Application Note · PlasmaQuant® PQ 9000 EliteSpecification Analysis of Gasoline by HR ICP-OES

Challenge
Determination of trace elements
in volatile organic compounds
such as gasoline with highrobustness and low limits ofdetection.

Solution
HR ICP-OES with high-resolution
optical system, superiorsensitivity and matrix tolerance
to achieve lowest detection limits
for trace element analysis involatile organic compounds.

Introduction
Volatile Organic Compounds (VOC) such as gasoline and naphtha are subject
to specification analysis in terms of trace elemental analysis of environmental
pollutants (e.g., lead) and compounds that are potentially harmful to engines andturbines. Elements such as vanadium, iron, nickel, or silicon contribute to corrosion
processes and therefore bear the risk of engine or sensor failure in automobiles
or aircrafts. The value of petroleum is often defined by the specification ofsuch elements. Due to the potential environmental hazard and risk of technicalbreakdown caused by trace elemental impurities, their analysis is subject of many
regulations, such as ASTM D5708, ASTMD7111, UOP389, and DIN EN 228,
requesting maximum levels in the ppb range for elements such as V, Fe, Al, K, Na,Ni, P, As, Si, and Pb.
VOC are among the most challenging samples to be analyzed by ICP techniques.
For specification analysis in the low µg/kg range, ICP-MS is often the method ofchoice as conventional OES instruments lack sensitivity due to sample dilution andspectral interferences. Typical analytical challenges are the poor short-term andlong-term stability due to the high volatility of the samples and to the high risk ofsoot formation caused by the high carbon content of the samples. Furthermore,
the carbon-based emission complicates the spectra as-recorded and bears the riskof severe spectral interferences. Conventional ICP-OES instrumentation is forced
to use less sensitive lines for many analytes, since interferences on primary lines
cannot be resolved properly.

2Specification Analysis of Gasoline by HR ICP-OES

Thanks to the vertical plasma geometry of the V Shuttle Torch and the unique plasma performance of its High-Frequency
Generator, direct analysis of undiluted gasoline proved feasible for the analytical routine on the PlasmaQuant® PQ 9000 Elite.
By avoiding soot formation and memory effects, frequent torch replacement becomes a relic of the past and detection limits
below 1 µg/kg for iron, nickel, and vanadium push the boundaries of ICP-OES detectability. Hence, the PlasmaQuant® PQ
9000 Elite represents a feasible alternative to ICP-MS instrumentation for the analysis of VOC.
Carbon-based spectral interferences are successfully removed thanks to the unmatched spectral resolution of the HighResolution Optics. However, for sodium and potassium analysis, oxygen addition is recommended. Excellent method
robustness was confirmed by a continuous aspiration test, which showed minor intensity drifting of all emission lines during
3.5 hours of uninterrupted submission of undiluted gasoline.
Materials and Methods
Samples and Reagents
Low-boiling point gasoline (straight-run) was submitted to direct analysis of Al, As, Fe, Ni, Na, K, P, Pb, Si and V on
PlasmaQuant® PQ 9000 Elite.
Sample Preparation/ Calibration
In the absence of a high-purity gasoline reference, a standard addition calibration routine was employed and organometallic
S-21+K (100 ppm) and As (100 ppm) from CONOSTAN were used to prepare calibration standards as shown in table 1.
Table 1: Concentration of calibration standards for standard addition
ElementUnit

Add.Cal.1

Add.Cal.2

Add.Cal.3

Add.Cal.4

Add.Cal.5

Al, Fe, K, Na, Ni, P, Pb, Si, V

mg/kg

0.051

0.102

0.220

0.305

0.540

As

mg/kg

0.050

0.100

0.216

0.300

0.530

Instrument Settings / Method Parameters
For the analysis, a PlasmaQuant® PQ 9000 Elite equipped with Organic-Kit and cooled spray chamber was used. The detailedsystem configuration is given in table 2.
Table 2: Plasma configurations and setup of the sample introduction system
ParameterSpecification

