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Instrumentation and Method Development for On-site Analysis of Helium Isotopes Kirk Richard Jensen, Toshinobu Hondo, Hirochika Sumino, and Michisato Toyoda Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.7b01299 • Publication Date (Web): 20 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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Analytical Chemistry

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Instrumentation and Method Development for On-site Analysis of Helium

2

Isotopes †

Kirk R. Jensen , Toshinobu Hondo , Hirochika Sumino‡, and Michisato Toyoda†

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†

4 5

†

Project Research Center for Fundamental Sciences, Graduate School of Science,

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Osaka University, Japan

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‡

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of Tokyo, Japan

Department of Basic Science, Graduate School of Arts and Sciences, The University

9 10

Corresponding author:

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Kirk R. Jensen

12

Mass Spectrometry Group

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Project Research Center for Fundamental Sciences

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Graduate School of Science, Osaka University, Japan

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1-1 Machikaneyama, Toyonaka-shi, Osaka-fu 560-0043

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[email protected]

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Phone: +81 6-6850-8244

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Fax: +81 6-6850-8230

19 20 21

Abstract

22 23

Helium isotope determination may be useful in measuring volcanic activity

24

and issuing earlier warnings of possible eruptions. A method is presented for

25

measuring the 3He/4He ratio in a gas sample using the multi-turn time-of-flight mass

26

spectrometer infiTOF-UHV (MSI-Tokyo Inc., Tokyo Japan) (infiTOF). In contrast to

27

conventional waveform averaging, peaks are determined by counting ion pulses from

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each time-of-flight trigger. Samples were also measured by conventional magnetic-

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sector mass spectrometry for comparison. Magnetic sector results were used to

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designate a standard for infiTOF measurement and to calculate a ratio for each sample

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measured by infiTOF. Mass assignment error for ultra-pure 3He+ standard was 4.30 x

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10-5 Da. Mass assignment error of 4He2+ and 3He+ for sample cylinders was 3.00 x 10-

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8

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found to be within 2% of the abundance ratios determined by magnetic-sector mass

Da and 2.25 x 10-4 Da, respectively. Abundance ratios determined by infiTOF were

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spectrometry. Mass drift was less than 50 x 10-6 Da over ten hours. Sample flow rate

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was not found to affect the results as long as the reference sample was analyzed under

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the same conditions. Results indicate that the infiTOF system may be a viable tool for

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measuring helium isotopes, which may eventually lead to earlier warnings of volcanic

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activity.

40 41

Introduction

42 43

Active volcanoes and earthquakes are two of many natural disasters that can

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cause devastating destruction. As such, it would be desirable to be able to predict

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these disasters before their occurrence. Generally, such activities are expected to be

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monitored by geophysical parameters, such as seismicity, ground deformation,

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electric potential, magnetic field, and resistivity1–3. Additionally, hydrological and

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geochemical parameters, such as changes in the water level of observation wells and

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radon emissions, can change in response to crustal deformations or magma activity

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preceding earthquakes and volcanic eruptions1,4,5. One possibility for monitoring

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such activities involves the two stable isotopes of helium, 3He and 4He. The ratio of

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these two isotopes in geochemical reservoirs, such as the atmosphere, ocean, crust,

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and mantle, are different depending on the balance of primordial (relatively enriched

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in 3He compared to the atmosphere) and radiogenic (dominantly 4He) helium6,7. The

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3

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magmatic (up to 1.1 x 10-5 or more) and crustal (less than 1 x 10-7) helium isotope

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ratios, the latter resulting from dissolution of radiogenic helium into groundwater,

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which then accumulated in crustal rocks8,9. When magma becomes active, the

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3

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contribution of magmatic helium is expected to be higher9. Such 3He/4He increases

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preceding volcanic eruptions have been reported for El Hierro Island, Canary10 and

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Ontake, Japan11. The 3He/4He ratio of hot springs/groundwater around a volcano has

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great potential for monitoring magmatic activity, particularly if such isotope

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anomalies could be used to evaluate possible volcanic eruptions. Additionally,

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enhanced release of radiogenic helium accompanying crustal deformation might be

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expected to coincide with a large earthquake12. While it has not been clarified yet

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when the 3He/4He ratio of groundwater changes in conjunction with an earthquake,

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3

He/4He ratios of hot springs and groundwater around a volcano have values between

He/4He ratios of nearby hot springs/groundwater may increase as the relative

He/4He changes after large earthquakes have been reported13–17. 2 ACS Paragon Plus Environment

