Multiscalar Signal Averaging in Spark Source Mass Spectrometry W. W. Harrison and W. A. Mattson Department of Chemistry, University of Virginia, Charlottesville, Va. 22903
Multiple scans are conveniently accumulated using the tnultiscale mode of a multichannel analyzer. Isotopic abundances with average relative % errors on the order of 5 % have been obtained to illustrate the improvement in precision that is possible with time averaging. Enhancement of sensitivity is also shown. Application to multielement analysis is demonstrated. No significant resolution loss is observed during the multiple scan averaging.
Electrical detection methods in spark source mass spectrometry using both the peak switch and scanning modes have been described ( 1 ). Scanning methods allow rapid qualitative analysis but are generally limited to a precision on the order of 2~35%( 1). Peak switch techniques can show precision to f 5 % by careful control of critical parameters (2, 3 ) and at the expense of a considerable increase in analysis time per element. For survey analysis of many elements per sample, time considerations may dictate use of the scanning mode, even given its less satisfactory precision. Averaging methods have been used to improve spectral signal-to-noise ratios by means of storing and adding repetitive scans. The background noise is random as compared to the unidirectional peak signals, resulting in a net improved spectrum. Such averaging techniques have been applied to mass spectrometry. A small time averaging computer has been used with a GC-MS system ( 4 ) to permit repetitive scanning and increased sensitivity. Computer controlled multiple scan averaging (5, 6 ) was shown to increase mass measuring accuracy in high resolution mass spectrometry. The multichannel analyzer (MCA), a versatile data readout device available in many laboratories, can act as a simple but effective averaging unit for spectra accumulation. The multiscale mode of the MCA has been used to enhance the measurement precision of isotopic abundances for uranium and plutonium ( 7 ) and to determine the isotopic abundance of europium produced by nuclear reactions (8). The fluctuating nature of the spark ion source and the poor ion statistics associated with the SSMS scanning mode suggested an opportune use of averaging techniques to enhance the precision of quantitative analysis and improve the attainable elemental detection limits. This study describes the use of a MCA in multiscale mode (9) to rap( 1 ) R. A. Binaham and E. M. Elliott. Anal. Chem.. 43. 43 (1971). C. W . Magee and W. W. Harrison, Anal. Chem., 45,852 (1973). G. H. Morrison and B. N. Colby, Anal. Chem., 44, 1206 (1972). F . J. Biros, Anal. Chem., 42, 537 (1970). R . J. Klimowski, R. Venkataraghavan, F. W. McLafferty, and E. E. Delany, Org. Mass. Spectrom., 4, 17 (1970). A. L. Burlingame, D. H. Smith, T. 0. Merren, and R. W. Olsen, 16th Annual Conference on Mass Spectrometry and Allied Topics, Pittsburgh, Pa., 1968. L. Nguyen, G. Goby, and B. Rosenbaum, lnt. J. Mass Spectrom. /on fhys. 11, 205 (1973). P. E. Moreland, Jr., C. M. Stevens, and D. B. Walling, Rev. Sci. hstrum., 38, 760 (1967). W. W. Harrison, W. A. Mattson, D. L. Donohue. and C. W. Magee, 21st
Annual Conferenceon Mass Spectrometry and Allied Topics, San Francisco. Calif., 1973.
idly and conveniently accumulate repetitive scans for subsequent analysis.
