OZONE SCIENCE & ENGINEERING 0191-9512/00 S3.004-.00
Vol. 22, pp. 551-561 International Ozone Association
Primed in the U.S.A. Copyright © 2000
Research Note
Interferences due to Ozone-Scavenging Reagents in
the GC-ECD Determination of Aldehydes and Ketones
as the CajSS
Edward T. Urbansky* Matthew L. Magnuson, Michael S. Elovitz,
David Freeman, and Jasmine Shauntee
United States Environmental Protection Agency (EPA), Office of Research and
Development, National Risk Management Research Laboratory, Water
Supply and Water Resources Division, Treatment Technology
Evaluation Branch, Cincinnati, Ohio 45268 USA
Received for Review: 5 My 1999
Accepted for Publication: 13 October 1999
Abstract
In order to study ozonation byproduct (OBP) formation as a function of
time, it is necessary to quench ozone and thereby fix the concentrations of
the byproducts. Reagents chosen for this purpose must not react with the
OBPs or otherwise adversely impact the analysis. Six potential ozone-
scavenging reagents were tested for possible interference in the GC-ECD
determination of aldehydes and ketones after derivatization with O-
(2,3,4,5,6-pentafluorohenzyl)oxylamine (PFBOA). All six— sodium
nitrite, sodium cyanide, sodium methanoate (formate), indigo-5,5'-
disulfonate disodium (Indigo Carmine), indigo-5,5',7-trisulfonate
tripotassium, and tin(n) chloride— were found to interfere in the analysis.
Introduction
The quantitative determination of compounds possessing a carbonyl moiety by GC-ECD
after derivatization with O-(2,3,4,5,6-pentafluorobenzyl)oxylamine to produce an
extractable oxime is now widely established (1-16). Because several classes of such
compounds, including aldehydes, ketones, and cc-oxocarboxylates are known to result
from the ozonation of potable water supplies, this particular method has been favored by
those studying OPB formation, and has been promulgated by the EPA as Method 556 for
certain analytes (16).
It is possible not to reduce residual ozone prior to analysis for OPBs, relying instead on
immediate derivatization with PFBOA or assuming that additional OPB formation is
negligible. However, based on experience in our laboratory, it is not uncommon to detect
551 '
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552 E. T. Urbansky et al.
ozone for an hour or longer following the initial exposure. In typical potable waters,
ozone has existed for up to 20 min (17). It is often not known how the composition of
OPBs varies during this time as a result of the residual oxidant. Data gathered under the
EP A's Information Collection Rule have been obtained using EPA Method 556, which
does not reduce residual ozone. When samples cannot be immediately analyzed and the
investigator is unwilling to assume that changes in OPB speciation are negligible as
would especially apply to a kinetics study of OPB formation, the matter of an ozone
scavenging reagent is of paramount importance.
Six possible choices were chosen for this work: nitrite, cyanide, methanoate (formate),
indigo carmine, indigotrisulfonate tripotassium, and tin(n) chloride. EPA Method 556
(16) indicates that both ascorbic acid and sodium thiosulfate are unsatisfactory in this
regard and we did not pursue them. Although Standard Method 6252 (15) suggests
potassium iodide, we consider this to be an unsatisfactory choice as ozone oxidizes I~ to
I2, which then has the potential to form iodinated byproducts. This is particularly
problematic in an OBP formation study where one may intentionally wish to quench large
concentrations of ozone and thus produce large amounts of iodine. Methanoate has a large
rate constant with O3(18) and must be oxidized to.carbonic acid or bicarbonate; any other
reaction would be a reduction. Indigo salts are used in ozone quantification because they
have rapid and stoichiometric reaction (19-20). We verified the reaction rate of cyanide
ourselves under these conditions.
In this communication, we report on the adverse impact of these ozone-scavenging
reagents on the determination of carbonyl-containing compounds.
Experimental Procedure
Materials
Derivatizing agent, PFBOA. O-(2,3,4,5,6-pentafluorbbenzyl)oxylaminehydrochloride
was purchased from Avocado/Johnson Matthey, Ward Hill, Mass., and prepared fresh at
15 mg mL"1 PFBOA*HCl prior to each set of experiments by dissolving a weighed
amount into doubly deionized water.
