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Last Update: 04/11/01

AutoProbe DEP Probe Tip Temperatures

Article by:   John J. Manura, Scientific Instrument Services, Inc., 1027 Old York Road, Ringoes, NJ 08551

Introduction:  

The DEP probe is a useful technique for the introduction of samples into the mass spectrometer.  The technique is popular because it permits the rapid analysis of samples with minimal sample preparation.  An automated version of the DEP probe technique has recently been developed by Scientific Instrument Services called the AutoProbe.

The DEP is heated by passing a DC current through a platinum coil.  The value for the current is accurately measured and displayed by the AutoProbe electronics and PC screen, however the actual DEP coil temperature is not displayed.  If would be useful to know the actual temperature of the DEP coil to perform optimum analysis of samples.  This paper describes the techniques and calculations used to determine the temperature of the DEP coil tip on the SIS AutoProbe. 

Instrumentation

Figure 1 – AutoProbeÔ attached to the Thermo Finnigan TRACEÔ MS

The AutoProbeÔ System from Scientific Instrument Services, Inc. is attached to the Thermo Finnigan TRACEÔ MS system.  The AutoProbe System is designed to inject samples directly into the MS source. The Thermo Finnigan XcaliburÔ software program is used for the operation of the MS.  The AutoProbe software for the operation of the AutoProbe is fully integrated into the Xcalibur software. 

Figure 2 - Plug in DEP Probe Tip for the AutoProbe

The AutoProbe uses a plug in Direct Exposure Probe (DEP) tip on the end of the auto injecting probe shaft.  The DEP coil is constructed from a 0.005" diameter x 0.375" long piece of pure platinum wire. The coil is spot welded to two stainless steel posts which are connected to two pins which plug into the electrical connectors on the end of the probe shaft. 

Figure 3 - DEP Tip attached to the AutoProbe  Shaft

Samples for analysis are dissolved in a suitable solvent and 0.125 to 1.0 ul of the prepared sample solution is injected onto the DEP coil on the tip of the probe. 

Figure 4 - Loading samples onto the DEP coil

Three levels of temperature control are entered into the AutoProbe method screen in the Xcalibur software. These current values are entered into the screen shown below and stored as part of the Xcalibur method.  Temperatures for each of the steps can either be isothermal or programmed at a ramp rate and the value held for a specified period of time in seconds. All values are entered in millamps of DC current  and the filament current is  measured by the AutoProbe electronics during operation.  

Figure 5 - AutoProbe Method Screen for the input of DEP Probe Current Values

After the sample is loaded onto the DEP coil, a small current is passed through the DEP coil to evaporate the solvent from the sample.  This solvent removal step can either be accomplished in air (outside the MS vacuum system) or inside the MS vacuum system (but not in the MS source).  This small current is needed to evaporate the solvent but not volatilize any of the sample of interest.  Either a constant current or a ramped current can be programmed into the AutoProbe software for this operation.  The current required for this step is measured by the AutoProbe electronics and displayed on a PC Window in the Thermo Finnigan XcaliburÔ software program for the operation of the MS. 

After the probe has been injected into the MS source, the sample is analyzed by passing a higher current, which was previously programmed into the AutoProbe method screen, to volatilize the sample into the MS source. 

As a final step, the DEP probe tip is removed from the MS source (but still in the MS vacuum system) and heated to a higher temperature to totally volatize any remaining sample residue from the DEP coil.  This assures that the DEP coil is clean and no carry over or "memory effects" will occur in the next sample analyzed by the AutoProbe. 

Experimental 

In the three steps of DEP coil heating describe above, only values for the filament current are entered into the setup screens and only the DEP filament current is measured by the AutoProbe software.  The actual temperature of the DEP coil is not measured or reported.  

This paper demonstrates the use of the Stefan Boltzman equation to determine the temperature of the DEP coil on the AutoProbe.  The Stefan Boltzman equation is as follows:

        Where:

•     P = Power dissipated in the filament coil in watts

•     S = the surface area of the filament coil in square meters

•       Π   = the Boltzman constant 5.67 x 10^-8

•     T₂ = the temperature of the filament (in degrees K)

•     T₁ = the temperature of the environment in which the filament is placed (in degrees K)

This formula determines the temperature of the filament by equating the power (P) emitted by the filament as a function of the surface area of the filament. 

The power dissipated by the filament is determined by measuring the current through the filament and then measuring the DC voltage drop across the filament.  Both the current and voltage can be measured for the filament when it is operating either in air or in vacuum inside the MS.

    Where:

•     P = Power dissipated in the filament coil in watts

•     V = the voltage drop across the filament in volts

•     I = the current through the filament in amps

The surface area of the filament coil is determined by measuring the diameter and length of the platinum coil used for the DEP coil tip.

