Eluents benzoate

Figure 4.1 Retention factors related to log P values. Column, 5/im octadecyl-bonded silica gel (LiChrosorb LC7) 25 cm x 4.1 mm i.d. eluent, 50% aqueous acetonitrile flow rate, 1ml min-1. Compounds , alkanols O, benzoates, O, polycyclic aromatic hydrocarbons, and A, alkylbenzenes.

As above, the log k values of the different types of compounds [alkanols (ROH), polycyclic aromatic hydrocarbons (PAH), alkyl benzenes (RB), and alkyl benzoates (ROB)] each demonstrated a linear relationship with their log P values and this relationship was observed with different ratios of acetonitrile and water. The experiment was then performed in eluents containing different organic modifiers and it was found that the behaviour of analytes containing different functional groups differed and the effect depended on the log P of the analyte. 

Example 1 Separation of an alkylbenzene and an alkyl benzoate. The logP values of butyl benzoate and isopropylbenzene are 3.74 and 3.52, respectively, from Table 4.1. The separation of these compounds in 50% aqueous acetonitrile therefore requires a high plate number column as the log k values will be nearly identical. The most suitable eluent is 40% aqueous THF. See compounds 9 and 19 in Figure 4.3B. 

The direct mode is employed with eluents with significantly lower equivalent conductance than the analyte ion. Increase in sensitivity is obtained as the degree of the ionization of the eluent decreases, that is, with more weakly dissociated eluents, and non suppressed conductivity methods have been extensively developed using benzoate, phthalate [246], oxalate [53] or other partially ionized species as mobile phases. A key factor in the success of this technique is the use of an ion exchanger of low-exchange capacity, which in turn permits the use of a very dilute eluent. 

Indirect detection exploits the absorbance of the eluent ion (e.g., NOj , phthalate, benzoate, meth-ylbenzylamine) that should absorb more than the analyte. The eluting analyte exhibits a negative peak. Nevertheless, this mode of detection has limited utilization since the detection limits are often inadequate. 

Figure 25-12 Isocratic HPLC separation of a mixture of aromatic compounds at 1.0 mL/min on a 0.46 x 25 cm Hypersil ODS column (C,8 on 5-jxm silica) at ambient temperature ( 22 C) (1) benzyl alcohol (2) phenol (3) 3, 4 -dimethoxyacetophenone (4) benzoin (5) ethyl benzoate (6) toluene (7) 2,6-dimethoxytoluene (8) o-methoxybiphenyl. Eluent consisted ot aqueous buffer (designated A) and acetonitrile (designated B). The notation 90% B in the first chromatogram means 10 vol% A and 90 vol% B. The buffer contained 25 mM KH2P04 plus 0.1 g/L sodium azide adjusted to pH 3.5 with HCI.

Fig. 1. Simultaneous separation and detection of anions and cations on a latex agglomerate column. Column Dionex HPIC-CS5 cation exchange column (250X2 mm) with precolumn HPIC-CG5 (50 X 4 mm) eluent 0.5 mM copper sulfate, pH 5. 62 flow rate 0.5 ml/min sample volume 20 gl containing 0.1 m M of each ion detection two potentiomet-ric detectors equipped with different ion-selective electrodes in series. Peaks (1) chloroacetate, (2) chloride, (3) nitrite, (4) benzoate, (5) cyanate, (6) bromide, (7) nitrate, (8) sodium, (9) ammonium, (10) potassium, (11) rubidium, (12) cesium, (13) thallium. Reprinted with permission from [10].

Polysaccharide-based CSPs also exhibit a chiral recognition for alcohols and a large number of resolutions have been reported. Chiral alcohols can usually be directly resolved with hexane containing a small amount of an alcohol as the eluent. For aliphatic alcohols, which cannot be directly resolved, their resolution is often efficiently attained as phenylcarbamate or benzoate derivatives on OD (Figure 17).85 For example, 2-butanol and 2-pentanol are completely resolved with a very high selectivity on OD as their phenylcarbamates. The derivatization of alcohols to phenylcarbamates and benzoates can be easily achieved by the reaction with phenyl isocyanates and benzoyl chlorides, respectively. In most cases, the phenylcarbamates are better resolved than the benzoates. For chiral compounds bearing phenolic hydroxy groups, the addition of a small amount of an acid to an eluent is recommended to depress its dissociation. 

