Laser-Enhanced Ionization for Trace Metal Analysis in Flames - ACS

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J. C. TRAVIS and G. C. TURK Center for Analytical Chemistry, U.S. National Bureau of Standards, Washington, D.C. 20234 R. B. GREEN Department of Chemistry, West Virginia University, Morgantown, WV

26506

I o n i z a t i o n of atoms in flames is more probable from an e x c i t e d s t a t e than a ground s t a t e . A dye l a s e r tuned to a d i s c r e t e atomic t r a n s i t i o n will sufficiently b i a s the e x c i t e d s t a t e p o p u l a t i o n of the atom to produce a change in i o n i z a t i o n r a t e which is e a s i l y measured w i t h conventional e l e c t r o n i c s . The excess i o n i z a t i o n i n the flame due to l a s e r e x c i t a t i o n has been g e n e r a l l y c h a r a c t e r i z e d as an optogalvanic e f f e c t but l a s e r enhanced i o n i z a t i o n (LEI) is more d e s c r i p t i v e , p a r t i c u l a r l y i n terms of the mechanism . Generalized p l o t s of the Saha equation3 (Figure 1) show that most elements ( i o n i z a t i o n p o t e n t i a l ≥ 5eV) are predominantly n e u t r a l at t y p i c a l flame temperatures. The a b s o r p t i o n of o p t i c a l energy, moving an atom c l o s e r to its i o n i z a t i o n l i m i t , will significantly increase the i o n p o p u l a t i o n in the flame. According to F i g u r e 1, an e l e c t r o n v o l t of e x c i t a t i o n energy will provide approximately one order of magnitude increase i n i o n i z a t i o n at 2500 K. LEI spectrometry is a h y b r i d technique which depends on both l a s e r e x c i t a t i o n and thermal i o n i z a t i o n . The process may proceed by p h o t o e x c i t a t i o n and thermal i o n i z a t i o n or a combination of thermal e x c i t a t i o n , p h o t o e x c i t a t i o n and thermal i o n i z a t i o n (Figure 2). The experimental system used i s illustrated i n F i g u r e 3. The sample i s a s p i r a t e d i n t o a f u e l lean a i r - a c e t y l e n e flame of a standard premix burner w i t h a 5 cm s i n g l e s l o t burner head. The atomized species are e x c i t e d w i t h a flashlamp-pumped tunable dye l a s e r wi£h c a p a b i l i t y f o r frequency-doubled o p e r a t i o n . The data reviewed- and presented here were obtained at l a s e r bandwidths of 0.05-0.1 nm, although f u r t h e r narrowing i s r e a d i l y p o s s i b l e . The l a s e r dyes used included Fluoral-7GA, Rhodamine 575, Rhodamine 6G, and Rhodamine 640, w i t h frequency doubling i n most cases. The s i g n a l i s detected with a p a i r of 1 mm diameter tungsten welding rods 1 cm apart and ^ 1 cm above the burner head. These dual cathodes run p a r a l l e l to the burner s l o t and are maintained at -500V to -1000V with respect to the burner head, which i s used I

2

This chapter not subject to U.S. copyright. Published 1978 American Chemical Society

Hieftje; New Applications of Lasers to Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1978.

NEW

Number

Density= 1 0

APPLICATIONS O F LASERS T O C H E M I S T R Y

1

°/cm

3

10

3000K

0.1 ^10-2

Lu Q Q_

ζ Ç2ioUJ >

4

_i

Lu 10"

6

10-

4

5

6

7

8

E N E R G Y TO IONIZE ( e V )

Figure 1. Relative degree of ionization of a generalized element (3) as a function of its ionization potential for flames of (a) 2000, (b) 2500, and (c) 3000 Κ

IONIZATION POTENTIAL

ΔΕ· I ΔΕ;

hi/ hi/

GROUND STATE USING A RESONANCE LINE

USING A NON-RESONANCE LINE

Figure 2. Typical avenues of User-enhanced ionization: (a) photo excitation from the ground state followed by collisional ioniza tion; (b) photo excitation from a thermally populated excited state followed by collisional ionization


6.

