27_Hyphenated - response Combination results in 3-D data...

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Unformatted text preview: response Combination results in 3-D data providing both quantitative and qualitative information. time One critical aspect of GC-MS and LCMS is the interface. Both chromatographic approaches have relatively large flows. GC - 1 - 50 ml / min LC - 0.1 - 5 ml / min - 50-5000 ml/min as a gas The mass spec requires a vacuum of about 10-5 - 10-6 torr - gas phase. flow from chromatograph Simplest approach would be to split the flow. MS waste Only allow what the MS can handle to actually enter. This can result in only 1/1000 or less of the sample to enter the MS - not good for trace work from GC to MS Vacuum source Most popular approach when packed columns must be used. Based on concept that larger molecules will diffuse more slowly so more will reach the MS entry jet. Relative simple and inexpensive approach. He in from GC liner He out to MS flow restrictor Somewhat similar to a jet separator. The MS pulls in about 1 ml/min through the flow restrictor. If column flow is above that - excess is vented. If flow is <1 ml/min, He from external source is pulled in. Best for sources that have flows close to 1 ml/min like capillary columns. end of column MS ionization source If we limit interface to capillary columns, the MS can actually use all column effluent. Now that we have an interface, how do we justify have a detector that costs more than the chromatograph? This is not a course in MS so we’ll limit our discussion to using the MS as a chromatographic detector. I’m assuming you know the basics of MS instrumentation and interpretation. If your scan rate is too high, you don’t obtain enough scans to properly define your peak. This results in a loss in precision. Maximum sensitivity and precision is obtained under conditions when minimum qualitative information is collected. We need some way to maximize both types of information - Do multiple runs - Target compound analysis A simple approach to obtain both quantitative and qualitative analysis. Works best if you are only dealing with a limited number of analytes. You also want to have an isotope labeled internal standard. Example Analysis of a drug in urine response 145 179 214 time 142 176 211 trideutrated internal standard Sample vapor jet •  Sample is heated and allowed to rapidly expand into a vacuum. •  Solvent is quickly pulled from component drops and a ‘static’ charge is produced. MS column •  Charged particles enter into the MS via a skimmer. to vacuum heaters high voltage A charge is transferred to the droplets. nitrogen from LC They are then sprayed into a vacuum chamber where the volatile solvent is ‘pulled off’. to MS nitrogen At some point, sample ions are ejected from the drop and enter the MS. to vacuum Thermospray and Electrospray both result in a solvent mist that is electrostatically charged. e- e- e- e- As the drops are pulled towards the skimmer, solvent is evaporated. e- From Source Towards Analyzer ee- This increases the charge density, causes the drop to further disperse and ultimately transfer the charge to the analyte. e- e- ee- Evaporation ee- e- eee- ee- e- e- e- e- e- ee- ee- e- ee- ee- e- ee- ee- ee- e- e- e- e- ee- ee- ee- ee- e- e- e- e- e- e- e- e- e- e- Breakup e- ee- ee- e- ee- interferometer injection port IR source flow cell alternate detector detector FTIR GC Two types of data are produced IR spectra for each scan Gram-Schmidt Chromatogram Full FT transformation of data is slow where as GS calculation is fast - it only results in one point per scan. This permits real time display of your data. This approach relies on doing plasma emission on your sample. Hewlett-Packard offers a small plasma emission detector based on a microwave plasma source. Relative expensive and sensitivity is very dependent on element and lines used vent holographic grating reagent and makeup gas water in plasma source window slit order sorter column movable photodiode array plasma water out •  Produces emission spectra for each element. •  A reagent gas may be needed to insure proper atomization and excitation. •  Can obtain elemental analysis and empirical formula. •  Limited at this point - can only look at a limited number of lines (due to small range of photodiode). condensing mirror ...
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