Artifact formation from the use of potassium-iodide-based ozone traps during atmospheric sampling of trace organic gases

Artifact Formation from the Use of Potassium-Iodide-Based Ozone Traps During Atmospheric Sampling of Trace Organic Gases Detlev Helmig" and Jim Greenberg National Center for Atmospheric Research, Boulder, Colorado 80307, USA

Key Words: Ozone removal Potassium iodide ozone trap Artifact formation Organic iodine compounds

Summary

Trace atmospheric gases have been sampled using potassium-iodide-based reactant traps for ozone removal. GC-MS analysis re- vealed the presence of organic iodine compounds which were present neither in the atmosphere nor as contaminants of the materials used, but more likely arose from reactions of organic gases or aerosols sampled with reactive iodine or hypoiodite released from reaction of ozone with potassium iodide in the traps. Losses of the individual organic trace gases measured were not, however, observed as a result of this mechanism. Several cautions in identifying unknown compounds from the atmosphere when using potassium iodide ozone traps are discussed.

1 Introduction

Ozone (03) has been shown to be a potential interference in the analysis of volatile atmospheric trace organic gases [l-141. A frequently used technique for removal of ozone from the sampled gas is reaction with potassium iodide (KI). Ozone traps relying on the reaction with KI have been used in several studies [6,9,15- 191 and were shown to be effective for removal of 0 3 from ambient air and in smog chamber experiments.

methanolic solution of KI (Fisher Scientific, Pittsburgh, PA, USA, typically at 0.05 g ml-', purified from organic contanii- nants by dispersion with activatedcharcoal [OKBO 32 cartridges, Supelco, Bellefonte, PA, USA]) was passed through the assem- bled denuder until all inner surfaces were wetted. The solvent was evaporated by blowing dry nitrogen through the denuder while heating to 100 "C for about 30 min to remove residual organic contaminants from the chemicals used.

During sampling the denuder efficiency was continuously moni- tored by operating a Dasibi ozone analyzer (Environmental Corp., Glendale, CA. USA) downctream of the denuder (total flow rate through the denuder was about 2 L min-'). For ambient ozone levels in the range 30 to 100 ppb, the readings on the Dasibi ozone analyzer generally fluctuated between 0 and 5 ppb, with most readings being zero. This concentration range is about the detection limit of the Dasibi instrument and considering the low precision at these low concentrations, the ozone removal rate is estimated at > 95 %. Consistent with findings reported in the literature [ 161, this lund of denuder showed efficient ozone removal for samples of ambient air up to several hundred liters.

The KI denuders were used for the sampling of gas phase standards of organic compounds during indoor smog chamber (Teflon bag) experiments and ambient measurements. Organic gases

The destruction of ozone by KI is believed to follow a surface reaction involving water from atmospheric moisture:

0 3 + 2KI + H2O -+ 0 2 + I2 + 2KOH (1)

although a different possible mechanism [20] does not include water:

were sampled on to Tenax and analyzed by thermal desorption GC-MS. A detailed description of the system used for the chemical analysis has been given elsewhere r221.

The denuders were also usedin a study to distinguish atmospheric degradation products of biogenic hydrocarbons present in ambient air from products that are possibly formed on the adsorbent during sampling. The following types of experiment were performed:

(2) @ + K I -+ 02+KOI

In some of our recent studies we employed such a system to remove ozone and found consistent evidence for formation of organoiodine reaction products through the use of these KI traps.

2 Experimental (i) Sampling of ozone-enriched hydrocarbon-free (zero) air (the range tested was 20 to 200 ppb ozone) on to blank cartridges.

2.1 Ozone Traps

Three different kinds of ozone trap were used during these studies. (a-pinene, P-pinene, limonene, tetrachloroethene).

2.1.1 The Annular Denuder Trap

The annular denuder system [21] consists of a 25 cm x 2.4 cm i.d. outside Pyrex glass tube and a 23.3 cm x 2.2 cm 0.d. inner glass tube. The walls bordering the annular space were sand- blasted to increase the retention of the coating solution. A

(ii) Sampling of zero air on to solid adsorbent cartridges which had previously been loaded with organic trace gases

(iii) Sampling of ozone-enriched zero air on to solid adsorbent cartridges loaded with the organic trace gases listed above.

(iv) Ambient air sampling in polluted environments (Riverside, CA, USA) and sampling of forest air (Whitaker Forest, CA, USA).

