Conventional SPE

Solid phase extraction (SPE) is the most important sample purification technique in residue analysis and has gradually replaced LEE. The objective of this section is to give a brief overview of SPE and sorbent materials. A number of books and review papers have already been written on this topic and can be consulted for more details.  

Solid phase extraction is used primarily to prepare liquid samples and extracts of semi-volatile or non-volatile analytes, but may also be used for solids pre-extracted into solvents. The choice of sorbent is the key factor in SPE, because this can control parameters such as selectivity, affinity, and capacity. This choice depends primarily on the analytes and their physicochemical properties, which should define the interactions with the chosen sorbent. However, results also depend on the kind of sample matrix and interactions with both the sorbent and the analyte. SPE sorbents range from chemically bonded silicas, such as with the C8 and C18 organic groups, to graphitized carbon. 

Given the diversity of chemistries involved, developing a viable method that could simultaneously extract different antibiotics (multi-analyte extraction) is challenging. It is essential to identify the right balance of analytical conditions through careful consideration of analyte physicochemical properties, such as solubility, pK, chemical and thermal stability, and polarity so as to maximize the analyte recoveries. Nevertheless, any such method may involve a compromise in optimal conditions for individual analytes.  

P-Lactams are sensitive to acids and bases, and this sensitivity varies with the nature of the sidechain. The maximum stability of monobasic compounds such as Pen G is exhibited in the pH range 6-7, whereas for ampicillin (an amphoteric compound), the maximum stability occurs at its isoelectric point of pH 5. The highly susceptible f)-lactam nitrogen is prone to attack by nucleophiles such as methanol. Furthermore, this nucleophilic attack is accelerated by acid catalysis and application of heat. They are also readily isomerized in an acidic environment. f)-lactams are typically extracted with water and/or polar organic solvents from solid matrices. 

Tetracyclines (TCs) may degrade under extremes of pH with strong acids as well as alkali through epimerization. 

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The use of SPE disks represents a relatively new SPE approach for the rapid and efficient isolation of pesticides from aqueous samples. Empore extraction disks have approximately 10 times the cross-sectional area of conventional SPE cartridges with comparable solvent mass (500 mg). The dense packing and uniform particle... 

Table 3.46 compares SPME and SPE. Although SPME has in common with SPE that the analytes are concentrated by adsorption into a solid phase, SPE involves absorbing the analyte from the sample onto a modified solid support. In practice, the two techniques are quite different. SPME differs from conventional SPE in that SPE isolates the majority of the analyte from a sample (>90%) but injects only about 1 to 2% of the sample onto the GC. SPME isolates a much smaller quantity of analyte (2-20%), but that entire sample is injected into the GC. SPME is easy-to-perform and often significantly more rapid and simpler than SPE, but its quantitative aspect is exacting. Both conventional SPE and SPME minimise the use of solvents for sample preparation and free analysts from tedious sample clean-up. Where SPE can replace LLE... 

Controlled-access sorbents are intended to be either inclusive or exclusive of large molecules and macromolecules. Wide-pore, or large-pore, sorbents are designed intentionally to allow accessibility of macromolecules to the internal pore structure of the sorbent such that they will be retained. Conventional SPE sorbents commonly have pores of 60 A, whereas wide-pore SPE sorbents have pores of 275 to 300 A [75]. 

SPME has the advantages of high concentrating ability and selectivity. Conventional SPE exhaustively extracts most of the analyte (>90%) from a sample, but only 1 to 2% of the sample is injected into the analytical instrument. SPME nonexhaustively extracts only a small portion of the analyte (2 to 20%), whereas all of the sample is injected [68,73,75], Furthermore, SPME facilitates unique investigations, such as extraction from very small samples (i.e., single cells). SPME has the potential for analyses in living systems with minimal disturbance of chemical equilibria because it is a non-exhaustive extraction system [51],... 

