Maximizing High Flow ESI Sensitivity

Today, met a general question: how to improve the ESI Sensitivity? So I would like to answer this question from the preparation of sample.

(1) Select appropriate chromatography grade HPLC solvents
(2) Avoid exotic solvent mixes (MeOH, MeCN, Water, 0.1% formic work best for 98% of LC/MS applications)
(3) Avoid adding excessive modifiers (eg amm.acetate @ 10mM not 50mM)
Choose the right column chemistry (C-8 vs. C-18); change column chemistry before changing solvent mix or composition.
(4) 2.1mm column or lower, flow rates of 200-400 uL/min
(5) Peak widths for quantification not greater than 8-10 secs
(6) Dissolve sample in start mobile phase solvent (weakest solvent possible).

Here is a schematic drawing of the Finnigan MAT 900S electrospray ion source (kindly provided by EPA/Dr. Helmut Muenster.)

Suitable Solvents for LC/MS

(1) Reverse-Phase LC/MS Solvents
ACN, MeOH, H2O, Isopropanol.

(2) Normal-Phase LC/MS Solvents (for APCI-MS)
Hexane, Methylene Chloride, Acetone, Ethanol.

(3) Compatible LC/MS Buffers and Modifiers:
Formic acid, Acetic Acid, Ammonium Acetate, Ammonium Formate,
Ammonium Hydroxide, Trifluoroacetic Acid (TFA) concentration should
be <0.1%.

Avoid Non-volatile Buffers, Alkali-metal phosphates, borates, etc. that is to say,

ØAcids

l Do not use inorganic acids (may cause source corrosion), Formic and acetic acid are recommended.

Ø Bases:l Do not use alkali metal bases (may cause source corrosion), Ammonium
hydroxide and ammonia solutions are recommended.

Surfactants (surface active agents): lDetergents and other surface active agents may suppress ionization.

Ø
Trifluoroacetic Acid (TFA): l May enhance chromatographic resolution, but causes ion suppression in both negative and positive ion modes.

Ø Triethylamine/Trimethylamine (TEA/TMA): May enhance deprotonation for Negative Ion Formation.

Steps for ESI Optimization (Electrospray Ionizatio)

Steps for ESI Optimization

1 If analyte’s pKa is unknown, evaluate 3 pH regions in positive and negative ion modes.

2 Acids – Negative Ion detection, adjust pH 2 units above pKa: Increase pH with NH4OH, TEA, TMA.

3 Bases – Postive Ion Detection, adjust pH 2 units below pKa* Decrease pH use formic acid , acetic acid, TFA. *In complex molecules, many exceptions to these rules are observed.

4 Remove salts which may cause ion suppression.

5 Adjust source temperature and source voltages to maximize signal

6 In negative ion mode, use lower spray voltage to minimize discharge.

The analyte is introduced to the source in solution either from the eluent flow from liquid chromatography. (Flow rate is 1µl min^-1) . The analyte solution flow passes through the electrospray needle that has a high potential difference (with respect to the counter electrode) applied to it (typically in the range from 2.5 to 4 kV). This forces the spraying of charged droplets from the needle with a surface charge of the same polarity to the charge on the needle. The droplets are repelled from the needle towards the source sampling cone on the counter electrode . As the droplets traverse the space between the needle tip and the cone and solvent evaporation occurs.
Once the droplet leaves the capillary and enters the nitrogen it continues to lose solvent until the charge density exceeds the surface tension, i.e. the Raleigh constant is exceeded, the droplet explodes resulting in smaller charged droplets. This process continues until the droplets are small enough for ion desorption. The creation of these ions facilitates the transfer of the sample molecules from the source into the MS as the ions are attracted and accelerated into the mass analyser.

Note: The first pioneering experiments on electrospray ionization (ESI) was conducted by Malcom Dole, Not me--- Malcolm. haha...

