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We present a new design of a robust cavity-enhanced frequency comb-based spectrometer operating under the continuous-filtering Vernier principle. The spectrometer is based on a compact femtosecond Er-doped fiber laser, a medium finesse cavity, a diffraction grating, a custom-made moving aperture, and two photodetectors. The new design removes the requirement for high-bandwidth active stabilization present in the previous implementations of the technique, and allows scan rates up to 100 Hz. We demonstrate the spectrometer performance over a wide spectral range by detecting CO2 around 1575 nm (1.7 THz bandwidth and 6 GHz resolution) and CH4 around 1650 nm (2.7 THz bandwidth and 13 GHz resolution). We achieve absorption sensitivity of 5 × 10−9 cm-1 Hz-1/2 at 1575 nm, and 1 × 10−7 cm-1 Hz-1/2 cm-1 at 1650 nm. We discuss the influence of the scanning speed above the adiabatic limit on the amplitude of the absorption signal.

Halogenated volatile organic compounds (HVOCs) play an important role in the photo-chemistry of the atmosphere, for example in ozone depletion [1] . They are produced naturally in the oceans but are also used in industrial and agricultural applications where they may pose a health-hazard due to their biological effects. Optical detection of these compounds would hence be of great value in, for example, atmospheric monitoring and leak detection in workplaces. Crucial for such detection schemes is access to accurate spectroscopic models, which in turn require high-precision laboratory measurements. Due to the combination of broad spectral coverage and high resolution, optical frequency comb Fourier transform spectroscopy is an excellent tool for providing the necessary spectroscopic data. We use a mid-infrared frequency comb and a Fourier transform spectrometer (FTS) to measure and assign high-resolution spectra of multiple absorption bands of two HVOCs: methyl iodide, CH 3 I [2] , and dibromomethane, CH 2 Br 2 , around 3.3 µm.

We use offset-frequency-free difference frequency generation comb sources and a Fourier transform spectrometer with comb-mode-width limited resolution to measure and analyze spectra of molecular species of atmospheric relevance: CH3I and CH2Br2 around 3000 cm-1, and 14N 162O around 1280 cm-1.

We report the design and characteristics of a simple and compact mode-locked Er-doped fiber laser and its application to broadband cavity-enhanced spectroscopy. The graphene mode-locked polarization maintaining oscillator consumes less than 5 W of power. It is thermally stabilized, enclosed in a 3D printed box, and equipped with three actuators that control the repetition rate: fast and slow fiber stretchers, and metal-coated fiber section. This allows wide tuning of the repetition rate and its stabilization to an external reference source. The applicability of the laser to molecular spectroscopy is demonstrated by detecting CO2 in air using continuous-filtering Vernier spectroscopy with absorption sensitivity of 5.5 × 10−8 cm−1 in 50 ms.

Broadband high-resolution spectra of two key atmospheric species, methyl iodide (CH3I) and dibromomethane (CH2Br2), are measured around 3 μm using a comb-based Fourier transform spectrometer and assigned with the help of the semi-automatic fitting in PGOPHER.

We present a robust cavity-enhanced comb-based spectrometer with a 6.6 GHz resolution and 60 Hz acquisition rate, based on the continuous-filtering Vernier principle, a fixed diffraction grating, a custom-made chopper wheel, and a low-bandwidth comb-cavity stabilization scheme. We measure a CO2 spectrum with 5 x 10-8 cm-1 sensitivity in 17 ms.

We use optical frequency comb Fourier transform spectroscopy to measure high-resolution spectra of iodomethane, CH3I, in the C[sbnd]H stretch region from 2800 to 3160 cm-1. The fast-scanning Fourier transform spectrometer with auto-balanced detection is based on a difference frequency generation comb with repetition rate, frep, of 125 MHz. A series of spectra with sample point spacing equal to frep are measured at different frep settings and interleaved to yield sampling point spacing of 11 MHz. Iodomethane is introduced into a 76 m long multipass absorption cell by its vapor pressure at room temperature. The measured spectrum contains three main ro-vibrational features: the parallel vibrational overtone and combination bands centered around 2850 cm-1, the symmetric stretch ν1 band centered at 2971 cm-1, and the asymmetric stretch ν4 band centered at 3060 cm-1. The spectra of the ν4 band and the nearby ν3+ν4-ν3 hot band are simulated using PGOPHER and a new assignment of these bands is presented. The resolved ro-vibrational structures are used in a least square fit together with the microwave data to provide the upper state parameters. We assign 2603 transitions to the ν4 band with a standard deviation (observed – calculated) of 0.00034 cm-1, and 831 transitions to the ν3+ν4-ν3 hot band with a standard deviation of 0.00084 cm-1. For comparison, in the earlier work using standard FT-IR with 162 MHz resolution [Anttila, et al., J. Mol. Spectrosc. 1986; 119:190–200] 1830 transition were assigned to the ν4 band, and 380 transitions to the ν3+ν4-ν3 hot band, with standard deviations of 0.00083 cm-1 and 0.0013 cm-1, respectively. The hyperfine splittings due to the 127I nuclear quadrupole moment are observed for transitions with J ≤ 2 × K. Finally, intensities of 157 isolated transitions in the ν4 band are reported for the first time using the Voigt line shape as a model in multispectral fitting.

