M. C. Valencia

Analysis of parabens in cosmetics by low pressure liquid chromatography with monolithic column and chemiluminescent detection

This paper presents an application of chromatographic separation based on an ultra-short monolithic column and chemiluminescent detection in an FIA type instrument manifold for the determination of four paraben mixtures: methylparaben (MP), ethylparaben (EP), propylparaben (PP) and butylparaben (BP). The separation is achieved in 150 s using two consecutive carriers: first 12% ACN:water that changes 75 s after injection to 27% ACN:water. The detection is based on the oxidation of the hydrolysis product of parabens, p-hydroxybenzoic acid, with Ce(IV) in the presence of Rhodamine 6G which evokes chemiluminescence of sufficient intensity to enable a sensitive determination of these species. After optimization of the variables involved, the analytical method is characterized, displaying the following values for concentration ranges, detection limits and precision, as relative standard deviation at low concentration (0.15 mg l(-1))-MP: from 9.9x10(-7) to 3.3x10(-4)M; 1.9x10(-8); 5.6%; EP: from 9.0x10(-7) to 3.3x10(-4)M; 2.8x10(-8); 3.5%; PP: from 8.3x10(-7) to 9.9x10(-5)M; 2.3x10(-8); 4.2%; and BP: from 7.7x10(-7) to 9.9x10(-5)M; 4.2x10(-8)M; 6.2%. The method was applied and validated satisfactorily for the determination of these parabens in cosmetic samples, comparing the results against a liquid chromatography reference method.

Flow injection analysis of the insecticide imidacloprid in water samples with photochemically induced fluorescence detection

A flow injection analysis (FIA) system, combined with photochemical induced fluorescence (PIF) detection is developed for the sensitive and rapid determination of imidacloprid. It is based on the conversion of imidacloprid into the fluorophore 1-(6-chloro-3-pyridyl-methyl)-2-(hydroxyimino)-3,4-didehydroimidalozolidene. In an aqueous medium, this compound shows native fluorescence with an excitation maximum at 334 nm and an emission maximum at 377 nm. The linear concentration range of application was 1.0–60.0 ng ml −1 of imidacloprid, with a relative standard deviation of 2.1% (for a level of 10 ng ml −1 ) and a detection limit of 0.3 ng ml −1 . The method was applied to check whether imidacloprid was present above this limit in waters from Cuenca and Granada (Spain). It was validated applying a recovery test (Student’s t-test). Recovery levels of the method reached around 100% in all cases.

Flow-injection method for the determination of tin in fruit juices using solid-phase spectrophotometry

An integrated solid-phase spectrophotometric-flow-injection method is proposed for the determination of tin in fruit juices. The complex formed between Sn(IV) and Pyrocatechol Violet in the flow system is concentrated on Sephadex QAE A-25 gel packed in a flow cell. The analytes are monitored by UV-visible spectrophotometry at 576 nm. The method shows a detection limit of 0.3 ng ml−1 and a linear range of 2–40 ng ml−1.

Monoparameter sensors for the determination of the antioxidants butylated hydroxyanisole and n-propyl gallate in foods and cosmetics by flow injection spectrophotometry

A flow-through optosensor with solid phase UV spectroscopic detection is proposed for the direct determination of single antioxidants, namely butylated hydroxyanisole (BHA) and n-propyl gallate (n-PG), without previous derivatization. The methods are based on the transient retention behaviour of these compounds in a flow-through cell packed with C-18 silica using ethanol-water mixtures as a carrier, and on the intrinsic absorbance monitored at 290 and 283 nm, respectively. After recording the analytical signal, the antioxidants were easily and quickly desorbed from the solid support by the same carrier. For BHA, calibration graphs were linear over the range 1.0-300.0 mg L-1 using area as the analytical parameter. The relative standard deviation (RSD) was between 0.5 and 1.6%. For n-PG, calibration graphs were linear over the range 1.0-300.0 mg L-1 in area and the RSD was between 1.4 and 1.5%. The methods were applied to the determination of these antioxidants in several food and cosmetics samples, and were validated using the standard additions method and an HPLC reference method.

Determination of trace amounts of beryllium in water by solid-phase spectrophotometry

A method for the determination of trace amounts of beryllium has been developed, based on solid-phase spectrophotometry. Beryllium reacts with Eriochrome Cyanine R in the presence of ethylenediaminetetraacetic acid to give a highly absorbent complex, which is fixed on a dextran-type anion-exchange gel. The absorbance of the gel, at 580 and 750 nm, packed in a 1 mm cell, is measured directly. The calibration graph is linear over the concentration range 0.1–1.2 µg l–1 and the relative standard deviation is 2.3%. The detection limit of the 1000 ml sample method is 6 ng l–1 for 0.030 g of anion-exchange gel. The sensitivity can be enhanced by increasing the sample volume. The method has been applied to the determination of beryllium in tap and natural waters.

