Once again about determination and detection limits

The papers takes stock of different methods for evaluating the detection (c _min) and determination (c _lim) limits of components described in the literature and presents a comparative analysis of the results of such evaluations. It is shown that use of the data on the fluctuations of the blank experiment, in spite of their wide application for such evaluations, gives ambiguous estimates for c _min and c _lim. The most correct method for evaluating the specified parameters is the experimental determination of the actual analyte concentration (content) from the empirical concentration dependence of the relative standard deviation. In case when this estimation method cannot be used, it is recommended to evaluate c _lim by the lower boundary of the calibration graph. The article may be useful for inexperienced analysts experimenters in choosing a method for evaluating the results obtained.     

The use of an inexpensive electronic filter for improving detection limits in the gas chromatographic-chemiluminescent determination of nitrosamines

The construction and use of an electronic noise filter is described. It is evaluated by use with a laboratory-built selective chemiluminescent detector used for the gas Chromatographie analysis of mixtures of volatile N-nitrosamines. A ten-fold reduction in the noise level was achieved with a corresponding decrease in detection limit to 4 nl l^-1.     

Assessment of limits of detection and quantitation using calculation of uncertainty in a new method for water determination

 An approach to the assessment of the limit of detection and the limit of quantitation using uncertainty calculation is discussed. The approach is based on the known evaluation of the limits of detection and quantitation as concentrations of the analyte equal to three and ten standard deviations of the blank response, respectively. It is shown that these values can be calculated as the analyte concentrations, for which relative expanded uncertainty achieves 66% and 20% of possible results of the analyte determination, correspondingly. For example, the calculation is performed for the validation of a new method for water determination in the presence of ene-diols or thiols, developed for analysis of chemical products, drugs or other materials which are unsuitable for direct Karl Fischer titration. A good conformity between calculated values and experimental validation data is observed. Received: 27 July 1998 · Accepted: 29 November 1998     

Development of a capillary electrophoresis-mass spectrometry method for the determination of rivastigmine in human plasma--optimization of the limits of detection and quantitation

A capillary zone electrophoresis-electrospray ionization-mass spectrometry (CZE-ESI-MS) method was developed for the analysis of the acetylcholinesterase inhibitor rivastigmine. Several electrophoretic and ESI-MS parameters were investigated in order to improve sensitivity. These parameters were categorized in three areas: (i) background electrolyte (BGE) parameters, (ii) sheath liquid parameters, and (iii) spray chamber parameters. The optimized results were obtained by using 40-mM ammonium acetate at pH 9 as BGE, a sheath liquid of 1% acetic acid in water:MeOH (50:50 v/v) at a flow rate of 10?μL/min, and a drying gas flow rate that was set at 6 L/min and at a temperature of 200°C. These parameters provided limit of detection and limit of quantitation of 2.8?ng/mL and 8.4?ng/mL, respectively. The optimal CZE-ESI-MS conditions were applied to a plasma sample obtained from an Alzheimer's disease patient following rivastigmine patch administration, and the mean (±standard deviation) plasma concentration was estimated to be 14.6 (±1.7)?ng/mL. Several sample preparation procedures were examined, and solid-phase extraction using a C18 cartridge proved to be the most effective procedure, since higher sensitivity and recovery were obtained. In addition, precision was evaluated based on migration time and peak area in plasma, and the relative standard deviations were in the range of 0.10-0.16% and 0.62-9.0%, respectively.     

Methodologies for detection of hemoglobin-based oxygen carriers

Blood substitutes based on hemoglobin or hemoglobin-based oxygen carriers (HBOCs) are oxygen-carrying therapeutic agents developed for use in operations and emergencies in place of donated blood. Increased oxygen-carrying capacity through the use of blood substitutes could help elite athletes to lengthen endurance capacity and improve their performance. As blood substitutes become more readily available, it is essential that a qualitative detection method for their abuse in sport is available. Ideally, such a method would be simple and inexpensive. This study investigates methods that could be used as screening procedures to easily detect HBOCs in plasma and develops tests that can unequivocally confirm their presence. The investigation into the screening method indicates that the direct visual screening of plasma discoloration is the most appropriate with detection limits of less than 1% HBOC in plasma. Two methods are shown to confirm the presence of exogenous hemoglobin in plasma samples, size-exclusion chromatography with photodiode array detection and high-performance liquid chromatography analysis of enzymatic digests with detection by electrospray mass spectrometry. This work emphasizes the need for cooperation between drug developers and sports testing laboratories to ensure that methods for the detection of putative doping agents are available prior to product release.     

