- Email: [email protected]
Molecular emission cavity analysis: principles and applications
78
trends in analytical chemistry, vol. 16, no. 2, 1997
ogy. Vol. 34. Metal Ions in Biological Systems, Marcel Dekker, New York, 1997, in press. [ 221 E.D. Stein, Y. Cohen and A.M. Winer, CRC Crit. Rev. Environ. Sci. Technol., 26 (1996) l-43. [ 231 L. Liang, M. Horvat and N.S. Bloom, Talanta, 41 (1994) 371-379. [ 241 P.J. Craig, D. Mennie, N. Ostah, O.F.X. Donard and F. Martin, Analyst, 117 ( 1992) 823-824. [ 25 J P.J. Craig, D. Mennie, M. Needham, N. Oshah, O.F.X. Donard and F. Martin, J. Organomet.
Chem., 447 (1993) 5-8. K. Kwetkat and W. Kitching, J. Chem. Sot. Chem. Commun., (1994) 345-347. [ 27 ] P.J. Craig, M.L. Needham, N. Ostah, G.H. Stojak, M. Symons and P. Teesdale-Spittle, J. Chem. Sot. [ 261
Dalton Trans., (1996) 153-156. [ 28 ] F.M. Martin and O.F.X. Donard, J. Anal. Atom.
Spectrom., 9 (1994)
1143-1151.
[ 291 F.A. Cotton and G. Wilkinson,
ganic Chemistry, 1988, p. 165ff.
Advanced InorWiley, New York, 5th edn.,
James Weber’s education includes a PhD degree in inorganic chemistry from the Ohio State
University (7963). In 7963 he became an assistant professor of chemistry at the University of New Hampshire and became a full professor in 1977. During his most recent sabbatical leave in 199192 at ENEA (La Spezia, Italy) he studied abiotic methylation of mercury(ll). He was an invited participant in the European Community workshop “Trends in Speciation Analysis in Environmental and Food Matrices” (Rome, 1994). Recent invited lectures include ones at the international Conference on Environmental and Biological Aspects of Main-Group Organometals (Bordeaux, 1994) and the 4th Symposium on Analytical Sciences (Brussels, 1996). He is on the editorial board of Applied Organometallic Chemistry. His research has been supported by the USA agencies NSF, EPA, and NOAA. He is now at the University of New Hampshire, Chemistry Department, Parsons Hall, Durham, NH 03824, where he directs research of graduate students on the biogeochemistry of methylmercury compounds in the estuarine environment. He published more than 90 papers in refereedjournals.
Molecular emission cavity analysis: principles and applications Antony C. Calokerinos Athens, Greece Molecular emission cavity analysis (MECA) is a chemiluminescence technique in which molecular emissions are generated within a cavity that is introduced into a hydrogen diffusion flame. The hydrogen flame acts as the environment of radicals, atoms and molecules which promote the chemical reactions producing excited species and the cavity acts as the environment for stabilizing the emitting species. The principles, development and applications of MECA are outlined and future possibilities are discussed.
1. Introduction: the genesis of a new analytical technique Molecular emission cavity analysis was discov0165-9936/97/$17.00 P//SO 165.9936
(96
)00096-9
ered in the early 1970s by Bogdanski, Townshend and (the late) Belcher at the Department of Chemistry of the University of Birmingham, UK. At that time, Belcher was interested in unusual flame tests which were mentioned in Feigl’s Spot Tests in Znorganic Chemistry. He asked Bogdanski to work for his PhD on what was later called candoluminescence, in which the luminescent emission is generated on the surface of a solid in contact with a hydrogen flame [ 11. Bogdanski used the cavity in the head of an ordinary Allen screw for introducing the solid into the flame environment. The screw was then cleaned with a mixture of phosphoric and sulphuric acids and washed with water. When the screw was brought into contact with a hydrogen flame, an intense blue emission followed by a green emission was noticed. It was soon realized that both emissions were arising from the extremely low residual amounts of the acids. When the experiment was repeated by injecting a solution of sulphuric acid into the Allen screw 8
1997 Elsevier Science B.V. All rights reserved.
trends in analytical chemistry, vol. 16, no. 2, 7997
Table 1 Emissions generated within the MECA cavity and the most commonly used wavelengths for analytical measurement (reproduced from Ref. [ 331 with permission of Academic Press)
79
over the years has been critically evaluated recently appeared in a monograph [ 3 1.
2. Principles Element Normal cavity Sulphur Tellurium Phosphorus Selenium Tin Gallium Germanium Lithium Cadmium Oxy-cavitp Antimony Arsenic Boron Carbon Nitrogen Silicon Thallium
Emitting particle
s2 Te2
HP0 Se2
SnO GaBr Gal GeCl Li Cd
SbO-0 continuum AsOcontinuum BO2
CH NO-O continuum SiO TI
Copper cavity or indium-lined cavitp Bromine CuBr InBr Chlorine CUCI InCl Iodine Cul Inl
and
and applications
Wavelength (nm)
384 500 528 411 485 350 391 455 670.8 326.1
355 400 518 431.5 500 540 377.5
494 375 532 360 510 410
“Slow flow of pure oxygen is supplied to the cavity in orderto promote molecular emissions from elements which require the presence of oxygen in the excited molecule or radical (such as BO?) or due to oxygen atom combination reactions (such as AsO+O + As02+hv). ‘Both cavities promote emissions from halide-containing molecules.
