Exclusive mode

Let us consider the separation of polymethylmethacrylate (PMMA) on a nonmodified silica column as an example. In THE (medium polar eluent) the PMMA eludes in size exclusion mode because the dipoles of the methylmethacrylate (MMA) are masked by the dipoles of the THE. Using the nonpolar toluene as the eluent on the same column, the separation is governed by adsorption because the dipoles of the carbonyl group in the PMMA will interact with the dipoles on the surface of the stationary phase. The separation of PMMA in the critical mode of adsorption can be achieved by selecting an appropriate THF/toluene mixture as the eluent. In this case all PMMA samples... 

The effect of pore size on CEC separation was also studied in detail [70-75]. Figure 9 shows the van Deemter plots for a series of 7-pm ODS particles with pore size ranging from 10 to 400 nm. The best efficiency achieved with the large pore packing led to a conclusion that intraparticle flow contributes to the mass transfer in a way similar to that of perfusion chromatography and considerably improves column efficiency. The effect of pore size is also involved in the CEC separations of synthetic polymers in size-exclusion mode [76]. 

The elution volumes of polystyrene and benzene in the size-exclusion mode were 0.98 and 1.78 ml, respectively (Figure 1.4A). This means that separations by molecular size can be achieved between 0.98 and 1.78 ml in this system. In the normal phase mode the elution volumes of octylbenzene and benzene were 1.98 and 2.08 ml, respectively, in n-hexane solution (Figure 1.4B). This type of chromatography is called adsorption or non-aqueous reversed-phase liquid chromatography. These are adsorption liquid chromatography and non-aqueous reversed-phase liquid chromatography. The elution order of the alkylbenzenes in the reversed-phase mode using acetonitrile was reversed... 

H585]. Lateral metalation actually becomes the exclusive mode of reaction when LDA is used as the base [76JCS(P1)994 83JOC4307]. 

FIGURE 16.12 Schematic representation of the LC LCD procedure (adsorption retention mechanism with a narrow barrier of adsorli injected in front of sample). Large volume of sample is depicted. The sample contains two polymers exhibiting different adsorptivities. The fast (SEC-like) elution of adsorbing polymer is hindered by the barrier of adsorli while the non-adsorbing species are freely eluted in the exclusion mode. Four stages of the process are shown. The peak focusing is demonstrated. 

Fig. 14 Size-exclusion chromatograms of standard PSs for monolith columns with and without the high-density PMMA brush (Mn = 15000, M /Mn = 1.2, a = 0.6 chains nm ) on an inner surface. The inset shows a cartoon illustrating the size exclusion mode by the brush...

In the range of 15-30% acetonitrile, the different components of the mixture were separated according to size, in a size-exclusion mode, with very little nonspecific adsorption and good peak symmetry. It was believed that the addition of TEA to the solvent contributed to these desirable effects by neutralizing residual silanol groups. Present supports should not suffer from the presence of such uncapped silanol groups. 

Samples were collected in midget bubblers containing dilute sodium carbonate solutions. Analysis of formate ion in these solutions was performed using a 500 mm anion separator column and sodium borate eluent. Analysis of formate in these solutions was also performed using the ion chromatography exclusion mode (ICE) using the Dionex IE-C-1 column with dilute hydrochloric acid eluent. 

Let us first examine the behaviour of macromolecules in the case of repulsing walls (we shall call this case the exclusion mode) and in the case of attracting walls (the adsorption mode). 

This is a well-known result obtained by Casassa46). In the exclusion mode, the longer the macromolecule the greater is the energy that has to be spent to place it in the pore. The distribution coefficient Kd exponentially decreases to zero with increasing size of the macromolecule. The larger the molecule, the smaller is VR in accordance with Eq. (3.1) in the exclusion mode 0VR/6N < 0. b. The case of attraction... 

As it was shown in Section 3.2, close to the critical conditions the distribution coefficient Kd is a function of chain length, pore size D and the energy of interaction of units with pore walls, 0. For a chosen molecule and adsorbent, Kd = Kd(0), and, therefore, by changing 0 one can successively achieve the transition from the adsorption to the exclusion mode and vice versa, finding in this way the critical conditions necessary for separation according to the functionality. 

For a given polymer with fixed X0 and Ax, Eqs. (3.15) and (3.16) describe a non-linear but monotonous variation of 0 with the composition of the mobile phase Cb (Fig. 10). The procedure of finding the critical conditions then becomes very simple it is necessary to find two solvents, in one of which (a) the adsorption and in the other one (b) the exclusion mode is operative and then, by changing their ratio, to find the point C where there is no retention volume dependence on the molecular weight of the polymer. The only requirement imposed on the choice of the solvents is that they both should be good solvents. In practice, however, it is... 

Figure 16 shows some examples of the separation according to functionality types of industrial PDEGA samples at critical conditions. It is not difficult to select fractions corresponding to the zones of different functionality and investigate in detail their MWD distribution by standard methods in the exclusion mode. 

