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

The main difference between C8 and C18 columns in HPLC is that the C8 column contains octyl carbon chain (C8) -bound silica as the stationary phase, but the C18 column contains octadecyl carbon chain (C18) -bound silica. In addition, the C8 column is less hydrophobic and has a lower density, but the C18 column is more hydrophobic and has a high density. Therefore, the C8 column has a low retention time while the C18 column has a high retention time. Therefore, the separation of the C8 column is little, but the C18 column gives a higher separation.  

In short, C8 and C18 are two types of straight chain alkyl groups used for surface modification of silica in reverse phase chromatography in HPLC. Reverse phase chromatography uses an aqueous, moderately polar mobile phase.  

Key areas covered  

1. What is a C8 column in HPLC - definition, properties, meaning 2. What is a C18 column in HPLC - definition, properties, meaning 3. What are the similarities between C8 and C18 columns in HPLC - Overview of the common features 4. What is the difference between C8 and C18 columns in HPLC - comparison of the main differences

key terms  

C8 column, C18 column, HPLC, hydrophobic stationary phase, reverse phase chromatography

Difference between C8 and C18 columns in the HPLC comparison summary

What is the C8 column in HPLC?  

The C8 column is a type of column used in reverse phase chromatography. In general, reverse phase chromatography uses a hydrophobic stationary phase. However, the stationary phase of the solid support or the column contains modified silica. Here, silica is a hydrophilic stationary phase. It also contains alkyl groups that are covalently bonded to silica, creating a hydrophobic stationary silica phase. At the same time, the mobile phase of the reverse phase chromatography is aqueous. This stationary phase therefore has a high affinity for non-polar compounds in the sample.

Difference between C8 and C18 columns in HPLC

Figure 1: Retention time depends on the relative strength of the analyte interactions with stationary and mobile phases

In particular, C8 columns contain octyl carbon chains with eight carbon atoms as alkyl groups for modification of the silica column. And these carbon chains are comparatively short. Therefore, this shortness reduces the hydrophobicity and the density of the stationary phase. Therefore, the retention time of the non-polar compounds in the C8 column is short, which leads to a lower degree of separation.  

What is a C18 column in HPLC?  

The C18 column is the cheapest and most common type of column in reverse phase chromatography. Also, the type of alkyl groups used in the modification are octadecyl carbon chains in the C18 column. In comparison, its carbon chain is longer. In addition, the density of the column in the C18 column is higher due to the higher number of carbons in the alkyl group. Accordingly, the hydrophobicity of the C18 column is also high. Thus, it extends the retention time for the non-polar compounds, which leads to better separation. It also reduces the tailing of the separation curve.

Key difference - C8 vs C18 column in HPLC

Figure 2: HPLC column

In addition, C18 columns are better suited for separating more hydrophobic compounds such as long-chain fatty acids. In contrast, C8 columns are better suited for separating small organic compounds.  

Similarities between C8 and C18 columns in HPLC   

  • C8 and C18 columns in HPLC are two types of straight-chain alkyl groups that are used to modify the surface of silica in reverse phase chromatography in HPLC.   
  • In general, silica is the most common type of stationary phase in normal phase chromatography. However, it is a polar stationary phase. Nevertheless, the above surface modifications make the stationary phase non-polar for use in reverse phase chromatography .   
  • In addition, the mobile phase of reverse phase chromatography is aqueous and moderately polar.   
  • The pore size of both columns is similar. In addition, both columns are provided with end caps.
  • In addition , the non-polar stationary phase increases the retention time of non-polar compounds.  

Difference between C8 and C18 columns in HPLC  

definition  

C8 column refers to one type of column used in reverse phase chromatography that contains octylsilane as the stationary phase, while the C18 column refers to another type of column used in reversed phase chromatography that contains octadecylsilance as the stationary phase.  

Type of alkyl groups  

The C8 column contains an octyl chain, but the C18 column contains an octadecyl chain .  

structure  

The structure of the C8 column is OH-Si-C8, while the structure of the C18 column is OH-Si-C18.  

Carbon length  

The C8 column has a short carbon chain, but the C18 column has a long carbon chain.  

Hydrophobicity  

The C8 column is less hydrophobic, but the C18 column, in contrast, is more hydrophobic.  

density  

The C8 column is less dense, but the C18 column is more dense .  

Retention period  

The C8 column has a low retention time, but the C18 column has a high retention time.  

escape  

The sample elutes rapidly in the C8 column, but the sample elutes slowly in a C18 column.  

Degree of separation  

The C8 column gives relatively little separation, but the C18 column gives greater separation.  

Types of samples  

The C8 column separates small organic compounds better, while the C18 column separates long-chain fatty acids well.  

diploma  

The C8 column is one of the columns used in reverse phase HPLC. Generally it contains octyl carbon chains that are shorter. Therefore, its hydrophobicity is comparatively low. This also leads to a short retention time of non-polar molecules in the column. Therefore, C8 columns are better suited for separating small organic compounds. In contrast, the C18 column is the most common column shape in reverse phase HPLC. However, it does contain octadecyl carbon chains that are longer. Therefore, both the hydrophobicity and the density of the stationary phase are high. As a result, the retention time and the degree of separation in the C18 column are high. The main difference between the C8 column and the C18 column in HPLC is the type of alkyl groups present and the retention time.  

References:

1. "High performance liquid chromatography". Wikipedia , Wikimedia Foundation, February 1, 2020, available here.

Image courtesy:

1. “Reverse Phase Gradient Elution Schematic” By Nategm - Own work ( CC BY-SA 4.0 ) via Commons Wikimedia 2. “Hplc kolom jsw” By Imgupl - Own work (Public Domain) via Commons Wikimedia

About the author: Hasa

Hasa holds a BA in English, French and Translation Studies. She is currently reading for a Masters in English. Her areas of interest include literature, language, linguistics and also food.