tandem mass spectrometry

Journal of Chromatography A, 1334 (2014) 118–125

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Journal of Chromatography A journal homepage: www.elsevier.com/locate/chroma

Rapid and sensitive method for the determination of polycyclic aromatic hydrocarbons in soils using pseudo multiple reaction monitoring gas chromatography/tandem mass spectrometry Dayue Shang a,∗ , Marcus Kim b , Maxine Haberl a a Pacific and Yukon Laboratory for Environmental Testing, Science and Technology Branch, Pacific Environmental Science Centre, Environment Canada, North Vancouver, British Columbia, Canada b Agilent Technologies Inc., Mississauga, Ontario, Canada

a r t i c l e

i n f o

Article history: Received 30 October 2013 Received in revised form 24 January 2014 Accepted 27 January 2014 Available online 3 February 2014 Keywords: GC/MS/MS GC/MS Polycyclic aromatic hydrocarbons Rapid extraction Pseudo MRM Soil and sediment

a b s t r a c t A method for the rapid determination of 18 polycyclic aromatic hydrocarbons (PAHs) in soil has been established based on a simplified solvent extraction and GC/MS/MS operated in pseudo multiple reaction monitoring mode (PMRM), a technique where the two quadrupoles mass monitor the same m/z. The PMRM approach proved superior to the classic single quadrupole technique, with enhanced sensitivity, specificity, and significant reduction in time consuming sample clean-up procedures. Trace level PAHs could be readily confirmed by their retention times and characteristic ions. The limit of quantitation in soil was observed to be 20 ng/g for 16 EPA-priority PAHs and 2 additional PAHs specific to Environment Canada. The developed method was linear over the calibration range 20–4000 ng/g in soil, with observed coefficients of determination of >0.996. Individual PAH recoveries from fortified soil were in the range 58.1 to 110.1%, with a precision between 0.3 and 4.9% RSD. The ruggedness of the method was demonstrated by the success of an inter-lab proficiency test study organized by the Canadian Association for Laboratory Accreditation. The present method was found to be applicable as a rapid, routine screening for PAH contamination in soil, with significant savings in terms of preparation time and solvent usage. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved.

1. Introduction Polycyclic aromatic hydrocarbons (PAHs) are ubiquitous hydrophobic compounds originating from natural or anthropogenic sources. These compounds are widely distributed in the environment and detected in soils and sediments, mainly due to atmospheric deposition processes [2]. All PAHs in the environment are an ecological and human-health concern. Of the one hundred and twenty-six Environment Protection Agency Priority Pollutants listed by the Clean Water Act, sixteen are PAHs, with seven being known carcinogens [1]. It is recognized that an increase in the relative amount of two to four ring compounds, such as naphthalene, fluoranthene, and phenanthrene, is usually a good indication of the presence of petrogenic hydrocarbons [1]. Larger PAHs such as the 5 and 6-ringed compounds are indicative or pyrogenic sources [3]. The reserves of oil sands bitumen in Northern Alberta, Canada, are estimated at 1.7 trillion barrels, with 173 billion estimated to be economically recoverable. Oil exploration in this region has been intensified over the past 20 years, with production increasing

∗ Correspondingauthor. Tel.: +1 604 903 4462; fax: +1 604 903 4408. E-mail address: [email protected] (D. Shang).

from 100,000 barrels per day to about 1.5 million barrels per day currently [2]. Close monitoring of PAH concentrations in soils and sediment has become critical, and large scale surveillance is being implemented by government agencies. The characterization and knowledge of PAH concentrations in soil and sediments can be instrumental in tracing an oil spill source and enabling remediation efforts. A rapid, sensitive, and robust analytical method for the determination the PAH concentrations in soil is urgently needed [2]. Traditional sample preparation techniques for the determination of PAHs in soil are time consuming and generally require large volumes of toxic solvents, together with multi-step extraction and silica gel or Florisil column clean-up procedures. To address these issues, and as an alternative to the classic Soxhlet solvent extraction methods, various techniques have been developed and used in the analysis of PAHs from soil. Alternative processing includes pressurized liquid extraction or accelerated solvent extraction (PLE or ASE), ultrasonic extraction, supercritical fluid extraction (SFE), and microwave-assisted extraction (MAE) [4–6]. An automated Soxhlet method has recently been developed with corresponding reduction in soil extraction time [7,8]. Despite intensive method development in this area, some of the referenced techniques suffer one or several shortcomings, including low recovery, expensive initial

