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Graphene Oxide Carburization Enhanced Ionization Efficiency for TIMS Isotope Ratio Analysis of Uranium at Trace Level Ling Zhang, Penghui Xiong, Hailu Zhang, Lumin Chen, Jie Xu, Haoxi Wu, and Zhen Qin Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b00543 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 13, 2019

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Analytical Chemistry

1

Graphene Oxide Carburization Enhanced Ionization Efficiency for

2

TIMS Isotope Ratio Analysis of Uranium at Trace Level

3

Ling Zhang, Penghui Xiong, Hailu Zhang, Lumin Chen, Jie Xu, Haoxi Wu*, Zhen Qin*

4

Institute of Materials, China Academy of Engineering Physics, Mianyang, 621900,

5

China.

6

*Corresponding author: Haoxi Wu, Zhen Qin.

7

Email: [email protected] (H. Wu). [email protected] (Z. Qin).

8 9

Abstract

10

Isotope analysis of trace uranium is important in nuclear safeguards and nuclear

11

forensics, which requires the analytical methodologies with high sensitivity, accuracy and

12

precision. As one of the most powerful techniques in isotopic measurement, thermal

13

ionization mass spectrometry (TIMS) usually suffers from its relatively low sensitivity in

14

ultra-trace measurement. To overcome this limitation, we have developed a new filament

15

carburization technique for TIMS, with graphene oxide (GO) as the ionization enhancer.

16

A high and steady ionization efficiency of ~0.2% for uranium was achieved in

17

single-filament mode, which was ten times of the classical double-filament method. With

18

total evaporation (TE) measurements, this method was validated with certified reference

19

materials (CRMs) at picogram, and the relative uncertainties for n(235U)/n(238U) was as

20

low as ~1% level. The enhancement mechanism of GO’s promoting effect on uranium

21

ionization was attributed to the uniform microstructure facilitating energy transfer and

22

formation of carbides. This approach provides an alternative simple and rapid method for

23

trace uranium isotope analysis with high sensitivity and excellent repeatability. Filament

24

carburization and uranium loading could be accomplished within 10 min. This technique

25

has great advantage in analysis of trace uranium isotope ratios and can be applied in the

26

researches of environmental analysis and nuclear forensics.

27 28

Keywords 1 / 31

ACS Paragon Plus Environment

Analytical Chemistry 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1

Graphene oxide, Filament carburization, TIMS, Ionization efficiency, Isotope ratio

2

analysis

3

Introduction

4

TIMS is one of the most powerful techniques for the determination of isotope ratios

5

of nuclear materials such as uranium and plutonium,1 which plays a significant role in the

6

fields of nuclear safeguards,2 nuclear forensics,3 geochemistry4 and environmental

7

analysis.5 However, traditional filament loading techniques are suffered from the low

8

ionization efficiency (ratio of atoms ionized to sample atoms loaded on a filament) for

9

actinides. As for uranium analysis, the ionization efficiency of TIMS with a double

10

filament setup is always less than 0.05%.6 The poor ionization efficiency greatly limits

11

the accuracy and precision of the isotope ratio measurements for actinides, which always

12

present at low amounts below nano-gram level in the environmental samples and swipe

13

samples for nuclear forensic and safeguards purpose.7-9 Some critical signature

14

informations in these trace-level samples would not able to be monitored, especially the

15

minor isotopes, which can reflect a more detailed picture inherent to the type of the

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undeclared nuclear-related activities.10 Thus, it is a critical demand on methods for

17

promoting the ionization efficiency in TIMS measurements. Any increase in the

18

ionization efficiency will result in higher sensitivity allowing the sample size to be

19

reduced and measurement precision to be improved.

20

Rhenium (Re) filaments have been widely used for sample loading and ionization in

21

TIMS analysis due to its high work function.11 And carburization of rhenium filaments

22

has been reported as a good choice to enhance the ion emitting of actinide samples,

23

because carbon was found to dissolve into rhenium to form a rhenium-carbon solid

24

solution.12,13 The electron work function 5.36 eV of this metal-carbon compound is

25

higher than 4.96~4.98 eV of pure rhenium, which will result in a better ion production

26

according to the Saha-Langmuir equation.14 Besides, the formation of actinide carbides

27

has been also thought to contribute to the enhanced emission of the actinide metal ions.

28

Carbon, as reduction agent, helps the formation of carbides at the expense of volatile

29

oxide species, from which it demands a higher temperature for the actinides to release as 2 / 31


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single charged ions. It could reduce the loss of actinide evaporation in an atom form

2

before the filament temperature is high enough for ionization.15,16

3

According to these two proposed mechanisms, various materials such as

4

graphite,4,5,17,18 resin bead,19,20 and benzene vapor10,21-24 were used as carbon sources in

5

developing carburization techniques for increasing ionization efficiency. The TIMS resin

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bead technique was firstly developed by Oak Ridge National Laboratory (ORNL) in

7

1970s.25 For uranium analysis, a resin bead was initially used to concentrates the uranium

8

from a bulk solution. After absorption, it was loaded into the Re cavity by using

9

collodion. Due to the enrichment of sample and the formation of U-carbide, it was

10

reported to have a high ionization efficiency of 0.58% for uranium.1 However, despite its

11

high sensitivity, the resin bead approach is still hindered by long pretreatment time and

12

tediousness of fixing a bead on the filament surface. Additionally, this technique is not

13

adaptable to single particle analysis. In recently years, benzene vapour-based filament

14

carburization technique has been widely used in applications of U&Pu single particle

15

analysis10,21,22 and age determination.24 Although this method could get an ionization

16

efficiency of 0.2~0.3% for uranium and was much more reproducible, it always required

17

a special vacuum instrument to expose the filaments to benzene vapour at high

18

temperature, which is time-consuming, expensive and has a potential risk of benzene

19

toxicosis. Recently, a porous ion emitter (PIE) technique was developed for the analysis

20

of trace quantities of actinides.26-28 Rhenium and platinum powders were mixed in a

21

gluing agent, and then heated to form a porous Pt/Re alloy as the thermal ionization

22

emitter. Although the preparation was a little time-consuming and required operation

23

skill, these works showed the porous micro- and nano-structures of ion emitter could

24

remarkbly enhance the ion yields.

