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Weekly Project

Page history last edited by PBworks 14 years, 2 months ago

Tenative Goals:

  • To research Gas Chromatography (GC)
  • To find out how it ties into finding the fingerprints of life
  • To pin down some valuable questions

 

Resources:

  • Steve Terry, Hal Jerman 1972 __Issues of GC for Mars__
  • Dick Zare

 

GC/MS Theory and Function

*(P.J. Baugh, 15) A gas chromatograph is essentially a device which enables a small amount of sample to be introduced into an inlet system where it is vaporized and passed into a chromatoraphic column. To provide suitable conditions for chromatography, the colum is held within an oven and a flow of inert carrier gas passes through it. A detector is fitted at the column exit to monitor the separated componentns as these elute from the column. The detector provides an electrical signal which is amplified and fed to a recording or data-processing device from which meaningful results can be obtained.

 

*(P.J. Baugh, 16) Most modern chromatographs, therefore, use a microprocessor at the heart of the control system. The use of such technology has the added benefits of an improved user interface, better programmability, method storage, external control, and intelligent system diagnostics.

 

*(P.J. Baugh, 31) Detector classification based on selectivity: (a) Non-selective (universal) detectors respond to compounds which differ from the carrier gas (b) Selective detectors respond to a range of compounds which have some common chemical or physical property (c) Specific detectors respond to a single chemical compounds.

 

*(P.J. Baugh, 32) Detector classification based on how they respond to the amount of analyte passing through: (a) Concentration-dependent detectors: the detectors produce a signal which is related to the concentration of solute in the gas stream present in the detector at some point in time. These detectors are normally non-destructive and so can be used in series with other detectors. Their response is expressed in units of signal per analyte concentration. Because of their sensitivity to analyte concentration, any dilution of the column effluent with a make-up gas will lower the response. This is unfortunate as many of these detectors will require a make-up gas when used with capillary columns in order to prevent peak-broadening effect. (b)Mass Flow dependent detectors produce a signal that is related to the rate at which solute molecules enter the detector. These detectors normally generate a signal as a result of some destructive process occuring in the solute molecules. Their response is expressed in units of signal per analyte mass flow. Response is generally unaffected by the addition of a make-up gas although this is normally unnecessary. (note) main detection systems used in GC are the mass spectrometer (MS) and Fourier transform infrared spectrometer (FTIR).

 

*(Imran - Chromatography concepts & contrasts):

Types of Detectors:

1) flame ionization detector

hydrogen-oxygen flame, samkple burned, produces ions

is detector of choice for organic compounds though can't detect co2, h20

 

2) thermal conductivity detector

uses transducers/hot wires

 

metastable ionization detector (MID) likely candidate because of its superior thermal stability and high sensitivity. combines universal response with high sensitivity (ppb), wide response range (>106). weighs about 1 g and "has been used in all breadboard systems for proposed exobiology flight experiments at NASA-Ames"

 

*Imran, Bada paper):

"The Mars Organic Detector (MOD) is an instrument that has been developed to search for traces of the key organic compounds, amino acids/amines, and PAHs, directly on the Martian surface (19). MOD is based on the following concepts: (i) amino acids and PAHs can be directly sublimed from natural samples by heating to 450°C under partial vacuum, thus eliminating the use of the aqueous reagents and organic solvents used in laboratory analyses; (ii) sublimed amino acids condensed on a cold finger coated with a reagent specific for amino acids can be detected at very high sensitivities by using UV fluorescence; and (iii) sublimed PAHs can be directly detected on the cold finger because they are naturally fluorescent when exposed to UV light.

consist of a delivery arm and a rock crusher; an organic detector, consisting of a sublimation cell, a chemical detector, and a fluorescence analyzer; two timable diode laser spectrometers (TDLSs), which each have a Herriott cell, a dual laser system; and a miniature capacitance manometer and Pirani gauge to measure absolute pressure"

 

GC/MS in Space (present and past)

 

