ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
SYMBOLS AND NOMENCLATURE V = accelerating voltage. E = electric sector voltage. B = magnetic field. V,, Eo = reference values of V and E for transmission of main beam ions by electric sector. mB = mass of singly charged ions transmitted by magnetic sector when V = Vo and E = Eo. P = (Vo/ V) (E/&). = (&/E)%. Conventional geometry = geometry of double focusing mass spectrometer for which the electric sector precedes the magnetic sector. Reversed geometry = geometry of double focusing mass spectrometer for which the magnetic sector precedes the electric sector. First field-free region = field-free region preceding the first sector in the flight tube. Second field-free region = field-free region preceding the second sector in the flight tube. Constant parent-ion spectrum = spectrum of peaks due to the generation of different daughter ions from the same parent ion. Constant daughter-ion spectrum = spectrum of peaks due to the generation of the same daughter ion from different parent ions. Constant neutral spectrum = spectrum of peaks for reactions related by a constant mass of the neutral fragment. P-scan = scanning mode in which all of the parameters V, E and B , except one (P),are kept constant. Linked scan = scanning mode in which at least two of the parameters V, E , and B , are varied simultaneously while a specified relationship is maintained between them. P,Qscan = linked scan in which P and Q, both functions of the parameters V, E , and B , are varied so that the ratio of P to Q remains constant. An example is an E,B2 scan, in which E and B are varied at constant V so that E is proportional to B2. Unlike the method of designating a linked scan by specifying a ratio that is kept constant (for example, an E / B 2 scan), the P,Q nomenclature for linked scans is consistent with the P nomenclature for simple scans.
695
LITERATURE CITED (1) M. J. Lacey and C. G. Macdonald, Org. Mass Specfrom., 12, 587 (1977). (2) T. W. Shannon, T. E. Mead, C. G. Warner, and F. W. McLafferty, Anal. Chem., 39, 1748 (1967). (3) R. W. Kisec, R. E. Sullivan, and M. S.Lupin, Anal. Chem.. 41, 1958 (1969). (4) J. Coutant and F. W. McLafferty, Int. J. Mass Spectrom. Ion Phys., 8 , 323 (1972). (5) W. Schonfeld, Inf. J. Mass Spectrom. Ion Phys., 18, 281 (1975). (6) W. Schonfeld, Org. Mass Specfrom., 10, 321, (1975). (7) W. Schonfeid, Org. Mass Specfrom., 10, 401 (1975). (8) C. Hwang and R. W. Kiser, Int. J. Mass Specfrom. Ion Phys., 27, 209 (1978). (9) M. J. Lacey and C. G. Macdonald, Org. Mass Spectrom., 13,284 (1978). (10) M. J. Lacey and C. G. Macdonald, Org. Mass Spectrom., 13,243 (1978). (11) M. A. Barber, W. A. Wolstenholme, and K. R. Jennlngs, /Vafure(LoMbn), 214, 664 (1967). (12) U. P. Schlunegger, Angew. Chem., I n f . Ed. Engl.. 14, 879 (1975). (13) D. H. Williams and I.Howe, "Principles of Organic Mass Spectrometry", McGraw-Hill, London, 1972, p 103. (14) F. W. McLafferty, "Interpretation of Mass Spectra", 2nd ed., W. A. Benjamin, Reading, Mass., 1973, Table A-5. (15) D. D. Speck, R. Venkataraghavan, and F. W. McLafferty, Org. Mass Specfrom.. 13,209 (1978). (16) J. R. Reeher, G. D. Flesch, and H. J. Svec, Org. Mass Spectrom., 11, 154 (1976). (17) F. B. Burns and T. H. Morton, J. Am. Chem. SOC.,9 8 , 7308 (1976). (18) R. G. Dromey, B. G. Buchanan. D. H. Smith, J. Lederberg, and C. Djerassi, J . Org. Chem., 40, 770 (1975). (19) T. Wachs, P. F. Bente 111, and F. W. McLafferty, Inf. J. Mass Spectrom. Ion Phys., 9, 333 (1972). (20) E. Stenhagen, S.Abrahamsson. and F. W. Mclafferty. "Atlas of Mass Spectral Data", Wiley, New York, 1969. Spectrum API-0109. (21) K. Levsen and H. Schwah, Ang9w. Chem., Inf. Ed. EngL 15, 509 (1976). (22) R. Robbiani, J. Kuster, and H. Seibl, Angew. Chem., Int. Ed. Engl., 