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Anal. Chem. 1984, 56, 382-385
Determination of Carbon, Nitrogen, and Oxygen in Solids by Laser Mass Spectrometry Zhao Shankai,' R. J. Conzemius,* and H. J. Svec A m e s Laboratory-USDOE
and Department of Chemistry, Iowa State University, Ames, Iowa 50011
Carbon, nltrogen, and oxygen are measured directly In metal speclmens wlthout standards by laser mass spectrometry. Accuracy Is wlthln the uncertalnty of NBS standard reference materlals used to test the technique. The results lndlcate that parts-per-bllllon level measurements of these elements will be stralghtforward. Contrlbutlons due to lnteractlons of samples and resldual gases are negated by Increased rates of laser cleaning In relatlonshlp to the background gas pressure.
Table I. Ranges of Laser Beam Parameters pulse width (at half peak height) wavelength beam divergence energy per pulse diffraction limited spot size ( 6 . 7 5 X Upcollimator and 60 mm lens) power density repetition rate
Improved methods are needed for determining nonmetallic elements present in solid materials (1). More than 45 papers have been published (2) describing the analysis of gaseous impurities in solids and thin films by laser mass spectrometry (LMS) since Honig (3) introduced the laser ion source. There are some vital advantages gained by using a focused laser beam combined with mass spectrometry to determine gaseous impurities in solids: 1. No background is introduced due to bulk heating of the sample as in methods, such as vacuum fusion, pulsed dc heating, etc. 2. Very low concentrations can be detected, possibily in the parts-per-billion range. 3. Only a few milligrams of sample is needed while most other methods require gram-sized samples. 4. Local analysis is possibile when spatial resolutions down to tens of micrometers or less are used. 5. No special sample preparation is required. 6. The possibility exists for absolute measurements, i.e., determination without requiring standard samples ( 4 ) . Heretofore some, but not all, of these advantages have been demonstrated (2, 5). Here we introduce a method for determining C, N, and 0 in solids without standards in which the relative errors approach the confidence levels for the concentrations known in reference materials.
EXPERIMENTAL SECTION Apparatus. The laser mass spectrometer has been described (6). Figure 1gives a schematic diagram of the instrument. X-Y galvoscanning mirrors, located just above the objective lens and used to raster the laser beam, are not shown to simplifythe layout. Laser. The laser system is a Model 255QT Nd-doped yttrium aluminum garnet (YAG) with an acoustooptic Q-switch manufactured by the Holobeam Corp. The ranges of beam parameters available with this system are given in Table I. Mass Spectrometer. A double focusing mass spectrometer constructed locally (7)based on Mattauch-Herzog (8) ion optics was used. The ion transmission through the energy limit slit was set to transmit -150 eV to the magnetic analyzer. The 150-eV slice of the beam was -40% of the beam transmitted by the electrostatic analyzer. The electrical detector consisted of a Faraday cup followed by ion signal amplification with a Cary Model 401 electrometer. A cryopump provides evacuation of the ion source chamber with a pumping speed of loo0 L/s, producing a clean vacuum and a rapid pump down of the chamber to 1 X torr within an hour.
Present address: Zhongshan University, Guangzhou, China.
100 ns 1.06 pm
diffraction limited 0.01-1.0 mJ 1 2 pm to 5 x 10'9 W/cm2 0-5000 HZ 10'6
Table 11. Effects of Power Density on C', N', and 0 ' Intensities
element C N 0
ion intensity (ppma) 1.9 x 1 0 9 3.2 x 109 W/cm2 W/cm2 7000 57 44
19000 160 260
ratio 2.7 2.9 5.9
Choice of OperatingConditions. Quantitative determination of trace gaseous impurities without standard samples requires careful control of operating parameters and choice of adequate operating conditions. There are three operating conditions especially important for determining gaseous impurities in solids: (a) complete cleaning of the sample surface before analysis; (b) stable operation affecting the interaction of the laser with the sample, such as laser power density, pulse repetition rate, laser beam focus position (focus point right on the sample, or above/beneath the sample), and laser scanning control; (c) residual gases in the ion source chamber and their contributions to the ion intensities. A series of experiments were designed to establish these operating conditions. Complete Cleaning. Preliminary experiments indicating that stable ion signals for carbon and nitrogen were attained very quickly whereas ion signals for oxygen were highly dependent upon the condition of the surface. Thus oxygen was chosen as the element most sensitive for observing the effect of cleaning a sample surface. This effect is nicely observed by plotting oxygen ion intensity vs. the number of laser shots. Such data are shown in Figure 2. One observes that the O+ intensity becomes stable after 11OOO shots rastered over the surface. This corresponds to about 12 times the number of laser shots necessary to erode a single layer 0.4 pm deep over a 600 X 600 pm area. If the scan area is minimized to 200 X 200 pm, the O+ intensity is seen to decrease further. The cause of this minimization will be discussed below. Laser Power Density. Almost all papers concerning analysis of solids by LMS have discussed this topic (2,5). Bykovskii et al. (9) show that relative sensitivity coefficients of the major constituents of inorganic compounds approach unity at about 5 X log W/cm2. We find similar results here for trace levels of C, N, and 0. However Of is more sensitive than Cf and Nf to changes in the laser power density. Table I1 shows that when the power density is raised from 1.9 X lo9 W/cm2 to 3.2 X lo9 W/cm2, the C+ and N+ ion intensities increase about 3-fold whereas the O+ intensity increases 6-fold. Thus for good analytical reproducibility, one must control the laser power density accurately and precisely, especially when determining oxygen.
