# Portable X-ray Fluorescence Analysis of Organic Amendments: A Review
## 3 Results and Discussion
### Determination of Elemental Content with pXRF
For Cu, Pb, and Zn, the LOQs were below the legal limit which indicates that these elements could be detected up to the regulatory level and accurately determined if an adequate calibration is available for amendments. It should be noted that for Cu, Cr, and Ni, our results in samples of organic amendments led to higher LOQs than the data reported in the literature, which could indicate a more intense interfering effect of organic matter depending on the measurement conditions (in our case, the Soil method, with a Niton XL3t 950s GOLDD +, Thermo Scientific, Billerica, MA, USA). The LOQs of pXRF or their respective LODs are poorer than those of large laboratory-based XRF instruments or of ICP-related techniques. Several of the previous studies [28; 32; 43] also evaluated the influence of other factors that will be outlined in the following sections. The above studies highlight the limited use of pXRF for the analysis of organic amendments. Despite the fact that the evolution of the technique has led to better detection limits that would have allowed the measurement of almost all the elements of interest in these ma trices, the results of the various studies are contradictory. In general, they show important divergences between the results obtained by pXRF and the reference analytical procedure used in each study. There may be different reasons to account for these divergences. One explanation could be the different calibration strategies they have used (Table 2). In a majority of studies, the pre-calibrated method called \"Soil\" has been used in the instruments, but in some studies other pre-calibrated procedures have been used. More information about pre-calibrated analytical modes in different pXRF instruments and their relevance and applicability to geochemical investigations can be found in a recent review by [PERSON] and [PERSON] [2]. On some occasions, calibration corrections based on measurements made in reference samples have been used. Measurement results should have been validated by the analysis of a suitable reference material but this procedure has not always been followed. On many occasions, the CRMs used are soils or sediments in the absence of similar organic materials certified for XRF. It is possible that the different pre-calibrations, by making use of different spectral peaks for the same element and different correction formulas, may give rise to different deviations due to the sample matrix, causing a good fit of some elements in some matrices and not in others. We must assume that the Soil method is adapted to materials such as soils and sediments. The Soil method is based on Compton normalization and provides the best results for a wide range of environmental applications. The Soil method is applicable to measure contents below 1% in samples composed mainly of light elements [2]. Soils and sediments are matrices that usually have contents of the organic fraction an order of magnitude lower than the organic amendments, and we do not know a priori the proper adaptation of the in-built method. In a following section, we will review the studies regarding the influence of the organic content. In most of the studies discussed, the comparison of methods (pXRF vs. wet chemistry method) was elucidated by calculating the correlation (r) or the regression (r or R\({}^{2}\) and slope) between both procedures. Especially for the case of comparing a simple or approximate method (as is the case of the pXRF measurement of the elements in concentrations closer to the LOQ) with a more precise one, these statistical parameters are inadequate to determine the similarity of methods, as [PERSON] and [PERSON] revealed in their classic work [46]. It would be necessary to use other methods for the statistical comparison of the procedures.