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Abstract
This is the first report of a simultaneous ultraviolet/visible/NIR and longwave infrared laser-induced breakdown spectroscopy (UVN + LWIR LIBS) measurement. In our attempt to study the feasibility of combining the newly developed rapid LWIR LIBS linear array detection system to existing rapid analytical techniques for a wide range of chemical analysis applications, two different solid pharmaceutical tablets, Tylenol arthritis pain and Bufferin, were studied using both a recently designed simultaneous UVN + LWIR LIBS detection system and a fast AOTF NIR (1200 to 2200 nm) spectrometer. Every simultaneous UVN + LWIR LIBS emission spectrum in this work was initiated by one single laser pulse-induced micro-plasma in the ambient air atmosphere. Distinct atomic and molecular LIBS emission signatures of the target compounds measured simultaneously in UVN (200 to 1100 nm) and LWIR (5.6 to 10 µm) spectral regions are readily detected and identified without the need to employ complex data processing. In depth profiling studies of these two pharmaceutical tablets without any sample preparation, one can easily monitor the transition of the dominant LWIR emission signatures from coating ingredients gradually to the pharmaceutical ingredients underneath the coating. The observed LWIR LIBS emission signatures provide complementary molecular information to the UVN LIBS signatures, thus adding robustness to identification procedures. LIBS techniques are more surface specific while NIR spectroscopy has the capability to probe more bulk materials with its greater penetration depth. Both UVN + LWIR LIBS and NIR absorption spectroscopy have shown the capabilities of acquiring useful target analyte spectral signatures in comparable short time scales. The addition of a rapid LWIR spectroscopic probe to these widely used optical analytical methods, such as NIR spectroscopy and UVN LIBS, may greatly enhance the capability and accuracy of the combined system for a comprehensive analysis.
© 2017 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Corrections
25 October 2017: A typographical correction was made to the author listing.
1. Introduction
Laser-induced breakdown spectroscopy (LIBS) is a promising laser-ablation based technology for rapid chemical composition analysis [1–6]. LIBS employs a short and intense laser pulse to create a hot, short-lived (sub-microsecond) micro-plasma that is made of electrons, atoms, ions, and molecules of the sample compound and the ambient environment on the sample surface. Important information concerning the identification, composition, and concentration of trace elements can be derived from the analysis of LIBS emission spectra. The compelling advantages of LIBS that have contributed to its increasing popularity include real-time and in situ analysis, little-to-no sample preparation, no laser excitation wavelength tuning, applicability to all sample phases, remote sensing capabilities, and relatively low cost and simple instrumentation. LIBS can provide an easy, fast, and in situ chemical analysis with a reasonable precision, detection limits, and cost. The compelling advantages of LIBS that have contributed to its increasing popularity in wide range of applications include real-time and in situ analysis, little-to-no sample preparation, no laser excitation wavelength tuning, applicability to all sample phases, remote sensing capabilities, and relatively uncomplicated instrumentation. For example in pharmaceutical industry, LIBS is able to rapidly assess the atomic composition, quantities, and the distribution of active pharmaceutical ingredient (API) and excipients (inactive ingredients) such as lubricants and coatings [7–10]. The assessment and analysis can take place at the manufacturing site, providing valuable data within a few minutes. The depth profiling capability can be utilized for coating analysis which comparing coating ingredients in a suspect pharmaceutical counterfeits tablet with an authentic one [11]. Other applications of LIBS in counterfeit identification and characterization include determining the quantitative spatial uniformity of the elemental constituents of pharmaceutical ingredients within a sample tablet or between batches of samples.
However, conventionally LIBS is a technique for elemental analysis in the UV to NIR spectral range (typically 170 nm to 1100 nm), and most of the information concerning molecular structure of the sample material is lost. The interference with ambient air (with abundant nitrogen and oxygen) and temperature changes in the LIBS plasma from shot to shot can infiuence the intensity ratios and lead to possible errors for quantitative analysis of compounds that is rich in nitrogen and oxygen such as many organic materials [12,13]. It is well known that molecules exhibit spectroscopic signatures in the Mid-IR to LWIR region due to vibrational and rotational transitions. Therefore, an extension of LIBS into the IR region promises to provide additional information concerning the identification and classification of substances, which can complement results obtained from conventional UVN LIBS measurements.
