Method Development for Direct Analysis of Mercury by Thermal Decomposition. David L. Pfeil

Transcription

1 Method Development for Direct Analysis of Mercury by Thermal Decomposition David L. Pfeil 1

2 Webinar Topics How does thermal decomposition work? When does it make sense to use this technique? Applicable methods Instrumentation Concerns about samples Method development strategies Some examples 2

3 Hydra C - Principle of Operation Solid or liquid samples are weighed and introduced in the Hydra C The sample is initially dried and then thermally decomposed in an oxygen flow Combustion products are carried off and further decomposed in a hot catalyst bed Mercury vapors are trapped on a gold amalgamator and subsequently desorbed for quantitation The mercury content is determined using atomic absorption

4 Hydra II C System Schematic Absorption Cells High Sensitivity Low Sensitivity C 600 C Decomposition Furnace Catalyst Furnace Drying Tube Amalgam Furnace Delay Tube O 2 Supply

5 When is Thermal Decomposition Most Useful? The analysis of solids such as: Soils, sediments & sludge Coal & fly ash Fish Food & feeds Plants & tissues Ores Difficult to digest samples Samples with known chemical interferences 5

6 Benefits of Thermal Decomposition Fast turnaround time More universal calibration Interference reduction Better sensitivity for solids No hazardous chemicals used 6

7 Time Required for CVAA Sample Digestion

8 Time Required for Direct Analysis

9 Reference Materials as Standards Sediment Sediments Coal Dogfish Oyster tissue Dogfish High Sensitivity Range Correlation Coef Low Sensitivity Range Correlation Coef

10 Standard Reference Materials SAMPLE No. Certificate Measured Recovery Tissue (ppm) (ppm) (%) Bovine Liver Blood Lypho Lypho Lypho Marine Dogfish Dorm Dogfish Dolt Oyster Tissue Sediments & Soils Coal HC

11 Interference Reduction Because the CVAAS technique requires a chemical reduction and the thermal decomposition technique does not, some interferences can be eliminated. Interference with KI Interference with Au Recovery (%) KI (%) CVAAS Thermal Decomp Recovery (%) Au Conc. (ppm) CVAAS Thermal Decomp 11

12 Detection Limits Comparison Typical published detection limits Classical CVAAS ~1 ng/l CVAFS ~0.1ng/L CVAFS with amalgam <0.05 ng/l Thermal Decomposition ng Detection Limits in concentration for thermal decomposition Divide the mass detection limit by the sample weight For Example: Assume the reported detection limit is 0.002ng and the sample weight is 0.2grams. Detection limit = 0.002ng/0.2g = 0.01ng/g (ppb) 12

13 Apples to Apples Typical published detection limits Classical CVAAS CVAFS CVAFS with amalgam ~1 ng/l or ~0.001ng/g (ppb) ~0.1ng/L or ~0.0005ng/g <0.05 ng/l - For solid samples dilution with required digestion is not taken into account. - Often X dilution is required and the method detection limit will be higher For Example: Assume 2.0g of sample is digested to a final volume of 100mls (or ~100g). CVAAS Detection limit = (0.001ng/g)(100mls) /(2g) = 0.05ng/g (ppb) Compare with 0.01ng/g by thermal decomposition for 0.2g sample 13

14 Chemicals Needed for CVAAS Acids Nitric Hydrochloric Sulfuric Oxidizers Potassium permanganate Potassium persulfate Reductants Hydroxylamine Stannous chloride Sodium borohydride Personnel safety Gloves Eye protection Ventilation Expense Purchase price Disposal costs Contamination Extra sample handling Reagent purity 14

15 Applicable Methods USEPA 7473 Mercury In Solids and Solutions by Thermal Decomposition, Amalgamation, and Atomic Absorption Spectrophotometry ASTM D6722 Standard Test Method for Total Mercury in Coal and Coal Combustion Residues by Direct Combustion Analysis 15

