K , = C31,d2
0.2 0
+
C4
(6 1
Kf and ECtare then empirically related by Equations 5 and 6. Usefulness of the Method. We have shown that GLC can be used to determine vertical ionization potentials where no anomalous (e.g., steric hindrance) effects occur. The values obtained are accurate to fO.l eV. This was illustrated in Table 111, where known ionization potentials were compared to the average of calculated values determined from the respective formation constants and equations in Figure l. I t should also be possible to determine electron affinities by the same method. Of potentially greater usefulness, however, is the unambiguous determination of formation constants, steric effects, and the partial deconjugation of systems. For example, we are now examining sterically hindered systems in an effort to quantify (at least on a relative basis) out-of-plane deformation angles.
0.15
5-
0.10
Y
CONCLUSIONS
0.05
0.00
7
Figure 1. Plot of Kfvs. .Id. The approximate equation constants are:
+
45 ‘ C : 4 = -9.075 X Ld2 0.750 50 ‘ C : 6 = -9.237 X loT3 kd2-k 0.750 55 ‘ C : & = -9.445 X hd2 0.750
+
constants probably should not be expected. I t is interesting to note that the energy of charge transfer, ECt,has been shown (22, 23) to fit the following empirical equation:
where C, is an empirical constant. In this research,
A final point to be made regards the nature of “charge transfer interactions.” These interactions may involve actual complex formation, loosely-bound contact pairing, or complexation of some intermediate strength (23).A further complication arises, since most of the available literature data has been obtained spectroscopically, yet Purnell has recently demonstrated that such methods yield questionable results (17). These and other disagreements lead the authors to speculate that what is presently termed “charge transfer” is not well understood. Such interactions may furthermore only be a reflection of solution phenomena which have yet to be elucidated. ACKNOWLEDGMENT
The authors acknowledge A. F. Isbell, Jr., R. S. H. Liu, and V. Ramamurthy for many helpful discussions and most of the dienes. We also very gratefully thank R. L. H. Williams for his Job-like patience, and glass-blowing skills. Received for review October 18, 1973. Accepted April 18, 1974.
Gas Chromatographic Detection and Confirmation of Volatile Boron Hydrides at Trace Levels E. J. Sowinski Western Electric Company, Allentown, Pa. 18103
I. H. Suffet’ Department of Chemistry and Environmental Engmeering and Science Program, Drexel Unlverslty. Philadelphia. Pa. 19104
A characterization of gas chromatographic detectors is presented in a plan which provides for airborne identification of specific boron hydrides of significance as environmental contaminants. Lower limits of detectability for electron capture, microcoulometry, and flame photomet-
To whom reprint requests should be sent. 1218
ric detection are described at ppb levels which are in the range of industrial air standards (Threshold Limit Values). Interferences of significance to environmental analysis are described and a GC-MS interface for confirming boron hydrides is presented. The advantages of combining GC detectors in environmental analysis for boron hydrides are discussed.