which was then introduced into the hydrogen flame, a bright blue emission was observed, attributed to the formation of excited Sz* molecules. Similarly, phosphoric acid generated an intense green emission due to excited HP0 species. In both experiments, the intensity of the emission was proportional to the amount of acid injected into the cavity. Hence, molecular emissions were generated within a cavity and this was the birth of molecular emission cavity analysis (MECA). The first publication appeared in 1973 [ 2 ] and since that time the technique has grown rapidly and has helped scientists to understand the principles of flame chemiluminescence. The plethora of information collected
The distinct features of MECA are the hydrogen diffusion flame and the cavity. The hydrogen flame acts as the environment of radicals, atoms and molecules which promote the chemical reactions producing excited species and the cavity acts as the environment for stabilizing the emitting species. 2.1. Hydrogen diffusion flame The flame normally used in MECA is the hydrogen diffusion flame, which is a flame of hydrogen diluted with nitrogen burning with atmospheric oxygen diffusing into the gas mixture. The main reaction of the flame is H2+HO’+H+H20 The maximum temperature of the hydrogen diffusion flame does not exceed 1000°C and since mixing of hydrogen and atmospheric oxygen is controlled by diffusion, it is obvious that from the edges to the centre of the flame body, the amount of oxygen which penetrates is drastically decreased. As a result of this difficulty in mixing of oxygen with hydrogen, the temperature in the centre of the flame is lower than that at the edge. The change in temperature is accompanied by a change in the nature and concentration of flame particles. The edge of the flame is rich in oxygen and also contains water vapour from the main chemical reaction of the flame. The inner part of the flame is rich in atomic and unburnt hydrogen. Hence the hydrogen diffusion flame is a cell for chemiluminogenic reactions (flame chemilumines-
Table 2 Typical t,,, values for some sulphur anions [ 81 and organic compounds [ 91 Compounda
till (s)
Compounda
tfll (s)
Na2S Na2S03 KSCN
2.0 2.4 18
Cystine Promethazine Sulphadiazine
0.5 0.2 2.0
“Experimental compounds.
conditions are the same for each group of
80
trends in analytical chemistry, vol. 16, no. 2, 7997
bamplc introduction
/
detector
Fig. 1. Conventional MECA cavity introduction Science B.V. from Ref. [ 41).
into flame by manual rotation
cence) which can generate excited species by direct mechanisms, e.g.
reprinted with permission
of Elsevier
temperature and rich in hydrogen atoms, favouring the generation of excited molecules such as S2 and
SnC12 + H + SnCl* + HCl or indirect mechanisms in which energy is transferred by or during recombination reactions: Na+H+H+Na*+Hz
sof-or s,o;-
Na+H+HO’-+Na*+H20 Nevertheless, the hot environment of the flame destroys the temperature-sensitive excited species. This difficulty can be overcome by carrying out the flame chemiluminescent reactions within a protected area, which is the cavity. The cavity provides an area sheltered from the hot environment of the flame and promotes chemiluminogenic reactions. The main characteristics of the cavity are: ( 1) The position of the cavity within the flame governs the nature and concentration of atoms, radicals and molecules which mix with the analyte. Thus, if the cavity is positioned at the centre of the flame, it operates within an environment of low
Time
(S)
Fig. 2. Sz responses from 5 ml of a mixture containing 20 mg S 1-r as Na2S03+20 mg S I--I as KSCN+40 mg S I-’ as (NH4)2S208 injected into a stainless-steel cavity (flame composition: 1.71 I HZ min-‘+4.0 I N2 min-’ ) ( reprinted with permission of Elsevier Science B.V. from Ref. [ 321).
81
trends in analytical chemistry, vol. 16, no. 2, 1997
Injection
Bubble breaker
k-
cw---
P
NaOH pellets
cavity
r/
Detector
fl
-Burner
head
Fig. 3. Schematic diagram of a gas generation MECA system for the determination of ammonium by generation of ammonia (reprinted with permission of The Royal Society of Chemistry from Ref. [ 241).
Sez. When the cavity is positioned at the edge of the flame, then the environment is rich in molecular oxygen and hydroxyl radicals at high temperature and oxygen-containing excited species are formed. This region can also generate atomic emissions by thermal excitation. (2) The temperature of the cavity can be lowered by allowing a constant flow of water to flow through its body. This allows continuous operation of the cavity within the flame and promotes emissions from species such as S2 and HP0 (Salet phenomenon). ( 3 ) The environment of the cavity can be altered by the introduction of a gas or from its material of construction in order to select and promote specific excitation species (Table 1). 2.2. Cavity Sample introduction
into the flame via the cavity
can be achieved by two different techniques: conventional MECA and gas generation MECA detection. 2.2.1.
Conventional
MECA
In conventional MECA the sample is deposited into the cavity (typical dimensions diameter 8 mm and depth 5 mm). The sample is usually a few microlitres of solution or milligrams of solid. The cavity is then introduced rapidly and reproducibly into the flame (Fig. 1). After introduction into the flame, the cavity temperature increases from ambient to its maximum value, which depends on material and conditions [ 4 1. During the heating up period of the cavity in the flame, a series of physical and chemical changes occur which are accompanied by the generation of the characteristic molecular emission. The cavity remains in the flame as long as required for the emission to reach maximum radiation and cease.
Table 3 Some representative
analytical
Technique
Mode
Compounds
Ref.
