Abstract:
A method and device for separating trace amounts of relatively large molecules in the gas or vapor state from a low molecular weight gas or vapor is disclosed. The method and device are particularly useful for interfacing a gas or liquid chromatograph to a mass spectrometer. The device comprises an enclosed chamber to which a vacuum system is attached. Two sample transfer columns are inserted through opposite sides in the chamber. The first column has an outer diameter smaller than the inner diameter or bore of the second column and the end of the first column may be moved into the bore of the second column. A second vacuum source is applied to the second transfer column. When a sample mixture in the gas or vapor state passes from the first column into the second column, the fractional split in the bulk or carrier stream between the two vacuum systems is accurately controlled solely by the diameters of the two columns and the extent to which the first column in inserted into the second. Also depending on the depth of insertion the larger trace sample molecules may be caused to preferentially pass into the second column and vacuum system. In general, sample enrichment of a gas stream can be accomplished at the expense of sample recovery. For chromatography/mass spectrometry applications trace sample recovery may be kept at acceptable levels while maintaining high vacuum conditions in the mass spectrometer.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a variable molecular separator and more particularly pertains to an interface between a gas chromatograph (&#34;GC&#34;) and a mass spectrometer (&#34;MS&#34;). GC&#39;s and MS&#39;s are used to analyze chemical properties of various substances. In addition, both usually have a common physical feature: the sample(s) may be carried through each analyzer in a continuous stream by an inert carrier gas (usually helium). Also the temperature throughout the system is high enough so that the sample(s) are (should be) in a gaseous state even though the original sample is typically a liquid or solid dissolved in a liquid solvent. 
     BACKGROUND OF THE INVENTION 
     An interface between a GC and MS is usually required since the carrier gas flow rate through a GC is relatively high and emerges from the GC at atmospheric pressure; while the MS must operate under high vacuum conditions and as a result has a relatively low intake capacity for carrier gas. An additional important requirement is that air from the atmosphere must be excluded from the entire system, especially the MS, in order to ensure good instrument performance. The main objective is to combine the sample separating capabilities of the GC with the sample identification capabilites of the MS into one continuous analytical operation with a minimum decrease in the performance levels of the combined instruments operating separately. This objective has not been fully realized. In general, quantitative analysis is not as sensitive nor as reliable with combined GC/MS analysis as with stand alone GC analysis. The reproducibility, accuracy and level of detection for the analysis of trace components or constituents of a given sample mixture has been sacrificed somewhat in order to be able to identify each component by the use of the MS. 
     The reason for this deterioration in performance, aside from some differences in the method of measurement, is that the effects of any GC/MS interface on the relative amounts of sample or carrier gas which passes through it are not well defined and are sensitive to slight variations in the experimental pressure/flow conditions for a given analysis. 
     Interface performance may be conveniently characterized by three definitions: (1) the percent transmission of sample through the interface or the &#34;percent yield;&#34; (2) the percent transmission of carrier gas or the &#34;split;&#34; and (3) the ratio of (1) to (2) or the sample &#34;enrichment factor.&#34; An obvious objective for any analysis is a 100% yield. A low split may be compensated for by a high enrichment factor. Also, the GC may be required to operate under low flow conditions in which the constituents of the sample are not separated from each other as well as they might be for stand alone GC analysis. 
     There are sevral methods currently used to interface a GC to an MS. One solution is the &#34;direct connect&#34; method. There is no actual interface device. Instead, the interface is eliminated and the output end of the GC column is inserted directly into the MS. The &#34;direct connect&#34; method when applicable, shares with the present inventions the capability of reducing the pressure at the end of the GC column to such a low level that a simplified pressure/flow relationship applies and the GC flow rate becomes independent of the pressure at the end of the GC column. This is important for reproducible quantitative analysis as discussed previously. However, this poses several problems. Most important, the flow rate of the sample through the GC column is usually too high for the MS to handle. This requires the GC to operate at a lower than optimum flow rate, which results in broader peaks on the strip charts produced by the GC. In addition, this type of connection makes it inconvenient to use either the GC or MS alone. 
     One further shortcoming of the direct method is that solvents in which the sample mixture is dissolved and also other major sample components may interfere with the MS analysis of the remaining sample components. While various types of interfaces can divert these objectionable sample components from the MS without disturbing the continuous GC/MS analysis (usually via a diversion valve), this is not possible with the &#34;direct connect&#34; method. 
     Another method of interfacing a GC to an MS is via an open split interface. The interface is essentially an enclosed chamber into which both the GC and MS columns are inserted into opposite ends along the same axis. An important feature of this interface is the very narrow bore of the MS transfer capillary, the impedance of which determines the fixed flow rate to the MS, provided atmospheric pressure is maintained in the interface. The interface also contains input and output make-up gas columns (vents). The excess flow from the GC exits through the output column. If the GC flow rate happens to be too low, make-up carrier gas must be added through the input column in order to maintain a constant MS flow rate and, more importantly, to exclude air from leaking in through the output vent and into the MS. 
