- Email: [email protected]
Internal and external checks in the NOSAMS sample preparation laboratory for target quality and homogeneity
Nuclear Instruments and Methods in Physics Research
B 92 (1994) 158-161
North-Holland
Beam Interactions with Materials 8 Atoms
Internal and external checks in the NOSAMS sample preparation laboratory for target quality and homogeneity E.A. Osborne *, A.P. McNichol, A.R. Gagnon, D.L. Hutton and G.A. Jones National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution, Woods Hole, h&4 02543, USA
In the NOSAMS sample preparation laboratory (SPL) we have developed rigorous internal procedures aimed at ensuring that sample preparation introduces as little error into our analyses as possible and identifying problems rapidly. Our three major CO, preparation procedures are: stripping inorganic carbon from seawater, hydrolyzing CaCO,, and oxidizing organic matter. For seawater, approximately 10% of our analyses are standards or blanks which we use to demonstrate extraction of virtually all the inorganic carbon. Analysis of the stable carbon isotopic composition of the CO* extracted from our standards indicates a precision of better than 0.15-0.20%0. We also routinely process 14C-free CO, in our stripping lines to demonstrate the absence of a significant process-dependent blank. For organic combustions and CaCO, hydrolyses, we use the carbon yield (% organic carbon (OC) or % CaCO, by weight) as a check on our sample procedures. We have analyzed the blank contribution of these procedures as a function of sample size. Our organic carbon blank is constant at approximately 0.4% modern for samples containing greater than 1 mg C and our carbonate blank is less than 0.2% modern for samples containing more than 0.5 mg C. We use a standard Fe/H, catalytic reduction to prepare graphite from CO,. We check the completeness of our reactions with the pressure data stored during the reaction as well as use a robot to determine a gravimetric yield. All graphite undergoes a visual inspection and is rejected if any heterogeneities are present. We have recombusted graphite made from CO, with 613C values ranging from - 42 to 1%0 and determined that the &13C of the recombusted carbon agrees with that from the pure gas to within 0.05%0, demonstrating little or no fractionation during the treatment of the sample. The 613C we measure on the CO, generated from more than 75% of our samples is compared to the 613C measured on the AMS as a further check of our procedures. As further external checks, we analyzed the International Atomic Energy Association (IAEA) samples during the establishment of our laboratory and are presently participating in the third international radiocarbon intercalibration (TIRI) exercise.
1. Introduction The NOSAMS sample preparation laboratory (SPL) is dedicated to ensuring sample quality throughout all sample preparation. Samples entering the SPL go through rigorous internal procedures before they are finally submitted to the AMS as graphite targets. There are two major steps in the processing of raw samples, CO, extraction and CO, reduction. The CO, extraction method varies with the type of sample. We are primarily concerned with three sample types: seawater, CaCO, and organic matter. Once the CO, is extracted, it is reduced to carbon graphite by a catalytic reaction. These procedures are well documented and linked numerically along with the sample data. Known standards are run intermittently with the samples to routinely monitor the performance of the line. We use the data obtained from these standards to check for fractionation of the carbon isotopes (deviations in 613C data), contamination (A14C), and total recovery of the
Corresponding author. Tel. + 1 508 457 2000 (ext. 3307), fax + 1 508 457 2183, e-mail [email protected]. 0168-583X/94/$07.00 0 1994 - Elsevier SSDI 0168-583X(94)00017-P
Science
sample during processing. In this paper, we detail our internal sample procedures and show calculations and data that we routinely use to validate our performance.
2. Internal checks 2.1. Seawater Seawater samples arrive at NOSAMS in 500 ml bottles. A stripping probe is inserted into the bottle in an inert atmosphere, the sample is acidified and placed on the seawater stripping line. To extract the CO,, nitrogen gas is bubbled through the acidified seawater, and the CO, is collected in liquid nitrogen traps [1,2]. We have established a systematic procedure for quality control on the seawater stripping line. Standards and blanks are run intermittently with the samples, and the data collected provide up-to-date information on the consistency and contamination of the process. For our quality control samples, we use sodium carbonate (Na,CO,), local surface seawater (from Buzzard’s Bay) and a carbon-14 free CO, gas (approximately 42 000 yr old).