Conventional
Manual
[lOI
Gas generation
Automated Flow injection
Thio-/dithioglycolic acids (8.0 x 10e5-1 .O x 1O-3 M thioglycolic acid) Trace Sod*- in high-purity NH3 40-90 mg 9-l of sulphur in selenium Inorganic and organic Sulphur anions and colloidal S with limits of detection in the range 0.1-0.8 mg 1-l Cephalosporins in the range 10.0-250.0 mg I-’ Organic sulphur compounds (5.0-30.0 mg I-’ ) Sulphur compounds 0.1-l 70 ng thiophene, 0.2-200 ng 3-methylthiophene
applications
Continuous flow Electrolysis Graphite cell Gas chromatography
of S2 emission
iI51 1161 [I71 1181
82
trends in analytical chemistry, vol. 16, no. 2, 1997
Table 4 Some representative
analytical
applications
of MECA (other than S2 emission)
Element
Application
Ref
Halides
Polychlorinated organic compounds, InCl emission (limit of detection 0.05 ng of chlorine) General studies with copper cavity, CuX (0.003-0.01 MCI-, 0.001-0.006 M Br-, 0.001-0.03 MI-) Organochlorine pesticides, polychlorinated biphenyls (1 O-l 50 mg of chlorine) 0.05-I 00 mg 9-l of Ge, As, Sb, Sn and Hg in coal or biological material by formation of volatile bromides Ge (0.2-10 kg), As (0.1-5 pg), Sn (0.5-10 pg) and Sb (l-40 l.tg) as hydrides, gas chromatography Generation of ammonia (l-100 mg I-’ of nitrogen), fertilizers Total nitrogen in tobacco leaves after Kjeldahl digestion Organophosphorus insecticides (5-100 ng of phosphorus), flow injection Phosphate (100-500 mg I-’ of phosphorus) in detergents After HPLC separation ( 1O-l 00 ng of phosphorus) Removal of cationic interferences by cation exchange 0.2-500 mg I-’ of Si, formation of volatile SiF4 0.1-40 mg I-’ of Sn, hydride formation
[ 191
Metals and metalloids
Nitrogen Phosphorus
Silicon Tin
It is then removed and cooled before repetition of the process. The time required for the appearance of maximum emission intensity after the introduction of the cavity into the flame is usually represented by t, and depends on the compound injected and the thermal properties of the cavity. Some examples of t, values are given in Table 2. Fig. 2 shows an example of how the t, value can be used to determine the components of a mixture containing sulphite, thiocyanate and sulphate (or peroxodisulphate). 2.2.2. Gas generation MECA detection In gas generation MECA detection, the sample must be in the form of a gas or vapour which is introduced into the cavity through a side duct (Fig. 3). The emission is generated during passage of the gas or vapour through the cavity. The analyte is converted into a gas by a chemical reaction in a carrier + analyte gas cooling water in p+
-7 Ll
cavity
Iszss&L a cooling water out
1
Fig. 4. Schematic diagram of a water-cooled cavity.
[201
[211 [221
[231 [241 [251
[261
[271
1281 1291 [301 [311
closed reactor, which is continuously purged by a carrier gas, such as nitrogen or argon. The cavity usually used is water-cooled to allow permanent stay within the flame. 2.3. Salet phenomenon:
recognition
after a
ten tury
In 1869, Salet observed that the blue emission from sulphur compounds and the green emission from phosphorus compounds within a hydrogen flame become more intense when a cool body is introduced into the hot environment of the flame. The importance of the discovery was realized during the development of the flame photometric detector for gas chromatography and is now known as the S&et phenomenon. The observation is due to the stabilization of the excited molecules on the cool surface. In MECA, the ordinary cavity acts as a cool body and enhances the S2 emission. Nevertheless, this ability is gradually lost as the cavity heats up. This disadvantage was overcome by designing the water-cooled cavity (Fig. 4). The cavity is cooled by a constant flow of water through the body of the construction in order to allow operation at a temperature lower than the flame. The temperature of the inner space of the water-cooled cavity can thus be maintained at a value of just above lOO”C, to avoid condensation of water vapour which is produced by the flame reactions. This cavity design can be used for continuous operation within the flame. The water-cooled cavity is the
83
trends in analytical chemistry, vol. 16, no. 2, 1997
best design for making full use of the advantages the Salet phenomenon 2.4.
in MECA
of
[ 5 1.
might be overcome by investigating hydrogen atom reservoir.
a non-flame
Sz emission
References The generation of excited diatomic sulphur molecules, Sz*, by flame chemiluminescence provides an extremely sensitive way for the determination of sulphur compounds. The S2 emission spectrum is a typical band spectrum with maximum intensity at 384 and 394 nm, which are used for quantitative work. In fact, the appearance of blue emission from a few mg 1-’ of sulphur visible to the human eye makes the MECA technique the most sensitive detector for sulphur compounds. Sensitivity is further increased by the Salet phenomenon. The mechanism of formation of S2 has been studied extensively [ 6,7 ] and the emission has been
used in a plethora (Table 3 ).
of analytical
applications
2.5. Other emissions Since a great variety of elements have been found to produce excited molecules within the cavity, a wide range of analytical applications by MECA have been developed (Table 4). Nevertheless, many other applications are still open for investigation.
