feature articles

Infrared Intensities of Liquids III: The Photometric Accuracy of FT-IR Transmission Spectra of C5Hlo, CH3NO2, CH2C12, C6H6, C6Hs'CH3, and C6HsC1 in the Liquid State at 25°C in 11- to 500-um Cells

JOHN E. BERTIE,* R. NORMAN JONESt and VICTOR BEHNAM$ Department o[ Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada

This paper completes the report of our FT-IR measurements of trans- mission spectra of organic liquids. These measurements were under- taken in order to estimate the photometric accuracy of our FT-IR spec- trometer for transmission spectra by comparing the results with those measured previously [Applied Spectroscopy 34, 657 (1980)l by a cali- brated dispersive spectrometer. We have studied 97 bands of chioro- benzene, toluene, and dichloromethane, in 500-~m cells and 54 bands of benzene, chlorobenzene, toluene, cyclopentane, and dichloromethane, in cells 11 to 53 ~m thick. The average value of the imaginary refractive index at the band peak, km~x, is reported for each band and compared with the calibrated value. When averaged over the 184 bands that are reported in this work and our earlier paper [Applied Spectroscopy 39, 401 (1985)], our measurements agree with the calibrated values to about 3% of km~, and have a 90% confidence limit of about 0.7 to 1% of km~. The calibrated values themselves have an estimated error of about 6%, so to obtain better knowledge of the accuracy of our measurements we need better standards and we need measurements to be made on other FT-IR instruments. We plan to publish simplified procedures that will encourage others to make such measurements. Our 90% confidence lim- it includes effects due to the use of different cells, daily realignment of the fixed mirror of the interferometer, realignment of the optics of the detector and sample compartments, change of beamsplitter, and a day- to-day variation of uncertain origin, in addition to the error sources that always contribute to typical analytical precision. Our data do not show these additional error sources to be systematic, so it is appropriate to include their effect in the evaluation of the confidence limit. We believe the main source of error to be the effect of the infrared cell on the path of the light beam through the instrument. The day-to-day variation of uncertain origin is thought to be due to very small changes in the beam path through the instrument, which in turn cause the effect of the in- frared cell on the beam to vary.

Index Headings: Infrared; Instrumentation, FT-IR; Interferometry, mid- infrared region; Spectroscopic techniques.

INTRODUCTION We have recently reported some preliminary values

for the peak intensities of selected absorption bands in the infrared spectra of CH2CI~, C~HG, and C~F~ in the

Received 4 September 1985. * Author to whom correspondence should be sent. I" Distinguished visitor to the University of Alberta, 1982-83. Present

address: Division of Chemistry, National Research Council of Can- ada, Ottawa, Ontario K iA 0R6, Canada.

:~ Present address: Department of Chemistry, McGill University, Mon- treal, Quebec H3A 2T6, Canada.

liquid state as measured on our FT-IR spectrometer, t These form part of a study to evaluate the photometric accuracy of FT-IR absorption measurements by trans- mission spectroscopy.

In this paper we present the remainder of our results on an extended range of liquids and we discuss their significance in assessing both the precision and the ac- curacy of intensity measurements on our FT-IR spec- trometer under conditions relevant to general practice in quantitative infrared spectroscopy.

A descriptive account of the principles which underlie this study and an overview of the results have been given elsewhere. 2 In this paper the principles are briefly men- tioned in order to orient the reader, and the detailed results are presented.

It is generally recognized that the precision of the in- tensity measurements on FT-IR spectrometers is high. This is because the random errors in FT-IR spectropho- tometry are small and a series of measurements under given operating conditions exhibit excellent reproduc- ibility. At present, however, little information is avail- able concerning the accuracy of such measurements and no method for determining the absolute photometric ac- curacy of an FT-IR spectrometer has yet been reported in the literature.

Accuracy expresses the correctness of a measurement and precision its reproducibility2 The need to deter- mine the accuracy of the wavenumber measurements of an infrared spectrometer has long been recognized by infrared spectroscopists, although the regularity with which this determination must be made has been re- duced by the use of FT-IR spectrometers. This deter- mination has been a subject of extensive study by the IUPAC Commission on Molecular Structure and Spec- troscopy, culminating in its Tables of Wavenumbers for the Calibration of Infrared Spectrometers, first pub- lished in 19614 and revised in 1977. 5 However, most in- frared spectroscopists are rarely called upon to consider that the intensity measurements of their spectrometers, however precise, may be significantly inaccurate.

The accuracy of a measurement reflects both the ran- dora errors and the systematic errors of measurement. The precision reflects only the random errors. For most

Volume 40, Number 4, 1 9 8 6 0003-7028/86/4004-042752.00/0 APPLIED SPECTROSCOPY 427 © 1986 Society for Applied Spectroscopy

analytical work one requires the highest precision to compare the intensity of the unknown with that of a standard reference sample. For such work the accuracy is of lesser importance; in fact, it is strictly of no impor- tance if one can keep it constant for both the sample and reference spectra. To achieve the highest precision and constant (but usually unknown) accuracy, one tries to keep the systematic errors uniform by keeping all instrumental factors as constant as possible. Thus one uses the same cell for all comparative measurements and, when possible, empties it and refills it in situ; the same beamsplitter is used for all measurements and the same source and detector are used. Any adjustment or realign- ment of the optics is avoided during the course of the measurements.

In order to explore the photometric accuracy of an instrument the experiments must be planned differently so as to assure that the systematic errors are revealed and do not remain constant. Only in this way can their effect on the accuracy be demonstrated. Accordingly, one should repeat the measurements using different sample cells and after realigning the optics of the interferome- ter, source, sample-chamber, and detector--of course, optimizing the system each time. Different beamsplit- ters should be used. The effects of systematic errors in the data processing should be explored by changing the apodization function, the phase correction procedure and the extent of zero filling. The electronic gain and filter settings should also be varied. Finally, the whole exper- iment should be repeated with the use of a different detector and, ideally, on a different spectrometer.

In summary, if one is seeking to evaluate the accuracy of the intensity measurement one should ensure that every possible instrumental factor is varied over the complete series of measurements. Only in this way can an assessment be made of the accuracy and of the kind of agreement that can be expected if the same measure- ments are carried out on a different spectrometer in a different laboratory.

As stated previously, ~ the objective of this work is to assess the photometric accuracy of our FT-IR spectrom- eter when used to measure the transmission spectra of a selected group of neat liquids. We have used a Bruker IFS 113V instrument, sold as the IBM Model 98 in the U.S.A. This is very well suited for this task because of the ease with which the beamsplitter can be changed and the fixed mirror of the interferometer adjusted to optimize the interferogram intensity.

