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第三節 穩定同位素的測量技術與方法 同位素測量基本原理 主機類型 前置設備 常用方法 數據解釋 同位素質譜儀 A mass spectrometer is a device that can separate atoms or molecules according to their mass. There are a number of different kinds of mass spectrometers operating on different principles. Most isotope ratio mass spectrometers are of a similar design, the magnetic-sector, or Nier mass spectrometer。 It consists of three essential parts: an ion source(離子 源), a mass analyzer(質量分析器) and a detector(探測器). 1/2MV2 = eV A mass spectrometer is an instrument which separates charged molecules by mass. An isotope ratio mass spectrometer (IRMS) works on this principle, but unlike other conventional mass spectrometers it has been specifically designed to measure the proportions of particular isotopes. An IRMS will be much more precise, but much less sensitive than other mass spectrometers. 對於一個在磁場中運動的帶電粒子,所受的力︰ F = q*v * B, q = 電荷, v = 運動速度, B = 磁場強度(高斯) F = m*v2/r, 運動粒子的勢能,r = 旋轉半徑 (cm) 所以,m*v2/r = q*v*B 對於動能, V*q = m*v2/2, V = 電壓(伏) 解得︰ m*v2 = 2V*q, v = (2V*q/m)1/2 合併上式︰2V/r = (2V*q/m)1/2*B 這樣,一個帶電粒子在電磁場場中運動的質-荷比就 取決於磁場強度、電壓和運動半徑 The mass spectrometers used for isotopic analysis generally comprise three basic sections: an ion source, a mass analyzer and an ion collection assembly. Gaseous molecules are introduced into the ionization chamber where interaction with a focused electron beam causes electrons to be stripped from the molecules, forming positive ions which are accelerated out of the chamber, down a flight tube placed between the poles of an electromagnet. Here, they are separated according to their mass-to-charge ratio (m/z). The ions are collected by a simple collector array consisting of three Faraday cup collectors. Only pure gases, or pure gas contained within a carrier gas, can be analyzed. Isotope ratio mass spectrometer Thermo Finnigan Delta XP Mass Range: 1-70 Daltons at 3 kV Resolution: CNOS: m/Δm = 95 (10 % valley), H/D: m/Δm = 10 (10 % valley) Sensitivity (absolute): 1500 molecules CO2 per mass 44 ion at the collector, also with high He load Ion Source Linearity: 0.02 %/nA ion current (m/z 44) Sample Consumption: 0.15 nmol/s CO2 for 10 nA signal at m/z 44 (10-2 mol/As) 3H+ Factor: Smaller than 10 ppm/nA, stability better than 0.03 ppm/nA/h John Dalton (September 6, 1766 – July 27, 1844) was an English chemist Dalton為質量單位, 等於原 子質量單位(amu)。碳 的同位素12C原子的質量 為12dalton,所以一Dalton 相當於1.661×10-24(= Avogadro數之倒數)克。 對分子來說,一個分子 的質量,用道爾頓單位 表示時,其值相當於分 子量。 DELTA V Plus The DELTA V Plus is the highest performance DELTA ever, offering the widest range of applications available. The ion optics have been refined, resulting in the highest possible ion transmission. The DELTA V Plus can accommodate up to 10 collectors, ensuring flexibility to cover all isotope ratio MS applications in the mass range up to m/z 96. MAT 253 MAT 253 takes over from the MAT 252 as the new world standard. It delivers the highest precision for the determination of H/D, 13C/12C, 15N/14N, 18O/16O, 34S/32S, 37Cl/35Cl, 81Br/79Br. General isotope measurements Dual inlet for air/gas Samples are prepared off-line. The pure gas is admitted into the IRMS by a variable volume, i.e. bellows. A reference gas is also admitted into the spectrometer via a bellows system. GasBench An innovative solution for high precision on-line isotope and molecular ratio determination of headspace samples, including water equilibration, carbonates, and atmospheric gases (e.g. CO2, O2/N2). The precision and accuracy of the dual viscous flow inlet system is achieved using modern ‘continuous flow’ techniques. TC/EA (High Temperature Conversion/Elemental Analyzer) is the technological breakthrough bringing the benefits of continuous flow IRMS to on-line oxygen and hydrogen isotope ratio analysis on solid and liquid bulk samples. ConFlo III The latest innovation in a series of continuous flow interfaces for coupling an Elemental Analyzer to an Isotope Ratio Mass Spectrometer. The ConFlo III allows attachment of any CFIRMS sample preparation unit which uses He carrier gas flows of 2 – 100 mL/min, including laser-aided combustion units, gas loop injectors and DOC analyzers. Continuous flow elemental analyzer (CFEA) Sample preparation, and subsequent analysis, is carried out on-line in a continuous flow of helium. An elemental analyzer is an automated sample preparation instrument in which samples are converted into pure gases via combustion, reduction, and pyrolysis reactions in the presence of catalysts. Gas Chromatography Mass Spectrometry (GC/MS) GC/MS is comprising a gas chromatograph (GC) coupled to a mass spectrometer (MS), by which complex mixtures of chemicals may be separated, identified and quantified. This makes it ideal for the analysis of the hundreds of relatively low molecular weight compounds found in environmental materials. In order for a compound to