ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
1465
Surface Acoustic Wave Probes for Chemical Analysis. 11. Gas Chromatography Detector Henry Wohltjen' and Raymond Dessy" Chemistry Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 2406 I
Quartz and ltlhium niobate surface acoustic wave devices have been examined for application as GC detectors. Polar and nonpolar materials have been successfully detected using both bare detectors and detectors partially covered with various coatings. Measurements of the resonance frequency of the devices gave best results. The dynamic range of such devices is limited because of damping of the oscillations to a few decades. The response is nonlinear. Amplitude measurements, although well understood theoretically, provided only modest sensitivity employing the current design (10 pg). However, the response is proportional to molecular weight and new interdigitizedfinger design could provide a factor of 1000 improvement in minimum detectable quantity.
ST-quartz and lithium niobate SAW devices have been utilized as detectors for a gas chromatograph. Amplitude and frequency measurements were performed and results indicated t h a t greater sensitivity could be obtained using frequency measurements. T h e lithium niobate device performed significantly better t h a n the quartz device. Selective coatings were successfully applied to the detector, thus enabling greater sensitivity and discrimination between compounds t o be realized. Evaluation of t h e minimum detectable quantity indicated that the compound o-chlorotoluene could be detected below the 100-ng level. T h e linearity and dynamic range of both t h e quartz and lithium niobate detectors was much poorer than other common GC detectors. Future research should focus on improving the sensitivity of t h e amplitude detection mode and unraveling the mechanism of operation of the frequency detection mode. The modest success achieved thus far suggests t h a t the surface acoustic wave device may indeed have a future as a detector in process control gas phase analysis.
EXPERIMENTAL Both quartz and lithium niobate were used as detectors on a BENDIX model 2200 gas chromatograph which contained a 6-ft by l/s-in. column of 3% OV-101 on Chromosorb h'. The inlet temperature to the column was 220 "C and helium was used as a carrier gas. The column was run isothermally at a temperature which depended upon the separation needs. The SAW detectors were attached to the column with a sleeve of Teflon tubing. The detectors operated at room temperature. The standard solutions used were prepared by a serial dilution method. Each mixture was separated on the GC and detected with a thermal conductivity detector to ascertain peak positions and solution purity. All experiments were conducted at least twice. Most experiments were repeated three times and many were repeated four or more times. Exploratory experiments with a quartz SAW detector indicated that the amplitude and frequency detection modes were significantly more sensitive than the phase measurements. As a result, phase measurements were neglected in these studies. Experiments were conducted with clean, uncoated quartz and lithium niobate detectors as well as with coated detectors. The Present address: IBM Watson Research Laboratory, Yorktown Heights, N.Y. 0003-2700/79/0351-1465$01 0010
coatings were applied to the quartz device with a cotton swab. The lithium niobate devices were dipped into a solution to prevent damage to delicate connecting wires. The coatings tried included Dow Corning 970 V vacuum grease, squalane, Apiezon L vacuum grease, di-n-decyl phthalate, and Carbowax 20 M. The DC 970 V, Apiezon L, and Carbowax 20 M were dissolved in a solvent (acetone or chloroform) to make a solution of several percent concentration by weight. The solution was applied to the device and the solvent was allowed to evaporate. Ordinarily this first coating would cause a significant attenuation of the surface wave and operation would cease. With the device connected in the frequency measurement system, the device would be rinsed with solvent until normal oscillations resumed. The di-n-decyl phthalate and squalane were liquids at room temperature and could be applied directly.
