Analysis of Alcohols and Phenols with a Newly Designed Gas

Anal. Chem. 1998, 70, 3488-3492

Analysis of Alcohols and Phenols with a Newly Designed Gas Chromatographic Detector Hiroshi Kishi

Oyama National College of Technology, 771 Nakakuki, Oyama, Tochigi 323, Japan Hiromi Arimoto

Shimadzu Corporation, Nakagyo-ku, Kyoto 604, Japan Toshihiro Fujii*

National Institute for Environmental Studies, 16-2 Onogawa, Tsukuba, Ibaraki 305, Japan

A new design of gas chromatographic detector, based upon hyperthermal negative surface ionization, was used in capillary gas chromatography to sensitively and selectively detect alcohols and phenols. The unique properties of this design allowed detection of these oxygen-containing compounds to which conventional nitrogen-phosphorus detectors are reported to have little response. The sensitivity to 1-pentanol and phenol was 8.5 × 10-1 and 2.6 × 100 C/g, respectively, with a linear range of operation greater than 1 × 105 for both these compounds. The minimum detectable level was in the range of 10-13 g/s. Compared with a flame ionization detector, the new detector was ∼100 times more sensitive to alcohols and phenols, depending on the particular species. The advent of the nitrogen-phosphorus detector (NPD) in combination with gas chromatography (GC) dates back to the early 1960s when Karmen and Giuffrida introduced such a detector.1-3 It functions by ionizing sample compounds at a hot, catalytically active solid surface immersed in a reactive H2/O2 gaseous boundary environment. The conventional NPD has an aluminosilicate glass bead at a negative potential that contains the alkali metal.4,5 Since the principle of this detector is negative surface ionization,6 its application is restricted to the detection of compounds with a high electron affinity or compounds containing fragments with high electron affinities. In general, NPDs have low levels of sensitivity toward compounds in which HCN bonding does not exist or toward most oxygen-containing compounds, such as alcohols and phenols. However, analysis of alcohols and phenols is important because they are widely used as raw materials in various chemical (1) Karmen, A.; Giuffrida, L. Nature 1964, 201, 1204. (2) Karmen, A. Anal. Chem. 1964, 36, 1416. (3) Giuffrida, L. J. AOAC 1964, 47, 293. (4) Kolb, B.; Bischoff, J. J. Chromatogr. Sci. 1974, 12, 625. (5) Patterson, P. L.; Howe, R. L. J. Chromatogr. Sci. 1978, 16, 275. (6) Fujii, T.; Arimoto, H. Anal. Chem. 1985, 57, 490.

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industries and some of them are of environmental concern.7,8 For instance, chlorophenols and nitrophenols constitute a major class of pollutants that extensively contaminate the ecosystem as well as accumulate in the food chain. Current analytical methods for these compounds are based on liquid/liquid extraction followed by gas chromatography with flame ionization detection (FID) or mass spectrometric detection. However, these methods sometimes lead to problems due to low sensitivity, low selectivity, or high cost. Recently, a new design of a gas chromatographic surface ionization detector (SID) based upon hyperthermal positive surface ionization was described.9,10 It was found10 that this new SID, which is sensitive in response to all organics, can be operated as a universal detector with much higher sensitivities toward some species having low ionization energies (IE). However, it is not as selective as a conventional SID. It was also found that the minimum detectable amount (MDA) of pyrene (at S/N ) 3) is ∼4.4 × 10-13 g/s with a linear range of operation greater than 1 × 106, and the MDA of toluene is ∼1 × 10-12 g/s. Other unique properties found were a very short response time and the ability to control the degree of selectivity by changing the molecular kinetic energy, the surface, and its temperature. These studies led to the possibility that sensitive and selective detection of specific compounds eluted from a gas chromatograph, such as phenols, could be explored by using the characteristics of hyperthermal negative surface ionization (HNSI) of organics, instead of hyperthermal positive surface ionization (HPSI). This report describes the application of HNSI to a conventional NPD. The newly designed detector, referred to as a new detector, was operated under experimental conditions where the surface was kept at a low work function state and the sample molecules carried kinetic energy. We report our evaluation of the new detector for alcohol and phenol analysis. (7) Helmi, A.; Luong, J. H. T.; Nguyen, A. Environ. Sci. Technol. 1997, 31, 1794. (8) Puig, D.; Silgoner, I.; Grasserbauer, M.; Barcelo, D. Anal. Chem. 1997, 69, 2756. (9) Danon, A.; Amirav, A. Int. J. Mass Spectrom. Ion Processes 1990, 96, 139. (10) Kishi, H.; Fujii, T. Anal. Chem. 1996, 68, 2776. S0003-2700(98)00138-3 CCC: $15.00

