Spectrochimica Acta Part B 86 (2013) 102–107

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Spectrochimica Acta Part B

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Review

Spectrometric methods for the determination of chlorine in crude oil and petroleumderivatives — A review

Adriana Doyle a, Alvaro Saavedra b, Maria Luiza B. Tristão b, Luiz A.N. Mendes b, Ricardo Q. Aucélio a,⁎a Department of Chemistry, Pontifical Catholic University of Rio de Janeiro, Rua Marquês de São Vicente 225, Gávea, Rio de Janeiro, RJ 22451-900, Brazilb Leopoldo Américo Miguez de Mello Research Center — Petrobras (CENPES), Cidade Universitária, Quadra 7, Ilha do Fundão, Rio de Janeiro 21949-900, Brazil

⁎ Corresponding author. Tel.: +55 21 3527 1319; faxE-mail address: aucelior@puc-rio.br (R.Q. Aucélio).

0584-8547/$ – see front matter © 2013 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.sab.2013.06.003

a b s t r a c t

a r t i c l e i n f o

Article history:Received 25 October 2012Accepted 6 June 2013Available online 14 June 2013

Keywords:Chlorine determinationCrude oilPetroleum derivativesSpectrometric methods

Chlorine determination in crude oil is made in order to guarantee that the oil does not contain levels of thiselement that might cause damages in the oil processing equipment. In petroleum products, the determina-tion of chlorine is made, for instance, to evaluate if there are proper concentrations of organochloride com-pounds, which are used as additives. Such determinations are currently performed following officialguidelines from the ASTM International and from the United States Environmental Protection Agency aswell as protocols indicated by the Universal Oil Products. X-ray fluorescence spectroscopy plays an importantrole in many of these official methods. In contrast, other spectrometric methods based on optical and massdetection are plagued by limitations related to both the fundamental characteristics of non-metals and tothe complex sample matrices, which reflects in the small number of articles devoted to these applications.In this review, the current status of the spectrometric methods, especially the role played by X-ray fluores-cence spectrometry, is evaluated in terms of the determination of chlorine in crude oil and petroleum deriv-atives. Comparison of the performance of the methods, limitations and potential new approaches to ensureproper spectrometric determinations of chlorine is indicated.

© 2013 Elsevier B.V. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1022. Sample preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1033. Reference materials and non-spectrometric official methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1044. Nuclear and optical emission methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

4.1. Neutron activation analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1044.2. Optical emission methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 104

5. Mass spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1056. X-ray fluorescence spectrometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1057. Perspective of the indirect determination of chlorine using enhanced molecular fluorescence spectrometry and molecular absorption spectrometry . 1068. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 106

1. Introduction

The prospected petroleum is a very complex mixture mostly com-posed of hydrocarbons (aliphatic and aromatic), smaller portions oforganic compounds containing heteroatoms such as S, O and N andmetal organic compounds (mostly of Fe, V, Ni and Cu). Water andsediments (sand, mud, silt and precipitates of dissolved solids) are

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also present and compose what is called the basic sediment andwater content (BSW). The water entrained in crude oil can be eitherfree or emulsified within the oil. The emulsified water, which ismore difficult to separate from the oil, may represent up to 60% ofthe total entrained water in crudes and in this water there are salts(mostly chlorides, with a major portion of NaCl, and lesser amountsof MgCl2 and CaCl2) [1]. These salts are incorporated during the for-mation of the oil inside the oil reservoirs, and a small part might beincorporated during prospection, especially in offshore operations[2]. There is a great variation of the salt content in crude oils, which

