Nov., 1953
TRACE ELEMENT DETERMINATION BY MASSSPECTROMETER
809
TRACE ELEMENT DETERMINATION BY THE MASS SPECTROMETER BY MARKG. INGHRAM University of Chicago and Argonne National Laboratory, Chicago, Ill. Received April 8, 1063
The application of mass spectrometric technique t o trace element determination in solids is reviewed. Three techniques which show promise are: (1) the Dempster vacuum spark method, (2) the ion bombardment method, and (3) the isotopic dilution method. Sensitivities of up to one part in ten to the tenth have been achieved.
The mass spectrometer as a tool for quantitative A typical photographic mass spectrum obtained analysis of the relative amounts of various hydro- with the Dempster vacuum spark method is shown' carbons in a gas sample is well known. The appli- in Fig. 1. This figure is a mass spectrum of the light cation of these machines to general inorganic quan- element impurities in a sample of uranium metal. titative analysis, however, has been restricted to a To obtain this spectrum the vacuum spark discharge very few laboratories. It is the purpose of this was maintained between two electrodes of the paper to point out some of the potentialities and uranium metal in question. For analysis of nonlimitations of the mass spectrometric methods of conducting solids the sample is packed in a forty thousandths inch diameter nickel tube and the disanalysis of inorganic solids. There are three basic methods of solid inorganic charge maintained between that tube and a second analysis: (1) the Dempster vacuum spark method, pure tantalum or nickel electrode. (2) the ion bombardment method, and (3) the isoThe fundamental question as to the sensitivity topic dilution method. The first two methods are of the spark method can be determined by comquite analogous to the optical spectrographic parison of Fig. 1, which is a calibration spectrum, methods in that they analyze for all elements pres- ie., a spectrum of a sample of known chemical coment in a solid sample in a single trace of the spec- position, with Table I which lists the composition trum. I n contrast t o this the isotopic dilution of the calibration sample used. The exposure used method, as it is most commonly used, analyzes for to obtain Fig. 1 was of three minutes duration duronly one element at a time. Its advantage over ing which time approximately ten milligrams of the the former methods is that it is several orders of metal was evaporated and sputtered by the spark magnitude more sensitive. Since the interest in onto the surrounding surfaces. this discussion is on trace elements in which ultiTABLE I mate sensitivity is of fundamental importance most E1ectrod.e of this paper will be devoted to the isotopic diluWt. of trace Known oonsumption element juet chemical in rng. to give tion method. The other two methods will be outdetectable detectable Ctqmposition Impurity in p.p.m. line in g. lined briefly to illustrate their limitations in trace element determination. Li? 5 ... ..* .. .. The three methods of analysis to be described Be9 0.02 2.0 0.4 X differ fundamentally only in the mechanism of proBO ' 1 . 2 x 10-10 0.3 0.4 ducing a charged beam characteristic of the sample C'S 4 .2 8 x 10-10 under investigation, Le., in the ion source. The "4 24 .Ol 2 . 4 x 10-lo type of mass sensitive analyzer to be used in con016 40 .002 0.8 x 10-10 junction with the source is dictated by the characF19 0 . 9 x 10-10 1.1 .08 teristics of the source used. It is apparent, thereNag* 10 .005 0 . 5 x 10-lo fore, that the discussion to follow must center Mgz4 14 ,025 3 . 5 x 10-10 around ion sources. The strong line at mass nine marked Be is comDempster Vacuum Spark Method.-The vacplicated by triply charged ions of aluminum. uum spark ion source as it is used in mass spectrosFrom this figure i t is clear that light element imcopy was developed by Dempster.' Its applicapurities of the order of one part per million are detion to quantiOative chemical analysis of solids was tectable with relatively short exposures. also proposed by Dempster. H e used the techThe difficulties involved in quantitative deternique in his work for a variety of applications. He mination of trace elements using the above method had sufficient confidence in the future of the techof analysis are very comparable to those met in nique to construct three instruments for " routine" use a t other laboratories. Shortly after the' war optical spectroscopy. Column four of Table I and just before his death he had one short note de- shows that the sensitivities vary from element to classified by the Atomic Energy Commission which element. This is due to the fact that different eleillustrated the method.2 The method has been im- ments have different ionization efficiencies. Thus, proved in the last six years by the addition of as in optical spectroscopy it is necessary to calibrate pulsed electronic RF supplies and electron record- with standards. It should be noted, however, that inga3 The illustrations I am using of this method the sensitivity for different elements varies by only about one order of magnitude, i.e., there are no are drawn from the work of Dempster.2 blind spots as in optical spectroscopy. Elements (1) A. J. Dempster, Am. Phil. Soc., 76, 755 (1935). which are difficult t o detect optically, for example, (2) A. J. Dempster. Atomic Energy Commission Paper, MDDCsulfur, phosphorus, oxygen, nitrogen and carbon, 370, April, 1946. appear in mass spectra with sensitivities comparable (3) J. G. Gorrnan, E. J. Jones and J. A. Hipple, Anal. Chew., 23, to other elements. There is at the present time no 438 (1951).
