1548
Anal. Chem. 1981, 53, 1548-1551 0
II /c
H,C -CH,CH,NH,
\
I
+
G
-
-CH,CH2NHCGCH,CH,COGH
(5)
/
HC ,, C
II 0
compare behavior on modified cobalt emitters were pyridine (PA 218 kcal/mol) (11),benzonitrile (PA 195 kcal/mol) (II), carbon disulfide (PA 168 kcal/mol) (II), and cyclohexane (PA unlisted in ref 11 but probably C168 kcal/mol). All these compounds were analyzed by field ionization. For field desorption, the compound whose behavior on modified cobalt emitters was known was anthracene. Results for the behavior of these compounds a t untreated and acid-surface carbon emitters are summarized in Table I. In Table I the results from the silanized and untreated emitters at no heating current are seen to be usually the same; it is only on heating that a difference appears. The second heating current value listed for each compound is the threshold value for protonation by the acid-surface emitter. At this value the base peak in the spectra of compounds with an easily discerned site of protonation becomes the (M + 1)+ion, and in the spectra of the cycloalkane becomes the (M - 1)+ion, in accordance with the behavior of alkanes in ordinary chemical ionization (eq 6) (13). At the protonation threshold
silanizing solution can produce emitters whose ionization efficiency is greatly reduced. The existence of a threshold for protonation is concordant with the endothermicity of the protonation reaction at zero field. It also has been universally observed with acid-surface metal emitters ( I , 2 )and although the onset of protonation varies with the geometry of each emitter to some extent, the threshold is lower for carbon than for cobalt emitters. This is a manifestation of the lower temperature of a cobalt emitter than a carbon emitter when the same heating current is passed through two otherwise identical emitters (19). In summary, polymerization of silanes by surface-bound water allows the extension of results found for metal emitters to the much more common carbon emitters. While protonation is incomplete because of incomplete surface coverage by the polymer, a peak resulting from protonation becomes the base peak, and this observation clearly establishes the identity of the (M + 1)+peak in a functionalized unknown, a problem which has required much attention (20) in field desorption studies because of the problem of distinguishing whether the FD peak of highest mass is M+. or (M 1)’ in an unknown. Extensions to other silanes, and the development of other methods for modifying functional groups at graphite surface, have begun.
+
ACKNOWLEDGMENT We are grateful to J. R. Hass for access to a VG Micromass emitter activating unit.
LITERATURE CITED current for the acid-surface emitter, or even slightly above it, the untreated emitter is seen to yield the same spectrum as with no heating current. These results are in accord with those obtained with metal emitters. The greatly increased acidity of the carboxylic acid group in the intense electric field at the emitter tip is accommodated by theory, because of the effect of an electric field on ionization of weak electrolytes (14). The pK of an acid in an electric field ought to decrease by a term proportional to the square of the electric field (2). Emitters of all types have tip radii of disperse dimension, which obscures the dependence of signal on field strength, and variation of field was not studied; controversy surrounding field effects has arisen from variations in the degree of importance attached to this point (15-18). Protonation is not complete here, in contrast to the protonation at the acid-surface metal emitter. We believe this is the result of incomplete surface coverage, and indeed, after an arc has struck a working acid-surface emitter of any type, the usual effect is to decrease the intensity of the (M + 1)’ peak. This would be expected if part of the acid polymer is destroyed by the arc, so that some ionization occurs by the classical mechanism at the now untreated surface. We have not found conditions for coating carbon emitters so that only (M 1)+ions are formed. Excessive periods of time in the
+
