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Different approaches of crude oil mineralisation for trace metal analysis by ICPMS
Microchemical Journal 106 (2013) 250–254
Contents lists available at SciVerse ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Different approaches of crude oil mineralisation for trace metal analysis by ICPMS Georgia Sanabria Ortega a, Christophe Pécheyran a,⁎, Gernot Hudin b, Edit Marosits b, Olivier F.X. Donard a a b
LCABIE, IPREM UMR 5254, CNRS, Université de Pau et des Pays de l'Adour, 64053, Pau cedex 9, France Anton Paar GmbH, Anton-Paar-Str. 20, 8054 Graz, Austria
a r t i c l e
i n f o
Article history: Received 9 May 2012 Received in revised form 20 July 2012 Accepted 22 July 2012 Available online 27 July 2012 Keywords: Extra-heavy crude oil Acid digestion Trace metal
a b s t r a c t Aqueous phase is the most common way to introduce samples into the ICPMS. However, due to the complexity of the crude oil matrix, harsh conditions are necessary to carry out complete acid digestion. In this work four microwave digestion methods based on maximum temperature and pressure programmes (ranging from 220 °C and 40 bar to ~ 1400 °C and 80 bar) were tested and compared with a digestion method in the High Pressure Asher (HPA-S) autoclave. Recoveries of between 80 and 120% were obtained for the different procedures evaluated. The lowest recovery and precision were obtained by the microwave-induced combustion method. The degree of decomposition of the organic material was directly related with the temperature and pressure used for the digestion. It was found that increasing the temperature and pressure by 40° and 20 bar, improved the digestion and allowed an increase in the sample size and therefore the detection limits. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recent developments in atomic spectrometry, particularly in inductively coupled plasma atomic emission/mass spectrometry (ICPOES and ICPMS) have opened new application fields in ultratrace, speciation and isotopic analysis, which represent new and valuable tools for the petroleum industry upstream and downstream. However, the organic sample matrix is still a challenge mainly because spectrometric techniques have been generally developed for the analysis of aqueous samples. Two main methods have been used so far for the analysis of high organic samples: direct injection or mineralisation (by acid digestion or dry ashing). For direct injection, many attempts have been made to introduce organic samples into the plasma while avoiding plasma overload or carbon deposition on the cones or the torch of the ICP spectrometer. Many studies have been devoted to the optimisation of the operating conditions and to the development of nebulisers [1–5] and micro-nebulisers [6–8], the latter being especially recommended when using ICPMS. Several types of micro-nebulisers, spray chambers and automated systems are commercially available, designed especially for the analysis of organic samples. Standard methods for direct injection by ICPOES, like the ASTM D 5708-11 [9] have also been developed. Hyphenated techniques, like ETV with ICPMS [10], LA with ICPMS [11–13] or the use of emulsion [14,15] offer other alternatives for direct injection. In addition, speciation analysis based on HPLC–ICP [16] or GC–ICP [17,18] coupling has recently received special attention.
⁎ Corresponding author. Tel.: +33 559407757; fax: +33 559407674. E-mail address: [email protected] (C. Pécheyran). 0026-265X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.microc.2012.07.012
The main disadvantages of direct injection are i) large sample dilution (×100, ×1000), especially when heavy or extra heavy crude oils are analysed, which hamper the analysis of ultratrace elements even when ultrasensitive techniques like ICPMS are used; ii) differences between sample and standard matrices (e.g. difference in viscosity) make mandatory the use of standard addition calibration method or matrix matching, which is time-consuming and impractical in laboratories with high sample throughput, and iii) memory effects imposing long rinsing times which again limit sample throughput. The second procedure widely used for the analysis of crude oil is the sample introduction in aqueous phase after decomposition of the organic matter by means of acid attack, combustion or dry ashing. Dry ashing is an old method, but still used in many petroleum laboratories. The advantage of this method is the possibility of increasing the amount of sample digested in order to increase the concentration in the solution (normally 5–20 g of sample is used). Besides, all the organic matter is destroyed, so there is no foam formation or viscosity differences due to organic matrix compounds. However, the procedure is time-consuming, loss of volatile elements can occur during sample burning (due to the formation of volatile species) and sample contamination during sample treatment is very critical [19]. Another method consists of using Parr combustion bombs in which large samples can be treated (~ 1 g). However, this procedure is not popular due to the risk of explosion. Acid decomposition is the procedure most commonly used as the sample preparation method for trace and ultratrace analysis. The sample is heated at atmospheric pressure with an oxidizing acid, such as H2SO4, HClO4, HNO3, or by the successive addition of different acids or reagents like HNO3 and H2O2 [20,21]. However, these procedures can take hours to obtain a clear solution and large quantities of
G. Sanabria Ortega et al. / Microchemical Journal 106 (2013) 250–254
reagents are used, which increases the risk of sample contamination. In addition, there is still the possibility of loss of volatile elements and safety must be considered. Microwave-assisted acid digestion is an alternative procedure that provides efficient sample decomposition in a relatively short time and increases safety in the laboratory by the use of electronic control of pressure and temperature [22,23]. Nevertheless, the amount of sample in the pressure vessels is limited due to the generation of a huge amount of gas during digestion that could cause a pressure rise in the reaction vessel and break the safety seal. As an alternative, the use of closed reactors operating at high pressure and temperature minimizes this problem. According to the standard method ASTM C 1234 [24], up to 0.7 g of oil can be handled in the HPA-S, but again, the limiting factor is the high pressure generated by the organic compounds. As can be seen, many different strategies have been proposed for the analysis of crude oil and by-products. In spite of the existence of standard methods and all the developments in analytical chemistry, there is still a lot of work to do to cope with the new requirements in petroleum research in terms of ultratrace, isotopes and speciation. Here we present the results of 3 different methods for the sample preparation of heavy and extra-heavy crude oils: a) microwave-assisted acid digestion with different temperature and pressure conditions, b) microwave-assisted combustion and c) acid digestion using the conventionally heated High Pressure Asher (HPA-S).
