heavy electrical insulating tape makes this extra safety measure well worth while. RESULTS AND DISCUSSION
Samples can be run in the modified probe tip much in the same way as before the modification. A sample is first put into a glass capillary closed at one end and then deposited inside the probe tip. The probe tip is then “sealed” with the screw (6). Generally, the screw is inserted about five or six turns, but the number of turns will depend upon the volatility of the sample. By not tightening the screw completely, the air inside the probe can be evacuated in the vacuum lock before insertion into the ion chamber. The probe is inserted into the ion source in a conventional manner. By monitoring the ion beam and the ion source pressure, one can quickly determine if the flow rate needs t o be changed. If an adjustment is in order, the probe rod is pushed forward until the hexagonal projection of the screw is inserted into the hexagonal passageway of the source block. The screw, thus, becomes restricted and this allows the depth of the screw to be adjusted by turning the exterior probe handle which turns the probe rod, the ceramic insulator, and the probe tip, but not the screw. Because the hexagonal projection blocks the sample passageway t o the ionization chamber, it is generally necessary to retract the probe about ‘116 inch to disengage the hexagonal projection from the passageway to allow the sample to travel uninterrupted (except for the screw) into the ionization chamber. If the sample is extremely volatile, however, the projection may be left in place to further suppress the amount of sample entering the ionization chamber. In addition to controlling the leak rate, the screw also prevents the sample from being accidentally blown out of the probe tip into the vacuum lock or ion source. As a temporary approach, requiring almost n o expenditure and no instrument downtime, the probe tip can be threaded as shown in Figure 1 and fitted with a stainless steel screw without an hexagonal projection, and the hole at the base of the tip can be plugged with a set screw as already described. This will provide a simple means of retarding the sample flow; however, the probe may have t o be retracted one or two times in order to adjust the screw t o obtain the optimum flow rate. This approach was used with good results in our laboratory for several weeks before the ion source was dismantled and the hexagonal passageway was machined. The capability to adjust the screw while it is in the ion source is obviously a convenience and well worth the trouble if the probe is used a great deal. If only an occasional probe
+-
I4 1 ’i_-----.
4
Figure 1. Direct probe assembly (1) External handle, (2) Injection block and vacuum lock assembly, (3) Ceramic insulator, (4) Heater, (5) Modified probe tip, (5A) Permanent screw, (6) Adjustable screw, (7) Modified hexagonal passageway to ionization chamber, (8) Ion source block
sample is run, the temporary approach may prove to be completely satisfactory. Although n o claim is made that a molecular leak is achieved with this modification, a relatively constant leak rate can be obtained for most samples and the advantages over the conventional probe tip cannot be overemphasized. Hundreds of samples have been run using the modified probe tip, many of which could not have been run otherwise using electrical recording. Samples collected in melting point capillaries from gas chromatographic separations are routinely run by cutting a 1i2-inch section of the capillary containing the sample and inserting the capillary into the modified probe tip. Some extremely volatile samples have been adsorbed on crushed charcoal prior to insertion into the tip with good success. Hygroscopic samples have been run after inserting the sample into the tip inside a dry box. The sample is sealed inside the tip with the screw and transferred to the vacuum lock under a nitrogen blanket. The probe tip and the screw are cleaned with an appropriate solvent and dried with a heat gun after each use, although glass capillaries are always used to keep contamination to a minimum. RECEIVED for review October 30, 1970. Accepted April 8, 1971.
ModificaRion of a Fraction Collector for Large Volumes and Use of Siphons in Fraction Collecting Norman S. Radin Mental Health Research Institute, University of Michigan, Ann Arbor, Mich. 48104 COMMERCIALLY AVAILABLE fraction collectors are designed for use with test tubes or with larger collection vessels, but none seem to be suitable for both types. We have modified our Technicon fraction collector (Technicon Instruments Corp., Tarrytown, N.Y.) for use with 125-nil Erlenmeyer flasks, 250-ml boiling flasks (Florence flasks), 16-02 Boston round bottles, and 32-oz Boston round bottles, while still retaining the capability of collecting in test tubes.