25

Graphene oxide (GO), a two-dimensional nanocarbon material, provides a good

26

choice of carbon source for TIMS filament carburization, with its fascinating properties

27

of high specific surface, good thermal & electrical conductivity and strong mechanical

28

strength.29-31 Graphene oxide could be well-dispersed in water to form a colloidal solution

29

and was proven to have sorption capabilities for radionuclides.32 Herein, the promoting 3 / 31



1

effect on uranium ionization using GO as the ion emitter was studied and the mechanism

2

was discussed. Based on GO loaded single Re filament, the isotope ratios of uranium

3

samples from pg to sub-ng level were determined accurately. A high and steady

4

ionization efficiency of 0.2% for uranium was achieved, with a fast filament

5

carburization process of less than 10 min. The analysis of swipe samples, river water

6

samples and human hairs was also accomplished without any sample enrichment.

7

Experimental section

8

Reagents

9

All chemicals were analytical grade except those noted otherwise, and were used as

10

received. Graphite powder (99.95%, 325 mesh, i.e. with an average size of ~78.2 μm)

11

was from Alfa Aesar. Other chemicals, H2SO4, H2O2, HF, K2S2O8, P2O5, KMnO4 and

12

HCl were bought from Beijing Chemical Company. All stock solutions were prepared

13

using ultrapure water.

14

Uranium isotopic certified reference materials (CRM) with the China National

15

Standards number GBW04493, GBW04492, GBW04483 were purchased from China

16

Institute of Atomic Energy. The certified isotope abundance values for these uranium

17

CRMs are displayed in Table S1. The materials were dissolved in 2% HNO3 (v/v, Merck,

18

Darmstadt, Germany) and prepared into uranium samples with different concentrations.

19

Purified reagent grade I water (Milli-Q Water Purification System (18 MΩ); Millipore,

20

Billerica, MA, U.S.A.) were used for all diluted HNO3 solutions.

21

Synthesis of graphene oxide

22

Graphene oxide (GO) was prepared by a chemical method based on the modified

23

Hummers-Offeman method.33 Graphite powder (3 g) was added into the mixture of

24

concentrated H2SO4 (12 mL), K2S2O8 (2.5 g) and P2O5 (2.5 g). Then it was treated under

25

80 ℃ for 4.5 h. After cooling to the room temperature, 0.5 L of distilled water was

26

added into the obtained mixture under an ice bath. After filtration, the prepared peroxide

27

graphite was dried under 30 ℃ and then added to concentrated H2SO4 (150 mL).

28

KMnO4 (15 g) was added gradually under stirring, and the temperature was kept under 20 4 / 31


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℃ by an ice bath. After stirring for 2 h at 35 ℃, 250 mL distilled water was added under

2

an ice bath and stirred for another 2 h. Successively, 700 mL of distilled water and 30 mL

3

of concentrated H2O2 were injected into the mixture. The mixture was filtered and

4

washed with an aqueous HCl solution (v/v 1:10) (1 L) followed by the same volume of

5

distilled water. After dialysis for a week, GO was filtered and dried under vacuum at 30

6

℃ for 12 h. Figure S1 displays the characterizations of GO.

7

Washing of graphene oxide for background reduction

8

Graphene oxide (10 mg) was dispersed in 50 mL HNO3 solution (1%, v/v) with

9

continuous ultrasonication for 30 min to get homogeneous solution. Then, the solution

10

was stirred for 15 min and centrifuged at 8000 rpm for 5 min at room temperature. The

11

supernatant was abandoned and the collected GO was washed with 1% HNO3 solution

12

again. After three repeated washing with HNO3, the collected GO was washed with

13

ultrapure water twice in the similar procedure. Finally, the collected GO was prepared

14

into a colloidal solution with different concentrations.

15

Filament Pretreatment

16

Prior to the loading of any ionization enhancers or samples, all Re filaments were

17

mounted in the filament bake-out unit and pre-treated in a filament degassing Instrument.

18

The filaments were slowly heated to 4500 mA (ca. 1750 ℃) resistively and held at this

19

temperature for 30 min under the vacuum of below 5×10−7 mbar, to remove any

20

impurities such as hydrocarbons and environmental uranium. The degassed filaments

21

were cooled down at room temperature and used for TIMS measurements.

22

GO and sample loading

23

2 μL GO colloidal solution (2.5 mg mL-1) after washing procedures was pipetted

24

onto the center of the filament surface carefully. A filament current of 1 A was conducted

25

for resistive heating until the GO solution was completely dried. After cooling for 5 min

26

at RT, the filament was loaded with uranium sample solution always at the same volume

27

of 1 μL, with varied concentrations. Extreme care was taken to localize the sample

28

solution exactly onto the surface of the GO layer on filament. Then, the sample solution 5 / 31



1

was dried electrothermally at 1 A for approximately 2 min. The prepared filaments were

2

then transferred to the sample turret of TIMS for analysis.

3

Swipe sample preparation

4

The swipe samples were prepared according to the ultrasound-assisted acid leaching

5

(UAL) method.34 Swipe samples were collected from a worktable in an accident-tolerant

6

fuel (ATF) laboratory of our academy. Three samples were collected by carefully wiping

7

the surface of different areas on the same worktable with three pieces of filter papers,

8

respectively. Each filter paper was then transferred into a clean polystyrene centrifuge

9

tube containing 10 mL of 2% HNO3 and kept in ultrasonic bath (with delivered power of

10

50 W) for 2 h. Then the filter paper was removed from the tube and the remaining

11

solution was centrifuged at 10000 rpm for 5 min to separate the undissolved sediments.

12

Finally, the supernatant was transferred to a clean polystyrene centrifuge tube and could

13

be introduced directly on the GO-Re filaments for TIMS measurements. 4 μL swipe

14

sample solution was loaded on each filament.

15

River water sample preparation

16

Two environmental water samples were directly collected from the Fu River near the

17

Moon Island Park in Jiangyou, Mianyang city. For each sample, ~10 mL clean surface

18

river water was collected in a clean polystyrene centrifuge tube and centrifuged at 8000

19

rpm for 5 min. The supernatant was transferred to a clean polystyrene centrifuge tube.

20

Then, 150 μL HNO3 (65%, Merck) was added in the water sample to make a ~1% HNO3

21

solution for TIMS measurements. 4 μL river water sample solution was loaded on each

22

filament.

