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use in our laboratory for 3 months without complications. During this period we have made more than 100 rock analyses with the reproducibilities indicated in Table 11, columns C and D. The usefulness and reliability of the method was tested on the standard rocks BR, GA, GH, GS-N, DR-N, and UB-N; the composition of these rocks has been discussed by de la Roche et al. (6, 7)and Roubault et al. (8). The results of this test are shown in Table 11. These standard rocks are extensively used in our analytical work, fresh standard solutions being prepared every week. T h e results demonstrate that the borate fusion advocated here is excellent for the analysis by ICP spectrometry of silicate rocks. The fusion and dissolution brings all material in solution, and the ICP-excitation leads to a marked improvement in the reproducibility and to lower detection limits than are possible with MWP excitation (2). As to the success of our digestion procedure, we can only speculate, but possibly the use of only a little metaborate leads to an incomplete destruction of the silicates. The silica and alumina may therefore exist as dissolved large complexes in the final acid solutions. These complexes cannot be broken down in a cold atomic absorption flame (3, 4 ) , but are completely decomposed in a hot plasma. The data for Si02 in Table I1 superficially suggest that the data here are somewhat inferior to the results by MWP spectrometry. However, the MWP-data in Table I1 for SiOz were obtained by four repeated readings in order to reduce the drift in the MWP system; this drift is particularly annoying for SiOz (2). The ICP data in Table I1 for SiOa on the other hand are based on only one set of readings. The spread in the data for T i 0 2 , MgO, Fe203,MnO, and BaO is considerably smaller than that reported in the literature for certified rocks, see last column in Table 11. The geochemical literature is rich in rock analyses that have poor Ti, Ba, and Mn determinations. T h e method presented here therefore represents a considerable improvement in rock analytical procedure. Our results demonstrate that several minor and trace elements can be determined in a satisfactory way by the
procedure outlined here. Studies in progress suggest that this conclusion is correct also for many trace constituents not reported here, such as Ni, Co, Sr, La, and Y. For traces present in low concentrations, however, (at the 1 ppm level or less) one will have to use HF-HC104 dissolution of larger quantities of sample to be able to measure low element concentrations. Other advantages with the present method are the small amounts of contaminants that are added by reagents, the reduced costs for high purity (or purification of) LiB02, and the reduced work in preparing it. The method is well suited for the analysis of very small geological samples (15-50 mg), which frequently are the only ones available, for instance when thin layers from sediment cores are studied. Comparisons of our MWP results (2) with the present ICP results suggest t h a t the MWP method is very sensitive to interelement effects; compare for instance the spread in T i 0 2 and Ba in this work and in (2). These observations will be discussed elsewhere.
ACKNOWLEDGMENT We thank K. Govindaraju, who cordially sent us the standard rocks GA, GH, UB-N, BR, DR-N, and GS-N. LITERATURE CITED (1) K. GovinQraju. G. Mevelle, and C. Cbuard, Anal. Chem., 48, 1325-1331 (1976). (2) J. 0. Burman, B. Bostrom, and K. Bostrom, Geol. Foeren. Stockholm Foehr., 99, 102-110 (1977). (3) N. H. Suhr, and C. 0. Ingamells, Anal. Chem., 38, 730-734 (1966). (4) C. 0. Ingamells, Anal. Chem., 38, 1228-1234 (1966). (5) 0. Joensuu, Rosenstiel School, University of Miami, Coral Gables, Fla., personal communication, 1976. (6) H. de la Roche, and K. Govindaraju, Rapport (1972) sur quatre standards gBochimiques (DR-N, URN, BX-N and DT-N), Bull. Soc. Fr. Gram., 100, 49-75 (1973). (7) H. de la Roche, and K. Govindaraju. Rapport pr6liminaire (1974) sur deux nouveaux standards g6ochimiques de 1'A.N.R.T. (GS-N, FK-H), (1975). ( 8 ) M. Roubauk, H. de la Roche, and K. Govindaraju, Etat actuel (1970) des Btudes coopBratives g6ochimiques, Sci. Terre, 15, 351-393 (1970).
RECEIVED for review November 4, 1977. Accepted December 27, 1977. This work was supported by the Swedish Board of Technical Development (STU) under a grant to K. Bostrom.
Size, Shape, and Position of a Spectrophotometer Light Beam Stephen D. Rains Bausch & Lomb Incorporated, Analytical Systems Division, 820 Linden Avenue, Rochester, New York 14625
Among the users of spectrophotometers, there is a great interest in testing instrument performance. Determining the size, shape, and position of the light beam as it passes through the cuvette location is a useful part of such testing, since it informs the user of the size and position requirements for the optical free aperture of the cuvette. This information may be photographically recorded by trimming a small piece of light sensitive material comprising a diazonium compound (for example, blueprint paper) t o fit in the cuvette holder just ahead of the cuvette itself. That is, the incident light should, for the purpose of the test, strike the photosensitive surface of the blueprint paper rather than the optical window of the cuvette. After an exposure a t approximately 425 nm of from 1 to 24 hours (depending on the light level used in the particular 0003-2700/78/0350-06~0501 .OO/O
instrument), a photographic image will be obtained of the size, shape and position of the cross-section of the light beam as it enters that location. The chief advantages of using blueprint paper are that it can be cut and mounted in room light if this is done without undue delay, and the development process (exposure to ammonia) is fast, simple, and dry. Although they may not be as convenient as blueprint paper, other photosensitive materials such as photographic emulsions could be used to test a t other wavelengths or to decrease the exposure time needed. All of the above variations share the advantages of creating a lasting record and of being usable in locations that are not accessible for direct visual observation. 1978 American Chemical Society
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Interested users could improvise many simple types of film holders t o photograph the cross-section of the energy beam a t other locations in a spectrophotometer or in other optical instruments. In some cases it is beneficial to devise a template (which could be as simple as a cardboard cutout) to exactly locate the photosensitive material with reference to some
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mechanical reference point in the instrument. This would enable comparison of the beam location observations taken at different times in the life of the instrument.
