HYBRID ICP TECHNIQUES AND FUTURE TRENDS

311 C h a p ter

15

HYBRID ICP TECHNIQUES AND FUTURE TRENDS 15.1 as

INTRODUCTION

Since the inductively coupled plasma (ICP) became established a reliable excitation source in atomic emission spectroscopy

(AES), there has been a growing tendency for the ICP to be combined with other instruments or techniques, i.e. there has been an upsurge techniques. ICP-AES various

of can

hybrid

analytical

be used

analytical

techniques

as the means

techniques

or

hyphenated

of final measurement

such

as

liquid

with

chromatography

(LC-ICP-AES) (Ref. 1), gas chromatography (GC-ICP-AES), ion chromatography (IC-ICP-AES), electrothermal atomization (ETA-ICP-AES) and, if flow-injection analysis (Ref. 2), hybrids such as LC-FIA-ICP-AES.

is

incorporated

Several workers have proposed that the ICP can be used in atomic fluorescence spectrometry (AFS) (Refs. 3 - 6). The ICP normally acts as a source of ground-state atoms. A commercial atomic fluorescence spectrometer (AFS) is available that uses an array of up to 12 hollow cathode lamps (Ref. 6). Despite its simplicity and low initial and running costs, the AFS technique has not proved popular mainly because of the drawbacks associated with the use of a weak primary source of

radiation.

Greenfield

(Ref. 5) uses the ICP as a source and an atomizer. A

technique

analys s

that

shows a

great

deal

of promise

for trace

is inductively coupled plasma-mass spectrometry (ICP-MS)

(Refs.

7 - 16).

source

of photons but

In this instance, as a highly

the ICP is

not used

efficient means of

as

a

producing

ions (Refs. 17, 18). Strictly speaking, therefore, ICP-MS falls outside the domain of emission spectroscopy. However, since it relies heavily on the development of the ICP, it is seen as an adjunct to plasma technology (Ref. 19). 15.2 HYPHENATED TECHNIQUES 15.2.1 Ele*. crothermal atomization - ICP-AES Several /orkers (Refs. 20 - 24) have atomization

(ETA)

- in

the form of

used electrothermal

a furnace normally

used in

312

conjunction with atomic-absorption spectrophotometry - as a sample introduction system for ICP-AES. It permits small solution samples (microlitres) to be analysed by injection into the electrothermal atomizer, and it also opens up the possibility that small solid pellets could be placed in the furnace for analysis. Increased sensitivity and lower detection limits are achievable. ETA

has been used for

the introduction of samples

electrode d.c. argon plasma (Ref. 25).

into a three-

An ETA sample-introduction

device is commercially available for ICP-MS (Ref. 26). been suggested (Ref. 27) that chemical matrix effects

It has can be

greatly reduced by the use of peak-area measurements. 15.2.2

ICP-MS

In from

1975, Gray (Ref.

28) demonstrated the

extraction of

ions

a d.c. plasma through a pinhole-size sampling orifice into a

pumped vacuum system leading into a quadrupole mass spectrometer. The microwave induced plasma (MIP) was tried as the source of ions.

Various workers then used the ICP as an ion source (Refs.

12 - 14). The main developmental work was undertaken in parallel by Gray and Date (U.K.) and Houk, Fassel and co-workers (U.S.A.). Two commercial firms (SCIEX in Canada and VG Isotopes in England) manufacture instruments that combine a low-power (1.5 kW) argon hybrid

ICP with a instrument

quadrupole mass spectrometer. has a

high capital cost

The

resulting

and, at the

time of

writing, there are nearly 100 ICP-MS instruments under evaluation, mainly in North America and Europe (Ref. 29). The initial application of ICP-MS was in the geochemistry field (Ref. 30), but its application has expanded into laboratories (Ref. 32).

nuclear (Ref.

31) and

other

Over the last decade and more, ICP-AES has gained prominence in elemental analysis as a comprehensive and versatile technique that is largely free of chemical matrix effects. However, ICP-AES suffers from severe limitations in regard to spectral interference, and its detection capabilities are limited. The need for greater sensitivity and an analytical system that is free of spectral interference led to the development of the ICP-MS system.

