Standard Deviation Of Replicate Measurements Research Articles

Picomolar-scale compound-specific isotope analyses.

We report modifications to compound-specific isotope analyses (CSIA) to enable high-precision isotopic analyses of picomoles of carbon for intact organic molecules. This sample size is two orders of magnitude below the amounts required for commercial systems. The greatly enhanced sensitivity of this system expands molecular isotope studies and applications previously prohibited by low concentrations and small samples. We utilize the resolving power and low volumetric flow rates of narrow-bore capillary gas chromatography to improve sample transfer efficiency while maintaining narrow peak widths. Post-column peak broadening is minimized using a micro-fluidic valve for solvent diversion, capillary combustion reactor, narrow-bore capillary transfer lines, and cryogenic water trap. The mass spectrometer was fitted with collector amplifiers configured to 25ms response times and a data logger board with firmware capable of rapid data acquisition. Carbon dioxide gas was introduced directly into the ion source to evaluate the dynamic range of the system and accuracy and precision of carbon isotope ratio (δ13 C value) measurements. The accuracy and precision for combusted compounds were evaluated using a suite of n-alkanes. For ≥30 pmol carbon introduced directly into the ion source, the mean difference between the measured and expected δ13 C values is 0.03‰ (1σ, n = 57) and the standard deviation of replicate measurements is 0.11‰ (1σ). The CO2 peak widths generated by the exponential dilution flask were 250ms and the peak widths produced by combusting n-alkanes were ca 500ms, less than 25% the width of conventional gas chromatography peaks. For a mixture of 15 n-alkanes (n-C16 to n-C30 ), the accuracy is 0.3‰ (1σ) and precision is 0.9‰ (1σ) for replicate δ13 C measurements with 100pmol carbon per compound on column. The pico-CSIA method described here offers improved chromatographic resolution and reduces sample size requirements by two orders of magnitude. These advances significantly broaden the available analytical window for CSIA in research areas frequently hindered by sample size limitations, such as forensics, paleoclimate, astrobiology, and biochemistry.

Determination of mercury in gasoline by photochemical vapor generation coupled to graphite furnace atomic absorption spectrometry

Quantitation of mercury in gasoline by graphite furnace atomic absorption spectrometry following its photochemical vapor generation is described. Samples were prepared as a mixture of gasoline and propan-2-ol. A high efficiency flow through 19W photochemical vapor generator was used to process prepared samples with in-situ collection of the resultant Hg0 vapor onto reduced palladium in the graphite tube. Response from inorganic and organomercury standards added to the samples did not differ significantly. A limit of detection and characteristic mass of 0.1μgL−1 and 0.6ng were obtained, respectively. Relative standard deviations of replicate measurements of 3.0ngmL−1 added Hg(II) were typically 7–8%. Recoveries of spikes from samples ranged from 90 to 97%. The concentration of mercury in all tested samples of gasoline was below the limit of quantitation (0.3μgL−1). The method was rapid, efficient, green, fit-for-purpose, responsive to both inorganic and organic mercury species and should also respond to any Hg0 endogenously present in the sample.

High Intercycle Variability of Day 3 FSH Levels is Useful for Predicting Ovarian Responses But Not Pregnancy Outcomes in IVF

OBJECTIVE: To determine if intercycle variability in day 3 FSH (D3FSH) levels is useful in predicting IVF outcomes. DESIGN: Retrospective analysis of patients undergoing IVF from January 2004 to March 2005 in a large private practice setting. MATERIALS AND METHODS: A total of 2504 fresh, nondonor IVF cycles were available for analysis. This group encompassed a broad spectrum of patients representing typical infertility patients presenting for assisted reproduction. The patients ranged in age from 21-44 and underwent controlled ovarian hyperstimulation by standard midluteal phase GnRH agonist down regulation or GnRH antagonist protocol using recombinant FSH or FSH in combination with HMG. Adjustments were made based on individual responses, and when two or more follicles had attained a minimum mean diameter of 19-20mm, follicular triggering was achieved with hCG and oocyte retrieval was performed 36 hours later. Standard laboratory protocols were followed, including intracytoplasmic sperm injection, assisted hatching for cleavage embryos and extended culture for blastocyst transfer, as clinically appropriate. Ultrasound guided embryo transfer was performed, and all patients received luteal progesterone support. Serum hCG levels were measured 15 days after retrieval, and a clinical pregnancy was defined as the presence of a gestational sac on ultrasound. An estimate of normal intercycle D3FSH variability was derived by regressing standard deviations of replicate measurements against mean FSH in patients 35 years old with D3FSH 10. Patients were placed in the high variability category when their calculated standard deviation was significantly higher than normal. Comparisons were made between patients with normal and high D3FSH variability as well as patients with a single treatment cycle. Continuous and categorical data were analyzed by ANOVA and Chi Square tests, respectively. A value of P 0.05 was considered to be statistically significant. Results are expressed as mean SD. RESULTS: Patients with replicate cycles were older. Relative to patients with normal variability, patients with high D3FSH variability had significantly diminished ovarian responses, with fewer retrieved and mature oocytes but similar fertilization, pregnancy and clinical pregnancy rates. All patients with replicate cycles had significantly lower pregnancy outcomes than patients with single cycles.

