Accurate Quantitation of the Phenyl Group in Methylphenylsilicone Oils by GPC-UV

Phenyl content is a crucial quality indicator for phenyl silicone oils, but the current 1H NMR method for determining it is still unreliable because of the possibility of interference from methylphenylcyclosiloxane or solvents such as toluene and xylene. Here, a novel method for the determination of the phenyl content in phenyl silicone oils for has been developed using gel permeation chromatography–ultraviolet detection (GPC-UV) analysis. Under optimized parameters, the standard curve has been established in the linear range of 5–1000 μg mL−1 with a squared correlation coefficient of more than 0.999, and a quantification limit of 0.972 μg mL−1 has been obtained. Potential interference-causing substances, such as methylphenylcyclosiloxane, toluene or xylene, and various silicone oils can be readily ruled out using the GPC-UV method. The phenyl content in 9 samples of phenyl silicone oil that were readily available was determined with a recovery in the range of 84.63–106.74%. The amount (0.613 mg g−1) of phenyl in sample H1 by GPC-UV was in good agreement with that (0.603 mg g−1) by the 1H NMR analysis.


Introduction
Phenyl silicone oil refers to a class of methyl silicone oil products in which part of the methyl group is replaced by the phenyl group (Scheme 1) [1,2]. Phenyl silicone oil is widely used as high-and low-temperature lubricants for electronics and electrical appliances [3][4][5], lubricants for various bearings, and matrix raw materials for personal care products and other high-end fields, because of its unique chemical structure, which gives its superior performances such as a high refractive index, radiation resistance, and excellent lubricity [6][7][8][9]. Of course, varied phenyl contents result in different properties and uses [10,11]. For further study on the structure-function relationship and product quality control, it is of great significance to determine the phenyl content in phenyl silicone oil [12].
Proton nuclear magnetic resonance ( 1 H NMR) is a commonly used method to determine the content of phenyl in phenyl silicone oil [13]. Nevertheless, the common CDCl 3 solvent contains a minor amount of CHCl 3 , which produces a signal of δ7.26 in 1 H NMR and thus interferes with the determination of the phenyl content in phenyl silicone oil. The NMR apparatus, on the other hand, is expensive to purchase and maintenance. It also is not good for quantitative analysis due to its relatively low sensitivity and narrow linear range. Additionally, the determination of the phenyl content in phenyl silicone oil will be hampered by the fact that commercially available phenyl silicone oil products frequently contain methylphenylcyclosiloxane (the raw material) and exist in toluene or xylene solutions, which will interfere with the determination of phenyl content in phenyl silicone oil. As a result, it is necessary to develop a new analytical method for the simple and accurate determination of the phenyl content in phenyl silicone oils.
Gel permeation chromatography (GPC), which uses cross-linked polystyrene gel particles with a controllable pore size as the stationary phase, separates samples according to the hydrodynamic volume and to determine the relative molecular mass and relative molecular mass distribution of polymers in samples [14][15][16]. Differential refractive index detectors, a typical detector in classical GPC instruments, have low sensitivity and a narrow linear range in quantitative analysis [17][18][19]. The UV absorption detector, however, is an ideal detector for quantitative analysis, with relatively high sensitivity, superior stability, a wide detection range, and good selectivity [20,21]. In addition, methylphenylsilicone oil is such a UV-sensitive polymer, and it is preferable to utilize an ultraviolet detector. Combination of UV with GPC has been used for quantifying substances and locating certain groups in polymers [22][23][24][25][26][27]. In 2005, Lai et al. [28] developed a new method to determine the degree of incorporation of surfactant in polystyrene latex particles and to quantitatively determine the degree of adhesion by using GPC-UV. Mork et al. [29] developed a GPC-UV method to rapidly determine the phenolic terminal in bisphenol A polycarbonate and analyzed the molecular weight of high-capacity PC samples.
This work aimed to establish an GPC-UV method for rapidly and selectively determining the phenyl content in phenyl silicone oils. All potential interference factors were investigated. In addition, the phenyl content was in several commercially available phenyl silicone oils.

Chemicals and Reagents
Deuterated chloroform (Sarn Chemical Technology Shanghai Co., Ltd.), 1,4-dioxane (Beijing OKA Biotechnology Co., Ltd.), tetrahydrofuran (THF, HPLC grade, ≥ 98% Sigma-Aldrich), toluene (Sigma-Aldrich), and xylene (Sigma-Aldrich) were obtained from commercial sources and used without further purification. Methylphenylcyclosiloxane, methyl silicone oil, hydroxy silicone oil, vinyl silicone oil, and methylphenylsilicone oils of H-1, H-2, and H-3 were purchased from Qingdao Fenghong Chemical Co., Ltd. Methylphenylsilicone oils of online shopping-1 and online shopping-2 were purchased online. Methylphenylsilicone oil standards, methylphenylsilicone oils of H-4, H-5, H-6, and H-7, were self-made in the laboratory as follows: methylphenyldimethoxysilane and octamethylcyclotetrasiloxane were stirred and mixed thoroughly. When their temperature reached 60 °C, NaOH solution was added to the mixture to catalyze the polymerization reaction for about 3 h. Then, the resulting methylphenylsilicone oil was produced by distilling the combination under reduced pressure at a temperature of 100-105 °C to obtain.

