Chemiluminescence in Analytical Chemistry

It has been shown that abstraction of an α or β hydrogen from the ozonide also can occur. In fact, α-hydrogen abstraction can occur up to 2.5 times faster than Criegee fragmentation [36]. As an example, the proposed mechanism for the reaction of cis-2-butene with ozone is shown in Figure 4.

Vibrational overtone bands within the ground electronic state of OH, known as Meinel bands, result from the reaction of H atoms with ozone,

Channels involving Criegee splitting and α-hydrogen abstraction can produce H atoms, as seen in Figure 4. Herron and Huie reported that 9% of the total reaction produces H atoms by the pathways shown [38]. The H O3 reaction, for which ∆H°298 322 kJ mol 1, is not sufficiently exothermic to produce electronically excited OH(A2 ), which requires an excitation energy of 390 kJ mol 1. The excitation energy could be provided from energy pooling of vibrationally excited OH or possibly through the reaction [39]

which can provide up to 665 kJ mol 1. This proposed reaction involves an unfavorable four-center intermediate, and the source of CH is unknown. However,

Figure 4 Proposed reaction mechanism for the reaction of cis-2-butene with O3. (Reprinted with permission from Ref. 36. Copyright 1974 American Chemical Society.)

emission from CH radicals has been observed in reactions of ozone with aromatics, allene, and acetylene, as reviewed by Toby [14].

As the reaction temperature is increased, chemiluminescence is observed in the reactions of ozone with aromatic hydrocarbons and even alkanes. Variation of temperature has been used to control the selectivity in a gas chromatography (GC) detector [35]. At room temperature, only olefins are detected; at a temperature of 150°C, aromatic compounds begin to exhibit a chemiluminescent response; and at 250°C alkanes respond, giving the detector a nearly universal response similar to a flame ionization detector (FID). The mechanisms of these reactions are complex and unknown. However, it seems likely that oxygen atoms produced in the thermal decomposition of ozone may play a significant role, as may surface reactions with O3 and O atoms.

4.1.4Other Chemiluminescent Reactions of O3

Chemiluminescence associated with the reactions of ozone with phosphine (PH3), arsine (AsH3), and stibine (SbH3) have been investigated by Fraser et al. [40, 41]. The reaction mechanisms are very complex. For example, in the AsH3/O3/O2 system, a 24-step mechanism was required for the computer simulation [41]. The emissions consist of a continuum in the wavelength range 360–700 nm, tentatively assigned to emission from the 2B1 state of AsO2, analogous to NO2, and a strongly banded ultraviolet emission corresponding to the transition AsO(A2 → X2Π). The relative contribution of these two emissions depends on the presence or absence of O2 and of externally added oxygen atoms. The PH3/O3/O2 system produced only analogous PO2* emissions, while the SbH3/O3/O2 reaction resulted in emissions of SbO from the B2 and A2Π1/2,3/2 states to the ground X2Π1/2,3/2 state in the wavelength regions 340–450 nm and 450–680 nm, respectively [41].

Electronically excited metal oxides have been produced in the reactions of ozone with molecular beams of metal atoms, A,

These studies have allowed the spectroscopic identification of a number of electronically excited states of the metal oxides, but there appear to have been no analytical applications of the reactions to date. The emitting states, as summarized by Toby [14], are CaO(A1Π), SrO(A1Π), PbO(a3 , b3 ), ScO(C2Π), YO(C2Π), FeO(C1), AlO(A2Πi, B2 ), and BaO(A1 , D1 ). Nickel carbonyl reacts with ozone to produce chemiluminescence from an excited electronic state of NiO, which is probably produced in the Ni O3 reaction [42, 43].

Tin, antimony, and selenium hydrides, produced by the borohydride reduction technique, were found to chemiluminesce with ozone in an analytical detection scheme. Limits of detection were 35, 10, and 110 ng of Sn, Sb, and Se,

respectively. Bismuth and mercury hydrides gave a nonlinear chemiluminescent response, while lead, germanium, and tellurium did not chemiluminesce with ozone under the conditions used [44].

4.2 Chemiluminescent Reactions of O Atoms

Although not frequently used in analytical applications, oxygen atoms are even stronger oxidants than ozone and produce chemiluminescence when reacted with a wide range of analytes. Chemiluminescent reactions of oxygen atoms were first studied in the ‘‘air afterglow’’ in the early 1900s by Lord Rayleigh, who passed air through a low-pressure electrical discharge and observed a faint green-yellow glow just downstream [45].

A background emission underlies all chemiluminescence spectra obtained in reactions of O atoms. This emission results from the recombination of O atoms,

Emissions from five different excited electronic states of O2 throughout the visible spectral region of 400–800 nm have been identified [17], placing a practical limit on the concentration of O atoms that can be used as a chemiluminescence reagent since the background emission increases quadratically in O atom concentration.

