# Portable X-ray Fluorescence Analysis of Organic Amendments: A Review
### Reference Procedures for Organic Amendments  
The standardized procedures for the determination of the content of heavy metals and other properties in compost and organic amendments usually imply the previous drying and grinding (at least to less than 0.5 mm) of the sample. The sample is then digested in strong acids. For most metals, two digestion procedures are available. Probably, the digestion with _aqua regia_ (EN 13650:2001 [11], ISO 54321 [12]) is the most widely used. The European procedures are equivalent to US EPA methods 3050B and 3051 [13] intended for sediments, sludges, soils, and oils. These procedures will not totally dissolve most soil improvers or growing media, and the efficiency of extraction for particular elements differs from element to element and might also differ for the same element in different matrices. The very strong acid digestion will dissolve almost all elements that could become \"environmentally available\", but elements bound in silicate structures are not normally dissolved by these procedures as they are not usually mobile in the environment. Therefore, the concentration of extractable elements in _aqua regia_, which is sometimes called \"_pseudo-total_ concentration\" cannot be considered as \"total\" but rather represent a fraction, greater or lesser depending on the chemical structure of the sample constituents. Complete digestion, including silicates, could be achieved by procedure EN 13656:2020 [14] which uses a mixture of hydrochloric (HCI), nitric (HNO3), and tetrafluorobic (HBF4) or hydrofluoric (HF) acids, or by the alternative US EPA method 3052 [13] which is applicable to the microwave-assisted acid digestion of siliceous matrices (ashes, sediments, and soils), and organic matrices (biological tissues, oils, oil contaminated soils, sludges, and other  
Figure 1: Functional schematic of a portable pXRF instrument (with permission of ThermoFisher, Waltham, MA, USA). complex matrices) if a total decomposition analysis is required. Elements extractable by these procedures can be described as \"total\". The following methods of the United States Composting Council [15] are equivalent to US EPA methods and also contemplate the use of hydrofluoric acid for siliceous matrices: 04.12-A (Microwave-Assisted Nitric Acid Digestion of Compost), 04.12-B (Nitric Acid Digestion of Compost and Soils), 04.12-C (Dry Ash Sample Digestion for Plant Nutrients), and 04.12-E (_Aqua Regia_ Procedure). After sample digestion, element concentrations can be measured by inductively coupled plasma optical emission spectrometry (ICP-OES) or other adequate techniques. Recently, spectroscopic methods such as near-infrared spectroscopy (NIRS) and Fourier-transform infrared spectroscopy (FTIRS) have been proposed for compost analysis [16]. While pXRF is an elemental detection and quantitation technique, the infrared spectroscopies identify molecules via quantized absorption of infrared light by molecules in their transitions between various vibration/rotational quantum states. The resulting spectra can be linked to several measured compost indicators. NIRS and FTIR can be used as an estimate of the compost indicator if good correlation between the spectral signal and the indicator is obtained. The spectroscopic measurements thus allow for fast screening of chemical properties and compost stability and are time and cost efficient. There are two standard European protocols for applying XRF to residues. The standard EN 15309:2007 [17] specifies the procedure for a quantitative determination of major and trace element concentrations in homogeneous solid waste, soil, and soil-like material by energy dispersive X-ray fluorescence (EDXRF) spectrometry or wavelength dispersive X-ray fluorescence (WDXRF) spectrometry using a calibration with matrix-matched standards. The procedure is therefore the standardized method for laboratory instruments and requires careful and time-consuming sample preparation.