Showing papers on "Detection limit published in 1992"

Analysis of Substituted Benzene Compounds in Groundwater Using Solid-Phase Microextraction

Abstract: rn Solid-phase microextraction (SPME) is applied to the analysis of benzene, toluene, ethyl benzene, and xylenes in groundwater. The inexpensive SPME method reduced the sample preparation time by 3-7-fold when compared to purge and trap methods. The relative standard deviation ranged from 3 to 5% for the single-operator relative standard deviation using a methyl silicone fiber. Limits of detection of 1-3 ppb (w/v) were obtained when using a fiber coated with 56-pm methyl silicone film and FID detection. The linear range extended from 15 to 3000 ppb (w/v). Solvents have been completely removed from the sample preparation step.

Improved speed, specificity, and limit of determination of an aqueous acid extraction thiobarbituric acid-C18 method for measuring lipid peroxidation in beef

Abstract: The objectives of this study were to improve the speed, specificity, and limit of determination of the aqueous acid extraction thiobarbituric acid (TBA) method for measuring malonaldehyde (MA) as a marker of lipid peroxidation in ground beef. The TBA reaction time was reduced from 30 to 5 min at 94±1 o C by increasing the concentration of TBA from 20 to 80 mM. The TBA-reactive substances (TBARS) in ground beef extracts and MA standard solution were stable for up to 2 and 12 days, respectively, in 5% (w/v) trichloroacetic acid (TCA) at 4 o C

Direct detection of trace levels of uranium by laser-induced kinetic phosphorimetry

Abstract: The kinetic phosphorimetric determination of uranyl ion in aqueous solutions at room temperature yielded a detection limit for UO 2 2+ of 1 ng/L. The response to uranium is linear from the detection limit up to 5 mg/L. The method is fast and accurate, with no separative pretreatment needed for most of the real samples investigated. In the analysis of biological samples, wet-ashing with HNO 3 /H 2 O 2 is required for measurements near the detection limit

Capillary electrophoresis of organic and inorganic cations with indirect UV detection

Abstract: The determination of alkali and alkaline earth metal ions by capillary electrophoresis using indirect UV detection is described. With the identical system it is also possible to determine short chain aliphatic amines and alkanol amines within 4 minutes. Indirect UV detection is achieved at 214 nm with a background electrolyte containing 5 mmol L−1 imidazole. Linear calibration curves could be obtained for peak areas between 0.5 and 10 ppm. The detection limits are around 0.1 ppm (corresponding to 10 fmol in about 10 nL sample volume) for all cations and amines and 0.05 ppm for lithium. Practical applications demonstrate the applicability of this system in routine analysis.

Optimization of comprehensive speciation of organotin compounds in environmental samples by capillary gas chromatography helium microwave-induced plasma emission spectrometry

Abstract: The operational variables are optimized for chromatographic resolution and detection limits. A comprehensive method for the determination of mono-, di-, tri-, and some tetraalkylated organotin compounds in water and sediments by gas chromatography-atomic emission detection (GC-AED) is developped. The absolute detection limit is about 0,05 pg (as Sn), and the calibration is linear over three decades.

Indirect UV detection of carbohydrates in capillary zone electrophoresis

Abstract: A new system for the rapid and sensitive analysis of underivatized carbohydrates has been established using capillary zone electrophoresis with indirect UV detection. At an applied potential of 28 kV, sugars and sugar acids could be separated by the combined effects of electroendosmosis and electrophoresis within 20 minutes in a fused silica capillary of 50 μm internal diameter and an effective length of 100 cm using 6mM sorbic acid, pH 12.1, as both carrier electrolytie and chromophore. The alkaline pH ensured ionization of the sugars and, hence, their detection by means of charge displacement. Furthermore, the chosen concentration of sorbic acid allowed the smallest fractional change in the background signal to be measured. While the electrophoretic mobilities of the sugars were found to increase within a pH range of 11.9 to 12.3, those of the sugar acids were not affected. Due to the increasing competition of hydroxide ions in the displacement of the chromophore with rising pH, a significant loss of sensitivity is observed at pH values higher than 12.1 and this pH was found to provide sufficient resolution, optimum sensitivity, and a acceptably short analysis time. Under these conditions, a lower detection limit of 2 pmol was obtained for glucose.