810
MARKG. INGHRAM
Vol. 57
.
F
evidence that any impurity evaporates preferentially with time. The ultimate sensitivity of the vacuum spark method is limited by two factors. The first is the problem of background. For example, if a discharge is maintained between two oxygen-free electrodes, a small line at mass sixteen due to oxygen will still appear due t o the residual gas pressure in the machine. This limitation becomes less serious as the vacuum conditions improve. The second limitation is due to scattering of the intense ion beams t o form a continuous background. This limitation is more serious when peaks are close together, i.e., at the higher masses. Again it should be noted that as vacuum conditions improve this limitation becomes less serious. The general usefulness of the Dempster vacuum spark technique is limited by the fact that a vacuum spark source can operate only in a double focusing mass spectrometer. There is at the present time no such commercial machine on the market, hence, the use of the technique is limited to those labs which have constructed their own equipment. The Ion Bombardment Method.--The Dempster vacuum spark method of analysis suffers from several practical problems which are largely removed by the ion bombardment source. These problems involve: (1) handling the large amounts of RF power necessary to run the source in proximity,to sensitive electrometers, (2) the inherent instability of a spark, (3) the high rate of consumption of sample due to “sputtering” of large pieces of the electrodes, (4) space charge defocusing due to pulsed nature of beam. These limitations are largely removed by converting to an ion bombardment type of source. Unfortunately, though the
,llic
source itself is old it has not been extensively applied to analytical problems, and, hence, only very general statements can be made concerning its usefulness. The first use of the secondary ions, produced by impact of high energy electrons on surfacea, was that made by D e m p ~ t e r . ~More recently electron bombardment of surfaces has been studied by Plumlee and Smith.6 The first study of ions produced by ion bombardment of solids was that made by Smith.6 Recent work has shown that the efficiency of ion production by ion bombardment greatly exceeds that due to electron bombardment. Thus, for the purposes at hand, the electron bombardment method can be eliminated. As the name implies the ion bombardment source is a source in which ions are directed against a surface which is t o be analyzed. Upon striking this surface they produce ions characteristic of the surface which can be accelerated and analyzed with a mass spectrometer to give the chemical constituents of that surface. Unfortunately the ion beam produced by the bombardment has considerable energy spread so that for high sensitivities a doubEe focusing machine i s again a necessity. Several alternative source arrangements are possible. Surfaces can be bombarded with either positive or negative ions, and either the positive or negative ions formed at the bombarded surface may be accelerated for analysis. Negative ion production is very selective, hence, should be used only for very special problems. The most general arrangement is positive ions formed by positive ion bombardment of surfaces. A high intensity source (4) A. J. Dempater, Phys. Rer., 11,316 (1918). (5) R. H. Plumlee and L. P. Smith, J . A p p . Phye., 21, 811 (1950). (6) 0. H. Smith, Phys. Rev., 7, 625 (1916).
