FIuorometric Determination of Submicrogram Quantities of Antimony T. D. Filer Health Services Laboratory, U.S . Atomic Energy Commission, Idaho Falls, Idaho A fluorometric procedure for the determination of antimony using 3,4',74rihydroxyflavone has been developed that is much more sensitive than other common methods. The fluorescence of the antimony(ll1) complex is measured in a perchloric acid solution using phosphate as a masking agent. The method has a detection limit of 0.04 pg, a precision to about 2% on 5 pg, and excellent tolerance to most common elements. After decomposition of the sample by pyrosulfate fusion, the antimony is extracted as the triiodide into methyl isobutyl ketone from a sulfuric acid solution.
METHODS FOR THE DETERMINATION of trace quantities of antimony in various sample types such as sea water, soils, biological materials, and air have been developed (1-5). The analysis of sea water for antimony has been used to determine the marine geochemical balance of this element (5). Even though antimony is not abundant, its determination in rocks and soils is of interest because its presence usually indicates a sulfide deposit (3). The determination of antimony in biological materials is of interest because of the use of antimony compounds in the treatment of several tropical diseases. It has been possible to study the distribution of antimony in animal organs and to follow blood levels and excretion rates in experimental animals and in patients undergoing therapy ( I ) . Also, antimony and its compounds are serious industrial hazards in the rubber industry, the printer's trade, and the storage-battery industry. Stibine, antimony hydride, is a particularly serious hazard which has a maximum allowable concentration of 0.1 ppm in air (6). Sensitive methods for the determination of trace quantities of antimony are not common. Until now, the most specific and sensitive methods available for the analysis of microgram quantities of antimony have been photometric methods using Rhodamine B as the reagent (1-5). The sensitivity of the Rhodamine B method is 1-2 pg of antimony. Tungsten, thallium, iron, gold, gallium, mercury, silver, arsenic, copper, and tin are potential interferences in the direct determination of antimony. The present method uses 3,4',7-trihydroxyflavone which is 25 to 50 times more sensitive to antimony than Rhodamine B. Most of the elements listed above do not interfere in the present procedure, notably iron, arsenic, copper, mercury, and thallium. In addition, antimony can be satisfactorily separated from most interfering ions by extracting the triiodide from a sulfuric acid solution by means of methyl isobutyl ketone. (1) T. H. Maren, ANAL.CHEM., 19, 487-91 (1947). (2) C. L. Luke, ibid., 25, 674 (1953). (3) F. N. Ward and H. W. Lakin, ibid., 26, 1168-73 (1954). (4) R. E. Van Aman, F. D. Hollibaugh, and J. H. Kamzelmeyer, ibid., 31, 1783-85 (1959). (5) J. E. Portmann and J. P. Riley, Anal. Chirn. Acta., 35, 35-41 (1966). (6) M. B. Jacobs, "The Analytical Chemistry of Industrial Poisons,
Hazards, and Solvents," Interscience Publishers, Inc., New York, N.Y.,1949, pp 253-61.