Journal of High Resolution Chromatography VOL. 18, JANUARY 1995 15

Artifact Formation during Atmospheric Sampling of Trace Organic Gases

2.1.2 Impregnated Glass Wool Trup

The second type of trap was made by soaking untreated glass wool (Alltech, Deerfield, 1L, USA) in a solution of KI (3 g), methanol (5 mL), and glycerol (2 mL; all chemicals from Baker, Phillipsburg, NJ, USA). The impregnated glass wool was sub- sequently dried by inserting it into a glass tube and pur ing with

were prepared by filling 5 cm x 0.53 cm i.d. stainless steel tubes with the impregnated glass wool.

The KI-coated glass wool traps were used in the course of the Mauna Loa Photochemical Experiment (MLOPEX-11) for meas- urement of organic trace gases in tropospheric background air [23-261. Before the first use of the traps for sampling and also between samples, traps on the GC-MS inlet system were back- flushed with clean helium at a flow of 200 mL min-' at about 75 "C (50" above the trap temperature used during ambient sampling).

Ozone removal efficiency was routinely checked by pulling ambient air through the traps and measuring the ozone concentration downstream of the denuder with a Dasibi ozone analyzer. For these measurements the ozone analyzer operated at flow rates of 1 L min-' or about 2-3 times the flow rates typically used for ambient air sampling. Under these conditions also, ozone readings fluctuated around 0 ppb and the ozone removal efficiency was estimated to have been > 95 %. The efficiency at the lower flow rates used during the sampling of organic trace gases was assumed to be at least this high, and probably higher.

ultra high purity zero air (at approximately 50 mL min- 5 . ). Tiaps

2.1.3 Crystalline KI Trap

The third kind of ozone trap tested consisted of 5 cm x 0.53 cm i.d. stainless steel tubing filled with crystalline Kl and plugged with silanized glass wool to retain the KI. This trap was used under the same conditions as the coated glass wool system described above.

2.2 Analytical Procedure

A closed, automated enrichment - thermal desorption injection system was used for analysis [23J. Ambient air was drawn through a temperature-controlled adsorbent trap and organic gases were then thermally desorbed into a GC-MS system. The solid adsorbents used during these experiments were Tenax TA and Tenax GR (Alltech, Deerfield, IL, USA) and Carbosieve SIII, Carbotrap C, and Carbotrap (Supelco). Ozone traps on the GC- MS system were replaced after maximum sampling volumes of approximately 300-500 L ambient air. The same kind of trap was used on a separate analytical system using cryogenic freeze-out and GC-FID analysis [25,26]. On this system the traps were shown to be efficient for ambient air sample volumes to at least 1.5 m3.

Linear temperature programmed retention indices (RI) were determined by analyzing standard samples containing n-alkanes as bracketing compounds; calculations were performed according to the relationship given in [271. Besides measurements on DB-1, several measurements were made on DB-5 and DB-1701; the columns used were (i) 0.33 mm x 60 m DB-l(O.25 pm film), (ii) 0.33 mm x 60 m DB-5 (0.25 pm film), and (iii) 0.33 mm x 60 m

DB-1701(0.25 pmfilm) (columnsfrom J&W Scientific, Folsom, CA, USA).

The oven temperature was reduced to -50 "C, held at this temperature during thermal desorption and sample transfer, and then linearly programmed to 180 "C at 6" min-'. RI standards and samples were measured in separate runs and in some instances several days apart; the precision of these measurements was, therefore, estimated to be within approximately -t 3 RI units.

Mass spectral data (mass selective detectors HP MSD 5970 for denuder measurements and HP MSD 597 1 A during MLOPEX) were averaged from approximately 5 scans around the peak maxima, performing a background correction, and normalizing the base peak abundance to 100 %. Only signals with intensities greater than 5 % are reported. Compound identifications were confirmed by use of literature mass spectra and RI data, where available. For selected species, RI were determined after the field experiments by preparing gas phase standards, collecting about 100 ng of alkyl iodides on to solid adsorbent cartridges and subsequent analysis by thermal desorption GC-MS using a two stage enrichment - injection system different from the one stage system used during the MLOPEX field measurements.

3 Results and Discussion

During the denuder experiments, which included the sampling of ozone-enriched standard gases as well a5 ambient air, anumber of organic iodine compounds were identified by GC-MS. Similar compounds were identified during the MLOPEX-I1 field cam- paign when KI-coated glass wool traps were used.

The organoiodine artifacts most frequently observed are summa- rized in Table 1. Compounds are listed in order of their retention sequence on DB-1, as measured during the MLOPEX-TI experiment. Available RT data on DB-5 andDB-1701 and standard and literature RI are also included in Table 1. Reasonable agreement was generally found between RI determined for Eamples and standards, and with literature RI data. The inexplicable exception was I -iodopropane; the good agreement observed between the mass spectra of sample and standard, however, confirmed the correctness of this identification.

