Repetitive scanning

Infrared (IR) investigations can be made on a sample of reactant previously heated to a known extent of reaction (a) and studied in the form of a mull or in an alkali halide disc. An alternative approach is to incorporate the reactant substance in a compact alkali halide disc [287] which is intermittently withdrawn from the reaction vessel for infrared measurements at appropriate intervals. Heated sample holders [288,289] permit repetitive scanning of the spectrum or continuous monitoring of a peak of interest during decomposition. 

What is the reason for the gradual increase of the cathodic and anodic cyclic-voltammetric peak currents observed upon repetitive scanning ... 

For thermal reactions a variable temperature probe is necessary since optimum polarized spectra are usually obtained in reactions having a half-life for radical formation in the range 1-5 minutes. Reactant concentrations are usually in the range normally used in n.m.r. spectroscopy, although the enhancement of intensity in the polarized spectrum means that CIDNP can be detected at much lower concentrations. Accumulation of spectra from rapid repetitive scans can sometimes be valuable in detecting weak signals. 

Figure 2. Repetitive scanning of m/e 310-330 (5 sec/scan). Standard conditions for this and all following figures ionizing voltage 70 eV, accelerating voltage 8 kV, trap current 300 /aA, multiplier 600, source 150°C.

NMR spectrometers have improved significantly, particularly in the present decade, with the development of very stable superconducting magnets and of minicomputers that allow measurements over long time periods under homogeneous field conditions. Repetitive scanning and signal accumulation allow H-NMR spectra to be obtained with very small sample quantities. 

Parent (unsubstituted) PF was first synthesized electrochemically by anodic oxidation of fluorene in 1985 [266] and electrochemical polymerization of various 9-substituted fluorenes was studied in detail later [220,267]. Cyclic voltammogram of fluorene ( r1ed= 1.33 V, Eox = 1.75 V vs. Ag/Ag+ in acetonitrile [267]) with repetitive scanning between 0 and 1.35 V showed the growth of electroactive PF film on the electrode with an onset of the p-doping process at 0.5 V (vs. Ag/Ag+). The unsubstituted PF was an insoluble and infusible material and was only studied as a possible material for modification of electrochemical electrodes. For this reason, it is of little interest for electronic or optical applications, limiting the discussion below to the chemically prepared 9-substituted PFs. 

Fig. 5.22. Illustration of the increase in signal to noise ratio when repetitive scans are added. Bottom trace represents a S/N ratio of 1.0. Successive traces represent summation of 4, 16, 50, 100, 1600 and 10,000 repetitive scans.

Full (repetitive) scan data collection vs. selected ion recording... 

CIR-FTIR spectroscopy provides a direct technique for studying in situ hydrous metal oxide surfaces and molecules adsorbed on these surfaces (37). By itself, FTIR spectrometry is a well established technique which offers numerous advantages over dispersive (grating) IR spectrometry (1) improved accuracy in frequency measurements through the use of a HeNe laser (2) simultaneous frequency viewing (3) rapid, repetitive scanning which allows many spectra to be collected in a small time interval (4) miriimal thermal effects from IR beam and (5) no detection of sample IR emissions (38). 

The operation of magnetic sector (Chap. 4.3), linear quadrupole (Chap. 4.4), or quadrupole ion trap (Chap. 4.5) mass spectrometers in the repetitive scanning mode is useful for the identification of the components of a mixture. If quantitation is a major issue (below), selected ion monitoring (SIM) is preferably employed the term multiple ion detection (MID) and some others are also in use. [33] In the SIM mode, the mass analyzer is operated in a way that it alternately acquires only the ionic masses of interest, i.e. it jumps from one m/z value to the next. [34-39] The information obtained from a SIM trace is equivalent to that from a RIC, but no mass spectra are recorded. Thus, the scan time spent on a diagnostically useless m/z range is almost reduced to zero, whereas the detector time for the ions of interest is increased by a factor of 10-100. [40] An analogous improvement in sensitivity (Chap. 5.2.3) is also observed. 

The sensitivity of a mass spectrometer is given in terms of the amount of sample consumed per second (e.g. a few pg/s or fmol/s) to give a signal with normalised intensity. Since the ion beam persists for some time, spectra are obtained by repetitive scanning. 

Combined GC-MS-computer systems with repetitive scanning can lead to the identification of GAs as minor components of complex extracts at levels down to 1CT- -1 g. In such cases mass fragmentograms can be constructed in which the distribution of ions of particular m/e values are plotted throughout a GC-MS run. Thus if the presence of a particular GA is suspected, characteristic ions in the mass spectrum of the derivatized GA are plotted. An identification can be made if the ions peak at the same retention time as the GA and have the same relative intensity as in the mass spectrum of the authentic compound (see Figure 6). In this way GAs can be detected which are masked in the GC trace by other compounds of similar retention time (cf. 33). 

The success of the polymerization depends on the solvent used for the process. Most studies of electropolymerized MPc have concentrated on the electrochemical polymerization of MPc(NH2)4 complexes [89-93], The polymerization process of these complexes involves the oxidation of the amino group forming radicals which attack phenyl rings of neighboring molecules [93], The formation of the polymers of (OH)MnPc(NH2)4 and OTiPc(NH2)4 on glassy carbon electrode (GCE) was successfully achieved via electropolymerisation of these complexes in DMF by repetitive scanning at a constant scan rate of 0.1 Vs-1. Simple adsorption of the monomer onto carbon electrodes (using MnPc derivatives) has been reported [94],... 

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