INSTRUMENTATION
Advisory Panel Jonathan W. Amy Glenn L. Booman Robert L. Bowman
Jack W. Frazer G. Phillip Hicks Donald R. Johnson
Howard V. Malmstadt Marvin Margoshes William F. Ulrich
CORRELATION SPECTROSCOPY J. H. DAVIES B a r r i n g e r Research L t d . , 3 0 4 C a r l i n g v i e w Dr., Rexdale, O n t a r i o , Canada
The potential applications of correlation
spectroscopy
techniques to new generations of laboratory instruments are great.
Benefits should result in certain fields
of absorption, emission, and fluorescent
spectrophotometry.
Correlation techniques can also be extended to spectral signa-*• ture of liquids, gases, and va pors, either in absorption or emis sion, can be used in electro-optical techniques to perform qualitative and quantitative measurements. One particularly powerful tech nique is that of correlation spec troscopy. In this technique, an in coming spectral signature is con tinuously cross-correlated in real time against a spectral replica of the spectrum sought. The stored replica is contained within the spec trometer and may be optical or magnetic. The technique is ap plicable from the far ultraviolet through the visible to the infrared ΠΠΙΙΕ DISTINCTIVE
spectral regions. If suitable back ground radiation sources are avail able, t h e technique can become a complete remote sensing system for both qualitative a n d quantitative measurements. Correlation spectroscopy h a s a significant role to play in both lab oratory analytical procedures and in real-time on-line process control. Initial development h a s concen trated on gas analysis, particularly those gases associated with com bustion processes and earth resource applications. T h e advantages of the techniques—high sensitivity, good selectivity, real-time readout —ensure a dynamic future for its
Figure 1. Absorption spectrum of iodine
analytical
interferometry applications to situations where op tical inputs are available. Basic Principle of Correlation Spectroscopy
T h e basic principle of correlation spectroscopy is t h e cross correla tion of a n incoming spectral signa ture against a replica spectrum stored within the instrument. T h e basic techniques have been well de scribed in the literature {1—5) a n d only a brief outline is given here. Figure 1 shows the regular absorp tion spectrum of iodine vapor and typically represents t h e t y p e of spectrum t h a t h a s been stored within t h e instrument. Figure 2
Figure 2. Dispersive system for vapor detection using spectrum correlation filter ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
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101 A
Instrumentation
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GENERAL
ELECTRIC
shows the type of correlation spec trometer which has been developed ; this is a dispersive Ebert spectro photometer. The incoming radiation to be ana lyzed is passed via reflection from the Ebert mirror to a grating which is the dispersive element. The radiation emerging from the grating is now dispersed into its various spectral components to be reflected a second time by the Ebert mirror and to pass through the exit aper ture of the spectrometer to a photodetector. The angular position of the grating determines the region of the spectrum that falls on the exit aperture. If the incoming radia tion is now made to vibrate periodi cally through a limited wavelength excursion, and a mask replica of the sought spectrum is positioned in the exit aperture, the incoming spec trum will be moved periodically in and out of correlation against the mask. A beat signal will result and will be detected by the photodetector. Phase-sensitive synchronous detection techniques can be used for the signal detection using, as the reference, the drive signal that causes the spectrum to vibrate. Signal Origin
The basic function of the correla tion spectrometer is the Beer-Lam bert law of absorption which de scribes the attenuation of a beam of monochromatic light transmitted through a medium which absorbs but does not scatter. Then where
H = H0e-acL
H0 == incident irradiance at a fixed wavelength H = emerging irradiance at same wavelength a = absorption coefficient at the same wave length c = concentration of me dium L = pathlength through medium
If the absorption coefficients are dif ferent at two known wavelengths, while H0 is the same, we have: Hi = H0-e~aicL and H, = H«e,-aich where αϊ and o 2 are the absorption coefficients at wavelengths 1 and 2, respectively.
Circle No. 52 on Readers' Service Card 102 A
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ANALYTICAL CHEMISTRY, VOL. 4 2 , NO. 6, MAY 1 9 7 0
Thus,
| p = e-(X[i'
J2 Ι
i lJ i If we assume the product 1 - cL l is\con i., stant over the acceptance solid angle of the spectrometer:
ΑΩ Σ R = aJ j 1 -
l
where α (λ) is the absorption cross section per molecule, and is a function of wavelength, temperature, and pressure X.u, λκ' are the beginning and end wavelengths of the mask corre sponding to slit (t) for the first position (1) of the spectrum with respect to the mask λ2ί, λ 2 / is the same for position 2 β (λ) is the spectrometer's transmis sion function η is the number of slits in the mask A is the spectrometer's aperture and Ω is the solid angle of acceptance The simplest response occurs when Νλ is constant throughout the wavelength of interest and is 2V0; that α (λ) is a periodic function of λ over the same waveband; that C is constant along the path L which is also constant, and that the instru ment's response is lineal constant over the same waveband such that βΜ =
β0.
