ADV01-05_1-298

56 / 298

• 6 • 800-356-1688 • 2005 vol. 1
Accurate analyses for
parts-per-million to
parts-per-billion levels of
organosulfur compounds
in petrochemical streams
are critical to meeting
new regulations for
lower levels of sulfur in
diesel fuel and gasoline.
Many sulfur compounds,
including hydrogen sul-
fide, methyl mercaptan, and ethyl mercaptan,
adsorb strongly to metal surfaces in sampling,
storage, and transfer apparatus. In addition to
causing inaccurate, falsely low values, adsorp-
tion can prolong analysis cycle times. To deter-
mine quantitative losses of active sulfur species,
we sampled, stored, and transferred low ppmv
to low ppbv concentrations of active sulfur
gases, using control (untreated) and Sulfinert®
treated system components.
Preventing Sulfur Compound Losses
During Storage
Figure 1a depicts results from a comparison in
which a gas containing 17ppbv of hydrogen sul-
fide was stored for 7 days in untreated or in
Sulfinert® treated stainless steel sample cylin-
ders. The response ratio for hydrogen sulfide,
relative to a stable reference material, dimethyl
sulfide, is steady at approximately 1:1 for at
least seven days in Sulfinert® treated cylinders.
The data show a Sulfinert ® treated system will
reliably store ppb levels of the active sulfur-con-
taining compound during transport from the
sampling site to the analytical laboratory. In
contrast, hydrogen sulfide degraded rapidly in
the untreated cylinder, and was lost totally with-
in 24 hours.
In a similar study in which gas containing
18.8ppbv methyl mercaptan was stored for 60
hours in Sulfinert ® treated sample cylinders,
recovery of the active sulfur compound was
equally high relative to the stable reference mate-
rial, dimethyl mercaptan, as shown in Figure 1b.
Sample Transfer: Adsorption of Sulfur
Compounds to Tubing
Comparison of Sulfinert® treated electropolished
stainless steel tubing (TrueTube ™ EPS tubing,
surface roughness average (RA): 5-10, O'Brien
Corporation, St. Louis, MO), untreated electrop-
olished stainless steel tubing (TrueTube™ EP tub-
ing, RA 5-10, O'Brien Corporation), and raw
commercial grade stainless steel tubing (RA 23-
27) showed only the Sulfinert® treated electrop-
olished tubing has the inertness necessary for
quantitatively transferring low ppmv to low ppbv
concentrations of sulfur compounds. Figures 2
and 3 depict the results (seamless 316L stain-
less steel, 1/8" OD, 0.020" wall). Tests were per-
formed at room temperature, using a gas flow
rate of 40cc/minute.
To confirm whether an active sulfur-containing
compound in a gas stream passing through 100-
foot (30.5-meter) lengths of tubing would adsorb
to active sites on the tubing surface, we meas-
ured the amount of time elapsed before values
for the sulfur content exiting the tubing were
stable and accurate, using helium containing
0.500ppmv methyl mercaptan. Figure 2 shows
Sulfinert® treated electropolished tubing did not
adsorb methyl mercaptan to any measurable
extent, delivering a representative sample with
no delay. Untreated electropolished tubing, in
contrast, totally adsorbed methyl mercaptan for
more than 75 minutes, and the sulfur gas level
did not stabilize until approximately 130 min-
utes. Conventional 316L seamless tubing totally
adsorbed methyl mercaptan for more than 90
minutes, and the sulfur gas level did not stabi-
lize until approximately 140 minutes.
When adsorption of sulfur-containing com-
pounds is prolonged, desorption from the sur-
face also is slow. This “memory” of adsorbed
compounds can cause long delays in equilibrat-
ing a sample stream. In Figure 3, Sulfinert®
treated tubing shows the lowest retention of
sulfur compounds, by several orders of magni-
tude. Samples can be evaluated, with accurate
results, with no delay between them.
Superior Storage and Transfer of Sulfur
Compounds
Sulfinert® Treated Systems Preserve ppb Levels of Active Compounds
Gary Barone, Restek Performance Coatings Division Manager, David Smith, RPC Chief Scientist,
and Martin Higgins, RPC Chief Engineer
• Improve analytical accuracy and reduce system cycle times, using Sulfinert® treated products.
• Increase analytical confidence for low parts-per-billion sulfur compounds, using Sulfinert®
treated sample cylinders.
• Transfer sulfurs in gas streams, without loss, using Sulfinert® treated electropolished tubing.
Figure 1 Sulfur compounds are stable in Sulfinert® treated stainless steel systems
a) 17ppbv hydrogen sulfide in 500mL cylinders
b) 18.8ppbv methyl mercaptan in 300mL cylinders
Figure 2 Sulfinert® treated electropolished
seamless stainless steel tubing (red) does not
adsorb methyl mercaptan (500ppbv).
Blue-untreated electropolished tubing;
violet-raw tubing.
Figure 3Sulfur memory is prolonged in raw
commercial grade stainless steel tubing (violet).
Red-Sulfinert® treated electropolished tubing;
blue-untreated electropolished tubing.
(500ppbv methyl mercaptan in helium)
Reference
1. Application of TrueTube™ in Analytical Measurement
Cardinal UHP August 2004
Available at or by contacting us at
800-356-1688, ext. 4. Request lit. cat.# 59088.
a) b)