Analytical Utility of an Inductively Coupled PlasmaâIon

Chapter 16

Analytical Utility of an Inductively Coupled Plasma—Ion Chromatographic System for the Speciation and Detection of Transition Metals 1

Downloaded by PENNSYLVANIA STATE UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: December 4, 1992 | doi: 10.1021/bk-1992-0479.ch016

Daniel J. Gerth and Peter N. Keliher

2

Department of Chemistry, Villanova University, Villanova, PA 19085 An overview of the c o n s t r u c t i o n and a p p l i c a t i o n of a coupled ion chromatograph-inductively coupled plasma instrument system i s presented. The instrumentation i s discussed i n terms of equipment and software requirements, and s p e c i e s s p e c i f i c d e t e c t i o n i n Fe ( I I ) / F e (II) and C r ( I I I ) / C r (VI) systems i s demonstrated. The advantages o f atomic s p e c t r o m e t r i c d e t e c t i o n f o r chromatography have been d i s c u s s e d many times. Recent p u b l i c a t i o n s (1.2,3) i n d i c a t e t h a t the technique i s r a p i d l y g a i n i n g acceptance as an important t o o l f o r both s p e c i a t i o n and d i s c r e t e sample i n t r o d u c t i o n . In order t o examine t h e p o t e n t i a l o f t h e technique, an i o n chromatography/inductively coupled plasma instrument system ( h e r e a f t e r c a l l e d IC-ICP), has been developed. A major o b j e c t i v e o f the p r o j e c t was t o i n t e r f a c e t h e instruments i n such a f a s h i o n as t o allow quick and simple changeover from t h e IC-ICP o p e r a t i n g mode t o continuous nebulization. Equipment The ICP u t i l i z e d i n t h i s study was an A p p l i e d Research L a b o r a t o r i e s QA-13700 Quantometric Analyzer (Sunland, CA) , equipped with a 24 channel polychromator, and 2 Kw RF generator o p e r a t i n g a t 27.4 MHz. Standard o p e r a t i n g c o n d i t i o n s a r e given i n Table I . Table I . ICP Standard Operating C o n d i t i o n s Forward Power Plasma Gas Flow Coolant Gas Flow C a r r i e r Gas Flow

1.1 kw 1.2 L/min 10.0 L/min 1.0 L/min

1

Current address: Lancaster Laboratories, 2425 New Holland Pike, Lancaster, PA 17601-5994 Deceased

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0097-6156/92/0479-0275$06.00/0 © 1992 American Chemical Society

