ANALYTICAL CHEMISTRY, VOL 50, NO. 14 DECEMBER 1978
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Figure 1. Sampling apparatus
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the rate of sampling and the amount of acetic anhydride. Test no. 4 was performed to evaluate the influence of humidity on the analysis. For this reason a known amount of acetic anhydride was added to the first absorber, which was connected on one hand to the second absorber and on the other to a gas washing bottle containing 10 mL of water; this provokes the saturation of the air flowing through the absorbers. No change in recovery was noticed. Interferences. Acetyl chloride reacts in the same way as acetic anhydride and therefore, if present, it is estimated as acetic anhydride. Anhydrides. different from the acetic one, do not interfere because, even if they react, their reaction products are chromatographically separated. Following the tests which have been carried out, both propionic and acetic anhydrides react with rn-aminophenol but their reaction products have different R, values (see Figure 2 ) . Precision, Accuracy, and Sensitivity. The precision of the method depends on t h e precision of the final chromaThe tographic estimation and can be considered about %E%. percentage accuracy, considering the results reported in Table 11, is estimated to be -10%. T h e detection limit, working as reported, is about 50 fig acetic anhydride in t h e absorber; therefore a sampling of 2.5 L of air is enough to reach the TLV value. T h e sensitivity can be increased by concentrating the absorbing solutions before t h e chromatographic estimation.
ACKNOWLEDGMENT T h e authors thank ACNA-Montedison for permission to publish this paper. LITERATURE CITED
Figure 2. Typical chromatogram of three absorbers containing: (A) acetic anhydride derivative, (B) a mixture of acetic anhydride and propionic anhydride derivatives, and (C) propionic anhydride derivative
T h e results are reported in the Table 11. In all the tests, the original volume of the absorbing solution was restored by adding 5 mL of toluene to the first absorber a n d 2 m L to t h e second one. T h e recoveries and t h e partition of acetic anhydride between the two absorbers were practically t h e same, varying
(1) "Threshold Limit Values for Chemical Substances and Physical Agents in the Workroom Environment with Intended Changes for 1977", The American Conference of Governmental Zndustrial Hygienists, Cincinnati, Ohio 45201 (2) W. M. Diggle and J. C. Gage, Analyst(London), 7 8 , 473-479 (1953). (3) E. K . Prokhorova, V. V. Mizgareva, A. A. Nazarenko, and E. N. Stepanov, Nauch. Rab. Inst. Okhr. Tr., Vses. Tsentr. Sov. Prof. Soyuzov, No. 77, 65-70 (1972); Chem. Abstr., 79, 69861e (1973).
RECEIVED for review May 22. 1978. Accepted July 25, 1978.
Ail-Glass Open-Split Interface for Gas Chromatography-Mass Spectrometry Hans-Jurgen Stan" and Bernd Abraham Institut fur Lebensmittelchemie der Technischen Universitat Berlin, Strasse des 17.Juni 135, D- 1000 Berlin 12, We.st Germany
One of the weak points in gas chromatography-mass spectrometry (GC-MS) work with glass capillary columns is the interface. Two techniques have found widespread use for coupling the glass capillary column to the mass spectrometer. One of the techniques involves direct connection of the glass capillary to the high vacuum of the ion source using a platinum capillary. T h e platinum capillary serves as a restriction, allowing only a definite quantity of the effluent from the capillary column t o enter the ion source. T h e advantage of this type of interface is its mechanical stability and the consequent ease with which it is handled. However, a major disadvantage is the activity of t h e platinum surface which requires proper deactivation. As Grob ( I ) demonstrated, the deactivation of the platinum surface depends to a large extent on the chromatographic conditions used, in particular, on the 0003-2700/78/0350-2161$01 .OO/O
liquid phase of the glass capillary column. T h e other technique used is the open-split connection according to Henneberg and co-workers ( 2 , 3 ) . In this type of interface, the inlet line to the mass spectrometer is a flow resistance which effects a constant flow into the ion source. T h e effluent from the capillary column is channeled so t h a t a constant stream of gas enters t h e mass spectrometer, and the excess gas escapes to the atmosphere. T h e inlet line and the restriction is usually a platinum capillary because of the ease with which the connection is made to the ion source. The device is often made simply by fitting a piece of shrinkable Teflon tubing on to the end of the column. The inlet capillary is inserted into the open end of the Teflon tube. T o avoid oxygen from the atmosphere entering the mass spectrometer (MS), the connecting device is surrounded by a mantling tube CZ 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1978
scavenger gas “-&&+He
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Figure 1. Open-split interface for GC/MS
