Analytische Chemie
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Research and Development in Non-Destructive Testing in Civil Engineering at BAM has de-veloped over more than two decades. Highlights of present research are ultrasound phase array probes, ground penetrating radar applications for the assessment of existing structures and Laser Induced Breakdown Spec-troscopy (LIBS) to determine which chemical elements on the surface of a specimen are. RFID sensor devel-opments for wireless moisture monitoring and data fusion for the combined analysis of sensor data are fast developing areas. In addition, a selection of application cases is briefly described.
Introduction: Laser Induced Breakdown Spectroscopy (LIBS) is an atomic emission analytical technique, wide spreading in laboratories and industries. One way to dramatically increase its analytical results is to deposit metal NPs on the sample surface, resulting in an better version called Nanoparticle Enhanced LIBS (NELIBS). In order to better know and use this technique, the evolution of the plasma has been studied with Tomography.
The majority of the built infrastructure is made of concrete, which is a multiphase system made of cement, aggregates, water and pores (every year nearly 4 billion tons of cement are produced which is largest mass flow generated by mankind). Concrete is often used in combination with steel as reinforced concrete. Environmental influences, especially the ingress of harmful ions in combination with the ingress of water, trigger different damage processes which reduce the designed lifetime of a structure. The ingress of chlorides from de-icing salt or sea water leads to corrosion of the reinforcement. Also the carbonation of the concrete may trigger the corrosion of the reinforcement. The ingress of alkalis from de-icing salts may cause the expansion of the amorphous silica aggregates (alkali-silica reaction) through formation of a swelling gel of calcium silicate hydrate if water is present. The ingress of sulfates may cause spalling of the concrete surface due to ettringite formation.
For the standard procedure in civil engineering cores are taken, cut in slices, grinded and the obtained homogenized powder is solved in acid and investigated by standard procedures.
BAM has developed the LIBS technique for the 2D evaluation of the chemical composition of concrete [1-11]. The technique is established for automated laboratory use with high numbers of samples to investigate transport processes of harmful species (Cl-, CO2, SO42- and alkalis) in concrete. Information about ingress depth and the quantitative values are important to estimate the remaining lifetime of the infrastructure. LIBS is a surface technique. To get information about the ingress depth, a core has to be taken and cut in the middle. The measurements are carried out at the cross section. The main advantages of LIBS are the direct measurement on the surface of the concrete, fast analysis (sample rate 100 Hz) with a spatial resolution of up to 100 µm, the consideration of the heterogeneity of the concrete and the possibility of automated measurements which save a lot of manpower and time. As an example the investigation of ingress profiles for standard diffusion and migration tests in civil engineering takes hours in comparison to just a few minutes using LIBS. At the same time a 2D-evaluation provides information about hot spots of elemental concentration which may not be found by standard methods.
Ingress of chlorides due to a crack in a repair mortar. Left: Photo of the cross section of a concrete core and the surface investigated by LIBS (area 70 mm x 70 mm). Right: Color coded chlorine intensity on the cross section of a concrete core, dark red represents high chlorine content.
The state of the art of LIBS technique for applications in civil engineering will be presented, including typical results of 2D investigation of concrete in laboratory. The performance is also demonstrated by examples for onsite applications using a mobile LIBS system. The road map to standardization is presented as well.