RESULTS The first test of a quartz SAW device as a Gt detector was accomplished using a frequency shift measurement and a 1.0-pL injection of a solution containing 0.1% dodecane in hexane as shown in Figure 1. T h e main peak was due t o hexane and the small peak occurring a t about 100 s was due to the dodecane. The dip which occurs in the base line a t the start of the run is an artifact introduced by capacitive coupling a t the time of injection. An attempt was made to enhance the sensitivity of t h e device by coating it with DC 970 V; Figure 2 shows the result. (The chromatogram of Figure 1 is shown a t the left with a different y axis scale factor.) The hexane peak causes the SAW oscillator to lose frequency lock owing to t h e amplitude attenuation. T h e magnitude of t h e dodecane peak is obviously greater. Figure 3 illustrates a similar effect with a 1% solution of o-chlorotoluene in hexane. The success achieved with frequency measurements prompted an investigation of the amplitude response. A comparison of the amplitude and frequency measurements is illustrated in Figure 4. Because of the relatively poorer response of t h e amplitude detector in a measurement that should be easy to make, all subsequent experiments were focused solely on frequency shift measurements. Since the lithium niobate SAW device performed very well in the frequency measurement mode (See Paper I of this series), attention was shifted to its application as a GC detector. One microliter of a solution containing 0.1% dodecane i n hexane was injected into the GC and the results are shown i n Figure 5 . The hexane peak caused t h e oscillator to lose frequency lock but the dodecane produced a signal over 40 times larger than that produced on the quartz device. A 1'70 solution of o-chlorotoluene in hexane produced a signal of 1.4 KHz for the o-chlorotoluene peak. Once again this signal was over 40 times larger than the quartz could provide under the same conditions. An injection of 1 pL of a solution of 0.1% o-chlorotoluene in hexanes was easily detected by a clean lithium niobate device. Coating the SAW detector with DC 970 V once again provided a dramatic increase in sensitivity and specificity. Figures 6, 7, and 8 illustrate a series of chromatograms obtained from the injection of 1 fiL of solutions containing 1% , 0.1 YO,and 0.01 ?&, respectively of o-chlorotoluene in pentane. T h e chromatogram in Figure 6 is particularly interesting because the o-chlorotoluene peak
C 1979 American Chemical Society
1466
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
E Q-~OROTOLUENE
IN
HWWE 1 , O p L LNJECTION
15 ~
M I N . FLCW
50"COLW
Ir
CLEAN
-7
10
c
20
oi
0
T I M E ( S E C ) *lo-'
K 37ov I
I
10
20
TIME(5EC)
Figure 3. Comparison of clean and DC 970 V coated quartz (1 % OCT) Figure 1. Clean quartz SAW chromatogram (0.1% C 12)
HWNE
Otl% CODEWE
N I
YEWE INJECTION
1,OyL
15 ML/MIN. FLOW 160"COLW
! ;i" Y)
C W
197ov
WLITa
I
1
-
10
TiME(5EC)
1
20
r10-l
0'
1
I
10
20
TIME(SEC> *lo-'
Figure 2. Comparison of clean and DC 970 V coated quartz
is greater than the pentane peak even though the pentane was present in 100-fold greater concentration. Figure 8 clearly shows the detection of about 100 ng of o-chlorotoluene. It also illustrates the loading that the coating experiences from the solvent peak which causes a step change in the base line. Attempts were made to coat the lithium niobate device with other coatings. The squalane and di-n-decyl phthalate coatings did not work well a t all. After coating. the device experienced a continuous base-line drift. The grease and wax coatings proved to be far superior and did not cause any base-line drift problems. The effects of different surface coatings on the specificity and sensitivity were measured by obtaining chromatograms of the relatively nonpolar compound
,
I
0
LO
,
20
T I M E C S E C ) e10-l
I
0
FREGUJEIXY
LO
20
TIMECSEC)
Figure 4. Comparison of amplitude and frequency detection
n-octane in hexane and of relatively polar o-chlorotoluene in hexanes. Figure 9 shows the relative effects on n-octane (C8) and o-chlorotoluene (OCT) on the frequency of a clean SAW device. The presence of an eluting compound caused the frequency to increase. (This was true of all chromatograms obtained from clean surfaces; however, some of the plots presented previously were inverted to make their appearance more familiar). A DC 970 V coating caused a substantial enhancement of the sensitivity of the device and exhibited a greater sensitivity for the o-chlorotoluene than for the n-octane (Figure 10). The frequency shift is in the opposite direction. Application of an Apiezon L coating resulted in a less impressive improvement in sensitivity and specificity compared to the DC 970 V coated detector. Figure 11 shows
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
1467
0,E D~DECANEIN
HWE 1,OpL INJECTION 15 ~ M I N FLOW ,
160" COLW
I
,
io
0 110
20
TIMECSEC) * l o - '
210
Figure 7. DC 970 V coated lihium niobats SAW chromatogram (0.1%
TIHE(SEC> * l o - '
Figure 5. Clean lithium niobate SAW chromatogram (0.1% C 12)
OCT)
Table I. Absolute Frequency Shifts for Coatings Investigated Absolute Frequency Shift, Hz surface clean Apiezon L Carbowax 20 M Dow Corning 970 V
c
N
1 0 @g 10 Pg o-chlorotoluene n-octane + 1 310
t 490 -1 250 i390 -2 190
-4630 -5 250 -12 500
2 L
Table 11. Specificity Indices for Coatings Investigated Specificity Indexu
5
C
1
1c
,
surface
o-chlorotoluene
clean Apiezon L Carbowax 20 M Dow Corning 970 V
1.0 -3.5 -4.0 -9.5
octane 1.0 -2.6 0.8
-4.5
Specificity index = peak height (coated)/peak height (clean ).
