Article pubs.acs.org/EF
Elemental Analysis of Crude Oils Using Microwave Plasma Atomic Emission Spectroscopy Jenny Nelson,† Greg Gilleland,† Laura Poirier,‡ David Leong,‡ Paul Hajdu,‡ and Francisco Lopez-Linares*,‡ †
Agilent Technologies, Inc., 5301 Stevens Creek Boulevard, Santa Clara, California 95051, United States Chevron Energy Technology Company, 100 Chevron Way, Richmond, California 94801, United States
‡
ABSTRACT: Trace elemental analysis using microwave-induced plasma (MP) generated by nitrogen gas was employed as an atomization and excitation source for emission spectrometric (microwave plasma atomic emission spectroscopy, MP-AES) analysis for crude oil samples. Nitrogen gas produced from air through a gas generator, and in combination with an external gas control module (EGCM) used to introduce air into the plasma, leads to a stable robust microwave-induced plasma, enough to perform the analysis of crude oil samples with different API values, sulfur (0.5−5 wt %) and nitrogen (500 −2500 mg/kg) by direct dilution in a o-xylene diluent. Excellent detection limits and spike recoveries at low and high concentration levels were determined for Ni, V, Fe, Ca, and Na in crude oil matrix. The recoveries obtained from the analysis of the three quality control (QC) test materials were within ±10% of the actual/certified values. Comparable results for Ni and V as well as the V/(V + Ni) ratio in crude oils between nitrogen plasma and conventional argon plasma such as inductively coupled plasma atomic emission spectroscopy (ICP-AES) and inductively coupled plasma mass spectrometry (ICP-MS) were achieved in terms of precision, relative standard deviation, and recoveries for the concentration range from 2 to 230 mg/kg.
1. INTRODUCTION Trace elemental analysis in the petroleum industry is crucial because the presence of metals and nonmetals in crude oils can impact the whole business from the production, the refinery, and up to the consumer. For example, nickel, vanadium, arsenic, and lead act as catalyst poisons during refinery operations; the presence of vanadium in fuel could result in formation of corrosive compounds during combustion; volatile organometallic compounds could be contaminants in the distillate fraction; and the presence of nitrogen and sulfur compounds in engine fuels could negatively impact the environment.1,2 Knowing the nature of trace elements present in crude oils is the link between its formation from basins and its refinement into final products.3 A clear understanding of the true nature of the role trace metals play in petroleum production will allow for faster progress in the petroleum business. For this reason, an accessible, robust, and costeffective analytical method is essential to the petroleum industry. Metals analysis is a routine practice in petroleum laboratories, and various strategies have been developed to fulfill the need to handle solid, aqueous, and organic matrices. Particularly for crude oil, most of the methods use some type of sample preparation followed by spectroscopic measurements.1,4 Advantages, disadvantages, and challenges of these methods have been reported recently.5−8 The use of conventional wet acid digestion methods is the most common choice today for trace elemental analysis of hydrocarbons using as detection technique inductively coupled plasma technology, OES/AES (optical emission spectroscopy/atomic emission spectroscopy; which effectively represent the same method and technology), and inductively coupled plasma mass spectrometry (ICP-MS) instruments.1,4−6 From here on in this work and for the © XXXX American Chemical Society
easiness of our discussion, ICP-OES and ICP-AES will represent the same thing. However, direct injection of organic solutions as an alternative has been increasing in popularity.9−20 It is an attractive alternative because it allows the handling of high numbers of samples, requires less time spent doing sample preparation, and provides results with good precision.16−20 Proper selection of the sample introduction system and RF power and use of oxygen as an auxiliary gas (eliminating carbon buildup on the torch and detector/cone interfaces) are all extremely important when considering running crude oils by direct dilution.6,7,16,17 The need for elemental analysis in the field or remote areas where the petroleum industry typically operates can be challenging depending upon the location. For most of these sites, it is limited and expensive, if not impossible, to obtain argon and run argon-based systems. As examples, there are certain places where argon gas in conventional cylinders are the only option available, where the constant supply is not necessarily guaranteed in a timely manner. Concomitant, the instrument needs to be operating at a specific time frame to reduce argon consumption; therefore, a possible alternative to overcome this situation is foreseen. Microwave-induced plasma (MP) for atomic emission technique has been demonstrated earlier as an efficient source for the determination of metals and nonmetals at different matrices,21−25 and particular for petroleum products and derivatives,26−28 gasoline and ethanol,29 as well as diesel and biodiesel.30 Furthermore, commercial units are nowadays Received: May 6, 2015 Revised: August 7, 2015
