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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
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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
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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
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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.