Elemental analysis of powders by glow discharge mass spectrometry

Analytica Chimica Acta 395 (1999) 165±171

Elemental analysis of powders by glow discharge mass spectrometry M. Inouea, T. Sakab,* a

b

Research and Development Lab., Daido Steel Co. Ltd., 2-30 Daido-cho, Minami-ku, Nagoya, 457-8545, Japan Department of Applied Electronics, Daido Institute of Technology, 2-21 Daido-cho, Minami-ku, Nagoya, 457-8531, Japan Received 5 January 1999; received in revised form 8 March 1999; accepted 11 March 1999

Abstract Glow discharge mass spectrometry (GDMS) which has been developed for bulk samples was applied to powder samples. Powders pressed on an indium sheet are used as samples. Ion beam intensities of elements in powders were measured by the conventional procedures for disk-shaped samples. The discharge condition which is common for bulk samples was employed. The surface contamination was removed by preliminary sputtering for 60 min. Grain sizes of powders and densities of powders pressed on an indium sheet do not affect ratios of ion beam intensities of analyzed elements to that of the matrix element. The relative sensitivity factors of 21 and 14 elements (isotopes) have been determined for Fe-based powders and Cr powders, respectively. The obtained relative sensitivity factors for powders correspond with those for disk-shaped samples and therefore, relative sensitivity factors for bulk samples are applicable to powders. The relative error of analytical results obtained by GDMS to the chemically analyzed results was about 25% in an average for trace elements of the concentration less than 10 mg kgÿ1 and smaller for elements of the higher concentrations. # 1999 Elsevier Science B.V. All rights reserved. Keywords: Elemental analysis; Glow discharge; Mass spectrometry; Relative sensitivity factor; Powder

1. Introduction For quantitative trace elemental analyses, glow discharge mass spectrometry (GDMS) has been applied extensively to conducting solid materials. This method provides a capability of simultaneous analyses for a wide range of elements from major concentrations to trace levels of mg kgÿ1. In analyses by GDMS, a glow discharge is struck between a body of a cell and a sample. Samples are sputtered by ions of the plasma and sputtered neutral atoms are ionized by both Pen*Corresponding author. Fax: +81-52-6125653; e-mail: [email protected]

ning ionization and electron impact mechanisms [1,2]. The ionized species are introduced into a mass spectrometer and ion beam intensities corresponding to both an analyzed element X and a matrix element M are measured and the ratio of these ion beam intensities is obtained. The concentration of the element X is determined by multiplying the observed ion beam ratio by a relative sensitivity factor (RSF) of this element in the relevant matrix. In GDMS analyses, quantitative accuracy is seriously affected by the accuracy of RSFs and much work has been carried out to obtain accurate RSFs of many elements in many matrices [3±5]. At the same time, investigations to clarify effects of many factors, such as sample shapes

0003-2670/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 3 3 4 - 7

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or matrices, on RSFs have been carried out [5±10]. However, most efforts have been focussed on bulk materials and very few investigations on powder samples have been reported. In this paper, GDMS is applied to powder samples. A method of sample preparation of powders is proposed. It is also con®rmed that most contaminants on powder surfaces can be removed by the conventional preliminary sputtering. The RSFs of many elements for powders have been determined and the in¯uence of sample sizes on RSFs is investigated. Furthermore, the determined RSFs for powders are compared with those for bulk samples. 2. Experimental 2.1. Mass spectrometer The used glow discharge mass spectrometer was VG9000 available commercially from F.I. Elemental Analysis. The used cell of ion sources was a Mega Flat Cell designed for disk-shaped samples [11]. The cell was cooled down to liquid nitrogen during the measurements. The operation was carried out in a constant current mode and the discharge voltage was adjusted by changing the pressure of injected argon gas of high purity (6N grade). The accelerating voltage to extract ions in the glow discharge into the mass spectrometer was 7.8 kV. The intensity of the ion beams was measured by a Daly photomultiplier for small currents (10ÿ19± 10ÿ13 A) and by a Faraday cup detector for large currents (10ÿ13±10ÿ9 A). The ef®ciency of both detectors was calibrated using a weak beam of 50 Ti from pure Ti metal. Scan points per peak were 60 channels, and at each point a measurement with an integration time of 200 and 160 ms was made for the Daly photomultiplier and for the Faraday cup detector, respectively. The width of a de®ning slit was 25 mm and the mass resolution at 5% valley de®nition was more than 4000. Measurements were carried out successively ®ve times for each peak and the averaged values were employed. 2.2. Sample preparation Powder samples used in the present research are tabulated in Table 1. Among these, two pure Cr

