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On the amplitude accuracy in fourier transform mass spectrometry
International Journal of Mass Spectrometry and Zen Processes, 72 (1986) 187-194 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
ON THE AMPLITUDE SPECTROMETRY
MARC
POREITI,
ACCURACY
JACQUES
RAPIN
IN FOURIER
and TIN0
TRANSFORM
187
MASS
GAUMANN
Department of PhysicaI Chemistry, Federal School of Technology, CH-101.5 busanne (Switzerland) (First received
9 October
1985; in final form 29 January
1986)
ABSTRACT Several examples are given to demonstrate that with a commercial Fourier transform mass spectrometer a dynamic range of 1: 30000 and an amplitude accuracy of - 1% can be obtained for routine high resolution work. The improvement over the best measurements obtained with one double focusing mass spectrometer is more than a factor of ten for speed, accuracy and resolution.
INTRODUCTION
Although the resolution of Fourier transform mass spectrometers (FTMS) is inherently very high, thus allowing sophisticated analytical applications, very little is known about the reproducibility and the accuracy that can be obtained with such instruments. Especially with isotopically labelled compounds, the accuracy is of crucial importance if one wants to distinguish between different reaction paths and to determine an isotope effect. In this publication, we present some results we obtained with a commercial FTMS instrument. It must be underlined that no special equipment was used. However, our experimental conditions were carefully chosen. The results were obtained on a daily routine basis over one year, and we think that they are representative of the kind of quality that can be obtained with a commercial instrument that is available today. EXPERIMENTAL
The measurements were performed with a Spectrospin CMS 47 instrument, equipped with an Aspect 3000 computer, a superconducting magnet of 3 T with 15 cm bore diameter at room temperature and a home-made vacuum and pressure-controlled inlet system. Two magnetically shielded 0168-1176/86,‘$03.50
0 1986 Elsevier Science Publishers
B.V.
188
5i.5
53.5
m/z
Fig. 1. The Fourier transform of the excitation chirp. Magnitude mode is shown.
turbomolecular pumps in series allowed a residual pressure of less than 2 x lo-” Tot-r to be attained within a short time. The background under these conditions consisted mainly of H,O, CO and CO, (and H,, that is not measurable). The working pressure was regulated to 6.0 x 10m9 Torr by means of a regulation valve and a hot filament ionization gauge. The pressure was checked by using a cathode ion gauge. A sample pressure of 10 Torr in the inlet system of about 5 ml volume allowed one to work for well over one week without any problem, corresponding to a sample consumption of less than 1 pg s-l. The ion source was the standard cylindrical ion source of the instrument, 6 cm in diameter and 8 cm long. The trapping potential was carefully adjusted so that the ion transient decay had a maximum symmetry while still maintaining a long lifetime. It might be mentioned that better mass resolution could be obtained with a more asymmetric trapping potential, but mass discrimination is the price to pay. The duration of the ionization pulse (70 eV) is 5 ms. Since all ions within the measurements range must be excited to exactly the same amplitude to obtain an even response, the careful adjustment of the excitation pulse is of crucial importance in FTMS. After considerable experimentation, we chose the single chirp conditions shown in Fig. 1. An eventual instability of the starting phase can cause unpredictable mass discrimination. Phase and amplitude stability are, in our opinion, the two factors responsible for the ultimate quality of the amplitude measurements. We found it convenient and necessary to add a second computer to
189 TABLE 1 Resolution needed for some doublets Doublet
Am (mu)
CH 13C >
4.5
2o@OO
CH, CD >
1.6
60000
0.5
200000
CD3
13CH > 5
lOO/Am
