Testing Method for Proficiency Testing of Sample’s Homogeneity and Stability

In 2007, Certification and Accreditation Administration of P.R.C (CNCA) organized the laboratory proficiency testing of “The Plastic Packaging Material Permeability Test - - the Nationwide Test of OTR and WVTR”. And the samples’ homogeneity and stability comparison are both accomplished all by Labthink.

The homogeneity of sample’s comparison is very significant. It’s critical for laboratory comparison and the laboratory proficiency testing project. When carrying out the project, the organizer must guarantee that the unsatisfactory results in proficiency testing will not impute to the changeability among the samples and the sample itself. Therefore, for the quantity of test characteristics, proficiency testing must be carried out on the homogeneity testing and stability testing. This paper will introduce samples of the plastic packaging material permeability proficiency testing project and the test condition of the homogeneity and stability.

1. Sample Preparation

　　This proficiency testing use “Split-level” sample design; sample is polyester film, which is divided into two groups of A and B, and has similar thickness and basic material. Whereas, there is a slight difference between the two groups and the testing results. This sample design could inspect the difference within and between different laboratories simultaneously.

2. The Testing of Homogeneity and Stability

　　2.1 The Testing of Homogeneity

　　For the proficiency testing plan of lot samples’ preparation, the homogeneity test is essential. According to the stipulation of CNAS-GL03 "Guidance on Evaluating the Homogeneity and Stability of Samples Used for Proficiency Testing", the sample’s homogeneity testing procedures are as follows.

　　1. Select samples at random from A and B groups for the homogeneity testing.

　　2. For each selected sample, reiterate testing 2 times under the same experimental condition at least.

　　3. Use VAC-V1 Gas Permeability Tester and TSY-T3 Water Vapor Permeability Tester for the homogeneity test. Testing principles are consistent with recommended method of this proficiency testing.

　　4. For the unusual values in the test, they could not be rejected before verification of their reason.

　　5. Use the single factor ANOVA method to proceed with the testing result statistically and the homogeneity analysis.

　　The single factor ANOVA method is the most common method to appraise material’s homogeneity. The basic method is extracts i samples (i=1, 2 ... m); and each sample tests j times (j=1, 2 ... n) under the same experimental condition. Proceed statistic F as follows:

　　Sum of squares 
     

　　Mean square

　　Statistics
     

　　If F＜F_α(f1，f2) (usually takes significant level α=0.05), then it indicates no special difference inside each sample and among samples. Therefore, the sample is homogeny.

　　2.2 Stability Test

　　In proficiency testing, the samples need to be transported and delivered to each participating laboratory, and it might affect the stability of OTR and WVTR of the sensitive testing samples. Therefore, stability of samples must be tested. The test method also adopts CNAS-GL03 “Guidance on Evaluating the Homogeneity and Stability of Samples Used for Proficiency Testing”.

　　1. Select samples at random from A and B groups for stability test; the quantities of samples should be representative enough.

　　2.   Use two kinds of environment, that is, 38℃/50％RH and 48℃/90％RH, to process the samples. It can effectively simulate summer's temperature and humidity environment of most districts in our country.

　　3.  The stability test also uses VAC-V1 Gas Permeability Tester and TSY-T3 Water Vapor Permeability Tester.

　　4. Use the T testing method to proceed with the statistical testing result and carry on the stability analysis.

　　T testing method includes two methods: one is the comparison between series of survey mean values and normal/reference values; the other is the consistence between two mean values.

      For the first method  

      For the second method  

　　And, t is the mean value which measures n times, n is the measurement time, S is the standard deviation of measured result. In the first method μ is the normal value or the reference value, and in the second method the subscripts express the category of the parameters, that is, the first or the second testing method. In order to ensure accuracy of mean value and standard deviation's, the testing time should not be less than 6 times.

　　Using T value is obtained by the computation to evaluate the stability of the samples. For the first method, the crucial value of t is tα (n-1) (usually takes significant level α=0.05); for the second method, the marginal value of t is tα (n1+n2-2) (usually takes significant level α=0.05). If t< the crucial value of t, then it indicates no great difference between the two mean values, and the sample is stable.

　　2.3 The sample test data of this proficiency testing

　　Through the single factor ANOVA method and the t testing method, Labthink obtained 108 homogeneity tentative data with 44 stability test data. These test data gained expert's consistent approval to the comparison sample and the material’s selection conform to the requirement completely. Therefore it could use in this proficiency testing project. Because the data is numerous, here has only enumerated the testing WVTR data of B group test specimen.

 Table1. The WVTR data of sample’s stability testing from B group  

　　Table2: The Data of Single Factor ANOVA Analysis

　　F marginal value is F0.05 (8, 18) =2.51. F value, after computation, is 0.984, which is smaller than the F marginal value -- this indicates that when the significant level is at 0.05, the sample's WVTR is very homogeneous.

3. The Significance of Sample Testing

　　Through great amounts of volume test data, Labthink proves the sample homogeneity and stability in “The Plastic Packaging Material Permeability Test - - The Test of OTR and WVTR” proficiency testing. They satisfy the request of CNAS-GL03 “Guidance on Evaluating the Homogeneity and Stability of Samples Used for Proficiency Testing” completely. Therefore, this proficiency testing can avoid the sample's changeability between inside and among each of them. It could reflect the whole level of current domestic permeability testing status accurately.