Application of high-resolution continuum source flame atomic absorption spectrometry to reveal, evaluate and overcome certain spectral effects in Pb determination of unleaded gasoline

Accepted Manuscript Application of high-resolution continuum source flame atomic absorption spectrometry to reveal, evaluate and overcome certain spectral effects in Pb determination of unleaded gasoline

Zofia Kowalewska, Hanna Laskowska, Michał Gzylewski PII: DOI: Reference:

S0584-8547(16)30210-5 doi: 10.1016/j.sab.2017.03.020 SAB 5233

To appear in:

Spectrochimica Acta Part B: Atomic Spectroscopy

Received date: Revised date: Accepted date:

14 September 2016 19 March 2017 20 March 2017

Please cite this article as: Zofia Kowalewska, Hanna Laskowska, Michał Gzylewski , Application of high-resolution continuum source flame atomic absorption spectrometry to reveal, evaluate and overcome certain spectral effects in Pb determination of unleaded gasoline. The address for the corresponding author was captured as affiliation for all authors. Please check if appropriate. Sab(2017), doi: 10.1016/j.sab.2017.03.020

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ACCEPTED MANUSCRIPT Application of high-resolution continuum source flame atomic absorption spectrometry to reveal, evaluate and overcome certain spectral effects in Pb determination of unleaded

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gasoline

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Zofia Kowalewskaa *, Hanna Laskowskab, Michał Gzylewskia

Faculty of Civil Engineering, Mechanics and Petrochemistry, Warsaw University of Technology,

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Łukasiewicza 17, 09-400 Płock, Poland

OBR JSC (Warter Group), Chemików 5, 09-411 Płock, Poland

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Abstract

High-resolution continuum source and line source flame atomic absorption spectrometry (HR-

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CS FAAS and LS FAAS, respectively) were applied for Pb determination in unleaded aviation or automotive gasoline that was dissolved in methyl-isobutyl ketone. When using

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HR-CS FAAS, a structured background (BG) was registered in the vicinity of both the 217 nm and 283.306 nm Pb lines. In the first case, the BG, which could be attributed to absorption by the OH molecule, directly overlaps with the 217 nm line, but it is of relatively low intensity. For the 283 nm line, the structured BG occurs due to uncompensated absorption by OH molecules present in the flame. BG lines of relatively high intensity are situated at a large distance from the 283 nm line, which enables accurate analysis, not only when using simple variants of HR-CS FAAS but also for LS FAAS with a bandpass of 0.1 nm. The lines of the 1

ACCEPTED MANUSCRIPT structured spectrum at 283 nm can have “absorption” (maxima) or “emission” (minima) character. The intensity of the OH spectra can significantly depend on the flame character and composition of the investigated organic solution. The best detection limit for the analytical procedure, which was 0.01 mg L-1 for Pb in the investigated solution, could be achieved using HR-CS FAAS with the 283 nm Pb line, 5 pixels for the analyte line measurement and

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iterative background correction (IBC). In this case, least squares background correction

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(LSBC) is not recommended. However, LSBC (available as the “permanent structures”

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option) would be recommended when using the 217 nm Pb line. In LS FAAS, an additional phenomenon related to the nature of the organic matrix (for example, isooctane or toluene)

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can play an important role. The effect is of continuous character and probably due to the simultaneous efficient correction of the continuous background (IBC) it is not observed in

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HR-CS FAAS. The fact that the effect does not depend on the flame character indicates that it is not radiation scattering. For LS FAAS, the determination of Pb using the 283 nm line, a 0.1

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nm bandpass and a fuel lean flame is strongly recommended. The analysis of certified

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reference materials, recovery studies and the analysis of real samples with low Pb content supported the satisfactory accuracy of Pb determination in automotive or aviation gasoline

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when the recommended analytical variants are applied. The studies in this work shed new light on spectral phenomena in air-acetylene flames. The

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structured background due to absorption by the OH molecules must be taken into account during Pb determination in other materials as well as in some other elemental determinations, especially at low absorbance levels. The usefulness of HR-CS FAAS for revealing and investigating a structured background was demonstrated. HR-CS FAAS does not reveal fully corrected spectral effects with a continuous character, which can be found in LS FAAS.

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ACCEPTED MANUSCRIPT Keywords: lead determination, high-resolution continuum source atomic absorption spectrometry, structured background, LSBC, gasoline *Corresponding author. Tel:+48 604569041; fax:+48 24 3653307 E-mail address: [email protected] (Z. Kowalewska)

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1. Introduction

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Lead is a very important element in the petroleum industry due to the application of tetraalkylleads (TALs) and especially the most widely used compound TEL (tetraethyllead) as

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an antiknock agent [1-3]. TEL, which was commercially introduced in 1923, turned out to be an efficient and useful additive to gasoline. It was estimated that the application of this

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compound significantly contributed to the development of transport and the global economy [2]. However, TEL has also been called “the mistake of the 20th century” [2,3]. Its poisoning

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effect on human health, first of all, its neurotoxicity, has been known since the beginning of

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TEL production, but the facts were not properly taken into account for many years. The broad application of TALs contributed to significant environmental pollution all over the world,

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including in Greenland snow and high alpine sites [4,5]. After the peak years of maximum leaded gasoline usage in the 1960s-70s, the process of phasing out TALs began. At present,

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only a few countries in the world allow the use of leaded automotive gasoline [2,3]. The aviation industry is more conservative than the automotive industry, and various grades of leaded aviation gasoline are still produced, although usually with diminished Pb content. The most popular aviation gasoline, Avgas 100LL, can contain up to 0.56 g L-1 Pb [6]. The complete phasing out of leaded gasoline is planned [3], and unleaded aviation gasoline [7,8] technologies are being developed.

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ACCEPTED MANUSCRIPT Automotive gasoline sold in European Union can contain up to 5 mg L-1 Pb [9]. The admissible Pb content in unleaded aviation gasoline is at a similar level, for example, 13 mg L-1 in 91 UL gasoline [7] or 5 mg L-1 in 85 UL gasoline [8]. Because the natural concentration of Pb (coming from the original crude oil) in middle distillates is usually 0.001-0.200 mg L-1 [10-22], a higher content of Pb in commercial unleaded gasoline indicates contamination.

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Lead is not only harmful to human health, but it also damages the catalytic convertors

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used in modern cars. For that reason, associations of cars producers have the aim of

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diminishing the threshold concentration of Pb (as other elements) to 1 mg L-1 for all grades of automotive gasoline [23].

