What is the Difference Between a Broadband and an Inverse Broadband Probe?

I am sometimes asked, what is the difference between a broadband probe and an inverse broadband NMR probe? Both probes typically have a fixed proton channel, a tunable broadband channel and a fixed ²H lock channel. The difference lies in the coil positions with respect to the sample. Broadband probes are optimized to observe an X nucleus (typically all nuclei with frequencies between ¹⁵N and ³¹P). The broadband, X coil is positioned closest to the sample to maximize the filling factor and hence the X nucleus sensitivity. The ¹H coil is outside of the X coil and can be used for either ¹H observation or ¹H decoupling. The filling factor for this coil is lower than that of the X coil. In an inverse broadband probe, the ¹H coil is closest to the sample maximizing the ¹H sensitivity. The X coil is outside of the ¹H coil and can be used for either X nucleus observation or decoupling. The X nucleus in an inverse broadband probe has much less sensitivity than in a broadband probe. The choice of probe depends entirely on whether the user desires more ¹H or more X sensitivity. For example, The ¹³C spectra of dilute samples should be run in a broadband probe whereas ¹H - ¹³C or ¹H - ¹⁵N HSQC spectra of dilute samples should be run in an inverse broadband probe.