High Mass Resolution Ion Microprobe Mass Spectrometry of Complex Matrices D. K. Bakale,‘ 6. N. Colby,? and C. A.
Evans, Jr.
Materials Research Laboratory, University of Illinois, Urbana, Ill. 6 180 1
The secondary ion mass spectra of complex materials such as minerals, glasses, and ceramics contaln a large number of molecular ions. If a low resolution mass spectrometer ( M I A M < 300-1000) is used for the characterlzation of these materials, the analysls of many elements Is precluded by molecular Ion Interferences. This study Investigated and evaluated the utility of high mass resolution and exact mass measurement techniques to reduce the effects of these Interfering species. Photographic plate and electrlcal detection resolutions of 6000-8000 and 2000-2500, respectively, were used for this study. The utillty of these techniques is IIlustrated by the analysis of a mineral, a ceramic bone Implant, an oxide thin film, and an NBS standard glass.
Characterization of a material using secondary ion mass spectrometry (SIMS) is accomplished by bombarding a sample with 1-35 keV primary ions and by mass analysis of the resultant sputtered secondary ions. In addition to the analytically important singly charged monatomic ions, there are several types of ions produced by the bombardment process which result in spectral interferences and dramatically complicate the qualitative evaluation and quantitative analysis of a material. Careful consideration must be given to the possible occurrence of these interfering ions before a mass spectral line can be unambiguously assigned to an element. In complex samples such as those of geological or biological origin, these ions can prohibit successful trace elemental analysis. Five general ion types which are potential sources of interference in ion probe spectra are: multiply charged positive ions of the matrix, polyatomic ions resulting from the interaction of the sample with the primary or bombarding ion beam, hydride ions, polyatomic combinations of the various elements in the sample including self polymers of the matrix, and hydrocarbon ions ( 1 ) . In some instances, sample characterization by other methods or knowledge of sample history can permit one to rule out the occurrence of a particular interference with an acceptable degree of confidence. However, in more complex samples, it becomes increasingly important to determine experimentally that the peak area assigned to an elemental ion does not contain any other ionic contributions and that the peak area is due to the isotope in question. Two basic approaches can be used to improve the analytical purity of a mass spectral peak. The first approach to the problem is to eliminate or reduce the relative contribution of interfering ions to the secondary ion mass spectrum of the sample (1-3). There are several instrumental operating modes which accomplish this. 1) Since the “tail” of the initial kinetic energy distribution for atomic ions extends to a much higher energy than that for molecular ions (21, Present address, Varian Associates, Vacuum Division, 25 Route 22, Springfield, N.J. 07081. Present address, E. I. DuPont de Nemours and Co., Inc., Instrument Products Division, 1500 s. Shamrock Ave., Monrovia, Calif. 91016. 1532
ANALYTICAL CHEMISTRY, VOL. 47, NO. 9, AUGUST 1975
an instrumental “energy window” may be used to discriminate against the lower energy interference ions (2, 3 ) . 2) The gas used to create primary ions may be changed to alter the molecular interferences which result from interactions’with the primary ion beam ( 1 ) . 3) The negative secondary ion spectrum which has a different interference pattern than the positive secondary ion spectrum may be analyzed. Specific examples are discussed in Ref. ( 1 ) . Any of the above procedures can also result in a significant reduction in the intensity of the analytical signal to such an extent that an analysis cannot be performed. There are, however, innumerable spectral interferences which will defy the use of the techniques discussed above. Thus, the analyst turns to a classic spectroscopic method to reduce interferences, high spectral resolution. Hernandez, Lanusse, Slodzian, and Vidal ( 4 , 5 ) first demonstrated the combination of ion sputtering with a double focusing mass spectrograph to obtain high mass resolution. Their work dealt primarily with interferences resulting from surface contaminations such as water vapor, ammonia, and hydrocarbons. The work described here presents some other applications of high mass resolution techniques using an ion microprobe mass spectrometer, the use of exact mass assignments to make accurate ion identification, and the application of high mass resolution to make both qualitative and semiquantitative analyses in the presence of complex molecular ions.
EXPERIMENTAL An AEI Scientific Apparatus, Inc. (Elmsford, N.Y.) IM-20 ion microprobe accessory attached to an MS-7 spark source mass spectrometer (AEI Scientific Apparatus, Inc.) was used for this study. T h e IM-20 accessory as manufactured is described below and is illustrated in Figure l. Primary Ion Column. The primary ion source is a duoplasmatron ( 6 )with a circulating fluid coolant, a variable magnetic field, and a molybdenum hollow cathode as its unique features. The primary ions are accelerated from the duoplasmatron by a 0-25 kV potential. The primary ion optics are the same as those described by Drummond ( 7 ) and Drummond and Long (8) with a symmetrical einzel condenser lens and an asymmetrical einzel objective lens (9) producing a working distance of 4 cm. Both lens voltages are derived from the primary ion accelerating potential via resistive voltage dividers. The primary ion beam spot size is variable from 2 to 300 pm and a current density of 100 mA/cm2 from 4 t o 35 fim has been demonstrated (10). The primary beam spot is moved over a 256- X 256-pm area of the sample with a digital scan generator which provides for x,y secondary ion beams, and x or y manual manipulation of the primary beam. Argon, oxygen, or nitrogen is metered into the duoplasmatron via a needle valve to form primary ions. The gas pressure in the duoplasmatron is monitored by a thermocouple gauge and is normally 0.1-0.2 Torr. Unionized support gas escaping from the duoplasmatron into the region before the condenser lens is removed via a 4-in. oil diffusion pump with a liquid nitrogen (LN2) trap. With the duoplasmatron operating, the pressure in this region is about 2 x Torr. A 3-in. oil diffusion pump and LNp trap differentially pump the region between the two lenses to maintain a pressure of 1 X Torr in this area. This part of the primary column is bakeable to further reduce the pressure between the lenses. Calculations indicate the partial pressure in the specimen chamber due to the duoplasmatron support gas in the specimen chamber is 3x Torr.
S E C O N D A R Y ON E X T R L C - I O N AND
BEAM
CENTERING
Q e s , ~
-
-_
-
7-~
Figure 2. Masslcharge 43 as a function of mass resolution
L
ASMA-RON
Figure 1. Diagram of IM-20 ion microprobe attachment for MS-7
spark source mass spectrometer
Specimen Chamber. The ion microprobe specimen chamber replaces the standard MS-7 spark source chamber and provides for spark source analyses as well as the ion microprobe analyses. The ion microprobe sample is held a t 17' to the horizontal with ion bombardment 17" to the sample normal and the final secondary ion path a t 60' to the normal. Three dimensional manipulation of the sample is provided by x, y , and z movements of micrometer drives through a bellows-sealed feedthrough. One inch of sample movement is presently available. The sample chamber is evacuated by a rotary pump with a zeolite foreline trap. Fine pumping is provided by the normal MS-7 specimen chamber vacuum system, a 3-in. oil diffusion pump, and LNz trap. The specimen chamber is bakeable, can be sealed with either a Viton or gold O-ring and has a LNz cooled cryosorption pump (11). With the Viton O-ring sealing the specimen chamber, 3 X Torr is obtained in the specimen chamber. Baking the chamber reduces the pressure
