A selective gas chromatography mass spectrometry method for

Subscriber access provided by Nottingham Trent University

Technical Note

A selective gas chromatography mass spectrometry method for ultratrace detection of selenocyanate Enea Pagliano, Kelly Lynn LeBlanc, and Zoltan Mester Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.9b02615 • Publication Date (Web): 29 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 15 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

Analytical Chemistry

A selective gas chromatography mass spectrometry method for ultratrace detection of selenocyanate Enea Pagliano,∗ Kelly L. LeBlanc, and Zoltán Mester National Research Council Canada, 1200 Montreal Road, K1A 0R6, Ottawa, Ontario, Canada

E-mail: [email protected]

Abstract The recent interest in the determination of selenocyanate in wastewater systems has spurred the development of analytical methods for its determination at the ultratrace level. Since most of current procedures require complex and costly instrumental congurations, we have developed a simple and rapid gas chromatography tandem mass spectrometry (GC-MS/MS) method able to detect SeCN in water samples with a LOD of 0.1 ng/g Se. 1 mL volume of aqueous sample was buered with sodium bicarbonate and treated with triethyloxonium tetrauoroborate for conversion of the analyte into volatile EtSeCN. The derivatization yield was higher than 90% and it could tolerate concentrations of chloride or sulfate up to 2%. The EtSeCN was extracted in chloroform and could be detected in electron ionization and also in negative chemical ionization mode with a further gain in signal-to-noise ratio by a factor of two. The method was applied for the analysis of natural waters with quantitation of SeCN in the low ng/g region. The Se13C15N internal standard could be used for isotope dilution. Quantitative spike recoveries of 1 ng/g Se were obtained from seawater and riverwater and 1 1

ACS Paragon Plus Environment

Analytical Chemistry 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

ng/g Se could be quantied within a standard uncertainty of 15%.

Introduction Although selenium (Se) is an essential nutrient for human health, it is typically considered a toxin in freshwater aquatic environments where, when present at concentrations slightly above background levels, it can have devastating eects due to its teratogenicity to predatory sh and waterfowl. 1 As we continue to improve our understanding of the biogeochemical cycling of Se, allowable limits laid out by various regulatory bodies have become increasingly strict. 2 The United States Environmental Protection Agency (USEPA), for example, has updated their water quality criterion for Se to include (sh) tissue-based limits and overall lower allowable aquatic Se concentrations. 3 As the toxicological eects of Se are dependent on its chemical form, speciation analysis is relevant for understanding the health of an aquatic system and to plan and implement remediation strategies. Selenocyanate (SeCN) is one of the Se compounds emerging as an analyte of interest in wastewater analysis. SeCN is formed through the combination of elemental Se0 and free cyanide (CN), and has been observed in various types of waters in concentrations that span a broad range. Trace levels of SeCN (0.2 ppb or 1% of total dissolved Se) have been found in a Se-contaminated eutrophic river system, 4 while this species accounts for up to 10% of the Se in some ue gas desulfurization waters, 5 and is the major form of Se in wastewaters from certain gold mining operations. 6 To comply with environmental regulations, industries must treat Se-containing wastewater before it can be released back into an aquatic system. The eciency of a removal procedure is largely species-dependent. For example, traditional precipitation methods for Se removal are not eective for SeCN. 7,8 On the other hand, methods specic for direct conversion of SeCN to Se0 are not applicable for Se(IV) and Se(VI). 9 Furthermore, novel biological treatment systems for Se seem likely to be inecient for SeCN removal from waters, based

2


Page 2 of 15

Page 3 of 15 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


on current knowledge. 1012 In the context of dierent requirements for wastewater treatment, there is a need for simple analytical methods capable of measuring SeCN at low concentrations and with high degrees of accuracy. Classical methods used for SeCN quantitation are not without shortcomings. For example, selective sequential hydride generation (with atomic absorbance or uorescence detection) involves the use of operationally-dened fractions: Se(IV), Se(VI), and reduced/organic Se; 1315 the last of which includes SeCN as well as a variety of other species. 16 Additionally, the presence of certain transition metals (as would occur in many industrial euents) has been shown to cause interferences in hydride generation through their action as catalyst in the decomposition of the analyte hydrides. 17 Therefore, separation methods have been developed involving anion exchange chromatography (AEC). SeCN is retained strongly on AEC columns, meaning that careful column and eluent selection is vital to ensure optimal chromatographic peak shape. Conductivity detection is commonly employed with AEC, but is a fairly insensitive for SeCN determination. 18 Inductively-coupled plasma mass spectrometry (ICP-MS) is a much more sensitive detection system and can be coupled to AEC either directly 19 or via a hydride generation system using online prereduction. 6,16 However, detection of Se by ICP-MS is penalized by polyatomic interference and requires the use of collision cell technology to reach the environmental concentrations. This type of specialized instrumentation can be prohibitively expensive for applications such as the monitoring of SeCN in industrial wastewaters. In this study, we developed a rapid and simple method for the quantitation of SeCN in water by gas chromatography mass spectrometry (GC-MS). The novel approach is based on a simple aqueous derivatization of SeCN into EtSeCN by triethyloxonium tetrauoroborate 20,21 which can be quantied by GC-MS at the sub part-per-billion level.

