I CORRESPONDENCE I
I
Plasma Sampling Mass Spectrometry for Trace Analysis of Solutions Sir, In the course of a study of the available techniques for solution analysis by mass spectrometry, it was concluded that the well established technique of mass spectrometric sampling of flames ( 1 ) could be applied to extract ions into a mass analyzer from an atmospheric pressure plasma. Inert gas plasmas maintained by a variety of electrical means have been extensively studied for emission spectrometry, and methods of sample introduction to them from solutions have been widely developed. The advantages of direct sample introduction to an atmospheric pressure ion source in terms of operational convenience and speed of analysis, if such a system were practicable, were appreciated and an experimental program to explore this was undertaken. A suitable plasma unit was available in the Capillary Arc Plasma developed by Jones (2). This is a small dc plasma in argon operating a t about 10 A and achieving core temperatures of some 5000K. This could be fed stably with nebulized solutions from either an ultrasonic or pneumatic nebulizer at rates of up to 0.25 ml per minute. Although arc stability was not affected by the water content, it was preferable to desolvate the sample by means of a heated chamber and water cooled condenser. If the whole sample could be subjected to the full core temperature, most elements of the periodic table when introduced from solution a t the level of a few wg ml-' should be completely ionized. However, very few have second ionization potentials low enough to contribute significant quantities of doubly charged ions a t 5000K. At atmospheric pressure, the ions present in the plasma are in equilibrium with the gas and thus have a small energy spread around 0.5 eV, to which the low field across the arc contributes very little. The ions present are therefore ideally suited for introduction into a simple mass analyzer.
a t about -200 V while neutral molecules were pumped away. The ions were focused by further electrodes through a second aperture into the second chamber pumped to a pressure of less than lop5 Torr. Here, they were directed along the axis of a small quadrupole mass analyzer. Those ions transmitted by the analyzer were then collected by a channel electron multiplier operated in the saturated mode. Pulses corresponding to the arrival of ions a t the multiplier mouth were fed to a standard pulse counting chain. The recorder output of a ratemeter was used to provide the Y drive for an X-Y plotter, while the X drive was derived from the quadrupole analyzer mass scan. The mass analyzer was fitted with 5-inch X 0.25-inch rods and scanned a range of mle values of 5-260, providing a resolution of about 200 (10% valley). A diagram of the system is shown in Figure 1. To determine the usefulness of the technique, a series of test solutions were prepared in distilled and deionized water a t concentrations of 1, 10, and 100 pg ml-I of a number of convenient elements. Once the system had been optimized, useful spectra could be obtained of the solution being nebulized.
RESULTS A typical spectrum obtained with the test solutions is shown in Figure 2. A mixture of equal parts of solutions containing lwg ml-l each of lead and cobalt was used. Substantial peaks for the isotopes of lead and for cobalt may be seen and no evidence of doubly charged lead ions is found. Contamination peaks due to sodium 23Na+and potassium 39K+, and 41K+ may be seen and also peaks due to 40Arf and 30[NO]+.Other major peaks in the spectrum may be identified as 19[OH3]+and its hydrates a t mass 37 and 5 5 , the hydrate of 39K+a t mass 57, and that of 5 g C ~a+t 77. Most of these ions arise from ion molecule reactions in the space between the plasma core and the sampling orifice and depend on sampling conditions and, particularly, on orifice temperature. Peaks from ions produced by these mechanisms are the principal source of interference, since hydrocarbons from the vacuum system do not become ionized with an external ion source. A further spectrum obtained under similar conditions from a mercury solution of concentration 100 pg ml-l is shown in Figure 3. The sensitivity obtained for mercury is appreciably lower than for lead or cobalt, as would be expected from its high ionization potential. However, it is also felt that its high volatility makes satisfactory sample injection into a plasma of this type more difficult.
