General Remarks on Sample Handling
17
4) easy to handle for chemical preparation and isotopic measurement,
5) in about the medium value of the naturally occurring isotopic
variation range.
Among the reference samples now used, relatively few fulfil all of the
requirements. For example, the most widely used carbon isotope standard, PDB, a belemnite, is now exhausted. The NBS distributes another
standard, Solenhofen limestone (NBS-20). Table 3 presents world-wide
standards now in use.
VI. General Remarks on Sample Handling
Isotopic differences between the samples to be measured are often
exceedingly small. Therefore, great care has to be taken in chemical
handling of the sample material, to avoid any isotope fractionation
there.
There are difficult chemical problems to be solved in the chemical
preparation of minerals and rocks for isotopic analysis. It is important to
carry out chemical reactions that convert rock samples to gas samples
without changing the isotopic composition by contamination (e.g., contamination through the reagents), by exchange with extraneous matter,
or by chemical isotope fractionation.
To convert the samples to a suitable form for analysis, several different preparation techniques are used in isotope geochemistry. Although
these preparation procedures are chemically very different, the various
methods have one general feature in common: Any preparation procedure providing a yield of less than 100 percent may produce a reaction
product that is isotopically different from the original specimen, since
the different isotopes exhibit different reaction rates.
A quantitative yield of a pure gas is necessary for the mass-spectrometric measurement in order to prevent not only isotope fractionations
during sample preparation, but also interference in the mass spectrometer. Contamination with gases having the same molecular masses and
having similar physical properties may be a serious problem. This is
especially critical with CO2 and N20 on the one hand (CRAIG and KEELING, 1963) and N2 and CO on the other. When CO2 is used, interference
by hydrocarbons and a CS + ion may also be a problem.
The chief contaminant of gaseous samples is air. Contamination may
result from incomplete evacuation of the vacuum system and/or from
degassing of the sample. How gases are transferred, distilled, or otherwise processed in vacuum lines is briefly discussed under the different
elements. All errors due to chemical preparation commonly limit the
precision to 0.1 to 0.3%0.
17
4) easy to handle for chemical preparation and isotopic measurement,
5) in about the medium value of the naturally occurring isotopic
variation range.
Among the reference samples now used, relatively few fulfil all of the
requirements. For example, the most widely used carbon isotope standard, PDB, a belemnite, is now exhausted. The NBS distributes another
standard, Solenhofen limestone (NBS-20). Table 3 presents world-wide
standards now in use.
VI. General Remarks on Sample Handling
Isotopic differences between the samples to be measured are often
exceedingly small. Therefore, great care has to be taken in chemical
handling of the sample material, to avoid any isotope fractionation
there.
There are difficult chemical problems to be solved in the chemical
preparation of minerals and rocks for isotopic analysis. It is important to
carry out chemical reactions that convert rock samples to gas samples
without changing the isotopic composition by contamination (e.g., contamination through the reagents), by exchange with extraneous matter,
or by chemical isotope fractionation.
To convert the samples to a suitable form for analysis, several different preparation techniques are used in isotope geochemistry. Although
these preparation procedures are chemically very different, the various
methods have one general feature in common: Any preparation procedure providing a yield of less than 100 percent may produce a reaction
product that is isotopically different from the original specimen, since
the different isotopes exhibit different reaction rates.
A quantitative yield of a pure gas is necessary for the mass-spectrometric measurement in order to prevent not only isotope fractionations
during sample preparation, but also interference in the mass spectrometer. Contamination with gases having the same molecular masses and
having similar physical properties may be a serious problem. This is
especially critical with CO2 and N20 on the one hand (CRAIG and KEELING, 1963) and N2 and CO on the other. When CO2 is used, interference
by hydrocarbons and a CS + ion may also be a problem.
The chief contaminant of gaseous samples is air. Contamination may
result from incomplete evacuation of the vacuum system and/or from
degassing of the sample. How gases are transferred, distilled, or otherwise processed in vacuum lines is briefly discussed under the different
elements. All errors due to chemical preparation commonly limit the
precision to 0.1 to 0.3%0.