- Email: [email protected]
Rapid determination of moisture content in paper materials by multiple headspace extraction gas chromatography
Accepted Manuscript Title: Rapid Determination of Moisture Content in Paper Materials by Multiple Headspace Extraction Gas Chromatography Author: Wei-Qi Xie Xin-Sheng Chai PII: DOI: Reference:
S0021-9673(16)30341-7 http://dx.doi.org/doi:10.1016/j.chroma.2016.03.059 CHROMA 357416
To appear in:
Journal of Chromatography A
Received date: Revised date: Accepted date:
7-2-2016 20-3-2016 21-3-2016
Please cite this article as: Wei-Qi Xie, Xin-Sheng Chai, Rapid Determination of Moisture Content in Paper Materials by Multiple Headspace Extraction Gas Chromatography, Journal of Chromatography A http://dx.doi.org/10.1016/j.chroma.2016.03.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Rapid Determination of Moisture Content in Paper Materials by Multiple Headspace Extraction Gas Chromatography Wei-Qi Xie and Xin-Sheng Chai* State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, China
1
Highlights
Rapid determination of moisture content in paper materials.
The method is based on multiple headspace extraction gas chromatography.
The present method is efficient, accurate, and suitable for the determination of the moisture content in various paper materials.
2
ABSTRACT This paper describes a new method for the rapid determination of the moisture content in paper materials. The method is based on multiple headspace extraction gas chromatography (MHE-GC) at a temperature above the boiling point of water, from which an integrated water loss from the tested sample due to evaporation can be measured and from which the moisture content in the sample can be determined. The results show that the new method has a good precision (with the relative standard deviation < 0.96 %), high sensitivity (the limit of quantitation = 0.005%) and good accuracy (the relative differences < 1.4 %). Therefore, the method is quite suitable for many uses in research and industrial applications. KEYWORDS:
Moisture
content;
Multiple
headspace
extraction;
chromatography; Paper materials.
*Corresponding author. Tel/fax: +86-20-87113713; E-mail: [email protected] (X.-S Chai)
3
Gas
1. Introduction The moisture content in paper materials can affect many physical properties of paper (such as stiffness, smoothness, and tensile strength), which are important to the performance of the products in their applications [1]. Therefore, analytical methods that can efficiently and accurately determine the moisture content in the paper materials and products play an important role in process control and optimization of industrial applications. Two approaches have been traditionally used for measuring the moisture content in paper materials; i.e., the oven-drying method and the toluene distillation method [2]. In the oven-drying method, the moisture content of the sample is obtained by simply measuring the weight loss of the sample before and after drying at 105℃ for > 6 h. Because other volatile substances (e.g., residual monomers in the coating chemicals) are also removed during the drying process, the moisture content of the sample can be easily overestimated. In the distillation method, toluene is used as a solvent to carry water out of the paper materials. Since there are two phases (i.e., organic and water phases) that clearly separate in the collected condensate, the amount of water can be measured. Although any volatile substances present in the paper have no effect on this distillation method, a significant amount of paper material (500 - 1000 g) is required in order to achieve the desired accuracy for the measurement. Moreover, toluene is a toxic, flammable chemical, and safety measures must be taken to protect the operators during the process. In more recent years, instruments such as near infrared spectroscopy (NIR),
4
microwave moisture meter and gas chromatography (GC), have been used to quantify the moisture content in various types of samples [3-5]. The major problems with the NIR and microwave techniques are the precision and accuracy of the moisture content measurement, especially for samples with complicated matrices. In the GC method, a sample extraction pretreatment, using a water-soluble organic solvent (e.g., methanol or ethanol) is required. Therefore, the method is complicated, time-consuming and can easily introduce significant errors in the pretreatment stages. Headspace gas chromatography (HS-GC) is an effective technique for analyzing volatile species in liquid samples that have complicated matrices [6]. However, the conventional HS-GC analysis is difficult to use for the determination of water content in solid samples because the vapor-solid partitioning of water varies among samples with different matrices, which introduces difficulties in the calibration of the method. Moreover, an internal standard calibration method is not suitable for the solid materials because it has a difficulty to spike a standard that can be evenly distributed in the sample. Multiple headspace extraction (MHE) [6-7] (i.e., repeatedly analyzing (in a stepwise fashion) the vapor phase in a given sample vial is now available in many commercial headspace auto-samplers. In each headspace extraction, the vapor phase (which includes the analyte(s)) is partly removed from the vial for GC testing. If the analyte in the sample is nearly completely vaporized from the vial, the total amount of analyte can be determined by integrating its contents during MHEs. In this way, the sample matrix effect is no longer a problem.