B.V. All rights reserved
159
E.A. Osborneet al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 158-161 2.1.1. Na,CO,
2.2. CaCO,
Na,CO, standards are prepared in the sample preparation laboratory by adding a known amount of Na,CO, to carbon-free H,O and analyzed (1 every 16 samples) on the stripping line. Knowledge of the original CO, concentration (Z CO,) allows us to calculate the percent CO, recovery of the stripping process by the following: % yield = I;CO, (final)/BCO,
(initial) X 100
= (CO, recovered [mmol] /water
weight [kg] X 100)
/(Na,CO,wt
[mg]/105.9887
[mg/mmol]
/water weight [kg]). The CO, extracted from the Na,CO, is run on a VG Optima mass spectrometer for a stable isotope analysis (S13C). Deviation from the original 613C value signals fractionation of the carbon isotopes during the water stripping process. Our results indicate an average precision better than 0.15%0. 2.1.2. Buzzard’s Bay seawater Buzzard’s Bay seawater is collected from local surface seawater in batches. Each batch is separated into approximately 30 samples, poisoned with HgCl, to stop biological activity, and sealed in 500 ml bottles. We use Buzzard’s Bay seawater as a standard (1 every 16 samples) because it is very similar to the WOCE seawater samples. For Buzzard’s Bay seawater we use CCO, (instead of % yield) and 613C data from Buzzard’s Bay seawater to provide us with another quality control check. Our results indicate ZCO, = 1.87 5 0.10 mmol/kg and 613C = 1.22 k 0.20%0 (data obtained for the first half of 1993). We also use Buzzard’s Bay seawater to ensure that our seawater samples do not change isotopically over the 2-3 yr they will be stored before analysis. Fraction modern vahres of Buzzard’s Bay seawater plotted over time indicate no storage contamination over a span of 1.5 yr. 2.1.3. Line blanks Line blanks, consisting of carbon-14 free CO,, are circulated through the stripping line (1 every 32 samples). A14C data of the line blanks give us information on the background i4C signal of the line. The line blank is run on the AMS in conjunction with seawater samples stripped at approximately the same time the line blank was run. To date, we have used them to ensure no samples were collected on contaminated ships. In the future, we will use the line blanks in our data analysis calculations.
CaCO, samples are hydrolyzed with 100% H3P04 acid within a closed evacuated reactor and placed in a 60°C water bath for the duration of the reaction, usually overnight. Carbon-14 free CaCO, (IAEA C-l) is run (1 every 10 samples) in conjunction with our samples. The data obtained from running this standard (% CO, recovery) give us a clear indication of the completeness of the hydrolysis reaction (99.12 k 1.01% in 1993). As a rigorous r4C check, we also compare our results from this standard and IAEA C-2 with the data supplied by the IAEA intercalibration program (refer to section 3). The size of the IAEA C-l standard is determined by the size of the corresponding samples, and as the sample size decreases, the effect of background on the sample increases. From our results, our carbonate blank is less than 0.2% modern for samples containing at least 0.5 mg carbon (4.2 mg CaCO,). 2.3. Organic matter Organic matter samples consist of a wide variety of sample types, each with a unique pretreatment and purifying procedure. For example, sediment samples (our most abundant) need only an acidification step to remove the inorganic carbon, while wood samples require a three-step acid-base-acid treatment to remove humic acid contaminants. With each pretreatment procedure, however, there is only one oxidation procedure. The cleaned organic sample is oxidized with CuO in a closed quartz combustion tube at 850°C with Ag added to remove the sulfur and chlorine by-products [31. As with CaCO,, the smaller the sample size the greater the effect of the background on the sample. We are also testing using a commercial carbon (Johnson Matthey Electronics, 200 mesh; 99.9999% Ultra Carbon Powder, “Alfa Powder”) as a substitute for the NBS-21 we use to test our blanks. Initial results suggest it has a lower background. We will also develop a carbon-14 dead sediment standard to test our entire procedure. 2.4. Graphite preparation The CO, extracted in any of the described methods is then reduced to graphite in a 10 ml reactor by the reaction [4]: CO, + 2H, Fe,625”C 2H,O + C(gr). Two mg Fe powder is placed at the end of a 2 in. horizontal quartz tube and the reactor is evacuated. The CO, sample (- 200 pmol) and an excess of H, are added to the reactor. The pressure of the reactor is II. AMS TECHNIQUES
160
E.A. Osborne et al. /Nucl.
