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Sample Preparation Product Guide: Find Effective Solutions Fast

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At Restek, we believe that the foundation for separation is preparation. A perfect chromatographic method can still be compromised by a sample preparation approach that does not properly match the sample. The matrix that the analytes of interest are in can present real problems, but overdoing sample preparation procedures can introduce other issues. The key is striking a balance so that you get the sample just clean or concentrated enough to successfully perform your work between scheduled maintenance events without investing more time and money into the sample preparation process than is necessary–or worse yet, losing analytes during the sample prep process and affecting the accuracy of the results!

Knowing how important sample preparation is to chromatographic analyses, Restek offers a wide variety of sample preparation products to make sure that your sample is ready to be separated, identified, and quantified successfully. This sample preparation product guide will explain the basics of each technique and help you quickly identify the best one for your sample matrix.

Table of Contents

  1. Solid Phase Extraction (SPE)
    1. Picking an SPE Sorbent (the Stationary Phase)
      1. Primarily Reversed-Phase Sorbents
        1. C8 and C18 Bonded Phases
        2. Styrene-divinylbenzene (SDVB)
      2. Primarily Normal Phase Sorbents
        1. Unbonded Silica Phases
        2. Alumina
        3. Florisil
      3. Multi-Functional Sorbents
        1. Diol
        2. Amino (NH2)
        3. Primary/Secondary Amine (PSA)
        4. Carbon Based
      4. Ion-Exchange Sorbents
        1. Anion Exchange
        2. Cation Exchange
        3. Weak Ion Exchange
        4. Strong Ion Exchange
      5. Method Specific Products
  2.  QuEChERS
    1. Extraction Salt Selection
    2. dSPE Cleanup Products
  3. Supported Liquid Extraction (SLE)
  4. Sample Filtration
  5. Solid Phase Microextraction (SPME)
  6. Accelerated Solvent Extraction (ASE)
  7. In-Line Sample Preparation
  8. Vials and Well Plates
    1. Vials
    2. Well Plates
  9. Syringes

In addition to this article, our Sample Preparation FAQs cover additional information about the products discussed here.

Solid Phase Extraction (SPE)

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SPE is a common sample preparation technique used to clean up and/or concentrate samples prior to GC or LC separation and detection. Even though SPE is a step before the analytical chromatography, it’s helpful to remember that SPE basically is chromatography. The principles that separate compounds in an analytical GC or LC column are the same ones that will separate unwanted matrix components from analytes of interest prior to analysis using an SPE product. Keeping this in mind will help you navigate the wide variety of SPE products that are available.

Picking an SPE Sorbent (the Stationary Phase)

With your sample in mind, select an SPE sorbent with characteristics that will be successful at separating the matrix from your analytes. To best do that, it’s helpful to know certain properties of your analytes and the matrix they are in. Are the target compounds polar or nonpolar? Are they (or can they be) in a charged state? Are they strong or weak organic acids or bases? Are they planar? The answer to each of those questions will help direct you to a family of products that is covered in this sample preparation product guide. Below is a brief description of the types of SPE products divided into broad separation mechanism categories.

