Strong cation exchanger , solid-phase

Fig. 13.18 Reversed phase HPLC-ESI-MS ion chromatograms of the strong cation exchange solid phase extract of an Echium vu/gore-derived honey, (a) Total ion chromatogram, (b) base ion m/z 200-500) chromatogram, and (c)-(f) reconstructed ion chromatograms displaying m/z 414, 398, 374, and 332, respectively.

J. X. Shen, R. J. Motyka, J. P. Roach, and R. N. Hayes, Minimization of ion suppression in LC-MS/MS analysis through the application of strong cation exchange solid-phase extraction (SCX-SPE),/ Pharm. Biomed. Anal. 37 (2005), 359-367. 

DMAE is extracted from simulants using solid phase extraction on a strong cation exchanger. The absorbed DMAE is removed from the absorbent with triethylamine and the resulting solution examined by GC using an internal standard... 

After hydrolyzate acidification with hydrochloric acid at pH values lower than 1, quinoxaline-2-carboxylic acid is quantitatively extracted into ethyl acetate, chloroform, or dichloromethane, since at these strongly acidic conditions the ionization of their carboxylate moiety is suppressed (pK, 2.88), and then back-extracted into aqueous buffered solutions at pH 6.0 or higher. These liquid-liquid partitioning procedures isolate quinoxaline-2-carboxylic acid from a complex mixture of tissue hydrolysates, and provide an aqueous extract suitable for further purification by solid-phase extraction. This has been accomplished either with the strong cation-exchange resin AG MP-50 (419, 420) or with a polar silica column (422). 

Initially, vidarabine was used as the internal standard for penciclovir in a method based on mixed-mode strong cation exchange (MCX) solid-phase extraction due to their similar properties, particularly hydrophobicity (Fig. 14a). Quite stable IS responses were obtained in a run consisted mainly of CS and QC samples with a CV of 13.02 % (Fig. 14b). However, 43 % of the CS and QC samples did not meet the acceptance criterion of accuracy. On the other hand, when penciclovir-d4 was used as the internal standard, all the CS and QC samples met the acceptance criterion in accuracy though the IS responses were more variable (the CV in IS responses was 23.81 %, Fig. 14c). 

Fig. 14 (a, top) Hydrophobicity (log D) vs. pH curves for penciclovir and vidarabine. (b, middle) Less internal standard response variation was observed while using vidarabine as the internal standard (CV= 13.02 %) but 43 % of the calibration standards (CS) and quality controls (QC) were rejected. Extraction MCX (mixed-mode strong cation exchange)-based solid-phase extraction, (c, bottom) More IS response variation was observed while using a deuterated internal standard, penciclovir-d4 (CV = 23.81 %) but 100 % of the CS and QC samples were accepted. Reproduced from ref. [36] with permission from Elsevier... 

Fig. 19 Randomly scattered low internal standard (IS) responses observed for incurred samples only, whose IS responses were within normal range during repeat analyses. Analyte olanzapine IS olanzapine-d3 sample pretreatment at clinic 25 % (w/v) L-ascorbic add added to plasma in a ratio of 1.25 100 (v/v) extraction MCX (mixed-mode strong cation exchange)-based solid-phase extraction. An incurred sample was coded for reassay when its IS response was outside 50 % of the mean IS response of the accepted calibration standards and quality controls. Reproduced from ref. [36] with permission from Elsevier...

Fig. 13.9 The benefits of strong cation exchange (SCX), solid phase extraction (SPE) clean-up of samples. The reversed phase HPLC-ESI-MS base ion (m/z 200-500) chromatograms of a reduced (zinc/sulfuric acid) extract of Senecio ovatus. (a) Methanolic solubles of the reduced extract and, (b) the SCX SPE of the methanolic solubles of the reduced extract.

A variety of protocols utilising combinations of liquid-liquid and solid-phase extractions (LLE and SPE) have been used to clean-up tissue extracts. Alkaline extracts are commonly made acidic, extracted into ethyl acetate and then back-extracted into aqueous buffer at alkaline pH. Acidic extracts have been extracted directly into ethyl acetate and then back-extracted into buffer. QCA and mQCA may act as acids or bases, and both of these properties have been utilized in the SPE clean-up of the buffered extracts from the initial liquid-liquid partitions. Extracts were acidified prior to clean-up on non-endcapped sex (strong cation exchange) SPE columns. The analytes of interest were eluted from the columns using a mixture of sodium hydroxide and methanol. Further clean-up and transfer to an appropriate solution for instmmental analysis was achieved by re-acidification... 

SeX-SPE Strong cation exchange with solid phase extraction... 

Several methods have been used to separate the lanthanides chemically solvent extraction, ion exchange chromatography, HPLC using Q-hydroxyisobutyric acid and, in limited cases, selective reduction of a particular metal cation.40-43 The use of di(2-ethylhexyl)orthophosphoric acid (HDEHP) for the separation of various rare-earth elements via solvent extraction has also been reported.44 16 This separation method is based on the strong tendency of Ln3+ ions to form complexes with various anions (i.e., Cl- or N03 ) and their wide range of affinities for com-plexation to dialkyl orthophosphoric acid. When the HDEHP is attached to a solid phase resin, the lanthanides can be selected with various concentrations of acid in order of size, with the smallest ion being the most highly retained. 

However, this method possesses several disadvantages such as long reaction time and complicated work-up procedure. For example, in the case of Sc(OTf)3-catalyzed reaction, the treatment required 72 h to get completed at the ambient temperature. After that a pure product was isolated from the reaction mixture by the capture of the solid phase by using strongly acidic cation exchange resin, followed by washing of the solvent and final treatment of resin with 2 M methanolic ammonia. 

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