A Simple in Syringe Low Density Solvent-Dispersive Liquid Liquid Microextraction for Enrichment of Some Metal Ions Prior to Their Determination by High Performance Liquid Chromatography in Food Samples

Abstract

A simple and highly sensitive method is developed for the simultaneous determination of Ni²⁺, Cr₂O₇^2-, Co²⁺, and Hg²⁺ by using in syringe low density solvent-dispersive liquid liquid microextraction (ISLD-DLLME), followed by high performance liquid chromatography with a UV detector. The four metal ions were derivatized with pyrrolidine dithiocarbamate (PDC) based on complexation before their enrichment by ISLD-DLLME in which 1-octanol and methanol were used as the extraction solvent and the dispersive solvent, respectively. The extraction was performed in a commercially available syringe under vortex agitation. Phase separation was achieved without centrifugation, and the extraction phase was easily collected by moving the syringe plunger. Parameters affecting the extraction efficiency were studied and optimized. Under the optimum conditions, the four metal-PDC complexes were detected within 18 min, and ISLD-DLLME could increase the detection sensitivity in the range of 64-230 times compared to the direct HPLC analysis. The obtained limits of detection (LODs) were found to be in the range of 0.011-2.0 µg L^-1. The applicability of the method is demonstrated for freshwater fish, shrimp, and shellfish samples. In addition, the results are in good agreement with those obtained by inductively-coupled plasma-optical emission spectrometry (ICP-OES).

Keywords: in-syringe microextraction; liquid phase microextraction; metal complex; pyrrolidine dithiocarbamate; simultaneous analysis.

Figure 1

(a) The effect of the types of extraction solvent. Extraction conditions: 0.1 mol L⁻¹ phosphate buffer at pH 5, 75 μL of extraction solvent, 300 μL of acetonitrile, and 20 s of vortex. (b) The effect of the extraction solvent volume. Extraction conditions: 0.1 mol L⁻¹ phosphate buffer at pH 5, 50–125 μL of 1-octanol, 250 μL of methanol, and 20 s of vortex.

Figure 2

(a) The effect of the dispersive solvent type. Extraction conditions: 0.1 mol L⁻¹ phosphate buffer at pH 5, 75 μL of 1-octanol, 300 μL of dispersive solvent, and 20 s of vortex. (b) The effect of the dispersive solvent volume. Extraction conditions: 0.1 mol L⁻¹ phosphate buffer at pH 5, 75 μL of 1-octanol, 100–300 μL of methanol, and 20 s of vortex.

Figure 3

The effect of type of salt on the extraction efficiency. Extraction conditions: 0.1 mol L⁻¹ phosphate buffer at pH 5, 50 μL of 1-octanol, 250 μL of methanol, 20 s of vortex, and 1 % w/v salt.

Figure 4

Chromatograms of the studied metals: (a) and (a) inset obtained from direct HPLC analysis PDC complexes of 0.5, 0.5, 0.5, and 1 mg L⁻¹ for Ni²⁺, Co²⁺, Cr₂O₇²⁻, and Hg²⁺, respectively); (b) obtained after ISLD-DLLME (PDC complexes of 0.2, 0.01, 0.01, and 0.2 mg L⁻¹ for Ni²⁺, Co²⁺, Cr₂O₇²⁻, and Hg²⁺, respectively).

Figure 5

Chromatograms of Esomus metallicus fish. (a) Blank sample; (b) spiked sample (2.5 μg kg⁻¹ for Ni²⁺, Cr₂O₇^2–, and Co²⁺, and 250 μg kg⁻¹ for Hg²⁺); and (c) spiked sample (25 μg kg⁻¹ for Ni²⁺, Cr₂O₇^2–, and Co²⁺, and 500 μg kg⁻¹ for Hg²⁺).

Figure 6

Schematic diagram illustrating ISLD-DLLME for the enrichment of the PDC complexes of Ni²⁺, Cr₂O₇²⁻, Co²⁺, and Hg²⁺.