Modification of a Beckman Model DU Quartz Spectrophotometer for Measurements to 192 Mp L. W. TAYLOR and L. C. JONES, JR. W o o d River
Research Laboratory, Shell O i l Co.,
Wood
River,
-Bechman i 3Iodel DL quartz spectrophotometer has been modified so that the absorption spectra of solutions can be measured accurately to wave lengths as short as 192 nip. The modification involves rotation of the collimating mirror, construction of a new walelength scale, substitution of a photomultiplier with a fused silica window ( E l l 1 6255) for the usual detector, replacement of the Beckman ultra,iolet source with an 4llen-Nester h?drogen discharge tuhe, use of thinspacer absorption cells, and installation of a \'? cor filter for measurement of the residual stra) energ). ProTision is also made for flushing the monochromator with nitrogen. Spectra obtained with the modified instrument are in excellent agreement with those measured with a recording Tacuum spectrometer over the entire spectral range. The modified instrument is an adequate substitute for a \acuum spectrometer in applications of practical importance where the conventional ultrav iolet spectrometer is not suitable.
(The manufacturer has recentl!- reconiniended reset,ting the prism drive arm on the prism shaft, rat,her than rotating the collimating mirror.) The short wave length end of the scale wa3 niade to (,orrespond to approximately 198 nip by rotation of thi. w r e x to its extreme position.
Table I .
True Ware Length, 31!.4a 194.17 197.3 200.2 223.0 225.9 230.2 232.3 244,7 257,6 275.3 289,4 296.7 313.2 334.2 364.3 366.3b 379.0 390.6 404.7 433.8 441.6 535 4 546.07 579 2 b ti07.3 671.6 208.2 (37.2
OR the past five >-ears this laboratory has been exploring
ALTERATIOA- OF S P E C T R A L R A N G E
Preliminary Mechanical Adjustment. The spectrophotometer romes equipped with a wave-length scale graduated from 200 to 2000 mp. The calibration can be adjusted by rotation of the collimating mirror with a micrometer screw accessible through a knock-out plug in the left end of the spectrophotometer case.
Calibration Data
(Ware lengths of lines froin high pressure mercury arc)
F
the use of far ultraviolet spectroscopy for the analysis of hldrocarbons. .An earlier public~ation(6) described a recording far ultraviolet spectrometer, presented the spectra of approximately 70 pure hydrocarhon~,and discussed correlations betm-een structiire and spectra. One application of these spectra has been in the determination of the types of aromatic hydrocarbons occurring in lubricating oil frartions. This method, which has bed (4), requires a measurement a t 197.5 mfl for the estimation of substituted lienzenes. As a result of this and other practical applications, it \vas desirable to have an inexpensive absorption spectrophotometer for routine measiiremcnts brlon- 200 m p . Tliercl itre niaiiy publishrd xcouiits of the nieasurement of n1)soiytion spectra to r a v e lengths as short as 185 mp n-ith unevacuihted quartz spectrometers. I t seemed likely, therefore, that the widely available Beckman DU quartz spectrophotometer would be suitable for this use after only minor alterations. -4hrief investigation revenld that this is indeed the case and th:it reliable measurements with the Beckman instrument are possible to a t least 192 mp. Estension of the useful range of the Beckman instrument involves solution of two problems. The first and most obvious of these is t o alter thc spectral range covered by the wive-length svanning mechanism and scale (normally 200 to 2000 nip). The second and more serious prohlem is t o increase the intensity and purity of the short v-ave-length radiation reaching the active surface of the detector.
