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PubMedDate not supplied

Effect of lipophylic salts on ise detection limit: application to calixarene-based highly efficient potassium-selective electrodes.

A new interpretative model for interpolation of potentiometric data from liquid membrane ion selective electrodes has been formulated; the model allows one to achieve information about partitioning equilibria establishing at the membrane-analysed solution interface of hydrophilic cations present in the membrane as counterions of exchanger lipophylic anions. The fundamental importance of such equilibria is related to their contribution to the detection limit of the electrodes. The soundness of the proposed model was positively verified by employing it in the characterization of new K(+)-selective electrodes based on a calixarenic ionophore. Although the validity of interpolation based on our model does not significantly differ from that obtained with the simpler model providing the inclusion of a constant parameter in the logarithmic argument of Nicolski-Eisenman's equation, the information obtained resulted to be analytically suitable for optimising the membrane composition and, as a consequence, the performance of the electrodes.

Calixarenes↗
PubMed1995

Assessment of data sets containing a considerable number of values below the detection limits.

Using real examples it is demonstrated that the concentrations of non-essential substances in biological systems always follow a statistical log-normal distribution. This is based on the assumption that in a first-order approximation the rate of excretion of the quantity of unwanted molecules or atoms in the biological system is only dependent on their concentration. Applying a natural logarithmic transformation, the resulting log-normal distribution always has a standard deviation of about 1, as shown by statistical analysis of more than 680 clusters of analytical results. Assuming a log-normal distribution with a standard deviation of 1, it is possible to derive factors for the estimation of percentiles, such as the median, even when this percentile is below the detection limit; quantifiable and higher percentiles only must be multiplied by a specific factor. This new method has the advantage that no arbitrary assumptions must be made about artificial parameters such as the numerical treatment of non-detectable analytical data. The assumptions used for the calculation of the factors are justified by proof with experimental data. The applicability of the new method is demonstrated for various contaminants in various plants and animals.

Animals↗
PubMed2004

Optimizing detection limits for the analysis of petroleum hydrocarbons in complex environmental samples.

To evaluate the sources, transport, bioremediation, fate, and effects of spilled petroleum and petroleum products, environmental studies often measure parent and alkylated polycyclic aromatic hydrocarbons (PAH), alkanes, and chemical biomarkers (e.g., triterpanes). Accurate data for low analyte concentrations are required when environmental samples contain hydrocarbons from multiple sources that need to be resolved and quantified. The accuracy and usefulness of the analyses can be improved by lowering the method detection limits (MDLs) for these compounds. Misidentification of hydrocarbon source can result when the MDLs are too high. Modifications to standard analytical methods (i.e., U.S. Environmental Protection Agency Method 8270) can lower MDLs by factors ranging from 10 to 1000. This reduction has important implications for ecological-risk assessments. Modifications having the greatest impact on the MDL include GCMS analysis in the selected-ion-monitoring mode (SIM), increased sample size, column cleanup of the extract, and decreased preinjection volume (volume of final extract prior to injection into instrument). In one study in which a benthic sediment sample was spiked with low levels of topped (heated to remove more volatile PAH that are naturally enriched in crude oil) Alaska North Slope crude, MDLs for individual PAH analytes and biomarkers were determined to be less than 0.5 ng/g (ppb) dry weight and less than 5 ppb dry weightfor individual alkanes. Similar results were obtained when the sediment was spiked with the 16 EPA priority pollutants. In addition, a method has been developed to estimate MDLs for source-specific alkylated PAH analytes and chemical biomarker compounds for which standards are not commercially available or are prohibitively expensive. These improved analytical techniques have been used to identify and quantify low levels of hydrocarbons, derived from both natural and anthropogenic sources, found in the benthic sediments of Prince William Sound, AK.

Environmental Monitoring↗
PubMed1978

Some considerations on quantitative methodology and detection limits in organic mass spectrometry.