TRAVIS

Trace Metal Analysis in Flames

93

as the anode. The e l e c t r o d e s remain v i s u a l l y outside the flame, although s t i l l i n e l e c t r i c a l contact with the flame. This c o n f i g u r a t i o n i s s l i g h t l y l e s s s e n s i t i v e than when the e l e c t r o d e s are immersed i n the flame, but i t avoids f o u l i n g and e r o s i o n which lead to long-term s i g n a l d e t e r i o r a t i o n . The burner head i s e l e c t r i c a l l y i n s u l a t e d from the burner body by a s t r i p of e l e c t r i c a l tape, so that the current may be monitored on the low v o l t a g e s i d e of the flame. The s i g n a l pulse i s separated from the dc background current with a high-pass f i l t e r , a m p l i f i e d , and pro cessed w i t h sample-and-hold c i r c u i t s and a minicomputer (or a boxcar s i g n a l averager w i t h d i g i t a l s t o r a g e ) . F i g u r e 4 shows LEI s i g n a l s from a s o l u t i o n of 25 ng/mL Mg and 7 ng/mL Na. The LEI i n t e n s i t i e s f o r these t r a n s i t i o n s are not i n d i c a t i v e of the r e l a t i v e concentrations and absorption c o e f f i c i e n t s alone. Indeed, the expected absorption r a t i o f o r Mg (285.2 nm) t o Na (285.3 nm) i s : 1900 f o r these c o n c e n t r a t i o n s . O s c i l l a t o r s t r e n g t h , i o n i z a t i o n p o t e n t i a l , and f r a c t i o n a l popula t i o n a l l p l a y important r o l e s i n determining the s i g n a l s t r e n g t h . The most s e n s i t i v e l i n e f o r atomic absorption, atomic f l u o r e s cence, and atomic emission, may not n e c e s s a r i l y be the best l i n e f o r LEI spectrometry. Table I shows the d e t e c t i o n l i m i t s f o r pure, aqueous s o l u t i o n s of s e v e r a l elements. These d e t e c t i o n l i m i t s were deter mined by using the average and standard d e v i a t i o n of the mean of 150 l a s e r pulses (^ 0.8 ys pulsewidth) a t a pulse r e p e t i t i o n r a t e of 5 pulses per second. For comparison, Table I a l s o contains d e t e c t i o n l i m i t s reported f o r other flame spectrometric t e c h n i ques. With one exception, LEI d e t e c t i o n l i m i t s a r e comparable to or b e t t e r than those reported f o r other flame spectrometric methods. A c l o s e r i n s p e c t i o n of the LEI d e t e c t i o n l i m i t s i s i n s t r u c t i v e . Table I I shows p e r t i n e n t parameters f o r the t r a n s i t i o n s used. One f a c t o r which i s p a r t i c u l a r l y important i s the energy d i f f e r e n c e between the l a s e r populated e x c i t e d s t a t e and the i o n i z a t i o n p o t e n t i a l . T h i s energy d i f f e r e n c e i s r e f e r r e d to as ΔΕ.. Sodium i s a s t r i k i n g example of the combined e f f e c t of t r a n s i t i o n p r o b a b i l i t y and proximity to the i o n i z a t i o n l i m i t . The d e t e c t i o n l i m i t s are comparable f o r both the strong l i n e a t 589.0 nm (which i s commonly used f o r s p e c t r o s c o p i c a n a l y s i s ) and the weak l i n e a t 285.3 nm, although the l a s e r output i s 100 times more powerful a t the strong l i n e . The e f f e c t of ΔΕ^ on s e n s i t i v i t y i s a l s o i l l u s t r a t e d by the low d e t e c t i o n l i m i t s obtained using e x c i t e d - e x c i t e d t r a n s i t i o n s of Cr, Cu, Mn, Pb, Sn, and T l . In these cases, the l o s s of s e n s i t i v i t y r e s u l t i n g from the small f r a c t i o n a l p o p u l a t i o n of the lower l e v e l i s s i g n i f i c a n t l y counter balanced by the decrease i n ΔΕ.. L E I a n a l y s i s of copper a t 324.8 nm i s r e l a t i v e l y i n s e n s i t i v e , s i n c e i t s ΔΕ^ i s l a r g e r than any of the other t r a n s i t i o n s attempted. Indium enjoys the dual advantages of a s m a l l ΔΕ. and a r e l a t i v e l y l a r g e a b s o r p t i o n coefficient, B . 0 t


