Hyphenated thermal

Thermal analysis is a group of techniques in which a physical property of a substance is measured as a function of temperature when the sample is subjected to a controlled temperature program. Single techniques, such as thermogravimetry (TG), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), dielectric thermal analysis, etc., provide important information on the thermal behaviour of materials. However, for polymer characterisation, for instance in case of degradation, further analysis is required, particularly because all of the techniques listed above mainly describe materials only from a physical point of view. A hyphenated thermal analyser is a powerful tool to yield the much-needed additional chemical information. In this paper we will concentrate on simultaneous thermogravimetric techniques. 

Apart from combined TA techniques (on-line or not) the actual trends in thermal analysis are the introduction of modulated and high-resolution techniques, hyphenated thermal analysis methods e.g. TG-FTIR, TG-MS, DSC-XRD, etc.), alternative heating modes, microthermal analysis methods, industrial standardisation and quality control. Modulation means a periodic perturbation of a temperature program. Temperature modulation finds application in DSC, TG, DETA, TMA and uTA. Temperature-modulated techniques, such as Modulated DSC (MDSC ) and Modulated TGA (MTGATM), broaden the insight into the material properties. The use of modulated temperature programs in thermal methods has been reviewed [37,37a]. 

As an alternative to wet ehemical routes of analysis, this monograph deals mainly with the direct deformulation of solid polymer/additive compounds. In Chapter 1 in-polymer spectroscopic analysis of additives by means of UV/VIS, FTIR, near-IR, Raman, fluorescence spectroseopy, high-resolution solid-state NMR, ESR, Mossbauer and dielectrie resonance spectroscopy is considered with a wide coverage of experimental data. Chapter 2 deals mainly with thermal extraction (as opposed to solvent extraction) of additives and volatiles from polymerie material by means of (hyphenated) thermal analysis, pyrolysis and thermal desorption techniques. Use and applieations of various laser-based techniques (ablation, spectroscopy, desorption/ionisation and pyrolysis) to polymer/additive analysis are described in Chapter 3 and are critically evaluated. Chapter 4 gives particular emphasis to the determination of additives on polymeric surfaces. The classical methods of... 

T. Provder, M. W. Urban, and H. G. Barth, eds.. Hyphenated Techniques in Polymer Characterisation Thermal-Spectroscopic and Other Methods, American Chemical Society, Washington, D.C., 1994. 

The combination of chromatography and mass spectrometry (MS) is a subject that has attracted much interest over the last forty years or so. The combination of gas chromatography (GC) with mass spectrometry (GC-MS) was first reported in 1958 and made available commercially in 1967. Since then, it has become increasingly utilized and is probably the most widely used hyphenated or tandem technique, as such combinations are often known. The acceptance of GC-MS as a routine technique has in no small part been due to the fact that interfaces have been available for both packed and capillary columns which allow the vast majority of compounds amenable to separation by gas chromatography to be transferred efficiently to the mass spectrometer. Compounds amenable to analysis by GC need to be both volatile, at the temperatures used to achieve separation, and thermally stable, i.e. the same requirements needed to produce mass spectra from an analyte using either electron (El) or chemical ionization (Cl) (see Chapter 3). In simple terms, therefore, virtually all compounds that pass through a GC column can be ionized and the full analytical capabilities of the mass spectrometer utilized. 

Hyphenation On-line GC, HPLC, SPE-SFE, SFE-SPE GC (most common), HPLC. Direct thermal desorption. 

Various ancillary GC techniques are headspace GC (Section 4.2.2), thermal desorption GC, pyrolysis GC, hyphenated methods (Chapter 7), multidimensional techniques (Section 7.4.1) and process GC. 

FD-MS is a very effective technique for determining molecular weights of thermally labile and nonvolatile compounds, such as polymer additives which do not give good molecular ion spectra during electron impact or chemical ionisation [108], In order to enhance the structural information of the technique, MS/MS approaches must be used [96], Hyphenated chromatography-FD/FT-MS techniques appear to be restricted to on-line GC-MS. 

