Rapid determination of carbamate pesticides in food using dual counter-current chromatography directly interfaced with mass spectrometry

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Abstract

Dual counter-current chromatography (dual CCC)–tandem mass spectrometry (MS/MS) was successfully performed with a newly designed spiral column for dual CCC. The small column capacity required for directly coupling with electrospray MS/MS was accomplished by forming a rectangular spiral groove on a plastic disk and sealing it with a PTFE sheet. This novel dual CCC–MS/MS technique was successfully applied for the rapid determination of methomyl, fenobucarb and carbaryl pesticides in food. A two-phase solvent system of n-hexane-acetonitrile-0.1% formic acid (45:45:10) was suitable for both good dual CCC separation and sufficient ionization of pesticides. Recoveries of these three pesticides from mandarin orange and spinach samples fortified at 0.05 mg/kg were in the range of 93–107% with relative standard deviations of 2.4–3.8%.

Introduction

Dual counter-current chromatography (dual CCC) performs genuine counter-current chromatography in which both lighter and heavier phases move through a coiled column in the opposite direction. It has a great advantage over the conventional counter-current chromatography, since the method allows successive sample injection where the target compound is always exposed to a fresh solution until it is eluted from the coiled column [1], [2], [3], [4]. In addition, there are no problems of loss of the stationary phase due to multiple sample injection. Unique CCC elution methods have been reported such as, dual mode elution CCC and elution-extrusion CCC [5], [6], [7], [8]. The advantage of these methods seems to be a simple system, using a standard CCC machine with a coil of tubing and one pump. However, the separations are phased, not continuous, and the processes are rather complicated. Using a multilayer coiled separation column we have successfully applied dual CCC for determination of residual carbamate pesticides in food. A food sample extract was cleaned up by dual CCC and then analytes were determined by electrospray (ESI) tandem mass spectrometry (MS/MS) [1], [2]. This dual CCC method is suitable for a sequential multiple sample analysis, since it needs only about 5 min for each sample. However, for more simple and rapid analysis, it seems effective to directly interface dual CCC with ESI-MS/MS.

Mass spectrometry (MS) is becoming a routine tool in separation science and many methods have been reported for structural confirmation of the compounds isolated by CCC. However, only a few methods have been reported for directly interfacing CCC with MS, and all of these methods use conventional CCC apparatus [9], [10], [11], [12], [13], [14]. They suggest that a short separation time, good sample resolution and a low column back pressure are required. Recently, we have built a novel dual CCC centrifuge holding a plastic disk with a narrow spiral channel. The plastic disk can alleviate the complexity of dual CCC, which requires connection of five flow tubes to the separation column. In addition, the use of this spiral disk separation column not only permits the reduction of the column volume, but also accelerates the counter-current process by a radial centrifugal force gradient generated by the spiral pitch, hence improving both the separation efficiency and analysis time. These advantages satisfy the above requirements and are most suitable for a simple and rapid analysis of pesticides in the complex sample matrix.

In the present paper, we describe interfacing this new dual CCC centrifuge to an ESI-MS/MS spectrometer and evaluate its performance in the analysis of carbamate pesticides in food samples.

Section snippets

Chemicals

Carbaryl and fenobucarb were purchased from Wako (Osaka, Japan), methomyl from Riedel-de Haën (Hanover, Germany) and [2H7] carbaryl (carbaryl-d7, chemical purity >99.7%), [2H3] fenobucarb (fenobucarb-d3, chemical purity >98.0%) and [2H3] methomyl (methomyl-d3, chemical purity >99.7%) from Hayashi (Osaka, Japan). These chemicals labeled with stable isotope were used as internal standards. Each stock solution of pesticides and their internal standards were prepared in acetone (1 mg/ml).

Dual counter-current chromatography

The J-type

New spiral column for dual CCC

Our dual CCC method previously reported was suitable for a rapid analysis, since it required only about 5 min for each sample. However, for more efficient and speedy analyses of samples, we considered the following improvements: first, the effective length of the coiled column should be made much shorter to further reduce the analysis time; and secondly, the solute partition process should be improved by reducing the cross sectional area of the channel. From these points of view, we have

Conclusions

A rapid method for analysis of residual carbaryl, fenobucarb and methomyl in food has been developed using dual CCC–MS/MS. A new spiral disk column gave good peak shapes for carbamate pesticides on the MRM profiles under dual CCC–MS/MS conditions. n-Hexane-acetonitrile-0.1% formic acid (45:45:10) was an effective two-phase solvent system for both good ionization and efficient CCC separation where the target compounds and interference compounds each elute from opposite ends of the dual CCC

Acknowledgement

The authors wish to thank Mrs. Y. Yoshimi for carrying out some of the experiments.

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