valuable for the analysis of wide boiling-range and high boiling mixtures. We observed no evidence of degradation in the analysis of some thermally less stable compounds which could not be chromatographed through a packed column without some decomposition. Not all parameters have been investigated to increase the range and applicability of this column. Desty, e.g., has pointed out the advantages of using hydrogen as carrier gas
for high-speed analysis; and in a number of applications, the use of this carrier gas may be of great value (3). ACKNOWLEDGMENT Thanks are due to E. J. Gallegos and R. M. Teeter for spectrometry to support SOme of the concluby sions in this work.
RECEIVED for review April 27, 1970. Accepted July 27, 1970
Effects of Spark Position in Spark-Source Mass Spectrometry W. H. Wadlinl and W. W. Harrison Department of’Chemistry, University of Virginia, Charlottesoille, Va. 22901 The position of the RF spark with respect to the entrance slit in spark-source mass spectrometry has been shown to affect resolution, line intensity, and relative sensitivity. Lines are shown quantitatively to be sharper at longer spark-to-slit distances. Line intensities also vary with this parameter. Relative sensitivity factors in copper, aluminum, and steel matrices are dependent upon the spark position. Ion profiles as a function of Y-deflector voltage are shown for +1 through +5 ions of copper.
As A GENERAL RULE, elemental sensitivities for spark-source mass spectrometry are considered to be approximately equal. This fortuitously allows the use of a single internal standard element of known concentration against which the concentration of all other elements in the sample may be computed. For best quantitative results, however, standard samples are used to obtain a relative sensitivity factor (RSF) for each element from the ratio of the “apparent” concentration, as determined by comparison to the internal standard element, to the “true” concentration of a particular element. These corrections may then be applied to the same elements in unknown samples. It is recognized that the RSF term will be affected to some degree by experimental parameters, such as spark voltage and sample matrix, Franzen and Hintenberger ( I ) showed that variation of ion accelerating potential could produce gross changes in elemental sensitivities. Halliday et al. ( 2 ) further demonstrated this as well as the effect of other spark source conditions, such as spark repetition rate and pulse length. Our interest in these reports arose from the need to determine RSF’s for a number of elements in different matrices. Every effort was made to maintain all conditions constant, including the use of an Ion Beam Chopper (3) to eliminate the need to vary spark repetition rate and pulse length. However, it was noted that unless very precise and reproducible electrode positioning was achieved, the value of the other precautions was 1 Present address, Department of Chemistry, Randolph-Macon College, Ashland, Va. 23005
(1) J. Franzen and H. Hintenberger, 2.Naturforsch. A , 18, 397
(1963).
(2) J. S. Halliday, P. Swift, and W. A. Wolstenholme, “Advances in Mass Spectrometry,” Vol. 3, p 143, London, 1964. (3) P. F. S. Jackson, J. Whitehead, and P. G. T. Vossen, ANAL, CHEM.,39, 1737 (1967).
negated. It was recently learned that Evans (4) has observed similar effects. The object of this investigation was to determine the effects produced by variation of the position of the R F spark with respect to the ion entrance slit of the spectrometer. EXPERIMENTAL Apparatus. A description of the equipment and experimental conditions has been given previously (5). Reagents. Electrodes, 1 mm X 10 mm, were prepared from standard aluminum and copper rods (Johnson, Matthey, and Co. Inc., London, England) and from NBS stainless steel rods. High purity acids (G. F. Smith Chemical Co., Columbus, Ohio) were used for a cleansing etch before prespark. Measurement of Electrode Position. In order to measure the distance from the electrodes to the accelerating slit, a small measuring device was fabricated. A mm scale was scribed upon a thin metal strip, which was cut to a shoulder on one end, allowing it to fit into the 1.6-mm diameter accelerating slit. To measure distances, this tool was placed against the slit, and one electrode was adjusted to lie on the scribe mark corresponding to the desired distance. Distances could be measured in this way to approximately 4~0.2mm. The second electrode was then brought to the same distance, directly below the first, and both were adjusted vertically and laterally so that a 1-mm overlap of the electrodes produced a spark on the beam axis. Location of the spark on the beam axis was achieved by sighting through a magnifying lens and adjusting the electrode gap to lie on the line of sight through the centers of the first two slits. The range of distance adjustment extends from 3 to 15 mm, limited by the electrode holders coming in contact with the accelerating slit holder in the near direction and the spark shield in the outer. RESULTS AND DISCUSSION
Highest ion flux is normally obtained by sparking the sample electrodes rather close to the accelerating slit. Critical parameters on our instrument (focus voltage, accelerating voltage, and Y-deflection setting) have been optimized for best response at a spark-to-slit distance of 4 mm on the ion beam axis. Results obtained at carefully selected increments from (4) C. A. Evans, Kennicott Copper Co., Lexington, Mass., 1970, personal communication. ( 5 ) J. P. Yurachek, G . G. Clemena, and W. W. Harrison, ANAL. 41, 1666 (1969). CHEM.,
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
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160-
120-
021 80-
6Spark-to-s'it,m m
40-
Figure 2. Effect of spark-to-slit distance on relative sensitivity factors in a copper matrix
h 2 4 6 8 10 12 14 Spark-to-slit,m m
