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The utilisation of a piezoelectric quartz crystal for measuring carbon dioxide in wine
ANALYTICA CHIMICA ACTA
ELSEVF’
Analytica
Chimica Acta 327 (1996) 95-100
The utilisation of a piezoelectric quartz crystal for measuring carbon dioxide in wine M. Teresa Games*,
Armando C. Duarte, JoZo P. Oliveira
Department of Chemistry, University of Aveiro, 3810 Aveiro, Portugal Received
12 October
1995; revised 10 January
1996; accepted
30 January
1996
Abstract A piezoelectric quartz crystal (PQC) is used for the quantification of carbon dioxide in still white wine. The methodology is discussed in terms of calibration stability and accuracy as a direct consequence of coating amount and absolute quantity of carbon dioxide in the samples. The method is compared, in terms of precision, with the titrimetric method. Standard deviations for several wine samples are, in the titrimetric method, between 4% and 12%, while using the PQC from 1% to 4%, for wines containing 300-1000 lg CO1 cmp3. Keywords: Piezoelectric
quartz crystal; Piezoelectric
crystals;
Carbon dioxide; Wine
1. Introduction Carbon dioxide is responsible for the freshness of taste in wines, and it enhances its fragrance. The importance of carbon dioxide, from the organoleptic point of view, is consubstantiated by the fact that wine-tasters are more sensitive to it than to acidity
L21. Carbon dioxide arises from fermentation. Although its content in wine decays with time, even still wine have hundreds of &cm3 carbon dioxide. Several methods have been used for the quantification of carbon dioxide in wine, although none of them is completely adequate from the analytical point of view. Three methods are currently recognised by the Association of Official Analytical Chemists [3]. The
* Corresponding
author.
0003-2670/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SOOO3-2670(96)00082-7
manometric method, which had retained its first action classification since 1960, is time consuming and complex [4]. The enzymatic method is based on the addition of hydroxide to the wine, followed by titration with acid to pH 8.45; the interference by other acids is corrected by a titration of a degassed wine sample with hydroxide to pH 7.75; poor reproducibility [4] and hysteresis phenomena [5] have impaired the application of the method on a routine basis. The titrimetric method, the most recent procedure, is based on the acid titration of an alkalinised wine sample between pH 8.6 and 4.0; the procedure is repeated for a degassed wine sample and the carbon dioxide concentration is calculated from the difference of &rant volumes. The titrimetric method was classified as first action in 1988 and has considerable advantages over the other methods. However, some problems and
96
M. Teresa Games et al./Analyrica
difficulties can be listed, namely, the difficulty in titrating to pH 8.6, where a single drop of acid causes a pH variation of more than 0.1 units, the low sensitivity at pH 4.0 arising from the buffer effect and the fact that 0.1 cm3 of an acid 0.025 mol/dm3 corresponds to 22 ug/cm3 C02. Besides, the method does not provide a way of checking the completeness of the decarbonation process. An overview of the wine analysis literature shows a lack of instrumentation beyond a pH meter for routine analysis. One can understand that price and complexity can be a drawback for the utilisation of the well-known Sveringhaus electrode, even after Lonvaud-Funel [5] found that use of the specific electrode was more reliable than the methods discussed above. As PQC provides an inexpensive technique, and its high sensitivity is well recognised, it can become an alternative method for the determination of carbon dioxide in wine. Both bulk acoustic and surface acoustic waves have been used for carbon dioxide detection and several amines, an imine and even a metal complex were tried as crystal coatings to interact with carbon dioxide [6-lo]. However, they have never been applied to real samples.
Fig. 1. Experimental layout: S, soda lime; B, automatic cell; C, coil; I, injection port; R, flowmeter.
