Analysis of Trace Metals in Orange Juice - ACS Symposium Series

16 Analysis of Trace Metals in Orange Juice JAMES A. McHARD, SUSAN J. FOULK, JEANETTE L. JORGENSEN, SAM BAYER, and JAMES D. WINEFORDNER

Downloaded by PURDUE UNIV on July 7, 2016 | http://pubs.acs.org Publication Date: December 15, 1980 | doi: 10.1021/bk-1980-0143.ch016

Department of Chemistry, University of Florida, Gainesville, FL 32611

Orange j u i c e h a s become o v e r t h e y e a r s one o f t h e most w i d e l y accepted n a t u r a l source beverages. I t s w i d e a c c l a i m i s due t o i t s i n v i g o r a t i n g f l a v o r , i t s source o f quick energy i n t h e form o f n a t u r a l sugars, i t s other n u t r i e n t q u a l i t i e s , e s p e c i a l l y as a s o u r c e f o r v i t a m i n C, and i t s h i g h p o t a s s i u m c o n t e n t r e l a t i v e t o o t h e r i n o r g a n i c m e t a l s , p a r t i c u l a r l y sodium. T h i s l a t t e r a t t r i b u t e ( h i g h p o t a s s i u m l o w sodium) makes o r a n g e j u i c e a h i g h l y d e s i r e d f o o d s o u r c e f o r t h o s e who must r e s t r i c t t h e i n t a k e o f s o d i um i n t h e i r d i e t . The a u t h o r s ' i n t e r e s t i n o r a n g e j u i c e i s , however, not i n t h e n u t r i t i v e area but i n f o r m u l a t i n g a complete des c r i p t i o n o f the elemental composition, e s p e c i a l l y the t r a c e i n o r g a n i c e l e m e n t a l c o m p o s i t i o n , and how t h i s c o m p o s i t i o n r e l a t e s t o geographical o r i g i n . Orange j u i c e i s m a r k e t e d i n t h e f o r m s o f s i n g l e s t r e n g t h ( u s u a l l y r e c o n s t i t u t e d from c o n c e n t r a t e ) and as f r o z e n concentrate. In F l o r i d a t h e s e forms a r e d e f i n e d i n degrees B r i x ( a v a l u e w h i c h r e l a t e s t o t h e s u g a r o r t o t a l s o l i d s c o n t e n t ) . I n most i n s t a n c e s , i n t h i s paper, c o n c e n t r a t i o n s o f c o n s t i t u e n t s w i l l be g i v e n based on " s i n g l e s t r e n g t h " j u i c e , a t e r m w h i c h d e n o t e s t h e c o n c e n t r a t i o n at w h i c h t h e j u i c e i s u s u a l l y consumed r e g a r d l e s s o f t h e f o r m i n w h i c h i t was p u r c h a s e d . There a r e v a r i o u s e x p r e s s i o n s r e l a t i n g t o e l e m e n t a l composit i o n t h a t d e s c r i b e t h e magnitude o r c o n c e n t r a t i o n range o f t h e p a r t i c u l a r e l e m e n t u n d e r c o n s i d e r a t i o n . I n t h e l i t e r a t u r e , one f i n d s e x p r e s s i o n s "major", " p r i n c i p a l " , "minor", " t r a c e " , " u l t r a t r a c e " and o t h e r s r e l a t i n g t o c o n c e n t r a t i o n l e v e l s . Inthis disc u s s i o n we w i l l adopt t e r m i n o l o g y f o r t h r e e c a t e g o r i e s ; macro ( t h o s e e l e m e n t s above 1%) m a j o r t r a c e ( e l e m e n t s f r o m 1 0 ppm t o 1%) and m i n o r t r a c e ( e l e m e n t s l e s s t h a n 1 0 ppm). These r a n g e t e r m s a r e p u r e l y a r b i t r a r y a n d a r e n o t meant t o b e o f f e r e d f o r a d o p t i o n as a g e n e r a l l y a c c e p t e d t e r m i n o l o g y s e t . T h e r e p o s s i b l y s h o u l d be one more c a t e g o r y s u c h a s u l t r a t r a c e f o r t h o s e e l e m e n t s w h i c h may b e p r e s e n t b u t a r e a t c o n c e n t r a t i o n s t o o l o w t o b e d e t e c t e d b y modern d a y i n s t r u m e n t a t i o n . F o r t h e p r e s e n t d i s c u s s i o n , h o w e v e r , t h i s range has l i t t l e v a l u e . 9

0-8412-0595-7/80/47-143-363$07.50/0 © 1980 American Chemical Society Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980.

364

CITRUS NUTRITION AND QUALITY

W a t e r , s u g a r s , o r g a n i c a c i d s a n d amino a c i d s make up a l l b u t a few t e n t h s o f 1 ? o f " s i n g l e s t r e n g t h " orange j u i c e . A breakdown o f t h e c o m p o s i t i o n o f o r a n g e j u i c e i s p r e s e n t e d i n T a b l e I . Table Principal

I . Components o f S i n g l e S t r e n g t h Orange J u i c e Constituents

P r i n c i p a l C o n s t i t u e n t s o f Major Elements (Column I ) a n d O t h e r E l e m e n t s (Column I I )

8 6 ?

Water

I 1 1 . 5 ?

Sugars


Amino a c i d s Organic

8 3 . 8 5 ?

Hydrogen

1 0 . 6 5 ?

1 ?

0 . 2 ?

Nitrogen

0 . 1?

Phosophorous

acids 1 ?

Carbon

Inorganic Ash 0 . 5 ?

TOTALS

Oxygen

Potassium

1 0 0 ?

5 . 1 5 ?

0 . 0 2 ?

Magnesium

0 . 0 1 ?

Calcium

0 . 0 1 ?

A l l Other Elements

0 . 0 1 ?

9 9 . 6 5 ?

0 . 3 5 ?

The v a l u e s i n T a b l e I a r e a p p r o x i m a t e a n d w i l l depend on g e o g r a p h i c a l l o c a l e , s o i l c o n d i t i o n s , f r u i t v a r i e t y and f e r t i l i z a t i o n practices. I t i s t h e group o f elements c o m p r i s i n g l e s s t h a n 0 . 0 1 ? o f s i n g l e s t r e n g t h o r a n g e j u i c e t h a t w i l l be t h e p r i n c i p a l s u b j e c t of t h i s d i s c u s s i o n . Historical The r e c o g n i t i o n i n t h e e a r l y p a r t o f t h e t w e n t i e t h c e n t u r y t h a t c e r t a i n m i n e r a l elements i n s m a l l c o n c e n t r a t i o n s had observa b l e e f f e c t s o n p l a n t , a n i m a l a n d human w e l l - b e i n g h a s l e d a h o s t o f i n v e s t i g a t o r s t o t r y t o measure t h e s e e f f e c t s q u a n t i t a t i v e l y . A r e m a r k a b l e and n o t e w o r t h y b e g i n n i n g l e a d i n g t o t h e c o n t i n u e d modern d a y i n t e r e s t i n a p p l y i n g a n a l y t i c a l s p e c t r o s c o p i c methods t o t h e a n a l y s i s o f a g r i c u l t u r a l p r o d u c t s was made b y H e n r i k L u n d e gardh i n t h e l a t e 1 9 3 0 s and e a r l y 1 9 U 0 s . L u n d e g a r d h was t h e f i r s t t o e s t a b l i s h t h e u t i l i t y o f f l a m e and spark s p e c t r o g r a p h i c methods f o r s o i l a n d l e a f a n a l y s i s ( l ^ f ^ ^ M Th results of t h i s work i n d i c a t e d a r e l a t i o n s h i p o f t h e s o i l c o n t e n t o f m a j o r t r a c e n u t r i e n t s l i k e c a l c i u m , phosphorus and n i t r o g e n and minor t r a c e n u t r i e n t s l i k e manganese, c o p p e r a n d i r o n t o optimum p l a n t and f r u i t d e v e l o p m e n t . I t h a s now b e e n w e l l e s t a b l i s h e d b y t h e C i t r u s Experiment S t a t i o n , Lake A l f r e d , F l o r i d a , ( 5 . ) , t h a t these e l e m e n t s a l o n g w i t h b o r o n , magnesium,molybdenum, p o t a s s i u m a n d z i n c a r e e s s e n t i a l elements i n t h e promotion o f s a t i s f a c t o r y growth o f o r a n g e t r e e s a n d t h e p r o d u c t i o n o f f r u i t . !