A mixture of 289 parts of cis-[2-(2,4-dichlorophenyl)-2-(lH-l,2,4-triazol-l-ylmethyl)-l,3-dioxolan-4-ylmethyl]benzoate, 200 parts of sodium hydroxide solution 50%, 1500 parts of 1,4-dioxane and 300 parts of water is stirred and refiuxed for 2 h. The reaction mixture is cooled and poured onto water. The product is extracted with dichloromethane. The extract is washed with water, dried, filtered and evaporated. The residue is purified by column-chromatography over silica gel using a mixture of trichloromethane and methanol (95 5 by volume) as eluent. The first fraction is collected and the eluent is evaporated, yielding 89 parts cis-2-(2,4-dichlorophenyl)-2-(lH-l,2,4-triazol-l-ylmethyl)-l,3-dioxolane-4-methanol melting point 138.2°C. 

Polysaccharide-coated CSP are prepared by coating of their benzoate or carbamate derivatives on substrate from a solution with the following evaporation of the solvent. These phases can be used under RP conditions with aqueous eluents or in normal-phase conditions with limited number of solvents to avoid dissolution of the coated layer [80, 81]. 

The adoption of an eluent having a very low conductivity, which can be passed directly through the conductometric detector. Typical eluents used are benzoate, phthalate, or other aromatic acid salts, with low limiting equivalent conductances (leading to direct detection) or potassium hydroxide eluent, with high conduc-... 

Fig. 3-4. Separation of fluoride, chloride, and bromide on a surface-aminated XAD-1-resin. — Separator 1000 mm X 2 mm I.D. XAD-1 (0.04 mequiv/g) eluent 6.5 10-4 mol/L potassium benzoate (pH 4.6) flow rate 2 mL/min detection direct conductivity injection volume 50 pL solute concentrations 4.8 ppm fluoride, 5.1 ppm chloride, and 26 ppm bromide (taken from [19]).

With benzoic acid as the eluent, generally higher sensitivities are obtained for the analyte anions in comparison to sodium or potassium benzoate, respectively. This is illustrated in the following reaction scheme ... 

Eluents based on sodium benzoate or benzoic acid, respectively, exhibit about the same elution power as bicarbonate and are thus used for the analysis of monovalent anions. When looking for less strongly adsorbing acids, Fritz, DuVal, and Barron [68] found other compounds that may also be employed as eluents. The properties of these compounds, that are important in this connection, are listed in Table 3-17. 

Fig. 3-67. Separation of anions derived from weak inorganic acids using a strongly basic eluent. — Separator column Wescan 269-029 eluent 0.004 mol/L NaOH + 0.0005 mol/ L sodium benzoate flow rate 1.5 mL/min detection indirect conductivity injection volume 100 pL solute concentrations 5 ppm borate (as B), 10 ppm silicate (as Si02), 10 ppm formate and sulfide, 20 ppm chloride and cyanide (system peak appears after 28 min.) (taken from [70]).

If this equilibrium is disturbed by a sample injection, a new equilibrium is established via relaxation i.e., the kind of relaxation process depends on the pH value of the sample injected. If the sample pH is lower than the pH value of the mobile phase, benzoate ions in the mobile phase are protonated due to the sample injection. Thus, the concentration of molecular benzoic acid in the mobile phase increases as does the concentration of the amount adsorbed to the stationary phase. The amount not adsorbed travels through the column and appears as a chromatographic signal the system peak. A qualitatively similar chromatogram is obtained when a sample containing the solute ions and the corresponding eluent component is injected into the system. However, only the position of the system peak is comparable, not its area and direction. 

Fig. 5-12. Ion-pair chromatographic separation of nitrite, nitrate, and benzoate. - Separator column IonPac NS1 (10 pm) eluent 0.002 mol/L TBAOH + 0.001 mol/L Na2C03 / acetonitrile (82 18 v/v) flow rate 1 mL/min detection suppressed conductivity injection volume 50 pL solute concentrations 10 ppm nitrite, 10 ppm nitrate, and 20 ppm benzoate.