Recent studies of LIBS emissions in the Mid-IR to LWIR (2-12 µm) region readily identified several emitting atomic and complex molecular species resulting from the laser-induced micro-plasma formation: neutral metal atoms, oxygenated combustion molecular byproducts (e.g. CO2, H2O), and intact sample molecules [14–19]. In these infrared LIBS spectra, majority of the emission signatures are not just atomic and simple molecular electronic transitions, as observed in UVN LIBS, but molecular vibrational transitions, which extends LIBS into a more extensive discipline of molecular spectroscopy. Intact sample target molecules not only survived the laser ablation, but were thermally excited by the laser-induced plasmas. Thus, LWIR LIBS is capable of providing direct information on the molecular nature of sample substances. A Mercury-Cadmium-Telluride (MCT) linear array detection system that is capable of rapidly capturing (~1–5 second) a broad spectrum of atomic and molecular LIBS emissions in the LWIR was recently developed by which a broad band (5.6 to 10 µm) emission spectrum of condensed phase samples can be acquired from just a single laser-induced micro-plasma. This setup offers the possibilities of a simultaneous UVN and LWIR LIBS measurement initiated by the same single laser pulse-induced micro-plasma which has capability of rapidly probing samples “as is” without the need of elaborate sample preparation. The combination of atomic emission signatures derived from conventional UVN LIBS and fingerprints of intact molecular entities determined from LWIR LIBS surmise to be a powerful spectral combination tool for chemical detection in various chemical analytical applications.
The work presented in this report is the first attempt of a simultaneous UVN and LWIR laser-induced breakdown spectroscopy measurement. Two different solid over-the-counter pharmaceutical tablets, Tylenol Arthritis Pain (Active Pharmaceutical Ingredient: acetaminophen) and Bufferin (Active Pharmaceutical Ingredient: acetylsalicylic acid), were studied. Beside LIBS, in numerous chemical analysis applications such as product sensing in pharmaceutical industry, NIR (1200 to 2200 nm) absorption spectroscopy is also a popular rapid optical analytical tool [20]. The NIR radiation penetrating into compacted materials such as pharmaceutical tablets and the diffusely reflected or transmitted NIR radiation will provide spectral information about the sample [21]. The NIR region of the spectrum contains overtones and combination bands mainly due to CH, OH or NH vibrations with large anharmonicity which are generally weaker than the absorptions of the corresponding fundamental vibrations in the mid-IR and LWIR region. Although not a very sensitive technique, NIR can generally penetrate much farther into a sample than mid infrared radiation and can be very useful in probing bulk material with little or no sample preparation. In this work, we probe the pharmaceutical target samples using both our newly developed rapid UVN + LWIR LIBS detection system and a high speed AOTF NIR spectral analyzer and hope to explore the feasibility of a combined rapid (less than five second sampling time), comprehensive, and versatile Visible, NIR, and LWIR analytic tool for wide-ranged chemical analysis applications that requires minimum sample preparation.
2. Experimental setup
We recently developed a MCT linear array detection system that is capable of rapidly capturing (~1-5 second) a broad spectrum of atomic and molecular LIBS emissions in the LWIR region [19]. This LWIR LIBS array system operates in a similar manner as conventional LIBS, but the detection spectral range is in the LWIR region, instead of UVN region. Emission signatures in both UVN (200-1000 nm) and LWIR (~5.6 to 10 µm) created by a single laser induced micro-plasma can be collected concurrently.
