Study on gas chromatography-mass spectrometrometrography for VOC, water-based coatings

0 Introduction
The detection of volatile organic compounds (VOCs) in water-based coatings is an important item in the mandatory national standard GB 18582-2008 "Limit of harmful substances in interior wall coatings of interior decoration materials" and GB 24408-2009 "Limits of harmful substances in exterior coatings for construction". The standard puts forward limit requirements for VOC content in water-based coatings on the inside and outside walls. During the implementation of the standard, we found that because VOC is volatile, the pre-processing method for extracting VOCs requires a higher
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. Considering the feasibility of laboratory operation and the accuracy and reliability of experimental results, it is necessary to explore the pre-treatment method of VOC in an efficient water-based coating.
voC detection methods are many, commonly used for gas chromatography (GC). In recent years, a joint gas chromatography-mass spectrometrometrometrometrometromety (GC-MS) detection method has emerged. Gas chromatography - Mass spectrometrety can quickly detect the unsealed chromatography peaks through the analysis of mass spectrometography, so as to qualitatively and quantitatively, the method is simple, fast, high separation efficiency, good effect, high sensitivity, reliable results. In this paper, GC-MS technology is used to detect 28 kinds of VOCs commonly found in water-based coatings, and on this basis, the pre-treatment methods of VOC detection are discussed.1 Experimental Part
1.1 Instruments and Reagents
Focus DSQ II Monopole Gas Chromatography - Mass Spectrometry Co., Ltd.; KH-3 00DB type CNC ultrasonic device, Kunshan Terronic Ultrasound Instrument Co., Ltd., magnetic mixer, Jintan City, Jiangsu Province, Sannan Instrument Factory, TG16-WS desktop centrifuge, Shanghai Luxiang Instrument Co., Ltd.
standard substances: methanol, benzene, ethylene glycol, ethylene benzene, inter-xylene, xylene, phthalates (color label, Tianjin Institute of Fine Chemicals); ethylamine, orthanol, ethyl glycol ether, toluene, 1,2- propylene glycol, butyl acetate, 1,3- propylene glycol, ethylene glycol ether acetate, diethylene glycol (color label, Tianjin City) Institute of Chemical Reagents); dimethyl ethanolamine, 2-amino-2- methyl-1-propylene alcohol, glycol methylene acetic acid, ethylene glycol monoclode, diethylene glycol ethyl ether acetate, diethylene glycol monool Butyl ether, 2,2,4- trimethyl-1,3-glycol, diglycol butyl ether acetate (chromatography pure, Tedia);
1.2 instrument analysis conditions
Agilent DB-624 Capillary column 60 m×0.25 mm×1.4 sm, high purity helium (99.999%); Chromatography heating program: sample The mouth temperature is 250 degrees C, the initial temperature is 40 degrees C is maintained at 2 min, the rate of 8 degrees C /min is heated to 230 degrees C, the temperature is maintained at 15 min, the constant flow mode is 2.0 mL/min, and the sample size is 1 μL. The severing in-sample has a severity ratio of 40. Mass spectrometrometer conditions: ion source temperature of 230 degrees C, transmission line temperature of 250 degrees C. Ionization is carried out by electron bombardment ionization (EI), ei ionization energy is 70 eV, scanning mass range (m/z) is 10 to 120 amu (atomic mass units), and the detector is switched off at 7.1 to 8.2 min.
1.3 standard solution configuration
the precise weighing of 10 to 20 mg or so of the standard substance in a 10 mL capacity bottle, with dichloromethane. The standard substance concentrations are: methanol 1.04 mg/mL, ethanol 1.18 mg/mL, isopropyl alcohol 1.48 mg/mL, iso butanol 1.65 mg/mL, ethylene glycol methyl ether 1.35 mg/mL, benzene 1.52 mg/mL, triamide 1.23 mg/mL, orthodol 1.57 mg/mL, ethylene glycol ether 1.39 mg/mL, dimethyl ethanolamine 1.46 mg/mL, toluene 1.56 mg/mL, glycol 2.06 mg/mL, 2- Amino-2- Methyl-1- Propylene alcohol 1.35 mg/mL, 1,2-propylene glycol 1.97 mg/mL, butyl acetate 1.41 mg/mL, ethylene glycol methyl ether acetate 1.9 mg/mL, ethylene 1.62 mg// mL, inter-xylene 1.64 mg/mL, xylene 1.52 mg/mL, phthalates 1.44 mg/mL, 1,3-propylene glycol 1.91 mg/mL, ethylene glycol ether acetate 1.55 mg/mL, Ethylene glycol monotylene 1.42 mg/mL, diglycol 2.05 mg/mL, glycol ethyl ether acetate 1.33 mg/mL, diglycol monotyl 1.05 49 mg/mL, 2,2,4- trimethyl-1,3-glycol 1.43 mg/mL, diglycol butyl ether acetate 1.44 mg/mL.2 Results and Discussion
2.1 Extraction
VOC extraction methods usually include stirring extraction, shock extraction, ultrasonic extraction, top-air solid phase micro-extraction, blow-sweeping capture, etc. Due to the low boiling point of VOC, if the use of more intense extraction methods, it is likely to make VOC excessive volatility, resulting in low detection results. Therefore, when choosing the extraction method, we should adopt a more moderate and efficient method. For the coating sample, two methods of stirring extraction and ultrasonic extraction were selected for investigation.
