Automatic Pretreatment Methods for Determination of Total Organic Carbon in Sedimentary Rocks
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摘要:
总有机碳(TOC)是衡量烃源岩有机质丰度的首要指标，实现其高效准确测定具有重要的现实意义。在TOC整体测试流程中，用稀盐酸去除无机碳的样品预处理过程目前主要采用手动法进行，存在周期长、氯化物残留等问题，成为制约整体测试效率的首要因素。学者们研究了溶样时间、溶样温度、离心洗样等对TOC测定值的影响，但尚未形成系统的预处理方法，预处理效率也未得到实质性改善。本文利用自主研制的有机碳自动预处理仪建立了两种自动预处理方法；根据国家标准 《沉积岩中总有机碳的测定》(GB/T 19145—2022)规定的预处理流程，采用多种岩性、不同TOC水平的国家标准物质和质量控制样品对这两种预处理方法进行了方法验证；并进一步分析了自动预处理方法的优势。结果表明，两种自动预处理的方法回收率总体为96.23%～102.12%，相对标准偏差为0.37%～3.23%，满足标准规定的数据准确性、重复性和再现性要求，数据质量较手动法得到进一步提升；自动预处理法的预处理时长大幅缩短至4～6h/批，提高了测试效率；氯离子活度对洗样次数的变化更为敏感，且可监测并有效降低样品中氯化物的残留量，建议作为洗样终点的定量监测指标。建立的两种自动预处理方法可替代手动法进行TOC测试的样品预处理，在保障数据质量和测试效率的前提下又可解放人力，使总有机碳的测试能力得以显著提升。
Abstract:BACKGROUND Total organic carbon is the primary indicator to measure the abundance of organic matter in source rocks. It is of great practical significance to achieve the measurement of TOC accurately and efficiently. In the whole process of determination of TOC, a manual pretreatment method was usually used to remove the inorganic carbon with hydrochloric acid, which has become the primary factor restricting the overall testing efficiency because of its long pretreatment cycles and chloride residues. The effects of parameters such as dissolution time had been studied, however, the systematic pretreatment methods had not yet been developed and the efficiency of pretreatment had not been substantially improved.
OBJECTIVES To improve the effect and efficiency of sample preparation for TOC test.
METHODS (1) To establish automatic pretreatment methods, a set of parameters were established. Dosage, rate, and interval of liquid addition were introduced to ensure stable operation, while reaction period and the activity of chloride ion were introduced to control the direction of the program automatically. (2) According to the pretreatment process specified in GB/T 19145—2022, the two pretreatment methods were validated using national reference materials and quality control samples with various lithologies and TOC levels. (3) Further comparison of two automatic pretreatment methods and a traditional manual method were conducted. Then, the activity of chloride ion was suggested to be a quantitative monitoring indicator for the end point of rinsing samples.
RESULTS (1) The established automatic pretreatment methods were verified to be reliable and effective. The test data showed that the overall recovery of the two automatic pretreatment methods was 96.23%-102.12%, and the relative standard deviation was 0.37%-3.23%. Both of the automatic pretreatment methods met the quality control requirements of data accuracy, repeatability and reproducibility. (2) The automatic pretreatment period was significantly reduced to 4-6h per batch compared with 22-36h of the manual method. This could be attributed to two factors, one was the shortened single rinsing cycle because of the local negative pressure around each crucible, the other was the faster approach to the target value for both pH and a(Cl−) in automatic pretreatment methods (Fig.4). (3) The activity of chloride ion a(Cl−) was introduced to be a quantitative monitoring indicator for the end point of rinsing samples, since it was not only more sensitive to changes in rinsed times than pH, but also better at monitoring the content of soluble chloride in samples and reducing the negative impact of residual chloride effectively.
CONCLUSIONS The two established automatic pretreatment methods could be replaced from the manual method for sample preparation in TOC test owning to better data quality and higher test efficiency. The activity of chloride ion was suggested to be a quantitative monitoring indicator for the end point of rinsing samples.
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表 1 实验样品
Table 1. Selected samples for comparison experiment.