. Nov., 1053
'
TRACE ELEMENT DETERMINATION BY M A S S SPECTROMETER
of this type has been described by Herzog and Viehboch.7 Another variation has been used by Wetherill and Inghram.8 The advantages of this source are as follows: (1) it ionizes all elements present just as the spark source; (2) it is a d.c. source, hence, RF and pulsed beams are not a problem as with the vacuum spark; (3) it is efficient, Le., sample consumption is very small. Its major disadvantages are: (1) it requires a special double focusing mass spectrometer; ( 2 ) it is limited in sensitivity-of about 1 p.p.m. with the best mass analyzers now in use. Isotopic Dilution Method.-The sensitivity and accuracy of the Dempster vacuum spark and the ion bombardment methods, though useful for many analytical problems, prove to be inadequate for problems requiring sensitivities of better than one part per million. Since this is the range which is of interest from the nuclear physics standpoint it is necessary for these purposes to look for another more sensitive method. The method chosen was that of mass spectrometric isotopic dilution. This method of analysis, which is an internal standard method, has been used for a number of years by the biologists9 for determination of the elements and compounds of hydrogen, carbon and nitrogen. Use has been limited t o these elements due t o the fact that these were the only elements for which samples of the element having isotopic compositions different from normal were available. As a result of the success of the Atomic Energy Commission in making available separated isotopes of many of the elements, this limitation has been almost eliminated and the mass spectrometric isotopic dilution method becomes a practical analytical tool with sensitivities for many elements far surpassing any previously known analytical method. The isotopic dilution method as used with the mass spectrometer can be described by outlining the procedure then giving an example of a typical analysis. The procedure is as follows. (1) A weighed portion of the sample t o be analyzed is dissolved in an appropriate solvent. (2) A weighed portion (usually an aliquot of a stock solution) of a separated tracer isotope of the impurity under consideration is added to the solution from step one. (3) The impurity element in uestion together with its dilutant is extracted chemically. &his step is optional depending on the element and the sensitivity desired.) ( 4 ) The change in isotopic composition of the impurity element due to dilution with the tracer is determined m:m spectrometrically.
From these four steps the concentration of the tracer element in the original sample is determined. A hypothetical example of such an analysis is the determination of the amount of vanadium in a metal sample. To a first approximation vanadium as it exists i n nature consists of a simple isotope of mass 51, as shown in A of Fig. 2 . Suppose to one gram of the original sample one microgram of the mass 50 tracer isotope of vanadium is added, B of Fig. 2. After mixing, extracting the vanadium chemically and measuring the resulting abundances with the mass spectrometer suppose one finds the ratio of (7) R. Herzog and F. Viehboch, Phvs. Rev.. 16, 855 (1949). ( 8 ) See 11. G. Inghram and R. J. Hayden, "Mass Spectroscopy," Preliminary Report of National Research Council (1953). (9) D. Rittenberg, J . A p p . Phva., 13, 561 (1942).
IA
VANAD I UM IN SAMPLE
IB
TRACER VANADIUM
I n
VANADIUM FROM SAMPLE PLUS
I ,
811
Fig. 2.-Hypothetical mass spectra illustrating the isotopic dilution method of analysis for the element vanadium. The vanadium in the sample consisting essentially of only mass 51 (A) is diluted with a known mass of the separated isotope of mass 50 (B). The ratio of 50/51 in the mixed sample (C) shows that the weight of vanadium in the sample under test is one half of that added as the diluting isotope.
