Simultaneous determination of triazine-based UV absorbers by first-order derivative spectrophotometry in different media

2.2.1. Configuration of standard and mixing solution

Preparation of single-component stock and standard solutions: A range of single-component standard stock solutions was prepared using different solvents. Specifically, precisely weigh and transfer 500 mg each of BEMT, EHT, and DBT into individual 500 mL brown volumetric flasks, then dilute them to the calibration mark to achieve standard stock solutions of the three triazine-based UV absorbers, each with a concentration of 1000 mg L^-1. The prepared standard stock solutions were securely sealed in a refrigerator at 4°C. Utilizing these standard stock solutions as the mother liquor, single-component standard solutions with concentrations of 1, 3, 5, 7, 9, and 11 mg L^-1 were obtained through a systematic stepwise dilution process.

Preparation of mixed stock and mixed standard solutions: Equal volumes of two single-component triazine-based UV absorber stock solutions, each having an initial concentration of 1000 mg L^-1, were combined to create a mixed stock solution containing each triazine-based UV absorber at a concentration of 500 mg L^-1. The dilution and preparation procedures for the mixed standard solutions followed the same methodology as that employed for the single-component standard solutions.

The solvents employed for preparing the above stock and standard solutions encompassed medium-polarity and polar organic solvents, including dioctyl carbonate, chloroform, ethyl acetate, diethyl ether, DMSO, dimethyl formamide (DMF), acetonitrile, 1,4-dioxane, isopropanol, tetrahydrofuran (THF), etc.

2.2.2. Determination by UV-vis spectrophotometry

The ultraviolet and derivative spectra of the binary mixture were acquired by scanning from 200 to 400 nm using ultraviolet spectrophotometry. A 1 cm quartz cuvette was used for the test solution, with the corresponding solvent serving as the reference. Absorbance data were collected at intervals of 1 nm.

2.3.1. Relative standard deviation

The intraday precision (repeatability) was assessed by analyzing the single-concentration mixed solutions of BEMT-EHT and BEMT-DBT. Three repeated measurements were conducted on the same day. To evaluate the diurnal-day precision, each mixed sample solution of ultraviolet absorber was re-prepared at the same concentration level, and response values were measured in three replicates. This procedure was carried out continuously over 3 days. Intra-day and inter-day precision are expressed as relative standard deviation (% RSD). Here, SD denotes the standard deviation of the sample, and μ represents the mean value of the sample. n indicates the total number of samples, and x_i refers to the samples as shown in Equation (1) and Equation (2).

2.3.2. Marked recovery experiment

The accuracy is determined by the recovery rate of a known quantity of UV absorbent that has been added to the sample, which serves to assess the reliability of the method. Equation (3) and Equation (4) are used to calculate the recovery rate and relative error between the measured concentration and the added concentration, respectively.

2.3.3. Determination of limits of detection and quantification

Limit of detection (LOD) and limit of quantification (LOQ) indicate the lowest concentration that can be detected by a detection method and the lowest concentration that can be accurately quantified, respectively. The following are standard formulas for calculating LOD and LOQ, where σ is the standard deviation of the Y-axis intercept, and S is the slope of the calibration curve as shown in Equation (5) and Equation (6).

2.3.4. Robustness and stability

The analytical conditions selected for assessing robustness encompass parameters including scanning step size, scanning speed, and temperature. Recovery rates were measured under varying analytical conditions to evaluate the sensitivity of the measurements to intentional variations in these parameters. Additionally, recovery rate measurements were conducted over a 36 h to assess the stability of the binary mixed triazine-based UV absorbers.

