Anal. Chem. 2004, 76, 1702-1707
Selective Detection of Volatile Aromatic Compounds Using a Compact Laser Ionization Detector with Fast Gas Chromatography Melissa J. Meyer,† Gregg M. Schieffer,‡ Eric K. Moeker,‡ Jeremy J. Brodersen,§ Orven F. Swenson,§ and Anthony J. Borgerding*,‡
Department of Chemistry, University of North Dakota, Grand Forks, North Dakota; Department of Chemistry, University of St. Thomas, St. Paul, Minnesota; Department of Physics, North Dakota State University, Fargo, North Dakota
We report results for a new gas chromatography detector that is comparatively sensitive and far more selective for aromatic compounds than the traditional photoionization detector. The detection means is multiphoton ionization at atmospheric pressure. The ionization source in these experiments is a diode-pumped passively Qswitched microchip laser operating at 266 nm. Experiments were conducted with the detector interfaced to a fast gas chromatograph. For 105 >105 >105 >105
>106 >106 >106 >106 >106 >106
>106 >106 >106 >106 >106 >106
>106 >106 >106 >106 >106 >106
>106 >106 >106 >106 >106 >106
a PID selectivity ) aromatic peak area/non-aromatic peak area. b ArSLID selectivity ) highest concentration of nonaromatic analyte studied/ LOD for aromatic compound (see text for details). c Benzene. d Toluene. e Ethylbenzene. f o-xylene. g Isopropylbenzene.
Table 2. Absorbance and Ionization Potential Values of Aromatic VOCs compd
absorbance at 266 nm
ionization potential (eV)a
benzene toluene ethylbenzene o-xylene isopropylbenzene MTBE methylcyclohexane 1-octene 2-heptanone heptanal 2-octanone
0.16 1.28 0.84 1.34 0.73 0.00 0.00 0.00 0.00 0.00 0.00
9.25 8.82 8.77 8.56 8.73 9.24 9.64 9.43 9.30 9.65 9.40b
a
See ref 15. b See ref 16.
Figure 4. Comparison of petroleum samples analyzed using GC with detection by either ArSLID or PID.
1-µL portion of a fuel was injected into a 1-L Tedlar bag and injected into the GC systems using the 10-µL injection loop. The results are shown in Figure 4. Although both detectors are able to generate distinguishable peaks for these complicated mixtures, even while pushing the limits of chromatographic resolution with these fast separations, the ArSLID data are cleaner. For example, in the unleaded gasoline chromatograms, benzene and toluene are not resolved from a number of other low-boiling-point compounds in the sample, and it would be difficult to quantify them using the PID. Using the ArSLID, however, the compounds that interfere using the PID are not detected, allowing baseline resolution of all of the BTEX analytes. The selectivity advantage
Figure 5. GC-ArSLID analysis of gas-phase sample containing 0.1 µg/L each of BTEXI (1 pg injected).
is even more obvious in the analysis of low-lead aviation fuel (Figure 4, right). The ArSLID detected no peaks from any aromatic compounds in this fuel. GC/MS analysis of this sample indicated a mixture of aliphatic compounds, and single ion chromatograms for common aromatic ions at m/z 91 and 77 detected no peaks, verification that the sample did not contain any aromatic compounds. Using a PID as a detector for analysis of the low-lead aviation fuel resulted in a number of peaks, including a very large, highly volatile component. These peaks were tentatively identified as one or more isomers of butene or pentene by our GC/MS analysis. This analysis is further evidence of a measurement in which analytes may be mistakenly identified as aromatic when using the PID and in which the added selectivity of the ArSLID is beneficial. Figures of Merit. In addition to enhanced selectivity, using MPI allows the ArSLID to be exceptionally sensitive. In the fast GC system used to make these measurements, our limit of detection for most analytes was 0.1 µg/L, defined as the lowest concentration at which a signal-to-noise ratio of 3:1 was achieved. Figure 5 shows a chromatogram for a gaseous sample of BTEXI at this concentration taken with the HSGC-ArSLID system. In this GC system, to perform this high-speed separation, the sample volume in the loop injector was only 10 µL, allowing narrow injection bands and good chromatographic efficiency. Note that this small volume corresponds to only 1 pg of each analyte being injected onto the column to achieve the chromatogram shown in Figure 5, further evidence of the sensitivity of the instrument. Strong signals were obtained for all of the analytes except for benzene. The detection limit for benzene was an order of magnitude higher (1 µg/L), which could be the result of two Analytical Chemistry, Vol. 76, No. 6, March 15, 2004
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factors. As shown previously in Table 2, benzene has a significantly lower absorbance than the other analytes at the wavelength of this laser (266 nm). In addition, its ionization potential of 9.24 eV is over 0.4 eV higher than any of the other analytes; the longest two-photon ionization wavelength of 268.4 nm for benzene14 is very close to the wavelength of our laser. That is, two photons of our 266 nm laser corresponds to ∼9.34 eV, barely exceeding the ionization potential of benzene. In MPI experiments, an aromatic dopant sometimes can be used to enhance ionization. Doing so in this detector may enhance the sensitivity of some analytes, but the detector would no longer be selective, because a dopant that is constantly present in the detector cell could charge-exchange with an nonaromatic analyte. This sort of charge-exchange could also occur with analytes that coelute, but as long as only one of them is aromatic, the overall signal will not change, because each ion results in one charge, even if it exchanges. However, it is important to recognize that the ionization efficiency of an aromatic compound may change if it coelutes with another aromatic compound. Since this could affect quantitation with the ArSLID, chromatographically resolving the aromatic analytes from each other is important. The system was calibrated by analyzing BTEXI standards ranging in concentration from 0.1 µg/L to 1 mg/L, and the signals for each analyte increased linearly with concentration, indicating a linear dynamic range of 5 orders of magnitude. Gaseous standards of this mixture containing higher concentrations were difficult to prepare at ambient temperature because the higher boiling analytes would not completely vaporize, resulting in peak areas that were lower than expected at higher concentrations. ArSLID