## 4 Discussion  
The aim of this study was to develop an SPME-LC-MS untargeted metabolomic method focused on isolating, separating, and analyzing lipids--with a specific focus on fatty acids and fatty acid-like molecules. These compounds were selected because they have previously been found to be implicated in psoriatic disease and may aid in classifying PsA patients from PsC patients [6; 10]. Additionally, the high prevalence of metabolic syndrome and cardiovascular events in PsA patients makes lipids of key interest in psoriatic disease. SPME-LC-MS was chosen as the analytical assay due to the streamlined, semi-automated workflow, the varying compound classes capable of being extracted, and previous research that has shown SPME to be comparable to common lipid extraction methods such as Folch extraction and Bligh and Dyer [43]. \begin{table}
\begin{tabular}{c c c c} \hline
**m/z** & **RT (min)** & **Tentative ID** & **Adduct** & **Biochemical Importance** \\ \hline
342.1558 & 1.5 & Tryptophyl-Histidine & [M+H]\({}^{*}\) & Dipeptide resulting from an incomplete breakdown in protein catabolism. \\ \hline
457.2308 & 9.1 & LPA(18:2(9Z,12Z)/0:0) & [M+Na]\({}^{+}\) & Intercellular lipid mediator. \\ \hline
473.1933 & 4.2 & Sofalcone & [M+Na]\({}^{+}\) & Mucosal protective agent. \\ \hline
491.2125 & 4.2 & Cymorcin diglucoside & [M+H]\({}^{*}\) & Phenolic glycoside. \\ \hline
489.2246 & 3.5 & Dolichyl b-D-glucosyl phosphate & [M+Na]\({}^{+}\) & Sesquiterpenoid. \\ \hline
374.147 & 3.8 & Phenylalanyl-Tryptophan & [M+Na]\({}^{+}\) & Dipeptide resulting from an incomplete breakdown in protein catabolism. \\ \hline
361.2007 & 5.1 & Aldosterone & [M+H]\({}^{*}\) & Aldosterone is a hormone that regulates sodium and potassium. \\ \hline
334.1588 & 14.3 & Arginyl-Histidine & [M+Na]\({}^{+}\) & Dipeptide resulting from an incomplete breakdown in protein catabolism. \\ \hline
780.5307 & 7.6 & (3\({}^{\prime}\)-sulfo)Galbeta-Cer(d18:1/16:0) & [M+H]\({}^{*}\) & Found in particularly high concentrations in myelin. \\ \hline
815.6991 & 22.1 & SM(d18:1/24:0) & [M+H]\({}^{*}\) & Sphingolipids were found in cell membranes. \\ \hline
204.123 & 0.9 & Acetyl-D-carnitine & [M+H]\({}^{*}\) & Long-chain acylcarnitine involved in beta-oxidation. \\ \hline
391.2089 & 4.8 & 19R-hydroxy-PGE2, 20-hydroxy-PGE2 & [M+Na]\({}^{+}\) & PGE2 stimulates bone resorption by osteoclasts, increases vasodilation, and increases cAMP production. \\ \hline \end{tabular}
\end{table} 
Table 10: Tentatively annotated features showing statistically significant differences between psoriasis and psoriatic arthritis patients during positive mode acquisition. \begin{table}
\begin{tabular}{c c c c c} \hline
**m/z** & **RT (min)** & **Tentative ID** & **Adduct** & **Biochemical Importance** \\ \hline
243.1715 & 4.1 & Isoleucyl-Isoleucine & [M-H]\({}^{*}\) & Dipeptide resulting from an incomplete breakdown in protein catabolism. \\ \hline
935.3194 & 0.8 & Bilirubin diglucuronide & [M-H]\({}^{*}\) & Glucuronidated derivative of bilirubin. \\ \hline
997.3346 & 0.8 & Sialyllacto-N-tetrarose a & [M-H]\({}^{*}\) & Oligosaccharide found in breast milk. \\ \hline
350.1517 & 4.4 & Tryptophyl-Phenylalanine & [M-H]\({}^{*}\) & Dipeptide resulting from an incomplete breakdown in protein catabolism. \\ \hline
179.0565 & 0.9 & 2.3-Dihydroxy-2-methylbutanoic acid & [M+Formate]\({}^{*}\) & Hydroxy fatty acid. \\ \hline \end{tabular}
\end{table} 
Table 11: Tentatively annotated features showing statistically significant differences between psoriasis and psoriatic arthritis patients during negative mode acquisition. Thirteen chromatographic methods in positive mode and twelve methods in negative mode, with varying solvents, additives, and gradients, were tested on two HPLC columns. Each method's performance was evaluated using instrumental quality control standards. The Accucore C30 column provided slightly narrower peaks and was thus chosen as the most suitable column for analysis. Chromatographic methods that used methanol instead of acetonitrile in their mobile phase B performed better.