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STUDIA UNIVERSITATIS BABES-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
ION OPTICAL SOLUTIONS INVOLVED IN FORENSIC MASS
SPECTROMETRY
D. Ioanoviciu1, C. Cuna1, A. Pamula1, I. Albert1, T. Neda2
1
National Institute for Research-Development of
Isotopic and Molecular Technologies Cluj-Napoca
2
Sapientia University Cluj-Napoca
ABSTRACT. The ion optical solutions to obtain a major detection limit
increase for explosive detectors is discussed. An ion source having high
sensitivity and focusing transversally will be combined with a very
efficient time-of-flight analyzer using cylindrical electrostatic mirror. A
limit of detection in the range of p. p. t. is expected for TNT and RDX.
1.
Introduction
Explosive detection is a major branch of the Forensic Science. A big number
of reviews and books deal with the methods of the forensic science. All those
include research directed to refine explosive detection methods in the present
context of the increased importance dedicated to fight terrorism. Reviews as:
Journal of Forensic Sciences, The American Journal of Forensic Medicine and
Pathology, Forensic Science International, Forensic Science Review, J. of Forensic
Science Society, Canadian Society of Forensic Science are may be the most
known, with the highest impact factor, to mention only a few of those publishing
articles belonging to forensic science. We cannot omit some fundamental works as
Forensic Science Handbook edited by Richard Saferstein, representative person for
this field, Analytical Methods in Forensic Chemistry by Ho (Harwood editions
1990).
The explosive detection is the field of application of various methods
but the obtained results did not offer an ultimate solution to the problem at a
satisfactory level. Absorption atomic spectrophotometry has been used to
detect antimony and barium from the residuum of gun shots [1] as well as
the inductively coupled plasma in an atomic emission spectrometer for the
determination of barium [2]. High-resolution liquid chromatography was
used to analyze the difeniamine traces from firing weapons [3]. Neutron
activation served to study bullets [4], [5]. Nitroglycerine derived explosives
were analyzed by gas chromatography coupled with a mass spectrometer
[6]. Magnetic nuclear resonance was also used with this purpose [7]. Raman
spectra were obtained from explosive particles [8]. Explosive traces were
detected in glow discharge connected to a quadrupole trap [9]. The detection
of trinitrotoluene was reported with a glow descharge ion source coupled to a
quadrupole-time-of-flight tandem mass spectrometer [10]. The same ion source
type was bound to a model MAT TSQ-700 triple quadrupole mass spectrometer to
D. IOANOVICIU, C. CUNA, A. PAMULA, I. ALBERT, T. NEDA
identify explosives [11]. In this way the spectra of TNT, PETN and RDV were
obtained [12]. The research in this project is directed to construct and test an
explosive detector of high sensitivity allowing to eliminate from the luggage flux
of those containing hidden bombs and to impede in this way of terrorist attentates
and the smugling of explosives. The detector will be designed to quickly examinate
explosion residues, giving hints to guide the forensic investigations. Such detectors
are useful for check points at the border, as well as all those which are investigating
crimes and events where firearms were used.
2.
General description of the new principle
The final goal of this research is to realize a functional model of an
ultrasensitive explosive detector having superior performances compared to other
existing detectors based on mass spectrometric analyzers. The ultrasensitive
detector will include in a compact assembly the ion source, the electrostatic mirror
and the ion detector. A sensitivity increase of several magnitude orders is expected
by combining the following procedures:
Fig. 1
The ion source will be so designed that the ions will be generated from a
variable thickness region, located between spherical, cylindrical or plane surfaces.
The ion extraction from the region of generation will allow their focusing in
transversal direction at the ion detector site. Because the explosive substance
molecules have a high electron attachment cross-section inside the ion source an
electron retarding system will be accommodated by using multiple ionizing
filaments. The electrons will be slowed down during the ionizing time lapse to ease
the electron attachment process. The ionization in this regime will be maintained
during the ion accumulation, its duration being established by tests.
ION OPTICAL SOLUTIONS INVOLVED IN FORENSIC MASS SPECTROMETRY
Fig. 2
The ion production time lapse will be followed by their extraction. The
voltages ensuring the ion extraction will be correlated with the static voltages
applied to the cylindric or spherical electrodes of the electrostatic mirror in such a
way that the longitudinal (time) focusing be accomplished simultaneously with the
transversal (space) one. An additional sensitivity increase will be added by this
focusing procedure. The use of multichannel plates with spherical surfaces will be
also considered with the same purpose.
The mass analyzer will be selected based on an in depth analysis of the ion
optical properties of various configurations which could include electrostatic
mirrors with one and two stages, with homogeneous field or with inhomogeneous
fields of parabolic or exponential type as well as the possibilities to use linear
analyzers with or without post source focusing.
The basic parameters of the electronic units are function of the mass analyzer
solution adopted.
The ion detection will be performed with channel multipliers or with electron
multipliers specially designed to have short time response. Most probably a double
channel plate chevron system will be adopted. The time response should be in the
range of picoseconds. The accumulation of the mass spectra will be obtained on a
personal computer by an interface transient integrator plate. A vacuum system will
be evacuated by turbomolecular pumps started by mechanical vacuum pumps, the
pressure inside the vessel being measured by vacuum gauges.
For the ultrasensitive explosive detector a sensitivity level in the p. p. t. range
is expected for trinitrotoluene (TNT) and cyclotrimethylene trinitramine (RDX).
3.
Ion optical solutions
The ion optical solution will be the result of comparison of various versions
of time of flight analyzers with electrostatic mirror operating the ion source in
impulse regime. The ion optical study of the ion motion inside the fields is based
D. IOANOVICIU, C. CUNA, A. PAMULA, I. ALBERT, T. NEDA
