1 / 72

Sample Collection and Preservation

Sample Collection and Preservation. Richard Sheibley Pennsylvania Dept of Env Protection. Sample Collection & Preservation. Entry Point Representative Composite Total Activity. Sample Collection & Preservation. Containers Sub-microgram Plastic or Glass Glass Only – tritium.

cyma
Download Presentation

Sample Collection and Preservation

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Sample Collection and Preservation Richard Sheibley Pennsylvania Dept of Env Protection

  2. Sample Collection & Preservation • Entry Point • Representative • Composite • Total Activity

  3. Sample Collection & Preservation • Containers • Sub-microgram • Plastic or Glass • Glass Only – tritium

  4. Sample Collection & Preservation • Preservation • HNO3 • HCl • Done by laboratory • Within 5 days • Hold 16 hours • None – tritium and iodine

  5. Sample Collection & Preservation • Holding time – Related to half life • 8 Days (131I) • 6 Months • Tritium • Alpha/Beta • Radium • Gamma • 1 – 4 Days (222Rn, 224Ra)

  6. Instrumentation & Methods: Gas Proportional Counters Richard Sheibley Pennsylvania Dept of Env Protection

  7. Instrumentation – Detectors • Gas proportional • Zinc sulfide (ZnS) scintillation • Liquid scintillation • Surface barrier • Lithium drifted germanium (GeLi) • High purity, germanium (HPGe)

  8. Instrumentation – Shielding • Low level measurement • Decrease background • Protect from environment • Lead • Steel • Copper

  9. Radioactivity Decay Review • Alpha Particles • Beta Particles • Photons

  10. Alpha • Particle • Heavy – helium nucleus • Highly charged

  11. Beta • Particle • Light – electron • Moderately charged

  12. Gamma • Wave • No mass • No charge • Photon – like light but higher energy

  13. Gas Proportional Counter • Alpha particles • Beta particles • Photons (gamma) • Optional detector

  14. Gas Proportional Counter • Ion Pair formation • Voltage Pulse • Proportional response

  15. Gas Proportional Counter • Components • Sample changer • High voltage power supply • Detector • Preamplifier • Amplifier • Scaler • Timer • Data collection & output device

  16. Gas Proportional Counter • Two Detector System • Sample • Guard

  17. Gas Proportional Counter • Sample Detector • Windowless • Sample inside counting chamber • Thin Window • Particle must penetrate window

  18. Gas Proportional Counter • Guard Detector • Anti-coincidence • Cosmic radiation • Background

  19. Gas Proportional Counter • Instrument Performance verification • Plateau • Instrument Background • Alpha Efficiency • Beta Efficiency

  20. Gas Proportional Counter • Plateau • Operating voltage • Consistent count rate • Alpha Plateau • Beta Plateau • “Knee”

  21. Gas Proportional Counter • Instrument Background • Cosmic radiation • Electronic noise • Natural radiation • Alpha • Beta • Background Subtraction

  22. Gas Proportional Counter • Instrument Efficiency • Counts / disintegrations • Detector area • Geometry • Particle energy

  23. Gas Proportional Counter

  24. Gas Proportional Counter

  25. Gas Proportional Counter • Method QC • Reagent Background • Efficiency • Method • Self adsorption • Alpha • Beta

  26. Gas Proportional Counter • Sample count rate factors • Distance to detector • Window absorption • Self absorption

  27. Statistics • Poisson Statistics • Random • Chi-square test • Standard deviation

  28. Statistics

  29. Statistics – Counting Error • Drinking water – defined in 40 CFR 141.25(c) • ± 100 % at 95% confidence interval • 1.96σ • Where σ = standard deviation of net counting rate of sample

  30. Statistics – Counting Error • Standard deviation σ =  where: Rs = sample counting rate Rb = background counting rate ts = sample counting time tb = background counting time

  31. Statistics – Counting Error Example Rs = 2.74 cpm Rb = 1.50 cpm ts = 50 min tb = 50 min C.E. = 1.96 [2.74/50 + 1.5/50]0.5

