General Research Interests:
I am interested in areas of analytical
chemistry,
primarily in analytical separations, and in environmental chemistry,
particularly
atmospheric chemistry.
Specific Research Projects:
1. Development of New Aerosol-based Detectors for High
Performance
Liquid Chromatography (HPLC)
Aerosol-based detectors function in three steps:
nebulization of the HPLC effluent, evaporation of the solvent leaving
aerosol
particles, and detection of the aerosol, with the
last
step depending on the type of aerosol-based detector. Evaporative
light scattering detection (ELSD), which measure the light-scattering
ability of the particles, has been commercially available for
decades. The
use of condensation nucleation counters, which "grow" and detect
particles, has resulted in a more sensitive
detection method known as condensation nucleation light scattering
detection
(CNLSD). More on CNLSD is available at John
Koropchak's internet site. Aerosol-based detection methods
are considered to be
"universal".
We have been investigating the construction of
sensitive
and potentially economical HPLC detectors using aerosol charging for
detecting the aerosol. We have used the name aerosol charge
detection
(ACD) although the commercial instrument (described more below) is
called the charged aerosol detector (CAD). The
first
ACD instrument
was built using nebulizers, a spray chamber,
heated tubing and an Electrical Aerosol size Analyzer (EAA) for aerosol
detection. In the flow injection analysis
(FIA)
mode (i.e. without
a chromatography column) and using water as the solvent, detection
limits
around 10 ppb were achieved with linear response observed from
0.1 to around 10 to 100 ppm (depending on the nebulizer used).
When
using typical HPLC solvents, the sensitivity often is limited by
non-volatile
impurities present in solvents. It is
possible to improve the signal to noise by using the EAA to remove
small particles. Detection limits of
about 20 ppb (similar to CNLSD and more sensitive than ELSD) and good
reponse
linearity were observed for weakly retained compounds separated
using
reverse-phase HPLC. This detection method is
documented in the Dixon and Peterson paper in the publications section
of my homepage. To see an example chromatogram set (using both AC
detection and UV detection) showing the separation and detection of 5
ppm
ammonium sulfate, uracil, sulfanilamide, nicotinamide, and adenosine, click
here.
A commerical instrument using aerosol
charging, but through the use of colliding ion and aerosol jets, has
been developed by ESA Biosciences. This instrument won a Silver
Editors' Choice Award at the
2005 Pittsburgh Convention and an R&D100 Award. More
information on the instrument
can be found here.
Sensitivity of the CAD instrument is similar to our first ACD
instrument. A number
of papers have come out documenting the CAD and some applications.
Current work is focused on creation of related
detectors utilizing other mechanism of aerosol charging and
involving Bill
DeGraffenreid (CSUS Physics and Astronomy Department). Work
also has been
undertaken to use ACD/CAD for practical applications. This has
involved projects describe below and is starting to involve other CSUS
faculty.
2. Application of ACD/CAD
a) Applications related to my research in
determination of aerosol chemistry.
Atmospheric aerosols are suspended particles
in the atmosphere. The collection of particles making up aerosols
is also know as particulate matter. The composition of organic
compounds found in atmospheric aerosols is not well known. The
focus on determining the composition in the past has focused on using
gas chromatography (GC) with more recent interest in real-time mass
spectrometry. Analysis of many polar compounds by GC is difficult
unless compounds are derivatized to less polar/more volatile
compounds. We have focused on developing methods for methods
using HPLC-ACD for analyzing such compounds. We have worked on
methods to analyze dicarboxylic acids and sugars. The work on
dicarboxylic acids has given relatively high detection limits (around 1
ppm) due to high baselines and volatility of the smaller dicarboxylic
acids. Application of HPLC-ACD to analysis of sugars,
specifically monosaccharide anhydrides (MAs) such as levoglucosan, have
been
more successful. We have been using a Ca impregnated cation
exchange column for the sugar separations with 100% water eluent.
