Gulf Coast Conference 2000,
Poster #75
Capillary Column
Selectivity and
Inertness for Sulfur
Gas Analysis in Light
Hydrocarbon
Streams by Gas
Chromatography
Allen K. Vickers, Jason Ellis and
Cameron George
91 Blue Ravine Road,
Folsom, CA, 95630-4714
Abstract:
Detector quenching and poor detection limits are problems frequently
encountered in applications involving the analysis of sulfur compounds
in light hydrocarbon streams. Both of these problems present obstacles
to the analyst hoping to accurately determine the concentration of reactive sulfur compounds in a hydrocarbon sample.
This paper will examine four different columns commonly used in
volatile sulfur compound analysis. These columns will be evaluated with
respect to elution order profile of light hydrocarbons and sulfur compounds, as well as inertness toward reactive sulfur compounds such as
hydrogen sulfide, sulfur dioxide and the mercaptans.
Introduction:
Unrefined C1 to C5 hydrocarbon streams typically contain significant
to trace amounts of volatile sulfur compounds as impurities. Analysis of
these sulfur compounds is critical to final product purity as well as avoidance of costly catalyst poisoning in the processing stream. Sulfur gas
analysis is important not only in the petrochemical industry but also in
environmental (EPA, CARB, etc.), industrial hygiene (NIOSH, OSHA)
and food products applications (e.g. sulfur compounds in beer or wine).
While low detection limits are certainly important in petrochemical test
methods, often times lower detection limits are required for the other
industries listed above due to very low odor threshold levels and Permissible Exposure Limits of sulfur-containing compounds. It is not uncommon to require quantitation of sulfur gases such as hydrogen sulfide
or methyl mercaptan down to low part-per-billion levels.
Sulfur selective detectors are typically employed for these applications because of their high selectivity and sensitivity for sulfur species.
Most commonly used are the Flame Photometric Detector (FPD), Pulsed
Flame Photometric Detector (PFPD) and Sulfur Chemiluminescence
Detector (SCD). More general selective detectors may also be employed,
such as the Atomic Emission Detector (AED) and Mass Spectrometer
(MSD). Regardless of the detector selected for the application, possible
quenching of response for low-level sulfur compounds may occur if they
elute underneath large hydrocarbon peaks. Quenching makes accurate
quantitation difficult due to detector response reduction and may also
cause difficulties in compound identification. Thus, column selectivity
is critical in these applications if accurate results are to be obtained.
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Due to the high vapor pressure of target analytes, columns used for
these applications must possess a high degree of retention if the analysis
is to be performed without using cryogenic oven temperatures. Thickfilm wall-coated open tubular (WCOT) columns and porous-layer open
tubular (PLOT) columns can provide high enough retention to separate
these volatile species at above ambient starting oven temperatures. While
WCOT columns can exhibit high enough retention, phase selectivity is
often not ideal. PLOT columns possess higher retention and selectivity
for these compounds, however many PLOT phases available that exhibit
acceptable elution patterns for sulfur and hydrocarbon compounds also
exhibit unwanted stationary phase surface activity. This activity can dramatically affect the sensitivity of the system for reactive sulfur compounds such as mercaptans and hydrogen sulfide, producing higher detection limits. Hence, column inertness is just as critical as stationary
phase selectivity in column selection for these applications.
This paper examines four different phases commonly used in volatile
sulfur compound analysis. PLOT columns examined include a silicabased, a carbon molecular sieve and a divinylbenzene porous polymer.
These PLOT columns are compared to a thick-film 100%
dimethylpolysiloxane WCOT column. Elution orders of common C1 to
C5 hydrocarbons will be given as well as six sulfur gases: hydrogen sulfide, carbonyl sulfide, sulfur dioxide, methyl mercaptan, ethyl mercaptan and carbon disulfide. Column inertness will be evaluated by examining peak shape and response for hydrogen sulfide.
Experimental:
A Hewlett-Packard 5890 GC (Avondale, PA) equipped with a Flame
Ionization Detector (FID; Hewlett-Packard, Avondale, PA) and a PulsedFlame Photometric Detector (PFPD; OI Analytical, College Station,
TX) was used to generate the chromatograms in this study. Injection
volume for each standard was held constant at 1.0 mL. Data acquisition
was performed via Hewlett-Packard Chemstation software.
