154 CHROMATOGRAPHY 



change columns becomes quite complex, and there is no complete agree- 

 ment on the principles involved. The synthetic resins are extremely 

 useful in preparing ion-free water and in separating mixtures of amino 

 acids, nucleic acids or nucleotides, and carbohydrate derivatives. The 

 trade-names Dowex and Amberlite have become very familiar terms in 

 the modern laboratory. 



Practical chromcftography 



A great variety of mixtures may be separated by chromatography. All 

 that is required is a combination of solvents and solid materials such 

 that there is a difference in the physical properties of the various com- 

 ponents of the mixture. Whenever a new or untried mixture is to be 

 chromatographed, the investigator must choose from among paper, 

 adsorbent columns, and ion exchange resins. The filter paper technique, 

 easiest and quickest, is usually tried first. The next question concerns 

 the type of filter paper to use. The manufacturers produce a variety of 

 papers, some of which may work better than others in any particular 

 chromatographic separation. It is convenient to keep samples of various 

 types in the laboratory, and a few preliminary trials usually will show 

 that one paper excels the others in speed or completeness of separa- 

 tion. 



The solvent to be used for development must also be chosen carefully. 

 Each mixture of materials has its own set of properties, a fact which has 

 an effect on the choice of solvents. If the general class of compounds to 

 which the mixture belongs is known, the literature or experience will 

 suggest several solvents. The materials to be separated must not be too 

 soluble in the solvent or they will move together almost as rapidly as 

 the solvent moves. If, in contrast, the solubility is too low, they will 

 remain at the spot where they were originally placed. 



Another problem that was not apparent in the earlier description of 

 pigment chromatography arises when colorless mixtures are separated. 

 Amino acids, sugars, and many other colorless compounds can be sep- 

 arated on paper, but the individual spots or bands must be found later. 

 Sometimes the paper is treated with a reagent that reacts with the sep- 

 arated materials to yield a colored product. Some materials are fluorescent 

 under ultraviolet illumination, while others absorb ultraviolet and appear 

 as dark spots on a slightly fluorescent paper. If some components in the 

 mixture are labeled with radioactive tracers, the paper chromatogram will 