!

e

Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980.


16.

MCHARD

ETAL.

Trace

Metals in Orange

Juice

365

Because o f t h e important n u t r i t i v e a t t r i b u t e s o f t h e e l e v e n e l e m e n t s s p e c i f i c a l l y l i s t e d a b o v e , most o f t h e a n a l y t i c a l e f f o r t i n t h e p a s t few decades h a s been d i r e c t e d toward t h e d e t e r m i n a t i o n o f t h e c o n c e n t r a t i o n s o f some o r a l l o f t h e s e e l e m e n t s i n f o o d s and p l a n t s o f i n t e r e s t . However, t h e r e c a n a l s o b e j u s t i f i a b l e i n t e r e s t i n what o t h e r e l e m e n t s a r e p r e s e n t . Some e l e m e n t s a r e , o f c o u r s e , t o x i c e v e n a t l o w c o n c e n t r a t i o n s . Some e l e m e n t s may h a v e a s y e t u n d i s c o v e r e d n u t r i t i v e v a l u e . Beyond t h e s e c o n s i d e r a t i o n s i t may b e i m p o r t a n t t o know i f t h e r e a r e e l e m e n t s whose c o n c e n t r a t i o n depends o n , a n d t h u s may b e i n d i c a t i v e o f , t h e g e o g r a p h i c a l l o c a l e i n w h i c h t h e p l a n t s were grown. I f s u c h e l e m e n t s a r e p r e s e n t a n d i f t h e i r c o n c e n t r a t i o n s c a n be m e a s u r e d , t h i s i n f o r m a t i o n , perhaps, c o u l d be used f o r " f i n g e r p r i n t i n g " o r charact e r i z i n g t h e j u i c e as t o i t s o r i g i n . The l i t e r a t u r e i s r e p l e t e w i t h s t u d i e s on t h e measurement o f a l i m i t e d number o f e l e m e n t s p r e s e n t i n s o i l , p l a n t , l e a v e s a n d f r u i t ; a n d e s t i m a t e s h a v e b e e n made o f t h e r e l a t i o n s h i p o f t h i s content t o growth, m a t u r i t y and q u a l i t y o f oranges. Inthe last 1 3 y e a r s ( t a k e n f r o m C h e m i c a l A b s t r a c t s ) , t h e a u t h o r s were a b l e to account f o r n e a r l y 1 0 0 r e f e r e n c e s r e l a t i n g t o t h i s s u b j e c t . Many o f t h e s e r e f e r e n c e s , h o w e v e r , were f r o m o b s c u r e f o r e i g n publ i c a t i o n s and o n l y a few p r e s e n t e d m e a n i n g f u l comparisons o f j u i c e s from d i f f e r e n t sources. I n a f a i r l y recent review, Kefford and C h a n d l e r {6) gave a c o m p r e h e n s i v e a c c o u n t o f g e o g r a p h i c a l c o m p a r i s o n s w h i c h h a d b e e n made t o t h a t d a t e ( S e e T a b l e I I ) . An e x a m i n a t i o n o f t h e s e c o m p a r i s o n s r e v e a l s t h a t t h e y d e a l m o s t l y w i t h t h e more common n u t r i e n t e l e m e n t s l i k e c a l c i u m , p h o s phorus, p o t a s s i u m a n d n i t r o g e n . An i n t e r e s t i n g a s p e c t o f t h i s c o m p i l a t i o n i s t h e wide range i n c o n c e n t r a t i o n v a l u e s f o r elements i n j u i c e s from d i f f e r e n t l o c a t i o n s throughout t h e w o r l d . Of t h e g e o g r a p h i c a l areas s t u d i e d b y u s , t h e s e wide v a r i a t i o n s have n o t been observed. A s e l e c t i o n o f r e f e r e n c e s from t h e e x t e n s i v e l i t e r a t u r e i s g i v e n a t t h e end o f t h i s c h a p t e r (j-22). T h e r e have b e e n o n l y t w o c o m p r e h e n s i v e s t u d i e s made o n t h e elemental content o f orange j u i c e s produced i n t h e U n i t e d S t a t e s . The f i r s t o f t h e s e was r e p o r t e d b y R o b e r t s a n d Gaddum ( 2 3 ) a l i t t l e o v e r hO y e a r s a g o . That s t u d y was a s p e c t r o g r a p h i c a n a l y s i s of t h e elemental contents o f s e v e r a l v a r i e t i e s o f F l o r i d a oranges and g r a p e f r u i t w h i c h were grown d u r i n g t h a t p e r i o d o f t i m e . E x c e r p t s f r o m t h e i r r e s u l t s a r e shown i n somewhat m o d i f i e d f o r m i n Table I I I . I t i s n o t e w o r t h y t h a t t h e r e s u l t s o f t h e R o b e r t s a n d Gaddum s t u d y compare r e m a r k a b l y w e l l f o r most e l e m e n t s w i t h modern d a y r e s u l t s o b t a i n e d b y u s u s i n g t h e more s o p h i s t i c a t e d i n s t r u m e n t a t i o n (see T a b l e V I I ) . T h i s c a n be observed b y comparing t h e v a l u e f o r V a l e n c i a o r a n g e s w i t h t h e modern v a l u e s s i n c e t h e B l o o d , S e e d l i n g , a n d L u e Gim Gong v a r i e t i e s a r e no l o n g e r c o m m e r c i a l l y p r o duced i n F l o r i d a . The o n l y o t h e r c o m p r e h e n s i v e s t u d y was r e p o r t e d i n t h e e a r l y s i x t i e s b y B i r d s a l l e t a l . (2k). B i r d s a l l and coworkers, l i k e



Extremes o f ranges o f f r u i t

5 0 - 1 U 7

. 5 - 6

U 2 - 2 U 0

5 7 0 - 1 8 0 0

1 2 0 0 - 2 U 6 0

Ν

Locales

c

2 0 - 5 5

Cl

and A r i z o n a .

from I s r a e l , S p a i n , S o u t h A f r i c a , U.S.A., West I n d i e s , a n d S o u t h A m e r i c a .

U 9 - 2 0 6

1 3 0 - 2 8 0

C o n s t i t u e n t s o f Orange J u i c e f r o m V a r i o u s (mg/kg) Ca Mg Fe Ρ

'Extremes o f r a n g e s f r o m I s r a e l , C a l i f o r n i a

D

2 - 3 0

Na

Inorganic

From K e f f o r d and C h a n d l e r ( 6 )

5 2 0 - 2 8 U 0

Juice

1 8 0 0 - U 6 0 0

9 2 0 - 2 7 8 0

Κ

Table I I .

Whole

Orange

Total Ash


>

c

> σ ο

δ

H

2

G H

C

Η

ο

as


Elemental

-

h

1 .h

3

-

N.D.

1 0 0

N.D.

2

-

-

1 .

0.13 1 . 3

1

0 . 3 9

2 . 8

-

70

-

2.3

N.D.

0 . 0 3

0 . 1 U -

-

.IU 0.

1 . 3

TRACE

-

0 . 3 3

0 .. 3 8

-

0 ., 3 8

N.D.

-

0 . 1 U

0 . 0 0 U

TRACE O.OOU - O.Oh N.D.

1 .,h

0 .,lh

g i v e n i n p a r t s p e r m i l l i o n based on s i n g l e s t r e n g t h j u i c e .

0 . U 5

N.D. N.D.

-

0 . 3 7

-

1 . 2

-

See r e f .

(23).

0 . 3 1

N.D.

-

TRACE

0 . 1 1

0 . 3 0

N.D.

0,

-

0 . 1 2

0 . 0 3

0 .12

-

-

N.D. 0 . 1 2

0 . 3 2

. 0 0 U - . 0 U

0 ,. 0 3

.03 -

.lh

0 . 0 3

0 . 0 0 U - 0 . 0 U 0

0

1 3.6

20

ho

50

30

0.03 -

60 60

70 70

90

0 . 0 U

TRACE TRACE TRACE

TRACE

80

N.D.

1 5 0 1 7 7 0

300 1 5 7 0

320 1 7 2 0

0 . 1 2

3 . 2

N.D. TRACE

1 ,. 2

0 . 1 2

0 . 3 0

1 . 2

1 2 . 1

TRACE

3 .. 2

-

N.D.