check the excitation output of our instrument during tuning in order to obtain the best results. The focusing conditions were not changed for more than one year except for a slight retuning after the ion source had been completely dismantled. We find that such an instrument can be operated during a long period on a 24 h per day routine basis without any maintenance, except for changing the filament and occasional baking out, if the sample inlet conditions are carefully controlled. The mass differences for some common mass doublets when working with D and ‘13C are shown in Table 1, together with the minimum resolution needed at m/z = 100. The actual resolution must be several times higher, since in FTMS it is expressed in line width at half height. The large base width of the Lorentz line obtained in FTMS demands an additional resolution if a large dynamic range for peaks close-by is needed. It is not particularly difficult to obtain a very high resolution in the mass range up to m/z = 200, but for routine work a compromise between the measurement time for one scan, the resolution needed, and the number of scans (which determines the dynamic range) has to be found if the ultimate capacity of the instrument is to be used. The only possible method for high-resolution work with present instruments is the narrow band or mixer mode measuring only one isobaric multiplet at a time. The transient was measured during 500 ms, the band width to 1 kHz and 1024 pts were taken for one scan. In the single-line method each multiplet has to be evaluated separately. In the samples cited below, usually about 30 m/z values were measured. 5 k scans accumulated for every multiplet corresponds to 21 h measuring time. The absolute sensitivity will shift slightly during this period, causing a systematic error. This is why the program was written, where each selected m/z value was scanned 100 times, then the next mass was chosen. The program went from the lowest mass to the highest one and then backwards
190
repeating this triangular cycle until it was stopped. Apart from the fact that systematic errors were minimized in this way, this method allowed the measurement cycle to be stopped whenever it was convenient, permitting a simple 24 h per day, 7 days per week operation since no refocusing is needed when a new sample is introduced. The transformation of the 1024 point transient was performed after a zero-filling to 32 k yielding about 180 pts per peak. With 5000 accumulated scans, a dynamic range of 1: 30000 can be obtained for a minimum signal/noise ratio (noise taken as peak-to-peak). The complex presentation of the Fourier transform was used. It turned out that the simplest way to obtain the amplitude value was by adjusting the phase manually to the pure absorption mode for each peak and measuring the peak height and the baseline, if necessary, on the screen with the cursor. The magnitude mode has the disadvantage of a larger width at the bottom of the Lorentz line. The products used were purified by gas chromatography. The labelling with 13C and D was obtained by standard synthetic methods which will be described elsewhere [1,2]. The isotopic purity of 13C used for labelling 2-methyl-2-hexene was 90%; the spectra were corrected for incomplete enlabelling. 13C and D-labelled toluene samples were 99% isotopically riched. The spectra for toluene were not corrected for incomplete labelling. RESULTS AND DISCUSSION
It is relatively easy to determine the reproducibility of an analytical method by repeating the experiment. With sufficient patience, a small value for the standard error can be obtained. However, it is more demanding to get an estimate of the accuracy of a method because of the difficulty of determining systematic errors. Here, we present a few attempts to assess a systematic error. Since any measurement can be repeated ad infinitum, we give only an estimation of the standard deviation s (except where noted), e.g. the average deviation of one experiment (which consists in most cases of about 5 k accumulated scans). The natural content of ‘:‘C It is generally assumed that the content of 13C is 1.08% [3]. Although there may be small variations with samples of different origin [4], we assume this value to be the true value. One possibility for checking the accuracy of any method is to check the natural abundance of 13C of different ions in mass spectra. This has the additional advantage that peaks of widely different intensities are compared, allowing an additional check on the linearity of the detection system. We did this for over a hundred ions chosen at random in
191