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depends not only upon the oil source but also on the production wellsand even zones within a field [3]. The non-emulsified water is readilyseparated from the oil (due to the differences in density and polarity)as the mixture is left to rest. The emulsified water, which is not easilyseparated from the oil, may account to up to 20% (in light oils) or 35%(in heavy oils) of the prospected material. Emulsified water is presentas small spherical droplets of water (usually no larger than 60 μm ofdiameter) dispersed and stabilized within the oil (continuousphase) forming a so called normal emulsion (water-in-oil emulsion),a heterogeneous system of two immiscible phases. The treatment ofcrude oil emulsions reduces the quantity of water and therefore thesalt content in the oil that is sent to further processing. The treatmentis made by heating the mixture in the presence of chemical agents toforce the breaking of emulsions and the coalescence of small waterdroplets, which then separates from the oil. Further desalting is gen-erally needed to decrease the salt levels in the oil to 1–5 mg kg−1 [4].The desalting is considered a critical operation at the refinery due tothe importance of meeting the specifications of the acceptable quan-tities of salt and water in the treated oil [2,4]. Such process is basicallya liquid–liquid extraction in a large apparatus where the heated crudeoil is stirred with water in order to take advantage of the favorablepartition, extracting the salts to the aqueous phase. Separation ofthe salty water from oil is improved by an electrostatic treatment [5].

The presence of high boiling point salts is very deleterious since,even when present in small concentration, they accumulate in stills,heaters and exchangers, leading to fouling, which requires extensiveclean-up and therefore longer downtime for equipment maintenance.Sodium and other metals might cause catalyst poisoning. In the caseof the chlorides, they are hydrolyzed at high temperatures forminghydrochloric acid vapors that corrode critical parts of the processingequipment, thus requiring the injection of basic compounds into theoverhead lines to minimize corrosion damage [6]. From the environ-mental point of view, the presence of halogens, such as bromineand to a lesser extent chlorine, contributes to the release of the oxi-dized form of mercury in the atmosphere, also referred to reactivegaseous mercury, especially in oils rich in mercury [6].

Organochloride compounds, known as organic bound chloride,originated from vegetal and animal sources, can also be present. Inaddition, organic bound chloride may be formed during processingand cleaning procedures at the producing site, pipelines and tanks.Organochloride substances may be intentionally introduced into pe-troleum derivatives, for instance, chlorinated paraffins are used asanti-wear additives in lubricating oils, which improves lubricatingperformance at high pressure. The mass fraction of chlorine from ad-ditives in lubricating oils may reach 0.25% or more [7].

High-octane gasoline blending components (also called reformates)are obtained by catalytic reforming of low-octane naphtha and relies onthe use of catalysts (generally composed of noble metals on a metaloxide support). The composition of the catalyst is adjusted to containa halide component (chlorine is preferred), which contributes to thenecessary acid functionality of the catalyst. In order tomaintain the chlo-rine content in the catalyst system within specific mass percent range,a chlorinated hydrocarbon/alcohol mixture is added continuously togenerate HCl to react with the catalyst. However, residual HCl reactswith the hydrocarbons producing halogenated organic compoundsthat appear in the final product (at low mg kg−1 levels) and might notbe totally removed [8].

A crucial step in the quality control of the crude oil is the determina-tion of the salt content, which, in turn,means the quantification of chlo-ride [6]. For petroleum products, the determination of chlorine innaphtha implicates the estimation of organic bound chlorine presentin crude oil [9]. In addition, the quality control of specific products re-quires the evaluation of the right quantities of organochloride additives.From the landmark review of Sychra et al. [10] to the most recent onesthat cover the methods developed to determine hetero-elements in lu-bricating oils [11], automotive fuels [12] and in crude oil and its heavy

fractions [13], no mention on the determination of chlorine is made.In this review, the peculiarities and difficulties related to the spectro-metric determination of chlorine in crude oil and petroleum productsare indicated. The spectrometric approaches are detailed and comparedto the official methods pointing out virtues, limitations and indicatingpotential new trends for such application.

2. Sample preparation

There is a clear advantage in the use of instrumental neutron acti-vation analysis (INAA) and X-ray fluorescence spectrometry (XRF)since their use requires virtually no sample preparation of samplesfor the determination of chlorine.