23

Hair sample preparation

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300~500 mg hairs from two healthy volunteers in our institute were used in

25

preparation of hair solution samples, respectively. The hairs were firstly washed with

26

distilled water, and then mixed with 5 mL of sub-boiled HNO3, 1 mL of H2O2 and 1 mL

27

of HF for microwave digestion. After digestion, the solution was evaporated to dryness.

28

The precipitate was finally dissolved in 10 mL 1% HNO3 for TIMS measurements. 2 μL 6 / 31


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1

hair sample solution was loaded on each filament.

2

Instrumental analysis

3

All the TIMS measurements were performed by total evaporation (TE) method using

4

a thermal ionization mass spectrometry with multicollector instrument (Including multi

5

faraday cups and ion-counting collectors). Zone refined rhenium filaments (0.75 mm

6

width and 0.035~0.04 mm thick) were purchased from Zhonglai New Material Co.

7

(Hunan, China) and degassed before each test. 5~200 pg of uranium were loaded on the

8

evaporation filament with the same volume of 1 μL. Analysis was conducted when the

9

pressure is less than 5×10-7 mbar. The loaded samples were heated to approximately 3000

10

mA at the incremental rate of 2000 mA min-1 in current, until the

11

monitored. When 238U signal reached 50 counts per second, the filament temperature was

12

recorded as the ‘initial ionization temperature’, which was monitored with an optical

13

pyrometer. Filament alignment and focusing parameters were optimized before TE

14

measurement. Then, the samples on the evaporation filament were heated to exhaustion

15

with increased filament current to 5300 mA. Data acquisition was performed by

16

recording the

17

classical method, smaller than 400 cycles), each cycle corresponding to ~1 s integration

18

time. The uranium ions of the isotopes

19

multi ion-counting collector array except those noted otherwise. The total ionization

20

efficiency were obtained by calculating the fraction of the U+ ions ionized from the

21

uranium sample atoms loaded on a filament.

234U, 235U, 236U

and

238U

238U

signal could be

signal about a period of 1000 cycles (as for

234U, 235U, 236U

and

238U

were measured with

22

ToF-SIMS was used to characterize the heated uranium-loaded GO-Re filament.

23

Before ToF-SIMS analysis, a degassed single Re filament was loaded with 5 μg GO and

24

1 μg uranium sample GBW04493 sequentially, then resistively heated for 30 sec at about

25

1600 ℃, under the vacuum of below 5×10-7 mbar. ToF-SIMS includes a 25 keV liquid

26

metal ion gun (LMIG) was used for both analysis and sputtering by providing a pulsed

27

(analytical mode) or focused (sputtering mode) beam of primary ions, respectively. A

28

vacuum of 1×10−9 mbar was maintained to prevent surface oxidation during the test.

29

Before the measurement, the filament surface was sputtered in situ to depths between 1 7 / 31



1

nm and 20 nm using an Au+ sputter ion beam to remove the oxide layer and obtain a raw

2

surface. Images of the positive ions including Re+, ReC+, U+, UC+ and UC2+ were

3

acquired using 25 kV Au+ ions with currents between 1.5 and 2 pA.

4

Scanning electron microscopy (SEM) images and energy dispersive X-ray

5

spectrometry (EDX) spectra were obtained using a LEO1530 field emission SEM system

6

(Germany). The transmission electron microscope (TEM) images were obtained with

7

Hitachi model H-800 TEM opened at an accelerating voltage of 100 kV. The IR spectrum

8

(500~4000 cm−1) was measured using a Perkin Elmer Fourier transform infrared

9

spectroscopy spectrometer with pure KBr as the background. Raman spectroscopy was

10

performed on a Renishaw RM2000 microscopic confocal Raman spectrometer

11

(Gloucestershire, United Kingdom) using green (514 nm) laser excitation. Scans were

12

taken on a range of 1000~2000 cm−1. TEM, IR and Raman spectroscopy were all

13

performed in Tsinghua University, Beijing.

14

Results and discussion

15

Filament characterizations

16

Graphene oxide (GO) colloidal solution was prepared by exfoliation of graphite

17

oxide synthesized according to the modified Hummers method.33 From the

18

characterizations of GO in Figure S1 in Supporting Infomation, the GO sheets had sizes

19

from sub-μm to ~40 μm with two-demensional layered structure. IR and Raman spectra

20

identified the degree of oxidation and existence of various oxygen-containing functional

21

groups in GO, such as -OH and -COOH. These groups and the layered structure allowed

22

GO to be well-dispersed in water, resulting in a GO suspension with good colloidal

23

stability.

24

As Scheme 1 shows, firstly, a drop of graphene oxide colloidal solution was carefully

25

pipetted onto the central surface of a pre-degassed single Re filament before uranium

26

sample loading. After resistive heating, the solvent vaporized and GO sheets deposited

27

gradually on the filament surface. From SEM images (Figure 1A, 1B), it is observed that

28

the dry GO layers on the filament possessed approximately uniform morphology over 8 / 31


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1

large areas with abundant wrinkles on the surface, except that the central zone had a little

2

thick GO deposition. The high dispersibility of GO in solution allowed to produce a more

3

reproducible and homogenous distribution of carbon on the filament, unlike the other

4

carbon additives such as graphite and collodion. The abundant wrinkles on the GO layer

5

surface would ensure the efficient interaction of uranium with GO, promoting the

6

formation of U-carbide. Then, 1 μg uranium sample was loaded on the GO layer via

7

resistive heating. Afterwards, this as-prepared filament was transferred into TIMS turret,

8

and resistively heated for 30 sec at about 1600 ℃ under the vacuum of below 5×10-7

9

mbar. It was proven that under this condition, uranium atoms would be initially ionized

10

(See later discussions). Figure 1C-E shows the SEM and EDX analysis of this shortly

11

heated uranium-loaded GO-Re filament. It was found that most amounts of GO were

12

burn out and some pieces of thin GO layers were remained on the filament surface, by

13

comparing Figure1A, 1B and 1C, 1D. The remaining GO layers still had abundant

14

wrinkles on the surface, suggesting that the high temperature and vacuum would not

15

affect the surface topography of GO. From EDX spectrum in Figure 1E, Re, C, U, and O

16

appeared to be the major elements on the heated filament surface. From EDX spot

17

analysis in Figure S2 in Supporting Infomation, it was observed that the EDX peaks of

18

uranium showed relatively larger intensity at the wrinkles of GO than the flat regions.

