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Chemical ionisation in the ion trap: a comparative study
international
Journal of Muss Spectrometry and Zen Processes, 99 (1990) 139-149
139
Elsevier Science Publishers B.V., Amsterdam
CHEMICAL STUDY*
IONISATION
SM. BOSWELL, R.E. MATHER
IN THE ION TRAP: A COMPARATIVE
and J.F.J. TODD**
Chemical Laboratory, University of Kent, Canterbury, Kent CT2 7NH (U.K.)
(Received 5 June 1990)
ABSTRACT The ion trap detector (ITD) is a mass spectrometer in which ions are first stored and then mass-selectively ejected into a detector. This ability to store ions provides a means whereby low pressure chemical ionisation (CI) reactions may be carried out. Such an application was first demonstrated in a quadrupole storage trap (Quistor), which was employed as the ion source for an AEI MS902 magnetic sector mass spectrometer. The aim of this paper is to compare the chemical ionisation mass spectra for a range of simple compounds obtained using three systems: a “conventional” high pressure source, the Quistor-MS902 combination and an ITD operating in the CI mode.
INTRODUCTION
Since its invention in the mid-1960s [I], chemical ionisation (CI) has become a standard technique in mass spectrometry. CI is a soft ionisation technique in which a reagent gas is allowed into the ion source at a pressure substantially in excess over that of the sample. This reagent gas is then ionised by electron impact (El), as in a conventional source, the ions thus formed interacting to produce the reagent ions which subsequently react with sample molecules to produced sample ions. This has the advantage that the amount of energy transferred into the sample molecule is signi~cantly less than would be the case from electron impact, resulting in less fragmentation and a consequent improvement in the amount of information at higher masses. A number of such reagent gases are in common use, including methane, isobutane and ammonia, imparting varying amounts of energy to the sample molecules, as well as providing different ionisation mechanisms such as proton donation, hydride abstraction or simple charge transfer. A more * Paper presented at the 10th Triennial International Mass Spectrometry Symposium, Salford, U.K., 3-5 July 1989. **Author to whom correspondence should be addressed. 0 168-l 176/90/$03.50
0
1990 Elsevier Science Publishers B.V.
140
detailed discussion has been published by Mather and Todd [2] and Harrison 131. The conventional method of utilising CI involves a high pressure of the reagent gas, of the order of 1 Torr. This is necessary because the residence time of ions in such a source is very short, about 10e5 s, and such high pressures are required to ensure that a sufficient number of collisions occur to generate an abundance of reagent ions. An alternative to such high pressure sources can be achieved by means of extending the lifetime of ions within the source region, thus increasing the chances of collisions. This is possible using ion storage instruments, such as the ion cyclotron resonance spectrometer, in which reaction periods in the millisecond range permit pressures as low as lop5 Torr to be utilised. Previously we have reported the use of a quadrupole ion storage trap (Quistor) as a CI source [4,5] for conventional mass analysers. The quadrupole ion trap, based as it is on similar principles to the Quistor but with the advantage of being its own mass analyser, provides another route to low pressure CI. With the advent of the Finnigan MAT ITDTM as a commercially available instrument with CI capabilities 16-91, it seems appropriate to compare the use of these two quadrupole devices, both with each other and with conventional high pressure sources. Here we report the results of studies undertaken on the use of isobutane as a CI reagent gas in the three types of source. INSTRUMENTATION
The initial work, comparing the chemical ionisation behaviours of the high pressure and Quistor sources, was carried out using an AEI MS902 magnetic sector mass spectrometer. To this was attached the respective sources, both of which were constructed in the workshops at the University of Kent. The high pressure source was a gas-tight modi~cation of a conventional EI source for the MS902, with the reagent gas being introduced into it via a hollow probe, the end of which also carried the sample under analysis. The reagent gas pressure was held at around 1 Torr, the pressure being measured by the source ionisation gauge. The Quistor has been described in detail elsewhere [5] but it basically comprises three stainless steel electrodes with polished hyperbolic surfaces. They are separated by boron nitride spacers, the ring electrode having an internal radius of 7.0 x 10p3m (Fig. 1). Both sample and reagent gas were introduced into the Quistor, from the same probe as used above, via a small hole drilled in the upper spacer. The reagent gas pressure was around lo-‘Tort-, much lower than in the high pressure source, whilst that of the sample was several orders of magnitude lower again. Figure 2(a) shows the timing diagram for the Quistor. An initial pulse of electrons, the creation pulse, causes ionisation of the reagent gas, the ions thus
141
BEAM
CENTRING
EL
Fig. 1. Central section through the assembled Quistor.
