Table 111. Calculated Protonated Molecular Ion (M(thd),Hf) Intensities for Various
2,2,6,6.Tetramethyl-3,5-heptanedionates Ion intensities m/e
Ni(I1)
425 426 427 428 429 430 431 432 433 434 435 436 437 438
52.6 13.2 22.3 6.3 3.6
600 601 602 603 604 605 606 607 608 609 610 611 61 2
0.8
0.9 0.2
Cu(I1) Zn(I1) VO(I1) Cr(II1) Fe(II1) Mn(II1) Co(II1)
from 10 ppb to 10 ppm with a correlation coefficient of 0.996, and the limit of detection was of the order of 1 ppb (8 X g of Cr). Currently, the CIMS is in the process of being computer controlled. This coupling will enable signal averaging to be improve this detection limit and done which fast analysis of mixtures of metal chelates.
LITERATURE CITED
53.4 13.4 25.9 37.8 6.2 9.5 0.9 22.9 0.2 0.1 8.7 77.0 16.0 19.4 3.8 3.1 1.0 0.4 0.2 2.9 1.1
57.2 28.0 8.7 1.9
0.2
4.0 1.5 68.0 62.6 25.6 25.0 5.5 5.9 0.9 1.0
68.0 25.6 5.5 0.9
jected or when injection sizes greater than 0.2 111 are used. If these limitations are exceeded, then signals are observed which are independent of mass in addition to the expected peaks which are due to protonated solvent ion and solvent fragment ions. This could be due to neutrals or long-lived excited species passing through the quadrupole mass filter to be ionized by collision immediately prior to the ion multiplier. This phenomenon is not unexpected since 0.5 111 of toluene will produce a partial pressure in the source of about 0.25 Torr which is comparable in pressure to the reagent gas. The relationship between the area under the ion current envelope and the concentration of the Cr(tfa)s was linear
(1) L. Fishbein, "Chromatography of Environmental Hazards", Voi. 11, Elsevier, 1973, Chapter 2. (2) G. E. Morris, AMA Arch. Dermatol., 78, 612 (1958). (3) A. M. Baetter, C. Damron, and V. Budacz, Arch. lnd. Health, 20, 136 (1949). (4) T. F. Manguso, lnd. Med. Surg., 20, 393 (1951). (5) H. P. Brinton. E. S. Frasier, and A. L. Koven, U S . Public Health Serv. Publ., No. 87, 385 (1952). (6) A. M. Baetter, Arch. lnd. Health, 2, 487 (1950). (7) W. Machle and F. Gregorius, U.S., Public Health Serv. Publ., No. 63, 1114(1948). (8) P. L. Bidstrup, Br. J. lnd. Med., 8, 302 (1951). (9) C. G. MacDonaid and J. S.Shannon, Aust. J. Chem., 19, 1545 (1966). (10) A. L. Clobes, M. L. Morris, and R. D. Koob, J. Am. Chem. Soc., 91, 3087 (1969). (11) J. L. Booker, T. L. Isenhour, and R. E. Sievers, Anal. Chem., 41, 1705 (1969) (12) ?.H.-Risby and L. G. Sanchez, unpublished results, 1970. (13) R. Belcher, J. R. Majer, R. Perry, and W. I. Stephen, Anal. Chim. Acta, 45, 451 (1968). (14) B. R. Kowalski, T. L. Isenhour, and R. E. Sievers. Anal. Chem., 41, 998 (1969). (15) R. J. Majer, Sci. Tools, 15, 11 (1968). (16) L. C. Hansen, W. G. Scribner, T. W. Gilbert, and R. E. Sievers. Anal. Chem., 43, 349 (1971). (17) W. R. Wolf, M. L. Taylor, B. M. Hughes, T. 0. Tiernan, and R. E. Sievers, Anal. Chem., 44, 616 (1972). (18) N. M. Frew, J. J. Leary. and T. L. Isenhour, Anal. Chem.. 44, 665 (1972). (19) K. J. Eisentraut, M. S. Black, F. D. Hiieman, and R. E. Sievers, Geochim. Cosmochim. Acta, 2, 1327 (1972). (20) T. L. Isenhour, E. R. Kowalski, and R. E. Sievers, Dev. Appl. Spectrosc., 8, 193 (1970). (21) T. H. Risby. P. C. Jurs, F. W. Lampe, and A. L. Yergey, Anal. Chem., 46, 161 (1974). (22) T. H. Risby, P. C. Jurs, F. W. Lampe, and A. L. Yergey, AnalChem., 46, 726 (1974). (23) R. W. Moshier and R. E. Sievers. "Gas Chromatography of Metal Chelates'', Pergamon Press, Elmsford, N.Y., 1965, references cited therein. (24) G. Guiochon and C. Pommier, "Gas Chromatography in lnorganics and Organometallics", Ann Arbor Science, 1973, references cited therein.
