Gasphase UV Spectrophotometry

The spectral region of the UV electromagnetic absorption of interest to the analytical chemists extends from about 160 to 330 nm. Molecules in which all valence shell electrons are involved in single bonds, such as the straight chain saturated hydrocarbons, show absorption maxima in the region of 160-170 nm. Their spectra are observed only as 'end absorptions' because of instrumental limitations. The other groups of nonaromatic hydrocarbons are bathochromic-shif-ted and possess their absorption edges in the range of 190-200 nm. This is one aspect of the importance of measuring the absorption of the UV light in the gas phase because there is no solvent wavelength cutoff at 190-200 nm. The other important aspect is that the spectra are not influenced by solvent effects and therefore well defined. There are also no shifts in ^max values and the bands are sharp in comparison with the relatively broad bands observed in the liquid phase, where their shapes depend on the degree of interaction between solvent and solute. Furthermore, in liquids, it is only very rarely that separation of vibrational bands is possible because the solvent effect generally tends to obscure the vibrational structure. By measurements in the vapour phase, these drawbacks are eliminated, which makes the UV spectra in the gas phase highly suitable for computer-based identifications of unknowns against a spectrum reference library. An example of the reproducibility of spectral details is shown in Figure 1, where a spectrum, taken from an analysis of cigarette smoke, is normalized and overlaid on the reference spectrum of isoprene. The absorbance spectra as well as the first derivatives are plotted. Derivatives of absorption spectra are preferably used in order to enhance spectral details. Derivatives of spectra can also be utilized in order to make selective determination of specific functional groups.

A study of molecular UV absorption spectra between 168-330 nm in the gas phase, for about 1000 organic compounds has demonstrated the importance of the short UV wavelengths for analytical purposes. About 70% of the 1000 spectra registered have absorption maxima at wavelengths shorter than 190 nm and their intensities are up to 100 times higher than those of the absorption bands at longer wavelengths. In addition to the high sensitivity of detection, the lower wavelength region is the spectral range, where the amount of details used for identification purposes increases considerably. Moreover, most of the functional groups in compounds not considered UV-ab-sorbing, such as alkanes, ethers, aldehydes and ketones, display detailed spectra in this region.

The influence of temperature on the shape of spectra has been studied in the range of 15-205°C. A slight broadening effect on spectral absorption bands (0.3 nm) and the vibrational structure (maximally 1.4 nm) with increased temperature was observed. These effects are however, considered to have

Table 1 The development of GC-UV: instrumental conditions

Author (year)

Instrument principle

Separation column

Separation conditions Flow cell configuration

Kaye (1962)

Kaye (1964) Novotny (1980)

Adams (1984)

Lagesson (1984) Kube (1985) Lagesson (1989)

Bornhop (1991) Bornhop (1992) Hackett (1995)

Sanz-Vicente (1996) Sanz-Vicente (1998)

Lagesson-Andrasko (1998)

Lagesson (2000)

Beckman DK-2, modified 1.8 m, i.d. 3.2 mm, and N2-purged packed column

Beckman DU, modified and N2-purged Perkin-Elmer GC-55 variable wavelength monitor Tracor Model 970 in a remote configuration. 240 nm and 260 nm bandpass filters Varian Cary UV-Vis spectrophotometer

HP 8450A, photodiode array

HP 8452A, photodiode array, 324 diodes

Rapid-scanning LC detector, Linear Instruments Rapid-scanning LC detector, Linear Instruments Remote detection using fibreoptics 206HR detector, Linear Instruments HP 8451, photodiode array

HP 8451, photodiode array

INSCAN 175 GC-UV spectrophotometer, photodiode array, 1024 diodes, N2-purged INSCAN 175 GC-UV spectrophotometer, photodiode array, 1024 diodes, N2-purged

1.8 m, i.d. 3.2 mm, packed column 30 m, i.d. 0.7 mm, UCON 50 HB2000, capillary column 2 m, i.d. 2 mm, packed column 3% 0V101 on 100/120, Supelcoport 8 cm, i.d. 1.5 mm, packed column (10 |im particles), DDP, 0V101 1.8 m, i.d. 6 mm, packed column 5% SP-1200, 1.75%, Bentone-34 8 cm, i.d. 1.5 mm, packed column (10 |im particles), DCQF1

30 m i.d. 0.53 mm, 1.0 |im, B-210, capillary column 25 m i.d. 0.32 mm, 0.4 |im, SE-52, capillary column 30 m, i.d. 0.32 mm, 0.25 | m, DB5, capillary column

4 m, i.d. 1 /8 in, packed column, 5% SE-30, Chromosorb W HP 4 m, i.d. 1 /8 in, packed column, 5% SE-30, Chromosorb W HP 8 cm, i.d. 1.5 mm, packed column

