Which Sorbent is Best For Your Application

Selecting the most appropriate sorbent is best approached from both a theoretical and empirical standpoint. Knowledge of the components in a specific sample will help you determine the general mode of separation, such as, normal phase, reversed-phase, or ion exchange, and more specific knowledge may help you select between the various types of sorbents in each mode. However, it is frequently necessary to determine the optimal separation by empirical means.

The text and tables that follow give you detailed information on the properties of the sorbents that Waters offers.

The Original C18 Sorbent

Waters original C18 sorbent is the most widely referenced SPE product, with over 2,300 published papers or articles specifying its use. This product is made using the most reproducible bonding technology available, utilizing monofunctional silane chemistry. Waters original C18 sorbent offers a unique selectivity and excellent recovery of analytes, and is recommended for most applications where HPLC is utilized for subsequent analysis.

tC18 (trifunctional) Sorbent

With the addition of tC18 sorbent, you can now choose between two highly reproducible C18 chemistries, monofunctional and trifunctional. Waters trifunctional tC18 cartridges are recommended for most applications involving GC, GC/MS or LC/MS especially if long sample contact times are involved. This new tC18 sorbent has also become increasingly popular in pharmaceutical applications when a high carbon load, very hydrophobic surface is required. (See Table 2)

New tC2 (trifunctional) Sorbent

Many applications require a less hydrophobic (relative to C18), short carbon chain sorbent to provide optimal sample processing of a wide range of analytes. However, finding a reproducible, stable C2 sorbent can be difficult. Waters has recently developed a new, state-of-the-art trifunctional bonding process that provides you with the most dependable sorbents (tC2 and tC18) of this type available. The tC2 is ideal for many types of applications such as environmental, pharmaceutical, etc.

Table 2: Recommendations for Using C18 and tC18 Sorbents

                                    C18               tC18

Sample Contact Time

long        (> 10 minutes)          NR                R

short       (< 10 minutes)          R                 O



Sample pH

< 4                                 NR                R

4-7                                 R                 O

> 7                                 NR                R



Detection Method                                      

HPLC                                                  

            Ultraviolet/Visible     R                 O

              Absorbance 

            Fluorescence            R                 O

GC                                                    

            Flame Ionization        O                 R

            Mass Selective          O                 R

            Electron Capture        O                 R

            Nitrogen-Phosphorous    O                 R



Selectivity                                           

Unique                              R                 NR

Similar to other                    NR                R

  vendors' C18

Key: R = recommended adsorbent

O = may also be applicable

NR = not recommended

Table 3: Physicochemical Characteristics of Sorbents

Sep-Pak        Surface   Activity    End-     Percent   Pore   Particle

Sorbent        pH^1      Grade       Capped   Carbon    Size   Size (m)



C18            7         N/A         Yes      12        125?   55-105

tC18           7         N/A         Yes      17        125?   37-55

C8             7         N/A         Yes      9         125?   37-55

tC2            7         N/A         Yes      2.7       125?   37-55

Amino Propyl   8         N/A         No       3.5       125?   55-105

  (NH2)

Cyano Propyl   7         N/A         Yes      6.5       125?   55-105

  (CN)

Diol           7         N/A         No       2         300?   37-55

Silica         7         High^2      N/A      N/A       125?   55-105

Florisil       8.5       High^3      N/A      N/A       60?    50-200

Alumina A      4-5       Grade 1^4   N/A      N/A       120?   50-300

Alumina B      9-10      Grade 1^4   N/A      N/A       120?   50-300

Alumina N      7-8       Grade 1^4   N/A      N/A       120?   50-300

Accell Plus    4         N/A         N/A      5.5       300?   37-55

  CM

Accell Plus    9         N/A         N/A      6         300?   37-55

  QMA

1 pH of 10% aqueous slurry

2 Moisture content <= 3.2%

3 Moisture content <= 2.5%

4 Moisture content <= 1.5%; activity <= 1 on Brockmann activity scale

Table 4: Chemistry of Bonded Phase Packing Sorbents

Sorbent                Bonding Chemistry     Surface Functionality

C18                    Monofunctional        -Si(CH3)2C18H37

tC18                   Trifunctional         -SiC18H37

C8                     Monofunctional        -Si(CH3)2C8H17

tC2                    Trifunctional         -SiC2H5

Amino Propyl (NH2)     Trifunctional         -Si(CH2)3NH2

Cyano Propyl (CN)      Difunctional          -Si(CH3)(CH2)3CN

Diol                   Trifunctional         -Si(CH2)3OCH2CH(OH)CH2OH 

Accell Plus CM         Polymeric^1           -CO2-Na^+

Accell Plus QMA        Polymeric^2            C(O)NH(CH2)+3N(CH3)^+CO-

1 An acrylic acid/acrylamide copolymer on Diol silica; ligand density: 350 ?moles/g; approximate protein binding capacity = 175 mg Cytochrome c/g of packing or 63 mg/Accell Plus CM Plus or Classic cartridge.

2 An acrylamide copolymer on Diol silica; ligand density = 220 ?mol/g; approximate protein binding capacity = 200 mg BSA/g of packing or 72 mg/Accell Plus QMA Plus or Classic cartridge.

Table 5: Cartridge Design and Nominal Weight of Sorbent (in milligrams)

Separation Mode        Plus     Classic     Light     General Surface

and Sorbent                                           Chemistry

  

Reversed-phase                                                          

C18*                   360      360         130       Neutral

Environmental          820                                                   

  C18*

tC18                   400      N/A         145       Neutral

Environmental          900                                                   

  tC18*

C8                     400      N/A         145       Neutral

tC2                    400      N/A         145       Neutral

Normal or Reversed   

  phase                                                       

Amino Propyl           360      360         130       Basic

  (NH2)**

Cyano Propyl           360      360         130       Neutral

  (CN) 

Diol                   360      360         130       Neutral

Normal Phase                                                            

Silica                 690      690         120       Neutral

Florisil               910      900         145       Basic

Alumina A              1710     1850        280       Acidic

Alumina B              1710     1850        280       Basic

Alumina N              1710     1850        280       Neutral

Ion Exchange**                                                          

Accell Plus QMA        360      360         130       Basic

Accell Plus CM         360      360         130       Acidic

*Sep-Pak Plus long-body cartridge.

**The amino propyl packing may also be used in the protonated form as a weak anion exchanger.

Sep-Pak Vac Cartridges Nominal Weight of Sorbent*

Syringe Size  New 50 mg  100 mg  New 200 mg  500 mg  1 g  2 g  5 g  10 g

1cc           X          X                                                                                                     

Vac RC                   X                                                                                                           

3 cc                             X           X                                                                       

Vac RC                                       X                                                                       

6 cc                                         X       X                                                     

12 cc                                                     X                                  

20 cc                                                          X                 

35 cc                                                               X

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