Anal. Chem. 1989, 6 1 , 1249-1252
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Microcolumn Gas Chromatography with Ultraviolet Detection and Identification Using a Photodiode Array Spectrophotometer Verner Lagesson*
Department of Occupational Medicine, University Hospital, S-581 85 Linkoping, Sweden
John M. Newman Newman-Howells Associates, Ltd., Wolvesey Palace, Winchester Hunts, SO23 9NB, U.K.
Gas chromatography comblned wlth ultravlolet (UV) detection and Identification using a photodiode array spectrophotometer has been investigated. The study concerned possible detection limits for UV-absorbing compounds at fixed wavelengths and possibilities for Identifications based on UV spectra collected “on the fly”. Detection limlts were found to be on the order of 100 pg using a detection spectral bandwidth of 5 nm on either slde of absorption maximum. Because of the high wavelength reproducibility for photodiode array type monochromators, and the absence of any solvent shift, the spectra were found to be accurately defined. Injected amounts of compounds, considered as relatively strong UV absorbers, at the 1-ng level resulted In clearly identifiable spectra including the first derivative of the spectra. When closely related compounds were grouped together, the shapes of the spectra at the near- and far-UV borderline showed clear similarities. However isomers were stili easily distinguished from each other.
INTRODUCTION Reports concerning gas-phase UV detection using gas chromatography are rare in literature. Kaye (1) made the first contribution to this area and was able to study absorption in the far-UV region down to 164 nm. This was carried out by means of a Beckman DK-2 spectrophotometer modified for far-UV operation and purged with nitrogen. Novotny and co-workers (2) have used a dedicated UV spectrophotometer and reported a detection limit for naphthalene that was about one-tenth of that for flame ionization detection (FID). Lagesson and Newman (3-9) have used a gas flow cell for UV spectrophotometers with a built-in microcolumn gas chromatograph and studied detection and identification of various compounds by spectral scanning of compounds trapped in the light beam. A UV-vis diode array spectrophotometer has been used by Kube and co-workers (IO) as a detector for gas chromatography. The detection limit given was 0.5 Kg for a number of components like benzene and toluene and thereby comparable to that obtained with a thermal conductivity detector. It is believed that this detection limit can be considerably improved. When considering the use of fast wavelength scanning ultraviolet spectrophotometry in conjunction with gas chromatography for detection and identification, there are two main comparisons that can be made with established analytical methods, namely, gas chromatography linked to Fourier transform infrared spectrometers (GC/FT-IR) and highperformance liquid chromatography (HPLC) with a fast scanning UV spectrophotometer of the photodiode array type as detector. A typical GC/FT-IR system uses a wide- or medium-bore capillary column with carrier gas flow rates at 3-5 mL/min. The light pipe is usually 10-15 cm long with
an inner diameter of approximately 1 mm. This geometric design is considered to give an optimum for sensitivity and resolution. The separations are carried out so that the volume of carrier gas between the half-height points (VI,*) of the GC peak is kept as close as possible to the dead volume of the light pipe. In an optimized GC/FT-IR system the minimum identifiable quantity (MIQ) ranges from 20 to 120 ng ( I I ) , depending on the absorptivity of the compound. Comparing the molar absorptivity for infrared (IR) and UV shows that UV, on average, has at least 2 orders of magnitude higher absorptivity than IR. This is an approximate estimation based on UVabsorptivity calculations which gave 110000 (at 210 nm) for naphthalene, ca. 63 000 for mesitylene (at 195 nm), and ca. 5800 for acetone (at 190 nm) representing high, medium, and low absorptivities for the UV region. This was compared with IR absorbance data for a Miran-1A (Foxboro Analytical) portable gas analyzer, and the molar absorptivities were calculated to 295 for isobutyl acetate, 125 for trichloroethylene, and 40 for xylenes representing the high, medium, and low absorptivities in the IR region. Corresponding order of MIQ improvement could be expected providing the same noise level expressed in absorbance. Ultraviolet absorbance measurements in the liquid phase have three main disadvantages in comparison with gas phase: (1)The “standard” spectra must be collected under mobile phase conditions identical with the sample, because of spectral shifts due to solvent effects. This problem is difficult to overcome particularly when gradients are used. (2) The low-UV region (below 220 nm) is usually the region where spectral differences are the largest, but the data in this region are often masked by the mobile phase cutoff. (3) The molecular vapor spectra are more informative about the structure of molecules, in comparison with spectra in solution, as solvation effects produce severe broadening of the absorption bands in the latter.
