Solid - Liquid Extraction

Table of Contents

Overview:

Solid-liquid extraction or leaching, is the method of removing a substance from a solid using a liquid solvent [1]. The substance to be extracted from the solid must be soluble in the liquid solvent. The rest of the insoluble solid will remain. At the end of this process, the extracted substance will be in a liquid phase [2].

Solid-liquid Extraction Considerations:

Solvent Selection:

The solute of interest must be soluble in the selected solvent. Physical properties of the solvent such as viscosity increase diffusion rates, which is important because the solvent must be able to penetrate the solid object or matrix [1]. In industry applications, the cost and recoverability of the solvent must be taken into consideration to minimize cost and waste. Often, the solvent is recovered through distillation or evaporation [1].

 Solid Preparation:

The solid may need to be treated to promote the diffusion of the solvent, such as dilating the pores of the matrix or solid [1]. The solid can also be crushed, grinded or cut in order to increase surface area and consequently increase the accessibility of the solvent to the solute. However, the size of the solid must not be so fine that the material becomes packed and inhibits diffusion.

Temperature:

The increase in temperature promotes the diffusion of the solvent through the matrix or solid and this will in turn increase the speed of extraction [1].

Pressure:

One advantage of solid-liquid extraction is the pressure. This extraction method is done at atmospheric pressure so if the sample cannot be pressurized, this extraction method excels.

Applications:

Chromatography [7]:

Chromatography is the separation of mixtures through ion exchange. A drop of ink is marked on a strip of paper. The tip of the paper is then dipped in a solvent. The solvent moves up the paper and past the drop of ink by capillary action. The solvent then continues travelling and dissolves the ink. The ink separates into different colours, with each colour displacing a different amount. The amount of displacement is dependent on many factors such as solubility or solvent flow. In this example, the paper is known as the stationary phase while the solvent is known as the mobile phase.

Solid-Phase Extraction:

Solid-phase extraction is a solid-liquid extraction technique where a solid adsorbent is placed into a cartridge so that solvents can be passed through it to isolate certain compounds. It works by loading the cartridge with a sample. Then a solvent can be passed through the cartridge to either remove the unwanted compounds or to remove the analyte [4].

Steps [4]:

1. 1. 1. Pre-treatment: Run a solvent or water through the cartridge to remove air-bubbles and adjusts the pH of the sample

      2. Condition: Rinse the sample with a solvent similar to the one that will be in the matrix/solid adsorbent of the cartridge.

      3. Load: Load the cartridge with a sample.

      4. Wash: Rinse the cartridge with the solvent.

      5. Elute (Optional): If the solvent removed the unwanted compounds, then the cartridge will hold the analyte and can be eluted.

Types of Solid Phase Extraction [4]:

Adsorption:

Normal Phase:

Normal Adsorption is used when the analyte has high polarity. The sample matrix should be non-polar. When the solvent is passed through the cartridge, the least polar compounds will elute first. This will leave the polar analyte within the cartridge.

Reversed Phase:

Reverse Adsorption is used when the analyte has a low polarity. The sample matrix should be aqueous. When the solvent is passed, polar compounds will elute first. The cartridge separates based on hydrophobicity and the non-polar analyte will remain within the cartridge.

Ion Exchange:

In ion exchange, solutes are separated based on electric charge [5].

Cation Exchange: 

In Cation exchange, the analyte of interest is positive. The matrix will be negatively charged. A buffer must be run through the cartridge first and will be attracted to the negatively charged matrix. The pH of the sample must be compared to the pH of the buffer. Compounds within the sample will have different strengths of charges. This means that when the sample solution is loaded into the cartridge, the more positively charged ions will displace the buffer ions and will be attracted to the matrix. The compounds within the solution that are not as positively charged will not displace the buffer ions and will instead elute. Therefore, the weakly ionized compounds will elute first.

Anion Exchange: 

In Anion exchange, the analyte of interest is negative. The matrix will be positively charged. A buffer must be run through the cartridge first and will be attracted to the positively charged matrix. The pH of the sample must be compared to the pH of the buffer. Compounds within the sample will have different strengths of charges. This means that when the sample solution is loaded into the cartridge, the more negatively charged ions will displace the buffer ions and will be attracted to the matrix. The compounds within the solution that are not as negatively charged will not displace the buffer ions and will instead elute. Therefore, the weakly ionized compounds will elute first.

Mixed-Mode:

In mixed-mode extraction, a combination of adsorption and ion exchange is used to isolate an analyte. 

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