# Portable X-ray Fluorescence Analysis of Organic Amendments: A Review
## 3 Results and Discussion
### Determination of Elemental Content with pXRF  
In addition to the results of the indicated searches, the oldest study that we know which uses pXRF in manure compost samples is that of [PERSON] et al. (2008) [42]. This article does not appear in the database searches performed, possibly because it does not include the search terms among its keywords or its title. The study evaluated the LOD of the pXRF technique on compost concerning EPA-regulated metals and concluded that the LOD of Ni, Cu, Zn, Se, Mo, and Pb were enough to use the technique as a screening tool. On the contrary, pXRF was not effective at detecting Hg, Cd, or As. Leaving aside the different legal regulations and the different instruments involved, the impossibility of determining the metals Cd and Hg in these matrices coincides with what is stated in Section 3.3. Chronologically, the following study on the subject is that of [PERSON] et al. [28] in 2012. The authors calculated LOD in compost samples for the elements As, Ca, Cd, Cr, Cu, Fe, K, Mn, P, and Zn and stated substantial improvements for all elements compared to the previous study by [PERSON] [42] attributable to improvements in technology. Their conclusion was that pXRF was able to acceptably quantify Ca, Cr, Cu, Fe, K, Mn, P, and Zn in dry samples but the divergences were important for As and Co.  
[PERSON] et al. [39] characterized metals in Irish sludges that had undergone treatment by thermal drying, lime stabilization, or anaerobic digestion. Correlation coefficients between pXRF and ICP-MS (_aqua regia_ digestion) results indicated the suitability and satisfactory use of the pXRF technique for the quantification of Fe, Cu, Se, Zn, and Pb (r > 0.90). However, it should be noted that the regulated levels in sludge are higher than for other amendments (Table 1). [PERSON] and [PERSON] [43] used pXRF for soil analysis but, being Scottish peat soils with very high OM contents, it is really equivalent to the matrices we consider in this review. [PERSON] and [PERSON] determined the LOD in the organic soils and found values below 5 mg kg\({}^{-1}\) for Cr, Ni, Zn, As, and Sr, values between 5-10 mg kg\({}^{-1}\) for Cu, and values between 10-15 mg kg\({}^{-1}\) for Mn and Pb. These LOD indicated better sensitivity for Cr, Ni, and Zn and worse for Mn and Pb (and also Fe and Ca) compared to the previous [PERSON] study [28]. The authors compared the concentrations of K, Ca, Ti, Cr, Mn, Fe, Ni, Cu, Zn, As, Sr, and Pb determined by pXRF with _pseudototal_ concentrations determined by _aqua regia_ extraction and ICP-OES. Their results indicated that the in-built calibrations of the instrument were not adequate for these organic soils and should be modified. The authors indicated that mineral or peat soils were not completely dissolved by the _aqua regia_ reflux procedure they used, leaving behind undissolved refractory minerals such as quartz and feldspars, so comparison of _aqua regia_ extraction with pXRF analysis is not straightforward. Nevertheless, the authors attribute the difference between the pXRF vs. _aqua regia_ extractable analysis to matrix effects in the pXRF and not to low recovery by _aqua regia_/ICP analysis. Although it does not appear in the database searches undertaken, it is necessary to cite the article by [PERSON] and [PERSON] [44]. The authors applied pXRF to litter analysis and found good results only for the major elements calcium and potassium. In 2018, two studies by [PERSON] et al. [30] and [PERSON] et al. [24] adapted methods developed for soils and used elemental data as a proxy for the prediction of derived properties, such as compost electrical conductivity (EC) and cation exchange capacity (CEC), respectively. To calculate the elemental contents, the recovery percentages based upon NIST certified reference material 2711a were used to establish correction factors for each element. [PERSON] et al.