Determination of humic substances in sediments by focused ultrasound extraction and ultraviolet visible spectroscopy

• Humic and fulvic acids were isolated and quantified in sediments.
• Process variables were optimized in FUSE pretreatment previous to UV–vis analysis.
• The new methodology avoids unnecessary steps included in traditional procedures.

The development of a new/fast methodology in order to determine the amount of organic matter (HSs) present in sediment samples was deeply optimized in this work. Emphasis was performed on alkaline extraction because traditional methods required high volumes and long extraction times. Focused ultrasound solid liquid extraction (FUSLE) followed by separation of HSs in acidic media and determination by means of UV–Vis turned out to be a very suitable analytical procedure to speed up HSs extraction, specifically humic acid (HA) and fulvic acid (FA). All variables affecting the extraction process (sodium hydroxide concentration and volume, ultrasound power, cycles and time) were screened by using the analysis of variance and multivariate design. These variables led optimum values of 29 min, continuous application power of 95% (10 cycles) and 15 ml of NaOH 1 mol l− 1. This methodology was found precise (RSD of 3.3% for HA and 3.7% for FA, for 5 determinations in 3 different days) and sensitive (LODs were 0.1 μg g− 1 and 0.3 μg g− 1 for HA and FA, respectively). Finally, the addition of Mn(II) was proven to be a good alternative to prevent the degradation of HSs in the extraction step.