6

In the work reported in this publication, experimental evaluation of 30-ml as well as 10-ml Vacutainers was considered to be of value since the larger size (actual capacity of 35 ml) provides for replicate sampling using a 1-ml gas chromatographic sampling loop. In the previous Bureau publication, it was concluded that the shelf life of evacuated Vacuatainers should be limited to 1-2 months owing to accumulation of significant levels of carbon monoxide. In our current investigation, it was considered advisable to determine the mechanism for accumulation of contaminant gases and, if pos sible, devise a means of limiting those accumulations, particularly those of CO.

Many of the samples currently analyzed by the Bureau are products of combustion generated by inhouse research programs involving saturated and unsaturated hydrocarbons in the C, -Cs range. Thus, it is of considerable interest to determine the stability of samples stored in Vacutainers for days or weeks prior to gas chromatographic analysis.

The objectives of the current experimental work can be summarized as follows:

1. Conduct shelf life tests of evacuated 10- and 30-ml Vacutainers to predict contaminant levels as a function of time.

2. Determine mechanism of gas accumulation or loss.

3.

Devise means of pretreating Vacutainer systems to eliminate or reduce gas transfer, desorption, or adsorption.

4. Study the behavior of samplers containing hydrocarbons in the C-C5

range.

EXPERIMENTAL WORK

Vacutainers are evacuated at the Bruceton, Pa., laboratories using a small evacuation chamber constructed to accommodate both 10- and 30-ml samplers. Both evacuated and filled Vacutainers are thermostated using an environmental chamber. Samplers are analyzed using the dual chromatograph employed for mine air analysis. The retention time for isopentane under the conditions employed is 9 minutes.

Equipment

An evacuation unit (fig. 1) is constructed for 10-ml and 30-ml VacuThis was done by modifying a plastic desiccator (11-1/4-in-OD flange) tainers. obtainable from several laboratory supply companies. The knob on the cover is replaced with a solid acrylic rod 1-in OD by 1-1/4 in high, drilled to accommodate a 3/8-in-diam by 7-1/2-in-long brass rod. Two internal ridges within

Work cited in footnote 5.

Lang, H. W., and R. W. Freedman. A Three Minute Gas Chromatographic Analysis

of the Main Constituents of Mine Atmospheres.

RI 7696, 1972, 7 pp.

the 3/8-in acrylic rod hole are provided for retention of O-rings. A knob is mounted on one end of the 7-1/2-in brass rod, and the other end is flanged to a fiberglass pressure plate 7-1/2 in. in diam by 1/4 in thick. Holders for the Vacutainers are built in the same manner as described in the previous publication, using 21/32-in holes for 10-ml samplers and 29/32-in holes for 30-ml samplers. The holder assembly consists of a 9-1/2- by 1/4-in backing plate spaced 1 in from the base plate. An inverted U-shaped press is constructed to assist in stopper insertion under vacuum. A photograph of the entire assemgly is shown in figure 2.

Both temperature and humidity can be controlled in the environmental chamber.

on a tee close to the evacuation unit. The procedure is similar to that used previously. In this case, additional pressure is exerted on the pressure plate by tightening the threaded knob of the press.

The samplers are filled with calibration gas by substituting the gas supply for vacuum by means of a 3-way valve and cutting the gas supply when the manometer indicates atmospheric pressure. Evacuated Vacutainers are filled with unknown gas samples either by injection through the septum or by withdrawing the stopper in the gas atmosphere. The sample is analyzed after it is transferred into the gas chromatograph sample loop. This is accomplished by injecting mercury through the septum to pressurize the sample, and then injecting gas into the loop with a gas-tight syringe.

Work cited in footnote 5.

Gas Permeation

Vacutainers evacuated or filled with test gas mixtures are thermostated at 25°-100°10 for days or weeks. Evolution of gases by Facutainer stoppers are evaluated by placing them in an evacuated 250-ml sidearm flask, which is sealed and maintained at constant temperature. Permeation of external gases through the stoppers is studied by placing evacuated samples within the flow-through desiccator in the environmental chamber. Gases are passed through at 100-200 ml/min.

RESULTS AND DISCUSSION

Shelf Life

The shelf life of evacuated Vacutainers was studied by storage in air over a 4-month period at 25°. For analysis, the samplers were filled with helium to atmospheric pressure and promptly subjected to chromatographic analysis. Results are given in table 1. Carbon monoxide accumulation becomes significant after about 4 weeks. This confirms the previous conclusion that the shelf life is 1-2 months. Carbon dioxide levels at this time are much higher than CO levels, but are still not significant for most analyses. Both sizes of Vacutainers show similar behavior.

Concentrations1 of gases accumulating in Vacutainers at 25° C

TABLE 1.

Time,

2

All concentrations shown are averages of 5 measurements.
Percent standard deviations are predominantly about +25%.

Pretreatment of Stoppers To Remove Accumulated Gases

Chemical Pretreatment

Stoppers were treated as indicated in table 2, and allowed to stand in evacuated filter flasks for 4 weeks. The filter flask atmospheres were analyzed. Flasks 1-3 show little change in CO level and a moderate change in CO2 level. Removing the glycerine lubricant by detergent wash (flask 2) had little effect, indicating indirectly that the source of CO was not glycerine at room temperature. The pure CO treatment (flask 4) yielded an expected rise in CO evolution based upon absorption-desorption behavior. The lower CO2 value is probably the result of chromatographic displacement of CO2 by CO, followed by lowered subsequent desorption of CO2 ·

10 All temperatures are Centigrade.

1Five 10-ml and five 30-ml stoppers were loaded into each flask. Concentrations 1 through 4 are net values (after concentrations in flask 5 are subtracted).

Eluted from rubber stopper of filter flask.

Thermal Pretreatment

Since gas desorption increases markedly with temperature, preheating to remove CO and CO2 was investigated. Two five-sample batches of evacuated 10-ml Vacutainers were held at 25° and 100°, respectively, for 4 weeks. The five-sample batch averages for accumulations at 25° for CO and CO2 were 2.3 ppm and 124 ppm, respectively. Accumulated concentrations at 100° were 360 ppm CO and 2,570 ppm CO2. Since some blackening of the rubber occurred owing to prolonged heating, a five-sample batch of fresh stoppers was preheated at 70° for 3 weeks under vacuum. These preheated stoppers were then inserted into Vacutainers that were evacuated and held for 3 weeks at 25°. Average concentrations of CO and CO2 were found to be 4.0 ppm and 33 ppm, respectively. It can be concluded that substantial CO2 was removed, but CO removal was unsuccessful.

Investigation of External Contamination

Batches of five evacuated Vacutainers were exposed externally at 30° to pure CO (2 days) and pure CO2 (5 days). In addition, blanks were run under these conditions using an external air atmosphere. Results are given in table 3.

From the net values, permeation of CO through the septum appears negligible and CO2 permeation appears very small considering the use of 100% gases