Laser induced mass spectrometry: Ion formation processes and recent developments

International

Journal &Mass

Spectrometry

LASER

INDUCED

F. P, NOVAK,

MASS

SPECTROtlETRY:

K. BALASANFIUGAH,

D. MATTERN

and

Department

of Chemistry,

53 (1983) 135-149

and Ion Physics,

Elsevier Science Publishers B.V., Amsterdam

-

135

in The Netherlands

Printed

ION FORMATION

PROCESSES

K. VISWANADHAH,

AND

RECENT

DEVELOPMENTS*

Z. A. WILK,

C. D. PARKER,

D. M. HERCULES University

of Pittsburgh,

Pittsburgh,

PA

15260 (U.S.A.1

ABSTRACT A summary of the ion formation processes in laser mass spectrometry presented. Particular emphasis is placed on the process of solid state Recent results on the direct analysis of materials chemical ionization. LMS are also presented.

is

using

INTRODUCTION During prolific

the

growth.

ionization" field

last

This

solid

desorption

state

spectrometry labile

(LWS).

spectrometry

is in a large

part

due to development

Each

of samples

techniques

(PD),

have

permitted

it is suited,

of molecular

ones

being

(FAB),

requirements

laser mass

of thermally

by conventional for sample

solution,

and fragment

spectrometry

and

analysis

or its ability

weight

"soft

ion mass

to analysis

to be a universal

of the

prominent

bombardment

has its own

has proved

has experienced

secondary

atom

not amenable

samples,

to which

of the more

desorption of fast

technique

both characteristic

some

techniques,

These

and no one technique types

mass

technique

or nonvolatile

spectrometry.

decades

(FD), plasma

the related

(SIMS),

several

mass

preparation

either

to provide

in the ions that

ions characteristic

are

of

structure. Although at least energy

a decade

transfer,

Similarities be some energy may

these

molecular

spectra

steps

transfer

differ

or more,

from

among

related

to the nature

the similarities

*Dedicated

OOZO-7381/83/$03.00

ionization

the various

and

and

involved

Although

energy

chemical

of the there

the nature

levels,

This

and the differences

may

of the

to the sample into

which

reactions,

for

are known.

that

packets)

of this energy

its environment.

the techniques

the same

or another

suggest

(or photon

phonon)

form

of the mechanisms processes

the dissipation

(exciton,

in some

techniques

particles

ionization,

among

to Professor

understanding

and

of the sample

employing

available

in the mechanisms.

the techniques,

or macromolecular

laboratories

from

involved

in volatilization,

been

no clear

the impinginq

results both

have

volatilization,

among

common

techniques

in turn

is most could

likely

explain

among

technique.

R.D. MacFarlane

on the occasion

0 1983 Elsevier Science Publishers

B .V.

of his 50th birthday.

136 The tried

use of a laser

in the early

as an ionization

1960's.

The&instruments

and metals,

and for a variety

is referred

to a recent

mass LMS

spectrometry. for

Heraeus)

commercial

of inorganic was

soon

species

in our

determining

Eleuzelaar

(2) among

species.

spectrometer

mass

although

Later

very

used

practical.

others

The reader of laser

reported

that year,

developed

s,ystems, the

on the

use of

the development

of

by Leybold-

for microprobe

utility

were

on inorganics

development

(3) (the LAMEIA-500R

originally

in bTologica1

not

primarily

historical

laser

laboratory

the types

mechanisms

the

of sample

influence

that

formation

paper

to the area

concerned

to which

give

of the

we will

processes

been

rise

preparation

an understanding

In this

has

of samples

fragmentation

of LMS

were

the

experiments

analysis

for organic.analysis

realized.

Work

gaining

(1) for

is not new;

were

of organic

reported;

was

of reasons

review

In 1978,

the analysis

the first

source

the

of direct

analysis.

spectra,

of spectra

of our

we will

aspects

of LMS.:

is suited,

of a laser

a summary

in addition

technique

types

interaction

in LHS;

several

to the observed

on the

provide

with

observed,

with

a solid

observations

present

evaluating

determining and sample.

on ion

several

new

applications

EXPERIMENTAL LMS ments.

spectra These

summary,

were

instruments

a Nd:YAG

Q-switched

(ca.

laser

15 ns)

W./cm2).

