What is the Difference between C8 and C18 columns in HPLC?

• High-performance liquid chromatography is a method used to separate, identify, and quantity of each analyte in the complex mixture using a mobile phase. Both C8 and C18 refer to the bonded face of the alkyl chain and both are used in HPLC separation. 
• There are numerous different kinds of reverse phases there in the market, including C8 and C18 columns. C18 is the most popular one than a C8 column. C8 and C18 are both reversed-phase columns, both columns refer to the alkyl chain length of the bonded phase. 
• The length of the chain affects the hydrophobicity of the sorbent phase and therefore increases the retention time of the component. C18 has the maximum amount of hydrophobicity, since the longer length of the carbon chain, C-18 is extra hydrophobic compared to the reverse phases.

What is the C8 Column?

• C8 column is a type of column used in reverse-phase chromatography. Generally, reverse-phase chromatography uses a hydrophobic stationary phase. However, the solid support the stationary phase or the column contains modified silica. Here, silica is a hydrophilic stationary phase.

• C8 column is a form of column present in some HPLC apparatuses, and it has Octylsilane as its stationary phase. And, this compound in the stationary phase has 8 carbon atoms in its alkyl chain. Further, it tends to retain components of the analyte less than that of the C18 column. Therefore, a compound will elute faster in a C8 column.
• However, it is less dense because it has a less number of carbon atoms; the length of the carbon chain of this compound in the stationary phase is short. Moreover, nonpolar compounds move down the column rapidly with the C8 column. It is mainly because of the lower hydrophobicity of the C8 compound.

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What is the C18 Column?

• C18 column is also a form of a column that we use in HPLC apparatuses, and it has Octadecylsilane as its stationary phase. Octadecylsilane (in the stationary phase) has 18 carbon atoms in its alkyl chain. Further, it tends to retain more components of the analyte when compared to C8 columns. The analyte will elute slower in this column.

What is the key difference between C8 and C18 Columns?

• The key difference between C8 and C18 columns is that the C8 column has Octylsilane as the stationary phase whereas the C18 column has Octadecylsilane.
• The C8 and C18 columns differ from each other according to the stationary phase. We use these columns in HPLC (high-performance liquid chromatography). The compounds we use in these columns have different alkyl chain lengths from silane compounds.
• The key difference between the C8 and C18 columns is that the C18 column consists of 18 carbons attached to the silica which is a longer carbon chain than C8. Because of the extra carbons chain, the C18 has a wide surface area which offers an additional time of reaction between the bonded phase and the compound. As a result, the compounds are eluted more gradually from the column, and there is more isolation.

What is the Difference Between C8 and C18 Columns?

• C8 column is a form of column present in some HPLC apparatuses, and it has Octylsilane as its stationary phase. The C8 column shows a low retention time. Moreover, the analyte elutes faster in this column. It is because it has a less dense stationary phase. 
• C18 column, on the other hand, is also a form of a column that we use in HPLC apparatus, but it has Octadecylsilane as its stationary phase. More importantly, this column shows a high retention time. In addition to that, the analyte elutes slowly in this column. It is because of the high dense stationary phase. 
• The following infographic presents a detailed difference between C8 and C18 columns as a side-by-side comparison.
• Moreover, C18 is denser than a C8 column. And this increases the path length of the analyte through the column. Also, this allows the separation of more complex compounds. The retention time of this column is high. In addition, the hydrophobicity of the stationary phase is high. It allows the slow elution of nonpolar compounds through the column.
• Applications of this type of column are mainly in environmental science, pharmaceutical industries, chemical analysis, etc.
• C8 column is used while small RT is desired, if hydrophobicity is low, there is less retention time for non-polar analytes, therefore, the non-polar analytes or compounds separate out more quickly with the C8 column. The C8 is selected over the C18, in the reverse phase matrix where the degree of hydrophobicity is low. But, the C18 column is more accepted and broadly used because C18 silica gel interacts with a broad range of analytes, hence it is used in separation, qualitative and quantitative studies in the pharmaceutical industries, chemical analysis, and environmental science.

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	C8 Column	C18 Column
Definition	C8 column is a form of column present in some HPLC apparatuses, and it has Octylsilane as its stationary phase.	C18 column is a form of column, we use in HPLC apparatus that has Octadecylsilane as its stationary phase
Stationary Phase	The stationary phase of C8 column contains Octylsilane	The stationary phase of C18 column contains Octadecylsilane.
Retention Time	C8 column shows a low retention time.	C18 column shows a high retention time
Carbon Atom	The number of carbon atoms in C8 column is 8.	The number of carbons atoms in C18 column is 18.
Elution	Analyte elutes faster in C8 column	Analyte elutes slower in C18 column.
Density	C8 column has a less dense stationary phase.	C18 column has a high dense stationary phase.
Hydrophobicity	C8 stationary phase has a low hydrophobicity.	C18 stationary phase has high hydrophobicity.
Movement of Nonpolar Compounds	Non-polar compounds move down the column rapidly.	Non-polar compounds  move down the column slowly.

Considering the above-stated differences, it is easy to identify some of the major ones.

1. C18 has 18 carbon atoms while C8 has only 8 carbon atoms.
2. C18 has a longer carbon chain, but C8 has a shorter one.
3. C18 has higher retention while C8 has shorter retention.
4. C18 has higher hydrophobicity, but C8 has lower hydrophobicity.

With the above-listed HPLC column comparison, it is easier to know the appropriate column that one should use when performing different types of HPLC.

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