A
DOI: 10.1021/acs.energyfuels.5b01026 Energy Fuels XXXX, XXX, XXX−XXX
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Energy & Fuels Table 1. 4200 MP-AES, ICP-MS, and ICP-OES Experimental Conditions instrument conditions
4200 MP-AES
EGCM setting pump rate tubing read time no. of replicates sample uptake delay stabilization delay background correction sample flow rate rinse flow rate peri-pump speed analysis peri-pump speed rinse forward power plasma gas flow rate auxiliary gas flow rate make up gas carrier gas flow rate optional O2 gas flow rate sampling depth/radial viewing height peri-pump speed rinse peri-pump speed analysis collision cell: He mode/H2 mode spray chamber temp elements isotope/wavelength (nm)
low 5 organic 3s 3 55 s 10 s auto 238 μL/min 4.5 mL/min 5 rpm 80 rpm
internal standard
Fe 259.940 V 311.070 Ni 341.476 Ca 396.847 Na 588.995 K 769.897 Sc 335.372
7700x ICP-MS
6500 ICP-OESa
1550 W 15 L/min 0.9 L/min 0.1 L/min 0.45 L/min 10% 8 mm 0.5 rps 0.04 rps (3.4 mL/min)/(5 mL/min) −2 °C 56 Fe 51 V 60 Ni, 62Ni 40 Ca 23 Na 39 K 45 Sc
1350 W 16 L/min 2 L/min n/a 0.4 L/min n/a 14 mm 85 rpm 40 rpm n/a 5 °C Fe 238.204 V 292.460 Ni 221.647 Ca 317.930 Na 588.995 K 769.897 Sc 227.318 Sc 255.237
89
a
Y
n/a = not available.
available,31 and the new developments in this field allow expansion of the potential of this technique in different businesses.28,31−33 Sustained nitrogen plasma produced by microwave technology channels the sample aerosol directly to the center of the plasma displaying the same characteristic as the argon ICP does.22,24,34−36 When nitrogen is used as the sustaining gas, interferences associated with Ar species formed in the plasma do not occur, so the determination of several more elements is feasible.24,25 The MP-AES is considered a mature technique for the detection of trace elements in gas chromatographic effluents.37,38 Considering that MP can be operated with different gases including oxygen, nitrogen and air, it is becoming a powerful tool for continuous emission monitoring (CEM) capable of withstanding extreme gas conditions during regular industrial process operations (see ref 32 and references therein). Most of the applications of MP emission or mass spectrometry have been oriented to aqueous solutions with less emphasis on organic solutions. Direct determination of several elements in methyl isobutyl ketone (MIBK) using nitrogen− oxygen mixed gas MP-AES has been reported recently.39 The present work is focused on petroleum crude oil samples dissolved in an o-xylene solution and analyzed by MP-AES. The metal concentration on selected samples could be below the range of limit quantification for ICP-AES; therefore the combination with ICP-MS was initially accessed. Considering
that the MP-AES instrument used in this work will generate a lower plasma temperature under fixed microwave power (at 1 kW, approximately 5000 K)40,41 in comparison with conventional ICP, increased matrix effect, higher carbon emission, and soot deposition are anticipated, when it is analyzing petroleum crude samples (high carbon content).11,29 This challenge can be resolved by incorporating an external gas control module (EGCM) which allows a continuing air/oxygen supply into the plasma, promoting carbon oxidation at such temperature, leading to carbon monoxide and carbon dioxide. As a consequence, improved plasma stability, elimination of carbon emission, and buildup at the torch are expected. It is important to take into account that a detailed comparison of the performance of MP-AES with conventional ICP instruments is beyond the scope of this work. Nevertheless, an initial evaluation of the quality of the results gathered in this work with ICP-OES/MS was carried out. Finally, we aim to present the first application of crude oil analysis by direct dilution using MP-AES, with crude oil samples with API gravity spanning between 7° and 38° and other differing properties in order to demonstrate that the direct dilution method is not limited by the type of crude oil that is analyzed.
