Weak Peak Enhancement by Selective Ion Trapping in a Quadrupole Ion Storage Source G. Lawson‘ and J. F. J. Todd” Chemical Laboratory, University of Kent, Canterbury, Kent, CT2 7NH, U.K.
The comblnatlon of a three-dlmenslonal quadrupole Ion storage source (QUISTOR) wlth a conventlonal quadrupole m a s filter Is shown to provlde a means of enhanclng the lntensltles of weak mass spectral peaks, and to be of potentlal use when samples are present In only trace amounts.
of a conventional quadrupole mass filter. This apparatus differs from the ICR instrument (16) in as much that a separate analyzer is incorporated. This combination of instruments effectively produces an analyzer with a mass selective ion source capable of storing ions over extended periods. In terms of the analytical problems discussed above, the ability to reject all ions except those of interest is a considerable advantage.
Current research trends, particularly in the biochemical and environmental fields have confronted chemists with the task of applying modern analytical techniques to the problem of detecting exceedingly low levels of particular species. Such concentrations may arise because the sample is either a low volatility (e.g., biochemical) compound or is in the form of trace amounts of substances in an excess of other gases, pollutants in air for example. This need to increase sensitivity has been particularly acute in mass spectrometry (1,2) and although the fundamental difficulty, insufficient density of sample molecules, is the same in both applications, two separate approaches have been employed. Substances of biochemical interest are generally highly polar, have high molecular weights, and possess low vapor pressures even at elevated temperatures; the general technique for dealing with such compounds is derivatization to increase the volatility. This procedure is not only time-consuming, but may be virtually impossible if only small quantities of the original material are available. In this regard, field ionization and desorption are among the new approaches which have proved successful for a large number of compounds of low volatility ( 3 , 4 ) ,as well as affording a means of reducing the extent of fragmentation, as does chemical ionization (5,6). On the other hand, direct admission of an air sample containing a pollutant into a mass spectrometer ion source leads to a situation in which virtually all of the ionization products are of no interest, e.g., N2+, 0 2 + , etc. This difficulty may be alleviated by pre-concentration techniques such as liquid nitrogen freeze-out (7,8), the incorporation of a membrane separator (9),or more commonly the direct combination of a gas chromatograph to the mass spectrometer (10). In all of the above methods there can be little or no direct control of either the ions that are formed in the source or the ions leaving the source prior to analysis, although use of different chemical ionization reagent gases does afford a degree of specificity. The ideal ion source, particularly one for monitoring pollutants, should be capable of creating, or at least preparing for analysis, only those ions of interest to the investigator. The retention of such species would allow the buildup of a sufficient number of ions to facilitate detection and analysis. The use of both the three-dimensional quadrupole ion storage (QUISTOR) (11-13) device and trapped ion cyclotron resonance (ICR) (14-16) makes it possible to retain ions of either a single or a range of m / e values. In this paper we describe the first of these techniques, in which a quadrupole ion storage device is used in place of the ion source
THEORETICAL In order to readily understand the chosen mode of operation of the Quistor/quadrupole combination, it is necessary to consider, in some detail, the behavior of a conventional quadrupole mass filter when scanning in the zero resolution, “total pressure mode”. Despite the large volume of research data published concerning the performance of the quadrupole mass filter, it is somewhat surprising that so little attention has been paid to this aspect. The operation of the quadrupole mass filter, comprising an array of four accurately parallel rod electrodes, can be explained (17)in terms of the “stability” of the ion trajectories, which themselves are described by Mathieu equations of the type
Present address, Rubber and Plastics Research Association, Shawbury, Shrewsbury Shropshire, U.K.
d2u - - ( a + 2q cos 2 y ) u = 0 dr
in which u represents the displacement of the ion from the origin. The values of the quantities a and q determine whether, by lying within the boundaries of the area shown in Figure 1,the trajectories are periodic such that an ion traverses the axis of the electrode structure, or whether the amplitudes of the oscillations continue to increase such that the ions collide with the electrodes. a, q, and y are related to the physical parameters of the mass filter by a=-
4e U
mr2&
, 4 =-
2e V ~.
mr2& =
, and y = at12
where U and V are the maximum potential differences, dc and rf, respectively, between opposing electrodes, ro is the radius of the inscribed circle tangential to the rods and 2nn is the angular frequency of the applied rf power. Normal (Le., mass selective) operation of the instrument is achieved by using scan line 1 (Figure 1)in which the values of U and V are scanned such that U / V remains constant. On the other hand, reference to Figure 1 suggests that operation along the q-axis (scan line 2), Le., with zero dc field (a = 0), would result in a range of masses satisfying the criterion 0 5 q I 0.9 and hence that these would be transmitted by the analyzer. If we limit this argument further then for a particular ionic species, provided q has a value within this range, there appears to be no obvious reason to suppose anything other than invariant transmission efficiency through the rod structure. Thus for a system having a constant rate of sample ion generation, the expected signal output as a function of the applied rf potential should be as shown in Figure 2a. However the observed signal, for example as measured for Nz+ formed from nitrogen containing traces ANALYTICAL CHEMISTRY, VOL. 49, NO. 11, SEPTEMBER 1977
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Figure 1.
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Stability diagram for a conventional quadrupole mass filter
rf
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Observed “total pressure” slgnal for a quadrupole mass fllter sampling a mixture of nitrogen and kryton at equal partial pressures together with the slgnals obtalned with each gas separately Figure 3.
