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Proficiency testing for determination of formaldehyde in leather in China
Accepted Manuscript Title: Proficiency testing for determination of formaldehyde in leather in china Author: Yang Zhao, Dong Guo, Zhiding Huang, Wenkai Chang, Xiaoxia Qi, Zenghe Li PII: DOI: Reference:
S0165-9936(16)30079-6 http://dx.doi.org/doi: 10.1016/j.trac.2016.06.014 TRAC 14786
To appear in:
Trends in Analytical Chemistry
Please cite this article as: Yang Zhao, Dong Guo, Zhiding Huang, Wenkai Chang, Xiaoxia Qi, Zenghe Li, Proficiency testing for determination of formaldehyde in leather in china, Trends in Analytical Chemistry (2016), http://dx.doi.org/doi: 10.1016/j.trac.2016.06.014. 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.
Proficiency testing for determination of formaldehyde in leather in China Yang Zhao a, b, Dong Guo c, Zhiding Huang b, Wenkai Chang b, Xiaoxia Qi b, Zenghe Li a, * a
Beijing Key Laboratory of Environmentally Harmful Chemical Analysis, College of
Science, Beijing University of Chemical Technology, Beijing, 100029, P. R. China. b
National Footwear Quality Supervision and Inspection Center (Beijing), China
Leather and Footwear Industry Research Institute, Beijing, 100015, P. R. China. c
Certification and Accreditation Administration of the People’s Republic of China,
Beijing, 100088, P. R. China.
* Corresponding author. Tel./fax: +86 10 64337798. E-mail address: [email protected] (Z. Li).
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Highlights
A new proficiency testing for determination of formaldehyde in leather.
The preparation of test materials was identical to the real tanning of leather.
The assigned values were established using the reported analytical results.
The robust z-scores were calculated to assess the participants’ performance.
The distribution of results from formaldehyde stock solutions was investigated.
2 Page 2 of 26
Abstract Formaldehyde can cause serious health hazards to human body. A proficiency testing scheme (CNCA-15-A09) for the determination of formaldehyde in leather was organized involving 162 laboratories in China. The test materials were prepared by adding melamine formaldehyde syntans to a blank sheepskin during tanning process. The tanning was identical to the real production process to assure the representativeness of test materials. The histograms of results for formaldehyde in the leather samples illustrated that the data sets approximated closely to the normal distribution. The mean value was in good agreement with the median value. Hence the assigned values of test materials for proficiency assessment were derived directly from the reported results of laboratories, and the measurement performance for determining formaldehyde in leather were assessed using robust z-scores. And the distribution characteristics of different preparation approaches for formaldehyde stock solution showed that it could greatly influence the process and results of analysis.
Keywords: Leather; Proficiency testing; Formaldehyde; Interlaboratory comparison; Reference material
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1. Introduction Leather is a kind of durable and flexible products came from animal or artificial synthesis. The leather industry has a long history, and the various requirements of people have promoted the development of leather manufacture. To achieve specific functions, a variety of synthetic chemical substances are used in the production procedure of leather. Nowadays, more and more countries and people have realized that the leather industry has a dramatic impact on the environment [1,2]. The environmental impacts mainly include two aspects, one is the heavy metal of polluting and poor quality chemicals used in the tanning process, the other is air pollution due to the transformation process of residual chemical reagent [3,4]. In addition to the environmental impacts, the long-term exposure to these leather products with toxic substances would increase the risk of physical damage [5,6]. Among these hazards caused by chemical materials, formaldehyde is a main matter and could not be ignored. Formaldehyde may occur in leather as a result of inadvertent migration and evaporation from products during the process of tanning, finishing and using [7,8]. The formaldehyde can cause serious health hazards such as sensitivity and diverse diseases to workers and consumers [9]. Formaldehyde is an irritating, reactive and toxic chemical. Inhalation of formaldehyde can irritate the eyes and mucous members, And direct contact with formaldehyde at high concentrations can cause skin irritation and allergic reactions. Furthermore, formaldehyde has been classified as a human carcinogen enlisted in carcinogens category 3 by European 4 Page 4 of 26
Union, and results in cancer of the throat, nose, and blood over a long period exposure [10,11]. The formaldehyde contamination in leather products has attracted worldwide attention in recent years [12]. And these formaldehyde safety issues have already caused serious impacts on international trade and the manufacture of leather products. Hence, many countries have developed standards and regulations to monitor and control strictly the formaldehyde concentration in raw materials and finished products of leather. Within Europe, leather products need to meet the requirements relative to the limit content in formaldehyde (≤ 150 mg/kg) according to the criteria of the European Eco-label for footwear set forth in the Commission Decision 2002/231/EC. In China, Many standards have strict limits for formaldehyde in leather products. The standards used for the limitation of formaldehyde mainly include GB 20400 Leather and fur – Limit of harmful matter, GB 30585 Safety technical specifications for children’s footwear, GB/T 16799 Furniture leather, GB/T 15107 Athletic footwear, QB/T 1284 Coney, QB/T 1873 Shoe upper leather and so on. General, formaldehyde is restricted depending upon the final application and the end consumer of the product. For formaldehyde in leather, the typical acceptable limits in leather are 20 mg/kg for infant products (36 months or under) and 75 mg/kg for adult products with direct contact to skin, and around 300 mg/kg if there is no direct contact to skin. The formaldehyde content of leather products was found to be below these risky limit values. Various approaches are available for the analysis of formaldehyde in leather [13-15]. Sample treatment is crucial for the determination of formaldehyde, and 5 Page 5 of 26
sample preparation procedures and experimental conditions largely determined the extracted amounts of formaldehyde. In addition to the standards, there has been a dramatic increase in the number of government and private testing laboratories in China. These laboratories mainly involve in the analysis of toxic heavy metals and compounds formaldehyde in raw materials and products of leather. A large proportion of these laboratories have been accredited, however, the accuracy, comparability and reliability of the determination procedures and the analytical results have never been assessed using practical techniques. The performance characteristics of different laboratories could be evaluated by interlaboratory comparison exercises or proficiency testing (PT) schemes [16,17]. PT is known to act as a unique external control for organization, performance and evaluation of participant testing on the same or similar items by means of interlaboratory comparisons [18-20]. In this study, a PT scheme for the determination if formaldehyde in leather (CNCA-15-A09) was organized by China National Accreditation Service for Conformity Assessment (CNAS) and China Leather and Footwear Industry Research Institute (CLF). And this PT scheme was designed in accordance with the general requirements of the ISO/IEC 17043 [21]. Moreover, the PT scheme belongs to class A program in China, that is to say, it is mandatory for the government leather quality supervision and inspection center laboratories and the entry-exit inspection and quarantine laboratories. Meanwhile, leather manufacture laboratories, testing laboratories, public research institutes and third-party laboratories, etc. can subscribe for participation in this PT program. The participants were required to test the 6 Page 6 of 26
contents of formaldehyde in leather samples at two levels. The proficiency of the participants was evaluated by statistical analysis of submitted results.