A common way to generate oxygen atoms is to flow either pure oxygen or oxygen dilute in a noble gas such as He or Ar through a microwave discharge [46, 47]; this produces mostly ground-state O(3P) atoms but also some O(1D). The species O(1D) is 189.5 kJ/mol more energetic than O(3P) and, as a result, is more reactive and can populate higher excited states in its chemiluminescent reactions than does O(3P). However, producing a constant concentration of O(1D) in a flow stream is extremely difficult due to wall deactivation and its rapid reactions with water vapor to produce OH radicals. Therefore, in a detector making use of oxygen atoms, it is generally best to remove O(1D) by providing adequate residence time between the discharge source and the reaction chamber. Significant quantities of O2(1∆g), the lowest bound electronic state of oxygen, also are produced in microwave discharges of O2. At high concentrations, O2(1∆g) can provide a background emission at 634 and 703 nm via the ‘‘dimol’’ emission process in which two singlet oxygen molecules cooperate to emit a single photon [48].

4.2.1O NO Chemiluminescence

The associative reaction of oxygen atoms with nitric oxide produces the yellowgreen chemiluminescence in the ‘‘air afterglow,’’ easily seen by the naked eye. The reaction has long been used to measure the concentrations of O atoms in kinetics experiments [49–51] and is so bright that it has been used to visualize

and photograph flow properties such as the mixing of gas streams in flow reactors. Kinetics studies have shown that the first step of the reaction mechanism is truly second order and reversible. A weakly bound electronically excited molecule, (NO2*), can undergo radiative emission to the ground state or be stabilized by collision with a third body M to form a more strongly bound excited-state molecule, NO2* [52]. The latter species may emit light or be collisionally quenched:

Chemiluminescence is believed to arise from the 2B1 and the 2B2 electronic states, as discussed above for the reaction of NO with ozone [17]. The primary emission is in a continuum in the range 400–1400 nm, with a maximum at 615 nm at 1 torr. This emission is significantly blue-shifted with respect to chemiluminescence in the NO O3 reaction (λmax 1200 nm), as shown in Figure 2, owing to the greater exothermicity available to excite the NO2 product [52]. At pressures above approximately 1 torr of O2, the chemiluminescence reaction becomes independent of pressure with a second-order rate coefficient of 6.4 10 17 cm3 molec 1 s 1. At lower pressures, however, this rate constant decreases and then levels off at a minimum of 4.2 10 18 cm3 molec 1 s 1 near 1 mtorr, and the emission maximum blue shifts to 560 nm [52]. These results are consistent with the above mechanism in which the fractional contribution of (NO2*) to the emission spectrum increases as the pressure is decreased, therefore decreasing the rate at which (NO2*) is deactivated to form NO2*. Additionally, the radiative lifetime and emission spectrum of excited-state NO2 vary with pressure, as discussed above for the NO O3 reaction [19–22].

4.2.2O SO Chemiluminescence

As with SO O3, discussed above, it is believed that sulfur monoxide reacts with ground-state O atoms to form SO2 in either the 3B1 or 1B2 electronic states, although the 1A2 and 1B1 may also contribute [24, 28, 29]. Standard third-order chemiluminescence reaction mechanisms and kinetics are thought to apply:

The emission spectrum consists of a series of weak bands starting at about 220 nm and then growing into a continuum from about 240 to 400 nm, with a maximum at approximately 270 nm as shown in Figure 5. Halstead and Thrush estimated that65% of the emission occurs from the 1B2 state, 15% from the 3B1, and 20% from a combination of the 1A2 and 1B1 states [24, 28, 29] with a rate constant of 2 10 31 cm6 molec 2 s 1 using argon as the bath gas at 300 K [53]. As with the reaction of SO O3 discussed above, collisional coupling results in a radiative lifetime that is pressure dependent.

It is important to note that the SO O chemiluminescence reaction can be surface mediated in addition to occurring in the gas phase. Here, the oxygen atoms are weakly bound to the surface and react with SO diffusing to the surface:

The emission maximum is red-shifted from about 340 nm to 380 nm and shows a more banded structure than in the homogeneous reaction [17]. This emission spectrum more closely approximates the spectrum of SO O3 than that of the homogeneous SO O reaction, indicative of energy loss to the surface [31].

Figure 5 Comparison of SO O and SO O3 chemiluminescence spectra. Note the blue shift of the SO O spectrum as compared to that of SO O3. (Adapted from Refs. 25, 152).

4.3 Chemiluminescent Reactions of Active Nitrogen

The use of active nitrogen as a chemiluminescence reagent results in an almost universal analysis technique for hydrocarbons; however, it also is possible to selectively detect organometallics, halogens, and compounds containing oxygen, phosphorus, or sulfur. Active nitrogen is generated in a similar manner to that for oxygen atoms and was first reported in the ‘‘nitrogen afterglow’’ by E. P. Lewis in 1904 [54]. Passing a stream of pure, ground-state N2(X1 g ) through a low-pressure microwave or electrical discharge generates a population of groundstate N (4S) atoms and N2 molecules excited to the A3 u and B3Πg states. It is the ability to readily react with or to transfer energy to a wide variety of molecules that gave this reaction mixture the name ‘‘active nitrogen.’’ Nitrogen atoms readily recombine to form nitrogen in either the A3 u state or higher B3Πg state, and molecules in the B3Πg state can decay radiatively to the lower A3 u state. Because the radiative transition of this triplet state to the singlet ground state is spin forbidden, the A3 u state is metastable with a lifetime on the order of seconds. The B3Πg → A3 u transition produces light in the yellow portion of the visible spectrum as a background to whatever chemiluminescent signal is sought above it [55]. The relative contributions of N atoms and N2* to chemiluminescent reactions depend on the source and pressure. At reduced pressure, high-energy discharges such as microwave discharges produce higher concentrations of N atoms than electrical discharges, while active nitrogen produced at atmospheric pressure consists almost entirely of N2(A3 u ) [39].

