Selective Identification of Organic Iodine Compounds Using Liquid

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Selective Identification of Organic Iodine Compounds Using Liquid Chromatography-High Resolution Mass Spectrometry Yijun Yang, Yue'e Peng, qing chang, Conghui Dan, Wei Guo, and Yanxin Wang Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/acs.analchem.5b03694 • Publication Date (Web): 10 Dec 2015 Downloaded from http://pubs.acs.org on December 13, 2015

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Analytical Chemistry

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structures were confirmed for four peaks, which were 3-iodo-L-tyrosine, 3,5-diiodo-L-tyrosine,

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4-iodophenol, and 2-iodobenzoic acid. This method is expected to lead the future discovery of

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new organic iodine compounds via LC-HRMS in different environmental samples, which is

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crucial for understanding the iodine biogeochemical cycling.

27 TOC

28 29

INTRODUCTION

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Iodine is an important trace element in biogeochemical and life processes, transferring among

31

different environmental media1,2 in various chemical forms3-6. The study of iodine chemical

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species is crucial for understanding the biogeochemistry cycling of iodine2,7,8, analyzing the

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relatively high toxic iodo-disinfection byproducts in drinking water9,10, and it also plays a very

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important role in the bioavailability of iodine to a host of diverse organisms, including

35

humans.11,12 Inorganic forms of iodine have been extensively studied13-15, while organic iodine is

36

little known about its composition and distribution in different samples.

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Although atomic spectrometry (i.e. high performance liquid chromatography-inductively coupled

38

plasma-mass spectrometry (HPLC-ICP-MS)) has been applied in quantifying some known organic

39

iodine compounds (i.e. thyroxine and triiodothyronine) and analyzing unknown organic iodine by

40

detecting the element iodine, it has difficulty in identifying the structure of unknown organic

41

iodine.8,16-18 In early years, unknown organic iodine compounds have been isolated and purified by 2

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classical natural products separation techniques, and identified by modern nuclear magnetic

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resonance (NMR) spectroscopy (both 1H and 13C), mass spectrometry (MS), together with infrared

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(IR) and ultra-violet (UV) absorption spectroscopy.19,20 However, the isolation procedure is

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time-consuming, and the isolated samples should be pure and at least milligram grade for NMR

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analysis.

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The evolution of high resolution mass spectrometry (HRMS) offers the prospect of the

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identification of unknown compounds in different matrices at low concentrations.21-23 In 2013, the

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Juliane group24 developed strategies for characterizing polar organic contamination in wastewater

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using HRMS and proposed a level system for the identification of small molecules via HRMS,25

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bringing chemists closer to the identification of unknown compounds.26 In 2014, N.S. Thomaidis

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et al.27 thoroughly discussed target, suspect and non-target workflows using HRMS/MS to identify

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new transformation products (TPs) of emerging pollutants. Yang group28 specially detected the

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unknown brominated disinfection byproducts (Br-DBPs) in artificial drinking water via ultrahigh

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resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) using full

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MS scan mode. The bromine- or chlorine-containing compounds were identified with the aid of

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characteristic isotopic peaks and software calculation, and confirmed by MS/MS. However, as

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iodine lacks characteristic isotopic peaks, the number of false detections would be increased when

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detecting unknown organic iodine compounds. More importantly, some organic iodine compounds

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would not be detected due to the restriction of elements assigned by the software.

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Specific, selective workflows that take into consideration the advantages of HR-MS instruments

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are still missing for unknown organic iodine compounds. The selective detection of unknown

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organic iodine compounds can be simplified if they are characterized by a functional group or 3


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heteroatom that can be selectively detected. Carbon-iodine bonds are more prone to homolytic

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fission because of the relatively weak covalent bond.29 Therefore, Putschew et al.30 reported that

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the iodine atom is characteristic for organic iodine compounds and used the negative ion (NI)

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in-source fragmentation MS for selective detection of iodinated benzene derivatives. However,

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this method is only suitable for quantification and the in-source fragmentation destroyed the

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compounds before they entered the mass spectrometer, making it difficult to selectively choose the

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compounds and determine their accurate mass. Zhang et al.31 found that unknown polar Br-DBPs

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could be selectively chosen, detected, and identified by using the precursor of a particular

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fragment ion (bromine ion m/z 79 or 81) with a precursor ion scan (PIS) method via electrospray

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ionization-triple quadrupole mass spectrometry (ESI-tqMS). This method could be extended to

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selectively identify organic iodine compounds. However, the unit mass resolution is too low to

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allow nominally isobaric ions to be mass-resolved, it may include several isobaric parent ions and

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bias the compositional interpretation of organic iodized compounds.

