A new He discharge-afterglow and its application as a gas chromatographic detector

Sp~ftrochm~c~ Am,

Vol

4OB. Nos 10-12. pp 1573-1584.

1985

PnntcdI” GreatBntaln

0

A new He discharge-afterglow and its application graphic detector G.

W. RICE,* A. P. D’SILVA, and V. A.

05868547/85 s3.00+ 00 1985. Pergamon PressLtd

as a gas chromato-

FASSEL+

Ames Laboratory and Department of Chemistry, Iowa State University, Ames, 10 50011, U.S.A (Received May 1985; in revised form July 1985) Abstract-A description is given for a low frequency. high voltage, electrodeless discharge-afterglow sustained in helium at atmospheric pressure When this discharge-afterglow was used as an element-selective, multi-element detector for gas chromatography. absolute limits of detection in the picogram range were obtained for F, Cl, Br, I, C, P, S, Si, Hg and As Because GC eflluents were introduced directly into the afterglow region of the discharge, extinguishment and contamination of the primary discharge were completely eliminated.

INTRODUCTION THERE IS an

increasing interest in element specific, multi-element detectors for gas chromatography that provide subnanogram limits of detection, wide linear response, tolerance to the passage of eluting solvent, minimum dead v,olume, and simplicity of operation and construction [l]. In response to this need, we have explored the scope of application of atmospheric pressure afterglow excitation to this task. We have earlier reported our results obtained with an atmospheric pressure nitrogen afterglow (N,-APAG) [2, 31, and more recently, we have demonstrated that an Ar-APAG provided powers of detection ranging from 0.04ng for P to long for Br [4]. Although both the Nz and Ar APAGs provided element-selective spectra, several limitations were apparent. In the case of the N,-APAG, element-selective detection was restricted to free atom emission from metal containing eluents and from carbon, the latter via the formation of CN emission from eluting organic compounds. Only molecular emissions from PN, Sir NC1 and NBr were observed when P, S, Cl and Br containing compounds were injected into the N,-APAG [5]. In the Ar-APAG, atomic spectra of Cl and Br were detectable, but the measured detection limits were marginal for GC applications. Significantly, of the spectral lines studied in the Ar-APAG, those of Br and Cl had the highest excitation potentials, with values of 9.72 and 10.62 eV, respectively. These observations suggested that the energy carriers in the Ar-APAG were either insufficient in energy or inadequate in number density (or both) to provide an acceptable level of excitation of the respective excited states. A comparison of the most likely energy carriers of an Ar-APAG with those of a He-APAG revealed that the latter had the potential capability of considerably higher collisional energy transfer. To take advantage of this potential, we have devised a simple and reliable primary discharge-afterglow system that possesses attractive possibilities as a versatile element selective, multielement GC detector. EXPERIMENTAL PROCEDURES Instrumentation A schematic diagram of the helium discharge-afterglow device is shown in Fig. 1. The electrodeless discharge was formed within a 3-mm o.d. by l-mm id. quartz tube that extended throughand

This paper was submitted for a special issue of Specrrochimica Acta dedicated to Professor Velmer Fassel. *Present address: Department of Chemistry, The College of William and Mary, Williamsburg, VA 23185, U.S.A. ‘Corresponding author. [I] [2] [3] [4] [S]

T. H. RISBYand Y. TALMI,CRC Crit. Rev. And. Chem. 14,231-265 (1983) and references contained therein. G. W. RICE, J. J. RICHARD,A. P. D’SILVA~~~ V. A. FASSEL, AM/. Chem. 53, 1519-1522 (1981). G. W. RICE,J. J. RICHARD, A. P. D’SILVAand V. A. FASSEL,Anal. Chim. Acta 142,47-54 (1982). G. W. RICE,A. P. D’SILVAand V. A. FASSEL,Anal. Chim. Acta 166,27-38 (1984). G. W. RICE, A. P. D’SILVAand V. A. FASSEL,Appl. Spectrosc. 38, 149-154 (1984). 1573

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G. W. RICE et al.

rs II

GROUNDING

ROD

QUARTZ TUBING (3mmOD/lmmID) STAINLESS STEEL ELECTRODE 1 2cm LENGTH) TO TRANSFORMER/ AC POWER SUPPLY

HEATER BLOCK

COLUMN-/

.

