ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
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Detection of Hydrogen Chloride Gas in Ambient Air with a Coated Piezoelectric Quartz Crystal J. Hiavay’ and G. G. Guilbauit * Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70 122
New methods for the selective detection of hydrogen chloride gas in ambient air in the ppm and ppb concentration range are presented. Triphenylamine and trimethylamine-HCI are the substrates used as coatings. The response times observed using either coatings are less than 30 s. Ammonia and moisture could cause interference problems using TMA*HCI as the coating material. The application of a gas chromatographic column packed with silica gel eliminates the effect of moisture.
Hydrogen chloride gas is a noxious byproduct of many industrial chloride processes, and a major exhaust product from the combustion of most solid rocket motors. In order to detect the content of this gas in the atmosphere, sensitive and selective monitors are needed. Several techniques are available for detection of hydrogen chloride in a dry environment a t concentrations of about 30 p p m or higher, but these techniques either do not have satisfactory response time, cannot monitor hydrogen chloride in ambient air because of t h e high reactivity of hydrogen chlorides in the presence of moisture, or d o not have sufficient detection limits. Analytical methods for hydrogen chloride in ambient air were summarized by Gregory ( I ) . T h e techniques discussed are: bubbler, pH, indicator tubes, microcoulometry, modified condensation nuclei counters, dual isotope infrared spectrometry, gas filter correlation method, chemiluminescent nitric oxide detection and chemiluminescent luminol oxidation. Techniques include both concentration (pprn) measuring instruments and dosage only (ppm-9) measurement methods. Data presented for each measurement technique include lower detection limit, response time, and in some cases, specificity. It is pointed out t h a t the primary shortcoming in each technique is in t h e specificity possible. In recent years, coated piezoelectric quartz crystal detectors have been developed as selective and sensitive air pollution sensors, as highly sensitive detectors for gas chromatography, and as a microbalance for rapid assessment of particulate mass concentration in the atmosphere. Applications of piezoelectric quartz crystal detectors in analytical chemistry were discussed by Hlavay and Guilbault ( 2 ) . T h e change in frequency of t h e oscillating quartz crystal is calculated according to t h e Sauerbrey equation ( 3 , 4):
A F = 2.3 X
lo6 X
AMs A
F2-
where i W = the change in frequency due to the coating (Hz), F = t h e frequency of t h e quartz plate (MHz), AMs = mass of the deposited coating (g), and A = area coated (cm2-). This equation predicts that a commercially available 9-MHz crystal would have a mass sensitivity of about 400 Hz/mg. T h e amount of air pollutants is mostly expressed as a volume ratio in the gaseous state; ppm and ppb. This has the Present address, Institute of Analytical Chemistry, Veszprem University of Chemical Engineering, 8201 Veszprem, P.O. Box 28, Hungary.
advantage of being independent of pressure and temperature. In a given experiment, change in the frequency can be expressed as:
A F = K.AC where K = constant which refers to the basic frequency of the quartz plate, area coated, and a factor to convert the weight of injected gas (g) into concentration (ppm). A C = concentration (pprn). In this paper we report new coatings for the specific detection of hydrogen chloride. Triphenylamine and trimethylamineehydrogen chloride, used as coating materials, enable the detection of hydrogen chloride a t ppb levels. A new coating technique, to deposit compounds on the surface of the crystals, is also discussed.
EXPERIMENTAL The experimental apparatus is shown schematically in Figure 1. Air, the carrier gas, was supplied by a vibrating diaphragm air pump. A GC column packed with silica gel was placed in the flow system with a Swagelok union, in order to adsorb the moisture from the air. The flow rate of air used was generally 30 mL/min. The crystals used in these studies are 9-MHz AT cut quartz crystals with gold-plated metal electrodes on both sides (International Crystal Mfg. Co., Okla.). The instrumentation consisted of a low frequency OX transistor oscillator (International Crystal Mfg. Co., Okla.) powered by a Heathkit Model IB-28 power supply. The voltage was a constant 9-V clc. The frequency output from the oscillator was measured by a Heath-Schlumberger Model SM-4100 frequency counter, which was modified by a digital to analog converter, so that the frequency could be recorded (Bristol Model 570 Dynameter). The frequency measured could be read on either the frequency meter or the recorder; the data resulting from the injection of a sample were recorded as a frequency change and a peak on the recorder. The schematic diagrams of the digital to analog converter and digital to analog converter power supply are of special design and can be obtained upon request. Reagents. Trimethylamine hydrochloride (TMA.HC1) (Matheson Coleman and Bell) and