rapidly or more rapidly than the original apparatus and recoveries after concentration with the nitrogen stream are as good as those reported previously with the glass ebullator (I). Thus, the gas-stream ebullator does not cause appreciable loss of the pesticides. The gas-stream ebullator may be particularly helpful when an inert or reducing atmosphere is needed during evaporation, since the appropriate gas can be used for ebullition. Unlike the original glass ebullator, the new ebullators are not easily broken, but they do have the disadvantage of requiring a source of gas supply. In a modification of the gas-stream ebullator that seemed to provide an added convenience, a stainless steel rod was silver-
soldered alongside the tip with its end extending about 1.7 cm beyond the gas exit. With this arrangement, the rod was allowed to rest on the bottom of the distilling tube, holding the gas exit 1.7 cm from the tube bottom. The need for adjusting each gas exit level by clipping the tubing to the column was thus eliminated.
RECEIVED for review June 30, 1972. Accepted July 31, 1972. Mention of a proprietary product is for identification only and does not constitute an endorsement of this product by the U.S. Department of Agriculture.
Advantages of Dual Electrode Mounts in Spark Source Mass Spectrometry Using Electrical Detection C. W. Magee, D. L. Donohue, and W. W. Harrison Department of Chemistry, University of Virginia, Charlottesville, Va. 22903
THECONVENTIONAL ANALYSIS technique in spark source mass spectrometry (SSMS) has involved photographic readout and the use of a single internal standard against which concentrations of the other elements in the sample are calculated. Relative sensitivity factors may be used to improve accuracy, but the general quantitative capabilities (120-30x) of the overall SSMS method did not warrent the use of multiple external standards, considering the large numbers of elements often determined. External standards were shown to be useful when considering a smaller number of elements (I, 2) particularly with careful parameter control (3). The recently developed electrical detection readout ( 4 , 5 ) has greatly increased the precision and accuracy of SSMS to a point where external standards are now necessary to fully utilize the peak switching integration mode, where a precision of better than 5 can be reached. However, slight changes in sparking parameters or source conditions may create significant effects on the integration readout value. Therefore, when in the usual manner one takes a series of integrations on elements in the standard electrodes, followed by breaking vacuum and loading the analytical samples for similar integrations, it is with the assumption that all conditions are similar over what may be a quite protracted period of time. It would obviously be more ideal to be able to switch between standard and analyte pairs more rapidly and, beyond that, to be able to come back to check the relative response of the two several times, if desired, over the period of the analysis. This investigation concerns the use of dual electrode mounts to study the advantages of using standard and analyte electrode pairs in the source at the same time. Data are presented for several different types of samples. (1) J. Kai and M. Miki, Mitsubishi Denki Lab. Rep., 5, 175 (1964); as described in “Mass Spectrometric Analysis of Solids,” A. J. Ahearn, Ed., Elsevier, London, 1966, p 105. (2) J. Franzen and K. D. Schuy, Z . Anal. Chem., 225,295 (1967). (3) W. W. Harrison and G. G. Clemena, ANAL.CHEW,44, 940 (1972). (4) C. A. Evans, R. J. Guidoboni, and F. D. Leipziger. Appl. Sprctrosc., 24, 85 (1970). (5) R . A. Bingham and R. M. Elliott, ANAL.CHEM., 43,43 (1971).
EXPERIMENTAL
Apparatus. The basic mass spectrometer and experimental parameters have been previously described (6). The standard AEI electrical detection system ( 5 ) has been added and used in the peak switch integration mode for all data in this study. Appropriate magnet current, acceleration voltage, electron multiplier voltage, and sensitivity settings were determined for each isotope measured. Modifications or additions to the standard system include : A spark gap monitor pickup coil mounted externally in a source port feeds an R F voltage to a Tektronix Model R564-B oscilloscope for display. This is used in conjunction with the AEI Autospark manual control to maintain a constant spark gap. A modification of a Kennicott telescope mount (7) is used with a 30 x optical system, cross-hairs, and calibrated reticle to carefully align the electrodes on the ion beam axis and to view the discharge during sparking. Rack and pinion focusing of the shallow depth of field optical system allows measurement of spark-to-No. 1-slit distance on a calibrated scale. Glass ports are used both above and below the spark discharge with a light source in each to allow lineup of the standard and analyte electrodes. The standard spark shield has been modified to provide an additional small light entrance from the top (for the overhead light source) and a 1-inch square front viewing window. The latter allows full view of both electrode pairs and is covered with a replaceable glass plate held in place by tabs cut into the spark shield. A continuously variable (0-0.012-inch) V-shaped collector slit is substituted for the standard two-position slit. Digital integration readout is accomplished with a Heath Model EU-805 Universal Digital Instrument which allows voltage to frequency conversion and subsequent counting in the Events Counting mode. Electrode Materials. Copper samples are from JohnsonMatthey, steels and orchard leaves from NBS. USP grade graphite (Ultra-Carbon Corporation) was used to form electrodes with the orchard leaves ash residue. (6) J. P. Yuracheck, G. G. Clemena. and W. W. Harrison, rbid., 41, 1666 (1969). , (7) P. R. Kennicott, General Electric Corporation, Schenectady, N.Y., personal communication, 1972.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
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Table I. Analysis of NBS 444 Steel us. NBS 442 as a Standard, Using the Dual Electrode Holders V, Cr, Mn, Co, Ni, Cu, Trial ppm % Z ppm % ppm 1 2 3 4 5 6
1043 1043 1043
19.1 19.8 19.2 20.1 20.0
1000
1043
19.7 Av 1034 NBScert. 12OOs 20.5 3.9% Re1 error 14% a Certified as 0.12p7,. 8 Certified as 0.22%. Certified a s 0 . 2 4 Z .
