Applications of Analytical Chemistry to Oceanic Carbon Cycle Studies (1993) / Chapter Skim
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Technologies for Chemical Measurements
Technologies for Chemical Measurements
Pages 33-72
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From page 33... ...
MASS SPECTROMETRY (FENN ET AL., 1990) Mass spectrometry (MS)
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From page 34... ...
Determining the sources and fates of individual organic compounds will be a major theme in marine organic geochemical research in the future, and its dependence on MS instrumentation is extreme. Marine laboratories in other countries are quickly entering this area, but laboratories in the United States are experiencing difficulty in finding funding to purchase the necessary instrumentation.
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From page 35... ...
Gas chromatography combined with MS is a mature technique for analyses of complex mixtures and isotopic analysis. Liquid chromatography can be combined with MS to examine polar, labile substances without chemical derivatization, and is already applied in a variety of marine laboratories.
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This AMS facility may also eventually be used for measuring Abe for marine geology research. Inductively coupled plasma-mass spectrometry (ICP-MS)
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From page 37... ...
. 1991 1996 2006 Overall sensitivity of detection 1-100 picomoles 1-100 femtomoles Attomole Molecular Weight Laser matrix time of flight Electrostatic quadrupole Time of flight Electrostatic-sector >3000 daltons LC-MS Flow probe Sector Flow electrostatic quadrupole Electrostatic-ion trap 300,000 daltons 130,000 daltons <500 daltons 5-50 picomoles <1200 daltons ~5000 daltons <1 picomole ~3000 daltons 106 daltons 106 daltons 10,000 daltons >10,000 daltons Femtomole near future: potentiometry, constant-potential techniques at steady state, pulse voltammetry, stripping voltammetry, and coulometric titrations (see Whitfield and Jagner, 1981, for review)
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From page 38... ...
Constant Potential Techniques at Steady State Electrochemical detection under convective conditions has been applied widely in freshwater measurements. In addition, seawater measurements have been combined with flow injection analysis (FIA)
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From page 39... ...
Stripping Voltammetry (Zirino, 1981; Shuman and Martin-Goldberg, 1984; Van der Berg, 1989) Voltammetry can also be employed in the stripping mode; that is, the material of interest is accumulated in or on the electrode, and once concen
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From page 40... ...
The technique of absorptive stripping voltammetry holds great promise for the determination of electroactive substances that can be adsorbed at an electrode. New developments in the use of microelectrodes for stripping voltammetry provide the possibility of extending the extensive body of laboratory procedures now employed to in situ measurements.
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From page 41... ...
Finally, it can be used for the generation of standards, particularly for substances that are difficult to prepare and store, and for in situ calibration. Coulometry may also be used for detection in liquid chromatography and flow injection analysis.
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From page 42... ...
This is an area of research that could pay off through qualitative improvements in accuracy, precision, and response time. Developments in electroanalytical chemistry are driven by technical advances in electronics, computers, and materials.
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From page 43... ...
Photoacoustic and thermal tensing spectrophotometries make use of the conversion of absorbed light into heat to detect absorbance indirectly by measuring sound evolved and deflection by changes in refractive index, respectively. For the purposes of chemical oceanography, spectrophotometry is limited to wavelengths from the UV to near-IA (250 to 1500 nanometers)
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From page 44... ...
Developments in traditional forms of spectrophotometry, as well as new methods, could find greater use in ocean measurements. Spectroscopy based on absorption of visible light may have reached a limit in its traditional form, however.
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From page 45... ...
In FT-IR, an entire spectrum is acquired simultaneously as an "interferogram," in which the absorbance data are encoded; the spectral data are obtained by calculating the Fourier transform of the interferogram. The advantages of this approach are its efficiency, in that it effectively scans the entire spectrum at once rather than serially in small increments; its generally higher resolution; and the capability for digital signal averaging and other manipulations that are otherwise difficult.
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From page 46... ...
Chemical reactions designed to attach fluorescent markers to amino acids allow measurement at natural concentrations by high-performance liquid chromatography. This is a model for techniques that might be developed for other types of organic compounds.
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Some recent research has shown that chemiluminescent methods may be adapted for in situ determinations of some metal ions (see the section on flow injection analysis)
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While inductively coupled plasma techniques are unsuited for in situ use, they have been integrated into flow injection and chromatographic systems. Incorporation of extraction and Reconcentration into a continuously operating system appears difficult.
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From page 49... ...
A substantial limitation arises from interference by photoluminescent impurities in the sample, generally constraining Raman spectroscopy to longer wavelength lasers, even though the strength of the scattering declines as the fourth power of the frequency of the incident light. At present, Raman spectroscopy for ocean measurements is limited by the weakness of the phenomenon and the type of equipment required.
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From page 50... ...
Fiber optics can be used to deliver light over long distances. Fiber optic sensors provide a means to measure properties of seawater continuously using changes in the characteristics of a transmitted light beam.
