Showing papers on "Detection limit published in 2003"

A confidence limit for the empirical mode decomposition and Hilbert spectral analysis

Abstract: The confidence limit is a standard measure of the accuracy of the result in any statistical analysis. Most of the confidence limits are derived as follows. The data are first divided into subsections and then, under the ergodic assumption, the temporal mean is substituted for the ensemble mean. Next, the confidence limit is defined as a range of standard deviations from this mean. However, such a confidence limit is valid only for linear and stationary processes. Furthermore, in order for the ergodic assumption to be valid, the subsections have to be statistically independent. For non‐stationary and nonlinear processes, such an analysis is no longer valid. The confidence limit of the method here termed EMD/HSA (for empirical mode decomposition/Hilbert spectral analysis) is introduced by using various adjustable stopping criteria in the sifting processes of the EMD step to generate a sample set of intrinsic mode functions (IMFs). The EMD technique acts as a pre‐processor for HSA on the original data, producing a set of components (IMFs) from the original data that equal the original data when added back together. Each IMF represents a scale in the data, from smallest to largest. The ensemble mean and standard deviation of the IMF sample sets obtained with different stopping criteria are calculated, and these form a simple random sample set. The confidence limit for EMD/HSA is then defined as a range of standard deviations from the ensemble mean. Without evoking the ergodic assumption, subdivision of the data stream into short sections is unnecessary; hence, the results and the confidence limit retain the full‐frequency resolution of the full dataset. This new confidence limit can be applied to the analysis of nonlinear and non‐stationary processes by these new techniques. Data from length‐of‐day measurements and a particularly violent recent earthquake are used to demonstrate how the confidence limit is obtained and applied. By providing a confidence limit for this new approach, a stable range of stopping criteria for the decomposition or sifting phase (EMD) has been established, making the results of the final processing with HSA, and the entire EMD/HSA method, more definitive.

Thin-film microextraction.

Abstract: The properties of a thin sheet of poly(dimethylsiloxane) (PDMS) membrane as an extraction phase were examined and compared to solid-phase microextraction (SPME) PDMS-coated fiber for application to semivolatile analytes in direct and headspace modes. This new PDMS extraction approach showed much higher extraction rates because of the larger surface area to extraction-phase volume ratio of the thin film. Unlike the coated rod formats of SPME using thick coatings, the high extraction rate of the membrane SPME technique allows larger amounts of analytes to be extracted within a short period of time. Therefore, higher extraction efficiency and sensitivity can be achieved without sacrificing analysis time. In direct membrane SPME extraction, a linear relationship was found between the initial rate of extraction and the surface area of the extraction phase. However, for headspace extraction, the rates were somewhat lower because of the resistance to analyte transport at the sample matrix/headspace barrier. It w...

Measurements of OH and HO2 radical concentrations and photolysis frequencies during BERLIOZ

Abstract: [1] This paper presents the measurements of OH and HO2 radical concentrations as well as photolysis frequencies of different molecules during the Berliner Ozone (BERLIOZ) field experiment in July/August 1998 at the rural site Pabstthum about 50 km NW of Berlin. Radical concentrations were measured using laser-induced fluorescence (LIF) spectroscopy, while filter radiometers and a scanning spectroradiometer were used to obtain photolysis frequencies. The radical data set covers the time period from 20 July to 6 August and consists of more than 6000 simultaneous measurements of OH and HO2 with a typical time resolution of about 90 s. The maximum OH and HO2 daytime concentrations were 8 × 106 and 8 × 108 cm−3, respectively. While nighttime values of OH were usually below the detection limit of our instrument (3.5 × 105 cm−3), HO2 did show significant concentrations throughout most of the nights (on average 3 × 107 cm−3). The OH concentration was mainly controlled by solar UV radiation and showed a high linear correlation with J(O1D). A deviation from this general behavior was observed around dawn and dusk, when OH concentrations well above the detection limit were observed, although J(O1D) was essentially zero. A comparison with data sets from previous campaigns revealed that even though the linear correlation is found in other environments as well the slope [OH]/J(O1D) differs significantly. The diurnal cycles of HO2 were less dependent on the solar actinic flux but were predominantly influenced by NO. During episodes of high NO, HO2 remained below the detection limit (1 × 107 cm−3) but started to rise rapidly as soon as NO started to decrease.

Determination of organophosphorus pesticides in fruit juices by matrix solid-phase dispersion and gas chromatography.