Mass spectrometric sensitivity data for the compounds cholesterol, cholestane, DDT, decafluorotriphenylphosphine, morphine, LSD and methyl stearate have been developed over a period of time and under a variety of conditions. This information is presented on both a relative and an absolute basis. With the exception of morphine, relative sensitivies were reproducible to within a factor of three over a six month period. This reproducibility facilitates quantitation and a single standard can be used for calibration of organic compounds from various classes. Detection limits of 50 femtograms for LSD are realized for samples introduced via the direct inlet probe.

Cholestanes↗
PubMed2006

Perchlorate in water via US Environmental Protection Agency Method 331 Determination of method uncertainties, lowest concentration minimum reporting levels, and Hubaux-Vos detection limits in reagent water and simulated drinking water.

US Environmental Protection Agency (EPA) Method 331 determines perchlorate in drinking water using non-suppressed ion chromatography with tandem mass spectrometry. This study reports the results of calibration and recovery studies in reagent water, as well as of a recovery study in simulated drinking water (i.e., total dissolved solids are 500 mg/mL each of chloride, sulfate, and bicarbonate). The perchlorate concentrations in the study ranged from 0.05 to 64 ng/mL. At 95% confidence, the Hubaux-Vos detection limit (H-V DL) was 0.04 ng/mL for the calibration study and the simulated-drinking-water recovery study, and 0.03 ng/mL for the reagent-water recovery study. The lowest concentration minimum reporting level was 0.03 ng/mL for reagent water and 0.0 7 ng/mL for simulated drinking water, again at 95% confidence.

Calibration↗
PubMed2006

A new approach to determine method detection limits for compound-specific isotope analysis of volatile organic compounds.

Compound-specific isotope analysis (CSIA) has been established as a useful tool in the field of environmental science, in particular in the assessment of contaminated sites. What limits the use of gas chromatography/isotope ratio mass spectrometry (GC/IRMS) is the low sensitivity of the method compared with GC/MS analysis; however, the development of suitable extraction and enrichment techniques for important groundwater contaminants will extend the fields of application for GC/IRMS. So far, purge and trap (P&T) is the most effective, known preconcentration technique for on-line CSIA with the lowest reported method detection limits (MDLs in the low microg/L range). With the goal of improving the sensitivity of a fully automated GC/IRMS analysis method, a commercially available P&T system was modified. The method was evaluated for ten monoaromatic compounds (benzene, toluene, para-xylene, ethylbenzene, propylbenzene, isopropylbenzene, 1,2,3-trimethylbenzene, 1,2,4-trimethylbenzene, 1,3,5-trimethylbenzene, fluorobenzene) and ten halogenated volatile organic compounds (VOCs) (dichloromethane, cis-1,2-dichloroethene, trans-1,2-dichloroethene, carbon tetrachloride, chloroform, 1,2-dichloroethane, trichloroethene, tetrachlorethene, 1,2-dibromoethane, bromoform). The influence of method parameters, including purge gas flow rates and purge times, on delta13C values of target compounds was evaluated. The P&T method showed good reproducibility, high linearity and small isotopic fractionation. MDLs were determined by consecutive calculation of the delta13C mean values. The last concentration for which the delta13C value was within this iterative interval and for which the standard deviation was lower than +/-0.5 per thousand for triplicate measurements was defined as the MDL. MDLs for monoaromatic compounds between 0.07 and 0.35 microg/L are the lowest values reported so far for continuous-flow isotope ratio measurements using an automated system. MDLs for halogenated hydrocarbons were between 0.76 and 27 microg/L. The environmental applicability of the P&T-GC/IRMS method in the low-microg/L range was demonstrated in a case study on groundwater samples from a former military air field contaminated with VOCs.

Journal Article↗
PubMed1984

Optimized acquisition parameters and statistical detection limit in quantitative EELS.