EXPERIMENTAL
Apparatus. The instrumentation used was a Beckman D U spectrophotometer with a fluorescence accessory modified as described (7). A combination of Corning Filters with color specification No, 3-74 and 5-58 having over 1 transmittance between 412 and 473 nrn with a maximum of 2 7 z was used for the primary. A combination of Corning Filters with color specification No. 3-71 and 4-97 having over 1 % transmittance between 463 and 680 nm with a maximum transmittance of 7 2 z was used for the secondary. A tungsten source was used in the present work but a medium pressure mercury lamp can be used with the same filter combinations with similar results. Reagents. STANDARD ANTIMONY(III) SOLUTIONS, 1 mg/ml AND 5 pg/ml. Dissolve 1.000 gram of antimony metal in 200 ml of concentrated sulfuric acid with heating as required. Cool and dilute slowly to 1 liter. Dilute 5.00 ml of the stock solution and 10 ml of concentrated sulfuric acid to 1 liter. The solutions contain 1 mg/ml and 5 pg/ml of antimony(III), respectively. HC104-Na2HP04-SuLrAMIc ACID SOLUTION. Dissolve 43.0 grams of Na2HP04.7H20,5.0 grams of sulfamic acid, and 86.0 ml of 72 perchloric acid in enough water to make 500 ml of solution. Cool and store in a glass-stoppered borosilicate glass bottle. 3,4',7-TRIHYDROXYFLAVONE SOLUTION, 0.02 The flavone may be prepared by the method of Roux and de Bruyn (8). Heat a-methoxyresacetophenone, cinnamic anhydride, and sodium anisate at 180 "C for 3 hours and follow with hydrolysis with potassium hydroxide in ethanol to produce 7-hydroxy-3,4'-dimethoxyflavone. Demethylation is accomplished by heating under reflux with excess hydroiodic acid for 30 minutes to yield 3,4',7-trihydroxyflavone. Other methods of preparation of the flavone have been described (9, 10). Transfer 20 mg of the flavone to a 100-ml volumetric flask and dilute to volume with 95 % ethanol. SULFURICACJD, 5kfL%DIUM IODJDE,0.01kf. Dissolve 0.75 gram of sodium iodide in about 300 ml of water; then add 139 ml of concentrated sulfuric acid and dilute to 500 ml with distilled water. Prepare and use the solution on the same day Procedure. The procedure given below for preparation and measurement of the fluorescence is that used in the development of the procedure using pure antimony solutions. It is also to be followed when antimony has been separated and can be obtained in concentrated sulfuric acid free of interfering elements. However, some applications of this procedure can be made without separations, provided the sample size is chosen so that the heavy metal content does not exceed the permissible levels described below. Place the antimony standard or other antimony solution into a 100-ml beaker. Add 1 ml of a 17% solution of sodium hydrogen sulfate, 5 drops of concentrated nitric acid, 2 drops of concentrated sulfuric acid, and evaporate the solution until fumes of sulfuric acid appear. Cool, add 1 ml
z
z.
(7) C. W. Sill and C. P. Willis, ANAL.CHEM., 31, 598 (1959). (8) D. 0.Roux and G. C. de Bruyn, Biochem. J . , 87 (2), 439 (1963). (9) Katsuzo Yamaguchi, Nippon Kagaku Zasshi, 1963, 148. (10) Z. I. Jerzrnanowska and M. Michalska, Rocrniki Chem., 35, 353 (1961).
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of water, 5 drops of concentrated nitric acid, and evaporate the solution again until fumes of sulfuric acid appear. Repeat this step and continue heating until all the sulfuric acid, including that condensed on the beaker walls, has been volatilized and fuming has ceased. The nitric acid treatment can be omitted if halides are known to be absent. Cool the sodium hydrogen sulfate residue, add 2 ml of water and 3 drops of 2 5 z hjdroxylammonium sulfate. Cover the beaker with a watch glass and boil the solution until the volume has been reduced to about 0.5 ml. Remove the cover glass and rinse with a few drops of water. Add 10.00 ml of the HC104-Na2HP04-sulfamic acid solution and 2 drops of 25 % hydroxylammonium sulfate. Cover the beaker with a watch glass and bring the solution to a boil. Cool and transfer the solution quantitatively to a 25-ml volumetric flask. Add 1.00 ml of 3,4‘,7-trihydroxyflavone solution, mix, and dilute to volume. Mix thoroughly and place in a constant-temperature bath at 25 “C for 30 minutes. Measure the fluorescence using the technique described previously (7, 11). Permanent glass standards (7) can be used to reproduce the same instrumental sensitivity from day to day. The time of measurement after addition of the flavone is very important