0021-9673/$ – see front matter. Crown Copyright © 2014 Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.chroma.2014.01.074

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investment, frequent equipment malfunction, and lack of robustness or ruggedness. Very recently, a new promising approach of “microextraction” has emerged using MAE combined with solvent bar [9]. While this approach is both “green” and effective, wide application of this method remains to be seen. An elegant approach to the issue would be to take advantage of the modern instrument’s enhanced capability of handling less processed sample extracts and use a “dilute and shoot” approach. Perhaps more importantly, simplified sample processing improves method ruggedness, which is critical for routine analysis. Presently the two most frequently employed techniques to determine PAHs are HPLC with fluorescence, UV, or diode array detection [10,11] and GC with MS detection [1,7,8,10]. The HPLC based methods are usually fast in comparison to the GC/MS methods; however, the disadvantages of the HPLC method are heavy dependence on chromatographic retention time for compound identification and the HPLC methods are typically an order of magnitude lower in sensitivity than GC/MS [12]. In complex matrices, such as soil extract, peak identification based solely on retention time is subject to interference from other components, making trace level PAH contamination difficult to characterize. For this reason, over 15 years the GC/MS technique has become established as the accepted method for PAH determination in the environment [7,8]. Despite numerous improvements to single quadrupole MS instrumentation however, performance cannot match the sensitivity and specificity offered by triple quadrupole MS. As a consequence, an increasing number of peer reviewed publications have applied GC/MS/MS techniques to PAH analysis. However, due to the unique structure stability of the PAH compounds, the traditional Multiple Reaction Monitoring (MRM) approach has been hampered by generally weak fragmentation ion responses for this group of compounds [13–15]. Considering the well-established GC/MS single quadrupole method, the application of the triple quadrupole presently does not provide adequate improvement in sensitivity and specificity to initiate a change from proven procedures. In this regard we challenged this conclusion and successfully applied GC/MS/MS techniques to PAH analysis. In this paper, we present a rapid analytical method for the analysis of PAHs in soil and sediments, based on a one step, low volume solvent extraction followed by GC/MS/MS in pseudo MRM mode. Long extraction time, large solvent volume consumption, and extensive silica gel column clean-up were eliminated. This was made feasible by the increased sensitivity and specificity achieved by pseudo MRM mode GC/MS/MS. Compelling results will be presented to support the favoring of this pseudo MRM mode GC/MS/MS over that of single quadrupole procedures, even for difficult-to-fragment compounds like PAHs. The present method was validated and applied successfully during an inter-lab proficiency study organized by The Canadian Association for Laboratory Accreditation Inc. (CALA).

NJ). This solution was stored at −20 ± 10 ◦ C in amber glass and had a shelf life of 12 months. An internal standard solution of Naphthalene-d8, Acenaphthene-d10, Phenanthrene-d10, and Perylene-d12 was purchased from Supelco (Oakville, Ontario). This internal standard was employed both in the preparation of calibration standards and in fortifying soil samples for spike recovery. Calibration standards were prepared in dichloromethane by serial dilution of primary standard to provide final concentrations of 10, 20, 40, 100, 500, 1000, 1500 and 2000 ng/mL. Internal standard at a final concentration of 200 ng/mL was added to all calibration standards. Disposable centrifuge filter tubes (15 and 50 mL, Polypropylene/Polyethersulfone) were supplied by Pall Corporation (Port Washington, NY). Disposable 50 mL polypropylene centrifuge tubes were purchased from Sarstedt (Numbrecht, Germany). Florisil® adsorbent (60–100 mesh) was from Fisher Scientific (Fairlawn, NJ. USA). OmniSolv solvents dichloromethane (DCM), acetone (ACE), hexane, isopropanol (IPA), acetonitrile (ACN), pesticide grade, were purchased from EM Science (Gibbstown, NJ. USA).