Split injection was used in this study to reduce residence time in the
inlet and thus reduce potential peak tailing associated with inlet-related
activity.
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J&W Scientific (Folsom, CA) manufactured all columns evaluated.
The columns used in this study were:
DB-1, 30 m x 0.32 mm I.D., 5.0 µm
GS-CarbonPLOT, 30 m x 0.32 mm I.D., 3.0 µm
GS-Q, 30 m x 0.32 mm I.D.
GS-GasPro, 30 m x 0.32 mm I.D.
A variety of hydrocarbon blend standards were used to generate retention times on each column. These standards were obtained from Scott
Specialty Gases (Plumsteadville, PA) and Hewlett-Packard and were
used for qualitative results only. The balance gas in each hydrocarbon
standard is nitrogen.
Three different sulfur gas standards were used in this study:
n
Sulfur dioxide standards were generated using a permeation
tube device obtained from GC Industries (Fremont, CA).
n
A mixture of five sulfur compounds was obtained from Scott
Specialty Gases at 1.0% each component, balance gas nitrogen.
The compounds represented in this standard were hydrogen sulfide, carbonyl sulfide, carbon disulfide, methyl mercaptan and
ethyl mercaptan. This standard was used for qualitative results
only. Due to the age of the standard (over two years old) it was
noted that reactive compounds were no longer at their stated
concentrations. This standard was diluted to a working level by
transferring an aliquot to a Tedlar bag containing a known volume of nitrogen.
n
Quantitative detection limit studies were performed with a
hydrogen sulfide standard obtained from Scott Specialty Gases.
The hydrogen sulfide concentration in this standard was 100
ppmV, balance gas nitrogen.
The sulfur compounds were not combined with the hydrocarbons to
produce one analytical standard. This allowed for investigation of true
retention times and peak shape without the influence of quenching on
the PFPD. Each column was first installed into the PFPD and detector
conditions were optimized for sulfur response. Retention time data was
then obtained for each of the sulfur compounds on the column. The column was then uninstalled from the PFPD, installed into the FID and
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retention time data was obtained for the hydrocarbons. Column headpressure was held constant during this procedure so as to not affect carrier gas linear velocity. This allowed for generation of retention times
using both detectors without introduction of possible activity associated
with a glass Y-splitter or deactivated fused silica tubing.
Results and Discussion:
Figure 1 and Table 1 illustrate selectivity for hydrocarbons and sulfur
compounds on DB-1, a 100% dimethylpolysiloxane stationary phase.
DB-1 was the only wall-coated open tubular (WCOT) column evaluated in this study. WCOT columns do not exhibit high retention at ambient operating temperatures for gaseous compounds such as ethane and
propane. The column used for this study possessed a very low phase
ratio (ß=16). This allowed for adequate retention at 40°C to resolve
hydrogen sulfide (H2S) from carbonyl sulfide (COS), however we can
note from the chromatograms that these sulfur gases are eluted amidst
the C2 and C3 hydrocarbons. Additionally, methyl mercaptan (MeSH) is
eluting in the C4 range. This elution pattern can create problems with
detector quenching by large amounts of hydrocarbons potentially present
in a sample. For example, if testing propylene streams for trace sulfur
gases, difficulty quantitating hydrogen sulfide, carbonyl sulfide and sulfur dioxide due to their close elution to the propylene peak would result.
Figure 1 also shows that carbonyl sulfide and sulfur dioxide (SO2)
coelute on this phase under these analysis conditions.
The peak shape for hydrogen sulfide and methyl mercaptan are both
excellent on DB-1, indicating little-to-no surface adsorption problems
with these reactive compounds in this column.
Figure 2 and Table 2 illustrate selectivity on GS-CarbonPLOT, a carbon-layer PLOT stationary phase. This column exhibits much higher retention than the DB-1 - the chromatograms in Figure 2 were generated
using a temperature program from 100° to 310°C. This higher retention
of GS-CarbonPLOT helps to resolve light hydrocarbons much better
than DB-1.
On GS-CarbonPLOT, H2S and COS do not coelute with the C3 hydrocarbons, however H2S does coelute with ethane and COS coelutes
with SO2 (Figure 2).
