The calibration curves for determining the concentration of unlabeled SCFA (C3, i-C4, C4, i-C5 and C5) were individually plotted from the GC/MS measurement at concentration levels from 0.5 to 10 mmol/L containing known amount of 2.1 mmol/L tert-penta-noic acid (t-C5) as internal standard (Table2). Whereas for acetate, the linearity was tested in the concentration ranged from 0.5 to 20 mmol/L. Correlations between the GC/MS responses and the amount of analytes were verified by plotting signal intensity ratio of individual SCFA to internal standard against the ratio of their concentrations. Good linearity was achieved in all cases with regression coefficients higher than 0.9978. The linearity range was wide enough to cover the amount of SCFA determined in the biogas digester samples (Fig. 5).