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Determination of Perchlorate at Trace
Levels in Drinking Water by Ion-Pair
Extraction with Electrospray lonization
Mass Spectrometry
Matthew L. Magnuson, Edward T. Urbansky, and
Catherine A. Kelty
National Risk Management Research Laboratory, Water Supply and
Water Resources Division, Treatment Technology Evaluation Branch
United States Environmental Protection Agency, 26 West Martin
Luther King Drive, Cincinnati, Ohio 45268
JN4LYTIG1L
C H E M I S T R Y
Reprinted from
Volume 72, Number 1, Pages 25-29
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Anal. Cham. 2000, 72, 25-29
Determination of Perchlorate at Trace Levels in
Drinking Water by Ion-Pair Extraction with
Electrospray lonization Mass Spectrometry
Matthew L. Magnuson,* Edward T. Urbansky, and Catherine A. Kelty
National Risk Management Research Laboratory, Water Supply and Water Resources Division, Treatment Technology
Evaluation Branch, United States Environmental Protection Agency, 26 West Martin Luther King Drive,
Cincinnati, Ohio 45268
Perchlorate has been added to the U.S. Environmental
Protection Agency's Drinking Water Contaminant Candi-
date list (CCL). The present work describes the analysis
of perchlorate in water by liquid-liquid extraction fol-
lowed by flow injection electrospray mass spectrometry
(ESVMS). Cationic surfactants, mostly alkyltrimethyl-
ammonium salts, are used to ion-pair aqueous perchlo-
rate, forming extractable ion pairs. The cationic surfactant
associates with the perchlorate ion to form a complex
detectable by ESI/MS. The selectivity of the extraction and
the mass spectrometric detection increases confidence in
the identification of perchlorate. The method detection
ILtnitfor perchlorate based on 3.14
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interfering species at the m/z of the most abundant ion, m/z 99,
i.e., by hydrated bromide Br(H20)-. The mass of perchlorate is
also in the region of "chemical noise", the m/z range with
abundant naturally occurring low-mass ions. Observation of
interfering ions in electrospray mass spectrometry without re-
course to chromatography or another on-line separation technique
is common, and considerations for experimental protocol have
been discussed in some detail.10 Consequently, to increase the
selectivity for perchlorate, we recently investigated the detection
of perchlorate using selective associative complexes of perchlorate
with organic bases and other substances.11 By observation of a
complex at a mass > 300 units higher than that of perchlorate,
the classical chemical noise region was avoided. The complexation
increased selectively, did not significantly decrease sensitivity
(LOD ~ 10 /ig/L-1), and was relatively free of common spectro-
scopic interferences. The quanthation of species, via association
of the ions in the electrospray process, has also been reported
for other analytes such as Cr3*, which has been electrosprayed
as a negative chloro complex.12 Horlick and co-workers refer to
electrospray of the complex as the "intermediate" mode of
observation.13 Recently, in this laboratory, haloacetic acids were
analyzed by electrospray mass spectrometry in a similar fashion.14
The use of a complexing agent for perchlorate increases
selectivity11 but does not necessarily improve sensitivity. One
approach to increasing sensitivity for perchlorate determination
is to add an ion-pairing agent to the aqueous solution and extract
the ion pair with an organic solvent6-7 Conventionally, the ion pair
is formed with a good chromophore, such as a dye, and the
perchlorate may be determined spectrophotometrically.6-7 It
seemed reasonable to combine the enhanced selectivity of
complexation in ESI/MS with the enhanced sensitivity of solvent
extraction. In the present work, cationic surfactants (alkyltri-
methylammonium salts) were investigated for their dual role in
the formation of a solvent-extractable ion pair and an electro-
sprayable selective complex for mass spectrometric detection.
Contaminated water from southern Nevada was analyzed by ion-
pair extraction with ESI/MS detection, and the results compared
favorably with those from ion chromatography.
EXPERIMENTAL SECTION
Reagents. Brilliant cresyl blue [81029-05-2 (CAS Registry
number)], brilliant green [633-03-4], and crystal violet [548-62-9]
were obtained from Spectrum (New Brunswick, NJ). Octyltri-
methylammonium bromide (C8) [2083-68-3], decyltrimethyl-
ammonium bromide (CIO) [2082-84-0], dodecyltrimethylammo-
nium bromide (C12) [1119-94-4], tetradecyltrimethylammonium
bromide (C14) [1119-97-7], and tributylheptylammonium bromide
(THAB) [85169-31-9] were used as received from Fluka (Buchs,
Switzerland). The organic solvents were obtained from Fisher
(Fairlawn, NJ) and were of Optima or similar quality. Aqueous
perchlorate fortifications were made with ammonium perchlorate
(10) Sharp, B. L.; Sulaiman, A. B.; Taylor, K A.; Green, G. NJ. Anal. At Spectrom.