and should be kept within 1 or 2 minutes of the recommended value of 30 minutes for blanks, standards, and samples for highest precision. Place 1 ml of water for a blank and 1 ml of the 5-pg/ml standard antimony solution in separate 100-ml beakers, add 2 drops of concentrated sulfuric acid and 1 ml of 17% sodium hydrogen sulfate solution. Evaporate carefully to dryness on an asbestos-covered hot plate until evolution of sulfuric acid fumes has ceased, and treat as described above. Subtract the blank from the standard and express the sensitivity as micrograms of antimony per net scale division. Correct the samples for an appropriate blank carried through the entire procedure including separations, if any, and calculate their antimony content from the sensitivity value obtained from the standard. Separation of Antimony. Although some samples can be analyzed directly following dissolution, most will require a means of separating antimony from other elements. This will be true whenever the given sample is of unknown composition, whenever trace amounts of antimony are present in a large excess of some interfering element, or whenever the sample is suspected of containing one or more of the particularly serious interferences such as tin, tungsten, or niobium. A convenient and very selective solvent extraction method in which antimony is extracted as the triiodide from a 0.01M solution of iodide in 5M sulfuric acid by means of methyl isobutyl ketone has been adapted to this procedure (5). After sample dissolution, place the antimony solution into a 100-ml beaker. Add 1 ml of a 1 7 z solution of sodium hydrogen sulfate, 2 drops of concentrated sulfuric acid, and evaporate the solution until the volume has been reduced to about 2 ml. Add 3 drops of 25$ hydroxylammonium sulfate. Cover the beaker with a watch glass and boil the solution until the volume has been reduced to about 0.5 ml. Remove the cover glass and rinse with a few drops of water. Add 10 ml of the 5M H?SOd-O.OlM NaI solution and transfer the solution quantitatively to a 60-ml separatory funnel using three 5-ml aliquots of the 5M H2SO4-0.01M NaI solution to wash the beaker. Extract the solution with 25 ml of methyl isobutyl ketone for 3 minutes and discard the aqueous phase. Wash the organic phase with 25 ml of 5M H2So4-0.O1M NaI and discard the wash. Extract the ketone phase by shaking with three 10-ml portions of 0.4Mhydrochloric acid for 3 minutes each and combine the three extracts in a 100-ml beaker. Add 1 ml of 17 sodium hydrogen sulfate, 2 drops of concentrated sulfuric acid, and 5 drops of concentrated (11) C.W. Sill, C. P. Willis, and J. K. Flygare, Jr., ANAL.CHEM., 33, 1671 (1961). 726
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
nitric acid to the combined extracts and continue as described under “Procedure.” Sample Preparation. SOILS AND ROCKS. A procedure for the decomposition of the refractory silicates which involves a potassium fluoride fusion and a transposition to a mixed alkali pyrosulfate fusion has been described (12). Pretreatment of the sample with concentrated nitric acid will be necessary in cases where chlorides, bromides, or iodides are present to eliminate loss of the antimony halide by volatilization. In cases where silicates are known to be absent, the potassium fluoride fusion can be omitted and the sample treated initially with nitric acid followed by a pyrosulfate fusion. The solution is then diluted to any desired volume and an appropriate aliquot separated as described above. BIOLOGICALSAMPLES. Conventional wet-ash methods using nitric, sulfuric, and perchloric acids are used for the digestion of organic material which is followed by pyrosulfate fusion. Procedures for the decomposition of whole blood, tissue, bone, feces, and urine -have been described (13). After the pyrosulfate fusion, the solution is diluted to any desired volume and an appropriate aliquot separated as described above. STIBINE I N AIR. A procedure for the collection of stibine in air samples by absorption in 0.05Npotassiurn permanganate and 0.1N sulfuric acid has been described (14). Destroy the potassium permanganate with hydrogen peroxide and treat as described under “Procedure”. In most cases, separations will not be necessary since manganese does not interfere with this procedure and sufficient quantities of other interfering ions are not likely to be found in air samples. RESULTS AND DISCUSSION