3. The future of MECA Since the discovery of MECA, the advantages of flame chemiluminescence have been fully investigated and established. The sensitivity for sulphur compounds is probably unsurpassed. Speciation by the different t,,, values in conventional MECA may be of great help in environmental applications, such as the study of acid rain. In fact, the application of MECA in environmental analysis has never been fully examined. An area of application which has never been investigated with MECA is the synchronous measurement of two or more emissions generated within the cavity. A challenging example would be the measurement of chloride, bromide and iodide by using a copper cavity, but many other examples also exist. The cavity has been used as a detector for a variety of analytical techniques. Hydrogen sulphide or sulphur dioxide generation manifolds can easily be automated. The disadvantage of using a flame
[ I] R. Belcher,
S.L. Bogdanski and A. Townshend, Talanta, 19 (1972) 1049. [ 21 R. Belcher, S.L. Bogdanski and A. Townshend, Anal. Chim. Acta, 67 ( 1973 ) 1. 31 D. Stiles, A. Calokerinos and A. Townshend (Eds. ), Flame Chemiluminescence by Molecular Emission Cavity Detection, Wiley, Chichester, 1994. [ 41 R. Belcher, S.L. Bogdanski, D.J. Knowles and A. Townshend, Anal. Chim. Acta, 77 ( 1975) 53. [ 5 ] S.L. Bogdanski, A.C. Calokerinos and A. Townshend, Can. J. Spectrosc., 27 (1982) 10. [ 61 K. Nakajima, K. Ohta and T. Takada, Anal. Chim. Acta, 309 (1995) 163. [ 71 S. Hauge, K. Maroy and A. Thorlacius, Anal. Chim. Acta, 291 (1994) 107. [81 M. Burguera, S.L: Bogdanski and A. Townshend, CRC Crit. Rev. Anal. Chem., 10 (1980) 185. [91 M.Q. Al-Abachi, Proc. Anal. Div. Sot., 14 ( 1977 ) 251. [ 101 M. Mclean, S. Van Wagenen, D. Wiedemann, Q. Fernando and S. Raghavan, Anal. Chem., 58 (1986) 965. [ill S. Sun and R. Tang, Fenxi Huaxue, 12 ( 1984) 283. [ 121 T.A. Kouimtzis, Anal. Chim. Acta, 142 (1982) 329. N. Evmiridis and A. Townshend, J. Anal. At. [I31 Spectrom., 2 (1987) 339. [I41 S. Hauge, K. Maroy and A. Thorlacius, Anal. Chim. Acta, 251 (1991) 197. [I51 N. Grekas and A.C. Calokerinos, Analyst, 115 (1990) 613. [I61 N. Grekas and A.C. Calokerinos, Anal. Chim. Acta, 202 ( 1987) 241. [17 G.P. Prostetsov, E.V. Prostesova, V.I. Belitskii and I.D. Karpovich, Zavod. Lab., 53 (1987) 41. [18 S.L. Bogdanski, A.C. Calokerinos and A. Townshend, Int. Lab., 12 (1982) 66. [I91 V.I. Rigin, Anal. Chim. Acta, 291 ( 1994) 121. [TOI 0. Osibanzo, A.O. Bankole and S.O. Ajayi, Analyst, 114 (1989) 1483. G. Persaud, R.B. Boodhoo, D.R. Budge11 and D.A. [211 Stiles, Anal. Chim. Acta, 177 ( 1985) 247. [=I V.I. Rigin, Zh. Anal. Khim., 42 ( 1987) 1778. u31 E. Henden, Anal. Chim. Acta, 173 ( 1985) 89. u41 R. Belcher, S.L. Bogdanski, A.C. Calokerinos and A. Townshend, Analyst, 106 (1981) 625. u51 I.M.A. Shakir and A.K. Kadhir, J. Univ. Kuwait Sci., 16 (1989) 261.
84
[ 26 ] J.L. Burguera and M. Burguera, Anal. Chim. Acta, 179 (1986) 497. [ 27 ] 0. Osibanjo, S.A. Al-Tamrah and A. Townshend, Anal. Chim. Acta, 162 ( 1984) 409. [ 281 M.J. Cope and A. Townshend, Anal. Chim. Acta, 134 (1982) 93. [ 291 R. Ajlec and J. Stupar, Vestn. Slov. Kern. Drus., 33 (1986) 87. [ 301 M. Burguera, S.L. Bogdanski and A. Townshend, Anal. Chim. Acta, 153 ( 1983 ) 4 1. [ 3 1 ] I.Z. Al-Zamil and A. Townshend, Anal. Chim. Acta, 209 (1988) 275.
trends in analytical chemistry, vol. 76, no. 2, 1997
[ 321 M.Q. Al-Abachi, A. Townshend, 139. I 331 Encyclopedia of Emission Cavity York, p. 3282.
R. Belcher, S.L. Bogdanski and Anal. Chim. Acta, 86 ( 1976) Analytical Science, Molecular Analysis, Academic Press, New
Professor Antony C. Laboratory of Analytical Chemistry, University miopolis, 157 71 Athens,
Calokerinos is at the Chemistry, Department of of Athens, PanepistiGreece.
Remote electrochemical sensors for monitoring inorganic and organic pollutants Joseph Wang Las Cruces, NM, USA Some recent developments in remote electrochemical sensors for in situ monitoring of priority organic and inorganic pollutants are considered. Such research addresses the challenges associated with a continuous submersible operation of electrochemical devices in unaltered environments and in ensuring that the field results are comparable to those of laboratory instruments. The new sensor technology offers fast and accurate return of the chemical data in a timely, safe and cost-effective manner. These and similar developments should lead to a more reliable assessment of pollutant gradient and fate in aquatic environments, and should have major impact upon the characterization of contaminated sites or hostile environments and on industrial process monitoring.
1. Introduction Contamination of water resources or waste sites with toxic metals or priority organic pollutants represents a major environmental problem. The traditional use of central laboratory measurement of pollutants is too expensive and time consuming.
Also, samples often change during their collection, 0165-9936/97/$17.00 P//SOl65-9936(96)00094-S
transport and delay, ultimately leading to unreliable results. In situ measurement, in which the target pollutant is determined in its own environment, is preferable since it affords the options of rapid warning and proper feedback, while avoiding the error, delay and cost incurred in the collection of individual samples for subsequent laboratory-based analysis. New real-time analytical methods, capable of monitoring priority pollutants in both time and location, are thus highly desirable. Such challenging demands for continuous monitoring have triggered a growing interest in the development of submersible chemical sensors for real-time monitoring of priority pollutants. Chemical sensors, based on the coupling of a recognition layer with a physical transducer [ 11, hold great promise for in situ environmental monitoring. Fiber optic sensor technology has been widely used for remote monitoring of organic [ 21 and inorganic [ 3 ] pollutants. In contrast, submersible electrochemical sensors have rarely been used for environmental surveillance, except for monitoring water quality indicators (such as pH, conductivity or dissolved oxygen [ 41). Recent advances in electrochemical sensor technology hold great promise for expanding the scope of these devices towards real-time monitoring of a wide range of inorganic and organic contaminants. These advances include the introduction of modified or ultramicroelectrodes, the design of highly selective biological or chemical recognition layers, of molecular devices 0
1997 Elsevier Science B.V. All rights reserved
trends in analytical chemistry, vol. 16, no. 2, 1997
ogy. Vol. 34. Metal Ions in Biological Systems, Marcel Dekker, New York, 1997, in press. [ 221 E.D. Stein, Y. Cohen and A.M. Winer, CRC Crit. Rev. Environ. Sci. Technol., 26 (1996) l-43. [ 231 L. Liang, M. Horvat and N.S. Bloom, Talanta, 41 (1994) 371-379. [ 241 P.J. Craig, D. Mennie, N. Ostah, O.F.X. Donard and F. Martin, Analyst, 117 ( 1992) 823-824. [ 25 J P.J. Craig, D. Mennie, M. Needham, N. Oshah, O.F.X. Donard and F. Martin, J. Organomet.
