Determination of Alkali Elements by Field Desorption Mass Spectrometry W. D. Lehmann and H.-R. Schulten" Institute of Physical Chemistry, University of Bonn, Wegelerstr. 12, 5300 Bonn, West Germany
The determination of cesium in sample sizes of 0.2 to 1 pL in which 0.3 to 1000 pg of the element was present has been achieved by field desorptlon mass spectrometry and single ion monitoring. The preclslon and accuracy of the method were f10% and about f20%, respectively. A sensitivity between 1.4 and 2.5 X lo-' C per pg was obtained for cesium. The sensitivity for detectlon of the other alkali elements was of the same order of magnitude but decreased slightly from cesium to lithium. Cesium was estimated In spectrograde solvents, in body fluids such as saliva and blood, and in environmental samples such as drinking water, seawater, and a natural aerosol.
T h e extraordinary high sensitivity of field desorption (FD) mass spectrometry (MS) for alkali ions was discovered during the early investigations of organic salts ( I ) . The FD spectra obtained always showed the signals for sodium and potassium ions t o be several orders of magnitude more intense than all organic ions when the spectra were recorded on photoplates and the emitter temperature was raised t o red heat ( 2 ) . Moreover, the signals for sodium and potassium were also observed with high relative abundance in the F D spectra of organic compounds that contained alkali salts as impurities (3-5). The presence of alkali cations [Cat]' in organic samples leads t o an attachment to the organic molecule (M) generating stable [ M Cat]' complexes (cationization) (6-8). This process is useful for molecular weight determination and direct isotope analysis of organic compounds. Although there is unanimous agreement between all groups working in the FD-MS field as t o the exceptional sensitivity for alkali cations, no analytical application of this phenomenon has been reported to date. This prompted us t o investigate t h e sensitivity and precision of field desorption mass spectrometry in the single ion monitoring mode for quantitative investigations of alkali ions and t o exploit the potential of the technique for trace analysis.
+
EXPERIMENTAL The FD ion currents were recorded on a homebuilt single focusing mass spectrometer of low resolution equipped with a FD source with micromanipulator (6). The FD emitter (at +8 kV) was positioned at a distance of 1 mm from the counter electrode (at -4 kV). Only the first lens (at approximately +2 kV) was used for focusing of the ion beam whereas all other deflection plates were a t ground potential. With an entrance slit width of 0.1 mm and an exit slit width of 0.5 mm, a resolution of about 300 (10% valley definition) was achieved. This experimental setup simplifies the operation of the FD mass spectrometer considerably because it allows an easy, fast, and reproducible optimizing of the FD ion currents which is particularly relevant for quantitation. The ions were detected using a channel electron multiplier (Valvo) and a combined counter/ratemeter registration unit (Ortec). The chamel electron multiplier was operated at -3 kV. Field anodes employed were 10-pm tungsten wires activated at high temperature ( I ) . The average length of the microneedles was about 30 pm. A linear emitter heating current (ehc) programmer (7) was used for the desorption of the samples. In all 1744
ANALYTICAL CHEMISTRY, VOL. 4 9 , NO. 12, OCTOBER 1977
cases the ehc was raised with 0.19 mA/s from 0 t o 100 mA (see Figure 2). All measurements for the calibration curve for [Cs]' (see Figure 1) were made with one FD emitter starting from small sample amounts. Before the registration of each evaporation profile (II), the emitter position and its field ionization efficiency were checked by monitoring the intensity of the molecular ion signal of isooctane ( m l e 114) and 0 mA ehc. For the determinations of unknown concentrations of cesium (see Table I), in each case a new field anode was employed. These emitters showed (within a deviation of about 20%) equal field ionization efficiency and control in the light microscope revealed that they had a similar morphology as the one used for the calibration curve. Standard solutions were prepared as follows: Cesium chloride (Suprapur, Merck AG, Darmstadt) was dissolved in methanol (Uvasol, Merck AG, Darmstadt) and aliquots of this solution were diluted by carbon tetrachloride (Uvasol) and carefully homogenized. For trace determination of alkali cations the procedure was executed in two steps: First, 3 pL of a standard solution containing a known amount of the alkali halide were applied to the emitter by the modified syringe technique (9)and a signal at the according mle value was recorded. Second, between 0.2 gL and 1 pL of sample were applied t o the same emitter and desorbed under identical conditions. From the peak areas of the evaporation profiles obtained in both measurements, the unknown amount of the alkali element present in the sample was calculated. Usually one analysis (calibration + sample analysis) could be performed within 30 min. The precision for repeated measurements of a standard solution was & l o % ;the accuracy of the technique for the determination of unknown concentrations was &20%.