Fig. 1. Schematic diagram of the discharge tube configuration utilized for generation of the helium discharge. approximately 3 cm above a 2cm long cylindrical, stainless-steel electrode. The latter was connected to a variable-frequency, A.C. power supply coupled to an impedance-matching, 20 KV transformer (Models EGRSOO and RS816T, respectively, ENI Power Systems, Rochester, NY. The newer HPG-2 high voltage plasma generator from the same company is a more appropriate power supply). The 3-cm extension served to contain the He afterglow. A stainless steel rod placed directly above the tip of the quartz tube outlet served as a grounding electrode for the afterglow. A 0.317-cm (l/8 in.) Swagelok fitting with a Vespel ferrule seal (Alltech Associates, Inc.) was used to mount the quartz discharge tube on a stainless steel heater block. The heater block maintained desired temperatures in the GC column between the GC oven and entry into the discharge tube. The inlet used for the He flow to sustain the discharge entered perpendicular to the quartz tube in the heater block. The capillary column from the GC was inserted through a Swagelok fitting at the bottom of the heater block. All three fittings connected to the heater block were arc welded to minimize possible atmospheric gas entrainment into the He flow. The entire block assembly and cylindrical electrode were thermally isolated by Fiberfax (Alltech Associates, Inc.) insulation and ceramic insulators respectively. The GC capillary column exit was positioned approximately flush with the top of the cylindrical electrode. Approximately 1Ocm of the polymeric coating was removed from the end of the capillary column to prevent deposition of the polymer onto the discharge tube walls. Optical emission from effluents exiting from the capillary column was observed between the stainless steel electrode and the grounding rod. A schematic diagram of the instrumentation utilized in this study is shown in Fig. 2. Matheson grade helium (99.9999% purity, Matheson, East Rutherford, NJ) was passed through separate molecular sieve/Drierite and oxygen traps (Alltech Associates, Deerfield, IL) to reduce the residual water. hydrocarbon, and oxygen containing impurities in the gas stream. A calibrated, stainless-steel, micrometering valve (Whitey, Co., Highland Heights, OH) was connected between the traps and the heater block to regulate the He flow rates. The injection system for the GC was completely replaced with an on-column injection system (SGE, Inc.: Austin, TX). All gas lines from the GC and discharge systems were verified to be leak-free down to the maximum sensitivity of the gas leak detector used ( < 1O-6 ml s-l He; Model 8016, Matheson). The entire discharge apparatus was mounted on top of a Hewlett-Packard Model 5710A Capillary GC in place of the standard detector location. Optical emission from the central 1 cm portion of the afterglow tube was focussed on the 1 cm high entrance slit of a 0.3-m monochromator (Model 218, McPherson/GCA, Acton, MA) by a 50-mm focal

Discharge-afterglow in helium

1575

He DISCHARG

Fig. 2. Schematic diagram of the instrumentation used for the GC detector system. length CaF, lens. A side-on type photomultiplier tube (Model R758, Hamamatsu, Middlesex, NJ) was used for the entire spectral region of interest (180-900 nm). An auxiliary Nz purge of the monochromator and optical path was used to observe emission below 190 nm. The photocurrents from the amplifier (Keithley Instruments, Inc., Cleveland, OH) could be recorded simultaneously through a strip chart recorder (Houston Instruments, Austin, TX) and an Apple/Isaac data acquisition system (Cyborg, Newton, MA). Operational characteristics The operating conditions for the discharge and the spectrometric and chromatographic systems are summarized in Table 1. The frequency of the power supply was tuned to minimum reflected power. No external means of discharge initiation, e.g. a Tesla coil, was required. During the first 10-15 min of operation, both the discharge and the afterglow appear light blue in color, but gradually become orange during that time period. This behavior appears to result from two phenomena. The appearance of the initialblue color is a result of the preferential excitation of the first negative band system of Nl . Such a preferential excitation can be explained with reference to the energy level diagram shown in Fig. 3, where the potential energies of the several species (He+, He:, He 2’s, 23s and HeJ presumed to be present in He afterglows are indicated. The range of energies for the He: and He, species are as given in Ref. [6]. Alternate values have been suggested in Ref. [7]. From Fig. 3 it is evident that there is a close energy match between the potential energy of He (2’S) metastable species or the ionic species He: and the vibrational-rotational manifold of the N: excited electronic state B2C+. Transitions from the B*E + state to ‘Zl state gives rise to the N: first negative emission observed in the 400-440 nm region as seen in Fig. 4. The selective transfer from the He species to N, occurs via the Penning type reaction He (2%)

