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Improving Sensitivity in the H.P. 5971 MSD and Other Mass Spectrometers - Part I of II

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SISWEB Technical Note C.

John J. Manura

Scientific Instrument Services, Inc. 1027 Old York Road, Ringoes, NJ 08551

(Part I | Part II - Increasing Mass Spec Sensitivity)

Introduction

The H.P. MSD instruments (Models 5970, 5971 and 5972) have proven to be very efficient, low cost, highly sensitive and versatile mass spectrometers. Due to their low cost as a complete GC/MS system, the HP MSD's have become the most widely utilized mass spectrometer. They are utilized for a large number of applications and routine methodology in instrumental laboratories. The MSD's have a wide dynamic range and are linear for quantitative analysis. They have proven sensitive in both the total ion scan as well as in the selective ion mode (SIM). We have been utilizing two 5971 MSD's in our laboratory for more than 4 years for use in developing applications for our Short Path Thermal Desorption System and for the development of mass spectrometer accessories. Like many mass spectrometer users, we are constantly searching for greater sensitivity and pushing the limits of detection in the instruments.

This article is the first of a series of two articles that will appear in this newsletter in which we will describe the improvements and changes that we have incorporated into our MSD's in order to improve their sensitivity. Many of the changes discussed here can also be applied to other mass spectrometers in order to improve their performance. This article will expand on a lecture that was presented at the 1994 EAS (1) and a poster presentation submitted for the 1995 ASMS Conference (2). Some of the same changes described below have been incorporated by HP in their new Model 5972 MSD, but we have made several additional modifications that are described. We have improved the performance of the 5971 MSD by increasing its sensitivity at least 10 fold by incorporating the improvements in the GC and the MSD as described in this and the subsequent article.

Mass Spectrometer sensitivity is normally expressed as an amount of analyte detected at a given signal-to-noise ratio. Sensitivity can be improved by decreasing the background or noise level, increasing the sensitivity of the mass spec or detector or by improving the resolution of the GC peak for the analyte of interest. (Table II)

In order to achieve these goals, improvements in both the GC and the MSD (mass spectrometer) must be investigated. Each instrument has a number of components that either contribute to the background noise, reduce peak resolution or effect sensitivity. (Table III.)

This first article will discuss the improvements incorporated into the mass spec or MSD component of our HP 5971 to improve its sensitivity. The second article in this series, scheduled for the April edition of this newsletter, will discuss the GC system components and steps that have been taken to increase the system sensitivity. Utilizing all these improvements and changes, we have increased the sensitivity of the HP 5971 by at least a factor of 10.

GC/MS Transfer Line

In the standard GC/MS transfer line on the HP 5971, the GC capillary column bottoms out inside the transfer line tip inside the MSD source and the GC column flow elutes at 90 to the normal GC flow. This design is advantageous for the quick and accurate positioning of the GC capillary column into the MSD source. The user just pushes the capillary column until it bottoms out and then backs it up about a half millimeter. However it does not provide for an efficient straight through flow of the GC effluent into the MSD source. In addition the analytes eluting off the GC capillary column are exposed to hot metal surfaces before their entry into the MSD source. Although this metal surface is gold plated to reduce surface reactivity, some sample decomposition can occur at this metal surface (Figure # 1).

We have redesigned the transfer line tip to eliminate this exposure of the GC analytes to this metal surface and provide for a straight through flow of the capillary column effluent into the MSD source (Figure # 1). This tip is similar to the design utilized in the HP Engine (5989A). The GC capillary column passes straight through the GC/MS transfer line and directly into the MSD source. Therefore the analytes being eluted off the GC column are purged directly into the MSD source and the analytes are not exposed to any metal surface before their entry into the MSD source. This new transfer line tip does make it more difficult to position the GC capillary column in the MSD source. Either the GC column must be installed into the GC/MS transfer line prior to attachment of the transfer line to the MSD source or the GC capillary column length must be premeasured and marked before it is fixed in place via the nut and ferrule on the GC/MS transfer line fitting.

