Fig. 1 shows computed graphs illustrating the general spin concentration dependence of the NMR noise signal amplitude according to the term in square brackets in Eq. (1). It has already been shown by Hoult and Ginsberg that spin noise can be detected with probe circuits of low quality factors Q   [16]. In the case of the detection of 13C NMR noise, the main challenge derives from the low gyromagnetic ratio leading to a much lower M  0. Together with Q   being smaller due to its dependence on the Larmor frequency [15] this has the consequence that the concentration, where the NMR noise signal has an intensity

maximum, varies according to equation(4) cmax=(ϑ-2)4kTλ21000πNAγ3B0μ0ηQϑAssuming

equal spin concentrations (which is difficult to achieve in practice), the maximum for 13C is encountered at Quizartinib a more than 128 times lower concentration than for 1H: equation(5) c13Cmax⩾128c1HmaxSince the influence of radiation damping on the NMR noise signal is very much reduced under these conditions, the find more contribution of absorbed circuit noise to the detected signal is expected to be much smaller than for 1H. In the limiting case of Eq. (1), i.e. λr0≫λr,λr≪λ2 the noise power signal amplitudes depend linearly on the spin concentration, equation(6) limλr0≫λr,λ2≫λrλ2(λ2+λr0)λ2+λr2-1=λr0λ2since λr0 as of Eq. (2),

Low-density-lipoprotein receptor kinase is also linearly dependent on the spin concentration. Thus this represents the limiting case of pure spin noise, arising from the statistical fluctuations of the magnetic moments [8] without “self-quenching” by radiation damping, as represented by the denominator in Eq. (1). Therefore, although the radiation damping rate has a major influence on the NMR noise signal for narrow lines and high magnetization, giving rise to the phenomenon of absorbed circuit noise (ACN) [7], with the pertinent setup (low γ and lower η) the contribution of ACN to the detected total NMR noise signal is very small and the signal intensities should be nearly linearly dependent on the spin concentration. A similar linear dependence is also found under gradient conditions [5], in paramagnetic solutions [11] and in static solid powders [12]. The range of concentrations and NMR noise signal amplitudes that can be covered for 13C in our experimental setup is indicated by the area shaded in grey in Fig. 1b. It has to be noted, that for inverse detection probes the 13C spin noise amplitude is expected to be even smaller than this rough estimate due to the lower filling factor η deriving from the coil geometry.