Column bleeding prevention

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. 

In preparing samples for GC/MS, one simple fact must be kept in mind, namely, that everything injected onto the gas chromatographic column will be deposited into the mass spectrometer with the exception of those sample components which remain in the injection port or on the column. For volatile components this is not a concern as they are pumped away by the spectrometer vacuum system without consequence, but semivolatile materials may deposit in the ion source of the spectrometer with resultant loss of sensitivity, increased maintenance, and other unfavorable results. It is not nncommon for normal column bleed to eventually degrade system performance. For particularly valuable samples, such as metabolite extracts, biological samples, or other samples obtained through extensive effort, the contamination threat must be tolerated as the cost of analysis. However, if sample cleanup is possible without significant sample alteration, then a reasonable effort should be made to prevent contamination of the spectrometer. 

The system can be made more sophisticated through the addition of fused silica restrictors acting as bleeds, thus preventing back diffusion of analytes towards the solenoid valves. Additional pre-concentration stages can also be provided between the primary and secondary columns to reduce the peak widths introduced to the secondary column. 

Washing the column with 0.35N nitric acid first will not only remove the chloride ions, but also prevents the formation of hydrolytic or polymeric species of plutonium in comparison with the water wash formerly used. The second wash has been shown to effectively displace the impurity cations however, some bleeding of plutonium and americium may occur and cause higher... 

It was found that some Mn bleeds off the Mn02 resin column, hence the requirement for the Fe/Mn co-precipitation. This could be prevented by placing a Chelex column immediately below the Mn02 column, to remove any Mn from the column eluent. This would add to the cost of the analysis however, and the since the Fe precipitation needs to be carried out anyway the Chelex column may be redundant. 

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]. 

Examples of troublesome reverse flow into columns that were shut down are in Sec. 11.4 (also see item 14 in Sec. 11.6). Blinding, double block and bleed, and line disconnection are used to prevent backflow where lines are operated intermittently or when a piece of equipment is shut down. Check valves (double check valves when high reliability is required), good piping and operating practices, trips, and alarms are commonly used for prevention of undesirable backflow in continuously operating lines. 

Capillary Columns. Many gas chromatographs sold today use capillary columns rather than packed colunms. Capillary colunms are made of very thin fused-silica with an inner diameter of about 0.25 mm. Typically the columns are very long, often 25 meters up to 100 meters in length. The stationary phase is coated as a film on the inner surface with a thickness of about 0.25 /xm. The film is bonded to the silica and cross-linked to improve thermal stability and to help prevent bleeding of the stationary phase from the column. Most of the capillary columns do not have any support material in them. 

---