0000000000222074
AUTHOR
R. M. Villanueva Camañas
The catalytic-thermometric determination of molybdenum in steels based on the hydrogen peroxide-iodide indicator reaction
Abstract The Mo VI -catalysed hydrogen peroxide-iodide reaction has been optimised for thermometry in an HCl medium using the initial slope method. The substitution of sodium perborate by H 2 O 2 and the conversion of the reaction to a process showing a Landolt effect are considered and discussed. The linear range extended from 0.2 μg Mo VI ml −1 to 1.2 μg Mo VI ml −1 with a detection limit of 0.06 μg Mo VI ml −1 . Interferences were also studied. The optimised procedure was applied to the evaluation of molybdenum in steels after extraction with α-benzoinoxime in chloroform.
Spectrophotometric determination of mercury(II) and silver(I) with copper(II) and diethyldithiocarbamate in the presence of triton X-100
Abstract Procedures for the determination of mercury and silver by displacement of diethyldithiocarbamate (DDTC) from its copper complex in the presence of 1% Triton X-100, and measurement of the decrease in the Cu(DDTC) 2 absorbance, are described. The use of the surfactant avoids the need for an extraction step. Reproducibility within 1% and detection limits of 0.25 ppm Hg(II) and 0.45 ppm Ag(I) have been obtained, and linear calibration ranges up to 13 ppm Hg(II) and 15 ppm Ag(I). In the presence of 0.1 M EDTA very good selectivity is achieved.
Spectrophotometric Determination of Cystine with O-Phthalaldehyde in the Absence of Thiol
Abstract A spectrophotometric method for the determination of cystine with o-phthalaldehyde (OPA) in the absence of thiol is described. When cystine is heated at 60[ddot]C for 30 min in a low excess of OPA (pH 9.5), a very stable derivative with 1:2 stoichiometry (cystine:OPA) and an absorption maximum at 335 nm (E = 4600) is formed. At pH < 1 the derivative is protonated (protonation constants: log K1 = 5.88 and log K2 = 3.70 at I = 0.1 and 20[ddot]C) and another absorption band at 440 nm (E = 3800) appears, which allows the determination of cystine in the presence of other amino acids.
Use of the o-Phthalaldehyde and N-Acetyl-L-Cysteine the Evaluation of Milk Proteins
Abstract o -Phthalaldehyde and N-acetyl-Lcysteine are used in the determination of milk proteins. Three procedures are proposed and compared. One of them is based on reaction of o -phthalaldehyde and N-acetyl-Lcysteine with the intact proteins and the two others on reaction of the reagents with the released amino acids after total acid hydrolysis of the protein samples. When the protein sample is hydrolyzed, calibration is performed either with a hydrolyzed protein standard or with isoleucine. A procedure for the measurement of the degree of enzymatic hydrolysis of milk proteins without separation of the unhydrolyzed protein, which makes use of the same reagents, is also described. In all c…
Some observations on the determination of sulfite by reduction to sulfide with sodium borohydride
Abstract Sodium borohydride is studied as an agent for reducing sulfite to sulfide. Rapid reductions are attained with greater than 97% recovery in the range 700–1200 μg of S/50 ml. Sulfate is not reduced.
Spectrophotometric determination of cystine by formation of an o-phthalaldehyde/N-acetyl-l-cysteine derivative
Abstract Cystine reacts with o -phthalaldehyde (OPA) in the absence and presence of a thiol compound to yield different compounds. The use of N -acetyl- l -cysteine as thiol leads to the formation of two derivatives, likely simple and double isoindoles, where the disulfide bond remains unbroken. In contrast, mercaptoethanol gives rise to the reduction of the amino acid to form a cysteine derivative. Obtaining cystine isoindoles makes it possible to spectrophotometrically determine the amino acid after Chromatographic separation and is further evidence of the large stabilization effect produced by N -acetyl- l -cysteine in the formation of OPA-thiol derivatives.
Quantitative retention—structure and retention—activity relationship studies of ionic and non-ionic catecholamines by micellar liquid chromatography
When ionic surfactants are used as mobile phases in micellar liquid chromatography, MLC, the retention of compounds is governed by hydrophobic and electrostatic forces. In the absence of electrostatic effects, the hydrophobicity of a compound is the predominant factor affecting its retention and its interaction with micelles. Because both interactions should be considered for ionic compounds, a novel retention model is proposed which includes the hydrophobicity of a compound and the molar fraction of its charged form. High correlations between the logarithm of the capacity factors and structural parameters were obtained for ionic compounds with different degrees of ionization. The effect of…
Determination of total free amino acids with o-phthalaldehyde and N-acetyl-l-cysteine
Abstract A spectrophotometric procedure is proposed for the determination of total free amino acids after reaction with o-phthalaldehyde and N-acetyl- l -cysteine, using isoleucine as the reference. The procedure was applied to the analysis of five samples of widely different composition. Recoveries were 98–105%.