Power1600 W

Plasma gas flow

15A (16B) L/min

Auxillary gas flow

1.2A (0.3B) L/min

Oxygen gas flow

0A (0.03B) L/min

Nebulizer gas flow

0.35 L/min

Nebulizer

0.4A (0.1B) mL/min concentric nebulizer, borosilicate

Spray chamber

IsoMist cyclonic spray chamber with dip tube, 50 mL, Peltier cooled to -14°C

Injector

Quartz, 1 mm; distance to coil set to - 2 mmC

Outer Tube/Inner Tube

Quartz / Quartz

Pump tubing

Viton (0.76 mm ID) black/black

Sample pump rate

0.1 mL/min

Rinse/ Read delay

180 s

A standard settings for undiluted gasoline
B settings for Oxygen addition to the plasma that reduces unspecific spectral interferences from gasoline matrix (see table 7)
C spacing between injector and coil further suppresses the carbon built-up at the injector tip (prolonging torch life time); not applicable with oxygen addition;
Please note, that change of plasma conditions does not require for the plasma to be turned off.

Specification Analysis of Gasoline by HR ICP-OES

Table 3: Plasma configurations and setup of the sample introduction system

ElementLine[nm]

Plasma viewIntegration
mode

Al

396.152axial

As

188.979

FeRead time[s]

Evaluation
No. of Pixel

No. of Pixel

No. of Pixel

Correction

Peak

103

ABC 2

auto-

axial

Peak

103

ABC

auto-

259.940

axial

Peak

103

ABC

auto

-

K

766.491 1

axial

Peak

103

ABC

auto-

Na589.592 1

axial

Peak

103

ABC

auto-

Ni

221.648

axial

Peak

103

ABC

auto-

P

213.618

axial

Peak

103

ABC

auto-

Pb

220.353

axial

Peak

103

ABC

auto-

Si251.611

axial

peak

103ABCauto-V

309.311

axialpeak

103

ABC

auto-

1 measured with Oxygen addition to the plasma
2 automatic baseline correction

Results and Discussion
Using a high-resolution ICP-OES enables the interference-free analysis of trace elements as displayed in table 4. Neither
matrix based nor inter-element interferences were observed for the majority of the analyzed trace elements in volatileorganic compounds. The investigation resulted in exceptional matrix-specific limits of detection in the sub-ppb range(< 5 µg/kg) for all investigated elements.
Table 4: Overview of results for gasoline sample
ElementLine[nm]

Content[µg/kg]Confidence Interval

Matrix specific LOD[µg/kg]

Al396.152

284.4

±7.8

1.83

As

188.979

21.6

±5.8

2.72

Fe

259.9407.1±5.7

0.45

K766.491 1<DL±Na589.592 1

99.2

±10.4

4.28

Ni

221.648

8.2

±

5.6

0.43

P

213.618

97±17.9

2.89

Pb

220.353

33.1

±6.5

1.83

Si

251.61177.5±5.70.56

V

309.311

24.6±5.60.38

1 measured with oxygen addition to the plasma

0.7634Specification Analysis of Gasoline by HR ICP-OES

Method validation as well as a long-term stability investigation was performed by measuring QC recovery of Add.
Cal.3 (undiluted gasoline) constantly for up to 3.5 hours. Recovery values between 97% and 107% at RSD values ofsignificantly less than 1.5% were achieved. Except for sodium and potassium, these results were achieved without theaddition of oxygen to the plasma, which demonstrates the exceptional plasma performance of the PlasmaQuant® PQ
9000 Elite for highly volatile samples. The detailed results are displayed in table 5. Figure 11 displays the results of theconstant signal reading of the QC standard Add.Cal.3, with minimum drift (97 to 103%) on the long-term measurementfor 3.5 hours.
Table 5: Overview of results for continuous aspiration test of gasoline matrix (Add.Cal.3)
After 30 min 1
Element

After 2 hours 3

After 3.5 hours 4

RSD [%] 2

Recovery [%]RSD [%]

Recovery [%]RSD [%]