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Currently, a commercially available magnetic-sector mass spectrometer

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composed of a large, heavy electromagnet (> 600 kg) and a large-radius (> 27 cm)

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flight tube is used to measure 3He/4He18, due to the mass resolution requirement

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(M/∆M > 600) needed to discriminate 3He from HD18–20. High sensitivity is also

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required because 3He accounts for only 0.1-10 ppm of total helium, and sampling

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intervals in previous helium studies have taken more than several days9,10,13 or even

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years11,14,15,17. These major hurdles limit helium isotope analysis to a suitable

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laboratory, and is at a clear disadvantage compared to other predictive geophysical

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and geochemical signals, where earthquakes and volcanic eruptions have been

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observed during continuous monitoring periods of less than an hour or a day1,4,5.

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Therefore, a technique is desired which allows practical, on-site, real-time monitoring

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of 3He/4He around a volcano.

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The “infiTOF” is a small, portable, time-of-flight (TOF) mass spectrometer

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derived from the MULTUM-S II multi-turn TOF mass spectrometer21, which is

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capable of high mass resolution and high mass accuracy. The applicability of

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infiTOF for helium isotope monitoring was investigated by using software-based ion

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counting and a high-speed digitizer (commonly used in modern TOF instruments

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instead of a traditional time-to-digital-converter (TDC)). The ion pulse counting

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method was needed because the 3He+ ion could not be observed using traditional

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waveform averaging due to the low count rate. Using an identified ion and rapid

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analyzer protocol switching, masses can be assigned accurately without using an

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external calibrant, as described in the Methods section. This is particularly

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advantageous for trace-level analysis, because no calibration sample has to be

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introduced prior to sample introduction. The abundance ratio of 3He compared to 4He

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in the expected sample is in range of 10-6 to 10-8, and because of this large difference,

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they cannot be monitored together without saturating the detector. Therefore, 4He2+

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was monitored as a quantitative reference for 4He+. The 3He+/4He2+ count ratio was

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measured and compared to results obtained using a conventional magnetic sector-type

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mass spectrometer at the Department of Basic Science, Graduate School of Arts and

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Sciences, University of Tokyo19. In this paper, a method is presented for determining

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the 3He+/4He2+ count ratio of a sample using the infiTOF mass spectrometer and a

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high-speed digitizer to count ion pulses from each TOF trigger waveform. A 3He

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standard was measured to identify the 3He peak and verify mass accuracy. Then the

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3

He+/4He2+ count ratio was determined for three different helium gas cylinders. Mass 3 ACS Paragon Plus Environment


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drift and ratio stability were also investigated. Results indicate this method may be a

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viable tool for on-site monitoring of magma activity and issuing earlier warnings of

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imminent eruptions.

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Methods

108 109

Measurements were collected on a miniature, multi-turn TOF mass

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spectrometer, infiTOF-UHV (MSI.Tokyo, Inc., Tokyo, Japan), with in-house

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modifications for timing control and data acquisition.21 Analyzer timing and digitizer

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acquisition delay were controlled by a delay/pulse generator system built in-house on

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a DE0-nano-SoC development kit (Terasic, Taiwan). This delay/pulse generator

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controls the analyzer protocol that determines the number of laps for each target ion

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and ion gate timing that excludes unwanted ions from each TOF trigger. Up to four

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protocols can be switched sequentially during a single analysis. The detector signal

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was passed through an ORTEC model 9301 high-speed preamplifier (Advanced

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Measurement Technology, US), followed by waveform acquisition by a U5303A

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Keysight 1 GS/s high-speed digitizer (Keysight, USA). Each waveform readout from

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the digitizer was labeled with an analyzer protocol number from the delay/pulse

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generator using hard-wired lines via parallel input/output, and then passed into

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concurrent waveform processing pipelines. Data acquisition and software counting

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were processed in 16 symmetric concurrent threads on a dual Intel® 8-core Xeon®

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processor PC with the Linux (Debian 8.6) operating system. The baseline of each

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waveform was adjusted to zero volts and ion counting was accomplished by adding

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one count for each event where the waveform voltage crossed the threshold set point

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of 10 mV. The most frequent pulse height of a single ion was about 25 mV or higher

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(supplemental Figure S1), while the observed electrical noise was less than 3 mV

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peak-to-peak without waveform averaging. The digitizer voltage resolution is 0.5

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mV and has very low systematic noise22, therefore, a 10 mV threshold is high

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enough to separate the ion pulse signal from other electrical noises.

