Model 5 6 0 0 Direct Frequency Synthesizer: 0.1 to 1 6 0 M H z
fectively, the investigator must be familiar with the spectra of the compounds or class of compounds sought. In rigorous reporting the scanned spectrum on which the selected ion monitoring is based should be made available. Although it varies with the sample, the sensitivity of selected ion monitoring is reported by many laboratories to fall in the range 1 0 - 1 2 to 1 0 - 9 g, between 1 pg and 1 ng. Often, sensitivity is limited by interference or by absorption on the GC column. Computer-Reconstructed Chromatograms
Here's A New Low-Cost Roule to NMR ENHANCEMENT ...Higher Resolution •••Improved S / N ...Oreoter Analytical Versotility •••Optimised Stobility Good news - now there's a direct frequency synthesizer that approaches ideal stability , purity, and digitalcontrol speed and response fidelity so closely that it can enhance the performance of most modern NMR spectrometers by an order o f magnitude. Unlike indirect synthesizers, it does not employ phase-locked loops; hence, it has significantly lower close-in phase noise, harmonics, and non-harmonic spurious outputs. For the same reason, its switching speed is two orders of magnitude faster, and that's great news if you are using advanced NMR architecture (e.g., computer-controlled frequency sweeping, or digital feedback for stabilization). The best news of all is that the new Rockland Model 5600 sells for less than the old indirect designs in this frequency range . . . at least a thousand dollars less (U.S.A.) than any other synthesizer in this class! Check these specs: a single 0.1 to 160 MHz frequency range, with con-
stant 1 Hz resolution; maximum switching time, 20 microseconds (that's 50 to 100 times faster than phase-locked designs); - 7 0 dB spurious (that's 10 to 15 dB better); —35 dB harmonic spurs (that's 6 to 10 dB better); - 7 0 dB phase noise; ±0.5 dB levelling (2:1 better); and 2 X 10" 9 /day stability (with built-in crystal reference). DATA? DEMO? BOTH? Use the inquiry number to request complete technical data — or call us for the name of your local Rockland field engineer. He'll be glad to arrange a demonstration of this advanced synthesizer. Rockland Systems Corporation 230 W. Nyack Road W. Nyack, N.Y. 10994 TEL: (914)623-6666 TWX: 710-575-2631
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Reconstructed Total Ion Chromatograms. A computer system can provide automated acquisition, normalization, and plotting of spectra; this kind of support is especially attractive when one is analyzing the compounds in a multicomponent mixture in a gas chromatograph/mass spectrometer. It is not necessary to wait until a peak elutes to scan the mass spectrometer. With computer control, spectra can be scanned and acquired repetitively every few seconds throughout elution of the entire gas chromatogram. This approach generates three-dimensional ion signals, each characterized by mass, intensity, and also by time (scan number) as shown in Figure 6. With this approach one can easily end up with several hundred spectra stored in the computer, and considerable ingenuity is required on the part of the investigator to retrieve useful information in an efficient manner. Figure 7 (top panel) shows a total ion chromatogram reconstructed by summing the ion intensities in each repetitive scan. The scan number is indicated on the axis. This kind of chromatogram looks very much like a total ion current chromatogram or flame ionization detector chromatogram, although peak shapes can be distorted if scans are not made frequently enough. Mass Chromatograms. With all of the repetitive scans stored in the computer, the intensities of any ion may be retrieved from each scan and plotted as a function of time. In Figure 7 in addition to the reconstructed total ion chromatogram, two mass chromatograms are shown, also reconstructed from about 250 repetitive scans made while a trimethylsilylated urine extract was eluted from the gas chromatograph. T h e mass chromatograms were reconstructed of ions prominent in the scanned spectra of trimethylsilylated glucuronides. These mass chromatograms resemble selected ion records, although they are less sensitive by as much as 10 4 , because of the shorter sampling times used in fast repetitive scanning. T h e limited mass range chromato-