© 1998 American Chemical Society Published on Web 07/03/1998

EXPERIMENTAL SECTION Detector Design. The new detector was constructed, mounted on a Shimadzu model 14B PrFFT gas chromatograph, and interfaced to the column exit by ∼5 cm of 1/16-in. stainless steel column tubing (i.d., 0.8 mm). The system had almost the same configuration as that reported previously for the new SID10 except that the ionizing surface was different. Briefly, the organic molecules from the gas chromatographic coupling, which were seeded in a helium supersonic beam, enter the vacuum chamber through a ceramic nozzle. In the vacuum chamber, the beam collided with a surface chosen for efficient negative ion production. The surface, a BaCO3/CaCO3/SrCO3 coating on a metal plate, was purchased from the Tokyo Cathode Corp. (Tokyo, Japan). It was square-shaped (4 mm × 4 mm), which could be aligned with the nozzle and electrically biased at a negative voltage. The surface temperature was raised to 1280 K. For supersonic jet operation, another helium gas was added to the carrier gas from the flow line coupling. The collector was a simple Faraday cup ion collector positioned at 45° to the surface. The detector’s vacuum chamber was pumped by a single 500 mL/min rotary pump (Alvac 2020T, Alvac, Tokyo, Japan). The vacuum was measured by a pirani gauge (PT-03, Alvac) that also served as a flux detector. The technique of aerodynamic acceleration11 was used in order to obtain molecular kinetic energies in the range of 1-4 eV. The ceramic nozzle had a 50-µm hole and was mounted on a 2-mm alumina tube; it has been described in detail elsewhere.12 The FID and the conventional NPD used for the comparison studies were commercial models (Shimadzu, Kyoto, Japan). Molecular samples for investigation of the performance were introduced through a diffusion cell.13 These test compounds were prepared with 1-pentanol and phenol. For the application study, both aliphatic alcohol and phenol mixtures were used. Chromatographic data were obtained with a glass column of 25 m × 0.32 mm i.d. fused silica capillary (FFS Ulbon HR-1, Shimadzu, Kyoto, Japan). The GC carrier gas was helium whose flow rate was at 1.5 mL/min. The analytical conditions are detailed in the figures. All the reagents were purchased from Nakarai Chemicals (Tsukuba, Japan). RESULTS AND DISCUSSION When compounds with excess kinetic energy, containing electronegative functional groups, strike a heated surface, they are efficiently ionized by the extraction of electrical charge from the surface. This so-called hyperthermal negative surface ionization process9 is controlled by the surface work function, the surface temperature, and the kinetic energy of the sample molecules. Before studying sensitivity optimization, common parameters that could be adjusted to optimize response characteristics were investigated: the shield plate shape and the applied voltage to the surface and shield plate. With a larger shield plate disk (a 10-mm disk with a 3-mm-diameter aperture in the center) than previously described, there was 4.4 times greater detectability (11) Miller, A. P. Free Jet Sources. In Atomic and Molecular Beam Methods; Scoles, G., Ed.; Oxford University Press: New York, 1988; Vol. 1, pp 1453. (12) Kishi, H.; Fujii, T. J. Phys. Chem. 1995, 99, 11153. (13) Altsuller, A. P.; Cohen, I. R. Anal. Chem. 1960, 32, 802.