Chem., 447 (1993) 5-8. K. Kwetkat and W. Kitching, J. Chem. Sot. Chem. Commun., (1994) 345-347. [ 27 ] P.J. Craig, M.L. Needham, N. Ostah, G.H. Stojak, M. Symons and P. Teesdale-Spittle, J. Chem. Sot. [ 261
Dalton Trans., (1996) 153-156. [ 28 ] F.M. Martin and O.F.X. Donard, J. Anal. Atom.
Spectrom., 9 (1994)
1143-1151.
[ 291 F.A. Cotton and G. Wilkinson,
ganic Chemistry, 1988, p. 165ff.
Advanced InorWiley, New York, 5th edn.,
James Weber’s education includes a PhD degree in inorganic chemistry from the Ohio State
University (7963). In 7963 he became an assistant professor of chemistry at the University of New Hampshire and became a full professor in 1977. During his most recent sabbatical leave in 199192 at ENEA (La Spezia, Italy) he studied abiotic methylation of mercury(ll). He was an invited participant in the European Community workshop “Trends in Speciation Analysis in Environmental and Food Matrices” (Rome, 1994). Recent invited lectures include ones at the international Conference on Environmental and Biological Aspects of Main-Group Organometals (Bordeaux, 1994) and the 4th Symposium on Analytical Sciences (Brussels, 1996). He is on the editorial board of Applied Organometallic Chemistry. His research has been supported by the USA agencies NSF, EPA, and NOAA. He is now at the University of New Hampshire, Chemistry Department, Parsons Hall, Durham, NH 03824, where he directs research of graduate students on the biogeochemistry of methylmercury compounds in the estuarine environment. He published more than 90 papers in refereedjournals.
Molecular emission cavity analysis: principles and applications Antony C. Calokerinos Athens, Greece Molecular emission cavity analysis (MECA) is a chemiluminescence technique in which molecular emissions are generated within a cavity that is introduced into a hydrogen diffusion flame. The hydrogen flame acts as the environment of radicals, atoms and molecules which promote the chemical reactions producing excited species and the cavity acts as the environment for stabilizing the emitting species. The principles, development and applications of MECA are outlined and future possibilities are discussed.
1. Introduction: the genesis of a new analytical technique Molecular emission cavity analysis was discov0165-9936/97/$17.00 P//SO 165.9936
(96
)00096-9
ered in the early 1970s by Bogdanski, Townshend and (the late) Belcher at the Department of Chemistry of the University of Birmingham, UK. At that time, Belcher was interested in unusual flame tests which were mentioned in Feigl’s Spot Tests in Znorganic Chemistry. He asked Bogdanski to work for his PhD on what was later called candoluminescence, in which the luminescent emission is generated on the surface of a solid in contact with a hydrogen flame [ 11. Bogdanski used the cavity in the head of an ordinary Allen screw for introducing the solid into the flame environment. The screw was then cleaned with a mixture of phosphoric and sulphuric acids and washed with water. When the screw was brought into contact with a hydrogen flame, an intense blue emission followed by a green emission was noticed. It was soon realized that both emissions were arising from the extremely low residual amounts of the acids. When the experiment was repeated by injecting a solution of sulphuric acid into the Allen screw 8
1997 Elsevier Science B.V. All rights reserved.
trends in analytical chemistry, vol. 16, no. 2, 7997
Table 1 Emissions generated within the MECA cavity and the most commonly used wavelengths for analytical measurement (reproduced from Ref. [ 331 with permission of Academic Press)
79
over the years has been critically evaluated recently appeared in a monograph [ 3 1.
2. Principles Element Normal cavity Sulphur Tellurium Phosphorus Selenium Tin Gallium Germanium Lithium Cadmium Oxy-cavitp Antimony Arsenic Boron Carbon Nitrogen Silicon Thallium
Emitting particle
s2 Te2
HP0 Se2
SnO GaBr Gal GeCl Li Cd
SbO-0 continuum AsOcontinuum BO2
CH NO-O continuum SiO TI
Copper cavity or indium-lined cavitp Bromine CuBr InBr Chlorine CUCI InCl Iodine Cul Inl
and
and applications
Wavelength (nm)
384 500 528 411 485 350 391 455 670.8 326.1
355 400 518 431.5 500 540 377.5
494 375 532 360 510 410
“Slow flow of pure oxygen is supplied to the cavity in orderto promote molecular emissions from elements which require the presence of oxygen in the excited molecule or radical (such as BO?) or due to oxygen atom combination reactions (such as AsO+O + As02+hv). ‘Both cavities promote emissions from halide-containing molecules.
