490
Anal. Chem. 1985, 57,490-493
Thermionic Ionization Detector with Lanthanum Hexaboride/SiIicon Dioxide Thermionic Emitter Material for Gas Chromatography Toshihiro Fujii* Division of Chemistry a n d Physics, National Institute for Environmental Studies, Tsukuba, Ibaraki 305, J a p a n
Hiromi Arimoto Analytical Application Department, Shimazu Corporation, Nishinokyo, Nakagyo, Kyoto 604, J a p a n
A thermionic ionization detector ( T I D) has been developed which employs the electrically heated filament with LaB, powder mixed with SiO, on the understanding that its ionization mechanism Is the thermionic emission of negativecharged particles from the hot surface of the low work function material. Mass spectrometric measurements indicate that CN-, PO-, PO,-, and PO3- are likely to be the initial chargecarrier species emitted from the hot LaB,/SiO, bead surface of the T I D in the presence of nitrogen- or phosphorus-containing sample compounds. Response characteristics have been examined in terms of filament surface temperature, composition of gaseous environment surrounding the filament, and so on, demonstrating that it provides no less sensitivity and specificity than the conventional alkali metal-giass bead TID.
Recently there has been a new type of thermionic ionization detector (TID) introduced to gas chromatography for the specific detection of nitrogen or phosphorus compounds ( I ) . This second generation N / P TID employs a flameless design which yields a favorable response under the condition of hydrogen air environment in which there is not enough hydrogen present to sustain a true flame. The alkali metal (usually rubidium) salt is mixed into a glass ( 2 , 3 )or a ceramic (4) which is heated electrically and is placed above the detector jet. In comparison with the old alkali flame ionization detector (AFID) ( 5 ) , this new detector shows a considerable improvement in stability (reliability) as well as in ease of operation. A critical review of the literature on the TID up to 1980 was published by Farwell e t al. (6). Several attempts to explain the ionization mechanism have been made. The detection mechanism proposed by Kolb et al. (7) for their Rb-glass bead detector in the nitrogen mode is the chemiionization reaction in the gas phase on the assumption of the excited rubidium. Rb*
+ CN
-
Rb+ + CN-
Such an ionization process should be possible because the energy resulting from the electron affinity (3.16-3.82 eV) of the CN radical (8)exceeds the calculated ionization potential (2.57 eV) of the excited R b atom observed most intensively in line spectrum, whose energy is 1.59 eV (9). A similar reaction mechanism was postulated for the phosphorus mode using POz as the intermediate radical. Most questionable in this theory is that it presumes the evaporation of the rubidium atom from the silicate source; this requirement is not consistent with the experimental result that the relatively good sensitivity of the sources containing nonvolatile alkali salt is given (10).
An alternative mechanism to the gas-phase process is the negative surface ionization process proposed by Patterson (11) for a rubidium-ceramic bead and by Olah (12)for a Rb-glass bead. According to this theory the alkali atoms do not leave the surface but catalyze the electron transfer taking place on the bead surface. The negative surface ionization phenomena is interpreted well by the classic Saha-Langmuir equation (13); for electronegative species, the degree of negative surface ionization a t thermal equilibrium is described as
where no and n- are the densities of neutral and negative species desorbed from a surface a t temperature T, 6 is the work function of the surface, Ea is the electron affinity of the species, and go and g- are the statistical weight of the ground state of the neutral and negative ion, respectively. As can be seen from this equation, the surface ionization efficiency depends strongly upon both the work function and the electron affinity. In the case of TID, the presence of the alkali metals in the bulk material which have work functions as low as 2.16 eV (Rb) (14) presumably causes the effective reduction of the work function, while the probable species to form the negative ion are the CN radical for N compounds and POz radical for P compounds, both of which are known to have high electron affinity. On the other hand it is well-known (15) that LaB, has a very low work function of 2.66 eV. This material is refractory with the melting points at 2210 "C. When this compound is heated to a sufficiently high temperature, the metal atoms a t the surface evaporate away. They are, however, immediately replaced by diffusion of metal atoms from the cell below. This property keeps evaporation losses a t a minimum and a t the same time provides a mechanism for constantly maintaining an active surface. Besides the borides are very stable chemically; moisture, oxygen, and even hydrochloric acid do not react with them. This, together with the low work function property and thermally stability, seems to give ideal properties to a bead material of TID detection on the assumption that the ionization mechanism is the thermionic emission of negative-charged particles from the surface of the hot bead. In the present study a TID with a new bead composed of a LaB, powder embedded in a silica matrix has been developed on the basis that the principle of the TID is negative surface ionization of the highly electronegative species. The most important performance and response characteristics of the LaBs/SiOz beads are given, demonstrating that its performance is comparable with that of conventional alkali sources for both phosphorus and nitrogen compounds while its stability is excellent, presumably due to the chemical durability of LaB, material.
