1771
Anal. Chem. 1985, 57, 1771-1772 600500 -
,
I
,
I
I
-Model
400 -
300 200 -
100 -
.
'6 5
10
15
20
25
'
30
Weight Percent Glucose
Figure 3. Frequency shift (glucose in water) vs. glucose concentration for a 5-MHz crystal. Circles are experimental data. Solid curve is prediction of model.
The differential equation describing the steady-state shear waves in the AT cut quartz resonator is the Helmholtz equation, yielding as solutions undamped sinusoidal shear strains traveling in the direction perpendicular to the crystal surface. When one surface of the resonator (the surface not in contact with the liquid) is unconstrained, standing waves result. In the liquid, the differential equation describing the shear waves is the diffusion equation, having as solutions highly damped sinusoidal shear waves traveling in the z direction, away from the resonator. This shear wave can be written in terms of the instantaneous velocity of the liquid layer located at z, u,(z,t) u,(z,t) = Ae-k(r-l)cos [K(z - 1) - u t ] (2) where A is the amplitude of the wave at the interface z = 1. The characteristic distance describing the envelope of the decay function is l / k , the reciprocal of the propagation constant. The propagation constant can be written in terms of the density p and the absolute viscosity 7 of the liquid K = (op/29)'/2 (3) Shown in Figure 1is the z profile of u,, where values of p and 11 are those for water at 20 "C, namely, 0.9982 g cm-3 and 1.0022 cP, respectively. The characteristic decay length is 2500 A. This wave is so strongly damped that ita sinusoidal character is not apparent. Requiring that the transverse velocity of the surface of the quartz resonator at z =: 1 be identical with that of the adjacent liquid, and that the force exerted by the liquid on the crystal be equal and opposite to the force exerted by the crystal on the liquid, leads to the condition for the shift Af in the resonant frequency f o Af = ~ O ~ / ~ ( ~ P / V U $ P Q ) ~ / ~ (4) ' Here PQ and /.LQ are the density and shear modulus of quartz
having the values PQ = 2.648 g cm-3 and PQ = 2.947 X 10" g cm-l s - ~ , respectively. We have tested this relation with Nomura's data (1). These are shown in Figure 2 where the frequency shift between pure water and water/sucrose solutions as reported by Nomura (circles) is plotted against the weight percent of sucrose. The solid line is calculated from eq 4 using values for the density and viscosity of sucrose/water solutions from ref 11and assuming an unloaded crystal frequency of 9 MHz. The agreement is remarkable, especially as there are no adjustable parameters. Results of experiments performed in our laboratory on solutions containing up to 23 wt % glucose are similar and are shown in Figure 3. They clearly demonstrate the importance of viscosity since the density changes only from 0.998 to 1.09 while the viscosity increases by a factor of 2.2. These experiments used a 5-MHz crystal, a frequency nearly half of that of Nomura's. Thus, the quantitative agreement also substantiates the frequency dependence indicated in eq 4. The oscillation frequency also seems to be sensitive to strains induced by mounting and hydrostatic pressure. We believe the deviations at high glucose concentration are due to an increase in the hydrostatic pressure. These measurements were taken in a flow cell driven by a constant velocity pump, and the driving pressure certainly increased as the viscosity increased.
Registry No. Glucose, 50-99-7. LITERATURE CITED (1) Nomura, T.; Minemura, A. Nippon Kagaku Kelshl 1980, 7980, 1261. (2) Konash, P. L.; Bastiaans, G. J. Anal. Chem. 1980, 52, 1929. (3) Bruckenstein, S.; Shay, M. 1983 Abstracts, 1983 Pittsburgh Conference and Exposltion, Atlantic City, NJ, March 1983, Paper 763. (4) Hiavay, J.; Guiibauit, G. G. Anel. Chem. 1977, 49, 1890. (5) Glassford, A. P. M. J . Vac. Sci. Techno/. 1978, 75, 1836. (6) Crane, R. A.; Fischer, 0. J . fhys. D : Appl. f h y s . 1979, 72, 2019. (7) Sauerbrey, G. Z . fhys. 1959, 755, 206. (8) Lu, Chih-shun; Lewis, Owen J . Appl. f h y s . 1972, 43, 4385. (9) Mecea, V.; Bucur, R. V. Thln Solid Films 1979, 60, 73. (10) Kanazawa, K. Keiji; Gordon, J. G., unpublished results. (11) "Handbook of Chemistry and Physics", 63rd ed.; CRC Press: Boca Raton, FL, 1982.
K. Keiji Kanazawa* Joseph G. Gordon I1 IBM Research Laboratory, K33-281 5600 Cottle Road San Jose, California 95193
RECEIVED for review September 29, 1983. Resubmitted October 4, 1984. Accepted October 29, 1984.