The majority of the built infrastructure is made of concrete, which is a multiphase system made of cement, aggregates, water and pores (every year nearly 4 billion tons of cement are produced which is largest mass flow generated by mankind). Concrete is often used in combination with steel as reinforced concrete. Environmental influences, especially the ingress of harmful ions in combination with the ingress of water, trigger different damage processes which reduce the designed lifetime of a structure. The ingress of chlorides from de-icing salt or sea water leads to corrosion of the reinforcement. Also the carbonation of the concrete may trigger the corrosion of the reinforcement. The ingress of alkalis from de-icing salts may cause the expansion of the amorphous silica aggregates (alkali-silica reaction) through formation of a swelling gel of calcium silicate hydrate if water is present. The ingress of sulfates may cause spalling of the concrete surface due to ett-ringite formation. For the standard procedure in civil engineering cores are taken, cut in slices, grinded and the obtained homogenized powder is solved in acid and investigated by standard procedures. BAM has developed the LIBS technique for the 2D evaluation of the chemical composition of concrete [1-5]. The technique is established for automated laborato-ry use with high numbers of samples to investigate transport processes of harmful species (Cl-, CO2, SO42- and alkalis) in concrete. Information about ingress depth and the quantitative values are important to estimate the remaining lifetime of the infrastructure. LIBS is a surface technique. To get information about the ingress depth, a core has to be taken and cut in the middle. The measurements are carried out at the cross section. The main advantages of LIBS are the direct measure-ment on the surface of the concrete, fast analysis (sample rate 100 Hz) with a spatial resolution of up to 100 μm, the consideration of the heterogeneity of the concrete and the possibility of automated measurements which save a lot of man-power and time. As an example the investigation of ingress profiles for standard diffusion and migration tests in civil engi-neering takes hours in comparison to just a few minutes using LIBS. At the same time a 2D-evaluation provides information about hot spots of elemental concentration which may not be found by standard methods.
The state of the art of LIBS technique for applications in civil engineering will be presented, including typical results of 2D investigation of concrete in laboratory. The performance is also demonstrated by examples for on-site applications using a mobile LIBS system. The road map to standardization is presented as well.
LIBS (laser-induced breakdown spectroscopy) is known as a fast and simultaneous multi-element analysis with little or no sample preparation. In the last few years there has been a growing interest in applications of LIBS in the field of agriculture. As part of the National Research Strategy BioEconomy 2030 the German Federal Ministry of Education and Research (BMBF) started an innovation programme called BonaRes. BonaRes consists of ten interdisciplinary research project associations which are dealing with soil as a sustainable resource for the bio-economy. One of these research projects is I4S (intelligence for soil) which has the goal to develop an integrated system for site-specific soil fertility management. This system includes a sensor platform, which contains different sensors, like XRF, VIS-NIR, Gamma and LIBS. The main task of LIBS measurements in this project is the real time determination of the elemental contents of nutrients in soils, like calcium, magnesium, potassium. For this purpose, a special setup has been designed. The sample uptake operates with the help of a rotatable sample plate which circulates with different velocities to simulate the application on the field. To provide a higher intensity and a better reproducibility of the obtained signal, a double-pulse Nd:YAG laser was used. In order to minimize dust formation from the soil during the operation of the laser, a dust removal by suction has been integrated. When using relative methods such as LIBS, a suitable calibration curve is needed for absolute quantification. With the help of 16 certified reference soils, calibration curves for different elements were initially calculated and used for the quantification of seven soil samples from different testing grounds in Germany. The complex matrix of soils, as well as the influence of moisture and grain size in soils makes the absolute quantification by LIBS challenging. To overcome these influences, a calibration curve based on multivariate analysis (partial least square regression) was generated.
The Laser-Induced Breakdown Spectroscopy (LIBS) is a laser spectroscopic method which allows a time efficient, minor-destructive, chemical analysis of materials. In principle all elements on the periodic table can be simultaneously analysed by using LIBS, regardless of the state of aggregation. LIBS offers numerous applications in the field of civil engineering; most importantly the analysis of building materials. This work will focus on the evaluation of concrete structures and harmful substances which can penetrate the concrete. A variety of information can be collected through LIBS. Determining the concentration of harmful substances like chloride, sodium or sulphur, the examination of the carbonation depth and the distinction between varying layers of materials (e.g. aggregates, cement paste, metals, etc.) are possible applications. All this information can be provided through one LIBS measurement in the form of a high resolution 2D element map, with resolutions up to 0.1 mm x 0.1 mm. To scan a concrete surface only an optical access is needed. To create a depth profile of an intruding substance the extraction of a drill core is necessary. Onsite measurements via LIBS can be conducted by using a mobile version of the LIBS system. Through using calibration curves LIBS allows not only the qualitative but also quantitative analysis of element concentrations. All those prospects make LIBS a trendsetting method to secure the integrity of infrastructures in a sustainable manner.
