Spark Gap Effects on Sensitivity in Spark Source Mass Spectrometry C. W. Magee and W. W. Harrison Department of Chemistry, University of Virginia, Char/ottesvi//e,Va. 22907
Changes in sensitivity with gap width were recorded for various elemental +1 and +2 ions and molecular ions utilizing electrical detection peak switching techniques. The peak switching was accomplished magnetically rather than electrostatically through use of a Hall probe magnetic field monitor, thus allowing coverage of the entire mass range at a constant accelerating voltage. For metallic matrices, gap effects are on the order of 5-10% for most elements. In graphite matrices, however, changes in sensitivity with gap of up to a factor of three are not uncommon. A correlation between gap effects and ionization potential is also drawn. The ratio of ionization potentials between the matrix and a particular measured species appears to exert a definite influence on the behavior of sensitivity changes with gap width.
Investigators have previously recognized that the width of the spark gap could have an effect on trace elemental analysis by spark source mass spectrometry (SSMS). One of the earliest studies of these effects was made by FranZen ( I ) , who showed how gap variations could affect elemental sensitivities in an iron matrix. He attributed these effects to changes in ion energy distributions upon varying the spark gap width. Woolston and Honig (2) demonstrated that the energy distribution of ions in the beam is also affected by gap width. Effects of spark gap on sensitivity have also been suggested by other workers as tangential observations to various studies. Bingham and Elliot (3) cited gap effects in high accuracy peak switching analysis. Colby and Morrison (4) have shown gap effects for vanadium and chromium. Konishi and Nakamura (5) have described spark gap effects on ion ratios. We have routinely monitored spark gap width (6) and recently described an automatic spark gap control unit (7). The present study was undertaken to obtain a clearer understanding as to the direction and magnitude of spark gap effects in a number of different matrices.
EXPERIMENTAL Apparatus. The basic AEI MS702 mass spectrometer used for this study has been previously described (8) as has the standard electrical detection equipment (3). Modifications or additions in our laboratory to the standard system have been detailed previously 16). An automatic spark gap control unit (7) was used to monitor all selected gap widths. Additional modifications used in this study include: (a) The electrical detection monitor head and amplifier are used for both log ratio scanning and peak switching. A wiring modification alJ. Franzen, Fresenius' Z. Anal. Chem., 197, 91 (1963). J. R . Woolston and R . E. Honig, Rev. Sci. Instrum., 35, 69 (1964). R . A . Bingham and B. M. Elliot, Anal. Chem., 43, 43 (1971). B. N . ColbyandG. H . Morrison,Ana/. Chem.. 44, 1263 (1972). (5) F . Konishi and N. Nakamura, Advan. Mass Spectrom., 5, 54 (1970). (6) C. W . Magee, D . L. Donohue, and W. W. Harrison, Anal. Chem., 44, 2413 (1972). (7) C. W. Magee and W. W. Harrison,Anal. Chem., 45, 220 (1973). (8) J. P. Yuracheck, G . G . Clernena, and W. W. Harrison, Anal. Chem., 41, 1666 (1969).