RECEIVEDfor review November 20, 1975. Accepted January 14, 1976. Part of this work was presented a t the 22nd Annual Conference on Mass Spectrometry and Allied Topics in Philadelphia, Pa., May 1974. Part of this work was supported by the U S . Environmental Protection Agency, Grant No. R803651-10.
Quantitative Analysis of Low Alloy Steels with the Ion Microprobe Mass Analyzer Kouzou Tsunoyama,* Yoshiharu Ohashi, and Toshiko Suzuki Research Laboratories, Kawasaki Steel Corp., Chiba, Japan
The effect of carbides on the relative Intensity of sputtered Ions In low alloy steels was investigated with the Ion microprobe mass analyzer. The relative Intensity varied with the formation of carblde In the matrix. The varlation of the relative intensity imposed a severe limitation upon the analysis of alloy elements by means of the callbration curve procedure. A method to correct for the effect of carblde formation was proposed and applied successfully to the analysis of low alloy steels. 832
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
The ion microprobe mass analyzer has recently been applied to various problems in steels. The spatial distribution of alloy elements and in-depth analysis of sheet surfaces were studied by several investigators (1-3). However, few reports have been published on quantitative analysis of alloy elements in steels. Andersen and Hinthorne ( 4 ) proposed a theoretical correction procedure for the quantitative analysis of sputtered ions. They demonstrated that the local thermal equilibrium (LTE) model worked well for
ID. A
Ar'
OiO
15
20
A c c e l . Volt , keV 20-
r-.
Ool
20
25
30
Atomic number
Figure 1. Relative sensitivities of alloy elements in steels. Sensitivity for Fe is taken as unity
many elements in a variety of matrices, including low alloy steels and stainless steels. Recently, Shimizu ( 5 ) and Rudenauer (6) applied the thermodynamic approach to analysis of low alloy steels, and Ishitani (7) proposed a modified model utilizing the dissociation of molecules. The thermodynamic approach is certainly a useful method for quantitative analysis of multielement samples, but the accuracy of the analytical value is not always satisfactory. More detailed researcli is required to improve the accuracy of the method and to answer the questions pointed by Castaing ( 8 ) ,Carter ( 9 ) ,and Werner (10). In contrast to the LTE model, the calibration curve approach is regarded to be a well established method when high accuracy is required. Leroy et al. ( 1 ) studied the analysis of irons by this technique and showed that the ratio of the ion intensity of an alloy element to the intensity of Fe was a linear function of the atomic concentration of the element. They applied calibration curves to the analysis of ternary iron alloys and found good agreement between ion probe microanalysis and chemical analysis. The authors ( 2 ) obtained similar results in analysis of several low alloy steels. Although the calibration curve procedure is a simple and convenient method, it is not applicable to all cases. A previous study ( 1 1 ) showed that the intensity ratio of alloy element ions to Fe ions sometimes increased when carbide was formed in the matrix. This observation indicates that the ion intensity ratio depends on the state of the matrix as well as the concentration of the element. In the present paper, the effect of carbides on the ion intensity ratio has been studied further, and a method to correct for the effect of carbides on the calibration curve procedure is proposed.