Owing to its high

ICP is an excellent source periodic table (Fig. 15.1).

of

ions

temperature (6000 to 10000

for most

elements in the

K),

313

0.1 H

He

100 75 Li Be

58 B

100 98 Na Mg

98 Al

100 99 100 K Ca Sc 100 96 98 Rb Sr Y 100 91 90 Cs Ba La

99 99 98 Ti V Cr 99 98 98 Zr Nb Mo 98 95 94 Hf Ta W

Fr Ra Ac

95 96 93 M n Fe Co 96 94 Tc Ru Rh 93 78 Re Os Ir

91 90 75 98 Ni Cu Zn Ga 93 93 85 99 Pd Ag Cd In 62 51 100 Pt Au Hg TI

5 0.1 0.1 C N O 85 33 14 Si P S 90 52 33 Ge As Se 96 78 66 Sn Sb Te 97 92 Pb Bi Po

degree of

ionization

of

the elements

The degree of ionization of an element can be the Saha equation: M + _ I l2wm,kT\l Q +

M

k

Ne 0.9 0.04 Cl Ar 5 0.6 Br Kr 29 8.5 I Xe At

Rn

98 99 99 97 100 93 99 100 99 91 92 Ce Pr Nd P m Sm Eu Gd Tb Dy Ho Er T m Yb Lu 100 100 Th Pa u Np Pu A m C m Bk Cf Es Fm Md No Lw

Fig. 15.1. The periodic table.

where

F

ni

h2

]

in

calculated

the

from

-(£ )

Q ’e

ne is the electron density, me is the mass of the electron,

is Boltzmann's constant,

constant,

Q+

and Q

T is the

are the partition

temperature, h is functions of the

Planck's ion and

neutral atom respectively, and IP is the ionization potential of the element. Taking ne = 5 X 1014 per cubic cm, it is readily calculated that mostelements of the periodic table are efficiently ionized under typical plasma conditions. In fact, ionic lines are often used in ICP-AESinstruments. Equation (1) assumes that the ICP is in thermal equilibrium. In fact, although there is considerable evidence that the ICP is not in thermal equilibrium, it is not far from local thermal equilibrium, and the degree of ionization is given approximately by Saha's equation.

314

Pump system

Fig. 15.2. Schematic ICP-MS system. Fig. 15.2 depicts the basic design of an ICP-MS system. The low-power (typically 1.2 kW) argon ICP is mounted horizontally against a sampling cone and 'skimmer7. Ions at atmospheric pressure in the ICP enter the small orifice (0.2 to 1.0 mm) of the water-cooled sampling cone, the tip of which is situated about 3mm beyond the radiofrequency (r.f.) coil, and pass through a multi­ stage vacuum system before entering the quadrupole spectrometer at a very low pressure (less than 10~5 torr) .

mass

Pneumatic nebulizers are normally used to feed the ICP with an aerosol of sample solution (at a flowrate of approximately 1.8 ml/min).

Beyond the

sampling cone

there is

a 'skimmer'

that

permits only the core of the molecular beam formed by the sampling cone to enter the next stage of the vacuum system. A pressure of about 1 torr is maintained in the sampling cone. The core of the molecular beam is then focused by an ion lens into the mass analyser. The ions are separated electronically by means of a quadrupole mass filter, and are registered by a high-sensitivity pulse-counting system. The spectra of the ICP-MS are fairly simple (Fig. 15.3). In the determination of trace elements, the mass spectra consist almost entirely of singly charged monatomic (M+ ) or oxide (MO+ ) ions. If spectral interference does occur, a less abundant isotope line can be used that is free of interference. However,

315

Mass/charge ratio (m

/z )

Fig. 15.3. Spectra of rare earth elements. this does mean a reduction in sensitivity and a

higher

detection

limit.