Table 1. Operating conditions of the Hedischarge-afterglow system (A) Discharge system He flow rate: Incident power: Reflected power: Frequency: (B) Spectrometric system Monochromator: Slit widths: Bandpass (FWHM) PMT (180-900nm) (C) Chrornatographic system Carrier gas: Carrier gas flow rate: cohunn:

80ml min-r 45w 0.2w 26-27

KHz

0.3m 50pm 0.2 nm 13OOv Helium 5ml min-’ Durabond 5 (J&W); 30m

[6] C. B. COLLINS and W. W. ROBERTSON, J. Chem. Phys. 40, 701-712 (1964). [7] C. I. M. BEENAKKER, Spectrochim.Acta 32B, 173487 (1977).

1576

G. W. RICE et al. NITROGEN

HELIUM

CARBON MONOXIDE

NEON

tie+ 24

-

=j2& N2+ T

XB2Y CO’

FIRST NEGATIVE

FIRST NEGATIVE

Fig. 3. Energy level diagram of He species presumed to be present in the He afterglow and the excited and terminal states of impurity species, (N2, CO, Ne) that are preferentially excited in the He afterglow. An asterisk indicates a metastable species.

CO

I

G

ax

z

; -

8Q-

-Y

60-

I

I

240

I

I

280

320

I

I

I

I

z&O

400

N,; He

G _1 g

He

He

40-

x.“-.“1 O-A_ 400

440

480 WAVELENGTH

520

560

600

(nmi

Fig.4. Background emission observed in the 19O-XMnm region from the helium afterglow. Amplifier gains: 190-300nm: 1 x lo-‘A, 30&4OOnm: 3 x lO_sA; 40(M00nm (bottom): 1 x IO-‘A; 40@6OOnm (top): 1 x IO-‘A. + Nz + N: (*Xi) + e + He or the charge transfer reaction He: + N, -+ N: (‘Z,’ ) + 2He. The latter has been proposed to be the more dominant reaction in high pressure He afterglows [S]. The gradual disappearance of this blue emission during the first l&15 min of operation suggests that N, is slowly [83 C. B. COLLINS and W. W. ROBERTSON,in Comptes Rendus de la VI ConjZrence Internatiomle Ph~notines d’lonisation dam les Gaz, Ed. D. HUBERT and E. CHEMEMIEU-ALEAN. Paris (1963).

sur les

Discharge-afterglow

in helium

1577

undergoing a “degassing” process in the early stages. The orange emission arises primarily from the Ne impurity in He (see Fig. 5). As shown in Fig. 3 there is near coincidence in the energy level of the He 2% metastable species and the Ne levels in the ‘S manifold, which results in a highly selective energy transfer [9]. Apparently, the energy transfer from He to Ne is enhanced in the absence of N,. The entire gas line assembly was sealed and pressurized when not in use to prevent contamination of the discharge tube and gas lines with air, which can result in considerably longer (> 1 h.) warmup times to remove residual surface contamination. The presence of the grounding electrode directly above the tip of the discharge tube had a profound effect on the afterglow emission. Without the electrode, the primary current pathway occurred through the stainless steel block used to mount the discharge tube, although a significantly attenuated afterglow was still visually observed above the discharge region due to the direction of the He flow. Some residual current flow through the steel block still persists with the grounding electrode in place. However, with the grounding electrode in place, emission intensities between the electrodes were markedly amplified, with uniform intensity throughout the entire 3-cm length region above the primary electrode. In the absence of the grounding electrode, the afterglow exists directly into the atmosphere, with an abrupt change in color caused by the selective excitation of the Nl first negative system.

