Determination of Submicrogram Quantities of Fluoride by a Rapid and Highly Sensitive Method

H. Tsunoda and M.-H. Yu (Editors) Fluoride Research 1985, Studies in Environmental Science, Volume 27, pp. 25-29 0 1986 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

25

DETERMINATION OF SUBMICROGRAM QUANTITIES OF FLUORIDE BY A R A P I D AND HIGHLY SENSITIVE METHOD

K A Z U Y O S H I I T A I AND HUM10 TSUNODA D e p a r t m e n t o f H y g i e n e and P u b l i c H e a l t h , U n i v e r s i t y , M o r i o k a , I w a t e 020, J a p a n

School o f Medicine,

Iwate Medical

ABSTRACT

A new m e t h o d h a s b e e n d e v e l o p e d f o r t h e d e t e r m i n a t i o n o f s u b m i c r o g r a m q u a n t i t i e s o f f l u o r i d e . It c o n s i s t s o f s e p a r a t i n g f l u o r i d e f r o m t h e t e s t sample b y t h e p y r o h y d r o l y t i c s e p a r a t i o n method and d e t e r m i n a t i o n o f t h e r e s u l t i n g aqueous HF s o l u t i o n b y e l e c t r o t h e r m a l m o l e c u l a r a b s o r p t i o n spect r o m e t r y as a l u m i n u m m o n o f l u o r i d e ( A l F ) . sensitive,

T h i s method i s rapid,

highly

and c a n be used f o r t h e d e t e r m i n a t i o n o f t o t a l ( o r g a n i c and

i n o r g a n i c ) f l u o r i d e i n v a r i o u s o r g a n i c compounds and b i o l o g i c a l m a t e r i a l s .

INTRODUCTION

A number o f a n a l y t i c a l m e t h o d s h a v e been d e s c r i b e d f o r t h e d e t e r m i n a t i o n o f f l u o r i n e i n o r g a n i c f l u o r i d e compounds

and b i o l o g i c a l m a t e r i a l s (1-3).

Many o f t h e s e m e t h o d s a r e r a t h e r c o m p l e x and t i m e - c o n s u m i n g .

I n addition,

when t h e f l u o r i n e c o n c e n t r a t i o n s o f t h e s a m p l e s a r e l o w , t h e y c a n n o t b e a c c u r a t e l y determined.

A r a p i d and h i g h l y s e n s i t i v e m e t h o d f o r f1uo:ide developed.

d e t e r m i n a t i o n h a s been

I t combines t h e p y r o h y d r o l y t i c s e p a r a t i o n method w i t h

electro-

t h e r m a l a l u m i n u m m o n o f l u o r i d e (A1F) m o l e c u l a r a b s o r p t i o n s p e c t r o m e t r y . The p y r o h y d r o l y t i c s e p a r a t i o n method i s v a l u a b l e f o r r a p i d i s o l a t i o n o f f l u o r i n e f r o m i n o r g a n i c r e f r a c t o r y compounds, many o r g a n i c m a t e r i a l s (4-7).

s u c h as c a l c i u m f l u o r i d e ,

o r from

I n a d d i t i o n , t h i s method i s f r e e f r o m conta-

m i n a t i o n by extraneous f l u o r i d e s .

By u s e o f an a u t o s a m p l e r ,

f l u o r i n e can

be a c c u r a t e l y and p r e c i s e l y d e t e r m i n e d a t t h e p p b l e v e l b y e l e c t r o t h e r m a l A1F m o l e c u l a r a b s o r p t i o n s p e c t r o m e t r y . MATERIAL AND METHODS Apparatus F i g u r e 1 shows t h e a p p a r a t u s u s e d i n t h e p y r o h y d r o l y s i s . t i o n tube,

A quartz reac-

550 m m l o n g w i t h an i n t e r n a l d i a m e t e r o f 1 3 m m i s m a i n t a i n e d b y

26 a main-heater

a t a p p r o x i m a t e l y 1100°C.

The t u b e i s g r a d u a l l y

heated t o

500°C w i t h a s u b - h e a t e r . O x y g e n g a s i s s c r u b b e d w i t h 0.2N KOH s o l u t i o n , s i l i c a g e l , a n d a c t i v e charcoal,

and p a s s e d t h r o u g h t h e t u b e w i t h w a t e r v a p o r a t a r a t e o f a p p r o x -

i m a t e l y 1 l/min.