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Reagent grade helium cylinders were obtained from two different distributers:

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two cylinders from Air Liquide Japan Ltd. (named in-house as Cylinder-1, and

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Cylinder-2) and one cylinder from Nihon Helium Co., Ltd. (named in-house as

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Cylinder-N). The 3He/4He abundance ratios of the three helium samples were

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determined with a conventional, single-focusing, magnetic sector-type mass 4 ACS Paragon Plus Environment

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spectrometer, MS-IV, which is equipped with a two-collector system for

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simultaneously measuring 3He and 4He19. Approximately 1 x 10-7 cm3 of gas sample

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at standard temperature and pressure was introduced into the MS-IV, which was

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operated in a static mode (isolated from the vacuum pumps to count helium ions

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without pumping them away for more than 20 minutes). The 3He and 4He ions were

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measured simultaneously by a secondary electron multiplier operated in a pulse-

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counting mode and a Faraday cup equipped with an amplifier and 1010 ohm feedback

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register. The count rates were about 3-12 counts/s and (0.6-1.0) x 107 counts/s for

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3

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by measuring a helium standard gas, HESJ, whose 3He/4He abundance ratio is (2.888

147

± 0.014) x 10-5. 23

He and 4He, respectively. The relative sensitivities for 3He and 4He were calibrated

The 3He+/4He2+ count ratios for the three helium cylinders were compared with

148 149

the 3He/4He abundance ratios determined by MS-IV. Ions were produced using

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electron ionization at 100 eV. Gas samples were introduced into the ion source at a

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flow rate of 1 mL/min (unless otherwise noted) by a mass flow controller (MFC 2022,

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Axetris AG, Switzerland) connected directly to the gas cylinder. Before data

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collection, the instrument was given 30 minutes to equilibrate. Ultra-pure (99.95%)

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3

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Hagiwara from the Center for Advanced High Magnetic Field Science at Osaka

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University.

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He (Cambridge Isotope Laboratories, Inc., UK) was provided by Dr. Masayuki

The time of flight for each ion of every trigger event was determined using the

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first local apex on the waveform following a threshold event (an event where the ion

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pulse is greater than 10 mV). Both 4He2+ and 3He+ were measured by alternating the

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detection protocol after each trigger (rapid analyzer protocol switching). The 4He+

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ion was excluded using an ion gate to prevent saturation of the detector. Data

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acquisition and analysis were performed using open-source software “QtPlatz”

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(https://github.com/qtplatz) with its plugin developed for the infiTOF system. Peak

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data for every TOF trigger was recorded into an SQLite database24. Some data

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plotting was done using GNU Plot25.

166 167

Results and Discussion

168 169

Mass Calibration



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Masses were assigned using the m/z and TOF relationship by inserting the known

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figure eight orbital length, Ln, and acceleration voltage determined from the TOF for

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an ion at different numbers of laps into the scan law. By using three analyzer protocol

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sequences, the 4He2+ profile waveform was monitored simultaneously at 10, 20, and

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30 laps. Time-of-flight was determined by calculating the centroid of the peak above

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50% of peak height as shown in Table 1.

176 = 1.139465 × 10 ∙ ∙ √ + 2.513965 × 10

(1)

177 178

Using the data from Table 1, we obtained the experimental formula Equation 1 and

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subsequently estimated the acceleration voltage as 3989.815 V. Masses for other ions

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were assigned by substituting the accelerator voltage and measured TOF values into

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the scan law. The accuracy of the obtained m/z and TOF relationship was validated

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by the analysis of 99.95% 3He as shown in Figure 1.

183 184

Verification of Mass Assignment

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To confirm the assigned mass of the 3He+ peak, two 3He standards were measured.

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Each cylinder was attached directly to the ion source without the use of a mass flow

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controller. After allowing the vacuum to stabilize, data acquisition was started.

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Following baseline measurement, the 3He cylinder was opened to the ion source. The

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3

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3.0155). Resolving power was approximately 5600 at 20 laps. For validation

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purposes, hydrogen gas was also analyzed; H2+ and DH+ were identified within 1.00 x

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10-4 Da of the corresponding accurate mass. Figure 2 is a spectrum of reagent grade

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helium gas from Cylinder-N at m/z 2; 4He2+ and H2+ were identified with mass errors

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of -3.00 x 10-8 Da and 1.22 x 10-5 Da, respectively. Figure 3 shows the m/z 3 region

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of Cylinder-N, in which 3He+ was identified with a mass error of 2.25 x 10-4 Da and

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was well separated from DH+.