which is constantly flushed with helium. T h e advantage t h a t this apparatus has over the direct connection interface is that the end of the gas chromatographic column is a t atmospheric pressure; therefore the chromatographic parameters are the same as those obtained outside t h e GC-MS combination. T h e high separating power of the capillary columns is not impaired. This splitting device can be completed with a second capillary which enters the open end of the Teflon tubing. An additional large flow of helium can be introduced by means of this scavenger capillary, which effectively dilutes the effluent and, as a consequence, the amount of substance which enters the MS is reduced. By these means, large concentrations of substances which might have a detrimental effect to the sensitivity necessary in trace analysis, are prevented from entering the MS. T h e major drawback of the open-split interface is that the inlet line and the restriction are constructed from long platinum capillaries which may result in the loss of labile substances. This is of great significance in trace analysis, particularly if silylated or acetylated derivatives are used. This paper describes an open-split connection in which substances transferred from the gas chromatographic column to the mass spectrometer come into contact only with deactivated glass.
EXPERIMENTAL Figure 1 shows the open-split interface that we have designed. We have worked only with glass capillary columns, but with this design, it is possible to work with packed columns as well. The interface has been developed for work with a quadrupole GC/MS 4000 (Finnigan, Sunnyvale, Calif.),but may be easily adapted for work with other mass spectrometers. The Finnigan 4000 is a fully integrated GC/MS system with a short distance between the GC and the ion source. The interface is built into a heated oven bhere the GC and MS are connected to each other. In the gas chromatograph, the columns are suspended in such a way that the end of the column is in a vertical position. The link with the horizontal interface is effected by a short, glass lined, stainless steel tube. At the end which is inside the GC oven, the glass capillary column is connected by means of a ‘/,,-inch Swagelok connection with a vespel seal. The other end is connected to the splitting device inside the interface oven. This junction can be made gas tight either with solder or with a simple ‘/,,-inch Swagelok fitting using a vespel or graphite ferrule. Facing the end of the glass lined tube, a glass capillary is installed which serves as the inlet line into the ion source with a suitable restriction. The dead volume in the open-split con-
nection is low because the distance between the two capillaries is only about 2 mm. The inlet line is mantled with a brass tube of inner diameter of 2 mm. The outer diameter of the inlet line is about 1.3 mm. Excess effluent gas, and helium, which is used as a flushing gas, flow through this intermediate space to the atmosphere. Helium is introduced into the intermediate space through a stainless steel tube which is soldered onto the mantel. The flow of the flushing gas is maintained at a level of between 10 and 15 cm3/min. To dilute undesired substances in the effluent, helium is also introduced through a bore directly into the space between the two capillaries using another soldered metal tube. The scavenger gas which is added, is regulated using a pressure controller. Using a solenoid valve or a toggle valve, the scavenger gas flow can be switched on or off with a rapid response. To avoid diffusion of substances into the scavenger gas line, it is recommended to maintain a low level of scavenger gas flowing permanently. This can be achieved by installing a small by-pass at the on/off valve. To ensure that the helium is pre-warmed to the interface temperature. both metal lines are coiled for 20 cm prior to the interface. The coils are assembled with the other parts in the interface oven. The glass capillary which serves as the inlet line lies freely in the interface oven. Its vacuum tight connection to the ion source inlet is effected using a ‘/,6-inch Swagelok fitting and a vespel ferrule. This restriction capillary can easily be made on a commercial drawing machine (Hewlett-Packard, Avondale, Pa.). We used Pyrex glass tubing with an external diameter of 6.0 mm and an internal diameter of 0.57 mm. The drawing ratio was adjusted empirically to produce a capillary with an internal diameter of less than 0.1 mm and an external diameter of 1.3 mm. The capillary which gave the required restriction was determined by measuring the flow of helium through pieces 27 cm long, under vacuum, using a soap film flow meter and a low vacuum pump. For our purposes, we selected a Pi-cm long capillary which allowed a flow of 2.5 cm3/min at room temperature. This figure is equivalent to a flow of 1.5 to 1.8 cm3/min at operational temperatures. A t this level, our mass spectrometer exhibits good operational performance. Deactivation of the restriction capillary can be effected using one of the various methods given for use with glass capillary columns. We used the carbowax treatment method ( 4 ) .