111
Observed Ware Length S e w scale change of (set at spectral range 3K1.0 mfi) 201.1 194.15 204.8 197.23 208.3 200.2 237.5 222.9 241 .O 225.7 246.7 ?30,2 232.2 249.4 244.6 266.4 2,,5.7.6 282.7 311.2 -,3.4 333.9 289.5 346.2 ?96.8 375.5 313.1 334.1 416 5 460,O 354.1 486.1b 366.2b 370.0 z21.5 554 0 3Y0.6 ,597 0 404 6 705C 435.7 940 491.3 G35.2 1165 ,146.0 1283b 579 36 ri06c n71 708 737
Old scale after
Error, lil/l
-0.02 -0.05 0.0 -0.1 -0.2 0.0 -0.1 -0.1 0.0 fO.l fO.l to.1 -0.1 -0.1 -0.2 -0.1 0.0 -0.1 -0.1 -0.1
-0.3 -0.2 -0.07 t0.1 -1.30 -0 6: -0 2 -0 2
t y a i - e lengths from ( 3 ) and ( 7 ) . Doublet ai'eraged. R-ave lengths greater than 600 iiip caniiot be read n-irh precision iiiuch better t h a n 1 iiip. a
b
C
Recalibration. .A preliminary c~alibr:itioii was then ohtained by observing the positions of easily recognized absorption bands of solutioris of benzene, naphthalenr, and anthratwie and of vapors of 1,3-butadiene. These data w x e smoothed and intermediate points obtained by plotting the correction against the wave length. From this preliminary calibration curve it was possible to predict the approximate positiou of modt of the lines in the mercury spectrum. The hydrogen discharge tube was then replaced n.it,h a high pressure mercury arc: (Hanovia =ilpine sun lamp S o . 5-311) and the exact positions of 49 mercury lines between 194.17 and 737.17 mp n-ere memured. These data were plotted on a large scale to permit acciirate interpolation of other wave lengths. Preparation of New Wave-Length Scale. -An accurate full scale transparency of the wave-length scale of the spectrophotometer was made by conventional photographic methods. The transparency was covered LTith tracing paper and mounted on a transparent celluloid turntable in a light box. The new wavelength scale was laid out on the tracing paper by interpolating the correct wave lengths (as read from the large scale plot of the calibration data) between the markings of the old scale. The
1706
V O L U M E 28, NO. 11, N O V E M B E R 1 9 5 6 markings were inked and lettered with a S o . 80 Leroy template and a 00 pen. I t was expected that the nen- wave-length scale (except for the additional section at the short wave lengths) would correspond to a rotation of the old one through a constant’ angle. This was very nearly the case. Holvever, a periodic error amounting t o about 0.1 inch occurred a t intervals of 180°, suggesting that the original scale niight have been incorrectly centered by about this distance. The new 13-ave-length scale was extended from 737 mp (the mercury line to longest wave length for vihich accurate (lata were readily available) to 1000 mp by rotating the tracing paper so that the 730-mp marks of the new and old scale coinc,ided exrept for the difference in radius. The portion of the old sc.;tle from T30 to 1000 nip was then traced. .4full scale, positive c~)iit:tctprint of the n e y scale was mounted on the spectrometer scroll. Calibration Adjustment. The calibration was completed by :rdjustirig the collimating mirror so that the scale was correct for any one wave length. The blue (486.13 mp) and red (656.28 ~ i i phydrogen ~ lines are probably the most convenient check points, although the mercury green line (546.07 mp) IT-as used in the present instance, because it -vas desired to measure a large number of additional mercurj- lines so thitt the accuracy of t,he sc.:tle niight be checked more completely. The calibration data (T:thle I) indicate n maximum error of 0.2 mp and an average ei’ror of 0.1 nip or less over the entire ultraviolet region. The amir:tcy above T3T nip has nbt been determined, but the new wale is expected to show a periodic error in this region, as disriissctl above.
1.00
z
0.75
a c
;
0.50
ENTRANCE SLIT WINDOW
-
z
/XIT
SLIT LENS
-
WINDOW FROM BECKMAN
4:
s 02s 0 I
200 2 10 WAVE LENGTH IN M
220
10
Figure 1.
