Article pubs.acs.org/ac
An Electrochemically Based Total Organic Carbon Analyzer for Planetary and Terrestrial On-Site Applications Shannon T. Stroble and Samuel P. Kounaves* Department of Chemistry, Tufts University, Medford, Massachusetts 02155, United States ABSTRACT: The search for organics on Mars began over 30 years ago. Neither the Viking GC/MS nor the more recent thermal and evolved gas analyzer (TEGA) aboard Phoenix were successful in detecting organics in the Martian soil. The most recent hypothesis for the “missing” Martian organics is thermal decomposition of organic material to CO2 during the pyrolysis step of these analyses caused by the recently discovered ∼1 wt % perchlorate in the Martian soil. To avoid this problem, an entirely different approach for the analysis of organics on Mars has been developed using an electrochemically based total organic carbon (TOC) analyzer, designated the Mars Organic Carbon Analyzer (MOCA). MOCA is designed as a small, lightweight, low-power instrument that electrochemically oxidizes organics to CO2. The CO2 is subsequently detected and quantified to determine the amount of TOC in the soil. MOCA can use the perchlorate present in the Martian soil to its advantage as an electrolyte, thus requiring only a buffered solution. Through a series of proof-of-concept tests, MOCA is shown to oxidize a variety of low-molecular-weight 1−5-carbon-containing molecules, including those containing carbon-13 using platinum and boron-doped diamond (BDD) electrodes at concentrations as low as 10 mg/kg. MOCA can also be used in terrestrial settings for on-site analysis of dissolved TOC.
T
nonvolatile salts or carbon dioxide, thus, preventing their detection by the respective mass spectrometers.12 Recently, Navarro-González et al.13 attempted to duplicate the Viking GC/MS analysis using Mars-like soils from the Atacama Desert in Yungay, Chile. The soils were mixed with 1 wt % magnesium perchlorate and processed using both the Viking13 and Phoenix14 heating protocols. The authors suggested that the CH3Cl and CH2Cl2 seen in the Viking experiments were not from Earthly contamination, as previous assumed, but rather, produced by the interaction of perchlorate and organics in the Martian soil when heated during the analysis.13 Although this reinterpretation of the results has been challenged,15 the presence of the perchlorate and the possibility of its interference with these analyses has engendered increased doubt in the results. The instruments aboard both the Viking and Phoenix missions used similar approaches to organics detection: volatilization via pyrolysis, followed by electron impact ionization and analysis by a magnetic sector mass spectrometer. The next organic detection instrument onboard the 2011 Mars Science Laboratory (MSL), called Sample Analysis at Mars (SAM),16 also uses a similar method for the detection of organics. SAM will differ from the previous instrumentation by use of a quadrupole mass spectrometer capable of a greater mass range and the additional capability of performing a single step derivatization of some nonvolatile compounds prior to
he search for the presence of life on Mars began in 1976 when the Viking Landers performed a set of experiments to detect microbial life1 and the presence of organic material in the Martian soil.2 Even though the Viking life detection experiments were positive (by predetermined criteria), the gas chromatograph/mass spectrometer (GC/MS) failed to detect any organics at the micrograms/kilogram level. This result provided a strong argument against the presence of life on Mars.3 In 2008, the Phoenix Mars Scout Lander became the first mission since Viking to include instrumentation capable of analyzing the soil for organics. Phoenix’s thermal and evolved gas analyzer (TEGA), a combination of a differential scanning calorimeter and a mass spectrometer with a detection limit of 10 μg/kg,4 also failed to detect any organics in the Martian soil.5 The negative results from both missions were surprising. Even if there were no biologically produced organics on Mars, both instruments should have detected the ∼2.4 × 108 grams of organic material that bombards the surface of the planet each year.6 In fact, it has been calculated the surface of Mars should contain 0.0026 g of carbon per gram of soil (or 3650 mg/kg organic carbon).7 There has been much speculation as to why the Viking and Phoenix analyses did not detect any organics.8,9 The most recent hypothesis for the “missing organics” is based on the discovery by the Phoenix’s Wet Chemistry Laboratory (WCL) that the martian soil at its landing site contains ∼0.6 wt % perchlorate (ClO4−).10,11 At the volatilization temperatures used during the Viking and Phoenix analyses, any organics present would have been oxidized by the ClO4− to either © XXXX American Chemical Society
Received: June 19, 2012 Accepted: June 23, 2012
A
dx.doi.org/10.1021/ac301704m | Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
Article
detection.16 In light of the findings by Navarro-González et al.13 and MSL’s use of high temperature volatilization, it is possible that SAM may also fail to detect organics on Mars, especially if perchlorate is as ubiquitous on Mars, as it is on Earth.17 For the above reasons, an entirely different approach to the detection of organics on Mars has been proposed, one based on total organic carbon (TOC) analysis. Although a TOC analyzer would not be able to determine the specific organics present in the soil, it would be able to determine the amount of organic carbon present and allow for an assessment of habitability and biological potential that does not depend on the presence of any specific organic molecules. However, commercially available TOC analyzers for Earth applications are typically large and heavy and require substantial power. In addition, they use corrosive reagents, ultraviolet light and/or high temperature ovens to convert the organics to CO2 before measurement . These factors limit the viability of using such TOC analyzers for in situ analyses on Mars or for on-site analyses on Earth. In this paper, we present a novel electrochemically based total organic carbon analyzer, designated as the Mars Organic Carbon Analyzer (MOCA), for the analysis of organics in Martian soil. By using an electrochemical approach,18 we have created a small, lightweight, low-power, proof-of-concept prototype instrument that does not require UV light, hightemperature ovens, or corrosive reagents. This instrument can also optionally use the perchlorate present in the Martian soil to its advantage as an electrolyte, thus requiring only a buffered solution. Here, we summarize the MOCA’s current state of design and demonstrate its ability to quantitatively oxidize (via platinum or boron-doped diamond (BDD) electrodes) several small, low-molecular-weight organic compounds at concentrations as low as 10 mg/kg.