2. EXPERIMENTAL SECTION 2.1. Reagents. All chemicals used were trace metal grade purity and used without further purification. The following reagents were B
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Energy & Fuels Table 2. Method Detection Limits (mg/kg) for Fe, V, Ni, Ca, Na, and K in a Blank Solution, Using MP-AES Fe
V
Ni
Ca
Na
K
blank solution (MP-AES)
259.940 nm
311.070 nm
341.476 nm
396.847 nm
588.995 nm
769.897 nm
MDL LOQ
0.01 0.09 Fe
0.00 0.04 V
0.00 0.03 Ni
0.00 0.03 Ca
0.07 0.72 Na
0.06 0.62 K
blank solution (ICP-OES)
238.204 nm
292.46 nm
221.65 nm
317.93 nm
588.99 nm
769.897 nm
MDL LOQ
0.01 0.05
0.01 0.05
0.01 0.05
0.01 0.05
0.02 0.15
0.01 0.25
Table 3. Recovery Efficiency for Fe, V, Ni, Ca, Na, and K in a 1.00 mg/kg Standard Solution, Using MP-AES Fe
V
Ni
Ca
Na
K
std soln 1.00 mg/kg
259.940 nm
311.070 nm
341.476 nm
396.847 nm
588.995 nm
769.897 nm
av recovery (%)
1.01 98−102
0.99 98−101
0.98 97−99
1.08 106−108
0.96 95−98
1.00 94−106
used, o-xylene (Fisher Scientific, Fair Lawn, NJ, USA), mineral oil (Fisher), Conostan (Quebec, Canada) S-21+K 10 mg/kg organosoluble standard, Conostan S-21+K 885 mg/kg organosoluble standard, dispersant (Chevron Oronite, Richmond, CA, USA), scandium and yttrium (100 mg/kg; Conostan), scandium (2000 mg/kg; Conostan), V23 organosoluble standard (VHG, Manchester, NH, USA), and NIST 1634c trace elements in fuel oil (Gaithersburg, MD, USA; Certificate of Analysis (C of A), expiration Dec. 31, 2020). 2.2. Instrumentation. An Agilent 4200 MP-AES (Agilent Technologies, Santa Clara, CA, USA), with an Agilent 4107 nitrogen generator (Agilent) and Agilent 7700x ICP-MS (Agilent), and a Thermo Radial ICAP 6000 Series ICP-OES (Thermo Fisher Scientific, Fremont, CA, USA) were all used for the direct injection of o-xylene solutions in this study. The sample introduction system used on the MP-AES consisted of a MicroMist nebulizer, glass double-pass cyclonic spray chamber, and solvent-resistant tubing (black−black for sample tubing and blue−blue for waste tubing), keeping a fast pump speed mode during uptake. An external gas control module (EGCM) accessory was used to inject a low flow of air into the plasma to prevent carbon deposits from building up in the torch, to overcome any plasma instability that may arise from the analysis of organic samples, and to reduce background emissions. For the Agilent 7700x ICP-MS, a MicroMist concentric pneumatic nebulizer, double-pass Scott quartz spray chamber, 1 mm injector torch, and platinum sampler and skimmer cones were used. Oxygen was mixed in at the high matrix introduction (HMI) port to prevent carbon deposits building up on the torch and cones. A Thermo Radial ICAP 6000 Series equipped with a Noordermeer V-groove nebulizer, IsoMist spray chamber (Glass Expansion, Pocasset, MA, USA), Thermo radial D-torch with a ceramic outer tube, and a quartz 1.0 mm injector were used. The summary of the conditions employed in this work is presented in Table 1. The wavelengths selection on ICP-OES and MP-AES for the analytes examined in this work was based according to signal intensity and lower spectral interference obtained while it was performing the analysis on such matrixes. Additionally, recommendation from open literature42,43 as well as established industrial standards such as ASTM D726044 and ASTM D570845 were considered. 