Chemical Papers 62 (3) 323–325 (2008) DOI: 10.2478/s11696-008-0030-2
SHORT COMMUNICATION
Packed column inlet as a direct injection capillary inlet Rafal Wawrzyniak* Department of Analytical Chemistry, Faculty of Chemistry, Adam Mickiewicz University Grunwaldzka 6, 60 780 Pozna´ n, Poland Received 25 July 2007; Accepted 13 November 2007
Discrimination is a known problem when analyzing high-boiling compounds, which considerably hinders their analysis at trace levels. This problem can be solved by using either direct or oncolumn injection. Modification of a standard injection port liner and a micro-syringe is presented, so that a direct injection with in-column evaporation can be easily done. The proposed changes are compatible with any packed column injection port, simple to implement, and inexpensive. The changes are described and the analysis of acyloglycerols in diesel oil spiked with 50 % FAME is used to demonstrate the utility of the proposed injection technique. c 2008 Institute of Chemistry, Slovak Academy of Sciences Keywords: direct injection, on-column injection, sample injector modification, capillary columns
A number of sample injection techniques have been used in capillary gas chromatography: split, splitless, on-column, direct, and programmed temperature vaporizing injections. The direct injection technique with in-column evaporation used in this work has been often combined with the on-column technique. These two different methods are discussed below (Rood, 1999). Direct Injection is a flash vaporizing injection method. The inlet system is heated independently from the column oven and evaporation occurs in the inlet. This inlet can be a glass liner (out-column evaporation) or a part of the column (in-column evaporation). The broad injection peak is caused by broadening in time and broadening in space. On-column Injection is a “cold” injection technique. The sample is injected as a liquid directly onto the column. The injection zone is cool to avoid hot needle discrimination. The broad of injection peak is caused only by broadening in space. The solutes are focused at the head of the column; evaporation occurs as the oven temperature increases. The first sample injectors permitting on-column injection were described by Schomburg et al. (1977), Grob (1978), and Grob and Grob (1978).
Direct injection onto a wide bore open tubular column, was first described by Zlatkis et al. (1959). After the market introduction of Megabore capillary columns which have an inner diameter of 0.53 mm, manufacturers of chromatographic equipment introduced special glass liners compatible with packed column injection ports. This modification enabled the use of Megabore columns with packed column instrumentation (Watanabe & Hashimoto, 1990). However, this solution permits only direct injection with outcolumn evaporation. Direct sample injection onto a column, offer the following advantages: elimination of sample discrimination, elimination of sample alteration, and high analytical precision. The disadvantage of the direct sample injection is the possibility of damaging the stationary phase film bonded to the inner surface of the column head. The best solution to this problem is the use of a pre-column. The paper presents a modification of the sample syringe used for direct injection with in-column evaporation. The proposed modifications are evaluated using a modified syringe/packed column injection port on a Hewlett-Packard 5890 series II equipped with a flame ionization detector (FID). The modifi-
*Corresponding author, e-mail:
[email protected]
Brought to you by | CAPES Authenticated Download Date | 9/24/15 11:23 PM
324
R. Wawrzyniak/Chemical Papers 62 (3) 323–325 (2008)
Fig. 1. Schematic illustration of particular stages of the direct sample injection with in-column evaporation with the use of the injecting device proposed.
cations require changing the glass insert (catalogue number: Agilent 5181-3382) into a direct injection with in-column evaporation liner. In the middle part of the open liner a constriction was made to facilitate insertion of the needle into the capillary column. The size of the constriction was chosen to match with the diameters of all presently available capillary columns, ranging from 0.25 mm to 0.53 mm inner diameters. For the direct injection onto the column a standard 10 µL syringe (catalogue number: Agilent 9301-0658) with needle 115 mm in length and 0.17 mm in diameter (catalogue number: Agilent 1909163000) was used. Also, the syringe was modified to facilitate the septum puncture. The modified syringe and injection port liner are shown schematically in Fig. 1. The applicator was prepared from a disposable syringe, 2 mL in capacity, with a centrally mounted LUER cone. The stainless steel needle has a length