Instrumental Analysis
Proton nuclear magnetic resonance ( 1 H NMR) analysis was carried out on an Avance-400 NMR instrument (Bruker AG, Germany). The analyte and the internal standard, 1,4-dioxane, were dissolved in CDCl3, and the solution was measured at room temperature.
UV analysis was carried out on UV-1200 (Thermofisher, USA). The analyte was dissolved in CHCl 3 , and the solution was measured in the wavelength range of 200-800 nm at room temperature.
All GPC-UV experiments were carried out using an Agilent 1100 series HPLC (Agilent, USA) with a FMH-1441-KONU gel permeation column (300 mm × 7.8 mm, Guangzhou Philomen Scientific Instrument Co. Ltd.) for separation. The analyte was dissolved in THF, toluene, and xylene, respectively. The analytical conditions were as follows: the mobile phase is THF, the flow rate was 0.8 mL min −1 , the detection wavelength was 265 nm, and the sample volume was 25 μL.

H NMR Analysis of the Phenyl Silicone Oil Standard
A silicone oil with high phenyl content was selected as the phenyl silicone oil standard, and its phenyl content was measured by 1 H NMR. Figure 1 shows the 1 H NMR spectrum of a CDCl 3 solution of 38.1 mg of phenyl silicone oil standard and 12.3 mg of 1,4-dioxane. In the 1 H NMR spectrum, two peaks at a low field of 7.2-7.6 ppm correspond to the protons on the phenyl ring, a signal at 3.64 ppm to methylenyl protons in the structure of 1,4-dioxane, and that at about 0.0 ppm to methyl proton in phenyl silicone oil. Thus, the phenyl content in the phenyl silicone oil standard was calculated to be 2.53 mmol g −1 , according to the peak area (5.22) of the phenyl proton and that (16.11) of the methylenyl proton.

Method Development for the Determination of the Phenyl Group
Toluene and THF (THF) are the common eluents for GPC analysis, and THF was chosen in this work because of its Scheme 1 Structure of phenyl silicone oil comparative low UV absorption. The UV absorption curve of the phenyl silicone oil standard solution was obtained in the wavelength range of 200-400 nm (the Supplementary  Fig. S1). As can be seen, a maximum absorption at 265 nm was observed, and thus this wavelength was chosen as the detection wavelength in the subsequent experiments.
A series of phenyl silicone oil standard solutions were subjected to GPC-UV analysis in the above-described optimum conditions at the mass concentrations of 5, 10, 50, 100, 500, and 1000 μg mL −1 (corresponding to phenyl concentrations of 0.972, 1.94, 9.72, 19.44, 97.2, and 194.4 μg mL −1 , respectively). As shown in Fig. 2, a broad peak at t R 7.50 min, corresponding to the polymer of phenyl silicone oil, was observed in the GPC-UV chromatography.
A calibration curve was constructed by plotting peak area vs. concentration (the Supplementary Fig. S2). Good linearity was achieved in the range studied with a correlation coefficient (R 2 ) at 0.9979. The limit of detection (LOD, S/N = 3) was measured to be 0.778 μg mL −1 , and the limit of quantification (LOQ, S/N = 10) was 0.972 μg mL −1 . Intraday reproducibility (RSD, n = 3) was determined to be 0.6%, and the interday reproducibility (RSD, n = 3) was 3.5%, indicating a good repeatability of the method.