2. Experimental 2.1. Organization of PT The scheme was organized by the Certification and Accreditation Administration of the People’s Republic of China (CNCA), in cooperation with China Leather and Footwear Industry Research Institute (CLF) and Sichuan Entry-exit Inspection and Quarantine Bureau Technology Center (SCTC). The PT coordinator and collaborators were responsible for designing the scheme, preparation of the test materials, distribution of samples and instructions to participating laboratories, and collection and statistical analysis of the data. A total of 162 laboratories from 27 regions of China agreed to participate, including Entry-exit Inspection and Quarantine laboratories, Quality Supervision and Inspection laboratories from national and provinces, and private laboratories. The local distribution in China of participates was shown in Figure 1. Participating laboratories were required to use the official analytical method consistent with their normal routine practice.
2.2. Preparation of test materials In general, the test materials for a PT program should identical to those used in compliance with external and internal tests in order to be sure the representativeness 7 Page 7 of 26
of each test material [22]. However, there is almost no native leather containing formaldehyde at appropriate concentration for a PT scheme. Therefore, the test materials were prepared by adding formaldehyde standard solution to a blank sample during leather tanning process. The production of leather materials for test involved three distinct processes: pretreatment; tanning; and finishing. In the pretreatment process, sheepskins were treated to remove salt and any foreign material by washing, to remove interstitial material to open up the collagen structure by soaking, and to remove wool and excess tissue by mechanical and chemical means. In the tanning process, sheepskins were firstly bated and pickled to create the condition; and then tanning process was carried out to establish and stabilized the basic properties of sheepskins. After that, a retanning program similar to tanning but containing formaldehyde was used to sheepskins. In this phase, formaldehyde was successfully introduced to test materials leather by using of melamine formaldehyde syntans as retanning agents. In the finishing process, dyeing and fat-liquoring and so on were used to make the leather closer to the real product and to attain the test material specifications. The details of preparation procedure of the test materials were summarized in Table 1. After cut into small pieces, aliquots of about 8 g of the as-prepared leather samples were packed into cleaned plastic bags and then vacuum sealed using aluminum foil bags. The organizer provided four test materials with different concentration level of formaldehyde in leather prepared by adding different amounts of formaldehyde standard substances in the tanning process. More than 150 packaged 8 Page 8 of 26
units at each level were prepared and stored at room temperature prior to being dispatched to the participants. These four samples with same matrices labeled as sample code A, B, C and D, were divided into two groups (code A, C and code B, D). These 162 laboratories were also divided into two groups according to the odevity after numbering at random. Samples code A and C were dispatched to 81 participants with odd numbers, and the other samples code B and D were dispatched to the rest 81 laboratories with even numbers. Among these four samples, samples code C and code D as interferences were not included in the statistical analysis of the results.
2.3. Homogeneity and stability of test materials For a PT program, the test materials must fulfill the homogeneity and stability, that is, all the aliquots of the test materials must be homogeneous and being stable during storage and transport to the participating laboratories [23]. All of the test materials underwent homogeneity testing for analyses prior to dispatch. Sample homogeneity was assessed according to the ISO/IEC 17043 and CNAS-RL02 procedures [21, 24]. The contents of formaldehyde in 12 randomly selected subsamples from samples code A and B respectively were analyzed in duplicate using colorimetric analysis methods. To determine the formaldehyde variations, 2.0 g leather from each of the 12 random subsamples were transferred into 100 mL Erlenmeyer flask. Subsequently, 50 mL of the sodium dodecylsulfonate solution were added. These samples were stirred smoothly for 60 min at a temperature of 40 oC, and then the extracted sample solutions were filtered immediately. After 9 Page 9 of 26
cooling down to room temperature, 5 mL extraction solution was mixed with 5 mL reaction solution (prepared by adding 150 g ammonium acetate, 3 mL glacial acetic, and 2 mL acetylacetone in 1000 mL deionized water). The mixed solution was heated in a water bath to 40 oC again and stirred for 30 min in the dark. Finally, the absorbance at 412 nm in spectroscopy was measured to identify the content of formaldehyde in the samples.. The blank absorbance was obtained from a mixture from 5 mL odium dodecylsulfonate solution and 5 mL reaction solution. The results of one-way analysis of variance (ANOVA) confirmed that these samples were sufficiently homogeneous. The stability study of the test materials was conducted at the beginning of PT program, during the storage of the test materials and after the receipt of test results from the participants. And the short-term stability of test materials was also carried out at 4 oC and 45 oC for seven days. The contents of formaldehyde in 3 randomly selected subsamples from samples code A and B respectively were analyzed in duplicate using colorimetric analysis methods. The average concentrations of the analyses in three random samples for duplicate analysis at each sampling interval time were compared with the mean values from the homogeneity test using the t-test. There were no statistically significant differences of these results, and the differences among the test values were no more than 0.3σ. This indicated that these samples were stable enough in the life-time. Homogeneity and stability indicated that the test materials were suitable for the proficiency testing samples.
