Effect of Tetrabutylammonium Cation on Solid-Phase Analytical Derivatization as a Function of Analyte Lipophilicity

Abstract

Analytical derivatizations (AD) can increase the sensitivity of analyses—including those with mass spectrometric detection—by as much as three orders of magnitude. The extra steps required, however, are a possible impediment to their use. To simplify AD we investigated solid-phase analytical derivatization (SPAD) of compounds with diverse structures by using pentafluorobenzyl bromide (PFBBr) as the reagent. Model compounds were organic acids (e.g. phenols, chlorophenols and carboxylic acids) which were simultaneously extracted and derivatized from 0.1 M NaOH onto a polystyrene–divinylbenzene resin (XAD-4) as their pentafluorobenzyl (PFB) derivatives. Test analytes ranged in molecular weight from 94 for phenol to 266 for pentachlorophenol and octanol–water partition coefficients (log P) values ranged from 1.48 for phenol to 7.15 for hexadecanoic acid. Under SPAD conditions, reaction rates rapidly increased with log P, but yields for less lipophilic compounds, although precise, were unacceptably low. Use of the tetrabutylammonium (TBA) cation as a phase transfer catalyst increased the yield of compounds with low log P; but, unexpectedly, as the log P of the analyte increased, the phase transfer catalyst caused a decrease in yield. The data from this study define the log P range of compounds that require TBA for optimal yield and the log P range of compounds for which TBA compromises yield. This insight led to a simple, two-step, one-pot technique that gave high yields of the PFB derivatives for the entire range of analytes studied. SPAD first extracted/derivatized the lipophilic analytes from aqueous solution onto the solid phase. Extraction/derivatization of the polar analytes followed upon addition of TBA to the reaction mixture. The PFB ethers and esters of the entire range of analytes were then eluted from the XAD-4. The two-step procedure was faster, used less reagent and required lower temperature than comparable methods in the literature. With the two-step procedure, pentafluorobenzylation of phenols and carboxylic acids from water gave yields in excess of 88% with the exception of phenol and pentachlorophenol which were recovered in 57 and 44%, respectively. For all analytes, relative standard deviations were below 15%. The effect of matrix on yield varied between zero and a decline in recoveries of approximately 20–30% depending on the analyte and concentrations of NaCl or humic acid. In the presence of matrix components relative standard deviations remained below 20%.