Download A system for precise sulfur isotope analysis by a small mass

GEOLOGINEN TUTKIMUSLAITOS
GEOLOGICAL SURVEY OF FINLAND
TUTKIMUSRAPORTTI N:o 20
REPORT OF INVESTIGATION No. 20
Markku Mäkelä and Jalle Tammenmaa
A system for precise sulfur isotope
analysis by a small mass spectrometer
Espoo 1977
GEOLOGINEN TUTKIMUSLAITOS
Tutkimusraportti n :o 20
Markku Mäkelä and lalle Tammenmaa
A SYSTEM FOR PRECISE SULFUR ISOTOPE ANALYSIS BY A SMALL MASS
SPECTROMETER
Espoo 1977
2
Mäkelä, M. and Tammenmaa, J. 1977. A system for precise sulfur isotope analysis by a
small mass spectrometer. Geological Survey 0/ Finland, Report o/Investigation No. 20.
23 pages and 7 figures.
A system for precise sulfur isotope ratio research being used at the Laboratory
of Economic Geology, Helsinki University of Technology is described.
Preparation technique of S02 gas' used in the mass spectrometric measurement
is presented. The mass spectrometric method utilizing the principle of double collection
applied to a small 180-degree mass spectrometer is elucidated and the measuring technique derived is given. Digitalization of the mass spectrometer output signals with an
online device and computer evaluation of the 8 34 S values conventionally used in
publications is presented and accuracy of those produced are examined.
The authors' address:
Markku Mäkelä
Helsinki University 0/ Technology
Dept. 0/ Mining and Metallurgy
lalle Tammenmaa
02150 Espoo 15, Finland
ISBN 951 - 690- 061 - 5
VTT OFFSETPAINO 19 77
599/3
3
CONTENTS
Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Preparation technique . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Mass spectrometer GD 150 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Principle of function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inlet system. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Analyzer. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Measuring bridge . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Measuring and calculation of Ö34 S values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Basic concepts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Measuring technique .... . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Calculation of Ö34 S values . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Accuracy of Ö34 S va lues . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
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5
INTRODUCTION
The application of stable sulfur isotopes to the problems of ore genesis has
provided additional information on the formation of numerous mineral deposits (Jensen, 1959; Stanton 1960; Ault and Kulp, 1960; Gavelin et al., 1960; Tatsumi, 1965;
Anger et al. , 1966; Sakai, 1968; Grinenko et al. , 1969; Sangster, 1971; Ohmoto, 1972).
On the basis of this and the results gained by the senior author at the Göttingen
Laboratory from the Outokumpu ore deposit (Mäkelä, 1972; and later on 1974) the
decision was made upon Professor Aimo K. Mikkola's initiative to create a sulfur isotope
laboratory in Finland. The Ore-geological Commission of Northern Finland instituted
by the Ministry of Trade and Commerce supporfed the intention and the Ministry financed 78 percent of the mass spectrometer proper. The rest of the financing is from
Helsinki University of Technology by grant from the Outokumpu Foundation. In the
fall 1973 the installation of a Varian GD 150 small mass spectrometer was completed
at the Laboratory of Economic Geology, Helsinki University of Technology.
Before this the preparative instrumentation was built up at the workshop of the
Department. The data system involving digitalization of the mass spectrometer output
signals in a special way, computer evaluation of the 834 S values conventionally used in
publications, and printing of the investigator's choice of data tables or analysis diagrams was drawn up prior to summer 1974.
In this paper a system for precise sulfur isotope ratio research using a small
mass spectrometer and the method of data handling is described.
PREPARATION TECHNIQUE
In stable isotope ratio research the element to be analyzed is introduced into
the mass spectrometer in a gaseous form. To prepare the volatile compound is one of
the most critical steps in stable isotope geochemistry, since a quantitative yield of a
pure gas is desired in order to prevent isotope fractionation during the preparation
and to prevent particle interference in the mass spectrometer.
There are two principal preparative techniques for sulfur isotope analysis;
either to convert the sulfur minerals into sulfur dioxide (Ricke, 1964), or into sulfur
hexafluoride (Putchelt et al. , 1971). Although sulfur hexafluoride has some unique
advantages for mass spectrometry, the difficulties in routine work arising from the
handling of the extremely reactive fluorinating agent, BrF3 , have caused us to reject
the method.
