Showing papers on "Detection limit published in 1995"

A Two-Component Model for Measurement Error in Analytical Chemistry

Abstract: In this article, we propose and test a new model for measurement error in analytical chemistry. Often, the standard deviation of analytical errors is assumed to increase proportionally to the concentration of the analyte, a model that cannot be used for very low concentrations. For near-zero amounts, the standard deviation is often assumed constant, which does not apply to larger quantities. Neither model applies across the full range of concentrations of an analyte. By positing two error components, one additive and one multiplicative, we obtain a model that exhibits sensible behavior at both low and high concentration levels. We use maximum likelihood estimation and apply the technique to toluene by gas-chromatography/mass-spectrometry and cadmium by atomic absorption spectroscopy.

Trace gas detection with cavity ring down spectroscopy

Abstract: Trace gas detection of small molecules has been performed with cavity ring down (CRD) absorption spectroscopy in the near UV part of the spectrum. The absolute concentration of the OH radical present in trace amounts in heated plain air due to thermal dissociation of H2O has been calibrated as a function of temperature in the 720–1125 °C range. Detection of NH3 at the 10 ppb level is demonstrated in calibrated NH3/air flows. Detection of the background Hg concentration in plain air is performed with a current detection limit below 1 ppt. The effect of the laser linewidth in relation to the width of the absorption line is discussed in detail. Basic considerations regarding the use of CRD for trace gas detection are given and it is concluded that CRD spectroscopy holds great promise for sensitive [(sub)‐ppb] and fast (kHz) detection of many small molecules.

Methylphenazonium-modified enzyme sensor based on polymer thick films for subnanomolar detection of phenols

Abstract: A methylphenazonium-zeolite-modified enzyme sensor based on a planar, screen-printed, two-electrode arrangement is described for the subnanomolar detection of phenols. Using tyrosinase (EC 1.14.18.1), a novel polyurethane hydrogel was applied for immobilization and stabilization of the enzyme, which forms a self-adhering layer on the active surface of the strip sensor. Performance and characteristics of the sensor were evaluated with regard to response time, detection limit, selectivity, and dependences on temperature and pH as well as operating and storage stabilities. The sensor shows marked sensitivity to eight phenolic compounds usually present in industrial waste waters. The detection limit for phenol was obtained with 0.25 nM

Development of a Submicrometer Optical Fiber Oxygen Sensor

Abstract: A submicrometer optical fiber oxygen sensor has been fabricated, based on the fluorescence quenching of tris-(1,10-phenanthroline)ruthenium(II) chloride in the presence of oxygen or dissolved oxygen. The Ru compound has been incorporated into acrylamide polymer that is attached covalently to a silanized optical fiber tip surface by photoinitiated polymerization. Leaching of the sensing reagent from the polymer host matrix has been minimized by the optimization of the ratio between the acrylamide monomer and the cross-linker, N,N-methylenebisacrylamide. The sensor is fully reversible and highly reproducible. A standard deviation of approximately 2% for 10 consecutive fluorescence measurements has been observed for several oxygen concentrations. The sample volume required for measurements is 100 fL. An absolute detection limit of 1 x 10(-17) mol is achieved. This is an improvement by a factor of 10(6) as compared to other existing optical fiber oxygen sensors.

Determination of organophosphorus, triazine and 2,6-dinitroaniline pesticides in aqueous samples via solid-phase microextraction (SPME) and gas chromatography with nitrogen-phosphorus detection

Abstract: Solid-phase microextraction (SPME) represents a very simple and rapid method for the extraction of organophosphorus, triazine and 2,6-dinitroaniline pesticides from aqueous samples without making use of any solvents. The same fiber can be used repeatedly. Moreover, a sample volume as small as 3 mL can be employed with no loss in sensitivity. 34 compounds have been extracted from aqueous samples by SPME using a 85 μm polyacrylate fiber. For organophosphorus pesticides, a 100 μm polydimethylsiloxane fiber has been used additionally for comparison. The fibers were directly introduced into the heated split/splitless injector of the gas chromatograph and determined using a nitrogen-phosphorus detector. The method was evaluated with respect to the limit of detection (LOD), linearity and precision. The limit of detection (LOD) depends on the compound and varies from 0.005–0.09 μg/L. The method is linear over at least three orders of magnitude with coefficients of correlation usually >0.999. For triazines and 2,6-dinitroanilines the coefficient of variation (precision) is <8% while for organophosphorus compounds it may reach values up to 18% (however, if the latter compounds are extracted using the polydimethylsiloxane phase considerably higher precision is achieved). The partitioning of the analyte between the aqueous phase and the polymeric phase depends on the hydrophobicity of the compound as expressed by the octanol/water partitioning coefficient (Pow). For triazines it was shown that there is a linear dependence of the logarithm of the analyte response on the log(Pow) i.e. the higher the hydrophobicity, the higher the affinity of the analytes to the polymeric phase of the fiber and the higher the response. Salt addition has a strong effect on the extraction efficiency. This effect increases with decreasing hydrophobicity (increasing polarity) of the compound. The triazines ametryn, atrazine, propazine, simazine and simetryn have been identified in a ground water well sample by SPMEGC/NPD.

