Polonium-210(²¹⁰Po) occurs in natural environment[JP] at trace levels as part of the ²³⁸U decay chain. As a pure α emitter,²¹⁰Po is one of the most radiotoxic nuclides. With wide dispersion in the environment,²¹⁰Po enters human body via food chains,thus being a serious threat to human health. ²¹⁰Po was known as one of the most important sources of the internal dose received by humans from foods^[1]. For example,it was reported that ²¹⁰Po contributed 75% of dose due to the α-radiation in marine organisms and contributed 50% of the internal α-ray irradiation dose to people^[2]. The maximum permissible human body-burden of ingested ²¹⁰Po was 1.1×10³ Bq^[3]. Concerning the public health,it is important to ensure that the contaminant levels in diet will not exceed the permissible limits^[4]. To address this heightened concern and effectively respond to ²¹⁰Po radiological emergency,the capability to rapidly analyze large numbers of samples in a variety of matrices is necessary for risk assessment and post incident decision making. ²¹⁰Po was detected in a large variety of samples,including soil,water,marine organisms,tobacco leaves,cigarettes,urine,and biological materials^[5,6],especially in seafood such as mollusc^[7,8]. The commonly used methods included α spectrometer measurement after source preparation through spontaneous deposition^[9]. However,the traditional methods of sample pretreatment process is complex and time consuming. So it is necessary to establish a simple routine environmental analysis method.

In this study,a new method was developed to determine ²¹⁰Po in seafood using large area grid ionization chamber α spectrometry.

Microwave digestion system (Preekem Scientific Instruments Co.[KG-*4],China),ultrasonic cleaner (Tianjin Science Instrument Co.[KG-*4],Ltd,China),²⁰⁹Po (1.178 Bq/ml,Beijing Research Institute of Uranium Geology),standard source of ²³⁷Np,²³⁹Pu and ²⁴¹Am (National Institute of Metrology,China),ultrapure water purification system (Milli-Q,USA),sodium dodecyl sulfate (CP,Tianjin Fuchen Chemical Reagents Factory),gelatin (CP,Xilong Chemical Co.[KG-*4],Ltd) and formaldehyde solution (AR,Xilong Chemical Co.[KG-*4],Ltd),HNO₃ (MOS,Tianjin Fengchuan Chemical Reagent Co.[KG-*4],Ltd) and H₂O₂ (CP,Tianjin Fengchuan Chemical Reagent Co.[KG-*4],Ltd). 2. Sample preparation

Clam samples were collected from market in Beijing. The soft tissues were taken out,dried at 105℃,and grinded into powder. 0.1-0.5 g of powder was weighted into Teflon vessel,HNO₃ (5 ml) and H₂O₂ (2 ml) were added into each vessel,and ²⁰⁹Po standard solution was added as the tracer. Samples were digested for 3 min at 105℃,using a microwave system with maximum power of 1 500 W,and then heated to 200℃ for 3 min. After the vessel being cooled to ambient temperature,the acid solution was transferred into weighted beaker,evaporated to dryness at 60-80℃^[1],and then beaker was weighed again. Then 50 ml of ultrapure water was added,disperse sample sdution under ultrasonic for 40-60 min,then 1‰ formaldehyde (0.5 ml) and 2×10^-3 gelatin (0.5 ml) were added into sample solution^[10]. Then the sample solution was transferred directly into the dish ( 25 cm),the surface of which was treated with 5% sodium dodecyl sulfate (SDS),and the sample dish were then put in the vacuum oven on a horizontal tray,evaporated to dryness at 50-60℃. The counting source prepared in this way can be put in large-area grid ionization chamber directly.

Following the above sample preparation procedure,0.1,0.2,0.3,0.4 and 0.5 g of clam powder was weighted,respectively,for the purpose of obtaining effective thickness of sample source.

3.Spectrometry

The sample sources were counted using large-area grid ionization chamber (Oradela Model 8210A). The chamber filled with commercially available mixed gas of 90% Ar and 10% CH₄ to about 48.26 kPa^[11]. Standard source (²³⁷Np- ²³⁹Pu- ²⁴¹Am mixed source, 25 cm) was used for energy calibration and efficiency determination (²³⁷Np 4 789 keV,²³⁹Pu 5 156 keV,²⁴¹Am 5 485 keV,and activity 136.4 Bq). The minimum detectable activity (MDA) was calculated by the following formula^[12]:

A comparative method for the measurement of ²¹⁰Po in clam samples was employed using the multi-channel α spectrometer (OCTE-Plus,USA),sample source was prepared through cupper disc spontaneous deposition^[13].