2.3. Calibration Standards Preparation. Calibration standards for the MP-AES, ICP-MS, and ICP-OES were prepared from different concentrations of Conostan organosoluble standards. For the MP-AES and ICP-OES, a diluent, containing o-xylene (Fisher), a matrix modifier (mineral oil; Fisher), a dispersant (oronite; Chevron), and scandium (2000 mg/kg; Conostan) spiked at 5 mg/kg as an internal standard was used to make the standards and dilute the samples. A dilution by weight of S-21+K 885 mg/kg standard (Conostan) with the previously mentioned diluent was used to create calibration standards of 2, 5, and 10 mg/kg. The diluent was used as the blank for the calibration. For ICP-MS, a similar diluent was used and procedure was followed. The only difference was scandium and yttrium (100 mg/
kg; Conostan) spiked at 0.1 mg/kg were used as internal standards. Due to the lower operating concentration range for ICP-MS, the stock solution S-21+K (10 mg/kg; Conostan) was used for the preparation of the ICP-MS calibration standards. Calibration standards ranging from 1 to 1000 μg/kg were prepared by weight with the o-xylene diluent containing a matrix modifier, and scandium and yttrium (Conostan) as internal standards. The diluent was used as the blank for the calibration. Continuing calibration verification standards (CCVs) were used on all instruments to check the quality of the calibrations. For MP-AES and ICP-OES, the CCV standard was diluted from 885 mg/(kg of S21+K) (Conostan; from a different lot number than that used for the calibration standards) and was prepared by weight with the diluent (containing o-xylene, a matrix modifier and internal standard). The CCV was diluted to approximately 5 mg/kg so that it would be at the midpoint of the calibration curve. Since the ICP-MS calibration curve was at lower concentrations, 10 mg/kg V23 (VHG) was diluted by weight in an o-xylene diluent containing a matrix modifier and internal standard to three different concentrations: 10, 50, and 600 μg/kg. 2.4. Samples. Fifteen crude oil samples were selected having broad API values (7−38°) and the following elemental compositions: 0.5−5 wt % S and 500−2500 mg/kg N. The main goal of this study was to determine MP-AES performance analyzing samples with this complex matrix by direct dilution. The selected samples may have the element concentration that would fall on the low limit of quantification for ICP-OES which can be accessible by ICP-MS. Therefore, the analysis of the samples with conventional plasma techniques (ICP-OES/MS) was carried out, to have additional reference. Then, ICP-MS values were used for comparison except in cases where the element’s concentration value exceeded the ICP-MS’s upper calibration limit; in those cases, the ICPOES value was used instead for the comparison with MP-AES. The crude oil samples were prepared by taking an aliquot and dissolving it in an o-xylene diluent; however, depending on the element concentration, some dilutions needed to be done to ensure the reading on the calibration range was covered. All samples were shaken for at least 30 min in a mechanical shaker to help the crude oil samples dissolve. On all instruments, o-xylene was used between each sample to rinse the sample introduction system. The samples were prepared less than 24 h before running, and separate dilutions were prepared for each instrument.
