Multi-Element Analysis of Petroleum
Crude Oils using an Agilent 7900 ICP-MS
Application note
Energy and fuels

Authors
Jenny Nelson,
Agilent Technologies, USA
Ed McCurdy,Agilent Technologies, UKIntroduction
ASTM International develops and publishes voluntary consensus standards formaterials, products, systems and services across a wide-range of industry sectors.
The purpose of the 12000+ ASTM standard test methods is to improve productquality and safety for consumers and to facilitate market access and trade between
businesses that adhere to the relevant standards [1].
In the petrochemical industry, certain elements are known to impact theperformance and value of the final product, while others have detrimental effects
on petroleum refining processes, as in the case of elements which can deactivate
or poison catalysts. Consequently, there are several ASTM methods on theelemental analysis of oils, lubricants and fuels.

Samples and sample preparation

One example is standard test method ASTM D7111-15a forthe determination of trace elements in middle distillate fuelsusing Inductively Coupled Plasma Atomic Emission
Spectrometry (ICP-AES). This method is widely used in theindustry, but as specifications for the levels of trace elements
in fuels become more stringent, some laboratories havemigrated their analysis to the more sensitive technique ofICP-MS, which provides significantly lower detection limits
than ICP-OES. To reflect this trend, the first ASTM ICP-MS
method for trace element analysis in petroleum has been
balloted in D0203. This is likely to be followed by an ICP-MS
method for elemental analysis in petroleum crude oils.

A total of 18 petroleum crude oil samples were measured in
the study, as detailed in Table 1. The samples were divided
into two groups according to properties including AmericanPetroleum Institute (API) gravity, elemental composition,hydrocarbon (H/C) content, and density.
The set of light crude petroleum oil samples and the multielement Conostan standard (10 elements at 2 mg/kg in crudeoil) were prepared by taking an aliquot of approximately 1 g,
which was dissolved in approximately 9 g of the o-xylene
diluent. For the heavier samples the sample aliquot and theamount of o-xylene diluent required was varied in order todilute the analyte element with the highest concentration towithin the calibration range. Sample dilutions ranged from
1:10 to 1:600. All samples were shaken in a mechanicalshaker for 30 mins to ensure that the crude oil samples werefully dissolved. The sample introduction system was rinsed
using o-xylene between each sample analysis.

This application note describes how the Agilent 7900 ICP-MS
was used to analyze different types of crude oil samples
following simple dilution in an organic solvent, o-xylene. The
aim was to develop a method that is suitable for routine usein the petroleum refining industry, particularly in high sample
volume facilities, where turnaround time is critical.

Experimental

Since Ca, Fe, Ni and V, are the most abundant metals in crudeoil, and are the most commonly studied elements within theindustry, they were the focus of this study.

Standards

All calibration standard solutions were prepared from
Conostan (Quebec, Canada) oil-base standards. The diluent
comprised trace metal grade purity o-xylene (Fisher
Scientific), a matrix modifier (made from mineral oil; Fisher
Scientific), an in-house dispersant, and scandium and yttrium
as internal standards, spiked at 0.1 mg/kg.

Instrumentation

An Agilent 7900 ICP-MS with an Agilent ASX-520 auto
sampler was used. The crude oil solutions were delivered tothe standard glass MicroMist concentric nebulizer throughsolvent-resistant peristaltic pump tubing with a diameter of
0.89 mm. Different pump speeds were used for rinsing andanalyzing the sample, as shown in Table 2. The standard
Peltier cooled quartz spray chamber was controlled at -2 °C tominimize the vapor pressure of the organic solvent. A plasma
torch with a 1 mm injector was used for stable sample
introduction to the plasma. The optional 5th gas mass flow
controller was used to add oxygen (20% in Ar) to the injector
gas stream after the spray chamber. O2 addition serves todecompose the carbon matrix and thereby avoid carbon
deposition on the interface cones. The more reactive plasma
environment with O2 addition requires the use of the morechemically-resistant optional Pt sampling and skimmer cones.
The 7900 was also fitted with an organics drain kit.

