1551
Anal. Chem. 1981, 53, 1551-1552
polarities was thus obtamed. This mixture (see Table I11 for components) was appliied to a 2-cm trap and eluted. The major portion of most of the compounds,eluted in the first ether fraction (fraction 11; see Figure 2). The smaller molecules, along with some of the more polar compounds, had significant portions in tlhe methanol fraction. The polycylic aromatics (anthracene and fluoroanthene) were not eluted at all with ether (even with up to 10 mL), although it was later discovered that they car1 be eluted with a small volume (less than 2 mL) of toluene. A single experiment in which this mixture was eluted from a 4-cm trap with 3 mL of methanol followed by 5 mL of ethier (see Table IV) resulted in most of the components again appearing in the first ether fraction, but with all compounds exhibiting a significantly greater degree of fractionation. All but two of the components were eluted in 2 mL or less. The present GCB elution procedure offers distinct advantages over the previously described method (1). By using methanol as the initial eluent, residual water on the trap is removed in the first 1-mL fraction. This allows the organic solvent which follows to act much more effilciently in desorbing the components from the GCB. An immediate concentration factor of 25- to 50-fold is thus realized over the previous
method by allowing adsorbates to be removed in 1-2 mL of solvent instead of 50 mL. As might be expected from previous HPLC applications of GCB ( 3 , 4 ) ,rudimentary fractionation of componentsis now possible with traps only 2 cm long, while a significant degree of fractionation is possible using longer (4 cm) traps. Employing a gradual solvent gradient for trap elution can improve the process even further. Results obtained in this study may differ slightly from those obtained when samples are actually adsorbed from water, since components would tend to migrate into and through the trap during the adsorption procedure. This could affect the resolution of the fractionation process and would probably be most noticeable in the early eluting components.
LITERATURE CITED (1) Bacaloni, Allessandro; Goretti, Gancarlo; Lagana, Aldo; Petronio, Blanca Maria; Rotatori, Mauro Anal. Chem. 1880, 52, 2033. (2) Petty, Robert L. Pacific Conference on Chemistry and Spectroscopy, San Franclsco. CA, 1978; Paper No. 84. Abstracts published by California Section, ACS, Berkeley, CA. (3) Colin, H.; Eon, C.; Gulochon, G. J. Chromatogr. 1878, 119, 41. (4) Colin, H.; Eon, C.; Guiochon, G. J. Chromatogr. 1978, 122, 223.
RECEIKED for review January 12,1981. Accepted May 18,1981.
Sample Introduction and Pressure Measuring System for Chemical Ionization Mass Spectrometers A. J. Illies and M. T. EBowers" Department of Chemlstty, University of California, Santa Barbara, California 93 106
G. G. Meisels Department of Chemistry, University of Nebraska -Lincoln, Lincoln, Nebraska 68588
Chemical ionization mass spectrometry (CIMS) is now well established as a valuable research tool in both analytical and fundamental applications (1-5). One of the difficulties with using CIMS in magnetic sector instruments has been that of electrical discharges through the gaseous sample between the ion source, which may be at potentials up to 10 kV, and ground. The discharges occur when the electric field causes electrical breakdown of the sample gas resulting in a flow of electrons toward the anolde (ion source) and of positive ions toward the cathode (ground) (6). The discharge is sustained by emission of secondary electrons at the cathode caused by impact of the fast moviing positive ions. The breakdown voltage, which is a function of the parameter E / N , where E is the electric field strength (V/cm) and N the gas density (molecules/cm3),varies considerably with gas sample. These differences depend upon the energy loss pathways available to the free electrons as tlhey travel through the sample gas toward the anode. A more poorly understlood characteristic of electrical discharges is the quenching of the discharge sit higher pressures (above ca. 50 torr) (4,6). 'Phis quenching of the discharge has been extensively used in most CIMS inlet systems where a large pressure drop is created through a nonconducting capillary leak. Disadvantages of using a capillary leak are (a) large sample backing pressures are required presenting a problem when small sample sizes or compounds with low vapor pressure are used and (b) the inability to measure the ion source pressure through a capillary leak. Futrell and Wojcik (7) solved this problem by using a chain of vacuum resistors
