Anal. Chem. 1987, 59,379-382
Table I. Retention Time ( t R )Reproducibility for Chlorophenols Separated in the Gradient Produced by an Open Tubular Mixera
tR (s) at the following peak no. 1
2
3
4
5
6
274.6 309.5 350.1 406.1 434 mean, n = 5 192 1.00 1.14 2.04 1.77 1.08 U 1.50 0.39 0.32 0.33 0.50 0.41 brei, % 0.78 "Gradient run flow rate 1.4 pL/s. Other conditions and peak identification as given for Figure 4. The influence of the gradient shape on the chromatograms is shown in Figure 4. It can be seen that the length of the chromatogram and the peak separation increases with increasing volume of solvent B introduced into the mixing device. Thus, the possibility for optimization of both resolution and analysis time is given. The reproducibility of a gradient profile is most conveniently evaluated by observing the retention time reproducibility of the components of some test mixture (5, 7, 8, 18). The gradient generator proposed in this paper was evaluated by using the conditions for the separation of chlorophenols by reverse-phase micro-HPLC presented in Figure 4iii. The flow rate during the gradient run was adjusted to 1.4 gL/s. The time gap between gradient switch (G) and sample injection (I) was always 90 s. The retention data obtained from five consecutive separations are summarized in Table I. It can be seen, that with the exception of the first peak, the RSD of the retention times varies between 0.3 and 0.5% RSD. The slightly higher variability of the first peak retention (0.78% RSD) can probably be explained by its higher sensitivity to flow rate changes caused by differences in the viscosities of solvents A and B. The reproducibility of the retention times can probably be further improved by electronically timing the delivery of solvent B. On the other hand, when the retention time reproducibility is not highly critical, solvent B can also be introduced by hand with a syringe. The small dependence of gradient profile on the flow rate of solvent B delivery (see Figure 2) can be an important advantage in such a case. Considering the solvent properties taken in the Theory, geometrical parameters presented in the Exerimental Section and the Poiseuille equation, it appears that for delivering 0.3 mL of solvent B within 0.3 min, a pressure of -0.35 atm is needed. Hence, the use of syringe or simple low-pressure pumps is also feasible from this point of view.
CONCLUSION The open tubular mixer proposed in this paper can effectively generate gradients for micro-HPLC. The duration and steepness of the gradient can be readily changed in a wide range by simply changing the volume of the initial solvent introduced in the mixing device. The design also has the potential for further minimization to fit packed or open ca-
379
pillary LC. Good reproducibility and simple control of gradient profiles render the proposed device suitable for automated chromatographic analysis. On the other hand, the simplicity of the design and its low cost are favorable for rapid optimization of chromatograms, even when operated manually and in a less well equipped laboratory.
ACKNOWLEDGMENT J. C. Gluckman is acknowledged for carefully reading the manuscript.
