Showing papers on "Detection limit published in 1994"

Limit of detection (LQD)/limit of quantitation (LOQ): comparison of the empirical and the statistical methods exemplified with GC-MS assays of abused drugs.

Abstract: The limit of detection (LOD) for any analytical procedure, the point at which analysis is just feasible, may be determined by a statistical approach based on measuring replicate blank (negative) samples or by an empirical approach, consisting of measuring progressively more dilute concentrations of analyte. The limit of quantitation (LOQ), or concentration at which quantitative results can be reported with a high degree of confidence, may likewise be determined by either approach. We used both methods to determine LOD and LOQ for forensic gas chromatographic-mass spectrometric (GC-MS) analyses of abused drugs. The statistically determined LOD and LOQ values for these assays underestimated the LOD because of the large imprecision associated with blank measurements and the inability of blank samples to meet typical GC-MS acceptance criteria. The empirical method provided much more realistic LOD values, supported by reasonable experimental data, and are 0.5-0.03 the magnitude of the corresponding statistical LODs. The empirical LODs and LOQs are identical for these GC-MS assays. The observations made here about the LOD/LOQ for specific forensic GC-MS procedures are generally applicable to any type of analysis.

Carbon-enhanced inductively coupled plasma mass spectrometric detection of arsenic and selenium and its application to arsenic speciation

Abstract: Addition of carbon as methanol or ammonium carbonate to the aqueous analyte solutions in combination with increased plasma power input enhanced the inductively coupled plasma mass spectrometry (ICP-MS) signal intensities of arsenic and selenium. In the presence of the optimum 3% v/v methanol concentration the signal intensities achieved were about 4500–5000 counts s–1 per ng ml–1 of arsenic and about 700–1100 counts s–1 per ng ml–1 of selenium (82Se), corresponding to enhancement factors of 3.5–4.5 compared with aqueous solution for the two elements. Differences in sensitivity (calculated on the basis of analyte atom) were observed between the individual arsenic species and between the selenium species in aqueous as well as in carbon-added solutions. The presence of 3% v/v methanol in the analyte solutions doubled the level of the background signal for arsenic and selenium, but its fluctuation (noise) was not increased. Therefore, the observed increase in analyte sensitivity led to a similar increase in signal-to-noise ratio. The addition of carbon as ammonium carbonate enhanced the arsenic signal by a similar factor but caused severe contamination of the ICP-MS instrument by carbon. In the 3% v/v methanol solutions of arsenic and selenium the signal intensity from antimony (internal standard) was enhanced by a factor of 1.5, which indicates that the enhancement effect of the arsenic and selenium signals by methanol is only to a limited extent caused by improved sample transport/nebulization efficiency. It is proposed that an increased population of carbon ions or carbon-containing ions in the plasma facilitates a more complete ionization of analytes lower in ionization energy than carbon itself. The enhanced detection power for arsenic was applied to arsenic speciation by high-performance liquid chromatography (HPLC)–ICP-MS, and made possible the detection of the arsenocholine ion (AsC) in extracts of shrimp at the 5–10 ng g–1 concentration level. The limit of detection was improved by a factor of 3.4 after addition of methanol and was 4.7 ng g–1 as the AsC ion.

Online Time-of-Flight Mass Spectrometric Analysis of Peptides Separated by Capillary Electrophoresis

Abstract: Time-of-flight mass spectrometry (TOFMS) is coupled online with capillary electrophoresis (CE) to analyze mixtures of biomolecules using an electrospray ionization (ESI) interface. The eluent is electrosprayed directly from the CE capillary using a thin gold wire to maintain its potential. The ions are extracted at right angles to the initial direction of the ion beam, and a complete mass spectrum is recorded every 100 μs. This CE/ESI-TOFMS apparatus achieves a separation efficiency of 50 000 theoretical plates with a concentration detection limit of (1-2)×10 -6 molar, which corresponds to a mass detection limit of approximately 40-80 fmol per component

Elemental speciation with liquid chromatography–inductively coupled plasma isotope dilution mass spectrometry

