The sample must contain only one protein, or else ambiguous data will be obtained. Peptides produced by digestion of a protein have unmodified N-termini and thus can readily be sequenced. Our instrument can readily sequence 10 pmole of standard protein, but we suggest supplying more sample if it is readily available, because sample losses during handling often reduce the amount we receive from what people estimate. The length of sequence obtained may be limited by sample amount, but is always limited by the properties of the protein. Side reactions increase the background of amino acids and decrease the amount of amino acid released from the protein.
Large proteins give more background signal than smaller proteins.
Our best run was about fifty amino acids. Edman chemistry can identify radiolabelled phosphorylation sites in peptides. Other modified amino acids may not be identified but their presence inferred by the lack of data. Cysteine cannot be positively identified unless it is alkylated. Available is a document with a description of sample preparation methods including many references and an explanation of the process of protein sequencing.