Analysis of Solids with the Mass Spectrometer AND J . A. HIPPLE ,Vationdl Bureau of Standards, Washington, D. C.
J. G. GORMAN, E. J. JONES,
Although the mass spectrometer has been strikingly successful in recent years in the analysis of mixtures of gases and vapors, there has been no parallel success in the routine analysis of solids. This is because the most versatile source, the high frequency spark, has been too erratic for the conventional mass spectrometric method. The aim in the present work was to develop a method whereby the spark source could be used in an instrument employing electrical recording. .4 method of compensating for fluctuations in the source is illustrated in the determination of the nickel and chromium content of six stainless steels. The results agree nicely with the composition determined chemically. This initial success makes it evident that the mass spectrometer should find wide application in the immediate future in the routine, rapid analysis of solids.
T
H E striking success attained with the mass spectrometer in recent years in analyzing complex gas mixtures has quickened interest in the possible application to the analysis of solids. Dempster ( 1 , s )has illustrated the determination of traces with his precision mass spectrograph, which had been developed for the measurement of packing fractions. However, the photographic method has inherent limitations in accuracy, convenience, speed, linearity, and range of concentrations that may be covered on one exposure. In addition, the sensitivity of the photographic plate decreases for heavier masses. Some of these limitations are discussed by Dempster, and it is possibly these limitations that led him to conclude that this application will be slow in developing, and to stress the comparison with optical spectroscopy where “fifty years elapsed before optical spectrographs were applied to any extent in industry.” This comparison is also stressed by Inghram ( 5 ) . A mass spectrograph for analyzing solids was constructed by Shaw and Rall ( 7 ) , using the conventional photcgraphic plate as detector, but apparently little application has been made to analysis, possibly because of the limitations just mentioned. Many of these limitations are removed by the use of electrical in place of photographic detection, as in the mass spectrometer for gases. A mass spectrometer of the conventional type requires a “quiet source” which produces an ion beam of very constant intensity. For some very specialized problems in analyzing solids, such sources are available. Hickam ( 4 ) has described a method of determining impurities in copper by completely vaporizing the sample in a furnace closely adjacent to the ionization region and integrating the ion current a t selected masses until the sample is exhausted. This method, although sensitive, is very limited in application. The best available source for solids is generally accepted to be the vacuum spark source (6,7 ) . Its very erratic nature has previously limited its application almost entirely to photographic detection-Le., the mass spectrograph rather than the mass spectrometer. In this paper is described a successful method of using the spark source with the mass spectrometer, in which the accuracy attained with chemically analyzed samples of stainless steels is comparable with that for gases and vapors on the conventional instrument. The results compare favorably with the best that has been done with the optical spectrograph.
two isotopes was increased by simultaneously focusing and measuring the ions corresponding to the two isotopes on separate collectors; their electronic circuit then measured the ratio of these two currents rather than the intensity of each separately. Thus, the accuracy obtainable with the gas-type source was improved. This principle was then employed in the Consolidated-Nier instrument, which was designed to measure isotopic ratios in gases and vapors. The isctope-ratio method must be revised in the application to general analysis, because a complete spectrum is required and it is not feasible to have a separate collector for each mass. It is most convenient for this work to have a single exit slit and ion collector and sweep the various successive maMes across this exit slit. In the authors’ work, a monitoring collector placed a t the entrance to the magnetic analyzer provides a measure of the ions of all masses that enter the analyzer. The magnetic field is then varied and the ions of various masses are scanned across the exit slit of the magnetic analyzer. The currents are amplified id the
STAINLESS STEEL - SAMPLE X3380 7/24/50
9
PRINCIPLE OF THE METHOD
After it has been decided that the most generally useful method would be the use of the spark source with electrical detection, the problem is to devise some means for compensating for the wild fluctuations in the current from the spark. The use of the “isotope-ratio” circuit of Nier, Ney, and Inghram (6) suggests itself. In their application, the effect of the source fluctuations was minimized and the precision of the determination of the ratio of
