AMD - Automated Multiple Development

The basis of automated multiple development (AMD) is the use of different modes of multiple development in whieh the mobile phase eomposition (5j and Sy values) is ehanged after eaeh, or several, of the development steps. Figure 8.11 illustrates the prineiple of AMD employing a negative solvent-strength gradient (deereasing 5-p values). 

Two-dimensional separations in planar chromatography are rather trivial to perform. All unidimensional multiple development techniques employ successive repeated development of the layer in the same direction, with removal of the mobile phase between developments. The main variants are multiple chromatography and incremental multiple development. The basis for automated multiple development (AMD) is the automation of unidimensional, incremental, multiple development with a reverse solvent strength gradient [998]. 2D TLC finds limited use, and is mainly a qualitative technique. 

Automated multiple development (AMD) has been successfully applied for the separation of compounds from plant extracts. AMD is a technique using the concentration gradient to separate substances differing... 

The thickness of the sorbent layer for analytical purposes lies between 50 and 100 (un (extra thin films for automated multiple development, AMD, are described in Section 11.1), 200 [un for HPTLC plates and 250 pm for the normal TLC plates. The layer thickness on aluminum foil is 200 pm, while layers that can be used for preparative work can be up to 2 mm thick. 

Multiple development was traditionally performed manually. It consists of repeated developments of a plate in the same direction with the same solvent over the same distance. The result is narrower bands and improved resolution and detection sensitivity. The Rf values become very precise and are adequate enough for identification. A variation of this method is automated multiple development (AMD) and shows promising future. The HPTLC/AMD method was used to monitor phenylureas, carbamates and triazines in drinking water. HPTLC can also be performed using polar modified stationary phases to separate pesticides in various foodstuffs such as triazines in corn, asparagus, tomatoes, grapes and potatoes [14]. 

Automated multiple development (AMD) is a new technique for determining pesticides suitable for several organic plant protection agents and some of their main metabolites in drinking water and groundwater with a quantification limit ranging from 0.05 to 0.1 /rg/1. 

It is described in ISO method 11370 entitled Determination of selected organic plant protection agents—automated multiple development (AMD) technique . 

ISO 11 370, Water Quality — Determination Of Selected Organic Plant Protection Agents — Automated Multiple Development (AMD) Technique, 1999. 

High performance thin-layer chromatography (HPTLC) has also been used for the determination of carbamate pesticides.Thus, TLC methods provide increased selectivity through silica derivatization, as well as higher analytical precision and sensitivity with high-performance plates. Butz and Stan reported an HPTLC system with automated multiple development (AMD-HPTLC) to screen water samples for pesticides. The method was applied to the determination of 265 pesticides in drinking water spiked with 100 ng/1 of each analyte. 

Conversely successful on-line coupling of HPLC to TLC is also possible. Hofstraat [15-17] and Baeyens and Lino[18] have described suitable apparatus. Burcsr, for instance, was able to demonstrate that adsorptive separation of selective cut fractions of an HPLC eluate from RP partition chromatography could be separated into up to 700 individual peaks [19, 20]. Here the thin-layer chromatographic step employed the Automated Multiple Development (AMD) technique. These investigations and the results of Kkoker, Funk and Eisenbeiss [21, 22] demonstrate the enormous power of such online coupling techniques in a very impressive manner. 

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... 

Lodi, G. Betti, A. Menziani, E. Brandohni, V. Tosi, B. Some aspects and examples of automated multiple development (AMD) gradient optimization. J. Planar Chromatogr. 1991, 4 (3/4), 106-110. 

Solid-phase extraction (SPE) using small, disposable cartridges, columns, or disks is employed for isolation and cleanup of pesticides from water and other samples prior to TLC analysis, especially using reversed-phase (RP) octa-decyl (C-18) bonded silica gel phases. Microwave-assisted extraction (MAE) is a time- and solvent-saving method for removing residues from samples such as soils. Supercritical fluid extraction (SEE) has been used for sample preparation in the screening of pesticide-contaminated soil by conventional TLC and automated multiple development (AMD). Ultrasonic solvent extraction (USE) and videodensitometry have been combined for quantification of pesticides in sod. Matrix solid-phase dispersion (MSPD) with TLC and GC has been used to determine diazinon and ethion in nuts. 

If required, for oxygen-prone, light-labile or heat-labUe compounds, the application (ATS4), development (Automated Multiple Development, AMD 2, CAM AG allows drying the plate under vacuum), and detection (TLC Scanner, CAMAG) can be performed in a dark, nitrogen-protected atmosphere. However, a column-based technique might be better suited for such compounds. 

Recently, automated multiple development (AMD) thin layer chromatography has been applied for the analyis of caffeic acid derivatives in E. angustifolia root extracts [45, 46]. The separation was performed on silica plates (Sil G-50 UV 254) and AMD was achieved in 25 steps using methanol, ethylacetate, toluene, 1,2-di-chloroethane, 25% ammonia solution, and anhydrous formic acid as modifiers. For the screening of echinacoside in crude plant extracts, a fast-atom-bombardment tandem mass spectrometry method has been developed [47, 48]. 

The first automated multiple development (AMD) technique was termed programmed multiple development (PMD). The commercial device for PMD originally manufactured by Regis is no longer available. 

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