Reference solutions All reference mixtures for the present elements of interest were prepared from the following: F, fluorobenzene; Cl, trichloroethane; Br, dibromomethane; I, 2-iodopropane; C, octane; S, thiophene; P, trimethylphosphite; Si, triethoxyethylsilane; Hg, diethylmercury; and As, triphenyl-arsine. Stock solutions containing 1Opg j.K’ of each respective analyte element were prepared in octane for the As and P compounds and in anhydrous diethylether for all other compounds. Serial dilutions containing from 1 pg j11-’ to 1Opg j4-’ of the elements of interest were also prepared. RESULTS AND DISCUSSION

Spectral characteristics of the He afterglow The most prominent features in the background spectra of the 190-600-nm Ni first negative bands, the relatively strong CO+ first negative emission in region and the OH and NH bands as shown in Fig. 4. The rather surprising again appears to be the result of the selective energy transfer processes [6, transfer pathway is evident in Fig. 3. Overall, the afterglow emission between was very similar to the background spectrum of a He supported, microwave [ll, 123 with the exception of the increased CO+ first negative emission emission. The absence of Si (I) emission in the present configuration strongly no devitrification of the quartz tubing occurs from the discharge. Another observation of He, molecular band system around 465.0 nm in the afterglow.

region are the the 190-250 nm CO+ emission lo]. The energy 190 and 600 nm induced plasma and no Si (I) suggested that exception is the Emission bands

He

Ne

-_.lL._ li.’_lL-L_Ne

H

He

Ne

Nf

I

II

”

I 600

I

I

I

I 650

I

I

I

II

I

703

I

I

I

I I

I

I

79 WAVELENGTH

I I

I

I

I I I Ill

800

850

IL 930

(nm)

Fig. 5. Background emission observed in the60&9OOnm region from the helium afterglow. Amplifier gains: bottom: 3 x IO-‘; top: 3 x IO-*A. R. T. YOUNG, JR, C. S. WrLtxTrand R. T. MAUPIN,.I. Appl. F. W. LEE and C. B. COLLINS,.I. Chem. Phys. 65, 5189-5197 S. A. ESTES,P. C. UDEN and R. M. BARNES,Anal. Chem. 53, K. TANAKE,H. HARAGUCHI and K. FUWA,Spectrochim. Acta

Phys. 41,293&2942

(1970).

(1976). 1829-1837 (1981). 36B, 119-127 (1981).

G. W. RICEet al.

1578

from CO+, OH, NH and Nz+ apparently arise from gas impurities remaining in the He after passage through the present trapping system. Emission in the 600-900 nm region consisted primarily of H (I), Ne (I), 0 (I), and He (I) emission lines as shown in Fig. 5. A summary of atomic and molecular background emission observed in the He afterglow is given in Table 2. Emission profiles of analyte elements To obtain characteristic atomic and/or molecular emission spectra from C, F, Cl, Br and S species, octane, trichlorofluoromethane, dibromomethane, or thiophene were continuously introduced into the discharge via vapor purge techniques. An auxiliary He flow of approximately 5 ml min - ’ was used to transport and inject the vapors into the afterglow region. For P, I, Si, Hg and As, the reference solutions were separated under the conditions summarized in Table 1. The elution-band peak heights were measured at the analyte wavelengths anticipated to give the highest emission response. Prominent emission lines observed for each element are given in Table 3 in the order of decreasing intensity. No significant spectral interferences were observed for the most intense analyte lines. Appropriate optical filters were used to reject lower order radiation. Although ion lines were observed for Cl, Br and S, their intensities were generally one to two orders of magnitude weaker than the atom lines. This observation is in direct contrast to the He-MIP, in which analyte ion emission lines for Cl, Br and S are observed to the most intense [ 111. Table 2. Atomic and molecular emissionobserved in the He afterglow Species He, N: co+ OH NH Ne* H 0 C