The use of oxygen as plasma auxiliary gas enables inductivelycoupled plasma optical emission spectrometry (ICPOES) and induc-tively coupled plasma mass spectrometry (ICP-MS) to be used to an-alyze crude oil and petroleum derivatives after a simple dilutionprocedure. However, the use of these atomic and mass spectrometrictechniques for the determination of chlorine without or with mini-mum sample preparation might be plagued by matrix interferences.

Liquid–liquid extraction using aqueous solutions (mixed withalcohols) is an alternative to remove chlorides from crude oil or fromother petroleum fractions after treatment with sodium biphenyl[14,15]. Such procedures, according to ASTM D6470 [14] recommenda-tions, involve heating at moderate temperatures (about 65 °C) andvigorous mixing to guarantee intimate contact between oil sample andthe extraction solution. Recently, Morigaki et al. [4] showed that the pro-cedure to extract chlorine from the oil to the aqueous phase is also effec-tive without heating. Then, the aqueous phase can be analyzed, aftersome dilution, by ion chromatography or by potentiometry.

Waste oil samples were diluted in kerosene and introduced into theplasma in order to enable the detection of several non-metals, includingchlorine, by ICPOES [16]. Amicrowave-induced combustionwas recent-ly proposed to convert the samples into aqueous solutions to be intro-duced into the argon ICP [17]. The procedure avoids the use of stronginorganic acids for sample dissolution since they increase the blankvalues of chlorine. Microwave-induced combustion involves the com-bustion of sample (less than 400 mg) in a closed quartz vessel pressur-ized with oxygen (20 bar) with ignition induced by microwaveradiation (1400 W for 5 min). A small content of NH4NO3 was used toaid the initial ignition. After combustion, the gaseous products areabsorbed in a suitable solution (25 mmol L−1 (NH4)2CO3 solution),which can be done under reflux conditions. Another combustion ap-proach to quantitatively transform all the chlorine into suitable watersoluble species was proposed by Howard and Vocke Jr [18]. This com-bustion is made on a Carius type combustion borosilicate tube placedinside a steel case, since the tube is submitted to high temperatureand pressure. The goal was to mix the sample with an excess ofAgNO3 together with a small volume of concentrated HNO3 in orderto convert all chlorine into solid AgCl. The tube was then heated to250 °C for about 16 h. Then the produced AgCl was washed andre-solubilized with ammonia solution.

Pyrohydrolysis was also proposed in order to transfer all chlorinefrom a complex sample to an aqueous solution [19]. The conversionwas made using a flow of hot water vapor (200 mL min−1) thatpasses through the heated sample placed inside a quartz tube heatedto 750 °C. The vapor converted all chlorine into gaseous HCl or HClOthat were carried out and condensed before being collected in a vesselcontaining an absorbing solution. This absorbing solution (0.75 mol L−1

NH4OH) was used to fix the acid vapor through an acid–base reaction.This combustion procedure is a variation of the one proposed in theASTM D4929 [9], in which the naphtha sample is burned at 800 °C (ina furnace) to convert organochloride compounds into acid vapor,which is collected in an aqueous phase to react with Ag+. A scrubbertube packed with CuO was used to minimize interferences from N, Pand S [20].

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3. Reference materials and non-spectrometric official methods

