1944
Anal. Chem. 1900, 6 0 , 1944-1947
Surface Characterization of a National Bureau of Standards Glass Reference Material by 252CfParticle Desorption Mass Spectrometry W. R. Summers, M. U. D. Beug-Deeb, and E. A. Schweikert* Center for Chemical Characterization & Analysis, Texas A&M University, College Station, Texas 77843-3144
National Bureau of Standards glass Standard Reference Materlals have been characterlzed by particle desorption mass spectrometry (PDMS). Surface cleanlng procedures, such as etching wlth HF or klloelectronvolt Ion sputterlng, allow the detection of low-level dopants. Dlfferences observed in the PDMS spectra before versus after sputter removal of surface hydratlon layers reflect the sensitlvlty of this technique to the chemlcal envlronment of the sample region from which the analytical signal orlglnates. The small number of partlcles (ca. I O 5 ) needed to obtaln a useful spectrum minlmlres sample charging problems often encountered In surface analysis of Inwlatlngmaterlals. Ll, Be, Rb, and Cs were detected in NBS glass, nominally doped at the 500-ppm level. Only Ll was detected In the 50-ppm doped glass. Therefore, the detection llmlts range between 50 and 500 ppm. The w e of a dual grid assembly for the acceleratlon of the desorbed ions resulted in Increased efficiency of the tlmeof-fllght mass spectrometer.
The surface characterization of insulating glasses and ceramic materials is of great importance in many applications (e.g., nuclear waste storage, dating of artifacts, and glass manufacturing). The complex chemistry a t the interface of the glass matrix and the atmosphere, as well as the insulating nature of these samples, renders their analysis difficult. During exposure to the atmosphere, a hydration layer can form on the glass surface, causing the selective partitioning of elements from the matrix. This layer is typically 50-60 nm deep ( I ) . Glass surface studies have been carried out with secondary ion mass spectrometry (SIMS) (2),ion scattering spectroscopy (3),Auger spectroscopy ( 4 ) ,X-ray photoelectron spectroscopy ( I ) , and the laser microprobe mass analyzer (5). A general concern with all techniques, except those that are instantaneously destructive, is the preservation of sample integrity during analysis (1,6). Static SIMS and fast atom bombardment mass spectrometry are among “low-damage’’procedures (7, 8 ) . A further significant decrease in sample damage is feasible with particle desorption mass spectrometry (PDMS), where the quantity of specimen consumed amounts typically to around 1 ppm of the sample analyzed. PDMS uses fast heavy ions, typically fission fragments from the radionuclide 252Cf,to induce the desorption of secondary ions, which are then identified by time-of-flight mass spectrometry. This technique was pioneered by Macfarlane (9) and has been used mainly for the study of large, fragile biomolecules (10). Yet, the technique may be equally well suited for the elemental and isotopic determination of complex surfaces. As already noted, minimal sample disturbances occur, since PDMS is based on single ion counting and simultaneous multielement detection. More specifically, only 105-106 primary ions are needed to generate a useful mass spectrum on a sample spot of 3-4 mm (11). Assuming that 1000 atoms are emitted per fission fragment (12) from the top 10 layers (13),then approximately W4%-lO”% of the surface is consumed. 0003-2700/88/0360-1944$0 1.50/0
The applicability of PDMS for the characterization of glass surface treatments has been demonstrated previously (14). In this study, the elemental sensitivity of PDMS has been evaluated by analyzing samples with known concentrations of several alkali elements. The surface contaminants on the glasses were removed by two methods: etching with HF and sputter cleaning. National Bureau of Standards glasses were chosen as samples for this study because they are well characterized insulating materials. EXPERIMENTAL SECTION Samples. National Bureau of Standards Reference Material glasses (SRM 611 to 617) were used in this study. The bulk composition of these glasses is 72% SO2,12% CaO, 14% Na20, and 2 % A1,03 (15). In addition, they are doped with 61 elements (as oxides) at the 500-ppm level for SRM 611, the 50-ppm level for SRM 613, the 1-ppm level for SRM 615, and the 0.2-ppm level for SRM 617. Two surface cleaning procedures were used: etching with HF and sputtering. The glasses were etched in 10% HF for 4 min. Another set of glasses was cleaned by sputtering at normal incidence with 1pA of 5-keV Xe+ on a beam spot 3 mm in diameter. The 30-min sputtering resulted in a removal of 660-990 A (11). Apparatus. The particle desorption time-of-flight mass spectrometer is shown in Figure 1. A 50-pCi 252Cfsource emits pairs of fission fragments, 180’ apart. One of the fragments strikes the sample and induces the desorption of secondary ions. These ions are identified by their flight times in a 20-cm drift tube, with a resolution ( m / A m )at full width at half-maximum (fwhm) of 220 at m/z 133 (16). The secondary ions are detected at the end of the drift tube by a microchannel plate detector. The other fission fragment from the pair strikes the s t a r t detector. For the analysis of the etched glasses, the fission fragment passes through an aluminized Mylar foil (1.4 wm thick) and induces the emission of electrons, which are then detected by microchannel plates. For the analysis of the sputtered glasses, the data was collected with a surface barrier detector, which decays over time under the constant fission fragment flux. The pressure in the experimental mbar during all analyses; data was chamber was ca. 5 x acquired for 12 h (etched) and 1 h (sputtered). Another experimental feature shown in Figure 1 involves the insertion of an electrically isolated grid between the sample and the acceleration grid at the beginning of the drift tube. The additional grid was maintained at a lower potential than that of the target, but with the same polarity. The physical dimensions of this assembly are given in Figure 1. RESULTS AND DISCUSSION Dual Grid Assembly. Multiple grid assemblies have been used both at the beginning (17) and end (18) of the field-free drift region in time-of-flight spectrometers to study metastable decomposition reactions. In this study, the dual grid assembly was used to increase the desorbed ion collection efficiency by improving the shape of the electrostatic field lines that control the secondary ion extraction. Additionally, this electrostatic field has a lens effect. Various voltage combinations were applied to the target and the first grid to determine their effect on the collection and transmission efficiencies. The optimum efficiency occurred with the target biased at 9 kV and the grid at 5 kV. Therefore, these voltages were used for all subsequent analyses. 0 1988 American Chemical Society
1945
ANALYTICAL CHEMISTRY, VOL. 60, NO. 18, SEPTEMBER 15, 1988
Table I. Mass Assignments for Positive Ion Glass Spectra
identification
mass
MICROCHANNEL PLATE DETECTOR
U S T A R T DETECTOR
Figure 1. Particle desorption time-of-flight mass spectrometer: a, target-grid distance, 1-4 mm; b, grid-grid distance 2.7 mm; c, aperture, 20 mm.
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