19

This result may suggest that the wrinkle structures of GO surface at micro- and

20

nano-level not only increased specific surface area substantially for U-C interaction, but

21

also promoted the absorption and accumulation of uranium on GO surface.

22

To directly demonstrate the formation of Re- and U-carbides during the measurement

23

of uranium using GO loaded Re filament, time-of-flight secondary ion mass spectrometry

24

(ToF-SIMS) analysis were performed. An uranium-loaded GO-Re filament were

25

resistively heated for 30 sec at about 1600 ℃, under the vacuum of below 5×10-7 mbar

26

before ToF-SIMS measurements. As previously described, this heating condition was

27

consistent with that during the TIMS measurement, at which the uranium ions were

28

initially emitted with the monitored

29

the images and spectra of Figure 2, the ions ReC+, ReCH+, UC+, UC2+ from the surface of

238U

signal at several thousands of cps. As shown in

9 / 31



1

this Re filament were observed, clearly demonstrating the simultaneous formation of Re-

2

and U-carbide species during filament heating, which were proven to have promotion

3

effect on the ionization efficiency of uranium.11-16 From the distribution of the surface

4

signals for these ions, it is found that the Re- and U-carbides were spatially uniformly

5

distributed on the filament surface, which contributed to the stability and precision for

6

uranium analysis. In these characterizations, all filaments had similar physical and

7

chemical properties and behaviours via the same GO carburization strategy, showing

8

excellent reproducibility.

9

Enhancement of uranium ionization efficiency

10

Generally, The background mainly from the intrinsic presence of ultra-trace level of

11

uranium in GO might interfere the measurement. About part-per-million (ppm) level of

12

uranium in GO (1.008 pg μg-1) was evaluated by total evaporation (TE) identical to the

13

sample measurement (Table S1 in Supporting Infomation). By using the washing

14

procedure decribed in the Experiment section, up to 99.8% of the residue uranium was

15

removed, and the background uranium was reduced to ppb level (0.002 pg μg-1).

16

Considering the deposition amount of GO (5μg) on the filament, the absolute background

17

was only about 10 fg of

18

background after GO purification would hardly interfere the isotopic measurement of

19

uranium of pg-level.

238U,

which was comparable to that of ICP-MS.35 Such a

20

Figure 3 shows that, for 5 pg uranium sample loading, GO on single filament

21

exhibited much better performance to generate a stable ion beam at a much higher

22

intensity than the traditional single- and double-filament modes. And GO also showed

23

greater ability to enhance the ionization efficiency compared to equivalent graphite as the

24

ion emitter. A high ionization efficiency of nearly 0.2% was obtained with GO, which

25

was enhanced with two orders of magnitude than the traditional single filament mode and

26

ten times larger than ~0.02% of double filament mode. It was only ~0.03% for uranium

27

ionization with equivalent graphite, illustrating that the nanostructure and larger surface

28

area of GO greatly contribute to the atom ioniztion. Furthermore, the initial ionization

29

temperature had been remarkbly decreased from ~1700 ℃ to ~1420 ℃ with GO, and 10 / 31


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1

to ~1560 ℃ with equivalent graphite. GO allowed the uranium to generate a stable ion

2

beam much more quickly. This should be attributed to the high work function of the

3

newly formed rhenium-carbon solid solution during continuously heating in the ion

4

source chamber of TIMS.12,13 This result also reveals that GO could interact with Re

5

more sufficiently to form Re-carbide than equivalent graphite, depending on its high

6

dispersibility and specific surface.

7

Optimization of filament carburization

8

In order to achieve the best ionization performance and analytical sensitivity, the

9

loading conditions and mass of GO onto the filament were optimised. It was found that

10

the filament heating current could not be larger than 1.5 A during the loading of GO

11

solution. Larger currents would make the filament in a high temperature and burn out

12

most GO on it rapidly, no matter in air or in a vacuum chamber. The loss of GO before

13

uranium sample loading would decrease the formation of U-carbides, resulting in low

14

ionization efficiencies (Figure S3 in Supporting Infomation). Therefore, 1 A current was

15

used for GO loading, which could vaporize the solvent in GO colloidal solution quickly

16

within 2 min, while avoiding the loss of carbon.

17

The ionization ability is highly dependent on the loading mass of GO. Different

18

amounts of GO from 1 to 40 μg were dropped onto the central surface of filament (2 μL

19

GO solution with different concentrations was loaded each), keeping the cover area of

20

GO layer almost the same, i.e. only layer thickness was different. Figure 4A shows that,

21

the uranium ionization efficiency sharply increased with the loaded GO increasing from 1

22

to 5 μg. This could be attributed to the increasing formation of Re- and U-carbides, which

23

promoted the ion emitting and reduced the loss of atoms during evaporation, respectively.

24

However, the total efficiency gradually decreased from 5 to 40 μg. This result suggests

25

that the overloaded GO would result in a thick carbon layer, hindering the contact of

26

Re-carbide formed at the GO/Re interface with uranium atoms loaded on top layer of GO.

27

The maximum ionization efficiency appeared at GO loading mass of 5 μg. As shown in

28

Figure 4B, as the GO loading mass increased, the initial ionization temperature of

29

uranium gradually increased and then reached a plateau at the GO loading mass of 30 μg. 11 / 31



1

This is because that the ionization temperature not only depends on the electron work

2

function of the metallic interface, but also on the fomation of U-carbides. The larger

3

amounts of GO would reduce uranium into more U-carbides, which required higher

4

temperature to be thermally evaporated and ionized. From these tests, 5 μg GO was used

5

as the optimal loading quantity for the following experiments.

6

To validate the good reproducibility and reliability of this filament carburization

7

method, independent TIMS measurements of the uranium ionization efficiency were

8

repeated for 50 times with 5 μg GO loaded single Re filaments. The ionization efficiency

9

ranged from 0.13% to 0.25%, with an average value of 0.17% (Figure 4C), which is close

10

to that from benzene vapour carburization method.21-24 The relative standard deviation for

11

ionization efficiency was calculated to be ~15%, which is much better than that from

12

experiments with equivalent graphite (Rel. S.D. ~78%, data from Figure 3B). Figure 4D

13

clearly illustrates that the GO ionization enhancer consistently produced steady uranium

14

ion yields, independent of the sample mass over the loading range from 5 to 200 pg.