The accuracy of an analytical application may be accessed by boththe existence of certified reference materials (CRMs) and the exis-tence of official methods of analysis. Only a few reference materialscontaining chlorine are offered, for instance, by the National Instituteof Standards & Technology (NIST), one with a certified values: (NISTSRM 1818a — chlorine in lubricating base oil) and one with a non-certified value (NIST SRM 1634c — trace elements in fuel oil). CRMof salts in crude oil cannot be found since these samples are not ho-mogeneous, a characteristic that is not suitable for CRM. Officialmethods to access the salt content in crude oil are indicated by theASTM International. These methods are used to estimate the amountof chloride in the sample as it is the major type salts in such samples.The ASTM D3230 [21] relies on the conductivity measurement of ahomogenized crude oil diluted with xylene and mixed with a solutionof alcohols and water. Standards are prepared the same way but usingmineral oil in place of the crude oil. The ASTM D3230 indicates salinestandard calibration solutions using the NaCl:MgCl2:CaCl2 proportionin the stock solution equal to 70/:20:10 (of a 10 g L−1 solutions), butadvises that if the saline proportion is known, it should be used in-stead for calibration since the proportion of salts affects the finalconductivity measurement and might produce systematic errors inthe analysis. As these proportions vary widely upon the oil field, itis a difficult task to find appropriate analytical curves for differenttypes of crude oil, which makes the implementation of the methodol-ogy laborious. The limit of detection (LOD) is 3 mg kg−1 reported aschloride.

In the ASTM D6470 [14], the salt content in crude oil is extractedafter diluting the oil in xylene and mixed with an extraction mixturethat is composed of water, acetone and alcohol. The solution is heatedunder vigorous stirring until boiling temperature is reached. Afterphase separation and cooling to room-temperature, the aqueouspart is collected and titrated with AgNO3. The method accounts allhalogen content as chlorides and the LOD is 3 mg kg−1. Liquid–liquidextraction of salts is strongly dependent upon the degree of contact ofthe extraction solvent with the sample. Thus, the use of standardizedrules for the determination of salinity, in most cases, does not guaran-tee good results since these procedures are not universal for all typesof crude oils. The success of these standard methods relies strongly onadaptations based on the experience of the analyst. Besides the risk ofheating flammable organic solvents, bromides and iodides are calcu-lated as chloride and may introduce systematic errors if the chloridecontent is being estimated by this method. The Universal Oil Products(UOP) method 588-94 [15] is a variation of the ASTM D6470 that isused also to quantify the organic bound chlorine. After the first ex-traction of the inorganic chloride, the residual oil is treated with sodi-um biphenyl reagent that reduces organic bound chlorine to chloridethat may also be extracted and titrated.

The ASTM D4929 [9] follows the same idea of converting organicbound chlorine into chloride to be titrated using potentiometric de-tection. The organic bound chlorine in crude oil is estimated by thecontent quantified in the naphtha cut (obtained at 204 °C) used inthe analysis. Alternatively, microcoulometry (after the combustionof sample) may be used for the quantification of chlorine. Combustionconverts all chlorine in the sample to acid vapors that are collected ina cell where they react with Ag+, forming low soluble precipitates.The reacted Ag+ is replaced in a redox process and the total electriccharge involved is directly related to the chlorine in the sample. TheLOD using microcoulometry is 1 mg kg−1. The presence of other ha-lides may impose positive systematic error in the chlorine determina-tion as HBr and HI also react with Ag+. Alternatively, Zanozina et al.[22] proposed the mercurimetric determination of chlorine after theextraction of chlorine into the aqueous phase. However, nowadays,due to the search for more environment-friendly approaches, suchprocedure is not recommended.

4. Nuclear and optical emission methods

4.1. Neutron activation analysis

Instrumental neutron activation analysis (INAA) was successfullyused for the determination of chlorine in petroleum products (gasoline,asphaltene, coke, etc.) [23], and in crude oil fromNigeria [24]. Measure-mentsweremade directly in the samples, requiring no sample prepara-tion. Chorine was determined at the gamma ray peaks at 2166 and1642 keV of the radionuclide 38Cl. The LOD value reported was0.020 mg kg−1 for 5 min sample irradiation time. This elementwas de-termined in bitumen (55 mg kg−1) and in crude oil (0.067 mg kg−1)[23]. For the analysis of seven Nigerian crude oils from different wells,a high purity Ge detectorwas used and the results, obtained by irradiat-ing the oil samples for 600 s, allowed the determination of Cl concentra-tions that were fairly constant (from 14.9 to 15.9 mg kg−1) [24]. Thesevalueswere correlatedwith the 24Na values, alsomeasured by INAA, in-dicating sea salt origin. Despite the advantages in terms of the easy wayto perform analytical measurements, INAA is only available in special-ized laboratories.

