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Evidence concerning the identity of secondary particles in post-acceleration detectors
International JournaI of Mass Spectrometry and Zon Processes, 69 (1986) 233-237 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
233
Short Communication EVIDENCE PARTICLES
CONCERNING THE IDENTITY OF SECONDARY IN POST-ACCELERATION DETECTORS
G.H. WANG *, W. ABERTH and A.M. FALICK ** Mass Spectrometty Resource, Department fornia, San Francisco, CA 94143 (U.S.A.)
of Pharmaceutical
Chemistry,
University
of Cali-
(Received 28 October 1985)
Post-acceleration detectors [l-3] and conversion dynodes [4] are widely used in organic mass spectrometry to improve the sensitivity of detection of high mass ions and to allow conventional electron multipliers to be used with low kinetic energy negative ions. When positive sample ions impinge on a post-acceleration electrode held at a high negative potential, secondary electrons are produced [l-4]. These electrons then enter an electron multiplier, producing a current pulse for each primary ion detected. In the recent literature dealing with post-acceleration detection, little attention has been paid to the possibility that negative ions may also be formed in large enough numbers to contribute significantly to the multiplier output. This phenomenon was described in earlier studies of electron and ion emission from metal surfaces bombarded with energetic particle beams [5], but the bombarding ions were usually monatomic. When negative sample ions strike a (positive) post-acceleration electrode, the nature of the secondary particles emitted has not been well studied. The possibilities include photons as well as positive ions originating from either the incident ion (via loss of two or more electrons or fragmentation) or the electrode surface [4]. Possible future improvements in the efficiency of post-acceleration detectors will depend upon a solid knowledge of the fundamental processes involved. In this note we describe some experiments on the effects of electric and magnetic fields on the secondary particles generated from both positive and negative sample ions. The experimental set-up is shown schematically in Fig. 1. Cesium ions are produced in the Csf gun [6], which is held at a potential 8 kV above that of the sample stage (A). The sample ions formed as a result of the Cs+ beam
* Present address: Institute of Chemistry, Academia Sinica, Beijing, China. ** To whom correspondence should be addressed. 0168-1176/86/$03.50
0 1986 Elsevier Science Publishers B.V.
234 SAMPLE
STAGE
*7.7kV-
,
/
,
G
FLECTRIC
OR MAGNETIC
FIELD
Fig. 1. Diagram of the experimental set-up. A, Sample stage; B, focus plates; C, grounded slit; D, post-acceleration electrode; E. grounded slit; F. CDEM entrance slit; G, deflection electrode.
impact are focused and accelerated by electrodes B and C, respectively. Either positive or negative sample ions can be selected, depending on the polarity of the voltages at A and B. The voltage applied to the (aluminum) post-acceleration electrode, D, is plus or minus 8 kV for negative or positive sample ions, respectively. Opposite the post-acceleration electrode are two grounded slits, E and F, separated by a distance of 30 mm. The distance between D and E is 25 mm. The electric field between D and E accelerates charged particles of appropriate polarity toward and through slits E and F, after which they strike a continuous dynode electron multiplier (CDEM). Neutral particles or photons may also pass through E and F and be detected by the CDEM. Between E and F, a deflector (G) is placed opposite a grounded plate (9 mm separation) and at right angles to E and F. For some experiments, a small electromagnet is mounted below the beam path between E and F, so that the principal component of the magnetic field is perpendicular to the plane of the figure. The CDEM is surrounded by a grounded metal box to minimize the effects of stray electric fields. Two sample materials were used in these experiments: Ultramark 1621, a fluorinated phosphazine (PCR, Gainsville, FL) and CsI. The CsI was deposited by wetting the sample stage with an aqueous solution of CsI and allowing it to dry. Both materials produce abundant positive and negative sample ions when bombarded with Cs’ ions. In Fig. 2 is shown the effect of varying the voltage applied to the deflector (G) on the CDEM output for both positive and negative sample ions generated from Ultramark. The electric field at 300 V is 333 V cm-‘. The results show that in both cases, more than 90% of the secondary particles
235 CDEM OUTPUT CURRENT (nA)
-300
-200
-100
0
loo
200
DEFLECTOR VOLTAGE
Fig. 2. CDEM
output
current
300
400
(V)
as a function
of voltage applied
to deflector
electrode.
