Application Note
Energy and Chemicals

Determination of Metals in Petroleum
Fractions using MP-AES
Including a comparison of sample preparation
methodsAuthors
Laura Poirier1, Jenny Nelson2,
Greg Gilleland2, Steve Wall2, Lidia
Berhane1, and Francisco LopezLinares1
Chevron Energy Technology
Company, USA1

2

Agilent Technologies, Inc., USA

Introduction
The determination of carbon, hydrogen, nitrogen, sulfur, and metals in feedstocks
is critical to decision making in oil refineries [1]. To analyze metals in heavyhydrocarbons, such as bitumen and heavy oil, each fraction is normally separated
from the hydrocarbon matrix. This separation is achieved by converting eachfraction into aqueous based samples using acid assisted digestion. Typically, nitricacid and hydrochloric acid combined with hydrogen peroxide are used to solubilize
metals via an ashing and refluxing method or microwave digestion. Flame AtomicAbsorption (FAAS), Inductively Coupled Plasma Optical Emission Spectrometry(ICP-OES), and ICP-Mass Spectrometry (ICP-MS) are often used for the analysis[2–6]. Microwave Plasma Atomic Emission Spectrometry (MP-AES) can also
analyze the digested samples [7]. MP-AES is attractive alternative technique for theapplication because of its relatively low operating costs and robustness.

The Agilent 4200 and 4210 MP-AES use magnetically coupledmicrowave energy to generate a robust and stable plasmausing nitrogen gas. The N2 can be extracted directly from airusing the Agilent 4107 nitrogen generator or from a Dewar,
allowing onsite testing in remote locations. Compared toFAAS and ICP techniques, MP-AES eliminates the need for
expensive and flammable gases like acetylene and argon.
This results in lower running costs, and safe, unattended
operation. To aid in the processing of petroleum and otherorganic samples the MP-AES can be fitted with an External
Gas Control Module (EGCM). The EGCM injects air into theplasma preventing the buildup of carbon in the torch. It also
reduces the background emissions caused by carbon species
during measurements.

Calibration standards were prepared for V, Ca, Fe, Ni, Na, andMo at 1, 2, 5, and 10 mg/kg. The standards were prepared byadding Conostan S21 standard by weight into pre-preparedo-xylene diluent. The o-xylene diluent containing a matrix
modifier and Sc ISTD was run as the blank for the calibration.

ASTM D5708, ASTM D5863, and ASTM D7876 methods relate
to the determination of metals such as iron, nickel, vanadium,and sodium in crude oils and residual fuels [8-10]. Eachmethod allows sample preparation using wet ash digestion(WAD), direct dilution (DD) in an organic solvent, microwave(MW) assisted acid digestion, and AAS or ICP-OES as thedetection technique.

Instrumentation

MP-AES was used as an alternative detection technique forthe elemental analysis of crude oils in a previous study [11].
The samples covered a wide range of American Petroleum
Institute (API) gravity, nitrogen and sulfur (N&S) content, anddensity. The study showed that MP-AES was suitable for themeasurement of samples prepared using DD.
In this work, iron, nickel, and vanadium were determined incrude oil residues diluted in o-xylene using an Agilent 4200
MP-AES. The Agilent 4210 MP-AES instrument can also be
used for this application.

To test recoveries of Ca, Fe, Na, Ni, Mo, and V, crude oilsamples were spiked with 885 ppm of the S21 standard.
Continuing calibration verification (CCV) standards wereused to check the quality of calibrations. The CCV standard
was diluted from the 885 ppm S21 standard (taken from adifferent lot number than the calibration standards) by weightof the diluent. The 885 mg/kg S21standard was diluted toapproximately 5 mg/kg (1:177), so the CCV would be at themidpoint of the calibration curve.
All measurements were performed using an Agilent 4200
MP‑AES, with nitrogen supplied from an Agilent 4107 Nitrogen
Generator. The sample introduction system consisted of aMicroMist nebulizer and double-pass glass cyclonic spraychamber. An External Gas Control Module (EGCM) accessory
and an SPS 4 autosampler were also used.
The MP-AES was controlled using the intuitive MP Expert
software, which recommends wavelengths for the selectedelements, and automatically sets the nebulizer flow rate andEGCM settings. Auto background correction was used toresolve the element emission line from the organic matrix.
Instrument operating conditions are given in Table 1.
Table 1. Agilent 4200 MP-AES operating conditions.
Parameter
EGCM setting

ValueHighSamples and standards

Pump rate (RPM)

5Ten crude oil samples covering a wide range of API gravity,
N&S content, and density were analyzed in this study.
Each crude oil sample was diluted from 1:20 up to 1:200
in o-xylene (Fisher Scientific) by weight, based on theestimated metal concentrations. A matrix modifier (mineraloil, Fisher Scientific), a dispersant (Chevron Oronite, US), andscandium (Sc), as the internal standard (Conostan®, SCPScience, Canada), were added to all standards, samples, andblanks. Compared to acid digestion, direct dilution reduces
sample preparation time to a minimum, which is beneficial
for laboratories that need to analyze high sample volumes.
Direct dilution also reduces the possibility of analyte loss orcontamination during sample preparation.