Excessive peak tailing is noted on the H2S peak on GS-CarbonPLOT
(Figure 2). Additionally, methyl mercaptan and ethyl mercaptan were
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not detected on GS-CarbonPLOT, even at very high concentrations. This
behavior indicates adsorption of active sulfur compounds on this phase.
The elution profile on GS-Q (a divinylbenzene porous polymer PLOT
phase) is shown in Figure 3 and Table 3. GS-Q shows a favorable elution profile of sulfur compounds relative to light hydrocarbons. Here we
see H2S, COS and the mercaptans pulled away from potential hydrocarbon interferences. This elution profile works well for analysis of sulfur
gases in propylene, LPG or LNG for example.
Peak tailing is noted on the H2S peak on GS-Q (Figure 3), indicating
some surface adsorption.
Sulfur dioxide was analyzed on the GS-Q however is not shown in
Figure 3 due to very poor peak shape. Sulfur dioxide exhibited Gaussian
peak shape, but was approximately 2 minutes wide with an apex at about
9.1 minutes. Obviously, this would not be an ideal column for quantitation
and identification of sulfur dioxide in a sample due to the extreme
broadness of the peak.
Figure 4 and Table 4 illustrate selectivity of GS-GasPro (a silica-based
PLOT phase) for these compounds of interest. GS-GasPro exhibits a
favorable elution profile without the tailing hydrogen sulfide problem
observed on GS-Q. This elution profile would also work well for analysis of sulfur gases in propylene, LPG or LNG.
GS-GasPro resolves carbonyl sulfide and sulfur dioxide (Figure 4),
making it the only column observed in this study to be useful for analysis
of these two compounds.
Peak shape exhibited by GS-GasPro for hydrogen sulfide and methyl
mercaptan is symmetrical, while only slight peak tailing is noted on the
sulfur dioxide peak shown in Figure 4. This indicates little-to-no surface
adsorption is occurring for these reactive compounds.
Figure 5 shows a comparison of approximate detection limits for hydrogen sulfide obtained on each column using this system setup. "Oncolumn equivalent" denotes the approximate concentration delivered to
the column after standard dilution and split ratio were accounted for.
Clearly, DB-1 and GS-GasPro exhibit the best inertness characteristics
for reactive sulfur compounds in this application, making them the best
choices for trace-level sulfur determination. Tailing peak shape observed
on GS-Q and GS-CarbonPLOT for hydrogen sulfide indicates presence
of surface activity. While there are obvious differences in signal-to-noise
present in the chromatograms shown in Figure 5, response was erratic at
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concentrations much lower than those shown on GS-Q and GSCarbonPLOT. These two columns exhibited an apparent quenching phenomenon, whereby response improved for hydrogen sulfide after several injections of high-level standards. This quenching effect can make
accurate quantitation difficult, as the effect is not long lasting.
The chromatograms shown in Figure 5 are meant to show relative
differences in inertness between the four columns studied, and are not
meant to be definitive examples of detection limits obtainable. Ultimately,
detection limits obtained on a given analytical system are extremely system dependent - lower detection limits may be obtained, however the
entire system must be finely "tuned" for trace-level analysis of reactive
compounds. Figures 6 and 7 show chromatograms from a system that has
been optimized for trace-level analysis of volatile sulfur compounds in
ambient air samples. Figure 6 shows the chromatogram of an injection
of 1.0 mL of a 6.0 ppbV standard of volatile sulfur compounds on a 5
meter GS-GasPro. The detection limit obtained by this analytical laboratory is 4.0 ppbV for each compound shown in the chromatogram, the
same detection limit obtained on the previously used WCOT column.
Figure 7 shows a chromatogram on the same system of a 100 ppbV sulfur dioxide standard. These chromatograms show the type of detection
limits obtainable on an optimized system.
