Ascorbic acid reduction of residual active chlorine in potable water
prior to halocarboxylate determination!
Edward T. Urbansky,* David M. Freeman} and F. Javier Rubio§
United States Environmental Protection Agency, Office of Research and Development,
National Risk Management Research Laboratory, Water Supply and Water Resources
Division, 26 West Martin Luther King Drive, Cincinnati, OH 45268, USA.
E-mail: Urbansky.Edward@EPA.gov; Fax: +1 513 569 7658; Tel: +1 513 569 7655
Received 7th February 2000, Accepted 23rd March 2000
Published on the Web 9th May 2000
In studies on the formation of disinfection byproducts (DBFs), it is necessary to scavenge residual active
(oxidizing) chlorine in order to fix the chlorination byproducts (such as haloethanoates) at a point in time.
Such research projects often have distinct needs from requirements for regulatory compliance monitoring. Thus,
methods designed for compliance monitoring are not always directly applicable, but must be adapted. This
research describes an adaptation of EPA Method 552 in which ascorbic acid treatment is shown to be a
satisfactory means for reducing residual oxidizing chlorine, Le., HOC1, CIO", and C12, prior to determining
concentrations of halocarboxylates. Ascorbic acid rapidly reduces oxidizing chlorine compounds, and it has the
advantage of producing inorganic halides and dehydroascorbic acid as opposed to halogenated organic
molecules as byproducts. In deionized water and a sample of chlorinated tap water, systematic biases relative to
strict adherence to Method 552 were precise and could be corrected for using similarly treated standards and
analyte-fortified (spiked) samples. This was demonstrated for the quantitation of chloroethanoate,
bromoethanoate, 2,2-dichloropropanoate (dalapon), trichloroethanoate, bromochloroethanoate, and
bromodichloroethanoate when extracted, as the acids, into tert-butyl methyl ether (MTBE) and esterified with
diazomethane prior to gas chromatography with electron capture detection (GC-ECD). Recoveries for
chloroethanoate, bromoethanoate, dalapon, dichloroethanoate, trichloroethanoate, bromochloroethanoate,
bromodichloroethanoate, dibromoethanoate, and 2-bromopropanoate at concentrations near the lower limit of
detection were acceptable. Ascorbic acid reduction appears to be the best option presently available when there
is a need to quench residual oxidants fast in a DBF formation study without generating other halospecies but
must be implemented cautiously to ensure no untoward interactions in the matrix.
Aim of investigation
Issues surrounding the decision to reduce residual oxidizing
disinfectants (Le., C12, HOC1, and NaOCl) have been discussed
previously as related to the analysis of halogenated disinfection
byproducts.1 The desirable characteristics of dechlorinating
agents and the relevant chemical concerns have also been
covered, including deficiencies of some reagents used in this
manner.1 In this report, we evaluate ascorbic acid as a
dechlorinating agent in the determination of haloacetic acids
containing bromine and chlorine. Previous work suggested that
ascorbic acid (HAsc) might prove suitable for the haloacetic
acids.1'2
EPA's Environmental Monitoring Systems Laboratory
(EMSL) established Method 552 for the quantitation of
halocarboxylates (often referred to as the haloacetic acids or
HAAs) in 1990.3 This version used diazomethane (CH2N2) to
esterify the extracted carboxylic acids. In 1995, concerns over
diazomethane led EPA's National Exposure Research Labora-
tory (NERL, previously EMSL) to change the esterification
reagent to acidic methanol.4 However, many laboratories,
including our own, have continued to safely use diazomethane
tThis paper is the work product of United States government
employees engaged in their official duties. As such, it is in the public
domain and exempt from copyright restrictions. © US government.
^Funding for this position was provided by EPA's Research
Apprenticeship Program.
§On leave from Departamento de Ingenieria Quimica y Energetica,
Universidad de Extremadura, Avda. de Elvas s/n, 06071, Badajoz,
Spain. E-mail: fjrubio@unex.es; Tel. 11 34 924 289385.
for methylation. In addition to changing the esterification,
Revision 1.0 prescribes ammonium chloride to convert
chlorine-based oxidants to chloramine.4 EPA's NERL demon-
strated that ammonium ion is satisfactory within a specified
limit of error and under a variety of drinking water conditions,
using multiple laboratory testing. Nonetheless, the search for
chlorine-oxidant scavengers that are generally useful for a wide
variety of analytes continues. Following from previous work,1
ascorbic acid was evaluated as a dechlorinating reagent prior
to the quantitation of certain halocarboxylates in aqueous
solution with particular interest in applying it to drinking water
analysis.
