NAT'L INST. OF STAND & TECH 1 83-2668

i AlllDb EtiDMTb jr Enhanced Ionization in

Flames

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center fcr Analytical Chemistry

Washington, DC 20234

Fecruary 1983

Final Report

Issued March 1933

|

J Preparea for: Office of Energy, Minerals, and industry

Office of Research and Development

U.S. Environmental Protection Agency

Washington, DC 20460

NBSIR 83-2668

i * i LASER ENHANCED INOIZATION IN

FLAMES

nation

A

i BvnrAn °r STANDAiS

Lru RAry APR * 7983

J. R. Devoe, J. C. Travis, G. C. Turk, and S. J. Weeks

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

Center for Analytical Chemistry

Washington, DC 20234

February 1 983

Final Report

Issued March 1 983

Prepared for: Office of Energy, Minerals, and Industry

Office of Research and Development

U.S. Environmental Protection Agency

Washington, DC 20460

U.S. DEPARTMENT OF COMMERCE, Malcolm Baldrige, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director
.
DISCLAIMER

In order to describe materials and experimental procedures adequately,

it has been necessary to identify commercial products by manufacturer's name

or label. In no instance does such identification imply endorsement by the

National Bureau of Standards nor does it imply that the particular product

or equipment is necessarily the best available for that purpose.

11
ABSTRACT

This report describes the discovery of laser enhanced ionization in

flames along with a description of research on the mechanisms of signal

production and collection. Early experiments on the method’s use for complex

sample analysis are described. Results of this research predict that two

photon-two wavelength laser enhanced ionization will produce detection limits

at the 10 parts per trillion level in solution for most elements. Laser

excitation provides high selectivity so that the method can be predicted to

have high accuracy.

Key words: Laser enhanced ionization; laser spectroscopy, optogalvanic

effect; spectroscopic analysis.
CONTENTS

Abstract iii

Figures v

Tables vi

1. Introduction 1

2. Conclusions 2

3. Recommendations 3

4. Description of the Experimental System 4

5. Preliminary Analytical Results 6

6. LEI Signal Production in Flames 12

7. LEI Signal Collection in Flames 21

8. Use of CW Laser for LEI 26

9. The Future of the LEI Technique 28

References 29

IV
FIGURES

Number Page

1 Experimental configuration 5

2 Calibration curves for elements in water 7

3 LEI spectrum of Ni-based alloy near the In analysis line

(4) 10

4 Processes considered in the theory of LEI signal

production 13

5 Comparison of experimental LEI sensitivities to the

theoretical sensitivity for twenty-one transitions in

fourteen elements (transitions are not identified) 16

6 Partial energy level diagram for Li and detection limits

for resonance (670.8 nm) , nonresonance (610.4 nm) and two

photon transitions 639.3 nm. Limits of detection are

0.001, 0.012, and 0.4 ng/mL 17

7 Stepwise excitation in LEI 19

8 Stepwise excitation of copper with partial energy diagram. . 20

9 Potential (upper) and field (lower) in a flame subjected to an applied potential for three values of the volume ionization rate. Dashed lines indicate effect without a flame . . 22

10 Percent recovery of 10 pg/mL In signal at 303.9 nm vs. sodium concentration using a tungsten plate cathode at

-1000 V and -1500 V 24

11 Potential and field behavior in a flame for four applied

potentials 25

v
TABLES

Number Page

1 Comparison of Detection Limits 8

2 Real Sample Measurements 9

3 LEI Detection Limits (ng/mL) for ^-pumped and Flashlamppumped Dye Lasers 18

4 Limits of Detection for LEI, Comparison of CW and

Flashlamp Pumped Dye Lasers 27

vi

SECTION 1

INTRODUCTION

In August 1976 our group was exploring the concentration of atomic

species produced in a commercial hollow cathode lamp by observation of laser

excited optical fluorescence. Dramatic changes in the voltage of and the

current drawn by the cathode were observed when the laser was tuned into

resonance with electronic transitions of the species being observed. Transitions producing signals of positive polarity as well as transitions producing

signals of negative polarity were observed. The magnitude of the effect was

such that transitions of low probability or transitions of high probability

but low numbers of atoms were easily observed (1,2).

An attempt was made to observe a similar effect in the flame source of an atomic absorption spectrometer. At the end of the first day of experimentation it was possible to detect a signal from sodium at a concentration

of 10 9 g/mL in a solution aspirated into the flame. Though the effect was initially not understood, subsequent studies have shown that the technique is

generally applicable to many elements and that in the flame, the signal is

produced by photon energy augmentation of a collisional ionization process;

hence the name laser enhanced ionization.

