Continuous online process analytics with multiplexing gas chromatography by using calibrated convolution matrices.

The development of fast and precise measurement techniques for process analytical technology is important to operate chemical processes safely and efficiently. For quantitative measurements of multiple components at a trace level, often gas chromatographic methods are used which have a response time of several minutes or of up to one hour. For fast changing processes, this can be too slow for efficient control. For reducing the dead time of a control loop by increasing the measurement frequency, a multiplexing gas chromatography (mpGC) technique for a chromatographic system exhibiting a systematic non-linear response has been developed. For mpGC, superimposed chromatograms are measured by injecting consecutive samples before all components of previous samples have eluted from the column. The deconvolution of a superimposed chromatogram yields a computed chromatogram which is an average over the single chromatograms forming the superimposed chromatogram. Such a computed chromatogram typically shows so called correlation noise depending on the degree by which the single chromatograms forming the superimposed chromatogram will differ from each other (non-linear response). A technique is presented to calibrate the convolution matrix in order to suppress correlation noise introduced by systematic errors of the chromatographic system. The remaining correlation noise in the computed chromatogram is then exclusively caused by changing concentrations in the sample stream. For the method presented here, the sample is injected five times during the run time of a single chromatogram. The computed chromatogram is obtained three times within this timespan while representing each time an averaged chromatogram over the last five injections. Therefore, the sample throughput is increased by a factor of three compared to conventional GC.

Online High Throughput Measurements for Fast Catalytic Reactions Using Time-Division Multiplexing Gas Chromatography.

Developing new catalysts is crucial for optimization of chemical processes. Thus, advanced analytical methods are required to determine the catalytic performance of new catalysts accurately. Usually, gas chromatographic methods are employed to analyze quantitatively the product distribution of volatile compounds generated by a specific catalyst. However, the characterization of rapidly changing catalysts, e.g., due to deactivation, still poses an analytical challenge because gas chromatographic methods are too slow for monitoring the change of the complex product spectra. Here, we developed a gas chromatographic technique based on the concept of multiplexing gas chromatography (mpGC) for fast and comprehensive analysis of the product stream from a catalytic testing unit. This technique is applied for the study of the catalytic reaction of methanol-to-olefins (MTO) conversion. For this method, the time distance between two measurements is chosen so that the chromatograms but not the peaks themselves are superimposed. In this way, stacked chromatograms are generated in which the components from successively injected samples elute baseline separated next to each other from the column. The peaks from different samples are interlaced, and for this reason, the method is referred to as time-division multiplexing gas chromatography (td-mpGC). The peaks are analyzed by direct peak integration not requiring a Hadamard transformation for deconvolution of the raw data as usual for many mpGC applications. Therefore, the sample can be injected equidistantly. The integrated peaks have to be allocated to the correct retention times. The time distance between two measurements for studying the reaction and regeneration cycles of MTO catalysts is 4.3 min and 38 s, respectively. Column switching techniques such as back-flush and heart-cut are introduced as general tools for multiplexing gas chromatography.

Online Continuous Trace Process Analytics Using Multiplexing Gas Chromatography.

The analysis of impurities at a trace level in chemical products, nutrition additives, and drugs is highly important to guarantee safe products suitable for consumption. However, trace analysis in the presence of a dominating component can be a challenging task because of noncompatible linear detection ranges or strong signal overlap that suppresses the signal of interest. Here, we developed a technique for quantitative analysis using multiplexing gas chromatography (mpGC) for continuous and completely automated process trace analytics exemplified for the analysis of a CO2 stream in a production plant for detection of benzene, toluene, ethylbenzene, and the three structural isomers of xylene (BTEX) in the concentration range of 0-10 ppb. Additional minor components are methane and methanol with concentrations up to 100 ppm. The sample is injected up to 512 times according to a pseudorandom binary sequence (PRBS) with a mean frequency of 0.1 Hz into a gas chromatograph equipped with a flame ionization detector (FID). A superimposed chromatogram is recorded which is deconvoluted into an averaged chromatogram with Hadamard transformation. Novel algorithms to maintain the data acquisition rate of the detector by application of Hadamard transformation and to suppress correlation noise induced by components with much higher concentrations than the target substances are shown. Compared to conventional GC-FID, the signal-to-noise ratio has been increased by a factor of 10 with mpGC-FID. Correspondingly, the detection limits for BTEX in CO2 have been lowered from 10 to 1 ppb each. This has been achieved despite the presence of detectable components (methane and methanol) with a concentration about 1000 times higher than the target substances. The robustness and reliability of mpGC has been proven in a two-month field test in a chemical production plant.

Microsolvation of molecules in superfluid helium nanodroplets revealed by means of electronic spectroscopy.