The accuracy determined by such experiments can be expected to be considerably lower than the best preci- sion obtainable in normal analytical work. It is also ex- pected to be much poorer than the intrinsic photometric accuracy of the spectrophotometer itself as measured with an open beam. One factor which contributes to this is that the conventional liquid sample cell contains about 12 mm of window material of refractive index about 1.5. Complex reflection effects may occur because of the part of the beam which is reflected by the cell. This reflected beam may pass through the interferometer for a second time before again becoming incident on the sample cell. 6 Further, the conventional liquid cell may change the position and character of all foci of the in- frared beam which occur after the beam has traversed

the cell, 6 including the focus of primary importance at the detector. In our experiments the cell is in the beam when the sample spectrum is being recorded but not when the reference spectrum is recorded, and a change in the position or definition of the image at the detector could affect the measurement. Such defocussing could also affect spectra measured dispersively, although the use of a matched pair of cells in a double-beam spec- trometer is expected to lessen the error from this source. The effect of the sample on the beam is probably smaller for emission or reflection spectra, and such measure- ments on either an FT or a dispersive spectrometer could have intrinsically higher accuracy on this account.

In accord with the above considerations, we have studied the accuracy of transmission spectra for three reasons. First, neither we nor the instrument manufac- turers with whom we discussed this problem in 1981- 1982 knew of any primary standard, such as a rotating sector, 7 that could be inserted into the beam of a rapidly scanning FT-IR spectrometer. The problem is the high rotation speed needed to exceed, by a factor >~ 10, the modulation frequencies of the radiation beam. High- speed rotating sectors have become available since our work was completed and such measurements of the ab- solute photometric accuracy of the open beam spectro- photometer may now be possible. Second, the literature contains absorption intensity data for a range of neat organic liquids that have been measured on a dispersive spectrometer that was calibrated with rotating sectors between measurements. 7 9 These data were obtained by two successive measurements through the same beam, one with the filled cell in place, and one with the sectors in place but no cell. Except for the sectors, this tech- nique can be duplicated exactly with our FT-IR spec- trometer, so that the earlier calibrated data can be used as provisional intensity reference standards. Third, since the vast majority of infrared measurements are made by transmission through the sample, it is desirable to have some estimate of the accuracy of such measurements independent of the instrument on which they were made.

The methods used to obtain the earlier results have been briefly reported? We have followed the dispersive study in calculating the real and imaginary refractive indices of the liquids from the transmission spectra. These entities n(~) and k(;) are related to the molar absorption coefficients, 8-I° which are more familiar to an- alytical chemists. They are preferred because, through the classical Fresnel equations, they correctly describe the transmission and attenuation of the light as it passes through a cell containing absorbing material. The imag- inary refractive index is maximal at the peak of the ab- sorption band and it is these values of k max that are com- pared between the spectra measured on the FT and dispersive spectrometers. An important advantage of the use of the imaginary refractive indices rather than molar absorption coefficients to describe absorption intensities is that the algorithm used to evaluate k from the trans- mittance largely eliminates errors that might otherwise occur through the use of an incorrect baseline. '1,12

The real and imaginary components of the refractive index are closely related to the real and imaginary com- ponents of the dielectric constant of the absorbing ma- terial. We intend to consider this aspect of these mea-

428 Volume 40, Number 4, 1986

surements in more detail in a later paper. We also intend to provide a tabulation of the molar absorption coeffi- cients of a selected group of bands of potential value as secondary intensity calibration standards for analytical chemists and to discuss some of the profiles and band areas.

The preliminary results reported earlier 1 were for ben- zene, dichloromethane, and hexafluorobenzene, liquids at 25 ± I°C in cells with a pathlength of 23 #m or, main- ly, 500 tim. The average over all bands of the precision of our results was 1.25% of km,. for peaks that trans- mitted less than 50%, and declined to about 5% for peaks that transmitted over 70%. The k .... values (in- tensities) of thirty-four out of thirty-eight bands which had no obvious defects lay within the error limits of the calibrated values (i.e., of the intensities found in the calibrated study 8 by dispersive spectrometry). The av- erage over all bands of the agreement between our mean value of km~ and the calibrated value 8 was about 3 %. The FT-IR values differed by about 2 V2 % when the Ge- on-KBr beamsplitter was replaced by Si-on-CaF2, sug- gesting a systematic error.

We note that the values reported in table III of Ref. 1 for band 36 of C6H6 should read "x 10 -~'' n o t " x 10-4. ''

In this paper we report the results of measurements of nearly 100 bands of liquid chlorobenzene, toluene, and nitromethane in cells with a 500-#m pathlength, and of measurements of 54 bands of benzene, chlorobenzene, toluene, cyclopentane, and dichloromethane in 11 #m to 50 um thick cells.

METHODS

The spectra were recorded on an evacuated Bruker IFS l13V FT-IR spectrometer (marketed as the IBM IR-98 in the U.S.A.) equipped with a deuterated trigly- cine sulfate detector. We used a water-cooled Globar source, 10-ram aperture, Si-on-CaF2 and Ge-on-KBr beamsplitters, 1-cm -I resolution, 1 level of zero-filling, triangular apodization, no optical filter, electrical filters set to the smallest bandpass that caused no distortion of the interferogram, ~3 an optical retardation velocity of 0.66 cm s -~, the average under high correlation of 128 interferograms sampled at 0.63 #m, and 512 data points zero-filled to a 2048-point double-sided interferogram to calculate the phase spectrum for use in Bruker's multi- plicative phase-correction routine. Empirically, the phase angles used to correct the interferogram were less than 15 ° . No gain switching was used, but the gain used for the sample spectrum was sometimes twice that used for the reference. The correction for the different gain by the Bruker ATS computer program was shown to be accurate to < 1 part in 2000.

The sample cells were several home-made, fixed-path, liquid cells (KBR windows), whose design was standard except for the 13-ram aperture, and one Perkin-Elmer, variable-path liquid cell (NaC1 windows). The windows were polished fiat to ~2 fringes of Na-D light across the aperture.

The purification of the benzene and dichloromethane has been described? The toluene, chlorobenzene, cyclo- pentane, and nitromethane were Fisher spectroanalyzed grade, British Drug Houses Reagent grade, Aldrich

TABLE I. Experimental and computational parameters.

Com- pound Region cm -~ nov a Beamsplitter

500-turn cells C6HsCI(t') 4000-3330 1.4974 Ge/KBr C6H~Cl(g) 2810-2010 1.4974 Ge/KBr C6H5C1(g) 2010-1600 1.4974 Ge/KBr CsHsCH3(g) 4190-3500 1.4750 Ge/KBr, Si/CaF~ C,HsCH3(g) 2760-2220 1.4750 Ge/KBr, Si/CaF2 C6HsCH3(g) 2220-1650 1.4700 Ge/KBr, Si/CaF2 C6H~CH3($) 640-505 1.5500 Ge/KBr CH3NO~(g) 4280-3400 1.365 Ge/KBr, Si/CaF2

1.3681 CHINO=(g) 2370-1800 1.355 Ge/KBr, Si/CaF2

1.365 11- to 50-/~m cells

C6H~(g) 4400-3500 1.476 Ge/KBr, Si/CaF2 C6H~(g) 3400-2730 1.475 Ge/KBr, Si/CaF2 C6H6(g) 2130-800 1.474 Ge/KBr C~Hs(g) 2130-1210 1.474 Si/CaF~ C~HsCH3(g) 3300-2670 1.4738 Ge/KBr C~HsCH3(g) 1700-800 1.4571 Ge/KBr C~HsCI(g) 3580-2810 1.4958 Ge/KBr C6HBCI($) 2010---630 1.4915 Ge/KBr C~H,o(8) 1810-690 1.3987 Ge/KBr CH2C12(g) 3510-2600 1.4124 Ge/KBr CH2C12(g) 1760-850 1.4080 Ge/KBr

"n= means in this case n at the highest wavenumber in the range. The kma. values were insensitive to the exact value used.