RESULTS A N D DISCUSSION After multiple use, the ionization efficiency of F D emitters decreases. Hence, for quantitative determinations by FD-MS, the method of internal standardization generally is preferred ( I O , 11). Using this technique, the sample (unknown amount) and standard (known amount) are desorbed from the FD emitter in one measurement under identical conditions. Previously, it has been shown that stable isotope dilution is the method of choice for quantitative determinations in
FD-MS (12-15). For quantitative mass spectrometric analysis of the elements of the first group of the periodic system, the technique of stable isotope dilution can be employed for lithium, potassium, and rubidium. However, quantitation of the monoisotopic elements sodium and cesium can only be performed by using a radioactive isotope or a different alkali element as internal standard or by establishing a calibration curve as external standard. In the case where very small amounts of sample are coated on the F D emitter the detrimental effect on the ionization properties after repeated measurements is minimized. In the F D analysis of alkali cations, because of the high sensitivity of the technique, only minute amounts of sample are required and thus a quantitative determination with an external standard is possible. Calibration and Sensitivity. As a basis for quantitative analysis by FD-MS, a calibration curve for the [Cs]' cation was established (Figure 1). This plot correlates the sample
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Single
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~~~~~
Table I. Determination of [ Cs]' in Spectrograde Solvents, Body Fluids, and Environmental Samples b y Field Desorption Mass Spectrometry
F I E L D DESORPTION MASS SPECTROMETRY [cs].
.i
Counts' Solvents
Body fluids
-1
~
lC.'*
'
Environmental samples
Water, dist., 8 100 0.9 1 PL Carbon tetra29000 3 chloride, 1 p L Methanol, 1 p L 6 100000 350 Human blood, 34000 18 0.2 p L Human saliva, 720000 300 0.2 p L Drinking water, 18 000 2 1 ClL Seawater, 666000 165 0 . 3 pL Natural aerosol, 760000 65 1-3
I
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,
,
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g
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I
,
I
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,
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,
,
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tetrachloride were prepared and analyzed. The values of the analyses are displayed with an error of 20% corresponding to 2~ (0= standard deviation) F I E L D DESORPTlON I SINGLE ION MONITORING of [Cs]'
, mle 2
L
6
pg/pL pg/pL pg/pL pg/pL pg/pL pg/pL pg/pL pg
pg
' The number of counts corresponds t o the intensities obtained for the calibration measurements.
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Figure 1. Calibration curve for quantitative determination of [Cs] correlating the sample amount with the peak area of the evaporation profile obtained by FD-MS and single ion monitoring of m l e 133. For these measurements, a number of standard solutions of CsCl in carbon
0
Concentration Cs
133
8
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Figure 2. Evaporation profile of about 350 fg [Cs]'. The emitter heating current was raised from 0 to 100 mA linearly with 0.19 mA/s. It can be derived that this sample amount corresponding to 2800 counts is by far more than the detection limit since the noise level is about 20-30 counts/s amount applied to the F D emitter with the peak area of the evaporation profile. T h e range of sample amounts covered by this curve extends from 1000 to 0.3 pg of cesium corresponding to 7 . 5 pmol to 2.3 fmol. When a linear extrapolation to smaller sample amounts is applied, a detection limit for [Cs]' of about 10 fg is derived from the calibration curve in Figure 1. However, measurements in this range of concentration could not be performed since no solvents containing less than about 1 pg/pL cesium were available. The original trace for the detection of about 350 fg of [Cs]' by field desorption and single ion monitoring is shown in Figure 2 . From Figures 1 and 2 , it can be derived that the sensitivity of the F D method for alkali ions exceeds the sensitivity for organic compounds. T h e observed sensitivity of about C I p g is of the same order of magnitude as the sensitivity of electron impact mass spectrometry for the detection of organic compounds. One of the reasons for this phenomenon is that t h e particles t h a t are t o be detected are already present as
