The structure and purity of a reference dye standard used for quantification of C.I. Solvent Red 164 in fuels Scott D. Harvey * , Garry W. Buchko, Richard B. Lucke, Cherylyn W. Wright, Angela M. Melville, Anthony J. Scott, Bob W. Wright Pacific Northwest National Laboratory, P.O. Box 999/MSIN P8-50, Richland, WA 99352, USA article info Article history: Received 3 November 2008 Received in revised form 23 January 2009 Accepted 26 January 2009 Available online 5 February 2009 Keywords: Diesel fuel dye Morton Red 26 C.I. Solvent Red 26 C.I. Solvent Red 24 NIST SRM 2037 Dye purity determination Dye structure verification abstract This study characterizes a primary reference dye standard, certified as being 99% C.I. Solvent Red 26, that is used in the United States to identify tax-free fuel, using nuclear magnetic resonance spectroscopy, thin-layer chromatography, gas chromatography/mass spectrometry and high-performance liquid chromatography/mass spectrometry. It was found that 72% of the primary reference dye is equivalent to National Institute of Standards and Technology Standard Reference Material 2037 (C.I. Solvent Red 24). Several of the impurities were tentatively identified and a number of additional impurities were partially characterized. Possible explanations and implications are briefly discussed. Ó 2008 Published by Elsevier Ltd. 1. Introduction The industrial preparation of azo dyes dates back to the 1870s [1], well before the advent of analytical tools for determining product composition. Commercial azo dyes often consist of a mixture of synthetic products, by-products, intermediates, and unreacted starting materials. Components in these mixtures may vary widely in their volatility, polarity, and molecular weights [2]. Even dyes that are purportedly pure are occasionally found to contain substantial amounts of impurities [3]. Dye compositions are difficult to completely characterize due to the broad range of possible products and, therefore, require several different analytical techniques, each with their unique capabilities, to obtain complete characterization. The work described herein applies to a number of diverse analytical approaches to characterize the composition of Morton Red 26, hereafter referred to as Red 26, a reference dye used in the United States to quantify the amount of dye added to tax-free fuel. C.I. Solvent Red 164 is a commercial dye that is added to off-road diesel fuel in the United States to identify its tax-free status. Federal regulations dictate the amount of C.I. Solvent Red 164 added to off-road diesel be spectrally equivalent to 11.1mg/L of C.I. Solvent Red 26 (Fig. 1a) [4]. Fortification at this concentration is verified by a visible absorbance spectroscopic method that references a stan- dard curve prepared with Red 26, a dye that is certified by the manufacturer as 99% C.I. Solvent Red 26. Researchers have assumed the equivalency of Red 26 and C.I. Solvent Red 26 based on this product certification; however, this identification has not been experimentally verified. Recently, a National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) (#2037, Chemical Abstract Service [CAS] #85-83-6) became available that has the structure shown in Fig. 1b (also commonly known as C.I. Solvent Red 24). C.I. Solvent Red 164 diesel fuel dye is a mixture of dyes with similar structures to those shown in Fig. 1 except they contain additional alkyl chains that enhance solubility in diesel fuel [4]. Quantification of C.I. Solvent Red 164 in fuel is performed by second derivative visible absorbance spectroscopy [4–6] refer- encing a standard curve prepared with Red 26. This determination assumes similar spectral properties between Red 26 and C.I. Solvent Red 164 (both wavelength maximum and molar extinction coeffi- cient), as well as use of pure Red 26 for preparation of calibration standards. During the course of various studies at Pacific Northwest National Laboratory (PNNL), it was observed that equivalent gravimetric concentrations of Red 26 absorbed approximately 25% less than NIST SRM 2037. These studies are described in more detail * Corresponding author. Tel.: þ1 509 372 1144; fax: þ1 509 376 5021. E-mail address: scott.harvey@pnl.gov (S.D. Harvey). Contents lists available at ScienceDirect Dyes and Pigments journal homepage: www.elsevier.com/locate/dyepig 0143-7208/$ – see front matter Ó 2008 Published by Elsevier Ltd. doi:10.1016/j.dyepig.2009.01.015 Dyes and Pigments 82 (2009) 307–315