Youngless, T. L.; Bursey, M. M.; Pedersen, L. G. J. Am. Chem. SOC. 1980, 102, 6881. Youngless, T. L.; Bursey, M. M.; Pedersen, L. G. Int. J . Mass Specfrom. Ion Phys. 1981, 38, 223. Ligon, W. V., Jr. Science 1979, 204, 198. Keough, T.; DeStefano, A. J. Anal. Chem. 1981, 53, 25. Neumann, G. M.;Rogers, D. E.; Derrick, P. J.; Paterson, P. J. K. J . Phys. D 1980, 13, 485. Murray, R. W. Acc. Chem. Res. 1980, 13, 135. Poole, C. F. “Handbook of Derivatives for Chromatography”; Blau, K., King, G. S., Eds.; Heyden: London, 1978; p 152. Beckey, H. D.; Hllt, E.; Schulten, H.-R. J. Phys. E 1973, 6 , 1043. Moses, P. R.; Wler, L. M.; Lennox, J. C.; Finklea, H. 0.; Lenhard, J. R.; Murray, R. W. Anal. Chem. 1978, 50, 576. Wlllman, K. W.; Greer, E.; Murray, R. W. Now. J. Chlm. 1979, 3, 455. Hartman, K. N.; Llas, S.; Ausloos, P.; Rosenstock, H. M.; Schroyer, S. S.; Schmidt, C.; Martinson, D.; Milne, G. W. A. “A Compendium of Gas Phase Baslclty and Proton Affinity Measurements, NBSIR 79-1777; U S . Department of Commerce: Washington, DC, 1979. Fraley, D. F. Ph.D. Dissertation, University of North Carolina at Chapel HIII, Chapel Hill, NC, 1981, Chapter 6. Munson, M. S. B. Anal. Chem. 1971, 43, 28A. Onsager, L. J . Chem. Phys. 1934, 2 , 599. Holland, J. F.; Sokmann, B.; Sweeley, C. C. Biomed. Mass Spectrom. 1976, 3 , 340. Beckey, H. D.; Rollgen, F. W. Org. Mass Spectrom. 1979, 14, 188. Holland, J. F. Org. Mass Spectrom. 1979, 14, 291. Beckey, H. D. Org. Mass Spectrom. 1979, 14, 292. Fraley, D. F.; Woodward, W. S.; Bursey, M. M. Anal. Chem. 1980, 52, 2290. Schulten, H A . ; Beckey, H. D. Org. Mass Spectrom. 1974, 9 , 1154.
RECEIVED for review March 2,1981. Accepted April 27,1981.
Elution of Adsorbed Organics from Graphltized Carbon Black Robert L. Petty Marine Science Institute, University o f California, Santa Barbara, Santa Barbara, California 93 106
In a recent article by Bacaloni et al. (I), the properties of graphitized carbon black (GCB)relating to adsorption of trace organics from water, and desorption with organic solvents, were examined. Several general classes of compounds were
studied. Most of the substances tested, although subject to eventual breakthrough, were shown to be completely retained by the GCB. For desorption of the compounds, several solvent systems were examined. A 1:l mixture of ether and hexane
0003-2700/81/0353-1548$01.25/00 1981 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981
was judged to be optimal, and their standard method specified a 50-mL elution volume for desorption from a 100-mg GCB trap. GCB has been used for some time in this laboratory for trace enrichment of hydrocarbons from seawater (2). The present paper reports a method which we have developed for desorption of trapped organics from GCB which requires only 2-4 mL of solvent and $whichprovides partial fractionation of the adsorbates as thisy are desorbed. EXPERIMENTAL SECTION Reagents and Equipment. Chlorinated'pesticides were obtained as a mixture from Applied Science. Aroclor 1254 was obtained from Monsanto,and individualPCBs were obtained from Supelco. Other compounds used as test subslmces were obtained from standard chemical suppliers and used without further purification. Methanol, methylenechloride, and hexane are "distilled in glass" grade, obtained from Burdick and Jackson. Ether is anhydrous "analytical realgent" grade from Mallinkrodt. Graphitized carbon black was obtained from Supelco (Carbopack B, 60/80 mesh) and cleaned by overnight Soxhlet extraction with methylene chloride. Gas chromatography was performed on a Hewlett-Packard 5840A chromatograph equipped with flame ionization and electron capture ("Ni) detectors and capillary injector system. GC columns used were Hewlett-Packard fused silica capillaries, either 12 m (column A) or 50 m long (column B) by 0.21 mm i.d. Procedures. Adsorptiion traps are prepared from dry GCB and 6 mm 0.d. by 4 mm i.d. Pyrex tubing. The tubes are dry packed, with tapping, using glass wool plugs t o hold the GCB in place. Approximate packing lengths of 2 cm and 4 cm are used, requiring about 100 and 200 mg of GCB, respectively. Traps are conditioned before their initial use, and regenerated after each subsequent use, by rinsing with 5--10 mL of either methylene chloride or ether, followed by 5-10 mL of methanol and approximately 5 mL of water. Solvents are added to the traps by means of a stainless steel reservoir (constructed from a length of in. tubing) having a vlolume of about 5 mL, which is attached via a Swagelok union. Samples are applied to a trap by syringe injection of 1-8 pL solutions of the test compounds into the upper glass wool plug, followed by a 2-mL water rinse of the trap. Samples are eluted by adding a measured volume of methanol to the reservoir and allowing the methanol to pass through the trap by gravity flow while collecting the effluent in 1-mLfractions. After the methanol stops dripping from the trap outlet the next solvent (usually ether) is added to the reservoir and more fractions are collected. For a 2-cm trap, 1.0 mL of methanol and 3.0 mL of ether are usually used. For a 