Anal. Chem. 1987, 59, 1819-1825
Moreover, the method is relatively rapid leas than 1min from sample introduction to cholesterol concentration value on screen. Certain modifications, in particular the automation of serum handling on the InfraAlyzer which is an apparatus for industrial use, could increase rapidity. The quality of the results obtained with serum cholesterol makes it possible to measure other lipid serum analytes such as total lipids (24), phospholipids, triglycerides, and other biochemical analytes on the same 100-pL sample and with the same method. Registry No. Cholesterol, 57-88-5.
LITERATURE CITED (1) Miettinen, M.; Turpeinen, 0.;Karvnen, L. J. Lancet 1972, 2 , 835-888. (2) Ducimetiere, P.; Claude, J. R.; Richard, J. L. Arferlal Wall 1977, 7 , 71-76. (3) Gordon, T.: Kannei. W. B. J . Am. Med. Assoc. 1972,227, 661-667. (4) Touks, D. B. Clin. Biochem. 1967. I , 12-17. (5) Fasce, C. F.: Vanderlinde, R. E. Clin. Chem. (Winston-Salem, N.C.) 1972, 78, 901-910. ( 6 ) Flegg, H. M. Ann. Clin. Biochem. 1973, 70, 79-84. (7) Albin, C. C.: Poon, L. S.; Chan, C. S. G.; Richmond, W.; Fu, P. C. Clin. Chem. (Winston-Salem, N.C.) 1974, 20, 470-475. (8) Haeckel, R.; Periick, M. J . Clin. Chem. Clln. Blochem. 1978, 14, 41 1-41 4.
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(9) Cooper, G. R.; Ulman, M. D.; Haziehunst, J. Clln. Chem. (Winston-Sa/em, N.C.)1979, 25, 1074-1079. (10) Bachoric, P. S.: Wood, D. S. In Hyperl/p&%smia: Diagnosis and Therapy; Rikind, B. M., Levy, R. I., Eds.; Grune and Stratton: New York, 1977; pp 49-51. (11) Massic, D. R.; Norris, K. H. Trans. Am. SOC.Agric. Eng. 1985, 8(1), 598-600. (12) Hart, J. R.; Norris, K. H.: Goiumbic, C. Cereal Chem. 1982, 3 9 , 94-98. (13) Ben-Gera. I.; Norrls, K. H. Isr. J . Agric. Res. 1968, 18(3), 117-124. (14) Hrushka. W. R.; Norrls, K. H. Appl. Spectrosc. 1982, 36, 261-285. (15) Honigs, D. E.; Freelin, J. M.; Hieftje, G. H.; Hirschelfd, T. 6. Appl. SWCWOSC. 1083, 37, 491-497. (16) Baker, D.; Norris, K. H. Appl. Spectrosc. 1985, 39(4), 618-822. (17) Watson. C. A. Anal. Chem. 1977, 49, 835A-840A. (18) Wetzel, D. C. Anal. Chem. 1983, 55, 1165-1176. (19) Mark, H.; Workmann, J. Anal. Chem. 1086, 58, 1454-1459. Reoression Analvsis: 2nd ed.; Wilev: (20) DraDer, N. R.: Mth, A. Amlied .. New York, 1981. (21) Tel, R. M.; Berends, G. T. J . Clin. Chem. Clin. Biochem. 1980. 78, 595-601. (22) Goddu, R. F.; Delker, D. A. Anal. Chem. 1980, 3 2 , 140. (23) Kisner, H. J.; Brown, C. W. Anal. Lett. 1985, 73, 377-397. (24) Jensen, R.; Lugan, I.; Peuchant, E. Bull. SOC.Pharm. Bordeaux 1986, 725, 223-232. '
RECEIVED for review October 27,1986. Accepted April 6,1987.
