Figure 2: Just enough sample preparation represents a continuum of methodologies.
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This just enough sample preparation process doesn't always provide the cleanest extract from the sample as more rigorous approaches such as
multimodal solid-phase extraction (SPE) or liquid–liquid back extraction might achieve but as long as the extractables do
not harm the separation or detection (and, of course, the column or instrument), that's okay. In reality, the sample preparation
time can be greatly reduced as long as the final outcome meets the needs of the analyst. Although the mass spectrometer still
represents a much higher priced detector than a UV or flame ionization detector, many laboratories are finding them to be
a cost-effective way to enhance and speed up their analyses, thereby improving overall productivity and lowering costs. Of
course, less-expensive selective detectors such as fluorescence in HPLC and electron capture in GC still allow the practice
of just enough sample preparation provided the analytes do not need derivatization.
The concept of just enough sample preparation does not imply one is cutting corners or that more sophisticated protocols are not required. It really
represents a continuum of sample preparation procedures as depicted in Figure 2. This figure represents just a few of the
many sample preparation methods that are in widespread use. Starting at the top of the figure with filtration, centrifugation
and "dilute and shoot", moving down the sample preparation protocols become more selective and more complex, sometimes requiring
a greater deal of effort and multiple steps to achieve just enough cleanup to meet the analytical needs. Minimizing the number of sample handling steps in any analytical technique is desirable
since the more times the sample is transferred, the greater the chance of analyte loss (or modification), thereby resulting
in poorer analytical precision and accuracy. If one or two steps meet the needs of the method that may be sufficient, but
in some cases additional sample preparation steps may be needed to get rid of interferences. The need to eliminate or minimize
interferences is no greater than that required for liquid chromatography–mass spectrometry (LC–MS) and LC–MS-MS (see below).
Figure 3: Striking the right balance in sample preparation.
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Figure 3 shows a pictorial representation of the just enough sample preparaton concept that actually applies to the entire analytical cycle, but is emphasized for the sample preparation
portion. It is here that many workers are faced with achieving the bulk of their selectivity enhancement. Ideally, in an analytical
method one always wants to achieve the best result with the least amount of effort and investment. On the other hand, the
actual data requirement may not require the ideal result but rather an acceptable result. For example, in screening hundreds
of urine samples for the presence of drugs of abuse most samples are negative. Thus, a qualitative analytical method may be
sufficient to rule out the presence of an illicit drug. However, if an illegal drug is spotted during the screening test,
then a more careful and perhaps more sophisticated look at a positive sample is required for quantitative analysis.
There are many other factors that may influence the choice of the sample preparation techniques used to provide just enough cleanup. An analyst's skill and knowledge are important. The availability of instrumentation, chemicals, consumables and
other equipment; the time available to develop a method and to perform the tasks at hand; the complexity and nature of the
matrix; the analyte concentration level and stability; the required sample size; the cost per sample (budget); and the safety
of the sample preparation technique are just a few of the many considerations that must be taken into account. It is the balance
of all of these and other considerations that come into play.
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