Chimica Acta 327 (1996) 95-100
2. Experimental 2.1. Apparatus The experimental set-up (Fig. 1) has beer described previously [ 1 l] and it consists mainly c a piezoelectric crystal housed in a cell that splits th gas flow in two and leads each of the streams to th centre of each of the crystal faces. Before reachin the cell the nitrogen flow, controlled with a variabl area flowmeter (Cole Parmer), enters the bottom c another glass cell where a medium size sintered glas plate sustains the wine sample and allows the gas t pass through it carrying carbon dioxide. The gas i then dried on a silica-gel column. The top of the wine cell has a silicone septur (G18) to introduce the wine samples via a tub connected to an automatic burette. For the injectio of pure carbon dioxide an Omnifit (ref. 3301) septur injector was used. 2.2, Reagents N,N,N’,N’-Tetrakis 2-hydroxyethyl mine (THEED) (Fluka 87600) was
burette; P, power supply; 0, oscillator;
X, crystal cell; F, frequency
ethylenedia dissolved i
meter; W, wine
M. Teresa Games et al./Analytica
ethanol (Merck 11727). Nitrogen was R grade and carbon dioxide was N45, both from “ArLiquido”. The wines were all still white wines produced in Portugal.
Chimica Acta 327 (1996) 95-100 8840380
-
8840380
-
8840300
-
97
2.3. Procedure The piezoelectric crystal was coated on both faces with THEED, with a spray previously described [ 121. THEED has already been shown to perform as a satisfactory coating for the detection of carbon dioxide [7, lo]. Standardisation consisted of injections of different quantities of pure carbon dioxide in a 30cm3/min nitrogen gas that flows through 10cm3 of decarbonated wine. The injection of the gas gives a positive increase in the crystal frequency, due to the sudden increase in pressure, but a long coil separates in time, this effect from the decrease in frequency resulting from the interaction of carbon dioxide with the crystal coating as shown in Fig. 2. After the introduction of the wine sample into the cell, the carbon dioxide causes a marked decrease in the crystal frequency, although the signal is not so sharp as the one obtained with pure carbon dioxide,
E -loo FM 3
-150 -
-5 8 -200 % i: -250 -300 i -350 -I -400
)I 0
50
100
150
200
250
300
350
400
8840260 0
I 100
I 200
I 300
I 400
I 500
time (s) Fig. 3. Series of positive frequency displacements after the introduction of 10cm3 of deaerated water (crystal coated with =27 pg THEED).
as shown in Fig. 2. Unfortunately, the wine introduction also causes a positive displacement in the frequency, which could not be time separated from the carbon dioxide signal, even with the insertion of a 528cm long empty coil between the drying column and the crystal cell. To correct this problem, the frequency changes obtained with the introduction of decarbonated water were subtracted from the wine signals (Fig. 3). All the experiments were performed under a constant nitrogen flow of 30cm3/min and at room temperature (ca. 20°C). Concerning the titrimetric method, the procedure of Goranov [ 131 was closely followed: a cooled volume of wine was carefully added to a beaker containing a known quantity of sodium hydroxide, always with the pipette tip submerged to avoid losses of the gas. The sample to be degassed was boiled and sodium hydroxide added afterwards; both were titrated with standardised sulphuric acid between pH 8.6 and 4.0.
450
time (s)
Fig. 2. Frequency changes of the same 9MHz Ag-plated crystal, coated with ~29 pg THEED, by (a) the introduction of 10cm3 of wine with H~02; (b) the injection of 2 cm3 of CO2 in the nitrogen that flows through 10cm3 of degas& wine, with H,O,.
3. Results and discussion Sulphur dioxide interacts with THEED to a greater extent than carbon dioxide, as shown in Fig. 4. This
98
M. Teresa Gomes et al./Analytica
Chimica Acta 327 (1996) 9.5-100
8637000
8836000
6835000
8837000
;t’
@I 8
6836000
\
3
8834000
(a) 1
& k
cm3SO2
P B
(b) 1 cm3 CO2
\
8635500
i!
8833000
8835000
_
8832000
without
H202
8834500 a
8831000
I
0
I
8834000
I
I
8000
4000
12000
0
3000
2000
1000
time (s)
time (s)
Fig. 4. Frequency changes of the crystal, caused by an injection of gas into the nitrogen that flows through a 10cms of degassed wine.