3



Experimental section Reagents and materials

Potassium selenocyanate (KSeCN, 99%) and isotopically enriched potassium selenocyanate (97% KSe13C15N with 99%

13

C and 98%

15

N), sodium bicarbonate (NaHCO3, 99.5%), any-

drous sodium sulfate (Na2SO4, 99%), chloroform (CHCl3, GC grade), hexane (C6H14, HPLC grade), acetonitrile (MeCN, HPLC grade), and triethyloxonium tetrauoroborate (Et3OBF4, 97%) were purchased from Sigma-Aldrich. The purity of KSeCN and KSe13C15N was veried by ICP-MS against the NIST 3149 primary standard. As reported in S2, purity for KSeCN and KSe13C15N was 77.9% and 78.3% respectively. Ultra-pure water was generated in-house with a Thermo Scientic GenPure UV xCAD plus system (18.2 MΩ cm at 25 °C). A solution of Et3OBF4 in MeCN was prepared by dissolving 5 g of reagent in 5 mL of MeCN precooled at -20 °C. This solution was stable for more then one month when kept at -20 °C. Tap water was obtained locally in Ottawa, river water was sampled from the Rideau River on April 13th 2019, and coastal seawater was sampled from the Atlantic Ocean (Halifax, February 23rd 2017). These matrices were ltered at 0.45 µm and spiked with SeCN (1 ng/g Se) before analysis. Other water samples were prepared in-house following the cultivation of Chlorella vulgaris in a growing medium containing 10-25 µg/L Se(VI) as described in S4. The Chlorella vulgaris could convert in situ some Se(VI) into SeCN which could be detected by GC-MS/MS.

Sample preparation

A single-step aqueous derivatization was employed to convert SeCN into volatile EtSeCN. A 60 mg aliquot of NaHCO3 was transferred in a 4 mL glass vial with 1 mL of aqueous sample (or standard) at 4 °C and 50 µL of Et3OBF4:MeCN solution. The reaction was completed within one hour at room temperature. Into the same vial, 0.5 mL of CHCl3 was added and the mixture was manually shaken for a few seconds. This simple liquid/liquid extraction 4


Page 4 of 15

Page 5 of 15 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


allowed the migration of EtSeCN into CHCl3. The organic layer was transferred with a glass pipette into a 4 mL vial containing 250-300 mg anhydrous Na2SO4 and analyzed by GC-MS.

GC-MS instruments

The EtSeCN derivative could be measured by GC-MS in both electron ionization (EI) and negative chemical ionization (NCI) mode. The EI experiments were performed using an Agilent 7000 triple quadrupole GC-MS in single ion monitoring (SIM, m /z 134.96 and 136.96) and in multiple reaction monitoring modes (MRM, m /z transitions 134.96 to 106.93 and 136.96 to 108.93). NCI detection was obtained on a Hewlett-Packard 5973 single quadrupole GC-MS in SIM mode (m /z 105.9 and 107.9). The separation was carried out on a DB-5.625 column (30 m length, 0.250 mm, 0.25 µm) using standard instrumental settings as reported in S1.

Safety considerations

Triethyloxonium tetrauoroborate is a strong ethylating reagent which was handled in a fumehood wearing nitrile cloves. 21 Leftovers of the Et3OBF4:MeCN solution were hydrolyzed before disposal.

Results and discussion Derivatization and extraction

The reactivity between Et3OBF4 and SeCN was studied in aqueous medium at room temperature. As reported for other inorganic anions, 20,21 this chemistry yields the analyte ethylderivative (EtSeCN) which is a molecule suitable for gas chromatographic analysis. It was observed that EtSeCN is not suciently volatile for static headspace sampling and required liquid-liquid extraction into an organic solvent. Derivatization conditions and extraction

5


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

MRM signal EtSeCN


Page 6 of 15

1900

1800

2300

1625

1725

2250

1350

1650

2200

1075

1575

2150

800

1500 0

20

40

60

80

NaHCO3 (mg)

100

2100 20

40

60

80

100

120

30

Et3OBF4:MeCN (mL)

60

90

120

150

180

Time (min)

Figure 1: Derivatization of 1 mL SeCN– standard 50 ng/g Se: variations of the analytical signal. Left: effect of NaHCO3 with 100 µL Et3OBF4:MeCN solution (180 min reaction time). Center: effect of Et3OBF4:MeCN solution with 60 mg NaHCO3 (180 min reaction time). Right: Effect of the derivatization time with 60 mg NaHCO3 and 50 µL Et3OBF4:MeCN.