94

N E W APPLICATIONS O F LASERS T O C H E M I S T R Y

PULSED LASER

PHOTODIODE

AMPLIFIER

AMPLIFIER SAMPLE

SAMPLE

HOLD

HOLD

&

&

A/D CONVERTER

OSCILLOSCOPE

COMPUTER

TTY

Figure 3. Block diagram of the instrument

Na V

j 0.1 nm

Mg t

\

285.3 285.2 WAVELENGTH (nm)

Figure 4. LEI spectrum of a solution of 25 ng/ mL Mg and 7 ng/mL Na in a C H /air flame 2

2


6.

TRAVIS

95


TABLE I Comparative D e t e c t i o n L i m i t s by LEI and Other Flame Techniques

Element Cr

a

2 b

4

10

4

5

8

60

10

0.9

100

0.2

Ga

0.07

50

In

0.008

30

Mn Na

0.3

0.4 .05

3 3

b

b

0.05

b

5

1

0.5

1

70

0.8

1

0.8

0.5

0.1

0.2

1

0.4 0.1

—

5

20

10

100

10

6

50

100

50

0.09

20

20

8

Pb

0.6

Sn Tl

b

30

—

—

3

Ni

,d Laser FAF*

2

2

o.i

C

2

Fe

Mg

FAF

FAE

ioo

1

C

FAA°

Cu

Κ

a

LEl

8

2 13

4

T h i s work, except as noted. A l l l i m i t s a r e i n ng/mL. o r i g i n a l l y i n r e f e r e n c e 2.

b

Reported

C

Taken from J . D. Winefordner, J . J . F i t z g e r a l d , and N. Omenetto, Appl. Spectrosc. 29, 369 (1975). FAA = Flame Atomic A b s o r p t i o n . FAE = Flame Atomic Emission. FAF = Flame Atomic Fluorescence.

d

S . J . Weeks, H. Haraguchi, and J . D. Winefordner, A n a l . Chem. _50, 360 (1978).



298.6 301.8 282.4 324.8 298.4 302.1 287.4 294.4 303.9 294.3 285.2 279.5 280.0 285.3 589.0 300.2 280.2 283.3 284.0 286.3 291.8

Wavelength (nm)

3

a

.065 .065 1.0 1.0 .4 .4 .2 .2 .6 .2 .6 .6 .6 .9 .9 1.0 .7 .7 .04 .04 .5

:

TT

2.53 4.35 .298 1.77 .223 .426 1.76 1.76 2.51 7.5x10 5.5 .76 1.11 „ 7.0x10 4.63 .65 4.76 1.03 2.42 3.13 2.35

Β χ 10 L -2„ -1,-1 ) S~ (W cm Hz

7

8308 8095 11203 0 0 0 0 826 0 0 0 0 17052 0 0 205 10650 0 3428 0 7793

\

1.3 1 3 1 1 1 1 2 1 1 1 1 1.7 1 1 .8 5 1 5 1 2

12788 13345 15719 31533 30193 30604 13606 13601 13778 1037 26620 24200 7212 6407 24476 28078 13491 24533 20603 24318 7218

the LEI Figure of M e r i t ΔΕ. 1 g (cm ) V o (cm )

2

4.2x10^ 2.6x10^ 1.6x10^ 5x10" 2x10 30 8x10^ 7x107 9.6x10 17 „ 9.5x10. 1.6xl0 29 32 z 6.2x10

2.6x10" 3.6x10 17 6.9 .78

Figure of Merit

Computed f r o m E q u a t i o n 1, u s i n g t h e f a c t o r s g i v e n and a n assumed s p e c t r a l i r r a d i a n c e o f -9 -2 -1 -7 -? -1 3 χ 10 W cm Hz f o r a l l uv t r a n s i t i o n s and 3 χ 10 W cm Hz f o r Na ( 5 8 9 . 0 ) .

Taken f r o m R e f e r e n c e 5.

