WO2000019193A1

WO2000019193A1 - Split flow electrospray device for mass spectrometry

Info

Publication number: WO2000019193A1
Application number: PCT/US1999/022323
Authority: WO
Inventors: Inc. Varian; Gregory J. Wells; Roger C. Tong; Peter P. Yee
Original assignee: Varian Inc
Priority date: 1998-09-28
Filing date: 1999-09-27
Publication date: 2000-04-06

Abstract

An electrospray apparatus for producing gas-phase ions from a liquid sample matrix. The electrospray needle (or capillary) (202) includes a main flow conduit through which the sample containing solution flows at a high flow rate (i.e., higher than which an electrospray tip can effectively ionize without nebulization assistance). A sampling tube with an electrospray tip intersects the main flow conduit. The sampling tube continuously pulls in a portion of the fluid sample from the main flow conduit. The flow through the sampling tube is formed into an electrospray (204) and provided to a mass spectrometer or other instrument. The inventive structure causes a portion of the main flow (200) to be split off prior to electrospraying, with the split flow rate being independent of the input flow rate through the main conduit. This permits setting the flow rate out of the electrospray needle to an optimized rate for input to a mass spectrometer, while permitting an input flow rate into the main flow conduit which is optimized for the output of a liquid chromatograph.

Description

SPLIT FLOW ELECTROSPRAY DEVICE FOR MASS SPECTROMETRY

FIELD OF THE INVENTION

The present invention relates to electrospray apparatus and methods, and more specifically, to a device which efficiently performs electrospray ionization at large liquid flow rates.

BACKGROUND OF THE INVENTION

Mass spectrometers have become common tools in chemical analysis. Generally, mass spectrometers operate by separating ionized atoms or molecules based on differences in their mass-to-charge ratio (m/e). A variety of mass spectrometer devices are commonly in use, including ion traps, quadrupole mass filters, and magnetic sector mass analyzers. The general steps in performing a mass-spectrometric analysis are: (1) create gas-phase ions from a sample; (2) separate the ions in space or time based on their mass-to-charge ratio; and (3) measure the quantity of ions of each selected mass-to-charge ratio. Thus, in general, a mass spectrometer system consists of an ion source, a mass-selective analyzer, and an ion detector. In the mass-selective analyzer, magnetic and electric fields may be used, either separately or in combination, to separate the ions based on their mass-to-charge ratio. Hereinafter, the mass-selective analyzer portion of a mass spectrometer system will simply be called a mass spectrometer. Ions introduced into a mass spectrometer are separated in a vacuum environment.

Accordingly, it is necessary to prepare the sample undergoing analysis for introduction into this environment. This presents particular problems for high molecular weight compounds or other sample materials which are difficult to volatilize. While liquid chromatography is well suited to separate a liquid sample matrix into its constituent components, it is difficult to introduce the output of a liquid chromatograph (LC) into the vacuum environment of a mass spectrometer. One technique that has been used for this purpose is the "electrospray" method. The electrospray or electrospray ionization technique is used to produce gas-phase ions from a liquid sample matrix to permit introduction of the sample ions into a mass spectrometer. Electrospray ionization has been used for providing an interface between a liquid chromatograph and a mass spectrometer. In the electrospray method, the liquid sample to be analyzed is pumped through a capillary tube or needle. A potential difference (of for example, three to four thousand volts) is established between the tip of the electrospray needle and an opposing wall, capillary entrance, or similar structure. The needle can be at an elevated potential and the opposing structure can then be grounded; or the needle can be at ground potential and the opposing structure can be at the elevated potential (and of opposite sign to the first case). The stream of liquid issuing from the needle tip is broken up into highly charged drops by the electric field, forming the electrospray. An inert drying gas, such as dry nitrogen gas (for example) may also be introduced through a surrounding capillary to enhance nebulization (droplet formation) of the fluid stream.

