or branch point was the only difference between the pre dicted and the correct structure. In one case (No. 6) the molecular formula was incorrect and hence there was no chance of a correct structure prediction. However, in this case the results were still useful, because the main structure (a substituted benzene ring) was correctly predicted even though the substituted groups were incorrect. In the final case (No. 5 ) the computational results were so contradictory as to make very little sense. Hence, no structure prediction was attempted in this case. It should be noted that even contradictory data of this sort is useful in that it constitutes a warning not to attempt to use it for prediction. Table V summarizes the results of testing the C H O N training set. In this case the results were even more successful than with the CH training set. All ten molecular formulas were correctly determined. Furthermore, the correct structure was uniquely determined in eight cases (No. 1-4,6-8, and 10) and was correctly predicted t o be one of the two structures in the other two cases (No. 5 and 9). The library use of the weight vectors is well demonstrated by this example. Sixtyfive trained weight vectors (each with one more component than a mass spectrum) have a very high degree of success in determining the formula and structure of compounds from a training set of 150. Hence the storage of this information will require less than 45z of the file space required by the actual mass spectra. Furthermore, the information retrieval amounts t o relatively simple calculations which might be performed on a desk calculator i n lieu of a computer. Table VI shows the results of the trial with the C H O N prediction set. This was the most difficult test, and, as expected, produced the poorest results. Four unique molecular formulas were correctly predicted; another was predicted as one of two, one more was predicted with a single wrong coefficient, three had two wrong coefficients, and one was so contradictory that prediction was not attempted. Of the structures, one (No. 3) was correctly predicted as one of two
possibilities, two others (No. 2 and 4) were classed as near misses, three others (No. 5 , 8, and 10) produced useful structural conclusions which were not sufficient for complete structural determination, three (No. 5, 7, and 9) were sufficiently contradictory that no attempt was made at structural prediction, and two (No. 1 and 6) were predicted quite incorrectly. Hence, using only the weight vectors trained from 150 compounds, prediction of C H O N structures had a relatively limited degree of success. It should be noted, however, that one is rarely limited t o only the low resolution mass spectrum for structural determination. Hence, other basic information such as a melting point, or boiling point, or N M R or infrared spectrum might be sufficient to distinguish among the various possibilities. For example, compound No. 3 of the C H O N prediction set was deduced to be either sec-butyl alcohol or tert-butyl alcohol, which can be easily distinguished in that their boiling points are 99.5 and 82.8 “C.,respectively. This work is not meant to imply that low resolution mass spectrometry can, by itself, solve complex structure problems for previously unknown compounds. It does show, however, that for known compounds, excellent results with a high degree of confidence may be simply obtained and, furthermore, useful information may be predicted for unknown compounds. In the case of unknown compounds, library search systems are of little use other than to establish that the compound is not included in the library. Hence, any information on prediction by the learning machine method is a n important gain. Studies are under way to improve predictive ability. Furthermore, as was demonstrated in earlier work, predictive ability often increases markedly with the size of the training set. Therefore weight vectors trained with a much larger training set may have notably greater success at prediction.
RECEIVED for review March 18, 1970. Accepted July 23, 1970. Research supported by the National Science Foundation.
~~
~
_
Versatile Short Capillary Column in Gas Chromatography T. H. GOUW,I. M.