Instr. and Meth. in Phys. Res. B 92 (1994) 158-161
monitored by a pressure transducer, with a total reactor pressure starting at - 2.5 atm and ending at u 0.2 atm, once the reaction is complete (when the pressure no longer changes). The CO, reduces to elemental carbon in the form of filaments, long threadlike fibers protruding from the Fe surface [4]. This Fe-C mixture becomes the AMS sample target for A14C analysis. The graphite process described above has stringent quality checks to ensure isotopically accurate and homogeneous targets. Total conversion of CO, to graphite is monitored by both pressure and gravimetric yield data, with yield calculations as follows:
O.‘O ~ 0.05
0.00
i
I
0
V
p s-O.05 $3 g-0.70
A 0
gravimetric yield [%I weight [mg][(Fe and C tube) - (Fe tube)] = x 100 CO, sample [mmol] X 12[mg/mmol] -50
pressure yield [ %] = P,,, reacted (actual)/Ppas = ((P
reactor (initial)
-30
-20
SlfC(unprocessed
reacted (theo)
- P reactor (final))
-40
X 100)
/{ 3 X (CO, sample [mmol] X 24.47[ml X atm/mmol]/l0.5[ml]}. Carbon fractionation during the graphite process is determined periodically by recombusting the graphite and comparing its S13C value with that of the original unprocessed CO,. Or, 62YC,Fe graphite -850”c,cUo CO, CO, J 4 PC 2 PC 1
-10
0
CO,) (o/00)
Fig. 1. Fractionation of the graphite reaction: carbon fractionation for the graphite reaction is determined by comparing the difference in the 613C values of samples before and after graphitization. A(6l”C) (i.e., 613C (recombusted graphite)613C (unprocessed CO,)) is plotted against 613C (unprocessed CO,) for a series of samples: (01 Mat Gas (commercially obtained from Matheson Gas Products, 613C = - 0.72%0), (Cl) Oxalic Acid I (613C = -19.11%0), (A) Oxalic Acid II (613C = - 17.59%0), and (v) Tank CO, (obtained from the Smithsonian Astrophysical Observatory, 613C = - 43.26%0).
2.0
where A(613C) = 613C, - 613C,.
g- 1.5
As seen in Fig. 1, fractionation process is less than 0.05%0.
during the graphite
3. External methods As a check on the entire AMS facility, a series of IAEA intercalibration samples were run through the entire system, and our results were compared to the final results of the intercalibration study [5]. The study included six different samples described below: IAEA IAEA IAEA IAEA IAEA
C-l C-2 C-3 C-4 C-5
IAEA
C-6
CaCO,, carbon-14 free CaCO, powder Cellulose Wood
preparation
0.0
II 0.5
1
”
I
1.0
0 IAEA C-5 0 IAEA C-6 c 1 I( 0 1 ’ 1.5
2.0
IAEA Consensus (fraction modern)
Wood ANU Sucrose.
in the sample
Y ;5 4 0.5
0.0
Our results vs. IAEA results (Fig. 2) further validate our methods
B E .-5 $j 1.0 &
laboratory.
We
Fig. 2. IAEA consensus vs. NOSAMS results: a series of IAEA samples were analyzed at NOSAMS to compare our results with those of the IAEA intercalibration study. Fraction moderm of NOSAMS is plotted against the IAEA consensus fraction modern value for a series of samples: (0) IAEA C-l (Carbon-14 free CaCO,), (0) IAEA C-2 (CaCO, powder), (A ) IAEA C-3 (Cellulose), ( v ) IAEA C-4 (Wood), (0) IAEA C-5 (Wood), and (0) IAEA C-6 (ANU Sucrose).
E.A. Osborne et al. /Nucl.
