Anal. Chem. 1983, 55, 145-146
solute, one structural parallel seems to dominate the responses of the acetyl and nitroxide radicals as the key functional groups in the two types of probe species. In both instances as the dipolar character of the solvent molecule increases there is a corresponding increase in the hyperfine splitting constant for the major functional group; and this condition accompanies the decrease in the spin density on the oxygen atom of the acetyl or nitroxide group with an increase in the net strength of the radical solute-solvent interaction. Registry No. 1-Methyl-4-acetylpyridinylradical, 64365-85-1.
LITERATURE CITED (1) Knauer, B.; Napier, J. J . Am. Chem. SOC.1978, 88, 4395. (2) Taft, R.; Abboud, J.; Kamlet, M. J . Am. Chem. SOC. 1981, 103,
145
(5) Kubota, S.;Ikegami, Y. J . Phys. Chem. 1978, 82, 2739. (6) Rlddlck, J.; Bunger, W. "Organic Solvents", 3rd ed.; Wiley-Interscience: New York, 1970; Chapter 5. (7) Yokoyama, T.; Taft, R.; Kamlet, M. J . Am. Chem. SOC. 1978, 98, 3233. (8) Kamlet, M.; Hall, T.; Boykin, J.; Taft, R. J . Org. Chem. 1979, 4 4 , 2599. (9) Chawla, B.; et ai. J . Am. Chem. SOC. 1981, 103, 6924. (10) It09 0.; Matsuda, M. J . Am. Chem. SOC. 1982, 1049 568. (1 1) Kamlet, M.; Taft, R.; Carr, P.; Abraham, M. J . Chem. Soc., Faraday Trans. 11982, 78, 1689. (12) Brady, J.; Carr, P. J . Phys. Chem. 1982, 86, 3053.
Orland W. Kolling Chemistry Department Southwestern College Winfield, Kansas 6?156
i.---. nm
(3) Abe, T.; Kubota, S.; Ikegami, Y. J . Phys. Chem. 1982, 8 6 , 1358. (4) Grossi, L.; Mlnlsci, F.; Peduill, G. J . Chem. Soc., Perkin Trans. 2 1977, 943.
for review August 3, 19B2* Accepted October 4,
1982.
Quantitative Analysis of Mixed Benzalkonium Chlorides by Laser Mass Spectrometry Sir: Laser mass spectrometry (LMS) has proved to be extremely useful for performing rapid qualitative analysis for a variety of nonvolatile and thermally labile compounds (1). Recently successful results have been obtained for qualitative analysis of a mixture of organic compounds (2). Though absolute quantification is difficult by LMS, the technique offers great potential for relative quantitative measureinents. Experiments using an epoxy resin doped with organometallic complexes of metal ions indicated that LMS has high potential with regard to both reproducibility and detection limits for metal ions (3). Promising results have been obtained for quantitative LMS analysis of metals in biological specimens (4, 5). Here we report for the first time, the quantitative analysis of a mixture of organic compounds. Two quaternary ammonium salts (I, 11) have been analyzed by LMS and compared with results for the same mixture from high-performed liquid chromatography (HPLC). The salts were present as chlorides. CHx
CH3
I
CH3
I1
EXPERIMENTAL SECTION The HPLC system consisted of a Waters 6000A pump, a Varian Varichrom detector, and B Rheodyne injection loop. The separation was carried out on a Zorbax CN column (250 X 4.6 mm, 5 pm) and a UV detector was used with 263 nm as the monitoring wavelength. The solvent yystem was acetonitrile-0.1 M acetate buffer solution (pH 5.0) (60/40 v/v) with a flow rate of 2.0 mL/min. The positive ion laser mass spectra were obtained with a LAMMA-500 instrument which has been described elsewhere (3). The output of a frequency quadrupled Q-switched Nd-YAG laser (265 nm, 15 nm pulse width) is focused onto sample using one of three microscopic objectives: lox, 32X, 1OOX. In this case tho -. .. .
32X objective was used. Changes in laser spot size and power had little effect on the mass spectra. Pulse power was varied with a set of fiters and was adjusted to give the optimum power density needed to obtain a mass spectrum (-los W/cm2). The absolute concentration ratio of components 1I:I was found to be 3 0 from HPLC. For LMS analysis, the sample was dissolved in methanol and evaporated on a formvar filmed grid t o give a uniform thin layer. The sample was scanned by the microprobe and 12 spectra were stored and averaged using a Hewlett-Packard 1000E-seriesdata system. The reproducibility of this experiment is &lo% relative standard deviation. Except for small intensity variations, there were no significant differences between the spectra that were averaged.
