Biomedical Mass Spectrometry 197% 2. 3 1 to 35
The Application of Quadrupole Mass Filters in Field Desorption Mass Spectrometry H. H. GIERLICH, H. J. HEINENand H. D. BECKEY Institut fur Physikalische Chemie der Universitat Bonn. 53 Bonn, Wegelerstralje 12. West Germany
(Receiced 1 1 December 1974) Abstract-The advantages and limitations which quadrupole mass filters afford to the field desorption technique with respect to use for routine work are discussed and experimentally confirmed by the analyses of some drugs using a field desorption quadrupole mass spectrometer. The possibility of fast identification of drug intoxication is demonstrated by the analysis of the chloroform extract of urine in a case of overdose of hypnotics.
Introduction
Experimental
FIELD desorption mass spectrometry (f.d.m.s.) has been applied successfully to a number of analytical problems over the last few years.'" It has been the general experience, however, that the use of slow scanning single focusing mass spectrometers is limited by the fact that the times for field desorption, which can range from a few minutes to only a few seconds, are often of the order of the scanning time of the magnet. This limitation can be overcome either by an integrating photographic recording system, as used in double focusing instrument^,^ or by a quadrupole mass spectrometer (q.m.s.) which can reach scanning speeds of up to 1000 atomic mass units per second, that are much faster than any average desorption rate observed previously. Moreover, a high degree of flexibility is given to the observation of the f.d. process, due to the fact that the operation mode of a q.m.s. concerning mass range, scanning speed and resolution is determined solely by electronics. Therefore, substances which field desorb very fast from the emitter surfacedue to their physical and chemical properties--can be detected by the q.m.s., as can very small amounts of substances which have to be flashed from the emitter surface in order to achieve a reasonable intensity for peak identification. It is well known that the q.m.s. discriminates against ions in the upper mass range if unit mass resolution is required. As the fragmentation level in fd. mass spectra is very low, the mass discrimination is not a serious restriction for the Ed. technique, so long as sufficient ion intensities for peak identification are obtained. Due to the fact that the intensities obtained by f.d. are lower than those achieved by electron impact (e.i.), the application of field desorption quadrupole mass spectrometry (f.d.q.m.s.) is limited to about mass 500 at present if the q.m.s. operates in the unit mass resolution mode. An earlier report has already discussed the combination of an f.d. source with a q.m.s., outlining the physical and technical problems involved.6 The present paper deals with the potentialities which the q.m.s. offers to the analyst in f.d. applications.
The mass spectra were obtained with an improved version of a field desorption quadrupole mass spectrometer described earlier.6 The q.m.s. used in these investigations was the Extranuclear Laboratories Model 162-8. High temperature activated 10 p tungsten wires were used as emitters which were heated by passing a direct current through the wire. The samples to be analysed were deposited on the emitter surface by dipping the emitter into solutions of the substances. The molarity of the solutions used was of the order of
Q Heyden & Son Ltd, 1975
10-2.
Results and discussion Substances for analysis by f.d.m.s. differ widely in their desorption behaviour. This does not only depend on the emitter temperature and the amount of substance ( to lo-'' g) adsorbed on the emitter surface, but also on the physical and chemical properties of the substances to be investigated. Often substances already desorb at room temperature when the electric field is applied to the emitter. In most cases, however, the desorption rates obtained at room temperature are too small for sufficient ion intensities in the d.c. measuring mode. When the emitter temperature is raised the desorption rate of some substances increases smoothly, whereas other substances show a steep increase in the desorption rate when a critical emitter temperature is surpassed, thus yielding high ion intensities over a short desorption time, Quadrupole mass spectrometers can deal with each different desorption behaviour individually, which is especially important if mixtures of substances are to be investigated. Furthermore, quadrupole mass spectrometers open up another possibility for f.d.m.s., as the transmission can be rapidly increased to a noticeable extent by decreasing the resolution. Thus, even with low desorption rates, valuable information about the pattern of the mass spectrum can be obtained. This mode reveals the mass ranges of interest as well as the best anode temperature' for the substance. With this advance information, peak identification with adequate higher 31
H. H. GIERLICH, H. J . HEINEN A N D H. D. BECKEY
32
M'
176
/
DMSO
Pemolin
20
LO
60
1W
120
140
1 60
180 m/e
FIG.1. The f.d. mass spectrum of Pemolin@(emitter heating current 10mA).