EXPERIMENTAL Apparatus. The AEI MS-702 mass spectrometer and electrical detection readout, including modifications added in our laboratory, have been previously referenced ( 2 ) .In addition, the standard AEI source chamber has been replaced by a larger unit, designed and constructed in our machine shops to accommodate a 1200 1./ sec oil diffusion pump, cold trap-baffle, and gate valve (all from Varian, Vacuum Division, Palo Alto, Calif.). The seldom used 2000 time constant for magnet scanning has also been replaced by a more useful 150 time constant. A linear ratio output is obtained (10) for peak area comparison by inputting the logarithmic output of the AEI ratio amplifier to an antilog converter, consisting of an inverting amplifier (Analog Devices, Model 141C) and a logarithmic function module (Teledyne Philbrick, Model 4350) wired in the antilog configuration. The linear ratio is input to a V-F converter and counter (Heath UDI, Model EU-805) and the output pulses are directed to a 4096 channel MCA (EDAX, Model 706) for spectra accumulation. Each channel of the MCA can accept up to lo6 counts. A visual readout module (EDAX, Model 871N) which will intensify and integrate operator selected channel bands of the MCA cathode ray tube display was used to obtain all peak areas. Reagents. NBS 468 and 461 standard steel samples and Johnson-Matthey CA8 standard copper sample were used as test electrodes of known compositipn. Procedures. The metal samples were machined to 'hs-in. diameter rods and ground flat a t the sparking surface to minimize changes in self-shielding. After being surface cleaned by rinses in acetone, ethanol, 1:l HCl and deionized-distilled water, the electrodes were dried, loaded into the source, aligned on the ion axis with the telescopic optical system a t 5 mm from the No. 1 slit, and presparked an appropriate interval. The desired number of scans were then accumulated on the MCA. A switch was installed on the mass spectrometer to synchronize the initiation of the exponential magnetic field decay with the MCA channel advance. Multiscale parameters such as channel group size and channel advance speed are selected based on the mass range scan of interest. Repetitive scans are taken by manually recharging the magnet circuit and once again initiating the exponential decay. A full mass range cyclic scan circuit is now in use (11 ) to allow automatic data accumulation, but the data in this study were taken with manual restarts.
RESULTS AND DISCUSSION Successful use of spectral averaging techniques requires the ability to reproduce scan conditions closely. In SSMS, scans from mass 250 to mass 10 which may require 8-10 minutes or longer, depending upon the magnet decay constant, put particularly severe demands on instrumental stability and reproducibility. Our object was to use the standard MS-702 and associated electrical detection scan circuitry; in question was whether successive scans could be made sufficiently reproducible to prevent resolution deterioration in the resultant averaged spectrum. Several factors must be considered in arriving a t conditions suitable for SSMS signal averaging: M a g n e t Conditioning. The decay of the magnetic field (10) C. W. Magee, P h D . Thesis, University of Virginia, Charlottesville. Va., 1973. (1 1) W. A. Mattson and W. W. Harrison, 22nd Annual Conference on Mass Spectrometryand Allied Topics, Philadelphia, Pa., 1974.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
1979
Mo Isotopes
(a)
Single Scan 98
9.6 95
Yr
+
97
C
92
'
3
0 100
0
9.4
.. _ ..-._
._..".
.
I
.......
.I"_ 9 .
.
..
._ . ......... ...___ ._
*
*
M C A Channels
i
Mo Isotopes
(b)
20 Scans
98 ln
+ c
96 95
-
. .-
* .
-
92
. . *-.
*_*
*-e
, . e
.
..
M C A Channels Resolution comparison of ( a ) single scan vs. ( b ) 20 scan accumulations of molybdenum isotopes on the MCA. Sample: NBS 468 steel Figure 1.
