Bruce Laprade is director of engineering at Burle Electro-Optics in Sturbridge, Massachusetts.

Bruce Laprade

 			For over 30 years, the mass spectrometer has been a trusted analytical tool in the laboratory. Today, mass spectrometers are used in such widely varying applications as food processing, semiconductor manufacturing, space exploration, forensics, drug discovery, energy exploration, and medical diagnostics. Mass spectrometers are even used as an early warning system for volcano eruptions and in homeland security and are playing a key role in the discovery of biomarkers for desease pathology.

Although other analytical approaches such as FT-IR, Raman, and UV–vis spectroscopy have been simpler to implement historically, they lack the specificity and sensitivity of mass spectrometry. Recent developments in sample inlet systems, improved software, and the advent of special purpose mass spectrometers have improved the utility of made instruments.  

The typical mass spectrometer has three key components: The ionization source, which converts the unknown material into ions; the mass filter, which separates the resultant ions based upon the mass-to-charge ratio; the ion detector, which detects and amplifies the ion signal at each mass, which is then used to produce a mass spectrum by the instrument computer. 

Applications in homeland security are now driving the development of field-portable instruments that can provide laboratory-grade analysis. To reduce the size, weight, and power consumption of a portable mass spectrometer, it is often necessary to deviate from the normal operating environment inside the vacuum system. 

Designers of field-portable mass spectrometers face three challenges: 					

A quick assessment of most existing systems reveals that the vacuum system is the most significant contributor in all three categories. 

For a mass spectrometer to operate, the unknown substance first must be ionized before it can be separated by mass and identified. All forms of mass separation require transporting the ions from the ionization source through the mass filter and onto the ion detector. Preventing multiple collisions with molecules within the spectrometer is essential for high sensitivity analysis. Therefore, the quality of the vacuum becomes an important consideration in designing portable instruments. 

According to the kinetic theory of gases, the average distance an ion can travel without a collision with a gas molecule (mean free path) is determined by chamber pressure, the temperature, and the collision diameter of the ions (1). It is plain to see, therefore, that to prevent significant ion molecule collisions in portable instruments, the distance the ions must travel from ionization through the mass filtering process and to the detector must be kept as short as possible. 

Conventional high vacuum pumps generally are not acceptable for portable instruments. Cryogenic vacuum pumps and oil diffusion pumps generally are not suitable because of weight and size, while ion pumps have a small pumping capacity.  

Reducing the vacuum requirements of the mass spectrometer by reducing the ion flight length would simplify the pumping system significantly and reduce cost, weight, and power consumption.  

Molecular drag pumps and low-cost oil-based roughing pumps can achieve vacuums as low as 5 × 10

  Torr, while dry scroll pumps can achieve vacuum levels into the 10

The most common method of ionization for field-portable instruments is electron ionization (EI). Filaments that operate near atmospheric pressure have been developed using yttrium coatings. In recent developments, some miniature systems now utilize electrical discharges to create the ions. 

Typical matrix-assisted laser desorption ionization (MALDI) time-of-flight (TOF) mass spectrometers require an ion flight path length of 1–2 m. These instruments must operate in vacuums of 10

Modern quadrupole based instruments typically operate at vacuum pressures in the high 10

  Torr range because ions need to travel 35 cm or more before they reach the detector. 

Most conventional ion traps and magnetic sector instruments require an operating vacuum of 10

The key factor in reducing the vacuum requirements is to reduce the ion flight path. If it were possible to shrink the ion flight path length to less than 5 cm (approximately 2 in), then it should be possible to operate a system in the millitorr range. The Inficon XPR

  instrument (Inficon, Syracuse, New York) utilizes a 17-mm-long quadrupole mass filter.  

Miniature Ion Trap-based instruments (Purdue University, Wes Lafayette, Indiana) fabricated from etched silicon substrates are reported to operate at or near atmospheric pressure. Although miniature quadrupoles and ion traps can operate at elevated pressures, their utility is very limited without the use of an electron multiplier to detect the ions.  

A Faraday cup is an ion detector in its simplest form. Ions colliding with the metal cup deposit their charge which is detected by a high gain electrometer. The advantage of this type of ion detector is that it operates well at all vacuum levels. This type of detector does not incorporate any gain feature, however, and therefore seriously limits the sensitivity of the mass spectrometer. 