Figure 1. Effect of spark-to-slit distance on line sharpness (see definition in text) this position indicate that, at least for our particular instrument, resolution, line intensity, and relative sensitivity are to varying degrees dependent upon the net distance of the spark from the slit. Resolution. The change in resolution (sharpness of lines) is noted when equal charge exposures are obtained at different spark-to-slit distances. The sharpness of lines is measured here as the ratio of peak height to its half-width (width at half of the peak height). For calculation comparisons, it is convenient to use the expression: Sharpness = (height) 2/area which is equivalent to the above definition, given the approximation that the peaks are triangular. Line sharpness is shown plotted as a function of spark-to-slit distance in Figure 1 for chromium and silver. This increase in resolution with distance from the entrance slit was confirmed for other elements in other metal samples, as well as for biological samples in a graphite matrix. To ensure that the resolution effects were not caused by the smaller ion flux rates at the longer distances from the slit, the Ion Beam Chopper was used to control, as best could be done, the ion flux for one sample to be essentially constant with distance, The same increase in line sharpness was noted as in the case of the variable ion flux. Another sample was shot at a constant slit-to-spark distance but with the ion flux varied over a larger range with the IBC for a series of exposures. No significant resolution differences were observed. Thus, the ion flux rate does not seem at least to be the major cause of the resolution increase with distance. To account for this phenomenon, the beam emerging from the ESA exit slit must become more well defined in either angular dispersion or energy distribution or both as the spark-toslit distance is increased. Greater angular dispersion of the beam would cause the lines to be more diffuse, and a greater energy spread would have the same net effect by ions having slightly different trajectories in the magnetic field. The beam defining slit (0.05 x 1.0 mm) is approximately 30 mm beyond the accelerating slit (1.6-mm diameter), allowing the net distance of the spark to the beam defining slit to be varied from about 34 mm to 45 mm, possibly changing somewhat the angular dispersion of the beam entering the ESA. The energy distribution of the ions entering the ESA may also be affected by the distance of the spark from the accelerating slit. The net energy of an ion in the ESA may be considered to result from the energy produced by the acceleration slit system, the initial energy obtained from the spark dis1400
charge, and any changes in energy prior to transmission through the entrance slit. The last factor might be dependent upon spark position, in which case a variation in the parameter could affect the selection of ion energies in the source that would subsequently be in register in the ESA. Users of spark-source mass spectrometers report at times what appear to be individual idiosyncrasies of particular instruments. Thus, we are not suggesting this as a general effect. However, after carrying out a complete set of focus adjustments several times to obtain the best spectra at our nominal 4-nm spark-to-slit distance, the increase in line sharpness has been reproducibly present at longer distances and has, in fact, been analytically useful. Intensity. It was noted for a series of replicate exposures as a function of spark-to-slit distance that the line intensities as determined by ball and disk integration of the ion-plate lines, often varied considerably in a systematic and reproducible manner. The further the spark is removed from the accelerating slit, the lower the total beam intensity at the slit will be, since an ever smaller angular sector of the ions emitted from the spark is subtended. However, a decrease in total beam intensity should not directly cause a change in the integrated line intensities, because individual exposures are determined by total charge accumulation as read from an electronic integrator which samples the ion beam continuously. It should only mean that a longer time period would be required to reach a particular charge, as measured in nanocoulombs. If we assume that the energy of the ions has not been altered, the decrease in plate darkening observed in our experiments would indicate that fewer ions are reaching the photoplate for the same indicated monitor charge accumulation. A decrease in intensity of a given trace spectral line might result from a decrease in intensity of that component relative to the total beam flux, primarily the matrix element. However, studies of the matrix ions have shown that even their line intensities have decreased with distance. If we consider the accelerating slit as a sampling point in the plasma of ions surrounding the spark, the observed decrease of intensities with distance corresponds to a decay of trace element ion concentrations. This could be accounted for by collisions of ions with one another, to form species which are not available for analysis (such as neutral species, or ions deflected out of register). Another possibility is a change in shape of the ion beam in the ESA as the spark-to-slit distance is varied. The electrometer monitor plate at the exit of the ESA determines ion beam current by intercepting approximately 50% of the incident ion beam (6), allowing the re( 6 ) Associated Electrical Industries, Ltd. ; Manchester, England,
Instruction Manual, MS-702.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1OOr
I r o n matrix
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Aluminum m a t r i x
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Figure 3. Effect of spark-to-slit distance on relative sensitivity factors in iron and aluminum matrices
mainder to pass through a 1.0 x 6.1 mm slit to the magnetic analyzer. To account for the effect observed, variations of the distance parameter may cause proportionately more charge to fall upon the slit plate to accumulate measured exposure and less to fall on the slit opening. This would yield an integrator indication of full exposure when actually a smaller number of ions had reached the photoplate detector. It does not seem unlikely that conditions could be produced which would alter this effect, such as the Y-deflection voltage variation, to be discussed in another section. Depending upon the Y-deflection setting, the intensities have been observed to increase or decrease. At the Y-deflector setting of maximum monitor reading, our intensities do decrease with distance. Relative Sensitivity. The variation of spark-to-slit distance was also shown to affect the relative sensitivity factor calculated against a selected standard element. We were interested in the compilation of RSF's in a number of different matrices and found a position dependent effect in each case. The variation of RSF for several elements in a copper matrix is shown in Figure 2. This variation may be due to the rates of decay of concentration in the plasma being somewhat different for each element, causing the RSF for the various elements to vary relative to one another as a function of distance.