In Element-Specific Chromatographic Detection by Atomic Emission Spectroscopy; Uden, P.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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The instrument e l e c t r o n i c s have been upgraded by LABCO, I n c . ( C a l i f o n , NJ) and c o n s i s t of t h r e e e i g h t channel 16 b i t A/D boards, stepper motor d r i v e r , autosampler c o n t r o l l e r , and power supply. The instrument i s c o n t r o l l e d by an IBM-PC/AT compatible computer, with 1 megabyte RAM, EGA g r a p h i c s , and a 20 megabyte hard d i s k d r i v e . Communications are through a standard RS-232 p o r t . A l l math i s performed with double p r e c i s i o n f l o a t i n g p o i n t numbers, which p r o v i d e 15 t o 16 s i g n i f i c a n t d i g i t s . The chromatographic equipment consists of a Beckman/Altex 110A HPLC pump, Rheodyne 9125 m e t a l - f r e e i n j e c t i o n v a l v e , and CS-5 separator column (Dionex, Sunnyvale CA) . Various sample loop volumes were used d u r i n g the course of the study. In keeping with our o b j e c t i v e of minimal hardware m o d i f i c a t i o n s , the s e p a r a t o r column was connected d i r e c t l y t o a standard Meinhard concentric n e b u l i z e r mounted i n a stock ARL c o n i c a l spray chamber. Software The c o n t r o l l i n g software was w r i t t e n i n P a s c a l (Borland I n t e r n a t i o n a l , S c o t t s V a l l e y CA) , which was chosen because of i t s f l e x i b i l i t y , speed, and modular programming approach (4) . The e n t i r e package c o n s i s t s of a s m a l l " d r i v e r " program, which i n i t i a l i z e s the system, and several s e l f - c o n t a i n e d modules, each c o n t a i n i n g a l l of the f u n c t i o n s and procedures r e l a t e d t o a s p e c i f i c t a s k ( i . e . c a l i b r a t i o n , a n a l y s i s ) . G l o b a l v a r i a b l e s were kept t o a minimum. T h i s approach has two main advantages : 1. Since the modules are used as o v e r l a y s , the amount of program code r e s i d e n t i n memory a t any p a r t i c u l a r time i s s m a l l , l e a v i n g more room f o r the manipulation of l a r g e data s t r u c t u r e s , and, 2. The modular approach allows new r o u t i n e s , such as the IC-ICP code, t o be e a s i l y added or modified a t any time. The IC-ICP module c o n s i s t s of two procedures, A c q u i r e and View. Acquire c o n t r o l s the a c t u a l data a c q u i s i t i o n , a l l o w i n g the operator t o s p e c i f y the i n t e g r a t i o n time, run length, data storage f i l e , and s e l e c t a channel f o r r e a l - t i m e monitoring o f the run. View allows the operator to display chromatograms from up to four channels simultaneously. Since the instrument i s equipped with a polychromator, the number of channels a c t u a l l y s t o r e d depends upon the a n a l y t i c a l program i n o p e r a t i o n a t the time; t h i s must be s e l e c t e d from the main menu before any other t a s k s may be performed. Up t o the f u l l compliment of 24 channels may be i n c l u d e d . For normal chromatographic runs, an i n t e g r a t i o n time of 0.5 seconds provides adequate sampling, and i s the d e f a u l t v a l u e . The a c t u a l time between data p o i n t s i s 1.4 seconds, due t o the 0.9 seconds the instrument r e q u i r e s t o read, t r a n s m i t , and process the 24 channels i n the polychromator a r r a y . In order t o minimize t h i s delay, we opted f o r a p o l l i n g r o u t i n e d u r i n g the read c y c l e . As the


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Speciation and Detection of Transition Metals

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instrument i s integrating, the software s t o r e s the p r e v i o u s l y acquired p o i n t s and updates the screen d i s p l a y . When f i n i s h e d , the software then c o n t i n u o u s l y p o l l s the s e r i a l p o r t u n t i l i t r e c e i v e s a data ready s i g n a l , a t which p o i n t i t accepts the new data and repeats the c y c l e .


D i s c r e t e Sample I n t r o d u c t i o n D i s c r e t e sample i n t r o d u c t i o n i s d e f i n e d here as the i n j e c t i o n o f s m a l l , f i x e d volume a l i q u o t s o f sample i n t o an instrument. The opportunity t o o b t a i n a complete ICP a n a l y s i s on as l i t t l e as 20 μΐ3 o f sample i s t o us as compelling a reason f o r examination o f IC-ICP as speciation. To a c e r t a i n extent, the c o n c e n t r a t i o n d e t e c t i o n l i m i t o f the IC-ICP system i s a d j u s t a b l e by simply u t i l i z i n g d i f f e r e n t sample loop volumes. The s i g n a l d e t e c t i o n l i m i t i s determined by the b a s e l i n e n o i s e , which was measured by simply l e t t i n g the instrument run without i n j e c t i n g a sample, and c a l c u l a t i n g t h r e e times the standard d e v i a t i o n o f the b a s e l i n e . The c o n c e n t r a t i o n d e t e c t i o n l i m i t was then obtained by r e l a t i n g the s i g n a l d e t e c t i o n l i m i t t o i n t e n s i t i e s obtained by s p e c i f y i n g a 1.0 mL sample loop, and i n j e c t i n g a mixed standard. Table I I presents the s i g n a l and c o n c e n t r a t i o n d e t e c t i o n l i m i t s f o r Fe ( I I I ) and Cr (III) obtained i n t h i s study, and the corresponding continuous n e b u l i z a t i o n d e t e c t i o n l i m i t . Table I I . D e t e c t i o n L i m i t s Analyte