2. Material and method

2.3. GC-MS analysis

2.1. Reagents and standards

A gas chromatograph (GC) HP 7890A from Agilent Technologies (Palo Alto, CA., USA) equipped with an Agilent 7693B automatic liquid sampler with 10 ␮L syringe was used for the separation of PAHs. Analysis employed a 1 ␮L sample injection in pulsed splitless mode (pulsed pressure at 50 psi with the split valve closed for 1 min). All analytes were separated on a Restek Rtx-5MS with Integra-guard column (30 m x 0.25 mm id, 0.25 ␮m). A 4 mm i.d. single tapered, deactivated inlet liner with glass wool at the bottom (Agilent Technologies) was installed into the injector. The oven temperature program was as follows: initial temperature at 50 ◦ C (hold 2 min), then 6 ◦ C/min to 310 ◦ C, hold for 20 min. The total run time was

The 18 PAHs analyzed in this study were Acenaphthene (ACE), Acenaphthylene (ACY), Anthracene (ANT), Benzo(a)anthracene (BAN), Benzo(a)pyrene (BAP), Benzo(e)pyrene (BEP), Benzo(b) fluoranthene (BBF), Benzo(g,h,i)perylene (BGP), Benzo(k) fluoranthene (BKF), Chrysene (CRY), Dibenz(a,h)anthracene (DBA), Fluoranthene (FLA), Fluorene (FLU), Indeno(1,2,3-cd)pyrene (IND), Naphthalene (NAP), Perylene (PER), Phenanthrene (PHE) and Pyrene (PYR). A certified standard solution of the 18 PAHs (2000 ␮g/mL each) was provided by SPEX CertiPrep (Metuchen,

2.2. Sample extraction and clean up Aliquots of 10 ± 0.1 g of air dried free flow homogeneous soil sample were weighed and placed into a 50 mL polypropylene centrifuge tube with screw caps. To the sample, 200 ␮L of 20 ppm internal standard mixture were added, followed by 5 g of sodium sulphate (pre-dried at 350 ◦ C). The mixture was then hand-shaken to mix sodium sulphate with the soil sample, with occasional spatula use to break any soil lumps to ensure homogeneity. After mixing, 15 mL of dichloromethane was added and the mixture was vortexed briefly. The slurry was further shaken for 10 min at room temperature using a mechanical wrist action shaker. The sample was centrifuged at 5000 rpm (4696 g) for 5 min. The supernatant was decanted and retained in a clean 50 mL polypropylene centrifuge tube. The remaining pellet was subjected to a second extraction in 5 mL dichloromethane (breaking up the pellet “cake” with a spatula if necessary), employing only a 5 min shaking time. Supernatant from both extractions were pooled and the volume adjusted to 20 mL with dichloromethane. An aliquot of the 20 mL extract was transferred to a 15 mL centrifuge filter tube with 0.2 ␮m filter device. Following centrifugation for 5 min at 5000 rpm (4696 g), the filtrate extract was ready for GC/MS/MS analysis. Refer to Fig. 1 for a flowchart of sample extraction steps. For soils contaminated with lube oil, vegetable oil, or animal oil and grease, the filter insert of the centrifuge tube may be pre-packed with approximately 3 g of Florisil to improve clean up. These materials may lower analyte recovery and the inclusion of an isotope dilution technique may be required to compensate (Supplementary materials).

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QC

Add 10 g bank soil to a 50 mL PP centrifuge tube. Spike in 0.2 mL of surrogate. Add 5 g sodium sulphate.

Add 10 g of sample to a 50 mL PP centrifuge tube. Spike in 0.2 mL of surrogate. Add 5 g of sodium sulphate.

Real Sample

Add 15 mL of DCM. Vortex mix.

Wrist-action shake the sample and solvent for 10 minutes.

Vortex mix. Centrifuge at 5000 rpm for 5 minutes. Decant into a clean 50 mL centrifuge tube.

