A standard deviation study was made by counting a resin column containing 137Cs,fifty times for periods of one minute. The average counting rate was 3491 cpm and the standard deviation found was =t53 cpm. The distribution of the individual counting rates was gaussian, an indication that the detector does not present the formation of spurious pulses. Sensitivity and Applications of the G.M. Counter. The background of the detector was found to be 38.8 cpm when placed in a conventional lead shield castle. Thus, this detector is capable of detecting 20 pCi of a source having 100 p-emission greater than 2 MeV and having a net counting rate equal to that of the background. It is assumed that the detector efficiency is 84.6z. The detector greatly increases the sensitivity limits in NAA. It can be used with those radionuclides having &particle emission of energies greater than 2 MeV with an absolute intensity of 6 0 z or greater. Such elements as W l , 4nK, 5BMn,65Ni,76As, S0Br,@Rb, gay, laITe, 1 9 , 139Ba, lz4Pr,and 1g41r have the previously mentioned characteristics and they can be produced by n,y reactions. These elements have half-lives greater than 18 min which allows the use of separa-
tion techniques such as ion-exchange resins where it may be possible to selectively retain an isotope. In special cases the radionuclides produced by NAA present no y-activity and the P-particles are sufficiently energetic to be counted with the G.M. counters. This is the case of alSi, 32P, and 210Bi. The latter elements have sufficiently long half-lives for isolation. There are other cases where a short-lived isotope formed by activation presents few possibilities of interferences and its decay can be studied with the G.M. counter. An example of this element is 20F which has a half-life of 10.7 sec and a P-energy of 5.42 MeV of 100% intensity. The useful reaction I6O(n,p)16N for determining oxygen presents the problem that the 16N can not be counted with good efficiency by y-spectrometry because of its gamma energies of 6.15 MeV and 7.1 MeV. However, the P-emission of I6N of 4.3 MeV and 10.3 MeV may be studied with high sensitivity with the G.M. counter.
RECEIVED for review September 27, 1971. Accepted February 11,1972.
A Thermo-Barometer Claude R. Reed Texaco Znc., Beacon, N . Y. 12508 MANYLABORATORY OPERATIONS require the measurement of gas volumes with subsequent correction of these observed volumes to volumes at standard temperature and pressure. Generally the standard temperature is 0 "C and the standard pressure is 760 mm of mercury absolute. These corrections require thermometer and barometer readings and the solution of the following equation: Volume at STP = 273 (Barometer reading) 760(273 "C)
[
+
1
-
rC TO APPARATUS MANIFOLD I
SEALEDJOINT
X Observed volume
Since the value for that part of the equation between the brackets is all that is needed for conversion of observed volume to volume at STP, a combination thermometerbarometer was designed so that this value (the gas law factor) can be read directly from a manometer scale. This "thermo-barometer" consists of a gas bulb of 300- to 500-ml volume attached to one leg of a manometer mounted on a meter stick. The other leg of the manometer is open to the atmosphere. The gas bulb is filled with dry air and the manometer fluid is dyed n-dibutylphthalate. After calibration, the scale from which the desired gas law factor value is to be read is inserted between the meter stick and the manometer. Figure l shows a thermo-barometer as used with a gas measuring buret. For calibration purposes, calculations were made to determine the theoretical manometer response of two thermobarometers, one with a gas volume of 378 ml and one with a gas volume of 526 ml. The theoretical meter stick readings for both manometer legs were calculated for all combinations of four temperatures and four pressures covering the temperature range of 21-32 "C and the pressure range of 740-770
GAS B U R m E
DYED BUTYLPHTHALAT E
i MANOMETER
Figure 1. A Thermo-Barometer
ANALYTICAL C H E M I S T R Y , VOL. 44,
NO. 11, SEPTEMBER 1972
1921
