Laser in Environmental and Life Sciences

The understanding of atmospheric processes has become an important scientific topic and economic factor: Weather forecasting influences wide areas of modern life; if certain industries are located in unfavorable sites, residential areas may suffer from industrial pollution. A comprehensive understanding of these processes requires theoretical models as well as extensive measurements.

Since many years lasers have been used in atmospheric diagnostics, especially in remote sensing. In particular the lidar method (described in Chap. 1) has become a powerful technique to monitor atmospheric parameters and has helped to understand a variety of atmospheric phenomena.

Digital holography presents a modern method for micro particle diagnostics, generating three dimensional snapshots of particle distributions in volumes in a rapid sequence.

Holographic techniques for micro particle analysis were already introduced by Silverman et. al. in 1964 (Silverman et. al. 1964), (Vikram 1992). Scientists pushed that technology forward to analyze atmospheric aerosol distributions in clouds. In 1994 the researchers Borman and Jaenicke made a comparison concerning the performances of a conventional holographic aerosol analysis system with the standard FSSP-100 and PVM-100 devices (Hohmann et al. 1994). The particle range in that investigation was in the range of 3–15μm diameter. With their new holographic method they could store 450 liters (0,45m³) of cloud volume in a hologram using one exposure with a recognizable particle size of 5–500μm (Uhlig 1995). However, the technical requirements for hologram exposure (ruby laser), hologram development (dark room), droplet reconstruction (reconstruction set-up in the lab) were enormous. Furthermore, analyzing the reconstructed cloud volume is extremely time consuming (Vössig et. al. 1997).

To overcome these disadvantages, a mobile system based on the method of digital holography has been developed and tested in a field campaign.

This innovative digital holographic measurement system uses the principle of Fraunhofer inline holography and provides a series of advantages compared to conventional holographic systems regarding time consumption and system design for both holographic storage and hologram reconstruction or hologram analysis.

Within the last years, the laser-based analysis of solids has developed so far that it can be applied generally and not only in the laboratory. The increasing number of applications prove this. In the following, projects shall be presented which make use of lasers concerning the analysis of solids as well as pollutants on solid surfaces in the field of basic and mainly applied research. Of course, this can only be a specific selection of the actual applications and research programmes due to the various utilized techniques and the numerous investigated substances.

Laser-induced fluorescence (LIF) spectroscopy is of great importance for environmental monitoring. Besides outstanding sensitivity and good selectivity, particular advantages of the LIF technique include the capabilities for in situ analysis and remote sensing. The major advantage of in situ LIF measurements is the lack of sampling and clean-up procedures preceding the analysis. Such procedures are error-prone, time consuming and expensive. The contamination of water or soil with petroleum products (oils) represents a major environmental risk. Since most petroleum products exhibit distinct native fluorescence it is promising to apply LIF analysis to the detection and characterization of oils in environmental coinpaitments. The LIF investigation of oil-polluted waters with LIDAR and fiber optical sensing techniques is well advanced and appropriate instrumentation is commercially available. The employment of fluorescence techniques for in situ analysis of soil contaminations has received considerable attention only during the last decade. The combination of LIF instrumentation with geotechnical drilling equipment for real-time subsurface detection of oil pollutions, as pioneered by Lieberman et al. (Lieberman 1990), and field demonstrations have recently been described in detail in technology reviews and a monograph (Hart 1997, Lieberman 1998, Balshaw-Biddle 2000). Importantly, LIF-based techniques find increasing regulatory acceptance and have been verified as field screening methods for petroleum products by the US Environmental Protection Agency (US-EPA). For assessment and control of the environmental damage imposed in Kuwait during the Gulf War LIF investigations play an important role in what may become the “biggest environmental remediation project ever attempted” (Shouse 2001, Quinn 1995). In Germany, various aspects of LIF analysis of soils have been addressed in works of, among others, Niessner et al., Schade et al., Marowsky et al., Zimmermann and Lucht (Baumann 2000, Schade 1996, Marowsky 2001, Zimmermann 1997, and references therein), as well as by our group (Löhmannsröben 1996, 1997, 1999, 2000).

