Practical Electron Microscopy and Database

The count of X-ray acquisition can be increased mainly by:
         i) Increase of acquisition time. The acquisition time may be increased if we have good stability of specimen stage and drift-correction software is used.
         ii) Increase of solid angle of EDS detector.
         iii) Thicker TEM specimens or SEM specimens are used. However, there are some drawbacks if such specimens are used.

The acquisition time of X-rays in EDS measurements should be optimized by considering the different factors as discussed below.

The data acquisition time should not be too short so that enough counts are recorded and thus the characteristic peaks can easily be separated from the background. In practice, there is no exact number of counts or amount of time that can be defined as "enough", but a good rule of thumb is to examine the spectrum, as it is being collected, until both the spectrum and background are steady.

However, X-ray acquisition time must be short enough to ensure that spatial resolution is not degraded by specimen drift and contamination and to ensure that artifact is not generated by beam damage. Spatial resolution can be improved by small probes and thin specimens while these conditions are exactly the opposite of those required to obtain high count rates.

Based on MoO₃ EDS measurements, Angeles-Chavez et al. [3] suggested that the EDS intensity and peak broadening increase with increase of acquisition time. The acquisition time has little effect in the chemical composition as shown in Table 2517. However, the increased time acquisition improves the accuracy of the experimental results.

Figure 2517a shows the SEM-EDS lower limits of detection (LLD) of Cr and Fe in glass (oxide) as a function of spectrum acquisition time. The detection limits can be improved at the beginning of increasing acquisition time but cannot do with further time increase.

On the other hand, Figure 2517b shows the behavior of peak-to-background ratio and minimum mass fraction (detection limit) as a function of overvoltage. In order to be able to detect minor elements, the overvoltage must be not less than 2, while bigger overvoltage and/or longer acquisition time should be used if the detection of trace elements is needed.

Automated peak identification process is sensitive to the noise in the spectral background because random groupings of background counts can mimic a characteristic X-ray peak when the threshold is set too low. Figure 2517c was obtained when a short data acquisition time was used. The four peaks (Fe K_α, Fe K_β, Fe L_α and C K) were really generated from the analyzing material; however, the other peaks (W, P, Pd, Ag, Ca and La) were false, which were misidentified from the noise in the background. Different artificial trace elements will show up because the noise in the background changes, when the data acquisition time increases. When the acquisition time is long enough so that P ≥ 3 (N_B)^1/2, the artificial peaks and thus the peak misidentification will then be eliminated. However, on many software, users are able to input their statistical threshold to define what constitutes a significant peak above the random noise in the background.

The EDS acquisition is normally faster than EELS acquisition. However, in general, long spectrum accumulation times should be used to acquire high-energy X-ray lines.

[1] Falcone, R., Sommariva, G. and Verita, M. (2006) WDXRF, EPMA and SEM/EDX quantitative chemical analysis of small glass samples, Microchim. Acta, 155, 137-140.
[2] Dale E. Newbury, Mistakes Encountered During Automatic Peak Identification of Minor and Trace Constituents in Electron-Excited Energy Dispersive X-Ray Microanalysis, Scanning, 31, 1–11 (2009).
[3] Carlos Angeles-Chavez, Jose Antonio Toledo-Antonio and Maria Antonia Cortes-Jacome, Chemical Quantification Mo-S W-Si Ti-V by Energy Dispersive X-Ray Spectroscopy, in X-Ray Spectroscopy, edited by Shatendra K Sharma, 2012.