Relative Standard Deviation Of Measurements Research Articles

Synthesis and analytical application of a novel tetradentate N 2O 2 Schiff base as a chromogenic reagent for determination of nickel in some natural food samples

A novel sensitive chromogenic reagent, N, N′-bis(3-methylsalicylidene)- ortho-phenylene diamine (MSOPD), has been synthesized and used in the spectrophotometric determination of nickel. At pH 8, MSOPD can react with nickel ion at room temperature to form a 1:1 complex. The apparent molar absorptivity is 9.5 × 10 4 l mol −1 cm −1 at 430 nm. Beer's low is obeyed over the range 0–1.0 × 10 −5 M of nickel with a detection limit of 1.36 × 10 −8 M. The relative standard deviation for measurement of 3.41 × 10 −6 M nickel is 1.3% ( n = 10). The method has successfully been applied to determination of trace amounts of nickel in some natural food samples.

Simplification and evaluation of a gold-deposited SPR optical fiber sensor.

The structure of the sensing element of a gold-deposited optical fiber sensor was simplified and quantitative analyses of various alcohols with the sensor showed improvement of the performance. The sensor uses surface plasmon resonance (SPR) at the interface of a sample solution and a thin (10 - 70 nm) gold film deposited on half of the exposed core of the optical fiber. The sensor with a film thickness of 45 nm can detect a small change of 5.6 x 10(-5) refractive index (RI) units in the refractivity. The response time is less than 0.5 min and the relative standard deviation for measurements is less than or equal to 1%. A straight line with a correlation coefficient of 0.9995 was obtained below 10%, v/v in the calibration curve for methanol solutions of benzyl alcohol. The minimum of the response curve due to the maximum excitation of SPR in the refractivity range from 1.33 to 1.44 RI units shifts to a lower refractivity as the film becomes thicker. The response curves of the sensors were calculated from SPR theoretical equations while considering of the distribution in the thickness of the deposited gold films. The improvement in the performance of the sensor is discussed.

A comparative evaluation of accelerated solvent extraction and Polytron extraction for quantification of lipids and extractable organochlorine in fish

Optimal conditions for extraction methods using an accelerated solvent extractor (ASE) and a Polytron homogenizer were established for quantification of (crude) lipids and extractable organochlorine (EOCl) in fish. The two methods, an ASE consecutive extraction at 55 and 100 °C for ground, freeze-dried filets, and a Polytron extraction at room temperature with four repetitions for ground, wet filets and with two repetitions for wet fish livers were evaluated in terms of reproducibility and comparability. With respect to lipid measurement, the two methods have comparable reproducibility and the experimental errors are relatively small compared to natural variations of fish to fish within a sampling site. For EOCl measurement, the relative standard deviation tends to increase with decreasing levels of EOCl in specimens. An important factor for the relative standard deviation of measurements of analytes present at trace levels in fish matrices is sampling error in taking aliquots. Statistical analysis using a paired comparison design showed that measurement values (crude lipids or EOCl) obtained from these two extraction methods are, in general, not equivalent. There is a good correlation between the two methods for lipid measurement; whereas for EOCl measurements, high degrees of correlation exist within the exposed fish group, but not in the reference group.

Simultaneous Determination of Residues of Chloramphenicol, Thiamphenicol, Florfenicol, and Florfenicol Amine in Farmed Aquatic Species by Liquid Chromatography/Mass Spectrometry

