Normal phase SPE uses polar interactions as retention mechanism. These interactions include hydrogen bonding, dipole-dipole, induced dipole, π-π, etc. The interactions are between polar functional groups of analytes and polar sorbent chemistries.Reversed phase uses nonpolar, hydrophobic interactions as retention mechanism. These interactions include Van-der-Waals, London dispersion, π-π, etc. These interactions are between carbon rich, hydrophobic parts of both analytes and sorbent.When dealing with sample matrices that are only soluble in organic solvents, one would use a normal phase method. Examples of such matrices are oil, grease, or butter.Ideally, the sample is diluted with a nonpolar solvent (i.e. hexane) prior to sample loading. The sample can then be washed with hexane or 5% methylene chloride in hexane (analogous to washing with 5% MeOH in water in RP SPE methods to remove matrix interferences). Elution will be done with a polar solvent that can interrupt polar interactions between analytes and sorbent.Fractionation can be done by applying solvents in the order of increasing polarity or mixtures of solvents with different polarities.Below is a quick reference of solvents in ascending polarity, for Normal Phase SPE methods.<h3>Solvents in Polarity Order</h3><h5>From low to high&nbsp;</h5><ul><li>Hexane</li><li>Isooctane</li><li>Carbon tetrachloride</li><li>Toluene</li><li>Benzene</li><li>t-butyl methyl ether</li><li>Chloroform</li><li>Dichloromethane</li><li>Diethyl ether</li><li>Ethyl acetate</li><li>Tetrahydrofuran (THF)</li><li>Acetone</li><li>Acetonitrile</li><li>40% MeOH in acetonitrile</li><li>20% MeOH in diethyl ether</li><li>20% MeOH in dichloromethane</li><li>Isopropanol</li><li>Methanol</li><li>Water</li><li>Acetic acid</li></ul>

How is normal phase (NP) SPE different from reversed phase (RP) SPE? How do I decide which one to use?

When considering the mass of sorbent required to perform a solid phase extraction it is critical to consider the sample matrix (serum, plasma, urine, saliva, food, river water, soil etc). This information will allow you, with the use of a suitable look up table to determine the mass of SPE sorbent to use. The tables are produced to reflect the relative levels of interfering endogenous material present in the samples.&nbsp;The SPE device must have enough capacity to retain all analytes of interest together with any endogenous materials capable of retaining on the sorbent with the same type of interactions as the analyte. If there is insufficient sorbent, it is possible some analyte may pass unretained through the SPE device during sample loading, leading to lower recoveries and a loss of consistency of recovery between samples.<h3>Sample Volume</h3><h4>Determines Bed Mass/ Solvent Volumes</h4><img src="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/Phenomenex_How_much_sample_can_I_load_onto_my_SPE_device_Is_it_linked_to_the_concentration_of_my_analyte_of_interest_12a9ab171c.jpg" alt="Phenomenex: How much sample can I load onto my SPE device? Is it linked to the concentration of my analyte of interest?" srcset="https://storage.googleapis.com/labrulez-bucket-strapi-h3hsga3/thumbnail_Phenomenex_How_much_sample_can_I_load_onto_my_SPE_device_Is_it_linked_to_the_concentration_of_my_analyte_of_interest_12a9ab171c.jpg 154w," sizes="100vw" width="983" height="992"><h3>Generic Loading Capacities:</h3><h4>Polymeric: 10 - 15% of Total Bed Mass | Silica: 5% of Total Bed Mass</h4>If we compare plasma and urine, we can load 500 µl of plasma onto a 60 mg polymeric sorbent such as <a target="_blank" rel="noopener noreferrer" href="https://lcms.labrulez.com/products?manufacturerItems=50&amp;search=Strata-X">Strata™-X</a>. Using the same 60 mg SPE tube 2 mL of urine could be loaded, indicating that urine contains significantly less endogenous material than plasma.For sample types not included in the table, the use of generic loading capacity, 5% of sorbent mass for silica materials, or 10-15% for polymeric materials are advised. The difference between silica and polymeric sorbent loading capacity is a function of available surface area, which is higher for polymeric materials and also ligand density – the entire surface of a polymeric material is essentially stationary phase, while on a silica material only the phase bonded to the silica takes part in the extraction mechanism for reversed phase or ion exchange extractions.

