Design of high pressure differential volumetric adsorption measurements with increased accuracy

High pressure adsorption measurements for light gases on volumetric equipment are prone to error. Differential units reduce the sensitivity to leakage, gas compressibility, and temperature gradients, but remain highly sensitive to volume uncertainties, the calibration of which is difficult in the presence of low-density, microporous samples. Calibration error can be reduced using a high initial pressure differential and large calibration volume; however, systematic error is prevalent in the literature. Using both analytical and multivariate error analysis, we demonstrate that calibration of the differential unit with the differential pressure transducer significantly decreases volume sensitivity. We show that hydrogen adsorption to GX-31 superactivated carbon at 298 K and 80 bar can be measured with a 7 % error in measurement (i.e. within 0.05 wt% for a 100 mg sample), even when experimental volume calibration is determined only within ~1 %. This represents approximately a 2–7 fold increase in sensitivity relative to previous reports using differential measurements. We also provide a framework for optimizing the design of a volumetric adsorption unit. For virtually any system design, the improved differential methods offer a significant increase in precision relative to the conventional volumetric measurement (from 10- to over 250-fold, depending on the precision of the pressure transducer). This improvement further enhances advantages of the differential unit, in addition to advantages that arise for treating gas compressibility and temperature fluctuations.     

Particle size measurements of heterogeneous film-forming latexes

Two-phase latex particles consisting of mainly polystyrene (PS) and polyisoprene (PI) in ratios varying from 7030 to 2080 were prepared using different polymerization techniques. Methacrylic acid (MAA) was used in small amounts as a comonomer. The latexes had narrow size distributions, and showed different particle morphologies depending on the monomer composition and the polymerization conditions used. In most cases the latexes were filmforming at room temperature. Particle size distributions and average particle sizes of the latexes have been determined using different particle sizing methods.Size determination by TEM was performed after staining with osmium tetroxide (OsO₄) or uranyl acetate (UAc). The staining methods showed no significant differences in particle size averages and particle size distributions when the ratio between the PI and PS phase did not exceed 5050. At higher phase ratios OsO₄ staining was preferred. The glass transition temperature of the PI phase increased after OsO₄ staining, which prevented deformation of the latex particles. Particle morphologies for the heterogeneous latex particles were also determined after OsO₄ staining.Particle sizes measured by TEM were generally smaller than the corresponding sizes measured by quasielastic light scattering (QELS). The difference in the measured diameters increased with increasing PI and PMAA content in the latex particles. The larger sizes observed by QELS are results of the presence of an immobilized water layer surrounding the particles in the aqueous environment, water absorption and swelling due to the presence of carboxylic acid groups, and adsorption of soluble carboxylated polymers forming a hydrophilic corona around the particles. By TEM the hard sphere diameters of dehydrated particles are measured.     

Recursive calculation of the standard deviation with increased accuracy

Summary When calculating the standard deviation of a series of measured data the analyst often underestimates the influence of rounding errors that lead to wrong results. This paper presents a procedure which avoids this disadvantage, being especially suitable for use in minicomputers.     

Factors affecting broadband acoustic emission measurements of a heterogeneous reaction

Broadband acoustic emission signals were obtained by attaching a piezoelectric transducer, sensitive up to 750 kHz, to the external wall of a 1 L jacketed glass reactor. Measurements were acquired of itaconic acid particles mixing in toluene; the total area of the acoustic emission signal from 55-500 kHz increased when the particle concentration, particle size or stir rate were increased. Signals at frequencies above 200 kHz were less sensitive to changes in particle size than those at lower frequencies. From calculation of the area of the signal in the range 55-200 kHz as a percentage of the signal area over the range 55-500 kHz, for mixtures of different size ranges of itaconic acid, it was possible to obtain an estimate of the mean particle size of a mixture. The heterogeneous esterification reaction of itaconic acid and 1-butanol was monitored non-invasively. A decrease in the overall acoustic signal area between 60 and 500 kHz was observed as the reaction progressed. Particle size and concentration information were contained in the amplitude of the acoustic emission signal, while the emission frequency yielded information on changes in the mean particle size.     

Mass spectrometry imaging of surface lipids on intact Drosophila melanogaster flies

