SWRHL-91 .GAS ANALYSIS CAPABILITIES OF THE SOUTHWESTERN RADIOLOGICAL HEALTH LABORATORY by Frederick B. Johns and Richard E. Jaquish Technical Services Southwestern Radiological Health Laboratory U. S. Department of Health, Education, and Welfare Public Health Service Environmental Health Service April 1970 ------- SWRHL-91 GAS ANALYSIS CAPABILITIES OF THE SOUTHWESTERN RADIOLOGICAL HEALTH LABORATORY by Frederick B. Johns and Richard E. Jaquish Technical Services Southwestern Radiological Health Laboratory U. S. Department of Health, Education, and Welfare Public Health Service Environmental Health Service Environmental Control Administration Bureau of Radiological Health April 1970 ------- ABSTRACT The Southwestern Radiological Health Laboratory collects and analyzes radionuclides which occur in the gaseous state in the environment. A variety of samplers are used depending upon the type of gas being sampled. Most of the sampling is from the atmosphere, but natural gas samples are also collected and analyzed. The mixtures of gases are separated in the laboratory using low temperature adsorption and gas chromatograph techniques. After separation, the quantity of radioactivity is determined by counting. The gases routinely sampled and analyzed are xenon, krypton, water vapor, carbon dioxide and radon. ------- TABLE OF CONTENTS Page ABSTRACT i LIST OF FIGURES iii LIST OF TABLES iv INTRODUCTION 1 SAMPLING EQUIPMENT 1 A. Freezing Traps 1 B. Molecular Sieve Sampler 3 C. Aerial Grab Sampler 5 D. Cryogenic Sampler 5 E. Radon Sampler 6 F. Natural Gas Sampler 7 SEPARATIONS 8 A. Water and Carbon Dioxide 8 B. Xenon and Krypton 9 C. Radon 11 COUNTING 12 REFERENCE 16 BIBLIOGRAPHY 16 ------- LIST OF FIGURES Figure Page 1. Freezing Trap in Field Installation 3 2. Small Molecular Sieve as Currently Used 4 3. Large Molecular Sieve Configured for Aerial Sampling 4 4. Large Molecular Sieve Configured for Ground Sampling 4 5. Disassembled Cryogenic Sampler 5 6. Cryogenic Sampler Installed in Aircraft 6 7. Pump Used for Integrated Radon Sampling 7 8. Combustion and Collection Apparatus for Natural 8 Gas Analysis 9. Schematic, Natural Gas Separation Apparatus 10 10. Gas Envelope Geiger-Mueller Tube 14 11. Radon Scintillation Cell 15 ------- LIST OF TABLES Table Page I. Radiological Properties of Gases 2 II. Mirimum Detectable Concentrations 13 IV ------- INTRODUCTION The Southwestern Radiological Health Laboratory has the capability of collecting and analyzing samples for radionuclides which occur in the gaseous state in the environment. Radionuclides which are found in the environment and are of significant health importance include tritium (3H), carbon-14 (^C), radon-222 (222Rn), and the isotopes of xenon, krypton and argon. A list of the nuclides of interest and their properties is found in Table 1. Sampling for radioactive gases is usually from the atmosphere but with recent Plowshare projects to stimulate natural gas fields with nuclear explosives, the analysis of radioactive gases in natural gas is of interest. SAMPLING EQUIPMENT Air sampling and analysis procedures are very much interrelated; the purpose of sampling is to collect only the gases of interest, thereby increasing the sensitivity of the method. Based on this criteria, several sampling devices have evolved. A. Freezing Traps - water for tritium analysis This device consists of two glass traps immersed in dry-ice acetone (DIA) slush. Air is passed through at a flow rate of three to six liters per minute. To check on collection efficiency,data are collected at the same time on the humidity in the ambient air. ------- TABLE I RADIOLOGICAL PROPERTIES OF GASES NUCLIDE Tritium 3H Carbon 14C Argon 37Ar 39Ar Krypton 85Kr 87Kr 88Kr Xenon 127Xe BOILING POINT °C HALF-LIFE 0 12.262a -78.6 5730a (as carbon dioxide) -185.7 35. Id 269a 1.83h -152 10.76a 4.4h 76m 2.80h -107 36. 4d ENERGY (MeV) TYPE OF DECAY AND ABUNDANCE 3- 0.0186 MAX 6- 0.156 MAX EC X-rays 3- 0.565 MAX B- 2.49 MAX Y 1.293 (99%) 3- 0.67 MAX Y 0.514(0/41%) 3- (77%) 0.82 MAX Y 0.150 (74%) 0.305 (13%) 3 3.8 MAX Y 0.403 (84%) 0.85 (16%) 2.57 (35%) 3- 2.8 MAX Y 0.166 (7%) 0.191 (35%) 0.56 (5%) 0.85 (23%) 1.55 (14%) 2.19 (18%) 2.4 (35%) EC, Y X-rays, 0.058 (1.4%) 0.145 (4.2%) 0.172 (22%) 0.203 (65%) 133X£ 135Xe Radon 222Rn -61.8 5.27d 9.14h 3.8229d 2 0.375 (20%) 0.346 MAX 0.081 (37%) 3- Y 0.92 0.250 0.61 5.49 MAX (91%) (3%) ------- >^*- Figure 1 - Freezing trap in field installation B. Molecular Sieve Sampler - water for 3H and carbon dioxide for 14C analysis Molecular sieves have the unique property of selectively absorb- ing atoms or molecules based on their diameter. This feature is used in the molecular sieve