suppression by presaturation. It con tains additional resonances from the acetate and ethylenic protons of free EDTA (EDTA(ac) and EDTA(en)) and the Ca(EDTA) complex and the ethylenic protons of Mg(EDTA); the resonance for the acetate protons of Mg(EDTA) overlaps the CaEDTA(ac) and EDTA(en) resonances (13). The concentrations of EDTA-chelatable calcium and magnesium in plasma have been determined from Ή NMR spectra of the type in Figure 5 (13). Suppression by differences in re laxation times. Several Ή NMR stud ies of human plasma have made use of differences between the T2s of the reso nances of interest and those of interfer ing resonances to selectively eliminate interfering resonances (13-17). These measurements are based on the differ ent rates of decay of the transverse magnetization from the various pro tons in a sample. Specifically, the shorter the spin-spin relaxation time, the faster the rate of decay, as illustrat ed in Figure 6. The effective spin-spin relaxation time T"2 for a particular pro ton is related to the width of its reso nance (ΤΙ = 1/τWi/2, where W1/2 is the width in hertz at resonance half height). Thus broad resonances can be selectively eliminated by delaying ac-
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Time (s) Figure 6. Dependence of the intensity of resonances having different spin-spin re laxation times on the length of the spin-spin relaxation period. The spin-spin relaxation period is equal to the time between the pulse and the start of data acquisition in pulse sequence Β (Figure 3) and is equal to 2τ in pulse sequence C and 2τη in pulse sequences D and E. These curves were calculated with the equation Ht) = HO) exp (—f/T2), which ignores the effect of diffu sion.
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