How much sample can I load onto my SPE device? Is it linked to the concentration of my analyte of interest?

<a target="_blank" rel="noopener noreferrer" href="https://lcms.labrulez.com/products?category=136&amp;manufacturerItems=50&amp;page=2">Solid phase extraction</a> is routinely selected as a sample preparation technique prior to HPLC and GC analysis. It provides:<ul><li>Clean-up;</li><li>Analyte pre-concentration;</li><li>Solvent switching.</li></ul>This works well for any homogeneous liquid samples, but clearly a solid sample cannot be applied directly to an SPE device. Here we will consider how best to approach the handling of solid samples prior to SPE.The first stage of sample pre-treatment involves homogenising the solid sample with a suitable solvent. The solvent selected for this purpose will:<ul><li>Be effective at solubilising the analyte.</li><li>Be effective at solubilising the sample matrix, although this is not necessary providing the sample is blended well with the solvent.</li><li>Ideally it will be a weak solvent when it comes to the SPE stage. This may not be possible, and it may be necessary to dry down and reconstitute the sample prior to loading on to the SPE cartridge.</li></ul>Having formed a suspension of the sample in a suitable solvent the sample should be vortexed to promote thorough mixing, and then centrifuged to pellet the insoluble components of the matrix. The solvent should then be removed for further treatment.At this point the SPE mechanism needs to be selected:&nbsp;<h3>Reversed Phase SPE Method</h3>Mechanism / Weak Solvent / Strong Solvent<ul><li>Reversed phase – Water/buffer – Acetonitrile/Methanol (hexane strongest)</li><li>Normal phase – Hexane/Heptane – Mid polar organic such as DCM, MTBE, IPA (strongest is water)</li><li>Ion exchange – Aqueous buffer at a pH which promotes ion exchange interactions between analyte and SPE sorbent – Organic solvent, typically methanol with pH adjustment to disrupt the ion exchange interactions.</li></ul>Analyte properties and sample diluent both need to be considered at this point. Reversed phase extractions are effective for moderately polar to moderately hydrophobic analytes, but only if they are dissolved in a weak solvent such as water. If the sample is dissolved in a mid-polar solvent the sample will either need to be diluted, or alternatively dried down and reconstituted. The same is true for normal phase, where samples need to be dissolved in hexane/heptane for analytes to be successfully retained during sample loading onto the SPE device.These examples will hopefully illustrate this concept.<h4>The extraction of PCBs from soil.</h4>PCBs are hydrophobic and will dissolve readily in a non-polar solvent. Soil can be dried, weighed and then extracted using hexane (or heptane). Once thoroughly mixed and centrifuged the resulting solution can be applied directed to a conditioned silica SPE tube allowing for a normal phase extraction to be conducted.<h4>Tylosin from meat.</h4>Tylosin is an antibiotic, with a log P of 1.0. Extracting from meat with a mixture of acetonitrile and water (80:20) will break any hydrophobic interactions releasing the tylosin. After centrifugation the resulting liquid sample will require further dilution with water to reduce the loading solvent strength prior to a reversed phase extraction.<h4>Enilconazole from oranges.</h4>Enilconazole is a fungicide widely used in agriculture and is often used with citrus fruits. It can readily be extracted from oranges using acetonitrile. As enilconazole is basic, the addition of HCl to the acetonitrile extract will promote protonation of the analyte, allowing for it to be directly retained on a strong cation exchange SPE material. This greatly simplifies the extraction process as no dilution of the sample is required (in contrast to reversed phase, or normal phase extraction where 100% acetonitrile would cause the analyte to be unretained during loading).

Considerations when working with solid samples and SPE

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