Dehalogenation Reactions in Californium-252 Plasma Desorption Mass Spectrometry Sir: A recent communication (1) calls attention to the extensive dehalogenation with concomitant incorporation of hydrogen encountered by a series of nucleosides and thyroxine during fast atom bombardment (FAB) mass spectrometry. Through the use of deuterated glycerol, all hydrogens of the matrix were shown to be involved in the reduction process. Ease of replacement followed the general order I > Br > C1
> F, i.e., inverse to the order of bond strength, and a radical mechanism was suggested. Californium-252 plasma desorption mass spectrometry (PDMS) (2) is a similar particle-induced ionization method with the distinction that 100-MeV fission fragments are employed rather than 6-keV xenon atoms. More important to this-discussion is that, because a time-of-flight spectrometer
This article not subject to U.S. Copyright. Published 1985 by the American Chemical Society
1772
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
Table I. Halogen Replacement on Selected Ions of Halouracils by FAB and 2azCfPDMS FAB % exchange" (M+H)+ (M-H)-
sample
molwt
5-iodouridine 5-bromouridine 5-chlorouridine 5-fluorouridine
370
33
322
21 12
278
'"Cf PDMS % exchange" (M+Na)+ ( B + H ) + (M-H)-
(M+H)'
11
13.8
8.6
8.7
7.1 5.9 3.6
11.9 9.3
(B-H)8.8 2.0
4.2 2.1 1.1 1.0
5.3 8.9 0.8 4.5 7.0 1.2 " Percent exchange (dehalogenation) is calculated from the corresponding halogenated and dehalogenated ions, e.g., MHHt/(MHH++ MxH+). The sums of 36Cland W l or 79Brand 81Brvalues were used. 5.3
262
Table 11. Halogen Replacement on Selected Ions of Thyroxine by FAB and 2s2CfPDMS
ion (M + H + 2Na - 2H)+ (M + H + Na - H)+ (M + H)' (M + H - HCOOH)+
'Wf PDMS % FAB % exchange exchange 38 19
15 25
47
allows the use of very weak "primary" ion currents (lo00 fission fragments/s), the glycerol matrix is unnecessary. Hydrogen replacement from this source at least was expected to be eliminated. EXPERIMENTAL SECTION Mass spectra were obtained with a %%f plasma desorption mass spectrometer built for NIH by R. D. Macfarlane (Texas A&M University, College Station, TX). A 45-cm flight tube was used with a 10-pCi californium-252 source. Data were collected and processed with a Perkin-Elmer Model 3220 computer. Intensities were determined as the actual number of counts (ions) in the channel of maximum intensity (channel widths were 1.25 ns). The samples were applied to aluminized Mylar film by the electrospray technique (2)using methanol as solvent. Data were acquired for 60 min in each case. RESULTS AND DISCUSSION Tables I and I1 compare the results when the same samples of uridines and thyroxine as those used previously were examined with the PDMS technique. The observed loss of halogen is less than half that observed in FAB and the order of loss is the same as found previously, Le., I > Br > C1> F. Although the most abundant ions in the molecular weight region both here and in FAB are due to (M H)+ ions, intense (M + Na)+ peaks are also observed in PDMS and they show somewhat less halogen loss, although the order is the same. Peaks due to the uridine and halouridine fragments (B + H)+ are typically 3-10 times as intense as the (M H)+;again they show a similar range of halogen loss. In the case of the POlyiodinated amino acid thyroxine (Table 11), iodine is lost from (M H)+ions as well as the various sodium cationized forms and a fragment resulting from the loss of the carboxyl group.
In the negative ion mode, the loss of halogen from (M - H)ions is less pronounced, again as observed in FAB. In this mode, we also note intense (B - HI- peaks showing similar halogen loss, except in the case of fluorouracil. Intense (M + X)- ions are also detected, except in the case of fluorouracil, and they are invariably accompanied by X-, Xz-, and, in the case of iodine, X3- ions a t lower mass. These halide attachment ions underscore the ability of the ionization process to mobilize halogen from the organic substrate. Considering the lower mobility expected of all radicals in the solid sample matrix of PDMS along with the lower hydrogen content (3-4.1%) of the samples compared to glycerol (8.7%),the diminished halogen loss is perhaps not surprising. These data call attention to the need for caution in interpretation of results obtained with all mass spectral methods depending on ionization or desorption from solid or liquid matrices. ACKNOWLEDGMENT We thank James A. McCloskey (University of Utah) for providing the samples of uridines and thyroxine and for helpful discussions. Registry No. 5-Iodouridine, 1024-99-3; 5-bromouridine, 957-75-5; 5-chlorouridine, 2880-89-9; &fluorouridine, 316-46-1; thyroxine, 51-48-9. (1)
LITERATURE CITED Sethi, S. K.; Nelson, C. C.; McCloskey, J. A. Anal. Chem. 1984, 56,
(2)
Macfarlane, R. D. Acc. Chem. Res. 1982,
1975.
' Current address: Laboratory of Bioorganic Chemistry, National Institute
of Arthritis,
+
Diabetes, and Dlgestive and Kldney Diseases, National Institutes
of Health, Bethesda, Maryland 20205.
Yi-Ming Yang Henry M. Fales* Lewis Pannell'
+
+
15, 268-275.
Laboratory of Chemistry National Heart, Lung, and Blood Institute National Institutes of Health Bethesda, Maryland 20205
RECEIVED for review February 15,1985. Accepted March 21, 1985.
