Trends in Environmental Analytical Chemistry 1 (2014) e2–e7

Analytical separations in harsh environments

Mihkel Kaljurand *

Faculty of Sciences, Tallinn University of Technology, Tallinn, Estonia

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e2

2. Definition of harsh environment. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e3

3. Going from the laboratory to the field: requirements for instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e3

4. Ways to confront problems imposed by harsh environments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e4

4.1. Environmental temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e4

4.2. Shock, impact and vibration. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e4

4.3. Ambient pressure and microgravity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e6

5. Detection: LIF versus electrochemical detectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e6

6. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e7

Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e7

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . e7

A R T I C L E I N F O

Keywords:

Environmental analytical chemistry

Extraterrestrial environments

Portable analysers

Chromatography

Capillary electrophoresis

A B S T R A C T

There is increasing interest in miniaturized systems for chemical analysis in harsh environments.

Detailed chemical analysis involves separation methods prior to detection. Depending upon the analytes

of interest, gas chromatography (i.e. for volatile species) or liquid chromatography (i.e. for polar organics

or large biomolecules) can be utilized. Over the last 5–10 years one can observe a strong drive towards

portable capillary electrophoresis systems, which has led to a variety of successful devices. This is due to

ease of miniaturization and robustness of capillary electrophoresis. This paper describes the

requirements of analytical separation instruments intended for operation in harsh environments, e.g.

at volcanoes, inside oil wells, in cold deserts of Antarctica, or even on the surfaces of other planets in our

solar system.

� 2013 Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Trends in Environmental Analytical Chemistry

jo u rn al ho m epag e: ww w.els evier .c o m/lo cat e/ teac

1. Introduction

For a host of applications, there is an increasing need to gatherdetailed chemical information about local environments withoutdelays associated with transferring samples to centralizedlaboratories. These analyses could possibly inform us of potentialhealth hazards in water or soil, or be of forensic value in a criminalinvestigation. Most importantly the desired chemical informationcould be acquired on site in real time. The environmental sample iscommonly a mixture of many components and this makes it

Abbreviations: AMD, amperometric detector; C4D, capacitively coupled contactless

conductivity detector; CE, capillary electrophoresis; GC, gas chromatography;

HPLC, high performance liquid chromatography; LIF, laser induced fluorescence;

MS, mass-spectrometry.

* Tel.: +372 620 4320; fax: +372 620 2828.

E-mail address: mihkel@chemnet.ee

2214-1588/$ – see front matter � 2013 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.teac.2013.11.001

difficult to perform exhaustive analysis of the sample as a wholeusing only spectroscopic methods. One possibility for analysis is touse an array of sensors where the sensors are designed to detectthe individual components (such as an array of different ionselective electrodes). While this approach has many advantages,the tailored nature of these systems tends to limit their generic use.A second approach is to separate the sample into its individualcomponents which can then be analysed individually withoutinterference from the other sample components. Depending on theseparation method used, it may be possible for the sample to beanalysed in a single run using a detector to discover and sometimesto identify species in the sample.

Instrumentation for analytical separations like gas chromatog-raphy (GC), high performance liquid chromatography (HPLC) orcapillary electrophoresis (CE) – either coupled with mass-spectrometry (MS) or not – is, in general, fragile. Commercialinstruments are engineered to work well in a safe laboratory

M. Kaljurand / Trends in Environmental Analytical Chemistry 1 (2014) e2–e7 e3

environment, following common analytical practice, whichdictates that samples are collected, catalogued, and transportedto the laboratory for analysis. However in certain situations, thisprocess is unnecessarily slow and will not meet the field analysismeasurement requirements of the end user. Examples of suchscenarios include situations faced by law enforcement agenciesthat require rapid information to make decisions by authorities(explosives detection, identification of toxic industrial chemicalsor chemical warfare agencies, etc.). Frequently, the analyticalseparation instrument must operate automatically under thecommand of a remote operator. Another interesting case of specialinterest is analysis in extraterrestrial environments. Also ofobvious interest are point-of-care medical tests in situations farfrom medical care facilities. Finally, process analyses in industry, orat oil and gas exploration or production or at undersea cabling sitesare also desirable.

