Residence lifetime

The dependence of (20 MHz and 25° C) on is graphically represented in Pig. 3 for monoaqua Gd(III) complexes with different values of the rotational correlation times. The limiting effect of the residence lifetime is small for low molecular weight complexes (xf> = 50 100 ps) and detectable only when On the other hand, for slowly tumbling... 

The mean residence lifetime of coordinated water protons represents one of the most important parameters that control the relaxivity of Gd(III) complexes 18). For several years it had been assumed that xm for the low molecular weight polyaminocarboxylate Gd(III) complexes was of the order of a few ns, as found for the octaaqua ion. In 1993, Merbach reported the first direct measurement of the water exchange rate for [GdDTPA(H20)] and [GdD0TA(H20)] and found that for these CAs the rate of water exchange was nearly three orders of magnitude lower than that for [Gd(H20)8] 49). 

The rate of water exchange is directly obtained by measuring the transverse relaxation rate and chemical shift of the water signal in a fairly concentrated (10-50 mM) aqueous solution of the Gd(III) complex. This subject will be discussed in detail in Chapter 7 of this volume. We just want to mention here that what is relevant for the analysis of NMRD curves is the mean residence lifetime of the water protons. In measurements we obtain... 

Fig. 7-28. Combined residence lifetimes of aerosol particles as a function of size. [Adapted from Jaenicke (1978c, 1980).] Important removal processes, active in various size ranges, are indicated. Coagulation and sedimentation time constants were calculated the time constant for wet removal is the residence time derived from 2,0Bi/210Pb and 222Rn/210Pb ratios (Martell and Moore, 1974). Curves 1 and 2 represent the background aerosol for rwel equal to 12 and 3 days, respectively. Curve 3 represents the continental aerosol with rwel = 6 days. The dashed line is calculated from a simple model for sedimentation equilibrium, as described in Section 7.6.3.

The lifetime of the crosslinks depend on the residence time for a polymer-bound surfactant unit in a micelle. The terminal relaxation times may be related to the residence lifetime of the suifactant unit in the micelle under shear conditions. 

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