Organic and Elemental Ion Mapping Using Laser Mass Spectrometry Zbigniew A. Wilk and David M. Hercules*
Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260
A commerclal laser mass spectrometer (LAMMA 1000) has been automated and programmed to produce Ion maps wlth high lateral resolutlon. The sample Is scanned In an x-y mode and tlme-of-fllght mass spectrometrlc analysls Is performed for each matrlx element. A user-selected mass is searched for the maximum lntenslty whlch Is denslty coded and displayed as a four-slded polygon. A matrix conslstlng of 31 X 31 data polnts can be completed in 20 mln. Ion maps were obtalned for control samples consistlng of organlc dyes depodted on a nitrocellulosesubstrate with transmlssion electron microscopy grids as a mask. Results from maps of molecular cations have shown that the maximum lateral resolution obtalnabie was 2.5 Mm, which was llmlted by the diameter of the laser beam. Gold and alumlnum Ion maps were obtained from the analysls of a bonding pad In an lnta grated circuit demonstratlng that elemental Information can also be obtalned. Additionally, Ion maps were obtalned for an inclusion in a coal maceral. The maps showed that the Inclusion was composed primarily of Iron and sulfur. Organic material was detected In locallzed areas on the perlphery of the Inclusion.
Laser mass spectrometry (LMS) is a valuable analytical tool currently used in many areas of research. The use of a laser as an ionization source has several important advantages over more conventional ionization techniques. The laser has the ability to ionize a wide variety of samples, including involatile and thermally unstable organics. Analysis usually can be performed without elaborate sample preparation; few charging
problems are encountered. Laser ionization produces both positive and negative ions such that unambiguous structure determination can often be accomplished. Another advantage associated with the laser is the ability to focus the beam to a small spot, on the order of micrometers. The focused beam can be positioned a t any location on the sample, and a mass spectral analysis can be obtained from that area. The ability of the laser to be accurately positioned onto the sample with a small beam diameter allows its use as a microprobe. The importance of microprobe mass spectrometric techniques lies in their ability to obtain chemical information from microvolumes through the analysis of molecular and/or structurally significant ions. Few methods currently available have the ability to analyze organic components in or on an organic matrix. Briggs et al. have used secondary ion mass spectrometry (SIMS) to map an organosilicon lubricant on an organic polymer (1). By use of an ion gun having a spatial resolution of 30 pm, it was shown that SIMS can be used effectively as an organic microprobe provided that sample charging can be neutralized. Another mass spectrometric microprobe is the Cameca ion microscope ( 2 , 3 ) . The principal use of the Cameca has been in the area of elemental analysis. Organic analysis using this technique is difficult due to sample charging (coating methods are used to circumvent this) and the high primary ion flux which is necessary. Possible use of the Cameca as an organic microprobe has been demonstrated by using an ion beam ( 4 ) and a laser ( 5 ) for sample ionization. Recently we have demonstrated the ability of the laser mass spectrometer to map organic ions from an organic matrix (6). These first experiments were designed to demonstrate the feasibility of imaging an organic molecular ion on an organic
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Flgure 1. Optical pholograph of a 100-mesh transmissbn electron microscopy grid taken using a magnification of 6 0 X . The hole distances are 205 Gm. The aspect ratio (ratio of the hole width to the bar width) is 3 : i .
sample surface. The experiments were performed by manual translation between analysis points. Ion maps were successfully generated providing information on compound identity, as well as compound location with respect to a larger area. It became clear from these experiments that a n automated mapping mode would contribute significantly to development of an analytical organic microprobe technique. We report here the initial results from a fully automated ion mapping system for laser mass spectrometry. T h e ion maps or images obtained by this system correlated well with a series of control samples. T h e spatial resolution appears to be limited by the laser beam diameter which is approximately 2.5 cm. T h e total analysis time needed t o generate a complete map depends on the sample area to be analyzed and the spatial resolution required. T h e time required to aceurndate an individual pixel for an ion map is approximately 1.25 s.
