The objective of this project is to evaluate the suitability of the ultrasonic flexural plate wave (FPW) device as the detector in a gas chromatograph (GC). Of particular interest is the detection of nitrous oxide (N2O). From experimental results we conclude analyte detection is achieved through two mechanisms: changes in gas density, and mass loading of the device membrane due to the sorption of gas molecules. Reducing the dead volume of the FPW chamber increased the FPW response. A comparison of the FPW response to that of the surface acoustic wave (SAW) detector provided with the GC (made by MSI, Microsensor Technologies, Inc.), shows that for unseparated N2O in N2, the FPW exhibits a sensitivity that is at least 550 times greater than that of the SAW device. A Porapak Q column was found to separate N2O from its carrier gas, N2 or He. With the Porapak Q column, a coated FPW detected 1 ppm N2O in N2 or He, with a response magnitude of 7 Hz. A coated SAW exhibited a response of 25 Hz to pure N2O. The minimal detectable N2O concentrations of the sensors were not evaluated

Black, Justin

Chen, Bryan

White, Richard M.

English

NASA Technical Reports Server

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NCC2-5185: "Evaluation of Relative Sensitivity of SAW and Flexural
Plate Wave Devices for Atmospheric Sensing"
Richard M. White, Justin Black, Bryan Chen - University of California at Berkeley
Marc Madou, Richard Quinn - NASA Ames
https://ntrs.nasa.gov/search.jsp?R=19980016026 2020-06-16T00:25:15+00:00Z
1. Acknowledgements
The NASA-Ames University Consortium funded this work under project number NCC2-5185.
2. Abstract
The objective of this project is to evaluate the suitability of the ultrasonic flexurai plate wave
(FPW) device as the detector in a gas chromatograph (GC). Of particular interest is the detection of nitrous
oxide (N20). From experimental results we conclude analyte detection is achieved through two
mechanisms: changes in gas density, and mass loading of the device membrane due to the sorption of gas
molecules. Reducing the dead volume of the FPW chamber increased the FPW response. A comparison of
the FPW response to that of the surface acoustic wave (SAW) detector provided with the GC (made by
MSI, Microsensor Technologies, Inc.), shows that for unseparated N20 in N2, the FPW exhibits a sensitivity
that is at least 550 times greater than that of the SAW device. A Porapak Q column was found to separate
N20 from its carrier gas, N2 or He. With the Porapak Q column, a coated FPW detected 1 ppm N20 in N2 or
He, with a response magnitude of 7 Hz. A coated SAW exhibited a response of 25 Hz to pure N20. The
minimal detectable N20 concentrations of the sensors were not evaluated.
3. Background
The flexural plate wave (FPW) device has been shown to function well as a gravimetric sensor for
measuring the concentration of a vapor (see "Acoustic Wave Sensors", Ballantine et al., Academic Press,
1997). As illustrated in Figure 1, the micromachined FPW device has a thin silicon nitride membrane
peripherally supported by a silicon frame.
Silicon Nitfide
(2.7 "am)
IDTs (AI, 0.3u.,a)
i ................................ ._ Zn0 (i.0 urn)
Figure 1. FPW Delay Line: Top View and A.A' Cross Section
Input and output transducers, which are formed by a piezoelectric film and patterned aluminum electrodes,
can generate and detect anti-symmetric ultrasonic plate waves in the megahertz frequency range. Sorption
of gas into a polymer film deposited on the membrane shifts the phase velocity of the plate wave. Using a
delay-line oscillator, which is formed by an electronic amplifier connected between the delay-line output
and input, a change in this wave velocity is readily measured as a frequency shift. Alternatively, a bare
FPW delay-line configuration measures the density of the surrounding medium.