He+ peak was observed at m/z 3.0155 with an error of 4.30 x 10-5 Da (exact mass

197 198

Determining the Optimum Number of Laps and Stray Counts

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A sample from Cylinder-N was analyzed for 4He2+ and 3He+ from 6 laps to 28 laps in

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2 lap increments for 15 minutes for each increment. Counts for each peak did not

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change significantly between 10 and 28 laps as shown in Figure 4. Figure 5 shows


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the total stray ion count, where a stray ion is an ion that appears on the spectrum, but

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is not synchronized to the trigger. At 20 laps, we observe a low number of stray ion

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counts and low noise while maintaining good signal, and there seems to be no

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quantitative disadvantage to using 20 laps over 10 (Figure 4). Additionally, the

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electrical noise at 20 laps was approximately 2-3 mV, well below the counting

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threshold of 10 mV (Supplemental Figure S1). Twenty laps were used for all

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subsequent measurements.

209 210

Measurement of 3He+/ 4He2+ and Comparison to Magnetic-Sector MS Results

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A sample of each cylinder was sent to the University of Tokyo for analysis by

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magnetic-sector mass spectrometry, and results of 3He/4He abundance ratio

213

determination are shown in Table 2. Cylinder-2 and Cylinder-N were also analyzed

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for 4He2+ and 3He+ ions by infiTOF. The 3He/4He abundance ratio was determined by

215

using Cylinder-N as a standard with respect to results obtained by the University of

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Tokyo (Table 2) in the following calculation: He

!He

=

[# He#$ ]&'( × [ He$ ]* × 3.326 × 10 [ He$ ]&'( × [# He#$ ]*

217

where Ref and Sam are the number of counts of the corresponding ion in Cylinder-N

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(the standard) and the sample cylinder, respectively. The obtained count ratio and

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determined results are shown in Table 3. Peaks for 4He2+ and 3He+ were detected at m/z 2.0007 and 3.0152,

220 221

respectively, with mass errors of 3.90 x 10-5 Da and 1.40 x 10-4 Da. Determination

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errors for Cylinder-2 were less than 2% (N=23), and day-to-day variance for

223

3

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provides strong evidence that ion counting coupled with an infiTOF could be used as

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a viable method for helium isotope detection in the field. At the moment, however, a

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flow rate of 0.1 mL/min of helium in a natural volcanic/hot spring gas sample would

227

be required to measure the 3He/4He abundance ratio with 2% precision, taking into

228

account that the 3He/4He abundance ratio of the sample would be two orders of

229

magnitude greater than the helium cylinders8. Because the helium concentrations in

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natural samples are in the ppm range, direct introduction of such a huge amount of

231

gas sample (more than 100 L/min) into the ion source of the infiTOF is unrealistic. In

232

order to apply the infiTOF for on-site helium isotope analysis in the future, removal

He+/4He2+ was less than 2% as shown in Table 3. This is excellent agreement and



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of other gas species and improvement of the sensitivity defined by (helium count

234

rate)/(helium flow rate into the ion source) is necessary. A simple helium separation

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system from a gas/water sample using a hot silica glass wall demonstrated by Bajo et

236

al.26 is a good prospect for this purpose.

237 238 239

Stability Study The ratio of the two helium isotopes and mass drift were measured over three

240

days to test the stability of the method. Figure 6 shows the mass stability of the two

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cylinders over three days. In order to keep data file size at a manageable level when

242

acquired over several hours, a new database file was created every 15 minutes. Data

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files were then combined into 60-minute intervals. Mass error over time was studied

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using 15 min averaged histograms for 10 total hours and no significant mass drift was

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observed (less than 50 x 10-6 Da over 10 hrs).

246 247

The effect of flow rate on the 3He+/4He2+

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In previous studies, it was noted that changing the helium pressure in the ion

249

source would change the observed 3He/4He abundance ratio19,20. In this experiment,

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changing the flow rate changes the partial pressure of helium gas in the ion source.