15

These results demonstrate the excellent reproducibility and stability of this GO-based

16

filament carburization technique. Compared to the current benzene vapour method,[21]-[24]

17

this GO based technique dramatically simplifies the carburization process and

18

equipments without decreasing the ionization efficiency and reproducibility. All the GO

19

and sample loading procedures could be rapidly completed within 10 min (2 min for GO

20

loading and drying, 5 min for air cooling and 2 min for uranium sample loading and

21

drying).

22

Furthermore, different amounts of Re powders were used to mix with GO solution

23

for filament carburization. As the mass ratio of Re powder and GO was raised, the

24

ionization efficiency increased slightly and then reached a plateau (Figure 5), illustrating

25

that Re powder could promote the formation of Re-carbide in the GO layers, generating

26

better property for ion emitting. It also provides another possibility for further

27

improvement of ionization enhancer, by intergrating other ion emitters with the this

28

porous layered nanomaterial.26-28

29

Isotope measurements of pg-level uranium standards 12 / 31


Page 12 of 31

Page 13 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

This GO carburization method for Re filament was then applied in isotope

2

measurements of some pg-level uranium isotopic certified reference materials. With a

3

multi-collector faraday cup array, the determined n(235U)/n(238U) 0.007285 for 200 pg

4

GBW04483 uranium standard was in agreement with the certified value 0.007264, within

5

the experimental uncertainty (Table 1). The relative uncertainty was obtained as 0.48%,

6

displaying excellent analytical precision for sub-nanogram uranium. As a comparison,

7

double-filament mode was also used to determine the ratio n(235U)/n(238U) of the same

8

200 pg GBW04483 uranium standard. The average ratio n(235U)/n(238U) and relative

9

uncertainty were both obtained to be larger, 0.007366 and 1.14%, respectively, compared

10

to using GO carburized filaments. It evidently shows the promotion effect of filament

11

carburization on analytical precision of isotopes, benefiting from the overall improvement

12

to the ionization efficiency. However, the value of the minor isotope ratio n(234U)/n(238U)

13

could not be accurately determined because the signal of

14

detected by faraday cups. Thus, multiple-ion counting (MIC) array was used for the

15

detection of minor isotope ratios in trace uranium samples.

234U

was too small to be

16

During TE measurements, correction factor (K) for isotopic measurements was

17

determined externally using the GBW04492 isotopic standard in order to correct the

18

isotope ratios, which was affected by the mass fractionation and the efficiency of

19

ion-counting collectors. For each turret, the K-factor for all isotope ratio corrections was

20

calculated using the measured major ratio value n(235U)/n(238U) and its certified value of

21

GBW04492 with the same loading amount of sample (shown in Table S2 and S3). The

22

corrected isotope ratio data for the GBW04493 sample in three separate turrets (6 runs in

23

each turret, totally 18 runs) are shown in Table 2. The resulting ratios from each turret

24

were in excellent agreement with the certified values. Even for 5 pg sample loads, the

25

relative uncertainty for not only the major isotope ratio n(235U)/n(238U), but also the

26

minor ratios n(234U)/n(238U) and n(236U)/n(238U) at 10-4 level, were found to be

27

approximately 1%. As expected, the relative uncertainties were found to increase with

28

decreased uranium amount. As for the GBW04483 sample, whose isotope ratios are close

29

to natural uranium, the MIC data obtained for each isotope ratio were also in agreement 13 / 31



1

with the certified values within the experimental uncertainties (Table 3). Compared to

2

GBW04493, the relative uncertainties of all ratios were observed slightly larger because

3

the real loaded amounts of 234U and 235U in GBW04483 were much lower (n(234U)/n(238U)

4

is at 10-5 level and n(235U)/n(238U) is at 10-3 level), which was easy to be affected by the

5

background and the ion counter dark noise. Fortunately, even for 5 pg loads, both isotope

6

ratios have relative uncertainties of less than 3%. From these results, the analytical

7

precision for pg-level uranium with GO carburization method is comparable to recently

8

reported TIMS results for trace uranium isotopic analysis,10,21,22,24 displaying great

9

potential for practical applications of single particle analysis for nuclear safeguards.

10

Real sample measurements

11

To initially test the feasibility of this technique for trace uranium isotope

12

measurement in real samples, the uranium isotope ratios of swipe samples, river water

13

samples and human hairs were measured under the same experimental condition as

14

previously described for the pg-level uranium standards. The three swipe samples shown

15

in Table 4 were collected from a worktable in the accident-tolerant fuel (ATF) laboratory

16

of our academy. Based on the

17

evaporation TIMS measurements, the uranium concentration of these swipe samples was

18

estimated to be ~0.8 ppb, while regarding the ionization efficiency as 0.2% (Table S4).

19

Generally, at this low concentration level, the isotope ratios could not be obtained

20

accurately with classical double filament TIMS measurements without any sample

21

enrichment. Table 4 shows the determined isotope ratios of trace uranium in these swipe

22

samples, with the proposed GO carburization method. For all three swipe samples from

23

the worktable, the values of n(234U)/n(238U) and n(235U)/n(238U) were obtained to be close

24

to 0.0004 and 0.05, respectively, while

25

background level) (Table 4). It indicates that low enriched uranium (LEU) had been

26

usually handled in experiments on this worktable. The relative uncertainties were found

27

to be slightly larger (up to 5% for major isotope) than those results in Table 2 and 3

28

because the loaded uranium contained in the real samples were less (~3 pg uranium

29

loaded for each swipe sample, Table S4). As the loaded uranium decreased, the precision

238U

ion counts and isotope ratios obtained in total

236U

could not be detected (i.e. close to

14 / 31


Page 14 of 31

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1

of isotope detection would be reduced with more effect from uranium background and

2

instrumental dark noise.

3

The environmental water samples were also tested in our method (Table 5). The river

4

water sample was pipetted onto GO-filament surface without any sample enrichment or

5

concentration. As expected, the determined n(234U)/n(238U) and n(235U)/n(238U) are

6

consistent with the values of natural uranium (~0.00005 and ~0.007, respectively). The

7

uranium concentration in the river water was evaluated to be with a mean value of ~0.5

8

ppb (Table S4). Moreover, measurements of hair samples from two workers in our

9

institute were carried out to extend the applications to biological samples (Table 6).

10

Similarly to the river water, the determined values including n(234U)/n(238U) and

11

n(235U)/n(238U) were also close to those of natural uranium. In these isotopic

12

measurements, the GO-based filament carburization technique showed enough accuracy

13

and precision, revealing its great potential for bulk analysis of swipe samples,

14

environmental samples, and biological samples, which is promising to be applied in

15

nuclear forensics.

