Additional Calibration Techniques Research Articles

Development and validation of a reverse phase High Performance Liquid Chromatographic method, using standard addition calibration, for determination of risperidone in human plasma

Risperidone is a drug of choice in the management of psychosis and high performance liquid chromatography (HPLC) is regarded as the most sensitive and accurate method for quantitative analysis of risperidone, as well as other drugs having low plasma concentrations in biological fluids. A simple and accurate HPLC method was developed for the determination of risperidone in human plasma using the standard addition calibration technique to augment the weak signals due to low plasma concentrations of risperidone. Risperidone was eluted using isocratic mode with a mobile phase of methanol: acetonitrile (60:40) on a Chemisil ODS C18 column (250 mm×4.6 mm i.d., 5 μ particle size). The sample injection volume, flow rate, column temperature and wavelength of detection were set at 10 µl, 1.0 ml/min, 35 ˚C and 280 nm respectively. The run time for the method was 7 minutes, with risperidone having a retention time of 3.566 minutes. The method was linear within the concentration 10-50 ng/ml (R2= 0.998), with LOD and LOQ of 0.13 and 0.40 ng/ml respectively. The precision and relative error was 3.44 % and 3.38 % respectively. The developed method can be employed in the quantitative determination of risperidone in pharmacokinetic and bioequivalence studies.

A newly modified QuEChERS method for the analysis of organochlorine and organophosphate pesticide residues in fruits and vegetables.

A newly modified QuEChERS method was developed, validated, and tested in-house for the multiclass, multiresidue determination of organophosphate (OPP) and organochlorine pesticide (OCP) residues in fruits and vegetables. The method incorporated a solvent extraction with acetonitrile followed by partitioning with magnesium sulphate (MgSO4) and sodium chloride (NaCl). The final step involved a dispersive solid phase extraction (d-SPE) clean-up prior to gas chromatography with electron capture detection (GC-ECD) analysis. Apples and lettuce were the selected matrices for fruits and vegetables, respectively in the method development stages. Various combinations of sorbents were tested in the clean-up step. Florisil and MgSO4 (FM) d-SPE proved to be the best combination for the clean-up step for both OCPs and OPPs in both the fruit and vegetable matrices. Recovery values fell within the acceptable range of 70 to 120% (RSD ≤20%). The new method is unique as it employs the standard addition calibration technique for the quantification of OCP and OPP residues. Limit of detection, limit of quantification, linearity, recovery analysis, and repeatability were validation parameters used to evaluate the accuracy, precision, and ruggedness of the method. The newly modified QuEChERS method provides a cheaper alternative for the analysis of pesticide residues in fruits and vegetables as samples can be analysed at a fraction of the cost of the original QuEChERS method. The efficacy of the method was tested on several fruits and vegetables from the Central Division of Trinidad and Tobago. OCP and OPP residues were found in 61% of the samples, most of which were above the maximum residue limits.

A 50–66-GHz Phase-Domain Digital Frequency Synthesizer With Low Phase Noise and Low Fractional Spurs

Digital phase-locked loop (DPLL) frequency synthesizers have become popular for wireless applications in the sub-10-GHz range. However, mm-wave synthesizers still rely on analog PLLs, predominantly of the sub-harmonic, integerN-type. This paper describes the design and implementation of a 50-66-GHz phase-domain DPLL that uses a fundamental frequency capacitively degenerated digitally controlled oscillator (DCO) with 40-kHz frequency step. Following frequency division with a modulus of only 4, a two-step 8-bit time-todigital converter (TDC) digitizes the phase of the 12.5-16.5-GHz divider output with 450-fs resolution. Digital calibration based on the statistical element selection technique augmented by mean adaptation is used to mitigate TDC nonlinearity that results from random mismatches. Additional digital calibration techniques are introduced to mitigate DCO non-linearity and phase mismatches in the digital phase extraction sub-system, and to ensure robust operation of the inductor-less 4× frequency divider over process, voltage and temperature (PVT) variations. A 65-nm CMOS prototype of the DPLL occupies 0.45 mm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> excluding pads and consumes 46 mA from a 1-V supply. The PLL achieves best (worst) case rms jitter of 220 (302) fs, best (worst) phase noise of -83/-94.5/-122 (-79/-88/ -116) dBc/Hz at 0.1/1/10 MHz offset, and -52.2(-48.3) dBc fractional spur.