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Herein, we develop a method for specific, step-wise, selective identification of unknown organic

78

iodine compounds via LC-HRMS. The retention time of organic iodine compounds on liquid

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chromatography was first confirmed by MS/MS experiments. Then, the possible accurate mass of

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organic iodine in the full MS spectrum was selected for validation. After the subsequent

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confirmation and identification scheme based on LC-HRMS, ChemSpider, and reference standards,

82

the organic iodine compounds were eventually identified. The method has two major features,

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firstly, it can easily discover and locate non-target compounds containing iodine by detecting their

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specific fragment ion, I- (m/z 126.9039), allowing selective identification of unknown organic

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iodine compounds in low concentrations and complex samples. Secondly, the retention time was 4



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regarded as an important parameter in detecting organic iodine compounds and distinguishing

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them from impurities with the same mass. The method was utilized to identify organic iodine

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compounds in a seaweed sample. Through the stepwise procedure, twenty eight possible organic

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iodine peaks were discovered. Of these, the accurate mass and element composition of the

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corresponding precursor ions were identified for twelve peaks, and molecular structures were

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confirmed for four peaks, which were 3-iodo-L-tyrosine, 3,5-diiodo-L-tyrosine, 4-iodophenol and

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2-iodobenzoic acid.

93 94

EXPERIMENTAL SECTION

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Chemicals and Standards. Ultrapure water with a resistivity of 18.2 MΩ cm-1 was obtained from

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a Milli-Q ultrapure water system (Millipore, USA). LC-MS grade methanol, acetic acid, and

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acetonitrile were acquired from Fisher Scientific (USA). 3-iodo-L-tyrosine 98% (MIT),

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3,5-diiodo-L-tyrosine dihydrate 98% (DIT), 2-iodobenzoic acid, 3-iodobenzoic acid, and

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4-iodobenzoic acid were purchased from Sigma-Aldrich (Shanghai, China). 2-Iodophenol 98%,

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3-Iodophenol 99%, 4-Iodophenol 99% were purchased from J&K Scientific Ltd (Beijing, China).

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Materials and Pretreatments. Commercially available, edible dried seaweed sample (nori as red

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seaweed) was obtained from an aquatic products market in Wuhan. The seaweed sample was

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smashed, weighed (about 0.5 g), and placed into screw cap tube. After adding 25 mL methanol,

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the seaweed sample was extracted for 10 min with the assistance of an ultrasonic wave at room

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temperature. The supernatants were collected and filtered through a membrane filter with a 0.45

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µm pore size.

5


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Instrumentation. A Dionex Ultimate 3000 Series liquid chromatography combined with Q

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Exactive hybrid Quadrupole-Orbitrap mass spectrometer (Thermo Scientific, Bremen, Germany)

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was used for the experiment. The liquid chromatography is equipped with Thermo Fisher Hypersil

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Gold C18 (3.0 µm particle size, 3×150 mm). The elution solvent A was water with 0.2% acetic acid,

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and B was acetonitrile. The gradient (A: B) was 90:10 at t = 0 min, linearly switched to 60:40

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from t = 0 min to 4 min and held until t = 9 min, then linearly switched to 90:10 from t = 9 min to

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11 min and held until t = 14 min. The gradient was operated at a flow rate of 0.25 mL min-1 and a

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column temperature of 40 °C. The sample injection volume was 5 µL. Mass spectra were

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processed using the Xcalibur 2.1 software (Thermo Scientific), and both positive and negative ion

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spectra were recorded. The mass tolerance of the precursor and fragment ions was below 5 ppm.

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Software Mass Frontier 7.0 SR1 was used to provide theoretical fragments and fragmentation

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mechanisms of compounds for the prediction of fragmentation patterns. The system was operated

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with a heated electrospray ionization (HESI) source in positive and negative mode with a spray

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voltage of +3.5 and −3.2 kV, respectively. An S-lens RF level of 50%, a capillary temperature of

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300 °C, and a mass resolution of 70000 were also used. Other parameters were set as default

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values.

