Sustainable Analytical Methods: The Challenge Is Assessing Methods Before and After Improvements

This first in a series of articles will introduce the background of the green chemistry goals of the American Chemical Society (ACS). There are 12 goals listed in the ACS’s green and sustainable science initiative. Please see them listed in Table 1.

Of these 12, half have a key role in analytical method development as marked by an × in the first column. They are green chemistry goals that we as chromatographers and analytical scientists have control over as we develop methods, from sample delivery to our labs to the time the result is shared. General principles of how to achieve these goals are in the last column. The ACS’s website for green chemistry, www.acs.org/greenchemistry, the source for these principles and goals, is an excellent starting point for understanding the direction of sustainability improvements that can be made.

The purpose of this series to educate by providing an overview of measurement techniques developed over the last ten to fifteen years that show potential for broad use and the capability to become the standard for greenness measurement in analytical methodology. Over the next decade, every analytical method published will ideally include a sustainability measurement. To achieve this, it is time for us to select the best system available. Improvements can always be made, but a uniform, globally accepted system is desired.

Following the selection of the best tool, we can expect greenness scores or sustainability measurements in all published manuscripts of analytical methods. Simply put, it behooves us to make sure the measurement system used is uniformly utilized If it is not, the information we gain from this endeavor will be minimal and eventually useless. Ideally, the chosen measurement system can be uniformly adopted by the analytical chemistry and separation science community in particular. This uniform rating of our analytical procedures will allow a viable comparison between analytical methods from two different laboratories. And if all analysis benefits are equal, the best method will be chosen because of its sustainablity advantages.

As mentioned, sample preparation, although not the only use of large solvent volumes in the analytical laboratory, is certainly a major contributor. In terms of strategies to improve sample preparation solvent use from a greener perspective, micro-extraction, automation, greener extraction solvents, and greener, more efficient energy sources are all potential options. There are five top measurement techniques for evaluating the greenness of sample preparation, as improvements such as these are made within an overall method. An overview of these green evaluation techniques follows. Following the overviews, a comparison of the individual features of each tool, describing what would make it the best candidate for a global long-term evaluation system, will be discussed. The individual tools under evaluation are: Analytical Eco-Scale, Green Analytical Procedure Index (GAPI), Analytical Method Greenness Score (AGMS), Analytical GREEnness Metric Approach (AGREE), and the Sample Preparation Metric of Sustainability (SPMS).

Analytical Eco-Scale

Published in 2012 by researchers from the University of Kielce and Gdańsk University of Technology (both in Poland),¹ this is an early system that used penalty points for hazards and energy. They tied their work to the ACS green chemistry goals but simplified the principles.

For reagents, they suggested eliminating or reducing them. Alternatively, they proposed replacing solvents with less toxic, safer solvents that could be obtained from renewable sources, or easily degradable choices. For methods, they proposed in situ, non-invasive, miniaturized methods that avoid derivatization. For energy, they simply recommended consuming less energy. For waste, they advocate elimination or reduction, decontamination online, or recycling.

Table 2a compares the Green Chemistry Criteria for three key steps from the Analytical Eco-Scale authors, highlighting the ideal green analysis and the least green analysis from left to right. Along with comparing the ideal green analysis to the least green analysis, the authors rated the green alternatives for all steps of the analytical procedure from the greenest option to the non-green option. For the sample, the three green options were given for collection, preservation, transport, storage, preparation, and analysis. Table 2b shows the remaining sample steps and the alternatives from the greenest option through the medium green option, then to the non-green option.

The system of penalty points presented by the Analytical Eco-Scale was divided into categories for reagents, where the amount of reagent used was given a separate point structure than the hazard of the reagent, based on its physical, environmental, and health-related characteristics. There can be more than one hazard category for a reagent in terms of points. Specifically, in terms of points for volumes, less than 10 mL(g), 10-100 mL(g), and greater than 100 mL(g) were assigned one, two, and three penalty points, respectively. In terms of hazard, the categories were none, less-severe hazard, and more-severe hazard, where the point assignments were 0, 1, and 2, respectively, although the total penalty points for a hazardous solvent can be greater than 2 if there is more than one hazardous component assigned to a reagent. In terms of instrument penalty points, the categories of energy, occupational hazard, and waste were rated. The energy points were 0, 1, and 2, for the categories of less than or equal to 0.1 kWh per sample, less than or equal to 1.5 kWh per sample, and greater than 1.5 kWh per sample. Although a wide variety of instrumentation was categorized for Eco-Scale use and application, chromatographic energy rates were <0.1 kWh per sample for ultra-performance liquid chromatography (UPLC) instrumentation, ≤ 1.5 kWh per sample for liquid chromatography (LC) and gas chromatography (GC) instruments, and 1.5 kWh per sample for LC-mass spectrometry (MS) and GC-MS.

There are just two classifications for occupational hazards. They are analytical process hermitization (isolation) and emission of vapor and gases to the air, assigned penalty points of 0 and 3, respectively. Waste, however, has a total of eight classifications, which were based on volume of waste generated and are as follows: No waste (0 points), < 1 mL(g) (1 point), 1-10 mL(g) (3 points), > 10 mL(g) (5 points), recycling (0 points), degradation (1 point), passivation (2 points) and no treatment (3 points).

