- Email: [email protected]
Static SIMS: metastable decay and peak intensities
Applied Surface Science 144–145 Ž1999. 26–30
Static SIMS: metastable decay and peak intensities I.S. Gilmore ) , M.P. Seah Centre for Materials Measurement and Technology, National Physical Laboratory, Teddington, Middlesex TW11 0LW, UK
Abstract The decay of a metastable ion to a daughter ion along the flight path of a time-of-flight ŽToF. mass spectrometer leads to well-defined peak structure in the mass spectrum. Through interference, these daughter ion peaks can reduce the detection limits in static SIMS and lead to uncertainties in both the true peak area intensity and the peak position. The area of the peak for the metastable parent ion is reduced to an extent which depends on its half life, the analyser design and the chosen instrument settings. These intensity changes directly affect the quality and reproducibility of spectra. The decay of metastable ions is analysed for the reference material, PTFE, used in a recent inter-laboratory study. A method is developed, for a ToF reflectron analyser, to characterise the decay process of the parent ion so that both parent and daughter are accurately identified. This method involves measuring the transit time shift of the metastable peak as a function of the reflectron voltage. To illustrate the method, the mass of the C 4 F4q parent ion is determined to an uncertainty of 0.6 amu. This parent ion decays by emission of CF20 to C 3 F2q. The choice of instrument parameters to avoid interference from metastable peaks and to improve data transferability is discussed. Crown Copyright q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Static SIMS; Ion fragmentation; Metastable decay
1. Introduction A molecular ion ejected from the surface, following a collision cascade, will possess both internal and kinetic energies from those collisions. If the internal energy of the molecule is sufficiently high, the inter-atomic bonds may subsequently disrupt and the ion may then fragment to form two daughter particles, one charged and one neutral. The half lives of such decays are in the range of a few microseconds to several hundred microseconds and are comparable to the typical flight times in time-of-flight ŽToF. mass analysers. Thus, these decays may be studied in )
Corresponding author. Tel.: q44-181-943-6922; Fax: q44181-943-6453; E-mail: [email protected]
ToF systems and conversely, these decays are relevant to the study of the ToF systems themselves. Analysers constructed especially to perform these studies are of the single stage mirror design with a detector added behind the ion mirror to count the neutrals as well as the usual detector to count the reflected stable and metastable daughter ions w1–3x. In static SIMS, the peaks arising from metastable decays are of concern because they may lead to both confusion in peak identification and inaccurate peak position determination w4x as well as completely concealing characteristic peaks of the analysed material. SIMS instruments are not usually equipped with the added detector behind the ion mirror and so a different method is developed here to identify metastable decays. The relative positions of the metastable peaks
0169-4332r99r$ - see front matter Crown Copyright q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 7 5 7 - 0
I.S. Gilmore, M.P. Seah r Applied Surface Science 144–145 (1999) 26–30
to the peaks for the stable ions may be changed by adjusting the reflectron voltage or any of the accelerating or decelerating potentials in the analyser, as discussed below.
2. Metastable decay In a simple single stage reflectron an ion is generated at the sample, is rapidly accelerated into the first drift region of length, Ld , into an ion mirror of length, Lr , and then out into a second drift region of length, Ld . The flight time, Tp , for a parent ion of mass, Mp , in this analyser with a drift voltage, Vd , and a reflector voltage, Vr , is given by, Tp s
2 Mp
(V
e
Ž Vr y Vd .