Add 5 mL of DCM to solid ‘cake’. Break up with manual agitation (with spatula if necessary). Shake for 5 minutes.

Vortex mix. Centrifuge at 5000 rpm for 5 minutes and decant into the same tube from the first extraction.

Make volume up to 20 mL with DCM. Vortex mix.

Add 5 mL of sample to a 15 mL disposable centrifuge filter tube and centrifuge at 5000 rpm for 5 minutes.

Discard filter insert and transfer 1.5 mL to a GC vial.

GC/MS/MS Analysis Fig. 1. Schematic of sample preparation procedure.

65.33 min. High purity helium gas (>99.999%) was used as carrier gas with the constant flow rate of 1.0 mL/min. Detection of the analytes employed an Agilent Technologies 7000 triple quadrupole mass spectrometer (MS) operated in electron impact positive mode ionization at 70 eV. Analyte ions were monitored in either “Classic” Multiple Reaction Monitoring (CMRM) or “Pseudo” MRM (PMRM) mode. The GC/MS transfer line and inlet temperatures were set at 300 and 320 ◦ C respectively. Ion source temperature was set at 325 ◦ C and quadrupole temperature at 150 ◦ C. A solvent delay of 4.5 min was employed. Table 1 lists the PAHs along with their observed retention times and their characteristic quantitation and qualifier ions. The N2 collision cell and He Quench Gas flows were both set to 1 mL/min. Quantification employed the integrated peak area ratio of the target ion to internal standard. A weighted (1/x) linear regression of the calibration standard responses was employed to define the calibration curve from which measured concentrations were calculated. The PAH analytes were identified by their target ions and retention time order. Retention times had to be within ± 0.1 min of the expected time for positive confirmation.

2.4. Method validation The linearity of the analytical GC/MS/MS PMRM method was assessed by analyzing duplicate calibration standards prepared at 10, 20, 50, 100, 500, 1000, 1500 and 2000 ng/mL (equivalent to soil samples spiked at PAH concentrations from 20 to 4000 ng/mL). Linearity using weighted (1/x) least-squares regression was considered acceptable when the correlation coefficient (r) was >0.995. The limit of quantitation (LOQ) and limit of detection (LOD) of the method were assessed based on the signal to noise (S/N) response of the relevant analyte peak response at the lowest standard concentration of 10 ng/mL. A S/N of >10:1 and >3:1 for LOQ and LOD respectively were considered acceptable for each analyte. Method accuracy (expressed as percent recovery) and precision (expressed as percent relative standard deviation (%RSD)) were determined by recovery studies in PAH-free soil samples spiked at low, mid, and high PAH concentrations. Eight replicate soil samples spiked with PAH standard at 20, 200 and 2000 ng/mL were processed and analyzed. Results showing an accuracy of 60% to 120% recovery from nominal concentration and a precision of <20% RSD were considered to be acceptable. The percent recovery was

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Table 1 List of PAHs and pseudo MRM acquisition parameters. Compound name Time Segment 1: 14.50 min d8-Naphthalene Naphthalene Time Segment 2: 21.00 min Acenaphthylene d10-Acenaphthene Acenaphthene Time Segment 3: 23.00 min Fluorene Time Segment 4: 27.00 min d10-Phenanthrene Phenanthrene Anthracene Time Segment 5: 31.00 min Fluoranthene Pyrene Time Segment 6: 37.00 min Benz(a)anthracene d12-Chrysene Chrysene Time Segment 7: 41.00 min Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(e)pyrene Benzo(a)pyrene d12-Perylene Perylene Time Segment 8: 45.00 min Indeno(1,2,3-cd)pyrene Dibenz(a,h)anthracene Benzo(g,h,i)perylene

ISTD

Indicative RT (min)

X

15.19 15.26

136 > 136 128 > 128

137 > 137 129 > 129

21.32 21.94 22.06

152 > 152 163 > 163 153 > 153

153 > 153 162 > 162 154 > 154

76 > 76

24.02

166 > 166

27.61 27.69 27.86

X

X

X

X

Precursor> product ion

Qual ion 1

Qual ion 2

Dwell (ms) quant/qual

CE (V)

