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9 Automated Flow Injection Analysis System for Formaldehyde Determination Downloaded by NORTH CAROLINA STATE UNIV on December 27, 2017 | http://pubs.acs.org Publication Date: August 8, 1986 | doi: 10.1021/bk-1986-0316.ch009

Mat H. Ho Department of Chemistry, University of Alabama, Birmingham, A L 35294

An automated and microprocessor-controlled flow injection analysis system was developed for formaldehyde emission measurements. This system was based on the modified pararosaniline method and a sampling rate of about 40 samples/hour was obtained. The relative standard deviations for sets of 15 repetitive measurements were 1.5% and 0.4% at concentrations of 1 and 10 μg/ml, respectively. The results obtained from this system correlated well with those obtained from the chromotropic acid. The simplicity, versatility, good precision, high sampling rate, and relatively low cost of the system make it attractive for the analysis of large numbers of formaldehyde samples. Formaldehyde i s a major component i n t h e m a n u f a c t u r i n g o f b u i l d i n g materials such as p a r t i c l e b o a r d , plywood and u r e a formaldehyde insulation. These materials c a n r e l e a s e formaldehyde vapor i n t o the a i r o f m o b i l e homes, o f f i c e b u i l d i n g s , and r e s i d e n c e s r e s u l t i n g in p o t e n t i a l formaldehyde exposure t o i n h a b i t a n t s and w o r k e r s . I t has been shown t h a t formaldehyde i n domestic a i r v a r i e s from near ambient c o n c e n t r a t i o n s ( 1 - 2 5 ppb) t o as h i g h as 4 ppm i n new m o b i l e homes (1). The h e a l t h effects and p o s s i b l e carcinogenicity associated w i t h formaldehyde exposure have c r e a t e d g r e a t c o n c e r n on the monitoring of this c h e m i c a l b o t h i n t h e w o r k p l a c e and i n d o o r environments ( 2 - 5 ) . The m o n i t o r i n g and toxicological studies o f formaldehyde e x p o s u r e , as w e l l as s t u d i e s on t h e e m i s s i o n o f t h i s c h e m i c a l from wood products generate large numbers o f samples t o be a n a l y z e d . Furthermore, i t i s n e c e s s a r y t o m o n i t o r t h e e m i s s i o n s on a r o u t i n e basis during production t o ensure t h a t t h e m a t e r i a l c o n t i n u e s t o release low l e v e l o f formaldehyde. I n homes, p a r t i c u l a r l y i n mobile homes, the amount o f formaldehyde r e l e a s e depends on t h e construction technoloy, ventilation, indoor temperature and relative h u m i d i t y , and age, s t r u c t u r e and p o r o s i t y o f b u i l d i n g materials. I t i s , therefore, necessary to study the emision o f formaldehyde from wood p r o d u c t s as a f u n c t i o n o f t h e s e p a r a m e t e r s . 0097-6156/ 86/ 0316-0107$06.00/ 0 © 1986 A m e r i c a n C h e m i c a l Society

Meyer et al.; Formaldehyde Release from Wood Products ACS Symposium Series; American Chemical Society: Washington, DC, 1986.

Downloaded by NORTH CAROLINA STATE UNIV on December 27, 2017 | http://pubs.acs.org Publication Date: August 8, 1986 | doi: 10.1021/bk-1986-0316.ch009

