In analytical chemistry, sample preparation refers to the ways in which a sample is treated prior to its analysis. Preparation is a very step in most analytical. Sample preparation, in analytical chemistry, the processes in which a representative piece of material is extracted from a larger amount and readied for analysis. hplc sample preparation I hope you found the last installment on Hydrophobic Interaction Liquid Chromatography (link to blog post) useful.
It is recommended that all samples be filtered before chromatographic analysis. Filtration can be performed manually and on an individual basis using syringe membrane filters, or it can be done on multiple samples simultaneously through plate configurations. Syringe filters are available in many membrane types to accommodate chemical and application compatibility and many membrane diameter sizes to accommodate varying sample volumes. You can choose either a 0.
Specifics of all filters should be reviewed with the vendor. A pre-column filter web link may be placed between the injector and the analytical column to trap particles present in the sample and particles frominjection valve wear. The filter usuallyconsists of a 5- or 2-micron frit heldin a cartridge. The frit can be easilyreplaced when the system pressure rises. There are also pre-column filters that are designed to remove matrix interferences web link.
This is a dispersive SPE offering that is increasingly becoming the technique of choice for extraction and clean-up of pesticide residues in food and other complex matrices. The robust procedure offers a number of compelling advantages: SPE is the passage of the sample through one or more cartridges packed with a stationary phase.
Performing this technique prior to injection will often selectively retain certain species within the homogeneous sample. Quite often, the retained species are substances that would interfere with the chromatography had they not been removed. However, they could also be the analytes of interest, in which case they would then be eluted from the cartridge.
There are a number of different cartridges whose suitability depends upon the type of chemistry undertaken. There are automated SPE systems web link that use cartridges to trap and concentrate your analytes of interest. In one run, you can perform solid-phase extraction on multiple samples in a short time.
You use sample preparation techniques because you want to get the most accurate analysis of your sample. Your sample matrix can reduce your ability to get an accurate result. Samples may have come from human fluids, waste water, food, or an array of other sources. These matrices can cause significant background signal and even interfere with the function of your instrumentation. To achieve better signal—to-noise, remove or reduce matrix interferences, and improve column and system performance, sample preparation is a must.
There can be limitations to the use of many sample preparation techniques. The use of any sample preparation device needs to be approached with an understanding of both the matrix you are trying to remove, and the analytes you are trying to retain for analysis. Using the wrong technique or the wrong method can cause issues like low recovery and poor reproducibility. Be sure to research techniques and applications website link for your compounds of interest in their matrices and test known standards in the matrix before moving to critical sample analysis.
There are many examples of sample preparation using a variety of techniques. Some examples of applications in different arenas are found below. I hope that this brief introduction to a few sample preparation techniques leads you to appropriate resources as you look into getting the most accurate results for your sample in your matrix.
Some of the benefits you will obtain are lower background interference, higher signal-to—noise, and accurate and reproducible results. Reproducibility can be achieved using the appropriate products and applications for your specific need.
Application notes in every area of study are abundant and available to you. For more application examples go to http: For additional information about automated solid-phase extraction techniques in any or all of the chromatography fields mentioned and some of the newest products available, review the material here: And finally, for an extensive guide to sample preparation, I would suggest purchasing Sample Preparation in Chromatography link to book purchase.
It is most useful to speak of the heterogeneity of a material as a scalar function that approaches perfect homogeneity in its limit. It is also essential to speak in terms of a given analyte or suite of analytes, since some components in a material may be much more heterogeneously distributed than others. The most comprehensive sampling theory was formulated by French chemist Pierre Gy in the second half of the 20th century.
Gy defined two types of material heterogeneity: While this dichotomy can be usefully applied to many material types, it is best described and understood in reference to particulate solid mixtures.
For example, if one considers a mixture of silt and sand to be sampled for the presence of calcium , the variation of that analyte among the silt and sand particles represents two forms of its constitution heterogeneity.
The degree of uniformity in the spatial arrangement of silt and sand particles then determines the distribution heterogeneity of calcium. Appropriate grinding of such a mixture to reduce the average particle size may diminish the constitution heterogeneity, and the correct blending of such a mixture may lower its distribution heterogeneity. Many commonly employed sampling practices are seriously flawed in that some constituents have a zero probability of being sampled. Such methods can never satisfactorily represent highly heterogeneous material.
In contrast, probabilistic sampling methods are techniques in which all constituents of the material have some probability of being included. However, it is only in a correctly designed sampling plan that probabilistic sampling achieves true representation. In a discussion of sampling it is useful to distinguish two forms of solids , monolithic and particulate, as well as liquids and gases and to treat each material type as a separate category.
At the same time, it is important to recognize that mixed phases also frequently need to be sampled; gases dissolved in liquids and solids, particles suspended in liquids, and solid and liquid aerosols are some examples.