This report highlights the studies that have been made on this technique,

describes its advantages and disadvantages, and indicates its future direction in our laboratory.
SECTION 2

CONCLUSIONS

Laser enhanced ionization (LEI) in flames is a new quantitative analytical spectroscopic method with a number of features that can result in major

improvements in accuracy and detectability when compared to existing methods.

The basic principle of the use of photons to produce ions selectively has

very broad applicability in both inorganic and organic chemical analysis.

LEI in flames has been demonstrated to be successful in analyzing practical samples. The major effort at this stage of the technique’s development

has been to understand the basic mechanisms of signal collection and production. The signal production mechanism is based upon collisional excitation

and ionization accompanied by photon excitation. The theory has been

sufficiently developed to allow prediction of experimental detection limits

for elemental analysis. Significantly more atomic transitions can be used

than in the case of other spectroscopic analyses.

The theory of signal collection has progressed to the point of explaining the phenomenon in terms of the steady state properties of the flame.

Alkali metal interference can be reduced in magnitude by producing the lasergenerated signal inside of the ion sheath near the cathode. When the signal

is produced by ion collection, electrical saturation, even in the CW laser

excitation regime, can be observed. This is responsible for the unexpectedly

high sensitivity relative to the higher absolute sensitivity of pulsed lasers.

A number of "spin-offs" from this research have been identified in the area

of flame property measurements such as diffusion coefficients of constituents

and flame gas velocity.

Current capabilities indicate part-per-trillion detection limits with

minimal interference effects limited primarily by the errors associated with

the flame-atom reservoir.

2
SECTION 3

RECOMMENDATIONS

The laser enhanced ionization method is very new and continued efforts

should be put into research on the technique. The following areas should be

thoroughly studied before a quantitative technique is established that can be

used to analyze a variety of samples with high accuracy:

1. Development of multiphoton stepwise excitation will result in low

detection limits for high ionization potential elements as well as providing for higher accuracy through improved selectivity.

2. Modified design of electrodes and optimization of geometry can improve

signal collection efficiency as well as reduce interference effects from

alkali metals. Improvement of signal-to-noise ratio should be possible.

3. Selection of appropriate laser systems as they become available can improve the ease of use and reliability. Considerable improvement can be made in reproducibility of intensity by using enough power to saturate

the transition. Efforts can begin to provide for sequential multielement analysis.

4. Once these areas have been thoroughly investigated, a quantitative analytical method can be developed and subjected to ruggedness testing and,

eventually, round robin tests on environmental samples.

3
SECTION 4

DESCRIPTION OF THE EXPERIMENTAL SYSTEM

Application of LEI to trace element analysis has been confined totally

so far to atmospheric flames. The "analytical burner" is a highly developed

and characterized atom reservoir, available commercially from a number of sources. Samples in liquid form are introduced as a fine spray into a laminar pre-mixed flame, in which atomization of the sample occurs. Such a burner with external electrodes bracketing the flame for the ionization measurement is illustrated in Figure 1. The burner is modified by

insulating the burner head from the burner body, so that the burner head may

also be used as an electrode. The added plates are wired in parallel and are held at high (^1000 V), negative potential, thus acting as a split cathode.

The burner head becomes the anode, from which the background current and

signal are drawn. The pulsed (or chopped) current component resulting from

the pulsed or chopped cw laser excitation is ac-coupled through a capacitor

into a pulse amplification and signal averaging system. The laser beam is

directed into the flame and may be conveniently adjusted with a beam

director

Cathode plates are recommended instead of rods for reasons which will be

explained later. Also, insertion of uncooled electrodes into the flame is

not recommended for analytical purposes because of rapid minutes-to-hours)

degradation of the probes and consequent drift in the measurement sensitivity.

For a C 2 H2 /air flame supported on a 5 cm slot burner. Mo or W plates separated

by ^ 1.0 - 1.5 cm and centered just outside of the width of the flow last

indef initely

The pre-amplifier design has been described (5) . Although signals

have been obtained with CW (2) , f lashlamp-pumped (3-7) , and N2 laser-pumped

dye lasers, the flashlamp system producing a 10 °s pulse seems to produce the

best sensitivity in atmospheric pressure flames. This is probably due to the ^ 10°s 1 collisional ionization rates of excited states normally populated by

single photon LEI. The bandwidth of the laser is nominally less than the

atomic linewidths in the flame.

For flashlamp dye laser excitation, signals are generally averaged at

^15 Hz in a boxcar averager with an effective one second time constant, and

the averaged signal is recorded on a strip chart recorder. The laser wavelength may be scanned linearly with time to give a spectral profile on the

recorder

4
5

Figure

1.