Most mass spectrometers today utilize some type of electron multiplier as the ion detector. These high-gain, low-noise devices can increase the sensitivity of the mass spectrometer by up to a factor of 100,000. There are four main types of electron multipliers: 					

Single channel electron multipliers (Channeltrons, BURLE)

Spiraltron (BURLE) channel electron multipliers

Conventional electron multipliers originally were designed to operate in vacuums of 10

Field-portable mass spectrometers will require ion detectors capable of operating reliably under the poor vacuum conditions that will be prevalent in these instruments. The ideal ion detector for portable mass spectrometers will have the following characteristics: 					

Operate at millitorr pressures or  higher

Produce high gain and maintain low noisewhile operating at elevated pressures

Electron Multipliers Operation in Poor Vacuum Conditions

When electron multipliers are operated in poor vacuum they can become noisy due to ion feedback. Ion feedback results when secondary electrons collide with residual gas molecules,resulting in the generation of positive ions. Positive ions are accelerated to the negative electrode of the multiplier. Upon collision with a dynode (discrete dynode multiplier) or channel wall (Magnums, single-channel electron multipliers, and microchannel plates), additional secondary electrons will be produced. This process results in the multiplier producing an output signal in the absence of any true ion signal. If the pressure is high enough, ions can be created by the strong electric field within the multiplier. The maximum operating pressure of an electron multiplier can be determined, therefore, by observing multiplier background noise under increasing pressure conditions. The operation of electron multipliers in the poor vacuum conditions that will be prevalent in portable instruments has not been widely studied.  

Several forms of electron multipliers have been characterized under abnormally high operating pressure conditions. Each form of electron multiplier was loaded into a specially designed vacuum chamber that was capable of modulating the chamber pressure. The multipliers were tuned to produce a gain of 1 million. (The output signal was 1 million times greater than the input ion signal.) Multiplier performance then was characterized at various operating pressures. Figure 1 summarizes how the noise of each type of multiplier varies with pressure. 

 Figure 1: Plot of electron multiplier noise as a function of vacuum pressure.			

The results indicate that the discrete dynode multiplier was the least tolerant of elevating the chamber pressure becoming noisy when the pressure entered the mid 10

The most tolerant multiplier was the 2-μm pore microchannel plate followed by the Spiraltron, which operated at high gain and low noise well into the 10

Operational lifetime is another critical performance parameter for electron multipliers operated at elevated vacuum pressures. Typical electron multiplier lifetime for benchtop instruments is about one year. Operational lifetime testing also has been investigated (2). Figure 2 illustrates some typical results for a microchannel plate ion detector operated at pressures above 10

Figure 2: Operational lifetime test results for a 5-Î¼m pore microchannel plate ion detector operated at pressures above 10â3 Torr.			

The results indicate that after the characteristic conditioning of the surface at the beginning of the test, the multiplier operation remained stable for hundreds of hours of operation. Other background gases have indicated similar performance (2). 

As our world continues to change, and more applications are identified, miniaturization of mass spectrometers will continue. The room-sized instruments of yesterday have been replaced by instruments the size of shoe boxes, while endusers dream of mass spectrometers the size of cell phones. Helping to maintain the sensitivity of these miniature instruments is a new generation of high-pressure tolerant electron multipliers. 

is director of engineering and quality assurance with BURLE Electro-Optics, Inc., Sturbridge, Massachusetts. He can be contacted at: 

Articles

Recent developments in sample inlet systems, improved software, and the advent of special purpose mass spectrometers have improved the utility of MS instruments. here, the authors discuss this new utility.

Performance Characterization of ion Detectors in Harsh Environments

Mass spectrometers are analytical instruments capable of identifying unknown materials in complex mixtures down to parts per billion. Once relegated to the research laboratories, mass spectrometers now are in use in a broad range of applications. They can be found screening for pesticides in canned foods, controlling semiconductor manufacturing processes, diagnosing disease, exploring for natural resources, discovering new pharmaceuticals, predicting volcanic eruptions, and in applications for securing our homeland. Indeed, these instruments have traveled beyond our world aboard the Galileo and Cassini spacecrafts to provide atmospheric analysis of neighboring worlds within our solar system. In fact one hardly can watch television or see a movie these days without hearing about mass spectrometers. 

Time-of-flight mass spectrometry (TOF-MS) rapidly is becoming the most popular method of mass separation in analytical chemistry. The development of low cost digitizers and extremely fast ion detectors has fueled this popularity. TOF-MS is deployed easily, and can produce very high mass resolution. This technique of mass separation can be adapted for all forms of sample introduction and ionization. Unlike quadrupoles and ion traps, TOF mass analyzers perform well at very high mass such as with analyses of molecules frequently found in proteomic applications. Wiley and McLaren (1) in 1955 followed by Cotter (2) in 1992 and Wollnik (3) in 1993 have described TOF analyzers. 

TOF mass spectrometers are produced in two main types - linear instruments and reflectron instruments. 

