Determination of acetylsalicylic acid and its major metabolites in bovine urine using ultra performance liquid chromatography

A new method based on ultra high performance liquid chromatography (UPLC) with photometric and fluorometric detection for the determination of acetylsalicylic acid and its main metabolites, namely gentisic, salicylic and salicyluric acids, in bovine urine samples is reported. Photometric detection was used for acetylsalicylic acid determination, whereas the native fluorescence of the metabolites was monitored using fluorometric detection. The separation was performed under isocratic conditions, using acetonitrile-phosphate solution (3.5mM, pH 3.5) (26:74, v/v) as the mobile phase. The retention times of the four compounds were lower than 2min, which are shorter than those achieved using conventional HPLC. Under the optimum separation conditions, the dynamic ranges and detection limits (ngmL(-1)) were: 0.2-2500, 0.09 for gentisic acid; 0.2-2500, 0.08 for salicylic acid and 2.5-15,000, 1.1 for salicyluric acid, using fluorescence detection, and 10-25,000, 2.2 for acetylsalicylic acid, using UV detection. Intra-day and inter-day precision data were assessed at two levels of concentration of each analyte using both detection systems. The selectivity of the method was checked by assaying different drugs of veterinary use showing that most of them did not interfere with the determination of the analytes. The method has been applied to the analysis of bovine urine samples, which only required a simple clean up step of the samples prior to injection in the UPLC system. A recovery study was performed, which provided values in the range of 80-100%. This fact proves the practical usefulness of this method as an ultrafast analytical tool for the therapeutic control of acetylsalicylic acid administration in bovine animals intended for food production.

Fluorometric determination of alkaline phosphatase activity in food using magnetoliposomes as on-flow microcontainer devices

Liposomes containing magnetic gold nanoparticles (AuNPs) and an enzymatic substrate (4-methylumbelliferyl-phosphate) have been used as on-flow microcontainers for reagent preconcentration in a flow injection method for the determination of alkaline phosphatase (ALP) activity. The dynamic range of the calibration graph was 6.4 × 10(-3)-0.25 U L(-1) ALP, and the detection limit was 1.9 × 10(-3) U L(-1). The precision, expressed as relative standard deviation (RSD%), was in the range of 0.7-2.4%. The overall method showed a sampling frequency of 10 h(-1). The method was applied to the determination of ALP in milk samples with recovery values ranging between 87.5 and 104.6%. The residual ALP activity in milk samples subjected to temperature treatments was also determined. The results obtained in the analysis of all milk samples were compared with those obtained by applying a previously described flow injection method.

Rapid chromatographic determination of caseins in milk with photometric and fluorimetric detection using a hydrophobic monolithic column

Reverse-phase liquid chromatographic methods using a hydrophobic C18 monolithic column and on-line photometric and fluorimetric detection for the determination of the major casein (CN) proteins in milk are presented. The separation of αs1-CN, αs2-CN, β-CN and κ-CN was achieved in only five minutes. Fluorimetric detection enabled better analytical results than photometric detection. Thus, the dynamic ranges of the calibration graphs and detection limits obtained using fluorimetric detection were (mgmL(-)(1)): αs1-CN (0.74-10.0, 0.22), αs2-CN (0.15-10.0, 0.045), β-CN (0.68-10.0, 0.20) and κ-CN (0.21-10.0, 0.06). The analytical features of the photometric method, which does not allow the quantification of β-casein, were (mgmL(-)(1)): αs1-CN (1.5-9.0, 0.45), αs2-CN (1.4-10.0, 0.43) and κ-CN (0.4-9.0, 0.12). Precision data, expressed as relative standard deviation, ranged between 0.6% and 5.3% for the fluorimetric method and between 2.4% and 6.2% for the photometric method. Both methods were applied to the analysis of three different milk samples, obtaining recoveries in the ranges of 86.6-103.2% and 92.0-106.5% using fluorimetric and photometric detection, respectively.

