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Chapter 10 Inorganic applications
CHAPTER 10
Inorganic applications
Over 50 isotopes may be assayed with varying degrees of efficiency in the liquid scintillation spectrometer. The emission of light however, may be derived from a number of different causes and the varying proportion of these utilizable sources of energy within each isotope will determine the efficiency with which the isotope may be measured. Such energy sources include gamma, positive electron, alpha, electron capture events, Auger and Cerenkov light. The quenching actions of different solvents also vary depending on the nature of the energy source, and hence each isotope will behave in a characteristic manner from this point of view. However, for many of the isotopes, the main contribution will be derived from the beta emission contribution much as in the case of tritium and carbon-14. Variation in efficiency will occur depending on the mean energy of the beta emission. Alpha-emitting isotopes have been studied in depth by Horrocks (1964). These include 217At,220Rh,232Th,233U, 236U,236Pu,239Pu and 242Cm.In addition, Basson (1956) has studied 211At. Other isotopes that are predominantly beta-emitters and can be assayed by liquid scintillation include 35S,55Fe, 63Ni, "Na, 24Na, 32P, 60C0, g o y , * O 6 R ~ ,'09Cd-'09Ag, 95Zr-95Nb,113Sn-'13mIn,I37C'37mBa, 5'Sm, 241Puand 252Cfand have been examined by Horrocks , and Studier (1961), Horrocks (1963), Steyn (1966), and Ludwick (1960). Isotopes capable of being studied by this technique are listed in Table 1, Appx. I. The usual object of any inorganic application involving metals is to 199
Subjei I indeh p 309
200
SAMPLE PREPARATION FOR LIQUID SCINTILLATION COUNTING
convert the inorganic ion into a form which is soluble in non-polar scintillant mixtures, in order to create as far as possible the conditions required by a homogenous counting technique. This is particularly essential in many inorganic applications, since quench correction techniques are more sensitive to changes in the structure of the counting system. The most usual procedure is to combine the metallic ion with an organic molecule to form a toluene-soluble ion-association complex. A colourless product is preferable but not always possible. A useful review of the complexing and solubilization methods for a large number of inorganic ions is by Dyer (1972). An alternative procedure is to employ an organic salt of the ion, in which the lipophyllic character is dominant and hence assists the solubility into a toluene based system. Octanoate and n-caproate salts are particularly suitable for this purpose. Provided the energy source is sufficiently penetrating, a suspension of a finely divided salt, held in gel suspension, can often be used and particular success has been achieved using this technique for those isotopes that can be concentrated by precipitation from very dilute solutions, as in the determination of 6oCo in environmental sources as the highly insoluble thio-cyanate complex (see chapter 12). In this chapter, only those isotopes that have been studied in relation to biochemical and biological problems will be considered, and which have not been included in the remaining parts of the book.
10.1. Solvent extraction methods Plutonium-241, which is produced from plutonium-239 by neutron capture in nuclear reactors, decays to americium-241 with a half life of 13.2 year and is accompanied by a predominantly 8- emission. Since these isotopes are being used in ever increasing quantities in nuclear reactors, possible contamination of biological systems become more and more likely. The detection of 241Puwas early desaribed by Horrocks and Studier (1958) who extracted the isotope in dibutyl
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201
phosphate and assayed the solution in a xylene-based liquid scintillation counting system. Using a similar procedure, Ludwick (1961) determined the isotope in urine to a minimum detectable level of 2.2 pCi using a 24 hr counting time. As the level of p- activity diminishes, the level of c1 activity will increase, since the half life of americium-241 is 458 years. By separating these two activities on the spectrometer and assessing the ratio at any given time, the time of origin of the isotope can be determined. Similar solvent extraction methods are also used for an ingenious assessment of the level of thorium in an Yttrium solution. The technique consists of extraction of thorium, from the mixed solution of ions, by means of a solution of di-2-ethylhexylphosphoric acid (20% v/v) in a solution of p-terphenyl (3.0 g) and dimethyl POPOP (0.1 g) in toluene. By adding known amounts of thorium a straight line can plot the amount of activity extracted, as expected from the partition rule between immiscible solvents. When the curve is extrapolated to the ordinate, the amount of thorium in the original solution can thus be determined (Fig. 10.1). It is claimed (Kim and McInnis 1972) that amounts as low as 10 ppm can be assessed by this technique. It is often not necessary to remove the aqueous layer from the scintillation vial when a scintillant extractant is used. For example, an aqueous sample containing trivalent transplutonium actinides can be extracted with a 0.01 M solution of an organic-soluble extractant dissolved in a Tpd-type scintillant mixture (Table 3, Appx. 11). The organic extractant used is chosen to be most suitable for the appropriate ion. In the series described by McDowell (1972) l-nonyldecylamine sulphate was most generally suitable, but di-(2-ethylhexyl) phosphoric acid is also useful in certain cases. The aqueous layer, after vigorous shaking, forms small droplets on the side of the vial but does not usually interfere with the counting efficiency. This latter solvent can also be used for the extraction of '47promethium from urine samples (Ludwick 1964). The method is as follows 1) Urine (250 ml) is treated with concentrated nitric acid (20 ml) in an
Erlenmeyer flask and boiled to near dryness in a fume cupboard. Sublert index p 309
202
SAMPLE PREPARATION FOR LIQUID SCINTILLATION COUNTING
5 10 WT. O F Th ADDED TO Y203ppm
15
Fig. 10.1. Method of estimating the level of thorium present in a sample of yttrium oxide by solvent extraction following known amounts of added thorium and extrapolation to zero (Kim and McInnes 1972) from ‘Organic Scintillators and Liquid Scintillation Counting’ (Academic Press Inc.).
