Pulsed Detection

A 90° Gaussian pulse is employed as an excitation pulse. In the case of a simple AX spin system, the delay t between the first, soft 90° excitation pulse and the final, hard 90° detection pulse is adjusted to correspond to the coupling constant JJ x (Fig- 7.2). If the excitation frequency corresponds to the chemical shift frequency of nucleus A, then the doublet of nucleus A will disappear and the total transfer of magnetization to nucleus X will produce an antiphase doublet (Fig. 7.3). The antiphase structure of the multiplets can be removed by employing a refocused ID COSY experiment (Hore, 1983). 

The reference scan is to measure the decay due to spin-lattice relaxation. Compared with the corresponding stimulated echo sequence, the reference scan includes a jt pulse between the first two jt/2 pulses to refocus the dephasing due to the internal field and the second jt/2 pulse stores the magnetization at the point of echo formation. Following the diffusion period tD, the signal is read out with a final detection pulse. The phase cycling table for this sequence, including 2-step variation for the first three pulses, is shown in Table 3.7.2. The output from this pair of experiments are two sets of transients. A peak amplitude is extracted from each, and these two sets of amplitudes are analyzed as described below. 

Bolometric signals, as we said, are modulated at a frequency co. Very rarely bolometers are used to detect pulsed signals or steady-state radiation levels. 

Pulseless electrical activity (PEA) is the absence of a detectable pulse and the presence of some type of electrical activity other than VF or PVT. 

The classical Jeener Broekaert sequence (133) is used to determine the dipolar-order relaxation time (in systems of spin 1/2 nuclides) and the Tiq relaxation time (in systems with spin 1 nuclides) of spin 1 nuclides with quadrupolar contributions to 7. Its FFC version is similar to the Inversion Recovery, except that the first 180° pulse is replaced by the sequence 90, — 5 — 45, the detection pulse becomes 45 and a special phase cycle is required. We shall not dwell on the details and purpose of the sequence since they go beyond the scope of this chapter. We wish to underline, however, the fact that sequences of this type require a close coordination of the preparatory sub-sequence with the signal-detection sub-sequence in order to isolate not just a particular magnetization component but a particular relaxation pathway. 

Looking at Fig. 6.10, we note that removing the pulse train (upper trace) triggers the detection pulses (lower trace). 

Figure 6.9 Pulse stream terminated, detection pulse triggered.

D experiments are devised in the assumption that the various times involved in the cycle of Fig. 8.1 (with the exception of when present) are small with respect to the nuclear relaxation times. When the latter are short for any reason, e.g. in the case of paramagnetic molecules because of the presence of unpaired electrons, the system of spins may have reached the equilibrium, or almost reached the equilibrium, before the detection pulse. Under these circumstances no memory is left for the state of the spins during the preceding steps. As a consequence, cross peaks may be decreased in intensity until below detectability. It is necessary, therefore, to match all the time intervals with the nuclear relaxation times, in order to detect the maximum possible cross peak intensities. The ideal case is that t ... 

Figure 12.4. Pictorial depiction of two common pulse sequences (a) a normal detection pulse sequence and (b) detection with decoupling.

There have been two further reports of sudden death after the use of droperidol to sedate agitation secondary to cocaine and phencyclidine intoxication (26). Both patients were restrained by the police and were then given droperidol, either 5 mg (a 33-year-old obese man) or 10 mg (a 22-year-old man). The first patient stopped breathing 10-15 minutes later, while being transported to the emergency department he was pulseless and couldn t be resuscitated. The other patient was unresponsive on arrival at the emergency department, with agonal respirations and no detectable pulse after 30 minutes of resusci-tative efforts he was pronounced dead. 

The product, 2-keto-3-deoxy-6-phosphogluconate, was separated by chromatography at room temperature and a flow rate of 1 mL/min on a Dionex CarboPac PA-1 column (4 mm x 250 mm). The mobile phase was composed of 24 mM NaOH and 300 mM sodium acetate for 5 minutes, followed by a linear gradient to 700 mM sodium acetate in 10 minutes. A linear gradient back to the initial conditions was run in 5 minutes. Pulsed amperometric detection was used with a pulse train consisting of a 480 ms detection pulse at +80 mV, followed by pulses of 120 ms at +600 mV and 60 ms at -600 mV. 