A N A L Y T I C A L CHEMISTRY, V O L . 46, NO. 9. AUGUST 1974
The "real time" analysis of specific volatile boron hydrides (B2-BI0) in reaction mixtures a t trace levels has always been a difficult analytical problem. Although methods of analysis for boron and its compounds are available, they lack selectivity and sensitivity for determining trace airborne concentrations related to air pollution and for determinations near the industrial air standards Threshold Limit Values (TLV's) ( I ) . Total boron analysis has been primarily used. Where some specific analyses have been performed, sensitivities were reported in the part per million (ppm) range, whereas health related levels are in the part per billion (ppb) range (2-4). The boron hydrides or boranes consist of a group of boron compounds of significance to air pollution and industrial hygiene for reasons involving: high toxicity, unknown environmental fate, and potential widespread use. The compounds of boron have been cited as one class of fourteen air pollutants which require increased control (1). The boron hydrides are the most toxic of the compounds of boron. The most common boron hydrides encountered industrially are diborane (BPHE-gas), pentaborane (BbHg-liquid) and decaborane (B10H14-solid). The relative health hazard associated with these compounds are reflected by their TLV's: B2H6, 100 ppb; B5H9, 5 ppb; and BlOHl4,50 ppb ( 5 ) . The boron hydrides have many industrial applications where analytical determinations in the environment are significant. Among these are uses as catalysts in synthetic reactions, as parent compounds in the synthesis of carborane polymers, and as dopant sources in the manufacture of silicon-based semiconductors. Diborane is the most common boron hydride used industrially. Notable in the chemistry of diborane is its tendency to form higher borane by-products when subjected to elevated temperature (6). Since diborane is commonly subjected to elevated temperatures in its industrial applications, concern exists for the identification and airborne fate of the more stable hydrides which may form as by-products. The use of programmed temperature gas chromatography, Kovats retention indices and flame photometric detection (FPD), previously reported by the authors, represent techniques of significant help in the separation and identification of volatile boron hydride mixtures (7, 8). However, improved analytical schemes which combine compound specificity, low limits of detectability (nanogram range) and which can confirm trace components of volatile boron hydride mixtures occurring as air pollutants or in reaction mixtures are needed. These requirements are especially important for environmental analysis as demonstrated in the analysis of pesticides and polychlorinated biphenyls (PCB's) (9, 10). Molecular characteristic methods are particularly necessary for confirmation of U.S. Department of Health, Education, and Welfare; Public Health Service: National Air Pollution Control Administration, "Preliminary Air Pollution Survey of Boron and Its Compounds," Raleigh, N.C.. October 1969. R. M . Adams, "Boron, Metallo-Boron Compounds and Boranes." Interscience Publishers, New York. N.Y., 1964. E. L. Muetterties, "The Chemistry of Boron and Its Compounds." John Wiley, New York, N.Y., 1967. R . L. Hughes, I . C. Smith, and E. W. Lawless, "Production of the Boranes and Related Research," Academic Press, New York, N.Y , 1967. American Conference of Governmental Industrial Hygienists, "Threshold Limit Values of Airborne Contaminants," Cincinnati, Ohio, 1973. A. Stock, "Hydrides of Boron and Silicon," Cornell University Press, Ithaca. N.Y.. 1933. E. J . Sowinski and I . H Suffet, J. Chromalogr. Sci., 9, 632 (1971) E. J. Sowinski and I . H. Suffet, Anal. Chem.. 4 4 , 2237 (1972) H. M . Gomaa. I . H. Suffet, and S. D. Faust, Residue Rev., 29, 171 (1969). C. G Gustafson. Environ. Sci. Techno/.. 10, 814 (1970)
identity especially a t trace levels (11). Although several dozen different detectors have been used in conjunction with gas chromatography, the requirements of low detectability limits and specificity have directed consideration to the combined use of electron capture, microcoulometry, flame photometry, and mass spectrometry as gas chromatographic detectors for confirming trace components of volatile boron hydride mixtures. Electron capture detection (EC) has been reviewed (12). The electron deficient character of boron hydrides suggests a high degree of sensitivity for detection with electron capture. The microcoulometric (MC) technique has been applied to materials which react with iodine (triiodide) (13). Boron hydrides are quantitatively oxidized by iodine, and analytical procedures have been reported based on this reaction (14): BIoH,,
+ 20 1 2 + 30 H?O
10 B(OH)j
+ 40 HI + 2 H?