18, 120 (1977). (23) R. W. Kondrats and R. G. Cooks, Anal. Chem., 50, 81A (1978). (24) R. K. Boyd and J. H. Beynon, Org. Mass Spectrom., 12, 163 (1977). (25) P.Krenmayr, F. KHica, and K. Varmuza, Monatsh. a m . , 109,823 (1978). (26) J. H. Beynon. W. E. Baitinger, and J. W. Amy, Inf. J. Mass Spectrom. Ion Phys., 3 , 5 5 (1989). (27) J. H. Beynon, J. E. Corn, W. E. Baitinger, J. W. Amy, and R. A. Benkesef, Org. Mass Spectrom., 3 , 191 (1970). (28) M. J. Lacey and C. G. Macdonald, Ausf. J. Chem., 31, 2161 (1978). (29) J. H. Beynon and R. G. Cooks, Inf. J. Mass Specfrom. Ion Phys., 19, 107 (1976). (30) F. W. McLafferty and F. M. Bockhoff, Anal. Chem., 50, 89 (1978). (31) W. V. Ligon and J. L. Webb, Int. J. Mass Spectrom. Ion Phys.. 22, 359 (1978). (32) A. Maquestmu, Y. Van Haverbeke, R. Fbmmang, and C. De Meyer, Bull. SOC. Chim. Belg., 86, 281 (1977). (33) R. P. Morgan, C. J. Porter, and J. H. Beynon, Org. Mass Specfrom., 12, 735 (1977).
RECEIVED for review November 27, 1978. Accepted January 23, 1979.
Determination of Carbon in Steel by Secondary Negative Ion Mass Spectrometry Naoharu Yarnaguchi, Ken-ichi Suzuki, and Kimitaka Sato' Fundamental Research Laboratories, Nippon Steel Corporation, 16 18, Ida, Nakahara-ku, Kawasaki 2 1 1, Japan
Hifumi Tamura Central Research Laboratory, Hitachi Ltd., 1-280, Higashi-Koigakubo, Kokubunji 185, Japan
The determination of carbon in steel by secondary negative ion mass spectrometry is described. Ions originating from adsorbates and gas phase were removed from solidphase ions by the dlfference of their energies. The ions were collected by a new method for monitoring total ion intensity. The technique of removing only negative ions from secondary electrons In a magnetic field before mass analyzing was used. As a result, excellent linearity was obtained between carbon contents (0.001 to 0.61 % ) and ion intensities.
Ion mass analysis (IMA) employing negative ions offers 0003-2700/79/0351-0695$01.00/0
advantages over Auger electron spectroscopy (AES) as already reported by Benninghoven (1). In particular, the use of negative ion improves detection sensitivity by 10 to 100 times over that using positive ions, especially for the determination of light elements such as carbon, oxygen, and sulfur. However, determination of light elements by secondary negative ions requires special consideration of the following factors: (1) influence of secondary ions originating from the specimen surface adsorbates and gas phase; (2) secondary negative ion yields of many matrix elements, for example, 56Fe-,are very low, so that a standard element is necessary, to establish a relative ion intensity ratio; (3) influence of secondary electrons 0 1979 American Chemical Society
696
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
Table I. Chemical Composition of Specimens Used in the Present Investigation (%) specimen C Si Mn P S Ti V pure iron 0.001 steel A 0.09 0.23 1.05 0.001 0.004
B C
D E F NBS 4 6 1 462 463 464 465 466 467 468
Cr
0.17 0.35 0.42 0.51 0.61
0.20 0.23 0.28 0.27 0.31
0.02 0.69 1.40 0.80 0.40
0.013 0.025
0.003 0.026
0.01 0.07
0.017 0.015
0.027 0.006
0.01 0.05
0.15 0.40 0.19 0.54 0.037 0.065 0.11 0.26
0.047 0.28 0.41 0.48 0.029 0.025 0.26 0.075
0.36 0.94 1.15 1.32 0.03 0.11 0.27 0.47
0.053 0.045 0.031 0.017 0.008 0.012 0.033 0.023
0.025 0.034 0.024 0.008 0.007 0.010 0.028
having the same charge as negative ions and emitted from the specimens. Here, the energy difference between the gas-phase and solid-phase ions is utilized to effect separation (2). In addition, a new method for monitoring total ion intensity (3) is used to make corrections. Furthermore, a novel approach was developed to ensure that only secondary ions are introduced into the mass analyzing system.