The analytical laboratory that generated the chromatograms shown in
Figures 6 and 7 (Air Toxics Ltd., Folsom, CA) was seeking a column
that could resolve sulfur dioxide from carbonyl sulfide, allow trace-level
quantitation and provide a relatively short analysis time. This laboratory
was able to obtain this by using a short (5-meter) GS-GasPro column in
their system, replacing a 30-meter thick-film 100% dimethylpolysiloxane
column. This column allowed for an analysis time of only 15 minutes,
compared with approximately 35 minutes on the previous column. A
subambient starting oven temperature (0°C) was required due to the time
required to transfer the large sample loop to the 0.32 mm I.D. GS-GasPro
column (approximately 20 seconds). Without a subambient starting temperature, peak shape suffered for both carbonyl sulfide and hydrogen
sulfide due to their chromatographic movement during the sample transfer time.
Sulfur dioxide and carbon disulfide switch elution order in Figure 4
and Figure 7. This elution order change can be attributed to differences
in column length and analysis conditions.
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Conclusion:
Detector quenching can be a significant problem when quantitating
volatile sulfur compounds in hydrocarbon streams such as bulk propylene, bulk ethylene or LPG. Column selectivity is important in these
applications to allow for elution of sulfur compounds away from large
hydrocarbon peaks in the sample matrix. As shown in this paper, the
GS-Q and GS-GasPro appear to have the best selectivity for these applications.
Quantitation of low-level sulfur compounds requires inertness in the
entire analytical system, including the capillary column. For truly tracelevel sulfur analysis this paper showed evidence that DB-1 and GSGasPro provided the most inert stationary phase surfaces for reactive
sulfur compounds such as hydrogen sulfide and the mercaptans.
Of all four columns evaluated, the GS-GasPro was the only column
that resolved sulfur dioxide from carbonyl sulfide. Sulfur dioxide was
eluted from the GS-GasPro with very good peak shape and response.
When elution pattern and inertness are both evaluated, it can be seen
that the GS-GasPro appears to be an excellent column choice for tracelevel quantitation of sulfur compounds in hydrocarbon streams.
Acknowledgements:
The authors would like to thank Wade Bontempo and Sandia Kao of
Air Toxics, Ltd. in Folsom, California for providing chromatograms
on the 5-meter GS-GasPro.
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Figure 1. DB-1
CS2
H 2S
COS/SO2
MeSH
EtSH
PFPD
1
3
2
Time (min)
4
FID
5
iC5
6
7
C5
C2B
MA iC4
C4 t2B
C1
C3/C3=
C4= /13BD
C2
C2= /acetylene
Table 1. DB-1
Column:
Injector:
Carrier:
Oven:
RT
1.61
1.72
1.78
1.97
2.13
2.17
2.20
2.75
3.10
3.25
DB-1
30 m x 0.32 mm I.D., 5.0 µm
200°C, 1:20 split
Helium, 10 psig, 2.0 mL / min @ 40°C
40°C for 5 min, 5°/min to 75°C and hold
Compound
methane
acetylene / ethylene
ethane
H2S
propylene
propane
COS / SO2
iso-butane
butene-1
n-butane
RT
3.42
3.47
3.71
5.05
5.77
5.96
7.32
10.56
Compound
trans-2-butene
methyl mercaptan
cis-2-butene
iso-pentane
ethyl mercaptan
n-pentane
CS2
n-hexane
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Figure 2. GS-CarbonPLOT
CS2
COS/SO2
H2S
PFPD
2
4
Time (min)
6
8
10
FID
C3=
C2=
MA
C4
C3
iC4 C4=
C1 acetylene C2
Table 2. GS-CarbonPLOT
Column:
Injector:
Carrier:
Oven:
RT
1.95
2.46
2.75
3.21
3.22
5.03
6.28
6.71
GS-CarbonPLOT
30 m x 0.32 mm I.D., 3.0 µm
200°C, 1:20 split
Helium, 10 psig, 1.7 mL / min @ 100°C
100°C for 2 min, 20°/min to 310°C and hold
Compound
methane
acetylene
ethylene
ethane
H2S
COS / SO2
propylene
propane
RT
9.57
9.85
10.08
11.06
12.50
12.75
15.44
Compound
iso-butane
butene-1
n-butane
CS2
iso-pentane
n-pentane
n-hexane
Mercaptans not detected
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Figure 3. GS-Q
CS2
COS
MeSH
EtSH
H2S
PFPD
1
2
4 Time (min)
3
FID
5
6
7
8
iC5
iC4
C5
C3=
T2B
MA
C1
C3
C4/C2B
13BD
C2
C4=
C2= /acetylene
Table 3. GS-Q
Column:
Injector:
Carrier:
Oven:
RT
2.07
2.38
2.47
2.83
3.21
3.60
3.75
5.31
5.48
** SO
2
GS-Q
30 m x 0.32 mm I.D.