Evaluation of the Anti-interference Performance of the Method
High selectivity is a key factor in a good analytical method. Potential interference factors for the determination of the phenyl group include the raw material (methylphenylcyclosiloxane), toluene solvent, and other types of silicone oil. Herein, the anti-interference performance of the established method is examined in the presence of the above interfering substance.
Methylphenylcyclosiloxane is the raw material, and thus every phenyl silicone oil product contains some of it. However, it can not be differentiated from phenyl silicone oil in 1 H NMR analysis. To our interest, the polymer of phenyl silicone oil can be easily separated from methylphenylcyclosiloxane with small molecular weight on a GPC column (Fig. 3). As shown in Fig. 3 and the Supplementary Fig. S3, the peak at 7.52 min is attributed to phenyl silicone oil, and that at 10.52 min corresponds to methylphenylcyclosiloxane. The resolution (R) of the two peaks is calculated using the e q u a t i o n o f R = which t R(A) and t R(B) stand for the retention time of phenyl silicone oil and methylphenylcyclosiloxane, and W 1/2(A) (3.41 min) and W 1/2(B) (2.00 min) refer to their peak width at half height. Thus, the calculated resolution is 1.52 for the two peaks, indicating a complete separation. By comparing the GPC-UV chromatogram of samples of phenyl silicone oil mixed with a different mass ratios of methylphenylcyclosiloxane in Fig. 3, the peak area of the signal at 10.51 min rises significantly with increasing the ratio of methylphenylcyclosiloxane, while that of the signal of phenyl silicone oil (7.52 min) keeps almost constant (2659 vs 2595). Therefore, the interference of methylphenylcyclosiloxane can be excluded due to the separation on a GPC column. Toluene and xylene are common solvents for phenyl silicone oil products, which cannot be distinguished from phenyl silicone oil in 1 H NMR analysis, either. Similarly, toluene and xylene are also small molecule compounds, and can be easily separated from the polymer of phenyl silicone oil on a GPC column (the Supplementary Figs.  S4 and Fig. S5). The R is 2.21 and 2.16, respectively, which indicated that the solvent peak could be completely separated from the peak of phenyl silicone oil. As can be seen, the peak area of the signal at 12.62 min rises with increasing the content of toluene. While that of the signal of phenyl silicone oil (7.52 min) keeps almost constant (3411 vs 3499). Similarly, and the peak area of the signal at 12.52 min rises with increasing the content of xylene, while that of the signal of phenyl silicone oil keeps almost constant (3411 vs 3446). Therefore, the GPC separation allows for the exclusion of the toluene or xylene interference on the detection of phenyl silicone oil.
However, other silicone oils are also polymer, which are co-eluted with phenyl silicon oil on a GPC column. These silicone oils include methyl silicone oil, hydroxyl methylsilicone oil and vinyl silicone oil, etc. Despite the fact that these silicone oils show almost negligible absorption at 265 nm (the Supplementary Fig. S6), their co-elution results in a different matrix for the phenyl silicone oil, which might exert an influence on the UV absorption of the latter.
Herein, solutions of phenyl silicone oil mixed with other type of silicon oil were analyzed by GPC-UV (Fig. 4, the Supplementary Figs. S7 and S8). The results are summarized in Table 1. Take Fig. 4 as an illustration. Almost the same peak area (2709 (Fig. 3a), 2728 and 2792) was obtained for the analyte of phenyl silicone oil, that mixed with methylsilicon oil at the mass ratio of 1:1 and 1:10. The RSD of the peak area is only 1.3% for the three samples. Thereby, the GPC-UV detection of phenyl silicone oil is unaffected by co-elution with methyl silicone oil. Similar results were obtained for hydroxyl methylsilicone oil and vinyl silicone oil (Table 1).
Overall, the established GPC-UV method for determining the phenyl content in phenyl silicone oil eliminated the interference of any potential substances, including the raw material (methylphenylcyclosiloxane), toluene solvent, and other types of silicone oil. In other words, this analytical method shows good selectivity.

Determination of the Phenyl Content of the Actual Sample
The established GPC-UV method was applied for the determination of the phenyl content in several phenyl silicone oil samples ( Table 2). As can be seen, the method had a good precision with an RSD (N = 3) ranging from 0.95 to 4.48%, and a good recovery ranging from 84.6 to 101.7%. The phenyl content in various phenyl silicone oil samples varied distinctively and ranged from 0.0 to 66.3%. Specifically, no phenyl content was found in several "phenyl silicone oils" purchased online. In view of the importance of the phenyl content in phenyl silicone oil, it is imperative to monitor its content in the relevant products.
At last, the validity of the GPC-UV method was further examined by comparing it to the 1 H NMR analysis. By using H1 as a model, the phenyl content was determined to be 0.603 mg g −1 by 1 H NMR analysis (the Supplementary Fig.  S9), which is in good agreement with the GPC-UV's result (0.613 mg g −1 ) with a relative error of just 1.63%. The GPC-UV method shows excellent potential for determining the phenyl content in phenyl silicone oil with exceptional sensitivity and selectivity, in comparison to the exorbitant price and maintenance for NMR.

Conclusion
In this work, a GPC-UV method has been established for the rapid and accurate determination of phenyl content in methylphenylsilicone oil. The result of the analysis is agreement with that of the 1 H NMR analysis. The established method has high sensitivity and selectivity, and it is possible to rule out the interference of solvents, methylphenylcyclosiloxane, and silicone oil could be excluded. The technique is appropriate for the quantitative analysis of phenyl content in academic work and the manufacturing of phenyl silicone oil.