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Molecular ion distributions in laser microprobe mass spectrometry of calcium oxide and calcium salts
Spcrrochimica
Am.
Vol.
Printedm Grcrl Brilain.
388.
No.
S/6. pp. 853-858.
0584~8547183
1983.
s3.00+
.oo
Q 1983.Perymon Preu Lid.
Molecular ion distributions in laser microprobe mass spectrometry of calcium oxide and calcium salts FRANKJ. BRUYNSEELS and RENAE.
VAN GRIEKEN
Department of Chemistry, University of Antwerp (UIA), B-2610 Wilrijk, Belgium (Received 9 Nouember 1982) Abstract-Laser Microprobe Mass Spectrometry (LAMMA) is used to examine micrometric particles of calcium oxyanion salts (CaCO,, CaSO,, CaS0,.2HIO) and calcium oxide, in both the positive and negative ion mode. The major molecular ions, appearing in the positive mass spectrum, can be divided into three series, namely Ca_O:_ , , (CaO): and (CaO), H + (m= 1 - 4). In the caseof the former two seriesthe relative intensities of the mass peaks as a function of the fragment valence K = (1 + 2n)/m, for Ca,Oi , can be fitted to a Gaussian distribution curve, as was earlier demonstrated for secondary ion mass spectrometry. The high stability of the (CaO)_H+ series can be explained by the favourable fragment valence of + 2 corresponding to the usual oxidation state of calcium.
IN LASERmicroprobe mass analysis (LAMMA) with the LAMMA@ -500 instrument, a pulsed focused laser beam is used for evaporating a microscopic spot from a sample or single particles with dimensions in the micrometric range [ 11. The analysis is performed by a timeof-flight mass spectrometer. Besides the application of LAMMA for quantitative analysis [2], it has also been used for the structural analysis of organic and inorganic materials [3]. Until now most attention has been paid to the characterization oforganic substances. Yet, the use of LAMMA for the identification of inorganic particles is also promising. Indeed the positive and negative LAMMA spectra from inorganic salts are indicative for the compound stoichiometry, but a careful examination of the type and the relative intensity of the molecular ions produced is necessary [4,5]. Qualitatively, LAMMA spectra are very similar to static SIMS spectra [6,7], but the superior lateral resolution of 0.5-1.0 pm and the high speed of LAMMA makes it potentially more interesting for single particle analysis. In laser probe excitation the irradiation energy can be varied by a 25 step optical filter system: for excitation energies below about 1 pJ, micrometric particles are not completely destroyed but only a part of the particle is evaporated, which makes LAMMA a surface sensitive technique if the focussing conditions are well chosen, while at higher energy complete destruction by laser irradiation can be accomplished, resulting in total particle analysis. In this study LAMMA spectra of calcium oxy-salts are examined. The experimental results show the presence of molecular ions, that originally were not present in the sample, which indicatesa certain degree of degradation and recombination of the compounds under pulsed laser irradiation. The composition and relative intensities of these molecular ions are examined in order to find the relationships between the mass spectral pattern and the original compound stoichiometry, and to contribute to the understanding of the laws that govern ion formation under laser irradiation.
EXPERIMENTAL In the LAMMA@instrument (Leybold Heraeus, Kiiln, F. R. Germany)a very intense laser pulse (109-10” Wcme2; T = 15 ns) from a frequency quadrupled Q-switched neodymium-YAG laser
[I] E. DENOYER,R. VAN GRIEKEN, F. ADAMS and D. F. S. NATWZH, Anal. Chem. 54,26A (1982). [2] Proceedings of the LAMMA Symposium held in Dusseldorf, F. R. Germany, October 8-lo,1980 (Organizers: F. HILLENKAMP and R. KAUFMANN), Fresenius’Z. Anal. Chem. 308, 193-320 (1981). [3] D. M. HERCULES,R. J. DAY, K. BALASANMUGAN, TUAN A. DANG and C. P. LI, Anul. Chem. 54 280A (1982). [4] L. SALVATI. JR, D. M. HERCULESand H. VCGT, Specrrosc. Lerr. 13, 243 (1980). [S] F. BRUYNSEELSand R. VAN GRIEKEN, Anal. C/tern. submitted. [6] J. D. GANJEI, R. J. COLTON and J. S. MURDAY, Int. J. Mass Spectrom. lon Phys. 37.49 (1981). [7] E. DE PAUW and J. MARIEN,lnf. J. Mass Spectrom. Ion Phys. 38, 11 (1981). 853
FRANKJ. BRUYNSEELS
a54 (,I= 265 nm)
is delivered
and RENBE.VANGRIEKEN
to the micrometric
particles,
that adhere
to a very thin Formvar
foil
(u 0.1
pm). The laser generated ions are accelerated by an einsel-type lens and analysed either in the positive or negative ion mode in a time-of-flight mass spectrometer including an ion reflector in the drift tube. The mass spectrum is detected in an open Cu-Be secondary electron multiplier, stored in a 100 MHz transient recorder and fed into a Digital Mint-I 1 computer for calibration, integration and further examination. A detailed description of the LAMMA@instrument has been given in e.g.
