A Brief Discussion on the Detection of Short-chain Chlorinated Paraffins in Plastic Track Surfaces

Chlorinated paraffins (CPs), also known as polychlorinated n-alkanes (PCAs), have the chemical formula CnH2n+2-mClm. They are a group of artificially synthesized chlorinated derivatives of straight-chain n-alkanes, with a carbon chain length (n) ranging from 10 to 38 carbon atoms and a chlorine content typically ranging from 30% to 70% by mass.

Chemical Structure Diagram of Chlorinated Paraffins (CnH2n+2-mClm)

Chemical Structure Diagram of Chlorinated Paraffins (CnH2n+2-mClm)

At room temperature, in addition to the 70% chlorinated paraffin is a white solid, the rest of the chlorinated paraffin is colorless or light yellow liquid. Chlorinated paraffins are generally categorized into three classes based on the carbon chain length: short-chain chlorinated paraffins (SCCPs) with carbon chain lengths of 10 to 13 carbon atoms, medium-chain chlorinated paraffins (MCCPs) with carbon chain lengths of 14 to 17 carbon atoms, and typical long-chain chlorinated paraffins (LCCPs) with carbon chain lengths of 20 to 30 carbon atoms.

In the industrial sector, chlorinated paraffins are commonly utilized as flame retardants and auxiliary plasticizers in the preparation of various polymer materials. They are also employed as additives in the preparation of plastic track surfaces. Currently, medium to long-chain chlorinated paraffins are primarily used in the production of plastic tracks. However, due to process influences, improper handling of medium to long-chain chlorinated paraffins often results in trace amounts of short-chain chlorinated paraffins remaining.

Short-chain chlorinated paraffins (SCCPs) are a group of derivatives formed by chlorination reactions on straight-chain normal alkanes, the length of carbon chain ranging from 10 to 13 carbon atoms and a chlorine content typically ranging from 30% to 70% (by mass). According to the European Chemicals Agency’s Chemical Substances Information System (ESIS), SCCPs (C10~C13) are classified as category 3 carcinogens (R40) and may cause long-term adverse effects in the skin upon prolonged exposure (R66). They are considered as a class of new compounds with PBT characteristics (persistent, bioaccumulative, and toxic substances).

Currently, the national standard adopts gas chromatography-electron capture negative ionization mass spectrometry (GC-ECNI-MS) for the detection of SCCPs in plastic track surfaces. However, this method tends to encounter peak overlap and interference in the final calculation results when analyzing samples containing chlorinated paraffins with different chain lengths.

The determination of SCCPs by carbon skeleton gas chromatography involves catalytically dehydrochlorinating SCCPs to straight-chain alkanes under high-temperature conditions for analysis. The reaction is represented as follows:

reaction

This method offers significant advantages in determining mixtures of chlorinated paraffins with different chain lengths, effectively resolving mutual interference during the detection of chlorinated paraffins with different chain lengths and avoiding false positives. This paper will introduce a carbon skeleton-gas chromatography method for detecting SCCPs in finished plastic track surfaces, providing an effective auxiliary method to eliminate interference in SCCPs detection on plastic tracks.

Experiment Section

1.1 Reagents and Instruments

Agilent 7890A gas chromatograph equipped with a flame ionization detector (FID). Standards of straight-chain alkanes: C10, C11, C12, C13, and standards of SCCPs: 1,2,4-trimethylbenzene.

Palladium chloride catalyst and carbon skeleton reaction lining: Prepared according to SN/T 2570-2010.

1.2 Sample Pretreatment

According to the pre-treatment methods outlined in Appendix G 5.1~5.2 of GB 36246-2018, the samples are subjected to pre-treatment and obtain the test solution.