2.4. Performance statistics 10 Page 10 of 26
In order to aid interpretation, proficiency test results often need to be transformed into a performance statistic. All data received from the participants were evaluated using a standard procedure that allowed direct comparison of the values. Result values were checked by running the Grubbs test and applying the Cochran test to identify the statistical outliers. The outline value was detected and removed from the data set, and these tests were iterated until no further outline value [25]. However, robust statistics are generally used to calculate consensus value without the need for eliminating outliers from the raw data sets, this may weaken the effects of the method on the results and provide a more reliable estimate of the measurement relating to the procedure [24,25]. In this work, robust statistics were used to assign a value and assess the participants’ results. The statistical analysis of the results submitted by the participants was carried out to measure the deviation from the assigned value. The participants were required to perform two independent measurements using their preferred analytical method and to report the mean for each of samples. For each result, the difference between the values was used to evaluate inter-laboratory variation by calculating the robust z-score, based on the median and Normalized Inter-Quartile Range (NIQR) [28, 29]. The robust statistics can routinely cope with near-normal distribution data and gives a perfect estimation of the consensus values. The z-scores are calculated using the following Equation: z = (x - X)/σ where x is the participant’s result, X is the assigned value (estimation of the true value 11 Page 11 of 26
that is used for the purpose of calculating scores), and σ is the standard deviation for PT assessment, defined as a target value for the acceptable deviation from the assigned value. Based on the ISO/IEC 17043 protocol, the participants’ performance was ranked into the following classes: — |z| ≤ 2.0
indicates “satisfactory” performance and generates no signal;
— 2.0 < |z| < 3.0
indicates “questionable” performance and generates a warming
signal; — |z| ≥ 3.0
indicates “unsatisfactory” performance and generates an action
signal.
3. Results and discussions 3.1. Histograms of results The histogram is a common statistical tool to compare one’s result with other participants and to describe the performance of proficiency testing. Histogram is particularly useful in the preliminary analysis stage for proficiency testing schemes. It could be used to check whether the statistical assumptions are reasonable, or if there is an anomaly - such as a bimodal distribution, a large proportion of outliers, or unusual skewness that was not anticipated [30]. The corresponding histograms of samples code A and B were provided using actual results submitted by participants without further calculation, as shown in Figure 2. In preparation for histograms, the scale and plot intervals were determined based on 12 Page 12 of 26
the standard deviation for the proficiency assessment. As can be seen in Figure 2, the results of formaldehyde residue analysis in two leather samples were approximately normally distributed date. So the robust method that can provide good performance was applied to statistical analysis.
3.2 Assigned values Assigned values affect the validity of performance statistics in PT schemes. Various procedures can be used to determine assigned values. As the number of participants is large and the dispersion among the results is small, a common procedure was to calculate the mean or median of the participants as the assigned value after elimination of the outliers. The assigned value was directly from the results of participants has been widely adopted by PT providers due to its statistical validity [31]. In this PT scheme, the test materials samples code A and B were analyzed respectively by 81 different laboratories including organizer and participants. The relevant statistics and data were displayed in Table 2. The homogeneity mean/median ratio values were 0.975 for sample A and 0.969 for sample B. And there was an average value of 0.972, which was close to 1.0. Hence the robust median value could be used as the assigned reference value for this PT scheme.
3.3 Evaluation of laboratory proficiency The data of test material samples code A and B were collected for statistical evaluation. The histograms of results reported values for formaldehyde in the leather samples illustrate that the data sets closely approximated a normal distribution after 13 Page 13 of 26
exclusion of outliers, as shown in Figure 2. And the mean value for each level of the samples is in good agreement with its median value, as listed in Table 2. Hence, the participants’ performances could be evaluated by robust z-scores by using of the median value and NIQR in the PT program. The results of robust statistical parameters are summarized in Table 3, including assigned median values, Normalized Inter-Quartile Range (NIQR), robust coefficient of variation (CV), number of results, number and percentage of satisfactory, questionable and unsatisfactory results. The percentages of satisfactory rates were 79.1% for sample code A and 84.0% for sample code B. As shown in Figure 3, the overall performance of the participants was comprehensively assessed using z-scores. The majority of laboratories performed well with |z| ≤ 2.0. But nine laboratories (code Lab014, Lab095, Lab128, Lab129, Lab135, Lab142, Lab145, Lab153 and Lab159) overestimated their results with extremely positive z-scores. Contrary to this case, another three laboratories (code Lab023, Lab098 and Lab139) underestimated their results with extremely negative z-scores.
