Sealed-tube Techniques

The enclosure of the sample in a sealed and sometimes evacuated chamber or tube is an old technique. The technique is useful for phase-diagram investigations (59), especially ihose involving corrosive materials (chalcu- 

Schouteten et al. (127) described a simple stainless steel DSC sample cell that is sealed by a resistance welding technique. The welded capsule could withstand pressures of over 80 atm. 

A high-pressure sample holder, capable of withstanding a maximum internal pressure of 150 atm, was described by Earnest et al. (128). The cell, constructed of stainless steel that could be gold plated, had a capacity of 40 pL and could be used to a maximum temperature of 4Q0CC. 

A sample holder, to be used for ignition studies, was described by Charsley et al. (129). The sample and reference materials were placed in flat-bottomed quartz crucibles, 6 mm in diameter and 20 mm in length. The crucibles were supported by plate-type Chromel-Alumel thermocouples, fitted with locating pins and having 0.5-mm-diameter wire leads. 

A miniature Li/S02 battery was enclosed in a nickel-plated steel can containing a thermocouple well so that it could be studied by DTA (130). 

---

The difficulties created by stopcocks and valves can usually be minimized. However, it is occasionally necessary to completely eliminate these sources of leakage and contamination by the use of break-seals and vacuum seal-offs. Typical situations in which sealed tube techniques are widely used are quantitative hydrolysis and oxidation reactions which require elevated pressures and temperatures, precise physical measurements on highly reactive organometallic compounds, long-term storage of reactive samples, and nonaqueous reactions under high pressure (for example, SO2 or NH3 at room temperature). Each piece of apparatus must be constructed to meet a specific need, so it is not possible to outline an apparatus which is of general use. Nevertheless, several examples will be presented here which serve to indicate the approach. 

Hot Tube and Sealed Tube Techniques, 42 General References, 44... 

S.R. Boyd, C.T. Pdtinger (1990) Determination of the abundance and isotope composition of nitrogen within organic compounds A sealed tube technique for use with static vacuum mass spectrometers. Meas. Sci. Technol. 1, 1176-1183... 

Wendlandt (79) found that by using the sealed-tube technique, heats of dehydration of metal salt hydrates could be obtained which would be impossible using conventional open tubes or crucibles. This approach is illustrated by the DTA curves for CuS04-5H20. In the open-tube curve, the peaks are... 

Critical temperatures of mixtures are usually determined in the same experiment as critical pressures. However, some workers > have used the sealed-tube technique discussed previously. The method is completely analogous to the method used for pure substances. Critical temperatures and critical pressures can be determined in the same experiment if an open-ended tube method is used. 

Iron oxychloride was prepared from Fe203 and FeQ3 by the usual sealed-tubes technique. All iron phosphonates were obtained according to the following chemical process n mmoles of phosphonic acid react with one mmole of FeOQ in 5 mL of solvent in sealed Pyrex tubes. Some reactions have been also realized in round-bottom flasks under nitrogen. 

Principle. This is essentially a small-scale modification of the macro piethod described on p. 416, the substance being completely oxidised in a sealed tube with fuming nitric acid in the presence of silver nitrate, the halogen being thus converted into silver halide. The collection and weighing of the silver halide require special techniques on the semi-micro scale. 

The earliest preparation of cellulose acetate is credited to Schiitzenberger in 1865. The method used was to heat the cotton with acetic anhydride in sealed tubes at 130-140°C. The severe reaction conditions led to a white amorphous polymer but the product would have been severely degraded and the process difficult to control. Subsequent studies made by Liebermann, Francimont, Miles, the Bayer Company and by other workers led to techniques for controlled acetylation under less severe conditions. 

Bisquinolizidine alkaloids have also been widely studied by this technique. For instance, the crystal structure of (—)-A16(17 ,-dehydrolupaninium perchlorate 23 was obtained from sealed-tube and synchroton X-ray diffraction data, and showed that the A, B, C, and D rings assume distorted half-chair, chair, distorted sofa, and chair conformations, respectively it was also used to determine the most precise dimensions so far known for the iminium group <1999JST245>. The crystal structure of quinolizinium hexafluorophosphate has also been studied <2001CSC174>. 

The advantage of trimethylsilyl (TMS) derivatives lies in the simplicity of the derivatization procedure, which is carried out by the addition of N,0-bis(trimethylsilyl)trifluoroacetamide (BSTFA) in acetonitrile and heating for approximately 2 h at 150 °C under anhydrous conditions in a sealed tube. However, there may be problems owing to the formation of multiple derivatives of each amino acid. Another technique involves the formation of n-butyl esters of the amino acids and their subsequent trimethylsilylation by a similar procedure. The n-butyl esters are formed by heating the amino acids for 15 min in n-butanol and HC1 and these are then converted to the A-TMS-n-butyl ester derivatives. A-acyl amino acid alkyl esters are commonly used. Acetylation of the butyl, methyl or propyl esters of amino acids,... 

There have been many studies for the optimization of conditions for the standard acid hydrolysis, but only a few of the more recent examples (24,27-29) are referenced here. These studies address the influence of various hydrolysis parameters on the accuracy of amino acid recoveries. Topics include acid-to-protein ratio, hydrolysis time, hydrolysis temperature, and the use of sealed tubes vs. open reflux. There is also evidence of a wide variety of techniques for the deaeration (very important ) of the sample, including vacuum, nitrogen purging, freeze/thawing, and combinations thereof. All of these issues have already been thoroughly reviewed in earlier... 

B. Sealed Tube Preparations. Sealed tube reactions and purifications afford good exclusion of the atmosphere and permit operations at moderate pressures. The latter capability makes it possible to work with highly volatile solvents, such as liquid ammonia, at room temperature. Since the sealed tube is often loaded on a high-vacuum line, the details on these techniques are given in Chapter 9. 

Mesuere et al. [99] and Gerringa et al. [100] have reviewed methods for the determination of copper in soils. Residual copper(II) complexes have been determined in soil by electron spin resonance spectroscopy. Fast neutron activation analysis has been studied [101] as a screening technique for copper and (zinc) in waste soils. Experiments were conducted in a sealed tube neutron generator and a germanium y-ray detector. 

Fast neutron activation analysis has been studied as a screening technique for zinc (and copper) in waste soils [247]. Experiments were conducted in a sealed tube neutron generator and a germanium X-ray detector. 

The current availability of small portable 14 MeV neutron generators and the future availability of high intensity 252Cf spontaneous fission neutron sources will certainly result in the wide spread use of activation techniques for non-destructive "on-stream" product analysis in industry. The cost of the required instrumentation for many types of activation analysis is not excessive, as compared to the cost of other modem analytical instrumentation. The simple off-on operation of the new sealed-tube neutron generators and minimal maintenance associated with the use of an isotopic Z5ZCf neutron source will permit operation of the analytical facility with technician-level personnel. The versatility of the activation technique justifies its inclusion among the other major analytical techniques employed in any modem analytical facility. 

Conjugate addition of indole to nitroolefins was carried out by thermal heating in a sealed tube or by addition of indolyl magnesium iodide or microwave irradiation. The results indicated that the microwave technique is most efficient with respect to time and yield (Scheme 26) [97]. 

---