Abstract: A rapid multiresidue method was developed for the determination of nine organophosphorus pesticides in fruit juices. The analytical procedure is based on the matrix solid-phase dispersion (MSPD) of juice samples on Florisil in small glass columns and subsequent extraction with ethyl acetate assisted by sonication. Residue levels were determined by gas chromatography with nitrogen−phosphorus detection. Spiked blank samples were used as standards to counteract the matrix effect observed in the chromatographic determination. The NPD response for all pesticides was linear in the concentration range studied with determination coefficients >0.999. Average recoveries obtained for all of the pesticides in the different juices and fortification levels were >70% with relative standard deviations of <11%. The detection limits ranged from 0.1 to 0.6 μg/kg. The identity of the pesticides was confirmed by gas chromatography with mass spectrometric detection using selected ion monitoring. The proposed MSPD method was ap...

Electrochemical determination of sulphide at nickel electrodes in alkaline media: a new electrochemical sensor

Abstract: The electrochemistry of a nickel hydroxide electrode has been studied both in the presence and absence of sulphide at microdisc and macroelectrodes. With sulphide present a new oxidative wave is observed. At a macroelectrode, this response produced a linear range from 20 to 200 μM with a corresponding limit of detection of 19 μM. Under the microelectrode regime, the response was found to be linear from 20 to 200 μM with a detection limit of 10 μM. The protocol has been developed into the design of a simple and cheap electrochemical sensing cell for the detection of sulphide in aqueous media.

Chemiluminescence detection for a microchip capillary electrophoresis system fabricated in poly(dimethylsiloxane).

Abstract: Chemiluminescence (CL) detection integrated with a microchip capillary electrophoresis (MCE) system that was fabricated in poly(dimethylsiloxane) was demonstrated for chemical and biochemical analyses. Two model CL systems were involved here: metal ion-catalyzed luminol-peroxide reaction and dansyl species conjugated peroxalate-peroxide reaction. Different strategies based on three chip patterns (cross, cross combining with Y, and cross combining with V) to perform on-line CL detection for MCE were evaluated and compared in terms of sensitivity, reproducibility, and peak symmetry. The chip pattern of cross combining with Y proved to be promising for the luminol-peroxide CL system, while the chip pattern of cross combining with V was preferred for the peroxalate-peroxide system where CL reagent could not be effectively transported by electroosmotic flow. A detection limit down to submicromolar concentrations (midattomole) was achieved with good reproducibility and symmetric peak shape. Successful separation of three metal cations such as Cr(III), Co(II), and Cu(II) and chiral recognition of dansyl phenylalanine enantiomers within 1 min revealed distinct advantages of combining MCE with CL detection for rapid and sensitive analyses.

Isolation of Arcobacter species from animal feces.

Abstract: A previously developed Arcobacter isolation protocol for poultry skin and meat was validated for the isolation of Arcobacter from feces of livestock animals. Good repeatability, in-lab reproducibility and sensitivity were achieved and the specificity was improved by additional incorporation of cycloheximide and increase of the novobiocin concentration in the selective supplement. The limit of detection of quantitative and qualitative analysis was 10(2) and 10(0) cfu g(-1) feces, respectively. From fecal samples collected at slaughterhouse, Arcobacter was isolated from 43.9% of porcine, 39.2% of bovine, 16.1% of ovine and 15.4% of equine samples. All three animal-associated Arcobacter species were isolated and levels up to 10(3) cfu g(-1) feces were determined.

Compound-specific carbon isotope analysis of volatile organic compounds in the low-microgram per liter range

Abstract: Compound-specific carbon isotope analysis (CSIA) has become an important tool in biological, archeological, and geological studies as well as in forensics, food sciences, and organic chemistry. If sensitivity could be enhanced, CSIA would further have an improved potential for environmental applications such as, for example, in situ remediation studies to assess contaminated environments, identification of pollutant degradation pathways and kinetics, distinction between degradation/formation mechanisms, or, verification of contaminant sources. With this goal in mind, we have developed methods to determine ∂13C values of commonly reported groundwater contaminants in low-microgram per liter concentrations. Several injection and preconcentration techniques were evaluated for this purpose, i.e., on-column injection, split/splitless injection, solid-phase microextraction (SPME), and purge and trap (P&T) in combination with gas chromatography−isotope ratio mass spectrometry. The ∂13C values of the target compou...

Characterization of electrode fouling and surface regeneration for a platinum electrode on an electrophoresis microchip.

