Wireless indoor air quality (iaq) monitoring in classrooms and laboratorie…
A new method for the
1. A New Method for ppb Analysis of
H2S in Air and Water
Pittcon: Environmental- LC/GC Techniques
Monday March 9, 2015
PM session 2:10
Paper # 770-3
Authors: Dr. Jack Driscoll & Jennifer Maclachlan
PID Analyzers, LLC Cape Cod, MA
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3. The authors, their avatars and
Twitter handles
Environmental LC/GC Techniques
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@pidguy @pidgirl
4. Introduction
• The photoionization detector (PID Model PI-52) is a very sensitive GC detector for sulfur
compounds with detection limits in the pico gram range for hydrogen sulfide. This detector
can be added to any commercial GC.
• Hydrogen sulfide can be efficiently collected in the field in an impinger with a basic solution
(0.1M sodium hydroxide), for both a
known period of time and flow rate.
• The solution should be kept out of sunlight and
analyzed quickly since the half life of sulfide is less than
24 hours.
Note: For field samples that won’t be analyzed right away, EPA Method 11, in our opinion, is the
best collection technique to use with 0.1 M cadmium sulfate adsorbent is a better alternative
since the cadmium sulfide precipitate formed is not easily oxidized.
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5. Introduction Continued
• Since the pKa for H2S is 6.9, the addition of 0.1
M acid will convert the sulfide (collected or
even cadmium sulfide) to hydrogen sulfide,
which can be swept out of a vessel with an
inlet, exhaust and septum (for the addition of
the hydrogen ion).
• Once the H+ is added, the solution is stirred for
several minutes, then the nitrogen is turned on
at 15 cc/min to remove O2 and the H2S is swept
into the sample loop of the six port GC injection
valve.
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Environmental LC/GC Techniques
6. Utilizing GC/PID for Specific H2S
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Photoionization Process
Schematic for PIDR + hv (10.6 eV) = R+ + e-
Where R is a molecule with an IP < 10.6 eV
hv= a photon with an energy of 10.6 eV
R+ = a positive ion that is measured at the
collection electrode
e- = an electron that completes the circuit
The ions created by the high energy photons in
the ion chamber are pushed to the collection
electrode by the + 125 V bias. The current
developed is proportional to concentration over a
range > 107
Environmental LC/GC Techniques
8. PeakWorksTM Software for Windows 7 OS
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Automation (Start,
Stop, AutoZero,
Integration,
Calibration),
Automatic Injection,
Export Data to Excel
Environmental LC/GC Techniques
9. Environmental LC/GC Techniques
How to use it:
Enter compound,
retention time,
width of the
window, then
put in up to 4
different
standards to
generate a
calibration curve
The user can
control injector
and detector
from this data
acquisition
Chromatography
Integration software
10. Weak Acids: H2S
• The strength of an acid refers to its tendency to lose a proton (H+). A strong
acid such as HCl is one that completely ionizes in a solution to H+ + Cl-.
• Weak acids only partially dissociate. An example, in water, is hydro
sulfuric acid. At equilibrium (pH = 6.9) both the acid (H+) and the conjugate
base (HS-) are present in solution.
At other pH’s the equilibrium is different. For example:
• pH < 4.0 pH >5- 8 pH >9
H2S <=> H+ + HS- <=> Sulfide ion
• This data shows that H2S can be trapped in solution at pH 9 or greater. It
also shows that H2S can be released at pH < 4 but we typically use pH
<1 to make sure we break up all the cadmium sulfide
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11. Formation of H2S gas from Sulfide
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Here’s the curve that
illustrates this:
If you are between pH 0-4
you have predominantly
H2S, H2S aqueous in
equilibrium with H2S gas
pH 5-9 is primarily H+ + HS-
At pH 9 you have sulfide in
solution
12. Static Headspace Technique
Simple Static Headspace Technique
Water samples are transferred
to a 40 mL EPA VOA vial.
Measure 25 ml of sample then,
after you’ve purged the oxygen
out of the headspace, you add 1
cc of concentrated HCl, allow it
to equilibrate, then a 1 ml
headspace sample is removed
and injected into the GC for
analysis of H2S.
Photo of VOA vials for H2S
Headspace Technique
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13. Static Headspace
Henry’s Law
• Now, according to Henry’s law, the
concentration in the gas phase is
proportional to the concentration in
the liquid phase and the gas sample is
injected into the GC.
Henry’s law can be stated (at constant
temperature) as :
P = Kh C
• where:
P= the partial pressure of the solute
in the gas above the solution,
C=the concentration of the solute, and
Kh=a constant with the dimensions of
pressure divided by concentration.
The constant, known as the Henry’s
law constant, depends on the solvent,
the solute, and the temperature.