Sample tubing

Orange-white

Waste tubing

Blue-blue

Read time (s)32Number of replicates3

Sample uptake delay (s)55Stabilization delay (s)10Fast pump during uptake

Yes

Rinse time (s)60Background correction

AutoResults and Discussion
The MDLs and LOQs show the efficiency of the MP-AES
nitrogen plasma for the analysis of petrochemical residues
following direct dilution with a solvent such as o-xylene.

MDLs and LOQs
Method detection limits (MDLs) (3σ) and limits ofquantification (LOQ) were calculated for Ca, Fe, Mo, Na, Ni,and V by measuring a blank solution (diluent containing amatrix modifier and internal standard) 10 times (10σ). The
results are shown in Table 2.

Table 2. Method detection limits and the limit of quantification (µg/kg) with a blank solution using MP-AES.
Blank solution (MP-AES)NaCaVFeNiMo

588.995 nm

396.847 nm

311.070 nm

259.940 nm

341.476 nm

313.259 nm

MDL

3.28

1.61

5.23

9.40

2.93

2.08

LOQ

10.95

5.37

17.44

93.97

9.78

6.94

Reprinted with permission from Laura Poirier et al, Energy Fuels, 2017, 31 (8), pp 7809–7815. Copyright (2017) American Chemical Society.

(28.19 mg/kg) and Ni (17.54 mg/kg) only. All recoveries werewithin ±10% (±12% for V in S21), as shown in Table 3.

Quality Control test
Three Quality Control (QC) tests were performed. A CCV
sample at the midpoint concentration of the calibration(5 mg/kg) was analyzed repeatedly. The S21 standard(885 mg/kg) was then analyzed. Finally, NIST 1634c
Trace Elements in Fuel Oil was analyzed seven times for V

As already demonstrated for crude oil analysis using MP-AES[11], these results indicate that MP-AES can also be used as
an alternative for residue analysis via direct dilution.

Table 3. QC average results of a CCV sample, S21 standard sample, and NIST 1634c Trace Elements in Fuel Oil CRM determined by MP-AES.
5 mg/kg (CCV)

885 mg/kg Standard

NIST 1643c (QC)

Element andwavelength(nm)

Meanconc
mg/kg

SDRSD
(%)

Recovery(%)

MeanConc
mg/kg

SDRSD(%)

Recovery(%)

MeanConcmg/kg

SDRSD
(%)

Recovery(%)

Na 588.995

4.98

0.08

1.61

100

938.97

8.9

0.95

106

-

---

Ca 396.847

4.96

0.11

2.2299918.34

12.72

1.39

104

-

-

--

V 311.070

5.09

0.19

3.73

102

987.02

7.99

0.81

112

30.03

0.21

0.7

107

Fe 259.940

5.11

0.24

4.7

102

965.38

8.94

0.93

109-

---

Ni 341.476

5.07

0.11

2.17

101940.4

7.41

0.79106

17.7

0.13

0.73

101

Mo 313.259

5.120.16

3.13

102

943.89

8.27

0.88

107

----

Petroleum fraction 1000 °F+ analysis
A variety of samples was analyzed by MP-AES to determine
if the properties of the different feed samples would impact
the plasma, causing a potential bias [7]. The samples included
a vacuum residue, heavy oil crude, deasphalted crude oil
containing different levels of sulfur (1.0 wt. % up to 5.5 wt. %),and nitrogen (200 mg/kg up to 14000 mg/kg).
The measured concentrations of the metals present in theresidue samples are shown in Table 4. The results show how
the metal concentrations differ among the samples. Thesamples were diluted 1:10 with the o-xylene diluent. When theconcentration of any element was >100 mg/kg, the dilution

factor was increased from 1:20 up to 1:200 depending on
the highest elemental concentration in the sample. The
measurement standard deviations did not exceed 1.5 mg/kg. For this set of samples, all samples were analyzed using
MP-AES.
However, it was not possible to quantify Na in samples S1,S2, S7, S8, and S10 because the dilution factor was too high
(1:50-1:200). The Na injected concentration was too low to bequantified at the same time as the high-level of Ca, V, Fe, andNi present in these samples. In some cases, applying a single
dilution factor will not keep all elements within the effective
calibration range.

