How Green Is Your Method? Evaluating the Sustainability of Analytical Methods

In previous installments, we have touched on various aspects of sustainability in analytical chemistry and chromatography, with some common means for thinking about and implementing sustainable ideas in our methods (1). In gas chromatography (GC), one simple change is to use nitrogen, where possible, as a replacement for helium as the carrier gas (2). Nitrogen is obviously more renewable than helium and does not require an expensive, energy-consuming generator device, as does hydrogen. Another possibility is to use miniaturized instrumentation, which requires smaller laboratory footprint and lower power consumption (3). Sample preparation remains an important area for improving sustainability, with solid phase microextraction (SPME) and quick, easy, cheap, effective, rugged, and safe (QueChERS) as example techniques offering low solvent and consumable consumption (4,5).

GC holds a special place among analytical methods because it can be argued that it is the original “green” technique. Invention of the electron capture detector (ECD) by Lovelock in the 1950s and the resulting analysis of chlorofluorocarbons (CFCs) in the atmosphere was among the seminal events that launched the environmental movement (6). Lovelock’s later book is among the first that described man’s impact on the overall climate and ecosystem of the Earth (7). Considering general characteristics, GC is naturally green, requiring small (typically 1 μL of liquid) samples, providing high-resolution separations and being relatively simple to set up, maintain and operate.

A challenge when developing new “green” methods or optimizing a current method for sustainability is determining how, or if, the sustainability of the method has been improved. The principles of green chemistry were developed in the 1990s and the principles of green analytical chemistry in the 2000s (8,9). Since then, several methods for evaluating the “greenness” of analytical methods have been developed. In the remainder of this article, we will make a brief comparison of three peer-reviewed and published methods: Red, green blue (RGB), analytical method greenness score (AMGS) and analytical greenness metric (AGREE). Like analytical techniques themselves, each of these scoring systems has strengths and weaknesses. For a comprehensive review of analytical chemistry greenness scoring systems and metrics, see the recent review by Yin and associates, in which fifteen systems are compared (10).

Table I lists the principles of green chemistry, as originally published by Anastos and Warner in 1999 alongside the principles of green analytical chemistry. While there are many similarities between the two, the original principles are heavily weighted toward synthesis and were modified and updated for characterization and analysis. Interestingly, principles of green analytical chemistry were codified in the late 2000s, yet many analytical chemists would (correctly) argue that analytical chemists have been practicing green chemistry for decades. Along with capillary GC itself, the seminal developments of supercritical fluid chromatography (SFC), supercritical fluid extraction (SFE), and SPME in the late 1980s are prominent examples of green chemistry being practiced well before the term was later coined. Analytical chemists are also trained to optimize methods for efficiency in the use of materials, consumables, instruments and analysis time. One challenge with implementing more green analytical methods is that many of our modern methods, like capillary GC, SFE, and ultra high performance liquid chromatography (UHPLC) are already green.

AGREE and the follow-up method AGREEprep are greenness calculators based on both visual and quantitative measures based directly on the 12 principles of green analytical chemistry shown in Table I (11,12). The analytical method is evaluated against each principle, with each assigned a numerical score between 0 (not green) and 1 (green) and a color score between red (not green) and green (green). Each of the 12 principles can be weighed equally or they can be assigned different weights based on the priorities or utilization in the method. The weighted average of the 12 scores between 0–1 is used to determine the final greenness score. A circular graphic is then used to present both the visual and quantitative greenness scores. Figure 1 shows an example of three chromatography-based procedures using the AGREE method.