which was then introduced into the hydrogen flame, a bright blue emission was observed, attributed to the formation of excited Sz* molecules. Similarly, phosphoric acid generated an intense green emission due to excited HP0 species. In both experiments, the intensity of the emission was proportional to the amount of acid injected into the cavity. Hence, molecular emissions were generated within a cavity and this was the birth of molecular emission cavity analysis (MECA). The first publication appeared in 1973 [ 2 ] and since that time the technique has grown rapidly and has helped scientists to understand the principles of flame chemiluminescence. The plethora of information collected
The distinct features of MECA are the hydrogen diffusion flame and the cavity. The hydrogen flame acts as the environment of radicals, atoms and molecules which promote the chemical reactions producing excited species and the cavity acts as the environment for stabilizing the emitting species. 2.1. Hydrogen diffusion flame The flame normally used in MECA is the hydrogen diffusion flame, which is a flame of hydrogen diluted with nitrogen burning with atmospheric oxygen diffusing into the gas mixture. The main reaction of the flame is H2+HO’+H+H20 The maximum temperature of the hydrogen diffusion flame does not exceed 1000°C and since mixing of hydrogen and atmospheric oxygen is controlled by diffusion, it is obvious that from the edges to the centre of the flame body, the amount of oxygen which penetrates is drastically decreased. As a result of this difficulty in mixing of oxygen with hydrogen, the temperature in the centre of the flame is lower than that at the edge. The change in temperature is accompanied by a change in the nature and concentration of flame particles. The edge of the flame is rich in oxygen and also contains water vapour from the main chemical reaction of the flame. The inner part of the flame is rich in atomic and unburnt hydrogen. Hence the hydrogen diffusion flame is a cell for chemiluminogenic reactions (flame chemilumines-
Table 2 Typical t,,, values for some sulphur anions [ 81 and organic compounds [ 91 Compounda
till (s)
Compounda
tfll (s)
Na2S Na2S03 KSCN
2.0 2.4 18
Cystine Promethazine Sulphadiazine
0.5 0.2 2.0
“Experimental compounds.
conditions are the same for each group of
80
trends in analytical chemistry, vol. 16, no. 2, 7997
bamplc introduction
/
detector
Fig. 1. Conventional MECA cavity introduction Science B.V. from Ref. [ 41).
into flame by manual rotation
cence) which can generate excited species by direct mechanisms, e.g.
reprinted with permission
of Elsevier
temperature and rich in hydrogen atoms, favouring the generation of excited molecules such as S2 and
SnC12 + H + SnCl* + HCl or indirect mechanisms in which energy is transferred by or during recombination reactions: Na+H+H+Na*+Hz
sof-or s,o;-
Na+H+HO’-+Na*+H20 Nevertheless, the hot environment of the flame destroys the temperature-sensitive excited species. This difficulty can be overcome by carrying out the flame chemiluminescent reactions within a protected area, which is the cavity. The cavity provides an area sheltered from the hot environment of the flame and promotes chemiluminogenic reactions. The main characteristics of the cavity are: ( 1) The position of the cavity within the flame governs the nature and concentration of atoms, radicals and molecules which mix with the analyte. Thus, if the cavity is positioned at the centre of the flame, it operates within an environment of low
Time
(S)
Fig. 2. Sz responses from 5 ml of a mixture containing 20 mg S 1-r as Na2S03+20 mg S I--I as KSCN+40 mg S I-’ as (NH4)2S208 injected into a stainless-steel cavity (flame composition: 1.71 I HZ min-‘+4.0 I N2 min-’ ) ( reprinted with permission of Elsevier Science B.V. from Ref. [ 321).
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trends in analytical chemistry, vol. 16, no. 2, 1997
Injection
Bubble breaker
k-
cw---
P
NaOH pellets
cavity
r/
Detector
fl
-Burner
head
Fig. 3. Schematic diagram of a gas generation MECA system for the determination of ammonium by generation of ammonia (reprinted with permission of The Royal Society of Chemistry from Ref. [ 241).
Sez. When the cavity is positioned at the edge of the flame, then the environment is rich in molecular oxygen and hydroxyl radicals at high temperature and oxygen-containing excited species are formed. This region can also generate atomic emissions by thermal excitation. (2) The temperature of the cavity can be lowered by allowing a constant flow of water to flow through its body. This allows continuous operation of the cavity within the flame and promotes emissions from species such as S2 and HP0 (Salet phenomenon). ( 3 ) The environment of the cavity can be altered by the introduction of a gas or from its material of construction in order to select and promote specific excitation species (Table 1). 2.2. Cavity Sample introduction
into the flame via the cavity
can be achieved by two different techniques: conventional MECA and gas generation MECA detection. 2.2.1.
Conventional
MECA
In conventional MECA the sample is deposited into the cavity (typical dimensions diameter 8 mm and depth 5 mm). The sample is usually a few microlitres of solution or milligrams of solid. The cavity is then introduced rapidly and reproducibly into the flame (Fig. 1). After introduction into the flame, the cavity temperature increases from ambient to its maximum value, which depends on material and conditions [ 4 1. During the heating up period of the cavity in the flame, a series of physical and chemical changes occur which are accompanied by the generation of the characteristic molecular emission. The cavity remains in the flame as long as required for the emission to reach maximum radiation and cease.
Table 3 Some representative
analytical
Technique
Mode
Compounds
Ref.
