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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About the authors

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The authors, their avatars and Twitter handles

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@pidguy @pidgirl

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 than24 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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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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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 eVhv= a photon with an energy of 10.6 eV R+ = a positive ion that is measured at the

collection electrodee- = 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

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Ionization Potentials (eV)

Name Formula Ionization Potential (eV)

oxygen O2 12.08

Hydrogen sulfide H2S 10.46

Arsine AsH3 10.5

Cadmium hydride CdH2 10.3

Plumbane PbH4 9.1

Butyl alcohol C4 H9OH 10.10

Iso butyl acetate C6H12O2 9.90

2Hexenal C6H10O 9.24

Water H2O 12.59

Nitrogen N2 15.6

Carbon dioxide CO2 13.79

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PeakWorksTM Software for Windows 7 OS

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Automation (Start, Stop, AutoZero, Integration, Calibration), Automatic Injection,Export Data to Excel

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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 system

Chromatography Integration software

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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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

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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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

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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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/concentrationCdS + H+ (conc.) => H2S (g) pH < 1 form H2S (g)

from CdSHeadspace gas sample from a sealed vesselH2S (g) + N2 => H2S (g) GC separationH2S (g) + hn (10.6 eV) => H2S + + e-

photoionizationIP H2S =10.46 eV

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Alternate Method: Dynamic Headspace for sulfide DH/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 orPurge 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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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.

Photos

PID PI 52-02

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Detector & Detector Character: “Detector Guy”

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The photoionization detector is a very sensitive GC Detector! 10 ppm H2S

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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

Speaking of sensitivity, here’s one of our Throwback Thursday #TBT features from Analytical Chemistry in 1977

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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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Water SamplesUsing 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

Plot of H2S Measurements by SH/GC/PID yields a nice linear relationship

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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 GCH2S (g) + N2 => H2S (g) GC separation

H2S (g) + hv (10.6 eV) => H2S + + e-

photoionization

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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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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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