Table 2 Operation conditions ICPMS. RF power (W)
1300
Plasma gas flow rate (L min−1) Auxiliary flow rate (L min−1) Nebulizer flow rate Interface cones Nebulizer Spray chamber Isotopes (m/z)
15 1.2 0.94 Ni Meinhard Cyclonic 51 V, 60Ni, 55Mn,
59
Co,
75
As,
95
Mo,
138
Ba,
208
Pb
the NIST SRM 1634c (USA) was digested by the different procedures described below. All materials were acid washed before use. 2.3. Sample preparation Three extra-heavy crude oils (crude oil 1, crude oil 2 and crude oil 3) from the Orinoco Belt, Venezuela, were digested using the 5 different methods. Microwave-assisted acid digestion: when the rotor HF100 was used, about 100 mg of sample was weighed into the Teflon liners. Then 6 mL of HNO3 and 2 mL of H2O2 were added. For the SXQ80 and the SFX100 about 300 mg of sample was weighted in the corresponding vessels and then 7.5 mL of HNO3 and 0.5 mL of H2O2 were added. In the three cases, samples were run with a pressure rate of 0.3 bar s −1. Microwave-induced combustion (MIC): for oxygen combustion, the quartz vessels SXQ80 were assembled with a special quartz sample holder (combustion kit). The sample holders were mounted with 2 filter papers and approximately 300 mg of sample was weighed onto the paper. The samples were covered with 2 additional filter papers and moistened with 55 μL of NH4NO3 to favour the ignition of the combustion. Then, 3.75 mL of HNO3, 0.25 mL H2O2 and 4 mL H2O were added into the quartz vessels and the sample holders were carefully placed into the vessels. The closed vessels were loaded into the rotor, and the rotor was placed into the Multiwave 3000. Then the vessels were filled with 20 bar oxygen. In this method, a pressure rate of 0.8 bar s −1 was used. The digestion/combustion programmes are shown in Table 1. High Pressure Asher (HPA-S): around 500 mg of sample was weighed into the quartz vessels, followed by the addition of 6 mL of HNO3 and 0.5 mL of H2O2. Samples were put into the HPA-S autoclave with a ramp of 1 h to increase the temperature from the ambient to 320 °C, followed by a dwell time of 30 min.
2. Materials and methods 2.1. Instrumentation Microwave-assisted acid digestion was carried out using the microwave digestion system Multiwave 3000 (Anton Paar, Graz, Austria) with three different configurations, namely HF100, SXF100 and SXQ80, providing different temperatures, pressure conditions and sample sizes. For the microwave-induced oxygen combustion (MIC), SXQ80 vessels were used with an additional combustion kit (Anton Paar GbmH, Graz, Austria). Table 1 summarizes the running programme used in this study. A High Pressure Asher instrument (Anton Paar GbmH, Graz, Austria) was also used. Acid digestion was carried out using quartz vessels (conversion kit 5 × 90 mL). The digestion programme consists of a 1 hour ramp to increase the temperature from ambient to 320 °C, followed by a dwell time of 30 min. Quantitative analysis was carried out in an inductively coupled mass spectrometer (ICPMS), Elan DRC II (Perkin Elmer, Ontario, Canada). Operating conditions are shown in Table 2. Quantification was carried out by external calibration, using Y and In as internal standards. 2.2. Reagents All solutions were prepared in de-ionized water (Milli-Q, Millipore, USA, 18.2 MΩ cm). HNO3 was Ultrex® grade from J.T. Baker® Chemicals Co. and H2O2 Optima® (30–32%) from Fisher Scientific, Germany was used. For quantitative analysis of V, Ni, Mn, Co, As, Mo, Ba and Pb the mono-elemental standard solution, 1 g.L−1, was purchased from SCP Science, Canada. To assess the accuracy and the precision of the methods,
251
3. Results and discussion After mineralisation differences in the appearance of solutions were observed. Sample solutions digested by the rotor HF100 were yellow-orange, with foam formation after agitation, slightly
Table 1 Microwave assisted acid digestion programme. Tmax, Pmax
HF100
SXF100
SXQ80
SXQ80 (MIC)
220 °C, 40 bar
260 °C, 60 bar
300 °C, 80 bar
1400 °C, 80 bar
Step
Power [W]
Ramp [min]
Hold [min]
Power [W]
Ramp [min]
Hold [min]
Power [W]
Ramp [min]
Hold [min]
Power [W]
Ramp [min]
Hold [min]
1 2 3
1200 1400 0
0 0 0
20 30 20
600 900 0
1 1 3
20 25 15
600 900 0
0 15 0
20 25 15
1400 1400 0
0 0 0
1:30 18:30 15
252
G. Sanabria Ortega et al. / Microchemical Journal 106 (2013) 250–254
Table 3 Qualitative comparison of 5 acid digestion procedures evaluated. System
Sample size (mg)
Time of procedure (min.)