0 ,, 0 3

TRACE N.D. TRACE

50

TRACE

0 . 1 9

o.ooU-o.oUo N.D.

2 8 0

- O.Ul N.D. N.D.

50

N.D.

1

-

N.D.

-

I 8 5 O

0 . 2 0

0 . 3 3

0 . 0 3

300

h

0 . lh

N.D. TRACE

0 . 3 8

60

1 .

h

0 .1 5

N.D.

-

0.03 -

0 . 3 8

o.oh

I 8 7 O

N.D. N.D.

.h

Λ

.lh

30

0

30

-

N.D.

N.D.

30

N.D.

-

30

0 ,. 1 2

N.D.

- 0 . 1 2

3

N.D.

0 ,. 3 2

30

0.03

N.D.

0 . 0 0 U - 0 . 0 H 0

N.D.

0 . 3 6 - 1

3 . 6

0 . 1 2

1 U 0

0 .U8

6 -

N.D. O.OUl - 0 . 2 1

0 . 0 3

Tangerine

1 1 0

-

N.D.

1 5 . 0

L u e Gim Gong

90

0 . 3 6 - 1

0 . 0 3

8 -

V a l e n c i a Orange

( 1 9 3 7 )

1 1 0

0 . 2 3

3 . 8

B l o o d Orange

C o n t e n t o f S e v e r a l V a r i e t i e s o f F l o r i d a Orange J u i c e s (PPM)

l 6 0

N.D. 0.13 - 0 . 3 7

0 . 3 6

S e e d l i n g Orange

Concentrations

aluminum barium bismuth boron calcium cadmium chlorine chromium cobalt copper iron lead magnesium manganese molybdenum nickel phosphorus potassium silver sodium sulfur strontium tin titanium vanadium zinc zirconium

Element

Table I I I .


368

CITRUS NUTRITION AND

QUALITY

R o b e r t s and Gaddum, r e p o r t e d on e l e m e n t s m e a s u r e d s p e c t r o g r a p h i c a l l y ; e x c e p t t h e i r measurements were done on o r a n g e s and lemons grown i n C a l i f o r n i a . T h i r t y - o n e e l e m e n t s , a l l o f w h i c h were meta l s , e x c e p t b o r o n and p h o s p h o r u s , were m e a s u r e d . The r e s u l t s were r e p o r t e d i n c a t e g o r i e s as p e r c e n t o f t h e a s h o f t h e j u i c e . Select i n g o n l y t h e j u i c e d a t a , r e a r r a n g i n g i t s l i g h t l y , and e x p r e s s i n g i t i n t e r m s o f p a r t s p e r m i l l i o n b a s e d on t h e o r i g i n a l j u i c e i n s t e a d o f as a p e r c e n t a g e o f t h e a s h , t h e v a l u e s a p p e a r i n T a b l e IV. Table IV. Elemental Content o f C a l i f o r n i a V a l e n c i a and N a v e l Orange J u i c e s (PPM) ( l 9 6 l )


a

Not Detected

J u i c e Type/PPM o f J u i c e Sample

Greater t h a n hO PPM

Concentrât i o n s between 0 . h PPM and hO PPM

Trace Less t h a n O.h PPM

Valencia

Ca, Κ

Mg,

P, Na, S i , Fe Β, S r , A l

Mn, Cu, T i , V, C r , N i , Mo, S n , Z r , Co, B a , Zn, Sr

Bi, Pb, Sb, Cb,

Navel

Ca, Κ

Mg,

P, S i , Na, A l , Fe

Ti, Cr, Zn, Sr,

B i , Ba, Co, Sb, A s , Cb, W, P b , Mo

a

Data

from B i r d s a l l et a l .

Cu, N i , V, Mn, L i , Zr, Ag, Β

Cd, Ag, As, W

(2h).

As one c a n r e a d i l y s e e , t h e q u a n t i t a t i v e s c a l e o f t h e r e s u l t s o f T a b l e I V i s l e s s d e f i n i t i v e t h a n t h e s c a l e i n T a b l e I I I . However, o r d e r o f m a g n i t u d e agreement i s a p p a r e n t f r o m a c o m p a r i s o n o f t h e two s e t s o f d a t a s h o w i n g t h a t t h e r e a r e p r o b a b l y no g r o s s d i f f e r e n c e s i n t h e m e t a l c o n t e n t s o f j u i c e s f r o m t h e two s o u r c e s - F l o r i d a and C a l i f o r n i a . I t w i l l be n o t e d t h a t t h e c o n c e n t r a t i o n r a n g e f o r p h o s p h o r u s i s an o r d e r o f m a g n i t u d e l e s s i n t h i s d a t a t h a n t h e R o b e r t s and Gaddum d a t a a n d , j u d g e d by t h a t and t h e v a l ues we o b t a i n , we c o n c l u d e t h e r a n g e f o r p h o s p h o r u s i n T a b l e I V t o be i n e r r o r . I t c o u l d be s t a t e d h e r e t h a t Lundegârdh d i d most o f h i s s t u d i e s on s o i l and l e a f a n a l y s i s c o n t e n d i n g t h a t t h e f r u i t ( i n t h i s case t h e orange i t s e l f ) v a r i e d l i t t l e i n composition. I n r e c e n t p u b l i c a t i o n s , we ( 2 5 , 2 6 , 2 7 , 2 8 ) h a v e d i s c u s s e d some modern a p p r o a c h e s t o t h e a n a l y s i s o f o r a n g e j u i c e , and t h e r e s u l t s o f t h e s e s t u d i e s w i l l be p r e s e n t e d i n t h e f o l l o w i n g s e c tions of t h i s chapter.


16.

MCHARD E T AL.

Trace Metals

in Orange

Juice

369


Methodology I n a n y a n a l y t i c a l p r o b l e m t h e a n a l y s t must o u t l i n e t h e g o a l s t o be a c h i e v e d b y t h e a n a l y s i s b e f o r e d e c i d i n g on m e t h o d o l o g y . O f t e n i t i s d e s i r e d t o measure t h e c o n c e n t r a t i o n o f p e r h a p s , o n e , o r a t m o s t , j u s t a few e l e m e n t s . I n t h e s e c i r c u m s t a n c e s , t h e i n v e s t i g a t o r may, f o r e x a m p l e , b e i n t e r e s t e d i n a p p l y i n g a n e l e m e n t l i k e manganese o r z i n c t o t h e s o i l o r f e r t i l i z e r and m o n i t o r i n g t h e r e s u l t i n g i n f l u e n c e on orange t r e e development o r f r u i t q u a l i ty. A l t e r n a t i v e l y , the o b j e c t i v e behind a p a r t i c u l a r a n a l y s i s m i g h t be t o measure t h e c o n t a m i n a t i o n l e v e l s o f e l e m e n t s l i k e cadmium and l e a d w h i c h r e s u l t f r o m t h e i n v a s i o n o f t h e g r o w i n g area by f o r e i g n environmental agents. I n s i t u a t i o n s l i k e t h e s e t h e method o f measurement o f t h e element ( o r elements) o f i n t e r e s t can be t a i l o r e d s p e c i f i c a l l y t o s u i t t h e p a r t i c u l a r n e e d s . As a m a t t e r o f f a c t , f o r a few m e t a l s l i k e m e r c u r y and t i n and t h e m e t a l l o i d s a r s e n i c , s e l e n i u m , a n t i mony, and t e l l u r i u m s p e c i a l methods o f sample p r e p a r a t i o n a r e u s u a l l y r e q u i r e d ( 2 9 _ , 3 0 _ , 3 1 _ , 3 2 , 3 3 _ ) . The r e a s o n s p e c i a l p r e c a u t i o n s and s p e c i a l methods a r e r e q u i r e d f o r t h e e l e m e n t s l i s t e d a b o v e , and f o r c e r t a i n o t h e r s , i s t h a t t h e y e i t h e r a r e r e a d i l y r e d u c e d t o v o l a t i l e f o r m s , r e a c t c h e m i c a l l y t o f o r m v o l a t i l e compounds, or are i n f l u e n c e d i n t h e i r a n a l y s i s by the matrices u s u a l l y present . Sample

