Evaluation of SPF of Marketed Sunscreen Lotion Using UV Spectrophotometry

Sunlight is an important natural source of energy and is essential for maintaining life and human health. However, prolonged and unprotected exposure to solar ultraviolet (UV) radiation can produce harmful effects on the skin. Solar radiation reaching the Earth’s surface mainly consists of ultraviolet rays, visible light, and infrared radiation. Among these, ultraviolet radiation is considered the most biologically active and harmful component. UV radiation is classified into UVA (320–400 nm), UVB (290–320 nm), and UVC (100–290 nm). UVC rays are absorbed by the ozone layer and do not reach the Earth’s surface, whereas UVA and UVB rays penetrate the atmosphere and significantly affect human skin. [1] UVB radiation is primarily responsible for erythema (sunburn), DNA damage, inflammation, and skin cancer. UVA radiation penetrates deeper into the dermal layer of the skin and contributes to photoaging, wrinkling, pigmentation, oxidative stress, and immunosuppression. Continuous exposure to these radiations may lead to severe dermatological disorders, including premature aging and carcinogenesis. Therefore, protection against UV radiation has become an important aspect of skin care and cosmetic science. [2] Sunscreen lotions are topical formulations developed to protect the skin from harmful UV rays by absorbing, reflecting, or scattering ultraviolet radiation. Sunscreens contain active ingredients known as UV filters, which may be classified as chemical absorbers (organic filters) or physical blockers (inorganic filters). These formulations are widely used to prevent sunburn, reduce skin damage, and minimize the risk of skin cancer. The effectiveness of sunscreen products is commonly expressed in terms of Sun Protection Factor (SPF). SPF is defined as the ratio of the minimum erythemal dose on protected skin to that on unprotected skin and indicates the degree of protection against UVB radiation. [3] Accurate determination of SPF is essential to ensure the efficacy, safety, and reliability of sunscreen products. Traditionally, SPF evaluation has been performed using in vivo methods involving human volunteers under controlled conditions. Although these methods are considered standard, they are associated with ethical concerns, variability in results, high cost, and lengthy procedures. To overcome these limitations, in vitro analytical methods have gained increasing importance in recent years. [4] Among various in vitro methods, UV-visible spectrophotometry is one of the most widely used techniques for SPF determination. UV spectrophotometry is based on the Beer-Lambert law, which states that absorbance is directly proportional to the concentration of the absorbing substance and the path length of light passing through the solution.

A = ε xlxc 

Where:

• A = Absorbance
• ε = Molar absorptivity
• l = Path length of cuvette
• c = Concentration of solution [5]

In this method, sunscreen samples are dissolved in suitable solvents such as ethanol or methanol, and their absorbance is measured in the UVB region (290–320 nm) using a UV-visible spectrophotometer. The obtained absorbance values are then used to calculate SPF values with the help of mathematical models such as the Mansur equation. [6]

SPF=CF×∑EE(λ)×I(λ)×Abs(λ)

This method offers several advantages, including simplicity, rapid analysis, reproducibility, cost-effectiveness, and minimal sample requirement. It is widely employed in pharmaceutical and cosmetic laboratories for routine quality control testing and formulation development. [7] Quality Assurance (QA) plays a major role in ensuring that pharmaceutical and cosmetic products comply with predefined standards of safety, quality, efficacy, and consistency. Evaluation of SPF values of marketed sunscreen lotions using UV spectrophotometry is an important QA parameter because it helps verify label claims, maintain batch-to-batch uniformity, and ensure regulatory compliance. [8] Any significant variation between labelled SPF and experimentally determined SPF may indicate formulation instability, degradation of active ingredients, or manufacturing inconsistencies. The present study focuses on the evaluation of SPF of marketed sunscreen lotions using UV spectrophotometric methods. Different commercially available sunscreen products are analyzed to determine their actual SPF values and compare them with manufacturer-labelled claims. The study aims to assess the effectiveness, quality, and reliability of marketed formulations and highlight the importance of accurate SPF labelling for consumer safety and public health. Thus, UV spectrophotometric SPF evaluation serves as a reliable, economical, and efficient analytical tool for sunscreen analysis and quality assurance in pharmaceutical and cosmetic industries.[9]

Table No 1: Concise Drug Profile of Sunscreen Formulations and Analytical Parameters Used for SPF Determination [10-14]