The research at this Laboratory is primarily concerned with Precambrian ore
deposits. Hence it follows that most of the sampies contain sulfur in the form of
sulfides. The separation of various sulfide species is done using a suction picking
apparatus under a zoom stereo microscope. The extraction of sulfur from country
rock sampies containing only minor amounts of sulfides is done by oxidating them
to BaS04. The sulfate is then reduced with Fe + Zn at 950°C and converted into
CdS by precipitation in a Cd-formiate solution. Pure sulfides are converted into S02
by reaction with vanadium pentoxide at a temperature of 1000 ° C in the manner described by Ricke (op. cit.).
Sulfide amounting to 0.14 mM (e.g. 8.3 mg FeS2) and 0.44 mM of the oxidizing agent, V20 S , are separately heated to 300°C in a vacuum of 133 mPa for
6
Fig. I . Set-up of preparative apparatuses for dispeling the volatile impurities from MeS - V2 0 S
pairs. Photo Erkki Halme.
12 hours to dispe1 the volatile impurities Fig. 1). Thereafter the MeS-V2 Os pairs are intimate1y mixed in mortars and placed in 5 cm 3 quartz tubes. The tubes, nine per
charge, are then evacuated to high vacuum of 133 x 10-2 mPa and heated to 150°C
for six hours (Fig. 1.). The quartz tubes are then sea1ed in order to maintain the
vacuum and transfered to a laboratory furnace where the reaction proper:
takes place at a temperature of 1000 oe. To prevent the formation of S03, which
would result in an isotope fractionation between S02 and S03 , the quartz tubes are,
after the reaction time of two and a half minutes, rapidly cooled to room temperature.
The tubes, now containing the S02 to be measured, are then introduced into plastic
sockets by which they are wnnected to the inlet system of the mass spectrometer.
7
MASS SPECTROMETER GD 150
Principle of function
A number of methods may be used for measuring the abundance of stable
isotopes; analysis by neutron activation, analysis of electromagnetic spectra, determination of the specific gravity of fluids , and analysis of mass spectra. The mass
spectrometric method utilizing the principle of double collection is, however, by far
the most effective for measuring the isotopic abundance ratios. Beams of two masses
are collected simultaneously and, after amplification , recorded in a manner dealing
with their ratio. In order to eliminate effect of instrument error and thus provide
the required accuracies the isotope ratio of a sam pie is successively compared with
the ratio of a known standard.
A diagram (Fig. 2) shows the principle of a GD ISO-type 180-degree mass
spectrometer for isotope ratio research.
The S02 gas to be analyzed is fed via an inlet system into the ion sour ce
region of the analyzer under apressure down to 133 x 10-1 mPa. Positive ions are
:.. •
$ •••••••
$:. •
••••
$
$
\..
•
•
e..
$
•••
$
..
$
•••
$
•• e
••
e
•••
$
.:
$
••
-------------------i.
$
e
COLLECTORS
ACCELERATION
ION IZATION
-- - -
Ue-pU A
POTENTIOMETRIC
RECORDER
Fig. 2. Diagram showing the principle of a GD 150 - type 180 - degree mass spectrometer equipped
with a double inlet system and a double collector arrangement.
8
provided by electron bombardment. In the electron beam where the singly charged
sot ions are formed they will acquire potential energy equal to eU, where e is the
charge of the ion and U the electrical potential of the electron beam. The ions are
focused upon an entrace slit by an electric lens system . On passing through the lens
system the consequent equal potential fall, means the singly charged ions possess
the same kinetic energy:
eU
=
mv 2
2
(1)
where m is the mass of the ion and v the velocity after passing through the electric
lens system.
The light 32 s 16 0 160+ ions will thus move with a higher velocity than the
heavy 34 S160 160+ ones. When the ions enter the magnetic fjeld in a direction perpendicular to the magnetic lines of force , they are subjected to a force perpendicular
to both the direction of the fjeld and the direction of their motion . The magnitude of
this so-ca lied Lorenz force depends on the field strength and on the charge and velocity
of the ions according to the equation:
(2)
where B is the strength of the magnetic fjeld, e is the charge, and v is the velocity of
the moving particle. Accordingly, the ions begin to move in semicircular paths, and
are then affected by centrifugal force,
mv 2
F2 =r-
(3)
where r is the radius of curvature. Combining the Eqs. (I), (2) and (3) the path of the
singly charged ions in the magnetic field is given by the formula :
(4)
The path followed by the ion in the magnetic field is thus a function of the mass
of that ion. This fact is the basis of mass spectrometry.
