Eluents hydroxybenzoate

The influence of NH., and CO, on the chromatographic behaviour of benzoic acid and its derivatives (o-, m-, p-hydroxybenzoic, nitrobenzoic, aminobenzoic, chlorobenzoic acids) was studied. The work was carried out by means of upgoing TLC on Sorbfil plates. Isopropanol- and ethyl acetate-containing water-organic eluents were used as mobile phases in the absence or presence of gaseous modifiers in the MP. The novel modification of TLC has been found to separate benzoic acids with different values of their dissociation constants more effectively than water-organic mobile phases. 

For ions that are UV transparent, detection is possible through the use of indirect detection. A wide variety of different eluent systems have been described in the literature. Eluents commonly used for indirect UV detection are similar to those used in non-suppressed conductivity detection phthalate and p-hydroxybenzoic acid along with other... 

Figure 6.1 UV chromatograms of the test mixture of four / -hydroxybenzoic acid esters (1, methyl 2, ethyl 3, propyl 4, butyl) after the column and after the NMR flow cell at flow rates of (a) 1.0 and (b) 0.1 ml/min conditions column, LiChrospher RP select B, 125 x 4 mm id, 5 Jim, spectrometer, Bruker DRX 600 probe head, 4 mm z-gradient LC probe, active volume 120 a1 eluents, acetonitrile (A) and D2O (B) gradient, t = Omin A/B (40/60), t = 8 min A/B (70/30) at a flow rate of 1.0 ml/min and t = 80 min A/B (70/30) at a flow rate of 0.1 ml/min...

Fig. 3-5. Separation of various anions on PRP-X100. - Eluent 0.004 mol/L sodium p-hydroxybenzoate (pH 8.6) flow rate 3 mL/min detection direct conductivity injection volume 100 pL solute concentrations 20 ppm each (taken from [21]).

Fig. 3-63. Separation of inorganic anions using p-hydroxybenzoic add as the eluent. - Separator column Wescan 269-029 eluent 0.004 mol/L PHBA, pH 8.7 flow rate 1.5 mL/min detection direct conductivity injection volume 100 pL solute concentrations 5 ppm fluoride, chloride, nitrite, bromide, and nitrate, and 10 ppm orthophosphate and sulfate.

Fig. 3-72. pH Dependence of the retention of various inorganic anions using p-hydroxybenzoic acid at a concentration c = 0.005 mol/L as the eluent (taken from [83]). 

For non-suppressed ion chromatography to be successful, the ion exchanger used in the separation column must have a low exchange capacity and a very dilute eluent must be used. In the separation of anions, the resin must have an exchange capacity between about 0.005 mequiv/g and 0.10 mequiv/g. Typical eluents are 1.0 x 10 M solutions of sodium or potassium salts of benzoic acid, hydroxybenzoic acid, or phtha-lic acid. These eluents are sufficiently dilute that the background conductivity is quite... 

Nonsuppressed IC methods for natural water samples are generally 1-2 orders of magnitude less sensitive for common inorganic anions than suppressed conductivity methods, but still find useful application for the determination of higher-concentration matrix anions, such as chloride and sulfate. Typical eluents used include p-hydroxybenzoic acid or phthalic acid, often used with small amounts of organic solvent to improve peak shapes. Such eluents can also be employed for indirect UV detection, although again sensitivity is somewhat less than for direct UV detection. 

Evans et reported the HPLC-ED of plasma salicylate (2-hydroxybenzoate). A LoD of 4 ng on column was claimed. The eluent used was methanol-aq. sodium acetate (approximately 60mmolL pH 6.0) (4 - - 46). However, the column used was not stated, a protracted sample purification procedure was needed, no internal standard was used and the applied potential advocated (GCE, -I-1.35 V vs Ag/AgCl) was very high by comparison with other methods involving the ED of phenolic hydroxyl groups. 

Indirect spectral detection requires the use of a strongly absorbing anion in the eluent. Eluents containing molybdate, p-hydroxybenzoate or phthalate have been used successfully. 

Benzoate or p-hydroxybenzoate, which is more soluble and therefore to be preferred, are two of the most useful carboxylate ions for eluents. Phalate or hydrogen phthalate is also widely used. Benzoate salts are useful for separation of acetate, bicarbonate, fluoride, chloride, nitrite, nitrate, and other early-eluting anions. Divalent anion and other late-eluting anions such as thiocyanate and perchlorate are not eluted effectively by benzoate. The concentration of a benzoate eluent that should be used depends on the type and capacity of anion-exchange resin used, but is typically 0.5-5.0 mM. 

Polyspher IC AN-1 from Merck AG (Darmstadt, Germany), consisting of a hydrophilic polymethacrylate packing with a particle diameter of 12 pm, exhibits a similar selectivity. Depending on the type of application, this column can be operated with eluents based on phthalic acid, / -hydroxybenzoic acid, tartaric acid, oxalic acid, or boric acid. Figure 3.19 shows a typical chromatogram obtained with a phthalic acid eluent. Under these chromatographic conditions (pH 4), orthophosphate elutes as a monovalent anion ahead of chloride. 

Figure 3.17 Separation of various inorganic anions on LCA A03. Eluent 4mmol/L sodium p-hydroxybenzoate (pH 8.5) flow rate 2 ml7 min detection nonsuppressed conductivity ...

The pH value of the mobile phase affects not only the dissociation of the organic acid used as an eluent but also the valency of some analyte ions. This is demonstrated in Figure 3.147, where the pH dependence of the retention of inorganic anions using a p-hydroxybenzoic acid eluent is displayed. As expected, the retention decreases as pH increases. At higher pH, not only the carboxyl... 

The hydroxybenzoic acids and the longer-chain aliphatic carboxylic acids can give poor detector response and peak shape for quantification. If they are important constituents of a sample, it is a good idea to rerun the sample with 1 to 5% methanol or isopropanol in the eluent to improve detector response and peak shape. It is imperative that the new relative elution times for the carboxylic acid anions be determined at this time so that the misiden-tification of peaks cannot occur. 

---