EXPERIMENTAL Quantitative analysis of low alloy steels was performed with the ion microprobe mass analyzer made by Applied Research Laboratories. The surface of the sample was ground on metallographic papers and polished with alumina powder. Details of the sample treatment adopted in our laboratories have been reported elsewhere (2). The main impurity elements on the surface of the sample were Na, K, Ca, Mg, and A1 and these were eliminated by sputtering with 02' ions for 10 min. The beam diameter was in the range of 300-600 wm and the accelerating voltage was 20 keV.
' e '
Z
I 101
0
"
n 0
0
u
z
L
0
100
10 Beam
Current
1
om
(nA)
Figure 2. Variation of the ion intensity ratio with ( a )primary ion accelerating voltage and ( b ) primary ion current intensity
After this preliminary sputtering, the primary ion beam of 100 /*m in diameter and current intensity of 100 nA was focused at the center of the pre-sputtered area, and the intensities of secondary ions were measured in vacuum of Torr. T h e diameter of the incident ion beam was decided to be 100 km so that the effect of the inhomogeneity of the samples used in this study could be reduced as low as possible. A preliminary experiment showed that the relative standard variation of the relative ion intensity of Mn, Ni, or Cr to Fe, which was obtained by bombarding 5 points in NBS 466 sample with an 0 2 + ion beam of 100 wm in diameter, was up to 4%. Oxygen primary ions have several advantages in the analysis of iron. They enhance the yield of sputtered positive ions (12), and erode the surface of iron quite uniformly (13). Moreover, the variation of the relative sensitivity of various alloy elements is reduced by 0 2 + bombardment. A typical example is shown in Figure 1. An NBS standard 466 was bombarded with 0 2 + , N2+, and Ar+ a t 20 keV. The relative sensitivities of seven elements (Ti, V, Cr, Mn, Co, Ni, and Cu) are plotted vs. the atomic number of the elements. Since the yield of sputtered ions may be very sensitive to the energy and current density of primary ions, the effect of the bombarding conditions on the ion intensity ratio was studied in advance. Figure 2 shows the relative ion intensity of Cr+ to Fe+ from stainless steel. One can state that the relative ion intensity obtained by 0 2 + bombardment is independent of the energy and current intensity of primary ions. Therefore, the relative ion intensity and calibration curves reported in this paper should be applicable in other ion microprobe laboratories.
RESULTS AND DISCUSSION Effect of Carbides on the Ion Intensity Ratio. The effect of carbides on the ion intensity ratio of alloy element ions to Fe ions was investigated using several Fe-X alloys. The concentration of alloy element X was about 0.2 at. %. Small sections were cut from the alloys and carburized in H2-CH4 mixtures a t 700 OC to provide carbon concentrations in the range of 0.002 to 0.9 wt 96. The specimens were then quenched after annealing at 710 "C for 30 min. Cementite and other carbides were precipitated homogeneously during this final heat treatment, when the concentration of C exceeded the solubility limit at 710 "C. Variation of the intensity ratio of X ions to Fe ions, Nx+l NF~+ with , carbon content is shown in Figure 3. In Fe-Nb = + constant in the carbon concentraalloys, N N ~ + I N F was ANALYTiCAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
833
I
0
1
10-3
Sample Current
200nA
Accel Voll
20kV
Mary kam
0;
10-1
m-2
c , w t '/. b t i o
d A l o m in Sarrple Nx/N~.~ 1 0 . ~
b
101 I
b
X
I
NI
1
Sample Current 2OOnA Accel Volt 20 kV Rimary Beam 0;
10.'
lo-'
c
I
0
I
.*
5.
