Gas chromatography determines the uncertainty of water content in water-based coatings

0 Introduction: With the
of the national energy conservation and emission reduction policy, water-based coatings have developed rapidly in recent years and have been widely used in construction, interior decoration and other fields. Since 2001, more and more environmental standards for coatings have been issued in China, strictly restricting the content of volatile organic compounds (VOCs), tribenzene, ethylene glycol ethers and other toxic and harmful substances. The accuracy and reliability of water content determination in water-based coatings have a greater

impact on VOC content. The five sources of uncertainty, such as test environment, measurement method, tester, test standard and test instrument, all have an impact on the measurement results, so the uncertainty of water content determination needs to be assessed. At present, there are three methods for measuring water content, such as gas chromatography, Karl-Fisher (KF) method, and air pressure method. In this paper, according to GB 18582-2008 Appendix B method, using gas chromatography, the various factors affecting the test results are systematically studied, and the assessment method of water content determination uncertainty in water-based coatings is described in detail, which provides a reference for assessing the impact of various uncertain sources on water content, so as to better evaluate the reliability of the measurement data.
1 measurement principle and method
in accordance with GB 18582-2008 "interior decoration material interior wall paint in the limit of harmful substances" appendix B method, called distilled water 0.2 g (m standard), isopropyl alcohol 0.2 g (m standard), accurate to 0.1 mg, put in a plug glass bottle, add 2 mL dimethyl methylamine seal, shake, spare. The specimen was measured under the same chromatography conditions as the relative correction factor, the chromatography and chromatography data of each group on the column were recorded, and the water content was calculated by the internal calibration method.
2 Establishment of mathematical models
among them:
r1 - average of the ratio of moisture to the area of the inner subject matter peak in the sample solution,
r2 - average of the ratio of moisture to the area of the inner specimen peak in the sample solution;
m mark - mass of the specimen, g;
m sample - mass of the specimen, g;
P - mass fraction of moisture in the specimen, % ;
A mark - peak area of moisture in the sample solution;
A sample - peak area of moisture in the specimen solution;
A marker - peak area of the specimen in the sample solution;
A sample - peak area of the inner subject matter in the specimen solution;
m sample - mass of the inner subject matter in the specimen solution;
m specimen - mass of the inner subject matter in the sample solution;
3 determination and quantification of the source of uncertainty
according to the above mathematical model, The source of uncertainty of water content in the coating is mainly composed of the following parts: (1) the mass m mark of the specimen; within the mass m sample of the inner subject matter; (5) the ratio of distilled water to the peak area of the inner subject matter in the sample solution, and the ratio of moisture to the peak area of the inner subject matter in the specimen solution; (6) the uncertainty introduced by the purity of the specimen.
4 standard uncertainty
the scale and specimen are weighed on the same scale, so the sensitivity of the scale is negligible. The uncertainty of the balance is limited to the linear uncertainty of the balance. The balance metering certificate proves that ± is 0.000 1 g, which is the maximum difference between the actual mass and the balance reading. The linear parting is quantified as standard uncertainty by rectangular distribution, so the standard uncertainty of the balance linear part can be expressed as:
(4) the uncertainty part introduced within the inner subject matter mass m sample in the specimen solution. The scale of the inner subject matter in the specimen solution is 0.3 g, and its relative standard uncertainty is: the determination result of the water content in the
table 1 specimen
in order to calculate the synthesis standard uncertainty of the multiplication notation, each part of the relative standard uncertainty is substituted into the lower formula:
5 synthetic uncertainty assessment
uncertainty source and relative standard uncertainty see table 2.
Table 2 Uncertainty List
6 Extended Uncertainty Assessment
Extended Uncertainty U can be obtained by using calculations that contain factors. According to experience, take the inclusion factor kp=2, then:
U=kp uc (o)=0.34%×2=0.7%
7 extended uncertainty report and
the mass score and determination of moisture in the water-based coating are as: (35.118±0.7), kp=2.
8 results
the discussion, different parameters and influences contribute differently to determining uncertainty. In this paper, the effects of sample quality, specimen quality, quality of the inner subject matter in the sample solution, quality, repeatability, sample purity and other factors on uncertainty are analyzed systematically. As can be seen from Table 2: Repetitive experiment is the most important part of all uncertain sources, so the quality of repetitive experiment is the key to correctly use gas chromatography to assess water content uncertainty in water-based coatings, which has practical guiding significance for future testing work.