Investigation of the sulfur speciation in petroleum products by capillary gas chromatography with ICP-collision cell-MS detection{ Brice Bouyssiere,*a,b Peter Leonhard,b Daniel Pro¨frock,b Franck Baco,c Clementina Lopez Garcia,c Steve Wilburd and Andreas Prangeb a
CNRS UMR 5034, He´lioparc, 2, av. Pr. Angot, F-64053 Pau, France. E-mail:
[email protected] b GKSS-Research Centre, Institute of Coastal Research/Physical and Chemical Analysis, Max-Planck Strasse 1, D-21502 Geesthacht, Germany c Institut Franc¸ais du Pe´trole, CEDI Rene´ Navarre, Vernaison, France d Agilent Technologies Inc., 3380 146th Place SE, Suite 300, Bellevue, Washington 98007, USA Received 28th August 2003, Accepted 2nd February 2004 First published as an Advance Article on the web 19th February 2004
Recent regulations concerning the low-sulfur gasoline require analytical methods able to provide specific information on sulfur containing compounds present in petroleum products at the ng g21 range. The on-line coupling of capillary GC with ICP-collision cell-MS was proposed for the speciation of sulfur in hydrocarbon matrices. The technique showed an absolute detection limit 0.5 pg for a 1 mL sample injected in the splitless mode which is about two orders of magnitude lower than the currently used techniques.
DOI: 10.1039/b310449d
Introduction
700
The international regulations on sulfur concentration in petroleum products in Europe and the US specify less than 0.5% sulfur in marine fuel in 2008 and less than 10 ppm sulfur in gasoil and petrol sold in Europe after 2005.1 In view of the considerable presence of sulfur in some crude oil products efficient desulfurization processes, such as, e.g., hydrodesulfurization must be developed. The available information on the total amount of sulfur (generally measured by X-ray fluorescence) begins to be insufficient for the optimization of the hydrodesulfurization process for the production of the demanded very low sulfur content fuels. Information on speciation of sulfur at low detection limits is becoming an important challenge. Gas chromatography has been the preferred separation technique for S-compounds in petroleum.2–9 Flame photometric (FPD), chemiluminescence and microwave atomic emission detectors (MIP AED) have been widely used,4–11 the particular popularity of the latter was due to the possibility for the simultaneous monitoring of carbon and sulfur.5,7,9,10 High resolution electron impact MS detection was also used in combination with chemiluminescence4 or MIP AED.6 Petroleum products are complex mixtures of a large number of compounds. The elution of large amounts of hydrocarbons during the chromatographic separation results in considerable noise in molecular-MS and even in S-specific detectors, such as MIP AED or chemiluminesence. ICP-MS is a more robust GC detector, presenting a very high selectivity and sensitivity.12–14 The applications in petroleum analysis concerned speciation of mercury,15–17 arsenic,18 lead,19 and the determination of metalloporphyrins in shale oil.20 Sulfur specific detection was much less investigated, the applications reported concerned plants21 and human breath.22 This was due to the necessity of high resolution ICP-MS22 because of the interference problem with the 16O16O and the 32S isotopes. The use of quadrupole ICP-MS implied the choice of the minor 34S isotope21 to eliminate the high background and the possible 16O16O interference on 32S isotope.14 However, polyatomic interferences can now be largely eliminated by the use of a collision cell as it { Electronic supplementary information (ESI) available: linearity, detection limits and retention times of six sulfur-containing compounds analysed by GC-ICP-MS and chromatogram of petroleum sample. See http://www.rsc.org/suppdata/ja/b3/b310449d/ J. Anal. At. Spectrom., 2004, 19, 700–702
was recently demonstrated for the analysis of traditionally ‘‘difficult’’ elements, such as phosphorus or chlorine.23–25 The aim of this research was to investigate ICP collision cell MS detection in gas chromatography of hydrocarbon matrices in order to achieve a substantial decrease of the detection limits in comparison with the currently available sulfur specific detectors (Table 1).
