Last Update: 12/23/99

Analysis of Sugars Via a New DEP Probe Tip For Use With the Direct Probe On the HP5973 MSD

By John J. Manura

INTRODUCTION

The Direct Insertion Probe (DIP) has been widely used to analyze single and multicomponent samples as described in previous application notes. However, for the analysis of high boiling point compounds or thermally labile compounds, another direct probe technique called Direct Exposure Probe (DEP) is routinely used. The DEP probe tip normally consists of a small heater wire with a loop on the end onto which the sample is placed. This heater wire can be heated to higher temperatures than the DIP Probe and can be heated very rapidly. Variations of the DEP probe tip included glass encapsillated wire tips. In order to use the DEP technique on the HP 5973 Direct Insertion Probe, a new DEP probe tip was designed which can be inserted into the HP5973 DIP Probe. This article describes this new probe tip and demonstrates its application

Experimental

A copper pin was machined to fit inside the tip of the direct insertion probe for the HP5973 MSD as shown in Figure # 1. The rounded tip of the pin extends beyond the probe tip by about 3 millimeters. On the surface of the pin a small hole, 0.030" diameter by 0.030" deep was drilled. This hole will hold 0.5 ul of liquid and is intended to allow the easy injection of samples onto the probe pin tip. After machining the probe tip was gold plated top provide a more inert surface.

Figure 1

Solution of Fructose and Sucrose were prepared in Methanol at a concentration of 500 nanogram per microliter. One half (0.5) microliter of this solution was then injected onto the tip of the DEP probe tip. 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 500^o per minute.

Three studies were done to evaluate this technique.

The copper tip proved to be quite satisfactory for the analysis of the sugar samples (Figure #2). 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 and, therefore, increased sensitivity.

Figure 2

Study the Effect of Different Heating Temperatures

In the first series of tests, the probe tip was ballistically heated to different temperatures from 250^o C to 450^o C (Figure # 3). 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 3

Study the Effect of Temperature Ramping

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 500^o per minute). In addition, it can be temperature programmed when this is required. The sugar samples were ramped at various rates from 50 to 500^o per minute ( Figure # 4 ). 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 4

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. In this study, sugar samples were analyzed in less than 1.5 minutes using the ballistic heating probe method. The direct probe technique for the HP 5973 MSD has been improved by the copper sample vial holder. This new tip had a small well for the sample with less surface area, which could be heated more rapidly - thereby producing greater sensitivity. Using these new copper 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. 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.

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