During the development of this procedure, recoveries of only 70 % were obtained when antimony standards were fumed to dryness in a mixture of sulfuric acid, nitric acid, and sodium hydrogen sulfate. Yet, when nitric acid was eliminated from this procedure, the recovery of antimony was always complete. Since trivalent antimony forms the fluorescent species, apparent losses can be attributed to the oxidation of antimony to a nonfluorescent species in the presence of nitric acid rather than to volatilization. The recovery of antimony was increased to only 90% when sulfur dioxide was used as a reductant following the fuming of the sample with sulfuric and nitric acids. Apparently, the niric-sulfuric acid mixture oxidizes trivalent antimony to a mixture of valence states, only one of which is reducible with sulfur dioxide. Maren (1) also observed that the sulfuric-nitric acid mixture oxidized trivalent antimony to a mixture of valence states which were found to be antimony(1V) and antimony(V). Antimony(1V) is easily reducible to the trivalent state with sulfur dioxide whereas pentavalent antimony is not readily reducible. Reduction of the pentavalent state can be accomplished, however, with hydroxylammonium sulfate. Effect of 3,4’,7-Trihydroxyflavone Concentration. The results obtained when various concentrations of 3,4‘,7-trihydroxyflavone were used are given in Figure 1. Curve 2 shows that the intensity of fluorescence of 5 pg of antimony increases with increasing flavone concentration to a maximum at about 2.7 X lO-3z. If the highest precision is desired, the higher concentration of flavone should be used because the maximum fluorescence signal is produced at this level and the instrument can be operated in the range that has maximum stability. Also, at the higher concentration of flavone, small changes in concentration will not produce significant varia(12) C. W. Sill, ANAL CHEM., 33, 1684 (1961). (1 3) C.W. Sill and C. P. Willis, ibid., 36,622-30 (1964). (14) E. V. Deyanova, Nou. 0bl.-Khirn Anal., 1962, 188-94.
f . . -
v I
I
O 3
2
.
I
I
0
f
1
2
IN HC104, ml IN NAOH, m l Figure 2. Effect of acidity
3,4’ 7 -TRIHYDROXY FLAVONE CONCENTRATION ( x 10-4oi0 I
1. Blank 2. 5 - p g Sb standard
Figure 1. Effect of 3,4 ‘;l-trihydroxyflavone concentration 1. Blank 2. 5-pg Sb standard
tions in fluorescence readings. However, the concentration offlavone can be adequately controlled so that it will not be a significant factor in precision, even on the steeper portion of curve 2. On the other hand, the intensity of the antimony fluorescence per unit blank fluorescence is greater at lower concentrations of the flavone. If the instrumental sensitivity can be increased so that the relatively weak fluorescence obtained at lower flavone concentrations can be spread over the full range of the instrument without significant loss of precision of measurement through instrument instability, smaller quantities of antimony can be detected. The minimum detectable quantity of antimony and the proper concentration of the flavone to be used are dependent on the value of the blank, and the stability and sensitivity of the instrument. The arrows shows that the recommended flavone concentration Lower concentrations result in smaller occurs at 9 x 10-4z:. blanks, but the instrumental instability at these levels cause a significant decrease in precision. Effect of Acidity. The effect of changes in acidity on the fluorescence of the antimony-3,4’,7-trihydroxyflavone complex was studied. Various amounts of 1N sodium hydroxide or 1N perchloric acid were added to the volumetric flask before the addition of the flavone and measurement of fluorescence. The excellent efficiency and high buffering capacity of the system is shown in Figure 2. The arrows mark the points on the buffer curves that result under the recommended conditions. Spectral Characteristics. Figure 3 shows the excitation and emission spectra for the reagent and its antimony complex in 1M perchloric acid. The fluorescence of 3,4’,7trihydroxyflavone exhibits its excitation maximum at 377 nm and a fluorescence emission maximum at 485 nm. The antimony complex shows its excitation maximum at 422 nm and fluorescence emission maximum at 475 nm. All values are uncorrected for emission characteristics of the light source or the response of the detector. Detection Limit and Precision. The detection limit and determination limit of this procedure were determined as
3
WAVELENGTH, nm Figure 3. Excitation and emission spectra of reagent and antimony complex in 1Mperchloric acid
+ reagent. + reagent.
