Valve switching applications are becoming popular in ion chromatography (IC). Example applications include matrix
diversion or matrix elimination prior to analysis of trace components.
A conventional ion chromatography system is usually
equipped with a single injection valve, and installing additional
valves for various applications such as two-dimensional
(2-D) separations and sample preparation applications is cumbersome.
The instrument detailed here has an integrated
automation management module supporting various valve
configurations and a reaction coil heater for postcolumn reaction
applications. Here, the authors will discuss the utility of
the system for various valve switching applications.
First, the instrumentation details of the system and the
automation module will be discussed. Next examples of
applications will be shown, including 2-D analyses of perchlorate
in high-ionic-strength matrices and a matrix neutralization
approach showing analysis of trace anions in concentrated
base. With the 2-D analysis method in
conjunction with suppressed conductivity detection, it is
possible to detect low levels of perchlorate without the use of
an expensive mass spectrometer.
System description
Figure 1 - The automation manager (AM) in the ICS-3000 detector/chromatography compartment (DC) is highlighted above. Mounted
as a sliding tray for easy plumbing, the AM is versatile enough to handle
difficult applications by supporting a variety of valve configurations.
The ICS-3000 Reagent-Free™ IC (RFIC™) system (Dionex Corp., Sunnyvale, CA) combines traditional
instrument improvements for signal enhancement with
valving and control for easy implementation of complex
applications. The system consists of a pump (single or dual,
SP or DP, respectively), eluent generator (EG), detector/chromatography compartment (DC), automation
manager (AM), autosampler (AS), and tablet PC. The system
can be configured as a single or dual system and incorporates
the latest advancements in RFIC system technology
(Figure 1). When configured as a single system, the additional
pump from the DP can be used for miscellaneous
applications such as pumping a deionized (DI) water stream
for sample preparation applications.
From ordinary DI water, RFIC systems generate high-purity
eluents that perform superior separations, and
then neutralize the eluent back to DI water in the suppression
step to facilitate sensitive detection using a conductivity detector. Using the EG with the continuously
regenerated-trap column (CR-TC) minimizes baseline
shift during the run, when pursuing eluent concentration
step changes or gradients.
The DC compartment consists of three sections for separation,
detection, and automation. The separator columns are
housed in the DC in a controlled thermal environment.
Capable of single or dual analysis, the DC houses conductivity
and electrochemical detectors. Two detector cells are
available for “plug-and-play” conductivity and electrochemical
operation. The DC also contains two valves in the
lower compartment for injection and sample preparation applications. All valves are on slide-out trays for easy installation and troubleshooting. An important feature within
the DC is the AM, a slide-out tray that is configurable with
up to two high-pressure valves, two low-pressure valves, and
a postcolumn reaction heater. The AM facilitates valve
switching applications.
The AM is mounted inside the DC compartment so that
sample preparation steps such as preconcentration and
matrix removal are in the near vicinity of the separator
columns. In conventional IC systems, mounting the sample
preparation valves close to the separator column is difficult
and often leads to excessive band dispersion. Due to the
close proximity of the AM to the separator columns, band
dispersion issues are minimized.
Two-dimensional perchlorate analysis
Although there are no federal drinking water regulations
for perchlorate, various states have adopted their own
advisory levels that range from 1 to 18 ppb. Trace-level
analysis in IC is typically done using concentrator
columns or large- volume injections. When high levels of
matrix ions are present, large-volume injections are preferred
over concentration methods since the matrix ions
can elute off the analyte of interest from the concentrator
column. It should be noted that large-volume injections
not only enhance the sensitivity of trace components, but
also enhance the sensitivity of matrix components. In
some cases, the matrix components interfere with the
analysis by coeluting or eluting the trace component into
a broad peak, leading to poor detection. This is the case
with perchlorate analysis.
Perchlorate in drinking water is typically analyzed in the ppb
range, whereas the matrix concentrations are at the hundreds-of-ppm level. U.S. EPA Method 314.0 was a direct injection
method but required an off-line matrix elimination step with
solid-phase extraction cartridges for samples containing high
levels of matrix ions. U.S. EPA Method 314.1 was published
as an update and allowed improved detection of perchlorate
in high-ionic-strength matrices. Alternatively, an automated
2-D heart-cutting method can be used as an in-line approach.
The ICS-3000 system was configured as a two-channel dual
system for this application. The injection valve in the first
dimension was fitted with a large-volume sample loop (4
mL). The injection valve in the second dimension was fitted
with a concentrator column that focused the heart-cut analyte
peak from the first dimension for further analysis in the
second dimension.
Figure 2 - Diagram highlighting components and configuration for
2-D IC in the ICS-3000.
The schematic for matrix removal and signal enhancement
is shown in Figure 2. There are several advantages of the 2-
D matrix diversion approach. Initial sample loading onto
the 4-mm column allows a large sample injection volume
(large amount of sample) due to the high capacity of the
analytical column and higher selectivity for perchlorate
relative to the matrix ions. Second, it is possible to focus
the perchlorate peak that is partially resolved in the first
dimension onto a concentrator column in the second
dimension. The suppressed effluent with hydroxide eluent
is water, which provides the ideal environment for ion-exchange
retention and focusing. Third, the second
dimension is operated at a lower flow rate relative to the
first dimension, thereby enhancing the detection sensitivity.
