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AMS measurements from microgram to milligram
Nuclear Instruments and Methods in Physics Research B 240 (2005) 474–477 www.elsevier.com/locate/nimb
AMS measurements from microgram to milligram Thomas Uhl *, Wolfgang Kretschmer, Wolfgang Luppold, Andreas Scharf Physikalisches Institut, Universita¨t Erlangen-Nu¨rnberg, 91058 Erlangen, Germany Available online 2 August 2005
Abstract A gas handling system was developed to establish radiocarbon measurements of carbon masses below 100 lg. In this system, CO2 originating from an elemental analyzer, is cryogenically stored and thereafter directly fed into a gas ion source. In Erlangen, we have a hybrid ion source (40 sample MGF-SNICS) for measuring graphite samples as well as CO2 samples. The advantage of using gas samples is the minimized carbon contamination during the target production due to the shortened sample preparation and the higher efficiency of produced negative carbon ions from CO2. This enables to measure samples with carbon masses down to 1 mg. Solid samples have almost no cross contamination during the measurement, whereas gas samples show a significant contamination from one sample to another which results in reduced precision. For this reason gas targets can be used where less precision is required (e.g. biomedical applications) or where only small samples are available (e.g. environmental investigations). Solid targets are produced where precise results are essential (e.g. archeological studies). We use gas targets for carbon masses from 1 to 100 lg and solid targets above 100 lg. The techniques to handle samples in the different mass regions for AMS measurements are described in the following. 2005 Elsevier B.V. All rights reserved. PACS: 01.30.Cc; 07.77.Ka; 06.60.Ei Keywords: Accelerator mass spectrometry; Radiocarbon dating; Gas ion source; Gas handling system
1. Introduction Our standard method to form solid targets is to combust carbonaceous samples with an elemental * Corresponding author. Tel.: +49 9131 8527119; fax: +49 9131 15249. E-mail address: [email protected] (T. Uhl).
analyzer (EA), to collect the CO2 cryogenically, to graphitize and to press it into cathode holders. As the contamination of samples rises with minor masses rapidly, a carbon mass of at least 100 lg is necessary. Moreover the step of graphitization is time consuming and labor intensive. Motivated by research in sectors of environmental science where only small samples are provided (e.g. 10 lg after a collection time of a year [1]) and
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.06.147
T. Uhl et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 474–477
intended biomedical applications [2] where many samples have to be measured, the ion source was modified to an hybrid ion source [3] that accepts solid samples as well as gaseous samples. Due to the possibility of the direct use of the CO2 the usual graphitization to form solid sputter targets is not necessary. Because of the higher efficiency of gas ion sources [4] and less contamination with carbon due to the minimized preparation line it is possible to measure samples with carbon masses down to 1 lm [5]. Comparable to solid samples every gas sample needs a new cathode. The CO2 is routed with a tube to the cathode surface. The surface material is titanium [5] and enables the sputtering of the CO2 to carbon ions due to the adsorption as it is quiet reactive. Since the CO2-gas is completely mobile in the ion source it is not restricted to the sputtering area. So the gas can also adsorb on neighbor cathodes. This adsorption process affects the results of the followings samples. Due to this behavior the precision will be less than the precision that is achieved with solid targets, if the sample amount is big enough so that the contamination during the graphitization process is less than the cross contamination in a gas ion source. So it will be reasonable to change from gas samples to solid samples at that carbon mass where the contaminations are nearly equal. This is assumed to be in the range of 100 lg. In fact we are also applying two different techniques for gas samples and one technique for solid samples to establish radiocarbon dating for carbon masses from microgram to milligram.
475
• The helium carrier flow for the ion source should be 0.1 sccm, but the EA provides a helium carrier flow of 100 sccm [5]. • The CO2 from the EA is provided only for 40 s. For the optimum CO2-flow into the ion source of 0.002 sccm for a sample size of 10 lg a constant flow should persist at least 10 min (Fig. 2). To solve these problems the CO2 out from the EA is stored cryogenically and then subsequently released into a helium flow with a flow rate of 0.1 sccm. Fig. 1(a) and (b) shows the arrangement and the principle of the Ôcryogenic storage and releaseÕ-technique. In Fig. 1(a) the CO2, coming from the EA with a high helium carrier flow of 100 sccm, is frozen in the Ôfreezing tubeÕ by continuously elevating a Dewar vessel with liquid nitrogen. The complete freezing is done within 90 s and CO2 is frozen over the whole inner surface
2. Techniques for radiocarbon dating from microgram to milligram 2.1. ‘Cryogenic storage and release’ technique to handle CO2-samples (1–10 lg) A direct coupling of the EA and a gas ion source [5,6] skips the step of graphitizing CO2 and pressing targets. Therefore a method had to be developed to link the EA and the ion source that accomplishes two problems:
Fig. 1. (a) Cryogenic storage of CO2 and (b) subsequently release of CO2.