Abstract: For the determination of elemental species, which normally exist at low concentrations in the environment, coupling of liquid chromatographic systems with inductively coupled plasma mass spectrometry (ICP-MS) is a powerful method with respect to detection limit. However, accurate results are still a problem in trace analyses but can be obtained by the application of isotope dilution mass spectrometry (IDMS). The developed LC–ICP-IDMS system consists of a high-performance liquid chromatography pump, a sample injection valve, a separation column (different types of chromatographic separation systems, e.g., ion or size-exclusion chromatography, were used depending on the separation problem), a non-destructive detector (e.g. a UV detector) for simultaneous determination of organic substances, and an element-specific ICP mass spectrometer. Isotope dilution is carried out by adding an isotopically enriched species-specific spike solution to the sample prior to the separation step in the case of the determination of well-defined species, or by continuous on-line introduction of a species-unspecific spike solution in the case of species with unknown composition and structure. The species-specific spiking method is demonstrated for the determination of iodide and iodate in mineral water using an ion chromatographic separation column. For example, iodate concentrations in the range of 0.5–20 ng ml–1 could be determined with relative standard deviations of about 2%. The species-unspecific spiking mode is used to determined heavy metal complexes with humic substances at a level of about 1 ng ml–1 as well as organo-iodine species in the concentration range 0.7–1.4 ng ml–1 in natural water systems. The accuracy of speciation could be verified by comparing the total element concentration with the sum of the different elemental species.

Attomole detection of proteins by matrix-assisted laser desorption/ionization mass spectrometry with the use of picolitre vials

Abstract: Significant improvements in the absolute detection limits for proteins in matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) are demonstrated by the application of so-called picolitre vials. By the reduction of the sample volume from a few μL down to 250 pL and simultaneous reduction of the sample spot area from a few mm2 down to 0.01 mm2, low attomole detection limits are obtained for bradykinin and cytochrome c. The detection limit in a single-shot mass spectrum of bradykinin is estimated to be as low as 250 zeptomol. These are currently the lowest amounts of protein and the smallest volumes analyzed by MALDI-MS.

Amperometric biosensor for phenols based on a tyrosinase–graphite–epoxy biocomposite

Abstract: A new biocomposite, based on the incorporation of the enzyme tyrosinase into a graphite-epoxy resin matrix, was used for the effective biosensing of phenolic compounds. The enzyme retains its bioactivity on confinement in the epoxy resin environment. This renewable (polishable) and rigid bioprobe offers convenient quantification for various phenolic substrates. The fast response (steady-state time = 25 s) accrues from the close proximity of the enzyme and graphite sites. The influence of various experimental variables was explored for optimum biosensing performance. Flow-injection monitoring of phenolic compounds at a rate of 50 samples h–1 yielded a detection limit of 1×10–6 mol l–1 and a relative standard deviation of 1.4%(n= 40).

Quantitative Measurement of Cyclosporin A in Blood by Time-of-Flight Mass Spectrometry

Abstract: Analytical methods have been developed for the detection and quantification of cyclosporin A (CsA) in blood using time-of-flight secondary-ion mass spectrometry (TOF-SIMS) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI/TOF-MS). Linear calibration curves were obtained for both methods in the range 25-2000 ng/mL in whole blood. The limit of detection (LOD) and limit of quantification (LOQ) were determined to be 7 and 23 ng/mL, respectively, for the TOF-SIMS method; the LOD and LOQ were 10 and 33 ng/mL, respectively, for the MALDI/TOF-MS method. Coefficients of variation ranged from 3 to 5% for the TOF-SIMS method and from 4 to 8% for the MALDI/TOF-MS method. Blood samples were also analyzed by a nonspecific method, fluorescence polarization immunoassay (FPIA), and by a specific method, high-performance liquid chromatography (HPLC). The TOF-MS results are in good agreement with the HPLC results, but not with the FPIA results. The TOF-MS methods can also provide information about CsA metabolites.

Determination of carbofuran by on-line immunoaffinity chromatography with coupled-column liquid chromatography/mass spectrometry.

Abstract: Development of a new technology for combined on-line sample preparation and analyte confirmation is presented. This technology involves the use of antibodies for trace analyte extraction and enrichment directly from a complex matrix. The antibodies are used in columns designed for use with ordinary high-pressure liquid chromatographic equipment. The immunoaffinity columns are combined with conventional reversed-phase LC columns by use of column-switching techniques and coupled directly to an atmospheric pressure ionization quadrupole ion trap mass spectrometer equipped with a pneumatically assisted electrospray (ion spray) interface. A column of aldehyde-activated silica was used to prepare a column specific to carbofuran. This column demonstrated excellent specificity toward carbofuran and showed no binding of another, unrelated compound, fluometuron. Direct extraction and detection of carbofuran was demonstrated at low levels (40 pg/mL) in spiked water, but the real utility of immunoaffinity chromatography (IAC) is demonstrated by the on-line extraction and detection of carbofuran from a chemically complex, crude potato extract. Samples extracted using an IAC column indicate that superior purification is obtained with IAC in comparison with samples pumped directly onto a reversed-phase trapping column. A detection limit for carbofuran of approximately 2.5 ng/g of potato was obtained using an atmospheric pressure ionization quadrupole ion trap mass spectrometer.

