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4.4.1Reactions of F Atoms with Hydrocarbons

The reactions of fluorine atoms with hydrocarbons are similar to those of active nitrogen in that they provide an essentially universal response. Fluorine atoms abstract H atoms from hydrocarbons at near-collisional reaction rates. Reactions with fluorine are highly exothermic, forming strong HEF ( 570 kJ mol 1) and CEF ( 485 kJ mol 1) bonds while breaking much weaker CEH ( 414 kJ mol 1) and CEC ( 368 kJ mol 1) bonds. The hydrogen abstraction reaction

F RH → HF† R

(23)

is exothermic by about 156 kJ mol 1, sufficient to produce vibrationally excited HF† in levels up to v 4. The (3,0) vibrational overtone band may be detected at 880 nm for most species containing a CEH bond and many other compounds containing hydrogen [62]. This is not a favorable wavelength for sensitive detection using photomultiplier tubes, and in order to be selective for hydrocarbons, emissions from electronically excited C2(d3Πg) at 470 nm and CH(A2∆) at 431 nm may be monitored. The detailed mechanisms that produce these species are unknown but must involve many reaction steps. The same emissions are observed in fluorine/hydrocarbon flames [63]. The excited state of diatomic carbon, C2(d3Π), has been observed in many systems, ranging from solar spectra to organic flames. It may be formed in the disproportionation reaction of CH radicals [64]:

CH CH → C2* H2

(24)

The chemiluminescent response of hydrocarbons reacting with an excess of fluorine atoms produced by multiphoton dissociation of SF6 is linear below 50 parts per million (ppmv) [61].

4.4.2Reactions of F2 with Sulfur Compounds

Certain reduced sulfur compounds such as thiols, sulfides, disulfides, and trisulfides, specifically those having the functionality

(25)

exhibit intense chemiluminescence with molecular fluorine, providing the basis of the fluorine-induced chemiluminescence detector (FCLD) [65]. Analogous selenium and tellurium compounds also respond with high sensitivity [66, 67]. Interestingly, the gases H2S, CS2, OCS, and SO2 do not provide significant responses in the FCLD, and the detector exhibits selectivities of 107 against alkanes. Compounds with weak CEH bonds, such as alkenes (allylic hydrogens),

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toluene, xylenes (benzylic hydrogens), aldehydes, and halocarbons, provide relatively weak responses [65]. Molecular fluorine also exhibits intense chemiluminescence with phosphines, alkyl phosphines, and monophosphinate esters [68].

Chemiluminescence is observed from several different emitting species, depending on the analyte and reaction conditions. Vibrational overtone bands of HF in the wavelength region of 500–900 nm are observed under nearly all conditions and are often the dominant spectral feature, the (3,0), (4,0) (5,1), and (6,2) bands being the most intense, while for some reaction conditions emissions from levels up to v 8 are observed [63]. It is likely that hydrogen atoms are produced in the reaction and form vibrationally excited HF in the reaction reported by Mann et al. [62]:

H F2 → HF†(v 9) F

(26)

While investigating the potential for an instrument to measure atmospheric dimethyl sulfide (DMS) [69], discussed below, Hills et al. investigated the possibility of adding H2 to the reaction cell to provide chemical amplification of the chemiluminescence signal via the catalytic chain reaction:

F H2	→ HF H	(27)
H F2	→ HF†(v 9) F	(28)
HF† → HF hν		(29)
Net: H2	F2 → 2 HF hν	(30)

In this case, chemiluminescence was monitored using a red-sensitive PMT to detect emissions from HF†. A factor-of-six enhancement in sensitivity to 1.1 parts per billion (ppbv) DMS was obtained. This is consistent with the fact that, based on the rate constant for the H F2 rate determining step [Reaction (28)], the reaction can cycle approximately 7 times during the cell residence time and confirms the observation by Turnipseed and Birks [7] that F atoms are produced in the F2 DMS reaction.