The

TOF mass

spectrometer

energy and

stored

This

ions

spreads

500 and

laser

laser

is brought normal

and

can

were

at a 45"

in

the

instrument;

with

signal The

at which 1.

~3 urn). from

In this

translation

from

laser

In the

case

while must

either

the ions

thus

stage

Elsewhere, being

are

be thin

kinetic

between

and

the

spectra

the the

any

solid

of the sample

chamber

(70 x 50 x 50 mn).

section were

from

for

are again

essentially

can make

the the

extracted

enough

ions

SEM

on the sample.

in the LAMMA-1000,

analysis

similar.

into the for

of the LAMMA-500,

is the size

to in the direct

%106-10'

by a 17 stage

impinges

In

and

accelerated

p'rimary difference

the

the front

sample

density

is detected

instru-

(4,5).

nm)

,to compensate

configuration

restriction

the LAHIIA-1000.

and

In contrast,

the only

referred

the results

The

Samples

angle

(power

focused,

of the sample,

to the front.

the spectra

sample

elsewhere

(h = 265

an ion reflector

in Figure

(generally

be analyzed;

obtained

back

the

recorder_ angle

to the surface.

the displacement All

is the

in detail

quadrupled

extracted,

formed.

transient

on the

normal

to penetrate

sample

ions

described

onto

employs

schematically

laser

extracted

are

which

in the

is focused

the front,

been

on the LAJltlA-500 or LAFtlA-1000

is frequency

is focused

versions

is shown

either

have that

generated

in a fast

1000

obtained

of the paper

obtained

on either

137

ion

optlom

&f ‘F

Ion optloo

Fig.

1.

Schematic representation impingement path

TLC-LrlS spectra of the dyes solvent

The

All

samples

of laser mechanism and

on this

situation

emission

and

to some

topic

independent;

the

energy.

vs.

(Whatman)

by standard

as received

TLC

LAMMA-

laser

Separations

plates.

methods

without

pulse

observed. laser

dealt

(ref.

further

shape The

of the

with

a solid

6, p. 38,

purification.

book

organic

have

however,

is the

by Ready molecules found

appears

in

of systematic

of lasers

understood.

been

important

resulting

A majority

interaction

less well

laser

process

density

the

in a recent

is even

with

understood.

with

of a laser

species

power

of a laser

is not well have

can be found

of molecular

of spectra

photon

ionization

summary

extent

HP-K

performed

used

for the interaction

the type

were

from

LAMNA-500

ionization

investigations A good

were

of the interaction

volatilization metals.

obtained

(Chem Service)

7).

Mechanism

were

contrasting

(7).

with

The

resulting

in

The duration

to influence

the

to be wavelength

parameter,

not

the single

138 There observed

are

undoubtedly

spectra.

and define

One

several

processes

several

can use

regions

the very

that

occurring

simplistic

are important

that

model

to the

give

rise

depicted

laser

to the

in Figure

2

interaction

phenomenon.

Lorw

beom

Region of “gas-phase’ moctionr I4 1 Surface ionization I3 1

-

ionization Fig.

2.

Regions

Region sample. of ca.

1 is the area

characterized of atomic a plasma thermal can some

solid

of which The

last

particles

into

could

occur.

since

this

peak

the The

which

sort

even

laser

with

Effective This

is

region

the

temperatures a region

of the formation

to the extent

this

ions

is depicted

vacuum_ collision

observed

has

interaction

been

can

be classified

as

This

area

reactions.

must

where

occur

cross-sections

in the LOIS spectra.

are

collisions

is also

where

region can occur, a majority

formed.

formed

is the region

however,

This

phase.

where

This

of high

to as the selvedge,

mobile

are presumably

as a cloud

is a region

referred

yet

is a likely

in chemical

reactions

laser

of condensed

phase

significant

region

of the

region.

on a LTE model.

densities,

of direct

and gas

can result

is where

symmetries

power

region,

as some

of the molecularly

interaction

sense.

This

state

based

interaction

in this

fragmentation,

to the region

upon

occurs

proposed

At high

true

gradient.

be looked

between

been

species.

Adjacent

of the direct

by extensive

in the

to laser/sample

undoubtedly

have

ianitotion

by loser 4 I 1

of importance

Ionization 7000°K

Direct

(2)

from

the emission

ion-mol

reasonably favorable;

ecu1 ar

close also

of reactions

to the

surface,

to retain

the

139 Given

the

difference

This

regions. direct

result

to have result

above

one might

model,

in the time

domain

is shown

qualitatively

of the intepaction

essentially

the same

of secondary

expect

that

of the formation in Figure

of the

time

as the

from

Fig.