Experimental

configuration.
SECTION 5

PRELIMINARY ANALYTICAL RESULTS

Much of the data collected to date with the LEI system must be considered

preliminary in the sense that over the past two years a number of new concepts

have been discovered about the basic mechanisms governing the operation of

the technique. As a result, major changes in the technology have placed the

sample measuring aspect in a constant state of change. Nevertheless, it is

possible to describe generally the capabilities of the method and to

indicate its potential once the major characteristics of the system have

been fully analyzed.

DYNAMIC RANGE AND DETECTION LIMITS

The dependence of instrument response on concentration has been determined for neat solutions of 18 elements (using 28 transitions) of which a few are plotted in figure 2. These relationships indicate a linear dynamic range

of four to five orders of magnitude, and a widely varying limit of detection

which may be correlated with the ionization potential of the element. When

compared with other flame techniques, the limit of detection (Table 1) may be seen to vary from orders of magnitude less sensitive (e.g., Cu)

,

to orders of

magnitude more sensitive (e.g., Li, In), depending on the ionization potential

of the element. Detection limits are reported as that concentration of

analyte which yields a signal amplitude three times greater than the noise

amplitude observed when the blank solution is aspirated. A more rigorous way

of reporting detection limits has been given by Currie (14) , but the purpose

of Table 1 is to show comparisons with other work not all of which provide

sufficient data for the more rigorous analysis. Moreover, the detection

limits obtained in neat solutions cannot be expected to be the same for more

complex samples. The data show only approximate and relative trends.

The initial instrumental system that has been used to date is capable of

making reasonably accurate measurements on those few samples which have been

studied. Table 2 illustrates three different types of samples. The measurement of indium in nickel-based alloy, which is a particularly difficult

analysis by most other methods, can be made directly using standards of

indium dissolved in water. Figure 3 shows a spectrum obtained by scanning

the laser. The nickel line at 303.79 nm has very low transition probability

and hence is comparable in intensity to the much less abundant indium. It can

be seen that the In line at 303.92 is well resolved from other potentially

interfering elements.

6
1VNDIS

7

i«wjxvGJLIMJJf\jUJ
TABLE 1. COMPARISON OF DETECTION LIMITS (ng/mL)

a

.

Element LEI FAA

b

FAE

b

FAF

b

LIF

b

Ag 1 1 2 0.1 4

Ba 0.2 20 1 — 8

Bi 2 50 20000 5 3

Ca 0.1 1 0.1 20 0.08

Cr 2 2 2 5 1

Cu 100 1 0.1 0.5 1

Fe 2 4 5 8 30

Ga 0.07 50 10 10 0.9

In 0.006 30 0.4 100 0.2

K 1 3 0.05 — —

Li 0.001 1 0.02 — 0.5

Mg 0.1 0.1 5 0.1 0.2

Mn 0.3 0.8 1 1 0.4

Na 0.05 0.8 0.

0.1

Ni 8 5 20 3 2

Pb 0.6 10 100 10 13

Sn 6 20 100 50 —

T1 0.09 20 20 8 4

Values taken from references 4 and 10.

b Flame atonic absorption (FAA) , emission (FAE) , fluorescence (FAF)

and laser induced fluorescence (LIF) in flames.

8
TABLE 2. REAL SAMPLE MEASUREMENTS.

Element Wavelength Sample LEI

a Expected3

Comments

In 303.9 Ni-based Alloy 35 37 1

Mn 304.5 River Sediment 786 770 2

Mn 304.5 SRM 1261-Steel 6700 6600 3

yg/g. Errors approximately 5% relative standard deviation for a single

determination.

Comments

1. No matrix effect; calibration via neat standard solutions.

2. High background current; calibration via standard additions.

3. Moderate background current; matrix effect compensated by using

fuel-rich (high electron content) flame for both sample and

neat standard solutions.

9
120

Ni




©

-» 80

<

CD

CO

In

<

—i

ULI QC 40

0

303.5

Cr

& ©

© ©

©

©

©

©

©

©

©

©

©

©

s> ©

© ^

1

Cr

s>

jd<£> * ®®

© fib

L

303.7 303.9

WAVELENGTH (nm)

304.1

Figure 3. LEI spectrum of Ni-based alloy near the In analysis

line (4) .

The Standard Reference Material (SRM) 1261 steel was found to produce a slight electrical background; however, it was possible to obtain an accurate

signal with neat solution standards using a fuel rich flame. Systematic

errors occur when the ion-electron density changes appreciably*. The fuel

rich flame increases the ion-electron density to a point where the addition

of the steel sample produces no additional effect.

This subject is treated partially in section 7. Studies subsequent to the

period of this report have resulted in electrode designs that eliminate this

type of problem.

10
The river sediment, which produced a rather high background, was accurately

analyzed by combining the fuel rich flame composition with a standard

additions procedure.