The electrospray drops consist of sample compounds in a carrier liquid and are electrically charged by the electric potential as they exit the capillary needle. The charged drops are accelerated in an electric field and injected into the mass spectrometer, which is maintained at a high vacuum. Through the combined effects of a drying gas and vacuum, the carrier liquid in the drops starts to evaporate giving rise to smaller, increasingly unstable drops which liberate surface ions into the vacuum for analysis. The desolvated ions pass through sample cone and skimmer lenses, and after focusing by a RF lens, into the high vacuum region of the mass spectrometer, where they are separated according to mass-to-charge ratio and detected by an appropriate detector (e.g., a photo-multiplier tube).

Although the electrospray method is very useful for analyzing high molecular weight dissolved samples, it does have some limitations. For example, commercially available electrospray devices utilizing only electrospray nebulization to form the spray are practically limited to liquid flow rates of 20-30 microliters/min, depending on the solvent composition. Higher liquid flow rates result in unstable and inefficient ionization of the dissolved sample. When used in conjunction with a liquid chromatograph, this acts as a limitation on the flow from the chromatograph. One method of improving the performance of electrospray devices at higher liquid flow rates is to utilize a pneumatically assisted electrospray needle. As shown in Figs. 1(A) and (B), one example of such a needle 10 is formed from two concentric, capillary tubes (elements 14 and 22 in the figure). In such a device the sample-containing liquid 12 flows through the inner tube 14 and a nebulizing gas 16 flows through the annular space between the two tubes. This improves the efficiency of the ionization process by improving the ability of the electrospray needle 20 to form drops 18 from the sample liquid. However, when operated at the high sample liquid flow rates typical for liquid chromatography, the drops formed are too large and degrade the performance of the mass spectrometer (by increasing the noise) if allowed to enter the device. This means such electrospray needles are still not capable of being efficiently used with a liquid chromatograph, which typically has a relatively high flow rate at its output.

Another conventional device utilizes a flow restrictor to control the portion of the main flow which is re-directed to a sampling needle. In such a device, the split ratio (the relative portion of the main fluid flow which is directed to the sampling needle) is variable, but once set it remains approximately constant over a wide range of input flow rates. Thus, as the input flow rate increases, the fluid flow rate through the sampling needle increases proportionally. At sufficiently high fluid flow rates into the main conduit, this can cause the output of the sampling needle to be greater than the optimal value for input to a mass spectrometer.

In addition to the noted disadvantages of conventional devices, there are several other disadvantages:

(1) the quality of the electrospray (i.e., the desired small particle size and uniform distribution) degrades as liquid flow rates increase. Pneumatic, mechanical, or thermal means are required to assist in nebulizing large liquid flow rates for electrospray. Finding the optimum operating conditions can be difficult, as the quality of electrospray is a function of multiple parameters (liquid flow rate, gas flow rate, geometry, and voltage);

(2) the efficiency of sample transport into the mass spectrometer decreases with increasing liquid flow rates. The background noise in the measurement due to large droplets increases with increasing liquid flow rate; and

(3) the hardware requirements for implementing pneumatically, mechanically or thermally assisted nebulization can be significant. For example, pneumatic nebulization requires gas plumbing and hardware to provide pressure and/or flow control. For example, disposal of significant amounts of vapor and condensate are issues when large liquid flows are nebulized for electrospray.

What is desired is an electrospray apparatus which can be used at high liquid flow rates, such as those typical of the output of a liquid chromatograph, to provide an ionized sample for introduction to a mass spectrometer, and which overcomes the noted disadvantages of prior art devices. SUMMARY OF THE INVENTION

The present invention is directed to an electrospray apparatus for producing gas-phase ions from a liquid sample matrix. The electrospray needle of the present invention includes a main flow conduit through which the sample containing solution flows at a high flow rate (i.e., higher than which an electrospray tip can effectively ionize without nebulization assistance). A sampling tube with an electrospray tip intersects the main flow conduit. In the absence of an electric field to cause droplet formation by the electrospray process, no liquid will flow through the sampling tube. In the presence of an electric field, droplets will be formed at the electrospray tip of the sampling tube. The sampling tube continuously pulls in a portion of the fluid sample from the main flow conduit. The flow through the sampling tube is formed into an electrospray and provided to a mass spectrometer or other instrument. The inventive structure splits off a portion of the main flow prior to electrospraying, with the split flow rate being independent of the input flow rate through the main conduit. This permits setting the flow rate out of the electrospray needle to an optimized rate for input to a mass spectrometer, while permitting an input flow rate into the main flow conduit which is optimized for the output of a liquid chromatograph or other desired instrument.