0
After travelling on semicircular paths at a deflection of 180 the relevant masses,
64 (32 S 16 0 16 0) and 66( 34 S 16 0 16 0), are simultaneously collected in two differen t co 1lectors and discharged. The charges given up to the collectors flow off via separa te
amplifiers to the measuring bridge where the amplifier output voltages generated by
both of the masses are so transformed that they balance each other. The readout is
recorded by a potentiometer recorder, a compensograph.
With regard to mass separation in any ion optical system it is a condition that
the ions do not turn aside from their prescribed paths as a consequence of molecule impacts causing particle interference. Therefore, the separation must occur in a high vacuum.
For this purpose the mass spectrometer is equipped with a pumping system capable of
producing a vacuum of around 133 x 10-3 mPa in the analyzer.
9
Iniet system
A double inlet system, i.e. separate inlet lines for sampIe and standard, allows a
rapid comparison of their isotope ratios and thus under nearly identical conditions. Both
sam pie S02 as weIl as that operating as the standard are fed in to the mass spectrometer
by breaking the quartz tubes in the plastic sockets already evacuated.
The isotope ratio to be measured is dependent on the pressure in the ion source.
Therefore an equal flow of sam pIe and standard into the ion source is required. To
produce this there are two containers each with volumes variable by 20 percent (Fig. 2)
so that the storage pressures of sampie and standard can be compensated.
In addition to the equal pressure requirement the gas flow between storage
containers and ion source chamber must be such that no mass discrimination occurs.
During the molecular gas flow where the mean free path length of molecules is large
compared with the dimensions of the tubing through which the gas flows the particles
do not influence each other. Therefore, the gas flow velocity of the lighter component
is greater than that of the heavier one . With time this means a pronounced enrichment
of the heavier component with the isotope 34S in the storage container.
To avoid the mass discrimination the measurements are carried out utilizing the
viscous gas inlet into the ion source region. In consequence of the relatively great pressure
difference between the storage containers and the ion source chamber, the former being
•
.......
....
"".
•
..
•
~
j-~~•
..,...
Sc:ho!teinsol1 1
--------~~--------------------~.
•
Fig. 3. Apart of the front panel of the mass spectrometer GD 150 showing the scheme of the
gas flow to the ion source. Photo Erkki Halme.
10
around 133 x 102 Pa, the mean path length of molecules is small, indicating that the
particles do influence each other. Due to this interaction the viscous gas flow is not
dependent on masses and there is no preference of light components as in the ca se of
molecu1ar gas inflow.
The inlet system is evacuated to forevacuum of around 133 x 102 Pa by a rotary
pump and the pumping line is equipped with and adsorption trap for eliminating the
adsorbed humidity.
The scheme of the gas flow con trolled by special va1ves and rotary buttons of the
variable storage volumes is shown in Fig. 3.
Analyzer
The analyzer (Fig. 4) comprises ion source, deflection magnet, ion collectors,
and analyzing chamber and is flange-connected as a compact unit to the vacuum system.
The control unit connected to the analyzer by cables contains all the electronic devices
required for operation of the ana1yzer.
Fig. 4. Analyzer of the GD 1SO mass spectrometer. Photo Erkki Halme.
11
Fig. 5. A schematic cross section of the electron impact ion sour ce of the mass spectrometer GD 150.
C = filament
E = electron collector
P = pusher lens
D =draw-out plates
L = ion lenses
A schematic cross section of the electron impact ion source is given in Fig. 5. The
cathode, rhenium wire filament emits electrons after being electrically heated. The electron emission of the cathode is stabilized to 250 p.A. The electrons thus produced are
focused and accelerated to an energy of 70 e V. They traverse the ionization zone as a
narrow beam parallel to the ion exit slit and reach an electron collector through a slit
in the ionization casing. When one of these energetic electrons collides with an S02
molecule it transfers its kinetic energy to an electron of the molecule. Both electrons
now leave the zone of interaction leaving behind a positively charged soi ion.
The ions thereby formed are accelerated by an electric field, a lens system, in
the direction of the draw-out plates. To push as many ions as possible forward to pass
through the slit of the draw-out plates the ion source is equipped with a pusher lens to
which a volta ge is applied which is negative with respect to the ionization region. The
force acting on the ions in the direction of draw-out plates corresponds to the difference between the voltages on the draw-out plates and on the pusher lens.
