Sample holder, vapor pressure

In all thermoanalytical investigations it is very important how the substance is placed and positioned on the sample holder. This was illustrated in simultaneous TG-DTA measurements in connection with gas analytical investigations by Wiedemann42. The three profiles shown in Fig. 37 correspond to the water vapor pressures recorded with a mass spectrometer during thermal dehydration of fine-grained calcium... 

In order to reach a low temperature, the sample may be placed inside a metal block which is flushed with a cooling medium. Such devices reach approximately -78°C. Even lower temperatures are achieved if the sample holder is in thermal contact with a heat exchanger. Liquid nitrogen is usually vaporized in the heat exchanger to cool the sample. To maintain the desired temperature, most sample holders can be heated electrically. Vaporization of liquid helium, typically in a closed cycle, also has a cooling effect. To avoid heat loss and to protect the cell windows from condensed water, the cells are surrounded by a vacuum chamber or a chamber containing a dry gas at atmospheric pressure. 

The reactor is a cylinder made of stainless steel (Fig. 1) (its length is 36 cm and its inner diameter is 10 cm). At the upper part it is connected to a vapor pressure reserve (4.5 L) containing a small amount of an ammonium sulfate supersaturated aqueous solution. This solution provides an 81% relative humidity atmosphere between toe temperature of 293 and 303 K. The vapor pressure reserve, connected to an exhaust valve, allows control of vapor pressure in the reacting area. The reactor, toe temperature of which is regulated at 398 K or 423 1 K, contains a sample holder with six specimens symmetrically disposed around toe axis. The sample holder is composed by two stainless steel disks (92... 

Another way to increase water vapor pressure in the reactor is to increase, without any exhaust valve, the amount of specimen. Table 1 shows that if all the six sample holders are filled, is considerably longer. The use of an exhaust valve appears necessary to get a constant independent of the amount of the specimen [2]. 

A vacuum of from 4-6 torr (0.53 - 0.80 kPa) is common, and the heat loss by the subliming water usually will keep the material frozen. During small-scale laboratory operations the sample holders are left in the open air, but in large-scale commercial applications, heat must be applied to provide the sublimation energy at the rate Just below that required to keep the material frozen. To sublime 1 g of ice at 0 C requires 666 calories (2.78 kJ). This heat is provided by warm-water trays placed under the sample trays, by radiation from heated walls surrounding the sample trays, or by microwave warming. Sublimation from commercial warm water-heated trays occurs at the rate of 0.1-1.0 kg HjO/hr/m. Few systems are cooled below -30 °F, because the vapor pressure is then too low for rapid sublimation. 

When the thermal decomposition reaction was carried out in a dynamic atmosphere of nitrogen, both the loss of water and the loss of carbon dioxide were affected by the geometry of the container. Likewise, the decomposition of CaC204 was unaffected. The marked effect of the geometry of the sample holder provides evidence that a significant pressure of water vapor and carbon... 

The sample holder should have as small a vapor volume as possible. A large vapor volume allows gas pressure gradients in the sample, may result in different reactions, and, it is claimed, may cause nonstoichiometric mass-losses. A large vapor volume will also make it more difficult to locate 7 and hence more difficult to compare results by different investigators. Also in this... 

Figure 3.6. Sample holders for vapor-pressure measurements (6).

The determination of the vapor pressure of various compounds was reviewed by Wiedemann (49). He discussed the determination of vapor pressure by TG techniques based on the Knudsen effusion method. The sample holder that was employed is illustrated in Chapter 3 (Figure 3.6). For some measurements, a Pyrex glass cell having a diameter of about 15 mm was used. Four organic compounds were studied p-chlorophenyl-AT. Af-dimethyl urea (Monuron. a herbicide), p-phenacetin, anthracene, and benzoic acid, in the temperature range of 250-400 K. The vapor-pressure curves of these compounds, in the range from 0 10 Torr, are shown in Figure 4.49. The AHs values calculated were Monuron. 27.4 p-phenacetin 27.6 anthracene, 20.1 and benzoic acid. 20.7 kcalmole. 

Gas Atmosphere. Both the thin film and powder sample holders were surrounded by glass tubes which were part of closed systems. The total pressure over a sample was always close to 1 atm. The flow rates of the gases and the compositions of gas mixtures were measured with Brooks Rotameters. To obtain the 1.3% o-xylene feed, air or oxygen was bubbled through the liquid hydrocarbon to saturate the gas with vapor at room temperature. 

The major disadvantage of the liquid matrix is high vapor pressure of the liquid, often too high for the vacuum systems of many current tof instruments. The use of atmospheric pressure ms in MALDI analyses (31) may lead to the increased use of liquid matrices. Furthermore, the liquid matrices seem to have poorer mass resolution than the same pol5nner in solid matrices. Finally, many instruments are designed with vertical sample holders, which are not impossible to work with but certainly this sample alignment does not favor the use of these matrices. Little work has been done on this promising method of sample introduction. 

To determine a gas-phase spectrum from a sample that is solid under normal conditions, the sample must be volatile. If the volatility is fairly high, the same inlet can be used as for liquids. For samples with low vapor pressure, a direct inlet system can be used. A small capillary is filled with the sample and placed in a heatable sample holder, and the opening of the capillary is then brought close to the PIR. Molecules evaporating from the capillary reach the PIR directly, and difficulties with deposition at narrow or cold parts of the inlet system do not arise. In addition, the molecules do not come into contact with heated metal parts, which often leads to catalytic decomposition. The amount of substance needed for the measurement of a gas-phase PE spectrum is about 20 mg, and it cannot be recovered. 

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