MATERIALS AND METHODS

MATERIALS

Marketed sunscreen lotions with different Sun Protection Factor (SPF) values such as SPF 15, SPF 30, and SPF 50 were procured from local pharmacies. The details of each sample, including batch number, manufacturing date, expiry date, and labelled SPF value, were recorded carefully prior to analysis. Ethanol (analytical grade) was used as the primary solvent for sample preparation, while distilled water was used for cleaning glassware and for dilution wherever required. The instruments used in the study included a UV-Visible spectrophotometer (double beam) with a wavelength range of 200–400 nm, an analytical balance with a sensitivity of ±0.1 mg, an ultrasonicator for proper dispersion of samples, and a magnetic stirrer for uniform mixing. The glassware used throughout the experiment consisted of volumetric flasks (10 mL and 100 mL), pipettes (1 mL, 5 mL, and 10 mL), beakers, quartz cuvettes with 1 cm path length, and standard laboratory accessories such as a funnel and Whatman filter paper.[15]

METHODOLOGY

 Principle

The sun protection factor (SPF) of sunscreen formulations was determined using UV spectrophotometry by measuring the absorbance in the UVB region (290–320 nm). The SPF value was calculated using the Mansur equation, which correlates absorbance values with erythemal effectiveness. This method provides a simple and reliable in vitro approach for estimating sunscreen efficacy. [17]

Preparation of Sample Solution

For sample preparation, 1 g of sunscreen lotion was accurately weighed and transferred into a 100 mL volumetric flask. Ethanol was added to the flask, and the mixture was sonicated to ensure complete dissolution of the formulation. The volume was then made up to 100 mL with ethanol. The resulting solution was filtered using Whatman filter paper to remove any undissolved particles. From this stock solution, 1 mL was further diluted to 10 mL with ethanol to obtain the final working solution used for analysis. [18]

 Instrument Calibration

Before analysis, the UV-Visible spectrophotometer was switched on and allowed to stabilize. Baseline correction was performed using ethanol as a blank to eliminate background interference. Wavelength accuracy of the instrument was verified according to standard calibration procedures to ensure reliable measurements. [19]

Determination of Absorbance

The prepared sample solution was transferred into a quartz cuvette, and absorbance was measured using a UV-Visible spectrophotometer in the wavelength range of 290–320 nm. Readings were recorded at regular intervals of 5 nm, including 290, 295, 300, 305, 310, 315, and 320 nm. Each sample was analyzed in triplicate to ensure reproducibility and accuracy of the results. [20]

Calculation of SPF

The SPF value was calculated using the Mansur equation:

SPF = CF × Σ [EE(λ) × I(λ) × Abs(λ)]

Where CF is the correction factor (10), EE(λ) represents the erythemal effect spectrum, I(λ) represents the solar intensity spectrum, and Abs(λ) represents the measured absorbance at each wavelength. Standard EE × I values were used for the calculations. [21]

Quality Assurance Parameters

The method was validated based on quality assurance parameters such as accuracy, precision, linearity, specificity, and robustness. Accuracy was assessed by comparing measured SPF values with labelled claims. Precision was evaluated using repeatability and expressed as %RSD. Linearity was considered where applicable, specificity ensured absence of interference, and robustness was checked under small variations in experimental conditions. [22]

Data Analysis

The experimental SPF values were calculated as mean values for each sample. Standard deviation and %RSD were determined to assess variability. The obtained SPF values were compared with the labelled SPF claims, and percentage deviation was calculated to evaluate compliance with marketed products. [23]

Precautions

All glassware used was properly cleaned and dried before use. Air bubbles in the cuvette were avoided during measurements. Cuvettes were handled carefully using tissue paper to prevent fingerprints and contamination. Fresh solutions were prepared for each analysis to ensure accuracy and consistency of results. [24,25]

RESULTS

The SPF value of marketed sunscreen lotions was determined using UV spectrophotometry based on the Mansur method. In this method, the SPF is calculated by correlating the absorbance of the sample in the UVB region (290–320 nm) with its erythemal effect. The Mansur equation used for the calculation is SPF = CF × Σ [EE(λ) × I(λ) × Abs(λ)], where CF is the correction factor (10), EE(λ) × I(λ) represents constant values at each wavelength, and Abs(λ) is the measured absorbance of the sample at different wavelengths ranging from 290 to 320 nm.

Observed Absorbance Data

The absorbance of the sunscreen sample was recorded at different wavelengths in the UVB region (290–320 nm) using UV spectrophotometry.

The absorbance values showed a gradual decrease after 300 nm, indicating reduced UV absorption capacity at higher UVB wavelengths, which is essential for SPF determination using the Mansur method.

EE × I Values (Standard Constants)

Calculation Table

Sum of Products = 0.84814

SPF Calculation

The SPF value was calculated using the Mansur equation:

SPF = CF × Σ [EE(λ) × I(λ) × Abs(λ)]

Where:

• CF = Correction factor (10)
• EE(λ) × I(λ) = Standard constant values
• Abs(λ) = Measured absorbance values

Substituting the values:

SPF = 10 × 0.84814 = 8.4814

Final SPF Value: ≈ 8.5

Fig no 1: Absorbance vs Wavelength Graph

Fig no 2 Graph: Wavelength vs EE × I Values