     Several problems, however, remain with the &#34;open split&#34; interface. The MS transfer column is generally an uncoated glass surface which effects the flow of sample in a different manner than the surface of the GC column. This effect is in addition to chemical interaction with highly reactive sample compounds (which is minimized by the use of relatively inert fused silica glass). The physical adsorption of the sample on the glass surface, unlike the liquid film surface of the GC column will effect the flow of the sample in a non-uniform and unpredictable manner. Condensation on the glass surface can also cause broad, skewed (non-symmetrical) sample peaks as detected by the MS. This effect becomes particularly important for very large molecular weight compounds which have a far greater tendency to condense at the maximum temperature of the interface. The inability to analyse chemically important large molecules is a critical restriction on GC/MS applications. 
     Further problems with the open split interface make it difficult to stabilize interface parameters, thus effecting the reproducibility of an analysis. If the GC flow is greater than the MS flow, the flow is split (the excess flow exits through the output vent), which results in sample loss. Although sample loss alone is not a critical problem in most instances, the open split interface does not provide a means for controlling the flow split. The flow split is dependent primarily on the GC flow rate. The MS flow rate is dependent on maintaining a fixed pressure in the interface. This requires that adjustments be made in make-up (input) gas flow and output vent gas flow for different GC flow rates. 
     In addition, if air is to be effectively purged from the interface by the output flow of the make-up gas, then the output vent must be relatively small or, alternatively, the flow must be relatively high, either of which increases the interface pressure to well above atmospheric levels. Since both the MS and GC gas flows are dependent on the square of the pressure in the interface (the GS flow goes down and the MS flow goes up with increasing pressure), the net result is an unstable condition which must be &#34;tuned&#34; by trial and error for each GC analysis. The problem is further complicated by the fact that the GC flow normally decreases by a gradual but significant amount during the course of analysis because of an increase in the viscosity of a gas as its temperature increases. Thus, interface parameters may require adjustment depending on the conditions for the GC analysis and as a result of normal changes of GC flow rate during the analysis. Altogether, there is a high probability of having significant differences in GC/MS flow parameters from one analysis to another. 
     A jet separator, such as that disclosed in Bradley, U.S. Pat. No. 3,936,374, can also be used to interface a GC to an MS. The jet separator is similar to the present invention except that a jet separator requires a short restrictor capillary or &#34;jet&#34; to be used at the end of the GC column in order to produce a supersonic jet in the interface between this restrictor and an almost equally narrow MS column across a very narrow gap. As a combined result of this very high linear velocity and the very high diffusion rate of the low molecular weight carrier gas into the interface chamber, a greatly reduced flow of carrier gas which has been &#34;enriched&#34; in the relative concentration of sample enters the MS column. 
     This has been the traditional GC/MS interface for high GC flow analysis, where the GC flow rate far exceeds the capacity of the MS. More recently with the development of much smaller capillary GC columns, high flow rate GC analysis is no longer required, though typically the flow rate is still too high for most MS instruments. However, the enrichment mechanism does not function at these relatively low flow rates. This problem may be solved by the addition of make up gas just in front of the interface in order to restore the enrichment mechanism to the extent that the net result may be a sample yield of roughly 40 to 60 percent depending on the sample molecular weight, with an optimized GC flow rate and an MS pressure at an acceptable level. The enrichment factor may then be at least approximately independent of the GC flow rate. 
     The primary problem with the jet separator interface is again, the active surface of the interface, which may have unpredictable interactions with reactive sample components and cause condensation of very high molecular compounds which results in skewed or asymmetric peaks. Also, isolation of the two instruments is not easily accomplished although there is usually solvent diverting capability. Nor is there a reliable or accurate mechanism to adjust carrier gas splitting in the interface. 
     Another significant problem with a jet separator interface, as with the &#34;open split&#34; interface, is that the relatively high exit pressure of the GC column still affects the GC flow rate and leads to unstable and difficult to reproduce pressure/flow conditions in the interface. The reliability of quantitative capillary GC/MS analysis with a jet separator, as with the &#34;open split&#34; interface, has not been clearly demonstrated. 
     OBJECTS OF THE INVENTION 
     One object of the invention is to provide a means by which the split of the GC carrier gas between the MS and an auxillary vacuum system (sepvac or SV) is conveniently and accurately controlled. 
     Another object of the invention is to provide a means for sample enrichment of the GC carrier gas stream passing through the interface into the MS. 
     Another object of the invention is to eliminate the problem of sample &#34;hold-up&#34; via condensation or chemical interaction between the sample and the surface of any interface between a GC and an MS. 
     Another object of the invention is to provide a means by which the percent composition of trace constituents in a natural or synthetic gas mixture may be quantitatively increased to more measurable levels. 
     Another object of the invention is to provide an interface for which the physical variables such as pressure and flow rate, throughout the GC/MS system including the MS ion source, may be conveniently and accurately monitored and controlled during the course of an analysis. 
     Another object of the invention is to provide a means by which either the GC or the MS may be operated independently of the other, including the ability to allow samples to be directly introduced into the MS ion source. 