0003-2700/85/0357-0490$0lSO/O@ 1985 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
, il!itir 1
Electro eter
El source
7
1 I
d ,IF 4
Inner collector
Ceramic insulator
tl
311
491
LaBe bead ssa :
T
analysis
Power controller
1
probe
1
%*I&
voltage power supply /
- 6 eV
power supply
Quartz n zzle
?nhT
Column
Figure 2. Schematics of experimental setup for the measurement of a negative ion species formed on the LaB,/Si02 bead. The design of the emitter probe is such that the LaB, bead on the Pt wire can be placed in the center of the electron ionization ion source chamber when the emitter probe is inserted.
!CN-
loo]
Figure 1. Construction of the thermoionic ionization detector with a
LaB,/SiO, bead on the
pt
wire.
EXPERIMENTAL SECTION Instrumentation. Experiments were conducted by using the TID with a newly developed LaB6/SiOz bead equipped on the Shimazu Model 7A PrFFt gas chromatograph. Chromatographic data were obtained by using a glass column of 1m X 2.6 mm i.d. packed with 5% SP 2250 on 80/100 mesh Chromosorb W-HP. Figure 1shows a schematic illustration of the present TID. The important component in this detector is the La& bead assembly which has an electrically heated, wire embedded within the body of the bead. The bead is placed so that it is directly above a quartz nozzle, which is biased at a negative voltage so that negativecharged species formed may move to the collector. A transformer coil of the radio frequency power controller is grounded at the center point. The bias of the bead with respect to the inner collector electrode is substantially equal, because the magnitude of the voltage drop across the bead is as small as 0.8 V. Preparation of the Beads. The mixture was prepared by weighing silicone dioxide (99.9%, Wako Chemical, Japan) and LaBS (99.9%, Stark, West Germany) powders. The SiOz powder was intimately mixed with La& powder. A s m d amount of water was then added to this mixture to make possible the mounting of this material on the platinum wire. The resulting pasty mixture was molded into the football-shaped bead (4mm long and 2-mm diameter) during the transfer to the Pt wire of the bead assembly. After installation to the main body of the detector, the P t wire and bead were heated in the air environment for an hour by passing sufficient current of 8.0 A to cause the bead to attach to the wire. Finally the bead was conditioned at the lower temperature in the operational gas atmosphere. Mass Spectrometric Identification of Negative Ion Species. In order to demonstrate the formation of negative ions on appropriately activated surfaces, the LaB6/Si02 bead was installed in a mass spectrometer so that ions formed could be mass-analyzed. However, the exact identification has not been made. A very simple and common experimental setup for negative surface ionization was employed for the mass spectrometric identification (Figure 2). A Finnigan 3300 quadrupole mass spectrometer with a conventional electron impact (EI) ion source was used. The emitter probe built and placed in the ion chamber of the E1 source has almost the same configuration as the solid inlet probe supplied by the manufacturer except that the solid sampler holder was replaced by the Pt wire with LaJ3,/SiOZ beads on it. The spot-welded Pt wire across the posts on the Kovar seal was heated using a commercial dc power supply. The emitter probe was maintained at the same potential as the E1 ion source chamber applied at -6.0 eV as the accelerating voltage. The instrumental details have been referred to elsewhere (16, 17). Sample. For the investigation of the performance, samples were prepared using n-hexane as the solvent with azobenzene,
01 10
i l
I i
20
30
-
I I
I 40
I
50
m/z
Figure 3. Negative surface ionization mass spectrum of acetonitrile on the LaB,/SiO, bead around 1300 K. OH-, 0-, and CI- are the background species. The assignment of HCN- and NCO- is not certain.
tri-n-butyl phosphate, malathion, and n-heneicosane as the test compounds. For the mass spectrometric study, acetonitrile and trimethyl phosphite were used. All the materials were purchased from Nakarai chemicals (Kyoto, Japan).