samplers. Two sampling devices are used. A large sampler containing 2000 grams of 13x molecular sieve operating at a flow rate of 120 liters is used in an air- craft or in semi-permanent installations. A small portable sampler is also used which contains 200 grams of molecular sieve and operates at a flow rate of 15 liters per minute. ------- Figure 2 - Small molecular sieve sampler as currently used. Figure 4 - Large molecular sieve con- figured for ground sampling. Figure 3 - Large molecular sieve sampler configured for aerial sampling ------- C. Aerial Grab Sampler - xenon and krypton analysis This sampling device is installed in the sampling aircraft. Air is pumped for thirty seconds from outside the airplane into a 30-liter plastic bag. The air in the bag is then pumped by means of a compressor into a steel gas bottle that has been previously evacuated. The volume of air is determined by weighing the evacu- ated bottle before and after it has been filled. Approximately 0.3m3 of air is collected as a sample for analysis for xenon and krypton. D. Cryogenic Sampler - xenon and krypton analysis This device utilizes a cannister of molecular sieve 5A immersed in liquid nitrogen at 12 psig (giving a temperature of -182°C) to Figure 5 - Disassembled cryogenic sampler ------- absorb xenon and krypton, although water vapor and carbon dioxide are also quantitatively collected. Air is pumped through the sampler and a dry gas meter measures the sample volume. The initial flow rate of 1250 liters per minute gradually decreases due to the freezing out of the water and carbon dioxide to a flow of 70 liters per minute. Average collection is 1.5 to 4m3. t * Figure 6 - Cryogenic sampler installed in aircraft. E. Radon Sampler - radon-222 analysis Two types of sampler are used to collect radon gas in air: 1. An integrated sampler consists of a small pump, Figure 7, which fills a plastic bag with a volume of 30 liters. The sample, collected over a 24-hour period, includes all gases in air although radon is the nuclide of interest. ------- 2. A grab sample for radon consists of a 2-liter gas bottle through which air is pumped until seven or eight volume changes of air in the bottle occur. This collection is made at atmospheric pressure for five to ten minutes. Figure 7 - Pump used for integrated radon sampling. F- Natural Gas Natural gas is collected in 8-liter steel gas bottles which are evacuated to a pressure of lO'^mm of mercury prior to use. In the field, the bottle is attached to the high pressure gas line. The valve is opened to allow gas to enter the bottle. The pressure is not allowed to exceed 1000 psi. The bottle is emptied and refilled eight times, then, under pressure, is returned to the laboratory. Before analysis, a measured volume of the natural gas sample is ignited with excess oxygen, Figure 8. Water vapor is collected and the remaining products of combustion and noble gases ------- are introduced into the gas analysis apparatus. Figure 8 - Combustion and collection apparatus for natural gas analysis i SEPARATIONS A. Water and Carbon Dioxide The collected gases from the molecular sieve, cryogenic and aerial grab samplers and products of combustion from natural gas are introduced into the separation apparatus, Figure 9, by a combination of heat and helium purge. In all cases, the water vapor is collected in the steel ball trap (Tj at liquid nitrogen (LN) temperature. A 8 ------- portion qf the carbon dioxide, xenon and krypton is also collected in the stjeel ball trap and the remainder on the charcoal in Cj at LN tempera ti^re. When only tritium and carbon-14 analyses are to be conducted!, T: is heated to approximately 30°C to melt the water and vaporize the carbon dioxide. The water is refrozen on the steel balls at -80°C with DIA and the carbon dioxide is transferred to C2. This step is repeated until all of the carbon dioxide is transferred. The water is distilled into a receiver at MI. Oxygen and nitrogen are removed from GI by purging with helium at DIA temperature. The carbon dioxide is desorbed from Ci at 350°C and transferred to T2 of LN temperature with a helium carrier gas. After removing the helium in T2 (by vacuum), the trap is allowed to come to room temperature and the carbon dioxide transferred to Section I for volume measurement and solution in Hydroxide of Hyamine (10 ) (a product of Rohm & Haas * A Chemical Corporation). The water, after distillation, and the carbon dioxide are prepared for liquid scintillation counting as described in Part IV below. B. Xenon and Krypton When analysis for xenon and krypton is to be performed, known volumes of xenon and krypton carrier are added to Tj. and C^ TI is then heated to 30°C and refrozen with DIA and the xenon, krypton and carbon dioxide transferred to Ci. The water is distilled to W^ After the oxygen and nitrogen on the charcoal in Ci are removed by purging with helium at ------- Figure 9 - Schematic, Natural Gas Separation Apparatus .