In the reactions of F2 with excess DMS and methane thiol, strong phosphorescence from triplet-excited thioformaldehyde, CH2S(3A2), dominates the chemiluminescence, while for conditions of excess F2, emission is predominantly that of HCF(A1A″ → X1A′) [5, 6, 70] as shown in Figure 7. The kinetics and mechanism of the reaction of (CH3)2S with F2 were investigated by Turnipseed and Birks using the flow tube technique with simultaneous detection by mass spectrometry and observation of the emission spectrum using a diode array spectrometer [7]. As DMS was titrated by addition of F2, a product having a m/e ratio of 80, corresponding to the mass of H2CCSFCH3, increased in proportion to the loss of DMS at m/e 62. As shown in Figure 8, once the endpoint of the titration

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Figure 7 Emission spectra obtained in the reaction of DMS with F2. (A) 155 mtorr CH3SCH3, 30 mtorr F2, 570 mtorr He; (B) 30 mtorr CH3SCH3, 20 mtorr F2, 350 mtorr He; (C) 30 mtorr CH3SCH3, 50 mtorr F2, 900 mtorr He. (Reprinted from RJ Glinski, EA Mishalanie, JW Birks, ‘‘Molecular emission spectra in the visible and near IR produced in the chemiluminescent reactions of molecular fluorine with organosulfur compounds,’’ Journal of Photochemistry 37:223, 1987, with permission from Elsevier Science.)

was reached, the ion current at m/e 80 began to decline with increasing F2 concentration. In agreement with the observations of Glinski et al. [6], the emission spectrum was found to change from CH2S* to HCF* and possibly some HF† near the titration endpoint. The rate constant for the reaction of F2 with DMS was found to be (1.6 0.5) 10 11 cm3 molec 1 s 1, which is extremely fast for the reaction of two closed-shell molecules [7]. By contrast, the rate constant for the F2 reaction with H2S, which does not respond in the FCLD, was found to have an upper limit of 6.4 10 16 cm3 molec 1 s 1. It was argued that the

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Figure 8 Changes in the DMS (m/e 62) reactant and CH3SFCH2 (m/e 80) product ion currents as a function of reaction time using the flow tube technique to study the reaction of DMS with F2. (Reprinted with permission from Ref. 7. Copyright 1991 American Chemical Society.)

fast rate of the F2 DMS reaction, occurring about once in every six collisions, may be due to the formation of a charge-transfer complex,

(31)

which is calculated, using the electron affinity for F2 and the ionization potential

˚

of DMS, to have a radius of 2.63 A. By comparison, the critical radius for forming

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˚

a charge transfer complex with H2S is 1.94 A. Turnipseed and Birks showed that the critical radius for forming a charge-transfer complex is highly correlated with the relative sensitivity of the FCLD to various compound classes [7].

Once the DMS-F2 complex is formed, atomic fluorine may be eliminated, analogous to the proposed mechanism for the reaction of F2 with olefins [71]:

(32)

This radical could then eliminate an H atom to form a CCS bond, also analogous to olefinic systems [71, 72], to form the species observed at m/e 80,

(33)

Subsequent reactions of the H and F atoms produced in these reactions may be responsible for HF† formation via Reactions (27–28) above and for CH2S* by means of reactions such as

H CH3SCH3 → CH3 CH3SH	(34)
CH3SH F → CH3S HF	(35)
CH3S H → H2 CH2S*	(36)
CH3S F → HF CH2S*	(37)

The reactions of H and F with CH3S are exothermic by 259 and 393 kJ mol 1, respectively, and either could be responsible for formation of CH2S(A3 ) [7].

Methylated sulfur compounds produce intense emission from HCF*, while the corresponding ethyl-substituted compounds produce only trace or no HCF* [6]. Emission from HF† was much stronger for the ethyl-substituted than for the methyl-substituted sulfides [6]. This suggests that methyl radicals, formed perhaps in Reaction (34), lead to the formation of HCF*. Emission of HCF* has been identified in F2/CH4 flames [70] where it is attributed to the association reaction

CH F M → HCF* M

(38)

Under excess F2 conditions, the reaction with DMS and other organosulfur compounds may produce CH as well, and at the low pressures ( 1 torr) of the FCLD, a more favorable reaction may be,

CH F2 → HCF* F

(39)

which has a more than adequate exothermicity of 326 kJ mol 1 for ground-state products.

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Carbon disulfide produces weak chemiluminescence relative to organosulfur compounds bearing hydrogen atoms. The absence of H atoms in CS2 precludes emissions from HF†, CH2S*, or HCF*. A banded emission in the wavelength region 600–900 nm was tentatively assigned to FCS [6] but later identified as arising from an excited electronic state of SF2 [73].