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0003-2700/84/0358-0382$01.50/00 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
4
383
YAG-I as e I
.-5 2 5
laser beam
a l i m i t slit
anode c h o rnber ion beom path
object slit magnetic onolyser
1111151111111 11111 0 15 N u m b e r of l a s e r s h o t s xl000
Figure 1. Schematlc diagram of laser and mass spectrometer.
Figure 4. Cleaning by laser shots scanning side-by-side across an entire 600 X 600 pm2 matrix.
30r
._
J
N u m b e r of l a s e r s h o t s xl000
Figure 2. Cleaning by laser shots with X-Y scanning mirror rates chosen randomly.
a
Flgure 5. Dependence of ' 0 intenslty on O2 pressure.
Figure 3. (a) Sawtooth wave form. (b) Triangular wave form
Laser Mirror Scanning. In the equipment originally supplied the laser beam was scanned on the specimen by X and Y scanning mirrors controlled by the electric wave form shown in Figure 3a. Damping of the mirror movement during the high slewing rate portion of this sawtooth wave form is difficult causing irregular laser beam control and overshooting of the scanned area. Thus some laser shots will impinge outside a precleaned area causing a heavily oxidized, uncleaned surface to release large amounts of oxygen and also causing the O+ intensity to fluctuate greatly. In addition, the X-Y mirror scanning rates were chosen randomly. This was found to provide irregular cleaning since some sites in an area to be cleaned always get fewer laser shots than others. These locations are exposed to residual gases for longer times without laser erosion and permit oxygen gettering thus causing the O+ intensity to fluctuate. An improvement in the Y axis scanning of the laser was made by providing for a triangular wave form control shown in Figure 3b. This eliminated the mechanical damping problem and allowed the laser to scan uniformly. The X axis scanning was placed under computer control so that each laser shot was placed side-by-side across the entire matrix being scanned, thus assuring a completely uniform erosion of the surface. This minimized background gas gettering by the surface due to the more efficient surface erosion. Figure 4 is a plot showing the cleaning effect using the improved methods. Comparison of Figures 2 and 4 shows the new cleaning technique to be more efficient. In Figure 4 the 0' ion signal becomes stable (within 10% of the asymtopic value) after only -7000 laser shots. Laser Focusing. The effects of laser focus position on ion intensity is discussed in only two references (10, 11). Detailed results of considerable experimentation here investigating these effects will be discussed elsewhere (12). Briefly, when the laser beam is focused at other than the surface of the sample, the matrix ion intensity and the impurity ion intensities are affected. For the elements studied here, highest ion intensity occurs when the specimen is placed 100 to 150 fim further away from the objective than the specimen location where a minimum diameter plasma is observed. In addition, manual focusing by observing the minimum visible plasma diameter or visual focusing at the surface
is unsatisfactory due to the subjective nature of the viewer's choice. Thus a computer program was created to set the focal point of the laser at the position yielding the maximum ion production. Residual Gases and Their Contributions to Ion Intensity. Clegg et al. (13) and Jansen et al. (14) used a liquid helium cryogenic pump within the ion source chamber to reduce the base pressure to about 1X lo* torr in spark source mass spectrometric studies. They obtained a very low background corresponding to about 0.02 ppma but did not establish the mechanism causing the lower blanks. Results of some studies here provide a better understanding of this effect. a. As shown in Figure 2, even when the sample surface had been cleaned implicitly and 0' intensity kept stable, reduction of the scanning range reduced O+ intensity and stabilized it at a lower level even though the base pressure remained constant. Reduction of the scanning range while maintaining the laser pulse repetition rate and the X and Z scanning rates reduces the time between repeated laser erosion of the same area. b. By introduction of oxygen into the ion source chamber with an iron specimen in place, O+ intensity increased rapidly with oxygen pressure. Reducing the scanning area yielded the same result as in (a). Figure 5 shows a plot of 0' intensity as a fraction of the total ion signal vs. oxygen pressure. It should be noticed that the slopes of these curves are different with the higher