Evaluation of the proteolysis degree with the o-phthalaldehyde/N-acetyl-L-cysteine reagent
The o-phthalaldehyde/N-acetyl-L-cysteine (OPA-NAC) reagent is applied to the spectrophotometric evaluation of the proteolytic activity of enzymes. The high stability of the OPA-NAC isoindoles makes a strict control of the time of reaction unnecessary. A mathematical expression is proposed to calculate proteolysis degrees, where the absorbance decrease of the OPA-NAC derivative of the protein itself during the hydrolysis process is taken into account. The method is applied to bovine serum albumine, caseine, lysozyme, lactoglobuline and protamine sulphate as substrates, and pronase, papaine, trypsin and chymotrypsin as enzymes.
A kinetic-catalytic method for iodide based on the simultaneous addition of inhibitor and substrate
Abstract A kinetic-catalytic method of analysis, based on the addition of a mixture of inhibitor and substrate, at a constant rate, over the catalyst in the presence of the other component of the indicator reaction is studied. The method is applied to the determination of iodide using the Ce(IV)As(III) catalytic reaction and Hg(II) as inhibitor. Under suitable experimental conditions the absorbance of the unconsumed Ce(IV) is inversely proportional to the initial iodide concentration.
Thermal Lens Spectrometric Detection of Catecholamines after Oxidation to Aminochromes
Abstract Experimental conditions for the spectrophotometric and thermal lens spectrometry (TLS) detection of catecholamines after oxidation to aminochromes with hexacyanoferrate (III) are optimized. At the low concentrations used in TLS, and in a 0.07 M citrate buffer, catecholamine oxidation can be performed at pH 7 and is immediate, whereas a lower pH value is required in spectrophotometry to avoid aminochrome polymerisation, the oxidation reactions being much slower. Similar TLS sensitivities are obtained for all catecholamines which facilitates HPLC evaluation. Sensitivity can be enhanced using a 50% ethanol-water medium. The linear dynamic range extends over two orders of magnitude, th…
Micellar Liquid Chromatography: A Worthy Technique for the Determination of β-Antagonists in Urine Samples
Several beta-antagonists (acebutolol, atenolol, celiprolol, labetalol, metoprolol, nadolol, propranolol) were determined in urine samples with fluorometric detection after direct injection, in less than 15 min, with a micellar mobile phase of 0.1 M sodium dodecyl sulfate (SDS), 15% propanol, and 1% triethylamine at pH 3. The limits of detection (38 criterion) were usually between 3 and 30 ng/mL. The addition of propanol and triethylamine and the reduction of the pH of the mobile phase improved the efficiency of the chromatographic peaks that was rather low in pure micellar eluents. The selection of the composition of the mobile phase was easily performed through the use of an interpretive p…
Determination of catecholamines as aminochromes by micellar liquid chromatography with thermal lens spectrophotometric detection
The determination of catecholamines (CAs) using micellar liquid chromatography with thermal lens spectrophotometric detection has been studied. CAs are oxidized with hexacyanoferrate(III) to aminochromes which are separated with a mobile phase of 0.05 M sodium dodecyl sulphate, 7% propanol and 0.03 M citrate buffer, pH 4.8, on a partially endcapped C18 column. The aminochrome-micelles and aminochrome-stationary phase association constants are evaluated. Using the 488 nm line of an Ar+ laser with 250 mW pump power the limits of detection are about 4 ng mL−1. The technique is applied to the determination of unconjugated CAs in urine using isoproterenol as internal standard.
On the preparation of simple and universal buffers including polynuclear species
A simple generalized procedure for the calculation of electrolyte concentrations in pH-buffers is proposed. Mixtures of acid-base systems and formation of polynuclear species at high ionic strengths are considered, and a diagram useful for the study and preparation of the buffers is shown.
Determination of phenoxy acid herbicides in drinking waters by HPLC and solid phase extraction
Abstract An HPLC procedure for determining phenoxy acid herbicides in waters is described. A LichroSpher RP select B octadecyl-silane analytical column and spectrophotometric detection at 230 nm were used. Adequate retention was achieved with a mobile phase containing MeOH/phosphate buffer 10−2 M pH 2.5/PnOH (50/42/8, v/v). The herbicides were isolated from water samples by using a single solid phase extraction procedure with C18 solid-phase columns. An enrichment factor of 500 is achieved. The coefficients of variation of the method were generally lower than 2.7% at 0.4 μg L−1 herbicide concentration levels. Recoveries ranged between 93 and 118%. The results obtained indicate that the prop…
Determination of catecholamines in urine by micellar liquid chromatography with coulometric detection
The determination of catecholamines by HPLC with a sodium dodecyl-sulphate (SDS), micellar mobile phase on a C18 column and with coulometric detection was studied. The eluate was conditioned at +0.25 and +0.00 V, and the current at −0.16V was recorded. A previously developed model which describes the chromatographic behaviour of solutes in HPLC with hybrid, micellar mobile phases was used to optimize the SDS and ethanol concentrations. A mobile phase of 0.15M SDS in a phosphate buffer of pH 3.4 and without ethanol is recommended. The limits of detection were 0.4–0.7 ng ml−1. The procedure was applied to the determination of unconjugated L-dopa, norepinephrine and dopamine in urine. Direct i…