样品编号 TOC含量
(%)岩性 样品类型 GBW(E)00314 0.53±0.06 黑色页岩 国家标准物质 GBW(E)00317 4.72±0.26 黑色页岩 国家标准物质 GBW(E)00318 6.61±0.34 泥岩 国家标准物质 GBW(E)00320 1.87±0.10 灰岩 国家标准物质 QC01 1.40±0.14 硅质泥页岩 质量控制样品 QC38 8.13±0.47 碳质泥页岩 质量控制样品 QC18 18.30±1.23 油页岩 质量控制样品 QC14 63.95±5.28 煤 质量控制样品 注：国家标准物质的TOC用“标准值及不确定度”表示；质量控制样品的TOC为参考值。
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表 2 自动预处理方法的TOC准确度评价满足国家标准要求
Table 2. Accuracy of tested TOC by automatic pretreatment method meeting the quality control requirements in GB/T 19145—2022
样品
编号岩性 TOC标准值及不确定度
(%)自动法A 自动法B TOC测试值
(%)TOC平均值
(%)方法回收率
(%)TOC测试值
(%)TOC平均值
(%)方法回收率
(%)GBW(E)
070314页岩 0.53±0.06 0.54 0.54 98.11～101.89 0.51 0.52 96.23～100 0.54 0.53 0.52 0.51 0.54 0.51 GBW(E)
070317页岩 4.72±0.26 4.74 4.74 100.21～100.85 4.69 4.75 99.36～101.27 4.73 4.76 4.76 4.78 4.74 4.75 GBW(E)
070318泥岩 6.61±0.34 6.56 6.67 99.24～102.12 6.62 6.58 98.18～100.15 6.72 6.62 6.63 6.49 6.75 6.60 GBW(E)
070320灰岩 1.87±0.10 1.86 1.87 98.93～101.07 1.85 1.84 97.86～98.93 1.89 1.83 1.87 1.83 1.85 1.84
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表 3 自动预处理方法的TOC重复性满足国家标准要求
Table 3. Repetitiveness of tested TOC by automatic pretreatment method meeting the quality control requirements in GB/T 19145—2022
样品编号 岩性 TOC含量
参考值
(%)自动法A测试的TOC含量(%) 自动法B 测试的TOC含量(%) 重复性限
(%)第1组 第2组 均值X1 均值X2 TOC D95 第1组 第2组 均值X1 均值X2 TOC D95 QC01 硅质
泥页岩1.40±0.14 1.38 1.41 1.36 1.40 0.04 1.35 1.31 1.33 1.37 0.04 ≤0.07 1.37 1.39 1.31 1.39 1.36 1.39 1.31 1.38 1.32 1.40 1.35 1.38 QC38 碳质
泥页岩8.13±0.47 8.06 8.05 8.14 8.09 0.05 8.03 8.04 8.03 8.03 0.00 ≤0.22 8.19 8.10 8.05 7.98 8.15 8.13 8.01 8.02 8.14 8.09 8.02 8.08 QC18 油页岩 18.30±1.23 18.31 18.09 18.29 18.13 0.16 18.00 18.13 18.03 18.11 0.08 ≤0.38 18.30 18.02 17.94 18.19 18.33 18.15 18.02 18.11 18.21 18.27 18.16 18.03 QC14 煤 63.95±5.28 64.50 64.17 63.92 64.88 0.96 65.88 65.35 64.99 65.28 0.28 ≤1.26 61.76 65.26 64.30 63.85 64.86 65.26 63.41 65.54 64.56 64.84 66.39 66.38
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表 4 自动预处理方法的TOC再现性满足国家标准要求
Table 4. Reproducibility of tested TOC by automatic pretreatment method meeting the quality control requirements in GB/T 19145—2022
标准物质编号 岩性 TOC含量标准值
及不确定度(%)自动法A测试的TOC含量(%) 自动法B测试的TOC含量(%) 再现性限
(%)第1组 第2组 均值Y1 均值Y2 TOC D95 第1组 第2组 均值Y1 均值Y2 TOC D95 GBW(E)070314 页岩 0.53±0.06 0.54 0.53 0.54 0.54 0.00 0.51 0.53 0.52 0.50 0.02 ≤0.15 0.54 0.54 0.53 0.50 0.52 0.55 0.51 0.48 0.54 0.53 0.51 0.50 GBW(E)070317 页岩 4.72±0.26 4.74 4.73 4.74 4.75 0.01 4.69 4.64 4.75 4.70 0.05 ≤0.61 4.73 4.76 4.76 4.65 4.76 4.77 4.78 4.78 4.74 4.72 4.75 4.73 GBW(E)070318 泥岩 6.61±0.34 6.56 6.75 6.67 6.74 0.07 6.62 6.63 6.58 6.55 0.03 ≤1.18 6.72 6.68 6.62 6.44 6.63 6.75 6.49 6.51 6.75 6.77 6.60 6.61 GBW(E)070320 灰岩 1.87±0.10 1.86 1.80 1.87 1.81 0.06 1.85 1.82 1.84 1.83 0.01 ≤0.27 1.89 1.81 1.83 1.83 1.87 1.81 1.83 1.81 1.85 1.83 1.84 1.85
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