4.3.1Chemiluminescent Reactions of Active Nitrogen with Hydrocarbons

The chemiluminescent products of the reactions of active nitrogen with organics produce cyanogen radicals (CN) in an excited B2 state, which radiatively decay to the X2 ground state. The chemiluminescence spectrum consists of a number of banded features between 350 and 420 nm, corresponding to the ∆v 1, 0, and 1 band progressions, as seen in Figure 6. The most intense feature is the ∆v 0 sequence in the narrow wavelength range of 383–388 nm. The source of CN* responsible for these emissions is not known. The reaction of CH radicals with N atoms has adequate exothermicity to produce CN*:

but the first step in the production of CH by successive hydrogen abstractions by N atoms is highly endothermic (e.g., ∆H0298 87 kJ mol 1 for N C2H6).

Low-pressure discharges tend to produce much more intense chemiluminescence from alkenes than from alkanes. This has been attributed to the addition of N to the double bond, as illustrated for ethylene,

Figure 6 CN(B2 → X2 ) emission in the reaction of active nitrogen with hydrocarbons. (Reprinted with permission from Ref. 58. Copyright 1979 American Chemical Society.)

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It has been proposed that this reaction intermediate could decompose to produce HCN and CH3 [55]. Chemiluminescence from alkanes can be greatly enhanced by addition of HCl. The proposed explanation is that energy transfer from active nitrogen dissociates HCl to produce chlorine atoms, which have rapid hydrogenatom abstraction reactions with alkanes,

This effect of adding HCl to the reaction cell has been used to advantage in distinguishing alkanes from alkenes and oxygenated hydrocarbons in a GC detector utilizing active nitrogen [56].

In atmospheric pressure discharges, where concentrations of N atoms are greatly reduced and possibly absent, the discrimination between alkanes and alkenes is not observed and an alternative means of initiating the reaction is required. In the absence of significant concentrations of N, the mechanism may consist of collisional energy transfer from N2(A3 u ), which lies 593 kJ mol 1

above the ground state, to the organic molecule, followed by fragmentation of the carbon skeleton [57]. Again, the complex reaction mechanism leading to CN and/or CN* is not known, but CN* could be formed via collisional energy transfer from N2* to ground-state CN.

4.3.2Other Chemiluminescent Reactions of Active Nitrogen

In addition to the ability to react nonspecifically with hydrocarbons, active nitrogen can readily participate in energy transfer reactions with volatile organometallic compounds, leading to atomic emission from the metal atom. By use of appropriate optical filters, selective detection of elements such as aluminum, lead, tin, and mercury has been achieved in the presence of large excesses of organics [58].

Molecules containing oxygen, sulfur, and phosphorus react with active nitrogen to produce excited states of NO, SN, and PN radicals, respectively [59]. Electronically excited OH also is observed in reactions of oxygenated compounds [56]. A disadvantage of selective detection of oxygen compounds using active nitrogen is that oxygen impurities also react with active nitrogen to form NO*. Finally, the reactions of active nitrogen with inorganic compounds containing boron, chlorine, bromine, and iodine have been found to chemiluminesce in characteristic spectral regions [57].

4.4 Chemiluminescent Reactions of F and F2

Being the most electronegative element and one of the strongest oxidants known, rich chemiluminescence is associated with reactions of F atoms, while F2 is surprisingly selective in its gas-phase reactions. Fluorine atoms can be generated by flowing a stream of F2 dilute in a noble gas through a microwave electrical discharge or, for those less courageous, SF6 or CF4 may be used as the source gas [60]. Because fluorine atoms react with silica at the high temperatures of the discharge, it is necessary to use a ceramic (alumina) discharge tube. The plasma produces a stream containing a mixture of F and F2; by providing adequate residence time in the connecting tubing, F atoms can recombine to form F2 for greater selectivity. Fluorine atoms may also be produced by multiphoton dissociation of SF6 using a highly focused infrared laser [61]. This approach has the advantage of producing the F atoms at the point where they react, rather than upstream, so that fewer atoms are lost to recombination on the vessel walls. Fluorine can be removed after the chemiluminescence cell and prior to entering the vacuum pump using a charcoal trap to produce nontoxic CF4, and the reaction product HF may be removed with an ascarite trap. Thus, it is possible to produce F/F2 online from innocuous starting materials and then remove it without any significant health risk.