New method for detection and spectrophotometric determination of quinones

The reaction between 3-phenylthiazolidine-2,4-dione (I) and p-benzoquinone (II), tetrachloro-p-benzoquinone (III), and 1,4-naphthoquinone (IV) in ammoniacal medium is applied for detection and spectrophotometric determination of quinones. The absorbance-concentration relationship is linear up to 18 μg/ml of quinone concentration. The lower limits of identification in the detection reaction are 2.5, 3.0, and 1 μg for (II), (III), and (IV), respectively, which reflect high sensitivity. The reaction between (I) and quinones is proved to be a condensation reaction and highly selective.     

A novel approach for the determination of detection limits for metal analysis of environmental water samples

Despite the widespread use of the USEPA method (U.S. Environmental Protection Agency, 40 CFR 136 Appendix B) for the determination of method detection limit (MDL), criticisms have been raised that the method does not account for measurement bias and outliers that subsequently lead to a common misunderstanding of the requirement for the determination of MDL. This paper demonstrates that it is difficult to follow the USEPA method for verifying the MDL for analysis involving multiple metals and proposes a precision and bias criterion for determining the MDL. A multiple-point fitted profile, based on the correlation between relative standard deviation (RSD) and concentration, is used to derive a robust MDL value. Representative examples of As, Ca, Cr, and Cu are used to illustrate this procedure. A procedure for identifying outliers is also discussed.     

RP-HPLC method with electrochemical detection for the determination of metoclopramide in serum and its use in pharmacokinetic studies.

A rapid and sensitive reversed-phase high performance liquid chromatographic method has been developed for the determination of metoclopramide in serum. The assay was performed after single extraction with ethyl ether using methyl parahydroxybenzoate as internal standard. Chromatographic separations were performed on C(18) stationary phase with a mobile phase composed of methanol-phosphate buffer pH 3 (30:70 v/v). Analytes were detected electrochemically. The quantification limit for metoclopramide in serum was 2 ng mL(-1). Linearity of the method was confirmed in the range of 5-120 ng mL(-1) (correlation coefficient 0.9998). Within-day relative standard deviations (RSDs) ranged from 0.3 to 5.5% and between-day RSDs from 0.8 to 6.0%. The analytical method was successfully applied for the determination of pharmacokinetic parameters after ingestion of 10 mg dose of metoclopramide. Studies were performed on 18 healthy volunteers of both sexes.     

Elaboration and use of a double channel detection ion chromatographic method for determination of ions in aqueous samples

Summary In this paper we report our research in developing a simple and reliable method for determination of various anions in aqueous samples. Among the several methods existing for determining ions in aqueous samples, ion chromatography is becoming more and more important, due to its reliability and ability to determine the concentration of more ions from one sample. Although several detection methods are available, in this work we used conductivity detection, and indirect UV photometric detection. Because in some cases it can be important to gather more detailed information on the composition of a sample, we present a way to meet this demand by using double channel detection. Presented at Balaton Symposium on High Performance Separation Methods, Siófok, Hungary, September, 1–3, 1999     

New automatized method with amperometric detection for the determination of azithromycin

A FIA-amperometric method for azithromycin determination was developed. A working glassy carbon electrode and a Ag/AgCl/NaCl (3 M) reference electrode were used. The determination is based on the electrochemical oxidation of the azithromycin at 0.9 V in Britton-Robinson buffer solution (pH 8.0). Due to the adsorption of the reaction products on the electrode surface, an effective cleaner cycle was implemented. By using the optimum chemical and FIA conditions, a concentration linear range of 1.0-10.0 mg L⁻¹ and a detection limit (LOD) of 0.76 mg L⁻¹ are obtained. The method was validated and satisfactorily applied to the determination of azithromycin in pharmaceutical formulations.     

The limits of decision,detection, and determination in anti-Compton gamma-ray spectrometry

The improvement in the limits of decision, detection, and determination effected by anti-Compton gamma-ray spectrometry are considered. A simple procedure for the experimental determination is presented.     

The use of polarizers to improve detection limits in fluorimetric analysis

The use of variously-oriented film polarizers to reduce scattered light interference in fluorimetric analysis is reported. Single horizontally-oriented polarizers in the excitation beam (to obtain fluorescence spectra) or the emission beam (to obtain excitation spectra) are efficient at removing first-order Rayleigh scattered light signals. Crossed polarizers are necessary to remove substantial second-order signals. With the aid of polarizers, fluorimetric detection limits can be considerably improved, even when excitation and emission maxima are well separated, and when deproteinized blood plasma is analyzed. Polarizers are also effective in reducing scattered light signals from macromolecular solutes.     