on the analysis of the trajectories inside the homogeneous, quadrupole,
exponential, spherical and cylindrical fields [13]. Besides the methods of ion path
simulation on computer, the matrix language will be also used by including a line
and a column with the time dependent elements. By this last procedure the
instrumental properties result from the multiplication of matrices expressing the
properties of various ion path portions.
This can be done also by computer programming. To describe ion motion in
accelerating and decelerating electric fields we start with the formula expressing
the flight time as function of the ion energy at the ion entry and exit inside and
from the applied field.
4.
Linear TOF
In a mass spectrometer with ion source operated with continuous extracting and
accelerating voltages position time focusing is obtained, called often “space
focusing”. The flight time differences due to different points of formation are
suppressed by using a field free space of D length, after the ions leave the source.
This distance results from the ion source geometric and operating parameters.
D = 22[da – db/(1 + )]
Where da and db are the distance over that the extraction and acceleration takes
place respectively,  the ratio between the reference ion velocity at the source exit
and when leaving the extraction region, ratio set by the applied electric fields.
The resolution of a time-of-flight mass spectrometer with ion optics focusing at
first order is given by the formula:
 = D /(to + tv + t2)
where D = t/2 is the temporal coefficient of mass dispersion, t the total flight
time of the reference ion from formation to detection, tv the “turn around” time of
an ion starting against the extracting field, t2 is the temporal aberration resulted
from the incomplete compensation of the initial position effects.. The detailed
expressions of the above enumerated quantities are:
D = [da(1 + 2) + db(1 - )/(1 + )]
tv = -22davo/vs2, t2 = [da(3 - 2) + db( - 1)( + 2)/(1+)]/(4vs)2
Here vs is the velocity of the reference ion in the field free space, vo the initial ion
velocity, resulted from the thermal molecular motion before ionization,  the
relative energy difference of the ions formed at different initial sites.
ION OPTICAL SOLUTIONS INVOLVED IN FORENSIC MASS SPECTROMETRY
5.
Homogeneous field reflectrons
The time-of-flight analyzers having electrostatic mirrors allow energy
focusing of the ions of the packets, of first or second order, depending upon the
number of used homogeneous field stages.
The time-of-flight mass spectrometer configuration assembled from an ion source
with a single accelerating field and an analyzer with mirror with two stages needs a
field free space of L length to be included in the ion path.
L = 2[22d1 + (2 – 1)ds]/(2 – 3)
The involved quantities are: d1 the depth of the mirror’s first stage
(decelerating), ds the depth of the extraction-accelerating space in the ion source
and  is the ratio between the reference ion velocity on the field free space and that
after the deceleration inside the first stage of the mirror. To find out the resolution
of the two stage mirror time of flight mass spectrometer in the formula we
substitute for D by the expression:
D  = L(1 – 1/2)/vs
Because the second order aberration disappears, we have to account for that of
third order t3 by a term of this order in the relative energy spread of the ions from
the packet p:
t3 = (2 – 1)Lp3/(8vs)
If for the ion packet energy focusing is used a single stage mirrors the temporal
focusing condition of first order is satisfied if:
L = 2(ds + 2d1)
The ion flight time being:
t = 2L/vs
The resolution formula contains in this case the second order
aberration:
t2 = p2(ds/2 + d1)/vs
the third order aberration being neglected.
D. IOANOVICIU, C. CUNA, A. PAMULA, I. ALBERT, T. NEDA
6.
Delayed extraction
The temporal focusing of the ions with respect to their initial velocities is
accomplished by applying the extraction of the ions delayed with respect to the
instant of their formation. This focusing procedure must be used for the detection
of ions result from matrix assisted laser desorption/ ionization. For a simple
geometry, composed from an ion source with two homogeneous electric fields, the
extraction one established after some delay with respect to the ionization instant,
and from a field free space, the delay  results from the formula:
 = 2da4/{vs[D/2 + 2(db/1+  - da)]}
The focusing is accomplished exactly only for ions of a given mass,
approximately for the neighbouring masses. This because  depends of the
reference ion velocity.
The presence of the term in vo2 severely limits the mass resolution accounting for
the initial velocity distribution widths resulted for ions generated by matrix
assisted laser desorption/ ionization. In the literature values between 450 m/s and
750 m/s were reported.
An original ion optical solution is that combining a hyperbolic electrode
ion source and a field free space [14]. The electrodes must be fed by high voltage
pulses which will be performed by high voltage commuters as those offered by
the company Behlke, Germany. This configuration insures complete initial
velocity focusing for the ions created from the tip of the end cap ion source
electrode.
During Tc the high voltage pulse the potential  inside the ion source is that of a
Paul ion trap:
 = o(z2 – r2/2)/zo2
where o is the height of the applied pulse, 2zo the distance between the end
electrodes. The perfect velocity focusing condition is satisfied for a field free
space of length L:
L = zo/[ sin(Tc) – cos( Tc)]
With the parameter  defined by the relation:
 = (2eo/m)1/2/zo
For the ions emitted from an area described for a circle of radius r on the
electrode tip, the length of the packet at the detector site will be:
td = [r2 L/(r2 + 4zo2) + L tovo + vo r2/(4zo2  sin(Tc))]/[zo sin( Tc)]
ION OPTICAL SOLUTIONS INVOLVED IN FORENSIC MASS SPECTROMETRY
The mass time dispersion coefficient of this geometry takes the form:
D = [Tc + sin(Tc)/ + Tc /sin( Tc)]L/(2zo)
7.
Electrostatic condenser TOF
To select the ion-optical solution the properties of the electrostatic
condensers must be also accounted for. The properties of the toroidal condensers
are given by the transfer matrix elements. The temporary matrix elements are
given and only those of first order.
(t/x) = 2 sin[e(2 -)1/2]/[vs(2 - )1/2] , (t/) = 2{1 – cos[ e(2 - )] Re/[vs(2 - )]
(t/ ) = Ree/(2vs) , (t/ ) = {2[ e - sine(2 - )1/2/(2 - )1/2]/(2 - ) - e/2}Re/vs
These elements form the last line of the first order transfer matrix of the toroidal
condenser.
x   x/x x/ x/
  /x / /
  =  0
0
1
   0
0
0
 t   (t/x) (t/) (t/)
8.
x/
/
0
1
(t/)
0
0
0
0
1





