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Chromatographia (2017) 80:1475–1481 DOI 10.1007/s10337-017-3355-6

SHORT COMMUNICATION

LC–MS/MS Method for Simultaneous Determination of Monoethanol‑ and Dimethylnitramine in Aqueous Soil Extracts

Cathrine Brecke Gundersen1 · Liang Zhu1 · Sofia Lindahl1,3 · Shiyu Wang1,2 · Steven Ray Wilson1 · Elsa Lundanes1 

Received: 21 March 2017 / Revised: 13 June 2017 / Accepted: 5 July 2017 / Published online: 12 July 2017 © Springer-Verlag GmbH Germany 2017

signal decrease over time when measuring DMA-nitramine alone, the use of polarity switching was beneficial, in addi-tion to frequent cleaning of the ion transfer capillary. The validated method can be used to determine nitramines in aqueous soil extracts, which is of importance as soil sorp-tion is a determinant of the compounds’ environmental fate.

Keywords LC–MS/MS · Carbon capture · Nitramine · Soil sorption

Introduction

CO2 capture technology is anticipated to contribute with 13% of cumulative carbon emission reductions to limit global warming by 2 °C (compared to pre-industrial values) [1]. Amine-based post-combustion CO2 capture (PCCC) is currently the most promising method for CO2 capture [2]. Regrettably, by-products of amine-based PCCC technol-ogy include nitramines (RR`NNO2), which are potentially carcinogenic and mutagenic [3]. Formation occurs atmos-pherically from volatile amines escaping the PCCC plant [4]. Key nitramines that can form from the “benchmark” solvent, monoethanolamine [2], include monoethanol (MEA)- and dimethyl (DMA)-nitramines (Fig. 1) [4–6]. Once formed, the nitramines are stable towards decay [4] and are expected to reach the ground by precipitation. Two individual modelling projects predicted the final nitramine concentration in sur-face waters to fall well below a set safety level of 4 ng L−1 [3, 7, 8]. There is, however, concern of nitramine accumulation in the terrestrial compartment via soil sorption, as MEA- and DMA-nitramines may possibly sorb to organic-rich soils [9]. Soil sorption is commonly determined indirectly by meas-uring potential loss of compound from the aqueous phase (hereafter called the aqueous soil extract) after equilibrium

Abstract Monoethanol (MEA)- and dimethyl (DMA)-nitramines are by-products of amine-based post-combus-tion CO2 capture (PCCC) processing, and are potentially carcinogenic. The compounds are challenging to measure, also with LC–tandem mass spectrometry (MS/MS), attrib-uted to their high polarity and extreme proneness to matrix effects. In contrast to related methods, the MEA- and DMA-nitramines were simultaneously determined in aqueous soil extracts in less than 10 min using a 1 mm × 150 mm Atlantis® T3 (3 µm) C18 column. A mobile phase of water/methanol (90/10, v/v) and 2  mM acetic acid allowed for electrospray ionization (ESI) of both analytes [in contrast to the need for both ESI and atmospheric pressure chemi-cal ionization (APCI) in related methods]. Polarity switch-ing electrospray was required for the simultaneous detec-tion of the analytes, and concentration limits of detection (LODs) in the aqueous soil extracts were ≤5.0 µg L−1 using an injection volume of 20 μL and no prior enrichment step. Matrix effects were compensated for using isotope-labelled internal standards, and satisfactory precision and linearity were obtained (within- and between-day precisions ≤19%, r2 ≥ 0.995 for concentrations up to 500.0 µg L−1). To avoid

