Analyte partitioning

Lord and Pawliszyn" developed a related technique called in-tube SPME in which analytes partition into a polymer coated on the inside of a fused-silica capillary. In automated SPME/HPLC the sample is injected directly into the SPME tube and the analyte is selectively eluted with either the mobile phase or a desorption solution of choice. A mixture of six phenylurea pesticides and eight carbamate pesticides was analyzed using this technique. Lee etal. utilized a novel technique of diazomethane gas-phase methylation post-SPE for the determination of acidic herbicides in water, and Nilsson et al. used SPME post-derivatization to extract benzyl ester herbicides. The successful analysis of volatile analytes indicates a potential for the analysis of fumigant pesticides such as formaldehyde, methyl bromide and phosphine. 

Principles and Characteristics Solid-phase microextraction (SPME) is a patented microscale adsorp-tion/desorption technique developed by Pawliszyn et al. [525-531], which represents a recent development in sample preparation and sample concentration. In SPME analytes partition from a sample into a polymeric stationary phase that is thin-coated on a fused-silica rod (typically 1 cm x 100 p,m). Several configurations of SPME have been proposed including fibre, tubing, stirrer/fan, etc. SPME was introduced as a solvent-free sample preparation technique for GC. 

Liquid-liquid extraction is a form of solvent extraction in which the solvents produce two immiscible liquid phases. The separation of analytes from the liquid matrix occurs when the analyte partitions from the matrix-liquid phase to the other. The partition of analytes between the two phases is based on their solubilities when equilibrium is reached. Usually, one of the phases is aqueous and the other is an immiscible organic solvent. Large, bulky hydrophobic molecules like to partition into an organic solvent, while polar and/or ionic compounds prefer the aqueous phase. 

In MEKC, the supporting electrolyte medium contains a surfactant at a concentration above its critical micelle concentration (CMC). The surfactant self-aggregates in the aqueous medium and forms micelles whose hydrophilic head groups and hydrophobic tail groups form a nonpolar core into which the solutes can partition. The micelles are anionic on their surface, and they migrate in the opposite direction to the electroosmotic flow under the applied current. The differential partitioning of neutral molecules between the buffered aqueous mobile phase and the micellar pseudostationary phase is the sole basis for separation as the buffer and micelles form a two-phase system, and the analyte partitions between them (Smyth and McClean 1998). 

Wennrich et al. [167] investigated the capabilities of coupling accelerated solvent extraction with water as the extraction solvent and solid-phase microextraction to determine chlorophenols in polluted soils. Subcritical water extraction was performed using a commercially available accelerated solvent extractor. This system solves the problem of the analytes partitioning back to the soil matrix, which can occur in straightforward subcritical water extraction because in the Wennrich et al. method [167] the aqueous phase and the soil are separated under the extraction conditions. 

Aqueous samples made alkaline with NaOH to pH >12 repeatedly extracted with methylene chloride analyte partitions into methylene chloride which is separated, concentrated, and analyzed by GC/NPD or GC/MS. 

Solid samples extracted with acetonitrile extract diluted with water the resulting solution mixed with methylene chloride-petroleum ether mixture (20 80) (A) and shaken analyte partitions into (A) solvent layer (A) repeatedly washed with saturated NaCl solution the extract then cleaned up on a florisil column (first eluted with 200 ml solution A and then with a mixture of methylene chloride 50% and 1.5% acetonitrile in petroleum ether eluant concentrated and diluted to desired volume with petroleum ether analyzed by GC-ECD (Pomer-antz et al., 1970). 

The sensitivity of the technique depends mainly on the value of the partition coefficient of the analytes partitioned between the sample and the liber stationary phase. The efficiency of preconcentration depends on both the type of liber used and its thickness (amount). This type of liber affects the amount and character of the sorbed species.51 The general rule like dissolves like applies here, that is, polar compounds are sorbed on polar libers, and nonpolar compounds on nonpolar libers. A broad range of standard libers is commercially available. 

The major factors that control headspace sensitivity are the analyte partition coefficient (K) and phase ratio (/ ). This was demonstrated by Ettre and Kolb [14] ... 

The sample volume also has an effect on both the rate and recovery in SPME extractions, as determined by extraction kinetics and by analyte partition coefficients. The sensitivity of a SPME method is proportional to n, the number of moles of analyte recovered from the sample. As the sample volume (Vs) increases, analyte recovery increases until Vs becomes much larger than the product of K, the distribution constant of the analyte, and Vf, the volume of the fiber coating (i.e., analyte recovery stops increasing when KfeVf Vs) [41]. For this reason, in very dilute samples, larger sample volume results in slower kinetics and higher analyte recovery. 

Solid-phase microextraction (SPME) — is a procedure originally developed for sample preconcentration in gas chromatography (GC). In this procedure a small-diameter fused silica optical fiber, coated with a liquid polymer phase such as poly(dimethylsiloxane), is immersed in an aqueous sample solution. The -> analytes partition into the polymer phase and are then thermally desorbed in the GC injector on the column. The same polymer coating is used as a stationary phase of capillary GC columns. The extraction is a non-exhaustive liquid-liquid extraction with the convenience that the organic phase is attached to the fiber. This fiber is contained in a syringe, which protects it and simplifies introduction of the fiber into a GC injector. Both uncoated and coated fibers with films of different GC stationary phases can be used. SPME can be successfully applied to the analysis of volatile chlorinated organic compounds, such as chlorinated organic solvents and substituted benzenes as well as nonvolatile chlorinated biphenyls. 