Whittemore, and R. E. Jentoft Chevron Research Company, Richmond, Calif. 94802
A 10-meter by 0.010-inch capillary column coated with OV-101 has proved to be a very versatile column for gas chromatography. It has been used to analyze heavy petroleum fractions with end points above 1000 O F and high boiling waxes up to n-C,,, yet it can easily resolve isobutane from n-butane at lower temperatures. It is the ideal column for rapid analyses. In conjunction with temperature programming, it is especially effective for the analysis of wide boilingrange and high boiling mixtures. A surprising uality is its exceptional ability to resolve isomers o? high molecular weight compounds. The column has been used to separate the diastereomers of 1,3,5-triphenyldecane, to resolve anthracene from phenanthrene, and to separate 1,2-benzopyrene from 3,4-benzopyrene. Another application of this column is its use for simulated distillation of wide boiling-range mixtures. By temperature programming the column, it i s possible to resolve C4 from C5 and yet obtain a linear relation between the retention time and the boiling points of the eluting n-hydrocarbons between Clo and Cd2. This run takes less than 40 minutes. 1394
CAPILLARY COLUMNS have found wide application in high resolution gas chromatography because their low pressure drop makes it feasible to use very long columns with resultant large numbers of theoretical plates. Columns up t o a thousand meters and longer in length with several million plates have been reported ( I ) . The typical capillary column in general use is around 100 meters long with approximately 5 x 104 plates. The emphasis in gas chromatography is usually on the separation of compounds which are difficult to resolve, and a short capillary column appears to be contrary to general practice. Compared t o the large amount of published information on long capillary columns, only a few papers have treated the merits of a short capillary column. Most of these publications are now several years old and were mainly concerned (1) R. P. W. Scott, “Gas Chromatography 1964,” A. Goldup, Ed., The Institute of Petroleum, London, 1965.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1 9 7 0
_
Figure 1. Synthetic mixture of n-hydrocarbons from n-butane to n-dotetracontane; pi = 10 psig; flow rate = 3.5 ml of He/min; T = 10-350 “C at 10 OC/min. Note the linear relationship between boiling point and retention time for all compounds eluting after n-decane
with the use of these columns for ultrahigh speed analysis (2-5). In their tour de force, Desty et a/. ( 4 ) managed to separate a 15-component Ca-C7hydrocarbon mixture in slightly less than two seconds using a column only 120 cm long. Another application which has been reported for these columns is in the analysis of wide boiling-range mixtures, but the number of references in the literature is very limited (2, 6, 7). A theoretical comparison of open tubular columns of different lengths concludes that a 100-meter column would always give more favorable results and can always match the actual performance of a shorter column in efficiency, speed, and sample capacity. The calculations were based on equivalent a-HETP curves for both long and short columns (8). I n a more recent paper, however, Grob and G r o b indicated that the efficiency of a capillary column per unit length increases with a decrease in length (9). (2) L. S . Ettre, “Open Tubular Columns,” Plenum Press, New York, 1965, p 48. (3) D. H. Desty, Admn. Chrornatogr., 1, 214 (1965). (4) D. H. Desty, A Goldup, and W. T. Swanton, “Gas Chromatonrauhy,” N. Bremner et a/.,. Ed.,. Academic Press, New York, 1962,‘~105. (5) D. H. Destv. A. GolduD. and B. H. F. Whvman. J. Znst. Petrol.. 45, 287 (1969). (6) J. R. P. Marco, “GC Newsletter,” Perkin-Elmer, Norwalk, Conn., 1 (3), l(1964). (7) D. H. Desty, Planta Med. (Stuttgart), Supplement 1967, p 25. (8) L. S . Ettre, J . Gas Chromatogr., 6,404 (1968). (9) K. Grob and G. Grob, J. Chromatogr. Sci., 7 , 515 (1969). 1
,
_
I
.
I
Notwithstanding the theoretical drawbacks, we find from our own practical experience that these short capillary columns have a much wider potential and applicability than have been recognized by the large majority of gas chromatographers. These columns are much more versatile than one would expect, and we have used them in a wide variety of applications. In conjunction with temperature programming, for example, these columns are very valuable for the analysis of wide boiling-range and high boiling mixtures. EXPERIMENTAL
Columns. To attain good results, a necessary prerequisite is to obtain a good-quality liquid coating. This aspect is very important in these short capillary columns. Columns were prepared from 0.010-inch “Chromat 1D”grade capillary tubing supplied by Handy and Harman Co., Norristown, Pa. The tubing was first cleaned by consecutive flushing with CH2Cl2, acetone, water, concentrated HNOI (until clear), water, NHdOH (until clear), N-methyl pyrrolidone, acetone, and CH2C12. This cleaning procedure also successfully reclaims used columns to new condition. [See also Teranishi et al. (IO).] Packing was carried out dynamically by coating once with 10 ml of a 5 OV-101 solution in CH2C12at 40 psig with a 6-meter by 0.010-inch pigtail at the end of the column. All the liquid from the packing procedure was collected, the solvent driven off, and the residue weighed in order t o calculate the coating thickness. (10) T. R. Mon, R. R. Forrey, and R. Teranishi, J. Gas Chromatogr., 5 , 497 (1967).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1395