Vso/Vslto be that shown in C of Fig. 2,i.e., two to one. Then since the peak a t mass 50 corresponds to the one part per million by weight of Vs0which was added, the peak at mass 51 which corresponds to the total vanadium in the sample before addition of tracer is just half that, i.e., 0.5 part per million. I n actual practice clean-cut cases such as that just cited seldom occur. Both the normal and tracer materials consist of mixtures of isotopes so that recourse to a simple set of two simultaneous equations is usually necessary. By consideration of the above description of the isotopic dilution procedure the following advantages of the procedure should be apparent. (1) The final result does not depend on the quantitative chemical recovery of the element in question. For
812
MARKG. INGHRAM u235 TRACER PLUS METEOR
-
x
U235 TRACER
i I; -x
100-
100-
c
270 269 ~238
268
267 u235
270 269 Urn8
268
267 u235
Fig. 3.-Mass spectrum illustrating the isotopic dilution method of determination of trace uantities of uranium. This element is analyzed as UOa so %at the apparent maw of U2a is 235 32 = 267. The peaks a t mass 268 and 269 are due to 0 1 7 and OL8,respectively. The change of 500% in the U2s abundance in the spectrum a t the left is due to a meteorite containing five parts per billion of uranium.
+
example, if in the foregoing example 95% of the VK0had been lost in processing so also, since isotopes cannot be separated by ordinary chemical procedures, 95% of the VK1would have been lost with the final answer remaining unchanged. (2) The method is absolute, Le., there are no undetermined additive factors by which the result must be corrected as in the usual wet chemical method where solubilities introduce subtractive errors. (3) Since the method does not depend on a particular mass spectrometer source as does the Dempster vacuum spark method, simpler and more sensitive mass spectrometric equipment can be used. Many of the elements can be detected with as little as a 10-l2 g. of material present. Four of the elements, namely, Na, E(, Rb and Cs, can be easily detected in the g. range. (4) The method is extremely sensitive to trace impurities. For example, uranium impurities in metals and stones have been quantitatively measured when they exist to only five parts per billion. Figure 3 shows an example of the determination of uranium in a stone meteorite where a change of 500% in the U235/U238 ratio represents five parts of uranium t o a billion of meteorite.lo Trace quantities of gases in solids have been measured with gas contents of less than one part per trillion.I1 ( 5 ) The range of concentrations detectable in a single experiment is very wide. For example, if the uranium content of the meteor whose isotopic dilution spectrum is shown in Fig. 3 had been twenty times lower or 10,000 times higher, the final result would still have had almost the full accuracy. Thus it is usually not necessary to make even a good guess as to the actual purity of the sample in question before adding the diluting isotope. (6) A final advantage is accuracy. With most of the elements which can be determined with the (10) H. Brown,M. Inghram, E. Lamen, C.Patterson and G. Tilton, Geol. SOC.Amer. Bul., Nov., 1951. Re%, 78, 822 (1950). (11) M. G. Inghram and J. H. Reynolds,
VOl. 57
method, the accuracy is of the order of one per cent. With a few elements accuracies ten times better than this are quite feasible. The isotopic dilution technique also has important disadvantages. (1) The first disadvantage of the method is that it is useful usually for only one element at a time. I n this respect it is very different from the Dempster vacuum spark method. It has been used in the rare earths and in the case of the alkalies t o determine as many as four elements in one run but these are unusual cases in that the chemistry of each of these sets of elements is similar. (2) A second disadvantage is that t o use the method samples of the element in question having isotopic compositions considerably different from normal are necessary. Some elements simply do not have a second isotope which can be used as the dilutant. The rest are available, with a few exceptions, only through the Atomic Energy Commission, and the Commission has at the present time very definite restrictions as to their use. Table I1 TABLETI SCOPEOF ISOTOPIC DILUTIONMETHODOF CHEMICAL ANALYSIS Elemanta
not
Elementa analyrable
H
Ni
Te
He Li
cu
(Be) B
Ga Ge Se
(1) Xe (Cs) Ba
C
N
Zn
Ne Mg Si
Br Kr Rb Sr Zr
S
0
c1
A
K Ca Ti
V Cr Fe
La Ce Nd (Pm)
Sm
analyzable
Ir Pt
F 'Na
Hg
AI
TI
P
Pb Th
su
U
co As
Mn Y Cb Rh
Mo
Eu Gd
Pr
(To)
DY
Tb
Ru Pd
Er Yb Lu Hf
Tm Ta
Ag Cd In W Sn Re Sb os Total 67