were optimized. The hydrolysis of Et3OBF4 releases H+(aq) turning the sample pH in the acid region. In this conditions, the SeCN is not stable and degrades to Se0. 4,20 For this reason, the samples were treated with NaHCO3 before derivatization. The analytical response was monitored as a function of NaHCO3. When the NaHCO3 was omitted, a 25% decrease in the signal was observed along with a RSD of 36% on the peak area of three replicates (Figure 1). When 1 mL standard was treated with 20 to 100 mg NaHCO3 and 100 µL Et3OBF4:MeCN, the signal was stable with an RSD below 5%. The response was further optimized by varying the amount of Et3OBF4:MeCN solution used for derivatization. As reported in Figure 1, no sharp variations in the signal intensity were observed when 1 mL standard with 60 mg NaHCO3 was treated with 20 to 120 µL Et3OBF4:MeCN and no signal was recorded if the reagent was omitted. A 50 µL volume of Et3OBF4:MeCN was adequate to ensure a large excess of reagent for complete derivatization. The eect of the derivatization time on the signal response was also investigated. Figure 1 shows that from 30 to 180 min, there were no signicant variations in the signal intensity. This optimization suggests that 1 mL sample should be treated with 60 mg NaHCO3 and 50 µL Et3OBF4:MeCN and left to stand for one hour at room temperature. After derivatization, the EtSeCN was extracted in 0.5 mL organic solvent. Although the

6


Page 7 of 15

volume of extracting solvent could be further reduced, a 0.5 mL ensured simple liquid transfers by common glass pipettes. Initially the extraction was performed in hexane; in a second moment it was noticed that chloroform allowed for a ve times better recovery (Figure S4). The method eciency was also evaluated. The derivatization procedure was applied to a 1 mL volume SeCN standard (750 ng/g Se). After chloroform extraction of the EtSeCN derivative, the aqueous phase was diluted and analyzed for residual Se by ICP-MS as reported in S2. Less than 6.5% of the total initial Se was found in the aqueous phase demonstrating that the overall eciency of the derivatization/extraction procedure is higher than 90%. 2.4 80

+

Et SeCN 134.9

2.2 2.0

Relative intensity

80

+

H SeCN 106.9

1.8 1.6 1.4 1.2

132.9

1.0

104.9

0.8 80

Se 80.0

0.6

80

108.9

+

CH Se 93.0

0.4

136.9

0.2 0.0

70

75

80

85

90

95 100 105 110 115 120 125 130 135 140

Mass−to−charge ratio, m/z

Figure 2: EI GC-MS mass spectrum of the EtSeCN derivative generated by aqueous reaction between Et3OBF4 and SeCN–. EI GC-MS/MS and NCI GC-MS spectra are reported in Figures S2 and S3.

180

a

1.0 ng/g

EI MRM

Signal SeCN-

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


1024

b

850

1.0 ng/g

EI SIM

c

1.0 ng/g

NCI SIM

5220

798

650

3945

110

572

450

2670

0.3 ng/g S/N = 13.5

0.3 ng/g S/N = 6.5

346

Blank

40

Blank

5.25

5.35

min

5.45

50 5.15

5.25

5.35

0.3 ng/g S/N = 23

1395

Blank

120 5.15

0.3 ng/g S/N = 23.8

250

1.0 ng/g

ICP-MS

145

75

d

5.45

5.02

min

Blank

120 5.12

5.22

min

5.32

10.5

11.5

12.5

13.5

min

Figure 3: Determination of SeCN– at detection limit. 0.3 ng/g Se was detected by: a. EI GCMS/MS on m/z transition 134.96 to 106.93 (S/N = 13.5), b. EI GC-MS on m/z 134.96 (S/N = 6.5), c. NCI GC-MS on m/z 105.9 (S/N = 23.8), and d. IC ICP-DRC-MS on m/z 77.9 (S/N = 23).

7



Page 8 of 15

Table 1: Quantitation of SeCN– in natural waters by GC-MS/MS (external calibration and isotope dilution) and by IC ICP-DRC-MS. Results are reported as ng/g Se and the standard uncertainty was evaluated by error propagation. Matrix

SeCN added

GC-MS/MS

ID GC-MS/MS

IC ICP-DRC-MS

Tapwater Tapwater Tapwater Riverwater Riverwater Riverwater Seawater Seawater Seawater Algee water Algee water Algee water Algee water

1.229 ± 0.001 1.229 ± 0.001 1.229 ± 0.001 1.188 ± 0.001 1.188 ± 0.001 1.188 ± 0.001 1.239 ± 0.001 1.239 ± 0.001 1.239 ± 0.001 n/a n/a n/a n/a

A selective gas chromatography mass spectrometry method for

Recommend Documents