1.Detection efficiency,MDA and recovery

The φ 25 cm blank dish was counted for 24 h,while mixed standard source for 30 min,to obtain detection efficiency 33% of the large-area grid ionization chamber α spectrometry. MDA was 9.870×10^-4 Bq. ²⁰⁹Po standard solution was spiked in clam powder and the present method was used to prepare source. After 24 h measurement,the recovery was 98.5%- 98.6%.

2. Alpha energy spectrum

All five sample sources could present complete shape of peaks at the same energy range in 4 880-5 330 keV. The α energy spectrums were compared,and the results showed that the peak shapes were both larger and more tailing while the sample source thickness increased. And thickness of sample source (258.14 g/cm²,0.3 g dry) had the sharpest peak than others.

3.The effect of sample source thickness

The counting results obtained from five different thicknesses of samples were compared in Table 1. The source thickness was equal to the sample residue mass divided by counting dish’s area. The count increased with the source thickness as a result of sample mass. At the same specific activity,the larger mass means higher activity. When the source thickness was 258.14 μg/cm²,the full width at half maximum (FWHM) was lower than others.

表 1(Table 1)

Table 1 Peak,full width at half maximum (FWHM) and counts for different thickness sources

Table 1 Peak,full width at half maximum (FWHM) and counts for different thickness sources

The dry clam samples were analyzed using both multi-channel α spectrometer and large-area grid ionization chamber α spectrometry. The spontaneous deposition source (φ 2 cm) measured using the multi-channel α spectrometer. The average activity of ²¹⁰Po was 0.056 4 Bq/g (dry sample). The clam powder sample 0.3 g was weighted,digested and detected for 36 h in the same chamber using the method presented in this study. The activity of ²¹⁰Po was 0.057 5 Bq/g (dry sample). The results indicated that ²¹⁰Po activities obtained using both methods were nearly identical.

In this study,a new method was developed to digest seafood samples using microwave system and determine ²¹⁰Po with large-area grid ionization chamber α spectrometry. This method features easy preparation and small quantity of samples,without separation step. In order to prepare the counting source as evenly as possible,the SDS was used to treat sample dish in attempt to achieve hydrophilia of inner surface,so as to allow homogeneous distribution and evaporation of sample solution by aid of horizontal tray^[14]. Compared with the plating method as adopted the National Standards (GB 14883.5\|1994)^[13],the quantity of sample was reduced from 5-50 g to 0.2-0.5 g. The digestion process was able to avoid large amount of foam and the loss of fat in some samples.

The developed method in this study,using microwave digestion,needs less than 1 g of sample. Digestion and cooling process could be completed in less than 1 h,which accelerated the digestion of fats and helped to obtain clear solution at last. Moreover,many samples could be processed in a parallel way. The literature reviewed the large-area cylindrical grid ionization chamber had higher sensitivity than semiconductor α spectrometer,because of the lower high quality coefficient,which was in direct proportion to MDA^[12] . Large screen grid ionization chamber’s lower limit of detection for a counting time 1 000 min at a confidence level of 95% was calculated to be 0.012 pCi(1 Ci=3.7×10¹⁰ Bq)^[11].

At the stage of sample source preparation,the disc diameter of 25 cm was used in the present method whereas in range of 2-2.5 cm in plating method. Thus,the probe area of the present method was 100 times bigger than that of plating method. About the sample source thickness,the increasing of sample mass lead to the increase of the source thickness^[14]. At the same specific activity the larger mass means higher activity. So,more counts could be obtained from the chamber. With the same activity and measurement time,its resolution decreased with source thickness. At the same thickness and measurement time,the counts increased with activity. For this reason,sufficient counts cannot be obtained from 0.1 and 0.2 g of samples,without peaks of ²¹⁰Po found. The 0.4 and 0.5 g of thicker samples resulted in higher self-absorption of α particles and α spectrum peaks tailing^[11]. Therefore,when using the large-area grid ionization chamber to measure α spectrum,the thickness of the source is main influencing factor.

Above all,the method developed for measurement of seafood could achieve higher recovery (above 98%) and lower MDA (9.870×10^-4 Bq),using small quantity of sample (0.2-0.5 g of dry sample). The present method is useful for measurement of other biological or environmental samples.

Acknowledgments

Dr. Wang Tiejian was specially acknowledged for his donation of facility to multi-channel α spectrometer measurement at Beijing Research Institute of Uranium Geology.