These three methods, as seen in the acronyms shown in the figure caption, involve a variety of chromatographic methods and techniques. All three avoid derivatization, so the wedge for principle #6 is green for all. Not surprisingly, metrics 7–11, which address energy, renewable reagents and operator safety or red for the Soxhlet extraction-based method and all of the methods have relatively high (not green) scores for energy usage metrics. In this example, all 12 principles are weighed equally, but they can be assigned different weights, based on the specific needs and goals of the analysis. This graphic illustrates some of the tradeoffs or compromises that may be necessary when choosing or optimizing a method for greenness. The stir bar sorptive extraction (SBSE)- high-performance liquid chromatography (HPLC)-based method has simplified sample preparation and instrumentation but lacks automation and integration of the instrumentation. The sorptive extraction headspace solid phase microextraction with gas chromatography tandem mass spectrometry (SE-HS-SPME-GC–MS/MS)-based method uses simplified and automated sample preparation but complex, energy-consuming instrumentation. Interestingly, none of these methods can be considered green, as evidenced by the color coding in the circles. Although the methods are operated very differently, there is not a dramatic overall greenness difference between the HPLC- and GC-based methods; not surprisingly, the Soxhlet extraction-based method is the least green, with both large solvent and energy consumption and energy intensive instrumentation.

The analytical method greenness score (AMGS) proposes the use of a single equation to determine a greenness score for an analytical method (13). This formula, shown in equation 1, focuses on an environmental impact evaluation of the solvents (mass, safety, health, and environmental friendliness), mobile phases, other reagents, instrument energy and sample throughput. While equation 1 below seems complex, especially for methods involving multiple solvents or sample preparation steps, a straightforward online calculator is provided by the American Chemical Society Green Chemistry Institute (14).

The two “m” terms are the mass of solvents and diluents consumed during sample preparation and the mass of mobile phase. The S, H, and E terms measure solvent safety, health, and environmental safety, with cumulative values n adding up for each solvent. These scores are derived from a solvent guide published by the American Chemical Society (ACS). The CED and E terms provide the energy involved with producing and disposing of the solvents and the energy consumption of the instrumentation. To facilitate solvent choice, the ACS provides a solvent selection guide based on the principles of green chemistry (15). This guide provides a wealth of information to assist in choosing alternative solvents for both GC and HPLC and it provides the basis for the solvent scores used on the AMGS method. R is the total run time for analyzing all samples in a batch. AMGS was originally developed for HPLC methods and has been applied to SFC.

Both AMGS and AGREE closely follow the principles of green analytical chemistry, with a focus largely on solvent and instrumentation related parameters and each produces a numerical greenness score that allows comparison between methods. In comparing methods or predicting the results of proposed method optimizations, users should look for dramatic overall changes; two of the methods described in Figure 1 are not very different in their overall score. Furthermore, users should be on the lookout for unintended consequences. For example, a change in instrumentation from HPLC to GC might result in higher energy consumption or less green sample preparation. Neither AGREE nor AMGS considers analytical performance aspects outside the principles of green analytical chemistry, which may have a major impact on the usefulness of the method.

The RGB system is shown schematically in Figure 2, with the idea that a method can be scored or described by the color combination of three fundamental principles: performance (Red), practicality (Blue) and greenness (Green) (16). Each of these fundamental principles is then supported by its various aspects. For example, performance includes measures such as limit of detection, linear range and reproducibility. A numerical score of 0–100 is applied to each of the principles and a final score (also between 0–100) is determined. RGB is the most holistic of these evaluation methods, as the others are focused more specifically on the green aspects of method development and optimization, while RGB includes analytical performance and practicality of the method in determining the final score.

The principles of green chemistry were largely conceived with an emphasis on synthesis; they have been adapted to analysis. For chromatographic methods, the major contributors to greenness (or lack of it) are solvents in sample preparation, mobile phases, either solvents or compressed gases and energy consumption by the instrumentation. For solvents and mobile phases, environmental concerns, including both production and waste, safety and energy consumption for production, use and disposal, must be considered. Clearly in GC, nitrogen, where applicable is the greenest mobile phase, followed by hydrogen and then helium, which is not a renewable resource. For energy consumption, especially for routine analysis, miniature or small-footprint instruments and automation should be considered. Finally, wherever possible automated, low sample and solvent volume sample preparation methods should be considered.

References

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