Interference-free determination of ultra-trace levels of Arsenic and Selenium using methyl fluoride

86 Pages·2014·2.14 MB·English

Department of Analytical Chemistry
Research group Atomic and Mass Spectrometry

Interference-free determination of ultra-trace levels of
Arsenic and Selenium using methyl fluoride as
reaction gas in ICP-MS/MS

Thesis submitted to obtain
the degree of Master of Science in Chemistry by

Elisabeth Nissen

Academic year 2013 - 2014

Promoter: Prof. Dr. Frank Vanhaecke
Copromoter: Dr. Lieve Balcaen
Supervisor: Eduardo Bolea-Fernandez

Acknowledgements/preface
During the academic year 2013/2014, Master student Elisabeth Nissen has produced the following
Master thesis to obtain the degree of Master of Science in Chemistry. All lab work was performed
from September 2013 to May 2014 at the Department of Analytical Chemistry, Atomic and Mass
Spectrometry group, Ghent University. It is expected that the reader has a basic knowledge of
analytical chemistry.
A great thanks to the staff and students at the Department of Analytical Chemistry, Atomic and
Mass Spectrometry group, Ghent University, for supplying the instrumentation, facilities and help
to execute this Master thesis project. I especially owe PhD student Eduardo Bolea-Fernandez, Dr.
Lieve Balcaen and Prof. Dr. Frank Vanhaecke a great thanks, since their help was invaluable in the
execution of this Master thesis project.

Abbreviations
AAS Atomic absorption spectrometry
AFS Atomic flourescence spectrometry
ICP-AES Inductively coupled plasma-atomic emission spectrometry
ICP-MS Inductively coupled plasma-mass spectrometry
LOD Limit of detection
LOQ Limit of quantification
SF-ICP-MS Double-focusing sector field ICP-MS
Q-ICP-MS Quadrupole ICP-MS
ICP-MS/MS Tandem mass spectrometry ICP-MS
CH F Methyl fluoride
3
DCP Direct current plasma
MIP Microwave induced plasma
IP Ionization potential
m/z-ratio Mass-to-charge ratio
ESA Electrostatic analyzer
TOF Time of flight
Q1 1st Quadrupole
Q2 2nd Quadrupole
MS/MS set-up Q1 followed by a collision/reaction cell and Q2
SQ set-up Collision/reaction cell followed by a single quadrupole
MFC Mass flow controller