4-cm trap, a 2- or 3-mL volume of methanol is followed by 4-6 mL of ether. Void volumes of the traps are approximately 0.4 and 0.8 mL, respectively, so the first fraction of a new solvent always contains some of the previous solvent. Sample recoveries are determined by GC analysis of 1-3 pL portions of the unconcentrakd fractions. In the w e of the Aroclor mixture, quantitation was by comparison of eluent fractions to a standard sample (external standard method), and in the case of the pesticide and miscellaneouscompound mixtures, it was by the internal standard method. The internal standard is added to the container in which the eluent fractions are to be collected. A mixture of three PCBs (2,3,6-trichloro-,2,3,5,6-tetrachloro-,and 2,2',4,5,5'-pentachlorobiphenyl) is used as the' internal standard for the pesticide mixture, while a series of even carbon straight chain esters (C8-Cmmethyl cssters) are used witlh the miscellaneous mixture. Chromatographic conditions are as follows: column A (12 m), initial temperature 75 "C held for 4 min, program rate 6 "C/min, final temp 240 "C, column inlet pressure 10 psig; column B (50 m), 90 "C for 1 min and then to 180 "C at 10 "C/min and then to 250 "C at 3 "C/min, column inlet pressure 20 psig. For either column, the injector temperature is 240 "C, IUD is 260 O C , and ECD is 300 "C. The splitless mode is used for sample injection, with a 0.6-min injection period. RESULTS AND DISCUSSION Three samples, each containing a mixture of compounds,
1549
Table I. Chlorinated Pesticide Mixture,a 2-cm Trap % in fraction I I1
compound
%recovery
CY-BHC 100 0 100 20 80 65 lindane 100 0 75 heptachlor 100 0 85 aldrin 100 0 100 heptachlor epoxide 90 10 100 endrin P,P-DDD 90 10 95 o,p-DDT 90 10 70 P,P-DDT 90 10 80 a Mixture of 65 p g of each component dissolved in 8 pL Percent of total recovered amount in first of hexane. (methanol) or second (ether) 1.0-mL fraction. Table 11. Aroclor 1254,' 4-cm Trap % of recovered amount in fraction peak __ IVc VC VIC no. I I ~ IIIC
1
0
70
25
2
0
0
90
3
0
80
4 5 6
0 0 0
85 90
15 15
15
75
7
0
100
0 70 10
8 9
10 11
12 13
14
0 5
0 0 0 0 0
0
10
5 10 5
0
95
0
80 90 100 100
0
0
0
0 10
0 0
O 30
O} 0 5 0
95
25 5
0
70
30 60
15
10 5 0 30 50
40
0
70 70
% recovery
0 0
95 100 90 115 100 100
5
85
0
110
2 fig in 2 pL of hexane applied to 4-cm GCB trap. Methanol eluent (1mL). Ether eluent (1mL fraction). a
were tested in the GCB desorption procedure. They included chlorinated pesticides, polychlorinated biphenyls (PCBs), and a miscellaneous mixture made up of several types of compounds. The components of the chlorinated pesticide mixture are listed, along with their elution and recovery data for a typical run, in Table I. In this run, an 8-pL portion of a solution containing 8 fig/pL of each component was applied to a 2-cm GCB trap. The trap was then eluted with water, methanol, and ether. Apparent absolute recoveries averaged about 85%, but precision on replicate runs was only about *15%, and all component recoveries were seen to fluctuate within the 60-100% range on replicate runs. The poor precision was due to problems with the capillary GC analysis procedure, rather than to a variability in actual elution recoveries. The major portion of all pesticide components, except lindane, was consistently eluted in the methanol fraction, with the major portion of the lindane always appearing in the first ether fraction. When this mixture was eluted from a 4-cm trap, using 3 mL of methanol, most of the components were split between fractions I1 and I11 (second and third milliliter of methanol). A portion of the a-BHC came off in fraction I, while lindane was again eluted mainly in the first ether fraction. The results of elution of a sample of Aroclor 1254 from a 4-cm GCB trap, using 2 mL of methanol, are presented in Table 11. Chromatograms of the eluted fractions are shown in Figure 1. Certain peaks in this complex mixture were selected and monitored through the elution sequence. It became clear upon analysis of the fractions that at least one,
1550
ANALYTICAL CHEMISTRY, VOL. 53, NO. 9, AUGUST 1981 78
Table 111. Miscellaneous Mixture,a 2-cm Trap % in fraction
peak n0.b 1 p-xylene 2 mesitylene 3 undecane S methyl octanoate 4 naphthalene 5 dodecane 6 6-undecanone 7 tridecane S methyl decanoate 8 1-chloronaphthalene 9 tetradecane 10 1-dodecanol S methyl dodecanoate 11 dibenzyl ether S methyl tetradecanoate 12 anthracene 1 3 octadecane 14 1-hexadecanol S methyl hexadecanoate 15 undecanophenone 16 fluoranthene 17 phytol S methyl octadecanoate S methyl eicosanoate
TEMPERATURE (‘C) 210 225 240 250
9090
130
o+--?