Further on the ions drawn out from the ionization chamber are accelerated in
the direction of the ion lenses. In the field of the lenses they are moreover bunched
together in such a way that they are focused onto the entrace slit and pass through it
as a slightly divergent bunch. In order to achieve high sensitivity in the mass spectrometer the ion beam is again focused by a further pair of slits which impart a high intensity to the bunch passing through.
From the Eq. (4) one sees that the separation of the ions according to their
masses can be regulated by the alteration of either the magnetic flux density or the
12
acceleration voltage. The analyzer of the GD ISO mass spectrometer is equipped with
a permanent magnet the magnetic flux density of which is 0.67 Wb/m 2 . Accordingly
the beams of the relevant masses (in sulfur research 64( 32 S 160 16 0) and 66e4S160160)
as weil as those 40Ar and 44(12C160 16 0) acting as checks for the measurement are
moved into the ion collectors at the end of their paths by adjusting the acceleration
voltage.
The double collector arrangement is evident from the Fig. 2. The ions of mass
66 representing the less abundant 34S isotope pass through a slit formed by two carbon
plates into the collector cup A, where they are discharged. The carbon plates arranged
symmetrically to the slit serve as the collector B for the ions of mass 64 related to the
more abundant 32S isotope. From the construction it follows that the mass 65 also
falls onto the carbon plate. Its minute effect can, however, be eliminated by mathematical correction treatment (p. 18).
The signal of the less abundant isotope from the A coHector is amplified with
a high ohmic resistor of 1 x 10 11 Ohm, while that of the more abundant isotope is
amplified with a working resistor of 3 x 109 Ohm.
Measuring bridge
In the measuring bridge the higher amplifier output voltage belonging to the
less abundant isotope 34 S is reduced by a very precise potentiometer with a linearity
of 5 x 10-6 in order to compensate the amplifier output voltage of the more abundant
isotope 32 S. The output signal y of the measuring bridge is thus equal to,
Y = VB - pV A ,
(5)
where VB and VA are the output voltages of the amplifiers Band A; p is the factor
which can be set between 0 and 1 with the potentiometer.
The aim of the measurement is to find a value Po which corresponds to ful
compensation. Then PO is a measure for the sought isotope ratio:
VB
Po
=U
A
34s
"v -
32S
(6)
By feeding successively the sampie and the standard gases into the analyzer the images
of the isotope ratios are recorded by a compensograph ( a plotter) respectively. The
differences in the isotope abundance ratios between the sampies are then caJculated as
folIows.
MEASURING AND CALCULATION OF 034S V ALUES
The double collection principle forms generally the basis of mass spectrometric
methods for measuring isotopic abundance ratios. A variety of techniques may, however,
be used to record the mass spectrometer output signals and to process the associated
data. The availability of a time sharing computer and on online device for digitalization
of graphs were determining factors' in building up our technique.
13
Basic concepts
In stable isotope geochemistry the differences in isotope ratios between the
sam pies are of interest not the absolute ratios themselves. In addition, the difference
can be measured more accurately and more easily than can the absolute ratio. This is
because of deficiences in the mass spectrometer itself at the level of accuracy demanded:
drift of parameters, background, mass discrimination etc.
Accordingly the isotope ratio is measured relative to an arbitrary standard and
given as per mille difference in a semiabsolute scale:
834S%o = (RsamPle
Rstandard
- l)X 1000
(7)
where R represents the isotope ratio, 34 Sj32 S. As a primary standard troilite sulfur of
the Canon Diabio meteorite with 34S/ 32 S = 0.0450045 (Macnamara and Thode, 1950;
Jensen and Nakai, 1963) is generally accepted.
In the Eq. (7) the 8 34 S value is calculated by means of absolute isotope ratios.
One can easily see that the 8 34 S value does not change even though the two isotope
ratios are multiplied by the same constant. From this it follows that only a value such as
p
= kR
(8)
proportional to the isotope ratio in question should be measured with the mass
spectrometer. Combining the Eqs. (6) and (8) it can be seen that p in fact corresponds
to the ratio of the output voltages and thus serves as a measure of the isotope ratio in
question. Actua11y p is a more complex function of the isotope ratio and includes the
dispersion caused by oxygen isotopes, background, and drift of the mass spectrometer
parameters, a11 represented by the constant k in Eq. (8).
Measuring technique
By recording the mass spectrometer output signal, y = UB - pUA, repeatedly
(p, y) pairs, y with different p values, are produced. With the aid of them the Po =
UB/DA value can be determined analytica11y.
