Capillary column bleed

CPSil-5CB low-bleed capillary column (25 mx0.25 mmx0.25 pm Chrompack). 

Active sampling using dual multisorbent (Carbopack C, Carbopack B and Carbosieve Sill) tubes over a period of eight hours was employed. The sampled VOCs were desorbed using an automated TD system and transferred via a heated fused silica line into a GC. The VOCs were separated on a low bleed capillary column, identified and quantified by their retention times and target and three qualifier ions using mass spectrometric procedures. The details of the analytical procedures can be found elsewhere (Zuraimi et al., 2006). 

An important problem with all liquid stationary phases is their tendency to bleed from the column. The temperature limits listed in Table 12.2 are those that minimize the loss of stationary phase. When operated above these limits, a column s useful lifetime is significantly shortened. Capillary columns with bonded or... 

For routine separations, there are about a dozen useful phases for capillary columns. The best general-purpose columns are the dimethylpolysiloxane (DB-1 or equivalent) and the 5% phenyl, 95% dimethylpolysiloxane (DB-5 or equivalent). These relatively nonpolar columns are recommended because they provide adequate resolution and are less prone to bleed than the more polar phases. If a DB-1, DB-5, or equivalent capillary column does not give the necessary resolution, try a more polar phase such as DB-23, CP-Sil88, or Carbowax 20M, providing the maximum operating temperature of the column is high enough for the sample of interest. See Appendix 3 for fused silica capillary columns from various suppliers. 

Column bleed is normally not a serious problem since capillary columns contain little stationary phase. 

Chromatographic Resolution. To optimize column-coating conditions and operating parameters glass capillary columns coated with various silicone-based stationary phases were tested with difficult-to-separate groups of PAH standards and Complex samples. The SE5 -coated columns performed excellently with respect to separation efficiency, column bleed and long-term stability. Other observers have had similar results with this... 

To meet the high demands of organic trace analysis,21 GC columns have been subject to continuous refinement. This refers not only to the reduction in diameter of the nowadays almost exclusively used capillary columns (separation efficiency increases with decreasing capillary diameter), but also reflects the development in stationary phase technology In order to reduce column bleed (which is essential for mass spectrometric detection), highly cross-linked stationary phases are used to... 

The most critical properties of a capillary column are resolution, support inertness, retention reproducibility, thermal stability, and column bleed. To provide fast, reliable, and accurate analysis, it is important that the stationary phase, internal diameter (ID) of the column, film thickness, and length of the column be chosen with a view to the particular application. CWC-related chemicals differ greatly from each other in their chemical and physical properties and thus the selection of the stationary phase is in most cases a compromise between resolution and analysis time. The most suitable stationary phases for the separation of chemicals related to the CWC are listed in Table 1, along with their structures and polarities (24). 

For capillary columns, linear flow velocity is often used instead of flow rate. This is conveniently determined from the length of the column and the retention time of a dilute methane sample, provided a flame-ionization detector is in use. Typical linear velocities are 20 to 60 cm/s for helium. At high operating temperatures there is sufficient vapor pressure to result in a gradual loss of liquid phase, a process called bleeding. ... 

Capillary columns, to be suitable to HT-HRGC, must be extremely robust and must be coated with a thin film of the stationary phase with the purpose of reducing the retention of the less volatile compounds and preventing stationary-phase bleed at high temperatures [7]. 

The use of highly effective capillary columns is essential for high-resolution Pyr-GC-MS, because pyrolysis products of polymers are generally very complex mixtures. The bonded-phase fused-silica columns are especially effective for Pyr-GC-MS because of their low level of stationary-phase bleed at elevated column temperatures. 

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 ... 

When working with capillary columns, this type of injector is also used for very dilutes samples in the splitless mode. In this mode a smaller volume of solution is injected very slowly from the micro-syringe during which bleeding valve 2 (Figure 2.5) is maintained in a closed position for 0.5 to 1 minute in order that the vaporized mixture of compounds and carrier solvent are concentrated in the first decimetre of the column. The proper use of this mode of injection, which demands some experience, requires a program that starts with a colder temperature in order that the solvent precedes the compounds onto the column. The re-opening of valve 2 provides an outlet for an excess of solvent-diluted sample. Some less volatile compounds are eliminated and that can interfere with the results of the analyses. 

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 above considerations, when translated into the practice of GC detection and quantitation mean that detectors such as the electron capture detector or the photoionization detector will greatly benefit from the columns of reduced flow-rates (provided that the detection cells can be manufactured correspondingly smaller). Further advantages of capillary columns include considerably reduced bleeding rates during the high-temperature operation as well as the already discussed column inertness. These practical gains may frequently be decisive in practical applications. 

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... 

The application of capillary columns became more abundant since the introduction of the fused silica polymer-enforced columns. Another development produced the chemi-bonded coat, which reduced bleeding and solvent stripping, giving the capillary columns longer life-time. For most analyses of alkanes the carrier gas used is He however, to optimize separation H2 is used. For very high temperature analyses super pure gases are required and the presence of traces of oxygen is detrimental to the life span of the column. 

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