Scintillation apparatus and method with surface-modified polyethylene sample vessels

Liquid scintillation vials are disclosed formed from polyethylene or polypropylene and fluorinated on at least the inner surface. The advantageous physical properties and disposability of polyethylene or polypropylene are retained, while reducing permeation of organic solvents of the scintillation cocktail into or through the walls. Therefore problems of vial swelling, resulting in jamming of scintillation counters, are avoided.

BACKGROUND OF THE INVENTION 
The present invention relates to scintillation counting, and especially to 
sample vessels (usually vials) for such counting. 
Automated instruments are well-known for measuring the radionucleide 
content of samples (frequently biological samples) placed in each of a 
series of vials and mixed therein with a scintillation cocktail. Since the 
scintillants which emit visible (countable) light in response to 
radioactive decay are organic-soluble and the samples are generally 
aqueous, the cocktail typically contains the scintillant, at least one 
organic solvent and at least one surfactant. 
Representative scintillation cocktails are described in U.S. Pat. Nos. 
4,443,356 (1984), 4,444,677 (1984), 4,438,017 (1984). 
Vials for such counters are traditionally glass, and must be carefully 
washed between uses to avoid various types of cross-contamination. While 
disposable polyethylene vials have been suggested and used, they have 
three major drawbacks compared to glass: (1) permeation of organic 
solvents leading to objectionable or even potentially toxic vapors in 
parts of the laboratory generally not equipped with proper airflow or 
hoods for such vapors, (2) swelling or deformation of the vials so as to 
jam or stick when being automatically inserted into or removed from the 
counting well, and (3) adherence of biological matter to the inner surface 
of the vials so as to either interfere with emitted light or to overcome 
the intimate mixing of organic and aqueous phases required for efficient 
energy transfer (scintillation efficiency). 
Various materials other than glass or polyethylene having been suggested 
for such vials. In U.S. Pat. No. 4,021,670 to Noakes (1977), various 
plastic such as "nylon, linear polyethylene and the fluoroplastics 
(Teflons)" are mentioned. Polytetrafluoroethylene (sold by Dupont under 
the registered trademark "TEFLON") is difficult to shape (is not melt 
processable) and is expensive, such that disposable PTFE vials would not 
be practicable. Nylons (polyamides) would probably swell worse than 
polyethylene, suffering from both the aqueous and the organic contents. 
See also Chem. Abstr. 93: 103429W. 
BRIEF DESCRIPTION OF THE INVENTION 
The present invention is based upon the surface fluorination of 
polyethylene or polypropylene scintillation vials, which overcome the 
problems of polyethlene vials without forfeiting their advantageous 
features of disposability, low cost, precise moldability and resistance to 
the aqueous contents. In particular, organic components of scintillation 
cocktails neither swell nor permeate through fluorinated polyethylene to a 
deleterious degree, while the problem of adherence of biological materials 
is also significantly reduced. Accordingly, the present invention provides 
an improved liquid scintillation counting apparatus having a sample vessel 
of the type adapted to be placed in a counter sample well opposite the 
face of a photosensing device and containing a scintillation mixture 
containing a volatile organic solvent and a biological sample, 
characterized by the sample vessel being polyethylene or polypropylene and 
at least the inner surface of the polyethylene or polypropylene being 
fluorinated. 
The present invention also provides an improved method of determining the 
radionucleide content of an aqueous sample wherein the sample and an 
organic scintillation cocktail are mixed in a sample vessel, the filled 
sample vessel is placed in a counter sample well and the light bursts 
emitted from the filled sample vessel in the sample well are counted, 
characterized by the sample vessel being polyethylene or polypropylene and 
at least the inner surface of the polyethylene or polypropylene being 
fluorinated. 
DETAILED DESCRIPTION OF THE INVENTION 
The shape and size of vials or other sample vessels used in the present 
invention are those known to the art of scintillation counting. For ease 
of introduction and removal from a sample well, cylindrical vials are 
preferred, with outside diameters in the range of 5-50 mm and heights in 
the range of 5-100 mm. Typical sizes are standard vials of 27.2 mm outside 
diameter and 59.7 mm height and mini-vials of 17.0 mm outside diameter and 
54.4 mm height. 
