Table V. Resultsa of Computer Searches for Compounds Having a Medium Intensity Peak a t 1050 cm-’ Using n o elimination of reference spectra b u t with simplest spectra srrategy
Using n o elimination of reference spectra Reference c o m p o u n d
Score
___~____
Source code
Reference c o m p o u n d
Isobutanol 50 V-325 Sodium 2-ethyl hexyl tripolyphosphate Ammonium alkyl naphthalene sulfonate 50 V-327 2-Ethyl butanol 50 V-328 Sodium heptadecyl sulfate Ai-Pentanol 50 v-330 Petroleum sulfonate 2,2,4-Trirnethyl pentanol 50 V-331 Petroleum sulfonate N-Hexanol 50 V-332 Petroleum sulfonate 2-Ethyl hexanol 0ct a n a1 50 v-333 Oleyl alcohol 12-Hydroxy stearic acid 50 v-128 Basic barium dinonyl naphthalene sulfonate 50 V-415 Butyl acid phosphate Propylene carbonate 50 V-416 Arabic gum Acetonitrile 50 V-423 1,2,6-Hexanetriol Dimethyl sulfoxide 50 V-654 Ethylene glycol Glyceryl triacetate 50 V-655 Isotridecanol Tributyl citrate 50 V-656 Sodium sulfo succinate Acetyl tributyl citrate 50 V-657 Chromium phosphate Dipropylene glycol benzoate 50 V-6 7 1 Tris(hydroxymethy1)aminoethane Toluene ethyl sulfonamide 50 Y-913 Diammonium phosphate Chlorinated biphenyl a Top 1 7 nonproprietary reference compounds excluding commercially branded products. A
~
-
nificant improvement could be obtained by introducing criteria for matching peak shapes ( I 2 ) . This sort Of evaluation can be done with t h e present program by visually checking t h e peak shapes after t h ei computer ’ , ” retrieves , d ” reference ’ , ~ ~ sDectra on t h e basis of matched Deak location and intensiHowever, computer experiments using peak shape triteria may show significant and unforeseen improvements.
ti.
LITERATURE CITED T. 0. Gronneberg, N. A. B. Gray, and G. Elington, Anal. Cbem., 47, 418 (1975). N. A. B. Gray and T. 0. Gronneberg, Anal. Cbem., 47, 419 (1975). S. R. Heller, D. A. Koniver, H. M. Fales. and G. W. A. Milne, Anal. Cbem., 46, 947 (1974). S. L. Grotch, Anal. Chem., 45, 2 (1973). G. Beech, R . T. Jones, and K. Miller, Anal. Cbem., 46, 714 (1974). A. G. Schoning, Anal. Chm. Acta, 71, 17 (1974).
~
Score
43 43 43 43 43 43 43 43
43
Source COdt.
2-128 z-112 2-4 5 u-42
u-33 u-34 T-29 u-57
43
Q-26 M-101
44 44 44 44 45 46 46
V-635 V-6 2 2 2-402 2-70 V-501 V-561 2-458
(7) s. L. Grotchl Anal. Chem.8 469 526 (1974). (8) G. Horlick, Anal. Cbem., 45, 319 (1973). (9) P. W. Liddell 111 and P. C. Jurs, Anal. Cbem., 46, 2126 (1974). (10) D. R. Preuss and P. C. Jurs. Anal. Chem., 46, 529 (1974). ~
{ii!F: ~ ~
(13) (14) (15) (16) (17) (18) (19) (20)
~ 7 ~n ~ ~ : ~ ~~a ~~ ~~~ ~o$ $ ~~ ~ d ; ~ Cbem,, ~ 46, 955 11974). k. W.’Sebesta and G. G. Johnson, Jr., Anal. Cbem., 44, 260 (1972) D. S. Erley, Anal. Cbem., 40, 894 (1968). D. H. Anderson and G. L. Covert, Anal. Cbem.. 39, 1288 (1967). D. S. Erley, Appl. Specbosc.. 25, 201 (1971). C. S. Rann, Anal. Cbem., 44, 1669 (1972). L. H. Gevantman, Anal. Cbem., 44 (7), 30 A (1972). P. R. Griffiths, Anal. Cbem., 46, 1206 A (1974). M. St. C. Flett, “Characteristic Frequencies of Chemical Groups in the Infra-Red”, Elsevier, Amsterdam/London/New York, 1963.
RECEIVEDfor review October 28, 1975. Accepted January 15, 1976.
