Direct probe

The measuring system composed of the Wirotest type equipment, the direction probe and reference sample are subject to calibration while using a strength machine. 

The dynamics of fast processes such as electron and energy transfers and vibrational and electronic deexcitations can be probed by using short-pulsed lasers. The experimental developments that have made possible the direct probing of molecular dissociation steps and other ultrafast processes in real time (in the femtosecond time range) have, in a few cases, been extended to the study of surface phenomena. For instance, two-photon photoemission has been used to study the dynamics of electrons at interfaces [ ]. Vibrational relaxation times have also been measured for a number of modes such as the 0-Fl stretching m silica and the C-0 stretching in carbon monoxide adsorbed on transition metals [ ]. Pump-probe laser experiments such as these are difficult, but the field is still in its infancy, and much is expected in this direction m the near fiitiire. 

EXAFS is a nondestructive, element-specific spectroscopic technique with application to all elements from lithium to uranium. It is employed as a direct probe of the atomic environment of an X-ray absorbing element and provides chemical bonding information. Although EXAFS is primarily used to determine the local structure of bulk solids (e.g., crystalline and amorphous materials), solid surfaces, and interfaces, its use is not limited to the solid state. As a structural tool, EXAFS complements the familiar X-ray diffraction technique, which is applicable only to crystalline solids. EXAFS provides an atomic-scale perspective about the X-ray absorbing element in terms of the numbers, types, and interatomic distances of neighboring atoms. 

In many respects the time-resolved pump-probe technique is similar to the CW counterpart. The use of pulsed laser light permits direct probing of both the magnitude of the PA and its dynamics. The experimental arrangement is practically the same as for the CW version, i.e., both pump and probe beams are focused and overlapped onto same spot on a sample. In addition, the pump and probe pulses are synchronized so that the lime interval t between them is constant and confined to a certain time range (in our case up to 3 ns). 

Purification of the radioactive tracer was modified to include a fractional sublimation before a single extraction—recrystallization cycle to conserve the tracer material. Microgram samples were prepared in melting point capillaries for assay by mass spectroscopic analysis (Table III), made by direct probe injection of the sample into the ion source (18). The probe was heated rapidly to 200°C, and mass spectra were obtained during vaporization of the sample. Tri-, tetra-, and pentachlorodibenzo-p-dioxins vaporized simultaneously with no observed fractionation. 

When the aim is isolation for identification by direct probe insertion mass spectrometry (MS), plastic materials, filter papers, and blenders should be avoided to prevent contamination during extraction and chromatography. It is also very important to avoid the cis-trans isomerization of carotenoids in solution, which is accelerated by heat, light, acids, and active surfaces. Therefore, a pure carotenoid or even a crude extract should never be stored in solution it should be kept completely dry in an inert atmosphere at low temperature. 

Other possible direct probes are optical experiments similar to studies [113] of vesicles but expanded towards shorter A (20-30 A). Alternatively neutron spin-echo studies of stacked bilayer arrays, which can probe the 10-30 A range [114], might possibly be applicable here. Finally, the x-ray grazing-incidence technique has been shown to be a powerful tool for studying short wavelength fluctuations at fluid interfaces [100]. The application of this technique to the investigation of membrane surface fluctuations can reasonably be expected in the near future [115,116]. 

One major interest in vibrational surface spectroscopy is the ability to directly probe lipid layers. Similarly to the previous case, the structure of the alkyl chains of phospholipids is readily determined from the ratio of the magnitude of the CH2 and CH3 symmetrical stretching modes [136,137]. At the D2O-CCI4 interface, a layer of... 

Thus we adopt the view (see suggested reading 1) that any structure along the reaction path is an intermediate. With modern laser spectroscopy (suggested reading 2), one can now directly probe such structures experimentally. Thus it is a reactive intermediate, because it can be seen spectroscopically, not because it can be put in a bottle for a short time. As we shall see, this revised view of an intermediate is particularly relevant for photochemical and other nonadiabatic processes. 

Identification. One compound isolated by TLC was found to be very inhibitory to morningglory seed germination and was identified using mass spectrometry. A LKB 2091 GC-MS was used for GC-MS analysis. In addition to GC-MS, the sample was also analyzed by direct probe. 