CREATION
PULSE
EJECTION
PULSE
(b)
ELECTRON
GATE
Fig. 2. Timing diagrams for (a) Quistor and (b) ITD. (See text for description.)
142
formed then being stored for a set period, allowing ionisation of the sample molecules. Finally a negative pulse was applied to the bottom end cap, expelling the ions from the device to be analysed in the normal manner by the MS902. The amplitude of the radio frequency (r.f.) voltage applied to the ring electrode was set at a level necessary to trap both the reagent and product ions. This, however, also sets an upper limit on the mass range of the device, to some extent restricting the choice of reagent gas available for higher mass samples. The ion trap used was the ion trap detector (ITD) manufactured by Finnigan MAT. The only modifications were the addition of the CI inlet system and the mounting of an ion gauge on the side of the manifold. Samples were introduced via the transfer line from a capillary gas chromatograph. The geometry and reagent gas pressure of the ITD were similar to that of the Quistor but it differs in that, being operated in the mass selective instability mode, the ions are sequentially expelled from the trap to the detector in order of increasing m/z ratio, rather than being pulsed out en masse into a separate analyser. In addition the ITD operates with a background pressure of Ca. lop3 Torr helium acting as a buffer gas. Figure 2(b) shows the scan function (timing sequence) used by the ITD software. The reagent gas is ionised and the resulting ions stored so that the required reagent ion distribution is set up (A). The r.f. is held so that any sample ions formed here are lost from the trap. Next the r.f. level is raised so that sample ions will be stored and this is maintained for the duration of the reaction time (B). The r.f. level is now rapidly increased to the start of the mass range of interest, expelling unwanted, for example, reagent, ions (C). Finally the r.f. is ramped to scan out sequentially the sample ions over the selected mass range. EXPERIMENTAL
The reagent gas chosen for this investigation was isobutane (2-methylpropane), a gas commonly employed in CI analyses owing to its restricted energy input to the sample molecule and the attendant limited fragmentation. It appears to act predominantly as a Bronsted acid [lo], i.e. it reacts by proton donation to the sample. Initial conditions within the three sources produce different isobutane ion distributions. Figure 3 shows the effects of storage time on reagent ions distributions within both the Quistor (Fig. 3a) and the ITD (Fig. 3b). As the storage time is increased the original low pressure spectrum, containing a large variety of ions, with m/z = 43 (C,HT) as the major one, changes to a much simpler spectrum with m/z = 57 (C,H,+) dominating. This is comparable with the effects of increasing the isobutane pressure within a high pressure source, where again the spectrum comes to be
143
(b)
0 A
3 A
‘00 F
0
0 A
4 Blo
v
0
3
QUISTOR
0 2
11.
j
0.0
“I
0.5
~
1.0
‘a
1.5
“‘*’ 2.0
0.1 2.5
0
3.0
STORAGE
5
10 15 20 25 30 35 40 45
TlME
Fig. 3. Plots showing the effects of storage time on the isobutane reagent ion distributions for (a) Quistor and (b) ITD: 0, m/z = 39; A, m/z = 41; V, m/z = 42; 0, m/z = 43; 0, m/Z = 57.
dominated by m/z = 57 at the approx. 1 Torr employed in our high pressure source (e.g. compare with Field’s Figure 1 in ref. 10). In neither type of source do adduct ions, i.e. ions above m/z = 58, appear in the spectrum. A number of compounds were originally used to compare the Quistor and high pressure sources [ 111, but only three of these were used to illustrate the comparisons with the ITD, heptan-l-01, benzaldehyde and di-n-butyl phthalate. In the Quistor and high pressure sources these were introduced using a probe whilst in the case of the ITD, samples were injected via the attached gas chromatograph. RESULTS
Bertzaldehyde
Figure 4 shows benzaldehyde spectra obtained from the various instruments, in the order (a) high pressure, (b) Quistor (9 ms storage time), and (c) ITD (11 ms storage time). All the spectra are displayed normalised to the base peak. The high pressure source produced only four ions, dominated by the (M + 1)” ion (m/z = 107), with the other three peaks at m/z = 105 ((M - I)‘“, 20%), 108 ((M + 2)+, 9%) and a tiny molecular ion peak at m/z = 106. The Quistor spectra for storage times of 5 and 9 ms had lost the molecular ion and some of the m/z = 105 intensity ~proportional to the (M - I)+ base peak) whilst gaining peaks at m/z = 79 and 80 (C,HT , 78% and C,Hz ,4% respectively). The zero storage time spectrum was signiIicantly different with the base peak at m/z = 106 and large peaks at m/z = 105 and
144
MASS Fig. 4. Isobutane CI mass spectra of benzaldehyde: (a) high pressure ion source; (b) Quistor source (9 ms storage time); (c) ITD (11 ms storage time).