N
c
0c"
0' e m
W
u" K
0K"
I 0
1
I
I
I
I
1
I
1
I
I
I
1
2
3
4
5 6 Minutes
7
8
9
10
11
Figure 2. Bareco microcrystalline wax; n-tetracontane added as internal standard; T = 280-350 "C at 20 "C/min The above procedure gives films about 0.5-micron thick. The column length in all our work described here is 10 meters. Gas Chromatographs. Two Perkin-Elmer 900 and one Perkin-Elmer 226 gas chromatographs with flame ionization detectors were used in these studies. The latter instrument is a single-column chromatograph where the column is coiled in a pancake configuration. The Perkin-Elmer 900's are regular dual-column instruments. Make-up helium gas was used in these chromatographs. RESULTS AND DISCUSSION
Figure 1 shows the chromatogram of a synthetic mixture of C4to C4?n-paraffins. Butane (bp, 31 OF) and pentane are resolved, while n-Cl2 (bp, 999 O F ) is eluted in 32 minutes before the end of the temperature-programmed section of the run. Note that the temperature was programmed from 10-350 "C, a range that is usually available as a standard accessory in most good commercial gas chromatographs. The short capillary column has the capability of giving good resolution while handling mixtures boiling over a range of about 1000 O F . Previously, very wide boiling-range mixtures could only be analyzed by the use of multiple columns (11, 12) or by the use of very wide range temperature programming (13). Figure 1 also shows the relation between the retention times and the boiling points of the eluted n-hydrocarbons. Between n-Clo and n-Cd2,the relation is essentially linear. This makes the output more amenable to digital computer operation for simulated distillation work because the correlation can be handled by fitting a polynomial function through the first few points and determining the best straight line through the remaining data points (14). Simulated distillation is usually carried out with short packed columns. Under condi(11) G. F. Harrison, P. Knight, R. P. Kelley, and M. T. Heath, "Gas Chromatography 1958," D. H. Desty, Ed., Academic Press, New York, 1958. (12) J. C. Winters, Oil GnsJ., 58 (24), 138 (1960). (13) C. Merritt, Jr., J. T. Walsh, D. A. Forss, P. Angelini, and S. M. Swift, ANAL.CHEM.,36, 1503 (1964). (14) T. H. Gouw, Ruth L. Hinkins, and R. E. Jentoft, J. Chrornatogr., 28, 219 (1967).
1396
pi
=
20 psig; flow rate
=
8.8 ml of He/min;
tions where butane and pentane are resolved, the maximum column temperature in a standard temperature-programmed run would be reached at the time when only a hydrocarbon in the CZO to C ~range O is being eluted. The retention times of the subsequently eluting n-hydrocarbons would then deviate considerably from the linear relation because of the isothermal operation of the column oven. Under conditions where the retention time-boiling point relation is linear up to 1000 OF for a packed column, there is seldom any resolution below n-Cs. This short capillary column is, therefore, especially applicable to the analysis of simulated distillation curves of wide boiling-range mixtures, such as petroleum crudes. The short capillary column is quantitatively superior t o packed columns in the analysis of heavy petroleum stocks. This type of material, with end points in the range of 8001000 O F + , is usually chromatographed on packed columns with low liquid phase loadings. A typical column might be 3 meters by l/s-inch outside diameter, packed with 0.5 SE-30 on 8OjlOO mesh Chromosorb G-HP. Using such a column, we chromatographed a heavy lube oil (260 SUS at 210 OF), programming from 150-300°C a t 6 OC/minute with a flow rate of 25 mliminute. n-Eicosane was added as a n internal standard. Only 40% of the lube oil was eluted from this column. When the same sample was chromatographed using the short OV-101 capillary column, 90% elution was obtained. This same stock could only be distilled t o about a 45-50% overhead in a 3-foot by '/*-inch spinning band distillation column a t 1-torr condenser pressure. The high gas velocity, which may be achieved at relatively low pressure drop, makes the column suitable also for the analysis of still higher boiling compounds. Figure 2 shows the chromatogram of a Bareco microcrystalline wax with a 190-195 O F melting range, where n-Clohas been added as internal standard. With an average linear gas velocity of around 165 cmisecond and a final isothermal operation a t 350 OC,the n-C5*peak is discernible in about 11 minutes after the injection. The column bleed is not excessive and could have been balanced out in a dual-column chromatograph.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
n, i I!