16 Instrumentation Hydra II C Modular Design Furnace Spectrometer Autosampler 16

17 Hydra II C - AA Spectrometer Folding Mirrors Reference Cell High Sensitivity Cell Low Sensitivity Cell Lamp Module

18 Low Concentrations Measurement of real samples Contamination can be significant Boats Analyst Environment Sample homogeneity 18

19 Dryer / Amalgamator Module Gold Amalgamation Trap Nafion Dryer Interface to Furnace Interface to Control Board

20 High Concentrations Long (2 x5 ) & Short Cell (1 ) come standard High Concentration option is unique 1,500ng max or 25,000ng max

21 Catalyst Sample boat enters empty part of catalyst tube First section captures catalyst poisons such as sulfur and halogens Second section provides oxygen to assist combustion Third section traps partially oxidized residuals 21

22 Sample Concerns Homogeneity Is the aliquot measured representative of the sample? Can I get acceptable precision? How much sample is needed? Flash point, if combustible We are placing fuel & oxygen in a sealed vessel Volatility Will evaporation on the sampler affect accuracy? Concentration High or low concentrations may not fall in the analytical range Moisture Samples must be dried before the decomposition step Physical condition of sample Is the sample contained in the boat 22

23 Do we need to perform sample preparation? You might want to grind your samples to get a more uniform consistency For something like soils or plant material a simple coffee grinder might suffice Cost ~$20 23

24 Combustible materials What is combustible In an oxygen atmosphere materials are more combustible than you might at first imagine Start small Control the amount of fuel added to the furnace Explosive conditions can exist when fuel and oxidant are present in the right ratio If you know the sample is combustible, try to keep the mass under 20ugs Use multiple deposits to increase the signal, if necessary 24

25 Multiple Injections To deposit more volatile sample mass multiple injections are typically offered in the software. Less mass of a sample is placed in each boat With each sample boat injection only the dry and combustion phases are carried out Only after the last injection the mercury is eluted and measured Concentration is calculated based on the total mass injected in all the boats 25

26 Example of Multiple Deposits Total deposit = 1.077gm 26

27 What is volatile? If the mercury species is volatile (or reactive), then results will be low if left on the sampler. Some acidic solutions can react with nickel boats, reducing mercury. If a solvent is employed with the sample but the mercury species is stable, loading the sampler should be acceptable. If unsure, run a sample directly after loading and repeat at various time intervals checking precision 27

28 Sample concentrations out of range Low concentrations Inject more sample Use multiple deposits to increase mass of mercury loaded High concentrations Inject less sample Analytical error during weighing Homogeneity concerns Dilution is difficult in solids Suitable blank material available? Digest the sample Liquid samples are easy to dilute 28

29 Moisture Control for Aqueous Samples As a rule of thumb allow 0.7 seconds of dry time for every microliter of solution. With a sample capacity of 1.4ml the dry time can be quite tedious 1400 x 0.7 = 980 seconds ( or >16 minutes) On the other hand solid samples may require little or no dry time 29

30 Physical limitations Difficult to contain samples types can be wrapped in a suitable material such as aluminum foil Hair Feathers Glass wool Sticky stuff needs to be transported carefully Previously frozen fish tissues 30

31 Method Development Strategies View from 30,000 feet Characterize your samples Determine sufficient dry time Run a small sample aliquot Estimate sample mass needed for mid-range concentration Evaluate response characteristics Determine precision Adjust decomposition values, if necessary Determine accuracy 31

32 The Pyrolysis Phase After the sample is dry, furnace temperature increases for sample pyrolysis or combustion. To satisfactorily complete this step sufficient oxygen must be present. Organic compounds require more oxygen More mass deposited requires more oxygen Multiple deposits may help reach complete oxidation Determination of Hg in rice is a good example 32

33 Coal & Fly Ash Coal and fly ash are typically two of the easier matrices to analyze by thermal decomposition; however, some coals high in sulfur may shorten catalyst lifetimes. Coal Phase Temperature ( C) Time (s) Drying Decomposition Catalyst 600 Wait Time 60 Amalgamator Notes: If moisture is high extend Drying Time Use nickel boats Calibrate on aqueous or matrix matched standards 33

