# Portable X-ray Fluorescence Analysis of Organic Amendments: A Review
### Fundamentals and Evolution of pXRF  
Portable X-ray fluorescence spectrometry allows the identification and quantification of elements from Mg to U in the periodic table. The primary X-rays are incident on the sample and, as a result, the absorption of an incident X-ray photon by an atom takes place as long as its energy is greater than the binding energy of the core-shell electron. The absorption of incident photons results in the emission of the core-shell/subshell electron (also known as a photoelectron), leaving a core-shell/subshell vacancy (i.e., the atom is in an excited state). XRF emissions occur in the aftermath of absorption by two competitive atomic de-excitation processes: (1) characteristic or X-ray fluorescence (XRF) and (2) Auger electron emissions. Both processes imply an electron transition from an upper subshell to a lower one within the atom. They are the basis of spectroscopic methods to identify the atom since both the XRF photons and the Auger electron have unique energies characterizing the electron transition in the given atom. XRF spectroscopy methods, however, are more practical than Auger electron spectroscopic methods due to the high attenuation of electrons in matter. The de-excitation transitions occur at characteristic energies unique to each particular element, so XRF can be used to determine the elements in the sample. In modern portable instruments, the primary X-ray radiation is generated in a vacuum-tight tube containing a cathode (Figure 1). The small coin-size of the field-emission X-ray tubes and their much simpler design than the large Coolidge-type tube used in industrial or diagnostic X-ray imaging applications allowed miniaturization. A high voltage of 20-60 kV accelerates the electrons towards a metal (such as Ag, Rh, Ta, Au, W, and others) target anode. When the electrons collide with the metal target, X-rays corresponding to characteristic K and L line fluorescence of the target atoms and a lower intensity continuum are produced.  
It is possible to find the fundamentals of X-ray spectrometry in the classic book of [PERSON] [3], detailed descriptions of technological aspects in the [PERSON] et al. handbook [4], recent developments of pXRF in reviews such as those of [PERSON] et al. and [PERSON] et al. [5; 6], and some others specific to fields such as geochemistry [7] and soils and environmental samples [8]. It is also possible to find interesting reviews focused on the historical development of the technique, such as that by [PERSON] [9], who reported on the 2012 James L. Waters Symposium. [PERSON]'s review described the development and commercialization of portable, hand-held X-ray fluorescence (XRF) spectrometers, the birth and the maturation of pXRF spectrometers, the performance improvements in miniature X-ray sources made by his company, Moxtek, the development of detectors, and the evolution of analytical capabilities of pXRF analyzers. In the symposium summarized by [PERSON] [9], [PERSON] described the birth and the maturation of hand-held XRF spectrometers. The development of hand-held XRF devices is mostly due to Dr. [PERSON], Professor of Physics at MIT, who founded the Niton Corporation in 1986. Among many awards, [PERSON] received the 2021 Birks Award for his contributions [10] to the development of portable XRF devices.