Design, testing, and simulation of microscale gas chromatography columns Page: 4 of 10
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ures the channel width and length. The patterned wafer
is etched using a plasma process developed by Bosch4
that cycles between etching and a polymer deposition to
maintain the vertical side walls of the channel while the
channel bottom continues to etch. After the etch, the
photoresist is removed from the wafer in the last step for
the silicon slice.
The etched channel area is anodically sealed with a
cover slip that has preprocessed through holes for access
to the channel. The Pyrex wafer, which is also 3 inch, is
machined by grinding through holes with a 15 mil burr
at appropriate locations, so that channel ends will align
to the holes. The Pyrex and silicon are prepared for an-
odic bonding by cleaning in a piranha solution, rinsing,
and drying. After the glass is aligned to the silicon, the
assembly is heated to about 300 C and then bonded un-
der a high potential. Whole wafers have been success-
fully fabricated through the bonding step before sawing
to separate individual devices. Finally, for each device,
capillaries for test fittings are attached to the holes in the
Channels must be coated to form the necessary sta-
tionary phase for separation optimization and to tailor
the separation mechanisms. We are currently in the pro-
cess of investigating various coating procedures for uni-
formity, thickness, and separation performance. One
spiral has been coated with OV- 1.
Since the width is a critical dimension in the analyti-
cal model, some spirals were sawed in half to precisely
determine their dimensions. The width of a nominal
40 pm spiral channel can actually range from 44 to
60 m whereas the length is fairly constant. The SEM
(Scanning Electron Microscope) image of the sawed
edge of a nominal 55 m wide spiral is shown in
Figure 2. Channel etch looked quite uniform from the
outer to inner channels of the device. The width at the
top of the channel was 55 pm from the side view (same
as the top view). The width at the bottom of the channel
was about 50 pm. Typically, the etch width at the bot-
tom of the channel is about 90% the width at the top.
Also from this sawed device, the depth of the channel is
280 pm compared to the nominal 250 m.
Flow in both the capillary and microscale spiral col-
umns was characterized using the system illustrated in
Figure 3. Gas flow into the column was controlled with
a low pressure (0-40 psi) regulator that was connected
through a short cross fitting to a pressure transducer, a
vent valve, and the column. A soap-bubble flow meter
was attached to the discharge end of the column to mea-
sure the volumetric flow rate. Three meters were avail-
able to cover five ranges of flow volume: 0.1 ml, 0.5 ml,
1 ml, 10 ml, and 50 ml. To minimize the influence of
pressure drift within the system, the volume was select-
ed to provide a collection time of about one minute. The
pressure transducer provided an uncertainty of about
1.5% after it was calibrated with a dead-weight compar-
ator. The accuracy of the flow meters was estimated to
be better than 1 %. Tests were conducted in ambient air
where the temperature was nominally 23 C and the
pressure at the discharge of the bubble flow meter was
nominally 83.4 kPa (12.1 psi). Precise measurements of
ambient conditions were made on each day of testing.
Experiments for determining the retention factors and
number of theoretical plates were performed on a
Hewlett Packard 6890 Gas Chromatograph (GC) using a
flame ionization detector (FID). Flow measurements
were taken at the exit of the FID with only the carrier
gas flowing (fuel gases and make-up gas turned off.)
The GC was equipped with a HP Automatic Liquid
Sampler and all injections were performed using a split
injection technique. The commercial columns used in
these tests were 5% phenyl-methyl polysiloxane coated
capillaries with 100 pm internal diameter and 0.1 jm
film thickness of Supelco SPB-5. Liquid samples were
prepared from reagent grade chemicals purchased from
Aldrich Chemical Company. A mixture was prepared of
n-octane, n-decane, n-dodecane and dimethyl methyl
phosphonate (DMMP). The concentration of each com-
pound in the carbon disulfide solvent was approximate-
ly 0.1% by weight. All carrier gases were Matheson
ultra high purity grade and all tests were run isothermal-
ly using constant carrier gas pressure during the run.
Column length, column temperature, carrier gas type
and carrier gas pressure were the experimental variables
in these tests. Test repeatability was indicated by two or
three runs at each condition.
The retention factor, k, was calculated using the car-
bon disulfide as the unretained peak, t, and the reten-
tion time of the various normal alkanes
k = R--
The number of theoretical plates, N, was calculated using
the peak retention time, tR, and the peak width at half
peak height, wh,5.
N = 5.545
Measurements were not taken on the DMMP peaks since
the polar compound "tailed" significantly on this column
and therefore would not present a symmetrical peak for
measuring consistent widths at half peak height.
IV. Analytical Model
A design tool based on analytical models was devel-
oped to compute the carrier gas velocity, the theoretical
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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/m1/4/: accessed December 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.