1.4 - Sample Preparation for α/β-Pulse Shape Discrimination
A variety of scintillation cocktails for all kind of applications (aqueous, organic, gelating, filter materials, etc.) are commercially available with modifying scintillator composition and solvents.
The "Bray"-solution as effective and frequently used classical cocktail contains 120 g naphthalene, 4 g PPO (2,5-diphenyloxazole), 0.1 g POPOP (1,4-bis-2-(5-phenyloxazolyl)-benzene), being filled up to 1 Liter with dioxane or toluene. Now user friendly and environmental safer cocktails containing di-isopropyl-naphtalene (DIN) as solvent and alcohol ethoxylate (AE) instead of nonyl phenol ethoxylate (NPE) as emusilfier are preferred. These ‘safe cocktails’ with DIN are heavily inflammable, environmental friendly, biologically degradable and allow as well an excellent α/β-PSD. A compilation of frequently used cocktails is given below and can be found with the appropriate properties in the suppliers leaflet.
Counting efficiency and quality of the α/β-discrimination depend mainly on cocktail composition, kind of counting vial and size.
Cocktail:
A variety of cocktails can be found from different chemical suppliers. The choice of the optimal cocktail depends on the application such as organic, aqueous, quench resistant, low level purposes, α/β- discrimination, emulsion or filter counting. A compilation of frequently used cocktails can be found in table 3. A comprehensive review on cocktails and their applications can be found in [L’Annunciata 2012] pp 580.
Volume:
We have investigated sample volumes of 20 mL and 10 mL in standard vials, 8 mL and 6 mL in mini-vials as well as 2 mL and 0.5 mL in Eppendorf plastic mini-vials for α/β-discrimination using Triathler (HIDEX). The best energy resolution and α/β-discrimination have been achieved with the latter ones in the provided adapters. The lowest limit of detection has been determined for 8 mL sample volume in diffused (scratched) mini-vials. Plastic and sand blasted glass vials with diffuse light are therefore preferable compared to normal glass vials (commercially available as "froasted scratched vials" by HIDEX Oy, Turku or FCI, Mainz).
Organic/Gel phases:
In organic scintillation cocktails like MaxiLight™, BetaPlate Scint™ or Extractive Scintillators the time duration of α-pulses is comprehensively longer compared to aqueous systems. This leads to a decisive better α/β-discrimination. According to our measurements less than 0.1 % of all high energetic 32P-pulses interfere in the α-channel of Triathler compared to 1 % in aqueous phases. Luminescence and low energetic β-emitters practically do not interfere in organic phases. A background of less than 1 count per hour in an optimized α-channel with respect to energy region and PLI for BetaPlate Scint in a mini-vial of 8 mL or less have been achieved.
With aqueous gel phases only a poor α/β-PSD separation is achievable, especially when quenching agents like EDTA are present.
From 8 different gelating cocktails measured in the Triathler device, we have found the best results for AquaLight™, Ultima Gold™ AB and OptiPhase HiSafe™ 3 in a ratio of 8 mL water to 12 mL cocktail (s. a. [Salonen 1993]).
The quality of α/β-PSD in aqueous phases is influenced by quenching from higher salt content or in EDTA-containing solutions (2.2.1.5.). A maximum of 6 mL could be mixed with 16 mL OptiPhase HiSafe 3 in order to form a clear gel. Milky opaque emulsions do not allow any α/β-separation capability.
L’Annunziata M.F. 2012: “Handbook of Radioactivity Analysis”, Chapter 15, 3rd Edition 2012, Elsevier
Salonen L. 1993: Measurement of low levels of 222Rn in water with different commercial liquid scintillation counters and pulse-shape analysis; in: J.E. Noakes et al. “Liquid Scintillation Spectrometry 1992”, pp 361-372, Radiocarbon 1993, Tucson