2) Several more additions of nitric acid are made and boiled off until the mixture is colourless. 3) The wet ash residue on completion of near drying procedure is dissolved in approx. 15 ml of dilute nitric acid and transferred to a 25 ml volumetric flask. 4) pH is brought to 3.4 with dilute NaOH, and the volume further adjusted with water. 5 ) The solution is extracted twice with 250 pl portions of 7% HDEHP, di(Zethylhexy1) phosphoric acid, in Tpd4 scintillant (Table 3, Appx. 11).The upper organic layer is removed with a micropipette. A final extraction with LOO pl of the HDEHP/Tpd4 mixture is made and the organic layers combined and transferred to a vial and assayed. Approximately 94% of the isotope can be removed by this procedure.
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10.2. Precipitation and complex salt formation The process of complex salt formation is closely allied to the solvent extraction procedures referred to in 0 10.1 since the formation of a complex salt of the metal is probably required before it becomes soluble in the complexing agent itself. However, whereas in the latter case, the complexing agent is the solvent by which the complex is isolated, in the present system, the complex is a precipitate which is either insoluble and is assayed in a heterogenous system or is soluble in a different solvent and thus assayable as a homogenous system. Both 241Puand 239Puhave been assayed simultaneously in tissue, by co-precipitating the two elements as the ferriphosphate complex as follows (Eakins and Lally 1972). 1) The plutonium isotopes are isolated by acid elution from an anion exchange column, the eluate is evaporated and treated with concentrated acid and again evaporated. 2 ) The residue is dissolved in water in 10 ml centrifuge tube. 3) 2 mg of an iron salt carrier (ferric ammonium sulphate) is added. 4) Ammonia is then added to precipitate the ferric hydroxide. 5 ) The precipitate is washed with water and the residue is dissolved in 0.25 ml orthophosphoric acid. 6) Ethanol (containing ammonium chloride, 0.01 M in absolute ethanol) is then added to precipitate the ferriphosphate complex. 7) The precipitate is centrifuged and the residue is washed with ethanol. 8) The complex salt is then slurried with a dioxane-based scintillator together with Cab-0-Sil and assayed as a suspension. The gain spectra for these two isotopes is shown in Fig. 10.2 and it can be seen that the separation that can be achieved is very good. 236Puand 238Pucan also be assayed by this procedure and nearly 100% efficiency can be obtained with each of the isotopes. Some isotopes, e.g. '47Promethium, behave very similarly to calcium; in one laboratory (Moghissi 1970), 10 mg praesodymium is used as carrier to co-precipitate this isotope as the oxalate. This salt may then Sublert index p 309
204
SAMPLE PREPARATION FOR LIQUID SCINTILLATION COUNTING
0
20
40
60 Gan f %)
80
100
Fig. 10.2. Gain spectra of 239Puand 241Pu(Eakins and Lally 1972). (From Liquid Scintillation Counting Vol. 2 Crook, M., Johnson, P. and Scales, B. eds., Heyden and Sons, Ltd.)
