Chemical analysis in space: the challenge of instrument design

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chemistry,

vol. 4, no.’ 6, 1985

trends

Chemical analysis in space: the challenge of instrument design R. J. Simpson Chislehurst, U.K.

Introduction So far very little chemical analysis has been carried out in space, as compared with the number of measurements of other kinds made on both manned and unmanned flights. It would, however, be useful to perform at least the simpler analyses in flight, rather than to store samples for analysis after return to earth. It is in the field of Space Life Sciences that there is the greatest need for chemical analysis. Foremost among the disciplines covered by this broad heading are the allied topics of human physiology and crew health monitoring. Experiments in plant or animal physiology, including future plans for closed ecological systems, in space, imply close control of conditions, leading to a requirement for the analysis of atmospheres or aquatic environments, and the substrate on which plants are to be grown. Monitoring the composition of nutrient solutions for hydroponic culture will also be important. Let us imagine what would happen in an analytical laboratory deprived of gravity. The most obvious effect is that all equipment would tend to float about unless fastened down. Liquids would not remain at the bottom of containers but might well form globules clinging to the sides; open beakers would be unusable. Some of the analyst’s traditional tools such as pipettes and burettes must also be discarded, along with measuring cylinders and volumetric flasks. pH and reference electrodes, partially filled with solutions, would become useless. Variable area flowmeters, using a ball or float in a tapered tube, would not function correctly. Mass measurement, instead of being simple and precise, would become difficult and imprecise, especially for non-rigid objects, such as liquids in containers. Many basic operations such as filtration, solvent extraction or distillation would require special equipment or might become impossible. The removal of bubbles from liq01659936/S/$02.00.

uids would be difficult. Air-segmented streams in analysers, the production of gas during reactions, and the release of dissolved gases on warming solutions are thus all likely to produce problems. The absence of gravity virtually eliminates thermal convection. Thus when liquids are heated, active mixing is required. A candle flame relies entirely on convection for the removal of burnt gas and will go out in the absence of gravity. Enclosed flames, fed with fuel and oxidant, would probably continue to burn, however, flame detectors for chromatographs would thus in principle be usable. Gas-filled radiation sources, such as quartz-halogen bulbs, will require less power under zero gravity. The same applies to thermal infra-red sources. A feedback loop to control the applied power is desirable. Selection of analytical techniques Manned spacecraft

The initial selection of candidate analytical methods to perform a specified task is guided by the principle that everything on a spacecraft is rationed. There are overall budgets to be met for parameters such as power, volume, mass, cooling and possibly the amount of magnetic material used. Other limited resources are the use of crew time and access to certain facilities such as data storage, processing or transmission to earth. Methods may be eliminated because of their effect on the spacecraft environment, such as the generation of vibration or external fields. The need for continuous supplies of gases requires consideration because of the stringent safety regulations regarding high-pressure gas cylinders, and their mass. The use of hazardous materials must be avoided; toxic or radioactive substances are obviously undesirable. Materials of construction are sometimes toxic; infra-red transmitting materials such as KRS-5 or arsenic trisulphide are examples. In such cases the risks are enhanced by zero-gravity, since small particles of the material, produced by abrasion 0

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or fracture, will float around and could be ingested. The chemical output from an analyser must be contained or made harmless. Unmanned spacecraft

The criteria for selecting instrumental methods for use on unmanned space vehicles differ somewhat from those applying to manned missions. The limitations on size, mass and power are likely to be more stringent. The instrument must, of course, be wholly automatic, and its requirements for calibration must be minimised. Ideally the instrument should be so stable that readings of acceptable accuracy can be obtained without recalibration during the mission. Care is needed to ensure that mechanisms exposed to space vacuum do not become immobilised by cold welding. Dry lubricants such as molybdenum disulphide can be used, and low friction materials such as PTFE may also be employed. Wherever possible, rolling motion is preferable to a sliding action. In manned spacecraft the environment in which the instrument operates is well defined. In contrast, temperature control on unmanned craft is a major problem. The temperature of the instrument must be maintained within limits which ensure that it operates correctly, reliably and with the required accuracy. The thermal design must ensure that at the correct operating temperature, heat inputs from power consumed or the sun are balanced by losses by conduction and radiation. In addition to conditions encountered in space, the instrument must withstand those found in the other phases of the mission, even though it need not operate during those periods. The ambient conditions during the period before launch must be considered, for example. Launch accelerations and vibration are generally more severe than those experienced on manned flights. With re-usable space vehicles, such as Eureca, recovery and re-entry accelerations must also be considered, though these are generally less severe than the launch stresses. Where equipment is destined to land on another planet, landing accelerations and the gravitational, thermal and atmospheric conditions expected must be taken into account. Potentially usable techniques .In the following sections we discuss some analytical techniques which are potentially suitable for space use. These are examples; obviously there are other methods which could be used. The choice of method will depend on the nature of the problem and the resources available, including money. As a rule it is advantageous to make the analytical equipment as simple as possible, using clever data process-

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ing in preference to complex instrumentation. Analysis of gases Quadrupole mass spectrometry.