EXPERIMENTAL SECTION Chromatography. A gas flow cell with a built-in microcolumn gas chromatograph (Gas Spec 011, Newman & Howells) was used. This instrument, and the various types of separation columns that can be made, has been described earlier (3-9). The injection mode is direct and the separation columns are just 80 mm long and packed with HPLC-packing materials as support with various ordinary liquid phases applied to it. The light pipe of the cell is made of glass and is 95 mm long with an internal diameter of 1.5 mm. For the chromatogram shown in Figure 2 the 80 mm long separation column was packed with Hypersil WP300 C8, 10-pm particles, as support and impregnated with DCQF1 as the liquid phase. The V l j Zof a peak hardly reaches the light pipe interphase dead volume of ca. 200 p L even under extreme conditions, because the carrier gas flows for the packed type of column used are usually between 15 and 30 mL/min. The transmittance of the cell with pure nitrogen carrier gas flow varied almost linearly from 60% at 190 nm to 40% at 290 nm. The overall characteristics
0003-2700/89/0361-1249$01.50/0 1989 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 61. NO. 11. JUNE 1, 1989
Fgun 3. Chromatogram of a fast separaHon of styrene. 700 pg. and 1,3,54ri~lbenzene,650 pg: range 1. wavelength range 190-200 nm: range 2. wavelength range 290-300 nm. Resuk range 1 minus range 2.
Flgue I. lnstrumenta arrangement for me gas flow cell wnh built-in microcolumn gas chromatograpn placed in lhe cuvene nousing of the HP8452 spectrophotometer
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Figum 2. Chromatograms fw a compound mixture wlth UV detection at six wavelengths (190, 205, 220, 235, 250. and 265 nm): injection volume. 1 pL: temperature start at 70 OC with a 15 'Ctmin ramp: Carrier gas flow. 16 mL/mln; separation column. DCQF-1 on Hypersil WP C8, 10 pm. Compounds used were as follows: (1) carbon disulfide, 125 ng: (2) benzene. 250 ng; (3)blchlwoethylene. 450 ng; (4) t o l m . 250 ng; (5) tebachloroethylene. 450 ng: (6) elhylbenzene, 250 ng: (7) methyl Isobutyl ketone. 4 pg; (8) styrene. 250 ng: (9) 1.3.5-
trimethylbenzene. 250 ng.
of the gas chromatographic system, using 80 mm long separation columns packed with HPLC materials, are that of fast separations at relatively low temperatures and with the capability of direct injection of relatively large sample volumes. Otherwise the efficiencies for these columns are limited hut the selectivities are high due to numerous possibilities for comhinations of various solid supports, liquid phases, and liquid phase loadings. The cell was temperature controlled with a variable constant current source and a temperature regulator. Nitrogen was used as carrier gas throughout the experiments. Figure 1 shows the gas flow cell firmly placed in the light beam of the spectrophotometer. This is arrar.ged so that, once aligned, the cell can easily he taken out for changing the separation column and replaced in the same aligned position. Spectrophotometry. A HewletbPackard Model 8452 opt.002 diode array spectrophotometer was used. It has a wavelength range of 190-510 nm covered by an array of 324 diodes and a spectral bandwidth of 1nm. A HP8452 instrument with a 2-nm spectral bandwidth was briefly used for studies concerning the influence of spectral resolution. Spectra were collected at 1-s intervals with an integrating period of 0.8 s throughout the investigations and all of these spectra were registered "on the fly". Data Handling. Control of the spectrophotometer and data collection was carried out by an Amstrad PC 1640 SDECD-20M
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Figure 4. Spectra for toluene recorded at 2-nm and l-nm spectral resolution including the first derivative 01 the spectra.
computer. Wavelength scans and chromatograms were registered in the "kinetic mode" of the software available and studies of individually collected speetra were made in the "general scanning" mode. This mode has an auto-Y-de function (normalized) which was used for comparing stored spectra with the "on the fly" collected spectra at various concentrations in the light beam.
