Ind. Eng. Chem. Res. 2005, 44, 10013-10015
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RESEARCH NOTES An Improved and Practical Headspace Gas Chromatographic Method for Determination of Carboxyl Acids Content in Wood Fibers X.-S. Chai,*,†,‡ R. W. Maurer,§ J. S. Hsieh,§ Q. X. Hou,‡ D. C. Zhang,§ and S. F. Wang†,‡ School of Light Industrial and Food Engineering, Guangxi University, Nanning, Guangxi 530004, People’s Republic of China, Institute of Paper Science and Technology, Georgia Institute of Technology, 500 10th Street, N.W., Atlanta, Georgia 30332, and School of Chemical and Biomolecular Engineering, Georgia Institute of Technology Atlanta, Georgia 30332
This paper reports an improved headspace gas chromatography (HS-GC) technique for the determination of carboxyl group content in wood fibers. The method is based on phase reaction conversion HS-GC technique developed previously [Chai et al., Ind. Eng. Chem. Res. 2003, 42, 5440-5444], but with major modifications in regard to sample preparation (i.e., sample acidification followed by water washing), reaction reagent concentration (i.e., minimization of bicarbonate decomposition), and the manner for sample addition (i.e., a direct sample addition). As a result, this technique greatly improves the measurement precision. The present method is simpler, more reliable, and more practical. It can be applied to the nonfiber samples. Introduction We have previously reported on a method for measuring the carboxyl group content in wood pulps using headspace gas chromatography (HS-GC).1 After reviewing this earlier method, we have found some points where significant error, both experimental and operational, may be added to the resulting data. Therefore, we have developed a revised method to eliminate much of this error. The major effort is to improve the method measurement precision and reliability. The other improvements such as the sample addition mode and measurement time are also described. Experimental Section Apparatus. All measurements were performed using a Hewlett-Packard model HP-7694 automatic headspace sampler and a Hewlett-Packard model HP-6890 capillary gas chromatograph that was equipped with a thermal conductivity detector. The detailed information on the headspace sampler and GC operation conditions was provided in our previous paper.1 Chemicals. All chemicals used in the experiment were from commercial sources. A standard bicarbonate solution that consisted of 0.0025 mol/L sodium bicarbonate and 0.1 mol/L sodium chloride, and a 0.100 mol/L hydrogen chloride acid (HCl) solution, were each prepared from source chemicals. Revised Procedures. Approximately 0.2 g of (ovendried) pulp sample was added to a beaker that contained * To whom correspondence should be addressed. Tel: 404 894 9992. Fax: 404 894 4778. E-mail:
[email protected]. † Guangxi University. ‡ Institute of Paper Science and Technology, Georgia Institute of Technology. § School of Chemical and Biomolecular Engineering, Georgia Institute of Technology.
200 mL of a 0.1 mol/L HCl solution and was mixed for 1 h at room temperature using magnetic stirring. The pulp sample then was washed thoroughly with deionized water (resistance of Ω > 17.4 ohm) with a filter to remove the residual HCl from pulps. The completeness of the washing can be checked by measuring the pH in the filtrates. The washed pulp was placed in a conditional environment (constant room temperature and humidity) for air-drying. Depending on the amount of carboxyl acids in the pulp, ∼0.03-0.08 g of sample is accurately weighed by an analytical balance and placed in a headspace sample vial. After adding 4 mL of bicarbonate solution, the vial is immediately sealed by a septum. By shaking the vial, a good uniformity of fiber dispersion in the solution is obtained. The closed vial is then placed on the tray of headspace sampler for automatic HS-GC testing. Calibration. Calibration was achieved through the reaction of a standard hydrogen chloride acid (HCl) with bicarbonate. Different volumes of a 0.100 mol/L of standard HCl solution, ranging from 1 µL to 30 µL, were injected using a microsyringe into a set of 4-mL 0.0025 mol/L sodium bicarbonate solutions in the closed headspace sample vials. The calibration factor (k), i.e., the slope of the calibration curve, was obtained and can be written as
k)
(Aexp - Ab) nHCl
(1)
The carboxyl group content in pulp sample can be calculated by
C (µmol/g) )
(Aexp - Ab) kw
10.1021/ie050921o CCC: $30.25 © 2005 American Chemical Society Published on Web 11/12/2005
(2)
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Ind. Eng. Chem. Res., Vol. 44, No. 26, 2005
