Novel bimodal porous N-(2-aminoethyl)-3-aminopropyltrimethoxysilane-silica monolithic capillary microextraction and its application to the fractionation of aluminum in rainwater and fruit juice by electrothermal vaporization inductively coupled plasma mass spectrometry

A novel bimodal porous N-(2-aminoethyl)-3-aminopropyltrimethoxysilane (AAPTS)-silica monolithic capillary was prepared by sol–gel technology, and used as capillary microextraction (CME) column for aluminum fractionation by electrothermal vaporization inductively coupled plasma mass spectrometry (ETV)–ICP–MS with the use of polytetrafluoroethylene (PTFE) slurry as fluorinating agent. The extraction behaviors of different Al species were studied and it was found that in the pH range of 4–7, labile monomeric Al (free Al³⁺, Al–OH and Al–F) could be retained quantitatively on the monolithic capillary, while non-labile monomeric Al (Al–Cit and Al–EDTA) passed through the capillary directly. The labile monomeric Al retained on monolithic capillary was eluted with 10 μL 1 mol L^− 1 HCl and the elution was introduced into the ETV for fluorination assisted ETV–ICP–MS determination. The total monomeric Al fraction was also determined by AAPTS-silica monolithic CME–fluorination-assisted electrothermal vaporization (FETV)–ICP–MS after the sample solution was adjusted to pH 8.8. Non-labile monomeric Al was obtained by subtracting labile monomeric Al from the total monomeric Al. Under the optimized conditions, the relative standard deviation (R.S.D) was 6.2% (C = 1 μg L^− 1, n = 7; sample volume, 5 mL), and the limit of detection was 1.6 ng L^− 1 for Al with an enrichment factor of 436 fold and a sampling frequency of 9 h^− 1. The prepared AAPTS-silica monolithic capillary showed an excellent pH tolerance and solvent stability and could be used for more than 250 times without decreasing adsorption efficiency. The developed method was applied to the fraction of Al in rainwater and fruit juice, and the results demonstrated that the established system had advantages over the existing 8-hydroxyquinoline (8-HQ) chelating system for Al fractionation such as wider pH range, higher tolerance of interference and better regeneration.