An Improved Method of Updating Retention Times for GCMS

Transcription

1 An Improved Method of Updating Retention Times for GCMS Richard Whitney, Ph.D., GC/GCMS Senior Product Specialist, Ronald D. Snelling, Ph.D., GC/GCMS Senior Product Specialist, Clifford M. Taylor, GC/GCMS Product Manager, Shimadzu Scientific Instruments, Columbia, MD, USA

2 Abstract One of the common issues in gas chromatography-mass spectrometry (GCMS) is updating of retention time windows when column maintenance is performed. Updating 6 to 8 retention times manually is usually not time-consuming, but many methods have 50 or more target compounds. Manually updating that many retention times is very time consuming and, if not done properly, target compounds will not be determined accurately. A new technique has been developed to update retention times automatically in the data processing software. The technique is based on the use of retention indices, referenced to a hydrocarbon standard (series of n-alkanes). This is a multipoint correction method, so target analytes will always be bracketed by the retention index marker compounds to ensure accuracy of updated retention times for target compounds. Examples of this technique illustrate the accuracy of this technique using a subset of US EPA Method 8270D analytes. 2

3 Method Retention Time Updating 1.5 Method retention times (quantitation windows) (x1,000,000) When a capillary column is cut for maintenance, retention times shift. 1.5 (x1,000,000) ????? Manual updating of retention times can be time-consuming, especially when the number of target analytes is large (>50-100). 3

4 Method Retention Time Updating Two approaches have been employed to deal with updating of retention times: 4 Adjust chromatographic conditions to keep retention times constant after column maintenance. Use retention time markers (retention indices) to easily adjust retention times with changing conditions (column length), keeping other chromatographic conditions constant. Based on fundamental chromatographic principles Maintains optimum chromatographic performance Easy, inexpensive, accurate

5 What is Retention Index? The retention index is information unique to a compound for a given analytical column phase. It is the retention time of the compound expressed on a scale based on n-alkane retention times. Kovats proposed it in 1958.* It is based on a thermodynamic principle and it is correlated to chemical structure. *Kovats, E Helv. Chim Acta 41: (1958) min Decane (C10) min Undecane (C11) 9.733min Decane (C10) min Undecane (C11) min Limonene Retention Index min Limonene Retention Index Instrument A Retention index is constant. Instrument B

6 Carbon number x 100 Linear Retention Index LRI (T) = 100 n (RT T - RT Cn ) / (RT Cn +1 - RT Cn ) (Temperature Program Condition) Carbon number vs RT RT Cn, RT Cn+1 - retention times of C n and C n+1 alkanes uv(x100,000) Chromatogram C10 T C RT min 6 Note: under linear temperature programming conditions, n-alkanes show a linear relationship between retention time and carbon number

7 Principle of Retention Index (1) Method information C17 C18 C R.I Target compound 21.5 min (Method value) n-alkanes Determining retention indices (done only once for each method) A series of n-alkane standards are run under the standard method conditions used for target compounds. From the method retention times for the target compounds and the n-alkanes, retention indices are calculated. 7

8 Principle of Retention Index (2) Retention indices are used to accurately predict method retention times for new conditions (done only once for new conditions). n-alkane standards are run under the revised conditions. New method retention times are calculated for target compounds After column trimming C17 C18 C R.I min Calculated value Retention time n-alkanes

9 Principle of Retention Index (3) Original method information C17 C18 C19 After column trimming min (original method value) C17 C18 C19 R.I. n-alkanes R.I Retention time 21.1 min new method value

10 Advantages of using Retention Index Automatic Adjustment of Retention Times (AART) Retention times of up to 1000 compounds can be updated simultaneously with one analysis of an n- alkane Reduced input by human errors Easy operation Less work, regardless of the total number of compounds Multiple n-alkane standards Correction is performed at multiple points from low to high retention times, enabling accurate correction over a wide range of retention times. SIM time updating can be performed simultaneously AART can easily be applied to any existing method 10 for Retention Time Updating

11 AART Operation Step 1 Prepare GCMSsolution Method for Target Compounds (including compound table) 11

12 AART Operation Step 2 Analyze n-alkanes (AART) standard and prepare AART compound table (for n-alkanes) Same conditions used for analysis of target analytes C15 C SHIMADZU

13 AART Operation Step 3 Download retention indices to GCMSsolution method Run Wizard (modify) under create compound table Retention indices are calculated for the target analytes 13

14 AART Operation Step 4 Run n-alkanes standard under revised conditions and update retention times for target analytes Execute AART function to update target compound retention times Target compound retention times are updated SHIMADZU

15 AART Results (1) AART results: Target compound retention times are accurately predicted for the revised conditions (with 2-3min retention time change for late-eluters). All target compounds properly identified; all isomer assignments correct, indicating accurate updating of quantitation windows SHIMADZU

16 AART Results (2) AART results: After executing AART, GCMSsolution quantitation properly identified all 95 target analytes. All isomer assignments were correct in the processed data (ie, benzo (b&k)fluoranthenes). Retention times predicted very accurately across the entire 25min GC run. 16

17 Library Searching with Retention Index (1) Many compounds show similar spectra (isomers, homologs, etc.) Spectra of compounds with multiple isomers show library search results with multiple hits difficult to interpret the results (six hits for this example) 17

18 Library Searching with Retention Index (2) Use retention index as library search criterion (Retention indices are included in many libraries) Set Retention Index scale with data from n-alkane standard Set Retention Index allowance to limit library search 18

19 Library Searching with Retention Index (3) Using retention indices can qualify the library search results Library search result: Benzo(a)anthracene Library search result: Chrysene 19 Note: the library must include accurate, isomer-specific retention indices, corresponding to the column phase used for analyses

20 AART Summary 20 AART function is unique to Shimadzu GCMSsolution Based on fundamental science retention index Maintains optimum chromatographic conditions Easy, fast, inexpensive Can be applied to any GCMS method with up to 1000 analytes Extremely accurate - based on use of multi-point retention index standards Retention index is useful in library searching 2007 SHIMADZU