Identified iodo compounds can be dwided into mono-substituted alkyl iodides, poly-substituted alkyl iodides, andmixed iodo-halo compounds. The interpretation of the mass spectra of organic iodides is straightforward. Most of the iodo compounds observed showed a distinct molecular ion. Common fragmentations observed are the fragmentation of M+ into I+ or HI' and an alkyl group. This fragmentation leads to abundant signals at mlz = 127 and 128, respectively; these signals are the characteristic features of the mass spectra of iodo compounds and the base peak for many of the compounds observed. With increasing molecular weight a reduction in the intensity of the 12711 28 signals and an increase in the intensities of alkyl fragmentations is observed. Compared with the literature mass spectra available [28], our measurements on the MSD quadrupole spectrometers gave sub- stantially higher relative intensities for the molecular ions and the 127/128 signals.

The only organoiodine species consistently detected in samples collected without the use of KI traps during the MLOPEX I1 measurements was methyl iodide. Methyl iodide has previously

16 VOL. 18, JANUARY 1995 Journal of High Resolution Chromatography


Table 1. Summary of the major organoiodine compounds identified by use of mas spectrometry and linear temperature programmed retention index data on DB-1, DB-5, and DB-1701 capillary columns.

Compound Mass spectrum') Retention indexb) DB- 1 DB-5 DB-1701

Iodomethane

Iodoethane

2-10dopr0pane

I-Iodo-2-propene

1-Iodopropane

Iodobutane isomer')

Iodobutane isomer')

Diiodomethane

Triiodomethane

142(100), 127(96), 141(21) 140(7)d'

127(100), 1S6(97), 128(16) 39( 14f'

127(100), 170(90), 41(59) 43(55), 39(S3), 50(22)d'

127(100), 168(96), 39(85) 41(36), 38(16), 40(12)d'

170(100), 127(92), 142(20) 42(17), 141(ll), 128(9f)

127(100), 184(95), 41(93) 39(41), 57(40), 155(16)

184(100), 127(63), 141(3Y) 42(29). 128(20), 43(17)

127(100), 141(91), 140(11) 139( 1 1 )d)

127(100), 140(42), 139(14)')

(523)')

601.7 [600]"

661.0 654.7h)

688.3 687.4h'

729.4 1700.0]D/698.2h'

795.0

828.8

876.4 [878.0]"

( 1 206)

(530)

6 I S.6e) [614.7Ig)

670.7e'

769.3/718.2') [716.8Ig'

8 15.1/821.7e'

867.8

914.1l924.6" [918.6Ig)

( 1268)/1 289.Se)

(560)

658.5

713.6

764.4

900.6

867.4

-

1017.3

(1 300)

"'mi? (Oh rel. abundance). b' Data in brackets are from the literature.

') Literature mass spectrum available [281. ') Retention index determined during denuder experiments on HP-5 column at 6" min8 [22].

h, Measured using chemical standard (Aldrich) on DH-1 at 6" min? ') Tenrdtively identified; available reference retention index data for iodobutane isomers (4 possible isomers): 2-iodobutane, 764.9"; 1-icdobutane, X01.3h), 800.0", 819.3";

Data in parentheses were determined by extrapolating C5 and C n n-alkane retention times from measured values for C~--CI.

Literature retention index on OV-1 at 8" m i d l 1341. '' Literature retention index on SE-54 at 8" min ' 1341.

during the denuder experimenls iodobutane isomers were identified at RI = 821.7, 831.1, and 879.4 on HP-5". ') Scan range does not include molecular ion at m/z 394

been identified in marine air [29-311; ambient methyl iodide background concentrations were typically found in the lower parts per trillion (ppt) range in the marine boundary layer.

The most abundant organic iodine species in all the experiments were diiodo- and triiodomethane. In addition to the compounds listed in Table 1, several other iodine compounds, e.g. c 4 - C ~ alkyl iodides, unsaturated and cyclic compounds, e.g. iodocyclo- hexane and iodobenzene, and a mixed halogenated compound (an isomer of iodochloroethene) were identified as minor peaks in a limited number of samples. Dichloroiodomethane and diio- dochloromethane were only observed in the denuder experiments. The peak areas of identified iodine compounds in the total ion chromatograms in the MLOPEX experiments were generally small compared with those of the other ambient organic trace gases identified; the levels were equivalent to lower ppt concentrations in ambient air.