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The bottom figure shows a fiber optic sensor for carbon dioxide.
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From page 52... ...
In systems where irreversible chemistry may be useful, reagent delivery systems may be employed to replenish reagents continuously (Luo and Walt, 19891. Sampling with fiber optic sensors can be continuous if needed; otherwise they can be operated discontinuously, with a lower duty cycle.
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From page 53... ...
Even immobilized enzymes have been employed as selective coating materials (for example, formaldehyde sensing using immobilized formaldehyde dehydrogenase; Guilbault, 19841. Indeed, the nature and selectivity of the coating material used to achieve chemical recognition of the target species is the key to fabricating useful analytical sensors based on piezoelectric devices.
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From page 54... ...
This will make the implementation of piezoelectric chemical sensing devices for ocean measurements rather difficult, but by no means impossible. Indeed, the coupling of pattern recognition techniques with an array of marginally selective piezoelectric transducers may, in the future, make these devices more useful for quantitative ocean measurements.
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From page 55... ...
A practical factor limiting the routine use of immunochemistry for environmental monitoring is that many U.S. Environmental Protection Agencyapproved methods require analyses by gas chromatography and mass spectrometry (GC-MS)
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This selectivity can be either intrinsic, that is, built into the chemical properties of the material, or coupled with selective carriers that allow a non-selective material to be converted into a selective one (see the section on recognition chemistry)
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Successful development and implementation of various chemical sensors for ocean measurements (based on optical, electrochemical, or mass transducers) requires concomitant advances in the design or discovery of organic or inorganic molecules that interact selectively with the important ocean analyses.
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From page 58... ...
Indeed, the design of metal ion binding ligands for use in sensors, as well as for preparative separation processes in metallurgy and related areas, is currently an active area of research. A wide range of crown ethers, cryptands, hemispherands, and acyclic molecules containing electronegative oxygen atoms or mixed oxygen and nitrogen electron donor atoms in appropriate geometric positions within these ionophore structures have been synthesized, and some of these compounds have been used for the development of relatively selective optical and electrochemical sensors for Na+, Ca2+, Li+, K+, and most recently Mg2+ (Figure 31.
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From page 59... ...
TECHNOLOGIES FOR CHEMICAL MEASUREMENTS <04 Cogoo: of To ,0 o 40W LooWo: Bis(Benzyl-1 5-Crown-5) Heptanedoate (K+)
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From page 60... ...
The use of large organic structures containing three-dimensional chemical recognition pockets is not limited to porphyrin-type molecules. Indeed, various cyclodextrins exist in which the orientation of the repeating carbohydrate structure forms a hydrophobic inner core that can interact selectively with hydrophobic chemical analyses of appropriate size (benzene, pyrene, phenols, and others)
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From page 61... ...
Electrophoresis separates materials on the basis of electrical charge and size as solvents flow through the plate. Gas chromatography and liquid chromatography are mature separation techniques that are used for many ocean measurements.
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From page 62... ...
Future developments that may facilitate ocean measurements from vessels or buoys include miniaturization of chromatographic equipment (so less solvent is needed per analysis) , new solvent transport systems, such as electrokinetic transport, to reduce power requirements on the pumps, and more sensitive detectors for liquid chromatography.
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From page 63... ...
There are hybrid methods that allow separation of neutral solutes by interactions similar to liquid chromatography. The power requirements for capillary zone electrophoresis are low, so it could be used to drive flow injection analysis.
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From page 65... ...
However, the mechanical simplicity of an osmotically pumped CFA system would certainly fall within the framework of a chemical sensor (Ruzicka and Marshall, 1990)
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From page 66... ...
Although Technicon Autoanalyzers~ have been taken to sea for 20 years, these fall into the dedicated automation category and are not good examples of the potential of flexible automation, which allows automation of numerous sequential steps. Flexible automation includes more sophisticated laboratory robots, with a spectrum of operational complexities and a moderate number of samples.
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From page 67... ...
In fact, robot-based laboratory procedures can provide a record of all steps for quality control purposes. Because of vibrations, power stability, and particularly corrosion, commercial laboratory robotic systems available today would have problems on ships.
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This brief discussion of Chemometrics is meant to provide sources of details about Chemometrics methods rather than giving the basic information itself. The books and review articles cited provide a thorough introduction to the major topics covered here that could be important for ocean measurements.
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From page 69... ...
The use of self-optimizing analytical instruments would be of great value for unattended instrumentation in ocean measurements. The application of standard experimental design methods to oceanic measurements should be a routine part of designing all experiments, particularly those that are expensive to perform and difficult to repeat.
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It is likely that software well suited to ocean science would have to be produced by combining commercially available software with custom-written software to gain the specificity needed. Communications One of the primary benefits of chemical sensors is the ability to obtain chemical data in real time.
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Chemometric procedures programmed into the instruments can be used to help in this data reduction process. A new generation of satellites that provide the equivalent of a global cellular communications network will be necessary to take full advantage of chemical sensor systems.
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