60
’
56
’
52
48 +MASS
Figure 1. Mass Spectrum of Stainless Steel Sample Obtained from Spark Source Retrace of original pen-and-inkrewrding
438
V O L U M E 23, N 0 . 3 , M A R C H 1 9 5 1
439
changed, a new clean disk was used to avoid contamination from the previous sample. Experience Relative Ion Current has shown that the analysis is Relative Corrected for Relative not criticall dependent on the Mass Ion Overlapping Isotope ConcentraDetermined Ionization position of t l e rod relative to the Element Used Current isotope abundance tion in Beam Chemically Factor disk, and for analytical work it Chromium 52 56.6 56.6 62.0 57.8 38.5 1.50 would usually be more convenIron 56 100.0 100.0 100.0 100.0 100.0 1.00 Nickel 58 14.2 13.9 18.7 19.7 22.1 0.89 ient to spark between two rods of the same material. The average monitor current obtained from Table 11. Determination of Composition of Sample X3522 this source was about 5 X lo-" Relative Ion Current ampere, and the average ion curRelative Corrected for Relative Concentration rent a t the final collector for the Mass Ion Overlapping Isotope In In Composition, largest peak was about 5 X lo-'* Element Used Currents isotope abundance beam sample % ampere. The recorder system 52 6.3 A 0.1 6.3 6.9 6.4 4.2 2.97 * 0.05 Chromium showed adequate precision for the Iron 56 100.0 a0.1 100.0 100.0 100.0 100.0 70.1 *O.l Nickel 58 23.3 A0.3 23.0 31 .O 32.6 36.6 25.6 ~ 0 . 3 larger peaks, but because of its Other .. ....... ... ... ... ... 1.33 limited amplification, the reproMean value and average deviation of 5 rum. ducibility of the smaller peaks was poor. For this reason, B galvanometer and manual ratio Dotentiometer were used for the analyses reported here. The iddition of an amplifier to the pen-and-ink recorder records the ratios of the various types of recorder should permit automatic recording with the desired preions emerging from the analyzer to the current measured by the cision on all peaks. monitoring electrode. Because the monitoring collector receives an ion current composed of all masses, and the final ion collector ANALYTICAL METHOD receives the current corresponding to only one mass a t a time, the two collectors will receive currents that tend to fluctuate in As in optical spectroscopy, an internal standard is requiredthe same manner-i.e., in the manner in which the source is Le., the absolute amount of an individual component is not deterfluctuating. mined but only its concentration relative to other components The appearance of the spectrum obtained in this way is illuspresent. For convenience, the authors have referred everything trated in Figure 1. This sample is a stainless steel and the to iron in the following presentation. major components are immediately evident. The record has the The first step is to calibrate the instrument by means of a ~ a m eappearance as that from the gas analyzer. In fact, the sample whose composition is known. Possibly the clearest same compensating method used here could also be applied to the explanation of the analytical procedure would be provided by mass spectrometers for analyzing mixtures of gases and vapors tracing through this procedure with an illustrative example. and for measuring isotopic ratios. This is done with the aid of Table I for the relative concentration of iron, nickel, and chromium. The trace components are not THE INSTRUMENT determined in this presentation, the purpose of which is to The instrument will be described in greater detail in another demonstrate the effectiveness of this new technique. The article and the analytical method is emphasized here. measurement of the trace elements appears to be a simple extenA mass spectrograph of the Dempster type ( 2 ) was available sion of the principles presented here. The means of doing this for this work. In this instrument, the ions undergo a deflection are at hand, but the curtailment of this project in the immediate of 90" in an electrostatic field and are then bent 180' in a magfuture makes it desirable t o make the present incomplete results netic field. The electrostatic deflection is necessary because of the generally available. wide distribution in energy of the ions emerging from the spark In making this analysis, mass 52 was used for chromium, mass source. The electrostatic deflectors allow only those ions in a 56 for iron, and mass 58 for nickel. In column 3 of Table I, the narrow energy band to enter the magnetic analyzer; this is necesrelative ion currents a t these masses are indicated. It is known sary for proper focusing a t the exit slit. The basic form of this that iron has an isotope a t mass 58 which is 0.3% of 56. This correction to the mass 58 peak is