  32. Statistics – Counting Error Example C.E. = 1.96 [2.74/50 + 1.5/50]0.5 C.E. = 1.96 [0.055 + 0.030]0.5 C.E. = 1.96 [0.085]0.5 C.E. = 0.80 cpm Result = 2.74 ± 0.80 cpm

  33. Statistics – Detection Limit

  34. Statistics – Detection Limit • LLD ~ (kα + kβ) σ o • kα = false negative • kβ = false positive • σ o = standard deviation of net counting rate of sample

  35. Statistics – Detection Limit • Generally use 95% Confidence • kα = kβ= k = 1.645 At the LLD • Sample count rate ~ background count rate

  36. Statistics – Detection Limit • σ o = [σ s2 + σ b2]0.5 • When Rs ~ Rb and ts = tb • σ s2 = σ b2 • σ o = [2]0.5σ b • LLD = 2[2]0.5 k σ b • LLD = 4.66 σ b • σ b = [Rb/tb] 0.5

  37. Statistics – Detection Limit • Time • Volume • Efficiency • Self absorption • Background

  38. Gas Proportional Counter • Counting interval • Time versus performance • Preset time • Preset count • Detection limit • Counting error

  39. Instrumentation & Methods: Gross alpha & beta Jeff Brenner Minnesota Department of Health

  40. EPA Method 900.0 • Prescribed Procedures for Measurement of Radioactivity in Drinking Water • EPA-600/4-80-032 August 1980 • Determination of Gross Alpha and Gross Beta Radioactivity in Drinking Water + + - • -

  41. EPA Method 900.0What we’ll cover • Scope of the method • Summary of the method • Calibration • Determining operating voltage • Determining system background • Determining efficiency calibration • Determining self-absorption factor • Quality control • Interferences • Application • Calculations • Activity

  42. EPA Method 900.0 Scope • The method is a screening technique for monitoring drinking water supplies • The solids are not separated from the sample • Solids concentration is a limiting factor in the sensitivity of the method

  43. EPA Method 900.0 Alpha and Beta Procedure Summary • Sample is preserved in the field or at the lab with nitric acid • Lab preservation • Within 5 days of collection • Hold for 16 hours after acidification • Homogeneous aliquot of preserved sample • Typically 250 mL or less

  44. EPA Method 900.0 Alpha and Beta Procedure Summary • Sample is evaporated to near dryness • If sample is evaporated to dryness in the beaker, re-start sample analysis • Add 10 ml 1N HNO3 to beaker to dissolve solids • Additional nitric acid is added to convert chloride salts to nitrate salts • Chloride salts attack the stainless steel planchet

  45. EPA Method 900.0 Alpha and Beta Procedure • Sample is quantitatively transferred to a tared planchet • Sample is reduced to dryness on planchet • Sample residue is dried to constant weight • Analyzed for beta emissions

  46. EPA Method 900.0 Alpha and Beta Procedure • Planchet is flamed and stored for 3 days to allow for the ingrowth • Flaming converts hygroscopic nitrate salts to oxides • Ingrowth for progeny of Ra-226 • Sample residue is reweighed to determine flamed residue weight • Analyzed for alpha emissions

  47. EPA Method 900.0 Alpha and Beta Procedure

  48. EPA Method 900.0 Calibrations (Determine Operating Voltage) • Calibration Order • Plateau • Spillover Correction or Crosstalk • Background • Efficiency • Sample Self Absorption or Mass Attenuation

  49. EPA Method 900.0 Calibrations (Determine Operating Voltage) • Determine appropriate (knee) operating voltage • alpha beta plateau • A plateau is generated by counting a source several times while increasing (stepping) the high voltage to the detector. • Alpha plateau = alpha activity • Beta plateau = alpha/beta activity • Generate an alpha/beta plateau after every P10 gas exchange • Quality of the gas affects the plateaus and instrument performance

  50. EPA Method 900.0 Calibrations (Determine Operating Voltage)

More Related