Packed column bleed

This uncatalyzed reaction occurs at an acceptable rate at room temperature (298 K), a = (15/14)aq/(15/14)gas = 1.055. The value calculated from spectroscopic data is 1.096 (Chapter 4), so parasitic bleed to species other than NO(g) and HN03(aq) is indicated (the chemistry shown in Equation8.13 is oversimplified). In the exchange aqueous acid trickles down through a packed column countercurrent to a rising NO stream. A two column cascade was employed in which 4% of the acid flow is... 

Packed column widely used in gas chrpmatography is valuable for many routine analyses and necessary in certain cases. However, recent developments in the manufacture of open tubular fused silica columns have resulted in a new product, the large bore column, which provides much better resolution, an inert system with less bleeding, and shorter analysis lime. This offers a very superior alternative for packed column in most cases, and with the same convenience in application. They have already opened a new horizon in gas chromatography especially for routine analyses. 

This paper addresses the operating characteristics and performance of a 30pi TCD when coupled with a high resolution capillary gas chromatograph. Differences in the peak shape, peak symmetry, electronic bandwidth, column flow rates, and column bleed Invoke different responses for the TCD with a capillary column as opposed to a packed column. These differences raise several questions which will be addressed in this paper ... 

One of the biggest problems in the early development of GC-MS was interfacing the column outlet to the mass spectrometer. Packed columns were used, and the high volumes of both sample and carrier gas overwhelmed the MS system, which operates under low pressure, and special interfaces had to be built. The advent of fused silica capillary columns meant that the GC-MS interface could be dispensed with, and the column eluent is introduced directly into the ion source. It is essential that column bleeding be minimized since the mass spectrometer will detect the stationary-phase materials. Bleeding is prevented by chemically bonding alkylsiloxanes to the column wall. Other low bleeding stationary phases are mentioned above. 

The extreme sensitivity of the electron capture detector is considerably more utilized with highly inert glass or fused silica columns, where minimum sample loss occurs and the separated electron-absorbing molecules can easily be detected and quantified. As capillary columns generate considerably less bleeding than the packed columns, the electron capture detector can be easily used under temperature programming conditions. 

OV-11, OV-17, OV-101, QF-1, SE-54, SP-2250, EGA, Dexsil-300, and others) have been used successfully in packed columns. Extensive reviews on detectors, column supports, stationary phases, reproducibility, and separation efficiency of TAB, HFB, and TMS derivatives have been published (Husek and Macek, 1975 Blackburn, 1978) and require no further elaboration. However, the capillary columns deserve further mention. The use of various stationary phases, especially SE-30, SE-54, SE-2100, OV-1, OV-17, OV-101, OV-210, EGA, and Carbowax 20M, have been utilized OV-101 (Chauhan et al., 1982 Chauhan and Darbre, 1982 Moodie, 1981 Husek, 1982 Desgres et al., 1979), SE— 54 (Gajewski et al., 1982), and SE-30 (Poole and Verzele, 1978) are apparently superior m terms of separability, low background noise, and low column bleed... 

Until two or three years ago, the GC of hydrocarbons was restricted by the highest temperature at which the liquid phase did not bleed . Hence with the exception of an inorganic salt Eutectic packed column (which could be used up to 400 °C), all standard columns were restricted to 325-350 °C maximum temperature. This temperature limit... 

Liquid phases for capillary columns are very similar to those used for packed columns. In both cases the liquid phase must show high selectivity, a, for the compounds of interest. In addition, they should be capable of operation at high temperatures with minimal column bleed. This is particularly important for sensitive detectors like FID, ECD, and MS which are used for trace analysis. 

Relative response factors of some common compounds have been published for the most common GC detectors, and some representative values from an early work by Dietz [16] are given in Table 8.3 for the FID and TCD. These values are 3%, and since they were obtained using packed columns they may contain some column bleed. For the highest accuracy, one should determine his/her own factors. 

The selection of the packed column depends on the application. If one wants to separate nonpolar compounds that vary only by boiling point, one often uses one of the polydimethyl siloxanes (methyl silicones) colunms such as SE-30 or EKI-200. For more polar compounds, chemists will select a silicone column that has attached methyl and phenyl groups on the silicone polymer (DC-710). Separating even more polar compounds calls for a polyethylene glycol (Carbowax) column or a column packed with diethylene glycol succinate (DECS). Chemists will need to carefully note the maximum temperature that can be employed with the columns (Table 22.1). Above the specified temperature, the liquid phase itself will begin to "bleed" off the column. 

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