* Cathrine Brecke Gundersen cathribg@kjemi.uio.no

* Liang Zhu liang.zhu@kjemi.uio.no

1 Department of Chemistry, University of Oslo, Blindern, P.O. Box 1033, 0315 Oslo, Norway

2 School of Pharmacy, Fudan University, Shanghai 201203, China

3 Present Address: Borregaard, P.O. Box 162, 1701 Sarpsborg, Norway

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has been established between the compound-containing aqueous phase and the soil. To study the soil sorption poten-tial of nitramines, a fast and reliable analytical method is thus needed to measure these compounds in aqueous soil extract. Key points to consider regarding measurement of MEA- and DMA-nitramines are their properties; small size and high polarity that create considerable analytical challenges. Meth-ods for larger nitramines exist (i.e. associated with the use of ammunition and explosives); however, no validated method for the determination of these small polar nitramines can be found in peer-reviewed literature. Moreover, the determina-tion of MEA- and DMA-nitramines have required separate methods [10–13], e.g. featuring graphite carbon-based LC and atmospheric pressure chemical ionization (APCI)-MS for MEA-nitramine and conventional reversed-phase LC and electrospray ionization (ESI)-MS for DMA-nitramine, respectively [11]. In addition, some methods rely on exten-sive sample preparation.

The aim of the present study was to establish and vali-date a fast and reliable LC–MS/MS method, without a prior enrichment step, for the simultaneous determination of MEA- and DMA-nitramines in challenging samples of aqueous soil extract to enable soil sorption assessment. Such samples are particularly challenging with regard to matrix effects and we address the need for isotope-labelled internal standard for each analyte.

Experimental

Nitramine Standards and Other Chemicals

3-Nitro-oxazolidin-2-one (CAS RN 85430-60-0) and DMA-nitramine (CAS RN 4164-28-7), >99% purity, standards were synthesized at the Norwegian Univer-sity of Life Sciences, Ås, Norway. MEA-nitramine (CAS RN 74386-82-6) was formed by hydrolyzation of 3-nitro-oxazolidin-2-one for ≥24  h [14]. Standard solutions of 2-nitroaminoethanol-1,1,2,2-d4 (d4-MEA-nitramine) and d6-DMA-nitramine, both in methanol and at a concen-tration of 1  mg  mL−1, were purchased from Chiron AS (Trondheim, Norway). Type II water (10–15  MΩ cm at 25 °C) was produced by a Purelabs Option-R, Elga-veolia

system (Paris, France). Solutions were made in type II water unless otherwise stated. Methanol (HiPerSolv Chro-manorm, HPLC grade) was purchased from VWR (Pro-labo, Fontenay-sous-Bois, France), LC–MS grade water was from Fisher Scientific (Waltham, MA, USA), and acetic acid (eluent additive for LC–MS) was from Fluka, Sigma-Aldrich (Germany).

Aqueous Soil Extract Matrix

To mimic the sample matrix in real samples from a soil sorption experiment, a solution of blank aqueous soil extract was produced, from an organic-rich soil (33%, w/w) in accordance with OECD Guideline 106, and used to make nitramine standard and validation solutions (specified in text). In short, air-dried soil was mixed with an aqueous solution (0.01 M CaCl2 and 0.1% NaN3) at 10 L kg−1 liquid-to-soil ratio. The soil sample slurry was rotated in an end-over-end tumbler (made in house) at 9 rpm for 24 ± 0.5 h followed by centrifugation of the supernatant (20,000g for 30 min, Heraeus™ Multifuge X3R, Thermo Scientific).

Standard Solutions

Individual stock solutions of 500.0  mg  L−1 MEA- and DMA-nitramines were prepared by dissolving appropriate amounts of 3-nitro-oxazolidin-2-one and DMA-nitramine. Individual stock solutions of d4-MEA-nitramine and d6-DMA-nitramine were prepared to 40 mg L−1. From the nitramine analyte stock solutions appropriate working solu-tions (at 10 and 1250 µg L−1) were prepared in water and in aqueous soil extract. An IS working solution was prepared by diluting d4-MEA-nitramine and d6-DMA-nitramines in water to a final concentration of 1250 µg L−1.