Viking (Mars) - GC/MS - surface and atmospheric measurements

Pioneer (Venus) - atmospheric measurements - min detectable level = 1-60ppm yashin ref

Huygens (Titan) - GC/MS

Vega Project (Venus) - http://solarsystem.nasa.gov/missions/profile.cfm?Sort=Target&Target=Venus&MCode=Vega_01&Display=ReadMore

 

 

Rosetta (comet) - http://www.spectroscopynow.com/coi/cda/detail.cda?id=683&type=Feature&chId=9&page=1

The Rosetta Lander will be the first spacecraft to land on a comet (Churyumov-Gerasimenko), scheduled for 2014, and will analyse organic material on the surface by GC/MS.

 

 

Mars Science Lab (SAM)

in the upper surface layer of Mars. This is likely inaccurate because organic molecules are not volatile and

would not vaporize at the relatively low temperatures that were applied. In this case, derivatization is a better method of extraction, and will be used on the MSL. Derivatization involves dissolving chemicals from a soil sample into a liquid, then applying a derivatizing agent to make the compound more volatile. Organic compounds contain labile H atoms, which tend to form hydrogen bonds that prevent the material from vaporizing. Replacing these H atoms with Si atoms makes the molecules less reactive, and then heating up the liquid only slightly will cause the organics to vaporize without being destroyed. Derivitazation makes it possible to detect extremely tiny amounts of organics in a sample.

 

  • The gas chromatograph separates a mixture of gas into its constituents, producing a graph of peak

intensity versus relative time. Different compounds exit the GC at different times, creating distinct peaks

which are analyzed separately by the MS. The MS uses Electron Impact ionization to produce molecular and

fragment ions, which are then separated by mass. The instrument we use is a quadrupole mass analyzer(Fig. 4), which consists of four parallel rods that have fixed DC and alternating RF potentials applied. Ions are able to traverse the area between the rods, and as the potentials are varied sinusoidally with time, the ions in the central region follow complicated trajectories. The specific path an ion follows depends on its m/z, and varying the voltage combinations creates a stable path to the detector for certain ions. All others will hit the quadrupoles and will not be detected. The end result is a mass spectrum: a plot of abundance

versus m/z for a given compound.Molecular ion peaks will be present, as well as fragment ion peaks. Mass spectra can be used to determine the chemical composition of the gas and to detect isotope ratios.

 

The SAM measurement objectives fall into six categories that are aligned with high-priority science goals of Mars exploration. These include measuring (1) abundances of trace atmospheric species such as noble gases and certain small molecules (CH4, H2S, etc.); (2) high-precision isotope ratios in key atmospheric species, such as the noble gases, C in CO2, N in N2, and D/H in H2O; identities, isotope ratios, and possible chirality of (3) relatively volatile organics and (4) pyrolyzable inorganic species, characteristic of specific mineralogies, in solid phase samples; (5) identities of moderate to high mass refractory organics; and (6) relative abundances of refractory elements, to trace levels, in solid phase samples.

  • Within these materials, large elemental fractionations (e.g., in H, C, N, O, P, and S), as well as trace organic compounds (in samples obtained at some depth), should be detected and spatially correlated. Particularly significant would be the detection of larger organics (100-1000 amu and beyond), including various aromatic and aliphatic hydrocarbons.
  • Masses up to 1000 amu have been detected despite the lack of wet chemical sample preparation. An intensive study of the elemental and organic analysis capabilities of the SAM/LDMS technique with a range of natural and synthetic Martian analog materials is underway.

 

Operational Advantages and Limitations

Advantages

 

Minimum Detectable Concentration-

thermal conductivity detectors - tens of ppm

helium ionization detectors - ppb

 

Disadvantages

*(Grob, 330) Temperature Problems with GC/MS - thermal degradation of components can occur in GC/MS. THis degradation is frequently catalyzed by active sites somewhere in the chromatographic system. The injection port is normally construted with a replaceable glas liner. The silanol groups normally present on galss surfaces can case degradation of sample components. Typically injection port liners are deactivated with silanizing reagents which convert silanol groups to trimethlysilyl ethers. The use of on-column injecters is also recommended to prevent thermal degradation, since the fused silica capillary column is coated with liquid phase and fewer active silanol groups are present and injections are usually performed at lower temperatures.