Refs [1,2]. RESULTSAND
DISCUSSION
Figure la shows the types of negative molecular ions emitted by a CaSO, particle under pulsed laser irradiation (Energy = 1 @). The characteristic mass peaks are 160-, r’OH_, 32S- and 320;, 48SO-, 64SO;, 8oSO;, g6SO; ,?aO-, 72Ca0;, ‘jCa02H-. The presence of the 0; ion is confirmed by its appearance in the negative CaC03 spectrum (Fig. lb). The other molecular ions in the CaC03 spectrum are 12C-, 160-, “OH-, 24C;, 72CaO;. CaO shows characteristic mass peaks (Fig. lc) at 160-, 320;, 2sC2H-, %aO-, s6CaO- and 72CaO;. Although the presence ofa sulfur oxyanion can easily be established, the distinction between CaC03 and CaO may be difficult because of the possible interference of the Formvar foil (polyvinylaldehyde) with the carbon and oxygen containing ion peaks. The positive ion spectra of the compounds under examination are quantitatively the same and offer the possibility of making a quantitative comparison of the molecular intensities. As seen in Fig. 2, three different cluster series are observed: Ca(CaO):, (CaO),+ and (CaO),H+. The calcium ions thus appear predominantly in the positive spectrum and are largely associated with oxygen and other calcium atoms. The relative intensities of the molecular ions can be evaluated with the empirical model as described by PLOGet al. [8] for ions with the general formula M,O;. This model, which was a/
CaSO,
SO;
_ b/
C i
Ion
t-lass
Ca CO,
0-
z
CaO-
i
OHI
C_w
l/L 1617
ICaOlO-
,~
2
A b 12
0;
,I. .
2L2526 32
1. 56
12
96 _
1 L 2 e c
c/
Ion Mass
CaO
0-
?!
CaOICaO)O-
I 1617
0; 1. 32
.
, 56
L 72
96 _
Fig. 1. LAMMA
IonMass
spectra in the negative ion mode of: (a) CaSO,; tb) QCO,; tc) QIO.
[8] C. PLOG, L. WIEDMANNand A. BENNINGHOVEN, Sur$~e Sci., 67, 565 (1977).
in calciumoxide and salts
Ion distributions
855
x
c_
E
Ca’ Ca(CaOl+
CaKaOl;KaO)~ 40
56
96
112
152
.-
168
208
.__--_ IonMass
224
1
Fig. 2. Positive ion mode LAMMA spectrum of CaSO,.
developed initially for the evaluation of static SIMS data, is also applicable to laser induced ion emission data as has been demonstrated via the relative ion distribution data of silica samples [9]. For describing the molecular ions with different atomic composition by a uniform parameter, the fragment valence K of a metal atom in an emitted fragment, i. e. its formal valence number, is of importance. K can be calculated by ascribing the valence number -2 to oxygen. For the ion M,O;, K is defined as [S]: K=q+2n -3
m
where q is the total charge of the molecular ion, n the number of oxygen atoms, and m the number of metal atoms. For the positive ions, the relative ion intensities I + (K) as a function of the fragment valence K can be fitted to a Gaussian curve of the form [S]: l+(K)=l&,,. exp[-(K-G+)2/2y2]
(1)
where I LXis the maximum of the distribution, G + the K-value of maximum of the curve, and y2 the variance of the intensities. In the spectra of the calcium compounds under investigation, for a given m number, only 2 molecular ions are present with different n number, namely n = m - 1 and n = m, so that a fitting for constant m-value as done for static SIMS-data [S] is not meaningful. However in LAMMA the m-value varies over a wider range and it becomes clear that the molecular ion distributions of the series Ca,Oz_ , and (CaO): are well represented by a Gaussian curve as a function of K i.e. by a parabola in a semilog plot. M, the mass of the molecular ions is, of course, equal to M = m x 40 + n x 16. For the major molecular ion series Ca,O;, the function K can be expressed as: 2m-1 K=----= m K
=2m+l -= m
2M-24 M+16 2M+56 M
2M-56
Kz2m-l -= m
M
2M+24
Kc2m+l -=
M_16
m
form=n+l,q= for
m =
+l
n, q =
for m = n,q
=
form=n-l,q=
+
1
-
1 -1
For both series, (CaO),H+ and (CaO),OH-, the fragment valence K is equal to 2, if one ascribes the valence number + 1 to hydrogen. For (CaO): and Ca,O;+ I and for Ca,O:_ 1and (CaO); , the same functions represented in Fig. 3, are described for equal m-values. In this regard, the two series are equivalent. However, from the experimental data, as shown in Figs 1 and 2 and Table 1, it is seen that higher molecular clusters appear almost exclusively in the positive mode spectrum. [9] E. MICHIELS, A. CELIS and R. GIJBELS,in Microbeam
Francisco Press, San Francisco (1982).
