Pulse induction detector

Before the advent of Fourier transform spectrometers, wide-line NMR was done by sweeping the magnetic field and observing the dispersion signal, or by pulsing the radiofrequency and observing the free Induction decay without transformation. The very broad spectral widths have caused problems with baselines and faithful representations of the entire llneshapes. Various techniques, such as the quadrupole echo (lA), progressive phase alternation of the excitation pulse and detector, short spectrometer dead times, and post-acquisition spectral correction (15) have circumvented most of these. 

The pulse induction (PI) type of detector pulses a radio frequency signal into the ground and is not affected by iron minerals (negative) and salt water (positive). It is good to use where the terrain varies from wet to dry or where uneven patches of mineral sand are found, as it does not have to be retuned. The PI detector will respond to hot rocks and is unaffected by salt water. The PI detector is often used in underwater configurations. Some larger and newer PI instruments are capable of determining the depth of an object fairly accurately. 

Generally, there are three types of instruments used to detect ordnance cesium vapor magnetometers, radio telemetry (common metal detectors), and pulse induction units. For many years, none could reliably distinguish between metal debris and ordnance items. Even size is difficult to tell because the signal is faint on a smaller, closer object and faint on a larger, deeper object. 

When the pulse is switched off, the excited nuclei return slowly to their original undisturbed state, giving up the energy they had acquired by excitation. This process is known as relaxation. The detector is switched on in order to record the decreasing signal in the form of the FID (free induction decay). You can observe the FID on the spectrometer s computer monitor, but although it actually contains all the information about the NMR spectrum we wish to obtain, it appears completely unintelligible as it contains this information as a function of time, whereas we need it as a function of frequency. 

Continuous wave instruments involve a considerable waste of time. A solution to the inefficiency of single - frequency observation is to excite all of the nuclei in a sample simultaneously and to observe the total response of the sample. This is done by periodical, intense, short RF pulses. A RF pulse excites a finite band width of frequencies. The detector observes a pattern called a free induction decay (FID). An example is presented in figure C.2. Fourier Transforming this FID, yields the classical NMR spectrum. 

To overcome the problem of detection in CE, many workers have used inductively coupled plasma-mass spectrometry (ICP-MS) as the method of detection. " Electrochemical detection in CE includes conductivity, amperometry, and potentiometry detection. The detection limit of amperometric detectors has been reported to be up to 10 M. A special design of the conductivity cell has been described by many workers. The pulsed-amperometric and cyclic voltametry waveforms, as well as multi step waveforms, have been used as detection systems for various pollutants. Potentiometric detection in CE was first introduced in 1991 and was further developed by various workers.8-Hydroxyquino-line-5-sulfonic acid and lumogallion exhibit fluorescent properties and, hence, have been used for metal ion detection in CE by fluorescence detectors.Over-... 

To separate sigrtal comportertts corresportdirtg ortly to the preparatory pulse, the phases of the prepulse and of the detector reference voltage were reversed at each repetition of the multi-pulse sequence. In this scheme, all free-induction and echo-signal components generated by sequence pulses other than the preparatory pulse are subtracted, while the echo signals related to the preparatory pulse are summed. 

Any of the methods of detection used in liquid chromatography can be used in IC, though some are more useful than others. If the eluent does not affect the detector the need for a suppressor disappears. Common means of detection in IC are ultraviolet (UV) absorption, including indirect absorption electrochemical, especially amperometric and pulsed amperometric and postcolumn derivatization. Detectors atomic absorption spectrometry, chemiluminescence, fluorescence, atomic spectroscopic, refractive index, electrochemical (besides conductivity) including amperometric, coulometric, potentiometric, polaro-graphic, pulsed amperometric, inductively coupled plasma emission spectrometry, ion-selective electrode, inductively coupled plasma mass spectrometry, bulk acoustic wave sensor, and evaporative light-scattering detection. 

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