Difference Between C18 Column and C8 Column

C8 and C18 Chromatographic Columns are Usually Divided into Three Parts:

C8 Chromatographic Columns

1. Silica Support: Provides the structural foundation and porosity for high-efficiency separations.
2. C8 Bonded Phase: Short hydrophobic chains for moderate retention of analytes.
3. End-Capping: Reduces unwanted interactions and improves column performance for a wide range of analytes.

C18 Chromatographic Columns

1. Silica gel: It is the filler matrix.
2. Octadecyl: It is the bonding phase; in addition to octadecyl, there are octadecyl, butanyl, phenyl, fluorophenyl, etc..
3. Silane bonding: that is, the bond and the method, usually the bonding phase is formed by the chemical reaction between the silanizing agent and the silanol group on the surface of the silica gel, so it will be called silane bonding.

There are often some doubts when choosing imported C18 chromatographic columns or C8 chromatographic columns.

C8 and C18 Columns in Common:

C8 and C18 are all silica-bonded octadecyl groups. Both C8 and C18 are reversed-phase columns suitable for analyzing weakly polar substances.

Difference Between C8 and C18 Column

C8 vs C18 column. In comparison, the polarity of C18 is smaller than that of C8. C8 is on the side with stronger polarity, and C18 is on the side with weaker polarity. That is to say, C8 is suitable for the analysis of slightly more polar substances in weakly polar substances, and C18 is ideal for the analysis of weaker polar substances in weakly polar substances. This is the difference in their polarity, but it is not particularly obvious. Generally, substances that C8 can analyze can also be separated on C18. This is because the latter has better retention properties than the former, which seems to be more closely related to the structure of the column packing.

C18 has a longer carbon chain than C8, bringing better retention properties. Therefore, C8 is more suitable for analyzing macromolecular substances, such as globulins, and is often used in conjunction with a buffer salt eluent. Conversely, substances with smaller molecular weights are often analyzed and separated using C18.

The retention performance of the C18 chromatographic column is stronger than that of C8, and the carbon content has a great influence on the separation effect and the retention time of the main components, so the higher the carbon content, the longer the retention time of the main components.

For the C8 column and the C18 column, I believe that many people are confused, sometimes even confused. The C8 column is indeed silica-bonded octadecyl, and the C18 column is silica-bonded octadecyl. Regardless of the difference between the words, the difference will also be different.

C8 vs C18 column. In contrast, C18 has a stronger analyte retention performance than C8, and the carbon content has a great influence on the separation effect and the retention time of the main components. Therefore, the higher the carbon content, the longer the retention time of the main components. Both C8 and C18 columns are types of reversed-phase columns. The principle of chromatographic separation is similarity and compatibility, and here is the difference in polarity.

C8 is on the weaker side with stronger polarity. The HPLC column features high surface coverage and thorough double end-capping, which improves the stability of column separation and optimal separation efficiency for the analysis of acidic, basic, and chelated compounds. C8 chromatographic columns have a variety of pH values ​​ranging from 0.5-8.0, 1.0-10.0, and 2-11.5. To meet the chromatographic analysis of different conditions, the tailing phenomenon usually occurs for strong basic compounds, the C8 chromatographic column still has a good symmetrical peak shape and high column efficiency for analysis; showing such greatest advantages, it is undoubtedly an excellent choice for C8 chromatographic column for routine analysis; excellent batch batch and column-to-column reproducibility; applicable to all sample size ranges from analysis to preparation for multicomponent systems.

The C18 chromatographic column is on the weaker side of the polarity, but on the issue of steric hindrance, due to the difference in the C chain, the C18 chromatographic column can play a better role and retain better characteristics.

Difference Between the C8 and C18 Columns in Detail

• Different filling materials

  • 1. C8 chromatographic column: The alkane bonded to the silica gel is 8 carbons, and the packing is C8. The stationary phase of a C8 column consists of octyl (C8) groups bonded to the silica support. These alkyl chains are eight carbons long. C8 columns are less hydrophobic than C18 columns.
  • 2. C18 chromatographic column: The alkane bonded to the silica gel is 18 carbons and packed with C18. C18 columns have octadecyl (C18) groups bonded to the silica support. These alkyl chains are 18 carbons long. C18 columns are more hydrophobic than C8 columns.

• Different polarities

  • 1. C8 chromatographic column: C8 is on the weaker side with stronger polarity.
  • 2. C18 chromatographic column: C18 is on the weaker side.

• Mobile phase:

  • C8: C8 columns may require a higher percentage of organic solvent in the mobile phase to elute compounds effectively due to their lower hydrophobicity.
  • C18: C18 columns can often be used with a broader range of mobile phase compositions due to their higher hydrophobicity. This flexibility can be advantageous in method development.

• Retention time and separation efficiency

  • 1. C8 chromatographic column: C8 is on the weaker side with stronger polarity.
    • Retention: Lower retention compared to C18 columns due to shorter alkyl chains.
      Analytes with shorter hydrophobic interactions will be eluted faster since the interaction with the C8 stationary phase is weaker.
    • Separation Efficiency: Moderate separation efficiency. While C8 columns can separate moderately hydrophobic compounds well, they may struggle with complex mixtures where stronger retention and separation are required.
  • 2. C18 chromatographic column: C18 is on the weaker side.
    • Retention: Higher retention is due to stronger hydrophobic interactions between the analytes and the C18 stationary phase. This longer alkyl chain increases the interaction strength, causing non-polar analytes to be retained longer.
    • Separation Efficiency: Higher separation efficiency because the increased hydrophobic interaction can provide better separation for a wide range of analytes, especially for compounds with higher hydrophobicity

• Effect is different

  • 1. C8 chromatographic column: analyze the slightly more polar substances among the weak polar substances. Since C18 has a longer carbon chain than C8, it brings better retention properties. Therefore, C8 is more suitable for analyzing macromolecular substances, such as globulins, and is often used in conjunction with a buffer salt eluent. C8 columns typically provide faster separation due to their lower hydrophobicity. They are ideal for analyses where speed is a priority, but some compromise on separation efficiency may be acceptable.
  • 2. C18 chromatographic column: suitable for analyzing weaker polar substances. Substances with smaller molecular weights are often analyzed and separated with C18. C18 columns provide higher separation efficiency, especially for complex mixtures and compounds with similar structures. They often produce sharper peaks and better resolution, but may require longer run times

• Selectivity:

  • C8: C8 columns may offer different selectivity compared to C18 columns. This can be advantageous when trying to resolve compounds that may not separate well on a C18 column.
  • C18: C18 columns are widely used and offer good selectivity for a broad range of compounds. They are often the default choice for routine HPLC analyses.

• Applications:

  • C8: C8 columns are commonly used for pharmaceutical analyses, environmental testing, and the separation of drug compounds. They are also suitable for analyzing peptides and proteins in some cases.
  • C18: C18 columns are versatile and are widely used in pharmaceutical, clinical, environmental, and food industry applications. They are the standard choice for many HPLC analyses, including the separation of pharmaceuticals, natural products, and biomolecules.

• Compatibility:

  • C8: C8 columns may be less forgiving when it comes to the compatibility of the mobile phase with the stationary phase. Extreme pH or solvent conditions can lead to phase separation or phase collapse.
  • C18: C18 columns are generally more robust and can tolerate a wider range of pH and solvent conditions.

• Uses

  • C8: The C8 column is suitable for lipids, steroids, analytes with high hydrophobicity, and samples with different hydrophobicity.
  • C18: The C18 column is suitable for acid, alkali, neutral, polar, medium polar, and non-polar sample analysis.