3.4 Assessment of formaldehyde stock solution All of these results deviated from the assigned values due to the existence of errors [32]. The most probable reasons were ineffective pretreatment of the test materials and nonstandard operation procedures. The leather contains a lot of complicated compounds that would influence the extraction under some condition, resulting in high variability in the values. And it was also found that there were some calculation errors involving equation in the report. In addition to these reasons, the 14 Page 14 of 26
preparation of formaldehyde stock solution may seriously affect on the final measurement results. A total of 162 laboratories from 27 provinces, municipalities and autonomous regions of China registered for the PT program, and all of them returned their results punctually to the organizers. Results obtained by using of formaldehyde stock solution prepared by reference material were reported from 126 laboratories, 19 laboratories prepared formaldehyde stock solution with calibration by themselves, and other 17 laboratories prepared formaldehyde stock solution by using of commercial formaldehyde solution. The results obtained by the participants were classified to calculate the z-scores and to compare the analytical variances according to the preparation approaches of formaldehyde stock solutions. As can be seen in Table 4, formaldehyde stock solution prepared by reference materials was the most favorable choice for of formaldehyde (84.1%) in test materials. But the satisfactory rate of participants adopted the other approaches was decreased to 73.7% and 70.6% respectively. There were some differences among the results obtained using diverse stock solutions preparation approaches. Therefore, these laboratories with questionable and unsatisfactory results should carry out an in-depth investigation of the pretreatment process, operation procedures and preparation of stock solution and take effective measures to correct and improve the detection level.
4. Conclusions A PT scheme was implemented for formaldehyde residue analysis of leather materials. For each result submitted from participant, the difference between the 15 Page 15 of 26
values was used to evaluate inter-laboratory variation by calculating the robust z-score, based on the median and NIQR. The approximated normal distribution characteristics of the results were observed in the histograms. The mean values were in good accordance with the assigned values after elimination of outliers. The ordered z-score graphs were used to compare different result values and to evaluate the performance of participating laboratories. Performance varied between laboratories. In addition to common important factors of ineffective pretreatment of the test materials and nonstandard operation procedures, the investigation of various preparation approaches of formaldehyde stock solution indicated that reference materials was the most favorable choice for analysis. This PT program is necessary and useful to help the laboratories especially without satisfactory to comply with quality assurance requirements and improve the detection level.
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21 Page 21 of 26
Figure Captions
Figure 1. Geographical distribution of participating laboratories in the proficiency testing program. Figure 2. Histogram of results for the proficiency testing of formaldehyde in (a) sample code A and (b) sample code B. Figure 3. Distribution of the participants’ z-scores for formaldehyde in (a) sample code A and (b) sample code B.
22 Page 22 of 26
Table 1. Procedure for preparation of the test materials. Step
Operation
Note
1
Soaking
20 r/min for 30 min, standing for 24 h
2
Unhairing
Na2S : H2O = 1 : 50 2.0% Ca(OH)2, 15 r/min for 3.5 h, static for
3
Liming overnight
4
Deliming
1.5% (NH4)2SO4, 15 r/min for 40 min 8.0% NaCl, 1.0% H2SO4, 0.3% CH2O2, 15
5
Pickling r/min for 1.5 h 8.0% tanning agent, 15 r/min for 2.0 h,
6
Tanning 1.0% NaAc 15 r/min for 1.5 h
7
Basifying
1.2% NaHCO3, pH ≈ 4.0 to 40 ℃ , 15 r/min for 2.0 h, static for
8
Temperature-rising overnight
9
Sammying
squeeze 50% water retanning
10
Retanning*
agent
containing
1.5%
formaldehyde, 1.0% NaAc 15 r/min for 2.0 h at 35℃
11
Neutralization
1.0% NaHCO3, pH = 6.0 ~ 6.5
12
Filling
3.0% filler
13
Dyeing
3.0% dye, 15 r/min for 1.0 h at 55℃
14
Fatliquoring
6.0% sulfited fish oil, 6.0% NCS, 15 r/min
23 Page 23 of 26
for 1.0 h at 55℃ 15
Fixation
1.0% CH2O2, 15 r/min for 1.0 h at 55℃
16
Softening
physical process to separate the fibers
17
Surface-coating
change the surface to close the real product
18
Packing
vacuum sealing, storage
*Retanning agent is melamine formaldehyde resin, 3.0% and 0.5% for sample code A and B, respectively.
Table 2. Results of statistical analysis of formaldehyde concentration in test materials. Analyte
Formaldehyde in leather
Sample
A
B
Total number of results
81
81
Number of results without outliners
75
73
Median value (mg/kg)
44.0
12.7
Minimum value (mg/kg)
32.7
6.9
Maximum value (mg/kg)
64.8
18.8
Homogeneity mean (mg/kg)
42.9
12.3
Homogeneity mean/median value
0.975
0.969
24 Page 24 of 26
Table 3. Robust statistical analysis of results in the PT scheme. Analyte
Formaldehyde in leather
Sample
A
B
Assigned value(mg/kg)
44.0
12.7
NIQR (mg/kg)
1.77
1.82
Robust CV (%)
4.02
14.3
Number of results
81
81
64 (79.1)
82 (84.0)
7 (8.6)
1 (1.2)
10 (12.3)
12 (14.8)
Number (percentage) of laboratories |z| ≤ 2.0 Number (percentage) of laboratories 2.0 < |z| < 3.0 Number (percentage) of laboratories |z| ≥ 3.0
Table 4. Comparison of results obtained using diverse preparation approaches of formaldehyde stock solution in the PT scheme. Formaldehyde stock solution Statistic results
Total Reference
Calibration
Commercial
material
by participate
solution
Number
106
14
12
132
Percentage (%)
84.1
73.7
70.6
81.5
Number
6
0
2
8
|z| ≤ 2.0
2.0 < |z| < 3.0
25 Page 25 of 26
Percentage (%)
4.8
0.0
11.8
4.9
Number
14
5
3
22
Percentage (%)
11.1
26.3
17.6
13.6
Number
126
19
17
|z| ≥ 3.0
Total
162 Percentage (%)
77.8
11.7
10.5
26 Page 26 of 26
S0165-9936(16)30079-6 http://dx.doi.org/doi: 10.1016/j.trac.2016.06.014 TRAC 14786
To appear in:
Trends in Analytical Chemistry
Please cite this article as: Yang Zhao, Dong Guo, Zhiding Huang, Wenkai Chang, Xiaoxia Qi, Zenghe Li, Proficiency testing for determination of formaldehyde in leather in china, Trends in Analytical Chemistry (2016), http://dx.doi.org/doi: 10.1016/j.trac.2016.06.014. 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.
