samples was determined by the wet digestion method and the proposed method using conventional flameless atomic absorption in order to show that this method is as good as the wet digestion method. The results are given in Table 111,Eind close agreement in the measured values by two different methods was obtained. In comparison with the wet digestion method (9),the present method afforded an excellent yield in high mercury levels in fish. In this method, methylmercury content is obtained by subtracting the value measured in acidic medium from total mercury value. Recovery of methylmercury added to fish was studied. As shown in Table IV, complete recovery was obtained. Precision and Accuracy. In order to examine the feasibility of this method, the reliability of the technique was tested and the results are shown in Table V. The relative standard deviation was acceptable for fish sample analysis. A survey of the mercury content could readily be accomplished,and this method could be applied to monitor changes in mercury
content during storage. With modifications of sample preparation, the present method may be applicable to determine mercury content in blood, hair, and foodstuffs.
LITERATURE CITED (1) T. Takeuchi, “Mlnamata Disease-A Study on the Toxic Symptoms by Organic Mercury”, Univ. of Kumamoto, Report of Department of Medical Sciences (1966). (2) K. Irukayama, Adv. Water follut. Res., 3, 153 (1967). (3) J. B. Willis, Endeavour, 32, 106 (1973). (4) F. D. Deitz, J. L. Sell. and D. Bristol, J. Assoc. Offic. Anal. Chem., 56,387 (1973). (5) K. Eichner, Lebensmkelchem. Gerlchtl. Chem., 26 240 (1970); Anal. Abstr., 25, 2598 (1973). (6) S. H. Omang, Anal. Chim. Acta, 63, 247 (1973). (7) L. MaQOS,Analyst(London), 06, 847 (1971). (8) Y. Umezaki, Jpn Analyst, 20, 173 (1971). (9) C. Uklta, Chief Editor, “Standard Methods of Analysis for Hygienic Chemists-with Commentary”, authorized by the Pharmaceutical Society of Japan, Kanahara Publ. Co., Tokyo, 1973, p 275.
RECEIVEDfor review October 28,1975. Accepted March 29, 1976.
Direct Coupling of Glass Capillary Columns to a Mass Spectrometer Frederick A. Thome* and George W. Young R. J. Reynolds Tobacco Company, Research Department, Winston-Salem, N.C. 27 102
The rapid advances made in the production of very good glass capillary columns for chromatographic work have led to the need for development of an interface to directly couple such columns to the mass spectrometer. The availability of large capacity pumps for mass spectrometers now makes possible the design of such an interface to accommodate up to 6 to 8 ml/min of carrier gas (helium, for example) to be taken directly into the ion source of the mass spectrometer. A variety of coupling techniques have appeared in the literature (1-5). In designing such an interface, several prerequisites are to be met. The interface should be capable of reducing the pressure from 1 atm a t the column outlet to less than Torr in the ion source of the mass spectrometer without affecting column performance. It should also produce an inert pathway by which sample molecules can be transferred into the ion source without loss of chromatographic resolution, at the same time offering some mechanical stability to the glass system. Although higher flow rates can be accommodated, the interface volume must be carefully controlled to avoid peak spreading from columns operating at less than 1ml/min flow rates. The interface, of course, must be situated so that it is readily accessible to the operator and permit ease of column change. The interface which we have designed and have been using for over a year now must be considered a variation of the “direct connection” type as described in the paper by Henneberg et al. (I).The interface incorporates an idea published by Neuner-Jehle et al. (2), that of a platinum capillary at the coupling point. The use of platinum a t this point gives good mechanical stability and favorable surface behavior. Our variation in the use of platinum as a buffer for the atmosphere-to-vacuum coupling of the column to the ion source, is in making this a closed system, taking advantage of the high yield from the column, and providing an opportunity for removal of large quantities of solvent during injection. The control of the solvent peak is very important since the splitless injection techniques employed in our laboratory would send large volumes of solvent directly to the ion source and cause excessively high pressure and contamination. When a sample
is not being run, the interface allows the source to be kept at Torr. The heart of the interface, shown in Figure 1, is a 50-mm piece of 0.3-mm 0.d. and 0.15-mm i.d. platinum capillary (Engelhard Industries) that is silver-soldered into a piece of glass-lined metal capillary tubing, 1.5-mm 0.d. and 0.5-mm i.d. The glass-lined metal tubing is threaded through the line of sight coupling directly into the ion source of the Varian CH-5 mass spectrometer. This CH-5 has been equipped with a 600-l./sec oil diffusion pump on the ion source. Enough glass-lined tubing is used to permit the platinum wire to be positioned inside the gas chromatograph oven. All exposed lines are heated by wrapping with micro heating tape from Clayborn Laboratories. The platinum capillary has been Torr with crimped to give a Pennig gauge reading of 9 X a helium flow rate of 5*ml/min. This is sufficient to handle all of the flow rate from a variety of column types used in our work. The gas chromatographic column is connected directly to the interface by means of a 20-mm piece of glass capillary of 0.31-mm i.d. prepared in quantity on a Hupe glass drawing machine. The platinum is inserted into the end of this glass