Fig. 5. Frequency changes of the same crystal caused by 1 cm3 01 SO1 passing through 10cm3 of degas& wine, with or without HA.
interference was eliminated by the addition of one drop of Hz02 (30 vol) to all the wine samples, degassed or not. Fig. 5 shows the effective lack of frequency decrease, due to sulphur dioxide, in the presence of H202. The sensitivity to carbon dioxide is dependent on the quantity of THEED sprayed onto the crystal. To test the accuracy of the method, COz was generated by reacting known amounts of Na&Os with an excess of citric acid. The results obtained are shown in Table 1. From the analysis of these data it is clear that the method can have an accuracy suitable for wine analysis providing that the working range is located within the portion of the calibration graph
Table 1 Results obtained
where the slope is greater than 60Hz./cm3. As the slope depends upon the amount of coating [lo], and a high load can prevent the crystal vibration, the obvious solution is to decrease the amount of the wine sample for the wines with larger amounts of carbon dioxide. The two methods were applied to different batches of wines. Table 2 shows the results of still white wine analysis by the titrimetric method, while Table 3 shows the results obtained by the PQC with a crystal coating in the range 22-27kHz (2632pg of THEED). For the titrimetric method the relative
using the PQC when excess of citric acid is added to different quantities
CO1 (cm3)
AF (average)
1.921 Blank 2.452 Blank 2.990 Blank 3.537 Blank 4.079 Blank
657 22 512 18 556 17 445 19 569 17
(Hz)
deviation(cm3)
of carbonate
(average of 5 determinations)
Slope (H.z/cm3)
Standard
162
0.05
Result (cm3) 1.85
Relative error (%) 3.1
68.4
0.04
2.49
1.6
82.9
0.03
2.96
1.0
45.5
0.05
2.92
17.4
48.9
0.06
3.24
20.6
M. Teresa Gomes et al./Analytica Table 2 Analysis of still white wines by the titrimetxic method (average of 5 determinations) Sample
A Blank B Blank C Blank iI= Blank a E Blank F Blank G Blank H Blank I Blank J Blank K Blank L Blank M Blank N Blank
Average
CO*
(cm3)
(pg/cm3)
5.92 4.94 6.20 5.06 5.89 5.08 7.18 4.87 5.14 4.66 5.34 4.58 5.85 4.63 6.26 5.50 6.83 5.07 6.20 5.06 6.23 5.07 5.79 4.86 6.03 4.88 7.26 5.40
Standard deviation (Wcm3)
Relative standard deviation
419
39
9.4
477
34
1.2
344
29
8.4
984
41
4.2
451
21
6.1
324
27
8.5
519
27
5.2
327
1.5
4.5
749
34
4.5
477
34
1.2
453
21
4.6
365
42
12
450
24
5.4
725
26
3.7
Chimica Acta 327 (1996) 95-100 Table 3 Analysis of still white determinations) Sample
AF (average) (Hz)
99
wines
using
CO1 (pg/cm3)
(%)
the PQC Standard deviation Wcm3)
223 323 669 201 382 415 280 280 309
377 317 366 718 392 698 298 306 317
(average
of 5
Relative standard deviation
(%)
14 13 2 8 6 7 11 6 5
640 620
“Average of 6 determinations.
1
460 1
I
1
I
I
4
2
3
4
5
6
co, (cm31
standard deviations ranged from 4% to 12% for wines with carbon dioxide from 320 to 980pg/cm3, while using the PQC the relative standard deviations ranged from 1% to 4%, for wines with carbon dioxide contents from 300 to 720 pg/cm3. A linear correlation test between standard deviation and concentration shows that, for both methods the slope is not statistically different from zero. As the standard deviations are independent of the carbon dioxide concentration an F-test was performed on the mean standard deviation of both methods. From the test it is possible to state, with more than 95% probability, that the proposed PQC method has higher precision. The major drawback of the proposed method is the short duration of validity of the calibration graph which imposes requirement for a new calibration for
Fig. 6. Calibrations for the same THEED, on consecutive days.
crystal
coated
with
r26pg
each batch of analyses. Fig. 6 shows two calibrations for the same crystal in consecutive days, and the decrease in sensitivity over this time. The currently available methods fail for wines with low concentrations of CO2 [5]. The PQC method, although time consuming, due to loss of sensitivity of the coated crystal, can play a role in these cases, since the method works particularly well for small amounts of carbon dioxide. Besides, the instrumental set-up is either home made, or cheap and widely available.