Analysis
1982
(Ed. K. F. J. HEINRICH),p. 383. San
FRANK J. BRUYNSEELS and RENL E. VAN GRIEKEN
856
0
for Ca,O,'andCa,O;..
o for Co,,O;-. and Ca,O,
I 1
I
2
I
3 -Number
I
1
4 5 of Co-atoms, m
I~
6
Fig. 3. Dependence of the fragment valence on the number of calcium atoms in the different molecular ion series.
Table 1. Average intensities of the positively charged molecular ions, normalized to the Ca+-peak Ion intensities, from Ion Ca+ Ca0+ (CaO)H+ Ca(Ca0)’ (CaW (CaO), H + wcao): (CaW (CaO), H + Ca(Ca0); (CaW (CaO), H + Ca(CaO); (CaO): (CaO)s H+
M/e
K
C&o,
40
1.00
1.0 lo”
1.0 10
56 57 96 112 113 152 168 169 208 224 225 264 280 281
3.00 2.00 1.50 2.50 2.00 1.67 2.33 2.00 1.75 2.25 2.00 1.80 2.20 2.00
5.8 10-l 1.7 10-l 6.3 10-l 3.6 lo- ’ 6.3 lo-’ 1.3 10-l 3.7 1o-2
5.9 lo-.’ 5.4 10-l 4.8 10-l 2.8 10-l 4.4 10-l 6.1 lo-* 2.3 lo-’ 9.2 10-r 1.7 10-Z 6.9 lo-’ 2.3 lo-’ 2.8 lo-’ 1.6 lo-’ 4.7 10-J
1.1 1o-2 6.3 1O-3 -
CaS04
CaO
1.0 10 5.5 10-l 2.2 10-l 5.6 lo- ’ 3.6 10-l 1.3 10-l 1.1 10-l 3.9 10-l 1.7 1o-2 1.7 10-r -
1.0 10 3.5 10-l 7.1 10-l 8.4 10-l 3.7 10-l 6.5 10-l 2.3 lo-’ 6.2 lo-’ 2.1 10-l 4.4 10-2 1.8 lo-’ 3.8 lo-* -
-
-
CaO, anhyd. 1.0 10
1.8 10-l 1.0 10-l 8.1 10-l 1.8 10-l 7.8 lo-* 1.4 10-l 1.4 10-r 1.0 lo-* 2.1 1o-3
The experimental data were fitted by polynomial regression analysis with the formula: In I = a, + a, K + a,K2. The resulting parameters of the Gaussian distribution of Eqn (1) are given in Table 2. A graphical representation of the data for CaSO,*2H,O is plotted in Fig. 4. The values for G [ of the Ca(Ca0): series and G i of the (CaO): series are constant for all three calcium salts, namely G ; = 1.227 + 0.007 (0.6 ‘%;) G; = 2.777 f 0.009 (0.3 2,) The “lattice valence” [8], here defined as G” = (G : + G: )/2, is equal to 2.002 + 0.003 (0.2 7”). The displacement parameter a, defined [8] as the difference between G” and G: or G :, equals 0.775 in both cases. It is clear that the parameters of the curves are the same for the
857
Ion distributions in calcium oxide and salts Table 2. Parameters of the fitting of a Gaussian according to. eq. (1) through the experimental data of Table 1, for the series: I = Ca(Ca0): and 2 = (CaO): Compound
taco,
vi+
G:
I+i ma
r
= 0.166
G: = 1.235
1+ , IMx= 2.68
0.987
y; = 0.157
G; = 2.771
I;,,
0.999
y; = 0.158
G: = 1.222
1+, IMx = 2.63
y; = 0.162
G; = 2.718
0.998
Case,
*y: = 0.171 y; = 0.166
G: = 1.225 G; = 2.773
0.999 = 1
CaO
y; = 0.180
G: = 1.248
y; = 0.175
G; = 2.746
y: = 0.147 y; = 0.145
CaS04.2H20
CaO. anhyd.