samples and three pure Fe samples, Fe(1)±Fe(3), were commercially available. They should be free from surface contamination and any contamination observed would be introduced in the process of the sample preparation and the measurements. On the other hand, four samples of Fe-based alloys, Fe(4)± Fe(7), were cast at Daido as high speed steel powders and their surfaces will be contaminated in the process of the production. The sizes of the commercial samples are noted in Table 1. For all the powders, the concentrations of components were determined by chemical analyses and the results are shown in Table 1. Powder samples were prepared for the present GDMS measurements by the following methods. The powders were 1. formed into a pin shape without any binder, 2. formed into a pin shape with a carbon binder, 3. pressed on a pure indium sheet. In the ®rst method, reproducibility of ion beam ratios was not attained. In the second method, the amount of impurities included in the binder were so high and this method is not applicable to trace analysis. Therefore the third method was employed. An indium block was rolled to a thin sheet. The indium used is 6N pure. First, an indium sheet was exposed to the plasma to con®rm the position of the exposure by the plasma and to remove surface contamination of the sheet which may be induced during the thinning process. Then, powders were put on the exposed area and they were pressed with a pressure of 50 kg cmÿ2 for about 5 s and the grains which were not pressed tightly in the indium sheet were blown away by pure dry nitrogen gas. Finally, the indium sheet with the powders was inserted into the cell. 2.3. Analysis of impurities in indium sheet Impurities in the used indium sheet were analyzed prior to elemental analyses of powders by mounting an indium sheet without any powders on the Mega Flat Cell. The discharge condition was the same as that previously employed for disk-shaped samples by the present authors: a current of 2.5 mA and a voltage of 0.8 kV using a tantalum mask of 12 mm diameter [5]. As the impurities 12 C and 208 Pb were detected, their concentrations being 90 and 3 mg kgÿ1, respectively.

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167

Table 1 Compositions of the used powder samplesa Element

Cr(1)

Cr(2)

Fe(1)

Fe(2)

Fe(3)

B C Mg Al Si P S Ca Ti V Cr Mn Fe Co Ni Cu Zn Ga As Nb Mo Sn Ta W Pb

0.0010 0.0084 0.0014 0.0181

0.0005 0.0163 0.0006 0.0009

0.0002 0.0123

0.0034 0.0043

0.0008 0.0045 0.0098 0.0112 0.0002

0.0003

0.0002 0.0049

0.0002 0.0011

0.0078 0.0075 0.0028 0.026

0.0019 0.0014 0.0005

a

0.0009 0.0002 0.0004

0.0029 0.0441 0.0021 0.0014 0.0009

0.0002

0.0078 0.205

0.0038 0.0005

0.0021 0.0006 0.0020 0.0002

0.0018

0.0003 0.0091 0.0012

Fe(4)

Fe(5)

Fe(6)

Fe(7)

0.0036

1.21

1.35

1.71

2.14

0.005 0.002

0.0021 0.22 0.020 0.014

0.0017 0.30 0.019 0.012

0.0023 0.29 0.014 0.0114

0.0024 0.24 0.015 0.0084

0.0045 0.0001 0.1800 0.0006 0.0031 0.0028 0.0062 0.0002 0.0012

0.002 0.0005

0.0018

3.18 4.29 0.29 79.2 0.22 0.12 0.051 0.014 0.005 0.0031 4.84 0.004 6.39

2.94 3.85 0.31 71.7 8.19 0.14 0.084 0.0088 0.004 0.0063 4.62 0.004 0.0036 6.39

4.73 3.98 0.27 65.8 7.22 0.11 0.046

5.09 4.01 0.28 58.1 11.28 0.16 0.067

0.012 0.004 0.0099 1.77 0.004 0.0089 14.11

0.0098 0.004 0.0031 5.55 0.010 0.0026 13.30

The size of Fe(1) is 100 mesh and those of Cr(1), Cr(2), Fe(2) and Fe(3) are 200 mesh.

