H 2 S interference on CO 2 isotopic measurements using a Picarro G 1101-i cavity ring-down spectrometer

Introduction Conclusions References Tables Figures


Introduction
Cavity ring-down spectroscopy is a relatively new method for making isotopic measurements of carbon dioxide, methane and water vapor at atmospheric concentrations (O' Keefe and Deacon, 1988).Applications for instruments using cavity ring-down spectroscopy include monitoring of greenhouse gas emissions (Chen et al., 2010;Crosson, 2008), monitoring carbon storage and sequestration (Krevor et al., 2010), studying plant respiration (Cassar et al., 2011;Munksgaard et al., 2013), and process monitoring in the automotive and pharmaceutical industries (Gupta et al., 2009).Recent attempts to apply this technique to monitoring of active volcanic centers have been successful (Lucic et al., 2014(Lucic et al., , 2015;;Malowany et al., 2014), but in some instances there have been anomalous responses from the Picarro G1101-i cavity ring-down spectrometers (CRDSs).Volcanoes emit a range of gases whose concentrations can be much higher than their concentrations in the ambient atmosphere.In particular, hydrogen sulfide gas is abundant in certain volcanic centers and can produce interference in the near-infrared spectrum in which the instrument operates.Our goal was to characterize and quantify this interference for future applications of the CRDS in volcanic environments.
Carbon isotopes are powerful tracers of volcanic gases and degassing processes (Gerlach and Taylor, 1990;Taylor, 1986) and are currently analyzed along with a suite of other geochemical tracers to monitor activity at active volcanoes (Carapezza et al., 2004).CRDS has a promising future monitoring activity at volcanic centers and tracking real-time changes in the isotopic composition of volcanic gases.However, interference of H 2 S with the isotopes of carbon diox- A sample bag containing a standard gas with known CO 2 concentration and isotopic composition was spiked with various amounts of H 2 S. The gas mixture was run directly into the CRDS to observe the interference, and then it was run through a copper tube filled with copper filings to ensure that H 2 S was removed and the isotopic value returned to that of the standard.Copper reacts with hydrogen sulfide, precipitating copper sulfide and releasing water.This can be observed by an increase in the water content measured by the CRDS after a sample has been run through the copper apparatus.
ide prevents accurate measurements of the 12 CO 2 and 13 CO 2 concentrations, resulting in erroneous δ 13 C measurements.
To use CRDS at volcanic centers, the interference of H 2 S gas needs to be characterized and removed.This paper reports the results of laboratory tests using carbon dioxide of a known isotopic composition spiked with different amounts of H 2 S to assess the nature of the H 2 S interference upon the CRDS.These controlled experiments were designed to qualitatively and quantitatively characterize the interference of H 2 S from low concentrations (1 ppb) to those observed at volcanic centers (> 10 000 ppb).To use these instruments for in situ measurements, a quick and efficient way of removing H 2 S from the sample gas prior to analysis is needed.Metals which have a high affinity for acid species, such as copper and zinc, react rapidly with H 2 S to form metal sulfides.If H 2 S can be removed from a sample gas without altering the isotopic composition of carbon dioxide, then the successful application of CRDS in H 2 S-rich environments will only require application of a simple metal scrub prior to analysis.

Experimental setup
Lab experiments were implemented to test the response of a cavity ring-down spectrometer over a range of H 2 S concentrations and then to remove all traces of H 2 S using a copper scrub.A Picarro G1101-i cavity ring-down spectrometer, S/N CBDS-086, designed for measuring the isotopic concentration of CO 2 , was set up in a lab at ambient conditions (25 • C, altitude = 100 m a.s.l., and a summer humidity index of 60-78).The instrument performs continuous measurements while in operation, and samples are run in series, always returning to background values between measurements.This instrument has an intake valve connected to a Tedlar ® gas bag containing a mixture of CO 2 and H 2 S gas.The internal pump in the CRDS actively pumps the gas at 30 mL min −1 into its cavity.Each gas mixture was first run directly into the instrument to observe the H 2 S interference at different H 2 S and/or CO 2 concentrations, and then it was run through 10 cm of copper tubing containing copper filings before entering the instrument (Fig. 1).Copper readily reacts with the H 2 S, removing it from the gaseous phase and leaving the pure CO 2 to be analyzed by the instrument.Copper filings were added to the copper tube to increase the surface area of copper available to react with the H 2 S. Both the Tedlar ® gas bags and the Tygon ® tubing used in these experiments are semi-permeable to CO 2 ; therefore, samples were prepared immediately prior to analysis to minimize the effects of diffusion.The time between sample preparation and analysis never exceeded 15 min.

