XtrackT® GRAVITY FLOW SPE COLUMNS. XtrackT® large particle bonded phases allow for uniform gravity flow for most blood and urine samples. A single column provides extraction for a broad spectrum of compounds with selective elution of acid neutrals, steroids and bases. XtrackT® large particle (80-200 µm) silica gels are available with hydrophobic, hydrophilic, ion exchange or copolymeric phases, including DAU mixed mode. XtrackT® is recommended for viscous sample matrices or for gravity flow applications.