Tl

Sn

Ni Pb

Na

In Κ Mg Mn

Ga

Fe

Cu

Cr

Element

1

Factors Determining

TABLE I I

2.0 2.0 100 100 4 2 0.07 0.1 0.008 1 0.1 0.3 5 0.05 0.1 8 0.6 3 6 10 0.09

Limit of Detection (ng/mL)

1

H O

W

t>

O

o

>

2 M

05

6.

TRAVIS

97


A zero-order f i g u r e of merit i s u s e f u l f o r p r e d i c t i n g the r e l a t i v e s e n s i t i v i t y of atomic t r a n s i t i o n s : F i g u r e of M e r i t = 31 Β

Here 3 i s the atomization e f f i c i e n c y of the element i n the flame; 1^ the l a s e r s p e c t r a l i r r a d i a n c e ; the E i n s t e i n c o e f f i c i e n t f o r the p r o b a b i l i t y of a b s o r p t i o n from s t a t e I t o u; E^ the energy of the lower s t a t e of the t r a n s i t i o n ; g^ and g a r e lower s t a t e and ground s t a t e s t a t i s t i c a l weights; Τ the flame tempera ture; and k the Boltzmann constant. The f i r s t exponential term expresses the r e l a t i v e p r o b a b i l i t y that a c o l l i s i o n i n the flame w i l l provide the thermal energy r e q u i r e d (ΔΕ.) to complete the i o n i z a t i o n process. The second exponential term, m u l t i p l i e d by the s t a t i s t i c a l weight r a t i o , i s simply the Boltzmann p o p u l a t i o n of the lower l e v e l of the t r a n s i t i o n , r e l a t i v e t o the ground state. The uv L E I d e t e c t i o n l i m i t s are p l o t t e d on a l o g - l o g s c a l e against the corresponding f i g u r e s of merit (shown i n Table II) i n F i g u r e 5. The trend of the data towards a slope of -1 r e s u l t s from the nominal r e c i p r o c a l r e l a t i o n s h i p between d e t e c t i o n l i m i t and s e n s i t i v i t y ( f o r constant l i m i t i n g noise) and supports the v a l i d i t y of the f i g u r e of m e r i t . Current i n v e s t i g a t i o n s using three and four l e v e l model c a l c u l a t i o n s — c a l l f o r s e v e r a l r e f i n e ments to the f i g u r e of merit, e s p e c i a l l y f o r e x c i t e d s t a t e t r a n s i t i o n s . The simple expression i s nonetheless more v a l i d than cross s e c t i o n s alone f o r p r e d i c t i n g the s e n s i t i v i t y of a t r a n s i tion. Given the f a v o r a b l e d e t e c t i o n l i m i t s of Table I , and the s p e c t r a l s e l e c t i v i t y a v a i l a b l e with tunable l a s e r s , i t i s worth while to study observed and p r e d i c t e d matrix i n t e r f e r e n c e s f o r r e a l sample a n a l y s i s by LEI. Such i n t e r f e r e n c e s may g e n e r a l l y be c l a s s i f i e d as chemical, s p e c t r a l , or e l e c t r i c a l . Chemical i n t e r f e r e n c e s w i l l g e n e r a l l y be the same as experienced by other flame s p e c t r o s c o p i c methods*-, and w i l l not be f u r t h e r discussed here. S p e c t r a l i n t e r f e r e n c e s , on the other hand, have some unique f e a t u r e s f o r LEI. Although atomic s p e c t r a l overlaps wjiich have been documented f o r other flame spectrometric methods- are s t i l l present, the r e l a t i v e degree of i n t e r f e r e n c e may d i f f e r d r a s t i c a l l y from o p t i c a l spectrometry due to the LEI f i g u r e of merit of the i n t e r f e r i n g t r a n s i t i o n . Thus, some of the documented i n t e r f e r ences may be i n c o n s e q u e n t i a l , because of a low f i g u r e of merit f o r the i n t e r f e r i n g l i n e , and conversely, some undocumented coincidences of weak s p e c t r a l l i n e s may become important due to a high f i g u r e of m e r i t . An example of t h i s l a t t e r case i s the "weak" 285.3 nm Na t r a n s i t i o n which y i e l d s a strong L E I s i g n a l and i n t e r f e r s s i g n i f i c a n t l y with the 285.2 nm Mg t r a n s i t i o n , (Figure 4 ) . Q