     Another object of the invention is to provide a design for a versatile interface, which can accommodate condensable carrier vapors (principally water) and much larger more condensable samples, and can thus provide a means for interfacing a liquid chromatograph to an MS for a continuous analytical operation. 
     Another object of the Invention is to provide the means along with MS analysis, by which the actual flow path or trajectory followed by sample molecules emerging from a column into a vacuum chamber under sub-sonic flow condition may be determined and controlled. 
     Still other objects and advantages of the invention will become apparent to those of skill in the art after reading the following description of a preferred embodiment. 
     SUMMARY OF THE INVENTION 
     The invention solves many of the problems of other interface methods noted above. Most importantly, the flows for both the GC and MS can be maintained at the optimum rates. The flow split can also be controlled very accurately. Since the sample that enters the MS column never enters the interface chamber and contacts only a chromatographic surface, the sample hold-up which results from chem-adsorption and condensation on interface surfaces is eliminated. These factors allow various parameters (e.g., flow rates and pressures) to be calculated. Also, the GC and MS columns can be easily removed to allow relocation of the two instruments if necessary. 
     The invention essentially comprises an enclosed chamber to which an auxillary (sep vac) vacuum system is applied. The input (GC) column and the output (MS) column are inserted through the chamber and the ends of the columns are in substantial axial alignment with each other. It is of crucial importance that the outside diameter of the input (GC) column be smaller than the inside diameter (bore) of the output (MS) column. For a variable form of molecular separator it is also important that one of the two columns be able to move along the axis of the other so that the input column can be inserted into the output column. Controlling the depth of insertion allows one to control the &#34;split&#34; of the input flow to the output column and also to control the level of sample &#34;enrichment&#34; of the carrier gas flowing to the output column. In the ideal configuration under ideal GC/MS conditions the sample enrichment will nearly compensate for the carrier gas split from the MS so that the net sample loss is small in spite of the greatly reduced flow to the MS. Thus, measurable amounts of the sample will enter the MS with the MS operating at acceptable pressure levels and with the GC operating at the high flow rates necessary for efficient separation of many sample constituents. 
     In effect, a system is created whose performance may be equivalent or superior to a direct connection to the MS without any of the disadvantages associated with this mode of GC/MS analysis. 
     An additional feature of the invention is that with the input column at any position within the output column, the sep vac flow can be stopped or reduced to a very low level so that a system equivalent to a direct connection is quickly produced. 
     An additional feature of the invention allows the flow rate and pressure at various stages of the system to be determined. In particular, the pressure in the interface itself and the pressure in the MS ion source can be calculated, which provides greater consistency from one analysis to another. 
     DESCRIPTION OF THE INVENTION 
     The device comprises an enclosed chamber to which a vacuum source (sep vac) is applied. Sample transfer columns are inserted through the chamber, the ends of which are in substantial axial alignment with each other. The input column (usually a GC column) has an outer diameter which is smaller than the inner diameter of the output transfer column (usually an MS column) and the end of the input column may be inserted into the bore of the output column. A second vacuum system (normally an MS) is applied to the output transfer column. When a gas flows through the input column, the fractional split (the percent transmission) of the gas flow between the two vacuum systems is dependent only on the relative dimensions of the connecting columns, independent of the actual size or capacities of the pumps involved. That is, both pumps are large enough so that the flow split is determined solely by the relative flow &#34;impedance&#34; of the columns or tubes between the pumps and the split point. With this configuration and the added condition that the pressure in the interface chamber is maintained at a level sufficient for molecular flow (a level which can be easily attained for reasonable GC flow rates) the split will be reproducibly and predictably determined by the annular space between the concentric transfer columns and the depth of insertion. 
     Another important dimension in determining the sample transfer characteristics of the interface is the overall length of the output column. A compromise must be made in choosing the length of the output column. If it is very short, a more accurate method of controlling the depth of insertion must be used. In GC/MS applications a very long MS column can cause slow passage of high molecular weight compounds, resulting in line broadening on the MS strip chart. Also the outside diameter of the GC column and the optimum GC flow rate must be taken into consideration. Furthermore, the inside diameter of the MS capillary may be changed. In any case, all of these factors can be controlled and accounted for in order to achieve optimum performance. 
     The interface should be designed so that the percent carrier gas transmission is approximately zero when the input column is just barely withdrawn from the output column so that a simplified Poiseuille equation applies and an accurate calibration curve can be constructed. This may be accomplished (for a given annular clearance) by having a relatively long MS column or by increasing the sep vac pumping capacity. Very long MS columns must be avoided since peak broadening for very large molcules will increase with the residence time in the interface. However, shorter MS columns will require more precise control of the depth of insertion of the GC column for reproducible interface performance. This will be particularly important for control of sample enrichment which is at a maximum at a point near the point of insertion. 