Laser induced breakdown spectroscopy, LIBS, is a method utilizing laser ablation and the subsequent atomic emission from the plasma for elemental analysis. Besides the acronym LIBS, today other designations such as LIPS (laser-induced plasma spectroscopy), LA-OES (laser ablation optical emission spectroscopy), or LSS (laser spark spectroscopy) can be found in the literature. Laser ablation is at present the only analytical method that offers direct sampling from any kind of material without sample preparation. So LIBS allows a multielement analysis of virtually all type of materials (gas, solids, liquids) through atomic emission spectroscopy. Today’s availability of reliable and less costly laser sources and improved detectors permits a rapid, on-line, and in-situ analysis with LIBS. This makes LIBS especially attractive for all kind of process analysis and environmental screening and monitoring.

This contribution presents three different experimental approaches for studying molecules in condensed phases: an intracavity Raman technique which is used to identify isomeric samples which could not be separated by standard HPLC settings, a laser desorption technique which provides the transport of large molecules into the gas phase under mild conditions and the cavity-ring-down spectroscopy for condensed phases. The latter method developed recently has been applied to questions in the field of nano-technology.

The majority of environmentally relevant substances can be quantitatively analyzed in various matrices — provided that the samples may be sent to an analytical laboratory. The analysis there involves an extraction and a clean-up step prior to the -mostly chromatographic — separation and analysis. Thus this procedure is quite time-consuming. Moreover, these laboratory based techniques are very costly if continuous monitoring is necessary or if hundreds of soil samples have to be analysed to assess the hazard potential of a particular site. For these screening and monitoring applications optical methods are much more suitable.

Chemical trace and ultra-trace analysis has reached an exceptionally high technological level during the last decades. This is mainly due to the combination of two or even more analytical methods which resulted in a “multi-dimensional” selectivity. The leading “two-dimensional” analytical technique is gas chromatography-mass spectrometry. The price for this high technological standard is a time-consuming sample preparation. Thus, species-selective detection of traces of organic pollutants (e.g. dioxins or pesticides) may take days or even weeks. Therefore, these conventional methods of trace analysis are not adapted to special problems such as: A prerequisite to solve these problems is the availability of rapid on-line or mobile on-site methods of selective detection. Their development is a challenge of modern technology and subject of modern research.

Unlike any other field of instrumental analysis, mass spectrometry is in a phase of highly dynamic development, in which new areas of applications and technical or methodical innovations stimulate one another. Its popularity arises from its universal applicability, its high sensitivity, the quickness of its measurements and their high information content. On the instrumental side, besides the continuous improvement of the conventional types of mass spectrometers, the development of novel ones on the basis of magnetic and electrodynamic ion traps (March and Todd 1995) has to be mentioned. However, the most important improvements in the field of mass spectrometry within the last decade are probably due to the development of new ionization techniques. These are, on the one hand side methods which allow the intact ionization of large, in particular biologically and medically relevant, molecules (e.g. Electrospray Ionization (Gaskell 1997) and Matrix Assisted Laser Desorption/Ionization (Karas et al. 1991) and on the other side techniques which involve a certain degree of selectivity in the ionization step. Selectivity is imperative for the analysis of complex mixtures and is usually added to a mass spectrometric measurement in form of chromatographic preseparation. However, chromatographic techniques eliminate one of the major advantages of mass spectrometry: the quickness of the measurement. Furthermore, they require a considerable effort for the sample preparation, which as far as time consumption is concerned in many cases exceeds that for the instrumental analysis step by far. Finally, sample preparation and clean-up are a major source of quantitative errors.

Optical sensors based on semiconductor lasers are at the threshold of routine applications in gas analysis and increasingly these sensors are used for industrial and environmental monitoring applications whenever sensitive, selective and fast in-situ analysis in the near- and mid-infrared spectral region is required. With the increasing complexity of processes, online gas analysis is becoming an issue in automated control of various industrial applications such as combustion and plasma diagnostics, investigations of engines and automobile exhaust measurements. Other challenges are online analysis of high purity process gases, medical diagnostics and monitoring of agricultural and industrial emissions (VDI 2002). The need to meet increasingly stringent environmental and legislative requirements has led to the development of analyzers to measure concentrations of a variety of gases based on near- and mid-infrared absorption spectroscopy.