  • Primarily Reversed-Phase Sorbents (preferentially retain nonpolar compounds and elute polar compounds)
    • C8 and C18 Bonded Phases are classic reversed-phase sorbents with the degree of nonpolar retention increasing with carbon chain length (C18 is typically more retentive of nonpolar compounds than C8 is). When the polarity distinction between compounds to be separated is clear, a C8 or C18 is a standard choice.
    • Styrene-divinylbenzene (SDVB) is a very nonpolar polymeric SPE sorbent that has found particular application in the analysis of pesticides in polar matrices (e.g., water samples and many food commodities).
  • Primarily Normal Phase Sorbents (preferentially retain polar compounds and elute nonpolar compounds)
    • Unbonded Silica Phases are widely used for the retention of polar compounds and have been available for decades. Silica surfaces are typically slightly acidic, in contrast to the more basic alumina normal phase sorbent (see below), so it will preferentially retain basic compounds over acidic compounds. Strong hydrogen bonding interactions with surface silanol groups provide a significant retention mechanism, but secondary interactions are also possible with other constituents in the silica.
    • Alumina sorbents are porous forms of aluminum oxide. Its native state is pH ≈ 10, but it can be treated to create neutral (pH ≈ 7.5) or acidic (pH ≈ 4) varieties. Alumina is another commonly used sorbent for a wide variety of matrices, including food, soil, and petroleum samples.
    • Florisil is a polar magnesium-silicate sorbent that is commonly used to separate polar analytes from nonpolar matrices. It is routinely used for the analysis of pesticides in a wide variety of environmental samples.
  • Multifunctional Sorbents (capable of acting in both reversed-phase or normal phase modes–or even ion exchange, depending on conditions)
    • Diol is primarily a polar sorbent, capable of hydrogen bonding with analytes because of the two alcohol groups at the end of the ligand. However, the hydrocarbon chain connecting the diol functional group to the silica lends this sorbent a nonpolar characteristic capable of retaining hydrophobic analytes.
    • Amino (NH2) phases use ligands bonded to silica particles, and they have a very versatile set of potential retention mechanisms. Primarily used to retain polar species, like the diol phase, the amino ligand also has some nonpolar characteristic for the retention of hydrocarbons. Amino sorbents have a weak anion-exchange potential, so they can be used to retain strongly anionic species like sulfonic acids.
    • Primary/Secondary Amine (PSA) sorbents are similar to amino sorbents, except they have two types of amine groups, a primary and a secondary amine. They are generally less polar than traditional amino phases, but PSA sorbents also typically have a higher ion-exchange potential than amino phases. This will result in less retention of polar compounds than the amino phase but a stronger retention of anions than the amino phase, providing another option when fine-tuning a sample preparation method to a given sample
    • Carbon-based sorbents (e.g., CarboPrep Plus products) are versatile in their ability to retain a wide range of compounds based on experimental conditions. CarboPrep Plus SPE cartridges are designed specifically to replace Florisil products (which are traditionally used to clean up environmental samples for organochlorine pesticides analysis) because of their ability to retain low-volatility matrix compounds that can contaminate instruments and result in unscheduled maintenance.
  • Ion-Exchange Sorbents (capable of retaining charged species)
    • Anion-exchange sorbents are capable of retaining negatively charged species (anions) in a sample, eluting neutral or positively charged species.
    • Cation-exchange sorbents are used to retain positively charged species (cations) in a sample, eluting neutral or negatively charged species.
    • Weak ion-exchange sorbents (anion and cation) are only charged under certain experimental conditions and are used to attract strongly ionic species in a sample. Their charged state can be neutralized with a change in conditions, resulting in the elution of the previously retained strongly ionic species. Remember, always pair “weak” with “strong,” (i.e., a weak ion-exchange sorbent should be paired with a strongly ionic species in a sample.)
    • Strong ion-exchange sorbents (anion and cation) are permanently charged sorbents that are used to retain weakly ionic species in a sample. When strong ion-exchange sorbents are used, experimental conditions are changed to “turn off” the charge state of the analyte, allowing it to be eluted.
  • Method-Specific Products
    • Restek also offers a wide array of method-specific products that are tailored to fit the requirements of particular applications (e.g., Resprep EPH cartridges, phospholipid removal products like Resprep PLR cartridges and well plates, and protein removal products like Resprep PPT3 well plates). Navigating to the SPE product offering on our website and using the “Application Specific” facet found in the “Application” filter option will list our wide variety of targeted SPE products.

Versatile, Clean, and Compatible – Restek SPE Vacuum Manifolds

Whether you are developing a method for SPE cartridges or 96-well plates, Restek’s reliable, clean, and configurable vacuum manifolds offer you a complete SPE workflow solution.

For More Information About SPE

We have additional resources if you want to find out more about the fundamentals of SPE or general guidelines for SPE method development along with product-specific instructions that can be found on a given product page. The following blogs are a good starting place.

QuEChERS

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Quick, Easy, Cheap, Effective, Rugged, and Safe–that’s QuEChERS (pronounced “catchers”)! The QuEChERS approach to sample preparation has become the technique of choice for the analysis of pesticides in a wide variety of food commodities. QuEChERS is a two-step process that starts with the use of specially formulated packages of extraction salts to partition target analytes from the sample matrix into an organic solvent (commonly acetonitrile). Extraction is followed by a quick cleanup using a technique called dispersive solid phase extraction (dSPE).

Extraction Salt Selection

The first choice to make when picking an extraction salt product is whether or not your target analytes need the final extract pH to be within a certain range to be stable. If so, one of the two buffered varieties of extraction salts would be a good place to start. The biggest difference between these two choices, which are identified by the methods that were developed using them (AOAC or EN), is the final pH range. AOAC salts buffer the final extract to a pH range just below 5, whereas the EN salt packages buffer to a range of pH 5-5.5. Unless you have analytes that would be better under one set of conditions or the other, either is a good starting place for pH-sensitive compounds. If your analytes are insensitive to final pH, the original “unbuffered” extraction salts would work just as well.

dSPE Cleanup Products

There are a wide range of dSPE products to choose from, but just like more conventional solid phase extraction, targeting the correct product for your application comes down to determining the nature and severity of the matrix components that need to be removed before analysis. These considerations will inform the type and quantity of sorbent used.