     Any of several mechanical means of changing the column positions is possible depending on the accuracy required and the degree to which pressure disturbances or atmospheric contamination can be tolerated by the system. With a bellows/micrometer drive mechanism, accuracy and resolution on the order of 0.05 mm is possible with no pressure disturbance at all. The limited range (about 1.5 cm) of this mechanism can be offset by the use of a shorter output column. A sliding seal method of moving the input column using a soft ferrule results in only a slight increase in the sep vac pressure with no measurable effect on the second (MS) vacuum system. 
     Another principal feature of the invention is that any secondary gases or vapors which are of such low concentration (about 5%) that the physical properties of the bulk gas are essentially unchanged may be caused to preferentially pass into the second column and vacuum system (the MS) at a level much higher than that of the carrier gas, thus producing &#34;enrichment&#34; of these trace constituents. This effect is the same whether the trace components are &#34;contaminants&#34; of the bulk gas and are enriched continuously or are in the form of discrete &#34;peaks&#34; in a &#34;gas chromatogram&#34; as a result of the prior separation in the GC column of a sample injected at the head of the GC column. This molecular enrichment effect is also controlled by the depth of insertion of the input column into the output column. Three characteristic configurations can be described with respect to the relative positions of the transfer columns. (See description of FIG. 1, below.) 
     When the input (GC) column is completely withdrawn from the output (MS) column, the split is so small (typically less than 0.1%) that the gas stream is effectively diverted from the second (MS) vacuum system. The enrichment factor is nearly one, though it rises gradually and then more sharply very near the output column orifice. The enrichment factor is defined as the ratio of the sample concentration at any given point compared to the sample concentration in the bulk gas or in the case of GC analysis within the GC &#34;peak&#34; prior to entering the interface. This reference value, taken as one (1.00) is the ratio of the sample to carrier gas concentration as measured by the MS detector response, when the GC column is removed from the MS column to such an extent that there is no preferential capture of sample molecules by the MS column (there are no molecular beam effects). This is noted experimentally by a gradual decrease to a constant minimum value as the GC column is withdrawn. 
     When the end of the input column is positioned just at the orifice of the output column, the split is still small (4% range) but the molecular enrichment factor is at a maximum. When the input column is inserted into the output column, the split gradually increases to 100% as the input column is fully inserted into the output column. There is a corresponding decrease in the enrichment factor to a value of one at the 100% insertion position. 
     The enrichment mechanism of this invention is entirely different from that of the traditional &#34;jet separator&#34; since the pressure/flow conditions under which it occurs are significantly different. Although the exact mechanism for the molecular enrichment effect in the jet separator in uncertain it is generally assumed that diffusion is the controlling factor. This is consistent with the requirement of viscous flow conditions and a supersonic linear velocity across the separator gap that has been noted experimentally. In the present invention there is no restriction to the flow at the end of the input column (the cross sectional area of the annular space between the columns is larger than that of the input column itself), thus the flow is effusive and subsonic. Also, simple geometry considerations dictate that diffusion cannot be a significant factor when the input column is inserted into the output column. 
     The mass flow rate (&#34;Q&#34;) of all gases at relatively high pressure through a circular tube is determined by the following form of the Poiseuille equation. This applies to all GC and GC/MS analysis. ##EQU1## where: J is a compression factor equal to ##EQU2##  and P=P i  /P e  ; and where: 
     P i  is the input column head pressure; 
     P e  is the output (exhaust) column pressure; 
     r is the inner radius of the column; 
     l is the length of the column; and 
     v is the viscosity of the carrier gas. 
     With conventional stand alone atmospheric exhaust gas chromatographs, the mass flow rate may be calculated (although somewhat inconveniently) by this equation. With either the jet separator or open split separator, the output (exhaust) pressure P e  fluctuates around some value near atmospheric pressure and cannot be easily measured or controlled, making this equation useless for accurate flow rate calculations. 
     However, with the current invention, the above Poiseuille equation is simplified because (1) the exhaust pressure may be maintained at a very low value (10 torr or less) when P i  is well above atmospheric pressure (760 torr.) (See discussion of FIG. 5 below.) Thus J can always be taken as equal to 3/4 since the limit for J as P becomes very large is 3/4; and (2) since P e   2  is a always very small compared to P 1   2 , (P i   2  -P e   2 ) becomes equal to P i   2 . 
     Thus the GC flow rate using the present invention can be computed accurately as follows by a simplified Poiseuille equation: ##EQU3## independent of the exhaust (interface) pressure for all useful GC flow rates at a given temperature. This simplified equation can also be used to compute the change in the GC flow rate with temperature in a much simpler manner than is possible with the first equation. 
     Since the inner radius (r) is accurately known for capillary columns and very accurate capacitance manometer gauges for GC head pressure control are (now) standard equipment for most GC&#39;s, the above equation is both a convenient and accurate way to calculate the GC flow rate in a GC/MS system that uses the present invention. 
