Evaluation of matrix effects on automated nutrient determination in seawater using segmented flow analysis

Enhancing the quality of nutrient data and improving the comparability of measurements across different laboratory instruments are critical. Matrix effects from seawater are known to impact the accuracy of nutrient determinations using segmented flow analysis (SFA)-based autoanalyzers. However, to date, few studies have specifically quantified the extent of these matrix effects or identified the contributing factors, particularly when ultrapure water is used for standard solutions, carriers, and washing solutions. This study systematically investigated and quantitatively assessed, for the first time, the factors and extent of matrix effects on the determination of Si(OH)(4), NO3-, NO2-, and PO(4)(3-)sing a four-channel nutrient autoanalyzer. The matrix effects were reclassified into eight categories based on their causes and sources. It was found that in addition to the welldocumented effects on Si(OH)(4), significant matrix impacts were also observed for NO3- and PO43-. For seawater with a salinity of 35, Si(OH)(4) was significantly underestimated by approximate 15%, while NO3- was overestimated by about 10%. PO43- as also overestimated. Results indicated that the ionic strength of sample affected Si(OH)(4), while PO43- as influenced by the static refractive index, which increased with salinity. Matrix effects on NO3- were attributed to both ionic strength and reduced efficiency of the coppered cadmium column due to chemical interactions.