2.1.1 Stirring Extraction method
is the simplest method of various extraction methods. Because the method is low cost and easy to operate, it is widely used in the extraction of various volatile organic compounds. The paint sample, which is accurately described as 2 g stirred well, is added to a 30 mL sealing bottle, 20 mL dichloromethane solvent is removed and stirred on a magnetic mixer. Stirring time has a great impact on the extraction effect, if the stirring time is too short, it is not possible to fully extract the VOC in the paint sample, and the stirring time is too long, it will lead to VOC volatilization and loss. Therefore, it is necessary to investigate the effect of stirring time factors on the efficiency of stirring extraction. Experiment with blank labeled paint samples, stirring 20 min, 30 min and 40 min, respectively, to calculate the recovery rate. The experimental process observed that after stirring 10 min, the paint samples were evenly dispersed in the dichloromethane solvent. As the stirring time increases, the sample is dispersed more evenly and the sample recovery rate is shown in Figure 1. When stirring 40 min, the mark-up recovery rate of 28 VOCs reached 94% to 116%, and all kinds of subjects to be tested were able to achieve better extraction results, indicating that 40 min was a more suitable time for stirring extraction. Try to extend the stirring extraction time, but there is no significant improvement in the extraction efficiency, but caused the loss of the object to be measured, so preferably 40 min for the stirring extraction time.
The effect of stirring extraction time on the recovery rate of 28 VOCs
2.1.2 ultrasonic extraction method
ultrasonic extraction method is one of the most commonly used organic compound extraction methods in the laboratory. Therefore, ultrasonic extraction method is also used as an important extraction method when selecting VOC extraction method. A coating sample with a precise 2 g mixing is added to a 30 mL sealed bottle, a 20 mL dichloromethane solvent is removed, and an ultrasonic extraction is selected at a frequency of 100 Hz. Similar to the stirring extraction method, the factors of ultrasonic extraction time have a great influence on the extraction effect. Experiments were conducted on blank labeled paint samples with ultrasounds of 10 min, 20 min, 30 min and 40 min, respectively, to calculate the mark-up recovery rate of VOC, as shown in Figure 2. As can be seen from Figure 2, when the extraction time is short, such as 10 min, the recovery rate of small molecular compounds, such as methanol, ethanol, etc., is relatively high. When the extraction time increased to 20 min, all 28 VOCs showed a high recovery rate, ranging from 73% to 116%. As the extraction time continues to increase, the recovery rate of VOC began to gradually decrease, when ultrasound extraction of 40 in, large molecular compounds, such as diethylene glycol monoclode, 2,2,4- trimethyl-1, 3-glycol, diethyl glycol, dibiphenyl butyl ether acetate and other losses are very serious, 28 VOCs recovery rate of only 42% to 98%. This is due to the loss caused by the volatility of the VOC in the coating sample as the ultrasonic extraction time increases. Obviously, the use of 100 Hz frequency and 20 min time is a better condition for ultrasonic extraction VOC.
Effect of ultrasonic extraction time on the recovery rate of 28 VOCs
Because dichloromethane is an oil-soluble organic solvent, and water-based coatings are water-soluble materials, and because the coating is very viscous and extended, it is easy to reunite in the ultrasonic extraction process, the coating can not be well in full contact with dichloromethane, resulting in reduced VOC extraction efficiency. The stirring extraction can disperse the water-based coating into fine particles, further increasing the surface area of the coating, so that it is fully in contact with the extraction solvent, to ensure that the target compound can be fully dissolved. From the experimental results, stirring extraction method is more likely to obtain high VOC recovery rate than ultrasonic extraction method, i.e. with higher extraction efficiency.
2.2 Purifying the solution after
extraction requires a static 0.5 h to allow the coating particles to settle as much as possible, and the coating particles need to be completely separated from the extraction solvent by centrifugation. The centrifugation of the upper solution after the sit-down is carried out using 6,000 r/min, 5 min, 9,000 r/min, 5 min, 6,000 r/min, 10 min, and 9,000 r/min, 10 min, which are tested at different speeds and centrifugal times. The results showed that when 6,000 r/min and 5 min condition centrifuges were selected, some paint samples were still suspended in the solution and could not form a liquid; Only after 10 min away from the
heart at a speed of 9,000 r/min can the extraction fluid form a complete solid separation to extract the clarified dichloromethane extract.
3 sample determination
the purchase of commercially available water-based coatings, reference to the above-mentioned optimized pre-treatment method for VOC content determination. Alcohol substances such as methanol, glycol, propylene glycol, 2,2,4- trimethylene-1, 3-glycol, etc. were detected in larger quantities, even more than 1,000 mg/L, in the paint samples purchased. In addition, ethers, esters, such as ethylene glycol methyl ether, butyl acetate, ethylene glycol monoclode, diethylene glycol butyl ether acetate and benzene products have also been detected. Figure 3 is a SIM spectrogram of a batch of water-based real stone paint samples, the main detections are methanol, 1,2-propylene glycol and 2,2,4-trimethyl-1,3-glycol.
VOC detection SIM spectrometry
4 conclusion of true stone paint samples
using gas chromatography-mass spectrometry to study the two commonly used water-based coating VOC extraction methods, such as stirring extraction method and ultrasonic extraction method, through comparison found that: stirring extraction method can make the paint sample and extraction solvent more fully contact, with higher extraction efficiency. Further,
condition optimization of centrifugation steps to obtain efficient and reliable pre-treatment methods. The sample was tested by optimized method, and the results were satisfactory, which showed that it was suitable for the detection of 28 VOC contents in the coating.