20
TIflE(SEC> *lo-'
Figure 6. DC 970 V coated lithium niobate SAW chromatogram (1 YO OCT)
a dramatic increase in specificity due to the presence of a Carbowax 20 M coating on the device. The nonpolar compounds (n-octane and the pentane solvent) experience frequency shifts in one direction while the polar o-chlorotoluene is shifted in the opposite direction. Table I summarizes these results in terms of the absolute frequency shift experienced by the SAW detector. I n order to facilitate the comparison of different coatings, a concept called the SPECIFICITY INDEX was developed. It is defined as the ratio of the peak height generated by a coated SAW device to the peak height generated by a clean
Table 111. Minimum Detectable Quantities for SAW GC Detectors Minimum Detectable Quantity, k g DC 970 V detector compound clean coating quartz
octane o-chlorotoluene
LiNbO,
octane o-chlorotoluene
10 5 1 0.5
2 0.5
0.2 0.05
device for a given compound. Table I1 lists the specificity indices for the coatings investigated. From this list it is clear that the DC 970 V provided the greatest increase in sensitivity
1468
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979 VI a
-1
0, U h O - t I L C r J TOLUE'IE IN
PEWWE
1 , O p L INJECTION
15 N ~ M I N .
120
C
FLOW
y.l
N
COLvn
I
k I VI
81
W
K
L
I
o +
W
P
I
k i n N
1
I
0
TIME(SEC) * l o - '
21
10
I
0
OCT)
1 20
TIMECSEC) * l o - '
TIME(SEC) 0 1 0 - l
Figure 8. DC 970 V coated lithium niobate SAW chromatogram (0.01 YO
I
10
Figure 9. Clean lithium niobate SAW chromatograms
Table IV. SAW GC Detector Calibration Curve Data
C-CKOROTOLJENE
DC 970 V coated quartz sample weight, pg 1.0
:F.
2.0 10.0 20.0 100.0
peak height, Hz in noise 25 217 320 freq. control lost
r:a b fi
N
I
DC 970 V coated LiNbO, 0.1 77 1.0 670 10.0 12600 20.0 18600 100.0 freq. control lost
*u 3 3
w a
t o o-chlorotoluene and n-octane while Apiezon L provided the smallest sensitivity increase. The Carbowax provides the greatest selectivity between the n-octane and o-chlorotoluene as is evidenced by t h e differences in sign of the specificity index. Estimates of t h e minimum detectable quantities are presented in Table 111. Evaluation of the linearity and dynamic range was achieved by plotting the peak frequency shift produced by samples of o-chlorotoluene against the weight of the sample injected. The raw d a t a for these evaluations are displayed in Table IV. Figures 12 and 13 illustrate the dynamic range plots for quartz and lithium niobate, respectively. The results are unimpressive. T h e Quartz SAW device had a dynamic range of
Lc K
0
10
20
TIMECSEC) * l o - '
0
I
I
10
TIME(SEC) * l o - '
Figure 10. DC 970 V coated lithium niobate SAW chromatograms slightly greater than one decade with poor linearity over that range. T h e lithium niobate device fared a bit better with
Table V. Some Physical Properties of Compounds Investigated compound
freq. shift Hz for 1%soln.
