Standardization lecture

Isotope Ratio Standardization

• we’ve seen in class that waters are standardized against
SMOW (or VSMOW), carbonates against PDB (or VPDB),
nitrogen against AIR, etc.. But what does this mean?

• we’ve seen in class that waters are standardized against
SMOW (or VSMOW), carbonates against PDB (or VPDB),
nitrogen against AIR, etc.. But what does this mean?
• each scale has its own arbitrary zero point. For waters
this is VSMOW. The definition of the delta value (in
terms of oxygen):
δ 18 O sample-VSMOW = ( ( 18 O/ 16 O) sample / ( 18 O/ 16 O) VSMOW – 1 ) * 1000‰

• we’ve seen in class that waters are standardized against
SMOW (or VSMOW), carbonates against PDB (or VPDB),
nitrogen against AIR, etc.. But what does this mean?
• each scale has its own arbitrary zero point. For waters
this is VSMOW. The definition of the delta value (in
terms of oxygen):
δ 18 O sample-VSMOW = ( ( 18 O/ 16 O) sample / ( 18 O/ 16 O) VSMOW – 1 ) * 1000‰
• If we plug VSMOW into this formula, we calculate a
value of zero. Hence this is the material that the
oxygen value of waters (plus other compounds) are
anchored to.

• we generally use a two point calibration that spans the
range of isotope values we analyze. In this way we
eliminate error that can occur from a one point
calibration, and we ensure inter-laboratory consistency
of analyses. We will see how this works in detail in
Tutorial 8 when we reduce the data for your hair
samples.

• we generally use a two point calibration that spans the
range of isotope values we analyze. In this way we
eliminate error that can occur from a one point
calibration, and we ensure inter-laboratory consistency
of analyses. We will see how this works in detail in
Tutorial 8 when we reduce the data for your hair
samples.
• what is the difference between SMOW and VSMOW, or
PDB and VPDB? The IAEA controls the dissemination of
stable isotopic standard material to the world. The
original water O and H scales were anchored to SMOW
and the carbonate C and O scales were anchored to
PDB. SMOW (Standard Mean Ocean Water) was an
actual batch of water samples that had δ 18 O = δD = 0‰,
by definition (no error). Likewise for PDB (Peedee
Belemnite, the rostrum of a squid-like animal that was
found in the Peedee Formation) δ 18 O = δ 13 C = 0‰

• but these original batches were exhausted over time,
and new batches had to be synthesized. Unfortunately
it is very difficult to make a new batch of water, or find
a carbonate sample, that has the exact same isotope
ratios as SMOW, or PDB, respectively.

• but these original batches were exhausted over time,
and new batches had to be synthesized. Unfortunately
it is very difficult to make a new batch of water, or find
a carbonate sample, that has the exact same isotope
ratios as SMOW, or PDB, respectively.
• IAEA synthesized a water sample that has isotope ratios
very close to SMOW. They also found a homogeneous
carbonate sample from the Toilet Seat Formation (TSL-
ST or NBS-19). Rather than associate an error with
these standards, the international community decided
to set the new water standard as δ 18 O = δD = 0‰.
Likewise the new carbonate standard NBS-19 was set to
δ 13 C = 1.95‰ and δ 18 O = -2.20‰. But this is not exactly true
as these new materials are slightly different. So we
now use VSMOW to refer to the new water scale and
VPDB to refer to the new carbonate scale. This way we
can track what particular standards samples were
calibrated against.

Uncertainty In Isotope Ratio
Measurements

• mass spectrometers are used to measure isotope ratios.
But a mass spectrometer is a precision machine, not an
accuracy machine. We must standardize in order to
achieve accuracy.

• mass spectrometers are used to measure isotope ratios.
But a mass spectrometer is a precision machine, not an
accuracy machine. We must standardize in order to
achieve accuracy.

• when we make our measurements we have two sources
of uncertainty to keep track of: measurement
uncertainty and uncertainty in standards used in
calibrations.

• when we make our measurements we have two sources
of uncertainty to keep track of: measurement
uncertainty and uncertainty in standards used in
calibrations.
• measurement uncertainty can arise from many sources:
-fluctuating conditions in the mass spectrometer
(and the peripheral devices that prepare our
samples) during the analyses. Ambient factors
such as temperature and humidity play a role, and
internal factors such as stability of electronics are
important.
-measuring very small ion beams such as the HD
beam in hydrogen isotope measurements, or very
small samples in general.
-how well tuned the mass spectrometer is.
-how clean the ion source is.
-etc.

• these factors all contribute to a scatter that occurs in
data, which we refer to as reproducibility or precision.
We make a measurement of this scatter for our
standards and report it with all data, to give the clients
a feel for how good our measurements are.

• standards from the IAEA may come as two types: those
that are set to exact values, so do not have any error
associated with them (e.g. VSMOW, VPDB), and others
that have an uncertainty associated with them. These
latter will introduce additional uncertainty into our
measurements.

• standards from the IAEA may come as two types: those
that are set to exact values, so do not have any error
associated with them (e.g. VSMOW, VPDB), and others
that have an uncertainty associated with them. These
latter will introduce additional uncertainty into our
measurements.
• in the case of nitrogen there exist no standards from
the IAEA that are set to any values by definition. The
nitrogen data in our lab is calibrated against two IAEA
standards: USGS25 (ammonium sulphate) δ 15 N = -30.4‰
± 0.4‰ AIR, and IAEA-305A (also ammonium sulphate)
δ 15 N = 39.8‰ ± 0.25‰ AIR. We must keep these
uncertainties in mind when we report the overall
uncertainty for our samples. The total uncertainty is a
combination of machine uncertainty (precision) and
standard uncertainty (accuracy).

