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.

Table 3413. Examples of signal-to-background (S/B) ratios in EELS. [2,3]

Element
Edge
Ratio*
C
 K 5.2 for 100 mrad, 16 for 50 mrad, 37 for 5 mrad
F
 K 0.39 ± 0.06
Na
 K 0.23 ± 0.04
Na
 L3,2 0.17 ± 0.03
P
 L3,2 0.016 ± 0.003
Cl
 L3,2 0.10 ± 0.02
* The numbers in unit of mrad are collection angles.

The decrease of the signal/background with larger collection semi-angles is shown in Figure 3413c. However, restricting the aperture means that a detectable signal becomes smaller, so that a compromise must be applied. It is then suggested that an optimum collection semi-angle is the order of 10 mrad.

K-signal/background and K-signal/noise ratios for different collection semi-angles

Figure 3413c. K-signal/background and K-signal/noise ratios for different collection semi-angles. [4]

 

 

 

 

 

 

 

 

 

[1] C. C.Ahn, Transmission Electron Energy Loss Spectrometry in Materials Science and the EELS Atlas, 2004.
[2] Richard D. Leapman and John A. Hunt, Comparison of detection limits for EELS and EDXS, Microsc. Microanal. Micronstruct., 2 (1991) 231-244.
[3] Egerton, R.F., Inelastic scattering of 80 keV electrons in amorphous carbon, Phil. Mag. 31, pp. 199-215 (1975).
[4] C. Colliex, V. E. Cosslett, R. D. Leapman, P. Trebbia, Contribution of electron energy loss spectroscopy to the development of analytical electron microscopy, Ultramicroscopy, 1(4), (1976), 301-305.

 
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