(i.e.

tens

of

milliseconds for the whole periodic table), and the spectra accumulated until the desired integration times are attained.

are

The

Instrumental

quadrupole

control

is

and

scanned

rapidly

spectral interpretation

are easily

accomplished by a computer of moderate size. Analytical measurements can be carried out by scanning

of

mass range from 3 to 300 atomic mass units (amu) or daltons. resolution of the quadrupole mass spectrometer is 1 Normally,

monovalent

measurements,

(M+ )

ions

are

but negative ions (M~)

monitored

for

the The amu.

analytical

can be monitored, which

is

useful for the analysis of fluorine and sulphur. Some elements form doubly charged ions, oxide species, or hydroxide species (Refs. 33 - 35). Unfortunately, as pointed out by De Galan (Ref. 36), even 0.01 per cent of molecules is a large excess compared with the trace constituents determined at the parts-per-million level. In ICP-AES, molecular species do not emit strongly, and cause no serious problems, but molecular-mass overlaps in ICP-MS pose a serious problem. However, optimization of the plasma conditions and of the pinhole interface may provide the solution to this problem.

316

Since an argon plasma is used, the major ion is Ar+ , atomic mass 40, with a number of minor peaks arising from water and entrained air (0+, 0H+, H20+, H30+, 02+, NO+ , ArH+). In addition, there are Ar2+ (atomic mass 80) and 36Ar+ , 36ArH+ , NH 3 + , NH4+, Ar4*2 , (02+).H, (H30+).H20, (ArH+) .Ar peaks. a

number of minor ions,

such as N+ , 38Ar+ ,

There are also

SiO+ , 54Fe+ , 40ArO+ ,

and 40ArOH+ . Background

spectral

features have

been

reported by

Tan and

Horlick (Ref. 37). The background ionsobserved in ionized water and the backgroundions observed in mineral acidsolvents are shown in Tables 15.1 and 15.2 respectively. Table 15.1 Ion

Background ions found in ICP--MS

Isotopic mass value

02+

n 2+

Ar2+

abundance (%)

3 2 s+

28

28Si+

92.2

80

8C>se+

49.8

76

76Se+ 78Se+

72 74

NO+

overlapping

Natural

32 34 36

78

ArO+

Ion

56 52 58 54

30

34S+ -

72Ge+ 7 4 Ge+ 7 4 Se+ 56Fe"*· 52Cr+ 58Ni+ 54Cr+ 54Fe+ 30Si+

95 4.2

9.0 23.5 27.4 36.7 0.9 91.7 83.8 67.8 2.3 5.8 3.1

317

Table 15.2 Acid

Background ions observed from mineral acid solutions

Ion

h 2s o 4

Isotopic mass value

S+

Ion Abundance overlapping (%)

32, 33, 34

SO+

48

48τί+

49

49Ti+ 50Ti+ 5C>cr+ 50v+

50

so2+

N+

65 66

65Cu+

1.2 30.9

66Zn+

27.8

ArN+ HC1,

Cl+

HC104

C10+

-

5.8 2.3

35, 37

54Fe+ 54Cr+ -

51

51v+

99.7

53

53Cr+

75

75As+

9.6 100

77

7?Se+

7.6

The background peaks interfere with only a few Ar+ 4 4 Ca ,

4.3 0.3 48.9

64Zn+

14 54

ArCl+

5.5 5.2

64

64Ni+

hno3

74

elements,

e.g.

interferes with Ca at mass 40, but a peak of lower abundance, can

be used

for analysis.

There

are more than

100 000

emission spectral lines, but there are only 211 mass spectral lines (Ref. 38), thus making spectral interference a rare event. The

ion characteristics of the

skimmer (Ni, Cu, Zn,

Fe) are not

seen in the background spectrum. Analysis is carried out sequentially but fairly rapidly, i.e. about 30 elements per minute. Oxide peaks do occur but are usually at very low levels. One commercial supplier maintains that the ratio of doubly charged ions to singly charged ions is less than 10"3 for all elements. The effects of concomitant

318

elements and

have been reported

in detail (Ref.

suppressions were observed and

out.

