Although Quality and SPC are sometimes used interchangeably, they are different. Quality is very broad and includes much more than just SPC, while SPC can be considered a tool in the Quality process.
A quick example may help to point out the difference. If product in the warehouse is going bad over time, then a process has to start to narrow in on the cause. It will involve brainstorming, perhaps creating fishbone diagrams to determine the possible causes. In the case of an off-color product, it could be rust building up in pipes, chemical formulation changes, or different raw materials being used. This part of the example refers to quality. Unlike SPC, quality requires more than installing software, collecting samples and analyzing the results.
Once the most likely causes of the off-color product have been determined, SPC can be used to monitor the attributes and narrow down and isolate the cause. It may be determined that when the pH of a sub-ingredient falls out of a certain range, the stability of the product color is degraded. With this knowledge, SPC can be used to monitor the pH so that if it falls outside of range, it can be corrected quickly. This prevents a bigger problem that may arise when the product stays in the warehouse for a period of time.