s,
-
O,Fm
-
... m
where Om is the peak area in mV-millimoks (Of eluent) and s, is then the F, is the carrier flow in millimOleS-min-'. peak height in mV observed for a given concentration of eluted material in the detector in moles Per mole, that is, parts per part of carrier. This perhaps allows a rational basis for comparison of gas and liquid chromatographic detectors. On this basispQo'" = 9.1 for our detector, where the super-
script m refers to the minimum detectable quantity in millimoles calculated from Sm. This is aooroximately a factor .. of 10 less than the value ofpQ." = 10.3 for the flame ionization detector given by Purnell. With voltage stabilization, temperature thermosWting, and signal amplification, it may be- nossible to reduce the noise and drift suficientlv to inr ~ ~ ~ creaSethe sensitivity another factor of five or more. . ~~
~
~~~~
~
Recmm, for review March 20, 1968. Accepted May 24, 1968. Work supported by a grant from the Office of Saline Water, No. 14-01-0001-1319.
Heat-Sealed Polyethylene Sample Containers for Neutron Activation Analysis W. D. Ehmann and D. M. McKown Department of Chemistry, University of Kentucky, Lexington, K y . 40506
HEATSEALED H~LYETHYLENE vials are most desirable for encapsulation of liquid or finely powdered solid samples to be assayed by nondestructive neutron activation analysis. Ordinary (lower density) polyethylene shows no appreciable radiation damage in 14-MeV neutron applications and can be irradiated for several hours in most research reactors without serious degradation. More important is the fact that the induced activity of the capsule is very low following both fast and thermal neutron irradiations. It is often difficult, however, to obtain a good heat seal on these vials because polyethylene tends to expand and flare when heated rather than contract to seal a cavity. This paper reports a unique method for constructing low cost sample irradiation vials from ordinary polyethylene tubing. The method ensures an air tight seal and may be used to encapsulate either solid or liquid samples for radioactive assay. The controlled shrinkage of heat-shrinkable insulation tubing is utilized as a means of compressing melted polyethylene to form a uniform seal. Irradiated polyolefin shrinkable tubing (Alphex FIT-221 shrinkable tubing, Alpha Wire Corp.) when heated to a temperature of about 120 "C, shrinks to one half its original diameter with very little longitudinal shrinkage. This minimum shrinkage temperature is slightly greater than the temperature necessary to cause lower density polyethylene to flow freely. Thus, when a section of shrinkable tubing jacketing the end of an ordinary polyethylene tube is beated locally to its shrinkage temperature, it will collapse the melted tubing walls to form an airtight seal.
Figure 1. Construction of heat-sealed polyethylene vials DISCUSSION
Sectional views of the sealed vials are shown in Figure 2, A and E. That shown in Figure 2A was constructed by inserting a polyethylene plug before sealing. This method is recommended for liquid- or heat-sensitive solid samples. The polyethylene plug prevents heat radiation down the tube and allows the seal to be made without increasing the temperature
PROCEDURE
The construction of the capsules is illustrated in Figure 1. A section of shrinkable tubing (1-2 cm) is placed over the end of a section of ordinary polyethylene tubing of desired size. The shrinkable tubing is heated gently over its entire length just long enough to allow it to shrink tightly on the polyethylene tube. The use of radiant heat from a hot iron or a finely directed hot air gun is recommended. The seal is then made by directing the heat obliquely onto the end of the tubing, as indicated in Figure I , until the melted polyethylene tubing walls are collapsed to a uniform melt. After cooling, the shrinkable tubing may then be stripped off. The other end of the capsule is sealed in the same manner. For making large capsules or sealing liquid samples, it may be necessary to insert a polyethylene plug before sealing (see Figure 1). 1758
ANALYTICAL CHEMISTRY
Figure 2. Sectional views of the sealed vials A . Using PE Plugs B. Without plugs C. Complete irradiation unit
inside the vial above 60 “C. Also shown in Figure 2 is a filled capsule positioned in a heat-sealed 2-dram snap-top vial (Olympic Plastics Co.). This irradiation unit is suitable for pneumatic tube transfer ( I ) . It should be noted that FIT-221 irradiated polyolefin shrinkable tubing will shrink to a predetermined diameter when heated to its shrinkage temperature. This controlled shrinkage ensures a good seal if sufficient heat is applied to melt the polyethylene. In addition to making sample vials, we have found that this heat-shrinking technique can be used to col(1) J. R. Vogt, W. D. Ehmann, and M. T. McEllistrem, Int. J. Appl. Radiat. Isotopes, 16,573-80 (1965).
lapse melted polyethylene tubing onto irregularly shaped samples, such as metal or rock fragments, to give a thick polyethylene coating. Such a sample is virtually free from atmospheric gases, which is an important advantage in nondestructive activation analysis for oxygen and nitrogen. The availability of both the shrinkable and polyethylene tubing in a wide variety of sizes makes this method useful for encapsulation of most any type of sample at minimal cost, as compared to machined polyethylene “rabbits”. RECEIVED for review April 4, 1968. Accepted May 20, 1968. Research supported in part by contract AT-(40-1)-2670 from the U.S. Atomic Energy Commission.