Multiple calibration standards ranging from 1 μg/kg to 1000μg/kg were prepared for the target elements by weight fromthe 10 mg/kg Conostan S-21+K multi-element organometallic
standard and o-xylene diluent. Separate calibration standards
were prepared for As and Se from Conostan oil-based
standards. The diluent solution was used as the blank forcalibration.
The standard reference material (SRM) NIST 1634c Trace
Elements in Fuel Oil (Gaithersburg, MD, USA) was used tovalidate the method for the certified elements Ni and V andthe reference elements As and Se. NIST 1634c was diluted
approximately 1:60 in the o-xylene diluent prior to analysis.

2

Table 1. Properties of the 18 crude oil samples analyzed in this study
Sample*

C (%)

H (%)

S (ppm)N (ppm)H/C

API gravity @15.56 °C/60 °F

Density (g/mL) @15.56 °C /60 °F

S1

84.84

13.08

1123

1093

1.84

36.20

0.8429

S2

85.90

13.31

5239

1078

1.85

35.40

0.8471

S3

83.60

12.92

4910

336

1.84

31.40

0.8685

S4

84.19

13.22

3886

310

1.87

41.40

0.8175

S5

85.24

12.76

4204

652

1.78

32.60

0.8616

S6

84.21

12.98

3350

500

1.84

33.60

0.8564

S786.1813.00

2741

498

1.80

33.60

0.8569S885.53

12.81

3129

514

1.78

33.10

0.8589S984.78

13.04

21000

508

1.83

35.00

0.8512

S10

83.15

10.30

49100

4050

1.49

9.67

1.0030

S11

85.52

11.23

18527

7580

1.58

13.50

0.9740

S1283.73

11.15

31980

3770

1.60

16.70

0.9532

S13

84.64

11.32

23821

2430

1.60

19.50

0.9364

S14

82.93

11.28

36809

3870

1.63

20.70

0.9286

S15

84.63

11.32

38500

1360

1.60

23.50

0.9121

S1687.39

12.67

10490

1470

1.74

30.90

0.8702

S1784.84

13.08

1123

1093

1.85

36.20

0.8429S1883.94

13.69

312

55.40

1.94

51.30

0.7733* S1-S9 (API gravity >30°) represent light crude oils samples with element concentrations <15 mg/kg. S10-S18 include all types of crudes with API gravity from 9° up to 51°.

The 7900 ICP-MS features a fourth-generation collision/reaction cell, the ORS4, which includes a standard He modecell gas line for removal of common polyatomic interferences
on most elements. The optional H2 cell gas line was used forthis application, as H2 mode gives superior removal of theintense plasma- or matrix-based polyatomic overlaps oncertain elements, notably Mg (m/z 24 – C2 interference), Si(m/z 28 – CO/N2), Ca (m/z 40 – Ar), Fe (m/z 56 – ArO) andSe (m/z 78 & 80 – Ar2). Instrument operating parameters are
given in Table 2.

Table 2. ICP-MS operating parameters

3Parameter

Setting

RF power (W)1550

Make up gas (L/min)

0.1

Carrier gas flow rate (L/min)

0.45

Option gas (O2:Ar – 20:80) flow rate (%)

10Sampling depth (mm)8Spray chamber temperature (°C)-2Peri-pump speed during rinse (rps)0.5

Peri-pump speed during acquisition (rps)0.04

Helium mode cell gas flow rate (mL/min)

3.4

Hydrogen mode cell gas flow rate (mL/min)5

Results and discussion
Calibration

The calibration data shown in Table 3 is typical of the performance of the 7900 ICP-MS. Linear calibrations were obtained for
all analytes as indicated by the calibration coefficients (>0.998 in all cases). Figure 1 shows representative calibration curves
for V, Ni, As, and Se.