BRIEF DESCRIPTION OF THE DRAWINGS

Figs. 1 A and IB are schematic diagrams showing a cross-sectional side view (1 A) and an end view (IB) of a prior art device for forming an electrospray from a liquid sample.

Figs. 2A and 2B are schematic diagrams showing a cross-sectional side view (2A) and an end view 2(B) of a first embodiment of the split flow electrospray device of the present invention. Fig. 3 is a schematic diagram showing a cross-sectional side view of a second embodiment of the split flow electrospray device of the present invention. Fig. 4 is a schematic diagram showing a cross-sectional side view of a third embodiment of the split flow electrospray device of the present invention.

Fig. 5 is a schematic diagram showing a cross-sectional side view of a fourth embodiment of the split flow electrospray device of the present invention.

Fig. 6 is a schematic diagram showing a fifth embodiment of the split flow electrospray device of the present invention. DETAD ED DESCRIPTION OF THE INVENTION

The present invention is a configuration for an electrospray needle or capillary which is designed for use in situations of a relatively high flow rate of a liquid sample matrix, such as from the output of a liquid chromatograph. The invention is especially useful in situations in which the flow rate is too high to obtain a satisfactory electrospray without the use of a nebulizing gas and the accompanying elements. The present invention is capable of forming an acceptable electrospray for relatively high input flow rates without the use of a nebulizing gas.

Figures 2A and 2B are schematic diagrams showing a side view (2A) and an end view 2(B) of a first embodiment of the split flow electrospray device 100 of the present invention. As shown in the figures, the inventive electrospray device includes a main flow conduit 102 through which the liquid sample matrix (i.e., the sample compound contained in a carrier liquid or liquids) 104 flows. As noted, this may be from the output of a liquid chromatograph (LC) which has a flow rate too large for an electrospray needle to effectively ionize without assistance.

A sampling tube 106 with an electrospray tip passes through the outer wall of main conduit 102 through which flows the sample matrix from its source, such as the output of a LC (not shown). Sampling tube 106 intersects the liquid flow in the conduit 102. Sampling tube 106 continuously diverts a portion of the sample liquid 104 from main flow conduit 102. The flow through the sampling tube is formed into an electrospray 110 and may, therefore, be provided to a mass spectrometer or another mass-analyzing instrument. The inventive arrangement results in a flow split off of the main flow, prior to electrospraying. This allows setting of the flow rate out of the electrospray needle to be made compatible with the optimized rate for input to a mass spectrometer, while permitting a higher flow rate in main conduit 102.

In the embodiment shown in Figure 2, the sample liquid is pulled into electrospray needle 106 by a combination of capillary action of the needle on the fluid flow in the main conduit and pressure in the main conduit. Initiation of the flow of fluid up the needle is caused by the pressure differential between the needle and the main conduit.

Alternate embodiments of the present invention include electrospray needles which utilize other mechanisms to initiate and maintain the flow through the sampling tube. The driving force(s) that produces flow through the tube can include any combination of capillary action, electro-osmotic flow, and pressure (which may be either actively or passively produced). In addition, sampling tube angles other than normal relative to the flow direction of the main flow conduit can be used to passively change the pressure drop across the sampling tube. Such an embodiment is shown in Figure 3 (in which sampling electrospray tube 106 is oriented at an angle φ with respect to the longitudinal axis of the main conduit). Similarly, back pressure in the main conduit may be adjusted by changing the resistance to the fluid flow downstream.

Construction techniques suitable for manufacture of the present invention include micromachining, and laminates made from metal, plastic and/or glass. Additional flow conduits and mixing features can also be incorporated into the inventive apparatus to permit the production of a desired fluid mixture to facilitate the formation of the electrospray. Figure 4 shows an embodiment of the present invention in which a mixing section 200 for liquids is provided prior to removal of a portion of the mixed flow through sampling tube/electrospray needle 202 to form the electrospray 204. Liquid ports 206 and 207 are used to introduce the liquid sample matrix and a desired fluid for mixing. Discharge port 208 provides an exit for the fluid flow not subjected to electrospraying.