4.2. Optical emission methods

In general, it is difficult to perform spectroscopic determination ofnonmetals because of the usually large energy gap between theground state and the excited states for these elements (for Cl, thisvalue is 8.9 eV from the ground state to the first excited state). As aconsequence, the most sensitive resonance lines appear in the vacu-um ultraviolet (for instance at 134.72 nm for chlorine), which re-quires special instrumentation (sealed instruments and purgedoptics). In addition, thermal population of the excited state is poor(about 1% at 7500 K for Cl considering the Boltzmann thermal distri-bution of population), which results in low intensity spectral emis-sion lines [25]. Hughes and Fry [26] reported partial Grotriandiagrams and tables of relative intensities of nonresonant emissionlines measured with an argon ICP for chlorine in the air accessible re-gion between 420 and 990 nm although these lines produce low in-tensities due to small population of the energy levels involved [26].The inefficient population of non-metals and non-metal ions in theexcited state is due to the tendency of these elements to form stable,difficult-to-dissociate molecules, which are often easy to vaporize butdifficult to excite.

Emission from chlorinewasmeasured at 134.72 nmusing an induc-tively coupled argon plasma optical emission spectrometer with opticspurged with dry nitrogen [16]. Waste oil was dissolved in kerosene andintroduced into the ICP using oxygen as auxiliary gas to minimize car-bon deposits in the torch and spectral lines from chemical speciescontaining the element carbon. The LOD reported was 0.9 mg kg−1.Other methods used an axially-viewed configuration and a cross-flownebulizer in combination with a Scott-type double-pass spray chamberto determine chlorine in four heavy oil samples (oAPI b 14) [19].Chlorine was removed from these samples as acid vapors afterpyrohydrolysis, which were solubilized in an aqueous solution to be in-troduced into the ICP. This procedure quantitatively removed the ana-lyte from highly complex matrices to an aqueous matrix, simplifyingconditions of the operation of the ICP. From the four samples analyzed,chlorine was measured in three of them with values of 66.4 mg kg−1,276 mg kg−1 and 13550 mg kg−1. In the fourth sample, the analytewas present in values below the limit of quantification (48 mg kg−1).These experimental values using ICPOES agreed with the ones obtainedusing the ASTM D6470 method.

Microwave-induced combustion was used to solubilize all thechlorine (present in extra heavy fuel oil samples and in CRMs suchas fuel oils and coal) in an aqueous solution containing a proper ab-sorbing reagent. This solution was introduced into the ICP resultingin satisfactory recoveries in the analysis of the CRM. Values of

105A. Doyle et al. / Spectrochimica Acta Part B 86 (2013) 102–107

chlorine in extra heavy crude oils varied from 62.1 to 9287 mg kg−1

and from 94.6 to 233 mg kg−1 in atmospheric and vacuum residueof crude oil distillation [27].

5. Mass spectrometry

Two mass spectrometric methods are reported for the determina-tion of chlorine in petroleum samples and its derivatives. The use ofICP-MS is impaired by the high ionization energies required for chlo-rine, the high background levels of this element in acids and solventscommonly used in such approaches and to severe polyatomic inter-ferences (36Ar1H+, 16O18O1H+ and 34S1H+). Despite these difficulties,quadrupole ICP-MS was used to quantify chlorine in heavy crude oilsafter pyrohydrolysis of the samples [19]. In order to minimize thepolyatomic interferences, a dynamic reaction cell filled with NH3

(0.35 mL min−1) was used with a rejection parameter q of 0.45.The success of the removal of chlorine from the oil to the aqueousphase introduced into the ICP and the minimization of the polyatomicinterferences were evaluated by the analysis of the NIST SRM 1634cwith the experimental value of 44.3 mg kg−1 close to thenon-certified value of 45 mg kg−1. The LOD was 3.6 mg kg−1 andthe results found for heavy crude oils varied from 12.9 to13432 mg kg−1 and agreed with the ones obtained using the ASTMD6470.