slope obtained when the scan area is larger. c. The same experiment was performed with a very active element, pure La, and with an inert element, pure Pt. Data similar to those in Figure 5 were obtained except that the slope for the La specimen was four times greater than that for Fe whereas the slope for the Pt specimen was approximately half that of Fe. d. Carbon monoxide was introduced into the ion source chamber containing an iron specimen while the C+ and O+ intensities were being observed simultaneously. The C+ intensity was unaffected even when the CO pressure was 5 X lo-' torr. At a CO pressure of 9 X lo-' torr the C+ intensity increased slightly and the increase was unaffected by reducing the scan area. The O+ intensity increased with CO pressure by less than that observed with the same oxygen pressures. These observations indicated that careful control of laser cleaning and site selection was necessary in order to optimize the signal from the bulk sample relative to the signal obtained due to surface contaminants from residual gases.
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
Table 111. Analysis of C, N, and 0 in NBS Standard 1040 Series N
Ca
sample
NBSb data, wt %
exptl; wt %
0.03
0.2 0.2 0.4
0.028 0.38 0.090 0.21 0.23 0.42
0.02
0.010
0.2
0.28
1040 1041 1042 1043 1.044
0.4 0.1
1045
1046 1047 a
h i
Carbon concentration is not certified. ) x i 0 - 4 / ( a t .WtFe).
0
cert,b wt %
exptl,
wt %
cert,b wt %
exptl, wt %
0.003
0.0031 0.0043 0.0142 0.0065 0.0053 0.0042 0.0056 0.0060
0.018 0.017 0.017 0.002 0.009 0.007 0.106 0.017
0.014 0.013
0.004
0.014 0.005
0.004 0.004 0.005 0.004
0.025 0.003 0.0075 0.0074 0.112 0.015
Exptl (wt % j = exptl (ppmaj x (at.
No uncertainty values given by NBS.
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Table IV. Analysis of 0 in Maraging Iron, NBS 1094 certifieda element value 0
exptl,b ppma
4.5 ppm by wt (16 ppma)
16.5 17.8 17.8 18.9 17.2
% re1
mean
std dev
std dev
17.6
0.89
5.0
Table V. Analysis of 0 in NBS Standard, Pure Platinum SRM 680a certifieda element
value
0
4ppmbyweight (50 m m a )
exptl, ppma mean 56 48
NBS reports a range in the determinations from 2.5 to Five separate determinations on same 7.5 ppmw. loading. Analytical Procedure. On the basis of the above experiments a special computer program was created for the determination of C, N, and 0 in solids. The program controls laser scanning such that slightly overlapping side-by-sidelaser shots are achieved across the entire matrix. The program allows the operator to choose the necessary cleaning cycles, it selects the suitable laser focus position to provide the highest ionization efficiency, it allows the operator to choose any desired scan area up to 1 mm2, and it reduces the scanning area by 70% relative to the cleaned area when the final analytical measurement is made. The actual technique used for analyzing the specimens was to load the sample, allow the ion chamber to evacuate to 1 X lo-' torr, and then to start taking data. The cleaning effect as shown in Figure 4 was always observed. As soon as the pertinent signal achieved a steady state, the scanning area was reduced to a 300 X 300 pm area. Enhanced cleaning would occur and a new, lower steady-state level would be reached within about five cleaning cycles. At this time a series of measurements were taken as shown in Tables I11 and IV and the average was used for the determination. Computer control permitted electrostatic peak switching between carbon, nitrogen, and oxygen for the NBS 1040 specimens. The analytical performance of this method was assessed by using NBS standard samples: iron and plain carbon steels SRM 1040,1041,1042,1043,1044,1045,1046, and 1047; NBS standard maraging steel SRM 1094; and NBS standard pure platinum SRM 680a. As an example, the following set of parameters were used to analyze NBS SRM 1094: laser power density, 3 X lo9 W/cm2; laser spot size, 15 pm (diameter); laser scanning area, 300 X 300 pm; pulse repetition rate, 333 Hz; laser shots for each measurement, 2000 shots; accelerating voltage, 15000 V at mass 16; torr. pressure during sampling, 1 X lom7
RESULTS AND DISCUSSION The experimental results (exptl.) were computed by using the relationship
total ion signal
/
iron ion signal
total ion signal
5.1
11
49
44 43
a
analyte ion signal
48
std % re1 dev std dev
1
x
lo6 ppma
The factor iron ion signal/total ion signal is an instrumental transmission calibration obtained by measuring the iron ion signal on the Faraday cup and the total ion signal on the
a NBS reports a range in the determinations from 3.2 t o 5.2 ppmw.