Ho Ao
Total 17
lists the elements in two columns, i.e., those which can and those which cannot be analyzed by the isotopic dilution method. Those elements which have been determined in solids by isotopic dilution are marked with an asterisk. Those elements in the not analyzable column, for which some future possibility of analysis exists by using radioactive tracers, are also parenthesized. (3) A third disadvantage results from problems of contamination. This difficulty is, of course, by no means unique to the isotopic dilution method of analvsis. For examde. the mass 238 Deak shown in Fig. 3 corresponds t o the 5 X 1 0 - 8 gram of uranium in the total original sample. If one milligram of ordinary dust got into the sample anytime during chemical processing it would give a 10%
'
Nov., 1953
TRACE ELEMENT DETERMINATION BY MASSSPECTROMETER
813
error in the result. Reagent purities when working a t different times so that a time integration of the in this submicrogram range are also a major con- spectrum is necessary as well as calibration with tamination problem. The reagent contamination standard samples. This is an inaccurate and painsproblem is generally taken into account by process- taking procedure. ing a blank along with the sample. There is, howAnother method developed by Muraca and Serever, no simple way to evaluate the erratic "dust" fass16 involves the use of a vacuum spark as the error. means of evaporating the sample into the ioniza(4) A fourth disadvantage which is common to tion chamber of a standard single focusing maall mass spectrometric methods of analysis is that chine. In such an evaporation, element fractionagood high vacuum technique is fundamental t o the tion with time is negligible so that integration is mass spectrometer. This introduces complications eliminated. A t the present time, however, the which are not met in the usual optical method of sensitivity of this technique is such that it cannot analysis. be considered a trace element technique. Table I11 shows an example of geologic materials Conclusion which were analyzed for uranium using the isotopic The isotopic dilution method is a method which dilution method. This table is taken from the work could be used today by many laboratories for trace of Brown, et aL10 The basic problem which was under consideration was the geologic age of differ- analysis of solids. I n contrast to the Dempster ent minerals within a given granite. This problem vacuum spark and ion bombardment methods for is discussed elsewhere. The table does, however, which no commercial machines are available, maillustrate the range, sensitivity and accuracy of chines are commercially available which w t h only minor modifications can handle some of the elethe method. ments using the isotopic dilution method. The TABLEI11 cost of the tracer materials is not high; Dr. Keim of Uranium content in p.p.m. Geologic material the Oak Ridge National Laboratory informs me Zircon 2614 & 25 1 that the cost of the separated lead isotopes used in Sphere 299 f 3 the geological studieslo was of the order of 0.2 Apatite 93.3 f 0 . 9 cent/sample. Cost of the tracer is, therefore, not Magnetite 4.12 i . I 3 a limiting factor. The thing which is important is Perthit e 0.22 .02 that there are only limited quantities of tracers Plagioclase .20 f .01 available. Thus, as far as cost goes, there is no Quartzite . I 3 i .01 reason why one should not use 100 times as much Modac stone meteor .0105 0.0003 tracer. However, such a waste of material, valuaNorton atone meteor .0054 f 0.0002 ble only because of lack of production, not cost of Ammonium nitrate .000075 f 0.000004 production, means that only one hundredth as many analyses can be made with the same amount The range shown is from below one tenth part of material. Thus, a t the present time experiments per billion to above two parts per thousand. The must be planned with care knowing that the use of error on the last sample indicates a sensitivity of the material precludes someone else doing an exfour parts per trillion. This, however, is not the periment which might be of more value. It is to ultimate limit since the errors in all cases, except be hoped that if the demand becomes great enough, the first three, are due to contamination problems adequate supplies can be made available through in the chemical procedure. Neglecting these con- private or other suppliers on a self-supporting basis. tamination errors the accuracy is of the order of one Two final words should be said concerning the apper cent., even at these low levels. plication of these mass spectrometric methods of Analyses similar to the above have been done for solid analysis. I n the author's opinion, the mass lead in rocks,'O thorium in rocks,1zxenon in rocks," spectrometric methods should not be made to comargon in rocks,12calcium in potassium,lanickel and pete with other presently used methods of analysis zinc in copper,14 selenium and krypton in sodium a t least until they are more completely developed bromide, l 4 gadolinium and samarium in europium, and better mass spectrometric equipment becomes etc.15 The limits of this technique have certainly available. They should be reserved for those probnot been reached. lems which cannot be solved by existing methods Alternative Methods.