The configuration of the combined UVN LIBS and LWIR LIBS MCT linear array detection system is shown in Fig. 1. The excitation laser is a flash lamp pumped, actively Q-switched Nd:YAG laser (Quantel Laser). The lasing wavelength is 1064nm and pulse energy can be adjusted between 70 to 300 mJ and pulse duration of ~10 ns. The pulse energy in this work was about 72 mJ. Solid pharmaceutical tablets, Tylenol Arthritis Pain and Bufferin, were mounted vertically on a xyz translational sample stage and a plano-convex lens of 1 meter focal length focused the laser pulses onto the sample. For the visible collection arm, a single lens focused the signal emission into the optical fiber of a compact UVN LIBS Czerny-Turner CCD spectrometer (Thorlabs CCS200). The FWHM spectral resolution of this UVN spectrometer is about 2 nm. For the LWIR collection arm, a set of two 90 degree protected gold parabolic reflectors focused the signal emission onto the entrance slit of a grating based monochromator. A 5.5 µm long pass filter was placed right in front of the entrance slit of the monochromator to prevent higher order leakage. The output slit was removed and replaced with the MCT linear array detector with integrated readout integrated circuits (ROIC). The MCT linear detector array has 332 pixels (photo diodes) along the direction of dispersion with a cutoff wavelength around 10 µm. The dimension of each MCT pixel is 50 µm × 50 µm. The single line resolution limit of this LWIR LIBS detection scheme, the FWHM of a single narrow LWIR emission line, is around 76 nm. Q-switch trigger of the Nd:YAG laser triggered the acquisition sequence of the LWIR MCT linear array and the control electronic of the LWIR detector subsequently triggered the visible spectrometer 20 µs after the Q-switch trigger, and the starts LWIR signal integration 0.1 µs afterwards . Every simultaneous UVN + LWIR LIBS emission spectrum studied in this work was initiated by the same single laser pulse-induced micro-plasma on the target surface in the ambient air atmosphere. The 20 µs delay time lessened the influences of the plasma-induced thermal backgrounds. 44 µs integration time was used for the LWIR and 10 µs for the visible emissions measurements. The accuracy of delay and integration time is within 0.1 µs.
For depth profile analysis of the coating of the solid pharmaceutical tablet samples in this report, the nanosecond pumping laser pulses were repeatedly directed to the same spot on the sample. For each tablet sample, seven consecutive laser pulses were fired on the same tablet surface location with approximately five-second time interval between each shot. Each pulse ablates a certain amount of the sample material, and consequently penetrates step by step deeper into the sample. Information on the depth profile of the sample tablet can be obtained from the simultaneous UVN + LWIR LIBS spectra of the successive laser pulses.
The NIR spectrometer used in this work is a Brimrose solid-state Luminar 5030 analyzer: a miniature, hand-held Acousto-Optic Tunable Filter-Near Infrared (AOTF-NIR) analyzer (Fig. 2). The Luminar 5030 Mini-Spectrometer consists of an integrated optical sensor module with light source, acousto-optic tunable filter, detector and electronics, battery-pack and operating/analytical software with PC interface. Utilizing the AOTF technology, this NIR analyzer without moving parts is capable of performing very high scanning speed (30 scans per second) over a broad NIR range between 1200 to 2200 nm. The solid tablet samples were placed on the probe of the spectrometer and whole NIR range (1200 to 2200 nm) of diffuse reflectance of the tablet sample can be acquired in less than five seconds. Afterward the same pharmaceutical samples were transferred to the combined UVN LIBS and LWIR LIBS MCT linear array detection system for further Simultaneous UVN + LWIR LIBS measurements.
3. Results and discussion
3.1 Tylenol arthritis pain (acetaminophen)
Tylenol Arthritis Pain is an over-the-counter pain reliever and fever reducer that contains 650 mg of API acetaminophen (C8H8NO2) and about 120 mg of inactive ingredients in each solid tablet. Each tablet is coated with a polymer coating.