( ž
d
2 Lr q
1
(V
Ld
d
/
Ž 1.
where the voltages are referenced to the sample and e is the electronic charge. If the ion decays into a charged daughter particle of mass m d and a neutral particle mass m o in the first drift space, the flight time of the charged daughter ion, Td , is given by Td s
(V
2 Mp
( ž e
d
2 md
Ž Vr y Vd . Mp
Lr q
1
(V
d
Ld
/
27
the SIMION w5x software. The analyser is of the single stage reflectron type with a 1.5 mm extraction gap from the sample into the first drift region and post-acceleration from the second drift region into the detector. The trajectories of both stable and decaying C 4 F4q ions are shown in Fig. 1. In this example the C 4 F4q decays in the ion mirror to a charged daughter, C 3 F2q, and a neutral daughter, CF20 with no kinetic energy involved in the disintegration. The charged daughter has only 60% of the mass and therefore the energy of the parent ion and is thus reflected earlier in the ion mirror and arrives at the detector before the parent. The flight time of the charged daughter, for decays at different positions in the spectrometer from the sample to the detector, is shown in Fig. 2. For a decay involving no fragmentation energy anywhere along the first drift region, the daughter ion arrival time is constant and results in an intense peak at a time shortly after the arrival time of the equivalent stable daughter. There is also a weaker peak from decays in the shorter second drift path which arrive shortly before the parent ion. The second peak may not be apparent if the metastable half life is low and insufficient decays occur in the
Ž 2.
and, if the time difference between parent and daughter ions, Tp y Td , is written D t 1
1 s
Dt
2 m o Lr
(
e Mp 2Vd
Ž Vr y Vd .
Ž 3.
As Vr is increased, D t decreases and the metastable daughter peak moves towards the parent peak. The reciprocal time difference increases linearly with Vr with an intercept-to-gradient ratio, R, equal to yVd . From this relationship M is calculated as follows by measuring the position of the charged daughter ion for different values of, Vr . Values of R are computed for different trial values of the parent mass, Mp which also allows us to define Tp and hence D t. The correct value of Mp is obtained for R equal to yVd . The mass of the neutral particle, m o , may now be deduced. This method is illustrated later in the inset to Fig. 4. To study the metastable decay in a more complex analyser an ion optical model was developed using
Fig. 1. The trajectories calculated by SIMION for a C 4 F4q ion which decays in the ion mirror to a charged daughter, C 3 F2q, and a neutral daughter, CF20 . The position of the decay is indicated by the ‘explosion’.
28
I.S. Gilmore, M.P. Seah r Applied Surface Science 144–145 (1999) 26–30
4. Results
Fig. 2. The ToF for the charged daughter C 3 F2q for decays at different times of the metastable parent C 4 F4q along the flight path calculated from the SIMION model.
second flight path. In practice we have not observed an intense second peak. 3. Experimental The instrument used in this study, a CAMECA TOF SIMS IV, is of the design described in Section 2 but with the addition of a lens and deflectors w6x. Spectra were recorded using a pulsed Gaq liquid metal ion gun at 25 keV impact energy with a repetition rate of 10 kHz, a pulse length of 650 ps and pulsed current of 0.5 pA. The ion beam was defocussed to 5 mm and rastered over an area of 150 mm = 150 mm giving a total ion dose per spectrum of 1 = 10 15 ionsrm2 . A fresh area of reference sample was analysed for each measurement. Secondary ions were extracted with a potential of 2000 V and the post-acceleration into the detector was 13.5 kV. This study is based on the PTFE bulk polymer reference material used in the recent interlaboratory study w7,8x. Charge stabilisation was achieved using a low energy electron beam pulsed between extraction pulses. The mass resolution was more than 7000, as measured on the 13 CFq peak. Spectra were recorded from fresh areas of the sample for Vr s 20, 100, 200, 300 and 400 V. As Vr increases there is a weak effect on the mass resolution with the 13 CFq broadening to a resolution of 5000 at Vr s 400 V.
