Electrokinetic Chromatography and Mass Spectrometric Detection Using Latex Nanoparticles as a Pseudostationary Phase

Abstract

The utility of novel latex nanoparticles as pseudostationary phases for electrokinetic chromatography with UV and mass spectrometric detection is demonstrated. The nanoparticles are synthesized using ab initio RAFT (reversible addition-fragmentation chain transfer) in emulsion polymerization, which yields small (63 nm) particles with a narrow size distribution, a hydrophobic core, and an ionic shell. The nanoparticles are shown to provide efficient and selective separations, with retention and separation selectivity dominated by hydrophobic interactions. The nanoparticles are highly retentive, such that they are effective at relatively low concentrations. Addition of the nanoparticles to the background electrolyte at these concentrations has a minor effect on the noise with UV detection, no measurable effect on the separation current, and minor effects on analyte ionization efficiency during electrospray ionization. The nanoparticles do not cause fouling or degradation of the electrospray-mass spectrometer interface even after several weeks of use. The combination of online sample preconcentration via sweeping and selective mass spectrometric detection yields low detection limits (10-16 ppb), particularly for more hydrophobic compounds.

Retention behavior and selectivity of a latex nanoparticle pseudostationary phase for electrokinetic chromatography

“…Similar ionic particles have been used successfully for electrokinetic ion exchange separations [62]. Previous results with the current latex NPs, however, indicate weak or non-existent electrostatic interactions [42]. This has been explored more fully in the current study.…”

“…LSER separations were conducted in 30 mM Tris(hydroxymethyl)aminomethane (Tris, 99.9%, Aldrich) buffer adjusted to pH 7.0070.05 using dilute hydrochloric acid (Fisher Scientific). The latex NPs were synthesized and characterized as described previously [42], and were stored at 41C in a 22.7% aqueous suspension. NP solutions were prepared by dilution of this stock suspension to the reported concentration in the BGE.…”

“…In a previous study with these NPs we reported the separation of a homologous series of alkyl-phenyl ketones and used these results to determine the electrophoretic mobility and methylene selectivity of the NPs [42]. The electrophoretic mobility is not significantly different from SDS micelles, but the reported methylene selectivity (3.25) is much greater than that of SDS micelles (2.33) [50], indicating much stronger hydrophobic interactions.…”

“…Extensive studies have shown that polymeric PSPs can offer significant performance advantages and selectivity differences relative to micellar PSPs [25][26][27][28][29][30][31]. More recently, polymeric nanoparticle (NP) PSPs have been introduced and their separation performance characterized [32][33][34][35][36][37][38][39][40][41][42]. Polymeric and NP PSPs have also been shown to have significant advantages for use with electrospray ionization and mass spectrometric (MS) detection [37,[43][44][45][46][47][48][49][50][51][52], as well as for the separation of hydrophobic compounds not easily resolved with micellar PSPs [22,[53][54][55][56][57].…”

“…Recently, we have introduced a novel polymeric NP PSP that is suitable for use in EKC with UV and MS detection [42]. The synthetic approach for these NPs, ab initio RAFT (Reversible Addition-Fragmentation chain Transfer) in emulsion polymerization [58,59], produces small (63 nm) and relatively monodisperse NPs with a poly(acrylic acid) shell and poly(butyl acrylate) core that can be used as PSPs without complicated post synthetic purification or modification.…”