be dissolved in EDTA solution and assayed in a colloid system. Efficiencies as high as 85% are reported with Y values of 1.8 pCi/ sample. Chlorine-36, present in gaseous chlorine, has also been assayed by the conversion of chlorine to SiCl, which can then be added up to 55% by volume to a toluene-based scintillant mixture. Y values of from 0.25 to 0.4 pCi/g chlorine were recorded by Ronzani and Tamers (1966). However, the conversion of chlorine-36 to sodium chloride and precipitation with ethanol and drying allowed about 11 g of sodium chloride to be added to a vial and assayed in Tpp4 with an efiiciency of 95 %. Under optimal conditions, Moghissi (1970)recorded a Y value of 0.43 pCi/g C1. The use of stainless steel cladding to encase the enriched uranium oxide in advanced gas cooled reactors results in the exposure of nickel in the stainless steel to neutrons and the conversion to nickel-63. This isotope has a soft beta emission (E,,, approx. 67 KeV) and a half life of 120 year. Liquid wastes from such reactors could result in the
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production of a potentially biologically important waste product which is difficult to detect. The isotope may however be complexed with pyridine and ammonium thiocyanate as tetra pyridine nickel dithiocyanate [Ni(C,H,N),] (CNS), which is soluble in a dioxanebased scintillant mixture (Harvey and Sutton 1970). The use of chelates labelled with carbon-14 to estimate inactive metal ions of the transitional type has been described (Chiriboga 1962), and provided that a suitable selective precipitating or complexing agent can be found for an appropriate metal ion there appears a considerable scope for analysis of both radioactive and non-radioactive metal ions by this technique.
10.3. Lipophyllic salts Many inorganic metallic ions form salts with long chain fatty acids, and these salts have the capacity of dissolving in polar scintillant mixtures and hence allow an otherwise insoluble material to be assayed in a homogenous system. There are clearly two ways in which the method can be used. Either for the estimation of labelled metal ions by synthesising the fatty acid derivative or alternatively by labelling the fatty acid and assaying the metal salt as an indirect assay of the concentration of the metallic ion. The octoate and n-caproate salts have been frequently employed for this purpose. Amongst the ions that have been assayed are caesium- 177/barium- 137m (Horrocks 1964), rubidium-87 (Flynn and Glendennin 1959) samarium-147 (Wright et al. 1961), plutonium-239 (Flynn et al. 1964), and nickel-63, using the n-caproate salt (Gleit and Jumot 1961). The estimation of fatty acids themselves can be made by using nickel-63 (Ho 1970).The free fatty acids in serum have been estimated by this procedure, and only 10 pl of serum need be used for the assay. The resulting nickel fatty acid salt is soluble in Bray’s scintillant mixture which is used for the radioassay.
Subject mdexp. 309
206
SAMPLE PREPARATION FOR LIQUID SCINTILLATION COUNTING
10.4. Noble gases The measurement of low levels of *'Kr in biological fluids and in gaseous phases is facilitated by the relative solubility of the gas in toluene-based scintillant mixtures. The ratio of the concentration of the noble gas in a toluene solution to that in the space above it also decreased as the temperature increases. Horrocks and Studier (1964) showed that at - 15"C, the ratio for Xe = 5, Radon = 32 and Kr = 0.9. At 27"C, the ratio for Xe = 3 at atmospheric pressure. The earlier method was to pre-evacuate the vial and inject the gas and scintillant. In the method adopted by Curtis et al. (1966) pressures are usually limited to 25 mm Hg or less, using a cathetometer to measure the exact volume added. The scintillator used in this case is Tpp7 (Table 3, Appx. 11) and the counting efficiency is 92.5 %. An alternative, more sensitive, procedure is to pump the gas from a krypton container into the vial at approx. 600 mm Hg. Since the level of krypton in dry air is of the order of 1.14 ppm, a cubic metre of air will be required to produce 1.14 ml. The scintillant used may be either scintillator plastic shavings ( 2 M mesh) or toluene-based scintillant (the plastic shavings are an inexpensive by-product of Pilot Chemicals Inc., Watertown, Mass., U.S.A.). In the first case, Sax et al. (1968) reported an efficiency of 94.4 k 0.7 % and a detection level of 1 pCi/m3 of air sample. In the latter method, described by Moghissi (1970),an evacuated glass vial fitted with a h e r stopcock is first filled with krypton to a pressure of 500 to 600 mm Hg. After filling, the stopcock is closed and by means of a 50 ml syringe, a toluene-based scintillant is allowed to enter the vial, dissolving the gas. The vial may almost be filled if the scintillant was previously efficiently deaerated. The krypton remains in solution, even though the vial is left open for a few minutes. The Y value by this procedure is reported to be 0.14 pCi/ ml Kr.
