Green Analytical Methodologies

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Chem. Rev. 2007, 107, 2695 27082695Green Analytical MethodologiesLawrence H. Keith,*,† Liz U. Gron,‡ and Jennifer L. Young§Environmental & Chemical Safety Educational Institute, 329 Claiborne Way, Monroe, Georgia 30655, Hendrix College, 1600 Washington Avenue,Conway, Arkansas 72032, and ACS Green Chemistry Institute, 1155 16th Street NW, Washington, DC 20036Received October 16, 2006Contents1. Introduction1.1. Green Chemistry1.2. Green Analytical Chemistry2. Trends in Green Analytical Chemistry2.1. Greening Pretreatment2.1.1. Solvent Reduction and Replacement2.1.2. Solvent Elimination2.1.3. Derivatization of Molecules and Surfaces2.2. Greening Signal Acquisition2.2.1. Spectroscopy2.2.2. Electrochemistry2.2.3. Bioanalytical Chemistry2.3. Greening with Automation and FlowTechniques2.3.1. Flow Injection Analysis (FIA) andSequential Flow Injection Analysis (SIA)2.3.2. Multicommutation2.4. Green by in-Situ3. NEMI and Greener Analytical Methods3.1. Background of NEMI3.2. Greenness Profiles of Greener AnalyticalMethods3.3. Application of Greenness Profiles3.4. Characteristics of Greener Analytical Methodsin NEMI4. Conclusion5. Acknowledgments6. 0627061. IntroductionFor over 10 years, the green chemistry movement has beenpromoting ways to reduce the risks of chemical use tohumans and the environment.1,2 An important goal is todevelop increasingly environmentally benign chemistries. Arelatively underexamined area of green chemistry is analyticalchemistry. However, analytical methods are not easilyidentified as being environmentally benign. Assessmentrequires careful examination of often complex analyticalmethodologies within the context of green chemistry. Thisarticle attempts to examine qualitatively the scope of greenanalytical chemistry with a survey of the recent analytical* Corresponding author.† Environmental & Chemical Safety Educational Institute.‡ Hendrix College.§ ACS Green Chemistry Institute.literature to discern common green analytical chemistrythemes while creating, and applying, a more quantitativeapproach to existing environmental methodologies. Theauthors set forth some basic characteristics, or “acceptancecriteria”, to which analytical methods should conform inorder to be called “green.” The application of these criteria,applied to over 800 methods in the National EnvironmentalMethods Index (NEMI), the largest available database ofenvironmental analytical methods, is discussed herein.1.1. Green ChemistrySimply stated, “Green Chemistry is the use of chemistrytechniques and methodologies that reduce or eliminate theuse or generation of feedstocks, products, byproducts,solvents, reagents, etc. that are hazardous to human healthor the environment.”1 Thus, an important goal of greenchemistry is to reduce hazards associated with products andprocesses that are essential to the world economy and tosustain the high quality of living that we enjoy throughchemistry. It seeks to achieve this goal by reducing oreliminating as much risk as possible associated with chemicalprocesses. If chemical hazards can be reduced, then risksfrom using or being exposed to chemicals are also reduced.Hazards from chemicals go beyond toxicity (acute andchronic) to include carcinogenicity, mutagenicity, explosivity,flammability, and corrosivity as well as including environmental impacts such as atmospheric damage and globalclimate change.1The Twelve Principles of Green Chemistry provide aframework for scientists and engineers to use when designingnew materials, products, processes, and systems.2 Theprinciples focus thinking in terms of sustainable designcriteria and have proven to be the source of innovativesolutions to a wide range of problems. Many, but not all, ofthese principles apply to green analytical chemistry. Thosethat are most relevant to, or most commonly encounteredin, analytical chemistry are marked in bold and with asterisksin Table 1. For analytical methods, green chemistry meansdesigning methods that reduce or eliminate the hazardoussubstances used in or generated by a method.1.2. Green Analytical ChemistryAs Anastas alluded to, it is an unfortunate irony thatenvironmental analytical methods often contribute to furtherenvironmental problems through the chemicals used in theanalysis.1 This is because many analytical procedures requirehazardous chemicals as part of sample preservation, preparation, quality control, calibration, and equipment cleaningseffectively creating wastes in larger quantities and with10.1021/cr068359e CCC: 65.00 2007 American Chemical SocietyPublished on Web 05/24/2007

