Effect of pH on lignin analysis by Raman spectroscopy

Abstract

Ultraviolet resonance Raman (UVRR) spectroscopy is a powerful tool for structural lignin analysis in situ: modification or isolation of lignin from biomass is not necessary. UVRR spectroscopy is equally applicable for samples with high lignin content and those with very low lignin content. Monomeric phenolic lignin model compounds and wood pulp samples have been studied at neutral and alkaline pH with UVRR spectroscopy. Concentration of guaiacol correlated well with the relative Raman band intensity, which indicates that lignin-containing solutions can be quantitatively measured with UVRR spectroscopy. A change in pH induced a recordable shift in the aromatic band position in the spectra, which was 25–35 cm-1 with phenolic model compounds without para substitution, 8–12 cm-1 with phenolic model compounds with para substitution, and about 2–7 cm-1 with pulp samples. No shift was detected with a non-phenolic model compound. Increasing the amount of phenolic hydroxyl groups increased the UVRR band shift in pulp samples. Additionally, increasing the pH enhanced the relative aromatic band intensity in the UVRR spectra in solution of the phenolic model compound. Accordingly, pH adjustment is relevant prior to any lignin analysis with Raman spectroscopy.

Recent innovations in analytical methods for the qualitative and quantitative assessment of lignin

“…Ultraviolet (UV) and resonance Raman (RR) spectroscopy have routinely been used for lignin analysis since lignin exhibits an UV absorption peak [105][106][107][108][109][110][111][112][113][114][115]. Beech and pine lignin radicals, key intermediate species in lignin biosynthesis, were measured using Kerr-gated RR spectroscopy [105,106].…”

Neutral Sulfite Treatment of Lignocellulosic Pulps

“…Previously it was reported that the pH of aqueous solutions of phenolic lignin monomers affect their Raman spectra (Warsta et al 2012). UVRR spectra of the kraft pulps confirmed that pH has a substantial effect on the Raman scattering of lignin (Fig.…”

“…2). While increasing pH appeared to increase the Raman scattering intensity of phenolic monomers (Warsta et al 2012), with the pulp samples the trend was mainly the opposite, although not always consistent. Most pulp samples (10 of 12) analyzed after wetting with 0.2 M NaOH gave slightly lower maximum intensities (normalized to the cellulose band at 1093 cm -1 ), than the same samples analyzed at pH 6.…”

“…2). This overlapping band is typical for phenolic guaiacyl units of lignin (Warsta et al 2012). Because the shoulder was relatively more intense for the neutral sulfite-treated pulp in comparison with the reference, the sulfite treatment increased, as well as the content of phenolic guaiacyl units in the residual lignin.…”

“…It was reported earlier that the content of phenolic hydroxyl groups in lignin can be quantified through measuring UVRR spectra at neutral and alkaline pH (Warsta et al 2006, Lähdetie et al 2007, Warsta et al 2012. Thus, the pH-induced aromatic band shift (pH 6 buffer vs. 0.2 M NaOH) was applied to calculate the actual phenolic hydroxyl group content according to Warsta et al 2006 (Table 1).…”