Detection Pulse-probe Pulse-probe High dynamic Pulse-probe Pulse-probe [30] Pulse-probe... 

Chromatographic measurements were made for the adsorption of benzene, toluene and m-xylene on molecular sieving caibon (MSC) in supercritical fluid CO2 mixed with organics. Supercritical chromatograph packed with MSC was used to detect pulse responses of organics. Adsorption equilibria and adsorption dynamics for organics were obtained by moment analysis of the response peaks. Dependences of adsorption equilibrium constants, K., and micropote difiiisivity, D, on amount adsorbed were examined. 

The rate of detected pulses equals the rate of photons striking the photocathode times Q. In other words, Q is the fraction of photons that ejects photoelectrons and results in an output pulse. The dark signal results from thermal ejection of electrons from the photocathode. Since Raman spectrometers often operate in the red and near infrared wavelength regions, the work function of the photocathode surface must be quite low. This small work function makes it difficult to prevent dark electron generation. Photon counting... 

Choose a proper dead time delay between the detection pulse and the beginning of the acquisition. An optimum choice is to set this duration to approximately 0.75 DW. This represents the very best that can be done and has to be set to avoid deadtime and ringing effects. 

Almost all of the experiments described in this chapter, and certainly all 2D experiments, require some time between scans during which the spin systems that have been irradiated or otherwise perturbed can return to equilibrium. There are two times during which relaxation processes occur (i) the relaxation delay time (DT), which is the period between the end of the acquisition of the signal (ta) and the first pulse of the pulse sequence being used and (ii) the repetition time (RT), which is the sum of DT + tg- Since relaxation occurs during as well as DT, especially for H-detected pulse sequences, it is important to consider RT, and not just DT, when deciding on experimental delay times between pulses. If, for example, it is determined that RT ought to be about 1 s and tg 200 ms, then DT should be set to approximately 800 ms. 

Measurements were done with the HEGRA CT1 telescope during November l-8th 2002. The standalone HEGRA CT1 telescope has a reflector area of 10 m2 and a camera of 127 PMs, each one with 0.25° FOV. Datasets were obtained with the Crab in the central pixel, and OFF source runs were also taken to check for any systematics. For optical observations the central pixel PM was modified. The DC branch, designed to monitor the DC current of the pixel, was adjusted to detect pulses of ms (timescale of the pulse width), and the AC branch, which is designed to transmit the ns fast signals generated by the Cherenkov showers was removed (De Ona-Wilhelmi et al.). 

Fig. 3-25. Gradient elution of different sugar alcohols and saccharides. - Separator column Ion Pac AS6A eluent (A) water, (B) 0.05 mol/L NaOH + 0.0015 mol/L acetic acid gradient 7% B isocratically for 15 min, then to 100% B in 10 min flow rate 0.8 mL/min detection pulsed ampero-metry on a Au working electrode (post-column addition of NaOH) injection volume 50 pL solute concentrations 15 ppm inositol (1), 40 ppm sorbitol (2), 25 ppm fucose (3), deoxyribose (4), 20 ppm deoxyglucose (5), 25 ppm arabinose (6), rhamnose (7), galactose (8), glucose (9), xylose (10), mannose (11), fructose (12), melibiose (13), isomaltose (14), gentiobiose (15), cellobiose (16), 50 ppm turanose (17), and maltose (18).

Fig. 3-105. Separation of various sugar alcohols and saccharides. - Separator column CarboPac PA-1 eluent 0.15 mol/L NaOH flow rate 1 mL/min detection pulsed amperometry at a Au working electrode injection volume 50 pL solute concentrations 10 ppm xylitol, 5 ppm sorbitol, 20 ppm each of rhamnose, arabinose, glucose, fructose, and lactose, 100 ppm sucrose and raffmose, 50 ppm maltose.

---