Separation Chemistry Factor

The separation factor, a (Fig. 4.7) is calculated by dividing the kf s for the two peaks under question. It measures the separation between the two peak centers. Components with an a = 1.0 overlap completely beyond a = 2.0, 

When we change retention with solvent polarity, all peaks show an equivalent shifting in the same direction. A variable producing an a change causes relative peak positions to shift individual peaks exhibit different amounts of shift, both in size and direction. Thus, k changes spread separations already present with a changes new separations are created. With an a change, relative peak positions can even reverse. 

Temperature is the first of the variables affecting separation. Increased temperature decreases retention time on the column, sharpens peaks, and produces the change in relative peak retentions typical of an a effect. At first, this appears to be the ideal variable, similar to temperature programming for GLC. However, temperature changes have some drawbacks. 

Recent changes in column stability with zirconium-based and hybrid silica columns have lead to resurgence in the use of column jackets to elevated temperature to speed analysis time. The problem of sample degradation at these higher temperatures remains a continuing problem as it does in GC separations. 

The most common variable used to control a is the stronger solvent in the mobile phase. The stronger solvent is the mobile phase component most like the column in polarity. Changing the chemical nature of this stronger solvent will produce shifts in the relative peak positions. For instance, if we are unable to achieve the desired separation on a C18 column using acetonitrile in water, we can produce an a effect by shifting to methanol in water an opposite effect occurs on switching to tetrahydrofuran in water (Fig. 4.8). 

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There are certain unique features to the chemical separations used in radiochemistry compared to those in ordinary analytical chemistry that are worth noting. First of all, high yields are not necessarily needed, provided the yields of the separations can be measured. Emphasis is placed on radioactive purity, expressed as decontamination factors rather than chemical purity. Chemical purity is usually expressed as the ratio of the number of moles (molecules) of interest in the sample after separation to the number of all the moles (molecules) in the sample. Radioactive purity is usually expressed as the ratio of the activity of interest to that of all the activities in the sample. The decontamination factor is defined as the ratio of the radioactive purity after the separation to that prior to the separation. Decontamination factors of 105-107 are routinely achieved with higher values possible. In the event that the radionuclide(s) of interest are short-lived, then the time required for the separation is of paramount importance, as it does no good to have a very pure sample in which most of the desired activity has decayed during the separation. 

Alpha (a)—(Separation or chemistry factor). A measure of separation between two peak maxima. Ratio of their k values. 

Separation Factor (a)—A measure of peak separation between peaks. Product of dividing one peak k by the other. Also called the chemistry factor because it is controlled by changes in the chemistry of the column, mobile phase, and the sample. 

The selectivity a, also known as the relative retention, the separation factor, or chemistry factor, of a chromatographic column is a function of thermodynamic of the mass-transfer process and can be measured in terms of the relative separation of the peaks ... 

They are very effective ways of retaining specific metal ions in a nonexchanging site. In effect, each M.porphyrin is a new element , different from the parent metal ion compare free Mg2+ with chlorophyll, and its organic part is different for each metal ion (see (5) below). Thus a metal element becomes like S or P in non-exchanging selectivity similar in a sense to that of organo-metallic chemistry (see Section 2.16). The concentrations of the complexes has then separate controls of synthesis based on novel transcription factors. 

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