Plasma Dynamics in Double-Pulse LIBS on Dicarboxylic Acids Using Combined 532 nm Nd:YAG and Carbon Dioxide Laser Pulses Staci R. Brown,* Charlemagne A. Akpovo, Jorge Martinez, Lewis Johnson Department of Physics, Florida A&M University, Tallahassee, FL 32310 USA Laser-induced breakdown spectroscopy (LIBS) was used as a method to monitor the evolution of C, hydrogen-a, carbon–carbon, and carbon–nitrogen spectral emissions from atmospheric recom- bination in a specific set of organic materials. Ablated samples were composed of a series of linear chain dicarboxylic acids with two to seven C atoms. Accumulated pulses of a focused neodymium-doped yttrium aluminum garnet (Nd:YAG) Q-switched laser beam operated at 532 nm generate a plasma in air at the sample surface. In this work, a dual-pulse LIBS technique was used to improve signal strength by enhancing the nanosecond LIBS plasma with CO 2 transverse-excited breakdown in atmosphere laser pulses with an operating wavelength of 10.6 lm. Through a time-resolved analysis, we demonstrate the correlation between the signal strength of selected emissions and the number of C atoms in the linear chain. We also illustrate the effects that these constraints, along with the presence of a chiral C in the chain, have on the peak intensities of the individual lines with respect to each other by comparing the increase or nonexistence of certain spectral lines as we increase the number of C atoms in the linear chain. Index Headings: Laser-induced breakdown spectroscopy; LIBS; Organic materials; Dual-pulse lasers; Enhancement; Dicarboxylic acids. INTRODUCTION Laser-induced breakdown spectroscopy (LIBS) is a spectroscopic technique that allows for the detection of the elemental constituents of materials in the liquid, solid, or gas phase. There are many potential applica- tions for LIBS that have been demonstrated in homeland defense, forensics, public health, pharmaceuticals, and environmental analysis. In a basic LIBS setup, focusing a pulsed laser near or on the sample surface creates plasma. Once the ionization threshold of the sample has been reached, the sample–air interface is ionized near the beam focus. After the laser pulse passes, free electrons and ions in the plasma quickly recombine and thermalize. As the plasma begins to cool, spectral lines representing ionized atoms, followed by neutral emis- sions, and lastly, molecular emissions can be detected using an intensified charged-coupled device. Almost like a fingerprint, the temporal progression of spectral data can be unique for each material and makes LIBS an analytical technique that allows for the fast and real-time verification of organic and nonorganic materials. 1–4 Although LIBS is minimally destructive, requires little or no sample preparation, and has the ability for depth profiling, there are a variety of physical and chemical effects that contribute or hinder signal strength and the repeatability of LIBS experiments. 1 Most LIBS experi- ments are conducted in air, leading to effects due to atmospheric recombination. 5–8 The accuracy and preci- sion of the measurement can be dependent on the homogeneity of the sample; yet, the chemical structure can aid in the use of LIBS as a discriminatory method through the study of matrix effects. 6,9 Sample roughness has been shown to affect the lens-to-sample distance. 5 Other conditions that have an impact on the quality of LIBS measurements include laser shot-to-shot variance and detector specifications, such as its dispersion, gain, gate delay, gate width, and slit width. 3,10 Also, the excitation modality (nanosecond or femtosecond) as well as the multiplicity of pulses (nanosecond with carbon dioxide (CO 2 ) transverse-excited breakdown in atmo- sphere (TEA) laser enhancement) in LIBS has been shown to be useful and valuable in enhancing the emission signal intensity. 11–15 Although there are a plethora of factors that influence the viability of LIBS measurements for quantitative and qualitative chemical analyses, some of these proper- ties (effects due to atmospheric recombination, sample matrix, nature of excitation, and multiplicity of pulses) can be exploited to improve and better understand detection limits. St-Onge et al. 6 studied the C and carbon–carbon (C 2 ) emissions by way of laser ablation in atmosphere of graphite and organic samples with varying molecular structures using cellulose as a binding matrix for the detection of aromatic rings containing double C bonds. They found that a clear relationship was established between C 2 emissions from the presence of compounds known to have aromatic rings (nicotinic acid and chlorobenzoic acid) versus those known to be without aromatic rings (oxalic acid and malic acid). However, with cellulose being C heavy, it is quite plausible that the clear relationship that the authors obtained would be affected by the C content of their binding agent. In this work, we chose simple, linear chain dicarboxylic acids having two to seven C atoms and no C-to-C double bonds. Through- out this study, we use LIBS as a spectroscopic technique for the analysis and discrimination of a specific series of linear chain dicarboxylic acids to emphasize the impact that the sample stoichiometry has on atomic (C and H) and diatomic (C 2 and carbon– nitrogen (CN)) emissions. Received 21 November 2013; accepted 3 June 2014. * Author to whom correspondence should be sent. E-mail: staci. brown@cepast.famu.edu. DOI: 10.1366/13-07397 1046 Volume 68, Number 9, 2014 APPLIED SPECTROSCOPY 0003-7028/14/6809-1046/0 Q 2014 Society for Applied Spectroscopy