Plates, for TLC

Fig. 46 Evaluation of the suitability of a hot plate for TLC by determination of the lemperature distribution. A) results of 25 thermal elements at temperature settings of 80 C, 100 C and 120 C, B) pattern of measuring points in five tracks (I—V) each with five measuring points...

In the second approach non-procedure related factors are considered. Factors such as e.g. different laboratories, different analysts, different instruments, different lots of reagents, different days, different columns for HPLC methods or different plates for TLC methods are then examined. 

We have already noted the availability of high performance plates for TLC. The other extreme is small plates made on microscope slides for use in fast screening.15 These slides are easily prepared from a slurry of silica gel in a mixture of methanol and methylene chloride. Directions are available in reference 11 as well as Peifer s original paper.16 A fourth type... 

To 1.0 volume of serum or resin-treated urine, add 3.0 volumes of absolute ethanol in a centrifuge tube. Mix well and allow to stand for 10 minutes. Centrifuge any precipitated protein at 2000 rpm for 10 minutes. Decant off the ethanolic extract and add 3.0 volumes of chloroform. Mix well and allow to stand for 5 minutes. The aqueous layer on top of the chloroform-alcohol mixture contains the free amino acids and is pipetted into a separate tube. Aliquots of the extract of 0.10 ml and 0.20 ml are used for quantitative total amino acid analysis (Section 4.4). The remainder is evaporated to dryness in vacuo (40°) and the dried material is redissolved in 0.1 volume of water for use in spotting the plates for TLC amino acid analysis (Section 4.5). 

Precoated plates for TLC have been commercially available since 1961. The sorbents may be coated on glass, plastic, or aluminum supports. Sorbents with and without binder, and with and without UV indicators, are available in a variety of layer thicknesses, ranging from 100 pm in the case of plastic plates and high-performance layers to 2311 pm for preparative layers. The largest selection is presented by glass plates, coated to a thickness of 250 pm in the case of analytical layers. ... 

Layer combinations described here are available as plates for TLC, HPTLC, and PLC. 

The alternative expression for resolution given in equation (7) demonstrates that the plate resolution, as in other forms of chromatography, depends on the number of theoretical plates, the selectivity and the capacity ratio of the solute for the particular plate concerned. In practice, however, the expression given in equation (7) appears to be the more practically useful for TLC. separations. 

This equation, although originating from the plate theory, must again be considered as largely empirical when employed for TLC. This is because, in its derivation, the distribution coefficient of the solute between the two phases is considered constant throughout the development process. In practice, due to the nature of the development as already discussed for TLC, the distribution coefficient does not remain constant and, thus, the expression for column efficiency must be considered, at best, only approximate. The same errors would be involved if the equation was used to calculate the efficiency of a GC column when the solute was eluted by temperature programming or in LC where the solute was eluted by gradient elution. If the solute could be eluted by a pure solvent such as n-heptane on a plate that had been presaturated with the solvent vapor, then the distribution coefficient would remain sensibly constant over the development process. Under such circumstances the efficiency value would be more accurate and more likely to represent a true plate efficiency. 

Apply 1 drop colloidal palladium solution to the [27, 28] start zone and dry at 80 to 90 °C for 60 mm Then apply the sample solution, store the TLC plate for 60 min in a hydrogen-filled desiccator, then dry and develop... 

Methods of sample application. Due to the lower sample capacity of the H PTLC layer, the amount of sample applied to the layer is reduced. Typical sample volumes are 100-200 nL which give starting spots of only 1.0-1.5 mm diameter after developing the plate for a distance of 3-6 cm, compact separated spots are obtained giving detection limits about ten times better than in conventional TLC. A further advantage is that the compact starting spots allow an increase in the number of samples which may be applied to the HPTLC plate. 

FIGURE 5.21 (a) Baron drying rack and (b) drying chamber for TLC plates. 

In addition to the aforementioned methods, TLC in combination with other instrumental techniques have also been used for quantification of inorganic species. For example, two-dimensional TLC coupled with HPLC has been utilized for the separation and quantification of REEs in nuclear fuel fission products using silaiuzed silica gel as layer material [60]. In another interesting method, REEs in geological samples have been determined by ICP-AAS after their preconcentration by TLC on Fixion plates [32]. TLC in combination with neutron activation has been used to determine REE in rock samples on Eixion 50 x 8 layers with the sensitivity limit of 0.5 to 10 pg/g for 10- to 30-mg samples [41]. A combination of TLC and A AS has been utilized for the isolation and determination of zinc in forensic samples [27]. 

A concentrated hexane extract (11.5 g/4 ml) of B. carterii resin and B. serrata resin, respectively, was applied to a column hlled with 80-g silica gel (Merck LiChroprep Si 60 No. 9390) conditioned by hexane. The fractionation was achieved by a gradient of 200-ml hexane followed by 200-ml hexane-dichloromethane (1 + 1 v/v), 200-ml dichloromethane and 200-ml dichloromethane-acetone (1+1 v/v) as eluents to give four subfractions of 200 ml each. These fractions were collected, concentrated, and applied to a TLC plate for a screening with the mobile phase dichlormethane-diisopropylether (9+1 v/v). 

It is sinple to denonstrate that it is easier to achieve a greater nuBber of theoretical plates in HPLC than for TLC. For difficult separations HPLC has the greater sepauration capacity. However, nost separations performed by HPLC are done with relatively few theoretical plates, typically < 20,000, with... 

Principles and Characteristics The prospects of Raman analysis for structural information depend upon many factors, including sample scattering strength, concentration, stability, fluorescence and background scattering/fluorescence from the TLC substrate. Conventional dispersive Raman spectroscopy has been considered as a tool for in situ analysis of TLC spots, since most adsorbents give weak Raman spectra and minimal interference with the spectra of the adsorbed species. Usually both silica and cellulose plates yield good-quality conventional Raman spectra, as opposed to polyamide plates. Detection limits for TLC fractions... 

Some objects may have a large size (gels, autoradiographic films, or TLC plates, for instance) and special techniques must be used to obtain a global image while keeping the high resolution constant. The usual way to treat the problem is to scan the object. 

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