Uncertainty in measurements of dermal absorption of pesticides.

Dermal absorption experiments form an important component in the assessment of risk from exposure to pesticides and other substances. Much dermal absorption data is gathered in rat experiments carried out using a certain standard protocol. Uncertainties in these data arise from many sources and can be quite large. For example, measurements of the systemic absorption of hexaconazole differed by more than an order of magnitude within a single experiment. Two diniconazole studies produced quite different results, due to minor differences in protocol and in chemical formulation. Limits of detection can also prevent accurate measurement when the amounts absorbed are small. These examples illustrate the need for measuring and reporting uncertainties in estimates that are based on these data. The most direct way to estimate uncertainty is to compute the sample standard deviations of replicate measurements. By pooling these estimates across dose and duration groups for which they are similar, the number of degrees of freedom is increased, and more precise confidence intervals can be obtained. In particular, the ratio of upper to lower 95% confidence limits was reduced by as much as ten-fold for hexaconazole, seven-fold for uniconazole, and nearly four-fold for propiconazole.

Variability in XRF-measured tibia lead levels

A few studies have examined the variability in 109Cd-based K-shellx-ray fluorescence (KXRF) bone lead measurements from replicate measurementsmade either at the same time or over a period of time, and one of thesestudies has shown that the uncertainty in an individual measurement is anunderestimate of the standard deviation of replicate measurements.Variability in KXRF tibia lead measurements was assessed from ten cadaverintact legs, from the bare tibiae dissected from nine of these legs, and fromfour in vivo volunteers. Cadaver legs underwent replicate measurementson multiple occasions. In vivo volunteers underwent singlemeasurements of the left tibia monthly for one year.Average tibia lead levels in the cadaver legs and in vivo volunteersranged from 6 to 50 and from 6 to 13 µg Pb per g of bone mineralrespectively.The factors influencing the standard deviation of replicate measurements wereinvestigated. Both cadaver and in vivo measurements confirmed that theuncertainty in an individual measurement is an underestimate of the standarddeviation of replicate measurements, suggesting a methodological deficiencyprobably shared by most current 109Cd-based K-shell XRF leadmeasurement systems.

Hydride Generation Laser-Induced Fluorescence of Arsenic and Selenium in the Inductively Coupled Plasma and Electrothermal Atomizer

The application of hydride generation (HG) sample introduction combined with laser-induced fluorescence (LIF) detection in the inductively coupled plasma (ICP) and electrothermal atomizer (ETA) is reported. Far-ultraviolet excitation of As at 193.696 nm and Se at 196.026 nm is accomplished by using a tunable dye laser system with frequency doubling and stimulated Raman scattering. HG-ICP-LIF and HG-ETA-LIF techniques have been developed and demonstrate good sensitivity and good linearity at the ng/mL to sub-ng/mL level for both elements. The relative standard deviation of replicate measurements is on the order of 3–10% at the 5 ng/mL level for the HG-ICP-LIF technique and 5–35% at the 0.6 ng/mL level for the HG-ETA-LIF technique. Detection limits of 1 ng/mL (5 ng absolute mass) for As and 0.06 ng/mL (0.3 ng) for Se are reported for the HG-ICP-LIF technique and 0.04 ng/mL (0.2 ng) for As and 0.16 ng/mL (0.8 ng) for Se are reported for the HG-ETA-LIF technique. Lower than expected sensitivity and precision are observed in the ETA, which may be due to inefficient trapping of the hydrides in the procedure. Selective HG procedures have been demonstrated for the speciation and recovery of selected oxidation states of As and Se. The analytical capabilities of the HG-LIF techniques are compared to those of other HG techniques, and recommendations are made for improved performance of future HG-LIF approaches.

A reagentless amperometric biosensor for hydrogen peroxide determination based on asparagus tissue and ferrocene mediation

A new reagentless biosensor for the amperometric detection of hydrogen peroxide was developed by co-immobilisation of ground asparagus tissue ( Asparagus officinalis) and ferrocene in a carbon paste matrix. The asparagus tissue acts as a source of peroxidase. Ferrocene is utilised as a mediator to facilitate efficient electron transfers between the electrode surface and hydrogen peroxide. The detection of hydrogen peroxide was based on the measurement of amperometric response due to the reduction at the electrode of ferricinium ion generated from the enzymatic reaction at 0.00 mV vs. SCE. The response characteristics and optimisation of the bioelectrode design are evaluated. The bioelectrode exhibits linear response up to a hydrogen peroxide concentration of 6 × 10 −5 M with a detection limit of 4.0 × 10 −7 M. Response times ( t 90) as low as 2 s and a relative standard deviation of replicate measurements of 1 × 10 −5 M hydrogen peroxide of 1.95% ( n = 15) were achieved. The bioelectrode sensitivity decreased to 50% of the original value within 30 days of continuous use.