capillary. The chromatographic column is butted to the glass capillary and sealed with a piece of shrink-type Teflon tubing. All of this is enclosed in a f/l~-inchSwagelok union cross, modified by tapping one side to 0.062-inch i.d. One side of the union cross is used to supply make-up helium gas, the other side of the union cross being connected to a vacuum pump (2-stage, 250 l./min, Edwards) through a fine control needle valve (Whitey, 22R52A). The column is sealed to the union cross through a Ks-inch dead volume fitting using a ferrule machined from high temperature septa material. With the vacuum pump valved off, the ion source is exposed to a constant flow rate determined by the crimping of the platinum wire. This flow rate can be satisfied completely by the flow rate from the column when intermediate i.d. capillaries are used. When small bore capillaries are used, preheated make-up gas from one side of the union cross obtained from a second injection port can be used to maintain constant flow. This is evidenced by the constant source pressure obANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
* 1423
-J
To bypassvacuumpump
Glass lined tubi
Figure 1. Coupling device construction details (not to scale) tained with a variety of column flow rates and the very low air peaks in the acquired mass spectra. Further evidence of the importance of the make-up gas is the constant source pressure over a wide temperature program (150 "C) of the column which would reduce the flow rate of the column. The control of the solvent is accomplished with the vacuum pump connected to the other side of the union cross. When the needle valve is open, the vacuum pump can completely handle the helium flow rates a t the platinum crimp and compete with the ion source pump, thereby reducing the Torr. When the needle source pressure to less than 1 X valve is closed, the original flow is created at the interface and the ion source pressure is adjusted accordingly. During injection of a sample, the needle valve is wide open, allowing the by-pass pump to evacuate the union cross and keep much of the solvent from the ion source. This arrangement also protects the ion source from unusually high concentrations of any components in a sample. I t has been used successfully to obtain excellent mass spectra on minor components eluting on the tail of major peaks by controlling the amount of flow directed to the ion source until the proper time. Figure 2 shows chromatograms of a sample run using an FID of the gas chromatograph and, for comparison, a run with the column coupled to the mass spectrometer with the described interface. The mass spectra taken to examine the background with the interface in use revealed no discernible contribution from any portion of the connecting materials. These background
Figure 2. Comparison between the FID chromatogram and the GUMS chromatogram of the same mixture
spectra were compared to those using a large bore capillary column that could be threaded over the platinum capillary, thereby omitting the Teflon tubing connection. This system as described has the following advantages: 1) A high yield of sample from the column as a result of the closed system. 2) Complete control of flow to the ion source. 3) Extremely leak-free atmosphere in terms of column connections. 4)Low dead volumes and comparable GC/MS traces relative to FID runs. 5 ) Relatively simple construction and ease of column change, yet providing suitable mechanical strength and inert path to the ion source. Even in the case of column breakage, the ion source is completely protected. The system has not needed any repairs or maintenance since installation over a year and a half ago.
ACKNOWLEDGMENT We thank Courtney I. Sadler for the drawing of Figure 1, and Jackie L. White for machining the ferrules from the septum material. LITERATURE CITED (1) D. Henneberg, U. Henricks, and G. Schomburg, Chromatographia, 8, (9), 449 (1975). (2) N. NeunerJehle, F. Etzweilar, and G. Zarske, Chromatographia, 6,5(1973). (3) K. Grob and G. Grob, J. Chromatogr., 62, 1 (1971). (4) P. Schultze and K. H. Kaiser, Chromatographia, 4, 381 (1971). (5) K. Grob and H. Jaeggi, Anal. Chem. 45, 1788 (1973).
RECEIVEDfor review February 10, 1976. Accepted April 2, 1976.
Bipolar Averaging Circuit for Enhancing Signal-to-Noise Ratios in Recorded Spectra John A. Wehrly," J. Fenton Williams, David M. Jameson, and David A. Kolb Department of Biochemistry, University of Illinois, Urbana, 111. 6 180 1
In spectrophotometric measurements, the signal-to-noise ratio often determines the practical sensitivity of an instrument. Since the SIN ratio is proportional to the square root of the number of samples, averaging data may effect a dramatic improvement in spectral quality. A circuit capable of averaging 10 or 100 positive voltage signals was described in the Designer's Casebook section of Electronics ( I ) . We describe here a circuit designed for a ratiometric spectrofluorometer ( 2 ) which permits bipolar averaging of 100 signals a t a variable sampling rate (See Figure 1). 1424
ANALYTICAL CHEMISTRY, VOL. 48, NO. 9, AUGUST 1976
Emitted light, normally collected a t right angles to the excitation direction, provides the sample signal while a small fraction of the exciting light is directed to a quantum counter to provide a reference signal. The bipolar capability of this circuit facilitates the precise adjustment of phototube dark currents and amplifier offsets. The ADC provides a digital output corresponding to the ratio of the two analog input signals. Although designed for an Analog Devices Model ADC-17141h digit BCD output analog-to-digital converter, the circuit is easily adapted to any
