Peak(Signal)-to-Background (S/B) Ratio in EELS
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Similar to EDS spectra, the peak-to-background ratio (P/B ratio), or so-called signal-to-background ratio (S/B ratio), in EELS spectra increase with the accelerating voltages from 100 kV to 400 kV, resulting in an increase in the detection sensitivity of any trace elements.

The P/B ratio in EELS also varies with specimen thickness. For instance, Figure 3413a shows that the background increases dramatically with increase of the thickness of TEM specimen, while the signal of Pt M4,5 edge decreases significantly. The highest signal in the range of these TEM specimen thicknesses for Pt occurs at 20 nm.

EELS of Pt M4,5 edg

Figure 3413a. EELS of Pt M4,5 edge.

On the other hand, as discussed on page2400, larger collection angles increase the background contribution quicker than the edge signal, resulting in a drop in the signal-to-background ratio as shown in Figure 3413b. Another consideration is that the magnitude of the entrance aperture is chosen as a compromise between maximizing both the signal-to-noise (SNR) and the signal-to-background ratio (SBR). In practice, a good compromise is to employ a collection angle of ~2-3ΘE. [1]

The signal-to-background (S/B) ratio of EELS as a function of the collection angle
Figure 3413b. The signal-to-background (S/B) ratio of EELS as a function of the collection angle. Optimized collection angle: ~2-3ΘE.









[1] C. C.Ahn, Transmission Electron Energy Loss Spectrometry in Materials Science and the EELS Atlas, 2004.