Capillary internal diameter

The side tube (length about 10 cm.) of a distilling flask (capacity 250 c.c.) is bent upwards at the junction with the neck of the flask. The end of the side tube, drawn out into a capillary (internal diameter 1-0-1-5 mm.), is then inserted through a cork into a piece of combustion tubing about 30 cm. long (Fig. 53). Within the tubing... 

Fused silica capillaries are almost universally used in capillary electrophoresis. The inner diameter of fused silica capillaries varies from 20 to 200 pm, and the outer diameter varies from 150 to 360 pm. Selection of the capillary inner diameter is a compromise between resolution, sensitivity, and capacity. Best resolution is achieved by reducing the capillary diameter to maximize heat dissipation. Best sensitivity and sample load capacity are achieved with large internal diameters. A capillary internal diameter of 50 pm is optimal for most applications, but diameters of 75 to 100 pm may be needed for high sensitivity or for micropreparative applications. However, capillary diameters above 75 pm exhibit poor heat dissipation and may require use of low-conductivity buffers and low field strengths to avoid excessive Joule heating. 

To reduce the elution speed of the analyte ions into the source, the electrophoretic voltage can be decreased just prior to elution of the first analyte of interest, minimizing the experimental analysis time while allowing more scans to be recorded without a significant loss in ion intensity [11]. Alternatively, the use of smaller-diameter capillaries than conventionally used for CE also increases sensitivity [11], A capillary diameter should, ideally, be commercially available, amenable to alternative detection methods, provide the necessary detector sensitivity, and be free from clogging. Capillary internal diameters of between 20 and 40 /u.m have been shown to be optimal and are compatible with microspray techniques. 

Figure 33-10 Electropherogram showing the separation of 30 anions. Capillary internal diameter 50 p-m (fused silica). Detection indirect UV, 254 nm. Peaks 1 = thiosulfate (4 ppm), 2 = bromide (4 ppm), 3 = chloride (2 ppm), 4 = sulfate (4 ppm),...

The left hand column gives possible capillary internal diameters in inches, and the second column gives the conversions to mm. The volumes of the capillaries in pl/cm are given in the right hand column. For example, the volume of a 40 cm long 0.25 mm diameter capillary is 20.4 p.1. If you use the common 0.13 mm capillary you are safe, at least concerning the capillary volumes. Be aware of additional capillaries such as a heat exchanger capillary in your detector, that has a volume that you cannot define. 

The small pathlength defined by the capillary internal diameter coupled with the need of a small injection volume to preserve the high-resolution features of CE place a strong demand on the detection capability. This is especially problematic for absorbance detectors (LOD in the order of 10 to 10 molL ), considered unsuitable for many environmental applications, where trace level occurrence or matrix complexity issues are generally of concern. 

P) in the capillary, such reactions can be carried out more systematically. Melting point capillaries (internal diameter 1—1.6 mm) are used, drawn out in the form of pipettes, and the desired amount of substance solution and of reagent solution are allowed to rise into two separate capillaries. The contents of one capillary are then transferred to the other by bringing the two pipette tips into suitable contact. Mixing is performed by tilting the capillary. Both ends of the capillary can now be sealed off and the contents warmed for a while if necessary. It is then cautiously opened at room temperature and the contents are directly applied to the layer. 

FIGURE 1 General schematic of a capillary electrophoresis system. Capillary internal diameters can range from 25 to 150 /xm, and capillary lengths can range from 20 cm to over 1 meter. In some cases, sample may be Injected at the capillary outlet, and the short segment between the outlet and the detection window serves as the separation path. 

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