Yasuyuki Shikamori and Kazumi Nakano
Agilent Technologies, Japan

Keywords

radioisotopes, radioactive, environmental, nuclear, strontium, ⁹⁰Sr, zirconium, cesium, ¹³⁷Cs, barium, abundance sensitivity, oxygen and hydrogen on-mass, nitrous oxide on-mass 

Introduction

ICP-MS can be an effective analytical tool for the analysis of long half-life radioisotopes due to its high sensitivity, speed of analysis, low sample consumption, and ease of sample preparation. The challenge for ICP‑MS analysis of radioisotopes arises from interferences; not only by polyatomic ions but also atomic isobar ions that cannot be separated even by high-resolution
(HR-) ICP-MS. 

Trace analysis of the radionuclide ⁹⁰Sr
(half-life = 28.74 years) in environmental samples is of great interest. ⁹⁰Sr is a main fission product that may be present in the environment following accidental releases from nuclear power plants. Geiger-Muller (GM) detectors or Liquid Scintillation Counters (LSC) are used to measure ⁹⁰Sr, though both techniques require complex chemical separation prior to analysis, or long integration times. ICP-MS is also used to measure ⁹⁰Sr, especially when a quick turn-around time is desired. However detection limits of quadrupole ICP-MS are compromised by a spectral overlap from ⁹⁰Zr; in common with all direct isobaric interferences, the ⁹⁰Zr overlap is too close in mass to the ⁹⁰Sr to be resolved using sector field HR‑ICP‑MS, which is limited to a maximum resolution (M/ΔM) of 10,000. This note describes a method for measuring trace ⁹⁰Sr in the presence of ⁹⁰Zr using ICP‑QQQ in MS/MS reaction mode. Since it isn't possible to obtain ⁹⁰Sr, a natural isotope of strontium (⁸⁸Sr) was used to estimate the DL for ⁹⁰Sr. A similar approach was applied to ¹³⁷Cs (half-life = 30.0 years).

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 and acquisition conditions: The following conditions were used for the analysis of ⁹⁰Sr and ¹³⁷Cs:

• For ⁹⁰Sr: MS/MS on-mass mode
  (Q1 = Q2 = 90) with O₂ + H₂ cell gas:
  1 mL/min of O₂ and 10 mL/min of H₂,
  Octopole bias = -5 V and
  KED = -13 V. 

• For ¹³⁷Cs: MS/MS on-mass mode
  (Q1 = Q2 = 137) with N₂O cell gas:
  7 mL/min of N₂O (10% N₂O balanced in He, introduced via the 3rd cell gas flow line), Octopole bias = -5 V and KED = -13 V. 

Results and discussion

Radioactive Sr-90
(O₂ + H₂ on-mass mode)

Figure 1 shows spectra of a solution containing Sr and Zr (natural isotopes) acquired on the 8800 ICP-QQQ operated in Single Quad mode (Q1 operated as an ion guide to emulate conventional quadrupole ICP-MS) with no cell gas (left), and in MS/MS mode with O₂ + H₂ cell gas (right). As can be seen in the left hand spectrum, the overlap of ⁹⁰Zr⁺ on ⁹⁰Sr⁺ precludes the low-level determination of ⁹⁰Sr by conventional quadrupole ICP‑MS. The spectrum on the right indicates that ⁹⁰Sr⁺ could be measured on-mass at
m/z = 90 free from interference by ⁹⁰Zr⁺, since Zr⁺ reacts readily with the O₂ + H₂ gas to form ZrO⁺ and ZrO₂⁺. The
signal-to-noise ratio for ⁹⁰Sr was improved by six orders of magnitude using
MS/MS O₂ + H₂ reaction cell mode.

Figure 2 is a spectrum of 100 ppm Sr acquired using MS/MS on-mass mode with O₂ + H₂ reaction gas. The excellent abundance sensitivity (peak separation) of MS/MS mode can be confirmed. The peak sides reach the baseline with no tailing from the intense peak of the natural isotope of ⁸⁸Sr⁺. In addition, no ⁸⁸SrHH⁺ at m/ z = 90 is formed in cell, even in a solution containing 100 ppm natural Sr.

Radioactive Cs-137 (N₂O on-mass mode)

Figure 3 shows spectra of a solution containing Cs and Ba (natural isotopes) acquired on the 8800 ICP-QQQ operated in Single Quad mode with no gas mode (left), and in MS/MS mode with N₂O cell gas (right). As can be seen in the left hand spectrum, the ¹³⁷Ba⁺ overlap on ¹³⁷Cs⁺ is a problem in conventional quadrupole ICP‑MS. As with ⁹⁰Sr, the right hand spectrum shows that ¹³⁷Cs⁺ could be measured on mass at m/z = 137, free from the ¹³⁷Ba⁺ interference. Ba⁺ reacts readily with N₂O to form BaO⁺ and BaOH⁺ while a part of the Cs⁺ analyte ion signal remains at its original mass (as shown by the substantial peak for ¹³³Cs in the right-hand spectrum). 

Estimated BEC and DL for Sr-90 and Cs-137

The BEC and DL for two radioisotopes,
⁹⁰Sr and ¹³⁷Cs, were estimated from these spectra as summarized in Table 1. This feasibility study demonstrates the potential of ICP-QQQ for the measurement of radioisotopes such as ⁹⁰Sr and ¹³⁷Cs.