1. Purpose

NMR signals are usually detected in quadrature mode with two phase detectors that are 90° out of phase (see Ch. 1, Section 1.1.2). These two audio signals are amplified and digitized either sequentially or simultaneously, then stored in different parts of the computer memory. This elegant scheme has many advantages. However, a drawback is that all components of the two channels involved must work identically. Failures that are often encountered include a d.c. offset between the two channels, wrong phase angle difference, and different amplification. Although small deviations can be eliminated by the usual quadrature phase cycle and by applying a baseline correction on the FID, it is important to know how well the two channels are matched to each other. The quadrature image test shown here gives a rapid indication of any wrong adjustment.

2. Literature

E. O. Stejskal, J. Schaefer, J. Magn. Reson. 1974, 14, 160–169.
T. D. W. Claridge, High-Resolution NMR Techniques in Organic Chemistry, Pergamon, Oxford, 1999, 59–65.
C. Anklin, D. v. Ow, H. Ruegger, Standard Test Procedures for Supercon Systems, Spectrospin AG, Fällanden, 1988.

3. Pulse Scheme and Phase Cycle

Single-pulse proton NMR sequence and phase cycle

4. Acquisition

Time requirement: 10 min

Sample: 10% CHCl₃ in [D₆]acetone.

Record a normal ¹H NMR spectrum (see Exp. 3.1) and note the offset of the CHCl₃ signal. You have to set: