Biomedical subjects

George C Rhoderick

Publications and source records attributed to George C Rhoderick.

6 recordsLinked to original sources

PubMed2006

Feasibility of preparing and analyzing gas standards containing heavy hydrocarbons (C10-C16).

State and federal agencies are beginning to monitor ambient air for compounds related to diesel exhaust. The National Institute of Standards and Technology (NIST) was asked to develop standards containing heavy (C10-C16) alkanes which could then be used in air monitoring and exhaust studies. Several primary gravimetric standards, containing heavy (C10-C16) alkanes in nitrogen, were developed and analyzed by gas chromatography (GC) with flame-ionization detection (FID). The results of this research indicate that accurate gas standards containing these hydrocarbons can be prepared. However, the analytical results show that the temperature of the transfer system from the gas cylinder to the GC column (including the gas-sample valve) must be heated in order to prevent adsorption of these compounds within the analytical system. The results indicate that even at elevated temperatures these compounds are being absorbed within the system. The results show that quantitative results cannot be obtained by using one compound such as hexane, as an internal standard to determine the concentration of other hydrocarbons. Quantitative and accurate results are best obtained if standards for each hydrocarbon of interest are used to determine concentrations of unknowns for the respective hydrocarbon.

Journal Article↗

PubMed2006

Development of a NIST standard reference material containing thirty volatile organic compounds at 5 nmol/mol in nitrogen.

Primary gravimetric gas cylinder standards containing 30 volatile organic compounds (VOCs) in nitrogen were prepared using a procedure previously developed to prepare gas mixture cylinder standards of VOCs at the 5 nmol/mol level. This set of primary standards was intercompared to existing gas cylinder standards, containing as many as 19 of the 30 volatile organics present in these new primaries, using gas chromatography with a hydrogen flame ionization detector coupled with cryogenic preconcentration. The linear regression analysis showed excellent agreement among the standards for each compound. Similar mixtures containing many of these compounds in treated aluminum gas cylinders have been evaluated over time and have shown stability for as much as 10 years. The development of these 30-component primary standards led to the preparation and certification of a reissue of Standard Reference Material (SRM) 1804 at the nominal amount-of-substance fraction of 5 nmol/mol for each analyte. A lot of 20 cylinders containing the mixture was prepared at NIST following previously demonstrated protocols for preparation of the cylinders. Each cylinder was analyzed against one cylinder from the lot, designated as the "lot standard," for each of the 30 compounds. As a result of the uncertainty analysis, the data showed that rather than declaring the lot homogeneous with a much higher uncertainty, each cylinder could be individually certified. The expanded uncertainty limits ranged from 1.5 to 10% for 28 of the 30 analytes, with two of the analytes having uncertainties as high as 19% in those SRM cylinders certified. Due to stability issues and some high uncertainties for a few analytes in 2 of the samples, 18 of the 20 candidate SRM samples were certified. These volatile organic gas mixtures represent the most complex gas SRMs developed at NIST.

Journal Article↗

PubMed2005

Long-term stability of hydrocarbons in NIST gas standard reference material (SRM) 1800.

A gas standard reference material (SRM) containing fifteen hydrocarbons in nitrogen at a nominal 5 nmol mol(-1) was issued in 1993. The certification period for SRM 1800 was assigned as 2 years, because of limited stability data. Over a period of 10 years reanalysis of the lot standard (a sample chosen from the SRM lot to which all other lot samples are compared), SRM samples remaining in stock for sale, and SRMs returned to the National Institute of Standards and Technology (NIST) for recertification, were compared with primary standards to assess the stability of the hydrocarbons. New primary standards were periodically introduced into the original primary standard suite to assess the stability and consistency of the primary standards. Over this ten-year period 11 SRM 1800 samples were reanalyzed, resulting in 210 amount-of-substance fraction (concentration) determinations performed for quality-assurance purposes. Of these measurements 209 (99.5%) agreed within the original 95% confidence interval of the +/-4% expanded uncertainty, demonstrating the stability of the standards. There was also agreement to within +/-2% of the original concentration for 204 (97%) of the measurements. This is well within the original +/-4% expanded uncertainty assigned to the hydrocarbon concentrations at the approximate 95% confidence interval demonstrating stability. These results will enable the expiry date to be increased for future restock issues of SRM 1800.

Environmental Monitoring↗

PubMed2004

Standards development of global warming gas species: methane, nitrous oxide, trichlorofluoromethane, and dichlorodifluoromethane.