that contribute to the CDEM output current can be deflected by the electric field. When CsI is used as a sample, the results are similar. Thus, ions or electrons are responsible for > 90% of the observed CDEM signal (in the absence of the deflecting field) for both positive and negative sample ions; and photons and/or other neutral species are not important. The effect of applying a weak magnetic field in the region between slits E and F is illustrated in Figs. 3 and 4. Figure 3 shows the CDEM output current as a function of the current applied to the electromagnet coil for both positive and negative sample ions from Ultramark. Figure 4 gives the analogous data when CsI is used as the sample. At a coil current of 10 mA, the maximum magnetic field in the beam path was approximately 25 gauss. This field is strong enough to deflect an 8 keV electron beam, but too weak to significantly affect 8 keV ions. In Figs. 3 and 4, the magnetic field has no measurable effect on the CDEM output current when negative sample ions are used. This result is as expected and is consistent with the conclusion that the secondary particles in this case are positive ions. When positive sample ions are produced from Ultramark, Fig. 3 shows that the CDEM output can be reduced by about 50% by activating the magnetic field. In the case of’ positive sample ions from CsI (Fig. 4) a maximum reduction of about 85% can be achieved. The fact that the magnetic field has no effect on the positive secondary ions (from negative sample ions) indicates that the field: (a) is not strong enough to deflect ions
236
z Fl U
NEGATIVE SAMPLE IONS >
I
I
I
5
10
15
MAGNET
COIL CURRENT
cl I
20 (mA)
Fig. 3. CDEM output as a function of coil current negative sample ions generated from Ultramark.
in deflecting
magnet
for positive
and
away from the multiplier entrance slit; and (b) does not affect the sensitivity of the CDEM. The experimental results described above allow one to draw several conclusions about those secondary particles produced in post-acceleration detectors which are responsible for the observed multiplier output signal. When positive sample ions are being detected, the secondary particles consist of electrons as well as up to 50% of negative ions. The negative ions may be sample ions or fragments or may originate from the post-acceleration electrode surface, either from the electrode material itself or surface contamination layers [5]. The fact that Ultramark and CsI give different proportions of negative ions suggests that the sample ions may be an
5 ii 5 U
z5
0
NEGATIVE
SAMPLE IONS
I 2 U
I
5 MAGNET
POSITIVE SAMPLE IONS
10
15
COIL CURRENT
20 (mA)
Fig. 4. CDEM output as a function of coil current negative sample ions generated from CsI.
in deflecting
magnet
for positive
and
237
important source of the negative secondary ions reaching the CDEM. When negative sample ions strike the post-acceleration electrode surface, positive ions are ejected and detected by the multiplier. These positive ions may come from sample ions or the electrode material [4]. Surface contamination on the electrode may also be important. Some X-ray photons must also be produced in both positive and negative cases, but they are evidently of only minor importance in generating the electron multiplier output. Further studies of the nature of the secondary ions are in progress in this laboratory. ACKNOWLEDGEMENT
This work was supported Grant RR 01614.
by the NIH
Division
of Research
Resources,
REFERENCES N.R. Daly, Rev. Sci. Instrum., 31 (1960) 264. R.J. Beuhler and L. Friedman, Int. J. Mass Spectrom. Ion Phys., 23 (1977) 81. K. Rinn, A. Muller, H. Eichenauer and E. Salzborn, Rev. Sci. Instrum., 53 (1982) 829. G.C. Stafford, Environ. Health Perspectives, 36 (1980) 85. G.C. Stafford, J.R. Reeher and M.S. Story, paper presented at 26th Annu. Conf. Mass Spectrom. Allied Top., St. Louis, MO, June, 1978. M. Kaminsky, Atomic and Ionic Impact Phenomena on Metal Surfaces, Springer-Verlag, Berlin, 1965, Chaps. 10 and 14. W. Aberth and A.L. Burlingame, in A. Benninghoven (Ed.), Ion Formation in Organic Solids, Springer-Verlag, Berlin, 1983, p. 167.