P l a t i n u m gauze,

p l a c e d a t t h e end o f t h e t u b e .

used a s a c a t a l y s t f o r t h e o x i d a t i o n .

is

A p i e c e o f q u a r t z wool i s placed behind t h e

p l a t i n u m gauze f o r t r a p p i n g v o l a t i l e m e t a l s and m e t a l o x i d e s (6).

A suit-

a b l e a m o u n t o f c o n d e n s a t e i s w e i g h e d w i t h an e l e c t r i c b a l a n c e i n a p o l y s t y r e n e b o t t l e w i t h cover. The a p p a r a t u s as f o l l o w s :

used f o r d e t e r m i n a t i o n o f f l u o r i n e i n t h e condensate a r e

a t o m i c a b s o r p t i o n s p e c t r o m e t e r m o d e l AA-855.

equipped w i t h a

d e u t e r i u m s i m u l t a n e o u s b a c k g r o u n d c o r r e c t i o n s y s t e m ; g r a p h i t e f u r n a c e atomi z e r , m o d e l FLA-100; sampler,

m i c r o p r o c e s s o r r e a d o u t s y s t e m , m o d e l MC-100;

model AS-301

( a 1 1 m a n u f a c t u r e d b y N i p p o n J a r r e l l - A s h Co.,

and a u t o Ltd.).

F i g u r e 1. A d i a g r a m s h o w i n g a p p a r a t u s u s e d f o r p y r o h y d r o l y s i s 1: 0.2" KOH s o l u t i o n : 2: S i l i c a g e l ; 3: A c t i v e c h a r c o a l : 4: F l o w m e t e r : 5: W a t e r f l a s k ( 9 0 ° C ) ; 6: R e a c t i o n t u b e ( Q u a r t z ) ; 7: S u b h e a t e r (0-500°C); 8: M a i n h e a t e r ( 1 1 0 0 ° C ) : 9: S a m p l e b o a t ; 1 0 : P l a t i n u m g a u z e ; 1 1 : Q u a r t z w o o l : 12: Condenser; 13: S t y r e n e b o t t l e : 14: E l e c t r i c b a l a n c e .

Reagents

A s t a n d a r d f l u o r i d e s o l u t i o n was p r e p a r e d b y d i s s o l v i n g a n a l y t i c a l - g r a d e s o d i u m f l u o r i d e (Wako P u r e C h e m i c a l s ) i n d i s t i l l e d w a t e r .

A 1% a l u m i n u m

s o l u t i o n w a s p r e p a r e d b y d i s s o l v i n g 5 g o f a l u m i n u m m e t a l p o w d e r (99.5%. Wako) i n 1 0 m l o f c o n c e n t r a t e d n i t r i c a c i d a n d d i l u t i n g t o 5 0 0 m l w i t h d i s t i l l e d water.

A 2% b a r i u m s o l u t i o n was p r e p a r e d b y d i s s o l v i n g 38.06 g

o f a n a l y t i c a l - g r a d e b a r i u m n i t r a t e (Wako)

i n d i s t i l l e d w a t e r and d i l u t i n g

t o 1000 m l . Procedure S a m p l e s r a n g i n g f r o m s e v e r a l mg t o g w e r e w e i g h e d i n t o t h e s a m p l e b o a t , and w e r e i n t r o d u c e d i n t o t h e s u b - h e a t e r a t u r e o f t h e sub-heater 4

p a r t o f t h e tube.

was g r a d u a l l y r a i s e d t o 500°C,

i n t r o d u c e d i n t o t h e c e n t e r o f t h e main-heater.

When t h e t e m p e r -

t h e s a m p l e b o a t was

F l u o r i n e i n t h e s a m p l e was

27 i s o l a t e d a s HF a n d w a s c o n d e n s e d w i t h w a t e r v a p o r b y t h e c o n d e n s e r .

AP

a p p r o p r i a t e a m o u n t o f t h e c o n d e n s a t e was w e i g h e d i n t o a p o l y s t y r e n e b o t t l e C o n d e n s a t e s r a n g i n g f r o m 5 t o 20 g ( m l ) w e r e u s u a l l y c o l -

w i t h cover. lected.

For very resistant materials,

tungsten t r i o x i d e ,

s u c h a s CaF2,

t h e a d d i t i o n o f some

w h i c h a c t e d a s an a c i d f l u x , was r e q u i r e d (7).

