Anal. Chem. 1982, 5 4 , 2114-2115
2114
Table I. Elution Data for Ethylenediamine concn,
ppm 100 15 50 25
0
IO
30
20
40
Flgure 1.
50
Ethylenediamine
Concentration
bo
80
70
90
100
I Q Q ~ ~
Callbration curve for ethylenediamine.
I ""I
0
5
""I
10
""i""1 15
20
TIME lMlNl
Determinatlon of ethylenediamine In llvestock iodine supplement: Dionex 4 X 50 mm catlon exchange column; eluent, 0.004 N HCI-0.0025 M ZnCI,; flow rate, 1.53 mL/mln; full scale = 3 pmho/cm; sample, 1.31 g/L iodine supplement. The resultlng concentration of ethylenediamine was determlned to be 12 ppm. Flgure 2.
ethylenediamine dihydrochloride to study linearity. A minimum of three points at each concentration was tested and a deviation from linearity above concentrations of 25 ppm was observed (see Figure 1). The data corresponding to integrated peak area vs. concentration (for concentrations of 0-25 ppm) were analyzed by linear regression and found to have a linear correlation of 0.99 with a relative standard deviation of 2.8% for eight measurements at 15 ppm. The minimum detectable limit was estimated to be less than 1 ppm by measuring signal to noise ( S I N ) at 1ppm. The signal was measured as max-
retention time, min
concn, ppm
retention time, min
6.53 6.81 1.20 7.80
20 15
8.01 8.24 8.27 8.60
10 5
imum peak height of ethylenediamine at 1 ppm, and noise was determined as the maximum peak to peak voltage variation in the base line for 1min before and 1 min after the peak. The resulting SIN ratio was calculated to be 105. It should be noted that using only a precolumn for separations greatly diminishes the capacity of the system. We observed that as concentration of sample increases, the retention times decrease slightly (see Table I). Small et al. have characterized a similar peak shift for many monovalent amines (3). For confirmation of the identity of the peak, the effluent was capturedafter the peak passed through the conductivity cell. This fraction was adjusted to a pH of approximately 9 using NaOH and extracted into chloroform. The organic solvent was evaporated down using a nitrogen stream at ambient temperature and placed into a solid probe cup for mass spectral analysis. The resulting electron impact mass spectra matched that of ethylenediamine. As a final experiment, a veterinary iodine supplement containing ethylenediamine dihydroiodide and sucrose was dissolved in deionized water and analyzed. Ethylenediamine was determined in amounts proportional to the therapeutic levels of iodine indicated (see Figure 2). Again the ethylenediamine was confirmed by mass spectral analysis. This determination is suited for a variety of applications for both solutions and salts containing ethylenediamine. ACKNOWLEDGMENT We wish to thank James Molnar, FBI Laboratory, for operating the mass spectrometer and interpreting the spectra. LITERATURE CITED (1) Wimberiey, J. W. Anal. Chern. 1981, 5 3 , 2137. (2) "Dionex Technical Note 4"; Dionex Corp.: Sunnyvale, CA, Aug 1980. (3) Small, H.; Stevens, T. S.;Bauman, W. C. Anal. Chern. 1975, 4 7 , 1801-1809. (4) Jaworski, M. Chromafographia 1980, 13, 2. (5) Douglas, B. E.; McDaniel, D. H. "Concepts and Models of Inorganic Chemistry"; Blaisdeli Publishing Co.: London, 1965; pp 391-397. (6) Gouid, E. S. "Inorganic Reactions and Structure"; Hoit, Rinehart and Winston: New York, 1961; p 335.
Richard C. Buechele* Dennis J. Reutter Forensic Science Research and Training Center FBI Academy Quantico, Virginia 22135
RECEIVED for review April 19,1982. Accepted June 7, 1982.
Exploding-Film Sample Introduction for Mass Spectrometry of Involatile or Thermally Labile Substances Sir: During the past decade, the search for techniques to obtain electron-impact mass spectra of thermally labile and/or involatile materials has become increasingly important. The usefulness of placing the sample near (I)and within ( 2 , 3 )the 0003-2700/82/0354-2114$01.25/0
electron beam in the mass spectrometer ion source has been demonstrated. Both of these techniques, however, require evaporation of the sample as a prerequisite to obtaining a mass spectrum. The technique described here does not depend 0 1982 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 54, NO. 12, OCTOBER 1982
2115
57C M'i
289 5 Ihl-Cll'
I
40
50
60
70
80
90
1
10C
110
'20
257
M A S S CHARGE
Figure 1. Exploding-fllm electron-impact mass spectrum of maleic acid. upon evaporation; we have used it to obtain electron-impact mass spectra of a wide variety of thermally labile and involatile compounds.
