Tapered capillary tips

Sheathless interfaces have also been developed to eliminate the dilution effect. To achieve a stable electrospray in the absence of nebulizer flow, the capillary tip is often tapered to provide a reduced orifice size. Electric connection is provided either by applying a conductive coating (e.g., gold, carbon black, or a conductive polymer) to the capillary tip, insertion of a wire... 

Glass needles are formed from borosilicate glass capillaries (outer diameter 1.2 nun, inner diameter 0.94 mm, length 10 cm Sutter Instrument Co.) having a thin, internal filament to aid filling on a horizontal needle puller (P-87, Sutter Instrument Co.). The needles are pulled to form a 3-mm tapered, sealed tip, which will be broken under halocarbon oil. 

This technique has the advantage of allowing the use of common CE capillaries without modification, which is of high practical importance. (The CE column exit may be tapered or pulled out to a fine tip in order to increase the local field strength.)... 

While the bath is heating up from one temperature setting to the next, carry out measurements of the dimensions of the capillary tubes. Since the top and bottom ends of these tnbes are tapered or ronnded, it is necessary to make two cathetometer readings at each end. As shown in Fig. 1, readings are made at the extreme tips of the inside space (a and d) and at levels where the tapering begins (b and c). The center section be) is a cylinder of uniform cross-sectional area the small ends ab and cd) can be described by cones or hemispheres. 

If two liquid layers are formed, they may be separated by removing the upper layer with a dropper having its tip drawn to a fine capillary. Extractions are handled in the same way. The extracting solution is added, the tube is shaken, and the upper layer is withdrawn. Solvent removal is effected by a rotary evaporator, the sample being placed in a tube Tvith a tapered bottom to facilitate removal of the remaining material with a fine-tipped dropper or a hypodermic syringe. 

The Mounting Pin. In this laboratory the best results have been obtained with a specially designed mounting pin, illustrated in Figure 1. The pin is a hollow copper rod with a diameter corresponding to the adaptor hole in the goniometer head (normally 3 mm or 0.125 in). The tip of the pin is tapered down to 0.5 mm. The bore diameter near the tip is -0.5 mm the rest of the bore is 1.5 mm. In preparation for use the fine bore hole is filled with standard electronic solder (by capillary action). A glass fiber of... 

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. 

Make microelectrodes—we use 1.2-mm diameter, thin-walled aluminosilicate glass with internal filament (A-M Systems, Everett, WA). Electrode tip should have a nice constant taper and be fine but not wispy. When back-filled with dextran and 1M potassium chloride, they should have a resistance of between 50 and 150MQ. In practice, the precise electrode resistance does not matter if the electrode penetrates a cell, records a stable membrane potential, and passes sufficient dye, then it is a good electrode (see Notes 7 and 8). Electrodes should first be back-fiUed with approx 0.5 0L of fluorescent dextran (lOOmg/mL in distilled water. Molecular Probes cat. no. D-3308 and D-3306 for tetramethylrhodamine and fluorescein fluorescence, respectively) and then with a little 1M potassium chloride. Don t worry about air bubbles. The capillary action of the internal filament will deal with them. 

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