different mass spectra, but covering a dynamic range of more than 1: 103. We obtained an average value of 1.05% with a standard error of 0.01%. Since we have no reason to believe that our samples were depleted in 13C, we have a small systematic deviation to lower values. This can be explained by the fact that the frequency of our excitation chirp is always centered at the value for the hydrocarbon fragment that consists only of 12C and ‘H. The fragment with 13C is always excited with a frequency that corresponds to a mass 4.5 mu lower. Because of the sin X/X form of our excitation, the lighter fragment obtains less excitation energy and therefore will cause a smaller response signal. This variation could be corrected; since our standard deviation s for one measurement is 0.1% we do not think that such a correction improves our results except in favourable circumstances (large peaks, etc). This accuracy cannot be compared with corresponding measurements on double focusing instruments, since the presence of background peaks, ion-molecule reactions and overlapping did not permit us to get a significant result over a comparable dynamic range. Loss of “‘C to a neutral fragment When a neutral fragment C,,H, is lost, there is a probability of losing a 13C label with this neutral fragment. If we have a sample consisting of a set of substances where every position 1 to n is labelled at least once with 13C, a probability can be calculated for the loss of every position to the neutral fragment. The sum of these probabilities must be n X 100% for the fragment C,, H,,,, neglecting any isotope effect. For 2-methyl-2-hexene, we measured 13 singly and doubly labelled isotopomers. For 17 ions between C,H: and C,H& and covering a range 0.14-25% of the total ion current (TIC) we obtained a value of 97% with a standard error of 1%. Referring to the preceding section, this value should be higher than lOO%, since the loss of a 13C to the neutral fragment yields the ion that contains one 13C less. But it is known from corresponding metastable measurements [5], that values between 96% and 98% are always obtained. This can be explained by an isotope effect (see, for example, ref. 6). The standard deviation of s = 4% compares well with results from metastable decays, where in favourable cases s = 1% can be obtained, increasing to 5% for less important ions. Very often overlap with other fragments precludes any reasonable measurement in the latter case. Toluene labelled simultaneously
with ‘:‘C and D
The fragmentation of the toluene cation is assumed to proceed mostly via a ring enlargement to tropylium where all positions become equivalent (see,
192
“CC&D
‘CC&ID. A/
B/
Cd%
WW ,
CIW C,H,D. ‘*C&H&
‘“C&Da
'=CC.HaD CIDI
I
W.
Fig. 2. The multiplets at (a) m/z = 42 and (b) m/z = 53 of C,H, .13CD,. The phase corrected absorption mode is shown (1024 pts measured, 512 ms transient, 12 k accumulated scans). SB indicates side bands.
for example, ref. 7). Two examples of multiplets are shown in Fig. 2. We measured a series of labelled toluenes with the idea in mind that any systematic deviation would show up as a deviation from an assumed random distribution. Labelling of either the phenyl ring or the methyl group with D should allow us to compare the intensities of corresponding fragments. Such a comparison is shown in Table 2, where the loss of methyl from C,D, . CH:’ (I) and C,H, - CD,+. (II) is compared. A D-atom in the neutral fragment from one ion corresponds to an H-atom in the fragment of the other ion; e.g. -CHD; from (I) should be compared with -CH,D’ from (II). Although no corrections for incomplete labelling and isotope effects are made, the correspondence is very good. It is also evident that no random distribution is observed. In Table 3 the probability for the loss of 13C to methyl from 13CHc and C,H, 13CD3 is compared. The loss of the methyl group GH, ’ as a neutral fragment is observed in only 56% of all methyl loss. We can therefore calculate two H/D distributions for the methyl groups originating from the ring and the side chain. This is done in the same table. For the methyl eliminated from the ring we might expect a H/D distribution that
193 TABLE 2 Comparison between toluene deuterated on the ring or the methyl group for the ion (M-methyl)+’ Values are given as %. C,Ds.CH,
C,H, *CD,
-CD; -CHD; -CH,D’ -CH;
10 27 26 37
-CH; -CH,D’ -CHD; -CD;
11 24 26 39
TABLE 3 (a) Probability of losing the methyl carbon atom to the methyl neutral fragment X = H or D. (b) Probabilities of losing deuterium atoms with a carbon atom from either the phenyl or the methyl group Random: calculated for random distribution (in W).
