Solid-phase extraction steps

Kennedy et al. developed a lasalocid immunoassay for application to residues in chicken meat and liver samples. The antibody was specific and did not cross-react with salinomycin, maduramicin, or monensin. Sample preparation consisted of homogenization in aqueous acetonitrile, removal of fat from an aliquot of the aqueous acetonitrile by hexane extraction, and evaporation of acetonitrile. The sample was then reconstituted with assay buffer. Liver required an additional solid phase extraction step. The LOQ was 0.02 xgkg for muscle and 0.15 agkg for liver. These workers were able to use the system to determine the half-life of lasalocid in the tissues. 

Most organic compounds may be best analyzed by GC/MS. Such GC/MS or GC analysis, however, is preceded by either a purge and trap concentration step or a liquid or solid phase extraction step using a suitable organic solvent. The purge and trap method for aqueous samples is applicable for volatile substances that have lower solubility in water. A mass spectrometer should be used wherever possible to identify the compounds more correctly. Although it has a lower sensitivity than other GC detectors, mass spectrometiy is, by far, the most confirmatory test for compound identification. 

As most of the product is secreted and is thus concentrated in the culture broth, the recovery process starts with filtration, usually with a rotary vacuum filter, followed by a cascade of solvent and solid-phase extraction steps. 

A common procedure for the analysis of nitrofuran metabolites involves hydrolysis of the protein-bound metabolites under acidic conditions followed by deriva-tization with 2-nitrobenzaldehyde (Eig. 7.5). After neutralization of the digest, solvent extraction is carried out with ethyl acetate. Residues are detected by LC-UV or LC-MS/MS In some cases an additional liquid-liquid extraction or solid-phase extraction step is applied to remove excessive matrix compounds. A broad overview of applied methods was published by Vass et al. in 2008. ... 

Figure 1 T5 pical solid phase extraction steps for sample preparation prior to analyte quantification.

FIGURE 21.5 Recovery of all detected endogenous components in plasma. Color indicates recovery, according to the scale on the right. All recoveries above 75% are shown in red (good recovery) and all recoveries below 25% are shown in white (poor recovery). Abbreviations ACN = acetonitrile FA = 0.1% formic acid MeOH = methanol SPE- solid-phase extraction step - stepwise addition of acetonitrile. OASIS SPE cartridges were all obtained from Waters Corp. and used per manufacturer instructions. Best overall recovery was observed using a methanol precipitant with 0.1 % formic acid. 

Zhu WX, Yang JZ, Wei W, Liu YF, Zhang SS. Simultaneous determination of 13 aminoglycoside residues in foods of animal origin by liquid chromatography—electrospray ionization tandem mass spectrometry with two consecutive solid phase extraction steps. J Chromatogr A 2008 1207 29-37. 

A solid phase extraction step for the reduction of the matrix oil components was optimized. Because of the hydrophobic characteristics of the additives, a reversed phase chromatographic separation method was used and optimized. The so developed method was used for the analysis of inhibited and passivated transformer oils (33). 

In recent decades the development of preconcentration steps to be implemented prior to analytical determinations of trace level compounds has been explored in considerable depth. With a view to eliminating or at least minimising the use of organic solvents used in conventional liquid-liquid extraction, other methodologies have been developed, such as membrane extraction, solid-phase extraction, solid-phase microextraction, etc. 

A method which uses supercritical fluid/solid phase extraction/supercritical fluid chromatography (SE/SPE/SEC) has been developed for the analysis of trace constituents in complex matrices (67). By using this technique, extraction and clean-up are accomplished in one step using unmodified SC CO2. This step is monitored by a photodiode-array detector which allows fractionation. Eigure 10.14 shows a schematic representation of the SE/SPE/SEC set-up. This system allowed selective retention of the sample matrices while eluting and depositing the analytes of interest in the cryogenic trap. Application to the analysis of pesticides from lipid sample matrices have been reported. In this case, the lipids were completely separated from the pesticides. 

To determine secondary alkanesulfonates in sewage wastewaters, solid phase extraction (SPE) and a single-step procedure which combines elution and injection port derivatization for analysis with GC-MS were developed [36]. Again a tetrabutylammonium ion pair reagent was employed both to elute the secondary alkanesulfonates as their ion pairs from CI8-bonded silica disks and to derivatize sulfonate ion pairs under GC injection port conditions. Secondary alkanesulfonates were effectively recovered from samples of raw sewage (>92%) and from primary (>98%) and secondary (>85%) effluents. No... 

Exudate collection in trap solutions usually requires subsequent concentration steps (vacuum evaporation, lyophilization) due to the low concentration of exudate compounds. Depending on the composition of the trap solution, the reduction of sample volume can lead to high salt concentrations, which may interfere with subsequent analysis or may even cause irreversible precipitation of certain exudate compounds (e.g., Ca-citrate, Ca-oxalate, proteins). Therefore, if possible, removal of interfering salts by use of ion exchange resins prior to sample concentration is recommended. Alternatively, solid-phase extraction techniques may be employed for enrichment of exudate compounds from the diluted trap solution (11,22). High-molecular-weight compounds may be concentrated by precipitation with organic solvents [methanol, ethanol, acetone 80% (v/v) for polysaccharides and proteins] or acidification [trichloroacetic acid 10% (w/v), per-... 

The method developer should identify critical points in the method. Frequently, the Youden test may be used to determine if temperature, time, flow rate for solid-phase extraction, weight, volume, and other variables in the method are critical. The developer needs to identify if it is acceptable to take a break during a procedure, length of the break, and steps that need to be completed quickly. Because of differences in background and training between analysts, method developers should not assume that other analysts will perform a technique in the same way as in the developer s laboratory. Often analysts will have different interpretations of simple terms such as shake , slow , complete , and fast . 

GC, utilizing flame ionization detection (FID), has been used to measure diisopropyl methylphosphonate in meat, grain, or milk (Caton et al. 1994). Sample preparation steps include homogenization, filtration, dialysis, and extraction on a solid sorbent. Two common solid phase extractants, Tenax GC and octadecylsilane bonded silica gel (C18 Silica), were compared by Caton et al. (1994). They reported 70% recovery when using Tenax GC and 85% recovery when using C18 Silica. Sensitivity was not reported. Equilibrium experiments indicate that 8-10 mg of Tenax GC are required to achieve maximum recovery of each g of diisopropyl methylphosphonate (Caton et al. 1994). By extrapolating these... 

Kobylinska et al. [62] described a high performance liquid chromatographic analytical method for the determination of miconazole in human plasma using solid-phase extraction. The method uses a solid-phase extraction as the sample preparation step. The assay procedure is sensitive enough to measure concentrations of miconazole for 8 h in a pharmacokinetic study of Mikonazol tablets and Daktarin tablets in human volunteers. The pharmacokinetics of the two formulations was equivalent. 

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