While polyethylene or polypropylene may be used in the present invention, 
polyethylene is preferred, and is referred to (without limiting the 
invention) in the following description. The polyethylene used for the 
vials may be of various density, molecular weight and melting temperature. 
Preferred are the injection molding grades of high density (specific 
gravity at least 0.95 g/cm.sup.3) polyethylene. Such polyethylene may be 
either a homopolymer or one of the various copolymers (typically with 
1-15% of a higher alkene comonomer) sold as polyethylene. 
If it is desired to fluorinate both the interior surface and exterior 
surface, then it is preferred to perform fluorination on the 
already-formed vials. Various treatments with elemental fluorine alone or 
with fluorine in combination with inert gases or other elemental halogenes 
(e.g., biomine) may be used. Exemplary treatments to fluorinate the 
surface of polyethylene are described in U.S. Pat. Nos. 3,862,284 (Air 
Products 1975), 4,142,032 (Union Carbide 1979) and 4,467,075 (Union 
Carbide 1984). 
The exact level of fluorination is not critical, and can best be determined 
empirically using tests for swelling, for solvent permeation and for 
retention of scintillation efficiency as shown in the present Examples. 
Once suitable conditions (especially of time and temperature) are 
established for a particular combination of fluorine-containing gas, 
fluorination apparatus and vial size, reproducibility should be good. In 
general, it is believed that a low level of surface fluorination is 
adequate for the present invention and that further treatment normally 
results in increasing the depth of polyethylene which is fluorinated. 
Thus, once a minimum level of treatment is obtained, further treatment for 
some finite period will not be disadvantageous so long as the majority of 
the mass of polyethylene remains unfluorinated. 
If it is desired to fluorinate only the interior surface of the vials, then 
it is preferred to use an elemental fluorine-containing gas at the end of 
a blow-molding or other molding operation while the exterior of the vial 
is still in contact with the mold. 
In use, the vial is filled with a scintillation cocktail and a sample which 
contains some or no amounts of a radionucleide. The cocktail normally 
contains an organic solvent in which a fluor is dissolved) and one or more 
surfactants for suspending the organic phase finely in an aqueous phase 
(which is either provided in the cocktail or provided by the sample). Such 
cocktails and methods for their use are described generally in D. L. 
Horrocks, Applications of Liquid Scintillation Counting (Academic Press 
1974); Liquid Scintillation Counting, Recent Applications And 
Developments, vols. 1 and 2 (Academic Press, C-T. Peng, et al, eds, 1980); 
Y. Kobayashi, LSC Application Notes 1-30 (1978) and 31-50 (1980) (New 
England Nuclear Corporation); and in the literature provided with various 
commerical cocktails. 
The vials of the present invention find particular application with 
scintillation cocktails containing alkylbenzene organic solvents such as 
toluene, xylene, cumene, pseudocumene (methyl, ethylbenzene), and 
paraxylene. Other suitable organic solvents include dioxane. The 
fluorination inhibits loss of the solvent into or through the vial wall. 
The vials of the present invention also find particular utility with the 
fluors PPO and POPOP or other oxazine-type fluors. The fluorination 
inhibits loss of scintillation efficiency which may proceed via: (1) 
adherence of fluors or sample material to the vial wall, (2) loss of 
solvent from the suspension into or through the vial wall or (3). 
Because of the long stability of filled vials in accordance with the 
present invention, both against solvent loss and against swelling, it 
becomes possible to make up filled vials and read them in a scintillation 
counter either immediately or after a delay period. This has special value 
if samples are collected in a variety of sites and analyzed on a central 
counter, or if the counter is used by laboratory groups with uneven 
workloads of vials to be counted. Furthermore, even after counting, the 
vials may be retained in their filled state and recounted later, 
especially as a quality control procedure.

EXAMPLES 
For the following experiments, two sizes of polyethylene vials were used. 
The larger vials had a 22 ml interior volume and were shaped as cylinders 
with a 27 mm outside diameter and 59 mm height. The polyethylene, which 
had been injection molded, was approximately 2.5 mm in average thickness. 
The smaller vials had a 7 ml interior volume and were shaped as cylinders 
with a 17 mm outside diameter, a 54 mm height and a 1.5 mm average 
thickness. 