Abstract: A method of electrochemically cleaning noble metal electrodes is presented and characterized for electrophoresis microchips with electrochemical detection. First, the loss of sensitivity due to electrode fouling by serotonin is characterized as a function of injection number and analyte concentration. Signal attenuation is observed to be greater at high concentrations (100 μM) and negligible at very low concentrations (∼1 μM). Next, an electrochemical treatment procedure is optimized to yield sensitive and reproducible amperometric detection of the highly adsorptive compounds, serotonin and histamine. Thus, the performance of the electrode is reproducibly regenerated following as much as a ∼99% reduction in surface activity. Utilizing the optimized three-level waveform, derived from that used for pulsed amperometric detection, detection limits as low as 78 nM and 17 μM have been obtained for serotonin and histamine, respectively. In the case of serotonin, this represents the lowest detection limit for a n...

Determination of perfluorooctane sulfonate in river water by liquid chromatography/atmospheric pressure photoionization mass spectrometry by automated on-line extraction using turbulent flow chromatography

Abstract: A simple, fast and sensitive liquid chromatography/atmospheric pressure photoionization mass spectrometry (LC/APPI-MS) method, with automated on-line extraction using turbulent flow chromatography (TFC), was developed for the determination of perfluorooctane sulfonate (PFOS) in river water. In this method, following an on-line extraction by injection onto a column under TFC conditions, PFOS is back-flushed onto a reversed-phase column via on-line column switching, and resolved chromatographically at a laminar flow rate of 1 mL min−1. Using this tandem LC-LC/APPI-MS system the extraction, separation and selective detection of PFOS in river water could be achieved with satisfactory selectivity and sensitivity. The limit of detection (LOD) (S/N = 3) and the limit of quantitation (LOQ) (S/N = 10)were 5.35 and 17.86 pg mL−1. The described procedure was very simple since no off-line sample preparation was required, total analysis time being 18.75 min. Copyright © 2003 John Wiley & Sons, Ltd.

Off-line solid-phase microextraction and capillary electrophoresis mass spectrometry to determine acidic pesticides in fruits

Abstract: A method based on solid-phase microextraction (SPME) and capillary electrophoresis/mass spectrometry (CE/ MS) is described for determining simultaneously five acidic pesticides (o-phenylphenol, ioxynil, haloxyfop, acifluorfen, picloram) in fruits. The CE device is coupled to an electrospray interface by a commercial sheath-flow adapter. Emphasis is placed on fulfillment of the speed and sensitivity requirements. The best separation is achieved using 32 mM ammonium formate/acid formic buffer at pH 3.1, with a working voltage of 25 kV. The MS detection of the five pesticides was performed in negative ionization mode. Full-scan spectra with base peaks corresponding to [M-H]- were obtained except for acifluorfen, which gives [M-H-CO2]- as most abundant ion. Compared with the conventional EC-UV, the limits of detection were lower for acifluorfen, haloxyfop, ioxynil, and picloram, by a factor of 20, 20, 50, and 2, respectively. Extraction involved fruit sample homogenization with an acetone-water solution (5:1), filtration, and acetone evaporation prior to fiber extraction. SPME conditions such as time, pH, ion strength, stationary phase of the fiber, sample matrix, and desorption solvents were examined. The recovery of the analytes ranged from 7 to 94%, and the relative standard deviation was between 3 and, 13%. The method was found to be linear between 0.02 and 500 mg kg(-1) with correlation coefficients ranging from 0.992 to 0.997. The limits of quantification were from 0.02 to 5 mg kg(-1). The optimized method was successfully applied to the analysis of acid pesticides in fruit samples.

ICP-MS determination of Pt, Pd and Rh in airborne and road dust after tellurium coprecipitation

Abstract: A method has been developed for the simultaneous determination of Pd, Pt and Rh (PGE) in environmental airborne and road dust samples by tellurium coprecipitation and ICP-MS. The Te coprecipitation was applied after digestion of the sample with aqua regia–HF in a microwave oven. This separation method removes more than 95% of the elements producing mass interference in PGE determination by ICP-MS. The methodology was validated with reference road dust samples CW7 and CW8. The detection limits are 0.3, 0.6 and 0.8 pg m−3 for Pt, Pd and Rh in airborne particulate matter, and 1, 1 and 0.4 ng g−1 for Pt, Pd and Rh in road dust. Application of the isotopic dilution method for Pt and Pd after their coprecipitation improves the results obtained for road dust samples. Rh (monoisotopic element) analysis was carried out by external calibration after Te coprecipitation.