Design and Performance. As shown in Figure 1, the flow of analytes eluting from the column runs perpendicular to the laser path. This minimizes the potential for chromatographic peak-broadening caused by molecules’ being ionized at times other than immediately after they elute from the column. However, it also requires that the laser be pulsed at a relatively high frequency to ensure that each eluted molecule encounters photons. For example, a flow of 5.0 mL/min using a 0.32-mm-i.d. column corresponds to a linear flow rate of 100 cm/s. For a 1-mm beam spot, an analyte in the carrier flow will intersect the laser beam for a period of 1 ms. To ensure that a laser pulse will strike an analyte in this flow, the laser pulse rate would need to be at least 1000 Hz, which the laser in our system easily attains. For our 8 kHz laser, we could theoretically operate with flow rates as high as 40 mL/min without a loss of ionization efficiency. Using ethylbenzene as a representative analyte, we made measurements over a range of flow rates to study its effect on detector efficiency. The detector efficiency, defined as the average actual peak height in millivolts divided by the theoretical signal, was calculated to be 0.002% for this system at flow rates ranging from 2 to 12 mL/min. The actual signal value was calculated taking into account the gain given by the electrometer, and the theoretical signal was calculated assuming 100% ionization of all analyte molecules injected. The constant value we calculated for the detector efficiency indicates that a constant fraction of analytes (14) Swenson, O. F.; Carriere, J. P.; Isensee, H.; Gillispie, G. D.; Cooper, W. F.; Dvorak, M. A. SPIE 1998, 3270, 216-225. (15) CRC Handbook of Chemistry and Physics, 75th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL: 1995. (16) Velesou, F. I. Dokl. Phys. Chem. 1960, 132, 521-527.
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Figure 6. Effect of purge gas flow rate on peak width. GC-ArSLID analysis of a 1 mg/L sample of BTEXI (10 ng injected). A, 100 mL/ min purge gas; and B, 500 mL/min purge gas.
are ionized and transported to the detector over this range of flow rates. In experiments for nonchromatographic systems in which we increased the flow rates, this calculated efficiency decreased above flow rates of 40 mL/min. At these high flow rates, a portion of the analyte stream will pass through the beam cross section between pulses, so fewer molecules will be ionized. Another possibility is that the eluent flow disperses into a wider pattern at high flow rates, due to a higher pressure drop, causing part of the stream of analytes not to interact with the laser beam. Reducing the half-cell radius to a dimension closer to that of the laser beam diameter should increase the percentage of analyte molecules that cross the beam path, increasing the detector efficiency and lowering detection limits even further. The half-cell radius is also an important factor in the amount of purge gas flow required to prevent peak-broadening. The volume of the half cylinder is 3.1 mL, so an eluting compound from a column with a carrier gas flow of 5.0 mL/min was calculated to reside in the half cell for 37 s (volume divided by flow rate, assuming a simplified scenario with laminar flow through the half cell). During this residence time, an analyte molecule could intersect the laser beam path and generate a signal, and initial experiments performed without a purge gas generated very broad chromatographic peaks. The purge gas was directed through the half cell in a direction perpendicular to the flow from the column to most efficiently flush un-ionized compounds from the detector. Figure 6 shows chromatograms taken using purge gas flow rates of 100 mL/min and 500 mL/min. At 100 mL/min, the calculated residence time is 1.9 s, and the peak widths are correspondingly broad (Figure 6A). The resolution of this chromatogram was much worse than the same separation performed using a commercial FID. However, using a purge gas flow rate of 500 mL/min, the peak shapes narrow to the same width as those using the commercial FID (Figure 6B). At this purge gas flow
rate, the calculated residence time is 0.37 s, so compounds are flushed from the detector cell before they can broaden the peaks. Note also that the signal is lower at the higher purge gas flow rate. This could be due to the absence of compounds being ionized at times other than immediately after they are eluted from the column or due to dilution of the analyte stream by the purge gas, which would reduce the detector efficiency. Using a half cell with a smaller radius, purge gas flow requirements will be lower and will hopefully reduce any dilution effects. It is also possible that the flow of purge gas redirects the analyte flow stream so that it becomes misaligned with the laser beam. However, experiments performed to optimize the signal at different laser positions always resulted in the signal’s being the highest when the beam was positioned directly beneath the point at which the column enters the half cell. We also performed experiments varying the diameter of the laser beam where it intersects the analyte flow. Although a wider diameter might be thought to allow a higher fraction of analytes to cross the beam path and be ionized, our best results occurred when the ion was as focused as possible at the analyte exit, indicating that the intensity of the beam is more important.
CONCLUSION This paper shows how new advancements in technology can result in powerful measurements being made more practical. Our laser cost was ∼$9000, and since there was no vacuum required, the total cost of this MPI detection system was less than $10 000. We expect both the cost and the size of the laser (and therefore, the detector) to get smaller in the future. ACKNOWLEDGMENT This work was supported by the U.S. Environmental Protection Agency under Grant No. R-82941501-0. The concept of a microchip laser-based aromatic-specific laser ionization detector (ArSLID) originated at Dakota Technologies, Inc. (DTI), which is developing a commercial version. We thank Greg Gillispie, President of DTI, for suggesting that we construct an ArSLID as a detector for fast GC. We also thank the staff of DTI for useful discussions on various technical issues. Received for review October 10, 2003. Accepted January 14, 2004. AC0352023
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