HNO3 (mL)
H2O2 (mL)
H2O (mL)
Tmax (°C)
P (bar)
Blanks quality
HF100 SXF100 SXQ80 Microwave induced combustion HPA
100 300 300 300
150 90 90 35
6 7.5 7.5 3.75
2 0.5 0.5 0.5
– – – 4
220 260 300 1400
40 60 80 80
Good Good Good Poor
Orange with foam formation Yellowish, no foam formation Yellowish, no foam formation Colourless transparent solutions
500
120
6
0.5
–
320
150
Good
Yellowish, no foam formation
viscous and sometimes a black residue was observed in the bottom of the vessel. Different temperature ramps and HNO3 volumes were tried (not showed) without any significant changes in the sample appearance. In contrast, yellowish-clear (almost transparent) solutions, without foam formation or any residue were observed after digestion at higher temperature and pressure conditions using the rotors SXQ80 and SFX100, and HPA-S (no difference was observed visually between these solutions). Visually, the best mineralisation was found when using the combustion method as solutions were completely transparent, without any colouration, foam formation or residues. Table 3 resumes the qualitative differences between the different procedures. The main difference between the procedures used is the degree of organic–material decomposition. The higher the temperature and pressure tolerated by the systems the higher the degree of decomposition of the organic matter. With the HF100 rotor, only 100 mg of crude oil can be digested at 220 °C and 40 bar; higher mass results in partial digestion of the sample. During digestion of organic samples, the pressure can rise abruptly owing to fast chain reactions, which can produce explosion, rupture of seals in the reaction vessels and loss of the sample. In monitored systems, when the pressure increases at a high rate or exceeds the pressure limit, the system stops to avoid overpressure and the digestion is thus not completed. When working at or above 260 °C and 60 bar (Rotor SXQ80 and Rotor SXF100) higher amounts of sample (more than 100 mg) can be treated, while obtaining clear solutions (almost transparent) there were no changes in viscosity and no foam formation, which indicates high degree of organic material decomposition. Higher pressure and temperature allowed the digestion of bigger samples. For example, HPA-S (320 °C and 150 bar) digested samples as big as 500 mg. Note that the microwave specific heating process (in depth agitation of polar molecules) was not shown to be more efficient for digesting crude oils compared to conventional heating, as HPA-S provided similar results, even with almost two times more sample. In previous work, the organic material remaining after acid digestion of the SRM NIST 1634c was studied. In this case, 500 mg of sample was acid digested in the HPA (most severe conditions) obtaining clear solutions, the acid was evaporated and the residue (yellow) was re-dissolved in acetonitrile. The sample was analysed by UV–visible-HPLC observing the presence of many organic compounds with different molecular weights, for more details see Sanabria et al. [25]. Identification of each compound was not carried out, because it was out of the scope of this work. However, this result indicates that even after severe digestion conditions, many organic compounds remain in solution, even in clear and non viscous solutions. In the work of Costa et al. [26], total carbon
Solution appearance
content was measured after mineralisation of oil with HNO3 and H2O2, giving a concentration of residual carbon of between 2.37 and 9.71%. On the other hand, the work of J.F.S. Pereira et al. [27], reports a carbon concentration up to 30 times higher in polymer samples treated by microwave-assisted digestion than by MIC. This result is not unexpected as, during combustion, the organic matter is transformed in CO2, but during acid digestion in addition to CO2, other types of compounds could be formed. Note that for determination by ICPMS, elements susceptible to carbon argide isobaric interferences could be affected [26] and matrix effects due to the sample viscosity could be significant when the organic matter is incompletely digested. From a quantitative point of view, the recovery for all methods is between 80 and 120%. A positive bias is observed for some elements (see Table 4), which we thought was caused by contamination of the reference material. Recovery obtained on Ni and V was, however, significantly lower with MIC than with the other techniques (− 8% to − 16% respectively, compared with the certified values). Table 5 shows the results obtained for the analysis of 3 extra-heavy crude oil samples. As can be seen, similar concentrations are obtained by the different procedures. However, here again, lower recovery was systematically obtained for V and Ni using the MIC. In addition, lower precision was observed when the MIC method was used. Also, high blanks were obtained when using this procedure maybe due to the purity of filter paper used for the combustion. When using this method a longer reflux is recommended in order to increase the recovery and the use of high purity ignition solution is recommended in order to avoid contamination of low-concentration elements (in this work an analytical grade ammonium nitrate salt was