Numbers on outputs are mg drnm3ammonia.
[NH$mg
dram3
Fig. 2. Calibration with ammonia gas of quartz crystal coated with pyroxidine hydrochloride: (a) alone; (b) in a matrix of Antarox CO-880. (X) Control on crystal coated with Antarox CO-880 on day 5. The correlation coefficient (Fig. 2b) for 6,12, 18 and 24 mg dm” NH, for days 3-12 together (from runs on day 3, 5, 6, 7, 8, 10 and 12) is 0.9912 (AHZ = 6.41 [NH,] + 20.0); the correlation coefficient for the same NH, concentration for day 53 is 0.9856 (AHZ = 6.27 [NH,] + 20.4).
the study period (day 53). The pyridoxine hydrochloride-coated crystal without the polymer support steadily and quickly lost its sensitivity, until at day 10 there was a mere 30 Hz decrease in frequency for the 30 mg dmm3 sample (Fig. 2a) compared with 198 Hz for the crystal with pyroxidine hydrochloride supported by Antarox CO-880. The response of the crystal with the Antarox support tended to approach a limiting value above 24 mg dmT3 ammonia. The blank of ca. 20 Hz for the crystal with the Antarox matrix containing pyridoxine hydrochloride is attributed to background moisture, for such a reading was obtained by injecting l-cm3 control samples of uncontaminated air equilibrated over water. Dry air gave blank readings of <3 Hz. A control experiment with Antarox CO-880 coated on the crystal without any pyridoxine hydrochloride confirmed that the polymer matrix was not significantly involved in ammonia sorption but did sorb water (Fig. 2b).
229
Conclusions A polyethoxylate polymer of high relative molecular mass, such as the Antarox CO-880 used here, is a suitable matrix for maintaining the useful life of piezoelectric crystal detectors coated with selective sorbents. Its humectant properties cause responses to sample moisture, but methods are available for overcoming this, such as the use of a gas chromatographic precolumn packed with an appropriate drying agent placed between the sample injection port and the crystal. The National University of Malaysia, Kuala Lumpur, Malaysia is thanked for financial support to M.A.Y. REFERENCES 1 2 3 4 5 6 7
G. G. Guilbault, Ion-Selective Electrode Rev., 2 (1980) 3. K. H. Karmarkar and G. G. Guilbault, Anal. Chim. Acta, 75 (1975) 111. L. M. Webber and G. G. Guilbault, Anal. Chem., 48 (1976) 2244. J. Hlavay and G. G. Guilbault, Anal. Chem., 50 (1978) 1044. A. J. Cannard, G. J. Moody, J. D. R. Thomas and M. A. Yarmo,unpublished K. H. Karmarkar and G. G. Guilbault, Anal. Chim. Acta, 71 (1974) 419. F. W. Karasek and J. W. Tienay, J. Chromatogr., 89 (1974) 31.
work, 1983.