1. Purpose

In this line-shape test, also often called the hump test, the ¹³C NMR signal of benzene is tested with regard to its line-width by measuring not only the width at half height (50%), but also at the heights of 0.55% and 0.11%. NMR signals should have a Lorentzian line-shape. Therefore the widths at the latter two heights should be 13.5 and 30 times the half-height line-width Δν_1/2. Deviations from these ratios indicate a non-Lorentzian line-shape: such a situation should be avoided. The ¹³C line-shape test not only detects bad shimming or a defective probe-head but also checks for sufficient ¹H decoupling power.

2. Literature

T. D. W. Claridge, High-Resolution NMR Techniques in Organic Chemistry, Pergamon, Oxford, 1999, 106–107.
C. Anklin, D. v. Ow, H. Ruegger, Standard Test Procedures for Supercon Systems, Spectrospin AG, Fällanden, 1988.

3. Pulse Scheme and Phase Cycle

Carbon-13 line-shape test with continuous-wave single-frequency proton decoupling

4. Acquisition

Time requirement: approx. 1 h, very dependent on the skill of the operator and the state of the system.

Sample: 80% benzene in [D₆]acetone, degassed and sealed.