Columns selectivity

The performances of column set A1 and B are similar, although column set B is more expensive. Using a specially selected column set for oligomer... 

FIGURE 9.4 Selecting columns according to molar mass range of sample the small pore size column has a separation volume of 1.3 ml (and partial exclusion above 200 kDa) and the other column is well selected with a separation volume of 2.7 ml. 

The lifetime for GPC columns used under high temperatures is lower than at room temperature. If higher temperatures are necessary, the user should look for special offers of high temperature columns or check which temperature interval is permitted for the selected column type. For a maximum lifetime a few precautions are advantageous ... 

These three criteria—molar mass interval, eluent, and working temperature—are fixed by the group of samples to be analyzed and considerably restrict the number of suitable columns. The selection has to be done from current lists of the manufucturers. It is useless to collect these data here, as such tables would be antiquated before this book is printed. This chapter deals with the quality of the selected columns. At this stage, columns of the same application profile are compared. The most important properties are (1) the number of... 

A better solution for preparative columns is the development of separation media with substantially increased selectivities. This approach allows the use of shorter columns with smaller number of theoretical plates. Ultimately, it may even lead to a batch process in which one enantiomer is adsorbed selectively by the sorbent while the other remains in the solution and can be removed by filtration (single plate separation). Higher selectivities also allow overloading of the column. Therefore, much larger quantities of racemic mixtures can be separated in a single run, thus increasing the throughput of the separation unit. Operation under these overload conditions would not be possible on low selectivity columns without total loss of resolution. 

Scheme 4.5 illustrates HPLCphase selection. Column manufacturers may have an applications database from which they can recommend a column and a method. Specific methods have been established for quite a large number of analytes, such as additives (e.g. antioxidants). Column selection and column technology have been reviewed [549]. Contrary to GC, and with the exception of SEC, selectivity in HPLC is determined not by the column alone but also by the mobile phase. There is therefore no one-for-one assignment between an analytical problem and the best column for this problem. 

Note how SAS Enterprise Guide defaults to row 1 containing column headings and then begins importing actual data in column 2. If you select Column Options in the left pane, you will see this window ... 

Column selection Column selection must lead to a minimum correlation between retention mechanisms (orthogonality must be maximized). 

FIGURE 3.13 Dependence on the resolution of two adjacent peaks from the separation selectivity, column efficiency, and capacity factors of peaks. Curves were calculated by keeping values of two parameters constant at the starting value and varying the third parameter. 

The time events are synchronized by one autosampler, and the diverting valve of the MS is used to select column effluent to monitor. Figure 4.12 shows an electronic diagram for this scheme. 

Figure 6.14 shows the results obtained from flow rate reproducibility studies. Results obtained for a randomly selected column over a total of three consecutive runs are presented. The inset highlights results obtained for pump B during these evaluations. As shown, the pump pulsation observed was consistently lower than 10 nL/min for each column. 

Select column with name VarName (only if x is a data frame)... 

In selecting columns, the general rule is that columns with a polar stationary phase are used to separate polar compounds, whereas columns with nonpolar stationary phases are used to separate nonpolar... 

In reversed-phase HPLC, column temperature is a strong determinant of retention time and also affects column selectivity. A column oven is therefore required for most automated pharmaceutical assays to improve retention time precision, typically at temperatures of 30-50°C. Temperatures >60°C are atypical due to concerns about thermal degradation of the analytes and column lifetimes. Exceptions are found in high-throughput screening where higher temperatures are used to increase flow and efficiency. Ambient or snb-ambient operation is sometimes found in chiral separations to enhance selectivity. Column ovens... 

Once the chromatographic method is developed, the selectivity of the optimized separation is checked on at least three different lots of the selected stationary phase for the selected column to determine whether the stationary phase is suitable for use in a final method. 

Figure 5. Development of knowledge base rules to select column and mobile phase constituents. indicates "direct hit".

The application of HPLC in routine environments, like pharmaceutical, food, or environmental analysis and particularly quality assurance, makes not only great demands on the robnstness of HPLC hardware, comprising pumps, column thermostats, and detection units, bnt in addition to the column reproducibility. Column reproducibility can be investigated at different levels of complexity Run-to-run reproducibility compares consecutive chromatographic runs, whereas long-term stability describes the column variance over several hundreds of injections. Column-to-column (batch-to-batch) reproducibility finally explores the match of independently fabricated chromatographic columns. Column characteristics that are routinely consulted for the determination of the robustness are retention, selectivity, column efficiency, and peak symmetry. 

Once the column is selected, click the Display button to change the display properties for the selected column ... 

Another use of this system is to select columns which have unique characteristics. The fluorosilicones, such as SP-2401, are the only phases for which the z value is greater than the y value, which indicates that ketones would be eluted after alcohols. With polyglycols such as Carbowax 20M, the reverse is... 

The use of two or more columns improves the probability that the identity of an unknown compound is the same as that of a compound with identical retention times however, these data alone are not conclusive proof. The reliability of the identification depends on the efficiency and polarity of the columns used. With efficient columns, the probability of having two or more components under one peak diminishes and the peaks are generally well resolved. Care must be taken in selecting columns to be certain the columns have different selectivities and not just different names. Thus, the McReynold s constants (Chapter 3) must be compared and should be quite different for each column. For example, in the analysis of pesticides, four different liquid phases might be chosen arbitrarily (e.g., OV-1, UCW-98, SE-30, and DC-200). However, the relative retention in this case will be the same for all four columns since they are all methyl silicones and have essentially the same McReynold s constants (2). 

Many column types are used for the separation and identification of foodstuffs. The nature of the separation dictates the parameters of the stationary phase as well as column and particle size. These last two characteristics are important, for they will determine the limits of sample loading and flow rate. Therefore, the specification of the selected column should be tailored es-... 

Finally, it must be indicated which data columns are the ANN inputs and which are the outputs. To do so, click the first of the ([NH4]) columns twice to select it and click /Edit/Change InputlOutput Column Types. A window will appear where it can be decided if the selected column is ANN input data or output data. Mark Output and press Change, to indicate that the [NH4 ] column is a network output. Press Find Next to select the next column, and go on in marking Output for a concentration column or Input for the ISE potential reading. 

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