Direct multi-elemental analysis ofcrude oils using the Agilent 4200/4210
Microwave Plasma-Atomic Emission
Spectrometer
Application note

Petrochemical, energy and fuelsAuthors
Jenny Nelson1, Greg Gilleland1,
Laura Poirier2, David Leong2,
Paul Hajdu2 andFrancisco Lopez-Linares2
1.Agilent Technologies, Inc., USA,
2. Petroleum and Material
Characterization Unit, Chevron
Energy Technology Co. USA

Introduction
Knowing the metal concentration of crude oil is vitally important to thepetroleum industry at all stages of the product cycle. Metals are usedto monitor crude migration within an oil reservoir; inform business
opportunities; affect the efficiency of catalysts or render them inactive; aswell as being integral to the final product quality and specification. Metals
are also closely monitored to avoid environmental issues such as soil
contamination from the decommissioning of petrochemical facilities andland reclamation.

Typically, techniques such as inductively coupledplasma-optical emission spectroscopy (ICP-OES),
ICP-mass spectrometry (ICP-MS), flame atomic
absorption (FAAS), X-ray fluorescence (XRF), andneutron activation analysis (NAA) are used in petroleum
labs for the identification and quantification of elements
in aqueous or organic samples. However, Microwave
Plasma-Atomic Emission Spectroscopy (MP-AES)
technology is a suitable alternative for this industry,
and is increasingly used for the multi-element analysis
of a wide range of sample types, including complex
organic matrices. In addition, elemental analysis isoften required in the field or remote areas where thepetroleum industry operates, which can be challenging
depending on the location. Agilent’s 4200 MP-AES uses
magnetically coupled microwave energy to generatea robust and stable plasma using nitrogen gas. Thenitrogen needed for the plasma is extracted straightfrom air using the Agilent 4107 nitrogen generator orfrom a nitrogen Dewar. Compared to conventional flame
AA and ICP techniques, MP-AES eliminates the needfor expensive and flammable gases like acetylene andargon gas to sustain the atomization/ionization source,
resulting in lower running costs and safe unattendedoperation.

Experimental

There are several ASTM methods for the determinationof elements in crude oil analysis. For example,
ASTM D7691 specifies ICP-OES for the analysis of S,
V, Fe and Ni in crude oil after dilution with an organicsolvent. Both ASTM D5708 and D5863 specify aciddigestion or direct dilution with an organic solvent forthe determination of Ni, V and Fe by ICP-OES and Ni, V,
Fe and Na by FAAS respectively.

Instrument operating conditions and analyte settings
are listed in Tables 1a and 1b.

Instrumentation
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 a MicroMist nebulizer, double pass
glass cyclonic spray chamber and solvent resistantsample tubing. An External Gas Control Module (EGCM)
accessory was used to inject a flow of air into theplasma to prevent carbon deposits from building up in
the torch, overcome any plasma instability that may
arise from the analysis of organic samples, and toreduce background emissions caused by carbon species
in the plasma.
The instrument is controlled by the intuitive MP Expert
software. The MP-AES features continuous wavelength
coverage and MP Expert software automatically adds
the recommended wavelength, nebulizer flow rate, andEGCM setting when elements are selected for simplified
method development. The powerful Auto backgroundcorrection mode easily handles any backgroundemission arising from the organic matrix, making
accurate corrections without any user intervention.

Table 1a. Agilent 4200 MP-AES operating conditions.

This application note describes the analysis of V,Ca, Fe, Ni, and Na in 20 crude oil samples using theAgilent 4200 MP-AES following a simple 1:10 dilution
in o-xylene. Results obtained using a combination ofICP‑OES and ICP-MS are given for comparison purposes.
This application is also applicable for Agilent’s 4210
MP‑AES instrument.

2Parameter

Value

EGCM setting

Low

Pump rate (RPM)

5

Sample tubing

Viton Organic black/black

Waste tubing

Viton Organic blue/blue

Read time (s)3Number of replicates3

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

Yes

Background correction

AutoTable 1b. List of analytes with wavelength and nebulizer flow.
ElementCa

Wavelength (nm)Nebulizer Flow (L/min)

396.8470.7

Fe259.9400.7

K769.8970.7

Na

588.995

0.7Ni

341.4760.7

V

311.0700.7

Sc (internal standard)

335.372

0.7

Standards and samples
Twenty crude oil samples covering a wide range ofAmerican Petroleum Institute (API) gravity, nitrogen andsulfur (N&S) content, and density were analyzed in thisstudy.