LITERATURE CITED Small Bore Liquid Chromatography Columns : Their Properties and Uses. Chemical Analysis, Scott, R. P. W., Ed.; Wiley: New York, 1984; Vol. 72. Novotny, M.; Ishii, D. "Microcolumn Separations"; J. Chromatogr. Libr., Vol. 30; Elsevier: Amsterdam, 1985. Sagallno, N., Jr.; Shih-Hsien, H.; Floyd, T. R.; Raglione, T. V.; Hartwick, R. A. J. Chromatogr. Sci. 1985, 2 3 , 238-246. Verzele, M.; Dewaele, C. I n "The Science of Chromatography", Brunner, F., Ed.; Chromatogr. Libr., Vol. 32; Elsevier: Amsterdam, 1985; pp 435-447. Schwartz, H. E.; Berry, V. V. LC Mag. 1985, 3, 1024 Scott, R. P. W.; Kucera, P. J. Chromatogr. 1979, 785, 27-41. Schwartz, H. E.; Karger, B. L.; Kucera, P. Anal. Chem. 1983, 5 5 , 152-1760. Schwartz, H. E.; Brownlee, R. Am. Lab. (Fairfield, Conn.) 1984, 161101. 43-58. Powley, C. R.; Howard, W. A.; Rogers, L. B. J. Chromatogr. 1984, 299, 4355. Ishii, D.; Asai, K.; Hibi, K.; Jonokuchi, T.; Nagaya, M. J. Chromatogr. 1977, 144, 157-168. Simpson, R. A.; Schachterle, S. D. "Design and Performance of Microbore System", presented at the Pittsburgh Conference and Exposition on Analytical Chemistry and Applied Spectroscopy, Atlantic City, NJ, 1984; abstract 650. Takeuchi, T.; Ishii, D. J. Chromatogr. 1982, 253, 41-47. Hirata, Y.; Nakata, F. J. Chromatogr. 1984. 294, 357-360. Karlsson, K. E.; Novotny, M. HRC CC,J. High Resolut. Chromatogr. Chromatogr. Commun. 1984, 7 , 411-413. Saito, M.; Wada, A.; Hibi, K.; Takahashi, M. Ind. Res. Dev. 1983, 2 5 ( 4 ) 102-106. Berry, V. V.; Takeuchi, T.; Ishii, D. 21st International Symposium "Advances in Chromatography", Oslo, Norway, 1985. Berry, V. V. Am. Lab. Fairfield, 1885, 77(10), 33-37, 39-41. Slais, K.; Preussler, V. HRC CC,J . High Resolut. Chromatogr. Chromatogr Commun, in press. Berry, V. V.; Ishil, D.; Takeuchi, T. HRC CC,J. High Resolut. Chromatogr. Chromatogr . Commun . 1985, 8 , 659-664. Glddlngs, J. C. Dynamics of Chromatography,Part I ; Marcel Dekker: New York, 1965; p 87-88. Taylor, G. I.Proc. R . SOC.London A 1953, 219, 186-203. Golay, M. J. E.; Atwood, J. G. J. Chromatogr. 1979, 186, 353-370. Atwood, J. G.; Golay, M. J. E. J . Chromatogr. 1981, 278, 97-122. Golay, M. J. E. J. Chromatogr. 1979, 186, 341-351. Tijssen, R. Sep. Sci. Technol. 1978, 73, 681-722. Hofmann, K.; Halisz, I. J. Chromatogr. 1979, 773, 211-228. Katz, E. D.; Scott, R. P. W. J. Chromatogr. 1983, 268, 169-175. Hofmann, K.; Halisz, I. J. Chromatogr. 1980, 799, 3-22.
.
RECEIVEDfor review June 23,1986. Accepted September 8, 1986.
Determination of Trace Levels of Trimethylamine in Air by Gas Chromatography/Surface Ionization Organic Mass Spectrometry Toshihiro Fujii* and Toshihumi Kitai National Institute for Environmental Studies, Tsukuba, Zbaraki 305, J a p a n The Occurrence and determination of aliphatic amines have received a great deal of attention in recent years (I,2). These foul-smelling compounds have been found in a number of ambient environments (3-5) and become a source of serious social and psychological problems. They are also involved in nitrosamine synthesis in air (6),because methylamines react with NO, and 02.
The necessity of determining these compounds a t low levels in complex matrices has resulted in a number of analytical schemes (7-10).Usually concentration prior to identification is necessary. Solvent absorption (7)and adsorption on porous are well-known concentration techniques. polymers (3,8,9) The final analysis has been done with a flame ionization detector or a nitrogen-selective detector combined with gas
0003-2700/87/0359-0379$01.50/00 1987 American Chemical Society
380
ANALYTICAL CHEMISTRY, VOL. 59, NO. 2. JANUARY 1987
surfaceionization source assembly
Figure 1. Schematic of GCISIOMS.