A region of the PTFE mass spectrum around the C 3 F2q ion is shown in Fig. 3 for five values of Vr . Each spectrum has been separately mass calibrated. As predicted, a peak due to a daughter ion from a metastable decay occurring in the first drift region appears close on the mass scale to a peak for a stable ion emitted from the surface. The metastable peak is too close to the stable C 3 F2q peak for a C 3 F2q daughter ion and, as we shall see, has a mass of 69 amu. Under the large metastable peak at 300 V is the beginning of a second group of daughter peaks which is for a 74 amu daughter ion. As Vr is increased the metastable daughter ion peaks move up the mass scale towards the mass of the parent ion, as described in Eq. Ž3.. The daughter peak has a width of 38.4 ns compared to the stable C 3 F2q ion which has a peak width of 2.06 ns. At a reflectron setting of 100 V the first metastable peak has moved to obscure the 13 CC 2 F2q peak and, at a setting of 200 V, completely obscures the 13 C 2 CF2q peak. This demonstrates the importance of understanding the effects of the instrument operating parameters for quantitative and qualitative analyses. The time centroid of each of the daughter ion peaks is measured from the spectra recorded for
Fig. 3. A region of the PTFE positive ion mass spectrum showing metastable daughter ion peaks for five values of Vr . For clarity all counts below 10 have been replaced by 10 counts and each spectrum displaced vertically. Isotopic peaks of carbon are shown bracketed.
I.S. Gilmore, M.P. Seah r Applied Surface Science 144–145 (1999) 26–30
Fig. 3 and the flight time of a range of possible parent ions in unit mass intervals is calculated from the mass calibration constants. The value of 1rDt is now calculated and plotted against the Vr for each case, as shown in Fig. 4. A least squares fit to the five data points gives the value of R which may be plotted against M, as shown in the insert to Fig. 4. The metastable parent ion mass is given where yR is equal to the drift voltage, in this case y2000 V. The plots shown in Fig. 4 are for three postulated parent ions, C 4 F3q, C 4 F4q and C 5 F4q for the second set of metastable peaks of Fig. 2. The reciprocal time difference has a linear dependence on Vr as expected. The calculated R-values of these parent ions with masses of 105, 124 and 136 amu are, respectively, 1438, 1979 and 2210 V. Since this should be 2000 V, we identify the metastable parent ion as C 4 F4q decaying with the loss of a neutral daughter CF20 . Analysis of the first set of metastable peaks of Fig. 2 now reveals that they are from a C 3 F5q metastable parent decaying to a charged daughter CF3q and a neutral daughter C 2 F20 . In this way it is possible to construct a decay map for all the metastable peaks in PTFE. Ions of the form C n F2qny1 decay by loss of a CF20 and C 2 F40 neutrals whereas ions of the form C n FŽ2qny3.,Ž2 ny4.,Ž2 ny5. decay with the loss of the CF20
29
neutral. Decays with the loss of F 0 and C 2 F20 are also observed. The ions with the general formula C n FŽ2qny2.,Ž2 n.,Ž2 nq1. are more stable and show fewer decays. The average R-value for all these measured ions is 1977 V with a standard deviation of 15 V. The parent mass may thus be determined here with a one standard deviation uncertainty of 0.6 amu. These neutral losses have similarities to those found using MS–MS ŽTandem SIMS. from collisionally activated dissociation ŽCAD. of PTFE w9x. However, the fragmentation pathways constructed in CAD can differ in many details w10x. The decay of a metastable ion and the fragmentation of a stable ion in a CAD cell are energetically very different. Additionally, results from the CAD system depend sensitively on target gas mass, pressure and the collision cell energy.
5. Conclusion A method to identify metastable parent ions in a SIMS spectrum from a single stage ToF reflectron mass analyser has been described. The mass of the parent ions can be determined with an uncertainty of around 0.6 amu so that the neutral fragments are clearly deduced. The effect of instrument parameters is described and it is recommended that, for a flight path of 2 m, the drift energy is kept below 3 keV to ensure that nearly all metastable ions have fully decayed. The decay pathways for PTFE have been characterised and it is found that most decays are by CF20 . The metastable ions in PTFE have all decayed by the end of the first drift path and so any variation in spectral intensities between different spectrometers is low. PTFE thus has excellent properties as a reference sample for characterising instruments.
Acknowledgements
Fig. 4. The reciprocal time difference for three trial values for the parent ions: C 4 F3q, C 4 F4q and C 5 F4q. Inset is a curve of R-values for all possible parent ions between masses 12 and 150 amu. The C 4 F4q ion is identified as the metastable parent ion with an uncertainty of 0.6 amu.
