The radioactive actinide element neptunium (Np) is formed by neutron

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Introduction

The radioactive actinide element neptunium (Np) is formed by neutron 

bombardment of uranium (U), with more than 50,000 kg of Np produced 

annually as a by-product of nuclear-power generation. Smaller quantities 

of Np can be found as a decay product of americium-241 (

241

Am) used in 

ionizing smoke detectors, with trace quantities being produced from nuclear 

bomb testing, and naturally (from natural neutron capture) in U ores. 

Accurate analysis of neptunium 237 in 

a uranium matrix, using ICP-QQQ with 

MS/MS

Application note

Authors

Garry Duckworth,

Springfields Fuels Ltd, UK,

Glenn Woods,

Agilent Technologies, UK,

Nuclear, environmental

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2

The predominant isotope 

237

Np is produced as follows:

237

Np sits at the top of the “neptunium decay series” 

which in turn produces a series of other radioactive 

elements with medium to very short half-lives, 

eventually forming Bi and Tl (Figure 1). 

237

Np is extremely mobile in the environment, as it 

readily forms aqueous solutions (more so than any other 

actinide), attaches to particles and colloids, and does 

not readily become trapped in humic substrates such as 

soil. 

Np also has a high affinity for calcium-rich materials 

including certain clays and concrete, so care is needed 

with its storage. Although Np is not readily absorbed in 

the human gut, once in the body, it will pre-concentrate 

in the bones. 

The relatively long half-life of 

237

Np (~2.14 million 

years) means that it is persistent in the environment, 

so it requires suitable containment, and needs to be 

monitored at low levels.

Abundance sensitivity of ICP-QQQ with MS/MS

Samples that contain Np usually contain U at far higher 

concentrations. The determination of ultra-trace 

237

Np 

in environmental samples and nuclear materials (fuel or 

waste) is difficult by ICP-MS because of the overlap due 

to peak tailing from the large 

238

U peak. 

Conventional quadrupole ICP-MS instruments (ICP-

QMS) operate at unit mass resolution to separate 

adjacent masses, meaning a nominal resolution 

(expressed as M/ΔM) of 237 at m/z 237. However, peak 

separation also depends on the abundance sensitivity 

(AS) of the spectrometer. AS is a measure of the peak 

tailing, calculated from the contribution that an intense 

peak makes to its neighbouring masses. 

Figure 1. Np-237 decay series. Adapted from “The NUBASE Evaluation of 

Nuclear and Decay Properties”. Nuclear Physics A 729: 3–128. DOI: 

10.1016/j.nuclphysa.2003.11.001

Quadrupole ICP-MS instruments can achieve AS of up 

to 1 x 10

-7

 (a peak of 1 x 10

7

 cps contributes 1 cps to 

the adjacent masses), so a peak with an intensity much 

higher than 10

7

 cps will make a significant contribution 

to the peaks either side. High Resolution-Sector Field 

(HR-SF)-ICP-MS has better resolution than ICP-QMS 

(M/ΔM of up to 10,000), but poorer AS. So adjacent 

peaks may appear to be separated on the mass scale, 

but the peak tail of an intense peak may still contribute 

to the masses above and below. The Agilent 8800 or 

8900 Triple Quadrupole ICP-MS (ICP-QQQ) use a unique 

configuration with two quadrupole mass analyzers (Q1 

and Q2) either side of the collision reaction cell. When 

both quadrupoles are operated as unit mass filters 

(MS/MS mode), this configuration delivers unmatched 

peak separation because the abundance sensitivity 

performance is the product of two mass separations – 

Q1 AS x Q2 AS – giving an overall AS of <<10

-10

. 

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3

ICP-QQQ is therefore able to successfully separate 

237

Np 

from the 

238

U overlap, even when the U is present at 

many orders of magnitude higher concentration. This 

is demonstrated in Figure 2, which shows the spectra 

of 100 ppt Np in a 10 ppm U matrix measured in Single 

Quad (SQ) mode (top) and MS/MS mode below. The 

ICP-QQQ spectrum shows the superior peak separation 

provided by MS/MS mode, and the clear elimination 

of the contribution on mass 237 from the adjacent 238 

peak. 

Figure 2. Spectra of 100 ppt Np in a 10 ppm U matrix sample solution 

obtained using ICP-QQQ in Single Quad mode (top) and MS/MS mode 

(bottom). MS/MS mode eliminates the peak tail on the low mass side of the 

intense 

238

U peak.

Experimental

Instrumentation

The Agilent 8800* ICP-QQQ was configured with an  

SPS 4 autosampler, and the standard sample 

introduction system consisting of a Micromist nebulizer, 

quartz spray chamber, quartz torch and Ni interface 

cones. Instrument operating parameters are given in 

Table 1. 