Gas mixture
Gas samples were prepared using mixtures of H 2 S, CO 2 and CO 2 -free air.A standard CO 2 gas of 995 ppm (± 20 ppm), certified according to Fourier transform infrared spectroscopy with reference to the NOAA X2007 CO 2 international standard and having an isotopic composition of −28.5 ± 0.5 ‰ relative to Vienna Pee Dee Belemnite (VPDB), was spiked with different volumes of a 100 ppm H 2 S gas to give H 2 S concentrations ranging from 1 ppb to 20 000 ppb (20 ppm).H 2 S concentrations were diluted from a gas cylinder containing 100 ppm H 2 mixed with air by adding an aliquot of the 100 ppm gas of up to 125 mL to 1 L of 995 ppm CO 2 in a Tedlar ® gas bag using a syringe.Dilutions were performed such that the CO 2 standard was not diluted to less than 900 ppm and yielded at least 1 L of gas mixture.CO 2 volumes were controlled by a flow meter at a rate of 500 ± 10 mL min −1 .
A second suite of gas mixtures comprised varying concentrations of both CO 2 and H 2 S to illustrate the effect of H 2 S upon different CO 2 concentrations.A 100 % CO 2 standard gas with an isotopic value of −16.0 ± 0.5 ‰ relative to VPDB was diluted to 500, 1000, 2000 and 3000 ppm by adding air that had been scrubbed using ascarite (NaOH) to remove background CO 2 ; 1 L of CO 2 -free air was added to the gas bag using a flow meter, while the CO 2 gas was added in different volumes using a syringe.The flow meter ran at a rate of 500 ± 10 mL min −1 and the syringe was accurate to ±0.05 mL.Uncertainties associated with preparing the CO 2 ranged from ±30 ppm at 3000 ppm CO 2 to ±45 ppm at 500 CO 2 .The diluted CO 2 gas was subsequently spiked with the 100 ppm H 2 S gas to concentrations of 100, 200 or 300 ppb H 2 S using the same technique as described above.The addition of H 2 S to the prepared CO 2 gas caused additional dilution of the intended CO 2 concentration of up to 100 ppm.Final CO 2 concentrations were calculated based on the effective dilution from the added volume of CO 2 -free air and the H 2 S, and were then compared to the CO 2 concentrations measured by the CRDS following the application of the H 2 S scrub.Uncertainties associated with the dilution of CO 2 upon the addition of H 2 S to the prepared sample gas ranged from ±28 ppm at 500 ppm CO 2 to ±119 ppm at 3000 ppm CO 2 .CO 2 concentrations were maintained at concentrations less than 3000 ppm because the instrument is not designed for CO 2 concentrations higher than this.H 2 S can generate interferences at concentrations less than 20 ppb; hence, samples were run at H 2 S concentrations of 1-20 000 ppb (0.001-20 ppm).