chromatography. However, some problems have been reported (11,12) in terms of detection limits, identification, interference, and peak tailing. Gas chromatography/mass spectrometry (GC/MS) has become the main method of confirmation (13) and improves the detection limits and reduces but does not eliminate the problems of interference. Surface ionization (SI) is defined as the ionizing process in which the chemical species ionize on hot metal or refractory metal oxide emitters. The S I technique has been used for analysis of chemical species with low ionization energy because of its high selectivity, sensitivity, and simplicity (14). However, its use has been restricted to metals; there have been very few attempts to apply the method to organic materials. Recent mass spectrometric studies of the SI of individual organic compounds (15) revealed that the SI gives greater output currents for certain kinds of organic compounds. The high sensitivity for trimethylamine (TMA) is surprising (16). This compound, which causes serious environmental problems as an odorous substance, yields much higher ionization in the SI mode than in the conventional electron ionization (EI) mode. These findings suggested that the combination of gas chromatography (GC) and SI organic mass spectrometry (SIOMS) could be used for the analysis of organic nitrogencontaining compounds. This paper describes the first demonstration of a GC/ SIOMS method. A newly developed method based on the GC/SIOMS with high detection capability can be very advantageous for the selective determination of volatile amines a t low concentration in air, since the low odor-threshold for volatile amines (only 0.002 ppm for TMA) (17) and the low hygienic threshold values (in the low parts-per-million ranges) make a trace method important. The present analytical procedure employs the direct gas sample inlet without complicated sample preparation and the selected ion monitoring. I t saves time and minimizes the risk of sample contamination and sample loss during manipulation procedures.
EXPERIMENTAL SECTION Apparatus. The experiments were performed with a Finnigan 3300 gas chromatograph/quadrupole mass spectrometer equipped with a home-built thermionic ion source. As can be seen in Figure 1, the design of the surface ionization source assembly (direct insertion probe) is such that the Re ribbon filament (6 mm X 0.75 mm X 0.025 mm) can be placed in the center of the conventional electron ionization ion source chamber when the probe is inserted. This combination-type ion source allows easily interchangeable operation both in the SI mode and in the electron ionization (EI) mode. The details have been reported elsewhere (16, 18).
The Re ribbon filament emitter was used in the presence of O2 at a controlled pressure of 2 X torr which was obtained by adjusting the Granville-Phillips series 203 variable leak valve (L.V.)in Figure 1. A continuous admission of O2allows an increase in work function up to 6.4 eV at a surface temperature of 1200 OC, which is essential for the high SI efficiency. Operating with the continuous oxygen flow caused no observable deterioration in the performance of the mass spectrometer. Gas chromatographic separations were performed by using the 3 m X 2 mm diameter glass column packed with 60/80 Gaschropack 54 + 5% KOH (Gaschro Industry Co., Tokyo) as a packing material, which provides base line separations and almost symmetrical peaks. The column was conditioned at 200 "C overnight. Before the measurements, two 10-pL portions of 10% KOH in water were injected to coat the injector liner. The column was then injected 20 times with 10-pL samples of distilled water ( 7 , 8, 19). Other GC/MS conditions were as follows: column temperature, 150 "C; injection-port temperature, 250 "C; carrier-gas flow rate, 30 mL/min He; GC transfer system of a single-stage jet separator, 250 "C. Procedure. Ambient air analyses were performed as follows. A 1-mL sample was injected into a GC with a 1-mL glass syringe (Pressure-Lok,C-type syringe with an open-end needle, Precision Sampling Corp.). The syringe with side port needle is not recommended, because it cannot eliminate the effect due to the inherent dead volume. The memory effect of the syringe from adsorption was reduced by washing with 10% KOH in water and then with distilled water. Although injecting a gas volume of more than 1 mL will extend the detection limit, it does not provide mass spectrometric stabilization because of the large quantity of nitrogen and oxygen in the gas samples. The SI spectra of monomethylamine (MMA), dimethylamine (DMA),and TMA are exclusively dominated by a few peaks (20), showing that the (M - H)+ intensity is highest for these compounds. The mass spectrometer was set to monitor selected mass 30 for MMA and TMA, 44 for DMA, and 58 for TMA. The resulting ion currents were recorded on a strip chart recorder. Ambient air samples were collected in 1-L glass cylinders closed with valves and membranes through which the gas samples were taken by the syringe. Washing with 10% KOH in water prior to storage was performed on all the containers to minimize adsorption on the glass surfaces. After several minutes of purging, all the sample containers were filled with an adjustable-flowpump (Sipin sample pump, SP-1, SIPIN Co., New York). Storage of samples for a period of 1 day caused no problem. Standarization. For standarization, a water solution of methylamines (from Wako Chemical, Tokyo), whose composition was determined gas chromatographically beforehand, was used; MMA is 3% water solution, DMA is 24%, and TMA is 11.5%. Stardard gases were made by vaporizing these amine solutions into the 1-L glass cylinder (treated by 10% KOH water solution). A series of standard concentrations from 0.85 to 48000 ppb (v/v) were made by further dilution. It was confirmed before the dilution process that the 1-L glass cylinder filled with air was free from methylamines. A plot of the peak area against the known standard concentrations gives a straight line over prepared concentration range with the uncertainty *12.8%, demonstrating that the present procedure was sound. Quantitation of the amines was done by comparing the single ion chromatogram peak areas for samples of standards in similar concentration ranges.
