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Note 66: Probe Tip Design For the Optimization of Direct Insertion Probe Performance

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By John J. Manura

Presented at ASMS, Orlando, FL, CA., June 1998

INTRODUCTION

The direct probe has been used for many years to introduce samples into the mass spectrometer. It has the advantages of permitting a quick analysis with minimal sample preparation. However, it has the disadvantage of making the identification of mixtures of compounds difficult due to the complexity of the resulting mass spectrum when multiple compounds are analyzed. Many users have overcome this obstacle utilizing the chemical ionization mass spec technique. This produces simplified spectra which can be interpreted with multiple component samples. However, the simplified spectra do not provide a conclusive identification of the analytes.

The purpose of this study is to evaluate the design of the direct probe tip and the direct probe sample vial to improve the direct probe technique. Different style probe tips as well as probe tips made from different materials were studied. Both the sensitivity of the technique for different styles of probe tips as well as the ability of the technique to separate multiple components in a single sample were studied. In addition, it would be beneficial to develop a probe tip that can analyze samples that could not be analyzed by other techniques due to the thermal instability of the compounds, high molecular weight or their inability to be chromatographed.

Figure 1

Figure 1 - Glass Sample Vial For the HP5973 Direct Insertion Probe

The standard probe uses flared glass sample vials (1.7mm diameter by 9 mm long) into which the sample is inserted. The sample vial is inserted into the probe tip where it is held in place by a small spring (Figure # 1). The glass sample vial is the standard vehicle for introduction of samples into the mass spectrometer. Solid matrix samples can be easily inserted into the glass sample vial for subsequent analysis. Most frequently, samples are dissolved or extracted into a volatile solvent, as methanol or chloroform and is then injected into the sample vial using a standard microliter syringe. The solvent is then evaporated to dryness, leaving the analyte residue in the glass vial for analysis. The glass sample vials can hold up to 3 uL of sample. The disadvantage of this technique is the design of the sample vial. The vial is quite deep, thereby making it difficult to volatilize the entire sample at the same time and introduce the volatile sample into the mass spec source in short time period. It is anticipated that the probe technique could be improved by making this vial cavity smaller and constructing the sample vial out of a material that would heat more quickly. These two design improvements would introduce the sample into the mass spec source in a shorter period of time, thereby improving the sensitivity of the technique.

Experimental

The Scientific Instrument Services Direct Insertion Probe was used in conjunction with the Hewlett Packard 5973 MSD. The mass spec probe tip can be heated ballistically up to 450oC at a ramp rate in excess of 400o per minute. The probe can also be temperature programmed for slower heating applications. The HP 5973 MSD was operated in the El mode and was scanned front mass 50 to 350 daltons. The mass spec was scanned at 1 scan per second. The GC control systems were disconnected via the software controls, and the analysis time was set for 3.0 minutes. After the sample analysis was complete, the total ion chromatogram was viewed using the ChemStation data analysis module. The resulting mass spectrum was recorded.

For this study, a mixture of the sugars fructose and sucrose were used. These two sugars were mixed and dissolved in methanol. This solution was then injected into the various probe tips for analysis. These sugars can not normally be analyzed via the direct probe technique using glass sample vials. The methanol solvent in each of the sample vials was allowed to evaporate to dryness before analyzing any of the samples. This was accomplished in about 30 seconds by allowing the solvent to evaporate at room temperature. Each sample was then inserted into the mass spec probe and inserted through the vacuum lock system of the probe inlet into the mass spec source of the HP 5973 MSD. Each sample was then heated ballistically, and the mass spec was scanned to analyze the thermally extracted sample. The total analysis time was 2 minutes or less per sample. As an alternative technique, the direct probe was temperature ramp at rates of 50 to 500o per minute.

Figure 2

Figure 2 - Gold Plated DEP Probe TIP For the HP5973 Direct Insertion Probe

The fist probe tip was constructed out of a copper rod. The copper rod fit snugly into the probe tip and was made long enough to extend fully into the direct probe cavity and extent out about 2.0 millimeters. The copper probe tip was tapered at the end protruding out of the probe, and a small well (0.03" diameter x 0.30" deep) was drilled into the tip (Figure # 2). Copper was chosen due to its ability to heat quickly. The small sample well holds less than 0.5 uL of sample. This probe tip was then tested by injecting the sugar sample into the sample well and then heating it ballistically to various temperatures from 250o C to 450o C. Next, the probe tip was studied by heating it at different ramp rates from 50o per minute to 500o per minute.

