Recent Developments of Stir Bar Sorptive Extraction for Food

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Recent developments of stir bar sorptive extraction for food applications: Extension to polar solutes Nobuo Ochiai, Kikuo Sasamoto, Frank David, and Pat Sandra J. Agric. Food Chem., Just Accepted Manuscript • Publication Date (Web): 25 Jun 2018 Downloaded from http://pubs.acs.org on June 25, 2018

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Journal of Agricultural and Food Chemistry

Recent Developments of Stir Bar Sorptive Extraction for Food Applications: Extension to Polar Solutes

Nobuo Ochiai 1*, Kikuo Sasamoto 1, Frank David 2, and Pat Sandra 2 1. GERSTEL K.K. 1-3-1 Nakane, Meguro-ku, Tokyo 152-0031 Japan 2. Research Institute for Chromatography, President Kennedypark 26, 8500 Kortrijk, Belgium

* Corresponding author: Dr. Nobuo Ochiai, GERSTEL K.K. Address: 1-3-1 Nakane, Meguro-ku, Tokyo 152-0031 Japan, Tel: +81-3-5731-5321, Fax: +81-3-5731-5322 email: [email protected]

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ABSTRACT

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Stir bar sorptive extraction (SBSE) is a miniaturized and solvent-less sample

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preparation method for extraction and concentration of organic compounds from

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aqueous samples. The method is based on sorptive extraction, whereby the solutes are

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extracted into a polymer such as polydimethylsiloxane (PDMS) coated on a stir bar.

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Using an apolar PDMS coating, SBSE provides high recoveries for apolar solutes,

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SBSE recoveries for polar solutes are however low. Although several more polar

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coatings for SBSE were developed, these extraction phases are mostly not compatible

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with thermal desorption (TD) and/or have inferior performance characteristics related to

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robustness, bleeding, stability, etc. compared to PDMS.

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In this perspective paper, two recently introduced SBSE approaches are described that

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can be used to extend the applicability of a PDMS coating to more polar solutes: (1)

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SBSE with freeze concentration (Ice concentration linked with extractive stirrer:

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ICECLES) which is based on the concentration of analytes by gradually reducing the

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phase ratio (sample/extraction phase) and (2) SBSE using a solvent swollen PDMS

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(solvent-assisted SBSE: SA-SBSE) which is based on a combination of polarity

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modification and volume increase by PDMS phase swelling using certain types of

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solvents, while maintaining the original characteristics of the PDMS phase.

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Keywords: Stir bar sorptive extraction (SBSE), Polydimethylsiloxane (PDMS), Food

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applications, Polar solutes, SBSE with freeze concentration, SBSE with solvent swollen

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PDMS.

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INTRODUCTION

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Miniaturized and solvent-less (or solvent minimized) sample preparation methods have

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been developed for isolation and extraction of trace organic compounds in food matrices

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prior to chromatographic analysis, as they provide many benefits such as small sample

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volume, high sample throughput, low operational cost, and low solvent consumption,

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while maintaining or even improving sensitivity. For aqueous food samples, classical

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solid phase extraction (SPE) is widely used for research and quality control, 1 however,

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the enrichment factor (original sample amount versus final extract volume) obtained

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with this technique is rather limited and either further concentration or large volume

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injection (LVI) are required to achieve adequate sensitivity. Consequently, a certain

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range of volatile compounds are also lost. Alternatively, also miniaturized forms of

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liquid-liquid extraction (µLLE), such as single drop micro-extraction, hollow fiber

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liquid-phase micro-extraction and dispersive liquid-liquid micro-extraction are currently

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developed.

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polarity and hydrophobicity of solutes. While enrichment factors can be significantly

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high, these liquid-liquid extraction methods are not applicable to a wide polarity range

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of volatile solutes and the extraction solvent can mask volatile solutes during gas

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chromatographic analysis. Both SPE and µLLE methods are therefore mostly used for

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target compound analysis, both in combination with gas chromatography-mass

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spectrometry (GC-MS) and liquid chromatography-mass spectrometry (LC-MS). 1, 2

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Since two decades, solid phase microextraction (SPME)

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extraction (SBSE)

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Similar to SPE, the extraction principle of µLLE is mostly based on

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3, 4

and stir bar sorptive

are well-established and widely used sample preparation methods

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for aqueous food samples. These solvent-less methods have identical principles and

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allow both extraction and concentration in a single step. Moreover, these methods

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provide enhanced sensitivity because the extracted fraction (on a coated fiber or a

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coated stir bar) can be introduced quantitatively into a GC system by thermal desorption

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(TD). However, extraction of polar/hydrophilic solutes at trace level is still a

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challenging task because these methods are generally more selective towards

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hydrophobic/apolar solutes, often resulting in a chromatographic profile which

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discriminates polar versus apolar solutes. This is a serious limitation, especially in

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aroma profiling of food samples whereby the delicate balance of relative concentrations

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of apolar and polar solutes is crucial.

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Although several phases with different selectivity, e.g. polydimethylsiloxane (PDMS),

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PDMS/divinyl benzene (PDMS/DVB), PDMS/Carboxen, and polyacrylate, are

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commercially available for SPME fibers, their sensitivities for polar/hydrophilic solutes

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are still considered to be limited because of large phase ratio conditions, either due to

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very small phase volume on the fiber (e.g. 0.5 µL for PDMS) or very large headspace

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sample volume (HS-SPME). For SBSE, it is known that the analyte recovery in the

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extraction phase (PDMS) is higher than that of SPME using the same phase because of

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50-250 times larger phase volume on the stir bar (and thus a much smaller phase ratio

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and more favorable recovery).

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have been developed to extend the applicability of SBSE for polar/hydrophilic solutes. 7,

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8

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(LD) and/or have inferior performance characteristics related to robustness, bleeding,

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Several alternative phases with different polarities

However, these extraction phases are mostly only compatible with liquid desorption

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stability, etc. compared to PDMS.

A commercially available stir bar coated with

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polyethyleneglycol-modified silicone (EG Silicone) can be used with TD-GC analysis

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but the stir bar tends to encounter physical change on the coating when re-used several

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times. Therefore, it is recommended to use this EG-silicone coated stir bar in

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combination with a classical PDMS for stirring in a multi-stir bar approach. 9

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Another approach to increase recovery of polar solutes with log Kow 3.0 can be extracted with high recovery, while polar/hydrophilic

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solutes with log Kow m/z 152

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TOC Graphics

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Recent Developments of Stir Bar Sorptive Extraction for Food

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