-“Cx; -cx;
-CD; -CHD; -CH,D’ -CH;
C,H, .13CH3
C,H,.13CD,
56.6 43.4
55.7 44.4
Random
-cx;
-“Cx;
2 27 54 18
2 26 52 20
62 23 6 1
TABLE 4 Comparison of data obtained by a double focussing instrument [7] and an FT-MS (in W) RMH-2 x -“CD; -13CHD; -“CH,D’ -“CH; -CD; -CHD; -CH,D. -CH;
43.9 16.5 3.1 0 1.9 9.3 17.2 8.0
FT-MS s 2.8 2.1 2.1 0.7 1.4 1.2 1.4 1.2 S=1.6
X
34.5 14.1 5.5 1.5 0.9 11.5 22.9 8.9
S
0.3 0.5 0.1 0.2 0.1 0.5 0.5 0.6 s = 0.4
194
corresponds to a random distribution. The data presented show this indeed to be the case. At the same time, it can be seen that the methyl group from the side chain underwent only little exchange. Double focusing instruments have a resolution that allows in many cases the separation of the doublets measured in this work. However, it is very cumbersome to obtain this high resolution that is often obtained with some mass discrimination. We know only of one publication where a systematic study with a resolving power of 1 : 50000 (10% valley) has been made [7]. Its results are compared with our data in Table 4. There is a systematic deviation between the results obtained by the two different instruments. One explanation could be that the FTMS instrument integrates the fragmentation over several ms, thus allowing more time for rearrangement. Further, the isotopic purity of the samples is different. The comparison of the standard deviations shows the large gain obtained by the FTMS method, especially when it is considered that the FTMS results were obtained overnight by an instrument that has not been retuned for over six months. ACKNOWLEDGEMENTS
We would like to thank Miss M. Rabier for experimental the Swiss National Science Foundation for a grant.
assistance
and
REFERENCES 1 2 3 4 5
M. Bensimon, A. Heusler, J. Rapin and T. Gaumann, Helv. Chim. Acta, to be published. J. Rapin, A. Heusler and T. Gaumann, Helv. Chim. Acta, to be published. J.H. Beynon, Mass Spectrometry, Elsevier, Amsterdam, 1960, p. 486. F.J. Winkler and H.-L. Schmidt, Z. Lebensm. Unters. Forsch., 171 (1980) 85. J.C. Antunes Marques, A. Falick, A. Heusler, D. Stahl, P. Tecon and T. Gaumann, Helv. Chim. Acta, 67 (1984) 425 and references cited therein. 6 M. Bensimon, T. Gaumann, C. Guenat and D. Stahl, 10th. Int. Mass Spectrom. Conf., Swansea, 1985; Helv. Chim. Acta, in press. 7 M.A. Baldwin, F.W. McLafferty and D.M. Jering, J. Am. Chem. Sot., 97 (1975) 6169.
ON THE AMPLITUDE SPECTROMETRY
MARC
POREITI,
ACCURACY
JACQUES
RAPIN
IN FOURIER
and TIN0
TRANSFORM
187
MASS
GAUMANN
Department of PhysicaI Chemistry, Federal School of Technology, CH-101.5 busanne (Switzerland) (First received
9 October
1985; in final form 29 January
1986)
ABSTRACT Several examples are given to demonstrate that with a commercial Fourier transform mass spectrometer a dynamic range of 1: 30000 and an amplitude accuracy of - 1% can be obtained for routine high resolution work. The improvement over the best measurements obtained with one double focusing mass spectrometer is more than a factor of ten for speed, accuracy and resolution.
INTRODUCTION
Although the resolution of Fourier transform mass spectrometers (FTMS) is inherently very high, thus allowing sophisticated analytical applications, very little is known about the reproducibility and the accuracy that can be obtained with such instruments. Especially with isotopically labelled compounds, the accuracy is of crucial importance if one wants to distinguish between different reaction paths and to determine an isotope effect. In this publication, we present some results we obtained with a commercial FTMS instrument. It must be underlined that no special equipment was used. However, our experimental conditions were carefully chosen. The results were obtained on a daily routine basis over one year, and we think that they are representative of the kind of quality that can be obtained with a commercial instrument that is available today. EXPERIMENTAL
The measurements were performed with a Spectrospin CMS 47 instrument, equipped with an Aspect 3000 computer, a superconducting magnet of 3 T with 15 cm bore diameter at room temperature and a home-made vacuum and pressure-controlled inlet system. Two magnetically shielded 0168-1176/86,‘$03.50
0 1986 Elsevier Science Publishers
B.V.