50 ppm Hydrogen Sulfide
headspace measurement
PID Analyzers LLC; URL- www.hnu.com 13
14. Collection of H2S
• There are several methods for collection and analysis of
H2S depending upon how quickly the sample will be
analyzed. These are primarily for air samples:
1. For samples to be analyzed quickly, within hours of
collection, collect in an impinger containing 0.01 M
sodium hydroxide and add ascorbic acid to prevent
oxidation of sulfide by oxygen then use static
headspace/GC/PID
2. For samples to be analyzed more than 24 hours later,
use the collection technique in EPA Method 11 where
the absorbing technique is 0.1M cadmium sulfate
which traps and fixes the sulfide as cadmium sulfide
then use the static headspace/GC/PID
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15. H2S Chemistry for Air Analysis
Use an impinger with 0.1 M cadmium in 0.0015 M sulfuric acid (about pH 3) for collection:
Hydrogen sulfide gas is bubbled into the impinger that contains .1 M cadmium sulfate which forms
cadmium sulfide precipitate, which is very insoluble, this is for collection and concentration , once
the sample is collected, put it in a sealed vessel and add it to the static or dynamic headspace and
after it’s purged with nitrogen you can add 1 cc of concentrated HCl, which breaks up the complex
and forms hydrogen sulfide. Then you sweep it out of the solution, then it’s injected into the GC -
we’re using a thick film capillary column (5 micron film), and it separates the hydrogen sulfide then
introduces it to the photoionization detector then forms the hydrogen sulfide ions which are
proportional to the concentration of hydrogen sulfide.
H2S (g) + CdSO4 => CdS (ppt) pH 3.0 collection/concentration
CdS + H+ (conc.) => H2S (g) pH < 1 form H2S (g)
from CdS
Headspace gas sample from a sealed vessel
H2S (g) + N2 => H2S (g) GC separation
H2S (g) + hn (10.6 eV) => H2S + + e-
photoionization
IP H2S =10.46 eV
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16. Alternate Method: Dynamic Headspace
for sulfideDH/GC/PID
Analytical Method
• Place the sample (be sure to get
all the precipitate) into the static
sparging vessel
• Add ascorbic acid to remove O2 or
Purge the vessel with N2 at
15cc/min. for 3-5 min. to remove
any residual O2
• Add 1 cc concentrated HCl
(11.65M), Ensure that the
sparger top is secure and the
solution is being stirred
• Start PeakWorks™ and inject
samples via 6 or 10 port valve as
required
Sparging Vessel
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17. Schematic Dynamic Headspace/GC/PID
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Gas sampling valve
can be controlled
by PeakWorks™ or
the GC itself
(depending on the
model)
The photoionization
detector (PI-52) is
non-destructive, so
it’s a good idea to
have scrubber on
the exit of the PID.
21. H2S Reproducibility Average: 845 ppb
Direct injection in air
H2S
ppb
662
852
735
750
735
490
Avg. 844.8
Std. dev 121.2
CV % 14.35
Chromatogram low ppm
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The above sample in air shows a
negative peak for oxygen and the
hydrogen sulfide is barely separated
from it-that’s why it’s a good idea to
sweep the oxygen out of the solution
before doing the injection!Environmental LC/GC Techniques
22. Speaking of sensitivity, here’s one of our Throwback
Thursday #TBT features from Analytical Chemistry in 1977
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23. H2S Reproducibility Average 20 ppm-air is quite
good
19.23 21.32
19.01 19.59
18.53 20.76
18.91 19.2
19.76 18.91
18.99 19.78
19.56 19.97
19.48
19.53 ppm Ave
0.73 ppm std. Dev
3.76 % CV %
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24. Water Samples
Using Static Headspace
300 ppm H2S sample in H+ 1000 ppm H2S in air
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Sweeping the oxygen before injection vs not
sweeping before injection
25. Plot of H2S Measurements by SH/GC/PID yields
a nice linear relationship
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26. Water Sample Collection
Samples of water from rivers, streams, marshes, waste water can be easily
collected. Since samples are to be returned to the lab for analysis, they have
to be preserved because the half life of sulfide in solution is < 24 hours. Add
ascorbic acid to the sample to remove any dissolved oxygen and prevent
oxidation of the sulfide before GC analysis.
S-2 + O2+ Ascorbic acid <=> S-2 sample preservation
S-2 + HCl (conc) =>H2S (g) pH<1 at lab- release
Headspace gas sample injected into the GC
H2S (g) + N2 => H2S (g) GC separation
H2S (g) + hv (10.6 eV) => H2S + + e-
photoionization
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27. Detection limit for Air Samples
• 300,000 counts = 300 ppm
• 30,000 counts = 30 ppm
• 3,000 counts = 3 ppm
• Use 25 cc of solution and a 100 ml of sample
• Detection limit for 100 ml sample is about 1 ppm
• For a typical air sample we might collect 2.5
liters-then the detection limit would be
• 1000/25 = 40 ppb
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28. Conclusions
• Air samples containing H2S can be efficiently collected in an impinger
containing 0.1 M cadmium sulfate in 0.0015M sulfuric acid. The cadmium
sulfide is converted to H2S using HCl and is analyzed by GC/PID.
• Samples which are to be analyzed quickly can be treated with ascorbic acid
then analyzed by SH/GC/PID.
• The H2S detection limit for a 2.5 L air sample is 40 ppb. The upper range is >
3,000 ppb for a 100 cc air sample.
• Water samples collected in the field containing sulfide should have ascorbic
added to remove residual O2. Then in the lab, conc. HCl is used to release H2S
for analysis by SH/GC/PID. With additional optimization of the PID, it was
possible to detect 100 ppb of sulfide.
• Analysis can be done by either static or dynamic headspace but the former is a
simpler method
• Want to learn more? Come to the Pittcon expo, we’ve got a booth, 1326.
• Check us out on the web at hnu.com
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