The

considered ionization place.

key

factor

in

the

39).Enhancements

a single mechanism was

matrix

effects

ruled

observed

was

to be the collision rate in the interface region where and the recombination of ions with electrons take Internal

elements)

standardization using

was found

effects found,

to compensate

background lines (light

only partly

for

and generally no corrective effect was

the heavier analytes.

the

matrix

exerted

on

More than one internal standard is needed,

and a heavy internal standard should be added to the sample prior to the analysis. Thompson and Houk (Ref. 40) have studied internal standardization in ICP-MS, and report that significant improvements in accuracy and precision are achievable if appropriate elements energy.

elements

are

that match the

selected as

internal

analyte element in

standards, i.e.

mass and

ionization

Uniformly low detection limits are obtainable, the detection limits for most of the elements being from 0.1 to 10 p.p.b. (micrograms per litre). These limits are one to two orders of magnitude lower than those obtainable with ICP-AES. The detection limits are shown in Fig. 15.4. In

Table 15.3, the ICP-MS

detection limits for some

are compared with those obtained by ICP-AES and flame AAS. Table 15.3

Comparison of detection limits Approximate detection limit (pg/1)

Element Boron Aluminium Chromium Manganese Arsenic Gold Titanium Uranium

ICP-MS

ICP-AES

Flame AAS

2 3

3

1000 20

1 1 30 0.4 2 1

15 4 1 40 10 3 200

3 3 100 10 50 6000

elements

319

Detection limits ng/ml

H 2 Li Be 2 1 Na Mg 3 2 Ca Sc Ti

1 1 4 K Cr M n Fe 0.8 0.8 0.8 1 1 2 4 Rb Sr Y Zr Nb M o Tc Ru 1 1 1 2 2 0.8 Cs Ba La Hf Ta W Re Os Fr Ra Ac

2 V

0.8 2 1.5 Co Ni Cu 1 0.4 1 Rh Pd Ag 2 1 0.4 Ir Pt Au

2 B

50 C

3 Al

300 Si P s

N

O

H

He

F

Ne

Cl Ar 2 50 80 100 Zn Ga Ge As Se Br Kr 2 1 2 0.4 2 Cd In Sn Sb Te I Xe 1.3 0.6 0.8 0.6 Hg TI Pb Bi Po At Rn

1 1 2 2 2 2 2 1 0.4 2 0.5 0.9 1 Ce Pr Nd P m Sm Eu Gd Tb Dy Ho Er T m Yb Lu 0.7 1 Th Pa U Np Pu A m C m Bk Cf Es Fm M d No Lw

Fig. 15.4. Detection limits in ICP-MS. One advantage of an ICP-MS system is that information can be obtained about isotope ratios (Ref. 41), since the different isotopes

appear as separate peaks on the mass spectrum.

Data on

the percentage abundance of natural isotopes can be obtained from reference books (e.g. Handbook of Chemistry and Physics by the Chemical Rubber Company).

A number of elements

occur

naturally

in only one isotopic form, e.g. 9Be, 27A1, 31P, 45Sc , 55Mn, 59Co, 75As, 89Y, 93Nb, 159Tbf 165fHo, 169Tm.

11 63

103Rh,

141Pr,

Boron exists as isotopes of atomic mass 10 (19.78 per cent) and (80.22 per cent). Copper is found as isotopes of atomic mass (69.09 per cent) and 65 (30.91 per cent). Iron occurs as

isotopes of atomic mass 54 (5.82 per cent), 56 (91.66 57 (2.19 per cent) and 58 (0.33 per cent).

per

cent),

320

ICP-MS

has been used for

the analysis of trace

impurities in

brines, natural waters, urine, faeces, hair, alloys, metals, and uranium ore, and for the determination

high of

purity

rare-earth

elements (Refs. 15, 42). Applications in food analysis are expected to increase, since it is possible to follow the path of low levels of metals in the body (Ref. 43). Although, the normal method of sample introduction in ICP-MS is by nebulization of the sample in solution form (Ref. 44), various methods for the introduction of solid samples are under investigation, e.g. laser ablation (Ref. 45) which the

advantages

of

solid

sampling,

ICP

would

excitation,

possess and

mass

spectrometric measurement.