O3-scavenging reagents. The following solutions were prepared in doubly deionized
water: 0.10 M sodium nitrite (GFS, Columbus, Ohio), 0.10 M sodium cyanide (Aldrich,
Milwaukee, Wis.); 0.010 M indigo-5,5'-disulfonate disodium (Indigo Carmine) (Fisher,
Pittsburgh, Pa.), 0.010 M indigo-S.S'.J-trisulfonate tripotassium (Aldrich). A solution of
0.10 M sodium methanoate was made by combining weighed portions of 88% w/w
methanoic (formic) acid (Mallinckrodt, Phillipsburg, NJ) and 50% w/w sodium hydroxide
(Fisher). Tin(II) chloride solution was made at 0.10 M by dissolving SnCl2 * 2H2O (GFS)
into 0.04 MHC1 (diluted from Aldrich l.OM);0.1 gL"1 mossy tin (Aldrich) was added to
protect against losses of stannous ion from atmospheric oxidation.
Aldehyde standards. A standard (ALD-554) was obtained from Ultra Scientific
(North Kingstown, RI), containing 1000 ug mL~' of each aldehyde/ketone in methanol.
The commercial standard was diluted 1/50 v/v into ethanenitrile (Fisher Optima®) to
produce a working standard at 20 fig mL"1. The following analytes are contained in this
standard: methanal, ethanal, propanal, butanal, frans-2-butenal (crotonaldehyde),
pentanal, hexanal, cyclohexanone, heptanal, octanal, nonanal, and decanal.
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Interferences due to Ozone-Scavenging Reagents in GC-ECD Determination 553
£?Se ^G coinmercial standard does not contain ethanedial (glyoxal) and oxopropanal
&£m ? Pymvaldehyde\a solution was prepared to contain 1000 ug mL'-by
into purge and trap grade methanol (Aldrich). This was* diluted IftoVffp&T
methanol to produce a working standard at 20 ug mL'1.
Methods
Part 1. Effects of O3-scavenging reagents on the analytical method
Snaf!?? i"8^ Wf dma5t0 EPA Method 556-Aliquots of 20 uL of each working
standard (vide supra) were dispensed via 25 uL syringe into 20 mL portions of doubly
detomzedwater m40 ml^glass EPA.rials. A1 -OmL portion of phosphate buffer (0 50 M
1r L- u Na¥lP°4> was added to each vial to give a post-mixing concentration
VI, WniCtl OrOVldeS adeouatR linffer fana^^, n~A ~ _rr -ff a -n" ±, ^-^^
is adequate buffer capacity and a pH of 6.8. Because the OBP
d^vrfrnaPn"J^T^ ^^^^^^°^^^^S^^^^
wSdS toPSaTt- °2 ^ alT0t, °f each ozon^cavenging reagent (vide supra)
was added to a vial. This produces a final scavenger concentration of 500 uM for 0.10 M
stock reagent (cyamde, methanoate, nitrite, and stannous ions) or 50 uM for 0 010 M
rSwSSr1^ Y6 T?^ °btain P«- * 5° ^ Becauseidigo sails react
f^(^ iml»sec°ndsX a stoichiometnc amount of these reagents is satisfactory
For the other reagents, there is a ten-fold excess primarily for kinetic reasons.
Four replicates were prepared for each reagent and one set of four was prepared without
any ozone-scavengmg reagent as acontrol. Two blanks (noRCHO standardised) were
also prepared. Vials were canoed, inverted to miv rti«, -i,j!i»/i _ T n u *-, '^ =7,
^
St luis Mn4'ri;t °-?5°<°w/w a^u?0?s FD&CBlueNo. 1 (Warner-Jenkinson,
t0 ""^o^eatod vials; and 3.00 ± 0.05 mL hexanes {Fisher
^ t0 Tr°Ve the Visibility of the Phase separation aSS
mdigo salts are present. After repeated inversion, thehexane
' ^ °f °'22 M H^/3'5 M NaC1 to
eess pBOArf t ' ' «
NJ ^rSSft ^? thetdned over fnhydrous sodium sulfate (EM Science, Gibbstown,
NJ, Tracepur®). The extracts were then transferred to 1.8 mL autosampler vials.
Hexane extracts were analyzed on a Hewlett-Packard (Palo Alto, Calif.) 6890 GC-ECD
oniTr^wT^^T^
n±n^ S?16^0 CPolsom, Calif.) DB-5 MS column (30 m x 0.250 mm i.d. x 0.25
urn nominal film) at constant (high purity) helium flow rate at 1 .0 mL min'1 (v = 25 cm if
SO ^ltef frature: 25VS de.te?tor temperature: 290 °C. Temperature program: £u
50 C for 1.0 mm; ramp 4 "C mm-' to 220 °C; ramp 20 "C min" to 250 "C, hold for 10 0
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554E.T.UrbanskyetaI.
allowed to react for? no » £Q,(aq) or 0.10 M NaCN(aq) were
extracted with an equal lolu^T^tST ^^^ The solu*>"s
(MTBE, Aldrich, Additlal
^
(Rohnert Park, Calif.) 7725 injector wSh Son .r" TT Injected into a Rheodyne
meas^e the sample volume; tSt SSwiK
Mass spectra were acquired inbornpoS'ion LH? ^ W&S ^^^^ted to 200 °C.