34 Plant tissue Plant material may be best handled by pre-desiccation and grinding to a consistent particle size; however, satisfactory results often may be obtained without any pretreatment. Plants Phase Temperature ( C) Time (s) Drying *mgs Decomposition Catalyst 600 Wait Time 60 Amalgamator Notes: Set dry time to 0.7 sec/mg sample weight if not desiccated. Use nickel boats Calibrate on aqueous or matrix matched standards 34

35 Soil Soil can contain plant roots and large rocks. It may make sense to sieve and/or grind the soils to exclude these materials. Soil Phase Temperature ( C) Time (s) Drying Decomposition Catalyst Temperature 600 Wait Time 60 Amalgamator

36 Soil Sample Untreated After Grinding 36

37 Fish Fish is recognized as the primary source of mercury adsorption in humans. The FDA has a recommended concentration limit for Hg at 1ppm. The limit in much of Europe is set at 0.5ppm Fish Phase Temperature ( C) Time (s) Drying Decomposition Catalyst Temperature 600 Wait Time 60 Amalgamator

38 Rice Many grains, such as rice, are dietary staples and while routinely low in Hg may deserve monitoring. Rice Phase Temperature ( C) Time (s) Drying Decomposition Catalyst Temperature 600 Wait Time 60 Amalgamator Note-limited mass required ( 50mg) 38

39 Oils We have looked at a variety of oils Used engine oils, Vegetable oil Marine oil The decomposition phase is hotter & longer than usual to prevent incomplete decomposition. Oils Phase Temperature ( C) Time (s) Drying Decomposition Catalyst Temperature 600 Wait Time 60 Amalgamator

40 Hair Hair can be an excellent indicator of past exposure to mercury. Clean sample before clipping Wrap in tin foil to control positioning Hair Phase Temperature ( C) Time (s) Drying Decomposition Catalyst Temperature 600 Wait Time 60 Amalgamator

41 Blood & Urine Urine can be used to determine recent exposure to inorganic mercury. Blood can be used to determine recent exposure to organic mercury Fluid Phase Temperature ( C) Time (s) Drying Decomposition Catalyst Temperature 600 Wait Time 60 Amalgamator

42 Waters Waters can be determined by thermal decomposition but: Slow because of moisture removal Less sensitive than classical CVAAS Water Phase Temperature ( C) Time (s) Drying 300 TBD Decomposition Catalyst Temperature 600 Wait Time 60 Amalgamator

43 A Single Platform AA C All run with Envoy 2.0 Software AF AFGold 43

44 Sodium Hydroxide Solution Sodium hydroxide solutions unstable in the sample boat. Acidified solutions are stable. In our experiments we have used 6N HCl NaOH Phase Temperature ( C) Time (s) Drying 300 TBD Decomposition Catalyst Temperature 600 Wait Time 60 Amalgamator

45 Gypsum Synthetic gypsum used in the production of wallboard a byproduct of many coal-fired power plants using flue gas desulfurization (FGD). As coal plant emissions become cleaner, concern is increasing in byproducts, like gypsum. Gypsum Phase Temperature ( C) Time (s) Drying Decomposition Catalyst Temperature 600 Wait Time 60 Amalgamator

46 Gypsum from one Power Plant A mean concentration of 170ng/g Hg 7.5 million tons of gypsum # is used in the production of wallboard About 2,500lbs of mercury re-introduced into the environment. Hg/yr = (0.170x10-6 lbs/lbs)(7.5x10 6 ton/yr)(2000lbs/ton) =2,550lbs Hg/yr # Based on ACAA 2006 Coal Combustion Product (CCP) Production and Use Survey 7,579,187 short tons used in wallboard production 46

47 In Summary Very short turn-around time for results. Greener technique with no digestion chemicals or waste products. Generic calibration for multiple matrices. Wide dynamic range but dilution is impractical. Multiple deposits for lower concentrations. Sensitivity better than classical CVAAS for solids, not quite as good for liquids. Combo systems 47

48 Questions????? 48