Analytes ionic

Gas-phase ion-molecule reactions involving ionic analytes would lead to their neutralization and, consequently these have to be minimized. This requires careful control of both the composition and the concentration of any buffer used. 

Enke, C. G. A predictive model for matrix and analyte in electrospray ionization of singly-charged ionic analytes. Anal. Chem. 1997,69,4885 4893. 

Online SPE LC/MS/MS is commonly used for bioanalytical applications in the pharmaceutical industry. Column switching systems and TFC systems are easy to build and control. Sophisticated commercial systems and SPE cartridges are readily available. Compared to offline sample preparation, the online approach can save time and labor. However, the development of online SPE bioanalytical assays remains analyte-dependent. Generic methods can be applied to many analytes. For extremely hydrophobic, hydrophilic, and ionic analytes at normal pH range and analytes with a variety of hydrophobicity and pKa values, analyte-specific methods must be developed. An understanding of the chemistry of the analytes and SPE is critical. 

A thermodynamic approach has also been employed on ion interaction chromatography (IIC) to predict the retention of neutral and ionic analyte species. The basic equations describing retention are... 

Analytes of very high polarity are not anymore ionized by field ionization. Here, the prevailing pathways are protonation or cationization, i.e., the attachment of alkali ions to molecules. [78] The subsequent desorption of the ions from the surface is effected by the action of the electric field. As [M-t-Na]" and [M-i-K] quasi-molecular ions are already present in the condensed phase, the field strength required for their desorption is lower than that for field ionization or field-induced [Mh-H]" ion formation. [37,79] The desorption of ions is also effective in case of ionic analytes. 

The precursor model of FAB applies well to ionic analytes and samples that are easily converted to ionic species within the liquid matrix, e.g., by protonation or deprotonation or due to cationization. Those preformed ions would simply have to be desorbed into the gas phase (Fig. 9.6). The promoting effect of decreasing pH (added acid) on [M+H] ion yield of porphyrins and other analytes supports the precursor ion model. [55,56] The relative intensities of [Mh-H] ions in FAB spectra of aliphatic amine mixtures also do not depend on the partial pressure of the amines in the gas phase, but are sensitive on the acidity of the matrix. [57] Furthermore, incomplete desolvation of preformed ions nicely explains the observation of matrix (Ma) adducts such as [M+Ma+H] ions. The precursor model bears some similarities to ion evaporation in field desorption (Chap. 8.5.1). 

FAB is well suited for the analysis of ionic analytes. In positive-ion mode, the spectrum is usually dominated by the cationic species, C which is accompanied by cluster ions of the general composition [CnH-An-i]". Thus, the distance between these signals corresponds to the complete salt CA, i.e., yields its molecular weight. This behavior is perfectly analogous to FD (Chap. 8.5.3). In negative-ion FAB, the anion A" will cause the base peak of the spectrum, and accordingly, cluster ions of the type [Cn.iH-AJ" are formed in addition. Consequently, both cation and anion are usually identified from the same FAB spectrum, irrespective of the chosen polarity. Nonetheless, it is common practice to select the polarity of the more interesting ion for the measurement. 

Electrospray ionization (ESI) is a soft ionization technique that accomplishes the transfer of ions from solution to the gas phase. The technique is extremely useful for the analysis of large, non-volatile, chargeable molecules such as proteins and nucleic acid polymers. [1] Different from fast atom bombardment (FAB, Chap. 9) the solution is composed of a volatile solvent and the ionic analyte at very low concentration, typically M. In addition, the transfer of ions... 

Redox electrodes are useful to the analyst because the metal, M, is a pure solid and invariably of a lower valence than the ionic analyte in solution, M"+. This means that the denominator in the logarithmic term in equation (3.8) is always unity, i.e. the Nemst equation for a redox electrode M can be simplified to the following ... 

To write down the electrochemical potential for the surface, we must consider that the ionic analyte changes the electrostatic surface potential, i.e.,... 

With TSP, ammonium acetate has emerged as the best general-purpose electrolyte for ionizing neutral samples. Improved ionization can be obtained by the use of a filament or discharge electrode to generate reactive ions for CI (87, 88). The processes involved in filament or discharge-assisted ionization must be used when operating in the absence of a buffer with nonaqueous eluents. With ionic analytes, the mechanism of ion evaporation is supposed to be primarily operative since ions are produced spontaneously from the mobile phase (89). Ion evaporation often yields mass spectra with little structural information in order to overcome this problem, other ionization modes or tandem MS have been applied (90). 

Mobile phase CH,CN/H20 for neutral analytes CH,CN/aqueous buffer 1 for ionic analytes 5 vol% CH, CN in H20 to 100% CH1CN for gradient elution... 

Analytes can be separated from complex matrices by sample preparation techniques that include liquid extraction, supercritical fluid extraction, and solid-phase extraction. Dilute ionic analytes can be preconcentrated by adsorption onto an ion-exchange resin. Nonionic analytes can be concentrated by solid-phase extraction. Derivatization transforms the analyte into a more easily detected or separated form. 

An MIP film is immobilized on the electrode surface and this electrode is maintained in the test solution (Case a in Scheme 4). Next, the MIP surface is thoroughly rinsed with abundant solvent to remove a physisorbed analyte and, then, directly used for analyte determination. Herein, impedimetric or potentiometric transduction can be used successfully for determination of either ionic and neutral, or ionic analytes, respectively. 

CEC has recently become an alternative to HPLC. A capillary is filled or its internal wall covered with a porous sorbent. The free volume remaining in the capillary is filled with an electrolyte. High voltage (on the order of ten kV) is applied across the length of the capillary. Sample plugs are introduced at one end. Sample components are carried to the other end due to electro-osmosis and - in the case of ions - also electrophoresis. In CEC the more important effect is electro-osmosis, which is essentially a flow mechanism of the electrolyte solution without the need for applied pressure. The separation of the sample components occurs mainly due to phase distribution between the stationary phase and the flowing electrolyte. Thus CEC is very similar to HPLC in a packed capillary except that the flow is not pressure driven and that ionic analytes undergo electrophoresis additionally to phase separation. 

This paper will discuss the development of SFE techniques for polar and ionic analytes based on two different approaches. The first approach uses the... 

Polar and ionic analytes can be extracted from a variety of sample matrices using in-situ chemical derivatization under SFE conditions followed by SFE extraction of the derivatized analytes. Derivatization of the analytes during a... 

Porphyrin dendrimers are suitable as sensors for small molecular and ionic analytes. Unsubstituted metal-free porphyrins often show poor solubility in water. However, if it proves possible to envelop them in hydrophilic dendrimers they can be used in water as fluorescent pH indicators because they exhibit distinct changes of their absorption and emission bands owing to protonation of... 

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