Instr. and Meth. in Phys. Res. B 92 (1994) 158-161
are participating in the third international radiocarbon intercalibration (TIRI) exercise, another intercalibration study, and will report our results when the study ends. We are also in the process of externally checking the standards’ (Oxalic Acid I, Oxalic Acid II, and a Reference Gas) performance on the graphite line. We sent aliquots of our CO, to the Toronto AMS laboratory, and the results received from Toronto match directly with our results.
161
IAEA samples and found that our results were consistent with the results of the IAEA consensus.
Acknowledgements
We would like to acknowledge support from the U.S. National Science Foundation, Cooperative Agreements OCE-801015 and OCE-802509. Also, we would like to thank our reviewers, R.J. Schneider and K.F. von Reden. This paper represents WHO1 contribution number 8582.
4. Conclusion We have set up a quality control protocol in the NOSAMS sample preparation laboratory. Internally, we check sample preparation methods for fractionation of the carbon isotopes, sample contamination, and total recovery by running known standards in conjunction with samples; 613C (mass spectrometer), A14C (AMS), and % yield data from the standards provide us with this information, respectively. In addition, we use this information to fine-tune the sample preparation methods, and are presently experimenting with new methods to further ensure the sample quality. Externally, we periodically check our AMS results against other AMS laboratories to validate our entire operation. We prepared and analyzed six different
References [l] A.P. McNichol and G.A. Jones, WOCE Operations Manual 68/91 (19911. [Z] A.P. McNichol, G.A. Jones, D.L. Hutton, A.R. Gagnon and R.M. Key, Radiocarbon (submitted). [3] A.P. McNichol, E.A. Osborne, A.R. Gagnon, B. Fry and G.A. Jones, these Proceedings (6th Int. Conf. on Accelerator Mass Spectrometry CAMS-61, Canberra-Sydney, Australia, 1993) Nucl. Instr. and Meth. B 92 (1994) 162. [4] J.S. Vogel, J.R. Southon and D.E. Nelson, Nucl. Instr. and Meth. B 29 (1987) 50. [S] R. Rozanski, W. Stichler, R. Gonfiantini, E.M. Scott, R.P. Beukens, B. Kromer and J. van der Plicht, Radiocarbon 34 (1992) 506.
II. AMS TECHNIQUES
B 92 (1994) 158-161
North-Holland
Beam Interactions with Materials 8 Atoms
Internal and external checks in the NOSAMS sample preparation laboratory for target quality and homogeneity E.A. Osborne *, A.P. McNichol, A.R. Gagnon, D.L. Hutton and G.A. Jones National Ocean Sciences Accelerator Mass Spectrometry Facility, Woods Hole Oceanographic Institution, Woods Hole, h&4 02543, USA
In the NOSAMS sample preparation laboratory (SPL) we have developed rigorous internal procedures aimed at ensuring that sample preparation introduces as little error into our analyses as possible and identifying problems rapidly. Our three major CO, preparation procedures are: stripping inorganic carbon from seawater, hydrolyzing CaCO,, and oxidizing organic matter. For seawater, approximately 10% of our analyses are standards or blanks which we use to demonstrate extraction of virtually all the inorganic carbon. Analysis of the stable carbon isotopic composition of the CO* extracted from our standards indicates a precision of better than 0.15-0.20%0. We also routinely process 14C-free CO, in our stripping lines to demonstrate the absence of a significant process-dependent blank. For organic combustions and CaCO, hydrolyses, we use the carbon yield (% organic carbon (OC) or % CaCO, by weight) as a check on our sample procedures. We have analyzed the blank contribution of these procedures as a function of sample size. Our organic carbon blank is constant at approximately 0.4% modern for samples containing greater than 1 mg C and our carbonate blank is less than 0.2% modern for samples containing more than 0.5 mg C. We use a standard Fe/H, catalytic reduction to prepare graphite from CO,. We check the completeness of our reactions with the pressure data stored during the reaction as well as use a robot to determine a gravimetric yield. All graphite undergoes a visual inspection and is rejected if any heterogeneities are present. We have recombusted graphite made from CO, with 613C values ranging from - 42 to 1%0 and determined that the &13C of the recombusted carbon agrees with that from the pure gas to within 0.05%0, demonstrating little or no fractionation during the treatment of the sample. The 613C we measure on the CO, generated from more than 75% of our samples is compared to the 613C measured on the AMS as a further check of our procedures. As further external checks, we analyzed the International Atomic Energy Association (IAEA) samples during the establishment of our laboratory and are presently participating in the third international radiocarbon intercalibration (TIRI) exercise.