It is quite simple to find another column which would yield the equivalent or improved resolution over that shown in Figure 3. It would be quite difficult, however, to find a column which would at the same time have the other qualities of this short capillary column. A surprising aspect of this column is its ability to separate isomers of high molecular weight compounds. The following hydrocarbon,
10
*
CH,(CHJ,-CH-
1
CHZ-CH-CH2-CHZ
688 \
Figure 3. Mixed hexanes. T = 0-100 "C at 13 "C/min; 1 = isobutane; 2 = n-butane; 3 = isopentane; 4 = n-pentane; 5 = 2.2 dimethylbutane; 6 = cyclopentane; 7 = 2,3-dimethylbutane; 8 = 2-methylpentane; 9 = 3-methylpentane; 10 = n-hexane; 11 = methylcyclopentane; 12 = 2,2-dimethylpentane; 13 = cyclohexane; 14-17 = 2-methylhexane, 2,3-dimethylpentane, 3-methylpentaneY lc3-, lt3-, and lt2-dimethylc yclopentane. The short capillary column is not only applicable to the analysis of wide-range and higher boiling compounds, but it can also show excellent resolution for low boiling hydrocarbons. Figure 3 shows a chromatogram of mixed hexanes. The analysis was carried out on a Perkin-Elmer 900 gas chromatograph with an average helium gas velocity of 18 cm/ second and the temperature programmed from 0-100 OC at 13 OC/minute. Make-up helium was added to the end of the column at a rate of 20 ml/minute. Isobutane and n-butane are resolved in three minutes; 2-methylpentane and 3-methylpentane are almost completely separated in only five minutes.
\
1,3,5-triphenyldecane, with a molecular weight of 370, has two asymmetric C atoms with no plane of symmetry. Hence, there are four stereoisomers of this hydrocarbon, consisting of two sets of enantiomers which are diastereomers of each other. A mixture of these compounds was isolated by supercritical fluid chromatography from a mixture of polystyrene oligomers (15). Figure 4 shows that isothermal analysis of this fraction results in two resolved peaks. Since this column will obviously not separate the enantiomeric pairs, these two peaks must represent the diastereomeric structures of this hydrocarbon. The possibility that these two peaks have different molecular weights is ruled out because the mass spectrum of the sample showed only one parent peak in the region around m/e = 370. Another application in the analysis of higher molecular weight isomers is in the separation and identification of polynuclear aromatic hydrocarbons. This has been the subject of numerous studies (16-18), especially in association with prob(15) R.E.Jentoft and T. H. Gouw,Polym. Lett., 7,811 (1969). (16) Katsuya Sato, Masami Matsui, and Nobuo Ikekawa, Bunseki Kagaku, 17, 639 (1968). (17) A. U. Solo and S . W. Pelletier, Chem. Znd. (London), 1961, 1755. (18) J. R. Wilmshurst, J. Chromatogr., 17, 50 (1965).
I
1
Y L
I
80
I
I
90 100 T i m e , M i n u t e s ---c
I
110
Figure 4. Separation of the diastereomers of lY3,5-triphenyldecane; isothermal at 125 "C ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1397
m-6P5O
1
50
I
,
40
30
4-
I
20
10
0
Time, Minutes
Figure 5. Separation of phenanthrene, anthracene, fluoranthene, l,Zbenzopyrene, 3,4benzopyrene, and perylene. Perkin Elmer 226 gas chromatograph; 10-meter by 0.01-inch stainless steel capillary column coated with OV-101; 15 minutes isothermal at 120 "C and temperature programmed at 3.75 "C/min to 220 "C; helium inlet pressure 10 psig, outlet flow rate 1.7 ml/min; sample size ~5 pl; split ratio 200:l amounts of polynuclear aromatic hydrocarbons detected approximately 10-8 g each
T i m e : M i n ut e s __c
Figure 6. Separation of five- and six-ring polyphenyl ethers. P = phenyl group; 0 = ether linkage; pi = 20 psig; flow rate = 8.8 ml/min; T = 150-300 "Cat 13 "C/min
