There is an emerging consensus that the way to search for life beyond Earth is to look for molecular biosignatures or biomarkers. Solvent extraction, a necessary step for the detection of trace concentrations of biomarkers, remains a critical technological gap. Current high TRL systems for solvent extraction are based on subcritical water, which can alter the structure and composition of biomarkers, or are subsystems of an analytical instrument, which makes it difficult to analyze the same extract with multiple instruments. A low temperature, solvent extractor would: (1) generate high value samples maintaining the integrity of biomarkers; (2) remove the burden of sample acquisition and processing from analytical units; and (3) allow to distribute portions of the same liquid extract to different analytical instruments, so results could be compared and correlated. The goal of this CIF project was to generate a high fidelity conceptual design (TRL 3) of a low temperature solvent extractor (MLT) with the following features: (1) is compatible with different types of samples (liquid, solid, ices); (2) operates at low temperature; (3) extracts polar and non-polar compounds; (4) maximizes user control over the extraction process; (5) provides a liquid extract to multiple analytical units; (6) is compatible with planetary protection requirements; and (7) is volume, mass and power optimized.
MLT achieves reliable liquid extraction by using Microsonic Systems patented Bulk Lateral Ultrasonic™(BLU™) technology, which allows to miniaturize the extraction unit. Because the intensity of the motion created by BLU technology is highly controllable, it has already been applied to a wide range of life science applications, from gentle mixing to compound solubilization, lysing cells and shearing of biomolecules. With a modular designed, MLT can be tailored to specific mission needs, and can implement different solvents to maximize the chemical range of extraction (e.g. water, organic solvents, surfactants). The pure organic extract can then be analyzed with both contact and non-contact instruments. MLT is designed for compliance with Category IV missions (life detection).
The three stages in solvent extraction are sample ingestion, liquid extraction and extract injection for analysis. In MLT, each stage has a dedicated subsystem: (1) Sample ingestion system, (2) Extraction chamber and (3) Extract outlet, respectively.
Sample Ingestion System: Sample ingestion is the transfer of the sample from a sample acquisition system (e.g. drill, scoop) into the extraction unit. MUSE interfaces with sampling acquisition systems through a funnel-shaped sample inlet that accepts up to 1 cc of powdered sample (i.e. soil, ice, salt, sediments), with a maximum grain size of 2 mm. The sample is transferred into the funnel through an air gap to avoid cross contamination between MLT and the sample delivery system. Each extraction cell (see below) has a dedicated funnel with an outer diameter of 2 cm and an inner diameter of 1 cm, providing ample margin for the powdered sample to fall through into the extraction chamber. Upon unloading the sample into the funnel, a piezoelectric motor (SQUIGGLE micro motor technology, http://www.newscaletech.com/technology/squiggle-motors.php) attached to the threaded end of the funnel creates a smooth bi-directional linear motion with sub-micron resolution. Thread friction drives the funnel, directly converting rotary motion to linear motion without the need for a gearbox. The speed and position of the threaded funnel can be precisely controlled. Downward displacement of the funnel causes a one-way check valve in the extraction cell to open, allowing the sample to gravity-fall directly into the extraction cell. Upon unloading the sample, the piezoelectric motor reverses the vertical motion of the funnel, which reseals the extraction cell, thus terminating the sample transfer sequence.
Extraction Chamber: The extraction chamber is the core component in MLT. The chamber consists of 9 single-use extraction cells, but thanks to the modular design, the total number of extraction cells can be customized according to mission requirements (see below). Because each extraction cell is a self-contained, closed system, the number of cells can change according to mission requirements with little impact on overall instrument design, which provides implementation and operational flexibility. Each extraction cell is a cylinder (1.5 cm diameter; 5 cm length; total volume 8.8 mL) coupled to a Fresnel Annular Sector Actuator (FASA) transducer at the bottom. The cylinder is sealed-capped with a silicon valve, and preloaded with 5 mL of extraction solution that can be customized for each cell to target different types of compounds (e.g. water, propanol, dichloromethane, etc.), or with a procedural blank. A critical aspect of the system will be its extraction efficiency (i.e. the amount of target compound that go into solution with respect to the original amount in the sample). UAE is a widely used technique due precisely to its high yields, and ample literature exists regarding its use, which commonly show extraction efficiencies upwards of 80%. However, placing a requirement of high extraction yields on a flight prototype can escalate the complexity of the system, for example by adding high temperature/pressure control. Instead, we opt for lowering the required extraction efficiency of the system (>50%) and assume that any loss in target compound during extraction will be compensated by the use of highly sensitive analytical systems and the higher quality of the extracted sample. In a mission scenario, the extraction solution would be frozen until a sample is deposited in the chamber. Preloading the extraction buffer in the chamber offers a series of important advantages:
Once the sample is in the extraction cell, the temperature of the cell is brought up to above freezing and the FASA transducer is activated to induce the mixing of the sample with the extraction buffer. Complete mixing is achieved within seconds. At that point the extraction sequence can be initiated. The FASA transducer at the bottom of each extraction cell creates an interference pattern in sound waves that, in turn, creates Bulk Lateral Ultrasonic BLUTM waves. The BLU waves propagate into the samples through mechanical coupling and can be tuned to varying intensities, which results in controllable mixing, heating, cooling lysing or shearing of samples depending on which task is needed. Typical extraction cycles last for 10-15 minutes but this function can be customized to meet science and operations needs. The design of FASA transducers grants a high degree of control over the extraction process.
Extract Delivery System: Each extraction cell is connected to a fluidic system that carries the solvent and can deliver the extracted sample to multiple analytical systems. After the sonication step, the sample can be allowed to settle for hours (or even 1 full day), after which a pump mechanism creates negative pressure that forces the extract into the fluidic system exiting the extraction cell through the top. It is important that the sample is allowed to settle completely before the extract is evacuated, so that no particles are carried into the microfluidic system. This will mitigate the risk of clogging and will also improve the quality of the extract. The number and distribution of extraction cells gives us flexibility to conduct blank analyses (i.e. circulating solvent from one cell in the absence of a sample) to assess for contamination and to conduct background analyses. The instrument requirement is to produce up to 5 mL of solvent extract for analysis, and each extraction cell will generate 8 mL of extract.More »
Flight instruments such SHERLOC and SuperCam (Mars2020), or RSL and MOMA (ExoMars) will conduct non-contact analyses of bulk samples to search for organic compounds and biomarkers, an important feature when considering the difficulties inherent to robotic sample processing. However, there is little doubt that the performance of these instruments would improve if they were to analyze extracts of organic compounds, instead of bulk samples. Wet chemistry is a requirement for more sensitive analytical systems currently being developed for flight, such as ion chromatography, mass spectroscopy, electrophoresis, or lab-on-a-chip technology. In the absence of a generic off-the-shelf solution to liquid extraction, these technologies remain orphan, or rely on their own customized extraction systems, which cannot be easily transferred or integrated into other analytical systems.
MLT is conceived as a multiuse liquid extractor that can service more than one analytical system at a time, and be easily integrated with current and future technologies with minimal TRL impact. This would separate the burden of sample processing from the analytical part, and critically, it would enable the concurrent analysis of the same sample with different analytical systems.
MLT could be the central element of a Discovery/New Frontiers class mission that seeks to detect and characterize organic compounds in regions of high Astrobiology potential, such as ice, lake sediments or evaporites.More »
|Organizations Performing Work||Role||Type||Location|
|Ames Research Center (ARC)||Lead Organization||NASA Center||Moffett Field, CA|
|SETI Institute||Supporting Organization||Academic||Los Altos, CA|