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The need f o r an automated and r e l i a b l e system f o r formaldehyde determination i s now c l e a r l y r e c o g n i z e d . I n response to t h i s need, an automated and m i c r o p r o c e s s o r - c o n t r o l l e d f l o w i n j e c t i o n a n a l y s i s (FIA) system was developed i n our l a b o r a t o r y . T h i s system i s based on the use o f the m o d i f i e d p a r a r o s a n i l i n e c o l o r i m e t r i c method ( 6 ) . The simplicity, versatility, good p r e c i s i o n , h i g h sampling r a t e , complete automation and r e l a t i v e l y low c o s t o f the system make i t attractive f o r the a n a l y s i s of l a r g e numbers of formaldehyde samples. In this c h a p t e r , s u f f i c i e n t background i n the p r i n c i p l e of FIA will be presented to allow the r e a d e r s to e v a l u a t e the technique and i t s p o t e n t i a l a p p l i c a t i o n to the r o u t i n e a n a l y s i s o f formaldehyde w i l l be e x p l o r e d . P r i n c i p l e o f Flow I n j e c t i o n A n a l y s i s Flow injection a n a l y s i s ( F I A ) , w h i c h was i n t r o d u c e d by R u z i c k a and Hansen (7-9) and by Stewart e t a l ( 1 0 ) , i s based on the concept o f controlled d i s p e r s i o n o f a sample zone when i n j e c t e d i n t o a moving and nonsegmented carrier stream. In continuous flow a n a l y s i s (CFA), successive samples a r e mixed and i n c u b a t e d w i t h r e a g e n t s on the way toward a f l o w through d e t e c t o r . The g r e a t e s t d i f f i c u l t y to overcome in CFA was i n t e r m i x i n g of adjacent samples during transport from the i n j e c t i o n v a l v e to the d e t e c t o r . I n the p a s t , it was widely believed that there a r e o n l y two ways to p r e v e n t carryover i n CFA: either by the use o f t u r b u l e n t f l o w or by a i r segmentation (±1*12.) • T u r b u l e n t f l o w y i e l d s a f l a t v e l o c i t y profile and therefore results i n a lower sample zone d i s p e r s i o n than the laminar f l o w where the v e l o c i t y p r o f i l e i s p a r a b o l i c . However, i t i s d i f f i c u l t to o b t a i n a t u r b u l e n t f l o w i n CFA. I n the segmented CFA, a i r bubbles were used t o d i v i d e the r e a c t i o n stream into a number o f compartments, thus p r e v e n t i n g e x c e s s i v e d i s p e r s i o n of the sample by the d i s p e r s i v e s o u r c e s i n h e r e n t i n the l a m i n a r flow ( 1 3 ) . From t h i s work the most p o p u l a r a u t o m a t i c a n a l y z e r , the T e c h n i c o n A u t o - A n a l y z e r , was developed. Although the presence of a i r bubbles i n the f l o w i n g stream creates several disadvantages, it was believed that a i r segmentation i s essential f o r s u c c e s s f u l CFA. However, i n 1975, Ruzicka and Hansen (7-9) and Stewart e t a l (10) demonstrated t h a t continuous flow a n a l y s i s can be performed i n an unsegmented stream and the absence of the a i r bubbles actually offers several advantages. The name f l o w i n j e c t i o n a n a l y s i s ( F I A ) was proposed for this technique. A s i m p l e FIA system t y p i c a l l y c o n s i s t s o f a pump or some o t h e r means to p r o p e l the c a r r i e r and/or r e a g e n t , a sample injector, a reaction c o i l , a f l o w through d e t e c t o r and a recorder or data h a n d l i n g d e v i c e . A p r e c i s e l y measured volume o f sample i s i n j e c t e d i n t o a c o n t i n u o u s f l o w i n g , nonsegmented c a r r i e r stream. The carrier stream transports the sample toward a f l o w through detector. Necessary reagents needed for a particular analysis are either p r e s e n t i n the c a r r i e r stream o r can be added further down stream on the way to the d e t e c t o r . As i t moves towards the d e t e c t o r , the sample d i s p e r s e s i n t o the c a r r i e r stream both longitudinally and radially by a combination of c o n t r o l l e d laminar flow and molecular diffusion. The sample i s mixed and reacted with reagents to form a d e t e c t a b l e p r o d u c t which i s then monitored by the detector. The response o f the d e t e c t o r can be



