Table 11. Retention Times Relatively -/-BHC, p,p-DDE, and p,p-DDT 7-BHC
Methyl ethers of 2,4,6-Trichlorophenol
+
OV-17 7 rnin at 160 o c
QF1 SF 96 9 min at 150 OC
0.096
2,3,4,6-Tetrachlorophenol
0.25
Pentachlorophenol
0.64 1 min at 200 o c
3 min at 180 o c
2.93
3.02
1.24 1.83
0.94 1.29
1.70 2.95
2.65 4 .OO
2.93
3.27
pp-DDE Methyl esters of 2,4-Dichlorophenoxy acetic acid 2,4,5-Trichlorophenoxy acetic acid 2-Chloroethyl esters of 2,4-Dichlorophenoxy acetic acid 2,4,5-Trichlorophenoxy acetic acid Methyl ether of 2-hydroxi-2 ',4,4 '-Trichlorodiphenyl ether
muscle liver gills 2. Pike muscle liver gills 3. Eel muscle 4. Ide muscle 5. Burbot muscle 6. Roach muscle
Dimethyl ether of Hexachlorophen
In extractable fat
12 23 19
1200 200 650
9.5 25 26 0.35 6.2 7.8 6.0
1100
160 710 1.9 350 1100
580
Water sample, taken two days after the discharge, contained 0.35 ng/ml.
SF 96 pp-DDT
In fresh tissue
1. Pike
0.14 0.32 0.70
r-BHC
Table 111. Levels of Pentachlorophenol (ppm) in Water and Fish Found Dead in a Contaminated Rivera
2 min at 200 o c
3.42
ier derivatives of the other chlorinated compounds are to be analyzed, the 1% SF 96 is preferable, because of short retention times a t relatively low temperatures. The use of internal standard in the gas chromatographic determination is recommended. Y-BHC (lindane), DDE, or DDT are suitable; the choice depends on the kind of substances to be studied. The relative retention times of the derivatives corresponding to these internal standards are shown in Table 11. The detection limits for the differ-
ent substances in 10 grams of an organic tissue or soil are 0.1-1 ppb and for one liter of a water sample 0.001--0.1 PPb. The method was tested on fish found dead in a river in the south of Sweden where a discharge of pentachlorophenol was suspected. Also a water sample, taken two days later was analyzed. The levels of pentachlorophenol were in accordance with those found to be lethal for the eel ( 1 ) . The results are shown in Table 111.
ACKNOWLEDGMENT The author is greatly indebted to Soren Jensen, head of the laboratory, for his valuable advice and constructive criticism of the manuscript. Received for review August 6, 1973. Accepted October 30, 1973.
I CORRESPONDENCE Hollow Cathode Ion Source for Solids Mass Spectrometry Sir: Hollow cathode discharge tubes are well known as sharp line spectral sources which have long been useful in physics ( I , 2) and more recently as source lamps for atomic absorption spectrometry. Demountable hollow cathode tubes may be utilized as atomic emission sources for trace element analysis (3-6) by placing the solid sample in the cathode or by evaporating a solution sample in the cathode cavity. The surface sputtering action of the hollow cathode discharge yields excellent elemental sensitivity (7) by moni(1) H. Schuler and H.Goiinow, 2. Physik, 93,611 (1935). (2) A . G.Shenstone. Trans. RoyalSoc. (London), A235, 195 (1936). (3) J. R . McNaily. G. R . Harrison, and E. Rowe, J. Opt. SOC.Amer. 37, 93 (1947) (4) G.Milazzo and N. Sopranzi. Appl. Spectrosc., 21,256 (1967). (5) W. W. Harrison and N. J. Prakash, Anal. Chim. Acta. 49, 151 (1970). (6) N. J. Prakash and W . W. Harrison, Anal. Chim. Acta, 53, 421 (1971 ) . (7) W. W. Harrison and E. H. Daughtrey, Anal. Chim. Acta, 65, 35 (1973).
toring the excited atomic species. A large neutral population is, of course, also formed.' It is further known, from ion microprobe mass spectrometry, that ion bombardment of a surface can produce secondary ions of the target species. Coburn and Kay (8, 9 ) have shown that both dc and rf discharges may be used in a planar diode sputtering system to determine major constituents in a surface film. Our initial intent in interfacing a hollow cathode tube to a mass spectrometer was to gain a better understanding of the reactions and plasma conditions which exist in our hollow cathode emission source. Subsequent experiments have shown that the hollow cathode discharge may offer advantages as a mass spectrometry surface ionization source for solids, such as metals and alloys. It can also be used to provide survey analysis of the trace elements in a (8) J. W. Coburn, Rev. Sci. lnsfrum., 41,1219 (1970) (9)J. W. Coburn and Eric Kay, Appl. Phys. Lett., 19,350 (1971).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 3, MARCH 1974
461
RESULTS AND DISCUSSION
Figure 1. Hollow cathode ion source, showing coupling mode to the MS-702 mass spectrometer
gas inlet; 2. stainless steel body; 3, Lava insulating cathode block: 4, hollow cathode: 5, anode plate; 6, cathode contact screw ( - ) ; 7, anode contact ( + ) ; 8, HCIS mount plate; 9, No 2 ion slit; 10, No. 3 ion slit; 11, slit housing: 12, insulators. and 13, mass spectrometer body. Note: Items 9-13 are partof the MS-702. 1,
solution deposited on the surface of the cathode and dried to a residual film. This is a preliminary report of the characteristics of such a device as a solids mass spectrometry source.