Optimizing the acquisition parameters for EELS recording has to be accomplished simultaneously from the physical and the statistical points of view; the statistical aspect of the question is covered here. Approximate probability density functions of the variables of interest are derived, which provide a global measure of signal-to-noise ratio taking into account every step of the EELS edge area estimation process. Qualitative and quantitative advice is given regarding the critical choice of the estimation and integration energy regions. The notion of visual contrast is presented; it permits the introduction of the concept of statistical detection limit. It is found that for typical experimental conditions, when other factors are equal, the required analysis time for the sample varies approximately as the inverse square of the concentration.

Electron Probe Microanalysis↗
PubMed2005

Reduction of detection limits of the direct deposition GC/FT-IR interface by surface-enhanced infrared absorption.

Even though the interface of gas chromatography (GC) and mass spectrometry (MS) is by far the most popular way of identifying molecules eluting from a GC in real time, the identification of compositional isomers by MS is equivocal at best. Much better results would be found by infrared spectrometry (IR) if the sensitivity of the GC/IR interface could be improved. In this paper, we show how the smallest quantity of molecules injected into a GC for which an identifiable infrared spectrum can be measured on-line has been reduced by a factor of 10 below the detection limit of the most sensitive current technique. A commercial direct deposition interface between a GC and a Fourier transform infrared spectrometer was modified by vapor-depositing an island film of silver on the surface of the zinc selenide substrate. Band intensities in the spectra of molecules located within approximately 4 nm of the surface of the silver islands were increased by at least 1 order of magnitude through surface-enhanced infrared absorption (SEIRA). The effectiveness of this approach was illustrated by comparing the limits of identification of butylbenzene isomers measured with and without the silver film. Comparison with the spectra of the same molecules measured by mass spectrometry showed the increased sensitivity and specificity of the GC/SEIRA interface.

Journal Article↗
PubMed2003

Improved selectivity and detection limit of the carbonate-selective electrode.

The properties of the carbonate neutral carrier 4-( n-hexadecyl)-3-nitro-1-trifluoroacetylbenzene were compared with those of a similar carrier, without a nitro group, studied previously. In spite of differences in the Hammett constant of the carbonyl group responsible for interaction with the analyte, the analytical characteristics of both carriers, measured under the same conditions, were comparable. Special care was taken to avoid the presence of an excessive carbon dioxide level in the diffusion layer at the membrane-solution interface. The internal reference solution was prepared without carbonate components; the external solution was protected from contact with atmospheric carbon dioxide. Under such conditions the detection limit of both electrodes was extended to 10(-11 )mol L(-1), and the selectivity towards salicylate, chloride, and acetate was significantly improved.

Journal Article↗
PubMedDate not supplied

Effect of different binding proteins on the detection limits and sensitivity of assays based on biotinylated adenosine deaminase.

The properties of binding proteins that control the nature and magnitude of inhibition of the enzyme-ligand conjugates in homogeneous enzyme-linked competitive binding assays were investigated. An assay for biotin that employed adenosine deaminase as the enzyme-label was used as a model system because of the availability of several biotin-specific binders with different characteristics. It was found that the association constant between the ligand and the binding protein, as well as the depth of the binding pocket, affect the response characteristics of the assay. In addition, the reported data suggest that steric-hindrance effects also contribute toward the sensitivity and detection-limit capabilities of the assay.

Adenosine Deaminase↗
PubMed1986

Molecular calibration in flow cytometry with sub-attogram detection limit.

A monoclonal antibody to complement receptor type 1 (CR1) has been double labelled with 125iodine and fluorescein isothiocyanate (FITC) and used for parallel radioimmune assay (RIA) and flow cytometric studies. Serial dilutions of the reagent with unlabelled antibody were used to obtain a linear correlation between radioactivity counts and fluorescence over the whole range of labelled antibody molecules bound per cell, namely 700-25,000. This has enabled us to calibrate our flow cytometer in terms of numbers of molecules per channel as opposed to the customary arbitrary fluorescence units. Normal human granulocytes from a single donor were found to have a mean of 25,000 CR1 molecules per cell with a normal distribution and standard deviation of 4775 molecules. The detection limit of the flow cytometer was 730 CR1 molecules per cell which is equivalent to between 125 and 150 molecules of free fluorescein. This represents less than 0.1 attogram (ag) of fluorescein per cell, where 1 ag = 10(-18) g. The resolution was 44 CR1 molecules per digitization step of the analogue-to-digital converters (ADC) which corresponds to the fluorescence from less than ten molecules of free fluorescein.