positive ions on the emitter surface. Field desorption of these monovalent ions from the matrix on the emitter obviously is much more efficient than field desorption of organic compounds. For organic compounds, the processes of thermal decomposition and evaporation of neutral molecules that do not undergo ionization compete effectively with the ionization process, whereas this is not valid for field desorption of alkali cations. T h e increase in sensitivity in the alkali ion series from lithium to cesium corresponds with the variation of the lattice energies and desolvation energies of the alkali elements. These energies decrease from [Li]+ to [Csl' thus favoring the desorption of [Cs]' as compared to the desorption of [Li]'. This effect is also reflected in the observation t h a t [Cs]' desorbs a t a lower emitter temperature than [Li]'. Investigation of Samples. In order to test the capacity of FD-MS for quantitative trace analysis, cesium was determined in a number of samples. We investigated solvents, body fluids, and environmental samples by single ion monitoring of mle 133 and measuring of the peak areas of the evaporation profiles by ion counting. The results of these analyses are compiled in Table I. T h e organic solvents investigated were of commercially available quality (carbon tetrachloride, Uvasol, E. Merck AG, Darmstadt, West Germany; methanol, analyzed reagent, J. T. Baker Chemicals B.V., Deventer, T h e Netherlands). Drinking water was taken from the t a p in our laboratory. Seawater was a sample taken from the North Sea near the coast of T h e Netherlands. The aerosol was directly sampled on a FD emitter (16, 17) for 2 h on the roof of our laboratory on May 13, 1977. Samples of body fluids were taken from the authors and analyzed without further treatment. Whereas the evaporation profiles of [Cs]' from t h e pure solvents showed a smooth shape, the investigations of the seawater, of the organic samples, and the aerosol showed stronger fluctuations of the ion current. For t h e analysis of the organic samples, the coated emitter was heated to about 50 mA for some minutes without t h e high voltage applied in order to evaporate and destruct t h e organic material. Then the analysis was performed as for the other samples. Performing this procedure with the high voltage applied to the emitter, we found by monitoring of m l e 133 that no significant loss of cesium occurs. In comparison with t h e method described, atomic absorption spectrometry has a lower sensitivity for cesium (18, 19) and requires special experimental conditions (20, 21). However, determinations of cesium by surface ionization mass ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
1745
spectrometry have been reported for inorganic cosmochemical (22) a n d radiochemical (23, 24) samples and revealed quantitative data in the femtogram range. Spark source mass spectrometry, which allows the simultaneous determination of a variety of metals, has been used for the quantitation of a number of metals including cesium in ashed mammalian blood samples (25). However, the results described above show that FD-MS has the unique feature of allowing the estimation of alkali elements in very small amounts of untreated biological a n d enuironmental samples. Quantitative analyses of lithium, potassium, and rubidium by FD-MS and stable isotope dilution are under way in our laboratory and are expected to enable determinations with similar sensitivity and higher accuracy.
LITERATURE CITED (1) H.-R. Schulten and H. D. Beckey, Org. Mass Spectrom., 6, 885 (1972). (2) H.-R. Schulten and F. W. Rollgen. Org. Mass Spectrom., 10, 649 (1975). (3) H.-R. Schulten, J . Agric. Food Chem., 24, 743 (1976). (4) H.-R. Schulten and D. Kummler, Fresenius’ 2 . Anal. Chem., 276, 13 (1976). (5) H.-R. Schulten, Methods Biochem. Anal., 24, 313-448 (1977). (6) H.-R. Schulten and H. D. Beckley, Org. Mass Spectrom., 7, 861 (1973). (7) F. W. Rollgen and H.-R. SchuRen, Org. Mass Spectrom., 10, 660 (1975). (8) F. W. Rollgen, U. Giessmann. and H.-R. Schulten, “Advances in Mass Spectrometry”, Vol. VII, N. Daly, Ed., Heyden & Sons, London, 1977, in press, and references cited therein. (9) H. D. Beckey and H.-R. Schulten, Angew. Chem., Int. Ed. Engl., 14, 403 (1975). (10) H.-R. Schulten, Cancer Treat. Rep., 60, 501 (1976).