Table 1. ICP-QQQ operating parameters.

Parameter

Value

RF power 

1550 W

Sampling depth

8.0 mm

Nebulizer gas flow rate

1.15 L/min

Spray chamber temp

2 °C

Cell gas

None

Calibration 

A blank and four neptunium calibration standards 

from 100 to 2000 ng/L (ppt) were prepared in nitric acid 

(2% v/v, Ultrapur, Merck, Germany ). The calibration 

curve obtained by ICP-QQQ showed excellent linearity 

over the calibrated range with a calibration coefficient 

of 1.0000 (Figure 3). As Np is essentially absent from 

the environment and does not occur as a typical reagent 

contaminant, the Background Equivalent Concentration 

(BEC) obtained was ~0.0009 ng/L (0.9 pg/L, ppq) and 

the Detection Limit (DL) was ~0.0031 ng/L (3.1 ppq). 

This illustrates the exceptionally low background and 

high ion transmission (sensitivity) of the ICP-QQQ when 

operating in MS/MS mode.

Figure 3. Calibration curve for 

237

Np obtained by ICP-QQQ.

235

U

234

U

238

U

237

Np cannot be measured 

due to overlap from the 

238

U peak

237

Np

* The Agilent 8800 ICP-QQQ has been superseded by the 8900 ICP-QQQ

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Results and discussion

Table 2. Measurement of 

237

Np in a series of U matrix samples using ICP-QQQ and ICP-QMS.

8800 ICP-QQQ

7900 ICP-QMS

Sample Name

Reported 

237

Np

conc., ug/L

CPS

Reported 

237

Np

conc., ug/L

CPS

1 ppm U - unspiked

0.0000__0.83__0.0154__5519.75'>0.0000

1.10

0.0016

570.48

1 ppm U, 0.1 ppb Np

0.1021

14942.85

0.1018

36525.19

1 ppm U, 1.0 ppb Np

1.0445

152806.38

1.0100

362304.66

10 ppm U - unspiked

0.0000

0.83

0.0154

5519.75

10 ppm U, 0.1 ppb Np

0.1029

15052.99

0.1152

41339.80

10 ppm U, 1.0 ppb Np

1.0486

153402.27

1.0196

365764.86

100 ppm U - unspiked

0.0000

3.97

0.1581

56728.76

100 ppm U, 0.1 ppb Np

0.0997

14586.02

0.2494

89482.09

100 ppm U, 1.0 ppb Np

0.9859

144228.95

1.0597

380137.27

The Abundance Sensitivity performance of the ICP-QQQ 

in MS/MS mode was tested using a series of spiked 

and unspiked uranium solutions. Three sets of uranium 

solutions were prepared, at concentrations of 1, 10 

and 100 mg/L (ppm). For each concentration level, the 

U matrix solutions were measured unspiked, and with 

Np spikes at 0.1 and 1.0 µg/L (ppb). For comparison 

purposes, the samples were analyzed using an Agilent 

8800 ICP-QQQ and an Agilent 7900 ICP-QMS, to assess 

the impact of the improved AS performance of the  

QQQ configuration.

The results in Table 2 show that accurate recoveries 

were achieved for both Np spike levels in all of the U 

matrix samples analyzed by ICP-QQQ—even with U:Np 

concentrations at a ratio of 1,000,000:1. In contrast, the 

Np results measured on the 7900 quadrupole ICP-MS 

show a small contribution from the U matrix. This U 

signal contributes to a false-positive result for Np in 

the higher U matrix samples, including the unspiked 

U matrices. While the U signal only increased the 

apparent Np concentration by a small amount  

(sub-µg/L), the low level at which Np must be 

monitored means that this false signal is significant. 

The ICP-QMS results show that the interference 

effect is more pronounced with increasing U matrix 

concentration, due to the relatively poor AS of ICP-QMS 

compared to ICP-QQQ.

Conclusions

ICP-MS is used successfully for the analysis of trace 

elements in a wide range of complex sample matrices. 

However, several challenging interferences remain, 

including the measurement of trace analytes that occur 

close to major or matrix element peaks. This study has 

shown that the superior abundance sensitivity provided 

by the ICP-QQQ’s tandem quadrupole mass analyzer 

configuration (MS/MS) has practical benefits for the 

analysis of trace concentrations of Np at m/z 237 in the 

presence of high concentrations of the adjacent major 

isotope of U at m/z 238.

Conventional quadrupole-ICP-MS cannot resolve the 

overlap/peak tailing from the 

238

U isotope sufficiently to 

allow ultra-trace level analysis of 

237

Np; SF-ICP-MS also 

has insufficient abundance sensitivity to resolve the 

adjacent masses well enough to perform this analysis.

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© Agilent Technologies, Inc. 2017

Published December 19 2017

 

Publication number: 5991-6905EN