lems of pollution control (19). Because of the high boiling points of most of these compounds, high separating temperatures are necessary to elute these hydrocarbons in reasonable times. The thermally stable liquid phases which can be employed in gas-liquid chromatography for this purpose are generally isotropic in nature. Under these conditions, it is difficult t o resolve 1,2-benzopyrene (bp, 493 "C) from 3,4benzopyrene (bp, 495 "C), so that long efficient columns are necessary t o carry out this separation (18, 20). Another approach is to use a n inorganic salt as the stationary phase (21). The analytical separation of these isomers is very important since these two compounds have markedly different physiological activities. Another difficult pair to resolve is anthracene (bp, 339.9 "C) and phenanthrene (bp, 336.8 "C). On common isotropic phases, such as silicone oils, silicone gum rubber, silicone grease, Apiezon L, and neopentyl glycol succinate, in the temperature range between 175-300 "C,the relative retention is close t o unity; and very little or no separation is observed in packed columns (22). To separate these two compounds, use has been made of very long capillary columns, such as the one employed by Grant with 5.105plates (23) or the one used by Cantuti et ai. with 4.104 plates (24). A slightly more successful approach is obtained by using gas-solid chromatography. Although no separation is observed using NaOHmodified alumina (25), separation is possible on potassium antimonate, potassium carbonate (213,or graphitized carbon black as the substrate. Figure 5 shows a chromatogram which we have obtained on a synthetic mixture of anthracene, phenanthrene, fluoranthene, perylene, 1,2-benzopyrene, and 3,4-benzopyrene. The two
isomeric pairs of interest are clearly resolved. In addition, perylene is also well separated from the benzopyrenes. The observed chromatogram is exceptional, especially if one considers that this analysis has been carried out on a very short column. It must be emphasized at this point that this separation has been demonstrated on a synthetic mixture of these compounds. Application of this technique to actual samples from pollution studies may not yield the same resolution as depicted here because of the expected greater complexity of real samples. Figure 6 shows a chromatogram of a polyphenyl ether, a liquid substrate often used in gas chromatography. The separation of these compounds by gas chromatography has been reported earlier by Smith and Gudzinowicz (27). Very high column temperatures, up to 400 "C, had to be employed t o elute these heavy compounds. We carried out this analysis with a n average flow rate of 8.8 ml/minute and with the temperature programmed from 150 to 300 "C at 13 "C/minute. The bulk of the sample consists of isomers of bis(phenoxyphenoxyphenyl)ether, the compound with six phenyl groups and five ether linkages (6P50). The main impurities are the isomers of bis(phenoxyphenoxy)benzene, the compound with five phenyl groups and four ether linkages (5P40). The chromatogram shows the large number of positional isomers which can be formed because the phenoxy groups can be linked together in the ortho, meta, or para position. Mass spectrometry has confirmed in this example that all compounds under 6P50 have the same molecular weight of 538 and all compounds with 5P40 have the same molecular weight of 446. The peak tentatively identified as 6 P 4 0 has a molecular weight of 522 when isolated and analyzed by mass spectrometry. The peak identified as m-6P50 is bis[m-(m-phenoxyphenoxy)phenyl]ether.
(19) D. Hoffmann and E. L. Wynder, "Air Pollution," Arthur C. Stern, Ed., Academic Press, New York, 1968, p 187. (20) J. H. Beeson and R. E. Pecsar, ANAL.CHEM.,41, 1978 (1969). (21) B. H. Gump, J . Chromatogr. Sci., 7 , 755 (1969). (22) S. T. Sie and G. W. A. Rijnders, Separ. Sci., 2, 770 (1967). (23) D. W. Grant, Anal. Abstr., 8, 3824 (1969). (24) V. Cantuti, G. P. Cartoni, A. Liberti, and A. G. Torri, J. Chromatogr., 17, 60 (1965). (25) A. Zane, Tobacco Sci., 12,23 (May 23, 1968). (26) A. Zane, J . Chromatogr., 38, 130 (1968).
SUMMARY
1398
The short capillary column is a highly versatile column applicable to a wide variety of assignments. It is particularly
(27) W. R. Smith and B. J. Gudzinowicz, 1969 International Gas Chromatography Symposium, Michigan State University, ISA Proceedings, 1961, p 111.
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
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 t o 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 t o 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. CHEM.,41, 1666 (1969).
ANALYTICAL CHEMISTRY, VOL. 42, NO. 12, OCTOBER 1970
1399
