‘ I
, IO
GAUSS
the only one that might be touchy is UV irradiation work, since irradiation of Teflon gives rise to detectable EPR signals. The Teflon tubing itself was tested for paramagnetic resonance signals by tuning the empty cell and scanning from 2400 to 4400 gauss at 9.502 GHz (employing a Varian E-3 spectrometer) at high sensitivity. No signal was observed above the normal noise level. The advantages of this cell, then, include ease of fabrication, low cost, and flexibility in that one can construct different cells for various applications at a minimal expense. Clearly these cells cannot be used for UV irradiation work or at elevated temperatures (they are, however, usable for low temperature studies). Other limitations that could probably be overcome with little effort are the reproduction and constancy of cell dimensions and the unsuitability for aqueous studies. With more frequent applications of EPR spectroscopy in analytical work beginning to appear, this versatile, inexpensive cell may be of considerable utility; in addition, it seems ideally suited for routine non-aqueous EPR studies.
,
Figure 2. EPR spectrum of p-nitrotoluene anion radical electrolytically generated in N,N-dimethylformamide-O.1F tetraethylammonium perchlorate at platinum
ACKNOWLEDGMENT
idea is simply that heat-shrinkable Teflon can be shaped to serve in whatever manner desired. The configuration can readily be adapted to a particular need, since the upper and lower glass portions can be constructed in any way desired before they are inserted into the Teflon tubing. It should be usable for many types of EPR applications; about
Special thanks are due to R. N. Adams and D. E. Smith for their support and encouragement. RECEIVED for review January 14, 1970. Accepted April 9, 1970. Work supported by NSF Grant No. GP-8941.
A Syringe Pycnometer for the Accurate Weighing of Milligram Quantities of Aqueous Solutions G . C. Lowenthal and Valdis Page Research Establishment, Australian Atomic Energy Commission, Lucas Heights, N.S. W. 2232, Australia THEACCURATE WEIGHING of open drops of an aqueous solution is complicated by evaporation effects. These effects are commonly allowed for by reading the drop mass at regular intervals after the ejection of the drop; the results being extrapolated to the time of ejection. However, the loss of heat from freshly formed drops is nonlinear for the first 5 to 10 seconds because of rapid evaporative cooling immediately after deposition ( I ) . The resulting temperature changes depend among others on the surface to volume ratio and are the greater for smaller drops. For a 10-mg drop the error from a linear extrapolation could reach 1%. The errors in small drop masses can be reduced substantially if a pycnometer is used for the weighings (1, 2 ) or if the temperature of the ejected aliquots is controlled so as to ensure the validity of a linear extrapolation ( 3 , 4 ) . The currently preferred method for the accurate weighing of milligram drops of aqueous solution is to use plastic ampoule pycnometers of about 8 ml air displacement with their opening (1) P. J. Campion, F. W. G. Dale, and A. Williams, Nucl. Znstrum. Methods, 31, 253 (1964). (2) J . S. Merritt and J. G. V. Taylor, A. E. C. L. Report, Chalk
River, Ontario, CRGP-1256 (1967). (3) A. E. Oakley and G. C. Lowenthal, Proc. I.A.E.A. Symp. Vienna STI/PUB/139SM 79/42, p 519 (1967). (4) W. van der Eijk and H. Moret, Proc. I.A.E.A. Symp. Vienna STI/PUB/139SM 79/43, p 529 (1967).
pulled out to a 5 to 8 cm long finely drawn tube (2, 5). However, it is then difficult to vary drop sizes except by varying the diameter of the outlet tube and the upturning of the ampoule during delivery disturbs the equilibrium conditions sufficiently to require a delay of 5 to 10 minutes before reproducible balance readings (within =k 2 pg on a microbalance) are reestablished. DESCRIPTION
To provide for increased flexibility and speed without loss of accuracy we have replaced the ampoule by a syringe pycnometer mounted as shown in Figure 1. The 2-ml capacity high quality disposable syringe is made from polypropylene and is supplied by the Boots Pure Drug Company of Sydney. The piston moves smoothly and the amount of vapor escaping between piston and wall is generally less than about 1 pg h-1; only about 10% of the syringes had leak rates past the piston in excess of 5 pg h-l. The syringe outlet is either the usual steel needle or, if acid solutions are used either 4-mm bore polyethylene tubing drawn at one end to about 0.3 mm i.d. or preferably glass needles. The outlet is heat-sealed to the conical end of the syringe to ensure a vapor tight fit. Figure 1 shows a glass needle of about 0.5-mm bore and 4 to 5 cm long; it was made as described elsewhere (3) and treated inside and outside with silicone fluid to ensure a clean break of the drop formed (5) A. P. Baerg, Metrologiu, 2, 23 (1967). ANALYTICAL CHEMISTRY, VOL. 42, NO. 7, JUNE 1970
815
Table I. Radioactive Concentration of Two W o Solutions Determined by Counting the Activity in Weighed Samples Solution 1 Radioactive : concentration at To(rCi g') P
L
12.67L 23.56: 16.79( 12.10t 12.972 16.97'i 15.335t
Figure 1. Dispensing of solution from the syringe pycnometer
mean
taining aqueous solutions was in the range 5 to 10 p g h-' at 40% relative humidity, about the same loss as from plastic .ampoule pycnometers. The syringe is attached to an aluminum strip 10 cm X 1.2 a n x 0.15 cm, using a Terry clip. One end of the strip had been turned up at right angles, drilled, and tapped to take a 2 cm long 4 BA aluminum alloy screw whose head was extended by a 5-mm diameter Teflon tube (see Figure 1). The assembly weighs 13 grams and its air displacement is 8.8 ml; errors resulting from corrections for ambient variations in temperature and pressure are always within *3 p g . We use the instrument for dispensing radioactive solutions when preparing radioactive counting sources. Solution from a storage vial is sucked into a syringe which is then clipped to the support and allowed to equilibrate inside the balance case, usually for 20 to 30 minutes. The pycnometer is handled by suitably shaped tweezers and during deposition by pliers as shown in Figure 1. After a drop has been dispensed the liquid is retracted from the needle and the syringe returned into the balance case; a 3-minute waiting period is sufficient for restoring the equilibrium state for the second weighing.
RESULTS The accuracy of weighings with the syringe pycnometer and a Mettler M5 microbalance has been checked hy the 45r 0-y coincidence counting of 6oCo sources prepared from stock solutions which is a well established method (1-3). The results for sources weighed out from two solutions whose radioactive concentrations differed by a factor of nearly 4.4 are summarized in Table I together with the overall errors calculated as the standard deviations of a single result, a, and the standard deviation of the mean of each group, urn. Also included are ur, the average of the standard deviations of the individual counting results and a,,,, the standard deviation of the mean of all counting results. Although the results inTable I are subject to other errors the counting and weighing errors are dominant for sources made from sufficientlysmall drops (< 20 mg). The weighing error owlcan he estimated, since the overall error, a, is the quadrature sum of a