RESULTS AND DISCUSSION Comparison of Sensitivities. Since the present ion source is a combination type, it is very easy to compare the mass spectrometric sensitivity in the SI and E1 modes. The effectiveness of the technique is demonstrated by a comparison of single ion chromatograms obtained with surface ionization and electron ionization of a known mixture of MMA, DMA, and TMA in Figure 2. The much greater sensitivity of surface ionization than that of electron ionization for TMA is clearly shown. The sensitivities of surface ionization and electron ionization for MMA are comparable. In addition, other experiments with TMA gave a peak with a signal to noise ratio
ANALYTICAL CHEMISTRY, VOL. 59, NO. 2, JANUARY 1987
SI
El
Table 11. Recovery Measurements on Spiked Air Samples with Varying Amounts of TMA Added
standard standard
381
a i r around garbage
I"'
TMA added, ppb
TMA found, ppb
TMA recovered, ppb
0 4.2
4.2
0
8.5
4.3
12.1 16.1
11.9
8.5 12.7
480 Ppb
7.9
Table 111. Trimethylamine Concentration Levels in Air Samplea TMA,
6
4
2
0
e a 21
T r n
d Time ( m i n )
Flgure 2. SIM chromatograms of aliphatic amines in a 1-mL air sample; spectrum of a 1-mL standard gas sample In E1 mode (left-hand column), spectrum of a 1-mL standard gas sample in SI mode (center column), and spectrum of an air sample from garbage site in the SI mode (right-hand column). All measuring ranges of ion currents are 1X A (full scale), except that of ion currents at m / z 58 in the S I mode, which are 5 X lo-' A (full scale).
Table I. Aliphatic Amines Examined in the Air Sample detection limitb retention" selected E1 mode, SI mode, compds time, min ion (rn/z) ppm ppb SI/EIc MMA DMA TMA
1.8 2.6 3.7
30 44 58
sampling site
0.5 0.62 0.14
1300 51 0.42
1-1in poultry building 1-2 around poultry building (5 m
date
PPbb
Feb., 1986 cold and dry
1.6 f 0.1 1.2 f 0.1
2-1 in cattle building Feb., 1986 2-2 around cattle building (5 m away) cold and dry
0.6 f 0.1 tr
3-1 in swine building March, 1986 3-2 around swine building (5 m away) cool and dry
0.8 f 0.1 tr
4-1garbage collection site (2 m away) March, 1986
0.6 f 0.1 tr
away)
4-2 1 m away from the above site 4-3 residence area 10 m away from 4-1 site
cool and dry
ND
Storage time is less than 1 day. tr = trace, detectable but less than 3 times detection limit; ND = not detected.