resolution can be easily performed even in more difficult cases. One must remember, however, that in the case of low ion intensity the scanning speed is limited by the fact that a minimum number of ions per mass must be registered in order to define the exact peak position. Thus, together with the time constant of the ion detection system and a suitable amplification of the signal, the scanning speed must be adapted to each specific desorption behaviour pattern for a given resolution mode, in order to optimize the information from the f.d. mass spectrum. If only information about the molecular weight of one or several components is of interest for routine procedures, the q.m.s. offers another quick procedure to the user. A wide mass range is oscillographically displayed with the resolution set low and with a high scanning speed. Then, simply by increasing the resolution and decreasing the scan width to one mass unit the peak of interest is matched on the oscilloscope screen. Since the transmitted mass is a linear function of the high frequency amplitude and hence of the internal feedback voltage of the q.m.s., this voltage supplies the m/e value immediately with a high degree of reliability. The f.d. mass spectra of some drugs are presented below showing the potentiality of the f.d.q.m.s. combination described above. the first substance The psychoenergeticum Pemolin @, discussed, has an entirely unproblematic desorption behaviour. When the high electric field is applied to the field anode, practically no desorption occurs at room temperature. With decreased resolution it was found that at the best anode temperature for Pemolin@ the
time of field desorption is in the order of a few minutes with reasonably high intensities in the molecular ion region. After determination of the best anode temperature and the desorption behaviour of Pemolin@in the low resolution mode, the spectrum was recorded with slow scanning speed and appropriate resolution (Fig. 1). The predominant peaks in the mass spectrum can be assigned to the parent ion of Pemolin@at m/e 176, the doubly charged parent ion at m/e 88 and the parent ion of the solvent dimethylsulphoxide at m/e 78. On the other hand, Clofedanol@,an antitussivum, desorbs rapidly at room temperature with very low desorption fields. Even though the f.d. technique seems to be inadequate for the investigation of volatile substances of this kind, it is still possible to record the molecular ion group with the fast scanning mode of the q.m.s., as can be seen in Fig. 2. Meclozin@, a hypnoticum and antiemeticum, also desorbs at room temperature, but only with a very low desorption rate. Optimal ion intensity is obtained at the best anode temperature of Meclozin@ while the desorption time is only a few seconds. Although the ion intensities obtained in the temperature range between room and best anode temperature are too small for peak identification, the low resolution mode can again reveal valuable information about the mass ranges of interest in the f.d. spectrum (Fig. 3). Consequently, the resolved spectrum of the molecular ion group and of the fragment ion groups in the mass range from 180 to 210, which had been determined by the low resolution run, was recorded at the best anode temperature (Fig. 4). The base peak in the spectrum is due to the protonated molecule at m/e 391. By cleavage of the C-N bond-
FIELD DESORPTION Q U A D R U P O L E MASS SPECTROMETRY
indicated in Fig. +one obtains the main fragments at mle 190 and mle 201. It is a significant feature of the f.d. technique that the fragmentation level in the mass spectra is very low. Therefore, f.d.m.s. is very suitable for the analysis of mixtures as the coincidence of molecular ions and fragment ions with the same m/e value is greatly decreased. If the determination of the molecular weight of the substances is sufficient for routine work, the q.m.s. not only offers a highly time resolved observation of the f.d. spectrum, but also a distinction between components with different desorptions behaviours. This is demonstrated by an example taken from forensic medical practice. Here the chloroformic urine extract of a case of overdose of hypnotics was examined by f.d.q.m.s. It was suspected that the patient had taken an overdose of the hypnotic 'Trisomnin'@',a combination of the barbiturates Propanal@ (5,5-dipropylbarbituric acid), Luminal@(5-ethyl,5-phenyl-barbituric
C Lofedanot
250
270
33
-
290 m/e
FIG.2. The f.d. mass spectrum of the molecular ion region of Clofedanol obtained at room temperature.