EXPERIMENTAL Source Modifications. An AEI MS-702 spark source mass spectrometer with electrical detection was used with previously described modifications (10, 11). A high speed pumping system is required t o provide differential pumping, allowing hollow cathode pressures of 1 to 5 Torr while maintaining source pressures of lo-* to Torr. Therefore, the standard AEI source was replaced by a larger source, designed and constructed in our machine shops, to accommodate a 1200 l./sec oil diffusion pump, cold trap-baffle, and gate valve (all from Varian, Vacuum Division, Palo Alto, Calif.). Gas input flows to the source of up to 30 ml/min may be used. The leads for the dc potential to produce the hollow cathode discharge were taken into the source through the internal rf spark connections. A Lambda Model 4'1 power supply was floated a t the acceleration potential and isolated by a transformer from earth. The discharge gas entered through a special source port and a needle metering valve. Hollow Cathode Tube. A drawing of the hollow cathode device, mounted on the mass spectrometer ion axis is shown in Figure 1. The gas enters a t 1, flows through a 0.04-in. hole drilled in the cathode 4, and the discharge occurs between the cathode cavity and the anode disk 5 . The cathode-anode distance may be varied by movement of the cathode. A 0.012-in. X 0.04-in. opening in the center of the anode acts as the entrance or No. 1 slit for the mass spectrometer. Dowel pins are used to align and mount the hollow cathode assembly to a conventional MS-702 kidney plate for accurate alignment on the ion axis. The cathode is insulated by a Lava block 3 from the anode. The dc electrical connections are made at 6 (negative) through a contact screw to the cathode and at 7' (positive) to the anode through the kidney plate. The stainless steel body 2 and gas inlet 1 are held a t the acceleration potential impressed upon the anode slit. This was limited to 10 kV by breakdown of the power supply isolation transformer. (10) C. W. Magee. D. L. Donohue, and W. W . Harrison, Anal. Chem., 44, 2413 (1972). (11) C. W. Mageeand W. W. Harrison,Ana/. Chem., 45,220 (1973).
462
Initial studies with the hollow cathode ion source (HCIS) showed that it exhibited discharge characteristics similar to previous optical HC tubes built in our laboratories. Breakdown of the gas, ususally argon, occurred at 200-400 volts, depending upon the pressure and cathodeanode distance. Source currents of up to 150 mA yielded an extremely stable ion beam of high intensity. The HCIS as yet has no cooling elements and becomes quite warm on extended operation, but this seems to have no effect on the discharge stability or the resultant mass spectrum, other than the removal of background hydrocarbon contribut ions. A mass spectrum may be obtained from a metal machined to act as a cathode, or the metal may be placed as a liner in the cavity of some other conducting cathode. Figure 2 shows a low gain mass spectrum of an argon discharge onto a pure copper cathode. Argon species, particularly 40Ar+ and 40ArH+, constitute the bulk of the ion beam. A t the conditions employed, only the copper matrix + be isotopes are observed. The ratio of 40Ar+ to 6 3 C ~ can varied between 20/1 and 10/1 by variation of experimental parameters. Doubly charged ions appeared only for the argon major isotope. Coburn and Kay (9) have suggested a Penning ionization process, Ar* M M + Ar e-, as a significant source of ions. In the high pressure, complex plasma region, other contributions such as electron impact, chemical ionization, and charge transfer may also be important ionizationroutes. A more complex sample, a stainless steel machined into a cathode, was next studied. A low gain scan of the major constituents is shown in Figure 3, indicating chromium, iron, and nickel along with the major argon species. A spark source mass spectrum of the same stainless material was run and compared to NBS stainless 442 to determine concentrations of the minor and trace elements in the HCIS sample. Figure 4 shows a HCIS mass spectrum illustrating several of these elements, indicating the sensitivity attainable a t moderate multiplier gains. Concentrations (atomic) were lead at 6 ppm, tungsten at 48 ppm, tin a t 38 ppm, molybdenum a t 1400 ppm, and niobium a t 45 ppm. Figure 5 shows a later run a t high sensitivity for the lead isotopes. The z04Pbconcentration is about 60 partsper -billion atomic. Neon, argon, and xenon have been used as discharge gases. Argon appears to offer the best compromise of sputtering, ionization, and spectral benignity, although more extensive experiments must follow to document this more completely. Xenon, in particular, complicates the spectrum with no apparent compensating advantages. A solution sample, consisting of a few drops of Ba and Sb a t 100 ppm each, was deposited and dried in the stainless steel cathode. Subsequent mass spectra showed prominent isotopes for each element. Advantages as an Ionization Source. The rf spark is the ionization source most in use for inorganic materials, such as metals, semiconductors, and general nonvolatile inorganic solids. Those who use this source recognize its many advantages but are also well aware of its limitations, particularly for quantitative analysis. The ion flux is sporadic, difficult to control, and often unreproducible. The HCIS data, in these early experiments, suggest several possible advantages of the unit as an ionization device for solid samples. 1. T h e ion flux is extremely stable. The monitor meter shows approximately f 1-2% fluctuation. Thus, ratio readout circuits which are required for the unstable rf spark are unnecessary with the HCIS. Excellent isotopic