used). In general, additional optimisation for the MIC method is necessary to increase the precision and to reduce the blank level. Close results are obtained when using rotors SXQ80 and SXF100. Sample preparations were carried out in the same laboratory, under repeatability conditions, and were analysed also in the same laboratory under repeatability conditions. An analysis of variance (not presented) was carried out excluding results obtained with MIC for which lower recovery was already clearly identified. The ANOVA comparison revealed significant difference for the determination of V, Ni, Mn, As, Mo and Ba for crude oil 2, and for the determination of Ni, Mn and Ba in crude oil 3. However, in this particular case, the relevance of this variance analysis might be biased due to the fact that samples were digested under reproducibility conditions (i.e. in the laboratories of Pau and Graz), while dilutions and measurements were carried out under repeatability conditions (i.e. in the same laboratory, the same day by the same analyst). As a consequence, the variance between digestion methods is considerably higher than the
Table 4 %Recovery obtained by the different procedures for the SRM NIST 1634c. (n = 3). Element HF100, μg g−1 % Recovery SXQ80, μg g−1 % Recovery SXF100, μg g−1 % Recovery MIC, μg g−1 V51 Ni60 Co59 As75
29.6 ± 0.5 17.7 ± 0.4 0.159 ± 0.005 0.13 ± 0.01
105.0 100.9 105.3 91.2
30.4 ± 1.3 18.1 ± 0.6 0.125 ± 0.001 0.167 ± 0.003
107.8 103.2 82.8 117.1
29.5 ± 0.8 17.9 ± 0.4 0.13 ± 0.001 0.17 ± 0.02
104.6 102.1 86.1 119.2
% Recovery HPA, μg g−1
26.1 ± 0.9 92.6 14.8 ± 0.1 84.4 0.15 ± 0.007 99.3 0.171 ± 0.007 119.9
29 ± 0.4 17.3 ± 0.2 – 0.179 ± 0.003
% Recovery Certified, μg g−1 102.9 98.6 – 125.5
28.19 ± 0.4 17.54 ± 0.21 0.151 ± 0.0064 0.1426 ± 0.0064
ND
0.16 ± 0.01
2.1 ± 0.066
0.16 ± 0.02
4. Conclusion The different procedures evaluated provide similar recoveries of the elements under study. The main difference is the degree of decomposition of the organic material, which is inversely proportional to the pressure and temperature reached by the digestion device. The maximum pressure accepted by the device also affects the quantification limits of the procedure. As a consequence, the larger the size of the sample, the higher the pressure reached inside the reaction vessel. Therefore, high pressure systems like HPA-S, which allow higher amounts of sample, are particularly interesting when low trace element concentrations have to be detected. All the evaluated systems are safe, at least 5 digestions can be done simultaneously and provide low blanks (least with the MIC procedure). However, microwave devices are faster and allow a larger number of samples to be prepared per cycle than the HPA-S. MIC is fast, efficient to decompose the organic material and small quantities of acids are required. However an increase in the size of samples and special care on the blank reduction would be important in order to improve the detection limits. The future of these devices could be directed towards the development of vessels which will allow an increase in the pressure and temperature tolerated and to increase the size of sample.
0.140 ± 0.004 0.0091 ± 0.0008 ND 0.09 ± 0.07
0.290 ± 0.002 0.22 ± 0.01
0.248 ± 0.003 ND 0.249 ± 0.003 0.008 ± 0.005 ND 0.09 ± 0.01 Pb208
ND = not detectable.
0.58 ± 0.03 Ba138
0.089 ± 0.002 0.090 ± 0.004
0.13 ± 0.08
0.082 ± 0.001
0.28 ± 0.02 0.57 ± 0.01 0.41 ± 0.05 0.54 ± 0.01
1.92 ± 0.01 1.99 ± 0.04 2.19 ± 0.05 2.24 ± 0.16 2.24 ± 0.02 2.32 ± 0.03 2.16 ± 0.11 Mo95
As75
References
0.54 ± 0.02
0.098 ± 0.004 0.104 ± 0.007
2.01 ± 0.03
0.279 ± 0.001 0.264 ± 0.009
0.214 ± 0.003 0.15 ± 0.01
474 ± 19 103 ± 2 0.036 ± 0.001 0.276 ± 0.002 0.137 ± 0.001 2.214 ± 0.004 0.122 ± 0.001 ND Co59
253
variance within a given digestion method. However, considering the 3 oil samples, the values obtained with these four digestion procedures agree within 5% for most of the elements except for Mn (b15%) which is quite acceptable in an analytical point of view. It is also worth noting that only HPA-S digestion allowed the determination of ultra-low Pb concentration in crude oil 3. This is certainly due to the fact that a higher amount of sample could be digested (500 mg compared to 100–300 mg).
0.0056 ± 0.0003
0.152 ± 0.002
2.13 ± 0.01
0.16 ± 0.01
0.279 ± 0.001
484 ± 6 105 ± 1 0.027 ± 0.003
MIC
386 ± 33 90 ± 10 0.04 ± 0.01 0.26 ± 0.03 0.20 ± 0.01 2.14 ± 0.13 0.15 ± 0.03 ND
SXF100 HF100
458 ± 3 99 ± 2 ND 497 ± 12 108 ± 3 0.043 ± 0.003 406 ± 29 95 ± 3 0.050 ± 0.002
470 ± 2 103 ± 1 0.045 ± 0.003 0.271 ± 0.003 0.082 ± 0.004 1.94 ± 0.02 470 ± 1 480 ± 7 103.8 ± 0.1 105 ± 2 0.035 ± 0.001 0.050 ± 0.003 0.26 ± 0.01 0.277 ± 0.002 0.086 ± 0.002 0.078 ± 0.002 1.96 ± 0.06 1.99 ± 0.03 487 ± 20 492 ± 16 473 ± 7 409 ± 28 490 ± 5 105 ± 3 106 ± 4 102 ± 2 88 ± 6 107 ± 1 0.044 ± 0.007 0.059 ± 0.004 0.054 ± 0.003 0.061 ± 0.016 0.047 ± 0.007 0.29 ± 0.01 0.335 ± 0.009 0.323 ± 0.001 0.301 ± 0.022 0.329 ± 0.009 0.138 ± 0.008 0.137 ± 0.010 0.134 ± 0.007 0.148 ± 0.020 – V51 Ni60 Mn55
SXQ80 HF100
Crude oil 2
HPA MIC SXF100 SXQ80 HF100
Element Crude oil 1
Table 5 Results obtained after digestion using 5 different procedures and measure by Q-ICPMS of 3 extra-heavy crude oil samples (μg g−1, n = 3).
