Anal. Chem. 1994,66, 2521-2528
Direct-Deposition Infrared Spectrometry with Gas and Supercritical Fluid Chromatography Donald F. Gurka,. Steven Pyle, Rlchard Titus, and Ellzabeth Shafter Environmental Monitoring Systems Laboratory-Las Vegas, Office of Research and Development, U.S. Environmental Protection Agency, Las Vegas, Nevada 89 193-3478 A direct-depositionFourier transforminfrared (FT-IR)system has been evaluated for applicability to gas chromatography (GC) and supercritical fluid chromatography (SFC) of environmental analytes. A 100-pm i.d. fused-silica transfer line was used for GC, and a 50-pm transfer line with an integral restrictor was used for SFC. Minimum identifiable quantities for GC/m-IR ranged from 0.5 to 2.0 ng. This is over an order of magnitude better than those reported for light-pipe FT-IR by this laboratory. However, some of this sensitivity improvement can probably be attributed to the smaller i.d. GC column used in this work and the ability to program the direct-deposition sample plate, thereby compensating for changes in analyte elution volume across the GC temperature ramp, or to the smaller detector area used in this work relative to that used for the light-pipeproject. Excellent SFC/FT-IR chromatography was obtained for poly (ethylene glycols) of average molecular weights 400, 600, 1000, and 1500. It was established that poly(ethy1ene glycol) 400 would not pass through a gas chromatograph. Newer supercriticalfluid chromatographs can attain almost twice the highest pressure used in this work, which should allow the separation of even higher molecular weight analytes. Hyphenated techniques’ may utilize a set of spectrometers [e.g., mass spectrometry/mass spectrometry (MS/MS); infrared/mass spectrometry (IR/MS)] or a separation device used in conjunction with a spectrometer [e.g., gas chromatography/Fourier transform infrared spectrometry (GC/FTIR), liquid chromatography/mass spectrometry (LC/MS)]. Regardless of the chromatography type (gas, liquid, supercritical fluid), an evacuated interface must remove the mobile phase before the separated analytes are introduced to the instrumental detection zone, unless the mobile phase is passed through a cell and the cell effluent is nor uented to the interface.2 This removal process may impede the achieval of maximum spectrometer performance and has been described by one spectroscopist as “the tail wagging the dog”.3 The need for mobile phase removal prior to the formation of gasphase molecules or ions may be, in part, responsible for the wide variety of liquid chromatography/mass spectrometry interfaces in use. These include thermospray, electrospray, particle beam, supercritical fluid, and fast atom bombardment.4 (1) Hirschfcld, T. Anal. Chem. 1980, 52, 297A-312A.
(2) Detectorsfor Capillary Chromatography; Hill, H. H., McMinn, D. G., Eds.; Wiley Interscience: New York, 1992. (3) Hirschfeld, T. Appl. Spectrosc. 1985, 39. 373-374. (4) Arpino, P. Fres. J. Anal. Chem. 1990, 337, 667-685.
This article not aubJectto US. Copyright. Published 1994 by the American Chemical Society
Over the range of temperatures employed for chromatographic separations, many chemicals can exist in more than one physical state. IR band positions and relative intensities can change with physical-state changes. Thus, a second problem created by chromatography type is the possibility for different physical states of the same chemical within the detection zone. This effect of physical state on infrared (IR) spectra is well documented.5 There is a shift to higher IR frequencies on progressing from solid to liquid to gaseous state. This progression is accompanied by a narrowing of IR bands and an intensity decrease for those bands which are hydrogen-bonded in the condensed state (alcohols, amides, carboxylic acids). Matrix isolation IR spectra, while qualitatively similar to spectra of isolated gasphase molecules, are still somewhat different.6 The result has been the creation of solid (KBr pellet), liquid, gas-phase, and matrix isolation IR reference libraries. This partially negates the sampling advantage of IR relative to mass spectrometry (requires gas-phase molecules or ions). This negation results from the cost of creating libraries of IR reference spectra, which may cost from $200 to $400 per spectrum, and may contain multiple spectra of the same chemical in different physical state^.^ This problem can be minimized by removing the mobile phase, prior to detection, and by depositing the analyte in the same physical state for each type of chromatography. A proposed solution to these problems is the universal IR interface.* This interface traps chromatographically separated eluites on a zinc-selenide plate. Gas and supercritical fluid applications of this approach have been reported.”’ Research is underway to adapt the system to liquid chromatography and eventually to capillary electrophoresis.*2 If successful, the interface will be amenable to the characterization of volatile, semivolatile, and nonvolatile analytes of a wide range of polarities. Previously, nonvolatile analytes have been almost inaccessible to IR characterization, in a dynamic sampling mode, and only recently to MS characterization by liquid chromatography/mass spectrometry. This has led to char( 5 ) Welti, D. Infrared Vapour Spectra; Heyden/Sadtlcr: London, 1970; p 14. (6)Schneidcr, J. F.; Demirgian, J. C.; Stickler, J. C. J. Chromator. Sci. 1986.24, 330-335. (7) Gurka, D. F.;Umaiia, M.; Pcllizzari, E. D.; Moseley, E.; de Haseth, J. A. Appl. Spectrosc. 1985, 39, 297-303. (8) Griffths,P. R.;Norton,K. L.;Lange,A. J.Microchem.J. 1992,46,261-270. (9) Bourne, S.; Haefner, A. M.;Norton, K. L.;Griffiths, P.R. Anal. Chem. 1990,
62,2448-2452.