When operating the GC/MS transfer line, its temperature should be kept as low as possible, especially if liquid phase coated capillary columns are taken directly through the interface to the MSD source. The GC/MS transfer line on the 5971 and 5972 instruments not only heats the capillary column between the GC and the MSD but it also heats the MSD source. It does this by direct thermal transfer of heat from the transfer line tip to the MSD source and source supporting housing. As a result the transfer line is normally kept at between 250C and 290C in order to maintain a source temperature of 180C. This source temperature is comparatively low compared to other mass spectrometers. However, higher temperatures would destroy the liquid phases on the capillary column inside the GC/MS transfer line. The problems with the capillary column liquid phases could be reduced by using deactivated fused capillary tubing with no liquid phase inside the GC/MS transfer line. However this would require the junction of the GC capillary column to the fused silica inside the transfer line using a low dead volume union (SGE type MVSU). Junctions of columns always present the potential for leaks or exposure of analytes to metal or reactive surfaces.

We are presently evaluating the design of the HP GC/MS transfer line in order to improve its performance and usefulness. One idea is to design the transfer line so that capillary columns can be changed without evacuating the MSD. Presently the entire MSD vacuum system must be shut down and vented in order to change columns.

Mass Spectrometer Source

Over the last 20 years we have continued to improve on our source cleaning methodology. These methods have been reported in our Mass Spec Handbook of Service (3) and in several of our previous newsletters (4) and are utilized in our source cleaning services (5). These cleaning methods have been perfected to provide the cleanest source possible and therefore the optimum sensitivity of the mass spectrometer. Highly polished metal (mirror like) surfaces help to maintain cleaner sources since they do not contain the small scratches and uneven surfaces that are prone to contamination. The source cleaning method is outlined above (Table IV) and described fully in the Mass Spec Source (5).

The alignment of the mass spec filament onto the mass spectrometer source is also of great importance. The filament must be accurately aligned over the ion entrance hole in the mass spec source in order to achieve the maximum number of electrons in the source and therefore the optimum mass spec sensitivity. This is not usually a problem on the EI versions of the MSD due to the large ion entrance hole on the source block. However on Chemical Ionization (CI) versions of the source the alignment is very critical due to the tightness of the source and the small ion entrance hole on the source block. In our laboratory we routinely align the filament wires over the mass spec source ion entrance hole utilizing a video optical system. This is a specialized system that has a 1.0 inch depth of field at 20X magnification that we use for a variety of functions including filament manufacture, filament alignment and quality control in our repair shops. Similar results can be obtained by most mass spec users by utilizing a good quality stereo microscope. This assures the optimum alignment of the filament and therefore the best system performance.

Electron Multiplier

Electron Multipliers (mass spec detectors) have improved significantly over the last few years. This has been mainly due to the increased competition among several manufacturers of these devices. The market presently is very crowded with at least 6 manufacturers of electron multipliers competing for a very small instrument market. Each manufacturer has been attempting to achieve the best, most sensitive, largest dynamic range or longest life electron multiplier. We have been associated with Galileo Electro-Optics Corporation for more than 15 years. They are the most widely known manufacturer of electron multipliers for mass spectrometers and in our opinion have the best technology for multiplier development as well as a strong market share in this field. We have been working with Galileo during the development of many of the new higher sensitivity multipliers and have served as a beta test site for these electron multipliers.

The latest technology utilized by Galileo is computer modeling of the optics in the design of the electron multipliers in order to achieve the optimum multiplier sensitivity and in order ion to reduce or eliminate extraneous noise generated either by the multiplier or by the mass spectrometer. The results have been several new introductions of multipliers with 2 to 10 times increase in sensitivity. In developing these new electron multipliers, Galileo has studied and improved the electron multiplier sensitivity, reduced extraneous noise, increased the dynamic range and increased electron multiplier life. Figure # 2 shows the improvement made in the Galileo multipliers for the HP 5971 over the last 5 years. The Model 4778 Galileo electron multiplier was the original multiplier supplied with the HP 5971 when it was introduced. The HP original specifications and sensitivity (Table I) were created utilizing this electron multiplier. The Model GHP71 was introduced by Galileo about 3 years ago in order to improve upon the performance of the 4778. The Model GHP71 provided for an increase in sensitivity of about 2 times as observed by the signal to noise levels shown in Figure # 2. The new Galileo Model 5778 for the HP 5971, 5972 and GCD and the corresponding Model 5772 for the HP 5970 series of MSD's have recently been introduced as the latest enhancements to sensitivity and performance. These newest multipliers improved sensitivity at least 5 fold as compared to the original multipliers supplied with these instruments (Figure # 2). In addition these new multipliers exhibit a larger dynamic range, increased lifetime, increased noise rejection and ease of installation.