HPLC grade, and Matheson Coleman-Bell Spectroqual- ity grade, respectively. The cyclopentane, chloroben- zene, and nitromethane were purified by thrice freezing two-thirds of the sample and discarding the remaining liquid. Gas chromatographic and infrared spectroscopic monitoring of this purification indicated no changes in the cyclopentane or chlorobenzene but significant changes in the nitromethane. The toluene was used as supplied. All liquids were stored over 4A molecular sieve.

The FT-IR instrument is kept evacuated. The sample cell is held snugly by a home-made holder which sits in a sample well that is purged free of water vapor by dry nitrogen in ~ 5 min. The daily routine for this work was to install the desired beamsplitter and tune the modu- lator, both without disturbing the vacuum. The pres- sure of nitrogen supplied to the air bearing was finely adjusted until the 100% lines were stable and flat to ±0.1% between 4000 and 400 cm -1 and ±0.3% to 5000 cm -1. Spectra were then recorded, in sequence, of the empty instrument, the empty cell, the full cell, and the empty instrument again to check for drift. The trans- mission spectrum of the empty cell was then calculated and, as described in more detail earlier, 1 processed via programs XXII or XLV of Ref. 14 to yield the thickness, or thickness and wedge factor, of the cell. The trans- mission spectrum of the cell-plus-sample was calculated, digitized at 0.5 cm -1 intervals, and processed via pro- gram XLVI of Ref. 14, which corrects it for reflection effects and calculates the real and imaginary refractive indices of the sample by calculating the transmission of the cell-plus-sample from Fresnel 's equations. The procedure 1,11,12 is iterative and starts from an approxi- mate set of imaginary refractive indices and a corre- sponding set of real refractive indices calculated from the imaginary indices via the Kramers-KrSnig trans- form.

APPLIED SPECTROSCOPY 429

TABLE II. Peak imaginary refractive indices of liquid C~H~CI at 25°C from 500-~m cells.

B a n d P e a k % k=~, number g/cm -1 T Ref . 8" T h i s w o r k b

7 3918.0 72 1.10 (09) x 10 4 1.03 (.01) x 10 -4 8 3899.0 70 1.15 (09) x 10 -4 1.09 (.01) × 10 -4 9 3785.0 36 3.52 (28) x 10-4 3.56 (.02) x 10 -4

10 3772.5 47 2.70 (20) × 10 -4 2.61 (.02) x 10 -4 11 3700.0 25 5.41 (37) x 10 -4 5.32 (.04) x 10 4 12 3654.0 70 1.09 (09) x 10 -4 1.07 (.02) x 10 -4 13 3637.0 65 1.44 (11) x 10 -4 1.38 (.02) x 10 -4 14 3613.5 64 1.46 (11) x 10 -4 1.45 (.01) x 10 -4 15 3490.5 74 8.3 (7) x 10 -~ 7.73 (.14) x 10 -5 16 3465.0 75 8.2 (7) x 10 -6 7.48 (.14) x 10 -5 17 3370.0 77 7.1 (7) x 10 -5 6.43 (.12) x 10 -5 35 2664.0 38 4.80 (38) x 10 -4 4.70 (.02) x 10 -4 37 2616.5 43 4.37 (33) x 10 -4 4.25 (.01) x 10 -4 38 2600.0 44 4.34 (33) x 10 -4 4.21 (.01) x 10 -4 39 2582.0 50 3.44 (25) x 10 -4 3.33 (.01) x 10 -4 41 2544.5 50 3.56 (26) x 10 -4 3.41 (.01) x 10 -4 43 2496.0 71 1.61 (13) x 10 -4 1.53 (.01) x 10 -4 44 2471.5 64 2.19 (10) x 10 -4 2.04 (.01) x 10 -4 47 2406.5 52 3.44 (25) x 10 -4 3.42 (.01) x 10 -4 48 2390.5 43 4.66 (35) x 10 -4 4.56 (.02) x 10 -4 50 2331.0 27 8.01 (55) x 10-4 7.75 (.01) x 10 -4 51 2320.0 27 8.42 (b) x 10 -4 7.73 (.01) x 10 -4 52 2271.0 55 3.31 (24) x 10 -4 3.25 (.01) x 10 -4 53 2240.5 38 5.98 (48) x 10 -4 5.73 (.01) x 10 -4 54 2194.0 70 2.06 (16) x 10 -4 1.92 (.02) x 10 -4 55 2179.5 51 3.76 (27) x 10 -4 3.66 (.01) x 10 -4 58 2089.0 74 1.54 (13) x 10 -4 1.47 (.01) x 10 -4 60 2045.5 74 1.64 (14) x 10 -4 1.50 (.01) x 10 -4 61 2023.5 65 2.66 (20) x 10 -4 2.49 (.01) x 10 -4 65 1881.0 5 2.43 (09) x 10 -8 2.393 (.003) x 10 -3 66 1861.0 0.5 4.65 (12) x 10 -8 4.45 (.03) x 10 -3 67 1787.0 1 3.57 (13) x 10 -8 3.55 (.02) x 10 -8 68 1730.5 1 3.47 (13) x 10 -3 3.48 (.01) x 10 -3 69 1686.0 44 6.73 (25) × 10 _4 6.517 (.008) x 10 -4 70 1645.0 8 2.33 (08) × 10 -~ 2.295 (.004) x 10 -a 71 1622.5 5 2.67 (10) × 10 3 2.642 (.006) x 10 -3

T h e n u m b e r in p a r e n t h e s e s is t he e v a l u a t e d u n c e r t a i n t y in t he t e r - m i n a l d igi t .

b T h e n u m b e r in p a r e n t h e s e s is t h e 90% conf idence i imi t .

R E S U L T S

The cell thicknesses were usually obtained with stan- dard deviations less than 0.1% of the thickness, for all thicknesses; on rare occasions the standard deviation was 0.5%. The wedge factors (half of the difference in thick- ness between the two optical sides of the cell) were ~2.5% of the thickness at l l -#m thickness, declined steadily to ~ 1% at 50/~m, and were negligibly small at 500 #m.