made and the relative peak instrument was retained, but considerable modifications were height due to NP8 is shown in column 4. In column 5, the data made to the various components. are adjusted to account for the fact that chromium and iron each The electrostatic deflection was changed to 45" in order that an have four stable isotopes and nickel has five. Although mass 56 electron multi lier could be placed a t the exit slit; in the oldei is used as a measure of the amount of iron present, the iron presarrangement t t e region around the ion receiver was crowded by ent a t other mass numbers must also be considered. From tables the proximity of the ion source. The electron multiplier has not of isotopic abundance, it is determined that Fe56accounts for yet been used, although it is ready for mounting on the unit. only 91.6% of the iron present in the sample, the rest being Feb4, This will be valuable in the study of trace components, but was Fe5', and Fes8. Similarly, Cr52 accounts for only 83.7% of the not necessary in demonstrating the practicability of this analytichromium in the sample, the rest being C P , Crs3, and Cr54. cal method. A sliding vacuum valve was inserted between the Therefore, the figure 56.6 in column 4 for chromium becomes ion source and the rest of the instrument, so that the spark elec(100/83.7) 56.6, and for iron (100/91.6)100. Nickel is similarly trodes could be changed without losing vacuum in the entire adjusted and these three revised values are normalized to Fe = system. A new housing was constructed for the region a t which 100 and entered in column 5 . This column now gives the relative the ions approach the magnet to permit the proper mounting and numbers of atoms of chromium, iron, and nickel in the ion beam positioning of the monitoring electrode. The analyzer housing received a t the ion collector. Using the values of the atomic was revised to allow for the insertion of a rotating fluxmeter into weights, the relative concentration by weight in the beam is obthe magnet gap in order that the ions reaching the final collector tained in column 6. This is not necessarily the same as the discould be identified according to their masses. The assembly in tribution of components in the sample, because the efficiency of the region of the ion receiver was modified so that a slit and ion creating ions in the source varies with the different elements in collector could be substituted for the hotographic plate. the sample. Likewise, the ions may be sorted preferentially by The source is the one described by Slaw and Rall ( 7 ) . In place the analyzer. The factor by which the relative concentration of a of the Tesla coil, the spark is powered by an interrupted oscillator component in the sample must be multiplied to give the corresubstituted several years ago by R. H. Britten. This oscillator sponding relative concentration in the beam is called the ionizaoperates a t about 1 megacycle and is turned on for an interval of tion factor. These factors are here determined by comparison 200 microseconds 60 times per second. The spark takes place with the composition of the sample determined chemically. between a rod and a disk. In the authors' work, the sample was The chemical determinations are shown in column 7. The ionisamachined to the form of a rod with a diameter of 0.025 inch, while tion factors given in column 8 are obtained by dividing the figures the disk was made from tantalum. Each time the sample was in column 6 by the corresponding ones in column 7. Table I.
Calculation of Ionization Factors Using Sample X3534
ANALYTICAL CHEMISTRY
440
with no significant change in the analysis. Sample X3380 was checked in this way because of Iron Other its deviation from the chemical Chemical. determination for chromium. % M.S., %a % This check run gave 18.9% chromium, as contrasted with 61.6 61.8d0.4 1.1 19.3% in Table 111. Further69.8 68.6 k 0 . 3 3.4 83.0 82.8 k O . 3 3.2 more, this check run was made 85.5 85.5*0.3 4.8 with both rod and disk in the 84.3 84.0 * 0.1 3.4 spark source fabricated from 70.0 70.1*0.1 1.3 sample X3380. In all previous runs. the disk waa made of tantalum and the rod was formed from the samde. The data ‘of Table I11 are plotted in Figures 2 and 3 to illustrate the linearity of the calibration curve and the nice check in most cases with the chemical determination. Perfect agreement between the ma53 spectromrtric arid the chemical method3 would be indicstcd by the measurement falling 011 the 45’ line in these figurrs. The vertical linr at t.:ich point indicates the range in which the measurement.< fell.
Table 111. Summary of Analyses of Stainless Steel Samples Chromium Chemical, % M.S., %”
Sample X3534 X3380 X32i5 X3532 x3533 X3322 a
Nickel Chemical, %
23.4==0.4 13.6 19.3 a 0 . 3 8.6 13.7 a 0 . 2 0.37 9.1 AO.2 0.57 5.5 s o . 1 6.8 2.97a0.05 25.7 Mean value and average deviation of 4,or 5 runs.