Validation Solutions

Aqueous (type II water) validation solutions (1  mL) were prepared with concentrations 15.0 and 10.0 µg L−1 (MEA- and DMA-nitramine, respectively)  =  low level (n  =  6), 125.0 µg L−1 (n = 1), 250.0 µg L−1 = medium level (n = 6), 375.0 µg L−1 (n = 1), and 500.0 µg L−1 = high level (n = 6). To these validation solutions 100 µL of deuterated IS work-ing solution was added to a final concentration of 125 μg L−1 of each IS. The concentration of IS was set in the lower con-centration range due to the cost of the standards.

Aqueous soil extract validation solutions (1  mL) were prepared with concentrations 15.0 and 10.0 µg L−1 (MEA- and DMA-nitramine, respectively) (n  =  1), 125.0  µg  L−1 (n = 1), 250.0 µg L−1 (n = 1), 375.0 µg L−1 (n = 1), and 500.0 µg L−1 (n = 1). To these validation solutions 100 µL of deuterated IS working solution was added to a final concentration of 125  μg  L−1 of each. Additionally, five

Fig. 1 The molecular structure of MEA- (left) and DMA-nitramines (right). MEA-nitramine has a pKa value of 6.2  ±  0.1 (https://sci-finder.cas.org)

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replicates of a solution containing 125.0 µg L−1 without IS were prepared.

Matrix Effects

Matrix effect indicates how much of the analyte signal is lost (or increased) because of ion suppression (or enhance-ment) caused by other components in the sample than the analyte(s). Investigation of matrix effect was carried out by comparing the signal obtained from five different concen-tration level validation solutions prepared in aqueous soil extract to those from identical concentration level solutions made by dilution in type II water. Matrix effect was quanti-fied as the relative difference in the linear regression slope value from the aforementioned two types of solutions:

Stability

The stability of the nitramine analytes in the aqueous soil extract was investigated over a 5-day period. This was done by placing five replicates of a 125.0 µg L−1 solution on the lab bench at room temperature. The replicates were analysed each successive day by first adding IS to a final concentration of 125.0 µg L−1. The signals were compared with the signals obtained from a sixth 125.0 µg L−1 solu-tion analysed at day 0.

LC–MS Instrumentation and Conditions

The LC–MS/MS equipment consisted of a Dionex Ultimate 3000 RS LC system (Thermo Scientific, USA) connected to a TSQ Vantage™ triple quadrupole mass spectrom-eter (Thermo Scientific) with either heated ESI or APCI

(1)

Matrix effect (%) =Slope in aqueous soil matrix

Slope in type II water matrix× 100.

interface. For data acquisition and processing Xcalibur ver-sion 2.2 was used.

Elution was carried out isocratically on a 1  mm ID  ×  150  mm Atlantis® T3 (3  µm) C18 column from Waters (Milford, MS, USA), at a flow rate of 50 µL min−1. The mobile phase composition was water/methanol (90/10, v/v) with 2 mM acetic acid. PEEK tub-ing (1/16″ × 0.0025″, Upchurch Scientific) was used for connections. The injection volume was 20  µL. The col-umn was operated at room temperature while the autosa-mpler was set at 4  °C. The total run time was set to 10 min.

The mass spectrometer was operated in polarity switch-ing mode with a cycle time of 0.3  s. For MEA-nitramine a negative voltage of 2500  V was used, while for DMA-nitramine a positive voltage of 3500  V was applied. The capillary temperature was set to 350 °C while the vaporizer temperature was 325 °C. The Q1 peak width was set to 0.7 full width at half maximum.

The monitored MS/MS transitions for quantification and confirmation of the nitramines are listed in Table  1. The optimized selected reaction monitoring collision energy (a.u.) and S-lens (a.u.) were 27 and 36 for MEA-nitramine and 11 and 33 for DMA-nitramine, respectively. The nitro-gen sheath gas pressure was set to 40 a.u. while the auxil-iary gas pressure was set to 10 a.u.