NEW

APPLICATIONS O F LASERS T O C H E M I S T R Y

! r

Cu Cu*

en :Fe Te

"'In'

Μη* Pb * » CrCr

Ξ~

Pb Μη Mg

Γ

Gaj|*Na Ga Na In

Γ

1

, .

juiiiil ι ι ι mill ι 10 1 ου

ι ι mill

Figure

10 of

ι ι ι mill

ι ι ι mill

ι ι ιini

3

4

s

10

10

l u

6

Merit

Figure 5. LEI detection limits as a function of predictedfigureof merit

Figure 6. Percent recovery of 100 ppb lead LEI signal as a function of sodium concentration for applied potentials of —600, —800, and —1000 V


TRAVIS


100

A P P L I E D POTENTIAL (Volts)

Figure 7. Percent recovery of 100-ppb lead LEI signal as a function of applied potential for so dium concentrations of 0,10, 20, and 30 ppm

TABLE I I I R e l a t i v e I n t e r f e r e n c e of D i f f e r e n t M a t r i c e s on 100 ppb Lead S i g n a l Matrix

I.P.

(10 ppm)

(eV)

Percent S i g n a l Recovery -50QV

-750V

-1000V

Κ

4.3

0

0

180

Na

5.1

45

90

110

Li

5.4

100

110

110

Ca

6.1

82

100

100

Cu

7.7

100

100

100

Analyte:

100 ppb Pb

λ = 280.2 nm


100

NEW

APPLICATIONS O F

LASERS T O

CHEMISTRY

Molecular s p e c t r a l i n t e r f e r e n c e s seem to be l e s s of a problem with LEI than conventional flame spectrometry, s i n c e most of the t r a d i t i o n a l i n t e r f e r i n g molecules have high (a 10 eV) i o n i z a t i o n p o t e n t i a l s . R e l a t i v e l y few molecular species have been observed i n flames to date using LEI . E l e c t r i c a l i n t e r f e r e n c e s are unique to LEI ( i n l i e u of such o p t i c a l background i n t e r f e r e n c e s as flame background, ambient l i g h t , and s c a t t e r e d source l i g h t , to which LEI i s impervious). Two types of e l e c t r i c a l i n t e r f e r e n c e may be i d e n t i f i e d : 1) the e f f e c t of ambient e l e c t r o n d e n s i t y i n the flame on i o n i z a t i o n / recombination r a t e s ; and, 2) the e f f e c t of the ambient e l e c t r o n and i o n d e n s i t y on the s i g n a l c o l l e c t i o n process. As an example, F i g u r e 6 i l l u s t r a t e s the e f f e c t of sodium on the s i g n a l from 100 ppb lead f o r s e v e r a l values of a p p l i e d p o t e n t i a l . At each p o t e n t i a l , the a d d i t i o n of sodium i s f i r s t seen to enhance the s i g n a l , then reduce, and f i n a l l y — at lower p o t e n t i a l s — completely e x t i n g u i s h the s i g n a l . F i g u r e 7 was obtained from the same data as F i g u r e 6, and i s u s e f u l f o r e x p l a i n i n g the s i g n a l behavior. The threshold v o l t a g e — below which no s i g n a l i s observed f o r a given matrix — i s r e l a t e d to the sheath, or space-charge, of p o s i t i v e ions which surrounds the two cathode w i r e s . For a given flame temperature and ambient i o n d e n s i t y , a corresponding e l e c t r i c a l s h i e l d i n g i s provided by t h i s sheath, reducing the magnitude of the e l e c t r i c a l p o t e n t i a l at the measurement s i t e . The s h i e l d i n g e f f e c t , and hence the t h r e s h o l d , i n c r e a s e s with i n c r e a s i n g matrix i o n c o n c e n t r a t i o n . Above t h r e s h o l d , e l e c t r o n s generated by enhanced i o n i z a t i o n are d r i v e n by the e l e c t r i c f i e l d toward the burner head with a v e l o c i t y p r o p o r t i o n a l to the a c t u a l e l e c t r i c f i e l d . E l e c t r o n s which reach the burner head before recombining w i t h a p o s i t i v e ion provide the LEI s i g n a l . Table I I I compares the r e l a t i v e degree of i n t e r f e r e n c e f o r matrices of v a r i o u s i o n i z a t i o n p o t e n t i a l s . Only the most e a s i l y i o n i z e d elements are seen to provide a problem. The e l e c t r i c a l i n t e r f e r e n c e s are subject to m o d i f i c a t i o n by instrumentation design and parameter o p t i m i z a t i o n , and an a c t i v e program to minimize i n t e r f e r e n c e s i s underway. Laser enhanced i o n i z a t i o n may be seen to be a s e n s i t i v e and s e l e c t i v e method. Although p r e s e n t l y subject to unique matrix i n t e r f e r e n c e s , these are subject to f u r t h e r instrument development or sample pre-treatment. The method i s " b l i n d " to such common o p t i c a l i n t e r f e r e n c e sources as flame background emission, ambient l i g h t , and s c a t t e r e d e x c i t a t i o n l i g h t . Because of the i n s e n s i t i v i t y to s c a t t e r e d l a s e r l i g h t , the a b i l i t y of l a s e r s to s a t u r a t e o p t i c a l t r a n s i t i o n s may be u t i l i z e d to the f u l l e s t advantage. F i n a l l y , the v a s t l y modified c r i t e r i a f o r s p e c t r a l s e n s i t i v i t y gives the method multi-element p o t e n t i a l , and the c a p a b i l i t y of avoiding t r a d i t i o n a l s p e c t r a l i n t e r f e r e n c e s . Implementation of LEI spectrometry r e q u i r e s minimal m o d i f i c a t i o n of a l a s e r induced f l u o r e s c e n c e (LIF) spectrometer. Though