Thermal ionization mass spectrometry was also proposed to de-termine chlorine in fossil fuels. In this case, coal samples (NIST SRM1632c trace elements in coal, NIST SRM 2692b and 2685b sulfur andmercury in coal) were used to test the method [18]. Samples weresealed in a glass ampoule containing AgNO3 in order to convert allchlorine to AgCl after heating. The samples were spiked with absoluteisotopic standard for chlorine in order to allow the determination byusing isotopic dilution. AgCl was re-solubilized in water by using am-monia solution and a fraction of this solution was placed in a rheniumfilament placed inside the mass spectrometer. Both the isotope dilu-tion negative thermal ionization mass spectrometry and isotope dilu-tion positive thermal ionization mass spectrometry were used toevaluate the method with good accuracy for the NIST SRM 1632cand the result was comparable with the one achieved by INAAand neutron-induced prompt gamma activation analysis. Resultsfor other materials were in agreement with the reported referencevalues. Blank measurements were high and limiting the sensitivityof the method.

6. X-ray fluorescence spectrometry

The use of X-ray fluorescence spectrometry (XRF) requires virtual-ly no sample preparation. This is a consequence of the characteristicspectroscopic phenomena which involves internal shell electron tran-sitions. The homogenization of the particle sizes would be necessaryfor quasi solid samples such as asphaltenes in order to minimize in-terferences in XRF. X-ray fluorescence is the characteristic X-ray radi-ation (secondary X-ray emission) emitted when an electron, in anionized excited state atom, jumps to fulfill a vacancy left by the ejec-tion of an electron from a more internal electronic layer. Despite thesimplicity of the technique and its capacity to perform analyses insamples in any form two main limitations restrict a broader applica-tion of this analytical technique: the severe matrix interferencesand, in general, the poor sensitivity when compared, for instance, tooptical emission spectrometry [28,29]. For light elements such aschlorine, the sensitivity of the determination is especially affecteddue to the poor efficiency of the absorption that results in photoelec-tric effect and to the weak fluorescence quantum yield (about 0.1 forchlorine). These efficiencies are severely decreased by the dispersedradiation phenomena (scattering of the incident X-ray primary radia-tion) and by the Auger effect [29].

The intensity of the signal measured by the detector can be relatedto the concentration of the analyte (given by the ratio between therelative density of the analyte and the relative density of the matrix)by a very complex equation that comprises three probability factors:(i) the probability of the primary X-ray to reach the analyte in an in-ternal layer of the sample without being absorbed by the matrix,(ii) the probability of the primary X-ray to produce a vacancy in theK shell (K lines are the only relevant ones for light elements) of theatom and this vacancy be fulfilled by another electron (involves selec-tion rules, true absorption efficiency and fluorescence quantum yield)and (iii) the probability of the secondary X-ray not to be absorbed bythe matrix and reach the detector (involves instrumental parameterssuch as the efficiency of the detector).