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recorded i n the form o f sharp peaks as shown i n F i g u r e 1. These peaks r e f l e c t both the p h y s i c a l d i s p e r s i o n and c h e m i c a l k i n e t i c s o f the r e a c t i o n that takes place between the injection p o r t and detection point. Dispersion i s a phenomenon o f g r e a t importance i n FIA. When a liquid stream flows through a tube, the v e l o c i t y o f the l i q u i d layer i n contact with the tube's s u r f a c e i s p r a c t i c a l l y z e r o and that at the center o f the tube i s t w i c e the mean v e l o c i t y o f the liquid (12,14). From t h i s s t a n d p o i n t o f the l a m i n a r f l o w , one can see that an i n j e c t e d sample bolus will result i n a parabolic velocity profile (Figure 1). I f a sample p l u g i s p l a c e d i n t o a moving stream, and i f the l o n g i t u d i n a l c o n v e c t i o n o f the l a m i n a r flow i s the o n l y means o f d i s p e r s i o n , i t would have an i n f i n i t e l y long tail by the time i t r e a c h e d the d e t e c t o r . As a r e s u l t , the carryover between a d j a c e n t i n j e c t e d samples becomes a serious problem i n CFA. Fortunately, l o n g i t u d i n a l c o n v e c t i o n i s not the only means of dispersion. Molecules can diffuse, both longitudinally (in the direction of flow) and radially (perpendicular to the d i r e c t i o n o f f l o w ) , between the sample b o l u s and carrier stream. In the narrow tube and f l o w i n g stream, the contribution of longitudinal diffusion to the d i s p e r s i o n i s l e s s important than t h a t of r a d i a l d i f f u s i o n . M o l e c u l e s a t the w a l l s o f the tubes d i f f u s e i n t o the c e n t e r o f the sample zone. As a r e s u l t , tailing of the sample due to p a r a b o l i c v e l o c i t y p r o f i l e i n the reaction tube i s minimized by radial diffusion (Figure 1). Diffusion of molecules between the sample and c a r r i e r , the l a t t e r including reagent, explains not only the low c a r r y o v e r and h i g h sample throughput but also the effective mixing o f sample and reagents. Mixing between the sample and c a r r i e r due to d i s p e r s i o n is always incomplete, but because d i s p e r s i o n p a t t e r n f o r a g i v e n FIA system i s p e r f e c t l y r e p r o d u c i b l e , FIA y i e l d s p r e c i s e r e s u l t s . The d i s p e r s i o n of the sample i n the c a r r i e r stream i s a f f e c t e d by several f a c t o r s such as f l o w v e l o c i t y , tube d i a m e t e r , tube l e n g t h and diffusion c o e f f i c i e n t o f the a n a l y t e . These parameters can be controlled i n o r d e r to g i v e an e x c e l l e n t r e p r o d u c i b l e d i s p e r s i o n . In FIA, d i s p e r s i o n i s a l s o f r e q u e n t l y used to d e s c r i b e the degree of d i l u t i o n o f sample i n the i n j e c t o r , r e a c t i o n tube and d e t e c t o r . When sample i s i n j e c t e d i n t o the c a r r i e r stream, i t t r a v e l s as a gradually expanding plug which i s s l o w l y d i l u t e d by the c a r r i e r . Dispersion i s r e q u i r e d to p r o v i d e adequate m i x i n g o f the sample and the reagent, however, i n c r e a s i n g d i s p e r s i o n w i l l decrease the analyte concentration and therefore reduces the sensitivity. Usually, d i s p e r s i o n i s d e f i n e d as a r a t i o o f the c o n c e n t r a t i o n o f the sample b e f o r e m i x i n g has o c c u r r e d t o the maximum c o n c e n t r a t i o n o f the sample a t the d e t e c t o r . Since the r e a c t i o n p r o d u c t s a r e measured b e f o r e s t e a d y - s t a t e conditions a r e e s t a b l i s h e d , the r e a d o u t i s a v a i l a b l e w i t h i n seconds of i n t r o d u c t i o n of the sample and FIA p o s s e s s e s the p o t e n t i a l f o r high sample throughput. This technique has proven to be f a s t , precise, inexpensive, h i g h l y v e r s a t i l e and c a p a b l e o f automating a wide v a r i e t y o f wet chemical procedures. I t i s a l s o p o s s i b l e to avoid or m i n i m i z e d the e f f e c t o f i n t e r f e r i n g s p e c i e s i n FIA because the r e a c t i o n i s not r e q u i r e d to r e a c h e q u i l i b r i u m . The tremendous interest i n FIA i n recent y e a r s i s r e f l e c t e d by i t s s u b s t a n t i a l growth b o t h i n i n s t r u m e n t a l development and a n a l y t i c a l a p p l i c a t i o n s


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(11). There are several e x c e l l e n t r e v i e w s (12,15-17) and a book (i!) that describe the c o n c e p t , p r i n c i p l e , instrumentation, a p p l i c a b i l i t y and l i m i t a t i o n o f FIA.