Calibration↗
PubMedDate not supplied

Synthesis, spectral properties, and detection limits of reactive squaraine dyes, a new class of diode laser compatible fluorescent protein labels.

We describe the synthesis and spectral characterization of two reactive long-wavelength fluorescence labels (Sq635-m and Sq635-b), having either one or two N-hydroxysuccinimidyl esters. Both are squaraine derivatives and consist of a cyanine-type chromophore and a central squarate bridge. To improve water solubility, we introduced two sulfonic acid groups into the heterocyclic ring systems, and for covalent attachment to proteins, a reactive N-hydroxy-succinimide ester (NHS ester) was synthesized. The squaraine markers exhibit low quantum yields in water (phi = 0.15) and high quantum yields (phi = 0.6-0.7) when bound to proteins. The absorption maxima at 635 nm in water and at approximately 645 nm when bound to proteins allow excitation with commercially available diode lasers. The detection limit of a representative squaraine dye in blood was estimated to be half that of a commonly used fluorophore.

Antibodies↗
PubMed2000

Reaction rate, activation energy, and detection limit for the reaction of 5-furoylquinoline-3-carboxaldehyde with neurotransmitters in artificial cerebrospinal fluid.

The fluorogenic reagent 5-furoylquinoline-3-carboxaldehyde (FQ) has proven valuable in the analysis of proteins and small neurotransmitters. We monitored the reaction rate between this reagent and five neurotransmitters at 40 degrees and 65 degrees C in artificial cerebrospinal fluid. The reactions followed pseudo-first order kinetics. The activation energy for the reaction of FQ was 10.6, 10.7, 22.0, 31.4, and 34.4 kJ mol(-1) for alanine, taurine, gamma-aminobutyric acid (GABA), glutamine, and glutamic acid, respectively. At 65 degrees C, the reaction rate was quite similar for alanine, taurine, glutamine, and glutamic acid (1.8 x 10(-3) s(-1)) but was twice as fast for GABA. A reaction time of nearly 1 h was required to quantitatively convert these neurotransmitters to their fluorescent products at 65 degrees C. Detection limits for the labeled neurotransmitters were 10(-9)-10(-8) M, which corresponded to 0.3-7 amol injected onto the capillary.

Animals↗
PubMed2004

Ferrocene-containing polyelectrolyte multilayer film-covered electrodes: electrocatalytic determination of ascorbic acid and use of inner blocking layers to improve the upper detection limit of the electrodes.

A multilayer film composed of ferrocene(Fc)-appended poly(allylamine hydrochloride) (Fc-PAH) and poly(potassium vinylsulfate) (PVS) has been prepared on the surface of a gold(Au) electrode by using a layer-by-layer self-assembly technique. Fc-containing polyelectrolyte multilayer (PEM) film-modified electrodes can electrochemically catalyze the oxidation of ascorbic acid successfully. For a 2 (Fc-PAH/PVS) bilayer-covered electrode the catalytic current increased linearly with increasing concentration of ascorbic acid over the concentration range 6 micromol L(-1)-3 mmol L(-1). To extend the dynamic range for ascorbic acid, the surface of the Au electrode was first covered with a (PAH/PVS)(2) film on which an additional (Fc-PAH/PVS)(5) film was coated. This strategy successfully extended the dynamic range of the electrode up to 25 mmol L(-1) ascorbic acid, because the (PAH/PVS)(2) layer blocked access of ascorbic acid to the electrode surface. The upper detection limit of the (PAH/PVS)(2) (Fc-PAH/PVS)(5) film-modified electrode is much higher than those of Fc-based ascorbic acid sensors reported so far. Electron transfer is diffusion-controlled within the (PAH/PVS)(2)(Fc-PAH/PVS)(5) film.

