Fused-silica capillary columns stainless steel

SFC chromatographs represent hybrids between GC and HPLC instruments (Fig. 6.4). In order to deliver the supercritical fluid, syringe pumps or reciprocal pumps are used and maintained above the critical temperature using a cryostat regulated at around 0 "C. In instances where an organic modifier is used, a tandem pump is employed which has two chambers, one for the critical fluid and one for the modifier. The liquid then passes through a coil maintained above the critical temperature so that it is converted into a supercritical fluid. Stainless steel packed columns like those used in HPLC (1 to 4 mm in diameter) or fused silica capillary columns like those used in capillary GC (2 to 20 m in length, internal diameters as low as 50 pm and stationary phase film thickness of at least 1 pm) are used in SFC. 

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. 

GC column stainless steel column packed with Porapak Q (50/80 mesh). A fused silica capillary column such as DBWAX or DB-624 may also be used. 

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. 

The original capillary column, invented and patented by Dr. Marcel Golay [1], consisted of a tube with a thin film of liquid phase coated on the inside surface. This is properly called a wall-coated open tubular column or WCOT, shown in Figure 6.2. The tube can be made of fused silica, glass, or stainless steel. Almost all commercial capillary columns are now made of fused silica. 

Figure 1.2 Various columns and materials employed for capillary gas chromatography (a) left 25 m x in. o.d. stainless steel capillary column in a pancake format, center 30 m X 0.25 mm i.d. aluminum-clad fused-silica column, right blank or uncoated stainless steel capillary tubing o.d. (b) 60 m x 0.75 mm i.d. borosilicate glass capillary column for EPA method 502.2 (c) 30 m x 0.25 mm i.d. fused-silica capillary column also pictured is a typical cage used to confine and mount a fused-silica column.

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 use of packed glass or stainless steel columns has been abandoned long ago and 30-60-m wall-coated open mbular fused silica capillary columns are used for essential oil analysis. For fast GC, shorter but more efficient columns are used to achieve analysis in a much shorter time. In capillary columns, gums or gum-like phases are preferred. Apolar silicone phases such as SE-30, SE-52, OV-1, OV-73, OV-101, DB-1, and DB-5 have become popular for essential oil analysis. Polyethylene glycol (PEG), polypropylene glycol, carbowax phases, and their nitroterephthalic esters also are used. PEG 20M and Car-bowax 20M are polar phases. 

Capillary columns are composed of fused silica which has shown remarkable properties of inertness and efficiency in chromatographic analysis. Glass capillaries and stainless steel are sometimes used too but their applications are nowadays limited. 

The coaxial Cf-FAB interface was originally designed to couple open-tubular LC and MS [42]. It consists of two coaxial fused silica capillaries a 10-pm-ID x 150-pm-OD open-tubular LC column surrounded by a 200-pm-ID x 350-pm-ID makeup or sheath tube. The matrix solvent is added close to the target. Either brass or stainless-steel targets were applied. 

Insert the exit column of the micro-capillary HPLC column into the 26-gauge stainless-steel electrospray needle until the fused silica capillary exits the... 

A third type of protective outer coating, stainless steel, for fused silica offers an alternative to aluminum-clad fused silica for elevated column temperatures. This technology is the inverse of that for polyimide-clad fused-silica capillary where a layer of fused silica is deposited on the inner surface of a stainless capillary. In Figure 3.19 scanning electron micrographs are displayed to compare the rough surface of stainless steel with the smooth surface of untreated fused silica and the surface of stainless steel after a micron meter layer of deactivated fused silica... 

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