230
k
.ihsorption spectra of components of spectrometer
I\lPROVEMENT OF SPECTRAL PURITY AND INTENSITY A T SHORT WAVE LENGTHS
Accurate analysis of multicomponent systems of compounds with overlapping absorption bands requires a reasonably linear relationship between absorbance and concentration of the absorbing constituents. This linear relationship can be expected only if the radiation used in the measurements is nearly “pure”that is, comprised essentially of a single narrow band of m-ave lengths. The Beckman DU quartz spectrophotometer with standard ultraviolet absorption accessories meets this requirement well for measurements above 220 mp. Below 220 mp the radiation reaching the receiver may be very impure, so that absorption data obtained by the conventional technique may be inaccurate. The explanation for this difficulty is simple. The hydrogen arc which serve3 as the source of continuous radiation in all modern ultraviolet qpectromrters is rich in energy throughout
1707 the 100- t o 600-nip region. According t o the liest publidicd measurements (8) the intensity actually increits mp. Hoivever, before reaching the active surface o the radiation must pass through the envelopes or windows of the discharge lamp, phototube, and absorption cells, a3 well as the sample solution or solvent and the prisms and lenses of the spectrometer. It miist also undergo reflections from a number of mirrors. -It each of these optical elements radiation of all wave lengths siiffers some attenuation due to stuttering. iinwanted (or incompli.tej reflections, or absorption. These losses tend to be selective, so that the shortest wive lengths :$re attenuated to a greater extent t,han the longer mive lengths. Scattering of a small fraction of the radiation striking th(. prism and collimating mirror of the spectrometer reaiilts in a certain amount of undispersed radiation reaching thr esit slit. In the normal operating range of a \?-ell designetl instrument such as the Beckman DU spectrophotometer the spiirioii.4 response due to this “spectral impurity” or “false energy” i:: negligible (>oiiipared to the signal originating from the dispersed radiation. Homver, at very short Tvave lengths, where the dispersed rx(1i:Ition has suffered great attenuation, t,hc signal from the uiitlispersed radiation may be large comparcd to that from the tliapersed beam. The radiation is then said t o I w inipure or to contain a large percentage of false energy. Measurement of False Energy. The ideal method 01’ nieasuriiig the falsr energy is to determine the apparent transmittance of a material known to be completely absorbing at the wave length of interest and completely transparent at all other wave lengths. Such materials cannot be found in practice. However, for nirasurements below 210 mp S’ycor (a high-silica g1;ti;s available from the Corning Glass Works) in a thickness of 2 mni. approximatcas the ideal material, as it is opaque below 210 nip :ind has a high transmittance at longer wave lengths. One of the empty openings on the filter slide of the Beckman DU spectrophotometer \vas fitted n-it,h a 2.05-mm.-thick, polished filter of VJ-cor. I n the experimental ~ o r kwhich followed, the percentage of false energy a t a given spect,ral position beloT1- 210 mp was assumed t o be given by 100 times the ratio of the apparent per cent transmittance of the 1-ycor (relative to ctir) a t the \ w v e length in question t o the apparent per cenr transmitt,ance :It 280 mp. The refcrence Twve length, 280 nip, w;ts ;.-elected arbitrarilv from t,he wide region v-here the tr:tnmiitt;uice of t,he filter is limited by reflection losses rather than 1)y uiiaorption. The value of the false energy comput,ed in this manner is actiialljsomewhat low, since the i‘ycor absorbs an appreciable amount of radiation above 210 mp. A further difficult!- arisw in applicntion of the false energy correction-namely, that practical ~ : i i i i ples may absorb more or less long viave length radiation than the r-ycor, so t’hat a different correction is required for each sample. As the spectral distribution of the false energy is not knowi, it is impossible t,o determine the correction Tvith a high degrce 01’ accuracy. It is t,hus imperative that the correction be rediiml to a minimum value. Selection of Components. It vi11 be obvious that reduction of the effect of stray radiation can most easily be achieved by reducing absorption of short Ti-ave-length radiation in the optical system of the spectrophotometer. Much iniprovenient can he achieved by careful selection of accessories. The most useful criterion of performance is the false energy at 192 or 200 nip obtained ivhen the component in question is instnlletl in the spectrometer. The simple blue-sensitive phototubes originally 3upplied \\-it11 the Beckman instrument have inadequate seriditivity for iise below 220 mp, so that a photomultiplier must he used. Esamination of five 1P28 photomultiplier tubes o n hand a t the time this work was initiated revealed a difference of a factor of 5 in the false energy a t 200 mp measured Tvith the hest and ivor.st of these tubes. The short wive-length response of the worst of these tubes was improved by a factor of about 2 hy grinding the
1708
ANALYTICAL CHEMISTRY
