Application Note
Energy and Chemicals

Simultaneous Total and Speciation
Analysis of Volatile Arsenic
Compounds in Gas and Liquefied Gas
Samples using GC-ICP-MSAuthors
Laura Freije-Carreloa, Mariella
Moldovana, J. Ignacio García
Alonsoa, Thuy Diep Thanh VOb,
Jorge Ruiz Encinara

Department of Physicaland Analytical Chemistry,
University of Oviedo, Spaina

Borealis Polyolefine GmbH,Linz, Austriab

Introduction
Volatile metal(loid) elements and compounds are present in many different typesof gas and liquefied gas samples, such as natural gas and biogas. Arsenic (As)is one of the most prevalent elemental impurities in such samples, and it is ofinterest in many industries, including gas exploitation and processing, and polymermanufacturing. In addition to its potential toxicity and effect as an environmental
contaminant, As must be carefully monitored and controlled in the petrochemicaland polymer industries. Its presence in monomer gases such as ethylene andpropylene affects the properties of polymers synthesized from these gases. Volatile
forms of As are also highly active catalyst poisons. Importantly, the toxicity of As
depends on the chemical species, with arsine being the most toxic form. Reliable
analytical methods are required, therefore, for both total and speciation analysis of
As compounds in gaseous samples.

Gas chromatography (GC) coupled with a mass spectrometerbased detector provides a powerful solution for As speciation
analysis. However, such systems are not always suitable fortotal As determination in gas samples, because species that
do not elute from the column (i.e. associated with particulatematter) cannot be detected.
In this study, a novel GC-ICP-MS method was used thatcombines As speciation analysis and total elemental As
determination in one measurement. The potential of themethod for the simultaneous determination of total elemental
concentrations and individual species of other elements, suchas Hg, in gases is also shown.

Experimental
Gas standards and samples
An arsine (AsH3) standard in nitrogen (N2) (Linde AG, Munich)
was used for quantification purposes. Liquefied butanepressurized under N2 and doped with AsH3 (ISGAS, USA) and areal propylene sample from a cracker plant (Borealis, Sweden)
were analyzed.

Instrumentation
An Agilent 7890B GC equipped with two interconnected
gas sampling valves and a J&W GS-GasPro column (60 m,0.32 mm id) was used. The GC was coupled to an Agilent
7900 ICP-MS using an Agilent heated GC-ICP-MS interface(G3158D). A gas dilutor GasMix Aiolos II and a vaporizer
chamber (Alytech, France) were used for online dilutionof the samples and standards. Optimized GC and ICP-MS
parameters are summarized in Table 1.
Table 1. Optimized GC and ICP-MS parameters.
ICP-MS parameters
Cell gas mode
RF power (W)GC parameters

No gasHe1600

Sampling depth (mm)8Dilution gas (L/min)

0.3–0.4

N2 flow (mL/min)

10

Extraction 1 lens (V)0Extraction 2 lens (V)Column flow (mL/min)

3

Transfer line flow (mL/min)

5Oven gradient40 to
250 ºCin 5 min,
2 min at
250 ºC-200

Injection mode

Split

Omega bias lens (V)-80

Split ratio

1:6

Omega lens (V)10

GC-ICP-MS interface
temperature (ºC)

250

Cell entrance lens (V)

-30

-40

Cell exit lens (V)

-50

-60

Deflect lens (V)100Plate bias lens (V)

-100

-60

2New instrumental set-up
A diagram of the GC-ICP-MS configuration is shown inFigure 1. The gas standard, sample, and dilution gas areconnected to the dilutor (top right). When a liquefied gas isanalyzed, a vaporizer chamber (fixed at 120 °C) is placed
between the sample and the gas dilutor inlet so the sample
can be vaporized before dilution. When gaseous samples areanalyzed, the vaporizer is bypassed and the gas is introduced
directly into the dilutor. A 3-way valve allows the dilutor outlet
gas flow to be directed to the GC through the gas samplingvalves (GSV) 1 and 2 (solid line in the lower part of Figure 1).
GSV1 and GSV2 are connected internally, so they are loadedat the same time.
Injection from the two valves can be controlled independently
via the ICP-MS MassHunter chromatography method settings,
but in this method, both valves were injected at the sametime.
When the GSVs are turned to the inject position, the gas
volume loaded into GSV1 is injected through the injection
port of the GC to the analytical column. Conventional GC
separation of the different species is then carried out toseparate the individual species as they pass through the GCcolumn to the ICP. Simultaneously, the gas volume loaded
into GSV2 is injected into the inert (fused silica deactivated,5 m, 0.32 mm id) transfer line leading directly to the GC-ICPMS interface. The transfer line is placed inside the GC ovenand heated to the initial temperature of the GC program. TheGSV2 gas volume bypasses the GC column and is quickly
transferred to the 3-way connecter at the column exit. Using
this configuration, the gas that was loaded into GSV2 enters
the GC-ICP-MS interface and is transferred to the ICP before
the dead volume of the GSV1 injection has passed throughthe GC column. Such direct transfer of the gas volume allows
a multi-element Flow Injection Analysis (FIA) to be performed
for any ICP-detectable elements present in the gaseous
samples. This option allows for fast total analysis and canbe used to assess overall recovery of the element from theGC column (i.e. some elemental species might not be eluted
from the GC column, for example, when elemental species areadsorbed onto particulate matter).
The ends of both the inert transfer line (for total elementalanalysis) and the analytical column (for speciated analysis)
are connected to the GC-ICP-MS interface with a two-holeferrule. At this point, the gas flows are mixed online withthe carrier Ar gas flow (previously heated in the pre-heating
pipe located inside the GC oven) and transferred throughthe heated GC interface to the ICP-MS plasma. Using thisapproach, total and speciation analysis of the elements
of interest can be performed simultaneously, on the same
sample injection, in a simple, and reproducible way.