Preparation

Orange j u i c e i n c o n c e n t r a t e f o r m ( t h e f o r m u s u a l l y s e l e c t e d for a n a l y s i s ) i s g e n e r a l l y about f o u r t i m e s the c o n c e n t r a t i o n o f s i n g l e s t r e n g t h j u i c e and t h e r e f o r e i s p r i m a r i l y s u g a r and w a t e r ( r o u g h l y 5 0 / 5 0 ; see T a b l e I ) . U s u a l l y , f o r a n a l y t i c a l p u r p o s e s , t h e o r g a n i c m a t t e r must b e d e s t r o y e d o r b r o k e n down i n s u c h a way t h a t i t does n o t i n t e r f e r e w i t h t h e a n a l y s i s . C o n s i d e r a b l e emphas i s has b e e n g i v e n t o t h i s a s p e c t i n t h e l i t e r a t u r e on f o o d a n d b i o l o g i c a l a n a l y s i s . No c l e a r c o n s e n s u s o f o p i n i o n seems t o p r e v a i l o n what t h e b e s t a p p r o a c h e s a r e . P r o b a b l y t h e s i m p l e s t method i s t o d r y a s u i t a b l e sample u n d e r a lamp o r i n a n open d r a f t o v e n u n t i l t h e w a t e r i s d r i v e n o f f and t h e n a s h a t t e m p e r a t u r e s up t o 5 5 0 ° C A l t h o u g h t h e t o t a l t i m e i n v o l v e d i n t h i s a s h i n g proc e d u r e i s u s u a l l y t h e b e t t e r p a r t o f one day, t h i s method r e q u i r e s l i t t l e o p e r a t o r a t t e n t i o n a n d a number o f s a m p l e s can b e c a r r i e d t h r o u g h t h e p r o c e s s a t one t i m e . An a d d i t i o n a l a d v a n t a g e t o a s h i n g i s t h a t r e l a t i v e l y l a r g e s a m p l e s can b e u s e d (up t o 2 0 g i n most i n s t a n c e s ) . D i s a d v a n t a g e s a r e t h a t v o l a t i l e e l e m e n t s ( o r e l e m e n t s f o r m i n g v o l a t i l e compounds) c a n b e l o s t t h r o u g h v o l a t i l ization. D e p e n d i n g o n what o t h e r i o n s a r e p r e s e n t , t h e l i s t o f elements g e n e r a l l y l o s t by ashing c e r t a i n l y w i l l i n c l u d e elements l i k e m e r c u r y , a r s e n i c , cadmium, s e l e n i u m , a n t i m o n y and t e l l u r i u m . Some l o s s e s , a l s o d e p e n d i n g o n t h e c i r c u m s t a n c e s , may be o b s e r v e d w i t h m e t a l s l i k e s i l v e r , l e a d and p o s s i b l y a l s o c o p p e r a n d z i n c



370

CITRUS

NUTRITION

AND

QUALITY

a l t h o u g h t h e a u t h o r s h a v e n o t o b s e r v e d s i g n i f i c a n t p r o b l e m s by d r y a s h i n g when d e a l i n g w i t h t h e s m a l l c o n c e n t r a t i o n s o f s i l v e r , c o p p e r and z i n c p r e s e n t i n o r a n g e j u i c e . T h i s i s p r o b a b l y due t o the r e t e n t i v e e f f e c t o f phosphate s i n c e phosphorus i s present i n o r a n g e j u i c e i n r e l a t i v e l y m a s s i v e amounts compared t o t h e c o n c e n t r a t i o n s of the l a t t e r t h r e e elements. A l s o , the high a l k a l i m e t a l content o f orange j u i c e , e s p e c i a l l y the h i g h potassium con c e n t r a t i o n r e n d e r s t h e f i n a l a s h a l k a l i n e , p r i n c i p a l l y as t h e c a r bonate. This m a t r i x i s a l s o conducive t o r e t e n t i o n of metals whi ch m i g h t o t h e r w i s e be v o l a t i l e o r be s u b s t a n t i v e on t h e a s h i n g crucible. S i l i c a c r u c i b l e s are l e s s d e s i r a b l e f o r ashing than p l a t i n u m o n e s . T h i s r e s u l t s f r o m two c o n s i d e r a t i o n s : l ) t h e s i l i c a s u r f a c e has open r e a c t i v e s i t e s f o r i o n exchange a n d / o r i o n a b s o r p t i o n and 2 ) s i l i c a t e n d s t o p i t and e r o d e i n t o t h e h i g h l y a l k a l i n e m a t r i x of the f i n a l ash. A l o w t e m p e r a t u r e a s h i n g t e c h n i q u e has r e c e n t l y begun t o r e c e i v e a t t e n t i o n ( 3^_, 3 5 . , 3 6 , 3 χ , 3 8 _ ) . The samples a r e p l a c e d u n d e r vacuum ( a f t e r r e m o v i n g t h e m o i s t u r e ) i n an a t m o s p h e r e o f o x y g e n o r a m i x t u r e o f o x y g e n and f r e o n . The l a t t e r gas seems t o s p e e d up t h e a s h i n g p r o c e s s . H e a t i n g i s p r o v i d e d by h i g h f r e q u e n c y c o i l s s u r r o u n d i n g t h e a s h i n g chamber. The chamber and a s h i n g con t a i n e r s must be made o f q u a r t z i f f r e o n i s t o be u s e d as a comp onent o f t h e gas a t m o s p h e r e . The a u t h o r s h a v e v e r i f i e d t h a t up t o 1 0 g o f f r o z e n c o n c e n t r a t e d o r a n g e j u i c e i n m u l t i p l e u n i t s may be a s h e d i n a s u i t a b l y s i z e d chamber. The t e m p e r a t u r e o f t h e ash i n g p r o c e s s c a n be v a r i e d up t o 2 0 0 ° C d e p e n d i n g on t h e f r e q u e n c y power. I t i s c l a i m e d t h a t many o f t h e m e t a l s c o n s i d e r e d t o be v o l a t i l e at h i g h e r temperatures are c o m p l e t e l y o r n e a r l y complete l y r e t a i n e d by t h i s a s h i n g method ( 3 8 ) . A number o f d i s c u s s i o n s ( 3 2 , 3 9 7 ^ 0 ^ 1 , U 2 ) can be founo. i n t h e l i t e r a t u r e r e l a t i n g t o t h e m e r i t s o f d r y v e r s u s wet a s h i n g . Wet a s h i n g e n t a i l s d e c o m p o s i n g t h e o r g a n i c components o f t h e sample by means o f m i x t u r e s o f o x i d i z i n g a c i d s . The u s u a l m i x t u r e s a r e n i t r i c and s u l f u r i c a c i d s ; n i t r i c and p e r c h l o r i c a c i d s ; n i t r i c , s u l f u r i c and p e r c h l o r i c a c i d s ; and s u l f u r i c a c i d and h y d r o g e n p e r oxide. T h e r e a r e two p r i n c i p a l a d v a n t a g e s t o a c i d a s h i n g ( s o c a l l e d wet a s h i n g ) : l ) t h e r e a r e l i t t l e i f any l o s s e s by v o l a t i l i z a t i o n ; and 2 ) t h e e l e m e n t s a r e k e p t i n an o x i d i z e d s t a t e and i n a s t r o n g l y a c i d medium; t h u s , d i m i n i s h i n g , i f n o t e l i m i n a t i n g , t h e chance of s u b s t a n t i v e r e a c t i o n s o r r e a c t i o n s w i t h the s u r f a c e o f t h e c o n t a i n e r . On t h e o t h e r h a n d , t h e r e a r e some v e r y s e r i o u s c o n s i d e r a t i o n s w h i c h t e n d t o r u l e a g a i n s t wet a s h i n g . The v o l u m e o f a c i d t o w e i g h t o f sample i s h i g h , u s u a l l y on t h e o r d e r o f 1 5 1 0 m l ( o r more) o f a c i d p e r gram o f sample e s p e c i a l l y when d e a l i n g w i t h orange j u i c e . T h i s means t h a t i f one w i s h e s t o decompose s a y 1 0 - 2 0 g o f sample f o r a s u r v e y o f many e l e m e n t s , t h e v o l u m e o f a c i d n e e d e d i s l a r g e , and b e c a u s e o f t h e h a z a r d o u s n a t u r e o f m i x t u r e s c o n t a i n i n g p e r c h l o r i c a c i d , c o n s i d e r a b l e t i m e and c o n s t a n t a t t e n t i o n i s demanded o f t h e o p e r a t o r . A l s o , w i t h o r a n g e j u i c e i n the presence o f s t r o n g a c i d s , c o n s i d e r a b l e foaming



16.