This paper focuses on two particular measurement scenarios:field analysis and analysis of extraterrestrial environments.Problems faced and solutions provided will be described.

2. Definition of harsh environment

Although associated with biology the following definition applieswell to analytical instrumentation too: a harsh environment meanssuch milieu in which it is difficult to live or survive. For aninstrument, a harsh or inhospitable environment could be consid-ered a set of conditions that can cause its malfunction over a periodof time. An electrical circuit can be easily damaged or destroyed ifintroduced to water or high humidity levels. Other potentiallydamaging conditions include extreme temperatures and tempera-ture cycles, penetration of particulates, electrostatic discharge,electromagnetic interference, vibrations, and physical impact.

Extraterrestrial analysers encounter especially harsh environ-ments. During its mission, the spacecraft, and, therefore, ananalyser on the board, is exposed to different sources of vibration,radiation, and also to variation of temperature under the vacuumpressure conditions of interplanetary space. Take for example achromatograph, whereas a potential breaking of the columns canbe prevented by using the proper wall material, vibration, as otherconstraints, can damage the stationary phases by (i) altering thebonding of the stationary phases with the internal capillary wall(vibration, radiation and temperature variation), which can lead toa bleeding of the stationary phase and (ii) modifying the nature ofthe stationary phases (radiation and thermal variation for liquidstationary phases) [1]. Such phenomena could degrade theanalytical properties of the columns, or damage the system byobstructing the columns. Damage to the detectors could also bepossible. In all these cases, the data integrity recorded duringmission operation would be compromised and could severely limitor invalidate any subsequent data interpretation.

When designing such an instrument, one must take theexpected environmental conditions into account prior to thedesign stage of development. The specific environmental condi-tions in which the product will be used will affect the productspecifications, and must be determined beforehand.

3. Going from the laboratory to the field: requirements forinstrumentation

Turl and Wood have discussed the efforts made to developanalytical equipment for field operations working in real-lifeoperational environments and the uncontrollable factors that canimpact their effectiveness and usability [2]. The followingdiscussion is based largely on their treatise. It is obvious thatsophisticated and effective instruments and methods that areproven in the laboratory can suffer from degraded performance, or

even failure, when set up in harsh conditions. Turl and Woods listmany specific technical challenges that should be consideredduring the design and development phases of the instrumentmeant to work in demanding or harsh environments. They are thefollowing:

� Performance: The instrument must meet performance standardsspecified by operational requirements across all of the potentialdeployment environments. Performance of an instrument isexpressed through many parameters such as sensitivity,selectivity, dynamic range, false alarm rate, ability for quantita-tion of target compounds, throughput, servicing and cleaningintervals, and robustness of operation. Depending on the targetanalytes and environment there will clearly be a trade-offbetween each of these factors.� Sampling: Sample collection is a critical stage of analysis which

can ultimately determine the overall effectiveness of the result. Apoor sampling procedure often cannot be recovered by enhancedsoftware processing or optimization of other parameters.Undesirable outcomes from poor sampling could result inreduction in performance and reduced operator confidence,which may increase the probability of false alarms. Furthermore,improper sampling may lead to reduction in sensor lifetime orinterruption of analysis due to maintenance. For extraction ofpolar organics or other molecules of interest present at very lowconcentrations in solid samples, an additional liquid extractionstep is also required.� Environmental issues were briefly discussed already above. More

specifically, the following are key factors: temperature of theenvironment (extreme cold or overheating) effects on efficiencyincluding changes over time, contaminants (dust or dirt) andweather conditions (e.g. rain) effect on instrument performance,vibration of mobile units causing system performance to degradeor fail completely.� Power consumption: Power requirements of analytical instru-

ments are frequently prohibitive. High current is rarely availablein external settings. Hence the availability of portable power canbe a significant obstacle to instrument operation. Issues toconsider are: power consumption (transient versus steady state),availability of mains supply and battery or fuel cell life, andrecharging time. A suitable battery pack may render theequipment too heavy to be a portable instrument.� Portability: Manual handling, vehicle mounting and use in