Measurement o f F l u o r i n e i n C o n d e n s a t e The p r o c e d u r e f o r t h e d e t e r m i n a t i o n o f f l u o r i n e i n t h e c o n d e n s a t e b y e l e c t r o t h e r m a l A1F m o l e c u l a r a b s o r p t i o n s p e c t r o m e t r y i s shown i n F i g u r e 2

(5).

To 800 p 1 o f t h e c o n d e n s a t e o r a s t a n d a r d s o l u t i o n i n a p o l y e t h y l e n e

c u p was added 200 p l o f t h e a l u m i n u m s o l u t i o n c o n t a i n i n g 0.1% a l u m i n u m and

1% b a r i u m a s n i t r a t e , r e s p e c t i v e l y . autosampler.

The p o l y e t h y l e n e c u p was s e t i n t h e

The m i x t u r e c o n t a i n i n g t h e c o n d e n s a t e and t h e a l u m i n u m s o l u -

t i o n was a u t o m a t i c a l l y i n J e c t e d i n t o t h e g r a p h i t e f u r n a c e . m e a s u r e m e n t a r e s h o w n i n F i g u r e 2.

Conditions f o r

T h e a b s o r b a n c e o f A1F g e n e r a t e d w a s

a u t o m a t i c a l l y m e a s u r e d and t h e f l u o r i n e c o n c e n t r a t i o n o f t h e c o n d e n s a t e was determined by a microprocessor readout system from a c a l i b r a t i o n curve obtained by using standard f l u o r i d e solutions.

Condensate

1 -

8 0 0 , ~l

; At I

U I

soIution'200p

e

P o l y e t h y l e n e cup

I

Autosampler (20,u Q )

I

D r y i n g (,l50V 30sec) I C h a r r i n g ( 8 3 O C 15sec)

I I

A t o m i z i n g (2800C 7 s e e ) Measurement F i g u r e 2. P r o c e d u r e f o r d e t e r m i n a t i o n o f f l u o r i d e b y A l F m o l e c u l a r a b s o r p t i o n spectrometry. A1 s o l u t i o n c o n s i s t s o f 0.1% A1 and 1% BA a s a n i t r a t e .

RESULTS AND D I S C U S S I O N Recoveries o f f l u o r i n e f r o m several standard m a t e r i a l s by use o f t h e p r e s e n t m e t h o d a r e shown i n T a b l e I. I n a l l t h e f l u o r i d e compounds t e s t e d , t h e f l u o r i n e contents found were i n agreement w i t h t h e c a l c u l a t e d values, a n d t h e r e c o v e r y w a s n e a r l y 100%.

As observed by Warf gt

r i d e s c a n be g r o u p e d i n t o t w o c a t e g o r i e s : and a s l o w l y p y r o h y d r o l y z a b l e group. alkaline-earth 1

aj.

(4).

fluo-

a r a p i d l y p y r o h y d r o l y z a b l e group,

The f l u o r i d e s o f t h e a l k a l i n e and

m e t a l s b e l o n g t o t h e l a t t e r g r o u p (6).

F l u o r i n e compounds

28 TABLE I

OF

RECOVERIES

FLUORINE FROM STANDARD MATERIALS

% F calculated

% F found

Recovery (%)

NaF

45.2

45.5

CaF2

48.7

48.2'

107 99.0

N ( CF2CF2CF2CF3)3

76.4

76.1

99.6

Cg IH 5 C6 H4 F

11.0

10.7

97.3

'W03 was added

o f the alkaline-earth

metals,

such as c a l c i u m f l u o r i d e .

are very heat

F l u o r i n e i n t h e s e m a t e r i a l s was s e p a r a t e d b y t h e u s e o f W03 as

resistant.

A c c o r d i n g t o Leuven Q t d l (7).

an a c i d f l u x .

i n t h e absence o f t h e f l u x

t h e r e c o v e r y o f f l u o r i n e i n t h e s e m a t e r i a l s was p o o r , b e l o w 50%, b u t i n t h e p r e s e n c e o f t h e f l u x t h e r e c o v e r y w a s n e a r l y 100%.

I n t h i s method,

the

r e c o v e r y o f f l u o r i n e i n CaF2 w i t h t h e u s e o f W03 was n e a r l y 100%. T a b l e I 1 shows t h e f l u o r i n e c o n t e n t s o f v a r i o u s o r g a n i c m a t e r i a l s d e t e r m i n e d b y t h i s method.

