RECOMMENDED PROCEDURE
The following procedure is recommended for determining vanadium(V) with 6-hydroxy-l,7-phenanthroline. Sample. Procure a representative sample and subject it to any necessary preparative treatment to obtain a usable solution. If necessary, remove any interfering ions to bring them within the tolerance limits listed in Tables I1 and 111. The solution should be sufficiently acidic to prevent precipitation of hydrous oxides or hydroxides. Calibration Curve. With a pipet transfer 0, 0.50, 0.75, 1.00, 1.25, and 1.50 ml. of stock vanadium solution, containing 0.100 mg. of vanadium per ml., to a series of 100-ml. beakers. T o each beaker add 7.0 ml. of 0.2y0 (w./v.) solution of reagent, 6.0 ml. of 0.4M potassium hydrogen phthalate buffer solution, and 10.0 ml. of 1,2propanediol. Adjust the pH to 3.0 with
a solution of hydrochloric acid or sodium hydroxide, pour the solution into a separatory funnel, and extract the colored complex with two 10-ml. portions of benzene. Allow the system to stand until clear each time. Drain the benzene layers into a 25-ml. volumetric flask, dilute to volume with benzene, and mix. Measure the absorbance of each solution a t 480 mp against a reagent blank. Plot a calibration curve with absorbances as ordinate and wavelengths as abscissa. Procedure. T o 2.0 ml. of the prepared solution, containing not more than 0.15 mg. of vanadium, develop the color as already described. Extract the reddish complex in the same way with benzene, and measure the absorbance of the extract a t 480 mp against a reagent blank. Determine the amount of vanadium from the calibration curve.
LITERATURE CITED
(1) Bach, J. >I Anales ., Asoc. Quim. Arg. 28, 108 (1940). (2) Blair, A. J., Pantony, D. A,, Anal. Chim.Acta 13, 1 (1955). (3) Blair, A. J., Pantony, D. A,, Minkoff, G. J., J. Znorg. Nucl. Chem. 5, 316 (1957). , (4) Buscarons, F., Marin, J. L., Claver, J. J., Anal. Chim.Acta 3,310,417 (1949). (5) Duswalt, J. M., Ph.D. Thesis, Purdue University, 1961. (6) I., &I.G.. ANAL. . , Duswalt. J. >Mellon. CHEM.33,’1782 (1961). ’ (7) Hollingshead, R. G. W., “Oxine and Its Derivatives,’] Vol. 11, p. 509, Butterworths, London, 1954. \ -
~
(8) Le Rosen, A. L., Moravek, R. T., Carlton, J. K., ANAL.CHEM.24, 1335 (1952). (9) Montequi, R., Gallego, M., Anales SOC.Espan. Fis.Quim. 32, 134 (1934). (10) Talvitie, N. A., ANAL. CHEM. 25, 604 (1953). (11) Willard, H. H., Young, Philena, IND. ENG. CHEM.,ANAL. ED. 6, 48 (1934).
RECEIVEDfor review March 22, 1965. Accepted June 4, 1965.
Mass Analysis of Su bnanogram Quantities of Iodine J. A. McHUGH and J. C. SHEFFIELD General Electric Co., Schenectady, N . Y
b A
mass spectrometric method employing an ion source that generates ions by sputtering has been used in obtaining the isotopic abundance analysis from nanogram quantities of iodine. Negative iodine ions are mass analyzed in a spectrometer composed of two magnetic sectors followed by an electrostatic analyzer. The high abundance sensitivity (>10’ : 1 ) permits one to measure minute quantities of IlZ9 in the presence of large amounts of normal Ilz7. This method has direct application in studies utilizing Ilz9 tracer. The sensitivity of the method i s sufficiently high that 1 0-13 gram of a given iodine isotope can be detected in a sample.