Figure 1. Calibration curve for Fe 259.940 nm.

Each crude oil sample was diluted ~ 1:10 in o-xylene(Fisher Scientific) by weight. A matrix modifier ofmineral oil (Fisher Scientific), a dispersant (ChevronOronite) and scandium (Conostan®), as the internalstandard were added to all standards, samples andblanks, to give a total oil concentration of 10% (w/w)
in each solution. Sample dilution reduces sample
preparation time to a minimum compared to aciddigestion ensuring that a wide range of samples can beanalyzed in a timely manner whilst minimizing analyteloss and contamination.

Table 2. Wavelength and working calibration concentration range.
Element &wavelength (nm)Concentration
range (ppm)

Correlation
coefficient

Calibration Fit

Fe 259.940

0-100.9999

LinearV 311.070

0-101.0000

Rational

Ni 341.476

0-100.9999

Rational

Ca 396.847

0-10

1.0000

Rational

Na 588.995

0-101.0000

Rational

K 769.897

0-100.9998

Rational

Method Detection Limit
Method detection limits (MDLs) (3σ) were calculated
by measuring 10 consecutive blank readings. The MDLs
(Table 3) demonstrate the ability of the Agilent 4200
MP‑AES to provide excellent detection limits.

Calibration standards were prepared for V, Ca, Fe, Ni,Na, and K at the 0, 2, 5 and 10 ppm level by diluting
by weight with Conostan® S-21+K standard intopre-prepared o-xylene diluent. The o-xylene diluent
containing a matrix modifier and Scandium internal
standard was run as the blank for the calibration.

Table 3. Method detection limits (ppm) for Fe, V, Ni, Ca, Na and K.
Element & wavelength (nm)

To test recoveries of Ca, Fe, K, Na, Ni and V, crude
oil samples were spiked with 885 ppm of Conostan®
S-21+K standard.

MDL (ppm)

Fe 259.940

0.016

V 311.070

0.009

Ni 341.476

0.008

Ca 396.847

0.015

Na 588.995

0.022

K 769.897

0.190

QC spike recovery tests
To check the validity of the method, three separate
Quality Control (QC) spike recovery tests were carriedout. First, a Continuing Calibration Verification (CCV)
sample at the midpoint concentration of the calibration(5 ppm) was analyzed 7 times, with a recovery within±10% of the target values.

Results and discussion
Working calibration range
The calibration curve for Fe is presented in Figure 1,
highlighting the working range for the MP-AES. All
correlation coefficients were greater than
0.999 (Table 2).

3

Secondly, a crude oil sample was spiked with S21+K at885 ppm to validate the method at a high concentrationlevel, with all recoveries within ±10%. Finally, a certified
reference sample, NIST 1634c Trace Elements in FuelOil (NIST, Gaithersburg MD) was analyzed 7 times forvanadium (certified at 28.19 ppm) and nickel (certifiedvalue of 17.54 ppm) only, with recoveries within ±10%.
A summary of the data is given in Table 4.

Analysis of crude oil samples
The developed method for MP-AES was applied to thedetermination of Fe, V, Ni, Ca and Na in 20 crude oilsamples. Displayed, in Tables 6a and 6b, are the results
for three of the crude oil samples; S1, S10 and S20. The
results presented show that the concentration level ofeach element varies between the different oil samples.
Sample “S20” was analyzed in duplicate, with good
agreement between the 2 sets of results.

Table 4. QC spike recovery results of a CCV sample, S21 + K spiked crude oilsample and NIST 1634c Trace Elements in Fuel Oil CRM.

Table 6a. Measured concentrations of metallic impurities in crude oil samples
S1, S10 and S20 (ppm).