Results and Discussion

Method Development

Detection

Both ESI (±) and APCI (±) were tested for their capability to provide sufficient MS signal (direct infusion studies), as

Table 1 Quantifier and qualifier MS/MS transitions of MEA- and DMA-nitramines as well as d4-MEA- and d6-DMA-nitramines

Nitramine Mode of ionization Parent ion (m/z) Product ion (m/z) Comment

MEA-nitramine Negative 105.1 43.1 Qualifier46.0 Quantifier59.9 Qualifier61.0 Qualifier

DMA-nitramine Positive 91.0 47.2 Qualifier59.2 Qualifier74.2 Quantifier

D4-MEA-nitramine Negative 109.1 46.0 Quantifier65.0 Qualifier

D6-DMA-nitramine Positive 97.1 47.2 Qualifier59.2 Qualifier65.2 Qualifier80.2 Quantifier

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both ESI and APCI have been used by others [10]. When using a TSQ Vantage™ MS, ESI provided higher signal/noise ratio for both nitramines. To include both nitramines in one method, polarity switching was needed, using negative mode (ESI−) for the primary nitramine (MEA-nitramine) and positive mode (ESI+) for the secondary nitramine (DMA-nitramine). The effect of mobile phase composition on the ionization efficiency was investigated by combining direct infusion of the aqueous nitramine standards and mobile phase using a T piece. For both ESI modes, methanol provided a far better signal/noise ratio compared to acetonitrile. An acidic mobile phase was favoured regarding DMA-nitramine detection, and thus an acidic mobile phase was chosen despite a reduced (ESI−) MS signal of the MEA-nitramine. Acetic acid was found to have the least negative impact on MEA-nitramine signal among the additives tested (e.g. formic acid and ammonia).

Selected reaction monitoring mode was chosen for selective and sensitive detection of the analytes. The tran-sitions (m/z) for the parent ion and the product ion of the nitramines, including the deuterated standards, are listed in Table  1. MEA-nitramine tends to produce abundant NO2

− (m/z = 46) upon the N–N bond cleavage. On the con-trary, protonated DMA-nitramine tends to eject an amino group (17  Da) to produce the fragment peak at m/z 74. Additionally, it is worth noting that for d4-MEA-nitramine the m/z 109.1 → 46.0 transition is the sum of two different product ions (NO2

− and OCDCD2−).

Chromatography

Even though the small nitramine analytes are of high polarity, reversed phase chromatography was chosen due to its general good chromatographic performance. Hydro-philic interaction liquid chromatography (HILIC) was considered, but was not chosen because the acetonitrile mobile phase used in HILIC was not compatible with nit-ramine ESI–MS detection, and because highly aqueous samples cannot be directly injected on to a HILIC column. Reversed-phase columns from different manufacturers were tested in preliminary experiments, and the Atlantis® T3 (3 µm) C18 column, which has a relatively high carbon load (14%) and is compatible with a high water content mobile phase, was chosen. This stationary phase has also been reportedly used by others for nitramine determination [13], although with a larger column inner diameter than in the present study. Since electrospray is a concentration-sensitive ionization technique, it is beneficial to use narrow columns for achieving high sensitivity and, therefore, a 1-mm-inner diameter microbore column was chosen. Due to the high polarity of the analytes and corresponding ISs, a high content of water in the mobile phase was needed to provide sufficient retention to minimise interference from

more polar matrix components. With 10% methanol in the mobile phase, up to 20 µL of an aqueous sample could be injected without significant band broadening on the 1-mm column. The MS run time was set to 10.0 min although all nitramines were eluted in less than 6.0 min. This allowed for sufficient flushing of the system between consecutive injections.

Challenges

Herein a few challenges during the method development are presented to serve as guidance to readers when estab-lishing LC–MS/MS methods for determination of nit-ramines. Small nitramines, especially those with a hydroxyl group, tend to adsorb to stainless steel tubing walls in gas phase (unpublished work). Even though no obvious carryo-ver effect was observed between injections in the present study with aqueous mobile phase (“Validation”), frequent cleaning of the ESI needle and the ion transfer capillary, both made of stainless steel, was highly necessary to keep the background signal at a minimum.