5
The objective of the present study is to develop a MHE-GC method for rapidly determining the of moisture content of paper materials. The major foci in this work were to establish the MHE-GC methodology and explore the preferred conditions, such as the headspace equilibration temperature, the extraction interval time, the number of extractions and the sample size. The precision and accuracy of the method were also evaluated. 2. Experimental 2.1. Samples The coated paper samples were collected from several laboratories and manufacturers in China. The moisture contents in these samples were determined by the TAPPI standard over-drying method [2]. 2.2. Apparatus and operations A GC system (Agilent GC 7890A, USA) and an automatic headspace sampler (DANI HS 86.50, Italy) were used for HS-GC measurement. The GC system was equipped with a thermal conductivity detector (TCD) and a GS-Q capillary column with an i.d. of 0.53 mm and a length of 30 m (J&W Scientific, USA) operating at a temperature of 105 ℃ with nitrogen carrier gas (flow rate = 3.8 mL/min). The headspace operating conditions were as follows: strong shaking for the sample vial at the temperature of 125 ℃; vial pressurization time = 0.2 min; and sample loop fill time = 0.2 min. The volume of each headspace sample vial was 21.6 mL. 2.3. Measurement procedures About 0.50 g of a coated paper sample was placed in an empty headspace vial.
6
By weighing the vial (including a PTFE/butyl and aluminum septum) before and after sample addition, the exact weight of the sample was determined, after which the sample vial was immediately sealed. The vial was equilibrated at 125℃ in the headspace sampler and allowed to proceed with a MHE-GC measurement at an interval time of 5 min. The GC signal (peak area) of water measured by GC at each headspace extraction was recorded. 3. Results and discussion 3.1. Methodology When a paper sample is placed in a container (headspace vial) and heated at an elevated temperature, some of the water in the paper sample is transfered to the vapor phase. Because the container is completely sealed, a phase equilibrium is established between the water in the vapor phase and the water remaining in the paper, described by vapor-solid partitioning coefficient (Kd);
K d Cs / C g
(1)
where Cs (g/g) and Cg (g/mL) are, respectively, the concentration of water in the solid sample and in the vapor phase at the equilibrium. In a headspace vial that contains a paper sample, the amount of water can be expressed as:
mw Cs w CgVg
(2)
where mw (g) is the total amount of water in the original sample. w and Vg are, respectively, the weight (g) of the paper and the volume (mL) of the vial (assuming that the volume of the solid sample is negligible). Merging Eqs. (1) and (2), we have,
7
at equilibrium,
mw ( K d w Vg )Cg
(3)
where the vapor-solid partitioning coefficient (Kd) is a parameter that is characteristic of a particular type of paper, reflecting differences in the specification and processing approaches used in making different kinds of paper . In the multiple headspace extraction (MHE) process, the amount of water extracted from the sample in each headspace extraction can be expressed as [8,9]
m φCgVg
(4)
where φ is the volume ratio constant of headspace sampling [8]. The total amount of water in the original sample is the sum of water extracted out of the sample vial; i.e., mw m1 m2 ... mn
n
m
(5)
i
1
According to Eqs. (4), (5) and the relationship between the GC signal (A) and Cg; i.e.,
Cg kA , we can obtain mw
n
n
1
1
mi K Ai
(6)
where K φkVg . The relationship between the GC signal A and its extraction number in MHE can be described as [6, 10] log( An ) log( A0 ) bn
(7)
or An 10 bn A0
(8) 8
where b is a ratio constant of the headspace extraction. Assuming that
10b q
(8-1)
Eq. (8) can be written as q
A A1 A2 n A0 A1 An1
(q < 1)
(9)
According to Eq. (9) and the approximation of infinite series, the integrated GC signal in the MHE can be expressed as n
A A A i
1
2
An A1 A1q A1q
n 1
1
A1 (1 q n ) A lim 1 n 1 q 1 q
(10)
Substituting Eq. (10) to Eq. (6), we have
mw K
A1 1 q
(11)
Since the ratio constant b in Eq. (7) is experimentally obtained by the MHE measurement, the total amount of water in the sample can be determined. Thus, the moisture content in the sample can be calculated by Cw
mw 100% w
(12)
where Cw is the moisture content in the sample and w is the sample weight. 3.2. The conditions in MHE-GC measurement 3.2.1. Use of chromatogram for water content measurement by HS-GC Fig. 1 shows a chromatogram resulting from the testing of a paper sample. It is clear that the water peak is well-separated from the oxygen (in air) enclosed in the headspace sample vial with the given capillary column and under the GC operating conditions. A feasibility study showed that there is basically no water peak observed if 9
