Abstract:

Gel nail polish has surged in global popularity， becoming a dominant force in the cosmetics market due to its compelling advantages. This rapid market expansion， however， is shadowed by significant and growing safety concerns related to the essential chemical agents enabling its functionality—photoinitiators （PIs）. Classified as a Category 1B CMR substance （carcinogenic， mutagenic， or toxic to reproduction） based on robust evidence of reproductive toxicity， 2，4，6-trimethylbenzoyl diphenyl phosphine oxide （TPO） has been comprehensively banned within the European Union （EU） under Regulation （EC） No 1223/2009， effective from September 1， 2025. Compounding this challenge was the conspicuous absence of validated analytical methods. Specifically， standardized， reliable， and accessible techniques for quantifying the complex spectrum of PIs in modern gel nail polish formulations were critically lacking. This included established PIs such as the now-banned TPO and its potential substitutes， all of which were embedded within the challenging， heterogeneous matrix of these formulations. This methodological gap severely hindered effective quality control during manufacturing， robust post-market surveillance by regulators， accurate consumer exposure assessment， and the essential safety evaluation of emerging replacement PIs. Consequently， there was an immediate and pressing need to develop dedicated analytical capabilities capable of monitoring both prohibited high-risk PIs and their prospective successors within this specific product category. To address the critical absence of standardized analytical methods for PIs in cosmetics， this study developed and validated a robust high performance liquid chromatography with diode array detection （HPLC-DAD） protocol. The protocol enables the simultaneous quantification of five strategically selected PIs in gel nail polish formulations. These PIs include TPO， its structural analogues ethyl （2，4，6-trimethylbenzoyl） phenylphosphinate （TPO-L）， ［bis（4-methylphenyl） phosphinyl］ （2，4，6-trimethylphenyl） methanone （TMO）， and ［phenyl（2，4，6-trimethylbenzoyl） phosphoryl］ （2，4，6-trimethylphenyl） methanone （PB-TMBPO）， as well as the α-hydroxy ketone 1-hydroxycyclohexyl phenyl ketone （HCHPK）. This target panel encompasses both the currently dominant PIs and prime TPO replacement candidates. The optimized sample preparation involved efficient extraction of target analytes from the complex gel nail polish matrix using acetonitrile under ultrasonication， eliminating the need for derivatization. Chromatographic separation was achieved on a Kromasil 100-5-C18 column （150 mm×4.6 mm， 5 μm） maintained at 30 ℃. A binary mobile phase gradient （water/acetonitrile） was delivered at 1.0 mL/min， with an injection volume of 5 μL. Selective detection leveraged dual wavelengths： 243 nm optimized for HCHPK and 375 nm for the four acylphosphine oxides. Quantitation employed external calibration curves. Method validation demonstrated exceptional performance： All five PIs exhibited baseline separation from matrix interferences. Linear calibration curves spanned 2–600 mg/L with outstanding correlation coefficients （r≥0.999 9 for each analyte）. Method sensitivity was confirmed with limits of detection （LODs） ranging from 3.6 to 45 μg/g and limits of quantitation （LOQs） from 15 to 141 μg/g. Accuracy， assessed through spiked recovery experiments at low， medium， and high concentration levels， yielded excellent results （91.6%–100.4%）. Precision， expressed as relative standard deviation （RSD， n=6）， was consistently high （0.3%–4.3%）， affirming method robustness and reliability for routine analysis. Application of the validated method to 30 commercially available gel nail polish samples revealed pervasive PI presence. Total PI content ranged from 2.95% to 9.66% （mass fraction）. Each batch contained 1 to 3 distinct PIs. None of these PIs were listed on the product labels. Critically， TPO， which has been banned in Europe， was present in 90% of the analyzed products at significant concentrations （1.42%–6.43%， mass fraction）. HCHPK was detected in 76.7% of the samples， with levels ranging from 0.83% to 4.84% （mass fraction）. TPO-L was present in 10% of the samples （6.73%–7.17%， mass fraction）， while TMO was detected at 0.25% （mass fraction） in a single sample. These findings indicate the prevalent use of prohibited substances like TPO and other PIs in the current gel nail polish market. Enhanced regulation， substitution efforts， and further safety assessments are urgently required. This study delivers a simple， specific， rapid， and economically viable HPLC-DAD method specifically tailored for the challenging gel nail polish matrix. It effectively addresses the analytical gap for monitoring key PIs， particularly during the critical transition period following the EU TPO ban. The methodology provides robust technical support for quality control laboratories （both industrial and regulatory）， facilitates compliance verification with evolving safety regulations， enables accurate risk assessment of consumer exposure， and empowers the development of safer gel nail polish formulations through reliable PI quantification. The alarming market data generated underscores the immediate necessity for regulatory action and industry innovation toward eliminating high-risk PIs like TPO.

Key words: high performance liquid chromatography （HPLC）, gel nail polish, photoinitiators （PIs）, 2, 4, 6-trimethylbenzoyl diphenyl phosphine oxide （TPO）, cosmetics, acylphosphine oxides