4.75 4.57 4.64 4.61 4.68
2340 2340 2300 2340 2320
4.65 4.62 0.75%
2320 22Wb 5.9%
9.84 9.96 9.84 10.01 9.70 9.70 9.85 10.1 2.5%
2510 2510 2480 2480
2495 24OOc 0.4Z
Table 11. Analysis of Johnson-Maithey C B Copper ~ us. CEO as a Standard (Values in ppm, by weight) N .i.. co Mn Fe Trial 1 2 3 4
Av J.-M. cert. Re1 error
16.7 16.5 16.5 16.0 16.3 15 9.3%
26.5 25.7 25.5 25.7 25.8 29 10.8%
17.4 17.4 17.0 16.7 17.1 18 5.0%
24.7 23.6 23.6 23.6 23.9 26 9.6%
Table III. Analysis of NBS Orchard Leaves, Sample No. 1571, Using a Duplicate Electrode Pair as Standard (Values in ppm, by weight) Trial Fe Mn Cr CU Zn 1 2 3 4 5 6 7 8 9
344 335 344 335 347 340 352 347
343 Av NBS value 300+ 20 Re1 error 14 * Not certified.
108 109 107 1 IO 107 105 101
107 109 107
(Imp
7.0%
2.44 2.26 2.39 2.48 2.35 2.30 2.44 2.30 2.35
13.4 12.6 12.6 13.3 14.1 12.8 13.1 13.3
2.36 (2.3). 2.6%
13.1 12 + 2 7.3%
24.9 25.8 23.7 24.4 25.8 24.9 23.3 24.1 24.1 24.5 (28p 12%
Dual Electrode Mount. The designed holder for the dual electrode pair approach is shown in Figure 1. Construction of the current units is from stainless steel and tantalum. Tantalum is used for the shields and clamps, those parts surrounding the spark discharge. Tantalum tension springs are used to mount compacted samples to avoid breakage, while set screws are preferred to hold metal samples securely. The design allows use of the standard AEI electrode holder A broad shield on one holder extends between the SuDDort. .. two electrode pairs and prevents direct deposition of material from one set onto the other while sparking. Geometry o f t h e holders is such that when one pair is sparking, the other pair is sufficiently separated to prevent sparking. Scale drawings and construction plans are available from the authors on request. Analytical Procedure. After loading the standard and electrode pairs in the holders, the standard pair is adjusted with the micromanipulators at 6 mm from the No. 1 slit and on the ion beam axis as set to the cross hairs in the viewing optics. Both electrode pairs may be viewed through 2414
Figure 1. Dual electrode holders for spark source mass spectrometry the soark shield window. but only the pair in the sparking position is visible through the optical system. The source is evacuated and, upon sparking, the analytical ion is brought to incidence on the collector slit by manipulation of the magnetic and electrostatic controls. After a brief prespark, several consecutive integrations are taken on the standard. The measurement signal for each mass of interest consists of the integrated current accumulated while a preset charge is collected a t the monitor. The analyte pair is then adjusted to the exact sparking position as used for the standard and briefly presparked, the total changeover time requiring less than one minute by an experienced operator. After a satisfactory number of integrations are obtained for the analyte pair, another set of integrations is taken from the standard and again for the analyte as a further check, if desired. Following subsequent determinations for other elements in the sample, standard-to-analyte integration ratios may be conveniently and simply rechecked if there is some uncertainty concerning sample homogeneity o r if there is a question of a possible change in an experimental variable. Sparking voltage has been at 30 kV, with a pulse length of 25 to 50 rsec and a repetition rate of 30 to 100 pulses-persecond. A 0.01-nC total monitor charge was used for each integration. These conditions provide adequate sensitivity while preventing serious sample erosion and changes in electrode shielding effects.