ThereactionoftheFPWdelay-linetoachangeinmassperunitarea,Am, can be characterized by
a gravimetric sensitivity factor Sin. This reaction is governed by the equation (Af/f) =Sm • Am,
where f is the resonant frequency of the oscillator and Af is the shift in resonant frequency induced by the
change in mass Am. The value of Sm may be approximated from the expression Sm = " 1/2"M, where M is
the mass per unit area of the membrane. With a high value of gravimetric sensitivity and a well-developed
fabrication process, the FPW delay-line offers several attractive features for gas detection.
When sensing gas density, the FPW device is characterized by a density sensitivity Sp. This mode
of operation is described analogously as (Af/f) = Sp. Ap, where AP is the change in gas density. The
density sensitivity, S0, is related to the gravimetric sensitivity, Sin, by the equation Sp = 8 • Sin. The
evanescent decay distance of the device, 5, is roughly X / 2n, where _, is the ultrasonic wavelength of the
structure.
In practice, the use of two sensors, a reference device and an active device, can minimize spurious
responses to environmental variables such as temperature and air pressure. As configured in this
experiment, both devices experience ambient conditions, but only the active device is exposed to the vapor
of interest (in some experiments a voltage source with constant frequency was used as the reference FPW
device).
4. Equipment
An MSI-301 gas chromatograph with SAW detectors was modified to incorporate FPW devices.
As illustrated in Figure 2, the MSI-301 is comprised of a sampling pump, a sample concentrator, a Porapak
column, an air pump and scrubber to generate cattier gas, and a pair of surface acoustic wave (SAW)
devices. As the bulk gas sample propagates through the column, column-molecule interaction gradually
separates the sample into its constituents. With the proper column, each constituent emerges at a different
retention time. Each component induces a shift in the resonant frequency of the active SAW or FPW
device.
Sample
Inlet
I Computer-
Data
_cquisition
Concentrator Module /
Valve System
Carrier Gas
Scrubber
20C,....oe]
"l CircuitBoard Hardware
H Carrier
Gas Pump [ - [
Carrier
Gas Inlet
FPW
Signal Procesx[nl
Hardware
I
Figure 2. Schematic of Modified MSI-301 Gas Chromatograph
To incorporate the FPW sensors into the MSI-30I, the gas flow from the GC column was
redirected over the active FPW sensor. A circuit board (developed by Bryan Chert) modifies the FPW
voltage output signal into a form indistinguishable from that of the SAW devices. Switching the GC
interface board leads between the FPW and SAW devices allows data collection with the MSI-301
software, GC_RAP.
The FPW housing, illustrated in Figure 3, was a modified version of the SAW housing. Unlike the
SAW device, in which the wave travels only on one side, the entire membrane of the FPW oscillates with a
sinusoidal displacement. To take advantage of this unique feature, flow from the GC column can be
directed over both sides of the FPW through two chamber inlets.
ReferenceFI:_/V_tL '-.._ _
N *--" Volume Plug
ActiveFPW
A
I I'"1 I
v
I I=::=1 I
1 •
v
Gas inlets
Top Inlet
BottomInlet
Gas Samples
Figure 3. FPW Housing: Perspective, top, and side views
The 100 ppm N20 in N2 or He experiments were conducted with gas canisters acquired from
Aldrich Chemical Company. The 1 ppm N20 in N2 results were obtained with gas samples from Scott
Specialty Gases. Richard Quinn provided the 300 ppb N20 in He samples.
5. Experiments
Coatings
In order to investigate the effects of coating on device sensitivity, the devices were coated with a
polymeric adsorbent, polyisobutylene (PIB). While the coated SAW devices were furnished by MSI, the
FPW devices were coated with an airbrush at U.C. Berkeley. The PIB thickness on the SAW devices was
nominally 200 kHz (resonant frequency 250 MHz), and the FPW coating was 50 kHz (resonant frequency
-3.5 MHz). For initial experiments, only the well of the FPW device was coated. Although the PIB
coating improved both SAW and FPW device sensitivity, it was not N20 selective.