A final probe tip was constructed from a solid gold rod. It was shaped the same as the copper tip above (.Figure # 2). Studies were conducted using this probe tip, as were done for the copper probe tip above. Additional studies were done on the length of the probe tip to see how this effected the sensitivity of the technique.

Results and Discussion

The copper tip proved to be quite satisfactory for the analysis of the sugar samples (Figure #3). The analysis of the sugar mixture produced two distinct peaks with distinctive mass spectra for each sugar. Similar results were obtained using mixtures of pharmaceuticals in previous studies of the HP 5973 probe using glass sample vials. However, this technique proved to be much faster and more sensitive than achieved using glass sample vials. The sample was contained in a smaller well with less surface area that was heated more quickly. The resulting total ion chromatograph produced peaks that had less peak width and were much higher, therefore, increased sensitivity.

Figure 3

Figure 3 - Analysis of Sugars Via MSD-DEP Probe Tip

In the first series of tests, the probe tip was ballistically heated to different temperatures from 250o C to 450o C (Figure # 4). The higher the temperature, the faster the analytes were desorbed off the probe tip producing less band width and greater sensitivity. Only a minor loss in resolution occurred at the higher temperatures. No decomposition of the sample was apparent at the higher temperatures.

Figure 4

Figure 4 - Analysis of Sugars Via Copper MSD-DEP Probe Tip

The second series of tests studied the copper tip to determine the effect of ramp rate. The new HP5973 probe was designed to ramp at high rates (greater than 500o per minute). In addition, it can be temperature programmed when this is required. The sugar samples were ramped at various rates from 50 to 500o per minute ( Figure # 5 ). These were compared to the ballistic heating of the probe tip described above. Higher ramp rates produced less band width and greater sensitivity. The ballistic method was still the most sensitive method of analysis.

Figure 5

Figure 5 - Analysis of Sugars Via Copper MSD-DEP Probe Tip

The gold probe tip was studied using the same sugar sample and studied as above. The gold tip produced even better results. Heating the gold tip ballistically to 450o C produced even less band width and greater sensitivity than the copper probe tip (Figure # 6). This is probably due to the better heat conduction of gold over copper. Additional studies were conducted on the length of the gold probe tip that extends from the probe rod. Probe tips that extend further from the probe rod and into the mass spec source lost some of the heating capacity the probe rod. A probe tip that extended 1.0 to 2.0 millimeters from the tip of the probe provided optimal results.

Figure 6

Figure 6 - Comparison of Gold and Copper DEP Probe Tip

Conclusion

The heated direct probe sample introduction system has proven to be a useful addition to new Hewlett Packard 5973 MSD for the analysis of solid matrix samples. Samples were analyzed in less than 1.5 minutes using the ballistic heating probe method. However, the probe can be temperature programmed at slower rates for the purpose of thermally separating mixtures of drugs in a single sample. The direct probe analysis of samples has proven to be a sensitive technique for solid sample analysis with detection levels down to 10 ng. The peak width is quite large as compared to a GC analysis and explains the loss in sensitivity compared to conventional GC analysis, where peak widths are normally less than 5 seconds. The highly sensitive HP 5973 MSD and the versatile ChemStation software permit the analysis of multiple component samples via the El mass spec probe technique to achieve fast and definitive method of analysis. The direct probe introduction system interfaced to HP 5973 will be a valuable technique for the analysis of many drug and pharmaceutical samples, where fast sample analysis is required.

The direct probe technique for the HP 5973 MSD has been improved throuh the design new gold sample vial holder. The gold probe tip used in the ballistic mode produced the best results of the various tips studied. This new tip had a smaller well for the sample with less surface area, which could be heated more rapidly - thereby producing greater sensitivity. Using these new gold probe tips, the sensitivity of the direct probe was increased 2 to 20 times over direct probe technique using glass sample vials. In addition, samples as sugars, high boiling pesticides and pharmaceuticals can be analyzed. Previously, these samples were difficult or impossible to analyze via the direct probe. In addition, mixtures of analytes in the same sample can also be analyzed. This analysis of mixtures is enhanced by the use of the HP ChemStation software to separate and identify the multiple component samples.

Additional work still needs to be done with this technique such as analyzing additional compounds and mixtures of compounds. The reproducibility of the technique also needs to be studied and improved. In addition, glass sample vials with smaller sample wells need to be studied and compared to the gold probe tips. The disadvantage of the gold tips is their cost. The tips must be cleaned and reused, but the glass sample vials are inexpensive and disposable.