We present a new robust approach to cavity-enhanced comb spectroscopy based on Vernier filtering, a fixed diffraction grating, custom-made chopper wheel, and a low bandwidth comb-cavity stabilization scheme. We measure a CO2 spectrum with a few GHz resolution and 5 x 10-8 cm-1 sensitivity in 9.4 ms.

We use broadband near-infrared continuous-filtering Vernier spectroscopy (CF-VS) for time-resolved detection of H

The need for fast and simultaneous detection of multiple combustion-related chemical species in various industrial environments has motivated the use of comb-based spectroscopic techniques in these applications. Dual comb spectroscopy has been employed to simultaneously measure CO2 and H2O concentrations in a gas turbine exhaust with 1% precision [1], and cavity-enhanced optical frequency comb Fourier transform spectroscopy has been used to detect H2O and OH radical in a flame [2,3]. However, the time resolution of these measurements has so far been of the order of seconds. Here, we present a cavity-enhanced continuous-filtering Vernier spectrometer (CF-VS) that allows detection of H2O and OH in a flame with a time resolution of 25 ms.

We report a photoacoustic spectroscopy setup with a high-power mid-infrared frequency comb as the light source. The setup is used in broadband spectroscopy of radiocarbon methane. Owing to the high sensitivity of a cantilever-enhanced photoacoustic cell and the high-power light source, we can reach a detection limit below 100 ppb in a broadband measurement with a sample volume of only a few milliliters. The first infrared spectrum of CH

A continuous-filtering Vernier spectrometer based on an Er:fiber femtosecond laser was developed to acquire broadband H2O and OH spectra in a premixed CH4/air flame with 25 ms time resolution and percent precision on concentrations retrieval.

We use broadband near-infrared continuous-filtering Vernier spectroscopy (CF-VS) for time-resolved detection of H2O and OH radical in a premixed CH4/air flat flame. The CF-VS spectrometer is based on a femtosecond Er:fiber laser, an external cavity that contains the flame, and a detection system comprising a rotating diffraction grating and photodetectors. Spectra of H2O and OH radical around 1570 nm are continuously recorded with 6.6 GHz spectral resolution, 4.0 × 10−7 cm−1 absorption sensitivity, and 25 ms time resolution, while the fuel-air equivalence ratio is periodically modulated with a square wave. The concentrations of the two analytes are retrieved with percent level precision by a fit of a Vernier model to each spectrum spanning 13 nm. The temporal profiles of both concentrations in each modulation cycle are repeatable and the steady-state concentration levels are in good agreement with predictions based on one-dimensional simulations of a static flat flame. The robust CF-VS spectrometer opens up for quantitative monitoring of multiple products of time-varying combustion processes with relatively simple data acquisition procedures.

This work presents an analytical method based on terahertz-time domain spectroscopy (THz-TDS) and partial least squares (PLS) regression models to quantify mebendazole (MBZ) polymorphs (forms A, B and C) in pharmaceutical raw material. Mebendazole polymorphs A, B and C were quantified with RMSEP values of 1.5% w/w, 1.2% w/w and 1.8% w/w, respectively. The limits of detection (LOD) ranges obtained with the best PLS regression models were 2.7-4.3% w/w, 2.9-4.0% w/w and 2.4-3.1% w/w, for polymorphs A, B and C, respectively. This analytical performance is better than those for the methods described in the literature using near (NIR) and middle (MIR) infrared spectroscopies. The main advantage of THz spectroscopy is its ability to access directly information related to crystal lattices. According to the results, the developed method is a powerful technique for the quantification of MBZ polymorphs in raw material. This methodology can be implemented as a Process Analytical Technology (PAT) tool for quality control of pharmaceutical feedstock.