15.3

FUTURE TRENDS

15.3.1 Speciation Most AES techniques allow the total elemental content in a sample to be determined, irrespective of the chemical form of the element, e.g. the ICP does not differentiate between organic and inorganic sulphur, chromium (III) and chromium (VI), (IV) and selenium (VI). However, the monitoring and protection of the

or

selenium

environment

has

become an important issue, and the demand for the determination of particular chemical species has grown (Ref. 46, 47). Inorganic arsenic

species,

organic

arsenic

less

toxic.

i.e. As species,

(III) and As

(V) are more

e.g. methylated

toxic than

arsenic compounds are

Chromium (VI) is ten times more toxic than chromium

(III). In gold-mining areas, arsenic trioxide is a byproduct of the roasting process used in the extraction of gold from pyritic ores, and it is consequently a major contaminant of the tailings material that is dumped. Methods for the separation of the species of elements (such as As, Cr, and Se) before analysis of the separated fractions by AES is a growing field for research and development. Environmental protection agencies are continually setting lower legal limits for toxic species, so that becoming greater.

the need for

lower detection limits

is

321

15.3.2 A

Fourier-transform Spectrometry (FTS) Fourier-transform spectrometer is a variation of a Michelson

interferometer (Ref. 48). Although FTS is an established technique in infrared spectroscopy, it has not yet been adopted for use in the visible and ultraviolet regions of the electro­ magnetic spectrum. In theory, FTS can be applied with advantage to any analytical problem in which high resolution and a high signal-to-background ratio are required. For short wavelengths, it is mechanically difficult to produce an FT spectrometer to acceptable tolerances, but if this problem can be overcome, should become a popular analytical technique (Refs. 26, 49).

15.4

FTS

CONCLUSION

Although it appears to lack the freedom from matrix effects of ICP-AES (Ref. 50), ICP-MS is a technique that offers reduced spectral

interference,

as

well

elemental

detection limits can

as

isotope

be an order

analysis.

of magnitude

The better

than those attainable by ICP-AES, which gives ICP-MS an assured future in trace analysis in many fields, e.g. the geological, biomedical, agricultural, environmental pollution. Generally, centred

nuclear,

future development

on improved methods of

MIP, and DCP excitation sources.

and nutritional

work in

AES is

fields, and

likely

to

sample introduction for the The onerous task

of

be ICP,

detecting

spectral interferences will eventually be handled automatically by highly

sophisticated

spectrometers

equipped

with

a

form

of

artificial intelligence, as well as an auto-optimizing capability. Possibly, electronic means for the detection and recording of spectra (incorporating Fourier transforms) will be developed that will match the undoubted advantages possessed by the photographic spectral plate. The coupling of atomic fluorescence spectroscopy and mass spectrometry to the ICP will expand the possibilities for use of the ICP in analytical work.

322 REFERENCES

1.

L. Ebdon, S. Hill and R.W. Ward, Directly coupled chromatography - atomic spectroscopy. Part 2. Directly coupled liquid chromatography - atomic spectroscopy. A review, Analyst (London), 112 (1987) 1-16.

2.

C.W. McLeod, I.G. Cook, P.J. Worsfold, J.E. Davies and J. Queay, Analyte enrichment and matrix removal in flow injection analysis-inductively coupled plasma-atomic emission spectrometry: determination of phosphorus in steels, Spectrochim. Acta, 40B (1985) 57-62.

3.

J.D. Winefordner, Multielement analysis: is atomic fluorescence spectrometry a viable method, ICP Inf. Newsl., 8 (1982) 173-177.

4.

D.R. Demers, D.A. Busch and C.D.Allemand, Simultaneous multielement analysis using ICP fluorescence spectroscopy, Am. Lab., 14 (1982) 167-176.

5.

S. Greenfield and M. Thomsen, Atomiser, source, inductively coupled plasmas, in atomic fluorescence spectrometry (ASIA). Some recent work, Spectrochim. Acta, 40B (1985) 1369-1377.

6.

D.R. Demers, Hollow cathode lamp-excited ICP atomic fluorescence spectrometry - an update, Spectrochim. Acta, 4OB (1985) 93-105.

7.

A.L. Gray and A.R. Date, Inductively coupledplasma source mass spectrometryusing continuum flow ion extraction, Analyst (London), 108 (1983) 1033-1050.

8.

D. Douglas, G. Rosenblatt and E. Quan, Inductively coupled plasma/mass spectrometry - a new technique for trace element analysis. Trace substances in environmental health - XVII. A symposium. D.D. Hemphill (ed.) University of Missouri, Columbia (1983) 385-390.