fl
β(λ)
}
(8)
70
,s
N^e-awJoc^dld\\
f'* β(λ) Nxe-aWeL f
SCIENCE EQUIPMENT FOR THE
\
(6)
Tl
^~fr,
ΑΏ Σ
*-
ai)cL
The basic instrument has already been given in the section on basic principle of correlation spectros copy. In the section on signal ori gins, a review of the physical pro cesses which produce the instru ment's voltage response was given. Equations 6 and 7 indicate the re sponse obtained when scattered sky light was used as the target illumi nating background; this is the true remote sensing role. One can nat urally see that the problems of
(5)
n
e-
{a
Instrument Techniques
If this current is passed through a high impedance load, the instru ment's response in volts is R
_ η β0 Δλ Noe-aicL Ati\ ηβ0 Δλ iV 0 e- aicL AQ)
( \
β(λ) Nxe-"McL
d\
|
(7)
d\ '
quantitative measurements rely upon a solution of these equations. This can be obtained once the re sponse of the spectrometer is known —i.e., its output voltage (R) against gas concentration product (cL). This is achieved by inserting known (cL) calibration cells into the opti cal path of the spectrometer. This system has been mounted in an Aero Commander aircraft and has suc cessfully undertaken many SO2/ N 0 2 airborne surveys over such cities as Toronto, Washington, Chattanooga, Los Angeles, and San Francisco. A typical record is shown in Figure 3. When control over the light source can be exercised, the problem becomes much simpler. This is the approach adopted by the ambient monitor technique (see Figure 4). Here the gas under analysis is passed through a multipass cell of fixed optical pathlength (L). Full control over the spectral radiance source (Νλ) is exercised. The am-
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(201) 377-9000 Circle No. 104 on Readers' Service Card
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
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103 A
Instrumentation
N 0 2 SURVEY 28 MAY 1968 FLIGHT NO. 2 A/C ALT 7500 ft. SURFACE AIR TEMP 25"C SURFACE AIR PRESS. 2 9 8 8 * Hg INVERSION HEIGHT 6 0 0 0 ft, TIME 17 03-17.4? PDT ι MX. ophrn/crn SAN FRANCISCO
Figure 3. Record of N0 2 airborne survey of San Francisco, May 28, 1968
bient monitor uses a quartz-iodine lamp as the source and multi- en trance and -exit slits. Movement of the spectrum with respect to the mask is achieved by vibrating the grating with the torque motor driven under closed loop control. This technique essentially produces a response function of the type shown in Equation 8. The optical length is 2.5 m and, by use of a
100-sec. integration time, noise levels of 15 ppb for S 0 2 and N 0 2 have been obtained. Another instrument technique is t h a t of long-path monitoring (see Figure 5 ) . This approach uses an actual quartz-iodide lamp posi tioned some distance away from the correlation spectrometer (up to 1000 m ) . T h e remote sensing spectrom eter is accurately boresighted into
the lamp, and the gas concentration between the lamp and sensor is thereby obtained. This is ob viously a more representative m e a sure of the average ambient gas concentration throughout the long p a t h t h a n can be obtained from single-point ambient monitors. T h e long-path system is further modified by allowing the incoming spectrum to be slowly moved p a s t the exit mask. This is achieved by slowly scanning the grating through a small angular movement. T h e net result is to cause the incoming spec t r u m to drift over the mask replica. There will be one grating position at which maximum cross correlation between mask and input spectrum occurs; a t this position all lines match, and is in effect the optical counterpart to an electronic high Q network. For all other grating po sitions, the spectral match is less t h a n optimum. Figure 6 well illus trates this for N 0 2 while Figure 7 shows the N 0 2 calibrations achieved for various increasing levels of gas in the test cell of a long-path sensor. A similar setup is used for S 0 2 . Table I shows the performance parameters for the long-path system. T h e basic performance of the spectrometer, as mathematically ex pressed in equations similar to 6 and 7, has been transformed into a computer program. I n this pro gram the gas absorption coefficients filter and spectrometer transmission functions, spectral radiance of background light source, number of slits, wavelength j u m p and absorp tion coefficients of interfering gases are variable parameters. T h e com puter output yields the cL product of the sought gas against B/aJ, Pu and Po as previously defined. B y this means the operating wave length position and correlation j u m p , correct number of slits and their widths can be optimized. Correlation Interferometry
Figure 4 .
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S 0 2 a m b i e n t monitor s c h e m a t i c
ANALYTICAL CHEMISTRY, VOL. 4 2 , NO. 6, MAY 1 9 7 0
Because m a n y gases and liquids have their fundamental absorption spectra in the near- and mid-infra red, where the luminous output of thermal sources is decreasing and the available detectors are generally less efficient and noisier t h a n those used in the visible and uv, it is necessary to use the available radia-
TABLE 1.
LONG-PATH SYSTEM PERFORMANCE
Gas Sensed
Spectral bandwidth
m'UflCH.ftNG!:ABiE'J MUSS ΑΡΪδ H U f 8 tJNif
I HAimG SCANNER
HiAVV D i m ÎRÎP0O
SOs
2800-3150 A
NOz
4130^500 A
1 ppb Threshold sensitivity 2 ppb (in 1000 m.) 2 ppb/ Chart resolution 1 PPb/ (1000 m.) minor div. minor div.