15 10

5 5

76 > 76

10 15 10

10 5 5

165 > 165

83 > 83

10

5

188 > 188 178 > 178 178 > 178

189 > 189 179 > 179 179 > 179

89 > 89 89 > 89

15 10 10

10 5 10

32.35 33.17

202 > 202 202 > 202

203 > 203 203 > 203

101 > 101 101 > 101

10 10

5 10

37.92 37.98 38.08

228 > 228 240 > 240 228 > 228

114 > 114 121 > 121 229 > 229

229 > 229

10 15 10

10 10 10

41.89 41.97 42.80 42.95 43.16 43.24

252 > 252 252 > 252 252 > 252 252 > 252 264 > 264 252 > 252

253 > 253 253 > 253 253 > 253 253 > 253 132 > 132 253 > 253

250 > 250 250 > 250 250 > 250 250 > 250 250 > 250

10 10 10 10 15 10

10 15 5 10 15 5

46.40 46.50 47.10

276 > 276 276 > 276 278 > 278

277 > 277 277 > 277 279 > 279

138 > 138 138 > 138 139 > 139

10 10 10

10 10 5

114 > 114

ISTD: Internal Standard; RT: Retention Time; CE: Collision Energy.

calculated from the equation: Mean (calculated)/Mean (spiked)) × 100. 2.5. Application to real samples The validated method was applied to a set of four soil samples (labeled C-18-01, C-18-02, C-18-03 and C-18-04) supplied by the Canadian Association for Laboratory Accreditation Inc. (CALA) as part of a proficiency testing program. The samples were analyzed by both a well-established GC/MS single ion monitoring method and by the new validated GC/MS/MS PMRM method. Results were compared as a demonstration of the performance of the new GC/MS/MS PMRM procedure.

with adequate sensitivity. A representative GC/MS/MS PMRM chromatogram of a blank soil sample spiked at 0.1 ␮g/g of PAHs standard and extracted under these conditions is shown in Fig. 2. For relatively clean soil samples, the main issue is the simultaneous extraction of interference components, particularly isobaric compounds. These interferences are significant in GC/MS analysis but could be overcome efficiently by the proposed GC/MS/MS PMRM approach. For this proposed procedure, the current elaborate routine sample clean-up and concentration steps, such as roto-vap and nitrogen gas blow down, were rejected in lieu of a “dilute and shoot” approach, with only a filter 0.2 ␮m centrifuge tube filtration step included to remove particulates. 3.2. GC/MS/MS determination

3. Results and discussion 3.1. Sample extraction and clean-up For PAHs analysis, the ideal extraction solvent(s) should have the characteristics of high extraction efficiency of the targeted compounds from soil, capability of application directly to wet soils (to prevent loss of low volatile PAHs during the drying process), and compatibility with GC/MS. To meet these requirements, and based on practical experience and literature review [1,8], several solvents and their combinations were examined during the present study (hexane, acetone, acetonitrile (ACN), dichloromethane (DCM) and isopropanol). A detailed discussion of solvent selection and cleanup method development can be found in Supplementary materials. The most practical approach was achieved by first reducing soil moisture and extraction in neat DCM solvent. The incorporation of sodium sulfate allowed for soil moisture reduction and DCM extraction yielded consistently high recoveries at over 60% for all 18 PAH compounds. Furthermore, all 18 PAH compounds exhibited acceptable chromatographic peak shape and were satisfactorily separated

In environmental analysis, co-eluting isobaric matrix interferences derived from soil and sediments usually make MRM the technique of choice to achieve high signal-to-noise ratios and method specificity. Polyaromatic hydrocarbons possess an exceptionally stable and rigid macrocyclic ring structure and yield very little fragmentation during collision induced dissociation. Therefore, in this work PAHs were analyzed by GC/MS/MS with pseudo MRM monitoring (GC/MS/MS PMRM), a technique where both resolving MS quadrupole mass filters monitor for the same molecular ion m/z that is employed for quantitation. In PMRM, the target ions are selectively transferred to the second resolving quadrupole mass filter without collision induced dissociation. In the present study, a systematic optimization of targeted PAH compound ionization and daughter ion fragmentation was conducted. Results supported observations by other groups [13–15] in that the most abundant peak in the electron impact ionization (EI+ ) fragmentation spectrum was consistently the molecular ion (M+ ). Other observed peaks were present at lower relative intensities despite efforts to optimize conditions for fragmentation and even