(IO) Visser, T.; Vredenbregt, M. J. Vib. Spectrosc. 1990, I ,
205-210.
(1 1) Pentoney, S. L.; Shafer, K. H.; Griffiths, P. R. J. Chromarogr.Sci. 1986.24, 236236. (12) CooperativeAgreementNo. CR819576020betwtcn thcEnvironmenta1Systems Monitoring Laboratory at Las Vegas and the University of Idaho.
Analyrcal Chemistry, Voi. 66, No. 15, August 1, 1994 2521
acterization of only volatile and semivolatile analytes in environmental samples, although it is known that many sample extractables are nonvolatile13J4 and that these nonvolatiles and their degradation products can be toxic15J6 or mutagenic. 1 6 ~ ~ This work describes the evaluation of a direct-deposition FT-IR interface for gas and supercritical fluid chromatography. Special emphasis was placed on environmentally important analytes. The analytes chosen for gas chromatography are on the EPA's Contract Laboratory Program target list. This work is an integral part of the U S . EnvironmentalProtection Agency's Surface Cleanup Program, which is dedicated in part to developing new field analytical methods to be used at Superfund sites. It is believed that the combination of gas and supercritical fluid chromatography will lead to a more complete sample characterization and, thus, to more reliable risk assessment.I8
EXPERIMENTAL SECT1ON Infrared System. The infrared system was a Biorad FTS45 Fourier transform infrared Tracer spectrometer equipped with a SPC-3200 work station and a 380-MB disk. The interferometer collected 4 scans/s at 8 cm-1 resolution using a triangular apodization function. A 0.1-mm mercurycadmium-telluride detector (750 cm-' cutoff) and the zincselenide direct-deposition plate were housed in an interface evacuated to Torr. The detector and the deposition plate were cooled to liquid nitrogen temperature for GC work. SFC was performed with the plate at ambient temperature. For GC, the transfer line was maintained at 230 OC while the transfer line tip was maintained at 280 "C. For SFC, the correspondingtemperatures were 150and 130 OC, respectively. The zinc-selenide plate was programmed to move 100 pm in 2 s for the first 5 min, in 3 s for the next 10 min, in 5 s for the following 10 min, and in 15 s for the final 20 min. Gas Chromatography. A Lee Scientific Model 501 gas chromatograph equipped with a duck-bill injector was used. Chromatography was carried out with a 4.5 m X 0.18 mm fused-silica column coated with 0.18 pm of DB-5. A 100-pm i.d. fused-silica transfer line was butted with a glass connector to the GC column. An 20-cm fused-silica transfer line was attached to the column head for on-column injection, and head pressure was maintained at 25 psi. Initial helium flow was 1 mL/min and was passed through a heated cartridge drier. For base-neutral analysis, the GC oven was ramped from 30 to 280 OC; for pesticide analysis, the GC was ramped from 35 to 250 OC; and for polynuclear aromatics, the GC was ramped from 35 to 280 OC. All ramp rates were 10 OC/min, and each ramp began with a 2.5-min hold. Prior to initiating GC/FT-IR work, the system sensitivity was checked by on-column injection of a 1-pL 300-pg solution of n-dodecane. The ratio of the dodecane absorbance at 291 6 (13) Rivera, J.; Caixach J.; Espadala, I.; Romero, J.; Ventura, F.; Guardiola, J.; Om,J . Water Supply 1989, 7 , 97-103. (14) Sikkema, S.;Dienzmann, E.; Ahlert, R.; Kowon, D. Enuiron. Prog. 1988, 7, 77-83. (15) Giger, W.; Brunner, P. H.; Schaffner, C. Science 1984, 225, 623-625. (16) Ballard, J. M.; Betowski, L. D. Org. Muss Spectrom. 1986, 49, 575-588. (17) Coleman, W.E.; Munch, J. W.; Kaylor, W. H.; Strclcher, R. P.; Ringhand, H. P.; Meier, J. R. Enuiron. Sci. Technol. 1984, 18, 674-681. (18) Menzel, D. B. Enuiron. Sci. Technol. 1987, 21, 944950.