Nitroprusside and methylene blue methods for silicone membrane differentiated flow injection determination of sulfide in water and wastewater.

Abstract: Hydrogen sulfide evolved from an acidified sample is pre-concentrated by permeation in a stationary alkaline acceptor solution enclosed in a silicone rubber sample loop. Depending on the sample volume pre-concentrated, the applicable analytical range spans low micrograms/L to tens of mg/L for both methods. The methylene blue method is more sensitive by a factor of approximately 30 and actually permits practical determinations in the sub-micrograms/L levels. The limit of detection (LOD) for the nitroprusside method ranges from 20 micrograms/L for a 20 microL sample by conventional flow-injection determination (no membrane, throughput 30 samples/h) to less than 2 micrograms/L for 12 mL sample pre-concentrated in the membrane system (throughput 5 samples/h). The membrane is highly resistant to fouling and permits analysis of untreated wastewater samples bearing suspended solids, oil, grease, etc. without any pretreatment. No significant interference is observed with either chemistry. Although the nitroprusside chemistry is less sensitive, it does not involve the use of concentrated aggressive reagents and is recommended unless ultratrace determinations are essential. Viable reaction mechanisms are proposed for both of these chemistries.

Determination of amino acids by capillary zone electrophoresis based on semiconductor laser fluorescence detection

Abstract: Chlorophyll is fluorescent in the deep-red resign and is determined by semiconductor laser fluorometryn after its separation with capillary zone electrophoresis. The separation efficency is several hundred thousand in the theoretical plate. Methylene blue is used as a chromophore in indirect fluorometry. The detection limit achieved is 1-pmol levels. A new labelling reagent is synthesized, which consists of a thiazine chromophore for fluorescence detection and a succinimidyl ester for combination with an amino acid. The labeled amino acids are clearly resolved by capillary zone electrophoresis, the detection limit being 10-pmol levels

Elemental speciation by liquid chromatography–inductively coupled plasma mass spectrometry with direct injection nebulization

Abstract: A new version of the direct injection nebulizer (DIN) is used to interface liquid chromatographic (LC) separations with element-selective detection using inductively coupled plasma mass spectrometry (ICP-MS). The DIN injects all of the sample into the ICP and has a dead volume of less than 1 µl. Charged species of arsenic and tin are separated as ion pairs on a micro-scale (1 mm i.d.), packed, reversed-phase column. Detection limits are 0.2–0.6 pg for arsenic and 8–10 pg for tin. For methanol + water eluents, the signal is highest at 25% methanol and stays within 25% of this maximum as the methanol fraction is varied from 20 to 80%. Compared with LC–ICP-MS with conventional nebulizers, the absolute detection limits and chromatographic resolution are substantially superior, and the dependence of analyte signal on solvent composition is somewhat less severe with the DIN.

Capillary zone electrophoretic analysis of carbohydrates by direct and indirect UV detection

Abstract: The analysis of carbohydrates has been always hampered by their lack of UV absorbance above 200 nm, which is an especially challenging problem in capillary electrophoresis due to the very small (nl) sample volumes injected. The introduction of 2-aminopyridine as derivatizing agent allows sensitive direct UV detection of saccharides in the fmol range. However, due to the requirement of the presence of a free aldehyde group only aldoses and uronic acids can be determined. This limitation was recently overcome by means of precolumn derivatization withp-aminobenzoic acid or ethylp-aminobenzoate, which permits the analysis of fructose with a lower mass detection limit of 0.3 and 0.14 pmol, respectively. The detection limits for aldoses were even as low as 15 and 7 fmol. A more universal approach is the use of indirect UV detection, which permits the analysis of carbohydrates, including (1–2)-linked disaccharides and aldonic acids, at the lower pmol level without the need for derivatization.