1. Introduction The NOSAMS sample preparation laboratory (SPL) is dedicated to ensuring sample quality throughout all sample preparation. Samples entering the SPL go through rigorous internal procedures before they are finally submitted to the AMS as graphite targets. There are two major steps in the processing of raw samples, CO, extraction and CO, reduction. The CO, extraction method varies with the type of sample. We are primarily concerned with three sample types: seawater, CaCO, and organic matter. Once the CO, is extracted, it is reduced to carbon graphite by a catalytic reaction. These procedures are well documented and linked numerically along with the sample data. Known standards are run intermittently with the samples to routinely monitor the performance of the line. We use the data obtained from these standards to check for fractionation of the carbon isotopes (deviations in 613C data), contamination (A14C), and total recovery of the
Corresponding author. Tel. + 1 508 457 2000 (ext. 3307), fax + 1 508 457 2183, e-mail [email protected]. 0168-583X/94/$07.00 0 1994 - Elsevier SSDI 0168-583X(94)00017-P
Science
sample during processing. In this paper, we detail our internal sample procedures and show calculations and data that we routinely use to validate our performance.
2. Internal checks 2.1. Seawater Seawater samples arrive at NOSAMS in 500 ml bottles. A stripping probe is inserted into the bottle in an inert atmosphere, the sample is acidified and placed on the seawater stripping line. To extract the CO,, nitrogen gas is bubbled through the acidified seawater, and the CO, is collected in liquid nitrogen traps [1,2]. We have established a systematic procedure for quality control on the seawater stripping line. Standards and blanks are run intermittently with the samples, and the data collected provide up-to-date information on the consistency and contamination of the process. For our quality control samples, we use sodium carbonate (Na,CO,), local surface seawater (from Buzzard’s Bay) and a carbon-14 free CO, gas (approximately 42 000 yr old).
B.V. All rights reserved
159
E.A. Osborneet al. / Nucl. Instr. and Meth. in Phys. Res. B 92 (1994) 158-161 2.1.1. Na,CO,
2.2. CaCO,
Na,CO, standards are prepared in the sample preparation laboratory by adding a known amount of Na,CO, to carbon-free H,O and analyzed (1 every 16 samples) on the stripping line. Knowledge of the original CO, concentration (Z CO,) allows us to calculate the percent CO, recovery of the stripping process by the following: % yield = I;CO, (final)/BCO,
(initial) X 100
= (CO, recovered [mmol] /water
weight [kg] X 100)
/(Na,CO,wt
[mg]/105.9887
[mg/mmol]
/water weight [kg]). The CO, extracted from the Na,CO, is run on a VG Optima mass spectrometer for a stable isotope analysis (S13C). Deviation from the original 613C value signals fractionation of the carbon isotopes during the water stripping process. Our results indicate an average precision better than 0.15%0. 2.1.2. Buzzard’s Bay seawater Buzzard’s Bay seawater is collected from local surface seawater in batches. Each batch is separated into approximately 30 samples, poisoned with HgCl, to stop biological activity, and sealed in 500 ml bottles. We use Buzzard’s Bay seawater as a standard (1 every 16 samples) because it is very similar to the WOCE seawater samples. For Buzzard’s Bay seawater we use CCO, (instead of % yield) and 613C data from Buzzard’s Bay seawater to provide us with another quality control check. Our results indicate ZCO, = 1.87 5 0.10 mmol/kg and 613C = 1.22 k 0.20%0 (data obtained for the first half of 1993). We also use Buzzard’s Bay seawater to ensure that our seawater samples do not change isotopically over the 2-3 yr they will be stored before analysis. Fraction modern vahres of Buzzard’s Bay seawater plotted over time indicate no storage contamination over a span of 1.5 yr. 2.1.3. Line blanks Line blanks, consisting of carbon-14 free CO,, are circulated through the stripping line (1 every 32 samples). A14C data of the line blanks give us information on the background i4C signal of the line. The line blank is run on the AMS in conjunction with seawater samples stripped at approximately the same time the line blank was run. To date, we have used them to ensure no samples were collected on contaminated ships. In the future, we will use the line blanks in our data analysis calculations.