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There was concern that flow rate fluctuations would also have an effect on the

252

measured count ratio, so various flow rates were tested on the infiTOF system. Flow

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rates from 1 to 5 mL/min were analyzed until at least 1000 counts of 3He+ ion were

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detected. Mass resolving power was 4500. Some flow rate dependency was observed

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for the count ratios of Cylinder-2 and Cylinder-N (Figure 7(A)). Similar results were

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seen for Cylinder-1 (Figure 7(B)). These observations are consistent with previously

257

published results19,20. The 3He/4He abundance ratio for each cylinder was then

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determined using the count ratio of a standard (Cylinder-N) measured at the same

259

experimental parameters. This result is shown in Figure 8. While there is a

260

noticeable dependency of the 3He+/4He2+ count ratio on flow rate in Figure 7, after

261

calculating the 3He/4He abundance ratio using a standard (Cylinder-N), there no

262

longer seems to be any dependence on flow rate (Figure 8), and it can be concluded

263

that as long as results are determined using a standard under the same conditions, flow

264

rate fluctuations will not adversely affect results.

265 266

Conclusions 8 ACS Paragon Plus Environment

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267 In this work, small amounts of 3He were measured in standard helium gas

268 269

cylinders using a small, portable mass spectrometer by counting individual ion signals

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in each TOF trigger. The mass of each ion measured was accurate to within 3.00 x

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10-8 Da for 4He2+ and 4.30 x 10-5 Da for 3He+, and remained stable over the course of

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ten hours. Ratios from the University of Tokyo were used to estimate the 3He/4He

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abundance ratio for another cylinder, which was then compared to the ratio

274

determined from infiTOF measruement. The error between magnetic sector and

275

infiTOF meassurements was less than 2%. Although helium separation from other

276

gas species and improvement in sensitivity are required, these results provide the

277

groundwork for a portable 3He detection method that could help in issuing earlier

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warnings of natural disasters, such as volcanic eruptions and earthquakes, which

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could, in turn, save lives.

280 281

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(25)

326 327

Williams, T.; Kelley, C.; Lang, R.; Kotz, D.; Campbell, J.; Gershon, E.; Woo, A. .

(26)

Bajo, K.; Sumino, H.; Toyoda, M.; Okazaki, R.; Osawa, T.; Ishihara, M.;

328

Katakuse, I.; Notsu, K.; Igarashi, G.; Nagao, K. Mass Spectrom. 2012, 1 (2),

329

A0009.

330 331

Acknowledgements

332

The authors wish to thank Dr. Masayuki Hagiwara from the Center for Advanced

333

High Magnetic Field Science at Osaka University for providing the Ultra-pure 3He

334

samples, Dr. Noriko Nakayama for providing Cylinder-N and for technical assistance 10 ACS Paragon Plus Environment

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335

with gas introduction, Dr. Hirofumi Nagao for his technical assistance with operating

336

and tuning the infiTOF, and Mr. Hisanori Matsuoka for his technical support.

337

Funding for this project was provided in part by the “Integrated Program for Next

338

Generation Volcano Research and Human Resource Development” of the Ministry of

339

Education, Culture, Sports, Science and Technology (MEXT), Japan, and by JSPS

340

KAKENHI Grant Number JP16H04165.

341 342 343

Conflict of Interest Disclosure The authors declare no competing financial interests.



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344 345

Figure 1: Mass spectrum for 99.95% 3He standard; the 3He+ ion was assigned with

346

5.62 x 10-5 Da error.

347 348 349


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350 351

Figure 2: Mass spectrum for reagent grade helium gas (Cylinder-N) around m/z 2;

352

4

He2+ and H2+ were identified with errors of -3.00 x 10-8 Da and 1.22 x 10-5 Da,

353

respectively.

354



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355

356 357

Figure 3: Mass spectrum for reagent helium gas (Cylinder-N) around m/z 3 showing

358

the separation of 3He+ and DH+.

359


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400000

4

350000

450

300000

400

350

4

Mean ( He=246627)

300

150000

250

3

He

200000

+

counts

250000

2+

500

2+

He 3 + He

4

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


He counts

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100000

200

3

Mean ( He=202.0)

50000

150

0

100 5

10

15

20

25

30

Number of laps

360 361

Figure 4: Relationship between number of laps and ion counts in 15-minute intervals.

362

Error bars indicate 1-sigma standard error.

363



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364 365 366 367 368

Figure 5: Effect of number of laps on stray (background) ion counts. Each count was obtained by counting all ions for a 10 mDa width at around m/z 3.0 (open circle). Closed circle represents each count normalized to a 6 ns (peak width in time domain) width.

369


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1.40e-03

1.20e-03

2+

ratio

Average (Cyl. N) 1.00e-03

+4

He / He

3

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


Average (Cyl. 2)

8.00e-04

6.00e-04 day 1

day 2

day 3

4.00e-04 0

5

10

15

20

25

Experimental number

370 371

Figure 6: The stability of 3He+/4He2+ measurements for Cylinder-2 and Cylinder-N on

372

three different days. Error bars indicate 1-sigma standard error.