2522
Analytical Chemistry, Vol. 66,
No. 15, August 7, 1994
cm-1 to that of the peak-to-peak noise a t 2050 cm-I was 456:l. The spectrum compares favorably to that reported earlier by Bourne et al. for 50 pg of d ~ d e c a n e . ~ Suprcritical Fluid Chromatography. A Lee Scientific Model 501 pump and chromatograph was used. A Valco pressure-actuated valve was used to inject the 0.2-pL sample volumes to a 5 m X 100 pm i.d. fused-silica column coated with a 0.25-pm film of SB-methyl-100. The column end was butted to a 50-pm i.d. fused-silica transfer line, which extended to the zinc-selenide sample plate. A 10 pm X 16 cm splitter was installed between the sample valve and the GC column. This splitter controlled carbon dioxide flow to the interface such that vacuum decreased from lo" to Torr across the pressure ramp. An integral restrictor was prepared at the transfer line end using the procedure of Guthrie et al.I9 After an initial hold of 5 min at 120 atm, the system was ramped at 8 atm/min to 400 atm. Analytical Solutions and Regression Analysis. Poly(ethylene glycols) of average molecular weight 400,600,1000, and 1500 were obtained from Aldrich Chemical. Methanol solutions were prepared to contain 3 nL/pL, 19.4 pg/pL, 15.3 pg/pL, and 11.3 pg/pL, respectively. Samples of 2 mg/ mL solutions of base-neutrals, pesticides, and polynuclear aromatics were obtained from Supelco. Regression analysis solutions were prepared from these by dilution to 24, 12, 6, and 2.5 ng/pL for pesticides; by dilution to 24, 12, 5, and 2.5 ng/pL for polynuclear aromatics; and by dilution to 12, 6, 2.5, and 1.0 for base-neutrals. Regression analysis was performed in duplicate on each stock solution dilution. The Gram-Schmidt and functional group chromatogram peak heights were used for computation using Lotus 1.2.3 software, revision 3.1. Minimum Identifiable Quantities. Minimum identifiable quantities were determined as previously reportedq20 Sequentially diluted stocksolutions were injected until the target analyte was not identified as one of the first five library search hits, obtained by searching a reference library of directdeposition spectra. The lowest concentration at which an identification does occur is the minimum identifiablequantity (MIQ).
RESULTS AND DISCUSSION Initial System Considerations. For maximum GC sensitivity, on-column injection was used. Regardless of the injection technique, a septum-equipped injector introduced fragments to the retention gap, which thermally decomposed during the GC thermal ramp. This problem was solved by using a septumfree, duck-billed injector.21 The deposition area of the zinc-selenide sample plate is limited. When its capacity is exhausted, the interface must be brought to ambient pressure, the plate cleaned and remounted, and the plate and optics realigned. To minimize instrumental downtime, the tradeoffs between run time, GC column, and chromatographic resolution were investigated. It was found that a 4.5 m X 0.18 mm fused-silica column coated with a 0.18-pm film of DB5 would adequately separate (19) Guthric, E. J.; Schwartz, H. E.J. Chromatogr. Sci. 1986, 24, 236-241. (20) Gurka, D. F.; Laska, P. R.;Titus. R. J . Chromarogr. Sci. 1982,20,145-150. (21) Freeman, R. R. High Resolution Gas Chromatography, 2nd 4.; HewlcttPackard Co.: Avondale, PA, 1981; p 69.
Stage position during deposition
30
20
0.121
Absorbance maximum 0.270
1
0
1
0
7
21
6
19
100
17
3
17
4
0
0
1 m
O.061
6
0
Flgure 1. Deposltlontest resultingfrom the Injectionof acetone vapor. 0.004
(c)
32 26-28 31 1 34
0.000 1
.
r
I
I
I
I
I
4
a
I
12
16
20
24
lime, min Figwe 3. GC functlonal group chromatograms (unsmoothed) of 1.2 ng of base-neutrals (a), 2.2 ng of polynuclear aromatics (b), and 2.0 ng of pesticides(c) recordedin the wave number regions of 950-1300, 700-900, and 700-900 cm-l, respectively. Chromatogram peak numbering corresponds to Table 2.
O"*l
(c)
1
3dOO
2dOO
2iOO
1dOO
ld00
I
Id00
Wavenumber (cm-1)
Flgure 2. QC/FT-IR spectra (unsmoothed) of 2 ng of DDT (a), 1.2 ng of dioctyl phthelate (b), and 2.2 ng of fluoranthene (c).