Capillary Electrophoresis of Inorganic Anions in Nonaqueous Media with Electrochemical and Indirect UV Detection

Abstract: Methanol and dimethylformamide were evaluated for the separation of a series of inorganic anions. As these solvents were added to an aqueous electrolyte, the electroosmotic flow went through a minimum at 70-80% (v/v) organic solvent and then increased as the composition approached that of the pure solvent. Reproducibility of electroosmotic flow was good (%RSD=1.1 for dimethylformamide), and Joule heating was not as important as in water systems. The effect of electrolyte concentration on electroosmotic flow was similar to that expected for aqueous solutions, but significant differences in separation selectivity were observed, with many ions showing reversed separation order compared to aqueous systems. In addition, there was some evidence of ion association effects that led to changes in selectivity as a function of concentration and nature of the electrolyte. The application of indirect UV detection in methanol was briefly evaluated with chromate, phthalate, and benzoate electrolytes. Phthalate electrons were slightly less sensitive than chromate, but chromate was not stable in methanol. Calibration of 11 inorganic anions separated in a phthalate electrolyte gave linear curves in the range of 5×10 -5 -8×10 -4 mol/L; detection limits were in the range of 2.0×10 -5 -3.4×10 -5 mol/L. Amperometric detection at a 25 μm Pt disk electrode gave detection limits in the range of 1×10 -9 -6×10 -8 mol/L for some anions, but calibration curves were linear only over small ranges of concentration. Peak shapes obtained with the electrochemical detection systems were good, and theoretical plate counts were in the range of 140 000-450 000

Application of the fast-evaporation sample preparation method for improving quantification of angiotensin II by matrix-assisted laser desorption/ionization

Abstract: The fast-evaporation method of sample preparation has been applied for quantitative analysis using matrix-assisted laser desorption/ionization (MALDI) mass spectrometry. An instrumental protocol focusing on improvement of shot-to-shot repeatability and compensation for signal degradation has been developed for quantification of angiotensin II using the fast-evaporation technique and an internal standard. The fast-evaporation method was compared to the standard method of sample preparation (using a multicomponent matrix) in the quantitative analysis of angiotensin II, and found to be superior in several respects. Improvement in sample homogeneity using the fast-evaporation method enhanced both point-to-point repeatibility and sample-to-sample reproducibility. The relative standard deviations of the analyte/internal standard ratios (point RSD) were decreased by a factor of three compared to those obtained using the multicomponent matrix method. The average point RSD was found to be ca. 5% for the fast-evaporation technique. Two internal standards were evaluated for quantification of angiotensin II. The better one, 1-SAR-8-Ile angiotensin II, yielded a relative standard deviation of the standard curve slope of ca. 2.2% over two orders of magnitude of concentration (45 nM to 3000 nM), an improvement by a factor of two over the standard preparation method. Renal microdialysate samples, spiked with angiotensin II and the internal standard 1-SAR-8-Ile angiotensin II, were also analyzed using the fast-evaporation technique. The detection limit was calculated to be in the high attomole range (675 amol). Furthermore, the accuracy for a single determination of angiotensin II concentration in these samples was found to be 13.9% with a relative error of 8.19%.

Determination of organometallic species by gas chromatography inductively coupled plasma mass spectrometry

Abstract: The development is described of a gas chromatography inductively coupled plasma mass spectrometry (GC–ICP-MS) coupling system for the determination of organometallic species of tin, mercury and lead. The compounds are separated by GC using a 30 m long capillary column. A heated transfer line, made from quartz, directs the capillary column to just in front of the argon plasma and serves as the sample introduction system. The ionized elements are simultaneously detected on-line by ICP–MS. The real-time transient signals of the different isotopes are recorded, and an integration system allows one to calculate the retention times and to quantify the species after calibration. With the help of standard solutions the instrumental limits of detection for GC–ICP-MS of various organometallic species such as methyl, butyl or phenyl compounds of tin, mercury and lead have been calculated. The detection limit for tin is about 50 fg, for lead about 100 fg and for mercury about 120 fg. The reproducibility of different runs is better than 5%. For the application of GC–ICP-MS to environmental samples a new method using sodium tetraethylborate for direct ethylation of the ionic organotin species in wet sediments was adopted for a simultaneously multi-element in-situ ethylation of the tin, lead and mercury species.

Determination of Chromium Species in Environmental Samples Using High-Pressure Liquid Chromatography Direct Injection Nebulization and Inductively Coupled Plasma Mass Spectrometry

Abstract: A sensitive technique for speciation and quantification of Cr(III), Cr(VI), and total chromium has been developed using high-pressure liquid chromatography (HPLC) combined with direct injection nebulization (DIN) and inductively coupled plasma mass spectrometry (ICP-MS). A characterization and optimization of a microcolumn specifically designed for chromium speciation has been conducted. The direct injection nebulizer (DIN) was optimized to produce the best possible sensitivity for the system. Parameters affecting the column component of the system such as sample pH, eluent strength, sample size, flow, and pressure characteristics were studied in addition to optimal DIN-ICP-MS parameters. The detection limit obtained was 30, 60, and 180 ng/L for total chromium, Cr(III), and Cr(VI), respectively. Total chromium and speciated chromium were determined in one measurement with an analysis time of approximately 500 s. Accuracy measurements for the two chromium species were within 5% of the certified value for NIST Standard Reference Materials 2108 and 2109. In order to demonstrate the capability of the technique to analyze a true environmental sample matrix, industrial effluent samples were analyzed, and the results were compared with an alternative conventional method.