In direct insertion techniques, reproducibility is the main obstacle in developing a reliable analytical technique. One of the many variables to take into account is sample shape. A compact sample with minimal surface area is ideal [64]. Direct mass-spectrometric characterisation in the direct insertion probe is not very quantitative, and, even under optimised conditions, mass discrimination in the analysis of polydisperse polymers and specific oligomer discrimination may occur. For nonvolatile additives that do not evaporate up to 350 °C, direct quantitative analysis by thermal desorption is not possible (e.g. Hostanox 03, MW 794). Good quantitation is also prevented by contamination of the ion source by pyrolysis products of the polymeric matrix. For polymer-based calibration standards, the homogeneity of the samples is of great importance. Hyphenated techniques such as LC-ESI-ToFMS and LC-MALDI-ToFMS have been developed for polymer analyses in which the reliable quantitative features of LC are combined with the identification power and structure analysis of MS. 

This chapter deals mainly with (multi)hyphenated techniques comprising wet sample preparation steps (e.g. SFE, SPE) and/or separation techniques (GC, SFC, HPLC, SEC, TLC, CE). Other hyphenated techniques involve thermal-spectroscopic and gas or heat extraction methods (TG, TD, HS, Py, LD, etc.). Also, spectroscopic couplings (e.g. LIBS-LIF) are of interest. Hyphenation of UV spectroscopy and mass spectrometry forms the family of laser mass-spectrometric (LAMS) methods, such as REMPI-ToFMS and MALDI-ToFMS. In REMPI-ToFMS the connecting element between UV spectroscopy and mass spectrometry is laser-induced REMPI ionisation. An intermediate state of the molecule of interest is selectively excited by absorption of a laser photon (the wavelength of a tuneable laser is set in resonance with the transition). The excited molecules are subsequently ionised by absorption of an additional laser photon. Therefore the ionisation selectivity is introduced by the resonance absorption of the first photon, i.e. by UV spectroscopy. However, conventional UV spectra of polyatomic molecules exhibit relatively broad and continuous spectral features, allowing only a medium selectivity. Supersonic jet cooling of the sample molecules (to 5-50 K) reduces the line width of their... 

VDU screen via suitable electronic amplifying circuitry where the data are presented in the form of an elution profile. Although there are a dozen or more types of detector available for gas chromatography, only those based on thermal conductivity, flame ionization, electron-capture and perhaps flame emission and electrolytic conductivity are widely used. The interfacing of gas chromatographs with infrared and mass spectrometers, so-called hyphenated techniques, is described on p. 114 etseq. Some detector characteristics are summarized in Table 4.11. 

Hyphenated methods can be divided into two types those that do and those that do not destroy the sample in the process of analysis. Spectrophotometric methods, thermal conductivity, and refractive index methods of detection do not destroy the sample. Chromatographic methods using flame ionization and similar detection methods destroy the sample as it is detected. Any hyphenated method that involves MS or thermal analysis (TA) will also destroy the sample. In most cases, the identification of the components in soil is most important, so the destruction of the analyte is of less importance. 

There are four basic hyphenated methods that result in the sample being destroyed. These are GC-MS, HPLC-MS, AAS/ICP-MS and TA/DTA-MS. All mass spectroscopic methods destroy the sample after separation however, both AAS and ICP destroy the sample no matter what follow-on method of analysis is used. In most cases, TA and differential thermal analysis (DTA) will also destroy the sample. The follow-on methods then analyze the components that result from this decomposition. DTA may also be used to follow transitions in the sample without destroying it. Because the sample is identified, there is typically no reason to collect the analyte of interest, and so destruction is not of concern. However, if there is a limited amount of sample, care should be taken in using one of these methods. 

HPLC has been applied to lipid analysis mainly in consideration of the necessity to avoid high temperatures, so at the very beginning, its applications dealt with thermally unstable molecules (e.g., tocopherols, phenolics, oxidation products) and often it was used as an ancillary technique, as a preparative step prior to MS analysis. The limits were in the high volume of the HPLC band that strongly limited the possibility to transfer it to a GC or to a MS. Only in the last 20 years or somewhat less, this kind of hyphenation has become commercially available. 

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