Nov 01, 2019
LCGC North America
Volume 37, Issue 11, pg 796–800
Although sample preparation is considered an enabling technology, it is among the most vital components of an analytical scheme. Given this apparent dichotomy, where does the analyst working at the laboratory bench learn about sample preparation approaches, especially chemical extractions? How is this situation exacerbated when these analysts have an AA, a BS, or no degree at all; or perhaps their training is in biology, physics, or any other laboratory science? This month, we will take a look at formal and informal training opportunities that may be available to educate all chemists in the fundamentals of these necessary laboratory skills.
Douglas E. Raynie
The question becomes “How do we train bench chemists, laboratory managers, regulators, and others in the performance of chemical extractions?” Analysts at all levels must have knowledge of chemical extractions as they develop new methods, consider new technologies for the laboratory, read and write standard operating procedures (SOPs) or draft regulatory methods, and so forth. There are few sources that combine a working knowledge of the performance of common extraction procedures, with an overview of new and emerging technologies, provide a basic understanding of common analytical extractions, or articulate a thorough theoretical basis for this mode of chemical separations. This situation is compounded by what is taught regarding chemical separations, including extractions, in the common chemistry curriculum. Griffiths (1) reports that, in a survey of analytical professors in the western United States, in the quantitative analysis course that typically represents the first, and often only, analytical chemistry course taken by undergraduate chemistry majors, the topic of “separation theory” is covered in an average of 1.7 lecture periods, and this mostly includes chromatography theory and the concept of distribution coefficients (discussed below), and 13% of courses do not address the topic. Furthermore, the nationally normed American Chemical Society analytical chemistry exams, summarized in Table I, barely touch the subject. Perusal of the more recent exams does not reveal anything significantly different.
I once heard a joke along the line of “How many biologists [or insert other type of nonchemical scientist] does it take to prepare a sample?” Answer: “None, that’s what chemists are for.” While the attempted humor stems from the realization that extraction and other sample preparation techniques are chemical processes, it also points to how little attention many scientists pay to these essential steps. Further, the ubiquitous nature of sample extractions also leads them to be less than fully appreciated. After all, we find extractions in brewing coffee or tea, laundering clothing, and elsewhere in everyday living. But the problem may not just be a lack of concern about sample preparation. The issue may be a lack of sufficient training.
Only an occasional, somewhat specialized tome (for example, reference ), provides a combination of the practice, applications, basic understanding, and detailed theory of analytical extractions. Another good source of such information is the short course program at any number of professional meetings. (For disclosure, I regularly teach sample preparation courses at Pittcon and the Eastern Analytical Symposium.) One advantage of professional short courses is the opportunity to directly interact with experts in the analytical extraction field.
Practice of Extraction
Analytical chemistry textbooks provide an overview, at best, of the practice of extraction. Additionally, laboratory technicians often have two-year degrees or bachelor’s degrees in laboratory sciences other than chemistry. Although the practice of chemical analysis is an emphasis of the curriculum of chemical technology AA degrees, we often must look beyond analytical education materials to get an understanding of practice. Keeping in mind that chemical extractions are not an exclusive domain of the analytical discipline, the practice of extraction is often covered in organic chemistry laboratory manuals, and most chemists, regardless of educational level, are required to complete an organic chemistry sequence.
Liquid–liquid extraction (LLE), including proper use of separatory funnels, is found in organic chemistry laboratory manuals. Solvents commonly used for extraction are presented, and although safety is mentioned, health and environmental concerns in the selection of solvents are typically not addressed, though the subject of green chemistry is becoming increasingly commonplace. The most basic understanding of chemical extractions, in the form of distribution, or partition, coefficients is presented:
K = Corg/Caq = analyte concentration in organic phase/analyte concentration in aqueous phase.
This leads to the ability to predict the ratio of solute (analyte) in the (aqueous) sample phase to that originally present in the sample phase after n number of batch extractions:
(Final solute amount)aq/(Initial solute amount)aq = (Vaq/(Vaq + VorgK)n 
where V is the aqueous (aq) or organic (org) solvent volume. This use of distribution coefficients makes some assumptions related to chemical potential that is beyond the scope of these laboratory manuals.
An example of a typical LLE, as presented in organic chemistry laboratory manuals, is shown in Figure 1. In this example, the (blue) aqueous sample contains three solutes (red and green spheres and squares). Based on solubility differences, reflected by the distribution ratio, the spherical molecules preferentially partition into the (pink) lighter-than- water organic extracting phase, which is then removed for subsequent work up and analysis.
Figure 1: Example of a a liquid–liquid extraction using a separatory funnel. An aqueous sample (blue) is extracted with a less dense organic solvent (pink).
The balance of the discussions is more practically focused. An overview of manipulation of pH relative to analyte pKa to protonate or deprotonate acid, amine, and related functional groups is often presented. The practice of extraction may include topics like glassware for cases of solvents more or less dense than water, washing the organic phase, salting out, and preventing or dealing with emulsion formation. Techniques other than liquid–liquid extraction are rarely presented, because most organic systems encountered in undergraduate organic experiments are solutions. Occasionally solid-phase extraction (SPE) is presented, but a comprehensive treatment of the intermolecular attractions that provide the basis of solute sorption is lacking.
In addition to the practical nature of organic chemistry laboratory manuals, the International Union of Pure and Applied Chemistry (IUPAC) (3), as well as Majors and Hinshaw (4), present guides to the terminology used in extraction and chromatographic separations. Such terms as batch versus continuous extraction, static versus dynamic conditions, solvent to feed ratio, raffinate, residue, extraction stage, diffusion, surface tension, adsorption, solubility, polarity scales, or isotherms are presented, and become the basis for the understanding and proper description of extractions.
Finally, although often not explicitly presented, knowledge of physicochemical parameters like polarity, volatility, molecular weight, and pKa of both the analyte and potentially interfering compounds is necessary in designing an extraction.
Posted by Akinbuli Opeyemi,
www.aasnig.com, [email protected]