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STUDIES ON LINEARITY IN AN INDIGENOUSLY DEVELOPED
REFLECTRON TIME-OF-FLIGHT MASS SPECTROMETER FOR
ATOMIC SPECTROSCOPY
R.C. Das, M.L. Shah, PP.K.
.K. Mandal, D.R. Rathod, V
as Dev
Vas
Dev,,
K.G. Manohar and B.M. Suri
Laser and Plasma Technology Division
This paper was adjudged the 2 nd Best Poster Paper in the
Instrumentation Session at the 11 th Indian Society for Mass
Spectrometry
Spectrometry-- Triennial International Conference on Mass Spectrometry
(ISMASTRICON-2009) held at Hyderabad, during Nov
(ISMAS-TRICON-2009)
Nov.. 24-28, 2009
Abstract
We have indigenously designed and fabricated a high temperature effusive atomic beam source based reflectron timeof-flight mass spectrometer (RTOF-MS). This atomic beam source can be operated at high temperatures of about 17002000 0C and is useful to generate dense vapours of most of the elements. The mass spectrometer can be operated in
linear mode as well as in reflectron mode. The resolution in linear mode for U I is >1200 and that of in reflectron mode
is >3200. The linearity of Sm I and U I photoionization (PI) signals were investigated. The non saturation or linearity of
PI signals is important from atomic number density, atomic transition absolute cross section measurements and instrument
dynamic range points of view.
Keywords: Time-of-flight, atomic beam, linearity, resolution and photoionization.
Introduction
A time-of-flight mass spectrometer (TOF-MS) is a valuable
tool for mass analysis. The important features of this mass
spectrometer are its ability to simultaneously determine
all the ionic masses with any mass-to-charge (m/q) ratio,
excellent speed (about few μ/s) of analysis and unlimited
mass range. Another advantage of this mass spectrometer
is its accuracy that depends on the electronic circuits rather
than on the mechanical alignment, that makes the
construction simple and easy. Wiley and McLaren in 1955
developed a two stage acceleration linear TOF-MS with
improved resolution. The most important advancement
in the form of reflectron TOF-MS (RTOF-MS) was
introduced by Mamyrin et al. in 1973.
A RTOF-MS having high temperature effusive atomic beam
source was indigenously developed in our laboratory. This
atomic beam source can be operated at high temperatures
of about 1700-2000 0C with uranium solid samples and
is useful to generate dense vapours of most of the
elements. The sample preparation and oven heating
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process was optimized with several experiments. The
linearity of U I photoionization (PI) signal variation with
detector bias was investigated. The non saturation or
linearity of PI signal with detector bias is important from
atomic number density, atomic transition absolute cross
section measurements and instrument dynamic range
point of view. The important parameters of the linearity
are ion density which is closely related to atomic density
and laser intensity and detector bias. Large ion density
will result in space charge related effects1 while high
detector bias will saturate the more abundant signal. So
the deviation in isotopic ratios will be resulted. In RTOFMS the linearity of square root of isotopic masses with
time-of-flight of Sm I was also obtained which can be
further used as a calibrated mass axis like other (quadrupole
or magnetic sector) mass spectrometers.
Experimental
The schematic diagram of the indigenously developed
RTOF-MS based experimental set up is shown in Fig. 1.
The mass spectrometer consists of an orthogonal atomic
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beam oven, a dual stage acceleration region (between
The SC three photon PI signal variation with detector bias
repeller, extractor and first Einzel lens plates), an Einzel
lens, a pair of steering plates, a single stage reflectron
for U I at fixed ion density was carried out to investigate
the linearity and dynamic range of the mass spectrometer.
and micro channel plate (MCP) detectors as described
in1. The Sm I was used in the oven as sample and the
Results and Discussion
oven was resistively heated to generate atomic beam. The
atomic beam reaching the interaction region located
between repeller and extractor plates was ionized by an
Nd: YAG pumped dye laser (DL). The single colour (SC)
signal produced by three step PI were detected by the
MCP detector, amplified by an one GHz fast amplifier,
displayed on a 500 MHz digital oscilloscope and recorded
by data acquisition. The laser was tuned to strong
transitions of Sm I at energy 17880.50 cm -1 that
simultaneously originates from 7F2 and 7F4 sub levels of
ground state as reported in2. Sm I was chosen for initial
experiment because of its high vapour pressure and natural
occurrence of large (7) number of its isotopes with
abundances (%): 144Sm (3.07), 147Sm (14.99), 148Sm
(11.24), 149Sm (13.82), 150Sm (7.38), 152Sm (26.75) and
154
Sm (22.75) as given in3. The atomic number density
and the laser intensity were optimized to avoid space charge
and saturation effects. The high temperature capability of
the oven and usefulness for most of the species was
established by generating U I atomic beam at temperatures
about 2000 0C.