remote locations are common considerations for field instru-ments, which necessitates their portability. Weight and size arekey considerations in instrument design. Miniaturization is away to make them truly portable. Rapid developments intechnologies such as lab-on-a-chip and micro-engineering haveallowed a number of methods that were previously laboratory-based to be miniaturized to a point of man-portability. However,it is not always possible to miniaturize an instrument withoutcompromising its performance. The frequently overlookedworld-to-chip problem [3] hinders the road to genuineminiaturization.� Continuity: In designing an instrument, consideration should be

given to flexibility in order to minimize the impact of failure orloss of capability. This is particularly relevant in situations wherethe financial consequences of disrupting commerce or missionbecome acute. A designer must consider the following: the effectof total failure or loss, the resilience of the equipment (mean timeto failure and mean time between failure, cost of consumables,and ease of servicing, acceptable downtime).

In addition to technical issues, there are several proceduralconcerns (such as educating operators and other human factors),which also need to be addressed.

M. Kaljurand / Trends in Environmental Analytical Chemistry 1 (2014) e2–e7e4

4. Ways to confront problems imposed by harsh environments

While there are well-known examples of the use of GC–MS inharsh extraterrestrial environments, there are virtually noexamples of using HPLC for the same purposes. To the presentauthor’s knowledge, there is only one work that aims (not in allaspects) to use HPLC for harsh environments [4]. Obviously HPLC isan elaborate technology to be used in harsh environments. CE is amore promising approach. Lewis et al. prepared an excellentcritical treatise on existing portable analytical separation instru-mentation with emphasis on CE [5]. The authors make a cleardistinction between two types of instrument relevant to the topicof this paper. According to their work, a truly portable CE systemshould fit comfortably in the palm of the hand; the weight shouldbe less than a few kg, and for field measurements, changing/charging batteries daily is acceptable. An instrument working insitu requires an autonomous control system; it must be capable offunctioning automatically without a user present. For in situmonitoring, lifetimes in the order of weeks are required. The deviceshould be made of a material that can be reliably cleaned and thatresists contamination. It must be autonomous, i.e. sample loading,microfluidic priming and pumping, cleaning, operation andanalysis must be performed automatically. Minimal/no samplepreparation is advisable and fast analysis should be performed, e.g.on microchip and displayed on the device.

In summary, portable systems exist that can be taken to thepoint of care where the researcher in the field may performmultiple CE runs in a single field trip, however, contrary to this: insitu instrument need not necessarily be portable, only autono-mous. When the two concepts strongly overlap the requirementsto the instrumentation might differ. Hence the in situ instrumentconcept is an extension of the portable instrument.

4.1. Environmental temperature

To operate an instrument at low ambient temperature one hastwo possible options. The obvious one is to use a probe/analyserpower supply to provide the energy needed for the heating of theanalyser. The other alternative is to use non-aqueous solvents (likealcohols or acetonitrile) that can be used at lower temperaturesthan the freezing point of water, which is one of the maincomponents of most of the wet chemistry-based analysis.

The heating of the interior of the instrument generates the needto increase the overall power capacity of the probe, however, sincecontemporary robotic laboratories are packed with the miniaturizedanalysers, the overall consumption of energy should not be too big.This seems to be a generic approach in the design of instruments thatare supposed to work at low temperatures. For example ‘‘Curiosity’’,the last car-sized robotic rover exploring Gale Crater on Mars as partof NASA’s Mars Science Laboratory mission (launched in November2011), incorporates a novel active thermal control system to keepthe sensitive instrumentation at safe operating and survivaltemperatures. While the 24-hourly temperature variations on theMartian surface range from �120 8C to +30 8C, sensitive equipmentis kept between �40 8C and +50 8C. This active thermal controlsystem is based on a single-phase, mechanically pumped fluid loopsystem, which removes or recovers excess waste heat and managesto maintain the sensitive equipment inside the rover at safetemperatures [6]. In another example, the Urey instrument wasplanned (but withdrawn by NASA) to be housed inside a shelteredcasing inside the European Space Agency ‘‘ExoMars Rover’’ payload[7]. The payload of the ExoMars Rover is expected to experiencetemperatures ranging between �100 8C and +50 8C during its transitto Mars and on the Martian surface [7].