Multiple, parallel sampling/electrospray tubes 105 may be utilized to obtain an increased ion generation rate, or for other purposes such as providing an electrospray to multiple analysis instruments. See Figure 5, which shows an embodiment of the present invention in which multiple electrospray needles 105 are provided to extract fluid from main flow conduit 102. Another embodiment of the present invention includes a reservoir 210 containing the liquid sample matrix 213, with a sampling tube/electrospray needle 212 inserted. This embodiment operates by adjusting the pressure inside reservoir 210 by means of pressure control port 214 in contrast to the flow sampling or splitting structure of the previous embodiments. This embodiment is shown in Figure 6. This embodiment of the invention is suited for introducing a mass calibrating fluid into a mass spectrometer without the use of a pump. The calibrating fluid does not have to flow through the LC column, and therefore it is possible to draw it from the reservoir by electrospraying it from the end of the sampling tube. The advantages of the present invention over prior art electrospray devices include: (1) pneumatic nebulization or mechanical (e.g. ultrasonic) assistance is not required to form an electrospray from large liquid flow rates; (2) the flow to the electrospray tip is only that which can be effectively electrosprayed without assistance of such nebulizing or assisting elements; (3) the device is relatively flow rate insensitive, i.e., changes in the flow rate through the main conduit have little or no effect on the electrospray flow rate; (4) excess flow (that which is split off and not electrosprayed) flows out of a discharge tube, and can easily be collected; and (5) the invention produces more efficient sample transfer to the mass spectrometer because of lower liquid flow rates which can be optimized to that most efficiently used by the spectrometer.

In general the sampling tube should be "primed" by some means prior to proper operation of the inventive system. Priming acts to fill the sampling tube with liquid and displace air within the tube. This is important because air bubbles within the tube will stop the electrospray process from occurring when the air is encountered. Priming can be done in several ways, e.g., temporarily increasing the restriction at the end of the main flow conduit, thus forcing liquid through the sampling tube.

Note that in the absence of a potential difference between the needle output end and an opposing structure, there is substantially no flow out of the needle. The flow rate out of the needle is approximately constant, and is substantially independent of the flow rate into and out of the main conduit, over a large range of input flow rates. This is in contrast to conventional devices which utilize a restrictor to control the portion of the main flow which is re-directed to a sampling needle. As noted, in such a conventional device, the split ratio, i.e., the relative portion of the main fluid flow which is directed to the sampling needle remains approximately constant over a wide range of flow rates so that at sufficiently high fluid flow rates into the main conduit, the output of the sampling needle can be greater than the optimal value for input to a mass spectrometer. However, this problem does not arise with the present invention, which means that the sampling needle output can be optimized for interfacing with a desired mass-analyzing instrument.

The terms and expressions which have been employed herein are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described, or portions thereof, it being recognized that various modifications are possible within the scope of the invention claimed.

Claims

WHAT IS CLAIMED IS;

1. An apparatus for performing electrospray ionization of a sample liquid, comprising: a main flow conduit for transporting the sample liquid; a sampling conduit intersecting the main flow conduit and extending exterior to the main conduit for transporting a portion of the sample liquid, with the portion of sample liquid transported being substantially independent of a flow rate of the sample liquid through the main conduit; and an electrospray ionization tip disposed at an end of the sampling tip exterior to the main flow conduit.

2. The apparatus of claim 1, further comprising: means for establishing a potential difference between the electrospray ionization tip and a surface opposing the tip.

3. The apparatus of claim 1, wherein the sampling tube is oriented substantially normal relative to a direction of fluid flow through the main conduit.

4. The apparatus of claim 1, wherein the sampling tube is oriented at an acute angle relative to a direction of fluid flow through the main conduit

5. The apparatus of claim 1, further comprising: a port for introducing a mixing liquid into the main flow conduit; and a mixing structure disposed in the flow of the mixing liquid and sample fluid.

6. ^' The apparatus of claim 1, further comprising: a plurality of sampling tubes intersecting the main flow conduit and extending exterior to the main conduit.