Detection of Nonexistent Molecular Ions Lewis A, Shadoff ChemicalPhysics Research Laboratory, The Dow Chemical Co., Midland, Mich. 48640
ONEDIFFICULTY ENCOUNTERED in the interpretation of mass spectra for the elucidation of molecular structure is the absence of molecular ion peaks in the mass spectra of some compounds, Using a high resolution mass spectrometer, the presence of metastable ion peaks resulting from transitions from molecular ions to abundant fragment ions has been observed for compounds which exhibit no detectable molecular ion peak. In this manner the mass of molecular ions may be obtained for many compounds. This method takes advantage of the energy filtering capability of a high resolution mass spectrometer ( I ) . If a metastable transition occurs between the ion source slit and the entrance to the electric sector, the energy of the metastable ion will be less than ions formed in the ion source. It is traveling with the velocity of its precursor but has a lower mass.
where Vis the ion accelerating voltage. The electric sector is tuned only for ions of energy eV so that the metastable ion does not emerge. If, however, the mass spectrometer is detuned by increasing the ion accelerating voltage and keeping all else the same, there will be some voltage, V’, at which the metastable ion will have the proper energy to get through the electric sector. This ion accelerating voltage depends on both the mass of the precursor and fragment ions. (4)
Thus, by determining the voltage required to allow “normal” ions formed in the ion source and the voltage required to allow metastable ions through the electric sector, the mass of the precursor ion may be determined. Consider the case where there is no observed molecular ion. That is, the molecular ion has decomposed to fragment ions before reaching the detector. If it decomposed to a relatively long lived fragment ion after leaving the ion source but before entering the electric sector, a metastable ion may be observed for this transition if the fragment lives long enough to be detected. Thus, the molecular ion need only live long enough to escape the ion source (-10-8 second) for its mass to be determined, even though the molecular ion is not observed in the mass spectrum. The experimental procedure is as follows. The mass spectrometer i s adjusted so that an intense ion peak in the mass spectrum is in focus on the detector, and a record is made of the ion accelerator voltage. The ion accelerator voltage is then increased slowly, whereupon the ion current becomes (1) M. Barber and R. M. Elliott, “Abstracts of Papers of 12th
Annual Conference on Mass Spectrometry,” Montreal, 1964, p. 150.
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ANALYTICAL CHEMISTRY
zero, When the ion current increases again at some higher voltage, this increase is due to a metastable ion, and a record is made of this voltage. Since there is no background interference, very small signals may be detected. Using Equation 4 the mass of the precursor ion may now be determined (usually 1 amu). This technique was employed with a CEC 21-l1OB high resolution mass spectrometer on some compounds which do not
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Static Extraction-A
exhibit molecular ions. The results are summarized in Table I. It appears to be useful for many different types of compounds, and may be useful for compounds which do display molecular ions which are obscured by background or impurities. RECEIVED for review June 28, 1967. Accepted October 16, 1967.
Novel Extraction Technique
Roy KO Battelle Memorial Institute, Pacific Northwest Laboratory, Richland, Wash. CONVENTIONAL LIQUID-LIQUID extraction techniques pose problems in the extraction of concentrated solutions or those containing insoluble residues, The former solutions frequently form second organic phases or emulsions; the latter may lead to the presence of the residue in the interface. Those problems have been avoided in this laboratory by making extractions without stirring or mixing of the two phases. In conventional extraction methods, the two phases are vigorously agitated to establish equilibrium in a short period of time. Equilibrium can also be approached without agitation but requires a much longer time. Extractions, then, are possible without vigorous mixing of the two phases but are normally not made that way because the time required is considered prohibitive. The times involved, however, are surprisingly not too long and can be considered practical especially in view of the advantages offered by the new technique. A novel liquid-liquid extraction technique termed static extraction is therefore proposed. A description of the method and some of its applications are presented. EXPERIMENTAL
Static extractions are made in polyethylene bottles by adding 10 ml of the aqueous phase and approximately 2 ml of the organic extractant solution. The bottles are capped and set aside overnight. The organic layer is then removed and a fresh layer added. The transfer is conveniently made by tilting the bottle and looking through the opening at the top of the solutions. A light shining through the bottle clearly shows the separation of the phases. The bottles are set aside overnight again. The process is repeated until the desired extraction efficiency is attained. In the case of dilute solutions, one overnight contacting is sufficient to give complete extraction. Concentrated solutions may require several contactings. Figure 1 shows the extraction efficiency as a function of time for the static extraction of grams/liter of gallium, 4 grams/liter of uranium, and 5 grams/liter of plutonium in which the organic phase was replenished every 24 hours. Gallium, as the Rhodamine B-chlorogallate complex, is completely extracted into carbon tetrachloridechlorobenzene in 20 hours; uranium as the chloro complex into tri-n-octylamine (TNOA) in xylene in 48 hours; and plutonium as the nitrate complex into TNOA in xylene in 48 hours. Static extraction has been applied in this laboratory to the determination of copper, gallium, tantalum, and zirconium in plutonium. For copper, tantalum, and zirconium determinations, the plutonium was separated by TNOA extraction. The copper and tantalum were then determined by emission spectrography; zirconium by x-ray fluorescence. For gallium determination, gallium was separated from plutonium by extraction of the Rhodamine B-chlorogallate complex and its absorbance in the organic phase measured.
Gal Iiu m Uranium PI utoni urn
0
25
50
75
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Time, Hours Figure 1. Per cent extraction as a function of time
DISCUSSION
In any liquid-liquid extraction process, the exchange of material between the two phases occurs by diffusion across the interface. If there is no stirring or agitation of the liquids, migration to and from the interface also occurs by diffusion. The organic layer immediately adjacent to the interface (in the usual case of transferring from the aqueous to the organic phases) becomes saturated with the extracting species and the rate of diffusion across the interface is hence slowed down. Replacing this layer with a fresh layer again brings the diffusion and hence the extraction rate to a practical level. Replenishing the organic layer every 16 hours is sufficient to maintain a practical extraction rate. Because the organic layer immediately adjacent to the interface builds up to saturation, only a small volume is necessary for each contacting. Static extraction offers some advantages over conventional methods. Equipment needs are simplified-only a container to hold the liquids is required. That is a real advantage in the analysis of radioactive materials. Stirrers or shakers need not be placed in hoods or glove boxes. Maintenance, decontamination, and removal of contaminated equipment are eliminated. In addition, the spread of radioactive contamination is minimized by not shaking or stirring radioactive solutions. VOL. 39, NO. 14, DECEMBER 1967
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