Meclozin ( H C O
I
100
I
150
200
250
-
300
350
1
Loo
m/e
FIG.3. The low resolution mass spectrum of Meclozin" obtained below the best anode temperature.
Meclozin ( H C I I
FIG.4. The peak resolved mass spectrum of the main fragments and the molecular ion group of Meclozin" obtained at the best anode temperature (14 mA emitter heating current).
H. H. GIERLICH. H. J. HEINEN A N D H. D. BECKEY
34
Luminal
232
a)
/
180
Urine extract Emitter heating current 13mA
/
-
200
150
250
m/e
Luminal
1
b ) Urine extract
Emitter heating current 15mA
1 \
L .
Eldoral
.
T
200
150
c )
l
250
Urine extract Emitter heating current 18mA
Luminal
-
mle
FIG.5 . The f.d. mass spectra of the chloroformic urine extract of a case of overdose of hypnotics obtained at different emitter heating currents.
FIELD DESORPTION QUADRUPOLE MASS SPECTROMETRY
acid) and Eldoral@ (5-ethy1,5(1-piperidyl)-barbituric acid) and attention was paid to the molecular ion regions of these barbiturates only. Figs. 5a to 5c show the f.d. mass spectra of the extract at different emitter temperatures. One can clearly see the different desorption behaviour patterns of the barbiturates present in the extract. Presumably, due to low concentration in the extract, Eldoral@ desorbed completely at low emitter temperatures with low intensities, whereas Luminal@ desorbed over a wide temperature range with high intensities. The desorption behaviour of Propanal@resembled that of Eldoral@,with higher ion intensities at a higher emitter temperature. Even though the concentration of Eldoral@ was very small, the consumption of ‘Trisomnin’@was confirmed. As the whole recording procedure for the mass spectrum takes only a few minutes, the application of f.d.q.m.s. to fast identification of drugs in extracts of blood, gastric contents and urine appears very promising. In conclusion, the investigations described above reveal the versatility of the f.d.q.m.s. Wide ranges of the mass spectrum can be displayed quasi-simultaneously on the oscilloscope screen due to the facility for a rapid mass scan. Hence changes in the operating mode
35
concerning resolution, mass range, emitter temperature and field strength can be visualized directly during the Ed. process. Should low resolution mass analysis not be sufficient and a high resolution f.d. mass spectrum with a double focusing instrument be necessary, a preinvestigation of the substance with the q.m.s. is of advantage in order to exploit the desorption behaviour of one or several components. ACKNOWLEDGEMENTS We would like to thank Professor Dr S. Goenechea from the Institut fur Gerichtliche Medizin der UniveritPt Bonn for the supply of the urine extract and for helpful discussions. We are indebted to ‘Extranuclear Laboratories’ for the loan of the quadrupole mass filter which was employed in these investigations.
REFERENCES I . Winkler, H. U . ; Beckey, H. D. O r g . Muss Spectrom. 1972,6,655. 2. Winkler, H. U.; Beckey, H. D. Biochem. Biophvs. Res. Commun. 1972, 46, 391. 3. Schulten, H.-R.; Beckey. H. D. J . Agric. Food Chem. 1973,21,372. 4. Schulten, H.-R. Biomed. Mass Spectrom. 1974, 1, 223. 5 . Schulten, H.-R.; Beckey, H. D. Org. Mass Spectrom. 1973, 7 , 861. 6. Heinen, H. J . ; Hotzel, Ch.; Beckey, H. D. Inr. J . Muss Spectrom. Ion Phvs. 1974, 13, 55.