EXPERIMENTAL SECTION A commercially available laser m a s spectrometer (LAMMA1O00, Leybold Heraeus GmbH) was used for the present work. The instrument makes use of a Nd-YAG laser to generate a fundamental wavelength of 1060 nm. Two frequency doubling crystals generate the 265-nm wavelength pulse of approximately 15 ns half-width which is used to interact with the sample. The instrument operates in the reflection mode in that the laser is brought onto the sample a t an angle of 45O with respect to its perpendicular; the ions are extracted normal to the sample plane. A more comprehensive description of the LAMMA-1000 is given elsewhere (7) The laser mass spectrometer utilizes a dedicated HewlettPackard 1M)O-E series computer for data acquisition, manipulation, and storage. Hardware, which was added to the LAMMA-loo0 and interfaced to the computer, includes stepper motors from Superior Electric Corp. which were attached to the sample stage manipulators, a Daedel. Inc.. Model PC410 stepper motor controller used to interface the stepper motors to the computer by means of an IEEE-488 standard bus, and an IEEE-488 bus compatible interface (listener only) designed in-house to trigger the laser. Dyes used for control samples were obtained from Chem Service; they were used without further purification. The nitrocellulose used as a substrate was obtained from Polysciences Inc. Transmisnion electron microsropy (TEM) grids were obtained from Structure Probe, Inc. The control samples were prepared in the following manner. A 5% solution of nitroeelldose was deposited onto a zinc substrate. With the nitrocellulose still tsctile, several TEM grids of varying mesh sizes were positioned onto the polymer and allowed to dry. A photograph of a 100-mesh TEM grid is shown in Figure 1 to illustrate the grid pattern. The width of the open holes is 205 pm. After the nitrocellulose dried (several minutes) dyes were electrosprayed on top of the "EM grids. Once the dyes had dried,
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Top of figure shows Hw timing sequence used fw ion mapping. Bebw is lhe block dlagram of automated LAMMA-I000 lon mapping system. Flgu~e2.
the TEM grids were removed from the nitrocellulose substrate. After removal of the grids, the sample was left with well-defined areas of deposited dyes. Masking by the TEM grids produced a pattern with known dimensions by which an ion map could be compared to evaluate the spatial resolution of the technique. System Hardware Description. Figure 2 illustrates the timing sequence of events and a block diagram of the hardware configuration of the LAMMA 1000 system modified for ion mapping. The computer controls all operations without operator intervention. Initially the laser is triggered from the computer by means of the laser trigger circuit. Firing of the laser initiates the ion mapping cycle (consisting of the steps shown in Figure 2) which is repeated until the ion map is complete. The pockels cell of the laser generates a voltage pulse which is used to trigger the transient recorder to begin collectingdata (the mass spectrum). The data are then transferred from the transient recorder memory to the main memory of the computer where it is searched for the intensity of a user-selected ion. The data (complete mass spectrum) are stored an a permanent file on hard disk. The intensity information obtained is color- or density-coded and displayed as a faur-edged polygon. The sample is then moved to the next analysis position, and the cycle is then repeated (heginning with the laser trigger) until the map is complete. Scanning of the sample is performed in a nonrandom fashion along the x and y axes. Initially the sample is stepped along the x axis beginning at the origin or x = 0,y = 0 position. After analysis is complete along the x axis, the stepper motors move the sample back to the origin with x = 0 and increment they axis by the required value. The x axis is then scanned again, and the procedure is repeated until the area is scanned completely. In this manner all steps are taken from the same direction to maximize reproduciblility of sample location. Auxiliary movements of the stepper motors have been incorporated into the software to minimize the nonreproducibility associated with placing the sample at the origin. The sample is always beyond the point of origin in the negative x direction before it is positioned back to x = 0. Such nonreproducibility arises from mechanical imperfectionsin the sample stage and micromanipulatorassembly. Major limitations to the speed of analysis include the time required for (1) the mechanical movements of the sample state, (2) the graphical display, and (3)the opening and closing of disk data files necessary for permanent data storage. Even with these limitations the current optimized software system can complete a full cycle in approximately 1.25s. Generating an ion map having a matrix composed of 961 points (such as those presented in this communication) therefore requires approximately 20 min. Ion maps having smaller mstrices can be completed in a relatively short period of time. System Software Description. The implementation of an automated ion mapping system for laser mass spectrometry requires relatively few hardware components (aside from the mass spectrometer) and is a software-intensive procedure. It is the function of the software to control all mechanical operations
ANALYTICAL CHEMISTRY. VOL. 59. NO. 14. JULY 15,
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Flgure 3. (A) Ion map of the quasi-molecular ion (M + H)+ of Gentian Violet at m t z 372. (FJ) Ion map of the fragment ion (M H CH,)+ of Gentian ViOlet at m / z 358. The hole wldth of lhe IOOmesh grld used to make this sample is 205 pm. The aspect ratio is 4.51. The step size is 25 pm.
