Mutual interference between 3,6-dichlorinated carbazole and p,p′-dichlorodiphenyltrichloroethane in gas chromatography mass spectrometry analysis

doi:10.21203/rs.3.rs-2954940/v1

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Mutual interference between 3,6-dichlorinated carbazole and p,p′-dichlorodiphenyltrichloroethane in gas chromatography mass spectrometry analysis

https://doi.org/10.21203/rs.3.rs-2954940/v1

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The widespread contamination of environment by polyhalogenated carbazoles (PHCZs) has been increasingly observed during the past decade. Among numerous PHCZ congeners, 3,6-dichlorocarbazole (36-CCZ) is often among the most frequently detected with higher concentrations. Although the environmental level of legacy pesticidep,p′-dichlorodiphenyltrichloroethane (p,p′-DDT) has been declining, it continues to be ubiquitously detected. These two compounds were found to interfere with each other during analyses using gas chromatographic (GC) couples with single- or triple-quadrupole low resolution mass spectrometry (MS or MS/MS). The base peak in mass spectra was that of m/z 235 for both compounds. In MS/MS with multiple reaction monitoring (MRM), the same transitions (235®200 and 235®165) were often used. Under the same GC operating conditions, one of the two GC columns used in this work did not resolve the two compounds at baseline. Pre-treatment using cleanup column chromatography can fractionate the sample extract, with the two compounds separated in different fractions before instrumental analyses. Reversed phase HPLC columns also work for resolving 36-CCZ and p,p′-DDT. But possible overlaps in GC retention and similarity in MS spectra might have caused data inaccuracy for 36-CCZ as well as DDT in some studies published to date, and steps to avoid the interference should be taken into quality control protocols in future research.

6-dichlorinated carbazoles

p′-DDT

GC-MS

GC-MS/MS

Since the revealing of large quantities of polyhalogenated carbazoles (PHCZs) in the sediment of Lake Michigan (Guo et al., 2014a, 2017; Zhu & Hites, 2005), research on environmental PHCZs has been steadily increasing. Among investigated congeners, 3,6-dichlorocarbazole (36-CCZ) is often among the most frequently detected with higher concentrations compared with others. For example, in the fish from four out of the five Great Lakes of North America, 36-CCZ was reported to contribute > 75% to the sum of 11 PHCZs (Wu et al., 2018). 36-CCZ is also among the three PHCZs with highest loads in sediment of Lake Erie (Li et al., 2022a). In soil samples from Germany, 36-CCZ was found to be as high as 68 ng/g (Mumbo et al., 2016). In sediment of a bay in the East China Sea, 36-CCZ reportedly contributed 83% to the sum of 11 PHCZs (Qiu et al., 2019). 36-CCZ is also the dominant PHCZ congener in drinking water from a populous city in central China (Wang et al., 2021).

Single dimensional high performance gas chromatography (GC) coupled with a single (MS) or triple (MS/MS) quadrupole mass spectrometry has been so far the dominant instrument used in the analysis of PHCZs. It includes GC equipped with a capillary column and coupled to MS or MS/MS which uses electron impact (EI) or electron capture negative (ECNI) ionization with selected ion monitoring (SIM) or multiple reaction monitoring (MRM). In their surveillance of pesticides in soils of Bavaria, Germany, Reischl et al. (2005) found a peak which was superimposed with that of p,p′-DDT on the chromatogram, and identified it as 36-CCZ using MS. Later, Zhou et al. (2019a) investigated the interference in PHCZ analyses by the presence of DDX (DDT, DDE, and DDD) as well as perfluorokerosene. Given the similarities between 36-CCZ and p,p′-DDT in the mass-to-charge ratio (m/z) of their base peaks as well as commonly used mass transitions in MRM data acquisition, their mutual interferences are very likely if their separation by GC is unsatisfactory. However, these phenomena have not been sufficiently recognized and taken into consideration in most published studies to date.

