"Just Enough" Sample Preparation: A Proven Trend in Sample Analysis

Dec 01, 2012
Volume 25, Issue 12

Sample preparation (and to a lesser extent data analysis) has often been considered the ratedetermining step and error-prone part of an analytical method. If selectivity can be achieved in other portions of the analytical cycle to meet the needs of the analyst, then the burden placed on sample preparation is decreased. The concept of "just enough" sample preparation is presented here and relies heavily on recent advances in tandem mass spectrometry detection that provides enhanced sensitivity and selectivity, which was unavailable in the past. Even so, more sophisticated sample preparation protocols may still be required, especially if ion suppression or enhancement result from coeluted interferences.

Figure 1: Steps in the analytical cycle. Adapted from reference 1.
In her February 2012 column instalment titled "It's All About Selectivity," guest columnist Diane Turner introduced the topic of how selectivity can be incorporated throughout the sample analysis cycle (1). Figure 1, borrowed from her article, nicely illustrates the workflow in a typical sample analysis. In most analytical processes, the chemist is looking for one or perhaps a few analytes of interest, often in a very complex matrix. Having an analytical method showing sufficient selectivity to analyse those few compounds of interest with the precision and accuracy required at the concentration level encountered is the desired outcome of method development. The selectivity can be achieved anywhere within the analytical cycle (Figure 1) during sampling, sample preparation, sample introduction, analyte separation, at the detector or even during data analysis. If the analytes of interest can be determined with good sensitivity, the presence of compounds from the sample matrix can be tolerated as long as those interferences do not cause harm (short-term or longterm) to the analytical instrument or column or if determined to be harmful, they can easily be removed. An example of the latter could be backflushing after each analysis to remove high-molecular-weight contaminants trapped at the head of a gas chromatography (GC) column. In Turner's column (1), she gave very nice examples of how selectivity can be achieved at each step of the analytical cycle for GC.

Having less selectivity in one portion of the analytical cycle can be made up for by having greater selectivity in another portion of the analytical cycle. For example, if an analyst has only a fixed-wavelength UV detector in his or her high performance liquid chromatography (HPLC) instrument or a thermal conductivity (TCD) or flame ionization detector (FID) for the GC system, there may not be sufficient detector selectivity to provide the necessary overall method selectivity to measure an analyte of interest without interference from undesired sample components. Therefore, additional sample preparation or finding a separation column that provides more selectivity during the separation may be required to make up for the limitations in the detector. In these cases, the analyst may spend a great deal of time and energy performing one or more sample preparation steps or optimizing the selectivity of the column and mobile phase system (HPLC) to rid it of potential interferences. On the other hand, if one has a very sensitive and selective detector, then perhaps spending a great deal of time optimizing the sample preparation or the analytical separation is unwarranted.

Because achieving selectivity for the separation column is not an easy task to predict, sample preparation often gets the brunt of the job to remove interferences from the sample of interest. It is sometimes unfortunate to burden analysts with this job, but there are time-proven sample preparation techniques available. However, with the advent and widespread use of tandem mass spectrometry (MS-MS) for both HPLC and GC with its high degree of selectivity and sensitivity, sample preparation as well as the chromatographic separation can sometimes be simplified as long as any interferences carried over from the sample matrix do not interfere with the separation or detection process. We term this simplified sample preparation process just enough sample preparation.

lorem ipsum