triphenylamine (TPA) (Aldrich Chemical Co.) were used as coatings. HC1, CO, SOn, HzS, NH3, NOz, and COz were obtained from Matheson Co., Inc., in lecture bottles. Syringe Dilution Method. A syringe dilution method was used to obtain very low concentrations of sample gases with air. One milliliter of pure test gas was sucked into a 10-mL gas tight syringe. Air was then sucked in from the laboratory so that the total volume became 10 mL. The tip of the syringe needle was closed by one’s finger. In about 30 s, the mixture in the syringe became homogeneous by diffusion. Thus the gas, for example, was 10 times diluted. In the second operation, 1mL of this diluted mixture was diluted to 10 mL, giving a mixture of 100 times dilution. By repeating this procedure, a mixture of any ppm or ppb level could be obtained. From 1 to 10 mL of this mixture of the required dilution could be injected into the cell for detection. This method of dilution is very fast, reliable, and avoids the use of big dilution flasks ( 5 ) . Methods of Application of Coatings. The method of coating the crystal with various substrates has been shown to be very important (6). TPA was dissolved in chloroform and this solution was applied over the entire surface of the electrode on both sides, with a tiny brush. Care must be taken to apply the coating as uniformly as possible; but the uniformity of coating was not
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Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 7, JUNE 1978
Recorder
Digital to Analog
Frequency Meter
'
Oscillator
u t
Injection
Figure 1. Block diagram of experimental apparatus
checked. The chloroform evaporated quickly leaving the TPA coating on the surface of the crystal. For placement of the TMA.HC1 on the surface of a crystal, a new technique was applied. A small amount (1-2 g) of the coating compound was put into a two-neck flask and the crystal was placed above this material. Using vacuum and heat, a vapor deposit of the TMA.HC1 on the crystal surface was obtained. The amount of the coating material deposited can be controlled by varying the heating time and temperature, and reproducible coatings are possible. An amount of coating was placed on the crystal, in each case, until a decrease of about 6000 Hz in the basic frequency of the crystal was observed. This amount will vary from experiment to experiment.
RESULTS AND DISCUSSION TPA, when applied to the electrode surface of the piezoelectric quartz crystal, showed a strong reaction with hydrogen chloride gas. T h e lone nitrogen pair of the T P A easily and specifically can bind the hydrogen chloride. Frequency change vs. concentration plots for TPA were found to be linear over t h e range of 1-100 p p m HC1. One hundred ppm of hydrogen chloride gave a 1 5 4 - H ~response while 1 ppm of hydrogen chloride gave a 5 2 - H ~response. This coating material can be applied for the detection of hydrogen chloride gas in the lower concentration range also; for 0.1, 0.01, and 0.001 ppm of hydrogen chloride 40,26, and 16 Hz responses were observed, respectively. Linear calibration curves were obtained a t the lower concentration levels, e.g., between 1ppm and 100 ppb, or betweeen 10 ppb and 100 ppb. The response time observed is only a few seconds, and a complete reversibility of response was observed in less than 1 min. T o test t h e specificity of the coatings, different gases were injected into the apparatus, and the resulting frequency changes were recorded. T h e results of these tests are listed in Table I. No serious interferences were observed for any gases a t 1000-ppm concentrations. T h e lifetime of the T P A coating is reasonable. Analysis of hydrogen chloride concentrations 20 days later demonstrated a 8-10% relative error obtained a t each HC1 concentration. If larger differences occur, one can easily use the
Table I. A Study of the Effect of Interferences in the Assay of HC1. All Gases Tested at 1000 ppm A. Triphenylamine coating A F , Hz Gas
H2S
14
so2
25 24
NO1
36
co 3"
27
30 41 TMA B. Trimethylamine,HCl coating AF, Hz' Gas
co 2
so2 H2 s co NO2 3"
CO 1
40 82
8 77
-537 irrev. 26 134
TMA a Response of the crystal to the gaseous pollutant in the presence of 1 0 ppm of HC1. crystal again, after cleaning and recoating with TPA. T h e coating procedure takes less than 5 min. Using TMASHCl as the coating material, a n extremely sensitive piezoelectric detector was obtained for the detection of hydrogen chloride in the atmosphere. At 100-ppm concentrations of hydrogen chloride, the response was about 480 Hz, whereas 1 ppb hydrogen chloride gave about 180-Hz response. T o plot the change in frequency vs. concentration over a large range, i.e., from 1 ppb to 100 ppm, in one calibration curve, the logarithm of both sides of Equation 2 was taken; a plot of log AF = log K + log AC was linear over t h e concentration range of 1 ppb-100 ppm. The response time observed with the TMA.HC1 coating was a few seconds, and complete reversibility of response was observed in about 30 s. In some experiments, even less than
ANALYTICAL CHEMISTRY, VOL. 50, NO. 7 , JUNE 1978
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from a few seconds to less than a minute.