m/z = 77 (C,H:). There were also smaller peaks at m/z = 78,79 and 107 and a tiny peak at m/z = 74. The ITD spectra are all dominated by the M + 1 ion, with smaller peaks (less than 20%) at m/z = 105, 106 and 108. At short storage times (less than 11 ms) the (M - 1)’ peak can increase in intensity to about 20%, while there are traces of a peak at m/z = 79. At extended ionisation times of 500~s m/z = 108 can be half the base peak height and m/z = 109 is also significant.
145
0.8
I I
(4
0.6
.:jI, 60
70
80
90
lea
110
120
130
140
150
lea
170
180
MASS Fig. 5. Isobutane CI mass spectra of heptan-l-01.
Details as for Fig. 4.
Heptan-I-01
Heptanol, in contrast with ~nzaldehyde, undergoes more extensive fragmentation in the ITD than in the Quistor, while the high pressure spectrum is again distinct from the other two, most noticeably in its much more limited fragmentation. There are no ions below m/z = 97 and it is dominated by two masses, the base peak at m/z = 99 (M + 1 - H,O)’ and m/z = 173 (89%, (M + 57)+). There are only two other peaks with intensities above 10% of the base peak, m/z = 115 (23%, (M - l)+) and m/z = 100 (1 I%, (M - H20)+). The spectrum obtained from the Quistor displays a number of peaks over 10% of the base peak intensity, whilst the location of this has shifted two mass
146
Fig. 6. Isobutane CI mass spectra of di-n-butylphthalate.
Details as for Fig. 4.
units to m/z = 97, although the m/z = 99 peak is of only slightly lower intensity. The peak at m/z = 98 is now over 10% but there is no longer a peak at m/z = 100, and the only mass seen above this was due to the (M - I)’ ion (3 1 %), there being no evidence of adduct ions. The most noticeable difference, however, was the appearance of small peaks at m/z = 85 and 83, and a group centred at m/z = 69, this peak being about 33% of the base peak intensity. The ITD spectra are similar to that obtained from the Quistor source but the fragmentation is more pronounced, with a greater proportion of the total ion count provided by the group of peaks at m/z = 69. The 97+/99+ ratio of ion intensities ranges from 2.5 to 4.5 (cf. approx. 1 for the Quistor) and the
147
m/z = 115 peak is less than 10% of the base peak height. There is again no sign of adduct ions above this mass.
Like heptan-l-01 above, the extent of fragmentation of di-n-butyl phthalate (DNBP) increases in the order “high pressure” < Quistor < ITD, although here the fragmentation pathways also differ, with the high pressure source producing mainly higher mass fragments compared with the low pressure instruments. The high pressure spectrum of DNBP shows a large (M + l)+ peak (m/z = 279) accounting for 65% of the total ion count. Other peaks can be observed at m/z = 280 (26%), 281, 261, 223 (lo%), 205 and 149, plus m/z = 317 has been observed. In contrast, although having a large peak at m/z = 279, the Quistor has its base peak at m/z = 205 and a significant signal due to m/z = 149. There are also peaks at one mass unit above each of these major peaks. Masses beyond m/z = 279 were, however, no~ally beyond the storage range of the Quistor. The ITD spectra are also dominated by the three peaks observed from the Quistor source, but the relative intensities are different: m/z = 149 accounts for over 50% of the total ion count whilst the peak at m/z = 205 represents two thirds of the remainder. There are again peaks at one mass unit above these but now m/z = 223 and 189 are also apparent, and m/z = 319 and 335 are sometimes observed. Possible reaction pathways leading to the formation of some of these ions within the high pressure source have been described elsewhere [ 111,but, as can be seen from the above-listed ions, these may not be applicable to the ITD. Further investigations are necessary to elucidate the fragmentation pathways within the ITT). Plots of ion intensities, measured relative to m/z = 149, against storage time are shown in Fig. 7 and indicate that, whereas m/z = 263 and 279 rise to a plateau intensity with increasing storage time, other masses decrease. Some of the differences between the high and low pressure sources may therefore be explainable in terms of residence time within the instruments, e.g. the abundance of m/z = 223 ions within the high pressure spectrum may be due to the short time spent in this source being insufficient for further fra~entation to occur. DISCUSSION
As can be seen from the results presented above, comparisons of the CI behaviour between the three types of source are to some extent compound dependent. However, the general picture appears to be one of increasing
148
Fig. 7. Plots of di-n-butylphthalate ion distributions (measured relative to m/z = 149) against storage time in the ITD. Ionisation Time: 0, 140~s; 0, 180~s; A, 500~s.
