externally. In addition, if permeation-rate equations are solved for concentration levels of accumulated gases using known parameters, extremely low values are obtained.

Mechanism of Gas Accumulation

The mechanism for accumulation of gases within evacuated Vacutainers is unknown. One possibility is the desorption of previously adsorbed gases from Transfer of external gases through the stoppers would be far

the stoppers.

too slow to account for measured accumulation.

Behavior of Stored Hydrocarbon Samples

Evacuated Vacutainers were filled to atmospheric pressure with the hydrocarbon mixture. One batch of five samples was analyzed immediately and other batches of five were analyzed at weekly intervals over the 4-week period. The results are shown in table 4. Precision is good based upon the four to five samples for each condition. In fact, the poorest single value for percent

standard deviation was 5.28% for n C4 H10 obtained for the five replicate samples analyzed after 4 weeks in storage. The much higher (above 25%) standard deviation obtained for CO and CO2 replicates reflect desorption behavior owing to previous exposure of the stoppers to these gases in the course of manufacture. Previous exposure to hydrocarbons does not appear to be the case Variation of stored hydrocarbon concentrations over

for Vacutainer stoppers.

a 4-week period is shown in figure 3.

TIME, weeks

FIGURE 3. Hydrocarbon concentrations during storage of 30-ml

Vacutainers at 25° C.

CONCLUSIONS

1. A shelf life of 1 to 2 months for evacuated Vacutainers has been confirmed for CO determination.

2. The mechanism for change of concentration of Vacutainers is unknown. It may be absorption-desorption into and from the stoppers.

3.

No practical means of pretreatment of stoppers were found to minimize CO accumulation.

4. Hydrocarbons in the C-C range can be stored for weeks with roughly predictable changes in concentrations.

*U.S. GOVERNMENT PRINTING OFFICE: 1976-603-755/154

INT.-BU.OF MINES, PGH., PA. 215 16