Chapter 24
Measurement of Atmospheric Gases by Laser Absorption Spectrometry H. I. Schiff, G. W. Harris, and G. I. Mackay Downloaded by EAST CAROLINA UNIV on December 31, 2017 | http://pubs.acs.org Publication Date: September 3, 1987 | doi: 10.1021/bk-1987-0349.ch024
Unisearch Associates, 222 Snidercroft, Concord, Ontario L4K 1B5 Canada
The advantages of Tunable Diode Laser Absorption Spectrometry (TDLAS) for measuring trace atmospheric gases are universality, positive identification, good sensitivity and rapid response time. An instrument is described which can measure two gases simultaneously under automatic computer control with detection limits better than 100 parts per trillion and with response times better than 5 minutes. Procedures have been established for the measurement of NO, NO , MNO, NH , H Q and HCHO. These species have been measured under a variety of conditions in smog chambers and in ambient air from mobile laboratories and from aircraft. 2
2
3
3
2
Tunable d i o d e l a s e r a b s o r p t i o n s p e c t r o m e t r y o f f e r s an a t t r a c t i v e method f o r a t m o s p h e r i c measurements atmospheric i n t e r e s t
( 1 ) . Almost a l l gases o f
a b s o r b s i n the 2 t o 1 5 m i c r o n r e g i o n . The v e r y
high s p e c t r a l r e s o l u t i o n of tunable diode l a s e r s permit s e l e c t i o n o f a s i n g l e r o t a t i o n a l - v i b r a t i o n a l l i n e which makes from o t h e r g a s e s v e r y u n l i k e l y .
I f an a c c i d e n t a l
interferences
interference
s h o u l d happen to o c c u r i t can r e a d i l y be i d e n t i f i e d by a change i n l i n e shape and another
l i n e c a n be c h o s e n .
Unequivocal proof of
the absence o f i n t e r f e r e n c e i s o b t a i n e d by measuring the c o n c e n t r a t i o n s at 2 d i f f e r e n t i n t e r f e r e n c e s at 2 d i f f e r e n t
l i n e s . The p r o b a b i l i t y o f
identical
l i n e s i s vanishingly small ( 2 ) .
0097-6156/87/0349-0274$06.00/0 © 1987 American Chemical Society
Johnson et al.; The Chemistry of Acid Rain ACS Symposium Series; American Chemical Society: Washington, DC, 1987.
24.
SCHIFF ET
AL.
Measurement
of Atmospheric
275
Gases
To get the d e s i r e d s e n s i t i v i t y and d e t e c t i o n l i m i t a l o n g a b s o r p t i o n p a t h can be o b t a i n e d by u s i n g a m u l t i - p a s s White The a b s o r p t i o n l i n e can be scanned
cell.
i n a f r a c t i o n o f a second
and
the response time o f t h e measurement i s n o r m a l l y l i m i t e d by t h e r e s i d e n c e time o f the sampled gas i n the White c e l l which i s t y p i c a l l y a few
Downloaded by EAST CAROLINA UNIV on December 31, 2017 | http://pubs.acs.org Publication Date: September 3, 1987 | doi: 10.1021/bk-1987-0349.ch024
Description
seconds.
of the Instrument
F i g u r e 1 shows t h e s c h e m a t i c o f a TDLAS system d e s i g n e d t o o p e r a t e i n f i e l d c o n d i t i o n s from a m o b i l e l a b o r a t o r y . Lead s a l t d i o d e s t y p i c a l l y o p e r a t e i n t h e 20 t o 80 Κ range and t h e wavelength r e g i o n over which they emit r a d i a t i o n depends on t h e temperature and c u r r e n t p a s s i n g through them.
the
Temperature c o n t r o l to + .005 Κ i s
p r o v i d e d by t h e c o m b i n a t i o n o f a c l o s e d c y c l e h e l i u m c r y o c o o l e r , a h e a t e r and a s e r v o temperature system. each One
Two
c r y o s t a t s a r e used*
h a v i n g a l a s e r s o u r c e assembly c o n t a i n i n g 4 l a s e r d i o d e s . l a s e r d i o d e from each assembly can be chosen t o p e r m i t
two
gases t o be measured s i m u l t a n e o u s l y . The e m i t t e d r a d i a t i o n from each o f the d i o d e s i s scanned over the s e l e c t e d a b s o r p t i o n f e a t u r e by changing t h e c u r r e n t through the d i o d e . The l a s e r beam from each head i s c o l l e c t e d and f o c u s s e d by an o f f - a x i s p a r a b o l i c m i r r o r , QAP
S
or 0AP
a
and t h e n d i r e c t e d t o a
s e l e c t i o n m i r r o r , S which f l i p s back and f o r t h t o p e r m i t the beam from each o f the d i o d e s t o e n t e r the White c e l l
i n turn.
The 45° a n g l e o f the e n t r a n c e window t o t h e White c e l l t h e l a s e r beam.
splits
Most o f t h e beam p a s s e s through the window i n t o
the White c e l l but about 5% i s r e f l e c t e d through a c e l l
containing
h i g h c o n c e n t r a t i o n s o f t h e t a r g e t gases onto a s e p a r a t e HgCdTe detector.
The o u t p u t from t h i s d e t e c t o r i s used t o l o c k the l a s e r
r a d i a t i o n wavelength t o t h e c e n t e r o f t h e a b s o r p t i o n l i n e . The beam e n t e r s the 1.75 m T e f l o n - l i n e d White c e l l a c o r n e r cube r e f l e c t o r to t h e d e t e c t o r .
containing
