Supercritical Fluid Extraction as a Sample Introduction Method for Chromatography
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Wood of Santalum album and resin of Boswellia carterii Birdw. were used to obtain their volatile oils by means of supercritical fluid extraction with carbon dioxide. Different extraction conditions were tested: 90 bar, 45°C; 120 bar, 60°C; and 120 bar, 45°C. On both matrices, a good process performance was obtained working at 120 bar and 45°C (density of CO 2 = 0.658 g cm −3 ) in the extraction vessel, at 20 bar and 15°C in the separator and at CO 2 flow of 1.5 kg/h. At these conditions the higher yields were obtained: 1.9% for S. album and 6.5% for B. carterii. The main compounds contained in the sandalwood volatile oil were: α α α α α -santalol (46.1%), β β β β β -santalol (20.4%), epi-β β β β β -santalol (6.8%) and trans-α α α α α -bergamotol (5.4%). In the corresponding HD essential oil the α-santalol and β β β β β -santalol contents were lower: 35.0% and 14.0%, respectively. The volatile oil of B. carterii were made up of incensole acetate (32.0%), octanol acetate (25.1%), incensole (17.8%) and phyllocladene (7.7%). The percentage of the main constituents in the oil obtained by HD was quite different. It contained larger amounts of octanol acetate (45.2%) and phyllocladene (13.2%) and lower amounts of incesole (6.1%) and incensole acetate (13.0%).
Wood of Santalum album and resin of Boswellia carterii Birdw. were used to obtain their volatile oils by means of supercritical fluid extraction with carbon dioxide. Different extraction conditions were tested: 90 bar, 45°C; 120 bar, 60°C; and 120 bar, 45°C. On both matrices, a good process performance was obtained working at 120 bar and 45°C (density of CO 2 = 0.658 g cm −3 ) in the extraction vessel, at 20 bar and 15°C in the separator and at CO 2 flow of 1.5 kg/h. At these conditions the higher yields were obtained: 1.9% for S. album and 6.5% for B. carterii. The main compounds contained in the sandalwood volatile oil were: α α α α α -santalol (46.1%), β β β β β -santalol (20.4%), epi-β β β β β -santalol (6.8%) and trans-α α α α α -bergamotol (5.4%). In the corresponding HD essential oil the α-santalol and β β β β β -santalol contents were lower: 35.0% and 14.0%, respectively. The volatile oil of B. carterii were made up of incensole acetate (32.0%), octanol acetate (25.1%), incensole (17.8%) and phyllocladene (7.7%). The percentage of the main constituents in the oil obtained by HD was quite different. It contained larger amounts of octanol acetate (45.2%) and phyllocladene (13.2%) and lower amounts of incesole (6.1%) and incensole acetate (13.0%).
The determination of trace analytes in complex natural matrices often requires extensive sample extraction and preparation prior to chromatographic analysis. Correct sample preparation can reduce analysis time, sources of error, enhance sensitivity and enable unequivocal identification, confirmation and quantification. This overview considers general aspects on sample preparation techniques for trace analysis in various matrices. The discussed extraction/enrichment techniques cover classical methods, such as Soxhlet and liquid-liquid extractions along with more recently developed techniques like pressurized liquid extraction, liquid phase microextraction (LPME), accelerated microwave extraction, and ultrasoundassisted extraction. This overview also deals with more selective methodologies, such as solid phase extraction (SPE), solid phase microextraction (SPME) and stir bar sorptive extraction (SBSE). The adopted approach considers the equilibriums involved in each technique. The applicability of each technique in environmental, food, biological and pharmaceutical analyses is discussed, particularly for the determination of trace organic compounds by chromatographic methods.
