Acidic analyte solvent

A sample contains a weak acid analyte, HA, and a weak acid interferent, HB. The acid dissociation constants and partition coefficients for the weak acids are as follows Ra.HA = 1.0 X 10 Ra HB = 1.0 X f0 , RpjHA D,HB 500. (a) Calculate the extraction efficiency for HA and HB when 50.0 mF of sampk buffered to a pH of 7.0, is extracted with 50.0 mF of the organic solvent, (b) Which phase is enriched in the analyte (c) What are the recoveries for the analyte and interferent in this phase (d) What is the separation factor (e) A quantitative analysis is conducted on the contents of the phase enriched in analyte. What is the expected relative erroi if the selectivity coefficient, Rha.hb> is 0.500 and the initial ratio ofHB/HA was lO.O ... 

The analytical method described is also used in following the consumption of peroxybenzoic acid or other peroxy acids during an oxidation reaction it has also been used in determining the conversion of other carboxylic acids to peroxy acids when solvent extraction has been used in the isolation. 

Figure 1.25 illustrates the principle underlying LLE in the solid-supported LLE format. In order to facilitate elution with a water-immiscible organic solvent, it is imperative that analytes are in their neutral form during sample load. Thus, for basic analytes, loading should be done in a high pH (9 to 10) buffer and for acidic analytes, a low pH (2 to 3) buffer. 

RP-HPLC has been employed for the determination of flavonoids and other phenolic compounds in cranberry juice. The neutral and acidic analytes were preconcentrated octadecyl silica SPE cartridges conditioned with distilled water (neutral analytes) or with 0.01 M HC1 (acidic compounds). Hydrolysis of samples was carried out in aqueous methanol solution acidified with 6 M HC1 at 35°C for 16h. Chromatographic separation was performed in an ODS column (150 X 4.6mm i.d. particle size 5/.an). Solvents A and B were water-acetic acid (97 3, v/v) and methanol, respectively. The gradient started with 0 per cent B (flow rate, 0.9 ml/min), reached 10 per cent B in lQmin (flowrate, 1.0 ml/min) and increased to 70 per cent B in 40min (flowrate, 1.0 ml/min). Analytes were detected at 280 and 360 nm. Some typical chromatograms are presented in Fig. 2.71. The concentrations of flavonoids and phenolic acids are compiled in Table 2.69. It was stated that the SPE-HPLC procedure makes possible the simultaneous determination of phenolic compounds and flavonoids, therefore, it can be employed for the measurement of these classes of analytes in other fruit juices [188],... 

Semiaqueous or Nonaqueous Solutions. Although the measurement of pH in mixed solvents (e.g., water/organic solvent) is not recommended, for a solution containing more than 5% water, the classical definition of a pH measurement may still apply. In nonaqueous solution, only relative pH values can be obtained. Measurements taken in nonaqueous or partly aqueous solutions require the electrode to be frequently rehydrated (i.e soaked in water or an acidic buffer). Between measurements and after use with a nonaqueous solvent (which is immiscible with water), the electrode should first be rinsed with a solvent, which is miscible with water as well as the analyte solvent, then rinsed with water. Another potential problem with this type of medium is the risk of precipitation of the KC1 electrolyte in the junction between the reference electrode and the measuring solution. To minimize this problem, the reference electrolyte and the sample solution should be matched for mobility and solubility. For example, LiCl in ethanol or LiCl in acetic acid are often used as the reference electrode electrolyte for nonaqueous measurements. 

Pederson described a specific HPLC method for the determination of dipyridamole in serum [74]. The HPLC system used was a Waters model 600 liquid chromatograph equipped with a U6K injector, a pBondapak Ci8 column (30 cm x 39 mm) (10 pm), and a model 440 dual channel filter absorbance detector in conjunction with a Tarkan W + W 600 recorder. The mobile phase was a 75 25 mixture of methanol and a 0.02 M solution of sodium acetate (adjusted to pH 4 with acetic acid). The solvent flow rate of 2 mL/min was produced by an applied pressure of approximately 2000 p.s.i. Detection of the analyte was made at the UV absorption maximum of 280 nm. 

Variations in the selectivity are sometimes observed with the change in the type of organic modifier due to the specifics of the analyte-solvent interactions (solvation) and the specific adsorption behavior of the organic modifier. In the following example the effect of type and concentration of methanol and acetonitrile modifiers on the retention of acidic, basic, and neutral analytes is discussed. 

In Figure 4-36 the acidic analyte wp a and IpKa values determined as a function of acetonitrile composition from 10 to 35 v/v% MeCN are shown. It is shown that even after correcting for the pH shift of the mobile phase upon addition of organic, the chromatographic IpA a values does not correlate to the pKa that was determined by titration in aqueous solvents, ZpK. An increase of 0.27 pKa units per 10% v/v MeCN for 2,4-dihydroxybenzoic acid was determined. A similar slope for other mono- and disubstituted aromatic benzoic... 

Figure 1.7 HPLC chromatogram of organic acids analysis of Prosecco grape must sample. 1. tartaric acid, 2. malic acid, 3. citric acid, 4. shikimic acid. Analytical conditions Lichrospher 100 RP-18 (250 x4mm, 5p,m) column (Merck, Darmstadt, Germany) at room temperature, detection at wavelength 210 nm, sample volume injected 20/xL solvent H3P04 5 x 10 3 M with isocratic elution at flow rate 0.6 mL/min...

Figure 1.11 Analysis of an organic acids standard solution. 1. citric acid, 2. tartaric acid, 3. malic acid, 4. succinic acid, 5. lactic acid, 6. acetic acid. Analytical conditions column Aminex HPX-87H (300 x 7.8mm, 9(jim) at 65 °C (Bio-Rad Laboratories, Richmond, CA) detection at wavelength 210 nm sample volume injected 10p,L solvent H2S04 0.026 N with isocratic elution at flow rate 0.8mL/min...

Cofre, P., and Sawyer, D.T. 1986. Electrochemical reduction of dioxygen to perhydroxyl (HOj ) in aprotic solvents that contain Bronsted acids. Analytical Chemistry 58, 1057-1062. 

As the selectivity properties are not always the only ones to be considered in practice, other solvents may also be used. For ecological reasons, halogenated ones should be only used if really necessary. For the preparation of the various solvent mixtures, which should be of similar strength. Figure 9.4 is a help. If the sample includes acidic analytes, it may be necessary to add a small amount of acetic, trifluoroacetic or formic acid to the eluent for basic analytes a possible additive is triethylamine. 

The quantification of the metabolites of CYPs 2D6, 3A4, and 2C19 reactions is carried out using a SCIEX API 3000 mass spectrometer, running in the positive ion, selected reaction monitoring (SRM) mode. The HPLC gradient system used consists of two solvent mixtures. Solvent mixture A (SMA) consists of 94.9% H20, 5% MeOH, and 0.1% formic acid and solvent mixture B consists of 94.9% MeOH, 5% H20, and 0.1% formic acid. The analytical column used is a Develosil Combi-RP5, 5 pm, 3.5x20mm (Phenomenex, Inc., Torrance, CA), with a mobile phase flow rate of 1.1 mL/min. A short run time (1 min) HPLC method is used because of the specificity of the mass spectrometer and the lack of matrix effects (this was thoroughly... 

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