Simultaneous UVN + LWIR LIBS measurements of a single Tylenol Arthritis Pain 650 mg tablet without any sample preparation are shown in Fig. 3 along with FTIR absorption spectra of key inactive ingredients. Several vibrational signatures of HPMC (Hydroxyproply methyl cellulose, C56H108O30), a common coating polymer, and starch ((C6H10O5)n) are readily observable: A broad ester COC stretching band at 9 µm, asymmetric bending vibration of methyl group in CH3O at 6.8 µm, and a cyclic COC stretching band at 7.23 µm [18]. The emission feature at 5.8 µm can be attributed to the CO stretching band of steric acid, a lubricant commonly used for boundary lubrication during pharmaceutical manufacturing [22]. Besides the emission signatures of C, N, and O, emission signatures of Titanium (Ti) atom at 500 nm and Magnesium (Mg) atom at 552 nm are identified in the UVN region [23]. The broad features at 625nm, 675 nm, and 714 nm are mainly due to the B3Π → X3Δ and A3Φ → X3Δ diatomic molecular transitions of TiO molecules [24]. Those Ti atoms and TiO molecules are most likely resulting from the breakdown of TiO2, a very common pharmaceutical additive, used in the coating. The Mg atoms are likely from Magnesium Stearate, another common lubricant used in the pharmaceutical manufacturing. From the LWIR emission spectrum, there is very little emission contribute from the API acetaminophen for the strong CO stretching bands of acetaminophen at 8 μm being absent in the spectrum. When the tablet probed as is, the dominant spectral features are from the inactive ingredients of the tablet coating.
Fig. 3 The emission spectra of Tylenol Arthritis Pain tablet (a) in the UVN region (shown between 400 to 900 nm) (b) in the LWIR region. The FTIR absorption spectra of key inactive ingredients are also plotted in (b): HPMC (red), steric acid (blue).
After removing the thin layer of the tablet including the coating using a metal blade, simultaneous UVN + LWIR LIBS measurements of the same tablet are shown in Fig. 4. The emission spectrum is dominated by signature emissions of acetaminophen between 5.6 to 10 µm: the C = O asymmetric stretching band at 6.1 μm, aromatic CC stretching at 6.2 μm, NH deformation at 6.4 μm, aromatic HCC and CCC deformation bands degenerate at 6.7 μm, CH3 deformation at 7 μm, OH deformation at 7.4 μm, CN stretching at 7.6 μm, and CO stretching at 8 μm [18]. The intense and distinct emission signatures from the inactive ingredients in the previous spectrum are mostly disappeared with removing of the coating. The strong and broad ester COC stretching bands of HPMC and starch at 9 µm are absent and so are the broad UVN signatures of the Ti and TiO.
Fig. 4 The emission spectra of Tylenol Arthritis Pain tablet with coating layer removed (a) in the UVN region (shown between 400 to 900 nm) (b) in the LWIR region. The FTIR absorption spectra of acetaminophen (red) is also plotted in (b).
NIR absorption spectra measured from the Tylenol tablet before and after coating removal are presented in Fig. 5 along with their second derivatives. Derivatives are mainly used to eliminate constant and linear baseline drift between samples. They are all basically shown spectral features of acetaminophen in the near infrared region (1200 to 2200 nm) [25]. The appeared difference may only come from the quantity variance of the acetaminophen probed in these two configurations of the tablet. The penetration depth of the near infrared radiation in a diffusive reflectance measurements can reach a couple of millimeters [21]. Thus the presents of the inactive ingredients in the vicinity of thin coating layer can only slightly perturb the NIR absorption spectrum that dominated by the bulk pharmaceutical materials.
Fig. 5 NIR spectra (a) and their second derivatives (b) measured from the Tylenol Arthritis Pain tablet before (black line) and after coating removal (red lines).