between the ion source and ground with stainless steel turnings at each potential to create a uniform, controlled potential gradient. We would like to present an alternate method. It was first used at the University of Nebraska-Lincoln on a modified Atlas CH-4 mass spectrometer operated at ion source potentials up to 3 kV and pressures up to 3 torr. The same technique has been extended at the University of California, Santa Barbara (UCSB),for use with a VG-Micromass ZAB-2F mass spectrometer operated at source potentials as high as 10 kV and pressures up to 1torr. The basic features of the design are shown in Figure 1. The sample leak valve which is at ground potential feeds into a glass tube which has been packed with approximately 4 in. of glass wool. The end of this glass tube is in contact with the ion source potential. We believe that the glass wool may prevent discharging by reducing the positive ion velocities below that required for the emission of secondary electrons and/or providing a very large surface area which may act as the third body in the ion-electron recombination reaction
I+ + e-
+M
+
I
+M
In our experiment the glass tube is passed through a Cajon ultra torr fitting (Cajon Vacuum Products, Macedona, OH) which has an O-ring seal separating the vacuum system and atmosphere and is bakeable to -250 "C. Connections to the ion source are made with VCR vacuum couplings and flexible stainless steel tubing (Cajon Vacuum Products, Macedona, OH). 0 1981 Amerlcan Chemical Society
1552
ANALYTICAL CHEMISTRY, VOL. 53. NO. 9. AUGUST 1981 55 FLEXIBLE TUBING
GUS$-METAL
57 SOURCE
where PLsand TISare the ion source pressure and temperature and PT and 'TT are the Dressure and temuerature a t the Baratron head.' At UCSB this sample introduction system is used for all easeous and volatile liauid m ~ l e in s both E1 and CI modes. The only disadvantage with the sample introduction system involves the removal of very polar samples from the g h wool. However, overnight pumping has removed a l l samples we have used to date.
CONNECTOR
LITERATURE CITED
ATMOSPHERE Flgun, 1. Schematic 01
(1) Illles. A. J.; Melsels. 0. 0. Anal. Chem. 1980, 52, 325. (2) Palley. C. W.: Illies. A. J.; Msl~&, G. G. Anal. Mem. 1980, 52.
VACUUM
,797
sample intmduction system.
M.: Flea F. H. J. Am. Wmm. SCO. 1075. 97, 5339. (4) Mather. R. E.; Todd. J. F. J. Int. J. k s 3 Specborn. Ion phyb. 1919, 30, 1. (5) Munson. 8. Anal. Mem. 1077, 49, 772A. (5) Gould, R. F.. Ed. A&. Chem. Ser. 1989. No. BO. (7) FutraII. J. H.;Wojclk. L. H. Rev. Scl. Inshm. 1971. 42, 244 (8) Panersan, G. N. '"Molecular Flow 01 Gam'': Wlley: 1956.
(3) &%ier.
We use the same method to measure the ion murce pressure with a MKS-Baratron capacitance manometer. At UCSB an additional glass ring is included in the line leading to the capacitance manometer. This results in an intermediate ground. Thus, if discharging should occur it would be to the intermediate ground and not to the capacitance manometer. One must, however, recognize that when glass wool is placed in the gas line leading to the pressure transducer thermal transpiration will occur (8). The ion source pressure is then given by
PIS = PdTIS/TT)"2
(2)
RECWW, for review Febmuy 17,1981. Accepted May 7,1981. The work done a t UCSB was supported by the National Science Foundation under Grant CHE77-15449 and a t the Univerisity of Nebraska by the Department of Energy under Contract DEAS02-76-ERO-2567. We gratefully acknowledge this support.
CORRECTIONS Smoothing of Digital X-ray Photoelectron Spectra by an Extended Sliding Least-Squares Approach Andrew Proctor and Peter M. A. Sherwood ( A d . Chem. 1980,52, 2315-2321).
There is an unfortunate typographical error in eq 8. The correct form is shown below: _ n= _2 (fitto a parabola, y = QU' + bu + e). Y(u).-z Y(U),-i + QzPz(u,Zm)= m
E ,
t=-m
({5[3t2- m(m
+ l)]u2 + (2m - 1)(2m + 3)tu +
m(m + 1)[3m(m + 1)- 1- 5t2])/NORMl)f(t) (8)
Determination of Atmospheric Sulfur Dioxide without Tetrachloromercurate(I1) and the Mechanism of t h e Schiff Reaction Purnendu K. Dasgupta, Kymron DeCesare, and James C. Ullrey (Anol. Chem. 1980,52, 1912-1922). On page 1912, under Experimental Section, the preparation of the pararosaniline working reagent should read 133.3 mL of the 0.2% commerciallv ~urifiedstock reazent (Harlem) and 113.9 mL of concentrated HCl were diluted 1L, rather'than 113.9 mL of 0.3% dye and 133.3 mL of HC1 being diluted to 1L.
