# Understanding Oxygen Concentrator Failures in Low Resource Settings: The Role of Dust and Humidity
## 3 Results and Discussion  
The XRD analysis of E-1, E-2, U-1, U-2, SA, and C-X zeolite adsorbents is shown in Figure 2. For all the samples, a FAU crystalline framework was observed. More precisely, zeolites were crystallized in a cubic structure (space group: \(Fd\overline{3}\)) with a refined lattice parameter of 25.0720 (3) A for sample E-1, 25.0339 (4) A for sample E-2, 24.9910 (3) A for sample U-1, 24.9888 (3) A for sample U-2, 24.9864 (8) A for sample SA, and 25.0043 (4) A for sample C-X. An additional faijasite X phase was found in samples E-2 and SA, matched to the literature as having the chemical formula \(Na_{1.84}Al_{2}Si_{4}O_{11.92}\cdot 7H_{2}O\)[28]. On the other hand, an additional Linde-Type A (LTA) phase was detected in samples U-1, U-2, and C-X (space group Fm\(\overline{3}\)c). It is worth noting that the XRD pattern of the SA sample showed a \begin{table}
\begin{tabular}{c c c c c} \hline
**Country** & **Acronym** & **OC Model** & \begin{tabular}{c} **Working Time** \\ **(h)** \\ \end{tabular} & \
\begin{tabular}{c} **Oxygen Purity** \\ **(\% \%/v)** \\ \end{tabular} \\ \hline Ethiopia & E-1 & JAY 10 L & N.S. & 41 \\  & E-2 & Yuwell 7F-10 & 14,360 & 33 \\ \hline Uganda & U-1 & DeVilbiss & N.S. & 40 \\  & U-2 &
\begin{tabular}{c} Drive \\ DeVilbiss \\ \end{tabular} & 17,842 & 50 \\ \hline South Africa & SA & N.A. & N.S. & 85 \\ \hline \end{tabular}
\end{table} 
Table 1: Zeolite samples main characteristics (N.S. stands for “Not Specified”). shift in the characteristic peaks of X zeolite (see insert in Figure 2), possibly resulting from differences in the Na\({}^{+}\) and Li\({}^{+}\) content, resulting in the contraction of the unit cell [29]. The zeolite samples were characterized through the WDXRF analysis to determine the Si/Al and Al/Na molar ratios for each sample which are presented in Figure 3. The Si/Al ratio falls within the range of 1.16 to 1.24 (mol/mol). It is well known that the adsorption of N\({}_{2}\) from the air in OCs is conducted using FAU-LSX-type zeolites with a Si/Al ratio close to one [30; 31]. A Si/Al ratio between 1 and 1.5 is consistent with the expected composition of X-type zeolites [32; 33], as confirmed by the XRD patterns. On the other hand, a strong presence of Na was observed, resulting in an Al/Na molar ratio between 1.09 and 1.69. This result was expected, as LS zeolites are typically associated with Na\({}^{+}\) as the charge-balancing cation, which enhances affinity for molecules with a large quadrupole moment, such as N\({}_{2}\)[34]. The variation range of elemental composition is attributed to the presence of additional fuj a a a (phases as well as the presence of other extra framework cations (e.g., Li, Mg, Ca, K, and Fe) that balance the anionic charge of the structure. Specifically, this is observed in sample SA, which is affected by the presence of a second phase and the occurrence of Li. Metal cations such as Mg, Ca, K, and Fe were found in the samples by WDXRF, as reported in Figure 4 (note that Li cannot be detected by this technique). Here, the results are presented by grouping the samples according to the geographical region of provenance. This was performed to highlight the potential occurrence of impurities on the surface of the materials and to correlate them with the environmental conditions of the country in which they were employed. No significant variation in the wt% of the above-mentioned elements was observed between the samples, with the exception of the K amount in the C-X one. In this case, the higher presence of K is linked to the zeolite framework probably due to an incomplete ion exchange procedure adopted during the conversion from KX to LSX zeolite [35]. Since this sample was not employed as an adsorbent in African medical environmental settings, the presence of some kind of contaminant accumulated on its surface is obviously to be excluded.