Bubble cell

FIGURE 2 Comparison of extended light path capillary versus a standard capillary. An improvement in signal is noted with the addition of the bubble cell capillary. Data is courtesy of Amy Guo (Amgen, Inc., Seattle, WA). 

Detection If a small capillary diameter is desired for efficiency purposes, the detection part of the capillary can be adapted for better detection sensitivity. Examples are the bubble cell capillary and the Z-cell. In the bubble cell capillary, the capillary diameter is enlarged at the detection window so that better concentration sensitivity is obtained. If you implement a bubble cell capillary in your pharmaceutical analysis method, it is important to test different batches. Test also whether you need a bubble cell capillary or whether you can gain similar sensitivity increase with a proper injection procedure. Also, check the effect of the bubble cell on band broadening. An approximately three-times sensitivity enhancement is possible. 

Since both the bubble cell and the Z-cell need high resolution in order to observe the sensitivity increase, test whether you can avoid the use by a clever injection procedure such as sample stacking or transient ITP (isotachophoresis) instead. Further information on detection approaches is provided in Chapters 3, 5, and 15. 

In comparison to UV-HPLC the sensitivity in UV-CZE is lower, resulting in poor concentration limits of detection (LOD). Increasing the detection pathway by means of wide-bore capillaries, bubble cells, and Z-shaped cells enhances the sensitivity only 2—40-fold. 

Busch et al. studied the applicability of CZE to the examination of hapten-antibody complex formation (11). The catalytic antibodies examined have been used to accelerate a Diels-Alder reaction. Association constants of two hapten-antibody complexes were investigated and compared to the ELISA method. The samples contained buffer, hapten, and antibody. The constants obtained with CZE are a factor of 3-5 larger than those found with the ELISA method. The free-hapten concentration is measured directly this allows confirmation of the stoichiometric model. Because of the poor concentration sensitivity of UV detection, the application of an extended optical path length such as a bubble cell is necessary to obtain reliable binding parameters. 

Such a design combines the bubble-cell characteristics, together with an U-type design of the glass tube employed as the NMR detector. The glass tube is positioned within a glass Dewar, thus enabling temperature-dependent measurements. Another feature is the direct attachment of the NMR radiofrequency... 

Figure 6.7 Illustration of a capillary in which a bubble cell has been blown. (Reprinted from Ref. 36 with permission.)...

The main advantage of OT CEC is that separation efficiency can be doubled using this type of column. The trade-off is that the OT columns can easily be overloaded and therefore require a sensitive detection system. The small diameter of these columns precludes the use of UV detection, and fluorometric detection or mass spectrometry (MS) needs to be used. The use of fused silica capillaries with a bubble cell at the detection window has been reported as an alternative to employ UV detection. This features limit, to a certain extent, the range of practical applications of OT CEC. 

Shamirzaev, 1999 G=200-900, =2-110 gapxSOO kWWC bubble, Cell, Annidar importmt in flow patterns, thin film suppresses nucleation, leads to convective boiling. 

Detection of neuropeptides in CE is usually performed by ultraviolet (UV) absorbance, fluorescence and mass spectrometry (MS). UV absorbance is widely used for detection in CE, often at 214 nm, because it is inexpensive, robust, and widely available on commercial instruments. Typically, concentration sensitivities lie in the micromoles per liter range, as shown in an application starting from 2.5-mL lumbar cerebrospinal fluid, where detection limits of neuroproteins with UV absorbance detection at 214 nm between 5 and 10 jUg/mL, corresponding to 0.1-1 mol/L, were reported [4]. In order to address the lack of sensitivity achieved with UV absorbance detection, path lengths in the detection window have been increased threefold by means of the bubble cell and 10-fold by applying the so-called z-cell. 

Type of flow pattern(s) involved in an adsorptive bubble separation system depends on the type of process used. For example, bubble fractionation involves two-phase (gas-phase and liquid-phase) bubble flow, while solvent sublation involves multiphase bubble flow in their vertical bubble cells. Foam fractionation involves a two-phase bubble flow in the bottom bubble cell, and a two-phase froth flow in the top foam cell. However, all froth flotation processes (i.e., precipitate flotation, ion flotation, molecular flotation, ore flotation, microflotation, adsorption flotation, macroflotation, and adsorbing colloid flotation) involve multiphase bubble flow and multiphase froth flow. 

All batch adsorptive bubble separation processes involve no net movement of liquid, but steady bubbling of gas through the stagnant liquid. The relative bubble velocity in the bubble cell is the function of buoyancy component and the superficial gas velocity. 

In the bubble cell of any continuous adsorptive bubble separation process, the relative bubble velocity is the function of buoyancy component, the superficial gas velocity, and the superficial liquid velocity. The superficial velocity of gas and liquid are caused by the continuous entry of the gas and liquid phases into the bubble cell. Figures 4 (A) and 4 (B) show the relative bubble velocity in various two-phase bubble flow systems (43). 

At the same time, there are some limitations in utilizing CE for library search routines. CE utilizes very small amounts of sample, nanoliter volumes, and thus very low levels of possible recognition elements. Because detection is usually through the capillary, except for MS, detection limits are very high (poor), making this a generally insensitive technique for trace analysis. There are some recent improvements in detector cell design, bubble cells, Z cells,... 

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