2 Applications of Tunable-Diode-Laser IR Spectroscopy to
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Chemical Analysis J. F. BUTLER, K. W. NILL, A. W. MANTZ, and R. S. ENG Laser Analytics, Inc., 38 Hartwell Ave., Lexington, MA 02173
Tunable diode l a s e r spectroscopy has become a widely used and important technique f o r u l t r a - h i g h r e s o l u t i o n i n f r a r e d mea surements (1). A r e s o l u t i o n in the order o f 10 cm makes it p o s s i b l e , f o r example, t o study fully r e s o l v e d lineshapes o f Doppler broadened l i n e s o f low pressure molecular gases. Such measurements, which can be made r a p i d l y and e a s i l y with a tunable diode l a s e r spectrometer, are virtually impossible by other t e c h niques. Tunable diode l a s e r s are a l s o being used i n s e n s i t i v e air p o l l u t i o n monitors i n both p o i n t (2) and long-path c o n f i g u r a t i o n s (3). The purpose o f the present paper is t o review and d i s c u s s new a p p l i c a t i o n s o f tunable diode l a s e r spectroscopy in s e v e r a l areas r e l a t i n g specifically t o chemical a n a l y s i s and measurement, i n c l u d i n g n o n l i n e a r spectroscopy and photochemistry. These l a s e r s have a number o f unique f e a t u r e s o f p a r t i c u l a r i n t e r e s t i n chemical a n a l y s i s . Some o f these f e a t u r e s and t h e i r general uses are summarized i n Table I . -4
-1
Low L e v e l Detection Concentrations o f molecular gases can be measured t o excep t i o n a l l y low l e v e l s using tunable diode l a s e r IR absorption spectroscopy. As one example o f t h i s a p p l i c a t i o n , a group a t McMaster U n i v e r s i t y has constructed a h i g h l y s e n s i t i v e a i r p o l l u t i o n monitor using the apparatus diagrammed i n Figure 1 (2^) . The a i r sample flows i n t o the m u l t i - t r a v e r s a l absorption c e l l a t r e duced pressures. An AC modulation superimposed on the DC b i a s current induces a small r e p e t i t i v e scan o f the l a s e r emission frequency, a l l o w i n g the use o f d e r i v a t i v e spectroscopy. The McMaster group was able t o measure absorbance l e v e l s as low as α = 10~ by using the second d e r i v a t i v e . Figure 2 i l l u s t r a t e s the use o f t h i s apparatus f o r the d e t e c t i o n o f atmospheric SO2. The McMaster group reported s e n s i t i v i t y l e v e l s as low as 3 ppb 5
0-8412-0459-4/78/47-085-012$05.00/0
© 1978 American Chemical Society In New Applications of Lasers to Chemistry; Hieftje, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
2.
Tunable-Diode-Laser
BUTLER E T A L .
IR Spectroscopy
13
TABLE I TUNABLE DIODE LASER FEATURES RELEVANT TO ANALYTICAL INSTRUMENTATION
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High S p e c t r a l P u r i t y (10
cm
)
High s p e c i f i c i t y e l i m i n a t e s or g r e a t l y reduces e r r o r s due t o i n t e r f e r i n g absorption l i n e s and the complexity o f a l g o rithms r e q u i r e d t o compensate f o r background absorptions. S p e c t r a l Region 3-30 ym r e g i o n contains absorption s p e c t r a o f n e a r l y a l l molecular substances. Tunability P r e c i s e s p e c t r a l l i n e s or regions may be s e l e c t e d . Information i n the lineshape can be u t i l i z e d . Ease o f Rapid
Modulation
High frequency operation reduces turbulence and p a r t i c u l a t e noise. Novel s i g n a l p r o c e s s i n g techniques can be employed. High Brightness Nearly opaque substances can be analyzed. Transmission over long o p t i c a l path i s f e a s i b l e . —6 Small E m i t t i n g Area (5 χ 10
2 cm )
Small, constrained regions can be probed. Long-path measurements are f a c i l i t a t e d .
In New Applications of Lasers to Chemistry; Hieftje, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
N E W APPLICATIONS O F LASERS T O C H E M I S T R Y
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CHOPPER
DIODE
PUMP
DIODE
LASER
CURRENT
POWER
MODULATOR
SUPPLY
REF. L O C K - IN
X-Y
AMPLIFIER
RECORDER
Figure 1. Diagram of a tunable -diode-laser pollution monitoring sys tem. Sensitivity is enhanced by the use of a multi-traversal, low-pressure cell and second-derivative detection.
Figure 2. Typical measurement data for the tunable-diode-laser instrument diagrammed in Figure 1. A calibration measurement of 20 ppb of S0 in Ν is followed by a measurement of air containing 8 ppb of S0 . 2
2
2
In New Applications of Lasers to Chemistry; Hieftje, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
2.
BUTLER
ET
AL.