Proficiency testing for determination of formaldehyde in leather in China Yang Zhao a, b, Dong Guo c, Zhiding Huang b, Wenkai Chang b, Xiaoxia Qi b, Zenghe Li a, * a
Beijing Key Laboratory of Environmentally Harmful Chemical Analysis, College of
Science, Beijing University of Chemical Technology, Beijing, 100029, P. R. China. b
National Footwear Quality Supervision and Inspection Center (Beijing), China
Leather and Footwear Industry Research Institute, Beijing, 100015, P. R. China. c
Certification and Accreditation Administration of the People’s Republic of China,
Beijing, 100088, P. R. China.
* Corresponding author. Tel./fax: +86 10 64337798. E-mail address: [email protected] (Z. Li).
1 Page 1 of 26
Highlights
A new proficiency testing for determination of formaldehyde in leather.
The preparation of test materials was identical to the real tanning of leather.
The assigned values were established using the reported analytical results.
The robust z-scores were calculated to assess the participants’ performance.
The distribution of results from formaldehyde stock solutions was investigated.
2 Page 2 of 26
Abstract Formaldehyde can cause serious health hazards to human body. A proficiency testing scheme (CNCA-15-A09) for the determination of formaldehyde in leather was organized involving 162 laboratories in China. The test materials were prepared by adding melamine formaldehyde syntans to a blank sheepskin during tanning process. The tanning was identical to the real production process to assure the representativeness of test materials. The histograms of results for formaldehyde in the leather samples illustrated that the data sets approximated closely to the normal distribution. The mean value was in good agreement with the median value. Hence the assigned values of test materials for proficiency assessment were derived directly from the reported results of laboratories, and the measurement performance for determining formaldehyde in leather were assessed using robust z-scores. And the distribution characteristics of different preparation approaches for formaldehyde stock solution showed that it could greatly influence the process and results of analysis.
Keywords: Leather; Proficiency testing; Formaldehyde; Interlaboratory comparison; Reference material
3 Page 3 of 26
1. Introduction Leather is a kind of durable and flexible products came from animal or artificial synthesis. The leather industry has a long history, and the various requirements of people have promoted the development of leather manufacture. To achieve specific functions, a variety of synthetic chemical substances are used in the production procedure of leather. Nowadays, more and more countries and people have realized that the leather industry has a dramatic impact on the environment [1,2]. The environmental impacts mainly include two aspects, one is the heavy metal of polluting and poor quality chemicals used in the tanning process, the other is air pollution due to the transformation process of residual chemical reagent [3,4]. In addition to the environmental impacts, the long-term exposure to these leather products with toxic substances would increase the risk of physical damage [5,6]. Among these hazards caused by chemical materials, formaldehyde is a main matter and could not be ignored. Formaldehyde may occur in leather as a result of inadvertent migration and evaporation from products during the process of tanning, finishing and using [7,8]. The formaldehyde can cause serious health hazards such as sensitivity and diverse diseases to workers and consumers [9]. Formaldehyde is an irritating, reactive and toxic chemical. Inhalation of formaldehyde can irritate the eyes and mucous members, And direct contact with formaldehyde at high concentrations can cause skin irritation and allergic reactions. Furthermore, formaldehyde has been classified as a human carcinogen enlisted in carcinogens category 3 by European 4 Page 4 of 26
Union, and results in cancer of the throat, nose, and blood over a long period exposure [10,11]. The formaldehyde contamination in leather products has attracted worldwide attention in recent years [12]. And these formaldehyde safety issues have already caused serious impacts on international trade and the manufacture of leather products. Hence, many countries have developed standards and regulations to monitor and control strictly the formaldehyde concentration in raw materials and finished products of leather. Within Europe, leather products need to meet the requirements relative to the limit content in formaldehyde (≤ 150 mg/kg) according to the criteria of the European Eco-label for footwear set forth in the Commission Decision 2002/231/EC. In China, Many standards have strict limits for formaldehyde in leather products. The standards used for the limitation of formaldehyde mainly include GB 20400 Leather and fur – Limit of harmful matter, GB 30585 Safety technical specifications for children’s footwear, GB/T 16799 Furniture leather, GB/T 15107 Athletic footwear, QB/T 1284 Coney, QB/T 1873 Shoe upper leather and so on. General, formaldehyde is restricted depending upon the final application and the end consumer of the product. For formaldehyde in leather, the typical acceptable limits in leather are 20 mg/kg for infant products (36 months or under) and 75 mg/kg for adult products with direct contact to skin, and around 300 mg/kg if there is no direct contact to skin. The formaldehyde content of leather products was found to be below these risky limit values. Various approaches are available for the analysis of formaldehyde in leather [13-15]. Sample treatment is crucial for the determination of formaldehyde, and 5 Page 5 of 26
sample preparation procedures and experimental conditions largely determined the extracted amounts of formaldehyde. In addition to the standards, there has been a dramatic increase in the number of government and private testing laboratories in China. These laboratories mainly involve in the analysis of toxic heavy metals and compounds formaldehyde in raw materials and products of leather. A large proportion of these laboratories have been accredited, however, the accuracy, comparability and reliability of the determination procedures and the analytical results have never been assessed using practical techniques. The performance characteristics of different laboratories could be evaluated by interlaboratory comparison exercises or proficiency testing (PT) schemes [16,17]. PT is known to act as a unique external control for organization, performance and evaluation of participant testing on the same or similar items by means of interlaboratory comparisons [18-20]. In this study, a PT scheme for the determination if formaldehyde in leather (CNCA-15-A09) was organized by China National Accreditation Service for Conformity Assessment (CNAS) and China Leather and Footwear Industry Research Institute (CLF). And this PT scheme was designed in accordance with the general requirements of the ISO/IEC 17043 [21]. Moreover, the PT scheme belongs to class A program in China, that is to say, it is mandatory for the government leather quality supervision and inspection center laboratories and the entry-exit inspection and quarantine laboratories. Meanwhile, leather manufacture laboratories, testing laboratories, public research institutes and third-party laboratories, etc. can subscribe for participation in this PT program. The participants were required to test the 6 Page 6 of 26
contents of formaldehyde in leather samples at two levels. The proficiency of the participants was evaluated by statistical analysis of submitted results.