No reports have been made of gas chromatography combined with electron capture or microcoulometry for detection of specific boron hydrides in mixtures as they may occur in industrial air environments. Mass spectrometry (MS) offers the greatest sensitivity (micro-nanogram range) for confirming trace boron hydrides in mixtures. Mass spectrometry has been used for the confirmatory identification of intermediates and products generated in the low pressure pyrolysis of diborane. Appearance potentials and molecular beam techniques have been generally employed for identifying boron hydrides generated in pyrolysis mechanism studies (15, 16). GC-MS interfacing methods have been evaluated primarily for organic materials, and applications involving GC-MS for confirming boron hydrides have not been reported. This paper, reports the response characteristics, including lower limits of detectability, linear response ranges, and the effect of some interferences for electron capture, microcoulometry, and flame photometric detection. A GC-MS interface for detecting boron hydrides as eluted from a gas chromatographic column is also described. The advantages of combining EC, MC, and FPD for confirming unknown boron hydrides are discussed. The development of a system which is capable of chromatographic separation of boron hydrides in the range B2-Blo with temperature programming, interfacing three chromatographic detectors, and GC-MS is presented. EXPERIMENTAL Apparatus. T h e gas chromatographic unit was a Tracor M o d e l MT 220 equipped w i t h a temperature programmer, a B e c k m a n 10i n c h recorder u t i l i z i n g a 1-mV range a n d m u l t i p l e detectors.
Electron Capture Detector. T h e electron capture detector used was a 10-mC Tracor 63Nidetector m a i n t a i n e d a t 280 "C. T h e detector was operated o n a d c voltage of 20 volts for best reproducib l e q u a n t i t a t i v e response w i t h t h e b o r o n hydrides. A m a x i m u m response was achieved a t 24 volts. T h e use of t h e 'j3Nisource was desirable since i t i s less susceptible t o deterioration resulting f r o m c o n t a m i n a t i o n t h a n tritium sources w h i c h are temperature limite d t o 220 "C. Microcoulometer. A D o h r m a n microcoulometer w i t h a n iodine (11) J W. Ralls, paper presented at the Western Experiment Station Collaboration Conference, WURDD, USDA. Albany, Calif.. 1962, and quoted by R Teranishi. P Issenberg, I . Hornstein, and E. L Wick "Flavor Research," Marcel Dekker, New York, N.Y., 1971, pp 79-80. (12) D. A. Leathard and E. C. Shurlock. "Identification Techniques in Gas Chromatography," Interscience. New York, N.Y., 1970. (13) J. A. Challacombe a nd J. A. McNulty, Residue Rev., 5 , 57 (1964). (14) A. E. Messner, Ana/. Chem., 30, 547 (1958). (15) A E. Baylis, G. A. Pressley. E. J. Sinke. and F. E Stafford. J. Amer. Chem. Soc., 86, 5358 (1964) (16) A B. Baylis. G. A. Pressley, and F. E Stafford, J. Amer. Chem. SOC.. 88, 2428 (1966).
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 9, A U G U S T 1974
1219
titration cell was used for microcoulometric analysis of the boron hydrides. A Hamilton long needle micro syringe was used for direct injections of decaborane in cyclohexane into the titration cell. For analysis of chromatographic effluent, a Whitey toggle valve OGS 2-A was used to divert carrier gas effluent from the FPD to the microcoulometer titration cell. Flame Photometric Detector. A Melpar FPD with a Baird Atomic selective interference filter of 70% transmission, 10-nm half-band width at 546 nm, as previously described by the authors, was used for the detection of boron (7). In addition, another 546-nm Baird Atomic interference filter of 60% transmission with a 3-nm half-band was compared to the 10-nm half-band filter. Mass Spectrometer. The mass spectrometer used in this study was a Varian MAT CH-790 magnetic focusing instrument operated at an ionizing potential of 70 eV, an ionizing current of 300 PA, and an average instrument pressure of