CaCO, samples are hydrolyzed with 100% H3P04 acid within a closed evacuated reactor and placed in a 60°C water bath for the duration of the reaction, usually overnight. Carbon-14 free CaCO, (IAEA C-l) is run (1 every 10 samples) in conjunction with our samples. The data obtained from running this standard (% CO, recovery) give us a clear indication of the completeness of the hydrolysis reaction (99.12 k 1.01% in 1993). As a rigorous r4C check, we also compare our results from this standard and IAEA C-2 with the data supplied by the IAEA intercalibration program (refer to section 3). The size of the IAEA C-l standard is determined by the size of the corresponding samples, and as the sample size decreases, the effect of background on the sample increases. From our results, our carbonate blank is less than 0.2% modern for samples containing at least 0.5 mg carbon (4.2 mg CaCO,). 2.3. Organic matter Organic matter samples consist of a wide variety of sample types, each with a unique pretreatment and purifying procedure. For example, sediment samples (our most abundant) need only an acidification step to remove the inorganic carbon, while wood samples require a three-step acid-base-acid treatment to remove humic acid contaminants. With each pretreatment procedure, however, there is only one oxidation procedure. The cleaned organic sample is oxidized with CuO in a closed quartz combustion tube at 850°C with Ag added to remove the sulfur and chlorine by-products [31. As with CaCO,, the smaller the sample size the greater the effect of the background on the sample. We are also testing using a commercial carbon (Johnson Matthey Electronics, 200 mesh; 99.9999% Ultra Carbon Powder, “Alfa Powder”) as a substitute for the NBS-21 we use to test our blanks. Initial results suggest it has a lower background. We will also develop a carbon-14 dead sediment standard to test our entire procedure. 2.4. Graphite preparation The CO, extracted in any of the described methods is then reduced to graphite in a 10 ml reactor by the reaction [4]: CO, + 2H, Fe,625”C 2H,O + C(gr). Two mg Fe powder is placed at the end of a 2 in. horizontal quartz tube and the reactor is evacuated. The CO, sample (- 200 pmol) and an excess of H, are added to the reactor. The pressure of the reactor is II. AMS TECHNIQUES
160
E.A. Osborne et al. /Nucl.
Instr. and Meth. in Phys. Res. B 92 (1994) 158-161
monitored by a pressure transducer, with a total reactor pressure starting at - 2.5 atm and ending at u 0.2 atm, once the reaction is complete (when the pressure no longer changes). The CO, reduces to elemental carbon in the form of filaments, long threadlike fibers protruding from the Fe surface [4]. This Fe-C mixture becomes the AMS sample target for A14C analysis. The graphite process described above has stringent quality checks to ensure isotopically accurate and homogeneous targets. Total conversion of CO, to graphite is monitored by both pressure and gravimetric yield data, with yield calculations as follows:
O.‘O ~ 0.05
0.00
i
I
0
V
p s-O.05 $3 g-0.70
A 0
gravimetric yield [%I weight [mg][(Fe and C tube) - (Fe tube)] = x 100 CO, sample [mmol] X 12[mg/mmol] -50
pressure yield [ %] = P,,, reacted (actual)/Ppas = ((P
reactor (initial)
-30
-20
SlfC(unprocessed
reacted (theo)
- P reactor (final))
-40
X 100)
/{ 3 X (CO, sample [mmol] X 24.47[ml X atm/mmol]/l0.5[ml]}. Carbon fractionation during the graphite process is determined periodically by recombusting the graphite and comparing its S13C value with that of the original unprocessed CO,. Or, 62YC,Fe graphite -850”c,cUo CO, CO, J 4 PC 2 PC 1
-10
0
CO,) (o/00)
Fig. 1. Fractionation of the graphite reaction: carbon fractionation for the graphite reaction is determined by comparing the difference in the 613C values of samples before and after graphitization. A(6l”C) (i.e., 613C (recombusted graphite)613C (unprocessed CO,)) is plotted against 613C (unprocessed CO,) for a series of samples: (01 Mat Gas (commercially obtained from Matheson Gas Products, 613C = - 0.72%0), (Cl) Oxalic Acid I (613C = -19.11%0), (A) Oxalic Acid II (613C = - 17.59%0), and (v) Tank CO, (obtained from the Smithsonian Astrophysical Observatory, 613C = - 43.26%0).
