Boron Analysis of Mehlich No. 3 Extractant with Modified Inductive Coupled Argon Plasma Techniques to Eliminate Iron Interference

Abstract

Abstract Correlation of Mehlich No. 3 (M-3) soil-extracted boron (B) to the traditional hot-water method has been high enough (r = 0.82) to warrant further investigation. The problem has been a lack of suitable analytical procedures to detect 0.01–0.02 ppm B in the extract with high precision. Instrumentation consisted of a dual-view, photodiode array charge-injection device fitted with a SeaSpray nebulizer with a mass controller set at 0.71 L/min. The plasma power was 1150 W, the pump speed was set at 100 rpm, and the integration time was at 15 s. These settings allowed the simultaneous measurement of phosphorus (P), potassium (K), calcium (Ca), magnesium (Mg), B, and zinc (Zn). The photodiode detection array has three sensitive B wavelengths that could be utilized; 208.959, 249.678, and 249.773 nm. The 249.678 and 249.773 lines are two to three times as sensitive and have twice the detection limit of the 208.959 line. Preliminary studies showed that iron (Fe) (249.782 line) interfered on the B 249.773 line, causing an inflated B extract reading of 0.06 µg/mL. The relative standard deviation for B concentrations <0.1 ppm in M-3 extractant (16 runs) was 1.17, and 1.15% for the 208.959, and 249.678 lines, respectively. Recovery of added B from 0.02 to 0.09 ppm ranged from 100 to 112%. Either of the two lines proved satisfactory, but the 249.678 line was more sensitive to low B concentrations. A field-soil greenhouse experiment showed that M-3 extracted B, measured by either the 208.959 or the 249.678 lines, correlated at r = 0.85, 0.84, and 0.67 with shoot-tissue B concentration for cauliflower, sweet corn, and tomato, respectively.