12
C-current [ µA ] / efficiency [%]
476
T. Uhl et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 474–477
12
C-current depending on CO2-flow + efficiency
4
12C-current [ µA ] efficiency
3.5 3 2.5 2 1.5 1 0.5 0 0
0.001
0.002
0.003
0.004
0.005
CO2-flow [sccm]
Fig. 2.
12
C-current and efficiency against CO2-flow.
of the Ôfreezing tubeÕ. Then the valve is switched to position b (see Fig. 1(b)). Now a low helium flow of 0.1 sccm is flowing through the Ôfreezing tubeÕ towards the ion source. By slowly lowering the Dewar vessel the CO2 is continuously released into helium and transported into the ion source. With this technique the CO2 fraction in the helium can be adjusted by changing the lowering speed and it is possible to feed CO2 continuously into the ion source up to 10 min. So this technique is optimal for carbon masses from 1 to 10 lg [5]. Due to flushing and evacuation of the system, no cross contamination between samples was observed in this system itself. If the carbon mass exceeds 10 lg, more than the desired 2 ll/min are dosed into the helium flow. But as you can see in Fig. 2 the efficiency is heavily dependent on the CO2 flow. If the CO2 flow is too high the efficiency drops dramatically and the advantages of using gas are lost. It would be possible to extend the system, but at higher carbon masses it is possible to dose the CO2 into the helium flow by a pressure drop. This more comfortable and easier technique is presented in the next section. 2.2. CO2 injecting into continuous helium flow (10–100 lg) For samples with carbon masses above 10 lg we are developing a new technique that already has passed functional testing. The CO2, out from the EA, is stored cryogenically in a gastight syringe (valves are in position ‘‘a’’ in Fig. 3). Thereafter the valves are switched to position ‘‘b’’ and the syr-
Fig. 3. CO2 injecting into continuous helium flow (10–100 lg).
inge is evacuated. Subsequently the plunge of the syringe is well positioned so that the syringe is leak-tight and the CO2 can be defrosted and warmed up to room temperature. The CO2 pressure is measured (the sample mass can be calculated – if needed) and adjusted with the syringe to 2–3 bar. With a glass capillary the CO2 is dosed into a helium carrier gas flow of 0.1 sccm that transports the CO2 into the ion source. The CO2pressure regulates the CO2 fraction in the helium flow. In this arrangement the CO2 fraction can be easily tuned to optimum setting. So a maximum efficiency of generated negative carbon ions from the CO2-flow can be reached. This technique cannot be applied for carbon masses beneath 10 lg because for the optimum settings we have to keep a pressure of 2–3 bars. The remaining internal volume of 12 ll inside the valve, the pressure sensor and the syringe does not allow to reach the needed pressure. The advantages of this system are: • The pressure is measured with the pressure sensor and regulated by the syringe. So the pressure can constantly be kept in optimum adjustment. • It can be completely automated.
T. Uhl et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 474–477
477
reduction room. Simultaneously, the bottom part of the reduction volume is cooled to a temperature of 25 C by a peltier element. Here, the water arising from the reduction is frozen and removed from the chemical reaction. With the help of an extra compression mold made especially for this purpose, the carbon–iron is pressed into an aluminum cathode with an inner diameter of 1 mm. Forty targets fit into a sample wheel, which is installed into the ion source. 3. Conclusion Fig. 4. Standard method to handle carbon samples (>100 lg).