0.38 12.2
333
Column temperature 150 "C with 55 mL/min at the He carrier flow rate. b A t the signal to noise ratio of 3. 'The ratio of the detection limit in the SI mode to that in the E1 mode. of 6 for a 225 pg/L (0.8 ppb by volume) sample, which could not be detected in a single ion trace using E1 mass spectrometry. These results are consistent with previous observations of a very high sensitivity for TMA with surface ionization (16). The relative sensitivity between three methylamines is different in the SI mode. This result can be easily interpreted by the Saha-Langmuir equation (14) that tells the SI-mode sensitivity for different organic compounds is strongly dependent on the ionization energy (IE) of the species. Since the electronic properties of the CH3substituents with electron donor characteristics lead to the decrease in IE, sensitivity varies in a wide range from MMA, DMA, to TMA. Therefore the limitation of the SI mode should be recognized; the method is not capable of detecting trace levels of DMA and MMA. Table I summarizes ionic species monitored, retention time, the detection limit of both E1 and SI modes (concentration of each compound producing a peak three times the noise level with a 1-mL injection of standard air sample), and SI/EI, the ratio of the detection limit obtained in the SI mode to that in the E1 mode. Although outside the scope of this paper, the E1 result suggests that the combination of the 1-L gas sample procedure and appropriate concentration processes make possible the determination of aliphatic amine compounds at the level of 1 ppb. Recovery and Precision. The injection of 1 mL of distilled water after the injection of methylamine standard in early experiments resulted in a ghost peak, which suggested sample loss due to adsorption. However, this problem was greatly diminished by coating the injector liner with KOH aqueous solution, in agreement with other studies (7,8,19). After KOH treatment of the packing material, the GC injector, and the gas syringe, the efficiency of the method after 1 day of storage was determined by making measurements on spiked
samples at various concentrations of TMA (Table 11). The percentage recovery of TMA standard fell in the range 102-91 %. The precision of the method was determined from five replicate analyses of a standard sample of 8.5 ppm concentration of TMA, with a relative standard deviation of 6.8%, using peak height (mm). Application to Air Sample. The method was applied to the determination of aliphatic amines in the air around cattle and poultry buildings and garbage sites. Table 111lists the results of these ambient sample measurements and Figure 2 shows a typical chromatogram from these sources in the extreme right-hand column. In air from the livestock house, TMA was successfully determined and neither DMA nor MMA was detected. Organic components did not interfere with these determinations because of the selectivity of the surface ionization. As can be seen, the level of TMA was highest for that air sample collected in the poultry house (i.e., site 1-1).Site 4-3 from a residential area gave no detectable TMA. Kuwata et al. (8)measured the aliphatic amines in the air of livestock houses. Their results ranged from 0.93 to 0.28 ppb for TMA. A comparison of our results with theirs shows fairly good agreement, considering the differences in method (and sampling procedure) and the places from which samples were taken.
CONCLUSION This is the first study on surface ionization organic mass spectrometry coupled with gas chromatography, which is based upon the recent discovery that some organics are very efficiently surface-ionized in either the molecular or radical form on the surface of an incandescent Re filament (16,20). The present method has achieved the determination, with 1-mL direct gas sample injection onto the GC-SIOMS at the threshold concentrations for odor detection. The method offers a few advantages over the flame ionization detection, which has been previously used. Because no preconcentration is needed, the analysis time is less than 10 min and is much shorter than the gas chromatographic method with the preconcentration procedure. Therefore, it may be useful in
Anal. Chem. 1907, 59,382-384
382
routine analysis for a large number of air samples. A second advantage is a high degree of specificity due to the characteristics of surface ionization. The present study is a succesul example of GC/MS combined with the surface ionization method. We believe that the GC/SIOMS method can be extended to analysis of other organic compounds in many fields that are able to be ionized efficiently on the SI emitter. These compounds are other alkylamines, diamines, and amino alcohols, hydrazines, and nitrosamines (20).
ACKNOWLEDGMENT The authors are grateful to D. S. Linton at Yokota Air Base for the manuscript preparation. Registry No. TMA, 75-50-3. LITERATURE CITED Fuselli, S.;Cerquiglini, C.; Chiacchierini, E. Chim. Ind. (Milan) 1978, 6 0 , 711. Brubaker, R . E.; Muranko, H. J.; Smith, D. B.; Beck. G. J.; Scovel, G. JOM, J . Occup. M e d . 1979, 21,688. Fuselli, S.;Benedetti, G.; Mastrangei, R . Atmos. Environ. 1982, 16, 2943. Mosler, A. R.: Andre, C. E.; Viets. F. G. Environ. Scl. Technol. 1973, 7, 642.