Although many other types of samples need to be analyzed to evaluate the

method, there has been sufficient encouragement from the results already

obtained to warrant an in-depth study. None of the parameters of the method

have been optimized. In fact, on intuitive grounds it seems that the shape

and relative position of the electrodes cannot be considered very close to

optimum. The best way to arrive at an optimum configuration is to study the

basic mechanisms of the method.

The electrical current pulse measured by the detection electronics

results from 1) the production of an excess (above the thermal equilibrium

value) of ions and electrons in the flame volume illuminated by the laser,

and 2) the transport of excess charges through the flame to the electrodes.

These distinct aspects of signal generation are referred to hereafter as the

"production" and "collection" stages, respectively. Since only the combined

effect of both processes is actually observed, simplifying assumptions have

been invoked to separate the two for individual study. These assumptions

are stated at appropriate points in the following discussions of ion

production and collection.

11
SECTION 6

LEI SIGNAL PRODUCTION IN FLAMES

The detection limit data in Table 1 show a wide variation among the

elements studied. We have observed that the detection limit is correlated

with the ionization potential of the elements. In the flame there is an ensemble of energetic species, mostly nitrogen, in which energy is transferred by collisional processes. One can calculate the probability of

ionization of species in the flame. The energy of the 2500 - 3000 K flame

proves to be sufficient to ionize only elements with low ionization potential

such as the alkali metals. Thus, the rate of ionization from collisional

processes in the flame is low due to the low number density of collision

partners with sufficient energy to cause ionization. However, with photon

absorption adding energy to the atom, it can be excited to an energy level

above its ionization potential.

A diagrammatic representation of the processes considered to be important

to the production of charged species in the flame is given in figure 4. The

case of a resonance transition is considered for simplicity. The rate

constants for the pertinent optical processes of absorption (E^ B 12 )

,

spontaneous emission (A21 ), and stimulated emission (E B 21 ) involve the

V

Einstein coefficients (A and B) , and the spectral irradiance of the laser

(E^) . The only collisional processes considered are collisional de-excitation

(k2 i) and collisional ionization from the excited state (k

? ^). Additional

collisional excitation and ionization processes are considered to be of

trivial magnitude, and ion-electron recombination is considered to be

negligible.

Assuming that the remaining processes are fast with respect to the laser

pulse length, the steady state population of the excited state is given by

B 12 \

(nT

- n

i

) (B

12 + B21) E

v + A21 + k

21 ( 1)

where n-, , n^ and n^ are the number densities (cm 3

) of excited state analyte

atoms, total (neutral and ionized) analyte atoms, and ionized analyte atoms,

respectively. The rate of ion production (n.) from the excited state, E

2

, is

then

12
ENERGY

POPULATION

STATISTICAL WEIGHT 03

cm

03 03

CM

C/D

-± UJ < C/D ^

C/D

C/D

i LU

< 00

S 00

z. uj

i— o

a. o

13

Figure

4.

Processes

considered

in

the

theory

of

LEI

signal

production.
n

i ( 2 ) " k

2i

n

2 * k

2i B12

E

v (n

T

~ n

l

)

^ B

12 + B

21 > E

v + A

21 + k21

Note that the rate of production of n^ decreases as increases. As the

number of neutral states (n^,-n^) decreases the rate of production of n^ must

d^cr^ase. This can lead to saturation of the ionization process. To evaluate

equation 2 analytically, one needs to know the functional relationship of

E with time, E (t) . To simplify the analysis, it is assumed that

( B

2 + B 2 i)E << (A21 + k2 i), that is, optical saturation does not occur.

The time dependent value

and substituting for N^-n^ in

of n^ is determined by integrating equation (2)

eq. 2 yielding

n. = —

1 A

: 2iB

12 "T

21+ k

21

E^(t) exp

k

2i

B 12 f E (t')dt'

A

21+ k

21 JO

(3)

The maximum value for the ionization rate is obtained by setting the

P x P derivative of equation (3) to zero. By assuming E^(t) = E (t/f ) where E^

represents the peak spectral irradiance of the laser at x , an empirical

expression for the (n.) is of the form:

1 max

[n.] = C n

1 max 1 T

k

2i

B

L2

L

A

21 + k

21

P 1

(A)

where Ci and C 2 are constants related to the pulse shape, where C 2 is less

than or equal to unity, 0 <_ t <_ x^, 0 <_ x <_ 1. The nonlinearity of ionization rate with laser irradiance, absorption coefficient, and ionization

coefficient (for C 2 < 1) reflects the partial conversion of neutrals to ions

before the laser peaks in power, and results directly from the appearance of

n_^ on the right hand side of equation (2) . For greater detail consult

reference 11.