Table VI. Analysis of NBS Standard SRM 1040 on Three Separate Days certified, element wt % C N 0 a
experiment, wt % J u l 30
Augl
Aug11
0.03' 0.003
0.028
0.0030
0.018
0.017
0.028 0.0031 0.017
0.0030
0.027 0.0097
Carbon concentration not certified.
monitor collector. The factor needs to be determined only once or remeasured when major changes are made in the instrumentation affecting the transmission from the total ion beam monitor to the Faraday cup detector. Tables 111, IV, and V show the results of a quantitative analysis of the NBS SRM 1040 series, NBS SRM 1094, and NBS SRM 680a, respectively. The precision of the readings from which the average was computed is shown in Tables IV and V and is normally within 10% Table VI shows the reproducibility with separate sample loadings using NBS 1040 as an example. The signal-to-noise ratio obtained for the data in Tables IV and V indicates that a detection limit of approximatly 1 ppmw was achieved. However these data were obtained by using a Faraday cup as an ion detector. Addition of an electron multiplier would greatly enhance the sensitivity. Furthermore experimental parameters such as the duration of measurement could be improved if necessary to obtain higher sensitivity. Of greater concern is the affect of residual gases on the background ion signal levels. The results of the experiments performed in choosing operating conditions indicate that the increase in the oxygen ion signal with increasing oxygen pressure is due mainly to the physical adsorption or chemisorption of oxygen on the surface of the sample during the time of sampling and is not due to gas-phase ionization. This observation agrees with some basic physical principles of absorption phenomenon under high vacuum (15). The following examples are pertinent to the subject of this paper and are given as an aid to the reader: a t los7 torr, 3.3 X lo9 molecules cm-3 is the gas density, 3.5 X l O I 3 molecules cm-2 I
ANALYTICAL CHEMISTRY, VOL. 56, NO. 3, MARCH 1984
s-l is the rate striking a sample surface at ambient temperature, 28.5 s are required to cover the entire sample surface (sticking coefficient equal to l),the average rate of intermolecular collisions is 0.65 s-l, about 10l6molecules (average size of a common gas molecule) can be accommodated on a 1-cm2 sample surface, thus the time required to cover 1 cm2 of surface with a monolayer is 1015/(3.5X 1013) = 28 s; in this experiment, the diameter of a crater caused by one shot of the laser is -20 ym, the average depth of the crater is estimated to be 0.4 pm, assuming an iron matrix the number of ions produced by one laser shot is about 1013,the number of O2molecules in a monolayer covering an area within a 20 ym diameter circle is 6 X lo9. Thus the contribution of a fully adsorbed monolayer of oxygen molecules to a single laser shot of total ions would be
(3 x 109)/(1 X
=3X
or 300 ppma
If the laser pulse repetition rate is 200 Hz and the scan area is 800 x 800 ym, the time required to scan the entire area with the laser spots placed side-by-side is 8 s, and for 400 X 400 pm the time is 2 s. Presuming a partial oxygen pressure torr during analysis, 290 s are needed to cover the of 1 X entire surface with oxygen molecules. Thus in the case of a 800 X 800 pm scan area, the fractional coverage of the surface should be 81290 or 0.028, and in 400 X 400 ym, it would be 2/290 or 6.9 X The contribution of the gettered oxygen in a single shot is 300 ppma X (2.8 X or 8.4 ppma and 300 ppma X (6.9 X or 2.1 ppma, respectively. With a laser repetition rate of 1000 Hz and a scan area of 100 x 100 ym the contribution of sorbed O2would be reduced to 0.03 ppma. Thus the background ion signals can be reduced to levels low enough to consider determining part-per-billion level constituents in solid samples. Although these calculations are based on estimates and assumptions of times and areas (the surface of the sample after laser erosion is very rough giving a higher specific area), they provide a basis for appreciating the contribution of residual gas to the a n a l e ion intensity and explain the results of the experiments well.
