An Automatic Sample Changer for Infrared Spectrometry Edward J. Bakker, J. Stuart Frost and Gordon D. Ogilvie Research Department, ICI Fibres Limited, Hookstone Road, Harrogate, England
MODERNINFRARED spectrometers are equipped with facilities for repetitive scanning and can be programmed to scan over a selected wavelength range, reset, and then either repeat the scan automatically or wait for a command signal to scan. When fitted with a device which automatically changes the sample, such spectrometers can be used to perform routine analyses or record spectra without attention and in overnight operation. Automatic sample changers for infrared spectrometers have been described previously. Johnson et al. (I) modified a 35-mm slide changer and McNiven, Hoffmann, and Scrimshaw (2) used a rotating disk driven by a stepping motor. We report a simple device using a rotating disk which was designed for use with a Beckman IR12 infrared spectrometer but is easily adaptable to other spectrometers having repetitive scan facilities. Although the disk design limits the number of samples which can be accommodated (30 in this device), it is much easier to enclose in a vacuum housing which may be desirable if hydrolytically unstable substances are to be examined. APPARATUS
Figure 1 shows an exploded diagram of the sample changer. The sample disk rotates within a housing consisting of a front plate which is attached to the spectrometer and a back plate on to which is fixed the Crouzet drive motor (type 395, 0.1 rpm), indent system, and a Burgess microswitch (type V4T4). (1) D. R. Johnson, J. W. Cassels, E. G. Brarne, and D. F. Westneat, ANAL.CHEM., 34, 1610-1614 (1962). (2) N. L. McNiven. P. Hoffrnann. and G. Scrimshaw, ibid., 37, 778-779 (1965). F I
FRYT RATE
Thirty slots are cut in the disk and the samples (in the form of polymer films) are mounted in front of these and secured by spring clips. Three screws fix the disk to the hub, allowing easy removal and replacement by an alternative disk designed to accommodate samples in the form of KBr pellets. At the bottom end of the housing are windows (NaC1) allowing the beam of the spectrometer to pass through the sample. The system is sealed by an O-ring at the periphery and a rubber disk at the drive shaft. Positive positioning of the disk is achieved by a ball-catch arrangement. At the correct position, a microswitch is operated by ball-headed pins set in the reverse side of the disk. One pin is omitted for shut-down purposes. A Torrington roller clutch (type RC/040708) allows the disk to be moved manually as well as allowing the disk to over-run the drive shaft and be pulled into position by the ball-catch. Figure 2 shows the control system which is based on the use of “Norbit 2” solid state logic modules described in the “Design Handbook for Norbit 2” by Mullard Ltd. Switch SI, is a contact in the repetitive scan circuit. When the spectrometer has reset itself, the rzlay energizes closing the contact and hence removing the logic “1” signal (+24 V) from the NOR gate 1. The output of this gate is now logic “1” so energizing relay R, through the relay driver LP1. Contact n closes, switching the sample changer motor on. As the disk moves, microswitch Sz operates, extinguishing the pilot light PI and removing the logic “1” signal from the timer TU and monostable M S . When the disk drops into the next position, Sz closes, the pilot light comes on, and the monostable fires operating relay R, for 5 sec. Contacts t close and start the spectrometer scan. Si now opens and the output of NOR gate 1 drops to logic 0 cutting off the power to the motor. The timer, NOR gate 2 , LP2 and relay Rb are used in automatically shutting down the spectrometer after the last
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Figure 1. Exploded diagram of sample changer ANALYTICAL CHEMISTRY, VOL. 42, NO. 9, AUGUST 1970
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* Figure 2. Control system based on “Norbit 2” solid state logic modules
sample. If S? opens for longer than 20 sec, the output of TU rises to logic ‘‘1’’ and through NOR gate 2, relay Rb is de-energized. Simultaneously relay R , opens stopping the motor. If the microswitch closes within 20 sec, the output of the timer remains at logic “0” and the auto shut-down does not operate. Thus by missing out the microswitch operating pin at the position after the last sample, auto shut-down is effected. The contracts of relay Rb are wired such that when the relay drops out, the spectrometer is switched to its normal standby state, i.e., chopper motors off, glower current reduced, etc. Additional switches provide certain override functions. Closing S, overrides the auto shut-down of the spectrometer and is useful during start-up and loading. It does not, however, override the stopping of the disk after the last sample. The samples may be rerun by using the manual “Drive” switch Sgto change the sample disk position (to No. 2 or greater) when the auto-scanning sequence will be picked up. Closing Sa prevents sample change except by the manual switch S 5 . The 6.8-kilohm resistors and 1-pf capacitors are for contact bounce suppression. The three relays are miniature plug-in types with 24-V dc coils. Relays R, and R, have 2-pole c/o contacts, Rb has 4-pole c/o contacts. Their drive units (LP) are Norbit 21A60 modules. A 2 NOR 60 module is used for NOR gates 1 and 2 and a T U 60 for the time delay. The monostable is wired from a 2 NOR 60 and a T U 60. Additional components are a 1-meg ohm resistor and a 20-pf capacitor for the time delay and a 1-meg ohm resistor and 5-pf capacitor
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ANALYTICAL CHEMISTRY, VOL. 42,
for the monostable. OA202 diodes are also wired across the relay coils. DISCUSSION
The automatic sample changer described has been used mainly for the routine measurement of carboxyl and hydroxyl end groups in polyethylene terephthalate for degradation studies and calculation of number average molecular weight. The analysis which has been carried out manually required 20-30 minutes of operator time t o position the sample and scan the spectrum for each determination. The analysis of 30 samples took at least ten hours to complete during which time the spectrometer was not available for other work. Application of the automatic sample changer has reduced the analysis time to five hours, allowed overnight operation and freed personnel and equipment for other work during the day. ACKNOWLEDGMENT
The authors gratefully acknowledge the help of Charles E. Walton in the construction of the equipment.
RECEIVED for review January 10, 1970. Accepted April 17, 1970.
NO. 9, AUGUST 1970
