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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-microgramPlastic or GlassGlass Only – tritium
Sample Collection & Preservation
Preservation HNO3
HCl Done by laboratory
Within 5 days Hold 16 hours
None – tritium and iodine
Sample Collection & Preservation
Holding time – Related to half life 8 Days (131I) 6 Months
Tritium Alpha/Beta Radium Gamma
1 – 4 Days (222Rn, 224Ra)
Instrumentation & Methods: Gas Proportional Counters
Richard Sheibley
Pennsylvania Dept of Env Protection
Instrumentation – Detectors
Gas proportional Zinc sulfide (ZnS) scintillation Liquid scintillation Surface barrier Lithium drifted germanium
(GeLi) High purity, germanium (HPGe)
Instrumentation – Shielding
Low level measurement Decrease background Protect from environment Lead Steel Copper
Radioactivity Decay Review
Alpha Particles Beta Particles Photons
Alpha
Particle Heavy – helium nucleus Highly charged
Beta
Particle Light – electron Moderately charged
Gamma
Wave No mass No charge Photon – like light but higher energy
Gas Proportional Counter
Alpha particles Beta particles Photons (gamma)
Optional detector
Gas Proportional Counter
Ion Pair formation Voltage Pulse Proportional response
Gas Proportional Counter
Components Sample changer High voltage power supply Detector Preamplifier Amplifier Scaler Timer Data collection & output device
Gas Proportional Counter
Two Detector System
Sample
Guard
Gas Proportional Counter
Sample Detector Windowless
Sample inside counting chamber
Thin Window Particle must penetrate window
Gas Proportional Counter
Guard Detector Anti-coincidence Cosmic radiation Background
Gas Proportional Counter
Instrument Performance verification Plateau Instrument Background Alpha Efficiency Beta Efficiency
Gas Proportional Counter
Plateau Operating voltage Consistent count rate Alpha Plateau Beta Plateau “Knee”
Gas Proportional Counter
Instrument Background Cosmic radiation Electronic noise Natural radiation Alpha Beta Background Subtraction
Gas Proportional Counter
Instrument Efficiency Counts / disintegrations Detector area Geometry Particle energy
Gas Proportional Counter
Beta Half life Energy (MeV)
Carbon 14 5730 yrs 0.156
Technetium 99 2.13X105 yrs 0.224
Strontium 90 29 yrs 0.546
Lead 210 22.26 yr 1.16
Gas Proportional Counter
Alpha Half life Energy (MeV)
Americium 241 432 yr 5.443, 5.486
Polonium 210 138 days 5.304
Thorium 230 75,400 yr 4.688, 4.621
Gas Proportional Counter
Method QC Reagent Background Efficiency
Method Self adsorption
Alpha Beta
Gas Proportional Counter
Sample count rate factors Distance to detector Window absorption Self absorption
Statistics
Poisson Statistics Random Chi-square test Standard deviation
Statistics
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
Statistics – Counting Error
Standard deviationσ = where:Rs = sample counting rateRb = background counting ratets = sample counting timetb = background counting time
b
b
s
s
t
R
t
R
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
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
Statistics – Detection Limit
Statistics – Detection Limit
LLD ~ (kα + kβ) σ o
kα = false negative kβ = false positive σ o = standard deviation of net
counting rate of sample
Statistics – Detection Limit
Generally use 95% Confidence
kα = kβ = k = 1.645
At the LLD Sample count rate ~
background count rate
Statistics – Detection Limit
σ o = [σ s2 + σ b
2]0.5
When Rs ~ Rb and ts = tb
σ s2 = 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 = [Rb/tb] 0.5
Statistics – Detection Limit
Time Volume Efficiency Self absorption Background
Gas Proportional Counter
Counting interval Time versus performance Preset time Preset count
Detection limit Counting error
Instrumentation & Methods: Gross alpha & beta
Jeff Brenner
Minnesota Department of Health
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
++
-
-
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
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
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
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
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
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
EPA Method 900.0 Alpha and Beta Procedure
EPA Method 900.0 Calibrations (Determine Operating Voltage)
Calibration Order Plateau Spillover Correction or Crosstalk Background Efficiency Sample Self Absorption or Mass Attenuation
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
EPA Method 900.0 Calibrations (Determine Operating Voltage)
EPA Method 900.0 Calibrations (Determine Operating Voltage)
EPA Method 900.0 Alpha and Beta Gas Proportional Counters
EPA Method 900.0 Alpha and Beta Gas Proportional Counters
EPA Method 900.0 Alpha and Beta Gas Proportional Counters
EPA Method 900.0 Alpha and Beta Gas Proportional Counters
EPA Method 900.0 Alpha and Beta Gas Proportional Counters
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
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
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
EPA Method 900.0(Determine Efficiency Calibration)
EPA Method 900.0(Determine Efficiency Calibration)
EPA Method 900.0Alpha/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 cm2) 0 and 100 mg for alpha 0 and 200 mg for beta
EPA Method 900.0Alpha Self-Absorption Factors
Th-230
Planchet # Solids (g) cpmDecay Corrected
Counts Efficiency
1 0.0087 72.03 375.14 0.1920
2 0.0092 72.83 375.14 0.1941
3 0.0116 69.38 375.14 0.1849
4 0.0143 64.32 375.14 0.1715
5 0.0180 61.32 375.14 0.1635
6 0.0202 53.61 375.14 0.1429
7 0.0241 50.75 375.14 0.1353
8 0.0260 43.36 375.14 0.1156
9 0.0300 46.74 375.14 0.1246
10 0.0316 44.32 375.14 0.1181
11 0.0335 46.00 375.14 0.1226
12 0.0389 39.47 375.14 0.1052
13 0.0659 27.23 375.14 0.0726
14 0.0834 26.34 375.14 0.0702
15 0.0980 21.11 375.14 0.0563
16 0.1087 17.96 375.14 0.0479
17 0.1219 16.39 375.14 0.0437
EPA Method 900.0Alpha Self-Absorption Factors
Self- Absorption Curve
0.0000
0.0500
0.1000
0.1500
0.2000
0.2500
0.0000 0.0200 0.0400 0.0600 0.0800 0.1000 0.1200 0.1400
S olids (grams)
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
EPA Method 900.0Interferences 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/cm2 (< 100 mg) alpha for gross alpha
Sample density on the planchet area should not be more than 10 mg/cm2 (< 200 mg) for gross beta
EPA Method 900.0Application
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
EPA Method 900.0Calculations
Alpha radioactivity Alpha (pCi/liter) = A * 1000
2.22 * C * VWhere:
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
EPA Method 900.0Calculations
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 * VWhere:
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
EPA Method 900.0Calculations
Beta radioactivity Beta counting in the presence of alpha radioactivity.
Beta (pCi/liter) = (B – AE)* 1000
2.22 * D * VWhere:
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
EPA Method 900.0Calculations
A (pCi/L) = (G-B)((SAF*g)+1)/(2.22*E*T*V)
Where: A = gross alpha/beta activity in pCi/LB = background counts per minuteE = efficiency of detectorG = gross counts per minute
SAF = alpha/beta self-absorption efficiency factor
T = count timeV = sample volume, (liters)g = net weight of solids, (grams)2.22 conversion factor, dpm/pCi
Alpha and beta radioactivity
EPA Method 900.0Method 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