of 15 mm and an internal diameter of 0.2 mm. The diameter of the needle was chosen to enable the insertion of a quartz needle and to ensure the tightness while maintaining the possibility of moving the quartz needle inside the steel one. The analyses of high-boiling point compounds, acylglycerols, are used to illustrate the advantages of the modified packed column injector and direct injector. The study was performed using diesel oil spiked with 50 % methyl esters of fatty acids (FAME) containing 0.33 % monoacylglycerols, 0.15 % diacylglycerols, and 0.30 % triacylglycerols. Chromatographic analysis was carried out on a capillary column DB-5HT of 15 m in length, internal diameter of 0.32 mm, coated with a stationary film phase of 0.1 µm in thickness. The carrier gas was helium at 50 kPa. The column temperature was programmed from 50 ◦C (held for 1 min) to 180 ◦C at 15 ◦C min−1 and then to 230 ◦C at 7 ◦C min−1 , and finally to 380 ◦C (held for 5 min) at 10 ◦C min−1 . The injector and detector temperature was 380 ◦C. The sample injection was performed with the “cold needle” technique (Supelco Bulletin 853B, 1999) and the injection time was 1 s. The amount of the sample injected onto the column was 2 µL. The paper presents three chromatograms (Figs. 2A– 2C) obtained using the sample described above. The first two were obtained using the direct type sample injector with a classical open liner and the injection was done using needles of different lengths. The third one was obtained using the modified liner/syringe described above. The poorest results are obtained using a syringe with a needle of 50 mm in length. When using this needle, the sample is evaporated in the middle of the open liner. Better results were obtained with a needle of 115 mm in length, permitting the sample introduction to a depth of about 1 cm above the inlet of the capillary column. With the direct, in-column evaporation the discrimination of the acylglycerols was even lower. Consequently, the signal of the compounds with higher boiling temperature was higher. The smallest signal increase was observed for the monoacylglycerols, while the greatest one for the triacylglycerols. The area of the peaks assigned to di- and triacylglycerols was measured relative to the monoacylglycerols. The values of percentual relative mass response collected in Table 1 were calculated as follows (Chauhan & Darbre, 1981)
Table 1. Discrimination of acylglycerols using different methods of injection with a 10 µL syringe Injection method Direct with short needle (50 mm) Direct with long needle (115 mm) Direct with in-column evaporation
Diacylglycerols
Triacylglycerols
55 (0.061a ) 70 (0.047a ) 83 (0.045a )
26 (0.089a ) 35 (0.049a ) 42 (0.025a )
a) Relative standard deviation (RSD) for n = 5.
Brought to you by | CAPES Authenticated Download Date | 9/24/15 11:23 PM
325
R. Wawrzyniak/Chemical Papers 62 (3) 323–325 (2008)
A
Detector response/a.u.
100 80 60 40 20
Detector response/a.u.
100
B
80 60 40 20
Detector response/a.u.
100
C
80 60 40 20
Time/min
Fig. 2. The chromatograms of the same sample obtained using the direct injector and a syringe with a needle of 50 mm (A) or 115 mm (B) length and the direct injector with in-column evaporation and a syringe with a quartz needle of 115 mm (C).
peak area of Cdi or tri /mass of Cdi or tri 100 = peak area of Cmono /mass of Cmono = relative mass response value/% (1) The results obtained using this newly proposed configuration clearly demonstrate that the degree of discrimination is significantly reduced when analyzing compounds with high boiling temperature. The modifications proposed are simple and do not require permanent changes in the chromatograph setup. The modifications are inexpensive and can be employed by any laboratory. The configuration proposed is not meant to and is not capable to substitute expensive injectors developed specifically for this type of analyses, but it permits to widen the use of existing standard GC equipment while improving the results quality. References Chauhan, J., & Darbre, A. (1981). Direct injection on capillary columns for gas chromatography. Journal of High Resolution Chromatography, 4, 260–265. DOI: 10.1002/jhrc.1240040603.
Grob, K. (1978). On-column injection onto capillary columns. Part 2: Study of sampling conditions – a practical recommendation. Journal of High Resolution Chromatography, 1, 263–267. DOI: 10.1002/jhrc.1240010510. Grob, K., & Grob, K., Jr. (1978). On-column injection on to glass capillary columns. Journal of Chromatography A, 151, 311–320. DOI: 10.1016/S0021-9673(00)88346-6. Rood, D. (1999). A practical guide to the care, maintenance, and troubleshooting of capillary gas chromatographic systems. Weinheim: Wiley-VCH. Schomburg, G., Behlau, H., Dielmann, R., Weeke, F., & Hushman, H. (1977). Sampling techniques in capillary gas chromatography. Journal of Chromatography, 142, 87–102. DOI: 10.1016/S0021-9673(01)92028-X. Supelco Bulletin 853B (1999). Capillary GC troubleshooting guide: How to locate problems and solve them (pp. 6–8). Retrieved from www.sigma-aldrich.com Watanabe, C., & Hashimoto, K. (1990). Direct injection of large sample volumes into capillary columns with packed column injector. Journal of High Resolution Chromatography, 13, 610–613. DOI: 10.1002/jhrc.1240130905. Zlatkis, A., & Kaufman, H. R. (1959). Use of coated tubing as columns for gas chromatography. Nature, 184, 2010. DOI: 10.1038/1842010a0.
Brought to you by | CAPES Authenticated Download Date | 9/24/15 11:23 PM