Sensor Chamber Volume
Early experiments showed that a bare FPW was sensitive to Nz0 in He. It was hypothesized that
the FPW device was responding to a change in the density of gas near its membrane, and that this response
could be improved by reducing the volume of the chamber. As shown above in Figure 3, a brass plug was
fabricated which eliminated most of the dead space around the active FPW sensor. The initial volume
around the active FPW was approximately 0.42 in3. With the plug, this volume was reduced by a factor of
seven to 0.06 in 3. The volume of the SAW chamber was less than 0.01 in 3.
Detector Noise
A problem encountered with both sensors is initial baseline frequency drift. This drift occurs when
the active device must equilibrate to the temperature of the column air flow, usually a few tenths of a degree
Celsius different than ambient air. Temperature equilibration takes twenty to thirty minutes, but lasts longer
for coated devices.
Measurements indicate that the noise levels of the SAW and FPW devices are comparable, and
that the PIB coating has variable effects. The minimum meaningful frequency shifts are shown below in
Table 1.
Table 1. Device Noise Levels (+ / - Hz per second)
Bare Coated
FPW O.4O 0,43
SAW 0.42 l .O6
Reproducibility
Results achieved with both the SAW and FPW devices are readily reproducible at high Nz0
concentrations, such as 100 ppm, and with other vapor samples such as ethanol. The reproducibility at
lower concentrations has not been sufficiently evaluated. Samples run successively (during the same day
for instance) exhibit highly reproducible results. Figure 4 shows two such N20 trials performed within
hours of each other. This chromatogram was produced with the Porapak P column, which does not
separate N20 from the carrier gas N2. External variables such as temperature and air pressure can also cause
shifts in peak size over time.
10000 ................
9000
• 8000
O
C
7000
a 60o0
5000
Q
O" 4000
14.
10 3000
2000
I
Trial 1
Trial 2
\
1000
z _-_ ___. =====____+___
0
0 50 100 150 2o0 Time250 [see]3°° 350 400 450 500 550
Figure 4. Coated Plugged FPW Response to Unseparated 100 ppm Nz0 in N2, Successive Trials
5.1 Porapak Q Column
Two GC columns were investigated -- the Porapak P and Q. Only the Porapak Q column
separated N20 from the carrier gases, N2 and He. The magnitude of the coated SAW response to pure Nz0
was 25 Hz (not shown). The SAW could not sense 1 ppm N20 in N2 or He (the detection limit of the SAW
was not investigated). As shown in Figure 5, the coated FPW detected 1 ppm N20 in He or N2, The
magnitude of the response was 7 Hz.
50
45
40
30
_ 2O
g
15
10
5
z 0
-5
-10
0 20 40 60 80 100 120 140 1601
Tim e [sec] j
.... J
Figure 5. Coated, Plugged FPW Response to 1 ppm Nz0 in Helium - Three Trials
5.2 Porapak P Column
With the Porapak P column the N20 and carrier gases, N2 and He, elute simultaneously. Thus the
results shown below are the aggregate response of the sensors to both the N20 and the carrier gases, N2 or
He. In this report, the term 'unseparated' is used to denote the simultaneous elution of N20 and the carrier
gases He or N2. Conclusions about N20 sensitivity can not be made with this unseparated data because the
FPW also shows a large response to both He and N2 gas.
5.2.1 FPW and SAW Response to Unseparated 100 ppm N20 in N 2
Table 2 summarizes SAW and FPW detection results for N_0 in N2. Results are expressed in terms
of peak height, in Hz, and peak area, which is the dimensionless product of Hz and seconds. The PIB
coating improved the SAW device sensitivity by a factor of 3, and the FPW sensitivity by a factor of around
40. The reason for this difference in improvement remains unclear.
Table 2. SAW and FPW Response to Unseparated 100 ppm N20 in Nz
Bare
Coated
Improvement (absolute value):
AArea,_t,_ Af=.t_
AA.l_ae_ted ' Afen_t_d
SAW Device
Peak Area Peak Hei_ht
25 5
75 -17
3.0 3.4
Unplu[_ed FPW Device
Peak Area Peak Height
17,800 730
Plugsed FPW Device
Peak Area Peak Heig_ht
3,300 2C0
123,000 9300
37,3 46.5
Figure 6 shows the coated SAW and coated, plugged FPW aggregate responses to 100 ppm N20 in
N2. The graph has a different axis for the SAW, with a scale of Hz, than for the FPW, with a scale of kHz.