Terahertz-time domain spectroscopy (THz-TDS) has the ability to probe the crystallinity of several materials, due to the interaction of THz radiation with optical phonons in crystal lattices. In this work, THz-TDS has been used to quantify the degree of crystallinity of microcrystalline cellulose (MCC) samples. The THz spectra of cellulose present absorption features which could be directly correlated with the crystallinity index (CI) obtained by means of the well-established powder X-ray diffraction (PXRD) technique. The effect of THz time-domain signal processing was investigated, and both univariate and multivariate, based on partial least-squares (PLS), regressions were carried out with the signal in the frequency domain to correlate the THz spectra with CI. Results show that the multivariate regression models based on spectral data, collected with the sample displaced from the focal plane of the THz optics to improve representativeness and measurement repeatability, present the best performance with external validation achieving an absolute root-mean-square error of prediction (RMSEP) of 4% for CI. This result compares well with the PXRD technique.

Oxidative stability is an important parameter for quality control of biodiesel samples. Autoxidation is the main cause of deterioration of this type of fuel, and yields hydroperoxides, peroxides and carboxylic acids as main products, in a similar way as in the raw materials employed for its manufacture (vegetable oils and animal fat). Usually, oxidative stability is determined by monitoring the formation of an autoxidation product until a significant change in its concentration can be detected. In this work, a near infrared emission (NIRES) method based on the hydroperoxide emission at 2900 nm (3450 cm -1) was evaluated for determining oxidative stability of biodiesel. A comparative study between the oxidative stability of biodiesel samples and their parent vegetable oils is also presented. Results show that the NIRES method provides oxidative stability values correlated to those obtained with the official Rancimat method (R2 = 0.88), and present equivalent precision. The photometer presents several advantages compared to the Rancimat method, including faster analysis, lower sample consumption and operational simplicity.

We report a photoacoustic spectroscopy setup with a high-power mid-infrared frequency comb as the light source. The setup is used in broadband spectroscopy of radiocarbon methane. Owing to the high sensitivity of a cantilever-enhanced photoacoustic cell and the high-power light source, we can reach a detection limit below 100x00A0;ppb in a broadband measurement with a sample volume of only a few milliliters. The first infrared spectrum of CH414 is reported and given a preliminary assignment. The results lay a foundation for the development of optical detection systems for radiocarbon methane.

Near infrared emission spectroscopy (NIRES) allows the determination of the induction time (IT) of edible oils in accelerated oxidation experiments by monitoring the emissivity of a band at 2900nm, which corresponds to the formation of hydroperoxides. In this work, a new near infrared emission photometer dedicated to the determination of oxidative stability is described. The photometer presents several advantages compared to the previously reported NIRES instrument, such as lower cost and extreme simplicity of design and maintenance. The results obtained in the evaluation of the proposed instrument were compared with the official Rancimat method and instrument. The significant advantages include: faster analysis, lower sample consumption and operational simplicity. It is demonstrated that the procedure for determination of oxidative stability of oils can be significantly simplified and performed by measuring the sample emission at only one spectral region centered at 2900nm. Also, the proposed instrument and method present precision equivalent to the Rancimat method (coefficient of variation=5.0%). A significant correlation between the methods has been found (R2=0.81).

