Raman spectroscopy coupled to high-resolution continuum source flame molecular absorption spectrometry for sequential determination of nitrogen species in fertilizers.

Raman spectroscopy (RS) was used to identify and quantify different nitrogen species in fertilizers. This is a fast and inexpensive method that requires no extensive sample preparation. Urea and nitrate were determined at 1000 and 1045 cm-1, respectively. Calibration plots obtained for these analytes showed adequate linearity, with regression coefficients (r) of 0.9989 and 0.9976, respectively. Ammonium was determined by difference after total N determination by high-resolution continuum source flame molecular absorption spectrometry (HR-CS FMAS), which provided a calibration plot with r = 0.9960. The inline coupling of RS and HR-CS FMAS allowed for a fast sequential determination of ammonium, nitrate, and urea, with limits of detection of 0.03 mg/L ammonium, 0.03 mg/L nitrate, and 0.01 mg/L urea. Relative standard deviations were ≤ 11 %, and the external standard calibration method provided accurate results for all analytes determined in certified reference materials, raw materials, and commercial samples of fertilizers. For comparison purposes, all samples were also analyzed by traditional Kjeldahl method. The RS HR-CS FMAS method was further validated by addition and recovery experiments, which provided recoveries in the 93 - 113 % range.

Simultaneous determination of phosphite and phosphate in fertilizers by Raman spectroscopy.

Raman spectroscopy is emerging and powerful technique for identifying oxyanions of phosphorus, but it still is not straightforward applied to identify and quantify simultaneously phosphite and phosphate species in solution. Herein, simultaneous determination of phosphate (PO4) and phosphite (PO3) in fertilizer were evaluated by Raman spectroscopy. The influence of pH on Raman spectra of species was evaluated at various solution pH values, and the results showed accurate and selective analysis for phosphate and phosphite by using pH = 10.0 and bands located at 874 cm-1 and 2321 cm-1, respectively. Linear working range in the 0.15%-6.20% (w/v) P concentration was consistently obtained with regression coefficients (r) of 0.9953 (PO4) and 0.9945 (PO3). The limits of detection were 0.10% (w/v) P (PO4) and 0.05% (w/v) P (PO3). Relative standard deviations were lower than 6% for samples containing 10% P2O5 (PO4) and 31% P2O5 (PO3). Commercial fertilizers were analyzed by the proposed method using external calibration and found concentrations of P were in agreement with those obtained by the comparative spectrophotometric method at the 95% confidence level (paired t-test). Recoveries of spiked samples were in the 92-110% range.