Trap continued monitoring

A second approach to gravimetry is to thermally or chemically decompose a solid sample. The volatile products of the decomposition reaction may be trapped and weighed to provide quantitative information. Alternatively, the residue remaining when decomposition is complete may be weighed. In thermogravimetry, which is one form of volatilization gravimetry, the sample s mass is continuously monitored while the applied temperature is slowly increased. 

One of the attractive features of SFE with CO2 as the extracting fluid is the ability to directly couple the extraction method with subsequent analytical methods (both chromatographic and spectroscopic). Various modes of on-line analyses have been reported, and include continuous monitoring of the total SFE effluent by MS [6,7], SFE-GC [8-11], SFE-HPLC [12,13], SFE-SFC [14,15] and SFE-TLC [16]. However, interfacing of SFE with other techniques is not without problems. The required purity of the CO2 for extraction depends entirely on the analytical technique used. In the off-line mode SFE takes place as a separate and isolated process to chromatography extracted solutes are trapped or collected, often in a suitable solvent for later injection on to chromatographic instrumentation. Off-line SFE is inherently simpler to perform, since only the extraction parameters need to be understood, and several analyses can be performed on a single extract. Off-line SFE still dominates over on-line determinations of additives-an... 

Dynamic headspace-extraction stripping and purge-and-trap methodology are used most often for determination of M-hcxanc in water and hazardous wastes. Dynamic headspace extraction techniques have been applied to water samples (Roberts and Burton 1994) and sediment (Bianchi et al. 1991). Detection limits of 0.5 g/L were reported for lake water (Roberts and Burton 1994) and 20 ng/kg (ppt) for sediment (Bianchi et al. 1991). Supercritical fluid extraction (SFE) is a relatively new technique that has been applied to -hcxane in soil (Yang et al. 1995). Membrane extraction of M-hexane from water samples has been developed to provide online, continuous monitoring (Wong et al. 1995 Xu and Mitra... 

A modification of this technique, applicable to many step polymerizations, involves the continuous monitoring of the small-molecule by-product. For example, for polyesterification between a diol and diacid above 100°C, water distills out of the reaction vessel and its volume can be measured by condensation and collection in a calibrated trap. 

Fat oxidative stability is measured by the active oxygen method (AOM, AOCS Method Cd 12-57). Oil or fat is held at 97.8°C while air is bubbled through it. The time required to develop a peroxide concentration of 100 meq/kg is the AOM stability of the sample. A closely related method, the oil stability index (OSI, AOCS Method Cd 12b-92), also bubbles air through hot oil. One of the breakdown products is formic acid, which is trapped in a water cell. The machine continuously monitors conductivity of the water, and records the time when it rises sharply. Rancimat times obtained at 110°C are 40-45% of the AOM times, so an OSI stability of 4 h is equal to an AOM stabihty of 10 h. 

Ciucanu 1 and Chiriac A. Membrane and trap system for continuous monitoring of volatile organic compounds using a portable gas chromatograph with thermal conductivity detector. J. Sep. Sci. 2002 25 447 52. 

Each separation is initiated by hlling the column with either the upper or lower phase of an equilibrated two-phase solvent system. In order to avoid trapping air bubbles in the column, the solvent should be introduced through the bottom of each column, which is kept in a vertical position. Then, the column assembly is tilted at a desired angle (25°-30°) from the horizontal plane. After the sample solution is introduced into the column, the mobile phase is eluted from the column while the apparatus is rotated at a desired rate (60-80 rpm). In order to retain a large volume of the stationary phase, the lower phase is eluted downward from the upper terminus and the upper phase upward from the lower terminus of the column assembly. The effluent from the outlet of the column is continuously monitored with an ultraviolet (UV) monitor and collected into test tubes using a fraction collector. 

The UV photometric ozone monitor is relatively maintenance free except for periodic calibration by iodimetry and changing of the inlet filter and catalyst. Hence, UV photometry is suitable for continuous monitoring of atmospheric ozone and has been adopted as a standard method along with the ethene-CL method in major counters. UV ozone monitors with higher concentration ranges (up to 10 vol%) are also available. In high-concentration ozone measurements, ozone-free reference should be prepared separately for calibration, because of insufficient activity of the catalyst for ozone destruction. For a humid air sample, a water trap (or humidifier) is necessary. 

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