(otherwise performed manually) in the proper sequence. Additionally the software must perform its function at the appropriate time. The various p r w a m modules must interact not only with individual mechanical devices but with each other as well. All software described waq written in Fortran 71 (HPimplementation) wing the Hewlett.Packard Real-Time Executive (RTE-fi VM) operating system. The following IS a brief description of the current mapping system software called LAMAP. The operator enters the mapping routine software after s mass calibration has been performed and an appropriate laser power has been determined. The appropriate laser power density is determined during a survey analysis of the sample; a power demsity yielding reprepentalive spectra for the sample is used. Upon entering the mapping routine. the user is presented with a parameter tahle requesting input parameters such as identification information and experimental information which are essential for the map. These parameters are used as an identification section for file storage. as well as for defining the experimental mnditions for the mnp. The experimental conditions include (1) the definition of the analysis area. (21 the step size in uniffi of micrometers between analysis points. (3) the mass to be mapped. and (1) the threshold v d u e ~to be used for graphical display. The analysis area is defined by locating the area of interest in the optical microscope and positioning the cross hairs at the corner of the sample area. l ' h e micrometer readings at the four corners are input into the computer by means of t he parameter tshle and define the area to be anal@. These are inwer values. T h e ~ t e si7e p in units of micrometers is then input as a real value. This determines how far the sample will he moved by the stepper motors between analysis points The stepper motor controller is currently canfigured for a minimum step of 2.50 r m : one may enter any multiple uf 2.50 (Le.. 5.00. 7.50,50.00.etc.). The combination of the analysis area hundaries and the step size determine the array size needed and. therefore, the number of polygons that will be displayed graphically. Currently the system ha9 an upper limit uf a 1W x 100 matrix but. if necessary. can easily be increased When very large matrices are used. the time required fur completion of an ion map becomes large and may be an important consideration for a given analysis problem. The next user input needed is the mass value to be searched. This is subdivided i n t o an initial low ma98 value and a final high mass value. If the user wishes to map an ion at mass AN,for example. a typical low maw value would be 199.50 and a typical high mass value would be 200.50. The user therefore has contrnl over the mass range (number of channels) to be searched. The ability to scan over a finite range of mawes ia important hecause the peaks in the mass spectrum at time3 shift by small amounts; shifts up to O..W mass unit are sometimes ohserved. These small shifts in mass. if not accounted for. may provide erroneous values for the maximum inwnsity of a given inn. Our experience has shown that searching a finite mass range is an important utility and one that may he uaed to deal with the shifting of peaks from
one spectrum to the next. Increasing the mass window will make certain that the entire peak is being searched. More reliable intensity values will therefore be obtained. The last input is that of the threshold value. This value is used t o set the background level of the map. Since each data point (intensity) in a mass spectrum is stored in &bit channels, the largest inkger value (intensity level) passible is 255. The threshold integer value may therefore range from 0 to a maximum of 255 intensity levels. Normally the threshold value corresponds to the intensity level of the background (Le., base line). A typical digitized value of the base line has been empirically determined to be approximately 40. Ten equal intensity ranges are then calculated from the difference between the largest integer value possible and the threshold value. These 10 intensity range are then coded so that each corresponds to a different density area fill in a four-edged polygon; a higher density corresponds to higher intensity values. Once the parameter table has been completed, the laser interface generates a signal that triggers the laser automatically. After the laser has fired, the computer performs the following operations in sequence: (1) transfers a spearum from the transient recorder to the main memory; (2) searches for the given mass value(s); (3) stores the complete m a s spectrum on hard disk; (4) displays the point on a graphics terminal; (5)moves the sample to the next analysis point by means of the stepper motors: (6) triggers the laser for the next analysis area. All of the above steps are performed without operator intervention. Current implementation is such that the operator is needed only for parameter input, After the data are transferred from the transient recorder to the main memory, the maximum intensity of the user-selectable mass range is obtained. The entire mass spectrum is then stored as a permanent file on disk. The intensity is then displayed on a graphics terminal either as a color-coded or density-coded POlygon (depending on output device). The sample is then moved by the stepper motors t o the next analysis position. Once the above operations have been completed, the computer again triggers the laser via the interface and the cycle begins again until the entire sample area has been analyzed. After a complete ion map is generated, all data for that map, including sample identification and experimental parameters, are permanentlystored on magnetic disk. The mapping routine is then terminated with all the spectra and map data file stored and a graphical display of the data (ion image) on the graphics terminal. An auxiliary program named REMAP may then be executed to generate a map of a different mass ion. The REMAP prcgam opens all the individual mass spectra (already stored for the initial ion map) data files created previously and searches each far the maximum intensity of a selected ion. The search for the maximum intensity is again performed over a finite mass range as selected by the user. The REMAP utility allows the user to reexamine the data files for any mass at a later time. Current implementation
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e 4. Ion maps of the quasi-moku!ar ion (M - Ci)+ of kiiiiant ('reen at m/z 385. (A) he hob wtdm of me 400-mesh grid used for mk sample k 4 2 pm. The aspect ratio is 2 1 . The Step sire is 5 pm. (B) The hole wldlh of the IOOOmesh grid is 12 pm. The aspect ratio is 1:l. The step size is 2.50 pm. w
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F@xe 5. (a) Optical photograph of a Wmn?SrClelly available EPROM integrated circun taken at a magnification of 107X. The dimensions of the contact points are approxhat&y 84 pm. (b)Gdd ion map of elecfical banding pad. (c) Aluminum h map generated from the same analysk as fM lhe map in b.