Cylindrical mirror TOF
The study of the electrostatic mirrors with cylindrical electrodes indicates
the possibility to obtain second order position temporal focusing with a single
grid. In this way a better sensitivity can be obtained by maintaining high
resolution.
To estimate the resolution the ion packet length at the detector t was
calculated:
T  b   2  b   3
for the first order focusing case with respect to the energy spread .
We distinguish two classes of configurations. One of them represents those
geometry's for rb, the radius where the reference ion stops inside the electrostatic
field, is greater than the radius r0 of the grid which limits the mirror.
Then:
 


b  a 2 ro a 2  1 / 2  2a 5 rb I c  3 L f 8 v
D. IOANOVICIU, C. CUNA, A. PAMULA, I. ALBERT, T. NEDA
With:
a 2  ln rb / ro 
a
I c   exp(  x 2 )d x
si
L f  4a 2 (ro  2arb I c )
o
the last relationship being the consequence of the first order energy focusing
condition. The configurations for rb>ro do not offer the possibility of second order
focusing, therefore the coefficient b calculation was omitted.
For the geometries with rb<ro, when the grid convexity is directed towards
the ion source the following expressions result for the temporal aberration
coefficients:



 v
b  3L f / 8  b2 2b3rb I d  rb b2  1/ 2
and


 b2  
b2 3 


b   5L f / 16    ro  b 4     2b5rb I d   v
2 4

 3  



where:
b 2   ln rb ro  ,
I d   exp( x 2 )dx ,
L f  4b 2 (ro  2brb I d ) .
A numerical exploration of the two aberration coefficients values allowed to
cancel the first for rb/ro=0,7013 when Lf/r0=0,6188. In these conditions also the
third order aberration coefficient is very small:
b= -0,0775 ro/v
9.
Conclusions
The ultrasensitive explosive detector will make an important step on the way
of refining the means to keep under control the traffic through the check points on
highways as well as through airports. By improving the detection limit of the
explosives it will help to better fight terrorism and to speed up forensic
investigation procedures.
ION OPTICAL SOLUTIONS INVOLVED IN FORENSIC MASS SPECTROMETRY
10.
References
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