The Application of Quadrupole Mass Filters in Field Desorption Mass Spectrometry H. H. GIERLICH, H. J. HEINENand H. D. BECKEY Institut fur Physikalische Chemie der Universitat Bonn. 53 Bonn, Wegelerstralje 12. West Germany
(Receiced 1 1 December 1974) Abstract-The advantages and limitations which quadrupole mass filters afford to the field desorption technique with respect to use for routine work are discussed and experimentally confirmed by the analyses of some drugs using a field desorption quadrupole mass spectrometer. The possibility of fast identification of drug intoxication is demonstrated by the analysis of the chloroform extract of urine in a case of overdose of hypnotics.
Introduction
Experimental
FIELD desorption mass spectrometry (f.d.m.s.) has been applied successfully to a number of analytical problems over the last few years.'" It has been the general experience, however, that the use of slow scanning single focusing mass spectrometers is limited by the fact that the times for field desorption, which can range from a few minutes to only a few seconds, are often of the order of the scanning time of the magnet. This limitation can be overcome either by an integrating photographic recording system, as used in double focusing instrument^,^ or by a quadrupole mass spectrometer (q.m.s.) which can reach scanning speeds of up to 1000 atomic mass units per second, that are much faster than any average desorption rate observed previously. Moreover, a high degree of flexibility is given to the observation of the f.d. process, due to the fact that the operation mode of a q.m.s. concerning mass range, scanning speed and resolution is determined solely by electronics. Therefore, substances which field desorb very fast from the emitter surfacedue to their physical and chemical properties--can be detected by the q.m.s., as can very small amounts of substances which have to be flashed from the emitter surface in order to achieve a reasonable intensity for peak identification. It is well known that the q.m.s. discriminates against ions in the upper mass range if unit mass resolution is required. As the fragmentation level in fd. mass spectra is very low, the mass discrimination is not a serious restriction for the Ed. technique, so long as sufficient ion intensities for peak identification are obtained. Due to the fact that the intensities obtained by f.d. are lower than those achieved by electron impact (e.i.), the application of field desorption quadrupole mass spectrometry (f.d.q.m.s.) is limited to about mass 500 at present if the q.m.s. operates in the unit mass resolution mode. An earlier report has already discussed the combination of an f.d. source with a q.m.s., outlining the physical and technical problems involved.6 The present paper deals with the potentialities which the q.m.s. offers to the analyst in f.d. applications.
The mass spectra were obtained with an improved version of a field desorption quadrupole mass spectrometer described earlier.6 The q.m.s. used in these investigations was the Extranuclear Laboratories Model 162-8. High temperature activated 10 p tungsten wires were used as emitters which were heated by passing a direct current through the wire. The samples to be analysed were deposited on the emitter surface by dipping the emitter into solutions of the substances. The molarity of the solutions used was of the order of
Q Heyden & Son Ltd, 1975
10-2.
Results and discussion Substances for analysis by f.d.m.s. differ widely in their desorption behaviour. This does not only depend on the emitter temperature and the amount of substance ( to lo-'' g) adsorbed on the emitter surface, but also on the physical and chemical properties of the substances to be investigated. Often substances already desorb at room temperature when the electric field is applied to the emitter. In most cases, however, the desorption rates obtained at room temperature are too small for sufficient ion intensities in the d.c. measuring mode. When the emitter temperature is raised the desorption rate of some substances increases smoothly, whereas other substances show a steep increase in the desorption rate when a critical emitter temperature is surpassed, thus yielding high ion intensities over a short desorption time, Quadrupole mass spectrometers can deal with each different desorption behaviour individually, which is especially important if mixtures of substances are to be investigated. Furthermore, quadrupole mass spectrometers open up another possibility for f.d.m.s., as the transmission can be rapidly increased to a noticeable extent by decreasing the resolution. Thus, even with low desorption rates, valuable information about the pattern of the mass spectrum can be obtained. This mode reveals the mass ranges of interest as well as the best anode temperature' for the substance. With this advance information, peak identification with adequate higher 31
H. H. GIERLICH, H. J . HEINEN A N D H. D. BECKEY
32
M'
176
/
DMSO
Pemolin
20
LO
60
1W
120
140
1 60
180 m/e
FIG.1. The f.d. mass spectrum of Pemolin@(emitter heating current 10mA).