dielectric constant
mol wt
boiling point, C
density
octane o-chlorotoluene p-chlorotoluene aniline benzyl alcohol 1-octanol nitrobenzene
550 1400 1400 2 000 8 2 000 108 000
1.95 4.45 6.08 6.89 13.1 10.3 34.8
114.2 126.6 126.6 93.1 108.1 130.2 123.1
126 159 162 184 205 194 210
0.7025 1.0825 1.0697 1.02173 1.0419 0.8270 1.2037
138 000
I
20
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
1469
0
N
L?.O-CHLCROTOLUEUE
':
c
N
"i
,
I
/
1
n
4
'i
,,,/'
0 0
0
I
50
I
100
,
150
I
eoo
I
250
SRMPLE UEIGHT (NRNOGRRMS>
Figure 13. Calibration curve for o-chlorotoluene on a DC 970 V coated lithium niobate SAW device
: I p.l
0
10
20
I
0
I
10
1
20
TIME(SEC)
TIMECSEC)
Figure 11. Carbowax 20M coated lithium niobate SAW chromatograms
0
/
0 I
10
I
20
SAMPLE U E I G H T
I
30
I
40
I
50
(MICROGRRMS)
Figure 12. Calibration curve for o-chlorotoluene on a DC 970 V coated quartz SAW device
usable signals being obtained over a range of two decades. The linearity was poor.
DISCUSSION In an attempt to elucidate the mechanism of operation, a variety of different compounds were tested on a clean lithium niobate SAW device and used in the frequency mode. Table V lists these compounds and some of their physical properties along with the signal they produced when 1 pL of a 1% solution containing these substances was injected into the GC. The frequency of an ideal SAW oscillator can be changed by two things; changes in the propagation distance or velocity. Changes in propagation distance result primarily from temperature and/or pressure variations. Changes in propagation velocity result from electric interactions between a surrounding layer and the surface wave, as well as temperature variations ( I ) . There is a third mechanism that can cause frequency changes to occur. The interdigital electrodes form part of a tuned circuit with the rf amplifier used to sustain
the oscillation. The presence of a layer of different dielectric constant surrounding the interdigital transducers could alter their capacitance and thus cause a shift in the tuning of the rf amplifier. A frequency shift would result. These three possible mechanisms are significantly different from the mechanism by which a bulk wave crystal oscillator operates. King ( 2 ) monitored the frequency of a bulk crystal vibration used as a GC detector. It was possible to relate frequency changes directly to effective mass changes of the crystal. Mass changes on the surface of a SAW device will have little effect if the wave velocity or propagation distance is not altered. Thus the two techniques are quite dissimilar. Originally, it was thought that the frequency shifts would be a direct result of wave velocity changes caused by variations of the dielectric constant of the ambient medium as a peak eluted from the column. These dielectric effects operate mainly, but not exclusively, ~n b e t u e e n the transmitter and receiver. Supporting this statement is the fact that the quartz device shows results that differ only in amount, not type, in comparison to the lithium niobate detectors, the quartz devices have coating only between, not on, the interdigitized electrodes. As material dissolves reversibly in the coating the electric wave associated with the SAW interacts with the surrounding dielectric, the long interaction path provides a lever which permits small environmental changes to affect large response changes. Classical plate capacitance would not be affected by such small absorbances. There is a definite relationship demonstrated in the data of Table V between dielectric constant and frequency shift. However, several anomalies cannot be easily explained. Within the limits of experimental error, o-chlorotoluene and p-chlorotoluene yield the same peak height even though their dielectric constants are very much different. Another anomaly occurs with the frequency shifts produced by benzyl alcohol and 1-octanol. It is also unclear why such a large jump in frequency shift occurs between aniline with a dielectric constant of 6.89 and benzyl alcohol with a dielectric constant of 13.1. Certainly, additional research will be required before the anomalous behavior of this detector in the frequency mode can be understood. The understanding of amplitude measurements rests on much firmer theoretical ground (See Paper I of this series). Unfortunately, the sensitivities achieved were not very impressive. The minimum detectable quantity for a typical organic compound with a molecular weight of about 100 was less than 10 pg with the apparatus used. Operation of a newly designed low loss detector at 300 MHz with a lower dead
1470
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
volume cell could conceivably provide a factor of 1000 improvement in this minimum detectable quantity. Amplitude measurements afford the possibility of measuring molecular weights directly at t h e output of the GC column, since amplitude response is a function of molecular weight. Improved sensitivity, especially for high molecular weight compounds, could easily rank the SAW device with thermal conductivity and flame ionization type GC detectors. T h e prospects for improving the amplitude measurement scheme are quite good when one considers t h a t the system noise level is presently about four orders of magnitude greater than the theoretical noise level generated by the detector itself. Compared to existing GC detectors, the lithium niobate SAW device is not presently competitive in terms of sensitivity, linearity, or dynamic range. A conventional thermal conductivity cell could probably do a factor of 10 better with little difficulty (3). In addition a thermal conductivity detector has a linear range of about 10' compared to about 10' for the SAW device. T h e sensitivity of the SAW device could be improved by operation a t higher frequencies. The linearity could be improved slightly with an electronic oscillator that was less amplitude sensitive.
