sample collection and preservation richard sheibley pennsylvania dept of env protection

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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

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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

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