The other option – separation at low temperatures – is morechallenging and of interest in the context of this review. When

specifications of contemporary CG promise to perform subzeroseparations there are very few examples of liquid phase separations.Some nonaqueous CE separations have been performed at subzerotemperatures [8–12]. Considering the possibility of keeping of theinstrument in the warm interior of the rover, the need for lowtemperature separation can be questioned altogether. Still, the needfor such analysis can be advocated in the case of, e.g. extraterrestrialanalysis of complex organic compounds in search of life signatures.Thus far, the only in situ technique employed to study these organicsamples was the rapid heating of samples to temperatures as high as600 8C, and following GC–MS analysis of degradation products. Thisapproach was used on the Huygens Probe [13,14] which is aEuropean contribution to the joint NASA�ESA Cassini�Huygensmission. However, the rapid heating of samples to temperatures ashigh as 600 8C, may lead to unpredictable side reactions, degradationof the sample components and secondary product formation,confusing efforts to interpret the original sample composition.

A recent Titan study [15] suggests that liquid based samplehandling and analysis on Titan will minimize disintegration of thesample. The use of organic solvents enables analysis over a broadertemperature range than aqueous separations. Willis et al. devel-oped a protocol for analysis of organic aerosols present on Saturn’smoon Titan, as well as the analogues of these aerosols (tholins)made on Earth [16,17]. The use of ethanol in their method meansthat long-chain primary amines can be dissolved. As the freezingpoint of ethanol is much lower than water, this protocol canperform separations at temperatures lower than 0 8C, which wouldnot be possible in aqueous phase. As was already pointed outabove, this is of particular importance when considering in situsampling of Titan aerosols. Unnecessary heating of the sample(even to room temperature) would lead to decomposition orunpredictable side reactions, which would make it difficult tocharacterize the sample appropriately.

Besides separation at low temperatures, sampling, samplepreparation and its transport from a planetary surface to ananalyser must also be addressed. Bearing that in mind, Willis et al.[7] addressed this problem for designing microfluidic diaphragmvalves and pumps capable of surviving in harsh conditions. A lab-on-a-chip CE analysis system developed in this work for the Ureyinstrument package required valving and pumping systems thatare robust under harsh conditions before and after exposure toliquid samples, which are to be analysed for chemical signatures oflife. The material which met this requirement uses membranesconsisting of Teflon1 and Teflon1 AF as a deformable material inthe valve seat region between etched Borofloat glass wafers. Theuse of Teflon instead of popular PDMS material has anotheradvantage: Teflon is inert material for nonaqueous buffers andsolvents. Thus, this technology could be applied to microfluidichandling systems for extremely cold, directly sampled nonaqueoussolvents, such as are present, e.g. on the surface of Titan. Williset al. also found that FluorocurTM perfluoropolyether material wasanother suitable candidate [18].

4.2. Shock, impact and vibration

Being shockproof and vibration proof is an essential feature ofanalytical separation instruments, which are supposed to work inthe harsh environments. Surprisingly, very few studies have beenpublished on the design of such instruments. Manufacturers ofportable and industrial gas chromatographs usually mention thattheir production is shockproof, without giving much detail abouthow this property is tested. However, there are common principlesof shock- and vibration proof design that should apply for theportable separation instruments as well. In general there arevarious ways known in the industry to design analysers that arehighly resilient against vibration and shock [19]:

M. Kaljurand / Trends in Environmental Analytical Chemistry 1 (2014) e2–e7 e5

� Vulnerable system components must be placed on the pivotpoint of the see-saw. The fulcrum of the system board remainsstable, components there will experience less vibration andshock no matter how much the board shakes.� An asymmetrical design is an effective way to reduce system

resonance and minimize vibrations and shock. A system that isasymmetrically balanced can disrupt energy wave transmissionsand reduce vibrations.� Simple, strong, and flexible mortise and tenon joints are a unique

strategy that bestows greater vibration and shock resilienceinstead of rigid screws, joints, and bindings which performpoorly in the vibrating environments. 3D printing – a process ofmaking a three-dimensional solid object from a digital model –might open new opportunities for manufacturing details forportable analytical instruments.� For exterior casing, aluminium alloy is a superior material

compared to sheet metal, because it is more rugged, stronger,and can endure more vibration and shock. Aluminium alloy islight, easy to shape, and also has high heat conductivity, all ofwhich provides a reliable operating platform for the entiresystem. Another popular casing material is acrylonitrile butadi-ene styrene plastic. The most important mechanical properties ofthe plastic are impact resistance and toughness. The globalleader in design and manufacture of high-performance casesolutions is Pelican Products, Inc. Their products are used in themost demanding environments like fire-fighting, law enforce-ment, military, aerospace, and industrial [20].

Fig. 1. Examples of some portable field CE instruments. Top left image: CE Resources

remotely controlled vehicle and base station including a notebook (a), the high-power w

(d), the IR distance sensor (e), and the window for air sampling (f) [33] (Courtesy of Wi

laboratory. (1) Sample injector, (2) shut-off valve, (3) split injector; (4) capillary, (5) con

image: CE instrument developed by Hauser’s group. (1) Membrane pump, (2) valves, (3) s

air [21] (Courtesy American Chemical Society).

� Damping material should be used to reduce the impact ofvibration and shock. As part of an overall anti-vibration strategy,it is particularly useful as a tool to fine-tune the characteristics ofthe anti-vibration and anti-shock design.

Although not common in designing instrumentation forchemical analysis, the strategies outlined above demonstrate thatit is possible to build a system that is highly resistant to vibrationand shock while still delivering excellent performance. CE is apromising technology for portability and over the last 5–10 yearsone can see a strong drive towards portable CE systems which hasled to a variety of successful devices. Many solutions andapplications have been provided by Hausers group [21–25] butalso by Hutchinson et al. [26], by the author of this paper [27,28], Liet al. [29–31], Kaigala et al. [32], da Costa et al. [33] and Lee et al.[34]. Some of the proposed instruments have been presented inFig. 1. Among the possible designs, one CE instrument is of specialinterest since it was designed to work with the Martian (i.e. harsh)milieu in mind [35]. This is the ‘‘Urey organic and oxidant detector’’designed to search for several classes of organic molecules in theMartian regolith (see Fig. 2).

It would be of interest to speculate on how those portable CEinstruments meet the requirements outlined above for shock- andvibration proof. Unfortunately, to the present author’s knowledge,no data is available about performance of those instruments in theconditions of shock and vibration. Frequently overlooked but oneof the most difficult aspects of the design of portable instruments is

CE-P1 instrument [31] (Courtesy Springer Verlag). Top right image: Picture of the

ireless access point (b), a high-gain Wi-Fi antenna (c), the wired network IP camera

ley). Bottom left image: close view of the electrophoresis system from the author’s

tactless conductivity detector; (6) waste outlet and (7) buffer storage. Bottom right

plitter, (4) detector, (5) safety cage for application of high voltage and (6) pressurized

Fig. 2. The Mars Organic Analyser, a portable microchip CE instrument. The

instrument houses a bank of 16 solenoids for pneumatic control of microfluidic

devices and a stationary optical system with a photomultiplayer tube for LIF

detection [46] (Copyright 2005, National Academy of Sciences, USA).

