Solid State Computer Interface and Update Unit for Existing Perkin-Elmer Double Beam Atomic Absorption Spectrophotometers Quentin Bristow Geological Survey of Canada, 601 Booth Street, Ottawa, K I A OES, Canada The interface unit described in this article was developed in the course of a project to automate the analysis of geochemical samples by atomic absorption spectrophotometry. Two Perkin-Elmer Corp., Model 303 instruments were held and, since only minor improvements in the mechanical and optical design have been made in more recent instruments, i t seemed worthwhile to design a solid state digital electronics package to bypass the original vacuum tube processing electronics and provide for easy interfacing to a modern minicomputer. At the same time, some of the convenience features found on more recent instruments have been incorporated for normal manual operation. The unit is also compatible with the other Perkin-Elmer double heam instruments which have appeared since the Model 303
Figure 1.
soha state update ana intertace unit Tor verKm-tvner aou-
DESIGN PHILOSOPHY Since computer operation was envisaged, there seemed little point in incorporating a digital display as such, although some form of digital output would, of course, he necessary for entry into a computer. The objectives of the design were as follows: No modification of the atomic absorption instrument itself other than the connection of a lead to hring the nnprocessed signal out from the photomultiplier tube. No disturbance of the existing circuitry in the instrument so that it would still be possible to operate it in the usual way with its own chart recorder, hut in parallel with the add-on unit; thus providing the possibility of a direct comparison a t any time, if required, between the old and new outputs on the same sample. Provision of an “Auto Zero” feature which would allow return of the signal to a predetermined hase line following a determination, a t the touch of a foot-operated switch. Provision of a single on-lineloff-line switch whereby the Auto Zero and other functions could he operated under manual or computer control. Incorporation of a method of analog to digital conversion that would result in a digital output linearly related to absorbance rather than ahsorption, and which would take a true ratio of samplelreference beam amplitudes; both of these functions being accomplished as a part of the A D conversion process without recourse to logarithmic amplifiers or analog multipliers. This was considered desirable as the latter techniques inevitably involve nonlinear devices which are temperature sensitive and prone to noise and drift problems. Provision of an analog output for chart recorder monitoring, derived from the digital one by digital to analog conversion. This approach provides an analog absorbance signal which has been accurately computed by a digital technique from the original reference and sample beam signals, and it also allows the auto zero feature to be imple2246
Figure 2. Simultaneous analytical records for Fe Upper set: Original P-E 303 contigumtion (absorption) Lower Set: Via interface unit (absorbance). Firs! three peaks at left represent 3. 6. and 9 Irglml. respectively
mented by a digital technique prior to reconversion for the analog output. Provision of the necessary computer interface inputs and outputs a t 5-volt logic levels on a rear panel connector.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
E---CHOPPED REFERENCE EEAV
SAMPLE BEAP.1 SEGMENTS
J
FLAME
/ I
PHOTOMULTIPLIER
I
\
REFLECTIlG
L I G H T CHOPPES
60
+I
1
1
4C
Nl R R O R
Figure 3. Principle of a double beam atomic absorption spectrophotometer
Employment of the most recently available multifunction digital and analog integrated circuits to minimize physical circuit complexity, while a t the same time taking full advantage of state of the art technology to provide high quality signal processing. Unit to be self-contained with all necessary regulated power supplies in a small instrument cabinet having the absolute minimum of front panel controls.
erence beams I s and I R , are related by the following equation which is a consequence of Beer’s absorption law: 1, = IRexp ( - p c )
where p is an absorption coefficient. The concentration c is thus given by:
OPERATION OF THE U N I T The photograph in Figure 1 shows the interface unit, while Figure 2 is a reproduction of simultaneous chart recordings taken for iron analyses. The upper chart is the normal Model 303 output proportional to absorption on the x1 scale expansion, while the lower one is the output from the interface unit proportional to absorbance with baseline stabilization by means of the auto zero feature. Manual operation in the “off-line” mode is straightforward, the operator touches the footswitch causing the READ light to come on, and aspirates the sample in the usual way. The chart recorder pen then generates a peak calibrated in absorbance units. After withdrawing the aspirator tube from the sample solution, the foot switch is again activated to return the chart pen to zero a t which time the READ light goes off. When under computer control in the “on-line” mode, the computer generates the necessary READ/ZERO signals instead of the footswitch, and accepts the data as a 12-bit binary word from the rear panel connector.
