Radiant energy – Ionic separation or analysis
Reexamination Certificate
2002-08-12
2003-05-20
Lee, John R. (Department: 2881)
Radiant energy
Ionic separation or analysis
C250S288000, C250S292000, C250S287000, C250S291000, C250S290000
Reexamination Certificate
active
06566651
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and apparatus therefor, for an improved laser-cooling fluorescence spectrometry as one type of ion trapping mass spectrometry.
Ion trapping mass spectrometry is a widely used method for trace analysis in environmental analysis and other fields. In the most typically utilized method, ions trapped in an ion trap are subjected to mass selection and extracted outside the trap, and these extracted ions are detected with an ion detector such as an electron multiplier. This type of mass spectrometry is still the most widely used method, and a large number of reference works are available (such as R. March et al., “Quadrupole Storage Mass Spectrometry”, Chemical Analysis: A Series of Monographs on Analytical Chemistry and Its Applications, Vol. 102, 1989, pp. xiii-xx (Contents), ISSN 0069-2883, John Wiley & Sons).
Laser-cooled fluorescence mass spectrometry relating to this invention is found in a method disclosed in 1995 (U.S. Pat. No. 5,679,950) as a novel ion trap mass spectrometry. In this method, laser-cooling and sympathetic cooling are utilized, and sensitivity, which has been limited to detection levels of about 100 ions in the ion trapping mass spectrometry of the prior art, is significantly improved by optical detection of the cooled sample ions so that even single ions can be detected in situ. In this method, the sample ions are trapped in the ion trap after mass analysis and can be measured repeatedly to detect ions in an “in-situ” (or non-destructive) manner.
SUMMARY OF THE INVENTION
The principle of laser-cooled fluorescence mass spectrometry is briefly described below.
Ions trapped in a radio-frequency-quadrupole ion trap possess a harmonic oscillation mode due to the potential of the ion trap (in a pseudopotential approximation). The oscillation of these trapped ions is known as secular (oscillation) motion. The frequencies of these secular oscillations are proportional to the charge of the ions and are inversely proportional to the mass of the ions. If the secular frequency can be detected, then the mass spectrometry of the ions can be performed (H. Dehmelt, “Radiofrequency Spectroscopy of Stored Ions I: Storage”, Advances in Atomic and Molecular Physics, Vol. 3, 1967, pp. 53-72, ISSN 0065-2199, Academic Press).
Firstly, the laser-cooled ions and sample ions are simultaneously trapped and cooled. The sample ions are sympathetically cooled by the laser-cooled ions. Next, an electrical oscillation is applied to make the sample ions resonate at the secular frequency, so that the sample ions are heated by the resonant oscillation. The sample ions are repeatedly undergoing Coulomb collision with the laser-cooled ions so that the sample ions transfer energy to the laser-cooled ions and the laser-cooled ions are sympathetically heated.
Increase of the temperature of the laser-cooled ions results in such effects as change of the fluorescence intensity, and change in the spatial distribution of the fluorescence. These changes give information on the mass and the amount of the sample ions.
The mechanism of fluorescence intensity change in the laser-cooled fluorescence mass spectrometry can be understood by the following brief theoretical analysis.
First, using a simplified theory of Doppler laser-cooling, the relation of laser cooling efficiency and fluorescence intensity is calculated with respect to ion temperature.
When a laser beam with a fixed wave vector irradiates free atoms (or ions), a force acts on the atoms in the direction of the wave vector of the light due to photon scattering. Laser cooling is performed using this force. Typically, an atom having a simple two-level transition is chosen to avoid optical pumping, and the laser wavelength is adjusted to the resonance transition of the two-level atom species.
When the momentum of the atom is counter to the direction of the wave vector of the laser light, the velocity of the atoms decreases after resonant absorption of photons due to momentum transfer of photons to the atom. Conversely, when the momentum of the atom is in the same direction of the wave vector of the laser light, then the velocity increases.
When the detuning frequency is negative, in other words, when the light has a wavelength slightly longer than the resonant wavelength, the probability of resonance scattering increases when the atoms are traveling counter to the direction of laser light wave vector, compared to when traveling in the same direction to the light wave vector due to the Doppler effect. In this case, consequently, the energy of the atom is lost and the atom cools down.
Spatially-uniform natural emission following absorption results in a random-walk increase of energy, whose balance with the cooling effect determines the ultimate attainable temperature h &Ggr;/2, where &Ggr; is the natural linewidth of the transition. As shown below, the average energy change due to resonant absorption is h&Dgr;&ngr;, where &Dgr;&ngr; is the detuning frequency, which is the deviation of the laser light frequency from the resonant frequency &ngr;
0
of the atoms at rest. When |h&Dgr;&ngr;| is greater than h&Ggr;/2, we can neglect the random-walk heating by natural emission, which is the case considered below to simplify discussion.
An average energy change h&Dgr;&ngr; due to resonance absorption of photons can be explained by simple kinetics. The wave vector direction of the laser light is set as the z axis. The velocity component along the z axis of the atom with mass m is defined as v
z
in the laboratory frame. In the center of mass system, the frequency of the light shifts &ngr;(1−v
z
/C) due to the Doppler effect. When &ngr; (1−v
z
/C) matches the resonant frequency &ngr;
0
, a resonant scattering occurs. The atom obtains momentum h&ngr;
0
/c from the laser light when the atom absorbs the light. Next, the atom emits photon by spontaneous emission process. This photon emission is uniform in all directions so net average change of the atom momentum does not occur. Consequently, the atom obtains momentum h&ngr;
0
/C by the resonant scattering of the laser light.
The velocity after the resonant scattering becomes v
z
′=V
z
+h&ngr;
0
/mc when observed in the laboratory frame. The change &Dgr;E of kinetic energy is
Δ
E
=
m
v
z
′
2
2
-
m
v
z
2
2
=
h
Δ
v
(
1
+
h
Δ
v
4
E
)
≅
h
Δ
v
(Equation 1)
When the atom is in the laser beam, the rate of resonant scattering per unit time is given by
sr
(
v
z
,
I
)
=
Γ
Ω
rabi
2
(
v
-
v
0
-
vv
z
c
)
2
+
Γ
2
+
Ω
rabi
2
(Equation 2)
Here, &Ggr; is the natural width of the transition. &OHgr;
Rabi
is the Rabi frequency, which depends on the light intensity.
Using these relations, an atom with velocity v
z
attains an energy change per unit time &Dgr;E
scatt
due to the resonant scattering;
(
Δ
E
scatt
Δ
t
)
(
v
z
,
I
,
Δ
v
)
=
h
Δ
v
sign
(
v
z
)
sr
(
v
z
,
I
)
(Equation 3)
Here, sign (V
z
) is a symbol that indicates the direction of the motion of the atom relative to the laser beam. It is negative when the directions of the atom and light beam are opposite, in which case the atom slows down. It is positive when the directions are the same, in which case the atom accelerates.
Next, the average energy and fluorescence intensity is calculated when laser-cooling is applied to the ions. Here, it is assumed that the ions are in a gaseous state (not in a Wigner-crystal state). The velocity distribution n (v
x
, v
y
, v
z
) can be written as a Maxwell distribution with an ion temperature T;
n
(
v
x
,
v
y
,
v
z
)
=
N
(
m
2
π
kT
Baba Takashi
Waki Izumi
Wang Dongbing
Hitachi , Ltd.
Lee John R.
Vanore David A.
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