MCHARD

ETAL.

Trace Metals

in Orange

Juice

371

o c c u r s a t t h e b e g i n n i n g s t a g e s o f t h e o x i d a t i o n , a n d i t becomes n e c e s s a r y t o a l l o w t h e samples t o r e a c t s l o w l y a t l o w t e m p e r a t u r e s u n t i l t h e f o a m i n g t e n d e n c y i s s u b d u e d , a p e r i o d o f t i m e t h a t may t a k e s e v e r a l h o u r s . The c o p i o u s amounts o f a c i d n e e d e d i n c r e a s e the l i k e l i h o o d o f m e t a l c o n t a m i n a t i o n from t h e a c i d s themselves s i n c e i t i s n e a r l y i m p o s s i b l e t o remove a l l t r a c e s o f i m p u r i t i e s during t h e i r manufacture. F u r t h e r m o r e , one r u n s t h e r i s k o f t h e a c i d anions r e a c t i n g w i t h t h e metals t o form r e l a t i v e l y s t a b l e but i n s o l u b l e p r o d u c t s , e.g., b a r i u m , c a l c i u m a n d o t h e r a l k a l i n e e a r ths form s u l f a t e s o f v a r y i n g degrees o f i n s o l u b i l i t y . I f one o r j u s t a few elements a r e t o be measured and i f i t i s determined t h a t v o l a t i l e l o s s e s o c c u r s u c h a s w i t h m e r c u r y , a r s e n i c , antimony and g r o u p V I e l e m e n t s , t h e n a c i d a s h i n g o f f e r s a n a c c e p t a b l e way f o r p r e p a r i n g t h e sample. I f a b r o a d r a n g e o f e l e m e n t s a r e t o be m e a s u r e d , a n d i n t e r e s t i n t h e above l i s t o f v o l a t i l e ones i s m i n o r t h e n we c o n s i d e r d r y a s h i n g t o be t h e method o f c h o i c e , e s p e c i a l l y f o r p r e p a r i n g orange j u i c e f o r a n a l y s i s . These c o n s i d e r a t i o n s b e a r o u t t h e t h o u g h t e x p r e s s e d e a r l i e r t h a t m e t h o d o l o g y must be t a i l o r e d t o t h e s p e c i f i c a n a l y t i c a l p r o b l e m . For a s e l e c t e d l i s t o f elements ( 2 6 ) t h e r e i s another a c i d treatment procedure which i s r e a d i l y a p p l i c a b l e t o the a n a l y s i s of orange j u i c e ; t h i s procedure i n v o l v e s h y d r o l y s i s w i t h moderatel y s t r o n g n i t r i c a c i d t o b r e a k d o w n most o f t h e s u g a r s a n d t o d e crease the s i z e o f the pulpy constituents. The s o l u t i o n i s t h e n f i l t e r e d , d i l u t e d , and measured by atomic a b s o r p t i o n . For elements t h a t c a n b e d e t e r m i n e d w i t h a n a i r - a c e t y l e n e f l a m e u s i n g a high s o l i d s (three s l o t ) burner, t h i s procedure o f f e r s a u s e f u l alternative. F r i c k e e t a l . ( 3 3 ) a l s o mentions the u t i l i t y o f t h i s method a n d g i v e s c o m p a r a t i v e r e s u l t s on t h e u s e o f t h i s method i n sample p r e p a r a t i o n . I f d r y a s h i n g h a s b e e n u s e d , o r i f t h e samples f o l l o w i n g a c i d a s h i n g have b e e n e v a p o r a t e d t o d r y n e s s , t h e f i n a l s t e p s i n t h e p r e p a r a t i o n o f t h e sample f o r a n a l y s i s w i l l depend on t h e t y p e o f end measurement t o b e u s e d ; t h e s e e n d measurements w i l l b e d e a l t w i t h i n some d e t a i l i n t h e d i s c u s s i o n t o f o l l o w . End Measurement

Techniques

The f i n a l measurement s t e p can b e done i n a number o f ways. A f e w o f t h e e l e m e n t s i n o r a n g e j u i c e a r e p r e s e n t i n l a r g e enough c o n c e n t r a t i o n s and/or have c h e m i c a l c h a r a c t e r i s t i c s which p e r m i t the u s e o f c l a s s i c a l a n a l y s i s . P h o s p h a t e a n d some h e a v y m e t a l s l i k e z i n c a n d i r o n may b e m e a s u r e d c o l o r i m e t r i c a l l y . C a l c i u m has b e e n m e a s u r e d a s t h e o x a l a t e , b u t s u c h methods a r e t i m e consuming and may r e q u i r e p r i o r s e p a r a t i o n f r o m t h e i n e v i t a b l e m a t r i x b e f o r e analysis. 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 r y . The most w i d e l y a c c e p t e d measurement t e c h n i q u e s i n modern d a y a n a l y s i s a r e f o u n d i n t h e f i e l d o f spectroscopy. A search o f the l i t e r a t u r e reveals that


372

atomic a b s o r p t i o n spectroscopy i s the most popular modern t e c h n i que f o r the measurement of t r a c e metals i n foods (26,30,32,33,^1, k2 k3 kk k5 k6). Crosby (k3) i n a comprehensive review a r t i c l e surveyed the l i t e r a t u r e over a f i v e - y e a r p e r i o d using two appropr i a t e j o u r n a l s (one B r i t i s h and one American) and found t h a t there was a t o t a l of 8 3 a r t i c l e s d e a l i n g with atomic a b s o r p t i o n as compared t o a combined number of 6 2 d e a l i n g with molecular absorption spectrometry, flame photometry, neutron a c t i v a t i o n , x-ray f l u o r e s cence, chemical t i t r a t i o n , e t c . Atomic a b s o r p t i o n c e r t a i n l y has advantages because of the extensive and a u t h o r i t a t i v e s t u d i e s that have been made to improve instrumentation and work out the d e t a i l s f o r the a n a l y s i s o f metals i n many types of chemical m a t r i c e s . Methods have been worked out f o r some 6 0 t o TO elements c o v e r i n g c e r t a i n l y the most important ones i n the p e r i o d i c t a b l e . Modern atomic a b s o r p t i o n instrumentation has been developed which almost completely e l i m i n a t e s the p o s s i b i l i t y of operator e r r o r , and with multi-element lamps a v a i l a b l e , a degree of multielement c a p a b i l i t y has been i n c o r p o r a t e d . T h i s minimizes the drudgery of r e p l a c i n g lamps and r e o p t i m i z i n g instrumental c o n d i t i o n s f o r each new metal to be analyzed. Atomic a b s o r p t i o n can a l s o be made very s e n s i t i v e by the use o f the g r a p h i t e furnace i n s t e a d o f a flame as a sample c e l l (30,^7,^8). Furnaces have been used i n a number o f d i f f e r e n t forms and c o n f i g u r a t i o n s ranging from c e l l s with open tops t o f i l a m e n t s and tube designs. Samples can be i n the form o f s o l i d s , p a s t e s , or l i q u i d s , and by programming the h e a t i n g o f the furnace, p r e l i m i n a r y ashing can u s u a l l y be avoided. 9



9

9

9

Atomic Emission Spectrometry. Emission spectroscopy was the e a r l i e s t developed multielement measurement technique (1+9,50,51, 52). I t s widest acceptance was by the metal i n d u s t r y where i t was p a r t i c u l a r l y u s e f u l i n determining a few elements r e p e t i t i v e l y i n a metal ( u s u a l l y some form of s t e e l ) matrix which was w e l l defined. I t was a l s o used i n the food and a g r i c u l t u r a l f i e l d and was r e s p o n s i b l e f o r much o f the e a r l y knowledge of the concentrations o f a number of t r a c e elements i n orange j u i c e (23, 2k). Recently, a very important development has been made t o enhance the ease o f manipulation and the range of a p p l i c a b i l i t y o f emission spectroscopy t o t r a c e metal a n a l y s i s . T h i s development i s the plasma source which can be employed as an. accessory source i n most d i r e c t reading emission spectrometers i n p l a c e o f the arc or spark or may be i n c o r p o r a t e d d i r e c t l y i n the design o f the spectrometer by the manufacturer. This development has been d i s cussed i n d e t a i l i n the recent l i t e r a t u r e ( U 9 , 5 3 - 5 8 ) . There are two popular types of plasma sources: l ) the d i r e c t current plasma (DCP), and 2) the i n d u c t i v e l y coupled plasma (ICP). In the commercial v e r s i o n of the former plasma source (marketed by Spectrometries, I n c . ) , the sample i s a s p i r a t e d with argon through a small o r i f i c e i n t o a chamber where the l a r g e d r o p l e t s s e t t l e out and the f i n e mist i s conveyed by the argon stream t h r ough a chimney to the v e r t e x of a plasma which i s i n the form of



16.

