Chromatographic separation and mass spectrometric analysis of N-acetyl-l-cysteine-protected palladium nanoparticles

Abstract

In this study, water-soluble N-acetyl-L-cysteine-protected palladium nanoparticles (NAC-PdNPs) were synthesised. The obtained NAC-PdNPs were well characterised by UV-vis absorption spectroscopy, infrared spectroscopy, X-ray photoelectron spectroscopy, and transmission electron microscopy. The as-synthesised NAC-PdNPs were found to be polydisperse in nature with an average particle size of 2.17 ± 0.20 nm. As such, a reverse-phase high-performance liquid chromatographic (RP-HPLC) methodology coupled with UV-vis absorption detection has been developed for separating and analysing this as-synthesised NAC-PdNPs product. A gradient elution program with mobile phase mixtures of methanol/water containing 50 mM tetrabutylammonium fluoride was successfully applied to separate the as-synthesised NAC-PdNPs sample. The spectral characteristics of each RP-HPLC-separated NAC-PdNPs fractions were captured online using a diode array detector. In addition, most major RP-HPLC fractions were collected and further characterised by matrix-assisted laser desorption/ionisation time-of-flight mass spectrometry (MALDI-TOF MS) in order to elucidate their chemical compositions. The mass spectra suggest that the NAC-PdNPs as-synthesised product is indeed a complex mixture of small PdNPs comprising Pd₁₀(NAC)₇, Pd₁₁(NAC)₇, Pd₁₁(NAC)₈, Pd₁₂(NAC)₉, Pd₁₃(NAC)₆, Pd₁₃(NAC)₉, Pd₁₄(NAC)₅, Pd₁₄(NAC)₉, Pd₁₅(NAC)₉, Pd₁₇(NAC)₁₁, and Pd₂₀(NAC)₁₁. The UV-vis spectral characteristics of the separated NAC-PdNPs are closely related to the number of Pd atoms. This study highlights the values of using high-resolution chromatography together with UV-vis absorption spectroscopy and MALDI-TOF MS for revealing the hidden properties of each individual PdNPs species in a PdNPs product.