Flow Field Thermal Gradient Gas Chromatography - Analytical

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Flow Field Thermal Gradient Gas Chromatography Peter Boeker* and Jan Leppert Institute of Agricultural Engineering, University of Bonn, Nussallee 5, D-53115 Bonn, North Rhine-Westphalia, Germany S Supporting Information *

ABSTRACT: Negative temperature gradients along the gas chromatographic separation column can maximize the separation capabilities for gas chromatography by peak focusing and also lead to lower elution temperatures. Unfortunately, so far a smooth thermal gradient over a several meters long separation column could only be realized by costly and complicated manual setups. Here we describe a simple, yet flexible method for the generation of negative thermal gradients using standard and easily exchangeable separation columns. The measurements made with a first prototype reveal promising new properties of the optimized separation process. The negative thermal gradient and the superposition of temperature programming result in a quasi-parallel separation of components each moving simultaneously near their lowered specific equilibrium temperatures through the column. Therefore, this gradient separation process is better suited for thermally labile molecules such as explosives and natural or aroma components. High-temperature GC methods also benefit from reduced elution temperatures. Even for short columns very high peak capacities can be obtained. In addition, the gradient separation is particularly beneficial for very fast separations below 1 min overall retention time. Very fast measurements of explosives prove the benefits of using negative thermal gradients. The new concept can greatly reduce the cycle time of high-resolution gas chromatography and can be integrated into hyphenated or comprehensive gas chromatography setups.

T

methods were modified by different thermal techniques, e.g., flows of fluids with heat exchange along the column, heat conduction from hot to cold zones, or heat production with variable heat density.5 Most of these techniques were only suitable for short packed columns but not applicable to long capillary columns. The transfer to long capillary columns resulted in elaborate and costly manual setups. To mention only a few: Fatscher and Vergnaud6 wrapped heating wire with decreasing wrapping density on a column. Phillips and Jain7,8 manually applied a conductive paint on a column with increasing coating thickness. Rubey9 used a sheath around the column to transfer cold gas along the column which heated up due to resistive collinear heating. All these concepts have in common that they depend on craftsmanship, are inflexible from a physical point of view, and require a distinct amount of effort to change a separation column in case of degradation. These days, the research group of Milton Lee is active in the thermal gradient GC (TG-GC) field. Contreras et al. have published two papers10,11 on the topic with two different technical approaches. In the first contribution a polyimide tube is used as a heat exchanger for a resistively heated column; the second approach utilizes a system with many independent wired heating zones to flexibly create gradients of different

he idea of the application of negative thermal gradients in the separation process goes back to Zhukhovitskii and Turkeltaub1 in the 1950s. The original idea was termed “chromathermography” by Zhukhovitskii. Originally, chromathermography was intended as a preconcentration method directly on packed separation columns to enhance the sensitivity for trace components.2,3 The ingenuity of the concept stems from two independent effects. If a stream of trace components flows through a separation column with decreasing temperature, every trace component will move along the column until the temperature is low enough to stop the movement. At this location on the column the whole amount of the respective trace component is absorbed in the stationary phase of the column during the preconcentration time. After this initial preconcentration time the temperature level is raised, whereas the negative gradient is maintained. The components then start to move toward the end of the column and the detector. During this movement the negative temperature gradient is focusing the components on the column. The front of a substance peak is in a colder region and therefore decelerated; the tail of this peak is in a warmer region and therefore accelerated, respectively. The net effect results in peak focusing and therefore sensitivity enhancement. Technically, the negative gradient was applied by a gradient heater which was moved along the column. In a certain setup the column was arranged in a circle with the heater repeatedly moving around thus stripping the analytes from the column at ever-higher temperature levels. 4 Later these mechanical © XXXX American Chemical Society

Received: June 12, 2015 Accepted: August 2, 2015

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DOI: 10.1021/acs.analchem.5b02227 Anal. Chem. XXXX, XXX, XXX−XXX