MCHARD ET

AL.

Trace Metals

in

Orange

Juice

373

an i n v e r t e d Y c o n f i g u r a t i o n . The v a p o r s a r e h e a t e d and a t o m i c emission takes p l a c e i n t h i s area ( v e r t e x of the Y ) . T h i s emiss i o n f l u x i s f o c u s e d on an e n t r a n c e s l i t t o t h e m o n o c h r o m a t o r and p r o g r e s s e s through t h e o p t i c a l path which c o n s i s t s o f a system o f m i r r o r s , an échelle g r a t i n g and a p r i s m t o s e p a r a t e t h e w a v e l e n g ths i n t o a two-dimensional p a t t e r n c o n s i s t i n g o f s e v e r a l wavelengt h o r d e r s . Measurement o f i n t e n s i t i e s c a n be s i m u l t a n e o u s ( i . e . , m u l t i e l e m e n t ) o r s e q u e n t i a l d e p e n d i n g how t h e i n s t r u m e n t has b e e n designed t o operate. The o t h e r t y p e o f p l a s m a s o u r c e , t h e i n d u c t i v e l y c o u p l e d p l a sma, has b e e n e n g i n e e r e d i n t o s p e c i f i c a l l y d e s i g n e d i n s t r u m e n t a t i o n m o d u l e s by A p p l i e d R e s e a r c h L a b o r a t o r i e s , J a r r e l l A s h ( a d i v i s i o n of F i s h e r S c i e n t i f i c Corp.), P e r k i n Elmer, Instrumentation L a b o r a t o r i e s , and B a i r d A t o m i c i n t h e U n i t e d S t a t e s ; f o r e i g n models are a l s o a v a i l a b l e . When t h e I C P p l a s m a i s i n o p e r a t i o n , t h e s a m p l e i s d i s p e r s e d as l i q u i d d r o p l e t s i n t o an a r g o n s t r e a m and d i r e c t e d t o t h e c e n t e r zone o f t h e h o t p l a s m a . This operating mode i s d i f f e r e n t f r o m t h e d i r e c t c u r r e n t c o m m e r c i a l v e r s i o n and i t means t h a t t h e e x c i t a t i o n r e g i o n i s p r o b a b l y c o n s i d e r a b l y h i g h er i n t e m p e r a t u r e . T h e r e a r e some a d v a n t a g e s and some d i s a d v a n t ages t o t h e I C P s y s t e m compared t o t h e DCP s y s t e m . Elements, l i k e v a n a d i u m , molybdenum and t u n g s t e n w h i c h t e n d t o f o r m h i g h l y r e f r a c t o r y compounds a r e p r o b a b l y more c o m p l e t e l y a t o m i z e d . Theref o r e , t h e r e i s l e s s i n t e r f e r e n c e from t h e m a t r i x , a t y p e o f i n t e r f e r e n c e which i s n e a r l y always a problem w i t h s p e c t r o s c o p i c analysis. The p r i n c i p a l d r a w b a c k s a r e : l ) n e b u l i z e r d e s i g n i s c r i t i c a l i n the o p e r a t i o n o f i n d u c t i v e l y coupled plasmas; the r e ason i s t h a t c l o s e c o n t r o l o f t h e r a t e o f f l o w o f sample t o t h e p l a s m a must be a c h i e v e d t o a v o i d p l a s m a i n s t a b i l i t y , 2 ) t h e v i e w ing ( o b s e r v a t i o n ) a r e a t o which t h e o p t i c s are exposed i s d i r e c t l y i n t h e c e n t e r o f t h e p l a s m a and b a c k g r o u n d e m i s s i o n i s h i g h comp a r e d t o t h e s i t u a t i o n w i t h t h e d i r e c t c u r r e n t p l a s m a where v e r y l i t t l e of the plasma r e g i o n i s observed d u r i n g a n a l y s i s , 3 ) mult i e l e m e n t f a c i l i t i e s , t h e t y p e o f i n s t r u m e n t a t i o n u s u a l l y accompa n y i n g t h e i n d u c t i v e l y c o u p l e d p l a s m a s , a r e e x p e n s i v e and may be o u t o f t h e f i n a n c i a l r e s o u r c e s o f many s m a l l l a b o r a t o r i e s , and k) when a m u l t i e l e m e n t f a c i l i t y i s c h o s e n t h e b u y e r must make a dec i s i o n as t o w h i c h e l e m e n t s he i s i n t e r e s t e d i n d e t e r m i n i n g ; i n d o i n g s u r v e y w o r k t h i s d e c i s i o n i s h a r d t o make b e c a u s e one s e l dom knows i n a d v a n c e t h e e l e m e n t s t h a t may be i m p o r t a n t . Because o f t h e d i s a d v a n t a g e s o f t h e ICP systems l i s t e d above, we c h o s e t o do o u r s t u d y on t h e s i m p l e r l e s s e x p e n s i v e s e q u e n t i a l d i r e c t c u r r e n t plasma system ( 2 7 , 5 9 - 6 5 ) · It w e l l to recognize t h a t i n any c h o i c e o f t h i s k i n d t r a d e - o f f s may become n e c e s s a r y . F o r e x a m p l e , t h e DC p l a s m a i s s u b j e c t t o more o r l e s s s e v e r e matr i x e f f e c t s , and t h e s e must be a c c o u n t e d f o r i n s e t t i n g up t h e m e t h o d o l o g y ( 2 8 ) . T h e s e e f f e c t s a r e i l l u s t r a t e d i n F i g u r e s 1 and 2 w h i c h show t h e i n f l u e n c e o f t h e l a r g e p o t a s s i u m c o n c e n t r a t i o n s on b o t h t h e atom and i o n l i n e s o f b a r i u m . i s


374



Ba

Concentration (ng/rnl) Analytical Chemistry

Figure /. Effect of analytical curve for ppm) in 0.1 M HNO ; K/Ba ratio (4000/L) A

potassium on barium calibration curves using DC P A ES: (a) barium in matrix containing constant Κ concentration (800 (b) analytical curve for barium in matrix containing constant in 0.1M H NO(c) analytical curve for barium in 0.1M HNOj (2S)



M C HARD E T AL.

Trace

100

Metals in Orange

Juice

200 300 Concentration (ng/ml)

Ba

375

400

Analytical Chemistry

Figure 2. Matrix effects on the calibration curve of barium using DCPAES at the atom line (553.55 nm) and the ion line (455.40 nm): (a) barium in 0.1M HNO , atom line; (b) barium in 0.1 M HNO containing 100 ppm K, atom line; (c) barium in 0.1M HN0 , ion line; (d) barium in 0.1M HN0 containing 1000 ppm K, ion line (28j s

