Acquisition field

B (the Zeeman field) must be made to commute rapidly between several predefined values, such as the polarization field Bp, the relaxation field and the acquisition field Ba-... 

In order to understand this, consider that in an FFC experiment the amplitudes of the acquired signals are approximately proportional to the signal acquisition field Ba- For example, in the case of the basic prepolarized sequence (described in Section VIII.C.), one can show (65) that... 

When the relaxation period t is zero, then S respectively the polarization field and the acquisition field. 

While the first factor in these expressions varies, the proportionality with respect to the acquisition field is always present. In order to maximize SjN, it is therefore advantageous to use always the largest possible acquisition field Ba and make the acquisition frequency match the corresponding Larmor frequency jBa of the measured nuclide. This, however, can be done only when using a broad-band console. 

For a given number of nuclides placed in the sample coil volume, the inherent S/N ratio p is a parameter depending only on the probe and preamplifier assembly. It is usually measured in terms of the maximum FID amplitude after a 90° pulse applied after the sample has reached its equilibrium magnetization Ma in the acquisition field Ba- Defined in this way, it is independent of the details of any FFC sequence. 

Extremely precise field settling (well below 0.1%) is required only when switching to the acquisition field where RE pulses are to be applied and/ or the NMR signal is to be collected. In all other switching periods (for example, switching from the polarization field to the relaxation field), field-settling precision of the order of 0.1% is quite sufficient. 

Once the acquisition field has been reached, the detection sub-sequence section of an FFC sequence is actually quite the same as in any other branch of NMR. This implies that the same categories of problems crop up and the same methods to solve them can be employed. We shall briefly mention two such cases. 

Figure 28 shows the diagram of the FFC version of the classical IR sequence. Notice that the first RF pulse has to be applied at the acquisition field Ba because the probe is tuned to the Larmor frequency at that field. This implies the presence of an extra field-switching interval unless, of... 

Fig. 28. FFC Inversion Recovery sequence. In the upper case the sample is first prepolarized in a filed Bp, then switched to the acquisition field Ba where the first RF pulse of 180° is applied and the sample magnetization is inverted. The field is then switched to B,. and the sample is allowed to relax for the variable time t. Finally, the field is switched again to the acquisition value and the magnetization is sampled by any of the sample-detection methods (here, a simple FID following a 90° RF pulse). Notice that, as shown in the lower diagram, in the special case when Bp = Ba it is possible to neatly avoid the extra switching interval prior to the inversion pulse.

A common objective of a data gathering programme is the acquisition of fluid samples. The detailed composition of oil, gas and water is to some degree reguired by almost every discipline involved in field development and production. 

REAL SIGNALS ACQUISITION or AE SIGNALS EROM THE FIELD (MAXIMUMll TRANSDUCERS)... 

Motion, and in particular diffiision, causes a further limit to resolution [14,15]. First, there is a physical limitation caused by spins diflfiising into adjacent voxels durmg the acquisition of a transient. For water containing samples at room temperature the optimal resolution on these grounds is about 5 pm. However, as will be seen in subsequent sections, difhision of nuclei in a magnetic field gradient causes an additional... 

Flow which fluctuates with time, such as pulsating flow in arteries, is more difficult to experimentally quantify than steady-state motion because phase encoding of spatial coordinate(s) and/or velocity requires the acquisition of a series of transients. Then a different velocity is detected in each transient. Hence the phase-twist caused by the motion in the presence of magnetic field gradients varies from transient to transient. However if the motion is periodic, e.g., v(r,t)=VQsin (n t +( )q] with a spatially varying amplitude Vq=Vq(/-), a pulsation frequency co =co (r) and an arbitrary phase ( )q, the phase modulation of the acquired data set is described as follows ... 

Other organi2ations have assumed important positions in the field of patent documentation. IFl/Plenum Data Corp. (formerly Information for Industry) began in 1955 to index U.S. chemical patents by the Uniterm Index system. Uniterm indexing was eventually extended back to 1950. The acquisition in 1971 of Du Font s in-house indexing system and staff resulted in a more powerful system, the Comprehensive Data Base (CDB), which now covers U.S. chemical patents frommid-1964 to date. 

NMR instrumentation consists of three chief components a magnet, a spectrometer console, and a probe. While in the past much solid state NMR research was conducted on home-built equipment, the current trend is toward the acquisition of commercial systems. The magnets used for solid state NMR applications generally are superconducting solenoids with a cylindrical bore of 89-mm diameter. The most common field strengths available, 4.7, 7.0, 9.4, and 11.7 Tesla, correspond to proton resonance frequencies near 200, 300, 400, and 500 MHz, respectively. 

In both electron post-ionization techniques mass analysis is performed by means of a quadrupole mass analyzer (Sect. 3.1.2.2), and pulse counting by means of a dynode multiplier. In contrast with a magnetic sector field, a quadrupole enables swift switching between mass settings, thus enabling continuous data acquisition for many elements even at high sputter rates within thin layers. 

Acquisition of practical knowledge in the field of explosion-indueed structural damage is still heavily dependent upon empirical data. Such data, however, usually give information only about those overpressure levels which relate to certain degrees of damage. Other parameters, such as duration, impulse, and shape of the blast wave are not taken into aecount. Tables containing such information are frequently published. The best known are contained in Glasstone (1966, 1977), a frequently cited reference. 

Most of the computer-based systems permit rearrangement of the data-acquisition sequence in the field. They provide the ability to skip through the route until the appropriate machine, or the measurement point of a machine, is located. However, this manual adjustment of the pre-programmed route is time consuming and should be avoided whenever possible. 

While serious research of the behavior of CA systems demands the use of dedicated hardware, the often high cost of such hardware sometimes prevents its acquisition, particularly by young researchers and graduate students just entering this field of study. It is therefore important to realize that there are many excellent software-only solutions that can often be used very effectively for research. While the list provided below is by no means exhaustive, it gives some idea of the kind of packages that are currently available either commercially or via anonymous ftp over the Internet. 

---