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Analytical Chemistry shape and velocity. A recent publication of Tolley et al.12 explores the theoretical background and derives some important conclusions about the resolution limits. In 1992, Blumberg had already outlined the theoretical background of thermal gradient GC.13 The full theory was published in 1994.14 Blumberg developed the idea of an “ideal basic separation” with an optimum resolution that cannot be exceeded by any approach including thermal focusing (Blumberg’s theorem). Although he argues against naive expectations, he also mentions the favorable effects of gradients in case of nonideal conditions. In 1997, a controversy was published15 between Blumberg and Jain and Phillips following their publications on TG-GC.7,8 Again, Blumberg stressed the validity of his theorem but also stated that “The focusing can be a great tool in helping to recover the losses in resolution and/or speed of analysis resulting from nonideal chromatographic conditions (poor sample introduction, column overloading, dead volumes, reversal of separation of some solute pairs during temperature programming etc.)”. In their reply to Blumberg, Jain and Phillips underline the existence of these nonideal conditions and the complexity of chromatography in the real world. Therefore, they conclude that thermal gradients in fact have positive prospects. In this contribution we attempt to prove this claim.

the outer layers of the bundle. Therefore, the uniformity of the temperature along the column length is poor. The temperature of the column inevitably will meander around the mean temperature over the length. The new concept of flow field thermal gradient gas chromatography (FF-TG-GC) is explicitly based on the control of the convective heat loss from a heated capillary.22 Unlike all previous concepts, a forced flow is utilized around the column. The equilibrium temperature is strongly dependent on the velocity of this flow. The gradient flow profile along the column is linked to a gradient temperature profile. From a theoretical point of view the new concept changes the focus from gradient heat production7,11 or gradient heat transfer9,10 to gradient heat loss. If the heat loss of the equilibrium can be controlled, the equilibrium temperature can also be controlled and varied over a wide range. The theory of such heat transport processes is an established field of engineering science.23 We have calculated the effects by the theory of nondimensional parameters and have found a reasonable parameter range to modulate the temperature of a resistively heated column by a controlled forced external flow onto the column. The basic principles of the calculation can be found in the Supporting Information. The next task is to generate a temperature gradient by increasing this external cooling flow along the column, i.e., the production of a flow field along a separation column of several meters length. We solved this problem by a controlled pressure drop (Figure 1). The separation column is positioned in a helical

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FLOW FIELD THERMAL GRADIENT GAS CHROMATOGRAPHY Resistive column heating is an established technology to avoid the high thermal mass ovens of conventional GCs. Mostly the heat is generated either directly in a metal separation column or in a separate metal sheath around a fused-silica separation column.16−18 The concept of resistive heating looks simple but in fact depends on a subtle energy balance between resistive heat production, convective heat transfer, and radiative energy loss. The general energy balance of resistive heating is energy production by dissipation of electrical energy = energy loss due to convection + radiative energy loss

It sometimes appears as if only the heat production part is taken into account by the technical concepts of resistive column heating. But, in fact, the heat loss due to convection has the main influence on the uniformity and stability of the temperatures; neglecting heat convection results in uneven temperatures and cold and hot zones. For a high-resolution separation the temperature stability and uniformity in conventional GC ovens are carefully controlled.19 The same requirements should be observed in resistive heating systems. Up to now the resistive heating systems depend on natural convection as the main mechanism of heat loss. Natural convection takes place by the ascending of warming air from a heated surface. This process cannot be controlled as it depends on the decreasing density of the warming air. Furthermore, the amount of heat loss, and therefore the equilibrium temperature, depends on the orientation of the warm surface. Horizontally oriented sections of heated columns become colder than vertical parts due to the different thermal conditions. Bundles of a column, heating wire, and temperature sensor wire are also used as fast resistively heated systems.20,21 Here, the situation is even worse. The heat production inside a bundle is balanced by the convective loss at the outer surface. Physically, a gradient will establish between the inner parts and

Figure 1. Schematics of the generation of a temperature gradient by a cooling gradient flow field generated by a pressure drop inside a tube with a helical channel and packed with porous material.

channel milled into a tube. Inside the tube porous material functions as a continuous flow resistance. Air pressed into the tube finds its way through the porous material and leaves the tube via the helical channel. Because of the increasing pressure drop or flow resistance the flow decreases as the flow path increases through the porous material. The result is a smooth decreasing flow from the bottom to the top of the helical channel. The variation of the intake flow via the gradient fan allows the variation of the temperature gradient. Measurements with a thermocouple inside the heated capillary have shown a nearly linear temperature gradient from the inlet to the outlet. B