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ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, M A Y 1973
lows the total ion beam integrator t o be fed from the electrical detection monitor amplifier, normally used only for log ratio scanning. In addition to eliminating the inconvenience of changing monitor heads between analysis modes, the modification also enables the ratiometer output (collector-to-total ion beam ratio) as observed on the recorder galvanometer to be used for centering the ion peaks on the collector slit for peak switch operation. Use of the ratio for precise alignment of the peaks is advantageous because its smoothing aspect allows easier signal maximization than does u8e of the collector output alone, due to the erratic nature of the ion beam. When this modification is used, the time constants of the collector amplifier and the total ion beam monitor amplifier are always matched. Such is not the case with the standard photographic detection monitor amplifier normally used for peak switching. It is conceivable that a mismatch in time constants could cause a deterioration in precision and accuracy. (b) A magnetic field monitor was constructed which greatly facilitates locating the mass peaks of interest. An inexpensive Hall probe ( F . W. Bell, Inc., Model BH206) provides an output voltage analogous to the magnetic field. No significant hysteresis effects are noted, and peak identification is normally to within zkO.1 mass unit. The probe is mounted in a non-magnetic shim of in. Plexiglas (Rohm & Haas) mounted between one magnetic pole and the side of the magnetic analyzer. The probe is outside the vacuum, readily accessible, and well into the magnetic field. Figure l a shows the position of the probe in relation to the ion flight path. Figure l b shows the associated equipment needed to construct the field monitor (power supply: Kepco model HB-525; DVM: Heath Universal Digital Instrument Model EU-805). A reference table of mass us. Hall voltage was compiled for a specific power supply current and ESA voltage. Reagents. Standard Johnson, Matthey aluminum AA1 and coppers CA4 and CC1 were used in the study of metal matrices along with an NBS standard low alloy steel sample No, 410. For the carbon matrix studies, USP grade graphite (Ultra Carbon Corporation) was used. Reagent or spectrographic grade chemicals were used to make a low impurity sample and NBS standard orchard leaves sample No. 1571 was used for a high impurity sample. High purity nitric acid ( G .F. Smith Co.) was used. Sample Preparation. The metal samples were machined to a diameter of 1.0 mm and the sparking surfaces ground flat to minimize changes in interelectrode self-shielding. After a nitric acid etch, the electrodes were mounted in the holders so the spark would strike between the flattened portions of the electrodes. For the low impurity graphite samples, reagent and spectrographic grade chemicals were used to make a multi-element standard, with all components at 200 ppm by weight except K which was present at 2000 ppm. The chemicals used for this standard solution were selected to ensure solution compatibility. The standard was mixed with graphite such that all concentrations in the electrode would be 200 ppm ( K = 2000 ppm). The slurry was freeze dried and mixed in a Wig-L-Bug (Crescent Dental Mfg. Co.) for 15 minutes. Electrodes of 5/64-in. diameter were then formed using the standard AEI sample moulding die. The polyethylene plugs were prepared with flat bottomed holes to produce a blunt electrode, one which normally erodes in such a way as to have the least adverse shielding effect on long term precision. For the high impurity graphite sample, NBS orchard leaves were dry ashed, and taken up in two drops of concentrated nitric acid to form a clear solution. This was diluted to one milliliter with distilled water and mixed with graphite in a 4 : l graphiteto-ash ratio by weight. The remainder of the preparation was identical to that of the low impurity sample. Procedure. The electrodes were loaded into the source, set at 5 mm from the No. 1 slit to minimize effects of unreproducible spark position 191, and aligned on the ion axis with the telescopic
(9) W. H. Wadlin and W. W . Harrison, Anal. Chem , 42, 1399 (1970)
‘
6 DIGIT DV M
CONSTANT CURRENT SUPPLY (0.01%)
Figure 1. (a) Placement locations of the Hall probe in the magnetic field. (b) Hall probe measurement system (adapted from a drawing of F. W. Bell, Inc., Columbus, Ohio)
optical system. All samples were pre-sparked for an appropriate interval. For the compacted graphite electrodes, this pre-spark was allowed to continue until the sparking surfaces were somewhat flattened so that changes in self-shielding during the actual analysis would be minimal. The gap width monitor was calibrated as follows. With the spark on, the electrodes were adjusted to a desired gap as determined by viewing the discharge through the calibrated 30X telescope. The rectified RF voltage as read on the gap monitor meter (7) allowed accurate calibration over a range of gap widths. This calibration procedure was carried out for each sample. Integrations were taken a t all selected gap widths for a specific mass before magnetically switching to the next mass of interest. This sequence allowed observation of any trend in gap effect for a specific species, and also allowed, before leaving that mass, a check of any integration which appeared anomalous. Spark conditions were 100 pulses per second of 25 lsecond duration a t 30 KV. All charge accumulations were 0.01 nanocoulombs, with three integrations taken at each analytical gap. This sequence was repeated two more times yielding a total of 9 integrations per gap width per mass.