.
23.7 18.2 13.4 9.1 5.5 2.95
~~~~
~
M.S., %“ 13.7k0.5 8.7 d 0 . 2 0 . 2 5 *0.01 0.56*0.02 7.1 * 0.1 25.6 d O . 3
_.
g?,::?,
COVCLCSION
15 IO , I N % OF M E N VALUE
5 V
I I
CHEMICAL I
Tlle results are better tli:in it had been anticipated would be possitde with a spark source. The interpretation is very easy because of the simplicity of the spectrum. The results that have h c w presented were tskcn over a period of 5 weeks, and there was 110 apparent shift in calibration during this period. The linearity as rrgards concentration is an attractive feature which eases the ralibration problem. The changing of the sample is tinieron-?umingin the present arrangenient, but with an arrangement ti&gntvl q)ecially for an?lytical work it nould be possihlr to intrrc.Iimige sornplrs i n R IIW minutes.
ANALYSIS(%) I
IO 15 20 25 Figure 2. Comparison with Chemical Determination of Chromium
0
5
The ionization factors that have been determined may now be used to analyze a sample. This is illustrated for sample X3522 in Table 11. The first six columns are the exact parallel of the corresponding ones in Table I. However, the ionization factors listed in Table I may now be used to obtain the relative concentration of chromium, iron, and nickel in the sample (column 7) from the meaaured concentration in the beam (column 6). The desired analysis for X3522 has now been attained. For convenient comparison with the chemical determination, the result ia expressed in percentage in column 8. The chemical analysis of 1.33% for the other components is assumed to be correct. This assumption is necessary only because the development of the mass spectrometric method was incomplete s t the writing of this manuscript. For routine analyses, the steps outlined above can be shortened. Once the correction for overlapping isotope is made to the relative ion current (Table I), an over-all calibration factor for each element can be obtained by dividing the ion current by the relative concentration determined chemically. Then in Table 11, the relative concentration in the sample (column 7 ) would be obtained directly by dividing the figure of column 4 by this calibration factor. Six stainless steel samples covering convenient concentration ranges were made available for this work by the bureau’s chemistry division. The samples had been prepared and the chemical compositions determined by the Uddeholms Aktiebolag in Hagfors, Sweden. If sample X3534 is used for calibration, the other five can be analyzed. Actually, the ionization factors were chosen BO that a good fit on all the samples was obtained, still assuming the calibration to be linear with concentration. This makes a very slight change in the calibration obtained with X3534 (Table I). The only ionization factor affected is chromium which b e comes 1.53 instead of 1.50. The data are summarized in Table I11 and compared with the chemical determination. To illustrate the reproducibility of the mass spectroscopic results, the average deviafions from the mean of four or five determinations in each case are shown. The sample waa not removed from the instrument between these runs, but on several other runs the sample was removed between runs
0
5 10 15 20 25 Figure 3. Comparison with Chemical Determination of Nickel
Although the method has been illustrated here with metallic samples, for which it is particularly well suited, nonmetallic samples have been studied photographically with the spark source in several mass spect’rographs. These studies indicate that analyses with the present method should be feasible, although possibly less accurate. The successful results on the stainless steel samples indicate that the mass spectrometer should have a wide field of application in the quantitative analysis of solids. LITERATURE CITED
(1) Dempster, A. J., U. S. Dept. Commerce, Office of Technical Services, Bibliography of Scientific and Industrial Reports, MDDC
370. (1946). (2) Dempater, A. J., Proc. Am. Phil. Soc., 75, is5 (19353. (3) Dempster, A. J.,Rea. Sci. Instruments, 7, 46 (1936). (4) Hickam, W.M., Phys. Rez,., 74,1222.4 (1948). (5) Inghram, M. G., “Advances in Electronics,” ed. by L . LIarton, Vol. 1, p. 263, New Yolk. Academic Press, 1946. (6) Nier, A,. O., N e y , E. P., arid Inghram, M. G . , Rew. Sci. Instruments, 18,294 (1947). ( 7 ) Shaw, A. E., and Rall, TI-.. Rei,. Sci. Instruments, 18,276 (1947). RECEIVED September 9, 1950.