Restek’s Q-sep Brand: Your One-Stop Shop for QuEChERS Methods

Whether it’s the extraction salts, dSPE products, reference standards, or any of the additional hardware used in most QuEChERS methods (e.g., centrifuge, solvent dispensers, racks, and other accessories), Restek has the gear and the technical know-how to help you support your existing QuEChERS methods or develop new ones to meet the wide diversity of target compounds and matrices.

There are also a number of additional resources available to help learn more about QuEChERS, including the ones listed below:

Supported Liquid Extraction (SLE)

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At first glance, SLE products look a lot like SPE products. They come in cartridges and 96-well plate formats, just like SPE, but instead of relying on adsorption onto a solid sorbent surface like SPE, SLE is a miniaturized, efficient form of liquid-liquid extraction. A highly porous diatomaceous earth is used as a support material in SLE. An aqueous sample is loaded on the SLE product and allowed to sit and diffuse throughout the high surface area support. After that wait period, a water-immiscible elution solvent is passed through the SLE product, and the analytes partition from the aqueous solution into the elution solvent and are then collected.

SLE or SPE?

SLE is quick and easy and may be just enough of a sample cleanup step to eliminate matrix components from interfering with your analysis or your instrument. SLE is great at removing salts, phospholipids, and other polar impurities while the analytes partition to the water-immiscible solvent and are collected for analysis. As we’ve discussed, SPE is chromatography in a cartridge or well-plate, so if your sample is particularly complex or contains matrix compounds that are just as likely to partition into the SLE elution solvent as your analytes, SPE is likely the answer.

Check out the answers to SLE-specific Frequently Asked Questions to find out more about SLE (and many other sample preparation products) and then navigate to our SLE product pages if it seems like a good fit for your application.

Sample Filtration

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One of the most convenient ways to filter a sample is to do the filtration in the autosampler vial, which can easily be done with Thomson SINGLE StEP filter vials. With a wide variety of filter membrane porosity and materials, all color coded for easy identification, the Thomson SINGLE StEP filter vial offers effective and efficient filtration for a wide variety of samples. Details on Thomson filter vial solvent compatibility can be found here.

In addition to the Thomson SINGLE StEP filter vials and the protein precipitation Resprep PPT3 96-well plates already mentioned above, Restek also offers an extensive array of syringe filters with luer type connectors with different filter materials, porosities, and sizes, all color coded to make it easy to tell one type of syringe filter from another.

Guidelines for selecting the correct syringe filter for your application as well as filter compatibility charts are available.

Solid Phase Microextraction (SPME)

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SPME is an automated sample preparation technique that is popular in environmental, food, and clinical laboratories. Restek is proud to offer our rugged approach to SPME–the Restek PAL Smart SPME Arrow. The new Smart technology offers the same application, performance, and lifetime as the products you’ve come to rely on with the added feature of a smart chip that can keep track of parameters, ranges, and usage history.

The robust, reproducible, and extremely sensitive Restek PAL Smart SPME Arrow allows you to quickly concentrate target analytes using a sorbent-coated fiber, which can then be analyzed directly via GC. With a growing selection of sorbents to choose from, the Smart SPME Arrow offers solutions for a wide range of volatile and semivolatile analytes in a variety of sample matrices.

If you are interested in learning more about the performance of the Smart SPME Arrow and the configuration options available, check out this sample preparation product guide, which focuses specifically on SPME. Using the Solid-Phase Microextraction (SPME) option under the Techniques filter in Restek’s blog, ChromaBLOGraphy, will also reveal a trove of tips, tricks, and applications for the SPME Arrow.

While the SPME Arrow is a more rugged option due to its protective sheath, if you’re happy with the performance of traditional SPME fibers, Restek still has you covered with a wide range of sorbents to choose from. Our SPME fibers are also available with Smart technology.

Accelerated Solvent Extraction (ASE)

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When it comes to supporting accelerated solvent extraction (ASE) methods, Restek offers extraction cells and accessories for a variety of ASE models. Simply filter the products by Instrument Model and restock your ASE consumables and replacement parts. We also have a convenient cross-reference chart for ASE products.

In-Line Sample Preparation

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Revive in-line sample preparation (ILSP) products integrate sample cleanup directly into the analytical flow path. A sample extract is injected into an HPLC and passes through an ILSP cartridge, which retains matrix components while eluting analytes onto an analytical column for separation and detection. Once the analytes have cleared the ILSP cartridge, a six-port valve and isocratic pump switch the flow so that a wash solvent can backflush the ILSP cartridge during the analysis, preparing it for the next sample injection. Primarily targeting the analysis of pesticides in food commodities, Restek’s Revive in-line sample preparation products offer an alternative to the traditional QuEChERS approach. You can find out more about ILSP here.

