ESI Charge State Calculator Application

Here is a neat program, created by Robert Winkler, which allows you to calculate the Mw of proteins using data from your electrospray ionization (ESI) mass spec source.

ESIprot 1.0    [http://www.bioprocess.org/esiprot/]

Reference this journal article:  Winkler R. "A universal tool for charge state determination and molecular weight calculation of proteins from electrospray ionization mass spectrometry data." Rapid Commun Mass Spectrom, 24(3),  285-294, 2010.

HPLC to UHPLC Conversion Notes (Column Dimensions, Flow Rate, Injection Volume & System Dispersion)

The use of ultra-high performance liquid chromatography (UHPLC) columns to reduce analysis times and sometimes improve detection limits is a hot topic. UHPLC presents a number of new issues. The incorporation of smaller 1.9 to 3.0 micron particles and smaller frits will raise backpressures and increase system wear and tear. Smaller diameter lines are often used (I.D. of 0.12mm or less) which can increase blockages and clogs if you do not filter your mobile phase and samples through a 0.45 or 0.2 micron filters. Piston seals and valve rotors can wear out early due to the very high pressures, heating and stress imposed on them. You should monitor your HPLC system carefully over time and consider increasing the frequency of preventative maintenance and inspection services as well. However, the smaller particle sizes can provide better resolution in some applications so they are well worth evaluating.

I must answer twenty or so questions each week in the area of UHPLC. The most common questions deal with selection of an UHPLC column and making adjustments to a method for the changes which effect: (1) Column Dimensions; (2) Flow Rate; (3) Injection Volume and (4) System Dispersion. The good news is that some of these questions can be answered with some basic math while others just require a basic understanding of how the system works.

(1) COLUMN DIMENSIONS: Let's start by making things as simple and brief as possible (this is supposed to be a "hint & tip", not a thirty page article). When initially converting from a convention HPLC column (e.g. with 5 micron particles) to an UHPLC column (e.g. with 1.9 to 3 micron particles), initially select a column with the same I.D. and length for the calculation. This way only the particle size changes. *I like to change one variable at a time. If you would like to change the column length to take advantage of some of the increased efficiency (and decrease the pressure!) which results from smaller particles, then please refer to the following equation.

     EQUATION A:  'Lc2' = ('Lc1' * 'p2') / 'p1'

[ 'Lc1' = Length of Column #1 in mm; 'Lc2' = Length of Column #2 in mm; 'p1' = particle size of Column #1 in microns; 'p2' = particle size of Column #2 in microns].
                    
   Example: Column # 1 is a standard HPLC column;  4.6 mm x 250 mm (5u) Column. You want to find out the length of an equivalent column which uses 1.9 micron particles instead of the 5 micron particles.

   'Lc2' = (250 * 1.9) / 5 ; Answer is: 'Lc2' = 95 mm. *So a 10 cm long column would be a good choice here.


(2) FLOW RATE: Flow rate is directly proportional to column diameter and as we saw above in Equation A, the particle size can also affect it too. If you keep the column length and internal diameter the same, then the linear flow will be unchanged with the same particle size. A change to the particle size alone will change the flow rate as follows: 'Fc2' = 'Fc1' x ('p1'/'p2').

A change to a smaller diameter column to compensate for the improved efficiency will require a change to the original flow rate to preserve the linear velocity. Please refer to the following equation.

     EQUATION B:   'Fc2' = ('d2' / 'd1')^2 * 'Fc1'

['Fc1' = Flow Rate of Column #1 in ml/min; 'Fc2' = Flow Rate of Column #2 in ml/min; 'd1' = Column #1 Diameter in mm; 'd2' = Column #2 Diameter in mm].
                     
   Example: Column # 1 is a standard 4.6 mm ID Column. You want to find out what the linear flow rate should be if you use a smaller diameter column (2.1mm in this example).

   'Fc2' = (2.1/4.6)^2 * 1.000 ; Answer is: 'Fc2' = 0.208 ml/min. *A flow rate of 200 ul/min would be fine. 

However, one other factor should be considered. The optimum flow rate for sub 2.5u particles are often about double that of the "normal" linear flow rate used with conventional particles (>2.5u). Evidence for this has been shown through analysis of the van Deemter curve with the tiniest particles showing much flatter curves. Retention (K prime) can often be maintained by combining twice the normal flow rate and speeding up the gradient time by a factor of 2. So a method utilizing std sized particles with a linear flow rate of 0.200 ml/min might benefit from a faster flow rate of 0.400 ml/min and a twice as fast gradient composition change.


(3) INJECTION VOLUME: A change in the column dimension may require a change to the injection volume (note: "volume" and concentration are two different things. If the solution concentration remains the same and you inject less, the on-column sample concentration will also be less). The smaller the internal volume of the column, the smaller the injection volume. To calculate the linear change in volume, please refer to the following equation.