the GC oven temperature is below 100℃. 3.2.2. Selection of the temperature and the interval time in headspace equilibration Because the present method is based on measuring the water released from the solid sample, it is important to conduct the determination at a temperature above the boiling point of water. Since too high a temperature could cause high water vapor pressure in the headspace vial and could distort butyl rubber septum that seals the vial (both of which could cause the vapor phase to leak during the test), we chose an equilibration temperature below 130℃. Fig. 2 shows the water losses from a given paper sample during the MHE at 105 and 125℃, respectively. It is clear that higher temperature accelerates the water removal from the sample and thus makes the testing more rapid. Therefore, we chose 125℃ as the equilibration temperature for the water content measurements in this study. Since the present method is based on the assumption that equilibrium partitioning of water between the vapor phase and the solid phase has been achieved before headspace sampling, the effect of time (i.e., interval time) for equilibration to take place between the headspace samplings at a given temperature (125℃) should be investigated. Fig. 3 shows the change of vapor water content during the process, and it is clear that the equilibrium (i.e., reaching the plateau) can be achieved within 5 min. Therefore, we chose 5 min as the interval time in the MHE-GC measurement. 3.2.3. Selections of the sample size and the number of headspace extractions Fig. 4 shows the decrease in GC signal decreasing for the samples of different
10
weights during the MHE. The decrease in GC signal for the lower weight sample is more rapid. On one hand, a larger sample size is useful for improving the accuracy in weighing and for obtaining a more representative sample of the solid material. On the other hand, as Fig. 4 shows, using a large sample size (weight) can make the testing less rapid due to a slower release of water from the solid. Table 1 shows that the accuracy of the method can be improved when the sample size is 0.50 g. Therefore, we selected a sample size of 0.50 g in the present study. If the moisture content in the sample is too high, a smaller sample size should be used. According to Eq. (11), q is an important parameter required for calculating the total amount of water in the tested paper sample, and it can be derived from the linear fitting of the logarithm of GC signals (A) vs. the number (n) in the MHE measurement using Eq. (7). The accuracy of the method can be improved by increasing the number of extractions, as shown in Table 2. However, more extractions mean a longer time to make the MHE-GC measurement. In this work, we chose five headspace extractions in the test because with n = 5 the water content measured by this method is very close to that value determined by the reference method; i.e., the oven-drying method [2]. 3.3. Calibration and evaluation 3.3.1. Method calibration The method calibration was performed by adding different weights (0 - 50 mg) of distilled water to a set of headspace sample vials and then analyzing the sample by MHE-GC. The standard calibration relationship thus obtained is summarized by Eq. (13).
11
A 0.42(1.73) 664.26 (2.65) mw
(13)
That is, A a(Δa) s(Δs) mw where A represents the integrated GC signals in the MHE calculated by Eq. (10) and mw represents the weight (mg) of distilled water spiked in the headspace sample vial. a, ∆a and s represent the intercept, the uncertainty of the intercept and the slope in Eq. (13), respectively. 3.3.2. Precision and limit of quantitation The repeatability of the MHE-GC method was investigated by quadruplicate tests of the coated paper samples, The results showed that the relative standard deviation (RSD) in these measurements was less than 0.96% (See Table 3). The limit of quantitation (LOQ) for the amount of water in a 0.50 g paper sample was 0.026 mg, which was calculated from the following equation. LOQ
10 a
(14)
s
If the sample weight is 0.50 g in the testing, the LOQ is about 0.005%. 3.3.3. Method validation To validate the performance of the new method, the moisture content of the coated paper samples were determined by both the MHE-GC method and the oven-drying method (TAPPI standard [2]), with the results listed in Table 4. The moisture content in the coated paper samples measured by the two methods has a relative difference of no more than 1.4%. These results demonstrate that the MHE-GC technique is a reliable method for the determination of moisture content of coated papers.