1 p p b hydrogen chloride was detected, but with less reproducibility. Dilutions a t this concentration level also become difficult t o reproduce, and can cause measurement errors. Interferences from other gases are listed in Table I. No interferences were obtained with CO, SO2,H2S, NO2,C 0 2 ,and T M A gas a t 1000 ppm. Ammonia a t 1000 ppm showed a strong irreversible reaction. Since hydrogen chloride reacts with ammonia in air, t h e presence of ammonia does cause interference problems. T h e lifetime of the TMA.HC1 coating is limited, because of the high reactivity of the coating material with moisture. T h e effect of moisture on the coating material was eliminated, by using a GC column packed with silica gel; no change in the frequency of t h e baseline was observed in measurements made using this silica gel precolumn. This piezoelectric quartz crystal detector was found to be useful for t h e measurement of concentrations of hydrogen chloride gas in t h e ppm and p p b range, with response times
ACKNOWLEDGMENT T h e authors gratefully acknowledge the technical assistance of Harry Rees and Steve Campbell in the design and construction of the digital to analog converter. LITERATURE CITED G. L. Gregory, NASA, TN D-8352 (1976). J. Hlavay and G. G. Guilbault, Anal. Chem., 49, 1890 (1977). G. 2. Sauerbrey, Z . Phys. 155, 206 (1959). G. 2 . Sauerbrey, Z . Phys., 178, 457 (1964). (5) K. H. Karmarkar and G. G. Guilbault, Anal. Chim. Acta, 71, 419 (1974). (6) R. B. W. Earp, Ph.D. Dissertation, University of Alabama, 1966. (1) (2) (3) (4)
RECEIVED for review November 7, 1977. Accepted April 3, 1978. T h e authors gratefully acknowledge the financial assistance of the Army Research Office in the form of Grant No. DAAG-77-G-0226.
Fluorescence Detection in High Performance Liquid Chromatographic Determination of Polycyclic Aromatic Hydrocarbons Bejoy S. Das" and Gordon H. Thomas Ontario Research Foundation, Sheridan Park, Mississauga, Onfario, Canada L5K 183
This paper describes a new routine method of P A H determination. T h e procedure is a combination of H P L C and a fluorescence detector with a deuterium lamp source. T h e system permits the use of dilute solutions, t h u s eliminating concentration and clean-up procedures. T h e extremely high sensitivity in fluorescence emission has reduced the minimum detectable concentration of most P A H close to subpicogram levels. T h e procedure is both rapid and simple, and the successful application of t h e new technique for routine monitoring of P A H in environmental, process, and occupational health samples is described.
The use of a variable wavelength fluorescence detector for the high performance liquid chromatographic determination of nine major polycyclic aromatic hydrocarbons (PAH) is described. The fluorometric detection involves a deuterium light source and excitation wavelengths below 300 nm. Limits of detection close to subpicogram levels were obtained (e.g., benzo[ alanthracene, 0.6 pg; benzo[klfluoranthene, 0.4 pg; benzo[a]pyrene, 1.1 pg). Precision studies gave a relative standard deviation from 0.32 to 2.66% (e.g. benro[a]anthracene, 0.33 %; benzo[k]fluoranthene, 0.70 %; benzo[alpyrene, 0.50%). The system allows the use of dilute solutions, thus eliminating the usual clean-up procedures associated with trace analysis. The application of the technique for the analysis of PAH In environmental, process, and occupational health samples is discussed.
EXPERIMENTAL Reagents. Spectro-grade cyclohexane and acetonitrile were obtained from Burdick and Jackson Lab. Inc. Distilled, deionized water shown to be free of fluorescent impurities was used. PAH standard samples were obtained from the following sources and were used without purification: fluoranthene, benzo[a] anthracene (B[a ] A), chrysene, benzo [ k] fluoranthene (B[ k ] F), benzo [alpyrene (B[ a ]P), benzo [ e ]pyrene (B[ e]P) from the stock of the laboratory of Air Pollution Control Directorate, Department of the Environment, Ottawa (the stock maintained previously for the International Agency for Research on Cancer, World Health Organization), and perylene, dibenz[ah]A and benz[ghi]perylene from Aldrich Chemical Company, Inc., Milwaukee, Wis. Stock solutions of the standards were prepared by dissolving in cyclohexane at kg/mL concentration. Appropriate dilutions with cyclohexane followed by replacing the cyclohexane with 7 5 % acetonitrile were made to obtain the concentrations of PAH of pg/pL prior to HPLC analysis of the standards. The stock solutions were stored at 4 "C when not in use. Apparatus. A Spectra-physics Model 3500B Liquid Chromatograph equipped with a variable wavelength fluorescence detector, Model FS 970 Spectrofluoro Monitor (Schoeffel Instrument Corp., Westwood, N.J.) and a variable wavelength UV detector, Model SF 770 Spectroflow Monitor (Schoeffel In-
Polycyclic aromatic hydrocarbons (PAH) are widespread contaminants of t h e environment. A significant number of these P A H are either known or suspected carcinogens (1). Many analytical techniques involving high performance liquid chromatography (HPLC) have been described in the literature (2-11) for the characterization of complex mixtures of P A H in environmental samples. None of t h e published methods has been developed into a reliable, sensitive, and routine procedure for the quantitative analysis of PAH. A major difficulty in the determination and measurement of P A H is their isolation from other interfering organic substances. To solve this problem, procedures for extraction, clean-up, a n d separation from interfering components are usually practiced. Extreme care has to be taken to avoid adventitious contamination or loss of P A H a t all stages of analysis. Elimination of clean-up steps in trace analysis is therefore desirable. 0003-2700/78/0350-0967$01.00/0
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1978 American Chemical Society
