Frequently Asked Questions
Q: Isn't mass spectrometry an expensive, difficult, non-quantitative, low sensitivity method that is dependent on the specific chemical composition of the biomarker?
A: Use of the term "mass spectrometer" in the bioanalytical community almost invariably evokes recognition of organic (protein) mass spectrometry. This is quite distinct from the present technology. The mass spectrometer used in this application is an inorganic (elemental) mass spectrometer, specifically an ICP-MS. Conventional organic mass spectrometry attempts to recognize proteins or peptides through mass fingerprints with reference to massive libraries, and is encumbered by sample-dependent sensitivity.
ICP-MS provides sample-matrix-independent sensitivity, absolute quantification, high dynamic range, and excellent resolution of mass channels. It has a reputation for great ruggedness and simplicity in the environmental analysis and metal-toxicology fields, but is new for the biosciences. It is not a simplification to say that the "mass spectrometer" in this instance is merely a detector that quantitatively informs on the metal composition of the tags in the sample.
Q: What is an Inductively Coupled Plasma Mass Spectrometer?
A: ICP-MS uses an inductively coupled plasma ionization source linked to a mass spectrometer. Briefly, a sample, most commonly an aerosol produced by nebulization, is injected into a high- temperature plasma obtained by heating a flowing argon gas stream with radio frequency (RF) energy. Under conditions approximating those at the surface of the sun, the sample is promptly vaporized, atomized and ionized as it flows through the plasma. High speed mass analysis provides a "mass fingerprint" that identifies the elements contained in the sample.
Q: Is compensation really not required in mass cytometry?
A: The abundance sensitivity (overlap of one detection, or mass, channel on the neighbor channel) is less than one part in 10000 so that correction for overlapping signals is not required for relative biomarker concentrations less than 4 or 6 orders of magnitude; no overlap is possible when alternate mass channels are used.
Q: But isn't the real limitation to multiplex analysis the fact that antibodies are cross-reactive?
A: This clearly remains the fundamental limitation to wide dynamic range, massively multiplexed immunoassay. In recent years, antibody manufacturers have responded to the need for multiplexing by taking a responsible approach to verifying the independence of suites of antibodies for specific diagnostic challenges, and this is expected to continue so that multiplexing becomes increasingly viable.
Q: What do the products look like?
A: MAXPAR® comprises a system of novel reagents designed specifically for massively multi-parametric biological assays. The core of the technology is a metalopolymeric tag that tightly binds (KD <10-16) the reporter stable isotopes and is conjugated to the affinity reagent via a linker group. The basic MAXPAR® kit contains enough material to label 100 µg of antibody with a customer-specified metal tag, as well as buffers and instructions; not included are TCEP and 30K spin filters.