ICP-MS plasma
ICP-MS collision cell

Dilution port (Ar)
ArHe

Injection port

N2
GC carrier gas

GasventAuxEPCInterface

1Column

Gas samplingvalves (GSV)TorchPre-heatingpipe

ICP-MS

Gas
introduced
direct toICP-MS
Gas
introduced
throughinjectionvalves

Gas for dilution
MFC 1

Dilutor
MFC 2

Vaporizer

Sample2

Inert transfer line

Gasstandard

GC Oven
He/Xe

HeFigure 1. Diagram of the GC-ICP-MS instrumental set-up for simultaneous total and speciation analysis (Reprinted with permission from Laura Freije-Carrelo et al,Anal. Chem., 2017, 89 (11), pp 5719–5724. Copyright 2017 American Chemical Society).

Using the same system configuration, the 3-way valve after
the gas dilutor can be used to divert the gas flow directly to
the GC-ICP-MS interface (dashed line in upper-right part ofFigure 1), bypassing the gas sampling valves and GC column.
Using a cross piece, this flow is mixed with the ICP‑MS Ar
dilution gas before entering the pre-heating pipe. This setup
supports:• Optimization of the ICP-MS operating conditions by
allowing monitoring of the continuous signal of theanalytes present in the gas standard/sample.
• Analysis using a fast, qualitative screening acquisition tofind out whether the sample contains the target elements
and if subsequent quantitative analysis is necessary.
Also, a second 3-way valve is installed (lower left part ofFigure 1) for selection of the GC carrier gas. The valve allows
the carrier gas to be switched from He to a mixture of Xe
in He for optimization of the torch position and ICP-MS
sensitivity. Finally, an auxiliary electronic pressure control (AuxEPC) controls the addition of a constant pressure of N2 into
the Ar dilution gas flow that is added to the column/bypass
flow at the entrance of the GC-ICP-MS interface. Addition ofN2 has been shown to enhance As sensitivity in GC-ICP-MS
applications [1].

Results and Discussion

An AsH3 in N2 standard was used to optimize the operating
conditions of the system. Chromatograms of a 68 ppb (v/v)
arsine standard in N2 and a N2 blank are shown in Figure 2.
The N2 blank chromatogram is shown on a y-axis scale thatis approximately two orders of magnitude lower than thescale used for the AsH3 sample. As can be seen, the first peakin the arsine standard chromatogram appears at 0.2 min,corresponding to the total As eluted from the inert transfer
line (column bypass flow). A second peak is observed at 3.8min, corresponding to the speciation analysis (AsH3 eluted
from the column). The chromatogram demonstrates that total
and speciation analyses can be performed simultaneouslywithin the same injection.
The difference in the peak area observed for total andspeciation analysis is due to the split ratio used (1:6) in theGC injector. No adsorption or loss of analyte is expected in
the inert transfer line. Therefore, the comparison between
the experimentally obtained and the expected theoretical GC
inlet split ratio can be used to assess if the total As observed
is exclusively in the form of AsH3 or if other As species arepresent in the sample. Integration of peak areas for the “totalAs” and AsH3 in each point of a calibration curve from 0.2 to68 ppb (n=13) showed a mean recovery for the AsH3 peak of
89 ±11 % of the total As in the standard. The results indicate

3that, as expected, As is exclusively present as arsine in thestandard. Note that an accurate and precise split ratio isassumed for such mass balance calculations. Also, the As
peak coelutes with the sample matrix in the total analysis(column bypass) measurement, so sensitivity may not be
consistent for both direct and GC conditions. These two facts
could explain the significant uncertainty (12% RSD) observed
for the calculated recovery.

