The mass spectrometer has preceded the gas chromatograph as an analytical tool by nearly fifty years. Prior to the hyphenated technique of GC/MS the most common method of sample introduction into the mass spectrometer has been the batch inlet. Because of economic considerations and perhaps technical and cultural resistance to the continued use of the mass spectrometer, gas chromatography supplanted the use of the batch inlet mass spectrometer. In recent years, however, the mass spectrometer has become cheaper, more reliable and easier to use than in the past. For this reason Merlin is reintroducing a batch inlet specifically designed for the Hewlett-Packard MSD (mass selective detector) and easily fitted to most other instruments. Where applicable the singular advantage of using this introduction is the speed of analysis. Analytical turnaround can be less than four minutes. Many analyses can be performed by simple use of response factors applied to the ions or masses of interest in a mixture--for example the amount of toluene (m/z 92) in Benzene (m/z 78). Things are rarely this simple, but more complex mixture can be resolved.

Example

The inverse is then taken and multiplied by peaks of interest in standard spectra

yielding

We can now calculate the spectra of an unknown mixture multiplying by the inverse

and dividing by response factors we get:

This is the principle is used in many ASTM Mass Spectrometer procedures specifically D-2789 Hydrocarbon Types in Low Olefinic Gasoline by Mass Spectrometry. The batch inlet used with the mass spectrometer is an ideal method of monitoring many processes such as a reformer very close to real time, but in a laboratory environment. With an autosampler, more than 200 samples could be processed in a 24 hour period. We have found the batch inlet useful in prescreening samples for oxygenates and aromatics in gasoline where more lengthy mandated methods are required. It is useful in screening any unknown for determination of gross composition. It is well suited for quantitation of highly aqueous organic solutions. The principle of the batch inlet is simple and is illustrated in the schematic below.

Prior to injection valves A and B are closed and sample introduced through injection port. After a short equilibration period valve C is closed and valve B is opened. A small leak between the batch inlet and the mass spectrometer facilitates the ‘steady state’ introduction of sample into the spectrometer. Spectra is recorded across a range from mass 35 to 165. After completion of the short collection period, all valves are opened--rapidly removing the sample via the auxiliary pump. This process is repeated for each sample.

As a test of efficacy the mass spectrometer was tuned with the mixture specified in ASTM D-2789 using the Merlin Batch Inlet. After this initial tuning two samples were analyzed over several weeks without further calibration. Listed below are results of analyses performed on a reformat according to method D-2789.

Independent analysis of the above sample resulted in an Aromatics concentration of 66.72 L.V.% by capillary gas chromatography. Analysis by capillary gas chromatography required approximately 2 hours while the same analysis on the Merlin batch inlet analysis takes 4 minutes!

The Merlin Batch Inlet offers the following features:

• Full Integration with Hewlett-Packard MSD hardware
• Full Integration with Hewlett-Packard MSD software
• Compatibility with Hewlett-Packard Autosamplers
• No GC--Eliminates need for carrier gases and GC pneumatics
• No GC column--Analysis time under 4 minutes
• Onboard ASTM D-2789 Software for low olefin naphthas
• Integrated software for olefinic naphthas

Applications include:

• PONA Analysis of naphthas and gasoline
• Oxygenate determination in naphthas and gasoline
• Hydrocarbon (PNA) types in Kerosene and Light Middle Distillates
• QA/QC of Organic and Aqueous mixtures