ANALYTICAL CHEMISTRY, VOL. 51, NO. 8, JULY 1979
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the same and that the half-peak widths should be 90 mV. Note in Figure 2 that the peak potentials do have a slight separation and the half-peak widths slightly exceed the theoretical values also. These deviations are scan rate dependent as shown and result from the finite resistance of the Ndion membrane separator. These deviations can be reduced but, of course, not eliminated by using a gold minigrid auxiliary electrode mounted concentric to the TLE assembly. It should be pointed out that any electrode material including mercury coated Pt or Au wires can be used as working electrodes and that the materials for auxiliary and reference electrodes are not limited either. The actual cost of making these cells is very small, especially as the electrode materials are reusable. Because of the permselectivity and small porosity of this material, electroactive anions and larger neutral and cationic species (Vitamin BIzaand BIZ, for example) have an unmeasurable rate of diffusion out of the TLE assembly during the course of typical electrochemical experiments. Small cations such as Fe3+ are taken up by the membrane and therefore cannot be used. This limitation can be circumvented by using a porous Vycor tube as a separator ( I O ) , but this material has a higher resistivity than the Nafion. Two alternative approaches using Nafion are being investigated and will be reported a t a later date along with experiments on volume calibration, flow cell applications, nonaqueous applications, concentration sensitivity effects of acidity and alkalinity, and biological uses.
1 Figure 2. Typical current-potential curves. 10 mM Fe(CN),3-in 0.5 M H,SO,. Volume = 0.25 pL. sweep rate: (1) 1 mV/s, (2) 2 mV/s. (A€)ip/2: (1) 90 m V , (2) 102 m V . - Epc): (1) 10 m V , (2) 20 mV
section can contain supporting electrolyte only. Therefore, very small amounts of the electroactive substance are actually needed for experiment. Solutions in both the T L E and 'I'L cell are drawn into by capillary action almost instantaneously. Because of the cationic permeability of the Nafion, it has a very low ohmic resistance and therefore it does not exhibit significant distortion in the electrochemical response even with relatively fast transient experiments. Typical current-potential curves for linear potential sweep voltammetry with this thin-layer electrode for the oxidation of Fe(CN)64- and subsequent reduction of Fe(CN)z- in 0.5 M H2S0, are shown in Figure 2. TLE theory (8)predicts that the peak potentials for the forward and reverse scans for reversible reactions are
LITERATURE CITED Yubbard, A. T.; Anson, F. C. Anal. Chem. 1964, 3 6 , 723. McClure, J. E.; Maricle, D. L. Anal. Chem. 1967, 3 9 , 236. Christensen, C. R.; Anson, F. C. Anal. Chem. 1963, 3 5 , 205. Sheaffer, J. C.; Peters, D. G. Anal. Chem. 1970, 42, 430. Anderson, L. B.; Reilley, C. N. J , Eleckoanal. Chem. 1965, 10, 295. (6) Propst, R. C. Anal. Chem. 1971, 43, 994. (7) Murray, R. W.: Heinernan, W. R.; O'Dom, G. W. Anal. Chem. 1967, 39, 1666. (8) Hubbard. A. T. Crlf. Rev. Anal. Chem. 1973, 3 , 201. (9) Lecuire, J. M.; Pillet, Y. J. Nectroanal. Chem. 1978, 97, 99. (IO) Tom, G. M.; Hubbard, A. T. Anal. Chem. 1971, 43, 671. ( 1 1) Caja, J.; Czerwihski, A. unpublished results, 1979. (1) (2) (3) (4) (5)
'On leave from the University of Warsaw, Poland.
Josip Caja Andrzej Czerwinski' Harry B. Mark, Jr.* Department of Chemistry University of Cincinnati Cincinnati. Ohio 45221
RECEIVEDfor review February 15, 1979. Accepted April 11, 1979.