9.

J.A. Olivares and R.S. Houk, Ion sampling for inductively coupled plasma mass spectrometry, Anal. Chem., 57 (1985) 2674-2679.

10.

D.J. Douglas and R.S. Houk, Inductively coupled plasma mass spectrometry (ICP-MS), Prog. Analyt. Atom. Spectrosc., 8 (1985) 1-18.

11. R.S. Houk, Mass spectrometry of inductively coupled plasmas, Anal. Chem., 58 (1986) 97A-105A. 12. R.S. Houk, V.A. Fassel, G.D. Flesch, H.J. Svec, A.L. Gray and C.E. Taylor, Inductively coupled argon plasma as an ion source for mass spectrometric determination of trace elements, Anal. Chem., 52 (1980) 2283-2289. 13.

A.R. Date and A.L. Gray, Plasma source mass spectrometry using an ICP and a high resolution quadrupole mass spectrometer, Analyst (London), 106 (1981) 1255-1267.

323

14.

A.R. Date and A.L. Gray, Development progress in plasma source mass spectrometry, Analyst (London), 108 (1983) 159165.

15.

A.R. Date and A.L. Gray, Determination of trace elements in geological samples by inductively coupled plasma source mass spectrometry, Spectrochim. Acta, 4OB (1985) 115-122.

16.

A.L. Gray, The ICP as an ion source - origins, achievements and prospects, Spectrochim. Acta, 40B (1985) 1525-1537.

17.

B.L. Caughlin and M.W. Blades, Analyte ionization in the inductively coupled plasma, Spectrochim. Acta, 40B (1985) 1539-1554.

18.

M.W. Blades, B.L. Caughlin, Excitation, ionization, and inductively coupled plasma, (1987) 57-109.

19.

A.L. Gray, Ions or photons - an assessment of the relationship between emission and mass spectrometry with the ICP, Fres. Z. Anal. Chem., 324 (1986) 561-570.

20.

A. Aziz, J.A.C. Broekaert and F. Leis, Analysis of micro­ amounts of biological samples by evaporation in a graphite furnace and inductively coupled plasma atomic emission spectroscopy, Spectrochim. Acta, 37B (1982) 369-379.

21.

G.F. Kirkbright and R.D. Snook, The determination of some trace elements in uranium by inductively coupled plasma emission spectroscopy using a graphite rod sample introduction technique, Appl. Spectrosc., 37 (1983) 11-16.

22.

H. Matusiewicz and R.M. Barnes, An electrothermal sample introduction system for ICP spectrometry, Appl. Spectrosc., 38 (1984) 745-747.

23.

H. Matusiewicz, F.L. Fricke and R.M. Barnes, Modification of a commercial electrothermal vaporisation system for inductively coupled plasma spectrometry: evaluation and matrix effects, J. Anal. At. Spectrom., 1 (1986) 203-209.

24.

H. Matusiewicz, Thermal vaporisation for inductively coupled plasma optical emission spectrometry. A review, J. Anal. At. Spectrom., 1 (1986) 171-184.

25.

W.G. Elliott, H. Matusiewicz and R.M. Barnes, Electrothermal vaporization for sample introduction into a three-electrode direct current argon plasma, Anal. Chem., 58 (1986) 12641265.

26.

J. Marshall, S.J. Haswell and S.J. Hill, Atomic spectrometry update - instrumentation, J. Anal. At. Spectrom., 2 (1987) 79R-131R.

27.

J. Alvarado, P. Cavalli, N. Omenetto, G. Rossi and J.M. Ottaway, Peak-area measurements in electrothermal atomisation inductively coupled plasma atomic emission spectrometry, J. Anal. At. Spectrom., 2 (1987) 357-363.

Z.H. Walker and L.L. Burton, spectral line emission in the Prog. Analyt. Spectrosc., 10

324

28.

A.L. Gray, Mass spectrometric analysis of solutions using an atmospheric pressure ion source, Analyst (London), 100 (1975) 289-299.

29. L. Ebdon and R.C. Hutton, Atomic spectrometry Anal. At. Spectrom., 1 (1986) 314-317.

viewpoint,

J.