2. Experimental 2.1. Organization of PT The scheme was organized by the Certification and Accreditation Administration of the People’s Republic of China (CNCA), in cooperation with China Leather and Footwear Industry Research Institute (CLF) and Sichuan Entry-exit Inspection and Quarantine Bureau Technology Center (SCTC). The PT coordinator and collaborators were responsible for designing the scheme, preparation of the test materials, distribution of samples and instructions to participating laboratories, and collection and statistical analysis of the data. A total of 162 laboratories from 27 regions of China agreed to participate, including Entry-exit Inspection and Quarantine laboratories, Quality Supervision and Inspection laboratories from national and provinces, and private laboratories. The local distribution in China of participates was shown in Figure 1. Participating laboratories were required to use the official analytical method consistent with their normal routine practice.
2.2. Preparation of test materials In general, the test materials for a PT program should identical to those used in compliance with external and internal tests in order to be sure the representativeness 7 Page 7 of 26
of each test material [22]. However, there is almost no native leather containing formaldehyde at appropriate concentration for a PT scheme. Therefore, the test materials were prepared by adding formaldehyde standard solution to a blank sample during leather tanning process. The production of leather materials for test involved three distinct processes: pretreatment; tanning; and finishing. In the pretreatment process, sheepskins were treated to remove salt and any foreign material by washing, to remove interstitial material to open up the collagen structure by soaking, and to remove wool and excess tissue by mechanical and chemical means. In the tanning process, sheepskins were firstly bated and pickled to create the condition; and then tanning process was carried out to establish and stabilized the basic properties of sheepskins. After that, a retanning program similar to tanning but containing formaldehyde was used to sheepskins. In this phase, formaldehyde was successfully introduced to test materials leather by using of melamine formaldehyde syntans as retanning agents. In the finishing process, dyeing and fat-liquoring and so on were used to make the leather closer to the real product and to attain the test material specifications. The details of preparation procedure of the test materials were summarized in Table 1. After cut into small pieces, aliquots of about 8 g of the as-prepared leather samples were packed into cleaned plastic bags and then vacuum sealed using aluminum foil bags. The organizer provided four test materials with different concentration level of formaldehyde in leather prepared by adding different amounts of formaldehyde standard substances in the tanning process. More than 150 packaged 8 Page 8 of 26
units at each level were prepared and stored at room temperature prior to being dispatched to the participants. These four samples with same matrices labeled as sample code A, B, C and D, were divided into two groups (code A, C and code B, D). These 162 laboratories were also divided into two groups according to the odevity after numbering at random. Samples code A and C were dispatched to 81 participants with odd numbers, and the other samples code B and D were dispatched to the rest 81 laboratories with even numbers. Among these four samples, samples code C and code D as interferences were not included in the statistical analysis of the results.
2.3. Homogeneity and stability of test materials For a PT program, the test materials must fulfill the homogeneity and stability, that is, all the aliquots of the test materials must be homogeneous and being stable during storage and transport to the participating laboratories [23]. All of the test materials underwent homogeneity testing for analyses prior to dispatch. Sample homogeneity was assessed according to the ISO/IEC 17043 and CNAS-RL02 procedures [21, 24]. The contents of formaldehyde in 12 randomly selected subsamples from samples code A and B respectively were analyzed in duplicate using colorimetric analysis methods. To determine the formaldehyde variations, 2.0 g leather from each of the 12 random subsamples were transferred into 100 mL Erlenmeyer flask. Subsequently, 50 mL of the sodium dodecylsulfonate solution were added. These samples were stirred smoothly for 60 min at a temperature of 40 oC, and then the extracted sample solutions were filtered immediately. After 9 Page 9 of 26
cooling down to room temperature, 5 mL extraction solution was mixed with 5 mL reaction solution (prepared by adding 150 g ammonium acetate, 3 mL glacial acetic, and 2 mL acetylacetone in 1000 mL deionized water). The mixed solution was heated in a water bath to 40 oC again and stirred for 30 min in the dark. Finally, the absorbance at 412 nm in spectroscopy was measured to identify the content of formaldehyde in the samples.. The blank absorbance was obtained from a mixture from 5 mL odium dodecylsulfonate solution and 5 mL reaction solution. The results of one-way analysis of variance (ANOVA) confirmed that these samples were sufficiently homogeneous. The stability study of the test materials was conducted at the beginning of PT program, during the storage of the test materials and after the receipt of test results from the participants. And the short-term stability of test materials was also carried out at 4 oC and 45 oC for seven days. The contents of formaldehyde in 3 randomly selected subsamples from samples code A and B respectively were analyzed in duplicate using colorimetric analysis methods. The average concentrations of the analyses in three random samples for duplicate analysis at each sampling interval time were compared with the mean values from the homogeneity test using the t-test. There were no statistically significant differences of these results, and the differences among the test values were no more than 0.3σ. This indicated that these samples were stable enough in the life-time. Homogeneity and stability indicated that the test materials were suitable for the proficiency testing samples.
