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Abstract
A study of two ion interaction reagents (IIRs) viz. n-octadecane sulphonate (C18-sulphonate) and eicosyl sulphate (C20-sulphate) was carried out for the separation of lanthanides by reversed-phase high performance liquid chromatography (RP–HPLC). The objective of the study was to identify a suitable IIR offering long term adsorption onto the RP column, thereby obviating the need to introduce the IIR in the mobile phase during the separation of lanthanides. This avoids the rigorous treatment of purified fractions before their mass spectrometric analysis. Resolution was used for comparing different IIRs for separation of lanthanides under identical chromatographic conditions, employing α-hydroxy isobutyric acid (α-HIBA) as an eluent. The volume and composition of IIR solution required for the modification of the column, as well as their long term adsorption, were studied. Columns coated with C18-sulphonate and C20-sulphate allowed the separation of lanthanides without introducing the IIR in the mobile phase. Between these two IIRs, C18-sulphonate offered higher resolution and provided good long term adsorption stability. A RP column modified with C18-sulphonate, as per the optimised procedure, was used for the separation of lanthanides from a geological reference sample without the need to include IIR into the mobile phase.
ACKNOWLEDGMENT
The authors are thankful to Dr. V. Venugopal, Director, Radio Chemistry & Isotope Group, BARC for his constant support and encouragement.
Notes
Resolutions calculated from the chromatograms recorded after modifying the RP column by passing 200 mL each of 2.5 × 10−4 M C20-sulphate solution prepared in 50, 40 and 30% of MeOH and 25% acetonitrile.
Chromatographic condition: C18 (100 mm × 4.6 mm) Chromolith column; α-HIBA of pH 5.0 concentration was changed from 0.03 to 0.18 M in 20 min.
Mobile phase flow rate: 1 mL min−1; Post-coloum reagent [0.15 mM Arsenazo (III) and 0.01 M urea in 0.1 M HNO3] flow rate: 0.3 mL min−1.
A RP column modified by passing different volumes of C18-sulphonate (2.5 × 10−4 M prepared in 30% of MeOH).
Chromatographic conditions: same as those given in Table 1.
A RP column modified using different volumes of C20-sulphate (2.5 × 10−4 M prepared in 30% of MeOH).
Chromatographic conditions: same as those given in Table 1.
A RP column modified by passing 600 mL of C18-sulphonate (2.5 × 10−4 M prepared in 30% of MeOH) followed by washings with different volume of water.
Chromatographic conditions: same as those given in Table 1.
A RP column modified by passing 1600 mL C18-sulphonate (2.5 × 10−4 M prepared in 30% of MeOH) followed by sequential washing with 700 mL of water and different volumes of mobile phase.
*Percentage change with respect to the resolution obtained by conditioning with 10 mL of the mobile phase.
Chromatographic conditions: same as those given in Table 1.
IIRs used: 0.007 M each of C8-sulphate and C8-sulphonate; 2.5 × 10−4 M each of C18-sulphonate and C20-sulphate (prepared in 30% MeOH).
Column: C18 (100 mm × 4.6 mm) Chromolith column; Mobile phase flow rate: 1 mL min−1; other chromatographic conditions: same as those given for Table 1.
Resolutions calculated from the chromatograms recorded under dynamically modifying condition by passing 0.007 M each of C8-sulphonate and C9-sulphonate.
Columns used: C18 (100 mm × 4.6 mm) Chromolith column and C8 (150 mm × 4.6 mm, 5 µm particle size) Purosher STAR column.
Gradient condition; α-HIBA of pH 5.0 concentration was changed from 0.03 to 0.18 M in 20 min.
Mobile phase flow rate: 1 mL min−1; Post-column reagent flow rate: 0.3 mL min−1.
ND: Not determined.
*Average and Relative standard deviation from triplicate runs.
Relative Standard Deviation (RSD) calculated from eight chromatograms recorded over a period of two months.
Chromatographic conditions: same as given in Table 1.