Orientadores: Celio Pasquini, Flávio Caldas da Cruz Tese (doutorado) - Universidade Estadual de Campinas, Instituto de Química Resumo: A região terahertz compreende a faixa do espectro eletromagnético com frequências entre 0,1 e 10 THz e corresponde a transições vibracionais de baixa energia ¿ interações intermoleculares, fônons ¿ e transições rotacionais puras em gases. Por muitos anos, essa região era conhecida como o THz gap, devido à ausência de instrumentação adequada para realizar medidas de qualidade. Com o advento dos lasers de femtossegundos e de semicondutores com tempo de vida de portadores da ordem de picossegundos, novas tecnologias, como a espectroscopia THz no domínio do tempo (THz-TDS), permitiram grandes avanços na área. No entanto, as aplicações analíticas são até hoje incipientes e, no cenário de pesquisa nacional em Química Analítica, não há ainda trabalhos realizados. Nesse sentido, o presente trabalho teve como objetivo empregar um espectrômetro THz-TDS com tecnologia asynchronous optical sampling (ASOPS) visando explorar o potencial da técnica e desenvolver métodos analíticos. Paralelamente, um trabalho de cunho instrumental foi realizado durante o estágio no National Institute of Standards and Technology (NIST), visando o desenvolvimento de um espectrômetro THz de alta resolução baseado em pentes de frequência. As aplicações mais promissoras encontradas foram aquelas envolvendo a investigação de cristalinidade de polímeros, tanto naturais ¿ celulose microcristalina (MCC) ¿ quanto sintéticos ¿ polietileno (PE) e politetrafluoroetileno (PTFE). Amostras com diferentes graus de cristalinidade foram produzidas a partir de materiais altamente cristalinos por moagem em moinho de bolas ¿ para MCC e polímeros sintéticos ¿ ou procedimentos de fusão e cristalização ¿ somente para os polímeros sintéticos. Um método analítico para a celulose foi desenvolvido com sucesso, resultando em uma publicação na Analytical Chemistry. Como principais vantagens, destacamse a rapidez de análise e o uso de radiação não ionizante. Resultados qualitativos foram obtidos para os demais polímeros. O desenvolvimento do espectrômetro THz de pentes de frequência também foi bem-sucedido e resultou em uma apresentação oral no Mid Infrared Coherence Sources (MICS) em Long Beach/CA, EUA Abstract: The terahertz band comprises electromagnetic waves whose frequencies range from 0.1 and 10 THz. Low energy vibrational modes ¿ such as intermolecular interactions and phonons ¿ and purely rotational transitions are the typical phenomena observed in this region. For many years, this band was known as the THz gap, due to lack of instrumentation that would allow good quality measurements. With the arising of femtosecond lasers and sub-picosecond carrier lifetime semiconductors, new technologies, such as time domain terahertz spectroscopy (THzTDS), were developed, and brought major improvements to the field. However, its analytical applications are still incipient, especially within the Brazilian scene. In that sense, the purpose of this work was to explore potential analytical uses of this kind of spectroscopy, by employing a THz-TDS spectrometer with asynchronous optical sampling technology to various applications. Additionally, during an exchange program at the National Institute of Standards and Technology (NIST), a terahertz dual frequency comb spectrometer (THz-DFCS) was developed, aiming at high resolution spectroscopy. Investigations of crystallinity in polymers ¿ particularly microcrystalline cellulose (MCC) and polytetrafluoroethylene (PTFE) ¿ were found to be promising. Samples with different crystallinity degrees were prepared from high crystalline substrates by either ball milling ¿ for both MCC and the synthetic polymers ¿ or melting and crystallization procedures ¿ just for the synthetic polymers. With cellulose, a quantitative analytical method was successfully developed, and it resulted in a paper published in Analytical Chemistry. The main advantages of this method are the short analysis time and the use of non-ionizing radiation. Qualitative results were achieved for the other polymers. The THz-DFCS spectrometer developed resulted in an oral presentation at the Mid Infrared Coherence Sources (MICS), which was held in Long Beach/CA, USA Doutorado Química Analítica Doutor em Ciências FAPESP 2013/00502-6 CNPQ CAPES