RESULTS AND DISCUSSIONS Detection. Figure 2 shows a chromatogram of a number of CompouncLs with detection presentations at six wavelengths. This type of result is similar to HPLC-photodiode array data presentation. A response for the solvent "hexane" is completely absent because the alkanes absorb first at around 170 nm thereby being excellent solvents for gas-phase UV detection. The first peak,carbon disulfide, was directly injected in an amount of 125 ng. The peak maximum shows an absorbance of 0.7 A at 196 nm, which points at a detection limit of less than 100 pg taking into account the noise level at this wavelength. Figure 3 shows a fast separation of 700 pg of styrene and 650 pg of 1,3,5-trimethylbenzene at a wavelength range of 196-200 nm, comparing it with the nonabsorbing range 29&300 nm, and showing the resultant subtraction of these two ranges. The saw-tooth-shaped noise for the two ranges is caused by variations in the temperature regulation which is canceled out in the subtraction. The detection limits, defined as three times the standard deviation of the noise ( 3 ~ ) . where calculated to 100 pg for styrene and 80 pg for 1,3,5trimethylbenzene. Identification. Spectral resolution is, for reasons mentioned earlier, more detailed in the gas phase than in the liquid phase. Figure 4 shows the difference between 2-nm and 1-nm resolution for toluene (overlaid). The first derivatives of the 2-nm and 1-nm resolutions are included. The first derivative, which amplifies the spectral details, exposes the shoulder at
ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989
characteristic shapes of the spemra for naphthalenes and, particularly, the extent of multiple bond or aromatic conjugation within molecules. At the borderline between near- and far-UV, which is of particular interest because of the usually high absorptivity in this region, there have been few gas-phase studies relating spectral relation to molecular structure. Figure 6 shows a number of spectra of compounds grouped together according to similarities in structure. These groups are toluene and ethylbenzene, four alcohols, the three isomers of trimethylbenzene, the isomers 2,4- and 2,4-dichlorophenol, trichloroethylene and tetrachloroethylene, and three ketones. As can be seen, the shapes of the curves for each compound are clearly unique. The spectra of toluene and ethylbenzene are considered virtually identical in the liquid phase but, in this m e , easily identifed and the f i t derivatives clearly show the dissimilarities of the shoulders a t around 213 nm. The spectra of methanol, 1-butanol and ethanol, 2-butanol, respectively, are very close. However the primary aliphatic alcohols have their maximum absorbance at around 180 nm and the complete spectrum down to this region would presumably expose the differences more clearly. Of particular interest are the two groups of isomers (trimethylbenzenes and dichlorophenols) which are similar but easily identifiable. Consequently, GC/UV like GC/FT-IR is complementary to GC/MS, which cannot distinguish between structural isomers. Derivative spectroscopy considerably enhances the ability to detect and to measure minor spectral features. The signal to noise situation is however deteriorated with increasing order of derivation. Figure 7 shows the fourth derivative of styrene. This was recorded "on the fly" after a direct injection of a 10-pL solution containing 1.4 mg/L styrene in hexane (14 ng) demonstrating the relatively large sample size that can be handled by the GC system used. Naphthalene is a compound that is known to have a strong UV absorption with a characteristic spectrum. Figure 8 shows the result of an injection of 500 pg of naphthalene where eight scans around peak maximum where added, thereby, to some extent, making an improvement of the signal to noise ratio.