Since (p, y) pairs have been measured and Po values determined for both the
standard and the sam pie, the calculation of the 8 34S value is straightforward using the
Eq. (7). This approach although not commonly used, has been found by us to be very
simple and flexible compared with many other techniques available.
Because of the strong time dependence of the mass spectrometer parameters
represented by the constant k in Eq. (8) the (p, y) pairs for standard and sampie are
measured successively. Consequently the S02 gases of the standard and the sampie each
in turn are allowed to flow into the analyzer and y values at different va lues of p are "
recorded in the manner shown in Fig. 6.
x
Signal drift is always present due to the change of the contamination degree and
the space charge in the analyzer during the measurement. In calculations a linear drift
is taken into account. Because of this the measurement is continued until the number
of the successive (p, y) pairs with linear drift is 12. The linearity is contro11ed visua11y.
V
14
OUTPUT
PLOTTER
PAPER
000000000000000000000000000000000000000000
-.t
M
11
11
a.. a..
y.O
a.. a..
• • • 11 10 9 8
7
6 5 4
3
Fig. 6. Stages of recording the mass spectrometer output signal y =
1 Y = 0 line
2 The sampie is allowed to flow into
the analyzer
3 stabilizing of the signal ysp for the
sampie
4 ysp at Pt
5 ysp at P2
2
Un -
1
pUA .
6 the sampIe valve is c10sed and the
standard is allowed to flow into the
analyzer
7 stabilizing of the signal yst for the
standard
8 Yst at P3
9 Yst at P4
10 the standard valve is c10sed and
the sampIe is allowed to flow into
the analyzer
The signal drift intro duces a new dimension, the time to the measurements. So the hole
record consists of tree variables p, y and t.
At the start of a measurement and at the end a (y = 0) line is drawn. This zero
point recording requires that no ion current strikes the collectors. The zero points of
amplifiers A and B are also illdividually recorded and eventually corrected. The pressure
for standard and sampIe is compared by alternate opening of the valves in the inlet
system. The pressures are adjusted to the same value by utilizing the output voltage
VA concerned as indicator.
Since the internal differences in the sulfur isotope composition between the
sampIes are of interest, groups of sampIes are linked together in a cycle of measurement
beginning and ending with working standard. Our working standards are linked with
the Canon DiabIo meteorite standard via the Göttingen Laboratory. The scheme of
the cycle measurement is used to conserve the standard material and to keep the time
the S02 gas remains in the storage containers the same for sam pIes as weil as for
standards.
The measurement itSelf is straightforward but sUITounded by a number of
control operations concerning the functioning of the mass spectrometer and the quality
of sam pIes.
15
The mass spectrometer is checked daily for background and leacks of valves in
the inlet system. The development of background is also controlled in the course of
each separate measurement. The tightness of the plastic sockets containing the quartz
tubes with S02 to be measured is checked by using the 40 Ar peak. The sampies and
the working standards prepared in the same way are checked for impurities by measuring the intensity of the 44( 12 CI60160) peak. This has been found by us to provide
a sensitive check on the condition of a sampie .
Ca1culation of
Ö 34S
values
The PO values, Eq. (6), for standard and sam pie are calculated using the
regression equation for the measurement :
(9)
where
forms an observation
(come from) Eq. (5)
Pi and Yj
is (a measure 00 the time of (event for) observation
tj
in relation to the others
(comes from) Eq. (6)
Po
denotes the sam pIe
sp
denotes the standard
st
if the observation refers to the sam pIe
A=l , B=O
if the observation refers to the standard
A=O, B=l
is an arbitrary parameter. The inverse of C being
C
sensitivity E = ßY/ßP
D
is (a measure of the) drift
After collecting the observations to the matrixes as;
(Pj ' Yj, t i )
p = (PI , P2 ' P3 " .. . . . . . . , PN)T
and
r
Al BI
x= I A 2 B 2
l ANBN
the parameter
K = (PO sp ' PO st ' C, D)T
are calculated using the method of least squares
16
(10)
where T denotes the transposition.
Goodness of fit tests are applied to improve the quality of the observations,
and those having a significant deflection from the fitted model are rejected.
The first approximation of the 8 34 S value in question is now calculated with
the aid of the Po values:
(8 34 St 1 ,
-2
"
-3
= ( POst
Posp - 1) x 1000
(11 )
where footnotes -1, -2, and -3 are referred to corrections needed; background
correction, correction of the distortion caused by oxygen and 33 S isotopes as weIl
as correction of the non-zero 8 34S of the laboratory standard used, respectively .