The spatial distribution of neutral lipids and semiochemicals on the surface of six‐day‐old separately reared naive Drosophila melanogaster flies has been visualized and studied using matrix‐assisted laser desorption/ionization‐time of flight (MALDI‐TOF) mass spectrometry and laser‐assisted desorption/ionization (LDI)‐TOF imaging (MSI). Metal targets were designed for two‐dimensional MSI of the surface of 3‐D biological objects. Targets with either simple grooves or profiled holes designed to accurately accommodate the male and female bodies were fabricated. These grooves and especially holes ensured correct height fixation and spatial orientation of the flies on the targets after matrix application and sample drying. For LDI‐TOF to be used, the flies were arranged into holes and fixed to a plane of the target using fast‐setting glue. In MALDI‐TOF mode, the flies were fixed as above and sprayed with a lithium 2,5‐dihydroxybenzoate matrix using up to 100 airbrush spray cycles. The scanning electron microscopy images revealed that the deposits of matrix were homogenous and the matrix formed mostly into the clusters of crystals (40–80 µm) that were separated from each other by an uncovered cuticle surface (30–40 µm). The MSI using target with profiled holes provided superior results to the targets with simple grooves, eliminating the ion suppression/mass deviation due to the 3‐D shape of the flies. Attention was paid to neutral lipids and other compounds including the male anti‐attractant 11‐cis‐vaccenyl acetate for which the expected distribution with high concentration on the tip of the male abdomen was confirmed. The red and blue mass shift (PlusMinus1 colour scale) was observed associated with mass deviation predominantly between ±0.2 and 0.3 Da. We use in‐house developed software for mass recalibration, to eliminate the mass deviation effects and help with the detection of low‐intensity mass signals. Copyright © 2014 John Wiley & Sons, Ltd.     

Precision and accuracy of radiotracer measurements of marine sand flow rates

In radiotracer measurements of the flow rate of marine surface sands, the chief difficulty is to determine the average thickness of the radiotracer spot. The simultaneous use of two detectors may appreciably reduce the uncertainty in the final result.     

Improvement of the accuracy of vapor pressure measurements by quartz membrane manometers

The sensitivity of and instrumental errors in high-temperature vapor pressure measurements by two quartz crescent-shaped manometers of enhanced sensitivity were determined; the thermal drift of their zero pints was studied. The contributions of instrumental errors and thermal drift to the total sample variance of pressure were found. The main source of the total error in pressure was shown to be zero point thermal drift and errors in the determination of the position of the membrane pin. Compared with the usual design of these membrane manometers, sensitivity was higher 14.7 and 21.3 times, and measurement errors were lower 5.4 and 19 times, respectively. The two manometers studied gave pressure errors of 0.13 and 0.32 torr, respectively, at a 0.95 confidence probability.     

Advantages of compound‐specific stable isotope measurements over bulk measurements in studies on plant uptake of intact amino acids

Increasing interest in the ability of plants to take up amino acids has given rise to questions on the accuracy of the commonly used bulk method to measure and calculate amino acid uptake. This method uses bulk measurements of ¹³C and ¹⁵N enrichment in plant tissues after application of dual‐labelled amino acids but some authors have recommended the use of compound‐specific stable isotope (CSI) analysis of the plants' amino acids instead. However, there has never been a direct evaluation of both methods. We conducted a field study applying dual‐labelled (¹³C, ¹⁵N) amino acids (glycine, valine, tyrosine and lysine) to soil of a Plantago lanceolata monoculture. Root and shoot samples were collected 24 h after label application and the isotope composition of the plant tissues was investigated using bulk and CSI measurements. Enrichment of ¹³C in the case of CSI measurements was limited to the applied amino acids, showing that no additional ¹³C had been incorporated into the plants' amino acid pool via the uptake of tracer‐derived C‐fragments. Compared with this rather conservative indicator of amino acid uptake, the ¹³C enrichment of bulk measurements was 8, 5, 1.6 and 6 times higher for fine roots, storage roots, shoot and the whole plant, respectively. These findings show that the additional uptake of tracer‐derived C‐fragments will result in a considerable overestimation of amino acid uptake in the case of bulk measurements. We therefore highly recommend the use of CSI measurements for future amino acid uptake studies due to their higher accuracy. Copyright © 2009 John Wiley & Sons, Ltd.     

A note on the accuracy and precision of137Cs measurements in foodstuffs

Formulas for the precision and accuracy of¹³⁷Cs measurements and of the consumer's and producer's risks in routine control are given. The limit of determination is determined by the combination of natural background, compton continuum of the sample aliquot, counting efficiency and the residual uncertainty in positioning.     

Assessing the accuracy of contact angle measurements for sessile drops on liquid-repellent surfaces

Gravity-induced sagging can amplify variations in goniometric measurements of the contact angles of sessile drops on super-liquid-repellent surfaces. The very large value of the effective contact angle leads to increased optical noise in the drop profile near the solid-liquid free surface and the progressive failure of simple geometric approximations. We demonstrate a systematic approach to determining the effective contact angle of drops on super-repellent surfaces. We use a perturbation solution of the Bashforth-Adams equation to estimate the contact angles of sessile drops of water, ethylene glycol, and diiodomethane on an omniphobic surface using direct measurements of the maximum drop width and height. The results and analysis can be represented in terms of a dimensionless Bond number that depends on the maximum drop width and the capillary length of the liquid to quantify the extent of gravity-induced sagging. Finally, we illustrate the inherent sensitivity of goniometric contact angle measurement techniques to drop dimensions as the apparent contact angle approaches 180°.     

