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Anal. Chem. 1081. 53, 1132-1 133
0.30volt
TIME [MINI
Flgure 4. Field desorption analysis of leucine enkephalin using the M Na' adduct ion signal for feedback control of the emmer current.
+
the M + H+ signal which is not part of the feedback loop. Since the amount of fragmentation is usually very dependent on the emitter temperature, the intensity/time profile for any given ion may not appear the same as that for the TIC. This is illustrated in Figure 3 which shows the M + H+ signal peaking in the middle of the run. Other measurements show that M+ predominates in the early period of desorption while fragment ions are most abundant during the later time of desorption. The TIC feedback mode may be suited to analysis of complex mixtures since the entire sample may be desorbed at a uniform rate. If, however, one wishes to analyze a mixture for one substance, feedback control may be derived from characteristic mass-selected ions. Figure 4 shows the m/z 570 (M+ Na+ adduct) and the emitter current as a function of time for a sample of the tetrapeptide leucine enkephalin. The electrometer range was set a t 0.03 and 0.3 V for the results given in a and b, respectively. In the first experiment, the sample is desorbed slowly over a long time; in the second experiment, it is desorbed rapidly over a short time. In this application the emitter current rapidly attains the appropriate level for the component of interest and then maintains desorption at a preselected rate.
Note that in Figure 4 the signal-to-noise ratio of the feedback control signal is substantially less than that of the emitter current. In this case, the plateau value of the m / z 570 ion current is only approximately 1 X A and contains substantial statistical noise. To smoothly control the emitter current when the feedback control signal is erratic, as is often the case in FD experiments, we employed a mechanism of self-dampening. The rate at which the emitter current approaches a null point (minimum difference voltage) is proportional to the difference between the instantaneous feedback signal and the reference voltage. Thus, the heating rate will be high when the system is far from the null point (either below or above) and will decrease as the electrometer signal approaches the reference voltage. This control method successfully minimizes the probability of overshooting the null point in the case of erratic and unstable desorption. A complete circuit diagram and construction notes may be obtained directly from the authors.
ACKNOWLEDGMENT The authors wish to thank Sidney Nanayakkara for his assistance in constructing the unit. LITERATURE CITED Shulten, H.R. rnt. J . ass Spectrom. ion mys. 1979, 32, 97-283. Barofsky, D. F.; Jacob, L.; Barofsky, E. 23rd Annual Conference on Mass Spectrometry and Allled Topics, Houston, TX, May 1975, pp 60-81. Wlnkier, H. U.; Neumann, W.; Beckey, H. D. Int. J. Mass Spectrom. Ion phys. 1976, 21, 57. Malne, J. W.; Soltrnan, 6.; Holland, J. F.; Young, N. D.; Qerber, J. N.; Sweeley, C. C. Anal. Chem. 1876, 48, 427. Shlralshi, H.; Otsuki, A.; Fulva, K. Bull. Chem. Soc. Jpn. 1979, 52, 2903. Schulten, H A . Cancer Treat. Rep. 1976, 60, 501. Schulten, H.-R.; Nlbberlng, N. M. M. Biomed. Mass Spectmrn. 1977, 4, 55. Fraley, D. F.; Woodward, W. S.; Bursey, M. M. Anal. Chem. 1960, 52, 2290. Schulten, H.-R.; Beckey, H. D. Org. Mass Spectrom. 1972, 6, 885. Beckey, H. D.; Heindrlchs, A.; Winkler, H. U. Int. J. Mass Spectrum. Ion Phys. 1970, 3, 9.
RECEIVED for review December 29,1980. Accepted February 27,1981. This work was supported by National Institutes of Health Grant GM 21584.
Automated Gel-Permeation System for Removal of Liplds In Gas Chromatography/Mass Spectrometric Analysis of Fatty Tissues for Xenobiotic Chemicals Oilman D. Velth" and Douglas W. Kuehl U S . Envlronmental Protectlon Agency, Environmental Research Laboratoty- Duluth, 620 1 Congdon Boulevard, Duluth, Minnesota 55804
Natural lipids remain one of the more difficult sources of interference in gas chromatography/mass spectrometry (GC/MS) analyses of tissue for hazardous organic contaminants. The gel-permeation chromatographic (GPC) method reported by Tindle and Stalling (1) and modified by Kuehl and Leonard (2) is widely used in GC/MS analysis of organic residues in tissues. The GPC technique in which methylene chloride is used as a solvent Dermits auantitative isolation of chemicals with molecular ;eights less than 600 from the heavier lipids regardless of the functional group of the molecule. The primary disadvantage of the technique is the500-mg limit to the amount of lipid that can be chromatographed as a single sample. A 1o(hZtissue Sample containing 8% natural lipid requires at least 16 aliquots of the sample extract sequentially chromatographed over a 20-h period. Although an
automated gel-permeation system which processes up to 23 individual samples is commercially available for approximately $9500, the unit requires that the sample be loaded into individual sample loops containing 0.5-1.0 g of lipid each. We describe herein a $300 controller for the automation of any GPC system which sequentially injects aliquots of large lipid samples and subsequently collects the fractions containing the trace contaminants in the lidds. EXPERIMENTAL SECTION Materials. A conventional GPC system for preparative scale chromatography can be automated by inserting three three-way Teflon solenoids and an electronic control mod& in the existing system. The following materials are needed (a) three Teflon three-way solenoids (General Valve Corp.) 12 V dc, $45 each; (b) three Model 328A time-delay relays with mounting socket (Automatic Timing and Controls Co., King of Prussia, PA), $46 each;
This article not subject to U S . Copyright. Published 1981 by the American Chemical Society
Anal. Chem. 1981, 53, 1133-1134