A PRACTICAL THEOREM ON USING INTERFEROMETRY TO MEASURE THE GLOBAL 21 cm SIGNAL

ABSTRACT The sky-averaged, or global, background of redshifted 21 cm radiation is expected to be a rich source of information on cosmological reheating and reionization. However, measuring the signal is technically challenging: one must extract a small, frequency-dependent signal from under much brighter spectrally smooth foregrounds. Traditional approaches to study the global signal have used single antennas, which require one to calibrate out the frequency-dependent structure in the overall system gain (due to internal reflections, for example) as well as remove the noise bias from auto-correlating a single amplifier output. This has motivated proposals to measure the signal using cross-correlations in interferometric setups, where additional calibration techniques are available. In this paper we focus on the general principles driving the sensitivity of the interferometric setups to the global signal. We prove that this sensitivity is directly related to two characteristics of the setup: the cross-talk between readout channels (i.e., the signal picked up at one antenna when the other one is driven) and the correlated noise due to thermal fluctuations of lossy elements (e.g., absorbers or the ground) radiating into both channels. Thus in an interferometric setup, one cannot suppress cross-talk and correlated thermal noise without reducing sensitivity to the global signal by the same factor—instead, the challenge is to characterize these effects and their frequency dependence. We illustrate our general theorem by explicit calculations within toy setups consisting of two short-dipole antennas in free space and above a perfectly reflecting ground surface, as well as two well-separated identical lossless antennas arranged to achieve zero cross-talk.

Use of Addition Calibration Technique for Determination of Acetaminophen and Hydrochlorothiazide in Human Urine by High-Performance Liquid Chromatography

The quantitation of target analytes in complex matrices such as biological samples requires special calibration approaches to compensate for additional capacity or activity in the matrix samples. A conventional calibration curve, obtained with standard solutions, is one of the most important calibration procedures for quantitation of target analytes in such matrices. However, these technique require a great number of reagents and material, and consume a considerable amount of time throughout the analysis. In this work, a new calibration procedure to analyze urine samples is proposed for the first time in chromatography procedures. The proposed calibration, called the addition calibration technique, was used for the determination of acetaminophen and hydrochlorothiazide in urine samples. The results obtained for the proposed calibration mode were compared to those obtained using standard addition and standard calibration techniques. The proposed addition calibration was validated by statistical studies between results obtained by the addition technique and conventional techniques, using the ANOVA test and linear regression. The results demonstrated good agreement among them. The performance of the analytical method was evaluated. Relative standard deviation, limit of detection, and limit of quantification are respectively 0.5-0.6%, 0.169-0.75 microg/mL, and 0.565-2.5 microg/mL. Linear range falls within the range of 0.3 to 63.8 microg/mL for both compounds. Accuracy ranged between 94% and 101%.

Development of a rapid and sensitive method for the simultaneous determination of 1,2-dibromoethane, 1,4-dichlorobenzene and naphthalene residues in honey using HS-SPME coupled with GC–MS

A new method for the simultaneous determination of 1,4-dichlorobenzene ( p-DCB), naphthalene and 1,2-dibromoethane (1,2-DBE) residues in honey has been developed. Analysis is carried out using gas chromatography–mass spectrometry (GC/MS) in selected ion monitoring mode (SIM), after extraction and preconcentration of target analytes by headspace solid-phase microextraction (HS-SPME), with a 100 μm film thickness polydimethylsiloxane (PDMS) fiber. Several parameters affecting the extension of the adsorption process (i.e., addition of salt, extraction time, extraction temperature) were studied. The optimal conditions for the determination of these analytes were established. The proposed HS-SPME method showed good sensitivity, without carryover between the samples. Linearity was studied from 5 to 2500 μg kg −1 for p-DCB, 0.5 to 500 μg kg −1 for naphthalene and 5 to 500 μg kg −1 honey for 1,2-DBE with correlation coefficients ( r 2) ranging from 0.9901 to 0.9999. Precision was assessed and both intra and inter-day R.S.D.s (%) were below 6.3%. The detection limits were found to be 1, 0.1 and 2 μg kg −1 honey for p-DCB, naphthalene and 1,2-DBE, respectively. The percentage recoveries that were evaluated with the proposed HS-SPME method and the standard addition calibration technique gave values among 72.8 and 104.3% for measurements in samples spiked with one target analyte or mixtures of the three. This method has been applied for the analysis of unknown honey samples. The results showed an excellent applicability of the proposed method for the determination of the target compounds in honey samples.

