Surface analysis data acquisition

Like XPS, the application of AES has been very widespread, particularly in the earlier years of its existence more recently, the technique has been applied increasingly to those problem areas that need the high spatial resolution that AES can provide and XPS, currently, cannot. Because data acquisition in AES is faster than in XPS, it is also employed widely in routine quality control by surface analysis of random samples from production lines of for example, integrated circuits. In the semiconductor industry, in particular, SIMS is a competing method. Note that AES and XPS on the one hand and SIMS/SNMS on the other, both in depth-profiling mode, are complementary, the former gaining signal from the sputter-modified surface and the latter from the flux of sputtered particles. 

Certainly, the inherent lack of depth resolution of the BMP techniques minimizes the utility of sputter profiling combined with BMP analysis. However, gross comparisons of the exterior versus interior composition can be obtained by recording X-ray spectra before and after a minimum of several thousand angstroms of material are sputtered from the particle surface (13,44). Similarly, BSCA is not very suitable when used in conjunction with sputter profiling for reasons that include a) data acquisition rates are very slow, b) potential chemical information is lost since sputtering may alter the chemical forms of the elements present, and c) individual particles cannot be depth profiled (11, 14, 26). 

As illustrated by the examples above, the analysis of DNA extracted from archaeological specimens can provide unique insights about the past. However, the study of aDNA is methodologically challenging, primarily due to the problematical state of its preservation. As such, protocols developed specifically for the extraction and analysis of ancient DNA have been developed, of which we have highlighted one that we have developed over the past few years. Our protocol offers solutions to two of the most common problems associated with the study of aDNA the possible presence of contamination on the surfaces of samples and/or the co-extraction of PCR inhibitors. We are optimistic that with attention to proper aDNA protocols, data acquisition, and authentication of results, DNA extracted from archaeological specimens will continue to provide a wealth of information about the past. 

Instruments of this type may also be used quite effectively to evaluate kinetics of time-dependent changes in foods, be they enzymatic or reactive changes of other types. The computerized data-acquisition capabilities of these instruments allow precise measurement of absorbance or fluorescence changes, often over very brief time periods ( milliseconds). This is particularly useful for analysis of fluorescence decay rates, and in measurement of enzymatic activity in situ. A number of enzyme substrates is available commercially which, although non-fluorescent initially, release fluorescent reaction products after hydrolysis by appropriate enzymes. This kinetic approach is a relatively underused capability of computerized microspectrophotometers, but one which has considerable capability for comparing activities in individual cells or cellular components. Fluorescein diacetate, for example, is a non-fluorescent compound which releases intensely fluorescent fluorescein on hydrolysis. This product is readily quantified in individual cells which have high levels of esterase [50]. Changes in surface or internal color of foods may also be evaluated over time by these methods. 

Data Acquisition. Frequently, data analysis takes as much or more time than data acquisition, and appropriate analysis can be done only if adequate data were taken initially. Because choosing an appropriate surface technique does not adequately define what is to be done, considerable thought needs to go towards determining the types of data to be taken and into possible control samples or other checks to insure that the data are meaningful. 

The purpose of this section is to show, by example, how the concerns of technique selection, potential problems, data acquisition and analysis have been applied for several different corrosion problems and techniques. Examples of fundamental research work and industrial problem solving have been included to show the range of applicability of the techniques. In most cases, more than one technique was used to solve the problem. Frequently, a surface analysis technique was used in combination with one or more other types of analysis method. These examples are not comprehensive it is hoped that sufficient references have been supplied to enable the reader to find other work of relevant interest. 

Consideration of Surface Tool Concerns. Reasonably fast data acquisition, small probing beam size (allowing both faster sputter profiling and spatial resolution) and semiquantitative data analysis, give AES a primary role in each of these two experiments. 

The available data from emulsion polymerization systems have been obtained almost exclusively through manual, off-line analysis of monomer conversion, emulsifier concentration, particle size, molecular weight, etc. For batch systems this results in a large expenditure of time in order to sample with sufficient frequency to accurately observe the system kinetics. In continuous systems a large number of samples are required to observe interesting system dynamics such as multiple steady states or limit cycles. In addition, feedback control of any process variable other than temperature or pressure is impossible without specialized on-line sensors. This note describes the initial stages of development of two such sensors, (one for the monitoring of reactor conversion and the other for the continuous measurement of surface tension), and their implementation as part of a computer data acquisition system for the emulsion polymerization of methyl methacrylate. 

Since its inception about 15 year ago, MALDI-IMS has been developed into a powerful and versatile tool for biomedical research. It allows for the investigation of the spatial distribution of molecules at complex surfaces. The combination of molecular speciation with local analysis makes a chemical microscope that can be used for the direct biomolecular characterization of histological tissue section surface. However, successful detection of the analytes of interest at the desired spatial resolution requires careful attention to several steps in the IMS protocol matrix selection, matrix coating, data acquisition, and data processing. MALDI-IMS is increasingly playing an important role in the drug discovery and development and disease treatment. 

Figure 66 shows the instrument (Qin and Liu, 1985) used for automatic tracking of the surface of the collapsing bed, inclusive of a computer for data acquisition and on-line analysis. The fluidized bed is 5 cm in diameter... 

SFM also enables us to measure specific interaction forces between a small silicon tip and the surface. The pull-off forces between the tip and the surface estimated from Force vs Distance Curves (FDC) can be correlated to the adhesive interactions between tip and surface [9]. Recording of FDCs line-by-line allows us to image surface topography and adhesive surface properties simultaneously [10]. This technique has some disadvantages, like the requirement for a large amount of data acquisition and analysis, which have been alleviated by the invention of the Pulsed Force Mode (PFM). The PFM simplifies and accelerates the measurements of adhesive properties with high lateral resolution [11, 12]. 

Intact-cell MALDI-TOF analysis offers several attractive features for rapid screening of bacterial collections. Analysis is performed directly on the cells after minimal sample preparation, and data acquisition is complete in only a matter of minutes. Intact biomarkers are introduced into the MALDI-TOF instrument under these conditions. Whether the observed biomarker molecules are desorbed directly from the surface of the cell wall or are extracted from the cells and co-crystallized with the matrix is currently unresolved, but MALDI spectra of intact bacteria generally contain a large number of peaks in the mass range 1-20 kDa [31]. For bacterial cells, proteins are the most often observed biomarkers. While this approach samples only a small percentage of the total proteins produced in the cells, these profiles have been reported by many groups to be suitable for taxonomic identification, down to at least the strain level. The wide availability of the MALDI-TOF instrumentation and its relative ease of use, coupled with relatively simple sample preparation procedures, have been key features in the rapid advancement of this approach. 

Spectra. The energy spectrum is collected from the particles emitted from all depths simultaneously using a silicon surface barrier detector, electronic amplifiers, an analog-to-digital converter and a multichannel analyzer. A reference pulse is fed into the electronics to monitor the stability of the system thus allowing corrections to be made should electronic drift occur during the course of the measurement. Specific systems are described in the references (1 -4,6,7,12-17). By using computer-based data acquisition systems, the depth profile can be displayed at the time of analysis. 

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