188
5i.5
53.5
m/z
Fig. 1. The Fourier transform of the excitation chirp. Magnitude mode is shown.
turbomolecular pumps in series allowed a residual pressure of less than 2 x lo-” Tot-r to be attained within a short time. The background under these conditions consisted mainly of H,O, CO and CO, (and H,, that is not measurable). The working pressure was regulated to 6.0 x 10m9 Torr by means of a regulation valve and a hot filament ionization gauge. The pressure was checked by using a cathode ion gauge. A sample pressure of 10 Torr in the inlet system of about 5 ml volume allowed one to work for well over one week without any problem, corresponding to a sample consumption of less than 1 pg s-l. The ion source was the standard cylindrical ion source of the instrument, 6 cm in diameter and 8 cm long. The trapping potential was carefully adjusted so that the ion transient decay had a maximum symmetry while still maintaining a long lifetime. It might be mentioned that better mass resolution could be obtained with a more asymmetric trapping potential, but mass discrimination is the price to pay. The duration of the ionization pulse (70 eV) is 5 ms. Since all ions within the measurements range must be excited to exactly the same amplitude to obtain an even response, the careful adjustment of the excitation pulse is of crucial importance in FTMS. After considerable experimentation, we chose the single chirp conditions shown in Fig. 1. An eventual instability of the starting phase can cause unpredictable mass discrimination. Phase and amplitude stability are, in our opinion, the two factors responsible for the ultimate quality of the amplitude measurements. We found it convenient and necessary to add a second computer to
189 TABLE 1 Resolution needed for some doublets Doublet
Am (mu)
CH 13C >
4.5
2o@OO
CH, CD >
1.6
60000
0.5
200000
CD3
13CH > 5
lOO/Am
check the excitation output of our instrument during tuning in order to obtain the best results. The focusing conditions were not changed for more than one year except for a slight retuning after the ion source had been completely dismantled. We find that such an instrument can be operated during a long period on a 24 h per day routine basis without any maintenance, except for changing the filament and occasional baking out, if the sample inlet conditions are carefully controlled. The mass differences for some common mass doublets when working with D and ‘13C are shown in Table 1, together with the minimum resolution needed at m/z = 100. The actual resolution must be several times higher, since in FTMS it is expressed in line width at half height. The large base width of the Lorentz line obtained in FTMS demands an additional resolution if a large dynamic range for peaks close-by is needed. It is not particularly difficult to obtain a very high resolution in the mass range up to m/z = 200, but for routine work a compromise between the measurement time for one scan, the resolution needed, and the number of scans (which determines the dynamic range) has to be found if the ultimate capacity of the instrument is to be used. The only possible method for high-resolution work with present instruments is the narrow band or mixer mode measuring only one isobaric multiplet at a time. The transient was measured during 500 ms, the band width to 1 kHz and 1024 pts were taken for one scan. In the single-line method each multiplet has to be evaluated separately. In the samples cited below, usually about 30 m/z values were measured. 5 k scans accumulated for every multiplet corresponds to 21 h measuring time. The absolute sensitivity will shift slightly during this period, causing a systematic error. This is why the program was written, where each selected m/z value was scanned 100 times, then the next mass was chosen. The program went from the lowest mass to the highest one and then backwards
190
repeating this triangular cycle until it was stopped. Apart from the fact that systematic errors were minimized in this way, this method allowed the measurement cycle to be stopped whenever it was convenient, permitting a simple 24 h per day, 7 days per week operation since no refocusing is needed when a new sample is introduced. The transformation of the 1024 point transient was performed after a zero-filling to 32 k yielding about 180 pts per peak. With 5000 accumulated scans, a dynamic range of 1: 30000 can be obtained for a minimum signal/noise ratio (noise taken as peak-to-peak). The complex presentation of the Fourier transform was used. It turned out that the simplest way to obtain the amplitude value was by adjusting the phase manually to the pure absorption mode for each peak and measuring the peak height and the baseline, if necessary, on the screen with the cursor. The magnitude mode has the disadvantage of a larger width at the bottom of the Lorentz line. The products used were purified by gas chromatography. The labelling with 13C and D was obtained by standard synthetic methods which will be described elsewhere [1,2]. The isotopic purity of 13C used for labelling 2-methyl-2-hexene was 90%; the spectra were corrected for incomplete enlabelling. 13C and D-labelled toluene samples were 99% isotopically riched. The spectra for toluene were not corrected for incomplete labelling. RESULTS AND DISCUSSION