Application of Tb(4)O(7) nanoparticles for lasalocid and salicylate determination in food analysis

The usefulness of Tb(4)O(7) nanoparticles (NPs) as analytical reagents using sensitized luminescence as a detection system is described for the first time, and the results obtained are compared with those obtained using Tb(III) ions. Two drugs used in veterinary practice, namely, lasalocid (LAS) and salicylate (SAL), have been chosen as model analytes to carry out this study. The experimental conditions for these systems have been optimized, and their analytical features were obtained. The detection limits obtained for LAS and SAL using Tb(4)O(7) NPs were 1.0 and 4.0 ng mL(-1), respectively, which were comparable to those obtained using Tb(III) ions: 1.8 and 1.0 ng mL(-1), respectively. However, precision data, with relative standard deviation values in the range 2.3-3.8% using the NPs and 3.5-6.5% using Tb(III) ions, were slightly better for LAS with Tb(4)O(7) NPs. The practical analytical usefulness of Tb(4)O(7) NPs as luminescent reagents has been shown by performing the determination of LAS in tap water, feed premix, and egg samples, obtaining recoveries in the range of 80.0-105.0%.

Long-wavelength fluorimetric determination of food antioxidant capacity using Nile blue as reagent

A method for the determination of the antioxidant capacity using long-wavelength fluorescence measurements is described for the first time. This method is a modification of the conventional oxygen radical absorbance capacity (ORAC) method that uses fluorescein or phycoerythrin and the generator of peroxyl radicals, 2,2'-azo-bis-(2-methylpropionamidine) dihydrochloride (AAPH). The long-wavelength fluorophor nile blue is proposed as an analytical reagent alternative to these conventional fluorophores. Kinetic curves have been obtained by monitoring the fluorescence variation (λex, 620; λem, 680 nm) with time, using the 96-well microplate format. The vitamin E analogue 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylic acid (Trolox) has been chosen as the model analyte, and the normalized area under the decay curve has been used as the analytical parameter. The dynamic range of the calibration curve is 0.8-8.0 μM, and the detection limit is 0.45 μM. The precision of the method, expressed as relative standard deviation and assayed using 1 and 5 μM Trolox concentrations, was 5.6 and 2.9%, respectively. The method has been applied to the analysis of fruit juices and wines, obtaining results that did not differ significantly from those provided using the ORAC method with fluorescein as reagent.

Luminescent determination of flavonoids in orange juices by LC with post-column derivatization with aluminum and terbium

A new post-column derivatization system is described and applied to the determination of flavonoids in citric beverages after their separation by LC using a monolithic column. The derivatization involves the formation of the chelates of the analytes with aluminum (III) and terbium (III) in the presence of the surfactant SDS and the measurement of the terbium sensitized luminescence at lambda(ex) 360 and lambda(em) 545 nm. Naringin, hesperidin, quercetin, naringenin, and kaempferol have been chosen as analyte models. The large Stokes shift and the relatively long wavelength emission of terbium(III) can minimize interferences from background sample matrix, which usually emit at shorter wavelengths. Calibration graphs were constructed in the intervals 6.0-1700 ng/mL naringin, 9.8-1700 ng/mL hesperidin, 2.1-2000 ng/mL quercetin, 5.2-1500 ng/mL naringenin and 2.5-2000 ng/mL kaempferol, with regression coefficients higher than 0.9935 in all instances. The precision of the method, expressed as RSD%, was established at two concentration levels, with values of 1.3 and 4.7%, which corresponded to the minimal and maximal error zones of the calibration graphs. The practical usefulness of the method is demonstrated by the analysis of orange juices, which were diluted and directly injected into the chromatographic system, obtaining recoveries between 86.9 and 108.2%.