The Laser-Induced Breakdown Spectroscopy (LIBS) is a laser spectroscopic method which allows a time efficient, minor-destructive, chemical analysis of materials. In principle all elements on the periodic table can be simultaneously analysed by using LIBS, regardless of the state of aggregation. LIBS offers numerous applications in the field of civil engineering; most importantly the analysis of building materials. This work will focus on the evaluation of concrete structures and harmful substances which can penetrate the concrete. A variety of information can be collected through LIBS. Determining the concentration of harmful substances like chloride, sodium or sulphur, the examination of the carbonation depth and the distinction between varying layers of materials (e.g. aggregates, cement paste, metals, etc.) are possible applications. All this information can be provided through one LIBS measurement in the form of a high resolution 2D element map, with resolutions up to 0.1 mm x 0.1 mm. To scan a concrete surface only an optical access is needed. To create a depth profile of an intruding substance the extraction of a drill core is necessary. Onsite measurements via LIBS can be conducted by using a mobile version of the LIBS system. Through using calibration curves LIBS allows not only the qualitative but also quantitative analysis of element concentrations. All those prospects make LIBS a trendsetting method to secure the integrity of infrastructures in a sustainable manner. A variety of examples which show the performance and results of LIBS analysis will be presented.
Data processing in the calibration-free laser-induced breakdown spectroscopy (LIBS) is usually based on the solution of the radiative transfer equation along a particular line of sight through a plasma plume. The LIBS data processing is generalized to the case when the spectral data are collected from large portions of the plume. It is shown that by adjusting the optical depth and width of the lines the spectra obtained by collecting light from an entire spherical homogeneous plasma plume can be leastsquare fitted to a spectrum obtained by collecting the radiation just along a plume diameter with a relative error of 10−11 or smaller (for the optical depth not exceeding 0.3) so that a mismatch of geometries of data processing and data collection cannot be detected by fitting. Despite the existence of such a perfect least-square fit, the errors in the line optical depth and width found by a data processing with an inappropriate geometry can be large. It is shown with analytic and numerical examples that the corresponding relative errors in the found elemental number densities and concentrations may be as high as 50% and 20%, respectively. Safe for a few found exceptions, these errors are impossible to eliminate from LIBS data processing unless a proper solution of the radiative transfer equation corresponding to the ray tracing in the spectral data collection is used.
Concrete is a multiphase material made of cement, aggregates and water. The heterogeneity of concrete is a result of mixing aggregates (grain size between 0.125 mm to 32 mm) and cement (grain size < 0.04 mm) together. There are always aggregates with a grain size below 0.125 mm (flour grains) therefore the cement matrix is always a mixture of these small particles. Different grain size distributions by making concrete are important to ensure the needed density and com-pressive strength. In order to estimate the remaining lifetime of concrete structures the quantification of element concentra-tions of alkali and chlorides regarding to the cement matrix only (1/3 of the total mass), is a major concern in civil engineer-ing. Due to two-dimensional scanning with LIBS the coarse aggregates can be evaluated and excluded from the analysis. In the case of particles (flour grains and cement) smaller than the laser spot size, the microscopic hetero-geneity influences the laser-plasma interaction and has therefore an impact on the results. In this study the influence of the micro-heterogeneity and the impact on the laser-material interaction is examined. The effect of changing ratios between cement particles and aggregate particles in the laser-induced plasma has been analyzed (see figure 1, right). Therefore, different samples with defined grain sizes are prepared and the obtained distributions were analyzed with x-ray diffraction (XRD). For the LIBS measurements, an automated system that operates with a low energy NdCr:YAG laser (pulse energy of 3 mJ, a wavelength of 1064 nm, a pulse width of 1.5 ns, a repetition rate of 100 Hz) and a NIR Czerny-Turner spectrometer has been used.
For concrete production aggregates, cement and water are mixed together and after 28 days of hydration a porous and multiphase material is formed. To consider the heterogeneity of concrete a 2D scanning system is used. The coarse aggregates (limit > 2 X d ) can be excluded. laser spot The remaining content is a mixture of flour grains and cement particles (micro heterogeneity). Harmful species like chlorides may penetrate together with water through the capillary pore space. A quantification of Cl regarding to the cement content only (European standard EN 206) is necessary for the damage assessment. A LIBS system operating with a NdCr:YAG laser (pulse energy of 3 mJ, a wavelength of 1064 nm, a pulse width of 1.5 ns, a repetition rate of 100 Hz) and two Czerny-Turner spectrometer (UV and NIR range) have been used.