HPA MIC SXF100
Crude oil 3
SXQ80
460 ± 10 99 ± 2 0.038 ± 0.005 0.27 ± 0.01
HPA
G. Sanabria Ortega et al. / Microchemical Journal 106 (2013) 250–254
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Contents lists available at SciVerse ScienceDirect
Microchemical Journal journal homepage: www.elsevier.com/locate/microc
Different approaches of crude oil mineralisation for trace metal analysis by ICPMS Georgia Sanabria Ortega a, Christophe Pécheyran a,⁎, Gernot Hudin b, Edit Marosits b, Olivier F.X. Donard a a b
LCABIE, IPREM UMR 5254, CNRS, Université de Pau et des Pays de l'Adour, 64053, Pau cedex 9, France Anton Paar GmbH, Anton-Paar-Str. 20, 8054 Graz, Austria
a r t i c l e
i n f o
Article history: Received 9 May 2012 Received in revised form 20 July 2012 Accepted 22 July 2012 Available online 27 July 2012 Keywords: Extra-heavy crude oil Acid digestion Trace metal
a b s t r a c t Aqueous phase is the most common way to introduce samples into the ICPMS. However, due to the complexity of the crude oil matrix, harsh conditions are necessary to carry out complete acid digestion. In this work four microwave digestion methods based on maximum temperature and pressure programmes (ranging from 220 °C and 40 bar to ~ 1400 °C and 80 bar) were tested and compared with a digestion method in the High Pressure Asher (HPA-S) autoclave. Recoveries of between 80 and 120% were obtained for the different procedures evaluated. The lowest recovery and precision were obtained by the microwave-induced combustion method. The degree of decomposition of the organic material was directly related with the temperature and pressure used for the digestion. It was found that increasing the temperature and pressure by 40° and 20 bar, improved the digestion and allowed an increase in the sample size and therefore the detection limits. © 2012 Elsevier B.V. All rights reserved.
1. Introduction Recent developments in atomic spectrometry, particularly in inductively coupled plasma atomic emission/mass spectrometry (ICPOES and ICPMS) have opened new application fields in ultratrace, speciation and isotopic analysis, which represent new and valuable tools for the petroleum industry upstream and downstream. However, the organic sample matrix is still a challenge mainly because spectrometric techniques have been generally developed for the analysis of aqueous samples. Two main methods have been used so far for the analysis of high organic samples: direct injection or mineralisation (by acid digestion or dry ashing). For direct injection, many attempts have been made to introduce organic samples into the plasma while avoiding plasma overload or carbon deposition on the cones or the torch of the ICP spectrometer. Many studies have been devoted to the optimisation of the operating conditions and to the development of nebulisers [1–5] and micro-nebulisers [6–8], the latter being especially recommended when using ICPMS. Several types of micro-nebulisers, spray chambers and automated systems are commercially available, designed especially for the analysis of organic samples. Standard methods for direct injection by ICPOES, like the ASTM D 5708-11 [9] have also been developed. Hyphenated techniques, like ETV with ICPMS [10], LA with ICPMS [11–13] or the use of emulsion [14,15] offer other alternatives for direct injection. In addition, speciation analysis based on HPLC–ICP [16] or GC–ICP [17,18] coupling has recently received special attention.
⁎ Corresponding author. Tel.: +33 559407757; fax: +33 559407674. E-mail address: [email protected] (C. Pécheyran). 0026-265X/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.microc.2012.07.012
The main disadvantages of direct injection are i) large sample dilution (×100, ×1000), especially when heavy or extra heavy crude oils are analysed, which hamper the analysis of ultratrace elements even when ultrasensitive techniques like ICPMS are used; ii) differences between sample and standard matrices (e.g. difference in viscosity) make mandatory the use of standard addition calibration method or matrix matching, which is time-consuming and impractical in laboratories with high sample throughput, and iii) memory effects imposing long rinsing times which again limit sample throughput. The second procedure widely used for the analysis of crude oil is the sample introduction in aqueous phase after decomposition of the organic matter by means of acid attack, combustion or dry ashing. Dry ashing is an old method, but still used in many petroleum laboratories. The advantage of this method is the possibility of increasing the amount of sample digested in order to increase the concentration in the solution (normally 5–20 g of sample is used). Besides, all the organic matter is destroyed, so there is no foam formation or viscosity differences due to organic matrix compounds. However, the procedure is time-consuming, loss of volatile elements can occur during sample burning (due to the formation of volatile species) and sample contamination during sample treatment is very critical [19]. Another method consists of using Parr combustion bombs in which large samples can be treated (~ 1 g). However, this procedure is not popular due to the risk of explosion. Acid decomposition is the procedure most commonly used as the sample preparation method for trace and ultratrace analysis. The sample is heated at atmospheric pressure with an oxidizing acid, such as H2SO4, HClO4, HNO3, or by the successive addition of different acids or reagents like HNO3 and H2O2 [20,21]. However, these procedures can take hours to obtain a clear solution and large quantities of
G. Sanabria Ortega et al. / Microchemical Journal 106 (2013) 250–254
reagents are used, which increases the risk of sample contamination. In addition, there is still the possibility of loss of volatile elements and safety must be considered. Microwave-assisted acid digestion is an alternative procedure that provides efficient sample decomposition in a relatively short time and increases safety in the laboratory by the use of electronic control of pressure and temperature [22,23]. Nevertheless, the amount of sample in the pressure vessels is limited due to the generation of a huge amount of gas during digestion that could cause a pressure rise in the reaction vessel and break the safety seal. As an alternative, the use of closed reactors operating at high pressure and temperature minimizes this problem. According to the standard method ASTM C 1234 [24], up to 0.7 g of oil can be handled in the HPA-S, but again, the limiting factor is the high pressure generated by the organic compounds. As can be seen, many different strategies have been proposed for the analysis of crude oil and by-products. In spite of the existence of standard methods and all the developments in analytical chemistry, there is still a lot of work to do to cope with the new requirements in petroleum research in terms of ultratrace, isotopes and speciation. Here we present the results of 3 different methods for the sample preparation of heavy and extra-heavy crude oils: a) microwave-assisted acid digestion with different temperature and pressure conditions, b) microwave-assisted combustion and c) acid digestion using the conventionally heated High Pressure Asher (HPA-S).