of the REMAP program allows searching only an individual mass with the subsequent display of intensities. Future vemions will incorporate the ability to generate maps by using Boolean op-
erators on as many as five different masses. Such an enhanced utility has the advantage of providing information as tu the presence or ahsence of several maws in only one map. this being a dramatic inrrease in information content.
RESULTS AND DISCUSSION Ion maps were obtained from a series of control samples
designed to test the automated LMS system capabilities. Figure 3A shows an ion map obtained for the (M - Cl)+ quasi-molecular ion ( m / r 372) of Gentian Violet. A 100-mesh transmission electron microscopy (TEM) grid having hole widths of 205 pm was used to prepare this sample. The distance between open hole areas of the grid is known to be 45.6 pm. From these known dimensions comparisons can be made with the dimensions calculated for the organic dye from the ion map. Use was made of 25-qn increments (or steps)
ANALYTICAL CHEMISTRY, VOL.
between analysis points to maximize the field of view of the sample and to ensure that the structure of the grid will be reproduced. The ion map shown in Figure 3A faithfully reproduces the structure of the TEM grid used. The dimensions of the deposited dye area have been calculated to be 206 17 pm, which compares favorably to the known value of 205 pm. The calculated value of 206 pm is very good when one considers that relatively large step sizes of 25 pm were used. Upon completion of the ion map presented in Figure 3A the REMAP software utility was invoked to generate an ion map of a fragment ion (M - CH4)* m / r 358 of Gentian Violet. Since the complete mass spectra have been stored from the previous analysis, no additional mass spectral analysis was required for generating a map for the fragment ion. Figure 3B shows the ion map obtained for the fragment ion of Gentian Violet. Since the sample and the experimental conditions for the map shown in Figure 3B are the same as those of the map shown in Figure 3A, it is expected that the map of the fragment ion should be identical with that of the map for the quasi-molecular ion. Comparison of the two maps shown in Figure 3 confirms this expectation. Only a few data points are different between the two maps owing to differences in ion intensities. Such a result demonstrates the ability to correlate the presence of more than one ion with specific locations on a sample. With the demonstrated ability to map organic ions from welldefmed locations, further samples were a n a l y d to obtain information about the highest lateral resolution posaible with the system. An ion map was obtained for the (M - CI)* ion ( m / r 385) of Brilliant Green. The sample was prepared by using a 200-mesh grid having hole widths of 94 pm and a distance between holes of 31.3 pm. The smaller dimensions of this sample required higher lateral resolutions to be used. Use was made of 10-pm incrementa between analysis points. The dimensions of the organic ion locations were calculated from the map (not shown) to be approximately 104 10 pm. This compares favorably with the known hole diameter value of 94 pm. The ion map presented in Figure 4A was obtained hy using still higher lateral resolutions. The quasi-molecular ion of Brilliant Green (M - CI)+ ( m / r385) was mapped from a sample prepared by using a 400mesh TEM grid. The known hole distance and the distance between holes are 42 and 21 pm, respectively. Five-micrometer increments were used to obtain the map in Figure 4A. The calculated dimensions of the ion location are approximately48 6 pm. This calculated distance aeain aerees very well with the known oven hole distance o r 4 2 ,I&. The ion map presented in Figure 4B was obtained by using 2.50-pm increments for Brilliant Green on a 1000-mesh TEM mid. The erid hole width and distance between holes were both 12 pm. The map of the (M - CI)+ quasi-molecular ion of Brilliant Green is presented in Figure 4B. A gridlike structure corresponding to the dye position on the sample is clearly evident in the map. Additionally, distortion of the grid is observed. This distortion is attributed to the elliptical shape of the laser beamsample interaction arises from the reflection geometry of the laser mass spectrometer, the angle between the sample and incoming laser beam being 45O. Despite the distortion, the structure of the grid is still readily discernible, and a rough calculation of the organic ion locations can be made. The dimension was calculated to be approximately 20 5 pm. Although this calculated value has a greater error relative to the previously calculated values, it is still a good estimate of the dimension of the organic dye on the sample. The m a p in Figures 3 and 4 demonstrated that organic ions m be mapped from very small areas of organic materials that are located on insulating organic substrates. The lateral resolution of the mapping system is approximately 2.50 pm
