Characterization of a microparticulate strong anion-exchanger in the HPLC of acidic drugs

Abstract

The use of Waters Spherisorb S5SAX for the HPLC of acidic compounds, including a number of non-steroidal anti-inflammatory drugs (NSAIDs), has been investigated. Adequate retention, separation, and peak efficiency and symmetry were obtained for most analytes on a 250 x 4.6 mm i.d. column using methanol containing ammonium perchlorate (10 mmol L(-1), pH 6.7 or pH 8.3) as eluent. The results of changes in (i) eluent pH (constant ionic strength); (ii) eluent ionic strength (constant pH); and (iii) adding water to the eluent (constant pH) were consistent with a retention mechanism dominated by ion-exchange with the bonded strong anion-exchange (SAX) moieties. However, there were some unexpected observations, including (i) a general decrease in retention at eluent pH values above 7.7; (ii) a marked increase in retention on adding 1% (v/v) water to the eluent; (iii) a subsequent marked decrease in retention on adding 5% (v/v) or more water; and (iv) decreased column activity with time. These observations may be due to (i) interaction between the charged SAX moieties and ionised surface silanols (with ionization increasing at higher eluent pH values) and (ii) influence of the solvation of silanols, analytes, SAX moiety, and counter-ion varying with both pH and water content. Nevertheless, the factors influencing separation of individual NSAIDs remain unclear especially as no relation between log k and pKa exists for these compounds. Hydrophobic interactions are unlikely to be important since basic and neutral compounds were hardly retained. Ease of accessibility of the counter-ion to the SAX moiety for analyte displacement may be a factor.

Robust open tubular layer of S‐ketoprofen imprinted polymer for chiral LC separation

“…For example, the solutes (L-and D-phenylalanine anilide) were positively charged while the MIP (made of MAA) was negatively charged in one case [43,45], and the solutes (N-carbobenzyloxytyrosine enantiomers) were negatively charged while the MIP (made of 4-vinylpyridine) was positively charged in another case [44] at the pH of solute pK a . Similar results were observed in non-MIP systems where an ion exchange retention mechanism applied [46,47]. In the MIP systems where an electrostatic interaction mechanism is operative, the interaction mechanism changed from relying mainly on ionic and hydrogen bonding to depending mainly on ionic and hydrophobic interactions on change of mobile phase from organic phase to aqueous media [44].…”

Artificial Neural Network Modelling of the Retention of Acidic Analytes in Strong Anion-Exchange HPLC: Elucidation of Structure-Retention Relationships

“…However, given appropriate conditions SAX-modified silica HPLC packings not only give efficient separations [24,31,32], but may also offer advantages over traditional alkyl-modified materials. For example, only negatively charged analytes are retained to any extent, which increases selectivity compared with alkyl-modified bonded phases.…”

“…A number of benzoic and phenylacetic acids were chosen as model analytes due to their relatively simple structures, and the availability of a number of analogues. Moreover, it was hoped that their resemblance to aspects of some non-steroidal anti-inflammatory drugs (NSAIDs) would help in interpreting the observed differential elution of these analytes on a Waters Spherisorb S5SAX packing [24] in terms of the contribution of various retention mechanisms, using methanolic eluents containing ammonium perchlorate. To enable this, the composition of the eluent with respect to ionic strength, and composition (% v/v water in methanol), was systematically varied over ranges wide enough to encompass conditions most likely to be useful in practice, whilst keeping the apparent pH ( S W pH) of the eluent constant.…”

HPLC of basic drugs using non‐aqueous ionic eluents: evaluation of a 3 μm strong cation‐exchange material

“…Microparticulate strong anion (SAX)-and cation (SCX)-exchange HPLC packings give effi cient separation of acidic (Law and Hussain, 2000;Morgan et al, 2006) and basic (Flanagan et al, 1988(Flanagan et al, , 2001Law and Appleby, 1996) drugs, respectively, given appropriate eluent conditions. The additional selectivity for ionized or protonated analytes off ered by ion-exchange chromatography using 100% methanol eluents containing an ionic modifi er minimizes the potential for interference from neutral species, or species of opposite charge to the analyte(s) of interest since they generally elute close to the void volume of the column (Croes et al, 1995;Morgan et al, 2006).…”

“…The additional selectivity for ionized or protonated analytes off ered by ion-exchange chromatography using 100% methanol eluents containing an ionic modifi er minimizes the potential for interference from neutral species, or species of opposite charge to the analyte(s) of interest since they generally elute close to the void volume of the column (Croes et al, 1995;Morgan et al, 2006). Improvements in effi ciency arising from the use of smaller diameter particles (<5 μm average particle size, aps) have long been predicted.…”