State-of-the-art EDS An introduction to elemental analysis with ChemiSEM Technology

Spectral Analysis Introduction Energy dispersive X-ray spectroscopy (EDS) in the scanning electron microscope (SEM) is one of the easiest methods for analyzing the composition of microscopic samples. • Sensitive to low concentrations—minimum detection limits below 0.1% in the best cases • Affords a high degree of relative precision — typically 2-4% • Nondestructive in most situations • Requires minimal sample preparation effort and time • Delivers complete analyses of complex samples quickly, often in under a minute Thermo Scientific™ ChemiSEM Technology offers a leap forward in usability, convenience,...

Spectral Analysis EDS Hardware Overview of EDS Analysis When excited under an electron beam, each element within a sample emits a unique and characteristic X-ray signal. The number of X-rays emitted by each element is loosely related to the concentration of that element and its atomic (Z) number. The role of the EDS system is to capture the X-rays as they are emitted from the sample, convert these X-rays into a series of electrical voltage pulses whose amplitudes reflect the energy of the detected X-rays, and pass these pulses to a multichannel analyzer that creates a histogram of the X-ray spectrum...

Spectral Analysis Digital Pulse Processing ChemiSEM Technology incorporates a set of proprietary digital pulse processing algorithms, tailored to provide the best possible performance with the specific hardware used in the Thermo Scientific detectors and microscopes. Figure 3: Cross section of the SDD. Concentric rings create a field gradient, along which the charge drifts towards the small collection anode and integrated FET integrated on the back surface. Image courtesy of PN sensor. Field Effect Transistor The next step in the signal-processing chain is the preamplifier, where current conducted...

Electron Beam Synchronization and Minimum Dwell Times For a traditional “discrete” EDS system, the final step of the EDS hardware chain is to correlate the electron beam position with the X-ray spectrum generated at that position. Positioning of the beam is done through an external beam control interface in a support box. This controls the microscope beam position via an analog control interface. Additionally, column, beam, and stage parameters must be read through an external digital interface between the support PC running the EDS software and the PC that runs the microscope. Each of these...

Spectral Analysis Spectral Analysis Qualitative Analysis Qualitative analysis is the process of identifying which elements are present in a sample above the minimum detection limit. The minimum detection limit arises from the statistical uncertainty of the peak intensity measurements.2 Qualitative analysis determines the energies of the peaks present in the spectrum and compares them with the known energies of X-ray emissions. This peak identification process happens automatically with ChemiSEM Technology, where sophisticated algorithms check for inconsistencies in the spectrum, such as a kβ...

Figure 8: Schematic depiction of the variation of interaction volume shape with average sample atomic number (Z) and electron beam accelerating voltage kV. By using a lower beam energy, high-resolution results can be obtained. In the case of Pt excited at 10 keV, the only X-ray line available is the M-line at 2.05 keV. Combining lowkV excitation with a quantification engine that is capable of reliably handling such signals, even nanoparticles can be analyzed with ChemiSEM Technology (Figure 9). Spectral Analysis Quantitative Analysis Quantitative analysis seeks to establish not only the identities...

The most reliable procedures add the models together in varying proportions until the result best fits the data. The quality of this fit is evaluated using a chi-square (χ2) test, with the idea being that χ2 is minimized when the proportions in which the peak models are added together reflect the proportions in which the constituent peaks are present in the overlapped data. A good approach is to consider the first and second derivatives of the standards in the least square fit to allow the algorithm to accommodate slight offsets in peak positions and width. For the most reliable peak-fitting...

Spectral Analysis The Z, A, and F correction terms depend on the geometry of both the sample and the EDS detector. Each of these terms is integrated along the path that the X-ray travels through the sample and to the detector. Miscalculations in the X-ray pathway, for example, due to sample topography, represent a significant source of uncertainty (Figure 14). Pathlength calculations are automatic in most modern SEM-EDS systems and consider beam conditions (kV, beam deceleration), stage position (working distance, tilt), and EDS detector position to provide accurate quantification over a range...

Spectral Analysis Modes of AnalysisIntroduction In SEM EDS, qualitative and quantitative information from EDS is usually combined with the electron beam position to provide insight into compositional variation across the sample. There are three principle modes for combining spatial and spectral information: point mode, linescan, and X-ray mapping. The X-ray mapping starts with simple region-of-interest gross count mapping and ends with full quantitative information for each and every pixel. ChemiSEM Technology goes one step further and combines progressive electron image segmentation with fully...

Spectral images are the extremely rich datasets; for example, a spectral image with 1024x1024 pixels contains one million distinct spectra and over two billion unique data points, assuming 2048 channels per spectrum (corresponding to a 10 eV channel width over 20 keV). By acquiring a full spectral image, you are able to perform full offline image processing, extracting any number of points or lines from within an X-ray elemental map. The drawback with X-ray spectral imaging is the long acquisition time needed to get statistically sufficient data, notwithstanding that modern EDS detectors offer...