The high-performance liquid chromatography (HPLC) of metal chelates is a powerful approach for the determination of ultratrace metal ions, providing advantages in sensitivity, toughness against coexistence matrix, and cost-effectiveness.
1A precolumn chelation HPLC system, which employs an eluent without a chelating reagent, is known as a kinetically differentiation mode (KD-)HPLC, and is both highly sensitive and selective. [1][2][3] We previously reported applications for monitoring V(V) in municipal air at ng/m 3 levels. 4,5 The high sensitivity enabled the detection of trace V(V) at natural occurrence levels in air samples 6 taken over one-day to onemonth periods.For further improvement of the sensitivity of KD-HPLC, the development of an effective preconcentration is required. That the excess chelating reagent becomes concentrated during the concentration step along with the complex has proved to be an obstacle in developing a more effective technique, since it results in poor resolution and peak deterioration in HPLC. The objective of this study is to develop an effective preconcentration method that enables selective separation of the V(V) complex with 4-(2-pyridylazo)resorcinol (PAR) from an excess of PAR in order to improve the sensitivity of KD-HPLC for V(V). We previously reported an off-line preferential preconcentration technique using solvent extraction and a onedrop concentration for the KD-HPLC of ultratrace Co(III) chelate with 2,2′-dihydroxyazobenezene (DHAB). 7,8 The excess DHAB was removed by extraction of its neutral form, and then an anionic Co(III) complex with DHAB was extracted as an ion-pair with tetrabutylammonium. This technique cannot be applied to [V V par] and PAR because both of them are mono anionic species.Solid-phase extraction has long been used for preconcentration and separation. 9 Although there are a number of solid-phase extraction techniques as well, [10][11][12] there are as yet no reports of an effective technique for the separation of metal chelates from a large excess of chelating reagent. In this study, a selective separation method for [V V par] in the presence of the excess chelating reagent was developed using a disposable ODS cartridge. The proposed method was successfully combined with an off-line preconcentration method consisting of an ionpair solid-phase extraction using another ODS cartridge and a subsequent one-drop concentration method. Its application to KD-HPLC for ultratrace V(V) has been demonstrated. A detection limit of V(V) at ppt levels was achieved. The proposed method was successfully applied to the determination of V(V) in an air-borne sample.
Experimental
Reagent and solutionsA commercially available PAR (Dojindo, Kumamoto, Japan) was purified by repeated precipitation from water-N,Ndimethylformamide, and then dissolving it in a slightly alkaline aqueous solution. The concentration of the PAR solution used was 1 mM. Metal ion standard solutions of Co(II), Fe(III), and A selective off-line preconcentration technique for the V(V) complex with ...