12
4. Conclusion A new method for the measurement of the moisture content in paper material samples has been developed, based on MHE-GC. The results show that the method is rapid, simple and accurate. With a commercial headspace auto-sampler and GC system, the method can perform automated measurements that are suitable for the accurate determination of the moisture content in various paper materials. Acknowledgements The authors acknowledge the financial support from the Natural Science Foundation of China (Project No. 21576105) and the team project of State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, China (Grant No. 2015ZD01).
13
References [1] A. C. Dimmick, Effects of sheet moisture and calender pressure on PCC and GCC coated papers. Tappi J. 6 (2007), 16. [2] TAPPI Test Method: Moisture in Pulp, Paper and Paperboard, T412om-11, TAPPI Press, Atlanta, 2011. [3] J. Mantanus, E. Ziémons, P. Lebrun, E. Rozet, R. Klinkenberg, B. Streel, P. Hubert, Moisture content determination of pharmaceutical pellets by near infrared spectroscopy: method development and validation. Anal. Chim. Acta. 642 (2009), 186-192. [4] M. Lewis, S. Trabelsi, S Nelson, Assessment of real-time, in-shell kernel moisture content monitoring with a microwave moisture meter during peanut drying. Appl. Eng. Agric. 30 (2014), 649-656. [5] J. N. Lin, L. L. Hong, L. Chen, J. Q. You, Determination of cigarette papers moisture content by gas chromatography. Chem. Pap. 69 (2015), 402-408.[6] B. Kolb, L.S. Ettre, Static Headspace-Gas Chromatography: Theory and Practice, 2nd ed., Wiley-VCH Press, New York, 2006. [7] M. Suzuki, S. Tsuge, T. Takeuchi, Gas chromatographic estimation of occluded solvents in adhesive tape by periodic introduction method. Anal. Chem. 42 (1970), 1705-1708. [8] X. S. Chai, J. Y. Zhu, Simultaneous measurements of solute concentration and Henry's constant using multiple headspace extraction gas chromatography. Anal. Chem. 70 (1998), 3481-3487.
14
[9] X. S. Chai, Q. X. Hou, F. J Schork, Determination of the solubility of a monomer in water by multiple headspace extraction gas chromatography. J. Appl. Polym. Sci. 99 (2006), 1296-1301. [10] B. Kolb, L. S. Ettre, Static Headspace-Gas Chromatography-Theory and Practice, 2nd ed., Wiley-VCH Press, New York, 2006.
15
Figure Captions Fig. 1. Chromatogram in HS-GC measurement of a coated paper sample.
16
Fig. 2. Effect of equilibration temperature on the water removal.
17
Fig. 3. The time required for the water headspace equilibrium.
18
Fig. 4. Effect of the sample size on the water removal during MHE.
19
Tables Table 1 Errors from the test with different sample size Sample weight, g
Moisture content, %
1.0 0.50 0.10
9.19 9.34 9.74
Relative difference, %* -1.8 -1.2 3.1
*Compared to the data (9.45%) measured by the oven-drying method.
20
Table 2 Errors from the test with different extraction numbers No. of extraction
Moisture content, %
2 3 4 5 6
4.75 5.19 5.33 5.39 5.45
Relative difference, %* -12.3 -4.2 -1.7 -0.6 0.5
*Compared to the data (5.42%) measured by the oven-drying method.