Sometimes the object of study is in one phase form, but the sample must be in another. Thus, molten steel is sampled by casting solid forms for analysis. Monolithic solids, even those with a very low order of heterogeneity, are very difficult to sample rationally.
However, as with all sampling, understanding the physical nature of the object of study can significantly improve the sampling plan. For example, a large ore body may extend for great distances underground in three dimensions, but mineralogical clues can direct sampling for the mapping effort.
Steel castings are commonly sampled at their cross-sectional mid-radius, where they are known to be free of edge effects and centre porosity. Sampling of particulate solids provides the model for much sampling theory.
In general, particulate system heterogeneity tends to be much greater than that of other phase systems. Thus, the single-grab sample is nearly always inadequate.
For this reason the sampling of contaminated soil , for instance, may employ random, systematic, or judgment-based sampling plans in order to achieve a given set of objectives e. In industry a particulate commodity may be either continuously or randomly sampled as it is being transported on a conveyer belt.
Very heterogeneous materials may need to be sampled in great bulk, amounting to 1 percent or more of the total. The resulting sample then needs to be reduced in size by some means that preserves its representative character. The original sample is formed into a cone-shaped pile and then flattened into a disk.
The disk is divided into four quadrants. Two opposite quadrants are shoveled into a second pile, mixed together, and then coned and quartered again. This sequence continues until the selected material has been reduced to a size small enough for a useful laboratory sample. Similar approaches are applied in river and ocean studies, and current and depth information are simultaneously recorded.
Chemical streams in pipes need to be sampled with specially designed diverter probes that avoid turbulence and wall effects. Liquid samples often require the immediate addition of analyte-specific preservatives.
For certain trace-level analyses the sample collection vessel must be composed of high-purity materials and rigorously cleaned before use. At ground level, automated monitoring sites are carefully located to avoid adventitious spikes from human activity and to obtain the most representative samples.
Atmospheric samples are also obtained manually with glass vessels using some displacement medium, such as water or mercury , or with a sealable airtight sampling syringe. Sometimes a syringe is used to fill a fluoropolymer gas-sampling bag. Smokestack gases or room air is sampled by pumping the atmosphere through a liquid or particulate-solid medium that absorbs and collects the gaseous analyte. Solid and liquid aerosols are often collected by drawing the atmosphere through microporous filters.
Pressurized gases can be sampled by means of a metal gas-sampling cylinder. Extreme care and special procedures are required in the case of asphyxiating, flammable, toxic, and corrosive gases. The laboratory sample usually needs to be further reduced and processed to what is frequently called the test sample. This is a much smaller, but still representative, subsample with an often finer particle size, from which test portions are selected for specific analyte determinations.
With a particulate material, if the analyte is associated with one or more constituents, it is possible to grind the laboratory sample to reduce the average particle size until the analyte can be regarded as a pointlike component of the entire laboratory sample. This particle diameter is called the liberation size and varies with the analyte and the type of material. Grinding more generally called comminution can be accomplished by various means, ranging from simple manual approaches to fully automated techniques.
Ground material is often sieved, but for chemical analysis purposes the retained fraction is always returned to the grinder until it all passes the desired mesh size. Excess grinding of some materials can lead to contamination from or analyte loss to the grinding tool. Also, overzealous grinding can result in the absorption of atmospheric gases including moisture by the sample and in the loss of fines.
In addition, very finely ground material is sometimes impossible to mix adequately. Mixing of the laboratory sample is another critical operation. If the particle size is reduced in a series of steps, generally each step is followed by an interval of mixing. This can be accomplished by hand with small laboratory samples, but other samples require some form of automation.
The effectiveness of any given mixing operation will be related to the particle size, shape, and density, as well as to external influences such as electrostatic or magnetic fields and air turbulence. Reducing the volume of the ground and mixed laboratory sample while keeping the sample representative is another concern.
The sample can be poured through a set of riffles that uniformly splits it into two or more streams. One is selected for further processing, with the other s discarded or archived for future reference.
If the laboratory sample is very large, the riffling process may be repeated several times. At the end of this process, the final selected stream is the test sample.
The simplest riffles design is a stationary arrangement of alternating chutes surmounted by a wide hopper, fabricated from sheet metal. Spinning rifflers use either a rotating carousel of collection vessels and a stationary vibratory feeder or a ring of stationary collection vessels and a rotating feeder. Another approach to sample size reduction involves a tabletop version of the coning and quartering operation see Sampling , usually conducted on a large sheet of glazed paper.
Monolithic solids also need to be converted to a suitable test sample. In the case of metal , surface oxides may need to be ground off or dissolved by acid. The piece may need to be cut to size for spectrometric work, or millings or drillings may need to be obtained. Liquid samples, even gases , require thorough blending. Liquids can be stirred or otherwise agitated.
Gases can be mixed by gently warming one end of the storage vessel. The selection of a test portion from the test sample is the first step in any specific analytical determination. In many cases the analyst has the freedom to weigh a mass or transfer a volume. This allows him to optimize the analyte response while controlling background and interference effects.
