Tubing microbore

The dimensions of concentric-tube nebulizers have been reduced to give microconcentric nebulizers (MCN), which can also be made from acid-resistant material. Sample uptake with these microbore sprayers is only about 50 xl/min, yet they provide such good sample-transfer efficiencies that they have a performance comparable with other pneumatic nebulizers, which consume about 1 ml/min of sample. Careful alignment of the ends of the concentric capillary tubes (the nozzle)... 

Columns are made from straight lengths of precision-bore stainless-steel tubing with a smooth internal finish. Typically they are 10-25 cm long and 4-5 mm i.d. Microbore columns, 20-50 cm long and with an i d. of 1-2 mm, are sometimes used where sample size is limited and to minimize solvent consumption because the volumetric flow rate through them is less than a... 

The IBW of a standard system can be reduced to 30-40 jL by using shorter lengths of 0.005-0.007" i.d. tubing and a semi-micro flow cell (2-3 tL).i° Further reduction might involve a low dispersion micro-injector or a redesign of the autosampler. Table 4 summarizes the typical IBW and other instrumental requirements of various column types from conventional (4.6 mm), Fast TC, minibore (3 mm), narrowbore (2 mm) and microbore (1mm) to micro TC (<0.5mm) columns. Note that the dispersion... 

Ideally, a sample is introduced into a chromatograph as a perfect plug. In practice, this is not the case, and diffusion occurs because of the injector. For narrow-bore and microbore applications, injectors capable of introducing the required sample volumes are commercially available and optimized to reduce dispersion. This is not the case for capillary LC, and homemade injection systems include the sample tube technique, in-column injection, stopped-flow injection, pressure pulse-driven stopped-flow injection (PSI), groove injection, split injection, heart-cut injection, and the moving injection technique (MIT). Of the injection techniques, only the split injector, MIT and PSI approaches can introduce subnanoliter sample volumes accu-... 

Microbore Columns. As the name implies, these column types have small inside diameters. The sizes vary, and some columns are packed while others are open tubes. A summary of the major types is presented in Table 13. The most common commercial columns are 1 mm i.d. and packed. The interest in these columns and the wide range of types has already resulted in the publication of three books on this topic alone.47-49... 

For LC/MS the main problem is the large amount of mobile liquid phase that must be removed to get the effluent reduced to the high vacuum of the MS. Microbore columns are desirable for this reason.22 The three most popular devices have been summarized by Majors23 direct liquid interface (DLI), moving belt transport, and thermospray.24 The thermospray device consists of a small bore capillary tube that is heated to produce a stable, high-velocity jet consisting mostly of vapor with a small amount of mist. It not only provides an interface to the MS, it also causes the ionization of analytes necessary for the MS. Some think it may find more widespread use as a transport device. 

Other Column Features. External Zone Broadening. If attention is not paid to the extracolumn volumes in connecting tubes, injection devices, and detectors, the efficiency of a separation may be decreased by zone broadening in these regions. This is especially important for the small and microbore columns just described when they are used at low flow rates. 

Basically, the flow control and sampling unit allows three alternative methods of operation. Firstly the eluent from the column can flow directly from the UV detector to the NMR sample tube and the spectra can be continuously monitored during the development of the separation. The success of this procedure will depend on the volume of the cell, the sample size, the column flow rate, the resolution of the NMR spectrometer and the rate of data acquisition by the computer. In general, unless the new micro-cell facilities mentioned above are exploited, this procedure will rarely be successful, particularly if microbore columns are used and multi-component mixtures are being examined. 

Column-liquid chromatography (CLC) can be conveniently divided into those systems which use packed columns and those which use open tubes (Figure 3.1). Capillary tubes (<4 < 350 pm) are used in open-tubular chromatography and the stationary phase is coated on the internal surface. Packed-column systems can be sub-divided arbitrarily into capillary columns, microbore columns, analytical columns and preparative columns according to the internal diameter of the column (Figure 3.1). 

Fig. 4 Syringe pump setup with drug administered separately of the primary i.v. fluid flow rate. Mixing of the drug and i.v. fluid occurs at a stopcock and for a short distance in microbore tubing. (From Leff, R.D. Roberts, R.J. Methods for intravenous drug administration in the pediatric patient. J. Pediatr. 1981, 98, 631-635.)...

The Finnigan chemical ionization source was modified by the addition of two cartridge heaters. Eluate entered the source through a heated 1/2" probe and excess solvent was removed by a mechanical vacuum pump connected directly opposite the eluate entrance. The interface consisted of 1 meter deactivated silica capillary tubing (ID, 60 OD, 0.008"), led from the outlet of microbore column and threaded through the probe. The probe design has been previously described (26). For all analyses, the probe was operated at 240 C and the source at 250 C. The analyzer pressure was 10 b torr. 

For use in the clinical laboratory, most analytical HPLC are fabricated from tubes made of 316 stainless steel that have IDs ranging from 0.3 mm to 5 mm and lengths from 50 mm to 250 mm (Table 6-3). Column end fittings, which have zero dead volume and frits to retain the support particles, are used to cormect the column to the injector on the inlet end and the detector on the outlet end. Generally, lower detection limits are achieved with columns having smaller IDs. These smaller ID columns are manufactured from narrow bore (approximately 2.1mm ID) and microbore (approximately 1.0 mm ID) tubes. In addition to providing improved efficiency, columns with smaller IDs use decreased volumes of mobile phase. For example, a 2-mm-ID column requires about fivefold less solvent than a 4.6-mm-ID column (see Table 6-3). 

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