Status report on sample preparation facilities for 14C analysis at the ...

4 J. Rethemeyer et al. / Nuclear Instruments and Methods in Physics Research B xxx (2012) xxx–xxx
Table 2
Results for reference materials (ca. 1 mg C) graphitized with the AGE.
Sample Material Measured value Reference value n AMS measurement
IAEA C5 a Wood 23.30 ± 0.12 pmC 23.05 ± 0.02 pmC 2 ETH
IAEA C6 Sucrose 150.41 ± 0.28 pmC 150.60 ± 0.10 pmC 5 ETH
IAEA C8 Oxalic acid 15.06 ± 0.09 pmC 15.03 ± 0.17 pmC 2 ETH
VIRI M b Wood 73.61 ± 0.41 pmC 73.90 ± 0.03 pmC 4 COL
VIRI O Cellulose 98.41 ± 0.55 pmC 98.46 ± 0.04 pmC 4 COL
VIRI S Barley mash 108.85 ± 0.61 pmC 109.96 ± 0.04 pmC 3 COL
VIRI T Humic acid 65.75 ± 0.37 pmC 65.82 ± 0.03 pmC 3 COL
VIRI U Humic acid 22.98 ± 0.15 pmC 23.08 ± 0.02 pmC 3 COL
VIRI H Whale bone 9618 ± 48 years BP 9528 ± 7 years BP 4 COL
VIRI I Whale bone 8373 ± 47 years BP 8331 ± 6 years BP 4 COL
Ox M c Mammoth bone 52,290 ± 1583 years BP >147k years BP 4 COL
a
IAEA Reference Products, Vienna, Austria.
b VIRI: fifth radiocarbon intercomparison [22,23].
c Oxford mammoth bone [24].
contamination that is difficult to quantify [14,20,21]. Initial tests
with GC standards of modern and fossil origin show compound
recoveries for the n-alkanes and n-carboxylic acids of intermediate
(C 18 ) and long chain length (C 30 ) between 72% and 99% (traps at
room temp.; Table 1). Lower recoveries were obtained for long
chain compounds initially trapped at 60 °C to avoid compound
crystallization in the intersection of the PFC capillaries into the
glass traps which apparently caused material losses. Contaminants
introduced during the gas chromatographic isolation were determined
by GC-FID analysis and were less than 0.6%. The 14 C depletion
of the isolated modern and the 14 C enrichment of the old/fossil
GC standards relative to the untreated material reflect the contribution
of exogenous carbon from both, modern and fossil sources
as also observed in previous studies [18,21]. The difference in 14 C
concentration between the untreated standard material and the
isolated compounds was 3.4 and 4.0 pmC for old/fossil material
and for modern material 4.4 and 1.5 pmC (n-carboxylic acids and
n-alkanes, respectively; Table 1). The 14 C concentration of the
exogenous carbon ( 14 C blank ) and its amount (C blank ) cannot be
determined directly. Thus, dated standard materials processed in
a similar way like the target compounds and isotopic mass balance
calculation are often used to determine both parameters. Using our
results for the isolated standards ( 14 C measured and C measured ) and for
untreated material ( 14 C sample and C sample ) we estimated C blank by
mass balance calculation (Eq. (1)) assuming that 14 C blank is either
modern (106 pmC) or fossil (0 pmC).
14 C measured C measured ¼ 14 C sample C sample þ 14 C blank C blank ð1Þ
with C measured =C sample +C blank .
The mass of exogenous carbon ( 14 C blank : 0 pmC) introduced during
GC isolation and subsequent sample preparation for the AMS
measurement was about 3.9 ± 1.6 lg C (octadecanoic acid) and
1.1 ± 2.4 lg C (squalane) for modern standard material. For the
old standards C blank ( 14 C blank : 106 pmC) was in a similar range with
2.1 ± 1.4 lg C (triacontanoic acid) and 3.8 ± 0.7 lg C (octadecane).
Possible source of contamination are column and septa bleed,
incomplete solvent removal and all steps of sample handling
including compound removal from the glass traps and transfer into
quartz ampoules, solvent evaporation and sealed tube combustion.
We could not remove contamination derived from column bleed
by eluting the isolated compound over a silica gel column as suggested
by Ohkouchi et al. [22]. Since GC analysis of the isolated
compounds shows very little contamination, we suppose that further
processing of the isolated compounds and sealed tube combustion
are major sources of contamination. Further analyses are
necessary to differentiate contaminants introduced during the different
processing steps including GC isolation, further sample handling
and combustion as well as to explain the relatively large
scatter of the 14 C results for the replicate isolations (n = 3–4) of
the different standards.
3.3. Standards and VIRI samples
To assure the quality of the sample handling and graphitization
procedure we measured a selection of organic and bone standard
materials from the Fifth Radiocarbon Intercomparison exercise
(VIRI; [23,24]), IAEA and the Oxford Radiocarbon Laboratory [25]
shown in Table 2. The results are in good agreement with the consensus
values. This also applies to the VIRI bone material and the
Oxford mammoth bone. The radiocarbon free mammoth bone
(>147k years) yields a very good blank value (52k years) without
ultrafiltration of the gelatinized collagen.
4. Summary
We presented first results for reference and standard materials
that have been processed and graphitized in the Radiocarbon laboratory
of the new CologneAMS facility, including organic materials
and bones, which agree well with consensus values. The process
blank for organic samples combusted in an elemental analyzer and
graphitized with the AGE for normal sized samples (ca. 1 mg C,
measured in Cologne) was 0.12 pmC. A constant contamination
of about 0.7 lg C with 75 pmC was determined, which derives
mainly from the combustion of organic samples in the elemental
analyzer including the tin containers. For small sample sizes of
about 150–350 lg C reasonable blank values (40,500 ± 500 years
BP for 150–160 lg C) were still obtained, while smaller sample
sizes (