The ionization rate constant k_ . may be shown to have the form

2i

k

2i

= k

0 6Xp ^“ (Ei “ E

2 )/kT ^ (5)

where E_^ and E 2 are the ionization potential and excited state energy, k is

the Boltzmann constant, and kg is a function of temperature (T) (7,12), the

masses of the analyte atom and the collision partner (probably N2 in our flame), the number density of the collision partner, and a collision cross

section. For present purposes, the variability of kq from one element to the

14
next is neglected as being much less important than the variability of the

exponential term in equation (5). For a 2500 K flame, kT = 1738 cm 1

, and

each eV (1 eV ~ 8,000 cm 1

) of optical energy increase of the atom may be seen from equation (4) to increase the ionization rate constant by a factor of

exp [8000/1738] 2 10 2

. Photons from a doubled dye laser range from about

2 to 5 eV, for implied ionization enhancements of 4 to 10 orders of magnitude.

Of course, saturation effects may place a limit to this range.

If a number of simplifying assumptions are made (11) , expected signal

levels for a number of transitions for the elements can be calculated.

Figure 5 is a correlation plot of the calculated vs. experimental signal.

Although a good deal of scatter is observed, the correlation is clearly

present

For a given photon energy one can see that those elements with the lower

E_^ have a larger rate constant, or conversely, if we can add energy via photon

absorption, the rate constant can be increased exponentially at 10 2 /eV of

difference in E.-Eo.

l

^

An interesting example solidifies these points and provides some insight

into the method. Inspection of the partial energy level diagram for lithium

(figure 6) indicates that the rate constant for ionization by thermal excitation (dashed lines; solid line is photoexcitation) is exp[ (31,283 - 14,904)/

1738] ^1.2 x 10 4 times larger from the 2 D level than from the 2 P° level.

However, the population of the 2 P° state by thermal excitation is

exp(-14, 904/1738) 'v 2 x 10 4 that produced by photoexcitation assuming

comparable cross sections for photoexcitation. This means that the detection

limits by either excitation process should be comparable which is experimentally proven to be so within a factor of 10.

The impact of the above for chemical analysis is that, for many elements,

infrequently used optical transitions will produce large signals in LEI.

This means that there will be a few more lines that can cause interferences.

On the other hand it may allow the analytical spectroscopist to use alternate

lines more conveniently than with other spectroscopies when an overlap of an interfering line with the normally used line occurs.

A more attractive possibility exists in the use of two-photon excitation.

If one uses one wavelength (such as 639.3 run for lithium in figure 6), two

photon excitation must couple with a virtual state and suffer the loss in

cross-section for this event to occur. On the other hand if two different

wavelengths are used and each wavelength excites to a real level, two benefits

are gained. First the selectivity of the photoexcitation process increases

by the product of the selectivity factors for each wavelength. Second, the cross sections are much higher than the virtual level two-photon excitation.

Two UV photons — from a suitably high-powered frequency-doubled laser — could

be used to excite energy levels up to ^10 eV above the ground state. The

effective single-photon wavelength of such transitions is in the vacuum UV.

Therefore, one can avoid the usual experimental difficulties and still excite

very high energy levels.

15
mVITY

(nA/|:pb)

r> TED :

9 O ST!W ; _ i i • J i 'Y (nA/ppb)

Figure 5. Comparison of experimental LEI sensitivities to the two-parameter

theoretical fit value for twenty-one transitions in fourteen

elements. The dashed lines are set at a factor of 10 away from

the center line. Different values for the same element are for

different optical transitions.

16
Ionization

Potential

17

Figure

6.

Partial

energy

level

diagram

for

Li

and

detection

limits

for

resonance

(670.8

nm),

nonresonance

(610. A

nm)

and

two

photon

transitions

639.3

nm.

Limits

of

detection

are

0.001,

0.012,

and

0.4

ng/mL.
Using instrumentation made available at the NBS Center for Fire Research,

the first LEI experiments using multiple wavelength, stepwise photoexcitation

were performed. A single nitrogen laser was used to pump two tunable dye

lasers. The two dye laser beams were directed colinearly through a pre-mixed

air-H2 flame, and LEI was detected in the usual manner (9)

Figure 7 summarizes the results obtained using both single and dual

wavelength N2 pumped dye laser excitation for several elements. The sensitivity enhancement factors resulting from the second step of laser excitation

range from 1 to 3 orders of magnitude. A variety of both instrumental and

fundamental parameters determine the degree of enhancement achieved including:

the power of each laser; the degree of optical and electrical saturation

involved for each transition; the photon energies; the ionization potential;

and the transition strengths of each line.