3s Background Signal (counts)

1.0 mL Injection (mg/L)

Continuous Nebulization (mg/L)

Fe (III)

33.3

0.14

0.005

Cr (III)

26.1

0.090

0.007

As mentioned above, the technique's c o n c e n t r a t i o n d e t e c t i o n l i m i t may be c o n t r o l l e d by v a r y i n g the sample loop volume. Figure 1 (a) and (b) both were obtained by i n j e c t i o n s o f the same 20 mg/L Cr (III) standard, u s i n g a 20 μ.1, and 1.0 mL sample loop, r e s p e c t i v e l y . The peak obtained with the 20 i n j e c t i o n i s e a s i l y seen, and while the peak shape has degraded somewhat, q u a n t i t a t i o n may still be performed v i a e i t h e r peak h e i g h t o r area measurement. Since the ICP a c t s as a mass s e n s i t i v e d e t e c t o r , t h e o r e t i c a l l y a f i v e f o l d i n c r e a s e i n the mass o f a n a l y t e i n j e c t e d should r e s u l t i n a f i v e f o l d i n c r e a s e i n the d e t e c t o r s i g n a l . As Table I I I i l l u s t r a t e s , peak area e x h i b i t s a 5% d e v i a t i o n from the t h e o r e t i c a l response, and i s the measurement o f c h o i c e .


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ELEMENT-SPECIFIC CHROMATOGRAPHIC DETECTION BY AES (a)


500

τ

200-1 0

1 1

1 2

1 3

1 4

1 5

1 4

1 5

TIME (min) (b)

0-1 0

1 1

1 2

1 3

TDfE (min)

FIGURE 1. S e n s i t i v i t y m o d i f i c a t i o n by changing sample injection loop s i z e . Both chromatograms are o f a 20 mg\L Cr (III) standard, (a) 20 M L sample loop, (b) 1000 M L sample loop.

Table I I I . A Comparison o f Peak Height and Peak Area Q u a n t i t a t i o n 20 μΐι Sample Volume

1000 M L Sample Volume

% Deviation from Theoretical

Peak Area (count*min)

1242

65656

5.0

Peak Height (counts)

140

6005

15.0


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GERTH & KELIHER


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3000

— Zn — · Cu

•\

+ 2000


• \ :

\

•

\

+ 1000 J 1

0

1

h

2

3 TIME (min)

1

4

5

FIGURE 2. Simultaneous d e t e c t i o n of 20 mg/L copper and 0.2 mg/L z i n c . The small peak i n the z i n c t r a c e i s due t o s p e c t r a l overlap of the copper 213.598 nm l i n e .

Figure 2 i l l u s t r a t e s another important aspect of the technique. T h i s chromatogram was obtained by i n j e c t i n g 100 ML of a mixed standard c o n t a i n i n g 20 mg/L copper and 0.2 mg/L o f z i n c , and simultaneously monitoring the Cu 324.754 nm and Zn 213.856 l i n e s . The small peak i n the z i n c t r a c e c o - e l u t i n g with the copper i s due t o s p e c t r a l overlap of the Cu 213.598 nm l i n e , while the a c t u a l z i n c peak i s completely r e s o l v e d from the copper i n t e r f e r e n c e . Because o f t h i s s e p a r a t i o n of analyte bands i n time, most s p e c t r a l overlap problems are r e s o l v e d , which g r e a t l y s i m p l i f i e s a n a l y s i s s i n c e monitoring a l l p o t e n t i a l l y i n t e r f e r i n g l i n e s i s not necessary.