High throughput bioassays using nanoparticles

An overview of the usefulness of different nanoparticles to improve the features of high throughput separation and individual and multiplexed detection bioassays is presented. Although the development of microarray and microfluidic systems has expanded the capabilities of these high throughput assays, the combined use of NPs and these devices has provided them with new applications in drug discovery, proteomic and genomic studies, and clinical diagnosis. This article reviews the wide application field of magnetic, gold, silver, semiconductor and other nanoparticles in high throughput bioassays. Also, the versatility of the detection systems described shows that NPs are useful alternatives to fluorescent dyes, which are often used in these assays.

Homogeneous immunoassay for soy protein determination in food samples using gold nanoparticles as labels and light scattering detection

A homogeneous aggregation immunoassay involving the use of gold nanoparticles (AuNPs) and light scattering detection is described for soy protein determination in food samples. AuNPs act as enhancers of the precipitate that appears when the antigen-antibody complex is formed. The AuNPs-antibody conjugate has been synthesized by physical adsorption of polyclonal anti-soy protein antibodies onto the surface of commercial AuNPs with a nominal diameter of 20nm. The direct assay is based on the reaction of the conjugate with soy protein, which reaches the equilibrium in about 10min, and the measurement of the light scattering intensity at 530nm, which is proportional to the analyte concentration. The dynamic range of the calibration graph is 0.2-20microgm L(-1) and the detection limit value is 65ngm L(-1). The precision, expressed as relative standard deviation, has been assayed at two different concentrations, 0.2 and 1microgm L(-1), giving values ranging from 4.7 to 5.9%. The interference of other proteins has been assayed. The usefulness of this method has been shown by its application to the analysis of fruit juice and "nonmilk yoghourt" samples. The results obtained with the proposed method are similar to those obtained by using a commercial ELISA kit, but the assay time is significantly shorter and the detection limit was about 10 times lower. A recovery study has been also performed, giving values in the range of 84.0-119.3%.

Nanostructures as analytical tools in bioassays

We critically evaluate the usefulness of different nanostructures described as labels, nanoscaffolds or separation media in immunoassays and nucleic-acid hybridization assays. Many of the great number of publications describe only theoretical aspects of using these nanostructures or nanoparticles, but do not verify their applicability in the presence of potential interferents that can be present in the sample matrix. We attempt a systematic study of the advantages and the limitations of using these new reagents in bioassays, the different assay formats for individual and multiplexed detection, and the capability of these assays in analyzing real samples.

Analytical innovations in the detection of phenolics in wines

A liquid chromatographic method with online photometric and luminescent detection for the determination of 18 phenolic compounds in wines is reported. Photometric detection is performed at four wavelengths, namely, 256, 280, 320, and 365 nm, using a diode array detection system. The luminescent detection is achieved by means of a postcolumn derivatization reaction of 10 of these compounds with terbium(III) in the presence of synergistic agents, such as ethylenediaminetetraacetic acid (EDTA) and n-octyltriphosphine oxide (TOPO). A micellar medium provided by the surfactants sodium dodecyl sulfate and Triton X-100 was used for the determination of the luminescent chelates at lambdaex 317 and lambdaem 545 nm. The long wavelength emission of lanthanide chelates can minimize interferences from background sample matrix, which usually emit at shorter wavelengths. The analytical features of the photometric and fluorometric methods, such as dynamic ranges of the calibration graphs, detection limits, and precision data, have been obtained. The practical usefulness of the developed methods is demonstrated by the analysis of Spanish and Italian wine samples (red, rosé, oloroso, and white), which were diluted and directly injected into the chromatographic system. The accuracy of both methods was checked by assaying a recovery study, which was performed at three different analyte levels for each type of sample.