• there are other factors that can contribute to the quality
of isotopic measurements. One extremely important
one is homogeneity. This has two facets to it: (1) is
your sample really a true representation of what you
want to measure; and (2) is your sample well mixed so
that if I take a small portion of it to measure it will
agree with other analyses from that same batch of
sample. Our overall precision can quickly be destroyed
from a poorly homogenized sample, and the accuracy
we may think we’ve obtained may be incorrect due to a
poorly chosen sample (i.e. the sample does not reflect
its source).

• hence all of the delta values that we have been
supplying for you in the labs have a certain amount of
uncertainty associated with them. A good lab will have
a solid QC/QA (quality control/quality assurance)
program in place to assure high data quality. For
example, when we run carbonate samples in our lab we
run the international standard NBS-19 directly to
calibrate all data. We also run an in-house standard as
a check on the calibration. We report the results of this
in-house standard to our clients as well as the accepted
value of this standard so they can see the quality of the
run for themselves. Also, and most importantly, we run
repeats of samples at random. This is the ultimate
check on how good the analysis is. Note that
homogeneity is crucial, and it is up to the client to send
homogeneous samples, otherwise we cannot guarantee
the quality of the analysis.

• Note: many isotope laboratories cheat when it comes to
reporting uncertainties, and they tend to report the
precision of the measurement as the overall
uncertainty. Not only that, but the precision they report
is based on many runs of a homogeneous standard. A
sample is not run many times to a statistically better
result for its isotope value, but run only once. So how
relevant is the precision of your standard you ran 20
times to a single sample analysis?

• Note: many isotope laboratories cheat when it comes to
reporting uncertainties, and they tend to report the
precision of the measurement as the overall
uncertainty. Not only that, but the precision they report
is based on many runs of a homogeneous standard. A
sample is not run many times to a statistically better
result for its isotope value, but run only once. So how
relevant is the precision of your standard you ran 20
times to a single sample analysis?
• in general the Saskatchewan Isotope Laboratory reports
the following uncertainties:
-C and O from carbonates: ±0.05‰ and ±0.10‰ VPDB,
respectively.
-D and O from waters: ±2‰ and ±0.2‰ VSMOW, respectively.
-C and N from organics: ±0.1‰ and ±0.4‰ VPDB and AIR,
respectively.

Sample Calculation

• how exactly does the software go about calculating a
delta value from measurements? Here is an example of
calculating δ 13 C for CO 2 generated from CaCO 3 :

• Given Info from Analysis (which we all should somewhat
understand from the lab tours):
Sample CO 2 Ion Beam Intensity (mV)
mass 44 mass 45 mass 46
4727.247 5646.864 6090.994
Ref CO 2 Ion Beam Intensity (mV)
mass 44 mass 45 mass 46
4703.024 5538.986 6054.629
Background CO 2 Ion Beam Intensity (mV)
mass 44 mass 45 mass 46
2.71 3.27 3.62
Ref Gas
δ 13 C -13.26‰, VPDB
δ 18 O -10.80‰, VPDB

Step 1
Subtract backgrounds from signal (done automatically by software).

Step 1
Subtract backgrounds from signal (done automatically by software).
Step 2
Calculate ratios:
Sample
45CO2 / 44CO2 46CO2 / 44CO2 1.194535424 1.288486512
Ref
45CO2 / 44CO2 46CO2 / 44CO2 1.17774989 1.287390624

Step 3
Calculate CO 2 delta values of sample wrt reference:
δ 45 CO 2 sample-ref = ((( 45 CO 2 / 44 CO 2 ) sample /( 45 CO 2 / 44 CO 2 ) ref ) - 1) * 1000‰
δ 46 CO 2 sample-ref = ((( 46 CO 2 / 44 CO 2 ) sample /( 46 CO 2 / 44 CO 2 ) ref ) - 1) * 1000‰
δ 45 CO 2 sample-ref = 14.25‰
δ 46 CO 2 sample-ref = 0.85‰

Step 4
Apply Boato equation to calculate delta values of the sample wrt
VPDB:
δ 45 CO 2 sample-VPDB = δ 45 CO 2 sample-ref + δ 45 CO 2 ref-VPDB + (δ 45 CO 2 sample-ref * δ 45 CO 2 ref-VPDB
/1000)
δ 46 CO 2 sample-VPDB = δ 46 CO 2 sample-ref + δ 46 CO 2 ref-VPDB + (δ 46 CO 2 sample-ref * δ 46 CO 2 ref-
VPDB/1000)
δ 45 CO 2 sample-VPDB = 1.31‰
δ 46 CO 2 sample-VPCB = -9.98‰

Step 5
Craig 17 O Correction
δ 13 C = (1.0676* δ 45 CO 2 ) - (0.0338 * δ 18 O)
δ 18 O = (1.0010 * δ 46 CO 2 ) - (0.0021 * δ 13 C)
δ 13 C sample-VPDB = 1.73‰

Step 5
Craig 17 O Correction
δ 13 C = (1.0676* δ 45 CO 2 ) - (0.0338 * δ 18 O)
δ 18 O = (1.0010 * δ 46 CO 2 ) - (0.0021 * δ 13 C)
δ 13 C sample-VPDB = 1.73‰
• Note: a two-point calibration negates all of this, and
makes data reduction much simpler to handle. But the
only way to do a two-point calibration is by having two
end-member standards that you know the delta values
of, and the above calculations are involved in
determining those values. So it is not really that these
calculations are negated, just hidden.

But why cheat ourselves out
of all of this fun???

Get lost.

Get lost.
Have a good break.