## 1 Introduction  
Volatile thiols are well known for possessing low sensory detection thresholds and contributing to the odor of a range of foodstuffs, including wine [1]. Although low-molecular weight thiols like methyl mercaptan are often associated with wine off-aromas [2], higher molecular weight polyfunctional thiols can contribute to the desirable varietal character of many wines, most notably Sauvignon blanc [3]. These polyfunctional thiols are largely lacking from winegrapes (_Vitis vinifera_) and are primarily formed during fermentation by microbial metabolism of non-volatile precursors, e.g., S-glutathione and S-cysteine conjugates [1]. While several supra-threshold polyfunctional thiols have been identified in wine, three are reported to be of particular relevance: 4-mercapto-4-methyl-2-pentanone (4-MMP, detection threshold = 0.8 ng/L), 3-mercaptoethanol (3-MH, 60 ng/L), and 3-mercaptoethanol acetate (3-MHA, 4.2 ng/L), described as having box tree, grapefruit, and passion fruit aromas, respectively [3, 4]. Analyses of polyfunctional thiols in wine is challenging due to their low concentrations, susceptibility to oxidation, and poor chromatographic behavior [1, 5]. Direct analysis of free thiols following a pre-concentration step such as solid phase extraction (SPE), solid-phase microextraction (SPME), stir-bar sorptive extraction (SBSE), or related techniques prior to gas-chromatography electron impact mass spectrometry (GC-MS) have been reported [6, 7], but analytical detection limits are well above sensory thresholds. As a result, the majority of studies on polyfunctional thiols in wines have utilized more specialized methods to remove interferences and/or improve GC-MS response. One general approach for pre-concentration and clean-up of polyfunctional thiols in wine involves performing mercuobenzoate derivatization followed by anion exchange chromatography [4]. Analytes are then released by addition of a competing thiol, and quantified by gas chromatography mass spectrometry (GC-MS). The drawbacks of this approach are that it utilizes toxic Hg salts, risks sample oxidation, and requires large sample sizes (1-2 L wine) to achieve acceptable limits of detection [8]. A second approach is to derivatize thiols prior to GC-MS analysis to improve their stability, chromatographic behavior, and mass spectrometric response. Several reports have reported that pentafluorylbenzyl (PFB) derivatives of wine thiols yields good linearity and sub-sensory threshold detection limits when GC-negative ion chemical ionization-MS (GC-NICI-MS) is used as a detector [9, 10, 11, 12]. Several sample preparation variants have been employed prior to GC-NICI-MS, including liquid-liquid extraction (LLE) followed by derivatization and liquid injection [9]; on-cartridge SPE derivatization followed by wash steps, elution with organic solvent and liquid injection [10]; and on-cartridge SPE derivatization followed by wash steps, elution, evaporation of the organic solvent, and SPME of the dried down residue [12]. For these methods, methoximation of the ketone group of 4-MMP prior to derivatization has been suggested to increase the PFB derivatization yield [10]. However, NICI-MS is not routinely available to most labs, and analysis of PFB-thiol derivatives by the more commonly available electron impact ionization MS (EI-MS) results in severe losses of sensitivity. In one report, a detection limit of 30 ng/L for 3-MH was achievable using GC-EI-MS detection after an initial LLE of a 200 mL wine sample into pentane followed by back-extraction into alkaline aqueous solution, PFB derivatization, and headspace solid phase microextraction (HS-SPME), [13]. Quantification of 4-MMP and 3-MHA was not reported. By comparison, a 2 ng/L detection limit was achieved for a 10 mL wine sample using on-cartridge SPE derivatization followed by GC-NICI-MS [10]. Ethyl propionate derivatization followed by GC-EI-MS detection has also been described but detection limits were 10-fold above sensory threshold for 4-MMP and 3-MHA [14].