3.1.1. Determination of quantification wavelengths in pure organic solvents

To evaluate the applicability of derivative spectrophotometry for the determination of triazine-based UV absorber mixtures across different media, we initially examined the zero-order UV absorption spectra of triazine-based UV absorbers in medium-polarity organic solvents, including chloroform, diethyl ether, ethyl acetate, and methyl tert-butyl ether (MTBE). The maximum absorption wavelengths and mass absorption coefficients for BEMT, EHT, and DBT in these solvents were documented (Table S2). The first-derivative UV spectra of the triazine-based absorbers and their binary mixtures showed suboptimal performance in diethyl ether and MTBE as solvents (Figures S1 and S2). In contrast, ethyl acetate and chloroform demonstrated optimal performance as solvents. At the zero-crossing points of interfering components, the first-derivative values for the mixtures were extremely similar to those of the analytes.

In ethyl acetate, the zero-order spectra of BEMT-DBT and BEMT-EHT binary mixtures (Figures 1a,b) exhibited significant spectral overlap between 260-320 nm, which hindered simultaneous quantification. As depicted in Figure 1(c) for the BEMT-DBT mixture, a zero-crossing point for BEMT was observed near 318 nm. At this specific wavelength, the first-derivative signal of the mixture remained independent of BEMT concentration and demonstrated a linear relationship solely with DBT concentration, effectively eliminating any interference from BEMT. Similarly, DBT displayed a zero-crossing point around 369 nm, where its first-derivative signal depended linearly on BEMT concentration without any interference from DBT. Consequently, wavelengths of 318 nm and 369 nm were selected for the simultaneous quantification of DBT and BEMT in the BEMT-DBT system. For the BEMT-EHT system (Figure 1d), wavelengths of 319 nm and 369 nm were designated for EHT and BEMT quantification, respectively.

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Figure 1.

Zero-order UV spectra of (a) BEMT and DBT, (b) BEMT and EHT in single and binary solution in ethyl acetate, First-order derivative UV spectra of (c) BEMT and DBT, (d) BEMT and EHT in single and binary solution (initial concentration of 5 mg L^-1 for each compound).

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In chloroform, the zero-order spectra of BEMT-DBT and BEMT-EHT mixtures have been presented in Figures 2(a,b). The corresponding first-derivative spectra (Figures 2c,d) provided quantification wavelengths of approximately 315 nm for DBT/EHT and 370 nm for BEMT in both binary systems.

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Figure 2.

Zero-order UV spectra of (a) BEMT and DBT, (b) BEMT and EHT in single and binary solution in chloroform, First-order derivative UV spectra of (c) BEMT and DBT, (d) BEMT and EHT in single and binary solution (initial concentration of 5 mg L^-1 for each compound).

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Subsequently, the applicability of derivative spectrophotometry for the determination of binary mixtures of triazine-based UV absorbers (BEMT-EHT, BEMT-DBT) was assessed across various polar organic solvents, including acetonitrile, DMSO, 1,4-dioxane, isopropanol, DMF, and THF. The maximum absorption wavelengths and mass absorption coefficients for BEMT, EHT, and DBT in these solvents were documented (Table S3). The method exhibited broad applicability in most solvents examined (Figures S3-S6). Utilizing isopropyl alcohol as the solvent, the zero-order UV absorption spectra for the BEMT-DBT and BEMT-EHT binary mixtures in isopropanol have been illustrated in Figures 3(a,b). The corresponding first-derivative spectra (Figures 3c,d) yielded quantification wavelengths near 322 nm for EHT/DBT and approximately 370 nm for BEMT in both systems.

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Figure 3.

Zero-order UV spectra of (a) BEMT and DBT, (b) BEMT and EHT in single and binary solution in isopropanol, First-order derivative UV spectra of (c) BEMT and DBT, (d) BEMT and EHT in single and binary solution (initial concentration of 5 mg L^-1 for each compound).

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Using DMSO as the solvent, the zero-order spectra of the BEMT-DBT and BEMT-EHT mixtures in DMSO have been shown in Figures 4(a,b). An analysis of their first-derivative spectra (Figures 4c,d) similarly revealed quantification wavelengths at approximately 322 nm for EHT/DBT and at 365 nm for BEMT.

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Figure 4.