For background correction the Po value in the Eq. (6) can be factored in the
following manner:
(12)
where
SB
denotes the signal of the output voltage UB
SA
denotes the signal of the output voltage UA
TB is the background of the voltage UB
TA is the background of the voltage UA
SB and SA are equal to zero when no gas is allowed to flow into the analyzer.
Provided that the conditions; TBsP = TBst = TB (sampie and standard are measured simultaneously), UAsp = UAst (the situation which is present at the beginning of
each measurement), and TB /SBst "'" TB /UB are valid, the background correction formula:
(13)
where
I} x 1000
can be derived. The effect of the background in the normal case is slight, TB /UB being
less than 0.002. When 834 S remains within the limits of 5 0/00 the correction does not
exceed 0.01 0/00.
As stated before from the construction of the ion collectors it follows that mass
65 in addition to mass 64 falls onto the carbon plate serving as the collector B. Thus
the signal SB is due to masses 64 and 65 the signal SA being d ue only to mass 66. Thus
it can be written;
(14)
17
where
Mi
is the fraction of molecule.§ with mass number i in the S02 gas
is a constant
k
As the abundances of the four stable sulfur isotopes (MacNamara and Thode,
1950) and those of the three stable isotopes of oxygen (Garlick, 1969) are:
32S :
95 .02
33S:
0.75
34 S :
4.21
36 S :
0.02
160 :
99.763
17
18
%
%
%
%
%
0.0375 %
0. 1995 %
0 :
0:
the combinations of the isotopes falling to the masses in question can be given as:
Mass number
Molecule
64
65
32S160170 ,
32S170160,
33S 16 0 16 0
66
34 S16 0 16 0,
32S 16 0 18 0 ,
32 S 18 0 16 0
Now the Eq. (14) can be factored in the following way:
The following notations are introduced in order to simplifity the derivation:
2
17
0 /
16
0
16
18
2 0/ 0
01
02
33S/ 32 S
34 S / 32S
=
SI
R the isotope ratio concerned
Then
1
Po
The
=
k
+ 01 + SI
R + 02
«'j 34SL2, _3 in the Eq. (13) can now be written as:
(15)
18
(16)
( P?st - 1) x 1000
POsp
Provided that the conditions; Olst = Olsp, 02st = 02sp (which means that oxygen
of the standard and the sample comes form the same source) and 33S/ 32 S = Sl
remains constant, are valid the Eq. (16) reduces to :
where
The isotope ratio Rst for the standard used is calculated utilizing its ö 34 S value as:
R st = ·'t:D
R_ (1
+
W· 3ö 34 Sst )
where ReD denotes the 34 SI 32 S of sulfur from troilite of the Canyon Diablo iron
meteorite as 0.0450045 (Macnamara , Jensen and Thode, 1963).
The influence of 33S/ 32 S = Sl (regarded as constant) is studied by differentiating
Eq. (16) with respect to the S lsp -term. It has been found that if ö 33 S stays within the
limits of 25 0 / 00 the error does not exceed 0.2 0 /00 .
The final correction formula for the oxygen and 33S isotope affection is thus
given as:
(17)
The samples are linked together in a cycle of measurements beginning and
ending with ~orking standard. The ö 34 S value of the working standards usually deviates
from the zero value of the Canyon Diablo meteorite standard. In order to relate the
ö 34 S values of the samples to the world wide accepted Canyon Diablo meteorite
standard a non-zero correction must be introduced. Since it can be proved that:
+
103
R
sp
- R
~t
st
+
19
10 3
~t
- ReD
ReD
where Rsp is
ReD
denotes the 34 S/34 S of the sampIe
denotes the 34 S/ 32 S of the Canyon DiabIo meteorite standard
Rst
denotes the 34 S/32 S of the working standard
then
where the subscripts show the bearing of the Ö 34S in question .
Thereafter the formula for the correction of the nonzero working standard can
be derived as:
(18)
Combining aH the correction formulae, Eq. (13), Eq. (17) and Eq. (18) the
final Ö 34S for the sampIe is calculated using the equation:
where ReD
202
= 0.0450045 and
= 2 18 0/ 16 0
=
0.0039995
by substitution it can be written:
tI 9)
In practice it is adventageous to re cord the images of the mass spectrometer
output signals, y values at different p, graphicaHy as the visible analog output can
easily be checked visuaHy.