Experimental section^
Part I. Effects at high analyte concentrations OSOngmL"1)
Standards and test solutions. Halocarboxylic acid standards
in tert-butyl methyl ether were obtained from Supelco
(Bellefonte, PA, USA). Aliquots of the commercial solution
were injected via gaslight syringes into 20.0 mL portions of
doubly deionized water in 40 mL EPA vials (polypropylene
screw caps and PTFE septa) to produce standard test solutions.
Twelve solutions at each concentration were prepared by
injecting 0,10,20, or 50 uL of the stock standard. This gave the
following concentrations: for chloroethanoic and dichloroetha-
noic acids, 0, 150, 300, TSOngmL"1; for bromoethanoic,
^Mention of specific brand names, manufacturers, or models
throughout this paper should not be construed to reflect endorsement
of firms or products by the United States government.
DOI: 10.1039/b001046o
7. Environ. MoniL, 2000, 2, 253-256
This journal is © The Royal Society of Chemistry 2000
253
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2,2-dichloropropanoic, bromochloroethanoic, and bromodi-
chlorocthanoic acids, 0, 100, 200, 500 ngmL"1; and for
trichloroethanoic, dibromoethanoic, and 2-bromopropanoic
acids, 0, 50, 100, 250 ngmL"1.
These were acidified with 1.0 mL of 0.25 M sulfuric acid to
minimize hydrolysis. Analyte concentrations for Part I are
about 100 times greater than what would be found in most
chlorinated potable water supplies. The concentrations were
intentionally elevated with the aim of magnifying any adverse
impacts as the concentrations of both the ascorbic acid and the
analytes are very large relative to the lower limits of detection.
A slow interaction, for example, might not be distinguishable
from the normal imprecision of the method at low concentra-
tions, but would be made more apparent if the concentrations
were increased.
Ascorbic add addition. Six of the standards at each
concentration were reserved for the addition of ascorbic
acid. A 2.00 mL aliquot of I.I M L-ascorbic acid solution was
pipetted into each test standard, producing a post-mixing
concentration of 0.10 M. This final concentration was chosen to
ensure excess HAsc relative to typical residual chlorine
concentrations found in tap water and to make any interactions
more apparent.
Sample treatment. Half of each set (3 with HAsc and 3
without) of standards was analyzed immediately. The other
half (3 with HAsc and 3 without) was retained for 16 d at
7±2"C. After addition of 2.0 mL of 98% w/w H2SO4, each
standard was extracted with 4.0 mL of pesticide residue
analysis grade rert-butyl methyl ether; dropwise addition of
0,25% w/w FD&C Blue No. 1 (Erioglaucine [3844-45-9]
CI 42090) aqueous solution improved the visibility of the
phase boundary. Extracts were drawn off from the aqueous
phase and dried with anhydrous environmental analysis grade
sodium sulfate. The carboxylic acid moieties were esterified
with diazomethane using a method described elsewhere,5 then
transferred to autosampler vials and stored in a freezer at
-80 °C pending GC-ECD analysis.
GC-ECD analysis. Extracts were analyzed on a Hewlett-
Packard (Palo Alto, CA, USA) 6890 GC-ECD system
equipped with an HP 7673 autoinjector. Volumes of
2.5-5.0 uL (splitless) were injected onto a J&W Scientific
(Folsom, CA, USA) DB-5MS column (30mx250um
id x 0.25 urn film) at constant (high purity) helium flow of
1.0 mL rain"1; inlet and detector temperatures: 270 °C.
Temperature program: hold 35 °C for 10.0 min; ramp
5.0°Cmin~' to 75"C, hold for 15.0min; ramp 5.0°Cmin"'
to 100 °C, hold for 5.0 min; ramp 5.0 "C min"1 to 135 °C; hold
for 2.0 min. Analytes, chemical formulae, and related informa-
tion are given in Table 1.
Recovery from fortified tap water. Cincinnati tap water
(source: Ohio River) was collected from a laboratory faucet
and represents a typical chlorinated municipal water supply.