The quadrupole mass spectrometer has the merits of being compact, low power, and very versatile. Paradoxically, one of the major problems is the provision of a vacuum. On manned spacecraft there is usually a vent line; this is a pipe, with appropriate valves, leading from the pressurised laboratory to the outer surface of the spacecraft. Unfortunately, access to the vent line is restricted. In addition, the vacuum available is poor; leakages from the spacecraft produce a gas pressure of 10-4-10-s Torr in its vicinity, and the pressure in the vent line may be substantially higher than this. Consideration of power, weight and vibration exclude many types of vacuum pump. There is, however, the possibility of using the vent line as a backing pump, and an ion pump for maintaining low pressure in the spectrometer. The type of sample inlet used is important. Capillary inlets are good in many ways but require high pumping capacity. Membrane inlets or controlled leaks are more suitable, but also have disadvantages. For both of these, stability may be a problem. The permeability of membranes depends on which gas is considered, and is humidity and temperature sensitive. On unmanned spacecraft, the vacuum nearby is said to be 10-s-10-6 Torr. If this is the case, it then becomes possible to operate the spectrometer without a pump. However, the tighter limitations on available power may preclude the use of a quadrupole mass spectrometer for such applications. For low-molecular-weight gases there are only a few instances of peak overlap, and even then the use of other peaks can often resolve the ambiguity. The use of the technique in conjunction with gas chromatography provides a method of great analytical power which could be exploited in future experiments. Infra-red spectrometry. This, too, is a versatile technique, its chief deficiency being that it cannot detect elemental gases, such as oxygen, which have no dipole moment. An important advantage, compared with mass spectrometry, is that vacuum is not required. It is possible to make compact infra-red (IR) gas analysers of low mass with small power consumption. There are, of course, many variations on the basic principle. Wavelengths may be selected by discrete filters, or a grating can be used as a dispersing element, either in a scanning mode with a single detector, or with an array of pyroelectric detectors. Fourier transform (FT) spectroscopy is an attractive possibility provided that the physical size of the optics can be kept down. The use of a helium-neon

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laser to monitor mirror position has the disadvantages of increased size and power consumption; a solid state laser source may be a better solution. An FTIR instrument for space use does not require the same rigidity in its optical construction that is needed for terrestrial use. It will only have to operate in a microgravity low vibration environment. It must, of course, survive launch acceleration and vibration without misallignment of optical components, but clamps could be provided to give protection during this period. Gas cells need careful design. The external dimensions must usually be kept small, and requirements for fast response time or long path length may be difficult to achieve, especially where several gas concentrations are to be measured simultaneously. In most instruments some form of source modulation is required. A conventional chopper requires power and is a potential source of vibration; it is also inherently wasteful of source power since only half of this is used. Switching a low thermal mass source on and off is a possible alternative, though changes in the spectral energy distribution during the switching cycle must be taken into account. Gas chromatography (GC). The versatility and sensitivity of this technique are attractive for many applications. A conventional laboratory column oven is too large and consumes too much power for use in space. Capillary columns could be coiled more compactly to reduce the oven volume required, but will need support to withstand launch accelerations. Alternatively, the sub-miniature columns fabricated by etching a spiral track on silicon show promise. Reduction in column dimensions is important in its own right, and also brings benefits in terms of the power required for temperature control, as well as reducing carrier gas requirements. The need for continuous supplies of gases is a disadvantage. The use of detectors needing auxiliary gas supplies is thus to be discouraged. A possibility is to use electrolytically generated hydrogen as a carrier; this gas can be used with thermal conductivity detectors and may be catalytically oxidised after use. The disposal of gases leaving the chromatograph is, of course, a matter to be considered carefully. Small amounts of certain gases may be safely removed by the cabin air purifiers. For some experiments, the length of time required for running a chromatogram precludes the use of the technique. This applies, for example, to fast breath composition measurements. In other applications, speed is not important, and gas chromatography could be a viable technique. The analytical power of GC-F’TIR is attractive where the application warrants the complexity of the equipment.

