Thanks for your zealous answer!I failed to reply your answer timely. For LIBS,I am a novice and lack of basic knowledge. You means that noise refers to the continuum emission in the early time of plasma propagation. From the spectra,how to distinguish noise form signal of Interest? You know that the spectra of trace elements is also weak.By choosing proper delay time,the continuum emission fades and the characteristic spectrum increase.The background varies from different delay time and integrating time. Why background occurs?In calculating the spectra intensity,should the background be subtracted?

When laser interact with the target surface it creates plasma. The plasma near focusing point is very hot and emits bremsstrahlung radiation, which is know as background emission. As the plasma expands away from the target surface it cools down along with decrease in the intensity of background emission. After plasma cooling first the line emission due to ions becomes observable and its intensity increases with time. After further cooling line emission from the neutral atoms becomes observable. Now one has to decide and optimize the gate delay and gate width of the detection system to get a better S/B signal either for ion or neutral line emission of interest.

Signal to noise ratio is dependent on many factors as has been discussed above. However a better detector can be able to reduce the noise. One has to take care to reduce the other noises coming from other sources.

S/N means peak signal to noise standard deviation while S/B means Signal peak to background ratio. By using the S/N ratio, It is important to consider whether the noise refers to the signal or to the background, in order to make it more clear.