Procedure
Prior to the start of every set of analyses, the 995 ppm CO 2 standard gas was analyzed to monitor instrumental drift and to use as a baseline for the subsequent analyses.A sample was run on the instrument by attaching a gas bag using Tygon ® tubing and allowing the CRDS to pump gas into the intake.Between measurements the instrument measured the background air in the lab (∼ 500 ppm), but when a sample bag was attached, there was an increase in the CO 2 concentration to 995 ppm.At this concentration level, the samples have lower instrumental noise than the background measurements.In order to obtain a reliable measurement, the gas bag was measured for 10-15 min.Using the statistical tools of the spectrometer's interface, the δ 13 C value of the gas sample was averaged using the raw delta value for the duration of the sample analysis.This yielded a time-averaged measurement of the isotopic composition, as well as the 12 CO 2 and 13 CO 2 concentrations.Slight variations in the background air were due to the respiration of one or more people in the lab during the analysis; however, this did not affect the outcome of the experiments as the instrument flushes the cavity with new gas every 1-3 s.
After a CO 2 gas sample spiked with H 2 S was analyzed, the sample bag was removed, and the instrument was allowed to return to background values.High H 2 S concentrations can cause large interferences with the isotopic measurements, and it sometimes took several minutes to return to background δ 13 C values, even after CO 2 concentrations had stabilized at ambient levels.After returning to background, the same sample was again connected to the instrument using Tygon ® tubing, then run through a copper tube filled with copper filings before entering the instrument.This procedure removed all H 2 S and allowed the instrument to measure the CO 2 gas without any interference from H 2 S. We used a 10 cm long utility grade copper tube with an outer diameter of 9.6 mm and an inner diameter of 7.5 mm filled with CHEM.57B copper filings which contain up to 10 % metal impurities.These materials are easily acquired, and the copper grade appears to be sufficient for scrubbing large H 2 S concentrations.The filings are necessary to provide a large surface area for reaction with H 2 S. Trials with only the copper tube did not remove all H 2 S.
With the attached copper scrub, data collection was similar to the previous run; the sample bag was analyzed for 10-15 min, and then the instrument was brought back to background values.With H 2 S removed, the instrument was able to visibly return to background levels of δ 13 C and CO 2 .A single 10 cm tube of copper filled with copper filings was used for all analyses and was effective for all H 2 S concentrations.Other trials (not included here) have shown that repeated measurements at H 2 S concentrations in excess of 1 ppm should use more copper (i.e., a longer tube and more filings) than used for these experiments.The deposition of copper sulfide on the filings is a good indication of the efficiency of the scrub; once a large portion of the copper is visibly reacted, the scrub should be changed.
The instrument also measures H 2 O and CH 4 concentrations continuously because of reported cross sensitivities with CO 2 for both water vapor (Rella et al., 2013) and methane gas (Vogel et al., 2013).We used the built-in water vapor correction to correct for variable water concentrations in each sample (Rella et al., 2013).Water concentrations were below 2 % H 2 O by volume in all samples; thus, the instrument correction factor remained valid at these concentrations such that the dry mole fraction of CO 2 was maintained within the Global Atmospheric Watch limits of ±0.1 ppm.The reaction of H 2 S produced water vapor, but the concentrations were not significant to the overall correction factor.Methane concentrations were monitored during each run for concentrations which would cause significant changes to the isotopic value using the sensitivity value of 0.42 ± 0.024 ‰ ppm −1 of methane (Vogel et al., 2013).CRDS-reported CH 4 levels were constant at 3.86 ± 0.21 ppm for all runs with the 995 ppm CO 2 standard and were much lower for samples run with the 100 % CO 2 standard (1.65 ± 0.1 ppm CH 4 ).Overall, variability in the methane concentration is negligible during all runs with a given standard, allowing for comparison of results; however, comparison of the runs using different CO 2 standard gases is not advised due to the different methane levels contained therein.

