Curie-point pyrolysers

Figure 11.1 shows the pyrogram of lead white pigmented linseed oil paint obtained at 610 °C with a Curie-point pyrolyser, with on-line methylation using 2.5% methanolic TMAH. The pyrolyser was a Curie-point pyrolysis system FOM 5-LX, specifically developed at FOM Amolf Institute (Amsterdam, the Netherlands), to reduce cold spots to a minimum. This means that the sample can be flushed before pyrolysis in a cold zone, and it also ensures optimum pressure condition within the pyrolysis chamber, thus guaranteeing an efficient transport to the GC injection system [12]. 

An alternative approach has been to use Curie-point pyrolysers. The use of the Curie point in accurately reproducing a temperature has already been discussed for the calibration of TG furnaces (p. 481). In a slightly different way the Curie point can be used for accurately reproducing pyrolysis conditions with the added advantage that the rise time is only about 0.4 s. The... 

Richards, J.M. and Bradford, D.C. Development of a Curie Point pyrolyser inlet from the Finnigan MAT ion-trap detector. Finnigan MAT IDT 25. 

Figure 11.25 A Curie-point pyrolyser. Reprinted from Irwin, Analytical Pyrolysis, Marcel Dekker Inc. NY (1982)...

Curie-point pyrolyser A pyrolyser in which a ferromagnetic sample carrier is inductively heated to its Curie point. 

Determination of volatiles at the trace level is also possible by pre-concentrating the headspace volatiles on a suitable adsorbent. The trapped compounds are subsequently recovered by thermal desorption in front of a cooled trap connected to the capillary column or by solvent elution followed by splitless or on-column injection. These methods, called dynamic headspace enrichment or purge-and-trap , have been applied to trace level analysis of volatiles, using conventional electrically heated systems [ 31, 32 ], a Curie-point Pyrolyser... 

The commonly used RF frequencies in Curie point pyrolysers are 400 to 1000 kHz, and the power outputs range from 100 to 1500 watts. The rate of temperature rise depends on the conductor mass and specific heat, as well as on the power consumption of the ferromagnetic conductor. This power consumption per unit surface is related to the amount of heat generated by the conductor and implicitly to the temperature. The power consumption per unit surface N (cal cm sec ) of a ferromagnetic conductor located along the axis inside a high frequency induction coil is given by the formula ... 

The Curie point pyrolysers have several advantages when compared to other systems. The TRT is usually short and the heating rate is reproducible. The Teq temperature is accurately reproducible for the same alloy. The contact between the sample and the heated alloy is good, which assures that the heat transfer to the sample is rapid and uniform. On the other hand, the set temperatures can only be discrete and are limited to the values offered by different alloys. Even though the direct contact of the sample with the ferromagnetic alloy offers the advantage of a good heat transfer, it can be a... 

Figure 4.2.3. Curie point pyrolyser with autosampling capability (DyChrom model JPS-330).

An important capability of Curie point pyrolysers should be that the sample does not suffer any modifications before the pyrolysis step itself. As previously indicated, the housing of the pyrolyser must be heated (commonly with electrical resistances) to avoid condensation or other modifications of the pyrolysate. However, because a waiting time is inherent between the moment of sample introduction in the pyrolyser and the start of the pyrolysis itself, the sample may be heated by radiation from the sample housing. Several Curie point pyrolysers [8b] have the capability to drop the ferromagnetic foil containing the sample from a cool zone into the induction area, which is pre-heated to avoid condensation. The pyrolysis takes place immediately after the sample is transferred into this induction area such that no uncontrolled preliminary sample decomposition takes place. 

Another problem with the furnace pyrolysers can be the difference in the temperature between the furnace and the sample. Again, due to the poor contact between the sample and the hot source, the sample may reach a lower actual temperature than the temperature of the furnace wall. It is interesting that in microfurnace systems there were reported variations in the pyrolysis products as compared to the results obtained in inductively or filament heated pyrolysers [7,18]. As an example, a study done on Kraton 1107 [7] decomposition found linearity between the oven temperature and the ratio of two decomposition monomers (styrene and dipentene) only in a narrow temperature range, namely from 450° C to 625° C. Kraton 1107 was found to decompose in filament or Curie point pyrolysers such that linearity can be noticed between temperature and styrene/dipentene ratio from 500° C to 850° C. The reproducibility of pyrolysis in a furnace was also found lower than for other pyrolysers [7]. 

The reproducibility of the results for heated filament pyrolysers (CDS Pyroprobe 1000) and Curie point pyrolysers (Horizon Instruments) was reported for several samples [34]. This included several synthetic polymers, dammar resin, chitin, an insect cuticle, a hardwood (cherry), a seed coat (water lily), lycopod cuticle (fossil Eskdalia), as well as several organic geological samples. All samples were pyrolysed at 610° C for 5 s in a flow of helium. The residence time in the pyrolyser before pyrolysis was kept constant and the temperature of the sample housing was 250° C. Other parameters such as the temperature of the transfer line to the analytical instrument were also the same. Both systems were connected to a GC/MS system for the pyrolysates analysis. 

Table 4.7.2. The area counts for the chromatographic peaks corresponding to several syringyl derivatives from cherry hardwood pyrolysate [34] for a filament pyrolyser (samples F1, F2, F3) and a Curie point pyrolyser (samples C1, C2, C3).