The program XLVI that calculates the optical con- stants requires certain input data and can operate in several modes. 14 Table I shows for each compound the spectral regions used in these computations, the value of n~o for each region, and the beamsplitter used. Tests showed that the calculated k values were insensitive to the exact value of n~ that was used, and two sets of values that gave identical results for nitromethane are included in Table I. Tests also showed that the program gave identical results in similar computation times whether used in the normal or in the subtractive Kra- mers-KrSnig mode, with noo taken from Ref. 8 and cor- responding to the highest wavenumber in the data. The program also gave identical results whether or not the

TABLE III. Peak imaginary refractive indices of liquid C6HsCH 3 at 25°C from $00-~tm cells.

Band km~ n u m b e r ~/cm -~ P e a k % T Ref . 8" T h i s w o r k b

1 4187.0 56 1.71 (12) x 10 -4 1.74 (.05) x 10 -4 2 4161.0 58 1.56 (11) × 10-4 1.62 (.05) × 10 -4 3 4132.0 56 1.74 (12) x 10 -4 1.74 (.05) x 10 4 4 4056.0 12 8.17 (31) x 10 _4 8.15 (.08) x 10 -4 5 4037.0 8 9.80 (37) x 10 -4 9.75 (.05) x 10 -4 6 3981.0 51 2.16 (15) × 10 -4 2.21 (.05) x 10 -4 7 3951.0 53 2.14 (15) x 10-4 2.16 (.04) x 10 4 8 3924.0 52 2.22 (16) × 10 -4 2.19 (.04) x 10 -4 9 3869.0 51 2.27 (16) × 10 -4 2.26 (.05) x 10 -4

10 3847.0 49 2.57 (18) × 10 -4 2.55 (.05) x 10 -4 11 3787.0 59 1.78 (13) x 10 -4 1.77 (.04) x 10 4 13 3724.0 65 1.42 (10) × 10 -4 1.38 (.04) × 10 -4 14 3707.0 66 1.44 (10) x 10 -4 1.36 (.04) × 10 -4 16 3650.0 48 2.69 (19) x 10 -4 2.68 (.04) x 10 -4 17 3624.0 62 1.65 (12) x 10 -4 1.61 (.04) x 10 -4 18 3587.0 69 1.27 (09) x 10 -4 1.20 (.09) x 10 -4 19 3551.0 71 1.22 (09) x 10 -4 1.08 (.04) x 10 -4 20 3519.0 76 9.3 (8) x 10 -5 8.4 (.4) x 10 -6 32 2734.5 5 1.71 (06) x 10 -3 1.71 (.01) x 10 -8 33 2672.0 62 2.29 (16) x 10 -4 2.26 (.03) x 10 4 34 2632.0 53 3.19 (23) x 10 -4 3.25 (.03) x 10 4 35 2604.5 46 4.06 (30) x 10 -4 4.10 (.03) x 10 -4 36 2586.0 27 7.87 (59) x 10 _4 7.66 (.02) x 10 -4 37 2540.0 59 2.77 (20) x 10 4 2.72 (.04) x 10 -4 38 2509.0 61 2.51 (18) x 10-4 2.52 (.03) x 10 4 41 2412.0 41 5.12 (39) x 10 -4 5.25 (.03) x 10 -4 42 2389.0 48 4.38 (32) x 10 -4 4.36 (.03) x 10 4 43 2360 32 7.69 (56) × 10 -4 7.05 (.03) × 10 -4 44 2335.0 29 8.18 (60) x 10 -4 7.84 (.04) x 10 4 45 2312.0 36 6.14 (49) x 10 -4 6.16 (.02) x 10 -4 47 2260.0 49 4.28 (31) x 10 -4 4.36 (.03) x 10 4 49 2207.0 63 2.74 (20) x 10 -4 2.70 (.03) x 10-4 50 2185.0 63 2.80 (20) x 10 -4 2.74 (.03) × 10 4 51 2163.0 47 4.84 (35) x 10 -4 4 .91( .03) x 10 4 53 2068.0 74 1.72 (14) x 10 -4 1.63 (.04) × 10 -4 55 1991.0 37 6.78 (25) x 10 -4 7.10 (.03) x 10 4 56 1942.0 <0 .5 4.62 (11) x 10 -8 4.41 (.05) x 10 -a 57 1872.0 4 2.70 (10) x 10 -8 2.68 (.01) x 10 -8 58 1857.0 <0 .5 4.60 (11) × 10 -8 4.58 (.15) x 10 -8 59 1802.0 1 3.90 (15) x 10 -3 3.90 (.04) × 10 -3 61 1736.0 8 2.22 (08) x 10 -8 2.20 (.02) x 10 -8 62 1697.0 37 8.09 (30) x 10 -4 8.32 (.04) x 10-4 63 1677.0 33 9.34 (34) x 10 -4 9.47 (.04) x 10 -4 93 622.0 25 3.20 (12) x 10 -3 3.29 (.01) x 10 -3 94 537.0 59 1.26 (01) x 10 -8 1.30 (.00) x 10 -8 95 521.0 9 6.67 (45) x 10 -8 6.83 (.05) × 10 -3

.,b See foo tno t e s to T a b l e II .

baseline correction was done before calculation of the apparent k, i.e., the first approximation to k.

The polarization discrimination and beam conver- gence angle were entered as 0.0 and 5.5 °, respectively. The calculated results were insensitive to these values; convergence angles between 3 and 8 ° gave identical re- sults, as did the measured polarization discrimination and no discrimination.

Chlorobenzene, toluene, and dichloromethane were each studied in four 500-#m cells, three fixed-path cells, and the variable-path cell. The average kmax values ob- tained and their 90 % confidence limits are in Tables II to IV for chlorobenzene, toluene, and nitromethane, re- spectively, together with the peak percent transmission of the band and the calibrated value s of k max. The wave- numbers reported are our values, which agreed with those of Ref. 3 within 2 cm -~ in nearly all cases. Before mea- surements were started on a liquid, it was decided which

430 Volume 40, Number 4, 1986

T A B L E IV. Peak imaginary refractive indices of liquid CH3NO2 at 25°C from 500-~m cells.