Another observation while measuring nitramines in aqueous soil extract was the loss of signal for DMA-nit-ramine during measurements in a long sequence. Quite interestingly, this observation only applied to DMA-nit-ramine being continuously measured in positive mode, without the application of electrospray polarity switch. This is most likely attributed to charging of the instrument [15], or more specifically, the ion transfer capillary at the atmos-pheric entrance of the mass spectrometer. An easy mean to avoid this effect is to include a short polarity switch to negative electrospray mode when setting up a method for measuring specific nitramines in aqueous samples under positive ESI mode only. Frequent cleaning of the ion trans-fer capillary was also performed to further avoid signal loss of DMA-nitramine.

Validation

The method was validated in terms of selectivity, matrix effect, concentration LOQ and LOD, linearity range, preci-sion (within-day and between-day), carryover, and stability of the analytes.

The selectivity of the method was confirmed by compar-ing the signal obtained from a sample of blank aqueous soil extract with those from a sample of fortified aqueous soil extract validation solution at the low concentration level (Fig. 2). The blank sample did not show signal at the MS/MS transition in Table 1 for any of the nitramines, demon-strating good selectivity for the method. Few other studies [11, 12] have used MS/MS for detection.

Matrix effect (Eq. 1) was observed when the response of the analytes from an aqueous soil extract validation

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solution was compared with that from an aqueous vali-dation solution. This was especially evident for MEA-nitramine (Fig.  3a). Different types of soils (e.g. min-eral- or organic rich) gave quite different values of the linear regression slope (results not shown) and the one shown here provided among the highest matrix effect. Normalizing the analyte signal to that of the corre-sponding IS corrected for the matrix effect (Fig.  3b). According to Eq.  (1), the linear regression slope value increased from 49 to 102% for MEA-nitramine (Fig. 3a, b), and decreased from 113 to 102% for DMA-nitramine (Fig.  3c, d). The issue of ion suppression/enhancement with regard to nitramine determination has not previ-ously been described. The mistake of not correcting for such severe signal loss as observed for MEA-nitramine will, in the case of soil sorption assessment, result in a false high soil sorption result. Using isotope-labelled IS for each analyte the reliability of the method is ensured. Previously, an isotope-labelled IS has been used only for MEA-nitramine [11]. Alternative ways of compensat-ing for matrix effects include the use of matrix-matched calibration solutions or the method of standard addi-tion, but arguably more labour-intensive compared to correction by IS when samples from different soil types are to be analysed. Due to the large variation in compo-sition of various aqueous soil extracts and the fact that

the IS compensated for matrix effects, aqueous (type II water) calibration solutions (with IS) can be used, and the method has been validated with regard to this.

The limit of quantification (LOQ) was defined as the lowest concentration that provided a signal with S/N >10 and with a precision <20% (n = 6). The LOQ in aqueous soil extract was found to be 15.0 and 10.0 µg L−1 for MEA- and DMA-nitramine, respectively. (S/N was 47 and 30 for MEA- and DMA-nitramine, respectively, and with a RSD of 16% for both nitramines). Based on these values, cor-responding LODs were estimated by extrapolation to be 5.0 and 3.3 µg L−1 for MEA- and the DMA-nitramine, respec-tively (S/N = 3). The LOQs, obtained with a single method and without any sample enrichment step, are within the same range as those reported in the open literature using nitramine individually optimized methods and some using enrichment steps (0.1–120 µg L−1) [11–13].

Thus, this method is applicable for determining the soil sorption potential of MEA- and DMA-nitramines at expo-sure concentrations down to ~30  µg  L−1, considering a highly conservative loss of 50% nitramines to the soil.

The method was linear over a concentration range from 15.0 and 10.0 µg L−1 (MEA- and DMA-nitramine, respec-tively) to 500.0 µg L−1 in aqueous soil extract. The corre-lation coefficient, r2 was estimated to be 0.995 for MEA-nitramine and 1.00 for DMA-nitramine.

