Sample stacking for the analysis of penicillins by microemulsion electrokinetic chromatography

Abstract

In this study, on-line sample concentration methods, which coupled field-amplified sample injection and sweeping technology with MEEKC, were used to detect and analyze eight common penicillin antibiotics (nafcillin, dicloxacillin, ampicillin, oxacillin, penicillin V, cloxacillin, penicillin G, and amoxicillin). During the optimization of field-amplified sample injection-sweeping MEEKC, the composition of sample matrix and the length of acidic plug were found to be the predominant influences for penicillin stacking. Both zwitterionic ampicillin and amoxicillin could only be stacked through cation-selective-exhaustive-injection sweeping, whereas the other six penicillin compounds were found to be concentrated by anion-selective-exhaustive-injection sweeping. Hence, in order to simultaneously concentrate the eight penicillins in a single-run sweeping step, a combination of successive anion- and cation-selective injections was used. When compared with previous CE-UV methods, the proposed on-line concentration MEEKC method provided better detection sensitivity and faster separation for these penicillins either in single ion-selective injection or in successive anion-/cation-selective injection where the LODs were in the range of 0.2-2.8 microg/L and 0.5-5.8 microg/L, respectively.

Advances in the theory and application of MEEKC

“…During ASEI the presence of an acid plug can prevent the sample analytes from partitioning with the ME droplets before sample concentration occurs [18]. During the ASEI stacking of penicillins [29], the presence of an acidic plug (pH 2) in front of the ME buffer converted the anionic analytes into neutral form and prevented their migration towards the detector end during sample injection. It was noted that increasing the acid plug from 30 to 90 s markedly enhanced the stacking ability and also improved separation.…”

“…Recently, the group has combined ASEI and CSEI injection with the ME sweeping method for the analysis of eight penicillin compounds (nafcillin, dicloxacillin, ampicillin, penicillin V, cloxacillin, penicillin G, amoxicillin and oxacillin) [29]. All penicillins had a pK a of approximately 2.6; however, ampicillin and amoxicillin existed as zwitterions in a weak acid or neutral solution.…”

“…Huang and Hsieh [29] assessed the role of water and acid plugs in ASEI and CSEI sample stacking of penicillins. It was found that increasing the length of water plug from 0 to 60 s improved signal intensities, but increasing to 90 s resulted in no further enhancement.…”

“…When employing a low pH, the EOF is suppressed and a negative voltage is applied across the capillary to move the ME droplets through the detector. Low pH running buffers have been used recently for the separation of aromatic acids [52], phenylalanine derivatives [47] and penicillins [29].…”

On-Column Preconcentration of Anti-Inflammatory Drugs in River Water by Anion-Selective Exhaustive Injection-Sweeping-MEKC

“…The properties of the sample matrix, including its acidity and conductivity, have significant impact on the sensitivity of ASEI-sweeping [23,24]. The influence of sample pH on NSAIDs stacking was first examined, since the amount of analytes that is introduced during the ASEI step depends on the dissociation degree of analytes in the sample matrix [20].…”