M. Teresa Gomes et al./Analytica
100
References Ul AS.
Curvelo-Garcia, Controlo da qualidade dos vinhos, Instituto da vinha e do vinho, Lisboa, 1988 (in portuguese). PI M.A. Amerine, H.W. Berg and W.V. Cruess, The Technology of Wine Making, 2nd edn., The AVI Publishing Company, Westport, Connecticut, 1967. [31 AOAC, Official Methods of Analysis, 15th edn., Arlington, VA toon III,
1/I”.
[41 A. Caputi, J. Assoc. Off. Anal. Chem., 54 (1971) 782. r51 A. Lonvaud-Funel, Recherches sur le gas carbonique vins, (These Doctorat Bordeaux II, 1976.
en Chimie
(mention
des Oenologie)),
Chimicu Acta 327 (1996) 95-100 [6] W.W. Fogleman and M.S. Shuman, Anal. Lett., 9 (1976) 751. [7] 0. Fatibello-F&o, J.F. Andrade, A.A. Suleiman and G.G. Guilbault, Anal. Chem., 61 (1989) 746. [8] M.S. Nieuwenhuizen and J. Nederlof, Sensors and Actuators B, 2 (1990) 97. [9] P.C.H. Li and M. Thompson, Analyst, 119 (1994) 1947. [lo] M.T. Gomes, A.C. Duarte and J.P. Oliveira, Sensors and Actuators, 26 (1995) 191. [l l] M.T. Gomes, AC. Duarte and J.P. Oliveira, Zeitscbrift fur Lebensmittel-Untersuchung und -Forschung, in press. [12] M.T. Games, A.C. Duarte and J.P. Oliveira, Anal. Chim. Acta, 300 (1995) 329. [13] N. Goranov, Feuillets verts de 1OIV 758, 1983.
ELSEVF’
Analytica
Chimica Acta 327 (1996) 95-100
The utilisation of a piezoelectric quartz crystal for measuring carbon dioxide in wine M. Teresa Games*,
Armando C. Duarte, JoZo P. Oliveira
Department of Chemistry, University of Aveiro, 3810 Aveiro, Portugal Received
12 October
1995; revised 10 January
1996; accepted
30 January
1996
Abstract A piezoelectric quartz crystal (PQC) is used for the quantification of carbon dioxide in still white wine. The methodology is discussed in terms of calibration stability and accuracy as a direct consequence of coating amount and absolute quantity of carbon dioxide in the samples. The method is compared, in terms of precision, with the titrimetric method. Standard deviations for several wine samples are, in the titrimetric method, between 4% and 12%, while using the PQC from 1% to 4%, for wines containing 300-1000 lg CO1 cmp3. Keywords: Piezoelectric
quartz crystal; Piezoelectric
crystals;
Carbon dioxide; Wine
1. Introduction Carbon dioxide is responsible for the freshness of taste in wines, and it enhances its fragrance. The importance of carbon dioxide, from the organoleptic point of view, is consubstantiated by the fact that wine-tasters are more sensitive to it than to acidity
L21. Carbon dioxide arises from fermentation. Although its content in wine decays with time, even still wine have hundreds of &cm3 carbon dioxide. Several methods have been used for the quantification of carbon dioxide in wine, although none of them is completely adequate from the analytical point of view. Three methods are currently recognised by the Association of Official Analytical Chemists [3]. The
* Corresponding
author.
