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. .
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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
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Figure 1. Schematic diagram of rotating sample cell assembly (all dimensions in cm) A. Quartz sample cell B. Aluminum sleeve C. Nozzle for air introduction D. Teflon cell holder and rotor E. Cover with air exit port F. Fins to let air out G. Grooves to collect air H. Nylon screw to secure sample tube I . Ring adapter J. Cover support of Aminco phosphoroscope attachment K. Quartz Dewar flask L. Bolts to hold removable cover
noise due to cell rotation. The exact speed of rotation is unimportant as long as it is sufficient to allow random sample cell positioning during the measurement (approximate speed is 300 rpm). The phosphorescence signal is then measured. Additional measurements of other samples require only the removal and positioning of the Teflon cylinder with sample
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cell in the aluminum sleeve and the replacing of the cover. The time required for running samples is approximately the same as for similar measurements made using the standard Aminco phosphorimetric instrumentation. The precision of phosphorescence measurements for several concentrations of benzoic acid in ethanol at 77 OK using the conventional Aminco sampling system and the rotating sample cell described in this paper is given in Table I. Several interesting results should be noted. In nearly all instances, an improvement of about IO-fold in precision results when the rotating sample cell is rotated as compared to when it is stationary-i.e., the sample cell is allowed to come to rest before taking measurements in the latter instance. The poor reproducibility of positioning the sample cell (and flask) in the conventional Aminco phosphorescence attachment is evident from the data in Table I. Even with careful alignment of the sample cell and Dewar flask when using the conventional Aminco system, the precision is still nearly 10-fold poorer than with the rotating sample cell system. In fact, at the lowest concentration the precision with the conventional system is extremely poor, no matter how much care is exerted in sample cell and flask alignment. On the other hand, the precision resulting with the rotating sample cell assembly for the lowest concentration is comparable to the precision for the highest concentration measured. No systematic errors were present when using either the conventional Aminco system or the rotating sample cell. Since the precision is improved, the confidence interval with 99% confidence for the average concentration obtained from the measured signals for each condition in Table I was about 10-fold smaller when using the rotating sample cell than when using the Aminco system. Although precision phosphorimetric measurements were made only on the Aminco phosphorescence system, it should be possible to improve the precision of phosphorescence measurements when using similar phosphorescence attachments for spectrofluorometers manufactured by other companies, as well as the precision of other types of luminescence measurements-e.g., fluorimetry-at low temperatures when using capillary sample tubes. ACKNOWLEDGMENT
The authors thank Arthur P. Grant for helping to design the rotating sample cell assembly. The authors also thank Maarten Van Swaay of Kansas State University, Manhattan, Kan., for helpful suggestions concerning this work.
RECEIVED for review March 19, 1968. Accepted May 20, 1968. This research was carried out as part of a study on the phosphorimetric analysis of drugs in blood and urine, supported by a grant from the U. s. Public Health Service (GM 11373-05).