 Figure 1.  Cross section of a linear TOF instrument.

In operation, the unknown sample first is converted to ions. Figure 1 illustrates a typical MALDI (matrix assisted laser desorption ionization) instrument. The resultant ions then are injected into the flight tube where they begin traveling towards the detector. 						

The motion of the ions within the flight tube can be described by equation 1:   

 is the mass to charge ratio of the ion, 

The lighter ions (low mass) will travel faster, while the high mass ions will move to the detector more slowly. If the flight tube is long enough, all the ions will be arranged according to mass (smallest to largest) when the ions arrive at the detector.  

When the ions arrive at the detector, they initiate a cascading of secondary electrons (4), this results in the generation of very fast voltage pulses, which precisely signal the arrival of the ions. A high-speed oscilloscope or transient recorder is used to record the arrival times. Figure 2 illustrates the arrival time spectrum of bradykinin collected on a linear TOF instrument.  

Figure 2.  Mass spectrum of bradykinin.			

Knowing the exact arrival times, equation 1 can be used to solve for the mass-to-charge ratio of the ions. 

The second type of TOF mass spectrometer is the reflectron instrument. Figure 3 illustrates this design. The reflectron design takes advantage of the fact that the further the ions are allowed to travel, the greater the distance between ions of slightly differing masses. Greater distances between ions with different masses will increase the arrival time differences between the ions and there by increasing the resolution at which ions of a similar 

 can be differentiated. In addition, this design corrects the energy dispersion of the ions leaving the source (5).   

In a reflectron analyzer, the ions are injected into the flight tube in the same manner as in a linear instrument. The ions traveling down the flight tube enter the reflectron lens. The electrostatic field created by applying separate high voltage potentials to each of the metal rings, slows the forward progress of the ions and eventually reverses their direction. They then exit the lens and are directed to the detector. This action effectively doubles the length of the flight tube, which improves mass resolution without adding any additional length to the flight tube.   

				The motion of ions in a reflectron lens has been described previously by Hoffman (5).			

				Most TOF instruments manufactured today incorporate reflectron analyzers. This device typically consists of a stack of alternating, precision ground metal rings and insulating spacers held together with threaded rods. This assembly may have hundreds of components which must be carefully assembled and aligned (typically by hand) in a clean, dust-free environment. Additionally, a voltage divider must be added to each row or layer in order to produce the electrostatic field gradient necessary to reverse the direction of the ion. 

An advanced variant of this classical design utilizes a single resistive glass tube to form the field gradient replacing hundreds of discrete components. 

One of the critical components of a TOF mass spectrometer for proteomics applications is the ion detector. Ion detectors are manufactured in three main types: microchannel plate (MCP), discrete dynode and continuous dynode (Chaneltron

 ). Modern high-performance TOF mass spectrometers used for proteomic research primarily utilize MCP-based detectors because of they have the largest collection area, the lowest time jitter, and the highest temporal resolution. Figure 4 illustrates some typical time-of-flight detectors manufactured by BURLE Electro-Optics. 

Figure 4.  Various MCP-based TOF ion detectors.						

The performance characteristics of the ion detector can have a profound effect on the performance of the mass spectrometer and the quality of the data produced. When selecting an ion detector, it is important to consider these key performance areas: gain, noise, temporal response, pulse shape, collection area, high mass sensitivity, positive and negative ion detection (bipolar), reliability, and serviceability. 

The gain of the TOF detector is a measure of the amplification factor produced when an ion strikes the input surface. Most instruments today require a gain of at least 10

. This means that a single ion will initiate a cascading of secondary electrons which result in a charge pulse of at least 1.6 Ã 10

 coulombs. This signal level is sufficient to be recorded above the electronic baseline noise. Most Chevron (a detector containing two microchannel plates in series) detectors are capable of producing gains of 10

. However, operating the detector at higher gains than necessary can reduce dynamic range and shorten the lifetime of the device. 

The noise characteristics of the detector also are very important. Noise is defined as spurious output pulses  which are not initiated by analyte ions. Because most spectra are created by averaging several "flights," occasional random pulses are removed from the data.  Repetitive noise pulses at given intervals will appear as extra peaks in the spectrum, which can lead to some exciting times when your colleague announces they have "discovered" a new molecule.  

Excellent temporal response of the detector is essential for high resolution and mass accuracy.  If the detector is too slow, then different mass ions arriving at the detector which differ slightly in mass will appear to the instrument to have the same mass. The resultant peak produced from these two ions will be one broad peak rather than two narrow ones. Because the instrument assigns mass based on the centroid of the resolved peak, it will improperly identify the mass of the ion.  