Transition sag-+ ‘Zz BsZ+ -rx%+ B2Z+ .+X2$ A2Z+ -+X2n A3ni-r X3x-

Ne (I) H (I) G (I) C (I)

Wavelength region (nm) -4650nm 290-440 180-260 280-315 336.0, 337.0 588.2, 640.2, 650.7 705.1, 724.5 486.1, 656.2 777.2, 844.6 193.0, 247.9

*Only most intense Ne lines are lised. Table 3. Prominent emission lines observed from analyte elements in the helium afterglow Element F

Cl

Br

I

Wavelength (nm)* Element 739.9 775.5 685.6 733.2 837.6 858.6 725.7 754.7 821.2 863.9 841.1 134.9 183.0 206.2

Wavelength (nm)

C

193.1 247.9

P

213.6 253.5 185.9

S

182.0 180.7 182.6 215.6 212.4 288.2 253.7 435.8 189.0 193.7 228.8

Si

Hg As

*Listed in order of decreasing intensity.

Discharge-afterglow

in helium

1.579

Flow, power and observation height dependencies

The effect of the helium discharge flow rate on the emission signals from 1 ml injections of 10 ng each of F, Cl and Br is shown in Fig. 6. In general, peak heights from all of the elements studied followed the same general trend as observed for Cl and Br, the lone exception being F, which showed peak emission at He flow rates of approx. 200 ml min-‘. A flow rate of 80ml min-’ was used for all subsequent analytical studies. Emission signals from analyte species were observed to steadily increase with increasing power, but signal to background scatter ratios did not improve above 45 watts. Power levels of 60 W or greater produced pinholes in the quartz tubing after only several minutes of operation, creating an arcing pathway from the discharge electrode to the grounding electrode at the end of the quartz tube and rendering the tube useless. Only slight variations in signal response were observed for all the elements studied for observation heights ranging from 1 to 2 cm above the capillary column exit. Therefore only the central l-cm region of the afterglow was imaged on the slit of the spectrometer. No attempt was made to optimize the length of the afterflow region or the electrode that produced the discharge. The relative position of the GC capillary column exit at the top of the cylindrical, primary discharge electrode had virtually no effect on the emission signals from analyte species within 2-3 mm of either direction of the upper end of the electrode. Signals were observed to steadily decrease beyond these boundaries. Placement of the capillary exit at the base of the cylindrical electrode, so that the efI%rents passed through the primary discharge, caused a reduction of the analyte signals by one to two orders in magnitude. Herein lies a key element of this discharge-afterglow detector. The analyte stream is not introduced into the primary discharge or plasma and hence cannot affect the various excitation and ionization processes that produce energetic species of He. It is also in this sense that this detector differs from a variety of He detectors described in the past, as, for example the direct current He discharge or He glow detectors [13,14], and the microwave induced plasmas [ll, 121. For all of these versions, the analyte is introduced into the sustaining plasma. Analytical performance

Representative chromatograms for Cl and Br from the injection of trichloroethene and dibromomethane, respectively, are shown in Fig. 7 and for S and Hg from thiophene and diethyhnercury, respectively, in Fig. 8. The quantities given are the amount of analyte element present in a l-p1 injection into the gas chromatograph. Both figures clearly demonstrate the

FLOW

RATE

(CC/MIN)

Fig. 6. Flow rate dependence on emission signals observed from F, Cl, and Br in the He afterglow.

[13] R. C. BRAMAN and M. A. TOMPKINS, Anal. Chem. 50, 1088-1093 (1978). [14] C. FELDMAN, Anal. Chem. 51, 664669 (1979).

1580

G. W. RICE et al.

3x10-BA

3x iO-7A

I ng Cl

10n --

_Cl

(I)

837.6nm

I x lO-8A IOOpg -_-

Cl

3opg Cl -rL

II

-

_

T 5 Y

~xIO-~A b(I)

long Br --

8272nm

d

i

IxIO-~A

, .