the 16 priority pollutant polynuclear aromatics in a run time of under 24 min. With the sample program rate used in this work, this translated to about 8 h of G C time before plate cleaning. This time can be increased by slowing the plate motion, but for maximum sensitivity, the plate must be programmed so that its speed compensates for the increased elution volumes resulting from the decrease in carrier gas flow (1 mL/min at 40 O C to 0.2 mL/min at 250 "C),which occurs across the GC program ramp. Deposition Tip Alignment. Prior to the collection of GC/ FT-IR analytical data, the alignment of the deposition tip relative to the zinc-selenide plate must be determined. This is accomplished by injecting acetone vapor to the cold plate. The spectrometer then collects 17 scans, each spaced by 100
pm, in a gridwork pattern around the tip and also collects a reference file. The carbonyl peak absorbance of acetone is used to generate a normalized pattern of absorbance ratios as seen in Figure 1. It is seen that 8 0 4 3 % of the acetone absorbance is at the gridwork center indicating a well-aligned tip. Infrared Spectra and Functional Group Chromatograms. Figure 2 shows the infrared spectra of 2 ng of DDT (a), 1.2 ng of dioctyl phthalate (b), and 1.2 ng of fluoranthene (c). Although the spectral signal noise is high, these results represent an order of magnitude sensitivity improvement, in real time, relative to earlier reports.22 Because the separated analytes are cold-trapped, sensitivity can be improved by postrun signal averaging; during this process, however, the carrier gas flow rate must be reduced to minimize interferences from carrier gas water vapor. Functional group chromatograms of 0.5 ng of base-neutrals, 2.2 ng of polynuclear aromatics, and 2.0 ng of pesticides are shown in Figure 3. These concentrations are close to the detection limit for each fraction, but most components are easily distinguished from baseline noise even though baseline smoothing was not employed. The advantage of using short, lightly loaded GC columns is evident from the polynuclear aromatic hydrocarbon chromatogram (Figure 3b), which shows that the 16component mixture is completely eluted in 24 min. This represents a run time that is 50% shorter than if the same separation were attempted with a 30 m X 0.32 mm column coated with 1 pm of DB-5. The base-neutral functional group sensitivity was similar to the Gram-Schmidt. This results from the presence of several different functional group types ~~
~
~
(22) Gurka, D. F.;Farnham, I.; Potter, B.B.;Pylc, S.;Titus, R.; Duncan, W.Anal. Chem. 1989,61, 1584-1589.
An@tlcalChemistry, Vol. 86, No. 15, August 1, 1994
2523
TaMo 1. Rogr.#kn Anatyrk on &am-Schmkn
compound
chrom. type
(aS) and Functlonal Group (Fa) stierror slope
slope (xio-3)
(xiv3)
Chromatogram Data
Y intercept
std error
Y intercept
(xiv3)
z
1.432 14.77 -0.090 9.470 -0.610 1.979 4.935 7.396 10.88 14.55 9.379 9.817 5.693 9.931 1.248 3.350 2.240 15.65 2.473 2.785 8.675 19.30 3.928 21.11 -2.090 11.63 -0.280 14.92 -5.930 -4.090 8.059 16.51
0.278 2.910 4.387 6.075 1.436 10.33 3.721 7.254 2.627 4.656 3.232 1.061 1.097 2.586 1.079 2.874 5.131 8.680 0.600 1.915 1.425 5.333 4.379 9.146 1.986 4.100 0.436 13.92 4.197 8.820 2.192 10.64