must be reproducible for each scan. Because the magnet response may be influenced by its previous conditioning, it is important to put the magnet through two or three pre-scan exponential decays, encompassing the field or mass range of interest. The magnet recharge must be initiated a t the same point in the exponential field decay. This is conveniently accomplished by starting the recharge at a set time from the scan initiation. For our system, the appearance of a designated channel number on the MCA visual readout module is used, but any other accurate timing unit (a stopwatch, for example) would be suitable. After the magnet recharge is begun, a specific and reproduced time is also selected, usually 30 seconds, for reinitiation of the exponential decay. A Hall probe (2) could be used to set the field to some initial value. However, the exact initial field is not particularly important, as long as it can be returned to consistently. It is the ability to reproduce a field decay pattern which is crucial to maintaining mass resolution for multiscan accumulation. Starting the magnet recharge after a nonreproduced time interval results in a hysteresis shift of the initial field. Monitoring this field with a Hall probe and setting the initial field to some set value is not only time consuming but also ineffective unless the overall decay time is reproduced. Scan Speed. The MS-702 provides six different scanning rate time constants. For repetitive, accumulated scans in a given time period, the operator may choose to average many scans (short time constant) with relatively poor ion statistics per scan or to average fewer scans (long time constant) with improved ion statistics per scan ( I ) . Shorter time constants provide a greater number of sampling intervals for a given time period and should average effects due to electrode erosion and shielding, which may be a factor 1980
for certain samples. Comparisons of individual scans for the 60,200, and 600 time constants showed, in general, that isotopic ratios were more accurately determined with the 600 time constant than with the 200 or 60. However, time considerations may limit signal averaging use of the 600, where a full mass scan may require 20-25 minutes each. The 200 time constant, with an 8-10 minute full mass scan, offered a convenient compromise scan rate. A later modification to provide a 150 time constant has also been useful to further reduce net scan time while showing no resolution deterioration. Signal Auerager. The conversion from an analog to a digital output and subsequent integration allows peak area measurement. To match the three decade log response of the ratio amplifier, the anti-log circuit, A-D converter, and MCA readout must be linear over the same range. Calibrated input signals have been used to demonstrate this capability. Resolution. A critical consideration in accumulating multiple spectra is whether a decrease in mass resolution occurs. For our readout, each of the mass peaks must be reproducibly located within the assigned MCA channels. With careful attention to magnet conditioning, this presents no problems. Figure l a shows a portion (Mo isotopes) of a single scan. Figure l b is the same mass region after collecting 20 scans on the MCA. If the number of channels comprising each peak and the region between peaks are compared for the single us. 20 scans, it can be seen that little peak broadening occurs. Data accumulated over a period of three hours showed no significant loss of resolution in the resultant averaged spectrum. The number of channels available per mass peak depends on the mass range, mass scan rate, channel advance rate, and MCA capacity. For the 4096 channels available with our unit, it was advantageous at times to deposit and store successive spectra in 1024 or 512 channel subgroups for comparison of individual spectra. Signal averaging has been successful with even a single channel ( 7 ) per peak. For full mass scans of mass 250 to 10, all 4096 channels are normally used in our studies. Increase in Precision. Signal averaging should produce improvements in both sensitivity and precision. I t is convenient to use isotopic abundance data as a measure of precision enhancement. As the number of accumulated scans increases, the experimental isotopic abundances should more closely approach the nominal values. Molybdenum at 2,000 ppm in NBS No. 468 stainless steel provided a set of 7 isotopes. For a lower concentration and a second matrix, lead at 3 ppm in J M CA-8 standard copper gave 4 isotopes, one a t very low concentration. We were interested in any intrinsic advantage in the use of peak areas over peak heights. Attributing increased precision in averaged peak area scans over conventional peak height scans to averaging alone could be misleading. Therefore, a comparison of averaged peak heights is included in Table I showing isotopic abundances computed from increasing numbers of scans. The recorder scans are from the same scans which were averaged on the MCA, i.e., the recorder run was simultaneous with MCA accumulation. Table I shows the increase in precision which can be attained by increasing the number of accumulated scans. The total number of scans taken would be dictated by such factors as time and required precision. From these data and other similar experiments, the improvement in precision beyond 15 scans is not normally significant. The use of 510 scans may be sufficient to produce the desired precision. The averaging of peak height dat,a from recorder scans yields results which are comparable overall with the averaged area data. The ratio amplifier time constant is impor-
ANALYTICAL CHEMISTRY, VOL. 46. NO. 13, NOVEMBER 1974
Table I. Isotopic Abundance for Molybdenum Isotopes Showing the Effect of Signal Averaging from Multiple Scans (Sample: NBS No. 468 Steel) Isotopes
KO. of Scans
...
