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Figure 1 Comparison of the prices of commercially available instruments used for element analysis

For instance, Cr(III) ions may coexist with Co(II) and Cu(II) ions in a complex sample. The latter two ions may produce CL emission under similar conditions as for Cr(III). Fortunately, the formation rate of the Cr-EDTA complex is relatively slower, making possible the selective determination of this metal ion in waste water [8], urine, blood, and hair [9]. Owing to the small number of CL reagents explored in recent years, the elements covered by CL techniques are still rather limited. Up to now only a few elements have been found to produce direct CL emission when reacted with CL reagents. Most of the publications so far involve indirect methods for the detection of elements.

3. PROGRESS IN ANALYTICAL CL METHODOLOGY

3.1CL Measurements Based on Direct Oxidation, Catalysis, and Inhibition Effects

Inorganic compounds are mainly determined based on their catalytic effect on CL reactions. Direct oxidation reactions are also available. Determination of ions based on the inhibition of the catalytic reaction was explored to extend the analytical applications. More than 20 metals ions and some nonmetal compounds were determined with very high sensitivity, though poor selectivity, employing such methods. Without doubt, the most useful, versatile, and efficient CL system currently available for the determination of inorganic ions still remains the luminol reaction although the system has poor selectivity for some typical metal ions. To some extent, lucigenin was also extensively studied for the analysis of inorganic compounds. As they may not directly produce a CL reaction, some inorganic

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ions could be determined by coupling an enzyme reaction with a luminol-based CL reaction. For example, phosphate ion was determined using the reaction of phosphate-ion-dependent pyruvate oxidase, the hydrogen peroxide produced being detected by luminol CL [10].

In the oxalate ester area, bis-(2,4,6-trichlorophenyl)oxalate (TCPO) belongs to one of the most important reagents for CL detection of, e.g., aminosubstituted polycyclic aromatic hydrocarbons and H2 O2. Only a few reports refer to inorganic compounds by using this CL system, however. For instance, Quass and Klockow [11] developed a method for the determination of Fe(II) and H2 O2 in atmospheric liquids by the CL reaction of oxygen with TCPO in the presence of the Fe(II) ion. The method gives detection limits below 100 nM Fe(II). Sato and Tanaka [12] reported a flow injection CL method for the determination of Al(III), Zn(II), Cd(II), and In(III). The method is based on the formation of fluorescent compounds of the metal ions with 8-hydroxyquinoline that produce CL emission in the reaction of TCPO and hydrogen peroxide. The detection limits for the metal ions were in the 20–70-ng/ml range.

An electrogenerated chemiluminescence (ECL) method was recently developed by several authors [13, 14] for the production of an (unstable) reagent for CL inorganic analysis. Zheng and Zhang [13] developed a novel electrolytic flowthrough cell for the production of BrO from KBr. The method was applied to the determination of sulfide based on the strong enhancing effect upon the CL emission intensity in the reaction between BrO and luminol. A similar method was developed by Qin et al. [14] for determination of the ammonium ion based on chlorine production from the CL ion. To improve the sensitivity and selectivity of CL detection, Zhang et al. [15] developed a stripping CL method for preconcentration and determination of metal ions. The method consists of a concentration (preelectrolysis) step, in which the analyte metal ion is reduced at a controlled potential while being deposited as the metal on a solid electrode placed in an ECL flow cell, a subsequent media exchange step, and finally the stripping-CL step, in which the CL emission on the electrode surface is detected in situ when the CL reagent solution is delivered into the cell to react with the metal ion, just stripped by oxidation. Copper ion as a representative analyte was investigated using constant potential stripping and CL detection of a luminol-copper(II)-cya- nide system. The low detection limit of 0.02 ng/mL could be achieved in a deposition time of 2 min. A similar design was also used for the development of a CL flow-through sensor for the copper ion [16].

Most of the described liquid CL systems are employed at ambient temperatures although CL emission may also be generated at very low temperatures. For instance, a low-temperature interaction of U(IV) and XeO3 in frozen aqueous H2 SO4 solutions accompanied by CL emission was studied by Lotnik et al. [17]. It was shown that the peak of luminescence at 195–200 K is related to CL of

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the excited uranyl ion (UO22 ) formed in the oxidation of U(V), an intermediate product of the interaction of U(IV) and XeO3. This reaction, however, has not yet been applied for analytical purposes.