The human contral is retained also in digitalization of the mass spectrometer
output signals. This is done using an online device combined via a teletype to a time
sharing computer (Fig. 7a, b). The coordinates of the images as weH as those of the
y = 0 line are picked up with the device (a co ordinate analyzer) and the digits are
trasferred to the computer. Additional data, ego readings of the p value, the series
of figures needed for the identification of the sampIe, as weH as TB and VB measurements of the background effect, are fed to the computer via the teletype. After
computation of a cycle of measurements beginning and ending with working standards,
the tele type lists the final ö 34S values of the sam pIes.
20
_.,
I
;,
" I
~
Fig. 7a. Set-up of apparatuses for digitalization of the mass spectrometer output signals. Photo Erkki
Halme.
Fig. 7b. The online device for picking up the coordinate digits of the signal images. Photo Erkki
Halme.
21
ACCURACY OF 834 S VA LUES
Since the working standards must be prepared in the same way as the sam pIes
proper no absolutely correct reference is avaiIable. Thus, the question of the accuracy
of the 8 34 S values produced cannot be answered exactly.
Concerning the errors in preparation the difficulties arise from the fact a known
sampIe dose can in reality be prepared only once. Therefore the differences between
the 8 34 S values of the repeated measurements f~r the same sam pIe can be caused
either by imhomogenity of the sam pIe material or by errors in preparation. According
to our experiments inhomogenity of the sampIe material is the main reason for the
repetition differences. The standard deviation of the 8 34 S values for the repeated
sulfur isotope analyses in one of the deposits studied is 0.08 %0, whiIe that in another
exceeds 0.18 %0. In contrast if synthetic ZnS is prepared by mixing a ninefold dose
of sulfide already purified for volatiIe impurities with a ninefold dose of the oxidizing
agent treated also for volatiles and the whole is then shared into nine parts which
subsequently are handled conventionally, their 8 34S values show a standard deviation
of 0.03 %0. In addition in proving the inhomogenities of the sampie material to be
the main cause of the repetition difference the experiment points out the crucial
nature of the mixing of MeS - V20S pairs in mortars.
Apart of the error in measuring can be studied simply by allowing the same
S02 gas to expand into both inlet lines and then measuring the difference of the mass
spectrometer output signals for both sides. The difference given as 8 34 S is found to
be of the order of 0.03 %0. On the other hand the systematic errors, as could be seen
from the correction formulas Eq. (13) and Eq. (17), tend to be proportional to the
Stanton, R.L. (1960) The application of sulfur isotope studies in ore genesis theory a suggested model. N. Z. J. Geol. Geophys. 3, 3, 375 ... 380.
Tatsumi, T. (1965) Sulfur isotopic fractionation between coexisting sulfide minerals
from some J apanese ore deposits. Econ. Geol. 60, 1645 ... 1659.
8 3~S difference. As previously stated their influence can, providing that certain
conditions are valid be decisively diminished.
The statistical upper accuracy limit of the 8 34 S produced with the system
desceibed is thus ± 0.03 %0 . One sees that the lower limit is dependent on several
factors of which some are bound up with the sampIe material. An estimate for
accuracy based on some 1500 sulfur isotope analyses hitherto done at the Laboratory of Economic Geology, Helsinki University of Technology can be given as
± 0. 1 °/00.
Manuscript received December 30, 1976
22
ACKNOWLEDGEMENTS
Professor Aimo K. Mikkola, the Head of the Laboratory of Economic Geology
at the Helsinki University of Technology, drew initially the senior author's attention to
sulfur isotope geochemistry as tool in ore genetical studies. His continued interest and
support have been decisive ever since.
The senior author is in person deeply aware of the fundamen tal importance of
training period at the Isotope Laboratory of the University of Göttingen for the establishment of a sulfur isotope laboratory in Finland. He is indebted to Professor K.H. Wedepohl, the Head of the Geochemical Institute of the University of Göttingen, for giving
him the opportunity to work in his Institute. The senior author is extremely grateful
to his friend Dr. Heimo Hielsen, the Head of the Isotope Laboratory of the same Institute, for introducing hirn into the basic principles of the mass spectrometry and isotope
geochemistry. Dr. Jochen Hoefs and the personnel of the Göttingen Laboratory are
gratefully acknowledged for the istructive cooperation.
23
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