Table 2 Operation conditions ICPMS. RF power (W)
1300
Plasma gas flow rate (L min−1) Auxiliary flow rate (L min−1) Nebulizer flow rate Interface cones Nebulizer Spray chamber Isotopes (m/z)
15 1.2 0.94 Ni Meinhard Cyclonic 51 V, 60Ni, 55Mn,
59
Co,
75
As,
95
Mo,
138
Ba,
208
Pb
the NIST SRM 1634c (USA) was digested by the different procedures described below. All materials were acid washed before use. 2.3. Sample preparation Three extra-heavy crude oils (crude oil 1, crude oil 2 and crude oil 3) from the Orinoco Belt, Venezuela, were digested using the 5 different methods. Microwave-assisted acid digestion: when the rotor HF100 was used, about 100 mg of sample was weighed into the Teflon liners. Then 6 mL of HNO3 and 2 mL of H2O2 were added. For the SXQ80 and the SFX100 about 300 mg of sample was weighted in the corresponding vessels and then 7.5 mL of HNO3 and 0.5 mL of H2O2 were added. In the three cases, samples were run with a pressure rate of 0.3 bar s −1. Microwave-induced combustion (MIC): for oxygen combustion, the quartz vessels SXQ80 were assembled with a special quartz sample holder (combustion kit). The sample holders were mounted with 2 filter papers and approximately 300 mg of sample was weighed onto the paper. The samples were covered with 2 additional filter papers and moistened with 55 μL of NH4NO3 to favour the ignition of the combustion. Then, 3.75 mL of HNO3, 0.25 mL H2O2 and 4 mL H2O were added into the quartz vessels and the sample holders were carefully placed into the vessels. The closed vessels were loaded into the rotor, and the rotor was placed into the Multiwave 3000. Then the vessels were filled with 20 bar oxygen. In this method, a pressure rate of 0.8 bar s −1 was used. The digestion/combustion programmes are shown in Table 1. High Pressure Asher (HPA-S): around 500 mg of sample was weighed into the quartz vessels, followed by the addition of 6 mL of HNO3 and 0.5 mL of H2O2. Samples were put into the HPA-S autoclave with a ramp of 1 h to increase the temperature from the ambient to 320 °C, followed by a dwell time of 30 min.
2. Materials and methods 2.1. Instrumentation Microwave-assisted acid digestion was carried out using the microwave digestion system Multiwave 3000 (Anton Paar, Graz, Austria) with three different configurations, namely HF100, SXF100 and SXQ80, providing different temperatures, pressure conditions and sample sizes. For the microwave-induced oxygen combustion (MIC), SXQ80 vessels were used with an additional combustion kit (Anton Paar GbmH, Graz, Austria). Table 1 summarizes the running programme used in this study. A High Pressure Asher instrument (Anton Paar GbmH, Graz, Austria) was also used. Acid digestion was carried out using quartz vessels (conversion kit 5 × 90 mL). The digestion programme consists of a 1 hour ramp to increase the temperature from ambient to 320 °C, followed by a dwell time of 30 min. Quantitative analysis was carried out in an inductively coupled mass spectrometer (ICPMS), Elan DRC II (Perkin Elmer, Ontario, Canada). Operating conditions are shown in Table 2. Quantification was carried out by external calibration, using Y and In as internal standards. 2.2. Reagents All solutions were prepared in de-ionized water (Milli-Q, Millipore, USA, 18.2 MΩ cm). HNO3 was Ultrex® grade from J.T. Baker® Chemicals Co. and H2O2 Optima® (30–32%) from Fisher Scientific, Germany was used. For quantitative analysis of V, Ni, Mn, Co, As, Mo, Ba and Pb the mono-elemental standard solution, 1 g.L−1, was purchased from SCP Science, Canada. To assess the accuracy and the precision of the methods,
251
3. Results and discussion After mineralisation differences in the appearance of solutions were observed. Sample solutions digested by the rotor HF100 were yellow-orange, with foam formation after agitation, slightly
Table 1 Microwave assisted acid digestion programme. Tmax, Pmax
HF100
SXF100
SXQ80
SXQ80 (MIC)
220 °C, 40 bar
260 °C, 60 bar
300 °C, 80 bar
1400 °C, 80 bar
Step
Power [W]
Ramp [min]
Hold [min]
Power [W]
Ramp [min]
Hold [min]
Power [W]
Ramp [min]
Hold [min]
Power [W]
Ramp [min]
Hold [min]
1 2 3
1200 1400 0
0 0 0
20 30 20
600 900 0
1 1 3
20 25 15
600 900 0
0 15 0
20 25 15
1400 1400 0
0 0 0
1:30 18:30 15
252
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Table 3 Qualitative comparison of 5 acid digestion procedures evaluated. System
Sample size (mg)
Time of procedure (min.)