3.

Time

or from

ion-molecule

broadened expected These

dependence

to some

extent.

have

emission

taken

Neutrals

are emitted

of the

laser

Ionization can

the gain

been

several

gradient

*

on the other pulse

by Cdtter

ionization

hundred

are a

of high thermal

to later

the laser

demonstrated

of by post

for

emission

be expected

that

in LriS

be offset

time after

times

hand, would

be

had terminated,

(8,9) and

by an electron

microseconds

and be

after

the neutral

beam

(8).

the termination

pulse.

4 summarizes

be classified

the types as being

or loss of an electron,

ionization solid

indeed

advantage

should

Neutral

for some

emission

as a

processes

Figure

Ions

reactions,

to continue

features

of particle

Species

laser.

Time

be a

the various

the solid would

the region

I

would

Ions that are formed

3.

laser with

profile

either

processes,

there

of ions from

state

of salts,

formed

from

the gain

ion-attachment

"CI" reactions

of ionization

observed

one of the following

or loss of a proton,

reactions,

or ion-molecule

processes

or what

reactions.

we shall

in

LW.

processes: the direct refer

to as

140

-K+n’

M+B M+ntl-

mf++B-

n+c+ -rc* H + A- -

Fig.

Summary

4.

Direct production found

in conventional process

is observed electron

rarely

result

The

that

loss

cation.

however, of molecular

aromatic

positive

this

which

are

electron

is not

the

ion

a very

by this

process

The

situation

for

anion

is rarely

seen.

known

to form

affinities,

stable

molecular

anions

observed_

most

common

species

observed

in LMS are due

Acids

tend

to loose

a proton

while

expected

from

chemical

properties.

properties,

their

that

quasimolecular also

and

typical

cations

molecular

proton.

Note

is the

hydrocarbons.

hydrocarbons, have

of an electron

This

production

in the odd electron

aromatic and

in the

spectrometry;

for polynuclear

polynuclear

in LMS

molecular

EI mass

resulting

in solution

The

would

in LMS.

mainly

capture

for

anions are

photoionization

processes

of the odd electron

prominent

Even

of ionization

M’

the

is they

anion

form

(M-H)-,

fragmentation

both

Many

to the

tend

species

the quasimolecular

as shown which

bases

for

could

a substituted

be useful

gain

or loss

to gain

one,

exhibit

amphoteric

cation

(M+H)+

chromone

of a

as and

the

in Figure

in identification

and

5.

141 structural basic

The charge

in solution

third

process,

separation.

the cation

C+ and

in the negative clusters

The

result

in many

that

exhibit

one would

properties

ionization

is that

instances

do not exhibit these

of salts,

in the positive

clusters observe

appreciable on laser

is brought ion mode

corresponding the anion

acidic

about

by

one observes

to C+(C+A-),,

A- and

or

irradiation.

and

the corresponding

A-(C+A-).

Another

important

situation,

organic

moiety.

are otherwise

5.

may

the direct

ion mode

In this

Fig.

Compounds

elucidation.

behavior

process

inorganic This

that occurs ions

can be utilized

obtainable.

The

in LMS

by some mechanism

sample

Positive and negative ion LMS 4H-1-benzopyran-3-yl)propanoate

in many

"attach"

situations

can be mixed

spectra

is cation

with

or anion

attachment.

themselves

where

to an

no LOIS spectra

an inorganic

salt,

of 2-(Acetylamino)-3-(4-oxo-

in

142 expectation most

of

important

forming in

forming

have

most

useful .

been

studied, The the

of

cul ar , are

parti

the

attachment drawback

species

does

Ion-molecular lastest

addition

process

is

in

or to

reacts

with

by

particular

limited

number

However, are

the

there

currently

Sol id

As part

would

systems,

is

great

occur

negative

ion

derivatives

In

C3N-,

addition

(M+15)-

for

demonstrates The

this

LMS.