Characterization of proteins by capillary electrophoresis in fused-silica columns: review on serum protein analysis and application to immunoassays.

Abstract: Protein mixtures can be characterized in terms of their separations by capillary electrophoresis (CE). The separation of proteins by CE is performed in untreated fused-silica columns. Model proteins and complex protein mixtures with pI values ranging from 4.0 to 11.0 are separated in such columns in less than 10 min in the presence of phosphate buffer with a pH between 4.0 and 9.0. The application of CE separation procedures for routine analysis of protein in serum, urine, and cerebrospinal fluid in borate-based buffer is also demonstrated. The detection of protein in CE is usually based on the intrinsic ultraviolet (UV) absorbance of the peptide bond at or near 200 nm, which provides a detection limit of about 10(-5) M. The same protein separation procedures can also be applied to immunochemical reaction systems in which one component is labeled. Thus, an antigen analyte, or the antibody to the analyte, may be labeled with a fluor and detected by laser-induced fluorescence (LIF). With a fluorescent-labeled reactant, the use of LIF detection further extends the detection limit to 10(-11) M. The CE separation technique for proteins provides a means to separate the bound and free species of the labeled antigen or antibody without the use of a solid support. The application of these separation techniques in conjunction with laser-induced fluorescence detection to make possible the homogeneous immunochemical measurement of species at concentrations in the range of 10(-9) to 10(-10) M is shown.

Continuous automated determination of atmospheric formaldehyde at the parts per trillion level

Abstract: 1,3-Cyclohexanedione was used for automated near real-time fluorometric determination of atmospheric formaldehyde. Although previous literature reports indicate this reagent to be nonselective, adaptation of a fast flow injection technique that does not rely on the completion of the reaction provided a method with a selectivity factor of >300 for HCHO over its nearest congener, CH 3 CHO. The system was optimized with respect to the reagent and scrubber liquid composition, reaction temperature, and pH. The limit of detection (LOD) in the aqueous phase is ∼6 nM (180 ng/L) HCHO for a 45-μL sample (270 fmol) with the linear dynamic range spanning >3 orders of magnitude, up to at least 14 μM

Novel, reagentless, amperometric biosensor for uric acid based on a chemically modified screen-printed carbon electrode coated with cellulose acetate and uricase

Abstract: Amperometry in stirred solution has been used for the systematic evaluation of modified screen-printed carbon electrodes (SPCEs) with a view to developing a reagentless biosensor for uric acid. The developed system consists of a base cobalt phthalocyanine (CoPC) electrode tailored to the electrocatalytic oxidation of H2O2 by means of a cellulose acetate (CA)–uricase bilayer. Uricase was immobilized by drop-coating the enzyme onto the CA membrane covering the CoPC-SPCE. The device exploits the near-universal H2O2-generating propensity of oxidases, the permselectivity of the CA film towards H2O2 and the electrocatalytic oxidation of this product at the CoPC-SPCE. The electrochemical oxidation of the resulting Co+ species was used as the analytical signal, facilitating the application of a greatly reduced operating potential when compared with that required for direct oxidation of H2O2 at unmodified electrodes. The time required to achieve 95% of the steady-state current (t95iSS) was 44 s [relative standard deviation = 7.5%(n= 10)]. Amperometric calibrations were linear over the range from 13 × 10–6 to 1 × 10–3 mol dm–3, with the former representing the limit of detection. The CA membrane extended the linear range of the biosensor by over two orders of magnitude, when apparent Michaelis–Menten constants (Km′) of immobilized and free enzymes are compared. This suggests that the process is diffusion-controlled and not governed by the kinetics of the enzyme. The precision of electrode fabrication was determined by cyclic voltammetry to be 4.9%(n= 6). Successive amperometric calibrations (n= 7) over 7 d using a single sensor revealed only a 14.0% diminution in sensitivity from the original response. Sensor stability and the dependence of the steady-state current on the pH, ionic strength and temperature of the supporting electrolyte were studied and the results are presented. The functioning of the biosensor is indifferent to a wide range of potential interferences studied in a synthetic sample and results correlate favourably with those of a standard hospital method.