EXPERIMENTAL The specimens used in this study were prepared so that the individual components would be present as a homogeneous solid solution. The chemical compositions of the specimens are shown in Table I. IMA analysis was performed using a modified Hitachi IMA-2 Ion Microprobe Analyzer (4-7). Argon positive ions were used as primary ions and accelerated to 15 keV. The ion beam was 0.5 mm in diameter, and the specimen current was 1 wA. The acceleration voltage for secondary ions was f3 keV. RESULTS AND DISCUSSION Solid- and Gas-Phase Ion Separation. The influence of gas-phase ions, originating from the specimen surface adsorption layer or the atmosphere, must be reduced to a minimum, in order to quantitatively analyze solid-phase ions accurately with IMA. For this purpose, two methods have been proposed. First, the specimen surface is irradiated by an electron beam. The gas phase and adsorbates are examined by analyzing the electron impact desorbed ions generated on the specimen surface. Then, the secondary ions emitted during irradiation with an ion beam are analyzed and the spectra compared to distinguish between the gas-phase and solidphase ions (8). In the second method, the energy difference between the gas-phase and solid-phase ions is utilized ( 2 ) . In the present study, the second method was employed since the quantitative analysis is difficult and involves specialized instrumentation for the former one. The concept of initial kinetic energy distribution for secondary ions has been discussed by Stanton (9)and Bradley (IO). They suggested that there is an energy difference between solid-phase and gas-phase secondary ions and that energy analysis could be used to discriminate between these two types of ions. The principle of the energy selection method is depicted in Figure 1. On the left, secondary ions with Eo(eV) energy at a secondary ion acceleration voltage of Vo(V) pass through the B slit. However, on the right, secondary ions with Eo AE, (eV) energy at a secondary ion acceleration voltage of Vo + AVO(V) pass through the @ slit. The results of energy distribution measurement for secondary ions from the steel specimens are shown in Figure 2.
0.01 0.037
0.010 0.004 0.20 0.057 0.26
0.011
0.024 0.058 0.10 0.29 0.002 0.007 0.041 0.17
SrJec'men
0.13 0.74 0.26 0.078 0.004 0.011 0.036 0.54
vo ( v ) Secondary ion accel voltage
Zr
Nb
Mo
0.005 0.063 0.20 0.010 0.002 0.005 0.094 0.005
0.011 0.096 0.19 0.037 0.001 0.005 0.29 0.006
0.30 0.080 0.12 0.029 0.005 0.011 0.021 0.20
z'
Vo + AVO ( v )
"m
Flgure 1. Principle of selection of secondary ion energy
102-
F-,'!