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Analytical Chemistry It is worth mentioning that without the gradient flow (e.g., with no cooling flow) such a system can also be operated as a classical temperature-programmed GC with uniform temperatures along the column. Therefore, a direct comparison between the classical and the gradient mode is possible by this technical realization. By modification of the porous material inside the tube, e.g., layers of different flow resistance, the shape of the resulting thermal gradient can also be modified, from linear to nonlinear gradients (e.g., concave or convex down). With this specific realization of the flow field TG-GC principle the gradient between the inlet and the outlet of the separation column can be controlled by the flow (pressure) generated by the gradient fan. Increased pressure affects the flow at the bottom more than at the top parts. Therefore, the thermal gradient increases. The overall temperature level of the separation column is controlled by the heating current. Simultaneous control of the gradient flow and of the heating current allows the free adjustment of temperature level and gradient slope.

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HEURISTIC THEORY OF THERMAL GRADIENT GAS CHROMATOGRAPHY The basic concept of the application of a negative thermal gradient is the modified transport mechanism in contrast to conventional gas chromatography. The movement of a certain substance on a separation column depends on the temperature level. This is caused by the partitioning of the substance and the temperature dependence between the absorbed part in the inner coating of the column and the concentration in the mobile carrier phase. In conventional temperature-programmed GC the temperature of the whole column is uniformly elevated (Figure 2A). Therefore, during the transport the individual substances experience a rising temperature. The chromatographic velocities of the substances rise until they reach the carrier gas velocity. From that point on no more separation takes place. With a negative temperature gradient along the column the transport conditions are changed substantially. The temperature program with negative temperature gradient typical for FF-TG-GC as shown in Figure 2B consists of three phases. First, the temperature at the inlet of the column is elevated at a higher rate than at the outlet. A negative temperature gradient is established. This effect is directly a result of increasing heat dissipation and the gradient (cooling) flow. In the second phase this gradient is maintained while the overall temperature level is enhanced. Physically this is done by a continuous decrease of the gradient flow in phase 2. In the last phase of the measurement cycle the gradient is lowered until the temperature is uniform along the separation column. Again this is done by a decrease of the gradient flow to zero flow. In a thermal gradient GC every substance gets “trapped” at a distinct temperature. The chromatographic velocity of a substance along the column is mainly a function of the temperature (given a certain separation column). The trapping condition is determined by the actual velocity of a substance in the column and the corresponding progression of the temperature along the column. If the progression of the temperature on the column is faster than the actual velocity of the substance, the substance lags behind this temperature progression and, due to the negative temperature gradient, subsequently experiences an increasing temperature. This temperature increase leads to a higher chromatographic velocity until the velocity of the progressing temperature is reached. At

Figure 2. Comparison of classical temperature-programmed GC (A) and negative temperature gradient GC (B). The red lines represent three substances with different elution temperatures. They show the trajectory of the substances along the column in terms of the temperature. Panel B shows the thermal gradient GC as performed in the measurements. During phase 1 the gradient is established, in phase 2 the temperature level is increased while a constant gradient is maintained, and in phase 3 the gradient is lowered and eliminated at the end. Dotted lines are isochronic lines which represent the column temperature along the column length.

this point the substance is trapped because a theoretical slight increase in velocity would result in a lower temperature thus decelerating the substance to the equilibrium value. The same holds true for a slight decrease of the substance velocity as this exposes the substance to a higher temperature and subsequently accelerates the substance again. This “trapping effect” is also connected with the focusing and peak shaping characteristic of the negative gradient. In a thermal gradient the front part of a chromatographic band is colder and decelerated, the peak tail is warmer and accelerated, and in effect the signal is compressed (Figure 3). Naturally, this compressing is counteracted by the diffusive spreading of a peak. In equilibrium the peak width converges to a distinct constant value as the substance moves further along the separation column. It is noteworthy that this causes constant chromatographic resolution beyond the equilibrium point, as Lee and his group have derived.12 Figure 2B also reveals another important feature of the separation with negative temperature gradient. The dashed lines of the substance transport in the case of classical temperature-programmed GC end at higher elution temperatures. This effect is the result of the continuous elevation of C

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Figure 4. Prototype FF-TG-GC system used for the measurements. Figure 3. Peak broadening in conventional GC compared to peak compression due to the negative temperature gradient. In equilibrium the velocity of a certain temperature on the column (“thermal” velocity) is identical to the chromatographic velocity of the substance.

