Quality Assurance/Quality Control

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δ²H replicate analysis of water by dual-inlet isotope-ratio mass spectrometry: Sample preparation by equilibration with gaseous hydrogen and automated analysis; requires 1 to 2 milliliters of water. Isotopic analysis by laser absorption spectrometry is avoided as a primary analytical technique for all USGS water samples because this method can produce anomalous results that will not hold up in court. The "gold standard" for δ²H determination on water is dual-inlet isotope-ratio mass spectrometry. For 2-mL samples, the estimated expanded uncertainty of replicate hydrogen-isotope measurements is 2 per mil. This means that if the same sample were resubmitted for isotopic analysis, the newly measured value would lie within the uncertainty bounds 95 percent of the time.

δ¹⁸O replicate analysis of water by dual-inlet isotope-ratio mass spectrometry: Sample preparation by equilibration with carbon dioxide and automated analysis; requires 0.2 to 2.0 milliliters of water. Isotopic analysis by laser absorption spectrometry is avoided as a primary analytical technique for all USGS water samples because this method can produce anomalous results that will not hold up in court. The "gold standard" for δ¹⁸O determination on water is dual-inlet isotope-ratio mass spectrometry. For 2-mL samples, the estimated expanded uncertainty of replicate oxygen-isotope measurements is 0.2 per mil. This means that if the same sample were resubmitted for isotopic analysis, the newly measured value would lie within the uncertainty bounds 95 percent of the time.

δ²H and δ¹⁸O replicate analysis of water extracted from soils and plants: Water is extracted from soils and plants by distillation with toluene; recommended sample size is that which will provide 1 to 5 milliliters of water per sample. Analysis is by dual-inlet isotope-ratio mass spectrometry. Isotopic analysis by laser absorption spectrometry is deprecated because the smallest remaining fraction of toluene in water will produce anomalous isotopic results.

δ¹⁵N and δ¹⁸O replicate analysis of dissolved nitrate in water: Dissolved nitrate in water is converted to nitrous oxide (N2O) by denitrifying bacteria, and the nitrous oxide is analyzed for nitrogen and oxygen isotopic abundance by continuous-flow isotope-ratio mass spectrometry. A minimum total amount of 0.002 milligrams of nitrate as nitrogen is required. The estimated expanded uncertainty of δ¹⁵N measurement results for samples with nitrate concentrations of at least 0.06 mg/L as nitrogen is ±0.5 per mil unless otherwise specified, and this value is conservative. The estimated expanded uncertainty of δ¹⁸O measurement results for samples with nitrate concentrations of at least 0.06 mg/L as nitrogen is ±0.5 per mil unless otherwise specified, and this value is conservative. If any given sample were resubmitted for nitrogen- and oxygen-isotope analysis, the measured values would fall within the uncertainty bounds of the previous results more than 95 percent of the time.

δ¹⁵N replicate analysis of dissolved ammonium in water: Dissolved ammonium in freshwater or saline water is converted to gaseous ammonia (NH₃), converted quantitatively to ammonium sulfate, and combusted in an elemental analyzer to gaseous nitrogen and sulfur dioxide. The gaseous nitrogen is separated by gas chromatography and analyzed for nitrogen isotopic composition with continuous-flow isotope-ratio mass spectrometry. The concentration of ammonium in sample must be at least 0.2 milligrams per kilogram as nitrogen. A minimum amount of ammonium in sample must be 0.2 milligrams as nitrogen. The estimated expanded uncertainty of δ¹⁵N measurement results is ±0.8 per mil unless otherwise specified. If any given sample were resubmitted for nitrogen-isotope analysis, the measured values would fall within the uncertainty bounds of the previous results more than 95 percent of the time.

δ¹³C of dissolved inorganic carbon: The method is modified from the ThermoFisher Scientific GasBench II Operating Manual (January 2018). The carbon isotopic composition of CO2 released from dissolved carbon in a sample by reaction with H3PO4 is measured. The expanded uncertainty is ±0.4 per mil, unless otherwise indicated. This means that if the same sample were resubmitted for isotopic analysis, the newly measured value would lie within the uncertainty bounds 95 percent of the time.

δ³⁴S replicate analysis of dissolved sulfate in water: Dissolved sulfate is precipitated to barium sulfate, combusted in an elemental analyzer to sulfur dioxide, and analyzed for sulfur isotopic composition with continuous-flow isotope-ratio mass spectrometry. The concentration of sulfate must be at least 20 milligrams per liter. The estimated expanded uncertainty of δ³⁴S measurement results is ±0.4 per mil unless otherwise specified. If any given sample were resubmitted for sulfur-isotope analysis, the measured values would fall within the uncertainty bounds of the previous results more than 95 percent of the time.

δ³⁴S replicate analysis of low-concentration sulfate in water: Dissolved sulfate having a concentration of less than 20 milligrams per liter is collected on an anion-exchange resin in the field, eluted in the laboratory with potassium chloride, precipitated with barium chloride to barium sulfate, combusted in an elemental analyzer to sulfur dioxide, and analyzed for sulfur isotopic composition with continuous-flow isotope-ratio mass spectrometry. The estimated expanded uncertainty of δ³⁴S measurement results is ±0.4 per mil unless otherwise specified. If any given sample were resubmitted for sulfur-isotope analysis, the measured values would fall within the uncertainty bounds of the previous results more than 95 percent of the time.

δ³⁴S replicate analysis of total sulfur in solids: A homogeneous powder is combusted with an elemental analyzer. Combusted gases are separated by gas chromatography and the sulfur dioxide is analyzed for sulfur isotopic abundance by continuous-flow isotope-ratio mass spectrometry. The estimated expanded uncertainty of δ³⁴S measurement results is ±0.4 per mil unless otherwise specified. If any given sample were resubmitted for sulfur-isotope analysis, the measured values would fall within the uncertainty bounds of the previous results more than 95 percent of the time.

δ¹⁵N and δ¹³C replicate analysis of total nitrogen and carbon in solids: A homogeneous powder is combusted to gaseous nitrogen and carbon dioxide with an elemental analyzer. These gases are separated by gas chromatography and are analyzed for nitrogen and carbon isotopic abundance by continuous-flow isotope-ratio mass spectrometry. A minimum required amount of nitrogen and carbon in a sample is 0.15 milligrams and 0.15 milligrams. The estimated expanded uncertainty of δ¹⁵N measurement results is ±0.5 per mil unless otherwise specified. The estimated expanded uncertainty of δ¹³C measurement results is ±0.5 per mil unless otherwise specified. If any given sample were resubmitted for nitrogen- and carbon-isotope analysis, the measured values would fall within the uncertainty bounds of the previous results more than 95 percent of the time.

δ¹⁵N replicate analysis of total nitrogen in solids: A homogeneous powder is combusted to gaseous nitrogen and carbon dioxide with an elemental analyzer. These gases are separated by gas chromatography and the gaseous nitrogen is analyzed for nitrogen isotopic abundance by continuous-flow isotope-ratio mass spectrometry. A minimum required amount of nitrogen in a sample is 0.15 milligrams. The estimated expanded uncertainty of δ¹⁵N measurement results is ±0.5 per mil unless otherwise specified. If any given sample were resubmitted for nitrogen-isotope analysis, the measured values would fall within the uncertainty bounds of the previous results more than 95 percent of the time.