The authors would like to thank Prof. D. Briggs for drawing attention to the MSrMS work. This work forms part of the Valid Analytical Measurement programme of the National Measurement System Policy Unit of the UK Department of Trade and Industry.
30
I.S. Gilmore, M.P. Seah r Applied Surface Science 144–145 (1999) 26–30
References w1x X. Tang, W. Ens, K. Standing, J. Westmere, Anal. Chem. 60 Ž1988. 1791. w2x D.F. Barofsky, G. Brinkmalm, P. Hakansson, B.U.R. ˚ Sundquist, Int. J. Mass Spectrom. and Ion Processes 131 Ž1994. 283. w3x G. Brinkmalm, P. Hakansson, J. Kjellberg, P. Demirev, ˚ B.U.R. Sundquist, Int. J. Mass Spectrom. and Ion Processes 114 Ž1992. 183. w4x S. Reichlmaier, J.S. Hammond, M.J. Hearn, D. Briggs, Surf. Interface Anal. 21 Ž1994. 739. w5x D. Dahl, SIMION v6.0, Idaho National Engineering laboratory, 1997, www.SRC.NETr ; KLACKrSIMION.HTML.
w6x M. Terhast, H.-G. Cramer, E. Niehuis, in: A. Benninghoven, B. Hagenhoff, H.W. Werner ŽEds.., Proc. SIMS X, Wiley, Chichester, 1997, p. 427. w7x I.S. Gilmore, M.P. Seah, in: G. Gillen, R. Lareau, J. Bennett, F. Stevie ŽEds.., Proc. SIMS XI, Wiley, New York, 1998, p. 999. w8x I.S. Gilmore, M.P. Seah, in: I. Olefjord, L. Nyborg, D. Briggs ŽEds.., Proc. ECASIA’97, Wiley, Chichester, 1997, p. 1081. w9x G.J. legget, D. Briggs, J.C. Vickerman, J. Chem. Soc. Faraday Trans. 86 Ž1990. 1863. w10x I.S. Gilmore, M.P. Seah, to be published.
Static SIMS: metastable decay and peak intensities I.S. Gilmore ) , M.P. Seah Centre for Materials Measurement and Technology, National Physical Laboratory, Teddington, Middlesex TW11 0LW, UK
Abstract The decay of a metastable ion to a daughter ion along the flight path of a time-of-flight ŽToF. mass spectrometer leads to well-defined peak structure in the mass spectrum. Through interference, these daughter ion peaks can reduce the detection limits in static SIMS and lead to uncertainties in both the true peak area intensity and the peak position. The area of the peak for the metastable parent ion is reduced to an extent which depends on its half life, the analyser design and the chosen instrument settings. These intensity changes directly affect the quality and reproducibility of spectra. The decay of metastable ions is analysed for the reference material, PTFE, used in a recent inter-laboratory study. A method is developed, for a ToF reflectron analyser, to characterise the decay process of the parent ion so that both parent and daughter are accurately identified. This method involves measuring the transit time shift of the metastable peak as a function of the reflectron voltage. To illustrate the method, the mass of the C 4 F4q parent ion is determined to an uncertainty of 0.6 amu. This parent ion decays by emission of CF20 to C 3 F2q. The choice of instrument parameters to avoid interference from metastable peaks and to improve data transferability is discussed. Crown Copyright q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: Static SIMS; Ion fragmentation; Metastable decay
1. Introduction A molecular ion ejected from the surface, following a collision cascade, will possess both internal and kinetic energies from those collisions. If the internal energy of the molecule is sufficiently high, the inter-atomic bonds may subsequently disrupt and the ion may then fragment to form two daughter particles, one charged and one neutral. The half lives of such decays are in the range of a few microseconds to several hundred microseconds and are comparable to the typical flight times in time-of-flight ŽToF. mass analysers. Thus, these decays may be studied in )
Corresponding author. Tel.: q44-181-943-6922; Fax: q44181-943-6453; E-mail: [email protected]
ToF systems and conversely, these decays are relevant to the study of the ToF systems themselves. Analysers constructed especially to perform these studies are of the single stage mirror design with a detector added behind the ion mirror to count the neutrals as well as the usual detector to count the reflected stable and metastable daughter ions w1–3x. In static SIMS, the peaks arising from metastable decays are of concern because they may lead to both confusion in peak identification and inaccurate peak position determination w4x as well as completely concealing characteristic peaks of the analysed material. SIMS instruments are not usually equipped with the added detector behind the ion mirror and so a different method is developed here to identify metastable decays. The relative positions of the metastable peaks
0169-4332r99r$ - see front matter Crown Copyright q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 7 5 7 - 0
I.S. Gilmore, M.P. Seah r Applied Surface Science 144–145 (1999) 26–30
to the peaks for the stable ions may be changed by adjusting the reflectron voltage or any of the accelerating or decelerating potentials in the analyser, as discussed below.