Use of Field Desorption Mass Spectra of Polystyrene and Polypropylene Glycol as Mass References up to Mass 10 000 Sir: With the rapid development of new ionizing techniques, for example, the field desorption ionization ( I ) , the means for obtaining mass spectra of high molecular weight substances are now widely available. T o extend the measurable mass range, improvements in the ion source and/or enhanced resolution of the mass spectrometer itself are important. However what is just as important is to prepare suitable mass references which give adequate ion peaks in the
mass range of interest. Perfluorokerosene, perfluorotertiarybutylamine and perfluorotriheptyltriazine (2) are now widely used in the mass range below lo00 amu with an electron impact ion source. Perfluoroalkoxyphosphazines ( 3 , 4 )give reference peaks up to 3700. Polyperfluoropropylene oxide has been recently reported as mass reference for the mass range 31-2000 ( 5 ) . This paper proposes the use of polystyrene and poly-
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very heavy molecules, those with molecular weight o f more than 10000, can still be singly ionized, mass analyzed. and detected. The rough mass number 10 000 was determined by the experimental conditions which were: 1.5-kV accelerating voltage, 0.5-m magnet radius and 1.1%Tesla magnetic field strength. The structure of polystyrene used for the study is as follows:
C H,(CH.2)3 [ C H, CH ( CGH,: ) ] .H
;A:
8°K
In this case, the molecular weight is expressed by (58+ I04n) amu. T h e mass number of major mass peaks up t a n = 10 (M = 1098) was calibrated by PFK, and the mass values expected from the formula (58 + 104n) were assigned to the strongest mass peak appearing with 104 mass intervals. Therefore a similar relationship was assumed to be valid in the region n = 11 (M = 1202) to n = 100 (M= 10458). This assumption was confirmed by observing the peak position matching in the region of overlap of the mass spectra as shown in Figure 1. Figure 1 clearly demonstrates that the repetition of strong peaks occurs in a regular interval. The accuracy of the nominal mass assignment is fO.5 amu up to m / p = 2000. f 2 up t o mle = 5000, and *5 amu up to n i / ~= I 1 000. The decrease in accuracy in the heavier region is caused hy two factors. The first is the problem of the apparatus iimitations: insufficient resolution due t o the use of a wide slit system to obtain better transmission, and the use of a 12-bit A/D converter for the mass marker. The second is the influence of isotope peaks. In the case of n = 100 ( M = 10458), for example, the molecular formula is expressed as CX(,IHB1,I. The intensity of isotope peaks becomes much stronger than that of a monoisotopic molecular peak because of the presence of I3C and *H. Considering a molecule with molecular formula C,Hj, the nominal mass of a monoisotopic molecule is given by M = 12i + j . The total probability of obtaining a molecule of mass (M+ K ) attributable to K mass contributions from isotopic replacements is calculated to be:
,*
0-4""
-,e
mass spectrum of polystyrene. Only significant peaks are bar plotted. The double bars mean overlapping of the peaks. The relative intensities are the averages of several measurements Figure 1. FD
propylene glycol as mass references. They offer direct internal calibration of mass scale in quite a wide mass range: from 266 to 11000. Since intense reference mass peaks are obtained by FD ionization, a mono FD ion source can be effectively used, thereby eliminating the complexities of a combined EI/FI/FD ion source.
EXPERIMENTAL Mass spectra were acquired using a Matsuda type double focusing mass spectrometer (6) at a resolution of MILW = 2000 (acceleration voltage, 1.5 kV; magnet radius, 0.5 m; main slit width. 0.2 mm). A 16-stage electron multiplier (Hamamatsu Co.. Ltd.) with 3-kV applied voltage was used. Post-acceleration of ions was not introduced. Output signals of a multiplier were amplified by a dc amplifier and recorded on an C V recorder (San-ei Instrument Co., Ltd.j with pulsed signals from the hole probe in the magnet. In all measurements, a silicon emitter with a 50-pm diameter tungsten core (7)was used as a field anode. The voltage between the anode and cathode was 10 kV. The emitter current was increased at the rate of 50 mA/min. Molecular ions lighter than 3000 amu appeared at about 400 mA of emitter current and those heavier than 5000 amu appeared a t about 500 mA. The temperatures corresponding to these currents, are roughly estimated to be lower t,han 400 "C. When the emitter current reached 700 mA, the temperature could be measured by an optical pyrometer (Yokogawa Co., Ltd.). The measured temperature was approximately 700 O C . Two microliters of solution (containing about 2 pg in acetone solvent) were loaded on the emitter by a syringe. Polystyrene and polypropylene glycol were purchased from Pressure Chemical Company.
j!
c I ' - K + 1 ) ! ( K - 1) ! where u = 0.01108 and c = 0.00015 are the natural abundance ratios of 13C and *H, respectively. T h e value of K corresponding to the strongest isotopic peak (K,,,,) is calculated from the above equation to be approximately K,,'=. ui. The contribution from the presence of 'H is calculated to he negligible. In addition to this, a mass shift of 0.007825j should be taken into account because the exact mass of hydrogen is 1.007825. In the case of CR04H810, for instance, ui + 0.007825j = 15.2. Therefore the nominal mass of the strongest isotopic peak is considered t o be 1 2 X 804 + 810 + 15 = 10473. R e have interpreted the mass number of the strongest observed peak in each group to correspond to the isotopic peak expected to be strongest from the above calculation. Polypropylene Glycol. T h e plotted FD mass spectrum
RESULTS AND DISCUSSION Polystyrene. T h e observed FD mass spectrum of polystyrene is shown in Figure 1 in the form of a bar graph plot. The spectrum was constructed by four mass spectra obtained from samples with different average molecular weight. The UV recording of the FD mass spectrum of polystyrene is shown in Figure 2. In particular it is worth noting that the
7000
8000
9000
10600
m e
11oOo
UV recording of FD mass spectrum of polystyrene of average molecular weight 8500. T h e voltage supplied for the 16-stage electron multiplier was 3 k V Figure 2.