 

Operational Requirements

*(P.J. Baugh, 1) GC is pre-eminent among analytical separation methods. It offers rapid and high resolution separations of a very wide range of compounds, with the only restriction that analytes should have sufficient volatility.

 

*(P.J. Baugh, 16) The performance of the chromatography and hence the quality of the results generated depends not only on the design of the components but also on how carefully they are controlled - particularly with respect to temperatures and gas flow rates.

 

  • (Imran - Akapo paper):

"small mass and size, low power consumption, high mechanical and shock strength, high sensitivity and accuracy, and reliability of operation under extreme space conditions" (ie high temperature & pressure range)

 

*

 

*only compounds with vapor pressure > 1e-10 torr can be analyzed (same as minimum detectable limit?)

 

*dynamic range?

 

  • Viking - had pair of radiosotope thermoelectric generators, stored in batteries, alternately charged & utilizes. 30V dc current

power consumption gcms 25-140W

 

  • (Imran - Akapo paper):

Viking had:

GC column (2 m×0.75 mm stainless steel) was filled with 60–80 mesh Tenax-GC (2,6-diphenyl-p-phenylene oxide) coated with polymetaphenoxylene and held isothermally at 50°C for 10 min. It was then ramped linearly to 200°C at 8.3°C/min and held at that temperature for a pre-determined period of 18, 36 or 54 min. A hydrogen stream was used to transfer the products into the gas chromatograph, the effluent of which was monitored every 10 s by the mass spectrometer. The mass range of the spectrometer was from m/z 10–220 with a resolution of ≈1:200.

 

Pioneer had:

300 g and had a mass of less than 10 kg

 

 

Huygens had:

The GC–MS instrument consists of a gas sampling system, a carrier-gas supply (hydrogen), a GC column assembly, and a mass spectrometer. The gas sampling system has three inlets: one for direct analysis of the atmosphere by MS, one for GC–MS analysis and a third for the analysis of the aerosol collector pyrolyzer (ACP) gaseous products. The GC column assembly is made of three parallel columns. One column, a Silcosteel tubing (Restek, Bellefonte, PA, USA) packed with carbon molecular sieve (Supelco, Bellefonte, PA, USA), is for the separation of permanent gases particularly nitrogen and carbon monoxide. The second column, a MXT capillary column (Restek) is for the analysis of nitriles and higher-molecular-mass compounds or gas mixtures released from the ACP experiments. The third column, which is a glassy carbon capillary column, is for the analysis of low-molecular-mass hydrocarbons, mainly C2–C3.

 

(P.J. Baugh, 358) Combined Gas Chromatography-Mass Spectrometry

Introduction

*GC-MS remains the most effective technique for separation, detection and characterization of the components of complex organic mixtures.

*GC and MS are regarded as the "natural combination", compared to the other combined chromatographic-mass spectrometric techniques as both display optimum performance with volatile or semi-volatile materials when smaple sizes per component are in the nanogram range.

*Considered in its simplest form, a MS is a device for producing and mass measuring ions.

 

GC-MS instrumentation:

*Sample Inlets -> Ion Source -> Mass Analyzer -> Ion Detector -> Data collection/analysis

 

*Imran:

minimum detectability or limit of detection

D = 2N/S, N noise level, S sensitivity

 

GC-MS Applications to solution of specific problems arising in mixture and trace analysis

  • Trace Anslysis
  • selected ion monitoring: the advantage of using MS in trace analysis derives from its ability to monitor pre-selected ion(s) in the spectrum of a compound. This technique, selected ion monitoring (SIM), leads to increases in both sensitivity, as the instrument does not then spend time scanning redundant regions of the mass spectrum, and selectivity, through the monitoring of an ion(s) (molecular or fragment) that is characteristic of the analyte of interest.

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