B a n d n u m - P e a k %

b e t ~ / cm- ' T

kin,,

Ref . 8" T h i s w o r k b

1 4159.0 3 1.13 (05) × 10 -3 1.17 (.04) × 10 -3 2 4080.0 49 2.33 (16) × 10 -4 2.30 (.03) × 10 -4 3 3960.0 72 7.8 (6) × 10 -~ 7.1 (.3) x 10 -5 4 3832.0 63 1.32 (09) × 10 ' 1.31 (.03) X 10 -4 5 ~ 3668.0 9 4.07 (25) × 10 -4 (7.0 ~ 1.25) X 10 -4 6 ~ 3573.0 24 3.32 (23) × 10 -4 (4.3 ~ 1.6) × 10 -4 7 ° 3447.0 49 2.44 (17) X 10 -4 (2.57 ~ 2.38) X 10 -4

15 d 2342.0 ~ 5 7 2.62 (18) × 10 -4 2.98 (.04) × 10 -4 16 2288.0 10 1.63 (07) × 10 -8 1.62 (.01) × 10 -3 17 ~ 2248.0 58 8.58 (05) X 10 -4 1.87 (.02) X 10 -4 18 f 2123.0 73 1.48 (12) × 10 -4 1.37 (.03) × 10 -4 19 2035.0 15 1.42 (07) X 10 -~ 1.438 (.006) × 10 -3 20 1984.0 47 5.29 (18) X 10 4 5.13 (.03) × 10 -4 21 1856.0 64 2.86 (11) × 10 -4 2.60 (.04) × 10 -4 22 1830.0 32 9.46 (32) × 10 -4 9.23 (.09) × 10 -4

" T h e n u m b e r in p a r e n t h e s e s is t h e e v a l u a t e d u n c e r t a i n t y in t he t e r - m i n a l d igi t .

b T h e n u m b e r in p a r e n t h e s e s is t h e 90% con f idence l imi t . ° T h i s b a n d is w a t e r - s e n s i t i v e . d Our w a v e n u m b e r d i s a g r e e s w i t h t h a t of Ref. 8. ° I m p u r i t y b a n d ; d e c r e a s e d on f r a c t i o n a l f reez ing.

k ~ is e r r o n e o u s l y g i v e n as 1.48 x 10 -3 in Ref . 8.

bands would be studied, and very intense as well as very weak bands were deliberately chosen to investigate the accuracy as a function of peak transmittance. All of the results we obtained for all of these bands are reflected in the tables.

Benzene, chlorobenzene, toluene, cyclopentane, and dichloromethane were studied in cells of thickness 11 to 53 tam. With two exceptions, each band was measured at least five times in at least three different cells. The two exceptions are the two most intense bands of tol- uene studied, band 92 at 694.5 cm ~, which was mea- sured twice in one 11-#m thick cell, and band 70 at 1495.5 cm -1, which was measured four times in 11- and 13-#m cells. The averaged results for these compounds in these cells are in Tables V to VIII, presented in the same format as in Tables II to IV with the addition of the cell pathlengths that were used.

In the tables, the evaluated uncertainties of the cali- brated values of Ref. 8 are given. These were obtained s,~,16 by the statistical propagation of estimates of the errors associated with each step of the procedure. They are, thus, attempts to define the accuracy of the values reported, and are not statements of the experi- mental precision. In contrast, the error estimates asso- ciated in Tables II to VIII with the average km,~ values of this work are statements of the precision of the mea- surement. They are 90% confidence limits, which were calculated by standard methods ~,~7,~s and are statistically accurate probability limits if all of the nonreproducibil- ity is due to random errors. As we note below, this may not be the case, and our 90% confidence limits (90% C.L.) may include contributions from errors that are really systematic.

It was noted in the introduction that many factors must be changed during the measurements in order to explore the accuracy of the measurements. During this work we have not realigned the source optics or the in-

TABLE V. Peak imaginary refractive indices of liquid C6H6 at 25°C, from 11- to 53-#m cells.

B a n d Cel l k~, , n u m - P e a k % p a t h

b e t ; / c m -1 T (in t~m) Ref. 8" T h i s w o r k b

3 4057.0 37 52 3.34 (12) x 10 -3 3.40 (08) × 10 -3 10 3696.0 75 52 7.71 (52) x 10-4 7.93 (41) × 10 -4 11 3642.5 73 52 9.61 (69) x 10 -4 9.27 (38) × 10 -4 16 3091.0 2 -25 33-11 3.01 (08) x 10 2 3.08 (04) × 10 -2 17 3071.5 4 - 3 3 33-11 2.28 (06) x 10 -2 2.34 (02) × 10 -5 19 3036.0 1 .5-10 33-11 5.20 (13) × 10 -~ 5.42 (07) × 10 -2 36 2325.0 65 52 2.31 (16) × 10 3 2.29 (05) x 10 8 38 2210.5 75 52 1.47 (11) × 10 -3 1.42 (05) x 10 -3 41 1960.0 25 52 9.92 (31) x 10-3 10.21 (09) × 10 -~ 43 1815.0 15 52 1.50 (04) × 10 -2 1.57 (01) × 10 -5 44 1754.5 78-84 52 -32 1.41 (05) × 10 -3 1.42 (04) × 10 -3 46 1671.0 79 -84 52 -32 1.29 (05) × 10 -8 1.32 (05) × 10 -3 47 1617.5 76-81 52-32 1.82 (07) × 10 -8 1.86 (04) × 10 -3 50 1479.0 1 -4 15-11 1.52 (03) × 10 -1 1.54 (01) × 10 -1 51 1393.0 57-68 52-32 5.28 (17) × 10 -3 5.28 (06) × 10 8 53 1249.0 77 -84 52 -32 1.76 (06) × 10 3 1.77 (07) × 10 -3 54 1176.0 58 -70 52-32 5.80 (19) × 10 -8 5.84 (12) × 10 -8 55 1148.0 70 -78 52-32 3.42 (15) × 10 -3 3.61 (12) × 10 -3 56 1036.0 4 -32 33-11 7.11 (14) × 10 -2 7.18 (04) × 10 -2 60 849.5 60 -76 52-31 5.68 (21) x 10-3 5.83 (15) × 10 -3

.,b See foo tno t e s to T a b l e II .

terferometer optics apart from one fixed mirror (one of the two fixed mirrors in the Genzel interferometer in the Bruker IFS 113V). We have done all calculations with the phase correction procedure supplied by Bruker (in the program ATS 83G) and have not explored the effect of the size of the interferogram transformed to yield the phase spectrum. Nor have we changed filter settings, or the detector or, or course, the spectrometer. The effects of zero-filling and apodization have been explored and found ~ to be less than 1%. The gain has no effect on the results. Change of the beamsplitter earlier I appeared to give a systematic error of ~ 21/2 %. In this work we were unable to show that the effect of changing the beam- splitter is systematic, so the results for different beam- splitters are included in the average value reported and any error from this source is included in our reported 90% confidence limits.

The interferometer fixed mirror was aligned each day. Half-way through the work the mirrors associated with the sample chamber were realigned, with no systematic

TABLE VI. Peak imaginary refractive indices of liquid C6HsCI at 25°C from 11- to 53-~m cells.