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Fig. 2. Combine extracted ion chromatogram of 18 PAHs spiked in DCM at 100 ng/mL (GC/MS/MS, EI ionization, pseudo-MRM acquisition).

under extreme collision energy conditions. Other weaker intensity ions that commonly formed were the multiply charge ions such as [M − 2H]+ or double-charged analytes M2+ . The results suggest that the highly condensed and stable PAHs are not easily amenable to routine MRM analysis without loss of sensitivity due to the limited number and low response of daughter ions; especially with respect to the low intensity qualifier ions. The lack of increased sensitivity for a triple quad over a single quad GC/MS analysis explains the paucity of peer reviewed papers published using triple quadrupole GC/MS/MS for PAHs analysis [13–15]. The high structural stability of PAH compounds was exploited as an advantage under the present PMRM approach. The characteristic PAH analyte parameters used in established single quadrupole GC/MS methods for PAHs were employed for monitoring in both the first and third quadrupole of the GC/MS/MS. No fragmentation of the target ions was attempted, while collision energy was fine tuned to achieve best signal to noise ratio by decreasing or eliminating co-eluting isobaric interference compounds. In effect, the collision energy was tuned to reduce interfering effects by fragmentation or creation of unfavorable energy transfer for interfering compounds, while targeted ions remained relatively intact. As a result, the PMRM technique provided two main advantages: (1) potential destruction and mass filtering of isobaric interferences; (2) the collisional focusing of the ion beam in a high pressure RF device as described by Douglas and French [16] to focus the ions toward the centre axis where they are better able to enter the acceptance aperture of the 2nd resolving quadrupole mass filter resulting in better transmission at a given resolution. To demonstrate the utility of PMRM approach, a systematic study of the relationship of collision energy to peak height and S/N was conducted. All 18 targeted PAHs produced a “sweet spot” collision energy where the peak area and S/N were optimized (Fig. 3, more to be found in Supplementary Fig. S1). In other words, at this optimized collision energy, one can expect strong peak and reduced signal to noise ratio for a particular compound. Occasionally, lower

or higher collision energy may be selected to increase either peak height (in the case of poorly ionized compounds and clean matrices) or S/N ratio (in the case of strong ions and dirty matrices). The present method for PAHs employed collision energies at the S/N “sweet spot” with a few exceptions. Under the determined optimized collision energy conditions for PMRM mode ion operation, it was theorized likely that the GC/MS/MS method would show improved limit of detection (LOD) and limit of quantitation (LOQ) for “dirty” samples when compared to routine GC/MS. To illustrate this point, a GC/MS single ion monitoring method was created based on a well-established in-house SOP for the analysis of PAHs in soil and sediments and using a single quadrupole instrument (7890A/5975C, Agilent Technologies). An experiment was carried out to compare the GC/MS/MS in PMRM mode to GC/MS in SIM mode. For this experiment, a series of samples were prepared in solvent laden soil extract spiked at various low PAH concentrations from 2 to 30 ng/g. The duplicated soil extract samples were analyzed by both GC/MS/MS in PMRM mode and GC/MS in a wellestablished SIM mode. A summary of the results is provided in Table 2. The advantages of PMRM over GC/MS SIM were clearly demonstrated. A number of PAHs at 10 ppb were not detected by the GC/MS SIM method due to severe matrix effects. The same samples produced strong peaks for all 18 PAHs in GC/MS/MS PMRM analysis, with excellent S/N at this trace level. Furthermore, at a 2 ng/g spiking level, minor peaks did not achieve an adequate S/N for quantitation of any of the PAHs by GC/MS SIM while, remarkably, GC/MS/MS PMRM still exhibited strong definitive peaks adequate for quantitation for the majority of the targeted compounds. In justification for the PMRM approach, one must answer the fundamental question: how does PMRM analysis compare with Classic MRM (CMRM) analysis in terms of specificity and sensitivity? With respect to specificity, the PMRM offers equivalent confirmatory identification of analytes by a combination of retention times, quantitative and qualitative ions, and ion ratio between quantitative and qualitative ions in the mass spectrum. In addition,

