Improving the Signal Strength and Detection Limits …
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and discriminate the ionic and atomic emission lines from the early continuum of
the plasma plume. At the initial stages, the plasma is generally dominated by a broad
‘white light’continuum mainly due to the bremsstrahlung (free-free) and recombination (free-bound) events. The continuum has little intensity changes as a function
of wavelength but eventually decays rapidly in comparison to the spectral lines with
time. This continuum at initial times often swamps the weaker emission lines from
many minor and trace elements. Proper gating helps to increase the signal-to-noise
(S/N) ratio and to discriminate the atomic/ionic lines from the background continuum, molecular bands, and other unwanted emission lines from the surroundings.
The feasibility of fast qualitative and semi-quantitative multi-elemental analysis
increases the relevance of LIBS in almost every arena, such as geology, archaeology,
artwork diagnostics, forensic, aerosol analysis, environmental, industry, medical,
marine, military, planetary applications, etc. [8–15]. However, similar to all other
techniques, LIBS also possesses some major limitations such as poor reproducibility and low signal strength, especially for the minor and trace elemental detection.
Moreover, LIBS alone cannot differentiate between the hydrates and polymorphism
of mineral samples [16]. The detection limits of LIBS are influenced by multiple
factors such as energy fluctuation, pulse duration, inhomogeneity of laser-induced
plasma, uneven surface of the sample, matrix effects, the contribution of environmental conditions, etc. [17]. These limitations often restrict the use of LIBS technique
in the vast area of applications ranging from biological, geological, organic samples
identification, trace elemental detections, etc. To mitigate this shortcoming and to
strengthen the analytical prediction capability of LIBS even in adverse environments,
a wide range of methodologies have been proposed by scientists. Among these, the
most popular approaches are the double pulse configuration (DP LIBS), spatial and
magnetic confinement, Nanoparticle enhanced LIBS (NELIBS), femtosecond LIBS
(fs-LIBS), and combining LIBS with other complementary spectroscopic techniques
[18–21].
Among the above-mentioned techniques, DP LIBS, spatial confinement, magnetic confinement, and NELIBS are the methods proposed for enhancing the signal
intensity of conventional single pulse LIBS (SP LIBS) techniques in order to improve
the detection limit. Accordingly, these methods can be classified into physical and
chemical enhancement methods, which are discussed briefly in the following sections.
1.1 Double Pulse LIBS
Conventional SP LIBS uses a single laser pulse to breakdown the material and produce plasma. Contrary to SP LIBS, in DP LIBS, two laser pulses having the same
pulse duration with an appropriate inter-pulse delay are used to improve the overall
sensitivity of the LIBS technique. This technique has been widely examined over
the past two decades for enhancing the LIBS signals [22, 23]. The signal enhancement in DP LIBS compared to conventional SP LIBS is mainly attributed to the
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and discriminate the ionic and atomic emission lines from the early continuum of
the plasma plume. At the initial stages, the plasma is generally dominated by a broad
‘white light’continuum mainly due to the bremsstrahlung (free-free) and recombination (free-bound) events. The continuum has little intensity changes as a function
of wavelength but eventually decays rapidly in comparison to the spectral lines with
time. This continuum at initial times often swamps the weaker emission lines from
many minor and trace elements. Proper gating helps to increase the signal-to-noise
(S/N) ratio and to discriminate the atomic/ionic lines from the background continuum, molecular bands, and other unwanted emission lines from the surroundings.
The feasibility of fast qualitative and semi-quantitative multi-elemental analysis
increases the relevance of LIBS in almost every arena, such as geology, archaeology,
artwork diagnostics, forensic, aerosol analysis, environmental, industry, medical,
marine, military, planetary applications, etc. [8–15]. However, similar to all other
techniques, LIBS also possesses some major limitations such as poor reproducibility and low signal strength, especially for the minor and trace elemental detection.
Moreover, LIBS alone cannot differentiate between the hydrates and polymorphism
of mineral samples [16]. The detection limits of LIBS are influenced by multiple
factors such as energy fluctuation, pulse duration, inhomogeneity of laser-induced
plasma, uneven surface of the sample, matrix effects, the contribution of environmental conditions, etc. [17]. These limitations often restrict the use of LIBS technique
in the vast area of applications ranging from biological, geological, organic samples
identification, trace elemental detections, etc. To mitigate this shortcoming and to
strengthen the analytical prediction capability of LIBS even in adverse environments,
a wide range of methodologies have been proposed by scientists. Among these, the
most popular approaches are the double pulse configuration (DP LIBS), spatial and
magnetic confinement, Nanoparticle enhanced LIBS (NELIBS), femtosecond LIBS
(fs-LIBS), and combining LIBS with other complementary spectroscopic techniques
[18–21].
Among the above-mentioned techniques, DP LIBS, spatial confinement, magnetic confinement, and NELIBS are the methods proposed for enhancing the signal
intensity of conventional single pulse LIBS (SP LIBS) techniques in order to improve
the detection limit. Accordingly, these methods can be classified into physical and
chemical enhancement methods, which are discussed briefly in the following sections.
1.1 Double Pulse LIBS
Conventional SP LIBS uses a single laser pulse to breakdown the material and produce plasma. Contrary to SP LIBS, in DP LIBS, two laser pulses having the same
pulse duration with an appropriate inter-pulse delay are used to improve the overall
sensitivity of the LIBS technique. This technique has been widely examined over
the past two decades for enhancing the LIBS signals [22, 23]. The signal enhancement in DP LIBS compared to conventional SP LIBS is mainly attributed to the