fragmentation in the order high pressure < Quistor < ITD. The comparisons between the high pressure source and the ITD are consistent with the results of Brodbelt et al. [7] who utilised methane as the reagent gas. They also found increased fragmentation in the ion trap and that the high pressure source can also produce adduct ions. As noted for DNBP, the differences may, at least in part, be due as much to contrasts in time scales as to variations in the reaction pathways. Residence times of ions within the high pressure source are of the order of IO-‘s whereas those in the storage devices are much longer, up to tens of milliseconds. Therefore within the latter there is a period between ion formation and detection during which any metastable ions may undergo further fragmentation, as is shown by the effects of storage time on DNBP ion distributions within the ITD. In competition with this, however, may be the effects of collisional stabilisation of ions by the helium buffer gas and possible ionisation by previously generated sample ions, proposed by Brodbelt et al. [7] as explanations of the observed increase in the relative abundance of m/z = 107 ions from benzaldehyde with storage time in the ion trap. There is some evidence of this from our investigations, e.g. there is a distinct increase in the relative abundance of the (M - l)- ion with storage time in our heptan-l-01 data, but, as these cover a much more limited time range (up to 44ms) compared with those of Brodbelt et al. (up to lOOOms), we are unable to confirm these ideas. It therefore appears that high and low pressure sources give different, and
149
possibly complementary, information about samples. The former tend to produce limited fragmentation whilst providing high mass information not available from standard EI analysis. The low pressure sources fall somewhere between these two, producing both high mass information whilst retaining some of the fragmentation necessary to elucidate structural details. Both the Quistor and ITD also provide the ability, by altering the storage times, to tune the CI conditions. The Quistor does, however, have the disadvantage that it is limited in the available mass range since the r.f. must be set so as to store both the reagent and sample ions. In contrast the ITD has a much wider range whilst the use of mass-selective instability to scan out ions, rather than pulse them from the source as is done with the Quistor, should provide enhanced sensitivity. The main advantage of the Quistor is that it can be fitted to conventional mass spectrometers, providing a low pressure source to complement already available ionisation techniques. REFERENCES 1 M.S.B. Munson and F.H. Field, J. Am. Chem. Sot., 88 (1966) 2621. 2 R.E. Mather and J.F.J. Todd, Int. J. Mass Spectrom. Ion Processes, 30 (1979) 1. 3 A.G. Harrison, Chemical Ionization Mass Spectrometry, C.R.C. Press, Boca Raton, FL, 1983. 4 R.F. Bonner, G. Lawson and J.F.J. Todd, J. Chem. Sot., Chem. Commun., (1972) 1179. 5 R.E. Mather, G. Lawson, J.F.J. Todd and J.M.B. Bakker, Int. J. Mass Spectrom. Ion Processes, 29 (1978) 347. 6 P.E. Kelly, G.C. Stafford, Jr., J.C.P. Syka, W.E. Reynolds, J.N. Louris and J.F.J. Todd, in J.F.J. Todd (Ed.), Advances in Ma& Spectrometry, 1985, Wiley, Chichester, 1986, p. 869. 7 J.S. Brodbelt, J.N. Louris and R.G. Cooks, Anal. Chem., 59 (1987) 1278. 8 S.A. McLuckey, G.L. Glish and P.E. Kelly, Anal. Chem., 59 (1987) 1670. 9 R.E. March and R.J. Hughes, Quadrupole Storage Mass Spectrometry, Wiley Interscience, New York, 1989. 10 F.H. Field, in J.L. Franklin (Ed.), Ion-Molecule Reactions, Vol. 1, Butterworths, London, 1972, p. 261. 11 R.E. Mather, Ph.D. Thesis, University of Kent, 1979.
