Chromatograms depicting sample purification (A and C), followed by sample concentration (B and D) using SPE techniques. Samples were extracted from B₁₂ medium cultured with strain J15 (A and B) and fortified Kefir (C and D). Vitamin B₁₂ concentration was measured (in μg·mL⁻¹): 0.77 (A) and 0.34 (C) before and 3.77 (B) and 3.36 (D) after sample concentration. Retention time (t _R) marks each B₁₂ peak.