4.4.3Reactions of F2 with Selenium and Tellurium Compounds

Intense chemiluminescence also accompanies the reaction of F2 with organoselenium and -tellurium compounds [67]. HCF and selenoformaldehyde emissions are observed in the reaction of F2 with dimethyl diselenide, (CH3Se)2 [66]. Analogous to the reaction of DMS, the emission spectrum changes from that of CH2Se* phosphorescence and some HF† under conditions of excess (CH3Se)2, to HCF* under conditions of excess F2. Selenium and tellurium compounds have even lower ionization potentials than the corresponding sulfur compounds, so they also are expected to form charge transfer complexes with F2 [7] in an manner analogous to Reaction (31).

4.4.4Reactions of F2 with Phosphorus Compounds

Phosphine, alkyl phosphines, and monophosphinate esters also produce intense chemiluminescence with F2 [68]. Again, these compounds have low ionization potentials and are expected to form charge transfer complexes with F2 [7]. The emission spectra obtained in the reaction of trimethyl phosphine (TMP) with excess F2 have contributions from HCF*, HF†, and an unidentified broad-band emission as shown in Figure 9. The relative contributions of HCF* and the un-

Figure 9 Emission spectra obtained in the reaction of TMP with F2. (A) 8 mtorr TMP, 7 mtorr F2, 59 mtorr He; (B) 20 mtorr TMP, 47 mtorr F2, 423 torr He. (Reprinted from Ref. 68, with permission from JW Birks.)

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known emitter change with the F2/TMP ratio, with the relative amount of HCF* increasing as this ratio increases. The HCF*/HF† ratio remains constant within a factor of two as the F2/TMP ratio is varied from 1 to 5. Interestingly, the emission spectrum for triethyl phosphine (TEP) contains HF† and the broad-band emitter, but no HCF*. This is the same trend observed for the sulfide reaction system discussed above and provides further evidence of the participation of methyl radicals in the formation of HCF*.

4.4.5Reactions of F and F2 with Iodine Compounds

Birks et al. reported chemiluminescence from the A3Π0 and B3Π1 states of IF in the reaction of F2 with I2 and suggested that the reaction kinetics were consistent with a four-center reaction forming the products IF* IF [74]. In a series of molecular beam studies, it was shown that the reaction actually forms a collision complex that decomposes to form two sets of products, IF* IF and I2F F [75–77]:

I2 F2 → [I2F F] → IF* IF (minor channel)	(40)
→ I2F F (major channel)	(41)

Those experiments suggest that chemiluminescence in the earlier work by Birks et al. was due principally to the secondary reaction

F I2F → IF* IF

(42)

The FCLD, described above for S, Se, Te, and P compounds, actually was first demonstrated for alkyl iodides [60]. Fluorine atoms (and almost certainly F2 through atom recombination) were produced by flowing a 1:1 mixture of SF6 and He through a microwave discharge. Limits of detection were of the order of 1 µg. Ironically, the detector was later found to be about five orders of magnitude more sensitive to organosulfur compounds in a study of possible interferences [65]. The formation of IF* is thought to occur through an expanded octet intermediate RIF, analogous to the I2 F2 reaction, which has been positively identified in the reaction of methyl iodide with F atoms [78]:

F RI M → RIF M	(43)
F RIF → RF IF*	(44)

4.5 Other Chemiluminescent Reactions

Other reactants that have been used to generate chemiluminescent reactions useful for chemical analysis include atomic sodium to detect halocarbons and chlorine dioxide to detect H2S and CH3SH.

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4.5.1Reactions of Atomic Sodium with Halocarbons

One fairly obscure chemiluminescent technique uses atomic sodium vapor to detect organic compounds containing more than one halogen atom. A piece of sodium metal is heated to approximately 400°C, enough to produce a significant vapor pressure of Na atoms in the reaction cell [79]. The mechanism, as it is hypothesized, produces sodium halides and a double bond in the carbon skeleton where the halogens were located, when structurally possible. Formation of the CCC double bond provides sufficient excess energy to populate high vibrational levels of the sodium halide:

RECXY Na → RECX NaY	(45)
RECX Na → RCC NaX†	(46)

Here, X and Y are halogens and R represents the remainder of the carbon skeleton. The reaction proceeds particularly well for the halogens chlorine and bromine, but much less so for fluorine, as the CEF bond is much stronger than either CECl or CEBr. However, the excited sodium halide, NaX†, is not the emitting species. Another collision between this molecule and another vaporphase sodium atom is needed:

NaX† Na(32S) → NaX Na*(32P)

(47)

The resulting doublet sodium atom can then emit light at the sodium D-line at 589 nm. With so many steps requiring atomic sodium, its concentration, which increases with temperature, significantly affects the chemiluminescent yield. Therefore, the temperature of the solid sodium must be tightly controlled. The excited sodium halide is readily quenched by ambient air molecules, so the system works best at reduced pressure.