• We have not seen cross contamination in the system itself. This is guaranteed due to the flushing and evacuation of the system between samples. 2.3. Standard method to handle carbon samples (>100 lg) The largest part of the CO2 gas out from the EA is routed into a cryo trap unit after a gas chromatographic separation, where it is frozen with liquid nitrogen (Fig. 4). A small fraction is routed in a mass spectrometer for stable isotopes. There, the ratio of the stable carbon isotopes (delta13C) can be ascertained. The carbon dioxide originating from the combustion must now be transformed into elementary carbon. This occurs through the catalytic balance reaction of carbon dioxide and hydrogen at iron powder: CO2 + 2 H2 ! C + 2H2O. There are 10 units where samples can be prepared simultaneously. In the reduction facility the carbon dioxide is frozen in a reduction room filled with iron powder. Hydrogen is introduced with a ratio of H2:CO2 = 2:1. The reduction takes place inside a small heater that is warmed up to a temperature of 620 C and moved across the
The use of a hybrid ion source enables the measurement of samples with carbon masses from microgram to milligram. To perform measurements from the lowest to the largest carbon masses three different techniques have to be applied. The technique that we apply is a question of the available carbon mass as well as of the desired precision. In our lab we already have used the cryogenic storage and release technique on environmental investigations [1]. Systematic investigations on cross contamination are in progress. Although the technique of the gas handling system is very easy, it has to be automated to use it for routine measurements. References [1] A. Scharf, W. Kretschmer, T. Uhl, K. Kritzler, K. Hunger, E. Pernicka, Nucl. Inst. and Meth. B, these Proceedings, doi:10.1016/j.nimb.2005.06.148. [2] B.J. Hughey, P.L. Skipper, R.E. Klinkowstein, R.E. Shefer, J.S. Wishnok, S.R. Tannenbaum, Nucl. Instr. and Meth. B 172 (2000) 40. [3] J.A. Ferry, R.L. Loger, G.A. Norton, J.E. Raatz, Nucl. Instr. and Meth. A 382 (1996) 316. [4] C. Bronk Ramsey, R.E.M. Hedges, Nucl. Instr. and Meth. B 123 (1997) 539. [5] T. Uhl, W. Kretschmer, A. Scharf, Radiocarbon 46 (2004) 65. [6] C. Bronk Ramsey, R.E.M. Hedges, Nucl. Instr. and Meth. B 172 (2000) 242.
AMS measurements from microgram to milligram Thomas Uhl *, Wolfgang Kretschmer, Wolfgang Luppold, Andreas Scharf Physikalisches Institut, Universita¨t Erlangen-Nu¨rnberg, 91058 Erlangen, Germany Available online 2 August 2005
Abstract A gas handling system was developed to establish radiocarbon measurements of carbon masses below 100 lg. In this system, CO2 originating from an elemental analyzer, is cryogenically stored and thereafter directly fed into a gas ion source. In Erlangen, we have a hybrid ion source (40 sample MGF-SNICS) for measuring graphite samples as well as CO2 samples. The advantage of using gas samples is the minimized carbon contamination during the target production due to the shortened sample preparation and the higher efficiency of produced negative carbon ions from CO2. This enables to measure samples with carbon masses down to 1 mg. Solid samples have almost no cross contamination during the measurement, whereas gas samples show a significant contamination from one sample to another which results in reduced precision. For this reason gas targets can be used where less precision is required (e.g. biomedical applications) or where only small samples are available (e.g. environmental investigations). Solid targets are produced where precise results are essential (e.g. archeological studies). We use gas targets for carbon masses from 1 to 100 lg and solid targets above 100 lg. The techniques to handle samples in the different mass regions for AMS measurements are described in the following. 2005 Elsevier B.V. All rights reserved. PACS: 01.30.Cc; 07.77.Ka; 06.60.Ei Keywords: Accelerator mass spectrometry; Radiocarbon dating; Gas ion source; Gas handling system
1. Introduction Our standard method to form solid targets is to combust carbonaceous samples with an elemental * Corresponding author. Tel.: +49 9131 8527119; fax: +49 9131 15249. E-mail address: [email protected] (T. Uhl).
