Eluents zwitterionic

In the most recent method described by Hu et al. [239] for the direct determination of ultraviolet-absorbing inorganic anions in saline matrixes, an octadecylsilica column modified with a zwitterionic surfactant [3-(N,N-di-methylmyristylammoniojpropanesulfate] is used as the stationary phase, and an electrolytic solution is used as the eluent. Under these conditions, the matrix species (such as chloride and sulfate) are only retained weakly and show little or no interference. It is proposed that a binary electrical double layer is established by retention of the eluent cations on the negatively charged (sulfonate) functional groups of the zwitterionic surfactant, forming a cation-binary electrical double layer. 

As an alternative to carbonate/bicarbonate systems, amino adds (a-aminocarb-oxylic acids) may be used as an eluent [47,48]. Their dissociation behavior is depicted in Fig. 3-36. At alkaline pH, amino acids exist in the anionic form due to the dissociation of the carboxyl group and, thus, may act as an eluent ion. The product of the suppressor reaction is the zwitterionic form with a correspondingly low background conductance. This depends on the isoelectric point, pi, of the amino acid. 

A second class of available eluents comprises the family of zwitterionic compounds (see Section 3.3.2), which exist in their anionic form at alkaline pH. The product of the suppressor reaction is the zwitterionic form with a correspondingly low intrinsic conductance. Promising experiments were carried out by Irgum [48] with N-substituted... 

Finally, it should be pointed out that chemical suppression in form of protonation reactions is also applicable to zwitterionic eluents [8,9],... 

When a small volume of an aqueous sample containing anions and cations is passed through a column with a zwitterionic stationary phase, neither the cations nor anions can get very close to the opposite charge on the stationary phase. The sample anions and cations are forced into a new state of simultaneous electrostatic attraction and repulsion interaction. Thus the sample ions are somewhat attracted to the zwitterionic stationary phase, but the attraction is weak enough that water alone can serve as the eluent. To preserve electroneutrality, an equal charge of anions and cations must be eluted. The authors have termed this separation method as EKC (electrostatic chromatography). 

An electrostatic ion chromatographic method was developed for the direct determination of iodide, bromide and nitrate in seawater (Hu et ai, 1999). An octadecylsifica column modified with a zwitterionic surfactant 3- NJV-dimethylmyristylammonio)propane-suifonate was used as the stationary phase, and an electrolytic solution was used as the eluent. The matrix species (such as chloride and sulfate) were retained weakly, and showed iitde or no interference. The method was applied to the determination of iodide, bromide, and nitrate in artificial seawater, giving detection fimits of 0.8 p,g 1 for iodide, 0.75 p.g 1 for bromide, and 0.52 p,g 1 for nitrate, and relative standard deviations of <1.2%. The real seawater samples were also analyzed successfully. Later, another electrostatic ion chromatographic method was developed for the determination of iodide in seawater by the same research group (Hu et ai, 2002). A reversed-phase ODS column was... 

Attempts have been made to use other surfactants, such as sodium decyl sulfate and sodium pentadecyl sulfate [8]. However, it was observed that none of these are able to quantitatively elute BSA when methanol is present in the eluents. In fact, sodium pentadecyl sulfate has limited solubility in eluents containing less than 30% methanol (v/v concentrations of organic solvents are given). One nondenaturing anionic surfactant, sodium desoxycholate was found to elute BSA from model serum injections. A zwitterionic surfactant known as C12 DAPS (3-(dimethyldodecyl-ammonium propanesulfonate) has also been successful in drug analysis [9]. 

Chapter 10 on zwitterion stationary phases is another new addition to this book. Separations are generally performed on an HPLC column coated with a zwitterion surfactant that contains both positive and negative sites. In some cases cations and anions can be separated in a single run using pure water as the eluent ... 

Sample ions can usually be eluted from a zwitterion column under milder conditions than are required with a regular ion-exchange column. A dilute eluent containing ions of low conductivity gives greater detection sensitivity for sample anions or cations. 

The ionic concentration of the mobile phase has much less effect on the retention times of sample ions on a zwitterion column than it does on a conventional IC column. At the eluent concentrations generally used with a zwitterion phase, an increase in eluent concentration results in only a slight decrease in the retention times. Sometimes longer retention times are observed as the eluent concentration increased. 

In 1981, Knox and Jurand [3] separated nucleotides, which have both positive and negative charges, on a reversed-phase column using an eluent containing a zwit-terion, (1.25 mM 11-aminodecanoic acid) in addition to 75 mM ammonium phosphate. The retention mechanism was attributed to the formation of a quadripole between the zwitterion and two oppositely charged sites on the nucleotide. The enhanced retention of nucleotides was due to the 11-aminodecanoic acid that was adsorbed by the stationary phase. 

In 1993, Hu, Takeuchi and Haraguchi [5] described a technique in which anions and cations were separated simultaneously on a zwitterion stationary phase with pure water as the eluent. An octadecyl silica column acquired a semi-permanent coating when a solution of a zwitterionic surfactant was passed through the column. It was demonstrated that analyte cations and anions were eluted together. The ability to separate ions by ion chromatography with a mobile phase containing no chemicals whatsoever is an ideal situation for detection of the ionic analytes. 

Macka and Haddad [8] separated the sodium salts of several inorganic anions on a column coated with a CHAPS zwitterion phase with only water as the eluent. 

Chapter 10 on Zwitterion Stationary Phases describes a fascinating variation of IC. Pure water can often be used as the eluent to separate sample ions when a zwitterion stationary phase is employed. Ions elute as cation-anion pairs. 

Fig. 2.23. Separation of nucleotides by zwitterion-pair chromatography, showing pH effect. Packing material, ODS-Hypersil eluent, H20-MeOH (88 12, v/v), 75 mM in phosphate and 1.25 mM in 11-amino undecanoic acid (212)...

Almost all of the above columns are alkyl phases, i.e. Cjg/Cg. For very polar compounds, one should also consider CN, phenyl or diol phases or even silica. In combinadon with RP eluents, the latter often yields very interesdng selecdvities. At last, stadonary phases with zwitterion functionality such as ZIC-HI LI C or, in the alkaline region, ZIC-pHILIC, are well suited for separadng strongly polar compoimds. 

With charged compounds (weak acids or bases), the adjustment of the pH of the aqueous mobile phase has the strongest influence on the change in selectivity of a chromatographic separation. This is due to the fact that organic acids, basic compounds, and hybrid ionic compounds (zwitterions) show very different behavior in response to pH variations of the eluent [7-9]. 

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