6.

TRAVIS


101

l e s s thoroughly developed than L I F a t t h i s time, LEI represents a complementary measurement which may be made simultaneously, i f d e s i r e d . Comparative LEI and LIF measurements w i l l o b v i o u s l y be r e q u i r e d f o r a wide v a r i e t y of samples and flames to a c c u r a t e l y e s t a b l i s h the dominant r o l e s o f the complementary methods of l a s e r e x c i t e d flame spectrometry.

Literature Cited 1. 2. 3.

4. 5.

6.

7. 8. 9.

Green, R. Β., R. A. K e l l e r , P. K. Schenck, J. C. T r a v i s , and G. G. Luther, J . Am. Chem. Soc. 98, 8517 (1976). Turk, G. C., J. C. T r a v i s , J . R. DeVoe, and T. C. O'Haver, A n a l . Chem. 50, 817 (1978). In order to render the p l o t s element-independent, the ratio of ion-to-atom p a r t i t i o n f u n c t i o n s has been s e t equal to u n i t y . The e r r o r in degree of i o n i z a t i o n should be l e s s than one order of magnitude f o r any given element. The n a t u r a l e l e c t r o n background p o p u l a t i o n o f the flame is assumed t o be negligible. See P. J. W. Boumans, Theory of Spectrochemical E x c i t a t i o n , H i l g e r and Watts, London (1966), p 161, for a more complete d i s c u s s i o n . T r a v i s , J . C., P. K. Schenck, and G. C. Turk, i n p r e p a r a t i o n . W i l l i s , J . B., in CRC Handbook of Spectroscopy, Volume I , J . W. Robinson, ed., CRC P r e s s , Cleveland (1974) p 799. Values from Table 12, p 814, w i t h c o r r e c t i o n s from Table 13 f o r Na and K. For a b r i e f d i s c u s s i o n , and p e r t i n e n t Tables, see M. L. Parsons, B. W. Smith, and G. E. Bentley, Handbook of Flame Spectroscopy, Plenum Press, NY (1975), p 61. See, for instance, R. J. L o v e t t , D. L. Welch, and M. L. Parsons, Appl. Spectrosc. 29, 470 (1975). Schenck, Peter K., W. Gary M a l l a r d , John C. T r a v i s , and Kermit C. Smyth, submitted f o r p u b l i c a t i o n . Weeks, S. J., H. Haraguchi, and J. D. Winefordner, A n a l . Chem. 50, 360 (1978).

RECEIVED August 7, 1978.


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