Iodine determination in biological materials by ICP-MS

Abstract: The present work describes an analytical procedure for the determination of the total iodine content in biological materials (serum, milk, plants, tissues etc.). Liquid samples can be directly analyzed after dilution, if necessary, by ICP-MS. Milk powder, plants and tissues were dissolved by using a modified simple Schoninger combustion, subsequently the residue was taken up in 0.1 mol/l NaOH and this solution was analyzed by ICP-MS. The detection limit is in the range of 0.01 μg/l. The method was tested using different CRMs certified for the iodine content.

Analytical investigations of tree rings by laser ablation ICP-MS

Abstract: Laser sampling inductively coupled plasma mass spectrometry has been used for the quantitative determination of trace elements to characterize annual growth rings of trees. The elements studied are Mg, Al, Ca, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Cd, Ba, Tl, Pb, Bi and U. Cellulose mixed with multielement standard solution pressed into pellets were used for the calibration. Using internal standardization with 12C the precision is typically better than 10% relative standard deviation. The limits of detection were limited by the purity of the cellulose. Analytical figures of merit and the analytical results are discussed in terms of environmental analysis.

Laser-Based Particle-Counting Microimmunoassay for the Analysis of Single Human Erythrocytes

Abstract: A particle-counting immunoassay system for ultrasensitive analysis of proteins in a capillary environment has been developed. The assay is based on the agglutination of antibody-coated particles in the presence of an antigen (usually a protein). The particles were electrophoretically migrated in a 20-[mu]m-i.d. capillary past a detection window where a laser beam irradiates continuously. The light scattering events generated by the agglutinated particles were counted while those produced by unreacted particles were electronically rejected. Glucose-6-phosphate dehydrogenase (G6PDH) was chosen as a test compound for the off-column as well as for the on-column versions of this method. A limit of detection of 620 molecules of G6PDH (1 zmol) was found in the on-column assay. The standard deviation between runs was approximately 6%, which is comparable to that of standard immunoassay methods. The application to the determination of G6PDH levels in individual human erythrocytes is presented. A 14-fold cell-to-cell variation was found which can be explained by the age distribution in the red blood cells. 42 refs., 5 figs.

Communication. A simple laser-induced fluorescence detector for sulforhodamine 101 in a capillary electrophoresis system: detection limits of 10 yoctomoles or six molecules

Abstract: High sensitivity fluorescence detection was generated for capillary electrophoresis based on a post-column sheath-flow cuvette. An inexpensive 8 mW He–Ne laser operating at 594 nm was used for excitation of Sulforhodamine 101. This dye, which is commonly used for both protein labelling and deoxyribonucleic acid (DNA) analysis, has molar absorptivity that is matched to the laser wavelength. Collection efficiency was doubled compared with previous work by using two microscope objectives, one located on each side of the illuminated sample stream. A series of injections of Sulforhodamine 101 was performed; the average detection limit (3σ) was 6 ± 1 molecule of dye injected onto the capillary. These injections ranged from 250 to 17 molecules of dye. Both the detection limits and the amount of injection are the lowest ever reported in capillary electrophoresis. The mass sensitivity of this detector is at least two orders of magnitude lower than the best on-column capillary electrophoresis fluorescent detector.

Analyses of Organic and Inorganic Mercury by Atomic Fluorescence Spectrometry Using a Semiautomatic Analytical System

Abstract: The present work describes an improved method for the determination of total and organic mercury by decreasing analysis time (use of a dual circuit), sample size, and handling procedures and eliminating the need for a clean room condition while maintaining the detection limit below the parts per trillion (pptr) level. Total mercury was analyzed after reduction by a one-stage gold amalgamation before transfer to an atomic fluorescence spectrometer. The detection limit was 0.07 ng L -1 for aqueous samples and 1 ng g -1 for biological tissue. Analysis time was 5 min. Organic mercury was first derivatized using sodium tetraethylborate and then trapped onto a graphitized carbon trap before transfer to an isothermally controlled GC column coupled to a cold vapor atomic fluorescence spectrometer