Long-wavelength fluorescence polarization immunoassay: determination of amikacin on solid surface and gliadins in solution

The versatility of the fluorescence polarization immunoassay (FPIA) is increased by using two long-wavelength labels, Nile Blue and a ruthenium(II) chelate. The first label has been used to study the potential of FPIA on a solid surface using dry reagent technology. The aminoglycoside antibiotic amikacin has been used as an analyte model, and the method has been applied to the analysis of serum samples. The second label has been used to show the practical application of FPIA to the determination of macromolecules, using gliadins as an analyte model, which have been determined in gluten-free food. Very low amounts of anti-amikacin antibodies and amikacin-Nile Blue tracer were immobilized onto nitrocellulose membranes, for the development of the amikacin method, and the consumption of reagents is lower than in conventional FPIA. Only the addition of the standard or sample extract at an adequate pH is required at the analysis time. The analyte displaces the tracer from the tracer-antibody immunocomplex, obtaining a decrease in the fluorescence polarization proportional to the analyte concentration. The gliadin-Ru(II) chelate tracer shows a relatively long lifetime, which allows the observation of differences in fluorescence polarization values between the tracer-antibody complex and the tracer alone. The dynamic range of the calibration graphs for both analytes is 0.5-10 microg mL-1 and the detection limits are 0.1 and 0.09 microg mL-1 for amikacin and gliadins, respectively. The study of the precision gave values of relative standard deviations lower than 5 and 1.5% for the amikacin and gliadin methods, respectively. Amikacin was determined in human serum samples using a previous deproteinization step with acetonitrile, obtaining recovery values in the range 83.4-122.8%. The gliadin method was applied to the analysis of gluten-free food samples by using a previous extraction step. The recovery study gave values between 94.3 and 105.0%.

Determination of fluoroquinolones in milk samples by postcolumn derivatization liquid chromatography with luminescence detection

A liquid chromatography (LC) method with luminescence detection for the determination of eight quinolone antibiotics is reported. The system encompasses three consecutive steps: (a) chromatographic separation using reverse-phase mode (RP-LC), (b) postcolumn derivatization reaction, and (c) luminescence detection by monitoring fluorescence (FL) and time-resolved (TR) signals. The derivatization step is based on the reaction between quinolones and terbium(III) to form luminescent chelates, which were determined at lambda(ex) 340 and lambda(em) 545 nm (FL mode) or at lambda(ex) 281 and lambda(em) 545 nm (TR mode). Dynamic ranges of the calibration graphs, obtained with standard solutions of analytes and FL and TR modes, respectively, were 190-3500 and 316-2000 ng mL-1 for marbofloxacin, 8-3500 and 8.1-1500 ng mL-1 for ciprofloxacin, 6.2-3500 and 13-1500 ng mL-1 for danofloxacin, 7.4-3500 and 8.4-1500 ng mL-1 for enrofloxacin, 14-3500 and 20-2000 ng mL-1 for sarafloxacin, 12.5-3500 and 13.9-1200 ng mL-1 for difloxacin, 7.6-3500 and 13-3000 ng mL-1 for oxolinic acid, and 9-2000 and 130-3000 ng mL-1 for flumequine. Limit of detection values obtained using FL and TR modes, respectively, were 60 and 95 ng mL-1 for marbofloxacin, 2 and 2.4 ng mL-1 for ciprofloxacin, 1.9 and 3.9 ng mL-1 for danofloxacin, 2.2 and 2.5 ng mL-1 for enrofloxacin, 3.8 and 7 ng mL-1 for sarafloxacin, 4 and 4.2 ng mL-1 for difloxacin, 2.3 and 4 ng mL-1 for oxolinic acid, and 2.7 and 40 ng mL-1 for flumequine. The precision was established at two concentration levels of each analyte and expressed as the percentage of relative standard deviation with values ranging between 1.9 and 7.8%. The validation procedure for the analysis of samples was carried out using European Community recommendations, and the decision limit and detection capability were calculated for bovine whole milk. The method was applied to whole, semiskimmed, and skimmed milk samples spiked with the target analytes, and the recoveries ranged between 93.3 and 106.0%.