Vials and Well Plates

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Every sample needs a place to live and, as detailed in this sample preparation product guide, Restek offers a wide range of sample vials, well plates, inserts, and racks to contain all manner of samples for both storage and analysis.

Vials

Autosampler Vials: These are some of our most popular products because a vast majority of chromatographic samples will eventually end up in one! Whether your sample is a liquid headed for a 2 mL autosampler vial, or a solid, liquid, or gas destined to inhabit a headspace vial, we have options for you. Some of the most common choices you will make when selecting a vial are below:

  • What kind of closure do you need?
    • Will snap-on or screw-on caps be sufficient, or does your sample or method call for a cap that is crimped on?
    • Watch out! Not all caps will fit on all types of vials, so make sure you’re matching your caps and vials correctly.
  • What type of vial cap septum is compatible with your sample and syringe?
    • A wide variety of septum materials are available, from silicon to natural rubber to just a simple perforated PTFE closure that doesn’t reseal at all. Make sure that the septum material you choose is compatible with your sample; otherwise, you may find that sample solvents can extract septum compounds, creating extraneous peaks, or the sample may even attack the septum, reducing its ability to seal properly, potentially allowing evaporation.
  • Clear or amber?
    • If your sample is light sensitive, look for amber-colored vials to protect your sample from unwanted photodegradation.
  • Do you want more out of your vial than just storage?
  • Do you need to use a limited-volume insert in your vial?
    • Sometimes you have a limited quantity of a sample, and adding 100 µL of a sample to a typical 2 mL autosampler vial won’t fill the vial enough for an autosampler syringe to reliably aspirate the sample for analysis. In these cases, a special insert can be used, but making sure the insert fits into the vial is important.
  • Vial racks
    • All those little vials need a place to hang out while they are being prepared and awaiting their day on the instrument, so don’t forget to pick up vial racks to make handling the vials during preparation and storage easy.

As you browse our selection of vials, you’ll find that they come in a wide variety of shapes, sizes, and materials with an almost equally wide range of caps to choose from. If you ever have any questions about what type of vial and cap you should use, don’t hesitate to reach out to your Restek sales representative, who will be happy to make sure you get the right vial and cap for the job.

Well Plates

For many customers, handling samples in individual vials just isn’t an option. For analysts dealing with large numbers of samples, the standard 96-well plate format is a staple. Restek offers a variety of 96-well plate products from empty collection plates and sealing mats to more application-specific products, such as our specially designed protein precipitation plates (Resprep PPT3 96-well plates), to our targeted phospholipid removal plates (Resprep PLR 96-well plates). Some SPE phases and our SLE products are also available in 96-well plate format.

Syringes

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Restek offers a comprehensive line of manual and autosampler syringes. Whether you are using syringes to inject a microliter of your sample into your chromatograph or have a heftier task requiring a 2000 milliliter SGE jumbo syringe, you can find the right product for your application.

Restek now offers Smart syringes. Each Smart Syringe is equipped with its own read/write chip with preset parameters, ranges, usage tracking, and a unique ID. The Smart syringe is automatically recognized by the PAL System—all important parameters are loaded to secure the highest precision and process safety.

There are a few things to keep in mind when selecting your syringe:

  • Try to use the smallest volume syringe for your application. For instance, if you are injecting 1 µL volumes as part of your GC method, use a 5 µL syringe instead of a 10 µL or greater volume syringe. The 10 µL syringe will work, but you’ll have greater accuracy and precision with the syringe whose total volume is closer to your required volume. Just make sure you have the correct syringe volume identified in your instrument software if you use autosampler syringes! Having a 10 µL syringe configured in the software but a 5 µL syringe installed in the actual autosampler will typically result in an incorrect volume being aspirated.
  • Keep your syringes clean and ready for the next injection, but don’t forget that they are consumables and ought to be inspected and replaced when necessary. Keeping materials compatibility in mind (especially when using syringes with cemented needles), make sure your syringes get good solvent washes after use to keep them free from contamination and to maintain a smooth action for the syringe plunger. A bent syringe plunger at the start of a batch of samples queued up on your autosampler will ruin anyone’s day!
  • Syringes have a coded language all their own when it comes to their point styles, needle dimensions, and barrel termination types. The good news is that there is help available if you need a good resource covering syringe designations and fundamentals

Conclusion

Restek’s foundation is chromatography, but we also appreciate that the foundation of chromatographic separations is sample preparation. So, to help ensure the most accurate and precise chromatography, we offer the wide range of products covered in this sample preparation product guide in addition to our industry-leading GC and LC columns, accessories, and certified reference materials.

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