y:
1
= 1.69
0.997
0.987 0.999
G; = 1.253
r;,, = 1.00 I+, IMx= 5.48
G; = 2.748
1;,,=0.81
0.999
2
0.980
3 -Fragment
valence
Fig. 4. Molecular ion intensities for the series Ca,O:_ , at left, and (CaO): , at right, as emitted for laser irradiation of CaSOI.2H20.
Ca(Ca0): and (CaO): series and that their distributions only differ in the intensity at the maximum, 1 A,. The average for the three calcium salts normalized to the Ca+-intensity amounts to: 1 T,, = 2.56kO.33 (13%) for Ca(CaO)z, and !i,.,,,, = 1.50+0.17 (11 %) for (CaO): . The spectra of CaO show basically the same features, but the ratio 1:,,/1:,,,,, is larger than for the salts, especially in the absence of H-atoms, as can be seen in Table 2. For the third series of molecular ions, (CaO),H+, the fragment valence is always equal to 2. This K value is equal to the experimental lattice valence and to the oxidation state of Ca in the considered compounds. The attachment of a proton gives rise to a relatively stable molecular ion that has a significant intensity even in the spectra of oven-dried samples. In the spectrum of CaSO,.2H,O, the (CaO), H+ intensity is higher or equal to the (CaO): intensity while for CaS04 the reverse phenomenon is observed. This means that the hydrogen is predominantly supplied by the water molecules. Also for the (CaO),H+ series the molecular ion intensity decreases about exponentially with the molecular ion mass. The (CaO):/(CaO),H+ intensity ratio remains essentially constant for the higher mvalues and this suggests that there is a correlation between the (CaO): and the (CaO),H+ formation. The high intensities of the (CaO),H+ molecular ions in the CaO-spectrum in comparison to the spectrum of anhydric CaO indicate the importance of the hydrogen atoms for the molecular ion distribution and the Ca-to-0 ratio of the molecular ions. For increasing m number the K(m) functions approach the fragment valence value of 2, which favours the formation of high molecular weight ions. The higher molecular ions are not a direct fragment
FRANK J. BRUYNSEELSand RENB. E. VAN GRIEKEN
858
of the original compound but are the results of chemical reactions that take place during the short laser irradiation. The appearance of the same molecular ion series in the spectra of CaSO,, CaCO, and CaO can be due to the rapid formation of CaO by the following reactions [lo]: CXO,
-+ CaO + CO2
CaSO, + CaO + SO,
T = 990°C in O2 T = 900 - 1034°C in CO2
For the negative ions of CaSO,, the following relative intensities were found on the average; %- : 1.04, 48SO- : 0.80, 64SO; : 1.00, *‘SO;: 0.24 and 96SO; : 0.01. The high intensity of the peak with mass 32 can partially be due to the presence of 320;. Applying the model of PLOG et al. [8] to the molecular ions of SO;, with n ranging from 1 to 4, yields the following parameters for the Gaussian distribution: G - = 2.29, y- = 1.53, Z&,x= 1.13 and r = 0.999. CONCLUSION LAMMA spectra are very similar to the static SIMS spectra, and the model of PLOG etal. [8] can be used for describing the molecular ion distribution for constant number of metal ions, but also for the series (CaO): and Ca,O,_ 1 with simultaneously varying numbers of metal and oxygen atoms. The series (CaO),H+ has a constant fragment valence value of 2, that is equal to the limit value of the K(m) function, to the lattice valence and to the oxidation state of Ca in the compounds. Acknowledgement-One of us (F.J.B.)is indebted to the Instituut ter Aanmoediging van het Wetenschappelijk Onderzoek in Nijverheid en Landbouw (IWONL) for financial support. This research was funded by the Interministrial Commission for Science Policy, Belgiuni through research grant SO-85/10.
[lo]
C. DUVAL, Inorganic Thermograuimetric Analysis, Elsevier, Amsterdam
(1963).