RESULTS AND DISCUSSION T h e new all-glass interface can easily be made in a laboratory without any special equipment, other than a glass drawing machine. Despite the fact t h a t the device is almost entirely constructed of glass, it demonstrates good mechanical
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stability. This is particularly important for the restriction capillary because a break at this point would lead to a collapse of t h e high vacuum conditions in the mass spectrometer, together with the possible damage of the filament and other oxygen sensitive parts. Restriction capillaries, made according to the instructions given in the Experimental section, show remarkable mechanical stability and flexibility. We have used these capillaries continuously with our MS for the past ten months without any breakage occurring. We can therefore see no reason to use platinum capillaries instead, merely because of their superior mechanical properties. Comparison of glass and platinum capillaries illustrates further advantages of glass capillaries other than the obvious better deactivation characteristics. Platinum capillaries are only available with fixed internal diameters. T h e restriction depends on the length and t h e internal diameter of the capillary. However, t h e length of t h e inlet line is laid down by the geometrical arrangement of t h e equipment. T h e sensitivity of t h e MS measurements is particularly dependent on the influx of the carrier gas in trace analysis. When platinum capillaries are used as inlet lines, the optimal influx is usually achieved by pinching the capillaries. This is a difficult procedure and t h e results achieved are difficult t o reproduce. Glass capillaries of a desired restriction are usually constructed using the drawing machine. Using the drawing ratio, a capillary is constructed with a calculated internal diameter of ca. 0.1 mm. During the drawing process, a piece of suitable length is cut from t h e continuously produced capillary and the flow of helium through this piece is measured using a soap film flow meter and a low pressure vacuum pump, (filter p u m p ) . Without interrupting the drawing process, the drawing ratio is adjusted to give capillary tubing of the required internal diameter. Because of the relationship between t h e gas viscosity and t h e temperature, t h e restriction is also affected by changes in temperature. T h e operational conditions must be borne in mind when constructing a capillary of this nature. Approximate calculations can be made with equations given in the appropriate text books for capillary gas chromatography. As a practical indication, we have given the flow rates of one of our restriction capillaries. Within the limits of accuracy of t h e simple measuring methods that we used, we obtained t h e following data: 2.5 cm3/min a t 25 "C a n d 1.7 cm3/min a t 220 "C. T h e merits of the all-glass interface for use in trace analysis of labile substances would appear to be obvious. In our laboratory we have experienced a great deal of difficulty in analyzing silylated estrogenic drugs used in animal production. Using the new interface, the detection limit improved by a factor of ten. B u t in its present state, the all-glass interface is not yet reliable because of t h e apparent adsorption of sensitive substances. However, there seems to be a good chance of improving t h e situation by optimizing t h e deactivation procedure. T h e production of the restriction capillaries in one's own laboratory allows one to use a variety of sorts of glass. All the well established methods for deactivating and coating glass capillaries can be used. T o avoid difficulties arising from the use of different coating materials in t h e column and the interface, both can be coated using the same procedure. I n this way the restriction lines can be adapted for use with various columns without any great difficulty, because t h e interchange of restriction lines is a simple procedure taking only a few minutes. Further advantages of using glass as the capillary material are the relative ease by which plugs or spoilage are detected and the much lower cost of glass in comparison to platinum. The following report briefly explains the advantages of using our open-split device in t h e residue analysis of biological or