The test portion size also bears a critical relationship to the subsampling error for a test sample with a given level of analyte heterogeneity. This means that, for any given test sample and analyte, there exists a minimum test portion size that achieves a truly representative sample.
For analytical methods whose precision with low-heterogeneity samples is good, it is possible to calculate a laboratory sampling constant K. The laboratory sampling constant, K , is then calculated: These relationships, while linked to the specific analyte and test sample, are independent of the test methodology employed. Unfortunately, the analyst who works with solid samples has little choice in selecting the test portion size.
With solid samples, spectrometric methods are used in which the test portion is that minute portion of a solid sampled by a spark, arc, or glow discharge to create a sample plasma. In X-ray fluorescence spectrometry the test portion is typically only a few atomic layers. In the trace concentration realm the use of such small test portions on even moderately heterogeneous solids can lead to a high subsampling error.
In glow discharge and X-ray fluorescence work it is similarly wise to regrind and repolish the sample and repeat the measurement several times. When a trace-level analyte is concentrated in a very heterogeneously distributed constituent of a test sample, a unique situation prevails.
Here, replicates of a selected test portion size that contain six or fewer particles of analyte-rich constituent will not produce a normal distribution of results. At a test portion size where z is between 1 and 6, the data behave erratically, with a large number of test results producing a skewed Poisson distribution.
However, at a test portion size where z is greater than 6, the data produce a normal or Gaussian distribution and an accurate mean. Unfortunately, at very minute test portion sizes, where z is zero, the data are also Gaussian because only the matrix analyte is being measured. In this case the mean is precisely wrong. This suggests a warning to trace analysts: The dissolution of inorganic samples nearly always means the preparation of an acidic aqueous solution from the test portion.
There are a number of ways to accomplish this, but the most common approach is the direct application of one or more mineral acids. Nonoxidizing acids, such as hydrochloric , hydrofluoric, and sulfuric acid , are particularly useful when oxidizing conditions would produce a protective oxide film on the sample surfaces.
Commonly employed oxidizing acids are nitric acid and perchloric acid. Nitric acid is not a strong complex former, but it does dissolve many metals , and it forms an impervious passive film on many others. Perchloric acid is completely noncomplexing. It is nonoxidizing at room temperature but becomes extremely oxidizing at elevated temperatures. Oxidizing acid mixtures are generally useful dissolution media for many inorganic samples.
For example, aqua regia is three parts hydrochloric acid and one part nitric acid. It is an effective solvent for platinum , palladium , and gold as well as a host of other metals and ores.
Despite the variety of acids and acid mixtures available, there are many sample types that require alternative measures. The material itself may be too acid resistant even for high-pressure acid bomb or sealed-vessel microwave approaches. The time for an acid dissolution may be excessive, or the only feasible acid approach may add interference or result in analyte loss. In these cases molten salt fusion is frequently the answer.
In this technique the finely ground test portion is mixed with a powdered flux in a crucible and heated until molten. The cooled melt is then dissolved in water or an aqueous acidic solution. With organic materials there are two distinct methodologies , depending on whether the analyte is inorganic or organic. For inorganic analytes the dissolution process generally requires the complete destruction of the organic matrix, and no single approach is universally applicable.
Ignition in a high-temperature laboratory furnace is the simplest technique, but it results in the loss of many elements. Ignition in a sealed oxygen atmosphere is a better approach when dealing with a volatile analyte.
The organic sample test portion is either directly applied to a filter paper or loaded into a gelatin capsule, which is then wrapped in a filter paper. The wrapped paper is secured in the platinum gauze clip attached to the flask stopper. The flask is flushed with pure oxygen. The filter paper is ignited, and the stopper is plunged into the flask.
The flask is inverted and held securely until the combustion is complete. The flask is then shaken for several minutes. Water or additional absorbing solution is added to the collar to aid in rinsing the flask neck when the stopper is withdrawn.
Combustion in an armoured metal oxygen bomb is another alternative. A small test portion is weighed into a sample cup suspended above a small volume of absorbing solution. An igniter wire lies across the sample. The lid is attached; the bomb is pressurized to 25 atmospheres with pure oxygen; and the sample is ignited electrically. The bomb is then cooled and is shaken periodically. The pressure is then slowly released, and the lid is removed.
This technique uses a low-pressure oxygen plasma generated by high-frequency induction coils to remove organic matter from the test portion.
FIGURE –Degree of error in laboratory sample preparation relative to On first impression, sample preparation may seem the most routine aspect of an. [Summary of sample preparation method]. Sample preparation is highly important to perform analytical procedures. Errors caused by sample preparation may be. Overview. Source: Laboratory of Dr. B. Jill Venton - University of Virginia. Sample preparation is the way in which a sample is treated to prepare for analysis.