0.9998 0.9980 0.9122 0.9774 0.9961 0.9417 0.9475 0.9789 0.9768 0.9833 0.9418 0.9985 0.9203 0.9912 0.9743 0.9868 0.9214 0.9613 0.9955 0.9934 0.9966 0.9925 0.9705 0.9905 0.9473 0.9234 0.9995 0.9465 0.9736 0.9535 0.9721 0.9679
1.760 -5.960 1.261 0.477 1.222 0.931 -0.520 -2.170 -1.720 7.081 -0.560 9.095 0.624 0.490 0.332 2.466 3.907 6.523 2.213 4.778 -4.430 7.256 0.214 2.175 0.249 13.65 -3.330 9.508
1.235 5.482 0.743 4.604 1.164 2.250 1.589 2.873 1.807 4.048 -0.539 6.443 0.454 0.191 3.474 4.775 6.756 12.71 6.942 2.209 9.121 1.145 3.560 0.152 3.883 2.155 5.207 0.915
0.9768 0.9466 0.9955 0.9839 0.9617 0.9790 0.9861 0.9821 0.9936 0.9751 0.9998 0.9822 0.9987 0.9999 0.9920 0.9810 0.9521 0.9345 0.9419 0.9958 0.9715 0.9952 0.9403 0.9999 0.9768 0.9980 0.9930 0.9993
-3.70 -0.560 5.510 -2.870 -3.080 -0.630 -17.49 3.137 -1.220 6.671 5.167 0.972
1.699 3.065 0.509 4.796 5.205 2.225 11.52 0.736 8.851 16.37 5.414 1.545
0.9592 0.9579 0.9750 0.9750 0.9229 0.9594 0.8408 0.9673 0.9796 0.9171 0.9514 0.9938
(xiv3)
PAHs acenaphthene acenaphthylene anthracene benz[a]anthracene benzo[b] fluoranthene benzo[k] fluoranthene bcnzo[g,h,i]perylene bcnzo[a]pyrene chrysene dibenz[u,h]anthracene fluoranthene fluorene indeno [ 1,2,3]pyrene naphthalene phenanthrene pyrene
a-BHC fl-BHC Y-BHC A-BHC aldrin dieldrin endosulfan I endosulfan sulfate eldrin endrin aldehyde endrin ketone dichlorodiphenyl dichloroethene dichlorodiphenyl trichloroethane methoxychlor bis(2-chloroethyl) ether
bis(2-ch1oroethoxy)methane bis(2-ethylhexyl) phthalate bis(2-chloroisopropyl)ether 4-bromophenyl phenyl ether butyl benzyl phthalate 2524
G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG G-S FG G S FG G S FG
1.620 0.020 5.506 0.174 1.120 0.263 3.388 0.364 1.689 0.106 3.522 0.620 1.163 0.274 4.187 0.435 1.254 0.193 3.031 0.279 0.957 0.238 3.879 0.152 0.274 0.810 2.328 0.155 0.488 0.079 2.104 0.172 1.293 0.378 3.804 0.520 0.656 0.044 1.986 0.115 1.785 0.105 5.191 0.320 1.849 0.322 7.914 0.548 0.620 0.146 1.207 0.246 1.319 0.032 4.964 0.835 1.876 0.309 2.983 0.649 0.952 0.161 4.955 0.638 Pesticides 0.617 0.095 1.997 0.335 0.961 0.045 3.115 0.282 0.505 0.071 1.330 0.138 1.031 0.123 1.844 0.176 1.731 0.139 2.192 0.248 3.600 0.033 4.146 0.394 1.098 0.028 1.977 0.015 5.679 0.551 2.968 0.292 2.606 0.413 4.154 0.778 2.418 0.425 2.944 0.135 4.605 0.558 1.428 0.070 1.223 0.218 2.410 0.012 2.182 0.238 4.231 0.132 5.385 0.319 3.083 0.056 Base-Neutrals 1.590 0.232 2.690 0.325 1.916 0.309 5.510 0.509 3.311 0.552 1.988 0.236 4.990 2.171 1.104 0.203 12.49 1.275 10.01 1.737 4.405 0.575 3.981 0.223
AnawicalChemistIy, Voi. 86, No. 15, August 1, 1994
Tabk 1 (Contlnwd)
chrom. type
compound 4-chlorophenyl phenyl ether
(%r3) 12.91 14.48 10.84 8.130 11.92 9.462 20.89 15.13 2.115 1.416 1.876 5.205 3.350 5.311 2.861 8.040
G S FG G S FG G S FG G S FG G S FG G S FG G S FG G S FG
di-n-butyl phthalate diethyl phthalate dimethyl phthalate di-n-octyl phthalate N-nitrosodimethylaminc N-nitrosodipropylamine N-nitrosodiphenylamine
10
0.020
0.016 Ir , r 7 , ~ . 7 - . . l , ~ l . , - - l . .
,..
..-.,, . . . . .. ..
H A
I
I?*
39
0.OOO 6
10
li
1's
22
Time, min.
Figure 4. GC functional group chromatograms (unsmoothed) of 0.5 ng of base-neutrals In wave number region 950-1300 cm-l (a) and 0.6 ng of baseneutrals In wave number region 1670-1800 cm-' (b). Chromatogram peak numbering corresponds to Table 2.