c
Cr Mn Si
x
\
P
A
0
0-
z P
- wt
'/a
0 Fe-Mn-C
AFeNi-C
2
4
Ratio of Atoms in Sample Nx/NF, x 1 6 '
Figure 4. Calibration curves for (a) AI, V, Nb, and Mo, ( b )Cr, Mn, Si, and Ni in dilute binary iron alloys
N F ~in+ Fe-Ni alloys remained constant in spite of the existence of cementite is not clearly explained a t present. In the calibration curve approach, it is assumed that the ion intensity ratio of N x + / N F ~is+given by the relation 0
10.3
10-2
10
-'
c, wt % Figure 3. Variation of the ion intensity ratio k c / & , + with the concentration of C in ( a ) Fe-Nb and Fe-V, ( b ) Fe-Cr, ( c ) Fe-Mn and Fe-Ni alloys
tion range 0.001-0.005 wt %. However when the concentration of c exceeded 0.005 wt %, the ratio "b+/NFe+ increased suddenly from 2.45 X to 3.32 X and continued to increase up to 6.11 X The same tendency was observed in Fe-V alloys. On the other hand NCr+/NFe+ in Fe-Cr alloys began to decrease when the concentration of C exceeded 0.04 wt %. Contrary to these results N M ~ + / N F ~in+ Fe-Mn alloys and N N ~ + / N F in~Fe-Ni + alloys did not depend on the concentration of C. The concentration of C a t which the ion intensity ratio increases suddenly in Fe-Nb and Fe-V alloys corresponds to the solubility limit of C a t 710 "C. So, the increase of ion intensity ratio may be due to the formation of carbide in the matrix. The decrease of N c ~ + / N Fin~ Fe-Cr + alloys may also be due to the same mechanism. The reason that the ion intensity ratio N M ~ + / N in F ~Fe-Mn + alloys and "i+/ 834
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
where Sx+is the sputtered ion yield of X, SF^+ is the sputtered ion yield of Fe, and N X I N F ,is the atomic fraction of X. The present observation confirms that Sx+is a function of the chemical bonding. This means that Sx+ should be rewritten as follows sX+
=
S X + , EP X +
(2)
where SX+,E is the sputtered ion yield characteristic of element X and Px+ describes the effect of the state of the matrix. The increase of the ion yield from clean metals to metal oxides reported by Benninghoven ( 1 4 ) is one indication of the variation of Px+due to chemical bonding. Combining Equations 1 and 2, one obtains (3)
It may thus be concluded that standard alloys used to obtain calibration curves should have the same value of Px+/ PF~+ and , that application of the calibration curves is restricted to those alloys which have the same Px+/PF~+. This restriction imposes a severe limitation upon the appli-
cation of the calibration curve procedure if a phase change occurs. Correction for the Effect of Carbide Formation. T o make quantitative analysis by means of the calibration curve procedure, it is necessary to obtain calibration curves using standard alloys which do not contain any precipitates, and then to correct the effect of the precipitates. The correction may be accomplished by dividing the observed relative ion yield by
(352)
PF~+ prec
I(&) PF=+sol
Calibration Curve o before Correction A
1
A
after Correction
I
/
(4)
The subscript "prec" denotes the matrix containing the precipitates and "sol" denotes the matrix without precipitates. The calibration curves reported in the previous paper ( 2 ) were obtained using a portion of the low alloy series of NBS standard steels. The concentrations of C in these alloys were in the range of 0.07-0.54 wt %, and some precipitates were observed in the matrices. Therefore, we cannot adopt these calibration curves as standards for matrices where the C concentration is below the solubility limit. Calibration curves not affected by secondary phases were obtained using several dilute binary iron alloys made by the Iron and Steel Institute of Japan. The concentration of C in these alloys was less than 0.002 wt % and precipitates were not observed in the matrix. X-ray fluorescence analysis of these alloys has been reported elsewhere ( 1 5 ) .Figure 4 shows the calibration curves of Al, V, Nb, Mo, Cr, Mn, Si, and Ni in these dilute alloys. For the correction of the effect of secondary