The FT Command

Fourier Transform: The FT command transforms the acquired data of the current data set, which may already have been processed using a filter function. Depending on the way a spectrum was recorded (e.g. simultaneous vs sequential sampling, quadrature settings, etc) the FT can be real or complex and some additional pre-processing may be required. All these options can be set in the Fourier Transform dialog box, following the main menu item 'Processing/More Processing/Fourier Transform' (shortcut: SHIFT+F).

If you click on the F1 icon (to select the indirect dimension) and then on the FT option, you will obtain the following dialog box:

 Protocol: This option corresponds to the data shuffling algorithm required to process phase sensitive 2D spectra

 None: If your spectrum is not phase sensitive, then the correct choice is: none protocol. (i.e. when only the

magnitude or power spectrum is to be obtained). It corresponds to the simpler 2D experiment in which a scan is collected with a 0º pulse of the mixing pulse, and then another is collected with a phase of 90º and finally combined before to storage in memory.

Bruker: QF mode

Varian:  p-type:   f1coef='1 0  0 -1 '  

 n-type:   f1coef='1 0  0  1 '

 Phase sensitive: this kind of experiments are used to get higher resolution and more information out of a 2D dataset. You can obtain better resolution and better S/N ratio by generating quadrature phase detection in the second dimension, but at the expense of longer acquisition times and larger datasets.

 Echo-antiecho: You should select this option for phase modulated experiments with gradient pulses (used for coherence pathway selection), where pairs of slices are added and subtracted to generate complex "fids" in the indirect dimension. Each slice contains both sine and cosine terms in t1.  This means that the data must be handled differently from hypercomplex data, in which odd numbered slices contain only cosine t1 terms, and even numbered slices contain only sin t1 terms.

Bruker: ECHO-ANTIECHO

Varian:  f1coef='1 0 -1  0 0  1 0  1'

 f1coef='1 0 -1  0 0 -1 0 -1'

Swap Halves (Quadrature): the negative frequencies are distinguished from the positive ones by the detection and summation of the in-phase and 90º out-of-phase magnetizations (two-channel mode).

Mirror Image (Invert): this command performs a complex-conjugate of the data before the protocol and FT is aplied. It is used to generate a mirror image of both the real and the imaginary parts of the spectrum, with respect to a vertical line positioned in the middle of the spectrum; i.e., the spectrum is reversed.

You will need to check/uncheck this option when you get the mirror image of your expected spectrum along any dimension.

Most Varian spectra will require this option to be checked whilst most Bruker spectra will not except those experiments acquired using the TPPI scheme.

Real FT: it allows to execute a real Fourier Transform as opposed to the conventional Discreet Fourier Transform (normally, this is only necessary for Bruker spectra acquired by sequential mode). Modern spectrometers acquire spectra in a way which avoids the use of this option.

Typically, quadrature detection involves the use of two digitizers (ADCs) to sample at the same time the real and imaginary components of the signal. However, quadrature detection can also be done with a single digitizer using the Redfield trick in which the real and imaginary parts are sampled sequentially in time but doubling the sampling rate in order to get the same spectral with. In this case, a Real Fourier Transform is needed. This option is typically used in 1D Bruker spectra acquired in sequential mode (QSEQ) and also it is also necessary along f1 for 2D spectra acquired in TPPI mode.

Swap IR/II Before FT: to cover Relay_CH coefficients 1 0 1 0 0 1 0 1

Change Sign II Before FT: to cover Relay_2CH coefficients 1 0 -1 0 0 1 0 -1