Electrospray Liquid Chromatography-Mass Spectrometry of Carotenoids

Abstract: The first method for electrospray liquid chromatography-mass spectrometry (LC-MS) of carotenoids is reported, which uses a C 30 reversed-phase HPLC column and a gradient solvent system containing methanol/methyl tert-butyl ether/ammonium acetate at a flow rate of 1.0 mL/min. The entire HPLC column effluent passes through a photodiode array absorbance detector and then into the electrospray LC-MS interface without solvent splitting. In this way, maximum sensitivity is achieved for both the photodiode array detector, which records the UV/vis spectra of each carotenoid, and the mass spectrometer, which measures the molecular ions of each carotenoid. Molecular ions, M .+ , without evidence of any fragmentation, were observed in the electrospray mass spectra of both xanthophylls and carotenes. In order to enhance the formation of molecular ions, solution-phase carotenoid oxidation was carried out by means of postcolumn addition of a halogenated solvent to the HPLC effluent Several different halogenated solvents were evaluated, including chloroform, 2,2,3,3,4,4,4-heptafluoro-1-butanol, 2,2,3,3,4,4,4-heptafluorobutyric acid, 1,1,1,3,3,3-hexafluoro-2-propanol, and trifluoroacetic acid. Among these halogenated solvents, 2,2,3,3,4,4,4-heptafluoro-1-butanol at a concentration of 0.1% (v/v) was found to produce the best combination of carotenoid molecular ion abundance and reproducibility. The limits of detection for lutein and β-carotene were between 1 and 2 pmol each, which was 100-fold lower than the detection limit of the photodiode array detector signal.

Quantitative analysis of cytokinins by electrospray tandem mass spectrometry

Abstract: A methodology was developed for the analysis of cytokinins from plant tissue using electrospray tandem mass spectrometry in combination with HPLC. A detection limit of 1 pmol zeatin-riboside injected on-column was achieved under multiple reaction monitoring conditions. Linearity was observed within a concentration range of 1–100 pmol injected on-column. In view of the analysis of a considerable amount of samples, the chromatographic conditions were selected which were a compromise between speed of analysis and resolution. This approach, in combination with multiple reaction monitoring, allowed the analysis of up to 150 samples a day, and quantitation of 16 different cytokinins per sample.

HPLC Determination of Atmospheric Organic Hydroperoxides

Abstract: A high-performance liquid chromatographic analytical technique has been developed for the separation of hydrogen peroxide and low molecular weight organic hydroperoxides. A total of 10 C1–C3 organic hydroperoxides have been separated. The detection system is based on postcolumn derivatization using an enzymatic reaction with p-hydroxyphenylacelic acid to form a fluorescent dimer. The detection limit for the technique is 9 × 10−8 M for hydrogen peroxide and 2 × 10−8 M for other hydroperoxides. This paper details the optimization of the analytical technique, examining column characteristics and the chemistry of the analytical reaction.

Ultrasensitive near-IR fluorescence detection for capillary gel electrophoresis and DNA sequencing applications.

Abstract: Electropherograms of oligonucleotides labeled with near-IR fluorescent dyes, separated by capillary gel electrophoresis and detected using an ultrasensitive near-IR fluorescence detection system, are presented. A universal M13 sequencing primer was labeled on the 5' end with a near-IR dye containing an isothiocyanate functional group. Comparison of the on-column detection limits in capillary gel electrophoresis for the near-IR dye-labeled sequencing primer to those obtained for a visible fluorescein-labeled primer indicated improved sensitivity for the near-IR case. The detection limit was found to be 3.4 x 10 -20 mol (SNR = 3) for the near-IR dye-labeled primer, while the on-column detection limit for the fluorescein analog was 1.5 x 10 -18 mol (SNR = 3). The sequence of nucleotide bases in an M13mp18 template was determined using a single lane, single dye technique. The molar concentrations of the ddNTPs used during chain extension reactions were varied to achieve a ratio of 4 :2 :1 :0 (A :C :G :T), which allowed the identification of each terminal base via fluorescence intensity measurements. Sequencing ladders were prepared from the M13mp18 template using standard Sanger dideoxy chain-terminating techniques, the modified T7 DNA polymerase, and the near-IR dye-labeled M13 universal primer. The data indicated reliable sequence determination by the 4 :2 :1 :0 (A :C :G :T) peak height identification method up to 250 bases from the annealing site. Comparison of the known sequence of the M13mp18 plasmid to that obtained using this protocol yielded a base-calling accuracy of 84% for the 4 :2 :1 :0 ratio.