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Various analytical techniques were used to develop methods of Pb determination in gasoline, including graphite furnace atomic absorption spectrometry [10-16,24], inductively

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coupled plasma mass spectrometry [17-20], inductively coupled plasma optical emission spectrometry [21,25], microwave plasma optical emission spectrometry [26] and various

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variants of voltammetry [27-28]. Some methods include Pb preconcentration [20-22].

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The officially approved methods for Pb determination in unleaded gasoline are the standard methods noted in gasoline specifications [7-9]. In the case of ASTM 7547

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specification [7], these are the ASTM 5059C standard method with the application of wavelength dispersive X-ray spectrometry for Pb determination in the range 2.6-132 mg kg-1

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[29] and the ASTM 3237 standard method with the application of flame atomic absorption spectrometry (FAAS) [30]. In compliance with European Union norm [9], the only officially approved method is the EN 237 standard method with the application of FAAS [31]. Two versions of this method are available, that is, the recommended one and the alternative one. The ASTM 5059C method is outside the scope of this work due to its relatively poor sensitivity and detectability. The primary features of the FAAS methods have been compared in Table 1. 4

ACCEPTED MANUSCRIPT FAAS has been used for Pb determination since the 1960s and many studies that were performed in the field (primarily in the 60s and 70s for leaded gasoline) led to the development of an effective samples and standards preparation procedure, which enabled an equalizing signal for various Pb species [32]. This procedure is still used in the FAAS standard methods [30-31] (Section 2.2).

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The direct causes for undertaking the topic of Pb determination in unleaded gasoline in

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this work were sporadic disagreements in the results of analyses that were performed

concerning the quality of the investigated fuel.

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according to official methods (Table 1 [30,31]), which led to contradictory conclusions

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The aim of this work was to find a source of error and to explain the inaccuracy of Pb determination in unleaded gasoline using FAAS. At present, apart from the classical version

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of FAAS with a Pb hollow cathode lamp as a line source of radiation (LS FAAS), a new version known as high resolution-continuum source flame atomic absorption spectrometry

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(HR-CS FAAS) [33-35] can be used. It was expected that HR-CS FAAS, which makes it

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possible to reveal the spectral neighbourhood of the investigated analyte line, would be helpful for performing the tasks in this study. It was also expected that the results of these

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investigations would make it possible to set recommendations for routine analysis using both

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HR-CS FAAS and LS FAAS.

2. Experimental

2.1. Instrumentation and its operation

The ContrAA 700 (Analytik Jena, Jena, Germany), a high-resolution continuum source absorption spectrometer, was equipped with a high-intensity xenon short-arc lamp as a 5

ACCEPTED MANUSCRIPT radiation source, a high-resolution double monochromator consisting of a prism and an echelle grating and a linear charge-coupled device array detector (200 pixels were available for analytical purposes). A spectral interval of approximately 0.245 nm (at 217.001 nm) or 0.314 nm (at 283.306 nm) was recorded simultaneously with a high dispersion of Δλ≈λ/175 000 per pixel. The resolution corresponded to 1.22 pm (at 217.001 nm) or 1.57 pm (at

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283.306 nm) per pixel. If it is not indicated otherwise, the experiments with the ContrAA 700

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were performed using 217.001 nm or 283.306 nm Pb lines (the abbreviations 217 nm and 283

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nm will be used further in this study), three pixels for Pb atomic absorption evaluation (CP±1; CP=central pixel) and iterative background correction (IBC) (more about the background

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correction can be found in references [33,36,37]).

The SOLAAR “M” (Thermo Electron, Waltham, USA), a line source atomic

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absorption spectrometer, was equipped with a hollow cathode lamp as a source of specific Pb radiation (the current of the lamp was 8 mA) and a deuterium lamp of the Quadline type was

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used for background correction. The spectrometer was operated at the 217 nm or 283 nm Pb

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line using a 0.2 nm or 0.1 nm bandpass.

In both HR-CS and LS FAAS, the sample introduction-waste removal systems were

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resistant to organic solvents. An air-acetylene flame was generated using burners with a 100 mm slit and an optimized burner height of 7 mm. In most of the investigated analytical

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variants, an oxidizing flame with a blue colour was used (a flame lean in acetylene, with the abbreviation “L”). In some experiments, a reducing flame with a yellow colour (which is rich in acetylene, with the abbreviation “R”) was generated. In LS FAAS, the maximum flow of additional air was always applied and the acetylene flow rate was 0.8 L min-1 (L flame) or 1.1 L min-1 (R flame). In HR-CS FAAS, an acetylene flow rate of 0.83 L min-1 and an additional air flow rate of 1.25 min-1 were applied to obtain an oxidizing flame while the fuel rich flame was obtained using an acetylene flow rate of 0.92 L min-1, but without additional oxidant. 6

ACCEPTED MANUSCRIPT The flow rate of the aspirated solutions was approximately 5 mL min-1. It was ensured that changes in the flow rate were lower than ± 6% when the flame character was changed as well as when solvent or various investigated solutions were aspirated. It concern not only typical calibration or sample solutions containing 90% solvent (1:10 dilution, v:v) but also certified reference material (CRM) solutions diluted in the solvent in a ratio of 1:5 or 1:2.5

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v:v.

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The list of analysis modes is presented in Table 2. In each analysis mode, the reference

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measurement [33,36] was the measurement of a blank solution.

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2.2. Standards, reagents, and samples

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The Pb standard, lead chloride (II) 99.999%, was from Sigma Aldrich (Saint Louis, USA). Iodine-sublimated and methyltrioctylammonium chloride (MTOACl) were obtained

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from Merck (Darmstadt, Germany). Methyl-isobutyl ketone (MIBK) 99% (POCH, Gliwice,

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Poland) was used as a solvent thoroughout the work. Toluene, isooctane and ethanol were purchased from Eurochem BGD (Tarnów, Poland). An isooctane:toluene mixture (1:1 v:v)

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was labelled as IT, while a 10% v:v solution of ethanol in IT was labelled as E10. The reagents used in this work were at least of analytical grade.