21
Table 3 The repeatability of the MHE-GC method Replica no. 1 2 3 4 Average RSD, %
Sample 1 7.83 7.78 7.75 7.71 7.77 0.65
Moisture content, % Sample 2 5.12 5.18 5.24 5.20 5.19 0.96
22
Sample 3 8.32 8.18 8.20 8.25 8.24 0.76
Table 4 Methods comparison Sample name Standard coated fine paper Starch coated paper Art paper Machine-finished coated paper Low coat weight paper Polyethylene coated paper
Moisture content, % Present Ref. method method* 6.63 6.59 7.78 7.71 5.39 5.42 8.25 8.14 4.52 4.57 5.18 5.12
*The oven-drying method
23
Relative difference, % 0.6 0.9 -0.6 1.4 -1.1 1.2
S0021-9673(16)30341-7 http://dx.doi.org/doi:10.1016/j.chroma.2016.03.059 CHROMA 357416
To appear in:
Journal of Chromatography A
Received date: Revised date: Accepted date:
7-2-2016 20-3-2016 21-3-2016
Please cite this article as: Wei-Qi Xie, Xin-Sheng Chai, Rapid Determination of Moisture Content in Paper Materials by Multiple Headspace Extraction Gas Chromatography, Journal of Chromatography A http://dx.doi.org/10.1016/j.chroma.2016.03.059 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Rapid Determination of Moisture Content in Paper Materials by Multiple Headspace Extraction Gas Chromatography Wei-Qi Xie and Xin-Sheng Chai* State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou, China
1
Highlights
Rapid determination of moisture content in paper materials.
The method is based on multiple headspace extraction gas chromatography.
The present method is efficient, accurate, and suitable for the determination of the moisture content in various paper materials.
2
ABSTRACT This paper describes a new method for the rapid determination of the moisture content in paper materials. The method is based on multiple headspace extraction gas chromatography (MHE-GC) at a temperature above the boiling point of water, from which an integrated water loss from the tested sample due to evaporation can be measured and from which the moisture content in the sample can be determined. The results show that the new method has a good precision (with the relative standard deviation < 0.96 %), high sensitivity (the limit of quantitation = 0.005%) and good accuracy (the relative differences < 1.4 %). Therefore, the method is quite suitable for many uses in research and industrial applications. KEYWORDS:
Moisture
content;
Multiple
headspace
extraction;
chromatography; Paper materials.
*Corresponding author. Tel/fax: +86-20-87113713; E-mail: [email protected] (X.-S Chai)
3
Gas
1. Introduction The moisture content in paper materials can affect many physical properties of paper (such as stiffness, smoothness, and tensile strength), which are important to the performance of the products in their applications [1]. Therefore, analytical methods that can efficiently and accurately determine the moisture content in the paper materials and products play an important role in process control and optimization of industrial applications. Two approaches have been traditionally used for measuring the moisture content in paper materials; i.e., the oven-drying method and the toluene distillation method [2]. In the oven-drying method, the moisture content of the sample is obtained by simply measuring the weight loss of the sample before and after drying at 105℃ for > 6 h. Because other volatile substances (e.g., residual monomers in the coating chemicals) are also removed during the drying process, the moisture content of the sample can be easily overestimated. In the distillation method, toluene is used as a solvent to carry water out of the paper materials. Since there are two phases (i.e., organic and water phases) that clearly separate in the collected condensate, the amount of water can be measured. Although any volatile substances present in the paper have no effect on this distillation method, a significant amount of paper material (500 - 1000 g) is required in order to achieve the desired accuracy for the measurement. Moreover, toluene is a toxic, flammable chemical, and safety measures must be taken to protect the operators during the process. In more recent years, instruments such as near infrared spectroscopy (NIR),
4
microwave moisture meter and gas chromatography (GC), have been used to quantify the moisture content in various types of samples [3-5]. The major problems with the NIR and microwave techniques are the precision and accuracy of the moisture content measurement, especially for samples with complicated matrices. In the GC method, a sample extraction pretreatment, using a water-soluble organic solvent (e.g., methanol or ethanol) is required. Therefore, the method is complicated, time-consuming and can easily introduce significant errors in the pretreatment stages. Headspace gas chromatography (HS-GC) is an effective technique for analyzing volatile species in liquid samples that have complicated matrices [6]. However, the conventional HS-GC analysis is difficult to use for the determination of water content in solid samples because the vapor-solid partitioning of water varies among samples with different matrices, which introduces difficulties in the calibration of the method. Moreover, an internal standard calibration method is not suitable for the solid materials because it has a difficulty to spike a standard that can be evenly distributed in the sample. Multiple headspace extraction (MHE) [6-7] (i.e., repeatedly analyzing (in a stepwise fashion) the vapor phase in a given sample vial is now available in many commercial headspace auto-samplers. In each headspace extraction, the vapor phase (which includes the analyte(s)) is partly removed from the vial for GC testing. If the analyte in the sample is nearly completely vaporized from the vial, the total amount of analyte can be determined by integrating its contents during MHEs. In this way, the sample matrix effect is no longer a problem.