100 (9.63%)
98 (23.78%)
97 (9.46%)
(16.53%)
95 (15.72%)
8.83 12.43 9.58 9.66 9.66
23.76 20.60 22.83 23.42 23.41
12.91 7.51 8.17 8.71 9.33
18.07 16.99 16.15 16.65 15.61
9.23 12.30 9.35 9.73 9.77
21.03 20.85 21.86 22.95 22.79
12.56 6.89 7.81 8.21 8.70
21.03 17.33 17.01 16.61 16.09
96
94 (9.04%)
92 (15.84%)
% error
16.87 17.01 18.49 16.89 16.56
4.91 9.95 9.21 9.27 9.56
14.65 15.51 15.57 15.44 15.88
16.38 12.31 5.95 3.28 2.88
16.92 18.42 19.31 17.66 17.53
5.13 10.59 10.09 9.55 10.00
14.10 13.63 14.57 15.29 15.11
19.65 17.19 10.54 5.67 6.15
Av re1
MCA
1 5 10 15 20 $%order
1 5 10 15 20
Table 11. Effect of Ratio Amplifier Time Constant On Signal Averaging of Recorder Peaks and MCA Areas Time constant
Siijnal
R e c o r d e r pk R e c o r d e r pk MCA a r e a R e c o r d e r pk MCA area R e c o r d e r pk
ht ht
0 40 0
ht
0
ht
40 40
Av
Scan mode
10 10 2 2 2 2
re1 dev
individual s c a n s individual s c a n s sets of 5 s c a n s s e t s of 5 s c a n s sets of 5 s c a n s sets of 5 s c a n s
43.1 28.6 14.3 16.5 14.0 11.1
-
Table 111. Comparison of Peak Averaging for Log Recorder and MCA on Trace Lead Isotope ( z04Pbat 13.6 ppba) in CAS Copper Standard ho. of scans
10 10 20 20
Mode
Recorder MCA Recorder MCA
.
Isotopic abundance (Normal = 1.48%)
5.81 1.70 4.67 1.52
M C A Channels Figure 2. Signal to background illustration for '04Pb at 13.6 ppba in CA-8 copper standard after 20 accumulated scans
tant in comparing areas to heights. Table I1 shows that the 40-msec time constant improves averaged peak heights; peak areas are relatively uninfluenced by time constant in terms of precision. Sensitivity, however, is favored at the 0 time constant. Bingham and Elliott ( I ) have shown the advantages of peak averaging. However, it requires no small amount of effort to gather and average peak height data from 20 scans. The MCA accumulation, by contrast, is conveniently stored and rapidly integrated by the visual readout module. Increase in Sensitivity. Multiple scan averaging may also be used to enhance the sensitivity of SSMS. For isotopes of low concentration (sub-ppm), where noise may become significant relative to the signal of interest, the accumulation of successive scans produces a more favorable signal-to-noise ratio compared to single scans. The CA-8 copper standard contains lead a t 3 ppm by weight. Single scans at high sensitivity showed poor isotopic abundance data for lead. The z04Pb peak a t approximately 44 ppb by weight (or 14 ppb atomic) was particularly inconsistent. Multiple scans were then averaged in an attempt t o improve the re-
sults. Figure 2 shows the net signal above background after 20 scans were accumulated. A S/N was calculated in two different ways. Using net integrated area under the 204 peak (7,472 net counts for 9 channels) against net background (70 counts) in the same number of channels, a S/N of 107 was obtained. More appropriate to detection limits was a comparison of net peak height (2,205 counts at maximum) against background variation, (17 counts) resulting in a S/N of 130. If a S/N of 2 is taken for minimum detectability, an extrapolation can be made to a detection limit for lead of 0.21 ppb atomic under these conditions. Table 111 shows the isotopic abundances obtained for *04Pb by the MCA method and by the averaging of peak heights over 20 scans. Application to Sample Analysis. The determination of isotopic ratios can be important in itself (isotopic dilution analyses, for example), but most SSMS investigators would find MCA signal averaging of greatest value for comprehensive elemental surveys by magnetic scanning, where improved precision and accuracy should result over single scans. To simulate the analysis of real samples, NBS No. 461 steel was selected and subjected to multiple scans over
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
1981
Table IV. Precision of MCA Averaging Techniques for Examination of NBS No. 461 Steel Sample Analysis results Ext. S t d b
I n t . Std‘
lsoto e weB
208 120 98 93
75 63 60 59 52 51 32 31
Element
Pb Sn Mo Nb As (std) cu Ni co Cr
V S
P Av rel% error = 0
75As
Wt. % cancn of element
(0.003) 0.022 0.30 0.011 0.028 0.34 1.73 0.26 0.13 0.024 (0.02) 0.053
0.00298 0.0171 0.334 0.0107
0.00268 0.0201 0.295 0.0123
0.342 1.74 0.266 0.123 0.0286 0.0201 0.0469 7.0
0.313 1.97 0.308 0.125 0.0295 0.0182 0.0478 10.8
...