were .subjected to electron impac^d SSSflS8^ m°dcs- MTBEextracts
spectrometronaV ionizat
anaataCalinr ionization mass
same as above, but the ternperatur?^^^^^
Results and Discussion
Part 1. Effects of O3-ScaVenging Reagents on the Ana«y«ca, Method
in mind the magnitude
differences which are not statistically
o
significant have been marked "nss."
speciesto ana^SS^^
would be possible; however, a ^Sa^f^S^aaasm ** which ««& reaction
subswuhon chemistry between *^1^^£^^>^ <* «*« or
As might be expected, cyanide had disastrous effects r^,,.; ^ ,
by 51%, and completely eliminating ethaneSaS gmethana.1!)y83%'ethanal
nucleoDhilically attack aldehydes and form cv ?°^r?pan.aI-Theabu'ifyof cyanide to
/ and requires no further discussion C21)nS 'S Wel1 known in organic
. However,
Melhanoate gave the mos
'«"« «n>« to analyte. This
8tllemseta- We therefore
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Interferences due to Ozone-Scavenging Reagents in GC-ECD Determination 55f
analyte present on a molar concentration basis, it is unsurprising that amide formatioi
would interfere: [HCO2"] = 500 (iM » [CH2O] = 0.67 \iM.
Table I. Analyte Peak Areas for Different Ozone-Scavenging Reagents"
analyte
methanal
ethanal
propanal
butanal
2-butenal
pentanal
hexanal
cyclohexanone
heptanal
octanal
nonanal
decanal
ethanedial
oxopropanal
no scavenger
50200 ± 400
6300 ± 400
13800 ± 800 ,
10000 ± 1000
14000 ± 2000
4900 ± 400
5600 ± 600
8900 ± 400
5100 ± 700
820 ± 90
1800 + 200
980 ± 90
8000 ± 1000
10100 ± 800
nitrite
47700 ± 400
-4.9%nssb
8900 i 100
+42%
16100 ± 200
+16%
11400 ± 300
+18% nss
11830 ±70
-14% nss
4900 ± 100
-0.70% nss
5800 ± 500
+3 .6% nss
8270 ± 80
-7.3%
5000 ± 100
-0.98% nss
860 ± 40
+6.2% nss
1110 ±70
-38%
580 ± 40
-40%
4200 ± 400
-49%
4900 ± 300
-51%
cyanide
8500 ± 300
-83%
5720 ± 70
-51%
13900 ± 200
-9.0% nss
9770 ± 90
-0.85% nss
12410 ± 50
+0.94% nss
5260 ± 40
-9.7% nss
6060 ± 60
+8.3% nss
7900 ± 100
-11%
5140 ± 60
+1.4% nss
660 ± 20
-19%
1200 ± 100
733%
570 ± 60
-41%
N.D.C
-100%
N.D.
-100%
"Uncertainty is the estimated standard deviation. 'Relative difference between the peak area obtained wit!
and without the scavenger; nss = not statistically significant given the errors in the measurements. "TM.D. =
not detected.
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556 E. T. Urbansky et al.
TABLE I CONTINUED
methanoate
44000 ± 5000
-13%
5000 ± 1000
-16% nss
11000 ± 2000
-20% nss
7000 ± 1000
-30%
8000 ± 2000
-40%
3000 ± 1000
-40%
3000 ± 1000
-40%
7000 ± 1000
-20%
3000 ± 1000
-50%
200 ±300
-70%
1000 ± 400
-40%
600 ± 200
-40%
4300 ± 900
-50%
4000 ± 1000
-60%
indigodisulfonate
47300 ± 400
-6.0%
5900 ± 200
-6.2% nss
11000 ±200
-20%
8110 ± 60
-16%
9500 ± 100
-31%
3280 ± 50
-33%
3430 ± 60
-39%
7000 ± 200
-21%
2860 ± 90
-44%
520 ± 40
-36%
900 ± 100
-52%
440 ±60
-55%
7700 ± 400
-7.6% nss
7100 ± 400
-30%
tin(Il)
223,000 ± 1000
+340%
5900 ± 300
-5.9% nss
12450 ± 60
-9.8%
7600 ± 100
-21%
10150 ± 70
-26%
3700 ± 20
-25%
3770 ± 40
-33%
7600 ± 200
-15%
2830 ± 50
-44%
N.D.
-100%
420 t 30
-76%
N.D.
