Desorption continued profile

Lasers can be used in either pulsed or continuous mode to desorb material from a sample, which can then be examined as such or mixed or dissolved in a matrix. The desorbed (ablated) material contains few or sometimes even no ions, and a second ionization step is frequently needed to improve the yield of ions. The most common methods of providing the second ionization use MALDI to give protonated molecular ions or a plasma torch to give atomic ions for isotope ratio measurement. By adjusting the laser s focus and power, laser desorption can be used for either depth or surface profiling. 

Imaging MS is and will become increasingly critical for many aspects of materials science. One example is in the semiconductor industry, where the ability to provide spatial and chemical information on the length scales of current integrated circuit fabrication (50 nm or better) with depth profiling to provide layer-by-layer maps of the fabricated layers is critical for the continued advancement of the computer industry. Maps of any heterogeneous surface are important in other areas of materials science. For example, using various laser desorption techniques, information about the molecules found in specific inclusions in meteorites or defects in reactive surfaces can be obtained. 

In the case of an unfavorable isotherm (or equally for desorption with a favorable isotherm) a different type of behavior is observed. The concentration front or mass transfer zone, as it is sometimes called, broadens continuously as it progresses through the column, and in a sufficiently long column the spread of the profile becomes directly proportional to column length (proportionate pattern behavior). The difference between these two limiting types of behavior can be understood in terms of the relative positions of the gas, solid, and equilibrium profiles for favorable and unfavorable isotherms (Fig. 7). 

We start with the case of d(z) injection profile into an isothermal column under laminar gas flow regime. The factors which cause broadening of the initial profile are longitudinal diffusion and radial diffusion across streamlines. These continuously change the concentration distribution and gradients. In the absence of any adsorption sojourn time of molecules on the wall (i.e. negligible adsorption-desorption energy), and only in this case, the two above diffusion processes yield ... 

For a dynamic system in which there is continuous removal of the product it can be shown that the rate -d[Aa]/df is equivalent to P,, the instantaneous pressure of product measured over the adsorbent, and so represents a relatively facile measurement. The type of desorption seen for such a simple first-order case is shown in fig. 17. The profile shows a peak which is due to the convolution of the rate constant (which increases with increasing... 

Conversely, as Xg-> 1.0 the desorption curves become increasingly diffuse and the desorption profile changes from two well-defined fronts separated by a plateau, for near linear systems, to a more or less continuous curve as the rectangular limit is approached. 

As another example, the case of air drying was calculated and compared with the complex calculation result obtained by the method introduced in the previous section (Chihara and Suzuki, I9 3a). The case of isothermal operation (Fig. 11.6, solid and open circles) is compared with the results of the continuous countercurrent flow model (solid and broken lines) in Fig. 11.14. Rigorous calculation was done for a throughput ratio of 0.01 and the change in the profiles of the amount adsorbed after adsorption and desorption steps found to be reasonably small. Thus the simple model simulated quite well the cyclic steady state profile of PSA. 

For most adsorption systems of industrial significance, the isotherm is favourable towards adsorption over the range of concentration of interest. Whilst this might be good for the adsorption step, the isotherm is of course unfavourable for the desorption step. TTierefore in desorption the MTZ is usually expected to be dispersive, thereby leading to a continuously spreading concentration profile. Ruthven (1984) provides further information for isotherms which have more complicated shapes, including those which have a point of inflection. 

Fig. 6.24 Schematic representation of a liquid-flow microcalorimetry system operating in continuous-flow mode, together with traces showing the thermal and mass exchange profiles for adsorption of solute from its solution in the solvent, followed by the desorption of the solute by flow of pure solvent 1 adsorbent bed, 2 inlet tube, 3 outlet tube, 4 toric seals, 5 aluminium block, 6 measuring thermistors, 7 syringe pomp, 8 downstream detector...

IGC measurements can be carried out using a pulse or continuous technique. The pulse of probe molecule is introduced into the carrier gas stream. This pulse is transported by the carrier gas through the system to the column with the solid sample. On the stationary phase, adsorption and desorption occur and the result is a peak in the chromatogram. The ratio of adsorption/desorption is governed by the partition coefficient. At fixed conditions of temperature and flow rate, the time of retention of a compound is characteristic of the system. An alternative is the fi ontal technique. This is carried out by injection into the carrier gas stream of a continuous stream of the probe molecule. When the sample enters into the column, there is a distribution between phases, and the concentration profiles takes the shape of a plateau, preceded by a breakthrough curve. The shape of this curve is characteristic of each system [3]. The benefit of the frontal technique is that equilibrium can be always established due to its continuous nature while pulse chromatography requires the assumption of a fast equilibration of the probe molecule adsorption on the surface. Between both techniques, the main part of publications describes pulse experiences, since they are faster, easier to control and more accurate, especially if interactions between probe molecules and the adsorbent are weak. 

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