RESULTS AND DISCUSSION This investigation was carried out using electrical detection peak switching techniques, allowing collection of more precise data (about 5% RSD) than is generally possible with photographic techniques. Thus it is now possible to measure by electrical detection small fluctuations in sensitivity which are not normally resolved from the fluctuations inherent in the photographic detection methods. In addition to more favorable detection methods, the close control of experimental parameters (9, 10) also leads to improved precision. In this context, work in our laboratories (11) has indicated the need for an awareness of spark gap widths effects and their possibly significant contribution to the overall precision and accuracy of an analysis. Gap effects may still be large enough to have importance even when less precise methods than peak switching are used. We have observed changes in sensitivity from 50 to 200% over not unduly large changes in gap. Normal precision figures for both photographic detection (12) and log-ratio scanning (3) methods are on the order of *30% RSD. This is within the range of error which may be incurred by inadequate spark gap control. ( 1 0 ) C. A .
Evans. R. J . Guidoboni, and F. D. Leipziger, Appl. Spectrosc.,
24, 85 (1970).
(11) C. W. Magee and W . W . Harrison, Twentieth Annual Conference on Mass Spectrometry and Allied Topics, Dallas, Texas, June 1972. (12) 0. H . Howard, Anal. Chem., 44, 1482 (1972).
Magnetic Field Monitor. Hall probes can be used to monitor the magnetic field in SSMS (13). The precision of the magnetic field monitor is in part due to the placement of the Hall probe well into the magnetic field. To obtain unambiguous mass definition, the probe must be placed in the magnetic field such that it intercepts a flux density truly representative of that which is deflecting the ions. Positioning of the probe outside the analyzer in this study allowed sampling of the field a t sequential locations over the magnet pole face in order to determine the most representative flux density, thereby reducing hysteresis effects to a very low level. Referring back to Figure la, the largest error (poorest reproducibility in locating mass peaks) was observed when the probe was placed in position 1. Hysteresis was smaller a t position 2 , but the measurements were still sufficiently imprecise to preclude positive mass identification. In position 3, hysteresis proved to be quite small, allowing 1 0 . 1 mass unit resolution. Even with a hysteresis-free probe, accurate peak identification would still not be possible, were it not for very precise measurements of the associated probe parameters. The transverse voltage developed in the Hall probe is governed by the following equation:
H ,= h ( i x B ) where H, is the Hall voltage, k is a constant which includes geometry, B is the magnetic field strength, and i is the control current, normally considered a vector quantity by nature of its polarity. To obtain a reproducible Hall voltage a t a given field strength, the current must be extremely stable. While our supply had good short term (hours) stability of 10.0170, normal long term drift necessitated measurement (with a 6 digit DVM) and subsequent fine adjustment of the current daily. A very accurate measurement and control of the ESA voltage is also necessary to hold the mass-Hall voltage relationship constant. This voltage was also precisely monitored on the DVM and adjusted as necessary. With these precautions, adjustment of the magnetic field to a calibrated Hall voltage can bring a desired mass to register on the collector slit to within fO.l amu. A brief fine tuning of the magnet current by the operator (using the log-ratio for signal maximization) is then usually necessary before peak switch integrations are begun. Selection of Sample Matrices. Graphite is a versatile matrix for use with non-conducting samples, and finds considerable application for use with biologicals (14), drugs and narcotics (15) and pollution samples (16). Graphite is available in high purity with very small particle size, thereby enhancing electrode homogenity. It also possesses excellent binding characteristics which allow its mixture with relatively large amounts of sample (often 1:1), while still forming a strong electrode. These properties make possible a rather extreme situation, whereby a “graphite” sample electrode may have an actual carbon content range from 99.99% (for analysis of relatively pure water samples) to less than 50%. Of interest was whether elements in graphite matrices of widely varying composition exhibit differences in spark gap effects. Both a high impurity sample and a low impurity sample were investigated. The high impurity electrodes contained about 20% added material, not an unrealistic amount for an ashed (13) G . H. Morrison, 6 .N. Colby, and J. R. Roth, Anal. Chem., 44, 1203 (1972). (14) W. W. Harrison and G . G. Clemena, Clin. Chim. Acta.. 36, 485 ( 1 972). (15) W. W. Harrison and M . A . Ryan, unpublished w o r k , University of Virginia. 1972. ( 1 6 ) R. Brown and P. G . T. Vossen, Anal. Chem.. 42, 1820 (1970).
ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, MAY 1973
053
1000
I 56Fe*z-
800
400 2,
200
i
+1 54
AI2
+
Fe
I
OdO
30
50
40
SPARK GAP
-
in m i c r o m e t e r s
35
25
15
Figure 2. Effect of spark gap width on elemental response in an aluminum matrix. Sample J M - A A 1
SPARK GAP
45
in micrometers
Figure 4. Effect of spark gap width on elemental response in a stainless steel matrix. NBS Sample 410 14001
6oo; 500 c >
2
1200-
400-
k
5
1000.
3002001
> k
800-
>
E
600-
Lo
"
z W
;0
25 SPARK GAP
30
35
Figure 3. Effect of spark gap width on elemental response in a copper matrix. Sample JM-CC1 biological or a geological sample. The low impurity electrodes were about 8000 ppm total. Aluminum, copper, and steel samples were selected as readily available standards of generally recognized homogeneity. They allowed comparison of certain elements not only from one metal to another but also with respect t c graphite. Gap Effects on Matrix Ions. The effects of intentional spark gap variation are shown in the data plots, Figures 2-8. It should be emphasized that these effects are not transient. The effects were reproduced within the same day and a t intervals extending over a period of weeks. Close control of other experimental parameters has indicated that, for our instrument at least, these are quite real and consistent phenomena. There is little doubt that these curve shapes would differ from one laboratory to another, given the concomitant non-uniform experimental parameters. While we cannot adequately explain the observed effects, they do bear discussion. The ordinate on all the data plots is purely relative. No effort has been made to maintain the same multiplier gain to allow for sensitivity comparisons from one species to another on the same plot. The data are only intended to demonstrate the general trend in sensitivity change with gap width for each specific element. An examination of the data plots, Figures 2-8, shows that changes in the matrix sensitivity may occur with gap width variation. The matrix element for a rather pure material might be expected to be relatively free of spark gap effects. Such is the case with aluminum (Figure 2) and copper (Figure 3) 054
400-
in micrometers
ANALYTICAL CHEMISTRY, VOL. 45, NO. 6, M A Y 1973
*OOi 0'
80
40 SPARK GAP
120
in micrometers
Figure 5. Effect of spark gap width on elemental response in a low impurity graphite sample and the low impurity graphite (Figure 5 ) . However, as the impurity level is increased, the matrix element shows a sensitivity change with gap width. This is illustrated in the low alloy steel (Figure 4) and the high impurity graphite sample (Figure 7 ) . In these cases, the electrode can no longer be thought of as pure matrix material. The impurities appear to contribute to the marked change in sensitivities with gap width variation. The ion beam monitor samples the mass unresolved ion beam, terminating the charge accumulation after a pre-set number of matrix and impurity ions have passed into the magnetic analyzer. If the gap is not changed and the other experimental parameters remain fixed, this matrix-to-impurity ratio will remain generally constant. However, as the gap is varied, the ratio appears to change, and, if the impurity level is large enough, a change in matrix sensitivity will be observed. This is the case to a small extent with the steel in Figure 4, (total impurity about 4%), and to a larger extent in the high impurity graphite in Figure 7 , (total impurity about 20%). Gap Effects on Non-Matrix Ions. The data, Figures 2-8, also show gap effects for trace constituents, those species most often of interest in SSMS. In some cases, sensitivity changes are small, but in other cases, they can be quite large, sometimes as much as a factor of 3.