RESULTS AND DISCUSSION Cross Contamination. The potential problem of cross contamination between the standard and analyte electrode pairs was first explored. Normally, the two are selected to be reasonably close together in concentration, if possible, but the large number of elements often determined by SSMS, coupled with the relatively small number of available stdndards, may make this impossible for certain elements and samples. The wide dynamic range over which the electron multiplier is linear may also lead to the use of standard and analyte pairs of significantly different concentration. The tantalum shield arrangement effectively isolates the electrode pairs on a line-of-sight basis, but there w d S still the question of indirect surface pickup. Repeated attempts to induce such cross contamination under reasonable analytical conditions have been negative. Even when a standard approximately 3 0 x the concentration of analyte is used, initial integrations on the latter have shown no tendency to be higher than ones. In addition to the shielding
employed, a further mitigating factor is the ability to use mild sparking conditions in the integration mode, thus reducing sample sputtering and distribution through the Source. The only case we have seen of pickup involved the sparking of pure magnesium electrodes as one pair, followed by monitoring trace levels of magnesium in a pair of aluminum electrodes
ANALYTICAL CHEMISTRY, VOL. 44,NO. 14, DECEMBER 1972
mounted in the other position. Even there a brief pre-spark brought the magnesium response down to its intrinsic level. Analytical Results. Tables I, 11, and I11 show data obtained on three different sample types using the dual electrode pair arrangement. Integrations were taken in the order shown under each element. Analyte integrations were alternated with those of the standard for repetitive comparison. The data are from consecutive analyses of the three materials and represent truly typical obtained data. Precision difficulties may appear more often in the compacted samples if extreme care is not taken in the preparation. The small amount of sample consumed puts severe demands on sample homogeneity. Because no other standard biological sample was available to compare to the NBS orchard leaves, duplicate electrode pairs were prepared from the orchard leaves with one pair assigned as the standard and the other as the analyte pair. Advantages of Dual Electrode Pairs. The quantitative analysis difficulties with SSMS in the past are well known. Electrical detection and integration now allow quantitative respectability for SSMS, but our experience has been that extremely close parameter control is necessary. It is not at all with slight unusual to see integration changes of 30-50 changes in electrode alignment and shielding. Changes in magnet current, acceleration-€SA voltages, or vacuum conditions may also produce time dependent differences. With dual electrode holders, the time between sampling of the standard and analyte is quite $mall and rechecks of any particular element us. the standard can be made at any time before the total analysis is completed. In our laboratory,
this has resulted in more precise and accurate data than using a standard pair to take integrations for a series of elements, followed by breaking vacuum, replacement with the analyte pair, and then obtaining integration values for each element in the analytical sample. This is not to suggest that accurate and precise data cannot be obtained by the conventional single electrode pair approach (3, but data can be obtained more rapidly with the dual-electrode holders and, of greater significance, more frequent cross checks can be made between sample and standard. For applications which d o not involve peak switch integrations, the dual holders are also useful. In photographic readout using an internal standard, overall turn around time for a series of samples can be cut in half by mounting two sets of analyte electrodes in each loading. The same would be true for the magnetic scanning survey mode (8). With respect to the latter, we have used the dual holders to mount a standard and analyte pair and scanned each set, thus using external standards rather than one internal standard. Specifically, the recently issued NBS orchard leaves standard has many elements at reasonable concentrations to compare t o drug samples, such as marihuana and opium, which are of interest in our laboratory. RECEIVED for review March 24, 1972. Accepted June 29, 1972. Thii study was supported by EPA Grant 16020 HGI and LEAA Grant 154. (8) R. Brown and P. G. T. Vossen, ANAL.CHEM.: 42,1820 (1970).
Economical Fluidized Drier for Gas Chromatography Packing Material P. C. Goodley and Marshall Gordon Department of Chemistry, Murray State Unicersitjq, Murray, K J . 42071
THEEFFICIENCY of gas chromatographic columns is a n important concern to the gas chromatographer who runs routine analysis. After many sample injections (especially where trace harmful materials might be included in the samples), the columns tend to lose resolution and become less efficient. Responding to this situation, the GC analyst can either purchase new columns or resort to their construction, which is more economical. For a n institution such as the authors’ laboratory, the more economical way is often the better. Also, it offers more versatility when one is choosing a column for a specific analysis (any length or diameter can be readily available in a short time). The construction and preparation of the column often presents t o the chromatographer the problem of being able to duplicate the previous column giving its exact analysis and retention times for specific compounds. This problem is present often, even though all steps involved in the preparation of the column were duplicated from past records. From past experience in our laboratory, this difficulty can be eliminated by the use of the fluidized drier ( I ) . (1) R. F. Kruppa, R. S. Henly, and D. L. Smead, ANAL.~ H E M . , 39, 851 (19h7).
FLUIDIZER BASE PLATE
A
MATERIAL AL
DRILL 8 TAP FOR PIPE
g
$DRILL
L
a
-r I
SEC. AA
Figure 1. Fluidizer base plate
ANALYTICAL CHEMISTRY, VOL. 44, NO. 14, DECEMBER 1972
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