CONCLUSION By carefully controlling laser parameters such as power density, pulse repetition rate, laser scanning, scan area, and focus position and by thoroughly cleaning the surface of the sample before taking data, one can determine C, N, and 0 quantitatively in solids without standard samples. The absolute error obtained by the difference between the reported values and the experimental results indicates that standards with higher accuracy are needed to evaluate the technique. The absolute error is expected to be improved by stabilizing the laser power using a computer feedback system and perhaps also by operating at higher laser power densities. It should be noted that the laser used here is pulsed possessing approximately a Gaussian distribution in time and
385
space. The analytical results obtained by using a laser with such temporal and spatial characteristics is very encouraging for further analytical development of laser mass spectrometry. The experimental results agree with a sample model for the contribution of residual gases to the observed ion intensity in which rates of adsorption of gas molecules onto the area being analyzed are overcome by adjusting the rates of cleaning by laser erosion. Parts-per-billion detection limits are expected with reasonable control of ion source operating conditions. The extension of this capability to matrices other than the test materials analyzed here is expected to provide similar results. These results indicate that LMS should be considered further in the determination of interstitial impurities in view of the growing understanding of the impact of these impurities on physical and chemical characteristics of materials, especially used in energy, solid-state, and space sciences.
ACKNOWLEDGMENT The authors thank Clarence Ness for aiding in the experimental measurements. Registry No. Carbon, 7440-44-0;nitrogen, 7727-37-9; oxygen, 7782-44-7.
LITERATURE CITED (1) Summary Report on Materials Preparatlon and Characterizatlon Capabilities; Oak Rldge, TN, Nov 16-18. 1982; CONF 821120. (2) Conzemius, R. J.; Simon, D. S.; Zhao, Shankal; Byrd, G. D. “Laser Mass Spectrometry of Solids: A Bibllography 1963-1962, Mlcrobeam Analysis-1983“; Gooley, R., Ed.; San Francisco Press Inc.: San FranCISCO, CA, 1983; pp 301-332. (3) Honig, R. E.; Woolston, J. R. Appl. Phys. Lett. 1963, 2 , 138. (4) Bykovskii, Yu. A.; Zhuravlev, G. I.; Belousov, V. I . ; Gladskoi, V. M.; Degtyarev, V. G.; Nevolin, V. N. Ind. Lab. (Engl. Transl.) 1978, 4 4 ,
799-804. (5) Conzemius, R. J.; Capellen, J. M. Int. J . Mass Spectrom. Ion Phys. 1980, 34, 197-271. (6) Conzemius, R J.; Svec, H. J. Anal. Chem. 1978, 5 0 , 1854-1860. (7) Foss, G. Ph.D. Thesis, Iowa State university, 1981. (8) Mattauch, J.; Herzog, R. Z . Phys. 1934, 89, 786-794. (9) Bykovskii, Yu. A.; Zhuraviev, G. I.; Belousov, V. I.; Gladskol, V. M.; Degtyarev, V. G.; Koiosov, Yu. N.; Nevolin. V. N. Sov. J . Plasma PIIYS.(Engi. Trans/.) 1978, 4 , 180-184. (10) Koshelev, K. N.; Dhekalin, S. V., Churilev, S. S. Sov. J . Quantum Nectron. (Engl. Transl.) 1975, 5 , 871-872. ( 1 1 ) Wurster, R.; Foas, U.; Wieser, P. Fresenius Z . Anal. Chem. 1981, 308, 206-211. (12) Conzemius, R. J.; Zhao, Shankai; Houk, R. S.; Svec, H. J., unpublished work, Ames Laboratory, Iowa State University, 1983. (13) Clegg, J. B.; Gale, I. G.; Millet, E. J. Analyst (London) 1973, 98, 69-74. (14) Jansen, J. A. J.; Wit, A.W. Frensenius Z . Anal. Chem. 1981, 309, 262. (15) Robinson, N. W. “The Physical Principles of Ultrahigh Vacuum Systems and Equipment”; Chapman and Hall Ltd.: London, 1968; Chapter 7.
RECEIVED for review October 11,1983. Accepted December 5,1983. Ames Laboratory is operated for the US. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. This research was supported by the Director for Energy Research, Office of Basic Energy Science.
