Techniques: Gate Width and Delay

LIBS is receiving considerable interest as a rapid, in-situ compositional analysis technique for a wide range of applications. These include analyses of thin films, metals and metal alloys, soil and other geological samples, biological samples, and environmental pollutants. LIBS delivers fast (~ a few seconds) elemental analysis with minimal sample preparation.

Thus researchers from various fields are shifting to LIBS as a replacement for more conventional atomic spectroscopic techniques that involve time-consuming acid dissolution (e.g., ICP-OES‚ "Inductively Coupled Plasma - Optical Emission Spectroscopy" and AA‚ "Atomic Absorption").

Timing is critical

For LIBS to be effective, one much achieve a high quality LIBS spectra, with good signal-to-noise and signal-to-continuum ratios. The timing of the LIBS spectra acquisition is critical to success.

Compared with the plasma used in ICP-OES analysis, LIBS plasma has notable different physical characteristics. With ICP-OES, Argon is ignited and ionized by the intense electromagnetic field created by the RF power of the ICP Torch coils. The prepared liquid sample is introduced into the plasma in the form of fine aerosols via a nebulizer and subsequently ionized inside the plasma for emission spectroscopy analysis (Figure 1). The plasma used in the ICP-OES is steady state in nature and has a stable temperature of 7000K ~ 8000K.

In contrast, LIBS plasma's physical properties are highly transient during its entire lifetime, which may last for a few hundreds of microseconds (Figure 2). With LIBS, a focused laser beam is irradiated on the sample surface, and the hot dense plasma is initiated on the sample surface due to interaction of initially ablated mass with the trailing part of the laser pulse. Plasma temperatures in excess of 30,000K can be reached early in the lifetime (<~100 nsec). These rapidly decay to the ambient condition as the plasma expands and cools. Additionally, the ion and electron number density of the plasma rapidly changes with respect to time.

Therefore, precise timing control of the LIBS spectrometer trigger with respect to the laser is essential due to different excited species that reside in the cooling plasma at different time scales.

Gate Delay

Although it is dependent on the laser fluence and the ambient environment, in general terms, most of the emission from the LIBS plasma early in the lifetime (<100 nsec) is dominated by the continuum emission that resembles the blackbody radiation. The continuum emission originates from kinetic energy adjustment of slowing free electrons in the presence of an electric field of ions (free to free transition) and electron-ion recombination (free to bound transition). The continuum emission tends to overshadow the emission lines from the excited species present in the plasma. Thus, the LIBS spectra acquisition must be delayed to improve the signal with respect to the continuum background. This delay is often termed as gate delay.

As the continuum emission decays, the emission lines from ions (usually singly charged) can be observed as early as a few hundreds of nanoseconds, followed by the emission lines from excited atoms that are strong on a microsecond time scale. Finally, as the plasma continues to cool, some excited neutrals and ions in the plasma may chemically react with others or the surrounding air to form molecular species. The emission bands from these molecular species can be observed as late as a few tens of microseconds after the laser pulse firing. Figure 3 illustrates the emission lines available from different emitting species in the LIBS plasma at different time scales. This figure also represents the time scales typical for laser ablation using nanosecond Nd:YAG laser with ~10 mJ to 100 mJ of energy per pulse with the laser spot size in the range of 10's to 100's microns. For laser ablation involving significantly lower laser energy, the plasma lifetime may decrease significantly and the times scales at which different excited species are available may change.

Applying Gate Widths and Gate Delays

Figures 4, 5, and 6 show sample LIBS spectra obtained by applying appropriate gate delays and gate widths (time duration for which the LIBS signal acquisition is continued from the start of the spectrometer triggering) for different sample matrices. These illustrate that by carefully controlling the timing of the LIBS signal acquisition, both ionic and atomic LIBS emission lines of interested analytes can be acquired with good signal-to-noise and signal-to-background ratios.

Optimum gate delay and width values may depend on the sample matrix, the interested analytes and the laser ablation environment. Properly selected gate delays/widths lead to useful LIBS data; however, poorly selected values yield the data of little value. For example, in Figure 6, using an Ammonium Hexafluorophosphate sample, several Phosphorous ionic lines between 520 to 550 nm VIS spectral range were captured by performing laser ablation in a low pressure environment and applying a short gate delay of 200 nsec.

Additionally, we used a short gate width of 100 nsec to exclude molecular emission bands that may present in this spectral region later in the plasma lifetime and would have interfered with P (II) lines during this signal acquisition (shown in red). Figure 6 also shows that by applying a longer gate delay and gate width of 1 ųsec (shown in green), these Phosphorus ionic lines disappear entirely from the captured LIBS spectra.

Conclusion

Accurate and precise LIBS analysis depends on several technical factors, including: selection of the correct laser wavelength, excellent instrument automation that ensures the consistency of the laser ablation, and appropriate sampling protocols, especially for spatially in-homogeneous samples. One of the most important considerations, though, is the ability to precisely control and flexibly alter the timing between the laser firing and the triggering of the spectrometer – because laser ablation creates a very transient plasma in which different excited species may be available at different time intervals. Optimizing this timing parameter is the first crucial step for meaningful LIBS analysis.

At Applied Spectra, we've perfected these techniques. We deliver LIBS solutions such as our RT100 series devices that can be configured with the 9520 series Digital Delay Generator from Quantum Composers to suit your elemental analysis needs.