Page 207 - Inorganic Mass Spectrometry - Fundamentals and Applications

Secondary Ion Mass Spectrometry I93

as shown in Fig. 4.30, unknowns could be quantified by determining their and
selecting the modified RSFs from the chart. He also suggested that E,~ could be es-
tablished by measuring ratios such as M2+/M+, ~O+/M+, MN+/M+, or others that
might be sensitive to and independent of element concentration. Ganjei et al.
E1021 applied this idea and made it practical by using changes in ambient oxygen
pressure to induce a range of sampling environment changes over which sensitiv-
ity factors could be measured from one standard. Sensitivity factors were plotted
versus a matrix ion species ratio such as 56Fec/54Fe+ in a steel standard. They called
this technique the matrix ion species ratio (MISR) method. Accuracies achieved
on a number of steels, aluminum alloys, and copper alloys were on the order of
10% relative. The best accuracy and precision were achieved with the introduction
of small additions of oxygen to the surface of the urknowns.
SNS, great care must be taken to reproduce
For successful quantification in
instrument conditions as closely as possible. These conditions include energy and
of
current density of the primary beam, type and purity the primary, vacuum con-
ditions, angle of incidence of the beam with the sample, energy bandpass of the
of
the
mass spectrometer, and detector efficiency. In addition, correct identification
secondary ions (i.e., without interference) is crucial to good quantification. As dis-
cussed in other chapters, this can be achieved with high mass resolution to sepa-
rate ions of interest from interfering species, through energy filtering, or by man-
ual or computer spectral stripping (i.e., subtraction). The sample and standards are
also important. A good standard is homogeneous on a micrometer scale and char-
acterized. These may be reference materials generated by organizations such as the
National Institute of Standards and Technology, standards from independent ma-
terial specialists, or ion implantation standards in which care is taken to measure
dose accurately and maintain purity of the implanted ion. Samples that have sec-
be
ond-phase inclusions are very difficult to quantify completely. Each phase must
treated separately as ion formation conditions are quite different. Where multiple
crystal orientations exist on the sample surface, each crystallite needs to be quan-
tified separately as ion emission varies greatly, depending on the orientation with
respect to the primary beam and the secondary ion extraction field.

At its simplest the secondary ion mass spectrometer needs only five elements.
These are an ion source to create the bombarding primary ions, a target or sample
holder, a mass analyzer for analysis of the secondary ions, a detector to transform
the ions to recordable form, and a recording device. Usually, the practical
SNS