A liquid chromatographic (LC)/mass spectrometric (MS) method was developed for determining the residues of chloramphenicol, thiamphenicol, florfenicol, and florfenicol amine in a number of aquatic species. The phenicols are extracted with acetone, the extracts are partitioned with dichloromethane, the aqueous layer is removed, and the organic layer is evaporated to dryness. The residue is dissolved in dilute acid and defatted with hexane, and the aqueous layer is prepared for analysis by LC. The phenicols are determined by reversed-phase LC by using a Hypersil C18-BD column with a water-acetonitrile gradient and MS detection using selected-ion recording. Calibration curves were linear for all analytes between 0.015 and 0.425 ng injected. The relative standard deviations for measurements by the proposed method were < 10% for all of the analytes studied, with recoveries ranging from 71% for florfenicol amine to 107% for florfenicol in salmon tissue spiked at the 2 ng/g level. Detection limits of 0.1 ng/g for florfenicol and chloramphenicol, 0.3 ng/g for thiamphenicol, and 1.0 ng/g for florfenicol amine are easily obtainable. The operational errors, interferences, and recoveries for spiked samples compare favorably with those obtained by established LC methodology. The proposed method is simple, rapid, and specific for monitoring residues of chloramphenicol, thiamphenicol, florfenicol, and florfenicol amine in a number of aquatic species.

Optical photothermal detection in HPLC.

A mode-mismatched parallel dual-beam thermal lens spectrometer with a far-field single-channel detector system was used as a detector in HPLC. An expert estimation of the measurement results was applied to optimize the optical-scheme configuration of the spectrometer to achieve the longest linear calibration range and highest repeatability under chromatographic flow conditions. Chelates with 4-(2-pyridylazo)resorcinol were separated and determined with the limits of detection of n x 10(-8)- n x 10(-7) mol L(-1); the relative standard deviation of measurements was 46%. Xylenol Orange, 4-(2-thiazolylazo)resorcinol, and dithizone were studied as post-column reagents in thermal lens detection in ion chromatography. The limits of detection were n x 10(-8)- n x 10(-7) mol L(-1); the linear calibration ranges were about three orders; the relative standard deviation of measurements was 3-7%. A combined photothermal-refractometric detector for HPLC based on a polarization interferometer is proposed. Metal complexes as 4-(pyridylazo)resorcinol chelates (limits of detection of n x 10(-8)- n x 10(-7) mol L(-1)) and sugars (limits of detection of 10-20 ng L(-1)) were investigated as model substances. Obtained results were compared with results for traditional detectors, which show that photothermal detection has higher sensitivity than photometric and other absorption detectors.

Evaluation and application of bismuth as an internal standard for the determination of lead in wines by simultaneous electrothermal atomic absorption spectrometry.

A method for the direct determination of Pb in wines by simultaneous multi-element atomic absorption spectrometry (SIMAAS) using a transversely heated graphite atomizer, Zeeman-effect background corrector and internal standardization is proposed. Bismuth was used as an internal standard and Pd(NO3)2 plus Mg(NO3)2 as chemical modifier to stabilize both the analyte and the internal standard. The implementation of two pyrolysis steps avoided any build-up of carbonaceous residues on the graphite platform. All diluted samples (1 + 1 v/v) in 0.2% v/v HNO3 and reference solutions (5.0-50 microg l(-1) Pb in 0.2% v/v HNO3) were spiked with 25 microg l(-1) Bi. For a 20 microl aliquot dispensed into the graphite tube, a good correlation (r = 0.9997) was obtained between the ratio of the analyte signal to the internal standard signal and the analyte concentration of the reference solutions. The electrothermal behaviour of Pb and Bi in red, white and rosé wines were compared. In addition, absorbance variations due to changes in experimental conditions, such as atomizer temperature, integration time, injected sample volume, radiation beam intensity, graphite tube surface, dilution and sample composition, were minimized by using Bi as internal standard. Relative standard deviations of measurements based on integrated absorbance varied from 0.1 to 3.4% and from 0.5 to 7.3% (n = 12) with and without internal standard correction, respectively. Good recoveries (91-104%) for Pb spikes were obtained. The characteristic mass was 45 pg Pb and the limit of detection based on integrated absorbance was 0.9 microg l(-1) Pb. Internal standardization increased the lifetime of the tube by 25%. Direct determinations of Pb in wines with and without internal standardization approaches were in agreement at the 95% confidence level. The repeatability and the tube lifetime were improved when using Bi as internal standard. The improvement in accuracy using an internal standard was only observed when the analytical results were affected by errors.