Overview
1. Introduction and aim ...................................................................................................................................... 1
2. Theory............................................................................................................................................................ 6
2.1 Instrumentation ........................................................................................................................................ 6
2.1.1 ICP-MS in general ............................................................................................................................ 6
2.1.2 Interferences in ICP-MS ................................................................................................................. 13
2.1.3 Triple quadrupole mass spectrometry (ICP-MS/MS) ..................................................................... 14
2.2 Contamination ....................................................................................................................................... 18
3. Experimental ................................................................................................................................................ 19
3.1 Instrumentation ...................................................................................................................................... 19
3.2 Samples and reagents ............................................................................................................................ 21
3.3 Sample preparation ................................................................................................................................ 22
3.3.1 Digestion procedure ........................................................................................................................ 22
3.3.2 Reconstitution procedure for urine ................................................................................................. 23
4. Results and discussion ................................................................................................................................. 25
4.1 Optimization of ICP-MS/MS protocol for the determination of As and Se .......................................... 25
4.1.1 As .................................................................................................................................................... 25
4.1.2 Se .................................................................................................................................................... 30
4.2 Optimization of SF-ICP-MS protocol for the determination of As and Se ........................................... 36
4.3 Calibration data and limits of detection ................................................................................................. 37
4.3.1 As .................................................................................................................................................... 37
4.3.2 Se .................................................................................................................................................... 39
4.4 Investigation of the improvement in sensitivity by the addition of MeOH ........................................... 40
4.4.1 As .................................................................................................................................................... 40
4.4.2 Se .................................................................................................................................................... 42
4.5 Results obtained for simulated matrices ................................................................................................ 44
4.5.1 As .................................................................................................................................................... 44
4.5.2 Se .................................................................................................................................................... 47
4.6 Results obtained for reference materials - As and Se ............................................................................ 52
5. Conclusion ................................................................................................................................................... 56
6. References ................................................................................................................................................... 57
7. Appendix ..................................................................................................................................................... 61
1. Introduction and aim
Arsenic (As) and selenium (Se) are two interesting elements to investigate, due to their presence in
different sample types e.g., environmental and biological samples, and due to the fact that As is
known to be a toxin, whereas Se is an essential element, but it becomes toxic at higher
concentrations, while the difference between an appropriate and an excessive concentration is
small.[1, 2]
Arsenic is a metalloid, which is present in the environment through both a natural route, with an
abundance in the earth's crust of 2.5 μg/g, and an anthropogenic route due to industrial, agricultural
and mining activities.[3,4] Arsenic is present in both organic and inorganic forms and in various
oxidation states (-III, 0, +III, +V). The toxicity is related to its chemical form and oxidation state
and inorganic species are known to be more toxic than organic species, with decreasing toxicity for
the species as follows: arsenite > arsenate > monomethylarsonate (MMA) > dimethylarsinate
(DMA).[1,2,5,6] For a human adult, inorganic As is lethal at an amount of 1 - 3 mg As/kg, whereas
long term exposure to inorganic As has been linked to adverse effects such as skin lesions, cancer,
developmental toxicity, cardiovascular diseases, neurotoxicity, abnormal glucose metabolism and
diabetes.[5]
Selenium is a non-metal, which is also present in the environment naturally, with an abundance in
the earth's crust of 0.05 μg/g, but it is also introduced through anthropogenic routes similar to those
of As, as a result of agriculture, mining, petrochemical and industrial activities.[7] Selenium also
exists in both organic and inorganic forms and in different oxidation states (+VI, +IV, 0, -II), and in
the environment, Se species such as Se (IV), Se (VI), dimethylselenide (dMeSe),
dimethyldiselenide (dMedSe) and dimethylselenone (dMeSeO ) can be found, with organic and
2
inorganic species having different toxicity. As mentioned before, Se is an essential element and it
can be linked to biological activities such as antioxidant actions, activation and degradation of
thyroid hormones, immunity enhancement, and reduction of colon cancer risk. However for Se, the
range between the deficiency level (< 40 μg/day for adults) and the toxic level (> 400 μg/day for
adults) is very narrow. A deficit in Se has been linked to some endemic diseases and the exposure to
too much Se can lead to selenosis.[2,8,9]
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Due to As being toxic even at low levels and the fact that for Se the range between toxic or essential
is very narrow and at low levels, it is important to be able to determine As and Se at ultra-trace
levels with high accuracy.
Arsenic and selenium have been investigated earlier using a variety of analytical techniques, such as
atomic absorption spectrometry (AAS)[10-12], atomic flourescence spectrometry (AFS)[13-15] and
inductively coupled plasma-atomic emission spectrometry (ICP-AES)[16-18]. However, inductively
coupled plasma-mass spectrometry (ICP-MS) has to be considered as the technique of choice due to
the advantages of very low limits of detection (LOD), a wide linear dynamic range, as well as
multi-element and isotopic capabilities.[1,2,19-22] However, the determination of As and Se within a
complex matrix by means of ICP-MS is not that straightforward, due to the following reasons: 1)
both As and Se have high ionization energies (9.82 eV and 9.75 eV, respectively), which means that
they are poorly ionized under normal ICP-MS conditions, and the ionization efficiency is in the
order of 52 % and 33 %, respectively[23], leading to poor sensitivity for the elements, and 2) spectral
overlap occurs for As and all Se - isotopes (Table 1) as a result of the occurrence of e.g., isobaric,
polyatomic and doubly charged ions of the same m/z-ratios, which means that interference-free
determination is a challenge.
Table 1 As and Se - isotopes with their natural isotopic abundance[24] and the most important isobaric, polyatomic and
doubly charged interferences[25,26] (non-restrictive list).
Abundance Isobaric Doubly charged
Analyte Polyatomic interference
(%) interference interference
40Ar35Cl+, 59Co16O+, 36Ar38ArH+, 38Ar37Cl+, 36Ar39K+,
75As+ 100 - 150Nd2+,150Eu2+,150Sm2+
43Ca16O+, 40Ar 23Na12C+, 12C31P16O+, 40Ca35Cl+
2 2
37Cl37Cl+, 36Ar38Ar+, 38Ar36S+, 40Ar34S+, 39K35Cl+,
74Se+ 0.89 74Ge+ 148Sm2+, 148Nd2+
58Ni16O+
76Se+ 9.37 76Ge+ 40Ar36Ar+, 38Ar38Ar+, 60Ni16O+, 39 K37Cl+, 41K35Cl+ 152Sm2+, 152Gd2+
38Ar39K +, 61Ni16O+, 59Co18O+, 40Ar37Cl+, 40Ca37Cl+
77Se+ 7.63 - 154Sm2+, 154Gd2+
36Ar40ArH+, 38Ar H+ 12C19F14N16O+
2 , 2
78Se+ 23.77 78Kr+ 38Ar 40Ca+, 62Ni16O+, 41K37Cl+, 40Ar 38Ar+ 156Gd2+, 156Dy2+
40Ar 40Ca+, 64Ni16O+, 64Zn16O+, 32S 16O+, 32S16O+,
80Se+ 49.61 80Kr+ 2 3 160Gd2+, 160Dy2+
40Ar40Ar+, 40Ca40Ca+,
82Se+ 8.73 82Kr+ 40Ar 42Ca+,34S16O+, 66Zn16O+, 12C35Cl +, 40Ar H+ 164Dy2+, 164Er2+
3 2 2 2
In literature, a variety of different approaches have been suggested to deal with these problems.
With regard to the problem of poor ionization, several studies have used the addition of organic
solvents, such as ethanol or methanol, or organic compounds to induce a signal enhancement of
these elements, denoted the carbon effect. The presence of carbon in a sample can influence the
ionization conditions in the plasma, the rate at which the aerosol is transported, the nebulization
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efficiency for the sample and/or the mass load of vapor. The signal enhancement is due to an
alteration of the region of maximum ion density in the plasma, an enhancement of the nebulization
of the sample and due to a charge transfer reaction from C+species to analyte ions: C+ + M → M+ +