Results
Interference was first observed with the addition of 20 ppb H 2 S, causing a change in δ 13 C of −0.5 ‰ from the 995 ppm CO 2 standard (δ 13 C = −28.5 ‰ ).As H 2 S concentrations increased, the δ 13 C decreased proportionally (Fig. 2).A sample without H 2 S returned a stable δ 13 C value, but with increasing amounts of H 2 S the δ 13 C value started to decrease over the course of a single run.This resulted in an increasingly negative slope in the raw δ 13 C signal with the addition of greater amounts of H 2 S.
The decrease in the measured δ 13 C resulted from changes in the 12 CO 2 and 13 CO 2 concentration measurements in the presence of H 2 S. Figure 3 shows an increase in the 12  There is a visible increase in the 12 CO 2 concentration and a decrease in the 13 CO 2 concentration with addition of H 2 S. The percentage decrease for 13 CO 2 is significantly greater than the percentage increase for 12 CO 2 .(b) Plot showing the 1000 ppm standard CO 2 gas with the addition of 3 mL of 100 ppm H 2 S and the subsequent response after the H 2 S was removed with the copper scrub.There is a small, yet visible, increase in the 13 CO 2 concentration and decrease in the 12 CO 2 concentration when H 2 S is removed.
and 12 CO 2 molecules in the absorption spectra.The gas samples prepared with the H 2 S and CO 2 mixture had elevated 12 CO 2 and depleted 13 CO 2 with respect to the same sample after the H 2 S had been removed with the copper scrub.Furthermore, the addition of H 2 S to the CO 2 standard gas to create our gas mixture resulted in a decrease in the true The addition of 3 mL of 100 ppm H 2 S to 1 L of the 1000 ppm standard gas resulted in a large drop in 12 CO 2 and 13 CO 2 concentrations.The observed concentrations are significantly lower than those predicted to result from dilution of the standard gas with the addition of 3 mL of H 2 S. When the copper scrub removed H 2 S, the CO 2 concentration remained anomalously low.It is likely that a reaction between H 2 S and CO 2 removes a portion of the CO 2 from the mixture before it is analyzed.
chemical concentration of both 12 CO 2 and 13 CO 2 .The real changes in CO 2 concentration are known by the large difference between the predicted dilution of CO 2 , and that measured after H 2 S had been removed.Figure 4 shows this decrease in both the 12 CO 2 and 13 CO 2 concentrations when H 2 S is added compared to the pure CO 2 standard gas.Dilution of the standard occurs by addition of 3 mL of H 2 S via syringe to 1000 mL of the 995 ppm CO 2 standard in a gas bag.This should result in a decrease of 12 CO 2 and 13 CO 2 concentrations of only 2.9 ppm and 0.032 ppm, respectively.However, the observed decreases in the 12 CO 2 and 13 CO 2 concentrations are much greater than the predicted dilution, 45 ppm for 12 CO 2 and 0.6 ppm for 13 CO 2 .This is on the order of 15 times greater than the predicted dilution.Since the decrease in CO 2 concentration cannot be explained by dilution when H 2 S is added, we propose that there is a compound reaction which consumes CO 2 with the addition of H 2 S to the gas mixture.Sample analyses with H 2 S concentrations from 1 to 500 ppb show a linear interference of −1 ‰ for every 23 ppb H 2 S added (Fig. 5).The interference was successfully eliminated by reacting samples with copper.Figure 6 shows a larger range of samples from 1 ppb to 20 000 ppb.The higher H 2 S concentrations still show a linear interference, but the interference is smaller at −1 ‰ for every 37 ppb H 2 S. We believe that this discrepancy is a result of diluting the CO 2 standard gas with larger volumes of H 2 S during sample preparation.The suite of samples from 1 to 500 ppb H 2 S had larger volumes of diluted H 2 S (1 ppm) added than the sample suite from 500 to 20 000 ppb.The larger dilutions resulted in lower CO 2 concentrations, suggesting that the H 2 S interference also depends on CO 2 concentration.Hence, we ran a further series of experiments to examine this effect.
The set of experiments performed at a range of CO 2 concentrations (500 to 3000 ppm CO 2 ) revealed that the H 2 S interference also depends strongly on the CO 2 concentration (Fig. 7).The interference from H 2 S is much smaller at high CO 2 concentrations and is quite large at atmospheric concentrations.For example, an interference of −1 ‰ resulted from the addition of 21 ppb H 2 S at 500 ppm CO 2 , whereas at 3000 ppm CO 2 an interference of −1 ‰ required the addition of 154 ppb H 2 S. Thus, the H 2 S interference is also dependent on the CO 2 concentration of the sample.During experiments performed at a fixed H 2 S concentration, it was found that the H 2 S interference with δ 13 C was inversely proportional to the CO 2 concentration of the sample.Black dots represent isotopic measurements after H 2 S has been removed with copper; here the isotopic composition is maintained at the standard value (−28.5 ‰).
the variation of the H 2 S interference at different CO 2 concentrations and shows this inverse relationship between CO 2 concentration and H 2 S interference.