0003-2670/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved PII SOOO3-2670(96)00082-7
manometric method, which had retained its first action classification since 1960, is time consuming and complex [4]. The enzymatic method is based on the addition of hydroxide to the wine, followed by titration with acid to pH 8.45; the interference by other acids is corrected by a titration of a degassed wine sample with hydroxide to pH 7.75; poor reproducibility [4] and hysteresis phenomena [5] have impaired the application of the method on a routine basis. The titrimetric method, the most recent procedure, is based on the acid titration of an alkalinised wine sample between pH 8.6 and 4.0; the procedure is repeated for a degassed wine sample and the carbon dioxide concentration is calculated from the difference of &rant volumes. The titrimetric method was classified as first action in 1988 and has considerable advantages over the other methods. However, some problems and
96
M. Teresa Games et al./Analyrica
difficulties can be listed, namely, the difficulty in titrating to pH 8.6, where a single drop of acid causes a pH variation of more than 0.1 units, the low sensitivity at pH 4.0 arising from the buffer effect and the fact that 0.1 cm3 of an acid 0.025 mol/dm3 corresponds to 22 ug/cm3 C02. Besides, the method does not provide a way of checking the completeness of the decarbonation process. An overview of the wine analysis literature shows a lack of instrumentation beyond a pH meter for routine analysis. One can understand that price and complexity can be a drawback for the utilisation of the well-known Sveringhaus electrode, even after Lonvaud-Funel [5] found that use of the specific electrode was more reliable than the methods discussed above. As PQC provides an inexpensive technique, and its high sensitivity is well recognised, it can become an alternative method for the determination of carbon dioxide in wine. Both bulk acoustic and surface acoustic waves have been used for carbon dioxide detection and several amines, an imine and even a metal complex were tried as crystal coatings to interact with carbon dioxide [6-lo]. However, they have never been applied to real samples.
Fig. 1. Experimental layout: S, soda lime; B, automatic cell; C, coil; I, injection port; R, flowmeter.
Chimica Acta 327 (1996) 95-100
2. Experimental 2.1. Apparatus The experimental set-up (Fig. 1) has beer described previously [ 1 l] and it consists mainly c a piezoelectric crystal housed in a cell that splits th gas flow in two and leads each of the streams to th centre of each of the crystal faces. Before reachin the cell the nitrogen flow, controlled with a variabl area flowmeter (Cole Parmer), enters the bottom c another glass cell where a medium size sintered glas plate sustains the wine sample and allows the gas t pass through it carrying carbon dioxide. The gas i then dried on a silica-gel column. The top of the wine cell has a silicone septur (G18) to introduce the wine samples via a tub connected to an automatic burette. For the injectio of pure carbon dioxide an Omnifit (ref. 3301) septur injector was used. 2.2, Reagents N,N,N’,N’-Tetrakis 2-hydroxyethyl mine (THEED) (Fluka 87600) was
burette; P, power supply; 0, oscillator;
X, crystal cell; F, frequency
ethylenedia dissolved i
meter; W, wine
M. Teresa Games et al./Analytica
ethanol (Merck 11727). Nitrogen was R grade and carbon dioxide was N45, both from “ArLiquido”. The wines were all still white wines produced in Portugal.
Chimica Acta 327 (1996) 95-100 8840380
-
8840380
-
8840300
-
97
2.3. Procedure The piezoelectric crystal was coated on both faces with THEED, with a spray previously described [ 121. THEED has already been shown to perform as a satisfactory coating for the detection of carbon dioxide [7, lo]. Standardisation consisted of injections of different quantities of pure carbon dioxide in a 30cm3/min nitrogen gas that flows through 10cm3 of decarbonated wine. The injection of the gas gives a positive increase in the crystal frequency, due to the sudden increase in pressure, but a long coil separates in time, this effect from the decrease in frequency resulting from the interaction of carbon dioxide with the crystal coating as shown in Fig. 2. After the introduction of the wine sample into the cell, the carbon dioxide causes a marked decrease in the crystal frequency, although the signal is not so sharp as the one obtained with pure carbon dioxide,
E -loo FM 3
-150 -
-5 8 -200 % i: -250 -300 i -350 -I -400
)I 0
50
100
150
200
250
300
350
400
8840260 0
I 100
I 200
I 300
I 400
I 500
time (s) Fig. 3. Series of positive frequency displacements after the introduction of 10cm3 of deaerated water (crystal coated with =27 pg THEED).