Other electron multiplier manufacturers have also been addressing the sensitivities of the electron multipliers. Figure # 3 demonstrates the comparison of the new Galileo Model 5778 with two other major manufacturers of multipliers for the HP 5971 MSD. The performance of all three manufacturers were near identical in sensitivity expressed as a signal-to-noise ratio. This data is the result of the examination of only one multiplier from each of the three different manufactures and therefore may not be 100% accurate. A more detailed study would have to investigate batch to batch variations of several multipliers and should investigate other factors such as the dynamic range and multiplier lifetime. We are not prepared to do this at this time.

We have been using a beta test version of the new Galileo Model 5778 for about a year. The sensitivity of the multiplier is still as good as when it was first installed. It has demonstrated a dynamic range over more than 4 decades for the quantitative work that we have done. Based on its performance to date we expect at least another year of operation since this multiplier still tunes at only 100 volts higher than when it was first installed.

Vacuum System

We have made numerous improvements in the vacuum system of the MSD in order to increase the vacuum level and reduce hydrocarbon background from the pump oils. The better the vacuum system, the lower the background signal in the instrument. When we first installed each of our MSD's in the lab, one of the first things that we did was remove the original rough pump that was supplied with the instrument. HP installed either an Alcatel UM2002 or a small Edwards E2M1 or E2M2 in the MSD's. Bothe of these pumps operate with small reservoirs of pump ils as compared to the larger rough vacuum pumps. As a result they cannot dissipate heat quickly and therefore run very hot. In addition the Alcatel Model 2002A operates at 3600 RPM. This high speed in this small pump causes it to run even hotter. The low lifetime of both of these small pumps is due to the high temperatures generated which causes increased wear and reduced lifetime of seals and other moving parts. The original pumps have a pumping speed of about 30 to 50 liters per minute. (Table V) We do not recommend any of these pumps for use on a mass spectrometer which needs to run 24 hours a day, 7 days a week with no downtime. We previously have recommended the Alcatel model ZM2004 or the newer Alcatel UM2005 rough vacuum pump. This pump has a pumping speed of about 100 liters per minute. These pumps will provide for decreased pump down time, lower vacuum and increased pump life. However in our lab, we wanted to move the rough vacuum pumps on the other side of the wall from the MSD in order to reduce the noise level in the mass spec lab. As a result the rough pumps were to be located about 6 to 10 feet from the MSD. Therefore we chose to use an even larger pump, the Alcatel UM2015. This pump has a pumping speed of 300 liters per minute. As a result of the pumps being in another lab, the noise associated with the vacuum pump is not even audible in the mass spec room. In addition all of these Alcatel rough pumps are extremely reliable. These larger rough pumps result in a much faster pumping speed which is especially noticeable during initial pump down after the system has been vented. In addition the system runs at a lower vacuum level due to this increased pumping efficiency. We have been using the same pumps for more than 4 years, with no down time associated with the rough pumps.

However as described in previous articles (6), proper care of the rough pumps is important. We recommend the changing of the rough pump oil every 3 to 6 months. Dirty or contaminated oils, increase the wear on the metal shafts of the pump and the pump runs much hotter. This higher temperature escalates the wear and compounds the problem. If the rough pump oils are routinely changed every 3 to 6 months, the rough pump should have a life of about 5 years before an overhaul is required.

Several new rough pump oils have been introduced in the last few years for use with rough pumps on mass spectrometers (Table VI). Seven or eight years ago, we were recommending Inland 19 as the best oil for rough pumps on mass spectrometers. This oil was equivalent to the standard oils used by Alcatel, Edwards, Welsh, Varian and Precision on their rough pumps. Inland 19 oil is still used by most pump manufacturers as the standard pump oil for their pumps. It is relatively cheap and is sufficient for most applications.