The signals were detected by a time-of-flight mass spectrometer (BenchTOF, MARKES International, Llantrisant, U.K.). Helium was used as carrier gas in constant pressure mode. The pressure was set to 180 kPa for all measurements except for the explosives measurements with 300 kPa. The pressure of 180 kPa results in a column flow of 0.8 mL/min at 35 °C and of 0.4 mL/min at 320 °C, respectively (1.6 and 0.8 mL/min with 300 kPa) calculated in consideration of the whole flow path (transfer lines, separation column) using our own serial flow calculator. The peak capacity used as separation metric in this contribution is calculated as the sum of the resolutions of all the peaks in a chromatogram. Bicchi et al.24 have used a similar method for the comparison of different GC modes. The resolution between two adjacent peaks 1 and 2 is calculated by the equation R1/2 = 1.18(tr2 − tr1)/(w0.5 1 + w0.5 2), with tr as the retention time and w0.5 equal to the full width at half-maximum (fwhm) value determined by peak fitting of all peaks with the scientific software Origin (OriginLab Corporation, Northampton, U.S.A.). The alkane measurements were done using an alkane standard C7−C30 from Supelco and the pyrethrine measurements with a pyrethrum extract from Sigma-Aldrich. ASTM D2887 reference gas oil from Supelco was used for the petroleum oil measurements. Explosives were synthesized and supplied by the Department of Chemistry/Energetic Materials at the Ludwig-Maximilian University of Munich.

the temperature beyond the chromatographically optimum conditions. This especially is a problem for very high speed separations where the limited traveling speed of the substances cannot follow the rapid increase of the temperature. From a certain point on the temperature is too high for chromatographic partitioning. The whole substance is then in the mobile phase and its velocity identical to the carrier gas velocity. This is an unfavorable effect as only the front parts of the column contribute to the separation. In contrast to this phenomenon in a thermal gradient GC every substance traverses the separation column at its own (constant) equilibrium separation temperature. Due to the gradient a wide range of temperatures coexist making such a gradient separation a quasi-parallel process.

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MATERIALS AND METHODS All measurements were performed on a prototype FF-TG-GC system22 as depicted in Figure 4. The separation column of 1.8 m length, 0.1 mm i.d., and a film thickness of 0.1 μm (CSChromatographie Service, Langerwehe, Germany, FS-Supreme5 ms) is inserted into a stainless steel capillary of 1 mm diameter and 0.1 mm wall thickness. Three turns of the capillary are inside a helical channel milled into a tube of 20 cm diameter and a wall thickness of 6 mm. The heating current is applied via special clamps. The temperature of the stainless steel capillary is monitored by infrared sensors at the inlet and the outlet. The temperature is controlled by a closed control loop using the outlet temperature as the input value. The PI temperature control algorithm is programmed on a small microcontroller with an attached laboratory power source. A conventional split/splitless injector (Fisons, Milan, Italy) was attached to the FF-TG-GC via a heated transfer line. All injections were done by manual injection with a split flow of 200 mL/min (1:250 split ratio at 0.8 mL/min column flow).

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EVALUATION OF SEPARATION CAPABILITY OF FF-TG-GC The basic criterion of a new GC concept is its separation performance. To this end we performed series of measurements with a standard alkane mixture ranging from C7 to C30. Optimum results for peak capacity were achieved using a flow of 0.8 mL/min of helium equal to the “speed optimized flow” derived by Klee and Blumberg.25 The measurements were done in constant pressure mode; therefore, the flow at the end of the D

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Analytical Chemistry Table 1. Results of Thermal Gradient GC Measurements with Different Heating Rates time interval (s)a

heating rate (°C/s)

heating rate (°C/min)

medium peak width (ms)

peak capacity

peak production (peaks/min)

C30 elution temp (°C)

20 40 35b 80 160

14.3 7.2 8.2 3.6 1.8

860 430 490 215 107

67 95 103 160 265

164 214 207 246 283

552 399 370 244 146

320 290 320 266 247

a

For heating ramp from 35 to 320 °C. bTemperature-programmed measurement.

