Ionisation probe

The detonation velocity is the rate of propagation of a detonation in an explosive if the density of the explosive is at its maximum value, and if the explosive is charged into columns which are considerably wider than the critical diameter, the detonation velocity is a characteristic of each individual explosive and is not influenced by external factors. It decreases with decreasing density of packing in the column. It is measured by ionisation probes or fibre optical sensors. 

The moving of the flame front may be followed using the ionisation probes (or other types of velocity probes)/electronic counter (or oscilloscope) technique. Optical and electrooptical methods may be used as well. 

The determination of the combustion rate using ionisation probes and an electronic counter resembles the determination of the detonation velocity. The same principle is applied in both cases. The main difference lies in the different values of the measured time interval In the detonation, the time interval is on the microsecond scale, while in combustion the time interval is of seconds or tenths of seconds. 

The determination of the combustion rate with two ionisation probes and an electronic counter is schematically represented in Figure 3.10. 

Figure 3.10. Measuring system for the determination of the combustion rate using an electronic counter and ionisation probes...

Ionisation probes. The operating principle of the ionisation probes is based on the fact that detonation products behind the detonation front are highly ionised, which makes them capable of conducting electric current. Thus, the arrival of the detonation wave at an ionisation prote (vsiiich is actually an electrical switch) enables the closure of the electric circuit. That allows a capacitor to discharge, and the associated voltage signal is used to start/stop the counting assembly of the electric counter. 

Figure 4.10. Ionisation probe made of two twisted copper wires...

The frequently used ionisation probe is the so-called pin ionisation probe (Figure 4.11). The electric conductor inserted into the explosive charge parallel with the charge s longitudinal axis serves as a common electrode. It is actually a noninsulated copper wire (or strip) 1 mm in diameter. The other two electrodes are steel pins placed perpendicularly to the common electrode at a distance 0.5-3.0 mm from it. When the detonation wave arrives at the pin, the electric contact between the common electrode and the pin is restored, due to the ionisation effect in the detonation wave. 

Using the optical fibre/high-speed camera technique, it is possible to overcome some difficulties that are present while applying the ionisation probes method. For example, if more than one signal arrives at the same time, a separate recording channel must be provided for each one—otherwise, confusion can arise from improper matching of signal-probe. 

The treatment of the data resembles the detonation velocity determination when ionisation probes and an electronic counter or an oscilloscope are used (Subsections 4.1,3 and 4.1.4). 

Figure 1.3. Real-time femtosecond spectroscopy of molecules can be described in terms of optical transitions excited by ultrafast laser pulses between potential energy curves which indicate how different energy states of a molecule vary with interatomic distances. The example shown here is for the dissociation of iodine bromide (IBr). An initial pump laser excites a vertical transition from the potential curve of the lowest (ground) electronic state Vg to an excited state Vj. The fragmentation of IBr to form I + Br is described by quantum theory in terms of a wavepacket which either oscillates between the extremes of or crosses over onto the steeply repulsive potential V[ leading to dissociation, as indicated by the two arrows. These motions are monitored in the time domain by simultaneous absorption of two probe-pulse photons which, in this case, ionise the dissociating molecule.

Figure 1.4. Experimental and theoretical femtosecond spectroscopy of IBr dissociation. Experimental ionisation signals as a function of pump-probe time delay for different pump wavelengths given in (a) and (b) show how the time required for decay of the initally excited molecule varies dramatically according to the initial vibrational energy that is deposited in the molecule by the pump laser. The calculated ionisation trace shown in (c) mimics the experimental result shown in (b).

Obviously, by far the best method of performing SIM is to use a means of sample introduction which generates sample peaks of relatively short peak widths (as in GC or LC) that can be integrated - as opposed to the probe methods of sample introduction which deliver the sample into the ionisation source at a near-constant rate over long periods of time. 

For non-volatile sample molecules, other ionisation methods must be used, namely desorption/ionisation (DI) and nebulisation ionisation methods. In DI, the unifying aspect is the rapid addition of energy into a condensed-phase sample, with subsequent generation and release of ions into the mass analyser. In El and Cl, the processes of volatilisation and ionisation are distinct and separable in DI, they are intimately associated. In nebulisation ionisation, such as ESP or TSP, an aerosol spray is used at some stage to separate sample molecules and/or ions from the solvent liquid that carries them into the source of the mass spectrometer. Less volatile but thermally stable compounds can be thermally vaporised in the direct inlet probe (DIP) situated close to the ionising molecular beam. This DIP is standard equipment on most instruments an El spectrum results. Techniques that extend the utility of mass spectrometry to the least volatile and more labile organic molecules include FD, EHD, surface ionisation (SIMS, FAB) and matrix-assisted laser desorption (MALD) as the last... 