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Lead chloride was dissolved into a 10% v:v solution of MTOACl in MIBK and further stepwise diluted [30,31]. The calibration solutions used throughout the work contained 0, 0.25, 0.5, 1.0, 1.5, and 2.0 mg L-1 Pb and 10% v:v the IT mixture, which was added for matrix matching. The first of the solutions was calibration blank as well as the procedure blank. All the measured solutions (that is the calibration solutions and the samples solutions) were pre-treated with iodine, which was used in a 3% m:v solution in toluene. After reacting

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ACCEPTED MANUSCRIPT with iodine (for a few minutes), MTOACl (as a 1% v:v solution in MIBK) was added, and the final measured solutions contained 0.006% iodine and 0.1% MTOACl [30,31]. Certified reference materials called “Lead in Gasoline Level 4”, containing 2.64 ± 0.13 mg L-1 Pb (AccuStandards, Inc., New Haven, USA) and “XRF petroleum CRM”

work, and they were labelled as CRM1 and CRM2, respectively.

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containing 13.21 ± 0.26 mg L-1 Pb in isooctane (VHG, Manchester, USA) were applied in this

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Isooctane, toluene, IT, ethanol and E10 (a 10% v:v solution of ethanol in IT) were pre-

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treated as samples and used as model samples. The real samples consisted of automotive gasoline EN 95 [9], aviation gasoline UL 85 [8], aviation gasoline UL 91 [7] and process

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solvents with the boiling range of gasoline.

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The investigated samples were diluted in a 1:10 v:v ratio, unless indicated otherwise.

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3. Results and discussion

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3.1. Investigation of the model solutions and CRMs using HR-CS FAAS

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Preliminary experiments that were performed using HR-CS FAAS showed significant differences in the spectra obtained for various samples and analysis conditions, which can

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indicate spectral phenomena as a source of inaccuracy in Pb determination. Therefore, systematic studies of spectral interferences were undertaken. For these studies, isooctane, toluene, IT mixture, ethanol, E10 and both CRMs were selected. The substances/materials represent groups of components/matrix of gasoline. They were prepared as samples using a 1:10 dilution factor [30,31]. In accounting for the alternative version of PN 237 [31] (Table 1), both CRMs were also diluted in a 1:5 ratio. Taking into account the low level of Pb in

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ACCEPTED MANUSCRIPT CRM1, and to better reveal the possible matrix effects, the CRM1 was also diluted in a 1:2.5 v:v ratio. The recommendation of both standard procedures [30,31] is the application of an oxidative flame with a light blue colour. However, the colour and its intensity are not an accurate specification. There are also limitations in the flame adjustment. On one hand, the

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amount of available air could be too small (a hardware limitation), and the flame would be of

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green or even yellowish colour due to the presence of non-burned incandescent carbon. On

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the other hand, in the case of a very lean flame, flashback is possible, which would result not only in an extinguished flame but also in a ruptured protective membrane. The determination

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of the influence of the flame character on the spectra/analysis results could be important in the research undertaken in this work.

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It was found that when a fuel lean flame was adjusted (blue colour), there were no perceptible changes of the flame appearance during aspiration of the calibration solutions and

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most of the sample solutions. Only for CRM solutions diluted in a 1:2.5 v:v ratio a yellowish

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layer of a height of approximately 12 mm was observed over the burner slit. For a fuel rich flame, the flame was always of an intense yellow colour. The height of the yellow part was

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30-75 mm and depended on the aspirated solution (which will be discussed later). Some of the obtained spectra are presented in Figures S1-S4 (Appendix). The Figures

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refer to measurements at the 217 nm or at the 283 nm Pb lines, performed using fuel lean or fuel rich flame. The Figures refer to the analysis of isooctane, toluene and ethanol as samples or to analysis of various dilutions of CRM 1. Because a structured background was registered in some experiments, the example spectra were more precisely characterised and specified in Table S1 (Appendix) and in Figures 1. Figs. 1a-1b were obtained for MIBK while Fig. 1c for CRM2.

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ACCEPTED MANUSCRIPT Generally, the spectra obtained for CRM1 and CRM2 were similar to one another, and the spectra obtained for ethanol (which was prepared as a sample in a 1:10 v:v dilution) were similar to the spectra obtained for MIBK (a direct solvent measurement). In considering Figures S1-S2 (Appendix), refereeing to the 217 nm Pb line, it can be stated that when a fuel lean flame is used , the baseline noise is very small. The application of

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a fuel rich flame causes a significant increase in the baseline noise, which is the highest for

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the lowest CRM1 dilution. The increase of noise can be attributed to the more intense

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radiation scattering due to the presence of large amount of incandescent carbon in the fuel rich flame.

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Additionally, a structured background can be clearly distinguished from the baseline noise in measurements in the vicinity of the 217 nm Pb line using the fuel rich flame (Figures

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S1d-S1f, S2d-S2f, Appendix). A similar background can be distinguished in the spectrum taken for MIBK (solvent) with the indicated characteristic lines (Fig. 1a).

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To identify the origin of the obtained structured spectra, spectra published in the

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literature were inspected. Because the pattern of the structured background depends on the molecule and does not depend on the analytical technique, both flame and graphite furnace

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atomization data could be taken into account. Generally, it is much more difficult to identify the molecules causing the structured spectra than the interfering atoms with single line spectra

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because tabulated spectroscopic data are available only for the interfering atoms [33,36,38]. For the structured spectra in the vicinity of the 217 nm line, most of the published data concern samples of complicated composition that contain huge concentrations of elements such as S, P, Si, N and some metals (e.g., Ca), which all can be blamed for the spectral interferences [33-35,39-45]. The samples investigated here do not contain these elements at a significant level, and none of the published spectra resemble the recorded spectra. An overview spectrum of the “pure” (without solution aspiration) air-acetylene flame should also 10

ACCEPTED MANUSCRIPT be considered [33,34]. In the 200-240 nm region, an absorption occurs in the flame, which can be attributed to absorption by an unknown molecule or by an NO molecule [33,34]. Careful inspection of the overview spectrum of the NO band system shows that the 217 nm Pb line is situated between two groups of rotational lines in the area of a very small NO absorption at the noise level [33]. Thus, the detected structured spectra are not caused by the

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NO molecule. Finally, only a suggestion offered by the ContrAA software [36] seems to shed

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light on the origin of the signals recorded in this work. It was found that the considered

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spectra are similar to the OH molecule spectrum [36]. Although the OH spectrum [36] has a less subtle structure, or is less disturbed by noise, it contains the characteristic lines, or groups

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of lines, described in Table S1 (Appendix) and depicted in Fig. 1a.. To the best of the authors’ knowledge, this study is the first in which the appearance of a structured spectrum in the

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vicinity of the 217 nm Pb line, attributed to the OH molecule, is reported to a wider audience. It is important to note that the direct overlap of the 217 nm Pb line with a less intense

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rotational line in the structured spectrum occurs (there is a group of rotational lines in the

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range of 216.978 to 217.005 nm, which correspond to the pixel number range of 99 up to 105). This result indicates the possibility of spectral interferences when using both LS FAAS

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and simple variants of HR-CS FAAS.