     In addition, the flow split can be determined solely by the column dimensions and the depth of insertion. The flow split or Carrier gas Percent Transmission at a given flow rate is determined by: ##EQU4## where: 
     F ms  and F sv  are respectively the pressure dependent conductance of the circular MS and annular SV sections of the output column whose separate lengths are independent on the depth of insertion of the input column into the output column. The flow split is given with good accuracy over all flow rates by: ##EQU5## where C sv  and C ms  are the pressure independent molecular flow conductances of the annular sep vac (SV) and circular MS columns which can be determined directly from the columns&#39; dimensions. ##EQU6## where k sv  and k ms  are constants which represent the rate of change of each conductance with the change in the interface pressure (P e ). (See FIG. 6.) 
     At very low GC flow rates and molecular flow conditions (&lt;0.1 ml/mm) the MS and SV conductances do not change with pressure and k ms  and k sv  =0 and K equals one. At higher pressures the rate of change for the flow conductances in both the MS and SV sections will be the same and K will still be equal to one. 
     The basic equation above can be used in a slightly different form to apply to any &#34;T&#34; splitter in which the lengths of the two capillaries may be varied: ##EQU7## where H is a constant determined by the radii of the capillaries and l ms  and l sv  are the lengths of the MS and SV capillaries, provided the exit pressures for both capillaries are small compared to the GC column exit pressure (P e ). The present invention may then be considered as a special case of the &#34;T&#34; splitter. Obviously an important and critical difference for this invention is the mechanical simplicity of the design which results from the concentric alignment of one leg of the &#34;T&#34; within the other, and which also provides and insures the locked interdependence of C sv  and C ms  since the sum of the lengths of the two capillaries must always be the same (L=l ms  +l sv ). An important related advantage is that l ms  or l sv  (along with the flow impedances, l/C sv  or l/C ms ) can be easily reduced to zero. 
     These very useful characteristics would be mechanically very difficult to duplicate in any other way. Mechanical simplicity comparable to the present invention can be achieved with a conventional &#34;T&#34; splitter only by including a variable conductance valve in the SV capillary. However, valve adjustments will cause correspondingly large changes in the value of K for the splitter. In general, for circular tube splitters K will be equal to one (1) for molecular flow conditions and then vary in a non-linear fashion between one and k sv  /k ms  for an approximately 100-fold increase in pressure above this level. At higher pressures the value of K will remain constant but the GC column exit pressure (P e ) then becomes so large that it begins to affect the GC flow itself as discussed previously. 
     The design of the present invention introduces what might at first seem to be a complication on this basis since the MS column has a circular cross-section, while the SV column an annular cross-section whose K value would be expected to be even less than the corresponding circular column (See FIG. 6). This would introduce an even greater error in the calibration curve when the value for K (K=K su  /k ms ) is assumed to be one. 
     However, it has been determined that over a pressure range well beyond the molecular flow level and for reasonable annular cross-sectional areas (within reasonable mechanical alignment tolerances) the rate of change of the annular SV conductance with pressure is the same as that for the circular MS capillary (k ms  =k sv ), and that the curve (A) in FIG. 6 represents both the MS and SV columns in the case of the present invention. 
     This slope (k sv ) is apparently determined by the radius (a 2 ), common to both MS and SV columns, and not by the annular separation (a 2  -a 1 ) as might be predicted (See FIG. 6). 
     Thus K is one (1) for the present invention for both molecular flow and non-molecular flow conditions and the same simplified equation for the carrier gas transmission holds for all reasonable capillary GC flow rates. ##EQU8## where: A=a 1  /a 2 , A&lt;1 
     L--the length of the MS capillary; 
     CEP is the GC column exit position; 
     a 1  is the outside radius of the GC column; 
     a 2  is the inside radius of the MS column; 
     (See FIGS. 3 and 4) 
     As CEP approaches its maximum value L the ratio (L-CEP/CEP) becomes very small and CPT approaches 100%. As CEP approaches its minimum value zero the above ratio becomes very large and CPT approaches zero. Negative CEP values (Case 1) may produce very small CPT values. It should be noted that perfect alignment of the GC column within the MS column along the entire column length is not a requirement. In the present invention, considerable variation of the relative position of the two capillaries will occur along the insertion length since both MS and SV capillaries are flexible fused silica glass. 
     In FIG. 6 a graphical method of determining the values of k ms , k sv  and K for a circular &#34;T&#34; splitter equivalent to the present invention is demonstrated. The curve A for the MS capillary applies to the present invention as well. 
     The curves (B) are for circular SV capillaries whose molecular flow conductances are the same as for the SV conductances resulting from the use of typical capillary GC columns with the present invention. 
     A further condition is that the column exit pressure is very low both when the GC column is fully inserted into the MS source and when fully withdrawn from the MS column (Case 1). There are two reasons for this. First, this insures that the CPT is 100% and zero percent in these two positions to a significant figure equal to the accuracy desired for a calibration curve. (FIGS. 3 and 4) Second, and most important, it is necessary that these pressures are small compared to the column exit pressure (P e ) for any intermediate position so that the GPT equations apply independent of these pressures (a similar argument as for GC column flow applies). This requirement is easily accomplished from the prior requirement of a relatively large SV vacuum pump and low impedance connecting tubing to the separator. On the MS side this requirement will more or less coincide with the requirement of low MS source pressure for satisfactory operation of the MS. 