HNO3 (mL)
H2O2 (mL)
H2O (mL)
Tmax (°C)
P (bar)
Blanks quality
HF100 SXF100 SXQ80 Microwave induced combustion HPA
100 300 300 300
150 90 90 35
6 7.5 7.5 3.75
2 0.5 0.5 0.5
– – – 4
220 260 300 1400
40 60 80 80
Good Good Good Poor
Orange with foam formation Yellowish, no foam formation Yellowish, no foam formation Colourless transparent solutions
500
120
6
0.5
–
320
150
Good
Yellowish, no foam formation
viscous and sometimes a black residue was observed in the bottom of the vessel. Different temperature ramps and HNO3 volumes were tried (not showed) without any significant changes in the sample appearance. In contrast, yellowish-clear (almost transparent) solutions, without foam formation or any residue were observed after digestion at higher temperature and pressure conditions using the rotors SXQ80 and SFX100, and HPA-S (no difference was observed visually between these solutions). Visually, the best mineralisation was found when using the combustion method as solutions were completely transparent, without any colouration, foam formation or residues. Table 3 resumes the qualitative differences between the different procedures. The main difference between the procedures used is the degree of organic–material decomposition. The higher the temperature and pressure tolerated by the systems the higher the degree of decomposition of the organic matter. With the HF100 rotor, only 100 mg of crude oil can be digested at 220 °C and 40 bar; higher mass results in partial digestion of the sample. During digestion of organic samples, the pressure can rise abruptly owing to fast chain reactions, which can produce explosion, rupture of seals in the reaction vessels and loss of the sample. In monitored systems, when the pressure increases at a high rate or exceeds the pressure limit, the system stops to avoid overpressure and the digestion is thus not completed. When working at or above 260 °C and 60 bar (Rotor SXQ80 and Rotor SXF100) higher amounts of sample (more than 100 mg) can be treated, while obtaining clear solutions (almost transparent) there were no changes in viscosity and no foam formation, which indicates high degree of organic material decomposition. Higher pressure and temperature allowed the digestion of bigger samples. For example, HPA-S (320 °C and 150 bar) digested samples as big as 500 mg. Note that the microwave specific heating process (in depth agitation of polar molecules) was not shown to be more efficient for digesting crude oils compared to conventional heating, as HPA-S provided similar results, even with almost two times more sample. In previous work, the organic material remaining after acid digestion of the SRM NIST 1634c was studied. In this case, 500 mg of sample was acid digested in the HPA (most severe conditions) obtaining clear solutions, the acid was evaporated and the residue (yellow) was re-dissolved in acetonitrile. The sample was analysed by UV–visible-HPLC observing the presence of many organic compounds with different molecular weights, for more details see Sanabria et al. [25]. Identification of each compound was not carried out, because it was out of the scope of this work. However, this result indicates that even after severe digestion conditions, many organic compounds remain in solution, even in clear and non viscous solutions. In the work of Costa et al. [26], total carbon
Solution appearance
content was measured after mineralisation of oil with HNO3 and H2O2, giving a concentration of residual carbon of between 2.37 and 9.71%. On the other hand, the work of J.F.S. Pereira et al. [27], reports a carbon concentration up to 30 times higher in polymer samples treated by microwave-assisted digestion than by MIC. This result is not unexpected as, during combustion, the organic matter is transformed in CO2, but during acid digestion in addition to CO2, other types of compounds could be formed. Note that for determination by ICPMS, elements susceptible to carbon argide isobaric interferences could be affected [26] and matrix effects due to the sample viscosity could be significant when the organic matter is incompletely digested. From a quantitative point of view, the recovery for all methods is between 80 and 120%. A positive bias is observed for some elements (see Table 4), which we thought was caused by contamination of the reference material. Recovery obtained on Ni and V was, however, significantly lower with MIC than with the other techniques (− 8% to − 16% respectively, compared with the certified values). Table 5 shows the results obtained for the analysis of 3 extra-heavy crude oil samples. As can be seen, similar concentrations are obtained by the different procedures. However, here again, lower recovery was systematically obtained for V and Ni using the MIC. In addition, lower precision was observed when the MIC method was used. Also, high blanks were obtained when using this procedure maybe due to the purity of filter paper used for the combustion. When using this method a longer reflux is recommended in order to increase the recovery and the use of high purity ignition solution is recommended in order to avoid contamination of low-concentration elements (in this work an analytical grade ammonium nitrate salt was used). In general, additional optimisation for the MIC method is necessary