Am.
Vol.
Printedm Grcrl Brilain.
388.
No.
S/6. pp. 853-858.
0584~8547183
1983.
s3.00+
.oo
Q 1983.Perymon Preu Lid.
Molecular ion distributions in laser microprobe mass spectrometry of calcium oxide and calcium salts FRANKJ. BRUYNSEELS and RENAE.
VAN GRIEKEN
Department of Chemistry, University of Antwerp (UIA), B-2610 Wilrijk, Belgium (Received 9 Nouember 1982) Abstract-Laser Microprobe Mass Spectrometry (LAMMA) is used to examine micrometric particles of calcium oxyanion salts (CaCO,, CaSO,, CaS0,.2HIO) and calcium oxide, in both the positive and negative ion mode. The major molecular ions, appearing in the positive mass spectrum, can be divided into three series, namely Ca_O:_ , , (CaO): and (CaO), H + (m= 1 - 4). In the caseof the former two seriesthe relative intensities of the mass peaks as a function of the fragment valence K = (1 + 2n)/m, for Ca,Oi , can be fitted to a Gaussian distribution curve, as was earlier demonstrated for secondary ion mass spectrometry. The high stability of the (CaO)_H+ series can be explained by the favourable fragment valence of + 2 corresponding to the usual oxidation state of calcium.
IN LASERmicroprobe mass analysis (LAMMA) with the LAMMA@ -500 instrument, a pulsed focused laser beam is used for evaporating a microscopic spot from a sample or single particles with dimensions in the micrometric range [ 11. The analysis is performed by a timeof-flight mass spectrometer. Besides the application of LAMMA for quantitative analysis [2], it has also been used for the structural analysis of organic and inorganic materials [3]. Until now most attention has been paid to the characterization oforganic substances. Yet, the use of LAMMA for the identification of inorganic particles is also promising. Indeed the positive and negative LAMMA spectra from inorganic salts are indicative for the compound stoichiometry, but a careful examination of the type and the relative intensity of the molecular ions produced is necessary [4,5]. Qualitatively, LAMMA spectra are very similar to static SIMS spectra [6,7], but the superior lateral resolution of 0.5-1.0 pm and the high speed of LAMMA makes it potentially more interesting for single particle analysis. In laser probe excitation the irradiation energy can be varied by a 25 step optical filter system: for excitation energies below about 1 pJ, micrometric particles are not completely destroyed but only a part of the particle is evaporated, which makes LAMMA a surface sensitive technique if the focussing conditions are well chosen, while at higher energy complete destruction by laser irradiation can be accomplished, resulting in total particle analysis. In this study LAMMA spectra of calcium oxy-salts are examined. The experimental results show the presence of molecular ions, that originally were not present in the sample, which indicatesa certain degree of degradation and recombination of the compounds under pulsed laser irradiation. The composition and relative intensities of these molecular ions are examined in order to find the relationships between the mass spectral pattern and the original compound stoichiometry, and to contribute to the understanding of the laws that govern ion formation under laser irradiation.
EXPERIMENTAL In the LAMMA@instrument (Leybold Heraeus, Kiiln, F. R. Germany)a very intense laser pulse (109-10” Wcme2; T = 15 ns) from a frequency quadrupled Q-switched neodymium-YAG laser
[I] E. DENOYER,R. VAN GRIEKEN, F. ADAMS and D. F. S. NATWZH, Anal. Chem. 54,26A (1982). [2] Proceedings of the LAMMA Symposium held in Dusseldorf, F. R. Germany, October 8-lo,1980 (Organizers: F. HILLENKAMP and R. KAUFMANN), Fresenius’Z. Anal. Chem. 308, 193-320 (1981). [3] D. M. HERCULES,R. J. DAY, K. BALASANMUGAN, TUAN A. DANG and C. P. LI, Anul. Chem. 54 280A (1982). [4] L. SALVATI. JR, D. M. HERCULESand H. VCGT, Specrrosc. Lerr. 13, 243 (1980). [S] F. BRUYNSEELSand R. VAN GRIEKEN, Anal. C/tern. submitted. [6] J. D. GANJEI, R. J. COLTON and J. S. MURDAY, Int. J. Mass Spectrom. lon Phys. 37.49 (1981). [7] E. DE PAUW and J. MARIEN,lnf. J. Mass Spectrom. Ion Phys. 38, 11 (1981). 853
FRANKJ. BRUYNSEELS
a54 (,I= 265 nm)
is delivered
and RENBE.VANGRIEKEN
to the micrometric
particles,
that adhere
to a very thin Formvar
foil
(u 0.1
pm). The laser generated ions are accelerated by an einsel-type lens and analysed either in the positive or negative ion mode in a time-of-flight mass spectrometer including an ion reflector in the drift tube. The mass spectrum is detected in an open Cu-Be secondary electron multiplier, stored in a 100 MHz transient recorder and fed into a Digital Mint-I 1 computer for calibration, integration and further examination. A detailed description of the LAMMA@instrument has been given in e.g.