Determination of selenium by inductively coupled plasma mass spectrometry utilizing a new hydride generation sample introduction system.

Abstract: An inductively coupled plasma mass spectrometer with a newly designed continuous flow hydride generator was used for the determination of Se in biological materials. The design of the hydride generator was important in minimizing interference from HCl and in maximizing analytical sensitivity. Two sample preparation procedures incorporating either 3.8 or 7.2 M HCl in the final sample solutions were compared. Interference from Cu was eliminated by the addition of 0.2 M NaI to the sodium borohydride solution (3.8 M method) or by maintaining a high concentration of HCl in the sample solution (7.2 M method). The 3.8 M method had the advantage of minimizing exposure of expensive equipment to corrosive HCl fumes, whereas the 7.2 M method did not contaminate equipment with I and had no measurable sample-to-sample cross-contamination. In practice, cross-contamination from sample to sample in both methods was negligible during analysis. An important factor in minimizing cross-contamination from sample to sample was the elimination of the air bubble normally entrained between samples. Determination of isotopic tracer enrichment was linear from 0 to 320% enrichment, which provided a broad range for isotope dilution analysis. A detection limit of 6.4 pg of Se was observed under optimum conditions, whereas a detection limit of 1.3 ng of Se was found for routine analysis of 1-g samples of plant material. Selenium was accurately determined by isotope dilution analysis in a variety of biological reference materials.

Sensitive and selective liquid-chromatographic assay of fluoxetine and norfluoxetine in plasma with fluorescence detection after precolumn derivatization.

Abstract: We determined fluoxetine (Prozac) and its major metabolite norfluoxetine in plasma by liquid chromatography with fluorescence detection. After liquid-liquid extraction from 1 mL of plasma, the extract was derivatized at room temperature with dansyl chloride, and the highly fluorescent derivatives were chromatographed with a reversed-phase C18 column and a mobile phase of phosphate buffer and acetonitrile. Dansylated fluoxetine, norfluoxetine, and the internal standard were eluted in less than 14 min with no interference from endogenous material. The calibration curve was linear over the concentration range 25-800 micrograms/L with inter- and intra-assay imprecision (CV) of less than 10%. Validity of the assay was checked by comparing results for 110 patients' samples with those by a liquid-chromatographic method with ultraviolet detection (r = 0.993 for fluoxetine, 0.957 for norfluoxetine). The identity of the dansylated derivatives was verified by positive chemical ionization mass spectroscopy. The lower limit of detection was approximately 3 micrograms/L. Because no major antidepressant, neuroleptic, or respective drug metabolites interfere with the quantification of fluoxetine and norfluoxetine, this is a useful procedure for pharmacokinetic studies and in clinical settings.

Elimination of the chloride interference on the determination of arsenic using hydride generation inductively coupled plasma mass spectrometry.

Abstract: In the determination of arsenic, attention has recently focused on the speciation of As(III) and As(V). Reversed-phase HPLC can be used to efficiently separate these two arsenic species. When inductively coupled plasma mass spectrometry is used for arsenic detection, an isobaric interference at m/z 75 is caused by the presence of chloride in the sample. These experiments describe the use of hydride generation in conjunction with a polypropylene-membrane gas-liquid separator to completely eliminate the transport of chloride to the plasma. A detection limit of 0.46 ppb for As(III) was achieved with this system. The chromatographic resolution of the system was not compromised by the addition of the gas-liquid separator. A determination of the arsenic content of a NIST urine sample was performed to demonstrate the effectiveness of the chloride elimination.

Specific and sensitive measurement of FK506 and its metabolites in blood and urine of liver-graft recipients.

