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The correlation experiments are readily carried out using the interro gation module in the data acquisition stream. The conditions of data acqui sition are established in the acquisi tion statement sent to the computer. The computer is only to sort data if ions within a particular TOF (or mass) range are included in the electronic word set for a particular fission track. With this option, it is possible to ob tain a mass spectrum using only those fission tracks that desorb a molecular ion. These correlations are telling us something, but at this point it is hard to understand what they mean or how to use them in interpretating mass spectral data. A c c u r a t e Mass M e a s u r e m e n t s . Accurate mass measurements are an important part of mass spectrometry because they can be used to obtain el emental compositions. The most accu rate mass measurements are those for isotopes of elements; accuracies of one part in 108 have been achieved by high-resolution magnetic sector MS and statistical methods to obtain an average value of atomic mass from a series of independent measurements. In 1976, we demonstrated the feasibil ity of this kind of measurement for 252 Cf-PDMS by measuring the mass of the SmO + ion with an accuracy of 0.18 mu using the results of 19 inde pendent measurements (17). This kind of measurement is readily adapt ed to our data acquisition software program using the interrogation mod ule. The calibration ions are selected in the manner discussed in the section on mass calibration. In addition, ap proximate TOF values are given to the computer for the ions whose masses we wish to determine accurately. As the electronic words flow through the interrogation circuit, only those words are processed whose TOF values fall within 500-ns-wide win dows encompassing the estimated TOF of the calibration ions and the ions whose masses are to be measured. Each ion type is allocated individual 6000-channel arrays in memory, and the interrogation circuit directs each word that falls within one of the win dows to the appropriate memory array with its full complement of significant figures. In effect, only preselected por tions of the mass spectrum are re corded—those that include the mass calibration ions and the ions of inter est—and these portions have been re corded using the full 78-ps precision of the TID. Data are accumulated for a predetermined time (e.g., 15 min). After this period lapses, a calibration equation is automatically calculated from the centroids of the calibration ions. Masses are calculated using the TOF centroids of the ions being stud ied. This measurement is repeated
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every 15 min. This is an ideal mea surement for an overnight run since the entire procedure is carried out un attended. In a 14-h period, ~80 deter minations of each ion mass will have been carried out, giving a good data set for determining precise masses with small standard deviations.
Summary The major effort in 2S2 Cf-PDMS de velopment is now being directed to studies of high molecular weight biomolecules. The recent results of Sundqvist suggest that there is little attenuation in the molecular ion yields of small proteins over the range mlζ 5000-14 000 (70). This implies that the mass limit for 252 Cf-PDMS has not yet been reached. There are now other ion sources that can be used to desorb high molecular weight ions, notably fast atom bombardment (FAB) (18) and liquid-phase secon dary ion mass spectrometry (liquid SIMS) (19). In addition to the new high-field magnets that have been in troduced recently there are other methods emerging for high mass mea surements. Besides the TOF method, ion cyclotron resonance (20) and the Wien filter (21) are being developed for this purpose. 252 Cf-PDMS is a comfortable sys tem in an academic environment where much of the labor of develop ment and measurement is carried out by students. A prototype system was built at Marburg within a two-week period from spare parts and the elec tronics normally found in a nuclear science laboratory. The current sys tems at Uppsala, Marburg, Darm stadt, and Erlangen were designed and constructed as PhD thesis projects. The Marburg system is a second-gen eration instrument featuring an HPLC interface, a new type of ion de tector for massive ions, and some ver satile ion optics for improved mass resolution (22). The basic simplicity and versatility of the instrument have been impor tant factors in the development of 252 Cf-PDMS for high mass studies. Following the important observations of Chait and Field (23) on the high fraction of metastable ions that are desorbed from the fission track, path lengths have been shortened from 1 m (the optimum for resolution) to 10-50 cm, so that ions are detected before they decay into fragments. By chang ing the acceleration voltage from 10 kV to 18 kV, the Uppsala group im mediately began detecting ions of small proteins in the ml ζ 10 00014 000 range because the extra energy increased the sensitivity for detecting massive ions. The Mamyrin reflection, which provides energy focusing to im prove mass resolution for TOF mea-