In all the measurements performed in the vicinity of the 283 nm Pb line, a structured

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background was registered, with primary peaks numbered in Figs. 1b - 1c and characterized in Table S1 (Appendix); considering the suggested spectra from ContrAA [36] and some spectra published in the literature [33,34,46], there is no doubt that the structured spectra registered in this work in the vicinity of the 283 nm Pb line are the OH molecule spectra. The spectra are a part of a band, which belongs to the Δ=+1 vibrational sequence in the electronic transition between the X2 and A2∑+ state [33].

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ACCEPTED MANUSCRIPT The fine-structured band registered in the vicinity of the 217 nm line could be attributed to a vibrational sequence of a higher change in the vibrational excitation in the same electronic transition. The spectra registered in the vicinity of the 283 nm line, using the fuel lean flame (Figs. S3a-S3c and S4a-S4c, Appendix), contain peaks of greater amplitude (deep minima,

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high maxima) than all the spectra registered in the vicinity of the 217 nm Pb line. Among the

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spectra registered in the vicinity of the 283 Pb line, some spectra contain peaks of relatively low amplitude (with a background “absorbance” range from approximately -0.020 up to

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approximately 0.020), and their peaks can have a “positive-negative character” (Figs. S3a-S3b

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and S4a, Appendix). Some OH molecule spectra contain only high maxima of absorbance up to approximately 0.040; for example, the spectra for ethanol (Fig. S3c, Appendix) and for

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MIBK (Fig. 1b). However, the OH molecule spectra contain only deep minima in the case of lower dilutions of CRMs (with a minimum “absorbance” of approximately -0.070 up to

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approximately -0.030, Figs. S4b and S4c, Appendix, as well as Fig. 1c). It is important to

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underline that the intensity of the OH spectrum registered in the vicinity of the 283 nm Pb line using the fuel lean flame significantly depends on the sample and its concentration.

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The application of a fuel rich flame instead of a fuel lean flame causes a significant decrease of the structured background that is registered in the vicinity of 283 nm Pb line,

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which is now in the range of absorbance of approximately -0.002 up to approximately 0.010. This finding would suggest that in the fuel rich flame, the population of OH molecules is decreased. This finding seems not agree with the observations for the 217 nm line and would dispute the origin of the structured BG at 217 nm as coming from an absorption by OH molecules. However, in fact, the appearance and intensity of a structured BG due to OH molecule depends mainly on the compensation capabilities for a given analysis, i.e. on a difference between a result of a measurement for sample solution and for a blank solution, not 12

ACCEPTED MANUSCRIPT on the total amount of OH molecules. The compensation capabilities may be different for various wavelengths as in different wavelength range various additional phenomena can occur and influence BG correction efficiency. For example, when using 217 nm Pb line and fuel rich flame a very intense radiation scattering by incandescent carbon occurs as scattering effect is irreversibly proportional to the fourth power of wavelength [47]. It is sure, that

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processes occurring within a flame are very complex.

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It is important to note that the rotational lines of the OH molecule spectrum do not

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directly overlap with the 283 nm Pb line and are mostly situated outside the 0.1 nm wavelength range around the 283.306 nm Pb line (Figs. 1b and 1c). Therefore, their effect is

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not expected to be observed in HR-CS FAAS and is not expected or is at least significantly diminished in LS FAAS when a narrow bandpass of 0.1 nm or less is used.

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The influence of the registered molecular spectra on the results of Pb determination can be verified by an evaluation of the results of the analysis of CRMs. The results of CRM1

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and CRM2 analyses, as presented in Figs. 2a and 2b, respectively, were obtained for various

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sample dilution and analysis modes. Another important evaluation was performed on the basis of the model sample analysis, the results of which are presented in Fig. 3 (Figs. 2 and 3 also

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contain data for LS FAAS, which will be interpreted later). The model samples, isooctane, toluene, ethanol, and E10, as well as the solvent, MIBK, did not contain an amount of Pb

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detectable by FAAS, and the results of their analysis, as expected, are close to zero. The results for the CRMs are mostly inside the confidence interval (all the results obtained with the lean flame are inside the interval). The results obtained using both the Pb lines in different ranges of the electromagnetic spectrum are mutually consistent as well, which additionally confirms the lack of spectral interferences in HR-CS FAAS. Spectral interferences are not present for the 283 nm Pb line because the lines of the OH molecule spectrum are distant from the Pb line. For the 217 nm line, the structured 13

ACCEPTED MANUSCRIPT background overlaps with the Pb line, but it does not have a practical effect when a lean flame is used. The higher intensity of the structured background in the vicinity of the 217 nm Pb line when using the fuel rich flame could be responsible for some of the decrease in the Pb determination results (Fig. 2a, M7 Procedure). In HR-CS FAAS, a tool to eliminate the effects of the direct overlapping of an

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interferent line and an analyte line is the least squares background correction (LSBC).

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Generally, to obtain an analyte spectrum that is free from overlap in LSBC, an interferent

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spectrum is generated and mathematically subtracted from the sample spectrum. In the only publication to address the OH structured absorption at the 283 nm Pb line [33], the efficiency

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of LSBC in eliminating the background absorbance in aqueous solution analysis (with an absorbance from -0.002 to 0.004) was demonstrated. At present, a convenient method, known

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as the permanent structures (PS) option, is available [36]. This option is a special version of LSBC that uses the interferent spectrum that is stored and ready to be used in ContrAA

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discussed in Section 3.3.

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software. The application of LSBC/PS for Pb determination in unleaded gasoline will be

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3.2. Investigation of model solutions and CRMs using LS FAAS

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In LS AAS, BG correction by the application of a deuterium lamp is not feasible to correct the structured BG due to the absorption by diatomic molecules [47]. However, the effect of the OH molecule in a flame is specific because it is usually a stable (permanent) effect, which should be eliminated by the blank correction, even in LS FAAS [33,36]. The effect of OH molecule absorption is not expected in graphite furnace AAS. The absorption by an OH molecule in an air-acetylene flame was investigated for the first time by H. Massmann [47,48] (for the 306.8 nm Bi line). His important finding, which 14

ACCEPTED MANUSCRIPT corresponds to the data from this work, is the dependence of the OH molecule background compensation efficiency on the flame character [48]. Massmann also found that the OH molecule underwent Zeeman effect (a Zeeman system would not correct the BG caused by the OH molecule). Generally, literature on the OH molecule absorption in FAAS is very scarce. This is

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probably due to the equalizing action of the flame, which eliminates the OH molecule

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absorption effect in most FAAS analyses.

