Mrs/Miss Stromer,

This you should know from your BSc basic cources (parts I and II) in 'Instrumental analysis' (or Analytical instrumentation). In many Institutions even there are parts I-II for undergraduate students plus part III for MSc-students. Or your question is associated with 'basic knowledge'.

ICP-MS is used for sensitive detection of compounds containing individual elements of interest, while ESI-MS is used for identification of compounds via determination of molecular weight. So, the data sets obtained will obviously vary and can not be compared.

The detection of an element in a particular compound definitely requires a higher mass detection sensitivity (that's ICP-MS) than does the detection of the whole molecule (case in point being ESI-MS).

Hence, the use of any of the two methods will entirely be dependent on the targeted outcome of your study.

Bobbi,

ICP = Inductively Coupled Plasma usually Argon which is pretty much the same temperature of the surface of the sun 10,000K - this is why it is good for atomic emission spectroscopy.  It pretty much vaporizes anything that comes in contact with it and you get elemental detection - for metals - Cadmium, chromium..etc. 

ESI - electrospray ionization is considered a "soft ionization" technique which creates a charged or ionized molecule that can be then manipulated into mass spectrometer using various voltages, vacuum, and eventually RF/DC currents on quadruples to isolate and detect certain masses.

Are you sure you mean ICP or are you referring to IC (Ion chromatography?)

Thank you for your responses everyone,

Peter, I do mean ICP. It is the only difference between the two instruments I could identify. If I am reading the procedure correctly, they both have HPLC columns in front of the ionizer and as mentioned the samples are from the same vials. 

HPLC is one of the sample introduction systems which both ICP-MS and ESI-MS can coupled to. To simplify, ICP-MS is for elements, up to mass 250, for those guys you find in the periodic table. ESI-MS is for molecules, capable of much higher mass range. This is simply because ESI is a "soft" ionization process compared to the ICP which changes your samples to atoms and then ions.

ICP is one of the most destructive ionization methods known to man, because it completely burns the sample at very high temperature. All the original speciation and oxidation states in your original liquid sample are wiped out under the conditions of ICP. Therefore it is well-suited only for metals detection and quantification (let's say for soil analysis, etc.). For many metals, ICP-MS is extremely sensitive. ICP-MS spectra are generally very simple (unless you have a complex mixture), and the heaviest ion you will see is pretty much UO+. No organic or biological samples can be analyzed by ICP with the expectation of getting meaningful results (unless of course you wish to detect the metal ions in metalloproteins and the like). So-called ESI on the other hand is a powerful "soft" ionization technique (as mentioned above) that can handle a very wide array of liquid samples of just about any origin, so long as they are soluble in water, acetonitrile, methanol or similar solvents and mixtures thereof, with or without additives such as buffers. The mass range can be up to hundreds of thousands of Daltons. If you wish to become a regular practitioner of MS, you should read a standard textbook on the subject, to see what it can do for you. See for example Jurgen Gross's book (of which there will be a third edition coming soon).

The question is only sensible if the sample contains an element that can be detected by ICP-MS, usually a metal (easily ionizable). It is an extremely sensitive technique (the detection limits are very low). In ESI-MS, the molecule should show up as (M+1)+ in positive mode, or (M-1)- in negative mode. If the molecule M contains only C, H, O, N, no specific signal will be generated in ICP-MS, because of the organic solvent of HPLC. The sensitivity of ESI-MS is highly dependent on the compound, pH of the solvent, additive or matrix. 

Thank you all for your answers. After reading all you have had to say, I am lead to conclude that there is a misunderstanding about which instrument was used as you all have confirmed my (rusty) understanding that ICP could not be used on these samples (all C,H,N, and O).