the dependence could be described in a form of a simple power equation of the following type:
I,=fcoP
( 11)
where f and m are experimental parameters. However, as Equation 11 is only an approximation to actually more complex dependence given by Equation 10, it is applicable for rather limited ranges of the flow rates, should f and m be constant. Equation 11 transforms into Equation 8 for the case of electrolysis with small limiting degrees of conversion. On the other hand, at high limiting degrees of conversion corresponding to almost complete electrolysis of the flowing substrate, the exponent m becomes close to 1, as was experimentally shown before ( 4 ) . In the interme-
diate range of the limiting degrees of conversion, both the exponent m and constant f change appreciably. The change of the exponent is illustrated by the experimental data obtained for a porous electrode of different heights built of graphite granules (6).
Roman E. Sioda Institute of Physical Chemistry Polish Academy of Sciences Warszawa, ul. Kasprzaka 44, Poland Received for review August 22, 1973. Accepted December 6, 1973. (6) R E Sioda, Electrochm Acta 13, 1559 (1968)
I AIDS FOR ANALYTICAL CHEMISTS Dissolution of Silicate Minerals and Glass for Trace Analyses Pieter Knoop Analytical Department of the Glass Development Centre, Production Division Glass, N. V. Philips' Gloeilampenfabrieken, Eindhoven, The Netherlands
In trace analyses, the problem with materials which are difficult to dissolve-e.g., glass, sand, and mineral silicates-is the impurity of the reagents to be used. Sometimes the amount of an element added together with the reagents is larger than the quantity originally present. To avoid these difficulties, we made solutions of pure sand and glass with the aid of Eppendorf micro-test tubes (Cat. No. 3810).
EXPERIMENTAL For t h e determination of trace elements, 50 mg of sample was weighed into the test tube. With the aid of a n Eppendorf pipet, 500 ~1 of' Merck supra-pure HF 40% was added and t h e lid firmly closed. After mixing, the test tube was placed in a Teflon-lined bomb ( I , 2 ) . These bombs, the dimensions of which are given in Figure 1, are easily made in a normal factory workshop. Ten test tubes ( e . g . , 8 samples and 2 blanks) are conveniently placed in a bomb of the size stated in Figure 1. About 10 t o 15 ml of the same hydrofluoric acid as is used in the test tubes is poured over these tubes t o balance the pressure build-up during t h e heating period. The Teflon lid of the bomb is replaced a n d t h e bomb is closed handtight. T h e whole is placed in a laboratory furnace for 1 hour a t a temperature of 130 "C (max. 150 "C). After cooling to room temperature, the bomb is opened. T h e test tubes are inspected for complete dissolution and rinsed with distilled water. Samples were directly injected by means of pl Eppendorf pipets into a flameless atomization device, in our case t h e Perkin-Elmer HGA 70. To protect the windows of t h e atomic absorption spectrophotometer Perkin-Elmer 403, a n exhaust system was installed on either side of the atomization device.
RESULTS AND DISCUSSION In this way we could determine in sand, 0.1 ppm Al, 0.1 ppm Cu, and 0.2 ppm Fe, with blanks of about of the ( 1 ) 6. Bernas,Ana/.Chem.. 40, 1682 (1968). (2) F. J. Langrnyhr and P. E. Paus, Anal. Chim. Acta, 43,397 (1968)
Figure 1. Physical dimensions of Teflon-iined bomb
above-mentioned values and a relative precision of 10%. In pure glasses we also determined Ni and Co in an order of 0.1 ppm. The micro-test tubes, together with the two types of tips for the Eppendorf ~1 pipets were left for at least 24 hours in a mixture of equal parts HC1 ( 1 M ) and acetone. Test tubes were handled with laboratory tongs made of a material not to be determined and rinsed with plenty of double-distilled water. The adhering water was removed by vigorous shaking. Then the tubes were put in a rack and left in a dust-poor environment for YZ hr to evaporate the last traces of water. If iron had to be determined. the A N A L Y T I C A L CHEMISTRY, VOL. 46, N O . 7, J U N E 1974
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outer mantle of the bomb was made of pure copper (99.9%), and for the determination of copper we used a stainless steel mantle. In determining sodium and potassium, all handling must be done with gloves. Marking or numbering was done with a green felt pen. In case of doubt, first test to see if the marking remains after the treatment.
We think that the above-mentioned method of dissolution can be adapted to other types of analysis, provided no reaction gases emerge from the samples to be dissolved. Received for review August 16, 1973. Accepted October 19, 1973.