     EQUATION C:   'V2' = 'V1' * {('d2'^2 * 'L2') / ('d1'^2 * 'L1')}

['V1' = Injection Volume #1 in ul; 'V2' = Injection Volume #2 in ul; 'L1' = Column #1 Length in mm; 'L2' = Column #2 Length in mm; 'd1' = Column #1 Diameter in mm; 'd2' = Column #2 Diameter in mm].
                     
   Example: Current injection volume is 10 ul. Column # 1 is a standard 4.6 mm ID x 250 mm Column. You want to find out what the equivalent injection volume should be for a 2.1 mm ID x 150 mm column.

   'V2' = 10 * (2.1^2 * 150) / (4.6^2 * 250) ; Answer is: 'V2' = 1.25 ul.



(4) SYSTEM DISPERSION: When converting HPLC methods to "UHPLC" methods, few parameters effects the results obtained more than the HPLC system's System Dispersion. The volume of liquid that is contained between the injector needle and flow cell (with the column removed or by-passed) is know as the system dispersion volume. This volume is determined by how the specific HPLC is designed and plumbed. On most HPLC systems, it can be easily changed and optimized to fit the specific application desired and only requires that you have a solid understanding of how the HPLC system works. The choice of connection tubing ID and length, how the autoinjector is programmed, its loop size and the detector's flow cell volume all contribute to the system dispersion volume. In the same way that changes to the total column volume can effect the peak shape and resolution, the internal system dispersion volume also contributes to the results. 

With standard sized analytical columns (i.e. 4.6 x 250 mm), the typical HPLC's system volume is so small relative to the volume of the column (e.g. 100 ul system dispersion vs 2900 ul column volume, or 3.5%) that it does not negatively impact the chromatography. However, anytime we utilize a tiny HPLC column whose column volume is a fraction of that found in a standard column (i.e. 100 ul system dispersion vs 2.1 x 50 mm column with 120 ul volume), diffusion and band spreading can quickly become so significant that effective plate numbers are quickly reduced below values found on a standard sized column. As column volume decreases (and approaches the system volume) the total system dispersion volume must also decrease. In general, try and keep the system dispersion volume at or below 10% of the column dead volume. This is most easily accomplished by reducing the number of connections and fittings used, reducing the lengths of all tubing used, using much narrower ID tubing (e.g. 0.12 mm vs 0.17 mm ID), reducing flow cell volume and reducing the flow-through volume in the autoinjector (i.e. loop size, needle seat, etc). The injector is often the largest contributor to the system dispersion so concentrate efforts here (e.g. after injection, switch the loop out of the flow path).

Some Stereochemistry Terms (Chirality); Structural Isomers, Stereoisomer, Enantiomer, Diastereomer, Meso

Some of the common terms used in chiral chromatography often need to be reviewed. Here are some of the most common ones we encounter in the chromatography lab each day. There are many detailed chemistry books on the topic, but I will try and present some simple explanations for each here.

"Structural Isomers": When two compounds have the same chemical formula, but different structures, then they are referred to as structural isomers. AKA "Constitutional isomers".

"Stereoisomers":  When two compounds have the same chemical formula and are connected to the same molecules, then they are referred to as stereoisomers.

"Enantiomers":  When two compounds have the same chemical formula and are connected to the same molecules (they are stereoisomers), but are also non-superimposable mirror images of each other, then they are referred to as chiral enantiomers. Enantiomers rotate the plane of plane polarized light to an equal extent, but in opposite directions.

"Diastereomer": When a stereoisomer compound has two or more chiral centers which are not mirror images of each other, then the compound is a diastereomer. IOW: Diastereomers are stereoisomers that are not enantiomers

"Meso Compounds": When a stereoisomer appears chiral due to the presence of more than two chiral centers (but is in fact not chiral), but is superimposable on its mirror image, then it is a meso compound. These are interesting forms and they are optically inactive because the two opposites cancel each other out (no +/- signal). 

Chiral HPLC and SFC Column Screening Strategies for Method Development:

We are experts in chiral separations and have learned a great deal about how to efficiently and quickly identify the best conditions to resolve a chiral sample by HPLC and SFC. You should always have a clear strategy in mind when developing an automated HPLC or SFC chiral method development screening system to separate racemic samples. Here are a few key points to focus on.

(1)  Make sure the chiral sample is as “chemically” pure as possible before you start. By “chemically” pure I mean it should only contain the two complementary enantiomers and not other impurities, starting materials or chemicals which might interfere with the identification or separation of the racemate. Often, many of the intermediate chemicals used in the synthesis of a compound have similar absorbance or retention characteristics. When this happens, you can be fooled into thinking you have resolved your chiral component apart when in fact you have simply resolved a non-enantiomer apart from the racemate. Chiral columns are very poor at discriminating the racemate from other non-chiral species. As such, you may find that the impurities or other components make it difficult to determine the actual HPLC or SFC purity of your chiral sample. Try and remember this phrase: “The chiral purity of a sample is only as good as the chemical purity”. So start with as clean a sample as possible when developing a chiral method.