     Thus, a complete calibration curve may only be obtained with relatively low GC flow (1 or 2 sccm) and the flow capacity of the particular MS will determine the maximum CPT allowed for a given GC flow rate both from the standpoint of split accuracy and satisfactory MS performance. 
     While the exact pressure at the end of the input column can be determined (FIG. 5), it is not necessary to control or determine this pressure exactly since both the input column flow rate and the split are independent of the input column exhaust or interface pressure under the proper conditions as previously discussed. However, the pressure at some arbitrary point in the interface exhaust tubing (this is usually considerably less than the pressure at the end of the input column) can be easily measured by an accurate capacitance manometer or other gauge. This pressure will be directly proportional to the mass flow rate to the SV pump provided molecular flow conditions are maintained in this tubing. This requires a large connecting tube dimension (ID/length) and a high pumping capacity. This pressure is then related to the MS source pressure through the value of the flow split. 
     Under these conditions, the change in the gauge pressure over the entire split range from zero to 100% SV flow will permit the split to be accurately calibrated over this range. With an accurate value for the GC flow rate the actual pressure in the MS source may then be easily determined. 
     The high vacuum ion gauge in the MS vacuum chamber may also be used to calibrate the flow split at the same time, but the capacitance manometer is more reliable, accurate and independent of the particular gas species. Also, if a double sided capacitance manometer is used in the SV exhaust tube, the differential pressure drop across a calibrated flow element may be used to determine directly the absolute mass flow rate of any carrier gas over all reasonable GC/MS flow rates. 
     When analyzing samples, especially very high molecular weight organic compounds, it is useful that the output transfer column be coated (preferably chemically bonded) with a thin liquid film with mass transfer or chromatographic properties similar to the coating of analytical GC columns. Thus, the sample will only be in contact with a &#34;chromatographic&#34; surface from the point of injection all the way to the MS. 
     The width and shape of the sample peak entering the MS may then be predictably controlled by controlling the temperature of the system and the flow rate through the system. The unpredictable and non-uniform (non-reversible) adsorption properties of glass and metal surfaces are thus avoided. 
     An isolation valve which allows the MS system to be operated without actually disconnecting the MS from the source of the gas flow (GC) is easily incorporated into the interface design. If the stainless steel column through which the GC column slides is reasonably long, it is sufficient that the GC column is in place, in order to move the column without maintaning a tight sliding seal and to shut the valve and then also to remove the GC column entirely with the valve shut; depending on the first vacuum system (sep vac) to maintain a less than atmospheric pressure against the valve and the MS high vacuum. Thus, a valve which is an absolute seal against atmospheric pressure is not necessarily required although it is possible (with slight design modifications). This choice of different valves permits considerable flexibility in the interface design, depending on the overall requirements of a particular system. It is important to note that an isolation valve in this system does not compromise the closed GC/MS flow system since only flow diverted from the MS comes into contact with any surface other than the liquid film surfaces of the GC and MS columns. This is not true of other isolation valve systems. 
     When the invention is used as a GC-MS interface, an additional feature of the invention allows liquid or solid samples to be introduced directly to the MS without GC analysis. This is accomplished by removing the GC (input) column entirely, and depositing a thin film of the sample in the open end of a sealed short fused silica capillary. The capillary can be easily inserted through the interface directly into the MS source without disturbing the source pressure. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a cross sectional elevational view of one form of variable molecular separator according to the invention, which discloses various positions of the input column. 
     FIG. 2 is a cross sectional elevational view of another form of variable molecular separator. 
     FIG. 3 is a graph illustrating how the relative sizes of the columns and the depth of insertion of the input column into a 19 cm. output column effect the percent transmission of a carrier gas in accordance with the present invention. 
     FIG. 4 is a graph illustrating how the relative sizes of the columns and the depth of insertion of the input column into a 3 cm. output column effect the percent transmission of a carrier gas in accordance with the present invention. 
     FIG. 5 is a graph illustrating how the relative sizes of the columns and the depth of insertion of the input column into the output column effect the pressure of the gas at the end of the input column. 
     FIG. 6 is a graph illustrating how the pressure at the end of the GC column effects the conductance of the interface. FIG. 6 also illustrates the difference between this invention and a conventional circular tube &#34;T&#34; splitter. For this invention, pressure does not affect the flow split since the pressure effects the conductance of both legs of the splitter in the same way. 
     FIG. 7 is a calibration graph illustrating how the sep vac gauge pressure may be used to accurately measure changes in the undiverted GC flow rate and also to accurately calibrate the splitter. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows three relative positions of the input column 17 and the output column 16. In case I the input column 17 is completely withdrawn from the output column 16. In case II the ends of the two columns are aligned. In case III the input column 17 is inserted within the output column 16. 