analyzer (EA), to collect the CO2 cryogenically, to graphitize and to press it into cathode holders. As the contamination of samples rises with minor masses rapidly, a carbon mass of at least 100 lg is necessary. Moreover the step of graphitization is time consuming and labor intensive. Motivated by research in sectors of environmental science where only small samples are provided (e.g. 10 lg after a collection time of a year [1]) and
0168-583X/$ - see front matter 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.06.147
T. Uhl et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 474–477
intended biomedical applications [2] where many samples have to be measured, the ion source was modified to an hybrid ion source [3] that accepts solid samples as well as gaseous samples. Due to the possibility of the direct use of the CO2 the usual graphitization to form solid sputter targets is not necessary. Because of the higher efficiency of gas ion sources [4] and less contamination with carbon due to the minimized preparation line it is possible to measure samples with carbon masses down to 1 lm [5]. Comparable to solid samples every gas sample needs a new cathode. The CO2 is routed with a tube to the cathode surface. The surface material is titanium [5] and enables the sputtering of the CO2 to carbon ions due to the adsorption as it is quiet reactive. Since the CO2-gas is completely mobile in the ion source it is not restricted to the sputtering area. So the gas can also adsorb on neighbor cathodes. This adsorption process affects the results of the followings samples. Due to this behavior the precision will be less than the precision that is achieved with solid targets, if the sample amount is big enough so that the contamination during the graphitization process is less than the cross contamination in a gas ion source. So it will be reasonable to change from gas samples to solid samples at that carbon mass where the contaminations are nearly equal. This is assumed to be in the range of 100 lg. In fact we are also applying two different techniques for gas samples and one technique for solid samples to establish radiocarbon dating for carbon masses from microgram to milligram.
475
• The helium carrier flow for the ion source should be 0.1 sccm, but the EA provides a helium carrier flow of 100 sccm [5]. • The CO2 from the EA is provided only for 40 s. For the optimum CO2-flow into the ion source of 0.002 sccm for a sample size of 10 lg a constant flow should persist at least 10 min (Fig. 2). To solve these problems the CO2 out from the EA is stored cryogenically and then subsequently released into a helium flow with a flow rate of 0.1 sccm. Fig. 1(a) and (b) shows the arrangement and the principle of the Ôcryogenic storage and releaseÕ-technique. In Fig. 1(a) the CO2, coming from the EA with a high helium carrier flow of 100 sccm, is frozen in the Ôfreezing tubeÕ by continuously elevating a Dewar vessel with liquid nitrogen. The complete freezing is done within 90 s and CO2 is frozen over the whole inner surface
2. Techniques for radiocarbon dating from microgram to milligram 2.1. ‘Cryogenic storage and release’ technique to handle CO2-samples (1–10 lg) A direct coupling of the EA and a gas ion source [5,6] skips the step of graphitizing CO2 and pressing targets. Therefore a method had to be developed to link the EA and the ion source that accomplishes two problems:
Fig. 1. (a) Cryogenic storage of CO2 and (b) subsequently release of CO2.
12
C-current [ µA ] / efficiency [%]
476
T. Uhl et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 474–477
12
C-current depending on CO2-flow + efficiency
4
12C-current [ µA ] efficiency
3.5 3 2.5 2 1.5 1 0.5 0 0
0.001
0.002
0.003
0.004
0.005
CO2-flow [sccm]
Fig. 2.
12
C-current and efficiency against CO2-flow.
of the Ôfreezing tubeÕ. Then the valve is switched to position b (see Fig. 1(b)). Now a low helium flow of 0.1 sccm is flowing through the Ôfreezing tubeÕ towards the ion source. By slowly lowering the Dewar vessel the CO2 is continuously released into helium and transported into the ion source. With this technique the CO2 fraction in the helium can be adjusted by changing the lowering speed and it is possible to feed CO2 continuously into the ion source up to 10 min. So this technique is optimal for carbon masses from 1 to 10 lg [5]. Due to flushing and evacuation of the system, no cross contamination between samples was observed in this system itself. If the carbon mass exceeds 10 lg, more than the desired 2 ll/min are dosed into the helium flow. But as you can see in Fig. 2 the efficiency is heavily dependent on the CO2 flow. If the CO2 flow is too high the efficiency drops dramatically and the advantages of using gas are lost. It would be possible to extend the system, but at higher carbon masses it is possible to dose the CO2 into the helium flow by a pressure drop. This more comfortable and easier technique is presented in the next section. 2.2. CO2 injecting into continuous helium flow (10–100 lg) For samples with carbon masses above 10 lg we are developing a new technique that already has passed functional testing. The CO2, out from the EA, is stored cryogenically in a gastight syringe (valves are in position ‘‘a’’ in Fig. 3). Thereafter the valves are switched to position ‘‘b’’ and the syr-
Fig. 3. CO2 injecting into continuous helium flow (10–100 lg).
