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Fig. 3. Concentrations of formic and acetic acids analyzed by the external calibration and standard addition methods in 3- and 6-mo-old compost samples (mean ± SD; for external calibration n = 14, for standard addition n = 4). Amounts of acids analyzed by the standard addition method are significantly higher than by the external calibration method (** indicates significance at the 0.01 probability level), especially in 6-mo-old compost, where the difference is three to four times.
