Raman Spectroscopic Analysis of Fertilizers and Plant Tissue for Perchlorate

Abstract

Raman spectroscopy, without the need for prior chromatographic separation, was used for qualitative and quantitative analysis of 59 samples of fertilizers for perchlorate (ClO₄⁻). These primarily lawn and garden products had no known link to Chile saltpeter, which is known to contain perchlorate. Perchlorate contamination is emerging as an important environmental issue since its discovery in water resources that are widely used for drinking, crop irrigation, and recreation in the Western U.S. Approximately 90% of the 32 fertilizer samples acquired between November 1998 and January 1999 contained perchlorate, and of these, the level was typically high (500–8000 mg/kg). However, only two of the 27 fertilizer samples acquired from June 1999 to May 2000 contained perchlorate and at a lower level than previously observed (i.e., ∼ 250–350 mg/kg). Raman spectroscopy was also applied to the qualitative identification of perchlorate in extracts of lettuce that had been grown in a greenhouse using irrigation water spiked (at 10 ppm) with perchlorate. Generally, with a fertilizer extract, detection limits were ∼ 50 ppm for perchlorate with short spectral acquisition times (∼5 min). Quantitation was typically accurate to within 2 to 14%, with an average %CV (coefficient of variation) of 6.3 among replicate runs. We compare Raman spectroscopy to ion chromatography, which is the currently recommended method for perchlorate analysis.