2696 Chemical Reviews, 2007, Vol. 107, No. 6Dr. Lawrence H. Keith is Director, Environmental Chemistry of theEnvironmental & Chemical Safety Educational Institute and also is president of Instant Reference Sources, Inc., a consulting company in Monroe,GA. He specializes in environmental monitoring, QA/QC, analytical methodsfor antiterrorism, training, and expert systems. He has over 40 years ofexperience and has served as Corporate Fellow at Radian International,LLC, in Austin, TX, and as a Vice President at the Waste Policy Institutein Blacksburg, VA. He is past chairman and secretary of the ACS Divisionof Environmental Chemistry and past editor of the division’s newsletter.He received a Ph.D. in chemistry from the University of Georgia, an M.S.in chemistry from Clemson University, and a B.S. in chemistry from StetsonUniversity. He has taught workshops in Australia, Asia, Canada, Europe,and South and Central America and has received four ACS awards andStetson University’s Outstanding Alumnus Award. He has published over50 electronic books and expert systems, 65 printed books and bookchapters, and more than 55 journal articles and government reports.Dr. Liz Gron earned her B.A. in chemistry at Colgate University and herPh.D. in inorganic chemistry with Arthur B. Ellis at the University ofWisconsinsMadison, 1987. She was a postdoctoral fellow and anIndustrial Research Liaison at the Department of Chemical Engineering,University of Delaware, before starting at Hendrix College in 1994. Sheis currently an Associate Professor of Chemistry at Hendrix College andhas taught courses in general, analytical, and inorganic chemistry. Herresearch interests focus on the parallel development of green chemicalreactions as well as the development of green pedagogical materials.Her chemical research investigates organic reactions in near-critical wateras a solvent replacement technology while also exploiting the uniqueproperties of extremely hot water to investigate underlying mechanisticinteractions between the reactants and the solvent. Her educationalmaterials development has focused on designing green experiments thatteach analytical and environmental chemistry, particularly to introductorychemistry students. During her latest sabbatical, Professor Gron held theposition of Visiting Assistant Professor of Chemical Engineering, Massachusetts Institute of Technology, while working as a Visiting Scientistwith Professor Jefferson Tester.greater toxicity than that of the original analyzed sample.For all of these reasons, green analytical chemistry isbecoming a new and important subarea of green chemistry.Keith et al.Dr. Jennifer L. Young is a senior program manager at the AmericanChemical Society Green Chemistry Institute (ACS GCI), where her workfocuses on the development of research tools to aid chemists andengineers in implementing green chemistry and engineering. Prior to joiningACS GCI in 2004, she held an industrial polymer research position atDuPont in the Ink Jet business. Jennifer earned a Ph.D. in polymer/organicchemistry in 2000 from the University of North Carolina at Chapel Hill byresearching polymerizations in supercritical carbon dioxide under thedirection of Joseph DeSimone. Her B.S. degree in chemistry is from theUniversity of Richmond. Jennifer has over 10 years of experience in greenchemistry and has presented and published papers and book chapterson numerous green chemistry topics.The goal of green analytical chemistry is to use analyticalprocedures that generate less hazardous waste and that aresafer to use and more benign to the environment. This goalmay be achieved by developing new analytical methodologies or, more often, simply modifying an old method toincorporate procedures that either use less hazardous chemicals or, at least, use lesser amounts of hazardous chemicals,if appropriate, safer chemical substitutions have not yet beendiscovered.For a long time, analytical chemists have been environmentally sensitive but have rarely used the word “green”,making the green developments a little harder to discern inthe literature. Since the first general reviews describing greenanalytical chemistry,1,3,4 more researchers are publishing ongreen or clean methodologies and using this terminology,with the trends in numbers of publications plotted in Figure1. The scope of this review will provide a literature reviewof recent advances in green analytical chemistry as well astouch on some traditional methodologies that have alwaysbeen environmentally benign, but perhaps not called green.2. Trends in Green Analytical ChemistryAnalytical chemistry provides the data necessary to makedecisions about human and environmental health. Fast,precise, and accurate results will always be the primarybusiness of an analytical chemist; the new green challengeis to meet the informational needs of chemists, industry, andsociety while reducing the human and environmental impactof the analyses.The natures of the analyte, the matrix, and the method ofsignal generation greatly influence the ease of creating agreen analytical method. Analysis schemes that do not requirepretreatment, use few reagents, or work with aqueoussolvents have a greenness advantage. This covers severalwell-established techniques measuring aqueous inorganicions, such as pH, ion chromatography, flame atomic absorption (FAA) spectroscopy, and graphite furnace atomicabsorption (GFAA) spectroscopy. Elemental analysis in solid

