Improving the Signal Strength and Detection Limits …
311
Ahmed et al. demonstrated that the laser wavelength also significantly affects the
signal enhancement apart from the different geometrical configurations [23]. With an
appropriate laser wavelength combination, inter-pulse delay, and energy, they have
observed a 30 times intensity enhancement of neutral iron lines in DP LIBS compared
to SP LIBS. However, the major limitations of these methods are the experimental
complexity and high cost due to the usage of two different lasers.
1.2 Spatial Confinement LIBS
Another promising method to improve the LIBS spectral intensity is the spatial
confinement of plasma. Compared to DP LIBS, spatial confinement is a simple and
cost-effective method to improve the sensitivity and detection limit of the LIBS technique, since it requires minimum physical modification of the conventional LIBS
system. During the plasma generation and expansion, shockwaves are also produced. The spatial confinement effect originates once the laser-produced shockwaves
encounter an obstacle and are reflected back to the central region of the plasma. The
reflected shockwave can deliver additional energy to the plasma plume and enhance
the spectral emission intensity [25, 26]. Commonly used obstacles to confine the
plasma plumes are parallel plates, hemispherical, cylindrical, and rectangular cavity.
In recent years, spatial confinement in plasmas has been studied in detail by various
groups. Guo et al. used a hemispherical cavity to confine the plasma and observed
that the accuracy of the quantitative analysis of low concentration elements in steel
samples is improved with spatial confinement [27]. Popov et al. used a miniaturized
chamber to confine the plasma and observed spectral intensity enhancement up to
10 times for iron in metallic samples and 3 to 5 times for arsenic in soils [28]. X.
Wang et al. investigated the spatial confinement effect on a femtosecond laser pulse
produced Cu plasma using time-resolved spectroscopy [29]. They have observed
enhancement of the Cu atomic line intensities when the femtosecond Cu plasma is
confined in a cylindrical cavity. The enhancement ratio depends on the cavity diameter and the atomic emission line selected. On the other hand, the cavity height did
not significantly affect the signal strength.
1.3 Magnetic Confinement LIBS
Similar to spatial confinement, another method for improving LIBS sensitivity is
magnetic confinement. A magnetic field device is introduced in the magnetic confinement method around the sample instead of a spatial constraint device [24]. Under
the external magnetic field, the movement of the charged particles (electron and ions)
in the plasma is influenced by the Lorentz force [30]. This may slow down the plasma
expansion and reduce the size of the plasma plume. As a consequence, the electron
density increases in the plasma, which leads to the enhancement of optical emission
311
Ahmed et al. demonstrated that the laser wavelength also significantly affects the
signal enhancement apart from the different geometrical configurations [23]. With an
appropriate laser wavelength combination, inter-pulse delay, and energy, they have
observed a 30 times intensity enhancement of neutral iron lines in DP LIBS compared
to SP LIBS. However, the major limitations of these methods are the experimental
complexity and high cost due to the usage of two different lasers.
1.2 Spatial Confinement LIBS
Another promising method to improve the LIBS spectral intensity is the spatial
confinement of plasma. Compared to DP LIBS, spatial confinement is a simple and
cost-effective method to improve the sensitivity and detection limit of the LIBS technique, since it requires minimum physical modification of the conventional LIBS
system. During the plasma generation and expansion, shockwaves are also produced. The spatial confinement effect originates once the laser-produced shockwaves
encounter an obstacle and are reflected back to the central region of the plasma. The
reflected shockwave can deliver additional energy to the plasma plume and enhance
the spectral emission intensity [25, 26]. Commonly used obstacles to confine the
plasma plumes are parallel plates, hemispherical, cylindrical, and rectangular cavity.
In recent years, spatial confinement in plasmas has been studied in detail by various
groups. Guo et al. used a hemispherical cavity to confine the plasma and observed
that the accuracy of the quantitative analysis of low concentration elements in steel
samples is improved with spatial confinement [27]. Popov et al. used a miniaturized
chamber to confine the plasma and observed spectral intensity enhancement up to
10 times for iron in metallic samples and 3 to 5 times for arsenic in soils [28]. X.
Wang et al. investigated the spatial confinement effect on a femtosecond laser pulse
produced Cu plasma using time-resolved spectroscopy [29]. They have observed
enhancement of the Cu atomic line intensities when the femtosecond Cu plasma is
confined in a cylindrical cavity. The enhancement ratio depends on the cavity diameter and the atomic emission line selected. On the other hand, the cavity height did
not significantly affect the signal strength.
1.3 Magnetic Confinement LIBS
Similar to spatial confinement, another method for improving LIBS sensitivity is
magnetic confinement. A magnetic field device is introduced in the magnetic confinement method around the sample instead of a spatial constraint device [24]. Under
the external magnetic field, the movement of the charged particles (electron and ions)
in the plasma is influenced by the Lorentz force [30]. This may slow down the plasma
expansion and reduce the size of the plasma plume. As a consequence, the electron
density increases in the plasma, which leads to the enhancement of optical emission