Instead, we opted to use ammonium formate (buffer pH ranges of 2.77-4.77 and 8.25-10.25) to reduce the issues observed with pure formic acid. As an eluent additive, suggested concentrations range broadly between 5-300 mM ; however, eluent additives above approximately 0.1 M are susceptible to precipitation and should be used cautiously. For this reason, we successively diluted recommended concentrations of ammonium formate eluent and attempted separation at the reduced concentrations. We found only a modest amount (0.143 mM reported) of ammonium formate was required for excellent peak shape and performance. The capacity factor for psilocybin with this method was measured as 1.23, on average. Peak shape for both analytes (psilocybin and psilocin) was separately measured by the average signal of triplicate QC sample injections. The Gaussian fit of this data agrees well (0.967 Pearson R 2 ) indicating psilocybin resides preferentially in the protonated state. The average QC retention time was measured at 0.615 minutes for psilocybin with a peak width of 0.0504 mins (FWHM). Peak shape and retention time for psilocin were also determined through the average of triplicate psilocin QC sample injections. The Gaussian fit of this peak also agrees well (0.939 Pearson R 2 ) and resolves suitably (0.125 mins FWHM). No additional peaks were observed for the psilocybin standards or QC samples, or the psilocin QC samples. It is worth noting that psilocybin may be monitored at 268 or 290 nm for enhanced selectivity (Psilocybin Product Information, 2021) at the cost of sensitivity. For purified samples and this eluent system (H2O / MeCN), 220 nm is effective for determining psilocybin and psilocin concentration . Both compounds absorb strongly at this wavelength and excellent accuracy may be expected at concentrations above a low (2.1 ppm) method limit of detection. The low concentration of ammonium formate used in this method minimizes spectral background, reduces reagent cost for high-throughput laboratories, and is also appropriate for gentle treatment of newer silica columns. For most analytical separations, the concentration and injection volumes used are low (ppm to ppb concentrations and µL scale volumes) and therefore it is possible to use dilute formulations and still provide adequate buffering capacity .