a species M,

of

when when

samples,

the

trace cause

in

type

of

are

The

solid

A+ is

forming

a new

We have

this

the

ionization formation

organic show

detail

ion

the

reaction,

as

for

a

and

next

CI which

MB+,

this

in

the state

generated

observed

of

processes of

are

a series

that

ectron

the of

in

LFlS,

odd-electron

molecules

odd-e1

behavior

the

was

these

section.

more

systems

we were

molecular contain

molecular

anions

electron

A class

anions.

ni trobenzene

derivatives;

dinitrobenzenes

species,

negative

molecular

observed,

o-dinitrobenzene,

and

the

base

an

additional

at

corresponding

the

ion

be

represented

were

LMS spectra

peak

substitution in

anion

nor

the

following

peak

generally intense

the

thus,

of the

related

quasimolecular species:

m/z to

reaction

substitution by

of

of = 183

an aromatic

(M+O-H)-.

is

and

most

initiated hydrogen

CN-,

was

to likely by

CNO-, to

observed

(M-NO}-. at

1 ,3,5-trinitro-

ni trobenzene

corresponds is

(M+l5)that

o-di

anion

corresponding peak

1,8-dinitronaphthalene, ion

The

results can

formation

reactions

in

be discussed

the

(Il-NO)- , with

and

species

and

studied.

this.

nucleophilic

the

most

ionization”

interest,

if

frequently

What

The

benzene.

the

observed

be misleading in

observed

of

neither

to

be

be disheartening

can

one:

in

EI-MS,

LMS behavior was

observed.

NO2-,

In

of

Surprisingly, were

study

LMS.

typical

species

to

ionization”

our

groups

compounds

only

study,

determining

in

withdrawing

potential

chloride

seems widely

ion

(10).

impurity

involved.

as wi 11

the

sodium

unintentionally.

conventional of

the

currently can

it

an

of

of sodium

not

more

that It

“chemical

species

of

of in

the

are

observed is

processes

reaction

“chemical

interested

species

one

halides,

are

also

is

instance, attachment

they

however,

which

solid-state

the

under

state

form;

sodium

to

defined

are

For the

cationization

predictable.

ionization

analogous

salts, sodium

reactions

not

a cationized

to

alkali

phenomenon

not

instance of

used

this

is

due

the

however,

reactions of

species

formation

Though

cationization

major

for

species. is

widely

(10);

Although

ion-attached

turn

!1+23.

most

utilized

ion-attached

the

attached reactions

mass

anion

impurities,

ion

cationization

a species

that

an

shown

in

Figure

6

(M+15)-. the the

result laser.

by oxygen;

the

of The

a process

resulting

143

Fig.

Negative

6.

ion

In contrast, benzene nitro

show

LMS spectrum

the negative

a negligible

groups

1,3,5_trinitrobenzenene high

at and

spectra

of meta

and para

dinitro-

some interaction

(M+O-H)-.

Thus,

ortho

substitution

although

(TNB)

nitrobenzene

is not

1,8_dinitronaphthalene

of the

essential

show

since

{Vl+l5)- with

intensity. In order

aromatics

was

formation

of

other also

o-di

ion LMS

peak

is important,

of

to study-this studied. ortho

facilitated

generally between

have only

aromatic positive

series

to make of PAH's

of dinitro

of these

groups

side

reactions,

carboxylic

is necessary

studies,

adjacent

and

in order

for

The

to each

The

is not mandatory.

for the

hydrocarbons electron

infrequently

aromatics

work

series

process

instance

is the

phenolic

to elucidate

the

reaction.

and

the above

The

donors

(RlPD)

use of these

(PAHIS)

affinities;

in LKS.

and electron

methyl-p-phenylelediamine attempt

more

this

nitro

of competing anion

a larger

the results

by two

however,

absence

Considerably of this

Polynuclear observed

or peri);

by.the

effect

7 summarizes

of the quasimolecular

compounds. mechanism

Figure

(11+0-H)- is facilitated

(either

formation

positional

electron

however,

formation

and

and TNB

reactions,

having

acceptors,

LMS spectra and

or more

molecular

rings

anions

of charge-transfer like

respectively,

donor

three

N,N,N',N'-tetra-

is well

were

acceptor

are

complexes

known.

obtained species.

with

In a

an

144

Fig.

Summary of study compounds showed

7.

