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Conditions that are less used today for the control of RPC selectivity include:. Buffer type is not commonly considered as a means of controlling selectivity for the separation of ionic samples. As discussed in Section 7.2.3, however, buffer type can affect the ‘‘effective’’ pK a of a solute, which is equivalent to a change in pH. 6 and/or older, type-A...
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For examples of these generalizations, see the discussion of Figure 7.12 in the following section.. It is possible to continuously vary the nature of retention, from RPC retention as in Figure 7.11a to IPC retention as in Figure 7.11b, by varying the concentration of the IPC reagent in the stationary phase. The concentration of the reagent in the stationary phase...
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346 IONIC SAMPLES: REVERSED-PHASE, ION-PAIR, AND ION-EXCHANGE CHROMATOGRAPHY. pH-2.5 phosphate buffer plus 2-mM octane sulfonate IPC reagent. if IPC separation is chosen, choose an IPC reagent and its initial concentra- tion as described in Section 7.4.2.1.. A prior knowledge of the composition of the sample, or previous exper- iments where pH is varied (step 3) should indicate the choice of...
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Figure 7.21 Separation of a mixture of carbohydrate standards by anion-exchange chro- matography with amperometric detection. Mixed mode 1-3. Figure 7.22 Separation of a nitrogen-mustard mixture by RPC (a) versus mixed-mode IPC (b). Figure 7.23 Separations of a mixture of acids, bases, and three amino acids by means of different columns. column, a commercial mixed-mode column (hydrophobic cation-exchanger), and the...
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Isomeric solutes are usually much better separated by NPC than by RPC, as seen in Figure 8.2b (C 1 or methyl-substituted anilines), Figure 8.2c (C 2 or dimethyl- plus ethyl-substituted anilines), and Figure 8.2d (C 3 or trimethyl-, methylethyl-, and n-propyl-substituted anilines). The greater isomer-selectivity of NPC versus RPC is also shown by the range in values of k for...
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The detection of sample bands in TLC as in Figure 8.8 normally requires the addition of a visualization agent to the plate. Changes in retention order as a result of a change in separation conditions can be quite pronounced when NPC is used with a silica column—often much more so than in RPC. This ability to exert a greater control...
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Figure 8.15 Use of seven experiments for the optimization of solvent-type selectivity in NPC.. Larger values of α for the solute to be purified are usually possible with NPC and silica columns, meaning that larger sample weights can be injected (other conditions the same). Finally, the use of TLC can be convenient for the preliminary assay of fractions from preparative...
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Figure 8.18 Separation of a mixture of derivatized oligosaccharides by HILIC with mobile phases of varying %-water. mobile phases are water-acetonitrile as indicated in the figure. Values of n in the figure for each peak refer to the number of saccharide units in the corresponding oligosaccharide. Although the ‘‘retention mechanism’’ for HILIC has received considerable attention in the literature, this...
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9.1.1 Other Reasons for the Use of Gradient Elution. Apart from the need for gradient elution in the case of wide-polarity-range samples like that of Figure 9.1, there are a number of other situations that favor or require the use of gradient elution:. High-molecular-weight compounds, such as peptides, proteins, and oligonu- cleotides, are usually poor candidates for isocratic separation, because...
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For gradient elution, the situation is more complicated—as values of k ∗ vary with column length and flow rate (Eq. For changes in column length L or flow rate F, a concomitant change in gradient time t G is the most convenient way of maintaining constant values of k ∗ and α. Just as a change in isocratic values of...
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Figure 9.11 Gradient separations of a peptide digest of recombinant human growth hormone.. (c) same as in (a), except a second gradient segment is added in order to accelerate elution of the last two peaks in the chromatogram. Thus the final gradient in Figure 9.11b is B in min.. Shortening run time is illustrated in Figure 9.11c for the sample...
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Figure 9.14 Plan for gradient method development.. [27]) The method-development requirements above are the same as discussed in Section 6.4 for isocratic separation, so we will not consider them further in this chapter.. the experiments used to arrive at a final separation (steps 3–8 of Table 9.2 and Fig. Ideally a full-range gradient (0–100% B) is preferred for the initial...
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As selectivity for a gradient method should have been optimized (step 5 of Table 9.2) prior to a change in column conditions, it is important to maintain the same values of k* (and α*) when changing column conditions and N*. This can be achieved by maintaining (t G F / L) constant (Eq. see Figure 9.6d – f. As long...
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Separation Conditions for Optimized Conditions in Figure 9.21a. The examples of Figure 9.21b can be more fully appreciated in terms of a relationship for gradient time. Similar plots as in Figure 9.21a,b result for the separation of higher molecular-weight samples, but with generally higher peak capacities and a need for still smaller particles.. Short run times are also needed in...
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9.1 or 9.26). (9.1) (k w ) value of isocratic k for. n increases with the number of polar groups in the solute molecule (Section 8.2). n increases with the number of polar groups in the solute molecule (Section 8.6). Thus, for a NPC separation with a 150 × 4.6-mm column, a flow rate of 2 mL/min, and gradient of...
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emphasize commercial computer-simulation software that can predict separation as a function of one or more experimental conditions, by means of experimental data from a few preliminary separations. for this application, two experimental runs are required prior to computer simulation. In the example of Figure 10.1, exper- iments using a mobile phase of 10 and 20% B (other conditions unchanged) are...
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Computer-Simulation Options. Peak tailing Can be simulated (Section 10.2.3.5). for maximum flexibility starting method development with gradient experiments often makes the most sense.. 10.2.3.2 Designated-Peak Selection. As an illustration of designated-peak selection, consider the example of Figure 10.4, but assume that it is required to assay only peaks 3, 8, and 10 in the presence of the remaining ‘‘interfering’’ peaks....
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Computer simulation can be used to check the effects of unin- tended variations in various conditions, for example:. The effect of a change in V D on a gradient separation is easily investigated by means of computer simulation. If computer simulation is used to optimize mobile-phase pH, it can also determine the robustness of the method with respect to small...
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Figure 11.5 Common integration errors. Rather than trying to find the peak-end point using the normal processes, the integrator will now automatically assign it to the forced time point for all chromatograms. 11.2.1.5 Additional Suggestions. The preceding discussion covers only a portion of the factors that contribute to a well-integrated chromatogram. For example, in the case of the adjustment made...
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(comprising all the constituents of the sample excluding the analyte. In applications where occasional over-limit samples are likely to be encountered, such as preliminary pharmacokinetic studies, it is wise to include dilution tests as part of the validation process. In other cases, concentration of the sample during sample pretreatment, or less dilution, can provide a solution. In any event, the...