Refs [1,2]. RESULTSAND
DISCUSSION
Figure la shows the types of negative molecular ions emitted by a CaSO, particle under pulsed laser irradiation (Energy = 1 @). The characteristic mass peaks are 160-, r’OH_, 32S- and 320;, 48SO-, 64SO;, 8oSO;, g6SO; ,?aO-, 72Ca0;, ‘jCa02H-. The presence of the 0; ion is confirmed by its appearance in the negative CaC03 spectrum (Fig. lb). The other molecular ions in the CaC03 spectrum are 12C-, 160-, “OH-, 24C;, 72CaO;. CaO shows characteristic mass peaks (Fig. lc) at 160-, 320;, 2sC2H-, %aO-, s6CaO- and 72CaO;. Although the presence ofa sulfur oxyanion can easily be established, the distinction between CaC03 and CaO may be difficult because of the possible interference of the Formvar foil (polyvinylaldehyde) with the carbon and oxygen containing ion peaks. The positive ion spectra of the compounds under examination are quantitatively the same and offer the possibility of making a quantitative comparison of the molecular intensities. As seen in Fig. 2, three different cluster series are observed: Ca(CaO):, (CaO),+ and (CaO),H+. The calcium ions thus appear predominantly in the positive spectrum and are largely associated with oxygen and other calcium atoms. The relative intensities of the molecular ions can be evaluated with the empirical model as described by PLOGet al. [8] for ions with the general formula M,O;. This model, which was a/
CaSO,
SO;
_ b/
C i
Ion
t-lass
Ca CO,
0-
z
CaO-
i
OHI
C_w
l/L 1617
ICaOlO-
,~
2
A b 12
0;
,I. .
2L2526 32
1. 56
12
96 _
1 L 2 e c
c/
Ion Mass
CaO
0-
?!
CaOICaO)O-
I 1617
0; 1. 32
.
, 56
L 72
96 _
Fig. 1. LAMMA
IonMass
spectra in the negative ion mode of: (a) CaSO,; tb) QCO,; tc) QIO.
[8] C. PLOG, L. WIEDMANNand A. BENNINGHOVEN, Sur$~e Sci., 67, 565 (1977).
in calciumoxide and salts
Ion distributions
855
x
c_
E
Ca’ Ca(CaOl+
CaKaOl;KaO)~ 40
56
96
112
152
.-
168
208
.__--_ IonMass
224
1
Fig. 2. Positive ion mode LAMMA spectrum of CaSO,.
developed initially for the evaluation of static SIMS data, is also applicable to laser induced ion emission data as has been demonstrated via the relative ion distribution data of silica samples [9]. For describing the molecular ions with different atomic composition by a uniform parameter, the fragment valence K of a metal atom in an emitted fragment, i. e. its formal valence number, is of importance. K can be calculated by ascribing the valence number -2 to oxygen. For the ion M,O;, K is defined as [S]: K=q+2n -3
m
where q is the total charge of the molecular ion, n the number of oxygen atoms, and m the number of metal atoms. For the positive ions, the relative ion intensities I + (K) as a function of the fragment valence K can be fitted to a Gaussian curve of the form [S]: l+(K)=l&,,. exp[-(K-G+)2/2y2]
(1)
where I LXis the maximum of the distribution, G + the K-value of maximum of the curve, and y2 the variance of the intensities. In the spectra of the calcium compounds under investigation, for a given m number, only 2 molecular ions are present with different n number, namely n = m - 1 and n = m, so that a fitting for constant m-value as done for static SIMS-data [S] is not meaningful. However in LAMMA the m-value varies over a wider range and it becomes clear that the molecular ion distributions of the series Ca,Oz_ , and (CaO): are well represented by a Gaussian curve as a function of K i.e. by a parabola in a semilog plot. M, the mass of the molecular ions is, of course, equal to M = m x 40 + n x 16. For the major molecular ion series Ca,O;, the function K can be expressed as: 2m-1 K=----= m K
=2m+l -= m
2M-24 M+16 2M+56 M
2M-56
Kz2m-l -= m
M
2M+24
Kc2m+l -=
M_16
m
form=n+l,q= for
m =
+l
n, q =
for m = n,q
=
form=n-l,q=
+
1
-
1 -1
For both series, (CaO),H+ and (CaO),OH-, the fragment valence K is equal to 2, if one ascribes the valence number + 1 to hydrogen. For (CaO): and Ca,O;+ I and for Ca,O:_ 1and (CaO); , the same functions represented in Fig. 3, are described for equal m-values. In this regard, the two series are equivalent. However, from the experimental data, as shown in Figs 1 and 2 and Table 1, it is seen that higher molecular clusters appear almost exclusively in the positive mode spectrum. [9] E. MICHIELS, A. CELIS and R. GIJBELS,in Microbeam
Francisco Press, San Francisco (1982).