Green Analytical MethodologiesChemical Reviews, 2007, Vol. 107, No. 6 2697Table 1. The Twelve Principles of Green Chemistry:2 Asterisks and Bold Type Indicate the Principles Most Applicable to AnalyticalChemistry*1.PreventionIt is better to prevent waste than to treat or clean up waste after it has been created.2.Atom EconomySynthetic methods should be designed to maximize the incorporation of all materials usedin the process into the final product.3.Less Hazardous Chemical SynthesesWherever practicable, synthetic methods should be designed to use and generatesubstances that possess little or no toxicity to human health and the environment.4.Designing Safer ChemicalsChemical products should be designed to effect their desired function while minimizingtheir toxicity.*5.Safer Solvents and AuxiliariesThe use of auxiliary substances (e.g., solvents, separation agents, etc.) should be madeunnecessary wherever possible and innocuous when used.*6.Design for Energy EfficiencyEnergy requirements of chemical processes should be recognized for their environmentaland economic impacts and should be minimized. If possible, synthetic methods should beconducted at ambient temperature and pressure.7.*8.9.10.Use of Renewable FeedstocksA raw material or feedstock should be renewable rather than depleting whenevertechnically and economically practicable.Reduce DerivativesUnnecessary derivatization (use of blocking groups, protection/deprotection, temporarymodification of physical/chemical processes) should be minimized or avoided if possible,because such steps require additional reagents and can generate waste.CatalysisCatalytic reagents (as selective as possible) are superior to stoichiometric reagents.Design for DegradationChemical products should be designed so that at the end of their function they break downinto innocuous degradation products and do not persist in the environment.*11.Real-time Analysis for Pollution PreventionAnalytical methodologies need to be further developed to allow for real-time, in-processmonitoring and control prior to the formation of hazardous substances.*12.Inherently Safer Chemistry for Accident PreventionSubstances and the form of a substance used in a chemical process should be chosen tominimize the potential for chemical accidents, including releases, explosions, and fires.samples can be done readily without any sample preparationusing X-ray fluorescence (XRF) spectrometry, where asample is bombarded with high-energy X-rays, causingemission of a secondary X-ray photon, fluorescence, uniqueto the element. Very simple techniques for organics have asimilar advantage of no sample pretreatment, such as gaschromatography (GC),5 attenuated total reflectance infrared (ATR) spectroscopy, and total organic carbon (TOC)analysis.Analytical schemes include a myriad of steps, and mostcan be separated into two broad categories: the pretreatmentsteps (including digestion, extraction, drying, and concentration) and the signal acquisition step. Although an ideal greenanalysis would obviate preconcentration steps, the evolvingunderstanding of the vanishingly low thresholds for thenegative biological activity of many environmental contaminants suggests that analytical chemists will continue to needsample pretreatment as a tool to take measurements fromdilute samples at, or below, the limit of detection.A survey of the recent analytical literature illustrates thatthe path toward greening analytical methodologies includesincremental improvements in established methods as wellas quantum leaps that completely rethink an analyticalapproach. Strategies used include changing or modifying thereagents and solvents, reducing chemicals used throughautomation and advanced flow techniques, miniaturization,and even eliminating sampling by measuring analytes insitu, on-line, or in the field.2.1. Greening Pretreatment2.1.1. Solvent Reduction and ReplacementA rich variety of greener methods have been developedto extract and concentrate analytes. As a rule, acceleratedsolvent extraction (ASE), ultrasound extraction, microwaveassisted extraction (MAE), supercritical fluid extraction(SFE), and membrane extraction reduce the use of organicsolvents and speed extraction times compared to traditionalliquid-liquid extractions. While ASE uses pressure and heatto speed extractions, up to 200 C,6 the other methods uselower temperatures, allowing for easier handling of thermallyfragile analytes and cleaner extractions. Ultrasonic andmicrowave extractions are relatively simple and inexpensivetechniques for greening extractions, while SFE is moreexpensive due to the equipment and requires careful controlof a wide variety of factors, making SFE more difficult tooptimize and validate.2.1.1.1. Ultrasound. Ultrasonic extraction uses highfrequency acoustic waves to create microscopic bubbles inliquids. The collapse of the small bubbles produces smallshock waves, cavitations, that are particularly well suitedfor breaking up or promoting the dissolution of solids.Ultrasonic extraction has been applied to a variety of organicextractions. These include the extraction of nicotine frompharmaceutical samples into heptane for GC analysis, whichreduced the amount of solvent required by 5/6 compared tothe conventional method,7 phthalates from cosmetics into