2. Metastable decay In a simple single stage reflectron an ion is generated at the sample, is rapidly accelerated into the first drift region of length, Ld , into an ion mirror of length, Lr , and then out into a second drift region of length, Ld . The flight time, Tp , for a parent ion of mass, Mp , in this analyser with a drift voltage, Vd , and a reflector voltage, Vr , is given by, Tp s
2 Mp
(V
e
Ž Vr y Vd .
( ž
d
2 Lr q
1
(V
Ld
d
/
Ž 1.
where the voltages are referenced to the sample and e is the electronic charge. If the ion decays into a charged daughter particle of mass m d and a neutral particle mass m o in the first drift space, the flight time of the charged daughter ion, Td , is given by Td s
(V
2 Mp
( ž e
d
2 md
Ž Vr y Vd . Mp
Lr q
1
(V
d
Ld
/
27
the SIMION w5x software. The analyser is of the single stage reflectron type with a 1.5 mm extraction gap from the sample into the first drift region and post-acceleration from the second drift region into the detector. The trajectories of both stable and decaying C 4 F4q ions are shown in Fig. 1. In this example the C 4 F4q decays in the ion mirror to a charged daughter, C 3 F2q, and a neutral daughter, CF20 with no kinetic energy involved in the disintegration. The charged daughter has only 60% of the mass and therefore the energy of the parent ion and is thus reflected earlier in the ion mirror and arrives at the detector before the parent. The flight time of the charged daughter, for decays at different positions in the spectrometer from the sample to the detector, is shown in Fig. 2. For a decay involving no fragmentation energy anywhere along the first drift region, the daughter ion arrival time is constant and results in an intense peak at a time shortly after the arrival time of the equivalent stable daughter. There is also a weaker peak from decays in the shorter second drift path which arrive shortly before the parent ion. The second peak may not be apparent if the metastable half life is low and insufficient decays occur in the
Ž 2.
and, if the time difference between parent and daughter ions, Tp y Td , is written D t 1
1 s
Dt
2 m o Lr
(
e Mp 2Vd
Ž Vr y Vd .
Ž 3.
As Vr is increased, D t decreases and the metastable daughter peak moves towards the parent peak. The reciprocal time difference increases linearly with Vr with an intercept-to-gradient ratio, R, equal to yVd . From this relationship M is calculated as follows by measuring the position of the charged daughter ion for different values of, Vr . Values of R are computed for different trial values of the parent mass, Mp which also allows us to define Tp and hence D t. The correct value of Mp is obtained for R equal to yVd . The mass of the neutral particle, m o , may now be deduced. This method is illustrated later in the inset to Fig. 4. To study the metastable decay in a more complex analyser an ion optical model was developed using
Fig. 1. The trajectories calculated by SIMION for a C 4 F4q ion which decays in the ion mirror to a charged daughter, C 3 F2q, and a neutral daughter, CF20 . The position of the decay is indicated by the ‘explosion’.