W
ITH
THE
INCREASED
INTEREST
in IlZ9as a tracer (1, S), the need for simple and sensitive detection capabilities for iodine has become evident. The existing activation analysis technique for 1 1 2 7 and 1129, although quite sensitive, is a rather lengthy procedure, and one that requires convenient access to a high flux reactor. A mass spectrometric method utilizing an ion impact solids source (2) has been used in obtaining the isotopic abundance analyses of nanogram iodine samples. Negative iodine ions are generated in
the source through bombardment of an iodine sample with an energetic positive gas-ion beam. The secondary negative ions are accelerated, mass analyzed, and detected. The lower limit for detection of any iodine isotope, based on the observed ion intensities from known samples, is 10-13 gram. EXPERIMENTAL
A detailed description of the ionimpact-solids source is being prepared for publication ( 2 ) . This source, henceforth referred to as Source I, is shown schematically in Figure 1. The primary ions are generated by electron bombardment of Hg vapor in SI.The ion beam follows the path indicated, and is focused onto the sample filament Sz. Typical primary ion currents to S2 are to A. The in the range of secondary ions produced from the sample (in this case I-) by the primary ion beam are extracted, focused, and mass analyzed. A second source, henceforth referred to as Source 11, has been developed exclusively for the production of negative ions. A schematic of Source I1 is shown in Figure 2. Positive ions generated from the gas molecules in the ionization chamber are drawn out and bombard the sample a t S. The negative secondary ions are accelerated and focused by the lens system. The negative ions pass through the region where the positive ions are generated, through
the collimators, and into the mass analyzer. Source I1 has the advantage of being much simpler than Source I , and thus is preferred for negative ion production. The ion detectors employed in the mass spectrometers were electron multipliers operated with the first dynode a t ground potential and the collector at positive high voltage (this eliminates the loss in energy the I - would experience when impinging on a conventionally operated electron multiplier). The pulses are taken off through a high voltage capacitor, amplified, and fed to a high speed counting system. In order to obtain a chart recording of a spectrum, a count rate meter is used in conjunction with the high speed counter. The output of the count rate meter drives a strip chart recorder. DISCUSSION
The first attempt at obtaining an I - beam was with Source I and a 12-inch radius, 60°-sector, single-direction focusing mass spectrometer. One microgram of IlZ7was evaporated onto a flat W ribbon filament from an aqueous solution of KI. A large negative I”’ ion beam was observed. In addition low intensity negative ion beams of other masses, due mainly to hydrocarbons and oxygen-containing species, were observed. Large beams of W03- are observed from the W sample filament. VOL. 37, N O . 9, AUGUST 1965
1099
I
Bc I b
s2'
A E C T R O N BEAM Figure 1 .
Schematic diagram of ion impact solids Source
I
x
A heavy line on the main black indicates a conducting coating. Source electrodes have been designated as follows: DO drawing out, BC beam centering, C collimator, EA electrostatic deflector-analyzer, and T the tilt plates
-
-
-
-
The sensitivity of this source for iodine was demonstrated by analyzing a n iodine sample which contained 4 x 10-9 gram of fission product A spectrum taken from this sample is shown in Figure 3. For a primary ion current of 5 X 10-9A. we observed a counting rate in excess of 10,000counts per second for the Based on this observation one should be able to achieve a detection capability of 10-13 gram for the nonnatural-occurring ItZ9, or for that matter, any other iodine isotope. The successful demonstration showing the high efficiency this source has for iodine warranted installation of this source in a mass spectrometer which is capable of measuring large isotope ratios (>IO6 : 1). With our source in such a machine, it becomes possible to measure small quantities of one isotope in the presence of quite large quantities
-
of another isotope. This feature is required if one is to successfully use IlZ9as a tracer. One can foresee instances when large quantities of normal IlZ7will be present in the iodine sample. The mass spectrometer used in the remainder of this work possesses two stages of magnetic analysis arranged in a C-configuration, followed by a go", 12-inch radius, electrostatic sector which precedes the electron multiplier detector. A schematic layout of the instrument is shown in Figure 4. Source I1 has been installed in this spectrometer, and the results that follow will be for this source. Comparable results were obtained with Source I. The ability to control sample contaminants will ultimately set the limiting sensitivity of this method. The sputtering process is not as highly
L --I
NEGATIVE ION BEAM TO MASS ANALYZER
I
I
T
-8.8
a--l
~~~
i
(-0
POSITIVE ION PRIMARY BEAM AND NEGATIVE SECONOPRY ION BEAM
Schematic diagram of Source II
Refer to Figure 1 for electrode designations
1100 *
POTENTIAL)
/
S/
Figure 2.