as shown in Fig. 2. Unfortunately, the wine introduction also causes a positive displacement in the frequency, which could not be time separated from the carbon dioxide signal, even with the insertion of a 528cm long empty coil between the drying column and the crystal cell. To correct this problem, the frequency changes obtained with the introduction of decarbonated water were subtracted from the wine signals (Fig. 3). All the experiments were performed under a constant nitrogen flow of 30cm3/min and at room temperature (ca. 20°C). Concerning the titrimetric method, the procedure of Goranov [ 131 was closely followed: a cooled volume of wine was carefully added to a beaker containing a known quantity of sodium hydroxide, always with the pipette tip submerged to avoid losses of the gas. The sample to be degassed was boiled and sodium hydroxide added afterwards; both were titrated with standardised sulphuric acid between pH 8.6 and 4.0.
450
time (s)
Fig. 2. Frequency changes of the same 9MHz Ag-plated crystal, coated with ~29 pg THEED, by (a) the introduction of 10cm3 of wine with H~02; (b) the injection of 2 cm3 of CO2 in the nitrogen that flows through 10cm3 of degas& wine, with H,O,.
3. Results and discussion Sulphur dioxide interacts with THEED to a greater extent than carbon dioxide, as shown in Fig. 4. This
98
M. Teresa Gomes et al./Analytica
Chimica Acta 327 (1996) 9.5-100
8637000
8836000
6835000
8837000
;t’
@I 8
6836000
\
3
8834000
(a) 1
& k
cm3SO2
P B
(b) 1 cm3 CO2
\
8635500
i!
8833000
8835000
_
8832000
without
H202
8834500 a
8831000
I
0
I
8834000
I
I
8000
4000
12000
0
3000
2000
1000
time (s)
time (s)
Fig. 4. Frequency changes of the crystal, caused by an injection of gas into the nitrogen that flows through a 10cms of degassed wine.
Fig. 5. Frequency changes of the same crystal caused by 1 cm3 01 SO1 passing through 10cm3 of degas& wine, with or without HA.
interference was eliminated by the addition of one drop of Hz02 (30 vol) to all the wine samples, degassed or not. Fig. 5 shows the effective lack of frequency decrease, due to sulphur dioxide, in the presence of H202. The sensitivity to carbon dioxide is dependent on the quantity of THEED sprayed onto the crystal. To test the accuracy of the method, COz was generated by reacting known amounts of Na&Os with an excess of citric acid. The results obtained are shown in Table 1. From the analysis of these data it is clear that the method can have an accuracy suitable for wine analysis providing that the working range is located within the portion of the calibration graph
Table 1 Results obtained
where the slope is greater than 60Hz./cm3. As the slope depends upon the amount of coating [lo], and a high load can prevent the crystal vibration, the obvious solution is to decrease the amount of the wine sample for the wines with larger amounts of carbon dioxide. The two methods were applied to different batches of wines. Table 2 shows the results of still white wine analysis by the titrimetric method, while Table 3 shows the results obtained by the PQC with a crystal coating in the range 22-27kHz (2632pg of THEED). For the titrimetric method the relative
using the PQC when excess of citric acid is added to different quantities
CO1 (cm3)
AF (average)
1.921 Blank 2.452 Blank 2.990 Blank 3.537 Blank 4.079 Blank
657 22 512 18 556 17 445 19 569 17
(Hz)
deviation(cm3)
of carbonate
(average of 5 determinations)
Slope (H.z/cm3)
Standard
162
0.05
Result (cm3) 1.85
Relative error (%) 3.1
68.4
0.04
2.49
1.6
82.9
0.03
2.96
1.0
45.5
0.05
2.92
17.4
48.9
0.06
3.24
20.6
M. Teresa Gomes et al./Analytica Table 2 Analysis of still white wines by the titrimetxic method (average of 5 determinations) Sample
A Blank B Blank C Blank iI= Blank a E Blank F Blank G Blank H Blank I Blank J Blank K Blank L Blank M Blank N Blank
Average
CO*
(cm3)
(pg/cm3)
5.92 4.94 6.20 5.06 5.89 5.08 7.18 4.87 5.14 4.66 5.34 4.58 5.85 4.63 6.26 5.50 6.83 5.07 6.20 5.06 6.23 5.07 5.79 4.86 6.03 4.88 7.26 5.40
Standard deviation (Wcm3)
Relative standard deviation
419
39
9.4
477
34
1.2
344
29
8.4
984
41
4.2
451
21
6.1
324
27
8.5
519
27
5.2
327
1.5
4.5
749
34
4.5
477
34
1.2
453
21
4.6
365
42
12
450
24
5.4
725
26
3.7
Chimica Acta 327 (1996) 95-100 Table 3 Analysis of still white determinations) Sample
AF (average) (Hz)
99
wines
using
CO1 (pg/cm3)
(%)
the PQC Standard deviation Wcm3)
223 323 669 201 382 415 280 280 309
377 317 366 718 392 698 298 306 317
(average
of 5
Relative standard deviation
(%)
14 13 2 8 6 7 11 6 5
640 620
“Average of 6 determinations.