within the same solution. Figure 4 demonstrates that if the functional group chromatogram is optimized for specific fraction components, better sensitivity can be obtained than by using the same functional group chromatogram for the entire fraction. This figure shows the base-neutral fraction recorded from 950 to 1300 cm-I (a) and from 1670 to 1800 cm-I (b). The latter chromatogram lowers peak-to-peak baseline noise by about a factor of 4, thereby enhancing sensitivity for the phthalates. Regression Analysis. Earlier light-pipe GC/FT-IR regression analysis studies indicated a linear dynamic range of about 2 orders of magnitude.22 The reported greater sensitivity of direct deposition relative to light pipe should result in a wider linear dynamic range; however, effects resulting from the increasing optical thickness of thedeposited eluitemight create
std error slope (xiv3)
Y intercept
Y intercept
std error
(~10-3)
(xiv3)
r2
0.971 1.563 0.953 1.184 0.973 0.634 3.098 0.771 0.347 0.253 0.349 0.786 0.613 1.711 1.089 0.253
4.704 2.847 13.06 5.381 19.77 14.33 -4.030 3.951 1.633 1.828 0.127 -3.650 -0.430 -26.19 30.29 -17.24
0.915 14.73 8.98 1 11.16 9.173 5.977 29.20 7.265 3.266 2.381 2.425 7.410 2.225 7.392 3.950 1.343
0.9833 0.9662 0.9773 0.9402 0.9804 0.9867 0.938 1 0.9923 0.9255 0.9128 0.9351 0.9359 0.8736 0.9990 0.9676 0.9060
nonlinear concentration/response curves. Results reported in Table 1 and the standard concentration data in the experimental indicate a linear dynamic range of 1 order of magnitude, although the direct deposition minimum identifiable quantities (MIQs) are about an order of magnitude better than the light-pipevalues reported from our labor at or^.^^ This results, in part, from the use of a thin-film GC column to speed sample throughput and thereby minimize the instrument downtime associated with cleaning the zinc-selenide sample plate. The summarized regression data in Table 1 reveal several trends. Except for polynuclear aromatics, the Gram-Schmidt and functional group mean slopes of these sample fractions are similar. This is consistent with the fact that most of the mid-infrared absorbance for the polynuclear aromatics occurs in the wave number region of 700-900 In contrast, the base-neutral fraction contains a wide variety of functional groups, including dialkyl and diary1 ethers, phthalates, and N-nitroso amines, each of which, for maximum sensitivity, should be studied in a different frequency region. The correlation coefficient (r2)data indicate that although the range is approximately independent of chromatogram and compound class type, in each case the functional group chromatogram mean r2 is superior to that of GramSchmidt. The GramSchmidt mean correlation coefficients for polynuclear aromatics, pesticides, and neutrals are 0.9653,0.977 1, and 0.9436, respectively. Thecorresponding mean correlation coefficients for the functional group chromatograms are 0.9705,0.9823, and 0.9578, respectively. This suggests that the GramSchmidt signal noise is generally poorer than that of the functional group. Even better functional group chromatogram signal noise could be obtained by using frequency regions optimized for each chemical, but this assumes a target compound rather than a screening approach. Detection limit results in Table 2 indicate that most of the 44 compounds studied in this work can be identified in the 1-3-ng range. When the light-pipe and direct-deposition GramSchmidt MIQs are compared, it can be seen that the (23) Gurka, D.F.;Qle, S. M.;Famham, I.; Titus, R.J. Chromatogr. Sci. 1991, 29, 339-344. (24) Scmmler,J.; Yang, P.W.; Crawford, G. E. Vib. Spectrow. 1991,2,303-310.
Analjltcal Chemistry, Vol. 66,No. 15, August 1, 1994
2525
Table 2. Mlnlmum IdentHlable Ouantlleo (MIO) from GramSchmldt and Functional Group Chromatogram
minimum identifiable quantities (ng) no. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 a
compound N-nitrosodimethy lamine bis(2-chloroethyl) ether bis(2-chloroisopropyl) ether N-nitrosodipropylamine bis( 2-ch1oroethoxy)methane naphthalene dimethyl phthalate acenaphthylene acenaphthene diethyl phthalate 4-chlorophenyl phenyl ether fluorene N-nitrodiphenylamine 4-bromophenyl phenyl ether CY-BHC @-BHC phenanthrene anthracene Y-BHC A-BHC di-n-butyl phthalate aldrin fluoranthene pyrene dieldrin endosulfan I endrin
dichlorodiphenyldichloroethene endrin aldehyde butyl benzyl phthalate endosulfan sulfate dichlorodiphenyl trichloroethane ethyl hexyl phthalate endrin ketone benz[a]anthracene chrysene methoxychlor di-n-octyl phthalate benzo[b] fluoranthene benzo[k]fluoranthene benzo [a]pyrene indeno[ 1,2,3-cd]pyrene dibenz [a,h]anthracene benzo [g,h,i]perylene
retention time (min) 2.20 3.31 3.80 4.1 1 5.12 5.76 9.10 9.18 9.58 10.53 10.64 10.75 11.01 11.42 11.91 12.55 12.73 12.86 13.26 13.85 14.60 15.22 15.27 15.70 15.82 16.31 16.48 16.64 17.23 17.44 17.60 17.84 17.89 18.29 18.5 18.5 18.96 20.47 20.69 20.72 21.20 23.10 23.30 23.56
direct deposition FG
GS 1.o 2.5 1.o 1.o 0.6 5.0 0.6 2.5 2.5 0.6 0.6 5.0 1.o 0.6 6.0 2.5 5.0 5.0 2.5 6.0 0.6 6.0 2.5 2.5 2.5 2.5 2.5 2.5 2.5 0.6 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 2.5 5.0 5.0 5.0 5.0
1.o 1.o 1.o 1.o 0.6 0.8 0.6 0.8 2.5 0.6 0.6 0.8 1.o 0.6 2.5 2.5 0.8 2.5 2.5 2.0 0.6 2.0 0.8 0.8 2.0 2.0 2.0 2.0 2.0 1.o 2.0 2.0 2.5 2.0 0.8 2.5 2.0 2.5 2.5 0.8 0.8 2.5 2.5 0.8
light pipe G S
LP DD"
5 10 10 5 10 25
5 4 10 5 16.6 5
50 25 5
20 10 8.3
50
10
50 50 50
5
dS
8.3 10 10 8.3
50 50
20 20
10
16.6
50 100 10 100
20 40 8.3 20
LP/DD indicates the quotient of the light-pipe and direct-deposition minimum identifiable quantities.