Finally, this approach also allows the potential to combine
two different chemistries in two dimensions, thereby
enabling a selectivity not possible using only a single
chemistry dimension.
Figure 3 - Chromatograms of a 1-D (a) and 2-D (b) analysis of perchlorate
in a high-salt matrix. Note that perchlorate is not detected due to
interference from the matrix ions in the 1-D separation. In the 2-D separation,
the interfering matrix has been removed and the perchlorate peak
is enhanced and also quantifiable.
Figure 3a shows the analysis of a sample in the first dimension
consisting of 5 ppb perchlorate in the presence of 1000
ppm matrix ions. As can be seen in this figure, the matrix
interferes with the detection of the perchlorate peak,
which is broadened and difficult to quantify. This same
sample was analyzed using the 2-D approach by using the 2-mm column in a second dimension. As shown in Figure 3b,
perchlorate is well resolved and free from any matrix
effects. Chromatography with matrix diversion and concentration
followed by analysis on a 2-mm column and a
concentration-sensitive detector yields sensitivity proportional
to the flow rate ratio of the first dimension versus
the second dimension: a fourfold gain. Due to the physical
proximity of the two dimensions in the system and minimal
delay volume, it was possible to achieve excellent peak
shape and recovery.
AutoNeutralization
Another example of valve switching involves a technique
known as AutoNeutralization™ (Dionex Corp.). This technique
is used when it is necessary to quantify anionic contaminants
in concentrated bases. The strategy most often
employed is to dilute the sample. This dilution reduces the
concentration of the interfering matrix ion to a level that
does not affect separation. However, dilution also reduces the
concentration of trace anions, compromising their detection.
AutoNeutralization solves the analytical problem of achieving
good detection limits of trace anions in concentrated
bases by neutralizing the base using a membrane-based neutralizer
device. The sample anions are in a water background
after neutralization and can be focused back onto a
concentrator column that is located on a third valve in the
lower compartment.
In this setup, the system was used in conjunction with an
AM module.
Two six-port valves on the AM module were used for neutralization.
The first valve was used for loading the sample onto
an ASRN™ neutralizer device (Dionex Corp.). The second
valve was used for holding the sample in the collection loop
and rerouting the sample back through the neutralizer and
diverting the neutralized stream onto a concentrator column.
Figure 4 - Plumbing configuration required for AutoNeutralization applications.
This plumbing configuration is shown schematically in
Figure 4. In the AutoNeutralization process, the concentrated
base sample is loaded into the 25-μL sample loop of
the sample valve. The sample loop is switched in-line and
flushed with a stream of deionized water (also known as the
carrier solution). The carrier solution transfers the concentrated
base from the sample loop to the ASRN, where the
sample is partially neutralized, then transferred to the 5000-μL loop. The recycle valve is then actuated to pass the sample
through the ASRN again so that it is completely neutralized.
The completely neutralized sample is finally delivered
to a concentrator column; since the anions are now in water,
they are concentrated as a tight band.
Finally, the anions are eluted from the
trap column to the analytical column,
separated, and detected.
Figure 5 - Chromatographic results of anion analysis with (a) and
without (b) AutoNeutralization, from caustic sample matrix.
Figure 5 shows the results of anion analysis
of a concentrated base sample with
(a) and without (b) neutralization.
Without neutralization, the anions are
broadened and show poor sensitivity in
a 1% sodium hydroxide sample matrix.
In the 50% sample matrix the presence
of the matrix causes a high background that makes detection
of additional anions virtually impossible. Figure 5a shows the
results after neutralization. Note that the large background
has been removed and detection of anions is now possible
with good recovery.
The AM allows easy implementation of the autoneutralization
application without any additional external
valves or controls. The system configuration for this
application was simplified with the AM installed. All
valves, plumbing, trap columns, and ASRN remain
inside the DC compartment, minimizing tubing lengths
and allowing all manipulation of sample to be contained
in a precisely controlled environment. In addition, all
valves are recognized through Chromeleon® control software
(Dionex Corp.), eliminating the use of external
TTL controls or triggers.
Conclusion
The ICS-3000 system provides an effective platform for pursuing
standard and multiple valve switching applications.
Applications have been shown that demonstrate the utility
of the system and the AM module for valve switching applications.
Due to the close proximity of the valves to the
columns and detector cells, this design provides a low-dispersion
platform for implementing multidimensional
sample preparation and analysis schemes. The inherent flexibility
and precise control of the system make such nonroutine
applications easier to implement.
Dr. Jack, Dr. Lin, Mr. De Borba, and Dr. Srinivasan are with Dionex
Corp., 1228 Titan Way, Sunnyvale, CA 94086-4015, U.S.A.; tel.: 408-737-0700; fax: 408-739-4398; e-mail: [email protected]. Dr.
Sekiguchi, Mr. Nakanishi, and Ms. Yoshimura are with Nippon Dionex,
K.K., Osaka, Japan.