1.3 Gas Chromatography Conditions

DB-1701 capillary gas chromatography column (30m × 0.25m × 0.25μm); carrier gas: high-purity hydrogen gas (purity 99.999%), flow rate 2 mL/min; FID detector temperature 300 ℃; injection port temperature 275 ℃; hydrogen gas flow rate for combustion 30 mL/min; air flow rate for support combustion 300 mL/min; splitless injection, injection volume 1 μL; column temperature program: initial temperature 50 ℃, ramped at a rate of 10 ℃/min to 240 ℃, held for 4 min.

1.4 Calculation of Short-chain Chlorinated Paraffins Content and Catalytic Efficiency

Refer to Chapter 7 of SN/T 2570-2010 for calculating the relevant content and lining catalytic efficiency.

Study on Catalytic Performance

2.1 Temperature of Injection Port

Temperature is an important parameter affecting the catalytic efficiency of palladium chloride. In carbon skeleton gas chromatography, the catalyst is placed in the reaction lining, and catalysis is achieved by raising the temperature of the injection port. Therefore, selecting the appropriate injection port temperature is crucial for the reaction efficiency. The results indicate that with increasing injection port temperature, the catalytic hydrogenation efficiency first increases and then gradually decreases. The highest catalytic hydrogenation efficiency is achieved at around 275 ℃, reaching approximately 88.3%.

2.2 Capability and Stability of Catalytic Hydrogenation

Different concentrations of SCCPs solutions (ranging from 20 μg/mL to 100 μg/mL) were prepared for catalytic hydrogenation experiments. The results showed that the catalytic efficiency of the reaction lining ranged from 84.3% to 87.6%, indicating the good stability of the carbon skeleton gas chromatography method for SCCPs determination. Using a standard solution of SCCPs with a concentration of 40 μg/mL, 100 consecutive catalytic experiments were conducted. The results revealed that the catalytic efficiency of the catalyst could still be maintained above 85%. It can be seen that within the specified number of uses, the catalytic effect of the reaction lining remains good.

Spiking Recovery Rate and Precision Test

Two samples, one of finished plastic track surfaces and one of raw materials, were selected, and each was prepared into spiked samples containing SCCPs at three different concentration levels. Recovery and precision tests were conducted. The average recovery rates and relative standard deviations are shown in the table below. It can be observed that the spike recovery rates ranged from 82.4% to 97.2%, with relative standard deviations of 3.4% to 4.9%.

Table 1: Spike Recovery Rate and Precision of the Method (n=6)

Concentration Addition (g/kg)

Finished Product

Raw Materials

Average Recovery Rate /%

RSD /%

Average Recovery Rate /%

RSD /%

20

87.6

4.2

82.4

4.9

50

90.1

3.6

88.6

4.1

100

97.2

3.4

93.6

3.6

Analysis of Actual Sample and Method Comparison

A sample of a known SCCPs content-containing track material was selected for detection using gas chromatography-mass spectrometry (GC-MS) and the method established in this study. The standard chromatogram of the obtained sample is shown in Figure a. From the graph, it can be observed that when using GC-MS for detection, the spectrum is prone to interference from other chain length chlorinated paraffins, affecting the quantitative results. However, when using the carbon skeleton-gas chromatography method (Figure b), the spectra are clear and visible for each alkane peak after reduction to straight-chain alkanes. Particularly, for chlorinated paraffins with different chain lengths, effective separation can be achieved, avoiding mutual interference.

Figure Chromatograms of SCCPs in Plastic Track Samples Determined by Different
Figure: Chromatograms of SCCPs in Plastic Track Samples Determined by Different Methods

Figure: Chromatograms of SCCPs in Plastic Track Samples Determined by Different Methods

(a. Gas Chromatography-Mass Spectrometry; b. Carbon Skeleton Gas Chromatography)

Conclusion

This study established a method for determining the content of short-chain chlorinated paraffins (SCCPs) in plastic track surfaces using carbon skeleton gas chromatography. The method effectively addresses the issue of mutual interference between chlorinated paraffins with different chain lengths in the samples. Moreover, the method has low detection costs and can be widely applied in different laboratories. It provides an effective auxiliary method for excluding interference in SCCPs detection on plastic tracks.

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