Ascorbic Acid↗
PubMed2006

Reliability of the cloned-enzyme donor immunoassay (CEDIA) for cocaine in human serum in the range between the detection limit and the cut-off.

Immunoassays are used worldwide for the rapid screening of drugs. Despite the fact that they are highly valuable tools for the testing of legal and illicit drugs, there is a non-negligable risk of false-positive and false-negative findings and many pitfalls must be taken into account when using these tests in an uncritical manner and without valid confirmation procedures. In order to check the correlation between cloned-enzyme donor immunoassay (CEDIA) readings and exact determined gas chromatography/mass spectrometry (GC/MS) values for benzoylecgonine, a total of 472 serum samples was measured with an immunoassay (CEDIA) as well as GC/MS. As a result, it was shown that in the lower area of concentration, up to approx. 300 ng equ. benzoylecgonine/ml, there is a semiquantitative useful correlation. With higher concentrations, the values shift more and more apart and many runaways are visible. Generally the use of tests for materials other than those declared is excluded from the manufacturer's warranty and is at the user's own risk, but the authors found that many immunochemical tests with a certification only for the use in urine are well suited to the screening of serum samples, and even low readings could be confirmed by GC/MS. With blood or serum as sample, it could be regularly observed that generally very small readings could not only be confirmed with mass spectrometry but could in most cases also be quantified. This was the key factor in determining the rate of confirmation in 495 cases. In total, an incredibly high reliability for the CEDIA cocaine test (via benzoylecgonine) was shown up to the area of detection (detection limit), far below the cut-off. These results confirm the extraordinary sensitivity and specificity of this test also for blood or serum as sample material to be tested.

Cocaine↗
PubMed2000

Development of a creatinine ELISA and an amperometric antibody-based creatine sensor with a detection limit in the nanomolar range.

Creatinine-specific antibodies have been generated and used for highly sensitive and specific immunochemical creatinine determinations. Creatinine was derivatized at N3 and coupled to KLH carrier protein. On the basis of this immunogen, monoclonal antibodies were developed by hybridoma technology. Antibodies from various clones have been characterized with BIAcore 2000 with respect to the dissociation constant and specificity. Antibodies of clone B90-AH5 exhibited the lowest dissociation constant (0.74 microM) and the highest specificity for creatinine and were chosen for the development of a competitive ELISA and an amperometric creatinine sensor. The creatinine sensor was constructed by fixing a creatinine-modified membrane on the top of a platinum working electrode which was then incorporated into a stirred electrochemical measuring cell. For creatinine determination the creatinine-containing sample was incubated with B90-AH5 and anti-IgG(mouse)-glucose oxidase conjugate and applied to the measuring cell. After a washing step glucose was added and the produced hydrogen peroxide was registered at Eappl = +600 mV vs Ag/AgCl. The measuring range was 0.01-10 microg/mL. The highest sensitivity for creatinine was achieved at 330 ng/mL (3 microM) and the lower detection limit at 4.5 ng/mL (40 nM). This is far below the relevant clinical range, which is 5-17 microg/mL (44-150 microM) and allows a reliable determination of very low creatinine concentrations in serum, where standard methods cannot be applied. After each measurement the sensor was regenerated with 10 mM HCl without any loss in binding activity.

Antibodies, Monoclonal↗
PubMed1987

Improved detection limits in an organic mass spectrometer using a combination of matrix free FAB and photodiode array detection.

Scanning mass spectrometers suffer from the disadvantage of monitoring only one mass at any particular time in contrast to mass spectrographs which allow the simultaneous detection of an extended mass range. Historically such mass spectra were recorded by directing the ion beam onto a photographic plate which was subsequently developed and interpreted. The introduction of electronic devices such as the photodiode array enabled the development of imaging detectors capable of real time readout, so that a range of masses can be detected and rapidly processed using a computer system. This work uses such a detector in combination with a double focusing MS50RF mass spectrometer for the parallel detection of a limited mass range.

Mass Spectrometry↗