TIME

Fig. 7. Peaks observed in the He afterglow detector from increasing amounts of Cl and Br reference solutions. Effects from the solvent are shown on the first chromatogram of each element. Amplifier gains used at each concentration are given above the amounts injected.

powers of detection, as well as the general reproducibility and linearity of the signal with respect to the gain of the amplifier. The emission signal changes caused by passage of the solvent through the afterglow are shown in the first chromatogram for each element. The wide intensity excursions that are typically observed during the passage of solvent through the afterglow appear to result from the interplay of two factors, namely, the temporary quenching of the normal afterglow background while concurrently there may be enhanced background contributed by characteristic emission of solvent fragments or from compounds formed from the solvent, such as CO. Scatter from emission of solvent fragments may also cause an elevation of the general background level. The relative contribution of each process may change markedly during the passage of the solvent causing wide intensity fluctuation in the solvent peak. Because normal afterglow background is established before the elution of analytes, these intensity fluctuations are not analytically bothersome or significant. Although the afterglow emission is quenched by excessive solvent load (> 0.01 jd), total extinguishment is prevented and the population of energetic afterglow species is quickly reestablished by continual replenishment from the primary discharge. A summary of the analytical wavelengths, limits of detection, linear range, and selectivity obtained for the elements studied thus far is given in Table 4. Limits of detection were based on measurements of the amount of analyte species required to produce chromatographic peak height signals that were three times greater than the background scatter. To accommodate the band broadening effects of column overloads at high concentrations, peak area measurements were used for linear response evaluations. Selectivity ratios were based on

Discharge-afterglow in helium 3x IO-*A I ng S

1581 3~10-~A long S -

-

S (1) 182.Onm

Hg (I 1 253.6

nm ~xIO-~A

lx 10-8A

I I

100P9 Hg

\ ‘aHa

-

low Hg -

1

L

TI ME Fig. 8. Peaks observed in the He afterglow detector from increasing amounts of S and Hg reference solutions. Solvent effects and amplifier gains are described as stated in Fig. 7.

Table 4. Analytical performance data for the He afterglow detector

Element F Cl Br I C P s Si Hg As

Analytical wavelength (nm)

Absolute limits of detection (PP)

739.9 837.6 827.2 183.0 193.1 253.5 182.0 251.6 253.6 189.0

20 8 15 2 10 30 5 50 0.5 20

Linear range

Sekctivity ratio

1 x 10”

400

5x 7x 5x 5x 1x 1x 5x 1x 5x

300 120 130

10’ lo3 10’ 10’ lo3 10” 102 10” lo2

80 80 40 7000 20

Test compound fluorobenzene trichloroethane dibromomethane iodopropane octane trimethylphosphite thiophene triethyoxyethylsilane diethylmercury triphenylarsine

the ratio ofconcentration of a carbon reference compound (octane) to analyte concentrations that gave signals of equal magnitude at the analyte wavelength. In general, limits of detection for all the elements studied thus far have been 50pg or less, with linear response to concentrations from two to three orders in magnitude. Selectivities have been observed to be poorer for elements with analytical wavelengths in the UV region, a factor which is attributable to CO emission as previously discussed. Further improvements in gas purity should improve selectivity factors significantly.