inge is evacuated. Subsequently the plunge of the syringe is well positioned so that the syringe is leak-tight and the CO2 can be defrosted and warmed up to room temperature. The CO2 pressure is measured (the sample mass can be calculated – if needed) and adjusted with the syringe to 2–3 bar. With a glass capillary the CO2 is dosed into a helium carrier gas flow of 0.1 sccm that transports the CO2 into the ion source. The CO2pressure regulates the CO2 fraction in the helium flow. In this arrangement the CO2 fraction can be easily tuned to optimum setting. So a maximum efficiency of generated negative carbon ions from the CO2-flow can be reached. This technique cannot be applied for carbon masses beneath 10 lg because for the optimum settings we have to keep a pressure of 2–3 bars. The remaining internal volume of 12 ll inside the valve, the pressure sensor and the syringe does not allow to reach the needed pressure. The advantages of this system are: • The pressure is measured with the pressure sensor and regulated by the syringe. So the pressure can constantly be kept in optimum adjustment. • It can be completely automated.
T. Uhl et al. / Nucl. Instr. and Meth. in Phys. Res. B 240 (2005) 474–477
477
reduction room. Simultaneously, the bottom part of the reduction volume is cooled to a temperature of 25 C by a peltier element. Here, the water arising from the reduction is frozen and removed from the chemical reaction. With the help of an extra compression mold made especially for this purpose, the carbon–iron is pressed into an aluminum cathode with an inner diameter of 1 mm. Forty targets fit into a sample wheel, which is installed into the ion source. 3. Conclusion Fig. 4. Standard method to handle carbon samples (>100 lg).
• We have not seen cross contamination in the system itself. This is guaranteed due to the flushing and evacuation of the system between samples. 2.3. Standard method to handle carbon samples (>100 lg) The largest part of the CO2 gas out from the EA is routed into a cryo trap unit after a gas chromatographic separation, where it is frozen with liquid nitrogen (Fig. 4). A small fraction is routed in a mass spectrometer for stable isotopes. There, the ratio of the stable carbon isotopes (delta13C) can be ascertained. The carbon dioxide originating from the combustion must now be transformed into elementary carbon. This occurs through the catalytic balance reaction of carbon dioxide and hydrogen at iron powder: CO2 + 2 H2 ! C + 2H2O. There are 10 units where samples can be prepared simultaneously. In the reduction facility the carbon dioxide is frozen in a reduction room filled with iron powder. Hydrogen is introduced with a ratio of H2:CO2 = 2:1. The reduction takes place inside a small heater that is warmed up to a temperature of 620 C and moved across the
The use of a hybrid ion source enables the measurement of samples with carbon masses from microgram to milligram. To perform measurements from the lowest to the largest carbon masses three different techniques have to be applied. The technique that we apply is a question of the available carbon mass as well as of the desired precision. In our lab we already have used the cryogenic storage and release technique on environmental investigations [1]. Systematic investigations on cross contamination are in progress. Although the technique of the gas handling system is very easy, it has to be automated to use it for routine measurements. References [1] A. Scharf, W. Kretschmer, T. Uhl, K. Kritzler, K. Hunger, E. Pernicka, Nucl. Inst. and Meth. B, these Proceedings, doi:10.1016/j.nimb.2005.06.148. [2] B.J. Hughey, P.L. Skipper, R.E. Klinkowstein, R.E. Shefer, J.S. Wishnok, S.R. Tannenbaum, Nucl. Instr. and Meth. B 172 (2000) 40. [3] J.A. Ferry, R.L. Loger, G.A. Norton, J.E. Raatz, Nucl. Instr. and Meth. A 382 (1996) 316. [4] C. Bronk Ramsey, R.E.M. Hedges, Nucl. Instr. and Meth. B 123 (1997) 539. [5] T. Uhl, W. Kretschmer, A. Scharf, Radiocarbon 46 (2004) 65. [6] C. Bronk Ramsey, R.E.M. Hedges, Nucl. Instr. and Meth. B 172 (2000) 242.