The initial work (see J. Chromatogr.
article in puclications section) gave detection limits of about 0.1 ppm
or 5 ng injected). This method has been applied to a number of
sample sets (described in section 3) with further improvements reducing
detection limits to 40 ppb or 2 ng injected (documented in the recent Atmos. Environ. article in the
publications section) and more recently to under 10 ppb or 1 ng
injected. The California Air Resources Board plans to use of a
variant of this method for measurement of levoglucosan.
b) Use of FIA-ACD for sensitive measurement of
non-volatile solute
A number of specific applications, such as
measurment of dissolved solids in water or hexane soluble materials in
soil, require the measurement of non-volatile solute in samples.
Traditionally this has been done by tedious gravimetric methods that
are not very sensitive (based on typical 0.1 mg uncertainties in
balances). While many of the specific applications have been
replaced by more modern methods that tend to correlate well with the
measured quantity, there is no universal method for non-volatile solute
measurement. For example, water quality often is determined by
electrical conductivity which generally correlates with dissolved solid
content. The FIA-ACD response is expected to be correlated with
non-volatile solute mass. In collaboration with other CSUS
faculty who are providing us with samples, we are investigating using
FIA-ACD for analysis of dissovled solids in river water, motor oil
contamination of soils, and lipids in fish eggs. We also have
analyzed aerosol samples to estimate the water-soluble aerosol
concentrations.
c) Quantitation of oligosaccharides - in
collaboration with Tom Peavy
(CSUS Biological Sciences)
Oligosaccharides are important constituents in
molecular
recognition in cells. We have been working to develop a method to
quantitate different oligosaccharides using
HPLC-ACD. Because of the complexity of oligosaccharides,
qualitative and quantitive analysis of individual oligosaccharides is
difficult. In this project, we are focusing on using hydrophillic
interaction HPLC-ACD to quantitate various oligosaccharides (actual
identification will require other techniques) present in amphibian egg
samples.
3. Analysis of Atmospheric Aerosols
Atmospheric aerosols are known to adversely
affect
health, to decrease visibility, and to affect the earth's
climate. Knowledge of the composition of atmospheric aerosols is
useful in understanding sources of particulate matter as well as for
understanding how particles behave (e.g. scatter light or nucleate
cloud droplets).
We have worked on using different techniques of
using ACD to analyze atmospheric aerosols. Initial interest was
in using FIA-ACD to determine water soluble aerosol mass
concentrations. When combined with other measurements this may be
useful for better characterizing water soluble organic mass.
Because our method for analysis of MAs has
been the most successful, we have applied this to the largest sets of
aerosol samples. Analysis has consisted of a small set of samples from
New Mexico, UC Davis, and CSUS for method development. We were
able to detect levoglucosan in all
winter samples collected, indicating the presense of woodsmoke.
We also were able to see forest fire smoke from samples collected in
August, 2002. Beyond these initial samples, we also have
investigated a set of samples from Montana
collected in August and September, 2003, which was a time of great
forest fire smoke influence, and a set of samples from cities in and
near the
Central Valley of California collected by the CA Air Resources Board in
January, 2001. The results from the set of samples from Montana
have been publised recently (see Ward et al. paper in publications
section). In the Central Valley samples, aerosol levoglucosan
concentrations
of up to about 10 µg/m3
were observed in winter in communities strongly affected by wood
burning. We have used the concentrations of MAs to estimate the
source of particulate matter from wood smoke in these
communities. We have started analyzing additional samples from
cities
affected by the August 2002 forest
fires in southern Oregon.
Some of the samples we have or are
analyzing are associated with the following forest fires:
Star Fire (Lake Tahoe Region), August/Sept.,
2001. This
should provide a link to a satellite picture showing this fire from a
NOAA website.
Southern Oregon fires of August, 2002. This
should provide a link to a satellite picture showing
these fires from a NOAA website.
Forrest fires around Missoula, Montana, August,
2003. This
should provide a link to a satellite picture showing
these
fires from a NOAA website.
Updated 10/1/07
