A chemometric approach exploring Derringer's desirability function for the simultaneous determination of Cd, Cr, Ni and Pb in micronutrient fertilizers by laser-induced breakdown spectroscopy

Abstract

Given the large variety of raw materials used in manufacturing micronutrient fertilizers, the quantitative determination of Cd, Cr, Ni and Pb in such complex matrices is challenging and often impairs the routine analysis even in modern laboratories. In the present work, a LIBS method for the direct analysis of pelletized test samples was developed to find a solution where the optimization relied on both analytical sensitivity and limit of detection by combining Doehlert experimental design with Derringer's desirability function. The test samples were analyzed in the form of pressed pellets prepared from a large variety of laboratory samples previously comminuted in a planetary ball mill. LIBS experiments were carried out with a Q-switched Nd:YAG laser at 1064 nm, and a spectrometer with Echelle optics and an intensified charged-coupled device. The optimized operating conditions were thus found with laser pulses of 25 J cm−2 (750 μm laser spot size), 30 accumulated laser pulses per site, 2.5 μs gate delay, and 10.0 μs integration time gate. Cd, Cr, Ni and Pb mass fractions, obtained by a validated reference method based on microwave-assisted acid digestion and ICP OES measurements, were used to build the calibration models. Linear correlation coefficients between the analyte mass fractions and the corresponding emission intensities ranged from 0.9980 for Pb to 0.9999 for Cr, and the limits of detection were 5 mg kg−1 Cd, 7 mg kg−1 Cr, 15 mg kg−1 Ni and 50 mg kg−1 Pb. The coefficients of variation of measurements (n = 10 sites per pellet, 30 laser pulses per site) varied from 0.5% (Ni, Pb) to 15% (Cd, Cr). The proposed method simplifies the analytical sequence and complies with the purpose of fast and accurate assessment of inorganic contamination profiles in micronutrient fertilizers.