Determination of cadmium and lead in reference materials by volatile species generation with in situ trapping flame atomic absorption spectrometry

The analytical performance of coupled volatile species generation-integrated atom trap (VSG-IAT) atomizer flame atomic absorption spectrometry (FAAS) system was evaluated for determination of Cd and Pb in reference materials. Lead using formation of PbH 4 and Cd 0 vapors are atomized in air-acetylene flame-heated IAT. A new design of VSG-IAT-FAAS hyphenated technique that would exceed the operational capabilities of existing arrangements (a water-cooled single silica tube, double-slotted quartz tube or an “integrated trap”) was investigated. A dramatic improvement in detection limit was achieved compared with that obtained using either of the above atom trapping techniques separately. The concentration detection limits, defined as 3 times the blank standard deviation (3 σ), were 0.05 and 0.40 ng mL − 1 for Cd and Pb, respectively. For a 120 s in situ pre-concentration time, sensitivity enhancement compared to flame AAS, was 500 and 575 folds for Cd and Pb, respectively, using volatile species generation-atom trapping technique. The sensitivity can be further improved by increasing the collection time. The precision, expressed as RSD, was 4.4% and 4.1% ( n = 6) for Cd and Pb, respectively. The designs studied include slotted tube, single silica tube and integrated atom trap-cooled atom traps. The accuracy of method was verified using certified reference materials (PLANTS 3 and IAEA/W-4) by standard addition calibration technique. The measured Cd and Pb contents in two reference materials were in satisfactory agreement with the certified values.

Analysis of Zinc in Biological Samples by Flame Atomic Absorption Spectrometry: Use of Addition Calibration Technique

The quantification of target analytes in complex matrices requires special calibration approaches to compensate for additional capacity or activity in the matrix samples. The standard addition is one of the most important calibration procedures for quantification of analytes in such matrices. However, this technique requires a great number of reagents and material, and it consumes a considerable amount of time throughout the analysis. In this work, a new calibration procedure to analyze biological samples is proposed. The proposed calibration, called the addition calibration technique, was used for the determination of zinc (Zn) in blood serum and erythrocyte samples. The results obtained were compared with those obtained using conventional calibration techniques (standard addition and standard calibration). The proposed addition calibration was validated by recovery tests using blood samples spiked with Zn. The range of recovery for blood serum and erythrocyte samples were 90-132% and 76-112%, respectively. Statistical studies among results obtained by the addition technique and conventional techniques, using a paired two-tailed Student's t-test and linear regression, demonstrated good agreement among them.

Electrochemical hydride generation atomic absorption spectrometry for determination of cadmium

An electrolytic hydride generation system for determination of another hydride forming element, cadmium, by catholyte variation electrochemical hydride generation (EcHG) atomic absorption spectrometry is described. A laboratory-made electrolytic cell with lead–tin alloy as cathode material is designed as electrolytic generator of molecular hydride. The influences of several parameters on the analytical signal have been evaluated using a Plackett–Burman experimental design. The significant parameters such as cathode surface area, electrolytic current, carrier gas flow rate and catholyte concentration have been optimized using univariate method. The analytical figures of merit of procedure developed were determined. The calibration curve was linear up to 20 ng ml −1of cadmium. The concentration detection limit (3 σ, n = 8) of 0.2 ng ml −1 and repeatability (relative standard deviation, n = 7) of 3.1% were achieved at 10.0 ng ml −1. It was shown that interferences from major constituents at high concentrations were significant. The accuracy of method was verified using a real sample (spiked tap water) by standard addition calibration technique. Recovery of 104% was achieved for Cd in the spiked tap water sample.

Dynamic Range Improvement of Multistage Multibit Modulator for Low Oversampling Ratios

This paper presents an improved architecture of the multistage multibit sigma-delta modulators (ΣΔMs) for wide-band applications. Our approach is based on two resonator topologies, high-Q cascade-of-resonator-with-feedforward (HQCRFF) and low-Q cascade-of-integrator-with-feedforward (LQCIFF). Because of in-band zeros introduced by internal loop filters, the proposed architecture enhances the suppression of the in-band quantization noise at a low OSR. The HQCRFF-based modulator with single-bit quantizer has two modes of operation, modulation and oscillation. When the HQCRFF-based modulator is operating in oscillation mode, the feedback path from the quantizer output to the input summing node is disabled and hence the modulator output is free of the quantization noise terms. Although operating in oscillation mode is not allowed for single-stage ΣΔM, the oscillation of HQCRFF-based modulator can improve dynamic range (DR) of the multistage (MASH) ΣΔM. The key to improving DR is to use HQCRFF-based modulator in the first stage and have the first stage oscillated. When the first stage oscillates, the coarse quantization noise vanishes and hence circuit nonidealities, such as finite op-amp gain and capacitor mismatching, do not cause leakage quantization noise problem. According to theoretical and numerical analysis, the proposed MASH architecture can inherently have wide DR without using additional calibration techniques.