RECEIVED for review September 16,1977. Accepted December 27, 1977.
Evaluation of the Dynamic Performance of Selected Ion Monitoring Mass Spectrometers D. E. Matthews,’ K. B. Denson, and J. M. Hayes* Departments of Chemistry and Geology, Indiana University, Bloomington, Indiana 4 740 1
Since the first reported use of selected ion monitoring with gas chromatography-mass spectrometry ( I ) , the technique has become an important tool in biomedical research, environmental trace analysis, and other fields of research. Mass selection is commonly accomplished by control of a voltage level-rod voltage in the case of a quadrupole mass filter, or ion-accelerating voltage in the case of a magnetic deflection mass spectrometer. Such voltage changes can be carried out rapidly, and the rate of “ion beam switching” is, accordingly, quite high, with the “dwell time” (time allowed for a single ion current measurement at a single mass) in some systems being much less than 1 second. There is a trend toward the use of increasingly higher beam switching frequencies, not only because electronic technical developments allow it, but also because it then becomes possible t o follow more rapidly varying ion-current signals while retaining accuracy in ratio measurements ( 2 ) or to select a wider variety of masses for observation. As these developments proceed, it becomes increasingly important to understand the dynamic characteristics of the mass spectrometer systems. Most importantly, once the spectrometer-control system has commanded a change in the selected mass, how much time must elapse before accurate ion-current measurements can begin? Not only does knowledge of such ion-current settling characteristics allow avoidance of outright errors in measurement, it also allows optimization of the pattern and rate of mass selection, thus increasing both the accuracy and efficiency with which analytical information can be extracted from the system. I t has long been recognized (3) that the stability of an accelerating voltage power supply can be conveniently tested by monitoring the ion current at some mass setting at which the ion current is strongly dependent on the accelerating voltage. This requirement is easily met by selecting a voltage which allows about half the maximum ion current for a given mass to reach the collector, i.e., by selecting a voltage on the “side” of a mass peak. In these circumstances, any subsequent drift in the accelerating voltage modulates the ion current, and knowledge of the slope of the ion current vs. voltage curve allows quantitative evaluation of the stability of the voltage source. T o make a similar observation of ion-current settling characteristics important in beam switching, it is necessary only to use such a half maximum point as the target to which the mass spectrometer is directed by the selected ion monitoring control system. The time required to reach the target, crucial details of the final approach to the selected mass, and the required settling time can all then be deduced from a recording of detector output vs. time. Present address, Department of Medicine, Washington University School of Medicine, St. Louis, Mo. 63110.
EXPERIMENTAL Mass Spectrometers. Results have been obtained using two different instruments. The first, a Varian CH-7 single-focusing magnetic deflection instrument, has been described in detail elsewhere ( 4 ) . The accelerating voltage power supply of this instrument has a maximum output of 3000 V and can be externally programmed in the range 2700-3000 V. The second mass spectrometer is a CEC 21-llOB double-focusing instrument (Mattauch-Herzog geometry) in which both the accelerating voltage and electric sector voltage power supplies have been replaced ( 5 ) . These supplies have maximum outputs of 10 000 and 500 volts, respectively, and can be externally programmed in any arbitrarily selected voltage range. Both instruments are controlled via a Varian 620i minicomputer. The computer communicates with the power supplies by means of digital-toanalog converters having 12-bit resolution. Ion-Current Measurements. Ion counting was used for measurements on the single-focusing instrument. The amplifier-discriminator-counter system, which has been described in detail elsewhere (6),has an effective deadtime of less than 50 ns. An electron multiplier-analog amplifier system was used on the double focusing instrument, the original amplifier having been replaced by a feedback electrometer based on an Analog Devices 435 operational amplifier. The electrometer employs a lo7 R feedback resistor, and has a bandpass, taking account of the effects of stray capacitance, of better than 3 kHz. A Teledyne Philbrick 470501 voltage-to-frequency converter (0-10 V input, 0-1 MHz output) was used to interface the amplifier with the same counting system used in ion-counting measurements. Procedure. Programs have been written allowing the entire experiment to proceed under software control, with the computer both controlling the power supplies and recording the ion-current signals. In a typical measurement, argon ( m / e = 40) is introduced to provide a single, prominent resolved ion-current signal, the magnetic field is adjusted to place the peak at either the high or the low end of the accessible selected mass range, and software control is initiated. Information required for effective control and data interpretation is acquired in the first steps, which vary the power supply voltages in order to locate the half maximum point on one side of the mass peak, measure the signal at that point, and then “map” the voltage vs. ion-current function on the side of the peak by systematically varying the voltages and recording observed ion currents. Data allowing determination of the settling characteristics are obtained by stepping to some voltage well removed from the half maximum point, allowing an adequate time for the system to stabilize, and commanding a step back t o the half maximum point while recording the ion current at a function of time. This process is repeated under do-loop control for a systematically varying sequence of voltage steps in order to determine the relationship between settling time and the magnitude of the commanded voltage change. RESULTS AND DISCUSSION Figure 1 shows the ion-current settling profile for an 8.5% increase in the accelerating voltage of the single-focusing
0003-2700/78/0350-0681$01.00/00 1978 American
Chemical Society