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373 374

Figure 7: The effect of flow rate on the 3He+/4He2+ count ratio for (A) Cylinder-2 and

375

Cylinder-N, and (B) Cylinder-1. Error bars indicate 1-sigma standard error.

376


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377 378

Figure 8: The effect of flow rate on the 3He/4He determination for Cylinder-1 and

379

Cylinder-2 after using a standard (Cylinder-N) to calculate the 3He/4He abundance

380

ratio from 3He+/4He2+. Error bars indicate 1-sigma standard error.

381 382



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Table 1: Time-of-flight and flight path length for m/z 2 at different lap conditions. Lap#

Ln (m)

Time(s)

10

7.1530 11.78037×10-6

20

13.7803 22.46154×10-6

30

20.4076 33.14347×10-6

383


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Table 2: The helium isotopic ratio determined for each cylinder measured by magnetic sector mass spectrometry at the University of Tokyo. Errors are one sigma errors (standard errors of the mean of 14 values). 3

Sample

He/4He

Cylinder-1

(4.79 ± 0.11) x 10-8

Cylinder-2

(2.444 ± 0.030) x 10-7

Cylinder-N

(3.236 ± 0.065) x 10-7

384



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Page 22 of 33

Table 3 The ratio of 3He+/4He2+ for Cylinder-N and Cylinder-2 on three different days. The 3He/4He abundance ratio was determined for Cylinder-2 by using Cylinder-N results from the University of Tokyo as a standard in a one-point calibration calculation. Cylinder Cyl-N Cyl-N Cyl-N Cyl-2 Cyl-2

Batch# 1 2 3 1 2

3

He+/4He2+

1.106 x 10-3 1.122 x 10-3 1.139 x 10-3 8.422 x 10-4 8.431 x 10-4

3

He/4He (est.)

CV%

n/a n/a n/a 2.457 x 10-7 2.460 x 10-7

1.625 1.768 n/a 1.93 1.93

385 386


Error (%) n/a n/a n/a 0.55 0.65

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387 388

Supplementary Figure S1:

389

A single triggered spectrum for 4He2+. Electrical noise level is ±3 mV (peak-to-

390

peak).

391



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392 393

For TOC Use Only

394


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Figure 1: Mass spectrum for 99.95% 3He standard; the 3He+ ion was assigned with 5.62 x 10-5 Da error. 130x61mm (300 x 300 DPI)

ACS Paragon Plus Environment


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Figure 2: Mass spectrum for reagent grade helium gas (Cylinder-N) around m/z 2; 4He2+ and H2+ were identified with errors of -3.00 x 10-8 Da and 1.22 x 10-5 Da, respectively. 212x117mm (300 x 300 DPI)


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Figure 3: Mass spectrum for reagent helium gas (Cylinder-N) around m/z 3 showing the separation of 3He+ and DH+. 189x115mm (300 x 300 DPI)



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Figure 4: Relationship between number of laps and ion counts in 15-minute intervals. Error bars indicate 1sigma standard error. 203x162mm (300 x 300 DPI)


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Figure 5: Effect of number of laps on stray (background) ion counts. Each count was obtained by counting all ions for a 10 mDa width at around m/z 3.0 (open circle). Closed circle represents each count normalized to a 6 ns (peak width in time domain) width. 199x164mm (300 x 300 DPI)



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Figure 6: The stability of 3He+/4He2+ measurements for Cylinder-2 and Cylinder-N on three different days. Error bars indicate 1-sigma standard error. 203x162mm (300 x 300 DPI)


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Figure 7: The effect of flow rate on the 3He+/4He2+ count ratio for (A) Cylinder-2 and Cylinder-N, and (B) Cylinder-1. Error bars indicate 1-sigma standard error. 302x476mm (300 x 300 DPI)



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Figure 8: The effect of flow rate on the 3He/4He determination for Cylinder-1 and Cylinder-2 after using a standard (Cylinder-N) to calculate the 3He/4He abundance ratio from 3He+/4He2+. Error bars indicate 1sigma standard error. 201x164mm (300 x 300 DPI)


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Supplementary Figure S1: A single triggered spectrum for 4He2+. Electrical noise level is ±3 mV (peak-topeak). 184x86mm (300 x 300 DPI)


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