Lasers have found a wide-spread application in life sciences, in particular in the field of biomedical research and clinical diagnostics. To date, these applications mainly involve laser-based instruments for imaging purposes or for therapeutic use, the latter exploiting the thermal or ablative effect of laser radiation interacting with biological tissue. However, modern laser systems get more and more useful also for analytical purposes in biomedical research. This contribution is intended to introduce the particular demands, advantages and problems of laser-based analytical techniques in life sciences, and to discuss in particular the medical and clinical aspects.

We have been heavily involved in the development of spectroscopic methods for the investigation of the IR spectra of gaseous free radicals and open shell molecular ions. Based on our expertise in molecular gas lasers, we have explored Laser Magnetic Resonance (LMR) particularly in combination with carbon monoxide (CO) lasers. LMR was first developed by K.M.Evenson and his co workers in 1968 in combination with FIR lasers for pure rotation spectroscopy of free radicals (Evenson 1968). Since these species are short lived and cannot be generated in high concentrations in general, the necessity of a very sensitive detection scheme is obvious.

Ambient air and the human breath both contain many different volatile organic and inorganic compounds. Most of them are present in very low concentrations. The analysis of these trace gases leads in case of ambient air to a better understanding of the atmospheric chemistry and in the case of breath tests to a deeper knowledge of the physiological processes in the human body. Amongst other sources these compounds are produced in the human organism and find their way via the blood through the lungs into the human breath. Most of the volume fractions of these trace gases are on the order of some ppb (parts per billion: 1:10⁹) down to several ppt (parts per trillion: 1:10¹²). This shows the need for ultra sensitive analytical methods for the in-vivo monitoring of human breath, which helps to understand various physiological and pathophysiological processes in the human organism.

Cavity ring-down spectroscopy (CRDS) is revolutionizing the sensitivity, speed, ease of use, robustness, and portability of trace chemical species detection. Today, CRDS is being commercialized across a broad range of application areas, including medical diagnostics, industrial process control, environmental monitoring, and civilian and military security. In this monograph we will address the application of CRDS to medical breath testing, a new field in medical diagnostics that promises to usher in a new era of testing methodology with improved specificity, sensitivity, and painless administration compared to the current generation of medical tests.

When talking about trace gas detection the usual association is that of monitoring environmental pollution, either as emission (from stack, car exhaust etc.) or immission control (ambient concentrations). A different field, but of growing importance, is the analysis of trace gas emissions from living organisms, either plants, animals or humans. These organisms synthesise different volatile compounds as part of their metabolism, in connection with developmental processes or as a reaction of the organism to external influence. Monitoring the emitted compounds in the air may thus yield important information about the organism and its interaction with the environment in a non-invasive way.

DNA adducts are the direct products of damage by endogenous or exogenous reactive agents to a critical macromolecular target such as DNA (Figure 17.1). Approximately 90% of the chemicals considered carcinogenic for humans form DNA adducts (Stiborovà et al. 1998). Although the majority of such DNA damage is eliminated by DNA repair processes, some persistent adducts often cause permanent mutations in important growth-controlling genes or loci, resulting in aberrant cellular growth and cancer (Hemminki et al. 2000). The amount of these reflects an integration of both the toxicokinetic properties of a genotoxic compound and the cellular DNA repair. Measurements of endogenous and exogenous DNA adducts are thus of interest in that they provide molecular, mechanism-based bridges between carcinogen exposure and disease end-points and can serve as biomarkers for carcinogenesis. In practice, investigations of such DNA damage can be viewed as a supplementary test that may, in some instances, help to facilitate the assessment of a compound with an unusual or puzzling profile of activity in statutory genotoxicity assays, animal bioassays, or both (Phillips et al. 2000).