AIDS FOR THE ANALYST Adaptation of Beckman DU Spectrophotometer to Direct Recording
I t is possible to return to ordinary null-balance operation on the spectrophotometer b!. means of a switch, for the most rareful and precise ~ ~ o r l . ; .
t e e Cohn, Beckman Instruments, Inc., Fullerton, Calif.
The amplifier is still of the direct current type, so a certain amount of drift is present. It should not esceed 0.1 to 0.2% 2' at 100% in 1 minute, if instrument and batteries are in good condit,ion. the null position of the null-record sa-itc)l operation is conventional (Figure 4). The second tube acts as :I current amplifier, and the indicating meter is nulled by operating the dark current, sensitivity of slit, and transmittance knobs in sequence. I n the record position, the second tube acts as a cathode follower, and its outputvoltage is fed klack to the first-stagegrid, to linearize the output, in place of the transmittance voltage. This feedback circuit is similar to the Beckman EriSE coniputer operational amplifier, n-hich is highly linear and stable. The cathode voltage is applied to any potentiometer recorder, with recommended full-scale range 50 mv. . I .elmrate sensitivity
has been developed for the Beckman DU I spectrophot,ometer \&ich converts the null-balance amplifier to a feedback amplifier capable of operating a conventional millivolt recorder or microanimet,er. The recorder records "energy" or phot,otube volt,age, as is common TVith the familiar single-beam t,ype of infrared spectrophotometer. \Then used with a n-avelength drive, cont'inuons spectrograms are obtained. The record thus includes the spectral response of source, detector, and optics. QSI_R~PLE circuit
... ". t
-
5 -. c
!
-
.. "?
"-
f n
"?
-. 0
il
s
i
Figure 1.
control, R21 is useful for the record position. The DU dark current control is still effective. It is sometimes neceiisary to readjust it in snitching betn-een null and record. .4ny potentiometer recorder may be used. If a 0- to 10- or 0- to 50-mv. recorder is used, slit widths narrower t,han those normally availahle are obtained a t the most sensitive position of the 100% adjustment control. The least sensi-
..
Emission spectrum of iron recorded from DU with adapter
When used a t fixed wave length, records may be made of absorption or emission us. time, either for time-varying reactions, or merely t,o permit visual averaging of detector noise. Recording is particularly helpful with flame spectrophot,ometry, providing qualitative inforniat,ion of the sort which recording previously made available in absorption studies. Examples of :in iron spectrum (Figure 1) and of a mixture of manganese chloride and pot,assium chloride (Figure 2) are shown. ;Iveraging of noise is also especially useful in flame work, t o extend t,he limits of detectability. I n flame n'ork the single-beam feature is generally found satisfactor>-.
-. _..- ...
...'
*.
,*-.,-
-.-'
I_
,
.*
. 7
*.'
4'
/..d
/'
..
.,,-.-'
f
d
Figure 3.
Benzene curve
Benzene vapor absorption, 270 to 225 mp. Hg lamp a n d photomultiplier, scanning time 10 minutes b. Air: background varies only threefold from 270 t o 220 inp S o t e adhesive ware-length scale attached t o strip chart 5.
tive position on 0- t,o 50-mv. span corresponds approximately to the least sensitive DU position. .4n indicating microammeter with a 0- to 100-pa. full scale may be substituted for R22, converting the spectrophotometer to a direct reading meter-type instrument. This may he convenient for lower accuracy work, where speed and operator fatigue are important. It is difficult to obtain meters linear to less than f1%, or about ten times worse than null balance operation. The circuit may be used with a photomultiplier and/or battery power regulator. The wave-length drive originally designed for the DK recording spect,rophotometer (Bull. 352, Beckman Instruments, In?,, Fullerton, Calif.) adapts readily to the DU instrument. It has five speeds, permitting scanning times of 1, 3; 10, 30, and 100 minutes for 288" of Lvave-length scale rotation. Because of the nonlinearity of the Tvave-length scale, the millimicron range covered in 288" varies Tvith the regiorl covered. The shorter times are suitable for liquid samples in the ultraviolet, the longer
Figure 2. High resolution obtainable with photoniultiplier attachment Resolves 403-mp .\In line a n d 404line completely; resolves doublets separated by 0.3 m p
~ n pR
ilbsorption studies over n-ide spectral ranges will usually require resett.ing t,he slits several times. NarroIv ranges can often be scanned wit,hout reset,ting slits. I n the ultraviolet, the background changes only threefold bet'ween 270 and 220 mp (Figure 3). While single-beam operation is less convenient, it, is often preferable t,o no recording Lit all. 141
ap&mimate wave-kngth scale on the strip chart: By the use of special chart drive gears (Part 4892, Beekman Instruments, Inc.,
Figure 5.
Wave-length drive for DU
Five speeds &reavailable, 5 to 500 minutes full scale. Loose gear on right fits on rave-length knob shaft, is thumbed for mhnual control
Altbougb such a device lacks many of the valuable features found in the double-beam automatic instruments.. i t a _. mears to be much less expensive, and may find a place in many laboratories