200°C, 1:20 split
Helium, 10 psig, 1.7 mL / min @ 100°C
100°C for 2 min, 20°/min to 250°C and hold
Compound
methane
ethylene / acetylene
ethane
H2S
COS
propylene
propane
methyl mercaptan
iso-butane
RT
5.75
5.98
6.02
6.16
7.42
7.67
7.69
8.00
9.06
9.62
Compound
butene-1
n-butane
cis-2-butene
trans-2-butene
ethyl mercaptan
CS2
iso-pentane
n-pentane
SO2 ** (not shown)
n-hexane
peak is 2 minutes wide
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Figure 4. GS-GasPro
COS
H2S
SO2
CS2
MeSH
EtSH
PFPD
4
2
Time (min)
6
8
10
FID
C2=
iC5 C5
C3=
C1
C2B
iC4
C4
13BD
t2B
C3
C2
MA/C4=
acetylene
Table 4. GS-GasPro
Column:
Injector:
Carrier:
Oven:
RT
1.82
2.35
2.62
3.34
3.39
3.63
4.01
5.34
6.14
6.35
6.45
GS-GasPro
30 m x 0.32 mm I.D.
200°C, 1:20 split
Helium, 10 psig, 2.0 mL / min @ 60°C
60°C for 2 min, 20°/min to 260°C and hold
Compound
methane
ethane
ethylene
COS
acetylene
H2S
propane
propylene
iso-butane
SO2
n-butane
RT
6.61
7.60
7.92
8.13
8.35
8.38
8.56
10.29
10.30
Compound
CS2
butene-1
trans-2-butene
cis-2-butene
iso-pentane
methyl mercaptan
n-pentane
ethyl mercaptan
n-hexane
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Figure 5. H2S Detection Limit and Peak Shape
GS-GasPro
GS-Q
DB-1
≈ 25-50 ppbV H2S
on-column equivalent
GS-CarbonPLOT
≈ 2500-5000 ppbV H2S
on-column equivalent
Figure 6. Trace Level Sulfur Determination
GS-GasPro
5 m x 0.32 mm I.D.
Gas sample valve, 1.0 mL fixed sample loop
Sievers SCD, reaction tube 800°C
Helium, 39 cm/sec at 0°C (determined by TR of COS)
0°C for 1 min, 30° / min to 160°C, hold at 160°C for 4 min,
50° / min to 260°C, hold at 260°C for 5 min
Column:
Injector:
Detector:
Carrier:
Oven:
Pk #
1
2
3
4
5
6
7
8
9
Compopund
COS
H2S
CS2
Methyl mercaptan
Ethyl mercaptan
Thiophene
Dimethyl sulfide &
Isopropyl mercaptan
n-Propyl mercaptan
t-Butyl mercaptan
3
Pk #
10
11
12
13
14
15
16
17
18
19
Compopund
Isobutyl mercaptan
3-Methylthiophene
n-Butyl mercaptan
Ethyl methyl sulfide
Dimethyl disulfide
Diethyl sulfide
2-Ethylthiophene
2,5-Dimethylthiophene
Tetrahydrothiophene
Diethyl disulfide
19
6.0 ppbV each component
15-18
7
11-14
4
5
6
8
2
9 10
1
2
3
4
5
6
7
8
Time (min)
9
10
11
12
13
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Figure 7. Sulfur Dioxide Determination
Column: GS-GasPro
5 m x 0.32 mm I.D.
Injector: Gas sample valve, 1.0 mL fixed sample loop
Detector: Sievers SCD, reaction tube 800°C
Carrier: Helium, 39 cm/sec at 0°C (determined by TR of COS)
Oven:
0°C for 1 min, 30° / min to 160°C, hold at 160°C for 4 min,
50° / min to 260°C, hold at 260°C for 5 min
CS2
MeSH
H2S
EtSH
SO2
others
= 100 ppbV
= 89 ppbV
SO2
COS
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
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