Inorganic applications
Over 50 isotopes may be assayed with varying degrees of efficiency in the liquid scintillation spectrometer. The emission of light however, may be derived from a number of different causes and the varying proportion of these utilizable sources of energy within each isotope will determine the efficiency with which the isotope may be measured. Such energy sources include gamma, positive electron, alpha, electron capture events, Auger and Cerenkov light. The quenching actions of different solvents also vary depending on the nature of the energy source, and hence each isotope will behave in a characteristic manner from this point of view. However, for many of the isotopes, the main contribution will be derived from the beta emission contribution much as in the case of tritium and carbon-14. Variation in efficiency will occur depending on the mean energy of the beta emission. Alpha-emitting isotopes have been studied in depth by Horrocks (1964). These include 217At,220Rh,232Th,233U, 236U,236Pu,239Pu and 242Cm.In addition, Basson (1956) has studied 211At. Other isotopes that are predominantly beta-emitters and can be assayed by liquid scintillation include 35S,55Fe, 63Ni, "Na, 24Na, 32P, 60C0, g o y , * O 6 R ~ ,'09Cd-'09Ag, 95Zr-95Nb,113Sn-'13mIn,I37C'37mBa, 5'Sm, 241Puand 252Cfand have been examined by Horrocks , and Studier (1961), Horrocks (1963), Steyn (1966), and Ludwick (1960). Isotopes capable of being studied by this technique are listed in Table 1, Appx. I. The usual object of any inorganic application involving metals is to 199
Subjei I indeh p 309
200
SAMPLE PREPARATION FOR LIQUID SCINTILLATION COUNTING
convert the inorganic ion into a form which is soluble in non-polar scintillant mixtures, in order to create as far as possible the conditions required by a homogenous counting technique. This is particularly essential in many inorganic applications, since quench correction techniques are more sensitive to changes in the structure of the counting system. The most usual procedure is to combine the metallic ion with an organic molecule to form a toluene-soluble ion-association complex. A colourless product is preferable but not always possible. A useful review of the complexing and solubilization methods for a large number of inorganic ions is by Dyer (1972). An alternative procedure is to employ an organic salt of the ion, in which the lipophyllic character is dominant and hence assists the solubility into a toluene based system. Octanoate and n-caproate salts are particularly suitable for this purpose. Provided the energy source is sufficiently penetrating, a suspension of a finely divided salt, held in gel suspension, can often be used and particular success has been achieved using this technique for those isotopes that can be concentrated by precipitation from very dilute solutions, as in the determination of 6oCo in environmental sources as the highly insoluble thio-cyanate complex (see chapter 12). In this chapter, only those isotopes that have been studied in relation to biochemical and biological problems will be considered, and which have not been included in the remaining parts of the book.
10.1. Solvent extraction methods Plutonium-241, which is produced from plutonium-239 by neutron capture in nuclear reactors, decays to americium-241 with a half life of 13.2 year and is accompanied by a predominantly 8- emission. Since these isotopes are being used in ever increasing quantities in nuclear reactors, possible contamination of biological systems become more and more likely. The detection of 241Puwas early desaribed by Horrocks and Studier (1958) who extracted the isotope in dibutyl
Ch. 10
INORGANIC APPLICATIONS
201
phosphate and assayed the solution in a xylene-based liquid scintillation counting system. Using a similar procedure, Ludwick (1961) determined the isotope in urine to a minimum detectable level of 2.2 pCi using a 24 hr counting time. As the level of p- activity diminishes, the level of c1 activity will increase, since the half life of americium-241 is 458 years. By separating these two activities on the spectrometer and assessing the ratio at any given time, the time of origin of the isotope can be determined. Similar solvent extraction methods are also used for an ingenious assessment of the level of thorium in an Yttrium solution. The technique consists of extraction of thorium, from the mixed solution of ions, by means of a solution of di-2-ethylhexylphosphoric acid (20% v/v) in a solution of p-terphenyl (3.0 g) and dimethyl POPOP (0.1 g) in toluene. By adding known amounts of thorium a straight line can plot the amount of activity extracted, as expected from the partition rule between immiscible solvents. When the curve is extrapolated to the ordinate, the amount of thorium in the original solution can thus be determined (Fig. 10.1). It is claimed (Kim and McInnis 1972) that amounts as low as 10 ppm can be assessed by this technique. It is often not necessary to remove the aqueous layer from the scintillation vial when a scintillant extractant is used. For example, an aqueous sample containing trivalent transplutonium actinides can be extracted with a 0.01 M solution of an organic-soluble extractant dissolved in a Tpd-type scintillant mixture (Table 3, Appx. 11). The organic extractant used is chosen to be most suitable for the appropriate ion. In the series described by McDowell (1972) l-nonyldecylamine sulphate was most generally suitable, but di-(2-ethylhexyl) phosphoric acid is also useful in certain cases. The aqueous layer, after vigorous shaking, forms small droplets on the side of the vial but does not usually interfere with the counting efficiency. This latter solvent can also be used for the extraction of '47promethium from urine samples (Ludwick 1964). The method is as follows 1) Urine (250 ml) is treated with concentrated nitric acid (20 ml) in an
Erlenmeyer flask and boiled to near dryness in a fume cupboard. Sublert index p 309
202
SAMPLE PREPARATION FOR LIQUID SCINTILLATION COUNTING
5 10 WT. O F Th ADDED TO Y203ppm
15
Fig. 10.1. Method of estimating the level of thorium present in a sample of yttrium oxide by solvent extraction following known amounts of added thorium and extrapolation to zero (Kim and McInnes 1972) from ‘Organic Scintillators and Liquid Scintillation Counting’ (Academic Press Inc.).