A N A L Y T I C A L CHEMISTRY, VOL. 51, NO.
Figure 3. FD mass spectrum of polypropyrene glycol Only significant
peaks
a r e bar
plotted
for a sample of average molecular weight 1220 is shown in Figure 3. Though the exact structure is unknown because it depends on the initiator and the catalyst for the polymerization process, the general structure is assumed to be:
HO[CH,CH(CH,)O],H The mass number of the manoisotopic peak of each group is expected to have (18 + 5 8,) amu from this structure. However in practice, it was (41 + 5 8,) amu which suggests that the base peaks were composed of ions containing Na cations. This assignment was confirmed by adding NaI to a solution of polypropylene glycol and observing a tremendous increase of the same base peaks. When KBr was added to the solution, ions containing K cations with corresponding (57 58m) amu become the base peak of each group. Polyethylene glycol was also investigated and it gave similar Na cation containing base peaks every 44 amu (CH2CH20).This sample is preferable for the mass calibration in high resolution measurements because the repeating unit is only 44 amu. The FD mass spectrum of polypropylene glycol of higher average molecular weight (mol w t = 4000) and that of polyethylene glycol (mol wt = 1000, 2000, 4000, 10000) will be reported separately.
+
In summary, polystyrene and polypropylene glycol complementarily offer a sufficient number of mass references in the high mass region (1000 < m / e < 10 000) for nominal mass determination. They can be also used as standards for high resolution measurements, when increased mass resolution and better A/D converters or photoplate detection are available. Another important aspect of this work is the direct determination of the molecular weight distribution of polymers. The mass spectrum of polypropyrene glycol shown in Figure 3 is in effect nothing but the distribution of the degree of polymerization of the sample. Though the separation of styrene-oligomer ( n = 1, 2, . . . 15) was accomplished for
8, JULY 1979
1331
convenience by liquid chromatography, the separation of polymers in the mass range of 10 000 amu is possible only by this method. The pattern of spectra shown in Figures 1 and 3 is expected to reflect the molecular weight distribution of the sample, because the temperature of the emitter is not high enough for thermal degradation. When the anode temperature becomes higher (the corresponding emitter heating current becomes more than 650 mA), then the mass spectrum no longer shows the repeated pattern but rather a continuous spectrum in all t,he mass range. In such a case the pyrolysis field desorption phenomena (8)may occur simultaneously with field desorption process. It is quite astonishing that singly charged ions of very heavy molecules ( m / e > l O O O O ) are detectable by mass spectroscopy. The investigation of such heavy mass regions will be interesting and significant in the future. ACKNOWLEDGMENT The authors acknowledge the helpful discussions of Ti. Izumi, Institute for Protein Research, Osaka Cniversity. The authors also thank the AID group of JEOI, Ltd. (Tokyo) for advice in the early stages of this work. LITERATURE CITED Beckey, H. D. "Principle of Field Ionization and Field Desorption Mass Spectrometry". Pergamon Press: New York, 1977. Aczel, T. Anal. Chem. 1968, 40, 1917 Fales, H. M. Anal. Chem. 1966, 3 8 , 1058. Olson, K. L.; Rinehart. K . L., Jr.: Cook, J. C.. Jr. Biomed. Mass Spectrom. 1977, 4 , 284. Ligon, W. V., Jr. Anal. Chem. 1978. 5 0 , 1228. Matsuda, H. At. Masses Fundam. Constants 1976, 5 , 185. Matsuo, T.; Matsuda, H.; Katakuse, I . Anal. Chem. 1979, 5 1 , 69. Schulten, H. R.; Becky, H. D.: Boerboom, A. J. H.: hleuzelaar, H. L. C. Anal. Chem. 1973, 4 5 , 2358.
Takekiyo Matsuo* Hisashi M a t s u d a Institute of Physics College of General Education Osaka University Toyonaka, 560, Japan Itsuo Katakuse Department of Physics Faculty of Science Osaka University Toyonaka, 560, Japan RECEIVED for review February 12, 1979. Accepted March 12, 1979.