phases, one must find the value of Px+/PFe+in Equation 3. I t is very difficult, however, to estimate the exact value of Px+, because Px+ is a function of various factors, such as secondary phases, and the size and number of the precipitates. In the present study, it was tentatively assumed that Px+/ P F ~was + a function of carbon content and independent of the concentration of the element X. This assumption may be valid only for the elements which tend to form carbides in low alloy steels, i.e., Nb, Mo, Mn, Ni, etc. In these circumstances, the relative ion intensity NX+/NFe+given in Figure 3 might be regarded as a representative of Px+/ PF~+ . Therefore one has
where (NX+lNFe+)prec is the ordinate in Figure 3 corresponding to the carbon content in the sample and ( N x + / N F ~ +is )the~ ion ~ ~intensity ratio obtained in the carbon concentration range of solid solution. To confirm the present correction method, NBS standards used in the previous paper ( 2 ) were re-analyzed. Figure 5 shows the relative ion intensity Nx+/NFe+before and after the correction vs. the ratio of atomic fraction Nx/NFe. The solid lines correspond to the dilute alloy calibration curves given in Figure 4. It is clearly shown that the ion intensity ratios of "t,+/NFe+, NV+/NFe+,and NCr+/NFe+ fail well on the calibration curves after the correction. For analysis of Mn and Ni, it was not necessary to make the correction because the ion intensity ratios of these elements did not change with carbon content. This correction procedure has, of course, certain limitations. One has to know the concentration of carbon by some other analytical method. Also, it cannot be applied to high alloy steels, in which alloy element X may form precipitates composed of Fe and X rather than carbides. The cor-
/* yI
i
Calibration Curve before Correction after Correction
10' Ratlo of Atoms
Ratio of
Nc, I Nc.
Atoms N x / N F ~
Figure 5. Ion intensity ratio b&/NFe+ before and after the correction vs. the ratio of atomic fraction NxINF, for analysis of ( a ) V and Nb, ( b )Cr, ( c )Mn and Ni in low alloy steels ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
835
rection method is applicable only to low alloy steels and to those elements which tend to form carbides in steels.
ACKNOWLEDGMENT The authors thank R. Shimizu for encouragement and helpful discussion during the course of this work. The authors also express their appreciation to D. B. Wittry for a helpful suggestions and for reading the manuscript. LITERATURE CITED ( 1 ) V. Leroy, J. P. Servaes, and L. Habraken, Metall. Rep. CRM, 35, 69 (1973). (2) K. Tsuruoka. K. Tsunoyama, Y. Ohashi and T. Suzuki, "Proc. 6th Int. Vacuum Cong., Kyoto, March 1974", Jpn J. Appl. Phys., Suppl. 2, Pt. 1, 391 (1974). (3) R. Matsumoto, K. Sato, and K. Suzuki, "Proc. 6th Int. Vacuum Cong., Kyoto, March 1974", Jpn J. Appl. Phys., Suppl. 2, Pt. 1, 387 (1974). (4) C. A. Andersen and J. R. Hinthorne, Anal. Chem., 45, 1421 (1973).
(5) R. Shimizu, T. Ishitani, T. Kondo, and K. Tamura, Anal. Chem., 40, 1020 (1975). (6) F. G. Rudenauer and W. Steiger, "Proc 6th Vacuum Cong., Kyoto, March 1974", Jpn. J. Appl. Phys., Suppl. 2, Pt. 1. 383 (1974). (7) T. Ishitani, H. Tamura, and T. Kondo, Anal. Chem., 40, 1294 (1975). (8) R. Castaing, "Proc. 6th Intern. Conf. X-ray Optics and Microanalysis", G. Shinoda, K. Kohra, and T. Ichinokawa, Ed., University of Tokyo Press, Tokyo, 1972, p 423. (9) G. Carter, Enrico Fermi Summer School Paper, 1973, in print. (10) H. W. Werner, "Proc. 6th Vacuum Cong., Kyoto, March 1974", Jpn. J. Appl. Phys., Suppl. 2, Pt. 1, 367 (1974). (11) K. Tsunoyama, Y. Ohashi, and T. Suzuki, Jpn. J. Appl. Phys., 13, 1039 (1974). (12) C. A. Andersen, "Third Natl. Electron Microprobe Conf., Chicago, Ill., 1968", lnt. J. Mass Spectrom. /on Phys., 2, 61 (1969). (13) K. Tsunoyama, Y. Ohashi, and T. Suzuki, Jpn J. AppI. Phys., 13, 1683 (1974). (14) A. Benninghoven and A. Muller, Phys. Letf. A 40, 169 (1972). (15) K. Kawamura, Trans. lron Steel Ins?.Jpn., in press.
