Sample Preparation for Volatile Compounds (VOCs) Sample Preparation for VOCs Organic compounds; P ≥ 0.1 mmHg at 20°C ~ 8% of total samples GC analyses Petroleum, petrochemical, food, flavor and fragrances, and environmental fields Collection/Transport Introduction w/o treatment Preparation/Introduction Measurement Overall accuracy Reliable result Typical Sampling and Sample Preparation Method for VOCs Sample preparation method Principle of technique Comments Grab sampling Gaseous sample is pulled or pumped into an evacuated metal bulb, canister, plastic bag, or syringe Mostly for VOCs in air, samples are returned to laboratory, analytes are isolated and concentrated by cold trapping techniques Solid-phase trapping (SPE) Gaseous sample is passed an adsorbent tube such as silica gel or activated carbon; trapped analytes are eluted with strong solvent For semivolatile organic compounds in air. Popular sorbents include silica gel, alumina, porous polymer (Tenax, PUF) and carbon Liquid trapping (Impinging) Gaseous sample is bubbled through a solution or solvent for which the analytes have a higher affinity Flow rate may cause foaming or aerosols Typical Sampling and Sample Preparation Method for VOCs Sample preparation Principle of technique method Comments Headspace sampling A solid or liquid sample is placed in a closed glass vial until equilibrium. Analytes partition themselves between a gas phase and a solid or liquid phase; gas phase is sampled and injected into a GC For determining trace concentrations of VOCs in samples that are difficult to handle by conventional GC. Increasing temperature, salting out, adjusting pH, would shift equilibrium of analytes from the matrix Purge & trap (dynamic headspace) A solid or liquid sample is placed in a closed container, VOCs are continually purged by an inert gas and subsequently trapped by SPE sorbent and then thermal desorbed into GC (Thermal desorption) For determining trace concentrations of VOCs in samples and for analytes that have unfavorable partition coefficient in static headspace sampling Typical Sampling and Sample Preparation Method for VOCs Sample preparation method Principle of technique Comments Thermal desorption Used with purge & trap and SPME to concentrate VOCs; sorbent is rapidly heated and analytes are transferred to a GC Typical sorbents include Tanex TA, glass beads, and Carbosieve, Carboxen, and Carbotrap Pyrolysis Nonvolatile large molecule samples such as polymers and plant fibers are thermally degraded to cleave linkages and produce smaller, more volatile molecules that are swept to GC Degradation have defined mechanisms and sample may break apart in a predictable manner providing structural info and fingerprint profiles about starting compound SPME Already discussed Already discussed Tedlar Air & Gas Sampling Bags Impringer Canister Headspacce Sampling Static Headspace (Equilibrium Headspace) – The sample, placed in a closed container may be in contact and in equilibrium with the extracting gas Dynamic Headspace (Purge & Trap) – The volatile compounds may be stripped off in a continuous flow of an inert gas ¾ Ideal for dirty samples, solid materials, samples with high boiling point analytes of no interest, samples with high water content, and samples that are difficult to handle by conventional GC Static Headspace GC Basic of Static Headspace Cg, Vg Partition Coefficient (K) = Cs/Cg Cs, Vs Phase Ratio (β) = Vg/Vs Cs=concentration of analyte in sample phase Cg=concentration of analyte in gas phase Vs=volume of sample phase Vg=volume of gas phase g Co C = K+β 9K and β are important variables in headspace analysis. K-Value Air-Water System (40oC) Compound Cyclohexane n-Hexane Tetrachloroethylene Chloroform o-Xylene Toluene Benzene Dichloromethane n-butyl acetate Ethyl acetate Methyl ethyl ketone n-Butanol Isopropanol Ethanol 1,3-Dioxane K Value 0.077 0.14 1.48 1.65 2.44 2.82 2.90 5.65 31.4 62.4 139.5 647 825 1355 1618 Boiling Point (oC) 81 69 121 61.1 145 111 80 40 126 77 79.6 117.7 82.4 78.3 106 Optimizing Static Headspace Extraction efficiency Sensitivity Quantitation Reproducibility Vial/sample volume (β) Temperature Pressure Matrix Partition Coefficient (K) Maximize the concentration of the volatile components Cg in the headspace Lower K by changing the temperature at which the vial is equilibrated or by changing the composition of the sample matrix. Cg Cg K β Headspace sensitivity 1. 