The Fig. 2 shows SC three photon PI signal of Sm I in
RTOF-MS and was observed at -1.35 kV. The abundance
ratios of peaks match to that of reported one with deviation
<10% and that for 149SmH”15%. The possible reasons of
deviations like the space charge and saturation effects
were eliminated. But, the other reasons for mismatch in
isotopic ratios like isotope shift (IS), the hyperfine structure
(HFS) splitting and coherence effect of magnetic sub levels
(M) of intermediate states for polarized light4,5 were not
taken care off. The use of high power intense (108-109 W
cm-2) broadband (1.5 G Hz) laser eliminates the effect of
IS in SC ionization by power broadening. The isotope shifts
of Sm I for few transitions in visible region are 0.2 to 0.5
GHz and 0.2 to 1.0 GHz for pairs (148,149) and (147,148)
respectively6. The 149Sm with nuclear spin (I) 7/2 shows
hyperfine splitting of energy levels hence the interference
with the nearby Sm I isotopic peaks of 148 and 150 are
possible. The dipole transitions for linearly polarized lasers
are limited to selection rules ΔM=0, but they are forbidden
if ΔJ (angular momenta)=0, ΔM=0 and M=0
simultaneously. As such, there will be loss (non excitation)
of population for J!J
transition for even
isotopes, but this
does not affect the
excitation of odd
isotopes
(Sm147,149), since the
grand total angular
momenta F = I+J
becomes a good
quantum number.
These two factors
HFS and polarization
of
laser
are
Fig. 1: Schematic of RTOF-MS based experimental set up. Here, PD: photodiode,
FPE: fabry perot etalon, HCDL: hollow cathode discharge lamp and Accl.:
Acceleration
October 2010
important
discrepancy
for
in
isotopic abundance
ratios.
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Fig. 4: Sm I SC signal vs. laser power
power..
Fig. 2: SC PI signal of Sm I and its oxides with
observed relative percentages
The linearity of square root of masses (m) of Sm I isotopes
and their oxides with respect to total time of flight (TOF)
at fixed ion density and detector bias was investigated. In
Fig. 3 linear fit to the experimental data was made and
using this equation, the isotopic masses were calculated.
The observed deviations in masses from actual masses7
were in the range ± 0.1%.
The Fig. 4 show power dependent SC signals of Sm I,
that shows lack of saturation for temperature about 1400
K with number density about 6.9x10 atoms/cc. Linearity
11
of SC three photon PI signals (Fig. 5) with detector voltage
at fixed atomic number density of U I was also studied.
The abundance ratio of 238U to 235U of 156 was measured
at detector bias -1.6 kV, oven temperature 1682 0C (1.06
x107 atoms/cc) and laser intensity 566 mW cm-2 of 591.5
nm resonant atomic transition.
The detector bias was reduced and the PI signals were
recorded down to -1.35 kV. At low bias -1.3 kV, the 235U
signal gets buried in noise, but the 238U signal reduces
linearly. This also proves the signal linearity for both 235
and 238 signals at low detector voltages about -1.35 kV.
The abundance ratio 156 is associated with error 11%
that is attributed to the non-excitation of all hyperfine
Fig. 3: Linearity of the observed Sm I and U I
TOF signals in RTOF-MS
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Fig. 5: U I SC signal vs. detector bias
components of 235U in the SC scheme used. The typical U
I TOF signals in linear and reflectron modes of operation
are shown in Figs. 6 and 7 respectively.
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laser intensity was obtained. The variation of U I SC three
photon PI signals with detector bias at fixed ion density
showed no saturation. These observations emphasize the
Conclusions
suitability of the system for absolute cross section
measurements and other spectroscopic applications.
An atomic beam source based RTOF-MS for studies of
atomic physics have been developed. This oven can
Acknowledgements
operate at temperatures in excess of 2000 0C using U I
and hence can be used for most of the samples. The
linearity of mass with respect to TOF was obtained. The
conversion TOF spectrum with time axis to mass axis like
other instruments is feasible. The optimization of SC three
photon PI signal of Sm I with respect to detector bias and
The authors sincerely thank Dr. L. M. Gantayet, Dir., BTD
Group and Dr. A. K. Das, Head, L&PTD for facilitating the
development of the experimental set up. We thank Head,
ED and his colleagues for their make fast amplifier; Head,
CDM and his colleagues for precision fabrication of RTOFMS components. We are also thankful to Dr. S. K.
Aggarwal, Head, FCD for important scientific discussions.
References
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Fig. 7: TTwo
wo colour three photon PI signal of
U I and its oxide in RTOF-MS
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