Bull. Nagoya Univ. Furukawa Museum
No.6, 2 l - 2 8, 1990

New Cu-bearing Glass Standards for Electron Microprobe Analysis
Kazuhiro SUZUKI 1), Izumi KAJIZUKA1), Koshi YAMAMOTO 2),
Masayoshi OHASHI3), Kenichiro SUGITANI1) and Setsuo YOGO1)
1) Department of Earth Sciences, Nagoya University, Chikusa,Nagoya 464-0l, Japan
2) Department of Earth Sciences, Kobe University, Nada, Kobe 657,
Japan3) Government Industrial Research Institute, Nagoya, Kita, Nagoya462, Japan

Abstract
Two Cu-bearing glass standards, NGCU-1 and NGCU-2, for electron microprobe analysis have been synthesized. The NGCU-1 glass contains 6.28-wt. % CuO in a base of Mg-CaAl silicate, and the NGCU-2 glass contains 1.74% CuO, 2.23% Ni2O3, 2.14% Co3O4, 1.06% SrO
and 2.70% BaO. The glasses are homogeneous and do not show a significant compositionalchange described in terms of "Lineweaver mechanismﾓ during electron beam bombardment.
The glasses are useful as standards for electron microprobe analysis.

Introduction
Electron microprobe analyses of manganese micronodules, usually less than 1 mm insize, have been recently carried out by many researchers (e.g. Ohashi, 1985; Sugisaki etal., 1987; Sugitani, 1987 and Sawada, 1988).

The manganese micronodules normallycontain NiO, CoO, CuO and BaO as much as 2-4wt. %. The reliability in analyses of suchminor elements by microprobe is dependent largely upon standard materials. The choiceof pure metal, oxide and sulfide as a standard causes an extremely high count rate during themeasurement of the standard intensity. This brings about some difficulties in the correction of counting-loss, because the dead time of the counting system may be extended or nonextended as pointed out by Reed (1975). Synthetic standards containing Cu and others insmall amounts are urgent for the microprobe analysis of manganese micronodules.

Thispaper describes two newly synthesized glasses for microprobe standards of Cu, Co, Ni, Srand Ba.
Received, 30th Oct., 1990.
Accepted for publication, 6th Nov., 1990.

- 21 -Synthesis
Two Cu-bearing glasses, NGCU-1 and NGCU-2, were synthesized. The source materials
are reagent powder of SiO2, Al2O3, MgO, CuO, Ni2O3, Co3O4, CaCO3, BaCO3 and SrCO3. Thesematerials are treated along the following lines and kept in a desiccator prior to mixing;1) SiO2, Al2O3 and MgO are dried in air at 950 C for 6 hours.
2) CuO, Ni2O3 and Co3O4 are dried in air at 200 C for 12 hours.
3) CaCO3, BaCO3 and SrCO3 are transformed into oxides through heating in air at 950-1000 C for 6 hours and ground with an agate mortar.
Thestarting mixture for the NGCU-1 glass is prepared by weighting SiO2, Al2O3, MgO,CaO and CuO on precision balance and mixing in an agate mortar, and that for the NGCU2 glass by mixing of SiO2, Al2O3, MgO, CaO, CuO, Ni2O3, Co3O4, SrO and BaO. Each ofthe starting mixture is placed in a Pt-crucible, and dried with a vertical quench electric furnace at 900 C for 2 hours to allow a complete degassing. The temperature is brought to 1500 Cand kept at this maximum temperature for an hour. The furnace is turned off, and the crucible is cooled to about 1350 C before removing. Quenching is done by dipping the crucible
into distilled water. We prepared the final glasses through multiple (three times) grinding andmelting at 1500 C in air.

Wet chemical analysis
The composition of the NGCU-1 glass was determined with a wet chemical method which
includes gravimetry for SiO2 and R2O3, and atomic absorption spectrometry for Al, Fe, Mg,Cu, Ca, Na and K. The analytical result is listed in column B of Table 1. Alkalis are notdetected but a substantial amount of Fe (the high oxygen pressure in the furnace is thought
to have resulted in ferric iron: 0. 17wt. % as Fe2O3) is detected from the NGCU-1 glass. Sinceno iron was added to the starting mixture (see column A of Table 1), Fe2O3 is interpreted tocome from impurities in thesource materials.

The difference in weight percent

CuO

between the starting mixture (column A) and the final glass (column B) supports thisinterpretation.

Anotherinterpretationisthat iron was introduced through contaminationfrom the crucible and/or the furnace. The microprobe check shows that the Pt-crucible
used for the melting had, indeed, been alloyed with iron.
The final glass is a little higher in the SiO2 and A12O3 contents, and lower in the MgO, CuO
and CaO contents than the Starting mixture. The discrepancies, at least in part, may beascribed to imperfect decarbonation of CaCO3, presence of impurity in CuO, and/or hydrationof MgO and CaO during grinding and mixing. We take the weight percent components
determined through the wet chemical method as the true composition of the NGCU-1 glass.