The article contains sections titled: 1. Introduction 1.1. A Strategy Appropriate to Trace Analysis 1.2. Avoidance of Systematic Errors 1.2.1. Trace Losses and Contamination 1.2.2. Uncertainty 2. Sample Preparation and Digestion in Inorganic Analysis 2.1. Sample Treatment after the Sampling Process 2.1.1. Stabilization, Drying, and Storage 2.1.2. Homogenization and Aliquoting 2.1.3. Requirements with Respect to Materials and Chemicals 2.2. Sample‐Preparation Techniques; General Considerations 2.2.1. Special Factors Associated with Microwave‐Assisted Digestion 2.2.2. Safety Considerations 2.3. Wet Digestion Techniques 2.3.1. Wet Digestion at Atmospheric Pressure 2.3.2. Pressure Digestion 2.3.2.1. Thermally Convective Pressure Digestion 2.3.2.2. Microwave‐Assisted Pressure Digestion 2.4. “Dry” Digestion Techniques 2.4.1. Combustion in Air 2.4.2. Combustion in Oxygen 2.4.3. Cold‐Plasma Ashing 2.4.4. Fusion 2.5. Illustrative Examples 2.5.1. Sample Preparation as a Function of Analytical Method 2.5.2. Combined Use of Multiple Decomposition Techniques 2.5.3. Comparative Merits of the Various Sample‐Preparation Techniques 2.5.4. Decomposition Procedures for Determining Nonmetals 2.6. Evaluation Criteria 2.6.1. Completeness 2.6.2. Uncertainty 2.6.3. Time Factors 2.6.4. The Final Result 2.7. Concentration and Separation of Inorganic Trace Materials 2.8. Automation and Direct Analysis 2.8.1. Automation 2.8.2. Direct Analysis 2.9. Analysis of Element Species 3. Sample Preparation in Organic Analysis 3.1. Sample Treatment after the Sampling Process 3.1.1. Stabilization, Drying, and Storage 3.1.2. Homogenization and Aliquoting 3.1.3. Requirements with Respect to Materials and Chemicals 3.2. Separation of the Analyte 3.2.1. Hydrolysis 3.2.2. Liquid ‐ Liquid Extraction 3.2.3. Soxhlet Extraction 3.2.4. Microwave‐Assisted Solvent Extraction 3.2.5. Supercritical Fluid Extraction (SFE) 3.2.6. Solid‐Phase Extraction (SPE) 3.2.7. Solid‐Phase Microextraction (SPME) 3.2.8. Stir‐Bar Adsorptive Extraction (SBSE) 3.2.9. Miscellaneous Techniques 3.3. Headspace Techniques 3.3.1. Static Headspace Technique 3.3.2. Dynamic Headspace Technique (Purge and Trap) 3.4. Determination of Trace Organic Materials in Air Samples 3.5. Analyte Concentration 3.6. Derivatization 3.7. Coupled Techniques Trace analysis is a very relevant and applications‐oriented branch of analytical chemistry. The sample preparation for trace analysis must be custom‐tailored to the problem at hand. Systematic errors can arise by contact with vessel materials, reagents, or the ambient atmosphere, as well as any change in chemical or physical state. In inorganic analysis, sample preparation has to meet the requirements for a substantially trouble‐free determination of the analyte. Digestion of the matrix (microwave digestion, wet digestion, dry digestion techniques) and subsequent careful comparison of several decomposition techniques is therefore an essentially important step. Some separation and concentration techniques of the analytes are liquid –liquid extraction, solid‐phase extraction, special precipitation reactions, and electrolytic deposition. The introduction of laboratory robots should make it possible to incorporate a significant degree of automation into the time‐consuming, labor‐intensive area of sample preparation as well, leading to more efficient, reliable, and reproducible sample work‐up. The goal of sample preparation in organic trace analysis is to isolate the analyte from the sample matrix (e.g., liquid‐liquid extraction, Soxhlet extraction, microwave‐assisted solvent extraction, steam distillation) and then concentrate it and convert it into a form suitable for analysis by the selected method. Separation and concentration of an analyte must often be followed by some type of derivatization. Various coupled sample preparation and determination processes are increasingly utilized in trace organic analysis.
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