1582

G. W. RICEet al.

Comparison

to the He-MlP Over the past twenty years the He-MIP has been developed into a versatile elementselective, multi-element detector for GC [l 1, 12, 15, 163. However, several constraints have limited the general acceptance and utilization of these plasmas as GC detectors. These constraints include: (a) extinguishment of the plasma when amounts of solvent or analyte exceeding l-l 0 pg s- 1are injected, thus requiring solvent venting, reignition of the plasma, or reduced sample loading at the expense of analyte throughput [l 1, 121; (b) devitrtication of the inner surface of the quartz tube by contact with the plasma; (c) requirement of periodic replacement or cleaning of the tube to reduce or eliminate memory effects caused by the devitrification [12]; and (d) the desirability of optimizing the observation window and He flow rates (50-700ml min- ‘) to obtain optimum sensitivity for each element to be determined [ 11, 161. The advent of the Beenaker TMolo microwave cavity has reduced sample overload effects significantly, but solvent venting or split sample injections are still required for large sample throughputs to maintain plasma stability [ 11,151. Recent advances in tangential flow torch designs have also been very successful in circumventing the above problems at the expense of significantly higher He consumption (2.5 1 min- ‘) [17]. Oxygen doped plasmas have been found to be effective in cleaning up the carbon deposits formed on the inside of the discharge tube [18]. A comparison of limits of detection from the present work to literature values for the HeMIP is tabulated in Table 5. As shown, detection limits obtained in this study were improved by factors of 3-l 20 over the He-MIP with the exception of C and Si, which were similar. Both systems exhibit the same overall range of linear response, although selectivity was generally better in the He-MIP [ll]. In addition to the excellent limits of detection and element-selective capabilities of the He afterglow, day to day reproducibility in the relative intensities of the spectral lines in the afterglow has not varied more than 10%. No devitrification or deposition has ever been observed in the quartz discharge tubes, even after one hundred hours or more of operation. A significant advantage of the He afterglow detector is that eflluents are introduced beyond the primary discharge region. Although temporary quenching of the afterglow region does occur from excessive solvent loads, the afterglow population is continually replenished from the primary discharge. Thus, the stability of the afterglow is maintained without resorting to the solvent venting or reignition steps commonly associated with the MIP. Finally, the system is easy to construct and maintain, reliable, and He gas consumption is relatively low.

Table 5. Comparison of absolute limits of detection (pg) for the present work and the He-MIP Element

Present work MIP (He)/

F Cl Br I C P S Si Hg AS

20 8 15 2 10 30 5 50 0.5 20

64

155 106 56 12 56 140 18 60 155

*Ref. [ll].

[15] B. D. QUIMBY, P. C. UDENand R. M. BARNES,Anal. Chem. 14,2112-2118 (1978). [16] K. J. SLATKAVITZ,L. D. HOEY, R. M. BARNESand P. C. UDEN,presented at the 1984 Pittsburgh Conference and Exposition, March 5-8, Atlantic City, NJ., paper No. 368. [17] S. R. GOODE, B. CHAMBERS and N. P. BUDDIN,Appl. Spectrosc. 37,439-443 (1983). [18] W. BRAUN,N. C. PETERSON, A. M. BASS and M. J. KURYLO,J. Chromutogr. 55,237-248 (1971).

Discharge-afterglowin helium

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Continuing efforts to improve the overall analytical figures of merit are presently being focused on: (a) improving the selectivity ratios for elements such as As, Si, S and P through application of background blank corrections, as has been demonstrated for GC-MIP [19, 201; (b) assessing the analytical performance of carrier gases other than He; and (c) reducing the level of the 0, N and H background emission sufficiently to allow the quantitative determination of these elements in typical column effluents. The origin of this background contamination, i.e. the source He or air diffusion into the afterglow detector, has not been unequivocably identified. Energy transfer considerations