1997, 12, 603-609.
(11) Urbansky, E. T.; Magnuson, M. L.; Freeman, D.; Jelks, C. /. Anal. At.
Spectrom., in press.
(12) Gwizdala, A. B.; Johnson, S. K; Mollah, S.; Houk, R S./. Anal. At. Spectrom.
1997,12, 504-506.
(13) Agnes, G. R.; Horlick, G. Appl. Spectrosc. 1994, 48, 649-655.
(14) Magnuson, M. L.; Kelly, C. A; Jelks, C. In preparation.
26 Analytical Chemistry, Vol. 72, No. 1, January 1, 2000
Table 1. Summary of Experimental Conditions
acquisition mode
applied ESI spray potential (optimized)
interface capillary temperature
sheath gas pressure
injection mode/injection volume
carrier liquid
flow rate
extractive ion-pairing agent
(optimized)
concentration of ion-pairing agent
in solution
extraction solvent
preconcentration factor
reconstitution solvent
negative ESI/MS
4.0 kV
200 °C
70psi(480kPa)
flow injection/50.0 ftL
methanol/dichloromethane
(70/30 v/v)
0.3 mL/min
decyltrimethylammonium
bromide (CIO)
1.0 mM
dichloromethane
500/1
methanol/dichloromethane
(70/30 v/v)
[7790-98-9] (Aldrich, Milwaukee, WI). Dilutions were made with
water deionized through reverse osmosis.
Synthetic Tap Water. A soft synthetic tap water was prepared
by adding appropriate ACS reagent grade salts to deionized water.
The synthetic tap water, which represents an extreme tap water
in terms of ionic strength, was prepared to contain the following
anion concentrations: 5.6 mM (200 mg/L) chloride, 0.97 mM (60
mg/L) nitrate, 53 fM (1 mg/L) fluoride, 0.078 ftM (10 fig/L)
bromate, 0.12 /iM (10/ig/L) chlorate, 0.10 mM (10 mg/L) sulfate,
0.16 mM (10 mg/L) carbonate, 63 fM (5 mg/L) bromide. These
concentrations were selected to be greater than the average
concentrations found in many source waters.15 Sodium salts were
used, except for bromide and bromate, which were prepared from
potassium salts.
Apparatus. Injections were made with a Rheodyne (Rohnert
Park, CA) model 7725 injector having a 200 ftL loop. The pump
for the carrier liquid was a Waters 600 (Waters, Milford, MA).
The mass spectrometer was a Finnigan MATTSQ-700 (Finnigan,
San Jose, CA) equipped with a Finnigan electrospray interface.
Mass spectra were acquired in the negative-ion mode by scanning
Q3 over appropriate mass ranges. Other experimental parameters
are listed in Table 1.
Procedure. A volume of 500 mL of the aqueous sample,
cationic surfactant, and 100 mL of the extraction solvent were
shaken together vigorously. A separatory funnel was used to
collect the organic phase. The organic phase was then reduced
through rotary evaporation to dryness at 60 °C (bath temperature).
The residue was redissolved in 5-7 mL of dichloromethane, and
the mixture was transferred to a disposable test tube and
re-evaporated at 45-50 °C in a heater block The localized residue
was then reconstituted in 1.00 mL of the chosen solvent, and the
solution was transferred to a 1.8 mL glass vial. Injections of 50
/J.L of this solution were then analyzed by flow injection (FD-
ESI/MS.
RESULTS AND DISCUSSION
1. Extraction Conditions. The extraction solvent for these
experiments was dichloromethane. Methyl isobutyl ketone (MffiK),
ethyl acetate, and fert-butyl methyl ether were also investigated.'
(15) U.S. Environmental Protection Agency. Chemical Analysis of Interstate Carrier
Water Supply Systems; EPA Document No. 430/9-75-005; GPO: Washington,
JJL^, iy/5.
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6S
^
B.