233
Short Communication EVIDENCE PARTICLES
CONCERNING THE IDENTITY OF SECONDARY IN POST-ACCELERATION DETECTORS
G.H. WANG *, W. ABERTH and A.M. FALICK ** Mass Spectrometty Resource, Department fornia, San Francisco, CA 94143 (U.S.A.)
of Pharmaceutical
Chemistry,
University
of Cali-
(Received 28 October 1985)
Post-acceleration detectors [l-3] and conversion dynodes [4] are widely used in organic mass spectrometry to improve the sensitivity of detection of high mass ions and to allow conventional electron multipliers to be used with low kinetic energy negative ions. When positive sample ions impinge on a post-acceleration electrode held at a high negative potential, secondary electrons are produced [l-4]. These electrons then enter an electron multiplier, producing a current pulse for each primary ion detected. In the recent literature dealing with post-acceleration detection, little attention has been paid to the possibility that negative ions may also be formed in large enough numbers to contribute significantly to the multiplier output. This phenomenon was described in earlier studies of electron and ion emission from metal surfaces bombarded with energetic particle beams [5], but the bombarding ions were usually monatomic. When negative sample ions strike a (positive) post-acceleration electrode, the nature of the secondary particles emitted has not been well studied. The possibilities include photons as well as positive ions originating from either the incident ion (via loss of two or more electrons or fragmentation) or the electrode surface [4]. Possible future improvements in the efficiency of post-acceleration detectors will depend upon a solid knowledge of the fundamental processes involved. In this note we describe some experiments on the effects of electric and magnetic fields on the secondary particles generated from both positive and negative sample ions. The experimental set-up is shown schematically in Fig. 1. Cesium ions are produced in the Csf gun [6], which is held at a potential 8 kV above that of the sample stage (A). The sample ions formed as a result of the Cs+ beam
* Present address: Institute of Chemistry, Academia Sinica, Beijing, China. ** To whom correspondence should be addressed. 0168-1176/86/$03.50
0 1986 Elsevier Science Publishers B.V.
234 SAMPLE
STAGE
*7.7kV-
,
/
,
G
FLECTRIC
OR MAGNETIC
FIELD
Fig. 1. Diagram of the experimental set-up. A, Sample stage; B, focus plates; C, grounded slit; D, post-acceleration electrode; E. grounded slit; F. CDEM entrance slit; G, deflection electrode.
impact are focused and accelerated by electrodes B and C, respectively. Either positive or negative sample ions can be selected, depending on the polarity of the voltages at A and B. The voltage applied to the (aluminum) post-acceleration electrode, D, is plus or minus 8 kV for negative or positive sample ions, respectively. Opposite the post-acceleration electrode are two grounded slits, E and F, separated by a distance of 30 mm. The distance between D and E is 25 mm. The electric field between D and E accelerates charged particles of appropriate polarity toward and through slits E and F, after which they strike a continuous dynode electron multiplier (CDEM). Neutral particles or photons may also pass through E and F and be detected by the CDEM. Between E and F, a deflector (G) is placed opposite a grounded plate (9 mm separation) and at right angles to E and F. For some experiments, a small electromagnet is mounted below the beam path between E and F, so that the principal component of the magnetic field is perpendicular to the plane of the figure. The CDEM is surrounded by a grounded metal box to minimize the effects of stray electric fields. Two sample materials were used in these experiments: Ultramark 1621, a fluorinated phosphazine (PCR, Gainsville, FL) and CsI. The CsI was deposited by wetting the sample stage with an aqueous solution of CsI and allowing it to dry. Both materials produce abundant positive and negative sample ions when bombarded with Cs’ ions. In Fig. 2 is shown the effect of varying the voltage applied to the deflector (G) on the CDEM output for both positive and negative sample ions generated from Ultramark. The electric field at 300 V is 333 V cm-‘. The results show that in both cases, more than 90% of the secondary particles
235 CDEM OUTPUT CURRENT (nA)
-300
-200
-100
0
loo
200
DEFLECTOR VOLTAGE
Fig. 2. CDEM
output
current
300
400
(V)
as a function
of voltage applied
to deflector
electrode.