T h e SAW device GC detector does have some properties that continue to make it interesting. I t is nondestructive and can theoretically be used with any carrier gas. T h e cost of the detector and electronics is low and it is possible to make the detector specific by the application of a selective coating.
ACKNOWLEDGMENT The authors thank H. M. McNair for advice and consultation on the chromatographic aspects of this research.
LITERATURE CITED (1) Famell, G. W. "SAW Propagationm Piezoe!ecVic SOW', Wave€k3mks, 1976, 2, 15. (2) King, W. H. "Using Quartz Crystals as Sorption Detectors . . . Parts 1 & 2 " , Res.lDev. 1969, 20(4 8 5), 28. (3) Hartmann, C. H. "Gas Chromatography Detectors," Anal. Chem. 1971, 4 3 ( 2 ) , 113A.
RECEIVED for review January 22,1979. Accepted May 1,1979. T h e authors thank the Gillette Charitable and Educational Foundation whose funds helped support this research, and Bendix Corpoation for t h e loan of a Model 2200 Gas Chromatograph. All plots were made on a Benson Lehner Plotter, a gift from Corning.
Surface Acoustic Wave Probes for Chemical Analysis. 111. Thsrmomechanical Polymer Analyzer Henry Wohltjen' and Raymond Dessy" Chemistry Department, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 2406 1
A surface acoustic wave device was developed to perform thermomechanical analysis of polymer films. Amplitude measurements of TBfor polycarbonate, polysulfone, and single and twephase copolymers of the above agree with Rheovibron measurements. Low order transitions (at 24 "C) have been measured in Teflon. Although the T, measurements using SAW devices were made at 30 MHz, the poor coupling with the surface made by normal disk samples resulted in the excellent agreement observed with classical low frequency methods. On the other hand, cast films, which have good surface contact with the substrate, show the shifts in T , predicted by the time-temperature principle.
T h e attenuation of the amplitude of a Rayleigh wave propagating in a quartz SAU' delay line has been used to determine the glass transition temperature of polymer films clamped to the surface. Agreement with low frequency dynamic mechanical measurements on the same films is good. T h e attenuation is a result of changes in the surface contact between the polymer film and the SAW device which occur when the elastic modulus of the polymer decreases a t the glass transition temperature. Measurements obtained by this measurement are not reversible. A crystalline transition occurring in Teflon was easily detected in a specimen clamped t o the SAW device. Attenuation of the surface wave was a consequence of the surface Present address: IBM Watson Research Laboratory, Yorktoan Heights, N.Y. 0003-2700/79/0351-1470$01 OO/O
contact increasing as t h e sample expanded. The transition is characterized by a significant change in the coefficient of linear expansion of Teflon. Observations of this transition are reversible. A cast film is mechanically coupled to the surface wave. Under these conditions, the measured glass transition temperature, T g ,is observed to be much higher than that found in experiments conducted a t low frequencies, as theory predicts. Measurements of the cast film properties are reversible. The utility of the SAW device in photoresist investigations has been demonstrated by monitoring the effects of solvent evaporation and photo induced cross-linking of the resist. The device affords a significant advantage in studies of this kind because it can monitor films of t h e same thickness used in industrial applications. T h e SAW device will undoubtedly find many other applications in polymer analysis where its high sensitivity and ability to handle small samples are essential.
EXPERIMENTAL A quartz SAW device was employed in all studies of the thermal behavior of polymer films in contact with the surface. The previously described temperature test apparatus ( I ) was used to provide temperature control over the range of 0 to 200 "C. The polymer films required a clamping mechanism to provide a reproducible contact force. The system shown in Figure 1 was the most effective. Owing to the very large attenuation of the surface wave amplitude which was produced by a polymer film a t its glass transition temperature, a modified amplitude measurement system was used. The sensitive balanced bridge system described previously was replaced by a much simpler apparatus shown in Figure 2 . The size of the polymer samples was kept small to F 1979 American Chemical Society