-In addition to the meth- but for which the mass spectrometric methods have ods outlined above, there are a number of alterna- been proved suitable. Secondly, in these relative procedures. One involves a slow evaporation tively early stages of applying the mass spectro.of the sample to be analyzed into the ion chamber metric methods to solid analysis the sensitivities of a conventional single focusing mass spectrome- and accuracies are directly proportional to the ter. In such cases different elements evaporate competence of the personnel assigned to them. The major problem with a mass spectrometer is that it (12) 0. Tilton, M. Inghram and C. Patterson, Cl'eol. SOC.Amer. Bul., Nov.. 1952. always gives an answer. It is up to the laboratory (13) M. Inghram, H. Brown,C. Patterson and D. Hess, Phua. Re%, worker to know what, if anything, that answer 80, 916 (1950). means. (14) J. H. Reynolds, ibid., 79, 289 (1950).
*
(15) R. J. Hayden. J. H. Reynolds and M. '76, 1500
(1949).
G. Inghram, ibid..
(18) R. F. Muraca and E. J. Serfass. Meeting of the ASTM Committee E-14 on Mass Spectroscopy, Pittsburgh. 1953.
MARKG. INGHRAM
814 DISCUSSION
F. SEITZ (University of Illinois).-Using the isotopic dilution method, how routine can measurements in a given part of the periodic system be made? Can it be turned into something that one would use in an ordinary laboratory for everyday analyses? M. G. INGHRAM.-The time required for an analysis depends on two major factors. One is the sensitivity desired and the other the particular elements under consideration. For example, if one wishes to know the amount of cesium or rubidium present in a potassium chloride sample to a sensitivity of one part in 108, it is only necessary to dissolve the sample, add tracers and run the mixture in the mass spectrometer. Using a vacuum lock equipped mass spectrometer, the total time for analysis would be less than one hour. I n contrast, if one wants to do the thorium content of a granite sample to the same sensitivity, about three days of time are required. The difference is due to the much longer time required for chemical processing. Of course, in routine use a number of samples could be carried along simultaneously. G. E. MOORE(Bell Telephone Labs).-In your preprint, you state that you can form ions of the surface impurities more efficiently by bombarding the surface with ions of the noble gases than by electron bombardment. Will you discuss this further?
M. G. INCrHRAM.-The statement refers only to ionization produced by ion and electron impact on surfaces. It is a very different problem from that encountered in the usual mass spectrometer where ionization occurs in the gas phase. To understand the reason for the greater efficiency of ions in producing ionization on surface impact one can use the qualitative picture of a two-body billiard ball collision. The electron, since it is very light, can’t’transfer enough momentum to the surface atom to break the chemical bond, while an atom, since it is much heavier, has no difficulty in accomplishing the task. The result is that ions are many orders of magnitude more efficient than electrons in surface bombardment ionization. E. G. JOHNSON (Minn. Mining and Mfg. Co.).-With the ion bombardment source what energies do you use to bombard the surface? M. G. INGHRAM.-we have used energies in the range of five to ten thousand volts. This range may not be o timum because as the energy is increased the ionization egciency increases but the energy spread in the ion beam produced also increases resultin in a decrease in transmission of the double focussing macfine. The optimum figure is thus a question of the particular mass spectrometer on which the source is used. P. GOLDBERG (Sylvania).-How is it that the oxygen peaks show up near mass 270 in the spectrum which you showed of uranium? M. G. INc+HRAM.-Using the simple surface ionization source, uranium evaporates as UOz+. Thus the peaks at 267 268 and 269 correspond to U236O162, U236016017 and U23b016018.
