Amino acids solid-phase extraction

The assembly of tetrapeptide 19 that contains all possible 0-dipeptide bonds, (03-03)-, (03-02)-, and (02-03), and also a turn inducing 03-(R)-Ala-02-(R)-Val element was achieved employing a Boc-strategy (Scheme 5). A fluorous benzyl group was incorporated in the first amino acid to streamline the purification procedure by fluorous solid phase extraction (LSPE) (Lilippov et al. 2002 de Visser et al. 2003). Thus, the assembly of the fully protected tetrapeptide commenced with the construction of the first 03-03-peptide bond by applying the previously established conditions. A residence time of 3 min at 90°C provided the Boc-protected dipeptide 15 in 91% isolated yield after LSPE. Notably, the product precipitated in the collection flask, which was kept at ambient temperature, indicating the poor solubility of this class of compounds (Hessel et al. 2005). 

In some instances, combinations of Cig and silica columns are also used for better purification of the crude extracts (431, 445). A combination of Cg, silica, and amino solid-phase extraction columns has been successfully employed to fractionate anabolic and catabolic steroid hormone residues from meat in polar and nonpolar neutral and phenolic compounds, and to purify further each fraction effectively (452). Another combination of two solid-phase extraction columns, one using a graphitized carbon black sorbent and the other Amberlite resin in the hydroxyl form, allowed neutral anabolics to be isolated and separated from acidic anabolics and their metabolites (453). A combination of basic alumina column placed in tandem with an ion-exchange column has also been applied for the purification of the crude extracts in the determination of diethylstilbestrol and zeranol (427), and estradiol and zeranol in tissues (450). 

Sample preparation for the determination of cyclamate by HPLC can be performed as described in Sec. I.C. (17,24,35,43). Potassium bromide can be used as an internal standard (17). In order to reduce interference, extracts can be clarified with Carrez reagents (24,35) or by solid-phase extraction. Nakazato et al. (43) added tetrabutylammonium bromide to the extract, adjusted pH to 4.0 -5.0, passed it through a Mega Bond Elut Cl 8 cartridge, and eluted cyclamate with a mixture of acetonitrile water (3 7, v/v). Lehr and Schmid (71) used an amino-anion-exchange column for the purification of cyclamate. After activation of the column with n-heptane, methanol, water, and 30% acetic acid, the sample was passed through the column and washed with water. Cyclamate was eluted with 2 M NaOH solution. 

Seeberger and coworkers prepared synthetically useful amounts of P-peptides (0.2-0.6mmol) by using a microreactor (reactor volume = 78.3 pi). The reaction of add fluoride and the TFA salt of amino acid benzyl ester in the presence of N-methylmorpholine (NMM) at 90 °C (3 min residence time) gave the dipeptide in 92% yield (Scheme 4.19). A fluorous tag method was used for an effident synthesis of tetrapeptides. Amino acid esters having fluorous tags were used to facilitate purification by fluorous solid-phase extraction (FSPE) (Scheme 4.20). 

A solution of the product from Step 6 (0.155 mmol) dissolved in 96% formic acid was heated to 40 °C 16 hours and concentrated. The residue was dissolved in CH2CI2 and passed through a 2 gram solid phase extracting tube, and isolated by solvent to provide the amino acid as a colorless foam in 90% yield (Note 2). 

Liquid-liquid extraction (LLE), solid phase extraction (SPE), and solid phase micro-extraction (SPME) are frequently applied for sample pretreatment. The type of extraction is highly related to the type of metabolites selected for determination. In previous metabolomics studies, extraction was focused on compounds of adequate stability that could be extracted together (carbohydrates, esters, amino acids, or organic acids). 

Solid phase extraction. With the availability of pre-prepared cartridges of silica-based adsorbents, the use of solid phase extraction has increased in the last few years although the technique has been in use for many years for the isolation of many biochemicals, e.g. amino acids, catecholamines. In essence it is a version of chromatography conditions for the selective adsorption of the analytes (column, solvent, pH, etc.) are chosen, the sample is applied to a column, washed and the analytes selectively eluted with appropriate solvents. Since the columns are disposable there is no need to worry about protein contamination or infection. The adsorbents available cover an even wider range than HPLC materials since they are not required to withstand high back pressures. It is possible... 

Buszewski, B., Kawka, S., and Wolski, T. 1993. Polyamide PA-6 and silica gel as a mixed packing material for isolating dabsyl amino acids using solid-phase extraction, LC-GC, 11 366-369. 

Chitosans find a wide variety of applications in chromatographic separations. The presence of free amino and hydroxyl groups in chitosan makes it a useful chromatographic support. Chitosan can be used in thin layer chromatography for separation of nucleic acid and solid phase extraction of phenols and chlorophenols [69]. 

Although Barcelo et al. [129] focused on the performance of solid phase extraction materials, the analyte base of 22 aromatic sulfonates (e.g., benzenesul-fonate, 2-amino-1,5-naphthalenedisulfonate, 1,3-benzenedisulfonate, l-hydroxy-6-amino-3-naphthalenesulfonate, 2-napthalenesulfonate, diphenylamine-4-sulfonate) is extensive. Separation is achieved on a Cjg column using a complex 30-min 100/0 25/75 water (5mM triethylamine with 5mM acetic acid to pH 6.5)/... 

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