Coupling sequential injection on-line preconcentration using a PTFE beads packed column to direct injection nebulization inductively coupled plasma mass spectrometry

Abstract

An automated sequential injection on-line preconcentration procedure for trace metals by using a PTFE bead-packed microcolumn coupled to ICP-MS is described, and used for simultaneous analyses of cadmium and lead. In dilute nitric acid (0.5%, v/v), neutral complexes between the analytes and chelating reagent, diethyldithiophosphate (DDPA), are formed and adsorbed onto the surface of the PTFE beads. The adsorbed complexes are afterwards eluted with 20% nitric acid and the leading part of the eluate (40 µl) is stored in a sample loop (SL), the contents of which are subsequently transported, via a direct injection high efficiency nebulizer (DIHEN), into the ICP-MS for quantification. The packed column (PC) generates considerably lower hydrodynamic impedance than other commonly used sorbent columns, while its preconcentration efficiencies, in terms of retention efficiency and enrichment factor, are significantly improved as compared to a knotted reactor (KR). Inherently, the column approach results in low blank values and consequently in low limits of detection (LODs). With a 40 s sample loading time at 4.5 ml min⁻¹, along with a sampling frequency of 18 h⁻¹, the retention efficiencies/enrichment factors for cadmium and lead were 96%/72 and 104%/81, respectively, versus 45%/34 and 54%/41 for a knotted reactor with similar internal surface area. In addition, the limits of detection (LODs) and precisions were at the same levels, i.e., the LODs were 2.9 ng l⁻¹ Cd (PC), 3.5 ng l⁻¹ Cd (KR), 6.0 ng l⁻¹ Pb (PC) and 4.7 ng l⁻¹ Pb (KR), while the RSDs were 1.9% (Cd, PC), 2.7% (Cd, KR), 2.2% (Pb, PC) and 2.5% (Pb, KR). The procedure was validated by determination of trace cadmium and lead in certified reference materials and urine samples.