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Figure 2. G U M S run of an extract from meat demonstrating the venting of the largest peak food samples. All these samples contain a great many substances from the matrix which lead to a rapid decrease in the sensitivity of the mass spectrometer if they are allowed to enter the ion source. Using the direct connection, there is no way of preventing the contaminating substances from entering the mass spectrometer. T h e open-split interface with a scavenger gas line makes it possible to protect the ion source from most of the spoilage substances present. As a rule, t h e solvent peak is always vented. Furthermore, the large quantities of substances co-extracted from the matrix of the biological sample are diluted. This may be effected by various methods. T h e simplest method is that in which the operator continuously observes the GC/MS run via the total ion current or the signal of the integrated mass spectra which result from t h e cyclic scans. T h e operator switches the on/off valve to open the scavenger gas line every time t h e signal exceeds a certain intensity. This brings about an immediate dilution or total venting (depending on the scavenger gas flow) of the substances in the peak, as shown in Figure 2. After a short time, which corresponds approximately to the peak width a t a given retention time, the scavenger gas is switched off for a moment to see if the undesired substance has passed. If this is in fact the case, the run continues further without dilution until the next peak of undesirably high concentration appears on the recorder or on the oscillographic screen. After a short training period, the operator is able to cut the peaks of high concentration in such a way that the mass spectral scans can be obtained both from t h e ascending and t h e descending part of t h e peak which may contain sufficient structural information to identify the substances in t h e diluted peak. T h e use of a pressure controller with a gage together with a n on/off valve to effect a rapid response are prerequisites for sharp peak cutting. The scavenger gas stream is operated a t a pressure of 0.1 to 0.5 bar to obtain t h e desired dilution which is determined empirically. If only a few substances,
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 14, DECEMBER 1G78
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e.g., the anabolic drug residues in meat, have to be estimated, the whole of the gas chromatographic run can be vented with the exception of short sections in which the substances to be analyzed are expected. T h e merits of this procedure are demonstrated in Figure 3. Working in this way, a lot of biological samples can be analyzed without any consequent loss in sensitivity. T h e new device offers a very important application in the trace analysis of samples which have to be derivatized prior to the GC/MS run. Until now it was necessary to remove the excess of derivatizing agent from t h e sample to avoid contamination of the ion source, which would have led to a rapid decrease in sensitivity. Major problems arise with t h e widespread use of silylation. Silylating agents are known to contaminate the ion source with a consequent drastic deterioration in sensitivity, T h e excess reagent is usually removed by drying the sample in a reaction vial under a stream of dry nitrogen. T h e sample is then dissolved in a dry solvent and injected into the gas chromatograph. T h e problem is t h a t most of the silylated derivatives are labile and are rapidly hydrolyzed in the presence of traces of water. Therefore, in t h e gas chromatography of silylated
compounds, it is good analytical practice to keep the sample in an excess of silylation reagent and t o always inject t h e mixture directly into the GC. Using t h e open-split interface with t h e scavenger gas line the same procedure can be applied in the G C / M S analysis of silylated compounds. It should be mentioned that silylation is only one prime example of derivatization in GC/MS analysis. Similar results are achieved with t h e various acylation and alkylation methods. Furthermore, on-column derivatization methods can now be adapted to GC/MS.
LITERATURE CITED (1) K . Grob, Chromatographia, 9, 509 (1976). (2) D. Henneberg, U. Henrichs, and G. Schomburg. J . Chromatogr., 112, 343 (1975). (3) D. Henneberg, U.Henrichs, and G. Schomburg, Chromatographia,8. 449 (1975). (4) E . Schulte, Chromatographia, 9 , 315 (1976).
RECEIVED for review August 7,1978. Accepted September 12, 1978. Financial support by the Bundesminister fur Jugend. Familie und Gesundheit and Deutsche Forschungsgemeinschaft is gratefully acknowledged.