Determination of some hydroxybenzoic acids and catechins in white wine samples by liquid chromatography with luminescence detection

A liquid chromatographic method with luminescence detection for the determination of eight phenolic compounds is reported. The method involves postcolumn derivatization with terbium(III). This derivatization is based on the reaction between phenolics and terbium(III) to form luminescent chelates, which were determined at lamda ex 295 and lamda em 545 nm using the fluorescence mode. The long wavelength emission of lanthanide chelates can minimize interferences from background sample matrix, which usually emit at shorter wavelengths. Also, the chromatographic separation allows the individual determination of phenolics, which cannot be done using the direct measurement of the fluorescence of their corresponding terbium chelates. Dynamic ranges of the calibration graphs and detection limits, obtained with standard solutions of analytes were (microg/mL): gallic acid (0.9-40, 0.3), protocatechuic acid (0.05-7, 0.016), catechin (0.2-40, 0.07), vanillic acid (0.25-40, 0.08), p-hydroxybenzoic acid (0.8-40, 0.25), syringic acid (0.17-40, 0.05), epicatechin (0.3-40, 0.09) and salicylic acid (0.07-12, 0.02). The precision was established at two concentration levels of each analyte and expressed as the percentage of RSD with values ranging between 1.0 and 6.5%. The practical usefulness of the method was demonstrated by the analysis of white wine samples, which were diluted two-fold and directly injected into the chromatographic system. The recovery values obtained ranged between 93.3 and 108.0%.

Long-wavelength fluorimetry as an indirect detection system in immunoaffinity chromatography: application to environmental analysis

The potential of long-wavelength fluorimetry when used as the detection system in immunoaffinity chromatography is assessed for the first time by applying this approach to the analysis of water and sludge samples. Nile blue (NB) was used to synthesize a long-wavelength fluorescent tracer for linear alkylbenzenesulfonates (LASs) using the carbodiimide method, in which the amino group of NB is covalently coupled to the activated carboxylic acid group of a LAS mimic with N-hydroxysuccinimide and dicyclohexylcarbodiimide. The method consists of the injection of a pre-incubated mixture containing linear sodium 4-dodecylbenzenesulfonate (LDS; used as the LAS model), anti-LAS antibodies, and the long-wavelength tracer into a commercial Protein G column. Free and bound tracer fractions are separated in the column, and the peak height of the immunochromatogram (corresponding to the free tracer) is directly measured at 626 nm (lambda (ex)) and 674 nm (lambda (em)), and then correlated to the analyte concentration. It is not necessary to perform an elution step immediately after every sample application. The dynamic range of the method is 0.05-2.5 microg ml(-1) LDS, and the detection limit is 15 ng ml(-1). The precision, expressed as the relative standard deviation, is 4.8-6.4%. Other surfactants (sodium dodecylsulfate and Triton X-100) do not cause interference. The recoveries obtained by applying the method to the analysis of water (ground- and wastewater) and sludge (primary and activated) samples ranged from 86.0 to 111.3%. Water sample analysis included an initial solid-phase extraction step, which cleaned up the samples and improved the detection limit fivefold.

Chromatographic determination of flumequine in food samples by post-column derivatisation with terbium(III)

The potential usefulness of terbium(III) as reagent for the luminescent determination of flumequine residues in food samples has been studied using both fluorescence (FL) and time-resolved (TR) modes and both batch (B) and integrated liquid chromatography (LC)/derivatisation approaches. The system was optimised in each instance to establish the analytical features of the four methods. The dynamic ranges of the calibration graphs, obtained with standard solutions of flumequine, were (ng mL(-1)): B-FL 0.18-600; B-TR 2.4-150; LC-FL 3.7-1000 and LC-TR 52-3000. The detection limits were also obtained giving the following values (ng mL(-1)): B-FL 0.055; B-TR 0.7; LC-FL 1.1 and LC-TR 15. The precision, expressed as the percentage of relative standard deviation, was equal or lower than 5.1% in all instances. The LC methods, which avoid the interference of other quinolone antibiotics, were applied to the analysis of chicken muscle and liver, and whole milk samples. The sample pre-treatment only consisted of a deproteinisation step. The validation procedure for the analysis of samples was carried out using EC recommendations, and the decision limit and detection capability were calculated. The recoveries obtained ranged from 95.0% to 103.8%.