One important aim of precision agriculture (PA) is the optimization of crop growth by means of site-specific measures, e.g. fertilizer application. Thus, PA should contribute to a resource efficient and ecofriendly soil management. Due to the expenses associated with traditional methods of soil analysis, requiring sample collection and laboratory analysis, PA technologies are still not in widespread use. Therefore, the aim of the project “I4S – Integrated System for Site-Specific Soil Fertility Management” is the development of a field-deployed, sensor-based analysis system offering rapid, cost-effective and spatially-resolved fertilizer recommendations.
In this system, laser-induced breakdown spectroscopy (LIBS) could be ideally suited to assessing elemental nutrient contents of soils. In addition to low cost, durability and small size, a reliable quantification procedure is a crucial requirement for such a system. However, the texture as well as the composition of the soil can affect the spectra. This matrix dependence is the key challenge to be addressed in the application of LIBS for soil evaluation. The focus of this work was the establishment of a LIBS method for soil analysis in a laboratory environment for future field application.
Natural soil samples of various origins, textures and compositions were used to characterize the matrix dependence of the LIBS spectra. Reference samples were prepared by adding defined amounts of the target elements to the soils (standard addition). Signals not superimposed by peaks of other elements were identified for each element. The reference samples also provided calibration curves for the respective soil type when the initial concentrations in the soils were taken into account. Additionally, the common laboratory method ICP-OES following aqua regia extraction was used to obtain reference values. Various approaches of calibration free evaluation of the data were also evaluated. In addition to traditional single-pulse experiments, the advantages of dual-pulse LIBS in relation to signal intensity, reproducibility as well as overcoming the matrix dependence of soil spectra were investigated. These methods were subsequently applied to validation samples collected on a dense grid within a field.
LIBS (laser-induced breakdown spectroscopy) is known as a fast and simultaneous multi-element analysis with little or no sample preparation. Because of that, over the last few years there has been a growing interest in applications of LIBS in the field of agriculture.
As part of the National Research Strategy BioEconomy 2030 the German Federal Ministry of Education and Research (BMBF) started an innovation programme called BonaRes. BonaRes consists of ten interdisciplinary research project as-sociations which are dealing with soil as a sustainable resource for the bio-economy. One of these research projects is I4S (intelligence for soil) which has the goal to develop an integrated system for site-specific soil fertility management. This system includes a sensor platform, which contains different sensors, like XRF, VIS-NIR, Gamma and LIBS. The main task of LIBS measurements in this project is the real time determination of the elemental contents of major and minor nutrients in soils, like calcium, magnesium, potassium. For this purpose, a special setup has been designed. The sample uptake operates with the help of a rotatable sample plate, on which the loose soil sample can be placed in form of a track. The sample plate circulates with different velocities to simulate the application on the field. To provide a higher intensity and a better re-producibility of the obtained signal, a double-pulse Nd:YAG laser (1064 nm) was used. In order to minimize dust formation from the soil during the operation of the laser, a dust removal by suction has been integrated When using relative methods such as LIBS, a suitable calibration curve is needed for absolute quantification. Within the I4S project a large number of soil samples from different testing grounds with known data of chemical composition, texture etc. is available. For the first calibration curves seven soil samples from different grounds in Germany were prepared as reference materials and four certified reference materials from China and Canada were purchased. With the help of these reference materials, calibration curves for different elements were initially calculated based on internal standard addi-tion. Copper was used as internal standard because of its low concentration in soils. The complex matrix of soils, as well as the influence of moisture and grain size in soils makes the absolute quantification by LIBS challenging. To overcome these influences, a calibration curve based on multivariate analysis was generated. Therefore, baseline correction on the second derivative with a Savitzky-Golay filter was followed by a partial least squares regression (PLSR). Multivariate analysis leads to noise reduction and neglection of interfering signals. Therefore, a better correlation between signal intensity and nutrient concentration is observed and a robust calibration curve is obtained.