Figure 1. Calibration curves of V, Ni, As, and Se4Recovery of certified reference elements V and Ni

Detection limits

NIST 1634c has certified values for vanadium and nickel,
which are important elements in the assay of crude oils as
they are usually present in the highest concentrations. Bothelements can adversely affect catalyst activity and productyields during the refinery process and vanadium compounds
can cause refractory damage in furnaces [1]. As aperformance check for the ability of the 7900 ICP-MS toprovide consistent, accurate determination of theseimportant elements, V and Ni were measured a total of 18times in the diluted NIST 1634c standard, withmeasurements spread over 6 separate days.

Typical limits of detection (LODs) and background equivalentconcentrations (BECs) for 23 elements are given in Table 3.
The LODs were calculated from three times the standard
deviation of 7 measurement results of the blank solution.
Table 3. ICP-MS performance data
Mass

Element

Tune ModeRLOD
µg/kg

BEC
µg/kg

23Na

He

0.999

1.64

26.67

24

Mg

H20.999

0.23

4.27

27

AlHe

0.999

0.48

2.1428Si

H21.000

1.19

15.54

31PHe

1.000

1.4224.7939K

H20.999

0.26

6.4040

Ca

H2

1.000

0.21

3.53

47

Ti

He

1.000

0.04

0.03

51

VHe

0.998

0.01

0.07

52

CrHe

0.999

0.08

1.9755

Mn

He

1.000

0.02

0.82

56

Fe

H2

1.000

0.02

95.67

60

Ni

He

1.000

0.03

0.1462

Ni

He

1.000

0.06

0.16

63

Cu

He

1.000

0.02

0.54

65

Cu

He

0.999

0.02

0.50

66

Zn

He

1.000

0.05

1.27

68

Zn

He

1.000

0.08

1.2175

As

He

1.000

0.01

0.02

78

Se

H21.000

0.090.0580

Se

H2

1.000

0.07

0.01

95MoHe0.999

0.010.1598

MoHe

1.000

0.03

0.07

107

Ag

He

1.000

0.01

0.01

111

CdHe

1.000

4.52

1.34

118

SnHe

1.000

4.65

1.32

120

SnHe

1.000

0.03

0.02

137

Ba

He

1.000

0.02

0.01

138

Ba

He

1.000

0.01

0.06

207

Pb

He

1.000

0.01

0.06

208

Pb

He

1.000

0.01

0.06

The results in Table 4 show excellent recoveries within 10%of the certified values for both elements. It is also apparent
that there was good agreement between the Ni results
calculated independently using the different isotopes, 60Ni
and 62Ni. This illustrates one of the key advantages of helium
cell mode on the 7900 ICP-MS, where a single set of cellconditions can address all common polyatomic interferences,giving access to secondary or qualifier isotopes which can beused to confirm the result measured using the primaryisotope.

Recovery of uncertified reference elements As and Se

NIST 1643c also has uncertified reference values for As andSe. Both of these elements were measured repeatedly overan extended period in order to create quality control charts
for the laboratory, as shown in Figure 2. The long-termperformance check was based on a total of 147 separate
measurements taken over the course of 12 months. Bothcharts show some variability within the upper and lower
control limits but, overall, the reproducibility of results for Asand Se in the control sample is very good. The average
recoveries for As and Se were also within +10% of thereference concentrations, as shown in Table 4. The statistical
data for the study, shown in Table 5, confirms the excellent
reproducibility of results for As and Se in the control sample
over the year.

The relatively high BECs and LODs for Fe, Zn, Cd and Sn are probably due tocontamination, most likely from the mineral oil component of the matrix modifier usedin the blank.