Experimental Instrumentation The GC used in this work was an Agilent Model 6890 Series gas chromatograph (Agilent, Wilmington, DE, USA) equipped with a split–splitless injection port with electronic pressure control. Injections were made, usually in split mode, using an HP 7683 series autosampler. GC separations were performed using a capillary column (HP-5MS, 30 m 6 0.25 mm id, 0.25 mm film thickness). The ICP-MS was an Agilent Model 7500cs ICP-MS system (Agilent Technologies, Tokyo, Japan). The interface between GC and ICP-MS (Agilent) was described elsewhere.13,14,21,24 In brief, the GC capillary column was threaded through a 1 m flexible heated transfer line and further through a heated 10 cm rigid transfer line to the end of the injector of the ICP-MS torch. Chromatographic data were handled using Plasma Chrome software (Agilent). Standards and reagents Sulfur containing compounds were all purchased from SigmaAldrich (Taufkirchen, Germany) with different purity levels (thiophene (99%), 2-methylthiophene (98%), 2-ethylthiophene (97%), benzothiophene (99%), 2-methylbenzothiophene (97%), dibenzothiophene (98%)). All standards were prepared in analytical grade hexane (purchased from Merck, Germany) at the sulfur concentration of 10 000 mg kg21. One crude oil and three products resulting from hydrodesulfurization of crude oil Table 1 Comparison of the experimental detection limit per sulfur component between currently available sulfur specific detectors and our work Detector
Detection limit per sulfur component
References
Chemiluminescence (SCD) Atomic Emission (MIP AED) ICP-MS
50 ng g21 500–700 ng g21 0.5 ng g21
26 27, 28 This work
This journal is ß The Royal Society of Chemistry 2004
were analyzed. Commercial petroleum products including diesel for boilers, diesel and 98 octane gasoline unleaded from a commercial gas station were also analysed as real petroleum samples. Glass vials were used for sampling. Helium (99.999%) and argon (99.999%) gases were obtained from Messer Griesheim (Krefeld, Germany) and were used, respectively, as a carrier gas for the GC and as a collision gas for the octopole reaction cell (ORC), additional carrier gas for the interface between GC and ICP-MS and plasma gas, auxiliary gas and carrier gas for the ICP-MS.
Table 2 Experimental conditions of the gas chromatography ICP-MS setup Instrumentation Chromatographic System Agilent 6890 GC Inlet Split/splitless Column HP-5MS (30 m 6 0.25 mm id 60.25 mm) Detector Agilent 7500 cs ICP-MS Interface Agilent Gas chromatographic conditions
Glassware Glassware was used for all the sample preparation of standards and petroleum products. This glassware was cleaned first with a common detergent for 1 h in hot water, thoroughly rinsed with tap water, then with MilliQ water and soaked for 24 h in 10% HNO3 solution. Finally, glassware was rinsed with MilliQ water and dried.
Injection mode Injection temperature Injection volume Carrier gas (flow rate) Oven temperature programme Transfer line temperature
Split 50:1, 10:1, 5:1 or splitless 250 uC 1 ml Helium (2.5 ml min21 at constant flow) 40 uC (3 min) to 320 uC at 10 uC min21 250 uC
Instrumental conditions
ICP MS detection conditions
The ICP-MS conditions were optimized using the continual signal of the 130Xe isotope naturally present in the GC helium carrier gas and in the ICP-MS argon carrier gas. The argon signal served to tune the MS spectrometer and to optimize ICP operating parameters such as the XYZ position, rf power, lens voltages, and auxiliary, plasma and carrier gas flow rates. These parameters were optimized daily with the objective of reaching the maximum sensitivity for the 130Xe isotope and the minimum background on the 32S isotope. The collision cell was optimized on line in order to obtain the minimum background on 32S isotope with the maximum signal on the 130Xe isotope. Dwell time was optimized, for each sulfur compound, so that intensity data for at least 10 points could be acquired for the narrowest peak in the chromatogram. The optimum GC-ICP-MS conditions are given in Table 2.