16

Conclusion

17

In summary, this work reports an initial investigation into the promoting effect on

18

uranium ionization using graphene oxide as the ionization enhancer in TIMS

19

measurements. Profiting from its high specific surface, excellent dispersing property, and

20

layered structure with abundant wrinkles, a high and reproducible ionization efficiency of

21

~0.2% for uranium was achieved in a single-filament mode, which is ten times of the

22

traditional double-filament mode. This ionization capability is comparable to the current

23

benzene vapor carburization technique while the carburization procedures were greatly

24

simplified and the operation time was shortened from several hours to only 10 min. This

25

method was successfully applied in the determination of isotope ratios for uranium

26

isotopic certified reference materials and real samples including swipe samples, water

27

samples and biological samples at pico-gram level. The detection accuracy and precision

28

shows advantage in trace uranium analysis, and could meet the requirement for nuclear 15 / 31



1

safeguards. The performance of GO ionization enhancer still has potential for further

2

improvement through introducing other ion emitters such as Re powders or platinum

3

particles into the GO layers.

4

Supporting Information

5

The supporting information is available free of charge on the ACS Publications website.

6

Supplementary figures (Figure S1-S3) and tables (Table S1-S4).

7

Acknowledgement

8

This work was financially supported by Science Challenge Project (No. TZ201600403),

9

National Natural Science Foundation of China (No. 21806150, No. 21701152) and TP

10

foundation of Institute of Materials, CAEP (TP02201802). The authors thank Dr. Dezhi

11

Zhang for his help in SEM measurement, and Dr. Wensheng Ren for his help in

12

providing river water and hair samples.

13

Notes

14

The authors declare no conflicts of interest.

16 / 31


Page 16 of 31

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1

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[1] Burger, S.; Riciputi, L. R.; Bostick, D. A.; Turgeon, S.; McBay, E. H.; Lavelle. M.

3 4 5

Int. J. Mass Spectrom. 2009, 286, 70-82. [2] Aggarwal, S. K.; Alamelu, D.; Khodade, P. S.; Shah, P. M. J. Radioanaly. Nucl. Chem. 2007, 273, 775-778.

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[3] Wallenius, M.; Mayer, K. Fresenius' J. Anal. Chem. 2000, 366, 234-238.

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[4] Delanghe, D.; Bard, E.; Hamelin, B. Mar. Chem. 2002, 80, 79-93.

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[5] Shinonaga, T.; Esaka, F.; Magara, M.; Klose, D.; Donohue, D. Spectrochim. Acta

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Part B 2008, 63, 1324-1328. [6] Saito-Kokubu, Y.; Suzuki, D.; Lee, C. -G.; Inagawa, J.; Magara, M.; Kimura, T. Int. J. Mass Spectrom. 2012, 310, 52-56.

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[7] Walther, C.; Denecke, M. A. Chem. Rev. 2013, 113, 995-1015.

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[8] Keegan, E.; Kristo, M. J.; Toole, K.; Kips, R.; Young, E. Anal. Chem. 2016, 88,

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1496-1505. [9] Shibahara, Y.; Kubota, T.; Fujii, T.; Fukutani, S.; Takamiya, K.; Konno, M.; Mizuno, S.; Yamana, H. J. Nucl. Sci. Technol. 2017, 54, 158-166. [10]Kraiem, M.; Richter, S.; Erdmann, N.; Kuhn, H.; Hedberg, M.; Aregbe, Y. Anal. Chim. Acta 2012, 748, 37-44. [11]Platzner, I. T. Modern isotope ratio mass spectrometry, vol. 145, John Wiley, Chichester, 1997.

21

[12]Pallmer, P. G. Jr.; Gordon, R. L.; Dresser, M. J. J. Appl. Phys. 1980, 51, 3776-3779.

22

[13]Kelly, J. M.; Roberson, D. M. Anal. Chem. 1985, 57, 124-130.

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[14]Wayne, D. M.; Hang, W.; McDaniel, D. K.; Fields, R. E.; Rios, E.; Majidi, V. Int. J.

24

Mass Spectrom. 2002, 216, 41-57.

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[15]Watrous, M. G.; Delmore, J. E. Int. J. Mass Spectrom. 2009, 286, 7-10.

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[16]Kraiem, M.; Mayer, K.; Gouder, T.; Seibert, A.; Wiss, T.; Thiele, H.; Hiernaut, J. P.

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Int. J. Mass Spectrom. 2010, 289, 108-118. [17]Shinonaga, T.; Esaka, F.; Magara, M.; Klose, D.; Donohue, D. Spectrochim. Acta, Part B 2008, 63, 1324-1328. 17 / 31



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[18]Shinonaga, T.; Donohue, D.; Ciurapinski, A.; Klose, D. Spectrochim. Acta, Part B 2009, 64, 95-98. [19]LaMont, S. P.; Shick, C. R.; Cable-Dunlap, P.; Fauth, D. J.; LaBone, T. R. J. Radioanal. Nucl. Chem. 2005, 263, 477-481. [20]Kurosaki, H.; Chang, D.; Inn, K. G. W. J. Radioanal. Nucl. Chem. 2006, 269, 279-281. [21]Kraiem, M.; Richter, S.; Kuhn, H.; Stefaniak, E. A.; Kerckhove, G.; Truyens, J.; Aregbe, Y. Anal. Chem. 2011, 83, 3011-3016. [22]Kraiem, M.; Richter, S.; Kuhn, H.; Aregbe, Y. Anal. Chim. Acta 2011, 688, 1-7. [23]Jakopic, R.; Richter, S.; Kuhn, H.; Aregbe, Y. J. Anal. At. Spectrom. 2010, 25, 815-821. [24]Sturm, M.; Richter, S.; Aregbe, Y.; Wellum, R.; Prohaska, T. Anal. Chem. 2016, 88, 6223-6230.

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[25]Walker, R. L.; Eby, R. E.; Pritchard, C. A.; Carter, J. A. Anal. Lett. 1974, 7, 563-574.

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1 2


[35]Zheng, J.; Tagami, K.; Homma-Takeda, S.; Bu, W. J. Anal. At. Spectrom. 2013, 28, 1676-1699.