The objective of this work is to examine the potential mutual interference of 36-CCZ and p,p′-DDT in the analysis of environmental samples using high resolution GC coupled with low resolution MS and MS/MS, and to make suggestions to enhance the reliability of the analyses for these two important pollutant chemicals in environmental samples.

Chemical standard of 36-CCZ was obtained from Wellington Laboratories (Guelph, Ontario, Canada), and that of p,p′-DDT was purchased from AccuStandards (New Haven, Connecticut, USA). Purities of 36-CCZ and p,p′-DDT were higher than 99%.

Shimadzu GC-MS/MS-TQ8040 was used to examine the separation and the identification of the two target compounds. The GC inlet temperature was set at 280℃ and injection was made in splitless mode. Two GC columns were separately used, including a Shimadzu SH-I-5MS and an Agilent DB-5MS, both were 30 m long × 0.25 mm i.d. × 0.25 µm film thickness. The carrier gas was high-purity helium at 1.2 mL/min. Three oven temperature programs were compared, including (A) initial temperature of 80℃ was kept for 3 min; then increased at 10℃/min to 150℃; then it was increased again at 5℃/min to 300℃ with lasted for 10min; (B) initial temperature was set at 40℃, and it was immediately increased at 10℃/min to 300℃ which as kept for 10 min.; and (C) initial temperature was set at 55℃, and it was immediately increased at 10℃/min to 215℃ which was kept for 4 min; then it was increased again at 5℃/min to 300℃ with lasted for 10 min. The MS/MS was equipped with an electron impact (EI) ion source with temperature set at 230 °C. The temperatures of both quadrupole 1 and quadrupole 2 were set at 150 ℃. Argon of 99.999% purity was used as the collision gas and the pressure in the collision cell was set at 4×10^− 3 mbar. The GCMS-TQ8040 NX software was used for instrument control, data acquisition and processing.

An Agilent 1260 Infinity II HPLC equipped with a diode array detector (DAD) was used to examine the separation of 36-CCZ and p,p′-DDT by liquid chromatography. The LC column was Hypersil ODS C₁₈ (25 cm long, 250 mm id, 5 µm particles), which was kept at 25°C. The mobile phase was mixture of 80:20 (v:v) acetonitrile:water, with flow rate of 1 mL/min. The DAD was set at 290 nm.

Similarities in mass spectra of 36-CCZ and p,p′-DDT

Figure 1a shows that, in EI-MS spectra, the base peak for both 36-CCZ and p,p′-DDT is that with mass-to-charge ratio (m/z) of 235. These match the MS spectrum of 36-CCZ we obtained years earlier using another GC-MS/MS system as well as the spectrum of p,p′-DDT from the NIST Chemistry WebBook (see Figure S1 of the Supplementary Material). For DDT, the molecular ion is rarely used, as it is readily fragmented in MS under the commonly used EI conditions (e.g. a nominal electron energy of 70 eV). Thus, the cluster with nominal mass of 235 dominates the mass spectrum of DDT, representing [M-CCl₃]⁺, which shows loss of trichoromethyl group from the molecular ion. For 36-CCZ, the base peak of m/z 235 leads its molecular ion cluster. This coincidence invalidates the use of the base peak to separate the two compounds. Unfortunately, the two next significant ions are also very similar between 36-CCZ and DDT, with m/z values around 200 and 165 (Figure 1a). This makes it impractical to select these ions to distinguish the two compounds in SIM. We noticed that the intensity ratio of m/z 200 to m/z 165 is much higher for 36-CCZ than p,p’-DDT (Figure 1a), suggesting that 36-CCZ tends to loss one chlorine to form [M-Cl]⁺ fragment while the parent DDT fragment [M-CCl₃]⁺ most likely losses both remaining para-chlorines to generate diphenylethane ion. However, in the case of difficult GC separations (see next section), such a difference may not facilitate quantitative analysis, although may hint on which compound might dominate the observed peak on a chromatogram. All other ions are so low that using them would greatly sacrifice detection sensitivity. In addition, the structures of these small ions may not be known with certainty. Nevertheless, discernable molecular ion M⁺ cluster (m/z 352 – 363) as well as the [M-Cl]⁺ (m/z 317 – 326) and [M-2Cl]⁺ (m/z 282 – 290) isotope clusters (Figure S1, lower panel) would indicate the presence of p,p′-DDT in the observed peak.

