10 h for Fe-PbO2 electrodes (15). A quaternary metal oxide
electrode consisting of a SnO2 film containing Ru, Ti, and Sb
in unknown oxidation states (“Ru-Ti-Sb-SnO2”) required
24 h to exceed a 99% decrease in COD (16).
TABLE 1. Chromatographic Components and Operating
Conditions
Chromatography Components
The present work focuses on the determination of ionic
compounds in the electrolysis solution during the course of
ECI of 4-chlorophenol. This study uses two LC-ES-MS
methods to determine the components of a complex mixture
without the need for chemical derivatization. Reverse phase
chromatography is useful in the determination of aromatic
molecules that are not easily ionized, and ion exchange
chromatography separates both organic and inorganic ions.
Ion exchange chromatography is used because it offers good
sensitivity when coupled to ES-MS, and these combined
analytical technologies enable identification and quantitation
of nearly all ionic products generated during the ECI process.
Xiang et al. (17) also have successfully coupled ion exchange
chromatography with ES-MS.
pum p
injection valve
Dionex m odel GMP-2 analytical pum p
Rheodyne 7010 high-pressure injector
(Cotati, CA)
injection volum e
colum n
50 µL
Reverse Phase Chromatography
Zorbax SBC18 (25-cm length,
3.0-m m diam eter)
m obile phase
liquid flow rate
50% water, 50% m ethanol
0.5 m L m in-1
Ion Exchange Chromatography
Dionex IonPac AS11 (25-cm length,
4-m m diam eter)
colum n
m obile phase
liquid flow rate
suppressor colum n Dionex ASRS-11 (4-m m diam eter)
0.25-26.5 m M NaOH in 50% m ethanol
0.5 m L m in-1
regenerent
25 m M sulfuric acid
Experimental Section
regenerent flow rate 5.0 m L m in-1
Reagents and Sam ples. 4-Chlorophenol, benzoquinone,
hydroquinone, phenol, NaOH (carbonate free), NaCl, NaClO3,
NaClO4, and methanol (HPLC grade) were obtained from
Fisher Scientific. 4-Chlorocatechol was obtained from Oak-
wood Products (West Columbia, SC). All carboxylic acids were
reagent grade (Aldrich), and water was distilled and deionized
(18 MΩ cm-1 at 25 °C) with a Barnstead Nanopure-II system
(Newton, MA). The preparation of reagents and standards
has been described (16).
ECI Apparatus. The ECI setup and preparation of
quaternary metal oxide films have been described (16). In
this study, the film was deposited on a 22 gauge platinum
wire producing an anode with an area of 5.3 cm2 tightly coiled
around a stainless steel counter electrode. The electrodes
were separated by a Nafion membrane (0.25-mm thickness).
The electrolysis was conducted with a constant current of
0.95 A, resulting in a current density of 0.18 A/ cm2. The
cathode was 3.2-mm o.d. × 4.4-cm and was drilled with 120
holes (1-mm diameter). A cathode of this diameter permitted
the use of Nafion tubing with an inner diameter of 4 mm
(Perma Pure, Inc., Toms River, NJ). The volume of solution
electrolyzed in this apparatus was 30 mL.
Reverse Phase Liquid Chrom atography. Electrospray
ionization performs best with some organic solvent, such as
methanol, in the sample flow to facilitate droplet formation
and solvent evaporation. A Zorbax column (SBC18, Rockland
Technologies, Chadds Ford, PA, 25-cm length, 3.0-cm
diameter) was used for reverse phase LC. The Zorbax column
is operated with a 1:1 methanol/ water mobile phase at a
flow rate of 0.5 mL min-1. These eluent conditions match the
appropriate ES-MS conditions, using TurboIonSpray, so the
effluent of the reverse phase column can be fed directly into
the ES-MS. The simplicity of the reverse phase LC-ES-MS
setup and the excellent match of mobile phase composition
and flow rates gave reverse phase ES-MS good detection limits
and separation quality compared to other LC-ES-MS systems.
Ion Exchange Liquid Chrom atography. A Dionex IonPac
AS11 anion exchange column (Sunnyvale, CA) was used with
a water/ methanol solvent. This separation also required a
strong base such as 27 mM sodium hydroxide in the mobile
phase. A self-regenerating (4-mm diameter) suppressor
(Dionex) was used to remove sodium cations and replace
them with hydrogen ions before entering the ES-MS. The
ion exchange system also included an IonPac ATC-1 anion
trap (Dionex) and an IonPac AG11 guard column (Dionex).
LC-ES-MS used with ion exchange chromatography can
accommodate an eluent gradient, and the following program
was used. Mobile phase A contained 100% water, mobile
phase B was made up of 1 mM sodium hydroxide, mobile
phase C was 100 mM sodium hydroxide, and mobile phase
D contained 100% methanol. The gradient was held at 25%
A, 25% B, and 50% D from t ) 0-2 min, progressed linearly
to 10% A, 40% B, and 50% D at t ) 5, progressed linearly to
10% A, 15% B, 25% C, and 50% D at 15 min, and was held
at that composition until the end of the separation. Note
that the percentage of methanol remained constant through-
out the gradient program. A small change in the concentration
of methanol would have had a large effect on the ES-MS
signal.
ES-MS. An API/ 1 (Perkin-Elmer SCIEX, Thornhill, ON,
Canada) single quadrupole mass spectrometer was used. The
API/ 1 used a curtain gas interface and has been described
previously (18, 19). The IonSpray source of the API/ 1 was
operated with a Perkin-Elmer SCIEX TurboIonSpray (Thorn-
hill, ON, Canada) attachment. The TurboIonSpray forces a
flow of nitrogen gas (5 L min-1, 500 °C) across the aerosol
stream exiting the IonSpray tube, increasing the collision
and evaporation rates involved in the ionization process and
allowing the use of larger liquid flow rates (1-2 mL min-1
)
into the ES-MS. The detection limits and signal-to-noise ratio
are improved with TurboIonSpray for many compounds
when compared to similar studies in this laboratory before
the addition of the TurboIonSpray (20). The attachment also
provides much more flexibility in chromatographic flow rates
and eluent compositions.
Other ES-MS conditions have been described (20). Horlick
reported that it was necessary to compensate for the variation
of electrospray signal with the total ionic composition of the
sample by using an internal standard ion (21). In the present
study, chloroacrylate and propionate were used as internal
standards for quantitative measurements. In the study
reported here, linear calibration curves with correlation
Experimental conditions for the liquid chromatography
methods are shown in Table 1. The Dionex Model GPM-2
analytical pump was used for the mobile phase of each
separation, and the Model AMP-1 analytical pump was used
to regenerate the suppressor when using ion- exchange
chromatography. A Cole-Parmer syringe pump (Nile, IL) was
used for direct infusion analysis.
2
coefficients (r ) of 0.980-0.999 were compiled for analyte
concentrations in the range of 0.1-50 ppm. Detection limits
were determined as the concentration of analyte required to
provide a net signal equivalent to three times the standard
deviation of the blank.
ICP-MS. ICP-MS measurements were taken using the
apparatus, operating conditions, and semiquantitative cali-
bration method described by Hu et al. (22) and Houk et al.
(16).
9
VOL. 33, NO. 15, 1999 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 2 6 3 9