Journal of Muss Spectrometry and Zen Processes, 99 (1990) 139-149
139
Elsevier Science Publishers B.V., Amsterdam
CHEMICAL STUDY*
IONISATION
SM. BOSWELL, R.E. MATHER
IN THE ION TRAP: A COMPARATIVE
and J.F.J. TODD**
Chemical Laboratory, University of Kent, Canterbury, Kent CT2 7NH (U.K.)
(Received 5 June 1990)
ABSTRACT The ion trap detector (ITD) is a mass spectrometer in which ions are first stored and then mass-selectively ejected into a detector. This ability to store ions provides a means whereby low pressure chemical ionisation (CI) reactions may be carried out. Such an application was first demonstrated in a quadrupole storage trap (Quistor), which was employed as the ion source for an AEI MS902 magnetic sector mass spectrometer. The aim of this paper is to compare the chemical ionisation mass spectra for a range of simple compounds obtained using three systems: a “conventional” high pressure source, the Quistor-MS902 combination and an ITD operating in the CI mode.
INTRODUCTION
Since its invention in the mid-1960s [I], chemical ionisation (CI) has become a standard technique in mass spectrometry. CI is a soft ionisation technique in which a reagent gas is allowed into the ion source at a pressure substantially in excess over that of the sample. This reagent gas is then ionised by electron impact (El), as in a conventional source, the ions thus formed interacting to produce the reagent ions which subsequently react with sample molecules to produced sample ions. This has the advantage that the amount of energy transferred into the sample molecule is signi~cantly less than would be the case from electron impact, resulting in less fragmentation and a consequent improvement in the amount of information at higher masses. A number of such reagent gases are in common use, including methane, isobutane and ammonia, imparting varying amounts of energy to the sample molecules, as well as providing different ionisation mechanisms such as proton donation, hydride abstraction or simple charge transfer. A more * Paper presented at the 10th Triennial International Mass Spectrometry Symposium, Salford, U.K., 3-5 July 1989. **Author to whom correspondence should be addressed. 0 168-l 176/90/$03.50
0
1990 Elsevier Science Publishers B.V.
140
detailed discussion has been published by Mather and Todd [2] and Harrison 131. The conventional method of utilising CI involves a high pressure of the reagent gas, of the order of 1 Torr. This is necessary because the residence time of ions in such a source is very short, about 10e5 s, and such high pressures are required to ensure that a sufficient number of collisions occur to generate an abundance of reagent ions. An alternative to such high pressure sources can be achieved by means of extending the lifetime of ions within the source region, thus increasing the chances of collisions. This is possible using ion storage instruments, such as the ion cyclotron resonance spectrometer, in which reaction periods in the millisecond range permit pressures as low as lop5 Torr to be utilised. Previously we have reported the use of a quadrupole ion storage trap (Quistor) as a CI source [4,5] for conventional mass analysers. The quadrupole ion trap, based as it is on similar principles to the Quistor but with the advantage of being its own mass analyser, provides another route to low pressure CI. With the advent of the Finnigan MAT ITDTM as a commercially available instrument with CI capabilities 16-91, it seems appropriate to compare the use of these two quadrupole devices, both with each other and with conventional high pressure sources. Here we report the results of studies undertaken on the use of isobutane as a CI reagent gas in the three types of source. INSTRUMENTATION
The initial work, comparing the chemical ionisation behaviours of the high pressure and Quistor sources, was carried out using an AEI MS902 magnetic sector mass spectrometer. To this was attached the respective sources, both of which were constructed in the workshops at the University of Kent. The high pressure source was a gas-tight modi~cation of a conventional EI source for the MS902, with the reagent gas being introduced into it via a hollow probe, the end of which also carried the sample under analysis. The reagent gas pressure was held at around 1 Torr, the pressure being measured by the source ionisation gauge. The Quistor has been described in detail elsewhere [5] but it basically comprises three stainless steel electrodes with polished hyperbolic surfaces. They are separated by boron nitride spacers, the ring electrode having an internal radius of 7.0 x 10p3m (Fig. 1). Both sample and reagent gas were introduced into the Quistor, from the same probe as used above, via a small hole drilled in the upper spacer. The reagent gas pressure was around lo-‘Tort-, much lower than in the high pressure source, whilst that of the sample was several orders of magnitude lower again. Figure 2(a) shows the timing diagram for the Quistor. An initial pulse of electrons, the creation pulse, causes ionisation of the reagent gas, the ions thus
141
BEAM
CENTRING
EL
Fig. 1. Central section through the assembled Quistor.