Table 1. Symbols and Definitions symbol Aexp Ab nHCl k C w Acarboxyl AHCl AAir CHCl VHCl F wHCl wwp
definition GC signal peak area in the actual measurement GC signal peak area obtained from the blank solution mass of HCl (µmol) calibration factor content of carboxyl acids in the fibers (µmol/g) weight of dried pulp (g) GC signal peak area from carboxyl acids reaction with bicarbonate GC signal peak area from excess HCl in the sample GC singal peak area from CO2 in air concentration of standard HCl solution (mol/L) volume of standard HCl solution used (L) density of HCl solution (g/L) weight of HCl dissolved in solution volume (g) weight of wetted pulp (g)
All symbols are defined in Table 1. These symbols are the same as those defined in the previous work1) Results and Discussion Measurement Uncertainty in the Previous Method. In the previous method,1 the carbon dioxide (CO2) generated from the reaction between carboxyl acids and bicarbonate is mainly caused by the H ions from carboxyl acids in the sample, the residual HCl, and the CO2 in the air in headspace of the headspace testing vial. The peak area measured by HS-GC on CO2 can be written as
Aexp ) Acarboxyl + AHCl + AAir
(3a)
Acarboxyl ) Aexp - AHCl - AAir
(3b)
or
The contribution by the CO2 in air is removed by blank testing. The contribution by the residual HCl solution is subtracted according to the following:
AHCl ) kCHClVHCl ) kCHCl
(
wHCl 1000F
)
(4)
where wHCl ) w - wwp. A relatively large contribution by the residual HCl solution will cause significant error in the measured CO2 contribution by carboxyl acids, especially for samples that contain a relatively low number of carboxyl acids. In addition to a wetted pulp sample (after acid pretreat-
ment and squeezing), a dried pulp sample should also be weighed, to determine the water content of the pulp. Because of the small sample size in the testing, a larger degree of uncertainty in sample weighing was easily introduced. Recently, we have discovered that the decomposition of a bicarbonate solution occurs at an elevated temperature,2 in which the reaction in a closed system, which can be written as
2HCO3- T H2O + CO32- + CO2
(5)
Therefore, one more item should be added in eq 3. It will cause a higher intercept in the blank testing, as shown in Figure 1a, which also affects the accuracy, especially for the low carboxyl acids content sample, although the effect can be minimized by a blank correction. However, it was determined that the decomposition effect can be reduced if a low concentration bicarbonate solution is used, as shown in Figure 1b. In Figure 1a, note that if the reaction solution contains a high bicarbonate content (0.030 mol/L), the measurement error is more significant when the amount of acid groups in the sample is very low. Because there is an excess amount of HCl in the wetted sample, it limits the use of a bicarbonate solution with a lower concentration. These procedures also make the method less reliable. Therefore, the method can be improved if the excess amount of HCl can be completely removed from the tested sample. Furthermore, the sample with an excess amount of HCl cannot simply be added into the headspace testing vial, which contains a bicarbonate solution; a rapid reaction does occur, so some CO2 will escape from the vial. In the previous work,1 a special method for sample addition is proposed, i.e., using a needle holding the sample to avoid contact with the bicarbonate solution before vial is sealed. Such a sample addition procedure is only suited to a fiber sample that can be held by the needle. Testing Pulp Sample Preparation. In the present work, we introduced the water washing after sample acidification pretreatment to remove the residual HCl completely from the sample. Deionized water (resistance of Ω > 17 mohm) was used in the sample washing until the filtrate pH is the same as that in water. The sample is then dried for further HS-GC testing. This leaves a negligible amount of strong acid remaining in the sample; consequently, the reaction rate of the sample with the bicarbonate solution has been slowed significantly, because the weakly dissociated carboxyl acids
Figure 1. Calibration curves used to determine the (a) high-content and (b) low-content species.
Ind. Eng. Chem. Res., Vol. 44, No. 26, 2005 10015 Table 2. Repeatability Testing by Two Testing Methods Pulp A (0.0300 g) replica number 1 2 3 4 5 average RSDa (%) a
Pulp B (0.0750 g)
present
previous
present
previous
51.5 48.6 52.1 50.4 49.7 50.5 2.77
49.9 49.4 53.9 51.4 53.2 51.6 3.83
126.0 131.6 129.8 126.5 127.1 128.2 1.87
120.5 127.8 126.9 130.3 131.4 127.4 3.34
Relative standard deviation.