Figure 1. Effect of kinetic energy (Ek) on the signal responses of 1-pentanol and phenol (left vertical axis). Also shown is the effect of the nozzle temperature (lower horizontal axis) on the background and noise level currents (right vertical axis). The kinetic energy (upper horizontal axis), which is controlled by the nozzle temperature, is calculated and shown for the phenol molecules. The surface temperature was set to 870 K, and the nozzle temperature was increased to 971 K.

response for the sample quantities. As the shield plate was withdrawn, the detectability decreased. The desirable characteristics of high sample response and low background signal were obtained with the surface and the shield plate applied at negative bias voltages against the ion collectorsthe remainder of data described in this paper were obtained using a bias of -4 V for the surface and -200 V for the shield plate. 1. Sensitivity Optimization. Sensitivity optimization was investigated in terms of the surface temperature, and the kinetic energy of the sample molecules, using 1-pentanol and phenol as test samples. In developing the present surface formulation, we investigated the surface coated with BaCO3/SrCO3/CaCO3 (tricarbonate) whose work functions are 2.0 eV.14 The tricarbonate serves as good surface as it is inert and can be used up to temperatures of ∼1280 K. Signal vs Ek. The signal response was studied as a function of incident energy for 1-pentanol and phenol to demonstrate the effect of translational energy. The translational energy was changed9,11 by varying the nozzle temperature (Tn). The results are shown in a semilogarithmic plot (Figure 1). The tricarbonate surface temperature was held constant at 870 K. Clearly, ioniza(14) Kishi, H.; Fujii, T. Chem. Phys. 1995, 192, 387.

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Table 1. Comparison of the New Detector with Conventional Ionization Detectors NPDc

new detector

sensitivitya (C/g of X) ionization efficiency (φ, %) MDAb (g/s of X) linear dynamic range

FIDc

1-pentanol

phenol

1-pentanol

phenol

1-pentanol

phenol

8.5 × 10-1 8 × 10-2 7.1 × 10-13 1 × 105

2.6 × 100 2.5 × 10-1 2.3 × 10-13 2 × 105

1.0 × 10-6 9.8 × 10-8 2.3 × 10-9 2 × 102

5.6 × 10-6 5.1 × 10-7 1.1 × 10-10 2 × 103

3.3 × 10-2 3.2 × 10-3 9.0 × 10-12 2 × 105

3.4 × 10-2 3.1 × 10-3 1.0 × 10-12 2 × 106

a Sensitivity was obtained with the optimum conditions at T ) 900 K and T ) 1130 K. X corresponds to 1-pentanol and phenol as a test sample. n s MDA, minimum detectable amount at S/N ) 3. c The measurements in both the NPD and FID modes were made under optimum conditions for MDA. The optimum conditions were checked with azobenzene and biphenyl, respectively, which give the catalog specifications.

b

Figure 3. Response vs sample size for 1-pentanol and phenol. The linearity levels off at higher sample sizes. This curve is measured under optimum conditions for the detection limit. Figure 2. Effect of surface temperature on 1-pentanol and phenol responses, noise level current, and background current. Tn ) 750 K.

tion due to impact was observed and the SI ions were enhanced exponentially by the increase in kinetic energy (Ek). The sensitivity could be increased by 3 orders of magnitude by increasing Ek to 3.5 eV. In general, the experimental trend was the same as that of many molecules examined in HPSI studies.12,15,16 Surface Temperature (Ts). The most important parameter determining response was the surface temperature, Ts (Figure 2). The signal response, the background current, and the noise level current each increased exponentially with Ts. Patterson and Howe reported similar temperature-related data for the conventional NPD.5 This result may be partially explained by the fact that, from the Saha-Langmuir equation, thermionic emission processes depend on the surface temperature. In addition, increased temperature alters the scattering conditions on the surface and hence the response. 2. Detection Characteristics. Detection characteristics of the new detector, in terms of sensitivity, selectivity, MDA, and (15) Danon, A.; Amirav, A. J. Phys. Chem. 1989, 93, 5549. (16) Kishi, H.; Fujii, T. J. Phys. Chem. B 1997, 101, 3788.