After 10 min of high flow to flush the pipes, water was collected
in glass vessels and used within 1 h of collection. Ten replicates
were fortified with 20.0 uL of the HAA standard and 1.00 mL
of 0.25 M sulfuric acid. Five of each were treated with 2.0 mL of
1.1 M ascorbic acid, the same as the standards. After capping
and mixing, samples were extracted and analyzed as described
above. Unspiked samples were used to determine the back-
ground levels of chlorination byproduct analytes.
Determination of residual active chlorine in tap water.
Chlorine was determined titrimetrically with ferrous ammo-
nium sulfate after reaction with 7V,A^-diethyl-/j-phenylenedia-
mine (DPD). Commercial DPD #1 Powder reagent mix
(LaMotte Co., Chestertown, MD, USA) was used. The
method was essentially that in Standard Methods.6 Reduction
of residual oxidant was demonstrated by the absence of color
when DPD reagent was added to tap water pretreated with
HAsc.
Part H. Effects at low analyte concentrations (2—10 ng mL"1)
Standards and test solutions. Halocarboxylic acid standards
in /ert-butyl methyl ether were obtained from Supelco.
Aliquots of the commercial solution were injected via gastight
syringes into 30.0 mL portions of Cincinnati tap water in
40 mL EPA vials to produce standard test solutions. Three
vials with 0.10 M HAsc (made by direct addition of the solid
reagent) and three vials without were prepared at the analyte
concentrations shown in Table 2. These concentrations reflect
those likely to be encountered in chlorinated potable water
supplies. The samples were analyzed immediately; therefore, no
additional acid was added for preservation.
Sample treatment and analysis. Other than the ascorbic acid
used to reduce the oxidizing chlorine, EPA Method 552
(microextraction)4 was used for these samples. The methylated
extracts were analyzed on a Tremetrics 540 GC (Austin, TX,
USA) with a J&W DB-1701 column (30 mx 322 urn
id x 0.25 jim film) at constant He pressure of 101 kPa
(ISpsi). Injections of 2.0 uL were used. The temperature
program was that of Method 552.
Results and discussion
Part I. Effects at high analyte concentrations (30-100 ng mL"1)
Data treatment. Chromatographic peak areas were plotted
against volume of standard used (uL) in 20 mL of water; see
Table 1 for further explanation. Each line represents 3
replicates at each of 3 concentrations for each analyte.
Table 1 Halocarboxylate analytes examined in this work
Analyte onion
Formula of acid
CAS RN"
(,/min
Concentration/ug mL~l
Chlorocthanoate
Bromoethanoate
2,2-Dichloropropanoate''
Dichloroethanoate
Tricbloroethanoate
Bromochlorocthanoate
Bromodichloroethanoate
Dibromocthanoate
2-Bromopropanoate
ClCHoCOiH
BrCH,CO-,H
CH3CC12C02H
C12CHCO2H
C13CCO,H
BrClCHCOH
BrCl,CCO,H
Br,CHCO2H
CH3CBrHCO2H
[79-11-8]
179-08-3]
[75-99-0]
[79-43-6]
[76-03-9]
[55589-96-3]
[7113-314-7]
[631-64-1]
[598-72-1]
5.2
8.4
8.8
9.2
12.8
14.8
17.4
17.9
18.3
300'
200
200
300
100
200
200
100
100
"Registry numbers are for the acids, not the anions. 'Retention times are for the methyl esters and were established using a methyl ester blend
standard purchased from Supelco and refer to the DB-1701 column. 'Concentrations refer to the concentration of each acid in the standard
purchased from Supelco; thus, a lOuL aliquot of standard diluted into 20 mL of water gives a chloroethanoate concentration of 150 ngmL"1
(ppb), etc. ^Parent acid is also known as dalapon.