Torr. A single stage Llewelyn silicone membrane separator was used as the GC-MS interface. Molecular leaks using stainless steel disks were also tested. Helium and nitrogen were evaluated as carrier gases. A Whitey toggle valve OGSA-2 was used to divert column effluent to the mass spectrometer. By opening this valve, the column effluent was split two ways. Approximately 90% was diverted to the mass spectrometer and the remaining 10% went to the FPD. This provided a simultaneous readout of column effluent on FPD and the mass spectrometer. An Yg-in. 0.d. stainless steel transfer line was used to transport column effluent to the mass spectrometer. This line was heated at 100 "C with resistance heating tape. The time required for an effluent peak to reach the mass spectrometer was approximately 4 seconds. This represented the time between an FPD response and a response on the mass spectrometer total ion monitor. Mass spectrometer scans were begun when ion current registered on a total ion monitor. Mass spectra were obtained on a nominally linear scan mode. Chemicals. The commercially available boron hydrides utilized were diborane (BzHs-gas), pentaborane (BSHg-liquid), and decaborane (B1OH1d-solid). Diborane was supplied as a pressurized gas (1000 ppm in nitrogen) by Air Products, Inc. This concentration of diborane was assayed by the supplier with a wet chemical technique and by mass spectrometry ( I T ) . The analytical accuracy was reported to be &2%. The stability of diborane in this concentration range was also reported by the supplier to be goode.g., less than 5% concentration loss up to one year. Pentaborane was supplied in a liquid pyrophoric dispenser cylinder by Matheson Gas Products. Decaborane was supplied by Strem Chemicals of Danvers, Mass. Hexaborane (BsHlo) was made by pyrolysis of diborane and confirmed by mass spectrometry (18). Standard hydrocarbon mixtures for the determination of interferences were supplied by Supelco, Inc. Cyclohexane used for preparing solutions of decaborane, was Matheson pesticide quality solvent. Silane (SiH4), which was evaluated as an interference with the flame photometric detection of boron, was supplied by Matheson Gas Products. Procedure. The detectors were used interchangeably through use of a four-port gas sampling valve which was placed between the gas handling system and the GC column oven. It was mounted inside a Carle valve oven Model 4300. The four-port valve directed samples from the gas handling system to either one of two GC columns which were attached to the FPD and electron capture detectors. Since electron capture detection of temperature programmed GC column effluent is undesirable because of column bleed and resultant detector contamination, the column leading to the electron capture detector was disconnected and capped off with a Swagelok fitting for temperature programming with the FPD. A Swagelok cap was placed on the FPD burner jet vent and the Whitey Toggle valve was opened to direct column effluent through a short transfer line to the microcoulometer cell. All boron hydride samples were injected into the chromatograph with a gas handling system. Diborane (1000 ppm in nitrogen) was metered directly through a gas sampling valve at 5 psig and 30 cm3/min flow rate. Pentaborane was transferred from the pyrophoric dispenser cylinder with nitrogen at 5 psig and 30 cm3/min flow rate. (Note: Extreme caution must be employed when handling pentaborane as it is spontaneously explosive when exposed to air). A Nupro check valve with a l/3 psi actuation pressure was placed directly adjacent to the carrier gas inlet of Yoder, Air Products a n d Chemicals, Inc., Emmaus, Pa., private communication,July 1971 E. J. Sowinski and I . H . Suffet. unpublished data, 1972.
(17) G. (18)
1220
the pyrophoric dispenser cylinder to prevent pentaborane backflow. Decaborane (approximately 50 mg) was placed inside a quartz tube and nitrogen at 5 psig and 30 cm3/min was used to carry decaborane vapors to the gas sampling valve.