Flgure 5. (a) Expanded view of the coelution peak for benzene and trichioroethylene (Figure 2). (b) Cdlected spectra at the time positions indicated by the arrows in Figure 5a wlth reference spectra for benzene and toluene overlaid.
around 213 nm for the 1-nm resolution while the derivative for the 2-nm resolution does not give very much additional information. The column used for the chromatographic separation shown in Figure 2 was deliberately chosen because of the very poor separation of benzene and trichloroethylene. An expanded view of this coelution peak is shown in Figure 5a. From individually registered wavelengths it is hard to see that it is a two-component mixture, but looking a t the spectra, shown in Figure 5b, collected at the times indicated by the arrows, there are clear individual spectra of the two compounds. The reference spectra are overlaid showing very good agreement. The UV spectra of organic compounds are considered not very informative with respect to the functional groups present or the structure of the molecule. However, certain features can be recognized in the liquid phase as, for example, the
CONCLUSIONS In our opinion the combined use of gas chromatography and ultraviolet absorption spectrophotometry deserves far more attention than it has received thus far. It is a sensitive method for both detection and identification and the cost for the instrumentation used for the present investigations is very low in comparison with GC/FT-IR and GC/MS systems. The . T .
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ANALYTICAL CHEMISTRY, VOL. 61, NO. 11, JUNE 1, 1989 I
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magnitude lower than for GC/FT-IR with a light pipe interface and is, thus, much closer to the identification limits for full-scan GC/MS. The drawbacks are that there are no libraries for gas-phase UV spectra and that some groups of compounds like alkanes and aliphatic ethers cannot be detected with low to medium priced UV spectrophotometers having a limited far-UV region. Also, for the spectrophotometer used in this case, no chromatography software is, to our knowledge, available. Registry No. Benzene, 71-43-2; trichloroethylene, 79-01-6; toluene, 108-88-3; tetrachloroethylene, 127-18-4;ethylbenzene, 100-41-4;methyl isobutyl ketone, 108-10-1; styrene, 100-42-5; 1,3,5-trimethylbenzene,108-67-8;1,2,4-trimethylbenzene,526-73-8; 1,2,3-trimethylbenzene,526-73-8;acetone, 67-64-1; methyl ethyl ketone, 78-93-3;2,6-dichlorophenol,87-65-0;2,4-dichlorophenol, 120-83-2.
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LITERATURE CITED
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Kaye, W. Anal. Chem. 1962, 3 4 , 287. Novotny, M.; Schvende, F. J.: Hartigan, M. J.: Purcell, J. E. Anal. Chem. 1980, 52, 736.
Lagesson, V.: Lagesson-Andrasko, L. Ana/yst 1904, 109, 867. Lagesson, V.; Newman, J. M. Chromatogr. Int. 1885, 6 , 21. Lagesson, V.; Newman, J. M. SpectroscopylChromatography 1988, 4 , 25.
3 -t e I I - t i t -6E-64 2 98 190' nm Figure 8. Spectrum for naphthalene recorded after a 500-pg injection. Eight spectra around peak maximum were added in order to improve the signal to noise ratio.
gas-phase UV spectrum is a unique property and although not as detailed as IR spectra also gives a "fingerprint" of the compound. As shown above the minimum amount that is required for a identifiable spectrum is about 2 orders of
Lagesson, V.: Newman, J. M. Chromatogr. Int. 1986, 16. Lagesson, V.; Newman, J. M. Chromatogr. Int. 1986, 30. United States Patent. 1987, May 26, No. 4.668.091. Lagesson, V.: Newman, J. M. HRC CC, J . H!gh Resolut. Chromatogr. Chromatogr. Commun. 1908, 1 1 , 577.
Kube, M.; Tierney, M.: Lubman, D. Anal. Chim. Acta 1985, 171, 375. Gurka, D. F.; Titus, R.; Griffiths, P. R.; Henry, D.; Giorgetti, A. Anal. Chem . 1907, 59, 2362.
RECEIVED for review November 16, 1988. Accepted March 1, 1989.