A rapid CE-potential gradient detection method for determination of quinolones

A rapid CE coupled with potential gradient detection (PGD) for the separation and detection of four quinolones, namely, enoxacin, ofloxacin (OFL), fleroxacin, and pazufloxacin, was described. Separation was performed in a fused-silica capillary (75 microm x 8.5 cm) using a buffer of 30 mM Tris and 4 mM phosphoric acid at pH 8.9. Under the separation voltage of 3 kV, the quinolones were separated within 2.8 min with good linearity (r(2) >or= 0.985). The method was successfully applied in determining OFL in a pharmaceutical formulation. Also, a liquid-liquid extraction (LLE) method was developed and coupled to CE-PGD in determining quinolones that spiked in milk samples. With dichloromethane and hexane for enrichment and purification, the LLE recoveries of the four quinolones were in the range of 77-106%. The detection limits of the quinolones with LLE-CE-PGD were from 23 to 65 ng/mL. The proposed CE-PGD method was validated with an HPLC method, and the results indicated consistency between the two methods.     

Reverse-FIA with spectrophotometric detection method for determination of vitamin C

Simple reverse flow injection analysis (rFIA) manifold with spectrophotometric detection was developed for an indirect determination of ascorbic acid. Parameters such as stability, accuracy and precision were established for the method and evaluated statistically to assess the applications of the method. Ascorbic acid in this procedure accelerates dediazoniation reaction of formed diazonium ions; hence its quantity can be determined by monitoring the derivatization product from coupling unreacted diazonium ion with phenol to give an azo dye (coupling reaction). The rFIA design was based on the injection of sodium nitrite into an acidic p-aminoacetophenon carrier stream in which diazonium ion was formed. This ion was inhibited by ascorbic acid stream before coupling with the phenol-Na₂CO₃ stream. Under optimum conditions, ascorbic acid acts in accordance with the Beer??s law at two concentration ranges 0.4?C6.5 ??g ml^?1 (R = 0.9995) and 7.0?C20.0 ??g ml^?1 (R = 0.9949), with detection limits of 0.25 ??g ml^?1. The developed method was applied to the determination of vitamin C in pharmaceutical formulations which produced satisfactory results compared with the standard methods reported in the British Pharmacopoeia.     

An easy and practical method for detection and estimation of microcrystalline cellulose in pasteurized milk

Microcrystalline cellulose (MCC) is suspected to be a new adulteration in pasteurized milk in China, yet an efficient method for MCC detection in dairy has not been established. This study presents a novel procedure to detect and estimate MCC in pasteurized milk using dialysis, cellulase hydrolysis, and a reducing sugar assay. The background value of reducing sugar was eliminated by dialysis, and cellulase activity toward MCC was stable in dialyzed milk. A criterion for MCC detection and an empirical formula for MCC estimation were summarized based on the reducing sugar variation after hydrolysis. The detection sensitivity was below 0.5 g/L. Reducing sugar distribution after cellulase-catalyzed hydrolysis was examined by HPLC, and revealed that most of the detected sugar was glucose. This paper describes a practical method for detection of MCC in pasteurized milk that might benefit dairy QC.     

Enzymatic method for the determination of phenols with the use of peanut peroxidase

Cationic peanut peroxidase was used for the first time for the determination of phenols at a level of 0.5–10μM. The examined phenols were found to be inhibitors or second substrates of peanut peroxidase in the indicator reaction of the oxidation ofo-dianisidine by hydrogen peroxide. The effect of phenols on the rate of the indicator reaction depends on their redox properties. The data on the effects of phenols on the catalytic activities of peroxidases isolated from different sources (peanut, horseradish roots,Medicago sativa alfalfa cells, and the xylotrophic fungusPhellinius igniarius) were compared     

Pushing the limits of NAA: Accuracy,uncertainty and detection limits

This paper describes some highlights from the author’s efforts to improve neutron activation analysis (NAA) detection limits through development and optimization of radiochemical separations, as well as to improve the overall accuracy of NAA measurements by identifying, quantifying and reducing measurement biases and uncertainties. Efforts to demonstrate the metrological basis of NAA, and to establish it as a “Primary Method of Measurement” will be discussed.