2698 Chemical Reviews, 2007, Vol. 107, No. 6Figure 1. Number of publications resulting from an ISI Web ofScience literature search for 1990-2006 for the keywords “greenchemistry” (9) and the combination of “green anal*” or “cleananal*” or “green method*” (2). Part a shows both keyword searchresults, and part b shows only the results for publications relatedto green/clean analytical methods.ethanol/water for high performance liquid chromatography(HPLC),8 and UV filters from sunscreens into ethanol forliquid chromatography.9 Ultrasound has also been used forinorganic analytes, most recently to extract mercury into aquaregia from milk samples.102.1.1.2. Microwave-Assisted Extraction (MAE). Microwave-assisted extraction (MAE) has proven broadly applicable for extractions from difficult sample matrixes,previously treated by time and solvent intensive Soxhletextractions or hydrodistillations. Microwave extractions canbe done in open or closed vessels, known as focused MAEand pressurized MAE, respectively. A recent review of theapplication of microwave techniques to environmentalsamples illustrates the wide range of matrices used, includingthe extraction of polycyclic aromatic hydrocarbons (PAH)from soil, polychlorobiphenyls (PCB) from coal, methylmercury from sediments, as well as trace metals and pesticide residues from plant materials.11 The use of MAE hasexpanded with adaptations that include using focused MAEfor Soxhlet extractions of thermally labile methylcarbamates12and acid herbicides13 from soil and using MAE to extractcamphor or borneol from fresh herbs into water and thencoupling to headspace solid-phase microextraction for reconcentration of the analytes.14 Microwave treatment canprovide a solvent-free separation technique by providingheating and dry distillation for essential oils versus hydrodistillation.15 Figure 2 shows a solvent-free microwaveextraction apparatus.2.1.1.3. Supercritical Fluid Extraction (SFE) and Superheated Water Extraction (SWE). A variety of solventtechnologies have been developed to replace non-renewablepetroleum solvents. Once exotic, SFE has become a routinemethod for handling thermally sensitive analytes. Solventsthat are heated and pressurized above their critical pointKeith et al.Figure 2. Solvent-free microwave extraction apparatus. Reprintedfrom Journal of Chromatography A, Vol. 1043, Lucchesi, M. E.;Chemat, F.; Smadja, J., Solvent-free microwave extraction ofessential oil from aromatic herbs: comparison with conventionalhydro-distillation, pp 323-327, Copyright 2004, with permissionfrom Elsevier.exhibit properties intermediate between those of liquids andgases, making them ideal for separations and extractions.Many SFEs are performed with carbon dioxide (SFE-CO2),which has a readily accessible critical point (31.1 C, 74.8atm) along with being inflammable and nontoxic. Theefficiency of SFEs is affected by the choice of extractionsolvent as well as the extraction pressure, temperature, fillermaterials (mixed with the sample matrix), modifiers (cosolvents), and collection solvent.16,17 The challenge of SFECO2 is the very low solubility of polar materials.18 Theapplicability of SFE-CO2 has been broadened by the use ofa modifier, primarily methanol,16,19 or the addition ofchelates.20A polar alternative to SFE-CO2 is superheated, subcritical,water extraction (SWE). SWE has the advantage of tunablepolarity, since the dielectric constant of the pressurized waterdecreases dramatically with increasing temperature (100373 C) due to reduced H-bonding; however, these highertemperatures limit the method to relatively thermally robustanalytes, for example organopesticides and triazine herbicides.21,22 High-temperature water is inflammable and nontoxic, but similarly to the SFE methods, many proceduresuse modifiers17 to optimize the extraction and the extractionyields a dilute aqueous sample which usually requires asubsequent concentration step. A truly unusual solvent systemunder development may provide the intermediate polaritiesthat mixtures of supercritical fluids with cosolvents are tryingto attain. These are gas-expanded liquids (GEL).23 In thisexample, CO2(g) is added to a pressurized solvent, decreasingthe dielectric constant and providing a truly tunable polarityat low temperatures.2.1.1.4. Membranes. Membranes, selective barriers between phases, provide an alternative for green analyteisolation and preconcentration. There are two primarymembrane techniques, filtration and extraction, which arethoroughly reviewed in a

of recent advances in green analytical chemistry as well as touch on some traditional methodologies that have always been environmentally benign, but perhaps not called green. 2. Trends in Green Analytical Chemistry Analytical chemistry provides the data necessary to make decisions about human and environmental health. Fast,