CREATION
PULSE
EJECTION
PULSE
(b)
ELECTRON
GATE
Fig. 2. Timing diagrams for (a) Quistor and (b) ITD. (See text for description.)
142
formed then being stored for a set period, allowing ionisation of the sample molecules. Finally a negative pulse was applied to the bottom end cap, expelling the ions from the device to be analysed in the normal manner by the MS902. The amplitude of the radio frequency (r.f.) voltage applied to the ring electrode was set at a level necessary to trap both the reagent and product ions. This, however, also sets an upper limit on the mass range of the device, to some extent restricting the choice of reagent gas available for higher mass samples. The ion trap used was the ion trap detector (ITD) manufactured by Finnigan MAT. The only modifications were the addition of the CI inlet system and the mounting of an ion gauge on the side of the manifold. Samples were introduced via the transfer line from a capillary gas chromatograph. The geometry and reagent gas pressure of the ITD were similar to that of the Quistor but it differs in that, being operated in the mass selective instability mode, the ions are sequentially expelled from the trap to the detector in order of increasing m/z ratio, rather than being pulsed out en masse into a separate analyser. In addition the ITD operates with a background pressure of Ca. lop3 Torr helium acting as a buffer gas. Figure 2(b) shows the scan function (timing sequence) used by the ITD software. The reagent gas is ionised and the resulting ions stored so that the required reagent ion distribution is set up (A). The r.f. is held so that any sample ions formed here are lost from the trap. Next the r.f. level is raised so that sample ions will be stored and this is maintained for the duration of the reaction time (B). The r.f. level is now rapidly increased to the start of the mass range of interest, expelling unwanted, for example, reagent, ions (C). Finally the r.f. is ramped to scan out sequentially the sample ions over the selected mass range. EXPERIMENTAL
The reagent gas chosen for this investigation was isobutane (2-methylpropane), a gas commonly employed in CI analyses owing to its restricted energy input to the sample molecule and the attendant limited fragmentation. It appears to act predominantly as a Bronsted acid [lo], i.e. it reacts by proton donation to the sample. Initial conditions within the three sources produce different isobutane ion distributions. Figure 3 shows the effects of storage time on reagent ions distributions within both the Quistor (Fig. 3a) and the ITD (Fig. 3b). As the storage time is increased the original low pressure spectrum, containing a large variety of ions, with m/z = 43 (C,HT) as the major one, changes to a much simpler spectrum with m/z = 57 (C,H,+) dominating. This is comparable with the effects of increasing the isobutane pressure within a high pressure source, where again the spectrum comes to be
143
(b)
0 A
3 A
‘00 F
0
0 A
4 Blo
v
0
3
QUISTOR
0 2
11.
j
0.0
“I
0.5
~
1.0
‘a
1.5
“‘*’ 2.0
0.1 2.5
0
3.0
STORAGE
5
10 15 20 25 30 35 40 45
TlME
Fig. 3. Plots showing the effects of storage time on the isobutane reagent ion distributions for (a) Quistor and (b) ITD: 0, m/z = 39; A, m/z = 41; V, m/z = 42; 0, m/z = 43; 0, m/Z = 57.
dominated by m/z = 57 at the approx. 1 Torr employed in our high pressure source (e.g. compare with Field’s Figure 1 in ref. 10). In neither type of source do adduct ions, i.e. ions above m/z = 58, appear in the spectrum. A number of compounds were originally used to compare the Quistor and high pressure sources [ 111, but only three of these were used to illustrate the comparisons with the ITD, heptan-l-01, benzaldehyde and di-n-butyl phthalate. In the Quistor and high pressure sources these were introduced using a probe whilst in the case of the ITD, samples were injected via the attached gas chromatograph. RESULTS
Bertzaldehyde
Figure 4 shows benzaldehyde spectra obtained from the various instruments, in the order (a) high pressure, (b) Quistor (9 ms storage time), and (c) ITD (11 ms storage time). All the spectra are displayed normalised to the base peak. The high pressure source produced only four ions, dominated by the (M + 1)” ion (m/z = 107), with the other three peaks at m/z = 105 ((M - I)‘“, 20%), 108 ((M + 2)+, 9%) and a tiny molecular ion peak at m/z = 106. The Quistor spectra for storage times of 5 and 9 ms had lost the molecular ion and some of the m/z = 105 intensity ~proportional to the (M - I)+ base peak) whilst gaining peaks at m/z = 79 and 80 (C,HT , 78% and C,Hz ,4% respectively). The zero storage time spectrum was signiIicantly different with the base peak at m/z = 106 and large peaks at m/z = 105 and
144
MASS Fig. 4. Isobutane CI mass spectra of benzaldehyde: (a) high pressure ion source; (b) Quistor source (9 ms storage time); (c) ITD (11 ms storage time).