2) Several more additions of nitric acid are made and boiled off until the mixture is colourless. 3) The wet ash residue on completion of near drying procedure is dissolved in approx. 15 ml of dilute nitric acid and transferred to a 25 ml volumetric flask. 4) pH is brought to 3.4 with dilute NaOH, and the volume further adjusted with water. 5 ) The solution is extracted twice with 250 pl portions of 7% HDEHP, di(Zethylhexy1) phosphoric acid, in Tpd4 scintillant (Table 3, Appx. 11).The upper organic layer is removed with a micropipette. A final extraction with LOO pl of the HDEHP/Tpd4 mixture is made and the organic layers combined and transferred to a vial and assayed. Approximately 94% of the isotope can be removed by this procedure.
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203
10.2. Precipitation and complex salt formation The process of complex salt formation is closely allied to the solvent extraction procedures referred to in 0 10.1 since the formation of a complex salt of the metal is probably required before it becomes soluble in the complexing agent itself. However, whereas in the latter case, the complexing agent is the solvent by which the complex is isolated, in the present system, the complex is a precipitate which is either insoluble and is assayed in a heterogenous system or is soluble in a different solvent and thus assayable as a homogenous system. Both 241Puand 239Puhave been assayed simultaneously in tissue, by co-precipitating the two elements as the ferriphosphate complex as follows (Eakins and Lally 1972). 1) The plutonium isotopes are isolated by acid elution from an anion exchange column, the eluate is evaporated and treated with concentrated acid and again evaporated. 2 ) The residue is dissolved in water in 10 ml centrifuge tube. 3) 2 mg of an iron salt carrier (ferric ammonium sulphate) is added. 4) Ammonia is then added to precipitate the ferric hydroxide. 5 ) The precipitate is washed with water and the residue is dissolved in 0.25 ml orthophosphoric acid. 6) Ethanol (containing ammonium chloride, 0.01 M in absolute ethanol) is then added to precipitate the ferriphosphate complex. 7) The precipitate is centrifuged and the residue is washed with ethanol. 8) The complex salt is then slurried with a dioxane-based scintillator together with Cab-0-Sil and assayed as a suspension. The gain spectra for these two isotopes is shown in Fig. 10.2 and it can be seen that the separation that can be achieved is very good. 236Puand 238Pucan also be assayed by this procedure and nearly 100% efficiency can be obtained with each of the isotopes. Some isotopes, e.g. '47Promethium, behave very similarly to calcium; in one laboratory (Moghissi 1970), 10 mg praesodymium is used as carrier to co-precipitate this isotope as the oxalate. This salt may then Sublert index p 309
204
SAMPLE PREPARATION FOR LIQUID SCINTILLATION COUNTING
0
20
40
60 Gan f %)
80
100
Fig. 10.2. Gain spectra of 239Puand 241Pu(Eakins and Lally 1972). (From Liquid Scintillation Counting Vol. 2 Crook, M., Johnson, P. and Scales, B. eds., Heyden and Sons, Ltd.)