5
The objective of the present study is to develop a MHE-GC method for rapidly determining the of moisture content of paper materials. The major foci in this work were to establish the MHE-GC methodology and explore the preferred conditions, such as the headspace equilibration temperature, the extraction interval time, the number of extractions and the sample size. The precision and accuracy of the method were also evaluated. 2. Experimental 2.1. Samples The coated paper samples were collected from several laboratories and manufacturers in China. The moisture contents in these samples were determined by the TAPPI standard over-drying method [2]. 2.2. Apparatus and operations A GC system (Agilent GC 7890A, USA) and an automatic headspace sampler (DANI HS 86.50, Italy) were used for HS-GC measurement. The GC system was equipped with a thermal conductivity detector (TCD) and a GS-Q capillary column with an i.d. of 0.53 mm and a length of 30 m (J&W Scientific, USA) operating at a temperature of 105 ℃ with nitrogen carrier gas (flow rate = 3.8 mL/min). The headspace operating conditions were as follows: strong shaking for the sample vial at the temperature of 125 ℃; vial pressurization time = 0.2 min; and sample loop fill time = 0.2 min. The volume of each headspace sample vial was 21.6 mL. 2.3. Measurement procedures About 0.50 g of a coated paper sample was placed in an empty headspace vial.
6
By weighing the vial (including a PTFE/butyl and aluminum septum) before and after sample addition, the exact weight of the sample was determined, after which the sample vial was immediately sealed. The vial was equilibrated at 125℃ in the headspace sampler and allowed to proceed with a MHE-GC measurement at an interval time of 5 min. The GC signal (peak area) of water measured by GC at each headspace extraction was recorded. 3. Results and discussion 3.1. Methodology When a paper sample is placed in a container (headspace vial) and heated at an elevated temperature, some of the water in the paper sample is transfered to the vapor phase. Because the container is completely sealed, a phase equilibrium is established between the water in the vapor phase and the water remaining in the paper, described by vapor-solid partitioning coefficient (Kd);
K d Cs / C g
(1)
where Cs (g/g) and Cg (g/mL) are, respectively, the concentration of water in the solid sample and in the vapor phase at the equilibrium. In a headspace vial that contains a paper sample, the amount of water can be expressed as:
mw Cs w CgVg
(2)
where mw (g) is the total amount of water in the original sample. w and Vg are, respectively, the weight (g) of the paper and the volume (mL) of the vial (assuming that the volume of the solid sample is negligible). Merging Eqs. (1) and (2), we have,
7
at equilibrium,
mw ( K d w Vg )Cg
(3)
where the vapor-solid partitioning coefficient (Kd) is a parameter that is characteristic of a particular type of paper, reflecting differences in the specification and processing approaches used in making different kinds of paper . In the multiple headspace extraction (MHE) process, the amount of water extracted from the sample in each headspace extraction can be expressed as [8,9]
m φCgVg
(4)
where φ is the volume ratio constant of headspace sampling [8]. The total amount of water in the original sample is the sum of water extracted out of the sample vial; i.e., mw m1 m2 ... mn
n
m
(5)
i
1
According to Eqs. (4), (5) and the relationship between the GC signal (A) and Cg; i.e.,
Cg kA , we can obtain mw
n
n
1
1
mi K Ai
(6)
where K φkVg . The relationship between the GC signal A and its extraction number in MHE can be described as [6, 10] log( An ) log( A0 ) bn
(7)
or An 10 bn A0
(8) 8
where b is a ratio constant of the headspace extraction. Assuming that
10b q
(8-1)
Eq. (8) can be written as q
A A1 A2 n A0 A1 An1
(q < 1)
(9)
According to Eq. (9) and the approximation of infinite series, the integrated GC signal in the MHE can be expressed as n
A A A i
1
2
An A1 A1q A1q
n 1
1
A1 (1 q n ) A lim 1 n 1 q 1 q
(10)
Substituting Eq. (10) to Eq. (6), we have
mw K
A1 1 q
(11)
Since the ratio constant b in Eq. (7) is experimentally obtained by the MHE measurement, the total amount of water in the sample can be determined. Thus, the moisture content in the sample can be calculated by Cw
mw 100% w
(12)
where Cw is the moisture content in the sample and w is the sample weight. 3.2. The conditions in MHE-GC measurement 3.2.1. Use of chromatogram for water content measurement by HS-GC Fig. 1 shows a chromatogram resulting from the testing of a paper sample. It is clear that the water peak is well-separated from the oxygen (in air) enclosed in the headspace sample vial with the given capillary column and under the GC operating conditions. A feasibility study showed that there is basically no water peak observed if 9
