Speciation of [Pt(IV) Cl6-n Brn]2- (n = 0-6) and some of their mono-aquated [Pt(IV) Cl5-n Brn (H2O)]- (n = 0-5) anions in solution at low concentrations by means of ion-pairing reversed-phase ultra-high-performance liquid chromatography coupled to electrospray ionization quadrupole time-of-flight mass spectrometry.
BACKGROUND
The speciation of the purely inorganic [PtCl6-n Brn](2-) (n = 0-6) anions and their corresponding mono-aquated [PtCl5-n Brn (H2O)](-) (n = 0-5) anions is of considerable importance to the precious metal refining and recycling industry, to ensure optimum recovery and separation efficiencies. Speciation of platinum complexes present in precursor solutions used for the preparation of precious metal nano-crystals of defined size and morphology appears also to be important. The various possible Pt(IV) complex anions in dilute aqueous can be characterized using ion-pairing reversed-phase high-performance liquid chromatography coupled to electrospray ionization quadrupole time-of-flight mass spectrometry (ESI-Q-TOFMS).


METHODS
Ion-pairing reversed-phase ultra-high-performance LC separation of the Pt(IV) complex anions present in aqueous solutions prior to detection by means of high-resolution ESI-Q-TOFMS using a low ESI source cone voltage (5 V) allows for the clear identification of all the platinum complexes from the characteristic pattern of fragment ions (m/z), presumably generated by 'reductive conversion' in the ESI source of the mass spectrometer. Sufficient chromatographic resolution for the series of Pt(IV) complexes is achieved using the (n-butyl)3 NH(+) ion generated in a formic acid/water/methanol (pH ~3.5) mobile phase. This mobile phase composition facilitates a low-background for optimal ESI-Q-TOFMS detection with enhanced sensitivity.


RESULTS
Direct-infusion mass spectrometry of the inorganic platinum complexes in aqueous solution is impractical due to their low volatility, but more importantly as a result of the very extensive series of fragment ions generated in the ESI source, which leads to virtually uninterpretable mass spectra. However, with prior separation, and by using low ESI cone voltages (5 V), the mass spectra of the separated analyte ions show simpler and systematic fragmentation patterns [Pt(IV) X5](-) → [Pt(III) X4 ](-) → [Pt(II) X3](-) → [Pt(I)X2 ](-) (X = Cl(-) and Br(-)), resulting in clear assignments. This methodology facilitates the characterization of the partially aquated [PtCl5-n Brn (H2O)](-) (n = 0-5) anions derived from the homo- and heteroleptic [PtCl6-n Brn](2-) (n = 0-6) anions, in equilibrated solutions at low concentrations.


CONCLUSIONS
Speciation of homo- and heteroleptic [PtCl6-n Brn](2-) (n = 0-6) anions, together with some of their partially aquated [PtCl5-n Brn (H2O)](-) (n = 0-5) species in dilute solution, can successfully be carried out by means of prior ion-pairing reversed-phase LC separation coupled to high-resolution ESI-Q-TOFMS at low ESI cone-voltage settings.
Department of Chemistry and Polymer Science, Stellenbosch University, Private Bag X1, Matieland, 7602, South Africa.
Pieter-Hugo
PH
van Wyk
Jacobus B
JB
van Dyk
Wilhemus J
WJ
Gerber
Marietjie
M
Stander
André
A
de Villiers
Klaus R
KR
Koch