B a n d Cel l kin,, h u m - P e a k p a t h

be t ; / c m - ' % T (in ttm) Ref . 8 ~ T h i s w o r k b

19 3083.0 24 52 6.70 (30) x 10 -3 6.89 (02) x 10 -3 20 3069.5 12 52 0.991 (20) × 10 -2 1.015 (04) x 10 -2 21 3058.5 20 52 7.96 (17) x 10 -3 8.00 (03) x 10 -3 22 3026.0 51 52 3.21 (14) x 10 -3 3.25 (01) x 10 -3 23 3016.5 56 52 2.78 (13) x 10 -3 2.79 (01) x 10 -3 72 1584.0 21 -56 13-11 8.11 (20) x 10 -2 8.05 (05) x 10 -2 75 1445.5 8 -13 13-11 1.09 (3) x 10 ' 0.976 (09) x 10 -1 86 1083.5 4 -7 13-11 1.79 (5) x 10 -1 1.76 (01) x 10 -1 87 1068.0 3 8 - 4 4 13-11 5.15 (10) x 10 2 5.17 (03) × 10 -2 88 1022.5 4 - 8 13-11 1.94 (5) x 10- ' 1.75 (02) x 10 -1 90 1002.0 46 -53 13-11 4.34 (8) x 10 -2 4.27 (04) x 10 -2 98 702.0 3 -6 13-11 3.11 (8) x 10- ' 2.93 (03) x 10 1 99 685.0 1 -3 13-11 3.89 (11) x 10- ' 3.55 (06) x 10 -1

..b See foo tno t e s to T a b l e II .

APPLIED SPECTROSCOPY 4:31

TABLE VII. Peak imaginary refractive indices of liquid C6H5' CH3 at 25°C from 11- to 53-;tm cells.

Cell Band Peak pa th h~,~

number ; / cm -~ % T (in ~m) Ref. 8" This work b

25 3086.5 8-55 52-11 1.12 (04) x 10 -2 1.19 (01) x 10 -2 26 3062.0 6-50 52-11 1.34 (04) x 10 -2 1.37 (01) × 10 -2 27 3027.0 1-24 35-11 3.24 (06) × 10 -~ 3.13 (07) x 10 2 29 2947.5 13-58 52-11 0.962 (30) × 10 -2 1.03 (01) x 10 -~ 30 2920.0 5-46 52-11 1.61 (05) × 10 -2 1.64 (01) × 10 2 31 2872.0 21-65 52-11 7.70 (21) x 10 -8 8.03 (11) × 10 -~ 66 1604.5 3-45 52-11 3.04 (10) x 10 -2 3.20(01) x 10-2 70 1495.5 5-13 13-11 1.16 (02) × 10 -~ 1.15 (03) x 10 -~ 71 1460.0 6-50 52-11 2.65 (09) × 10 -~ 2.80 (02) x 10 -2 72 1379.0 28-70 52-11 1.37 (03) × 10 -2 1.33 (02) x 10 -2 78 1178.5 43-77 52-11 1.03 (03) x 10 -2 1,00 (03) x 10 -2 81 1081.0 10-56 52-11 3.05 (10) × 10 -2 3.11 (02) × 10 -2 83 1030.6 6-50 52-11 4.00 (03) x 10 -2 4.11 (06) x 10 -2 87 895.5 53-80 52-11 10.0 (5) x 10 -3 9.68 (33) × 10 -3 92 694.5 3.5 11 3.80 (06) × 10 -~ 3.42 (04) × 10 -~

~,b See footnotes to Table II.

effect on the results. The detector and its optics were aligned three times. On two occasions the same distri- bution of results was obtained, while the third alignment gave incorrect results. It is worth noting that this third alignment gave a larger open-beam signal than we have ever had, but gave the transmission of the cell full of liquid as ~95% in regions where the liquid does not absorb. This baseline transmission of 95 % is impossible, because reflection effects limit the transmission to ~ 89 % when the liquid does not absorb. Accordingly, the results obtained with this detector alignment were ignored. All other results, are, however, included in the average.

It should be noted that the instrument was used for other work while this work was in progress, and that we were careful to avoid making all of the measurements on one compound on the same day. In general, only one measurement was made on any one compound on one day, and checks were frequently made several months after the first spectra of a compound were recorded. No systematic changes were noted and all of the results are included in the average.

Thus the 90 % confidence limits include the effects of detector and sample optics realignment, change of beamsplitter, daily realignment of the fixed mirror of the interferometer, change of sample cell, and day-to- day variation of unidentified origin, in addition to the error sources that contribute to typical analytical pre- cision.

DISCUSSION

In all of this work, the bands to be studied were cho- sen before the spectra were recorded and the tables show all of the results obtained.

Tables II to IV show that 97 bands were studied in 500-~m cells and our k .... value lies within the error limits of Ref. 8 for 84 of them, including band 51 of C6HsC1, for which Ref. 8 gives no estimate of the error. Among the 13 bands which disagree are 4 bands in the spectrum of CH3NO2 which are clearly due to impuri- ties. Thus, bands 5, 6, and 7 varied from spectrum to spectrum and are associated with traces of water which the CH3NO2, which was kept over molecular sieve, ab-

T A B L E VIII. Peak imaginary refractive indices of liquid CsH,o and CH2CI, at 25°C from 32- to 53-urn ceils.

Cell Band Peak pa th k ....

number ; / cm -~ % T (in ttm) Ref. 8 ~ This work b

CsH10

25 1454.5 4-11 29 893.5 26-40

CH2Cl~

8 3055.0 21 9 2987.0 43

25 1422.0 37 30 896.0 32

52-32 3.30 (05) x 10 -2 3.39 (02) x 10 -5 52-32 2.19 (04) x 10 -2 2.19 (01) × 10 -~

51 7.05 (16) x 10 -~ 7.31 (08) × 10 -~ 51 3.82 (11) x 10 -~ 3.90 (03) × 10 3 51 9.74 (20) x 10 -3 9.73 (04) × 10 -~ 51 1.82 (04) x 10 -2 1.84 (01) x 10 2

a,b See footnotes to Table II.

sorbed while the cell was filled. Further, band 17 de- creased in intensity during purification and is, thus, at least partly due to impurity. These bands are not suit- able for further study and have been omitted from the statistical analysis of the results. Band 15 of CH3NO2 has been included in the analysis, although it is suspect because our frequency is 16 cm -1 lower than that in Ref. 8. Thus, 2 bands out of 36 for C6H~C1, 5 out of 46 for C6H~' CH3, and 2 out of 11 for CHINO2 (including band 15) have intensities outside the limits of Ref. 8. Two of these bands, 66 of C~HsC1 and 56 of C6Hs'CH3, trans- mitted <0.5 %, and band 20 of C6H5" CH3 was very weak and nonreproducible in this work and in Ref. 8. No ob- vious reason for disagreement exists for the 5 remaining bands which disagree (namely 44 of C6H~C1; 19, 43, and 55 of C6HsCH~; and 21 of CH3NO2).

Our statistical analysis uses three quantities. AER, the average error of Ref. 8, is the estimated error of Ref. 8 expressed as a percentage of k max and averaged over all of the bands we included in the analysis. AA, the average agreement, is the difference between our aver- age value and the calibrated value, expressed as a per- centage of k max and averaged over all bands. AP, the average precision of this work, is the 90% confidence limit expressed as a percentage of km,x and averaged over all bands.