Blank experiments were performed by sampling hydrocarbon- free, dry zero air or liquid nitrogen blow-off through the whole analytical system, including the KI denuder or traps. Iodine compounds were not observed at significant levels in any of these blank runs.

No significant reduction in the abundance of iodine compounds was observed with prolonged use of traps and denuders, as would have been expected if the iodine compounds were system con-

taminants. This observation and the absence of iodine compounds from blank experiments suggest that iodine compounds are not associated with contaminants in the materials and chemicals used.

Most of the major iodine compounds were observed during the use of all of the adsorbents tested. Since these adsorbents have quite different chemical structures and physical properties (e.g. organic polymer for Tenax, activated charcoal for Carbotrap) it appears unlikely that artifacts are formed through a reaction involving the adsorbent itself.

Laboratory experiments performed with the denuder system showed an increase in artifact formation with increasing ozone levels in the samples. This observation suggests that ozone ndst be present for artifact formation. In the presence of ozone, a plausible artifact formation mechanism includes the formation of molecular iodine according to eq. (1). This mechanism would be consistent with observations during prolonged use of KI- coated glass wool traps - durin continuous sampling of am-

after a few days the trap inlet side showed some yellow decol- oration, which may indicate the presence of molecular iodine. The possible formation of molecular iodine during testing of KI-coated annular denuders has also been described previously [ 16,17,32]. This iodine release was found not to interfere with

bient air at approximately 2 L min -5 with a Dasibi ozone analyzer,

Journal of High Resolution Chromatography VOL. IX, JANUARY 1995 17


the analysis of nitrogen oxides [17]. In the analysis of organic trace gases described here, however, this effect is of concern because the released molecular iodine may react with traces of organic gases, acrosols or residues, or materials in the analytical device, to form organic artifact compounds.

A number of possible reaction pathways for the formation of organic iodine compounds can be postulated [ 33 ] . Because of thermodynamic considerations, reactions of molecular iodine with hydrocarbons, such as hydrogen substitution, the addition of iodine to double bonds, or halogen substitution of halogenated organics, are expected to be slow. Under the heterogenous conditions encountered in the analytical system, however, and also for trace organic gases adsorbed on solid adsorbents, the iodine reactivity may present other possibilities. If, according to eq. (2), hypoiodite is formed in the ozone trap, a number of further reaction pathways, including iodoform reactions, may occur. In the presence of water, furthermore, hydrogen iodide can be formed; this can add to double bonds of unsaturated compounds, leading to the formation of alkyl iodides.

Comparison of measurements made without the ozone traps did not indicate measurable loss of any individual compounds through the use of the traps or denuder. Parallel quantitative ambient air measurements made during MLOPEX on two GC- FID systems with cryogenic freeze-out [26] also gave consistent quantitative data for ethene (concentration range approximately 3-100 ppt) and propene (0.5-30 ppt) analyzed using a KI trap on one channel and an Ascarite trap (sodium-hydroxide-coated sil- ica; A.H. Thomas Co., Philadelphia, PA, USA; an efficient 0 3 scrubber) on the other. The occurrence of the C2 and C3 iodides (Table 1) was, consequently, probably not the result of reactions of ethene and propene in the KI traps. We have not yet identified particular precursor compounds and mechanisms for any of the suspected artifacts listed in Table 1.

4 Conclusion

The findings presented here indicate that reactive iodine compounds (e.g. I2 or 01-) may be formed in KI-based ozone traps and lead to the formation of organic iodine species. Caution should, therefore, be exercised when identifying organic gases obtained from atmospheric samples using KI traps. For quantitative measurements of trace organic gases, it is recommended that recovery experiments be performed with ozone added to the test gas sampled. It is possible that the organic iodine compounds may be artifacts of reactions involving trace atmospheric organic gases or aerosols sampled. At present, no measurable losses of a number of specific volatile trace organic gases investigated in these studies have been observed. As a possible means of elimi- nating the liberation of 12, Possanzini et al. [32] used the addition of sodium arsenite to the reacting agent for I:! scavenging. This technique has not been investigated in this study.

Acknowledgment

The National Center for Atmospheric Research is sponsored by the National Science Foundation. The results on the denuder experiments reported in this paper were obtained during D.H. ’s affiliation with the Statewide Air Pollution Research Center at the University of California, Riverside. We thank the City of Boulder Wastewater Treatment Plant for allowing us to use their GC-MS system for some of the data analysis reported here.

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18 VOT,. 18, JANUARY 1995 Journal of High Resolution Chromatography

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Artifact formation from the use of potassium-iodide-based ozone traps during atmospheric sampling of trace organic gases