to increase the precision and to reduce the blank level. Close results are obtained when using rotors SXQ80 and SXF100. Sample preparations were carried out in the same laboratory, under repeatability conditions, and were analysed also in the same laboratory under repeatability conditions. An analysis of variance (not presented) was carried out excluding results obtained with MIC for which lower recovery was already clearly identified. The ANOVA comparison revealed significant difference for the determination of V, Ni, Mn, As, Mo and Ba for crude oil 2, and for the determination of Ni, Mn and Ba in crude oil 3. However, in this particular case, the relevance of this variance analysis might be biased due to the fact that samples were digested under reproducibility conditions (i.e. in the laboratories of Pau and Graz), while dilutions and measurements were carried out under repeatability conditions (i.e. in the same laboratory, the same day by the same analyst). As a consequence, the variance between digestion methods is considerably higher than the
Table 4 %Recovery obtained by the different procedures for the SRM NIST 1634c. (n = 3). Element HF100, μg g−1 % Recovery SXQ80, μg g−1 % Recovery SXF100, μg g−1 % Recovery MIC, μg g−1 V51 Ni60 Co59 As75
29.6 ± 0.5 17.7 ± 0.4 0.159 ± 0.005 0.13 ± 0.01
105.0 100.9 105.3 91.2
30.4 ± 1.3 18.1 ± 0.6 0.125 ± 0.001 0.167 ± 0.003
107.8 103.2 82.8 117.1
29.5 ± 0.8 17.9 ± 0.4 0.13 ± 0.001 0.17 ± 0.02
104.6 102.1 86.1 119.2
% Recovery HPA, μg g−1
26.1 ± 0.9 92.6 14.8 ± 0.1 84.4 0.15 ± 0.007 99.3 0.171 ± 0.007 119.9
29 ± 0.4 17.3 ± 0.2 – 0.179 ± 0.003
% Recovery Certified, μg g−1 102.9 98.6 – 125.5
28.19 ± 0.4 17.54 ± 0.21 0.151 ± 0.0064 0.1426 ± 0.0064
ND
0.16 ± 0.01
2.1 ± 0.066
0.16 ± 0.02
4. Conclusion The different procedures evaluated provide similar recoveries of the elements under study. The main difference is the degree of decomposition of the organic material, which is inversely proportional to the pressure and temperature reached by the digestion device. The maximum pressure accepted by the device also affects the quantification limits of the procedure. As a consequence, the larger the size of the sample, the higher the pressure reached inside the reaction vessel. Therefore, high pressure systems like HPA-S, which allow higher amounts of sample, are particularly interesting when low trace element concentrations have to be detected. All the evaluated systems are safe, at least 5 digestions can be done simultaneously and provide low blanks (least with the MIC procedure). However, microwave devices are faster and allow a larger number of samples to be prepared per cycle than the HPA-S. MIC is fast, efficient to decompose the organic material and small quantities of acids are required. However an increase in the size of samples and special care on the blank reduction would be important in order to improve the detection limits. The future of these devices could be directed towards the development of vessels which will allow an increase in the pressure and temperature tolerated and to increase the size of sample.
0.140 ± 0.004 0.0091 ± 0.0008 ND 0.09 ± 0.07
0.290 ± 0.002 0.22 ± 0.01
0.248 ± 0.003 ND 0.249 ± 0.003 0.008 ± 0.005 ND 0.09 ± 0.01 Pb208
ND = not detectable.
0.58 ± 0.03 Ba138
0.089 ± 0.002 0.090 ± 0.004
0.13 ± 0.08
0.082 ± 0.001
0.28 ± 0.02 0.57 ± 0.01 0.41 ± 0.05 0.54 ± 0.01
1.92 ± 0.01 1.99 ± 0.04 2.19 ± 0.05 2.24 ± 0.16 2.24 ± 0.02 2.32 ± 0.03 2.16 ± 0.11 Mo95
As75
References
0.54 ± 0.02
0.098 ± 0.004 0.104 ± 0.007
2.01 ± 0.03
0.279 ± 0.001 0.264 ± 0.009
0.214 ± 0.003 0.15 ± 0.01
474 ± 19 103 ± 2 0.036 ± 0.001 0.276 ± 0.002 0.137 ± 0.001 2.214 ± 0.004 0.122 ± 0.001 ND Co59
253
variance within a given digestion method. However, considering the 3 oil samples, the values obtained with these four digestion procedures agree within 5% for most of the elements except for Mn (b15%) which is quite acceptable in an analytical point of view. It is also worth noting that only HPA-S digestion allowed the determination of ultra-low Pb concentration in crude oil 3. This is certainly due to the fact that a higher amount of sample could be digested (500 mg compared to 100–300 mg).
0.0056 ± 0.0003
0.152 ± 0.002
2.13 ± 0.01
0.16 ± 0.01
0.279 ± 0.001
484 ± 6 105 ± 1 0.027 ± 0.003
MIC
386 ± 33 90 ± 10 0.04 ± 0.01 0.26 ± 0.03 0.20 ± 0.01 2.14 ± 0.13 0.15 ± 0.03 ND
SXF100 HF100
458 ± 3 99 ± 2 ND 497 ± 12 108 ± 3 0.043 ± 0.003 406 ± 29 95 ± 3 0.050 ± 0.002
470 ± 2 103 ± 1 0.045 ± 0.003 0.271 ± 0.003 0.082 ± 0.004 1.94 ± 0.02 470 ± 1 480 ± 7 103.8 ± 0.1 105 ± 2 0.035 ± 0.001 0.050 ± 0.003 0.26 ± 0.01 0.277 ± 0.002 0.086 ± 0.002 0.078 ± 0.002 1.96 ± 0.06 1.99 ± 0.03 487 ± 20 492 ± 16 473 ± 7 409 ± 28 490 ± 5 105 ± 3 106 ± 4 102 ± 2 88 ± 6 107 ± 1 0.044 ± 0.007 0.059 ± 0.004 0.054 ± 0.003 0.061 ± 0.016 0.047 ± 0.007 0.29 ± 0.01 0.335 ± 0.009 0.323 ± 0.001 0.301 ± 0.022 0.329 ± 0.009 0.138 ± 0.008 0.137 ± 0.010 0.134 ± 0.007 0.148 ± 0.020 – V51 Ni60 Mn55
SXQ80 HF100
Crude oil 2
HPA MIC SXF100 SXQ80 HF100
Element Crude oil 1
Table 5 Results obtained after digestion using 5 different procedures and measure by Q-ICPMS of 3 extra-heavy crude oil samples (μg g−1, n = 3).
HPA MIC SXF100
Crude oil 3
SXQ80
460 ± 10 99 ± 2 0.038 ± 0.005 0.27 ± 0.01
HPA
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