It is relatively easy to determine the reproducibility of an analytical method by repeating the experiment. With sufficient patience, a small value for the standard error can be obtained. However, it is more demanding to get an estimate of the accuracy of a method because of the difficulty of determining systematic errors. Here, we present a few attempts to assess a systematic error. Since any measurement can be repeated ad infinitum, we give only an estimation of the standard deviation s (except where noted), e.g. the average deviation of one experiment (which consists in most cases of about 5 k accumulated scans). The natural content of ‘:‘C It is generally assumed that the content of 13C is 1.08% [3]. Although there may be small variations with samples of different origin [4], we assume this value to be the true value. One possibility for checking the accuracy of any method is to check the natural abundance of 13C of different ions in mass spectra. This has the additional advantage that peaks of widely different intensities are compared, allowing an additional check on the linearity of the detection system. We did this for over a hundred ions chosen at random in
191
different mass spectra, but covering a dynamic range of more than 1: 103. We obtained an average value of 1.05% with a standard error of 0.01%. Since we have no reason to believe that our samples were depleted in 13C, we have a small systematic deviation to lower values. This can be explained by the fact that the frequency of our excitation chirp is always centered at the value for the hydrocarbon fragment that consists only of 12C and ‘H. The fragment with 13C is always excited with a frequency that corresponds to a mass 4.5 mu lower. Because of the sin X/X form of our excitation, the lighter fragment obtains less excitation energy and therefore will cause a smaller response signal. This variation could be corrected; since our standard deviation s for one measurement is 0.1% we do not think that such a correction improves our results except in favourable circumstances (large peaks, etc). This accuracy cannot be compared with corresponding measurements on double focusing instruments, since the presence of background peaks, ion-molecule reactions and overlapping did not permit us to get a significant result over a comparable dynamic range. Loss of “‘C to a neutral fragment When a neutral fragment C,,H, is lost, there is a probability of losing a 13C label with this neutral fragment. If we have a sample consisting of a set of substances where every position 1 to n is labelled at least once with 13C, a probability can be calculated for the loss of every position to the neutral fragment. The sum of these probabilities must be n X 100% for the fragment C,, H,,,, neglecting any isotope effect. For 2-methyl-2-hexene, we measured 13 singly and doubly labelled isotopomers. For 17 ions between C,H: and C,H& and covering a range 0.14-25% of the total ion current (TIC) we obtained a value of 97% with a standard error of 1%. Referring to the preceding section, this value should be higher than lOO%, since the loss of a 13C to the neutral fragment yields the ion that contains one 13C less. But it is known from corresponding metastable measurements [5], that values between 96% and 98% are always obtained. This can be explained by an isotope effect (see, for example, ref. 6). The standard deviation of s = 4% compares well with results from metastable decays, where in favourable cases s = 1% can be obtained, increasing to 5% for less important ions. Very often overlap with other fragments precludes any reasonable measurement in the latter case. Toluene labelled simultaneously
with ‘:‘C and D
The fragmentation of the toluene cation is assumed to proceed mostly via a ring enlargement to tropylium where all positions become equivalent (see,
192
“CC&D
‘CC&ID. A/
B/
Cd%
WW ,
CIW C,H,D. ‘*C&H&
‘“C&Da
'=CC.HaD CIDI
I
W.
Fig. 2. The multiplets at (a) m/z = 42 and (b) m/z = 53 of C,H, .13CD,. The phase corrected absorption mode is shown (1024 pts measured, 512 ms transient, 12 k accumulated scans). SB indicates side bands.
