The Column - November 2008 - (Page 17)

November 2008 www.thecolumn.eu.com The Column Incognito Method “Non”-Specifications! I come across many situations in which clients, from a variety of industries, assure me that methods are validated and/or registered and that they cannot be altered — usually when I’m trying to assist them with problems or transferring the method into my laboratory for application development or troubleshooting. I invariably respect this perspective, however, I’m often unpleasantly surprised at the lack of essential detail or just plain inaccuracy in the way chromatographic methods are specified — often rendering the validation exercise pointless because of the large amount of subjectivity possible when implementing the method. My fear is that these “missing” instrument or method parameters may be simply invented or system defaults accepted without due consideration of the impact on the data produced — possibly resulting in unnecessary repeat analyses or, in the worst cases, results that may not be fit for purpose. So what is required to properly specify a set of instrument or method parameters? This month I intend to cite what might be contained in the “ideal” GC method specification and discuss some of the many inaccuracies and omissions that can adversely impact the quality of data produced. We will investigate the issue of HPLC method specification in next month’s Incognito. It is simpler to specify a method in terms of the workflow of the system — therefore, GC methods might cite conditions in the order: Sample introduction conditions (headspace, thermal desorption etc.)/carrier gas settings/inlet conditions/column description/oven temperature programme/detector conditions/data analysis settings. Of course, there are a myriad different GC instrument configurations, but let’s consider how methods using the most popular inlets and detectors might be specified and the potential pitfalls of not doing so. moisture, hydrocarbons and oxygen from the carrier for optimum performance. Flow-rate or carrier linear velocity should be specified for the carrier rather than the column head pressure (or pressure drop across the column). The pressure drop across the column is a function of the carrier flow, oven temperature and column dimension and any slight deviation in these parameters (poorly calibrated oven thermocouple, trimmed column length) will require an alteration in the column pressure drop to achieve a given flow-rate (or linear velocity). Further, the column dimensions and carrier gas type must be correctly entered into the instrument for flow-rate to be correctly calculated when using systems with computerized pneumatics — this being especially important after column maintenance where the column inlet has been trimmed to improve chromatographic performance. Specifying the mode of carrier operation is also important with modern computerized pneumatics. In this instance we have specified constant flow, which raises the carrier gas pressure as the column temperature increases to achieve a constant carrier gas flow-rate into the detector. Of course this mode has several advantages such as increasing the signal-tonoise ratio of late eluting peaks, decreasing overall analysis time and, in some situations, giving rise to flatter baseline profiles when using mass-flow sensitive detectors. Sample Introduction Injection volume: Injection solvent: 1 µL Dichloromoethane (cite also grade and purity) Syringe size: 10 µL, cone tipped Autosampler routine: Wash needle: Solvent A (3 10 µL) pre-injection Sample priming: Aspirate sample (3 10 µL) and dispense to waste Sample pumps: (3 10 µL) with 5 s viscosity delay Wash needle: Solvent B (3 10 µL) post-injection Carrier Gas Carrrier gas: Flow-rate: Mode: Helium (99.999% purity) 1 mL/min (37 cm/sec) Constant flow Ideally the required carrier gas purity should be specified and I have also seen statements relating to the necessity to filter out Author: Incognito E-mail: incognito@thecolumn.eu.com 15 http://www.thecolumn.eu.com

Table of Contents for the Digital Edition of The Column - November 2008

The Column - November 2008
Contents
Smoke Signals
News
Incognito Method "Non" - Specificiations!
Market Trends and Analysis Market Profile: Capillary Electrophoresis
Simple Differentiation of Black Tea Samples Using a Chemometric Approach
Q&A: The Rise and Rise of Multidimensional GC

The Column - November 2008