envelope in front of the photocathode to about half its original thickness. This tube was later cut apart and the absorption spectrum of a part of the envelope determined (Figure 1). It will be seen that transparency is very poor below 230 mp. Large differences in short wave-length performance were also found for different Beckman hydrogen lamps. The absorption spectrum of the window from a typical lamp is also shonQ in Figure 1. It is apparent that the combination of a poor photomultiplier and a poor hydrogen lamp can lead to catastrophic loss of energy below 230 mp. False energy may be further reduced by substitution of hydrogen lamps and photomultipliers with m-indows of crystal quartz or fused silica. The Bllen-Nester lamp ( 1 ) is a commercially available hydrogen discharge tube of this type which can be installed in the Beckman instrument without much difficulty, although a special power supply must be provided. The circuit described by Allen ( 1 ) is satisfactory if used in conjunction with a good voltage regulator. Photomultiplier tubes with fused silica windows are available from EM1 Research Laboratories, Middlesex, England, and from Dumont in this country. A tube of the former type (EM1 6255) %-as installed in the present instrument. The electrical circuit used by Huke, Heidel, and Fassel (5) with the 1P28 tube was satisfactory for this purpose. The entrance slit window and the collimating lens which covers the exit slit of the monochromator account for loss of another 45% of the energy a t 192 mfi (Figure 1). Kear 200 mp absorption by atmospheric oxygen and ozone produced near the lamp also
0.75
a
I-
WAVE LENGTH IN
contributes appreciably to attenuation of the monochromatic beam (Figure 2). Substantial reduction in false energy was obtained when both slit windows were removed and the monochromator was flushed ivith dry nitrogen. The nitrogen was introduced in the following manner: The machine screw which fastens the cast-iron monochromator case to the sheet metal base just below the collimating mirror m-as removed. A hole was drilled through this screw, a tubulation attached, and the screx reinserted in the base. A short length of Tygon tubing leads from the tubulation to a 1-liter Dewar flask containing fresh liquid nitrogen. The principal remaining source of difficulty is selective short n-ave-length absorption by the common solvents in the usual 1-em. absorption cells. The extent of such absorption depends, of course, on the purity of the solvent. The curves shown in Figure 3 are for secondary reference fuel grade hydrocarbons (8) further purified by percolation through silica gel (1 pound of gel per liter of hydrocarbon). Solvents purified in this manner have been found to be as transparent as the corresponding American Petroleum Institute spectroscopic standards. The transmittance of 1-em. thicknesses of these solvents is, however, still poor a t 200 mp and is influenced by dissolved oxygen. The absorption by both solvent and dissolved oxygen can be reduced to a satisfactory level by reduction of the absorption cell length to less than 0.5 mm. Absorption cells of this length are not available for the Beckman instrument from commercial sources. Cells of the design used with th6 research vacuum spectrometer (6) have therefore been adopted for the present application. Fused silica-e.g., Hanovia Ultrasil-of 1-mm. thickness is satisfactory for windom in such cells, as it combines freedom from fluorescence with high transparency and good mechanical properties. Synthetic sapphire could also be used but is less suitable, since it is much more difficult to fabricate and its high refractive index leads to excessive losses by reflection. Lithium fluoride is unsatisfactory, because it fluoresces appreciably. Calcium fluoride is also very transparent in this region but has not been tested in this investigation, and in any event is much more expensive than optical grade fused silica. The absorption cells have an amalgamated lead or Teflon spacer approximately 0.1 mm. thick. The exact length of these thin cells is calculated by Beer's law from measurements of the absorbance
Mp
Figure 2. Absorption spectrum of 1-meter length of air at 1 atm.
0.50
190
N- HEPTANE,
195
200
205
210
WAVELENGTH IN M p
0.25
Figure 4.
ISOOCTANE n
190 200 210 WAVE LENGTH IN Mp Figure
3.
220
Absorption spectra of plus absorption cells
230 solvents
Effect of modifications on spectral purity
A . Spectrophotometer Serial No. D-277 with selected 1P28, selected Beckman hydrogen lamp, and 1-om. cell filled wlth air-saturated n-heptane B. Spectrophotometer Serial No. 1336 with same lamp, detector, and absorption cell. slit windows removed C Same as B b u t with'0.0lPcm. cell filled with n-heptane 0: Spectrophotometer Serial No. ,1336 with EM1 6255, AllenNester lamu. 0.018-om. cell with n-heptane; monochromator flushed with nitrogen
V C L U M E 2 8 , N O . 11, N O V E M B E R 1 9 5 6
60,000
I
1 I **MODIFIED BECKMAN -REFERENCE 6
1709
1
8000
30,000
0
Figure 5 .
I90 200 210 WAVELENGTH IN MP
220
230
Absorption spectrum of 1,3-dimethyl-sethylbenzene
0 190
200
210
WAVE LENGTH IN M p of a solution in the thin cell and the absorbance of a quantitatively diluted aliquot of the same solution in a standard 1.OOO-cm. absorption cell. It has been found practicable to match lengthe of pairs of cells of this type to about 1 to 3%. A cell compartment of conventional design was constructed of black Phenolite to accommodate two cells of this type. The performance which was achieved with two Beckman DU spectrophotometers modified in varying degrees is shown by Figure 4.