3. RESULTS AND DISCUSSION 3.1. Accessing MP-AES Instrument Performance: Method Detection Limit (MDL), Limit of Quantification (LOQ), and Recovery Efficiency. In order to access the instrument sensitivity, method detection limits (MDL (3σ)) and limit of quantification (LOQ)46,47 were determined using a C
DOI: 10.1021/acs.energyfuels.5b01026 Energy Fuels XXXX, XXX, XXX−XXX
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Table 4. QC Spike Recovery Results of a CCV Sample, S-21+K Standard Spiked in Crude Oil Sample and NIST 1634c Trace Elements in Fuel Oil CRM, Determined by MP-AES
av of 7 analyses of 5.00 mg/(kg of CCV sample) SD RSD (%) recovery (%) av of 7 analyses of S-21+K SD RSD (%) certified value (mg/kg) recovery (%) av of 7 analyses of NIST 1634c SD RSD (%) certified value (mg/kg) recovery (%)
Fe
V
Ni
Ca
Na
K
259.940 nm
311.070 nm
341.476 nm
396.847 nm
588.995 nm
769.897 nm
4.83 0.04 0.79 95−98 862.90 4.62 0.54 885.00 97−98
4.94 0.02 0.47 98−99 894.41 2.14 0.24 885.00 100−102 30.95 0.16 0.52 28.19 109−110
4.96 0.01 0.18 99−100 876.46 0.75 0.09 885.00 99−100 18.03 0.02 0.14 17.54 102−103
5.03 0.02 0.36 100−101 853.04 3.65 0.43 885.00 96−97
4.94 0.06 1.31 98−100 834.89 11.80 1.41 885.00 93−96
5.04 0.06 1.19 100−102 940.93 8.46 0.90 885.00 105−107
Table 5. Results of Low- and High-Spike Recovery Test, Using MP-AES sample\analyte
Fe
V
Ni
Ca
Na
K
341.476 nm
396.847 nm
259.940 nm
311.070 nm
588.995 nm
769.897 nm
low matrix spike (av, mg/kg) spiked concn (mg/kg)
11.85 12.10
13.20 12.10
12.13 12.10
12.26 12.10
9.30 12.10
10.93 12.10
recovery (%) high matrix spike (av, mg/kg) spiked concn (mg/kg)
97.94 74.24 76.89
109.12 78.23 76.89
100.28 75.82 76.89
101.33 76.45 76.89
76.86 72.43 76.89
90.35 75.60 76.89
96.55
101.74
98.60
99.42
94.20
98.32
recovery (%)
First, a CCV sample at the midpoint concentration of the calibration (5 mg/kg) was analyzed 7 times and the determined recovery was within ±10% of the true value. Second, a crude oil sample was spiked with S-21+K at the 885 mg/kg level, and the spikes were measured to validate the method at a high concentration level. Finally, certified reference sample NIST 1634c Trace Elements in Fuel Oil was analyzed 7 times for vanadium (certified at 28.19 mg/kg) and nickel (certified value of 17.54 mg/kg) only, with recoveries within ±10%. A summary of the data is given in Table 4. Particularly for the NIST 1634c material, the sodium concentration was also monitored as a reference and the average value determined was 21.31 ± 3.72 mg/kg, which initially does not agree with the information value of 37 mg/kg provided by NIST. However, the value determined by MP-AES is comparable with the reported value given by Botto16 of 23.2 ± 5.2 mg/kg, done by 10 replicate dilutions in tetralin (1:20) analyzed by ICP-AES. Connected to the previous, our internal QC value performed by ICP-MS, done by 10 replicate dilutions (1:50) in o-xylene, give us a value of 22.03 ± 3.72 mg/kg, which is in agreement with the previous data. Therefore, we have a good confidence in the data obtained for this material analyzed by this technique. Additionally, the low RSD obtained in all elements evaluated (less than 1.5%) is an indication of the good instrument stability due to the low drift observed during the length of the run (approximately 8 h). The results presented in Table 4 clearly indicate that the MPAES is capable of achieving a high level of recovery (100 ± 10%), using all QC standards, indicating it is as an alternative for hydrocarbon analysis via direct dilution.
blank solution (diluent containing a matrix modifier and internal standard) 10 times, and the recovery efficiency was determined by measuring a 1 mg/kg standard solution (Conostan S-21+K, 885 mg/kg) 10 times. The results gathered from both measurements are presented in Tables 2 and 3. Table 2 shows the six elements most commonly found in crude oils. All native petroleum contains at least 28 elements,3 where the most abundant are Ni, V, and Fe while Ca, Na, and K are to a lesser extent and could be incorporated into the matrix during the oil production. As is shown in Table 2, it is interesting that a very low MDL can be achieved with this instrument by direct dilution method which is comparable to those obtained in our facilities by ICP-OES. Meanwhile for LOQ values, both instruments have comparable results, having Na and K higher values for MP-AES than ICP-OES but low enough to cover crude oil analysis requirements (100 mg/kg, the dilution factor was between 1:20 and 1:30. The results presented therein show, for the crude samples evaluated, all metals were quantified and, in some cases, the element concentration was below the limit of quantification (BLOQ). The standard deviation associated with the measureE
DOI: 10.1021/acs.energyfuels.5b01026 Energy Fuels XXXX, XXX, XXX−XXX
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V as well as for the V/(V + Ni) ratio for all samples between nitrogen plasma and conventional plasma (ICP-OES and ICPMS) in terms of precision, relative standard deviation, and recoveries for the concentration range from 2 to 230 mg/kg were achieved. The results presented here illustrate the potential of this technique as an alternative for performing elemental analysis on these types of samples; the instrumentation used in this work can handle a high content of carbon without any limitation, and it operates at a very low cost due the nitrogen required being produced from air compressor with a low-cost nitrogen generator associated with low maintenance cost in both of them.
