Table I11 lists cstw-. art1 their R values, which were uscd to tleterniine whether or not branching or substitution in the hydrocarbon chain affected adsorption on the column. Because the carbonyl oxygen is considered the principal adsorbing atom, through hydrogen bonds to electron acceptor sites on the adsorbent, the presence of bulky atoms or groups of atoms on iicighboring carbon atonis could partiall:- block adsorption of the adsorptive on the column. The chlorine atom could be involved in steric hindrance of the carbonyl oxygen, it could itself be adsorbed, or by virtue of its electronegativity it may exert inductive influences on the adsorbing atom which n-oultl lessen
affinity of the molecule for the adsorbent. The size of the chlorine atom does not suggest any particular potential for it as a blocking atom, nor is there evidence that the chlorine atom has more than very slight affinity for adsorption columns. Electronegativity of the chlorine atom would reduce the electron density about the carbonyl oxygen, thus lowering adsorption of the adsorptive. This is reflected in the R values measured for the halogenated esters, and i n particular ethyl trichloroacetate. ACKNOWLEDGMENT
The authors wish to express their
appreciation to the Research Corp. for financial assistance niaking this work possible. LITERATURE CITED
(1) Feigl, F.,, “Spot Tests in Organic Analysis,” p. 173, Elsevier, Ken-
York, 1957. ( 2 ) Zbid., p. 237. (3) Goddu, R. F., LeBlanc, F., Wright, C. hl., ANAL. CHEW 27, 1251
(1955).
(4) LeRosen, A. L., Monaghan, P. H., Rivet, C. -4,, Smith, E. D., Zbtd., 23, 730 (1951).
RECEIVEDfor review August 14, 1957. Accepted January 27, 1958.
Use of Substitute Standards in Infrared Differential Spectrophotometry W. H. WASHBURN and M. J. MAHONEY
Abbott laboratories, Norfh Chicago, 111. ,The accuracy in infrared differential analysis for a specific impurity is often limited b y the presence of small amounts of the impurity in the standard material. This limitation can b e overcome to a great extent by substituting for the reference standard a known pure compound of different chemical structure, but having nearly identical absorption characteristics in the area of analytical interest. The important factors in selecting a proper substitute are discussed.
can be largely eliminated if another compound, obtainable in a pure state and possessing the same absorption characteristics as the major component in the region of analytical interest, is substituted for it in the reference beam. The choice of a suitable substitute reference is dependent upon a number of factors. The relative importance of these factors was investigated and is reported in this study. RECOMMENDED PROCEDURE
D
.IXALTSIJ has frequently been applied to the determination of small amounts of known impurities in relatively pure materials. I n the differential method. employed when there is overlapping of absorption bands of major and minor components, the absorption due to the major component is essentially subtr:tcted from the total absorption. The remaining absorption is due to the minor component (1-9). If, however, sonie of the impurity to be measured-for example, lyc-is prebent in the niaterial to be used as standard, all determinations based on this standard will be 1% lower than the true value. The accuracy is thus dependent on the availability of a standard sample free of the impurity to be measured. Frequently, due to difficulties in purification, such a standard is not available. This problem is often encountered when the major Component and the impurity are isomeric. This limitation of differential analysis
IFFERESTIAL
-4 11 qualitative and quantitative measurements were obtained using a PerkinElmer Model 21 double beam infrared spectrophotometer equipped with sodium chloride optics. \Then a reference library of spectra is scanned to select a possible substitute standard, i t is necessary to consider the following points. The peak absorption of the substitute should be within 1 0 02 micron of the peak absorption of the major component. The slope of the absorption band of the substitute standard should be closely parallel to that of the absorption band of the major component through an interval a few hundredths of a micron on either side of the analytical peak of the minor component. These rather close specifications require that one obtain the pure reference compounds and check the absorption requirements on one’s own spectrometer. Solvent, cell thickness. and concentration, if not already decided upon, should be established a t this point as though the determination n-ere to be a conventional differential analysis (1-9).
A further examination of the possible substitutes is now undertaken by espanding the abscissa or wave length scale from the usual 2 inches per microii to 16 inches per micron. With nothing in the reference beam, the sample (standard in question) is now run through the interval of interest, a t the concentration and cell thickness determined above. I n the case where the impurity peak lies on the short wave length side of the major band, this interval is roughly from a point 0.10 micron shorter wave length than the peak of the minor component, to the peak of the major component. When the impurity peak lies on the long wave length side of the major band, the interval of interest is reversed. Next, the several substitutes being considered are superimposed on this run, a t concentrations such that all substitutes absorb somewhat less intensely than the sample (about 0.10 t o 0 . 2 5 absorbance unit less than the sample in the range of 0.02 to 0.03 micron on either side of the peak absorption point of the minor component). The best substitute will be the material absorbing most nearly parallel to the sample run over this 0.04- to 0.05-micron portion of the interval, and also absorbing more strongly than the sample a t a point about midway between the minor component peak and the major component peak of the sample. The slight undercompensation of the major component from 0 . 0 2 t o 0.03 micron on either side of the peak absorption of the minor component ensures against loss of the minor component absorption band in the differential curve. The slight overcompensation of the major component at a point a few hundredths of a micron VOL. 30, NO. 6, JUNE I958
1053
Table 1.
