INSTRUMENTATION
Advisory Panel Jonathan W. Amy Glenn L.Booman Robert L.Bowman
Jack W. Frezer G. Phillip Hicks Donald R. Johnson
I
Howard V. Malrnstadt Marvin Margoshes William F. Ulrich
CORRELATION SPECTROSCOPY J. H. DAVIES Barringer Research Ltd., 304 Carlingview Dr., Rexdale, Ontario, Canada
The potential applications of correlation spectroscopy techniques to new generations of laboratory analytical instruments are great. Benefits should result in certain fields of absorption, emission, and fluorescent spectrophotometry. Correlation techniques can also be extended t o interferometry THE
DISTIKCTIVE
spectral signa-
t u l e of liquids, gases, and va-
pors. either in absorption or emission, can be used in electro-optical technique. to perform qualitative nncl quantitative measurements. On(. particularly powerful technique i. that of correlation spectrowopg. I n this technique, an inroming spectral signature is continuously cross-correlal ed in realtime against a spectral replica of the spectrum sought. T h e stored replica is contained witliin the spectrometer and may be optical or magnetic The technique is applicable froiu the far ultraviolet through the visible to the infrared
spectral regions. If suitable background radiation sources are available, the technique can become a complete remote sensing system for both qualitative and quantitative measurements. Correlation spectroscopy has a significant role t o play in both laboratory analytical procedures and in real-time on-line process control. Initial development has concentrated on gas analysis, particularly those gases associated with combustion processes and earth resource applications. The advantages of the techniques-high seii>itivity, good eelccti\-ity. real-time readout -ensure a dynamic future for it\
applications to situations where optical inputs are avai1:tble. Basic Principle of Correlation Spectroscopy
The basic principle of carrel(‘it’ion spectroscopy is the cross correlation of an incoming spectral signature against a replica spectrum stored within the instrument. The basic techniques have been well described in the literature (1-5) and only a brief outline is given here. Figure 1 shows the regular absorption spectrum of iodine vapor and typically represents the type of spectrum that has been stored within the instrument. Figuire 2
3 u
E m
.-5
E
c
e
Figure 1. Absorption spectrum of iodine
Figure 2 . Dispersive system for vapor detection using spectrum correlation filter ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1.970
101 A
Instrumentation
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1
shows the type of correlation spectrometer which has been developed ; this is a dispersive Ebert spectrophotometer. The incoming radiation t o be analyzed 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 t o pass through the exit aperture of the spectrometer to a photodetector. The angular position of the grating determines the region of the spectrum t h a t falls on the exit aperture. If the incoming radiation is now made to vibrate periodically through a limited wavelength excursion, and a mask replica of the sought spectrum is positioned in the exit aperture, the incoming spectrum 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 correlation spectrometer is the Beer-Lambert law of absorption which describes the attenuation of a beam of monochromatic light transmitted through a medium which absorbs but does not scatter.
H = Hoe-aCL (1) where H o = incident irradiance a t Then
a fixed wavelength
H = emerging irradiance a t same wavelength a = absorption coefficient a t the same wavelength c = concentration of medium L = pathlength through medium
If the absorption coefficients are different a t two known wavelengths, while H o is the same, we have: HI = H o.e-alcL and H 2 = Hoe-a2CL where al and a2 are the absorption coefficients a t wavelengths 1 and 2, respectively.
Circle No. 52 on Readers' Service Card
102A
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
I n general, the exit plane slits have a finite width and H o is not constant with wavelength ; similarly al and a2 vary with wavelengths. Thus, a more general power expression is:
L, L?
Xl+dX
5-
e-(ai-az)cL
.
Nx.dA
X?fAX
p2
Nx.dA (3)
I n this expression al and a2 are the average cross sections per molecule in the bands ( h l , XI Ahl) and (A2, hg A h 2 ) . PI and P2 have the dimension of watts, t h a t is, power passing through the slits. The passing power can be analyzed by means of a mask or array of slits for several exit slits a t two different positions rather than for one slit in two positions. One would expect t h a t if the slits are properly chosen, the effect of the spectral distribution of the light source and influences due t o other absorbing gases, could be minimized. I n general, not only the quotient of P , and P2 can be used to determine cL, but also any other suitable relations between them. I n this instrument, the physical effect selected to measure the product CL is the difference in power received by the phototube a t two different wavelength positions on the mask. The power difference naturally results in a phototube current difference. The actual procedure used in the remote sensing spectrometer is as follows: (1) At one position of the spectrum with respect to the mask, the incident poFer, P1,on the phototube derives a current, J , which is then held fixed. The voltage, V , across the phototube dynode chain is variable and adjusted by an AGC loop so that the response to PI watts is always J amps. (2) For the second position of the spectrum with respect to the mask, the incident power will be P2 and will consequently generate J' amps. The correlation spectrometer measures the difference D between J and S-i.e.,
+
+
Instrumentation
When the two spectral positions are within the linear response of the phototube J f / J = P 2 / P 1 Then
D = J (1 -
g)
(4)
If this current is passed through a high impedance load, the instrument’s response in volts is
where CY is the load impedance (ohms) and J is the phototube response (amps) . I n general, the response of the spectrometer to a target of spectral radiance N , is:
i.e., R
= aJ
{I - e-(az--al)cL1 (8)
Instrument Techniques
T h e basic instrument has already been given in the section on basic principle of correlation spectroscopy. I n the section on signal origins, a review of the physical processes which produce the instrument’s voltage response was given. Equations 6 and 7 indicate the response obtained when scattered sky light was used as the target, illuminating background; this is the true remote sensing role. One can naturally see t h a t the problems of
If we assume the product CLis constant over the acceptance solid angle of the spectrometer:
where a ( A ) is the absorption cross section per molecule, and is a function of wavelength, temperature, and pressure A z ~ ,hi’ are the beginning and end wavelengths of the mask corresponding to slit (i) for the first position (1) of the spectrum with respect to the mask AZi, heel is the same for position 2 p ( A ) is the spectrometer’s transmission function n is the number of slits in the mask A is the spectrometer’s aperture and R is the solid angle of acceptance T h e simplest response occurs when iYA is constant throughout the wavelength of interest and is No; t h a t a ( A ) is a periodic function of h over the same waveband; t h a t C is constant along the path L which is also constant, and t h a t the instrument’s response is lineal constant over the same waveband such t h a t P(A)
= Po.