The temperature programming rate of 860 °C/min or 14.3 °C/s in Figure 5 is much higher than the value of 10 °C per hold-up time that is recommended as an optimum in classical programmed GC by Blumberg and Klee.26 We have calculated the dead time for the gradient GC with a proprietary calculator for series of columns based on the equations derived in our previous papers on the variable dome splitter and the Deans’ switch.27,28 The hold-up time of the measurements at 35 °C is ∼2 s. This is equal to a maximum recommended heating rate of 5 °C/s. Although the heating rate is nearly 3 times higher than the 10 °C/hold-up time rule, even the C30 signal already elutes near the end of the temperature ramp (see Figure 5). To compare these results with the classical temperatureprogrammed (TP) mode a measurement without thermal gradient is presented in Figure 7. The heating rate has been

temperature program is only one-half of the initial value. Table 1 lists the chromatographic parameters of the measurements. The measurements revealed high peak capacities, even for very fast GC cycle times. Figure 5 shows a very fast

Figure 5. Measurement of an alkane mixture with a 20 s heating interval between 35 and 320 °C. There is a 10 s solvent delay after liquid injection and 10 s hold time.

measurement where the temperature (outlet temperature) was ramped from 35 to 320 °C in 20 s. This is equal to a rate of 14.3 °C/s or 860 °C/min. Despite the suboptimal manual injection the mean peak widths (fwhm) are only 67 ms. The sum of the chromatographic resolutions between all peaks from C8 to C30 adds to a peak capacity of 164. A measurement with a less fast heating rate of 40 s for the interval between 35 and 320 °C is presented in Figure 6. The heating rate here is 430 °C/min or 7.2 °C/s. The mean peak widths in this measurement are 95 ms and the peak capacity is 215 for the alkane series.

Figure 7. Temperature-programmed measurement of an alkane standard with a 35 s heating interval between 35 and 320 °C. There is a 10 s solvent delay after liquid injection and 10 s hold time.

adjusted to 490 °C/min (8.2 °C/s) in order to obtain the same elution times for the alkane series. Due to the optimized technical setup also this classical measurement has a high peak capacity of 207. The peak width of the C20 alkane is slightly broader with 92 ms. The most pronounced effect of the thermal gradient in this specific setup is the reduction of the elution temperature. Figure 8 shows the peak width and the elution temperatures of both measurement modes. The reduction for C28 is ∼45 °C. The peak widths between C10 and C21 are up to 10% lower with the thermal gradient. The widths of the late eluting substances (C28−C30) are much higher with TP-GC because of the unfavorable isothermal elution for these compounds in the classical temperature-programmed mode. Table 1 shows the results of a series of FF-TG-GC measurements with decreasing heating rates. The medium peak width increases with slower heating rates. Nevertheless, the peak capacity also increases because of the better separation of the peaks. A peak capacity of 283 is obtained for the

Figure 6. Measurement of an alkane mixture with a 40 s heating interval between 35 and 320 °C. There is a 10 s solvent delay after liquid injection and 10 s hold time. E

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Table 2. List of Explosives and Taggants Ordered According to Elution on the FF-TG-GC System

Figure 8. Elution temperature and peak widths of the alkanes from C8 to C30 compared for the TG-GC and the TP-GC measurements of Figures 6 and 7, respectively.

separation region between C8 and C30 in only 160 s. The rate of peaks per time (peak production) is at a very high level for the fast measurements. A maximum value of 552 peaks/min is achieved with the fastest heating rate. It is worth mentioning that the speed of the separation phase is complemented by a very fast cooling down time, too. Within 10 s the column cooled down from 320 to 35 °C. Therefore, the alkane measurements had total measurement cycle times of 50 s (for the 20 s heating interval) and 190 s (for the 160 s heating interval), respectively.

no.

category

name

CAS

MW

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

explosive explosive ex-taggant taggant substance taggant explosive explosive explosive explosive explosive explosive explosive explosive explosive

EGDN TATP 2-MNT, o-MNT DMDNB 3-MNT, m-MNT 4-MNT, p-MNT NG 2,6-DNT 2,4-DNT ETN TNT PETN RDX picramic acid tetryl

628-96-6 17088-37-8 88-72-2 3964-18-9 99-08-1 99-99-0 55-63-0 606-20-2 121-14-2 7297-25-8 118-96-7 78-11-5 121-82-4 96-91-3 479-45-8