M. Kaljurand / Trends in Environmental Analytical Chemistry 1 (2014) e2–e7e6

packaging – how all the electronics, pumps, detectors and displaysare located in the most compact and ergonomic way as pointed outby Turner [36]. He stresses that ‘‘The case has to provide electricalscreening for both radiofrequency emissions from the instrumentand to prevent external fields from affecting its operation. It has tobe sealed against the elements, and it must be easy to assemble. Ithas to be as light as possible, and yet it must be robust’’ [36]. Inconclusion, one can expect that if the portable instrument isencased in the shock-, dust-, and waterproof case (like PeliTM) thenit should well withstand environmental disturbances if theinternal mount is performed in accordance with the requirementslisted above. A challenge for the CE research community now is tofurther develop and test portable and in situ CE analysis systems,keeping in mind the demands imposed by the conditions of theharsh milieu in which the instrument will operate.

For the design of portability or in situ CE the use of propermaterial choice for manufacturing instrument components is animportant issue. For in situ monitoring the material must be robustto ensure a good device lifetime. Portable instruments are not soconstrained. Robustness is desirable but it may be traded againstcost/convenience. For chip-based CE systems, particularly in recentyears, the popularity of polymer materials compared to glass-basedsystems has grown significantly. This trend is attributed to the lowercost of the polymers and their ease of fabrication. Also, polymers canbe more robust than glass, which is often too fragile for field use.Greater mechanical robustness of polymeric chips is even moreimportant for in situ monitoring especially in the case of harshenvironments. However, a big disadvantage of polymeric materialsis the fact that they are not compatible with organic solvents and donot offer the same chemical inertness as glass or silicon and as suchare prone to surface contamination. This affects their performanceand lifetime. One approach to overcome this is to design the systemin such way that the fluidic component is replaceable. This yieldssome flexibility in the design of non-chip based portable CEinstruments. On the contrary, for in situ instruments based on amicrofluidic chip, channel replacement is not an option and so acleaning regime for the selected material needs consideration.

4.3. Ambient pressure and microgravity

There is no info on how the low/high ambient pressure mayinfluence the performance of the analytical separation instrument.This is not a problem on terrestrial environments but this may posean issue on extra-terrestrial environments: e.g. solvents mightrapidly evaporate. This challenge can be tackled by using solventsthat do not evaporate like dimethyl sulfoxide [37] or ionic liquids.Use of ionic liquids probably makes good sealing of the instrumentunnecessary but at the moment it is not clear how suitable ionicliquids are to process the analytes of interest. Another solution is tosimply create a pressurized shell that surrounds the CE instrumentin a gas blanket, in order to control the pressure [37]. In this way,the spacecraft would create the necessary conditions for main-taining the working buffer in the liquid state.

Microgravityisaconditionthatishardlyencounteredonearthbutlow gravity is encountered in environments of extra-terrestrial smallbodies (like moons and comets). Physical processes that depend onthe weight of a body act differently without a certain amount ofgravity. Itcan causeseriousproblemsonthe workof instrumentsandcan influence their performance. On the other hand, e.g.microgravityenvironment allows the observation of continuous-flow electropho-retic separations without the interference of natural convection thatdisturbs this process on earth [38]. Thus, the possible influence ofmicrogravity on the work of analytical instrument used in extra-terrestrial environment requires attention. Testing the effects ofmicrogravityonearthispossibleinspecialplanesexecutingparabolicflights for a short period of time. No info of such tests on a CEinstrument has been available to the present author.

5. Detection: LIF versus electrochemical detectors

Contemporary sensitive analytical instruments are frequentlybased on laser-induced fluorescence (LIF). The detector has thelowest detection limit among the others implemented in separa-tion science (85 pM for Pacific Blue labelled amino acids in theUrey instrument [39]). LIF lacks robustness because it involveslaser as a radiation source, optical set-up and photo detector. Oneobvious disadvantage of LIF is that it requires the use offluorophores for detection analytes, which do not naturallyfluoresce to be marked. However, for portable and in situ systemsit is desirable to minimize or remove the requirement for anysample pretreatment. In addition, fluorophors tend to have largemolecular mass, and consequently can cause the analytes to havesimilar mobilities. This increases the difficulty of separating theanalytes and may need longer separation channels. A significantimpact on instrument design and operation has long-term storageof fluorophors during spaceflight. One fluorophor, Pacific BlueTM

succinimidyl ester, has of particular importance in this respect. Itshydrolysis was studied by Stockton et al. [40].