Major efforts have been undertaken in recent years for developing new CL systems. These studies extend the application of CL analysis to inorganic species. It should be noted, however, that there still exists no new CL system that exceeds the sensitivity offered by luminol for current metal ions. Table 2 lists some typical examples of inorganic analyses using different CL systems.

3.2 CL Measurements Based on Reduction Reactions

Several authors observed CL emission based on reduction reactions. Lu et al. [59] developed a method by applying a Jones reductor for producing unstable reductants. A column (100 3 mm i.d.) filled with Zn-Hg particles was inserted into the flow stream of a flow injection system. CL was measured using a homemade CL analyzer. Although the Jones reductor was more effective for the species studied in 0.5–5 mol/L H2 SO4 solution, the authors found that a lower acid concentration improved the CL emission. The optimal pH was 6.5 for V(II), 2.5 for Mo(III), 3.5 for U(III), 3.0 for W(III), 3.0 for Cr(II), 2.5 for Ti(III), and 2.5

for Fe(II). The methods allowed determination of the above-mentioned species at g/mL to ng/mL levels. It was assumed that the CL reactions were related to

the production of superoxide radicals by dissolved oxygen in the solutions. The proposed methods could be successfully applied to the determination of V [60], Mo [61], and U [62] in water or steel samples.

It is well known that lucigenin produces CL emission when reacted with organic reducers such as ascorbic acid. Lu et al. [63–66] extended the reaction to inorganic ions. In the series of their reports they explored the reactions between lucigenin and reducers such as Cr(II), Fe(II), Mo(III), Ti(III), U(III), V(II), and W(III), among others, and found that CL emission can be produced by the reaction between the reductant and lucigenin. With this method, the ions mentioned above could be sensitively determined by using an online flow injection system applying a Jones reductor. It should be noticed that the detection limits for most ions studied were lower than those obtained using luminol reactions. Table 3 lists a comparison of the results.

3.3 CL Measurements Based on Coupling Reactions

Although many inorganic compounds cannot directly produce CL emission when reacting with the current CL reagents, they may be determined by coupling a CL reaction with another reaction, the latter producing a product playing a key role in the CL emission. A typical example is the determination of molybdenum(VI) ions based on the accelerating effect of the ions on the KBO3-KI reaction [67].

Table 2 Analysis of Inorganic Species Using CL Systems

		Detection limit
Analyte	CL system	(ng/mL)	Ref.
Ag(I)	Luminol-K2 S2 O8	10	18
	Lucigenin-H2 O2	100	19
	Gallic acid-H2 O2	500	20
As(V)	Luminol	10	21
As(III)	Luminol-Cr(VI)-H2 O2	1	22
Au(III)	Luminol-H2 O2	0.04	1, 23
	Tween-40-H2 O2	0.2	24
	Coumarin-KOH-H2 O2	0.6	25
Co(II)	Luminol-H2 O2	0.0074	26
	Lucigenin-H2 O2	0.02	27
	Gallic acid-H2 O2	0.01	28
	Sulfosalicylic acid-H2 O2	0.001	29
Cr(III)	Luminol-H2 O2	0.0015	30
	Rhodamine-H2 O2	28	31
Cu(II)	Luminol-KCN	0.009	32
	Luminol-H2 O2	1	33
	4-Hydroxy-coumarin-H2 O2-CTMAB	0.3	34
	o-Phenanthroline-H2 O2-Sn(IV)	0.16	35
Fe(III)	Luminol-H2 O2	0.04	36
Fe(II)	Luminol-O2	0.1	37
	Gallocyanin-H2 O2	0.07	38
Hg(II)	Luminol-Cu(II)-KCN	2	39
Ir(III)	Luminol-KIO4	1.0	40
Mn(II)	Luminol-H2 O2	0.08	41
	Gallic acid-H2 O2	400	20
	MnO4 -Na2 CO3-KOH	0.1	42
Ni(II)	Luminol-H2 O2	0.01	43
	Alizarine violet-CTMAB	0.1	44
	Anthracene green-H2 O2	0.01	45
Os(VIII)	Luminol-H2 O2	0.2	46
	1,10-Phenanthroline-H2 O2	0.0004	47
Pb(II)	Luminol-H2 O2	1000	33
	Gallic acid-H2 O2	1000	20
Ru(III)	Luminol-H2 O2	0.3	48
	Luminol-KIO4	3	48
	Luminol-K2S2 O8	60	48
Rh(III)	Luminol-KIO4	40	49
Si(IV)	Luminol	0.1	50
	Luminol-heteropoly acid	10	51
Cl2	Luminol	0.2	52
	Luminol-H2 O2	4	53
Br2	Luminol	6	54
I2	Luminol	10 9 (M)	55
NO2	Luminol	0.46	56
CN	Luminol	1.2	57
	Luminol-Cu(II)	2	58

