Basic Understanding of Extraction
It is reasonable to assume that the next level of understanding of analytical extractions can be found in undergraduate quantitative analysis textbooks. But, as previously discussed, this assumption may be suspect. Perusal of three of the most widely used analytical texts showed that treatment of separations theory and extractions varied. Strong emphasis is placed on equilibria. This is important, because concepts like partitioning are presented in terms of equilibria. However, it should be noted that most extractions are operated away from equilibrium conditions (that is, kinetics must also be considered), and, although thermodynamic understanding is needed at the most advanced theoretical level, extractions occur through the application of heat and work in opposition to the second law of thermodynamics. Although distribution coefficients, as defined above, are presented, this information may be used to more closely determine the role of solute solubility, as measured by the partition coefficient, solvent-to-sample volume ratios, and number of extractions needed to isolate a given amount. For example, Figure 2 compares the quantitative nature of extractions where the distribution coefficient is 2.0 (meaning the solute is twice as soluble in the organic phase as in the aqueous sample phase) and 50 mL (green line), 100 mL (black line), and 200 mL (blue line) of organic extracting solvent is used to isolate the analyte from 100 mL of aqueous sample. This is in comparison to the red line, which demonstrates an increase in the distribution coefficient to 10 and 100 mL of organic solvent is used.
Figure 2: Effect of repeated extractions on the amount extracted as a function of distribution coefficient (K = 2.0 in all cases, except the red line), and amount of organic extracting solvent (100 mL sample and 100 mL solvent, except the cases of 50 mL solvent [green] and 200 mL solvent [blue]).
In cases where the analyte may dissociate or decompose, the distribution coefficient may be replaced by the distribution ratio, which accounts for all forms of the analyte species in each phase. For example, when carbon dioxide is dissolved in water, the distribution ratio, D, must account for CO2, H2CO3, HCO3–, and CO32– levels in each phase. With this understanding, the percent extracted, %E, is expressed as:
%E = 100D/(D + (Vaq/Vorg)
Beyond this, analytical textbooks may begin to delve into SPE and related techniques with a more thorough treatment of the relationship between intermolecular attractions, solute adsorption, and sorptive phase selection. Extraction of solids, such as Soxhlet extraction, is begun to be discussed and, in some occasions, a cursory definition of advanced methods, such as supercritical fluid extraction (SFE), microwave-assisted extraction (MAE), or ultrasound-assisted extractions (UAE), may be presented. Some textbooks discuss topics like complexation and post-extraction sample clean-up.
Separations Theory Applied to Analytical Extractions
A more extensive treatment of theory in general found in the engineering literature, in graduate courses, and in specialized texts. In such training aids, treatment of the extraction mechanism may be broken down into the two major components, phase contact and phase separation. A discussion of molecular transport, typically via diffusion explained using Fick’s Laws, can be presented. Distribution ratios, rather than distribution coefficients, are preferred so that mass balance can be discussed alongside energy flow. In discussing mass balance, the potential for intentionally or unintentionally hydrolyzing or otherwise reacting the analyte is considered, as is thermal degradation. For extracting solid samples, topics like solvent wetting of the solid surface, solvent penetration into the solid particle, the impacts of a static solvent layer around the sample particle, particle-size and porosity, and extraction temperature should be a focus. Such presentations can demonstrate how particle size reduction can increase both internal transport by reducing the length of the diffusion pathway and external transport by enlarging the surface area of particle–solvent contact.
Treatment of extraction distribution coefficients or ratios can lead to Gibb’s phase rule:
F = C – P + 2
where F is the number of degrees of freedom describing the system, C, is the number of system components, and P is the number of phases. If temperature and pressure are held constant during extraction (and they generally are), the factor of two is valid, and the equation can be used to explain the function of other extraction parameters. For example, in a headspace extraction, the volume of the headspace, as well as the sample phase, is shown to be vital. In another instance, if an analyte may be protonated or deprotonated, the number of system components increases, and it can be shown that the concentration of analyte in the extracting phase depends not only on its concentration in the sample phase, but also on the pH of the sample phase.
Other advanced treatments may include interfacial tension or solubility parameters. Low interfacial tension is needed to drive mass transfer across the phase boundary and predicts initial solvent disruption and distribution in solvent-entrained solutions. When solute solubility increases, interfacial tension is generally lower, or more favorable. Other solubility treatments like the Hildebrand solubility parameter is another worthwhile discussion. The Hildebrand solubility parameter is a measure of the cohesive interaction energy of solute-solvent mixtures related to the heat of vaporization on the system. It is described by the hydrogen-bonding ability, dispersion coefficient, and polarity and if the Hildebrand solubility parameter of a solvent is close to that of the analyte (solute) of interest, extractions become more favorable. Other, similar solvent descriptors are available. The Hildebrand solubility parameter and other treatments are the basis for various modeling simulations of chemical extractions; they are widely used in the engineering literature.
Some of the theoretical treatments (with or without a presentation of Gibb’s phase rule presented earlier), for isolating chemical compounds from the analytical sample, provide the basis for specific, often more advanced extraction techniques like SPE, solid-phase microextraction (SPME), related sorption-based technologies, SFE, MAE, UAE, or pressurized-fluid extraction. Practice and detailed descriptions of the basis for these techniques are usually found in more specialized literature.
While knowledge is power, unfortunately the formal education of most analysts centers on education in advanced analytical techniques, good laboratory practice, SOPs, regulatory compliance, laboratory safety, and other items. Common laboratory procedures, like extractions, are often excluded from the educational process. Awareness and knowledge of advanced extraction and sample preparation procedures is even more scarce. In this column, we presented an overview of the type of information that should be included in training protocols, and discussed where this information can be found.
- P. R. Griffiths, Anal. Bioanal. Chem. 391, 875–880 (2008).
- R. E Majors, Sample Preparation Fundamentals for
- Chromatography (Agilent Technologies, 2013).
- L. S. Ettre, Pure and Appl. Chem. 65, 819–872 (1993).
- R. E. Majors and J. Hinshaw, LCGC North Am. 34(S2) (2016).
Posted by Akinbuli Opeyemi,
www.aasnig.com, [email protected]