No other elements could be detected. As the purity of the used indium block was 6N, the sheet would be contaminated by carbon in the process of thinning. 2.4. Influence of powder density on indium sheet on the ion beam ratio In the present method of sample preparation, the density of powder samples on a sheet is not controllable and the density may change in each measurement. Therefore, in order to clarify the in¯uence of sample density on the ion beam ratio, ®ve samples with different powder densities were intentionally prepared using the Cr(1) sample by changing the densities of the powders. Cr(1) was selected because the size was most uniform among the present powders and thus the effects of the size can be excluded. The ion beam intensities of all the impurity elements in Cr(1) as well as 52 Cr of the matrix and 115 In of the sheet were measured. The ratios of the ion beam intensity of 115 In to that of 52 Cr were 197%, 50%,

25%, 7% and 5% for the respective samples. The ion beam ratios of the impurity elements to Cr matrix were determined for each of the ®ve samples individually. The coef®cients of variation of the ion beam ratios among the ®ve samples were less than 10% for the impurities whose concentrations were higher than 10 mg kgÿ1 and this result is comparable to the usual internal reproducibility for bulk samples. Therefore, it is concluded that ion beam ratios are not in¯uenced by sample density. 2.5. Measurements 2.5.1. Determination of discharge condition The ion beam ratios of all the elements in Fe(1) to the matrix were measured ®ve times successively using one sample under the conditions of the combination of the voltage of 0.8, 0.9, 1.0 and 1.1 kV with currents of 1.5, 2.0 and 2.5 mA. A tantalum mask of 12 mm diameter was used. The coef®cients of variation under the above conditions were almost equal to

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M. Inoue, T. Saka / Analytica Chimica Acta 395 (1999) 165±171

Fig. 1. Time dependence of ion beam ratios of the impurities in pure Fe(1).

each other and less than 10% except for the combination 0.9 kV and 1.5 mA. The condition of 0.8 kV and 2.5 mA has been employed for further studies as this condition was previously used for measurements of bulk materials with disk shapes [5]. The employment of the same experimental condition is inevitable when the present results for powder samples are to be compared with the previous ones for bulk samples. 2.5.2. Preliminary sputtering The ion beam ratios of the impurities in Fe(1) to the matrix were measured every 10 min during 2 h. The changes of the ion beam ratios are shown in Fig. 1. For some elements, the ion beam ratios are high at ®rst and gradually fall down. This is caused by surface contamination. For the elements except carbon and nitrogen, the ion beam ratios reach stable values after 60 min. Therefore, the preliminary sputtering was conducted for 60 min in the present measurements. The carbon may come from the contamination of the indium sheet and nitrogen may be due to the atmospheric contamination of the cell. 2.6. Evaluation of relative sensitivity factors Relative sensitivity factors (RSFs) are related to the ion beam ratios by the following equation [12]: RSFX ˆ …CX =CM †=…IX =IM †;

(1)

where the suf®ces X and M indicate that the quantities are relevant to an analyzed element and a matrix

element, respectively, and I and C are ion beam intensities and concentrations by weight, respectively. For the Fe-based powders, gradients of calibration lines composed from IX/IM and CX/CM passing through the origin have been employed as the RSFs. Both pure Fe-powder samples (1), (2) and (3) and the Fe-based alloy powders cast at Daido have been used. Furthermore, in order to investigate the in¯uence of powder size on RSFs, the Fe-based alloys were separated into two groups of different sizes of 200 and 100 mesh, the average sizes of each group being 59 and 110 mm, respectively. For the Cr powders, only one sample was available for some elements and the RSFs of these elements were determined by Eq. (1). For other elements where two samples were available, RSFs have been determined from the gradients of the calibration lines. 3. Results and discussion 3.1. Influence of powder size on RSFs Using the two groups of different sizes of the Febased alloys independently, the RSFs of 19 elements (isotopes) are determined. The results are shown in Table 2. For 181 Ta the relative difference is 6.0% but for all the other elements it is less than 5% and the average value of the relative difference is 1.7%. Furthermore, no systematic differences between the two groups were recognized. Therefore, the RSFs are not in¯uenced by the size of the powders. The above observation reveals the fact that surface contamination is removed almost perfectly by preliminary sputtering because the in¯uence of surface contaminants should be predominant for powders of a smaller size causing systematic differences of the RSFs. 3.2. Relative sensitivity factors The RSFs for the Cr powders and the Fe-based powders are determined. In the case of the Fe-based powders, the RSF of carbon was determined by using only the alloys cast at Daido where so much carbon is included whereas the inclusion of carbon in the indium sheet is negligible. For 27 Al, 44 Ca, 70 Ge, 107 Ag and 109 Ag, the calibration lines did not pass through the origin. For 27 Al, 107 Ag and 109 Ag, almost the same

M. Inoue, T. Saka / Analytica Chimica Acta 395 (1999) 165±171 Table 2 Comparison of relative sensitivity factors determined using Febased powders of different sizes Element 12

C 28 Si 31 P 32 S 51 V 52 Cr 55 Mn 56 Fe 59 Co 60 Ni 63 Cu 65 Cu 75 As 93 Nb 98 Mo 100 Mo 117 Sn 119 Sn 181 Ta 184 W