3

Table 4. Measured concentrations of metals present in residue samples in mg/kg using MP-AES.
Solution labelNaCaVFeNiMo

588.995 nm

396.847 nm

311.070 nm

259.940 nm

341.476 nm

313.259 nmS1<13.00

508.9

311.37116.76176.08<8.21S2<43.20

40.90

995.46<24.9204.01<27.00S314.43<2.05

166.29

16.79

63.13

<2.55

S4

14.67

<2.58

165.63

16.99

63.97

<3.21

S5

17.21

<2.09

180.22

14.67

70.73

<2.60

S69.44<1.85

126.72

10.93

37.91

<2.31

S7

<2.15

<1.09

56.21

30.58

28.73

<1.35

S8

<12.4

<6.26

287.59

42.57

68.93

<7.79

S9

14.54

<5.80

406.5816.22

147.02

18.58

S10<28.20<14.30427.3921.32

197.77151.93NIST 8505<15.1017.33

424.67

<8.74

50.66

<9.54

* Results are the mean of triplicate measurements with RSD lower than 10 % < indicates that the concentration was less than the method detection limit
Reprinted with permission from Laura Poirier et al, Energy Fuels, 2017, 31 (8), pp 7809–7815. Copyright (2017) American Chemical Society.

Comparison of sample preparation techniques
Previously, we reported the analysis of crude oil samples
by MP-AES following dilution in an organic solvent [7, 11].
Due to the complex nature of the samples selected for thiswork, different solubility behavior and the potential presenceof insoluble solids was anticipated. Therefore, the direct
dilution (DD) method was compared to a conventional wetash digestion (WAD) method for the most useful elements
found in these samples: V, Fe, and Ni. Samples prepared by
DD were measured by MP-AES and the samples prepared by
WAD were measured by ICP-OES. The results are presented inTable 5.
Table 5. Sample preparation method and measured concentration of V, Fe,and Ni in mg/kg.
DD-MP-AES
SolutionLabelV
311.070nm

Fe259.940
nm

S1311.37

S2995.46

S3
S4WAD-ICP-OES
Ni
341.476nm

V309.31nmFe238.20nm

Ni231.60nm

116.76176.08289.00117.00166.00<24.90204.01884.0017.60

203.00166.29

16.79

63.13

156.00

32.10

64.90

165.63

16.99

63.97

156.00

32.40

64.90

S5

180.22

14.67

70.73

168.00

27.00

72.00

S6

126.72

10.93

37.91

120.00

18.50

39.80

S7

56.21

30.58

28.73

89.40

169.0045.30

S8287.59

42.57

68.93

262.00

61.80

66.00

S9406.58

16.22

147.02335.0025.30

145.00S10427.3921.32

197.77383.0029.50

187.00NIST 8505

424.67<8.7450.66

386.00

6.49

51.37

Reprinted with permission from Laura Poirier et al, Energy Fuels, 2017, 31 (8), pp 7809–7815.
Copyright (2017) American Chemical Society.

4

In most cases, the DD results correlate well with the WAD
results for V and Ni, while there is greater variability in the Feresults. The DD/WAD ratios for Ni and V span from 1.00 up
to 1.14, which implies that DD can be used as an alternative
to WAD for analyzing these metals in a residue. Solid
microparticulate minerals containing Fe could be presentin the heavy oil fractions and deposits [1], contributing to
the low bias for this metal. Fe is reportedly the dominantelement present in such minerals [12]. If this particulate was
not adequately dispersed in the organic solvent, lower results
would be expected in comparison with an acid digestion
method.
V and Ni are the most common elements in petroleum
fractions. They also tend to be present at a higher
concentration in the heaviest and non-distillable fractions,such as residues, resins, and asphaltenes [13]. Additionally,
they are the two most important elements measured in a
crude oil assay. Assays are carried out to determine if a crude
oil feedstock is suitable for a particular petroleum refinery[14]. Figure 1 presents a comparison of ratios between thesample methods evaluated (DD and WAD) for V and Ni.

References
1.

K. H. Altgelt, M. M. Boduszynski, Composition andAnalysis of Heavy Petroleum Fractions, Marcel Dekker:
New York, 1994, Chapter 2, 19-40

2.

C. Hardaway, J. Sneddon, J. N. Beck, Determination ofMetals in Crude Oil by Atomic Spectroscopy, Analytical
Letters, 2004, 37, 2881–2899.