S p e c i a t i o n Studies Due t o the s i m i l a r i t i e s i n s i z e and e n t h a l p i e s of h y d r a t i o n , most t r a n s i t i o n metals e x h i b i t very s i m i l a r a f f i n i t i e s f o r c a t i o n exchange r e s i n (5) . In order t o e f f e c t a u s e f u l separation, c h e l a t i n g agents are employed as e l u e n t s . By v a r y i n g the chelant and e l u e n t pH, the r e t e n t i o n times of the v a r i o u s metals can be adjusted as r e q u i r e d . Some c h e l a t i n g agents commonly employed are oxalic, c i t r i c , and t a r t a r i c a c i d s , EDTA, and, more r e c e n t l y , p y r i d i n e - d i c a r b o x y l i c a c i d , or PDCA. Lithium


280


hydroxide i s normally employed t o a d j u s t the e l u e n t pH, because of the low a f f i n i t y the L i * i o n has f o r the i o n exchange r e s i n .


Iron. Iron s p e c i a t i o n was c a r r i e d out on a CS-2 c a t i o n separator column (Dionex, Sunnyvale, CA) u s i n g a 10 mM o x a l i c / 7 . 5 mM c i t r i c a c i d eluent, adjusted t o pH 4.3 with LiOH. Under these c o n d i t i o n s , Fe (III) forms a t r i p l y charged a n i o n i c complex, and i s u n r e t a i n e d by the r e s i n . Fe ( I I ) , however, i s not s t r o n g l y complexed, and e l u t e s much l a t e r , a t 11.5 minutes. F i g u r e 3 demonstrates t h i s separation. Chromium. The importance of s p e c i e s s p e c i f i c i n f o r m a t i o n i n environmental Chromium analyses cannot be understated. Since the t y p i c a l chrome waste treatment employs a r e d u c t i o n step t o convert the c a r c i n o g e n i c Cr (VI) t o the more benign Cr ( I I I ) , i t i s imperative t o know a l l s p e c i e s c o n c e n t r a t i o n s i n order t o determine the e f f i c a c y of the treatment program. D i r e c t chromatography of the two forms i s impossible, however, due t o the presence of v a r i o u s Cr (III) - hydroxyl complexes. Due t o the s l i g h t l y d i f f e r e n t a f f i n i t y of each complex f o r the i o n exchange r e s i n , these complexes e l u t e as a s e r i e s of low, broad unresolved bands under the chromatographic c o n d i t i o n s used i n t h i s study. F i g u r e 4 i s the chromatogram r e s u l t i n g from the i n j e c t i o n of a s o l u t i o n c o n t a i n i n g 20 mg/L of Cr (III) u t i l z i n g a PDCA based eluent. As can be seen, i t i s impossible t o q u a n t i t a t e the chromium. I t i s known, however, t h a t Cr (III) forms a s t a b l e , mononegative complex with PDCA near n e u t r a l pH (6). While t h i s r e a c t i o n i s slow a t room temperature, h e a t i n g the sample t o b o i l i n g f o r a b r i e f p e r i o d (under one minute) allows the r e a c t i o n t o proceed t o completion. The r e s u l t i n g complex can then be e a s i l y separated from the Cr (VI) , which, a t the same pH, i s predominantly present as the d i n e g a t i v e chromate anion. In order t o e f f e c t the complexation, we used a microwave d i g e s t i o n oven, and determined the optimum h e a t i n g time a t 100% power. A l l complexation r e a c t i o n s were c a r r i e d out i n open p o l y e t h y l e n e Erlenmeyer f l a s k s t o avoid overheating and pressurization. F i g u r e 5 shows the chromatograms a f t e r 10, 20, 30, 40, 50, and 60 seconds of microwave exposure. From a graph of peak h e i g h t v s . time, 30 seconds exposure was chosen as optimum. The f i r s t peak (seen most c l e a r l y i n 5a) i s the monopositive Cr-PDCA cation. Since o x i d a t i o n of excess PDCA by chromate would lead t o h i g h Cr (III) r e s u l t s , we subjected a 20 mg/L s o l u t i o n of chromate t o the same complexation procedure. F i g u r e 6 c l e a r l y demonstrates t h a t no o x i d a t i o n takes p l a c e . F i g u r e s 7 and 8 are chromatograms of mixtures of Cr (III) and Cr (VI), demonstrating the f i n a l s e p a r a t i o n . Future work planned i n c l u d e s examining industrial


16. GERTH & KELIHER



400

100 +

Β

10

12

14

16

TIME (min)

FIGURE 3. Separation o f Fe (II) and Fe (III) u t i l i z i n g an oxalate based eluent.