RESULTS Electron Capture Detection. The response range for electron capture detection of decaborane is linear in terms of peak height in the range of 0.1 ng B10H14 to approximately 3.0 ng, Figure 1. The response in terms of peak height does not vary sufficiently for use as a calibration curve in the range greater than 3.0 ng. However, in the range greater than 3.0 ng B10H14, the response was found to vary in terms of peak area, Figure 2. Figure 3 shows a comparative response for diborane and decaborane in terms of peak height. Diborane responds over a greater linear range, in terms of peak height, than decaborane. The lower limit of detectability for the commercially available boron hydrides by electron capture detection is in the 0.10-ng range. Table I shows a comparison of the values obtained for detectability limits with electron capture and the FPD. Lower limits of detectability were determined by the procedure of Skogerboe, Heybey, and Morrison (19). This approach has been detailed by the authors ( 7 ) . Figure 4 shows a direct comparison of responses for electron capture and flame photometric detection of diborane made under the same experimental conditions. For the particular sample of diborane depicted, two small additional peaks are seen for the electron capture response, while only the primary diborane peak is seen for the flame photometric response. The additional responses observed in the electron capture chromatogram are possibly due to organic impurities in the diborane. The presence of dimethylether as a n impurity from the synthesis of diborane is one possibility ( 2 ) . Microcoulometric Detector. A typical microcoulometric response for &OH14 is shown in Figure 5 . This response was obtained with a direct injection of decaborane as a cyclohexane solution into the microcoulometer titration cell. No significant difference in response character was observed for B10H14 which was eluted from the GC into the microcoulometer titration cell. For both modes of sample entry, the microcoulometric response was dependent on gain settings. Peak shapes improved with increasing gain; however, this was accompanied by a corresponding base-line deterioration. A response range for the microcoulometric detection of B10H14 is shown in Figure 6. The response is linear in the range of 1.0 ng to at least 30 ng B10H14 as plotted in terms of peak area. The lower limit of detectability for B10H14 with microcoulometry was determined to be 1.2 ng. The lower limits of detectability for B2H6 and B5H9 are also in the nanogram range, Table I. Interferences. Interferences which may be encountered in the analysis of industrial boron hydride air contaminants with the chromatographic detectors are reported in terms of a specificity factor which is the ratio of interference detectability to boron hydride detectability. &OH14 was used as the reference for determining specificity factors. Programmed temperature GC conditions as previously reported by the authors were used (8). Silicon as silane is of interest as a n interference in the determination of boron hydrides since it is commonly present in industrial air samples containing boron hydrides from use in the semiconductor industry. Silicon exhibits primary flame emission in the 200 n m region (20). (19) R. K Skogerboe. A . T. Heybey. and G . H . Morrison, Anal. Chem., 38. 1821 (1966). (20) R . C P a r k h u r s t . Proc. Phys. SOC., 52, 707 (1940).
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 9. AUGUST 1974
b
ilOn,,
~AYOGIIMS
1,
I
Figure 1. Electron Capture calibration c u r v e for
0
in t e r m s
B10H14
L
I
1
1
1
I 0
TIME (minut.')
,
I
1
2
TIME ( m i n u l w )
of p e a k height
Figure 4. Isothermal separation o f B2H6 on with Electron Capture and FPD
3% OV-1 at 40 "C
(a) BzH6detected with Electron Capture (3.2 X A full scale) (6)BZH6 detected with FPD (6.4 X l O - ' A full scale) 0.
0.
RESPONSE (mvl 0.
I
1
4
8
1 I1 NANOGRAMS
I
1
I
1
16
zo
24
18
0
nIOn14
Figure 2. Electron Capture calibration c u r v e for of p e a k a r e a
B30H14
in t e r m s ,
,
,
#
I
,
0
1
2
3
4
5
TIME ( m i n u t e s )
Figure 5. M i c r o c o u l o m e t r i c detection of BlOHl4 (50 ng) injected as a c y c l o h e x a n e solution directly into the titration cell ( 1 - n V full scale response)
/ NANOGRAMS B
~
+H
16
8 1~0 n 1 ,
1 / , , , , , , , , , , , , ,
Figure 3. Electron Capture calibration c u r v e s for B2Hs and B l o H l l in t e r m s of peak height
2
4
b
8
10
12
14
NANOGRAMS
16
10
18
12
24
26
28
0
BloHl4
Figure 6. M i c r o c o u l o m e t r i c calibration c u r v e for B1oHl4
Table I. Experimentally Determined Lower Limits of Detectability for Boron Hydrides i n Nanograms"
Table 11. Experimentally Determined GC Detector Interference Specificity Factors'
Lower limit of detectability, ng Gas chromatographic detector
B2Hs
BrH9
BlOHl4
FPD Electron Capture Microcoulometry
1 .oo 0.10 1 .oo
1 .o 0.1 1 .o
0.71 0.10 1.20
Procedure of Skogerboe, Heybey, and Morrison (19).
