Automatic Pretreatment Methods for Determination of Total Organic Carbon in Sedimentary Rocks

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.

To improve the effect and efficiency of sample preparation for TOC test.

(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.

(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.

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.