mean sensitivity improvementis about a factor of 13. However, flow decrease directly affected the earlier light-pipe sensitivity part of this sensitivity improvement results from using a GC studies. It is believed that a -light-pipe GC/FT-IR system column with an inside diameter of 0.18 mm as opposed to the equipped with a pressure-programmable GC and a 0.18-mm 0.32-mm column used for the earlier light-pipe work.22 This GC column may achieve sensitivities comparable with those results in a smaller GC elution volume because of a smaller of this direct-depositionsystem. This presumes the availability inside cross-sectional area, which results in a higher number of light pipes with volumes of about 25 pL to match the elution of column plates and leads to a sensitivity e n h a n ~ e m e n t . ~ ~ . ~volumes ~ obtained from 0.18-mm GC columns. It should be However, the increase in column plates may be offset by the noted that SFC/IR flow cells with volumes as small as 500 need to inject less sample with smaller bore columns to reduce nL have been reported.27 In addition, a 0.1-mm mercurycolumn overloading. In addition, part of the sensitivity cadmium-telluride (MCT) detector was used in this study enhancement results from the ability to program the zincwhile a 1-mm detector was used in the earlier light-pipe work. selenide plate position. This compensates for the increasing The noise equivalent-power (NEP)in IR systems is equal to elution volume, which results from the decrease in carrier gas A'/2/D* where A is the detector area and D* is its specific flow associated with the GC temperature ramp and is suggested detectivity.28 Thus, a t constant detector D*, this should lead in Table 2 by the trend to higher light-pipe/direct-deposition for this work relative to a sensitivity enhancement of ( MIQ ratios as a function of increasing retention times. This (25) Lee, M. L.; Yang, F. J.; Bartle, K. D. Open Tubular Gas Chromotography: Theory and Practice; Wiley-Interscience: New York, 1984; Chapter 2. (26) Hyver, K. J.; Phillips, R. J. J . Chromatogr. 1987, 399, 34-46.
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(27) Jenkins, T. J.; Kaplan, M.; Davidson, G.; Healy, M. A,; Poliikoff, M. J. Chromotogr. 1992,626, 53-58. (28) Henry, D.E.; Giorgetti, A.; Haefner, A. M.; Griftlths, P. R.; Gurka, D.F. Anal. Chem. 1987,59, 2356-2361.
1
124
.03 3g40
43
.02
.01
.oo .04 4
,
8
12
,
16
,
20
- 1 -
24
? n e vir
Flguro 5. GC functional group Chromatograms (unsmoothed) for 20 ng of polynuclear aromatics In the wave number region 700-900 cm-’ with the rinc-selenkb plateat amblent temperature(a)and liquidnitrogen temperature (b). Chromatogrampeak numberingcorresponds to Table 2.
to light-pipe studies (all other spectrometer parameters for the direct deposition and light pipe systems being approximately equivalent). To achieve the maximum sensitivity enhancement, the light-pipe optical system should be optimized for a smaller detector area. SFC/FT-IR Evaluation. The majority of SFC work reported uses carbon dioxide as the mobile phase with and without added modifiers.29 However, both carbon dioxide and modifiers would be expected to freeze on a liquid nitrogencooled sample plate. This would interfere with the analyte determination. Therefore, a sample plate that is warmer than that used for GC/FT-IR should be used for SFC/FT-IR. Figure 5 shows a functional group chromatogram for a 16component polynuclear aromatic mix acquired with the plate at ambient temperature (a) and at liquid nitrogen temperature (b). Hydrocarbons with vapor pressures equal to or less than that of chrysene are efficiently trapped at ambient plate temperature. This seemed adequate for this project, which was concerned with SFC/FT-IR studies of poorly volatile environmental contaminants. By the simple expedient of changing the transfer line from 100- to 50-pm i.d. and making an integral restrictor at the transfer line end, the interface was converted for SFC work. The restrictor is required to lower the mobile-phase flow to the interface such that the system pumps can maintain adequate vacuum. Figure 6 shows the SFC/FT-IR functional group chromatograms for the wave number region 2800-3000 cm-l for 0.6 nL of poly(ethy1ene glycol) 400 (a), 3.9 pg of poly(ethy1ene glycol) 600 (b), 3.1 pg of poly(ethy1ene glycol) 1000 (c), and 2.3 pg of poly(ethy1ene glycol) 1500 (d). Prior to SFC/FTIR, the poly(ethy1ene glycol) 400 solution was analyzed by gas chromatography but no peaks were detected, verifying that these analytes are not amenable to gas chromatography. Assuming that each solution contained about 20 components (by visual inspection of chromatograms) at equal concentration, detection limits would be about 100 ng each (excludes injection split). Figure 7 shows the FT-IR spectra of the chromatogram peaks for poly(ethy1ene glycol) 400 at 12.3 and 23.1 min. The decreasing ratio of 0 - H to C-H stretch, ( 2 9 ) Fields, S.M.; Markides, K. E.; Lee, M. L. J. Chromatogr.1987,406,223-235.
