Distinction of n-alkanols in biodiesel fuel by diffusion-ordered NMR spectroscopy

Recently, higher alkanols have attracted the attention of researchers because, compared with other widely studied lower alkanols (such as ethanol and methanol), they have higher energy density, higher cetane number, better mixing stability and lower hygroscopicity. In biodiesel, alkanols are often added to improve the combustion performance of diesel engines. The qualitative of alkanol components is particularly important for the quality control of biodiesel. A reliable analysis method is demanded to distinguish the various alkanols in biodiesel. This study employed diffusion-ordered nuclear magnetic resonance spectroscopy (DOSY NMR) to establish a quantitative relationship model between diffusion coefficients and molecular weights of n-alkanols in DMSO‑ d 6 . It is found that the deviation from theoretical molecular weight (ΔMW) of the external calibration curves (ECCs) using dimethyl sulfoxide (DMSO) (0.46–1.88) as internal standards can significantly reduce the MW error than the internal calibration curves (ICCs) (1.05–2.65). The calibration curve of ECCs shows excellent fitting degree (R 2  = 0.999) and effectively mitigates the impact of external variables: ΔMW remains within ±7 when the concentration ranges from 0.05 to 100 mg/mL or the temperature varies from 288 K to 308 K. Based on the molecular weight determined by this method, n-alkanols that differ by only one carbon atom could be distinguished accurately, and the method has been validated in actual biodiesel samples containing C 4 -C 8 n-alkanols.