the GC oven temperature is below 100℃. 3.2.2. Selection of the temperature and the interval time in headspace equilibration Because the present method is based on measuring the water released from the solid sample, it is important to conduct the determination at a temperature above the boiling point of water. Since too high a temperature could cause high water vapor pressure in the headspace vial and could distort butyl rubber septum that seals the vial (both of which could cause the vapor phase to leak during the test), we chose an equilibration temperature below 130℃. Fig. 2 shows the water losses from a given paper sample during the MHE at 105 and 125℃, respectively. It is clear that higher temperature accelerates the water removal from the sample and thus makes the testing more rapid. Therefore, we chose 125℃ as the equilibration temperature for the water content measurements in this study. Since the present method is based on the assumption that equilibrium partitioning of water between the vapor phase and the solid phase has been achieved before headspace sampling, the effect of time (i.e., interval time) for equilibration to take place between the headspace samplings at a given temperature (125℃) should be investigated. Fig. 3 shows the change of vapor water content during the process, and it is clear that the equilibrium (i.e., reaching the plateau) can be achieved within 5 min. Therefore, we chose 5 min as the interval time in the MHE-GC measurement. 3.2.3. Selections of the sample size and the number of headspace extractions Fig. 4 shows the decrease in GC signal decreasing for the samples of different
10
weights during the MHE. The decrease in GC signal for the lower weight sample is more rapid. On one hand, a larger sample size is useful for improving the accuracy in weighing and for obtaining a more representative sample of the solid material. On the other hand, as Fig. 4 shows, using a large sample size (weight) can make the testing less rapid due to a slower release of water from the solid. Table 1 shows that the accuracy of the method can be improved when the sample size is 0.50 g. Therefore, we selected a sample size of 0.50 g in the present study. If the moisture content in the sample is too high, a smaller sample size should be used. According to Eq. (11), q is an important parameter required for calculating the total amount of water in the tested paper sample, and it can be derived from the linear fitting of the logarithm of GC signals (A) vs. the number (n) in the MHE measurement using Eq. (7). The accuracy of the method can be improved by increasing the number of extractions, as shown in Table 2. However, more extractions mean a longer time to make the MHE-GC measurement. In this work, we chose five headspace extractions in the test because with n = 5 the water content measured by this method is very close to that value determined by the reference method; i.e., the oven-drying method [2]. 3.3. Calibration and evaluation 3.3.1. Method calibration The method calibration was performed by adding different weights (0 - 50 mg) of distilled water to a set of headspace sample vials and then analyzing the sample by MHE-GC. The standard calibration relationship thus obtained is summarized by Eq. (13).
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A 0.42(1.73) 664.26 (2.65) mw
(13)
That is, A a(Δa) s(Δs) mw where A represents the integrated GC signals in the MHE calculated by Eq. (10) and mw represents the weight (mg) of distilled water spiked in the headspace sample vial. a, ∆a and s represent the intercept, the uncertainty of the intercept and the slope in Eq. (13), respectively. 3.3.2. Precision and limit of quantitation The repeatability of the MHE-GC method was investigated by quadruplicate tests of the coated paper samples, The results showed that the relative standard deviation (RSD) in these measurements was less than 0.96% (See Table 3). The limit of quantitation (LOQ) for the amount of water in a 0.50 g paper sample was 0.026 mg, which was calculated from the following equation. LOQ
10 a
(14)
s
If the sample weight is 0.50 g in the testing, the LOQ is about 0.005%. 3.3.3. Method validation To validate the performance of the new method, the moisture content of the coated paper samples were determined by both the MHE-GC method and the oven-drying method (TAPPI standard [2]), with the results listed in Table 4. The moisture content in the coated paper samples measured by the two methods has a relative difference of no more than 1.4%. These results demonstrate that the MHE-GC technique is a reliable method for the determination of moisture content of coated papers.