FPD
SiH, Hvdrocarbons
(c6-c8)
Trichloroethylene a
900 5.000
1,500
EC
1 100
0.1
MC
10 10,000
7,500
Ratio of concentrations of interference to B G M .
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 9, AUGUST 1974
1221
However, silane has been found to interfere in the FPD detection of B& and B10H14 a t the boron line of 546 nm in the concentration range of approximately 900 to 1, Table 11. The use of the 3-nm half bandwidth filter a t 546 nm reduced the silane interference proportionately to a reduction in sensitivity observed for boron; e.g., by a factor of three. In addition to silane, a mixture of hydrocarbons in the range c6-cS and trichloroethylene were evaluated as potential interferences since they are solvents commonly encountered as contaminants in industrial air environments. The retention times of trichloroethylene and hydrocarbons c6-cS (3, 1, 4, and 5 minutes, respectively), on a 6% OV-17 column under programmed temperature conditions did not interfere directly with any known boron hydride. However, they may interfere with intermediate boron hydride species not directly identified from relative retention times and Kovats indices. Hydrocarbons (c6-cS)and trichloroethylene interfered to a lesser extent than silane in the flame photometric detection of B10H14. Specificity factors for these were 5000 and 1500, respectively (Table 11). Specificity factors for other materials, as related to pentaborane, have been reported for a 550-nm filter with a 50% transmittance and a half bandwidth of approximately 25 nm (21). These include nitrogen dioxide, ammonia, benzene, carbon tetrachloride, and methanol, all of which have specificity factors in the range 1000 to 8000. Silane and trichloroethylene were detectable in the same range as B10H14 with the electron capture detector. The hydrocarbons (c6-Cs) provide a lesser interference with a specificity factor of approximately 100 (Table 11). A measure of relative sensitivities of some 27 compounds with electron capture detection have been reported (22). Compounds of industrial significance in this group have the following relative sensitivities: chlorobenzene 2.6, dichloroethylene 20,000, tetraethyl lead 30,000, DDT 2,000,000, fluoroethane 16,000,000, and carbon tetrachloride 400,000,000. Silane interfered in the microcoulometric analysis of &OH14 with a specificity factor of approximately 10 (Table II). However, hydrocarbons and trichloroethylene provided a minimum interference with specificity factors of approximately 10,000 for each. In general, strong oxidants can interfere in the detection of boron hydrides with the iodine microcoulometric reaction by releasing 13- from I- solutions. One particular material of concern is SO2. The removal of strong oxidants by addition of NaK3 to the cell electrolyte has been reported. (23). Acetone and peroxides have been reported to interfere in the coulometric system while methanol, methyl borate, boric acid, and benzene did not interfere when injected into the titration cell ( 2 4 ) . Ozone and nitrogen oxides have a reverse effect on the microcoulometer since they can oxidize the iodide-containing electrolyte to iodine. G a s Chromatography-Mass Spectroscopy. Several pertinent factors were established in the process of developing an appropriate GC-MS interface for confirming the identity of boron hydrides. 1) A single stage Llewyllen silicone membrane was sufficient to maintain an adequate mass spectrometer vacuum of approximately Torr. 2) Sample size for adequate boron hydride membrane permeability is critical. For example, a 10-cm3 sample (21) R . S. Brarnan and E. S. Gordon, Proc. Ann. Inst. Autom. Conf. Exhibit. 17, 13 (1962) (22) J . L. Radornski and A. Rey.J. Chromatogr. Sci., 8, 108 (1970). (23) "Encyclopedia of Industrial Chemical Analysis." Vol. 7, John Wiley and Sons, New York, N Y . . 1968. (24) R . S . Braman, D. D. DeFord. T . N. Johnston, and R . Kuhns, Anal. Chem.. 32, 1258 (1960)
1222
Table 111. Threshold L i m i t Values (TLV) and Experimentally Determined Detectability L i m i t s (LLD) for Boron Hydrides LLD, ppm" TLV bpm) ( 5 )
B2Ha BEHs BBHl0b BioHia
0.1 0.005
Not determined 0.05
FPD
EC
MC
0.1
0.01
0.05 = O .05 0.025
0.005 < O .05 0.0025
0.1 0.05 = O .05 0.025
*
a LLD's related to a total gas sample volume of 10 cma. Values are approximate as estimated from correlation with BsHo.