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Procedure for Organic Iodine Identification. A systematic procedure was established for the

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detection and identification of organic iodine compounds in the seaweed sample (Figure 1). Step 1:

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choosing a certain mass range to analyze in negative t-MS2 scan mode. Since the collision induced

126

dissociation (CID) easily dissociates iodide ions (I-) from organic iodine, the peaks of extracted

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ion chromatograms of m/z 126.9039 (I-) implied the peaks of organic iodine. Step 2: the same

128

mass range as Step 1 was analyzed in the full MS scan mode, all possible precursor ions were 6



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selected in the full MS mass spectra according to the retention time of peaks from Step 1, and

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accurate mass of each selected precursor ion was obtained. Step 3: all precursor ions selected in

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Step 2 were reconfirmed using the t-MS2 scan mode, and spectra of fragments ions were obtained.

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Only the precursor ions for which their fragments contained I- (m/z 126.9039) were used in the

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next step. Step 4: all possible element compositions of the precursor ions were calculated within 5

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ppm mass tolerance using the QualBrowser of XCalibur from accurate masses allowing the

135

elements C, H, N, O and I. The possible structures of organic iodine were acquired from

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ChemSpider database by typing their molecular formula, and further by matching retention times,

137

MS/MS data with literatures or reference standards. It is worth noting that, in practice, a full MS

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and t-MS2 combination mode was used in one step to avoid the retention time error between Step

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1 and Step 2.

140 141

Figure 1. Stepwise procedure for unknown organic iodine compounds identification

142 143

RESULTS AND DISCUSSION

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Selection of Organic Iodine Peaks. Starting without any information about the compounds

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present in the sample, the first step was the selection of potentially relevant peaks for

146

identification. Since the organic iodine fragmentations will contain iodide ions (I-) at an adequate 7


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collision energy (NCE=30 in this study), assuming that the characteristic fragment ion is selected

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as I- (m/z 126.9039), it should be possible to find organic iodine in the seaweed sample via

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LC-HRMS using the t-MS2 scan mode. In the t-MS2 scan mode, all precursor ions within the

150

selected mass range are fragmented in the collision cell. For instance, the product ion spectra of

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m/z 125~150 were obtained by setting isolation width as 25.0 m/z and inclusion list as m/z

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137.5000 in the t-MS2 scan mode. In this study, we aimed to demonstrate the procedure for the

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identification of unknown organic iodine compounds, so the mass and charge ratio (m/z) value of

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target ions was restricted between 100 and 500, all ions within m/z 100~500 were fragmented in

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sections (25 m/z width as a section), and the ions over m/z 500 were not considered. Every section

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is scanned in a single run respectively, and one scan requires 14 minutes (see Instrumentation).

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For all these 16 sections, the total time will be 16 multiply 14 minutes, that is, 224 minutes. Peaks

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in extracted ion chromatograms of m/z 126.9039 (I-) indicate the existence of organic iodine, and

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the retention times of these peaks reveal the organic iodine retention times. The numbers of peaks

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within 5 ppm mass tolerance in different sections are listed in Table 1 (the results of every two

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sections were summarized in one line), 28 peaks were selected in total for further structure

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identification. One peak (m/z 126.9039) was excluded, because this peak was verified to be iodide

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ions (I-) itself but not the organic iodine. Another peak (m/z 380.7129) corresponding to triiodide

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ion (I3-) was also excluded (see SI Figure S1).

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8



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Table 1. Results of peaks selection for unknown organic iodine compounds identification m/z

number of

number of confirmed

number of confirmed

number of confirmed

range

selected peaks

accurate masses

molecule formulas

structures

100~150

2(1)

0

0

0

150~200

1

1

1

0

200~250

3

3

3

2

250~300

4

2

2

0

300~350

3

1

1

1

350~400

5(4)

3

3

0

400~450

6

2

2

1

450~500

4

0

0

0

28(26)

12

12

4

total

167

Determination of Accurate Mass and Molecular Formula. LC-HRMS was used to acquire

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information about the accurate masses and possible elemental compositions of the selected organic

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iodine peaks using the full MS scan mode. Every precursor ion of organic iodine found in the full

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MS scan was confirmed by matching the retention time of each selected peak in the t-MS2 scan.