Element and wavelength (nm)Fe259.940

V311.070

Ni341.476

Ca396.847

Na588.995

K769.897

5 ppm CCV
sample(mean, n=7)

4.83

4.94

4.96

5.03

4.94

5.04

% Recovery

97999910199101

885 ppm S21+K spiked
sample(mean, n=7)

862.9

894.4

876.5

853.0

834.9

940.9

Certifiedvalue (ppm)

885.0

885.0

885.0

885.0

885.0

885.0

% Recovery

98101

999694

106

NIST 1634c
CRM(mean, n=7)

-

30.95

18.03

-

-

-

Certifiedvalue (ppm)

-

28.19

17.54

-

-

-% Recovery-110103--Sample

Na588.995

K769.897

Average(ppm)

74.24

78.23

75.82

76.45

72.43

75.60

Spiked conc.
(ppm)

76.89

76.89

76.89

76.89

76.89

76.89

% Recovery

9710299999498

Ni
341.476(nm)

SD

S1

0.46

0.13

13.86

0.03

9.51

0.05

S10

9.65

0.57

0.13

0.03

0.87

0.03

S20

130.99

1.05

87.48

0.21

68.08

0.18

*S20b

130.81

0.86

87.08

0.32

68.15

0.06

Ca396.847 nm

SDNa
588.995 nm

SD

<LOD

-

<LOD

-

S10

5.05

0.33

5.99

0.39

S20

58.29

0.27

26.59

0.41

*S20b59.43

0.6225.89

0.22* Duplicate sample of S20<LOD indicates the result was below the limit of detection

Comparative results with ICP-MS/ICP-OES
Quantitative measurements of Ni, V, Fe and Ca in the 20crude oil samples by MP-AES were in good agreementwith the data obtained using a combination of ICP‑OESand ICP-MS, showing good correlation between
techniques (Figure 2).

Element and wavelength (nm)Ca
396.847

SDS1

Table 5. Results of 79.89 ppm concentration spike recovery test in a crudeoil sample.

Ni341.476

V311.070(nm)

SampleSpike recoveries of a crude oil sample
Accuracy of the method was checked by spiking acrude oil sample in triplicate with S21 +K at 79.89
ppm for all analytes. The spike recovery results areshown in Table 5. For all analytes, recoveries were
within the ±10% range of the target values. The results
demonstrate the ability of the 4200 MP-AES to analyze
Fe, V, Ni, Ca, and K spikes accurately.

V311.070

SD

Table 6b. Measured concentrations of metallic impurities in crude oil samples
S1, S10 and S20 (ppm).

-Fe259.940

Fe259.940(nm)4

Figure 2. Comparison of measured values for Ni, V, Fe and Ca in crude oil samples obtained using a combination of ICP-MS & ICP-OES and MP-AES, including
correlation coefficients.

Conclusions

Productivity is further improved with the new generationof MP Expert software that facilitates simultaneous
auto background correction and a series of ease-of-use
features, including application-specific software applets
plus plug-and-play hardware, that simplify method
development and torch alignment, with minimal training.

The Agilent 4200 MP-AES fitted with the EGCM
accessory was used successfully for the analysis of
20 crude oil samples following a simple 10x dilution ino-xylene, comparatively to preparation methods andprocedures as defined in ASTM D7691.

ReferenceExcellent detection limits and spike recoveries wereachieved for Ni, V, Fe, Ca, Na and K in this difficult
matrix.

J. Nelson, G. Gilleland, L. Poirier, D. Leong, P. Hajdu,andF. Lopez-Linares, Elemental Analysis of Crude Oils Using
Microwave Plasma Atomic Emission Spectroscopy,Energy & Fuels, 2015, 29 (9), 5587–5594.

The recoveries obtained from the analysis of the three
QC test materials were within ±10% of the actual/certified values. A wide-API range of crude oil samples
was evaluated and the MP-AES results compared wellwith measured values obtained by ICP-OES or ICP-MS.
The 4200’s use of a nitrogen-based plasma eliminates
the need for hazardous gases such as acetylene,ensuring a significant reduction in running costs
associated with ongoing gas supply and greatly
improving laboratory safety for unattended operation.

5

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© Agilent Technologies, Inc. 2016
Published September 1 2016
Publication number: 5991-7104EN