Nevertheless, many of the difficulties with LIF systems can beovercome. A fully integrated four-layer microchip electrophoresisdevice for end-to-end mCE analysis of amino acids is reported byWillis et al. [41]. The device consists of bonded layers of Borofloatglass wafers and a flexible PDMS membrane. The objective of thework was to develop a complete system that could thus be used forfuture in situ extraterrestrial exploration. Authors demonstratelabelling, dilution, and separation of amino acids with this devicewith minimal operator intervention. The solutions were placed inthe appropriate reservoirs at the beginning of the experiment,and all subsequent fluidic manipulations were performed via amicrovalve circuit designed for autonomous investigations. Allthat would be required of the spacecraft probe would be to deliveran aqueous sample to the device.

An alternative to LIF would be the use of more robust detectors.One such detector that is lightweight, consumes little energy, and

M. Kaljurand / Trends in Environmental Analytical Chemistry 1 (2014) e2–e7 e7

does not require sample derivatization and pre-processingmeasures contactless conductivity (impedance) of analytes.Contactless conductivity detectors (C4D) were developed by doLago and Fracassi da Silva [42] and Zemann [43]. Considering thegeneric construction of the C4D – two pieces of metallic tubes laidon the capillary, it may be safely claimed that this is the mostrobust and suitable detector for working in a harsh environment.Its disadvantage is higher detection limit than that of for LIF(100 nM, a generalization from [44]) and it requires specific lowconductivity separation electrolytes for better detection.

Another detector to be considered could be an amperometricdetector (AMD) [44]. The main challenge of coupling CE to AMD isto manage highly sensitive redox current measurements in thepresence of a high-voltage electrical field. The detector limit ofAMD is between LIF and C4D being about 10 nM [44]. Unlike LIF,electrochemical detection systems are not inherently electricallyisolated from the large voltages, which is an obvious disadvantageof this detector. Still, miniaturized CE systems, especially thosedesigned for portability have moved away from LIF, which wasonce the most widely used detection method in CE. As we sawabove, the reason for the shift from LIF to other detection systems,(such as electrochemical or C4D), was because the electrochemicalmethods are better suited to miniaturization. A very commondetector in CE – UV–vis – is not considered here because itssophistication is comparable to that of LIF but other performanceparameters are far inferior to LIF, electrochemical or C4D detectors.

Detection of biogenic compounds from the soil of the Atacamadesert revealed amino acid concentrations of 1–100 ppb (i.e. 5–500 nM) [45]. It follows from the above that despite the fact thatAMD and C4D are inferior to LIF in terms of detection limits, theycan also meet requirements encountered in realistic situations.However the concentrations of amino acids present on extrater-restrial targets may very well be far lower than that encounteredon any naturally occurring terrestrial samples.

6. Conclusions

Despite the fact that harsh environments provide a complicatedand interesting topic of research, there are only a few instrumentsdesigned for these purposes. Of special interest is the extraterrestrialmilieu, since its analysis could provide answers to problems that areof fundamental interest in astrobiology and planetary science.Analysis of extraterrestrial, harsh environments presents complete-ly new challenges to instrument designers and analytical chemiststhat are not comparable to those encountered on the earth.

By identifying gaps in the literature that require attention, onecan conclude that the influence of extreme temperatures onseparation instruments/protocols have been studied and under-stood at the present as well as the ways to tackle these extremetemperatures. At the same time, the effects of shock/vibration andextreme ambient pressures and microgravity have not been studiedat all. Despite the fact that the ways of dealing with the shocks andvibrations are generally known in theory it is not well known howthose measures block the vibration and shock influence on theinstrument’s performance in practical operation. Separation sciencein harsh environments emerges as an exciting, new field of researchwhere emphasis should be placed on an holistic approach forportable and in situ CE instrumentation design.

Acknowledgement

For valuable criticism and suggestions on the manuscript, theauthor wishes to acknowledge his indebtedness to Dr. Peter Willisfrom Jet Propulsion Laboratory, California Institute of Technology(Pasadena, CA, USA).

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