Commercial 2-stage pressure regulators have been used to control the gas pressure. The seat should be leak-proof or a build-up of pressure will ensue. Ambient temperature should be maintained relatively constant (=t1") during a run. Flow rates are constant as long as sufficient Nz remains in the tank to register the required pressure on the gauge. Polyethylene tubing (0.060-inch 0.d. x 0.034-inch i d . ) is used to conduct the solutions. Where necessary, connections are made with pieces of 20-gauge needle. The rate of flow can be altered by variation in gas pressure. Variation in pressure in the range 20 to 40 psi resulted in efflux of 70% glycerol through tubing from the sphere of 4.8 to 9.2 ml/min, respectively. The flow rate was reduced in the complete system, which includes displacement of the gradient from the centrifuge tube and through the flow cell. Thus, 8 ml/min was obtained from the sphere with 35 psi, but this was reduced to 4.5 ml/min in the complete system. Although the device has not been tested with gradients of smaller volumes, we see no reason why equally satisfactory
results should not be achieved. It should also be applicable to puncture type systems in which the gradient is displaced from the bottom (3). One limitation that deserves mention occurs with 10-40 % sucrose gradients. With this solution, 70 glycerol mixes with the 40 sucrose. A clean junction can be obtained with 100% glycerol but this solution requires greater pressure for feasible flow rates than the system can handle.
x
ACKNOWLEDGMENT
We thank Helene Davis for her help in the evaluation experiments.
RECEIVED for review March 5,1970. Accepted June 18,1970. Support for this research was provided by United States Public Health Service Research Grant No. AM-00803. (3) M. K. Brakke, Anal. Biockem., 5,271 (1963).
Atomic Absorption Spectrometry for Direct Determination of Metals in Powders M. A. Coudert and J. M. Vergnaud Dkpartement de Chimie, Facultk des Sciences, Alger, Algbrie
SAMPLES FOR ATOMIC ABSORPTION analysis must usually be in some type of solution. Venghiatis, however, has mixed solid samples with flammable powders, the combustion of which resulted in vapors suitable for atomic absorption analysis (I). In another technique, the metal to be determined is adsorbed from solution into a flammable powder which is then burnt (2). The method presented here, which is quantitative and continuous, consists of a device feeding the powder at a constant rate inside the burner. The powder is then pushed into the flame by the gas mixture. EXPERIMENTAL Apparatus. A Techtron model AA-4 atomic absorption spectrometer was used. The burner was removed and replaced by a round one with a feeding device constructed by the authors (Figure 1). The powder first goes through the spiral conveyor from the hopper to the center burner channel and is moved up to the flame by the gaseous current. The screw rod which is rotated by a motor is kept vibrating in order to improve the flow of the powder and its regularity. A Perkin-Elmer palladium hollow cathode lamp was used. Lamp current was 20 mA. Air flow rate was 4 liters/minute and acetylene flow rate about 0.3 liter/minute (3). Reagents. The powders of the catalysts used were first diluted with calcium carbonate. Regardless of the metal concentration in the powder, it will then behave like pure calcium carbonate through the feeding device. The mass flow rate of the powder will remain constant. (1) A. A. Venghiatis, Ar. Absorption Newsleft., 6(1), (1967). ( 2 ) P. Maud and J. Robin, Bull. SOC.Chim. Fr., 1968,854. (3) M. A. Coudert and J. M. Vergnaud, C. R. Acad. Sci. Paris, Ser. C , 268, 1225 (1969).
I
i
Figure 1. Diagrammatic section of burner and feeding device for powders (1) burner, (2) hopper, (3) powder, (4) screw rod, (5) electric motor, (6) acetylene inlet, (7) air inlet, (8) capillary, (9) drain
tube Procedure. The adjustable prop of the burner and the feeding device for gases are kept in place; however, when powders are used, the capillary of the sprayer and the drain tube (for liquid not transformed to an aerosol) are closed. Damping of the galvanometer is maximum in order to avoid absorbance fluctuations. The amount of powder required for one measurement is about 1 gram-i.e., 10 mg of powder to analyze. The time required is about 10 seconds. Calibration curves were prepared for Pd on activated carbon, Pd on alumina, and the molecular sieve Siliporite for which the sodium had been exchanged for palladium. All materials had been previously diluted 100 times in calcium carbonate.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
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ders with those obtained with a liquid, Figure 3 shows the absorbances as a function of the mass flow rate of Pd in the flame (so that they can be plotted on the same graph). The mass flow rate of metal in the flame is expressed by Equations 1and2:
For powder
X=
QPcp
For liquid
X
QlC&
=
mass flow rate of Pd in flame feed rate of powder feed rate of liquid into the sprayer Pd concentration of powder Pd concentration of solution ratio of number of cm3 of solution in the form of an aerosol which reaches the burner to the aspiration rate of sample into the capillary.
X
= = QZ = C, = CZ = R =
Q,
0
i d 2 3 4 S 6 7 PALLADIUM WTO THE POWDER : ppm
8
b
?O'
Figure 2. Variation of absorbance as a function of the concentration of palladium in powder, expressed in ppm Siliporite A Carbon 0 Alumina
+
According to Figure 3, the sensitivities for powders are a little lower than those obtained with a liquid. Because the sensitivities for powders depend upon the nature of the carrier, the calibration curves should be performed with a carrier, the nature of which should be the same as that of the powder in which the metal is to be determined. The Pd standards were made by adding varying amounts of the Pd catalyst (5 Pd) to calcium carbonate. We could use two different methods for the preparation of standards.
To prepare different catalysts with a varying amount of Pd deposited on the same carrier and then to dilute each of them in CaC03at the same rate of dilution. To use the same catalyst (containing 5 % Pd in our experiments) and prepare the standards by varying the dilution rate in CaC03.
0
50 100 FLOW RATE oF
150
zoo
~wmw(yg/m)
Figure 3. Variation of absorbance as a function of mass flow rate of palladium in flame, expressed in pg/min
+
Solution Siliporite
We have chosen the second method to be sure that only one parameter is varying (weight Pd). With this second method, the size of Pd crystals might vary with the amount of Pd deposited and therefore the absorbance might vary at the same time. The particle size of Pd catalyst is much lower than the particle size of calcium carbonate. The Pd catalysts are diluted 100 times, so that the Pd standards behave like pure calcium carbonate through the feeding device, and the mass flow rate is then constant, It is important to always use calcium carbonate of the same particle size.
A Carbon 0 Alumina
Sensitivities
The origin of the calibration curve was determined by zeroing the instrument with the flame alone. The point corresponding to zero concentration is determined by a measurement with a powder of the carrier alone diluted in calcium carbonate. For such powders, the absorbance is negligible, so that the curves begin at the origin. The curves are almost linear for concentrations of Pd of less than 5 % (Figure 2).
Concentration required to produce 1 absorption, ppm
Carrier Siliporite Carbon Alumina Liquid
5.5 6 8.5 7.8
Mass flow rate of Pd in flame required to produce 1 absorption, &min 1.22 1.32 1.84 1.12
RESULTS AND DISCUSSION
According to Figure 2, the sensitivity is a function of the nature of the carrier. To compare the sensitivities for pow-
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RECEIVED for review December 3, 1969. Accepted April 27, 1970.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 11, SEPTEMBER 1970