RESULTS AND DISCUSSION Mass Spectrometric Study. The mass spectrum, shown in the Figure 3, was taken under the condition that the LaB,/SiOz bead emitter in the E1 source was maintained around 1300 K and the acetonitrile was admitted into the mass spectrometer from a reservior via a variable leak to be 5 X IO-5 torr as a partial pressure. The spectrum is dominated by the peak a t m / e 26 which is presumably assigned as the CN- ion. Also seen are smaller quantities of HCN-(?) while OH- and much smaller quantities of C1-, 0-, and NCO- are the background ion species. The introduction of trimethyl phosphite as a test sample for phosphorus compounds gives rise to the formation of the abundant PO- as well as PO, and PO3-. These results provide circumstantial identification of certain negative ions that have previously been conjectured to prevail in the TID environment. However, the mass spectrum results do not necessarily confirm that these are the only or the most important negative ion species that are created in the TID, because of the difference in chemical environment between the mass spectrometer and the TID. In the vacuum environment of the mass spectrometer, sample molecules interact directly with the hot bead surface, and ions formed are the result of thermal pyrolysis process. In the TID, the hot bead is immersed in an atmospheric pressure “flame plasma” which contains substantial concentrations of very chemically active species such as H atoms, 0 atoms, and OH radicals, as well as substantial amounts of water vapor. This is a thermal and combustion environment as opposed to the thermal pyrolysis prevailing in the mass spectrometer. For comparison, the similar measurement was made by using the commercial RbzS04/SiOzbead emitter which is the
492
ANALYTICAL CHEMISTRY, VOL. 57, NO. 2, FEBRUARY 1985
Table I. Comparison of Performance of LaB,/SiOz Bead with That of RbzSOd/CerarnicBead" compound
sensitivity,*C/g of X LaB6 Rb2SOd
selectivity,cg of C / g of X La& RbZS04
azobenzene malathion
0.17 0.8
5.2 x 104 2.4 x 105
0.2 0.4
7 x 104 105
detection limit, g of X/s
LaB, 1.2 x 10-14 2.6 x 10-14
Rb2SO4 5 x
10-14
2.5 x 10-14
"From ref 11. * X corresponds to the N atom for azobenzene and to the P atom for malathion. 'C stands for carbon atom.
-14/
10
7.4
,
,
,
7.5
7.6
7.7
,j 7.8
Bead Heating C u r r e n t ( A )
Figure 4. Bead heating current dependence of azobenzene response
(soli line), noise current (dotted line),and background current (dashed line)at a hydrogen flow rate of 5.5 mL/min: azobenzene sample size, 0.43 ng.
replacement for the LaBs/SiOz bead emitter. The observed species are exactly the same as those of LaBs/SiOz, and their relative and absolute intensities are practically unchanged, indicating that both bead surfaces function essentially in the same manner; they operate as low work function surfaces for the efficient occurrence of the negative surface ionization. Response Characteristics. Since the preliminary performance test showed a promising result, the detailed response characteristics were investigated in terms of the bead surface temperature, the composition of the gaseous environment, and bead composition, and performance was then accessed using the criteria of sensitivity, selectivity, minimum detectable amount, and dynamic range. Surface Temperature Effect. Figure 4 illustrates the response effect caused by varying the bead surface temperature. Bead temperature is varied by changing the bead heating current. This figure shows that both the signal and the background currents increase with the heating current. Patterson reported similar data on the temperature behavior for the alkali metal-ceramic bead. This result may be partially explained by the fact that from the Saha-Langmiiir equation, thermionic emission processes depend on the surface temperature. Besides, temperature rise alters the combustive conditions around the beads and hence the response. At the higher bead current a slight decrease in the ratio of sample peaks to the detector noise level occurs. This result indicates that an optimum bead current has to be found for the detection limit. Generally favorable operation is achieved with bead currents a t 7.63 A that generate a noise level of 1 X A in the present case. Under the optimum conditions, the minimum detectable amount a t the signal-to-noise ratio of 2 was measured using azobenzene and malathion. The results are listed in the second extreme-right column of Table
I. Effect of Gaseous Atmosphere. Kolb (7) showed that the sensitivity toward nitrogen compounds increased with the decreasing hydrogen flow when the condition of constant background current was maintained. The constant back-
