Design of a Direct Exposure Probe and Controller for use ona Hewlett-Packard 5989 Mass Spectrometer

Thermal Desorption System Problem Solving - June 1994

by Christopher Baker and John J. Manura,

Scientific Instrument Services, Inc.,

Presented at the 1994 ASMS Meeting, Chicago, IL. JUNE 1994

Introduction

The most common method for the direct introduction of samples into a mass spectrometer is routinely accomplished with the use of a direct insertion probe (DIP). This technique is rapid and easy to use but it has maximum temperature limitations as well as temperature ramp rate limitations. The DIP design relies on the transfer of heat from a heating element through the glass wall of the sample vial to volatilize the sample. This probe has a maximum temperature of 350 degrees C and a maximum ramp rate of 50 degrees C/min on the H.P. Engine mass spectrometer. This article describes a new direct exposure probe (DEP) and controller which were designed for use on the HP5989 instrument to expand the range of compounds that can be analyzed via the mass spec probe. The DEP probe tip consists of a small heater wire onto which the sample is directly loaded. The DEP wire can be heated almost instantaneously as well as under controlled ramp rates to temperatures from room temperature up to pyrolysis temperatures (> 1200 degrees C).

Probe Design

A drawing of the DEP probe is shown in Figure 1. It can be seen that the probe tip consists of a plug in connector with a replaceable filament wire for holding and heating of the sample. A glass sample vial is not used in this technique. The sample is dissolved in a solvent, such as methanol, and a drop is placed directly on the filament wire. A current is passed through the filament to directly heat the sample and volatilize it into the mass spectrometer source. Temperatures of over 1200 degrees C can be attained ballistically with this design. This rapid heating can minimize thermal decomposition of some samples as well as vaporize samples quickly into the mass spectrometer to increase the sensitivity of sample analysis. In this technique the actual temperature of the sample is not monitored but it can be varied and controlled through changing of the current levels passed through the filament. A current controller was developed which controls the amount of current passed through the filament from 0.01 amps up to 3.00 amps. It can be programmed to ramp the current through the filament as a function of time at rates from 0.01 amp per minute up to 5.0 amps per minute. No modifications of the HP5989 mass spectrometer were required to utilize the new DEP probe.

Figure 1 - The DEP Probe

Experimental

All experiments were carried out on a Hewlett-Packard 5989A mass spectrometer in the chemical ionization (CI) positive ion mode of operation. The CI reagent gas utilized was isobutane at a source pressure of 0.9 torr. Sucrose and Fructose, along with the pesticide Avermectin were analyzed to demonstrate the usefulness of the probe. Two different styles of filament tips were studied. Due to the plug in style of tips for the DEP probe many other modifications of probe tip design are possible. A rhenium ribbon filament and a glass encapsulated rhenium wire filament were used for these analyses. These two filament tips are shown in Figure 1. These two filament designs were ramped at various rates to determine the effect of both the filament design and ramp rates on the analysis.

Results

Figure 2 shows the separate DEP analyses of Fructose and Sucrose utilizing the rhenium ribbon style DEP filament. The sugar samples were dissolved in Methanol at a concentration of 1.0 ug per ul of solvent and 1.0 ul of this solution was introduced to the center of the rhenium ribbon filament. After the samples were placed onto the rhenium ribbon filament and inserted into the mass spectrometer source, the DEP filament current was ramped from 0 to 1.3 amps at 1.0 amps/min. I t can be seen from the total ion profile that the samples come off in a narrow band but at different times. Although the mass spectra of both sugars are near identical, they can be separated by their ease of volatilization into the mass spec source. This is verified in Figure 3 which demonstrated a total ion profile and mass spectral analysis of a mixture of both sucrose and fructose. The conditions are the same as those of the separate analyses of these two compounds. The ramping of the probe allows the separation of these two compounds and their identification.

Figure 2 - DEP Analysis of Fructose and Sucrose

Figure 3 - DEP Analysis of Mixture of Fructose and Sucose

The comparison of the various ramp rates on the analysis of sucrose on the rhenium ribbon style DEP filament is shown in Figure 4. The fastest ramp rate (3.0 amps/min.) provides the sharpest ion profile and also the greatest sensitivity since the analyte is vaporized into the mass spec source in a shorter period of time. When the ramp rate is slowed to 1.0 amp/min. the profile becomes very broad and less sensitivity for the analyte is achieved due to the introduction of the same amount of sugar into the source over a longer period of time. This corresponds to the GC analysis of a sample, in which the highest sensitivity is achieved by optimizing the resolution or peak width of the analyte of interest. Figure 5 shows the same set of experiments except that the DEP filament is a glass encapsulated rhenium wire. The sucrose comes off earlier from the glass encapsulated wire than from the rhenium ribbon. Slowing the ramp rate of the analysis has the same effect of widening the ion profile and loss of sensitivity as for the rhenium ribbon style filament.

Figure 4 - Comparison of Ramp Rates

Figure 5 - Comparison of Ramp Rates

The analysis of a high molecular weight pesticide via the DEP probe technique is shown in Figure #6. The pesticide Avermectin was analyzed using the glass encapsulated rhenium wire filament at a ramp rate of 2.0 amps per minute from 0 to 1.5 amps total. Avermectin has a molecular weight of 872. This drug can not be analyzed via standard low temperature direct probe analysis due to the 350 degrees C temperature limitation of these probes. However the DEP style filaments are able to ramp up to the high temperatures needed to volatilize this high molecular weight drug into the mass spectrometer source. The major peaks are at 111,145 and 305 and a small peak was observed at 874.

Figure 6 - DEP Analysis of Avermectin

Conclusions

A direct exposure probe (DEP) and current ramp controller were designed which allow the analysis of a wide range of compounds via direct introduction into the HP5989 mass spectrometer. The fast ramp rates achievable with this design minimized the thermal decomposition of the samples analyzed and permit the maximum sensitivity of analysis. Samples of sugars and a high molecular weight pesticide were analyzed with this probe which would have otherwise been impossible to accomplish with conventional DIP probe designs due to their temperature limitations as well as their limited ramp rate capabilities. Two different DEP filament tip designs were used, however minimal differences were observed for the samples analyzed. Additional probe tip designs can be easily tested with this probe due to the plug in design of the probe tip. Additional studies should be performed on the design of the probe tips. The impact of filament design on the analysis may be important for some samples in order to control the sensitivity of the sample analysis as well as the thermal decomposition or surface reactivity of the analyte with the filament tip.