1
460 1
I
1
I
I
4
2
3
4
5
6
co, (cm31
standard deviations ranged from 4% to 12% for wines with carbon dioxide from 320 to 980pg/cm3, while using the PQC the relative standard deviations ranged from 1% to 4%, for wines with carbon dioxide contents from 300 to 720 pg/cm3. A linear correlation test between standard deviation and concentration shows that, for both methods the slope is not statistically different from zero. As the standard deviations are independent of the carbon dioxide concentration an F-test was performed on the mean standard deviation of both methods. From the test it is possible to state, with more than 95% probability, that the proposed PQC method has higher precision. The major drawback of the proposed method is the short duration of validity of the calibration graph which imposes requirement for a new calibration for
Fig. 6. Calibrations for the same THEED, on consecutive days.
crystal
coated
with
r26pg
each batch of analyses. Fig. 6 shows two calibrations for the same crystal in consecutive days, and the decrease in sensitivity over this time. The currently available methods fail for wines with low concentrations of CO2 [5]. The PQC method, although time consuming, due to loss of sensitivity of the coated crystal, can play a role in these cases, since the method works particularly well for small amounts of carbon dioxide. Besides, the instrumental set-up is either home made, or cheap and widely available.
M. Teresa Gomes et al./Analytica
100
References Ul AS.
Curvelo-Garcia, Controlo da qualidade dos vinhos, Instituto da vinha e do vinho, Lisboa, 1988 (in portuguese). PI M.A. Amerine, H.W. Berg and W.V. Cruess, The Technology of Wine Making, 2nd edn., The AVI Publishing Company, Westport, Connecticut, 1967. [31 AOAC, Official Methods of Analysis, 15th edn., Arlington, VA toon III,
1/I”.
[41 A. Caputi, J. Assoc. Off. Anal. Chem., 54 (1971) 782. r51 A. Lonvaud-Funel, Recherches sur le gas carbonique vins, (These Doctorat Bordeaux II, 1976.
en Chimie
(mention
des Oenologie)),
Chimicu Acta 327 (1996) 95-100 [6] W.W. Fogleman and M.S. Shuman, Anal. Lett., 9 (1976) 751. [7] 0. Fatibello-F&o, J.F. Andrade, A.A. Suleiman and G.G. Guilbault, Anal. Chem., 61 (1989) 746. [8] M.S. Nieuwenhuizen and J. Nederlof, Sensors and Actuators B, 2 (1990) 97. [9] P.C.H. Li and M. Thompson, Analyst, 119 (1994) 1947. [lo] M.T. Gomes, A.C. Duarte and J.P. Oliveira, Sensors and Actuators, 26 (1995) 191. [l l] M.T. Gomes, AC. Duarte and J.P. Oliveira, Zeitscbrift fur Lebensmittel-Untersuchung und -Forschung, in press. [12] M.T. Games, A.C. Duarte and J.P. Oliveira, Anal. Chim. Acta, 300 (1995) 329. [13] N. Goranov, Feuillets verts de 1OIV 758, 1983.