be dissolved in EDTA solution and assayed in a colloid system. Efficiencies as high as 85% are reported with Y values of 1.8 pCi/ sample. Chlorine-36, present in gaseous chlorine, has also been assayed by the conversion of chlorine to SiCl, which can then be added up to 55% by volume to a toluene-based scintillant mixture. Y values of from 0.25 to 0.4 pCi/g chlorine were recorded by Ronzani and Tamers (1966). However, the conversion of chlorine-36 to sodium chloride and precipitation with ethanol and drying allowed about 11 g of sodium chloride to be added to a vial and assayed in Tpp4 with an efiiciency of 95 %. Under optimal conditions, Moghissi (1970)recorded a Y value of 0.43 pCi/g C1. The use of stainless steel cladding to encase the enriched uranium oxide in advanced gas cooled reactors results in the exposure of nickel in the stainless steel to neutrons and the conversion to nickel-63. This isotope has a soft beta emission (E,,, approx. 67 KeV) and a half life of 120 year. Liquid wastes from such reactors could result in the
Ch. 10
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205
production of a potentially biologically important waste product which is difficult to detect. The isotope may however be complexed with pyridine and ammonium thiocyanate as tetra pyridine nickel dithiocyanate [Ni(C,H,N),] (CNS), which is soluble in a dioxanebased scintillant mixture (Harvey and Sutton 1970). The use of chelates labelled with carbon-14 to estimate inactive metal ions of the transitional type has been described (Chiriboga 1962), and provided that a suitable selective precipitating or complexing agent can be found for an appropriate metal ion there appears a considerable scope for analysis of both radioactive and non-radioactive metal ions by this technique.
10.3. Lipophyllic salts Many inorganic metallic ions form salts with long chain fatty acids, and these salts have the capacity of dissolving in polar scintillant mixtures and hence allow an otherwise insoluble material to be assayed in a homogenous system. There are clearly two ways in which the method can be used. Either for the estimation of labelled metal ions by synthesising the fatty acid derivative or alternatively by labelling the fatty acid and assaying the metal salt as an indirect assay of the concentration of the metallic ion. The octoate and n-caproate salts have been frequently employed for this purpose. Amongst the ions that have been assayed are caesium- 177/barium- 137m (Horrocks 1964), rubidium-87 (Flynn and Glendennin 1959) samarium-147 (Wright et al. 1961), plutonium-239 (Flynn et al. 1964), and nickel-63, using the n-caproate salt (Gleit and Jumot 1961). The estimation of fatty acids themselves can be made by using nickel-63 (Ho 1970).The free fatty acids in serum have been estimated by this procedure, and only 10 pl of serum need be used for the assay. The resulting nickel fatty acid salt is soluble in Bray’s scintillant mixture which is used for the radioassay.
Subject mdexp. 309
206
SAMPLE PREPARATION FOR LIQUID SCINTILLATION COUNTING
10.4. Noble gases The measurement of low levels of *'Kr in biological fluids and in gaseous phases is facilitated by the relative solubility of the gas in toluene-based scintillant mixtures. The ratio of the concentration of the noble gas in a toluene solution to that in the space above it also decreased as the temperature increases. Horrocks and Studier (1964) showed that at - 15"C, the ratio for Xe = 5, Radon = 32 and Kr = 0.9. At 27"C, the ratio for Xe = 3 at atmospheric pressure. The earlier method was to pre-evacuate the vial and inject the gas and scintillant. In the method adopted by Curtis et al. (1966) pressures are usually limited to 25 mm Hg or less, using a cathetometer to measure the exact volume added. The scintillator used in this case is Tpp7 (Table 3, Appx. 11) and the counting efficiency is 92.5 %. An alternative, more sensitive, procedure is to pump the gas from a krypton container into the vial at approx. 600 mm Hg. Since the level of krypton in dry air is of the order of 1.14 ppm, a cubic metre of air will be required to produce 1.14 ml. The scintillant used may be either scintillator plastic shavings ( 2 M mesh) or toluene-based scintillant (the plastic shavings are an inexpensive by-product of Pilot Chemicals Inc., Watertown, Mass., U.S.A.). In the first case, Sax et al. (1968) reported an efficiency of 94.4 k 0.7 % and a detection level of 1 pCi/m3 of air sample. In the latter method, described by Moghissi (1970),an evacuated glass vial fitted with a h e r stopcock is first filled with krypton to a pressure of 500 to 600 mm Hg. After filling, the stopcock is closed and by means of a 50 ml syringe, a toluene-based scintillant is allowed to enter the vial, dissolving the gas. The vial may almost be filled if the scintillant was previously efficiently deaerated. The krypton remains in solution, even though the vial is left open for a few minutes. The Y value by this procedure is reported to be 0.14 pCi/ ml Kr.