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dynamic range, were compared with those of the conventional NPD and FID. Table 1 gives a comparison of the detection characteristics of the new detector with those of conventional NPD and FID. Sensitivity and MDA. As already mentioned, the sensitivity (S) of the detector, which is defined as coulombs per gram of sample, can be varied depending on the operating conditions, such as surface temperature (Ts) or kinetic energy (Ek). Our results indicated that sensitivity increased with higher Ek and Ts, but at the higher Ek and Ts, there was a slight decrease in the ratio of sample peaks to the detector noise level. Thus, the optimum Ts and Ek have to be found to determine the detection limit. As can be seen from Figures 1 and 2, favorable operation was achieved with Ts at 1130 K and Ek at ∼2.7 eV. In the case of phenol, these conditions generate a noise level of 2 × 10-13 A. From the chromatograms taken under these optimum conditions, the sensitivity for 1-pentanol was determined to be 8.5 × 10-1 C/g, and the corresponding MDA (i.e., 3 times the noise level) was 7.1 × 10-13 g/s. The sensitivity for phenol was 2.6 C/g, and the corresponding MDA was 2.3 × 10-13 g/s. Selectivity. The sensitivity for benzene was measured to be 8.3 × 10-4 C/g. The selectivity of this detector, which is defined as the ratio of sensitivity, was as follows: S1-pentanol/Sbenzene ) 1.0

Figure 4. Comparison between (A) new detector and (B) flame ionization detector (FID). The detected gas chromatograms were of a formulated mixture of 0.1 ng (for the new detector) and 1 ng (for the FID) of each alkyl alcohol in a xylene solution: (1) methanol; (2) ethanol; (3) 2-propanol; (4) 2-methyl-2-propanol; (5) 1-propanol; (6) 2-butanol; (7) 2-methyl-1-propanol; (8) 2-methyl-2-butanol; (9) 1-butanol; (10) 2,2-dimethyl-1-propanol; (11) 3-methyl-2-butanol; (12) 3-pentanol; (13) 2-methyl-1-butanol; (14) 1-pentanol. The 1-µL solution was injected into a Shimadzu gas chromatograph with a splitless-type injector. The capillary column was held 5 min at 40 °C and programmed to increase to 300 °C at a rate of 7 °C/min. New detector conditions: Tn ) 980 K and Ts ) 1010 K. FID conditions: H2 flow rate 30 mL/min and air flow rate 600 mL/min.

× 103, Sphenol/Sbenzene ) 3.1 × 103. These results indicate that this detector is selective to OH-containing compounds. Dynamic Range. Figure 3 shows a graph of signal versus sample size for 1-pentanol and phenol, with data points for sample sizes extending over 5 orders of magnitude. This working curve was taken under the optimum conditions for the detection limit. The detector noise for these measurements was ∼2 × 10-13 A, which was more than 1 order of magnitude lower than has been previously reported for the detector in the HPSI mode.10 The data illustrate that both compounds exhibit a linear response to sample sizes ranging over at least 5 orders of magnitude. A useful factor for comparing different types of ionization detectors is ionization efficiency, φ, defined as the ratio of the number of ions giving the signal current to the number of incoming sample molecules. For 1-pentanol and phenol in the accompanying sensitivity data, the φ values of the new detector were of the order of 0.08 and 0.25%, respectively. The corresponding φ value of the widely used FID was ∼0.003% (Table 1), which was very close to the literature value of 0.004% for decane, a representative hydrocarbon.17 Thus, the new detector is highly efficient as far as ionization detectors are concerned, and this is the basis for its ability to detect compounds at the femtogram level. 3. Chromatograms of Aliphatic Alcohols and Phenols. A new detector chromatogram of a series of n-alkyl alcohols was (17) Kolb, B.; Auer, M.; Pospisil, P. J. Chromatogr. Sci. 1977, 15, 53.

