Spectrometer filter time constant

In general, the longer the sweep time the better the sensitivity since the filter time constant parameter can be set longer with consequent improvement in signal-to-noise ratio. In practice, however, sweep times are usually set in accordance with the expected lifetime of the radical species, the stability of the instrument, and the patience of the operator. Decay of the radical or drift of the spectrometer during a scan is clearly undesirable. The sweep time is most commonly set in the range 4-10 min. 

Fig. 6. 3 P-NMR spectra of sarcoplasmic reticulum phospholipids using the spectrometer described in Fig. 1. (A) Sarcoplasmic reticulum (12 mg of protein per milliliter) dissolved in cholate, pH 8. Peak assignments 1, P 2, PE 3, PI 4, PC. 6000 transients were collected with an acquisition time of 2 s per transient, no delay between transients, and a filtering time constant of 1 s (B) lipid extract of sarcoplasmic reticulum (10 mg of lipid per milliliter) dissolved in cholate. Peak assignments as in (A) 600 transients were collected with other spectrometer settings as in (A). From London and Feigenson (1979), but note that the original figure has been altered to now show positive chemical shifts in the direction of decreasing field strength.

Equation (41.11) represents the (deterministic) system equation which describes how the concentrations vary in time. In order to estimate the concentrations of the two compounds as a function of time during the reaction, the absorbance of the mixture is measured as a function of wavelength and time. Let us suppose that the pure spectra (absorptivities) of the compounds A and B are known and that at a time t the spectrometer is set at a wavelength giving the absorptivities h (0- The system and measurement equations can now be solved by the Kalman filter given in Table 41.10. By way of illustration we work out a simplified example of a reaction with a true reaction rate constant equal to A , = 0.1 min and an initial concentration a , (0) = 1. The concentrations are spectrophotometrically measured every 5 minutes and at the start of the reaction after 1 minute. Each time a new measurement is performed, the last estimate of the concentration A is updated. By substituting that concentration in the system equation xff) = JC (0)exp(-A i/) we obtain an update of the reaction rate k. With this new value the concentration of A is extrapolated to the point in time that a new measurement is made. The results for three cycles of the Kalman filter are given in Table 41.11 and in Fig. 41.7. The... 

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