Paul G. Boswell

Fluorous Polymeric Membranes for Ionophore-Based Ion-Selective Potentiometry: How Inert Is Teflon AF?

Fluorous media are the least polar and polarizable condensed phases known. Their use as membrane materials considerably increases the selectivity and robustness of ion-selective electrodes (ISEs). In this research, a fluorous amorphous perfluoropolymer was used for the first time as a matrix for an ISE membrane. Electrodes for pH measurements with membranes composed of poly[4,5-difluoro-2,2-bis(trifluoromethyl)-1,3-dioxole]-co-poly(tetrafluoroethylene) (87% dioxole monomer content; known as Teflon AF2400) as polymer matrix, a linear perfluorooligoether as plasticizer, sodium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate providing for ionic sites, and bis[(perfluorooctyl)propyl]-2,2,2-trifluoroethylamine as H+ ionophore were investigated. All electrodes had excellent potentiometric selectivities, showed Nernstian responses to H+ over a wide pH range, exhibited enhanced mechanical stability, and maintained their selectivity over at least 4 weeks. For membranes of low ionophore concentration, the polymer affected the sensor selectivity noticeably at polymer concentrations exceeding 15%. Also, the membrane resistance increased quite strongly at high polymer concentrations, which cannot be explained by the Mackie-Meares obstruction model. The selectivities and resistances depend on the polymer concentration because of a functional group associated with Teflon AF2400, with a concentration of one functional group per 854 monomer units of the polymer. In the fluorous environment of these membranes, this functional group binds to Na+, K+, Ca2+, and the unprotonated ionophore with binding constants of 10(3.5), 10(1.8), 10(6.8), and 10(4.4) M(-1), respectively. Potentiometric and spectroscopic evidence indicates that these functional groups are COOH groups formed by the hydrolysis of carboxylic acid fluoride (COF) groups originally present in Teflon AF2400. The use of higher ionophore concentrations removes the undesirable effect of these COOH groups almost completely. Alternatively, the C(=O)F groups can be eliminated chemically, or they can be used to readily introduce new functionalities.

Fluorophilic Ionophores for Potentiometric pH Determinations with Fluorous Membranes of Exceptional Selectivity

Ionophore-doped sensor membranes exhibit greater selectivities and wider measuring ranges when they are prepared with noncoordinating matrixes. Since fluorous phases are the least polar and least polarizable liquid phases known, a fluorous phase was used for this work as the membrane matrix for a series of ionophore-based sensors to explore the ultimate limit of selectivity. Fluorous pH electrode membranes, each comprised of perfluoroperhydrophenanthrene, sodium tetrakis[3,5-bis(perfluorohexyl)phenyl]borate, and one of four fluorophilic H(+)-selective ionophores were prepared. All the ionophores are highly fluorinated trialkylamines containing three electron withdrawing perfluoroalkyl groups shielded from the central nitrogen by alkyl spacers of varying lengths: [CF(3)(CF(2))(7)(CH(2))(3)](2)[CF(3)(CF(2))(6)CH(2)]N, [CF(3)(CF(2))(7)(CH(2))(3)](2)(CF(3)CH(2))N, [CF(3)(CF(2))(7)(CH(2))(3)](3)N, and [CF(3)(CF(2))(7)(CH(2))(5)](3)N. Their pKa values in the fluorous matrix are as high as 15.4 +/- 0.3, and the corresponding electrodes exhibit logarithmic selectivity coefficients for H(+) over K(+) as low as <-12.8. The pKa and selectivity follow the trends expected from the degree of shielding and the length of the perfluoroalkyl chains of the ionophores. These electrodes are the first fluorous ionophore-based sensors described in the literature. The selectivities of the sensor containing [CF(3)(CF(2))(7)(CH(2))(5)](3)N are not only greater than those of analogous sensors with nonfluorous membranes but were of the same magnitude as the best ionophore-based pH sensors ever reported.

A 14 Parameter Study of UHPLC’s for Method Development Transfer and Troubleshooting

Five ultra-high pressure liquid chromatography (UHPLC) instruments were compared to one another by examining the overall system performance and key functions of the system parts including: pump, auto-sampler, thermo-stated column compartment, and the detector. The five UHPLC systems used in this study were: ThermoFisher Vanquish, Agilent 1290 Infinity I, Agilent 1290 Infinity II, Waters Acquity I-Class, and Shimadzu Nexera X2. The identities of the systems were blinded in the results and discussion section to use this study for scientific purposes only rather than for competition and marketing. The following tests were performed to evaluate and compare the five UHPLC systems: injector linearity and precision, sample carryover, sample (autosampler) temperature accuracy, column temperature accuracy and precision, pressure ripple, pump mixing accuracy, flow rate accuracy, detector drift and noise, detector linearity, wavelength accuracy, extra-column volume, and dwell volume determination. This study presents an approach on how to test the performance of UHPLC systems along with potential problems that analysts may face when using the UHPLC systems, examples of such issues are: retention time irreproducibility, low sensitivity, method transfer failure, etc.