Chromatography is a technique used to separate and analyze different components of a mixture. However, there are certain substances that cannot be separated using chromatography.

Option 1: The correct answer is option 1, radio-isotopes. Chromatography is not suitable for separating radio-isotopes because these substances are radioactive and can be hazardous to handle. Different techniques, such as radioactive decay and mass spectrometry, are employed to separate and analyze radio-isotopes.

Option 2: Chromatography is commonly used to separate colors from dyes. This option is incorrect.

Option 3: Chromatography is also effective in separating pigments from natural colors. Therefore, option 3 is not correct.

Option 4: Although chromatography can be used to separate drugs from blood, it is not the only method available. Other methods, such as immunoassays and liquid-liquid extraction, are commonly used for drug separation. Therefore, option 4 is not correct.

In summary, the correct answer is option 1 because chromatography is not suitable for separating radio-isotopes.