Analysis
1982
(Ed. K. F. J. HEINRICH),p. 383. San
FRANK J. BRUYNSEELS and RENL E. VAN GRIEKEN
856
0
for Ca,O,'andCa,O;..
o for Co,,O;-. and Ca,O,
I 1
I
2
I
3 -Number
I
1
4 5 of Co-atoms, m
I~
6
Fig. 3. Dependence of the fragment valence on the number of calcium atoms in the different molecular ion series.
Table 1. Average intensities of the positively charged molecular ions, normalized to the Ca+-peak Ion intensities, from Ion Ca+ Ca0+ (CaO)H+ Ca(Ca0)’ (CaW (CaO), H + wcao): (CaW (CaO), H + Ca(Ca0); (CaW (CaO), H + Ca(CaO); (CaO): (CaO)s H+
M/e
K
C&o,
40
1.00
1.0 lo”
1.0 10
56 57 96 112 113 152 168 169 208 224 225 264 280 281
3.00 2.00 1.50 2.50 2.00 1.67 2.33 2.00 1.75 2.25 2.00 1.80 2.20 2.00
5.8 10-l 1.7 10-l 6.3 10-l 3.6 lo- ’ 6.3 lo-’ 1.3 10-l 3.7 1o-2
5.9 lo-.’ 5.4 10-l 4.8 10-l 2.8 10-l 4.4 10-l 6.1 lo-* 2.3 lo-’ 9.2 10-r 1.7 10-Z 6.9 lo-’ 2.3 lo-’ 2.8 lo-’ 1.6 lo-’ 4.7 10-J
1.1 1o-2 6.3 1O-3 -
CaS04
CaO
1.0 10 5.5 10-l 2.2 10-l 5.6 lo- ’ 3.6 10-l 1.3 10-l 1.1 10-l 3.9 10-l 1.7 1o-2 1.7 10-r -
1.0 10 3.5 10-l 7.1 10-l 8.4 10-l 3.7 10-l 6.5 10-l 2.3 lo-’ 6.2 lo-’ 2.1 10-l 4.4 10-2 1.8 lo-’ 3.8 lo-* -
-
-
CaO, anhyd. 1.0 10
1.8 10-l 1.0 10-l 8.1 10-l 1.8 10-l 7.8 lo-* 1.4 10-l 1.4 10-r 1.0 lo-* 2.1 1o-3
The experimental data were fitted by polynomial regression analysis with the formula: In I = a, + a, K + a,K2. The resulting parameters of the Gaussian distribution of Eqn (1) are given in Table 2. A graphical representation of the data for CaSO,*2H,O is plotted in Fig. 4. The values for G [ of the Ca(Ca0): series and G i of the (CaO): series are constant for all three calcium salts, namely G ; = 1.227 + 0.007 (0.6 ‘%;) G; = 2.777 f 0.009 (0.3 2,) The “lattice valence” [8], here defined as G” = (G : + G: )/2, is equal to 2.002 + 0.003 (0.2 7”). The displacement parameter a, defined [8] as the difference between G” and G: or G :, equals 0.775 in both cases. It is clear that the parameters of the curves are the same for the
857
Ion distributions in calcium oxide and salts Table 2. Parameters of the fitting of a Gaussian according to. eq. (1) through the experimental data of Table 1, for the series: I = Ca(Ca0): and 2 = (CaO): Compound
taco,
vi+
G:
I+i ma
r
= 0.166
G: = 1.235
1+ , IMx= 2.68
0.987
y; = 0.157
G; = 2.771
I;,,
0.999
y; = 0.158
G: = 1.222
1+, IMx = 2.63
y; = 0.162
G; = 2.718
0.998
Case,
*y: = 0.171 y; = 0.166
G: = 1.225 G; = 2.773
0.999 = 1
CaO
y; = 0.180
G: = 1.248
y; = 0.175
G; = 2.746
y: = 0.147 y; = 0.145
CaS04.2H20
CaO. anhyd.