(2)  Column choices are very important when using a fully automated Chiral Screening System such as the LC Spiderling™ Column Selection System. Here are some popular questions we are asked on this topic.

How do you know how many and which types of columns to include in your chiral screening system?

Keep the goal of creating a "screening system" in mind. Don't lose sight of this basic strategy. You want to select the smallest number of different columns which have the widest specificity for your expected sample types. Often five (5) to nine (9) different types of chiral columns will do the job. Screening more columns than that is often a waste of time. Leave a “test” position in your screening system so you can evaluate new columns all of the time. The key is to identify the best ones first.

Normal or Reverse Phase Columns?

Most chiral screening is performed in the normal phase mode (NP provides easy solvent removal for scale-up and there are a wide range of quality columns available which can be used to generate useful data). You can mix reverse phase and normal phase columns in the same system as long as you incorporate a bypass and flush step between the different methods to wash out the old solvent and bring in the new solvent safely. For this brief discussion, we will assume you have a dedicated normal or reversed phase screening system. 

How many different mobile phase systems should I use?

Quick answer is as few as possible. The concept of using a screening system is to quickly identify the column (1^st) and mobile phase (2^nd) which baseline resolves the racemate. Select three or four different mobile phase types, with and without modifier systems, which span the range of polarity needed to increase your chance of retaining the sample on the column, but for no longer than 30 minutes. Keep it simple! The goal is to retain the sample, hold, then elute it.

Which types of chiral columns are best?

Well, if you ask the different column suppliers this question, then they will most likely answer that the columns “they sell” are the best. At last count, there are over two hundred different chiral columns advertised on the market today. Most are advertised to be the 'best', but in fact they all can not be… In reality, you should evaluate as many different kinds as possible with your own samples to determine which the “best” are. Do not be fooled by examples of the column separating out very simple compounds such as racemic trans-Stibene oxide as this is one of the easiest compounds to separate even if 90% of the chiral stationary phase is missing! Consider also if the column type can separate compounds without the use of fancy modifiers or complex mobile phase mixtures. Simple is better and usually more easily reproduced too. Some of the most commonly used types of chiral columns are the: cellulose/amylase, Protein, Pirkle type and cyclodextrin based columns. All these columns have different preferred mobile phase choices (Most protein columns are run in reverse phase, while all of the others mentioned have versions which can be run in either normal or reverse phase). You should consult with the appropriate manufacturer about how to best use these columns. Do not necessarily select a column because the column has been “reported in the literature to be used by the largest number of people”. Who cares how many people used a particular column. This is not a scientifically valid argument that the column is useful. We routinely read published papers which describe a "novel" chiral method run on a specific column which clearly shows worse results than could have been obtained with another commercial column using a simpler mobile phase. Many try and force a column to resolve a sample apart because it is the only chiral column they have. The purpose seems to be directed at publishing a paper and not at developing high quality chiral HPLC or SFC methods at all. Just because someone tried it before does not mean that their method or column choice was a good one. Remember, very few chromatographers have practical experience developing CHIRAL methods. It takes special training to be successful at it. Invest some time learning about and evaluating the different chiral column types with your own samples to find out which ones are most applicable. Just as with achiral analysis, make sure the sample is fully soluble in the selected mobile phase before analysis. Select a chiral column based on your own scientific evaluation and testing. Start with a "full sized" columns to maximize the amount of stationary phase the sample comes in contact with. 

 

A Note about AVOIDING "Short Length" Chiral columns: We do not use or recommend any short "scouting" columns. Using SHORT columns will often result in you miss-identifying a column type that actually works. You will not see retention when in fact you would have if you used a standard sized column. Please don't make this novice mistake with chiral columns. Use chiral columns that are LONG, not short for screening (the goal is to maximize the amount of support the sample comes in contact with). The use of short chiral columns in HPLC / SFC column screening is often a waste of your money and time. If you want to identify which chiral columns will resolve your samples, stay away from short columns.

Can you provide some free advice as to which columns are the “best”?

OK, we have tried them all (but not for your samples), but here are two of our favorites (in no order):

The Pirkle based Whelk-O 1 (and/or Whelk-O 2) is the only Pirkle based column we have ever used which can produce a significant quantity of racemic separations using simple mobile phase systems. No other Pirkle based chiral column has ever proven to be as good as this one in real world chiral pharmaceutical drug method development. Every other one tried has disappointed us, but this one has been responsible for a number of successful separations in normal and reversed phase modes.

The coated polysaccharide chiral stationary phase made by Daicel, known as "Chiralcel OD" (OD-H) is one of the best chiral columns on the market. Note that these are the non-covalently bound coated versions of the column. This support type has a broad range of selectivity not seen in any other types of columns available. At this time, the more stable covalently bound versions are also very good, but just do not measure up to the high success rate this one has. BTW: It is normal to see different results between the covalently bound and coated versions of the same column. They are completely different supports and if budget allows, you can have both types of columns available for screening.