2.4. Performance statistics 10 Page 10 of 26
In order to aid interpretation, proficiency test results often need to be transformed into a performance statistic. All data received from the participants were evaluated using a standard procedure that allowed direct comparison of the values. Result values were checked by running the Grubbs test and applying the Cochran test to identify the statistical outliers. The outline value was detected and removed from the data set, and these tests were iterated until no further outline value [25]. However, robust statistics are generally used to calculate consensus value without the need for eliminating outliers from the raw data sets, this may weaken the effects of the method on the results and provide a more reliable estimate of the measurement relating to the procedure [24,25]. In this work, robust statistics were used to assign a value and assess the participants’ results. The statistical analysis of the results submitted by the participants was carried out to measure the deviation from the assigned value. The participants were required to perform two independent measurements using their preferred analytical method and to report the mean for each of samples. For each result, the difference between the values was used to evaluate inter-laboratory variation by calculating the robust z-score, based on the median and Normalized Inter-Quartile Range (NIQR) [28, 29]. The robust statistics can routinely cope with near-normal distribution data and gives a perfect estimation of the consensus values. The z-scores are calculated using the following Equation: z = (x - X)/σ where x is the participant’s result, X is the assigned value (estimation of the true value 11 Page 11 of 26
that is used for the purpose of calculating scores), and σ is the standard deviation for PT assessment, defined as a target value for the acceptable deviation from the assigned value. Based on the ISO/IEC 17043 protocol, the participants’ performance was ranked into the following classes: — |z| ≤ 2.0
indicates “satisfactory” performance and generates no signal;
— 2.0 < |z| < 3.0
indicates “questionable” performance and generates a warming
signal; — |z| ≥ 3.0
indicates “unsatisfactory” performance and generates an action
signal.
3. Results and discussions 3.1. Histograms of results The histogram is a common statistical tool to compare one’s result with other participants and to describe the performance of proficiency testing. Histogram is particularly useful in the preliminary analysis stage for proficiency testing schemes. It could be used to check whether the statistical assumptions are reasonable, or if there is an anomaly - such as a bimodal distribution, a large proportion of outliers, or unusual skewness that was not anticipated [30]. The corresponding histograms of samples code A and B were provided using actual results submitted by participants without further calculation, as shown in Figure 2. In preparation for histograms, the scale and plot intervals were determined based on 12 Page 12 of 26
the standard deviation for the proficiency assessment. As can be seen in Figure 2, the results of formaldehyde residue analysis in two leather samples were approximately normally distributed date. So the robust method that can provide good performance was applied to statistical analysis.
3.2 Assigned values Assigned values affect the validity of performance statistics in PT schemes. Various procedures can be used to determine assigned values. As the number of participants is large and the dispersion among the results is small, a common procedure was to calculate the mean or median of the participants as the assigned value after elimination of the outliers. The assigned value was directly from the results of participants has been widely adopted by PT providers due to its statistical validity [31]. In this PT scheme, the test materials samples code A and B were analyzed respectively by 81 different laboratories including organizer and participants. The relevant statistics and data were displayed in Table 2. The homogeneity mean/median ratio values were 0.975 for sample A and 0.969 for sample B. And there was an average value of 0.972, which was close to 1.0. Hence the robust median value could be used as the assigned reference value for this PT scheme.
3.3 Evaluation of laboratory proficiency The data of test material samples code A and B were collected for statistical evaluation. The histograms of results reported values for formaldehyde in the leather samples illustrate that the data sets closely approximated a normal distribution after 13 Page 13 of 26
exclusion of outliers, as shown in Figure 2. And the mean value for each level of the samples is in good agreement with its median value, as listed in Table 2. Hence, the participants’ performances could be evaluated by robust z-scores by using of the median value and NIQR in the PT program. The results of robust statistical parameters are summarized in Table 3, including assigned median values, Normalized Inter-Quartile Range (NIQR), robust coefficient of variation (CV), number of results, number and percentage of satisfactory, questionable and unsatisfactory results. The percentages of satisfactory rates were 79.1% for sample code A and 84.0% for sample code B. As shown in Figure 3, the overall performance of the participants was comprehensively assessed using z-scores. The majority of laboratories performed well with |z| ≤ 2.0. But nine laboratories (code Lab014, Lab095, Lab128, Lab129, Lab135, Lab142, Lab145, Lab153 and Lab159) overestimated their results with extremely positive z-scores. Contrary to this case, another three laboratories (code Lab023, Lab098 and Lab139) underestimated their results with extremely negative z-scores.
