Ion Microprobe Mass Spectrometric Determination of Oxygen in Copper C. A. Evans, J r . Ledgemont Laboratory, Kennecott Copper Corporation, Lexington, Mass. 02173 An ion microprobe mass spectrometer has been used to determine oxygen concentration in a copper matrix. The oxygen blank was 3-5 ppmw when extreme care was taken to maintain a low residual vacuum and clean surfaces in the target chamber. A working curve was established over the range of 20-2300 ppm of oxygen. Precisions were 20-25% RSD for concentrations greater than 100 ppm. The technique has been used to determine oxygen concentration gradients across a surface with a resolution of 300-500 1. In addition, depth concentration gradients can also be examined.
used to reduce the contamination from the residual oxygen pressure in the vacuum and from surfaces within the sample chamber, analyses could be performed in the 10-20 ppmw range. A precision of 2 0 4 5 % RSD has been realized a t oxygen concentrations above 100 ppmw oxygen where sufficient 1 6 0 + intensity is found. A working curve of 160+/63Cuf us. ppmw oxygen has been established and used for the analysis of oxygen concentration gradients in copper.
THEANALYSIS OF OXYGEN in metals has been a continuing problem for analytical chemists. If a large sample ( 2 1 gram) is available, techniques such as neutron activation, inert gas fusion, and vacuum fusion can provide excellent detection limits and accuracy. However, when an analysis is required on a small sample or for concentration gradients over a short distance, the above techniques are not easily applicable. If the oxygen is present above 1%, as in a n oxide inclusion, a specially equipped electron microprobe can provide an analysis. Spark source mass spectrometry has been used by several workers to analyze for oxygen in metals with ppm detection limits and moderate spatial resolution ( I , 2). Although only a few milligrams of material are consumed during an analysis, a larger sample is generally used. Concentration gradients can be analyzed by sectioning and self-electrode techniques or by the use of a probe electrode (3). The ion microprobe mass spectrometer provides another method for the analysis of impurities in solids. This technique has been applied to a variety of materials, including thin films (4-6), metals (7), and insulators (8). The sample is bombarded with an energetic beam of ions, usually 5-15 keV Ar+. The resultant sputtering causes neutral atoms as well as positive and negative ions to be ejected from the sample. Commonly, positive ions are extracted into a mass spectrometer and recorded by the appropriate detector. Since the primary ion beam erodes the sample layer by layer, concentration variations with depth can be measured in addition to surface inhomogeneities. Movement of either the primary beam or the sample permits the measurement of concentration gradients along the sample. Spatial resolution is determined by the ion current homogeneity of the primary beam, primary beam diameter, and the penetration rate of the sample by the ion beam. This paper discusses the application of the ion microprobe to the analysis of oxygen in copper. When extreme care was
The GCA Ion Microprobe Analytical Mass Spectrometer (GCA Technology Division, Bedford, Mass.) was used in this study. Research grade argon was used as the duoplasmatron supply gas. Either Airco research grade or Lif-0-Gen research grade argon was used. Table I details the instrumental operating conditions. The calibration standards were rods of 3.0 to 8 mm in diameter and approximately 5 cm in length. A groove was cut in the rod a t least 2.5 cm from the spot to be analyzed. Low oxygen (
