N-Hydroxy Compounds as New Internal Standards for the 31P-NMR Determination of Lignin Hydroxy Functional Groups

TMDP (2–chloro–4,4,5,5–tetramethyl–1,3,2–dioxaphospholane) is a very powerful reagent for tagging hydroxy groups as the 31P-NMR active phosphite derivative (Fig. 1). TMDP derivatization/ 31P–NMR spectroscopy has been effectively applied to analyze lignin structural changes during kraft pulping (Ahvazi et al. 1998; Froass et al. 1998) and various bleaching operations (Froass et al.1996; Wang et al. 1997; Sun and Argyropoulos 1996; Sun and Argyropoulos 1995 ). Usually, cyclohexanol is selected as an internal standard for the TMDP/31P–NMR method (Granata and Argyropoulos 1995). In our experience, we have found that occasionally the cyclohexanol–phosphite product overlaps with derivatized aliphatic and phenolic lignin structures (Fig. 2, also see Zawadzki and Ragauskas 1999). Cholesterol has also been used as an internal standard (Smit et al. 1997), but unfortunately, the chemical shift for the phosphite product is very similar to that of cyclohexanol (Table 1). Clearly, an internal standard that is baseline resolved would be useful when investigating lignin samples displaying disperse aliphatic– and phenolic–phosphite regions. TMDP is known to react with a variety of heteroatom functional groups containing labile hydrogen; for example, OH, CO2H, NH, and SH (Wroblewski et al. 1988). Because of the range of TMDP reactivity, we have investigated the phosphite reaction products arising from a variety of substrates. In this paper, we propose that N-hydroxy compounds may be useful new internal standards for TMDP/31P–NMR analysis of isolated lignins.