AsH3 in nitrogen standard
Nitrogen blank

Synthetic sample analysis
Liquefied butane doped with a non-certified level of arsine
was analyzed as a synthetic sample. Helium mode was used
for analysis of the samples to minimize the occurrence ofany polyatomic interferences. The vaporization chamber wasset at 120 ºC and connected online with the gas dilutor tovaporize the sample. A chromatogram of butane diluted 1:7 in
Ar is shown in Figure 4.
Results obtained for total and arsine speciation analysis
are given in Table 2. The synthetic butane sample was also
analyzed by the producer (ISGAS) and an independent lab,with results of 218 and 40 ppb arsine, respectively. The wide
range of these results illustrates the difficulty of developinga method for the reliable analysis of gaseous and liquified
gas samples. The results obtained in this study are given in
Table 2. The measured concentration of ~95 ppb arsine iswithin the range of concentrations obtained by the other twolaboratories, giving confidence in the applicability of the new
GC-ICP-MS method.

Figure 2. Chromatogram of arsine in nitrogen (68 ppb, v/v). (Reprinted withpermission from Laura Freije-Carrelo et al, Anal. Chem., 2017, 89 (11), pp5719–5724. Copyright 2017 American Chemical Society).

Calibration curves were generated for “total As” andspeciation (AsH3) analysis based on AsH3 standard levels
from 0.2 to 68 ppb, as shown in Figure 3. The method showed
good linearity within the tested range of concentration forboth total (r2=0.9996) and speciation analysis (r2=0.998).
Detection limits of 2 ppt for total As and 12 ppt for AsH3
by speciation analysis were obtained. To the best of ourknowledge, these are the lowest DLs for direct total As andAsH3 speciation analysis in gaseous samples ever reported inthe literature.
Figure 4. As chromatograms of blank and arsine-doped liquefied butane
diluted 1:7. Intensity scale of the butane blank chromatogram is 7.5x lower.
Table 2. Quantitative results (ppb As, v/v) obtained for arsine-doped liquefied
butane and propylene samples. Expanded uncertainties (95% confidencelevel) are given (not including the uncertainty of the arsine in N2 standard).
LiquefiedbutaneFigure 3. Calibration curves for “total As” (red points) and AsH3 (blue points)
using arsine standards.

4

Propylene(gas phase)

Propylene(liquid phase)

Total analysis92 ± 454 ± 434 ± 3Speciation
analysis

95 ± 453 ± 1331 ± 5(Reprinted with permission from Laura Freije-Carrelo et al, Anal. Chem., 2017,89 (11), pp 5719–5724. Copyright 2017 American Chemical Society)Mercury in butane
The butane sample was also doped with mercury. Hg wasmeasured simultaneously in the same acquisition as Asto show the multi-elemental capability of the system. The
chromatogram for Hg is shown in Figure 5. As can be seen,
signals for both total Hg (first peak) and Hg species (secondpeak) were clearly detected. However, the peak profile
observed for Hg shows much more tailing than the As peak.
The peak shape is likely due to unspecific adsorption of Hg
species along the valves and devices from the gas cylinder tothe ICP. The peak tailing could probably be reduced using amethod and conditions optimized for Hg analysis.

The use of ICP-MS for detection ensures the GC-ICP-MS
setup is suitable for simultaneous quantitative analysis (totaland speciation) of any ICP-detectable element present in gasand liquefied gas samples.

References

1. Yang Song, Yuhong Chen, Determination of Arsine
in Ethylene and Propylene by GC-ICP-MS, Handbook ofHyphenated ICP-MS Applications, 2nd Edition, pp 75–76,
Agilent publication 2015, 5990-9473EN.

More Information

This study was originally published in:
Laura Freije-Carrelo et al., Instrumental Setup for
Simultaneous Total and Speciation Analysis of Volatile
Arsenic Compounds in Gas and Liquefied Gas Samples,Anal. Chem. 2017, 89, 5719−5724. DOI: 10.1021/acs.
analchem.7b01082

Figure 5. Hg chromatograms of blank and mercury-doped liquefied butane
diluted 1:7.

Real sample analysis
Finally, a real propylene sample containing both gas andliquefied phases was analyzed. The vaporization chamber
was set at 120 ºC and connected online with the gas dilutor
for vaporization of the liquid phase of the sample. For the gas
phase analysis, the sample was introduced directly to the gas
dilutor. Quantitative results are given in Table 2. The results
demonstrate that the setup could also be used to assess thepartition of total As and its species between gas and liquid
phases of liquefied gas samples.

Conclusions

The novel GC-ICP-MS method has been shown to be suitable
for the direct, simultaneous quantitative analysis (total andspeciation) of ICP-detectable elements such as As and Hg
present in gas and liquefied gas samples.
Detection limits in the low ppt levels were obtained forboth total As and speciated As analysis without the needfor a preconcentration step. The speed of the new GC-ICPMS method (5 min per injection) makes it convenient for
implementation in industrial laboratories. The fast analysis
times allow the determination of contaminants to support
acceptance or rejection of a stock of gas.

5

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© Agilent Technologies, Inc. 2018
Printed in the USA, April 8 20185991-8799EN