<descriptor thcode="2"><thesaurus thcode="2" name="USGS Thesaurus" scope="Topics and methods of scientific study carried out by USGS, with product types, scientific disciplines, geologic time, and types of institutional structure and activities.  Broad and shallow, used to help people find scientific information." creator="U.S. Geological Survey" rights="Public domain" edition="2.0" date="2020-01-09" codetype="number" prefix="topic" uri="https://www2.usgs.gov/science/USGSThesaurus" tblname="term" nonpref="nonpref" relterm="relterm" root_code="1" contact="pschweitzer"><category><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/><term thcode="25" code="29" name="distinct labels" parent="26" scope="Preferred labels (descriptor texts) are distinct within the thesaurus. A concept can be specified either by its unique identifier (code) or by the text of its descriptor."/><term thcode="25" code="16" name="hierarchical" parent="11" scope="Substantive broader-narrower relationships exist among the terms."/><term thcode="25" code="13" name="transitive" parent="12" scope="Narrower terms have an &quot;is a&quot; relationship with their broader terms, and generally can be described as part-of, type-of, or instance-of the broader term."/><term thcode="25" code="21" name="disciplines" parent="2" scope="Broad categories of human inquiry or activity, such as scientific disciplines."/><term thcode="25" code="20" name="methods" parent="2" scope="Actions taken to obtain or analyze information"/><term thcode="25" code="19" name="organizational functions" parent="2" scope="Things that organizations do, typically including various types of programs, projects, and services."/><term thcode="25" code="7" name="temporal" parent="2" scope="Terms indicate periods of time, such as chronostratigraphic units, named historical periods or intervals surrounding known historical events, or seasons."/><term thcode="25" code="5" name="theme" parent="2" scope="Terms indicate domains of human concern, such as scientific discipline, societal problems, or characteristics of earth, space, oceans, and living things."/></category><altlabel><altlabel>USGSThesaurus</altlabel><altlabel>USGSThesaurus</altlabel></altlabel></thesaurus><term thcode="2" code="153" name="chemical analysis" parent="619" scope="Chemical techniques used to identify the composition of substances."/><uf><usefor name="detection (chemical)"/><usefor name="geochemical surveys"/><usefor name="leaching (analytical method)"/><usefor name="mineralogical analysis"/><usefor name="nuclear magnetic resonance spectroscopy"/><usefor name="sediment geochemistry"><usewith><term thcode="2" code="1199" name="unconsolidated deposits" parent="1005" scope="Loosely bound sediments such as sand, gravel, and silt which tend to accumulate in low areas or valleys."/></usewith></usefor></uf><bt><term thcode="2" code="619" name="laboratory methods" parent="734" scope="Techniques to analyze and test samples in a place equipped and designed for the work."/><term thcode="2" code="734" name="methods" parent="1" scope="Techniques, methods, procedures, or strategies for research, management, collection, or analysis of scientific information in USGS."/><term thcode="2" code="1" name="Categories" scope="Thematic terms for categorizing information resources.  This thesaurus is intended to support finding and understanding scientific information."/></bt><nt><term thcode="2" code="276" name="DNA sequencing" parent="153" scope="Biochemical procedure to determine the order of nucleotides in genes."/><term thcode="2" code="69" name="atomic absorption analysis" parent="153" scope="Technique used to identify chemicals based on the measurement of the spectra produced by atoms and molecules with absorption of electromagnetic radiation."/><term thcode="2" code="1342" name="atomic emission spectroscopy" parent="153" scope="Chemical detection of element concentration by examining the intensity and wavelength of light emitted from the sample when gaseous sample is ionized and maintained in a plasma state."/><term thcode="2" code="158" name="chromatography" parent="153" scope="Analytical techniques used to separate or partition the components of a gaseous or liquid mixture by differences in absorption rates during flow around or over surface absorbents such as columns of silica, filter papers, or gels."/><term thcode="2" code="328" name="electrophoresis" parent="153" scope="Analytical technique to separate and identify components in a mixture using the differential migration of molecules based on their net charge through a paper or gel medium."/><term thcode="2" code="400" name="flow cytometry" parent="153" scope="Laboratory technique to determine the amount of DNA in cells tagged by fluorescent dye by measuring the intensity of fluorescence under a laser beam."/><term thcode="2" code="711" name="mass spectroscopy" parent="153" scope="Instrumental technique to separate and identify molecules.  Gaseous ions are formed, with or without fragmentation.  Their mass/charge ratios and relative electrical abundance are then measured or the spectra are recorded."/><term thcode="2" code="783" name="neutron activation analysis" parent="153" scope="Sensitive laboratory technique for both qualitative and quantitative analysis of elements in samples.  Target nuclei are bombarded with neutron beams to start nuclear reactions which emit characteristic gamma ray radiations."/><term thcode="2" code="872" name="particle-beam spectroscopy" parent="153" scope="Analytical technique using a concentrated beam of charged subatomic particles."/><term thcode="2" code="919" name="polymerase chain reaction" parent="153" scope="Biochemical technique that uses enzymes to replicate new nucleotides of DNA from DNA triphosphates using DNA as a template."/><term thcode="2" code="1337" name="x-ray diffraction" parent="153" scope="Study of crystals which uses short wave electromagnetic radiations (x-rays). The xrays, scattered by the crystal atoms, show a characteristic interference pattern that is dependent upon the structure of the crystal."/><term thcode="2" code="2001" name="x-ray fluorescence" parent="153" scope="Estimation of chemical element concentration by measuring the energy level of secondary X-rays generated when the sample is bombarded with high-energy X-rays or gamma rays."/></nt><rt><term thcode="2" code="89" name="biochemistry" parent="650" scope="Study of chemical processes that occur within or are mediated by living organisms."/><term thcode="2" code="2299" name="biomarkers" parent="317" scope="Biochemical, physiological, morphological, or histopathological characteristics of organisms that signify exposure to contaminants.  Also used for biogeochemical measurements of rocks to detect biological origin."/><term thcode="2" code="437" name="geochemistry" parent="291" scope="Study of the distribution of chemical elements and natural compounds on the earth and in the atmosphere and the chemical processes that affect the earth."/></rt></descriptor>