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Once the precursor ion was determined, an accurate mass was provided by XCalibur. It is a

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remarkable fact that some selected precursor ions were not organic iodine ions (i.e., arising from

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peaks with coincidently the same retention time, while not containing iodine). Therefore, all

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selected precursor ions should be reconfirmed using t-MS2 scan mode. In the t-MS2 scan mode,

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these selected precursor ions were fragmented by setting their masses as the target ions with a

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mass width of 0.4 Da. Only the precursor ions that contained dissociated iodine ions were 9


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reconfirmed as the likely organic iodine compounds.

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179 180

Figure 2. Workflow for compound 1 identification. (a) Extracted ion chromatograms of m/z

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126.9039 in negative t-MS2 scan mode, target parent ions were set from m/z 300.0000 to m/z

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325.0000, (b) extracted ion chromatograms of m/z 305.9629 in negative full MS scan mode, (c)

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mass spectra acquired at 6.11 min in full MS scan mode and t-MS2 scan mode (inset, (d)

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deprotonated compound 1, (e) protonated compound 1).

185 186

All possible molecular formulas for the potential organic iodine compounds were calculated

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within 5 ppm mass using Xcalibur from their accurate masses tolerance allowing the elements C,

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H, N, O and I. The plausibility of the calculated molecular formulas was evaluated by basic

189

chemical criteria32 and the ChemSpider database33 (see SI Note N1). By applying these two

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approaches, the numbers of likely molecular formulas were restricted to one or two for each

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accurate mass. Figure 2 shows the workflow of the identification of compound 1, all iodine ion’s

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peaks were extracted from the MS/MS spectrum (Figure 2a) and the one at 6.11min were selected 10



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for identifying, and a corresponding precursor ion m/z 305.9629 was extracted from full MS scan

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spectrum (Figure 2b), then, this precursor ion was fragmented to confirm that it contains the iodine

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and to help elucidating the structure (Figure 2c).

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Finally, the accurate mass of 12 organic iodine precursor ions was determined among 28 selected

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peaks, and their molecular formulas were also calculated (Table 1). Figure 3 shows extracted ion

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chromatograms of the 12 organic iodine compounds. Their mass spectra in the full MS and t-MS2

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scan modes are shown in Figure S2, determining the accurate mass of the organic iodine

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compounds and verifying that their product ions contained I- (m/z 126.9039).

201

202 203

Figure 3. Extracted ion chromatograms of organic iodine compounds in negative full MS scan

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mode. Graphs (a)-(l) correspond to compounds 1-12 listed in Table 2, respectively.

205 206

Structure Proposals and Identification. ChemSpider searching, MS/MS experiments, and

207

reference standards matching were performed to facilitate the proposal of possible structures for

208

unknown organic iodine compounds.

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For example, in the negative mode, the accurate mass of deprotonated compound 1 ([M-H-]) was

210

305.9629 and its most probable elemental composition was C9H10O3NI. Other possible elemental 11


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compositions were excluded by basic chemical criteria and the ChemSpider database as mentioned

212

above. When searching C9H10O3NI in the ChemSpider database, 45 hits were found and the top

213

candidate was 3-Iodo-L-tyrosine (MIT). The fragment ions m/z 244.9455, 248.9405, 261.9712,

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272.9397 and 290.9500 were observed in the positive MS/MS spectrum of compound 1 (Figure

215

2c), indicating the neutral loss of NH3, H2O and CO, and their fragmentation mechanisms were

216

listed in the supporting information (see SI FigureS3). Similarly, the top candidate for compound 2

217

according to ChemSpider was 3,5-Diiodo-DL-tyrosine (DIT). By matching the retention time of

218

reference standards with that of compounds 1 and 2, compounds 1 and 2 were confirmed to be

219

MIT and DIT (see SI FigureS4 and FigureS5). For compound 3, the molecular formula was

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C6H5OI, and 8 hits were on the ChemSpider. The top 3 hits were isomerides of iodophenol,

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namely 2-Iodophenol, 3-Iodophenol, and 4-Iodophenol. By comparing with reference standards,

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this compound was confirmed to be 4-Iodophenol (see SI FigureS6). Similarly, compound 4

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(C7H5O2I) was confirmed to be 2-Iodobenzoic acid (see SI FigureS7). For other eight compounds,

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we listed the three most likely candidates searching from the ChemSpider for each molecular

225

formula (see SI FigureS8). The further verification was restricted because reference standards are

226

unavailable. Organic iodine compounds without reference standards can be isolated and purify for

227

further structure elucidation by NMR analysis.