■
MATERIALS AND METHODS Reagents. Organic salts of sodium formate, sodium acetate, acetone, DL-alanine, aspartic acid, glycine, glutamic acid, and serine (Sigma-Aldrich) were used as received. Magnesium perchlorate hexahydrate (Mg(ClO4)2·6H2O) (Sigma-Aldrich) was prepared as a 1% supporting electrolyte solution. Sulfuric acid and phosphoric acid prepared in 0.1 M concentrations (∼1% H2SO4 and ∼1% H3PO4) were also used as supporting electrolytes. The water used in all solutions was purified to a specific resistivity of 18.2 MΩ·cm (Barnstead Nanopure). Ultrahigh-purity N2 gas (AirGas East) was used to purge the system of atmospheric CO2 prior to oxidation. Instrument Design. The proof-of-concept MOCA consists of a custom-built electrolytic flow-through cell, commercially available solution reservoir, pump, power supply, and a carbon dioxide detector (Figure 1 top). The electrolytic flow-through cell, fabricated from polyethylimide (PEI), measures 8 × 3 × 1 cm (Figure 1 bottom). A flow-through type system was used to aid in the oxidation of the organics since the rate is masstransfer-limited and only molecules reaching the electrode surface will be oxidized. Other possible designs to reduce the complexity, including a thin layer cell, are being considered for the final instrument. Inside the cell, either a platinum or boron-doped diamond was used as the anode, and platinum was always used as the cathode. Both electrodes consisted of a wire (4.2 cm long with a diameter of 0.5 mm) with ∼0.7 cm2 geometric area (Figure 1 bottom) each and with an interelectrode gap of 1 cm. The BDD electrodes (obtained from the Diamond Lab at Case Western Reserve University) were fabricated by hot filament
Figure 1. Prototype MOCA system (top) and close-up of the electrolytic flow-through cell (middle) and BDD electrode (bottom).
chemical vapor deposition (HF CVD) of boron-doped diamond on a tungsten wire substrate. The electrolyte was stored in a 100 mL glass solution reservoir and circulated through the electrolytic cell by means of a peristaltic pump (Fisher Scientific) at a flow rate of 0.3 dm3/hour. Power was supplied by an EG&G model 263A galvanostat (Princeton Applied Research) capable of producing a range of currents from −200 to 200 mA (or current density from −173 to 173 mA/cm2). Two different CO2 detection methods were employed: a Stanford Research Systems Quadrupole Mass Spec QMS200 Residual Gas Analyzer (RGA) or a Li-Cor Non-Dispersive Infrared Analyzer LI-820 CO2 Gas Analyzer (NDIR). Experimental Protocol. Specific parameters, such as background electrolyte, type of organic, and applied voltage/ current, were varied depending on the experiment, but the overall procedure remained the same. The analysis consisted of adding 25 mL of an aqueous solution containing the electrolyte and organic molecule of interest to the solution reservoir. The reservoir was sealed and connected by silicone tubing to the pump and electrochemical cell in a closed-loop configuration. The detector was plumbed into the system at the headspace of the reservoir. Ultrapure N2 gas was bubbled through the solution inside the solution reservoir before the start of the B
dx.doi.org/10.1021/ac301704m | Anal. Chem. XXXX, XXX, XXX−XXX
Analytical Chemistry
Article
rate between the BDD and Pt electrodes increased with increasing starting concentration of organic. When oxidizing the 90 mg/kg formate sample, the rate of oxidation was 4.8times faster with the BDD electrode. This increased to a 7times faster rate of oxidation for the 180 mg/kg sample and ∼10-times faster for the 360 mg/kg sample. There was, however, one drawback to the BDD electrodes used in these experiments. The BDD coating on these electrodes began to delaminate during use, exposing the tungsten wire underneath. A tungsten substrate without any coating was tested as a possible working electrode, but it was unable to oxidize formate or the other organics tested. Thus, as the BDD coating delaminated, the electrode became less efficient until it eventually ceased to function. This resulted in the inability to perform oxidations for more than a few hours with the BDD electrodes, and therefore, it was not possible to determine which electrode would provide the most complete oxidation. To prevent delamination, all experiments were kept to