Resonator-based multi-stage modulator for wideband applications with improved dynamic range

A new design methodology for wideband, multi-stage, multi-bit /spl Sigma//spl Delta/ modulators (/spl Sigma//spl Delta/Ms) with improved dynamic range, is presented. The key to improving dynamic range is to have the first stage oscillated, then the coarse quantisation noise vanishes and hence circuit non-linearities do not cause a leakage quantisation noise problem. Based on the proposed methodology, a fourth-order four-bit /spl Sigma//spl Delta/M can achieve the dynamic range of 80 dB at the OSR of 8 without using additional calibration techniques.

Determination of napropamide in technical formulations, soil and vegetable samples by sensitised fluorescence: validation of the method

A fluorimetric method was developed for the determination of napropamide in aqueous micellar solutions of sodium dodecyl sulphate (SDS). The determination was performed at pH 7.2, obtained by a 0.5 M phosphate buffer solution, and 0.08 M SDS. In micellar solutions the fluorescence emission intensity was measured at 337 nm (excitation at 288 nm). The linear calibration ranges for napropamide were 10–100 ng ml −1 and 0.1–2.0 μg ml −1. The detection limit, according to the error propagation theory, was 4.2 ng ml −1. The relative standard deviations ( n=10) were 0.88 and 0.78%, respectively, for 60 and 1600 ng ml −1 napropamide. The fluorimetric method was applied to the determination of napropamide in a technical formulation and spiked soil and vegetable samples (pepper and tomato). Dichloromethane was used for the extraction of the herbicide from soil, pepper and tomato samples. Owing to matrix interference, vegetable samples were analysed using a standard addition calibration technique.

Determination of arsenic in sediments, coal and fly ash slurries after ultrasonic treatment by hydride generation atomic absorption spectrometry and trapping in an iridium-treated graphite tube

A simple and reliable method for the determination of arsenic in sediment and in coal without sample digestion, based on hydride generation from slurry samples is proposed. After grinding the samples to a particle size of ⩽50 μm, the sample powder was mixed with aqua regia and hydrofluoric acid in an ultrasonic bath for 30 min. After diluting the mixture with hydrochloric acid, the slurry was allowed to stand for 48 h, and an aliquot was used for hydride generation with sodium borohydride. More than 80% of the arsenic was leached to the aqueous phase under these conditions, except for one sediment sample with very high silica content. The generated arsine was collected in a graphite tube, treated with 0.5 mg of iridium as a permanent modifier, and the arsenic determination was carried out by electrothermal atomic absorption spectrometry. The same tube could be used for at least 160 cycles without any re-treatment. The greatest advantage of the method was that only a minimum of reagents and sample handling were required, reducing the risks of contamination and/or analyte loss. However, the addition calibration technique had to be used in order to obtain results within the 95% confidence level for 11 certified reference materials, 5 sediments, 5 coals and one coal fly ash. One certified sediment slurry was spiked with the analyte and the resulting addition calibration curve was used for the analysis of the certified sediments. Similarly, one certified coal was used to obtain the addition calibration curve for the coal and coal fly ash samples. The recoveries of the certified values, except for one sediment, were between 91 and 115%. The limits of detection in the samples were 0.54 and 0.7 μg g −1 for the coal and sediment samples, respectively, obtained for 1 ml of slurry containing 1 mg of sample.

Indirect measurement of desferrioxamine and its chelated compounds aluminoxamine and ferrioxamine by Zeeman atomic absorption spectrometry.

This indirect method for determination of desferrioxamine (DFO, deferoxamine, Desferal) and its chelated compounds ferrioxamine (FO) and aluminoxamine (AIO) in biological fluids involves metal chelate formation and solvent extraction, the metal being determined by Zeeman atomic absorption spectrometry (Zeeman-AAS). Special attention was paid to the pH and recovery of extraction, iron-aluminum interactions, extraction times and volumes, centrifugation rates, and instrument settings. Precautions were taken to avoid contamination with aluminum and iron and to prevent co-extraction of iron and aluminum not chelated to DFO. The addition calibration technique was used for standardization. Mean +/- SD recoveries of AIO and FO after extraction were respectively 98.3% +/- 3.7% and 101% +/- 3.7%. Sensitivity was 45 pg/0.0044 A.s for aluminum and 10 pg/0.0044 A.s for iron. Detection limits were 0.22 mumol/L for AIO, 0.06 mumol/L for DFO and FO.