One of the major advantages of the LIBS technique is its ability to depth profile a target by repeatedly discharging the laser pulses in the same sample position, effectively going deeper into the target material with each laser shot. The depth profiling capability of the UVN LIBS has been leveraged for coating analysis [26]. The number of laser pulses required to penetrate a pharmaceutical coating is directly proportional to the coating thickness [27]. We measured the average depth per shot under the experimental conditions of this work to be approximately 35 μm. With the newly extension of analytical spectral range to LWIR, we explore the LIBS depth profiling of the Tylenol tablet. The simultaneous UVN + LWIR LIBS spectra of each of the seven consecutive laser pulses firing on the same tablet surface location are shown in Fig. 6. Except for the diminishing of the K atomic lines at 766 nm and 769 nm in seventh shot spectrum, other elemental emission features of C, N, O in the UVN changed little from first to seventh shot. However, from the LWIR spectra, by monitoring the COC stretching bands of HPMC and starch at 9 µm and the CO stretching bands of the acetaminophen API at 8 μm, one can tell the acetaminophen starts to appear at third shot and the cellulose inactive ingredient contribution gradually decreases and become insignificant after the fifth shot and the whole spectrum resemble a fully acetaminophen one. The difficulty in monitoring the C, N, O-rich pharmaceutical compounds using UVN LIBS in ambient air due to significant interferences and emission contributions from atmospheric C, N, and O is noted in earlier studies [12]. On the other hand, to induce detectable LWIR atomic emissions from the transitions between high-lying Rydberg states of non-metal atoms such as N and O would require much higher laser pumping energy intensity than this current detection system employed [19]. Besides, the vibrational and rotational modes of homonuclear molecular nitrogen (N2) and oxygen (O2) in the ambient air are generally infrared inactive. Therefore, similar to those of the N and O atoms, emission spectra of atmospheric N2 and O2 molecules in the infrared region also arise from transitions between upper electronic states [28]. Under current experimental conditions, no LWIR emission features from atmospheric N, O atoms and N2, O2 molecules were observed. This absence of atomic and molecular nitrogen and oxygen LWIR emissions under current setup greatly reduces the atmospheric interference in the LWIR spectral region of LIBS spectra measured in ambient air environment [19]. A detailed study of mechanism would require further investigation employing higher pumping laser power and replacing the ambient air environment with various inert gas atmosphere such as Argon and Helium.
Fig. 6 The simultaneous UVN (a) + LWIR (b) LIBS spectra of each of the seven consecutive laser pulses (from 1st: black line to 7th: purple line) firing on the same Tylenol tablet surface location.
3.2 Bufferin (aspirin)
Bufferin (buffered aspirin) is a combination of API aspirin (acetylsalicylic acid, C8H8O4) and antacid buffers, such as calcium and magnesium carbonate. The antacid helps reduce upset stomach that aspirin often causes. Besides the API and the antacid buffers, the Bufferin tablet is coated with a polymer coating. This coating barrier insulates drugs from the acidic pH of the stomach. In addition, such coatings facilitate delivery of a drug to its optimal absorption site in the intestine, provide delayed action, or for delivering the drug to its local site of action in the intestine.
Simultaneous UVN + LWIR LIBS measurements of a single Bufferin 325 mg tablet without any sample preparation are shown in Fig. 7 along with FTIR absorption spectra of key inactive ingredients. Similar to the spectra of the Tylenol tablet, several vibrational signatures of coating polymer HPMC (Hydroxyproply methyl cellulose) are clearly visible in the LWIR emission spectrum: A broad ester COC stretching band at 9 µm, asymmetric bending vibration of methyl group in CH3O at 6.8 µm, and a cyclic COC stretching band at 7.23 µm [18]. The emission feature around 5.8 µm can be attributed to the CO stretching band of citric acid [29]. Besides the emission signatures of C, N, and O, emission signatures of Ti at 500 nm, and broad TiO features at 625nm, 674 nm, and 714 nm are also identified in the UVN region. Similar to Tylenol, these Ti atoms and TiO molecules are most likely from the TiO2 used in the coating of the Bufferin tablets. While there might be a small contribution of the buffer materials (carbonates at 7 µm) to the LWIR spectrum, there is no trace of any aspirin signature in the LWIR region.
