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346	Calokerinos and Palilis

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13

Gas-Phase Chemiluminescence Detection

James E. Boulter and John W. Birks

University of Colorado, Boulder, Colorado

1.	INTRODUCTION		350
2.	CHARACTERISTICS OF GAS-PHASE CHEMILUMINESCENCE		350
3.	REACTOR DESIGN AND OPTIMIZATION		351
4.	GAS-PHASE CHEMILUMINESCENT REACTIONS		354
	4.1	Chemiluminescent Reactions of O3	354
	4.2	Chemiluminescent Reactions of O Atoms	360
	4.3	Chemiluminescent Reactions of Active Nitrogen	363
	4.4	Chemiluminescent Reactions of F and F2	365
	4.5	Other Chemiluminescent Reactions	372
5.	APPLICATIONS TO GAS CHROMATOGRAPHY		375
	5.1	Requirements for GC Detection	375
	5.2	Flame Photometric Detector	375
	5.3	Fluorine-Induced Chemiluminescence Detection of S, Se, Te,
		and P Compounds	378
	5.4	Sulfur Chemiluminescence Detector	379
	5.5	Thermal Energy Analysis and Nitrogen-Selective Detectors	381
	5.6	Redox Chemiluminescence Detector	382
	5.7	Active Nitrogen Detectors	383
6.	APPLICATIONS TO ATMOSPHERIC RESEARCH		384
	6.1	‘‘NOx Box’’ Detector for NO, NO2 and NOy	384
	6.2	Fast Ozone Detector	385

349

350		Boulter and Birks
6.3	Isoprene Detector	385
6.4	Dimethyl Sulfide Detector	386
7. CONCLUDING REMARKS AND FUTURE TRENDS		386

1. INTRODUCTION

The use of fire, being the first chemical reaction controlled by humans, is an important marker for the beginning of civilization. The thermal and visible radiation of fire was used to great benefit for warmth, cooking, nighttime visibility, and warding off predators. For millennia, our ancestors must have marveled at the luminescence and other unique properties of fire; indeed the Greeks considered fire one of four basic elements comprising all matter. Chemiluminescent emissions from flames served as one of the first analytical tools used for qualitative identification of specific elements, and flame emission spectrometry is still one of the most sensitive and selective means for quantification of alkali metals and alkaline earths. The discovery of the ability to produce gas-phase chemiluminescence without the assistance of the thermal energy provided by flames led to many new analytical applications. This chapter focuses primarily on the analytical utility of these so-called ‘‘cold’’ chemiluminescent reactions, where reactants are formed and molecular excitation is achieved without the assistance of the thermal energy associated with a flame.

Chemiluminescence techniques have several inherent advantages, including high sensitivity, high selectivity, and simplicity of instrumentation, making them the methods of choice for select applications. The high quantum efficiency of photon detection, in combination with a dark background in the absence of any analyte, can result in very low limits of detection. Additionally, because very few chemical reactions produce intense chemiluminescence in the UV/Vis region, the selectivity of chemiluminescence is often very high. For example, nitric oxide is measured in the atmosphere at low parts per trillion (pptv) levels in the presence of hundreds of other compounds at higher concentrations without any separation step, based on its chemiluminescent reaction with ozone [1–4]. Finally, the instrumentation for chemiluminescence is simple, robust, and inexpensive, making it well suited for both field and laboratory applications.

2.CHARACTERISTICS OF GAS-PHASE CHEMILUMINESCENCE

As in the condensed phase, gas-phase chemiluminescence consists of a chemical reaction forming an excited-state product that then undergoes one or more relax-

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351

ation processes to attain its ground state. In theory, any reaction sufficiently exoergic to generate a significant fraction of products in an excited state has the potential to be chemiluminescent. In such a reaction, the excess energy is partitioned into a combination of translational, rotational, vibrational, and electronic states of the products. Emission of a visible or ultraviolet photon, which can be detected with a high quantum efficiency, generally requires that the reaction form a product in an excited electronic state. Vibrational overtone emission such as that of HF produced in the reaction of fluorine with certain sulfur compounds is a rare exception to this rule [5–7]. Once a product molecule is formed in an excited electronic state, the mechanism by which it relaxes to its ground state also contributes to the chemiluminescent character of the reaction. Excess energy within a molecule may be redistributed by vibrational relaxation, internal conversion, intersystem crossing, fluorescence, or phosphorescence [8]. In gas-phase chemiluminescence detection, radiative emission is usually competitive with nonradiative processes with the result that both the quantum yield of the reaction and the emission spectrum vary with physical conditions such as bath gas composition, temperature, and pressure.

