Bidimensional columns

Large potentiality of the combination of different separation mechanisms within the same gel bed offer bidimensional columns [30], in which the bed is formed by a layer (as in TL GPC) in a sandwich arrangement. In one direction, the sample can be eluted in the steric exclusion mode and, using another eluent in the perpendicular direction, the elution is completed (as in the mode of sorption liquid chromatogra-phy). 

Kato et al. [43] used the pressure profile method and the electroconductivity probe in a bidimensional column. The liquid hold-ups obtained by the two methods agreed well except near the top of the fluidized bed. Because this study involved experimentation with air, aqueous solutions of carboxymethyl cellulose at different concentrations and glass particles of different sizes (0.42 mm, 0.66 mm, 1.2 mm, 2.2 mm) the effects of superficial liquid velocity, liquid viscosity and particle size on the liquid hold-up were considered. With this data a correlation similar to the one derived by Richardson and Zaki for liquid-solid fluidized beds was proposed [43]. 

However, it is highly possible that error was introduced in the measure of bubble sizes. In a bidimensional column, 25mm thick, parabolic shaped bubbles (side view) can be expected. Therefore, the bubble diameters measured should be smaller than their actual sizes. This effect is more significant for the smaller bubbles. Consequently, the apparent higher bubble velocities could in fact correspond to a situation where the observed are related to bubbles that were, as a result of the measurement technique, assessed with a reduced size. 

With regard to comprehensive LC data elaboration, the acquired data is commonly elaborated with dedicated software that constructs a matrix with rows corresponding to the duration of the second-dimension analysis and data columns covering all successive second-dimension chromatograms. The result is a bidimensional contour plot, where each component is represented as an ellipse-shaped peak, defined by double-axis retention time coordinates. When creating a 3D chromatogram, a third axis by means of relative intensity is added. The colour and dimension of each peak is related to the quantity of each compound present in the sample. Figure 4.9 illustrates an example of data elaboration in comprehensive LC. 

An alternative for achieving a lower column load and enough analyte in the detector is to perform an additional separation before the analytes reach the analytical column. In this separation, part of the sample that is not of interest can be eliminated, and at the same time the important analytes can be kept. This preliminary separation can be done using bidimensional chromatography (see further), but simpler techniques are also reported, such as programmed temperature vaporization (PTV) injection, etc. 

The first possible path does not use the second column, and the eluted compounds are diverted through the restrictor loop to the detector (Detector 1). For this case, the valve to Detector 1 is open. At the same time enough make-up gas is sent to the mid-point restrictor in order to have flow in the second column (Column 2) and through Detector 2. Gas chromatographic columns must have a flow of inert gas when heated to avoid the degradation of the stationary phase. No bidimensional separation takes place when this path is activated. 

The second possible path uses the second column and leads to bidimensional separation. In this case, the valve to Detector 1 is closed, and the flow is forced to the... 

As an example, a bidimensional separation was performed for a segment in a separation of a cellulose pyrolysate. Figure 5.2.13(A) shows the chromatogram of a pyrolysate obtained at 600° C from microcrystalline cellulose. The results were obtained on a 30 m X 0.32 mm Carbowax column with 0.5 pm film thickness as a first dimension separation in a bidimensional system similar to the one shown in Figure 5.2.12. 

A heart-cut of this interval was taken in the GC bidimensional system, and the separation was performed on a 30 m x 0.32 mm DBS column with 0.5 um film thickness. The oven temperature for the second separation was initially kept at 45° C for 5 min. and then heated with a ramp of 3° C/min. to 240° C. The detector used for the second dimension was also an MS system. The chromatographic results are shown in Figure 5.2.13(C). 

In linear development of a chromatogram, unidirectional or bidirectional developments of the chromatogram are possible. Similarly, as in liquid column chromatography, there are possible, in this case, either on-line or off-line techniques of sample application, separation, and detection, as well as various modifications (e.g., partly off-line method). Bidirectional development can also be vertical. Using vertical bidimensional development, applying different eluents, components of complex, difficult mixtures can be separated. The separation of such mixtures is also possible by means of this technique using multiple automatic development of chromatogram. 

While symbols for common column parameters in GC are maintained with the same meaning in GCxGC, their relationship with the columns in the first or second dimension must be distinguished in some way. The use of a superscript prefix (1 for the first column, 2 for the second column) is recommended for instance, and (r are the s5anbols for the retention time in the first- and the second-dimension columns ( D and columns), respectively. Other considerations useful in GCxGC nomenclature apjjear in "Multidimensional Confusion," an editorial of the Journal of Separation Science [7]. Monodimensional and bidimensional procedures can be abbreviated as ID and 2D, or as 1-D or 2-D. The use of a hyphen (as in two-dimensional) is recommended when "two" is used to distinguish a procedure from others having a different number of dimensions. 

In GCxGC, definition of resolution must include the fact that separation is measured for peaks that appear in a bidimensional plot. Supposing elliptical shapes for GCxGC peaks, 2D resolution is defined as the square root of the sum of the squares of the resolution of the two (A and B) columns [45,46] ... 

The discotic phase is distinctive from the aforementioned phases because the shapes appear like disks instead of rods. This mesophase consists of stacked columns of molecnles. These columns are packed together to form a bidimensional crystalline array. The characteristics of this type of mesophase depend not only on the molecules disposal in the columns but also on the columns arrangement. The latter aspect can lead to new mesophases. 

Whenever it is possible to extract the chromophore, or better, to isolate the integral photoreceptor unit, all chemical, biochemical, and physicochemical assa) can be used to carefiiUy characterize the structural and functional properties of the chromophore. The chromophore, for instance, can be purified by HPLC, and its molecular weight and structure can be determined by mass, IR, and NMR spectroscopy."- Information on the apoprotein-chromophore complex, as another example, can be obtained by means of column chromatographies, mono- and bidimensional gel electrophoresis, and enzymatic assays. When reliable hypotheses are available on the chemical nature of the sensing chromophore, specific inhibitors of its biosynthesis can advantageously be employed. ... 

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