Curie point standards

Figure 16.5 The TGA Curie point method records for each standard an apparent sharp weight change at a well-defined temperature, which corresponds to a known transformation in the standard s ferromagnetic properties at that temperature. The figure shows the relative temperature precision from three replicate calibration runs using alumel and nickel Curie point standards. (Courtesy of TA Instruments, New Castle, DE, www.tainst.com.)...

Accurate temperature calibration was necessary for the example above, and this leads on to another advantage of TG-DTA instruments the temperature calibration can be properly carried out as the sample temperature is measured directly, which is not so in conventional thermobalances in which the temperature sensor is not in contact with the sample. A TG-DTA instrument was used to provide accurate transition temperatures for the ICTAC Curie point standards for TG calibration. 

Since there is no contact between pan and furnace, the thermal lag is higher than in DSC. The standards recommended by ICTA and distributed by NBS are ferromagnetic standards exhibiting loss of ferromagnetism at their curie point temperature within a magnetic field Nickel (354°C), Permanorm 3 (266°C),... 

MS with ionization at lower energies, etc.) were proven less reproducible than the work done on synthetic polymers and using either Curie-point or filament pyrolysis and standard GC/MS or MS analysis. 

Besides standard Py-GC/MS, a faster procedure to identify the type of caramel color using a pyrolytic step has been developed [4,4a] using comparisons of cumulative mass spectra (see also Section 5.3). These were generated by adding the spectra obtained from a Curie point Py-GC/MS analysis. 

Thermal analysis systems require calibration prior to routine use. In TGA, calibration for mass is carried out by calibrating the microbalance using a set of standard weights, as for any balance system. Temperature calibration is effected by measuring the Curie point temperatures of a suite of International Confederation for Thermal Analysis and Calorimetry (ICTAC) Certified Reference Materials, which have well-defined Curie points. ... 

A Perkin-Elmer TGA-7 instrument calibrated by Curie points of several metal standards has been employed for non-isothermal thermogravimetric analysis. The measurements were carried out at a desired heating rate (in the range of 3 - 40 K/min) in both inert (argon) and oxidizing (oxygen) atmospheres, as appropriate. 

Norem et al. (14, 15) used the third method, as previously discussed, to calibrate the temperature of their type of furnace and/or sample container, A ferromagnetic material was placed in the sample container and suspended within a magnetic field. At the material s Curie point temperature, its equivalent magnetic mass diminishes to zero and the thermobalance indicates an apparent mass-loss. For calibration over the temperature range from ambient temperature to lOOOC, it is obvious that a number of ferromagnetic materials must be used. The criteria which were considered characteristic of an ideal standard were the following (15) ... 

At temperatures considerably higher than the Curie point (320 K), we can use the standard expression for the magnetic entropy R ln(fi + 1), where ju is the atomic magnetic moment,... 

Curie-point material that has been distorted. To encapsulate a sample completely, some systems use a foil of ferromagnetic material instead of a wire. The sample is placed in the center of a piece of metal foil, which is then wrapped around it and dropped into the high-frequency coil chamber, just as a standard wire would be. 

The weight calibration of thermobalances is done using standard weights. The temperature calibration is more difficult. The method using the Curie point temperature, as... 

In Situ Methylation and Py-MS The sample (standards of known concentration or the cells to be analyzed) and 5 uL of 0.1 M tetramethylammonium hydroxide (TMAH) is added to the Curie-point wire and dried in hot air. The resulting dimethyl-DPA-coated wire is introduced in a quadrupole ion trap equipped with microtube furnace pyrolyzer capabilities, using a 200-460 C temperature ramping program. Alternatively, a triple quadrupole mass spectrometer may be employed using a Curie-point pyrolysis inlet with a 70 eV El source. Figure 19.6 shows the pyrolysis mass spectra of B. anthracis with and without in situ methylation obtained after a brief 10 min sample preparation and analysis from a 2.2 x 10 CFU sample. 

Curie point pyrolysis has been used to carry out quantitative analysis of monomer units in polyhexafluorpropylene-vinylidene fluoride [95]. The polymer composition is calculated from the relative amounts of monomer regenerated and the trifluoromethane produced during pyrolysis. A calibration curve is obtained using samples whose compositions are measured by NMR as standards and a least squares fit calculated. The reproducibility... 

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