Experimental Apparatus. F i g u r e 2 shows the b l o c k diagram o f the FIA system used for the d e t e r m i n a t i o n o f formaldehyde. The system c o n s i s t s o f a sampler (Technicon, Tarrytown, NY), a peristaltic pump, a microprocessor-controlled solution handling unit (Model SHS-200, Fiatron I n c . , Milwaukee, WI), a s p e c t r o p h o t o m e t r y d e t e c t o r (Model LC 55, P e r k i n E l m e r , Norwalk, CT) and a s t r i p c h a r t r e c o r d e r . The SHS-200 u n i t consists of a sample valve and a reagent v a l v e systems. The optical e n c o d e r , which i s use f o r c o n t r o l l i n g the pump speed, i s mounted on the pump motor s h a f t to ensure p r e c i s e pump speed m o n i t o r i n g and r e g u l a t i o n . The sample and r e a g e n t v a l v e systems consist o f f i v e t h r e e ways T e f l o n s o l e n o i d v a l v e s . A l l o f these a r e under s o f t w a r e c o n t r o l and can be programmed v i a a f r o n t p a n e l keyboard (18). A l l parameters such as mode, pump speed, washing time, sample injection t i m e , time interval between injections were programmed i n t o the m i c r o p r o c e s s o r c o n t r o l u n i t . Several o p e r a t i o n a l modes such as f i x e d sample volume, programmable sample volume, programmable r e a g e n t volume, s t o p f l o w , merging stream and on stream d i l u t i o n can be o b t a i n e d by programming the pump speed, t i m i n g , and v a l v e s t a t e s ( 1 8 ) . I n t h i s s t u d y , mode 20 was used and pararosaniline was a l l o w e d to f l o w c o n t i n u o u s l y as carrier stream. Formaldehyde samples were a u t o m a t i c a l l y f e d i n t o the FIA system v i a a sampler which was a l s o under m i c r o p r o c e s s o r control. The reaction coils consist o f 650 cm o f 0.8 mm i . d . Teflon tubing and the temperature was c o n t r o l l e d a t 50 C by a thermostated water b a t h . The f l o w r a t e was k e p t a t 1.0 ml/minute. This a l l o w e d about 196 seconds f o r the r e a c t i o n t o o c c u r b e f o r e reaching the d e t e c t o r . The sample i n j e c t i o n time was programmed i n o r d e r t o i n j e c t 250 u l formaldehyde i n t o the c a r r i e r s t r e a m . Reagents. A l l c h e m i c a l s were ACS a n a l y t i c a l r e a g e n t grade and were used w i t h o u t further purification. D e i o n i z e d d i s t i l l e d water was used for solution p r e p a r a t i o n s . The s t o c k p a r a r o s a n i l i n e r e a g e n t was o b t a i n e d as an 0.2% (W/V) solution i n 1M HC1 from CEA Instruments, Emerson, NJ. The w o r k i n g p a r a r o s a n i l i n e s o l u t i o n (0.9 mM pararosaniline i n 0.5 mM HC1) was p r e p a r e d from the s t o c k solution and s u f f i c i e n t HC1 was added to b r i n g i t s c o n c e n t r a t i o n t o 0.5 mM. The second r e a g e n t , which i s 1.60 mM sodium s u l f i t e , was prepared by dissolving 0.2 g o f anhydrous sodium s u l f i t e ( F i s h e r Scientific Co., F a i r Lawn, NJ) i n d e i o n i z e d water and d i l u t i n g t o 1 liter. This r e a g e n t must be made f r e s h d a i l y . Formaldehyde s t o c k solution, a p p r o x i m a t e l y 1 mg/ml, was p r e p a r e d by d i l u t i n g 2.7 ml o f 37% formaldehyde s o l u t i o n ( F i s h e r S c i e n t i f i c Co., F a i r Lawn, NJ) t o 1 liter with d e i o n i z e d water. The s t o c k s o l u t i o n was s t a n d a r d i z e d using the s u l f i t e method ( 1 9 , 2 0 ) . T h i s s o l u t i o n remained s t a b l e for several months. Formaldehyde s t a n d a r d s o l u t i o n s were p r e p a r e d daily from the s t o c k s o l u t i o n . A c h r o m o t r o p i c a c i d s o l u t i o n , 0.01 g/ml, was p r e p a r e d f r e s h by d i s s o l v i n g 4,5-dihydroxy-2,7-naphthalenedisulfonic acid d i s o d i u m s a l t (Eastman Kodak, R o c h e s t e r , NY) i n d e i o n i z e d water.


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Sample Bolus Resulting From Laminar Flow and Molecular Diffusion o

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To Waste F i g u r e 2. Schematic diagram o f the m i c r o p r o c e s s o r c o n t r o l l e d FIA system f o r f o r m a l d e h y d e . ( 1 ) Formaldehyde s t a n d a r d s o r samples; (2) 0.9 mM p a r a r o s a n i l i n e i n 0.5 M HC1; ( 3 ) 1.60 mM sodium s u l f i t e ; ( 4 ) p e r i s t a l t i c pump ( 5 ) m i c r o p r o c e s s o r c o n t r o l u n i t ; ( 6 ) sample i n j e c t i o n v a l v e s system; ( 7 ) r e a c t i o n c o i l s ; ( 8 ) Y connector



