Matrix Isolation Fourier Transform Infrared Spectrometry of Polycyclic Aromatic Hydrocarbons Gleb Mamantov,” E. L. Wehry,* R. R. Kemmerer, and E. R. Hinton Department of Chemistry, University of Tennessee, Knoxville, Tenn. 379 16
The applicationof matrix isolationFourier transform infrared spectroscopy to qualitative and quantitative analysis of polycyclic aromatic hydrocarbons (PAH) Is described. Samples , In nitrogen matrlces at -15 K are produced by mlxing the PAH vapors effusing from a Knudsen cell with a large excess of nitrogen gas. From the results obtained with nearly 20 PAH, analysis of moderately complex mlxtures (-10 components) appears feasible. The applicability of Beer’s law to matrixisolated PAH samples over a large concentration range has been demonstrated. A detection llmit of 0.5 pg of perylene (2 X mol) has been achieved.
In the technique of matrix isolation (hereafter denoted “MI”), sample species, in the gas phase, are mixed with a large excess of inert diluent gas (the matrix); the gaseous mixture is then deposited onto an optical surface for spectroscopic examination a t low temperature. Since the initial description of the technique by Whittle, DOWS,and Pimentel(1) in 1954, MI has been widely used, particularly for spectroscopic characterization of transient species; infrared (IR) absorption spectroscopy has often been employed in such studies (2-14). However, MI-IR spectroscopy has received very little consideration as a technique for the qualitative and quantitative (15)analysis of stable species, though Rochkind (16,17) has demonstrated that a variant of the conventional MI procedure can be successfully employed for the quantitative IR analysis of specific compounds in mixtures of gases. Two principal reasons exist for the lack of utilization of MI in analytical spectroscopy. First, very low temperatures (120 K) are required for successful MI (5);until relatively recently, such temperatures could be obtained only by cryostats using liquid refrigerants. Development of closed-cycle refrigerators, which can attain temperatures of 15 K or below without use of liquid cryogens, has greatly enhanced the facility with which MI experiments can be executed (18).Second, MI-IR spectroscopy has been hampered by the source energy limitations characteristic of many dispersive IR spectrometers; a typical cryostat contains three windows, a t each of which reflection losses occur, and the deposited sample is often highly scattering. Consequently, attainment of adequate S/N ratios for analytical purposes is often difficult when conventional IR spectrometers are employed. This difficulty should be alleviated by use of interferometric Fourier transform (FT) spectrometers (19-24). Thus, recent technological developments appear to have vitiated the principal obstacles to development of MI-IR as a useful analytical technique. The objective of MI is to minimize interactions of solute molecules with each other and with the “solvent” (matrix), so that spectra observed in the deposited solid resemble, as closely as possible, those of isolated molecules in the gas phase at low pressure. Under these conditions, highly characteristic IR spectra, useful for “fingerprinting”, should be readily acquired, and ideal absorbance-concentration relationships (Beer’s law) should be valid, provided that the spectra are measured a t sufficiently high resolution. Furthermore, the rotational fine structure observed in IR spectra of most gas86 * ANALYTICAL CHEMISTRY, VOL. 49, NO. 1, JANUARY 1977
eous samples should be “frozen out” in the low-temperature matrix. Thus, by comparison with the “classical” IR sampling techniques (KBr disks, mulls, liquid solutions, gas cells, ATR of evaporated solutions), MI appears highly advantageous for both qualitative and quantitative IR analysis. In particular, combination of the MI and FT techniques should significantly expand the applicability of IR spectroscopy in the analysis of complex mixtures of constituents present in small quantities. Identification and quantitative determination of specific constituents of “polycyclic organic matter” is currently receiving strong emphasis (25).While the applicability of conventional IR spectroscopy to the analysis of polycyclic aromatic hydrocarbons has been discussed (26), the technique has been virtually ignored in practical analysis of polycyclic compounds. “NOspecial techniques have been developed for IR analysis of polycyclic compounds.. . The disadvantages of vibrational spectroscopy are the relatively weak bands, the fact that IR band strengths are not proportional to concentration, the requirement for a vibrationally transparent medium, and the lack of unique polycyclic structural features. The disadvantages far outweigh the advantages of these techniques” (25).The work reported here was motivated by the belief that the combination of M I with FT-IR spectroscopy should be capable of alleviating the problems cited in the preceding sentences, such that IR can play a very useful role in the analysis of complex samples of closely related compounds, such as polycyclic aromatic hydrocarbons (PAH).