Plasma gas (flow rate) Carrier gas Aux gas Isotopes monitored (dwell time) Forward power Sampling depth Reaction cell Collision gas
Argon at 15 l min21 Argon 0.8 l min21 He, 40 psi added to Ar carrier via GC additional controller 32 S (80 ms), 34S (80 ms), 130Xe (10 ms), 13 C (10 ms) 600 W 7 mm On Helium (2.5 ml min21)
Results and discussion Optimization of the ICP-MS forward power for the sulfur signal Fig. 1 shows the ratio between the peak area and the 32S background. On this basis and optimum forward power of 600 W was chosen for future experiments. Analysis of a mixture of sulfur containing compounds The chromatogram of a mixture of sulfur containing compounds in hexane at the concentration of 500 mg kg21 under the GC conditions of Table 2 is presented in Fig. 2. The disturbance at the retention time of 3.5 min is due to changing ionization conditions because of the elution of the solvent. Each compound shows the same response (average peak height ~ 24.8 ¡ 0.9) and the peaks show a Gaussian shape. The dwell time of the ICP-MS was chosen in order to obtained a sufficient number of points per peak for the peak definition.
Fig. 1
Study of forward power efficiency on ionization of
32
S.
GC-ICP-MS of a mid oil distillate sample Fig. 3a and b show example chromatograms obtained for the isotopes 32S and 13C, respectively during an analysis of a petroleum product ‘A’ (mid oil distillate) with a total sulfur concentration of 17400 mg kg21. Due to the high sulfur concentration a split ratio of 50:1 was used for this analysis. The 13C signal demonstrates the complexity of the hydrocarbon matrix. Fig. 3c and d show the zooms of the chromatogram in Fig. 3a allowing the identification of certain peaks by their retention time matching with standards. This shows a distribution of the C1, C2, C3 and C4 benzothiophenes (BT) and dibenzothiophenes (DBT), similar to that described elsewhere.4 The compounds present in the C1, C2, C3 and C4 clusters correspond to different isomers. Fig. 3c and d demonstrate that GC-ICP-MS was able to separate and identify all the main groups of thiophenic compounds in less than 30 min.
Fig. 2 Chromatogram of sulfur containing compounds in hexane with a sulfur concentration of 500 ppb. 1: Thiophene, 2: 2-methylthiophene, 3: 2-ethylthiophene, 4: benzotiophene, 5: 2-methyl-1-benzothiophene and 6: dibenzothiophene.
Figures of merit Table S1{summarizes the parameters (slope and R2) of the calibration curves obtained in the 0.5–100 mg kg21 range for the different S-containing compounds. These data show good J. Anal. At. Spectrom., 2004, 19, 700–702
701
Fig. 3 Chromatogram obtained for a gasoline with 17400 ppm of S (use of split ratio 50:1): (a) sulfur 32S signal; (b) carbon 13C signal; (c) chromatogram obtained for a gasoline with 17400 ppm of S (use of split ratio 50:1) from 0 to 14 min with the correspondence of thiophenic compounds; (d) chromatogram obtained for a gasoline with 17400 ppm of S (use of split ratio 50:1) from 12 to 30 min with the correspondence of thiophenic compounds.
linearity and independence of the response of the sulfur compounds of their chemical formulae. Due to the coelution of thiophene with hexane during a splitless injection, a value could not be obtained for these compounds in this injection mode. This effect does not occur when an undiluted petroleum sample is analysed. However, in the case of undiluted samples analysis the presence of some major hydrocarbon compounds can affect the response factor and the stability of the 32S signal during the solvent elution (cf. Fig. 2). Table S2{ shows the stability of the retention time for the analysis of the six thiophenic standards. No degradation of the detector sensitivity was observed. The independence of the sulfur response of the compound’s chemical formula allows the quantification of S-containing compounds by the use of an internal standard. For the analysis of samples with a high sulfur concentration it is better to apply a split ratio than to use dilution with a solvent. The technique showed an absolute detection limit of 0.5 pg for a 1 mL sample injected in the splitless mode (the detection limit was calculated, in accordance to the IUPAC guideline, by 3 6 standard deviation on the blank baseline divided by the slope of the calibration curve). This is about two orders of magnitude lower than that of the currently used techniques (cf. Table 1).
6 7 8 9 10 11 12 13
14 15 16 17 18 19 20
Analysis of petroleum samples
21
The chromatograms of the six different samples analyzed are shown in Fig. S1. It can be seen that the distribution of the sulfur compounds can be significantly different among the samples depending of the origin and the pretreatment.
22
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