Signal-to-noise relationship

Figure 4.7. Consequences for the case that the proposed regulation is enforced The target level for an impurity is shown for several assumptions in percent of the level found in the official reference sample that was accepted by the authorities. The curves marked A (pessimistic), B, and C (optimistic) indicate how much the detected signal needs to be below the approved limits for assumptions concerning the signal-to-noise relationship, while the curves marked 1-3 give the LOQ in percent of this limit for LOQs of 0.02, 0.01, resp. 0.005. The circle where curves B and 1 intersect points to the lowest concentration of impurity that could just be handled, namely 0.031 %. The square is for an impurity limit of 0.1%, for which the maximal signal (<= 0.087%) would be just a factor of = 4.4 above the highest of these LOQs.

Whereas the plasma background signal (except at high levels) is predominantly shot noise limited, the net analyte signal is dominated by flicker noise (except at the detection limit region) as demonstrated by the signal to noise relationship data in Tables V and VI. Source flicker noise dependence of the net analyte intensity is a multiparameter phenomenon which may stem from 120 Hz power beats of the plasma, variations in gas flow rates or variations in the rate of nebulization. 

Table V. Signal-to-noise relationship demonstrated with Be 313.04 nm line...

Table VI. Signal -to-noise relationship demonstrated with Zn 213.8 ...

In the earliest SFG experiments [Tadjeddine, 2000 Guyot-Sionnest et al., 1987 Hunt et al., 1987 Zhu et al., 1987], a first-generation data acquisition method was used, and, because of the limited signal-to-noise ratios, IR attenuation by the electrolyte solution was a substantial handicap. So, in earlier SFG studies, as in IRAS studies, measurements were performed with the electrode pressed directly against the optical window [Baldelli et al., 1999 Dederichs et al., 2000]. With the in-contact geometry, the electrolyte was a thin film of uncertain and variable depth, probably of the order of 1 p.m. However, the thin nonuniform electrolyte layers can strongly distort the potential/coverage relationship and hinder the ability to study fast kinetics. 

As a rule of thumb, we have found that, in the range of resolution enhancements from 2 to 5, the minimum required signal-to-noise ratio can be approximately represented by an empirical relationship... 

The steady-state current depended on the concentration of ammonia. A linear relationship was observed between the current decrease and the ammonia concentration below 42 mg l-1 (current decrease 4.7 iA). The minimum concentration for the determination of ammonia was 0.1 mg 1" (signal to noise, 20 reproducibility, 5 %). The current decrease was reproducible within 4 % of relative error when a sample solution containing 21 mg 1 of ammonium hydroxide was employed. The standard deviation was 0.7 mg 1 in 20 exper-ments. The response time of the sensor was within 4 min. 

SAR structure activity relationship SNR signal-to-noise ratio... 

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