Two ASTM methods [30,31] are indicated to determine chlorine(as organochloride additive) in unused lubricating oil together withsulfur and other five elements (Ca, Cu, Mg, P, Zn and Mo), also asadditives. These samples are homogeneous and contain relevantquantities of different elements that may interfere with the X-ray de-termination of chlorine (especially the spectral interference from Mo,which can spectrally overlap the chlorine line). The ASTM D7751 [30]uses energy-dispersive X-ray fluorescence spectrometry (EDXRF),where photons from the source of all energies are focused on thesample and the energies of the secondary emission are discriminatedby the detector (a detector with high sensitivity and spectral resolu-tion such as the Si(Li) semiconductor detector). The method indicatesa range of application for chlorine between 10 and 4000 mg kg−1. Acalibration curve was established using 1-chlororoctane in baseoil. Due to the relatively high concentration of the other elements,present as additives in the unused lubricating oil, a matrix correctionwas applied to compensate inter-element effects (attenuation or en-hancement of fluorescence of the analyte of interest). The determina-tion was made by the comparison of intensities against a calibrationcurve using a fundamental parameters approach (combined with cor-rections from backscatter), which uses a theoretical model for thecorrection of matrix effects. The correction term is calculated from ex-pressions derived from basic physical principles and contains physicalconstants and parameters that include absorption coefficients, fluo-rescence quantum yield, primary spectral distribution as well assolid angle characteristic from the instrument geometry. The calcula-tion of concentrations in samples is based on making successivelybetter estimates of the composition of the sample by an iteration pro-cedure, which is, nowadays, included in the software routine of theinstrument.

The ASTM D6443 uses the wavelength-dispersive X-ray fluores-cence spectrometry (WDXRF) [31] and enabled LOD and LOQ valuesfor chlorine (using a Ge crystal to select the excitation wavelengthsfor this analyte) of respectively 4 and 15 mg kg−1. An oil solublechlorine-containing standard was recommended for calibration. Acalibration model to compensate interferences is indicated by usingestimates of the influence coefficient for the effect of each absorbingelement on the analyte.

The United States Environmental Protection Agency (EPA), Method9075, describes the determination of total chlorine in petroleum prod-ucts (used oils, fuel oils) [32]. Themethod does notmention the analysisof crude oils. The applicable range of this method is from 200 mg kg−1

to percent levels. Themethod uses 1-chlorodecane as chlorine standardin mineral oil. The method indicates a procedure to cope with potentialinterferences from sulfur by the use of a calibration curve with correc-tion term specific for S.

Yao and Porsche [33] describe a method to determinate chlorinein liquid petroleum derivatives (gasoline, distillate fuels, residualfuels, lubricating oils, additives) by WDXRF enabling the detectiondown to 200 mg kg−1. The standards were prepared with carbon tet-rachloride dissolved in xylene.

Kendall et al. [34] determined chlorine in used oil byWDXRF usinga germanium crystal detector. The standards of chlorine were

106 A. Doyle et al. / Spectrochimica Acta Part B 86 (2013) 102–107

prepared with 1-chlorodecane. The chlorine in the analyzed samplesvaried from 900 to 12000 mg kg−1.

Recently, an EDXRF method was developed exclusively for the ap-plication in crude oils [35]. A careful study was made to enable homo-geneous samples and representative subsamples for analysis. Themethod allowed the determination of the analyte at lower concentra-tion levels (LOD of 8 mg kg−1). A simple strategy for calibration,using a mixture of an aqueous solution NaCl and glycerin, enabledbetter signal correlation and avoided the use of expensive organicchloride standards and toxic and volatile organic solvents. The instru-ment corrected automatically the influence of oxygen in the matrix(glycerin). The method was applied for the determination of chlorinein different crude oils with results varying from 230 to 3720 mg kg−1

and agree with the results obtained using the ASTM D6470.Total reflection X-ray fluorescence spectrometry (TXRF) is an ap-