The best way to avoid this situation is to select an ion detector with a temporal response which is significantly faster than the gap in the arrival times for the ions. The highest temporal resolution detectors available today incorporate microchannel plates with extremely small pores. The ultrafast TOF instrument uses 2-Î¼m pore microchannel plates and produce single-ion pulse widths of less than 350 ps. Increasing the speed of the detector improves mass resolution and reduces the required length of the flight tube, leading to smaller, less expensive instruments. 

Detector manufacturers go to great lengths to design ion detectors with excellent raw pulse shapes. The ideal pulse shape is symmetrical, with the amplitude proportional to the abundance of the ions impinging on the detector. Frequently, however, these ideal pulse shapes are distorted by impedance mismatches in system hardware and electronics. In a typical TOF-MS system, the signal from the detector must travel 10 cm to the vacuum feed through, through the vacuum wall, and then up to 2 m to the instrument digitizer. Any impedance mismatch can cause distortion in the pulse shape leading to degradation in mass resolution and accuracy. Many experimenters attempt to fabricate detectors by assembling microchannel plates into hardware assemblies only to observe distorted pulses, which suffer from tailing and ring. Figure 5 shows examples of good and distorted pulses. Pulses are oscilloscope traces displayed in the raw form from the detector.  

Figure 5.  Examples of good and distorted pulses: (a) pulse reflection causes what looks like a second peak; (b) impedance mismatch causing ringing (2 ns/div.) (10 ns/division); and (c) raw ion pulse from a well matched detector and electronics package  (500ps/div).			

The active diameters for TOF detectors typically range from 8 to 75 mm. The active diameter of the ion detector need only be as large as the ion beam. Initially, ion detectors tended to be very large (40-75 mm) to capture the entire ion beam. As ion optics improved, the required collection area was reduced. Today the most widely used time-of-flight detector, the APD APTOF 18, has an 18-mm active diameter. 

Proteomics research often times requires the analysis of high mass materials. Conventional TOF detectors loose detection efficiency as the mass of the ion increases. Laprade and Owens (6) have described in detail the development of a patented bipolar TOF detector that significantly increases the detection efficiency for high mass ions. In this device, the ions pass through a grid at the end of the flight tube and then are subjected to post acceleration using up to 10 kV before they collide with the microchannel plate and initiate the cascading of electrons. This detector also incorporates a low work function coating on the converting surface to further increase the detection efficiency.  

Most mass spectrometers produce spectra using only positively charges ions. Although this is adequate for most applications, running a mass analysis in both the positive and negative ion modes frequently reveals valuable information about the structure of the original molecule that produced the ions. Detecting negative ions necessitates reversing the polarity of the detector and capacitively coupling the signal to the system digitizer. 

The bipolar TOF detector  utilizes electro-optical isolation in the form of a fast scintillator and photomultiplier tube to prevent the system digitizer from being exposed to the 10 kV post acceleration voltage. This unique structure makes it easy to detect positive or negative ions and eliminates the need for capacitive coupling the signal to the digitizer.  

The active elements in a time-of-flight detector are not intended to last the life of the instrument. Periodically, the microchannel plates will need to be replaced with fresh ones. The service period will depend upon the types of samples analyzed. Discounting vacuum failures, a typical TOF detector used in a proteomic application should function for at least two years. Many manufacturers of TOF instruments now incorporate a cartridge design which enables the end user to easily replace the MCPs. 

The future for mass spectrometry is a bright one. Teams from academia, national labs, and industry are hard at work on the next generation of instruments which promise even better performance and value. Driven by a near fever pitch of applications development in drug discovery, homeland security and energy exploration, these instruments will continue to improve our quality of life.     

5. E. De Hoffmann, J. Charette, and V. Stroobant, 

Mass Spectroscopy Principles and Applications

6. B.N Laprade, R. Cochran, and K.G. Owens, poster 1415P presented at the 2002 Pittsburgh Conference. ? 

 is director of Engineering and Quality Assurance for BURLE Electro-Optics, Inc. (Sturbridge, MA). Phone: (508) 347-4044. E-mail: 

Time-of-Flight (TOF) is rapidly becoming the most popular method of mass separation for proteomics and conventional analytical chemistry. The development of very high temporal resolution ion detectors and low-cost high-speed digitizers has rendered this technique easily deployed and able to produce very high mass resolution. The performance of a TOF mass spectrometer is dependent upon a number of critical components. This article will focus on the ion detector.

Bruce Laprade

Articles

Performance Characterization of ion Detectors in Harsh Environments

Time-of-Flight Mass Spectrometry for Proteomics Research