O
>
16
14
12
10
8
CJ
s
u
Q
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100
80
I
• !•(
0>
60
20
C10-Bi*ClO«
380
C10-Br-Br
360
ClO-CICVCIO,
388
340
360
380
m/z
400
420
Figure 2. Mass spectrum of 100 /
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which is often seen in the electrospray of solutions with many
ionic species.16 Thus, the synthetic tap water, with its high ionic
content, would have a more suppressed signal. Careful use of the
separatory funnel did not alleviate this problem. A third possible
source of matrix effects is the species that are probably extracted,
e.g., sulfate, but are not detected by ESI/MS. Although not a
spectroscopic interference, their presence in the electrospray may
chemically interfere with the association and/or electrospray of
the perchlorate complex. Likewise, the presence of other detected
species, namely the chloride complex and the bromide complex,
may interfere with the association and/or electrospray of the
perchlorate complex. Whatever the cause of the matrix effect, the
slopes of the calibration plots (Figure 4) vary between waters, so
accurate determination of perchlorate must be made through the
use of standard additions. The high correlation coefficients of the
calibration plots suggest that standard addition should result in
accurate perchlorate determination.
For the plots in Figure 4, a blank was subtracted from the data.
The background is probably due to the presence of natural organic
matter (NOM) in the water. This material typically has a large
m/z distribution range but is present in very small quantities and
is not normally detected. Due to the large concentration factor
(500-fold) in this experiment, the NOM is concentrated and results
in a false positive of 0-2 /tg/L depending on the water. Since the
types and quantities of NOM vary with source water, it is
necessary to determine the blank for each source water. This
determination can be made on the basis of the following
considerations: The cationic surfactant is essential for the extrac-
tion of perchlorate. Because the cationic surfactant (ion-pairing
agent) is present below its critical micelle concentration,19 it was
assumed that the surfactant should not sufficiently affect the
extraction of the natural organic matter. Experimentally, the
background was determined to be the same with and without the
surfactant Therefore, when the extraction procedure is performed
(18) For the reduction, 5 g of zinc dust and 100 fiL of glacial acetic acid were
added to 500 mL of the water sample. The sample was allowed to react
12-24 h, and the zinc dust was then filtered off at 0.45 fM (cellulose
acetate). These quantities were optimized and mass spectrometrically
observed to eliminate the nitrate signal. Acetic acid performed better than
mineral acids at the same molarity.
(19) Mukcrjcc, P., Myscls, K. J., Eds. Critical Micelle Concentrations of Aqueous
Surfactant Systems; U.S. Department of Commerce, National Standard
RcTercnce Data System, National Bureau of Standards: Washington, DC,
1971; Vol. 36, passim.
without the cationic surfactant, the blank value for the water is
obtained (Figure 4).
To demonstrate the capabilities of ion-pair extraction with ESI/
MS detection for determining perchlorate in contaminated water,
drinking water was obtained from a source in southern Nevada.
The perchlorate concentration in this drinking water was deter-
mined using ion chromatography at the water utility to be 8-9
ftg/L. According to the procedure outlined above, the concentra-
tion of the perchlorate was determined to be 8.4 ± 0.2 fig/L (n =
3) by standard addition. The agreement between the two inde-
pendent techniques increases confidence in the results, namely
that the peak in the ion chromatograph is for perchlorate and not
for an interfering species. The FI-ESI/MS determination, made
at ~40 times the detection limit, shows higher precision than the
ion chromatography determination, made at 2-3 times the 1C
detection limit
CONCLUSION
A sensitive technique for the analysis of perchlorate is
demonstrated for measuring trace levels in a variety of water
matrixes. The use of an ion-pairing agent for extraction reduces
the spectroscopic interference for the nonchromatographically
separated sample. The results for a drinking water sample
obtained by FI-ESI/MS compare well to those obtained by another
technique, ion chromatography with conductivity detection. The
ion-pairing/extraction agent is selective for perchlorate among
common ions. The method detection limit is among the lowest
reported in the literature. Therefore, results from this technique
could be compared with those from ion chromatography, as well
as other emerging low-level techniques, for the purpose of
increasing the confidence in the accuracy of the reported per-
chlorate concentration.
ACKNOWLEDGMENT
The authors thank microbiologist Peggy Roefer of the Southern
Nevada Water Authority for supplying samples of raw and finished
waters used in this work.
Received for review August 12, 1999. Accepted October
21, 1999.
AC9909204
Analytical Chemistry, Vol. 72, No. 1, January 1, 2000 29
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