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

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Presentation on theme: "Sample Collection and Preservation Richard Sheibley Pennsylvania Dept of Env Protection."— 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 HNO 3 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 ( 131 I) 6 Months Tritium Alpha/Beta Radium Gamma 1 – 4 Days ( 222 Rn, 224 Ra)

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 BetaHalf lifeEnergy (MeV) Carbon 145730 yrs0.156 Technetium 992.13X10 5 yrs0.224 Strontium 9029 yrs0.546 Lead 21022.26 yr1.16

24 Gas Proportional Counter AlphaHalf lifeEnergy (MeV) Americium 241432 yr5.443, 5.486 Polonium 210138 days5.304 Thorium 23075,400 yr4.688, 4.621

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 – d efined in 40 CFR 141.25(c) ± 100 % at 95% confidence interval 1.96 σ Where σ = standard deviation of net counting rate of sample

30 Statistics – Counting Error Standard deviation σ = where: R s = sample counting rate R b = background counting rate t s = sample counting time t b = background counting time

31 Statistics – Counting Error Example R s = 2.74 cpm R b = 1.50 cpm t s = 50 min t b = 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 LLD ~ (k α + k β ) σ o k α = false negative k β = false positive σ o = standard deviation of net counting rate of sample

35 Statistics – Detection Limit Generally use 95% Confidence k α = k β = k = 1.645 At the LLD Sample count rate ~ background count rate

36 Statistics – Detection Limit σ o = [σ s 2 + σ b 2 ] 0.5 When R s ~ R b and t s = t b σ s 2 = σ b 2 σ o = [2] 0.5 σ b LLD = 2[2] 0.5 k σ b LLD = 4.66 σ b σ b = [R b /t b ] 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 900.0 What well 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 HNO 3 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)

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52 EPA Method 900.0 Alpha and Beta Gas Proportional Counters

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57 EPA Method 900.0 Calibrations (Spillover Correction or Crosstalk) Alpha beta discriminators should be adjusted to minimize false readings Alphas counted as betas and betas counted as alphas

58 EPA Method 900.0 (Determine System Background) Contribution of the background must be measured Measure under the same conditions, counting mode, and geometry as the samples Count background longer than samples Establish good statistics Background determination is performed every time the P10 gas cylinders are changed

59 EPA Method 900.0 (Determine Efficiency Calibration) Calibrate to obtain relationship of count rate to disintegration rate. Natural uranium and thorium-230 are approved as gross alpha calibration standards for evaporation methods and co-precipitation methods Americium-241 is only approved for the co- precipitation methods. 40CFR part 141.25 Analytical methods for radioactivity. Footnote 11 Strontium-90 and cesium-137 are approved as gross beta calibration standards. Cesium-137 is volatile NIST traceable standards

60 EPA Method 900.0 (Determine Efficiency Calibration)

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62 EPA Method 900.0 Alpha/Beta Self-Absorption Factors Determined by graphing residue weight (mg) vs. the efficiency factor (dpm/cpm) Multiple aliquots Constant alpha and beta activity using calibration standards Varying solids concentration 2-inch diameter counting planchet (20 cm 2 ) 0 and 100 mg for alpha 0 and 200 mg for beta

63 EPA Method 900.0 Alpha Self-Absorption Factors Th-230 Planchet #Solids (g)cpm Decay Corrected CountsEfficiency 10.008772.03375.140.1920 20.009272.83375.140.1941 30.011669.38375.140.1849 40.014364.32375.140.1715 50.018061.32375.140.1635 60.020253.61375.140.1429 70.024150.75375.140.1353 80.026043.36375.140.1156 90.030046.74375.140.1246 100.031644.32375.140.1181 110.033546.00375.140.1226 120.038939.47375.140.1052 130.065927.23375.140.0726 140.083426.34375.140.0702 150.098021.11375.140.0563 160.108717.96375.140.0479 170.121916.39375.140.0437

64 EPA Method 900.0 Alpha Self-Absorption Factors

65 EPA Method 900.0 Quality Control Instrument efficiency check Analyzed daily Control chart Establish action limits Low background check Analyzed daily Control chart Establish action limits Analytical Prep Batch Laboratory Reagent Blank (LRB) Laboratory Fortified Blank (LFB) Sample Duplicates at a 10% frequency Sample Spikes at a 5% frequency Control chart Establish action limits

66 EPA Method 900.0 Interferences Moisture obstructs counting and self–absorption characteristics Non-uniformity of the sample residue in planchet accuracy precision Sample density on the planchet area should not be more than 5 mg/cm 2 (< 100 mg) alpha for gross alpha Sample density on the planchet area should not be more than 10 mg/cm 2 (< 200 mg) for gross beta

67 EPA Method 900.0 Application The National Primary Interim Drinking Water Regulations (NIPDWR) require the following detection limits Gross Alpha 3 pCi/L Gross Beta 4 pCi/L Maximum Contamination Level (MCL) Gross alpha 15 pCi/L >15 pCi/L run uranium determination

68 EPA Method 900.0 Calculations Alpha radioactivity Alpha (pCi/liter) = A * 1000 2.22 * C * V Where: A= net alpha count rate (gross alpha count rate minus the background count rate) at the alpha voltage plateau C= alpha efficiency factor, read from graph of efficiency versus mg (cpm/dpm) V= volume of sample aliquot, (ml) 2.22= conversion factor from dpm/pCi

69 EPA Method 900.0 Calculations Beta radioactivity If there are no significant alpha counts when the sample is counted at the alpha voltage. Beta (pCi/liter) = B * 1000 2.22 * D * V Where: B= net beta count rate (gross beta count rate minus the background count rate) at the beta voltage plateau D= Beta efficiency factor, read from graph of efficiency vs. mg (cpm/dpm) V- volume of sample aliquot, (ml) 2.22= conversion factor from dpm/pCi

70 EPA Method 900.0 Calculations Beta radioactivity Beta counting in the presence of alpha radioactivity. Beta (pCi/liter) = (B – AE)* 1000 2.22 * D * V Where: B= net beta count rate (gross beta count rate minus the background count rate) at the beta voltage plateau A= net alpha count rate (gross alpha count rate minus the background count rate) at the alpha voltage plateau E= alpha amplification factor, read from the graph of the ratio of alpha counted at the beta voltage/alpha counted at the alpha voltage vs. sample density thickness D= Beta efficiency factor, read from graph of efficiency vs. mg (cpm/dpm) V- volume of sample aliquot, (ml) 2.22= conversion factor from dpm/pCi

71 EPA Method 900.0 Calculations A (pCi/L) = (G-B)((SAF*g)+1)/(2.22*E*T*V) Where:A=gross alpha/beta activity in pCi/L B=background counts per minute E=efficiency of detector G=gross counts per minute SAF=alpha/beta self-absorption efficiency factor T=count time V=sample volume, (liters) g=net weight of solids, (grams) 2.22conversion factor, dpm/pCi Alpha and beta radioactivity

72 EPA Method 900.0 Method SOP Main Sections Scope and Application Summary of Method Definitions Regulatory Deviations Interferences Safety Equipment and Supplies Reagents and Standards Calibration and Standardization Procedure Data Analysis and Calculations Method Performance Pollution Prevention Waste Management References Diagrams, Flowcharts, Validation Data


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