provide the majority of the H ions in the heterogeneous reaction system at room temperature. The error introduced by the addition of the sample to the bicarbonatecontaining headspace sample vial is minimal, because the slow reaction will generate little to no CO2. Method Comparison. A comparison study was conducted, based on two pulp samples. The repeatability tests were conducted using five 0.030-g lots of pulp sample A and five 0.075-g lots of pulp sample B. Both sample pretreatment approaches (i.e., that described in the previous1 and present work) were applied. Table 2 lists the measured GC detector signal peak areas of the measurements for the testing. As shown in the table, consistent results from the sample with a water washing were obtained, which matched those from the previous method. However, the relative standard deviations in the two pulp sample sets A and B with the present sample pretreatment approach are 2.77% and 1.87%, respectively, which are smaller than those (3.83% for sample A and 3.34% for sample B) from previous method. It clearly shows that the present method improves the measurement precision. For the sample with a low carboxyl acid content, a larger sample size is recommended, to improve the measurement precision. Table 2 also shows that the larger GC signals in the pulp B testing are due to a larger sample size (0.075 g), which reduces the relative standard deviation (RSD) in the measurement. Other Considerations. 1. Physical Geometric Constraints. One of the most important aspects of the improved testing method is elimination of physical constraints on the sample itself. In the previous method, the sample was placed on a needle and separated from the bicarbonate solution until the vial was sealed to prevent introduction of error via the rapid reaction of HCl with the bicarbonate. However, the method was essentially limited to testing only samples that could be held by the needle; particulate or powdered samples were incapable of being accurately tested using the previous technique. However, this improved method allows the testing of any type of solid sample without the introduction of appreciable error into the measurement. Powders or particles can now be added directly to the bicarbonate without the accuracy of the testing being determined by the speed of the user. However, if there are strong acids (such as sulfonate acids) in the solid sample, special precautions should be taken when adding bicarbonate solution, because CO2 generation for this reaction is much faster than that for the weak (carboxyl) acids in fibers. Consequently, the following method is recommended for measurement of samples that contain strong acid groups. The solid sample is first placed in the sample vial. Next, an open tube containing the bicarbonate
Figure 2. Schematic diagram showing the method of placing the sample and bicarbonate solution in the testing vial for HS-GC measurement.
solution is placed inside the sample vial. Make sure the outside of the vial is free of any residual bicarbonate. Note that the bicarbonate solution and solid sample are not yet in contact. After the sample vial is sealed tightly, it is shaken thoroughly, to mix the solid sample and bicarbonate solution. Rapid CO2 generation should occur; however, this method prevents any lost CO2 and, thus, any introduction of error into the measurement.2. Headspace Measurement Time. Although ∼10 min is required for the pulp reaction, the experiment efficiency can be improved by use of the overlapping constant mode of thermostating.3 The equilibration time for a sample is usually much longer than the time needed for analysis. However, the overlapping constant mode uses this fact to maximize throughput efficiency with the current headspace sampler system. All of the samples are loaded simultaneously. The first begins its thermostating time (TT) to bring the sample to equilibrium; once equilibration is reached, the chromatographic cycle beings. To minimize throughput time, the second sample enters the TT phase with a time lag equal to the chromatographic cycle time. Hence, the second sample enters the analysis phase immediately after the first sample finishes. Thus, the experiment time for multiple samples can be drastically reduced. Conclusions This paper has introduced a modified approach to the headspace gas chromatographic technique for the determination of carboxyl acids on the wood fibers. The repeatability of the method is greatly improved by this approach; furthermore, this method can be applied to areas where the sample is not necessarily a fiber. The present method is simple, reliable, and practical. Acknowledgment This work was partly supported by the Guangxi University Key Program for Science and Technology Research. Literature Cited (1) Chai, X.-S.; Hou, Q. X.; Zhu, J. Y.; Chen, S.-L.; Wang, S. F.; Lucia, L. Carboxyl Groups in Wood Fibers. 1. Determination of Carboxyl Groups by Headspace Gas Chromatography. Ind. Eng. Chem. Res. 2003, 42, 5440. (2) Chai, X. S.; Maurer, R. W.; Hsieh, J. S.; Zhang, D.; Wang, S. F. Determination of Acidic and Basic Species by Headspace Gas Chromatography. J. Chromatogr. A 2005, 1093, 212. (3) Kolb, B.; Ettre, L. S. Static Headspace-Gas ChromatographysTheory and Practice; Wiley-VCH: New York, 1997.
Received for review August 9, 2005 Revised manuscript received October 27, 2005 Accepted November 3, 2005 IE050921O