be analyzed by GC/MS it must be sufficiently volatile and thermally stable. In addition, functionalized compounds may require chemical modification prior to analysis, to eliminate undesirable adsorption effects that would otherwise affect the quality of the data obtained. Samples are usually analyzed as organic solutions consequently materials of interest (e.g. soils, sediments, tissues etc.) need to be solvent extracted and the extract subjected to various 'wet chemical' techniques before GC/MS analysis is possible. The sample solution is injected into the GC inlet where it is vaporized and swept onto a chromatographic column by the carrier gas (usually helium). The sample flows through the column and the compounds comprising the mixture of interest are separated by virtue of their relative interaction with the coating of the column (stationary phase) and the carrier gas (mobile phase). The latter part of the column passes through a heated transfer line and ends at the entrance to ion source where compounds eluting from the column are converted to ions. Two potential methods exist for ion production: electron ionization (EI) and chemical ionization (CI). For EI a beam of electrons ionize the sample molecules resulting in the loss of one electron. A molecule with one electron missing is called the molecular ion and is represented by M+. When the resulting peak from this ion is seen in a mass spectrum, it gives the molecular weight of the compound. Due to the large amount of energy imparted to the molecular ion it usually fragments producing further smaller ions with characteristic relative abundances that provide a 'fingerprint' for that molecular structure. This information may be then used to identify compounds of interest and help elucidate the structure of unknown components of mixtures. CI begins with the ionization of methane, creating a radical which in turn will ionize the sample molecule to produce [M+H]+ molecular ions. CI is a less energetic way of ionizing a molecule hence less fragmentation occurs with CI than with EI, hence CI yields less information about the detailed structure of the molecule, but does yield the molecular ion; sometimes the molecular ion cannot be detected using EI, hence the two methods complement one another. Once ionized a small positive is used to repel the ions out of the ionization chamber. The next component is a mass analyzer (filter), which separates the positively charged ions according to various mass related properties depending upon the analyzer used. Several types of analyzer exist: quadrupoles, ion traps, magnetic sector, time-of-flight, radio frequency, cyclotron resonance and focusing to name a few. The most common are quadrupoles and ion traps. After the ions are separated they enter a detector the output from which is amplified to boost the signal. The detector sends information to a computer that records all of the data produced, converts the electrical impulses into visual displays and hard copy displays. In addition, the computer also controls the operation of the m a s s s p e c t r o m e t e r . Gas Chromatography Combustion Isotope Ratio Mass Spectrometry (GC/C/IRMS) GC/C/IRMS is used to ascertain the relative ratio of light stable isotopes of carbon (13C/12C), hydrogen (2H/1H), nitrogen (15N/14N) or oxygen (18O/160) in individual compounds separated from often complex mixtures of components. The ratio of these isotopes in natural materials varies slightly as a result of isotopic fractionation during physical, chemical and biological processes resulting with the relative isotopic ratio of specific compounds being highly diagnostic of key environmental processes. Furthermore, use of growth substrates incorporating compounds artificially enriched in the heavier isotope can aid in the deconvolution of often highly complex and obscure biogeochemical pathways. The primary prerequisite for GC/C/IRMS is that the compounds constituting the sample mixture are amenable to GC, i.e. they are suitably volatile and thermally stable. Polar compounds may require further chemical modification and in such cases the relative stable isotope ratio of the derivatization agent must also be determined. For carbon and nitrogen compounds eluting from the chromatographic column then pass through a combustion reactor (an alumina tube containing Cu, Ni and Pt wires maintained at 940 ºC) where they are oxidatively combusted. This is followed by a reduction reactor (an alumina tube containing three Cu wires maintained at 600 ºC) to reduce any nitrogen oxides to nitrogen. For hydrogen and oxygen a high temperature thermal conversion reactor is required. Water is then removed in a water separator by passing the gas stream through a tube constructed from a water permeable Teflon membrane. The sample is then introduced into the ion source of the MS by an open split interface.