28
I.S. Gilmore, M.P. Seah r Applied Surface Science 144–145 (1999) 26–30
4. Results
Fig. 2. The ToF for the charged daughter C 3 F2q for decays at different times of the metastable parent C 4 F4q along the flight path calculated from the SIMION model.
second flight path. In practice we have not observed an intense second peak. 3. Experimental The instrument used in this study, a CAMECA TOF SIMS IV, is of the design described in Section 2 but with the addition of a lens and deflectors w6x. Spectra were recorded using a pulsed Gaq liquid metal ion gun at 25 keV impact energy with a repetition rate of 10 kHz, a pulse length of 650 ps and pulsed current of 0.5 pA. The ion beam was defocussed to 5 mm and rastered over an area of 150 mm = 150 mm giving a total ion dose per spectrum of 1 = 10 15 ionsrm2 . A fresh area of reference sample was analysed for each measurement. Secondary ions were extracted with a potential of 2000 V and the post-acceleration into the detector was 13.5 kV. This study is based on the PTFE bulk polymer reference material used in the recent interlaboratory study w7,8x. Charge stabilisation was achieved using a low energy electron beam pulsed between extraction pulses. The mass resolution was more than 7000, as measured on the 13 CFq peak. Spectra were recorded from fresh areas of the sample for Vr s 20, 100, 200, 300 and 400 V. As Vr increases there is a weak effect on the mass resolution with the 13 CFq broadening to a resolution of 5000 at Vr s 400 V.
A region of the PTFE mass spectrum around the C 3 F2q ion is shown in Fig. 3 for five values of Vr . Each spectrum has been separately mass calibrated. As predicted, a peak due to a daughter ion from a metastable decay occurring in the first drift region appears close on the mass scale to a peak for a stable ion emitted from the surface. The metastable peak is too close to the stable C 3 F2q peak for a C 3 F2q daughter ion and, as we shall see, has a mass of 69 amu. Under the large metastable peak at 300 V is the beginning of a second group of daughter peaks which is for a 74 amu daughter ion. As Vr is increased the metastable daughter ion peaks move up the mass scale towards the mass of the parent ion, as described in Eq. Ž3.. The daughter peak has a width of 38.4 ns compared to the stable C 3 F2q ion which has a peak width of 2.06 ns. At a reflectron setting of 100 V the first metastable peak has moved to obscure the 13 CC 2 F2q peak and, at a setting of 200 V, completely obscures the 13 C 2 CF2q peak. This demonstrates the importance of understanding the effects of the instrument operating parameters for quantitative and qualitative analyses. The time centroid of each of the daughter ion peaks is measured from the spectra recorded for
Fig. 3. A region of the PTFE positive ion mass spectrum showing metastable daughter ion peaks for five values of Vr . For clarity all counts below 10 have been replaced by 10 counts and each spectrum displaced vertically. Isotopic peaks of carbon are shown bracketed.
I.S. Gilmore, M.P. Seah r Applied Surface Science 144–145 (1999) 26–30
Fig. 3 and the flight time of a range of possible parent ions in unit mass intervals is calculated from the mass calibration constants. The value of 1rDt is now calculated and plotted against the Vr for each case, as shown in Fig. 4. A least squares fit to the five data points gives the value of R which may be plotted against M, as shown in the insert to Fig. 4. The metastable parent ion mass is given where yR is equal to the drift voltage, in this case y2000 V. The plots shown in Fig. 4 are for three postulated parent ions, C 4 F3q, C 4 F4q and C 5 F4q for the second set of metastable peaks of Fig. 2. The reciprocal time difference has a linear dependence on Vr as expected. The calculated R-values of these parent ions with masses of 105, 124 and 136 amu are, respectively, 1438, 1979 and 2210 V. Since this should be 2000 V, we identify the metastable parent ion as C 4 F4q decaying with the loss of a neutral daughter CF20 . Analysis of the first set of metastable peaks of Fig. 2 now reveals that they are from a C 3 F5q metastable parent decaying to a charged daughter CF3q and a neutral daughter C 2 F20 . In this way it is possible to construct a decay map for all the metastable peaks in PTFE. Ions of the form C n F2qny1 decay by loss of a CF20 and C 2 F40 neutrals whereas ions of the form C n FŽ2qny3.,Ž2 ny4.,Ž2 ny5. decay with the loss of the CF20
29
neutral. Decays with the loss of F 0 and C 2 F20 are also observed. The ions with the general formula C n FŽ2qny2.,Ž2 n.,Ž2 nq1. are more stable and show fewer decays. The average R-value for all these measured ions is 1977 V with a standard deviation of 15 V. The parent mass may thus be determined here with a one standard deviation uncertainty of 0.6 amu. These neutral losses have similarities to those found using MS–MS ŽTandem SIMS. from collisionally activated dissociation ŽCAD. of PTFE w9x. However, the fragmentation pathways constructed in CAD can differ in many details w10x. The decay of a metastable ion and the fragmentation of a stable ion in a CAD cell are energetically very different. Additionally, results from the CAD system depend sensitively on target gas mass, pressure and the collision cell energy.