BRELL(KAPL).-What is done in working with rare gases? I am thinking particularly of fission krypton and xenon, when these are present with large amounts of other gases, air, carrier, etc.
M. G. INCrHRAM.-Dr. Wetherill has been working with spontaneous fission product krypton and xenon. He extracts these gases from old minerals. He obtains of the order of 10-8 S.T.P. cc. of the rare gases in the presence of large amounts of contaminants. Some of the reactive contaminants are removed by a gas train purifier, largely by hot
VOl. 57
calcium vapor. The remaining gas consisting of perhaps 10-3 S.T.P. cc. is introduced to the mass spectrometer. With this total amount of gas he is able to detect a rare gas peak present to as little as 10-13 S.T.P. cc., ie., the spectrometer itself acts as the final purifier since it picks out one unique mass in the presence of large amounts of impurities present a t other masses.
V. P. GUINN(Shell Development Co.).-Could you elaborate a bit upon the type of ion beam detector that you mentioned? This is of the electron multiplication type, I presume? M. G. INQHRAM.-TO achieve maximum sensitivity, it is, of course, necessary that each component of the machine be of the most sensitive design. The most sensitive detection is certainly an electron multiplier in which each individual ion gives a detectable signal. We use an internal electron multiplier for this purpose, i.e., the ion beam itself strikes the first stage of the multiplier where it gives rise to secondary electrons which are then multiplied in the usual manner. The technique of allowing the ion beam to strike a scintillator and then detecting the light pulse produced with a standard photomultiplier has been tried but has not worked out very successfully. With the internal multi lier it is quite easy to count every single ion that gets tfrough the machine. One can either use a scalor and counting rate meter or direct integration with a DC amplifier to record the multiplied beam. GUINN.--IS the internally placed electron multiplier which you use commerciaIly available, or is it one which you made yourself?
M. G. INGHRAM.--NO, this type of detector is not yet commercially available. We have made our own. There is however, a company which will have them available within a few months, I believe. A. E. MILCH(Mellon Inst.).-You mention that isotope ratios are not affected in any way by the chemical Separation, for example, precipitations. How do you reconcile that with the fact that in many cases isotope separation is successively effected by chromatographic absorption of solids in solution?
M. G. INGHRAM.-It is only a question of accuracy. It is exceedingly difficult in any one stage chemical process to get a separation of more than a few tenths of a per cent. The accuracies I have quoted are of the order of 1-2%, thus the isotope separation effects which you mention are all below the limits of detection of the method outlined. C. H. GREENE(Corning Glass Works).-In the ion bonibardnient method is there any variation of the mass spectrum with ion beam intensity?
M. G. INGHRAM.-\tTe haven’t done enough with the method to be convinced about this point. The major y t i o n is: are you looking a t the surface composition or t e true composition of the solid sample? Certainly, with small bombardment beams one sees largely the gases absorbed on the surface. With larger beams and higher sample tem eratures the surface is removed fast enough SO that I thin[ one should be able to obtain the true composition of the sample. W. D. COOKE.-HO~about contamination on walls and sources, ie., in metal analysis as compared to gas analysis.
M. G. INGHRAM.-MYfeeling is that wall contamination is less serious in solid analysis than with gases. When gases like hydrogen chloride acid, etc., are introduced into a gas machine, there is a detectable vapor pressure left days later. I n contrast, when solid samples are evaporated in the surface ionization source a t high temperatures, one to two thousand degrees, they make one collision with the walls and they are down.