254 J. Environ. Monit., 2000, 2, 253-256
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Table 2 Difference in analyte recovery near the detection limit for samples dechlorinated with ascorbic acid relative to untreated samples"
Analyte Concentration/ng mL'1 Relative recovery (%) Concentration/ngmL"1
Relative recovery (%)
Chloroethanoate
Bromoethanoate
2,2-Dichloropropanoate
Dichloroethanoate
Trichloroethanoate
Bromochloroetlianoate
Bromodichloroethanoate
Dibromoethanoate
2-Bromopropanoate
5.0
3.3
3.3
5.0
1.7
3.3
3.3
1.7
1.7
-2±10
-17+4
+ 8±4
+ 1+3
-16±4
+ 19±5
-20+20
+ I4±6
+ 6±2
10
6.6
6.6
10
3.3
6.6
6.6
3.3
3.3 -
— 5+fi
JX o
— 11+4
1 1 3-*t
+ 3 + 2
+ 3±2
+ 10±5
+ 10+5
-S0±10
+5±5
-9±2
Differences were calculated from the average of three fortified test solutions of Cincinnati tap water with HAsc and three without. Relative
difference=(^w/HA5e-/4w/?)//4w;0, expressed as a percent. Uncertainty was propagated from the estimated standard deviations of the means No
data were rejected as outljers. Other than the use of ascorbic acid, Method 552 (microextraction) was strictly followed.
Slopes and y-intercepts were determined by unweighted least
squares linear regression; the ^-intercepts were either statisti-
cally indistinct from zero based on the standard errors or less
than 10% different from their standard errors. Propagated
error was computed in the standard way.7
Initial effects. The effect of ascorbic acid on method
performance is examined by computing the relative difference
in the slopes of calibration curves made in deionized water with
and without ascorbic acid treatment: (fn^^^^-m^lm^i,,,
where mv/0 is the slope without ascorbic acid and TOW/HA*; is the
slope with ascorbic acid. Relative differences are shown in
Table 3. Comparison of the slopes on day zero shows that
ascorbic acid has minimal, but quantitatable impact directly on
the analytical procedure in deionized water. Although the
difference for 2,2-dichloropropanoate is of debatable meaning,
bromochloroethanoate and bromodichloroethanoate are
demonstrably affected, with signal reductions of 11% and
17%, respectively. It is unclear whether this is due to a loss of
sensitivity or a loss of the analytes themselves. Regardless, the
standard errors of the least squares parameters and the
regression coefficients indicate that the effect is quantitatable
and precise; therefore, it should represent a correctable bias in
this instance. For bromochloroethanoate with HAsc:
m=383±16; J?2=0.982; for bromodichloroethanoate with
HAsc: m=258±13; .R2=0.976; ^-intercepts were statistically
indistinguishable from zero based on their standard errors.
Effects of extended exposure to HAsc. After 16 d of exposure
to ascorbic acid, all analytes showed a signal reduction relative
to samples stored without ascorbic acid except for trichloro-
ethanoate, which showed a gain of questionable significance,
5 ±4% (see Table 3). The greatest loss that was distinguishable
from the propagated error was 6% for chloroethanoate.
Consequently, we conclude that this reagent could be readily
used for reducing residual chlorine, provided that suitable
Table 3 Relative differences in slopes of unweighted least squares
calibration lines showing the effect of ascorbic acid treatment"
DayO*
Day
Chloroethanoate
Bromoethanoate
2,2-Dichloropropanoate
Trichloroethanoate
Bromochloroethanoate
Bromodichloroethanoate
+ 0.2 ±2%
+4±4
-4±4
-3±7
-11±4
-17 + 5
-6 ±2%
-3±2
-4±3
+ 5±4
-8±8
-6±9
"Calibration lines were plots of peak area vs. volume of standard;
each line was determined from duplicate test solutions prepared at
five concentrations by diluting specific volumes of standard into
20.0 mL of water: 0 (blank), 10, 20, 50, 100 uL. 'Relative differen-
ce=(mWrtiAsc—mw/0)/mw/0, expressed as a percent; uncertainty was
propagated from the standard errors of the least squares slopes.
"Sets with and without HAsc were stored for 16 d in a cold room at
7±2°C.
standards are prepared concurrently with sampling. It is not
possible to state whether the effects on bromochloroethanoate
and bromodichloroethanoate that are suggested by the day
zero data are (1) mediated by other matrix elements over time,
(2) rapidly brought about so that no additional effects are
observable over 16 d, or (3) masked by the uncertainties of the
experimental measurement. Any of these or some combination
thereof is possible.
Recovery from fortified tap water. The concentration of
residual active chlorine, [Cl2]+[HOCl]+[CIO~], in the Cin-
cinnati tap water samples found by DPD Fe" titrimetry was
13 UM=890 ug LT1 C12. Blank correction was made by deter-
mining background levels of chlorination byproduct analytes
in unspiked tap water samples and these were subtracted prior
to comparison of HAsc-treated and untreated samples. Fig. 1
shows the results for the HAAs. Analyte response in fortified
tap water is actually increased a few percent after ascorbic acid
treatment; however, this is certainly within acceptable limits.