Fig. 7 The emission spectra of Bufferin 325 mg tablet (a) in the visible region (b) in the LWIR region. The FTIR absorption spectra of key inactive ingredients are also plotted in (b): HPMC (red), citric acid (green), dibasic sodium phosphate (blue), and poly-ethylene glycol (magenta).
Simultaneous UVN + LWIR LIBS measurements of the same tablet after removing the thin layer of coating by a metal blade are shown in Fig. 8 along with FTIR absorption spectra of buffer materials. There is little or no trace of any aspirin and coating ingredients signature emission in the LWIR region. The spectrum is dominated by the very strong CO3 stretching bands of the antacid buffer carbonates at 7 µm [30]. The UVN spectrum shows very intense emission signature of Mg (from the antacid buffer MgCO3) at 552 nm and multiple signatures of Ca (from the antacid buffer CaCO3) between 612 to 617 nm [23]. All the Ti and TiO signatures are absent in the UVN spectrum indicating there is no inactive (coating) ingredient left just as the LWIR emission spectrum suggested.
Fig. 8 The emission spectra of Bufferin 325 mg tablet with coating layer removed (a) in the UVN region (shown between 400 to 900 nm) (b) in the LWIR region. The FTIR absorption spectra of key antacid buffers are also plotted in (b): MgCO3 (red) and CaCO3 (blue).
Vibrational emission signatures of aspirin, which is very different from those of the coating ingredients and antacid buffers, dominated the spectrum of Bufferin in the LWIR region after we removed half of the tablet to probe its center core (Fig. 9(b)). The emission spectrum shows aspirin signatures between 5.6 to 10 µm: the C = O asymmetric stretching band encompassing the separate 5.7 and 5.9 µm bands, aromatic CC asymmetric stretching at 6.2 µm, combination bands of CH3CO torsion and CH3 asymmetric deformation at 6.8 µm, CH3 symmetric deformation at 7.3 µm, combination bands of OH asymmetric deformation and aromatic CC asymmetric stretching at 7.7 µm, combination bands of aromatic CH deformation and Ph-OCOCH3 (carboxylate ester) asymmetric stretching at 8.2 µm, strong combination bands of CH3 asymmetric deformation and OCOCH3 asymmetric stretching bands at 8.6 µm, combination band of aromatic CH deformation and aromatic CC stretching at 9.2 µm, combination of CH3 and OCOCH3 deformation bands at 9.8 µm [18]. The UVN LIBS emission spectrum of this aspirin dominate site (Fig. 9(a)) bears a resemblance to the UVN LIBS spectrum of acetaminophen (Fig. 4(a)) probably due to the ambient atmosphere interferences. Both UVN emission signatures of Mg and Ca from the antacid buffers are greatly reduced in intensity and UVN emission signatures of Ti and TiO from coating ingredient TiO2 are absent in the core site of the tablet.
Fig. 9 The emission spectra of the center core of a Bufferin 325 mg tablet (a) in the UVN region (shown between 400 to 900 nm) (b) in the LWIR region. The FTIR absorption spectra of aspirin (acetylsalicylic acid) (red) is also plotted in (b).
NIR absorption spectra measured from the Bufferin tablet at these same three different sites: “as is”, with thin coating removed, and half tablet removed to reveal the center core, are presented in Fig. 10 along with their second derivatives. Similar to the Tylenol (acetaminophen) tablet, NIR (1200 to 2200 nm) absorption spectra and their second derivatives measured from Bufferin tablet before and after coating removal are very alike and mostly shown no features of aspirin. Only the NIR absorption spectrum of center core and its second derivative display typical aspirin NIR spectral features at 1660 and 2145 nm [31, 32]. The NIR absorption spectra seems to confirm what we have learned from the UVN-LWIR LIBS observation, that is, the Bufferin tablet consist of three distinct layers: an outer coating, an aspirin API core, and an antacid buffer layer in-between. The NIR radiation penetration depth of the Bufferin tablet should not be more than 2 mm, for without removing the coating and the buffer layer (< 2 mm in thickness) the NIR radiation could not reach the aspirin core.