These three quantities are tabulated in Table IX for the 93 accepted bands of the three compounds that were studied in 500-#m cells. These quantities are given for different peak transmissions in Table IX, and can be summarized as follows: For bands that transmit more than 50% the average precision is ~ 1.8% of h . . . . while it is 0.7% for bands which transmit between 50% and 0.5%. For peak transmissions below 0.5% the precision is decreased (Table IX), but this is at least partly be- cause we failed to use sufficient significant figures for these bands when we transferred the transmittance val- ues from the Aspect 2000 computer to the program XLVI on the Amdahl computer.'

This precision is impressive in view of the several fac- tors (see above) that contribute to our calculated 90% confidence limits.

The only available estimate of the accuracy of our results is the comparison with those of Ref. 8, namely the average agreement, AA. Thus on average they are accurate to 3 % of kmax for peaks which transmit between 10 and 70 % and to 1.5 % of kmax for peak percent trans- missions between 10% and 0.5%. If it is recalled that

432 Volume 40, Number 4, 1986

Table IX. Stat is t ica l information for 93 bands of C~HsCI(e) , C6HsCH3(£), and CH3NO2(e), studied in 500-~m cells. ~

Peak percent transmission

>70 70 to 50 50 to 10 10 to 0.5 <0.5

C6H~C1 Number of bands 10 9 11 6 0 AER 8.4% 7.0% 7.3% 3.5% ... AA 6.5% 3.5% 4.3% 1.6% ' " AP 1.2% 0.5% 0.4% 0.3% ' "

C~H~CH~ Number of bands 3 19 16 6 2 AER 8.0% 6.8% 6.2% 4.2% 2.4% AA 9.0% 2.1% 2.2% 0 .75% 2.5% AP 3.5% 2.2% 0.7% 0.7% 2.2%

CHINO2 Number of bands 2 3 5 1 0 AER 7.9% 5.8% 4.6% 4.4% AA 8.6% 7.7% 2.0% 3.4% AP 3.2% 1.7% 0.8% 3.4%

Total Number of bands 15 31 32 13 2 AER 8.2% 6.8% 6.3% 3.9% 2.4% AA 7.3% 3.0% 2.9% 1.5% 2.5% AP 1.9% 1.6% 0.6% 0.7% 2.2%

• AER, AA, and AP are the Average Percent Error of the calibrated values in Ref. 8, the Average Percent Agreement between our values and the calibrated values, and the Average Percent Precision (aver- age 90% confidence limit expressed as a percentage of k~..) of this work, as defined in the Discussion section of the text.

the program T E R P O L , 1 the 0.5 cm -~ digitization inter- val of our results, and the effect of the apodizat ion func- tion may 1 yield errors of the order of 0.5 to 1%, it be- comes clear tha t this accuracy is very good.

We next apply the statistical analysis to all of the bands of the six compounds tha t we have studied in 500- am cells in this s tudy and in Ref. 1. We measured 130 bands tha t had no obvious deficiencies and had peak transmissions between 80% and 0.5%. For 117 of these bands the average kma x value agreed with the calibrated value s to within the es t imated error s of tha t value. The es t imated error s was, on average, 6% of km,x. For bands which t ransmi t ted less than 70% the average agreement was 2.8%, with some bands in disagreement by 8% or more and some agreeing perfectly. The average precision of our results was 1.2% of k~,~.

We turn now to the results f rom 11- to 53-am cells. Tables V to VIII show tha t 54 bands of benzene, chlo- robenzene, toluene, cyclopentane, and dichloromethane were studied in cells 11- to 53-am thick, and tha t our average k m,x value lay within the es t imated error limit of the calibrated value s for only 35 of them. Toluene appeared to be a part icularly bad case, with only 6 of 15 bands meeting this test. But the statistical analysis in- dicates tha t our results f rom these cells are no worse than those from 500-am cells, and tha t the average error of Ref. 8 is smaller for these bands than for those stud- ied in 500-am cells. The average error of Ref. 8, average agreement, and average precision of this work are: for the entire 54 bands s tud ied- -3 %, 3 %, and 1.5% of kmAx; for the 46 bands which t ransmi t ted less than 70 % - - 3 %, 3 %, and 1.1% of k .... ; and for the 15 bands of t o luene - - 2.8%, 3.5%, and 1.4%. Again some bands agreed per- fectly while others disagreed by 10%. These statistics

show tha t the agreement for toluene is worse than for the other liquids, bu t not by much.

If we take all of the 184 bands which have no obvious deficiency tha t we measured in cells of thickness be- tween 11 and 500 am, the average agreement with the calibrated values is ~3 % of km~x and the average pre- cision of our results is ~0.7 to 1.5% of kmax.

We draw at tent ion to bands 10, 11, 36, and 38 of ben- zene. They were measured in 52-am cells (Table V) 6 to 10 months after they were measured in 500-am cells (ta- ble III of Ref. 1). Our two sets of results for the four bands agree to 0.5%, exactly, 0.3% and 1.4%.

The 1036 cm -1 band of benzene has been measured by many workers by a t t enua ted total reflection and by transmission spectroscopy. These earlier results are summarized in Ref. 19 and range from 0.069 up to the calibrated value s kmax = 0.0711 ± 0.0014. In our early work, 1 which used 23-am cells, we obta ined 0.0714 with a 90% confidence limit of 0.0010. In this work, 12 to 20 months after our earlier work, we measured the band fourteen times in cells 11, 13, 14, 16, and 33 am thick, and found kmax to be 0.0718 with a 90% confidence limit of 0.0004.

C O N C L U S I O N

We conclude from this data tha t the average of four to six measurements of the transmission spectrum of a liquid tha t are made with an FT- IR spectrometer , using different cells and realigning the fixed mirror of the in- terferometer each day, will probably be accurate to ~ 3 % and yield a 90 % confidence limit of ~ 1%.

We have tr ied in this work to obtain an overall picture of the accuracy of our measurements . We have not made a systematic de terminat ion of the sources of error. We have explored the effect of various possible error sources, but have been unable to identify systematic errors ex- cept to note tha t the use of just one cell will almost certainly give a biased result.

The same cell in independent experiments on the same day with the same in ter ferometer al ignment often gave 0.3% reproducibili ty. Different cells on different days with different alignments of the in ter ferometer gave sta- tistically dist inct sets of values, differing by _< 1.5 %. The evidence does not show statistically tha t the cell tha t gave the highest value for one compound on one day always gave the highest values. There was an addit ional day- to-day variation of near 1%, which we th ink was due to small daily differences in the beam pa th through the instrument . These differences could be caused by the cell. However in view of the single-day results it is more likely tha t these daily differences originate before the cell is reached, and cause the effect of the cell on the beam path, and hence on the detector focus, to vary from day to day.

Thus we believe the main source of the variat ion tha t causes our 1% average precision to be the interact ion of the cell with the beam. This must be similar in FT- IR and dispersive instruments. Fur ther , the precision of our transmission measurements on the Bruker FT- IR in- s t r u m e n t is c o m p a r a b l e to t h a t of re f lec t ion measurements 2° on a Nicolet 7199 FT- IR instrument , for which the sample surface and position are known 2° to be responsible.