20
;2 do ’ i 5 TIME (minutes]
45
40
amtc Id
Flgure 1. Capllbry gas chromatogram of Aroclor 1254 standard (STD) and eluent fractions (111, IV, V) from k m GCB trap. Chromatographed on column B (see Experimental Section for details). See text for explanation of peak numbers.
11e
%
recovIIIe ery
50 70 30 50 100 0 12 100
0 100 0 115
25 12 95 12
100 100 95 5
0 15 0 70 0 100 0 70
50 12 50
0 100 0 100
45
55
0 85 0 75 0 100
50
40
60
0
25 50 50
0 0 0
0
0
95 100
5
50 25 50
0 75 0 0 0 100
0 0
0 100
55
95
of
0
80 95
25
95
0
0
of
90
a Mixtures of five multicomponent solutions applied in a total of 1 3 p L of hexane. Numbers corresponding to chromatographic peaks in Figure 2. Amount applied Methanol eluent. e Ether eluent. f Comto trap, pg. ponents were later eluted with toluene.
Table IV. Miscellaneous Mixture, 4-cm Trap peak no.a l b 20 2 3 0
1
3 4
5 6 7 8 9
I! r 3
10 11
12 13 14
TEMPERATURE ( “ C ) 75
0
75
105 I
,
5
10
135
165
15 20 TIME (minutes)
195
225 240
/
,
25
30
I
C
0 0 0
% in
2b
3b
75 70 6 0 4 0 6 0 7
5 0
0 5 0
0 1 0 0 0 0 0 0 1 0 0 0 0 0 0 1 0 20 70 0 0 1 5 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 1
fraction 4c 5c 0 0 0
35 30 0 0 0
25 0 0 10 85 0
0 0 0 0 0
0 0 75 0 0 0 0
6 5 3 5 8 0 2 0
6c 0 0 0 0 0 0 0 0’ 0 0 0 0 0 0
7c 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0 0 0 0 0 0 0 0 0 0 0 0 0 0
15 0 0 20 25 55 0 0 0 0 0 16 17 0 0 0 0 0 0 a See Table I11 for components. Methanol eluent. Ether eluent. d Ethylene dichloride eluent.
35
Flgure 2. Capillary gas chromatogram of “Miscellaneous Mlxture” standard (STD) and eluent fraction I1 from 2-cm GCB trap. See Table I11 for identification of numbered peaks. Peaks labeled S are internal standards (see Table HI). Chromatographed on column A (see Ex-
perimental Sectlon for detalls).
and probably several, of the peaks present in the chromatogram of the whole mixture (STDin Figure 1)was composed of more than one component. For example, fraction I11 exhibited a peak with retention time slightly shorter than that of peak 718 in the standard, while fraction IV had a peak with
a slightly longer retention time. Also, peak 13 has a recovery pattern which exhibits two maxima, and peak 9, although eluting in an apparently smooth pattern, extends over five fractions. All of the other selected peaks are spread over only two or three fractions. In contrast to the pesticide mixture components, the PCBs are eluted almost entirely in the ether fractions. A “miscellaneous mixture” was made up from several “off the shelf” chemicals, including aliphatic and aromatic hydrocarbons, alcohols, ketones, an aromatic chloride, and an ether. A range of molecular weights, structural types, and
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