2. 3. 4. 5. EtOH Methyl ethyl ketone Toluene N-hexane Tetrachloroethylene High K -- Temp B. Kolb, L.S. Ettre, “Static Headspace- Gas Chromatography: Theory and Practice,” Wiley-VCH, New York. 1997. Headspace sensitivity 1. 2. Cyclohexane 1,4 dioxane β = 21.3 β= 3.46 With salt β = 3.46 Low K -- β B. Kolb, L.S. Ettre, “Static Headspace- Gas Chromatography: Theory and Practice,” Wiley-VCH, New York. 1997. Static Headspace Sampling Gas tight syringe Autosampler – Balance-pressure system – Pressure-loop system Tekmar 7000HT Static Headspace autosampler Gas Tight Syringe Step 1 Sample reaches equilibrium Step 2 Sample is extracted from headspace Step 3 Sample is injected Gas Tight Syringe Needle Point Style #2 22/20-degree beveled needle point recommended for septum penetration. #3 90-degree needle point for use with HPLC injection valves and for sample pipetting #5 Conical needle with side port for penetration of septa. Gas Tight Syringe Advantage – Simplicity Disadvantage – The loss of the substances – No reproducibility Autosampler - Balance Pressure System Sample reaches equilibrium Pressurization of injection Sample is extracted and injected Autosampler - Pressure-Loop System Inlet 1 Step 3 2 Sample reaches is extracted equilibrium/pressurization from injected headspace Loop To Column Dynamic Headspace Continuous gas extraction Volatilized (purged) analytes can be trapped by an adsorbent or cryogenic trapping For substances which are too low in concentration or have unfavorable partition coefficients for their determination by static headspace Purge and Trap Purging gas is bubbled below the surface of a liquid sample using a fritted orifice to produce finely dispersed bubbles The VOCs are transferred from the aqueous phase to the vapor phase The gas flow sweeps the vapor through trap containing adsorbent materials which retain the VOCs The retained VOCs are thermally desorbed and analyzed by GC Schematic of Purge & Trap To GC column Heat Carrier gas Purge gas Valve Trap Vent HP-7675A Purge and Trap System Tekmar 3100 Purge and Trap Sample Concentrator 2016/2032 Purge and Trap Autosampler Purge and Trap Glassware Trapping Adsorbent resins – Purge & Trap – Direct sampling Sufficient capacity – Breakthrough volume – Bed volume – Flow rate Affinity of resin for water Back pressure Trap Polymers – Tenax – Polystyrene – Polyurethane foams Carbon – Graphitized carbon black – Charcoal – Carbon sieves Silica gel Alumina Tenax TA – 2,6-diphenylene oxide Tenax GR – Tenax TA + 30% graphite Carbotrap, Carbotrap C – Graphitized carbon blacks Carboxen 569, Carbosieve SIII – Carbon molecular sieves Glass Beads Tenax TA A porous polymer resin based on 2,6-diphenylene oxide High temperature limit of 350 oC – Tenax degrades if react with O2 at high temperature – abundance of phenolic compounds and oligomers Low affinity for water – Useful for high moisture content samples including the analysis of volatile organic compounds in water Selection of Adsorbents Types of analytes The physical properties of the adsorbent Breakthrough info. John J. Manura, Manura Selection and Use Of Adsorbent Resins For Purge and Trap Thermal Desorption Applications Scientific Instrument Services, Inc. http://www.sisweb.com/referenc/applnote/app-32.htm Scientific Instrument Services, Inc. http://www.sisweb.com Breakthrough The breakthrough volume for a compound on a