Relative sensitivity factor 59 mm

110 mm

Relative Error (%)

4.023 1.784 2.246 2.615 0.619 1.889 1.278 1 0.889 1.406 3.579 3.528 3.286 0.645 1.147 1.201 1.709 1.699 1.131 1.445

3.920 1.800 2.217 2.619 0.613 1.855 1.276 1 0.891 1.388 3.653 3.597 3.280 0.639 1.187 1.222 1.750 1.741 1.067 1.430

2.628 ÿ0.889 1.308 ÿ0.153 0.979 1.833 0.157 ÿ0.224 1.297 ÿ2.026 ÿ1.918 0.183 0.939 ÿ3.370 ÿ1.718 ÿ2.343 ÿ2.412 5.998 1.049

calibration lines were obtained for each group of the different sizes of the Fe-based alloys. Therefore the scatter of observed points may be due to the uncertainty of the concentrations determined by the chemical analyses. The uncertainty of the chemical analyses is a probable explanation in the case of 44 Ca and 70 Ge because the concentrations are very small for these samples. For 69 Ga, a value of 17.0 was obtained. The reason for such a large RSF has not been clari®ed. This large value is probably not correct and 69 Ga is excluded. In the case of the Cr powders, 12 C and 208 Pb are excluded as these elements are included in the indium sheet. The ®nal results are shown in Table 3.

169

powders. In the previous analysis on the bulk samples of different matrices, the RSFs of these three elements in the Fe-matrix were larger than the RSFs averaged over the matrices. Therefore, there is a possibility that the RSFs of these three elements in the bulk Fe-matrix may have relatively large errors. Another point that should be noted is that segregation is often observed for P and As [10]. The large discrepancy may be caused by some segregation in the case of P and As. Nevertheless, the average of the magnitudes of the relative differences for all the elements is 10.0% which is a fairly good agreement. For the Cr matrix, we have not determined RSFs for bulk samples and no reports on the determination of RSFs for Cr disk-samples have been found. However, in the previous work, it has been revealed that the RSFs for minor components are independent of the matrix [5]. Therefore, it would be pro®table to compare the present RSFs for the Cr powders to the RSFs which are the averages for different matrices. A presentation of the RSFs of the Cr powders and the RSFs for the bulk sample is shown in Fig. 2. There exists a linear correlation except for P. The reason why the RSF of P deviates from other elements is not clear. As pointed out in the case of the Fe-matrix, segregation of P is one possibility. As the deviation is observed only for P, P has been excluded in the present analysis. We assume that this correlation can be expressed by a simple proportional relation. The proportionality factor was determined by the method of least squares, and all the RSFs for the Cr powders have been multiplied by this proportionality factor. The results are shown in

3.3. Comparison of RSFs for powders and bulk samples For the Fe matrix, the RSFs for bulk samples have been determined by the present authors under the same experimental condition [5]. The results are also shown in Table 3. Only for three elements (31 P, 75 As and 93 Nb) the RSFs differ more than 20% and the RSFs for the bulk samples are always larger than those for the

Fig. 2. Comparison of relative sensitivity factors for Cr powders and for bulk samples.

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Table 3 Relative sensitivity factors for powders RSFs for Fe-based samples

11

B C 24 Mg 27 Al 28 Si 31 P 32 S 44 Ca 48 Ti 51 V 52 Cr 55 Mn 56 Fe 59 Co 60 Ni 63 Cu 65 Cu 66 Zn 69 Ga 75 As 93 Nb 98 Mo 100 Mo 117 Sn 119 Sn 181 Ta 184 W 12

RSFs for Cr-based samples

Powder

Bulk

Relative error (%)

Powder

Bulka

Powderb (Fe-based)

1.233 3.967

1.276 3.811

ÿ3.37 4.09

0.873

1.633

2.094

28.2

1.791 2.227 2.659

1.880 3.019 2.773

ÿ4.73 ÿ26.2 ÿ4.11

0.489 0.526

1.173 1.262

ÿ11.9 27.0

0.616 1.872 1.278 1 0.890 1.396 3.618 3.562 3.691

0.567 2.052 1.392 1 0.834 1.397 3.988 3.890 3.540

8.64 ÿ8.77 ÿ8.19

1.332 0.994 1.648 2.812 2.820 0.461 0.523 0.644 1.950 1.479 1.004 0.856 1.281 3.911

