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 13.9 100 21.3 150 27.1 200 31.4 250 34.4 300 37.1 350 38.8 400 40.0 450 41.2 500 42.1 lo00 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). 37, 537 (1965). (7) S. Ainsworth, ANAL.CHEM., (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
-
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
Brass plate thickness 4.5m.m.\
Tcavity
I
Figure 2. Front face view of brass cell
qiameter 8m.m.
front face 16 m.m. c
back face
18m.m.
*
materials and/or have required extensive modification of the optical train through the sample compartment. For example, Hirschfeld (2-5) described a multiple internal reflectance system for use with turbid or highly colored solutions, but the design required a system of mirrors and a sample carriage. Measurement of fluorescence of chromatographic strips has been studied (&IO), and commercial scanners are currently available. Again, these have specific and limited usefulness and require instrument redesign. Commercial instruments for front surface illumination (of liquids) require special housings, and the cells are difficult to clean and manipulate-particularly when the samples are pastes or solids. In this communication, a novel, simple, windowless, fluorometric sample cell is described which fits directly into the sample compartment (Instrument Brochure No. 768F, American Instrument Co., 1964) of the Aminco spectrophotofluorometer (SPF). The sample cell assembly (see Figures 1 and 2) consists of a brass plate (32 mm X 19 mm X 4.5 mm) bevelled on the edges to slide diagonally into the standard 1-cm2holder of the SPF (see Figure 1). The sample is placed into a circular cavity (8-mm diam X 3.75-mm depth) positioned laterally in the brass plate so that the cell window is centered at the center of the incoming excitation beam. A circular (10-mm diam) monel screen (50 mesh) with an opening (5 mm X 3 mm) is press-fitted into a 10-mm diameter counter bore which is just deep enough to accommodate the thickness of the screen. Solutions are placed into the cell through the opening by means of a 250-p1 hypodermic syringe. The liquid is kept in the cavity by surface tension. The geometry of the bevelled edges is such that the cell, when in place, rests at an angle of about 40" to the incoming excitation beam. The angle is displaced from 45" to minimize specular reflection. Specular reflection is negligible as long as the angle is 40" and the excitation and emission peaks are separated by at least 50 nm. The cell holds about 150 p1 depending on the depth of the cavity. If the cavity is too
Arrow indicates possible extension of curve to higher concentrolions.
I
I
0.I LOG
1.0 OF CONCENTRATION
I
1
10 OF QUININE
100 IN
PPM
Figure 3. Analytical curve (plot of log fluorescence signal cs. concentration) of quinine in 25 (aqueous) glycerine (0.05M in
H804 shallow, specular radiation from the rear surface results in appreciable background signals. By selecting appropriate slit arrangements, the analytical curve for quinine sulfate (see Figure 3) is linear over a concentration range of three decades (from about 100 ppb to about 100 ppm). The upper concentration is well above the upper limit attainable with standard 1-cm2 fluorometric cells. The lower concentration limit of linearity is also above the lower value obtained with the standard 1-cm2fluorometric
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
1923
cell; however, the absolute limit of detection is quite good with the novel front surface cell because of the need of only about 150 MI of solution as opposed to several milliliters of solution in the standard 1-cmz cells. The major advantages of this new front surface cell d o not rest with detectability but rather with simplicity of filling, durability, adaptability to existing instruments, usefulness with turbid and highly colored solutions, and usefulness with pastes (in this case, the front surface screen is removed to allow easy filling and filling of the sample cell cavity).
ACKNOWLEDGMENT
The authors acknowledge the assistance of Arthur Grant and D. J. Burch in helping to design and construct the front surface cell. RECEIVED for review April 7 , 1972. Accepted May 25, 1972. Research carried out as a part of a study on the phosphorimetric analysis of blood and urine, supported by U S . Public Health Service Grant G M 11373-09.