d
-12.m
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Figure 4. Portion of a mass spectrum showing the increase in ionic radii as the deflection setting is made more positive
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DEFLECTOR SETTING Figure 5. Integrated intensity beam profiles as a function of deflector setting for 1 to +5 T u ions Fourth order polynomial curve fitting program used for data analysis, Calcomp plotter display
------L-12.m Ed Y"
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Figure 6. Beam profiles of T u 5 + as a function of deflector setting and spark-to-slitdistance Data treatment as in Figure 5
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
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Our data suggest that elements of similar chemical nature may also be similar in RSF’s. In the copper matrix (Figure 2), iron and cobalt have nearly the same sensitivities at all distances and the nickel sensitivity is the same as for iron and cobalt at small distances. In this case, the assumption of a common RSF for iron, cobalt, and nickel at small distances would be reasonable. Silicon, however, deviates considerably from the sensitivity observed for iron, cobalt, nickel. Figure 3 shows the relative sensitivities of several elements in an iron matrix, wherein copper sensitivity is very nearly that of cobalt and nickel. RSF’s in an aluminum matrix are also shown. Beam Profiles. It appeared that a study of energy distributions in the ion beam might be useful in regard to the variation in line intensity and sharpness effects previously described. If the ion beam were not homogeneous in energy distribution as the beam expanded during transit, the results obtained from the photoplate would be critically dependent upon the particular portion of the beam allowed to exit the slit system. The MS-702 has a Y-deflector control located in the beam suppress assembly. This control is achieved using a horizontal slit (3.2-mm diameter) consisting of two half plates, the lower one earthed and the upper connected to a variable voltage source. Application of the appropriate voltage to the unearthed plate allows deflection of the ion beam as it passes toward the ESA entrance slit. Thus, a beam profile may be obtained by taking a series of exposures as a function of Y-deflector setting. The data presented for these settings are in the units printed on the instrument control, a full range of -10 to +lo, corresponding to a linear 250 V actually applied to the unvariation of -250 V to earthed half plate. The dispersion of the beam in the ESA may not be due entirely to the energy distribution, as the angular dispersion might also make a contribution. The energy distribution does seem to be a significant factor, however, considering the spectra shown in Figure 4, which represents equal exposures at increasingly more positive Y-deflector settings. The position shifts of the isotopes show that at progressively higher Y-deflector settings, larger radii are defined for the ions, indicating that higher energy ions are now coming into register at the ESA exit slit. This effect is most noticeable in the high mass lines.
+
1402
Figure 5 shows the beam profiles of +1 to $5 63Cuions obtained from a pure copper sample. The shape of these profiles varies as the charge increases. The singly charged ion has a nearly constant intensity across the entire beam, but the multiply charged ions show a pronounced minimum in the center of the beam. This is possibly caused by the expansion of the beam due to mutual electrostatic repulsion of the ions. The higher charged ions experience a greater repulsion and are thus more concentrated in the outer edges of the beam. There also appears to be a slight shift of the minima toward higher energy for the higher charged ions. Beam profiles of Wu5+ at 4 , 7 , and 14 mm in Figure 6 show increasingly deeper minima with greater spark-to-slit distance. The beam appears to be more heterogeneous in energy the farther the spark is removed from the ESA, possibly indicating that a large part of the beam expansion occurs in the small distance traversed in the source, prior to the 20-kV acceleration. The minimum in the center relates to the earlier discussion of line intensities as a function of the spark-to-slit distance. This deepening of the central minimum with increasing distance could cause the observed effect of decreasing integrated intensity with distance when the center of the beam is focused on the ESA exit slit, because high rate of charge accumulation on the slit plate would not be representative, with changing spark position, of the intensity of the fraction of the beam which passes through the slit and on to the photoplate. In Figure 6, the decrease of line sensitivity with distance is not a general effect, but, depends upon the Y-deflector setting. In fact, subsequent experiments studying the effect of spark distance on line intensity at various Y-deflector settings show complex functions of spark distance in the outer portions of the beam. The standard Y-deflector setting of +2.0 was selected for normal operation to give the highest monitor current, which is equivalent to aiming the beam so that it is centered on the monitor slit. Investigators using a different Y-deflector setting could well see different effects of line density with spark-to-slit distance than those observed in this study. It is, however, important that the various effects of these parameters be recognized and defined, if necessary. RECEIVED for review April 6, 1970. Accepted July 24, 1970
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