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Procedure. Formaldehyde sample from the sampler was i n j e c t e d i n t o the carrier stream where i t was mixed w i t h p a r a r o s a n i l i n e and then sulfite to form an alkylsulfonic acid chromophore which can be monitored spectrophotometrically at 570 nm. For calibration, standard formaldehydes were s e q u e n t i a l l y i n t r o d u c e d a f t e r a s t a b l e baseline was obtained. At least f i v e consecutively reproducible peaks were r e c o r d e d f o r each c o n c e n t r a t i o n . A f t e r each s t u d y or each day o f o p e r a t i o n , the FIA system was c l e a n e d t o remove any pararosaniline film, alkylsulfonic acid colored product, or particulate matters. This reduced the scattered light i n the absorption c e l l and the s t a i n i n g o f the t u b i n g w a l l s . The c l e a n - u p procedure was initiated by running d i s t i l l e d deionized water through the system for five minutes f o l l o w e d by another f i v e minutes washing with 0.1 N n i t r i c a c i d and then f l u s h i n g the u n i t for 30 minutes w i t h d e i o n i z e d w a t e r . The c h r o m o t r o p i c a c i d method was used f o r comparative s t u d i e s , and the a n a l y t i c a l p r o c e d u r e f o r the c h r o m o t r o p i c a c i d method was based on the p r o c e d u r e recommended by the American P u b l i c H e a l t h A s s o c i a t i o n ( 1 9 ) . R e s u l t s and

Discussion

The pararosaniline method has been used w i d e l y f o r the determination of formaldehyde i n aqueous solutions and i n the atmosphere. I n t h i s procedure mercury ( I I ) - s u l f i t e and a c i d i f i e d pararosaniline r e a g e n t were sequentially added to an aqueous formaldehyde s o l u t i o n (21,22). I n 1965, an automated procedure f o r formaldehyde was d e s c r i b e d by L y l e s e t a l ( 2 1 ) . L a t e r , Lahmann and Jander (22) modified the reagent c o n c e n t r a t i o n s to enhance sensitivity. This method has been adapted to the CEA 555 formaldehyde a n a l y z e r (CEA Instruments, I n c . , Emerson, N J ) . The major drawback of the p a r a r o s a n i l i n e method i s the use o f poisonous tetrachloromercurate t o s t a b i l i z e the s u l f i t e r e a g e n t . I n o r d e r t o avoid the t o x i c h a z a r d and d i s p o s a l problem o f mercury, a m o d i f i e d pararosaniline method f o r formaldehyde d e t e r m i n a t i o n was d e v e l o p e d by M i k s c h et a l ( 6 ) . To a n a l y z e a formaldehyde s o l u t i o n , the acidified pararosaniline r e a g e n t was added f i r s t and then sodium sulfite. Formaldehyde reacts with p a r a r o s a n i l i n e and s u l f i t e t o produce alkyl sulfonic acid which can be d e t e c t e d a t 570 nm. Studies on the reagent stability, temperature dependence and i n t e r f e r e n c e o f t h i s method have a l s o been p u b l i s h e d (23.24). Concentrations of pararosaniline (0.9 mM), h y d r o c h l o r i c a c i d (0.5 mM) and sodium s u l f i t e (1.60 mM) were s e l e c t e d to p r o v i d e the same final concentrations after mixing as i n the o p t i m i z e d conditions described by M i k s c h e t a l ( 6 J . No attempt was made t o determine the pH of the reaction inside the flow system. Formaldehyde was injected into the stream of acidified pararosaniline and then merged w i t h sodium s u l f i t e to produce a c o l o r e d p r o d u c t . The r e s u l t s were r e c o r d e d as sharp peaks. In the determination of formaldehyde using pararosaniline method, the temperature of the r e a c t i o n s h o u l d be c o n t r o l l e d i n order to o b t a i n reproducible results (6,24). The r a t e o f the reaction i s also temperature dependent ( 6 ) . I n t h i s s t u d y , the temperature of the reaction c o i l was k e p t c o n s t a n t a t 50 C. Since Teflon i s not a good t h e r m a l l y c o n d u c t i v e m a t e r i a l , i t i s expected that the temperature o f the r e a c t i o n was about 40 C.



