Quadrature detection methods

Figure 3.23. Data sampling schemes for the two common quadrature detection methods, (a) Simultaneous sampling the two quadrature channels (representing x and y magnetisation) are sampled at the same point in time, (b) Sequential sampling the two channels are sampled alternately at twice the rate of method (a), and the phase inverted for alternate pairs of data points (see text).

Quadrature detection A method for detecting NMR signals that employs two phase-sensitive detectors. One detector measures the jc-component of... 

Proton-proton homonuclear decoupling has been performed by the ESLG decoupling sequence [46]. Quadrature detection in coj was achieved by using the time proportional phase increment method (TPPI) [47]. During the acquisition period, two pulse phase modulation (TPPM) heteronuclear decouphng ]48] was applied (Figure 7.6). 

Bayesian probability theory157 can also be applied to the problem of NMR parameter estimation this approach incorporates prior knowledge of the NMR parameters and is particularly useful at short aquisition times158 and when the FID contains few data points.159 Bayesian analysis gives more precise estimates of the NMR parameters than do methods based on the discrete Fourier transform (DFT).160 The amplitudes can be estimated independently of the phase, frequency and decay constants of the resonances.161 For the usual method of quadrature detection, it is appropriate to apply this technique to the two quadrature signals in the time domain.162-164... 

Figure 13 Pulse sequence for the solid-state 31P—1H frequency-switched Lee-Goldburg (FSLG) experiment. Proton-proton homonuclear decoupling was performed by using the FSLG decoupling sequence. Quadrature detection in Wi was achieved by using the TPPI method. During the acquisition period, TPPM heteronuclear decoupling was applied.

As indicated in Section 3.4, both methods of quadrature detection are used in commercial instruments. It is important to be aware of the method being used in a particular experiment, because foldover gives quite different results in the two approaches. 

NMR samples contained 0.6 ml receptor (0.5-2.0 mM) dissolved in refolding buffer (vide supra) with 10% DjO. One-dimensional F NMR spectra were obtained at 470 mHz on a General Electric GN 500 spectrometer fitted with a 5 mm F probe. Parameters included 16K data points, 3.0 second relaxation delay and 25 Hz linebroadening for processing spectra. T, relaxation times were measured by the inversion recovery method. The two-dimensional F NOESY NMR spectrum was obtained on a Varian Unity Plus 500 using the standard Varian pulse sequence. A total of 128 experiments with a mixing time of 0.3 seconds were performed with collection of 1024 data points. Quadrature detection in the second dimension was obtained through the method of States and Haberkom. C ( H NMR spectra were obtained on a Varian 500 Unity Plus fitted with a 10 mm broadband probe. 

The spectra were acquired with 2048 t2 complex data points and 256 ri increments in the phase sensitive mode with quadrature detection using the method described by States et al. (25). Water resonance was supressed during the 1.5s relaxation period used in the NOESY, DQF-COSY and TOCSY experiments and the mixing period of the NOESY experiments by irradiating continuously at its resonance frequency. The amide exchange experiments were carried out by... 

Quadrature detection is universally employed in all modem spectrometers. However, there exist two experimental schemes for implementing this and which of these you are likely to use will be dictated by the spectrometer hardware and perhaps by the age of the instrament (on some modem instmments the operator can choose between the two methods). 

Figure 5.17. The States method of fi quadrature detection requires two data sets to be acquired per increment to generate separate sine and cosine modulated data sets.

All modern spectrometers use a method know as quadrature detection, which in effect means that both the x- and y-components of the magnetization are detected simultaneously. 

After a single excitation pulse or a series of pulses the signal detection period begins where the response of the spin system to the pulse sequence is recorded. The basic configuration for quadrature detection [2.32, 2.33] is two detectors in the x,y-plane with a 90° phase difference where each detector may be assigned to the x- or y-axis. As illustrated in Fig. 2.6 there are two detection methods, in simultaneous detection both detectors are sampled at the same time whilst in sequential detection the detectors are sampled alternatively. 

The quadrature detection mode is given as a suffix to the main sequence name me magnitude calculated, TPPI time proportional phase increment, E/A Echo / Antiecho The term "selective" is reserved exclusively for sequences using selective pulses. If selectivity is achieved using other methods this is defined using a different term. 

The ID version of the 2D- H/H-N0ESY sequence shown in Figure 6B is perhaps one of the most widely chosen suppression schemes. The first increment of the 2D is acquired while the delay increments, phase cycles, and quadrature detection related to the indirectly detected dimension are not used (Exorcyle and other phase cycling is still performed ). This sequence also seems to be the present method of choice for metabonomics studies. 

In summary, quadrature detection is an extremely advantageous detection scheme for FT NMR and should be incorporated as a standard item in any FT spectrometer. Without it, 1) the transmitter power is used inefficiently leading to larger frequency dependent tip angles, 2) less than optimum S/N, the amount of which depends on the method of implementation and 3) inconvenience in doing experiments requiring irradiation of a part of the spectrum such as selective saturation experiments. 

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