resolution can be easily performed even in more difficult cases. One must remember, however, that in the case of low ion intensity the scanning speed is limited by the fact that a minimum number of ions per mass must be registered in order to define the exact peak position. Thus, together with the time constant of the ion detection system and a suitable amplification of the signal, the scanning speed must be adapted to each specific desorption behaviour pattern for a given resolution mode, in order to optimize the information from the f.d. mass spectrum. If only information about the molecular weight of one or several components is of interest for routine procedures, the q.m.s. offers another quick procedure to the user. A wide mass range is oscillographically displayed with the resolution set low and with a high scanning speed. Then, simply by increasing the resolution and decreasing the scan width to one mass unit the peak of interest is matched on the oscilloscope screen. Since the transmitted mass is a linear function of the high frequency amplitude and hence of the internal feedback voltage of the q.m.s., this voltage supplies the m/e value immediately with a high degree of reliability. The f.d. mass spectra of some drugs are presented below showing the potentiality of the f.d.q.m.s. combination described above. the first substance The psychoenergeticum Pemolin @, discussed, has an entirely unproblematic desorption behaviour. When the high electric field is applied to the field anode, practically no desorption occurs at room temperature. With decreased resolution it was found that at the best anode temperature for Pemolin@ the
time of field desorption is in the order of a few minutes with reasonably high intensities in the molecular ion region. After determination of the best anode temperature and the desorption behaviour of Pemolin@in the low resolution mode, the spectrum was recorded with slow scanning speed and appropriate resolution (Fig. 1). The predominant peaks in the mass spectrum can be assigned to the parent ion of Pemolin@at m/e 176, the doubly charged parent ion at m/e 88 and the parent ion of the solvent dimethylsulphoxide at m/e 78. On the other hand, Clofedanol@,an antitussivum, desorbs rapidly at room temperature with very low desorption fields. Even though the f.d. technique seems to be inadequate for the investigation of volatile substances of this kind, it is still possible to record the molecular ion group with the fast scanning mode of the q.m.s., as can be seen in Fig. 2. Meclozin@, a hypnoticum and antiemeticum, also desorbs at room temperature, but only with a very low desorption rate. Optimal ion intensity is obtained at the best anode temperature of Meclozin@ while the desorption time is only a few seconds. Although the ion intensities obtained in the temperature range between room and best anode temperature are too small for peak identification, the low resolution mode can again reveal valuable information about the mass ranges of interest in the f.d. spectrum (Fig. 3). Consequently, the resolved spectrum of the molecular ion group and of the fragment ion groups in the mass range from 180 to 210, which had been determined by the low resolution run, was recorded at the best anode temperature (Fig. 4). The base peak in the spectrum is due to the protonated molecule at m/e 391. By cleavage of the C-N bond-
FIELD DESORPTION Q U A D R U P O L E MASS SPECTROMETRY
indicated in Fig. +one obtains the main fragments at mle 190 and mle 201. It is a significant feature of the f.d. technique that the fragmentation level in the mass spectra is very low. Therefore, f.d.m.s. is very suitable for the analysis of mixtures as the coincidence of molecular ions and fragment ions with the same m/e value is greatly decreased. If the determination of the molecular weight of the substances is sufficient for routine work, the q.m.s. not only offers a highly time resolved observation of the f.d. spectrum, but also a distinction between components with different desorptions behaviours. This is demonstrated by an example taken from forensic medical practice. Here the chloroformic urine extract of a case of overdose of hypnotics was examined by f.d.q.m.s. It was suspected that the patient had taken an overdose of the hypnotic 'Trisomnin'@',a combination of the barbiturates Propanal@ (5,5-dipropylbarbituric acid), Luminal@(5-ethyl,5-phenyl-barbituric
C Lofedanot
250
270
33
-
290 m/e
FIG.2. The f.d. mass spectrum of the molecular ion region of Clofedanol obtained at room temperature.