Plasticiser/oil in rubber is usually determined by solvent extraction (ISO 1407) and FTIR identification [57] TGA can usually provide good quantifications of plasticiser contents. Antidegradants in rubber compounds may be determined by HS-GC-MS for volatile species (e.g. BHT, IPPD), but usually solvent extraction is required, followed by GC-MS, HPLC, UV or DP-MS analysis. Since cross-linked rubbers are insoluble, more complex extraction procedures must be carried out. The determination of antioxidants in rubbers by means of HPLC and TLC has been reviewed [58], The TLC technique for antidegradants in rubbers is described in ASTM D 3156 and ISO 4645.2 (1984). Direct probe EIMS was also used to analyse antioxidants (hindered phenols and aromatic amines) in rubber extracts [59]. ISO 11089 (1997) deals with the determination of /V-phenyl-/9-naphthylamine and poly-2,2,4-trimethyl-1,2-dihydroquinoline (TMDQ) as well as other generic types of antiozonants such as IV-alkyl-AL-phenyl-p-phenylenediamines (e.g. IPPD and 6PPD) and A-aryl-AL-aryl-p-phenylenediamines (e.g. DPPD), by means of HPLC. 

It is of interest to examine the development of the analytical toolbox for rubber deformulation over the last two decades and the role of emerging technologies (Table 2.9). Bayer technology (1981) for the qualitative and quantitative analysis of rubbers and elastomers consisted of a multitechnique approach comprising extraction (Soxhlet, DIN 53 553), wet chemistry (colour reactions, photometry), electrochemistry (polarography, conductometry), various forms of chromatography (PC, GC, off-line PyGC, TLC), spectroscopy (UV, IR, off-line PylR), and microscopy (OM, SEM, TEM, fluorescence) [10]. Reported applications concerned the identification of plasticisers, fatty acids, stabilisers, antioxidants, vulcanisation accelerators, free/total/bound sulfur, minerals and CB. Monsanto (1983) used direct-probe MS for in situ quantitative analysis of additives and rubber and made use of 31P NMR [69]. 

Direct probe FI-MS of PP samples containing various phosphites was used to study the effect of a sequence of extrusion passes at 260 °C [109]. 

Consequently, higher-molecular-weight (less-volatile) additives can generally be detected more readily with direct-probe introduction. 

In an acetone extract from a neoprene/SBR hose compound, Lattimer et al. [92] distinguished dioctylph-thalate (m/z 390), di(r-octyl)diphenylamine (m/z 393), 1,3,5-tris(3,5-di-f-butyl-4-hydroxybenzyl)-isocyanurate m/z 783), hydrocarbon oil and a paraffin wax (numerous molecular ions in the m/z range of 200-500) by means of FD-MS. Since cross-linked rubbers are insoluble, more complex extraction procedures must be carried out (Chapter 2). The method of Dinsmore and Smith [257], or a modification thereof, is normally used. Mass spectrometry (and other analytical techniques) is then used to characterise the various rubber fractions. The mass-spectral identification of numerous antioxidants (hindered phenols and aromatic amines, e.g. phenyl-/ -naphthyl-amine, 6-dodecyl-2,2,4-trimethyl-l,2-dihydroquinoline, butylated bisphenol-A, HPPD, poly-TMDQ, di-(t-octyl)diphenylamine) in rubber extracts by means of direct probe EI-MS with programmed heating, has been reported [252]. The main problem reported consisted of the numerous ions arising from hydrocarbon oil in the recipe. In older work, mass spectrometry has been used to qualitatively identify volatile AOs in sheet samples of SBR and rubber-type vulcanisates after extraction of the polymer with acetone [51,246]. 

Although the feasibility of direct probe MS for the analysis of additives in complex polymeric matrices has been demonstrated (Section 6.4), application is limited, difficult and requires above-average mass-spectroscopic expertise. Direct desorption in the MS probe is usually limited to screening of volatile components. Direct multicomponent spectroscopic analysis has other hurdles to overcome (UV/VIS lack of spectral discrimination IR/R functional-group recognition only, with no discriminative power for additives with similar functionalities NIRS unsuitable for R D problems NMR sensitivity). 

If we consider only a few of the general requirements for the ideal polymer/additive analysis techniques (e.g. no matrix interferences, quantitative), then it is obvious that the choice is much restricted. Elements of the ideal method might include LD and MS, with reference to CRMs. Laser desorption and REMPI-MS are moving closest to direct selective sampling tandem mass spectrometry is supreme in identification. Direct-probe MS may yield accurate masses and concentrations of the components contained in the polymeric material. Selective sample preparation, efficient separation, selective detection, mass spectrometry and chemometric deconvolution techniques are complementary rather than competitive techniques. For elemental analysis, LA-ICP-ToFMS scores high. 

---