Zero-order UV spectra of (a) BEMT and DBT, (b) BEMT and EHT in single and binary solution in DMSO, First-order derivative UV spectra of (c) BEMT and DBT, (d) BEMT and EHT in single and binary solution (initial concentration of 5 mg L^-1 for each compound).

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3.2.1. Simultaneous determination of triazine-based UV absorbers in the sunscreen solvent of dioctyl carbonate

To validate the practical utility of the method, the simultaneous determination of triazine-based UV absorber mixtures was investigated in dioctyl carbonate, a solvent commonly used in sunscreen formulations. Firstly, quantification wavelengths and calibration curves were established. The zero-order UV absorption spectra for the BEMT-DBT and BEMT-EHT binary mixtures in dioctyl carbonate have been presented in Figures 7(a,b), with their corresponding first-derivative spectra shown in Figures 7(c,d). The quantification wavelengths were determined to be 316 nm for DBT and 318 nm for EHT, where the first derivative of BEMT approached zero, thereby eliminating its interference. Similarly, 372 nm was selected for BEMT as the derivatives of DBT and EHT approached zero at this wavelength, thus removing their interference.

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Figure 7.

Zero-order UV spectra of (a) BEMT and DBT, (b) BEMT and EHT in single and binary solution in dioctyl carbonate, First order derivative UV spectra of (c) BEMT and DBT, (d) BEMT and EHT in single and binary solution (initial concentration of 5 mg L^-1 for each compound).

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In dioctyl carbonate, the first-derivative spectra for both BEMT-DBT and BEMT-EHT systems at various concentrations have been presented in Figures 8(a and b), respectively. Figures 8 (c and d) illustrate the corresponding standard curves at the specified wavelength. Within the concentration range of 1 mg L^-1 to 9 mg L^-1, the concentrations of single-component ultraviolet absorbers in both BEMT-DBT and BEMT-EHT exhibit a good linear relationship with the derivative values of absorbance at a specific wavelength. The quantitative statistical parameters derived from this method have been summarized in Table 1. The standard curves yielded regression coefficients of R² (BEMT) = 0.996 and R² (EHT) = 0.989 for BEMT-EHT, as well as R² (BEMT) = 0.999 with R² (DBT) = 0.984 for BEMT-DBT, thereby confirming a strong linearity between derivative response and concentration through high correlation coefficients and negligible intercepts.

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Figure 8.

First-order derivative UV spectra and calibration curves in dioctyl carbonate. (a) UV spectra of the binary mixture of BEMT and DBT (1-9 mg/L). (b) UV spectra of the binary mixture of BEMT and EHT (1-9 mg/L). (c) Calibration curves for the binary mixture of BEMT and DBT. (d) Calibration curves for the binary mixture of BEMT and EHT.

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The sensitivity was determined using Eqs. (5) and (6) to calculate the values for LOD and LOQ, as presented in Table S4. LODs for BEMT and EHT in the BEMT-EHT binary system were 0.43 mg L^-1 and 0.25 mg L^-1, respectively, while the LOQs were determined to be 1.30 mg L^-1 and 0.76 mg L^-1, respectively. LODs for BEMT and DBT in the BEMT-EHT binary system were 0.38 and 0.22 mg L^-1, respectively, while the LOQs were determined to be 1.15 mg L^-1 and 0.67 mg L^-1, respectively (Table S4). These values show that the proposed method has good sensitivity.

Subsequently, we conducted method validation to confirm the accuracy, reliability, and applicability of this approach. The precision evaluation data for the first-derivative spectrophotometric determination of BEMT-EHT and BEMT-DBT binary mixtures in dioctyl carbonate were evaluated, as summarized in Table 1. As illustrated in Table 1, both intra-day and inter-day precision studies demonstrated relative standard deviations (RSD) below 2.47% (n=3), indicating excellent reproducibility and high precision of the method.