loop was required for sampling diborane pyrolysis products originating from a 1000 ppm B2H6/N2 gas mixture. The amount of boron hydride entering the mass spectrometer was in the nanogram range. 3) The use of helium us. nitrogen as the chromatographic carrier gas had little apparent effect on membrane efficiency in separating sample from carrier gas while maintaining low vacuum in the spectrometer ion source. 4) A jet-type separator made from two stainless steel disks with 10-micron diameter holes was evaluated as a means of direct GC-MS interfacing. Two of these disks in series did not allow sufficient sample entry into the spectrometer ion source and one disk was not sufficient to maintain adequate ion source vacuum. The versatility of the GC-MS system developed has been shown by its ability to analyze mass spectrometrically individual boron hydrides occurring as mixtures from the pyrolysis of B2Hs (28).
DISCUSSION A general consideration of the three GC detectors (FPD, electron capture, and microcoulometry) indicates the FPD to be the best overall detector for analysis of the boron hydrides. This relates to combined advantages of selectivity and sensitivity for analysis of the boron hydrides. The FPD, with the 70% transmittance, 10-nm half bandwidth filter, is the least susceptible of the three detectors to interferences, Table 111. In addition, if sensitivity is not critical and selectivity is, a more narrow bandpass filter with a 2- to 3-nm half bandwidth can be used in place of the 10-nm filter. These factors plus the ability of the FPD to detect boron hydrides a t TLV levels makes it the primary choice as the chromatographic detector for analysis of the boron hydrides. Although the electron capture detector is the most sensitive, it is not amenable to programmed temperature chromatography. Also, calibration curves for the electron capture detector have a smaller linear range as compared to either the FPD or microcoulometer. Additionally, a comparison of the FPD and microcoulometer indicate the FPD is less susceptible to interferences. The FPD and microcoulometer can provide from a simple 10-cm3 gas syringe sample, a direct quantitative determination in the range of the industrial air standards (TLV's) for specific boron hydrides, except BsHg. For example, with the FPD, a direct determination can be made a t the TLV of 0.050 ppm for B10H14. This analysis can be made with a chromatographic injection of 2.5 ng of &OH14 in 10 cm3 of air collected with a gas syringe. Table 111 compares TLV's and lower limits of detectability on an airborne concentration basis for the three detectors. The use of a gas sampling syringe which can collect up to 100 cm3 of gas for chromatographic analysis may provide still greater sensitivity for detection of the boron hydrides. Confirmation of analysis is of prime importance in environmental analysis. The three detectors, used conjunctively, can assist in confirming the identity of boron hy-
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 9, AUGUST 1974
drides which may be encountered in environmental analysis. This is because of the different physiochemical response characteristics of the three detectors. For example, an FPD response a t 546 nm indicates the presence of boron, an electron capture response indicates a n electron deficient compound, and a microcoulometric response using the iodine system indicates the presence of a reducing material. This combined information can indicate the presence of a boron hydride as opposed to some other boron compound type as, for example, a saturated organo boron compound. For tentative identification purposes, the three detectors can be used simultaneously to detect the effluent from a single GC column. This can be done by splitting the column effluent with a toggle valve and diverting sample to the FPD and electron capture detector. Since the electron capture detector is a nondestructive detection system, the sample effluent from this detector can be passed on to the microcoulometer. This approach can provide a readout from one sample on three detectors with a dual split. Quantitative analysis for the boron hydrides requires a verification of standard curves due to potential variability in detector response as a function of slight changes in operating conditions. This can be accomplished by using an external standard ratio. In this method, a known amount