3 4

Figure Legends

5

Scheme 1 Schematic representation of the filament carburization and sample loading

6

procedures for TIMS isotope ratio measurement of uranium, with graphene oxide (GO) as

7

ionization enhancer. Left: the TIMS and single-filament setups. Right: the filament

8

carburization and sample loading. The use of GO-carburized filaments improved the

9

ionization efficiency for total evaporation TIMS measurements of uranium.

10

Figure 1 Scanning electron microscope images of 5 μg GO-loaded rhenium filament (A)

11

and surface topography of filament-supporting GO layer (B). (C-D) SEM images

12

showing thermal treated uranium-loaded GO-Re filament (C) and topography of the

13

remaining GO (D). (E) EDX spectrum of the indicated area on thermal treated

14

uranium-loaded GO-Re filament (dashed-line box, inset, scale bar: 20 μm). The Re

15

filament was loaded with 5 μg GO and 1 μg uranium sample GBW04483 sequentially,

16

then heated for 30 sec at about 1600 ℃ electrothermally, under the vacuum of below

17

5×10-7 mbar.

18

Figure 2 ToF-SIMS ion images and spectra generated from a heated uranium-loaded

19

GO-Re filament. (A) ToF-SIMS images of (a) overlay, (b) Re+, blue, (c) ReC+, green, (d)

20

U+, yellow, (e) UC+, orange, (f) UC2+, orange. Scale bar: 50 μm. (B) ToF-SIMS spectra

21

of Re and Re related compounds. Ions from ReC are red marked. (C) ToF-SIMS spectra

22

of U and U related compounds. Ions from UC and UC2 compounds are red marked. The

23

Re filament was loaded with 5 μg GO and 1 μg uranium sample GBW04493 sequentially,

24

then resistively heated for 30 sec at about 1600 ℃, under the vacuum of below 5×10-7

25

mbar.

26

Figure 3 (A) The

27

filaments during TIMS measurements of 5 pg uranium sample GBW04483. Each cycle

28

represents an integration time of about 1s. Re(s): single Re filament (black),

238U

count rate versus the cycle number obtained from different

19 / 31



1

Graphite-Re(s): 5 μg graphite-loaded single Re filament (red), GO-Re(s): 5 μg graphene

2

oxide-loaded single Re filament (blue), Re(d): double Re filament (purple). (B) Total

3

ionization efficiency (red histograms, left y coordinate) and initial ionization temperature

4

(blue circles, right y coordinate) obtained from Re(s), Graphite-Re(s), GO-Re(s) and

5

Re(d) during TIMS measurements of 5 pg uranium sample GBW04483. Error bars are

6

standard deviations from three parallel experiments.

7

Figure 4 (A-B) Total ionization efficiency (A) and initial ionization temperature (B) of

8

100 pg uranium versus GO loading mass on Re filament. (C) Histograms of uranium

9

ionization efficiency. Data were obtained from 50 independent experiments with 5 pg

10

uranium sample. (D) Total ionization efficiency of varied uranium sample loading mass,

11

5, 50, 100, 200 pg. All data were obtained from TIMS measurements of uranium sample

12

GBW04483 with 5 μg GO-loaded filament. Error bars are standard deviations from three

13

parallel experiments.

14

Figure 5 Ionization efficiency versus the mass ratio of Re powder and GO. Different

15

amounts of Re powders were mixed with 2.5 mg mL-1 GO solution. In each test, the

16

loaded Re powder/GO mixture solution contains 5 μg GO.

17 18

20 / 31


Page 20 of 31

Page 21 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1

Table 1 Multiple faraday cup collecting measurements of 200 pg loads of GBW04483

2

isotopic standard uranium solution with GO-single Re filaments and double Re filaments,

3

respectively. No.

n(235U)/n(238U) (GO-single

n(235U)/n(238U) (Double Re,

Re, N=6a)

N=6a)

1

0.007287

0.007396

2

0.007274

0.007420

3

0.007269

0.007385

4

0.007241

0.007294

5

0.007293

0.007462

6

0.007273

0.007237

Measured valueb

0.007285

0.007366

Uc (k = 2)

0.000035

0.000084

0.48

1.14

Rel. Uc (k = 2) (%)

4

aN

5

bCorrected

represents number of replicate analyses. relative to a linear regression of multiple measurements of GBW04492.

6 7

21 / 31



1

Page 22 of 31

Table 2 Multiple ion counting (MIC) measurements of pg-level loads of GBW04493

5 pg Isotope ratio

n(234U)/n(238U)

Turret 1

Turret 2

Turret 3

Turret 1

Turret 2

Turret 3

1

0.000672

0.000685

0.000684

0.0006868

0.0006794

0.0006880

2

0.000659

0.000688

0.000686

0.0006739

0.0006898

0.0006866

3

0.000678

0.000686

0.000680

0.0006830

0.0006894

0.0006816

4

0.000663

0.000671

0.000673

0.0006710

0.0007060

0.0006988

5

0.000678

0.000708

0.000694

0.0006861

0.0006823

0.0006786

6

0.000689

0.000670

0.000702

0.0006854

0.0006907

0.0006796

Certified value

0.0006889

0.0006889

0.0006889

0.0006889

0.0006889

0.0006889

Uncertainty (k = 2)

0.0000044

0.0000044

0.0000044

0.0000044

0.0000044

0.0000044

Measured value

0.000673

0.000685

0.000686

0.0006810

0.0006896

0.0006855

Uc (k = 2)

0.000011

0.000014

0.000010

0.0000068

0.0000092

0.0000075

1.63

2.04

1.46

1.00

1.33

1.09

1

0.08695

0.08736

0.08897

0.08691

0.08868

0.08988

2

0.08746

0.08894

0.08947

0.08796

0.08912

0.08833

3

0.08854

0.08950

0.08897

0.08760

0.08819

0.08812

4

0.08777

0.08892

0.08765

0.08816

0.08892

0.08916

5

0.08878

0.08914

0.08766

0.08840

0.08806

0.08806

6

0.08856

0.08853

0.09002

0.08790

0.08948

0.08882

Certified value

0.088456

0.088456

0.088456

0.088456

0.088456

0.088456

Uncertainty (k = 2)

0.000051

0.000051

0.000051

0.000051

0.000051

0.000051

Measured value

0.08801

0.08873

0.08879

0.08782

0.08874

0.08873

Uc (k = 2)