given adsorbent and at a given temperature is defined as the calculated volume of carrier gas per gram of adsorbent resin which causes the analyte molecules to migrate from the front of the adsorbent bed to the back of the adsorbent bed. ( t R × F) = (L/g) VB mA tR = Retention time (min) F = Flow rate (L/min) mA = Adsorbent mass (g) Breakthrough Scientific Instrument Services, Inc. Calculation of Breakthrough Volume tR VB ( t R × F) − DV = mA VS = VB* 0.5 VF = VB* 2 VS VF Breakthrough Volumes of Alcohols (C1 – C11) on Tenax TA 0.4 5 20 120 Breakthrough Volumes of Alcohols by Tenax TA Desorption Chart Breakthrough Data Hydrocarbons Alcohols Alkenes Alcohols & Glycols Acetates Acids Aldehydes Ketones Halogens Amines Aromatics and Terpenes Water Scientific Instrument Services, Inc. http://www.sisweb.com Thermal Desorption (TD) Thermally desorb analytes from the adsorbents (P&T, direct sampling) Direct Thermal extraction – Volatiles from solid samples Thermal desorption – To GC – Transfer line – Focusing – Improved resolution Thermal Desorption/Extraction 0.4 mm i.d. Glass wool plug Solid sample Adsorbent resin Temp. ~300 oC 10 cm Glass wool plug Direct Thermal Extraction Temp. < 200 oC Thermal Desorption Short Path Thermal Desorption Tempearture/Time Desorption temperature – Enough to volatilize the organic compound without degrading them and without producing unwanted artifacts – Temperature rate Desorption time – Sample matrix – Sample size – Interaction strength between analyte and the solid surface – Desorption temperature – Diffusion time of analyte out of the sample Reduce Band Broadening: Focusing PTV Retention Gap Cryogenic focusing Cryo-Trapping Scientific Instrument Services, Inc. Cryo-focusing Cryo-focusing is a technique for introducing samples from purge-andtrap concentrators into capillary GC columns It enables samples desorbed from adsorbent traps to be introduced into narrow-bore columns without losing any resolution of the column Single Stage Thermal Desorption GC Detector Sample Tube GC Analytical Column Two-Stage Thermal Desorption Split Carrier Inlet Cold Trap Hot Sample Tube Desorb Flow Split GC Detector Hot Trap Carrier Inlet Carrier Inlet GC Analytical Column Cryo-Trapping Scientific Instrument Services, Inc. Thermal Desorption Analysis without the use of solvent – Analysis of 100% instead of aliquot – Elimination of solvent peak in the chromatogram enables the analysis of early eluted volatile analytes that are not masked by the solvent – Elimination of solvent reduction, no evaporation of solvent to the environment, and no waste Applications VOCs from water/soil (EPA Method 502.1, 503.1, 8030A, SW-846 Method 5030A) VOCs from biological fluids (urine, plasma, saliva, tissues) Fragrances, flavors Forensic investigation (arson accelerants) Pyrolysis Next stage in thermal extraction technique Bond dissociation at very high temperatures (600-800 oC) and break apart into smaller and simpler volatile molecules in a predictable manner By measuring the fragments, the molecular composition of the original sample can be reconstructed Polymer defects, variations, and degradation mechanisms Pyrolysis a H b H As sample is heated, the weaker bond breaks first forming free radicals C C If C-C bonds are the weakest, the polymer will break into oligomeric fragments including monomers d H Cl eH Polyethylene Polyvinyl chloride Polystyrene If the side group bond is weaker, the group is removed from the chain before fragmented, so the monomeric identity is lost The materials can be heated to a relatively low temperature for desorption of intact small molecules including solvents, excess reagents, residual monomers, and additive such as plasticizer Pyrolysis - Applications Synthetic polymers (Polyvinyl chloride, Polystyrene, Polyester) Natural polymers – Plant fibers (Cellulose, Cotton) – Animal fibers (Wool, Silk) Dried paints Cosmetic samples