Tandem MS/MS greatly enhances the detection sensitivity over single quadrupole MS and was found to have detection limits close to those using high-resolution MS (HRMS) (Sun et al., 2022; Zhou et al., 2019a). However, MS/MS with automated MRM is unable to distinctively resolve 36-CCZ and p,p′-DDT. Figure 1b shows the similarity in EI-MS/MS total ion chromatograms obtained in this work. The major transitions from the parent ion of m/z 235 produced ions with m/z values around 200 and 164 for both 36-CCZ and p,p′-DDT. The transitions 235®200 and 235®164 represent loss of one and two, respectively, of the phenyl chlorines from the parent ion for both compounds. Our finding supports the statement by Zhou et al. (2019a) that “even though EI-MRM mode has strong anti-interference ability, p,p′-DDT influenced the identification and quantification of 36-CCZ”.

Given the difference in elemental composition between 36-CCZ molecular ion [M]⁺ (C₁₂H₇NCl₂) and DDT’s moiety ion [M-CCl₃]⁺ (C₁₃H₉Cl₂), their exact masses differ. The former has a mass of 234.99555, and the latter 235.00814, as we calculated using online Mass Spec Tools (Isotope distribution calculator and mass spec plotter, 1996). Several studies have used HRMS coupled to GC or LC for PHCZ analyses (Guo et al., 2014; Li et al., 2023; Zhou et al., 2019a, 2021). Using an EI-HRMS with a resolving power 10,000, Zhou et al. (2019a) successfully resolved 36-CCZ and p,p′-DDT. The second most abundant ion for 36-CCZ (M+2, m/z = 236.9926) was found to be affected by perfluorokerosene (PFK), which was used for MS calibration; therefore, the less sensitive ion (M+4, m/z = 238.9896) had to be used as the confirmation ion (Zhou et al., 2019a).

GC separations of p,p′-DDT and 36-CCZ

Co-elution is common on one-dimensional GC; for example, a number of PHCZs had similar (<0.05 min) retention times with some halogenated flame retardants from a 30 m DB-5MS column (Guo et al., 2014). Among PHCZ congeners, 2,7-dibromocarbazole (27-BCZ) and 3,6-dibromocarbazole (36-BCZ) have very close retention times (Zhou et al., 2019a) . Such coelutions are not problematic in most cases where the coeluting compounds do not share their characteristic m/z or transitions in MS or MS/MS analyses. However, in the cases of p,p′-DDT vs. 36-CCZ and 27-BCZ vs. 36-BCZ, efforts are needed to separate the coeluting pair as much as possible before they reach MS or MS/MS, for the reasons discussed in the previous section.

Figure 2a shows that p,p′-DDT and 36-CCZ cannot be baseline-separated on the SHI-I-5MS column of this work with the oven programs A, B, or C. Reischl et al. (2005), who first reported the overlap of the two compounds, used Agilent HP-5 column of 25 m long ´ 0.2 mm i.d. ´ 0.32 mm film thickness. Zhou et al. (2019a) investigated the issue in more details using an Agilent DB-5MS column of 30 m long ´ 0.25 mm i.d. ´ 0.1 mm film thickness. However, in this work, 36-CCZ and p,p′-DDT were sufficiently separated on the DB-5MS column (Figure 2b). Additionally, in our early research on PHCZs, the GC retentions of 36-CCZ and p,p′-DDT were found to be >2 min apart with the Restek Rxi-XLB column of 30 m long ´ 0.25 mm i.d. ´ 0.1 mm film thickness (Figure S2). Another study, which analyzed both 36-CCZ and DDTs in soil samples but did not report separation issues, used a Restek Rtx-Dioxin2 column (40 m ´ 0.18 mm i.d. ´ 0.18 mm film thickness) as well as cool injection (Mumbo et al., 2016). All the columns mentioned above are commonly used non-polar capillary columns with stationary phases of either 5%-phenyl / 95%-methylpolysiloxane or 5%-diphenyl / 95% dimethylpolysiloxane. The column dimensions (length, inner diameter, and film thickness) also affect separations; and the efficiency of a specific column may decrease with extended used. Data obtained to date are insufficient to discuss how these parameters affect the separation between 36-CCZ and p,p′-DDT.