Fig. 10 NIR spectra (a) and their second derivatives (b) measured from the Bufferin 325 mg tablet “as is” ((a): black solid line; (b): black dashed line), after coating removal (red lines), and revealing the center core (blue line).
Similar to the depth profiling study of the Tylenol tablet, the Bufferin tablet was also studied by recording simultaneous UVN + LWIR LIBS spectra of each of the seven consecutive laser pulses firing on the same tablet surface location (shown in Fig. 11). Due to the relative emission intensities of each laser shot, the laser shots are presented in reversed order comparing to those of Tylenol tablet in Fig. 6. The strong CO3 stretching bands of the antacid buffer carbonates at 7 µm start to appear and dominate after two shots and the Ti and TiO emissions at 500 nm, 674 nm, and 714 nm from the coating ingredient TiO2 start to reduce intensities after fifth shot. The strong atomic emission features of Mg and Ca between 550 to 620 nm from the antacid buffer layer remain strong from second shot to the fifth shot spectrum. At seventh shots, without reaching the core region of the tablet which lies thousands of micrometers beneath the coating, there is no sign of aspirin API in either spectrum.
Fig. 11 The simultaneous UVN (a) + LWIR (b) LIBS spectra of each of the seven consecutive laser pulses (from 1st: purple line to 7th: black line) firing on the same Bufferin 325 mg tablet surface location.
4. Conclusions
In our first attempt to combine the newly developed rapid LWIR LIBS linear array detection system to existing rapid analytical techniques such as conventional for a wide range of chemical analysis applications, two different solid pharmaceutical tablets, Tylenol Arthritis Pain and Bufferin, were studied using both a recently designed simultaneous UVN + LWIR LIBS linear array detection system and a fast AOTF NIR spectrometer. Pharmaceutical ingredients/constituents were readily detected and identified at various sites of the solid pharmaceutical tablets without the need of employing complex data processing. In LIBS depth profiling studies of these two pharmaceutical tablet without any sample preparation, one can easily monitor transition of the dominant emission signatures from coating ingredients gradually to the pharmaceutical ingredients underneath the coating. The addition of a rapid LWIR Spectroscopic probe to those widely used optical analytical method (i.e. NIR spectroscopy and UVN LIBS) greatly enhance the capability and accuracy of the combined system for a comprehensive analysis. This is due to the fact that most of organic and inorganic compounds have distinctive characteristic infrared spectra, which allows the discrimination of one substance from another. LIBS techniques in both UVN and LWIR spectral region are more surface/thin film specific while NIR spectroscopy has the capability to probe more bulk materials with its greater penetration depth. Both UVN + LWIR LIBS and NIR absorption spectroscopy shown the capabilities of acquiring useful target analyte spectral signatures in comparable short time scales (within five seconds) with little to no sample preparation. The combination of NIR characteristic absorption spectrum, atomic emission signatures derived from conventional UVN LIBS in conjunction with fingerprints of intact molecular entities determined from LWIR LIBS has the potential to be a powerful spectral analytical tool for chemical analyte detection and chemical component monitoring.
It would further enhance the capability of this combined spectroscopic analytical tool by extending its LWIR spectral range. The spectral range of the current LWIR LIBS detection scheme is limited by the 5.5 μm long pass filter placed in front of the monochromator for higher-order leakage prevention and the 10 μm cutoff wavelength of the MCT detector currently employed. Varying the composition of the MCT alloy can change its cutoff wavelength. Future efforts to replace the 5.5 μm long pass filter with a 5 μm high performance long pass filter and employ an MCT array detector designed with its cutoff wavelength extended to 11 µm and beyond would effectively broaden the operational spectral range of the LWIR LIBS array detection system in both directions.
Funding
Small Business Technology Transfer (STTR) Phase III (W911SR-C-0022); the Defense Threat Reduction Agency (CB4059); National Science Foundation (NSF) (HRD-1137747); Army Research Office (ARO) W911NF15-1-0050.
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