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4. Conclusion A new method for the measurement of the moisture content in paper material samples has been developed, based on MHE-GC. The results show that the method is rapid, simple and accurate. With a commercial headspace auto-sampler and GC system, the method can perform automated measurements that are suitable for the accurate determination of the moisture content in various paper materials. Acknowledgements The authors acknowledge the financial support from the Natural Science Foundation of China (Project No. 21576105) and the team project of State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, China (Grant No. 2015ZD01).
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References [1] A. C. Dimmick, Effects of sheet moisture and calender pressure on PCC and GCC coated papers. Tappi J. 6 (2007), 16. [2] TAPPI Test Method: Moisture in Pulp, Paper and Paperboard, T412om-11, TAPPI Press, Atlanta, 2011. [3] J. Mantanus, E. Ziémons, P. Lebrun, E. Rozet, R. Klinkenberg, B. Streel, P. Hubert, Moisture content determination of pharmaceutical pellets by near infrared spectroscopy: method development and validation. Anal. Chim. Acta. 642 (2009), 186-192. [4] M. Lewis, S. Trabelsi, S Nelson, Assessment of real-time, in-shell kernel moisture content monitoring with a microwave moisture meter during peanut drying. Appl. Eng. Agric. 30 (2014), 649-656. [5] J. N. Lin, L. L. Hong, L. Chen, J. Q. You, Determination of cigarette papers moisture content by gas chromatography. Chem. Pap. 69 (2015), 402-408.[6] B. Kolb, L.S. Ettre, Static Headspace-Gas Chromatography: Theory and Practice, 2nd ed., Wiley-VCH Press, New York, 2006. [7] M. Suzuki, S. Tsuge, T. Takeuchi, Gas chromatographic estimation of occluded solvents in adhesive tape by periodic introduction method. Anal. Chem. 42 (1970), 1705-1708. [8] X. S. Chai, J. Y. Zhu, Simultaneous measurements of solute concentration and Henry's constant using multiple headspace extraction gas chromatography. Anal. Chem. 70 (1998), 3481-3487.
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[9] X. S. Chai, Q. X. Hou, F. J Schork, Determination of the solubility of a monomer in water by multiple headspace extraction gas chromatography. J. Appl. Polym. Sci. 99 (2006), 1296-1301. [10] B. Kolb, L. S. Ettre, Static Headspace-Gas Chromatography-Theory and Practice, 2nd ed., Wiley-VCH Press, New York, 2006.
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Figure Captions Fig. 1. Chromatogram in HS-GC measurement of a coated paper sample.
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Fig. 2. Effect of equilibration temperature on the water removal.
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Fig. 3. The time required for the water headspace equilibrium.
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Fig. 4. Effect of the sample size on the water removal during MHE.
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Tables Table 1 Errors from the test with different sample size Sample weight, g
Moisture content, %
1.0 0.50 0.10
9.19 9.34 9.74
Relative difference, %* -1.8 -1.2 3.1
*Compared to the data (9.45%) measured by the oven-drying method.
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Table 2 Errors from the test with different extraction numbers No. of extraction
Moisture content, %
2 3 4 5 6
4.75 5.19 5.33 5.39 5.45
Relative difference, %* -12.3 -4.2 -1.7 -0.6 0.5
*Compared to the data (5.42%) measured by the oven-drying method.
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Table 3 The repeatability of the MHE-GC method Replica no. 1 2 3 4 Average RSD, %
Sample 1 7.83 7.78 7.75 7.71 7.77 0.65
Moisture content, % Sample 2 5.12 5.18 5.24 5.20 5.19 0.96
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Sample 3 8.32 8.18 8.20 8.25 8.24 0.76
Table 4 Methods comparison Sample name Standard coated fine paper Starch coated paper Art paper Machine-finished coated paper Low coat weight paper Polyethylene coated paper
Moisture content, % Present Ref. method method* 6.63 6.59 7.78 7.71 5.39 5.42 8.25 8.14 4.52 4.57 5.18 5.12
*The oven-drying method
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Relative difference, % 0.6 0.9 -0.6 1.4 -1.1 1.2