Results of Analyses on Synthetic Mixtures
C0ncn.a Major Peak, Xlinor Peak, and Cell Component p Component p Thickness 1 Tridione 10.04 Diphenyl 9.90 lOYc, 0 . 5 mm. 2 -4spirin 9.89 Tridione 10.04 5%, 0 3 mm. 3-Heptene 10.32 Caffeine 10.23 5%, 3 0 . 5 mm. 4 Butyn base 8.97 Colchiceine 8.77 2%, 0 5 mm. Chloroform concentration.
Mixture
a
Substitute Reference 7-Picoline p-Dichlorobenzene Acrylonitrile Procaine base
Known Concn.0 of Content Reference of Minor PfePeak, and Cell Component, Found, cision, p Thickness % % % 10.03 15%, 6.0 6.8 2 ~ 0 . 6 0.5 mm. 9.88 25%, 1.5 1.1 1 0 . 1 0 . 3 mm. 7.0 8.9 h0.5 10.38 27,, 0 . 5 mm. 10 5 10.3 10.0 8.98 1.8%, 0 . 5 mm.
.3
panded, uncorn p e n s ated spectra of Sample versus substitute references A, 8, and
3 n 8
C
m a
10%
Figure 2. Expanded, yncomp e n s a t.e d spectra of sample versus severa! concentrations of substitute reference
l
,5
A 1.0
PEAK
MALOR
MINOR PEAK
PEAK 9.8
PEAK
10.0
9.9
10.04
WAVE LENGTH IN MICRONS
past the impurity peak serves to remove all distortion effects of the major component immediately after the impurity band has been resolved. Figure 1 s h o w that substitute A is the preferred one in the case illustrated. Having selected a substitute reference compound, its optimum concentration is now determined. The expanded spectrum of the sample is run again in the interval of interest. Upon this are superimposed several dilutions of the substitute in the general concentration range used for this particular substitute in the previous series of runs. The best concentration here is the one most nearly parallel to the sample for about 0.02 micron on either side of the peak of the minor component, and also absorbing more strongly than the sample a t a point about midway between the minor component peak and the major component peak of the sample. I n Figure 2 the 15% level represents the best compromise between these two factors in the case illustrated. The concentration of the substitute is now adjusted to its optimum level by running a series of differential runs with sample in the sample cell and substitute in the reference cell, and further adjusting the concentration of the substitute reference until the observed minor component peak occurs within = k O . 005 micron of its true absorption maximum. With the concentration of substitute thus determined, known small amounts 1054
ANALYTICAL CHEMISTRY
' V
STANDARD DIFFERENTIAL
i
SUBSTITUTE
REFERENCE
DIFFERENTIAL
Figure 3. Typical absorbance measure- Figure 4. Typical absorbance measment in differential analysis urements using substitute references in differential analysis
of minor component may now be added to the sample in question, a Plot made of concentration vs. absorption, and the curve extrapolated through Ier0 to determine the amount Of minor 'Omponent present in the original sample. RESULTS AND DISCUSSION
The factors pertinent in selecting substitute reference materials were in-
vestigated using mixtures of a number of known pure substances. When these factorshad been worked out, arbitrary mixtures were supplied to one of the authors. Of these mixtures, trimethadione (3,5,5-trimethyl-2,4-osazolidinedione) in aspirin represents the case of an impurity occurring on the long wave length slope of a major band. ,4 substitute reference for this type of case is
clioaeii in the same manner as outlined above, but the expanded scale spectra then appear to be roughly mirror images of Figures 1 and 2, which represent the case of an impurity occurring on the short wave length slope of a major hand. The individual to n hom the mixtures nere submitted was given the problem or" determining the amount of minor coniponent. For this investigation the iiiisture and pure minor component nere supplied but not the pure major component. In each case the unexpaiided spectrum of the mixture showed no distortion of the major band that nould suggert the presence of an impurity.