Usefulness of ytterbium(III) as analytical reagent for total sulfite determination in white wine samples

Ytterbium(III) is used as reagent for the determination of sulfite by measuring the formation of the Yb(III)-sulfite complex through the variation of the light scattering intensity with time. The low solubility of this complex causes an efficient dispersion of the radiation at 490 nm, which is measured at 980 nm. Each kinetic datum is automatically obtained in only 0.5 s by stopped-flow mixing technique. The application of the initial rate method using a long emission wavelength minimizes the potential interference of fluorescent background signals from the sample matrix. The dynamic range of the calibration graph is 1-250 microg/mL, and the calculated detection limit is 0.35 microg/mL. The precision, expressed as relative standard deviation, is <6%. The method has been applied to the determination of total sulfites in white wine samples, which requires only the sample dilution and the use of two aliquots to improve selectivity. However, the matrix effect found for red wines precludes the application of the method to the direct analysis of these samples. Analytical recoveries ranged from 96.0 to 106.7%. The results obtained with the proposed method agreed with those provided by the p-rosaniline method. Unlike this method, in which toxic reagents are required, the use of ytterbium(III) as analytical reagent shows the advantage of its low acute toxic rating.

Selective kinetic determination of amikacin in serum using long-wavelength fluorimetry

A simple and rapid method for the determination of the antibiotic amikacin, involving the use of a long-wavelength fluorophor, namely indocyanine green, (ICG) is presented. The dye is oxidised by cerium(IV) in acidic medium, resulting in a sharp decrease of the fluorescence, but this fluorescence quenching is inhibited in the presence of amikacin, which can be ascribed to the formation of an ion pair between the fluorophor and the analyte. The initial rate of the system is monitored at lambda(ex): 765 nm and lambda(em): 812 nm as excitation and emission wavelengths, respectively, using the stopped-flow mixing technique, which makes the method applicable to automatic routine analysis. Each measurement is obtained in only 2-3s. The method presents a detection limit of 0.02 microg m1(-1) in standard solutions, which corresponds to 2.5 microg ml(-1) in serum samples. The precision is in the range 4.8-6%. The good selectivity of the method allows amikacin to be determined in the presence of other antibiotics, including other aminoglycoside antibiotics, in serum. The recoveries obtained from the analysis of different samples were in the range 89.4-104.7%.

Stopped-flow fluorescence polarization immunoassay

A general survey of the analytical application of kinetic methodology in fluorescence polarization immunoassay (FPIA) is presented. Stopped-flow mixing technique (SF) allows the initial rate of the immunochemical reaction between the tracer and the antibody to be obtained, which is used as the analytical parameter instead of the equilibrium signal used in conventional FPIA. The instrumentation required is described and the features of the analytical methods proposed are compared with those obtained by conventional FPIA. The usefulness of SF-FPIA for routine screening in clinical, environmental and food analysis is discussed.

Determination of methyl anthranilate in food samples by coupling stopped-flow mixing technique and time-resolved luminescence detection

A sensitive and fast approach for the determination of methyl anthranilate in grape must and honey samples, using time-resolved luminescence measurements, has been reported for the first time. The method involves the alkaline hydrolysis of the ester to anthranilic acid and the formation of a chelate with terbium(III) and tri-n-octylphosphine oxide in presence of Triton X-100. Kinetic and equilibrium measurements were obtained in 0.1 and 15 s, respectively, by using a stopped-flow mixing technique. The dynamic ranges of the calibration graphs of the kinetic and equilibrium methods were 21.9 nmol/L-29.2 micromol/L and 19.7 nmol/L-21.9 micromol/L, respectively, and the detection limits were 7.3 and 6.6 nmol/L, respectively. The precision, expressed as relative standard deviation, was less than 3%. Although both-kinetic and equilibrium methods exhibited very similar analytical features, only the better selectivity of the former allowed the content of methyl anthranilate to be determined in the samples, as the initial rate measurements avoided the negative effect that the sample matrix caused in the equilibrium measurements. The analytical recoveries obtained by applying the kinetic method to the analysis of grape must and flower honey samples were in the range 92.5-105.0%.