Computer Resolution of Infrared Spectra of Unknown Mixtures Tomas Hirschfeld Block Engineering, lnc., 19 Blackstone Street, Cambridge, Mass. 02 739
The spectrum of a mixture of an unknown small number of unknown constituents in unknown proportions can be resolved into the spectra of these constitutents without isolating them. By computer manipulation of repeat spectra of partially fractionated samples, an array matrix inversion may be constructed that gives recognizable spectra of the mathematically separated constituents.
T h e IR spectrum of a mixture where all b u t one constitue n t is known can be resolved if reference spectra of t h e known constituents are available (this of course implies near perfect registration and very good S/N ratios). This procedure. known as “spectrum stripping” ( I ) operates by the successive subtraction of the reference spectra, multiplied by appropriate concentration factors, from t h a t of t h e mixture.
T h e extension of this technique to unknown concentrations of a known constituent is trivial in a computerized spectrometer, where trial and error adjustments of the concentration factors are used until the known interferent peaks just disappear. If a nonoverlapped peak of t h e known constituent exists, calculation procedures can be used to accelerate the convergence of the trial and error procedure (one-step operation is unlikely, however, because of polychoric mixing, refractive index changes, and Beer’s law departures). In this note, a procedure will be described t h a t extends “spectrum stripping” to the general case of a mixture of unknown concentrations of a n unknown number of unknown constituents. T h e method involves ratioing the absorbance spectra of the mixture before and after a partial fractionation procedure. Such a procedure might be standing a few hours in a ANALYTICAL CHEMISTRY, VOL. 48, NO. 4 , APRIL 1976
721
HEXANE-CYCLOHEXANE-TOLUENE
MIXTURE
100,
!
" 3 d 0 0 '
2000 "
"
'
I O '0 0
7 , %7
C r - ''
!
1
I)'
"
" U N M I x I NG "
, I
c OE FF I c I E N T S
,C
Flgure 3. Ratio of absorbance spectra of two successive mixtures
I 0 !
L..
3000
'
'
2000
I
'
'
'
000 cm-
Figure 1. Reference spectra of pure components of ternary mixture
(heated) watch glass, partial dissolution in a solvent, extraction, filtration through absorbents, use of leading and trailing edges of unresolved chromatographic peaks, etc. Only slight separation is required, and one need not know its extent or the behavior of the individual components in it. Mathematically, we have for the initial mixture spectrum
i 3000
2000
1000 cm-1
"
mc,
2000
CYCLOHEXANE
"
! I I I I000 c m -
and after partial separation "HE X A ~ E "
We can now see t h a t for t h e ratio spectrum
will be constant and equal to a, in any range of Y where f n ( u ) dominates the spectrum. R(u) will therefore consist of a number of flat regions linked by curves and slopes. T h e number of flat regions of differing height will give the number of constituents, and allow assembling a set of (unassigned) a, 's. A flat region will occur lcherever a single constituent dominates t h e spectrum, whether it has a peak there or not. T h e maximum number of constituents that may be studied is limited by t h e requirement t h a t at least one flat region must exist in R ( v ) for each constituent, which becomes statistically unlikely as the number of overlapping spectra increases. A weaker limitation of N is set by the requirement a, # a,, as constituents having the same a, will not be detected separately, and will give a single, summed spectrum during t h e separation procedure. However, accidental coincidences of an's will evidence themselves as changes in the size of the set of flat region heights as the measurements are repeated with a different or more intense fractionation. One must remember here that a, # 1 even for a compound unaffected by the fractionation procedure, owing to the changes in the balance of the mixture. 722
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
Figure 4. Computer resolved spectra of mixture components
In practice, this procedure will practically always work for two-component mixtures, and most of the time for three-component ones. For mixtures with N I 4, the procedure will onl~7exceptionally be successful. R ( u ) will be quite noisy in both the M ( Y ) 0 (no absorption) and M ( Y ) ,M ' ( Y ) m ("bottoming out") spectral regions. In the former case, addition of a very small constant to the spectra before ratioing will eliminate the noise but produce a false flat region a t 1 wherever there is no absorption. I t is preferable to avoid the problem completely by thresholding out low (or very high) absorbance regions in the spectrum. Having determined N from R ( o ) ,the fractionation is repeated a t different intensities or with different procedures a further iV - 2 times, and a set of N - 1 spectra
-
-
is obtained, from which one calculates (for cross checking purposes)