Curve A was obtained with an instrument (Serial No. D-2Ti) that had been completely overhauled about 2 years previously, and was fitted with the best Beckman hydrogen lamp and the best IP28 photomultiplier available in the laboratory. The slit windows were still in place and standard 1-em. cells filled with +heptane were used in the measurements shown. The instrument could not be balanced a t wave lengths below 203 mp with a 2-mm. slit width. False energy was about 8% a t this wave length and about 1.7% a t 210 mp. The other curves of Figure 4 v-ere obtained with a second instrument (Serial No. 1336), which had been in use about 7 years prior to the work described here. Curves B and C were obtained with the same selected photomultiplier and hydrogen lamp which had been used in the other instrument. The windows had been removed from the monochromator for this experiment, however. The false energies obtained with the 1-em. cell were similar to those observed with the first instrument. With a cell 0.012 em. in length filled with nheptane, a tenfold reduction in false energy a t 200 mp was obtained and the measurements could be extended from 200 to 192 mp, although the false energy a t 192 mp was about 11%. The maximum in false energy near 195 mp in curve C corresponds to one of the Schuman-Runge absorption bands of oxygen (Figure 2). By flushing the monochromator with nitrogen and using the lamp and multiplier with silica windows it was possible to reduce the false energy a t 192 mp to about 4Oj,. As the mirrors in this instrument were old and the Allen-Nester lamp and the EM1 multiplier were randomly selected, it is possible that performance could have been improved further by selection of these components. Below 220 mp, the mirrors rapidly lose reflectivity and can deteriorate significantly in a few months. A practical criterion of adequate spectral purity is that calibration curves obtained with a spectrophotometer should follow Beer’s law. The data of Table I1 show that the fully modified instrument satisfied this criterion reasonably well. The absorbance data have been corrected for the 1.4% false energy a t 200 mp as measured with the Vycor filter. It is apparent that there is a small residual nonlinearity which results in a 5% spread of absorptivities over the 0.2 to 1.0 range of absorbances. This is probably due in part to underestimation of the false energy by the Vycor filter technique and in part to nonuniformity of the thickness of the spacer over the free aperture of the absorption cells. The average molar absorptivity of 52,200 for 1,3-dimethyl5-ethylbenzene at 200 mp is in good agreement mith the value of 54,000 f 2700 obtained with the vacuum spectrometer (6).
Figure 6. Absorption spectrum of trans-2-hexene
Figure 5 shows that the spectrum of this compound measured with the modified Beckman DU is in good agreement with that from the vacuum spectrometer over the range of the former instrument, Figure 6 indicates similar agreement in the case of the spectrum of trans-Zhexene.
Table 11. Test of Beer’s Law (1,3-Dimethyl-5-ethylhenzene~ in 0.0188-om.cell a t 200 mp) concentration, Corrected Absorbance, Molar Moles/Liter log10 Io - 0.014Io Absorptivity Liters/Mole cL. I - 0.014 Io x 10’ 2.06 4.12 6.17 8.23 10.29
0.204 0.396
54,100 52,500 51,600 0 771 51,200 0.970 51,500 Av. 52,200 a American Petroleum Institute standard hydrocarbon No. 566-55. Purity, 99.89 i 0.06 mole %. 0,682
It has been found practical to transfer quantitative methods from the vacuum spectrometer to the modified Beckman DU spectrophotometer without recalibration and without significant loss of accuracy. The modified instrument has now been in routine use for about 2 years. LITERATURE CITED
(1) Allen, A. J., Franklin, R.G., J Opt. SOC.Amei.. 29, 453-5 (1939). (2) Am. SOC.Testing Materials, Philadelphia, ”ASTN Manual of Engine Test Methods for Rating Fuels,” p. 116, 1952. (3) Brode, W. R., “Chemical Spectroscopy,” 2nd ed., p. 520, Wiley, New York, 1943. ( 4 ) Burdett, R. -4.. Taylor, L. W., Jones, L. C., Jr., “Molecular Spectroscopy,” pp. 30-41, Institute of Petroleum, 26 Portland Place, London, W.1, 1955. (5) Huke, F. B., Heidel, R. H., Fassel, V. A., J . O p t . SOC.Amer. 43, 400-4 (1953). (6) Jones, L. C., Jr., Taylor, L.W., ANAL.CHEM.27, 228-37 (1955). (7) Kayser, H., “Handbuch der Spektroscopie,” vol. 5 , p. 538, S. Hirzel, Leipaig, 1910. (8) Packer, D. M., Lock, C., J . Opt. SOC.Amer. 41, 699-701 (1951). RECEIVED f o r review
March 16, 1956. Accepted July 25, 1956. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., 1966.