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Analysis of liquids Liquid chromatography.

vol. 4, no: 6,1985

Under this heading we will consider not only high-performance liquid chromatography but also ion chromatography and ionexchange chromatography, since in many respects the instrumental requirements are similar. The use of a liquid as the carrier eliminates some of the problems associated with gases. However, it must be remembered that the liquid to be pumped through the column will not reside in the bottom of a reservoir bottle. Instead it must be stored in a vessel whose volume can change as liquid is withdrawn. The carrier liquid could be contained in a collapsible bag, or in a cylinder with a moveable piston. At the other end of the chromatography system similar considerations apply; the column effluent will not flow conveniently into a waste bottle but must be collected in a bag or other flexible vessel. For some analyses, not using gradient elution, the contamination of the carrier liquid by sample is only slight, and it may be possible to re-use it, making corrections if necessary for the accumulation of sample material. Safe containment of the solvents used is essential and in some cases several levels of containment may be called for; the enclosure of a bag in a solvent proof cylinder is an example. Solvent outgassing as the pressure diminishes on passing through the system is a familiar problem, as is the presence of bubbles in detector flow cells. While it is true that buoyancy will not be present to help to dislodge bubbles, it is also true that there will be no tendency for them to lodge at the highest point in the cell. Liquid chromatography detector techniques such as refractive index, conductivity and ultraviolet absorbance should all work well. Indeed the absence of convective atmospheric turbulence may reduce noise in optical detectors, though power regulation and cooling of radiation sources need care. As in GC, column dimensions are being reduced. Microbore columns at present being developed are attractive because of their small size and low carrier consumption. Thin-layer chromatography (TLC). The inherent simplicity and versatility of this technique make it an attractive prospect for space use. The conventional developing chamber with liquid solvent at the bottom cannot, of course, be used. Instead, the solvent must be held in a sponge or similar porous structure. Because of the stringent restrictions on the release of solvent vapour into the spacecraft air, the developing chamber must be completely sealed. If the plate is to be removed for examination, then solvent vapour must be extracted by the vent line, held in cold trap or removed with an adsor-

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bent, before the chamber is opened. Fragile glass plates represent a hazard and those on aluminium or plastic substrates are preferable. Development of TLC plates by spraying reagents presents safety problems even if this is done in a sealed chamber, since droplets which do not impact on a surface will float indefinitely. Electrostatic attraction of droplets onto the plate would be one way of reducing the risk, but it would be better to avoid spraying altogether if possible. TLC equipment can be small, and power requirements will be low. Only small volumes of solvents or reagents are needed. Information can be obtained from the plate in a variety of ways, including UV or visible spectrophotometry, fluorescence and FTIR. The plate can be stored after use, and separated compounds can be removed and analysed after the mission. If the complexity of the application demands it, two-dimensional TLC gives enhanced separation power. Electrochemical sensors. Ion selective electrodes and the related potentiometric dissolved gas sensors are well suited for analysis in space. Their small size, inherent safety, sensitivity and the low power required by the associated equipment are all attractive features. As in normal applications, stability and selectivity are potential problems. Any electrodes with. internal solutions, such as pH or reference electrodes, will need modification, since the solution may float inside the electrode. Solutions can be rendered viscous or made into gels to eliminate this problem, or they may be contained in a collapsible vessel within the electrode. The behaviour of reference electrode liquid junctions will be altered, since gravity-driven convective flow will be absent and only diffusion will operate, unless means are provided for applying pressure to the internal solution. The use of ion selective electrodes in a flow injection analysis system is an attractive possibility. The continuing development of ion-selective field effect transistors (ISFETs or ChemFETs) is leading to interesting possibilities in analysis. Their small size is attractive, particularly where sample volumes are low. These devices give a current output related to ion activity and should thus be less prone to electrostatic pick up effects than ion selective electrodes. Any form of electrochemistry involving mercury is likely to be forbidden in space applications. However, it is possible that voltammetry using carbon or other solid electrodes may be usable, either in its own right or in chromatography detectors. Conductivity measurements have the merit of simplicity, although they lack specificity. Measurement of urine conductivity, for example, would indicate total electrolyte excretion, and even though the in-

formation given is limited, it would be better than having no measurement at all. Dry reagent chemistries. Much work has been done on so-called dry reagent analysis, mainly by firms in the field of clinical analysis. All the reagents required for the analysis are immobilised, often as a series of layers on an inert substrate. Immobilised enzymes are sometimes incorporated into one of the layers. The analytical result may be detected colorimetrically or by fluorescence. In some systems thin film electrodes are incorporated, and the indication is potentiometric or conductimetric. Attractive features of this approach are that the reagents are totally contained; the associated equipment could be made small and with low power consumption. Dry calorimetric methods, in the form of paper tapes, are also used for the determination of certain gases. Common to all these techniques are the problems of liquid handling in zero gravity. Total containment is necessary throughout the operation. Obtaining the sample to be analysed, conveying it to the analyser and introducing it, are operations needing particular attention. Conclusions In order to give an idea of the problems and constraints facing the instrument designer developing analytic equipment for use in space, we have discussed in outline a few potentially viable techniques, as examples. Many familiar instrumental techniques can be readily rejected for reasons which have been discussed above. Examples are atomic adsorption (flame or furnace), magnetic sector mass spectrometry, X-ray fluorescence and Raman spectroscopy. The reader may care to consider why it is unlikely that any of these techniques is likely to be used in space in the foreseeable future. It is of course possible that in years to come the restrictions on power, mass, size and crewtime usage may be relaxed to some extent, but at present these factors are among those that govern the activities of the analytical chemist with his eyes on space. References The following papers, are of course, only a very small selection from the vast number of papers on analytical instrumentation. They have been picked out because they describe developments which are, or may in the future be, significant in the field of chemical analysis in space. Chromatography