y:
1
= 1.69
0.997
0.987 0.999
G; = 1.253
r;,, = 1.00 I+, IMx= 5.48
G; = 2.748
1;,,=0.81
0.999
2
0.980
3 -Fragment
valence
Fig. 4. Molecular ion intensities for the series Ca,O:_ , at left, and (CaO): , at right, as emitted for laser irradiation of CaSOI.2H20.
Ca(Ca0): and (CaO): series and that their distributions only differ in the intensity at the maximum, 1 A,. The average for the three calcium salts normalized to the Ca+-intensity amounts to: 1 T,, = 2.56kO.33 (13%) for Ca(CaO)z, and !i,.,,,, = 1.50+0.17 (11 %) for (CaO): . The spectra of CaO show basically the same features, but the ratio 1:,,/1:,,,,, is larger than for the salts, especially in the absence of H-atoms, as can be seen in Table 2. For the third series of molecular ions, (CaO),H+, the fragment valence is always equal to 2. This K value is equal to the experimental lattice valence and to the oxidation state of Ca in the considered compounds. The attachment of a proton gives rise to a relatively stable molecular ion that has a significant intensity even in the spectra of oven-dried samples. In the spectrum of CaSO,.2H,O, the (CaO), H+ intensity is higher or equal to the (CaO): intensity while for CaS04 the reverse phenomenon is observed. This means that the hydrogen is predominantly supplied by the water molecules. Also for the (CaO),H+ series the molecular ion intensity decreases about exponentially with the molecular ion mass. The (CaO):/(CaO),H+ intensity ratio remains essentially constant for the higher mvalues and this suggests that there is a correlation between the (CaO): and the (CaO),H+ formation. The high intensities of the (CaO),H+ molecular ions in the CaO-spectrum in comparison to the spectrum of anhydric CaO indicate the importance of the hydrogen atoms for the molecular ion distribution and the Ca-to-0 ratio of the molecular ions. For increasing m number the K(m) functions approach the fragment valence value of 2, which favours the formation of high molecular weight ions. The higher molecular ions are not a direct fragment
FRANK J. BRUYNSEELSand RENB. E. VAN GRIEKEN
858
of the original compound but are the results of chemical reactions that take place during the short laser irradiation. The appearance of the same molecular ion series in the spectra of CaSO,, CaCO, and CaO can be due to the rapid formation of CaO by the following reactions [lo]: CXO,
-+ CaO + CO2
CaSO, + CaO + SO,
T = 990°C in O2 T = 900 - 1034°C in CO2
For the negative ions of CaSO,, the following relative intensities were found on the average; %- : 1.04, 48SO- : 0.80, 64SO; : 1.00, *‘SO;: 0.24 and 96SO; : 0.01. The high intensity of the peak with mass 32 can partially be due to the presence of 320;. Applying the model of PLOG et al. [8] to the molecular ions of SO;, with n ranging from 1 to 4, yields the following parameters for the Gaussian distribution: G - = 2.29, y- = 1.53, Z&,x= 1.13 and r = 0.999. CONCLUSION LAMMA spectra are very similar to the static SIMS spectra, and the model of PLOG etal. [8] can be used for describing the molecular ion distribution for constant number of metal ions, but also for the series (CaO): and Ca,O,_ 1 with simultaneously varying numbers of metal and oxygen atoms. The series (CaO),H+ has a constant fragment valence value of 2, that is equal to the limit value of the K(m) function, to the lattice valence and to the oxidation state of Ca in the compounds. Acknowledgement-One of us (F.J.B.)is indebted to the Instituut ter Aanmoediging van het Wetenschappelijk Onderzoek in Nijverheid en Landbouw (IWONL) for financial support. This research was funded by the Interministrial Commission for Science Policy, Belgiuni through research grant SO-85/10.
[lo]
C. DUVAL, Inorganic Thermograuimetric Analysis, Elsevier, Amsterdam
(1963).