Cl and El are both limited to materials that can be transferred to the ion source of a mass spectrometer without significant degradation prior to ionisation. This is accomplished either directly in the high vacuum of the mass spectrometer, or with heating of the material in the high vacuum. Sample introduction into the Cl source thus may take place by a direct insertion probe (including those of the desorption chemical ionisation type) for solid samples a GC interface for reasonably volatile samples in solution a reference inlet for calibration materials or a particle-beam interface for more polar organic molecules. This is not unlike the options for El operation. 

To use the DCI probe, 1-2 xL of the sample (in solution) are applied to the probe tip, composed of a small platinum coil, and after the solvent has been allowed to evaporate at room temperature, the probe is inserted into the source. DCI probes have the capability of very fast temperature ramping from 20 to 700 °C over several seconds, in order to volatilise the sample before it thermally decomposes. With slower temperature gradients, samples containing a mixture of components can be fractionally desorbed. The temperature ramp can be reproduced accurately. It is important to use as volatile a solvent as possible, so as to minimise the time required to wait for solvent evaporation, which leaves a thin layer of sample covering the coil. The observed spectrum is likely to be the superposition of various phenomena evaporation of the sample with rapid ionisation direct ionisation on the filament surface direct desorption of ions and, at higher temperature, pyrolysis followed by ionisation. 

The DCI probe is particularly attractive for samples that are susceptible to thermal decomposition, although it can equally well be used as a general means of introducing samples into the ionisation source, i.e. as an alternative to the direct insertion probe. The types of sample which benefit most from DCI probing are higher-molecular-weight, less-volatile compounds, organometallics, and any thermally sensitive compounds [40,67]. DCI is considered to be a soft ionisation technique. 

Quantitative analysis using FAB is not straightforward, as with all ionisation techniques that use a direct insertion probe. While the goal of the exercise is to determine the bulk concentration of the analyte in the FAB matrix, FAB is instead measuring the concentration of the analyte in the surface of the matrix. The analyte surface concentration is not only a function of bulk analyte concentration, but is also affected by such factors as temperature, pressure, ionic strength, pH, FAB matrix, and sample matrix. With FAB and FTB/LSIMS the sample signal often dies away when the matrix, rather than the sample, is consumed therefore, one cannot be sure that the ion signal obtained represents the entire sample. External standard FAB quantitation methods are of questionable accuracy, and even simple internal standard methods can be trusted only where the analyte is found in a well-controlled sample matrix or is separated from its sample matrix prior to FAB analysis. Therefore, labelled internal standards and isotope dilution methods have become the norm for FAB quantitation. 

Desorption/ionisation techniques such as LSIMS are quite practical, as they give abundant molecular ion signals and fragmentation for structural information. In the conditions of Jackson et al. [96], all the molecular ion and/or protonated molecule ion species were observed in the LSIMS spectrum when only 1 pmol of each additive was placed on the probe tip. However, as mentioned above, in LSIMS/MS experiments the choice of the matrix (e.g. NBA, m-nitrobenzylalcohol) is very important. Matrix effects can lead to suppression of the generation of molecular ions for some additives. LSIMS is not ideal for the quantitative detection of polymer additives, as matrix effects are very important [96]. 

Principles and Characteristics Continuous-flow (or dynamic) FAB/FTB [102] and frit FAB/F1B [103] offer a means of introducing samples in solution into a continuous flow of solvent which terminates at the modified FAB/FIB probe tip, and they extend the applicability of FAB. Samples are injected through a conventional HPLC injection valve, or solutions are simply drawn in by the high vacuum in the ionisation source of the mass spectrometer. These very similar techniques are particularly amenable to coupling with HPLC columns, and ionisation of the sample is unchanged with respect to conventional FAB and FIB/LSIMS. 

Principles and Characteristics Thermospray ionisation (TSP) involves introduction of a relatively high flow (0.2-2mLmin ) of solvent into the ion source of a mass spectrometer, and is therefore suitable as an interface for HPLC-MS, using standard bore columns. A vaporiser probe (essentially a resistively heated capillary tube of about 100 xm i.d.) acts as a transfer line for taking solvent and solute into the source. The source is heated to prevent condensation of the solvent, and the temperature of the capillary is chosen so as to ensure vaporisation of the solvent. In this way, a vapour jet is generated, which contains small, electrically charged droplets if the solvent is at least partially aqueous and... 

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