APPLIED SPECTROSCOPY 4:33

Our measurements are, on average, accurate to ~3 %, as judged by calibrated measurements tha t themselves are est imated s to be accurate to 6 % on average. To ob- tain bet ter knowledge of the accuracy of our measure- ments we need better standards, and we need measure- ments to be made on different F T - I R instruments. In order to encourage such measurements, and to facilitate the search for the sources of systematic errors, we intend to simplify the procedure for processing the data for certain bands.

ACKNOWLEDGMENTS

We thank the Natural Sciences and Engineering Research Council of Canada for their support. J.E.B. thanks F. F. Cantwell and R. Kra- tochvil for valuable discussions about errors and statistics. And we thank H. H. Mantsch and his coworkers for valuable criticism of an early draft of this paper.

1. J. E. Bertie, R. N. Jones, and V. Behnam. Appl. Spectrosc. 39, 401 (1985).

2. J. E. Bertie, H. H. Eysel, V. Behnam, and J. C. M. Zwinkels, "Quantitative intensity measurements by Fourier transform in- frared spectroscopy (FT-IR)," in 1985 International Conference on Fourier and Computerised Infrared Spectroscopy, Jeanette G. Grasseli, David G. Cameron, Eds., Proc. SPIE 553, 34 (1985).

3. I. M. Kolthoff and E. B. Sandell, Textbook of Quantitative Inor- ganic Analysis, (The Macmillan Company, New York 1952), 3rd ed.

4. Tables of Wavenumbers for the Calibration of Infrared Spec-

trometers, I.U.P.A.C. Commission on Molecular Structure and Spectroscopy (Pergamon, Oxford 1961), 1st ed.

5. A. R. H. Cole. Tables of Wavenumbers for the Calibration of Infrared Spectrometers, I.U.P.A.C. Commission on Molecular Structure and Spectroscopy (Pergamon, Oxford 1977), 2nd ed.

6. T. Hirschfeld, in Fourier Transform Infrared Spectroscopy, J. R. Ferraro and L. J. Basile, Eds. (Academic Press, New York, 1979), Vol. II, Chap. 6, pp. 193-242. id, Appl. Spectrosc. 32, 509 (1978).

7. R. N. Jones. Pure Appl. Chem. 18, 303 (1969); J. Jpn. Chem. (Ka- gaku no Ryoiki) 21, 609 (1967) (in English).

8. T. G. Goplen, D. G. Cameron, and R. No Jones, Appl. Spectrosc. 34, 657 (1980).

9. J. P. Hawranek and R. N. Jones, Spectrochim. Acta 32A, 111 (1976). 10. J. E. Bertie and H. H. Eysel, Appl. Spectrosc. 39, 392 (1985). 11. J. P. Hawranek, P. Neelakantan, R. P. Young, and R. N. Jones,

Spectrochim. Acta 32A, 75 (1976). 12. J. P. Hawranek, P. Neelakantan, R. P. Young, and R. N. Jones,

Spectrochim. Acta 32A, 85 (1976). 13. J. E. Bertie, in Vibrational Spectra and Structure XIV, J. R.

Durig, Ed. (Elsevier, Amsterdam. 1985), Chap. 4, p. 221). 14. "Computer Programs for Infrared Spectroscopy," N.R.C.C. Bul-

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34, 652 (1980). 16. J. P. Hawranek and R. N. Jones, Spectrochim. Acta 32A, 99 (1976). 17. W. E. Harris and B. Kratochvil, An Introduction to Chemical

Analysis (Saunders College Publishing/CBS, Philadelphia 1981). 18. D. P. Shoemaker, C. W. Garland, and J. I. Steinfeld, Experiments

in Physical Chemistry (McGraw Hill, New York, 1974), 3rd ed. 19. D. G. Cameron, D. Escolar, T. G. Goplen, A. Nadeau, R. P. Young,

and R. N. Jones, Appl. Spectrosc. 34, 646 (1980). 20. J. C. M. Zwinkels, Ph.D. Thesis, University of Alberta, Alberta,

Canada (1984).

Oxide, Hydroxide, and Doubly Charged Analyte Species in Inductively Coupled Plasma/Mass Spectrometry

M . A. V A U G H A N and G. H O R L I C K * Department of Chemistry, University of Alberta, Edmonton, Alberta T6G 2G2, Canada

In inductively coupled plasma/mass spectrometry analyte, M may be distributed among several species forms including doubly charged ions (MS+), singly charged ions (M+), mono-oxide ions (MO+), and hydrox- ide ions (MOH+). Detailed data are presented for Ba to illustrate the dependence of the ion count of these species and their ratios (M2+/M +, MO+/M +, and MOH+/M +) on nebulizer flow rate, plasma power, and sampling depth. Although these data are representative of most ele- ments, many form oxides to a much greater degree than Ba; data are presented for Ti, W, and Ce to illustrate this fact. These various analyte species are important in that serious interelement interferences can oc- cur because of spectral overlap. An extensive pair of tables indicating potential spectral interferences caused by element oxide, hydroxide, and doubly charged ions is presented. Index Headings: Mass spectroscopy; ICP.

I N T R O D U C T I O N

Induct ively coupled plasma/mass spectrometry (ICP- MS) is a sensitive technique for trace and ultra-trace

Received 2 December 1985. * Author to whom correspondence should be sent.

elemental analysis. In this technique, singly charged el- emental analyte ions generated in an ICP are extracted into and measured with a quadrupole mass spectrome- ter. For a large number of elements, the singly charged elemental analyte ion (M ÷) is by no means the only an- alyte species tha t is observed. Many elements form mono- oxides (MO+), some form doubly charged ions (M2+), and a few form hydroxide species (MOH+).

The presence of these species has been noted in a number of papers. Date and Gray 1 reported observing 18SBa2+ and show spectra for Ba +, BaO ÷, BaOH ÷, Ba ~+, AsO ÷, AsOH ÷, UO ÷, and UO2+. 2 They also reported the presence of ThO÷2 Houk and Thompson 4 reported seeing Sr 2÷, SrOH ÷, and PrO +, UO ÷, and UO2 ÷. They also dis- cussed the formation of MO ÷ and MOH ÷ species in gen- eral. Gray and Date ~ reported the formation of rare earth oxides, in part icular HoO ÷, T m O ÷, and LuO +. The be- havior of Sr ~+, Ba 2÷, MoO +, WO ÷, and CeO ÷ with power and nebulizer flow rate was discussed by Horlick et al. 8 Douglas and Houk measured ThO+/Th ÷, Ba2÷/Ba +, and UO+/U + ratios and discussed the potential of MO ÷ in-

434 Volume 40, Number 4, 1986 0003-7028/86/4004-043452.00/0 APPLIED SPECTROSCOPY © 1986 Society for Applied Spectroscopy