Abstract: A specific and sensitive assay for quantifying the immunosuppressant FK506 and its metabolites in blood and urine was developed. 32-O-Acetyl FK506 was synthesized and used as internal standard. FK506 and its metabolites were purified from the samples by solid-liquid extraction and were injected into a high-performance liquid chromatographic (HPLC) system linked to a mass spectrometer (MS) by particle-beam interface. The FK506 derivatives were separated from interfering material by use of a 100 x 4 mm C8 analytical column and water/acetonitrile or water/methanol gradient elution; they were detected by negative chemical ionization with methane as reagent gas. The limit of detection was 25 pg in a standard solution, and the limit of quantification in blood was 250 pg (extracted from 1 mL of blood). The CV was 11.3% at 5 ng, and no interferences with other drugs were found.

Measurement of ultratrace levels of chromium by adsorptive-catalytic stripping voltammetry in the presence of cupferron

Abstract: An ultrasensitive adsorptive–catalytic stripping (voltammetric and potentiometric) procedure for determining trace levels of chromium in the presence of cupferron is described. The method offers higher sensitivity than previous adsorptive stripping schemes for chromium. Optimum experimental conditions were found to be a 1 × 10–3 mol l–1 piperazine-N,N′-bis(ethanesulfonic acid)(PIPES) solution (pH 7.0) containing 1 × 10–4mol l–1 cupferron and a preconcentration potential of –0.82 V. A preconcentration time of 1 min results in a detection limit of 1.0 ng l–1. The relative standard deviation at 100 ng l–1 is 2.4%(n= 20, for 20 s preconcentration). The stripping potentiometric scheme allows convenient measurements of chromium in the presence of dissolved oxygen. The identical stripping response for chromium(III) and chromium(VI) solutions makes the method applicable to the measurement of the total chromium content. The simultaneous determination of chromium and uranium is illustrated. The merits of the proposed procedure are demonstrated by the analysis of soil and groundwater samples.

Capillary electrophoretic analysis of flavonoids

Abstract: Combining the effects of electrophoresis and electroendosmosis, flavonoids were separated in less than ten minutes in a fused silica capillary tube with a borate buffer adjusted to pH 10. An increase in the concentration of borate from 0.1 to 0.2 M resulted in longer migration times due to a decrease in electroosmotic flow, but also in improved selectivity and higher resolution of flavonoids. The calibration curve of rutin showed a detection limit of 0.02 mg/mL and linearity over its pharmaceutical concentration range. Using an internal standard of known concentration, the content of rutin in a methanolic extract of Sambuci flos could be determined with a coefficient of variation as small as 3.8% by the molar ratio-peak area ratio method.

Critical evaluation of three analytical techniques for the determination of chromium(VI) in soil extracts

Abstract: Three different analytical techniques [1,5-diphenylcarbazide spectrophotometry, chelating ion-exchange chromatography (Chelex-100), and ion-pairing reversed-phase high-performance liquid chromatography (RP-HPLC) combined with electrothermal atomic absorption spectrometry (ETAAS)] were critically evaluated for the determination of CrVI in soil extracts Spectrophotometry was not applicable to the analysis of most soil extract samples owing to its high limit of detection (LOD = 30 ng cm–3), and the possibility of the instantaneous reduction of CrVI under the acidic conditions employed A Chelex 100 column, although adequately sensitive (LOD = 15 ng cm–3), is inclined to give higher results as inhert and moderately labile CrIII complexes partially passed through the resin together with CrVI In addition, very small particles (<045 µm) carrying chromium can produce severe positive systematic errors In order to avoid this, filtration employing a 01 µm filter is recommended Ion-pairing RP-HPLC was found to be the most sensitive technique (LOD = 03 ng cm–3) It might also give high chromate results if negatively charged CrIII complexes form ion pairs with tetrabutylammonium phosphate and their elution partially coincided with that of CrVI Fulvate ligands showed this type of interference Reversed-phase HPLC is not suitable for analysis of extracts obtained from soils with freshly added tannery waste owing to the effects of the undestroyed tannery waste matrix This study showed that each method investigated was vulnerable to some type of interference