However, as Table 3 shows, despite the sensitivity enhancements, the

detection limits obtained are, with the exception of sodium, inferior to those

reported earlier for LEI using single photon LEI with a flashlamp pumped dye

laser

TABLE 3. LEI DETECTION LIMITS (ng/mL) FOR N 2 -PUMPED AND FLASHLAMP -PUMPED

DYE LASERS

Element

N2 Laser

3

Single Wavelength Dual Wavelength Flashlamp

Ca 30,000 100 0.1

Cu V H* O

cn 500 100

Fe 2,000 100 2 In 100 0.6 0.006

Li 4 0.04 0.001

Mn 30 5 0.3

Na 6 0.04 0.05

Taken from reference (9)

This observation is consistent with preliminary studies on the time

dependence of signal production and collection. Production suffers when the

characteristic excited state ionization time, 1/k,, , is much longer than the

pulse length. Also, the obtainable peak current suffers when the time

required to extract an ion from the flame is longer than the laser pulse, as

is discussed in more detail in the next section.

18
STEPWISE

EXCITATION

LEI

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CM

CD

CD

CO

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CD

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CD

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CD

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COA

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CD CD A ”

CT> COLn

CN

CM

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go

a

a: g>

19

Figure

7.

Stepwise

excitation

in

LEI.
An attempt was made to use two flashlamp pumped dye lasers which produce

a 1 ys pulse. Figure 8 depicts a partial energy diagram of copper. Radiation

at A] was produced by a Chromatix Corp. model CMX-4* flashlamp pumped dye

laser while radiation at A 2 was produced by a Phase-R Corp. model DL-1400*

flashlamp pumped dye laser. Both lasers were fired synchronously. The

sensitivity (slope of the calibration curve) of the technique is about 10 2 times greater when the stepwise excitation is done than when only one wavelength is used. This improvement did not carry over to a similar improvement

in detection limit because of an inordinate amount of radiofrequency

interference. This reduced the improvement in limit of detection to a factor

of 25.

Copper

Figure 8. Stepwise excitation of copper with partial energy diagram.

See disclaimer on page ii.

20
SECTION 7

LEI SIGNAL COLLECTION IN FLAMES

To begin the description of the signal collection process, it is important to describe the properties of a flame when a potential is applied

between electrodes within or in close contact with the flame. The response

of flames to external fields has been extensively studied and documented by

Lawton and Weinberg (12). At the instant a field is applied to a flame,

electrons and positive ions begin to move toward opposite electrodes with

different drift velocities (V) , given by

V = -y E

e e V. = y .E 1 l (5)

where E is the electric field, y the species "mobility", and the subscripts

distinguish electrons (e) and positive ions (i) . The mobility depends

inversely on mass, and is thus much greater for electrons than positive ions.

Since electrons and ions are generated at the same rate in the flames, the

greater electron extraction rate results in a build-up of net positive charge

in the flame. This charge is localized around the cathode, and continues to

build up until the negative potential of the cathode is effectively neutralized

beyond the net positive charge cloud. The net positive charge region around

the cathode is called the "sheath" or "cathode fall". The latter designation

refers to the fact that virtually the entire potential difference between the

cathode and anode is "dropped" across the sheath.

Figure 9 illustrates the behavior of the potential and the field in

three idealized flames of uniform temperature and composition between plane

parallel electrodes, with a "sub-saturation" negative potential applied to

the cathode (12). The term saturation refers to the condition where there is no field free region between the electrodes. The dashed lines represent the

behavior of the potential (upper line) and field (lower line) in the absence

of a flame. The solid lines show the effect on the potential and field of

three flames which differ in volume ionization rate, r_^ (ions produced per

cm* per second) . This rate is a function of the temperature and composition

of the flame as well as aspirated ionizable species such as K, Na, etc. The

sheath is the region of non-zero field and non-constant potential, i.e., to

the right of the vertical line. Between the anode and the edge of the

sheath, no field exists, and the potential takes on the value of the anode

potential (zero, for a grounded anode). Therefore, a laser beam placed in

the region to the left of the boundary (using condition B) in figure 9 will

21
?2

Distance

(cm)

Figure

9.

Potential

(upper)

and

field

(lower)

in

a

flame

subjected

to

an

applied

potential

for

three

values

of

the

volume

ionization

rate.

Dashed

lines

indicate

effect

without

a

flame.
produce ions and electrons, but no signal will be observed at the electrodes.