ment did not exceed 1.5, and for the replicate sample (S15), it was observed that the precision did not exceed 2%. Finally, it was shown there were no limitations on the different crude oil types that can be analyzed on the MP-AES instrument. 3.5. Initial Assessment of the Performance of Nitrogen-Based Plasma vs Conventional Argon-Based Plasma. Table 7 presents the concentration in milligrams per kilogram for Ni and V as detected by ICP-OES/ICP-MS and MP-AES in the 15 crude oil samples. The selection of these metals is due to their inherent implication to the oil business; the V/Ni ratio has been used in geochemistry as a correlation parameter and genetic indicator,48,49 and additionally, these elements can modify catalyst activity and act as a poison during refining operations.50 The idea was to get initial information on how closely the data obtained by MP-AES compared with the conventional technology in samples with element concentrations where all instruments could perform the analysis accordingly. It is observed in Table 7 that a successful analysis of 15 crude oil samples, with Ni and V concentration ranges between 2 and 230 mg/kg, can be performed with the MP-AES technology, following a simple 10× dilution in o-xylene (or a 20×−30× dilution as needed for higher concentrations). The precision obtained from measurements between the nitrogen plasma and the traditional argon plasma instruments were within 100 ± 10%. This provides confidence that the nitrogen plasma can handle difficult matrixes such as crude oil without major limitation. An excellent reproducibility for both metals is also demonstrated with the duplicate measurements of sample S15. Excellent precision was obtained for the determination of the V/(V + Ni) ratio in the samples; in general the values are 100 ± 10%, which is very good for crude oil analysis by direct dilution.15,51 Considering that, initially, we just wanted to determine the performance of MP-AES performing elemental analysis in crude samples with a limited number of samples, initial t test was performed for both elements and the outcome suggested that it cannot be proven with at least 95% certainty that the methods employed give different results that would be attributable to nonrandom errors. Nevertheless, a more detailed study which is beyond the scope of this work would require an expansion of the sample universe as well as more data acquisition with all techniques involved. Finally, no indication of carbon buildup or other residue on the torch was observed after the run (more than 8 h), which is a great indication of the capacity of this instrument to handle efficiently these types of samples.
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AUTHOR INFORMATION
Corresponding Author
*Tel.: +1 (510) 2423694. E-mail: fl
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Agilent Technologies, Santa Clara, CA for the use of their facilities and MP-4200 AES and Chevron Energy Technology Company for providing funding and permission to publish this work.
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REFERENCES
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4. CONCLUSIONS It was demonstrated the suitable application of nitrogen plasma generated using MP-AES for the analysis of crude oil samples with different characteristics by simple direct dilution in oxylene diluent. Excellent detection limits and spike recoveries at low and high concentration levels were determined for Ni, V, Fe, Ca, and Na in crude oil matrix. The recoveries obtained from the analysis of the three QC test materials were within ±10% of the actual and or certified values. Additionally, the low RSD obtained in all elements evaluated, less than 1.5%, provides a good indication of low instrument drift during a long period of run time. The Ni and V analysis of 15 crude oil samples was successfully achieved; in general, comparable results for Ni and F
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DOI: 10.1021/acs.energyfuels.5b01026 Energy Fuels XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.energyfuels.5b01026 Energy Fuels XXXX, XXX, XXX−XXX