Precise Measurement of the G+C Content of Deoxyribonucleic Acid by High-Performance Liquid Chromatography

High-performance liquid chromatography is a promising alternative for determining the G+C content of bacterial deoxyribonucleic acid (DNA). The method which we evaluated involves enzymatic degradation of the DNA to nucleosides by PI nuclease and bovine intestinal mucosa alkaline phosphatase, separation of the nucleosides by high-performance liquid chromatography, and calculation of the G+C content from the apparent ratios of deoxyguanosine and thymidine. Because the nucleosides are released from the DNA at different rates, incomplete degradation produces large errors in the apparent G+C content. For partially purified DNA, salts are a major source of interference in degradation. However, when the salts are carefully removed, the preparation and degradation of DNA contribute little error to the determination of G+C content. This method also requires careful selection of the chromatographic conditions to ensure separation of the major nucleosides from the nucleosides of modified bases and precise control of the flow rates. Both of these conditions are achievable with standard equipment and C18 reversed-phase columns. Then the method is precise, and the relative standard deviations of replicate measurements are close to 0.1%. It is also rapid, and a single measurement requires about 15 min. It requires small amounts of sample, and the G+C content can be determined from DNA isolated from a single bacterial colony. It is not affected by contamination with ribonucleic acid. Because this method yields a direct measurement, it may also be more accurate than indirect methods, such as the buoyant density and thermal denaturation methods. In addition, for highly purified DNA, the extent of modification can be determined.

Accuracy and precision of determinations of chlorinated pesticides and polychlorinated biphenyls with automated interpretation of mass spectrometric data

Performance characteristics were determined for an analytical method that includes automated interpretation of mass spectra of polychlorinated biphenyls (PCBs) and 21 chlorinated pesticides. Determination of PCBs was by level of chlorination. After three capillary column injection techniques were evaluated, a splitless, cold-trapping technique was used. Reproducibility of mass spectrometer response to calibration compounds relative to two internal standards was evaluated over a period of 3 months. In the three sets of fortified water extracts, the largest relative standard deviation of replicate measurements was 8.4% for individual pesticides (concentrations of 3-10 ..mu..g/L), 16% for PCB isomer groups (concentrations unknown), and 2.9% for total PCBs (concentrations of 27-130 ..mu..g/L). Method bias was calculated for each pesticide and for total PCB concentrations in each of the three data sets. The highest method bias was observed with river water extracts. The detection limit for each pesticide in reagent water is approximately 1 ..mu..g/L.

Automatic emission spectrometer for the determination of nitrogen-15

An automatic nitrogen-15 analyser, employing a novel use of a rhodium-platinum catalyst for the generation of nitrogen and capable of analysing 60 samples per hour, is described. Nitrogen compounds of biological origin are first converted into ammonium chloride by conventional Kjeldahl digestion and distillation methods. The ammonium chloride sample (about 5 µl containing about 10 µg of nitrogen) is injected into a soda-lime reactor at 590 °C through which flows a stream of purified helium. Ammonia that is released passes directly into the catalyst tube and the generated nitrogen and hydrogen are separated by passage through a gas-chromatographic column, which also retains the water.After passing through a pressure restrictor the nitrogen flows in the helium stream through a Spectrosil discharge tube located in a microwave cavity. The emitted radiation is analysed by means of a specially constructed dual-wavelength monochromator and the intensities of the 14N14N (297.7 nm) and 14N15N (298.3 nm) bands are measured simultaneously by two photomultipliers. Amplified signals, proportional to the peak intensities, are fed through phase-sensitive detectors into a ratiometer, the output from which is fed to a digital voltmeter and printed out in terms of nitrogen-15 abundance. A peak detector indicates the total nitrogen content of each sample and actuates the nitrogen-15 print-out.The response of the instrument is slightly curvilinear but may be regarded as linear over limited ranges. Calibration can therefore be achieved by running suitably chosen standards to fix upper and lower set points. Carryover between samples is very small and is eliminated by running duplicates. Standard deviations of replicate measurements of natural abundance and enriched standards are less than 0.01 atom-%, while determinations of nitrogen-15 in biological samples were shown to be accurate to ± 0.01 atom-% by comparison with a Statron NOI-4 nitrogen-15 analyser.