SIGNAL PROCESSING FUNDAMENTALS The basic principle of operation of a double beam atomic absorption spectrophotometer is illustrated in Figure 3. The hollow cathode lamp emits the particular wavelength that will be strongly adsorbed by the element sought when the sample solution is aspirated into the flame. The resulting change in intensity of the beam is seen by the photomultiplier tube, and its electrical output is a measure of the element concentration in the sample. The double beam principle involves the use of a rotating mirror, segmented in such a way that the beam from the hollow cathode lamp is passed alternately through the flame and via a path which bypasses the flame. The photomultiplier tube output is thus a composite of the so-called “sample” and “reference” beam signals, which are present during alternate half cycles of the 6 0 - H ~power frequency, as shown in Figure 3. (The monochromator is omitted for clarity). The element concentration “c” and the intensities of the sample and ref-
The amplitudes of the electrical signals corresponding to ZR and Is must therefore be processed according to this equation to provide an output which is linearly proportional to the element concentration c.
SAMPLE A N D REFERENCE BEAM DEMODULATION The chopped or multiplexed signal a t the photomultiplier tube anode shown in Figure 4 is synchronously demodulated by means of a digital phase locked loop integrated circuit, locked into and operating a t 64 times the ac line frequency, thus allowing each cycle of ac to be divided into 64 parts for accurate “sorting” of the signal contents. A counter is decoded by logic gates to open and close three solid state analog switches in the correct sequence to reconstitute three dc levels via sample and hold circuits. These levels correspond to the sample and reference beam intensities, and the zero or dark current level which is subsequently subtracted from the other two. This digital demodulation of the composite signal provides well defined and precise levels corresponding to the sample and reference beam amplitudes with the minimum of noise and crosstalk. ANALOG/DIGITAL CONVERSION Conversion is achieved by measuring the time required for a capacitor charged to the reference beam signal level to discharge down to the sample beam signal level. This time is linearly proportional to sample absorbance and is independent of anything which does not affect the ratio of the reference signal to sample signal. I t is thus independent of variations in the hollow cathode lamp output and also of variations in photomultiplier tube gain (provided they do not cause signal levels to move right outside the operating range). The capacitor discharge time intervals are measured by counting clock pulses of an appropriate frequency into a 12-bit binary counter, the contents of which become the digital representation of absorbance in arbitrary units. Under manual control, the completkm of each conversion
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14. DECEMBER 1974
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v
D A R K CURRENT SUBTRACTION CIRCUITS
-
t REFERENCE BEAM SIGNAL
Figure 4. Digital demodulation scheme for reconstituting separate sample and reference beam signals
OUTPUT BUFFER
REFERENCE B E A M SIGNAL
Presetto511 d u r l n Q Z E R 0 mode operation Presents 1 2 b i t value 01 previous A 0 conversion while next one I " p r o ~ r e ~
Analogue
SAhlPLE BEAM S I G N A L
t ANALOGUE OUTPUT
LOGARITHMIC ANALOGUE DIGITAL CONVERTER
1L
UP COUNTER READ ENABLE ,
Prebet w i t h conlents 01 down counter before each new c o u n t in REAO m o d e Or!g$nal Zero signal level 1 8 t n u s subtracted f r o m each A 0 convers8On
D C Level t o balance o f f Presel 51 1 m u n t
1L
D O W Y COUNTER
. eacn ~ o ~ i v e r s ~proportional on lo tog, tReIeience1 - Log, ISarnplel
ZERO E Y A B L E
Preset t o 5 1 1 before each new count A D conveitei values corresponding t o zero signal are effectively c o m p l e w n t e d ~n here Inhlblted in REAO mode
I
Figure 5. Digital implementation of auto zero feature
triggers the start of the next, resulting in a free running variable conversion rate which depends on the absorbance signal present. Under computer control, the conversion rate is fixed by the computer, and successive 12-bit numbers are entered in core for subsequent processing. 2248
ANALYTICAL CHEMISTRY, VOL. 46,
AUTO ZERO IMPLEMENTATION The reference and sample dc signal levels have an arbitrary initial difference between them when the main instrument and interface unit are turned on, which is reduced to within acceptable limits by means of a front panel
NO. 14, DECEMBER 1974
control and null meter. When the interface unit is set t o ZERO, the digital representation of this offset is stored in the lowest of the three digital registers shown in Figure 5, a new value being stored after each analog to digital conversion. Meanwhile the top register, marked output buffer, remains reset to zero. When the unit is set to READ, all digital conversion numbers are entered into the middle register, while the last value obtained for the offset in the lowest register remains stored there and is subtracted from all subsequent values obtained during the READ period. Each of these corrected values is then transferred to the output buffer for acceptance by the computer and for conversion to an analog signal for the chart recorder. The net result is that when the unit is a t ZERO, the output (both digital and analog) is held a t zero and, when it is set to READ, the outputs continue to show zero (due to subtraction of the offset) unless and until significant drift occurs, or a true signal is recorded. The true zero is always automatically restored however when the instrument is again set to ZERO.