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mure E. Oplical pholqraph of coal sample at 250X magnlhlion. Mawral Is w a n as lhe bright circular area in lhs wnler. The square area that was analyzed is vlslble In lhe photograph as track marks. The marks arise from the ablation of the sample by the laser beam. and is limited by the laser beam diameter. In addition to mapping organic molecular ions, laser maSS spectrometry has the ability to map elemental ions. Although element maps can be obtained by using high lateral resolution techniques such as scanning electron microscopy and the ion microscope, the LMS mapping technique has the capability of obtaining both elemental and organic ion information at the same time. T o illustrate elemental ion mapping, ion maps are presented for gold and aluminum on an eraseable, programmable read only memory (EPROM) integrated circuit. Figure 5a shows an optical photograph of a portion of the integrated circuit. The lead wires are connected to the circuit by means of solder joints on a bonding pad. A 100-pm area around a bonding pad was analyzed. The resulting ion maps are shown in Figure 5b,c. Figure 5b shows a map of the gold ion, and Figure 5c shows a map of the aluminum ion. The width of the solder joint, as calculated from the gold ion map, is 71 10 pm. This corresponds well with the actual measured value of approximately 84 pm (each bond has slightly different dimensions). The map of the aluminum ion shown in Figure 5c reinforces the location and dimension of the bond. It also clearly demonstrates that aluminum is used as the bonding pad for the gold solder joint. Ion maps were also obtained for a microscopic component of a coal sample. Inspection of the coal by optical microscopy revealed the presence of an inclusion located within the much larger coal maceral sample. An optical photograph of the coal maceral is shown in Figure 6. Preliminary LMS analysis revealed that the content of the inclusion was much different than that of the surrounding coal maceral matrix. Spectra obtained from the inclusion showed primarily iron and sulfur
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ANALYTICAL CHEMISTRY. VOL. 59. NO. 14. JULY 15. 1987
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(e) Map of FeS;
(b) Map of Fe,S+ ion at m l z 144. (c) Map of S~ ion at m l z 32. (d) Map of S; bn a1 m l r 64. lon at mlz 120. (0Map of C; carbon cluster ion at m l r 84.
ions, whereas the surrounding maceral region was primarily organic showing predominantly carbon clusters. The region containing the inclusion was then mapped. Positive ion maps were obtained for Fe' ( m / r56) and Fe&+ ( m / z 144) and are
shown in parts a and b of Figure I, respectively. Both the Fe+ and F e , S maps clearly define the position of the inclusion. Although the intensity of F e a t ions is significantly lower than that of the Fe+ ions, the F e 2 S ion can still he correlated
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Anal. Chem. 1987, 59, 1825-1830
with the position of the inclusion. The exact nature of the iron- and sulfur-containing species comprising the inclusion is very difficult to determine by LMS. The iron and sulfur ions observed are the same, independent of the compound type. Therefore, a second analysis of the same area was performed by using negative ions. Maps were generated for S- (Figure 7c), S2-(Figure 7d), FeSf (Figure 7e), and C7(Figure 70. Theae four maps of different negative ions confirm what was observed in the positive ion maps; the iron and sulfur components are localized in the inclusion. With the C7-carbon cluster mapped it can be shown that there is very little carbon-containing material within the inclusion. All carbonaceous materials are located in the adjoining maceral on the periphery of the inclusion. Measurements of the inclusion size were calculated from the maps of Fe+ and Cf. The calculated value of the size of the inclusion was approximately 150 pm and correlated very well with the dimensions observed for the maceral with an optical microscope. Exact determinations are difficult due to the irregular shape of the maceral. The ion maps from the coal maceral demonstrate again that positional information can be obtained from elemental and cluster ions. Additionally, these maps demonstrate that both positive and negative ion maps can be obtained (separate
analysis must be performed), thereby providing the ability for corroborating evidence as to the presence or absence of specific ion species in the sample.