ANALYTICAL CHEMISTRY, VOL. 46, NO. 3, MARCH 1974
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Figure 2. Low gain mass spectrum of a copper cathode using the HClS Tube current, 20 mA at 400 V ; 10 kV acceleration: source pressure: 1 Torr of argon: 1.5-kV multiplier voltage
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35
40
45
50
55
60
65
75
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80
85
m/e Figure 3. Low gain mass spectrum of a stainless steel cathode using the HCIS. Conditions as in Figure 2 1000
la+
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+ >,
ro
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rnle Figure 4. Upper mass region of a HClS stainless steel cathode spectrum taken at Figure 2 conditions, except 2.5-kV multiplier voltage
xx)
mle Figure 5. HCIS sensitivity demonstrated for lead isotopes. 204Pb concentration, 60 ppba. Figure 2 conditions, except 3.0-kV mul-
tiplier voltage ratios can be obtained from a single scan. 2. Relatively nonselective ionization occurs. Interelement sensitivity factors appear to be quite small, within a factor of three. This can possibly be attributed to the Penning ionization process. The nonselective ionization is a significant advantage for both qualitative and quantitative analysis. 3. A small energy spread is observed. Background contamination can be seen as doublets and sometimes triplets which resolve even in the electrical scanning mode. Quadruplets may be observed with photoplate detection. Lines which normally interfere with inorganic spectra may not be a problem with this source. The Penning ionization
mode probably explains this low energy spread. 4. Good sensitiuity is obtained. Trace elements may be detected and determined to the low ppm level or below. 5. A large ion f l u x is produced. Monitor currents of up to 50 nanoamperes require replacement of the standard AEI readout monitor, which often pegs on its least sensitive position, or the use of beam chopping. 6. A variety of discharge gases ma3 be used. The inert gases vary in available ionization energy and sputtering capacity. Reactive gases might also be useful. 7. I t is a surface technique. Surface films, at least of conductors, may be sputtered and analyzed with relatively simple apparatus. 8. Solution residues m a y be examined. Prior experience with optical hollow cathodes has shown that thin films may be deposited reproducibly and sputtered for quantitative elemental analysis. Very little work has been done with the HCIS to optimize discharge parameters for optimum sensitivity, stability, and precision. It is unlikely that the conditions selected in these initial experiments will prove to be optimum. Thus, with further evaluation of such factors as tube current, discharge gas, gas pressure, and cathode-anode geometry, improved performance of the HCIS is expected. Another model has been constructed which allows simultaneous ion and optical sampling of the discharge to study the hollow cathode reactions. The HCIS has also been useful in our department as a filamentless source for chemical ionization mass spectrometry. W. W. Harrison C. W. Magee Department of Chemistry University of Virginia Charlottesville, Va. 22901
Received for review July 5 , 1973. Accepted October 4, 1973. Research supported in part by KIH Grant No. GM14569 and EPA Grant No. R-801829.
AIDS FOR ANALVTICAL CHEMISTS Digital Automatic Attenuator with Displays, Peak Memory, 1/0 Interface, and Scale Expansion-For Use with Potentiometric Chart Recorders Stephen R . Pareles’ Department of food Science, Rutgers-The
State University, College of Environmental Science, New Brunswick, N.J. 08903
Most analytical instruments, gas chromatographs, for example, provide an attenuation or range-change switch on the output of the detector which the operator uses to center both major and minor peaks on the potentiometric chart recorder with respect to the vertical axis. Thereby, component purity and retention time can be accurately ascertained in addition to an estimate of quantity, sometimes closely related to peak height. Frequently, the manipulation of the switch is an exercise in perception and dexterity and demands the operator’s attention for much of the analysis. This is especially so when the analyzed 1 Present address, project sponsored p r i v a t e l y .
464
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 3, M A R C H 1974
mixture is a complex unknown such as a natural product isolate. The result is an attenuated chromatogram that is nonreproducible and difficult to interpret. Automatic attenuators, intended to obviate these problems, have been described for several years. These require fixed mechanical attachments t o the recorder, such as microswitches (1-3) or gears ( 4 ) , to detect pen position; or ( 1 ) D. J. Darling, F. D. Miller, R . C. Bartsch. and F. M . Trent, A n a / .
Chem., 32, 144 (1960). (2) F. Bauman, F. A. White, and J. F. Johnson. Anal. Chem.. 34, 1331 (1962). (3) R . R Lowry, J . Chromatogr. Sci.. 7 , 383 (1969) ( 4 ) K . Abel and W. B. Dabney, A n a / . Chem.. 3 5 , 1335 (1963).