EXPERIMENTAL Matrix Isolation. The key to use of MI for PAH is the observation (27, 28) that polycyclic hydrocarbons (even those having high molecular weights) can be vacuum sublimed at temperatures not greatly above ambient. Consequently, the Knudsen cell method (29, 30), wherein a “molecular beam” of solute is formed by effusion of the vapor through an orifice, can be employed for sample deposition. The Knudsen cell employed in this work consists of a ca. 2-cm length of 7-mm i.d. glass tubing, equipped at one end with a 7/25 female joint and at the other end with a -0.5-mm orifice. Either solid or liquid samples can be placed in the cell (the latter via GC syringe). The sample tube is wrapped with nichrome heating wire, and is attached to an evacuable cryostat head via a 29/42 male joint. The leads to the heating wire are connected to a variable autotransformer for control of temperature; the oven is calibrated periodically by means of a chromel-alumel thermocouple that enters the cell through the effusion orifice. The cryostat head in turn is attached to a commercial closed-cycle helium refrigerator (“Spectrim”, CTI Cryogenics, Waltham, Mass.), which operates via the Gifford -McMahon refrigeration cycle (31). This refrigerator is capable of attaining temperatures as low as 14 K with load. An especially convenient feature of the refrigerator is the provision for heating the cesium iodide window, to remove a deposited sample, without breaking the vacuum. Thus, a sample containing compounds which sublime at different temperatures can be examined by the successive formation of several deposits, providing a measure of fractionation of a complex sample without recourse to use of liquid solvents. A more detailed description of the experimental apparatus has recently appeared (32). In a typical MI experiment employing the continuous deposition method (33), the total deposition time was 1-2 h, depending on sample size. The matrix-to-sample ratio varied from 30O:l to 250,OOO:l (the quantity of matrix was controlled by an external vacuum line
Table I. Strong and Medium MI-IR Absorption Bands of Polycyclic Aromatic Hydrocarbons (Resolution 2 cm-1)
Sublimation Temp ("C)
IR bands
Compounds 1. Triphenylene
1502,1438, 746" cm-l
70-75
2. Chrysene 3. Benz[a]anthracene 4. Pyrene 5. Anthracene 6. Phenanthrene 7. Carbazoleb
1365,1234, (819,816 doublet), 765" 1504,1365,886,809,785, 752"
70-75 70-75
8. Tetracene 9. Fluoranthene
10. Fluorene 11. Acenapthene
12. Perylene 13. Benzo[b]fluorene 14. Benzo[a]fluorene 15. 1,3,5-Triphenylbenzene 16. Benzo[a]pyrene 17. 1,2,5,6-Dibenzanthracene
1436,1185,846," 715 881, (733,730 doublet)," 603 1504, 1462,870,818, 740.5," 617 1496, 1464.5, 1454, 1397.5, 1338," 1328,1241,754, 729.5, 618,569 900.5," (747,744.5 doublet), 551.5 1461, 1455.5,1444, 1138,830 (782, 778 doublet)," 750, 620 (1483.5, 1480 doublet), (1456.6,1451.5 doublet) 1408, 1301,1190,1005,958,746," 696,622 (1431, 1422 doublet), 1374, 1017.5,843, 792," 750 1498, 1385,818," (776, 774 doublet) 957, 872, 775,a 764,728,570,475 1223, 1020, (828,826 doublet), (763,760.5 doublet)"
45 45 45 50 90-100 40