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Miskch e t a l (J5) showed t h a t the absorbance tends t o d e c r e a s e as the temperature increased above 25 C, p r o b a b l y because o f the evaporation of s u l f u r d i o x i d e from the a c i d i c s o l u t i o n s . However, such sulfur d i o x i d e or formaldehyde l o s s e s a r e not p o s s i b l e i n our flow system due to containment o f the sample and r e a g e n t s w i t h i n the T e f l o n t u b i n g . The sensitivity of the system, which was measured as peak heights, can be enhanced by increase the c h e m i c a l development period following the a d d i t i o n of a c i d i f i e d pararosaniline and sodium sulfide. This can be done by i n c r e a s e the l e n g t h o f the reaction coil. The i n c r e a s e i n r e s i d e n c e time i s c o u n t e r b a l a n c e d , however, by an i n c r e a s e i n the d i s p e r s i o n o f the sample zone. The reaction coil o f 650 cm was chosen f o r the FIA system. It is important to r e a l i z e t h a t o n l y a r e l a t i v e l y s h o r t r e s i d e n c e time i s achieved i n FIA. T h e r e f o r e , the F I A t e c h n i q u e was o r i g i n a l l y not though to have a v e r y wide scope o f a p p l i c a t i o n s , s i n c e many colorimetric methods performed m a n u a l l y u s u a l l y r e q u i r e d 30 minutes or more f o r optimum c o l o r development. In the present case, optimum c o l o r development f o r formaldehyde d e t e r m i n a t i o n u s i n g the modified p a r a r o s a n i l i n e p r o c e d u r e r e g u i r e s about 60 minutes a t room temperature and 10-15 minutes a t 40 C (j6). I n the FIA System, the chemical r e a c t i o n never r e a c h e d the s t e a d y s t a t e due t o s h o r t residence time. However, the time i s c o n t r o l l e d p r e c i s e l y and excellent reproducible results can be obtained. Furthermore, mixing between formaldehyde and r e a g e n t s due t o d i s p e r s i o n may be incompleted, but because d i s p e r s i o n p a t t e r n f o r a g i v e n FIA system i s p e r f e c t l y r e p r o d u c i b l e , the system y i e l d s p r e c i s e r e s u l t s . Teflon t u b i n g was used to c o n s t r u c t the system. T h i s reduced the staining of the tubing walls by p a r a r o s a n i l i n e and c o l o r e d product. The staining p r o c e s s may increase the background or contribute to the memory e f f e c t following the a n a l y s i s o f h i g h formaldehyde concentrations and therefore d e c r e a s e the s a m p l i n g frequency. Since the i n t e r f e r e n t studies has been reported elsewhere ( 6 . 2 5 ) . i t was not r e p e a t h e r e . However, i t i s e x p e c t e d that the s e l e c t i v i t y i n the FIA w i l l be much b e t t e r as compared t o the manual p r o c e d u r e because FIA i s a k i n e t i c t e c h n i q u e and the s t e a d y s t a t e i s not a l l o w e d to a c h i e v e d . Figure 3 shows the t y p i c a l r e s p o n s e peaks o f the FIA system for f o r m a l d e h y d e . The p r e c i s i o n o f a l l measurements was v e r y good. The r e l a t i v e s t a n d a r d d e v i a t i o n f o r s e t s o f 15 i n j e c t i o n s were 1.5% and 0.4% at concentrations o f 1 and 10 ug/ml, r e s p e c t i v e l y . Aqueous formaldehyde s t a n d a r d s were used f o r the c a l i b r a t i o n . Linearity was observed f o r the c o n c e n t r a t i o n range from 1 to 15 Ug/ml. The equation describing the linear portion of the calibration plot i s given by Y * 0.098 X + 0.031 where Y i s the peak h e i g h t i n absorbance unit and X i s the c o n c e n t r a t i o n o f formaldehyde i n yg/ml. The c a l i b r a t i o n p l o t i s shown i n f i g u r e 4. Comparison studies between the FIA and the c h r o m o t r o p i c a c i d were performed. Fifteen samples w i t h formaldehdye c o n c e n t r a t i o n s ranging from 1 to 10.8 yg/ml were determined by b o t h methods and a correlation coefficient o f 0.994 was o b t a i n e d . T h i s i n d i c a t e s a good c o r r e l a t i o n between two methods. The flow i n j e c t i o n system described here can be used f o r automated analysis o f l a r g e numbers o f formaldehyde samples. The



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5 min. F i g u r e 3. T y p i c a l r e s p o n s e peaks o f F I A system f o r formaldehyde, (a) 13.5 yg/ml; (b) 10.5 yg/ml; ( c ) 8.0 yg/ml; ( d ) 5.0 yg/ml

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Automated Flow Injection Analysis System for ... - ACS Publications

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