What is RAST's METHOD? What is principle of Rast method for determining molecular weight of compounds?

What is RAST

RAST (Radioallergosorbent test) is a blood test used to measure the amount of specific IgE antibodies in a person's blood in response to a particular allergen. RAST was developed as an alternative to skin prick testing, which can be uncomfortable and can cause allergic reactions.

The RAST method involves taking a blood sample from a person and exposing it to specific allergens in the laboratory. If the person has IgE antibodies to that allergen, the antibodies will bind to the allergen and form a complex. The complex is then captured on a solid phase, such as a paper disc or microplate, coated with the allergen.

RAST's METHOD

In 1922, K. Rast developed a very simple and convenient method for determining the molecular weight of soluble substances in camphor. The melting point of camphor is 176°C. The melting point is an important physical constant if the substance is pure.

w = weight of the solute in g.

m = Molecular weight of solute

W = Weight of solvent in g.

ΔTf = Observed depression in freezing point.

 Kf =Molal depression constant.

If a pure substance A is mixed with another substance B, the melting point of A is lowered. This lowering in melting is commonly known as the depression of the Freezing point. The molecular weight of substance B and other factors such as molecular depression depends on stability, the weight of solvent, etc. Rast made use of the fact that the molal depression constant of camphor is very high (39.7 degrees C) as compared with that of water (1.8) and benzene (5 / 12 degrees  C) for 1000 gm. 

Therefore this method is applicable for those solutes only which dissolve in camphor. The melting points of the resulting solutions are quite lower than that of camphor and can be measured accurately using an ordinary 0.1°C thermometer. The melting points of camphor and mixture of solute and camphor may be determined by the ordinary capillary tube method.

Because of its simplicity and convenience, this method is widely used in organic chemistry Further, the method requires a very small amount of solute and the results are obtained fairly quickly. Therefore this method is also known as the micro method or micro cryoscopic method.

Remembering  factors

• This method cannot be applied if the unknown solute is not soluble in camphor or reacts with camphor or decomposes when heated up to the melting point of camphor. 
• It is assumed that no eutectics or compounds separate and only pure camphor crystallizes when cooled.
• The chemical composition of camphor varies according to the source of availability, so it is necessary to determine the molecular depression stability for a sample of camphor provided using a calibration experiment with known molecular weight solubility.
• Solutes that undergo dissociation or association give abnormal values of molecular weight. 

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