Probe design and sensitivity

When discussing sensitivity in the context of real laboratory samples as opposed to instrument testing, it becomes useful to consider two definitions of this. The classic measurement of instrument sensitivity uses a fixed solution concentration (0.1% ethyl benzene in CDCI3, equivalent to 14 mM) under standard conditions and so measures the concentration sensitivity of the system. 

Probe diameter (inverse configuration) Sample volume (pi) Relative mass sensitivity 

The mass sensitivity of an rf coil scales inversely with the diameter of the coil, d (S/N oc l/d), to first approximation, meaning that greater intrinsic sensitivity can be achieved by narrowing the coil. This is the basis for the development of the so-called microprobes. This terminology is used somewhat loosely and there exists no formal definition of a microscale probe, but it is generally taken to mean a probe geometry that is small when compared to a standard probe. Nowadays, the standard remains a probe designed to accept a 5 mm diameter maximum NMR tube, colloquially referred to as a 5 mm probe, so commercial microprobes would now include the 3.0, 1.7 and 1.0mm probes. 

One further approach to miniaturisation in the Varian NanoProbe has been the use of small rotors of 40 pL total volume inside the specially constructed probe head that spins the sample at the magic angle (54.7° from the axis of the Bq field) at severtd kilohertz to eliminate lineshape distortions that would arise from sample discontinuities close to or within the rf coil (magic angle spinning is described in Section 10.7). This allows the whole sample volume to be retained within the coil s active volume, unlike tube-based approaches where up to 50% of the sample may sit outside of this volume, and so improves sensitivity. The use of MAS also makes the probe suitable for the analysis of heterogeneous samples such as those attached to resins from combinatorial synthesis [32], but it lacks the convenience of tube-based sample handling and appears to be less widely adopted for work with mass-limited samples. 

The gains arising from cryogenic cooling of probes may be described more formally by considering the noise contributions arising within the hardware. This may be summarised 

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