It is seen from this figure that in the exclusion mode the presence of the functional groups increases the resolution of separation according to the molecular weights because the functional groups are more strongly adsorbed than the backbone units. Thus the MWD even for very narrow polybutadiene samples (Mw/Mn = 1.05) can be investigated in detail. 

For the investigated type of telechelic polymer (preferably a sample containing only nonfunctional macromolecules so that retention volumes are close to V0 + Vp), it is first of all necessary to choose two solvents, in one of which (polar solvent b with E0b) the exclusion mode is operative and in the other (nonpolar solvent a with s ) the adsorption mode. The solvents must be able to dissolve readily the analyzed samples and satisfy the detection conditions. 

Other aquo-cobalt(III) complexes, such as Co(en)2(H20)2 3, react with nitrite in the same way and for the reaction of the chromium complex, Cr(NH3)5H20+3 with nitrite, this seems to be the predominant, if not the exclusive, mode of substitution. 

High-performance liquid chromatography (or less common, high-pressure liquid chromatography, HPLC) is a preferred method of analysis for many compounds because it does not require the high temperatures used in gas chromatography. Separations in HPLC can be based on either a size exclusion or on an adsorption principle. The size exclusion mode is useful for separating fatty acids from... 

The opposite approach is a descriptor that is calculated in exclusive mode for a certain atom type with the inverse condition... 

RDF. The distance mode dehnes the mode for distance calculation available modes are Cartesian distances, bond-path distances, and topological distances. Descriptors may be calculated on particular atoms. Exclusive mode restricts the calculation to the atom type, and with ignore mode the selected atom type is ignored when calculating the descriptor. In partial-atom mode an atom number has to be given instead of the atom type. The second atom property is available if 2D RDF is selected as code method. 

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). 

The regiochemistry of exohedral addition reactions is governed by the minimization of [5-6] double bond within the fullerene framework. The exclusive mode for typical... 

This paper describes the use of poly(styrene-divinylbenzene) copolymer, PRP-1, as a reverse-phase adsorbent in the assay of the antibiotic aztreonam and related compounds. Comparisons are also made for similar assays using silica-based columns. None of the shortcomings described earlier, associated with bonded phase columns, is observed. In addition to the reverse-phase mode, the PRP-1 columns are tested in ion-pair as well as in size exclusion modes of separation. Superior resolutions are obtained in the reverse-phase chromatography of ionic compounds without the use of lon-palring agents. In addition to the normal adsorption and/or partitioning,... 

In a more general sense, the size exclusion mode of liquid chromatography relates to a separation regime, where entropic interactions are predominant and TAS > AH. In the reverse case, AH > TAS, separation is mainly directed by enthalpic interactions. As both separation modes in the general case are affected by the macromolecule size and the pore size, a certain energy of interaction may be introduced, characterizing the specific interactions of the monomer unit of the macromolecule and the stationary phase, e is a function of the chemical composition of the monomer unit, the composition of the mobile phase of the chromatographic system, and the temperature. 

The critical diagram M vs retention time for a short RP-18 column of 60 mm length is shown in Fig. 7A. At acetonitrile concentrations > 47 vol.% in the solvent mixture, the retention time decreases as the molar mass of the PEO calibration sample increases retention corresponds to a size-exclusion mode. The reverse behaviour, i.e. the adsorption mode, is obtained at acetonitrile concentrations < 45 vol.%. The critical point of adsorption is operating at an eluent composition of acetonitrile-water 46 54 v/v, where separation is accomplished exclusively with respect to the functional endgroups- (see Fig. 7B). For the alkoxy-terminated PEOs, two distinctively different fractions are obtained, due to the formation of polyethylene glycol as an unwanted by-product. In the case... 

To explain the separation procedure of polymer blends using chromatography at the critical point of adsorption, the behavior of blends of polystyrene (PS) and polymethyl methacrylate (PMMA) in different chromatographic modes is shown in Fig. 24. With silica gel Si-100 as the stationary phase, the mobile phase comprised mixtures of MEK and cyclohexane. In pure MEK a size exclusion mode was operating for both components. Under these conditions PS and... 

PMMA may be separated only if their molar masses are different. For low molar mass samples (PS, PMMA 30000 g/mol) the two components of the blend were identiiied, but separation was poor. For higher molar masses (PS, PMMA 150000 g/mol), however, one symmetric elution peak, similar to the elution profile of a homopolymer, was obtained. When cyclohexane was added to MEK, the elution behavior of PMMA changed dramatically, whereas for PS it remained nearly constant. In the near-critical region (75 vol.% of MEK) separation of blend components of equal molar mass was improved, compared to the size exclusion mode. At the critical point of PMMA, which corresponds to a mobile phase composition of MEK-cyclohexane 73 27 vol.% (see Fig. 24, upper right corner), a complete separation of the elution zones of PMMA and PS was achieved. Regardless of the molar mass, all PMMA fractions were eluted... 

In a similar approach, the molar mass distribution of the PMMA fraction in the blends may be determined when chromatographic conditions are operating corresponding to the critical mode of PS and the size exclusion mode of PMMA. 

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