Connecting thermo balances to Pfeiffer Vacuum gas analyzers

PFEIFFERVACUUMAPASSIONFOR PERFECTIONPfeiffer Vacuum GmbHHeadquarters· GermanyT +49 6441 802-0info@pfeiffer-vacuum.dewww.pfeiffer-vacuum.com Vacuum Solutions Analytics Connecting thermo balances toPfeiffer Vacuum gas analyzers Thermal gravimetric analysis (TGA) is an analytical method bywhich the change in mass of a specimen is measured as afunction of temperature and time. The aim of the analysisin most cases is to determine the composition of a material，including in the field of rubber and plastics analysis，the analysis of mineral substances (ceramics)， as well as inthe chemical and pharmaceutical industry. Thermal gravimetric analyzers can be coupled with otheranalyzers such as a mass spectrometer， to allow easierinterpretation by determining the mass-to-charge ratio. Inthis case， it is referred to as an evolved gas analysis (EGA).This is demonstrated by the following example of use - thepyrolysis of polyvinyl (PVC) chloride. Polyvinyl chloride is a thermoplastic material which isused for a wide range of applications. Due to its excellentresistance to weathering， PVC is particularly used in theconstruction industry， for instance as a material for piping，profiles or cables. This property， however， is also at the sametime one of the great disadvantages when it comes to dispo-sing of PVC. In many cases， nothing can be recovered otherthan the energy yielded through incineration. However， duringthis process the gaseous acid hydrogen chloride and variouschlorobenzenes are formed， some of which are suspected ofbeing carcinogenic. Thermal gravimetric analysis in conjunction with evolved gasanalysis can help us to understand this combustion process.In this case， a type TGA/DSC 1 Mettler Toledo thermobalancewas connected to a Pfeiffer Vacuum ThermoStar. The speci-men consisted of 21.971 mg of PVC in powder form and washeated at a rate of 20 K/min from 100℃ to 600°C. The resultis shown in Figure 1. The measured data show that at 308.0℃ slightly morethan half of the specimen has evaporated. It is mainlyhydrogen chloride that is released (m/z=36). However，small amounts of benzene (m/z = 78) are also given off atthis stage. At 466.6 ℃ a further percentage of almost 21 % ofthe specimen is lost. With the aid of the mass spectrometer，it can be seen that this weight loss is caused by theevaporation of benzene as well as monochlorobenzeneand dichlorobenzene (m/z=112 and 146). Solution： In the trial shown above， a Pfeiffer Vacuum benchtopmass spectrometer was used. Figure 2 shows a sampleconfiguration and Figure 3 describes the configuration ina schematic diagram. It can be seen that only approximately 1% or 1-2 sccm of thegas flow from the thermobalance is led into the ThermoStar.The remaining gas is suctioned off. The thermal analysisis conducted as a rule at atmospheric pressure， while thequadrupole mass spectrometer (QMS) in the vacuum worksat a pressure of<1·105hPa. The ThermoStar is a special solution which was specificallydesigned for coupling with thermo balances. It has a quartzcapillary which can be heated up to 200 ℃ (with a 350 ℃option) with a heated hose. This prevents vapors fromcondensing during the process analysis. With the ThermoStar，Pfeiffer Vacuum has an analytical system which enables128 different masses to be monitored. Unknown gases arealso reliably identified by the analyzer with the aid of spectrallibraries. The analyzer consists of a heated and temperature-controlledgas inlet system with a quartz glass capillary and a platinumscreen， the PrismaPlus quadrupole mass spectrometer， a dryMVP 020-3 diaphragm vacuum pump and a HiPace turbo-pump. The ThermoStar is designed for qualitative and quantitativegas analysis. The analyzer can be used in the 1-100 amu，1- 200 amu or 1-300 amu mass range. Its capillary inletallows a practically segregation-free gas supply. The ThermoStar is impressively easy to handle. It can bedirectly connected to thermo balances and is ready for usestraight away. Its compact size allows the ThermoStar to beused in practically every location and it is also lightweight andportable. The user-friendly interface and backlit LCD displaymake it easy to read off all instrument-specific parameters. mCCompact， user-friendly analyzer Inert inlet， no change in gas composition GGas inlet heatable up to 200 ℃ (with a 350°℃ option) Multigas analysis Low detection limit (<1 ppm) even for condensable gases Monitors up to 128 masses FFor 1-100 amu， 1-200 amu or 1-300 amu mass range Figure 1： TGA-MS measurement of pyrolysis of PVC Figure 2： Mettler-Toledo TGA/DSC 1 connected to aPfeiffer Vacuum ThermoStar GSD 320 Pressure：<1.10 hPa Atmospheric pressure