Techniques such as Py-MS or Py-GC/MS are not always suitable for the identification of less volatile or highly polar compounds. HPLC analysis with MS detection provides a better tool in such cases, and it was successfully applied for lignin pyrolysate analysis (see Section 5.7). Pyrolysis products from a lignin sample from mixed hardwood, obtained using the organosolv procedure with ethanol/water and generated at 510° C in an on-line Curie point pyrolyser and analyzed by HPLC [8], indicated the presence of a series of compounds shown in Table 9.1.7. 

Table 9.1.7. Pyrolysis products from lignin from mixed hardwood generated at 51(f C in an on-line Curie point pyrolyser and analyzed by HPLC.

Mass spectra of most of these compounds are not reported and therefore their identification by Py-GC/MS analysis is difficult. Only a small number of them were recognized in Curie point pyrolysates of peptides. A study on glycyl dipeptides [11a] showed that a common electron impact ionization shows the molecular ion of dipeptides, and also the ion m/z =113 generated by the loss of substituents that takes place prior to ring fragmentation (except for Ala-Gly). 

The pyrolysis generating the compounds indicated above was done at 510° C in a Curie point pyrolyser [1]. 

The heating time of the wire is usually from 1 sec [63] to a few tenths of a second [61, 64], or even two or three hundredths of a second, depending on the pyrolysis conditions for the Curie-point pyrolyser. The kinetics of heating or cooling of the wire depends on its diameter and the power output of the high frequency oscillator [57, 65, 66] (see Fig. 3.4D [65] and E [57]). 

A curie-point pyrolyser can also be used with insoluble polymers, the samples being pyrolysed in the form of solid pieces. Such a sample, whose size may reach 0.1—0.5 mg, is placed in a recess specially made in the wire. To increase the amount of the sample to be pyrolysed, which is used in the form of a piece of weight up to 1 mg, it has been proposed to wind a 0.5-mm diameter wire as a tight coil around another wire of the same diameter with a piece of wire being placed on the bottom of the resulting spiral... 

The drawbacks of the Curie-point cells include the necessity to work at strictly fixed temperatures, which means that step-by-step pyrolysis is impossible. Also, until recently no provision was made in known Curie-point pyrolysers for heating the cell walls to prevent possible condensation of heavy pyrolysis products on the cold walls, nor was any attention paid to ensuring conditions for rapid entry of the pyrolysis products into the chromatographic column. 

It should be noted that all of the above-described pyrolytic devices suffer from a serious drawback. Although relatively good reproducibility of the results can be attained on the same device, devices of the same model from the same manufacturer often show poor reproducibility. Until about 1970 it was considered that the best reproducibility as regards the composition of the pyrolysis products could be achieved on a Curie-point pyrolyser [77]. However, a comparative study of the results obtained in 18 laboratories on the same sample, conducted by the Py—GC subgroup of the Chromatography Discussion Group of the Institute of Petroleum in London, has shown that Curie-point cells are characterized by the same scatter of data as cells of the other types [78]. 

Statistical analyses based on crystallographic data have been conducted on the cyclodextrins and the results were used to compile contact surfaces/molecular lipophilicity patterns. Pyrolyses of cyclodextrins have been studied by use of a Curie-point pyrolyser. ... 

Gas-Liquid Chromatography - Acetylated glycosyl fluorides have been identified by this procedure, and capillary gas chromatography has been employed in pyrolysis studies of disaccharides using a Curie-point pyrolyser. ... 

Curie-Point Pyrolyser Instruction Manual, Horizon Instruments Ltd. 

These experiments were carried out under a flow of helium at a thermolysis temperature ranging from 450 °C to 900 °C with a Curie-point pyrolyser. Data were obtained on the pyrolysis products of the pentadiene and acrylonitrile homopolymers, blends and various copolymers. By applying the both-side boundary effect theory on the molar amounts of these degradation products, which depend both upon copolymer composition and triad sequence distribution in the chain, the relative values of the monomer formation probability constants were calculated as shown next. Methyl-3-cyano-4-cyclohexene-l or methyl-3-cyano-5-cyclohexene were identified in the breakdown products formed at temperatures as low as 110 "C. 

Juvet and co-workers [4] have used this technique for the preparation of fingerprint pyrograms of polymers. They claim that this technique yields considerably more simple and reproducible decomposition patterns than filament, furnace, and Curie point pyrolysers and claim that this is due to greater control of energy input and a more predictable manner in which the polymers decompose photolytically. Photolysis is carried out using a pure thin film of the polymer which is then irradiated using a medium-pressure mercury source. The photolysis products are swept onto a GC to produce a pattern characteristic of the polymer. The retention indexes of the photolysis products of some common polymers are given in Table 6.2. 

Blackwell [32] used a Curie point pyrolyser to carry out quantitative analysis of monomer units in polyhexafluoropropylene-vinylidene fluoride. The polymer composition is calculated from the relative amounts of monomer regenerated and the trifluoromethane (CHFj) produced during pyrolysis. The exact mechanism by which trifluoromethane is produced during pyrolysis is not known, but it is presumed that the free trifluoromethyl group is cleaved from the polymer backbone. The trifluoromethyl group then extracts a proton from the polymer chain to form trifluoromethane. 

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