ground current condition could be done by adjusting the electric heating. This was explained by the experimental result that with reduced hydrogen flow, the yield of cyan radicals is increased, demonstrating the function of the flame as a CN generator. In spite that his proposed mechanism of chemiionization in the gas phase is quite different from ours, his description of the flame as the CN generator may hold for our detector since the response of the La& bead against hydrogen flow is found to be very similar to that of Kolb's bead. Bead Composition. As noted by Patterson (4),it is hardly possible to choose exactly the optimal bead composition which gives the best response a t the desired operational conditions. With extremely weak response a t no LaB, content, the bead richer in La& provided the greater response. However, one of the problems encountered with the increase in LaB is the fact that the handling of the bead formation becomes more difficult as the La& concentration increases. The bead with LaB,/Si@ = on a weight basis is used for all the present studies. Performance Characteristics. Linearity. The measurement of the linear range has been made with azobenzene and malathion as a test sample, showing that the straight portion of the working curve covers nearly 5 orders of magnitude for the nitrogen-containing compound whereas it is least 4 orders of magnitude for the phosphorus compound with the uncertainty of f1.7%. These results were obtained for a hydrogen flow of 5.5 mL/min and a bead bias voltage of -200 V when the bead heating current was such that a background current of 4 X lo-" A was given. Sensitivity and Selectivity. The sensitivity (S) of the detector can be expressed as Coulombs per gram of nitrogen. As already mentioned, the sensitivity of the detector can be varied, depending on the operating conditions such as bead temperature, gaseous environment, and so on. The sensitivity increases with the higher noise level, and, therefore, an optimum has to be found for the detection limit. Under the optimum conditions for the detection limit, sensitivity, S, was 0.17 C/g for nitrogen as represented by azobenzene, 0.8 C/g for phosphorus by tri-n-butyl phosphate and 3.3 X lo4 C/g for carbon by n-heneicosane. Consequently selectivity of this detector which is defined as the ratio of sensitivity was as follows:
sN/sc = 5.4 x 104 g of c l g of N Comparison of the La& Bead with the Conventional Alkali Metal Bead. Table I gives a comparison of performance characteristic of the La& bead with the alkali metal bead in terms of sensitivity, selectivity, and detection limit for nitrogen in azobenzene and phosphorus in malathion. All the values of the alkali metal bead are cited from Patterson's literature (11). This result demonstrates that the LaB6/silica source provides high sensitivity t o both nitrogen and phosphorus as the well-established alkali metal source. In conclusion, the excellent performance specifications for nitrogen and phosphorus compounds obtained with a TID and a LaB,/SiOz bead are consistent with a process of negative surface ionization. This work demonstrates that a low work function bead composition for the TID does not necessarily
Anal. Chem. 1985, 57,493-495
require the use of an alkali metal compound. Also mass spectrometric studies demonstrate that the negative ion species CN-, PO-, POz-, and PO3- are prominently formed when nitrogen and phosphorus compounds impact the surface of the LaB,/SiO, bead in the vacuum environment of a mass spectrometer ion source. These mass spectrometric data are consistent with prior speculations that these specific ion species are also formed in the TID environment.
493
(3) Lubkowitz, J. A.; Semonian, B. P.; Galobardes, J.; Rogers, L. B. Anal. Chem. 1978, 50,672. (4) Patterson, P. L.; Howe, R. L. J . Chromatogr. Sci. 1978, 16. 275. (5) Karmen, A.: Guiffrida, L. E. Nature (London) 1964, 201, 1204. (6) Farwell, S. 0.; Gage, D. R.; Kagel, R. A. d . Chromafogr. 1981, 19, 358. (7) Kolb, B.; Auer, M.; Pospisil, P. J . Chromafogr. Sci. 1977, 15, 53. (8) Franklin, J. L.; Harland. P. W. Annu. Rev. Phys. Chem. 1974, 25, 485. (9) Reader, J.; Corliss. C. H. I n “CRC Handbook of Chemistry and Physics”; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1979/1980; p E-303. IO) Brazhnikov, V. V.: Gurev. M. V.; Sakodynsky, K. I. Chromatogr. Rev. 1980, 72, 1. 11) Patterson, P. L. J . Chromatogr. 1976, 167,381. 12) Olah, K.; Szoke, A.; Vajta, Zs. J . Chromatogr. Sci. 1979, 17,497. 13) Moon, P. B. Proc. Cambridge Philos. Soc. 1932, 28,490. 14) Lazarev, V. B.; Malov, Y. I. Fiz. Met. Metalloved. 1967, 24. 565. 15) Lafferty, J. M. J . Appl. Phys. 1951, 22,299. 16) Fujii, T. I n t . J . Mass Spectrom. Ion Processes 1984. 57,63 17) Fujii, T. J . Phys. Chem. 1984, 8 8 , 5228.
ACKNOWLEDGMENT I am grateful t o D. S. Linton a t Yokota Air Base for the manuscript preparation procedure.
Registry No. LaB,, 12008-21-8;S O z , 7631-86-9. LITERATURE CITED (1) Kolb, B.; Bishoff, J. J . Chromafogr. Sci. 1974, 12. 625. (2) Burgett, C. A,; Smith, D. H.;Bente, H. B. J . Chromatogr. 1977, 134, 57.
for review July 12, 1984. Resubmitted September 11, 1984. Accepted October 22, 1984.