H. Wohlten, Anal. Chem., 56 (1) (1984) 87A. (Microsensors and microinstrumentation.)

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142 Electrochemical gas sensors

W. J. Albery and P. Barron, J. Electroanal. Chem., 138 (1982) 79. (A membrane electrode for the determination of CO, and Oz.) Mass Spectrometry NASA report: Contract No NAS9 - 15817. (Gas analyser mass

spectrometer.) I. Soda1 and G. D. Swanson, EMB Magazine, March (1982) 32. (Making the mass spectrometer an efficient anaesthetist’s aide.) Flow Injection Analysis

J. Ruzicka and E. H. Hansen, Flow Injection Analysis, Wiley, Chichester, New York, 1981. Liquid Chromatography M. B. Masters, Anal. Proc., 21 (1984) 322. (The latest devel-

opments in ion chromatography.) Piezoelectric sensors

J. F. Alder and J. J. McCallum, Analyst (London), 108 (1983) crystals for mass and chemical measurement. A review.) 1169. (Piezoelectric

Spectrophotometry

W. L. Truett and J. P. Dybwad, Znt. Lab., 14 (1984) 26. (A portable FTIR spectrometer using a rotating refractor.) Fibre Optics

W. R. Seitz, Anal. Chem., 56 (1) (1984) 16A. (Chemical sensors based on fibre optics.)

J. I. Peterson and S. R. Goldstein, Anal. Chem., 52 (1980) 864. (Fibre optic pH probe for physiological use.) Potentiometric sensors

A. Sibbald and A. K. Covington, Clin. Chem., 30 (Simultaneous on-line measurement of blood ionic calcium, sodium and pH with a 4-function ChemFET circuit sensor.) J. Koryta and K. Stulik, Ion Selective Electrodes, University Press, New York, 1983.

(1984) 135.

potassium, integrated Cambridge

Dry reagent analysts B. Walter, Anal. Chem., 55 (1983) 498A. (Dry reagent chem-

istries.)

R. .I. Simpson received his chemistry degree at the Queen’s College, Oxford, 1961. He joined Sira in 1964. His main interests are electroanalytical techniques, applications of UV, visible and IR spectroscopy to analysis, and gas and liquid chromatography. He has done extensive work on development of a novel blood analyser for use in renal dialysis and open heart surgery, and is a consultant on applications of ion selective electrodes and related sensors to clinical analysis. He has recently completed an ESA Study into Gas Analysis Techniques for Human Physiological Measurements in Space, and is currently working on the Eureca Exobiology and Radiation Assembly, and on a survey project examining the needs for Space Life Sciences instrumentation. His address is Sira Ltd., South Hill, Chislehurst, Kent BR75EH, II. K.

Modulated surface vibrational spectroscopy at the electrode-solution interface Kevin Ashley and Stanley Pons Salt Lake City, UT, U.S.A. Infrared spectroelectrochemistry can be used to examine reactions occurring at the electrode-solution interface. This vibrational probe can be applied to study the structure and orientation of molecules and the dynamics of adsorptiondesorption processes at the electrode suflace. The technique can reveal valuable information about the mechanisms and kinetics of surface reactions.

In the last ten years a great deal of interest has centered on chemical reactions occurring at solid-liquid and solid-gas interfaces, owing to their control in energy production and catalytic devices such as fuel cells. To study these reactions in a complete manner it becomes necessary to learn more about the energetics and structure of reactants, intermediates, and products at or near the interface, as well as their kinetics. Once such information becomes available, this knowledge can be utilized in the design of cus0165-9936/85/$02.00.

tom devices for specific reactions. There are thus many practical as well as fundamental reasons to be able to closely study surface processes. Until recently it has not been possible to use an infrared vibrational probe to study the structure and orientations of molecular species and the dynamics of adsorption-desorption processes at or near the electrode-electrolyte interface, due to the large absorbance by the solution. Early work consequently centered on internal reflectance methods which precluded the passage of radiation through the solventelectrolyte system’ (Fig. la). This method is limited since there are few choices of infrared transparent and electrically conductive materials. Not long ago2 it was demonstrated that it was feasible to reflect light from the electrode through the strongly absorbing solution and obtain vibrational spectra of species near the electrode surface (Fig. lb). This was made possible through the application of spectroelectrochemical potential modulation, the use of thin layer 0 Elsevier Science Publishers B .V .