With a selected column, changing oven temperature program is a common way to alter the relative retention time and achieve better separations. It is not uncommon that the elution orders of compounds are changed when oven temperature program is altered. In Figure 2a-B with the SHI-I-5MS column, the peaks of 36-CCZ and p,p′-DDT are partially resolved, with a resolution of about 1.3, which was estimated from the difference between the two retention times by the average width at half peak heights. This resolution makes separate quantitation possible for the two target compounds. The observations to date suggest that complete separation of 36-CCZ and p,p′-DDT is possible with a carefully selected column and fine-tuned GC operation parameters.

In addition to oven temperature programming, the injection mode as well as inlet temperature and/or pressure programming have not been sufficiently explored for PHCZ analysis. Parametric optimization of large volume injection (LVI) techniques has been done for pesticides, polychlorinated biphenyls (PCBs), polycyclic aromatic hydrocarbons (PAHs), polybrominated diphenyl ethers (PBDEs), and probably others (Björklund et al., 2004; Mol et al., 1996). Injecting as much as 100 microliters in each run would lower the detection limits significantly for most compounds. In our previous PHCZ studies, 60 mL was injected at 40 °C inlet temperature for each run (Guo et al., 2014a, 2017; Li et al., 2022a). Compared with the more often used injection volumes of 1 to 2 microliters, LVI technologies have the potential to enhance the analytical sensitivity significantly.

The log Kow values are 5.4 for 36-CCZ (computed by XLogP3 3.0) and 6.9 for p,p′-DDT (ATSDR, 2019). Given the differences, we had expected complete separation of 36-CCZ and p,p′-DDT on high-performance reverse phase HPLC columns. Our HPLC-DAD analysis showed that the retention times for 36-CCZ and p,p′-DDT were 3.78 and 8.22 min, respectively, meeting our expectation. The number of reports on PHCZs using HPLC is limited (Li et al., 2022b, 2023; Peng et al., 2023; Zhou et al., 2019b, 2021). Based on the result of this work, the HPLC-MS data for 36-CCZ are unlikely to be affected by the presence of p,p′-DDT. Additionally, LC often offers shorter run time than GC (Zhou et al., 2019b), making it attractive where high throughput of samples is needed. However, incomplete separations of PHCZs from other chemicals are likely, thus purity of the target PHCZ peaks should be carefully verified.

Separation of p,p′-DDT and 36-CCZ during sample extract cleanup

Several promulgated standard methods, such as the U.S. EPA SW846 Methods 3600s, have established various cleanup methods to eliminate unwanted substances from the sample extracts before instrumental analyses. Such cleanup methods not only reduce interferences but also reduce inlet contamination and help extend column life. Fractionation, in the context of this paper, is to collect a few fractions containing different targeted analytes during the cleanup. These fractions can then be analyzed using different types of instruments or under different operating conditions for enhanced selectivity and sensitivity (Guo et al., 2014b; Jang & Li, 2001; Liu et al., 2006).

For PHCZ analysis, various cleanup methods used to date have been reviewed and compared (Sun et al., 2022; Zhou et al., 2019a). Low- or high-performance liquid-solid sorption as well as gel permeation chromatography have been the most used in the cleanup of sample extracts. Silica gel, Florisil, alumina, and others have been used alone or, in most cases, in combination, in column chromatography. Zhou et al. (2019a) found good recoveries and repeatability for eleven PHCZs with the use of Florisil and silica gel in combination and eluting by 3:1 hexane:DCM mixed solvent. One or more portions of the eluted solution are often discarded, thus potential interferences from untargeted chemicals and substances were largely reduced. Sorbents should not be acid- or base-treated, as such treatment leads to the loss of PHCZ congeners which is not substituted in the 1,3,6,8 positions (Riddell et al., 2015).