0.00073

0.00074

0.00096

0.00052

0.00055

0.00071

0.83

0.83

1.08

0.59

0.62

0.80

1

0.000928

0.000933

0.000927

0.000922

0.0009439

0.0009510

2

0.000931

0.000941

0.000965

0.000932

0.0009392

0.0009453

3

0.000917

0.000929

0.000945

0.000926

0.0009328

0.0009307

4

0.000944

0.000959

0.000934

0.000943

0.0009427

0.0009357

5

0.000929

0.000953

0.000936

0.000932

0.0009450

0.0009297

6

0.000944

0.000948

0.000945

0.000950

0.0009561

0.0009452

Certified value

0.0009418

0.0009418

0.0009418

0.0009418

0.0009418

0.0009418

Uncertainty (k = 2)

0.0000062

0.0000062

0.0000062

0.0000062

0.0000062

0.0000062

Measured value

0.000932

0.000944

0.000942

0.000934

0.0009433

0.0009396

Uc (k = 2)

0.000010

0.000012

0.000013

0.000010

0.0000077

0.0000088

1.07

1.27

1.38

1.07

0.82

0.94

b

Rel. Uc (k = 2) (%)

n(235U)/n(238U)

b

Rel. Uc (k = 2) (%)

n(236U)/n(238U)

b

Rel. Uc (k = 2) (%)

2

50 pg

GBW04493 (N=6a)

isotopic standard uranium solutions with GO-Re filaments. 22 / 31


Page 23 of 31 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1 2 3 4

represents number of replicate analyses included in the average ratios shown from each turret. And filaments on three separate turrets were tested. The total replicate number is 6epl18. bCorrected relative to a linear regression of multiple measurements of GBW04492.

5

Table 3 Multiple ion counting (MIC) measurements of pg-level loads of GBW04483

6

isotopic standard uranium solutions with GO-Re filaments.

aN

5 pg Isotope ratio

n(234U)/n(238U)

Turret 1

Turret 2

Turret 3

Turret 1

Turret 2

Turret 3

1

0.0000571

0.0000543

0.0000565

0.00005702

0.00005560

0.00005336

2

0.0000591

0.0000555

0.0000549

0.00005610

0.00005469

0.00005459

3

0.0000553

0.0000565

0.0000547

0.00005594

0.00005406

0.00005488

4

0.0000562

0.0000537

0.0000575

0.00005585

0.00005304

0.00005564

5

0.0000561

0.0000544

0.0000533

0.00005427

0.00005376

0.00005335

6

0.0000556

0.0000558

0.0000543

0.00005504

0.00005339

0.00005429

Certified value

0.0000548

0.0000548

0.0000548

0.0000548

0.0000548

0.0000548

Uncertainty (k = 2)

0.0000022

0.0000022

0.0000022

0.0000022

0.0000022

0.0000022

Measured value

0.0000566

0.0000550

0.0000552

0.00005570

0.00005409

0.00005435

Uc (k = 2)

0.0000014

0.0000011

0.0000016

0.00000094

0.00000093

0.0000090

2.47

2.00

2.90

1.69

1.72

1.66

1

0.00762

0.00737

0.00739

0.007434

0.007126

0.007278

2

0.00757

0.00744

0.00732

0.007309

0.007281

0.007167

3

0.00726

0.00709

0.00723

0.007294

0.007274

0.007351

4

0.00739

0.00724

0.00718

0.007298

0.007216

0.007257

5

0.00730

0.00725

0.00744

0.007333

0.007318

0.007237

6

0.00726

0.00730

0.00727

0.007186

0.007201

0.007276

Certified value

0.007264

0.007264

0.007264

0.007264

0.007264

0.007264

Uncertainty (k = 2)

0.000051

0.000051

0.000051

0.000051

0.000051

0.000051

Measured value

0.00740

0.00728

0.00730

0.007309

0.007236

0.007261

Uc (k = 2)

0.00016

0.00012

0.00010

0.000079

0.000069

0.000060

2.16

1.65

1.37

1.08

0.95

0.83

b

Rel. Uc (k = 2) (%)

n(235U)/n(238U)

b

Rel. Uc (k = 2) (%)

7 8 9 10

50 pg

GBW04483 (N=6a)

aN

represents number of replicate analyses included in the average ratios shown from each turret. And filaments on three separate turrets were tested. The total replicate number is 6×3=18. bCorrected

relative to a linear regression of multiple measurements of GBW04492.

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Page 24 of 31

1

Table 4 Multiple ion counting (MIC) measurements of swipe samples from a worktable

2

with GO-Re filaments. Swipe 1 (N=3a)

Swipe 2 (N=3a)

Swipe 3 (N=3a)

Mean value

0.000389

0.000375

0.000396

Uc (k = 2)

0.000033

0.000023

0.000012

8.48

6.13

3.03

Mean value

0.0478

0.0484

0.0476

Uc (k = 2)

0.0021

0.0025

0.0010

4.39

5.17

2.10

Isotope ratio

n(234U)/n(238U)

Rel. Uc (k = 2) (%)

n( U)/n( U) 235

238

Rel. Uc (k = 2) (%)

3

aThe

4

Table 5 Multiple ion counting (MIC) measurements of water samples from Fu River with

5

GO-Re filaments.

values are based on data from three separate experiments.

Water 1 (N=3a)

Water 2 (N=3a)

Mean value

0.0000549

0.0000563

Uc (k = 2)

0.0000019

0.0000056

3.46

9.95

Mean value

0.00749

0.00772

Uc (k = 2)

0.00036

0.00041

4.81

5.31

Isotope ratio

n( U)/n( U) 234

238

Rel. Uc (k = 2) (%)

n( U)/n( U) 235

238

Rel. Uc (k = 2) (%)

6

aThe

7

Table 6 Multiple ion counting (MIC) measurements of hair samples from two volunteers

8

with GO-Re filaments.


Hair 1 (N=3a)

Hair 2 (N=3a)

Mean value

0.0000644

0.0000581

Uc (k = 2)

0.0000062

0.0000073

9.63

12.56

Mean value

0.00772

0.00751

Uc (k = 2)

0.00057

0.00066

7.43

8.79

Isotope ratio

n(234U)/n(238U)

Rel. Uc (k = 2) (%)

n( U)/n( U) 235

238

Rel. Uc (k = 2) (%)

9

aThe


10 24 / 31


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Scheme 1

2

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Figure 1

2

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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