acquisition rates > 2 MHz (22). Acoustic emission as a mecha n i s t i c probe. Different emission producing mechanisms can be detect ed by s e p a r a t i n g t h e i r acoustic
H202
Immobilized catalase
Figure 4. Selective chemical sensor based on passive AE.
signals into classes on the basis of their shapes, frequency domain char acteristics, and amplitude (23). These classes are associated with mecha nisms by visual or other observa tions. Lee and co-workers (16) were able to distinguish crystal growth from crystal fracture during experi m e n t s t h a t c o m b i n e d AE w i t h dilatometry. Wade and co-workers (21) suggested t h a t the dominant emission mechanism during recrystallization of salts such as potassium nitrate may be crystal fracture occur ring during growth. Wentzell and co workers (15) found t h a t different bubble evolution sites on an enzyme catalytic surface produce repeatable emission waveforms, and each has its own particular acoustic signature. The hydration of silica gel is a pro cess in which violent audible emis sion from gel granule fracture is ac companied by considerable emission at ultrasonic frequencies (14). A de tailed study of this reaction (23) used five descriptors (maximum ampli tude, variance, half-life, median fre quency, and bandwidth) to character ize each of 80 AE s i g n a l s . Five principal components (eigenvectors) were then extracted from the covari-
ance matrix of these data. These or thogonal (uncorrelated) linear combi nations of the original five variables were then ranked. Most of the (vari-
Figure 5. Principal-component projection of data from hydration of silica gel granules. Groups marked 1 and 4 are higher energy signals that were visually related to the cracking of the sil ica gel granules. Groups 2 and 3 are of lower en ergy and associated with gas evolution processes. Group 5 is of uncertain origin. (Adapted from Ref erence 23.)
NEW GENERATION IMAGING TTME-OF-FLIGHT S M S Provides chemical mapping with high spatial and high mass resolution. rime-of-flight secondary ion mass spectrometry is a powerful technique for providing a detailed analysis of the top monolayer of surfaces. A true surface sensitive technique, T0F-S1MS uses a low dose, pulsed primary ion beam to eject secondary ions from a sample with minimal surface damage. The result is extreme surface sensitivity, on the order of femtomolar concentrations, for the identification of molecular compounds in corrosion, adhesion, and contamination studies. The TFS line of modular TOF-SIMS instruments from CHARLES EVANS & ASSOCIATES offers configurations * which include static SIMS, direct ion imaging SIMS, microprobe imaging SIMS, and laser desorption/ablation SIMS. These 90 ν 90 '·( micrometer %mieroprohe 1images of AI MBaterconnects on a semi-conductor device shows a large area of contamination on 2 micron wide AI lines. The left and right images are ofNa ions and Κ ions respectively.
CHARLESEVANS&ASSOCIATES® SPECIALISTS IN MATERIALS CHARACTERIZATION
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Look for us at the ASMS meeting in Nashville, TN. CIRCLE 32 ON READER SERVICE CARD ANALYTICAL CHEMISTRY, VOL. 63, NO. 9, MAY 1, 1991 · 503 A
