Sample conditions to avoid pH distortion in RP‐LC

Abstract

Band deformations might take place for acids and bases in preparative applications and adsorption studies where it is necessary to use overloaded injections. In this study, we focus on how deformations can be prevented without using highly concentrated buffers that may precipitate in the eluent. We have systematically investigated how the elution zones depend on which protolytic form the analyte has when it is dissolved. Basic and acidic model compounds are investigated using eluents with different pH values and the resulting elution profiles are compared when the analytes are dissolved in their protonated and deprotonated form, i.e., in uncharged form or as different kinds of salts. Depending on the analyte's protolytic form, a sample zone is created at the column inlet whose pH deviates, more or less, from the bulk eluent's. If the local adsorption strength in this sample zone is greater than the bulk eluent's, the elution profiles are compressed. Under opposite conditions, the eluted bands are more or less deformed and may even be split; completely different deformations can even take place for different kinds of salt combinations. Explanations of these, and other, effects, together with detailed guidelines for proper sample preparation to avoid peak deformations, are given.

pH-Dependent Degradation of Diclofenac by a Tunnel-Structured Manganese Oxide

“…HPLC-MS was used to determine the M/Z ratio of parent DIC, oxidation intermediates (reaction time of 2 h), and products (reaction time of 24 h), and Figures 3-5, respectively, present their MS chromatograms. Figure 3a has a peak with a very pronounced tailing and this phenomenon should be due to the pH mismatch effect mentioned in a previous study [35]. This fact should not affect the identification of the oxidation intermediates and products because the pronounced tail did not appear after reaction (because the concentration of DIC decreased significantly) and most of the compounds did not appear in this region.…”

“…The extent of adsorption is, as with anions of a weak acid, strongly dependent on pH and favored by lower pH values [34]. In HPLC if the pH of the sample solution and the eluent is not well-matched with each other and around the pKa of the organic acid deformed peaks will appear and then mislead the HPLC analysis conclusions [35,36]. Therefore, the pH could influence the mechanism of the antibiotic interaction with the manganese oxide while those issues have not been systematically addressed.…”

A closer study of peak distortions in supercritical fluid chromatography as generated by the injection

“…The assumptions above gives a simple model that is similar to the "plug model", which was previously used to describe how pHmismatch between sample solution and eluent affects the elution profile [13]. In the pH study the chromatography was modeled using the ideal model (efficiency is infinite) solved using characteristic lines approach.…”

“…These combined effects are often simply denoted as "plug effects" [11,12]. It is well-known from LC that injecting the solute in an injection solvent with stronger elution strength as compared to the bulk mobile phase leads to severe and complex band distortions especially at large injection volumes [13][14][15]. It can be suspected that the underlying reason for these "plug effects" are even more complex in SFC and might also be due to viscous fingering effects.…”

Exploring the Efficiency of UHPLC-Orbitrap MS for the Determination of 20 Pharmaceuticals and Acesulfame K in Hospital and Urban Wastewaters with the Aid of FPSE

“…Obtaining narrow peak widths is especially challenging for basic pharmaceutical compounds. Recently, Samuelsson et al comprehensively explained that the tremendous different peak deformation and peak splitting effects are dependent on the solute species form and how band distortions due to pH mismatch can be effectively avoided by careful control of the protolytic species form in the sample preparation [57]. Moreover, such splitting effects get worse in complex matrices where sample solvent interferes with eluent analytes [58].…”