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.

RATIONALE: 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.

A robust method for speciation, separation and photometric characterization of all [PtCl(6-n)Br(n)]2- (n=0-6) and [PtCl(4-n)Br(n)]2- (n=0-4) complex anions by means of ion-pairing RP-HPLC coupled to ICP-MS/OES, validated by high resolution 195Pt NMR spectroscopy.

A robust reversed phase ion-pairing RP-HPLC method has been developed for the unambiguous speciation and quantification of all possible homoleptic and heteroleptic octahedral platinum(IV) [PtCl(6-n)Br(n)](2-) (n=0-6) as well as the corresponding platinum(II) [PtCl(4-n)Br(n)](2-) (n=0-4) complex anions using UV/Vis detection. High resolution (195)Pt NMR in more concentrated solutions of these Pt(II/IV) complexes (≥50 mM) served to validate the chromatographic peak assignments, particularly in the case of the possible stereoisomers of Pt(II/IV) complex anions. By means of IP-RP-HPLC coupled to ICP-MS or ICP-OES it is possible to accurately determine the relative concentrations of all possible Pt(II/IV) species in these solutions, which allows for the accurate determination of the photometric characteristics (λ(max) and ɛ) of all the species in this series, by recording of the UV/Vis absorption spectra of all eluted species, using photo-diode array, and quantification with ICP-MS or ICP-OES. With this method it is readily possible to separate and estimate the concentrations of the various stereoisomers which are present in these solutions at sub-millimolar concentrations, such as cis- and trans-[PtCl(4)Br(2)](2-), fac- and mer-[PtCl(3)Br(3)](2-) and cis- and trans-[PtCl(2)Br(4)](2-) for Pt(IV), and cis- and trans-[PtCl(2)Br(2)](2-) in the case of Pt(II). All mixed halide Pt(II) and Pt(IV) species can be separated and quantified in a single IP-RP-HPLC experiment, using the newly obtained photometric molar absorptivities, ɛ, determined herein at given wavelengths.