...
A
0.00264 0.0151 0.295 0.0940 0.0247 0.302 1.54 0.235 0.109 0,0252 0.0177 0.0415 13.1
B
0.00245 0.0184 0.270 0.0112 0.0256 0.286 1.80 0.282 0.115 0,0270 0.0166 0.0439 11.8
internal standard. * See text. c A is same day data, B is consecutive days.
the approximate mass range of 230 to 13. One set of accumulated scans was used to calculate a relative sensitivity coefficient (RSC) for each analyzed element us. an element selected as an internal standard. The RSC’s were then used to compute elemental concentrations from the data collected in a second set of scans for the same sample. Results were also calculated using one set of scans as external standard data us. another set as the “sample” to calculate direct element-to-element concentrations, eliminating the need for RSC’s. For demonstration purposes here, a single electron multiplier-amplifier sensitivity was selected to place the greatest number of sample elements into proper register ( i e . , some elements will be in saturation and others not detected a t this sensitivity). Interferences eliminated certain elements, leaving 12 elements for analysis. Table IV shows the results of two different determinations, with both internal and external standard data indicated. Ten-scan sets were taken a t a scan time constant of 150. Column A in each case represents comparison scan sets taken on the same day. Column B data were calculated from scan sets taken on consecutive days (RSC or external standard set one day, “sample” set the next day). The results using RSC’s appear somewhat better than data using external standards, but more work would be required to verify this. In addition to 75As as the internal standard, 6Wu was also used for comparison and the net % errors ranged from 8.0 to 12.8%, essentially the same as for 75As. For long term confidence in RSC’s, a reliable internal standard is required and its selection is critical. An average of about 10-12% error appears to be the limitation as opposed to average single scan variations of 25-40% for this sample type in our laboratory.
1982
BC
AC
The improvement from signal averaging is obtained a t the expense of time. For a set of 10 accumulated scans as shown in Table IV, total scan time is close to 50 minutes. It is possible that satisfactory results could be obtained in fewer accumulations, but this would be a function of sample type, sparking conditions, and precision requirements. Comprehensive elemental analysis using RSC’s can be done faster by MCA averaging than by peak switching to obtain results in the f10% error range. For a small number of elements, peak switching could be preferable. Based on our results with these manual scans, we are convinced that further data improvement can be obtained by averaging more scans of shorter duration. We are presently studying the use of an automatic cyclic scan unit to allow such accumulations over the entire mass range for broad elemental analysis or over smaller selected regions for specific elements.
CONCLUSIONS The MCA is used in our laboratories as a versatile readout unit for several types of analytical instrumentation, including neutron activation analysis, X-ray fluorescence, and mass spectrometry. The averaging capabilities of the multiscalar mode make it useful for fluctuating, unstable signals such as from SSMS. The improvement in SSMS precision and sensitivity attainable by use of this method will, in our opinion, justify the cost of the MCA, which is small compared to the SSMS equipment investment.
RECEIVEDfor review January 15, 1974. Accepted July 29, 1974. Research supported in part by EPA Grant R-801829.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 13, NOVEMBER 1974
