Packed columns density

Five types of columns are routinely used in gas chromatography classical packed columns with internal diameters greater than 2 mm containing particles in the range 100 to 250 micrometers micropacked columns having diameters less than 1 mm with a packing density similar to classical packed columns (dp/d less than 0.3, where dp is the particle diameter and d the column diameter) packed capillary lumns have a column diameter less than 0.5 mm and a packing density less than classical packed columns (dp/d 0.2-0.3) SCOT columns (support-coated open... 

Slurry packing techniques are required for the preparation of efficient columns with rigid particles of less than 20 micrometers in diameter. The same general packing apparatus. Figure 4.8, can be used to pack columns by the balanced-density slurry, liquid slurry, or the viscous slurry techniques. Down-fill slurry packing is the method of choice for small bore columns and packed capillary columns. 

Mlcropacked column, which have internal diameters less than 1 am and a packing density similar to the columns described above (see section 1.7.1), are packed by special methods, since they are generally used in longer lengths and operated at higher inlet pressures than conventional packed columns [128,129]. The colled... 

Implementation of SFC has initially been hampered by instrumental problems, such as back-pressure regulation, need for syringe pumps, consistent flow-rates, pressure and density gradient control, modifier gradient elution, small volume injection (nL), poor reproducibility of injection, and miniaturised detection. These difficulties, which limited sensitivity, precision or reproducibility in industrial applications, were eventually overcome. Because instrumentation for SFC is quite complex and expensive, the technique is still not widely accepted. At the present time few SFC instrument manufacturers are active. Berger and Wilson [239] have described packed SFC instrumentation equipped with FID, UV/VIS and NPD, which can also be employed for open-tubular SFC in a pressure-control mode. Column technology has been largely borrowed from GC (for the open-tubular format) or from HPLC (for the packed format). Open-tubular coated capillaries (50-100 irn i.d.), packed capillaries (100-500 p,m i.d.), and packed columns (1 -4.6 mm i.d.) have been used for SFC (Table 4.27). 

The selection of the column type is mainly determined by the composition of the sample. In general open-tubular (capillary) columns are preferred for low-density (gas-like) SFC, whereas packed columns are most useful for high-density (liquid-like) SFC. Open-tubular columns can provide a much larger number of theoretical plates than packed columns for the same pressure drop. Volumetric flow-rates are much higher in packed column SFC (pSFC) than in open-tubular column SFC (cSFC), which makes injection and flow control less problematic. 

A new generalised correlation for pressure drop in packed columns, similar to Figure 11.44, has been published by Leva (1992), (1995). The new correlations gives a better prediction for systems where the density of the irrigating fluid is appreciably greater than that of water. It can also be used to predict the pressure drop over dry packing. 

Because the halogenated hydrocarbons that have to be used for this are both toxic and expensive, the use of balanced density slurries for packing columns is declining. 

Nuclides, reaction with monomers, 14 248 NuDat database, 21 314 Nukiyama-Tanasawa function, 23 185 Null-background techniques, in infrared spectroscopy, 23 139-140 Number-average molecular weight, 20 101 of polymers, 11 195, 196 Number density, of droplets, 23 187 Number of gas-phase transfer units (Nq), packed column absorbers, 1 51 Number of overall gas-phase transfer units (Nog), packed column absorbers, 1 52 Number of transfer units (Nt, NTU), 10 761... 

Column pressure usually has little effect on enantioselectivity in SFC. However, pressure affects the density of the mobile phase and thus retention factor [44]. Therefore, similar to a modifier gradient, pressure or density programming can be used in fast separation of complex samples [106]. Later et al. [51] used density/temperature programming in capillary SFC. Berger and Deye [107] demonstrated that, in packed column SFC, the effect of modifier on retention was more significant than that of pressure. They also showed that the enhanced solvent strength of polar solvent-modified fluid was nof due fo an increase in densify, caused by fhe addition of fhe liquid phase modifier, buf mainly due fo fhe change in composition. 

Although efficient columns have reportedly been obtained by packing at presrares not exceeding 2000 psi (136 atm), the use of higher pressures appeaf to be necessary to optimize not only the efficiency bu also the stability of the column. It is not unusual to pack columns t 10,000 to 12,000 psi (680 to 816 atm) final inlet pressure. With silica rased bonded phases the packing density is about 0.39 ml of dry column material per milliliter of column volume. 

Figure 1335. Packed column and internals, (a) Example packed column with a variety of internals [Chen, Chem. Eng. 40, (5 Mar. 1984)]. (b) Packing support and redistributor assembly, (c) Trough-type liquid distributor, (d) Perforated pipe distributor, (e) Rosette redistributor for small towers. (0 Hold-down plate, particularly for low density packing.

The two main methods of packing columns for HPLC are dry packing, which is suitable for particles of diameter > 30 jim, and balanced-density slurry packing which is best for small particles of diameter 5-30 jum. 

Columns consisting of particles of less than 30-50 jum in diameter are prepared most efAciently by slurry packing. Balanced-density slurry packing [11,12] is the most successful of such methods. In this technique, a supporting liquid is used which has the same density as that of the particles. This eliminates sedimentation problems. A typical balanced-density slurry-packing apparatus is shown in Fig.3.42. For the preparation of a... 

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