I n order f o r rapid pyrohydrolytic separation o f

f l u o r i n e i n t h e s e m a t e r i a l s a s HF,

W03 w a s a d d e d a s a n a c i d f l u x .

f l u o r i n e c o n t e n t o f orchard leaves,

NBS S t a n d a r d R e f e r e n c e M a t e r i a l 1571,

w a s r e p o r t e d t o b e 4 ppm.

The

W i t h t h i s m e t h o d , i t w a s f o u n d t o b e 4.6 ppm.

T h e a v e r a g e f l u o r i n e c o n t e n t i n t h e same s a m p l e d e t e r m i n e d b y A.O.A.C. m e t h o d ( 1 ) was 5.320.61

11.5%.

ppm (n=7),

and t h e 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 was

The a v e r a g e v a l u e o f u n p o l i s h e d r i c e o b t a i n e d b y t h i s m e t h o d was

0.57 ppm.

The a v e r a g e f l u o r i d e v a l u e i n t h e same s a m p l e a s d e t e r m i n e d b y

t h e A.O.A.C.

m e t h o d ( 1 ) was 0.7020.13

ppm ( n = 5 ) ,

and t h e r e l a t i v e s t a n d a r d

TABLE I 1 DETERMINATION

OF

Sample

FLUORIDE I N V A R I O U S MATERIALS n

x f S.D. (uLl/g)

0.21

c.v

(%)

O r c h a r d 1e a v e s

8

4.6

f

Unpolished r i c e

6

0.57

2 0.02

3.4

Human h a i r

5

2.4

f 0.24

10.0

Human serum

4

0.043 f 0.006

4.6

14.0

29 d e v i a t i o n was 18.6%.

Sakurai

& &.

(9) r e p o r t e d t h a t t h e average f l u o r i n e

c o n t e n t i n u n p o l i s h e d r i c e i n J a p a n was 0.89 ppm. The f l u o r i n e c o n t e n t o f human h a i r , available,

f o r which l i t t l e information i s

was d e t e r m i n e d b y t h i s method.

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 w a s 10%. t h o s e f o r t h e p l a n t s a m p l e s shown,

The a v e r a g e was 2.4 ppm and t h e

The l a r g e v a r i a t i o n , c o m p a r e d t o

i s p r e s u m a b l y due t o t h e h a i r b e i n g

s c a t t e r e d b y a n e l e c t r o s t a t i c f o r c e w h i l e t h e s a m p l e was p l a c e d i n t h e r e a c t i o n tube.

The f l u o r i n e c o n c e n t r a t i o n o f human s e r u n i s s o l o w t h a t

a c c u r a t e d e t e r m i n a t i o n h a s been d i f f i c u l t .

A l a r g e volume o f serum sample

i s needed f o r t h e d e t e r m i n a t i o n .

W i t h t h i s method,

done w i t h o n l y 1 m l o f sample.

The r e l a t i v e l y h i g h 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 seen i n T a b l e I 1 i s due t o

t h e d e t e r m i n a t i o n was

the extremely low concentration o f

f l u o r i n e i n t h e sample.

CONCLUSION T h e c o m b i n e d u s e o f p y r o h y d r o l y s i s a n d e l e c t r o t h e r m a l A1F m o l e c u l a r a b s o r p t i o n s p e c t r o m e t r y h a s made i t p o s s i b l e t o d e t e r m i n e f l u o r i n e i n many m a t e r i a l s r a p i d l y and a c c u r a t e l y .

I n t h e presence o f a c a t a l y s t such as

W03, a r a p i d s e p a r a t i o n o f f l u o r i n e f r o m i n o r g a n i c r e f r a c t o r y c o m p o u n d s (e.g.

CaF2) c a n b e a c c o m p l i s h e d .

Aluminum m o n o f l u o r i d e (AlF) m o l e c u l a r

a b s o r p t i o n s p e c t r o m e t r y c a n be u s e d t o d e t e r m i n e t h e l e v e l o f f l u o r i n e i n a s a m p l e o f 20 g l w i t h h i g h a c c u r a c y . hydrolysis,

When s a m p l e s a r e o b t a i n e d f r o m

pyro-

c o n t a m i n a t i o n b y extraneous f l u o r i d e i s avoided.

ACKNOWLEDGEMENT T h i s research i s supported by Grant-in-Aid R e s e a r c h (No.

60480193) f o r 1985-1987

f o r Co-operative S c i e n t i f i c

from t h e M i n i s t r y o f Education,

S c i e n c e and C u l t u r e o f t h e G o v e r n m e n t o f Japan

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