EL spectra

Electron ionization occurs when an electron beam crosses an ion source (box) and interacts with sample molecules that have been vaporized into the source. Where the electrons and sample molecules interact, ions are formed, representing intact sample molecular ions and also fragments produced from them. These molecular and fragment ions compose the mass spectrum, which is a correlation of ion mass and its abundance. El spectra of tens of thousands of substances have been recorded and form the basis of spectral libraries, available either in book form or stored in computer memory banks. 

Chemical ionization produces quasi-molecular or protonated molecular ions that do not fragment as readily as the molecular ions formed by electron ionization. Therefore, Cl spectra are normally simpler than El spectra in that they contain abundant quasi-molecular ions and few fragment ions. It is advantageous to run both Cl and El spectra on the same compound to obtain complementary information. 

Standard El spectra are obtained with an electron energy of 70 eV (electrons accelerated through 70 V). For most compounds, it is easier to produce positive ions than negative ones, and most El mass spectrometry is concerned with positive ions. 

The electroluminescence spectra of the single-layer devices are depicted in Figure 16-40. For all these OPV5s, EL spectra coincided with the solid-state photoluminescence spectra, indicating that the same excited states are involved in both PL and EL. The broad luminescence spectrum for Ooct-OPV5-CN" is attributed to excimer emission (Section 16.3.1.4). 

Polymer 55 exists as three different regioisomers head-to-head, head-to-head, and random. The various isomers have been synthesized, and it has been found they have different absorption. PL, and EL spectra 93[. The effective conjugation... 

A comparison of the electron impact (El) and chemical ionization (Cl-methane) mass spectra of 1//-azepine-1-carboxylates and l-(arylsulfonyl)-l//-azepines reveals that in the El spectra at low temperature the azepines retain their 8 -electron ring structure prior to fragmentation, whereas the Cl spectra are complicated by high temperature thermal decompositions.90 It has been concluded that Cl mass spectrometry is not an efficient technique for studying azepines, and that there is no apparent correlation between the thermal and photo-induced rearrangements of 1//-azepines and their mass spectral behavior. 

It has been previously noted (see Section 4.2 above) that use of the moving-belt interface allows El spectra to be obtained from compounds that do not yield spectra when analysis is attempted using a conventional El probe. The same is true when the dynamic-EAB probe is used in that spectra may be obtained from compounds that do not yield spectra when a static-FAB probe is used. This has been attributed to the presence of the mobile phase. 

Arguably the ultimate LC-MS interface would be one that provides El spectra, i.e. a spectrum from which structural information can be extracted by using famihar methodology, and this was one of the great advantages of the moving-belt interface. There is, however, an incompatibility between the types of compound separated by HPLC and the way in which electron ionization is achieved and therefore such an interface has restricted capability, as previously discussed with respect to the moving-belt interface (see Section 4.2 above). 

Examples of such compounds include anionic surfactants whose analysis had previously been limited to desorption techniques such as FAB and thermospray but which yielded interpretable El spectra when using a parUcle-beam interface... 

The particle-beam interface provides El spectra from HPLC eluates and this is of great advantage over other interfaces which provide only molecular weight information. Why then, is it of advantage to be able to generate Cl spectra from the particle-beam interface ... 

The particle-beam interface has been developed primarily to provide El spectra from HPLC eluates but may be combined with other ionization techniques such as CL If quantitative studies are being undertaken, a detailed study of experimental conditions should be undertaken. Isotope-dilution methodology is advocated for the most accurate results. 

A number of thermally labile and relatively involatile compounds which do not yield El spectra when using more conventional inlet methods do so when introduced via the particle-beam interface. 

Moore [231] has used hybrid MS/MS (EBQQ geometry El spectra) for the direct analysis of DODPA in unvulcanised rubber. Particular advantages of the technique are ... 

If measurements are made in thin oxide films (of thickness less than 5 nm), at highly polished Al, within a small acceptance angle (a < 5°), well-defined additional maxima and minima in excitation (PL) and emission (PL and EL) spectra appear.322 This structure has been explained as a result of interference between monochromatic electromagnetic waves passing directly through the oxide film and EM waves reflected from the Al surface. In a series of papers,318-320 this effect has been explored as a means for precise determination of anodic oxide film thickness (or growth rate), refractive index, porosity, mean range of electron avalanches, transport numbers, etc. 

In agreement with the NMR results, the bulky alkyl-alkoxysilanes RSi(OC2H5>3 (R = i-C4H9 and n-CgH ) are not able to form polymeric or spherical siloxanes under the conditions used for this study. The observed TOF-SIMS spectra are similar to the El spectra of the pure liquids and, therefore, underline the presence of physically adsorbed species [5,9]. 

EL spectra (peaked 820 nm) recorded from the crossed NW LEDs exhibit blueshifts relative to the bulk band gap of InP (925nm). The blueshifts are caused in part by quantum confinement of excitons, although other factors may also contribute. Furthermore, PL studies have demonstrated that the PL peak can be systematically blueshifted as the NW diameter is decreased,53 and... 

Figure 11.9. NW LED. (a) Crossed InP nanowire LED. (top) Three-dimensional (3D) plot of light intensity of the electroluminescence from a crossed NW LED. Light is only observed around the crossing region, (bottom) 3D atomic force microscope image of a crossed NW LED. (inset) Photoluminescence image of a crossed NW junction, (b-c) Multicolor nanoLED array, (b) Schematic of a tricolor nanoLED array assembled by crossing one n-GaN, n-CdS, and n-CdS NW with a p-Si NW. The array was obtained by fluidic assembly and photolithography with ca. 5- xm separation between NW emitters, (c) Normalized EL spectra obtained from the three elements. [Reprinted with permission from Ref. 59. Copyright 2005 Wiley-VCH Verlag.]...

Only the particle-beam interface produces El spectra for direct comparisons with computerized library spectra of fragmentation patterns. The other systems enable the relative molecular mass (RMM) of analytes up to 105 and above to be established. An example of an HPLC-APCI separation and identification of some benzodiazepine tranquillizers is shown in Figure 4.39. The most appropriate choice of LC-MS interface for a particular... 

Ortho-Effect. The ortho-effect is one of the most widely known structural phenomena in organic chemistry. It is widely used in organic chemistry for synthetic purposes. The mass spectra of the majority of ort/jo-substituted aromatic compounds possess significant differences in comparison with the spectra of their meta- and para-isomers. A classic example of the ortho-effect in mass spectrometry involves fragmentation of alkylsalicylates. The intense peaks of [M - ROH]+ ions dominate in the El spectra of these compounds. These peaks are absent in the spectra of their meta- and para-isomers. The reaction leading to these ions may be represented by Scheme 5.12. 

Lahti and coworkers [212] reported a series of meta-linked oligo(phenylene vinylene) block copolymers 181a-c. A mem-linked phenylene unit imposed an additional hypsochromic shift on the emission of these segmented polymers. The PL maxima were found at 399-416 nm, but a significant (ca. 70 nm) red shift was observed for EL spectra (ITO/polymer/Ca/Al) (Chart 2.39). 

A similar increase in the green PL peak was observed on photooxidation of the dialkyl-PF 199 film (Figure 2.13) [293]. Infrared (IR) spectra also indicate an appearance of fluorenone C=0 peak on photooxidation [246,293] and the same peak in the green region appears in the EL spectra during the device operation. 

---