Meclozin ( H C O
I
100
I
150
200
250
-
300
350
1
Loo
m/e
FIG.3. The low resolution mass spectrum of Meclozin" obtained below the best anode temperature.
Meclozin ( H C I I
FIG.4. The peak resolved mass spectrum of the main fragments and the molecular ion group of Meclozin" obtained at the best anode temperature (14 mA emitter heating current).
H. H. GIERLICH. H. J. HEINEN A N D H. D. BECKEY
34
Luminal
232
a)
/
180
Urine extract Emitter heating current 13mA
/
-
200
150
250
m/e
Luminal
1
b ) Urine extract
Emitter heating current 15mA
1 \
L .
Eldoral
.
T
200
150
c )
l
250
Urine extract Emitter heating current 18mA
Luminal
-
mle
FIG.5 . The f.d. mass spectra of the chloroformic urine extract of a case of overdose of hypnotics obtained at different emitter heating currents.
FIELD DESORPTION QUADRUPOLE MASS SPECTROMETRY
acid) and Eldoral@ (5-ethy1,5(1-piperidyl)-barbituric acid) and attention was paid to the molecular ion regions of these barbiturates only. Figs. 5a to 5c show the f.d. mass spectra of the extract at different emitter temperatures. One can clearly see the different desorption behaviour patterns of the barbiturates present in the extract. Presumably, due to low concentration in the extract, Eldoral@ desorbed completely at low emitter temperatures with low intensities, whereas Luminal@ desorbed over a wide temperature range with high intensities. The desorption behaviour of Propanal@resembled that of Eldoral@,with higher ion intensities at a higher emitter temperature. Even though the concentration of Eldoral@ was very small, the consumption of ‘Trisomnin’@was confirmed. As the whole recording procedure for the mass spectrum takes only a few minutes, the application of f.d.q.m.s. to fast identification of drugs in extracts of blood, gastric contents and urine appears very promising. In conclusion, the investigations described above reveal the versatility of the f.d.q.m.s. Wide ranges of the mass spectrum can be displayed quasi-simultaneously on the oscilloscope screen due to the facility for a rapid mass scan. Hence changes in the operating mode
35
concerning resolution, mass range, emitter temperature and field strength can be visualized directly during the Ed. process. Should low resolution mass analysis not be sufficient and a high resolution f.d. mass spectrum with a double focusing instrument be necessary, a preinvestigation of the substance with the q.m.s. is of advantage in order to exploit the desorption behaviour of one or several components. ACKNOWLEDGEMENTS We would like to thank Professor Dr S. Goenechea from the Institut fur Gerichtliche Medizin der UniveritPt Bonn for the supply of the urine extract and for helpful discussions. We are indebted to ‘Extranuclear Laboratories’ for the loan of the quadrupole mass filter which was employed in these investigations.
REFERENCES I . Winkler, H. U . ; Beckey, H. D. O r g . Muss Spectrom. 1972,6,655. 2. Winkler, H. U.; Beckey, H. D. Biochem. Biophvs. Res. Commun. 1972, 46, 391. 3. Schulten, H.-R.; Beckey. H. D. J . Agric. Food Chem. 1973,21,372. 4. Schulten, H.-R. Biomed. Mass Spectrom. 1974, 1, 223. 5 . Schulten, H.-R.; Beckey, H. D. Org. Mass Spectrom. 1973, 7 , 861. 6. Heinen, H. J . ; Hotzel, Ch.; Beckey, H. D. Inr. J . Muss Spectrom. Ion Phvs. 1974, 13, 55.
