Detection of calcium phosphate species in soil by confocal μ‐Raman spectroscopy^#

Abstract

Abstract Background Raman spectroscopy is a promising but largely underexplored tool for the detection of phosphates (P) in soil. Although it requires minimal sample preparation, it has been demonstrated mainly in test matrices or substrates to circumvent the typical signal interference caused by fluorescence of organic matter in actual agricultural soils. Aims The aim of this study was to highlight the Raman spectroscopic detection and identification of distinct calcium phosphate species amended in contrasting soil matrices—including a real arable soil. Methods Pure calcium dihydrogen phosphate [Ca(H 2 PO 4 ) 2 ·H 2 O], calcium hydrogen phosphate (CaHPO 4 ), and β‐tricalcium phosphate [β‐Ca 3 (PO 4 ) 2 ] were each amended in Luvos ® healing earth, loess from a C‐horizon, and a loam arable soil from an Ap‐horizon at a dose of 1 mmol (10 g soil) –1 . The unique Raman signature of each pure calcium phosphate species was determined by confocal μ‐Raman spectroscopy and used subsequently as a reference spectrum to identify the compound in each soil matrix. Controls without added P were also analyzed. Results Ca(H 2 PO 4 ) 2 ·H 2 O, CaHPO 4 , and β‐Ca 3 (PO 4 ) 2 were each unambiguously detected in the treated soils. Native hydroxyapatite [Ca 5 (PO 4 ) 3 (OH)], quartz (SiO 2 ), feldspar (NaAlSi 3 O 8 ), calcite (CaCO 3 ), and dolomite [CaMg(CO 3 ) 2 ] were also identified, for example, in the Raman microscopic image of the control Luvos ® healing earth sample. Intrinsic β‐Ca 3 (PO 4 ) 2 and Ca 5 (PO 4 ) 3 (OH) present in Ap‐horizon loam were detected and distinguished from each other by a Lorentzian fitting, which deconvoluted the individual Raman signals from an unresolved peak. Conclusions The usefulness of confocal μ‐Raman spectroscopy to detect distinct P species present in agricultural soil could be shown as a proof of concept.

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