The original collector hes a support with four concentric rings, each holding 50 test tubes. The activating motor rotates the rack ‘/SO of a circle after each fraction is collected. My modification involves adding an d a p t e r rack to the top of the test tube rack, and inserting a stepping switch between the collector motor and the sample sellsing circuit, which ~ large bottles are gives 3 or 4 steps per coritaiiier. ‘ I h u the handled by a 12-place adapter rack, each place requiring ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
e
1143
A
Figure 1. Fraction colIector adapter for 32-02bottles, bottom view A = spacer, ’/*-inchthick. B = spacer into which Is glued a dowel, turned down on a lathe to fit snugly into one hole in the metal rack. Note the gap between the top and upper left holes near B , due to the asymmetry of the adapter
4 steps by the changer motor. The other containers are handled by a 16-place rack and a 3-step movement between flasks. The adapter racks (Figure 1) are made from l/r-inch thick hardboard (“Masonite”), with a hole in the center to permit centering on the Technicon’s center post. The racks are aligned by gluing a wooden dowel to the underside, positioned so that the dowel enters test tube position number one. The rack for the large bottles is raised ‘/*-inch above the Technicon rack by wood spacers to give better centering. In order to center each hole properly, the Technicon rack is laid down o n the Masonite disk and hole positions are marked. The holes are cut with a fly cutter mounted on a drill press and the dimensions are chosen to give a moderately snug fit with the largest container within any one group. Because 50 is not integrally divisible by 3 or 4, there is an end space in each adapter disk which makes the disk asymmetric. This means the collector can be left unattended for only 16 collections with the 3-step disk and for only 12 collections with the 4-step disk. However, preparative chromatography ordinarily does not require collecting a large number of fractions so this limitation is not too annoying. The stepping circuitry is not very complex but does depend on the nature of the sample sensing circuit. We use the Rinco siphon system (Schaar Scientific, Inc., Chicago, Ill.), which gives fractions of specific volumes. This system uses a self-starting siphon which is connected through a sidearm to a simple pressure sensor. As the siphon fills with column effluent, the pressure rises in the side arm and eventually closes an electrical circuit. This closure causes the stepping circuit to move one step. If the stepper is set by the operator for one step, the changer motor becomes activated to rotate the rack 1 step (for test tube collections). If the stepper is set for 3 or 4 steps, the motor is activated 3 or 4 times.
-__-
1144
ANALYTICAL CI-lEMISTRY, VOL. 43, NO. 8, JULY 1971
As the siphon continues to fill, it eventually empties through the siphoning arm, resetting the pressure sensor. The siphon portion of a Soxhlet extractor can be used in this system by simply placing a glass tube in the liquid and connecting the tube to the pressure sensor. Our shop has built a n additional stepping circuit which allows one to collect up to 10 fillings per vessel. This circuit is interposed between the pressure sensor and the motor circuit. Thus one siphon can deliver multiple volumes and one need not purchase a wide variety of siphons. Such a stepping system is standard equipment with some fraction collectors, such as the Golden Retriever (Instrumentation Specialties Co., Lincoln, Neb.). Readers desiring to use our circuit can write to the author for a copy. On request, our shop will fabricate the entire electrical control system. We have run into two kinds of trouble with siphon dispensers. In certain aqueous systems containing detergents and tissue extracts, lipid precipitated o n the siphon walls, making them water-repellent. This stopped the self-starting siphon action, causing the siphon to overflow continuously through the top mouth. The only cure seems to be: avoid such columns. The other problem has arisen with certain organic solvents, from lipid columns, in which the density and surface tension conspired to block the self-starting or selfstopping siphon action. This problem was solved by bubbling a slow stream of air bubbles through a fine polyethylene tube, the exit of which is placed just a t the entrance to the siphon sidearm. As the siphon fills, the bubbles rise through the main body of the liquid. As the siphon fills to the overflow point and liquid leaves through the overflow sidearm, bubbles are drawn into the flowing liquid. The bubbles help the flowing stream break up when the siphon runs dry, thus stopping the siphon action. The polyethylene tube is held in place by a bit of “masking” tape at the top of the siphon. It should be noted that the size of the siphon must be related to the flow rate of the column. With a large siphon and slow column, the liquid may simply dribble out the sidearm, without starting the siphon action. With a small siphon and high flow rate, the liquid may enter the sidearm during the drainage stage so fast that siphoning fails to stop. These limitations prevent the use of many portions per collection vessel; possibly 6 is the maximum for organic solvents. It is also important to calibrate the volume of one siphon portion while the column is running, as this volume is higher than the static volume. ACKNOWLEDGMENT
I a m grateful to Mrs. Inez Mason for her assistance in making the devices work, and to our shop engineers, Paul Yoder, James Mullison, and Xenophon Pesaros, for designing and building the adapters. RECEIVEDfor review February 15, 1971. Accepted April 22,1971. Work supported by USPHS Grant NS-03192.