Limits of detection as low as 9.1 10 14 g for C2H4Cl2 have been measured with a relative standard deviation of 2% and a linear dynamic range of 4–5 orders of magnitude [79]. Weak interferences were found for oxygenand nitro- gen-containing organics. This technique also produces a chemiluminescent response for compounds with a single carbon and/or halogen by an unknown mechanism, and has been applied to the detection of atmospheric N2O at sub-ppm levels through a mechanism similar to Reactions (45–47) [80].

4.5.2Reactions of OClO with Sulfur Compounds

The chemiluminescent reaction with chlorine dioxide provides a highly sensitive and highly selective method for only two sulfur compounds, hydrogen sulfide and methane thiol [81]. As in the flame photometric detector (FPD), discussed below, atomic sulfur emission, S2(B3 u → X3 g ) is monitored in the wave-

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length range 250–400 nm, as shown in Figure 10. Although population of S2* is thought to proceed through the recombination of sulfur atoms, the reaction mechanism is not understood; however, the overall stoichiometry of the reaction is

2 H2S OClO → S2*	1		Cl2 2 H2O	(48)
2

Surprisingly, despite requiring two analyte molecules to produce one S2 molecule, the kinetics of the chemiluminescent reaction are first order with respect to the sulfur compound. This can be explained if every H2S or CH3SH molecule is consumed in the reaction and every S atom recombines to form S2, through the use of an excess of OClO to maintain pseudo-first-order reaction conditions [81]. The limit of detection for this analysis was found to be 3 ppbv for H2S.

Figure 10 Chemiluminescence spectrum of S2 observed in the selective reaction of OClO with H2S. Note that this is the same excited-state species observed by the FPD. (Reprinted with permission from Ref. 81. Copyright 1982 American Chemical Society.)

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5. APPLICATIONS TO GAS CHROMATOGRAPHY

Gas chromatography is one of the most powerful analytical techniques available for chemical analysis. Commercially available chemiluminescence detectors for GC include the FPD, the SCD, the thermal energy analysis (TEA) detector, and nitrogen-selective detectors. Highly sensitive detectors based on chemiluminescent reactions with F2 and active nitrogen also have been developed.

5.1 Requirements for GC Detection

A number of demands are placed on detectors applied to gas chromatography owing to the specific nature of this separation technique. The time resolution must be greater than approximately 10 Hz in order not to degrade the resolution of the separation. Ideally, the sensitivity should be sufficient to detect quantities as little as picograms of an analyte as it elutes from the column but should have a linear response up to nearly microgram quantities. The detector must be able to accommodate flow rates of typically 1 cm3 min 1 of gas at atmospheric pressure or slightly higher and at temperatures ranging up to 400°C. It must be stable, robust, and simple enough for routine use. Often, the design must be adapted to meet these requirements. For example, it may be necessary to reduce the volume of the detection cell at the expense of sensitivity to shorten the response time. A vacuum pump may be used to optimize the sensitivity and/or reduce the residence time in the detector. Either the GC effluent can be thermally equilibrated before reaching the detector or the detector temperature held constant at some temperature above the oven temperature. The mobile phase can have additional constituents in addition to helium if necessary for detection. Computers can easily correct for some of the complexities inherent to some detectors, such as a nonlinear response.

5.2 Flame Photometric Detector

The most commonly used and widely marketed GC detector based on chemiluminescence is the FPD [82]. This detector differs from other gas-phase chemiluminescence techniques described below in that it detects chemiluminescence occurring in a flame, rather than ‘‘cold’’ chemiluminescence. The high temperatures of the flame promote chemical reactions that form key reaction intermediates and may provide additional thermal excitation of the emitting species. Flame emissions may be used to selectively detect compounds containing sulfur, nitrogen, phosphorus, boron, antimony, and arsenic, and even halogens under special reaction conditions [83, 84], but commercial detectors normally are configured only for sulfur and phosphorus detection [85–87]. In the FPD, the GC column extends