Rotating Sample Cell for Low Temperature Phosphorescence Measurements H. C. Hollifield and J. D. Winefordner Department of Chemistry, University of Florida, Gainesville, Fla. 32601
PRECISION OF MEASUREMENTS in phosphorimetry using an Aminco spectrophotofluorometer with phosphoroscope attachment (American Instrument Co., Inc., Silver Spring, Md.) is primarily limited by sample cell positioning errors. ( I ) . The Aminco sampling device consists of a quartz Dewar flask containing liquid nitrogen and a quartz sample cell held by two O-rings in a bakelite holder which is held in place in the Dewar flask by means of spring clips. Previous attempts to minimize sample cell positioning errors (1) have proved to be tedious and only moderately successful. Because of the success of a spinning sample cell in NMR to minimize magnetic field inhomogeneities, it was felt that a similar design might be useful for phosphorimetric measurements to average out optical inhomogeneities and minimize sample positioning errors. In Figure 1, the rotating sample cell holder developed for phosphorimetric measurements in our laboratory is given. The holder consists simply of an air-driven cylinder of (Du Pont) Teflon which holds the sample cell and which is in an aluminum sleeve with a nozzle for directing the air flow on blades of the Teflon cylinder. The sleeve is held in place on the “cover support” of an Aminco phosphoroscope attachment (see Figure 1) by means of a ring adapter. The sample cell is held in place in the Teflon cylinder by means of a nylon screw. A removable cover containing the air exit port completes the design. The authors do not intend that the design shown in Figure 1 be considered optimum and, therefore, all specifications are not given in detail. Most any design which will result in a smoothly rotating sample cell is suitable. To carry out a phosphorimetric measurement with the rotating sample cell assembly, the aluminum sleeve with nozzle is mounted permanently to the “cover support” and connected to a source of air. The Dewar flask is filled with liquid nitrogen and placed in the phosphoroscope attachment. A quartz sample cell of the proper length is placed in the Teflon holder and held firmly by a nylon screw. Sample is placed (1) J. D. Winefordner, W. J. McCarthy, and P. A. St. John, “Phosphorimetry as an Analytical Approach in Biochemistry,” Chap. in “Methods of Biochemical Analysis,” David Glick, Ed., Interscience, New York, 1967.
Table I. Precision of Phosphorescence Measurementsa Using Conventional Sampling System* and Rotating Sample Cell. Number of detns 5 5 13
Concn of phosphor“ (mgb-4
Re1 std dev, ConvenRotating sample celi tional Stationaryd Movinge ... 14.9 1.08
1.00 1 .OO ... 19.2f 4.15f 1 .OO ... 18.8f lr32f 10 1.00 ... 12.0 0.77 9 1 .oo 21.39 ... ... 10 1.00 9.7h ... ... 1.00 10 7.1i ... ... 5 0.100 ... 18.3 1.54 0.0100 5 ... 12.2 1.34 0.00100 5 ... 19.4 1.84 10 0.00100 ... 9.6 2.14 10 0.00100 35.2g ... 10 0.00100 17.2h ... ... 0.00100 10 18.2i ... ... Phosphorescence measurements made on benzoic acid in ethanol at 77 “K. The solvent medium at 77 “K formed a clear, rigid glass except where designated. * The conventional Aminco sampling device consists of a quartz Dewar flask with liquid nitrogen and a quartz sample cell held by two O-rings in a bakelite holder which is held in place in the Dewar flask by spring clips. c The system described in the paper (see Figure 1). d Measurements were taken with the sample cell not rotating and in the random position which results upon terminating the air flow. e Measurements were taken with the sample cell rotating as described in the paper. f These particular samples formed a cracked matrix at 77 OK. 0 Measurements were taken with the sample cell and flask placed into position in a random manner. h Measurements were taken with the flask carefully aligned but the sample cell randomly positioned. i Measurements were taken with careful alignment of the flask and sample cell each time. .
I
.
Q
in the sample cell, and the Teflon holder with cell is placed in the aluminum sleeve. The cover is then fixed firmly into place, and air is introduced into the nozzle until the sample cell is turning at a speed just sufficient to prevent electrometer VOL. 40, NO. 1 1 , SEPTEMBER 1968
1759