FIGURE 4. Chromatogram obtained by i n j e c t i n g 1000 /iL of a 20 mg/L Cr ( I I I ) standard without prior complexation.


281

282


(a)


6000+

OA

0

1

1

1

1

1

2

3

4

1 5

TIME (min)

(b)

o-l

1

1

1

1

1

0

1

2

3

4

5

TIME (min)

FIGURE 5. Optimization o f microwave h e a t i n g time f o r the formation o f the Cr (III)-PDCA complex. (a) 10 seconds, (b) 20 seconds, (c) 30 seconds, (d) 40 seconds, (e) 50 seconds, (f) 60 seconds.


16.

GERTH & KELIHER



(c)

-I 0

1

1

1

1

1

1

2 3 TIME (min)

4

5

(d)

o-l

0

1

1

1

1

1

1

2

3

4

5

TIME (min)

Figure 5. Continued. Continued on next page.


283



284

Figure 5. Continued.


16. GERTH & KELIHER


285

8000

6000 4-


4000 +

2000

THEE (min) F I G U R E 6. A 20 mg/L Cr ( V I ) standard a f t e r undergoing the PDCA complexation procedure. The l a c k o f a Cr ( I I I ) peak i n d i c a t e s t h a t no o x i d a t i o n o f the PDCA by the Cr ( V I ) i s taking place.

6000

4000 +

2000 +

7. Chromatogram obtained from a 1000 M L i n j e c t i o n o f a 10 mg/L Cr ( I I I ) - 10 mg/L Cr ( V I ) mixed standard. Cr ( I I I ) i s e l u t e d as the Cr(PDCA) a n i o n i c complex, Cr ( V I ) as the chromate anion.

FIGURE




286

TIME (min) FIGURE 8. Chromatogram obtained from a 1000 μΐί i n j e c t i o n of a 0.2 mg/L Cr (III) - 20 mg/L Cr (VI) mixed standard, (a) s c a l e expansion showing complete r e s o l u t i o n of the Cr (III) peak, (b) both peaks on s c a l e , i l l u s t r a t i n g the dynamic range of the technique.


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b o i l e r and c o o l i n g tower waters, a comparison o f d e t e c t i o n l i m i t s obtained with a l t e r n a t e n e b u l i z a t i o n systems (e.g. Hildebrand G r i d n e b u l i z e r ) , and f u r t h e r augmentation o f the software t o allow complete post run p r o c e s s i n g o f t h e chromatogram.


LITERATURE CITED 1.) K e l i h e r , P . N . ; G e r t h , D.J.; Snyder, J.L.; Wang, H.; Zhu, S . F . Anal Chem. 1988, 60, 342R-368R. 2.) K e l i h e r , P.N.; Ibrahim, H.; G e r t h , D . J . Anal Chem, 1990, 62, 184R-212R. 3.) Environmental A n a l y s i s u s i n g Chromatography I n t e r f a c e d with Atomic Spectroscopy; H a r r i s o n , R.M.; Rapsomanikis, S . , Eds., Ellis Horwood Ltd.: C h i c h e s t e r , U . K . , 1989. 4.) Turbo Pascal Reference Manual, Borland I n t e r n a t i o n a l , S c o t t s V a l l e y CA. 5.) Smith, R. E . Ion Chromatography A p p l i c a t i o n s ; CRC Press: Boca Raton F L , 1988; V o l . 1, pg 77. 6.) Ion Chromatography A p p l i c a t i o n Note 26; Dionex C o r p o r a t i o n , Sunnyvale, CA, 1986; pp 1-2. RECEIVED April 26, 1991


Analytical Utility of an Inductively Coupled PlasmaâIon

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