.02
.00
10
20 30 MINUTES
40
Figure 8. SFC functlonal group chromatograms of 0.6 nL of poly(ethylene glycol) 400 (a), 3.9 pg of poly(ethyleneglycol) 600 (b), 3.1 pg of poly(ethy1eneglycol) 1000 (c), and 2.3 pg of poly(ethy1eneglycol) 1500 (d). The same SFC pressure ramp was used for each glycol solution.
with increasing retention time, is consistent with the increasing ratio of recurring molecular chain residue to terminal unit -OH. Kalinoski et al. have previously reported supercritical ammonia SFC/MS analysis of poly(ethy1ene glycol) 4OOa3O They reported a range of molecular mass from 194 to 678 Da across the full oligomer separation. CONCLUSION Real-time sensitivity of the GC/direct-deposition IR system is over an order of magnitude greater than that of light-pipe (30) Kalinoski, H. T.; Udseth, H. R.; Wright, B. W.; Smith, R. D. J . Chromorogr. 1987,400, 307-316.
AnatyticalChemistt-y, Vol. 88, No. 15, August 1, 1994
2527
I
w
0
2
m CC
%m a
0.20
0.00 4000
I
I
I
3000
2000
1000
WAVENUMBER (cm.’) Flgve 7. SFCIFT-IR spectra of poly(ethy1eneglycol) 400 chromatogram peaks at 12.3 (a) and 23.1 min (b).
systems. These identification limits are consistent with those predicted for direct deposition by Griffiths et They predicted limits of a few hundred picograms for strong infrared absorbers with limits of 1 or 2 ng for weak absorbers like (31) Griffiths, P. R.; Norton, K. L.; Bonnano, A. S.Involving Elimination of the Mobile Phase. In Hyphenated Techniques in Supercrifical Fluid Chromatography and Extracfion;Jinno, K., Ed.;Elsevier Science: Amsterdam, 1992; Chapter 6. (32) Samoiloff, M. R.; Bell, J.; Birkholz, D. A.; Webster, B.; Amott, E. G.; Pulak, R.; Madrid, A. Environ. Sci. Technol. 1983, 17, 329-334. (33) Yonker,C. R.; Frye,S. L.;Lalkwarf,D. R.;Smith,R. D.J. Phys. Chem. 1986, 90, 3022-3026. (34) Duffus, J. H. Mefabolism of Toxic Substances by Animals. Environmental Toxicology; John Wiley and Sons: New York, 1980; Chapter 2.
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polynuclear aromatics. It is believed, however, that light pipe systems can be modified to achieve these higher sensitivities by using small-bore capillary columns and smaller area MCT detectors. Additional sensitivity can be gained for the lightpipe approach by using GC pressure programming. This would compensate for the increase in elution volumes occurring across the GC ramp, which is caused by decreasing carrier gas flow rates. Pressure programming the light-pipe system should produce a similar effect as programming the sample deposition plate. In the SFC mode, using carbon dioxide as the mobile phase, the direct-deposition approach can determine neutral analytes with molecular mass of at least 1500 Da. Previous work with environmental sediments suggests that most of the sample toxicity can be in theneutral fraction.32 However, it is expected that toxicity and mutagenicity will be highly sampledependent. The addition of polar modifiers to the carbon dioxide should extend the direct deposition approach to polar nonvolatile compounds.29 Even without the addition of modifiers, supercritical carbon dioxide, which is reported to have the solvating power of hexane,33is expected to dissolve and separate many toxic sample components. This is because membrane transport can be a prerequisite to toxicity manifestation, and lipid- solubility is favored for passive diffusion across the mammalian cellular membrane.34 This work demonstrates the ease of converting a direct-deposition IR interface from gas to supercritical fluid chromatography. The possibility of chromatographic degradation on changing from SFC to GC will be investigated in future work at this laboratory.
ACKNOWLEDGMENT The US.Environmental Protection Agency (EPA), through its Office of Research and Development (ORD), funded and performed the research described here. It has been subjected to the Agency’s peer review and has been approved as an EPA publication. The U.S. Government has the right to retain a nonexclusive, royalty-free license in and to any copyright covering this article. Received for review August 4, 1993. Accepted May 6, 1994.’ Abstract published in Aduance ACS Abstracts. June 15,
1994.