30. A.R. Date, Y.Y. Cheung and M.E. Stuart, The influence of polyatomic ion interferences in analysis by inductively coupled plasma source mass spectrometry, Spectrochim. Acta, 42B (1987) 3-20. 31.

P.D. Blair, The application of inductively coupled plasma mass spectrometry in the nuclear industry, Trends Anal. Chem., 5 (1986) 220-223.

32. D. Beauchemin and J. McLaren, Highlights of the workshop on applications of ICP-MS, ICP Inf. Newsl., 11 (1986) 613-638. 33. M.A. Vaughan and G. Horlick, Oxide, hydroxide, and doubly charged analyte species in inductively coupled plasma/mass spectrometry, Appl. Spectrosc., 40 (1986) 434-445. 34. D.A. Wilson, G.H. Vickers and G.M. Hieftje, Spectral and physical interferences in a new, flexible inductively coupled plasma mass spectrometry instrument, J. Anal. At. Spectrom., 2 (1987) 365-368. 35. G. Zhu and R.F. Browner, Investigation of experimental parameters with a quadrupole ICP/MS, Appl. Spectrosc., 41 (1987) 349-359. 36.

L. de Galan, New directions in optical Anal. Chem., 58 (1986) 697A-707A.

atomic

spectrometry,

37.

S.H. Tan and G. Horlick, Background spectral features in inductively coupled plasma/mass spectrometry, Appl. Spectrosc., 40 (1986) 445-460.

38.

L. Ebdon, S. Greenfield and B.L. Sharp, The versatile inductively coupled plasma, Chem. Brit., 22 (1986) 123126,130.

39.

D. Beauchemin, J.W. McLaren and S.S. Berman, Study of the effects of concomitant elements in inductively coupled plasma mass spectrometry, Spectrochim. Acta, 42B (1987) 467-490.

40.

J.J. Thompson and R.S. Houk, A study of internal standardization in inductively coupled plasma - mass spectrometry, Appl. Spectrosc., 41 (1987) 801-806.

41.

J.R. Dean, L. Ebdon and R. Massey, Selection of mode for the measurement of lead isotope ratios by inductively coupled plasma mass spectrometry and its application to milk powder analysis, J. Anal. At. Spectrom., 2 (1987) 369-374.

42.

H.P. Longerich, B.J.Fryer, D.F. Strong and C.J. Kantipuly, Effects of operating conditions on the determination of the rare earth elements by inductively coupled plasma - mass spectrometry (ICP-MS), Spectrochim. Acta, 42B (1987) 75-92.

325

43.

S. Munro, L. Ebdon and D.J. McWeeny, Application of inductively coupled plasma mass spectrometry (ICP-MS) for trace metal determination in foods, J. Anal. At. Spectrom., 1 (1986) 211-219.

44.

A. Gustavsson, Characterization of an interface for sample introduction into an inductively coupled plasma, Spectrochim. Acta, 42B (1987) 111-118.

45.

A.L. Gray, Solid sample introduction by laser ablation for inductively coupled plasma source mass spectrometry, Analyst (London), 110 (1985) 551-556.

46.

W.D. Spall, J.G. Lynn, J.L. Andersen, J.G. Valdez and L.R. Gurley, High-performance liquid chromatographic separation of biologically important arsenic species utilizing on-line inductively coupled argon plasma atomic emission spectrometric detection, Anal. Chem., 58 (1986) 1340-1344.

47.

J.J. Thompson and R.S. Houk, Inductively coupled plasma mass spectrometric detection for multielement flow injection analysis and elemental speciation by reversed-phase liquid chromatography, Anal. Chem., 58 (1986) 2541-2548.

48.

L. Faires, Fourier transforms for analytical spectroscopy, Anal. Chem., 58 (1986) 1023A-1034A.

49.

A. Thorne, Novel instrument design in atomic spectroscopy: Grating or Fourier spectrometers: which and why? Anal. Proc., 22 (1985) 63-65.

50.

J.A. Olivares and R.S. Houk, Suppression of analyte signal by various concomitant salts in inductively coupled plasma mass spectrometry, Anal. Chem., 58 (1986) 20-25.

atomic