Yasuyuki Shikamori, Kazumi Nakano and Naoki Sugiyama
Agilent Technologies, Japan

Keywords

radionuclide, iodine, ¹²⁹I, environmental, nuclear, xenon, NIST 3231 Level I and II, abundance sensitivity, oxygen on-mass

Introduction

Iodine-129 is a long-lived radionuclide
(half-life of 15.7 My) which has been released into the environment as a result of human activities such as nuclear weapons testing, accidents at nuclear power plants and especially by emissions from spent nuclear fuel reprocessing plants. The determination of iodine-129 in environmental samples is very difficult by ICP-MS due to the element’s relatively low sensitivity, the very low concentrations at which ¹²⁹I must be determined, relative to potentially high levels of ¹²⁷I, the high background caused by ¹²⁹Xe impurities in the argon plasma gas, and possible polyatomic interference from ¹²⁷IH₂⁺. Iodine analysis is further complicated by the fact that it is rapidly volatilized from samples prepared using the acid digestions that are normal for ICP-MS analysis, so an alternative, alkaline sample solubilization and stabilization strategy is required. The isobaric interference from ¹²⁹Xe⁺ can be significantly reduced using ICP-QMS with an Octopole Reaction Cell operated in O₂ reaction mode, resulting in a measured ratio for ¹²⁹I/¹²⁷I of 10^-7  in NIST 3231 SRM Level I^[1] However, the problem of potential overlap due to tailing from ¹²⁷I and ¹²⁷IH remains, as the relative abundance of the ¹²⁹I to ¹²⁷I will typically exceed 10^-7, which is of the same order as the abundance sensitivity (ability to separate adjacent peaks) of quadrupole ICP-MS (ICP‑QMS). In order to overcome these challenges, ICP‑QQQ operating in MS/MS mode with O₂ reaction gas was applied to determine ultratrace levels of iodine-129 in aqueous samples.

Experimental

Instrumentation: Agilent 8800 #100.

Plasma conditions: Preset plasma/Low matrix.

Ion lens tune: Soft extraction tune:
Extract 1 = 0 V, Extract 2 = -190 V.

CRC conditions: O₂ gas at 0.8 mL/min,
Octopole bias = -18 V and KED = -1.5V. MS/MS O₂ on-mass mode was applied to measure iodine-127 and iodine-129
(Q1 = Q2 = 127 for iodine-127,
Q1 = Q2 = 129 for iodine-129). 

Reference materials and calibration standards: Calibration standards were prepared by diluting ¹²⁹I isotopic standards NIST SRM 3231 Level I and II
(NIST, Gaithersburg MD, USA) with
0.5% TMAH in deionized water. The
Level I Certified Value for
¹²⁹I/¹²⁷I = 0.981x10^-6 ± 0.012x10^-6,
Level II = 0.982x10^-8 ± 0.012x10^-8. These reference materials were used to check the calibration linearity of the iodine isotopes and to validate the isotopic ratio of iodine-129 and iodine-127. 

Results and discussion

Optimization of oxygen cell gas flow

The oxygen gas flow rate was optimized by varying the O₂ flow over the full range of the mass flow controller (0–1.12 mL/min), while monitoring the ^127,129I signal and blank intensity, as shown in Figure 1. As the flow rate of O₂ increases, the background signal (due to ¹²⁹Xe) at m/z = 129 decreases rapidly, and the iodine signal remains high, dramatically improving the DL for ¹²⁹I.

Abundance sensitivity 

Scan spectra over the mass range 127 to 129, covering both ¹²⁷I and ¹²⁹I, were acquired for the two SRMs, NIST 3231 Level I and II, using the Agilent 8800 ICP-QQQ in MS/MS on-mass mode with O₂ reaction gas. The overlaid spectra are shown in Figure 2. Excellent abundance sensitivity can be seen, with the sides of the intense (>10⁹ cps) ¹²⁷I peak reaching baseline with no tailing of ¹²⁷I⁺ or ¹²⁷IH⁺ on ¹²⁹I⁺. 

Calibration curves for ¹²⁷I and ¹²⁹I 

In order to check the linearity of both iodine isotopes, different concentration solutions of NIST 3231 SRM Level I were prepared in 0.5% TMAH and analyzed as calibration standards, as shown in Figure 3. The BECs for ¹²⁷I and ¹²⁹I were 2.9 µg/L and 0.04 ng/L respectively, and the detection limits (3σ, n=10) were 0.26 µg/L for ¹²⁷I and 0.07 ng/L for ¹²⁹I.

Analysis of NIST 3231 SRM Level I and Level II 

The ¹²⁹I/¹²⁷I ratio in 10x diluted
NIST 3231 SRM Levels I
(¹²⁹I/¹²⁷I = 0.981 x 10^-6) and
II (¹²⁹I/¹²⁷I = 0.982 x 10^-8) was measured using ICP-QQQ in MS/MS on-mass mode with O₂ cell gas. The results are summarized in Table 1. After subtracting the ¹²⁹I blank, the measured ¹²⁹I/¹²⁷I ratio of NIST 3231 SRM Levels I and II corresponded well with the certified values of 0.981x10^-6 and 0.982x10^-8 respectively. The good agreement with the certified ratio indicates that the potential interference of ¹²⁷IH₂⁺ on ¹²⁹I⁺ is completely removed by O₂ reaction with MS/MS mode. 

Reference

1. The ultratrace determination of iodine 129 in aqueous samples using the
   7700x ICP-MS with oxygen reaction mode, Agilent application note, 5990-8171EN. 

More information

1. The ultratrace determination of iodine 129 using the Agilent 8800 Triple Quadrupole ICP MS in MS/MS mode, Agilent application note, 5991-0321EN.