EXPERIMENTAL SECTION The gases generated by rapidly heating a thin film of nitrocellulose to a temperature above its decomposition point (ca. 168 "C) are used to propel thic sample to be analyzed into the electron beam of the mass spectrometer. Procedure. A conventional field-desorption emitter wire (with the carbon dendrites preeient to increase the surface area) is dipped into a 2.5% solution of nitrocellulose in acetone (w/w) and allowed to dry. The concentration of nitrocellulose is not critical; 0.5% to 5% solutions have been used successfulily, but the mo8t consistently reliable results were achieved with a 2.5% solution. The film-bearing emitter is then dipped into a solution of the sample to be analyzed, allowed to dry, and inserted into the mass spectrometer ion source to a position within a few millimeters of the electron beam. Heating the emitter wire results in an intense ion signal of short duration (seconds) when the decomposition point of the nitrocellulose film is reached. The nitrocellulose decomposition producta interfere significantlywith the sample ions only at m / e 18 (H,O), 28 (CO), 30 (NO),and 46 (NO,). Apparatus. The mass spectrometer was a MAT731 (Bremen, Germany) operated in tlhe electron-impact mode at 70 eV with the combined field desorption/electron impact ion source. The originally supplied 4-mm spacer screw was installed on the end of the field-desorptionprobe, permitting use of the field-desorption chassis to heat the emitter wire while the ion source was being operated in the conventional electron-impact mode. The field desorption emitter wire was maintained at accelerating (ion source block) potential. Reagents. Nitrocellulcme (type Rs,viscosfty 18-25 c P Hercules Inc., Wilmington, DE) was dissolved in acetone to make a 2.5% (w/w) solution. Maleic acid, aluminum phtlhalocyanine chloride, and riboflavin were obtained from Kodak Laboratory Chemicals, Rochester, NY. RESULTS AND DISCUSSION The mass spectrum of maleic acid obtained by the new technique is shown in Figure 1. An intense molecular ion ( m / e 116) is observed. When conventional sample introduction techniques are used this ion is of low intensity and a molecular ion appears a t m l e 98, generally attributed to virtually complete conversion of maleic acid to maleic anhydride. The protonated molecular ions typical ( 4 ) for compounds of this kind are not observed. The mass spectrum of aluminum phthalocyanine chloride (Figure 2) is an example of the utility of the exploding-film technique for the analysiii of an involatile material. In addition to the molecular ion, unusually large signals corresponding to (M - C1)2+were recorded. The mass spectrum of riboflavin
zw
601 173
c
401
:iI .
156
!
1
MASS C H A R G E
Figure 3. Exploding-film electron-impact mass spectrum of riboflavin. (Figure 3) provides intense signals for the molecular ion and fragmentation of the saccharide entity as compared with previously reported data (5). The mass spectra obtained by this technique are of high quality, but their duration is generally a few seconds. We have found a scanning speed of 4 s/mass decade with scan initiation at the first indication of rapidly increasing total ion current to be most useful. The field-desorption emitter normally is not damaged and, with reasonable care, can be reused many times. No problems with abnormally rapid ion source contamination have been observed. Use of conventional direct introduction or in-beam probes has not been explored. Although the number of compounds examined by this technique is not large, we have observed no instance-including sucrose-in which the protonated form of the molecular ion was produced preferentially. In some cases, the ions generated have been recorded by photoplate detection. The exploding-film technique has also been used to introduce samples into a Fourier transform mass spectrometer (Nicolet Instruments, Madison, WI). Results will be described elsewhere.
LITERATURE CITED Baldwln, M. A.; McLafferty, F. W. Org. Mass Specfrom. 1973, 7 , 1353-1 356. Dell, A.; Williams, D. H.; Morris, H. R.; Smith, G. A.; Feeney, J.; Roberts, G. C. K. J. Am. Chem. SOC.1975, 97, 2497-2502. Anderson, W. R.; Frlck, W.; Daves, G. D., Jr. J. Am. Chem. SOC. 1978, 100, 1974-1975. Reed, R. I.; Reid, W. K. J. Chem. SOC. 1983, 5933-5944. Brown, P.; Hornbeck, C. L.; Cronin, J. R. Org. Mass Specfrom. 1972, 6 , 1383-1399.
R. S. Gohlke* L. S. Wakeman Research Laboratories Eastman Kodak Company Rochester, New York 14650
RECEIVED for review April 22,1982. Accepted June 17,1982.