Conventional
Manual
[lOI
Gas generation
Automated Flow injection
Thio-/dithioglycolic acids (8.0 x 10e5-1 .O x 1O-3 M thioglycolic acid) Trace Sod*- in high-purity NH3 40-90 mg 9-l of sulphur in selenium Inorganic and organic Sulphur anions and colloidal S with limits of detection in the range 0.1-0.8 mg 1-l Cephalosporins in the range 10.0-250.0 mg I-’ Organic sulphur compounds (5.0-30.0 mg I-’ ) Sulphur compounds 0.1-l 70 ng thiophene, 0.2-200 ng 3-methylthiophene
applications
Continuous flow Electrolysis Graphite cell Gas chromatography
of S2 emission
iI51 1161 [I71 1181
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trends in analytical chemistry, vol. 16, no. 2, 1997
Table 4 Some representative
analytical
applications
of MECA (other than S2 emission)
Element
Application
Ref
Halides
Polychlorinated organic compounds, InCl emission (limit of detection 0.05 ng of chlorine) General studies with copper cavity, CuX (0.003-0.01 MCI-, 0.001-0.006 M Br-, 0.001-0.03 MI-) Organochlorine pesticides, polychlorinated biphenyls (1 O-l 50 mg of chlorine) 0.05-I 00 mg 9-l of Ge, As, Sb, Sn and Hg in coal or biological material by formation of volatile bromides Ge (0.2-10 kg), As (0.1-5 pg), Sn (0.5-10 pg) and Sb (l-40 l.tg) as hydrides, gas chromatography Generation of ammonia (l-100 mg I-’ of nitrogen), fertilizers Total nitrogen in tobacco leaves after Kjeldahl digestion Organophosphorus insecticides (5-100 ng of phosphorus), flow injection Phosphate (100-500 mg I-’ of phosphorus) in detergents After HPLC separation ( 1O-l 00 ng of phosphorus) Removal of cationic interferences by cation exchange 0.2-500 mg I-’ of Si, formation of volatile SiF4 0.1-40 mg I-’ of Sn, hydride formation
[ 191
Metals and metalloids
Nitrogen Phosphorus
Silicon Tin
It is then removed and cooled before repetition of the process. The time required for the appearance of maximum emission intensity after the introduction of the cavity into the flame is usually represented by t, and depends on the compound injected and the thermal properties of the cavity. Some examples of t, values are given in Table 2. Fig. 2 shows an example of how the t, value can be used to determine the components of a mixture containing sulphite, thiocyanate and sulphate (or peroxodisulphate). 2.2.2. Gas generation MECA detection In gas generation MECA detection, the sample must be in the form of a gas or vapour which is introduced into the cavity through a side duct (Fig. 3). The emission is generated during passage of the gas or vapour through the cavity. The analyte is converted into a gas by a chemical reaction in a carrier + analyte gas cooling water in p+
-7 Ll
cavity
Iszss&L a cooling water out
1
Fig. 4. Schematic diagram of a water-cooled cavity.
[201
[211 [221
[231 [241 [251
[261
[271
1281 1291 [301 [311
closed reactor, which is continuously purged by a carrier gas, such as nitrogen or argon. The cavity usually used is water-cooled to allow permanent stay within the flame. 2.3. Salet phenomenon:
recognition
after a
ten tury
In 1869, Salet observed that the blue emission from sulphur compounds and the green emission from phosphorus compounds within a hydrogen flame become more intense when a cool body is introduced into the hot environment of the flame. The importance of the discovery was realized during the development of the flame photometric detector for gas chromatography and is now known as the S&et phenomenon. The observation is due to the stabilization of the excited molecules on the cool surface. In MECA, the ordinary cavity acts as a cool body and enhances the S2 emission. Nevertheless, this ability is gradually lost as the cavity heats up. This disadvantage was overcome by designing the water-cooled cavity (Fig. 4). The cavity is cooled by a constant flow of water through the body of the construction in order to allow operation at a temperature lower than the flame. The temperature of the inner space of the water-cooled cavity can thus be maintained at a value of just above lOO”C, to avoid condensation of water vapour which is produced by the flame reactions. This cavity design can be used for continuous operation within the flame. The water-cooled cavity is the
83
trends in analytical chemistry, vol. 16, no. 2, 1997
best design for making full use of the advantages the Salet phenomenon 2.4.
in MECA
of
[ 5 1.
might be overcome by investigating hydrogen atom reservoir.
a non-flame
Sz emission
References The generation of excited diatomic sulphur molecules, Sz*, by flame chemiluminescence provides an extremely sensitive way for the determination of sulphur compounds. The S2 emission spectrum is a typical band spectrum with maximum intensity at 384 and 394 nm, which are used for quantitative work. In fact, the appearance of blue emission from a few mg 1-’ of sulphur visible to the human eye makes the MECA technique the most sensitive detector for sulphur compounds. Sensitivity is further increased by the Salet phenomenon. The mechanism of formation of S2 has been studied extensively [ 6,7 ] and the emission has been
used in a plethora (Table 3 ).
of analytical
applications
2.5. Other emissions Since a great variety of elements have been found to produce excited molecules within the cavity, a wide range of analytical applications by MECA have been developed (Table 4). Nevertheless, many other applications are still open for investigation.