1400J 12001000-
1000-
>
zk 8 0 0 -
800-
E
v7
u 600cn
2 2ooG 0
40
80
SPARK GAP in micrometers Figure 6. Effect of spark gap width on +2 ions ions in a low impurity graphite sample
0
120
60
120
SPARK GAP
and molecular
Considering first the metal matrix samples, the elemental +1 trace constituents generally do not show a very large sensitivity dependence on gap width. changes of this magnitude would ordinarily not affect results obtained by photographic techniques, or log-ratio scanning, but might have significance when peak switching. Arsenic and phosphorous sensitivities show effects sufficiently large as to be reflected in any of the three detection methods. Although of little analytical interest, the + 2 ions also tend to decrease significantly with gap. The graphite matrix samples tend to show a very large sensitivity dependence on gap width, with changes of 200% being common. In these cases, a gap variation of only a relatively few micrometers would be enough to make peak switching analysis with 5% precision impossible. Gross changes such as these would obviously affect photographic detection, and log-ratio scanning as well. Ionization Potential Correlation. There was some suggestion from the data that the ionization potential (I.P.) of the impurity element relative to the I.P. of the matrix might have some correlation to the observed gap effects. The data are not entirely consistent, but elements with an I.P. close to that of the matrix showed generally less severe gap effects than did those elements with an I.P. greatly divergent from the matrix. This seems to be in keeping with certain relationships proposed previously by investigators with regard to the calculation of relative sensitivities. It was shown (1 7) that relative sensitivities are dependent on the ratio of the ionization potential for a specific element to that of the reference standard (matrix, or internal standard impurity). Farrar (18) indicates that when the elements and standards have similar ionization potentials, a dependence of sensitivity on I.P. is not obvious. Such is the case with some of our data showing gap width effects on sensitivity. However, when the I.P.'s of the trace constituent and the matrix are considerably different, a large dependence of sensitivity on gap width is apparent. This same ionization potential effect was noted by Willardson and Socha in a study of relative sensitivities (19). They formulated an expression based entirely on ionization potential which, in addition, depended on whether the I.P.'s of the impurity elements were greater than or less than that of the matrix. Further analogy to these studies is hazardous considering that spark gap is a (17) Harry Farrar i V , in "Trace Analysis by Mass Spectrometry." Arthur J. Ahearn, Ed., Academic Press, New York, N. Y., 1972, pp 274-9. (18) Ibid.. p 278. (19) Ibid.. p 276.
Effect of spark gap high impurity graphite sample Figure 7.
180
in micrometers
width
on elemental response in a
14001
1000-
600>-
t:
$
600.
vl
z
3
400-
200,#'
systematic variable in the present study and not in those referenced above, but it does provide a basis for the observed irregularities in response between ions of considerably different I.P. Considering the metal matrix samples, the transition metal +1 ion sensitivities show gap width effects which average about *5%. As seen in Table I, the first I.P.'s for these elements are very close to those of the matrix elements aluminum, copper, and iron. As previously noted, As and P in both steel and copper tend to be much more sensitive to gap variation. These elements have a considerably higher I.P. (Table I ) than the containing matrix. Doubly-charged species, also with a higher I.P. than the matrix, tend to respond with the same general trend as do As and P, decreasing in sensitivity as the gap is widened. The matrix dimer in all samples (Figures 2-4, 6, 8) is seen to increase markedly with gap. In our experience, molecular ions are invariably produced in greater abundance at wider gaps, where higher voltage is required to break down the inter-electrode distance. The graphite samples present a somewhat different situation than the metal samples, considering the high I.P. of the carbon matrix. In the low impurity sample (Figure 5 ) , the sensitivities of Mn, Fe, and P b show a large dependence on gap width. It is tempting to point out the much lower I.P.'s of these elements relative to that of the maANALYTICAL C H E M I S T R Y , VOL. 45, NO. 6 , MAY 1973 * 855
Table I. First and Second Ionization Potentials (20) of Selected Elements Analyzed in This Study by SSMS Ionization potential in volts Element
AI
As Ba C
Ca
co Cr
cu Fe
Ga In K Mn
I
5.96 10.5 5.19 11.2 6.09 7.81 6.74 7.68 7.83 5.97 5.76 4.32 7.41 10.9 7.38 4.16 7.30 5.67 12.5 (27) 14.4
II 18.7 20.1 9.95 24.3 11.82 17.3 16.6 20.34 16.16 20.43 18.79 31.7 15.7 19.6 14.9 27.4 14.5 10.98
the monitor in that it measures the mass unresolved ion beam. Thus, changes in sensitivity of the matrix will be reflected in the sensitivity variations of the other species. Arsenic, from its I.P. relative to that of carbon, might be expected to be insensitive to gap. Instead, as seen in Figure 7, it increases considerably. Other +1 ion sensitivities rise much more than would be anticipated, up to 400%. The matrix leveling effect is also seen in Figure 8 for the + 2 ions. The Ba+2 plot might be expected to remain rather level from its I.P. but instead rises. The unresolved doublet of + 2 ions from Sr and Rb might be expected to drop due to the very high second I.P. of Rb, but instead it rises slightly. Production of molecular ions again increases with gap width.