Addition of micelle-forming agents to the buffers can help improve selectivity. The agents can be either cationic (e.g., CTAB) or anionic (e.g., SDS). As mentioned before, this technique is called MECC. It is similar to reversed-phase HPLC in that an analyte partitions between a mobile phase,... 

Almost 30 years ago, Colin and Guiochon mentioned in an excellent review [10] that there are essentially three possible ways to model separation mechanism. The first one is analyte partitioning between mobile and stationary phases, the second one is the adsorption of the analyte on the surface of nonpolar adsorbent, and the third one has been suggested by Knox and Pryde [11], where they assume the preferential adsorption of the organic mobile-phase modifier on the adsorbent surface followed by the analyte partitioning into this adsorbed layer. 

Partitioning is the first and probably the simplest model of the retention mechanism. It assumes the existence of two different phases (mobile and stationary) and instant equilibrium of the analyte partitioning between these phases. Simple phenomenological interpretation of the dynamic partitioning process was also introduced at about the same time. Probably, the most consistent and understandable description of this theory is given by C. Cramers, A. Keulemans, and H. McNair in 1961 in their chapter Techniques of Gas Chromatography [12]. The analyte partition coefficient is defined as... 

In gas chromatography the analyte partitioning between mobile gas phase and stationary liquid phase is a real retention mechanism also, phase parameters, such as volume, thickness, internal diameter, and so on, are well known and easily determined. In liquid chromatography, however, the correct definition of the mobile-phase volume has been a subject of continuous debate in the last 30 years [13-16]. The assumption that the retardation factor, i /, which is a quantitative ratio, could be considered as the fraction of time that components spend in the mobile phase is not obvious either. 

Because of the assumption made above, the solution is limited to the linear region of analyte partitioning and adsorption isotherms. The analyte distribution between two liquid phases (eluent and adsorbed phase) at equilibrium could be described as follows ... 

Assuming that only neutral and ion-paired analytes can partition into the acetonitrile adsorbed layer, corresponding partitioning constants should be introduced. The is the constant of nonionic analyte partitioning in the acetonitrile adsorbed layer is the constant for ion-pair partitioning into the acetonitrile adsorbed layer. Similar adsorption constants should be introduced for the description of the surface adsorption processes. The is the constant... 

SPME is a patented sample preparation method for GC applications (32-36). The solvent-free technique was developed in 1989 by Janusz Pawliszyn (http. /Avww.science.uwaterloo.ca/ -janusz/spme.html) at the University of Waterloo in Ontario, Canada, and a manual device made by Supelco, Inc. has been available since 1993. In 1996, Varian Associates, Inc., constructed the first SPME autosampler. SPME involves exposing a fused silica fiber that has been coated with a non-volatile polymer to a sample or its headspace. The absorbed analytes are thermally desorbed in the injector of a gas chromatograph for separation and quantification. The fiber is mounted in a syringe-like holder which protects the fiber during storage and I netration of septa on the sample vial and in the GC injector. This device is operated like an ordinary GC syringe for sampling and injection. The extraction principle can be described as an equilibrium process in which the analyte partitions between the fiber and the aqueous phase. 

Solvent interaction model for normal-phase liquid chromatography. The solvent-interaction model of Scott and co-workers (Scott and Kucera, 1979) assumes that the analyte partitions between the bulk mobile phase and a layer of solvent absorbed onto the stationary phase. The quantitative description of the relationship between retention and the composition of the mobile phase in the solvent-interaction model requires the definition of the void volume corrected retention volume (V), which is related to the retention volume (F ) and the void volume (Fq) by... 

Planning to optimize slurry preparation. The slurries must have analyte concentrations that are appropriate for the analyte line selected. The factors of interest include homogeneity of the solid, distribution of the analyte in the solid, density, particle size and analyte partitioning in the slurry. If the analyte distribution in the solid is heterogeneous, one must strive for very small (< 10 pm) particles. The minimum mass required for analysis based on particle size and density should be computed. The volume-to-volume ratio (solid volume/liquid volume ratio) should be computed in order to ensure that it is lower than 0.25. 

In aqueous matrices, most compounds have quite small values (< 0.25), so the headspace has a low analyte trapping capacity. As a result, the sensitivity of headspace SPME is almost the same as that of direct SPME. The sensitivity loss is only significant when the target analytes partition well into the headspace (i.e. when they possess large... 

Extension of the fiber into the sample or in the headspace. Organic analytes partition into the stationary phase of the fiber until equilibrium is reached. 

A related technique, called solid-phase microextraction, uses a fused silica fiber coated with a nonvolatile polymer to extract organic analytes directly from aqueous samples or from the headspace above the samples. The analyte partitions between the fiber and the liquid phase. The analytes are then desorbed thermally in the heated injector of a gas chromatograph (see Chapter 31). The extracting fiber is mounted in a holder that is much like an ordinary syringe. This technique combines sampling and sample preconcentration in a single step. 

A.26.14 (a) The partition coefficient ratio is the ratio of die time the analyte spends in the mobile phase compared stationary phase. Tlreoretical plates refers to the number of times an analyte partitions between the phases. Retention time is the time required for die analyte to travel past the stationary phase, (b) Answers will vary. 

FIGURE 17.2 Schematic diagram of the liquid-liquid extraction procedure in which analyte partitions from an aqueous phase into an organic phase. (Reprinted from Wells [54]. Copyright 2003, with permission from Elsevier Science.)... 

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