Excitation at 422 nm Emission at 475 nm C. Reagent alone. Excitation at 377 nm D. Reagent alone. Emission at 485 nm A . Sb B. Sb
defined by Currie (15). To determine the precision obtained with larger quantities of antimony 10, blanks and ten 5-pg antimony standards were analyzed under the recommended conditions, including the evaporation of antimony solutions to dryness in the presence of sulfuric acid and the transfer from (15)
L.A. Currie, ANAL.CHEM., 40, 586-93 (1968).
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Element Sn
A1 Ga In
Ti Ge W Mo
Nb Ta Cr Ba Si Ag Pb Bi Au Pt Zr Hf
Quantity, mg
Table I. Effects of Other Substances Error, scale division. Blank 5-pg Sb $46.1 $21.9 +8.3 $2.0 -4.8 -8.3 $1.1 -1.5 -14.0 -0.6 $0.8 f7.0 -1.5 +10.1 $4.6 $7.4
0.01 1 .o 0.01 1 .o 0.1 0.1 0.1 0.1 0.1 0.1 0.1 1 .o 1.0 1.0 1 .o 1.0
0.1 0.1 0.1 0.1 1.0 1 .o 0.1 10.0 10.0
-0.7 -1.1 -1.0 -1.9 -2.6 0.0 -0.7 -5.5 -1.5
> +10 +8.2 +9.3 +1.5
-31.9 -79.3 -48.1 -40.0 -89.1 -37.2 -19.9 -27.7 -6.0 +7.1 -9.8 -13.8
Remarks Fluor; 0.22-pg sc. div. Fluor; 46-pg sc. div. Fluor; 1.2-pg sc. div. Fluor; 500-pg sc. div. Yellow complex with flavone Yellow complex with flavone Yellow complex with flavone Yellow complex with flavone Yellow complex with flavone Yellow complex with flavone Turbid due to anhydrous Crz(S04)r Turbid Flocs of Si O2 Turbid Turbid . Turbid, yellow complex with flavone Elemental gold precipitates Elemental platinum precipitates
-22.8 - 12.5 $4.3 -0.4 Faint yellow color U -18.8 Se -22.9 V -18.8 Added as Naz HP04before fuming P -7.4 Added as Naz HPOI after fuming P -3.2 a Blank, 17.0 sc. div.; 5-pg Sb standard, 96.5 sc. div.; sensitivity, 0.0630 Mg/sc. div. Differences larger than f0.4 sc. div. on blanks or f 2 . 0 sc. div. on standards probably indicate significant effect of added substance.