30 m, i.d. 0.32 mm, 0.25 |im, Hp-5, capillary column

Injection volume: 30 |L Flow (He): 189 mL min"1 Injection volume: 30 i L

Injection volume: 5 |L Flow (N2): 30 mL min-150°C, 2 min; 10oCmin^1, 250°C Injection volume: 1 |L Flow (N2): 10 mL min-1 & 90°C Injection volume: 0.2-0.5 |L

Flow (He): 60 mL min-1 Injection volume: 1 | L Flow (N2): 15 mL min-1 start 70°C, ramp 15°Cmin-1 Flow (He): 5 mL min-1

Injection volume: 0.5 |L, 1 : 100 Flow (He) Injection volume: 1 i L splitless

Injection volume: 50 i L

Injection volume: 1 |L Flow (N2): 3 mL min~ make-up flow 7 mL min-

Path length: 1 cm 1 Volume: 1.5 mL Path length: 1.12 cm Volume: 50 | L

Path length: 9.5 cm Volume: 170 |L

Path length: 12.5 cm Volume: 22 mL

Path length: 9.5 cm Volume: 170 |L

Path length: 9.4 cm Volume: 170 |L

Path length: 9.4 cm Volume: 170 i L

References:

Kaye W (1962) Analytical Chemistry 34: 287-293.

Kaye W and Waska F (1964) Analytical Chemistry 36: 2380-2381.

Novotny M, Schwende FJ, Hartigan MJ and Purcell JE (1980) Analytical Chemistry 52: 736-740.

Adams KA, Debra L, Van Engelen and Thomas LC (1984) Journalof Chromatography 303: 341-349.

Lagesson V, Lagesson-Andrasko L (1984) Analyst 109: 867-870.

Kube M, Tierney M and Lubman DM (1985) Analytica ChimicaActa 171: 375-379.

Lagesson V (1989) Analytical Chemistry 61: 1249-1252.

Bornhop DJ and Wangsgaard JG (1991) Journal ofHigh Resolution Chromatography 14: 344-347.

Bornhop DJ, Holousek L, Hackett M and Wang H (1992) G.C., Rev. Sci. Instrum. 63: 191-201.

Hackett M, Wang H, Miller GC and Bornhop DJ (1995) JournalofChromatographyA 695: 243-257.

Sanz-Vicente I, Cabredo S, Sanz-Vicente F and Galban J (1996) Chromatographia 42: 435-440.

Sanz-Vicente I, Cabredo S and Galban J (1998) Chromatographia 48: 542-547.

Lagesson-Andrasko L, Lagesson V and Andrasko J (1998) Analytical Chemistry 70: 819-826.

Lagesson V, Lagesson-Andrasko L, Andrasko J and Baco F (2000) JournalofChromatographyA 867: 187-206.

Table 2 The development of GC-UV: instrumental specifications

Author (year)

Bandwidth (nm)

Noise level Detection limits

Data collection and handling

Novotny (1980)

Adams (1984) Lagesson (1984)

Kube (1985) Lagesson (1989) Bornhop (1991) Bornhop (1992) Hackett (1995)

Sanz-Vicente (1996)

Sanz-Vicente (1998)

165-220 160-210

0.08

Recordings at wavelengths: 205, 210, 220, 260 and 280 Recordings at & 20

wavelengths: 240 and 260 Single wavelength 0.25-3.5

recordings Spectral scanning after carrier gas stop

226-350 190-510 195-360 192-360 192-360

190-300

190-250

Lagesson- 168-330

Andrasko (1998)

Lagesson (2000) 168-330

3x10~4 AU 3x10~4 AU

10 ng naphthalene 10 ng naphthalene

0.3 ng naphthalene

Strip chart recorder Sanborn Model 60-1300 dual-channel high speed recorder; scanning speed: 6 s Strip chart recorder

1.6x10 4AU 43 ng naphthalene Digital integrator

8-94 ng polycyclic aromatics 0.5 ng carbon disulfide

2 x 10~4 AU 2 x 10~4 AU 2 x 10~5 AU 2 x 10~5 AU 2 x 10~5 AU

530 ng benzene 80 pg mesitylene

0.2 ng coumarin 90 pg naphthalene

3x10 4 AU 15 ng mesitylene

3 x 10~4 AU 40 ng phenol

0.7 and 1.7 4x10~5AU

4 x 10~5 AU 0.5-3 pg naphthalene

Strip chart recorder

Printer

PC data handling IBM model 50 IBM model 50

IBM model PS2 55 SX, 206 software,

Linear Instruments Integration time: 0.5 s Data handling by means of a program in BASIC Integration time: 0.5 s Data interpretation by means of a program in BASIC Data collection: Instaspec II, Oriel Corp

Data handling: Grams/386, Galactic Industries Data collection: Instaspec II, Oriel Corp

Data handling: Grams/386, Galactic Industries

References: As shown in Table 1.

negligible influence on spectral searching for unknowns, especially if the reference spectrum and the unknown spectrum are registered at similar temperatures.

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