Van Langenhove, H. R.; Van Wassenhove, F. A,; Coppin, J. K.; Van Acker, M. R.; Schamp. W. M. Environ. Sci. Technol. 1982, 16,883. Fine, D. H.; Rounbehler, D. P.; Sawicki, E.; Krost, K. Environ. Scl. Technol. 1977, 1 1 , 577. Audurrsson, G.;Mathiasson, L. J . Chrornatogr. 1984, 315,299. Kuwata, K.; Akiyama, E.; Yamagali, Y . ; Yamagali. H.;Kuge, Y. Anal. Chem. 1883, 55,2199. Bouyoucos. S. A.; Melcher, R. G. Am. Ind. Hyg. Assoc. J . 1983, 44, 119. Kashihira, N.; Makino, K.; Kirita, K.; Watanabe, Y. J . Chrornatogr, 1982, 239, 617. Casselman, A. A.; Bannard, R. A. R. J . Chromatogr. 1974, 88, 33. Fujimura, K.; Kiranaka, M.; Takayanagi, H.; Ando, T. Anal. Chem. 1982, 54,918. Pons, J. L.; Rimbault, A.; Darbord, J. C.; Leiuan, G. J . Chrornatogr. 1985, 337, 213. Zandberg, E. Ya.; Ionov, N. I. Surface Ionization; Israel Program for Scientific Translations: Jerusalem, 1971. Zandberg, E. Ya.; Rasulev, U. Kh. Russ. Chem. Rev. (Engl. Trans/.) 1982, 51, 819. Fujii, T. Int. J. Mass Spectrom. Ion Processes 1984, 57,63. Billings, C. E.; Jonas, L. C. Am. Ind. Hyg. Assoc. J . 1981, 42,479. Fujii, T. J. Phys. Chem. 1984, 88, 5228. Dunn, S. R.; Simenhoff, M. L.; Wesson, L. G. Anal. Chem. 1976, 48, 41. Fujii, T.; Kitai. T. I n t . J . Mass Spectrom. Ion Processes 1986, 71, 129.
RECEIVEDfor review July 8, 1986. Accepted September 17, 1986. This work was supported in part by the Education Ministry in Japan, Contract No. 60880026.
Controtted Back Pressure Valve for Constant Flow and Pressure Programming in Packed Column Supercritical Fluid Chromatography K. R. Jahn and B. W. Wenclawiak*' Anorganisch Chemisches Institut, Westfalische Wilhelmsuniversitat, Corrensstrasse 36, 0-4400 Munster, FRG In supercritical fluid chromatography (SFC) it is necessary to maintain the entire eluent system at high pressure under supercritical conditions. It is common in capillary SFC to use small capillary tubes as flow restrictors at the outlet. Adjusting these small bore tubes (5-10 pm) to a certain length (I) or providing nitrogen back pressure (2)results in the desired flow rates. In packed-column SFC with its higher flow rate the use of a regulation valve is advantageous (3-6). In order to maintain the same pumping rates at different eluent pressure, the valve must be adjusted very carefully. Motorized valves are mentioned in ref 3 and 6, but the devices are either technically complicated (3) or still need manual adjustment. Adjusting the valve by hand is time-consuming, especially when using binary eluent mixtures with long equilibration times in the column (7). Because of the density effects in SFC, any supercritical system is very sensitive to any alterations in pressure. In praxis, the mass transport into and out of the system should be the same, thus avoiding any change in the system. This can be achieved by using a self-controlled back pressure valve, which is installed after the detection cell. The major criterion for its precise working is the reproducibility of retention times. The valve system described here uses a device where any changes in pressure are used to control the valve.
EXPERIMENTAL SECTION A dual piston pump (Altex 100) has been used and was operated in the constant flow mode. It was equipped with a cooling device (-10 "C) to avoid evaporation of liquid COz. As the density of solvents is pressure and temperature dependent, the pump had
Present address: De artment of Chemistry, University of ToToledo, OH 43606.
ledo, 2801 w. Bancroft
&.,
Ma
II
I If-
Figure 1. Back pressure control unit with valve: Ma, manometer; P, pressure puge; V, capillary needle valve (Chrompack); M, motor; PID,
(can be connected to a microcomputer). been calibrated prior to its use. The pump is equipped with a compressibility compensation control, which was adjusted according to the procedure as described in the manual. The obtained flows for C 0 2 have been checked after recalculation to normal temperature and pressure (NTP) conditions. A Rheodyne 7126 injection valve, with a 20-pL sample loop, was used throughout all experiments. The column, 250 X 4.6 mm id., was packed with 7-pm silica (Lichrosorb,Si 60). It was kept in an oven at the desired temperature. A Kontron 8-pL flow-through cell, modified for use at pressures above 15.0 MPa, was installed in a Zeiss PMQ I1 spectrophotometer, operated at 254 nm. A data system, started by the Altrex programmer at the same time the sample was injected, recorded simultaneously the output control unit
0003-2700/87/0359-0382$01.50/0 @ 1987 American Chemical Society