Additional tubing between the MSI-301 column to the FPW housing causes the longer elution time for the
FPW.
25 10.0
20
•_ 15 8.0 ,-=-
U) ¢)
10
,'- 6.0 ==
_. s g
._
-_ -5
[
o -10
-15
-20
4.0
._n
M
2.0 E
0
Z
0.0 _
................ FPW Response
P eakHeight-9kHz , i =
] 1 \
-25 ; -2.0
0 10 20 30 40 50 60
Time (see)
Figure 6. Coated SAW and FPW Response to Unseparated 100 ppm Nz0 in N2
Table 3 shows the relative sensitivities of the SAW and FPW sensors (FPW response divided by
SAW response). Using peak area, the response to unseparated N20 in N2 samples of a coated and plugged
FPW is up to three orders of magnitude greater than the response of the SAW. Using peak height, the FPW
device is 550 times more sensitive to unseparated N20 in N2 samples than the SAW device. It is presently
unclear why the PIB coating affects the relative sensitivities.
Table 3. Relative Sensitivities of SAW and FPW Detectors to Unseparated N20 in Nz
Bare Devices:
Coated Devices:
AArearpw
AAreasAw
Unplugged FPW
Afvew
AfSAW
Plugged FPW
AAreaFPW Afvew
AAreasAw Afs^w
.... 130 --
240 40 1640 550
Figure 7 illustrates FPW sensitivity before and after the plug volume reduction. Table 4 provides
these results in numerical format. With a coated FPW, the unseparated N20 in N2peak area jumped from
17,800 to 123,000 as a result of the volume reduction. Thus a seven-fold reduction in volume produced a
seven-fold increase in FPW sensitivity. Since the volume of the SAW chamber is less than 0.01 in3, further
reductions in the FPW volume promise to yield even greater sensitivities.
10000 L
 ooo r :
+++0
_ 7000
= :!
o :: i
_ 5000
o-
_ 4000
i1
N
_ 000
,_0 1000
0
0 50 100 150 200 250 300 350 400 450 500 550 600
Time [see]
Figure 7. Coated, Plugged and Unplugged, FPW Response to Unseparated 100 ppm N20 in N2
Table 4. Improvement in FPW Sensitivity to Un eparated Nz0 in Nz with a Reduction in Chamber Volume
Unplu,g_ed FPW Plugged FPW Improvement
Peak Area Peak Height Peak Area Peak Height AAreapl.._ AfnluE_
AArea_vt_wo Af_#._g.o
Coated FPW 17,800 730 123,000 9300 6.9 12.7
5.2.2 FPW and SAW Response to Unseparated 100 ppm N20 in He
Experiments analogous to those presented above were also conducted with 100 ppm N20 in He.
Although the detection of N20 in He has little practical use, this gas mixture was initially used because it
was readily accessible. The reader is reminded that results shown below represent the aggregate response of
sensors to N20 and the carrier gas He. Tables 5 and 6 gives SAW and FPW response to unseparated 100
ppm N20 in He. The PIB coating improved the sensitivity of the SAW device by a factor of 2 or 3.