s

3

3



376

CITRUS NUTRITION AND

QUALITY

Atomic F l u o r e s c e n c e Spectrometry. A spectroscopic technique r e l a t e d t o some o f t h e t y p e s m e n t i o n e d above i s a t o m i c f l u o r e s cence spectrometry (AFS). L i k e atomic a b s o r p t i o n s p e c t r o m e t r y ( A A S ) , AFS r e q u i r e s a l i g h t s o u r c e s e p a r a t e f r o m t h a t o f t h e h e a t ed f l a m e c e l l . T h i s c a n be p r o v i d e d , as i n AAS, by i n d i v i d u a l ( o r m u l t i e l e m e n t l a m p s ) , o r by a c o n t i n u u m s o u r c e s u c h as x e n o n a r c or by s u i t a b l e l a s e r s o r c o m b i n a t i o n o f l a s e r s and d y e s . The l a s er i s s t i l l p r e t t y much i n i t s i n f a n c y b u t i t i s l i k e l y t h a t f u t u r e d e v e l o p m e n t w i l l c a u s e t h e l a s e r , and c o n s e q u e n t l y t h e many s p e c t r o s c o p i c i n s t r u m e n t s t o w h i c h i t c a n be a d a p t e d t o , t o b e come i n c r e a s i n g l y p o p u l a r . C o m p l e t e f r e e d o m o f w a v e l e n g t h s e l e c t i o n s t i l l remains a problem. U n l i k e AAS t h e l i g h t s o u r c e i n AFS i s n o t i n d i r e c t l i n e w i t h t h e o p t i c a l p a t h , and t h e r e f o r e , t h e r a d i a t i o n e m i t t e d i s a r e s u l t o f e x c i t a t i o n by t h e lamp o r l a s e r source. Atomic f l u o r e s c e n c e spectrometry p r o v i d e s h i g h s e n s i t i v i t i e s f o r e l e m e n t s whose a n a l y t i c a l l y u s e f u l e n e r g y t r a n s i t i o n s a r e i n the u l t r a - v i o l e t r e g i o n s of the spectrum. This s e n s i t i v i t y dimini s h e s as one a p p r o a c h e s t h e u p p e r end o f t h e v i s i b l e r e g i o n (i+00700 nm). We made a c o m p a r a t i v e s t u d y (28) d e t e r m i n i n g s e v e r a l e l ements i n s e v e n b r a n d s o f F l o r i d a o r a n g e j u i c e b y i n d u c t i v e l y coupled plasma atomic emission spectrometry (ICPAES), d i r e c t c u r r e n t p l a s m a a t o m i c e m i s s i o n s p e c t r o m e t r y (DCPAES), f l a m e a t o m i c f l u o r e s c e n c e s p e c t r o m e t r y ( F A F S ) , and f l a m e a t o m i c a b o s r p t i o n s p e c t r o m e t r y (FAAS). T h e s e c o m p a r i s o n s a r e shown i n T a b l e V. The s a m p l e s f o r t h i s s t u d y were d r y a s h e d and d i s s o l v e d i n e i t h e r n i t r i c o r h y d r o c h l o r i c a c i d s , i . e . , t h e s e were s e p a r a t e s e t s d i s s o l v e d i n e a c h a c i d ( t h e a c i d c o n c e n t r a t i o n d u r i n g a n a l y s i s was 0.1 M). I t i s o b s e r v e d t h a t t h e n i t r i c and h y d r o c h l o r i c a c i d m e d i a show some d i f f e r e n c e s , b u t t h e d i f f e r e n c e s do n o t seem t o be particularly consistent. The p r e c i s i o n o f r e p l i c a t i o n u s i n g t h e i n s t r u m e n t t y p e s l i s t e d was g e n e r a l l y t h e b e s t w i t h FAAS. I n s p i t e o f t h e v a r i a b i l i t y n o t e d between some o f t h e m e t h o d s , t h e r e s u l t s seem w e l l w i t h i n s i m i l a r r a n g e s o f c o n c e n t r a t i o n s and t h e degree of c o n f o r m i t y g i v e s c o n f i d e n c e t o the g e n e r a l v a l i d i t y of the r e s u l t s . Neutron A c t i v a t i o n Spectrometry. Another i n s t r u m e n t a l t e c h n i q u e w h i c h has a p p l i c a b i l i t y t o a w i d e r a n g e o f e l e m e n t s i s n e u tron activation analysis. I n t h i s method t h e sample ( w h i c h c o u l d be o r a n g e j u i c e w i t h o u t any p r i o r sample t r e a t m e n t ) i s i r r a d i a t e d with a strong neutron f l u x . The e l e m e n t s o f a n a l y t i c a l i n t e r e s t are thus c o n v e r t e d t o u n s t a b l e i s o t o p e s which decay w i t h c h a r a c t e r i s t i c e n e r g i e s and t h u s measurement o f t h e i n t e n s i t i e s r e s u l t s i n a n a l y t i c a l v a l u e s f o r t h e elements o f i n t e r e s t . T h e r e a r e some s e r i o u s d r a w b a c k s t o t h i s m e t h o d , however. The m a t r i x c a n c a u s e s e v e r e b a c k g r o u n d e f f e c t s e s p e c i a l l y when t h e sample c o n t a i n s l a r g e amounts o f an e l e m e n t , l i k e p o t a s s i u m , w h i c h i s t h e s i t u a t i o n w i t h orange j u i c e . I n t h i s event t e d i o u s c h e m i c a l s e p a r a t i o n s must be c a r r i e d out t o a c h i e v e a d e q u a t e s e l e c t i v i t y , a c c u r a c y



16.

MCHARD

ET AL.

Trace Metals

in Orange

Juice

377

and p r e c i s i o n . F a c i l i t i e s f o r neutron a c t i v a t i o n are v e r y expens i v e a n d , t h u s , most o f t e n t h e a n a l y s t w o u l d g e n e r a l l y h e o b l i g e d t o send samples t o a s e r v i c e l a b o r a t o r y equipped t o p e r f o r m such analysis. A sample o f F l o r i d a o r a n g e j u i c e h a s b e e n a n a l y z e d b y n e u t r o n a c t i v a t i o n ( 6 6 ) a n d t h e r e s u l t s a r e g i v e n i n T a b l e V I . F o r some elements, a c t u a l c o n c e n t r a t i o n s a r e shown a l o n g w i t h t h e p r e c i s i o n o f t h e d a t a , b u t f o r most e l e m e n t s o n l y m a x i m a l c o n c e n t r a t i o n s are l i s t e d . T h i s means t h a t v a l u e s c a n n o t b e h i g h e r t h a n t h o s e g i v e n b u t c o u l d be much l o w e r . A g a i n , t h e s e d a t a a r e i n c l u d e d i n t h i s d i s c u s s i o n o n l y f o r c o m p a r i s o n w i t h t h a t t o be p r e sented i n subsequent paragraphs and i t should be emphasized t h a t t h e v a l u e s l i s t e d as maxima a r e r e l a t i v e l y m e a n i n g l e s s . F o r many o f t h e e l e m e n t s , t h e d a t a a g r e e w e l l w i t h i n an o r d e r of magnitude w i t h the d a t a gathered by us and w i t h t h e d a t a o f R o b e r t s a n d Gaddum. T h e r e a r e some o b v i o u s d i s c r e p e n c i e s . The u p p e r l i m i t s g i v e n f o r s u l f u r (UOOO ppm), p h o s p h o r u s (2200 ppm) s i l i c o n (UOOO ppm), a n d p r o b a b l y Nb (108 ppm) a r e f a r t o o h i g h . A l s o , t h e r e seems t o be no r e l i a b l e e v i d e n c e t h a t y t t r i u m , t i t a n ium a n d n i c k e l a n d z i r c o n i u m a r e above 1 ppm i n s i n g l e s t r e n g t h orange j u i c e . O t h e r w i s e , t h e d a t a a r e c r e d i b l e , a n d i t s h o u l d be n o t e d t h a t t h e s e v a l u e s w e r e o b t a i n e d f r o m o n l y one sample a n d no a t t e m p t was made t o i m p r o v e t h e v a l u e s b y r a d i o c h e m i c a l s e p a r a t ions . S p a r k S o u r c e Mass S p e c t r o m e t r y . A n o t h e r method f o r t r a c e a n a l y s i s p r o b a b l y s h o u l d be mentioned and t h a t i s spark source mass s p e c t r o m e t r y . I n t h i s t e c h n i q u e , t h e sample i n t h e f o r m o f a s o l i d s e r v e s as an e l e c t r o d e and v a p o r s , formed by s p a r k i n g , a r e a t o m i z e d a n d i o n i z e d t o m e t a l i o n s w h i c h a r e s e p a r a t e d b y a mass s p e c t r o m e t e r and m e a s u r e d . The equipment i s e x p e n s i v e a n d good r e s u l t s r e q u i r e t h e a t t e n t i o n o f a s k i l l e d o p e r a t o r . Even under t h e b e s t c o n d i t i o n s o r d e r o f m a g n i t u d e agreement o f r e s u l t s i s about t h e b e s t t h a t can be a c h i e v e d . C o m p a r i s o n o f F l o r i d a Orange J u i c e w i t h J u i c e s f r o m O t h e r phic Locales