h
.-. 3
lo:
:
A
c,
\ I
AVO (Arbit. Unit) Flgure 2. Energy distribution of secondary negative ions. Specimen, carbon steel. Primary ion, 4oAr+ (15 keV, 1 FA). Beam diameter of primary ion; 0.5-mmi.d. Vacuum; 1 X lo-' Torr In all cases, gas-phase ions are distributed to a greater extent on the low energy side. Furthermore, the influence of gasphase ions is held to a minimum and solid-phase ions are measured accurately. This is possible only if the specimen potential is set to permit detection of secondary ions with high initial kinetic energies. As in the Figure, there is a difference in the energies of solid-phase and gas-phase ions, and the former are said to be 100 eV greater. This probably results from the fact that the energy of an ion depends on its potential a t the point of
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979 Primary ion beam
Electric sector
I
I
697
Primary ion beam
Mc gnetic sector
1I
I
Detector
electron
B
w
0
bla
/
.-..C 3
.-c
Flgure 3. Principle of total ion monitoring method
ionization. The gas-phase species are ionized in a region where the field value is lower than for ions sputtered from the specimen surface. Standard Relative Ion Intensity Ratio. In IMA analyses, the ionization efficiency or yield of secondary ions varies with specimen surface conditions. It is therefore necessary, especially in the case of quantitative analysis: to correct individual secondary ion intensities. At present, the following two methods are employed for this purpose: (1) The secondary ion intensity of a matrix element is taken as the basis for determining the relative ion intensity ratio. In the case of steel, 56Fe or 54Fe is frequently used for this purpose. (2) The current of total secondary ions introduced into the mass analyzing system is measured and used as the basis for determining the relative ion intensity ratio. This method is commonly known as the “total ion monitoring method” ( 3 ) . Since secondary electrons have the same charge as negative ions, they must be eliminated to obtain accurate total negative secondary ion measurements. The solution to this problem is explained in the next section. When performing negative ion spectroscopy, the ability to detect heavy elements sensitively deteriorates markedly. Therefore, when steel is analyzed, it is necessary to depend completely on the total ion intensity monitoring method. It is impractical to use iron (Fe) as a standard element. The total secondary ion intensity monitoring method, as its name implies, has an important role of correcting individual secondary ion intensities for variations in extraction efficiency. Using this method, it is also possible to correct, to some extent, the influence of specimen surface variations or apparent surface work functions on secondary ion emission capacity. Namely, its purpose is to detect a part of the total ion current emitted from the specimen, which is, of course, proportional to the total ion current. In addition, this monitoring method determines the ratio of the total to the mass resolved ion current, reduces the effect of the above influences as much as possible, and then provides as correct a value as possible. The principle of the total ion monitoring method is shown in Figure 3. Here, the ion current striking the p slit (resolving slit), located between the electric and magnetic sector fields, is used to represent the current. The reason for placing the P slit in this position is to avoid the influence of reflection ions. Concrete application of this method for negative ions calls for the separation of secondary negative ions and secondary electrons. This is explained in the next section. Separation of Secondary Electrons from Negative Ions. Secondary electrons having the same charge as total
a
SAMPLE CURRENT (?A) Flgure 5. Effect of correction of ion intensity by total ion monitoring
method
f3KeV
Q
2 + v)
c
a,
c
.-I=
200
t I
0
I
0.2
I
I
I
0.4
I
0.6
C a r b o n content ( w t %) Flgure 6. An example of calibration curve for carbon In steel and effect
of primary ion species on sensitivity of carbon ion
secondary negative ions are also detected by the P slit. Therefore, accurate measurement of secondary negative ions becomes difficult. In the present study, secondary electrons were prevented from entering the mass analyzing system by the adoption of a magnetic field. The magnetic field is formed at right angles to the secondary ion path near the extraction electrode. The implementation of this separation method is shown in Figure 4. An example of total secondary negative