Table 3 shows an example of the penalty point scale for the application of polychlorinated biphenyls (PCBs) determination in sewage sludge by GC-MS. It is a straightforward rating system that can give a concrete numerical value for evaluating the greenness of an analytical method.

The basis of the analytical Eco-Scale is that the ideal green analysis has a value of 100. Then, as illustrated in Table 3, given the point scales described above, each of the analytical procedure steps, including amounts of reagents, hazards, energy, and waste, is assigned penalty points if it deviates from the ideal green analysis. The number of penalty points evaluated for a given procedure (65 for the determination of PCBs in sewage sludge by GC-MS), is subtracted from 100. The Eco-Score for this analytical method is then 35. Following the ranked scale of greater than 75 representing an excellent green analysis, greater than 50 an acceptable green analysis, and less than 50 an inadequate green analysis, the PCB determination is a less-than-adequate green analysis. The authors suggest that the PCB determination rated low in greenness because of the use of hazardous solvents. The score could be improved by choosing greener solvents. The analytical Eco-Scale is a semi-quantitative tool for laboratory practices and is simple and fast to use. Organic compound determinations generally result in a higher number of penalty points and, therefore, lower Eco-Scale scores. It does, however, readily indicate the weakest points in an analytical procedure and those that could be improved to achieve a higher score.

Green Analytical Procedure Index

Plotka-Wasylka developed the Green Analytical Procedure Index in 2018.² The procedure evaluates the green character of an entire analytical methodology from sample collection to final determination. GAPI uses a five-pentagram symbol to evaluate and quantitate the environmental impact of each analytical step. Green, yellow, and red colors are used to illustrate low, medium, and high impact. General method type is illustrated by the central pentagram; the outer pentagrams represent 1) sample preparation; 2) sample collection, preservation, transport, and storage used; 3) reagents and compounds used; and 4) instrumentation. Table 4 shows the Green Analytical Procedure Index Parameter Description used to establish the color coding of GAPI, which shows some consistencies with the Analytical Eco-Score in terms of ratings. For example, for sample collection, an in-line collection receives a green rating, an on-line or at-line sample collection receives a yellow rating, while a red, or least desirable rating, is given to an offline sample collection.

Figure 1 shows the GAPI symbols for four separate procedures for determining biogenic amines in wine. Procedures 1 and 2 both use GC-MS analysis, but with different solvents; procedure 1 used seven reagents (sodium hydroxide, phosphate buffer, hydrochloric acid, acetonitrile, toluene, isobutyl chloroformate, and methanol), and procedure 2 used only five (pyridine, hydrochloric acid, chloroform, isobutyl chloroformate, and methanol). Because of the reduced number of reagents, the penalty points for procedure 2 resulted in a lower score for these, as well as for waste. Total penalty points for procedures 1 and 2, respectively, were 43 and 31. Using capillary electrophoresis with diode-array detection, a drastic reduction in penalty points and a much greener method was given in procedure 3, where the total penalty points were only 15. Four reagents were used in procedure 3, two of them received no penalty points (20 mM borate buffer and sodium dodecyl sulfate); the other two reagents were methanol and an internal standard. Procedure 3, using capillary electrophoresis, was the greenest method, with a total analytical Eco-Scale score of 85. The fourth procedure had the worst score. It was a high-performance liquid chromatography (HPLC) method using fluorometric detection. Solid-phase extraction was also required; eight individual reagents were used, and the reagent penalty score was the highest of all four procedures at 38. Figure 1 gives the penalty scores for each of these procedures, as well as the GAPI color-schemed symbols. It is obvious from the green pentagons in Procedure 3 that it is the greenest method, and from the red pentagrams for Procedure 4, that some improvements are needed.

Summary and Series Completion

The GAPI evaluation methodology provides an easy-to-understand pictorial comparison of several methods, in which it is clear which is the greenest method. Consistent with the analytical Eco-Score evaluation, the two techniques are complementary and can be used together. Additional evaluations of greenness evaluation techniques will be presented in the continuing series. Expect overviews of the Analytical Method Greenness Score, Analytical GREEnness Metric Approach, and the Sample Preparation Metric of Sustainability in the next article.

The intent of this overview is to get the opinions of the analytical, chromatography, and sample preparation experts on the optimum technique for evaluating our own methods. For that endeavor, we will be concluding the series with a poll. Asking your opinions on which technique you would prefer to use to evaluate the literature methodology, the overall objective is to evaluate our methods for the greenness as we publish and use them.

References

1. Gałuszka, A.; Migaszewski, Z. M.; Konieczka, P. et al. Analytical Eco-Scale for Assessing the Greenness of Analytical Procedures. Trends Anal. Chem. 2012, 37, 61-72. DOI: 10.1016/j.trac.2012.03.013
2. Płotka-Wasylka, J. A New Tool for the Evaluation of the Analytical Procedure: Green Analytical Procedure Index. Talanta 2018, 181, 204-209. DOI: 10.1016/j.talanta.2018.01.013