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Fig. 3. Signal to noise ratio in relation to collision energy for 3 typical PAHs. All standards were prepared in DCM and at 1000 ng/mL. The dotted line indicates the optimized and selected collision energy used in the method. The symbol () indicates signal to noise ratio of the targeted compound at this collision energy (v). The symbol () indicates peak area of the targeted compound at this collision energy (v).

the PMRM method was shown to be superior to CMRM with respect to sensitivity, as a result of the reduction of interfering isobaric compounds. A comparison of S/N for the PAH analyte peaks from soil extracts fortified 1 ppm PAH standard mixture is shown in Fig. 4 and clearly demonstrates the improved sensitivity of PMRM over CMRM for the majority of the analytes (12 out of 18 PAHs). Although

detection of some individual PAH compounds could be significantly improved by the PMRM method, the elimination of isobaric compounds for all PAH analytes as a group was not complete. In certain cases, notably Acenaphthene, Benzo(e)pyrene and Perylene, interfering compounds were observed that possessed similar chemical stability to the target ion and the original CMRM remained the

Table 2 Instrument comparison: area counts at PAH concentrations from 2–20 p p b. Agilent 7000 triple quadrupole

Agilent 5975 single quadrupole

Compound

2 ppb

10 p p b

20 p p b

2 ppb

Napthalene Acenaphthylene Acenaphthene Fluorene Phenanthrene Anthracene Fluoranthene Pyrene Benzo(a)anthracene Chrysene Benzo(b)fluoranthene Benzo(k)fluoranthene Benzo(e) pyrene Benzo(a) pyrene Perylene Indeno(1,2,3-cd) pyrene Benzo(g,h,i)perylene Dibenz(a,h)anthracene

999 537 343 ND 1372 930 1325 1641 ND 1029 ND ND 405 355 813 202 455 ND

3909 2619 1190 1551 4810 2046 3659 4056 955 2791 1859 652 1841 1159 1976 587 1531 804

8121 5758 2877 2790 6462 4213 7925 9323 2521 6306 4778 1411 3922 3230 4363 2237 3963 2293

ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND ND

*ND = Not detected.

10 p p b 29 25 33 22 ND ND 46 46 ND ND 35 35 42 33 49 17 29 0

20 p p b 57 45 33 42 ND ND 86 94 ND ND 70 85 85 74 102 47 63 45

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Fig. 4. Signal to noise comparison for pseudo-MRM and classic MRM in relation to collision energy for 3 typical PAHs. The results were obtained from a soil sample spiked at 1000 ng/g of 18 PAHs, extracted with DCM and run duplicated. The symbol () indicates signal to noise ratio of the targeted compound using pseudo MRM. The symbol () indicates signal to noise ratio of the targeted compound using classic MRM.

optimal choice for analysis. Nevertheless, many well established GC/MS single quad methods may benefit from this straightforward PMRM approach without extensive new method development process. 3.3. Method validation The linearity of calibration curves was determined by analysis of duplicate preparations of calibration standards over the nominal concentration range of 10–2000 ng/mL for all PAH analytes (equivalent to soil samples spiked at PAH concentrations from 20 to 4000 ng/g). The MS response for all analytes was observed to be linear in this concentration range, with correlation coefficients of >0.996. The limit of quantitation (LOQ) of the method was determined to be 20 ng/g for all the targeted compounds. Furthermore, prepared stock standard solutions and working solutions were found to be stable when stored at –20 ± 10 ◦ C for up to 6 months. The precision and accuracy of the analytical GC/MS/MS-PMRM method was confirmed employing eight soil samples spiked at low (20 ng/g), mid (200 ng/g), and high (2000 ng/g) concentrations of PAHs (Supplementary Tables S1, S2 and S3). Accuracy for

determination of the PAH compounds was demonstrated by a percent recovery in the range 58.1–110.1%, with an observed precision of <5%RSD for each of the individual compounds. Results for recovery of the PAH compounds at low, mid and high levels were in line with those reported by other authors using sonication [6,17], ASE [7,8], SFE [4,8] or an automated Soxhlet procedures [7]. The method showed a limit of detection limit (LOD) of between 5–10 ng/g for PAH compounds extracted from the soils.