The accuracy assessment data for the BEMT-EHT and BEMT-DBT binary systems have been presented in Table 2. The results indicate mean recoveries of 98.13% for BEMT and 99.54% for EHT within the BEMT-EHT system, accompanied by corresponding mean relative errors of 1.87% and 0.46%. In the case of the BEMT-DBT system, mean recoveries were found to be 98.18% for BEMT and 98.01% for DBT, with mean relative errors of 1.82% and 1.99%. The close alignment between theoretical values and experimentally determined values validates the accuracy of the proposed method.

3.2.2. Simultaneous determination of triazine-based UV absorbers in DMSO/H₂O (4:96, v/v) mixed solvent

Due to its low toxicity and high stability, DMSO is extensively used as a cosolvent in toxicity experiments involving hydrophobic pollutants. Consequently, we concentrated on investigating the quantitative analysis of binary triazine-based UV absorbers in DMSO/H₂O (4:96 v/v) mixed solvents using derivative spectrophotometry. Figures 9(a, b) present the first-derivative spectra for both BEMT-DBT and BEMT-EHT systems at various concentrations in this medium, respectively. The corresponding standard curves are depicted in Figures 9(c,d). The regression analysis confirmed excellent linearity within the established concentration range, with R² values of 0.997 (BEMT) and 0.998 (DBT) for the BEMT+DBT system, as well as R² values of 0.999 (BEMT) and 0.997 (EHT) for the BEMT+EHT system, thus validating the method’s efficacy for precise quantification of individual components in DMSO/H₂O (4:96, v/v) mixed systems.

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Figure 9.

First-order derivative UV spectra of the binary mixture of (a) BEMT and DBT, (b) BEMT and EHT within the concentration range of 1-11 mg L^-1 in DMSO/H₂O (4:96 v/v), Calibration curves for binary mixture of (c) BEMT and DBT, (d) BEMT and EHT.

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Table 3 shows the linear relationship between the concentrations of two two-component mixed ultraviolet absorbers in DMSO/H₂O (4:96, v/v) solution and dA/dλ, which includes details on the concentration range, measurement wavelength, regression equations, and correlation coefficients for regression analysis. The precision evaluation data for the first-derivative spectrophotometric determination of BEMT-EHT and BEMT-DBT binary systems in this medium have been presented in Table 3. Both intra-day and inter-day precision studies demonstrate RSD below 2.86% (n=3), thereby confirming the high precision and reproducibility of the proposed first-derivative method.

LODs for BEMT and EHT in the BEMT-EHT binary system were 0.52 and 0.35 mg L^-1, respectively, while the LOQs were determined to be 1.58 and 1.06 mg L^-1, respectively. LODs for BEMT and DBT in the BEMT-EHT binary system were 0.54 and 0.25 mg L^-1L, respectively, while the LOQs were determined to be 1.64 and 0.76 mg L^-1, respectively (Table S4). These values show that the proposed method has good sensitivity.

The accuracy assessment data for the BEMT-EHT and BEMT-DBT binary systems have been presented in Table 4. The results indicate mean recoveries of 97.85% for BEMT and 100.98% for EHT within the BEMT-EHT system, accompanied by corresponding mean relative errors of 2.15% and 0.98%. For the BEMT-DBT system, mean recoveries were found to be 99.00% for BEMT and 100.69% for DBT, with mean relative errors of 0.90% and 0.69%. The close agreement between nominal and experimentally determined values substantiates the accuracy of the proposed method.

In addition, the robustness of the above two methods (as shown in Table S5) indicates that the analytical results are unaffected by minor variations in key experimental parameters, including scanning step size (Δλ), scanning speed, and temperature, thereby confirming the method’s reliability under typical operational conditions. The stability of the method was confirmed by monitoring changes in the recovery rate of three UV absorbers-BEMT, EHT, and DBT in a binary mixture across different solvents at 25±2°C over various time intervals (0, 12, 24, and 36 h), as presented in Table S6. These supplementary experiments and data enhance the method’s credibility and practical value.