According to the above definition, direct detection is feasible when using carefully selected eluents such as phthalate [5] or benzoate [6], which exhibit a low equivalent ionic conductance (see Table 6-1). This results in a conductivity increase when a solute ion passes the conductivity cell. 

The retention times of 17 monovalent anions on resins with different functional groups but with almost identical exchange capacities (average 0.027 mequiv/g) were compared with the use of a solution of a monovalent anion (sodium benzoate) as the eluent [8]. Relative retention times were calculated by dividing the measured retention times by that of chloride. Data for resins with various trialkylammonium groups are presented in Table 3.1. 

Lithium, sodium, potassium, or other salts of benzoic acid, phthalic acid, sulfoben-zoic acid, citric acid, and others are useful eluents for anions. These are rather large organic anions that are less mobile than most inorganic anions and therefore have lower equivalent conductances. For example. Table 4.1 shows that the benzoate anion has a limiting equivalent eonductance of 32 S cm equiv, while ehloride, nitrate, sulfate, and other typieal sample anions have higher equivalent conductances (approximately 70 S cm equiv ). If a sodium benzoate eluent is used, the equivalent conductance is the sum of sodium ion (50) and benzoate (32), or 85 S cm equiv". The equivalent conductance of an anion is the sum of equivalent eonductances of the sodium ion (50) and the anion (70), or 120 S cm equiv. On an equivalent basis, this amounts to almost a 50 % increase in conductance. 

The benzoate salt is one of the two most useful carboxylic acid salts for eluents. The other is phthalate. 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 to... 

Although sodium hydroxide can be used as an eluent for cyanide, acetate, arsenitc, fluoride, and other easily eluted anions, the hydroxide ion is a rather weak eluent for many anions. More recent work has shown that a sodium hydroxide solution containing small amounts of sodium benzoate (in about 1 10 molar ratio) behaves similarly to hydroxide (the peaks are usually still of decreasing conductance), but is a more powerful eluent than sodium hydroxide alone. 

X 10 M solution of benzoic acid is 20% ionized. Thus this eluent has a hydrogen ion and a benzoate ion concentration each of 2.50 x 10 M. Comparison of this benzoic acid solution with an eluent containing 2.50 x 10 potassium benzoate shows that the two eluents give very similar retention times for inorganic anions (see Table 6.7). 

Table 6.7. Comparison of anion retention times with benzoic acid and potassium benzoate eluents that have the same benzoate ion concentrations. (From Ref. 113]).

Thus the concentration of anion A in an elution peak reduces the benzoate concentration by an equivalent amount. However, the eluent equilibrium is dynamic, and it is always 20 % ionized. This means that 80 % of the hydrogen and benzoate ions in the exchange reaction come from molecular benzoic acid and are converted into highly ionized H A. This effect enhances the detection sensitivity considerably because the counterion of the A has an unusually high equivalent conductance. A discussion of the phenomenon is given in Section 6.3.3.6. 

Anions of strong acids may be separated in acidic solution (as in Fig. 6.10) or at a basic pH. Weak acid anions require a basic solution to exist in the anionic form. Separation of borate, silicate, sulfide, cyanide as well as the anions of two stronger acids is shown in Fig. 6.11. An alkaline solution of sodium benzoate was used as the eluent. Carbonate may also be separated under alkaline conditions. Separation of these anions by suppressed IC is usually not attempted because they arc converted to the non-conducting molecular form by the acidic suppressor. 

Figure 6.11. Separation of weak acid anions. Conditions Alltcch Anion/R column, sodium hydroxide/ sodium benzoate eluent, 1.5 mL/min, conductivity detection. Hydroxide is the primary eluent for this separation and the weak acid anions are detected with indirect conductivity. A small amount of benzoate is added to speed the elution of the anions (Courtesy Alltcch).

D. T. Gjerde and J. S. Fritz, Behavior of various benzoate eluents for ion chromatography. Anal. 

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