    Where:

•     S = the surface area of the filament coil in square meters

•     d = the diameter of the filament wire in meters

•     l = the length of the filament wire in meters

•     π (pi)   =  3.14

Substituting the components for power and surface area and rearranging the Stefan Boltzman equation to determine the temperature of the filament (T₂ ) yields the following equations:

Solving for T₂⁴:

Solving for T₂ - the filament temperature in degrees K:

For these studies a measured current was passed through the DEP filament coil and the voltage drop across the filament was measured with a DC digital voltmeter.  When the probe was in vacuum inside the MS vacuum system, DC current values from 16 milliamp (0.016 amp) up to 1200 milliamp (1.200 amp) were applied to the filament and the voltages measured.  More than 20 different current values were measured throughout the specified range.  When the probe was in air, DC current values from 16 milliamp (0.016 amp) up to 150 milliamp (0.150 amp) were applied to the filament and the voltages measured.  The values measure in air and vacuum were near identical for all current values.  The DEP probe could not be taken to higher values in air, because the filament would oxidize in air at temperatures in excess of 300 degrees C.  These values for current and voltage were then substituted into the formula above to determine the temperature of the filament at the specified current level.  Since the calculated temperature is in degrees K, 273 was subtracted from the calculated values to determine the filament temperature in degrees C.

The following chart displays the values for the temperature of the AutoProbe DEP filament as a function of the filament current when the filament is exposed to air at  normal room temperature (20 degrees C).  A linear line was approximated to the data for the purpose of developing an easy formula for the determination of filament temperature as a function of filament current. The temperature of the filament can be read from the chart or calculated from the formula in the chart. 

The following chart summarizes this data and can be used to determine the current required to volatilize the solvent from the sample which has been applied to the DEP probe tip at the first step of analysis. These current values can be used in the method screen to set the DEP coil temperature for the solvent removal step. The same currents will achieve this temperature in air as well as in the MS vacuum.  However lower temperatures are required to vaporize solvents at the lower pressures in the MS. When the filament is in air, the current through the DEP coil should not exceed 100 milliamps, because this could cause the filament to prematurely burn out due to oxidation of the filament in the presence of oxygen. 

The chart below displays the values for the temperature of the AutoProbe filament as a function of filament current when the filament is in the mass spec vacuum and inserted into the MS source at various source temperature from 20 to 300 degrees C.  This chart can be used to determine the DEP filament temperature for any MS source temperature.  The curves are not linear over the entire filament current range due to the contribution of the environment temperature to the filament temperature. However at filament temperatures above 700 degrees the plots for all the source temperatures are nearly identical. 

For the above plots, a linear formula could be determined to approximate the temperature of the filaments as a function of filament current.   This formula for the plot at 150 degrees C is as follows:

     T = 0.886 * I (mA) + 131

However any data calculated with this formula is not exact.  Better data for the various source temperatures is more accurately extrapolated from the plots above. The following chart  summarizes this data and can be used to determine the current required to use the AutoProbe and to desorb  sample analytes off the DEP probe coil and into the MS source.  These current values can be used in the method screen to set the DEP coil temperature for the sample analysis step.

Conclusion

The temperatures of the DEP coil on the AutoProbe can be determined from the measured DEP filament currents.  Three different current values are required for the operation of the AutoProbe which can be determined from the data reported above.

(1)  Solvent Removal Step.  From this data the MS user can select filament current values which will evaporate the sample solvent without desorbing the analytes of interest off the DEP filament wire.  Typically temperatures between 40 and 100 degrees C are used for this step which equate to current values between 29 and 79 milliamps.

(2)  Sample Analysis Step.  For the analysis of MS samples, filament currents can be selected to desorb the analytes of interest off the DEP wire and into the MS source.  Excessive temperatures which could pyrolyze the analytes can be avoided.  Typically temperatures between 100 and 800 degrees C are selected for this purpose.  However the user must take into account the temperature of the MS source when selecting the correct current value to achieve the required temperature of the DEP coil.

(3)  Filament Cleaning Step.  For the cleaning of the DEP coil after sample analysis, temperatures between 800 and 1200 degrees can be determined to completely remove residues of samples from the DEP wire and thereby avoid cross contamination or "memory effects" in subsequent samples that will be analyzed.  Current values between 750 and 1200 milliamps can be used for this purpose.  However it must be kept in mind that the higher the temperature, the shorter the filament life.  Baking out filaments at currents of 900 milliamp provided a filament life of more than 2000 analysis in our previous studies.  Bake outs of more than 1100 milliamps will greatly shorten this DEP filament lifetime. 

The AutoProbe technique has been proven to be a quantitative MS injection technique that can reproducibly and quantitatively analyze higher molecular weight samples in about 3 minutes per sample.  The technique will be invaluable in the analysis of samples not only in a quality control laboratory but also in a laboratory where high volumes of samples need to be analyzed and quantitated in a short period of time.

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