We note that EPA Method 552 allows for up to 20% variation
in analyte recoveries for quality control test samples in
deionized water, and up to 30% variation in analyte recoveries
for fortified (spiked) tap water samples.
bromodichloro
bromochloro
trichloro
2.2-dichloro-
prapanoate
chloro
C
1
^^
utetaa.
UAUW.
ffff/ffft
w
££6Ut£t
Dlwat
Lapwat
y
ST
5 10 15 20 25 3C
area/1000
Fig. 1 Relative recoveries (average chromatographic peak areas) for
several haloethanoates and 2,2-dichloropropanoate in deionized (DI)
water and Cincinnati tap water. Areas are based on a concentration of
300ngmL~' relative to chloroethanoate (see Table! to determine
other analytes) and are blank-corrected. Recovery in tap water falls
within the permissible 30% window given in Method 552 relative to DI
water when not treated with ascorbic acid (black bars). Hatched bars
show effect of treatment with 0.10 M ascorbic acid. In general, ascorbic
acid lowers the recovery in both DI water and tap water; however,
bromochloro- and bromodichloroethanoate showed increased recovery
in tap water. Although the allowable variation in recovery is 20% in DI
water for Method 552, we infer that this concentration of ascorbic acid
works well enough to allow quantitation, but probably influences
conditions enough to warrant careful consideration before it is used in
the matrix under study.
/. Environ. Monit., 2000, 2, 253-256 255
-------
Based on the results obtained under our conditions, we
found that ascorbic acid was suitable for the reduction of
residual oxidizing chlorine, even when used in very large excess.
We speculate that a more conservative addition of this reagent
might result in even smaller effects. Because the data show
precise biases, the effects we observed could be corrected for by
the appropriate treatment of standards and spiked samples.
Part n. Effects at low analyte concentrations (2-10 ng mL"1)
In order to determine the effect of ascorbic acid near the lower
limit of detection, relative differences were computed using the
average peak areas for the three treated and three untreated test
solutions made by spiking Cincinnati tap water. Table 2 shows
that there is a reproducible and quantitatable effect for all but
the most highly oxidized species, specifically, bromodichloro-
ethanoate and trichloroethanoate. Bromodichloroethanoate
quantitation is intrisically imprecise and this effect is somewhat
magnified by the ascorbic acid, especially near the detection
limit. At 6.6 ng mLT1, the mean peak area for three replicates
demonstrated a 16% RSD without HAsc, but the imprecision
grew to 42% RSD for the HAsc-treated samples. Such effects
were previously observed1 for other analytes with a highly
oxidized carbon atom (especially a trihalocarbon).
Bromochloroethanoate experiences a rather curious gain in
response upon addition of ascorbic acid. As with the
trihalocarbon effect, this has also been observed1 for certain
analytes. The mechanisms through which these effects occur
remain unstudied. Based on these and previous results,1 it is
reasonable to speculate that ascorbic acid interacts with
trihalogenated carbon atoms. The magnitude of the effect
appears to vary with analyte concentration. This variability is
especially noticeable for bromodichloroethanoate. Caution is
therefore warranted when using ascorbic acid to dechlorinate
water samples prior to the quantitative determination of
chlorination byproducts containing trihalo (haloformyl)
carbon atoms. In a DBF formation study, it would be
necessary to carefully assess the side effects of ascorbic acid
(used as a chlorine reductant) within a particular matrix. In
routine monitoring of drinking water, ascorbic acid may be
suboptimal for bromodichloroethanoate, but it has minimal or
no effects on the monohalogenated species. It is worth pointing
out, however, that Method 552 was never validated for
bromodichloroethanoate, which we included in this study.
Furthermore, we have found that methylation of brominated
trihaloethanoates with diazomethane suffers from complicat-
ing side reactions;8 this implies that ascorbic acid treatment
should be verified separately using acidic methanol (Method
552.2) for the brominated trihaloethanoates. The reaction
chemistry of the dihalo- and trihalocarboxylates under
drinking water conditions is not well-developed. Consequently,
it is difficult to predict what conditions will promote
dccarboxylation, hydrolysis, reduction, or substitution and
thereby complicate the analytical chemistry. Under the best
conditions, the quantitation of these species is challenging;
thus, careful attention must be paid to avoid introducing
additional sources of error.