of boron hydride (for example, 1.0 p1 of 10-4M B10H14 in cyclohexane which is equivalent to 12 ng of B10H14) is injected and detected chromatographically before and after an unknown boron hydride sample. This response is then compared with a previously determined standard curve for the same compound and detector, and an adjustment in the standard curve intercept, if necessary, is made corresponding to the ratio in response observed. A confirmatory analysis should be made of any boron hydride which is tentatively identified by relative retention times on different columns, by Kovats retention indices, or which is detected by FPD, electron capture, or microcoulometric detectors. For this purpose, a direct GC-MS interface utilizing a single stage Llewyllen silicone separator can be used. This experimental system is designed to confirm the identity of boron hydrides which can occur as by-products from industrial operations in which diborane is used a t elevated temperatures and a t atmospheric pressure or the reaction products of boron hydrides under environmental conditions as, for example, in the presence of oxygen and water vapor a t atmospheric pressure. Received for review December 31, 1973. Accepted March 21, 1974. This research was made possible by a grant from the Western Electric Company to Drexel University.
Neutron Capture Gamma Ray Spectrometry for Determination of Sulfur in Oil A. Reza Pouraghabagher and A. Edward Profio Department of Chemical and Nuclear Engineering, University of California, Santa Barbara, Calif. 93706
Neutron capture gamma ray spectrometry has been applied successfully for instrumental analysis of the sulfur content of fuel oil in the per cent range. Source-associated background and detection efficiency were studied to select a 7-cm source-detector spacing, using a 0.24-microgram 252Cf source and 7.6-cm by 7.6-cm Nal(TI) scintillation detector, with no gamma ray attenuator. This is probably the weakest neutron source that has been used for capture gamma ray analysis. The signal-to-background ratio was about 0.04 at 1 % sulfur; hence, background had to be measured precisely in a run with a blank (pure fuel oil), and subtracted. Calibration against standard solutions gave 22.9 cpm per wt % sulfur. The lowest concentration measured with this source was 0.48%. The statistical precision of the net count rate was 4 1 % at this concentration, decreasing to 8 % error at 3.53 wt % sulfur, for a 33.33-minute run. A stronger source, e.g., 5 p g 252Cf, will permit shorter data accumulation times or better sensitivity and precision.
Crude petroleum may contain 0.1% to 6% sulfur. depending on origin. Desulfurization of fuel oil and other distillates is often necessary to meet air quality standards or to reduce corrosion of equipment. Blending may be undertaken to provide a range of product concentrations
from about 0.1% on up, according to regulations. Power plants may have to burn low-sulfur fuel on certain days, while favorable meteorological conditions may permit combustion of cheaper and more abundant high-sulfur oil on other days. Thus, a rapid and reliable analyzer for sulfur content of oils is needed a t refineries and possibly a t electric power plants. On-line, instrumental analytical techniques offer practical advantages over standard chemical analyses. X-Ray absorption instruments are in use, but are expensive and suffer from interference from the nickel and vanadium also present in most petroleum ( I ) . Neutron activation analysis ( 2 ) is not well-suited for determination of sulfur (especially with the weak neutron sources available for field use) because most captures in sulfur lead to stable isotopes or a pure beta-ray emitter. The 5-min 37Sgamma ray activity can be counted, but a nuclear reactor is required because of the small abundance and cross section of 36S. However, neutron capture gamma ray spectrometry, in which the gamma rays emitted promptly upon neutron capture are measured, is suitable. The present work shows that sources emitting on the order of lo6 neu(1) 0. I . Milner, "Analysis of Petroleum for Trace Elements," The Mac-
millan Company. New York, N . Y . , 1963. (2) Paul Kruger, "Principles of Activation Analysis." Wiley-lntersoence, New York. N . Y . , 1971
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 9, AUGUST 1974
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