EXPERIMENTAL The exhaust gas from the plasma emerges as a tail flame about 2 cm long and this was allowed to impinge on a small orifice, 7 5 bm in diameter, in the tip of a metal cone. The interior of the cone formed part of the first chamber of the vacuum system. This chamber was pumped to a pressure below Torr. Ions entering by the orifice were collected by a cylindrical electrode maintained SAMPLING ORIFICE, CAPILLARY
VACUUM CHAMBERS
SAMPLE
TO VACUUM PUMPS
200
PULSE
206
POWER
CONTROL
204
50
150
100
I
207
1
200
Ve
Figure 1. Schematic of analyzer system 600
Figure 2. Spectrum of aqueous solution containing 0.5 pg ml-I each of cobalt and lead
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
Table I. Aqueous Solution Detection Limits (2 g ) Element
I
Ag As Cd co Cr cu Fe Hg Mg Mn
I
I
Pb Se Zn
/I
Figure 3. Spectrum of aqueous solution containing 100 k g ml'' mercury
Detection limit,
urj
ml-1
0.0001
0.0005 0.001 0.00004 0.00008 0.0008 0.00006 0.14 0.001 0.0009 0.00005 0.02 0.008
suggest that this technique of Plasma Sampling Mass Analysis (3)has a useful place in multielement analysis.
of
LITERATURE CITED By setting the mass analyzer to the peak of the element of interest, counts may be integrated for the sample and for background on a blank. In this way, a series of values may be obtained for detection limits. Limits obtained for 30second integrations for a number of elements, expressed as the level equivalent to 2 cr background, are shown in Table I. Although much remains to be done in optimizing the system, these detection limits, the ease with which the sample may be introduced with little preparation, and the simplicity of programming element selection with a quadrupole
Knewstubb, "Mass Spectrometry of Organic tons," Academic Press, New York, N. Y., 1963, Chapter 6, p 255. (2) J. L. Jones, R. L. Dahlquist, and R. E. Hoyt, Appl. Spectrosc. 25, 628 (1) P. F.
(1971). (3)
Patents Applied for.
Alan L. G r a y Applied Research Laboratories Ltd. Wingate Road Luton, England
RECEIVEDfor review July 29, 1974. Accepted October 21, 1974.
I AIDS FOR ANALYTICAL CHEMISTS Separator Tubes for Electrolysis Cells Made from Fluoropolymer Ion-Exchange Membranes J. E. Harrar' and R. J. Sherry2 Lawrence Livermore Laboratory, University of California, Livermore, Calif. 94550
One of the critical components in most cells for controlled-potential electrolysis and coulometry is a semipermeable separator or diaphragm for isolating the counterelectrode compartment from the working-electrode compartment. Various kinds of materials and assembly designs have been used (1-3) to obtain the ideal properties of low electrical resistance, negligible flow of the solvent and the species of interest, chemical inertness, and good mechanical stability under diverse conditions. Ion-exchange membranes have performed satisfactorily in many electrolysis cells ( I , 2 ) ;however, they have not always possessed the desirable long-term chemical inertness, and it has not always been possible to fabricate these membranes in the configurations required for optimizing the cells. This Aid describes the fabrication and use of counterGeneral Chemistry Division. Research Engineering Division.
electrode separator tubes incorporating a relatively new type of ion-exchange membrane, the base polymer of which has properties similar to those of Teflon. The membrane is in the form of tubing, which is unique because it permits the fabrication of a cylindrical separator that is optimum for metal-gauze working-electrode cells. Two techniques for fabricating the counter-electrode separator tubes are described: one using a Kel-F holder for the membrane, and the other employing a new fluoropolymer tubing that is heat-shrinkable at a relatively low temperature that does not damage the membrane. EXPERIMENTAL The material tested for use as a separator is the Nafion-brand perfluorosulfonic-acid membrane manufactured by E. I. du Pont de Nemours and Company. The method of manufacture of this membrane and some of its uses are described in Reference 4; and its physical, mechanical, and general electrochemical characteris-
ANALYTICAL CHEMISTRY, VOL. 47, NO. 3, MARCH 1975
601