m/z = 77 (C,H:). There were also smaller peaks at m/z = 78,79 and 107 and a tiny peak at m/z = 74. The ITD spectra are all dominated by the M + 1 ion, with smaller peaks (less than 20%) at m/z = 105, 106 and 108. At short storage times (less than 11 ms) the (M - 1)’ peak can increase in intensity to about 20%, while there are traces of a peak at m/z = 79. At extended ionisation times of 500~s m/z = 108 can be half the base peak height and m/z = 109 is also significant.
145
0.8
I I
(4
0.6
.:jI, 60
70
80
90
lea
110
120
130
140
150
lea
170
180
MASS Fig. 5. Isobutane CI mass spectra of heptan-l-01.
Details as for Fig. 4.
Heptan-I-01
Heptanol, in contrast with ~nzaldehyde, undergoes more extensive fragmentation in the ITD than in the Quistor, while the high pressure spectrum is again distinct from the other two, most noticeably in its much more limited fragmentation. There are no ions below m/z = 97 and it is dominated by two masses, the base peak at m/z = 99 (M + 1 - H,O)’ and m/z = 173 (89%, (M + 57)+). There are only two other peaks with intensities above 10% of the base peak, m/z = 115 (23%, (M - l)+) and m/z = 100 (1 I%, (M - H20)+). The spectrum obtained from the Quistor displays a number of peaks over 10% of the base peak intensity, whilst the location of this has shifted two mass
146
Fig. 6. Isobutane CI mass spectra of di-n-butylphthalate.
Details as for Fig. 4.
units to m/z = 97, although the m/z = 99 peak is of only slightly lower intensity. The peak at m/z = 98 is now over 10% but there is no longer a peak at m/z = 100, and the only mass seen above this was due to the (M - I)’ ion (3 1 %), there being no evidence of adduct ions. The most noticeable difference, however, was the appearance of small peaks at m/z = 85 and 83, and a group centred at m/z = 69, this peak being about 33% of the base peak intensity. The ITD spectra are similar to that obtained from the Quistor source but the fragmentation is more pronounced, with a greater proportion of the total ion count provided by the group of peaks at m/z = 69. The 97+/99+ ratio of ion intensities ranges from 2.5 to 4.5 (cf. approx. 1 for the Quistor) and the
147
m/z = 115 peak is less than 10% of the base peak height. There is again no sign of adduct ions above this mass.
Like heptan-l-01 above, the extent of fragmentation of di-n-butyl phthalate (DNBP) increases in the order “high pressure” < Quistor < ITD, although here the fragmentation pathways also differ, with the high pressure source producing mainly higher mass fragments compared with the low pressure instruments. The high pressure spectrum of DNBP shows a large (M + l)+ peak (m/z = 279) accounting for 65% of the total ion count. Other peaks can be observed at m/z = 280 (26%), 281, 261, 223 (lo%), 205 and 149, plus m/z = 317 has been observed. In contrast, although having a large peak at m/z = 279, the Quistor has its base peak at m/z = 205 and a significant signal due to m/z = 149. There are also peaks at one mass unit above each of these major peaks. Masses beyond m/z = 279 were, however, no~ally beyond the storage range of the Quistor. The ITD spectra are also dominated by the three peaks observed from the Quistor source, but the relative intensities are different: m/z = 149 accounts for over 50% of the total ion count whilst the peak at m/z = 205 represents two thirds of the remainder. There are again peaks at one mass unit above these but now m/z = 223 and 189 are also apparent, and m/z = 319 and 335 are sometimes observed. Possible reaction pathways leading to the formation of some of these ions within the high pressure source have been described elsewhere [ 111,but, as can be seen from the above-listed ions, these may not be applicable to the ITD. Further investigations are necessary to elucidate the fragmentation pathways within the ITT). Plots of ion intensities, measured relative to m/z = 149, against storage time are shown in Fig. 7 and indicate that, whereas m/z = 263 and 279 rise to a plateau intensity with increasing storage time, other masses decrease. Some of the differences between the high and low pressure sources may therefore be explainable in terms of residence time within the instruments, e.g. the abundance of m/z = 223 ions within the high pressure spectrum may be due to the short time spent in this source being insufficient for further fra~entation to occur. DISCUSSION
As can be seen from the results presented above, comparisons of the CI behaviour between the three types of source are to some extent compound dependent. However, the general picture appears to be one of increasing
148
Fig. 7. Plots of di-n-butylphthalate ion distributions (measured relative to m/z = 149) against storage time in the ITD. Ionisation Time: 0, 140~s; 0, 180~s; A, 500~s.