WI TE
LlblO
COLLECTION
E
CMLECTION
Flgwe 1. Electronic control module and modification of gel-permeatlon
chromatograph. (c) 4-A, 12-V dc power supply (Straco, Inc., Dayton, OH), Model PS-4 or equivalent, $19.05; (d) 12-V relay, toggle switches, power cords. Construction. A moclifcation of the conventional GPC system used in our laboratory ia shown in Figure 1. The basic system consists of a metering pump (Milton Roy), a six-position sample injector fitted with 5O-lriL sample loop (Cheminert), a 900 X 25 mm gel column (Ace Gk),and a W detector (254 mm) (Varian). To automate the sample injection, we inserted two three-way solenoids in the sample ltmp (Figure 1). The sample is still loaded into the loop through the six-position injection valve; however, with the injector in the “hject” pasition, flow through the sample loop occurs only when the inject solenoids are activated. At a constant solvent flow rate, controlling the time the solenoids are activated precisely controls the volume of injection. Automation of the fraction collection is accomplished by adding a three-way solenoid to ithe effluent line from the column. The normally open side of the solenoid is connected to the waste solvent receiver and the normally closed side to the sample receiver. The electronic control module is constructed without soldering by connecting the three variable-time-delay relays in series as shown in Figure 1. The terminals (Figure 1)correspond to the
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terminals on the mounting bracket of the timer. Functionally, the timers are connected as a momentary-start interval timer and two on-delay timers in series. The momentary-start timer activates the injector solenoids when the “inject start” switch is depressed. The solenoids remain activated for the duration of the timing cycle, after which power is applied to the next timer through terminal 1. The second timer is connected as an on-delay timer which activates the sample-collection solenoid at the end of the timing cycle. Thus,the adjustment of this timer controls the time between sample injection and sample collection during which the lipids elute from the column and into the waste receiver. At the end of the timing cycle the collect solenoid is activated, and power is applied to the third timer. This timer is a reset delay which allows collection of the sample until the end of the timing cycle when it activates the 12-V relay and sends a reset pulse to the first timer. When the first timer is reset, the others return to the initial conditions, and the next aliquot of the sample is injected into the column. This process cycles until the power is turned off. Procedure. The tissue extract is concentrated to less than 50 mL in methylene chloride and centrifuged to remove particulates. After the GPC system has been flushed thoroughly and a flow rate of approximately 3.5 mL/min is attained, the injector block is moved to the load position and the 50-mL sample is drawn into the sample loop. The injector block is moved to the “inject” position, which transfers control to the inject solenoids. Initially the three timers may be set at 1.2 min, 36 min, and 30 min, respectively,which corresponds to a 1.2-min injection (4.2 mL), a 36-min waste cycle (126 mL), and a 30-min collection cycle (105 mL). After the first injection, the waste timer is adjusted so that sample collection coincides with the end of the elution of the lipids. RESULTS AND CONCLUSIONS The cleanup of large tissue samples for GC/MS analyses frequently requires approximately 1 week of technician time/sample when manual GPC systems are used. Automating this procedure allows each sample to be cleaned up unattended within 24 h. Moreover, if the GPC columns are adequately matched with respect to performance, a single control model will control several GPC systems simultaneously. We have used this system for the past 2 years without a single failure because of the solenoids or timing network. LITERATURE CITED (1) Tlndle, R. C.; Stalling, D. L. Anal. Chem. 1972, 44, 1768-1773. (2) Kuehl, D. W.; Leonard, E. N. Anal. Chem. 1978, 50, 182-185.
RECEIVED for review July 18,1980. Resubmitted January 22, 1981. Accepted January 22,1981.
Qualitative Determlnatlon of Volatfie Compounds In Solids by Vaporizatlon/Mass Spectrometry Alan J. Power Materials Research Labonntorles, Defence Science and Technology Organlsation, Melbourne, 3032, Australia
In many branches of materials science it is sometimes necessary to identify gases and low molecular weight organic compounds in solids. Sophisticated equipment for quantitative thermal volatilization analysis of solids has long been available, and interfacing such instrumentation to a mass spectrometer for identification of volatiles is quite common. At these laboratories there is a need for rapid, qualitative analysis of low molecular weight compounds in solids, particularly polymers, and a simple, low cost device to be used in conjunction with a gas chromatograph/mass spectrometer (GC/MS) system has been developed to meet this requirement. 0003-2700/81/0353-1133$01.25/0
The device (Figure 1) comprises a brass container fitted with a removable stainless steel lid and is connected by stainless steel tubing to the injection port and molecular separator inlet inside the GC oven, thereby replacing the GC column. Substitution of a “dead volume” device in place of the GC column has been described previously (1, 2). A wire gauze filter (Figure 1,item A) covers the outlet tubing to prevent entry of any fine solid material into the molecular separator. A custom-built, stainless steel holder (Figure 1,item D)is used for loading powders and other particulate solids. The tolerance between the holder disks and the container walls is minimized to ensure that most of the helium carrier gas flows through 0 1981 American Chemical Society
