Thermally extracted mobile-phase

Seven Argonne Premium coal samples ranging from lignite to low volatile bituminous in rank were analyzed by Pyrolysis-Field Ionization Mass Spectrometry (Py-FIMS) in order to determine the existence and structural nature of a thermally extractable "mobile phase". In addition, Curie-point Pyrolysis-Low Voltage Mass Spectrometry (Py-LVMS) was employed to demonstrate the importance of mild oxidation on the thermally extractable mobile phase components. 

Py-FIMS results clearly reveal the existence of a thermally extractable, bitumen-like fraction which is chemically distinct from the remaining coal components. In lignite, several biomarker compounds were noticeable in the mobile phase components while bituminous coals contain various alkylsubstituted aromatic compounds in the mobile phase. Blind Canyon coal, which contains 11% resinite, exhibits mobile phase components believed to originate from terpenoid aromatization. Curie-point Py-LVMS results illustrate the importance of the oxidation status of coal for studying the mobile phase since mild air oxidation severely changes the structural characteristics of the thermally extractable mobile phase. 

Figures 5-7 are the spectra exhibiting the thermally extracted mobile phase components over different temperature intervals. All five coals mentioned in Figures 5-7 show distinct mass spectra in the mobile phase compared to the spectra of the macromolecular structure (25) and contain alkylsubstituted naphthalenes in the mobile phase although their relative amounts are dependent on coal characteristics. In general, (the temperature where the maximum rate occurs) in Py-FIMS was in the range of 430-470 C for bituminous rank coals. Around T, the macromolecular structure of bituminous rank coals is decomposed to yield FI spectrum showing the dominant peaks of alkylsubstituted phenols. Detailed FI spectra of the macromolecular structure for the ANL-PCS coals mentioned in Figures 5-7 are illustrated elsewhere (25).

Figure 5. Thermally extracted mobile phase components over different temperature intervals from Illinois 6 and Blind Canyon coals.

The following physico-chemical properties of the analyte(s) are important in method development considerations vapor pressure, ultraviolet (UV) absorption spectrum, solubility in water and in solvents, dissociation constant(s), n-octanol/water partition coefficient, stability vs hydrolysis and possible thermal, photo- or chemical degradation. These valuable data enable the analytical chemist to develop the most promising analytical approach, drawing from the literature and from his or her experience with related analytical problems, as exemplified below. Gas chromatography (GC) methods, for example, require a measurable vapor pressure and a certain thermal stability as the analytes move as vaporized molecules within the mobile phase. On the other hand, compounds that have a high vapor pressure will require careful extract concentration by evaporation of volatile solvents. 

Solid-phase microextraction (SPME) consists of dipping a fiber into an aqueous sample to adsorb the analytes followed by thermal desorption into the carrier stream for GC, or, if the analytes are thermally labile, they can be desorbed into the mobile phase for LC. Examples of commercially available fibers include 100-qm PDMS, 65-qm Carbowax-divinylbenzene (CW-DVB), 75-qm Carboxen-polydimethylsiloxane (CX-PDMS), and 85-qm polyacrylate, the last being more suitable for the determination of triazines. The LCDs can be as low as 0.1 qgL Since the quantity of analyte adsorbed on the fiber is based on equilibrium rather than extraction, procedural recovery cannot be assessed on the basis of percentage extraction. The robustness and sensitivity of the technique were demonstrated in an inter-laboratory validation study for several parent triazines and DEA and DIA. A 65-qm CW-DVB fiber was employed for analyte adsorption followed by desorption into the injection port (split/splitless) of a gas chromatograph. The sample was adjusted to neutral pH, and sodium chloride was added to obtain a concentration of 0.3 g During continuous... 

Fluorescent light microscopy distinguishes between the extractable liquids, which fluoresce strongly, and the matrix, which does not. The disruption of weak bonding effected by thermal treatment, which increased the extract yield, was paralleled by changes in fluorescence intensity. The fluorescence spectra of the extracts also reflect the compositional differences between the mobile phase and the solubilised coal network. 

In the context of the present discussion the term "mobile phase will be used to describe those components which can be thermally extracted ("distilled", "desorbed ) under vacuum at temperatures below the thermal degradation range of the coal. The residue, designated as the nonmobile ("network ) phase, is thermally degraded in the pyrolysis temperature range. Of course, the onset of pyrolysis may vary considerably, depending on heating rate, rank and coal type (10). 

Temperature-programmed vacuum pyrolysis in combination with time-resolved soft ionization mass spectrometry allows principally to distinguish between two devolatilization steps of coal which are related to the mobile and non-mobile phase, respectively. The mass spectrometric detection of almost exclusively molecular ions of the thermally extracted or degraded coal products enables one to study the change of molecular weight distribution as a function of devolatilization temperature. Moreover, major coal components can be identified which are released at distinct temperature intervals. 

With regard to the central question whether there exists a chemically and/or physically distinct mobile phase in coals, mass spectrometric data on ANL-PCS coals strongly support the presence of a thermally extractable, bitumen-like fraction which is chemically distinct from the remaining coal components. 