quantitative measurements rely upon a solution of these equations. This can be obtained once the response of the spectrometer is known --i.e., its output voltage ( R )against gas concentration product ( c L ). This is achieved by inserting known ( c L ) calibration cells into the optical path of the spectrometer. This system has been mounted in an Aero Commander aircraft and has successfully undertaken many SOa/ X02 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 (iY,) is exercised. The am-
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Circle No. 104 on Readers’ Service Card ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970 103A
Instrumentation
the lamp, and the gas concentration between the lamp ancl sensor is thereby obtained. This is obviously a more representative measure of the average ambient gas concentration throughout the long path than can be obtained from single-point airihient rnoiiitors. The long-path system is further modified by allowing the incoming spectrum to be slowly moved past the exit YO, S c l R V f Y mask. This is achieved by slowly scanning the grating through a ?H \ M Y 1968 small angular movement. The net FLIGHT NO 2 result is to cause the incoming specAIC Ail 7500ft trum to drift over the mask replica. There mill be one grating position a t SURFACE AIR TEMP 25 C which maximum cross correlation bURFACE AIR PRESS 29 88” Hy between mask and input spectrum occurs; a t this position all lines “dVER5ICjN HEIGHT 6000 ft match, and is in effect the optical “\1k ~ - 1 ~ - 1 7 4PDT 7 counterpart to an electronic high Q * . network. For a11 other grating po1‘ cDt !,?/‘.l sitions, the spectral match is less than optimum. Figure 6 well illusSAN FRANCISCO trates this for NO2 while Figure 7 shows tht. 9 0 2 calibrations Figure 3. Record of NO, airborne survey of San Francisco, May 28, 1968 achieved for various increacing levels of gas in the test cell of a long-path sensor. A similar setup ic used for SO, Tahle I ; h o w the hient monitor uses a quartz-iodine performance parameters for the 100-sec. integration time, noise lanip as the source and mult,i- enlevels of 15 ppb for SO, and 90, long-path system. The basic performance of the trance and -exit ;slits. hlovement of have heen obtained. s p ec t r oinet er as math err1a t i ca 11y exthe spectrum with respect to the Snother instrument technique is inask is achiered by vibrating the pressed in equations similar to 6 that of long-path monitoring (Pee grating with ihe torque motor and 7. has heen transfoimcd into a Figure 51. This approach uses an tlrivcn under closed loop control, computer program I n this proactual quartz-iodide lamp posiThis technique essentially produces grain the gas nbsorption coefficients tioned some distance away from the :i response function of the type filter and spectrometer transmission correlation spectrometer (up to 1000 functions, spectral radiance of s2ion.n in Equati.on 8. The optical m ) . The remote sensing spectrornlength is 2.5 rn and, by use of a background light cource. number of eter is accurately boresighted into slits, wavelength jump and absorption coefficients of interfering gases are variable parameters The computer output yields the cL product of the sought ga. ngainct R’crJ, P , , and P, as previously defined. By this means the operating wavelength position and correlation jump, correct number of slits and their widths can be optimized.
,
~
Correlation Interferometry
Figure 4. SO, ambient monitor schematic 104A
ANALYTICAL CHEMISTRY, VOL. 42, NO. 6, MAY 1970
Because many gases and liquids have their fundamental absorption spectra in the near- ancl mid-infrared, where the luminous output of thermal sources is decreasing and the available detectors are generally less efficient and noisier than those used i n the 7:isible and uv. it is necessary t o use the available radia-
1
TABLE I. LONG-PATH SYSTEM PERFORMANCE Gas Sensed
Spectral bandwidth Threshold sensitivity (in 1000 rn.)
so0 2800-3150 A
4130-4500 A
2 ppb
1 ppb
NOo
Chart resolution 2 PPb/ 1 PPb/ (1000 m.) minor div. minor div. Response linearity 2% 3% error (9-2500 ppb in 300 m.) 40 sec. to F.S. 20 sec. to F.S. Speed of response Grating oscillation: Speed, A/minute constant. Span, adjustable 5 A to 500 A usually set -15 A Sensor acceptance angle: Azimuth 3 rnR. Elevation adjustable 0 to 30 mR Light source beamwidth: 15 mR Permissible source to sensor misalignment: 6 mR max.