Environmental scientists from federal agencies, such as the National Oceanic and Atmospheric Administration (NOAA), and academia have long suspected that increasing anthropogenic inputs of various trace gases into the atmosphere can cause changes in the earth's climate and protective ozone layer. Nitrous oxide and methane, cited in the Kyoto Protocol, as well astrichlorofluoromethane (CFC-11) and dichlorodifluoromethane (CFC-12), cited in the Montreal Protocol, are all greenhouse gases and are implicated in the destruction of the stratospheric ozone layer. The lack of national standards prompted research to determine the feasibility of preparing accurate and stable standards containing these four compounds. Development of these standards would support the measurement of these species by those in the atmospheric research community not having their own source of standards. A suite of eight primary gas standards containing methane, nitrous oxide, CFC-11, and CFC-12 in a balance of air were prepared gravimetrically to bracket the ambient atmospheric concentrations. The combined uncertainties (uc) were calculated from error propagation analysis that included the weighing data from the gravimetric preparation and other sources of error such as the purity analysis of the compounds and air matrix. The expanded uncertainties (U) for the gravimetric standards were < 0.5% as calculated from the equation U = kuc, where the coverage factor k is equal to 2 for a 95% confidence interval. Analyses of the suite of standards by gas chromatography with flame-ionization and electron capture detection resulted in average absolute residuals of < 0.25% from regression models. The NIST suite of eight gravimetric standards was used to determine the concentrations in two standardsfrom NOAA. Those analyses resulted in bias across the two laboratories of < or = 2.1%.

Air Pollutants↗

PubMed2004

Gas-phase databases for quantitative infrared spectroscopy.

The National Institute of Standards and Technology (NIST) and the Pacific Northwest National Laboratory (PNNL) are each creating quantitative databases containing the vapor-phase infrared spectra of pure chemicals. The digital databases have been created with both laboratory and remote-sensing applications in mind. A spectral resolution of approximate, equals 0.1 cm(-1) was selected to avoid degrading sharp spectral features, while also realizing that atmospheric broadening typically limits line widths to 0.1 cm(-1). Calculated positional (wave- number, cm(-1)) uncertainty is </=0.005 cm(-1), while the 1sigma statistical uncertainty in absorbance values is <2% for most compounds. The latter was achieved by measuring multiple (typically >/=9) path length-concentration burdens and fitting a weighted Beer's law plot to each wavenumber channel. The two databases include different classes of compounds and were compared using 12 samples. Though these 12 samples span a range of polarities, absorption strengths, and vapor pressures, the data agree to within experimental uncertainties with only one exception.

Calibration↗

PubMed2003

Analysis of natural gas: the necessity of multiple standards for calibration.

The importance of natural gas as an international trading commodity and the cost to consumers has made the accuracy of determinations for the components of natural gas very important. Pricing of natural gas is based on the heating value of the gas determined from either calorimetry measurements or calculations based on individual component concentrations determined by gas chromatography (GC). Due to the expense of accurate calibration standards, many analysts and laboratories will use a single calibration standard to perform natural gas determinations. Therefore, the purpose of this study was to determine whether an analyst could accurately measure the components of natural gas, in particular methane, using a single standard, or whether a suite of standards is necessary to calibrate the analytical instrument. A suite of eight gravimetric primary standards was prepared covering a concentration range for methane of 64-94 mol%, with uncertainties of +/-0.05% relative (95% confidence interval). These natural gas primary standards also contained nitrogen, carbon dioxide, ethane, propane, iso-butane, n-butane, iso-pentane, n-pentane, and n-hexane with varying concentrations from 0.02 to 14%. A single analytical method was used in which only the amount of sample injected onto the column was altered. The results show that when injecting a 0.5 ml sample volume a second-order regression through the standards is necessary for the determination of methane. The results for nitrogen, ethane and propane also show the same trend. Only those individual standards whose methane concentration is within 1% of the test mixture predicted a concentration within 0.05% of the regression value. Those individual primary standards whose methane concentration is different by more than +/-1% of the test mixture predicted values differing by +/-0.5 to +/-2.0% from the regression value. These differences lie well outside the predicted concentration uncertainty interval of +/-0.20%. A smaller sample volume, 0.1 ml, resulted in a set of data that could be fit using linear regression. Each of the eight primary standards individually predicted the methane in the test mixture to be within +/-0.11% of the predicted value from linear regression. The data confirm that it is imperative to fully characterize the analytical system before proceeding with an analysis.

Calibration↗