T h e set of fr,(u)’s can now be obtained by matrix inversion of the set of spectral arrays. Here we use the set of coefficients a, rI plus a line of 1’s (corresponding to M ( u ) )in t h e denominator and substitute a sliding row of spectral arrays M , ( u ) in t h e denominator to obtain t h e set of component spectra. In addition to SO.01 cm-’ spectral registration (forcing the use of a laser referenced interferometer type spectrometer), and a very good S/N ratio plus a built-in computer (again favoring the use of Fourier systems), an array oriented software is required to make this procedure convenient. ( T o the author’s best knowledge, these requirements are met only by t h e Digilab FTS spectrometers). T h e procedure used would then typically be: 1) T h e sample‘s spectrum is measured, t h e mixture left to evaporate for a while, and the measurement repeated. 2) T h e spectra are array ratioed, t h e number of flat regions of different height counted ( N ) ,and their height measured ( u n ) .3) Assuming .V = 2. one obtains the pure component spectra via the operations fI(U)
1 = Ml(Ui ____ U I-a>
f?(V)
= M , ( v , -- M ( u ) -
a2
- M ( u )a1 - a2
1
a2
- a1
scaling trail and errors. This procedure for “unmixing” the spectra of mixtures requires enough memory space to store a minimum of N 1 spectra and a signalhoise ratio D K N times higher than desired in t h e final spectrum of the components, where D is t h e dynamic range between the absorption of t h e component and t h a t of the mixture a t the frequency for which t h e noise is specified (first-order approximation). To experimentally demonstrate t h e procedure, a toluene-cyclohexane-hexane mixture was used. Figure 1 shows t h e separate reference spectra of the components, and Figure 2 that of their mixture. Figure 3 shows the ratio of the absorbances of this and another one where the more volatile constituents have become depleted, with only t h e flat regions shown. T h e insert shows in more detail the appearance of the complete plot and that of one such flat region. T h e end result of t h e procedure described here shows t h e three separate spectra obtained for the components, Figure 4. Minor cross-contamination is evident in t h e spectra, b u t t h e components could readily be identified through them.
+
LITERATURE CITED (1) Ken Kizer, Digilab, Inc., 237 Putnam Avenue, Cambridge, Mass., 02139, private communication, 1975.
a1
a2
- a1
and plots them under automatic scale expansion to avoid
RECEIVEDfor review J u n e 2, 1975. Accepted January 5 , 1976.
Identification of Characteristic Chromophores in Gas-Phase Ions by Photodissociation Spectroscopy Robert C. Dunbar Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44 706
Photodissociation spectra are shown for parent ions of a number of hydrocarbons containing various patterns of unsaturation, and it is shown that the spectra are highly characteristic of structural features. Classes of ions readily identified include saturated hydrocarbons, compounds containing single (or isolated) double bonds, conjugated dienes, and benzene derivatives. Spectral differences arising from substituents and cyclic geometry are smaller but still significant. For all except the saturated hydrocarbon ions, the spectra are well understood in terms of previous theoretical work on r -* R* transitions in radical ions.
Optical spectroscopic approaches to characterizing gasphase ionic species, of which t h e new technique known as ion photodissociation spectroscopy is notable, are interesting in their potential applicability to analytical problems. Photodissociation spectroscopy has the extremely low sample-quantity requirements and the automatic discrimination of components of mixtures which are common to mass spectrometric analytical methods; a t the same time, it possesses the cardinal advantage of optical spectroscopic analysis in that the spectra are characteristic of the optical chromophores of the molecule. and therefore of its chemical structure. This approach thus unites attractive features
of mass spectrometry and optical spectroscopy in a way which may be complementary t o both. T h e actual instrumental capability for achieving this is very recent, and it is t h e purpose here to describe the principles and techniques involved, and to illustrate with some examples t h e kinds of analytically useful information which are now available. Direct determination of the optical spectrum of a gasphase cation by observing its absorption spect,rum is a straightforward and obvious approach, but, in practice, t h e difficulty of building u p a usable concentration of gasphase ions has restricted such measurements t o a handful of very simple ionic species ( I , 2 ) . However, it has been increasingly recognized t h a t a more indirect, and more tractable, approach to this problem through ion photodissociation gives spectra which are for many purposes equivalent to direct absorption spectra (3-10). T h e principle involved is that the dissociation of an ion by photon absorption, which can be readily observed, necessarily implies t h a t the photon wavelength corresponds to an optical absorption of t h e ion. T h u s t h e wavelength dependence of photodissociation is expected to show peaks corresponding to some or all of the allowed optical transitions, and this expectation is borne out in practice in a very satisfactory way. Numerous other approaches have been described to the problem of identifying and distinguishing gas-phase ions in a more selective and powerful way than inspection of 70-eV ANALYTICAL CHEMISTRY, VOL. 48,
NO. 4,
APRIL 1976
723