proach of energy dispersive analysis using the incident primaryX-ray in a way that it reaches the surface of a sample support (witha specific refraction index and containing a thin film of the sample)at an angle that total reflection of the beam, with a specific energy, oc-curs in a way that the X-ray interacts only with the near-surfacelayers of the sample film [36]. In such configuration, only a verysmall part of the primary beam penetrates into the sample leadingto the drastic reduction of both the spectral background producedby scattering of radiation and the interferences imposed by the ma-trix of the sample. As the incident beam is totally reflected, the sam-ple is excited by both the incident and the reflected beam whichresults in the amplification of the excitation radiance. In addition,the extreme grazing incidence geometry allows placing the detectorvery close to the sample surface, which results in a very efficient sig-nal collection, which potentially improves the sensitivity in the deter-mination of metals and also for non-metals. TXRF has been used forthe direct determination of chlorine present in acidic solutions [37]and in nuclear materials after chlorine separation by pyrohydrolysis[38] with measurements being easily made at 1 mg kg−1. The deter-mination of chlorine in petroleum and derivatives by TXRF has notbeen reported. However, some degree of sample dilution is expectedin order to enable a proper thin film of sample over the sample sup-port. For crude oils, the required minute size sample may not be rep-resentative of the crude oil to be analyzed.

7. Perspective of the indirect determination of chlorine usingenhanced molecular fluorescence spectrometry and molecularabsorption spectrometry

An alternative approach to overcome disadvantages inherent toatomic optical spectrometric methods is to take advantage of thespectrum of small molecules in the gas phase formed by the quantita-tive reaction of chlorine with other elements such as indium (InCl),and aluminum (AlCl) and gallium (GaCl) [39]. Early approacheswere potentially more successful by measuring fluorescence usinglaser excitation (using nitrogen pumped dye laser) and a graphitefurnace where chlorine was supposed to be quantitatively convertedto InCl [40]. The fluorescence scheme at 267.21/359.92 nm allowedan absolute LOD of 15 pg Cl. Although never applied to petroleumproducts, this approach is potentially interesting for crude oils be-cause of the high sensitivity and selectivity of the laser excited fluo-rescence and the capability of the furnace to easily deal withsamples with high organic content.

The advent of modern high-resolution continuum source atomic ab-sorption spectrometers that use a high-intensity continuous excitationsource, a high-resolution double monochromator and a multichanneldetector made possible the absorption measurement of halogens [41].Chlorine was determined as InCl (at 267.24 nm) in a flame [42] with aLOD of 3 mg L−1, however, serious interferences were found when in-organic acids were present. The indirect determination of chlorine asAlCl (at 261.42 nm) in the graphite furnace was also attempted with

an absolute LOD value of 70 pg [43]. The success of these methodsfor the determination of chlorine requires the efficient conversion ofthe analyte into a specific metal chloride vapor, which might be prob-lematic when analyzing complex samples. No application on the deter-mination of Cl in petroleum samples is found in the literature so far.However, the use of high-resolution continuum source molecular ab-sorption spectrometry in such applications might bring advantagesfrom the point of view of sensitivity, selectivity and also practicaladvantages.

8. Conclusions

The determination of chlorine is crucial in the petroleum industry,especially in crude oils. Spectrometric methods play an importantrole in the analysis of petroleum and petroleum derivatives due totheir simplicity and reliability but limitations imposed by the charac-teristics of non-metals degrade the performance of the direct deter-mination of chlorine by optical and mass spectrometric methods.X-ray fluorescence spectrometry is an interesting approach, especial-ly with the minimization of interferences based on mathematical cor-rections and chemometrics. Limits of detection for the light elementchlorine achieved by EDXRF and WDXRF are relatively poor. Howev-er, the total reflection approach might minimize such limitation. Indi-rect determinations using molecular fluorescence or absorption ofeither AlCl or InCl, formed in a graphite furnace, might lead to the de-sired performance and replace current official methods based onprevious cumbersome extractions and conversion of organic boundchlorine into chloride. However, matrix interferences must be mini-mized and quantitative conversion of chlorine into metal chloridevapor guaranteed and reproducible.

Acknowledgments

Authors thank Petrobras, CNPq, FAPERJ and MCT-FINEP for the fi-nancial support. Aucelio thanks CNPq and FAPERJ for the scholarships.

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