Kinetic model of stimulated emission created by resonance pumping of aluminum laser-induced plasma
(2017)
Stimulated emission observed experimentally in an aluminum laser induced plasma is modeled via a kinetic approach. The simulated emission at several cascade transitions is created by a pump laser guided through the plasma at several microseconds after its creation and tuned in resonance with the strong 3s²3p-3s²4s transition at 266 nm. A two-dimensional space-time collisional radiative plasma model explains the creation of the Population inversion and lasing at wavelengths of 2100 nm and 396.1 nm. The population inversion for lasing at 2100 nm is created by depopulation of the ground 3s²3p state and population of the 3s²5s state via the absorption of the resonant radiation at 266 nm. The population inversion for lasing at 396.1 nm occurs during the laser pulse via the decay of the population of the pumped 3s²5s state to the excited 3s²4s state via cascade transitions driven optically and by collisions. In particular, efficient are the mixing transitions between neighboring states separated by small gaps on the order of kT at Plasma temperatures of 5000–10 000 K. The model predicts that the population inversion and corresponding gain may reach high values even at very moderate pump energy of several lJ per pulse. The efficiency of lasing at 2100 nm and 396.1 nm is estimated to be ~3% and 0.05%, correspondingly with respect to the pump laser intensity. The gain for lasing at 396.1 nm can reach as high as ~40 cm⁻¹.
The polarization effect that the pump radiation at 266 nm imposes on the stimulated emission at 396.1 nm is discussed. The calculated results are favorably compared to experimental data.
2D evaluation of spectral LIBS data derived from heterogeneous materials using cluster algorithm
(2017)
Laser-induced Breakdown Spectroscopy (LIBS) is capable of providing spatially resolved element maps in regard to the chemical composition of the sample. The evaluation of heterogeneous materials is often a challenging task, especially in the case of phase boundaries. In order to determine information about a certain phase of a material, the need for a method that offers an objective evaluation is necessary. This paper will introduce a cluster algorithm in the case of heterogeneous building materials (concrete) to separate the spectral information of non-relevant aggregates and cement matrix. In civil engineering, the information about the quantitative ingress of harmful species like Cl−, Na+ and SO2−4 is of great interest in the evaluation of the remaining lifetime of structures (Millar et al., 2015; Wilsch et al., 2005). These species trigger different damage processes such as the alkali-silica reaction (ASR) or the chloride-induced corrosion of the reinforcement. Therefore, a discrimination between the different phases, mainly cement matrix and aggregates, is highly important (Weritz et al., 2006). For the 2D evaluation, the expectation-maximizationalgorithm (EM algorithm; Ester and Sander, 2000) has been tested for the application presented in this work. The method has been introduced and different figures of merit have been presented according to recommendations given in Haddad et al. (2014). Advantages of this method will be highlighted. After phase separation, non-relevant information can be excluded and only the wanted phase displayed. Using a set of samples with known and unknown composition, the EM-clustering method has been validated regarding to Gustavo González and Ángeles Herrador (2007).
For the damage assessment of reinforced concrete structures the quantified ingress profiles of harmful species like chlorides, sulfates and alkali need to be determined. In order to provide on-site analysis of concrete a fast and reliable method is necessary. Low transition probabilities as well as the high ionization energies for chlorine and sulfur in the near-infrared range makes the detection of Cl I and S I in low concentrations a difficult task. For the on-site analysis a mobile LIBS-system (k = 1064 nm, Epulse ≤ 3 mJ, t = 1.5 ns) with an automated scanner has been developed at BAM. Weak chlorine and sulfur signal intensities do not allow classical univariate analysis for process data derived from the mobile system. In order to improve the analytical performance multivariate analysis like PLS-R will be presented in this work. A comparison to standard univariate analysis will be carried out and results covering important parameters like detection and quantification limits (LOD, LOQ) as well as processing variances will be discussed (Allegrini and Olivieri, 2014 [1]; Ostra et al., 2008 [2]). It will be shown that for the first time a low cost mobile system is capable of providing reproducible chlorine and sulfur analysis on concrete by using a low sensitive system in combination with multivariate evaluation.