Discussion
Carbon isotopic measurements of CO 2 using cavity ringdown spectroscopy have a clear interference in the presence of H 2 S that is dependent on both the H 2 S and CO 2 concentrations.At lower CO 2 concentrations, the H 2 S interference was more pronounced due to the relatively higher proportions of H 2 S contained within the sample.This may explain the discrepancy between the slopes of Figs. 5 and 6, where there was more dilution of the CO 2 standard gas at lower H 2 S concentration (Fig. 5) than at higher H 2 S concentration (Fig. 6), resulting in a larger H 2 S/CO 2 ratio in samples with lower CO 2 concentrations.The H 2 S interference with the G1101-i CRDS is an inherent property of the spectral lines that are fitted to determine the 12 CO 2 and 13 CO 2 concentrations (Fig. 9).The specific spectral lines used in the Picarro G1101-i were chosen to avoid overlapping ambient levels of common gas species encountered in atmospheric air (i.e., H 2 O, CH 4 , NH 3 , etc.).In the case of water vapor for example, where it is not possible to choose CO 2 lines that are free from overlap, the system measures and corrects for such species to the extent that they interfere with either the 12 CO 2 or 13 CO 2 spectral features.For H 2 S specifically, the chosen spectral lines avoid the strongly absorbing H 2 S spectral lines, but there are weaker lines that partially overlap with both spectral features of the CO 2 used in the system.At typical ambient levels of H 2 S for which the spectroscopy of the G1101-i was designed, these weak lines have no measurable effect on the reported CO 2 concentrations or carbon isotope ratio.However, at elevated levels, they begin to cause the observed measurement bias.
Since the isotope ratio measurements in CRDS use ratios of the absorption peaks of the two spectral lines of the CO 2 isotopologues, it is the relative concentration of H 2 S to the CO 2 that determines the effect of the H 2 S on the isotope ratio (Fig. 9).The more pronounced effect of H 2 S on the isotope ratio at lower CO 2 concentrations is due to a weak H 2 S spectral line slightly overlapping the 13 CO 2 line such that when the ratio of CO 2 to H 2 S concentration is low, the measured 13 CO 2 line will be more affected by the H 2 S since it makes up a larger proportion of the overall measured line shape.There is a similar overlapping H 2 S line The relative magnitude of H 2 S interference is much larger for 13 CO 2 than for 12 CO 2 .Note the logarithmic scale.near the 12 CO 2 peak that has a similar (but opposite sign) concentration-dependent effect on the reported 12 CO 2 concentration as compared to the 13 CO 2 concentration.The reason for the sign difference of these two H 2 S concentrationdependent effects is related to how the independent spectroscopic fitting algorithms used for each peak to calculate the isotopologue concentrations interpret the change in line shape imparted by the interfering H 2 S signal.
In addition to the H 2 S interference, there was an unanticipated decrease in both the 12 CO 2 and 13 CO 2 concentrations with the addition of H 2 S to the standard gas that could not be accounted for solely by dilution (Fig. 4).We propose that a reaction between CO 2 and H 2 S is occurring to consume CO 2 upon combination in the Tedlar ® bags.However, we have not directly measured any products; hence we are uncertain as to what reaction will be consuming these reactants at atmospheric conditions.Isotopic readings of our gas mixture indicate that the effects of any reactions are small compared to the effects of H 2 S, so the major concern for these and future experiments is the removal of H 2 S from all samples prior to analysis.
Although all H 2 S experiments conducted in this study use an older model (G1101-i) of the carbon isotope analyzer from Picarro, all subsequent models have maintained the same spectral lines for CO 2 , and their H 2 S performance is presumed to be equivalent.We have verified that no spectroscopic corrections for H 2 S have been applied to any model of the carbon isotopic analyzer, and as such the copper scrub proposed here is a simple and effective solution to the H 2 S interference for all current models.The operating lines of the instrument were chosen to minimize strong overlap of spectral lines from ambient levels of small molecules found in ambient air such as ammonia, water vapor, H 2 S etc., and as such the H 2 S interference only occurs at concentrations > 1 ppb.Normal atmospheric concentrations are much less than this amount, and no correction for the H 2 S overlap has previously been warranted.In non-atmospheric conditions, such as those on active volcanoes or sour gas plants, these concentrations are more common and H 2 S should be considered as an interferant.