beaker to volumetric flask. The mean for the 5-pg antimony standards was 96.5 scale divisions with a standard deviation of 1 1 . 0 scale divisions. The mean for the blanks was 17.0 scale divisions with a standard deviation of f0.2 scale division. The results indicate a detection limit of 0.04 pg of antimony. The minimum quantity of antimony that can be determined with a precision of 10% is 0.13 pg. Linearity. The effect of antimony concentration on the fluorescence was investigated at a 3,4',7-trihydroxyflavone concentration of 7.50 X 10-7 mole per 25 ml to determine the linearity under analytical conditions. The instrument response for samples of up to 10 pg of antimony is linear within the precision of the procedure--i.e., about As the antimony concentration is increased beyond 10 pg, deviation from linearity becomes more pronounced. Effects of Other Substances. A detailed investigation was made of the effect of many other substances on both blanks and 5-pg antimony standards. The element or compound investigated was added before fuming with sulfuric acid to determine its effect under the recommended conditions. No error could be detected on blanks and less than 2z on 5-pg antimony standards in the presence of 1 mg of fluoride, chloride, bromide, iodide, lithium, potassium, rubidium, cesium, beryllium, magnesium, zinc, cadmium, mercury, boron, thallium, iron, cobalt, nickel, scandium, yttrium, lanthanum, calcium, strontium, cerium, copper, arsenic, gadolinium, lutetium, and thorium. Errors produced by other substances are shown in Table I. The use of the extraction procedure described previously increases the tolerance of the procedure to the serious interferences listed in Table I. Antimony can be determined in the presence of lo5times as much phosphorus (as phosphate), l o 4 times as much aluminum, silver, gold, platinum, hafnium, vanadium, and 103 times as much gallium, titanium, germanium, molybdenum, niobium, tantalum, chromium, barium,
2z.
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lead, bismuth, zirconium, uranium, selenium, and 102 times as much tin and tungsten. FLUORESCENT COMPLEXES.Tin, aluminum, gallium, and indium also form fluorescent complexes with the flavone under the given conditions. Tin, because of its sensitivity to the flavone, and aluminum, because of its abundance, are potentially the most serious interferences of this group. The extraction procedure, described previously, will effectively separate antimony from 95-97 % of these particular interferences. COLORCOMPLEXES.Titanium, germanium, tungsten, molybdenum, niobium, tantalum, and bismuth are serious interferences because of the colored complexes these elements form with the flavone. All of these elements can be detected quite sensitively by the appearance of a yellow color when the flavone is added. The extraction procedure will effectively separate antimony from all of these interferences with the exception of bismuth. HALIDES.Iodide, bromide, and to a lesser extent, chloride can be serious interferences in this procedure if they are present during strong heating of the sample because of the volatility of the antimony halides. Oxidation of the halide to the halogens with nitric acid before such treatment will eliminate this interference. The presence of fluoride in the final solution used for fluorometric determination produces serious negative interference, probably due to the complexation of antimony by the fluoride. Fortunately, fluoride will be eliminated from the original mple by the pyrosulfate fusion. PHOSPHATE. Phosphate is used in the procedure as a masking agent for zirconium and hafnium, both of which form strongly fluorescent complexes with the flavone. AS seen from Table I, additional phosphate introduced by the sample is a more serious interference when it is carried through the fuming step, probably because of the formation of pyrophos-
phate which is generally a more powerful complexing agent for antimony than orthophosphate. The extraction procedure described previously eliminates this source of potential negative error. IRON. In the absence of sulfamic acid and hydroxylammonium sulfate, iron produces serious negative interference because of the strong absorption of both the emitted antimony-3,4‘,7-trihydroxyflavonefluorescence and the exciting radiation by the ferric flavonate complex. However, reduction to ferrous ion and complexation with sulfamic acid virtually eliminates this interference. OTHER INTERFERENCES. Most other interferences are caused by elements that produce turbidity in the final solution used for fluorometric determination. Elements such as c h r om i 11 ni , ba r i u m , si I L c r , Icad, ii nd bismuth t h ii t form i nso lu-
ble sulfates or phosphates will interfere. Gold and platinum cause turbidity because they are reduced to the elemental state during the procedure. None of these, however, seem to remove antimony from the solution by absorption of occlusion. Therefore, removal by filtration will eliminate the interference caused by the light-scattering particles. ACKNOWLEDGMENT
The author acknowledges the assistance of his associates during many helpful discussions. Special thanks are due to E. G . Paul for preparation of the flavone used in the present investigation. RECEIVED for review December 2, 1970. Accepted February 24, 1971.