CTMAB, cethyltrimethyl amonium bromide.

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Table 3 CL Determination of Inorganic Ions Based on Reduction Reaction
			Linear range
Analyte	Reduced form	CL reagent	(g/mL)	Ref.
Cr(VI)	Cr(II)	Luminol	1.0–10	59
		Lucigenin	0.1–10	63
Fe(III)	Fe(II)	Luminol	0.001–2.0	59
		Lucigenin	0.00001–10	64
Mo(VI)	Mo(III)	Luminol	0.001–0.1	61
		Lucigenin	0.0001–1.0	65
Ti(IV)	Ti(III)	Luminol	0.1–1.0	59
		Lucigenin	0.001–6.0	63
U(VI)	U(III)	Luminol	0.001–2.0	59
		Lucigenin	0.001–10	66
V(V)	V(II)	Luminol	0.0001–0.01	60
		Lucigenin	0.001–90	63
W(VI)	W(III)	Luminol	0.01–0.04	59
		Lucigenin	0.001–10	63
Sn(IV)	Sn(II)	Luminol	0.1–1.0	59

The CL signal arises from the reaction of luminol with I2 produced by the latter reaction. Other reactions are also available including KIO3-KI [68], H2O2-KI [69], and H2 O2-Na2 S2 O3 reactions [70] catalyzed by Mo(VI), among others. Similar reactions could also be used for the determination of trace vanadium(V) ions [71, 72]. The developed methods were applied to the determination of trace elements in natural waters and other matrices with detection limits below the ng/mL levels for traces of molybdenum and vanadium.

Another example for the determination of an element based on CL coupling reactions is the arsenic determination by coupling K2 Cr2 O7-AsH3 to the luminol- H2 O2-Cr(III) reaction [22]. Arsine was produced by hydride generation and oxidized by K2 Cr2 O7 in acidic medium. The Cr(III) ions produced in the first reaction could be reacted with luminol and H2 O2 to generate CL emission. A similar method was used for the determination of SO2 in air [73] and Na2S in water [74].

An indirect method for the determination of lead by coupling reactions was developed based on the replacement of Fe(II) by Pb(II) from the Fe(II)-EDTA complex. The subsequent CL reaction was based on the Fe(II)-luminol-O2 system. The method was used to determine lead in polluted water samples [75]. Such methods may be extended to other ions with proper complex constants as compared to the Fe(II)-EDTA complex, after HPLC separation. Analysis of elements based on indirect reactions is summarized in Table 4.