Large numbers of each vial size was fluorinated in fluoropolymer racks for 
varying lengths of time in accordance with the Linde SMP process (Union 
Carbide Corporation, see U.S. Pat. No. 4,467,075). In the following 
Examples, Level I refers to the least degree of fluorination, Level II to 
the intermediate level of fluorination and Level III to the greatest 
degree of fluorination. Unfluorinated vials were used as controls. The 
polyethylene screw caps were fluorinated separately on the same trays, 
such that the interior and exterior surfaces of both vials and caps were 
fluorinated. 
EXAMPLE 1 
Sixteen of the larger vials (four each of Level I, Level II, Level III and 
Controls) were filled with a xylene-based scintillation cocktail 
(ScintiVerse I from Fisher Scientific) and stored at 25.degree. C. for 72 
days. Outside diameters of the vials were measured initially and after 21 
and 72 days. The increase in outside diameter (expressed as a percentage 
change, was, on average for each group: 
______________________________________ 
Control Level I Level II Level III 
______________________________________ 
21 d 1.2% 0.05% 0.05% 0.05% 
72 d 1.3% 0.05% 0.3% 0.4% 
______________________________________ 
EXAMPLE II 
Example I was repeated using a pseudocumene-based scintillation cocktail 
(ScintiVerse II from Fisher Scientific; see U.S. Pat. No. 4,444,677). The 
percentage changes in outside diameter were: 
______________________________________ 
CONTROL LEVEL I LEVEL II LEVEL III 
______________________________________ 
21 d 1.5% 0.17% 0.17% 0.17% 
72 d 1.7% 0.3% 0.3% 0.3% 
______________________________________ 
EXAMPLE 3 
Example 1 was repeated using the smaller vials and the xylene-based 
scintillation cocktail. The resulting increases in outside diameter were: 
______________________________________ 
CONTROL LEVELS I, II, III 
______________________________________ 
21 d 1.0% 0.05% 
72 d 0.95% 0.05% 
______________________________________ 
EXAMPLE 4 
Example 1 was repeated using the smaller vials and the pseudocumene-based 
scintillation cocktail (ScintiVerse II cocktail). The resulting increases 
in outside diameter were: 
______________________________________ 
CONTROL LEVEL I, II, III 
______________________________________ 
21 d 1.2% 0.05% 
72 d 1.5% 0.05% 
______________________________________ 
EXAMPLE 5--Counting Efficiency 
Sixteen of the smaller vials filled with the xylene-based scintillation 
cocktail (as in Example 3) were also charged with 100 microliters of a 
radioactive sample, the sample having 22,000 disintegrations per minute 
(dpm). The samples were added immediately before counting. The vials were 
then read on a Beckman Model LS-1800 Scintillation Counter initially and 
after 21 and 72 days. For each vial, the loss of cpm registered was noted 
and converted into a percentage loss of counting efficiency. The average 
results for the four replication were: 
______________________________________ 
CONTROL LEVEL I LEVEL II LEVEL III 
______________________________________ 
21 days 
-6% -3% -4% -3% 
72 days 
-24% -7% -9% -7% 
______________________________________ 
EXAMPLE 6--Counting Efficiency 
Example 5 was repeated using the small vials and the pseudocumene-based 
scintillation cocktail of Example 4. The average losses in counting 
efficiency were: 
______________________________________ 
CONTROL LEVEL I LEVEL II LEVEL III 
______________________________________ 
21 days 
-11% -2% -2% -2% 
72 days 
-31% -9% -9% -7% 
______________________________________ 
EXAMPLE 7 
One of the 20 ml vials was filled with xylene and placed individually in a 
four liter glass chamber at 25.degree. C. After 45 minutes, air samples 
were taken of the chamber around the vial and analyzed for ppm xylene 
(microliters xylene/liter air). Readings were 51 ppm for control vials, 18 
ppm for Level I vials, 13.5 ppm for Level II vials and 13 ppm for Level 
III vials. 
EXAMPLE 8 
Example 7 was repeated except that the vials were filled with pseudocumene. 
The measured pseudocumene levels were 38 ppm for the control vials, 8 ppm 
for the Level I and Level II vials and 7 ppm for the Level III vials.