Retention variation with temperature

The variation of the retention volume with temperature is such that a plot of the logarithm of the retention volume versus reciprocal of the absolute temperature, called a retenticm di am, is linear. The slope of this straight line is related to the enthalpy of the process, solution in the stationary phase (GLQ or adsorption on the surface (GSC), with. 

If the HPLC mobile phase is operated close to the pA of any solute or if an acidic or basic buffer is used in the mobile phase, the effects of temperature on retention can be dramatic and unpredicted. This can often be exploited to achieve dramatic changes in the separation factor for specific solutes. Likewise, the most predictable behavior with temperature occurs when one operates with mobile phase pH values far from the pA s of the analytes [10], Retention of bases sometimes increase as temperature is increased, presumable due to a shift from the protonated to the unprotonated form as the temperature increases. As noted by Tran et al. [26], temperature had the greatest effect on the separation of acidic compounds in low-pH mobile phases and on basic compounds in high-pH mobile phases. McCalley [27] noted anomalous changes in retention for bases due to variations in their pA s with temperature and also noted that lower flow rates were needed for optimal efficiency. 

Many factors can influence the Kovats index which can make it unreliable at times for the characterization of gas chromatographic behavior, although it generally varies less than the relative retention with temperature, flow, and column variation. However, for many it is the preferred method of reporting retention data. 

All possible multicomponent reactions were carried out by the combinatorial variation of between two and ten starting materials (2-CR to 10-CR) in parallel in methanol at room temperature and using a robotic dispensing system. With the aid of automated liquid chromatography and data evaluation, products were searched that are unique to a specific mixtures by comparing the retention times with the starting materials and over all other mixtures that contain the respective sub-combinations. Thus, for example, a novel product of a four-component reaction should not be contained in all three possible 3-CR mixtures. 

From eqn.(3.6) we conclude that there are two solute-dependent factors that affect retention. In the first place, this is the vapour pressure of the pure solute. p° is a strong function of the temperature (see below) and therefore, temperature may be used as a parameter to influence retention. However, the vapour pressure is a pure component property and it cannot be changed at will. Differences in the vapour pressure of two solutes (or differences in variation of vapour pressure with temperature) may or may not provide us with a means to achieve separation. When the vapour pressure is not sufficiently different, we need to create differences in the second solute-dependent factor. 

Eqn.(3.6) provides a good insight into the variation of retention with temperature in GLC. Both the activity coefficient and the vapour pressure of the solute vary with temperature in an exponential way. For the activity coefficient we can write... 

Figure 6.4 shows a schematic example of the variation of retention with temperature in GC for a number of solutes, which could, for example, form part of a homologous series. 

Figure 6.4 Schematic example of the variation of retention with temperature in gas chromatography. Retention lines are drawn for a group of 8 solutes (e.g. homologues). Vertical dashed lines (a and b) correspond to chromatograms (a and b) in figure 6.1. Horizontal dashed lines indicate the range of optimum capacity factors.

In this equation fg is the retention time under isothermal conditions at the temperature T and A, B and C are constants. An analogous expression is used to describe the variation of the peak width at the location of half the peak height (w1/2) with temperature. 

Magnetic properties of nanoparticles of transition metals such as Co, Ni show marked variations with size. It is well known that in the nanometric domain, the coercivity of the particles tends to zero. 23 Thus, the nanocrystals behave as superparamagnets with no associated coercivity or retentivity. The blocking temperature which marks the onset of this superparamagnetism also increases with the nanocrystal size. Further, the magnetic moment per atom is seen to increase as the size of a particle decreases 25 (see Figure 7). 

Retention factors in CEC are also reduced by increasing column temperature because of increased partition into the mobile phase van t Hoff plots of In k versus T l are generally [7,55] linear, and the slopes of such plots may differ sufficiently for column selectivity to be changed by temperature variation. For example, in the CEC of a number of diuretic drugs on ODS-bonded silica at temperatures between 15 and 60°C, the resolution of chlorothiazide and hydrochlorothiazide increases [7] with decreasing temperature, and the relative retention of chlorothalidone and hydroflumethazide is reversed with increasing temperature variations of k with temperature, which may make [56] temperature programming a useful technique in CEC. 

Stein, Gray and Guillet 82, 83) demonstrated the suitability of the GC method to study crystallization kinetics, through the variation of the retention volume with time. When the pcdymer stationary phase is cooled from the melt to a temperature below Tj, the retention volume decreases with time at the rate at which crystdline domains are being formed. The maximum possible crystallinity at a given temperature is obtained from the relation... 

Two main factors that cause retention-volume variations with column temperature are assumed an expansion or a contraction of the mobile phase in the column and the secondary effects of the solute to the stationary phase. When the column temperature is... 

Fig. 3-2. Variation of the retention of bromide and nitrate with temperature. — Separator IonPac AS3 eluent 0.0028 mol/L NaHC03 + 0.0022 mol/L Na2C03 flow rate 2.3 mL/min detection suppressed conductivity injection volume 50 pL anion standard.

The variation in the shape of the retention diagram with polymer film thickness was accounted for by a quantitative analysis of the retention data above and below Fg. It was shown that at temperatures below Tg the retention volume per unit area of the coated support was constant, irrespective of film thickness, indicating the existence of a single surface retention mechanism. At temperatures above Tg, equilibrium retention data were suitably described by the relation... 

Perhaps the RIs vary more with temperature because the analytes are aromatic and the reference compounds are aliphatic. If you have the spreadsheet set up, measure the retention times in the figures and put in the appropriate values for the triplets of branched hydrocarbons CIOs or C14s and see whether their RIs are more consistent with variation of temperature. 

LRIs (linear retention indices) depend on programming and flow conditions, but changes in their values are not very marked, particularly when the variation of RI with temperature is small [9] in this case, LRI and RI values for a compound are similar. 

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