|
|
Opinion Polls & Market Research
Introduction
Sample preparation is a key procedure in modern chemical analysis. By some estimates, 60-80% of the work activity and operating cost in an analytical lab is spent preparing samples for introduction into an analytical device.The importance of sample preparation stems from three major concerns:
Sample Concentration
Frequently, the component of interest is present in levels too low for detection. Sample preparation can concentrate the component to adequate levels for measurement.Contaminants
The presence of interfering matrix elements can mask the analysis of the component of interest. Sample preparation can remove excess contaminants to yield clean, informative chromatograms.In Solution
For most analyses (HPLC, GC, spectrophometery, RIA, etc.), the sample must be properly prepared in solution for subsequent analysis.Common methods for sample preparation include liquid-liquid
extraction,
centrifugation, and solid phase extraction. Solid phase extraction
(SPE)
is one of the simplest, yet most effective and most versatile, methods
of sample preparation.
Utilizing low cost, prepacked, disposable cartridges, a sample
component
of interest is separated from other species by applying the sample
mixture
to a solid chromatographic sorbent and selectively eluting the desired
component.
Convenient, Easy-to-Use
Convenience and ease-of-use are primary benefits of Sep-Pak(R) SPE cartridges, to maximize the productivity of your analytical operation, while saving you time and money. When compared to other sample preparation techniques, Sep-Pak cartridges also offer:Faster sample prep - average time cut by 2/3
Lower cost - less solvent and reagent consumption means less hazardous waste
Greater recoveries - minimal sample transfer
Greater accuracy - no cross contamination
Less sample handling - no emulsion problems
Reduced harm to labile samples - minimal evaporation
Improved safety - due to reduced solvent/sample exposure and glassware
Easy automation - simultaneous batch processing of
multi-samples
Strategies for Solid Phase Extraction
There are two simple SPE strategies for sample preparation. You can choose a cartridge sorbent and sample solvent to cause the component(s) of interest to be:1 - unretained while matrix interferences are adsorbed
2 - retained while matrix interferences pass through
unretained
The first strategy is usually chosen when the desired sample
component
is present in high concentration. When components of interest are
present
at low levels, or multiple components of widely differing polarities
need
to be isolated, the second strategy is generally employed. The second
strategy
may also be used for trace enrichment of extremely low level compounds
and concentration of dilute sample. A complex matrix may be treated by
both elution strategies to isolate different target analytes.
With either strategy, there are three different
chromatographic
modes
to choose from. You can select :
- normal phase
- reversed-phase
- ion exchange
For example, the first strategy may be used prior to the analysis of organic acids in red wine. By passing the wine sample at the appropriate pH through a C18 reversed-phase cartridge, the interfering wine pigments are removed by adsorption while the organic acids elute unretained.
With a similar sample but a different goal, the second
strategy can
be used to isolate phenolic pigments in red wine, also with a C18
cartridge.
In this case, the sugars and acids pass directly through the cartridge,
while the pigments are retained and subsequently eluted with a stronger
mobile phase. Of course, it is possible to combine two such
applications
into a single, multi-step protocol to prepare several classes of
compounds
for analysis.
Table 1: Chromatographic Modes and Sorbent Types
Chromatographic Mode
Normal Phase Reversed-Phase C18, Ion Exchange
Silica, Florisil, tc18, C8, tC2, Diol, Accell Plus QMA,
Alumina, Diol, NH2, CN Accell Plus CM,
NH2, CN NH2
Sorbent polarity High Low High
Typical solvent Low to medium High to medium High
polarity range
Typical sample Hexane, Toluene, H20, Buffers H20, Buffers
loading solvent CH2CI2
Typical elution Ethyl acetate, H20/CH3OH, H20/CH3CN Buffers, salt
solvent acetone, CH3CN solutions
Sample elution Least polar sample Most polar sample Sample components
order components first components first most weakly
ionized first
Solvent change Increase solvent Decrease solvent Increase ionic
required to polarity polarity strength or
elute increase
retained
compounds
pH (AX) or
decrease pH
(CX)
|
|
Medtechnica-analytical department
Who else has been visiting this website? רוצים לראות מי עוד מבקר באתר