the direct detection of liquid-phase products of electrocatalytic
reactions such as methanol oxidation, since, due to the
membrane-inlet system in conventional online DEMS, the latter
is limited to gaseous/volatile product analysis. In addition, since
there is little or no fragmentation of the molecules during the
soft ionization in ESI-MS, this could avoid the interference
between the mass fragments occurring in electron impact
ionization mass spectrometry.
On the other hand, a number of fundamental problems have
to be solved to enable the application of ESI-MS for online
monitoring the products of electrocatalytic reactions. This is the
topic of the present paper, where we describe the design,
operation, and performance of an ESI-MS setup for online
quantitative ESI-MS detection of the liquid-phase MOR products
formaldehyde and formic acid.
For this purpose, the following problems have to be solved.
First, electrospray ionization does not allow ionizing carbonyl
groups (aldehydes and ketones), since these functional groups
can hardly add or lose a proton. Therefore, the detection of
formaldehyde by ESI-MS requires an appropriate derivatization.
2,4-Dinitrophenyl hydrazine (2,4-DNPH) is commonly used for
derivatizing aldehydes to form a hydrazone, which provides a lone
electron pair at the nitrogen atom to add/lose a proton.24 The
online derivatization of analytes, however, is not widespread. The
difficulties mainly concern the compatibility of the derivatization
reagents and derivatized products with the mobile phase used
for separation. For instance, Herra´ez-Herna´ndez et al. reported
about the online derivatization into precolumns for the determi-
nation of drugs by liquid chromatography, using both a precolumn
and analytical column for the derivatization and separation of
analytes, which required electrically controlled switching valves.25
The latter can be avoided for the derivatization of formaldehyde,
if the derivatized product does not have to be separated from the
2,4-DNPH for detection. In the present work, we tested the online
derivatization reaction of 2,4-DNPH with the analyte (formalde-
hyde), both in 0.5 M sulfuric acid solution, in a first step.
Furthermore, severe complications arise from the high acidity
of the supporting aqueous electrolyte (sulfuric acid solution),
which not only affects the ionization probability of organic
molecules but also leads to severe corrosion of the instrument.
Therefore, sulfuric acid must be fully removed before the analyte
solution reaches the ESI-MS, while the aliquots of the molecules
of interest should still be present in the analyte solution. Appropri-
ate strategies need to be developed to achieve this goal. Here,
we present an approach for the online extraction of organic
molecules from a strongly acidic aqueous phase into an immiscible
organic phase, which after phase separation is piped to the ESI-
MS for quantitative analysis, while sulfuric acid solution remains
in the aqueous phase waste. To achieve this objective, it is
necessary to design an online extraction device, which can be
operated continuously.
Online extraction has attracted considerable attention for
various applications.26,27 In general, online extraction devices are
based on passing two or more phases through a capillary or
through microfluidic systems (laminar flow). Alternatively, online
extraction can be achieved via segmented flow of the immiscible
phases.28-30 In both cases, mass transfer between the phases
occurs via diffusion, and the devices allow a high overall
throughput. Recently, Kralj et al. reported an approach for
continuous liquid-liquid extraction, which was based on a
microfluidic device.31 The complex setup, which includes three
basic units, a mixer, an extractor, and a microporous membrane
liquid-liquid separator, was applied for quantitative analysis of
polar and chargeable compounds such as organic amines and
acids in different matrixes.32-34
In the present work, a flow system for online liquid-liquid
extraction was designed and constructed, which allowed for the
effective removal of sulfuric acid from the analyte. The overall
analytical procedure contained a sequence of mixing, derivatiza-
tion, extraction, separation, and detection processes. Due to the
low cross section of formaldehyde for ESI ionization, formaldehyde
was first derivatized by 2,4-DNPH. After that, online extraction of
formic acid and derivatized formaldehyde was performed in the
second module. Then, the organic and aqueous phases were
separated based on their specific weight, and only the organic
phase eluent was admitted to the ESI-MS for detection. Following
the mixing-reaction-extraction-separation sequence, quantita-
tive online detection of formic acid and formaldehyde in 0.5 M
sulfuric acid was achieved using ESI-MS. The method developed
was tested for the quantitative analysis of the MOR products by
ESI-MS.
EXPERIMENTAL SECTION
Equipment and Chemicals. For the mass spectrometric
measurements, we used an electrospray ionization mass spec-
trometer model 1200 L (Varian Inc.). Since both analyte molecules,
formic acid and the derivatized product of formaldehyde (2,4-
Dinitrophenyl hydrazone), have functional groups that readily lose
a proton, the negative ion ESI mode was used for the ionization.
This also avoids possible oxidation of analyte during the electro-
spray process in the positive ion mode. The detector voltage was
1 kV, and the needle voltage was -4.5 kV. A Rheodyne LC
switching six-port valve, located before the spray chamber of the
ESI-MS, was used for the manual injection of the analyte from
the sample loop (5 µL). The latter was filled with organic analyte
in the load mode; subsequently its content was injected into the
continuously flowing mobile phase (pure water) during the
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