Comparison Between Reversed Phase C18 and C8 SPE Cartridge

Sample Pretreatment Technology

SPE cartridge is a widely used and popular sample pretreatment technology. It uses solid adsorbent to adsorb the target compounds in liquid samples and separate them from the matrix and interference compounds of the samples. Then elution or pyrolysis adsorption was used to separate and enrich the target compounds. On the basis of traditional liquid-liquid extraction, it adopts the principle of similar phase dissolution between substances and combines the basic knowledge of liquid chromatography and gas chromatography stationary phase.

The difference between reversed phase C18 and reversed phase C8 SPE cartridge

Reversed-phase Solid Phase Extraction (SPE) cartridges, including C18 and C8, are widely used in sample preparation for chromatographic analysis. Here’s a comparison between C18 and C8 SPE cartridges:

Reversed phase C18 SPE cartridge is of 40-75μm particle size, 0.8cm³/g pore volume, and 480m²/g surface area. The retention mechanism is that moderate hydrophobic retention provides negligible secondary polar interactions and keeps a less retentive alternative for the non-polar compound. Reversed-phase C8 SPE cartridge has a high retention rate for non-polar to moderately polar compounds. And most non-polar compounds can remain in the hydrophobic reversed phase.

1. Stationary Phase:

  • C18: Contains octadecylsilane groups, providing strong hydrophobic interactions. It is suitable for non-polar and moderately polar compounds.
  • C8: Contains octylsilane groups, offering slightly weaker hydrophobic interactions compared to C18. It is suitable for a broader range of polarities.

2. Retention Mechanism:

  • C18: Provides stronger retention for non-polar compounds due to the longer carbon chain.
  • C8: Offers intermediate retention, making it suitable for a wider range of compounds with varying polarities.

3. Selectivity:

  • C18: Well-suited for separating hydrophobic compounds with subtle differences in structure.
  • C8: Offers good selectivity for both polar and non-polar compounds.

4. Sample Compatibility:

  • C18: Best for non-polar to moderately polar analytes. Less suitable for highly polar compounds.
  • C8: Suitable for a wider range of polarities, making it more versatile.

5. Water Wettable:

  • C18: Less water-wettable, which means it may require special conditioning before use.
  • C8: More water-wettable, making it easier to use without extensive conditioning.

6. Retention of Highly Polar Compounds:

  • C18: May have limited retention for highly polar compounds.
  • C8: Can retain moderately polar to highly polar compounds more effectively.

7. Analyte Recovery:

  • C18: Efficient for non-polar and moderately polar analytes.
  • C8: Offers good recovery for a wider range of polarities.

8. Applications:

  • C18: Commonly used for separating hydrophobic and non-polar analytes. Suitable for pharmaceuticals, environmental analysis, and lipid analysis.
  • C8: Widely used for a broad range of compounds, including pharmaceuticals, environmental samples, and natural products.

9. Matrix Interference:

  • C18: May be more susceptible to interference from complex matrices due to its stronger hydrophobic interactions.
  • C8: Can offer reduced matrix interference in complex samples.

10. pH Sensitivity:

  • Both C18 and C8 are generally stable over a wide pH range, but extremes of pH can affect their performance.

11. Back Pressure:

  • C18: May generate higher back pressure due to the stronger hydrophobic interactions.
  • C8: Tends to have lower back pressure compared to C18.

12. Cost:

  • Typically, C8 cartridges may be slightly more cost-effective than C18 cartridges.

In summary, the choice between C18 and C8 depends on the specific analytes being extracted and the complexity of the sample matrix. C18 is preferred for non-polar to moderately polar analytes, while C8 provides a more versatile option for a wider range of polarities. It’s recommended to perform method development and optimization to determine which column is best suited for a particular application.

Similarity on reversed phase C18 and reversed-phase C8 SPE cartridge

Moreover, the reversed phase C8 SPE cartridge is of strong hydrophobic selectivity and a wide range of pH, which fits for retaining organic compounds in biological matrices and aqueous substances. And reversed phase C8 SPE cartridge can be used to separate biomolecules such as lipids, bile acids, and antibiotics, or applied to analyze poisons, pollutants, and their metabolites in biological matrices.

Compared to the C8 SPE cartridge, the reversed phase C18 SPE cartridge is of multiple cartridge sizes and modes for choice. Its high carbon loading can greatly enhance recovery and retention ability. High chemical compatibility and strong acid tolerance are guaranteed by medical PP-grade cartridges. What’s more, different structures in the same samples are easily conducive to extract multiple analytes.

Reversed Phase SPE CartridgesOperation Steps of SPE Cartridge

The operation steps of the SPE cartridge are as follows:
1. The polar samples are extracted from the nonpolar matrix;
2. Then activation: 3-5 ml nonpolar solvent elution column;
3. Cleaning: eluting adsorbed sample with 5 ml of appropriate nonpolar solvent;
4. Elution: the sample to be tested is eluted into the collection container in batches with 1-5 ml polar solvent;
5. Rinse the packing bed with 3-5ml methanol, do not let the packing dry. And apply the sample solution to the top of the packing bed. Samples for analysis have freely passed through the bed without being retained. Moreover, if the desired compound was retained, wash off weakly retained interfering compounds with a polar solvent.

There are two questions about using the SPE cartridge.

Sample loading

Due to the complexity of the sample matrix, the sieve has a pore size of 20um, and particles need to be filtered out before loading. For example, a protein-containing sample needs to be treated with acid, salt, organic solvent, heating, etc.; for general matrices, it should be processed by filtration, centrifugation, or high-speed centrifugation, changing to large pore fillers, etc.

Now many third-party testing units have a lot of samples, but the pre-processing steps that should be done cannot be saved, otherwise, the instrument will be damaged and the data will be affected.

Rinsing and elution

Generally, there are relatively few cases of slow flow rate in the two steps of rinsing, and elution, because rinsing, and elution are pure solvents without matrix. Generally, there is no blockage in the sample loading step, which will not affect it.

It is possible that the slow flow rate is after rinsing the cartridge and drying because once the cartridge is dried, air will enter the packing, which returns to the sixth cause of the slow flow rate caused by activation equilibrium, which can be solved by applying a little pressure.