Table I. Manometer Scale Length Factors Internal gas bulb Scale length, mm per 0.01 volume, ml increment of gas law factor Manometer tube cross sectional area of 0.29 cm* 50 100 150 200 250 300 350 400 450 500 lo00
13.9 21.3 27.1 31.4 34.4 37.1 38.8 40.0 41.2 42.1 47.9 a 54.4 Manometer tube cross sectional area of 0.25 cm2 300 38.8 400 40.8 Manometer tube cross sectional area of 0.30 cm2 200 31.3 500 41.7
mm. The calculated values for meter stick readings, plotted against calculated values for gas law factors, were within 0.2z of a straight line. The calculated open leg meter stick readings were used to make the scales necessary for direct reading of the gas law factor from the open legs of the manometers. These scales are inserted between the manometers and meter sticks so that 0.911 on the scales was at the
same level as the 500-mm mark on the meter sticks. The volumes of air in the gas bulbs were adjusted to give nearly correct gas law factor readings from the scales. Final adjustments were made by sliding the scales up or down to give correct readings. The scales were then fixed in these positions. Observed gas law factor values obtained using these two thermo-barometers were within 0.2 of the calculated values. This slight deviation from calculated gas law factor values is due to thermal expansion and contraction of the manometer fluid. Additional calculations were made to obtain data for use in making gas law factor scales for thermo-barometers with a wide range of gas bulb volumes. These calculations were made for pressures of 740 mm and 770 mm at 26.7 “C and assumed a balanced manometer at 26.7 “C and 760 mm. The use of one temperature avoided the effect of temperature change on the volume of manometer fluid. As shown in Table I, most calculations were for manometer tubes with a cross sectional area of 0.29 cm*. Data are also listed for manometer tubes with cross sectional areas of 0.25 cm2 and 0.30 cm2. Note that an increase or decrease in cross sectional area requires a corresponding increase or decrease in gas volume for the scale length to remain constant. Also note that scale length does not increase rapidly for gas volumes above 300 ml. A similar instrument could be made by filling the capsule of an aneroid barometer with an inert gas and calibrating for read-out of gas law factors.
RECEIVED for review April 26, 1972. Accepted May 25, 1972
New, Simple Windowless Cell for Front-Surface Fluorometry J. A. McHard and J. D. Winefordner’ Department of Chemistry, University of Florida, Gainesville, Fla. 32601
THE STANDARD 1-cm2 fluorometric cells have several disadvantages : they are fragile; surfaces are readily contaminated by fingerprints, dust, dirt, etc., in normal handling; prefilter and postfilter effects occur at high concentrations of analyte when the cells are utilized in the common right angle method of illumination observation; and turbid, highly colored solutions and pastes cannot be measured. Front surface illumination-observation minimizes several of the above problems ( I ) . However, all previous front surface cells (2-12) have been constructed from fragile
Front view of
screen with window I0m.m.
I
Window centered in s c r e e d 1
Author to whom reprint requests should be sent.
(1) C . A. Parker, “Photoluminescence of Solutions,” Elsevier New York, N.Y., 1968. (2) T. Hirschfeld, Caiz. Spectrosc., 10, 128 (1965). (3) &id., 11, 102 (1966). (4) Zbid., p 115. (5) T. Hirschfeld,Appl. Opt., 6, 715 (1967). (6) E. Kuntz, F. Bishai, and L. Augenstein, Nature, 12, 980 (1966). (7) S. Ainsworth, ANAL.CHEM., 37, 537 (1965). (8) F. Bertram and W. Kobisch, Chem. Ztg., 91, 809 (1967). (9) H. Jork, “Quantitative Paper Thin-Layer Chromatography Symposium,” E. J. Shellard, Ed., Academic Press, London, 1969, P N. (10) B. L. Hamman and M. M. Martin, Anal. Biochtm., 15, 305 (1966). (11) C. A. Parker, Aiinlyst (London),94, 161 (1969). (12) G. Winkelman and J. Grossman, ANAL. CHEM.,39, 1007 (1967). 1922
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Scale in m.m. L “ ‘
0
2
4
6
v
Figure 1. Top section view of cell mounted diagonally in standard 1 cm X 1 cm Aminco fluorometer cell housing
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