The

presence

exception

of

corresponding

to the

results; hydrocarbon) results

were Since

must

of

samples

contain

no effect

that

had

benzyl

The

presence

of

a peak

as many

observed the

TMPD had

(M-H)-.

presence just

of [M+O-HIformation in nitrobenzenes. the nucleophilic substitution reaction

reactant an oxygen

for

of all

species atom,

on

the

aromatic

i c hydrogens

at

of

(!1+15)-

TNB however, (where

nitro-substituted of

the in

we attempted

compounds, showed showed

had

Boxed LMS analysis

with

surprising to

shown.

the These

studied.

nucleophilic to

use

substitution other

oxygen

the

peaks

tl corresponds

samples

PAH’s

the

; these

on

reaction containing

aromatic

145 species, All

Na20,

e.g.

of the above

observed

gave

the same

of the potential

the relative

amounts

observed.

combination

The

and

nitrocellulose

result;

reactants

of these

formation

ion LMS

O-',

species

polymer,

the formation

The negative

for all reagents.

formation was

NaN02

NaN03,

02-'

and the

of the species

as CI reagents.

of a peak

spectra

(M+15)-

of the reagents

and N02-; intensity

at

show

no correlation of the

was

with

(M+O-H)-

(M-+0-H)- can be the result

ion of any

of the following:

ArH + O-'

+

ArO-+H-

(1)

ArH+02-'+

ArO-+OH.

(2)

ArH + NO2- +

ArO-

(3)

+ HNO

It is difficult of solution brought

about

variety extend

existing

of samples

of selective

We have

important

quasimolecular formation

reactions

What

can be produced

can-be

of completing

analyzed

(M+H)'

or simply

from

a detailed The

and

of the quasimolecular

"plasma"

states.

are analogs

of reactions is hoped

that will

by LMS,

in the LMS characterization

compounds.

ions,

reactions

is that

a

not only

but also

to add

the

of amino

acids.

We

transfer

interested

in the process

of these

in excited

that

these

or are the result

ionization.

proton been

to say whether

thermally,

of chemical,ionization

Intermolecular

the

point

induced

by species

the range

dimension

are

at this

chemistry

amino

(M-H)-. cation

study

acids

of the fragmentation

show

high yields

The question results

intermolecular

from

proton

arises

as to whether

a protonation

transfer

Datterns

of the reaction

among

amino

in

acid

molecules.

In

order

mechanism, exchanged valine-d3 appeared

to distinguish

a study

was

with

deuterium.

were

obtained.

that

functionality

positive

In the case

the proton and not

plasma

out where

The

at m/z = 122 while

indicates

between

carried

from

and the

hydrogens

ion LMS spectra of valine-d3

for valine

responsible

protonation

the labile

it appeared chain,

of both

valine

the quasimolecular at m/z

for protonation

the aliphatic

transfer

on valine

must

= 118. come

substantiating

were and

catio n

This from

the am ine

the transfer

mechanism. One

could

labile,

and

density

until

conditions

still

are the

only

for valine-d3,

both

argue only

that

ones

the

ionized.

H+ and D+ were

the This

quasimolecul shows

hydrogens

that

ar

the amino

We therefore

observed ion

from

gas-phase

increased

in the spectrum.

corresponding protonation

group

to

the Even

(bI+D)+

was

are more laser under

power these

observed

is not responsible

for

146 the quasimolecular whether

this

multimer Direct

ion

(M+H)"

in amino

is due tb a "solid

over

the

acids.

state"

It is not

reaction

clear

however,

or to the dissociation

of a

surface.

analysis

The

LAMMA-1000

potential

for

summarize

some

not

configured

of solids.

in our

analysis.

practical,

similarly

analysis

experiments

LMS to direct though

and other

the direct

laboratory

dealing

A few of the examples demonstrate

LMS

instruments

have

In this last section

the variety

have

with

great

we

the application

ootential

of samples

of

use, while

from

which

LMS

others, spectra

can be obtained. The shown

coupling

itself

of separation

to be very

techniques

useful.

The

coupling

HPTLC

with

LMS.

We will

analysis

of samples

from

an HPTLC

We have be analyzed

been

groups

triphenylmethane method

Dyes

plate

characterized rosaniline

hydrochloride,

separated

by LMS.