Geogra-

I n an attempt t o update the e a r l i e r s t u d i e s on orange j u i c e and t o t r y t o a r r i v e a t some q u a n t i t a t i v e v a l u e s f o r a b r o a d r a n ge o f e l e m e n t s i n F l o r i d a a n d o t h e r s o u r c e j u i c e s a m p l e s , we u n dertook the a n a l y t i c a l p r o j e c t described i n the f o l l o w i n g d i s c u s sion. Orange j u i c e s o u r c e s o f p r i m a r y i n t e r e s t t o u s were f r o m F l o r i d a a n d B r a z i l b u t some v a l u e s were o b t a i n e d o n j u i c e s f r o m o t h e r l o c a l e s , i . e . , M e x i c o a n d C a l i f o r n i a a r e i n c l u d e d f o r comparison. Experimental Twenty-four

F l o r i d a j u i c e s , s e v e n t y - f o u r B r a z i l i a n a n d some



0.. 0 1 8

1 . 6 9

Iron

Manganese

Magnesium

c

C ?RSD LOD

Copper

O.6U9

%AD

C ?RSD

1 . 0 5

0 . 1 7 9 7 . 5 ^

Ο.169 6 . 9 2 5 . 8 9

0 . 1 6 1

6 . 5 3

0 . 3 0 2

3 .31

0 .26U

1 . 0 2 0 . 6 0 3

0.5lh

0 . 5 2 6

0 . 5 9 2

0 . 2 1 9

Ο.196

0 . 1 5 1

0 . 0 8 0

0 . 1 9 5

U.60

1 2 1 .

2 . 2 7

0 . 1 6 5

',60

1 . 2 0

0 . 1 8 3 . 8 0

0..180 8 ,. 6 1

0 . 2 ^ 0

1 . 8 3

1 1 5 .

0 . 3 0 2

0 . 0 0 0 5

1,. 0 2

0,,187

5.

1 1 0 .

1.,68

0..18U

1 . 8 3

0 . 9 2 0.,83 1 1 . .7

1 . 0 6

lU.7

1 . 8 5

1 0 3 .

3 . 3 6

0 . 0 6 8

3.hO

0 . 8 1 5

U.07

9 7 .

1 . 2 U

0 . 0 8 1

1 6 . 1

U.98

1 . 0 3

0 . 9 8 5

0. 256

2 . 3 7 0 . 0 0 3

0. 0 2 8

0 . 0 2 6

0 . 0 2 7

0 . 0 2 5

0 . 5 6 7

0 . 3 3

0. 3 0 3 6. 83

5 . 9 1

0 . 2 8 7

8 . 5 7

0 . 3 3 2

0 . 3 8 0

2 . 8 0

O.1U3 1 . 0 1

02h

97

0. 267

0 .

5. 73

c

3

FAAS HN0

b,d

8 . 5 3

O.hlh

0 . 6 2 0

0 . 7 3 8

0 . 0 5 0

3 . 5 8

9 5 .

0. 591

1 0 0 .

0.

0 . 0 1 0

Oil

1 2 . 8

0. 0 3 5

HNO

b,d

1 . 1 2

5 . 8 8

1 1 9 .

F AFS 0 . 0 5 0

HC1

5 .25

l U 2

0 . 0 6 3

0 .207

C ?RSD LOD ?AD

U.50

0.kh6

1 . 3 0

9 .20

c

O.UUU

1 . 8 6 0

.hoh

C ?RSD LOD

%AD

0

0 . 1 U 7

0 . 1 6 6

0 . 0 5 0

0 .060

7 9 . 6 . 1 5

87.

0 . 0 0 7 1 . 2 7

1 . 2 6

3 . 5 2

0 . 0 5 3

3

0 . 0 0 3

U.77

O.OU7

HN0

9 . 3 3

88.

0 . 8 3 3

0.013

1 0 . 1

DCPAES HC1

2 . 5 8

%AD

3

0 . 0 3 9

HN0

b,d

b,d

C o m p a r a t i v e A n a l y t i c a l D a t a From F o u r I n s t r u m e n t a l Methods (28)

1 .22

1 0 1

C ?RSD LOD

.6

%ΑΌ

0 .027

lU

c ?RSD LOD

Calcium

Barium

a

ICPAES HC1

T a b l e V.


H

> r

G

α o

>

O

G H 50

c/3

G

Η SO

00

Nagy and Attaway; Citrus Nutrition and Quality ACS Symposium Series; American Chemical Society: Washington, DC, 1980. 0.01+3 2 . 5 9 6

8 . 3 1

0 . 1 2 8 0 . 5 ^ 2

9 . 6 U

0 . 3 8 0

0 . 6 3 3

%AD

1.1+1 0 . 0 8 9

1.13 5.1U

6 . 3 6

%ΑΏ

c

0 . 3 7

0 . 2 8 1

2 . 7 2

0 . 0 7 6

0.061+

c

1 5 . 6

0 . 0 0 0 9

8 . 2 5

2.91+

1.1+0

0.131+

O.29I+

0.301+

0 . 3 5 3

C ?RSD LOD

0 . 0 1 8 1 . 0 5

0 . 0 1 6

0.01+5

0 . 9 8 7

1 . 0 0

2 . 2 2

5 . 2 5

c

6 . 8 5

0 . 0 0 6

0 . 0 0 5

Ο.516

3 . 9 U

0 . 0 0 8

6 . 5 9

3 . 2 7

0 . 7 2

5 . 5 5

0 . 0 1 7

1+.57

1.0U

1 0 . 1

0.037

12.1+

c

7.03

c

%AD

5.76

2 . 1 1

C #RSD LOD

11.0

8.81+

1+.52

9 . 8 0 0 . 5 2

0 . 3 7 6

O.Ul+2

0 . 3 7 6

8 . 1 3

0 . 6 7 6

C ?RSD LOD

3 . 7 8

8 . 3 6

1 . 5 2

0 . 2 1

1.06

0 . 9 6

2 . 1 6

%AD

2 . 1 9

2.1+3

2 . 0 2

2 . 7 6

1 . 0 3

1 . 9 5

2 . 9 3

3.39

1925.

1+. 5 7

2 . 5 9

1859.

0 . 0 0 2 1 . 7 6

3

2 . 0 1

3

0 . 0 1 7

HN0

I+.9U

I 8 l 8 .

1.U5

I85I+.

0 . 0 2 8

HC1

0 . 0 0 8

3

1 . 2 8

HN0

,b,d FAAS HN0

h.99

2 2 8 8 .

1 . 5 9

0 . 0 0 7

HC1

FAFS

(Continued)

1 . 9 3

l 6 5 2 .

0.1+79

0 . 0 1 2

3

DCPAES

b,d

1.5U

2109.

0 . 2 0 3

%AD

HN0

b,d

b,d

A n a l y t i c a l D a t a From F o u r I n s t r u m e n t a l M e t h o d s

C ?RSD LOD

0 . 0 1 2

LOD

ICPAES

Comparative

Analytical Chemistry

C = A v e r a g e c o n c e n t r a t i o n , ppm; ?RSD = | 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 ; LOD = L i m i t o f D e t e c t i o n , ppm; %ΑΌ = A v e r a g e D e v i a t i o n o f M e a s u r e m e n t . A l l c o n c e n t r a t i o n s a r e c a l c u l a t e d o n t h e b a s i s o f s i n g l e ICPAES = I n d u c t i v e l y C o u p l e d P l a s m a s t r e n g t h orange j u i c e (SSOJ). °Values n o t m e a s u r e d , s e e t e x t . E m i s s i o n S p e c t r o m e t r y ; DCPAES = D i r e c t C u r r e n t P l a s m a E m i s s i o n S p e c t r o m e t r y ; FAFS = F l a m e A t o m i c F l u o r e s c e n c e S p e c t r o m e t r y ; FAAS = F l a m 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 r y .

Zinc

Sodium

Rubidium

Potassium

1

Manganese (Contd )

HC1

T a b l e V.



< 0 . 0 0 1

< 0 . 3 3

< 0 . 0 2 8

< 0 . 6 8

0 . 0 0 9

< 0 . 1 9

< 0 . 0 1 9

Analysis of Trace Metals in Orange Juice - ACS Symposium Series

Recommend Documents