Preparation of Thin Layers on Plates - Thin Layer Chromatography (TLC)

 PREPARATION OF THIN LAYERS ON PLATES 

The paramount importance with regard to the preparation of thin layer is that it must be uniform and consistent throughout. Various means have been put forward to apply thin layers of powdered or their suspen-sions or their slurries to the carrier plates with a view to achieve an uniform layer throughout the length of the plates. These are namely :

 

(a) Pouring of Layers : 

In order to obtain layers of equal thickness, a measured amount of the suspen-sion or slurry is placed on a given-size plate that is rested on an absolutely labelled surface. The plate is subsequently tipped backward and forward to permit the slurry (or suspension) to spread uniformly on the surface of the plate.

 

(b) Dipping : 

Peifer* in 1962, was pioneer in introducing this technique, whereby two plates at a time back-to-back are dipped together in a slurry of the adsorbent in either chloroform or chloroform-methanol. However, this particular methods is not much in use now-a-days.

 

(c) Spraying : 

Reitsema** was first to develop this method by making use of a small paint-sprayer for the distribution of the suspension or slurry onto the surface of the glass-plate.

 

Disadvantages : There are mainly two major disadvantages of this technique, namely :

 

(i) Non-uniformity of layers on a single-plate, and

 

(ii) Variation observed from one plate to the other was significant.

 

Belgian Patent No : 625012 : It essentially consists of spraying either molten or partially molten absorbent onto a glass plate, for instance : an alumina film prepared by melting and aluminium rod with an oxyacetylene flame and subsequently spraying the molten adsorbent onto a glass plate.

(d) Spreading : 

In this particular case, the suspension or slurry is put in an ‘applicator’, which is subsequently moved either over the stationary glass-plate or vice-versa i.e., it is held stationary while the glass plate is pulled or pushed through. This technique termed as ‘spreading’ usually yields uniform thin layers on the glass plates.

 

Kirchner’s* technique essentially consists of :

 

·              selecting uniform surfaced glass plates,

 

·              placing them between glass or metal guides which are thicker than glass plates by the amount that is desired for the adsorbent layer, and

 

·              spreading the slurry on the glass plate with the help of a glass rod.

 

Egon Stahl’s apparatus exclusively designed for the application of thin-adsorbent layers which broadly comprises of two major parts, namely :

 

(i) Aligning Tray : It is a tray on which the glass plates are placed in a series or in-a-line, and

 

(ii) Spreader : It holds the spreading mixture (as a slurry or suspension) and applies it uniformly in a thin-layer.

 

Figure 28.1 (a) shows the operation of thin-layer spreader ; while Figure 28.1 (b) depicts the aligning tray with glass-slide partially coated.


In Figure 28.1 (a)                  

 

A = Glass plate,

 

B = Spreader, and

 

C = Slurry of the adsorbent.

 

Here, the slurry (C) is put in the spreader (B) and then moved along the direction of application onto the surface of the glass plate (A) to obtain an uniform layer,

 

In Figure 28.1 (b)                  

 

A = Glass plate,

 

B = Layer of adsorbent, and

 

C = Aligning tray

 

The slurry of the adsorbent is introduced into the aligning tray (C) which is then moved onto the glass plate (A) to obtain an uniform layer of adsorbent (B).

 

In 1962, a well known West German firm DESAGA introduced a much improved and simplified ver-sion of a TLC-spreader that could conveniently give uniform layer thickness raging from 0.00 to 2,0 mm.

 

(e) ‘TLC-Plates ready-for Use’ (or Pre-coated Plates)

 

E. Merck AG (Darmstadt, West Germany) was pioneer in introducing two types of ready-for-use TLC plates either on glass or polysheets (ethyleneterephthalate), namely :

 

(i) Ready-for-use TLC Plates with Cellulose-F, and

 

(ii) Ready-for-use TLC Plates with Silica Gel F-254.

 

Interestingly, these precoated TLC-plates essentially have : first, a special abrasive-resistant layer con-taining no gypsum ; and secondly, the layer contains a reliable fluorescent indicator that is excited to emit a fluorescence under either a short-wave or a long-wave UV light.

 

Advantages : These ‘ready-for-use’ TLC-plates have the following advantages namely :

 

·              It may be safely activated at 110-120° C, before, use,

 

·              The properties of the layer minimise spot-diffusion that helps both more strong concentration of spots and more distinctive separations with higher sensitivity,

 

·              It may accept more corrosive spray-reagents, for example : conc. sulphuric acid, phosphoric acid, phosphomolybdic acid, perchloric acid on antimony trichloride ; and the sprayed plates could be heated upto 110-120 °C without any darkening whatsoever,

 

·              The migration rate is slightly enhanced when compared to hand coated plates, and

 

·              The TLC plates may be cut into strips by the aid of a glass cutter applied on the reverse side.

Comments

Post a Comment

Popular posts from this blog

Karl Fischer Method for Determination of Water

Image
  INTRODUCTION A plethora of chemical compounds for the determination of small amounts of water present in organic solids, pharmaceutical substances and organic solvents have been devised over a length of time. But unquestionably the most important of these is the one proposed by Karl Fischer (1935), which is considered to be relatively specific for water*. It essentially makes use of the Karl Fischer reagent which is composed of iodine, sulphur dioxide, pyridine and methanol.   Note : Both pyridine and methanol should be anhydrous.   THEORY Water present in the analyte reacts with the Karl Fischer reagent in a two-stage process as shown below : From Eq. ( a ) step l, it is obvious that the oxidation of sulphur dioxide takes place by iodine to yield sulphur trioxide and hydrogen iodide thereby consuming one mole of water. In other words, each one molecule of iodine disappears against each molecule of water present in the given sample. It is pertinent to mention here that in the presenc
Read more

Volumetric Analysis

Image
  VOLUMETRIC ANALYSIS Volumetric analysis may be broadly defined as those analytical methods whereby the exact volume of a solution of known concentration actually consumed during the course of an analysis is considered as a measure of the amount of active constituent in a given sample under determination (assay).   1. THEORY   According to the  official method of analysis , hydrochloric acid can be determined by  first  weighing a given sample accurately, and  secondly , by adding carefully a solution of known strength of sodium hydroxide in the presence of an appropriate indicator unless and until the exact equivalent amounts of HCl and NaOH have undergone the following chemical reaction : Analyte (or Active Constituent)  is the chemical entity under assay   e.g. , HCl.   Titrant  is the solution of known strength (or concentration) employed in the assay   e.g. , NaOH.   Titration  is the process of adding and then actually measuring the volume of titrant consumed in the   assay. Thi
Read more

Thermoanalytical Analysis

  INTRODUCTION Thermoanalytical methods  essentially encompass such techniques that are based entirely on the con-cept of heating a sample followed by well-defned modified procedures, such as : gravimetric analysis, differ-ential analysis and titrimetric analysis. In usual practice, data are generated as a result of continuously re-corded curves that may be considered as ‘thermal spectra’. These thermal spectra also termed as  ‘thermograms , often characterize a single or multicomponent system in terms of :   ( a ) temperature dependencies of its thermodynamic properties, and   ( b ) physicochemical reaction kinetics.   Broadly speaking the thermoanalytical methods are normally classified into the following three categories, namely :   ( i ) Thermogravimetric Analysis (TGA),   ( ii ) Differential Thermal Analysis (DTA), and   ( iii ) Thermometric Titrations.   All the above mentioned techniques shall be discussed briefly with specific reference to their theory, instrumentation, methodo
Read more