Determination of nickel by a new chromogenic azocalix[4]arene

A novel azocalix[4]arene, 5,17-bis(quinolyl-8-azo)-25,26,27,28-tetrahydroxycalix[4]arene 1, was prepared by linking 8-aminoquinoline to calix[4]arene through a diazo-coupling reaction. Based on it, a simple, sensitive and selective spectrophotometric method was developed for the determination of Ni 2+. In basic media, Ni 2+ formed a 1:1 complex with 1. Beer’s law was obeyed in the range 1.7×10 −7–5.1×10 −6 mol l −1 of Ni 2+. The apparent molar absorptivity (ε) of 1-Ni 2+ complex was 1.28×10 5 l mol −1 cm −1 at 580 nm, and the detection limit was 1.4×10 −7 mol l −1 ( S/ N=3). The relative standard deviation for measurement of 8.5×10 −7 mol l −1 Ni 2+ was 2.7% ( n=10). The proposed method was applied to the determination of trace amount of Ni 2+ in certified samples with satisfactory results.

Direct Analysis of Coffee and Tea for Aluminium Determinationby Electrothermal Atomic Absorption Spectrometry

A method for direct analysis of tea and coffee samples by using electrothermal atomic absorption spectrometry is described. Coffee and tea from different sources were analyzed without digestion step. For slurry analyses the samples were ground, sieved at 105 μm and then suspended in 0.2% v/v HNO3 and 10% v/v Triton X-100 medium. For liquid phase aluminium determination the samples were prepared in the same way and only the liquid is introduced directly into the graphite furnace. Calibration was performed by aqueous standards for both cases and the determinations were carried out in the linear range between 50 and 250 μg L−1. The characteristic mass of aluminium and the detection limit were 45 pg and 2 μg L−1, respectively. Using a typical 0.1% m/v coffee slurry sample, the relative standard deviation of measurements (n=15) for repeatability was about 8.2%.

Prediction of Measurement Precision by Chemometric Invalidation of Flow Pulsation in Refractive Index Detection of Liquid Chromatography.

The standard deviation and relative standard deviation of measurements in the refractive index detection of liquid chro-matography are predicted based on the 1/f fluctuation model which is made up of white noise and the Markov process. First, repeated measurements and Monte Carlo simulation corroborate the reliability of this prediction in the absence of flow pulsation. Pulsation, if any, can be distinguished in the frequency space from the 1/f noise which is common to most analytical instruments. In this case, the uncertainty prediction is also available without modifying the prediction theory, if a zero window is set at the time period of a fundamental tone of the pulsation (virtual invalidation of pulsation). The zero window is used for determining the zero level relative to which the signal intensities are summed for a signal area (measurement). A maltose determination is taken as an example.

Phosphorimetric Detection of Polycyclic Aromatic Hydrocarbons on Solid-Phase Extraction Membranes

Solid-surface room-temperature phosphorimetry (SS-RTP) has been employed for the determination of polycyclic aromatic hydrocarbons (PAHs) adsorbed on solid-phase extraction (SPE) membranes. Three types of membranes commercially available were tested as solid substrates for phosphorescence emission. The effects of humidity and oxygen quenching were studied on SPEC® membranes. Calibration curves with linear dynamic ranges varying from two to three orders of magnitude were obtained in the presence of 0.05 M sodium dodecyl sulfate and 0.1 M TlNO3. The relative standard deviations of measurements varied between 5 and 10%. The absolute limits of detection were estimated at the ng to sub-ng level. On the basis of the performance of SPE membranes as solid substrates, the potential application of SPE-RTP for the analysis of PAHs is discussed.

Deductive Prediction of Precision in Measurement, Calibration, and Standard Addition Method in Atomic Absorption Spectrometry for Cadmium

The relative standard deviation of measurements in atomic absorption spectrometry for cadmium determination is predicted by the signal fluctuation model, which is made up of the white noise and Markov process. The standard deviation (SD), w, of the white noise and SD, m, and autocorrelation parameter, ρ, of the Markov process are determined from the power spectrum of the actual signal fluctuation in the system and in turn are put to use for the uncertainty prediction of measurements in the time space. The confidence intervals of the conventional calibration and standard addition method are calculated from the predicted SD of measurements.

