INDUCTIVELY COUPLED PLASMA–ATOMIC EMISSION SPECTROMETRY

ICP-AES is a method of qualitative and quantitative analysis for elemental composition of samples. ICP analysis is based on the measurement of wavelengths and intensities of spectral lines emitted by secondary excitation. ODP continually evaluated the analytical capabilities of the shipboard laboratories and decided to upgrade the analytical capability of the XRF system by installing the ICP-AES instrument. In addition to analyzing hard rocks, the ICP system was used to analyze interstitial waters and sediments.

The Jobin-Yvon JY2000 instrument was installed in the Chemistry Laboratory on the JOIDES Resolution prior to Leg 187. Extensive testing was done during Leg 187 to evaluate the ICP system, create the ODP procedures for analyzing hard rock samples, and compare the results with duplicate analyses on the XRF. Overall, the analytical results were within the margin of error. Some advantages of using ICP included: samples could be analyzed more quickly, less sample material was required, and more trace elements could be measured reliably at lower detection limits. Additional procedures were developed during Legs 188 and 189 for analyzing interstitial water samples and sediments.

Data Acquisition

ICP-AES works on the concept that excited electrons emit energy in narrow, well-defined wavelengths as they return to ground state. Characteristic wavelengths have been identified for many oxides and elements. The intensity of the energy at a given wavelength is proportional to the concentration of that element in the sample. The constituents of an unknown sample can be quantified by comparing the measured intensities to standards with known composition.

Sample Preparation

ICP-AES analysis requires that the sample be in solution. Sediments and hard rocks had to be dissolved. A method similar to XRF sample preparation was developed to avoid dealing with hydrofluoric acid in the shipboard environment. A washed and dried sample was powdered by crushing the sample between two plastic disks in a hydraulic press. Powder was produced by grinding pieces <1 cm in diameter in a Spex Shatterbox, using a tungsten carbide grinding vessel, the same procedure for producing powdered samples for XRF analyses. Typically, 0.1 g of sample powder was mixed with 0.4 g ultrapure-grade LiBO2 flux and LiBr wetting agent in a Pt-Au crucible. This mixture was fused at 1050°C for 10–12 min. After the bead cooled, it was dissolved in nitric acid. A small amount of filtered solution was diluted by additional nitric acid. This method was preferable and resulted in a stable sample solution that could be safely transported to scientists' home laboratories for additional study.

Interstitial water samples were much easier to prepare. The filtered interstitial water sample was acidified with dilute nitric acid and diluted again with deionized water. Undiluted interstitial water samples could be run, but care was needed to not clog the nebulizer on the ICP-AES instrument.

Calibration

ICP-AES analytical methods are based on comparison of the unknown sample's line intensities to one or more well-characterized standard reference materials. The calibration for ICP-AES needed to be performed on each run. For hard rocks normally collected by ODP, a number of standards have been used, as the variability of sediment compositions precluded using a single standard. Sediments are often combinations of shales, carbonates, and siliceous deposits, so combinations of standards can be used to cover much of the spectral range. Table T9 contains many of the hard rock and sediment standards used by ODP for XRF and ICP-AES analyses.

The standards used for calibration for interstitial water analyses must be constructed. The recommended method was to spike filtered surface seawater in order to create a master standard solution. The master standard solution could also be created using IAPSO standard seawater.

Analyses Data

The analytical run began within 10 min of the end of the calibration run. The first sample was the master drift sample, analyzed at the beginning and again at the end of the run. Preliminary results were calculated by the system software, but the complete data reduction had to be done by the scientists. Data were loaded into an Excel spreadsheet with many of the necessary calculations previously set up.

A more complete discussion on procedures for collecting ICP-AES data can be found in ODP Technical Note 29 (Murray et al., 2000).

Archive

Janus ICP-AES Data Format

Because the data collected on the ICP-AES system were very similar to data collected on the XRF system, ICP-AES data for hard rocks and sediments were archived in the same tables as the XRF data. Interstitial water data were entered into the tables with the other interstitial water analyses.

The data model for ICP-AES element composition can be found in "Janus ICP-AES Data Model" in "Appendix D." ICP-AES major oxide and trace element data can be retrieved from Janus Web by using a predefined query. The ICP-AES query Web page allows the user to extract data using the following variables to restrict the amount of data retrieved: leg, site, hole, core, section, depth range, or latitude and longitude range. Occasionally, replicate samples were analyzed for the major oxides. Those data, when available, were uploaded separately. The Web query reports the replicate data on separate lines. In addition, the trace element data will also be reported on a separate line, even though trace element data were collected at the same time as the major oxide data.

Table T10 lists the data fields retrieved from the Janus database for the ICP-AES predefined query. The first column contains the data item, the second column indicates the Janus table or tables in which the data were stored, and the third column is the Janus column name or the calculation used to produce the value. "Description of Data Items from ICP-AES Query" in "Appendix D" contains additional information about the fields retrieved using the Janus Web ICP-AES query and the data format for the archived ASCII files.

Data Quality

The quality of the ICP-AES data was considered very good, even though the ship environment was a difficult environment in which to get accurate measurements. ICP-AES analyses performed on volcanic, mafic, and ultramafic samples are the highest quality because there are standards that characterize the range of elemental concentrations most often found in these types of rocks. It is more difficult to create standards that would encompass the range of elemental concentrations found in sedimentary environments.

In order to aid the interpretation of ICP-AES analyses, an additional column of information was added to the XRF_SAMPLE table after ICP-AES data were being archived in Janus. This column, Sample_type, was added to allow scientists to describe the type of rock that was analyzed, and in turn, allow the extraction of data based on specific rock types. These data were not usually stored in the original data files, and it was necessary to extract that information from the Initial Reports volumes. Because of constraints of time, it was not possible to complete entering this information.

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