3. REACTOR DESIGN AND OPTIMIZATION

As shown schematically in Figure 1, a gas-phase chemiluminescence detector consists of a reaction chamber, inlets for the analyte and reagent gas streams, a vacuum pump to lower the pressure in the reaction chamber (typically to a few torr), and a transducer such as a photomultiplier tube (PMT) to monitor the light produced in the reaction. The reagent gas, usually present in large excess, reacts with a trace concentration of analyte to produce an excited product that subse-

Figure 1 Schematic diagram of a generalized gas-phase chemiluminescence detector.

352	Boulter and Birks

quently emits light. Of course, the actual design and optimization of a chemiluminescence instrument is somewhat more complicated [9]. For example, any stray light entering the reaction chamber provides a radiation background, so the reaction chamber must be well sealed from ambient light and from any light produced in the reagent gas source, such as a microwave discharge, if atomic or radical species serve as the reagent gas. The pressure within the reaction chamber depends on the flow rates of the analyte and reagent gas streams and on the speed of the vacuum pump. Finally, the reaction chamber volume affects both the signal intensity and response time of the detector.

The interdependence of the flow rates, pumping speed, total pressure, reaction chamber volume, residence time, reaction rate, quantum yield, and chemiluminescence signal is given by simple equations if one assumes a large excess of reagent gas and plug-flow conditions, characteristic of most gas-phase chemiluminescence detectors in the pressure range 1–10 torr. As an example, we will consider the measurement of the analyte NO using the well-known chemiluminescent reaction between nitric oxide and ozone, the mechanism of which includes the following steps [10]:

NO O3	→ NO2* O2	(1a)
NO O3	→ NO2 O2	(1b)
NO2* M → NO2 M		(2)
NO2* → NO2 hν		(3)

where M is the bath gas, and the rate constant for Reaction (2) depends on its concentration and identity. The rate-limiting step for this mechanism is the reaction of NO with O3, the total rate constant (k1 k1a k1b) of which is 1.8 10 14 cm3 molec 1 s 1 at 298 K [11]. The chemiluminescence efficiency, defined as the fraction of reactions of NO with O3 that result in the production of a photon, is determined by two factors, the branching ratio for the formation of the electronically excited product NO2*, and the quantum efficiency for radiative deactivation of NO2*. The branching ratio (fraction of reactions that produce NO2*) is given by

	φb.r.	k1a		(E1)
		k1a k1b

and the chemiluminescence quantum efficiency (fraction of NO2* molecules that emit a photon) is given by

	φq.e.	k3	(E2)
		k3 k2[M]

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353

The fraction of NO molecules that react during the residence time, tres, within the reactor is given by,

	[NO]0 [NO]	1 exp( k1[O3]tres)	(E3)
	[NO]0

where [NO]0 is the concentration of NO in the absence of O3, and [NO] is the concentration of NO in the presence of O3. The light intensity, I, expressed in photons s 1, is given by the flow rate of analyte molecules into the reaction chamber multiplied by the fraction of molecules that react during the residence time within the reactor, the branching ratio to form excited-state products, and the quantum efficiency for emission of light:

I XNOFNOφb.r.φq.e.{1 exp( k1[O3]tres)}

(E4)

where FNO is the flow rate of the gas stream containing NO in units of molec s 1, and XNO is the mixing ratio (mole fraction) of NO in the gas stream. The residence time is given by

	tres	V		(E5)
		S

where V is the volume of the reaction chamber in units of L and S is the pumping speed in units of L s 1 after considering the conductance of the connecting tubing and any intervening valves. Of course, the measured signal in photon counts s 1 will be reduced from Eq. (E4) by the collection efficiency (fraction of emitted photons falling on the detector) and quantum efficiency of the PMT or other detector averaged over the emission spectrum.