Guidelines for Choosing Ionization Method

Guidelines for Choosing Ionization Method for LC-MS

Note: This July, will focus on LC-MS and GC-MS@Pesticides.

So Do not go, I will be back --<(:) smile>- I learned this from FX TV channel, can not endure up to 10 times annoying TV Advertisements during a Movie... So I disconnected the cable services @ June-2008. Am I stupid? yes, maybe, but whatever....

Why 371,445 and 519 in my LCMS system?

So sweet question, today, remind me 2005 Fall, The First Question I asked at Tech.
All the optimal conditions were set up, where were they from?

One reasons could be they are from Contaminant Peaks from C18 Column Bleed. The contaminant ions, at M/Z 371, 445, and 519, when seen on the LC/MSD and LC/MSD Trap systems in positive ion mode, are believed to be C18 column bleed peaks of polysiloxane [O-Si(CH3)2-]n

Methylsulfonylmethane (MSM, or Dimethylsulfone)

Methylsulfonylmethane (MSM, or dimethylsulfone) is an organosulfur compound with the formula (CH3)2SO2. It occurs naturally in some primitive plants and is present in small amounts in many foods and beverages and it is marketed as a dietary supplement, although its benefits are disputed.

Purity and Limit of Dimethyl Sulfoxide ( DMSO) by Gas-Chromatography with FID.

Reference Standard: USP DMSO and MSM RS.
Column: 3m m x 0.5 mm capillary column coated with a 5 um phase G2.
Oven Temp: 120 degree C.
Inlet/ FID temp: 250 degree C.
Inlet Split ratio is 2:1.
He ( carrier gas) = 5 ml/min.
C-Air/H2 = 400:40 ml/min.

Result: MSM Purity is 101.03%, Dimethyl Sulfoxide ( DMSO) is 0.041%, Chromatography purity is 99.91%.

Good to go for Lunch.

How to Use ChemStation to Calculate System Suitability

How to Use ChemStation to Calculate System Suitability

Cool.....Work Hard, Think Smart........

Basic Principles of Headspace Analysis 2

PS: Thanks for the PerkinElmer

Basic Principles of Headspace Analysis

A headspace sample is normally prepared in a vial containing the sample, the dilution solvent, a matrix modifier and the headspace. Volatile components from complex sample mixtures can be extracted from non-volatile sample components and isolated in the headspace or gas portion of a sample vial. A sample of the gas in the headspace is injected into a GC system for separation of all of the volatile components.

Phases of the Headspace Vial

G = the gas phase (headspace)
The gas phase is commonly referred to as the headspace and lies above the condensed sample phase.

S = the sample phase
The sample phase contains the compound(s) of interest. It is usually in the form of a liquid or solid in combination with a dilution solvent or a matrix modifier.

Once the sample phase is introduced into the vial and the vial is sealed, volatile components diffuse into the gas phase until the headspace has reached a state of equilibrium as depicted by the arrows. The sample is then taken from the headspace.

From USP <467>

Headspace Operating Parameters Sets
	1	2	3
Equilibration Temp (oC)	80	105	80
Equilibration Time ( Min)	60	45	45
Transfer-line Temp (oC)	85	110	105
Carrier Gas: N2, He @ appropriate pressure
Pressurization Time ( s)	30	30	30
Injection Volume ( ml)	1	1	1

The FID Detection Limitation ( g/s)

One more question for the FID.

The FID detection limitation ( g/s) is : 2NW/A.

Where,
N is the Baseline noise (A)
W is the Mass of n-Hexadecane (g)
A is the Average area amount of n-Hexadecane ( A• s)

So, how to calculate the Baseline Noise (A)?

(1) The Noise (N: peak to peak) is measure over a representative section of baseline equal to 20 times the width of the analyte peak ( Wi).

(2) When the instrument is under optimal conditions that produce a continuous, electronic output, the Noise ( N ) may be measured from the Peak to Peak Variation ( N: peak to peak) in the baseline signal.

N = N:peak to peak

Nice....