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Figure 2. (a) “Expected” and (b) observed dependence of the transmission of N2+ ions, formed from nitrogen-containingtraces of oxygen, argon, and krypton, through a quadrupole mass filter operating in the “total pressure mode” (Le., with zero dc blas) upon the applied rf potential. With the mass filter employed in this work, an E.A.I. Quad 250A, the potential corresponding to the stabllity limlt (Indicated by the dashed line) was found to be ca. 760 V peak-peak.
of oxygen, argon, and krypton (Figure 2b), shows that a definite maximum ion transmission occurs at a potential corresponding to a q value in the range 0.3 Iq S 0.4. A further aspect of this phenomenon is that when the ion signal from a mixture of two gases of widely differing masses, e.g., nitrogen and krypton, is plotted as a function of the applied rf potential, two humps are seen, the maxima of which occur a t potentials having the same ratios as the masses. Calculation shows that the maxima occur over the same range of q values for each of the gases. Thus the total pressure mode output, for equal partial pressures of nitrogen and krypton, has the form shown in Figure 3, a feature which is also apparent for 02’)Ar’, and Kr’ seen as fine structure in Figure 2b. Operation of the quadrupole at an rf potential corresponding to point A would result in only nitrogen ions being transmitted by the analyzer. Similarly for conditions corresponding to point B, only krypton ions will reach the detector despite equal numbers of both ions being created in the ion source. An analogous effect has found to occw in the Quistor. Thus the number of ions stored within the device, as indicated by the number collected following pulsed ejection from the trap after a given storage time, exhibits a maximum at a constant value of q regardless of the m / e value of the ion concerned. An explanation of this has been advanced (18) on the basis that the maximum number of ions which may be contained within the Quistor (i.e,, ignoring ion-loss processes) is a 1820
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Figure 4. Schematic layout of the Quistor/Quadrupolesystem together with a representation of the timing sequence
compromise between the probability of an ion when initially formed having a stable trajectory lying within the bounds of the electrode structure, which decreases as q increases, and the efficiency of ion trapping, which is favored at high q values (18, 19). This behavior inevitably, therefore, gives rise to a degree of mass selectivity in the Quistor, even in the “zero resolution” mode, and is particularly pronounced when a mixture of ions contains species of widely differing masses.
EXPERIMENTAL The experimental apparatus and timing sequence were essentially the same as that designed in this laboratory and developed over a number of years (20-23);they are shown schematically in Figure 4. Ions,created within the device by a pulse of nominally 50-eV electrons, are subjected to oscillating electric fields generated by applying radiofrequency power to the ring electrode only. After a predetermined storage time, the ions are ejected from the trap into the quadrupole mass filter by differentially biasing the end-cap electrodes. The ion pulse arriving from the detector is gated, at a given delay time within the boxcar amplifier, so that spuriously ejected ions from the Quistor are not recorded. The system is operated at a repetition frequency of 100 Hz and under these conditions, a slow mass sweep of the
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Mass sp-a of nbobenzene recorded with a magnetic sector instrument (A) and with the present system having peak-to-peak rf Potentials applied to the Quistor of (B) 700 V, (C) 1100 V, and (D) 1750 F w e 6.
i Flgure 5. Mass spectra of ions eJected from the ion storage source operating at the potentials shown for a sample mixture comprislng equal partial pressures of air, argon, and kryton at a total pressure of ca. 1 X Torr
quadrupole, sampling ca. 100 ion ejection events per atomic mass unit, allows the ion “beam” from the Quistor to be analyzed. In this work the rf potential at 0.75 MHz was scanned over the range 0-2000 V (peak-to-peak) and the electron pulse width was 20 1 s . The sample investigated initially consisted of equal partial pressures of air, argon, and krypton at a total pressure of ca. 1 X Torr. Mass spectra of the ejected ions were recorded for the Quistor operating at different points along the rf axis of Figure 3. The data reproduced in Figure 5 do in fact show that the postulated mass selective ion storage occurs. To emphasize the selective capability of this experimental configuration, similar spectra were recorded using nitrobenzene as the sample. Several experimental difficulties were encountered since the inlet system was designed only for gaseous sample input and as a result only a low partial pressure of nitrobenzene could be admitted. In order to compensate for this, the sensitivity of the detector was increased by operating the analyzer at very low resolving power, but the resultant spectra when compared with the conventional sector instrument data (Figure 6) do in fact show the possibility of monitoring any chosen group of peaks from a particular compound. Again the system was operated in the pulsed mode but with the storage time increased to 2 ms. At this point it must be emphasized that in these experiments a certain degree of mass selectivity has been achieved without recourse to the use of any dc potentials as is customary in quadrupole applications. There are two advantages to be gained in this mode. The first relates to the maximum number of ions which may be stored since the application of a dc potential restricts the dimensions of the region in which ions when created remain stable within the trap and this therefore reduces the number of ions which may be retained (19) (cf. above). The second advantage is in the reduction in complexity of the electronic circuits required for the apparatus. With the selective ion storage capabilities thus available, the possibility of building up an increased concentrationof a particular species was investigated. The approach adopted was to set the Quistor to selectively retain krypton ions and introduce an air sample artificially enriched with a small quantity (ca. 0.1%) of krypton. Under conditions of continuous ionization and increasingly long storage times, a buildup in the number of krypton ions present in the packet of ions ejected from the Quistor is to be expected. The increase in signal observed for the krypton peak
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