Elemental Analysis

Abstract

Elemental analysis is the measurement of certain elements within a sample. A variety of techniques used for this purpose, including mass spectrometry, X-ray fluorescence spectrometry, atomic absorption, inductively coupled plasma tech-niques, and instrumental neutron activation analysis, were described previously. The automatic elemental analyzers discussed in this chapter are typically used to measure elements commonly found in organic and certain inorganic substances, specifically carbon, hydrogen, nitrogen, oxygen, and sulfur. When applied to archae-ological materials, carbon–hydrogen–nitrogen (CHN) or carbon/nitrogen (C/N or C:N) ratios can be used to characterize an unknown substance as organic or inor-ganic or determine if it is a protein. Most textbooks do not explain in detail how these instruments work so the descriptions below are based on product informa-tion from Leco Corporation (2007) and PerkinElmer, Inc. (2005). Techniques used for the analysis of data generated by the instruments are outlined in chemistry and instrumentation textbooks such as Pasto and Johnson (1979), Skoog et al. (1998), Solomons (1980), and Solomons and Fryhle (2004). Instrumentation The processes performed during elemental analysis vary according to the targeted elements but typically involve sample combustion, reduction to simple gases, and separation of combustion gases and detection. A portion of the substance is care-fully weighed into a quartz combustion tube, or a metal vial made of either tin or aluminum, and placed inside the instrument. The system is then completely purged with the carrier gas, usually helium or argon, to remove all interfering gases. Combustion is performed in a stream of oxygen at around 950 • C or in the pres-ence of metal oxides, such as cobalt oxide or a mixture of manganese dioxide and tungsten oxide. The combustion gases are then exposed to reagents, usually copper and copper oxide, to convert carbon monoxide, CO, into carbon dioxide, CO 2 , and different nitrogen oxides into N 2 ; sulfur is measured as sulfur dioxide, SO 2 , and hydrogen is measured as water vapor, H 2 O.