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

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(H2PO4)2·H2O], calcium hydrogen phosphate (CaHPO4), and β‐tricalcium phosphate [β‐Ca3(PO4)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(H2PO4)2·H2O, CaHPO4, and β‐Ca3(PO4)2 were each unambiguously detected in the treated soils. Native hydroxyapatite [Ca5(PO4)3(OH)], quartz (SiO2), feldspar (NaAlSi3O8), calcite (CaCO3), and dolomite [CaMg(CO3)2] were also identified, for example, in the Raman microscopic image of the control Luvos® healing earth sample. Intrinsic β‐Ca3(PO4)2 and Ca5(PO4)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.

Shifted excitation Raman difference spectroscopy for soil component identification and soil carbonate determination in the presence of strong fluorescence interference

“…[ 15,20 ] This finding is in accordance with our previous Raman and SERDS studies on selected soil types where the detection of hydroxyapatite has already been demonstrated. [ 5,12 ] This result is not surprising as all calcium orthophosphates, for example contained within commercial fertilizers, convert to hydroxyapatite in soil over time. [ 9 ] It is important to note that the intense and spectrally broad feature above 1140 cm −1 in the shown SERDS spectrum of hydroxyapatite is not a Raman signal as it does not shift with the shift in excitation wavelength as applied for SERDS.…”

“…In the case of micro-Raman spectroscopy with spot sizes on the order of a few micrometers, identification of individual soil constituents is rather easy as for each measurement spot mostly pure component spectra can be acquired. [5,11,12,17] It should be noted that in our study, an excitation and collection spot size of approximately 100 μm has been selected to average out soil spatial heterogeneities on the micrometer scale. The recorded spectra from individual measurement spots will thus very likely contain molecule-specific information from more than a single species in most cases.…”

“…The major reason for the scarce use of Raman spectroscopy is the interference of fluorescence [4] mainly arising from soil organic matter [5,6] and clay minerals. [7,8] In some cases, strong fluorescence can even prevent successful Raman analysis of soil samples.…”

“…[11] However, Raman microscopic investigations conducted by our group have demonstrated that residual fluorescence interference is still an issue potentially masking weak Raman signals even when a confocal geometry is chosen. [5,12] Due to the severity of fluorescence interference in many cases, additional approaches were required to extract useful Raman spectroscopic information from soil specimens. One of the simplest techniques is a mathematical removal of background contributions as applied for the analysis of Chinese farmland soils, [13] soil characterization at archeological sites [14] and the identification of phosphates in soil.…”

“…[ 11 ] However, Raman microscopic investigations conducted by our group have demonstrated that residual fluorescence interference is still an issue potentially masking weak Raman signals even when a confocal geometry is chosen. [ 5,12 ]…”