Furthermore, for a given laser beam position between the electrodes which

currently gives a signal, an increase in r , for example caused by an increased concentration of an alkali metal, may cause a decrease in signal

intensity. This is observed experimentally. Figure 10 shows a substantial

decrease in signal above 200 pg/mL matrix sodium concentration. The reader

will note a considerable enhancement in signal at intermediate concentrations

of sodium. The cause of this has not been completely verified, but current

data suggest that it results from the position in the flame that the signal

is generated which is consistent with the foregoing discussion. For example,

the position marked E in figure 9 suggests a maximum in the curve as shown in

figure 10. A number of experiments have been performed in an attempt to

reduce this alkali metals interference effect. Major improvements have been

realized by making it possible for the laser active volume to exist within

the sheath. Electrodes in the shape of rods cause the LEI signal to be more

affected than parallel plates. This is because the field gradient is larger

which produces a narrower sheath or region where the laser signal can be

collected than with flat plates. A recent improvement has been made by

placing a water-cooled electrode in the flame. Close proximity of the laser

beam to the electrode allows the laser signal to be produced within the small

sheath that is generated at high alkali metal concentrations [approx. 5 mg/mL

(13)]. Increasing the voltage between the plates up to breakdown in the

plasma also helps by increasing the size of the sheath. This can be seen in

figure 11 which shows calculations of field and potential for different

voltages.

A number of diagnostic techniques have been developed that provide data

on a number of the basic processes in the LEI signal production and collection. For example, it has been found possible to measure an image of the

laser-produced ion cluster which generates a signal as it is collected on the

cathode (13) . From this and similar techniques it may be possible to measure

a number of electromigration properties of species in a flame, such as diffusion coefficients and velocities of gaseous species. It has even been

possible to observe LEI of certain molecular species in the flame (8)

.

23
PERCENT

RECOVERY

0 30 60 90 120 150 180 210 240 270

SODIUM CONCENTRATION (|4g/mL)

Figure 10. Percent recovery of 10 pg/mL In signal at 303.9 nm vs. sodium concentration using a tungsten plate cathode at

-1000 V and -1500 V.

24
Field

(V/

cm)

Potential

(V) E0Oc

03

<-* 00 a MM Q

25 l

Figure

11.

Potential

and

field

behavior

ina

flame

for

four

applied

potentials.
SECTION 8

USE OF CW LASER FOR LEI

The initial experiment on LEI four years ago was done using a continuous

wave (CW) argon- ion-laser-pumped , dye laser. Such lasers possess a number

of attractive features when compared to pulsed laser systems. Among these

are: high average power, high stability of both wavelength and intensity,

laser signal processing requiring less sophisticated electronics, and the

ability to modulate the wavelength. However, the use of these lasers has

been limited by the relatively narrow wavelength range over which they have

operated. With a 5 W argon ion laser system only Na and Ba could be measured,

and for this reason the majority of LEI research has been done using pulsed

laser systems. In addition, the CW laser provides an equilibrium condition

not possible with pulsed lasers (except on ys time frames) . This enhances

comparison of theory with experiment and has proven to be most useful in

developing the diagnostic technique mentioned in the previous section.

With the goal in mind of obtaining wavelength coverage from 407 to

750 nm, a 6 W, krypton-ion-laser-pumped, dye- laser system was assembled. The

laser beam is mechanically chopped and the signal is detected using a lock-in

amplifier. Excellent linear dynamic range has been demonstrated for three

elements. Detection limits have been measured for seven elements and are presented in Table 4 along with corresponding detection limits for LEI using

the flashlamp pulsed dye laser. With the exception of Na, the LEI detection

limits obtained with the CW laser do not compare favorably with those for the

flashlamp laser. This may be due in large part to the lower photon energies

(higher wavelengths) being utilized for the CW measurements. In the only

case where the same wavelength has been used for both lasers, Na at 589.0 nm,

the CW detection limit (0.025 ng/mL) is better by a factor of four than that

of the flashlamp laser. Since the peak power of the flashlamp laser is 10°

times the average power of the CW laser, one might wonder how the limits of

detection can be comparable. Recent data have shown that with the CW laser,

the relatively long time available for both signal production and collection

makes it possible to observe saturation of LEI at the low power of the CW

laser (13)

.

The CW LEI method has much merit for chemical analysis. The wavelength

and intensity can be kept more stable than pulsed lasers. Both amplitude and

frequency modulation can be used to enhance the signal to noise ratio and

reduce the magnitude of the background.

26
TABLE 4. LIMITS OF DETECTION FOR LEI. COMPARISON OF CW AND FLASHLAMP

PUMPED DYE LASER.

Element CW

ng/mL (wavelength)

nm

Flashlamp

ng/mL (wavelength) nm

Ca 1 (422.7) 0.1 (300.7)

Ga 300 (417.2) 0.07 (287.4)

In 0.1 (451.1) 0.006 (303.9)

Li 20 (460.3) 0.001 (670.8)

Rb 16 (420.2)

Na 0.025 (589.0) 0.1 (589.0)

Sr 4 (460.3)

27
SECTION 9

THE FUTURE OF THE LEI TECHNIQUE

The LEI technique has the potential for the detection and quantification

of most of the elements at or below the part-per-trillion level with high

selectivity and accuracy. The technique already appears to be a substantial

improvement over competing techniques, and therefore, more research should be

conducted

Research on the signal production and collection processes will result

in further improvements in the design of the system. The use of two photon

excitation where each photon populates a resonance energy level in the atom

can greatly enhance the sensitivity and selectivity of the technique. While

the flame is the best medium for LEI at the present time, future research on atom reservoirs such as vaporization of solids into a gaseous discharge may

be of interest for LEI.