About 5 years ago, Invoil 20 was introduced as a new pump oil for high vacuum applications. It has a lower vapor pressure and therefore can achieve a lower vacuum than the Inland 19 pump oil. Due to its increased performance and lower ultimate vacuum level, we recommended its use in mass spectrometers and electron microscopes. It is more expensive than the standard oils, but we felt that it was worth it. The label on the manufacturers bottle labels Invoil 20 as a diffusion pump oil, since that is why it was originally developed. Actually it is a very low grade diffusion pump fluid and a high grade rough pump oil. Although the manufacturer claims that the oil will last three to five times longer than the lower grade oils in rough pumps, we still recommend changing all pump oils every six months. This will maximize the life of the vacuum pump.

In the last two years, another new oil has been introduced called Inland 45. This again is a more highly refined and distilled oil with a lower vapor pressure that will provide for increased performance in the rough pumps used on mass spectrometers. It is highly recommended for use on high resolution mass spectrometers where the ultimate vacuum level is required. Again this oil is more expensive, but will provide a much lower vacuum level, reduced hydrocarbon background and increased performance from the vacuum pump and mass spectrometer. It is presently used by several mass spectrometer manufacturers and is shipped with many mass spectrometers including the HP 5972.

Santovac™ 5 has a history of being the best vacuum pump oil for use in diffusion pumps (Table VII). This oil is a highly refined polyphenyl ether fluid that can achieve vacuum levels of 1 x 10-10 torr in research grade mass spectrometers. Santovac 5™ is routinely used in the diffusion pumps on most mass spectrometers manufactured over the last 25 years. Its only drawback is its price, it is quite expensive. Other lower grade diffusion pump fluids are available such as the Dow Corning diffusion pump fluids. They are less expensive but will not provide the vacuum levels or cleanliness achievable by Santovac™ 5.  Santovac™ is a registerd trademark of Santovac™ Fluids, Inc.

A new higher grade of Santovac™ 5 is now available, called Santovac™ 5P Ultra. This oil is a more highly refined grade of Santovac to provide even better performance. We are recommending that all mass spectrometer users switch to this new oil. It is not much more expensive than the standard Santovac™ 5 and the improved performance and lower background justifies its use.

Molecular Sieve or Coaxial Foreline Traps are recommended for use is most mass spectrometer vacuum systems. The purpose of these traps is to trap the hydrocarbons and water vapor originating from the oils in the rough pump. They are typically placed in the vacuum line between the rough vacuum pump and the diffusion or turbomolecular pump to prevent backstreaming of oil into the high vacuum system. The foreline traps normally use synthetic zeolite or a coarse metal mesh (either copper or stainless steel) with a high surface area per unit volume for the condensing of vapors originating from the rough pump. Several models are available which can be periodically baked out to rejuvenate them for continued use. Using these traps will reduce or eliminate hydrocarbon oils contamination originating for the rough pump vacuum system. The result will be less contamination in the mass spec source and therefore less mass spec background and greater mass spec sensitivity.

Conclusion

The above article describes many of the changes that have been made to the MSD side of an HP 5971A MSD in order to improve performance. These changes will increase the sensitivity by a factor of between 5 to 10 fold over the original instruments performance. All of these improvements were undertaken in order to increase the sensitivity of the MSD, reduce the sources of MS background noise and improve the ultimate vacuum level and cleanliness of the vacuum system. The areas of improvement include the GC/MS transfer line, the mass spec source and filament, the electron multiplier, the vacuum pumps, rough pump oil, diffusion pump oil and the overall cleanliness and maintenance of the total MSD system. However additional improvement can still be achieved by addressing the sample that enters the MSD from the GC. Most of the background levels or noise that is observable in the total ion scan on mass spectrometers originates from the gas chromatograph. This will be addressed next month in part II of this article.

(Continue to Part II)

References
(1) Manura, John, Thermal Desorption GC/MS using the HP MSD, Eastern Analytical Symposium, Somerset, NJ, November 1994

(2) Manura, John, Improving Sensitivity in the HP MSD, American Society of Mass Spectroscopy, Atlanta, GA, June 1995. (submitted for approval)

(3) The Mass Spec Handbook of Service, Vol. 1, (1983), pp 3-19.

(4) The Mass Spec Source, Mass Spec Source Cleaning, January 1994.

(5) SIS 1995-1996 GC/MS Catalog

(6) The Mass Spec Handbook of Service, Vol. 1

(Part I | Part II - Increasing Mass Spec Sensitivity)