Methods may include emplacing a wellbore tool in a wellbore, the wellbore tool including a gas chromatograph and a mass spectrometer, wherein the mass spectrometer is configured to operate at a pressure greater than 10−2 Torr, measuring a sample from the wellbore using the wellbore tool, and determining a molecular weight of one or more components of the sample from the measured response of the wellbore tool. Methods may also include establishing a library of one or more chemical components, emplacing a wellbore tool in a wellbore, the wellbore tool including a gas chromatograph and a mass spectrometer, wherein the mass spectrometer is configured to operate at a pressure greater than 10−2 Torr, measuring a sample from the wellbore using the wellbore tool, comparing the measured response from the wellbore tool for the sample with results from the library of one or more chemical components, and determining a molecular weight of one or more components of the sample.