Direct Determimnation of Mercury in Sediments by Atomic Absorption Spectrometry

A pyrolysis chamber is described for the direct determination of mercury in sediments by atomic absorption spectrometry. The finely ground and dried sample is weighed (2.0–60.0 mg), transferred to a quartz cup, inserted in a pyrolysis chamber and heated at 1100 °C for 30 s by means of three focused infrared lamps in the presence of an air stream as the carrier gas. Interfering substances are destroyed by a precolumn containing alumina, silica and copper(II) oxide, and the released mercury is amalgamated on to a gold-coated collector. Mercury vapour is later measured by AAS after thermal desorption. Calibration is easily performed by flow injection, in which known volumes of mercury-saturated air are introduced into the same air carrier stream. The absolute detection limit was 0.26 ng Hg and the relative standard deviation of measurements (n=12) from a typical 4.0 mg sample was about 7%. By applying a t-test, there was no significant difference at the 5% probability level between the results obtained with the proposed method and those certified or recommended for NIST materials. One analysis takes only 2 min, including sample weighing.

Use of nitrous oxide as a purge gas for automated nitrogen isotope analysis by the Rittenberg technique

An apparatus that operates with an isotope-ratio mass spectrometer to automatically perform nitrogen isotope analyses by the Rittenberg technique was modified to permit the use of nitrous oxide (N2O) instead of Freon (CCl2F2 or CHClF2) for the purging of air prior to hypobromite oxidation of ammonium-N to N2 in a plastic microplate. Analytical performance was unaffected by the modifications. Up to 768 samples can be processed in a single loading, at a rate of 6 to 12 samples/h. Within the range of 0.2 to 20 atom % 15N, isotope-ratio analyses of 50 to 200 μg of N using the automated Rittenberg apparatus (ARA) with a double-collector mass spectrometer were accurate to within 0.7%, as compared to manual Rittenberg analyses of 1 mg of N using the same mass spectrometer with a dual-inlet system. Automated analyses of 20μg of N were accurate to within 2%, and automated analyses of 10 μg of N were accurate to within 7%. The relative standard deviation for measurements at the natural abundance level (10 analyses, 20-200 μg of N) was < 0.04 %.

Measurement precision and [formula omitted] noise in analytical instruments

The recently promulgated uncertainty theory of instrumental analyses was used to minimize the relative standard deviation of measurements. Naphthalene, acenaphthene, pyrene and perylene in HPLC were used as illustrations. For a Gaussian model peak with a width (S.D., σ) of fifteen data points, the optimum integration domain was about ±6 data points (±0.4σ) around the peak centre on the HPLC baseline examined. Two commonly used integration modes were examined: horizontal zero line and oblique zero line. The precision was almost the same for both modes in the HPLC analysis. The signal shape at the limit of detection was also shown for the optimum integration domain. The error accompanying the use of the uncertainty theory was evaluated using a Monte Carlo simulation. The practical applicability and limitations of the theory are discussed.

Detection of low levels of trichloroethylene vapor with Raman spectrometry

The response of a novel fiber-optic Raman probe to low levels of trichloroethylene (TCE) vapors is characterized. The detection limit of the current probe for TCE vapor is 34 mg/L, and the probe exhibits a fully reversible response. The probe uses an organic-polymer, low-density polyethylene to concentrate TCE vapors in the optical path of the fiber-optic Raman spectrometer. The relative standard deviation for measurement of 677 mg/L of TCE in the vapor is 0.3%.