3.144 0.686 0.518 0.403 2.398 1.461 1.156

11.5 48.8 ÿ1.0 ÿ37.4 23.0 ÿ1.2 15.1

1.473 3.067

15.0 ÿ21.6

3.830 0.642 1.116 1.212 1.729 1.720 1.097 1.437

4.861 0.806

ÿ21.2 ÿ20.3

2.789

ÿ10.1

1.102 2.071 2.044 1.239 1.334

9.98 ÿ16.5 ÿ15.9 ÿ11.5 7.72

6.71 ÿ0.07 ÿ9.28 ÿ8.43 4.27

2.480 1.311 0.286 0.216 0.168 1 0.609 0.482 0.614 1.279 1.163

4.075 3.102 3.818 0.733

Relative error (%)

0.355

1.095

0.851

ÿ22.3

0.610

2.138 1.310 1.482

1.463

ÿ1.3

a

Relative sensitivity factors for bulk samples determined using five different matrices of Fe, Ni, Cu, Al and Ti, where the reference is Fe [5]. Relative sensitivity factors for Cr powders after being multiplied by the proportionality factors which relate the relative sensitivity factors for bulk samples and for Cr powders.

b

Table 3 where the relative difference between the averaged RSFs for the bulk samples are also shown. The differences are large in comparison with the case of the Fe powders. However, except Ca and V, these are within 30% and the average of the magnitudes of the relative differences is 18.4%. No systematic deviation is obtained. If the fact that the impurities in the Cr powders are trace amounts is taken into consideration, it is concluded that the RSFs for Cr powders agree with the RSFs for the bulk samples within the conventional reproducibility. Therefore, it can be concluded that in cases of powder analyses, RSFs determined using bulk materials are applicable. The RSFs for powders of Cr and Fe matrices have been compared directly in the same manner. In this comparison, no dependency of RSFs on the matrix

was observed. The averaged magnitude of the relative difference is as large as 22.9% due to the smaller number of the elements. 3.4. Accuracy of analyzed values For the Fe powders where the range of concentrations was wide, the analyzed values were compared with the chemical ones. As an example, the case of the Fe-based alloy (4) is shown in Fig. 3. The agreement between the analyzed values by the present GDMS and those by the chemical method is clearly demonstrated. Furthermore, the accuracy was determined for different orders of concentrations. The average values of the relative errors are shown in Table 4 where the average concentrations and the number of the ana-

M. Inoue, T. Saka / Analytica Chimica Acta 395 (1999) 165±171

171

Fig. 3. Comparison of the present analyzed values with those by chemical analyses for the elements in the sample of Fe(7).

Table 4 Averaged magnitudes of relative errors of analyzed values for different orders of concentrations Order of concentration (w/w)

Averaged concentration (w/w)

Number of elements

Relative error (%)

1±10 ppm 10±100 ppm 100 ppm±0.1% 0.1±1% 1±10% 10±100%

3.1 ppm 43.5 ppm 349 ppm 0.23% 4.14% 44.8%

7 42 37 26 40 14

24.2 20.1 6.2 2.6 3 1

lyzed elements are also shown for the respective orders of concentrations. The relative errors are less than 30% at mg kgÿ1 level making the present method widely applicable to powder samples. Acknowledgements We are grateful to Ms T. Takahashi of Marubun (now, Nippon Jarrell-Ash) for advice on the present sample preparation.

References [1] K.R. Hess, W.W. Harrison, Anal. Chem. 60 (1988) 691. [2] R.L. Smith, D. Serxner, K.R. Hess, Anal. Chem. 61 (1989) 1103. [3] W. Vieth, J.C. Huneke, Spectrochim. Acta 46B (1991) 137. [4] A.P. Mykytiuk, P. Semeniuk, S. Berman, Spectrochim. Acta Rev. 13 (1990) 1. [5] T. Saka, M. Inoue, N. Okamoto, Proceedings of the Fifth International Conference on Progress in Analytical Chemistry in the Steel and Metals Industries, Luxembourg, 1998, in press. [6] P.M. Charalambous, Steel Res. 58 (1987) 197. [7] K. Robinson, E.F.H. Hall, J. Metals 39 (1987) 14. [8] S. Itoh, F. Hirose, R. Hasegawa, Spectrochim. Acta 47 (1992) 1241. [9] M. Saito, Anal. Chim. Acta 274 (1993) 327. [10] M. Saito, Spectrochim. Acta 50B (1995) 171. [11] D.M.P. Milton, J. Clark, D. Potter, R.C. Hutton, Anal. Sci. 7 Suppl. (1991) 1243. [12] N.E. Sanderson, E. Hall, J. Clark, P. Charalambous, D. Hall, Mikrochim. Acta 1 (1987) 275.