Eluent nicotinic acid

The chromatogram shown in Fig. 3-66, obtained using nicotinic acid as the eluent, serves as an example of the variety of organic adds that can be used as eluents. Nicotinic acid is most suitable for the separation of monovalent anions. In contrast to other monocarboxylic adds, it is characterized by a good water solubility and a low equivalent conductance. In addition, its propensity to adsorb at the stationary phase is very low, which mostly prevents the formation of the usual system peak. However, divalent anions are strongly retained under these chromatographic conditions. For their fast separation, eluents based on salicylic add or trimesic add are more suitable. 

Fig. 3-66. Separation of monovalent anions using nicotinic acid as the eluent. — Separator column Wescan 269-031 eluent 0.01 moI/L nicotinic acid flow rate 2.7 mL/min detection direct conductivity injection volume 100 pL solute concentrations 20 ppm azide, 10 ppm formate and fluoride, 20 ppm orthophosphate, 10 ppm nitrite and chloride, 15 ppm bromate (taken from [70]).

Fig. 8-87. Analysis of water-soluble vitamins. - Separator column Spherisorb ODS 2 (5 pm) eluent (A) 0.1 mol/L KOAc (pH 4.2 with HOAc), (B) water/methanol/acetonitrile (50 10 40 v/v/v) gradient linear, 6% B in 30 min to 100% B flow rate 2 mL/min detection UV (254 nm) injection volume 50 pL solute concentrations 5 nmol each of ascorbic acid (1), nicotinic acid (2), thiamine (3), pyridoxine (4), nicotinic add amide (5), p-aminobenzoic add (6), cyanocobala-mine (7), and riboflavine (8).

Although only monovalent anions are included in Table 6.8, similar effects are observed with some divalent anions. Salicylic acid will elute sulfate in 5.2 min and thiosulfate in 7.3 min under the conditions in Table 6.7. However, the weaker eluents such as succinic or nicotinic acid require an excessive amount of time to elute divalent anions. 

Table 6.9 lists the relative retention time of the chloride sample anion with the acid eluents chosen for this study. The adjusted retention time of chloride decreases as the retention time for the eluent anion increases and it also decreases as the amount of ionization of the eluent increases. The most satisfactory separations were obtained with either nicotinic acid or succinic acid as the eluent acid. 

Working principle. The electrolytic conductivity of the eluent is measured in a flow cell. The metallic inlet and outlet tubes act as electrodes across which an alternating current voltage at a frequency of 1-10 kHz is applied. Constructions with cell volumes from 0.1 to about 10 pL are achievable. Weakly dissociated aqueous electrolytes such as phthalic acid and nicotinic acid are frequently used in IC. They allow ppm levels to be deteaed after electronic compensation of the background conductivity. 

Chromatographic retention times of a number of anions were compared, with 6.0 mM nicotinic acid and 2.0 mM phthalate (pH 6) as the eluents. In many cases, the length of the spacer arm had very little effect on the relative retention times (relative to Cl ). However, the selected data in Table 3.4 show that the relative retention times of bromide, nitrate, chlorate and iodide decreased, while that of sulfate increased slightly. 

Separation of common anions, each at the 1 ppm level, is now routine. Detection at low ppb range is possible when a hydroxide eluent and a large sample volume are used (Figure 6.14). However, detection of anions at the low ppb level has also been realized with nonsuppressed conductivity with a nicotinic acid eluent [14]. 

In practice, eluents of widely varying polarity have been used. Very weakly polar steroids (oestrenols) can be quantitatively eluted with hydrophobic solvents like methylene dichloride (cf Table 14) [217], whereas difficulties are reported in the elution of the more polar progesterone under the same conditions [691]. Chloroform has been used to elute dinitrophenylhydrazones from silica gel and opium alkaloids from alumina [470, 535]. Methanol and ethanol are often used for elution of substances of all types of compound class from silica gel [213, 215, 259, 434] or alumina [122, 437]. Butyl acetate has been chosen as the most suitable eluent for penicillin V [486]. Acetone has proved suitable for recovering ubiquinones [733]. Polar neutral, acid and basic aqueous eluents have also been employed, e.g., water for mucic acid derivatives [451] (cf Table 14), 1% Tween 80 solution for cobalamin [128], 0.2 N sulphuric acid for Vitamin Bg factors [702], 34% ammonium persulphate solution for nicotinic acid [702] and ammonium hydroxide for azo dyes [638]. Nitro-4-acetaminophenetoles have been extracted from alumina with the highly polar dimethylformamide [489]. 

Table 2 shows Rf values of nicotinic acid and nicotinamide and their derivatives in different eluents. The detection is performed by illumination under short-wavelength (257.3 nm) UV light. 

Figure 4.37. Effect of eluent pH on selectivity in reverse-phase BPC. Column, 30 x 0.4cm, Bondapak-C, 10/jm mobile phase, 0.025M NaH2P04-Na2HP04 wiih 40%v methanol flowrate, 2ml/min temp., ambient detector, UV, 220nm, samples. 1—lOpg. (1) Salicylic acid (2) phenobarbitone (3) phcnacctin (4) nicotine ...

Fig. 7.5. Effect of variation of the eluent pH upon elution volume. Chromatographic conditions column, pBondapak Cjg (10 pM) (300 mmX4 mm I.D.) mobile phase, eluent solutions (pH 3.0, 5.0, 7.0 and 9.0) were made up from 0.025 M NaH2P04 and/or 0.025 M Na2HP04 solutions plus 40% methanol (solutions were adjusted to final pH by the addition of 5% sodium hydroxide or phosphoric acid solution) detection, UV at 220 nm. , salicylic acid a, phenobarbitone a, phenacetin O, nicotine , methylamphetamine. Reproduced from Twitchett and Moffat (1975), with...

Several other organic acids have been used as eluents in anion chromatography. These include succinic, nicotinic, salicylic, fumaric and citric acid [14]. One objective was to find an eluent that is not adsorbed by the resin matrix and that attains equilibrium faster than benzoic acid. The following organic acids, listed in order of their increasing eluting power for sample anions, attain equilibrium fairly rapidly nicotinic, succinic, citric, fumaric, salicylic. 

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