Gas chromatographic determination of residual monomer content in synthetic resin emulsions - Master's thesis - Dissertation
Abstract: This paper presents a gas chromatography method using an internal standard for determining the residual monomer content in synthetic resin emulsions. The precision and accuracy of the technique were evaluated, confirming its reliability and applicability across various types of synthetic resin emulsion samples. The procedure is straightforward and highly accurate, making it suitable for routine analysis in both laboratories and small to medium enterprises (SMEs).
0. Preface
The introduction of national mandatory standards has made it essential to monitor hazardous substances in coatings available on the market. Unreacted residual monomers in synthetic resins can pose health and environmental risks, necessitating their control. Although instrumental methods like gas chromatography are effective, they often require high investment and complex sample preparation. For SMEs with limited resources, simpler and more cost-effective techniques are needed. In this context, a reliable and efficient method utilizing a small-diameter capillary column with a hydrogen flame ionization detector (FID) and an internal standard approach was introduced. This method offers good separation, high recovery rates (between 94% and 103%), and meets all necessary testing requirements.
1. Experimental Part
1.1 Instruments and Reagents: Electronic balance (with 0.0001 g sensitivity), GC7800 gas chromatograph (with a split device), 1.0 μL micro-injector, 20 mL plastic bottles with rubber stoppers, 1 mL and 2 mL medical syringes, a small-diameter capillary column DB-17HT (0.25 mm × 30 m × 0.15 μm), and an integrator or chromatography workstation. Reagents include vinyl acetate (VAM), methyl methacrylate (MMA), styrene (ST), butyl acrylate (BA), 2-ethylhexyl acrylate (2-EHA), and cyclohexanone as the internal standard, along with acetone and distilled water.
1.2 Principle of Determination: A known amount of internal standard is added to the sample, followed by dilution with acetone (4:1). The diluted solution is injected into the gas chromatograph, where the components are separated based on their retention times. The hydrogen flame ionization detector records the chromatogram, and the residual monomer content is calculated using the internal standard method.
1.3 Determination Conditions: The temperature program includes an initial temperature of 30°C, held for 3 minutes, then increased at 10°C/min to 140°C, followed by a rapid increase at 50°C/min to 260°C for 4 minutes. The injection volume is 0.2 μL, with a split ratio of 45:1. Carrier gases include nitrogen, hydrogen, and air, each with specific pressure settings.
1.4 Relative Correction Factor Determination: Standard solutions are prepared by weighing precise amounts of monomers and the internal standard, followed by mixing with acetone. The relative correction factors (Fi) are calculated using peak areas and masses of the monomers and the internal standard.
1.5 Parallel Deviation and Recovery Test: The relative deviation of the correction factor should be less than 0.05. Recovery tests are conducted to validate the method's accuracy by analyzing spiked samples and calculating recovery rates.
1.6 Sample Analysis: After thorough mixing, a sample of approximately 2 g is weighed, and a small amount of internal standard (cyclohexanone) is added. The mixture is diluted with acetone and analyzed under the same conditions as the calibration.
1.7 Result Calculation: The mass fraction of each residual monomer is calculated using the formula involving the relative correction factor, peak areas, and sample mass. The results are averaged from two parallel measurements.
1.8 Reproducibility: The relative deviation between two measurements is less than 0.5%, indicating high reproducibility of the method.
2. Results and Discussion
(1) The use of a small-diameter, high-temperature capillary column allows for efficient separation of esters, even when dealing with high-boiling-point monomers. Injection volumes must not exceed 0.2 μL, and the split ratio plays a key role in ensuring accurate detection.
(2) The vaporization chamber is equipped with a quartz glass liner filled with treated glass wool to prevent residue from entering the column. Regular replacement of the liner is recommended to maintain performance.
(3) When preparing standard solutions, the mass of each monomer may vary depending on the internal standard response. The goal is to make the peak area ratio close to one for better quantification.
(4) Some samples may form emulsions after dilution with acetone. It is advisable to analyze only the supernatant to avoid interference.
(5) VAM may hydrolyze in the presence of water, so dehydration methods such as hydrazine acetone are used during recovery analysis. However, in the test samples, an aqueous acetone solution (4:1) was used, which helped dissolve certain samples effectively.
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