ACKNOWLEDGMENT We wish to thank Bob Muha of the Department Electronics Shop for his design of the laser trigger interface, Paul Lyons of the U.S. Geological Survey for the coal samples, and John Morelli for the parameter table.
LITERATURE CITED Briggs, D.S I A , Swf. Interface Anal. 1983, 5(3),113. Morrison, G. H.; Slodzian, G. Anal. Chem. 1975, 4 7 , 932A. Furman, 0 . K.; Morrison, G. H.Anal. Chem. 1980, 52, 2305. Simko, S. J.; Grlffis, D. P.; Murray, R. W.; Llnton, R. W. Anal. Chem. 1985. 5 7 , 137-142. Furrnan, B. K.; Evans, C. A. Springer Ser. Chem. Phys. 1982, 79. Novak, F. P.;Wllk,’Z. A.; Hercules, D. M. J . Trace Microprobe Tech. 1985, 3(3). 149. Heinen, H. J.; Meler, S.; Vogt, H.; Wechsung, R. Int. J . Mass Specfrom. Ion Phys. 1983, 4 7 , 19.
RECEIVED for review January 7,1987. Accepted April 1,1987. This work was supported by the National Science Foundation under Grant No. CHE 85-41141.
Performance of a Glow Discharge Mass Spectrometer for Simultaneous Multielement Analysis of Steel Norbert Jakubowski, Dietmar Stuewer,* and Wojciech Vieth’ Znstitut fur Spektrochemie und Angewandte Spektroskopie, Postfach 778,D-4600 Dortmund I, Federal Republic of Germany
The analysis of steel has been performed on a previously described laboratory prototype of a new glow discharge mass spectrometry (GDMS) system equipped with a quadrupole Mer In order to study lis anaiyilcal performance. Operational parameters of the discharge have been optimized and seven NBS standards and one USS standard have been analyzed. A weighted regresslon has been applied for calibration. The results Include sensltivlty factors and preclslon data for 26 elements. I n multleiement analysls, the detection limit was about 0.1 pmd/mol while single-element determination was possible at a 5 times lower detection ilmlt. Analysis results for two test samples wlth 30 elements each agree within 10% of the certified values. Consequently the method proves to be a valuable tool for simultaneous multielement trace analysls of solids.
Mass spectrometry applied to the direct analysis of solids has several advantages. All elements can be determined simultaneously and detection limits are low and nearly the same for all elements. In practical applications however, analytical methods often show a great discrepancy between their performance for real samples and principal figures of merit. Traditional spark source mass spectrometry (SSMS), for inPermanent address: Institute of Electronic Materials TechnolPoland.
ogy, Konstruktorska 6, Warsaw,
0003-2700/87/0359-1825$01.50/0
stance, displayed low detection limits (1) but did not gain widespread attraction because of its rather poor accuracy and precision, due to the evaluation procedure involved. In recent years, there was considerable progress using stationary discharges rather than the radio frequency (rf)spark discharges. Mass spectrometry with an inductively coupled plasma as ion source (ICP-MSJ,as developed mainly by Gray and Date (2), has gained greater attention for the analysis of solutjons. When applied to the analysis of solids however, its power of detection is restricted by the maximum salt concentration in the solution. Moreover, signals arising from cluster ions may cause systematic errors. Sputtered neutrals mass spectrometry (SNMS), as introduced by Oechsner et al. (3), is particularly promising for surface and in-depth analysis because of its low sputtering rates. Compared to secondary ion mass spectrometry (SIMS) (4),SNMS has lower matrix effects. For all these techniques quadrupole filters are suitable mass analyzers. For a laser microprobe mass analyzer with transient analytical signals, a time-of-flight mass spectrometer (5) is the analyzer of choice. Of all the techniques summarized so far, none up to now could prove to be specifically applicable to bulk analysis of solids. Coburn and Harrison have pointed out in a comprehensive review (6) that glow discharges with reduced pressure are well suited as ion sources for direct mass spectrometric analysis of conducting and semiconducting bulk materials. In particular, the simplicity and stability look advantageous. Harrison et al. (7)first applied a direct current (dc) glow 0 1987 American Chemical Society