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In this study, the results of Pb determination in gasoline using HR-CS FAAS showed differences in the OH spectra, depending on the measured solution. Similar experiments, to

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those using HR-CS FAAS were performed using LS FAAS (Table 2, Figs. 2 and 3). To obtain additional information on the BG in LS FAAS, signals for the total absorbance (T, the

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measurement of the Pb lamp intensity), the background absorbance (BG, the measurement of the deuterium lamp intensity) and the atomic absorbance (AA, obtained by subtraction of BG

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from T) were inspected for a few seconds of measurement time. The BG absorbance values

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read from these plots are presented in Table 3, and some examples of the evolution of the AA, BG and T signals, as obtained for model solutions using 217 or 283 nm Pb lines, are depicted

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in Figs. S5 and S6, respectively (Appendix). In these figures, in some situations the wrong BG correction is indicated by negative BG and/or T and/or AA signals. A negative AA

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signal/result is evidence of an overcorrection, for an understated result (the average BG measured using the deuterium lamp is higher than the actual BG at the analyte line). An undercorrection presents the opposite situation. Taking into account the data from Table 3, Figs. 2 and 3 as well as Figs. S5 and S6 (Appendix), it can be stated that the BG registered in LS FAAS has a wide range (from a highly negative “absorbance” of approximately -0.22 up to a highly positive absorbance of approximately 0.5-0.6) and that an incorrect BG correction can occur even though the BG is 15

ACCEPTED MANUSCRIPT relatively small. The BG absorption is usually much smaller when using the 283 nm line than when using the 217 nm line. A very important finding is the different behaviours of the various substances tested as model samples. The most spectacular difference occurs when a lean flame, 217 nm line and 0.2 nm bandpass are applied (M3 procedure): an undercorrection occur in the isooctane

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analysis and an overcorrection, in the toluene analysis (Fig. 3 as well as Figs. S5a and S5b,

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Appendix), when performed against standards prepared using an IT mixture (1:1, v:v) for

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matrix matching [31]. These differences occur even though isooctane, toluene and the IT mixture are diluted in a 1:10 v:v ratio.

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Clearly, gasoline can have a highly variable hydrocarbon composition. For example, automotive gasoline can contain up to 35% v:v of aromatic hydrocarbons and up to 18% v:v

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of olefins [9], while 85 UL gasoline can contain up to 35% aromatic hydrocarbons and only 4% olefins [8]. The application of an IT 1:1 v:v mixture for matrix matching is a

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simplification. A better solution seems to be the recommendation to apply lead-free gasoline

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for matrix matching/calibration curve preparation [30], but it should be a gasoline of appropriate composition (known and similar to that of the samples).

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The spectral effect for a sample can significantly depend on the analysis condition. For example, for isooctane, in always using the 217 nm Pb line, an undercorrection (M3

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Procedure with a 0.2 nm bandpass and fuel lean flame), a right correction (M4 Procedure with 0.1 nm bandpass and fuel lean flame) or an overcorrection (M9 Procedure with 0.2 nm bandpass and fuel rich flame) can occur, which can be observed in Figs. S5a, S5d and S5g (Appendix), respectively, as well as in Fig. 3. As shown in Fig. 3, the positive and negative errors for the model samples are much lower when using the narrower bandpass of 0.1 nm, which concerns both the 217 and the 283

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ACCEPTED MANUSCRIPT nm Pb lines. The better accuracy when using the 0.1 nm bandpass and the 283 nm Pb line can be explained by the cutting off of the OH molecule line regions. The results for E10, the 10% v:v solution of ethanol in IT, were correct (Fig. 3) in the investigated cases upon application of the lean flame (M3-M6 Procedures). This finding indicates that a 10% v:v addition of ethanol to automotive fuel [9] does not influence the

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accuracy of Pb determination.

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The investigations using LS FAAS show severe difficulties in BG correction when

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using a rich flame. The difficulties occur at both Pb lines, although they are much greater at the 217 nm line. The signals can be unstable when using rich flame (e.g., Fig. S5g,

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Appendix). However, the primary problems are from severe BG overcorrection. Among the various compounds, the highest overcorrection was observed for ethanol as a sample or for

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solvent (MIBK) measurement. The BG absorbance readings were -0.15 or -0.22, respectively, and an absurd reading of a Pb concentration of “-5 mg L-1” in the recalculation for the initial

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sample can occur. The recommendation of both standard procedures [30,31] to maintain the

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fuel lean flame is strongly confirmed here. The BG registered for both CRMs is similar and decreases similarly with sample

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dilution. The BG registered for both CRMs at the 217 nm Pb line does not depend significantly on the bandpass (0.2 nm or 0.1 nm) and on the flame character (Table 3).

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Erroneously high results are obtained for low dilutions of CRMs using the 217 nm Pb line (Figs. 2a), which can be correlated with high BG as well as with CRM hydrocarbon composition. CRM1 contains approximately 85% v:v isooctane, according to a chromatographic analysis in OBR JSC, while CRM2 contains approximately 100%, according to the certificate. Thus, for CRMs, a behaviour similar to the behaviour of isooctane can be expected, and the behaviour was actually observed. The effect is lower when using the narrower bandpass of 0.1 nm, similarly to the case of isooctane. The effect that occurs at the 17

ACCEPTED MANUSCRIPT 217 nm Pb line due to the BG, which is not fully compensated for, is clearly observed in the isooctane analysis as overstated results. This effect was not found in HR-CS FAAS. HR-CS FAAS has the basic option of continuous event correction, which is performed using correction pixels in WRC mode or absorption minima in IBC mode [33,36,37]. The IBC option used here probably corrected for some effects that occurred during various model

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samples analyses. Therefore, BG absorbance of the 0.6 level and undercorrection were not

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observed in HR-CS FAAS. It is important to add that analysis by HR-CS FAAS (IBC and

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WRC options) is simultaneous, while LS FAAS measurements of the BG and T are performed sequentially. For quickly changing BGs, a time shift can lead to errors.

