Instrument pulse calibration

This section is primarily intended for those who need to set-up experiments or those who have new hardware to install for which new calibrations are required. As with any analytical instrumentation, correct calibrations are required for optimal and reproducible instrument performance. All the experiments encountered in this book are critically dependent on the application of rf and gradient pulses of precise amplitude, shape and duration and the calibrations described below are therefore fundamental to the correct execution of these sequences. Periodic checking of these calibrations, along with the performance tests described in the following section, also provides an indication of the overall health of the spectrometer. 

Having determined the necessary pulse duration, the transmitter power must be calibrated so that the pulse delivers the appropriate tip angle. As already alluded to, this can be avoided on instruments with linearised amplifier outputs, provided accurate hard-pulse calibrations are known. The calibration of soft pulses differs from that for hard pulses where one uses a fixed-pulse amplitude but varies its duration. For practical convenience, amplitude calibration is usually based on previously recorded calibrations for a soft rectangular pulse (as described below), from which an estimate of the required power change is calculated. Table 10.3 also summarises the necessary changes in transmitter attenuation for various envelopes of equivalent duration, with the more elaborate pulse shapes invariably requiring increased rf peak amplitudes (decreased attenuation of transmitter output). 

Since a change in flow rate will change the pulse duration and amplitude, the sampling flow rate must be kept the same as that at which the instrument was calibrated. This can be achieved by facing one end of the sample inlet into the airflow path and using isokinetic sampling to control the velocity at the inlet. If the counter is not located directly in the sampled air flow, a probe may be located at the end of a sample transport tube with residence time in the tube kept as short as possible to avoid partiele loss. In most gas flow control systems the volumetric gas flow rate is defined at ambient pressure and... 

A Q-switched, frequency-quadrupled Nd—YAG laser (X, = 266 nm) and its accompanying optical components produce and focus the laser pulse onto the sample surface. The typical laser spot size in this instrument is approximately 2 pm. A He-Ne pilot laser, coaxial with the UV laser, enables the desired area to be located. A calibrated photodiode for the measurement of laser energy levels is also present... 

The instruments include an ionization chamber, the charcoal-trap technique, a flow-type ionization chamber (pulse-counting technique), a two-filter method, an electrostatic collection method and a passive integrating radon monitor. All instruments except for the passive radon monitor have been calibrated independently. Measurements were performed... 

The first photoelectric fhiorimeter was described by Jette and West in 1928. The instrument, which used two photoemissive cells, was employed for studying the quantitative effects of electrolytes upon the fluorescence of a series of substances, including quinine sulfate [5], In 1935, Cohen provides a review of the first photoelectric fluorimeters developed until then and describes his own apparatus using a very simple scheme. With the latter he obtained a typical analytical calibration curve, thus confirming the findings of Desha [33], The sensitivity of these photoelectric instruments was limited, and as a result utilization of the photomultiplier tube, invented by Zworykin and Rajchman in 1939 [34], was an important step forward in the development of suitable and more sensitive fluorometers. The pulse fhiorimeter, which can be used for direct measurements of fluorescence decay times and polarization, was developed around 1950, and was initiated by the commercialization of an adequate photomultiplier [35]. 

Robustness Even if the NMR instrument is not properly calibrated (for example, the probe tuning and pulse length calibration are not optimized), as... 

Verify a True Peak measurement instrument by first establishing an initial calibration nsing a sine wave. Then verify the pulse response of the instrument nsing a nonsymmetrical duty cycle pulse and compare the results to the sine wave calibration. Follow these steps ... 

Another approach to a source of vapors to calibration of instruments, and similar to that described above, was that of Davies et al. [67] who used a computer-controlled pulsed vapor generator with TNT, RDX, and PETN. The explosive solid was coated on quartz beads, which were then packed into a stainless steel tube. The tube was coiled and placed into a temperature-controlled chamber. Ultrapure air was passed through the coil at temperature and vapors of explosives were vented from the coil at rates or concentrations governed by coil temperature, airflow rate, and pulse width. Calibrations could reach the picogram to nanogram range when an IMS analyzer was used as the calibrating instrument. 

Use the protocols set out in the instrument manual for a pulsed NMR unit to perform the recommended self-tests and internal calibrations. 

Spherical particles of known diameter (e.g., 5% to 20% of the diameter of the aperture in the glass tube) are used to calibrate the electrical pulse counting instrument. The particles are suspended to an appropriate concentration in electrolyte solution (see recipe). Monodisperse latex particles are commercially available, which can be used for this purpose. Particle size calibration standards can be obtained from a number of chemical suppliers or from the National Institute of Standards and Technology (e.g., NBS 1003b). Lines (1996) lists a number of standards that are appropriate for this purpose. 

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