that contribute to the CDEM output current can be deflected by the electric field. When CsI is used as a sample, the results are similar. Thus, ions or electrons are responsible for > 90% of the observed CDEM signal (in the absence of the deflecting field) for both positive and negative sample ions; and photons and/or other neutral species are not important. The effect of applying a weak magnetic field in the region between slits E and F is illustrated in Figs. 3 and 4. Figure 3 shows the CDEM output current as a function of the current applied to the electromagnet coil for both positive and negative sample ions from Ultramark. Figure 4 gives the analogous data when CsI is used as the sample. At a coil current of 10 mA, the maximum magnetic field in the beam path was approximately 25 gauss. This field is strong enough to deflect an 8 keV electron beam, but too weak to significantly affect 8 keV ions. In Figs. 3 and 4, the magnetic field has no measurable effect on the CDEM output current when negative sample ions are used. This result is as expected and is consistent with the conclusion that the secondary particles in this case are positive ions. When positive sample ions are produced from Ultramark, Fig. 3 shows that the CDEM output can be reduced by about 50% by activating the magnetic field. In the case of’ positive sample ions from CsI (Fig. 4) a maximum reduction of about 85% can be achieved. The fact that the magnetic field has no effect on the positive secondary ions (from negative sample ions) indicates that the field: (a) is not strong enough to deflect ions
236
z Fl U
NEGATIVE SAMPLE IONS >
I
I
I
5
10
15
MAGNET
COIL CURRENT
cl I
20 (mA)
Fig. 3. CDEM output as a function of coil current negative sample ions generated from Ultramark.
in deflecting
magnet
for positive
and
away from the multiplier entrance slit; and (b) does not affect the sensitivity of the CDEM. The experimental results described above allow one to draw several conclusions about those secondary particles produced in post-acceleration detectors which are responsible for the observed multiplier output signal. When positive sample ions are being detected, the secondary particles consist of electrons as well as up to 50% of negative ions. The negative ions may be sample ions or fragments or may originate from the post-acceleration electrode surface, either from the electrode material itself or surface contamination layers [5]. The fact that Ultramark and CsI give different proportions of negative ions suggests that the sample ions may be an
5 ii 5 U
z5
0
NEGATIVE
SAMPLE IONS
I 2 U
I
5 MAGNET
POSITIVE SAMPLE IONS
10
15
COIL CURRENT
20 (mA)
Fig. 4. CDEM output as a function of coil current negative sample ions generated from CsI.
in deflecting
magnet
for positive
and
237
important source of the negative secondary ions reaching the CDEM. When negative sample ions strike the post-acceleration electrode surface, positive ions are ejected and detected by the multiplier. These positive ions may come from sample ions or the electrode material [4]. Surface contamination on the electrode may also be important. Some X-ray photons must also be produced in both positive and negative cases, but they are evidently of only minor importance in generating the electron multiplier output. Further studies of the nature of the secondary ions are in progress in this laboratory. ACKNOWLEDGEMENT
This work was supported Grant RR 01614.
by the NIH
Division
of Research
Resources,
REFERENCES N.R. Daly, Rev. Sci. Instrum., 31 (1960) 264. R.J. Beuhler and L. Friedman, Int. J. Mass Spectrom. Ion Phys., 23 (1977) 81. K. Rinn, A. Muller, H. Eichenauer and E. Salzborn, Rev. Sci. Instrum., 53 (1982) 829. G.C. Stafford, Environ. Health Perspectives, 36 (1980) 85. G.C. Stafford, J.R. Reeher and M.S. Story, paper presented at 26th Annu. Conf. Mass Spectrom. Allied Top., St. Louis, MO, June, 1978. M. Kaminsky, Atomic and Ionic Impact Phenomena on Metal Surfaces, Springer-Verlag, Berlin, 1965, Chaps. 10 and 14. W. Aberth and A.L. Burlingame, in A. Benninghoven (Ed.), Ion Formation in Organic Solids, Springer-Verlag, Berlin, 1983, p. 167.