Fig. 11 HPLC of carotenoids solvent-extracted from (A) raw and (B) thermally processed carrots. Column, 5-/um polymeric C1(J (250 X 4.6-mm ID) mobile phase, methyl tert-butyl ether/methanol (11 89), 1 ml/min absorbance detection, 453 nm. Tentative peak identifications (1) all-trans-lutein (2) 13-cis-a-carotene (3) a cis-a-carotene isomer (4) 13 -cA-a-carotene (5) 15-cis-/3-carotene (6) 13-cis-/3-carotene (7 and 8) cis-fi-carotene isomers (9) all-frans-a-carotene (10) 9-cis-a-carotene (11) all-frans-/3-carotene (12) 9-ci. -/3-carotene. (Reprinted with permission from Ref. 192. Copyright 1996, American Chemical Society.)...

The widespread agricultural use of the OPPs and their potential mammalian toxicity has dictated the development of several methods for isolating and detecting the parent compounds. A method that allows the extraction and simultaneous detection of OPPs and their primary and secondary metabolites from beef tissue was developed using HPLC-DAD with a gradient mobile phase (96). Information on the thermal decomposition of OPPs in meat and other food products is still very limited. The temperature and pH stabilities of OPPs were investigated in lean beef... 

Once a chromatographic system has been identified for a preparative purpose, the stability and work-up procedure of the product-containing fractions should be investigated. Sometimes it turns out that the products cannot be isolated by simple removal of the solvents, because of thermal instability or too basic or acidic conditions in the mobile phase. In such a case an appropriate extraction procedure from the mobile phase may help to isolate the products. 

TSP spectra have much in common with the DLI spectra. Again the protonated or deprotonated aglycon is the major peak in the mass spectrum and thus fits the general characteristic of glucuronic acid conjugates (lfi). Also present were adduct ions, this time with NH4 + or acetate Ac from the 0.1M aqueous ammonium acetate in the mobile phase, which also contained 20% acetonitrile. These adducts were formed from the molecular ion, thermally derived BaPOH and a glucuronic acid derivative. The adduct ions have much in common with chemical ionization spectra (,17.18 ). This may be expected from the dissociation of ammonium acetate and extraction of the ions from the liquid. 

SFC is used for the separation of relatively nonpolar analytes, thermally unstable analytes and analytes with an elevated molecular weight. The key feature differentiating SFC from conventional techniques is the use of a significantly elevated pressure in the column. This allows the use of mobile phases which are either impossible or impractical to obtain under conventional LC and GC conditions. SFE is the ideal way to introduce a sample in SFC, since the same SF used as a solvent for the extraction acts as the mobile phase in the chromatograph. 

Recent work has used the yield of the chloroform extract of coal as an indication of the extent of the mobile phase (Derbyshire et al., 1991). In untreated coals, only a portion of the mobile phase appears to be removable but after mild preheating there are sharp increases in the yield of the extract. It has also been noted (Brown and Waters, 1966a,b), perhaps to no one s surprise, that there is an increase in the yield of chloroform-extractable material with treatment temperature. This has been interpreted to mean that there is a gradation in the manner in which the smaller molecules are associated with the network. Thermal studies also tell us that the thermal chemistry of chemical bonds can also vary with temperature. 

Thus, the results of such work, as mentioned here (and there are many other examples), may provide strong evidence that the thermal extraction (in many cases, thermal decomposition is a more appropriate term) of coal produces evidence for the molecular species that constitute the mobile phase, but, the thermal chemistry of coal is much more complex than these data would suggest. 

That coal contains low-molecular-weight extractable species is a fact. That these species may form a mobile phase within the macromolecular network of coal is effective in explaining some of the many facets of coal behavior, including physical phenomena such as porosity and solvent diffusion (Rodriguez and Marsh, 1987 Hall et al., 1992). However, that the constituents of this network can be extracted (unchanged) by thermal means or by solvent treatment after exposure of the coal to high temperatures where the stability of many organic species is suspect and that only the weak bonds are broken is open to question. 

TLC with Merck silica gel 60 TLC plates and hexane-methyl ethyl ketone-ethyl acetate (80 3 17) mobile phase was used successfully to analyze 81 different samples, which included 54 thermal transfer printer samples (43 photographic prints on paper and 11 plastic card samples) and 27 printer ribbons, with excellent resolution of the colors. Pyridine was used for extraction of 5 mm hole punches from paper samples, 5x5 mm cuttings from ribbons, and scalpel scrapings from cards. Developed plates were examined in daylight and under 254 and 366 nm UV light, and a Foster and Freeman, Ltd. (Evesham, Worcestershire, UK) Video Spectral Comparator 2000 High Resolution (VSC 2000 HR) was used to document and record the TLC plates in black and white and color modes, and to observe and record IRL properties on the TLC plate produced from the D2T2 (dye difrusion thermal transfer) process dyes and/or overlays. 

Solid-phase microextraction is an adsorption/desorption technique used to analyze the volatile and non-volatile compounds in both liquid and gaseous samples used as an alternative to the headspace, purge-and-trap, solid-phase extraction, or simultaneous distillation/extraction techniques. Analytes are thermally desorbed and directly introduced into any gas chromatograph or GC/mass spectrometry (MS) system. When coupled to HPLC with the proper interface, the analytes are washed out of the fiber by the mobile phase. 

HPLC has been used increasingly in the analysis of food samples to separate and detect additives and contaminants. HPLC can separate a large number of compoimds both rapidly and at high sensitivity, reduce separation times, and reduce the volume of sample needed. HPLC is ideally suited for compounds of limited thermal stability, but requires sample pretreatment such as extraction and filtration. In addition, HPLC requires careful selection of mobile phase and sample pumping rate [24]. 

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