3.4. Analysis of proficiency testing samples The validated method was applied to a set of four soil samples (labeled C-18-01, C-18-02, C-18-03, and C-18-04) supplied by the Canadian Association for Laboratory Accreditation Inc. (CALA) as part of a proficiency testing program. Due to the high concentration of spiked of PAHs in the samples, only 1 g of each material was processed and analyzed. Table 3 presents the typical results for one of the samples. The results were compared with the assigned value and found to be satisfactory (Supplementary Table S4). The accuracy for this test with the current method ranges from 63 to 139%.

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Table 3 CALA Proficiency testing sample C-18-04: measured vs. assigned concentrations. Compound

Sample

Actual concentration (p p b)

Calculated concentration (p p b)

Accuracy (%)

Acenaphthene Acenaphthylene Anthracene Benzo (a) anthracene Benzo (a) pyrene Benzo (b) fluoranthene Benzo (g,h,i) perylene Benzo (k) fluoranthene Chrysene Dibenzo (a,h) anthracene Fluoranthene Fluorene Indeno (1,2,3 - cd) pyrene Naphthalene Phenanthrene Pyrene

C-18-04 C-18-04 C-18-04 C-18-04 C-18-04 C-18-04 C-18-04 C-18-04 C-18-04 C-18-04 C-18-04 C-18-04 C-18-04 C-18-04 C-18-04 C-18-04

1119 1343 1224 6076 4007 7869 4452 4119 6784 1029 19517 1322 5045 36372 16835 13368

1161 1501 1543 5363 3222 5198 3843 4127 5947 1202 14658 1095 4400 33325 13059 14165

104 112 126 88 80 66 86 100 88 117 75 83 87 92 78 106

Measured concentrations vs. assigned concentrations in CALA PT-C-18-04.

4. Conclusions

Appendix A. Supplementary data

A sensitive and rapid GC/MS/MS method for the determination of PAHs in soil samples has been developed. Sample extraction was reduced to a rapid mechanical shaking procedure using just 20 mL of solvent, and further sample clean-up was eliminated for most samples as a result of the application of GC/MS/MS in pseudo MRM mode. An additional Florisil clean-up was all that required in the case of more heavy contaminated soils. A series of experiments were conducted to compare GC/MS/MS PMRM and GC/MS/MS CMRM detection modes for soil extract sample analysis. Results for the recovery of PAHs demonstrated the advantage of PMRM triple quad analysis over single quad SIM mode GC/MS. In the present study, the highly stable nature of PAH compounds was taken advantage of and the collision cell fragmentation was optimized for fragmentation of potential interference ions, rather than for fragmentation of the highly abundant parent ions. In summary, the PMRM provided additional mass filtering due to dual quadrupole ion focusing and potential reduction or destruction of isobaric interferences in the collision cell, thus improving sensitivity for most of the targeted PAH compounds. Considering the significant saving in time and solvent volume by PMRM approach, we suggest that labs currently using GC/MS SIM move to GC/MS/MS PMRM to improve method productivity and sensitivity, especially in the case of structurally stable compounds such as alkylated PAHs, oil biomarkers, dioxins and furans, and some pharmaceutical drugs.

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chroma. 2014.01.074.

Acknowledgements The authors gratefully acknowledge the support and input of their colleagues, notably Randy Englar, Liane Chow, Oxana Blajkevitch, Lauretta Liem and Norman Berke of the Pacific Environmental Science Centre of Environment Canada, North Vancouver, BC.

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