fragmentation in the order high pressure < Quistor < ITD. The comparisons between the high pressure source and the ITD are consistent with the results of Brodbelt et al. [7] who utilised methane as the reagent gas. They also found increased fragmentation in the ion trap and that the high pressure source can also produce adduct ions. As noted for DNBP, the differences may, at least in part, be due as much to contrasts in time scales as to variations in the reaction pathways. Residence times of ions within the high pressure source are of the order of IO-‘s whereas those in the storage devices are much longer, up to tens of milliseconds. Therefore within the latter there is a period between ion formation and detection during which any metastable ions may undergo further fragmentation, as is shown by the effects of storage time on DNBP ion distributions within the ITD. In competition with this, however, may be the effects of collisional stabilisation of ions by the helium buffer gas and possible ionisation by previously generated sample ions, proposed by Brodbelt et al. [7] as explanations of the observed increase in the relative abundance of m/z = 107 ions from benzaldehyde with storage time in the ion trap. There is some evidence of this from our investigations, e.g. there is a distinct increase in the relative abundance of the (M - l)- ion with storage time in our heptan-l-01 data, but, as these cover a much more limited time range (up to 44ms) compared with those of Brodbelt et al. (up to lOOOms), we are unable to confirm these ideas. It therefore appears that high and low pressure sources give different, and
149
possibly complementary, information about samples. The former tend to produce limited fragmentation whilst providing high mass information not available from standard EI analysis. The low pressure sources fall somewhere between these two, producing both high mass information whilst retaining some of the fragmentation necessary to elucidate structural details. Both the Quistor and ITD also provide the ability, by altering the storage times, to tune the CI conditions. The Quistor does, however, have the disadvantage that it is limited in the available mass range since the r.f. must be set so as to store both the reagent and sample ions. In contrast the ITD has a much wider range whilst the use of mass-selective instability to scan out ions, rather than pulse them from the source as is done with the Quistor, should provide enhanced sensitivity. The main advantage of the Quistor is that it can be fitted to conventional mass spectrometers, providing a low pressure source to complement already available ionisation techniques. REFERENCES 1 M.S.B. Munson and F.H. Field, J. Am. Chem. Sot., 88 (1966) 2621. 2 R.E. Mather and J.F.J. Todd, Int. J. Mass Spectrom. Ion Processes, 30 (1979) 1. 3 A.G. Harrison, Chemical Ionization Mass Spectrometry, C.R.C. Press, Boca Raton, FL, 1983. 4 R.F. Bonner, G. Lawson and J.F.J. Todd, J. Chem. Sot., Chem. Commun., (1972) 1179. 5 R.E. Mather, G. Lawson, J.F.J. Todd and J.M.B. Bakker, Int. J. Mass Spectrom. Ion Processes, 29 (1978) 347. 6 P.E. Kelly, G.C. Stafford, Jr., J.C.P. Syka, W.E. Reynolds, J.N. Louris and J.F.J. Todd, in J.F.J. Todd (Ed.), Advances in Ma& Spectrometry, 1985, Wiley, Chichester, 1986, p. 869. 7 J.S. Brodbelt, J.N. Louris and R.G. Cooks, Anal. Chem., 59 (1987) 1278. 8 S.A. McLuckey, G.L. Glish and P.E. Kelly, Anal. Chem., 59 (1987) 1670. 9 R.E. March and R.J. Hughes, Quadrupole Storage Mass Spectrometry, Wiley Interscience, New York, 1989. 10 F.H. Field, in J.L. Franklin (Ed.), Ion-Molecule Reactions, Vol. 1, Butterworths, London, 1972, p. 261. 11 R.E. Mather, Ph.D. Thesis, University of Kent, 1979.