5Table 5. Repeatability data (mg/kg) for As and Se in NIST SRM 1634c, basedon 147 measurements taken over 12 months

Table 4. Recoveries of V, Ni, As and Se in NIST SRM 1634c
Element and[cell gas mode]

Average conc. & SD(mg/kg)

Certified (C) or
Reference (R) value(mg/kg)Recovery(%)

51 V [He]a30720 ± 835

28190 ± 400 C

109

60 Ni [He]a18897 ± 380

17540 ± 210 C

10862 Ni [He]a17544 ± 287

17540 ± 210 C

10075 As [He]b128.1 ± 5.3

142.6 ± 6.4 R

9078 Se [H2]b106.8 ± 5.8

102 ± 3.8 R

105AsSe

Repeatability variance

2σ(r)

2.118

0.401

Repeatability standard deviation

σ(r)

1.455

0.633

Repeatability relative
standard deviation, %RSD(r)

1.083

0.604

r

4.075

1.773

Repeatability value = 2.8 x σ(r)Calculated from 18 separate measurements taken on 6 separate daysab

Quantitative results

Calculated from 147 separate measurements taken over 12 months

Quantitative results for Ca, Fe, Ni and V in 18 crude oilsamples measured using the Agilent 7900 ICP-MS are shownin Table 6. The results show large variations in concentration
for these critical elements between the different samples,with the greatest variation being observed for vanadium,
ranging from 0.07 to 301 ppm.
Table 6. Quantitative results for Ca, Fe, Ni and V in 18 crude oil samples
measured using ICP-MS (all data mg/kg)

Figure 2. Quality control charts for As (top) and Se (bottom) based on a totalof 147 separate measurements of NIST 1634c taken over 12 months

Crude oil sample

40

Ca56

Fe62

Ni51V

S1

9.59

2.13

4.92

0.45

S25.12

2.46

4.83

4.83

S3

<LOQ*

1.54

6.02

0.07

S4<LOQ*

0.96

0.99

0.85

S50.87

1.05

5.86

12.98

S60.86

1.10

3.33

7.64

S70.75

2.33

3.41

7.64

S8

1.24

1.13

2.81

6.75

S9<LOQ*

0.63

6.25

9.29

S10

41.21

8.39

67.82

194.44

S118.22

4.39

32.83

41.96

S128.23

2.21

43.82

234.03

S138.94

2.14

39.09

209.30

S14

<LOQ*

0.26

49.42

301.09S15<LOQ*0.40

18.68

47.89

S16<LOQ*

1.22

9.67

25.29

S179.59

2.13

4.92

0.45

S180.521.23

0.210.42*Limit of Quantitation (LOQ) was calculated from 10 measurements of ablank solution (diluent containing oil matrix and internal standard).

6Conclusions
The Agilent 7900 ICP-MS with ORS4 collision/reaction cell(CRC) was used for the direct multi-elemental analysis of
crude oil samples following dilution in o-xylene. The efficient,frequency-matching rf generator of the Agilent 7900 provides
a robust and stable plasma, facilitating reliable, accurate
analysis of organic samples over an extended analyticalperiod. Interferences on all analytes were reduced using He
or H2 cell gas mode. Fast gas switching in the ORS4 allows
the use of multiple gases in a single acquisition, without
significant impact on acquisition time. The high sensitivity of
the 7900 ICP-MS, combined with its low background andsimple spectra in organic matrices, ensured that significantly
lower detection limits were achieved for a wider range ofelements compared to more traditional techniques, such as
ICP‑OES.
The method delivered excellent accuracy for certified andreference elements in NIST 1634c Trace Elements in Fuel Oil,with measurements taken on separate days over an extendedperiod. The reproducibility of the methodology makes itsuitable for routine use in the petroleum refining industry,increasing the likelihood that ASTM will issue an ICP-MS
method for petroleum. It is anticipated that this will befollowed by a further ASTM method for trace elements inpetroleum crude oils.

Reference1. ASTM International, www.astm.org

7www.agilent.com/chem
Agilent shall not be liable for errors contained herein or forincidental or consequential damages in connection withthe furnishing, performance or use of this material.
Information, descriptions, and specifications in this publication
are subject to change without notice.
© Agilent Technologies, Inc. 2017
Published March 17, 2017
Publication number: 5991-7826EN