Separation and sequencing of isomeric oligonucleotide adducts using monolithic columns by ion-pair reversed-phase nano-HPLC coupled to ion trap mass spectrometry

An ion-pair reversed-phase nano-high-performance liquid chromatography (IP-RP-nano-HPLC) method using a monolithic poly(styrene-divinylbenzene) (PS-DVB) column coupled to nanoelectrospray ionization mass spectrometry (nano-ESI-MS) was evaluated to separate and identify isomeric oligonucleotide adducts derived from the covalent binding of (+/-)-anti-7r,8t-dihydroxy-9t,10-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene [(+/-)-anti-BPDE] to double-stranded (ds) 5'-PO4--ACCCGCGTCCGCGC-3'/5'-GCGCGGGCGCGGGT-3' oligonucleotide. The influence of three different nanospray emitters on electrospray signal was evaluated in terms of analyte ion sensitivity. The best nanoelectrospray performance for the oligonucleotides was observed with the distal metal-coated emitter. The performance of three different stationary phases was also investigated. The chromatographic separation performance of the polymeric monolithic PS-DVB stationary phase significantly surpassed that of columns packed with the microparticulate sorbents C18 or PS-DVB. Different mobile phase organic solvents and ion-pairing reagents were also evaluated. An optimized mobile phase consisting of methanol and 25 mM triethylammonium bicarbonate resulted in the best chromatographic resolution and increased MS sensitivity of the oligonucleotides. By using a monolithic PS-DVB stationary phase fabricated in a nanocolumn, four positional isomeric (+/-)-BPDE-oligonucleotide adducts were separated and identified. In addition to four of the possible five positional isomers, three positional isomers were also resolved to several diastereoisomers, although their stereostructures could not be identified in the absence of reference standards.