Any interpretation, or more correctly, speculation on the mechanistic pathways that lead to the detection of atomic spectral lines of the halogens, C, P, Si, Hg and As at the picogram level must be consistent with the following key experimental observations: (a) the electronic transitions leading to the atomic emission originate from excited states ranging from 4.86 to 14.37eV above the ground state; (b) the picogram powers of detection are observed even when these elements are introduced as stable molecular compounds; and (c) introduction of the analyte compounds into the primary discharge, rather than into the afterglow, leads to a decrease in analyte emission of an order of magnitude or two. Obviously, the number density of effective energy carriers, and the nature and extent of various fragmentation, atomization, excitation, and radiative and dissociative recombination, and other de-excitation processes occurring in the primary discharge and in the afterglow must be vastly different. To appreciate this difference, it is important to note that in the absence of sample introduction into the primary dielectric discharge, the latter occurs in a very pure He gas environment. Unless the energies received by free electrons from the electrical field are sufficiently high to cause excitation and ionization, the collisional interactions of the electrons with the much heavier He atoms are primarily elastic, with the fractional loss of electron energy per collision being only 2m/M, where m and M, respectively are the masses of the electron and He atoms [l, 21,223. In a stable discharge, the electrons continue to gain net energy from the field to assure that there are a sufficient number in the high energy tail of the electron energy distribution curve to not only populate excited states in He atoms but also to form He+ ions and electrons through inelastic collisional processes [21,22]. In this way, excited He atoms, atomic and molecular He ions, and other energetic He species, such as He2 may be formed [6, 22,231. Some of these species are sufficiently long lived (metastable) to be transportable to the afterglow region during their lifetimes. The most likely energy carriers are: He (2%, 19.81eV), He+ (1S’S ,,2, 24SeV), He* (a%,‘, 14.6-17.4eV) and He: (X’C,‘, 18%21.6eV) [6,21-261. Of these He (23S) has an extremely long lifetime of N 150min at low pressure [27]. The characteristic atomic emission from He (2%) at 388.9 nm and the molecular band emission around 465.0nm from Hez(3n, +“Zl ) is clear evidence of the presence of the above metastables in the afterglow. The experimental observations (a) to (c) discussed above can best be reconciled by assuming that the number densities of one or more of these energy carriers and the relevant cross sections for particular fragmentation, atomization, and excitation processes are high enough to generate the very sensitive atomic emission spectra that are observed. The key consideration is that in a pure He dielectric discharge, the electron energy distribution is high enough to generate relatively long-lived energetic species through inelastic collisional processes [28]. In the presence of significant amounts of solvent and analyte in the primary dielectric discharge, the environment is that of a molecular gas diluted M. A. ECKHOFF, T. H. RIDGWAYand J. A. CARUSO,Anal. Chem. 55, 10041009 (1983). D. F. HAGEN,J. BELISLE and J. S. MARHEVKA, Spctrochim. Acta 3SB, 377-385 (1983). M. A. BIONDI.Elect. Eng. 00, 806 (1950). A. T. ZANDERand G. M. HIE~~E, Appl.Spectrosc. 35, 357-371 (1981). C. B. COLLINSand W. W. ROBERTSON,J. Chem. Phys. 40,2202-2208 (1964). C. B. COLLINSand W. W. ROBERTSON, J. Chem. Phys. 40,2208-2211 (1964). C. F. BAUERand R. K. SKOOERGOE,Specrrochim. Acta 3BB, 1125-l 134 (1983). F. W. LEE, C. B. COLLINSand R. A. WALKER,J. Chem. Phys. 65, 1605-1615 (1976). J. R. W~~DWORTHand H. W. Moos, Phys. Rev. 12A, 2455-2463 (1975). R. DELOCHE,P. MONCHICOURT, M. CHERET and F. LAMBERT, Phys. Rev. 13A, 1140-l 176 (1976).

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in He. Now the energy transfer processes are primarily dictated by the molecular nature of the plasma. The parent analyte molecule and their fragments, be they molecular or atomic, possess a multitude of low lying electronic, vibrational, and rotational degrees of freedom able to accept and/or transfer a broad range of energies. Thus, in a molecular gas discharge, inelastic collisions leading to the population of a multitude of excited states in the electronic, vibrational and rotational manifolds of the analyte molecules and their fragmentation products, are the primary modes of energy transfer. At atmospheric pressure this process leads to the equilibration of the electron and gas temperatures, a consequence of which is a marked decrease in the electron energy distribution. Stated another way, the rate of ionization needed to sustain the discharge can be maintained by electrons that only have enough energy to ionize analyte molecules or their fragmentation products. These energies are not high enough to generate the energetic He species. Because these species are then not formed in the primary discharge, it is not surprising that their impact in the afterglow region is not as evident. Acknowledgements-The Ames Laboratory is operated for the U.S. Department of Energy by Iowa State University under Contract No. W-7405-Eng-82. This research was supported by the Division of Chemical Sciences, Budget Code KC-03-02-02, Office of Energy Research. The authors wish to thank L. C. HANSENand T. H. GIBBONSfor their active participation in the data collection phase of this research project.