C-species (1), where M is the analyte ion. This signal enhancement is especially observed for some
species, such as As and Se, so-called hard-to-ionize elements, since carbon has an even higher
ionization energy of 11.36 eV, and it can transfer its positive charge according to process (1).[23,27]
In literature, studies can be found that report an enhancement of the As signal of 240 % and of the
Se signal of 250 % due to the presence of glycerol, and as a results of methane being present an
enhancement of 500 % and 300 % for As and Se, respectively, was reported[28], whereas other
papers have also reported an enhancement of 150 % for As and Se using methanol[21]. However, if
the presence of carbon affects the total intensity of As and Se in a sample, this can also cause a
problem, since standards and samples may have a different C-content, and then, external calibration
fails. Thus, it is important to use an appropriate internal standard to correct for this enhancement,
thus one that also experiences this carbon effect or use standard addition for calibration.[29]
In order to overcome the problem of spectral interference, two main approaches have been
proposed. 1) Using a quadrupole-based ICP-MS instrument (Q-ICP-MS) equipped with a
collision/reaction cell to overcome the problem of spectral interference by chemical resolution,
where either the interference is removed or reduced due to collision or reaction with the cell gas or
where the analyte reacts with the cell gas and forms a reaction product that can be measured
interference-free at another m/z-ratio.[22] By using a collision gas mixture, such as H /He, one can
2
aim at reducing the spectral background at the mass of As and at the masses of the Se - isotopes.
However, this will also substantially reduce the transmission of As+ and Se+ ions. Detection limits
of 0.15 μg/L and 0.03 μg/L have been reported for As and Se (via 80Se), respectively, using this
method.[19] Another option is to use a reaction gas such as O , in order to convert As+ and Se+ ions
2
into AsO+ and SeO+ ions, and measure in the mass-shift mode, where the reaction product can be
detected at the new mass (m/z 91 for 75As+ and m/z 90, 92, 93, 94, 96 and 98 for 74Se+, 76Se+, 77Se+,
78Se+, 80Se+ and 82Se+, respectively).[30,31] Other typical gases that could be used, are CH and H .[32]
4 2
2) A second option is to use an ICP-MS instrument equipped with a double-focusing sector field
mass analyzer, denoted SF-ICP-MS, where the mass resolution is high enough for interference-free
determination. However, this instrument comes at a high purchase price and the use of the high
resolution mode results in the loss of sensitivity of 2 orders of magnitude, due to a reduction in the
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