228

Table 2 gives an overview of all organic iodine compounds identified in the analyzed seaweed

229

sample. All the compounds were confirmed their accurate mass and molecular formula. Four

230

compounds of them had their structures elucidated (Figure 4), while the other eight compounds

231

were listed their possible structures from ChemSpider. For the four validated organic iodine

232

compounds, the proposed procedure offered limits of detection (S/N=3) of MIT, DIT, 12



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4-iodophenol, and 2-iodobenzoic acid were 1.0, 0.5, 0.02, and 0.1 µg L-1, respectively, according

234

to the serial dilution method. Of the four compounds, 4-Iodophenol and 2-Iodobenzoic acid have

235

not been reported in seaweed before. 2-Iodobenzoic acid has antifungal activity34, it indicates that

236

seaweed could be important materials for producing antifungal medicine. Additionally, the

237

accurate mass of organic iodine compounds is likely to be used as an indicator for understanding

238

their environmental behavior, as it is convenient to be acquired via LC-HRMS.

239

Table 2. Overview of identified organic iodine compounds in seaweed tR [min]

240

Accurate mass

Molecular Formula

CAS No. Mass tolerance

of [M-H]-

Identification

[ppm]

1

6.11

305.9629

C9H10INO3

70-78-0

0.815

3-Iodo-L-tyrosine

2

7.24

431.8600

C9H9I2NO3

66-02-4

0.376

3,5-Diiodo-DL-tyrosine

3

8.76

218.9308

C6H5OI

540-38-5

2.106

4-Iodophenol

4

11.46

246.9256

C7H5O2I

88-67-5

2.210

2-Iodobenzoic acid

5

4.55

196.9094

C3H3O2I

1.552

6 hits*

6

7.83

380.9953

C11H15IN2O5

0.347

8 hits*

7

8.25

432.9891

C14H15O6N2I / C15H11O2N6I

1.096

3 hits/ 4 hits*

8

8.26

350.9845

C10H13IN2O4

0.721

22 hits *

9

8.39

395.0105

C12H17IN2O5

0.562

3 hits *

10

8.88

234.9257

C6H5O2I

1.897

17 hits *

11

9.46

262.9211

C7H5O3I

0.039

26 hits *

12

8.05

294.9470

C8H9O4I

0.593

6 hits *

*numbers of structures when searching ChemSpider database by typing molecular formula.

241 13


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242 243

Figure 4. Structures of identified organic iodine compounds in the seaweed sample

244 245

CONCLUSIONS

246

A new method for the selective and sensitive identification of unknown organic iodine compounds

247

using LC-HRMS was developed in this study, providing valuable information about the retention

248

time, accurate mass, molecular formula and structure of organic iodine compounds. Twenty eight

249

organic iodine compounds were successfully detected and twelve of them were identified in the

250

seaweed sample. Although the structures of some organic iodine compounds are not fully

251

identified, the communication about their retention time and accurate mass in the scientific

252

community is of great importance, especially if the results are accompanied by an uncertainty or a

253

confidence level. This method could be applied in the detection and identification of unknown

254

organic iodine compounds in other environmental samples, such as water (groundwater, surface

255

water and wastewater), soil and plants. The information about the retention time, accurate mass,

256

molecular formula and structure about the identified organic iodine compounds will in turn

257

improve our understanding of the biogeochemical behavior and its fate of iodine in the

258

environment.

259 260

ASSOCIATED CONTENT

261

Supporting Information

262

Evaluation rules of calculated molecular formulas, mass spectra of identified organic iodine 14



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263

compounds, theoretical fragmentation pattern, identification with reference standards for

264

compounds1-4, and table of three mostly likely candidates for compounds 5-12. This material is

265

available free of charge via the Internet at http://pubs.acs.org.

266 267

AUTHOR INFORMATION

268

Corresponding Author

269

*(Y.Peng) Phone.:+86-27-67883998. E-mail: [email protected].

270

*(W.Guo) E-mail: [email protected]

271 272

Notes

273

The authors declare no competing financial interest.

274 275

ACKNOWLEDGMENTS

276

This research was financially supported by the National Natural Science Foundation of China (No.

277

21305130 and No.41120124003), China Geological Survey (12120113103700), and the

278

fundamental research funds for the central universities, China University of Geosciences

279

(No.CUGL140415).

280 281

REFERENCES

282

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(2) Osterc, A.; Stibilj, V.; Raspor, P. In Encyclopedia of Environmental Health, Nriagu, J. O., Ed.; 15


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