3.

R. A. Kishore Nadkarni, J. D. Hwang, L. Young,
Multielement Analysis of Crude Oils Using Inductively
Coupled Plasma-Atomic Emission Spectrometry, Journalof ASTM International, 2011, 8, Paper ID JAI103837

4.

R. Sanchez, J. L. Todoli, C-P. Lienemann, J. M. Mermet,
Determination of trace elements in petroleum
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5.

P. A. Mello, J. S. F. Pereira, M. F. Mesko, J. S Barin, E. M.
M. Flores, Sample preparation methods for subsequent
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6.

L. Poirier, J. Nelson D. Leong, L. Berhane, P. Hajdu, F.
Lopez-Linares, Application of ICP-MS and ICP-OES on theDetermination of Nickel, Vanadium, Iron, and Calcium
in Petroleum Crude Oils via Direct Dilution. Energy Fuels,
2016, 30 (5), pp 3783–3790

7.

L. Poirier, J. Nelson, G. Gilleland, S. Wall, L. Berhane, andF. Lopez Linares, Comparison of Preparation Methodsfor the Determination of Metals in Petroleum Fractions(1000 °F+) by Microwave Plasma Atomic Emission
Spectroscopy, Energy Fuels, 2017, 31 (8), 7809–7815

8.

Standard Test Methods for Determination of Nickel,
Vanadium, and Iron in Crude Oils and Residual Fuels
by Inductively Coupled Plasma (ICP) Atomic Emission
Spectrometry, ASTM D5708-12e1; ASTM International:West Conshohocken, PA, USA, 2012

9.

Standard Test Methods for Determination of Nickel,
Vanadium, Iron, and Sodium in Crude Oils and ResidualFuels by Flame Atomic Absorption Spectrometry. ASTM
D5863; ASTM International: West Conshohocken, PA,USA, 2012

V DD-MP-AES/
V WAD-ICP-OES

Ni DD-MP-AES/
Ni WAD-ICP-OES

Figure 1. Comparison of sample preparation methods and detection
techniques for vanadium and nickel.

Conclusions

Vanadium, nickel, and iron can be analyzed in residue samples
following direct dilution in an organic solvent, in combination
with measurement using MP-AES. While V and Ni are notsensitive to the sample preparation method, Fe recovery
using the direct dilution method is often biased low because
microparticles of iron may not distribute well into organicsolutions. For Fe, better results are obtained using wet ash
digestion.
A comparison of sample preparation techniques showed
good agreement for Ni and V in samples prepared using
wet ash digestion and direct dilution. The samples were
measured by ICP-OES and MP-AES, respectively. Direct
dilution is convenient for laboratories that need to analyze
large numbers of samples. It eliminates the need for time
consuming acid digestion processes and reduces thepossibility of analyte loss or contamination during sample
preparation.
The Agilent 4200/4210 MP-AES represents a low-cost
alternative technique for the analysis of petrochemical sample
types. When fitted with a nitrogen generator, the technique
can be safely used in remote areas or field locations.

More Information

Laura Poirier, Jenny Nelson, Greg Gilleland, Steve Wall,
Lidia Berhane, and Francisco Lopez Linares, Comparisonof Preparation Methods for the Determination of Metals inPetroleum Fractions (1000 °F+) by Microwave Plasma AtomicEmission Spectroscopy, Energy Fuels, 2017, 31 (8),pp 7809–7815. DOI: 10.1021/acs.energyfuels.7b00654.

10. Standard Practice for Practice for Sample Decomposition
Using Microwave Heating (With or Without Prior Ashing)for Atomic Spectroscopic Elemental Determination inPetroleum Products and Lubricants. ASTM D7876 – 13.
ASTM International: West Conshohocken, PA, USA, 2013511. J. Nelson, G. Gilleland, L. Poirier, D. Leong, P. Hajdu, F.
Lopez-Linares, Elemental Analysis of Crude Oils Using
Microwave Plasma-Atomic Emission Spectroscopy,Energy Fuels, 29, 2015, 5587–5594
12. L. Carbognani, M. Orea, M. Fonseca, Complex Nature ofSeparated Solid Phases from Crude Oils, Energy Fuels,
13, 1999, 351–358
13. T. F. Yen, The role of trace metal in petroleum. Ann Arbor
Science Publisher Inc.1975, Chapter 1, 1-30
14. H. N. Giles, Crude Oils. The significance of Test
for Petroleum Products, Salvatore J. Rand, editor.
ASTM Manual Series: MNL 1, Seventh Edition, ASTM
International, 2014, West Conshohocken, PA. Chapter 5,51-62

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