     FIG. 2 discloses another version of the invention. The variable molecular separator contains a valve body 24 which serves to position the three main component parts of the interface, a bellows valve drive assembly described below, the first flange 19, and the second flange 22. The bellows valve drive assembly is composed of an upper valve drive housing 4 which is attached to a lower valve drive housing 32 by a bonnet nut 9. The upper valve drive housing 4 contains a leak detection orifice 5. A rotating valve stem 3 is threaded into the upper valve drive housing 4 via threads 31. A set screw slot 2 allows a valve drive handle 1 to be attached to the rotating valve stem 3. At the lower end of the rotating valve stem 3 is a sliding stem connection 6 which houses the upper end of a nonrotating valve stem 7. A bellows 8 is connected to the lower end of the nonrotating valve stem 7. The bellows valve drive assembly is attached to the valve body 24 by means of flange screws 12 with a silver gasket 13 placed between the valve body 24 and the bellows valve drive assembly. A separator vacuum tube 25 leads from the valve body to a vacuum pump. A sidearm 26 allows connection to a capacitance manometer pressure gauge. 
     A movable stem insert 11 is connected to the lower end of the nonrotating valve stem 7. By rotating valve drive handle 1, the stem insert 11 can be positioned between a first jet 21 and a second jet 18, or alternatively, the stem insert may be withdrawn to permit the gap between said jets 21 and 18, to be open. 
     A first flange assembly comprises a first flange 19, which holds a stainless steel capillary tube 20, and can accommodate a vespel ferrule 28 which is held in place by bonnet nut 30. The first flange assembly connects the first flange 19 to the valve body 24 via screws 14. The first flange assembly abuts a first jet 21 which can be screwed into valve body 24. A silver vacuum jacket 29 is placed between the first flange 19 and the valve body 24. The first jet 21 along with the stainless steel capillary tube 20 provides radial alignment of the input column 17 in the interface. The first jet 21 and stainless steel capillary tube 20 may be easily removed and replaced with a jet and capillary tube having different sized bores to accommodate various sizes of input columns 17. 
     The second flange assembly 22 attaches the valve body 24 to the MS body (not shown). A second jet 18 screws into the valve body and provides radial alignment of output column 16. The output column 16 is fitted to the second jet 18 via a vespul ferrule 27 and bonnet nut 33. A silver vacuum gasket 15 is placed between the second flange assembly 22 and the valve body 24. The second flange assembly 22 is attached to the valve body by screws 23. 
     FIG. 3 shows how the relative sizes of the columns and the depth of insertion of the input column into the output column affects the percent transmission of a carrier gas (CPT) into the output column in accordance with the present invention. The x-axis represents the input column exit postion (CEP) or how many centimeters of the input column are inserted into the output column. In this case, the output column is 19 centimeters long. The y-axis represents the percentage of carrier gas flowing through the input column that exits through the output column. The carrier gas not exiting via the output column is discharged through the sep vac. In the graph shown, an output column with an inside diameter of 0.053 cm was used. The four lines represent four different input column sizes. For line A, the outside diameter of the input column is 0.032 cm, for line B, 0.036 cm, for line C, 0.040 cm, and for line D, 0.045 cm. As the graph illustrates, as the outside diameter of the input column is increased while keeping the inside diameter of the output column constant, a higher percentage of carrier gas passes into the output column. Also, more carrier gas passes into the output column as the input column is inserted deeper into the output column. As the input column approaches its maximum depth of insertion (L) which is the length of the MS column, the ratio L-CEP/CEP becomes very small and the value of CPT approaches 100%. As the input column is withdrawn and CEP approaches zero, this ratio becomes very large and the transmission through the output column approaches zero. 
     FIG. 4, like FIG. 3, also discloses how the depth of insertion of the input column into the output column effects the percent transmission of a carrier gas into the output column. The lines A, B, C and D represent the same input column sizes as in FIG. 3. However, in FIG. 4, the output column is only 3 cm long, as opposed to 19 cm long. FIG. 3 shows that with a 19 cm output column length, a small change in depth of insertion of the input column, such as 1 cm, may not have a dramatic effect on the carrier gas percent transmission. However, the same 1 cm change has a dramatic effect on the carrier gas percent transmission if the output column is only 3 cm long, as in FIG. 4. Thus, the ability to precisely control the depth of insertion becomes more important as the output column length becomes shorter. 
     It is necessary that there is a a corresponding increase in the SV pumping capacity as the MS column is reduced in order to insure that there is sharp drop in pressure as the GC column is withdrawn from the output column. This insures that the carrier percent transmission (CPT) is essentially zero at this point and also that the equation for the CPT applies. If the output column length (MS transfer column) is reduced further the added impedance of the column orifice (edge effects) becomes significant and the theoretical CPT curve will be only approximate. A point by point calibration curve must then be constructed for each particular interface. 
     FIG. 5 discloses how the carrier gas pressure at the end of the input column changes as the input column is inserted into the output column. The lines A, B, C and D also represent the same input column sizes as in FIGS. 3 and 4. The relationship between the coluumn exit pressure (P e ) and the column exit position (CEP) is as follows: ##EQU9## Q is the input column mass flow rate; A and B are constants dependent on column radii; 
     L is the output column length; 
     D is a constant dependent on temperature. 
