Cationic analytes

The participation of cations in redox reactions of metal hexacyanoferrates provides a unique opportunity for the development of chemical sensors for non-electroactive ions. The development of sensors for thallium (Tl+) [15], cesium (Cs+) [34], and potassium (K+) [35, 36] pioneered analytical applications of metal hexacyanoferrates (Table 13.1). Later, a number of cationic analytes were enlarged, including ammonium (NH4+) [37], rubidium (Rb+) [38], and even other mono- and divalent cations [39], In most cases the electrochemical techniques used were potentiometry and amperometry either under constant potential or in cyclic voltammetric regime. More recently, sensors for silver [29] and arsenite [40] on the basis of transition metal hexacyanoferrates were proposed. An apparent list of sensors for non-electroactive ions is presented in Table 13.1. 

Self-regenerating suppressor cartridge incorporating an electrolysis cell, (a) For anionic analytes, (b) For cationic analytes. 

The electrophoretic mobilities of cationic analytes in water, in non-aqueous methanol and acetonitrile were measured and the influence of ionic strength and ion-pair formation on the mobility was determined. It was assumed that the mobility n of the ion at infinite dilution (jxQ ) and the viscosity of the pure solvent are constant ... 

S.P Porras, M.-L. Riekkola and E. Kenndler, Electrophoretic mobilities of cationic analytes in non-aqueous methanol, acetonitrile and their mixtures. Influence of ionic strength and ion-pair formation. J. Chromatogr.A 924 (2001) 31 12. 

The HR-ICPMS cation analytical results are a robust dataset that is remarkably free of data qualifiers for 32 of the 63 cations analyzed an additional 14 cations have <20% censored data. Further, the low-level data have consistent map distribution patterns that make sense geologically. Described below are patterns for several possible porphyry-related elements. 

Temsamani, K. R., Ceylan, O., Yates, B. J., Oztemiz, S., Gbatu, T. R, Stalcup, A. M., Mark, H. B., Jr. and Kutner, W., Electrochemically aided solid phase microextraction conducting polymer film material applicable for cationic analytes, /. Solid State Electrochem., 6,494-497 2002. 

Fig i Mechanisms of ion-selective optode response in aqueous samples containing a cationic analytes, M+, or b anionic analytes, X-. L represents the ionophore ligand, C the chromoionophore and Y" and Z+ the corresponding ionic additives (counterions)... 

Bidlingmeyer, B.A. Separation of ionic compounds by reversed-phase liquid chromatography an update of ion-pairing techniques. J. Chmmatogr. 1980,18, 525-539. Sarzanini, C. et al.. Retention model for anionic, neutral and cationic analytes in reversed-phase ion interaction chromatography. AnaL Chem. 1996, 68,4494-4500. 

While a hydrophobic ion-pair is retained on hydrophobic stationary phases better than an ionized analyte, the retention of the duplex on normal phases is easily predicted to be lower than that of the ionized analyte because polar interactions are reduced. Actually the trend of k versus IPR concentration under normal phase IPC is the opposite of reversed phase IPC [34]. An aminopropyl, a cyanoethyl, and a silica stationary phase were compared for the analysis of alcohol denaturants. The cyanoethyl phase was selected and anionic IPRs were used to reduce retention of cationic analyte, suppressing their interactions with negatively charged silanols... 

The influence of the nature of the IL anion confirms the importance of ion-pairing for fine tuning retention. The lower the hydrophobicity and ion-pairing ability of the IL anions, the lower the retention. Ion pairing of the cationic analyte and the IL anion decreases silanophilic interactions and this in turn results in better peak shapes, and may eventually increase analyte retention. Actually, even if most research related the retention decrease upon IL addition in the mobile phase to decreased silanophilic interactions, it must be noted that positively charged analytes and IL cations also undergo repulsive electrostatic interactions. 

To deconvolve the silanophilic effect from the electrostatic repulsion, a nonsilica-based stationary phase may be suitable in research work. On a polystyrene-divinylbenzene reversed phase column, an ethylammonium formate RTIL was not able to produce effective ion-pairing interactions with acidic and basic model compounds, and baseline resolution was only obtained in the presence of classical IPRs (tetrabutylammonium and dodecylsulfate ions, respectively). However, the RTIL was able to mimic the methanol role [123,126]. In summary, IL cations reduce positively charged analyte retention since they (1) screen free silanols and (2) electrify the stationary phase with a positive surface charge that is repulsive for cationic analytes. The hydrophobic character of IL anions is responsible for possible analyte retention increases via ion-pairing. 

The mixed mechanism outlined above can be elucidated by analyzing the effects of different salts at the same concentration on the retention of a cationic analyte. The... 

---