3.4 Assessment of formaldehyde stock solution All of these results deviated from the assigned values due to the existence of errors [32]. The most probable reasons were ineffective pretreatment of the test materials and nonstandard operation procedures. The leather contains a lot of complicated compounds that would influence the extraction under some condition, resulting in high variability in the values. And it was also found that there were some calculation errors involving equation in the report. In addition to these reasons, the 14 Page 14 of 26
preparation of formaldehyde stock solution may seriously affect on the final measurement results. A total of 162 laboratories from 27 provinces, municipalities and autonomous regions of China registered for the PT program, and all of them returned their results punctually to the organizers. Results obtained by using of formaldehyde stock solution prepared by reference material were reported from 126 laboratories, 19 laboratories prepared formaldehyde stock solution with calibration by themselves, and other 17 laboratories prepared formaldehyde stock solution by using of commercial formaldehyde solution. The results obtained by the participants were classified to calculate the z-scores and to compare the analytical variances according to the preparation approaches of formaldehyde stock solutions. As can be seen in Table 4, formaldehyde stock solution prepared by reference materials was the most favorable choice for of formaldehyde (84.1%) in test materials. But the satisfactory rate of participants adopted the other approaches was decreased to 73.7% and 70.6% respectively. There were some differences among the results obtained using diverse stock solutions preparation approaches. Therefore, these laboratories with questionable and unsatisfactory results should carry out an in-depth investigation of the pretreatment process, operation procedures and preparation of stock solution and take effective measures to correct and improve the detection level.
4. Conclusions A PT scheme was implemented for formaldehyde residue analysis of leather materials. For each result submitted from participant, the difference between the 15 Page 15 of 26
values was used to evaluate inter-laboratory variation by calculating the robust z-score, based on the median and NIQR. The approximated normal distribution characteristics of the results were observed in the histograms. The mean values were in good accordance with the assigned values after elimination of outliers. The ordered z-score graphs were used to compare different result values and to evaluate the performance of participating laboratories. Performance varied between laboratories. In addition to common important factors of ineffective pretreatment of the test materials and nonstandard operation procedures, the investigation of various preparation approaches of formaldehyde stock solution indicated that reference materials was the most favorable choice for analysis. This PT program is necessary and useful to help the laboratories especially without satisfactory to comply with quality assurance requirements and improve the detection level.
16 Page 16 of 26
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21 Page 21 of 26
Figure Captions
Figure 1. Geographical distribution of participating laboratories in the proficiency testing program. Figure 2. Histogram of results for the proficiency testing of formaldehyde in (a) sample code A and (b) sample code B. Figure 3. Distribution of the participants’ z-scores for formaldehyde in (a) sample code A and (b) sample code B.
22 Page 22 of 26
Table 1. Procedure for preparation of the test materials. Step
Operation
Note
1
Soaking
20 r/min for 30 min, standing for 24 h
2
Unhairing
Na2S : H2O = 1 : 50 2.0% Ca(OH)2, 15 r/min for 3.5 h, static for
3
Liming overnight
4
Deliming
1.5% (NH4)2SO4, 15 r/min for 40 min 8.0% NaCl, 1.0% H2SO4, 0.3% CH2O2, 15
5
Pickling r/min for 1.5 h 8.0% tanning agent, 15 r/min for 2.0 h,
6
Tanning 1.0% NaAc 15 r/min for 1.5 h
7
Basifying
1.2% NaHCO3, pH ≈ 4.0 to 40 ℃ , 15 r/min for 2.0 h, static for
8
Temperature-rising overnight
9
Sammying
squeeze 50% water retanning
10
Retanning*
agent
containing
1.5%
formaldehyde, 1.0% NaAc 15 r/min for 2.0 h at 35℃
11
Neutralization
1.0% NaHCO3, pH = 6.0 ~ 6.5
12
Filling
3.0% filler
13
Dyeing
3.0% dye, 15 r/min for 1.0 h at 55℃
14
Fatliquoring
6.0% sulfited fish oil, 6.0% NCS, 15 r/min
23 Page 23 of 26
for 1.0 h at 55℃ 15
Fixation
1.0% CH2O2, 15 r/min for 1.0 h at 55℃
16
Softening
physical process to separate the fibers
17
Surface-coating
change the surface to close the real product
18
Packing
vacuum sealing, storage
*Retanning agent is melamine formaldehyde resin, 3.0% and 0.5% for sample code A and B, respectively.
Table 2. Results of statistical analysis of formaldehyde concentration in test materials. Analyte
Formaldehyde in leather
Sample
A
B
Total number of results
81
81
Number of results without outliners
75
73
Median value (mg/kg)
44.0
12.7
Minimum value (mg/kg)
32.7
6.9
Maximum value (mg/kg)
64.8
18.8
Homogeneity mean (mg/kg)
42.9
12.3
Homogeneity mean/median value
0.975
0.969
24 Page 24 of 26
Table 3. Robust statistical analysis of results in the PT scheme. Analyte
Formaldehyde in leather
Sample
A
B
Assigned value(mg/kg)
44.0
12.7
NIQR (mg/kg)
1.77
1.82
Robust CV (%)
4.02
14.3
Number of results
81
81
64 (79.1)
82 (84.0)
7 (8.6)
1 (1.2)
10 (12.3)
12 (14.8)
Number (percentage) of laboratories |z| ≤ 2.0 Number (percentage) of laboratories 2.0 < |z| < 3.0 Number (percentage) of laboratories |z| ≥ 3.0
Table 4. Comparison of results obtained using diverse preparation approaches of formaldehyde stock solution in the PT scheme. Formaldehyde stock solution Statistic results
Total Reference
Calibration
Commercial
material
by participate
solution
Number
106
14
12
132
Percentage (%)
84.1
73.7
70.6
81.5
Number
6
0
2
8
|z| ≤ 2.0
2.0 < |z| < 3.0
25 Page 25 of 26
Percentage (%)
4.8
0.0
11.8
4.9
Number
14
5
3
22
Percentage (%)
11.1
26.3
17.6
13.6
Number
126
19
17
|z| ≥ 3.0
Total
162 Percentage (%)
77.8
11.7
10.5
26 Page 26 of 26