Significant insight has been obtained on the basic processes of the LEI

technique. After several of the above mentioned areas have been studied, a number of developments need to be made to result in a quantitative analytical

instrument. For example, multiple element capability might be investigated by

using jump tuning of the laser system. In addition, the technique needs to be

evaluated for sources of error including ruggedness, reproducibility, and

accuracy

To date, our studies indicate that LEI is an important analytical

technique because of its sensitivity and specificity for the analysis of

trace constituents.
REFERENCES

1. Green, R. B. , R. A. Keller, G. G. Luther, P. K. Schenck, and J. C. Travis

Appl. Phys. Lett., 29:727, 1976.

2. Green, R. B. , R. A. Keller, P. K. Schenck, J. C. Travis, and G. G. Luther

J. Am. Chem. Soc., 98:8517, 1976.

3. Turk, G. C. PhD Dissertation, University of Maryland, 1978.

4. Turk, G. C., J. C. Travis, J. R. DeVoe, and T. C. O'Haver. Anal. Chem.,

51:1890, 1979.

5. Turk, G. C., J. C. Travis, J. R. DeVoe, and T. C. O'Haver. Anal. Chem.,

50:817, 1978.

6. Van Dijk, C. A. PhD Dissertation, Rijksuniversiteit te Uttrecht,

Netherlands, 1978.

7. Travis, J. C., P. K. Schenck, G. C. Turk, and W. G. Mallard. Anal.

Chem., 51:1516, 1979.

8. Schenck, P. K., W. G. Mallard, J. C. Travis, and K. C. Smyth. J. Chem.

Phys., 69:5147, 1978.

9. Turk, G. C., W. G. Mallard, P. K. Schenck, and K. C. Smyth. Anal. Chem.,

51:2408-2410, 1979.

10. Weeks, S. J., H. Haraguchi, and J. D. Winefordner. Anal. Chem., 50:360,

1978.

11. Travis, J. C., P. K. Schenck, G. C. Turk, and W. G. Mallard. Anal.

Chem., 51:1516, 1979.

12. Lawton, J., F. J. Weinberg. Electrical Aspects of Combustion. Oxford

University Press, London, 1969, pp. 319-322.

13. Schenck, P. K. , J. C. Travis, G. C. Turk, and T. C. O'Haver, J. Phys.

Chem., 85:2547-57, 1981.

14. Currie, L. A. Anal. Chem., 40:586-593, 1968.

29

NBS-M4A 'REV. 2-FC) U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBS I R 83-2668

2. Performing Organ. Report No 3. Publication Date March 1983

4. TITLE AND SUBTITLE

Laser Enhanced IcDnization in Flames

5. AUTHOR(S)

J. R. DeVoe , J. C. Travis, G. C. Turk, S. J. Weeks

6. PERFORMING ORGANIZATION (If joint or other than NBS, see in struction s)

national bureau of standards

DEPARTMENT OF COMMERCE

WASHINGTON. D.C. 20234

7. Contract/Grant No.

S. Type of Report & Period Covered

9. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State, ZIP)

10. SUPPLEMENTARY NOTES

Q~l Document describes a computer program; SF-185. FlPS Software Summary, is attached.

11. ABSTRACT |A 200-word or less factual summary of most significant in formation. If document includes a significant

bi bhography or literature Survey, mention it h ere)

This report describes the discovery of laser enhanced ionization in flames

along with a description of research on the mechanisms of signal production and

collection. Early experiments on the method's use for complex sample analysis

are described. Results of this research predict that two photon-two wavelength

laser enhanced ionization will produce detection limits at uhe 10 parts per

trillion level in solution for most elements. Laser excitation provides high

selectivity so that the method can be predicted to have high accuracy.

12 . KEY WORDS . S i * tc twelve entries; clonabeticai oraer; capitalize only proper ncmes; ana seoa'Ote key

Laser enhanced ionization; laser spectroscopy, optogalvanic efrect;

spectroscopic analysis.

words by semicolon s)

13. AVAILABILITY

Uni i mi tea

RE?; For Official Distribution. Do Not Release to NTlS

r

; Order From Superintendent of Documents. U.S. Government printing Office. Washington, D.C.

‘ 20402.

Order From National Technical Information Service (N TiS) , Spnn gf i eld, VA. 4 2 1 6 1

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