Triac Switching Circuitry for Eliminating Interfering Transients in Digital Logic Automated Systems E. S. Iracki,’ M. B. Denton,Z and H. V. Malmstadt School of’Ciiemical Sciences, Department of Chemistry, Unicersity of’lllinois at Urbanri-Ciiat,ipc~i~n, Urbana, Ill. 61801
THEUTILIZATION OF DIGITAL LOGIC (1-3) and computers (4-6) for automation of analytical instrumentation is continuously increasing. Quite often, it is necessary for the associated circuitry to switch various 120-volt ac line operated devices, such as solenoid valves, motors, etc. When logic driven relays and motor-driven cam operated switches are employed, transients are often encountered which interfere with both digital and analog electronic systems. These transients are the result of arcs between opening switch contacts caused by energy stored within inductive loads. A triac is useful for switching ac loads; it ceases conduction at zero current, causing the circuit to be opened at the zero voltage crossing point. This results in no energy being stored and no transients are generated (7). Circuit. Figure 1 gives a circuit of four triacs and associated circuitry which can be driven by the popular TTL (zero, and +5 volt) current sinking logic levels. Any number of these switching circuits can be added together as long as the isolation transformer is capable of handling the combined load. This form of isolation was most convenient for the authors because of the “spare parts” availability of the transformer; however, one might wish to utilize photoconductors ( I ) or other devices in alternative isolation techniques. The basic simplicity of the transformer isolation technique renders its use rather appealing; however, the cost of devices with higher volt-ampere ratings (e.g., several kilovolt-amperes) might preclude their use in a designer’s circuit. Operation. When the input is a t ground level (zero logic level) the 2N5138 transistor is forward-biased, causing its collector to be positive; and the triac gate diode is reversebiased. Therefore, the triac does not conduct. When a +5 volt logic level is applied, the 2N.5138 is no longer forwardbiased and the collector approaches -15 volts. The gate 1 Present address, Environmental Systems Engineering, Clemson University, Clemson. S.C. * Present address, Department of Chemistry, University of Arizona, Tucson. Ariz.
(1) H. V. Malmstadt and C. G. Enke, “Digital Electronics for Scientists,” W. A. Benjamin, New York, N.Y., 1969. (2) H . V. Malmstadt and C. G. Enke, “Computer Logic,” W. A . Benjamin, New York, N.Y., 1969. (3) J. S. Springer, AXAL.CHEM., 42 (8), 22A (1970). (4) J. W. Frazer. ;bid.. 40 (8), 26A (1968). ( 5 ) G. Lauer and R. A. Osteryoung, ;bid., 40 (101, 30A (1968). (6) S. P. Perone, ihitl., 43, 1288 (1971). (7) “Silicon-Controlled Rectifier Manual,” 4th ed., General Electric, Syracuse, N. Y . , 1967. 1924
4
II
/ I
,
ny
~
4 7K
-
- 15V
2N5l38
-
lr(
I
Gnd.
120 VAC
Figure 1. Schematic of triac switching circuit diode is forward-biased and the triac conducts. The SC45B triacs are rated a t 10 anipercs, and loads LIP10 this valuc may be applied on any individual circuit. However, total power consumption should not exceed the rating of the isolation transformer. Transformer TI is an isolation transformer to prevent the ground level of the logic system from becoming 120 volts ac. This device should not be operated without some form of isolation as severe electrical shock could easily result. Construction. The unit was constructed on a 7- X 9- X 2-inch aluminum chassis. A small front panel holds ChinchJones 300 series plugs for connecting the solenoids. Special plugs are recommended to prevent someone from rashly plugging in a large load. Banana plugs are used for the inputs. For general use, it is suggested that a cover of some type be placed over the triacs and output plugs to prevent any possible shock hazard. Application. Any source of TTL logic levels can be used. The authors have found two types of driving circuits particularly useful. When a load is going to bc turned on for a fixed time up to approximately 40 seconds, a monostable such as the SN74121 (Texas Instruments, Inc., Dallas, Texas 75222) or the Heath dual monostable multivibrator card EU-800LA (Benton Harbor, Mich. 49022) can be triggered by a computer or controlling logic (see Figure 2 4 . In both cases, the monostable is triggered by the trailing edge of the
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