152.06 222.24 137.05 176.17 137.05 137.05 227.09 182.14 182.14 302.11 227.13 316.13 222.12 199.12 287.14

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VERY FAST HIGH PEAK CAPACITY SEPARATIONS OF EXPLOSIVES AND TAGGANTS The new technology of flow field thermal gradient GC has been developed in the context of the security research project ChemAir funded by the German Ministry of Education and Research (BMBF). Many of the explosives are thermally labile components. Therefore, it was suspected that capillary gas chromatography may not be the best choice, especially in combination with fast measurement cycles. At least three effects inherent to the FF-TG-GC now enable very high speed measurements of thermally labile components: (1) the reduction of elution temperatures due to the modified transport in the column; (2) the high chromatographic efficiency of the gradient setup even with short columns; (3) the reduced residence times at elevated temperatures inside the short capillary columns. Table 2 shows the list of explosives and taggants (explosive markers) used for the measurements. For explosives measurements all temperatures in the analytical flow path have to be carefully controlled. The injector temperature was set to 175 °C and the transfer lines to 200 °C. Figures 9 and 10 show the chromatograms of a 20 and a 40 s heating interval, respectively. Although all substances could be identified even with the short temperature program (Figure 6), the individual substances show different amounts of degradation. Especially PETN is labile. A slightly slower 40 s heating program (Figure 10) reveals the benefits of the thermal gradient on the measurement of explosives. The PETN signal is now nearly of the same height as the TNT signal in its vicinity. For both measurements the corresponding measurements without the application of a thermal gradient were done. Figures 11 and 12 show the comparison of the TG and the TP mode in a mirrored view. Compared to the 20 s measurement with gradient the peaks of ETN and RDX are reduced to

Figure 9. Explosives measurement with a 20 s heating interval (35− 320 °C). Because of the enhanced elution temperatures of the very fast measurement the signals of ETN, PETN, and RDX are reduced by degradation. The thermal gradient was set to ∼40 °C.

Figure 10. Measurement with a 40 s temperature ramp and a thermal gradient of ∼50 °C after the initial phase. The explosives (especially PETN) show a low amount of degradation as the elution temperatures are lower compared to the hyperfast measurement of Figure 9.

approximately one-third of the height with TP-GC. PETN degrades completely in the temperature-programmed mode. Compared to the slower 40 s measurement the TP-GC measurement shows nearly similar peak heights, again except for PETN which has only ∼50% of the TG-GC peak height. F

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Figure 13. Comparison of two measurements of natural pyrethrins in a mirrored fashion. The FF-TG-GC measurement is drawn upward, the TP-GC measurement downward, respectively. The reduction of the elution temperatures for pyrethrin I and II is indicated as 30 and 33 °C, respectively.

Figure 11. Comparison of a TP-GC and a TG-GC measurement with temperature ramps of 17 and 20 s, respectively: reduction of the peak height of ETN and RDX to one-third, complete degradation of PETN with TP-GC.

classical GC mode due to the higher elution temperatures and the corresponding degradation.

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MEASUREMENT OF ASTM GAS OIL Fast simulated distillation analysis with the ASTM method D288731 is an established tool to measure the boiling point distribution of petroleum samples. According to this method wide-bore columns (0.52 mm i.d.) of 10 m length should be used. The run time of an analysis is ∼25 min. With the new FFTG-GC such measurements can be made within 60 s as demonstrated in Figure 14. The resolution with the 1.8 m

Figure 12. Comparison as in Figure 10 with ramps of 35 and 40 s for TP-GC and TG-GC, respectively: peak height of PETN reduced to one-half with TP-GC; peak capacity improved with TG-GC from 94 to 105.

The peak capacities are higher with TG-GC, slightly higher for the hyperfast measurement, and ∼11% higher for the less fast measurement.