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It is interesting to notice that differences in the flame appearance for various model samples can be distinguished (especially for the fuel rich flame, Table 3). As the flow rate of

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the aspiration of various solutions was always similar (due to similarity of their properties as viscosity and surface tension, among others), the relationship between the flame appearance

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and the combustion differences of the investigated substances could be considered. For

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isooctane, toluene and ethanol, the lower heating value (LHV) differs significantly as it is equal to 44.3, 40.6 and 28.9 MJ kg-1 (25°C), respectively. Thus, the higher the heating value

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of a sample, the more heat is released when the sample solution is combusted, the temperature of the flame is higher, and the zone of the yellow luminous flame is larger. This agrees well

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with the data from Table 3 in terms of both, flame appearance and BG absorbance in LS FAAS at the 217 nm line. For example, for isooctane, the LHV is higher than that for IT (blank); thus, the yellow luminous zone is larger and the BG absorbance is also higher. On the other hand, for toluene and ethanol, the LHV is lower than that of IT (blank), the yellow zone is smaller, and the BG absorbance is lower (for ethanol, it is even “negative”). The phenomena described above cannot be simply/only correlated with higher or lower scattering of radiation. Undoubtedly, the radiation scattering is higher in the fuel rich 18

ACCEPTED MANUSCRIPT flame due to the greater amount of incandescent carbon, but this cannot be simply correlated with the BG observed at the 217 nm line. For example, a similar BG was registered for CRM1, diluted 1:2.5 v:v, independently of the flame character (217 nm line, Procedures M3 and M9, Table 3). All the above data demonstrate the complexity of the processes occurring when

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organic solutions are aspirated into an air-acetylene flame. It is not possible to fully explain

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all the findings in this work. It appears that apart from radiation scattering and the structured

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background observed at both wavelengths and interpreted as absorption by the OH molecule (283 nm) or probable absorption by the OH molecule (217 nm), other phenomena can occur

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due to the variety of organic constituents present in the samples.

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3.3. Figures of merit, analysis of real samples and recommendations

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The parameters that characterize the sensitivity (characteristic concentration), linearity

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of calibration (correlation coefficient) and detectability (detection and determination limits) of the Pb determination were determined for HR-CS FAAS (Table 4) and LS FAAS (Table S2,

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Appendix). The detection and determination limits were calculated as three or seven standard deviation values for ten blank solutions prepared in parallel.

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In all the investigated options, the linearity of the calibration curve is good, and the sensitivity is satisfactory compared to expectations [47,50-54]. Usually, in HR-CS FAAS the best detection limit for narrow lines is achieved using 3 pixels, while for broad lines, the application of 5 pixels is more advantageous [49]. The experimental data (Table 4) show that the detectability is similar when using 3 or 5 pixels for analyte signal evaluation. This similarity occurs at both Pb lines. This finding can partially

19

ACCEPTED MANUSCRIPT explain differences in the choices undertaken in various works concerning Pb determination using HR-CS FAAS, e.g., 3 pixels [50,51] or even 7 pixels [52-54]. Another factor that can influence the detection is the applied background correction mode, i.e., the basic mode of background correction (WRC, an approximation of the baseline using correction pixels, and IBC, an approximation of the baseline using the absorption

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minima), as well as LSBC (the PS option) [36,37,49]. The data in Table 4 are supported by

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Figs. S7-S10 (Appendix), obtained for ethanol and CRM1, using the 217 nm or 283 nm Pb

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lines, with the application of IBC or WRC, and with or without the additional option of PS. It can be seen that the detectability depends significantly on the BG correction mode, that is, the

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baseline fitting mode (it is always better to use the IBC version than the WRC version) and the application of the PS option (the PS option is recommended when using the 217 nm line,

line, especially in the IBC version).

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especially in the WRC version, and it is strongly not recommended when using the 283 nm Pb

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The data correspond very well to the findings concerning molecular absorption at both

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wavelengths. PS eliminates the effect of the structured background directly overlapping with the 217 nm line; therefore, its application is advantageous at this line. The application of PS

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worsens the determination at the 283 nm wavelength because the Pb line is not overlapped at all by the OH molecule spectrum, and treatment of the high-intensity rotational lines of the

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OH molecule in the PS option increases the noise level and influences the baseline (Figs. S9c and S10c, Appendix).

The detectability of the optimized versions of LS FAAS and HR-CS FAAS makes it possible to detect 0.010-0.020 mg L-1 Pb in a measured solution. Taking into account the 1:10 v:v dilution ratio, the detection limits in the original gasoline sample are 0.1-0.2 mg L-1. This is satisfactory for the control of Pb level in automotive or aviation gasoline. There is also a

20

ACCEPTED MANUSCRIPT room to decrease the lower range of the standard methods of Pb determination [30-31]. In this way, the expectations of car producers would be met [23]. According to the findings from the previous sections, the results of CRMs analysis would not be enough to provide good accuracy because both investigated CRMs as well as the CRMs available on the market contain primarily isooctane, and their analysis would not

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reveal some difficulties associated with the variable composition of gasoline samples.

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Therefore, analyses of real samples were performed. Careful inspection of the area in the

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vicinity of the 217 and 283 nm Pb lines did not reveal any unexpected effects/lines, apart from those reported in the previous sections. Because the Pb content in all the investigated

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aviation and automotive gasoline samples was below the best detection limit (0.010 mg L-1), spiking tests were performed. The recovery that was achieved in the spiking tests for the

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gasoline samples, as well as for the model samples, was satisfactory (Table S3, Appendix). However, in the spiking test measurements, the same matrix is investigated and background

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compensation is relatively easier.

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Some real samples (process solvents) contaminated with a low level of Pb were analysed as well (Table 5). Sample 2 was analysed twice, with the second analysis occurring

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within a month of the first one. The sample was analysed at various dilutions, and each time, the dilutions were prepared on the day of analysis. As shown in Table 5, all the results

LS FAAS.

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obtained using HR-CS FAAS are mostly consistent, but there are discrepancies when using

In view of all the findings in this work, the recommendations for Pb determination using LS AAS are as follows: the 283 nm line, a 0.1 nm bandpass, a fuel lean air-acetylene flame and a 1:10 v:v sample dilution. For HR-CS FAAS, the best option is as follows: application of the 283 nm line, IBC baseline fitting without LSBC, 5 pixels for signal evaluation and a fuel lean air-acetylene flame. In HR-CS FAAS, lower sample dilutions (1:5, 21

ACCEPTED MANUSCRIPT 1:2.5 v:v) are possible, which would enable the method to reach a 0.04 mg L-1 detection limit in the original sample.