Simple Temperature Programmer for a Mass Spectrometer Direct Insertion Probe Jouko J. Kankare Department of Chemistry, University of Turku, 20500 Turku 50, Finland A standard procedure for recording mass spectra of relatively involatile solids is to insert them directly into the ion chamber of the mass spectrometer using a specially designed solids inlet probe. Temperature of the sample is raised either by using the ion chamber heater or a separate heater of the solids inlet probe. Heating rate is either manually controlled or “ballistic”--i.e., controlled only by the rate of heat transfer from the environment. Majer et al. (1, 2 ) have shown that certain isomers have sufficiently different evaporation rates so that their separation is possible in the mass spectrometer solids inlet. They used the ballistic heating method. In some cases, they mentioned that the temperature of the ion chamber was rather critical in order to achieve satisfactory separation. It can be anticipated that if a smooth temperature control were available, more reproducible results might be obtained. The Perkin-Elmer double-focusing midresolution mass spectrometer Model 270 has a simple manual temperature control for the solids inlet system. Temperature is adjusted by controlling the voltage over the heater spool by a simple transistor circuit and a potentiometer whose knob is on the front panel of the instrument. This knob must be turned very slowly in the beginning of the heating process in order to avoid a sudden rise in the pressure of the ion chamber. Every user of this instrument also knows the “ghost peaks” in the total ion current which arise when the potentiometer is not turned sufficiently smoothly. With this particular instrument, there is, however, a very simple method for overcoming these difficulties. The instrument is a combined GC-MS, and the gas chromatograph has a temperature programmed oven heater controller. The temperature ramp is produced by a potentiometer driven by a stepper motor. It is rather easy to connect an additional potentiometer mechanically to the shaft of the gear-box. This potentiometer delivers the reference voltage to the control circuit of the solid inlet heater. There are now, in principle, two methods for producing a smooth temperature rise in the solids inlet system. One is a complete control system similar to that used in the GC oven heater control. Another possibility is to use the original circuit. The first method involves a complicated circuitry, but the temperature rise obtained is strictly linear. Fortunately, this linearity is not required in most applications, and in this work the latter method was employed. In the original circuit, the control potentiometer was nonlinear, but in this work, a linear precision 10-turn potentiometer was used. The heater voltage delivered by the circuit is proportional to the deviation of the potentiometer slider from (1) J. R . Majer and M . J. A. Reade, Chern. Cornrnun., 1970, 58. (2) J. R. Majer and R. Perry, J. Chern. SOC. A , 1970, 822.
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A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 7, J U N E 1974
di#r,n b y ”‘DQ“’rnl,
rnOl0,
Figure 1. Temperature programmer for the solids inlet heater. The added circuit is enclosed by dotted line
the starting point unless some kind of nonlinearizing method is employed. Because the heating power is proportional to the square of voltage, it is desirable to have a voltage which is proportional to the square root of the deviation of the slider. This can be approximated to some degree of accuracy by connecting two resistors between the slider and both ends of the potentiometer. The values of the resistors shown in Figure 1 have been calculated so that below one-fourth of the total length of the potentiometer, the electrical power delivered to the solid sample is very nearly a linear function of dial position. The total resistance of the resistor circuit is not constant and thus the voltage over the potentiometer must be held constant using a zener diode. As previously mentioned, the different evaporation rates of some isomers have been employed for their analytical separation in the direct insertion probe of a mass spectrometer. It is clear that the method is not restricted to the analysis of isomer mixtures, and, indeed, it seems that it is highly useful, e.g., in evaluating the purity of synthetic products. A real example of this application is shown in Figure 2. 1-Naphthonitrile was allowed to react with salicoyl hydrazide p-toluenesulfonate in order to synthesize 3-(l-naphthyl)-5-(o-hydroxyphenyl)-1,2,4-triazole. The product was crystallized several times from benzenepetroleum ether, but the rather wide melting range showed that it was impure. In order to analyze this persistent impurity, a 10% solution in chloroform was made from the product. Five microliters of this solution was injected into a solids inlet sample capillary, the solvent was evaporated, and the tube was transferred into the direct insertion probe. The temperature of the probe was raised a t the rate of 5 centigrades per minute (GC panel setting). Total ion current was continuously monitored on a strip chart recorder and mass spectra were recorded a t peak current values. The spectra showed that the first peak in Figure 2 corresponded to a component with a molecular