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Table 4 Analysis of Elements Using Coupling Reactions
Element	Coupling reactions	Detection limit	Ref.
As(III), (V) Cr2 O72 AsH3 → Cr3 AsO43		1 ng/mL As	22
	Luminol H2 O2 → CL light
	MoO42 I → I2	40 pg/mL As	76
	Luminol I2 → CL light
Cr(VI)	CrO42 Fe(CN)62 → Cr3 Fe(CN)63	20 pg/mL Cr	77
	Luminol Fe(CN)63 → CL light
Ge(IV)	MoO42 I → I2 MoO32	0.3 ng/mL Ge	78
	Luminol I2 → CL light
Hg(II)	Hg2 CN → Hg(CN)42	2.9 ng/mL Hg	79
	Luminol Cu2 CN → CL light
Mo(VI)	BrO3 I → I2 Br	3 ng/mL Mo	67
	Luminol I2 → CL light
	IO3 I → I2 Br	3 ng/mL Mo	68
	Luminol I2 → CL light
	S2 O32 H2 O2 → SO42 H2 O	0.12 ng/mL Mo	70
	Luminol H2 O2 → CL light
Pb(II)	Pb2 Fe-EDTA → Pb-EDTA Fe2	—	75
	Luminol Fe2 O2 → CL light
PO43	MoO42 I → PO43 I2 MoO32	4 ng/mL PO43	80
	Luminol I2 → CL light
Rh(III)	BrO3 Mn2 → MnO4 Br2	5 pg/mL Rh	81
	Luminol MnO4 → CL light
V(V)	ClO3 I → I2 Cl	2.8 ng/mL V	71
	Luminol I2 → CL light
	S2 O32 H2 O2 → SO42 H2 O	0.43 ng/mL	72
	Luminol H2 O2 → CL light
S2	Fe3 S2 → Fe2 S	0.4 nmol/L	82
	Luminol Fe2 O2 → CL light
	Cr2 O72 S2 H → Cr3 S	0.25 ng/mL	74
	Luminol H2 O2 → CL light

3.4CL Measurements Based on Time-Resolved Techniques

Time-resolved CL analysis is based on the measurement of the difference of dynamic rates of CL reactions as proposed by Zhang et al. in 1989 [83]. It aims at improvement of the selectivity for analysis of metal ions in real samples, as it is known that generally such analyses suffer from poor selectivity since various metal ions may catalyze the same CL reaction—e.g., luminol/H2 O2—under simi-

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Figure 2 Typical dynamic profile of CL emission of Au(III) and Ag(I). (From Ref. 84, with permission.)

lar conditions. A typical example is the determination of silver in an environmental sample containing cobalt [84] or gold. Although it is well known that the latter two ions may produce strong CL emission similar to Ag(I) by catalyzing the reaction of luminol with persulfate in alkaline medium, a well-resolved Ag peak of CL emission was observed from the peaks of Co(II) or Au(III) due to a slower dynamic reaction of Ag with persulfate. Based on the kinetic distinction of Au(III) and Os(IV) in the Tween 80–KOH CL system, a time-resolved CL analysis for the simultaneous determination of traces of Au(III) and Os(IV) was proposed by Han et al. [85] offering detection limits of about 1 ng/mL for Au(III) and 10 ng/mL for Os(IV). The method was applied with satisfactory results to the determination of Au(III) and Os(IV) in metallurgical materials samples of noble metals [85]. Figure 2 shows the typical dynamic profile of CL emission of Au(III) and Ag(I).

3.5 CL Measurements on Solid Surfaces

In recent years, a series of luminol-based CL methods were developed for the determination of trace elements on a solid surface.

An early study referred to the determination of traces of gold in ores by absorbing this element on the surface of foamed plastic [23]. The sample containing the gold traces was first dissolved by a HCl-HNO3 mixed solution and a piece of foamed plastic was then placed therein. After vibration for 30 min. on a vibrator, the solid material was removed, washed with tap water, and placed in a CL cell. A 2-mL volume of 1.0 mmol/L EDTA was added to the cell and the lid was closed; then 2 mL of 0.01 mmol/L luminol solution was injected into

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Figure 3 Schematic diagram of a typical instrument for measuring CL signals on a solid surface.

the cell and the CL signal was measured by using a laboratory-constructed CL photometer. With this procedure, traces of gold could be selectively determined in the range of 0.1–10 g with an RSD of less than 10%.

Determination of silver in ores was also carried out on filter paper as developed by Zhang et al. [86]. Ag(I) was first separated by using a ring oven technique and determined by luminol-based CL measurement directly on the filter paper. The method permits selective determination of silver in the range of 0.5–50 ng with an RSD of 8.4%. Similar methods were also developed for the CL-based determination of Co(II) [87], Ni(II) [88], and Cr(VI) [89] on filter paper or on ion-exchange resins. The studies showed that solid-surface CL analysis (SSCL) has advantages over the suggested solid-surface fluorescence analysis (SSF) and solid-surface room temperature phosphorescence analysis (SSRTP) because the latter methods suffer from a high background emission from the solid substrate resulting from scattering of excitation light [90]. Figure 3 shows the schematic diagram of a typical instrument for measuring CL signals on a solid surface.