5. Conclusion A method to identify metastable parent ions in a SIMS spectrum from a single stage ToF reflectron mass analyser has been described. The mass of the parent ions can be determined with an uncertainty of around 0.6 amu so that the neutral fragments are clearly deduced. The effect of instrument parameters is described and it is recommended that, for a flight path of 2 m, the drift energy is kept below 3 keV to ensure that nearly all metastable ions have fully decayed. The decay pathways for PTFE have been characterised and it is found that most decays are by CF20 . The metastable ions in PTFE have all decayed by the end of the first drift path and so any variation in spectral intensities between different spectrometers is low. PTFE thus has excellent properties as a reference sample for characterising instruments.
Acknowledgements
Fig. 4. The reciprocal time difference for three trial values for the parent ions: C 4 F3q, C 4 F4q and C 5 F4q. Inset is a curve of R-values for all possible parent ions between masses 12 and 150 amu. The C 4 F4q ion is identified as the metastable parent ion with an uncertainty of 0.6 amu.
The authors would like to thank Prof. D. Briggs for drawing attention to the MSrMS work. This work forms part of the Valid Analytical Measurement programme of the National Measurement System Policy Unit of the UK Department of Trade and Industry.
30
I.S. Gilmore, M.P. Seah r Applied Surface Science 144–145 (1999) 26–30
References w1x X. Tang, W. Ens, K. Standing, J. Westmere, Anal. Chem. 60 Ž1988. 1791. w2x D.F. Barofsky, G. Brinkmalm, P. Hakansson, B.U.R. ˚ Sundquist, Int. J. Mass Spectrom. and Ion Processes 131 Ž1994. 283. w3x G. Brinkmalm, P. Hakansson, J. Kjellberg, P. Demirev, ˚ B.U.R. Sundquist, Int. J. Mass Spectrom. and Ion Processes 114 Ž1992. 183. w4x S. Reichlmaier, J.S. Hammond, M.J. Hearn, D. Briggs, Surf. Interface Anal. 21 Ž1994. 739. w5x D. Dahl, SIMION v6.0, Idaho National Engineering laboratory, 1997, www.SRC.NETr ; KLACKrSIMION.HTML.
w6x M. Terhast, H.-G. Cramer, E. Niehuis, in: A. Benninghoven, B. Hagenhoff, H.W. Werner ŽEds.., Proc. SIMS X, Wiley, Chichester, 1997, p. 427. w7x I.S. Gilmore, M.P. Seah, in: G. Gillen, R. Lareau, J. Bennett, F. Stevie ŽEds.., Proc. SIMS XI, Wiley, New York, 1998, p. 999. w8x I.S. Gilmore, M.P. Seah, in: I. Olefjord, L. Nyborg, D. Briggs ŽEds.., Proc. ECASIA’97, Wiley, Chichester, 1997, p. 1081. w9x G.J. legget, D. Briggs, J.C. Vickerman, J. Chem. Soc. Faraday Trans. 86 Ž1990. 1863. w10x I.S. Gilmore, M.P. Seah, to be published.