The

obtained

from

on a HPTLC spectra

the neat

lower

mass

shown

in Figure

end

These determine determine The analysis black

due

samples,

to silica

of

on the direct

violet, The

the One

layer

an assignment

This

spot.

of seven green,

would

be

plate was

TLC-LMS

their blue

and

was

were

ethyl

then

B, violet)

analyzed

analogous

a contribution

spectra

location

previously

Victoria

green,

TLC

there

since

dyes

the plates

that The

are

obtained.

is by thin

to confirm

cannot

of new dyes.

malachite

from

except

dyes

dyes

of ionic

of dyes

of TLC-LMS

air dried

species.

class

are readily

the TLC

study

Most

to those to the

of two dyes

are

8.

initial

results

are

of samples

detection

limits.

encouraging

results

possibilities.

pen mark

from

brilliant

directly

the types

Reproducible marker,

obtained

these

A mixture

violet,

gentian

results

to the presence

such

structures

noted.

plate.

has already

the possibility

by LMS.

to be able

spectrum

for an initial

(methyl

due

however,

identifying

in authenticating

by LMS

of dyes

One

spectra

be advantageous

is readily

initial

spectrometry

and

spectrometry

affords

plate.

LHS

a mass

are convenient

our

volatility.

Their

It would

helpful

on the TLC

was

limit

on Rf by obtaining

especially

mass

of separating

chromatography. based

that dyes.

present

in the analysis

by conventional

functional common

interested

to mass

LAMMA-1000

on paper.

spectra

were

and of a variety

very

prompted

Figure The

promising

for which

spectrum

of colors.

and further technique

further

9 shows

obtained

the

from

investigation

the positive is that

technique

is underway

of pen may

and

of other

ion LMS

of gentian

a variety

This

work

is suitable,

spectra

to

direct of a

violet. types,

thus

roller

have

and

forensic

to

147

m

I)

l VI

GENTIAN

n-

VIOLET

1

’ I

mL

.

Fig. 8.

Positive

ion TLC-LMS

of brilliant

green

and gentian

violet

3 I

Fig.

9.

Positive

ion LMS spectrum

of pen

mark

directly

from paper

148 application;

work

conventional

analytical

One Figure

further

is currently example

the spectrum

in lob is that

to caffeine,

detectable

amount)

does

not have much

one of a variety

Fig.

of the direct

10a is the negative

correspond

10.

to compare

caffeine potential

analysis

ion CMS spectrum

this method

potential

of a brand

of the decaffeinated

and that

of samples

Negative coffee

underway

with

methods.

as expected,

as an analytical from

which

ion LMS spectra

coffee

crystal.

there

in the decaffeinated

crystal.

of a) regular

that

less

it does

while the spectra

this example rcpiesent

directly.

and b) decaffinated

ACKNOWLEDGEMENTS This National

work

was

Science

supported

Foundation

by the Office (Grant

of Naval

Research

and-by

the

No. CHE-8108495).

REFERENCES 1. 2. 3.

R. J. Conzemius and J. 34 (1980) 179. W. A. Posthumus, P. G. Noever de Brauw, Anal. R. L. Kaufman, et al.,

M. Capellen,

Int. J. Mass

Spectrom.

10.

(yet a

Though

can be obtained

coffee

in Figure

crystal

Note

is much

technique,

spectra

Ss shown

Ion

Phys.,

Kistemaker, H. L. C. Meuzelaar and M. C. Ten Chem., 50 (1978) 985. Hicroscopica Acta, Vol. 73, 1972, p. 1.

149 4. 5. 6. 7. 8. 9. 10.

E. Denoyer, R. Van Grieken, F. Adams and D. F. S. Natusch. Anal. Chem., Vol. 54, No. 1, 1982, p. 26-41A. H. J. Heinen, S. Meier, H. Vogt, R. Wechsung, Int. J. Mass Spectrom. Ion Phys., 47 (1983), p. 19-22. K. Venkataraman (Ed.), The Analytical Chemistry of Synthetic Dyes, J. Wiley, New York, 1977. John F. Ready, Effects of High Power Laser Radiation, Academic Press, New York, 1971. R. J. Cotter, Anal. Chem., Vol. 52, No. 11, 1980, 1767-1770. R. 8. Van Breemen, M, Snow and R. J. Cotter, Knt. J. Mass Spectrom. Ion- Phys,, 49 (1983) p. 35-50. K. Balasanmugam, T. A. Dang, R. J. Day and D. f1. Hercules, Anal. Chem., Vol. 53, No, 14, 1981, p. 2296-2298,