- 22 -

Table 1. Compositions (weight per cent) of synthesized glasses(NGCU-1 and NGCU-2) for microprobe standards
NGCU-1

NGCU-2SiO2
Al2O3
Fe2O3MgO
Ni2O3
Co3O4CuOCaOSrO
BaO

A
52.59
13.17
8.83
6.47
18.94-

B
52.69
13.22
0.17
8.77
6.31
18.73
-

C
52.7
13.2
0.15
8.70
6.28
18.8
-

Total

100.0

99.72

99.83

(0.2 )(0.1 )(0.02 )
(0.1 )(0.07 )
(0.1 )

D
46.8
22.5
0.82
4.41
2.23
2.14
1.74
15.7
1.06
2.70

(0.2 )(0.1 )(0.04 )
(0.03 )
(0.03 )
(0.02 )
(0.02 )
(0.1 )(0.03 )
(0.03 )

100.10A : expected composition of NGCU-1
B : wet chemical analysis of NGCU-1
C : average and standard deviation of 170 microprobe
analyses on 17 chips of NGCU-1
D : average and standard deviation of 150 microprobe
analyses on 30 chips of NGCU-2

Stability of glasses under electron bombardment
Silicate glasses are known to show changes in X-ray intensities with time of electron
beam bombardment during microprobe analyses (e.g. Varshneya et a1., 1966; Borom andHanneman, 1967). A question arises: Are the present glasses sufficiently stable underelectron beam bombardment to permit quantitative microprobe analyses? To know how
the X-ray intensities vary with time, we monitor X-ray intensities for a long duration byirradiating a probed beam on a given spot of the glasses. Experimental conditions are
3mm diameter beam of 0.1µA current, 3µm diameter beam of 0.05µA current and 20µm
diameter of 0.1µA current. Accelerating voltage is 15kV. X-ray intensities are recorded in every
40 second intervals during the irradiation, and normalized to zero time as;

It / It=0 =Average count rate between (t - 40) and t
Count rate at t = 0

where t is time in second. The count rate at t=0 is indeterminable; we measure the countrate through moving the sample at a constant rate, and take this as the count rate at t=0.
The measurement is repeated more than ten times, and the arithmetic average of the- 23 --It / It=0 values is taken.

A typical set of results is shown in Fig. 1, and the time-dependentchanges of CuKα intensities under different current densities are compared in Fig. 2.

Fig. 1. Time-dependent changes of X-ray intensities from the NGCU-1 glass subjected toelectron beam bombardment. The average count rate between (t-40) and t is taken
as that at t, and normalized to zero time (see text).

Fig. 2. Time-dependent changes of CuKα intensities from the NGCU-1 glass under different current densities of the electron beam. The X-ray intensities are normalized withthose at zero time. The observed decrease in the CuKα intensity is fitted with anequation of the formula It / It=0 = A + B exp (-t / K).

- 24 -

Figure l shows that the normalized intensity for CuKα decreases exponentially, increases
for SiKα and AlKα, and is kept nearly constant for MgKα and CaKα. The intensitychanges appear to be consistent with the Lineweaver mechanism (Lineweaver, 1963) that
explains the effect of high electron flux on the evolution of oxygen from silicate glasses.
Lineweaver suggested that incident electrons from the microbeam stop at some finite depth
in glasses and form a zone of net negative charge underneath the surface. The field,
established between the charged zone and the carbon conductive film, causes positively
charged network-modifying cautions (e.g. Cu) to migrate toward the charged region. This
process is accompanied by migration and evolution of nonbridging oxygens. The blistering
effect of the carbon conductive film on the NGCU-l glass under the probed beam supports
this interpretation. Migration of an ion from probed region to the charged zone causes a
decrease in the fraction of that ion and an increase in the fraction of all other ions. Copper
should be the most mobile cation in the NGCU-l glass; migration of Cu ions away fromthe probed region toward the charged region is probably a major cause of the observed
decrease in the CuKα intensity. Network-forming silicon and aluminum should be theleast mobile cation; their fractional concentrations in the probed region increase as theresult of Cu migration and oxygen evolution. The compensation of the Mg- and Cu-migration effect with that of other ions should result in the approximate constancy of the MgKαand CaKα counts.
The NGCU-l glass yields more than 0.97 of the zero time CuKa intensity even after 600
second irradiation of a high density electron beam. The ca. 3% difference in X-ray counts
between at t=0 and t=600 may be indistinguishable from the uncertainty owing tocounting statistics on routine measurements. The NGCU-1 glass is practically acceptable
for the microprobe standard. The time-dependent change of CuKα X-ray intensities ofNGCU-2 glass is nearly the same as that of the NGCU-l glass. Figure 2 suggests that theinevitable intensity changes are minimized by keeping the electron density as low as
possible while retaining sufficient X-ray intensity for the analysis, and by defocusing
the electron beam to about a diameter of 20µm.