Results of this Rtudy, including choice alveiit, concentration, and cell thicknesses, are shown in Table I. In mixture 2 the minor component is a strong absorber. I n mixtures 1, 3, and 4 the niinor component in each case ahsorbs rather n-eakly. The determination of intensity value iq necessarily in terms of two points rather than the usual three points employed in the standard base-line measurement. The two significant points are the peak absorption of the minor component and the shoulder on the side farthest from the major band. The shoulder lying clorest t o the major band 01
of course, displaced from its normal position because of deliberate overcompensation. Figures 3 and 4 illustrate this situation. The degree of success in choosing a substitute reference for a specific problem is dependent on the size of the collection of reference spectra and the availability of the pure reference compounds. With large collections and a degree of patience and experience. the major absorber can be closely simulated. However, after examining a large number of bands under expanded scale, the authors believe that nature will seldom, if ever, exactly duplicate a particular absorption band contour. By expanding the wave length scale, it is ea3y to detect differences in a series of absorption bands that a t the standard 2 inches per micron appear to be virtually superimposable. Though a qaniple run a t standard scale may shonno indication of any distortion of the major band, it \vi11 often show a definite flattened area near the peak point of the minor component with the espanded scale. Thus, merely expanding the abscissa may be sufficient in many cases to detect the presence of a suspected impurity and to make a semiquantitative determination. The practicing spectroscopist is acutely aware of the limitation imposed
upon him by the degree of purity of thr best standard material available. T-irtually all determinations are in ternis of comparison to a like standard. In differential work particularly, this standard all too often leaves something to be desired regarding freedom from the contaminant being analyzed. Selection of R substitute reference known to be free of the contaminant to be determined should help to emancipate the infrared spectroscopist from dependence on questionable standards. The authors hope that this approach nil1 be considerably expanded in the near future, not only in quantitative studies but also in the study of overlapping bands in structcral investigations. LITERATURE CITED
Bard, C. C., Porro, T. J., Rees, H. L., h s . 4 ~ .CHEY. 27, 12 (1955). Beroza, M., I b i d . , 25, 112 (1953). Freeman, S. K., Ibid., 27, 1268 (1955). Ibid., 29, 63 (1957). McDonald, I. R. C., Watson, C. C., Ibid., 29, 339 (1957). Powell, H., J . A p p l . Chem. 6, 488 (1986). . 24, 619 Robinson, D. Z., ~ A L CHEX (1952). Washburn, W,H., A p p l . Spectroscopy 11, 46 (1957). Kashburn, W. H., Scheske, F. .4., -4s.4~.CHEM.29, 346 (1957). RECEITEDfor review rlugust 16, 1957. .kcepted January 8, 1958.
Determination of Carbon Dioxide in Automotive Exhaust by Means of Infrared Filter Photometer 1. L. PARSONS, J. C. NEERMAN, J. R. LIFSITZ, and F. R. BRYAN Scientific laboratory, Ford Motor Co., Dearborn, Mich. ,An infrared filter photometer has been designed for the determination of carbon dioxide in automotive exhaust. The instrument utilizes filters of the multilayer interference type which limit transmittance to the 4.29micron wave length a t which carbon dioxide absorbs radiation. Carbon dioxide concentrations from 1 to 18 volume % are determined within 10% of the amount present. Results obtained with the photometer on exhaust gases tend to b e slightly higher than Orsat results.
S
the recent advent of interference filters for use in colorimetry. there have been continued efforts t o extend the usefulness of such filters into the ultraviolet and infrared regions. Important applications of interference filter photometers have been reported for the 1- to Cmicron region of the inIh-cE
frared (2, 4). Improved evaporation techniques and new substrate materials are n o v providing interference filters capable of isolating narrow bands above -1 microns (1). An interference filter photometer for the measurement of carbon dioxide a t 4.29 microns has been designed and utilized as a monitor to indicate the carbon dioxide content of gasoline engine exhaust. DESCRIPTION OF ANALYZER
Interference Filter Assembly. The filter provides high transmittance at 4.29 microns and minimum transmittance a t all other wave lengths ( 3 ) through a combination of elements consisting of narrow-band-pass and long-wave-length-pass interference filters on a substrate which serves as a short-wave-length-pass element. Filter combinations of this type were obtained from A. F. Turner, Bausch & Lomh Optical Co.
The narrow-band-pass element consists of a multilayer dielectric interference filter of germanium and cryolite deposited on one side of a thin glass substrate. Transmitting side bands of longer wave length than 4.29 microns are suppressed by this glass substrate. The side bands a t lower wave lengths ( I to 3 microns) are masked by an interference-type long-wave-length-pass filter used in conjunction with the narrowband-pass filter. The spectral transmittance of this combination of filters, as measured on a Perkin-Elmer Model 13 spectrometer, is shown in Figure 1. Peak transmittance a t 4.29 microns is 37yc and pass band half-width is 0.069 micron. Sample Cell. The relatively high concentrations of carbon dioxide in automotive exhaust necessitate a sample cell of short path length. Absorption measurements on carbon dioxide indicate t h a t a cell path length of 1 cm. is appropriate for the concentrations encountered. VOL. 30, NO. 6, JUNE 1958
1055