Orientador: Célio Pasquini Dissertação (mestrado) - Universidade Estadual de Campinas, Instituto de Química Resumo: A autoxidação de lipídeos é uma das principais causas de degradação de óleos e gorduras. Este processo gera uma série de compostos indesejáveis, tais como, peróxidos, hidroperóxidos e ácidos carboxílicos de cadeia curta. A determinação da estabilidade oxidativa de um óleo, é, portanto, essencial para estimar o tempo de prateleira - um parâmetro importante para a indústria alimentícia. Para quantificar o grau de oxidação de um óleo, uma série de parâmetros pode ser utilizada, como os índices de peróxido e de anisidina, o teor de dienos conjugados, dentre outros. Métodos acelerados foram desenvolvidos para a estabilidade oxidativa - uma medida da variação do grau de oxidação ao longo do tempo -, em particular o método Rancimat. Nele, uma amostra de cerca de 3 g de óleo e aquecida em temperaturas na faixa de 100-130 °C, e um fluxo de ar constante e passado através da amostra e carreado para um recipiente contendo água deionizada. O monitoramento da condutividade elétrica desta solução permite identificar o período de tempo necessário para que a taxa de formação de ácidos carboxílicos de cadeia curta se torne significativa. Este tempo é denominado tempo de indução (TI), e corresponde ao índice de estabilidade oxidativa (OSI - Oxidative Stability Index) do óleo. A espectroscopia de emissão no infravermelho próximo (NIRES), por outro lado, permite o monitoramento de hidroperóxidos, devido ao aumento e alargamento de uma banda em 2900 nm que ocorre ao longo do processo oxidativo. Neste trabalho foi desenvolvido e avaliado um fotômetro NIRES, dedicado para a determinação de estabilidade oxidativa. No método NIRES, uma amostra de 10 mL e aquecida a uma temperatura fixa (140 - 180 °C), sendo mantido um fluxo constante de ar purificado ao redor da amostra. A emissividade do óleo em 2850 e 2675 nm e continuamente monitorada com um detector de PbS, sendo a seleção dos comprimentos de onda realizada por dois filtros de interferência. A formação de hidroperóxidos ocasiona um aumento significativo da emissividade a 2850 nm, permitindo a determinação de TI de forma análoga ao método Rancimat.. O equipamento construído apresenta uma série de vantagens com relação ao método Rancimat, tais como: rapidez na análise, simplicidade operacional, menor consumo de amostra e a possibilidade de determinar a estabilidade oxidativa em temperaturas de fritura (140-180 °C). A precisão do método NIRES encontrada é equivalente à precisão do método Rancimat, e existe uma correlação entre eles (R = 0,90). Abstract: Lipid autoxidation is the main cause of fats and oils deterioration. This process generates a series of undesirable compounds, such as hydroperoxides and low chain carboxylic acids. Thus, determination of oilfs oxidative stability is essential for estimation of the shelf life - an important parameter for the food industry. Several parameters are used for quantifying the degree of oxidation of an oil sample, such as the peroxide value, anisidine value, and conjugated dienes. Accelerated methods have been developed for measuring oxidative stability - the variation of the degree of oxidation with time -, particularly the Rancimat method. In this method, a constant air flow passes through a 3 g oil sample, heated at 110 - 130°C, and then is collected in a vessel containing deionized water. By monitoring the deionized waterfs conductance, an induction time (IT) - which corresponds to the Oil Stability Index (OSI) - can be determined. On the other hand, near infrared emission spectroscopy (NIRES) allows the determination of IT by monitoring the formation of hydroperoxides. As the oil undergoes autoxidation, a band at 2900 nm becomes broader and more intense. In this work, a near infrared emission spectroscopy photometer, dedicated to determination of oxidative stability, has been developed and evaluated. In the NIRES method, a 10 mL oil sample is heated at a constant temperature (140 - 180 °C), and a purified air flow is maintained around the sample. The oilfs emissivity at 2675 and 2850 nm is continually monitored with a PbS detector, and the wavelengths are selected with two interference filters. Formation of hydroperoxides increases significantly the emissivity at 2900 nm, allowing the determination of IT in a way analogous to the Rancimat method. The developed equipment presents several advantages towards the Rancimat, including: faster analysis, lower sample consumption, operational simplicity, and the possibility of measuring oxidative stability at frying temperatures. Also, the methodfs precision is equivalent to the Rancimatfs precision, and a correlation between them has been found (R = 0,90). Mestrado Química Analítica Mestre em Química

The standard method for sensitive measurements of long-lived radioactive isotopes, such as 14C, is accelerator mass spectrometry (AMS). The AMS facilities are indispensable for applications like radiocarbon dating, but too expensive and massive for in-situ measurements. Laser spectroscopy, on the other hand, is a promising solution for isotope-selective detection of gas-phase compounds in applications that require low-cost instrumentation and on-site measurement capability. The recent work on laser spectroscopy of 14CO2 has led to impressive detection limits below the natural 14C abundance [1,2], which encourages us to develop similar techniques also for other radiocarbon compounds. Here, we report the first spectroscopic study of radioactive methane, 14CH4. Our work is motivated by a number of important applications, such as determining the biofraction of methane fuel mixtures and in-situ monitoring of radioactive gas emissions at decommissioning sites of light water nuclear reactors.

Sensitive in situ detection of multiple atmospheric species at fast acquisition rates is needed for environmental monitoring. For field applications, robust and compact design is also demanded. Continuous-filtering Vernier spectroscopy (CF-VS) [1] is a cavity-enhanced frequency-comb-based technique that provides broad spectral bandwidth and high absorption sensitivity in short acquisition times. In CF-VS, groups of comb lines (Vernier orders, VOs) are transmitted through the cavity when its free spectral range (FSR) is slightly detuned from the comb repetition rate ( f rep ) and continuously swept across the broadband laser spectrum (by scanning the FSR). In previous implementations [1] - [3] , a diffraction grating rotating on a galvo scanner was used to image one VO on the detector during the spectral scan, limiting the acquisition rates to 20 Hz. Moreover, high-bandwidth stabilization was needed to synchronize the scans of the galvo and the cavity FSR. Here we present an improved design of CF-VS based on a compact Er:fiber laser and a moving aperture that follows and selects one VO. This removes the requirement of tight active stabilization and enables faster acquisition rates.