3. The future of MECA Since the discovery of MECA, the advantages of flame chemiluminescence have been fully investigated and established. The sensitivity for sulphur compounds is probably unsurpassed. Speciation by the different t,,, values in conventional MECA may be of great help in environmental applications, such as the study of acid rain. In fact, the application of MECA in environmental analysis has never been fully examined. An area of application which has never been investigated with MECA is the synchronous measurement of two or more emissions generated within the cavity. A challenging example would be the measurement of chloride, bromide and iodide by using a copper cavity, but many other examples also exist. The cavity has been used as a detector for a variety of analytical techniques. Hydrogen sulphide or sulphur dioxide generation manifolds can easily be automated. The disadvantage of using a flame
[ I] R. Belcher,
S.L. Bogdanski and A. Townshend, Talanta, 19 (1972) 1049. [ 21 R. Belcher, S.L. Bogdanski and A. Townshend, Anal. Chim. Acta, 67 ( 1973 ) 1. 31 D. Stiles, A. Calokerinos and A. Townshend (Eds. ), Flame Chemiluminescence by Molecular Emission Cavity Detection, Wiley, Chichester, 1994. [ 41 R. Belcher, S.L. Bogdanski, D.J. Knowles and A. Townshend, Anal. Chim. Acta, 77 ( 1975) 53. [ 5 ] S.L. Bogdanski, A.C. Calokerinos and A. Townshend, Can. J. Spectrosc., 27 (1982) 10. [ 61 K. Nakajima, K. Ohta and T. Takada, Anal. Chim. Acta, 309 (1995) 163. [ 71 S. Hauge, K. Maroy and A. Thorlacius, Anal. Chim. Acta, 291 (1994) 107. [81 M. Burguera, S.L: Bogdanski and A. Townshend, CRC Crit. Rev. Anal. Chem., 10 (1980) 185. [91 M.Q. Al-Abachi, Proc. Anal. Div. Sot., 14 ( 1977 ) 251. [ 101 M. Mclean, S. Van Wagenen, D. Wiedemann, Q. Fernando and S. Raghavan, Anal. Chem., 58 (1986) 965. [ill S. Sun and R. Tang, Fenxi Huaxue, 12 ( 1984) 283. [ 121 T.A. Kouimtzis, Anal. Chim. Acta, 142 (1982) 329. N. Evmiridis and A. Townshend, J. Anal. At. [I31 Spectrom., 2 (1987) 339. [I41 S. Hauge, K. Maroy and A. Thorlacius, Anal. Chim. Acta, 251 (1991) 197. [I51 N. Grekas and A.C. Calokerinos, Analyst, 115 (1990) 613. [I61 N. Grekas and A.C. Calokerinos, Anal. Chim. Acta, 202 ( 1987) 241. [17 G.P. Prostetsov, E.V. Prostesova, V.I. Belitskii and I.D. Karpovich, Zavod. Lab., 53 (1987) 41. [18 S.L. Bogdanski, A.C. Calokerinos and A. Townshend, Int. Lab., 12 (1982) 66. [I91 V.I. Rigin, Anal. Chim. Acta, 291 ( 1994) 121. [TOI 0. Osibanzo, A.O. Bankole and S.O. Ajayi, Analyst, 114 (1989) 1483. G. Persaud, R.B. Boodhoo, D.R. Budge11 and D.A. [211 Stiles, Anal. Chim. Acta, 177 ( 1985) 247. [=I V.I. Rigin, Zh. Anal. Khim., 42 ( 1987) 1778. u31 E. Henden, Anal. Chim. Acta, 173 ( 1985) 89. u41 R. Belcher, S.L. Bogdanski, A.C. Calokerinos and A. Townshend, Analyst, 106 (1981) 625. u51 I.M.A. Shakir and A.K. Kadhir, J. Univ. Kuwait Sci., 16 (1989) 261.
84
[ 26 ] J.L. Burguera and M. Burguera, Anal. Chim. Acta, 179 (1986) 497. [ 27 ] 0. Osibanjo, S.A. Al-Tamrah and A. Townshend, Anal. Chim. Acta, 162 ( 1984) 409. [ 281 M.J. Cope and A. Townshend, Anal. Chim. Acta, 134 (1982) 93. [ 291 R. Ajlec and J. Stupar, Vestn. Slov. Kern. Drus., 33 (1986) 87. [ 301 M. Burguera, S.L. Bogdanski and A. Townshend, Anal. Chim. Acta, 153 ( 1983 ) 4 1. [ 3 1 ] I.Z. Al-Zamil and A. Townshend, Anal. Chim. Acta, 209 (1988) 275.
trends in analytical chemistry, vol. 76, no. 2, 1997
[ 321 M.Q. Al-Abachi, A. Townshend, 139. I 331 Encyclopedia of Emission Cavity York, p. 3282.
R. Belcher, S.L. Bogdanski and Anal. Chim. Acta, 86 ( 1976) Analytical Science, Molecular Analysis, Academic Press, New
Professor Antony C. Laboratory of Analytical Chemistry, University miopolis, 157 71 Athens,
Calokerinos is at the Chemistry, Department of of Athens, PanepistiGreece.
Remote electrochemical sensors for monitoring inorganic and organic pollutants Joseph Wang Las Cruces, NM, USA Some recent developments in remote electrochemical sensors for in situ monitoring of priority organic and inorganic pollutants are considered. Such research addresses the challenges associated with a continuous submersible operation of electrochemical devices in unaltered environments and in ensuring that the field results are comparable to those of laboratory instruments. The new sensor technology offers fast and accurate return of the chemical data in a timely, safe and cost-effective manner. These and similar developments should lead to a more reliable assessment of pollutant gradient and fate in aquatic environments, and should have major impact upon the characterization of contaminated sites or hostile environments and on industrial process monitoring.
1. Introduction Contamination of water resources or waste sites with toxic metals or priority organic pollutants represents a major environmental problem. The traditional use of central laboratory measurement of pollutants is too expensive and time consuming.
Also, samples often change during their collection, 0165-9936/97/$17.00 P//SOl65-9936(96)00094-S
transport and delay, ultimately leading to unreliable results. In situ measurement, in which the target pollutant is determined in its own environment, is preferable since it affords the options of rapid warning and proper feedback, while avoiding the error, delay and cost incurred in the collection of individual samples for subsequent laboratory-based analysis. New real-time analytical methods, capable of monitoring priority pollutants in both time and location, are thus highly desirable. Such challenging demands for continuous monitoring have triggered a growing interest in the development of submersible chemical sensors for real-time monitoring of priority pollutants. Chemical sensors, based on the coupling of a recognition layer with a physical transducer [ 11, hold great promise for in situ environmental monitoring. Fiber optic sensor technology has been widely used for remote monitoring of organic [ 21 and inorganic [ 3 ] pollutants. In contrast, submersible electrochemical sensors have rarely been used for environmental surveillance, except for monitoring water quality indicators (such as pH, conductivity or dissolved oxygen [ 41). Recent advances in electrochemical sensor technology hold great promise for expanding the scope of these devices towards real-time monitoring of a wide range of inorganic and organic contaminants. These advances include the introduction of modified or ultramicroelectrodes, the design of highly selective biological or chemical recognition layers, of molecular devices 0
1997 Elsevier Science B.V. All rights reserved