098
ANALYTICAL CHEMISTRY, VOL. 51, NO. 6, MAY 1979
300
-Art
15KV
0
- 3 KV 0 . 5 r n m Q , lyA
464
''
462
'465 I
I
I
0.2 CARBON
I
0.4
I
I
0.6
(O/o)
Figure 7. Relationship between carbon content in NBS standard steel and ion intensity of '*C-
ion monitoring carried out after removal of secondary electrons is shown in Figure 5. Quantitative Analysis of Carbon i n Steel. After solving the problems discussed in the above sections, the feasibility of determining the carbon content in steel was studied. Here, the effect of primary ion species on the sensitivity to carbon ions was examined using steel specimens. It was found that 40Ar+was the most effective ion species as shown in Figure 6. Negative ions (12C-)show very good linearity in the carbon content range of 0 to 0.6%. Moreover, the two sets of measurement, taken on different days, reveal no differences as shown in Figure 6. A similar study using positive ions, showed that sensitivity was lower than with negative ions. This consistency in day-to-day measurements tends to substantiate the use of total ion monitoring. Next, a similar experiment was carried out on commercially available standard steel specimens, NBS 461 to 468. The analysis conditions were the same as above and the results are shown in Figure 7. The NBS 461,465, and 466 specimens are on the standard calibration line shown in Figure 6, but all other specimens show higher values. It can be seen from Table I that all high value specimens contain significant amounts of so-called carbide formers such
as vanadium and niobium. A similar phenomenon was also observed for positive ions by Tsunoyama et al. (11). They attributed the phenomenon to the presence of precipitation phases in the specimens and resultant variations in the ionization efficiency of alloy elements. Furthermore, they measured the ion intensity ratio of target elements to iron (Fe) where the concentration of target elements remained constant and the carbon content varied in order to study the influence of precipitation phases on ion intensity ratio in detail. In addition, they investigated these relationships with regard to the precipitation of carbides. As a result, Fe-V alloys, when annealed at 710 "C, released V4C3and (Fe-V)& precipitates. On the other hand, more complex precipitation phases were observed with Fe-Nb alloys. This fact suggests that such changes in the ion intensity ratio are due to the concentration of alloy elements present in the ferrite matrix and precipitation phases, to the actual state of combination with iron and carbon, and to the crystal structure. As a result, the present approach using negative ions appears to parallel the phenomena observed with positive ion analysis. Light Elements O t h e r t h a n Carbon. Sulfur is another element that plays an important role and is contained in appreciable amounts in steel. The role of sulfur was investigated employing the linearity of the calibration curve. Approximate linearity was obtained, but more detailed studies will be necessary along with study of nitrogen, oxygen, and other light elements before meaningful conclusions can be proposed. ACKNOWLEDGMENT The authors express appreciation to S. Nagashima, Yokohama National University, for his encouragement and valuable suggestions. They are also indebted to H. Okada, the Director of the Nippon Steel Fundamental Research Laboratories, and H. Watanabe, the Director of the Hitachi Central Research Laboratory for their continued interest. LITERATURE C I T E D (1) A. Benninghoven, Appl. fhys., 1, 3 (1973). (2) K. Nakamura, H. Tamura, annd T. Kondo, Jpn. J . Appl. Phys., 13, 917 (1974). (3) H. Tamura, T. Kondo, I. Kanomata, K. Nakamura, and Y. Nakajima, Jpn. J . Appl. Phys., Suppl. 2 , F't. 1, 379 (1974). (4) K. Sato and H. Tamura, J . Vacuum SOC. Jpn., 17, 385 (1974). (5) R. Matsumto, K. Sato, and K. Suzuki, Tetsu To Mgane, 80, 1980 (1974). (6) R. Matsumoto, K. Sato. and K. Suzuki, Jpn. J . Appl. fhys., Suppl. 2 , Pt. 1, 387 (1974). (7) K. Sato, K. Suzuki, R. Matsumoto, and S. Nagashima, Trans. Jpn. Inst. Met., 18, 61 (1977). (8) M. Stark, Opfik, 38, 139 (1972), and papers cited therein. (9) H. E. Stanton, J . Appl. fhys., 31, 678 (1960). (10) R. C. Bradley, J. Appl. Phys., 30, 1 (1959). (11) K. Tsunoyama, Y. Ohashi, and T. Suzuki, Anal. Chem., 48, 832 (1976).
RECEIVED for review March 13, 1978. Accepted January 22, 1979.