2.0
where A(613C) = 613C, - 613C,.
g- 1.5
As seen in Fig. 1, fractionation process is less than 0.05%0.
during the graphite
3. External methods As a check on the entire AMS facility, a series of IAEA intercalibration samples were run through the entire system, and our results were compared to the final results of the intercalibration study [5]. The study included six different samples described below: IAEA IAEA IAEA IAEA IAEA
C-l C-2 C-3 C-4 C-5
IAEA
C-6
CaCO,, carbon-14 free CaCO, powder Cellulose Wood
preparation
0.0
II 0.5
1
”
I
1.0
0 IAEA C-5 0 IAEA C-6 c 1 I( 0 1 ’ 1.5
2.0
IAEA Consensus (fraction modern)
Wood ANU Sucrose.
in the sample
Y ;5 4 0.5
0.0
Our results vs. IAEA results (Fig. 2) further validate our methods
B E .-5 $j 1.0 &
laboratory.
We
Fig. 2. IAEA consensus vs. NOSAMS results: a series of IAEA samples were analyzed at NOSAMS to compare our results with those of the IAEA intercalibration study. Fraction moderm of NOSAMS is plotted against the IAEA consensus fraction modern value for a series of samples: (0) IAEA C-l (Carbon-14 free CaCO,), (0) IAEA C-2 (CaCO, powder), (A ) IAEA C-3 (Cellulose), ( v ) IAEA C-4 (Wood), (0) IAEA C-5 (Wood), and (0) IAEA C-6 (ANU Sucrose).
E.A. Osborne et al. /Nucl.
Instr. and Meth. in Phys. Res. B 92 (1994) 158-161
are participating in the third international radiocarbon intercalibration (TIRI) exercise, another intercalibration study, and will report our results when the study ends. We are also in the process of externally checking the standards’ (Oxalic Acid I, Oxalic Acid II, and a Reference Gas) performance on the graphite line. We sent aliquots of our CO, to the Toronto AMS laboratory, and the results received from Toronto match directly with our results.
161
IAEA samples and found that our results were consistent with the results of the IAEA consensus.
Acknowledgements
We would like to acknowledge support from the U.S. National Science Foundation, Cooperative Agreements OCE-801015 and OCE-802509. Also, we would like to thank our reviewers, R.J. Schneider and K.F. von Reden. This paper represents WHO1 contribution number 8582.
4. Conclusion We have set up a quality control protocol in the NOSAMS sample preparation laboratory. Internally, we check sample preparation methods for fractionation of the carbon isotopes, sample contamination, and total recovery by running known standards in conjunction with samples; 613C (mass spectrometer), A14C (AMS), and % yield data from the standards provide us with this information, respectively. In addition, we use this information to fine-tune the sample preparation methods, and are presently experimenting with new methods to further ensure the sample quality. Externally, we periodically check our AMS results against other AMS laboratories to validate our entire operation. We prepared and analyzed six different
References [l] A.P. McNichol and G.A. Jones, WOCE Operations Manual 68/91 (19911. [Z] A.P. McNichol, G.A. Jones, D.L. Hutton, A.R. Gagnon and R.M. Key, Radiocarbon (submitted). [3] A.P. McNichol, E.A. Osborne, A.R. Gagnon, B. Fry and G.A. Jones, these Proceedings (6th Int. Conf. on Accelerator Mass Spectrometry CAMS-61, Canberra-Sydney, Australia, 1993) Nucl. Instr. and Meth. B 92 (1994) 162. [4] J.S. Vogel, J.R. Southon and D.E. Nelson, Nucl. Instr. and Meth. B 29 (1987) 50. [S] R. Rozanski, W. Stichler, R. Gonfiantini, E.M. Scott, R.P. Beukens, B. Kromer and J. van der Plicht, Radiocarbon 34 (1992) 506.
II. AMS TECHNIQUES