The application of such procedures to more complex spin systems is problematic. Every additional spin doubles the spin dynamical complexity and decreases the transformation efficiency. For example, [1-13 C]-succinate contains 5 coupled spins-1/2, and has 4 times as many quantum states as [1-13 C]-fumarate. In general field-sweep methods perform poorly for multiple-spin systems, since it is hard to maintain control of the highly complex nuclear spin dynamics, especially through the multiple level crossings associated with field sweeps .