RECEIVEDfor review August 28, 1975. Accepted December 8, 1975.
Optimization of Arsine Generation in Atomic Absorption Arsenic Determinations Darryl D. Siemer," Prabhakaran Koteel, and Vlbhakar Jariwala' Chemistry Department, Marquette University, 535 North 14th Street, Milwaukee, Wis. 53233
Experiments undertaken with simple apparatus to optimize the arsine generation-atomic absorption determination of arsenic are described. The effects of variations in atomizer design and operating conditions, matrix elements, oxidation states of an analyte, and conditions of the arsine generation procedure are investigated using three different signal readout modes. The procedure described has a sensitivity of 3 X g with RSD at signal levels corresponding to those used In analytical work of 2 to 3%. The analytical results achieved on dry ashed samples of NBS No. 1571 Orchard leaves agree well with literature values.
Volatile hydride generation combined with atomic absorption determination of the metal hydride serves as a simple and extremely sensitive analytical method for several metalloids. Varieties of this technique have been in use for several years (1-8). All of the major atomic absorption instrument manufacturers as well as several accessories firms now offer equipment permitting more or less rapid modification of standard instruments for these analyses. It is the purpose of this paper to describe experiments performed with several handmade models of these devices. The purpose of the work was to develop as sensitive and accurate a system for the determination of arsenic in foodstuffs as is consistent with a reasonable degree of ease of use and simplicity. Any hydride atomic absorption analytical method can be thought of as being composed of three steps: first, generation of the hydride; second, transfer of the hydride to the atomizer; and finally, decomposition of the hydride to gas phase metal atoms within the optical axis of the atomic absorption spectrometer. In order to increase the number of free-metal atoms present at a given time within a unit cross section of the atomizer and, hence, the sensitivity of the Present address, Chemistry Department, Milwaukee Area Technical College, Milwaukee, Wis.
836
ANALYTICAL CHEMISTRY, VOL. 48, NO. 6, MAY 1976
method, it is necessary to generate the hydride quickly, "strip" it from the solution with a minimum of dilution with other gasses, and get as large a fraction into the atomizer in one time as is possible. It is also desirable to use sample preparation and analysis techniques which minimize reagent blanks. In order to satisfy these requirements, we investigated several apparatus, all of which combined homogeneous borohydride reduction, use of the fuel gas, hydrogen, for stripping the hydride from the solution, and a long, highvolume, quartz-tube atomizer burning an extremely rich hydrogen-air or oxygen mixture. The argon or nitrogen recommended for solution stripping by makers of several of the commercially available generators seriously dilutes the hydride stream and reduces sensitivity. The tube atomizer contains a higher fraction of the total hydride generated in any given instant than does a typical premixed flame from a slot burner.
EXPERIMENTAL Both an Instrumentation Laboratories (IL) Model 251 and a GCA McPherson M,odel EU 703-D atomic absorption spectrometer were used for this research. Conventional hollow cathode lamps run at currents from 10 to 12 mA were used on both of these instruments and a hydrogen continuum lamp was used t o check for nonatomic absorbance interference. Figure 1 shows an example of the Vycor arsine generators used. I t differs from that described in our earlier paper (9) in that it is somewhat smaller and in that all solutions are added and removed from it with syringes through a rubber septum. Figure 2 depicts an example of the burner atomizer used. It differs from earlier versions in that it features an oxidant inlet which can be freely moved relative to the optical axis of the atomizer. This feature was found to be desirable after it was observed that some of our initial atomizers built with fixed oxidant inlets would often refuse to burn smoothly with either oxygen or air and were very sensitive to oxidant flow variations. These burners have atomizer tubes 1 2 cm long and 11 mm internal diameter. Commercial grades of hydrogen and oxygen were used and their flows measured with Gilmont and Manostat flowmeters. The flow rates read on the flowmeter used for hydrogen were corrected for