4. Conclusions

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The application of HR-CS FAAS for Pb measurement in solutions of gasoline in

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methyl-isobutyl ketone enabled us to reveal that the absorption of the investigated radiation

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by OH molecules (which are always present in air-acetylene flames) can significantly differ when the blank/standards and the samples are aspirated into the flame. Therefore, the

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absorption can remain uncompensated for, despite blank subtraction, and can be a source of error.

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OH molecule absorption occurs as a structured BG at the 283 nm Pb line (previous literature on the topic was very scarce). The structured background at the 217 nm Pb line,

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which can also be attributed to OH molecule absorption, was announced for the first time in

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this work. Both effects were carefully investigated here, in terms of matrix variability as well as analysis conditions, and this research led to proposed conditions in which spectral

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interferences due to absorption by OH molecules do not exist or are at least significantly reduced in both HR-CS FAAS and LS FAAS.

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In LS FAAS, apart from the OH molecule effect, additional phenomena could be distinguished by the differences in the results obtained for model samples, for example, isooctane and toluene, representing different groups of hydrocarbons. In some analyses, a higher concentration of isooctane leads to a high background (with absorbance up to 0.6) and the overstating the Pb determination results. The background, unlike OH molecule absorption as well as radiation scattering, does not significantly depend on the flame conditions. This absorbance is not observed in HR-CS FAAS. 22

ACCEPTED MANUSCRIPT The studies in this work are on a very specific topic but have more general meaning and shed new light on spectral phenomena that occur in air-acetylene flames. The structured background due to absorption by OH molecules should be taken into account during the determination of other analytes in other materials, especially at low absorbance levels. Some correlations between lower heating values of model samples, flame appearance

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and BG intensity or BG character have been found. The topic will be developed in future

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works.

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Acknowledgements

The experimental part of this work was performed in OBR JSC (Warter Group), and

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the authors are grateful for the opportunity to perform the research. The authors also thank Bożena Słupska from OBR JSC for providing technical assistance.

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This work was in part financed by the Warsaw University of Technology (statutory funds no. 504/02351/7191/40.000101), and its support is gratefully acknowledged.

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Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/...

References:

23

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Figure captions

1. Characterisation of the OH molecule absorption spectra: the 217 nm Pb line and fuel rich

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flame for the measurement of MIBK (a), the 283 nm Pb line and fuel lean flame for the measurement of MIBK (b) and the 283 Pb line and fuel lean flame for the measurement of

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CRM2 1:5 v:v (c).

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2. Results of CRM1 (a) and CRM2 (b) analysis under different conditions. For a given procedure, the average result of measurements for two sample solutions (prepared in parallel

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at a given dilution ratio) is shown as a bar with marked uncertainty (± standard deviation, n=2). The central horizontal lines represent the certified values. For the CRM1, the distance

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between the central line and the border line represents the square root of the sum of the squares of the standard uncertainties of the certified values and the analysis values. For CRM2, the border lines are distant from the central line by the reproducibility of the EN 237 method [31]. 3. Readings of Pb concentrations for the solvent (MIBK) and solutions of organic compounds, prepared as samples (dilution ratio in MIBK 1:10 v:v). No Pb addition. For a given procedure and a given model sample two results for two solutions prepared in parallel are shown as bars 30

ACCEPTED MANUSCRIPT with marked uncertainty (± standard deviation, n=2). For E10, only one solution was prepared and measured. The lines parallel to X axis are distant from the X-axis by determination limit (assumed to represent confidence interval of results for analyses of samples which do not contain Pb). In the case of the main figure it is determination limit of the M2 Procedure (the lowest between M1-M8 Procedures). In the case of the sub-figure it is determination limit of

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the M9 Procedure (the lowest between M9-M10 Procedures). The readings of Pb content

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outside horizontal lines are assumed to suffer from systematic errors.

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Figure 1a

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ACCEPTED MANUSCRIPT Table 1 Primary features of standard methods of Pb determination in unleaded gasoline using FAAS. ASTM 3237 [34]

EN 237 [ 35],

EN 237 [ 35],

recommended mode

alternative mode

2.5-25

2.5-10

3.0-10

Wavelength, nm

283.306

217.000

283.306

Sample used for matrix

Lead-free gasoline

Isooctane :toluene

Isooctane :toluene

mixture, 1:1 (v:v)

mixture, 1:1 (v:v)

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Range of the method, mg L-1

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matching in calibration curve

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preparation 1:10

1:10

1:5

Standard

PbCl2

PbCl2

PbCl2

Repeatability, mg L-1

1.3

0.12

0.08

Reproducibility, mg L-1

2.6

0.62

0.40

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Sample dilution ratio, v:v

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Table 2 List of analysis modes. Analysis Apparatus mode

Wavelength , nm

Bandpass,

Flame character L - fuel lean

nm

HR-CS

217.001

HR-CS

283.306

LS FAAS LS FAAS LS FAAS LS FAAS HR-CS

217.0 217.0 283.3 283.3 217.001

L

0.2 0.1 0.2 0.1 -

L L L L R

283.306

-

R

217.0 283.0

0.2 0.2

R R

NU

SC

FAAS M3 M4 M5 M6 M7

HR-CS FAAS LS FAAS LS FAAS

AC

CE

PT E

D

M9 M10

MA

FAAS M8

L

-

FAAS M2

-

R - fuel rich flame

RI

M1

PT

flame

39

ACCEPTED MANUSCRIPT Table 3 Readings of background absorbance in measurements using LS FAAS and some observation of flame appearance. Sample

Readings of background absorbance

solution

dilution

M3

M4

M5

M6

M9

M10

the yellow

(217

(217

(283

(283

(217

(283

layer

nm,

nm,

nm,

nm,

nm,

nm,

of rich flame,

0.2 nm,

0.1 nm,

0.2 nm,

0.1 nm,

0.2 nm,

0.2 nm,

mm*

L)

L)

L)

R)

R)

RI

SC L)