trix, which would fall in line with the previous data. However, K and Ca, which also have I.P. much smaller than the carbon matrix, do not follow this trend. Doubly charged ions again show a gap width effect which may be dependent on I.P. Sr+2 has an I.P. close to that of the matrix and is relatively insensitive to gap above a width of 80 microns. In and Ga have very high second I.P.'s and show a definite decrease in sensitivity with gap. The molecular ion sensitivities again rise sharply with gap. In the high impurity graphite sample, any effects which I.P. differences might produce are apparently swamped out by the 20% change in the matrix element sensitivity. This swamping out effect may be directly attributable to
CONCLUSION In the process of systematically studying parameter variation in SSMS, we have observed that sensitivities of many species change with spark gap width. These changes are reproducible on both a long and short term basis, and seem to have some dependence on the ratio of the ionization potential of the matrix to that of the species being measured. However, the final effects of the gap width on sensitivity must be considered the product of a number of contributing factors, of which ionization potential may be one. Differences in spark gap width produce changes in spark breakdown voltage, spark current, electrode temperature, interelectrode self-shielding, ion energy distribution, and ion extraction efficiency, all of which probably play some part in determining how an individual species changes sensitivity with gap width. In an experiment such as this, the effects contributed from each variable cannot be resolved. What has been shown and is most important is that many elements do change sensitivity with gap, and that in attempting to collect precise and accurate data with the best electrical detection methods, less than satisfactory results may be obtained without the recognition and attention to control that the spark gap parameter demands.
(20) "Handbook of Chemistry and Physics," Chemical Rubber Co., Cleveland, Ohio, 47th ed., p €65. (21) J. Drowart, R. P. Burns, G. De Maria, and M . G. Inghram, d. Chem. Phys.. 31, 1131 (1959).
Received for review October 5 , 1972. Accepted December 11, 1972. This study was supported by research grants from EPA and LEAA.
P Pb Rb Sn Sr c5
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Photometric Determination of Trace Bromide in Alkali Metal Chlorides E. F. Joy, J. D. Bonn, and A. J. Barnard, Jr.
Analytical Services, J. T. Baker Chemical Co., Phillipsburg, N.J. 08865
High-purity alkali metal halides serve as standards and find use in nutrition and materials science. The determination of trace bromide in such salts has been studied. The rosaniline method has excellent sensitivity and involves hypochlorite oxidation of bromide to bromate, conversion to bromine, bromination of the dye, and photometric measurements. Previous work has not noted that chloride markedly enhances the bromide color development. Consequently, for chloride salts, use of a standard curve developed in water and an aqueous blank yields high results. The procedures presented circumvent these 856
ANALYTICAL C H E M I S T R Y , VOL. 45, NO. 6, M A Y 1973
difficulties and take advantage of this chloride enhancement. Bromide-free sodium chloride is added to the weighed sample, a bromide standard, and a blank so that the chloride concentration in each solution is in the favorable range. A chloride salt low in bromide can itself serve to provide the chloride requirement. The procedures allow the determination of 0.2 to 200 ppm of bromide in lithium, potassium, and sodium chlorides.
In recent years this company has been working toward a line of high-purity chemicals distinguished by extensive