Thus, the selective enrichment of the target analyte was successfully demonstrated using the imprinted polymer. Conventional SPE sometimes needs to be combined with a different type of SPE or other separation steps to complete pre-purification, because compounds with similar chemical properties may accompany the analyte as impurities. On the other hand, imprinted polymer is an affinity-type SPE sorbent that exhibits specificity for an analyte therefore, the imprinted polymer-based SPE is able to streamline the whole procedure of analysis. Although aqueous conditions were employed here, it is also notable that the utility in organic solvents is one of the useful characteristics of imprinted polymers as SPE sorbents [18,19]. 

Mastovska and Lightfield have improved a previous analytical method replacing conventional SPE by dSPE for the determination of 11 P-lactams in bovine kidney samples. The method involves solvent extraction/deproteinization with H2O/ACN (20 80, v/v), followed by dSPE clean-up with C18 sorbent and final LC-MS/MS determination. Taking into account the simplicity of this protocol, accuracy was satisfactory with recoveries ranging from 87% to 103% and RSD <16%. Moreover, the use of dSPE increased the number of samples that can be prepared in a day by a factor of 3-4. 

A chapter in a popular book on HPLC nicely presents SPE (57). Two texts have recently been published on the principles and practice of SPE (58). Recently, a special issue of LC-GC The Magazine of Separation Science was devoted to sample preparation and included articles that addressed not only conventional SPE but included automated SPE, martix solid-phase dispersion, membrane filtration, solid-phase microextraction, and polymeric RP-SPE sorbents (59). Cahners Business Information, that publishes R D Magazine has recently started a monthly newsletter titled Sample Preparation largely as a means to showcase products related to SPE and related techniques. 

The efficiency of sample clean-up of lAC columns in determination of MC has been compared by several authors to conventional SPE sorbents.For cleanup of tap water samples, the use of C18 sorbent did not remove numerous coconcentrated impurities that make identification of MC signals more difficult. The functioning of two different I AC columns, silica-based and Sepharose-based ones, has been compared to that of... 

SPME is a very simple and efficient sample preparation method, requiring no solvent (at least for GC applications). Following its introduction (Belardi 1989 Arthur 1990), SPME has been widely used in different fields of analytical chemistry since its first applications to environmental and food analysis. A recent extensive review (Vas 2004) covers the principles and applications of the technique. Other reviews (Queiroz 2004 Hinshaw 2003 Wang 2004 O Reilly 2005 Ouyang 2006) describe special aspects. All conventional steps of extraction, concentration, possibly derivatization, and transfer to a GC are integrated into one step and one device, considerably simphf5dng the sample preparation procedure. (If HPLC is to be used, SPME more closely resembles conventional SPE in a special miniaturized format.)... 

Figure 4.17 Comparison of HPLC-MS analyses of (a) melatonin standard, (b) a human serum sample processed by conventional SPE (Cj8 sorbent, the arrow indicates the melatonin peak), and (c) by an anti-melatonin immunosorbent. Reproduced from Rolbfk et al, J. Chromatogr. B 775, 9 (2002), copyright (2002), with permission from Elsevier.

Effect of nano particles of Al Oj on conventional SPE films have been examined by FTIR, DSC and B-G spectroscopy. The dispersal of Al O nano particles to the SPEs shows dechnation in the glass transition and melting temperature as established from DSC analysis. The FUR spectra show possible interactions between Al O nano particles and host SPE films. The optimum room temperature ionic conductivity of the order of 7 x 10 S/cm having minimum activation energy (E 0.22eV) is observed for NCPE films. This shows one order increment in the conductivity over the conventional SPE films. The temperature dependent conductivity shows Arrhenius type thermally activated behavior before as well as after glass transition temperature. Maximum value of ion transference number is found to be 0.96 which is indicative of predominant ionic (protonic) transport in the SPE and NCPE thin films. It has been observed that dielectric constant for SPE and NCPEs increases with temperature while it decreases with frequency. 

If the desired column/cartridge is available commercially, its application is unlikely to be much more difficult than the performance of a conventional SPE. Figure 2 shows the general working steps. 

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