Simultaneous stopped-flow determination of butylated hydroxyanisole and propyl gallate using a T-format luminescence spectrometer

A simple and fast luminescent method is used for the first time to resolve a mixture of two synthetic antioxidants, propyl gallate (PG) and butylated hydroxyanisole (BHA), by the joint use of the stopped-flow mixing technique and a T-format luminescence spectrometer. The determination of these compounds involves two different and independent reactions. On the one hand, PG determination is based on an energy transfer process that involves the formation of a lanthanide chelate with terbium in the presence of Triton X-100 and tri-n-octylphosphine oxide. On the other hand, BHA is determined using a reaction between the oxidized form of Nile Blue and the antioxidant. Both systems are excited at the same excitation wavelength (310 nm), and the emission wavelengths are 545 and 665 nm for PG and BHA, respectively. The absence of overlap in the emission spectra makes it possible to measure separately the analytes in each channel of the instrument. Initial rate and equilibrium signal are used as analytical parameters and measured in 0.1 and 1 s for PG and BHA, respectively. Calibration graphs are linear over the range 0.09-3.5 microg mL(-)(1) for PG and 0.3-15 microg mL(-)(1) for BHA. The relative standard deviations of both systems are close to 2%. The proposed method is applied to the determination of these two antioxidants in several commercial food samples with recoveries ranging between 94.8 and 102.9% for PG and between 94.1 and 102.1% for BHA.

Selective kinetic determination of paraquat using long-wavelength fluorescence detection

The reaction between paraquat, ascorbic acid, and Cresyl Violet in alkaline medium and in the presence of sodium dodecyl sulfate has been applied for the first time to the development of a kinetic-fluorometric method for the determination of paraquat. The reaction rate of this system is measured by using the stopped-flow mixing technique, which makes the method applicable to automatic routine analysis. Analytical data are obtained in approximately 30 s. The calibration graph is linear over the range 6-500 ng mL(-)(1), and the detection limit is 1.8 ng mL(-)(1). The relative standard deviation is <3%. The use of dynamic measurements at long wavelength favors the high selectivity of the method. Diquat behaves in this system similarly to paraquat, but its interferent effect is easily avoided by using cysteine. The proposed method has been applied to the determination of paraquat in tap water, milk, and white wine samples with recoveries of 89-104%.

Simultaneous determination of benzoic acid and saccharin in soft drinks by using lanthanide-sensitized luminescence

A simple and fast approach is used for the first time to develop a time resolved lanthanide-sensitized luminescence method for the simultaneous determination of a preservative and a sweetener, namely benzoic acid (BZ) and saccharin (SC), respectively, in food samples. The method involves the formation of the corresponding ternary chelates with terbium(III) and trioctylphosphine oxide (TOPO) in the presence of Triton X-100, and the measurement of the initial rate and equilibrium signal of this system, which were obtained in 0.1 and 5 s, respectively. The dynamic ranges of the calibration graphs, obtained by using kinetic and equilibrium measurements, were 0.2-36 micrograms ml-1 and 0.15-30 micrograms ml-1, respectively, for BZ, and 3.3-24 micrograms ml-1 and 4-36 micrograms ml-1 for SC and the detection limits were 0.07 and 0.04 microgram ml-1, respectively, for BZ, and 1.1 and 1.2 micrograms ml-1, respectively, for sodium SC. The relative standard deviation ranged between 2.3 and 3.0%. Both compounds were determined simultaneously by using a system of two equations which were resolved by using the calibration data obtained individually for each analyte and by multiple linear regression. Mixtures of BZ and SC in ratios between 3:1 and 1:9 were satisfactorily resolved by using both approaches. The method was applied to the direct analysis of several soft drinks. Analytical recoveries ranged between 89.3 and 108.5%.