for example, ref. 7). Two examples of multiplets are shown in Fig. 2. We measured a series of labelled toluenes with the idea in mind that any systematic deviation would show up as a deviation from an assumed random distribution. Labelling of either the phenyl ring or the methyl group with D should allow us to compare the intensities of corresponding fragments. Such a comparison is shown in Table 2, where the loss of methyl from C,D, . CH:’ (I) and C,H, - CD,+. (II) is compared. A D-atom in the neutral fragment from one ion corresponds to an H-atom in the fragment of the other ion; e.g. -CHD; from (I) should be compared with -CH,D’ from (II). Although no corrections for incomplete labelling and isotope effects are made, the correspondence is very good. It is also evident that no random distribution is observed. In Table 3 the probability for the loss of 13C to methyl from 13CHc and C,H, 13CD3 is compared. The loss of the methyl group GH, ’ as a neutral fragment is observed in only 56% of all methyl loss. We can therefore calculate two H/D distributions for the methyl groups originating from the ring and the side chain. This is done in the same table. For the methyl eliminated from the ring we might expect a H/D distribution that
193 TABLE 2 Comparison between toluene deuterated on the ring or the methyl group for the ion (M-methyl)+’ Values are given as %. C,Ds.CH,
C,H, *CD,
-CD; -CHD; -CH,D’ -CH;
10 27 26 37
-CH; -CH,D’ -CHD; -CD;
11 24 26 39
TABLE 3 (a) Probability of losing the methyl carbon atom to the methyl neutral fragment X = H or D. (b) Probabilities of losing deuterium atoms with a carbon atom from either the phenyl or the methyl group Random: calculated for random distribution (in W).
-“Cx; -cx;
-CD; -CHD; -CH,D’ -CH;
C,H, .13CH3
C,H,.13CD,
56.6 43.4
55.7 44.4
Random
-cx;
-“Cx;
2 27 54 18
2 26 52 20
62 23 6 1
TABLE 4 Comparison of data obtained by a double focussing instrument [7] and an FT-MS (in W) RMH-2 x -“CD; -13CHD; -“CH,D’ -“CH; -CD; -CHD; -CH,D. -CH;
43.9 16.5 3.1 0 1.9 9.3 17.2 8.0
FT-MS s 2.8 2.1 2.1 0.7 1.4 1.2 1.4 1.2 S=1.6
X
34.5 14.1 5.5 1.5 0.9 11.5 22.9 8.9
S
0.3 0.5 0.1 0.2 0.1 0.5 0.5 0.6 s = 0.4
194
corresponds to a random distribution. The data presented show this indeed to be the case. At the same time, it can be seen that the methyl group from the side chain underwent only little exchange. Double focusing instruments have a resolution that allows in many cases the separation of the doublets measured in this work. However, it is very cumbersome to obtain this high resolution that is often obtained with some mass discrimination. We know only of one publication where a systematic study with a resolving power of 1 : 50000 (10% valley) has been made [7]. Its results are compared with our data in Table 4. There is a systematic deviation between the results obtained by the two different instruments. One explanation could be that the FTMS instrument integrates the fragmentation over several ms, thus allowing more time for rearrangement. Further, the isotopic purity of the samples is different. The comparison of the standard deviations shows the large gain obtained by the FTMS method, especially when it is considered that the FTMS results were obtained overnight by an instrument that has not been retuned for over six months. ACKNOWLEDGEMENTS
We would like to thank Miss M. Rabier for experimental the Swiss National Science Foundation for a grant.
assistance
and
REFERENCES 1 2 3 4 5
M. Bensimon, A. Heusler, J. Rapin and T. Gaumann, Helv. Chim. Acta, to be published. J. Rapin, A. Heusler and T. Gaumann, Helv. Chim. Acta, to be published. J.H. Beynon, Mass Spectrometry, Elsevier, Amsterdam, 1960, p. 486. F.J. Winkler and H.-L. Schmidt, Z. Lebensm. Unters. Forsch., 171 (1980) 85. J.C. Antunes Marques, A. Falick, A. Heusler, D. Stahl, P. Tecon and T. Gaumann, Helv. Chim. Acta, 67 (1984) 425 and references cited therein. 6 M. Bensimon, T. Gaumann, C. Guenat and D. Stahl, 10th. Int. Mass Spectrom. Conf., Swansea, 1985; Helv. Chim. Acta, in press. 7 M.A. Baldwin, F.W. McLafferty and D.M. Jering, J. Am. Chem. Sot., 97 (1975) 6169.