RESULTS AND DISCUSSION Figure 1shows the HPLC separation of the benzalkonium chloride mixture (I, 11). Because the components comprising the mixture are chemically similar and detection depends on the UV absorption of the benzyl moiety, no external standards were used for quantitation. Integration of the peak areas was done by height times width-at-half-height. This technique yielded a ratio, 111, of 3.0. No significant amounts of the Clo and CI6derivatives similar to I1 and I could be detected by HPLC. Thus the sample was assumed to be a two-component mixture. Figure 2 shows the positive ion laser desorption mass spectra (averaged over 12 spectra) of the benzalkonium chloride mixture. Intact cations appear at m / z = 304 (11) and m / z = 332 (I). Loss of toluene from both cations is consistent with the following fragment ions:
*hHz
N-c14H19
I
m / z = 240 derived from I
m / z = 212 derived from 11. 0 1982 Amerlcan Chemical Soclety
Anal. Chem. 1983, 55, 146-148
146
R : Cj4HZg INTACT CATION I
h H 2 5 II
II
I
HPLC
BENZALKONIUM CHLORIDES
Figure 1. HPLC separatlon of benzalkonium chloride mixture (I, 11).
, ~ o
C14H29
INTACT CATION
51
BENZALKONIUM CHLORIDES
POSITIVE ION LDMS
I
spectrometry can be applied to analysis of mixtures of organic compounds using quasi-molecular ions. It is interesting to note that if one attempts mixture analysis from the fragment ions a t mlz 240 and 212 a ratio of 1I:I = 4.0 results. This clearly differs from the HPLC ratio, indicating that one may have to exercise caution when using peaks other than those for quasi-molecular ions for quantitation. One might argue that the intensity of the mlz 304 intact cation may have a contribution from the other (mlz = 332), as a result of a neutral loss of 28 mass units. Such a loss can be excluded, because fragmentation through a four-membered transition state, which is usually expected for quaternary ammonium salts, would not lead to loss of neutral fragment of 28 mass units. Even if it did occur, one would expect the same type of fragmentation from I1 which is chemically similar and of higher intensity than compound I. Such is not observed. The analysis of the benzalkonium chloride mixture by LMS is in excellent agreement with the HPLC result, within the limit of experimental error (&lo%). This first attempt of quantitative analysis of organic compounds shows that LMS using the LAMMA 500 has potential for quantitative analysis of organic mixtures. LMS has several advantages over other techniques for quantitation. First, it requires only a small amount of sample (micrograms or less). Second, no special sample preparations are required. Third, analyses are fast. Fourth, the microprobe capabilites of the LAMMA-500 have the potential for quantitative analysis of organic inclusions in an organic matrix.
ACKNOWLEDGMENT We thank N. Brake and K. Cornelius for their help with the HPLC work. Registry No. I, 139-08-2;11, 139-07-1.
C12H25 II
'I
LITERATURE CITED
Figure 2. Laser desorptlon mass spectrum of benzalkonium chloride mixture (I, 11).
The base peak at mlz = 91 is the familiar benzyl (tropylium) ion, as would be expected. We have used the intensities of the intact cation peaks for quantitative calculation. The ratio measured by LMS for the benzalkonium chlorides is 111 = 2.9 & 0.3. This is within experimental error of the value of 3.0 derived from HPLC data. This is a clear demonstration that laser desorption mass
(1) Hercules, D. M.; Day, R. J.; Balasanmugarn. K.; Dang, T. A.; Li, C. P. Anal. Chem. 1982, 5 4 , 280A. (2) Talrni, Y.; Dutta, K. P. Anal. Chlm. Acta 1981, 132, 111-118. (3) Kaufmann, R.; Hillenkamp, F.; Wechsung, R. Med. Prog. Techno/. 1979, 6, 109-121. (4) Schroder, W. H. Z . Anal. Chem. 1981, 308, 212-217. (5) Seydel, U.; Lindner, B. Z . Anal. Chem. 1981, 308, 253-257.
Kesagapillai Balasanmugam David M. Hercules* Department of Chemistry University of Pittsburgh Pittsburgh, Pennsylvania 15260 RECEIVED for review August 2, 1982. Accepted October 14, 1982. This work was supported by the National Science Foundation under Grant CHE-8108495.
Fiber Optic Probe for Remote Raman Spectrometry Sir: When combined with UV-Vis spectrophotometry, optical fibers are useful waveguides for directing radiation of the sample and returning the partially absorbed light for detection. A recent report discussed the application of fibers for remote fluorescence, where the sample may be located a great distance from the spectrometer ( I ) . However, the poor transmission characteristics of silica fibers in the infrared region prevent their use in IR absorption spectrometry. We report here a probe for Raman spectrometry, where the advantages of fiber optics are combined with the structural
information inherent in Raman spectra. Fiber optics have been used previously for collection of Raman scattering ( 2 ) and for holding samples for Raman spectrometry ( 3 , 4 ) . In the present work, both the excitation beam and scattered light are carried by fibers, so the sample may be located far away from the spectrometer, in a hostile environment if necessary. In addition the probe itself is very simple and rugged and may be used for routine analysis. The apparatus is shown in Figure 1 and is based on 200 pm diameter multimode fibers of common use in communications.
0003-2700/83/0355-0146$01.50/00 1982 American Chemical Society