Fig. 2 Chromatograms of blank aqueous soil extract (red line), and fortified aqueous soil extract validation solution at low concentra-tion level (black line) displaying the transitions of a MEA-nitramine (m/z = 46.0), b DMA-nitramine (m/z = 74.2), c d4-MEA-nitramine

(m/z = 46.0), and d d6-DMA-nitramine (m/z = 80.2). The separation was performed on a 1 mm × 150 mm Atlantis T3 C18 column with water/methanol (90/10, v/v) and 2 mM acetic acid as mobile phase at a flow rate of 50 µL min−1. The injection volume was 20 µL

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The within-day precision of the method was determined by analysing six replicates of aqueous (type II water) vali-dation solutions at the low (15 and 10  µg  L−1 for MEA- and DMA-nitramine, respectively), medium (250 µg L−1), and high (500 µg L−1) concentration levels. The between-day precision was determined by analysing one replicate of each low, medium, and high aqueous (type II water) vali-dation solution concentration level during six subsequent days. Both the within-day and between-day precisions were satisfactory (≤19%) for both nitramines.

No carryover was observed, neither for the two ana-lyte nitramines nor for their corresponding ISs. This was demonstrated by analysing a blank sample following the analysis of the highest concentration validation solution (500 µg L−1). No detectable peaks were observed at the m/z transitions at the retention times of interest.

The stability of both MEA- and DMA-nitramines in the aqueous soil extract sample matrix was investigated over a 5-day period. IS was added to the validation solutions just prior to analysis (125 μg L−1). No significant degradation of any of the two nitramine analytes was observed under the applicable conditions. Bias from day 0 ranged from 92 to 104% for MEA-nitramine and from 95 to 114% for DMA-nitramine.

Concluding Remarks

A fast method for simultaneous determination of MEA- and DMA-nitramines in aqueous soil extract has been developed and validated. The use of isotope-labelled ISs corrected for matrix effects ensured a reliable method. Determination of μg  L−1 concentrations of MEA- and DMA-nitramines in 20  µL of aqueous soil extracts can be performed within 10 min without any enrichment step using a 1-mm-inner diameter column. This can allow for analysis of soil sorption exposure concentration down to 30 µg L−1 for soils of varying organic content. Good selec-tivity was achieved on a conventional C18 reversed-phase column with a high content of water in the mobile phase, and using polarity switching ESI–MS/MS detection. The established method should be readily applicable also to other types of aqueous environmental matrices (e.g. lake water, etc.).

Acknowledgements This research was funded by VISTA—a basic research programme funded by Statoil, conducted in close collabora-tion with the Norwegian Academy of Science and Letters. This work has, moreover, been carried out during the tenure of a Ph.D. schol-arship (first author) at the Department of Chemistry, University of Oslo, Norway, funded by the Norwegian Ministry of Education and

Fig. 3 a Peak area of MEA-nitramine, b peak area ratio of MEA-nitramine/IS (d4-MEA-nitramine), c peak area of DMA-nitramine, and d peak area ratio of DMA-nitramine/IS (d6-DMA-nitramine) as a

function of nitramine concentration in type II water (white fill) and in aqueous soil extract (black fill)

1481LC–MS/MS Method for Simultaneous Determination of Monoethanol‑ and Dimethylnitramine in Aqueous…

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Research. The research is part of a project entitled amine research and monitoring (ARM), funded by Technology Centre Mongstad and the University of Oslo. We would like to acknowledge the exchange pro-gram at the Fudan University. A special thanks is extended to pro-fessor Yngve Stenstrøm at the Norwegian University of Lifesciences, Ås, Norway, for synthesizing the nitramine analyte standards, and to Cecilie Mathiesen at the Centre for Integrative Microbial Evolution (CIME), University of Oslo, Norway for kindly giving access to their centrifuge. Lena Foseid is thanked for conducting some of the pre-liminary experiments.

Compliance with Ethical Standards

Funding This study was funded by VISTA—a basic research pro-gramme funded by Statoil (Project Number 6160). The authors declare that they have no conflict of interest. This article does not contain any studies with human participants or animals performed by any of the authors.

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