Table 5. SAW Response to Unseparated 100 ppm N20 in He
100 ppm N:0 in He
Peak Area Peak Height
Bare SAW 400 110
Coated SAW 1310 250
Improvement (absolute value): 3.3 2.3
AArea,_tcd Af_mt,:d
AAreaun_,..,d ' Af_.t_
300 ppb N_0 in He
Peak Area Peak Hei_ht
100 30
not perfome..,d not performed
Table 6. FPW Res
Bare FPW
Coated FPW
_kfa_ted
Improvement: Afu._t,_
_onse to Unseparated 100 ppm N20 in He
Unplugged FPW
Peak Area Peak Height
-- 2100
330,000 7250
-- 3.5
Plugged FPW
Peak Area Peak Height
Plug Improvement
AAreao_=,,_
t_kAl'_aunpl_gged t_f_plu_t._l
21000 -- 10
1.68"10 _ 34000
-- 1.6
5.1 4.7
Figure 8 shows the coated SAW and plugged, coated FPW responses to unseparated 100 ppm N20
in He. The SAW, with a scale of Hz, is given on a different axis than the FPW, with a scale of kHz.
225 40
200
= 3517s
,.30
7s -
z=
0
,]
-25 -5
0 20 40 60 80 100 120 140 160 180 200 220
Time [sec]
Figure 8. SAW and FPW Response to Unseparated 100 ppm Nz0 in He
The relative sensitivities of the SAW and FPW devices to unseparated 100 ppm N20 in He are
given in Table 7. The plugged and coated FPW device is considerably more sensitive than the SAW.
Table 7. Relative Sensitivities of SAW and FPW Detectors to Unseparated 100 ppm N20 in He
Unplugged FPW Plu_;_ed FPW
AAreav_ afo...._w ,_AteaF_w
AAreasAw AfsAw AAreasAw
Bare Devices: -- 20 --
Coated Devices: 250 _30 1280
Afs^w
190
130
The response of the FPW sensor to unseparated 100 ppm N20 in He is given in Figure 9. The bare
FPW exhibits what is believed to be an N20 peak embedded in a helium peak. Using peak height instead of
area, the PIB coating improved the FPW device sensitivity by a factor of 1.6 or 3.5.
ili_iiii_;i_i!)i_!!F_i!ii_!,!i!i,i i_i_i i_ii_i_i ii_ii:!i:ii_iii_ii_] _ ! _!; _i_iiiiiii_i:iii_ii!;i
Uncoated FPW
;!!iiiiiiiiiiii_i;'!i_i!iii_ii_!i!,ii,!  iiiii !ii ;iii!,iii  !i !iiiiiii!
;iili_ii!!iiiiiii_ii_!!&iii!ii
ii!iiiiii i!i!ii ii! iii,li  iiij ! iiii!i
li_ii_!!_i_i IJii!ii!iii!ii!ii_!ii!i
ii_ii__ii_ii!iiii!i'fi,iiiii!i!iil
0 20 40
Figure 9. Bare and Coated FPW Response to Unseparated 100 ppm Nz0 in He
As shown in Figure 10 and Table 6, a reduction in the dead volume around the FPW sensor
significantly improved sensitivity. With the addition of the plug, the 100 ppm N20 in He peak jumped from
an area of 330, 000 to 1.68 x 106. Thus the seven-fold reduction in volume produced a five-fold
improvement in sensitivity.
10.0
9.0
N
i:iiiil i:!!ill8.0
i!i_ii_i!iii_iiii!!i_iii!iil
7.0 !ii!!!ii!iii_:i!i:i¸
iii
4.0
.= 3.0
fi 2.0
1.0
0.0
Plugged Volume
I:i II:
0 10 20 30 40 50 60 70 80 90 100
Time [see]
Figure 10. Coated, Plugged and Unplugged, FPW Response to Unseparated 100 ppm N:0 in He
Conclusions
With a Porapak P column, the FPW is several orders of magnitude more sensitive than the SAW to
unseparated samples of N_0 in He or N_. With the Porapak Q column, which separated N_0 from the carrier
gases, a PIB-coated, plugged FPW exhibited a response of 7 Hz to 1 ppm N_0 in He or N_. The magnitude
of the SAW response to pure N_0 was 25 Hz. It should be noted that the sensor coating, polyisobutylene,
was chosen as a "starting point", and is more suited for the detection of organic volatiles.


Evaluation of Relative Sensitivity of SAW and Flexural Plate Wave Devices for Atmospheric Sensing

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