-100%
3000 ± 400
-64%
3500 ± 400
-65%
indigotrisulfonate
45000 ± 4000
-10%
6000 ± 200
-5.1% nss
10000 ± 1000
-27%
7200 ± 800
-25%
8500 ± 900
-38%
2900 ±400
-42%
3200 ± 300
-43%
6400 ± 400
" -28%
2600 ± 300
-48%
800 ± 100
-4% nss
1100 ±300
-38%
600 ± 400
-35% nss
6000 ± 1000
-20% nss
6000 ± 1000
-40%
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Interferences due to Ozone-Scavenging Reageats in GC-ECB Determination 557
pursuing any further.
Part 2. Investigation into Reactions of O3-Scavenging Reagents With
PFBOA
Combining solutions of PFBOA'HCI and nitrite or cyanide resulted » some loss of fl»
mnan A BST-MS signal and the formation of what has been tentatively lflenn.ne~j^._J:. ,
FFBOAESI-:
^^^^m^^^^^^^1^ is a general base catalyzed
phenomenon as well.
• 1 I. .Z. _
model analyte.
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558 E. T. Urbansky et al.
postulated as steps in the redox reactions of hydroxylamine and aqueous dihalogens (26-
28). We did observe the formation of bubbles when nitrite is combined withPFBOA. We
expect that these are composed of nitrous oxide formed subsequent to thenitrpsylation of
the PFBOA by nitrous acid. Thus, some of the reactions observed in this work are
probably similar to the reaction that occurs between other alkoxylamines and nitrous acid
(25). With the alkoxylamines, the net result is a nitrosylation of the amine moiety of the
alkoxylamine or formation of nitroxyl in the case of hydroxylamine (eventually leading to
hyponitrous acid and on to nitrous oxide) (25).
If any hydroxylamine is produced in our reactions, it appears to be totally consumed in
subsequent reactions that do not involve aldehydes or ketones since hydroxylamme-
derived aldoximes and ketoximes are not seen by GC-MS even when test solutions are
spiked with substantial amounts of pentanal or hexanal. We verified that we could in fact
see the mass spectra of the oximes of these aldehydes by combining hydroxylamine with
the aldehydes in aqueous solution and then extracting.
Alternative reductants, such as bisulfite are excluded due to reaction with the carbonyl
functionality (21,30-31) or other adverse impacts (32). Any nucleophilic species must be
considered not only in terms of potential for undesirable reaction with the electrophihc
analytes, but also with the derivatizing agent. Electrophilic effects must also be taken into
account, such as that suspected for methanoate.
Conclusion
Of the six reagents chosen as possible OSRs, not one was found to give a truly
satisfactory result Instead, all led to adverse impacts on the quantitation of aldehydes and
ketones. Consequently, how to quench residual ozone for kinetics studies of OBP
formation remains an unsolved problem. The ability to obtain good kinetic data is thus
restricted by the inability to eliminate ozone and thus fix speciation of OBPs at a point in
time. Perhaps it is possible to treat these effects as systematic error. As long as the
precision remains high and the error is truly determinate, said error can be corrected for
mathematically. We plan to explore whether such an approach is valid.
From our data and results, it is not possible to say what mechanisms are responsible for
interference by these reagents in the GC-ECD determination of aldehydes and ketones as
the pentafluorobenzyloximes; nevertheless, it is clear that interferences do in fact occur.
Any investigator planning to use an ozone scavenger must be cognizant of these potential
impacts on OPB analysis and must not assume that the effects will be negligible.
Furthermore, these results suggest that matrix effects may be more pronounced than
commonly believed (given that we expected all of the OSRs tested to behave as
spectators) and further underscores the need for careful determination of analyte recovery.
Acknowledgments
We recognize the assistance of the following individuals: H. Paul Ringhand, J.J. Yao, and
Catherine Kelty. In addition, we appreciate the complimentary sample of FD&C Blue No
1 provided by Wamer-Jenkinson Co., Inc. EPA's research apprenticeship program (RAP)
is recognized for its financial support by DF and JS; we note Andy Avel and Steve James
for their roles in administering the RAP.
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Interferences due to Ozone-Scavenging Reagents in GC-ECD Determination 559
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Interferences due to Ozone-Scavenging Reagents in GC-ECD Determination 561
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Key Words
Ozone; Pentafluorobenzyloxylamine; Pentafluorophenylmethyloxylamine; Pentafluorp-
benzylhydroxylamine; Pentafluorobenzyloxime; Disinfection Byproducts Analysis;
Ozonation Byproducts Analysis; Ozone Scavenger; Nitrite; Cyanide; Formate; Indigo
Carmine; Indigodisulfonate; Indigotrisulfonate; Aldehyde; Ketone;
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