     In this equation (P e  *) gives an accurate value of the pressure only when the pressure is low enough for molecular flow conditions to exist. This pressure can be calculated from the input and output column dimensions. For many useful applications including GC/MS analysis the pressure will be above this level and P e  * will not be the correct value, said additional information will be required in order to determine the actual pressure. ##EQU10## where k is not equal to zero. In this equation, P e  * is the molecular flow pressure just discussed and k is the rate of change of the ratio of the actual conductance to the molecular flow conductance with the change in pressure (see FIG. 6) for the output column. Combining these two equations the actual pressure (P e ) can be determined as a function of the column exit position (CEP) as shown in the plot. The dotted horizontal line represents the line of demarcation between molecular and non-molecular flow conditions for these particular input and output columns. Below the line the first equation can be used to compute the pressure. Above the line the second equation must be used. It is assumed that k is the same for both MS and SV output columns as will be demonstrated later. 
     The graph in FIG. 5 shows that when the input column is positioned just at the end of the output column (the CEP is zero), the gas pressure is very low, since there is no restriction between the end of the input column and the separator vacuum. As the input column is inserted into the output column the pressure at the end of the input column increases quickly, and then decreases back to zero as it is fully inserted into the output column. This graph is important because it shows that even when the pressure at the end of the input column is greatest, the pressure still does not get high enough to require that the complex Poiseulle equation (noted above) be used to compute the input column flow rate. This is a marked difference from the jet separator type of interface, which requires a restriction at the end of the input column in order to function, which greatly increases the pressure at the end of the input column. 
     FIG. 6 discloses how the ratio between the actual conductance and the molecular flow conductance of the interface is dependent on the pressure at the end of the input column. As noted above, when molecular flow conditions exist, the carrier gas percent transmission (or &#34;split&#34; of the gas) is determined solely by the sizes of columns and the depth of insertion. Since these parameter are known, the split for molecular flow conditions can be easily calculated. However, when the pressure increases, the pressure itself effects the split, so the more complicated equation noted above applies. The plot in FIG. 6 also gives a clear illustration of the difference between the performance of this invention and the performance of a conventional flow splitter. An empirical equation is used which relates the actual conductance of a circular tube to its molecular flow conductance (F/C) as a function of the average pressure in the tube. The resulting curve which depends on the radius of the tube can be graphically reduced to a simplied form: 
     
         F/C=1+kP.sub.1 
    
     where P 1  is the head pressure and where K is the slope of the straight line portion of the curve and has the units of reciprocal pressure. In the molecular flow region k is equal to zero and F=C. At higher pressures the value of F may be determined for a given pressure (P 1 ) since C can be determined from the column dimensions. Thus, the percent transmission of all &#34;T&#34; splitters of which the present invention is a special case, using either circular or annular columns whose lengths are variable may be determined using the basic equation for determining CPT of &#34;T&#34; splitters. A circular tube splitter could (in theory) be constructed to perform identically to the present invention provided the pressure (P 1 ) (which in this case is the GC column exit pressure, P e ) is maintained in the molecular flow region. Circular SV columns would have to be used whose conductance was equal to the conductance of the corresponding annular SV column of the same length. This is achieved by using circular columns with the radius equal to a*. 
     
         a*=[(a.sub.2.sup.2 -a.sub.1.sup.2)(a.sub.2 -a.sub.1)].sup.1/3 
    
     where 
     2a 2  is the inner diameter of the MS column; 
     2a 1  is the outer diameter of the GC column. 
     At low molecular flow pressure levels K is equal to one (1.0) for both splitters. At higher pressures K is equal to one for circular column splitters only when the radii of both columns are the same which limits its usefulness considerably for most applications including GC/MS analysis. At intermediate pressures up to a 100-fold increase over the molecular flow maximum, K varies in a non-linear fashion between one and k sv  /k ms . At higher pressures K is constant and equal to k sv  /k ms , but these high column exit pressures begin to affect the GC flow rate itself. 
     For the present invention, it has been determined experimentally that the same equation for carrier gas transmission (CPT, FIG. 3) holds for pressures well behond the molecular flow region and for GC columns of different outside diameters. Thus it may be reasonably concluded that for all useful GC/MS flow rates using typical size GC columns, k ms  =k sv  and K=1 using the present invention as a GC/MS interface or as a gas stream splitter in general. 
     FIG. 7 shows the undiverted GC flow rate plotted against the SV gauge Pressure (P o ). The GC flow rate is calculated by the following equation as discussed previously: ##EQU11## 
     A straight line plot indicates that this equation is valid and also that the gauge pressure P o  may then be used to generate the calibration curve which is a check on the theoretical curve (FIGS. 3 and 4). A corresponding calibration plot may be generated with the MS high vacuum ion gauge which may also be used to calibrate in the very low transmission range when the GC column is completely withdrawn. 
     The pressure P o  is the corrected pressure; that is, the gauge pressure minus the gauge pressure when there is no flow. 
     Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limiting sense.