Automated multiple development instrument

Fig. 3 Automated multiple development instrument (AMD 2). (Photo courtesy of Camag Scientific, Inc.)...

Fig. 2 The steps in the process of thin-layer chromatography that have been instrumentalized and automated to a large degree in the recent past. PMD = Programmed Multiple Development, AMD = Automated Multiple Development, DC-Mat or ADC = Automatic Development Chamber.

A technique that can achieve the maximum attainable resolution in TLC on a given separation distance without forced flow is automated multiple development (AMD). This step-gradient technique was developed by Burger. With respect to peak capacity the technique can be compared to HPLC, but it still maintains all benefits of planar chromatography. The heart of the instrument is a specially designed vacuum-tight chamber. Following sample application... 

Automated multiple development (AMD), providing automatic chromatogram development and drying, is a novel form of the PMD technique. Automated multiple development as an instrumental technique can be used to perform normal-phase chromatography with solvent gradients on HPTLC plates. Most of the AMD applications use typical gradients Starting with a very polar solvent, the polarity is varied by means of base solvent of medium polarity to a... 

Jaenchen, D.E. Proceedings 3rd International Symp. on Instrumental HPTLC, Kaiser, R.E., Ed. Institute for Chromatography Bad Duerkheim, Germany, 1985 71. Butz, S. Stan, HJ. Screening of 265 pesticides in water by thin-layer chromatography with automated multiple development. Anal. Chem. 1995,67, 620. 

AUTOMATED MULTIPLE DEVELOPMENT (AMD). Linear, ascending, multiple developments are carried out in an automated instrument. The development distance is increased and the strength of the mobile phase is reduced (stepwise gradient elution) for each step. For silica gel, the mobile phase composition changes from more polar to less polar. AMD leads to zones with reduced diffusion and increased resolution because of a concentration effect. 

PMD. Programmed multiple development. The repeated development of a TLC plate with the same mobile phase or different mobile phases in the same direction for gradually increasing distances, using an automated commercial instrument. Also termed automated multiple development (AMD). 

Automated multiple development, mentioned above in Section C, involves development in a special instrument with a universal gradient (125) starting with the most polar (strongest) solvent and becoming increasingly weaker. Zones are focused into well-resolved, narrow bands. 

Important new instruments were developed after the UM chamber, aimed at increasing the efficiency of TLC through improvement of the separation mechanism. Programmed multiple-development TLC as elaborated by Perry (12) combines the techniques of continuous multiple development and evaporation. Recently this technique was improved by Burger (13). In this system the chomato-plate is developed several times in the same direction with various eluents of decreasing elution power. Between developments the chromatoplate is dried by vacuum. This method is termed automated multiple development (AMD) (14). High-performance TLC (HPTLC) is based on the use of chromatoplates coated with fine particles of narrow particle size distribution sorbent and is carried out with sophisticated instrumentation (15,16). 

Steroids have been extensively investigated using instrumental techniques such as overpressured layer chromatography (2,4,17,34,55), parallel development TLC (56), programmed multiple development [57], and automated multiple development/AMD (58). Each technique has been reported to furnish a significant improvement in separation selectivity in comparison with development in conventional TLC chamber systems. 

In recent years, TLC has gained attention for pesticide residue analysis by virtue of its evolving into an automatable instrumental technique, especially after the advent of the automated multiple-development (AMD) technique [1,156-159]. High-performance thin-layer chromatography (HPTLC) has been used for the determination of pesticides in water (Table 18.2) [113,159-161]. Thus, Butz and Stan developed an AMD-HPTLC method for the determination of 265 pesticides in drinking water [161]. 

Photometric detection, 208-210 Photomultipliers, 378-379 Physical methods of detection, 206-211 photometric detection, 208-210 visual detection, 206-208 Physical phenomena in TLC, 49-53 broadening of chromatographic spots, 50-53 capillary flow, 49-50 volatility of solvents, 53 Pigments. See Natural pigments Planar chromatography (instrumental TLC), 3, 129-148,373-385 automation in, 131,382-384 chromatogram development, 135-140 automated multiple development (AMD), 138-140... 

As we shall see, the solution conductivity depends on the ion concentration and the characteristic mobility of the ions present. Therefore, conductivity measurements of simple, one-solute solutions can be interpreted to indicate the concentration of ions (as in the determination of solubility or the degree of dissociation) or the mobility of ions (as in the investigations of the degree of solvation, complexation, or association of ions). In multiple-solute solutions, the contribution of a single ionic solute to the total solution conductivity cannot be determined by conductance measurements alone. This lack of specificity or selectivity of the conductance parameter combined with the degree of tedium usually associated with electrolytic conductivity measurements has, in the past, discouraged the development of conductometry as a widespread electroanalyti-cal technique. Today, there is a substantial reawakening of interest in the practical applications of conductometry. Recent electronic developments have resulted in automated precision conductometric instrumentation and applications... 

All approaches to method optimization based on multiple experiments have the requirement that all components be detected and that they be tracked between runs. For complex samples, this is typically the most labor-intensive aspect of method development. For unattended method development, the instrument is required to monitor the change in retention of each component automatically. The historical limitations to this technology have been a key stumbling block in the widespread adoption of automated method development. 

Because of the inherent ability of solid-phase synthesis to be integrated and automated, numerous instruments were built from the onset of solid-phase chemistry, and this development culminated after the introduction of combinatorial chemistry methods. Operational simplicity of solid-phase synthesis contributed to the development of multiple solid-phase synthesis, where numerous reaction vessels are handled at the same time. In 1989, Schnorrenberg and Gerhardt" introduced the automated multiple synthesis of peptides in parallel fashion. Multiple synthesis in a continuous flow manner was also later reported. 

The importance of these surface-analysis techniques has resulted in the development of a range of highly automated instruments. In the effort to obtain multiple analytical data, a trend has occurred during the last ten years to build combined instruments, that is apparatus which will permit measurements by several techniques, in a single vacuum system. In this way, greater utilization of the complex instrumentation involved and a more economic use of the functional parameters of the instruments are ensured. 

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