ANALYTICAL CHEMISTRY
127 129 MASS Figure 3. Mass spectrum of an iodine l o p 9 gram of sample containing 4 fission product 1 l Z 9 Spectrum taken with Source I in single-stage, 1 2-inch radius, mass spectrometer
selective as the thermal or surface ionization process, and thus it is possible to generate ions from many different materials residing in or on the sample surface. There is selectivity, though, in that the electronic structure of sample atoms, or the atomic composition of sample molecules determines the ratio of positive to negative secondary ions produced. Because of this selection, the negative secondary ion mass spectra are freer of contaminant peaks than the corresponding positive ion spectra. This is because the major contaminants encountered are from hydrocarbons present as residual gases in the spectrometer vacuum system. Pure hydrocarbons produce a greater preponderance of positive ion species. However, contaminants containing oxygen or any highly electronegative species generally will yield appreciable negative ions. We have observed what appears to be an oxygen-containing hydrocarbon of mass 129. It has been observed in all analyses to date. Figure 5 shows a spectrum taken from an analysis of a sample containing 3 X 10-l1 gram of ILZ9.The impurity peak is quite large. The resolution of the spectrometer is sufficient to detect and make an accurate measurement of the P 9 :1 1 2 7 ratio in the presence of 100 times more impurity. The contaminant comes mainly from the vacuum system environment. This conclusion is reached from our observations that the ratio of hydrocarbon-129 to 1'29 can be changed (and reduced considerably) by raising the temperature of the sample and by pro-
C )y,
0.18 AMU
\ \
12in,
f V ION GUN 2 V ElECTROSTLITlC ANAlYZER PLATES
THIRD SLIT
Figure 4.
PLATE
ELECTRON MWTlPLlER
Figure 5. aining 3
Schematic layout of 3-stage mass spectrometer
longed bombardment of the sample with a large primary ion beam. The contaminants are either vaporized or sputtered from the surface with this treatment. Control and elimination of vacuum system contamination can be achieved through the use of ultra-high vacuum techniques--all metal systems, metal gaskets, bake-outs, etc. The abundance sensitivity of the mass spectrometer is such that large isotope ratios can be measured without any interference from the more abundant isotope. Ratios of adjacent mass isotopes exceeding 107: 1 can be measured. The mass peak produced by an ion impact source tails to the high mass side because some ions of that mass possess more energy than the average. The three-stage spectrometer (magnetic, magnetic, electrostatic) eliminated the interference a t the adjacent higher masses caused by this tail. .-I spectrum taken for a 1-pg. natural iodine sample is shown in Figure 6. The 112' beam intensity was in excess of IO6 counts/ second (beams in exress of lo7 counts/ second have been obtained for I samples of 1 to 5 pg.) and a t the 1 1 2 9 mass position the intensity mas 0.5 count per second. This intensity a t the 129 position corresponds to slightly less than gram of 1129. Whether this is contamination of the natural sample, or an impurity of the same mass, has not yet been ascertained. Ions like C#&-, BrO3-, and H2IIz7- could To date, not be differentiated from experiments designed to prove that the small peak a t 1 1 2 9 is due to molecular species have yielded negative results. Most of our later discussions concerned the detection, and the problems
I29
130
128
MASS
Spectrum taken from analysis of a sample con-
X lo-" gram 1129
Impurity peak ( a hydrocarbon containing oxygen) Is reduced to 1/5 the Ilz9 peak by heating and prolonged bombardment of the sample
r
1.1 x 10s C/IOC
I lea
Figure 6. iodine
*
II?
Spectrum taken for a l - ~ g sample . of natural
Source II and the 3-stage mass spectrometer employed
encountered with the detection, of very small iodine samples,