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MEASUREMENT OF THERMALLY LABILE NATURAL COMPOUNDS To additionally evaluate the improvements in connection with the lower elution temperatures of FF-TG-GC we performed measurements with an extract of natural pyrethrins. Pyrethrum is a naturally occurring insecticide extracted from the plant Chrysanthemum cinerariaefolium. The active ingredients are pyrethrin I and II, cinerin I and II, and jasmolin I and II. Especially the two pyrethrins are thermally unstable and degrade under normal GC conditions.29 Harynuk and Mariott30 have demonstrated the fast measurement of the pyrethrins using a short first column and high carrier flow rates in a GC × GC setup. Figure 13 shows a fast measurement of an analytical pyrethrin standard with a 1.8 m column with the FF-TG-GC. The classical temperature-programmed GC mode and the new thermal gradient GC mode are compared. The heating rate of the temperature-programmed measurement has been adjusted to a 35 s ramp interval in order to obtain the same elution times for the pyrethrins. The signals of the pyrethrins are lower in the

Figure 14. Fast FF-TG-GC measurement of ASTM D2887 reference gas oil with a 40 s heating interval (35−320 °C).

narrow-bore column (0.1 mm i.d.) is even higher than that of the standard 10 m column. The key benefit of using the thermal gradient GC for petroleum or other fractions of crude oil is the reduction of the elution temperatures. Even higher boiling compounds than present in the ASTM D2887 mix are compatible with gas chromatography if the elution temperatures can be kept below the columns temperature limits. The demand for faster gas oil measurements recently has been complemented by a new ASTM standard32 (ASTM D7798) especially covering very fast GC measurement.

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CONCLUSIONS AND OUTLOOK The new development of a thermal gradient GC has shown the prospects of this measurement mode. Although “Blumberg’s theorem” of an inherent resolution limit of partition G

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Engineering. The head of the workshop, Roland Lutz, developed the electronic control and the control software. Prototypes were made by Wilfried Berchtold with high mechanical sophistication. Walter Petriwsky machined many of the parts with great dedication. We appreciate the help from our partners in the ChemAir project. The group of Thomas M. Klapötke (especially Martin Härtel) at the LMU in Munich, Germany synthesized and prepared the explosives. The Bundeswehr Research Institute for Protective Technologies and NBC-Protection defense sciences (WIS) in Munster, Germany (especially Julia Rothe) assisted in the method development for the explosives detection. Our project partner five technologies GmbH, Munich, supported us in interfacing the FF-TG-GC to the time-of-flight mass spectrometer. We are also grateful to our partner institute of Bioanalytics and Food Science at the University of Bonn. Matthias Wüst and his group have supported our research and development to a great deal.

chromatography has to be observed, we see specific benefits of the flow field thermal gradient GC mode: (1) the pronounced reduction of the elution temperatures by the modified separation mechanism; (2) increased chromatographic resolution by exploitation of the optimization potentials toward an “ideal separation”; (3) suitability for hyperfast GC measurements below 60 s (according to the terminology of Tranchida and Mondello33) at high resolution; (4) very short measurement cycles by the minimal thermal mass of only the capillary and column and the inherent active cooling. In this contribution first measurements with a prototype instrument are presented. All measurements have been performed with helium as carrier gas. Switching to hydrogen will further improve the chromatographic resolution. For thermally labile components a programmable temperature injector would be beneficial. We were restricted to constant pressure mode in our experiments. As we are further improving this technology we will reveal more properties and preconditions of FF-TG-GC. More experiments and studies are required to evaluate the full potential of this technology. Experimental data show that an additional degree of freedom in form of the thermal gradient is introduced to gas chromatography. This must be substantiated by a sound theoretical GC model and simulation of this method of chromatography.34,35 Method development would benefit from the possibility to make simulation experiments with faster parameter optimization. Thermal gradient gas chromatography may also be used as a second column in GC × GC setups.36 Here the focusing property of the gradient can support the peak sharpening. The FF-TG-GC can either be directly connected to the first column via a switching device or attached after a modulator.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.analchem.5b02227. Additional information on the calculation of the heat transfer of horizontally orientated cylinders as noted in text (PDF)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Author

*Fax: +49-228-732596. E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research work was funded by the German Research Agency (DFG), Grant BO 51/1-2 (New methods for odor measurements) and the German Federal Ministry of Education and Research, BMBF (security research projects EXAKT and ChemAir). We would like to express a special thanks to our advisors Sandra Muhle (VDI/BMBF) and Andreas Engelke (DFG) for their ongoing assistance. The development of a new technology requires special technical skills. We are very grateful to the staff of our workshop in the Institute of Agricultural H

DOI: 10.1021/acs.analchem.5b02227 Anal. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.analchem.5b02227 Anal. Chem. XXXX, XXX, XXX−XXX