The empirical model explaining microsolvation of molecules in superfluid helium droplets proposes a non-superfluid helium solvation layer enclosing the dopant molecule. This model warrants an empirical explanation of any helium induced substructure resolved for electronic transitions of molecules in helium droplets. Despite a wealth of such experimental data, quantitative modeling of spectra is still in its infancy. The theoretical treatment of such many-particle systems dissolved into a quantum fluid is a challenge. Moreover, the success of theoretical activities relies also on the accuracy and self-critical communication of experimental data. This will be elucidated by a critical resume of our own experimental work done within the last ten years. We come to the conclusion that spectroscopic data and among others in particular the spectral resolution depend strongly on experimental conditions. Moreover, despite the fact that none of the helium induced fine structure speaks against the empirical model for solvation in helium droplets, in many cases an unequivocal assignment of the spectroscopic details is not possible. This ambiguity needs to be considered and a careful and critical communication of experimental results is essential in order to promote success in quantitatively understanding microsolvation in superfluid helium nanodroplets.

Rotational spectroscopy of single carbonyl sulfide molecules embedded in superfluid helium nanodroplets.

The pure rotation spectrum of carbonyl sulfide embedded in superfluid helium nanodroplets was measured in the frequency range from 4 to 15.5 GHz. Four lines, corresponding to the J = 1-0, J = 2-1, J = 3-2, and J = 4-3 transitions, were found. The line widths of the transitions increase with increasing rotational quantum number J, which is indicative of a distribution of the effective B rotational constant. A comparison of the pure rotational spectrum with the microwave-infrared double resonance spectrum [S. Grebenev, M. Havenith, F. Madeja, J. P. Toennies, and A. F. Vilesov, J. Chem. Phys., 2000, 113(20), 9060] reveals that the double resonance measurement scheme probes predominantly rotational transitions within the vibrationally excited state.

Structure and dynamics of phthalocyanine-argonn (n = 1-4) complexes studied in helium nanodroplets.

Van der Waals clusters of phthalocyanine with 1-4 argon atoms formed inside superfluid helium nanodroplets have been investigated by recording fluorescence excitation spectra as well as emission spectra. The excitation spectra feature a multitude of sharp lines when recorded in superfluid helium droplets in contrast to the respective spectra measured in a seeded supersonic beam (Cho et al. Chem. Phys. Lett. 2000, 326, 65). The pickup technique used for doping of the phthalocyanine and the argon into the droplets allows for nondestructive analysis of the cluster sizes. Alternation of the pickup sequence gives information on the binding site of the argon atoms. The investigation of dispersed emission spectra in helium droplets can be used as a special tool for the identification of 0(0)0 transitions within the variety of sharp lines seen in the excitation spectra. Thus, different isomers of the clusters can be distinguished. Moreover, the emission spectra reveal information on dynamic processes such as vibrational predissociation of the van der Waals complexes and interconversion among isomeric species. The binding energy of the phthalocyanine-argon1 complex in helium droplets was estimated to be at most 113 cm-1.

Evidence for an energy level substructure of molecular states in helium droplets.

The pure tunneling inversion transition of ammonia embedded in (4)He droplets was investigated in the microwave frequency range. We observed a spectrum that consists of a sharp peak, only 15 MHz wide, on top of a broad feature. The peculiar line shape could be simulated with an empirical model and is a clear experimental evidence for an energy level substructure of molecular states in doped helium droplets.

Fine structure of the (S(1)<--S(0)) band origins of phthalocyanine molecules in helium droplets.

The laser-induced fluorescence (LIF) excitation spectra of free base phthalocyanine (Pc), Mg-Pc, and Zn-Pc molecules in superfluid helium droplets at T=0.38 K have been studied. The spectra reveal the rich vibronic structure of the S(1)<--S(0) electronic transitions. The band origins of the transitions consist of zero phonon lines accompanied by phonon wings, which originate from simultaneous electronic excitation of the molecule and excitation of the collective modes of the helium surrounding it. The phonon wings have discrete structures suggesting localization of some helium atoms in the neighborhood of the molecules. Zero phonon lines of Mg-Pc and Zn-Pc molecules are split into three components, which are separated by 0.2-0.4 cm(-1). Possible mechanism of splitting involves static or dynamic Jahn-Teller interaction of metal-phthalocyanine molecules in the twofold degenerate S(1)((1)E(u)) state with the helium shell.

Microsolvation of phthalocyanines in superfluid helium droplets.

Experimental and theoretical investigations of the spectroscopy of molecules in superfluid helium droplets provide evidence for the key role of the first helium layer surrounding the dopant molecule in determining the molecule's spectroscopic features. Recent investigations of emission spectra of phthalocyanine in helium droplets revealed a doubling of all transitions. Herein, we present the emission spectra of Mg-phthalocyanine and of phthalocyanine-argon clusters in helium droplets, which confirm the splitting as a general effect of the helium environment. A scheme of levels is deduced from the emission spectra and attributed to quantized states of the first helium layer surrounding the dopant molecule.