Concluding remarks
Isotopic measurements using this particular implementation of CO 2 spectroscopy in cavity ring-down spectrometers have a clear and quantifiable interference resulting from the presence of H 2 S in excess of a few parts per billion.Laboratory experiments using controlled amounts of H 2 S mixed with a CO 2 gas of known concentration and isotopic composition show that the interference is linear and dependent on both the H 2 S and CO 2 concentrations of the sample.The H 2 S interference arises as a result of the line choice for this type of spectrometer (Picarro © G1101-i), which avoids interference with other common atmospheric species such as H 2 O, CH 4 , NH 3 , etc., but it has some small lines remaining in the range of H 2 S that causes the interference observed at high H 2 S concentrations.All models of the carbon analyzer from Picarro © use the same spectral lines and, thus, are susceptible to the same type and magnitude of interference with H 2 S. The most practical approach to eliminating H 2 S interference when measuring the δ 13 C value is the use of a metal scrub, for example copper, to remove all H 2 S before the sample is run through the CRDS.Removing this interference is an important step to making real-time measurements of δ 13 C of CO 2 with cavity ring-down spectrometers in environments with high sulfur concentrations, such as actively degassing volcanoes.Volcanoes have a range of CO 2 concentrations (400-1 000 000 ppm), and the H 2 S interference is significant in the operational range of the CRDS (0-3000 ppm).Therefore, the most practical approach to eliminating the interference is with a simple scrub for all samples containing H 2 S in excess of 1 ppb.

Figure 1 .
Figure 1.Diagram showing the H 2 S experimental setup.A sample bag containing a standard gas with known CO 2 concentration and isotopic composition was spiked with various amounts of H 2 S. The gas mixture was run directly into the CRDS to observe the interference, and then it was run through a copper tube filled with copper filings to ensure that H 2 S was removed and the isotopic value returned to that of the standard.Copper reacts with hydrogen sulfide, precipitating copper sulfide and releasing water.This can be observed by an increase in the water content measured by the CRDS after a sample has been run through the copper apparatus.

Figure 5 .
Figure5.Isotopic signal from the Picarro G1101-i CRDS for 995 ppm CO 2 with H 2 S concentrations ranging from 0 to 500 ppb.Black dots represent isotopic measurements after H 2 S has been removed with copper; here the isotopic composition is maintained at the standard value (−28.5 ‰).

Figure 8 .
Figure 8.The H 2 S interference is inversely related to the CO 2 concentration.(a) The isotopic signal from the CRDS varies with changing CO 2 concentration when the H 2 S concentration is held constant at 300 ppb.(b) Isotopic value vs. 1/CO 2 illustrating the change in δ 13 C with the addition of H 2 S to a standard gas (−16.0 ‰) at different concentrations.

Figure 9 .
Figure 9. HITRAN model for 400 ppm CO 2 and 1 ppm H 2 S (45 • C, 140 Torr) illustrates the overlapping of H 2 S lines with CO 2 lines.The relative magnitude of H 2 S interference is much larger for 13 CO 2 than for 12 CO 2 .Note the logarithmic scale.
CO 2 Raw carbon isotope signal from the Picarro G1101-i CRDS with varying amounts of H 2 S. Addition of H 2 S causes an increasingly negative response for the isotopic value.The raw isotopic signal at each H 2 S concentration does not stabilize, but instead starts to slowly decrease resulting in a "sloped" response.Variations in background levels can be attributed to variations in laboratory conditions (i.e., respiration).
The end result is that H 2 S causes a large negative interference on the δ 13 C value measured by the instrument, predominantly governed by a negative interference with the 13 CO 2 concentration.This apparent decrease is a result of instrument interference between the H 2 S molecule and the 13 CO 2