H.-R. Schulten. W . D. Lehmann, and M. Jarman. in “Quantitative Mass Spectrometry in Life Sciences“, A . P. De Leenheer and R . R. Roncucci, Eds., Elsevier Scientific Publishing Company, Amsterdam, p 187. W. D. Lehmann, H. D. Beckey, and H.-R. Schulten, Anal. Chem., 48, 1572 (1976). W . D. Lehmann, H. D. Beckey. and H.-R. Schulten, Ref. 11. p 177. W. D. Lehmann and H.-R. Schulten, Angew. Chem., Int. Ed. Engl., 16, 184 (1977). W. D. Lehmann and H.-R. Schulten, Horned. Mass Spectrom., in press. H.-R. Schulten and U. Schurath, J . Phys. Chem., 79, 51 (1975). H.-R. Schulten and U. Schurath, Atmos. Environ.. 9. 1107 (1975). D. A . Segar and J. G. Gonzalez, Anal. Chim. Acta, 58. 7 (1972). K. Govindaraiu. R. Hermann. G. Mevelle. and C. Chourad. At. Absoro. Newsl, 12, j 3 (1973) G E Janauer, F E Smith, and J Mangan, At Absorp Newsl, 6 , 3 11967) k . ~ AV. . Derschau and H. Prugger, Fresenius’ Z . Anal. Chem., 247, 8 (1969). B. M.Gordon, L. Friedman, and G. Edwards, Geochim. Cosmochim. Acta. 12. 170 (1957). B. M. Gordon and L. Friedman, Phys. Rev., 108, 1053 (1957). G. Friedlander, L. Friedman. B. Gordon. and L. Yaffe. Phvs. Rev.. 129. 1809 (1963) D F Ball, M Barber, and P G T Vossen, Biomed Mass Spectrom , 1, 365 (1974)
RECEIVED for review May 27, 1977. Accepted July 19, 1977. Presented in part a t the International Symposium on Microchemical Techniques, Davos, Switzerland, May 22-27,1977. This work was supported by the Deutsche Forschungsgemeinschaft, Ministerium fur Wissenschaft and Forschung des Landes Nordrhein-Westfalen and the Fonds der Deutschen Chemischen Industrie.
Plasma Chromatography of Phosphorus Esters J. M. Preston” Defence Research Establishment Ottawa, National Defence Headquarters, Ottawa, Ontario, Canada K 1A 024
F. W.
Karasek and S. H. Kim
Department of Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3G 1
Mobility and diffusion coefficients, in air, of 47 ions formed by atmospheric ionic reactions of 16 phosphorus esters and thloesters are reported. Very careful assignment of ions to the peaks of the mobility spectra produced considerable confidence in these assignments. With some compounds, asymmetric peaks indicative of limlted hydration reactions were noted.
Phosphorus esters vary tremendously in toxicity. Diethyl methyl phosphonate, for example is essentially innocuous ( I ) while for a substitution product such as parathion the average lethal subcutaneous dose for a 0.5-kg mouse is only 8 mg. The corresponding figure for GB is 0.1 mg (2). Thus phosphorus esters have found wide use as insecticides and are potential weapons of chemical warfare. Their detection in the atmosphere is therefore of considerable interest. Organophosphorus compounds have high proton affinities and thus their detection can be accomplished by ionizing air suspected of containing such compounds, discarding ions of no interest, and recording the resulting current. The ultimate in selectivity is attained by directing t h e ions into a mass spectrometer with, for example, unit mass resolution u p t o the highest masses of interest. Since the ionization process 1746
ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
has usually been chosen to produce minimal fragmentation, there exist mass numbers which serve as good diagnostics for the organophosphorus compound. For example, one may monitor the mass of the major ion of the mass spectrum of the compound, or may use algorithms which calculate the ratios of the intensities of several ions. Such systems have been used for detection of many atmospheric contaminants (3). For many applications, however, the economic and logistic burden of such a system may be excessive. Interest is therefore growing in developing detectors which exhibit limited selectivity, since these will be adequate in many situations. The prime reason why the requirements for selectivity can be relaxed is t h a t the extremely nucleophilic nature of phosphorus esters reduces the number of interferents capable of garnering an appreciable fraction of the available charge in the atmospheric-pressure ionic reactions. An example of a suitable detector, offering high sensitivity and acceptable selectivity, would be a portable plasma chromatograph ( 4 ) . Actually even simpler instruments which, like the plasma chromatograph, separate ions on the basis of differences in transport coefficients will probably also be acceptable ( 5 ) . Such instruments separate ions according to their mobility coefficients, diffusion coefficients, or both. Actually all such instruments, including the plasma chromatograph operate in low-field conditions; that is, the energy