RECEIVED
Lipophilic Lithium Ion Carrier in a Lithium Ion Selective Electrode Victor P. Y. Gadzekpo,’ James M. Hungerford, Azza M. Kadry,* Yehia A. Ibrahim,3 and Gary D. Christian*
Department of Chemistry BG-10, University of Washington, Seattle, Washington 98195
A lithium ion selective electrode was constructed with N,N’-diheptyi-5,5-dimethyi-N,N’-bis( 3-oxapentyi)-3,7-dioxanonanediamide as a lithium carrier. Dioctyi azeiaie, tris( 2ethyihexyi) phosphate, and triisononyi trimelitate were used as plasticizers. Selectivity coefficients of 0.063, 0.104, 7.1 X and 2.5 X are reported for sodium, potassium, calcium, and magnesium, respectively, relative to lithium ion. A new and simpler procedure is reported for the synthesis of the lithium carrier.
Several acyclic polyethers (5),macrocyclic crown ethers (6, 7), and acyclic dioxadiamides (8-10) have been investigated as carriers in lithium ion selective electrodes. Selectivities reported for most of these neutral carriers in ion-selective electrodes for lithium over other cations, however, are not high enough to enable them to be used in most lithium determinations.
EXPERIMENTAL SECTION Synthesis of Lithium Carrier [N,N’-Diethyl-5,5-dimethyl-N~-bis(3-oxapentyl)-3,7-dioxanonanediamine] ( 1). The synthesis (Figure 2) of compound 1, described recently by Shanzer et al. ( 4 ) ,is based on the use of the cyclic stannoxane (3),prepared by reacting 2,2-dimethyl-1,3-propanediol (2) with dibutyltin diethoxide. Subsequent reaction of 3 with N heptylbromoacetamide was described to give N,N’-diheptyl-5,5dimethyl-3,7-dioxanonanediamide(4) which was then alkylated with 2-bromoethyl ethyl ether to yield 1 ( 4 ) . Attempts to repeat the above synthesis of the intermediate 4 showed the complexity of the reaction product, which needed a tedious chromatographic separation (11). We have investigated two alternative and simpler routes toward the synthesis of the tetraoxadiamide 1. Thus, treatment of the diol 2 with ethyl diazoacetate gave the ester 5 (12). Reaction of the ester 5 with n-heptylamine cleanly yielded the amide (4)in an almost quantitative yield. Amide 4 was then converted into the desired ionophore 1 following the same procedure described by Shanzer et al. ( 4 ) . More direct and efficient synthesis of 1 has been achieved by reacting the acid chloride 7 with N-(2-oxapentyl)heptylamine(10). The acid chloride (7) was obtained from ester 5 by hydrolysis to the corresponding acid (6) followed by reaction with thionyl chloride. N-(3-Oxapentyl)heptylamine(10) used in this investigation was readily obtained by lithium aluminum hydride reduction of N-heptylethoxyacetamide (9). The latter compound was synthesized by the condensation of ethyl ethoxyacetate (8) with n-heptylamine. a. N,N’-Diheptyl-5,5-dimethyl-3,7-dioxanonanediamide (4). A mixture of ester 5 (5.52 g, 0.02 mol) and n-heptylamine
Lithium carriers are very important in lithium transport through the blood brain barrier and across other membranes (1-3). Shanzer et al. ( 4 ) reported the transport properties of N,Nr-diheptyl-5,5-dimethyl-N,N’-bis( 3-oxapentyl)-3,7-dioxanonanediamide (Figure 1). This tetroxadiamide is reported to be an efficient carrier for lithium ion. It is said to wrap around the lithium ion in an octahedral arrangement using six binding sites and is reported to carry lithium ions as efficiently as valinomycin carries potassium ions, possibly for structural reasons. The carrier selectivity of the tetroxadiamide for lithium ion estimated by equilibrium partition experiments performed in a water/methylene chloride system with lithium chloride and sodium chloride gave values of more than 40 ( 4 ) . The reported efficient transporting capability of the tetroxadiamide has led us to investigate its characteristic behavior as a carrier in a lithium ion selective electrode. Present address: Department of Applied Chemistry, The University of Wales Science and Technology, P.O. Box 13, Cardiff CF1 3x1, United Kingdom. *On leave from Department of Chemistry, Faculty of Pharmacy, Za azig University, Zagazig, Egypt. !Fulbright Scholar, on leave from Department of Chemistry, Faculty of Science, Cairo University, Giza, Egypt.
e
0003-2700/85/0357-0493$01.50/0 1985 American Chemical Society