Recovery [%]Al0.56

97.3

0.46

105

1.26

102

As

0.53

102

0.43

101

0.9

101

Fe

0.02

101

0.38

102

0.17

102

K

0.71

104 50.72

104 5

1.33

102 5Na0.46102 5

1.32

101 5

0.2

99.9 5

Ni

0.34

98.4

0.44

98.9

0.17

98.9

P

2.25

103

1.46

103

0.21

107

Pb0.37

101

0.65

101

0.73

99.7

Si

0.52

99.3

0.38

98.2

0.52

98.4

V

0.2

101

0.46

101

0.3

102

1 results were obtained after 30 minutes of continuous aspiration of undiluted gasoline (including warm-up and calibration)
2 RSD values from three replicate measurements
3 results were obtained after 2 hours of continuous aspiration of undiluted gasoline
4 results were obtained after 3.5 hours of continuous aspiration of undiluted gasoline
5 measured with oxygen addition to the plasma

Figure 11: Plot of relative signal intensities obtained from continuous aspiration of undiluted
gasoline (long-term stability)

Specification Analysis of Gasoline by HR ICP-OES

Interference-free detection of elements on their most sensitive emission lines, e.g. As 188.979, Si, 251.611, V 309.311 or Pb
220.353 is feasible resulting in exceptionally low matrix specific limits of detection in the ppt to ppb range (0.38 µg/kg forVanadium). A selection of high-resolution emission spectra including the applied automatic background correction (ABC) canbe found in table 6.
Table 6: Collection of High-Resolution spectral data for undiluted gasoline (red), Add.Cal.3 (black)and ABC (green)
As 188.979 nm

Si 251.611 nm

V 309.311 nm

Only the analysis of sodium and potassium required oxygen addition to the plasma in order to avoid interferences due tocarbon-based unspecific emission. For regular plasma conditions, unspecific spectral interferences from the gasoline matrix
gave rise to about 3.5 million counts in close spectral vicinity of Na 589 (table 7, top, left). The sodium signal cannot be
resolved from that of the gasoline matrix (table 7, middle, left), which leads to significant line drifting of Na 589 duringcontinuous aspiration (table 7, bottom, left) and hence false quantification. Oxygen addition in turn reduces unspecificspectral interferences by about 90% (table 7, top, right), hence allowing for sodium detection with good reproducibility (table7, middle, right). Besides, hardly any line drifting of Na 589 upon continuous aspiration of undiluted gasoline occurs (table7, bottom, right), which safeguards high precision of the trace analysis. Oxygen addition further improves matrix-specificdetection limits from 8.0 to 4.28 µg/kg (for sodium) and from 4.0 to 0.76 µg/kg (for potassium).
Table 7: Impact of Oxygen addition on signal intensity, spectral interferences and long term signal stability for sodium and potassium in undiluted
gasoline
Parameter
Na 589.592 nm
red sample
black sample, Add.Cal.3

Na 589.592 nm(detail-spectra/zoom-in)
red sample
black sample, Add.Cal.3

Na 589.592 nm
K 766.491 nm

Plot of relative signal
intensities obtained
during continuous
aspiration of gasoline
for more than 3 hregular plasma conditions

with oxygen addition5

6

Specification Analysis of Gasoline by HR ICP-OES

Conclusion
The analysis of volatile organic compounds such as naphtha or gasoline regarding the trace elemental contents represents
one of the most demanding tasks for ICP techniques. The high volatility of the samples often leads to unstable plasma
conditions, resulting in severe signal drifts and poor accuracy and precision of the obtained results. Dilution in less volatilesolvents or alterations to the plasma gas composition is required for conventional ICP-OES instrumentation. Thanks to theHF generator in combination with the vertical torch orientation of the V-Shuttle Torch, the PlasmaQuant® PQ 9000 Elite canmeasure undiluted VOC with great precision, accuracy, and long-term stability.
The high resolution of the PlasmaQuant® PQ 9000 Elite is able to resolve spectral interferences originating from carbonbased emission and makes it possible to use the most sensitive lines for the majority of the investigated analytes. The use of
the most sensitive lines and the possibility to run VOC undiluted result in detection limits in the ppt to ppb range and great
long-term stability and make the PlasmaQuant® PQ 9000 Elite superior to conventional ICP-OES and a feasible alternative toICP-MS instrumentation.

This document is true and correct at the time of publication; the information within is subject to change. Other documents may supersede this document, including technical modifications andcorrections.

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