Simultaneous determination of aluminium and beryllium at the subnanogram per millilitre level by solid-phase derivative spectrophotometry

Abstract The applicability of derivative solid-phase spectrophotometry is demonstrated for the resolution of mixtures of aluminium and beryllium with closely overlapping absorption profiles. A spectrophotometric method for the simultaneous determination of beryllium and aluminium based on first-derivative solid-phase spectrophotometry is proposed. Both metal ions were fixed on a dextran-type anion-exchanger gel at pH 4.4, as coloured complexes with Eriochrome Cyanine R in the presence of ethylenediaminetetra-acetic acid. The absorption spectrum of the gel, packed in a 1 mm cell, was recorded directly. The application range is up to 60 ng ml −1 for aluminium and up to 4.0 ng ml −1 for beryllium, and the RSD is 3.5% and 3.8%, respectively. The detection limit is 0.04 ng ml −1 for beryllium and 2.25 ng ml −1 for aluminium. The relative error obtained in the analysis of a synthetic aqueous solution is 1.50% for beryllium and 1.75% for aluminium. The method was applied to the determination of both analytes in natural water, orange juice, geological and botanical samples.

Improved Multianalyte Determination of the Intense Sweeteners Aspartame and Acesulfame-K with a Solid Sensing Zone Implemented in an FIA Scheme

Abstract A multianalyte flow‐through sensor is proposed for the simultaneous determination of aspartame (AS) and acesulfame‐K (AK) in tabletop sweeteners. The procedure is based on the transient retention of AK in the ion exchanger Sephadex DEAE A‐25 placed in the flow‐through cell of a monochannel flow injection analysis (FIA) set‐up using pH 2.70 ortophosphoric acid/sodium dihydrogen phosphate buffer 0.06 M as carrier. In these conditions AS is very weakly retained, which makes it possible to measure the intrinsic ultraviolet (UV) absorbance of first AS and then AK after desorption by the carrier itself. The applicable concentration range, the detection limit, and the relative standard deviation were the following: for AS, from 10 to 100 µg mL−1; 5.65 µg mL−1; 3.4% (at 50 µg mL−1); and for AK, between 40 and 100 µg mL−1; 11.9 µg mL−1 and 1.61% (at 50 µg mL−1). The method was applied and validated satisfactorily for the determination of AS and AK blends in tabletop sweeteners. The results were compared against an HPLC reference method.

Flow-through spectrophotometric sensor for the determination of saccharin in low-calorie products

A simple, rapid and inexpensive monoparameter flow-through sensor has been developed for the determination of saccharin in low calorie and dietary products. The method is based on the transient adsorption of the sweetener on Sephadex G-25 solid phase packed to a height of 20 mm in the flow cell. The optimal transient retention of the synthetic sweetener, in terms of sensitivity and sampling frequency, was obtained when pH 2.75 citric acid-sodium citrate buffer 5 × 10−3 M was used as a carrier at a flow-rate of 1.5 ml min−1. Saccharin was determined measuring its intrinsic absorbance at 217 nm at its residence time. Calibration graphs for peak height and peak area were linear over the range 5.0–200.0 μg ml−1, RSD 1.18%, and 1.0–200.0 μg ml−1, RSD 0.78%, respectively. Saccharin was determined in several food samples measuring height or area peak, obtaining recoveries ranging between 98–104 and 99–102% for height and area peak, respectively. The procedure was validated for use in the determination of saccharin in low calorie and dietary products giving reproducible and accurate results.

Flow-through sensor for determination of butylated hydroxytoluene in cosmetics

ABSTRACT A flow-through optosensor with solid phase UV spectroscopic detection is proposed for the direct determination of the antioxidant Butilated Hydroxytoluene (BHT) without previous derivatization. The method is based on the transient retention of this compound in a flow-through cell packed with C18 silica using ethanol : water mixture as a carrier, and its intrinsic absorbance monitored at 274 nm. Afterwards, the recording of the analytical signal BHT was easily and quickly desorbed from the solid support by the same carrier. The calibration graphs were linear over the range 2.0–300.0 mg L−1 using area as the analytical parameter. The relative standard deviation (RSD) was between 0.5 and 1.6%. The method was applied to the determination of this antioxidant in several cosmetics samples and was validated using the standard addition methodology and an HPLC reference method.

Speciation of selenium (IV) in natural waters by solid phase spectrophotometry.

A method for the speciation of selenium (IV) based on solid-phase spectrophotometry (SPS), has been developed. In acidic conditions selenium (IV) oxidizes potassium iodide and the I(3)(-) forms an ionic association with Rhodamine B (RB) which is fixed on a dextran type lipophilic gel. The gel phase absorbances at 590 and 800 nm are measured directly, and allows for the determination of selenium (IV) in the range of 0.7-18.0 microg l(-1), with a relative standard deviation (RSD) of 2.8%. The method has been applied to the determination of Se(IV) in natural waters.