Homogeneity
The final glasses do not show any crystalline phase under the microscope. The homogeneity was examined by means of microprobe analyses. The operating conditions ofelectron microprobe are 15kV accelerating voltage, 3mm beam diameter and 0.0150.03µA beam current. The X-ray intensity is integrated for 20 to 100 seconds. The
measurement is repeated three times, and the arithmetic average is taken. The background is estimated from the offset readings on both side of each line-peak position.
Analytical uncertainties are statistically estimated to be about 0.2wt. % for SiO2, 0.1wt. %for Al2O3 and CaO, and 0.06-0. 03 wt.% for other components.

- 25 -Fig. 3.

Frcquency distribution diagrams of the CuO, SiO2, A1203, MgO and CaO
contents of 170 spot analyses on 17 different chips of the NCTCU-1 glass.

A total of 170 spot analyses were made on 17 chips of the NGCU-1 glass. The results are presented as frequency distribution diagrams (Fig. 3),

and the average and standard devia-

tion are listed in column C of Table 1. The frequency distribution of individual components shows a high kurtosis. The CuO contents range from 6.12 to 6.49 wt.%; the average is
6.27 wt.%. The analytical result coincides well with the wet chemical analysis (6.3l wt.%,column B in Table 1), and the standard deviation for CuO, 0.07, is approximately
equal to the statistically estimated uncertainty (0.06 wt.%). The average compositions of
SiO2, Al2O3, Fe2O3, MgO and CaO also coincide with those determined withthewetchemical analysis, and the standard deviations for these components are nearly thesame as the statistically estimated uncertainties. The NGCU-1 glass can be regarded as homogeneous one.
A total of 150 spot analyses were made on 30 chips of the NGCU-2 glass. The results are presented as frequency distribution diagrams (Fig. 4), and the average and standard deviation are listed in column D of Table 1.

The oxidation states of Fe, Ni and Co are notdetermined, and hence they are conveniently expressed as Fe2O3, Ni2O3 and Co3O4, respectively. As shown in Figure 4, the 150 measurements on different chips do not much diverge.
suggests that the NGCU-2 glass is also homogeneous.

- 26 -This

Fig. 4. Frequency distribution diagrams of the SiO2, Al2O3, CaO, CuO, Co3O4, Ni2O3,SrO and BaO contents of 150 spot analyses on 30 different chips of the NGCU-2glass.

Conclusion
ThesynthesizedCu-bearing

Mg-Ca-Al silicate glasses, NGCU-1 and NGCU-2, arehomogeneous. They do not show a significant compositional change described in terms of"Lineweaver mechanism" during electron beam bombardment and are acceptable as themicroprobe

standards.

These silicate-based

Cu-standards

makeiteasyto apply thequantitative data-reduction procedures employed by most microprobe laboratories. TheNGCU-1 and NGCU-2 glasses are available upon request.
Acknowledgements
we thank Professors K. Suwa and R. Sugisaki of Nagoya
University for their usefull advice.

References
Borom, M. P. and Hanneman, R. E. (1967): Local compositional changes in alkali silicate glasses
during electron microprobe analysis. J. Appl. Phys. , 38, 2406 - 2407.
Lineweaver, J. L. (1963): Oxygen outgassing caused by electron bombardment of glass. J. Appl.
Phys., 34, 1786 - 1791.
Ohashi, M. (1985): Depositional environments and chemical compositions of manganese micronodules.
Jour. Geol. Soc. Japan, 91, 787 - 803. (In Japanese with English abstract).
Reed, S. J. B. (1975): Electron Microprobe Analysis. Cambridge University Press, London. 400 p.
Sawada, S. (1988): Internal texture and chemical composition of manganese micronodules in deepsea sediments from the Central Pacific Basin. Mining Geol., 38, 27 - 37.

- 27 -Sugisaki, R., Ohashi, M., Sugitani, K. and Suzuki, K. (1987): Compositional variations inmanganese micronodules: A possible indicator of sedimentary environments. Jour. Geol., 95,433 - 454.
Sugitani, K. (1987): A geochemical study of hydrothermal manganese micronodules from marine
sediments and sedimentary rocks on land. Jour. Geol. Soc. Japan, 93, 555 - 574. (In Japanesewith English abstract).
Varshneya, A. K., Cooper,. A. R. and Cable, M. (1966): Changes in composition during electron
microprobe analysis of K2O-SrO-SiO2 glass. J. Appl. Phys., 37, 2199.

- 28 -