Copyright © 2005 John Wiley & Sons, Ltd. Biomed. Chromatogr. 20: 369–376 (2006) Evaluating residual monomer in dental acrylic resins 369 ORIGINAL RESEARCH ORIGINAL RESEARCH Published online 21 September 2005 Copyright © 2005 John Wiley & Sons, Ltd. BIOMEDICAL CHROMATOGRAPHY Biomed. Chromatogr. 20: 369–376 (2006) Published online in Wiley InterScience (www.interscience.wiley.com). DOI: 10.1002/bmc.575 Development and application of methods for determination of residual monomer in dental acrylic resins using high performance liquid chromatography V. M. Urban, 1 Q. B. Cass, 2 * R. V. Oliveira, 2 E. T. Giampaolo 1 and A. L. Machado 1 1 Department of Dental Materials and Prosthodontics, São Paulo State University, UNESP, Araraquara, São Paulo, Brazil 2 Department of Chemistry, São Carlos Federal University, UFSCar, São Carlos, São Paulo, Brazil Received 24 June 2005; accepted 21 July 2005 ABSTRACT: Two high-performance liquid chromatographic methods for determination of residual monomer in dental acrylic resins are described. Monomers were detected by their UV absorbance at 230 nm, on a Nucleosil ® C 18 (5 µm particle size, 100 Å pore size, 15 × 0.46 cm i.d.) column. The separation was performed using acetonitrile–water (55:45 v/v) containing 0.01% triethylamine (TEA) for methyl methacrylate and butyl methacrylate, and acetonitrile–water (60:40 v/v) containing 0.01% TEA for isobutyl methacrylate and 1,6-hexanediol dimethacrylate as mobile phases, at a flow rate of 0.8 mL/min. Good linear relation- ships were obtained in the concentration range 5.0–80.0 µg/mL for methyl methacrylate, 10.0–160.0 µg/mL for butyl methacrylate, 50.0–500.0 µg/mL for isobutyl methacrylate and 2.5–180.0 µg/mL for 1,6-hexanediol dimethacrylate. Adequate assay for intra- and inter-day precision and accuracy was observed during the validation process. An extraction procedure to remove residual monomer from the acrylic resins was also established. Residual monomer was obtained from broken specimens of acrylic disks using methanol as extraction solvent for 2 h in an ice-bath. The developed methods and the extraction procedure were applied to dental acrylic resins, tested with or without post-polymerization treatments, and proved to be accurate and precise for the deter- mination of residual monomer content of the materials evaluated. Copyright © 2005 John Wiley & Sons, Ltd. KEYWORDS: residual monomer; acrylic resins; HPLC; method validation; polymerization reaction INTRODUCTION The use of the autopolymerizing reline acrylic resins permits direct intra-oral relining of complete and re- movable partial dentures, thus producing an accurate adaptation of the denture bases to the mucosa covering the residual ridges (Matsumura et al., 2001; Haywood et al., 2003). These hard chair-side reline resins have been used to improve the fit of denture bases, thereby providing better retention and stability for removable prostheses. The composition of the autopolymerizing hard chair-side resins is different from the conventional autopolymerizing acrylic resins, which are based on methyl methacrylate (MMA), particularly with regard to the constituents of the liquid (Arima et al., 1996). Analysis of the composition of hard autopolymerizing reline resins has demonstrated that the liquids may con- tain butyl methacrylate (BMA), isobutyl methacry- late (IBMA), and cross-linking agent 1,6-hexanediol dimethacrylate (1,6-HDMA) (Arima et al., 1995; 1996). During the polymerization reaction of the acrylic resins, the conversion of monomer into polymer is not complete and varying amounts of free or unreacted monomer remain in the polymerized resin (Vallittu et al., 1998; Araújo et al., 2002; Lee et al., 2002). Residual monomer is a well-known plasticizer and affects the physical and mechanical properties of the acrylic resins (Arab et al., 1989; Dogan et al., 1995; Lee et al., 2002; Azzarri et al., 2003). In addition, studies have demon- strated that the residual monomer leached out from the polymerized resins has a potentially cytotoxicity effect (Tsuchiya et al., 1994; Kedjarune et al., 1999; Rose et al., 2000) and can be the cause of mucosal damage (Ali et al., 1986; Barclay et al., 1999). Therefore, the reduction of residual monomer content could improve the properties and reduce the cytotoxicity effects of the polymerized acrylic resins, and some post-polymerization treatments have been suggested, such as microwave irradiation and immersion in hot water (Tsuchiya et al., 1994; Yunus et al., 1994; Blagojevic and Murphy, 1999; Machado et al., 2002). *Correspondence to: Q. B. Cass, Department of Chemistry, São Carlos Federal University, UFSCar, Rodovia Washington Luís (SP- 310), km 235, São Carlos, São Paulo 13565-905, Brazil. E-mail: quezia@dq.ufscar.br Abbreviations used: BMA, butyl methacrylate; D, Duraliner II; 1,6- HDMA, cross-linking agent 1,6-hexanediol dimethacrylate; IBMA, isobutyl methacrylate; K, Kooliner; L, Lucitone 550; MMA, methyl methacrylate; TEA, triethylamine; TRF, Tokuso Rebase Fast; UGH, Ufigel Hard. Contract/grant sponsor: FAPESP; Contract/grant number: 01/01406-3; 02/06559-5. Contract/grant sponsor: FUNDUNESP; Contract/grant number: 00413/02-DFP.