Fused-silica-lined stainless-steel capillary

Fused-Silica-Lined Stainless-Steel Capillary Columns... 

Figure 3.12 Metal-clad capillary columns (a) aluminum-clad capillary column (b) fused silica-lined stainless steel capillary column (lower) and polyimide-clad fused-silica capillary columns (upper), [(a) Courtesy of the Quadrex Corporation (b) courtesy of the Restek Corporation.]...

The combination of nebulizing and desolvation gases (probe parameters) described above is suited to liquid flows of -0.2 ml/min. Because mass spectrometry is a concentration-dependent technique, reducing solvent flow will increase the residence time of the analyte in the source. The consequent enhancement of absolnte sensitivity can reach 1,000-fold. The potential for snch inCTeases in sensitivity led to the development of instrumentation and techniqnes for LC where solvent flows are 0.3-1.0 til/min, nano-LC. At such flow rates fused silica and stainless steel lines (with tips tapered to 10-20 ti) can be used as the ESI source. The flow rates in nano-LC also permit the reduction of the nebulization gas pressure to -250 Torr (<1 bar), sometimes even to zero. Furthermore, a drying gas is no longer required. When there is no need to separate the components of the sample, small volumes of material (-1 al) can be loaded into a capillary and placed in the ESI housing. In this case no gases are needed and the potential difference between the liquid in the capillary and that in the instrument chamber is sufficient to create an electro-osmotic flow of nebulized droplets from the tip of the capillary. 

One can find 1-mm glass-lined stainless steel columns or fused silica capillary columns. Fused silica is available with 75-yU.m, 100-/xm, or 320-/xm internal diameter by 20-cm, 25-cm, or 30-cm length. Fused silica is a glass tubing with an outside polyimide coating to prevent breakage. 

With the use of very low diameter columns, dead volume becomes a key problem, so manufacturers have gone to great lengths to try and minimize these effects. There are various column formats that have been employed for micro-LC, including fused silica capillary columns, glass-lined stainless steel, and glass-lined peek such as peeksil. 

Stainless steel columns are not lined with fused silica, which can crack or flake off when flexed or bent, thus exposing active sites. As a result, these columns can be tightly coiled to fit small GC ovens. Stainless steel capillary columns are available in three internal diameters, the standard 0.25-mm, 0.50-mm, and 0.80-mm I.D., and in lengths up to 60 m. 

Figure 3.10 Scanning electron micrographs of (a) untreated fused silica b) the rough inner surface of stainless steel capillary tubing (c) the smoother inner surface of the stainless steel capillary tubing after deposition of a thin layer of fused silica (c) also illustrates regions where fused-silica lining was removed selectively to expose untreated stainless steel surface below. (Courtesy of the Restek Corporation.) (Continued)...

Along the same line, an aluminum capillary lined with quartz coated with carbon black has been evaluated for the analysis of amines. Volatile Organic Compounds (VOCs), and oil products (25). In an alternative stainless steel column format (non-glass-lined stainless steel) intfoduced by Agilent Technologies, the metal column is deactivated and thus provides inertness similar to that of fused silica. Columns of this nature, termed DB-ProSteel series, have the same o.d. as that of a standard megabore column (0.53 mm i.d.) and require no special ferrules. 

The catalyst testing was carried out in a gas phase downflow stainless steel tubular reactor with on-line gas analysis using a Model 5890 Hewlett-Packard gas chromatograph (GC) equipped with heated in-line automated Valeo sampling valves and a CP-sD 5 or CP-sil 13 capillary WCOT colunm. GC/MS analyses of condensable products, especially with respect to O-isotopic distribution, was also carried out using a CP-sil 13 capillary column. For analysis of chiral compounds, a Chirasil-CD capillary fused silica column was employed. 

Microwave induced plasma mass spectrometry has also been used as a detector for supercritical fluid chromatography (SFC) [113] for the separation of halogenated hydrocarbons. The design of an SFC-MIP interface must ensure that the frit restrictor temperature remains at a high temperature to prevent condensation of analytes. Stainless steel transfer lines may be used. The frit restrictor should be connected to a length of deactivated fused silica capillary, inserted through the transfer line, and positioned flush with the aluminum MIP torch inset (Fig. 10.21). 

The transfer line consists of an electrically heated stainless steel tube, through which an uncoated, yet deactivated fused silica transfer capillary is passed until the end of the plasma injector. All parts of the stainless steel transfer tube are heated, including the part inside the torch box. The ICP-MS instrumentation is prone to signal suppressions and/or instrumental drift. These problems can be compensated by the use of internal standards. In the case of GC-ICP-MS the internal standard can be added to the carrier gas of the GC apparatus. A suitable internal standard is Xenon (Xe) [41]. The 126Xe signal is monitored simultaneously with the other isotopes of interest. In this way instrumental drift and signal suppression can be corrected. 

Coupling GC to ICP-MS is easily accomplished by connecting the column to the inner tube of torch using a transfer line between the GC oven and the plasma torch (Fig. 2). The transfer line usually consists of an electrically heated stainless-steel tube through which a piece of deactivated fused silica is passed. The transfer line capillary ends at the tip of the ICP injector. Generally, the stainless-steel tubing is maintained at a temperature that prevents the condensation of the GC effluent in the transfer line. Fluctuation in the transfer line temperature can affect GC peak shape and resolution. 

Capillaries are also made of stainless steel. Stainless steel interacts with many compounds and so is deactivated by treatment with dimethyl dichlorosilane (DMCS), producing a thin lining of fused silica to which stationary phases can be bonded. Stainless steel columns are more robust than fused silica columns and are used for applications requiring very high temperatures. 

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