NU

Model samples

The height of

PT

Sample/

1:10

0.07

0.08

0.007

0.008

0.07

-0.020

50

Toluene

1:10

0.006

-0.024

0.008

0.002

-0.010

0.020

40

Ethanol

1:10

-0.001

-0.05

0.007

-0.002

-0.15

-0.016

30

MIBK

-

-0.002

-0.06

0.002

-0.002

-0.22

-0.013

30

E10

1:10

0.020

0.015

0.005

0.005

0.040

0.002

45

D

0.007

0.006

0.05

-0.012

45

0.2

0.2

0.015

0.019

0.3

-0.024

60

0.6

0.6

0.020

0.037

0.5

-0.034

75

1:10

0.05

0.05

0.007

0.009

0.05

-0.015

45

1:5

0.2

0.2

0.017

0.021

0.3

-0.020

60

AC

1:2.5

0.05

CRMs

0.05

1:5

CRM2

PT E

1:10

CE

CRM1

MA

Isooctane

*for the calibration solutions (IT 1:1 v:v matrix) the height of the yellow luminous layer was approximately 45 mm; all measurements were performed at the height of 7 mm over the burner slit

40

ACCEPTED MANUSCRIPT

Table 4 Sensitivity and detectability data for the determination of Pb using HR-CS FAAS. 217 nm Pb line, background correction: IBC Number of pixels

1

3a

3

5

5

off

off

On

off

on

0.245

0.0887 0.0887 0.0649 0.0650 0.0591 0.0572 0.0569c

7

9

11

off

off

evaluation Permanent structure

SC

Characteristic concentration, mg

NU

L-1 Correlation

off

RI

correction

PT

taken for signal

0.9983 0.9990 0.9990 0.9993 0.9993 0.9995 0.9996 0.9996

coefficient, R 0.018

0.016

solution, mg L-1 Determination limit

0.016

0.014

0.016

0.017

0.023

0.033

1

PT E

D

for a solution, mg L-

0.016

MA

Detection limit for a

217 nm Pb line, background correction: WRC

taken for signal evaluation

1

AC

Permanent structure correction

3

3

5

5

7

9

11

off

off

On

off

on

off

off

off

0.242

0.0876 0.0880 0.0640 0.0643 0.0580 0.0559 0.0553

CE

Number of pixels

Characteristic

concentration, mg L-1 Correlation

0.9981 0.9986 0.9987 0.9990 0.9991 0.9989 0.9985 0.9981

coefficient, R Detection limit for a

0.041

0.041

0.027

0.046

0.030

0.055

0.064

0.075

41

ACCEPTED MANUSCRIPT solution, mg L-1 Determination limit

0.062

for a solution, mg L1

283 nm Pb line, background correction: IBC Number of pixels

1

3b

3

5

5

off

off

On

off

on

0.412

0.156

0.157

0.112

0.113

7

9

11

off

off

Permanent structure

Characteristic concentration, mg

NU

L-1 Correlation

0.101

0.0993 0.0992

SC

correction

off

RI

evaluation

PT

taken for signal

0.9998 0.9997 0.9998 0.9996 0.9997 0.9996 0.9996 0.9996

coefficient, R 0.021

0.012

solution, mg L-1 Determination limit

0.062

0.010

MA

Detection limit for a

0.078

0.013

0.016

0.021

0.022

1

PT E

D

for a solution, mg L-

283 nm Pb line, background correction: WRC

taken for signal evaluation

1

AC

Permanent structure correction

3

3

5

5

7

9

11

off

off

On

off

on

off

off

off

0.409

0.1542 0.1544 0.1108 0.1109 0.0995 0.0975 0.0971

CE

Number of pixels

Characteristic

concentration, mg L-1 Correlation

0.9998 0.9997 0.9997 0.9996 0.9996 0.9995 0.9995 0.9996

coefficient, R Detection limit for a

0.028

0.018

0.019

0.019

0.021

0.027

0.036

0.046 42

ACCEPTED MANUSCRIPT solution, mg L-1 Determination limit

0.042

for a solution, mg L1

the characteristic concentrations were 0.086 and 0.13 mg L-1 and the detection limits for a solution were 0.051 and 0.025 mg L-1 for the analysis when using a fuel rich flame; cthe best parameters are marked by bold letters.

AC

CE

PT E

D

MA

NU

SC

RI

PT

a,b

43

ACCEPTED MANUSCRIPT Table 5 Results of the analysis of contaminated process solvents, mg L-1 (average results of 2-3

Analysis M1

M2

M3

M4

M5

M6

dilution,

number

(CS,

(CS,

(LS,

(LS,

(LS,

(LS,

217 nm)

283 nm)

217 nm,

217 nm,

283 nm,

283 nm,

0.2 nm)

0.2 nm)

0.1 nm)

Sample 1

2.38±0.04 2.26±0.06 2.31±0.10 2.33±0.07 1.81±0.10 -

NU

1:10 *

0.1 nm)

SC

v:v

PT

Sample

RI

parallel analyses ± standard deviations). Fuel lean flame, PS option not applied.

Sample 2 analysis

1.55±0.08 1.44±0.07 1.14±0.03 1.57±0.09 1.67±0.09 1.58±0.08

MA

1:10

1

1.58±0.20 1.52±0.02 1.73±0.02 -

PT E

2 analysis

1.45±0.05 1.50±0.08 1.26±0.10 -

1.43±0.01 -

1

-

-

-

1.19±0.08 -

-

CE

1:5

1.59±0.33 -

D

analysis

analysis

1:2.5

AC

2

analysis

1.61±0.11 1.57±0.06 1.01±0.21 -

0.98±0.02 -

1

-

-

-

<0.4

-

-

analysis 2 Sample 3

44

ACCEPTED MANUSCRIPT 1:10

0.55±0.02 0.54±0.03 0.66±0.07 0.63±0.05 1.12±0.11 0.60±0.05

*for sample 1, the fuel rich flame was also used, and the results of the Pb determination were 1.91±0.20 and 0.33±0.21 mg L-1 for the 217 and 283 nm Pb lines, respectively (0.2 nm

AC

CE

PT E

D

MA

NU

SC

RI

PT

bandpass in both cases, procedures M9 and M10, respectively).

45

ACCEPTED MANUSCRIPT

HIGHLIGHTS

Inconsistencies in FAAS determination of Pb in unleaded gasoline are explained Structured background due to absorption by OH molecules disturbs Pb determination Absorption by OH molecules significantly depends on the gasoline components

PT

Absorption by OH molecules depends on the Pb line, bandpass and flame conditions.

AC

CE

PT E

D

MA

NU

SC

RI

LSBC is not recommended for the 283 nm line and is advantageous for the 217 nm line

46