3.6Hyphenated Techniques: Coupling HPLC/CE to CL Detection

High-performance liquid chromatography (HPLC) and, more recently, capillary electrophoresis (CE) [6] have been coupled to CL detection. These hyphenated techniques greatly improve the sensitivity and selectivity of analysis. For instance, Cr(III) and Cr(VI)-species were well separated on a Dionex AS4A anion exchange column containing a small portion of cation exchange groups. Very high sensitivity was achieved using a CL postcolumn reaction detector based on the catalytic oxidation of luminol. Linear calibrations were obtained over the range 0.1–500 ng/mL with detection limits of 0.05 and 0.1 ng/mL for Cr(III) and Cr(VI), respectively [91]. Similar work was also carried out by Gammelgaard

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et al. [92], who developed a method for Cr-species separation by using two Dionex ion-exchange guard columns in series. Cr(VI) was reduced by potassium sulfite, whereupon both species were detected by use of the luminol–hydrogen peroxide CL system. The detection limit was 0.12 ng/mL for chromium(III) and 0.09 ng/mL for chromium(VI). Although it was possible to speciate Cr(III) and Cr(VI) using a FIA system [8], Cr-species separation using the HPLC technique offers the advantage of high selectivity especially in waste water samples. Figure 4 shows a typical chromatogram of Cr species, separated and measured by coupling HPLC with CL detection.

Some inorganic anions were also separated and detected by coupling HPLC to a CL detector. For instance, Sakai et al. [93] developed a method for the determination of inorganic anions in water samples by ion-exchange chromatography with CL detection based on the neutralization reaction of nitric acid and potassium hydroxide (Fig. 5). The faint CL from the neutralization reaction was enhanced by addition of Fe(III) to the acid. The enhanced CL emission was suppressed by adding inorganic anions such as chloride, bromide, nitrite, nitrate, and sulfate to the base, which were separated by anion-exchange chromatography. For each anion, linear calibration ranges extending from 100 ng/mL to 100 g/ mL were obtained.

Fujiwara et al. [94] found that, when present as a heteropolyacid complex, molybdenum(VI), germanium(IV), and silicon(IV) produced CL emission from the oxidation of luminol, and similar CL oxidation of luminol was observed for arsenic(V) and phosphorus(V) but with the addition of the metavanadate ion to the acid solution of molybdate. A hyphenated method was therefore proposed for the sensitive determination of arsenate, germanate, phosphate, and silicate, after separation by ion chromatography. The minimum detectable concentrations of arsenic(V), germanium(IV), phosphate, and silicon(IV) were 10, 50, 1, and 10

Figure 4 Chromatogram of Cr(VI) and Cr(III) in the range 1–5 g/L (correlation coefficients 0.999 based on peak height measurements). Cr(VI) appears at 2.8 min, Cr(III) appears at 4.6 min. (From Ref. 92, with permission.)

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Figure 5 Chromatogram of a synthetic mixture of chloride (a), bromide (b), nitrate (c), and sulfate (d) separated on an anion-exchange column. (From Ref. 93, with permission.)

ng/mL, respectively. The method was applied to analyses of seaweed reference material, rice wine, and water samples. A gas-phase CL detector was also employed for selective detection of liquid-phase sulfur compounds (Fig. 6). This detector operates by converting sulfur-containing compounds in the liquid phase into sulfur monoxide. The photo emission resulting from the SO O3 reaction is monitored by a photomultiplier tube (PMT). Application of the detector to reversed-phase HPLC analysis of standards of sulfur-containing pesticides, proteins, and blood thiols, and to the FIA of acid-soluble thiols in rat plasma is presented [95].

Several authors have developed highly sensitive CL detection coupled to CE for ion analysis. Huang et al. [96] used luminol as a component of the separation electrolyte that prevented loss of the light signal. Detection limits of 20 zmol, 2 amol, 80 amol, 740 amol, and 100 fmol for Co(II), Cu(II), Ni(II), Fe(III), and

Figure 6 HPLC separation of a 4.0-nmol mixture of blood thiols detected by CL. (1) N-acetyl-L-cysteine; (2) reduced glutathion; (3) cysteine; (4) methionine; (5) oxidized glutathion. (From Ref. 95, with permission.)