Potassium-selective optrode based on an immobilized fluorogenic crown ether

A new optical sensor phase for potassium ions has been developed based on the immobilization of the pH-dependent fluorogenic crown ether 4-acryloylamidobenzo-18-crown-6 on the non-ionic polymeric resin Amberlite XAD-2. Two different optical designs, a flow-through sensor and a fibre optic probetype sensor (optrode), have been constructed and their analytical performance characteristics have been evaluated. The resulting fluorimetric sensors for K+ ions exhibited detection limits of 0.4 or 0.8 μM of K+ (16 μg/l or 31 μg/l), depending on the design, while the linear response occurred from 1 to 25 μM of the metal concentrations. The precision, evaluated as the relative standard deviation of measurements of K+ levels at around ten times the detection limit (e.g. 5 μM), turned out to be around ±2%. Advantageous features of this fluorimetric sensing phase and optrode include ease of construction, simplicity of use, reversibility, short response times (ca. 1 min full scale deflection) selectivity and operational stability, suitable for sensing potassium at low levels in complex matrices such as biological fluids. The fluorimetric optical sensor has been successfully applied to the direct determination of potassium in clinically important samples (serum and urine) and in natural waters. Very good accuracy has been obtained just using adequate synthetic aqueous potassium standards for calibration.

High-performance liquid chromatographic determination of pyrophosphate in the presence of a 20,000-fold excess of orthophosphate

An HPLC method was based on anion-exchange separation of pyrophosphate (diphosphate) and orthophosphate and postcolumn spectrophotometric detection at 140°C with a molybdenum(V)-molybdenum(VI) reagent. The reagent was easy to prepare, stable for at least 6 months at room temperature, and ready for the determination of pyrophosphate and orthophosphate by the so-called heteropoly blue method without use of any reducing agent. A photodiode-array detector for HPLC indicated the spectral characteristics of the heteropoply blue complex that was detectable at 330–800 nm. The HPLC method had a wide dynamic range from 3 × 10 −7 to 5 × 10 −4 m for both pyrophosphate and orthophosphate with a relative standard deviation of measurement of 10 ∼ 2%. Pyrophosphate of 5 × 10 −7 and 5 × 10 −6 m, respectively, could be determined in the presence of a 20,000-fold excess of orthophosphate; 0.01 and 0.1 m.

Titrimetric and gravimetric determination of the specific pore volume of microparticulate silica gels

The titrimetric and gravimetric methods of determination of the specific pore volume were studied unmodified and modified chromatographic silica gels of mean particle size above 5 μm. The apparatus and titrimetric procedure suggested elsewhere were adapted so that the relative standard deviation of measurement did not exceed 2%. Water, n-octane, and methanol were used as titrants, and methanol was found to suit best from the point of view of accuracy and rapidity of analysis and versatility of use. This solvent is also suitable for the gravimetric measurements, which can be carried out without thermostatting and at barometric pressure. The titrimetric procedure is less time consuming than the gravimetric procedure which, on the other hand, is less tedious and less instrumentation-demanding.

Determination of Pheophorbide a and Methylpheophorbide a by Fluorescence High Performance Liquid Chromatography: Availability of Narrow Baseline Method

A narrow baseline method for the simultaneous determination of pheophorbide a and methylpheophorbide a in core sediment samples has been developed by high performance liquid chromatography. A linear relationship between the concentration of methylpheophorbide a and the peak height on the narrow baseline was obtained. The relative standard deviation of measurement was found to be 2.4% (n=6) for samples containing 8.1×10-8mol dm-3 ofmethylpheophorbide a.

Liquid Chromatographic Determination of Hypoxanthine Content in Fish Tissue

A liquid chromatography (LC) method for determining the hypoxanthine content in fish tissues has been developed. Hypoxanthine is extracted with 0.6M perchloric acid, and determined by LC on a reverse phase microparticulate column with UV absorbance detection. The mobile phase is 0.01M potassium phosphate buffer (pH 4.5). The percent relative standard deviation for measurements by the recommended method was less than 7% with a detection limit of 10 ng. Recoveries of hypoxanthine added to various fish tissues were better than 90%. The operational errors, interferences, and recoveries for spiked samples have been investigated and compare favorably with an established xanthine oxidase enzyme method. The described LC method is simple, rapid, and specific for measuring hypoxanthine content in various fish tissues. Some post-mortem studies have indicated the method may also be used for the determination of adenosine monophosphate, inosine monophosphate, and inosine.