Retention prediction system

Baba, Y., Fukuda, M., and Yoza, N., Computer-assisted retention prediction system for oligonucleotides in gradient anion-exchange chromatography, /. Chromatogr., 458, 385, 1988. 

In this contribution, we will describe the basic approach to construct the retention prediction system in reversed-phase LC for alkyl-benzenes, polycyclic airomatic hydrocau bons (PAHs) aind polau group substituted benzenes, baised on the use of sudi established relationships between retention and physicodiemlcal parameters of these compounds. The system has been constructed on a 16—bit microcomputer, and the application for optimization of sepairation conditions will be demonstrated. 

This equation means that, if X, the concentration of organic modifier in the mobile phase, and and P, descriptors ofa compound are given, the logarithm of the capacity factor, log k can be determined for any chromatographic conditions. This is the basic concept of the retention prediction system investigated in this study. 

To simplify the explanation, and as the typical example for constructing the retention prediction system, only the case of the CIS column has been focused in the following discussions. 

According to the above mentioned procedures, the computer-assisted retention prediction system (RPS) for the CIS column wascon-structed on the 16-bit microcomputer. The flow-chart of thisfunction of RPS is shown in Figure 2. In the use of RPS as the system to predict retention times of solutes, the following data eure input with the interactive style after accessing the function on the CRT of the computer (1) the compound name or the chemical formula of interesting solutes, (2) experimental conditions (mobile phase (M), volume fraction of organic modifier in the mobile phase (X), flow rate of the mobile phase (F)). 

A great number of stationary phases are listed in catalogues and it is sometimes difficult to choose the best column for a particular analysis. The chemical nature of the phases and their polarities do not always allow one to predict which column will be optimal for a given separation. Therefore, a technique called the retention index system has been developed with the use of reference compounds whose retention factors differ with different stationary phases. Using retention indices obtained on columns of different stationary phases, it is possible to characterise a compound and facilitate its identification. 

The above-given Martin equation form the basis for the Kovats retention index system in gas chromatography as well as for several HPLC retention prediction schemes. It must be noted here that the relationships between retention parameters and carbon numbers are usually linear at some limited range of the aliphatic chain length up to 6-8 carbon atoms in reversed-phase HPLC [491. 

Substituent electronic constants used to derive simple QSRR for real retention prediction potency have seldom succeeded. A wider application in that respect found the Hansch substituent hydrophobic constants, n 8], and Dross et al. [64] or Hansch and Leo [65] fragmental hydrophobic constants, /. The sums of these constants (plus corrections due to intramolecular interactions) account for the retention in reversed-phase liquid chromatographic systems [7,12). 

QSRR are employed by analytical chemists to help to identify unknown members of individual classes of analytes of pharmacological, toxicolr ical. environmental or chemical interest. At the same time. QSRR of g(K)d retention prediction potency helps to identify. structural descriptors of analytes, which also provide an efficient prediction of properties other than the chromatographic ones. This way the chromatographic. systems are identified which allows for a fast and convenient evaluation of analyte hydrophobicity. 

Multiple regression analyses were performed by the use of aJlEECOM 800 computer (Mitsubishi Electric, Co., Ltd., Osaka, Japan). The computer system for retention prediction was a 16-bit mioixjcomputer NEC 9801 (Nippon Electric, Co., Ltd., Tokyo, Japan), and the programs were written in BASIC language. 

Although it is often possible to predict the effect of the solvent on retention, due to the unique interactive character of both the solvents and the enantiomers, it is virtually impossible to predict the subtle differences that control the separation ratio from present knowledge. Nevertheless, some accurate retention data, taken at different solvent compositions, can allow the retention and separation ratios to be calculated over a wide range of concentrations using the procedure outlined above. From such data the phase system and the column can be optimized to provide the separation in the minimum time, a subject that will be discussed later in the treatment of chromatography theory. 

When the relationship between the distribution coefficient of a solute and solvent composition, or the corrected retention volume and solvent composition, was evaluated for aqueous solvent mixtures, it was found that the simple relationship identified by Purnell and Laub and Katz et al. no longer applied. The suspected cause for the failure was the strong association between the solvent and water. As a consequence, the mixture was not binary in nature but, in fact, a ternary system. An aqueous solution of methanol, for example, contained methanol, water and methanol associated with water. It follows that the prediction of the net distribution coefficient or net retention volume for a ternary system would require the use of three distribution coefficients one representing the distribution of the solute between the stationary phase and water, one representing that between the stationary phase and methanol and one between the stationary phase and the methanol/water associate. Unfortunately, as the relative amount of association varies with the initial... 

Inclusion of thermography into a predictive maintenance program will enable you to monitor the thermal efficiency of critical process systems that rely on heat transfer or retention electrical equipment and other parameters that will improve both the reliability and efficiency of plant systems. Infrared techniques can be used to detect problems in a variety of plant systems and equipment, including electrical switchgear, gearboxes, electrical substations, transmissions, circuit breaker panels, motors, building envelopes, bearings, steam lines, and process systems that rely on heat retention or transfer. 

From the general framework of the Snyder and Soczewinski model of the linear adsorption TLC, two very simple relationships were derived, which proved extremely useful for rapid prediction of solute retention in the thin-layer chromatographic systems employing binary mobile phases. One of them (known as the Soczewinski equation) proved successful in the case of the adsorption and the normal phase TLC modes. Another (known as the Snyder equation) proved similarly successful in the case of the reversed-phase TLC mode. 

Apart from enabling rapid prediction of solute retention, the Soczewinski equation allows a moleeular-level scrutiny of the solute — stationary phase interactions. The numeiieal value of the parameter n from Equation 2.14, which is at least approximately equal to unity (n 1), gives evidence of the one-point attachment of the solute moleeule to the stationary phase surface. The numeiieal values of n higher than unity prove that in a given chromatographic system, solute molecules interact with the stationary phase in more than one point (the so-ealled multipoint attachment). 

Table 4.45 shows the main features of SEC. This technique has become an indispensable tool for polymer characterisation. SEC has some advantages over other LC methods, such as the predictability of the end of a chromatographic run and of the retention times in a calibrated chromatographic system. SEC is an attractive technique for prefractionation or sample clean-up prior to a more sensitive RPLC technique. This intermediate step is especially interesting for experimental purposes whenever polymer matrix interference cannot be separated from the peak of interest [647]. Disadvantages are that the whole separation must be eluted within the... 

The citrate would be strongly retained by the stationary phase, so that the retention of the two acids would decrease. However, if mobile phase buffers are formed using polyvalent salts, there is a strong probability of complex formation, which will alter the predicted behaviour of the system. 

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