LC MS: choosing an analyser, and scoping an outsourced method

Choosing the analyser, or the provider

1. Targeted quantitation wants a triple quadrupole. For measuring known compounds at low concentrations in complex matrices, a triple quadrupole monitoring specific transitions gives the sensitivity and the dynamic range, and it is the workhorse of quantitative bioanalysis and residue testing.
2. Discovery wants high resolution. Identifying unknowns needs accurate mass and fragmentation, which means a time-of-flight or orbital trap analyser. These also quantify, less well at the very bottom of the range, and they generate far more data that somebody has to handle.
3. Ionisation follows the analyte. Electrospray suits polar and ionisable compounds; atmospheric pressure chemical ionisation suits less polar ones. Some analytes ionise poorly in both and need derivatisation. This is decided by chemistry rather than by instrument brand.
4. Matrix effects are the real difficulty. Co-eluting matrix components suppress or enhance ionisation, which moves results without any obvious sign. Stable isotope labelled internal standards are the proper defence, and a method without one in a complex matrix should be treated with caution.
5. Scoping an outsourced method. Give the provider the analyte, the matrix, the concentration range and the required limit, and ask what internal standard is used, what validation has been performed in that matrix, and what the certificate reports. A quote without those is not comparable with another.

The real cost of owning one, against mass spectrometry services

The instrument is the smaller part. Service contracts on mass spectrometers are substantial, solvents and standards recur, and the instrument needs an experienced operator to produce data anyone should rely on. Laboratories that buy one without the person get a very expensive instrument producing plausible numbers.

For occasional work, an accredited service laboratory is almost always cheaper and better than ownership. Ownership makes sense when the workload is continuous and the method is yours.

Records and validation

Where results are reported, complete laboratory records covering the method, the calibration, the quality controls and any reprocessing are required, and the data system's audit trail is part of that. Reprocessing without a recorded reason is the thing that draws attention.

Method validation in the actual matrix, not in solvent, is what makes a quantitative result defensible. Ask which matrix a provider validated in before accepting a quote.

Which technique the sample decides, and the types of mass spectrometry it names

Gas phase separation needs an analyte that is volatile and thermally stable, or that can be made so by derivatisation. Liquid phase separation handles everything else, including large, polar and thermally fragile molecules, which is why it dominates biological and pharmaceutical work.

So the choice is made by the analyte, not by preference. Volatile organics, solvent residues, fatty acid profiles and many environmental contaminants go to the gas instrument; peptides, drugs, metabolites and most everything polar go to the liquid one.

What tandem detection on a triple quadrupole LC MS buys

Selecting a precursor ion, fragmenting it and measuring a specific fragment removes almost everything else in the sample from the measurement. That is where the selectivity and the very low detection limits of quantitative work come from, and it is why a triple stage instrument is the standard for quantification in a complex matrix.

It is a targeted technique: you measure what you decided to measure. Discovery work, where the compounds are unknown, needs a high resolution instrument that records everything, and many laboratories own one of each for exactly that reason.

Method development is most of the work

Developing a method means choosing the ionisation, finding the precursor and product ions, tuning the energies, developing a separation that resolves interferences, choosing an internal standard and establishing a sample preparation that recovers the analyte from the matrix.

The instrument is bought in a week and the method takes months. Where a method already exists for the analyte and the matrix, buying the instrument that runs it is far cheaper than developing one; where it does not, the development effort belongs in the business case.

Solvents such as acetonitrile for LC MS are part of the method

Solvent grade matters more here than anywhere else in chromatography, because impurities and additives ionise, suppress the analyte or add background across the spectrum. A grade specified for mass detection is tested for exactly that.

Vials, caps and septa contribute extractables that appear as background, and plasticisers from the wrong tube are a classic source of a peak nobody can place. Fix the consumables with the method and treat a change of supplier as a change of method.

Field instruments and what they are for

Transportable instruments exist for on-site screening: site characterisation, security, emergency response. They trade sensitivity, resolution and quantitative accuracy for the ability to answer a question where the sample is.

That trade is right when a fast presence-or-absence answer changes what happens next, and wrong when a reportable number is needed. Most programmes use them to triage and send the important samples to a laboratory.

What oligonucleotide mass spectrometry asks for that small molecules do not

These are large, highly charged, phosphate rich molecules, so the ion source sees them as a ladder of charge states rather than one peak, and the sodium and potassium that small molecule methods tolerate produce adducts that bury the signal. The method therefore starts with the ion pairing agent and the cleanliness of everything upstream of it, and a column dedicated to the work rather than shared.

Interpretation is the other half: the answer is a mass that has to be deconvoluted from the charge envelope, and a truncation or a single missing base is a small difference on a large number. Expect the supplier to report the deconvoluted mass and the expected one, not a chromatogram with a purity figure.

Mass spectrometry standards, and what each kind is for

Three different things travel under the word. A calibration standard sets the mass axis and is run on a schedule; a system suitability standard proves the instrument and the method are working today; an internal standard, ideally the isotopically labelled version of the analyte, corrects for everything the sample does to the ionisation. The third is what makes a quantitative result defensible, and it is the one most often left out of a quotation.

Where a method will be transferred or audited, ask which standard is used for what and how each is sourced, because a certificate that cannot say where the reference value came from is not traceable to anything.

Common questions

Which mass analyser do I need?

A triple quadrupole for targeted quantitation of known compounds at low levels; a high resolution analyser for identifying unknowns. They are different instruments for different questions and each does the other's job poorly.

What are matrix effects?

Suppression or enhancement of ionisation by co-eluting sample components, which shifts results without any obvious sign. Stable isotope labelled internal standards are the proper defence in complex matrices.

Should I buy an LC-MS or use a mass spectrometry analysis service?

Outsource occasional work: the instrument is the smaller cost beside service, consumables and an experienced operator. Ownership makes sense when the workload is continuous and the method is yours.

What should an LC-MS quote specify?

The analyte, matrix, concentration range and required limit, the internal standard used, the validation performed in that matrix, and what the certificate reports. Without those, two quotes are not comparable.

Which technique does my sample need?

Gas phase separation if the analyte is volatile and thermally stable or can be derivatised; liquid phase for everything polar, large or fragile. The analyte decides, and many laboratories own both.

Is method development included when I buy an instrument?

Rarely, and it is most of the work. If a validated method exists for your analyte and matrix, buy the instrument that runs it; if not, put the months of development into the business case.

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