Concluding remarks
Our results suggest that, under some conditions, ascorbic acid
can be used to reduce oxidizing chlorine compounds in order to
fix the concentration of chlorination byproducts in tune. Such
quenching is particularly useful in studies on the formation of
chlorination byproducts where very substantial concentrations
of oxidizing disinfectants are likely to be present. In these cases,
the ammonium ion used in Method 552.2 would be inadequate
as it reacts stoichiometrically with hypochlorite to give
chloramine (except under breakpoint conditions). If samples
are to be drawn as a function of time, it is clearly necessary to
prevent subsequent chlorination of carbonaceous material.
Obviously, any reductant chosen must not adversely interfere
in the analysis. These preliminary experiments showed that
ascorbic acid met the criteria1 for the following analytes:
chloroethanoatej bromoethanoate,|| 2,2-dichloropropanoate,||
dichloroethanoate,|| trichloroethanoate,|| bromochloroethano-
ate,|| dibromoethanoate,|| and 2-bromopropanoate in deionized
water and a local chlorinated potable water supply. None-
theless, care must be taken to evaluate any method modifica-
tion in a particular matrix, as we pointed out previously when
we used ascorbic acid in the Ames assay.9 In light of the other
reductants that we have considered, ascorbic acid has been the
most successful in meeting both the bioanalytical and
chemoanalytical needs of disinfection byproduct formation
studies. As always, there is a balance to be struck between the
objectives of a particular study and the requisite levels of
accuracy and precision in analytical measurements. We believe
that judicious use of ascorbic acid can serve an important role
in drinking water analytical chemistry and warrants further
consideration in this capacity.
Acknowledgements
We thank Warner-Jenkinson Co., Inc. (St. Louis, MO), for
providing a complimentary sample of FD&C Blue No. 1. We
mention key staff, without whom, the Research Apprenticeship
Program would not be possible: Andy Avel, Steve James,
Milton Wiggins, Randy Parker, Tiffaney Livingston (EPA) and
Jim Wade (University of Cincinnati). FJR acknowledges a
grant from the Junta de Extremadura.
References
1 E. T. Urbansky, /. Environ. Monti., 1999, 1, 471, and references
therein.
2 Y. Takahashi and M. Morita, Kankyo Kagaku, 1997, 7, 495.
3 E. T. Urbansky and W. J. Bashe, J. Chromatogr. A, 2000, 867, 143.
4 J. W. Hodgeson, J. Collins and R. E. Barth, Method 552.
Determination of Haloacetic Acids and Dalapon in Drinking Water
by Liquid-Liquid Extraction, Derivatization, and Gas Chromato-
graphy with Electron Capture Detection, US Environmental
Protection Agency, Cincinnati, OH, July 1990.
5 J. W. Hodgeson, D. J. Munch, J. W. Munch and A. M. Pawlecki,
Method 552.2. Determination of Haloacetic Acids and Dalapon in
Drinking Water by Liquid-Liquid Extraction, Derivatization, and
Gas Chromatography with Electron Capture Detection, Rev. 1.0, in
Methods for the Determination of Organic Compounds in Drinking
Water, Supplement III, US Environmental Protection Agency,
Cincinnati, OH, August 1995, EPA/600/R-95/131.
6 Standard Methods for the Examination of Water and Wastewater,
American Public Health Association/American Water Works
Association/Water Environment Federation, Baltimore, MD,
20th edn., 1998, Method 4500-C1:G, pp. 4.63^.64.
7 P. R. Bevington and D. K. Robinson, Data Reduction and Error
Analysis for the Physical Sciences, WCB/McGraw-Hill, Boston, 2nd
edn., 1992, p. 43.
8 F. J. Rubio, E. T. Urbansky and M. L. Magnuson, J. Environ.
Monit., 2000, 2 (DOI: 10.1039/b000674m).
9 E. T. Urbansky and K. M. Schenck, /. Environ. Monit., 2000, 2,
161.
UThesc analytes are often collectively referred to as HAA6 (six
haloacetic acids included in EPA's Information Collection Rule).
256 J. Environ. Monit., 2000, 2, 253-256
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