Standards
15
inlet of the standard and unknown samples produces a step-voltage
track shown in the top right of Fig. 6. After calibration, these steps give
the difference, .1, between the ion beam ratios of the standard and the
unknown samples directly. From the numerical value of .1 the desired bvalue (for the definition, see below) of the sample is calculated by the
equation
bX(Sample) = bX(standard) + .1 (1 + bX(standard) + C) .
X refers to the rare isotope in question, "standard" refers to the working
standard with b =1= 0, and C is a correction for superposition of the rare
isotope mass number with other molecules, e.g., 13C1602 with 12CI60170.
In the analysis of carbon we collect masses 44 and 45 simultaneously,
where the ratio is:
13C160160 + 12C170160
R =
12C160160
The component 12C 1 70 16 0 constitutes about 7 % of the total mass 45.
The correction factor for carbon analysis contains a constant term
reflecting the 17 O-content of the standard and a variable term reflecting
the difference in the 17 O-content of the sample and the standard. The
latter correction is obtainable directly from an 180-analysis of the gas
being analyzed for 13C (fractionations for various isotopes increase in
proportion to the mass difference, i.e., b 18 0 = 2b 17 0). For a more
detailed discussion see CRAIG (1957).
The digitization equipment, giving a direct readout of .1, is shown in
the lower part of Fig. 6. The main components are two voltage-tofrequency converters and an electronic counter equipped for frequency
ratio measurements.
The converters provide pulse frequencies directly proportional to
the input voltages, i.e., fB represents - pUB' and fA represents the small
difference signal obtained from amplifier A in the measurement of the
unknown sample. fA is fed to the normal counter input, whereas fB is
fed to the external input of the time base decades. In this way a direct
frequency ratio measurement is provided.
The main feature of this digitization method is its true integration
of signal voltage during the entire counting time.
v. Standards
In stable isotope geochemistry we are concerned with measuring
small changes in isotope ratios, where the isotope ratio must be measured
relative to an arbitrary standard. The accepted unit of isotopic ratio
measurement is the delta-value (b), given in per mil (%0). The b-value is
defined as
s:. 0/
R(sample) - R(standard) X 1000
u In /00 =
R(standard)
15
inlet of the standard and unknown samples produces a step-voltage
track shown in the top right of Fig. 6. After calibration, these steps give
the difference, .1, between the ion beam ratios of the standard and the
unknown samples directly. From the numerical value of .1 the desired bvalue (for the definition, see below) of the sample is calculated by the
equation
bX(Sample) = bX(standard) + .1 (1 + bX(standard) + C) .
X refers to the rare isotope in question, "standard" refers to the working
standard with b =1= 0, and C is a correction for superposition of the rare
isotope mass number with other molecules, e.g., 13C1602 with 12CI60170.
In the analysis of carbon we collect masses 44 and 45 simultaneously,
where the ratio is:
13C160160 + 12C170160
R =
12C160160
The component 12C 1 70 16 0 constitutes about 7 % of the total mass 45.
The correction factor for carbon analysis contains a constant term
reflecting the 17 O-content of the standard and a variable term reflecting
the difference in the 17 O-content of the sample and the standard. The
latter correction is obtainable directly from an 180-analysis of the gas
being analyzed for 13C (fractionations for various isotopes increase in
proportion to the mass difference, i.e., b 18 0 = 2b 17 0). For a more
detailed discussion see CRAIG (1957).
The digitization equipment, giving a direct readout of .1, is shown in
the lower part of Fig. 6. The main components are two voltage-tofrequency converters and an electronic counter equipped for frequency
ratio measurements.
The converters provide pulse frequencies directly proportional to
the input voltages, i.e., fB represents - pUB' and fA represents the small
difference signal obtained from amplifier A in the measurement of the
unknown sample. fA is fed to the normal counter input, whereas fB is
fed to the external input of the time base decades. In this way a direct
frequency ratio measurement is provided.
The main feature of this digitization method is its true integration
of signal voltage during the entire counting time.
v. Standards
In stable isotope geochemistry we are concerned with measuring
small changes in isotope ratios, where the isotope ratio must be measured
relative to an arbitrary standard. The accepted unit of isotopic ratio
measurement is the delta-value (b), given in per mil (%0). The b-value is
defined as
s:. 0/
R(sample) - R(standard) X 1000
u In /00 =
R(standard)