Tunable-Diode-Laser
15
IR Spectroscopy
f o r SO2. T h e i r p r e d i c t e d s e n s i t i v i t y l e v e l s f o r other p o l l u t a n t s measured by t h i s technique are summarized i n Table I I . As another i l l u s t r a t i o n o f the p o t e n t i a l low l e v e l d e t e c t i o n c a p a b i l i t y of tunable diode l a s e r spectroscopy, consider the mea surement o f CC>2 c o n c e n t r a t i o n . Researchers from Laser A n a l y t i c s and the New York State Department of Health (£,_5) have shown t h a t the i s o t o p i c s h i f t s i n CO2 absorption l i n e s are e a s i l y r e s o l v e d with diode l a s e r s (Figure 3) and have experimentally v e r i f i e d an absorption s t r e n g t h f o r strong l i n e s of about S = 2.1 χ 10"" cm" molecule~ cm . From the AFGL l i n e l i s t i n g s (6), i t i s found t h a t the optimum CC>2 l i n e f o r d e t e c t i o n purposes i s the Ρ (18) l i n e a t 2210.885 cm"" , and t h a t the p r i n c i p a l i n t e r f e r i n g species i n a i r i s N2O. Assume a d e t e c t i o n system i n c o r p o r a t i n g a 6 - l i t e r m u l t i t r a v e r s a l c e l l , a 200 m o p t i c a l path, and α£ = 10" . In a back ground of 7.6 Torr of a i r , the minimum d e t e c t a b l e c o n c e n t r a t i o n of C02 i s found to be 2.6 χ 10^ molecules/cm^. T h i s c o r r e sponds to a t o t a l mass of 1.16 χ 10~ picograms i n the c e l l , or 1.73 χ 10~ p i c o c u r i e s i n e q u i v a l e n t u n i t s o f r a d i o a c t i v i t y . In pure CO2 without a background of a i r , c o n c e n t r a t i o n as low as 975 molecules/cm corresponding to a t o t a l mass of 4.36 χ 10"^ p i c o grams or 6.48 χ 10""4 p i c o c u r i e s can, i n p r i n c i p l e , be detected. 14
18
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1
1
2
li+
1
5
llf
2
2
3
P a r t i a l Pressure
Determination
Laser s p e c t r o s c o p i c a n a l y s i s o f f e r s a d i r e c t , non-perturbing and general method o f determining p a r t i a l pressures i n gaseous mixtures. As i l l u s t r a t e d i n the H2SO4 study discussed below, p a r t i a l pressures can be measured by t h i s new technique to much lower l e v e l s and under more severe c o n d i t i o n s (e.g., 200°C hot s u l f u r i c acid) than would be p o s s i b l e by other methods. A Laser A n a l y t i c s group has i n v e s t i g a t e d p a r t i a l pressures of H2O, SO3 and H2SO4 vapors over a z e o t r o p i c aqueous s o l u t i o n o f H2SO4 under a program sponsored by the U.S. Environmental P r o t e c t i o n Agency (7_). The p a r t i a l pressures were determined by mea s u r i n g absorption l i n e strengths of SO3 and H2O above the s o l u t i o n and comparing them with l i n e strengths of c a l i b r a t i o n sam p l e s . The H2SO4 pressure was then deduced from a t o t a l pressure measurement using a U-tube manometer f i l l e d with l i q u i d s u l f u r i c acid. F i g u r e 4 shows a diode l a s e r scan of the vapor over an H2SO4 bath near 1416 cm" f o r a sample temperature of 165°C. The broad absorption d i p i n the center i s due t o atmospheric water vapor i n the 2.2m unpurged o p t i c a l path e x t e r n a l to the sample cell. One low pressure water l i n e i s apparent a t the center o f t h i s d i p ; the other l i n e s are due to SO3. The use of diode l a s e r spectroscopy allows s i n g l e l i n e s of SO3 or H2O t o be s e l e c t e d and t h e i r strengths measured. P a r t i a l pressures o f SO3, H2O and H2SO4 obtained i n t h i s study are summarized i n Table I I I . The d i s s o c i a t i o n constant Κ was c a l c u l a t e d from the data of Table I I I and i s i n c l u d e d i n 1
P
In New Applications of Lasers to Chemistry; Hieftje, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
NEW
APPLICATIONS
O F LASERS T O C H E M I S T R Y
TABLE I I
Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 20, 2013 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0085.ch002
MINIMUM DETECTABLE CONCENTRATION FOR A TUNABLE DIODE LASER SYSTEM
Pollutant
Approximate Frequency (cm" )
S0
1140
3
3
1050
0.5
N0
1150
2
C0
1075
300
H0
1135
50
NH
1050
0
2
2
2
2
3
1
Sensitivity (ppb)
0.05
PAN
1150
-0.3
CH
4
1300
0.03
S0
2
1370
0.3
N0
2
1600
0.02
NO
1880
0.03
CO
2120
0.01
C0„
2350
0.001
In New Applications of Lasers to Chemistry; Hieftje, G.; ACS Symposium Series; American Chemical Society: Washington, DC, 1978.
Downloaded by UNIV OF SOUTHERN CALIFORNIA on June 20, 2013 | http://pubs.acs.org Publication Date: June 1, 1978 | doi: 10.1021/bk-1978-0085.ch002
2.
BUTLER
Tunable-Diode-Laser
ET AL.
17
IR Spectroscopy
Figure 3. Τ unable-diode-laser transmission spec trum near 4.5 ^m showing the C 0 1-0 band R(23) absorption line at 2224.713 cm' , the C 0 ρ s band Τ(62) line at 224.033 cm , the C 0 [ΟΙ !