Inelastic incoherent neutron scattering measurements of intact cells and tissues and detection of interfacial water

We have previously used inelastic incoherent neutron scattering spectroscopy to investigate the properties of aqueous suspensions of biomolecules as a function of hydration. These experiments led to the identification of signals corresponding to interfacial (hydration) water at low water content. A prediction from these studies was that in the crowded environment inside living cells, a significant proportion of the water would be interfacial, with profound implications for biological function. Here we describe the first inelastic incoherent neutron scattering spectroscopy studies of living cells and tissues. We find that the interfacial water signal is similar to that observed for water interacting with purified biomolecules and other solutes, i.e., it is strongly perturbed in the librational and translational intermolecular optical regions of the spectrum at 20-150 meV. The ratio of interfacial water compared to total water in cells (approximately 30%) is in line with previous experimental data for hydration water and calculations based on simple assumptions.     

Improving mass accuracy of high performance liquid chromatography/electrospray ionization time-of-flight mass spectrometry of intact antibodies

The glycosylation profile of intact antibody due to the galactose and fucose heterogeneity in the N-linked sugars was determined with instrument resolution of 5000 and 10,000. After deconvolution of electrospray ionization mass spectra to complete convergence, several extra peaks appeared in addition to the peaks observed in the original mass spectra. The artificial peaks were avoided if deconvolution was stopped after a smaller number of iterations. A standard antibody was used as an external calibrant to minimize mass measurement errors during long-period experiments. Precision of four consecutive LC/MS measurements of the same antibody was 10 ppm (+/-1.5 Da). By using this approach, the masses of 11 intact antibodies were measured. All antibodies containing N-terminal glutamines had a negative mass shift due to the formation of pyroglutamate (-17 Da). Although the pyroglutamate variant of intact antibody was not resolved from the unmodified variant, this modification led to a mass shift proportional to the percentage of N-terminal pyroglutamate. By accurately measuring the mass shift we were able to quantify the abundance of pyroglutamic acid on intact antibodies. Mass accuracy in measuring different antibodies was below 30 ppm (+/-4 Da). The accurate mass measurement can be an effective tool for monitoring chemical degradations in therapeutic antibodies.     

Mass spectrometric study of heterogeneous catalytic monooxidation of the S-position of thiophenes with hydrogen peroxide

A biomimetic catalytic reaction system is developed experimentally for heteromonooxidation of heteroaromatic compounds. The system consists of two interacting simultaneous reactions, decomposition of H₂O₂ and oxidation of the substrate. This leads to the synthesis, isolation, and identification of the S-monoxide of thiophenes.     

The assessment of electronic balances for accuracy of mass measurements in the analytical laboratory

The accuracy of mass measurements when using electronic balances is considered within the frame of analytical assays in terms of the uncertainty budget according to the internal quality control routine, the calibration process, the balance specifications, and the weighing scenarios. Buoyancy corrections for both conventional and true mass are fully discussed. The procedure is illustrated with a worked example.     

Studies relating to the accuracy of in vivo measurements of liver and kidney cadmium

A greater understanding of the inaccuracies involved inin vivo measurements of liver and kidney cadmium by neutron activation was necessary. Studies were therefore carried out, using phantoms, into the effect onin vivo measurement accuracy of organ depth and organ mass. In addition an independentin vitro technique was developed, and used to analyse autopsy samples from three people previously measuredin vivo, thus making direct comparisons possible. No evidence of bias in thein vivo results was found, but the need to correct for organ mass and depth was highlighted. Current measurement procedures result in residual uncertainties of ±25%.     

Warning: false sense of certainty from the illusion of accuracy and precision in measurements

The illusion of accuracy and precision in testing is often associated with overconfident safety checks before catastrophes: National Aeronautics and Space Administration (NASA) “no safety-of-flight issues,” Deepwater Horizon site in Gulf Mexico reported as “industry model for safety” a year before its explosion, and Fukushima nuclear plant safety test found “satisfactory” at seismic magnitude viewed highest possible. In health care, testing overuse and misuse play significant roles in overconfidence and mistakes. Gaps between systems are most susceptible to errors, such as in miscommunications between the laboratory and the clinic. A safety culture is characterized by ability to communicate, anticipation of misunderstanding and resilient learning from failures. The discussion will show how messages from the laboratory may reduce the false sense of certainty from a test result; how results of tests are meaningless without pre-test estimates; how multiplicity of routine tests increases chances of false positivity; and how reporting measurement limitations can educate all users, including laboratory technicians, clinicians and patients, about the importance of quality assurance and about the pervasive nature of uncertainty beyond analytical performance. In conclusion, an illusion of accuracy and precision in testing evokes among clinicians a false sense of certainty leading to overuse and misuse of tests. Messages from the laboratory may reduce overconfidence from test results and lead to safer use of testing.