compared with that from a conventional FID, which has been the detector of choice for alcohol analysis (Figure 4); clearly, the former is more sensitive than the latter. The sensitivity of the new detector in detecting the various alkyl alcohols did not strongly depend on the alkyl chain length; there was a relatively uniform response to many alcohols. This behavior suggests a similarity in the production rates of the OH radicals, which were formed in the dissociation process on the surface, whose negative ions are assumed to be the charge carriers in the detector. Overall, the new detector chromatograms show little tailing, confirming that the intrinsic response time is short. One class of samples, which exhibit very large new detector responses, is the chlorophenols and nitrophenols. Figure 5 shows typical chromatograms obtained from the analysis of a sample which was a formulated mixture of various phenol compounds measured by the new detector and FID detectors. (For the quantities of the individual components, refer to the figure caption.) The pentachlorophenol result is striking. This compound yields a MDA that is smaller in the new detector than in FID by a factor of 28.4. That the new detector is very sensitive to the molecular structure of the sample is illustrated by comparing the response to the two isomers, o-nitrophenol and p-nitrophenol. The area of response to o-nitrophenol was ∼3 times larger than that of p-nitrophenol, although their concentrations were equal. In addition to providing specific responses to certain compounds in Analytical Chemistry, Vol. 70, No. 16, August 15, 1998

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this mixture, the new detector provided substantially larger ionization signals than the FID. For example, the new detector gave current signals to pentachlorophenol that were twice as high as those from FID, even when the sample was one hundredth the size. The poor FID sensitivity may be partially explained by the well-known fact that when heteroatoms, such as O, Cl, P, etc., are present in the sample compounds, the FID response is lower than its response to simple hydrocarbon compounds. CONCLUSION A possible analytical application of the state-of-the-art, supersonic free jet technique was studied in a new detector for gas chromatography. The excellent performance specifications for alkyl alcohol compounds and phenol compounds were consistent with the process of hyperthermal negative surface ionization, and therefore, the low work function surface was desirable for sensitive analysis by the new detector. The response of the new detector to aliphatic alcohols and phenols is ∼100 times greater than the response obtained from the same compounds when a conventional FID was used. Thus, the detector sensitivity can feasibly be enhanced 10-fold and potentially 100-fold. In its present state of development, the detector has a minimum detectability of 7.1 × 10-13 and 2.1 × 10-13 g/s for 1-pentanol and phenol, respectively, and a linear range of operation of greater than 1 × 105. Because of its superior detection capability, this instrument should find wide applicability in alcohol and phenol analyses and other applications where specific and sensitive detection is required. While only selected applications have been considered in this paper, this detector is likely to be useful in analyzing compounds of environmental concern, such as nitropolyaromatic hydrocarbons and Bisphenol A. A final note is that the conversion to a hyperthermal SID is simple; that is, the low work function surface is merely interchanged with a high work function surface.

Figure 5. Gas chromatograms of a formulated mixture of phenol compounds in methanol solution. The injection volume is 1 µL: (1) methanol (solvent); (2) 1-pentanol; (3) phenol; (4) o-nitrophenol; (5) 2,4-dinitrophenol; (6) 4-chloro-m-cresol; (7) 2,4,6-trichlorophenol; (8) 2,4-dinitro-o-cresol; (9) p-nitrophenol; (10) 2,6-dinitro-p-cresol; (11) pentachlorophenol. Each peak of the new detector profile corresponds to 10 pg, while each peak of the FID profile corresponds to 1 ng. The gas chromatographic conditions are the same as those of Figure 4, with the exception of the new detector settings; Ts ) 1053 K and Tn ) 893 K.

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ACKNOWLEDGMENT This work was supported in part by the Japanese Ministry of Education, Science, and Culture; Grant-in-Aid for General Scientific Research (No. 07804050). Received for review February 9, 1998. Accepted May 28, 1998. AC980138O