First, the stability and performance of 1-octanol with plasma samples was attempted improved by mixing it 1:1 (weight ratio) with 2-nitrophenyl octyl ether (NPOE), based on previous reports . However, this did not improve the stability. Alternatively, tri(butyl) phosphate was reported by Wu et al. as an efficient liquid membrane for EME of three barbiturates (acids) , despite not offering any HBD interactions that otherwise previously was assumed as required for EME of acidic analytes. The Kamlet-Taft values are α = 0.00, β = 0.88, π = 0.65, and the solvent is assumed to extract acids primarily by dipole interactions. Tri(butyl) phosphate was highly efficient in extractions from a neat standard sample, but performance deteriorated in plasma samples and current increased with time. Additionally, tri(butyl) phosphate is a suspected carcinogen and was therefore considered an unsuitable solvent. Instead, other phosphates were investigated. Tri(pentyl) phosphate (α = 0.00, β = 0.80, π = 0.63) yielded a stable and low current (30 µA) and moderate extraction recoveries of analytes of 2 ≤ log P ≤ 4. The solvent has recently also been identified as suitable for EME of hydrophobic peptides , as well as some basic small-molecule substances .