Note that increasing the flow rate of the analyte NO results in an increased signal according to Eq. (E4), but a complication arises in that the pressure (and therefore [M]) also increases. For a fixed pumping speed, the residence time within the reaction chamber is constant, and the pressure in the reaction cell will increase linearly with increasing flow rate of gas into the cell:

P	FRT		FRTtres		(E6)
	NAS NAV

where F is the total flow rate (sum of analyte and reagent streams) into the cell in molec s 1, R is the molar gas constant, T is the cell temperature, and NA is Avogadro’s number. Thus, as the throughput of analyte increases, the pressure also increases. The corresponding increase in molecular concentration ([M] NAP/RT) results in quenching of NO2* and a decrease in the chemiluminescence quantum efficiency, according to Eq. (E2). These competing effects result in the existence of an optimal flow rate for the gas stream containing the analyte, above which no improvement in signal is obtained. Ultimately, the sensitivity is limited

354	Boulter and Birks

by reactor considerations such as the size of the vacuum pump and the flow rates of the analyte and reagent gas streams.

Optimization strategies and a number of generalized limitations to the design of gas-phase chemiluminescence detectors have been described based on exact solutions of the governing equations for both exponential dilution and plugflow models of the reaction chamber by Mehrabzadeh et al. [12, 13]. However, application of this approach requires a knowledge of the reaction mechanism and rate coefficients for the rate-determining steps of the chemiluminescent reaction considered.

4. GAS-PHASE CHEMILUMINESCENT REACTIONS

Typically, intense chemiluminescence in the UV/Vis spectral region requires highly exothermic reactions such as atomic or radical recombinations (e.g., S S M → S2* M) or reactions of reduced species such as hydrogen atoms, olefins, and certain sulfur and phosphorus compounds with strong oxidants such as ozone, fluorine, and chlorine dioxide. Here we review the chemistry and applications of some of the most intense chemiluminescent reactions having either demonstrated or anticipated analytical utility.

4.1 Chemiluminescent Reactions of O3

The most widely used gas-phase chemiluminescence reagent is ozone. Analytically useful chemiluminescence signals are obtained in the reactions of ozone with NO, SO, and olefins such as ethylene and isoprene, but many other compounds also chemiluminesce with ozone. Ozone is conveniently generated online at mixing ratios of 1–5% by electrical discharge of air or O2 at atmospheric pressure [14].

4.1.1O3 NO Chemiluminescence

The chemiluminescence associated with the reaction of NO with O3 is perhaps the best known and most analytically useful gas-phase chemiluminescent reaction [15]. The mechanism of the reaction is summarized by Reactions (1–3), given above. The exothermicity of the reaction, 200 kJ mol 1, corresponds to the shortwavelength cutoff of the reaction [16]. Emission is observed in the range 600– 3000 nm, as shown in Figure 2. Although the spectroscopy of NO2 is still not well understood, the emission appears to originate in a combination of the 2B1 and 2B2 states [17]. As seen in Figure 2, only a small fraction of emission occurs below 800 nm where photons can be detected with high quantum efficiency. Furthermore, only a small fraction of the reaction of NO with O3 results in ex-

Gas-Phase Chemiluminescence Detection

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Figure 2 Comparison of NO O3 and NO O chemiluminescence spectra with blue and red optical filter transmissions and the response of a blue-sensitive photomultiplier tube. Note the blue shift of the NO reaction with O compared to that with O3 and that the addition of a blue filter effectively removes emission from NO O3, while the red filter effectively removes emission from NO O.

cited-state NO2 [10]. Assuming that the emitting species was 2B2, Clough and Thrush estimated the rate constant for formation of electronically excited-state molecules to be given by k1a 1.26 10 12 exp( 2100/T) cm3 molec 1 s 1 [10]. Comparing this with the now well-established rate constant for the overall reaction of k1 2.0 10 12 exp( 1400/T) cm3 molec 1 s 1 [11], the fraction of reactions that produce excited states is only 6.0% at room temperature.

The radiative lifetime of NO2* is calculated from its integrated absorption spectrum to be 0.3 µs [18], but the actual measured lifetime is at least two orders of magnitude longer. This anomaly, which was investigated by numerous research groups [17], has been attributed to coupling of an excited 2B2 electronic state to high vibrational levels of the 2A1 ground state [19–22]. Those states with a larger fraction of 2B2 character show shorter lifetimes.

4.1.2O3 SO Chemiluminescence

The chemiluminescent reaction of SO with ozone is the basis of the sulfur chemiluminescence detector (SCD) [23] discussed later in this chapter,

SO O3 → SO2* O2

(4)