In the analysis of lake sediment, we previously developed a cleanup/fractionation procedure using a glass low-pressure chromatographic column with combined sorbents of neutral silica gel and alumina (Guo et al., 2014b). From this column, four fractions of the sample extract were eluted for different GC-MS analyses (Guo et al., 2014b). The column and the fractionation method are briefly described in the footnote of Table S1. From this cleanup/fractionation column, DDT was found to elute almost completely in the second fraction (F2) as shown in Table S1. PHCZs were eluted in not only F2 but also the third (F3) and fourth (F4) fractions. However, 36-CCZ was eluted >99% in the third (F3) and fourth (F4) fractions, along with most other PHCZ congeners with less than four halogens. These results indicate that 36-CCZ and p,p′-DDT can be sufficiently separated before instrumental analyses. In our previous studies of PHCZs (Guo et al., 2017; Li et al., 2022a), concentrated F3 and F4 were combined with equal volumes for injections to EI-MS/MS (Agilent 7890-7000 GC-MS/MS) for three smaller PHCZ congeners including 36-CCZ. For other congeners, F2 and combined F3+F4 were separately analyzed using an ECNI-MS (Agilent 6890-5973 GC-MS) for better sensitivity.

We recommend an evaluation of published data for 36-CCZ, concerning potential inaccuracy due to the interference caused by p,p′-DDT, if the data were generated by GC-MS or GC-MS/MS. We are not here to question the possibility that 36-CCZ dominates environmental PHCZs. However, the concentration data reported to date may not be confidently used if the associated quality control protocols did not take into consideration interferences which are known to be likely. In these cases, findings based on data generated using GC-MS or GC-MS/MS may need to be reconsidered. For example, concentrations of 36-CCZ and DDT in fish from the Great Lakes were reportedly strongly correlated (Wu et al., 2018), but their possible mutual interference during the analyses is unclear. In addition to the pair of 36-CCZ and p,p′-DDT, the pair of 3-bromocarbazole (3-BCZ) and p,p′-DDE may also need attention, as mentioned by Zhou et al. (2019a).

The widespread applications of DDT in the past have severely contaminated natural waters, and it has been continuously detected in the environment despite the overall declining trends. With the increasing number of studies on PHCZs, avoiding major artifact in the data is critical and urgent. For future studies, we suggest using fractionation of sample extracts, comparing GC columns, and fine tuning the GC inlet and oven temperature programs, in order to achieve complete separation of 36-CCZ and p,p′-DDT before MS or MS/MS analyses. Two-dimensional GC×GC with two columns of different stationary phases could greatly improves the separation; however, it requires costly time-of-flight MS detector and has only been used in limited studies for the analysis of PHCZs. High-resolution mass spectrometer (HRMS) was found to be not only superior over LRMS in sensitivity but also outperforms MS and MS/MS in anti-interference capacity.

Author contribution Shanshan Zhou procured funding, developed the experiment, reviewed and edited the manuscript; Qi Su conducted the research, collected and analyzed the data; Dan Zhong created the figures; Jiehong Guo offered figure S2 and Table S1; An Li contributed to conceptualize the experiment, wrote the original draft and reviewed; Jinsong Liu contributed to explain the data; All authors reviewed and approved the final manuscript.

Funding This work is funded by the National Natural Foundation of China (No. 41877494).

Data availability Data presented in this study may be available on reasonable request from the corresponding author.

All authors have read, have understood, and have complied as applicable with the statement on “Ethical responsibilities of Authors” as found in the Instructions for Authors.

Ethics approval and consent to participate Not applicable for that section.

Consent for publication Not applicable for that section.

Competing interests The authors declare no competing interests.

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No competing interests reported.

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Mutual interference between 3,6-dichlorinated carbazole and p,p′-dichlorodiphenyltrichloroethane in gas chromatography mass spectrometry analysis

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