Trace identification of organic moleculses in ultrapure water using Ion Mobility Spectroscopy Page: 2 of 6
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Compatibility with on-line or in-line
operation.
IMS has been used for both on-line and in-line
monitoring of gas streams. With some
modifications to the hardware and software, an
IMS could perform on-line monitoring of a
slipstream taken from a UPW line through one
of the following methods: 1.) Continual
sampling of the liquid. The IMS can accept a
continual stream of water provided the flow rate
is low enough. Capillary restrictors would be
used to control this flow. Sensitivity and
specificity could suffer due to the high
concentration of water vapor in the system. 2.)
Discrete sampling of the liquid. A liquid
sampling valve would be used to make periodic
injections into the IMS. This method would
have several advantages over continual
sampling, including i) Semi-quantitative data
could be obtained for each specie, ii) larger
sample injections could be used, and iii)
provisions for on-line calibration could easily be
incorporated. 3.) Continual sampling of a
vapor. The partitioning coefficients between
acetone or isopropanol and water is such that
headspace analysis is not a viable approach.
However, vaporizing a small amount of water in
a furnace, and sampling the resulting vapor
would be a relatively easy way to monitor the
water. Sample flows, temperatures, and other
parameters would have to be closely controlled.
4.) Membrane separation technologies.
Membrane technologies, which have been used
extensively in mass spectrometry, could be used
for in-line monitoring. As the water passes over
a silicone membrane, the organics, which are
soluble in polymers, pass through the membrane
and into the IMS.
Instrumental
The data presented here was acquired using a
PCP Model 110 IMS (West Palm Beach, FL).
Sample introduction was by direct injection of
test solutions using a 1.0p1 syringe. Sample
volume ranged from 0.1 1 to 1.041. It should be
noted that sample volumes up to 101 can be
used. The IMS temperature was held at 200*C.
Zero air was used as a carrier and drift gas at
flow rates of 100 and 500m1/minute,
respectively. No dopants were added to the
carrier gas stream.Experimental
The organics of primary interest as impurities in
UPW are acetone, isopropyl alcohol, ethylene
glycol and n-methyl pyrrolidone (NMP).
Compounds such as H2SO4, NH40H,
chloroform, H2O2, NH4F, HF, KOH, and
surfactants (dodecylsulfate sodium salt,
dodecyl-trimethylammonium bromide, and
hexadecyl trimethylammonium bromide) are of
secondary interest. Using Class A volumetric
glassware, solutions of acetone, isopropyl
alcohol, ethylene glycol, n-methyl pyrrolidone
(NMP), methylene chloride, dodecylsulfate
sodium salt, dodecyl-trimethylammonium
bromide, and hexadecyl trimethylammonium
bromide were prepared at concentrations
ranging from 10 parts-per-million to 1 part-per-
trillion. Appropriate volumes were injected into
the IMS and the responses recorded.
Typical responses are shown in figures 1 and 2.
Figure 1 shows the response obtained by
injecting 50 nanograms of ethylene glycol into
the IMS. The x-axis is the drift time (function
of molecular weight and shape), and the y-axis
is the signal intensity. The z-axis is time, with
"0" at the rear of the plot. The ethylene glycol
is responsible for the two indicated peaks at the
right side of the plot. Note that these peaks are
well-resolved from the reactant ion peaks (left
side of plot), which makes identification
relatively easy. Figure 2 shows the response
obtained by injecting 15.7 nanograms of
acetone into the IMS. Note that the acetone
peak is not well-resolved from the reactant ion
peak, which makes both identification and
quantitation much more difficult. Isopropanol
yields a peak very similar to acetone, while
NMP produces a well-defined peak similar to
ethylene glycol. The drift time of each peak is
unique, allowing speciation as well as
quantitation. Dodecylsulfate sodium salt,
dodecyl-trimethylammonium bromide, and
hexadecyl trimethylammonium bromide were
not detected by IMS.
Detection Limits
Figure 3 shows a calibration curve obtained
from ethylene glycol. Although not linear, the
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Rodacy, P.; Leslie, P.; Klassen, S. & Silva, R. Trace identification of organic moleculses in ultrapure water using Ion Mobility Spectroscopy, article, February 1, 1996; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc672656/m1/2/: accessed April 19, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.