ANALOG O U T P U T The analog output is derived from the output buffer register by means of a 12-bit D/A converter. The implementation of this is greatly simplified by the use of COSMOS digital logic in conjunction with 4-bit R-2R ladder networks prepackaged in plastic dual-in-line packages. The COSMOS devices use F E T elements rather than bipolar transistors, so that their outputs closely approach perfect analog switches. The other bonuses of using this type of logic are the negligibly low power consumption and very high noise immunity. A suitably low impedance output for driving a
chart recorder is obtained by means of an operational amplifier. When the unit is set to READ, the initial reference-sample beam offset is subtracted from all digital values subsequently entered into the output buffer as was explained earlier. If, however, negative signal drift occurred during the READ cycle, the subtraction would result in numbers which would give near full scale readings. This could cause the analog output to swing back and forth continuously between close to full scale and zero. T o avoid this situation the digital “zero” is set to be 511, which allows in effect an eighth full scale of negative signal to occur before the problem referred to above would be encountered. A dc level is, of course, incorporated in the analog output to offset the 511 count and provide a ground level output when the unit is reset.
CONCLUSION A unit has been described which allows older PerkinElmer Inc., double beam atomic absorption spectrophotometers to yield considerably improved performance, and to have some of the convenience features found on more recent instruments, while providing simple and straightforward interfacing to a computer for automated operation. The unit is essentially an add-on device with no modification required $0 the original instruments, other than the connection of a single lead to a point which is easily accessible, and with no effect on the original mode of operation. The Ottawa-based Canadian Government Agency, Canadian Patents and Development Limited, are arranging for commercial manufacture and sale of the unit. RECEIVEDfor review May 8, 1974. Accepted July 30,1974.
Coating Reservoir for Making Porous Layer Open Tubular Gas Chromatography Columns J.
C.Nikelly
Department of Chemistry, Philadelphia C o b g e of Pharmacy and Science, Philadelphia, Pa. 79 704
It was recently shown that porous layer open tubular (PLOT) columns can be conveniently made in the analytical laboratory by coating the inside walls of capillary tubing using a relatively simple dynamic coating procedure ( I , 2 ) . The only special apparatus required is a reservoir for holding the coating mixture which is forced through the capillary tubing. Although PLOT columns are generally made with higher liquid loads and consequently have higher capacity factors than wall-coated columns, this advantage may not always be achieved when using the dynamic coating method; as increasingly more viscous coating mixtures are used, the risk of clogging during the coating step is increased. I t was apparent, however, that if the larger diameter reservoir is used and if the coating mixture is forced upward rather than downward into the capillary tubing, the likelihood of clogging would be reduced. Actually, such a tee-design reservoir which is used primarily for coating wall-coated rather than porous layer columns is commercially available (Scientific Glass Engineering, Ltd., N. Melbourne, Victoria, Australia 3051). It has,
(1) J. G. Nikelly. Anal. Chem.,44, 623 (1972). (2) J. G. Nikelly and M. Blurner. Amer. Lab., 6 , 12 (1974)
however, a serious limitation: made of glass and using septum-type seals, it would not be suitable for coating PLOT columns which normally require relatively viscous coating mixtures and higher coating pressures. This report describes an all-metal reservoir of the same basic tee design which was found to be very practical for coating PLOT columns and which can be easily assembled from commercially available components (tubing and brass fittings). These are listed in Table I and the assembled reservoir is shown in Figure 1. The main component of the assembly is a Swagelok heat exchanger tee with a bored through reducer. The upper arm of the tee, “process tube” in heat exchanger terminology, is a yle-in. tube fitting while both the side arm and the lower arm (jacketing tube) are I/4-in. tube fittings. The capillary column is inserted through the upper arm to within 2 mm of the reservoir bottom. The side arm is connected to the pressure source (air or nitrogen), a “Quick-Connect” Swagelok fitting being very suitable for this purpose. The low arm is fitted with a 11/4-in.long x Y4in. 0.d. metal tube which is connected to a 2-in. long X Kin. 0.d. metal tube (reservoir) via a %-in. to %-in. reducing union. The bottom of the reservoir is a ‘&in. cap fitting. T o coat a column, the reservoir is first disassembled a t the reducing union, filled with a few milliliters of chloro-
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