Electron Spectroscopy of Quaternary Nitrogen Compounds John J. Jack and David M. Hercules Department of Chemistry, University of Georgia, Athens, Ga. 30601 Electron spectroscopy offers great potential as a technique for chemical structure determination. The N(ls) binding energies of tetra-alkylammonium, and mono-, bi-, and tricyclic aromatic quaternary nitrogen systems have been determined. The order found in terms of decreasing binding energy was R4N+,pyridinium > isoquinolinium > quinolinium > acridinium, benzoquinolizinium. The effect of counterion on the binding energy was determined in the tetra-alkyl series; ring substituent effects were investigated in the pyridinium series. The results are discussed in terms of molecular orbital calculations, electronegativity, crystal structure, and effects on basicity and electrophilic attack. The trend of the data indicates that certain structural information can be obtained from a single-atom correlation. However, the greater than 5 eV spread observed for quaternary compounds indicates that single-atom correlations will be of limited value. The anion effects are sufficiently large that the counterion must be determined to correct for its effect. Some adjustment must also be made for substituent effects. The conclusion is that a multipleatom correlation must be the approach used.
ELECTRON SPECTROSCOPY (ESCA) has great potential as a technique for chemical structurai determinations. A review discussing such possibilities has appeared recently ( I ) . It is desirable to determine the electron binding energy of an atom of interest, and then to be able to correlate this energy with a specific structure or structures, based on an empirical correlation chart. To that end this work represents the first systematic, extensive investigation into a particular type of functional group-i.e., quaternary nitrogen-in an attempt to determine whether or not a one-atom correlation is feasible. The binding energies of tetra-alkylammonium, and mono-, bi-, and tricyclic aromatic quaternary nitrogen systems have been determined. The effect of the counterion on binding energy was determined for a variety of anions in the tetraalkyl series. Ring substituent effects were investigated in the ( I ) D. M. Hercules, ANAL.CHEM., 42 (l), 20A (1970).
pyridinium series. Data for a variety of nitrogen compounds have been reported previously (2-5). EXPERIMENTAI.
A block diagram of the ESCA apparatus is given in Figure 1. The vacuum apparatus consists of standard oil diffusion pumps with liquid nitrogen trapping. The X-ray power supply is a Norelco Model 12215 constant potential X-ray generator. The Helmholtz coil system consists of two pairs of circular coils for cancellation of the vertical magnetic field, and two sets of three square coils for cancellation of the horizontal magnetic field. Current (10 amperes, maximum) for the monochromator coils is provided by a current regulated power supply with a relay operated voltage-divider (ROVD) programmer (Alpha Scientific, Inc.). The ROVD unit provides 217 (= 131,072) incremental steps of the 10-ampere supply by means of manual switches or 17-bit computer signal. The 17-bit computer signal is supplied by the relay contacts of a Hewlett-Packard (HP) Model 12551 B relay output register card. The current output is monitored by determining the potential across the internal reference registor of the Alpha supply with a H P 3450 A multifunction meter. Electrons are detected by a MM-1 focused mesh electron multiplier (Johnston Laboratories, Inc.). The MM-1 is operated in the pulse counting mode. The MM-1 output is fed into a PAD-1 (Johnston Laboratories) charge sensitive preamplifier-amplifier discriminator for further amplification. The output of the PAD-1 is fed into a H P H70-5325 B counter subsystem for counting. The current stepping and electron counting are Apparatus.
(2) K. Siegbahn et a/., “ESCA ‘Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy,’ ”
Almquist and Wiksells, Uppsala, 1967. (3) J. M. Hollander, D. N. Hendrickson, and W. L. Jolly, J . Chem. Phys., 49, 3315 (1968). (4) K. Siegbahn ef al. “ESCA Applied to Free Molecules,” NorthHolland Publishing Co., Amsterdam, 1969. (5) R. Nordberg, R. G. Albridge, T. Bergmark, U. Ericson, J. Hedman, C. Nordling, K. Siegbahn, and B. J. Lindberg, Arkio Kemi,28, 257 (1968). ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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