^lomzotion Chamber & Thermal Conductivity yRough Vacuum C, M, C2 Wj - Water Trap M. T! - Steel Ball Trap Ci - Charcoal M! - Molecular Sieve (5A) T2 - Steel Ball Trap C2 - Charcoal Trap T3 - Empty Trap ------- DIA temperature, heat is applied to Ci and the gases of interest are transferred to the molecular sieve column (l^) which is at LN tempera- ture. The temperature of the molecular sieve column is raised to wet ice temperature (0°C) which will elute the krypton more rapidly than the xenon. The thermal conductivity cell and the ionization chamber indicate the elution of the various gases. After krypton has passed through the column to C3, the temperature of the column is raised to boiling water temperature (100°C) to speed the removal of the xenon. The xenon is transferred to T3 and the temperature is raised to 350°C to remove the carbon dioxide to T2. The xenon is then transferred to Section I by heating T3. The recovered volume is determined for yield determination and a portion of the xenon is then transferred to a gas envelope Geiger-Mueller tube for counting. The krypton is transferred to Section I in the same manner prescribed for the xenon. The volume is determined and the gas transferred to a counting cell. The carbon dioxide is transferred to Section I, the volume determined, and a portion dissolved in Hydroxide of Hyamine (10X) for liquid scintillation counting. C. Radon When radon-222 analysis is required from the cryogenic sample, a portion of the xenon is transferred to an alpha scintillation counting cell for 11 ------- radon-222 determination. The radon remains with the xenon fraction. Radon in natural gas is determined by direct transfer of the natural gas to an alpha scintillation counting cell. The cell is then alpha counted. Radon in air is determined by adsorbing the radon in a known volume of air on charcoal and then desorbing with helium into an alpha scintillation counting cell. COUNTING Tritium is determined by liquid scintillation counting. A measured volume of sample water is transferred to a liquid scintillation count- ing cell containing the scintillator cocktail. (1) Since gamma-emitting nuclides interfere with the tritium counting, all water samples are gamma scanned and, when gamma emitters are present, the water is re- distilled with holdback carriers. All samples are counted on the liquid scintillation spectrometer for a minimum of 100 minutes (See Table II). Carbon-14 is also determined by liquid scintillation count- ing. The carbon dioxide is dissolved in 5 ml of Hydroxide of Hyamine (10X) which will dissolve 4.5 millimoles of gas. This solution is then added to a liquid scintillation cocktail and counted in the liquid scintillation spectrometer. 12 ------- TABLE II MINIMUM DETECTABLE CONCENTRATION (MDC) Tritium Carbon-14 Xenon and Krypton Radon AIR 0.4 pCi/ml H20 collected3 0.02 pCi/ml C02 collected 100 pCi/total sample 0.04 pCi/liter NATURAL GAS 1 pCi/liter 20 pCi/ liter 100 pCi/total sample 0.04 pCi/liter a = based on 5 ml sample counted for 100 minutes The radioxenon and -krypton are presently counted in a gas envelope Geiger-Mueller tube, Figure 10, to determine the total beta activity. A portion of the sample is also gamma scanned for isotopic identification and relative concentration. Radon-222 is determined by alpha counting the radon gas in a scintilla- tion counting cell, Figure 11, the interior surface of which is coated with ZnS. The counting cell containing 125 ml of gas is placed on a photomultiplier tube for scintillation counting. The counting time is usually 60 minutes. 13 ------- G. M. TUBE GAS ENVELOPE Figure 10. Gas Envelope Geiger-Mueller Tube 14 ------- 67mm 90mm Corning No. 2 or Equivalent Brass Collar Kovar Metal Clear Silica Window 50 mm Figure 11. Radon Scintillation Cell 15 ------- REFERENCE A. A. Moghissi, H. L. Kelley, J. E. Regnier, M. W. Carter. Low-Level Counting by Liquid Scintillation - I. Tritium Measurement in Homogeneous Systems. International Journal of Applied Radiation and Isotopes, Vol. 20, pp 145-156. Pergamon Press. (1969) BIBLIOGRAPHY Breck, D. W. and Smith, J. V. Molecular Sieves. Scientific American. (January 1959) Linde Molecular Sieves Non-hydrocarbon Materials Data Sheets. Union Carbide Corporation. Momyer, F. F. The Radiochemistry of the Rare Gases. National Academy of Sciences, Nuclear Science Series. 16 ------- |