Design, testing, and simulation of microscale gas chromatography columns Page: 1 of 10
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K. A. Shollenberger, D. R.
A microscale gas chromatography column is one
component in a microscale chemistry laboratory for de-
tecting chemical agents. Several columns were fabricat-
ed using the Bosch etch process which allows deep, high
aspect ratio channels of rectangular cross-section. A de-
sign tool, based on analytical models, was developed to
evaluate the effects of operating conditions and column
specifications on separation resolution and time. The ef-
fects of slip flow, channel configuration, and cross-sec-
tional shape were included to evaluate the differences
between conventional round, straight columns and the
microscale rectangular, spiral columns. Experimental
data were obtained and compared with the predicted
flowrates and theoretical number of plates. The design
tool was then employed to select more optimum channel
dimensions and operating conditions for high resolution
Sandia National Laboratories is currently developing
a microscale chemistry laboratory (gChemLab). The
goal is to build a handheld unit for detecting chemical
agents such as sarin and soman nerve agents. Part of the
system will be a gas chromatography (GC) column that
will separate chemical species for detection. Typical GC
columns used today for separating chemicals are silica
tubes ranging from 50 to 500 microns in diameter and
30 m in length. These columns have a thin liquid layer
absorbed onto the walls, typically 1-10 nm thick, where
chemical separation occurs due to differences in the
equilibrium absorption coefficients for each chemical
species flowing through the column. For the Sandia
gChemLab project, it is desired to reduce the dimen-
sions of the column to fit on a 1 cm2 silicon chip while
maintaining good separation efficiency in a reasonable
Several design issues result from the microscale con-
straint. The columns will be etched on silicon wafers.
Thus, the geometry will be more complicated than con-
ventional straight columns with circular cross-sections.
*Sandia National Laboratories, P.O Box 5800, Albuquerque, NM 87185-0
Phone: 505-844-9018, Fax: 505-844-4523, email: email@example.com
DITRBIUON OF THIS DOCUMENT IS UNUMD 1
Adkins, and C. C. Wong J>J 2
New Mexico OS T I
First, the column width will be on the order of tens of
microns while the depth will be hundreds of microns
thus yielding high aspect ratio channels. Second, the
column will have a rectangular cross-section. Third, the
column will be in a spiral configuration as shown in Fig-
ure 1. System design constraints, such as the gas injec-
tion system and pump design, affect the operation of the
GC column. Thus, an optimum GC column design must
address these fabrication and operating constraints. De-
sign parameters must be selected based on an obtainable
flowrate, which is limited by the pump performance,
and yield the best separation resolution in a reasonable
A modeling tool has been developed to aid in the de-
sign of microscale GC column configurations. The de-
sign code to predict GC column performance employs
the compressible flow equations with slip at the wall
and the Golayl equation with Giddings2 pressure cor-
rection. The Golay equation describes the quality and
time required for a particular separation for both round
and rectangular cross-section columns. Experimental
measurements of flowrates and separations in conven-
tional columns and in the microscale spiral columns are
compared with model predictions. These comparisons
validate the model and allow us to understand and char-
acterize the flow and separation phenomena.
The microscale GC columns are fabricated at Sandia
using a reactive ion etch process which etches deep, nar-
row, rectangular cross-section channels in a spiral
shaped pattern as shown in Figure 1. Several spiral con-
figurations have been fabricated with a width of 10 to
40 m, depth of 80 to 250 pm, and length of 0.3 to 1 m.
The separation distance between the rings varies such
that the entire column fits in a 1 cm2 area. In 1975,
Terry etched 200 pm wide and 20 tm deep channels.
The Bosch4 etch process enables our deep channels and
thus, smaller chip areas.
Starting with a commercial 3 inch silicon wafer, a
mask is prepared for the deep etch by patterning a rela-
tively thick photoresist of 10 to 12 m, which config-
Sandia is a multiprogram laboratory
327 operated by Sandia Corporation, a
Lockheed Martin Company, for the
United States Department of Energy
under contract DE-AC04-94AL850Q,
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Draft Paper submitted to 1998 International Mechanical Engineering Congress and Exposition, Nov. 1998
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DESIGN, TESTING, AND SIMULATION OF COAJ P -9 ') J 0 7--
MICROSCALE GAS CHROMATOGRAPHY COLUMNS
M L Hudson* R Kottenstette CM Matzke G C P re-Mason RS(EIvED
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Hudson, M. L.; Kottenstette, R.; Matzke, C. M.; Frye-Mason, G. C.; Shollenberger, K. A.; Adkins, D. R. et al. Design, testing, and simulation of microscale gas chromatography columns, article, August 1998; Albuquerque, New Mexico. (digital.library.unt.edu/ark:/67531/metadc710213/m1/1/: accessed October 17, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.